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Sample records for neoproterozoic sedimentary successions

  1. Sedimentary talc in Neoproterozoic carbonate successions

    NASA Astrophysics Data System (ADS)

    Tosca, N. J.; MacDonald, F. A.; Strauss, J. V.; Johnston, D. T.; Knoll, A. H.

    2010-12-01

    Talc, Mg3Si4O10(OH)2, is normally considered a high temperature mineral and usually forms from hydrothermal alteration or metamorphism of ultramafic rocks. Talc has also been identified in evaporites, but is more scarce in association with carbonate rocks. Here we report the unusual occurrence of early diagenetic talc associated with early Neoproterozoic (~800-700 Ma) carbonate deposited on two separate platform margins. In the Akademikerbreen Group of Svalbard, the talc occurs as nodules that pre-date microspar cements, filling molar tooth structures and primary porosity in stromatolitic carbonates. In the upper Fifteenmile Group of the Ogilvie Mtns, NW Canada, the talc is present as nodules, coated grains, rip-up clasts and massive beds that are several meters thick and can be traced laterally over 150 km. All of the Neoproterozoic talc samples exhibit a high degree of stacking disorder and are essentially turbostratic, with no mixed layering or occurrences with sepiolite. Sedimentary talc requires a specific set of chemical conditions for its precursors and formation, and its occurrence on two margins suggests a link (direct or indirect) to contemporaneous ocean chemistry. At low temperatures, the Mg-silicate system is controlled mainly by kinetic phenomena and constraints on early diagenetic chemistry are difficult to derive based on thermodynamics alone. To address this problem, we designed an experimental program to evaluate the roles of pH, Mg2+ and SiO2(aq) on the formation of Mg-silicates from low-SO4 (~2.8 mmol/kg), Neoproterozoic-like seawater. Our experiments reveal a sharp and reproducible pH boundary (at ~ 8.66), only above which does poorly crystalline Mg-silicate precipitate. Increasing Mg2+ and/or SiO2(aq) alone is insufficient to nucleate the material; the pH control can be explained by Mg-silica complexing activated by the deprotonation of silicic acid above pH ~8.6-8.7. FT-IR, TEM and XRD analyses of the primary precipitate reveal a talc-like 2

  2. Sedimentary talc in Neoproterozoic carbonate successions

    NASA Astrophysics Data System (ADS)

    Tosca, Nicholas J.; Macdonald, Francis A.; Strauss, Justin V.; Johnston, David T.; Knoll, Andrew H.

    2011-06-01

    Mineralogical, petrographic and sedimentological observations document early diagenetic talc in carbonate-dominated successions deposited on two early Neoproterozoic (~ 800-700 million years old) platform margins. In the Akademikerbreen Group, Svalbard, talc occurs as nodules that pre-date microspar cements that fill molar tooth structures and primary porosity in stromatolitic carbonates. In the upper Fifteenmile Group of the Ogilvie Mountains, NW Canada, the talc is present as nodules, coated grains, rip-up clasts and massive beds that are several meters thick. To gain insight into the chemistry required to form early diagenetic talc, we conducted precipitation experiments at 25 °C with low-SO 4 synthetic seawater solutions at varying pH, Mg 2+ and SiO 2(aq). Our experiments reveal a sharp and reproducible pH boundary (at ~ 8.7) only above which does poorly crystalline Mg-silicate precipitate; increasing Mg 2+ and/or SiO 2(aq) alone is insufficient to produce the material. The strong pH control can be explained by Mg-silica complexing activated by the deprotonation of silicic acid above ~ 8.6-8.7. FT-IR, TEM and XRD of the synthetic precipitates reveal a talc-like 2:1 trioctahedral structure with short-range stacking order. Hydrothermal experiments simulating burial diagenesis show that dehydration of the precipitate drives a transition to kerolite (hydrated talc) and eventually to talc. This formation pathway imparts extensive layer stacking disorder to the synthetic talc end-product that is identical to Neoproterozoic occurrences. Early diagenetic talc in Neoproterozoic carbonate platform successions appears to reflect a unique combination of low Al concentrations (and, by inference, low siliciclastic input), near modern marine salinity and Mg 2+, elevated SiO 2(aq), and pH > ~ 8.7. Because the talc occurs in close association with microbially influenced sediments, we suggest that soluble species requirements were most easily met through microbial influences on

  3. Biostratigraphic and chemostratigraphic correlation of Neoproterozoic sedimentary successions: upper Tindir Group, northwestern Canada, as a test case

    NASA Technical Reports Server (NTRS)

    Kaufman, A. J.; Knoll, A. H.; Awramik, S. M.

    1992-01-01

    Recent advances in Proterozoic micropaleontology and sedimentary isotope geochemistry suggest that improved interbasinal correlation of Neoproterozoic (1000-540 Ma) successions is possible. Because widely varying interpretations of its age have been suggested and no reliable radiometric dates or paleomagnetic data are available, the upper Tindir Group of northwestern Canada provides an opportunity to test this hypothesis. The age of these strata is of paleontological importance because silicified carbonates near the top of the group contain disc-shaped-scale microfossils that may provide insights into the early evolution of biomineralization. A reinterpretation of upper Tindir microfossil assemblages suggests a late Riphean age. Although diagenesis and contact metamorphism have altered the isotopic compositions of some carbonates, least altered samples indicate that delta 13C of contemporaneous seawater was at least +4.7%, typical of Neoproterozoic, but not Cambrian, carbonates. Strontium isotopic compositions of the least altered samples yield values of approximately 0.7065, which can be uniquely correlated with late Riphean seawater. Together, micropaleontology and the isotopic tracers of C and Sr constrain the upper Tindir carbonates and their unique fossils to be late Riphean, likely between 620 and 780 Ma.

  4. Biostratigraphic and chemostratigraphic correlation of Neoproterozoic sedimentary successions: upper Tindir Group, northwestern Canada, as a test case.

    PubMed

    Kaufman, A J; Knoll, A H; Awramik, S M

    1992-02-01

    Recent advances in Proterozoic micropaleontology and sedimentary isotope geochemistry suggest that improved interbasinal correlation of Neoproterozoic (1000-540 Ma) successions is possible. Because widely varying interpretations of its age have been suggested and no reliable radiometric dates or paleomagnetic data are available, the upper Tindir Group of northwestern Canada provides an opportunity to test this hypothesis. The age of these strata is of paleontological importance because silicified carbonates near the top of the group contain disc-shaped-scale microfossils that may provide insights into the early evolution of biomineralization. A reinterpretation of upper Tindir microfossil assemblages suggests a late Riphean age. Although diagenesis and contact metamorphism have altered the isotopic compositions of some carbonates, least altered samples indicate that delta 13C of contemporaneous seawater was at least +4.7%, typical of Neoproterozoic, but not Cambrian, carbonates. Strontium isotopic compositions of the least altered samples yield values of approximately 0.7065, which can be uniquely correlated with late Riphean seawater. Together, micropaleontology and the isotopic tracers of C and Sr constrain the upper Tindir carbonates and their unique fossils to be late Riphean, likely between 620 and 780 Ma. PMID:11537751

  5. Sedimentary genesis and lithostratigraphy of Neoproterozoic megabreccia from Mufulira, Copperbelt of Zambia

    NASA Astrophysics Data System (ADS)

    Wendorff, Marek

    2005-07-01

    The Lufilian arc is an orogenic belt in central Africa that extends between Zambia and the Democratic Republic of Congo (DRC) and deforms the Neoproterozoic-Lower Palaeozoic metasedimentary succession of the Katanga Supergroup. The arc contains thick bodies of fragmental rocks that include blocks reaching several kilometres in size. Some megablocks contain Cu and Cu-Co-mineralised Katangan strata. These coarse clastic rocks, called the Katangan megabreccias, have traditionally been interpreted in the DRC as tectonic breccias formed during Lufilian orogenesis due to friction underneath Katangan nappes. In mid-90th, several occurrences in Zambia have been interpreted in the same manner. Prominent among them is an occurrence at Mufulira, considered by previous workers as a ≈1000 m thick tectonic friction breccia containing a Cu-Co-mineralised megablock. This paper presents new results pertaining to the lower stratigraphic interval of the Katanga Supergroup at Mufulira and represented by the Roan Group and the succeeding Mwashya Subgroup of the Guba Group. The interval interpreted in the past as tectonic Roan megabreccia appears to be an almost intact sedimentary succession, the lower part of which consists of Roan Group carbonate rocks with siliciclastic intercalations containing several interbeds of matrix-supported conglomerate. A Cu-Co-mineralised interval is not an allochthonous block but a part of the stratigraphic succession underlain and overlain by conglomerate beds, which were considered in the past as tectonic friction breccias. The overlying megabreccia is a syn-rift sedimentary olistostrome succession that rests upon the Roan strata with a subtle local unconformity. The olistostrome succession consists of three complexes typified by matrix-supported debris-flow conglomerates with Roan clasts. Some of the conglomerate beds pass upwards to normally graded turbidite layers and are accompanied by solitary slump beds. The three conglomeratic assemblages are

  6. Provenance of Neoproterozoic sedimentary basement of northern Iran, Kahar Formation

    NASA Astrophysics Data System (ADS)

    Etemad-Saeed, Najmeh; Hosseini-Barzi, Mahboubeh; Adabi, Mohammad Hossein; Sadeghi, Abbas; Houshmandzadeh, Abdolrahim

    2015-11-01

    This article presents new data to understand the nature of the hidden crystalline basement of northern Iran and the tectonic setting of Iran during late Neoproterozoic time. The siliciclastic-dominated Kahar Formation represents the oldest known exposures of northern Iran and comprises late Ediacaran (ca. 560-550 Ma) compositionally immature sediments including mudrocks, sandstones, and conglomerates. This work focuses on provenance of three well preserved outcrops of this formation in Alborz Mountains: Kahar Mountain, Sarbandan, and Chalus Road, through petrographic and geochemical methods. Mineralogical Index of Alteration (MIA) and Chemical Index of Alteration (CIA-after correction for K-metasomatism) values combined with A-CN-K relations suggest moderate weathering in the source areas. The polymictic nature of Kahar conglomerates indicates a mixed provenance for them. However, modal analysis of Kahar sandstones (volcanic to plagioclase-rich lithic arkose) and whole rock geochemistry of mudrocks suggest that they are largely first-cycle sediments and that their sources were remarkably late Ediacaran, intermediate-felsic igneous rocks from proximal arc settings. Tectonic setting discrimination diagrams also indicate a convergent plate margin and continental arc related basin for Kahar sediments. This interpretation is supported by the phyllo-tectic to tectic composition and geochemistry of mudrocks. These results reveal the presence of a felsic/intermediate subduction-related basement (∼600-550 Ma) in this region, which provides new constraints on subduction scenario during this time interval in Iran, as a part of the Peri-Gondwanan terranes.

  7. Derivation of S and Pb in phanerozoic intrusion-related metal deposits from neoproterozoic sedimentary pyrite, Great Basin, United States

    USGS Publications Warehouse

    Vikre, P.G.; Poulson, S.R.; Koenig, A.E.

    2011-01-01

    The thick (???8 km), regionally extensive section of Neoproterozoic siliciclastic strata (terrigenous detrital succession, TDS) in the central and eastern Great Basin contains sedimentary pyrite characterized by mostly high d34S values (-11.6 to 40.8%, <70% exceed 10%; 51 analyses) derived from reduction of seawater sulfate, and by markedly radiogenic Pb isotopes ( 207Pb/204Pb <19.2; 15 analyses) acquired from clastic detritus eroded from Precambrian cratonal rocks to the east-southeast. In the overlying Paleozoic section, Pb-Zn-Cu-Ag-Au deposits associated with Jurassic, Cretaceous, and Tertiary granitic intrusions (intrusion-related metal deposits) contain galena and other sulfide minerals with S and Pb isotope compositions similar to those of TDS sedimentary pyrite, consistent with derivation of deposit S and Pb from TDS pyrite. Minor element abundances in TDS pyrite (e.g., Pb, Zn, Cu, Ag, and Au) compared to sedimentary and hydrothermal pyrite elsewhere are not noticeably elevated, implying that enrichment in source minerals is not a precondition for intrusion-related metal deposits. Three mechanisms for transferring components of TDS sedimentary pyrite to intrusion-related metal deposits are qualitatively evaluated. One mechanism involves (1) decomposition of TDS pyrite in thermal aureoles of intruding magmas, and (2) aqueous transport and precipitation in thermal or fluid mixing gradients of isotopically heavy S, radiogenic Pb, and possibly other sedimentary pyrite and detrital mineral components, as sulfide minerals in intrusion-related metal deposits. A second mechanism invokes mixing and S isotope exchange in thermal aureoles of Pb and S exsolved from magma and derived from decomposition of sedimentary pyrite. A third mechanism entails melting of TDS strata or assimilation of TDS strata by crustal or mantle magmas. TDS-derived or assimilated magmas ascend, decompress, and exsolve a mixture of TDS volatiles, including isotopically heavy S and radiogenic Pb

  8. Chemostratigraphy of early Neoproterozoic sedimentary rocks of Yenisei ridge (Siberia, Russia)

    NASA Astrophysics Data System (ADS)

    Vishnevskaya, Irina; Pisareva, Natalia; Kanygina, Nadejda; Proshenkin, Artem

    2014-05-01

    One of the biggest Proterozoic sedimentary basins in Russia is around the Siberian platform. This study about little part of them - Neoproterozoic sedimentary rocks of Yenisei ridge (Southwestern margin of Siberian Platform). Thise geological structure is ancient and very difficult for reaserch. It is a collage of different blocks: volcanic arcks, ophiolite complexes and sedimentary rocks of various ages and degrees of metamorphism. Sedimentary complexes of Siberian platform are outcropping along Angara River and its tributary. Neoproterozoic ones are presented by terrigenous-carbonate rocks of Tungusik and Oslyan groups. Despite the long study history of the area is still controversial question of time of formation of these rocks. As determination of the age of Precambrian sedimentary rocks is very difficult, Sr isotopic chemostratigraphy appears to be the only approach to establish the age of carbonate sequences. All Rb-Sr author's data was investigated by the method of selective dissolution with the preliminary removal of epigenetic carbonate phases. The isotope dilution method with mixed 87Rb + 84Sr spike was used to determine Rb and Sr concentrations in both fractions on the MI 1201AT mass spectrometer. Sr isotope ratios were measured on the Finnigan MAT-262 (BAC CU, Irkutsk, Russia) and Triton Plus (IGG UB RAS, Ekaterinburg, Russia). The C-O isotopic composition in carbon samples was measured on the Finnigan MAT-253 equipment. The main criteria for integrity were correlations of impurity-elements (Mn, Fe, Sr) and stable isotopes (C, O) with each other. The less altered rocks of the Tungusik Group are characterized by 87Sr/86Sr ratio of 0.7055-0.7058, and wide variations in the δ13CPDB values from 0 to +5o [1]. The primary 87Sr/86Sr of Dashka Formation (Oslyan Group) is 0.7057 - 0.7060 and δ13CPDB value varies in interval 3.7-4.3o like in upper part of Tungusik Group. High positive values of δ13CPDB indicate that carbonates had accumulated in warm sea

  9. The paleoclimatic significance of deformation structures in Neoproterozoic successions

    NASA Astrophysics Data System (ADS)

    Arnaud, Emmanuelle

    2012-01-01

    This paper reviews the different types of soft sediment deformation structures that can form in glacial and non-glacial settings and explores the potential use of these structures in resolving long standing debates in paleoenvironmental reconstructions of Neoproterozoic glacigenic successions. Soft sediment deformation structures are created when compressional, gravitational or shear stress is applied to unlithified sediments during or shortly after deposition. In subglacial or ice marginal glacial settings, shear and compressional stress imparted by ice moving on top of a deformable substrate or advancing ice buldozing unlithified ice marginal sediments can result in a wide range of folding, faulting and shear structures. In glaciofluvial or stagnant ice marginal setting, gravitational collapse and remobilization of sediments associated with the melting of buried ice can result in normal faulting and broad folding. In glaciolacustrine or glaciomarine settings, compressional, shear and gravitational types of deformation structures can occur as a result of grounding ice or icebergs, rapid sedimentation and reworking downslope associated with high sedimentation rates. In non glacial settings, similar deformation structures can form as a result of slope instability and reworking of sediments downslope, rapid sedimentation, seismic shaking, wave induced shearing or loading. In this context, two case studies are presented to demonstrate the type of paleoenvironmental information that an analysis of deformation structures can provide. In the first case study, analysis of deformation in the Port Askaig Formation (Scotland) reveals a distinctive stratigraphic distribution of deformation structures. The types of deformation observed together with their recurrence over several 100s of meters and their basinal context are used to infer a seismic origin for the deformation, which in turn suggests a significant tectonic control on sedimentation atop a record of ice margin

  10. Neoproterozoic active continental margin in the southeastern Yangtze Block of South China: Evidence from the ca. 830-810 Ma sedimentary strata

    NASA Astrophysics Data System (ADS)

    Wang, Wei; Zhou, Mei-Fu; Zhao, Jun-Hong; Pandit, Manoj K.; Zheng, Jian-Ping; Liu, Ze-Rui

    2016-08-01

    The Jiangnan Fold Belt in the South China Block has been traditionally assumed to be Mesoproterozoic in age and related to the global Grenville orogeny. Sedimentary successions in the Jiangnan Fold Belt archive direct record of tectonic evolution; however, they have not yet been evaluated properly. The Lushan massif, comprising Kangwanggu and Xingzi groups, is the major basement complex in the Jiangnan Belt. Regional correlation of these two groups is poorly constrained, such as with the Shuangqiaoshan group, and thus their role in the regional tectonic evolution is not clear. Detrital zircon U-Pb ages suggest that the Xingzi and Kangwanggu groups were deposited at 820-810 and ca. 830 Ma, respectively. They are composed of dominantly felsic to intermediate volcanic detritus, as indicated by the relatively high Th/Cr (0.24-0.06) ratios and radiogenic Nd isotopes (εNd(t) values = + 1.5 to - 2.9) of the sedimentary rocks. An overwhelming abundance of Neoproterozoic (ca. 860-810 Ma) angular, detrital zircon grains in both the groups indicates derivation chiefly from locally distributed syn-sedimentary igneous rocks. A predominance of zircons with ages close to the time of deposition implies a convergent plate margin setting for Kangwanggu and Xingzi groups. Geochemical signatures, such as La-Th-Co and Th-Sc-Zr/10 plots for Xingzi and Kangwanggu sedimentary rocks also underline tectonically active settings, consistent with the arc affinity of the associated mafic and felsic volcanic rocks. In contrast to the dominant Neoproterozoic detritus in the Kangwanggu sandstone, argillaceous rocks of the Xingzi group received additional input of pre-Neoproterozoic detritus. Moreover, the Xingzi argillaceous rocks have εNd(t) values (+ 0.9 to - 2.9) slightly lower than those of the Kangwanggu sandstones (+ 1.5 to 0.0), indicating contribution from mature crustal materials exposed during progressive uplift of continental basement during orogenesis. These features suggest the

  11. The Mintom Formation (new): Sedimentology and geochemistry of a Neoproterozoic, Paralic succession in south-east Cameroon

    NASA Astrophysics Data System (ADS)

    Caron, V.; Ekomane, E.; Mahieux, G.; Moussango, P.; Ndjeng, E.

    2010-06-01

    This paper presents a lithologic and stratigraphic description of the Neoproterozoic (ante- or syn- Pan-African orogeny) Mintom Formation (new) of southeastern Cameroon, and provides a new facies and geochemical analysis of the sedimentary succession, formerly referred to as the upper Dja series. The Mintom Formation can be subdivided from base to top into four members that record a general increase in carbonate content. The members (all new) from lower to upper are: Kol Member (diamictite and pelite), Metou Member (dolostone), Momibolé Member (calcareous pelite), and Atog Adjap Member (limestone). Although the lithostratigraphic architecture looks very similar to that of well-documented syn- and post-glacial Neoproterozoic deposits, physical evidence of glacial influence is absent. By contrast with other Central African Neoproterozoic carbonates deposited in ramp settings, the succession does not contain open marine facies. Limestones consist of monotonous subhedral microsparitic calcite mosaics and display occasional microbial laminae. These observations force reevaluation of both previous paleoenvironmental interpretations of the deposits and their comparison with neighboring Ediacaran carbonates. We assume that the graded basal succession from diamictite to laminated pelitic facies is compatible with emplacement of mass flow deposits in toe-of-slope setting during regional uplift. Interpretation of the overlying Métou dolostone is uncertain though sedimentological and geochemical properties point to a likely quiet depositional setting. The upper part of the Formation, including the Momibolé and Atog Adjap Members, is conspicuously laminated, in places rhythmically and ripple-bedded, suggesting shallow subaqueous and calm depositional conditions only interrupted by occasional slumps indicative of a locally steepened bottom topography. Evaporitic fabrics and fenestral pores further indicate shallow water, possibly peritidal, environmental conditions. In spite

  12. Sedimentary and tectonic history of the Holowilena Ironstone, a Neoproterozoic iron formation in South Australia

    NASA Astrophysics Data System (ADS)

    Lechte, Maxwell Alexander; Wallace, Malcolm William

    2015-11-01

    The Holowilena Ironstone is a Neoproterozoic iron formation in South Australia associated with glacial deposits of the Sturtian glaciation. Through a comprehensive field study coupled with optical and scanning electron microscopy, X-ray fluorescence, and X-ray diffraction, a detailed description of the stratigraphy, sedimentology, mineralogy, and structure of the Holowilena Ironstone was obtained. The Holowilena Ironstone comprises ferruginous shales, siltstones, diamictites, and is largely made up of hematite and jasper, early diagenetic replacement minerals of precursor iron oxyhydroxides, and silica. These chemical precipitates are variably influenced by turbidites and debris flows contributing clastic detritus to the depositional system. Structural and stratigraphic evidence suggests deposition within a synsedimentary half-graben. A model for the Holowilena Ironstone is proposed, in which dense oxic fluids expelled during sea ice formation in the Cryogenian pool in the depression of the half-graben, allowing for long-lived mixing with the ferruginous seawater and the deposition of iron oxides. This combination of glacial dynamics, tectonism, and ocean chemistry may explain the return of iron formations in the Neoproterozoic.

  13. Geology of East Egypt greenstone field in Neoproterozoic isoand arc: Reconstruction of Iron formation sedimentary environment.

    NASA Astrophysics Data System (ADS)

    Kiyokawa, S.; Suzuki, T.

    2015-12-01

    Geology of East Egypt greenstone-granit belt which is northern part of Nubia shield was identified neoproterozoic island arc amalgamated sections. There are several iron formation within these greenstone belt. Age data shows this iron formation may be overlaped during 700 Ma Snowball period, how ever, there is no detail report of well preserved ice related evidences. We now started detail field work for identified tectonic reconstruction, original stratigraphy around Iron formation and sedimentary environment during the iron formation sedimentation area. East Egyptian shield was divided three geology, Proterozoic greenstone complex, 700-600 Granitic domes and cover sequence (Hammamet Group). We focus three area to identified sedimentary environment of iron sedimentation. Along the north-south trend of Wadi EL Dabban area are, we named Wadi branch as West site is RW-0 ~ 12, East site is RE-0 ~ 12 from north to south. Northern area is structurally moderate, southern portion is north dipping. Southern portion was intruded by granite and several place contain granitic dikes. Northeast to eastern area are identified younger sedimentary sequence (Hammamat Group) which is unconformablly overlay on the other iron formation bearing greenstone belt. Structurally these area is divided four units. Wadi was divided by right-lateral strike-ship fault. The displacement are more than 3 km. Also north dipping faults are identified.East-West trend fault are divided two units. It is divided NE, SE, NW and NS units.SW unit is most well preserved thick sequence of the Iron formation. SW unit is well preserved iron formation sequence within thick volcaniclastics. This unit mostly north dipping around 40-60 degree. Structural repetition in not well understand. Reconstract stratigraphy in this unit is at least 4000m in thickness. 5 member is identified in this sequence. Several thin iron formations are observed with in pillow lava and volcaniclastic sequence. These very thick

  14. Neoproterozoic diamictite-cap carbonate succession and δ13C chemostratigraphy from eastern Sonora, Mexico

    USGS Publications Warehouse

    Corsetti, Frank A.; Stewart, John H.; Hagadorn, James W.

    2007-01-01

    Despite the occurrence of Neoproterozoic strata throughout the southwestern U.S. and Sonora, Mexico, glacial units overlain by enigmatic cap carbonates have not been well-documented south of Death Valley, California. Here, we describe in detail the first glaciogenic diamictite and cap carbonate succession from Mexico, found in the Cerro Las Bolas Group. The diamictite is exposed near Sahuaripa, Sonora, and is overlain by a 5 m thick very finely-laminated dolostone with soft sediment folds. Carbon isotopic chemostratigraphy of the finely-laminated dolostone reveals a negative δ13C anomaly (down to − 3.2‰ PDB) characteristic of cap carbonates worldwide. Carbon isotopic values rise to + 10‰ across ∼ 400 m of section in overlying carbonates of the Mina el Mezquite and Monteso Formations. The pattern recorded here is mostly characteristic of post-Sturtian (ca. ≤ 700 Ma), but pre-Marinoan (ca. ≥ 635 Ma) time. However, the Cerro Las Bolas Group shares ambiguity common to most Neoproterozoic successions: it lacks useful radiometric age constraints and biostratigraphically useful fossils, and its δ13C signature is oscillatory and therefore somewhat equivocal.

  15. Tectonostratigraphy and depositional history of the Neoproterozoic volcano-sedimentary sequences in Kid area, southeastern Sinai, Egypt: Implications for intra-arc to foreland basin in the northern Arabian-Nubian Shield

    NASA Astrophysics Data System (ADS)

    Khalaf, E. A.; Obeid, M. A.

    2013-09-01

    This paper presents a stratigraphic and sedimentary study of Neoproterozoic successions of the South Sinai, at the northernmost segment of the Arabian-Nubian Shield (ANS), including the Kid complex. This complex is composed predominantly of thick volcano-sedimentary successions representing different depositional and tectonic environments, followed by four deformational phases including folding and brittle faults (D1-D4). The whole Kid area is divisible from north to south into the lower, middle, and upper rock sequences. The higher metamorphic grade and extensive deformational styles of the lower sequence distinguishes them from the middle and upper sequences. Principal lithofacies in the lower sequence include thrust-imbricated tectonic slice of metasediments and metavolcanics, whereas the middle and upper sequences are made up of clastic sediments, intermediate-felsic lavas, volcaniclastics, and dike swarms. Two distinct Paleo- depositional environments are observed: deep-marine and alluvial fan regime. The former occurred mainly during the lower sequence, whereas the latter developed during the other two sequences. These alternations of depositional conditions in the volcano-sedimentary deposits suggest that the Kid area may have formed under a transitional climate regime fluctuating gradually from warm and dry to warm and humid conditions. Geochemical and petrographical data, in conjunction with field relationships, suggest that the investigated volcano-sedimentary rocks were built from detritus derived from a wide range of sources, ranging from Paleoproterozoic to Neoproterozoic continental crust. Deposition within the ancient Kid basin reflects a complete basin cycle from rifting and passive margin development, to intra-arc and foreland basin development and, finally, basin closure. The early phase of basin evolution is similar to various basins in the Taupo volcanics, whereas the later phases are similar to the Cordilleran-type foreland basin. The

  16. Neoproterozoic diamictite-bearing sedimentary rocks in the northern Yili Block and their constraints on the Precambrian evolution of microcontinents in the Western Central Asian Orogenic Belt

    NASA Astrophysics Data System (ADS)

    He, Jingwen; Zhu, Wenbin; Zheng, Bihai; Wu, Hailin; Cui, Xiang; Lu, Yuanzhi

    2015-12-01

    The origin and tectonic setting of Precambrian sequences in the Central Asian Orogenic Belt (CAOB) have been debated due to a lack of high resolution geochronological data. Answering this question is essential for the understanding of the tectonic framework and Precambrian evolution of the blocks within the CAOB. Here we reported LA-ICP-MS detrital zircon U-Pb ages and in-situ Hf isotopic data for Neoproterozoic sedimentary cover in the northern Yili Block, an important component of the CAOB, in order to provide information on possible provenance and regional tectonic evolution. A total of 271 concordant U-Pb zircon ages from Neoproterozoic sedimentary cover in the northern Yili Block define three major age populations of 1900-1400 Ma, 1300-1150 Ma and 700-580 Ma, which are quite different from cratons and microcontinents involved in the CAOB. Although it is not completely consistent with the local basement ages, an autochthonous provenance interpretation is more suitable. Some zircon grains show significant old Hf model ages (TDMC; 3.9-2.4 Ga) and reveal continental crust as old as Paleoarchean probably existed. Continuous Mesoproterozoic zircon age populations exhibit large variations in the εHf(t) ratios, suggesting the long-time involvement of both reworked ancient crust and juvenile material. Similar Mesoproterozoic evolution pattern is identified in many continental terranes involved in the CAOB that surround the Tarim Craton. Based on our analysis and published research, we postulate that the northern Yili Block, together with Chinese Central Tianshan, Kyrgyz North Tianshan and some other microcontinents surrounding the Tarim Craton, once constituted the continental margin of the Tarim Craton in the Mesoproterozoic, formed by long-lived accretionary processes. Most of the late Neoproterozoic zircons exhibit significant positive εHf(t) ratios, suggesting the addition of juvenile crust. It is consistent with the tectonic event related to the East Africa

  17. The tectonic evolution of the Neoproterozoic Brasília Belt, central Brazil, based on SHRIMP and LA-ICPMS U-Pb sedimentary provenance data: A review

    NASA Astrophysics Data System (ADS)

    Pimentel, Márcio M.; Rodrigues, Joseneusa B.; DellaGiustina, Maria Emilia S.; Junges, Sergio; Matteini, Massimo; Armstrong, Richard

    2011-04-01

    The Brasília Belt is a Neoproterozoic orogenic belt in central Brazil, developed between the Amazon, São Francisco-Congo and Paranapanema cratons. It consists of a thick sedimentary pile, made up of several stratigraphic units, which have been deformed and metamorphosed along the western margin of the São Francisco Craton during the Brasiliano orogenic cycle. In the western part of the belt, a large, juvenile magmatic arc is exposed (the Goiás Magmatic Arc), consisting of calc-alkaline plutonic suites as well as volcano-sedimentary sequences, ranging in age between ca. 860 and 650 Ma. Regional-scale, west-dipping thrusts and reverse faults normally mark the limits between the main stratigraphic units, and clearly indicate tectonic transport towards the east. The age of deposition and tectonic significance of the sedimentary units comprising the Brasília Belt have been a matter of continuous debate over the last three decades. In the present paper, recent provenance data based on LA-ICPMS U-Pb ages of detrital zircon grains from several of these units, are reviewed and their significance for the age of deposition of the original sediments and tectonic evolution of the Brasília Belt are discussed. The Paranoá, Canastra and the Vazante groups, in the central part of the Belt, have detrital zircon grains with ages older than ca. 900 Ma and are interpreted as representative of the passive margin sequence deposited on the western margin of the São Francisco Craton. On the other hand, samples from the Araxá and Ibiá groups have a much younger population of Neoproterozoic zircon grains, as young as 650 Ma, and have been interpreted as syn-orogenic (fore-arc?) deposits. The Bambuí Group, exposed in the easternmost part of the belt and covering large areas of the São Francisco Craton also has young zircon grains and is interpreted, at least in part, as the foreland basin of the Brasília Belt.

  18. Acadian dextral transpression and synorogenic sedimentary successions in the Appalachians

    SciTech Connect

    Ferrill, B.A.; Thomas, W.A.

    1988-07-01

    The successive Seboomook-Littleton (northern Appalachians) and Catskill-Pocono (central Appalachians) clastic wedges suggest oblique convergence and southwestward migration of Acadian orogeny beginning in Early Devonian and continuing into Early Mississippian. Wrench-fault movement in Maritime Canada coincided with deposition of all but the earliest components of the Catskill-Pocono clastic wedge and continued into the Pennsylvanian. Contrasts between a thin, Lower to Middle Devonian shallow-shelf facies in the Alabama Appalachian fold-thrust belt and a time-equivalent, thick, shallowing-upward sedimentary to volcanic succession in the adjacent Talladega slate belt are interpreted to reflect a wrench-fault basin. A wrench-fault setting for Devonian rocks in Alabama integrated with manifestations of oblique convergence during the Acadian orogeny in the central and northern Appalachians can be accommodated in dextral transpression along the entire length of the Acadian Appalachian orogen.

  19. Chemostratigraphy of predominantly siliciclastic Neoproterozoic successions: a case study of the Pocatello Formation and Lower Brigham Group, Idaho, USA

    PubMed

    Smith, L H; Kaufman, A J; Knoll, A H; Link, P K

    1994-05-01

    Isotopic chemostratigraphy has proven successful in the correlation of carbonate-rich Neoproterozoic successions. In successions dominated by siliciclastic rocks, chemostratigraphy can be problematic, but if thin carbonates punctuate siliciclastic strata, useful isotopic data may be obtained. The upper Pocatello Formation and lower Brigham Group of southeastern Idaho provide an opportunity to assess the potential and limitations of isotopic chemostratigraphy in overwhelmingly siliciclastic successions. The 5000 m thick succession consists predominantly of siliciclastic lithologies, with only three intervals that contain thin intercalated carbonates. Its depositional age is only broadly constrained by existing biostratigraphic, sequence stratigraphic and geochronometric data. The lowermost carbonates include a cap dolomite atop diamictites and volcanic rocks of the Pocatello Formation. The delta 13C values of these carbonates are distintly negative (-5 to -3), similar to carbonates that overlie Neoproterozoic glaciogenic rocks worldwide. Stratigraphically higher carbonates record a major positive delta 13C excursion to values as high as +8.8 within the carbonate member of the Caddy Canyon Quartzite. The magnitude of this excusion is consistent with post-Sturtian secular variation recorded elsewhere in the North American Cordillera, Australia, Svalbard, Brazil and Nambia, and exceeds the magnitude of any post-Varanger delta 13C excursion documented to date. In most samples, Sr-isotopic abundances have been altered by diagenesis and greenschist facies metamorphism, but a least-altered value of approximately 0.7076 supports a post-Sturtian and pre-Marinoan/Varanger age for upper Pocatello and lower Brigham rocks that lie above the Pocatello diamictite. Thus, even though available chemostratigraphic data are limited, they corroborate correlations of Pocatello Formation diamictites and overlying units with Sturtian glaciogenic rocks and immediately post

  20. Geochemistry, geochronology, and Sr-Nd isotopes of the Late Neoproterozoic Wadi Kid volcano-sedimentary rocks, Southern Sinai, Egypt: Implications for tectonic setting and crustal evolution

    NASA Astrophysics Data System (ADS)

    Moghazi, Abdel-Kader M.; Ali, Kamal A.; Wilde, Simon A.; Zhou, Qin; Andersen, Tom; Andresen, Arild; Abu El-Enen, Mahrous M.; Stern, Robert J.

    2012-12-01

    The Kid Group is one of the few exposures of Neoproterozoic metavolcano-sedimentary rocks in the basement of southern Sinai in the northernmost Arabian-Nubian Shield. It is divided into the mostly metamorphosed volcaniclastic Melhaq and siliciclastic Um Zariq formations in the north and the mostly volcanic Heib and Tarr formations in the south. The Heib, Tarr, and Melhaq formations reflect an intense episode of igneous activity and immature clastic deposition associated with core-complex formation during Ediacaran time, but Um Zariq metasediments are relicts of an older (Cryogenian) sedimentary sequence. The latter yielded detrital zircons with concordant ages as young as 647 ± 12 Ma, which may indicate that the protolith of Um Zariq schist was deposited after ~ 647 Ma but 19 concordant zircons gave a 206Pb/238U weighted mean age of 813 ± 6 Ma, which may represent the maximum depositional age of this unit. In contrast, a cluster of 11 concordant detrital zircons from the Melhaq Formation yield a weighted mean 206Pb/238U age of 615 ± 6 Ma. Zircons from Heib Formation rhyolite clast define a 206Pb/238U weighted mean age of 609 ± 5 Ma, which is taken to approximate the age of Heib and Tarr formation volcanism. Intrusive syenogranite sample from Wadi Kid yields a 206Pb/238U weighted mean age of 604 ± 5 Ma. These constraints indicate that shallow-dipping mylonites formed between 615 ± 6 Ma and 604 ± 5 Ma. Geochemical data for volcanic samples from the Melhaq and Heib formations and the granites show continuous major and trace element variations corresponding to those expected from fractional crystallization. The rocks are enriched in large ion lithophile and light rare earth elements, with negative Nb anomalies. These reflect magmas generated by melting of subduction-modified lithospheric mantle, an inference that is further supported by ɛNd(t) = + 2.1 to + 5.5. This mantle source obtained its trace element characteristics by interaction with fluids and melts

  1. Provenance analysis and tectonic setting of late Neoproterozoic metasedimentary successions in NW Argentina

    NASA Astrophysics Data System (ADS)

    Do Campo, M.; Guevara, S. Ribeiro

    2005-06-01

    Major, trace, and rare-earth element data for Puncoviscana Formation metasediments were used to constrain their provenance and tectonic setting. This unit, which crops out in northwest Argentina, consists mainly of pelite-greywacke turbidite sequences. Incipient regional metamorphism and a polyphase deformation, with a main deformation during the latest Proterozoic-earliest Cambrian Braziliano orogeny, affected the sedimentary sequences. The enrichment in light rare-earth and other incompatible trace elements over compatible ones, as well as the high and uniform Th/Sc ratios, indicate a predominance of upper-crust acid rocks as parental material. Some chemical characteristics of these rocks, such as their high Th/U, Rb/Sr, and Zr/Sc ratios, imply sedimentary recycling. On the basis of tectonic discriminant diagrams that employ trace elements considered relatively immobile during low-grade metamorphism, a passive margin setting can be inferred. Moreover, the comparison of the trace element contents of Puncoviscana metapelites with those of mudstones deposited in known tectonic settings shows the closest matching with passive margin shales.

  2. Early Neoproterozoic Global Change Through the Lens of the Tambien Group, Northern Ethiopia

    NASA Astrophysics Data System (ADS)

    Swanson-Hysell, N.; Maloof, A. C.; Condon, D. J.; Park, Y.; MacLennan, S. A.; Schoene, B.; Tremblay, M. M.; Alene, M.; Anttila, E.; Haileab, B.; Tesema, T.

    2015-12-01

    The early Neoproterozoic is a crucial period in the evolution of life and climate on Earth. Basins that developed during the time contain a record of the diversification of eukaryotic life as well as large-scale changes to the carbon cycle and paleogeography during the period leading up to Cryogenian glaciation. Understanding global change leading up to Cryogenian glaciation is key for interpreting the boundary conditions that resulted in the beginning of dramatic climate and geochemical oscillations during this critical interval. Existing age models for Neoproterozoic nonglacial intervals, such as the time leading up to Cryogenian glaciation, largely have been based on correlation of carbonate δ13C values, but there are few tests of the assumed synchroneity of these records between basins. In contrast to the ash-poor successions typically targeted for Neoproterozoic chemostratigraphy, the Tonian to Cryogenian Tambien Group (Tigray region, Ethiopia) was deposited in an arc-proximal basin where volcanic tuffs suitable for U-Pb geochronology are preserved within the mixed carbonate-siliciclastic sedimentary succession. We use physical and isotopic stratigraphic data sets from Tambien Group sedimentary rocks in concert with high-precision U-Pb dates from intercalated tuffs to establish global synchroneity of large scale carbon isotopic change. These new temporal constraints strengthen the case for interpreting Neoproterozoic carbon isotope variation as a record of large-scale changes to the carbon cycle. Furthermore, these dates strengthen the temporal framework for interpreting paleogeographic change, geochemical cycling, and environmental evolution during the radiation of early eukaryotes.

  3. Sedimentary process control on carbon isotope composition of sedimentary organic matter in an ancient shallow-water shelf succession

    NASA Astrophysics Data System (ADS)

    Davies, S. J.; Leng, M. J.; Macquaker, J. H. S.; Hawkins, K.

    2012-11-01

    Source and delivery mechanisms of organic matter are rarely considered when interpreting changing δ13C through sedimentary successions even though isotope excursions are widely used to identify and correlate global perturbations in the carbon cycle. Combining detailed sedimentology and geochemistry we demonstrate how organic carbon abundance and δ13C values from sedimentary organic matter from Carboniferous-aged mudstones are influenced by the proportion of terrestrial versus water column-derived organic matter. Silt-bearing clay-rich shelf mudstones that were deposited by erosive density flows are characterized by 1.8-2.4% organic carbon and highδ13C values (averaging -22.9 ± 0.3‰, n = 12). Typically these mudstones contain significant volumes of terrestrial plant-derived material. In contrast, clay-rich lenticular mudstones, with a marine macrofauna, are the products of the transport of mud fragments, eroded from pre-existing water-rich shelfal muds, when shorelines were distant and biological productivity in the water column was high. Higher organic carbon (2.1-5.2%) and lowerδ13C values (averaging -24.3 ± 0.5‰, n = 11) characterize these mudstones and are interpreted to reflect a greater contribution by (isotopically more negative) amorphous organic matter derived from marine algae. Differences in δ13C between terrestrial and marine organic matter allow the changing proportions from different sources to be tracked through this succession. Combining δ13C values with zirconium (measured from whole rock), here used as a proxy for detrital silt input, provides a novel approach to distinguishing mudstone provenance and ultimately using δ13C to identify oil-prone organic matter in potential source rocks. These results have important implications for using bulk organic matter to identify and characterize global C-isotope excursions.

  4. Paleomagnetic study on the Neoproterozoic mafic dikes and Early Permian volcanic-sedimentary rocks from NW Yili Block (NW China): Implications for post-orogenic kinematic evolution of the SW CAOB

    NASA Astrophysics Data System (ADS)

    Zhu, Xin; Wang, Bo; Chen, Yan; Liu, Hongsheng; Shu, Liangshu; Faure, Michel

    2016-04-01

    As one of the largest accretionary orogens of the world, the Central Asian Orogenic Belt (CAOB) has been the focus of geological studies in the last decades. However, several key points are still in hot debate, such as the formation process of the Paleo-Asian Ocean, the intracontinental movements among constituent blocks of the CAOB. In order to better understand these questions, we conducted a paleomagnetic study on the Neoproterozoic (ca. 780 Ma) mafic dikes and Early Permian (ca. 268 Ma) volcanic and sedimentary rocks from NW of the Yili Block (NW China). Ten sites have been sampled from three mafic dikes. The thickness of dikes varies from 10 to 40 meters. At about 15 km west of the mafic dikes, 4 sites were drilled in the Lower Permian basalts and limestones that unconformably overlay the Neoproterozoic mafic dikes. Mineralogical investigations show the titanium-poor magnetite as the major magnetic remanence carrier. Stepwise alternating field (AF) and thermal demagnetizations reveal two-component magnetizations. The low temperature (coercivity) component shows a viscous and unstable magnetic remanence, whereas the high temperature (coercivity) component stably decays toward to the origin and is considered as the characteristic remanent magnetization (ChRM). All ChRMs isolated from both the mafic dikes and volcanic-sedimentary samples exclusively show a reversed magnetic polarity. Based on the following 3 arguments, we suggest that the Neoprotozoic mafic dikes have been remagnetized in the Early Permian. 1. International reference of magnetostratigraphic polarity shows a dominance of the normal polarity for the Neoproterozoic period and a superchron of the reversed polarity for the late Carboniferous-Permian; 2. Two groups of sampling show coherent paleomagnetic poles with an undistinguishable angular difference; and 3. The widespread Early Permian magmatism in the sampling area could be the cause of the remagnetization. Consequently, an Early Permian

  5. The Neoproterozoic.

    PubMed

    Butterfield, Nicholas J

    2015-10-01

    The Neoproterozoic era was arguably the most revolutionary in Earth history. Extending from 1000 to 541 million years ago, it stands at the intersection of the two great tracts of evolutionary time: on the one side, some three billion years of pervasively microbial 'Precambrian' life, and on the other the modern 'Phanerozoic' biosphere with its extraordinary diversity of large multicellular organisms. The disturbance doesn't stop here, however: over this same stretch of time the planet itself was in the throes of change. Tectonically, it saw major super-continental reconfigurations, climatically its deepest ever glacial freeze, and geochemically some of the most anomalous perturbations on record. What lies behind this dramatic convergence of biological and geological phenomena, and how exactly did it give rise to the curiously complex world that we now inhabit? PMID:26439347

  6. Neoproterozoic variations in the C-isotopic composition of seawater: stratigraphic and biogeochemical implications

    NASA Technical Reports Server (NTRS)

    Kaufman, A. J.; Knoll, A. H.

    1995-01-01

    The recent proliferation of stratigraphic studies of delta 13C variation in carbonates and organic C in later Neoproterozoic and basal Cambrian successions (approximately 850-530 Ma) indicates a strong oscillating trend in the C-isotopic composition of surface seawater. Alone, this trend does not adequately characterize discrete intervals in Neoproterozoic time. However, integrated with the vectorial signals provided by fossils and Sr-isotopic variations, C isotope chemostratigraphy facilitates the interbasinal correlation of later Neoproterozoic successions. Results of these studies are evaluated in terms of four stratigraphic intervals: (1) the Precambrian/Cambrian boundary, (2) the post-Varanger terminal Proterozoic, (3) the late Cryogenian, and (4) the early Cryogenian. Where biostratigraphic or radiometric data constrain the age of Neoproterozoic sedimentary sequences, secular variations in C and Sr isotopes can provide a level of stratigraphic resolution exceeding that provided by fossils alone. Isotopic data place strong constraints on the chemical evolution of seawater, linking it to major tectonic and paleoclimatic events. They also provide a biogeochemical framework for the understanding of the initial radiation of macroscopic metazoans, which is associated stratigraphically, and perhaps causally, with a global increase in the burial of organic C and a concomitant rise of atmospheric O2.

  7. Syn-rift, syn-glacial and syn-orogenic sedimentary mélanges as indicators of tectonic and palaeoclimatic evolution of the Lufilian Belt, Neoproterozoic-Lower Palaeozoic of Central Africa

    NASA Astrophysics Data System (ADS)

    Wendorff, Marek

    2010-05-01

    The Lufilian belt is an important segment of the Neoproterozoic-Lower Palaeozoic orogenic network within southern and central Africa. It deforms a sedimentary suite of the Katanga Supergroup (880-500 Ma). Mélange occurrences, traditionally called the Katangan breccias/megabreccias, are a prominent feature of the belt architecture. Some mélange bodies reach thickness of 2000 m and contain huge blocks of Katangan rocks. They were previously considered as tectonic mélanges ("friction breccias") marking regional decollement zones related to thrusting during the Pan-African orogenesis. However, these fragmental rocks were recently shown to be of sedimentary origin. They form two regionally extensive olistostrome bodies and one glaciogenic unit. The main lines of evidence for the olistostrome genesis are following: (1) lack of pervasive shearing that would point to tectonic fragmentation; (2) textures and structures diagnostic for subaqueous sediment gravity flows ranging from debris flows to turbidites; (3) roundness and provenance of clasts, and lateral facies gradients implying erosion, abrasion and unroofing of the Katangan source rocks elevated in the source areas; (4) lower boundaries of fragmental bodies are not tectonic but stratigraphic; (5) injections of unconsolidated conglomeratic matrix filling open joints in allochthonous blocks embedded in olistostrome lithosomes. The oldest mélange is a disorganised to locally organised syn rift olistostrome complex with olistoliths reaching 5 metres across. The clasts were derived from the uplifted rift margin and redeposition resulted from mass-wasting (rockfalls producing sedimentary breccias), sliding of solitary blocks, and pebbly to cobbly debris flows. The succeeding glaciogenic mélange complex originated during the Grand Conglomerat glaciation (correlative to the Sturtian glacial). It consists of disorganised clast-in-matrix facies that resulted from glacial erosion of the uplifted rift margin and

  8. Geochemistry of the Neoproterozoic Johnnie Formation and Stirling Quartzite, southern Nopah Range, California: Deciphering the roles of climate, tectonics, and sedimentary process in reconstructing the early evolution of a rifted continental margin

    NASA Astrophysics Data System (ADS)

    Schoenborn, William A.

    sediments. A total of 104 detrital zircon grains from two stratigraphically distinct samples of the Neoproterozoic Johnnie Formation in southeastern California were analyzed by SHRIMP. Samples were taken from quartz arenites in the lower and middle Johnnie Formation, which overlay sediments of rift-basin origin in the Kingston Peak Formation, to ascertain the position within the succession of the rift-to-drift transition. A 207Pb/206Pb age profile of detrital zircons from the lower Johnnie Formation has major peaks at 1749 Ma, and 1658 Ma, a subordinate peak at 1461 Ma, a lesser peak at 1239 Ma, and a few older Paleoproterozoic and Archean grains. A sample from the middle member has peaks at 1428 Ma, 1319 Ma, and 1074 Ma; a number of Paleoproterozoic peaks, and a number of peaks of Archean age similar to the Stirling Quartzite. The middle Johnnie Formation has a greater proportion of late Grenville age detritus, lesser amounts of older ˜1400 Ma Mesoproterozoic grains than the either the lower Johnnie Formation or Stirling Quartzite. When combined with detrital zircon data from the overlying strata, these data indicate a general increase upsection of late Grenville age detritus from ˜30% in the middle Johnnie Formation to ˜5% in the upper Stirling Quartzite to ˜60% in the Wood Canyon Formation in response to erosion of source areas, which temporarily shifted the influx of sediments from distal to local sources. These data support a Laurentian provenance for Johnnie Formation sediments consistent with contributions from both distal sources in the cratonic interior and local basement sources in the rift shoulder. Thus the Johnnie Formation-middle Wood Canyon Formation succession formed an early passive margin partly constrained by the continental edge consistent with recent sequence stratigraphic interpretations. A statistical comparison of Johnnie Formation and Stirling Quartzite detrital age distributions to those from Mesoproterozoic successions in the western United

  9. Newly discovered Cryogenenian Diamicties and Cap-carbonate in the Neoproterozoic successions from the Tarim Craton, NW China

    NASA Astrophysics Data System (ADS)

    Wen, B.; Evans, D. A.; Li, Y.; Wang, Z.

    2014-12-01

    Neoproterozoic cap carbonates associated with glacial deposits have been the unique lithofacies for establishing regional or interregional correlations and understanding the environmental conditions in the postglacial oceans. Here, we report a newly discovered Cryogenian tillite (in Aksu section), and a diamictite and cap-carbonate couplet (DCC) (in Wushi section) from the northwestern margin of Tarim Craton, NW China. The Aksu tillite is represented by reddish conglomerates, comprising angular to subrounded, poorly sorted clasts with diameters of 5-30 cm. Notably, the glaciogenic features, such as striations on clasts can be observed. The DCC member in Wushi section, sits atop an angular unconformity, and represents the end-Cryogenian strata according to the regional sequence stratigraphy (at the base of the Sugetbrak Fm, redbeds with interbedded 615-Ma mafic magmatism). The upper cap-carbonate of the DCC member consists of a < 2m-thick pink dolomite, which directly and conformably overlies the lower red diamictite. Given the striking similarity in both sedimentological characteristics and stratigraphic positions, the Aksu tillites and the Wushi DCC member may represent the same glaciation, most likely the Marinoan glaciation. To further test the correlation, high-resolution C-O isotopes and representative samples for geochemistry (Strontium isotope and trace element ratios) from the cap-dolomite are measured. As for the Sr isotope analyses, an incremental leaching technique was employed to extract the information about the chemical compositions of post-glacial ocean water. The δ13CPDB results consistently show moderate negative values of ca. -2.5‰. The minimum 87Sr/86Sr ratio from the leachates of each sample defines the contemporaneous seawater value, with the Mg/Ca ratio close to 1 (0.8). The 87Sr/86Sr ratios of all representative samples vary from 0.70680 to 0.70855. All the chemostratigraphic and cap-dolomite characteristics may suggest that our newly

  10. Early Permian volcano-sedimentary successions, Beishan, NW China: Peperites demonstrate an evolving rift basin

    NASA Astrophysics Data System (ADS)

    Chen, Shi; Guo, Zhaojie; Qi, Jiafu; Zhang, Yuanyuan; Pe-Piper, Georgia; Piper, David J. W.

    2016-01-01

    The Lower Permian volcano-sedimentary Zhesi Group has been investigated in the Hongliuhe and Liuyuan areas in Beishan, China, which is significant for the reconstruction of Late Paleozoic evolution in the southern part of the Central Asian Orogenic Belt. A variety of volcanic facies were distinguished in the Upper Zhesi Group: pillow basalt with interstitial limestone, thin-interbedded limestone and basalt, closely packed pillows, pillow-fragmented hyaloclastite breccia, and peperite. Laser 40Ar/39Ar whole-rock dating of the basalt yielding an age of 277 ± 11 Ma, as well as Early Permian brachiopod fossils in the limestone interbedded with the basalt, indicate that basalt was erupted in the Early Permian. The identification of the peperite and other facies originating from magma-sediment mingling reveals that the basaltic lava flows were derived from autochthonous basaltic magmatism and formed as part of the Lower Permian succession. The peperite also indicates that these subaqueous basaltic lava flows are not dismembered ophiolitic components, but formed in an autochthonous extensional setting in the Early Permian. The clastic rocks in the Lower Zhesi Group underlying the basaltic flows and peperites in the Hongliuhe and Liuyuan areas show a general fining-upwards sequence, indicating that they were deposited in a progressively deepening basin overlying the Devonian Hongliuhe suture zone. Subaqueous volcanism in a rift basin or basins, accompanied by coeval deposition of carbonate sediment and mud, built up the peperite-bearing volcanogenic-sedimentary successions. From among the various tectonic hypotheses for the Beishan region, this study demonstrates that by Early Permian the region was developing post-collisional rift basins.

  11. Implications for the Neoproterozoic Biological and Climatic History from Dating of the Doushantuo Phosphorites, S. China

    NASA Astrophysics Data System (ADS)

    Barfod, G. H.; Albarede, F.; Knoll, A. H.; Xiao, S.; Frei, R.; Baker, J.

    2002-12-01

    Models such as "Snowball Earth" suggest a link between Neoproterozoic glaciation events and the animal diversifications that preceded the Cambrian explosion. To evaluate such hypotheses, it is critical to resolve the precise chronology of the Neoproterozoic. As of now, ages of many Neoproterozoic sedimentary successions are only weakly constrained by bio- and chemostratigraphical correlation to successions that contain dated ash layers. Furthermore, attempts to radiometrically date sedimentary successions have had limited success. Pb-Pb and Lu-Hf dating of phosphates yield consistent ages close to 600 Ma for the Doushantuo phosphorites, South China. These age constraints are consistent with a depositional age estimate of 580 Ma +/- 20 Ma based on C-isotopic and biostratigraphic correlations (Knoll and Xiao, 1999, Acta Micropalaeontol. 16). The dates are however significantly younger than earlier reported Rb-Sr and Sm-Nd ages (610-700 Ma) for the Doushantuo sediments, which have been ascribed to the influence from detrital clay on the measured isotope compositions. The Lu-Hf and Pb-Pb ages indicate that the exquisitely preserved animal remains found in the Doushantuo Formation predate diverse Ediacaran fossil assemblages and thus represent the earliest animal ancestry assemblage known. The Doushantuo formation lies directly above the Nantuo Tillite deposits, which is generally correlated with the "Marinoan" glaciation - the younger of the two major Neoproterozoic ice ages. Age constraints for "Marinoan" glaciogenic deposits only exist in Newfoundland and Massachusetts, where radiometric dating constrain the glaciation to be younger than 595.5 +/- 2 Ma. Compared to this date, the Pb-Pb age of 599.3 +/- 4.2 Ma for the Doushantuo phosphorites indicates that the Nantuo glaciation predates the glaciation recorded in Eastern North America and therefore support the occurrence of a distinct post-Marinoan glaciation. The combination of Pb-Pb and Lu-Hf dating applied to

  12. De Long Islands: sedimentary history and provenance

    NASA Astrophysics Data System (ADS)

    Ershova, Victoria; Prokopiev, Andrei; Khudoley, Andrei; Sobolev, Nikolay; Petrov, Eugeniy

    2014-05-01

    The De Long Islands are an archipelago located in the East Siberian Sea, represent one of the few exposures of the Neoproterozoic and Early Paleozoic rocks in this part of the Arctic Ocean, and therefore are a very important area for study. It consists of 5 islands: Jeannette, Henrietta, Bennett, Vil'kitsky and Zhokhov. Vil'kitsky and Zhokhov Islands are covered by Cenozoic basalts, therefore are not considered here, whilst the Paleozoic rocks of interest for this study outcrop on Jeannette, Henrietta and Bennett islands. Jeannette Island is the smallest, containing exposures of a highly deformed and tectonized sedimentary succession. This succession is represented by siltstones and argillites, with beds of gravel to cobble conglomerates. The defining characteristic of these deposits is the abundance of tuffaceous beds, along with volcanic pebbles within the conglomerates. On Henrietta Island, four different units have been identified. The oldest one is very similar to the rocks which outcrop across Jeannette Island. The second unit consists of sandstones with lenses and layers of polymictic conglomerates. The third unit is represented by red-colored sandstones, whilst the youngest unit comprises basalt flows of an assumed Middle Paleozoic age. Accordingly detrital zircons data the age of sedimentary succession of Henrietta and Jeannette islands is the Neoproterozoic. On Bennett Island, Cambrian and Ordovician strata mainly consist of carbonates with minor interbedded clastics. We determined U-Pb ages for detrital zircons from 4 samples, from Jeannette and Henrietta Islands. Three samples have similar age populations, although there are some variations in the abundance of each population. The samples are dominated by Neoproterozoic and Mesoproterozoic grains with distinct peaks at ca. 550, 660, 1000, 1150, 1450, 1665 Ma. The youngest sedimentary unit on Henrietta Island has a very different detrital zircon distribution. The 550 Ma zircon population prevails (60

  13. Contribution of Grenvillian events to the formation of most complete Riphean sedimentary successions in Northern Eurasia

    NASA Astrophysics Data System (ADS)

    Maslov, A. V.; Podkovyrov, V. N.; Gareev, E. Z.; Ronkin, Yu. L.

    2014-03-01

    The Grenvillian orogeny (˜1250 to 980 Ma) was one of the most significant Riphean events. It determined the formation of many structures observable now in North and South America, northwestern Europe, South Africa, Western Australia, Antarctica, and other regions. Nevertheless, its reflections in the most complete and relatively well investigated Upper Precambrian sedimentary sections of northern Eurasia such as the Riphean stratotype (Bashkir meganticlinorium) and hypostratotype (Uchur-Maya region) still remain unknown. This is primarily true of the petrographic and chemical compositions of terrigenous rocks. This work is dedicated to the analysis of peculiar features in variations of the whole-rock chemical composition of sandstones and fine-grained clastic rocks (shales, mudstones, fine-grained clayey siltstones) that constitute Middle-Upper Riphean boundary layers of the Bashkir meganticlinorium, Kama-Belaya aulacogen, and Uchur-Maya region. The analysis reveals no tendency for the decrease in the degree of the chemical and, consequently, mineralogical maturity in the upward direction through the Middle-Upper Riphean sections in the above-mentioned regions. The whole-rock compositions of fine-grained clastic rocks associated with sandstones correspond mostly to that of "common" Upper Precambrian clayey rocks. The formation of practically the entire Yurmatinian-Karatavian succession in the Bashkir meganticlinorium proceeded under relatively stable TDM and ɛNd(T) values. The period of 1250 to 980 Ma in the central and eastern parts of the Siberian Platform was marked by repeated rifting episodes alternating with accumulation of mature platformal sediments, although repercussions of Grenvillian collisional processes are missing from this region as well. The performed analysis provides grounds for the conclusion that contribution of the Grenvillian events to the formation of most complete Riphean successions in northern Eurasia was insignificant.

  14. Uncovering the Neoproterozoic carbon cycle.

    PubMed

    Johnston, D T; Macdonald, F A; Gill, B C; Hoffman, P F; Schrag, D P

    2012-03-15

    Interpretations of major climatic and biological events in Earth history are, in large part, derived from the stable carbon isotope records of carbonate rocks and sedimentary organic matter. Neoproterozoic carbonate records contain unusual and large negative isotopic anomalies within long periods (10-100 million years) characterized by δ(13)C in carbonate (δ(13)C(carb)) enriched to more than +5 per mil. Classically, δ(13)C(carb) is interpreted as a metric of the relative fraction of carbon buried as organic matter in marine sediments, which can be linked to oxygen accumulation through the stoichiometry of primary production. If a change in the isotopic composition of marine dissolved inorganic carbon is responsible for these excursions, it is expected that records of δ(13)C(carb) and δ(13)C in organic carbon (δ(13)C(org)) will covary, offset by the fractionation imparted by primary production. The documentation of several Neoproterozoic δ(13)C(carb) excursions that are decoupled from δ(13)C(org), however, indicates that other mechanisms may account for these excursions. Here we present δ(13)C data from Mongolia, northwest Canada and Namibia that capture multiple large-amplitude (over 10 per mil) negative carbon isotope anomalies, and use these data in a new quantitative mixing model to examine the behaviour of the Neoproterozoic carbon cycle. We find that carbonate and organic carbon isotope data from Mongolia and Canada are tightly coupled through multiple δ(13)C(carb) excursions, quantitatively ruling out previously suggested alternative explanations, such as diagenesis or the presence and terminal oxidation of a large marine dissolved organic carbon reservoir. Our data from Namibia, which do not record isotopic covariance, can be explained by simple mixing with a detrital flux of organic matter. We thus interpret δ(13)C(carb) anomalies as recording a primary perturbation to the surface carbon cycle. This interpretation requires the revisiting of

  15. Quantifying time in sedimentary successions by radio-isotopic dating of ash beds

    NASA Astrophysics Data System (ADS)

    Schaltegger, Urs

    2014-05-01

    , e.g., Schoene et al. 2012), which then becomes transferred into volcanic ashes as excess dispersion of 238U-206Pb dates (see, e.g., Guex et al. 2012). Zircon is crystallizing in the magmatic liquid shortly before the volcanic eruption; we therefore aim at finding the youngest zircon date or youngest statistically equivalent cluster of 238U-206Pb dates as an approximation of ash deposition (Wotzlaw et al. 2013). Time gaps between last zircon crystallization and eruption ("Δt") may be as large as 100-200 ka, at the limits of analytical precision. Understanding the magmatic crystallization history of zircon is the fundamental background for interpreting ash bed dates in a sedimentary succession. Ash beds of different stratigraphic position and age my be generated within different magmatic systems, showing different crystallization histories. A sufficient number of samples (N) is therefore of paramount importance, not to lose the stratigraphic age control in a given section, and to be able to discard samples with large Δt - but, how large has to be "N"? In order to use the youngest zircon or zircons as an approximation of the age of eruption and ash deposition, we need to be sure that we have quantitatively solved the problem of post-crystallization lead loss - but, how can we be sure?! Ash bed zircons are prone to partial loss of radiogenic lead, because the ashes have been flushed by volcanic gases, as well as brines during sediment compaction. We therefore need to analyze a sufficient number of zircons (n) to be sure not to miss the youngest - but, how large has to be "n"? Analysis of trace elements or oxygen, hafnium isotopic compositions in dated zircon may sometimes help to distinguish zircon that is in equilibrium with the last magmatic liquid, from those that are recycled from earlier crystallization episodes, or to recognize zircon with partial lead loss (Schoene et al. 2010). Respecting these constraints, we may arrive at accurate correlation of periods of

  16. Quantifying time in sedimentary successions by radio-isotopic dating of ash beds

    NASA Astrophysics Data System (ADS)

    Schaltegger, Urs

    2014-05-01

    , e.g., Schoene et al. 2012), which then becomes transferred into volcanic ashes as excess dispersion of 238U-206Pb dates (see, e.g., Guex et al. 2012). Zircon is crystallizing in the magmatic liquid shortly before the volcanic eruption; we therefore aim at finding the youngest zircon date or youngest statistically equivalent cluster of 238U-206Pb dates as an approximation of ash deposition (Wotzlaw et al. 2013). Time gaps between last zircon crystallization and eruption ("Δt") may be as large as 100-200 ka, at the limits of analytical precision. Understanding the magmatic crystallization history of zircon is the fundamental background for interpreting ash bed dates in a sedimentary succession. Ash beds of different stratigraphic position and age my be generated within different magmatic systems, showing different crystallization histories. A sufficient number of samples (N) is therefore of paramount importance, not to lose the stratigraphic age control in a given section, and to be able to discard samples with large Δt - but, how large has to be "N"? In order to use the youngest zircon or zircons as an approximation of the age of eruption and ash deposition, we need to be sure that we have quantitatively solved the problem of post-crystallization lead loss - but, how can we be sure?! Ash bed zircons are prone to partial loss of radiogenic lead, because the ashes have been flushed by volcanic gases, as well as brines during sediment compaction. We therefore need to analyze a sufficient number of zircons (n) to be sure not to miss the youngest - but, how large has to be "n"? Analysis of trace elements or oxygen, hafnium isotopic compositions in dated zircon may sometimes help to distinguish zircon that is in equilibrium with the last magmatic liquid, from those that are recycled from earlier crystallization episodes, or to recognize zircon with partial lead loss (Schoene et al. 2010). Respecting these constraints, we may arrive at accurate correlation of periods of

  17. Newly Discovered Exposures of Neoproterozoic Diamictite within the Samre Fold-Thrust Belt of Northern Ethiopia

    NASA Astrophysics Data System (ADS)

    Park, Y.; Anttila, E.; MacLennan, S. A.; Swanson-Hysell, N.; Maloof, A. C.; Schoene, B.; Haileab, B.

    2015-12-01

    Life and climate evolved dramatically during the early Neoproterozoic - sedimentary rocks from this period record both the diversification of eukaryotic life as well as large scale fluctuations of the carbon cycle and paleogeography during the lead up to Cryogenian glaciation. Understanding global change leading up to this glaciation is critical for interpreting the conditions that initiated dramatic climate and geochemical oscillations. The Tonian-Cryogenian Tambien Group (Tigray region, northern Ethiopia) is a mixed carbonate-siliciclastic sedimentary succession deposited in an arc proximal basin that culminates in the Negash diamictite interpreted to represent the ca. 717-662 Ma Sturtian Glaciation. The presence of intercalated tuffs suitable for high precision U-Pb geochronology makes these sedimentary rocks an ideal target to temporally constrain physical and isotopic stratigraphic data sets of the early Neoproterozoic. The lower Tambien Group has been temporally constrained and used to establish global synchroneity of large scale carbon isotopic change ca. 800 Ma (Swanson-Hysell et al., 2015). We report the discovery of extensive exposures of upper Tambien Group successions southeast of the town of Samre within a newly mapped fold-thrust belt. Stratigraphic study across these exposures opens an opportunity to document environmental change across the basin during the apparently conformable transition from a mixed carbonate-siliciclastic platform into the Negash diamictite of the Sturtian Glaciation. The presence of ashes within the sediments holds the promise of combining high-precision dates with chemostratigraphic data to constrain global change before and during the onset of Snowball Earth glaciation.

  18. Accelerating Neoproterozoic Research through Scientific Drilling

    NASA Astrophysics Data System (ADS)

    Condon, Daniel; Prave, Anthony; Boggiani, Paulo; Fike, David; Halverson, Galen; Kasemann, Simone; Knoll, Andrew; Zhu, Maoyan

    2014-05-01

    The Neoproterozoic Era (1.0 to 0.541 Ga) and earliest Cambrian (541 to ca. 520 Ma) records geologic changes unlike any other in Earth history: supercontinental tectonics of Rodinia followed by its breakup and dispersal into fragments that form the core of today's continents; a rise in oxygen that, perhaps for the first time in Earth history, resulted in the deep oceans becoming oxic; snowball Earth, which envisages a blanketing of global ice cover for millions of years; and, at the zenith of these combined biogeochemical changes, the evolutionary leap from eukaryotes to animals. Such a concentration of hallmark events in the evolution of our planet is unparalleled and many questions regarding Earth System evolution during times of profound climatic and geological changes remain to be answered. Neoproterozoic successions also offer insight into the genesis of a number of natural resources. These include banded-iron formation, organic-rich shale intervals (with demonstrated hydrocarbon source rocks already economically viable in some countries), base and precious metal ore deposits and REE occurrences, as well as industrial minerals and dimension stone. Developing our understanding of the Neoproterozoic Earth-system, combined with regional geology has the potential to impact the viability of these resources. Our understanding of the Neoproterozoic and early Cambrian, though, is overwhelmingly dependent on outcrop-based studies, which suffer from lack of continuity of outcrop and, in many instances, deep weathering profiles. A limited number of research projects study Precambrian strata have demonstrated the potential impact of scientific drilling to augment and complement ongoing outcrop based studies and advancing research. An ICDP and ECORD sponsored workshop, to be held in March 2014, has been convened to discuss the utility of scientific drilling for accelerating research of the Neoproterozoic through early Cambrian (ca. 0.9 to 0.52 Ga) rock record. The aim is to

  19. Flood Basalts and Neoproterozoic Glaciation

    NASA Astrophysics Data System (ADS)

    Halverson, G. P.; Cox, G. M.; Kunzmann, M.; Strauss, J. V.; Macdonald, F. A.

    2014-12-01

    Large igneous provinces (LIPs), which are commonly associated with supercontinental break-up, are the product of the emplacement of >106 km3 of mafic rocks in less than a few million years. LIP magmatism, in particular continental flood basalt (CFB) volcanism, perturbs global climate on shorter time scales through the radiative effects of degassed SO2 and CO2. On longer time scales, CFBs alter climate through the effect of the high weatherabilty of mafic rocks (5-10 times greater than average continental crust) on global silicate weathering. A link between flood basalt weathering, Rodinia break-up, and Neoproterozoic snowball glaciation has been postulated. Here we present a new compilation of Nd isotope data on Neoproterozoic mudstones from Laurentia, Australia, and South China along with a new seawater strontium isotope record from well preserved carbonates that support this hypothesis. These datasets are consistent with an outsized role of basalt weathering on the global silicate weathering budget during the second half of the Tonian period (~850 to 725 Ma). Along with Os isotope data, they also suggest that an additional pulse of basalt weathering at the end of the Tonian may have initiated the Sturtian snowball glaciation. CFBs have relatively high concentrations of phosphorous. Hence, the drawdown in atmospheric CO2 required to trigger the Sturtian snowball Earth was likely accomplished through a combination of increased silicate weathering rates and enhanced biological productivity driven by greater nutrient supply to the oceans. CFBs were also the likely source of the iron in Neoproterozoic iron formation (IF), all significant occurrences of which are restricted to Sturtian-aged glacial successions. Dramatic declines in ɛNd following the Cryogenian snowball glaciations are mirrored by stepwise increases in 87Sr/86Sr, reflecting the scouring of the continents by global ice sheets. This continental resurfacing removed the extensive basalt carapace as well as

  20. Provenance analysis and tectonic setting of the Neoproterozoic sediments within the Taoudeni Basin, Northern Mauritania

    NASA Astrophysics Data System (ADS)

    Nicoll, Graeme; Straathof, Gijs; Tait, Jenny; Lo, Khalidou; Ousmane, N'diaye; El Moctar Dahmada, Mohamed; Berndt, Jasper; Key, Roger

    2010-05-01

    We have dated over 800 detrital zircon grains from the Neoproterozoic sediments within the Taoudeni Basin of Mauritania on the West African craton. This sequence of sediments preserves a relatively condensed mixed continental and marine succession as well as Neoproterozoic glacial and glacially influenced deposits. The underlying Archaean and Birimian basement of the West African craton is exposed on the Reguibat shield in the north, and on the Leo shield in the south although smaller inliers occur scattered along the Bassaride and Mauritanide belts, as well as in the core of the Anti-Atlas belt. The large West African craton is totally surrounded by Pan-African fold belts. Sedimentation within the Taoudeni basin started around 1000Ma and lasted until the end of the Carboniferous. The basin is 1000-1500 km in diameter and the sedimentary pile is on average 3000 m thick. All dated zircons in the stratigraphically lowest Char and Atar Groups are older than ~1800Ma. These groups show a strong input of 2950 and 2075Ma ages, indicating sourcing from the local underlying granitic and gneissic basement. These basal sediments also include a large input from a rare 2475Ma source. Samples from the upper Assebet El Hassiane Group contain numerous zircons of 2000-900Ma. While the Neoproterozoic Marinoan glaciogenic "Triad" Jbeliat Group and stratigraphically above formations show a large range of 3200-595Ma ages. We have also undertaken a detailed Carbon isotope profile study through the carbonates which cap the Glacial Jbeliat Group. The upper part of the Jbeliat cap carbonate displays a distinct and pronounced rise from -4.3 to +3.8 13C, followed by the final demise of carbonate productivity. This positive trend is consistent with the upper part of the globally extensive Ghaub/Nantuo/Marinoan cap carbonate sequences. This world-wide sequence is characterized by composite negative-to-positive trends up section and so this isotope stratigraphy along with the zircon data helps

  1. Unusual occurrence of some sedimentary structures and their significance in Jurassic transgressive clastic successions of Northern Ethiopia

    NASA Astrophysics Data System (ADS)

    Dubey, N.; Bheemalingeswara, K.

    2009-04-01

    Mesozoic sedimentary successions produced by marine transgression and regression of sea in northeastern part of Africa are well preserved in Mekelle basin of Ethiopia. Here, a typical second order sequence is well developed and preserved overlying the Precambrian basement rocks or patchy Palaeozoic sedimentary successions. Initiation of Mesozoic sedimentation in Mekelle basin has started with deposition of Adigrat Sandstone Formation (ASF). It is a retrogradational succession of siliciclastics in coastline/beach environment due to transgression of sea from southeast. ASF is followed by Antallo Limestone Formation (ALF)- an aggradational succession of carbonates in tidal flat environment; Agula Shale/Mudstone Formation (AMF); and Upper/Ambaradom Sandstone Formation (USF)- a progradational succession formed during regression in ascending order (Dubey et al., 2007). AMF is deposited in a lagoonal evaporatic environment whereas USF in a fluvial coastal margin. ASF is an aggregate of cyclically stacked two lithologies ASF1 and ASF2 produced by sea-level rise and fall of a lower order mini-cycle. ASF1 is a thick, multistoried, pink to red, friable, medium to fine grained, cross-bedded sandstone deposited in a high energy environment. ASF2 is a thin, hard and maroon colored iron-rich mudstone (ironstones) deposited in a low energy environment. ASF1 has resulted during regressive phase of the mini-cycle when rate of sedimentation was extremely high due to abundant coarser clastic supply from land to the coastal area. On the other hand, ASF2 has resulted during transgressive phase of the mini-cycle which restricted the supply of the coarser clastic to the coastal area and deposited the muddy ferruginous sediments in low energy offshore part of the basin where sedimentation rate was very low. Apart from these two major lithologies, there are also few other minor lithologies like fine-grained white sandstone, carbonate (as bands), claystone and mudstone present in ASF. ASF is

  2. Sedimentary facies and progradational style of a Pleistocene talus-slope succession, Northern Calcareous Alps, Austria.

    NASA Astrophysics Data System (ADS)

    Sanders, Diethard

    2010-05-01

    In mountain ranges, talus slopes are ubiquitous and typically represent the highest deposystem. The style of talus buildup from a low-dipping, immature slope to a high and steep, geomorphically mature slope, however, to date was not documented. Near Innsbruck city (Austria) a lithified talus-slope succession records progradation and downlap via talus-associated alluvial fans along its toe-of-slope. The considered succession ('Hötting Breccia' Auct.) probably accumulated during the terminal Riss-Würm interglacial to early Würmian, and became lithified before the Last Glacial Maximum. The Hötting Breccia consists of alluvial-fan deposits which, in turn, are locally downlapped by a succession deposited from aggrading to prograding talus slopes. Up-hill, the fossil talus slopes pinch out in onlap onto former rock cliffs. In the eastern part of outcrop, talus buildup is well-exposed along the flanks of a canyon; there, facies and depositional geometries record: (a) a basal, low-dipping alluvial-fan interval that accumulated near the toe-of-cliff, overlain and downlapped by (b) a steeper-dipping talus-slope succession. In the steep-dipping (25-35°), proximal slope segment hundreds of meters in length, the talus succession consists mainly of: (i) clast-supported breccias of cohesive debris flows, intercalated with (ii) openwork breccias from grain flows and particle creep. Progradation of the steep-dipping segment of talus slopes took place via shingling of alluvial-fan depositional units along the toe-of-slope; the fan depounits linked the progradation of the steep-dipping, proximal talus-slope segment with the lower-dipping substrate ahead of the slopes. The change from alluvial-fan deposition along the toe of initially high cliffs towards climbing onlap and progradation of talus slopes occurred when a slope segment dipping with the mean angle of residual shear of talus material had formed at the apex of the fan. Because the free cliff face supplying talus

  3. Sedimentary facies and progradational style of a Pleistocene talus-slope succession, Northern Calcareous Alps, Austria

    NASA Astrophysics Data System (ADS)

    Sanders, Diethard

    2010-07-01

    In mountain ranges, talus slopes are ubiquitous and typically represent the highest deposystem. The style of talus buildup from a low-dipping, immature slope to a high and steep, geomorphically mature slope, however, to date was not documented. Near Innsbruck city (Austria) a lithified talus-slope succession records progradation and downlap via talus-associated alluvial fans along its toe-of-slope; the fans linked progradation of the steep-dipping talus-slope segment over a lower-dipping substrate. The considered succession ('Hötting Breccia' Auct.) probably accumulated during the terminal Riss-Würm interglacial to early Würmian, and became lithified before the Last Glacial Maximum. The Hötting Breccia consists of alluvial-fan deposits which, in turn, are locally downlapped by a succession of aggrading to prograding talus slopes. Up-hill, the fossil talus slopes pinch out in onlap onto former rock cliffs. In the eastern part of outcrop, talus buildup is well-exposed along the flanks of a canyon; there, facies and depositional geometries record: (a) a basal, low-dipping alluvial-fan interval that accumulated near the toe-of-cliff, overlain and downlapped by (b) a steeper-dipping talus-slope succession. In the steep-dipping (25-35°), proximal talus-slope segment hundreds of meters in length, the talus successions consist mainly of: (i) clast-supported breccias of cohesive debris flows, intercalated with (ii) openwork breccias from grain flows and particle creep. Progradation of the steep-dipping segment of talus slopes took place via shingling of alluvial-fan depounits along the toe-of-slope. The fans linked the progradation of the steep-dipping, proximal talus-slope segment with the lower-dipping substrate ahead of the talus slope. The change from alluvial-fan deposition along the toe of initially high cliffs towards climbing onlap and progradation of talus slopes occurred when a slope segment dipping with the mean angle of residual shear of talus material had

  4. A colluvium - travertine sedimentary succession from the Tibetan Plateau: dating and climatic significance

    NASA Astrophysics Data System (ADS)

    Meyer, Michael; Wang, Zhijun; Schlütz, Frank; Hoffmann, Dirk; May, Jan-Hendrik; Aldenderfer, Mark

    2014-05-01

    Morphodynamics and sedimentation on the Tibetan Plateau are strongly controlled by cold-arid climate conditions and distinct freeze-thaw cycles. In such a periglacial environment mass-wasting processes are dominant on mountain slopes, causing thick successions of talus and colluvium to accumulate. While periglacial slope dynamics are ubiquitous on the plateau today, they were probably much more intense during the various cold stages of the Late Pleistocene. However, the exact nature as well as the timing and duration of such temperature controlled slope dynamics on the Tibetan plateau are not well constrained. Travertines are secondary carbonates precipitated from hydrothermal springs. On the Tibetan Plateau these types of spring deposits form along neotectonic faults, where super-saturated ground water can penetrate onto the surface, facilitating degassing and carbonate precipitation. Spring carbonate formation further requires non-permanently frozen ground and reasonable humid conditions in order to recharge the ground water aquifer. Travertines hold potential for palaeoenvironmental reconstruction, because they are dateable via U-series techniques and their geochemical, biological and petrographic signature can be used to extract high resolution palaeoenvironmental information. Due to a dense network of neotectonic faults on the Tibetan plateau, travertines are relatively common. Nevertheless, the potential of these hydrothermal spring deposits as an archive for palaeoenvironmental change on the plateau has yet to be explored. Here we present the first results obtained for an unusual, non-continuous sediment sequence encountered in southern Tibet at an altitude of 4200 m asl. near Chusang village, i.e. a ca. 200 m thick succession of periglacial colluvium alternating with travertine deposits. Preliminary data indicate that travertine deposition at the Chusang hydrothermal spring occurred periodically throughout the Late Pleistocene and extensive travertine

  5. Sedimentary and chemostratigraphic record of climatic cycles in Lower Pliensbachian marl-limestone platform successions of Asturias (North Spain)

    NASA Astrophysics Data System (ADS)

    Bádenas, Beatriz; Aurell, Marc; Armendáriz, Maider; Rosales, Idoia; García-Ramos, José Carlos; Piñuela, Laura

    2012-12-01

    A combined sedimentological, lithological and chemostratigraphical (Mg/Ca, δ13C, δ18O) analysis of the Lower Pliensbachian marl-limestone platform successions exposed along the Asturias coastline (northern Spain) has resulted in the characterization of high-frequency cycles. The highest-order sedimentary cycles (i.e. elementary cycles) are centimeter- to deciemeter-thick alternations of bioclastic and muddy laminated/burrowed facies, which do not match the marl-limestone couplets. They encompass three sedimentary stages: deposition from storm-density currents (bioclastic facies), dominant lateral advection of continental terrigenous mud accumulated on to an oxygen-deficient seafloor (laminated facies), and recovery of bottom oxygenation involving the burrowing of laminated sediments (burrowed facies). The close match between the number of elementary cycles recorded during the Jamesoni Subzone in Asturias and Yorkshire (Northern England) gives support to the idea of the influence of a regional climatic factor (i.e. millennial-scale cyclicity). Decimeter- to meter-scale cycles formed by bundles of elementary cycles are thought to record orbitally driven climatic changes (precession or obliquity, depending on the time calibration considered). Lower hemicycles of bundles are dominated by marls/calcareous mudstones, with decreasing burrowing and eventual preservation of laminated facies. They formed during humid periods, which controlled an increase in freshwater and terrigenous input to the platform and quasi-estuarine circulation promoting bottom-anoxia. Upper hemicycles of bundles are dominated by burrowed and bioclastic limestones, thought to be formed under arid conditions with anti-estuarine circulation and an increase of shallow carbonate production and offshore resedimentation. Chemostratigraphic data from belemnites recorded in the muddy laminated and burrowed facies indicate that significant concomitant shifts in δ13C and δ18O occurred during the lower

  6. 'Zipper-rift': a tectonic model for Neoproterozoic glaciations during the breakup of Rodinia after 750 Ma

    NASA Astrophysics Data System (ADS)

    Eyles, Nicholas; Januszczak, Nicole

    2004-03-01

    The 'Snowball Earth' model of Hoffman et al. [Science 281 (1998) 1342] has stimulated renewed interest in the causes of glaciation in Earth history and the sedimentary, stratigraphic and geochemical response. The model invokes catastrophic global Neoproterozoic refrigerations when oceans froze, ice sheets covered the tropics and global temperatures plummeted to -50 °C. Each event is argued to be recorded by tillites and have lasted up to 10 million years. Planetary biological activity was arrested only to resume in the aftermath of abrupt and brutal volcanically generated 'greenhouse' deglaciations when global temperatures reached +50 °C. The 'Cambrian explosion' is regarded by some as a consequence of post-Snowball glacioeustatic flooding of continental shelves. We shall show by a systematic review of the model that it is based on many long standing assumptions of the character and origin of the Neoproterozoic glacial record, in particular, 'tillites', that are no longer valid. This paper focusses on the sedimentological and stratigraphic evidence for glaciation in the light of current knowledge of glacial depositional systems. By integrating this analysis with recent understanding of the tectonic setting of Neoproterozoic sedimentary basins, an alternative 'Zipper-rift' hypothesis for Neoproterozoic glaciations is developed. The 'Zipper-rift' model emphasises the strong linkage between the first-order reorganisation of the Earth's surface created by diachronous rifting of the supercontinent Rodinia, the climatic effects of uplifted rift flanks and the resulting sedimentary record deposited in newly formed rift basins. Initial fragmentation of Rodinia commenced after about 750 Ma (when the paleo-Pacific Ocean started to form along the western margin of Laurentia) and culminated sometime after 610 Ma (with the opening of the paleo-Atlantic Ocean on the eastern margin of Laurentia). Breakup is recorded by well-defined 'tectonostratigraphic' successions that were

  7. Ancient sedimentary structures in the <3.7 Ga Gillespie Lake Member, Mars, that resemble macroscopic morphology, spatial associations, and temporal succession in terrestrial microbialites.

    PubMed

    Noffke, Nora

    2015-02-01

    Sandstone beds of the <3.7 Ga Gillespie Lake Member on Mars have been interpreted as evidence of an ancient playa lake environment. On Earth, such environments have been sites of colonization by microbial mats from the early Archean to the present time. Terrestrial microbial mats in playa lake environments form microbialites known as microbially induced sedimentary structures (MISS). On Mars, three lithofacies of the Gillespie Lake Member sandstone display centimeter- to meter-scale structures similar in macroscopic morphology to terrestrial MISS that include "erosional remnants and pockets," "mat chips," "roll-ups," "desiccation cracks," and "gas domes." The microbially induced sedimentary-like structures identified in Curiosity rover mission images do not have a random distribution. Rather, they were found to be arranged in spatial associations and temporal successions that indicate they changed over time. On Earth, if such MISS occurred with this type of spatial association and temporal succession, they would be interpreted as having recorded the growth of a microbially dominated ecosystem that thrived in pools that later dried completely: erosional pockets, mat chips, and roll-ups resulted from water eroding an ancient microbial mat-covered sedimentary surface; during the course of subsequent water recess, channels would have cut deep into the microbial mats, leaving erosional remnants behind; desiccation cracks and gas domes would have occurred during a final period of subaerial exposure of the microbial mats. In this paper, the similarities of the macroscopic morphologies, spatial associations, and temporal succession of sedimentary structures on Mars to MISS preserved on Earth has led to the following hypothesis: The sedimentary structures in the <3.7 Ga Gillespie Lake Member on Mars are ancient MISS produced by interactions between microbial mats and their environment. Proposed here is a strategy for detecting, identifying, confirming, and differentiating

  8. Chapter 2: Sedimentary successions of the Arctic Region (58–64° to 90°N) that may be prospective for hydrocarbons

    USGS Publications Warehouse

    Grantz, Arthur; Scott, Robert A.; Drachev, Sergey S.; Moore, Thomas E.; Valin, Zenon C.

    2011-01-01

    A total of 143 sedimentary successions that contain, or may be prospective for, hydrocarbons were identified in the Arctic Region north of 58–64°N and mapped in four quadrants at a scale of 1:11 000 000. Eighteen of these successions (12.6%) occur in the Arctic Ocean Basin, 25 (17.5%) in the passive and sheared continental margins of the Arctic Basin and 100 (70.0%) on the Circum-Arctic continents of which one (<1%) lies in the active margin of the Pacific Rim. Each succession was assigned to one of 13 tectono-stratigraphic and morphologic classes and coloured accordingly on the map. The thickness of each succession and that of any underlying sedimentary section down to economic basement, where known, are shown on the map by isopachs. Major structural or tectonic features associated with the creation of the successions, or with the enhancement or degradation of their hydrocarbon potential, are also shown. Forty-four (30.8%) of the successions are known to contain hydrocarbon accumulations, 64 (44.8%) are sufficiently thick to have generated hydrocarbons and 35 (24.5%) may be too thin to be prospective.

  9. Age and Correlation of Late Paleoproterozoic Sedimentary Successions in Northwestern Canada and Their Bearing on the Paleogeography of Laurentia

    NASA Astrophysics Data System (ADS)

    Rainbird, R. H.; Davis, W. J.; Hahn, K.; Furlanetto, F.; Thorkelson, D.

    2009-05-01

    Nearly 40 years ago, Fraser et al. (1970) proposed that thick basinal deposits of late Paleoproterozoic age along the western paleo-continental margin of Laurentia might represent the marine, deep-water complement of thinner but broadly correlative terrestrial sandstone deposits preserved today in intracontinental basins of the Canadian shield such as the Hornby Bay, Athabasca and Thelon. These basins exhibit comparable geometry, lithology, stratigraphy and overall paleocurrent patterns, which suggested they were initially co- extensive. Regional paleocurrents derived from crossbedded sandstone units interpreted as braided river deposits are dominantly west-directed in the mid-upper parts of all basins but are variable in the lower parts, supporting distinct initial basins that were later joined by broad fluvial braidplains originating from sources along active orogenic uplands located to the east (e.g. Trans-Hudson orogen). The sediment from these rivers was shunted westward across the craton and ultimately deposited along Laurentia's western margin. Geophysical data suggest that the distal parts of these river systems are preserved in the subsurface of northwestern Canada and are contiguous with fine-grained siliclastic and carbonate rocks of the Wernecke Supergroup and Muskwa Assemblage. One way to test this paleogeographic model is to compare the provenance of different parts of this sedimentary system using detrital zircon geochronology. Previous studies of the Muskwa assemblage (Ross et al. 2001) were compared with data for the Athabasca Group of the Athabasca Basin (Rainbird et al. 2007). A prominent peak of ages between 1.9-1.8 Ga is present in both successions and suggests common provenance from the Trans-Hudson orogen and delivery of detritus to the western margin of Laurentia by a >1000 km long drainage system. Based on correlation of seismic sections, MacLean and Cook (2004) proposed that the Wernecke Supergroup is equivalent to the lower part of the

  10. A new age within MIS 7 for the Homo neanderthalensis of Saccopastore in the glacio-eustatically forced sedimentary successions of the Aniene River Valley, Rome

    NASA Astrophysics Data System (ADS)

    Marra, Fabrizio; Ceruleo, Piero; Jicha, Brian; Pandolfi, Luca; Petronio, Carmelo; Salari, Leonardo

    2015-12-01

    Field observations as well as borehole, sedimentological and geochronologic data allow us to reconstruct the geologic setting of the Aniene River Valley in northern Rome, framing it within the recently recognized picture of temporally constrained, glacio-eustatically forced aggradational successions of this region. The sedimentary successions cropping out in this area include those described in the literature of the early 20th century in Saccopastore, where two skulls of Homo neanderthalensis were recovered. Based on the geometry, elevation and sedimentologic features of the investigated sedimentary deposits, the stratigraphic record of Saccopastore is correlated with the aggradational succession deposited in response to sea-level rise during glacial termination III at the onset of MIS 7 (i.e. ˜250 ka), corresponding to the local Vitinia Formation, as opposed to previous correlation with the MIS 5 interglacial and a locally defined "Tyrrhenian" stage (˜130 ka). This previous attribution was based on the interpretation of the sedimentary succession of Saccopastore, occurring between 15 and 21 m a.s.l., as a fluvial terrace formed around 130 ka during the Riss-Würm interglacial, ca. 6 m above the present-day alluvial plain of the Aniene River. In contrast to this interpretation, a 40Ar/39Ar age of 129 ± 2 ka determined for this study on a pyroclastic-flow deposit intercalated in a fluvial-lacustrine sequence forming a terrace ˜37 m a.s.l. near the coast of Rome constrains the aggradational succession in this area to MIS 5, precluding the occurrence of an equivalent fluvial terrace at lower elevation in the inland sector of Saccopastore. We therefore interpret the stratigraphic record of Saccopastore as the basal portion of the aggradational succession deposited in response to sea-level rise during MIS 7, whose equivalent fluvial terrace occurs around 55 m a.s.l. in this region. We also review the published paleontological and paleoethnological records recovered

  11. The Neoproterozoic Abu Dahr ophiolite, South Eastern Desert, Egypt: petrological characteristics and tectonomagmatic evolution

    NASA Astrophysics Data System (ADS)

    Gahlan, Hisham A.; Azer, Mokhles K.; Khalil, Ahmed E. S.

    2015-10-01

    The Neoproterozoic Abu Dahr ophiolite, South Eastern Desert, Egypt, is one of the best preserved and least dismembered ophiolite successions in the Arabian-Nubian Shield. It contains a Penrose-type ophiolite sequence from mantle section below mafic crust upward to oceanic sedimentary cover overlying mafic volcanics, although the original magmatic (stratigraphic) contact between the mantle and crustal sections is disrupted by tectonism. The Abu Dahr ophiolite is metamorphosed under greenschist facies conditions, and low-temperature alteration is widespread. Petrography reveals that: (i) the mantle is homogenous, serpentinized, and dominated by harzburgite and less abundant dunite; (ii) the cumulate ultramafics are represented by wehrlite and pyroxenite; and (iii) the crustal section is represented by metagabbros, meta-anorthosites and metabasalts. The Abu Dahr serpentinized peridotites show high Mg# (0.92-0.93), with enrichment of Ni, Cr and Co, and depletion of Al2O3 and CaO, and nearly flat and unfractionated REE chondrite-normalized pattern. Major and trace element characteristics of the Abu Dahr metagabbro and metabasalt (crustal section) indicate a tholeiitic to calc-alkaline affinity. Units of the crustal section have low-Nb and Zr concentrations, low Dy/Yb and relatively elevated La/Yb ratios, high U/Yb and Th/Yb ratios, and LREE enriched chondrite-normalized pattern. All of the Abu Dahr ophiolite units have trace-element signatures characterized by enrichment of LILE over HFSE. Rare and trace element patterns indicate a genetic link between the Abu Dahr mantle, cumulate ultramafics, and crust. Chromian spinel has survived metamorphism and is used as a petrogenetic indicator in the Abu Dahr serpentinized peridotites. The spinel is homogeneous with a limited composition, and shows high-Cr# (>0.6) combined with low-TiO2 character (mostly <0.1 wt.%). The Abu Dahr ophiolite is interpreted as a fragment of depleted oceanic lithosphere that experienced high degrees

  12. Seismic velocities within the sedimentary succession of the Canada Basin and southern Alpha-Mendeleev Ridge, Arctic Ocean: evidence for accelerated porosity reduction?

    NASA Astrophysics Data System (ADS)

    Shimeld, John; Li, Qingmou; Chian, Deping; Lebedeva-Ivanova, Nina; Jackson, Ruth; Mosher, David; Hutchinson, Deborah

    2016-01-01

    The Canada Basin and the southern Alpha-Mendeleev ridge complex underlie a significant proportion of the Arctic Ocean, but the geology of this undrilled and mostly ice-covered frontier is poorly known. New information is encoded in seismic wide-angle reflections and refractions recorded with expendable sonobuoys between 2007 and 2011. Velocity-depth samples within the sedimentary succession are extracted from published analyses for 142 of these records obtained at irregularly spaced stations across an area of 1.9E + 06 km2. The samples are modelled at regional, subregional and station-specific scales using an exponential function of inverse velocity versus depth with regionally representative parameters determined through numerical regression. With this approach, smooth, non-oscillatory velocity-depth profiles can be generated for any desired location in the study area, even where the measurement density is low. Practical application is demonstrated with a map of sedimentary thickness, derived from seismic reflection horizons interpreted in the time domain and depth converted using the velocity-depth profiles for each seismic trace. A thickness of 12-13 km is present beneath both the upper Mackenzie fan and the middle slope off of Alaska, but the sedimentary prism thins more gradually outboard of the latter region. Mapping of the observed-to-predicted velocities reveals coherent geospatial trends associated with five subregions: the Mackenzie fan; the continental slopes beyond the Mackenzie fan; the abyssal plain; the southwestern Canada Basin; and, the Alpha-Mendeleev magnetic domain. Comparison of the subregional velocity-depth models with published borehole data, and interpretation of the station-specific best-fitting model parameters, suggests that sandstone is not a predominant lithology in any of the five subregions. However, the bulk sand-to-shale ratio likely increases towards the Mackenzie fan, and the model for this subregion compares favourably with

  13. Fe Isotope Composition of Neoproterozoic Post-Glacial "Cap Dolostones"

    NASA Astrophysics Data System (ADS)

    Halverson, G. P.

    2005-12-01

    The largest variations in the Fe isotope composition in the geological record are found in sedimentary rocks, presumably as the result of redox transformations of iron during mineral precipitation, microbial processing, and diagenesis (Johnson et al., Cont. Min. Petrol., 2003). Systematic trends in the variability of the Fe isotope composition of sulfide minerals formed in ancient marine black shales broadly mirror patterns in sulfur isotope data (Δ33S, Δ34S), which are consistent with geological and other geochemical evidence for the progressive oxidation of the earth's surface during the Precambrian (Rouxel et al., Science, 2005). Therefore, the record of the Fe isotope composition of minerals formed in the marine environment appears to be a promising proxy for the redox evolution of the ocean. We have developed a method to extract the marine Fe isotope composition from carbonates in an attempt to establish higher resolution records of changes in marine redox changes than permitted by black shale geochemistry. We have applied this method to the study of ca. 635 Ma iron-rich dolostones, which are found in Neoproterozoic successions worldwide and immediately post-date a purported snowball (Marinoan) glaciation during which time the deep ocean is thought to have become anoxic (Hoffman et al., Science, 1998), allowing its Fe isotopic composition to evolve towards the composition of relatively light (δ57Fe vs. IRMM-14 ~ -0.6‰) hydrothermal iron (Beard et al., Geology, 2003). Fe isotope compositions were measured relative to IRMM-14 in medium-resolution mode on a Neptune MC-ICP-MS with a long-term external (2σ) reproducibility of < 0.04‰/amu. Preliminary data on dolomite samples from Svalbard, northern Namibia and northwest Canada show a range in δ57Fe values from -0.65 to 0.04‰, similar to the range found in siderite and Fe-rich dolomite in ancient BIFs (Johsnon et al., 2003) and to values for the Namibian cap dolostone reported by Leighton et al

  14. Strontium isotopic variations of Neoproterozoic seawater: Implications for crustal evolution

    SciTech Connect

    Asmerom, Y.; Jacobsen, S.B.; Knoll, A.H.; Butterfield, N.J. ); Swett, K. )

    1991-10-01

    The authors report high precision Sr isotopic data on carbonates from the Neoproterozoic Shaler Group, Victoria Island, Northwest Territories, Canada. Samples with low {sup 87}Rb/{sup 86}Sr ratios (<0.01) were selected for Sr isotopic analysis. {delta}{sup 18}O, Mn, Ca, Mg, and Sr data were used to recognize altered samples. The altered samples are characterized by high Mn/Sr ({ge}2) and variable {delta}{sup 18}O; most are dolomites. The data indicate that between ca. 790-850 Ma the {sup 87}Sr/{sup 86}Sr ratio of seawater varied between 0.70676 and 0.70561. The samples show smooth and systematic variation, with the lowest {sup 87}Sr/{sup 86}Sr value of 0.70561 at ca. 830 Ma. The low {sup 87}Sr/{sup 86}Sr ratio of carbonates from the lower parts of the section is similar to a value reported for one sample from the Adrar of Mauritania ({approx}900 Ma), West African Craton. Isotopic ratios from the upper part of the Shaler section are identical to values from the lower part of the Neoproterozoic Akademikerbreen Group, Spitsbergen. Although a paucity of absolute age determinations hinders attempts at the precise correlation of Neoproterozoic successions, it is possible to draw a broad outline of the Sr isotopic composition of seawater for this period. Data from this study and the literature are used to construct a curve of the {sup 87}Sr/{sup 86}Sr ratio of Neoproterozoic seawater. The Sr isotope composition of seawater reflects primarily the balance between continental Sr input through river input and mantle input via hydrothermal circulation of seawater through mid-ocean ridges. Coupling of Nd and Sr isotopic systems allows the authors to model changes in seafloor spreading rates (or hydrothermal flux) and continental erosion. The Sr hydrothermal flux and the erosion rate (relative to present-day value) are modeled for the period 500-900 Ma.

  15. Geological evolution of the Neoproterozoic Bemarivo Belt, northern Madagascar

    USGS Publications Warehouse

    Thomas, Ronald J.; De Waele, B.; Schofield, D.I.; Goodenough, K.M.; Horstwood, M.; Tucker, R.; Bauer, W.; Annells, R.; Howard, K. J.; Walsh, G.; Rabarimanana, M.; Rafahatelo, J.-M.; Ralison, A.V.; Randriamananjara, T.

    2009-01-01

    The broadly east-west trending, Late Neoproterozoic Bemarivo Belt in northern Madagascar has been re-surveyed at 1:100 000 scale as part of a large multi-disciplinary World Bank-sponsored project. The work included acquisition of 14 U-Pb zircon dates and whole-rock major and trace element geochemical data of representative rocks. The belt has previously been modelled as a juvenile Neoproterozoic arc and our findings broadly support that model. The integrated datasets indicate that the Bemarivo Belt is separated by a major ductile shear zone into northern and southern "terranes", each with different lithostratigraphy and ages. However, both formed as Neoproterozoic arc/marginal basin assemblages that were translated southwards over the north-south trending domains of "cratonic" Madagascar, during the main collisional phase of the East African Orogeny at ca. 540 Ma. The older, southern terrane consists of a sequence of high-grade paragneisses (Sahantaha Group), which were derived from a Palaeoproterozoic source and formed a marginal sequence to the Archaean cratons to the south. These rocks are intruded by an extensive suite of arc-generated metamorphosed plutonic rocks, known as the Antsirabe Nord Suite. Four samples from this suite yielded U-Pb SHRIMP ages at ca. 750 Ma. The northern terrane consists of three groups of metamorphosed supracrustal rocks, including a possible Archaean sequence (Betsiaka Group: maximum depositional age approximately 2477 Ma) and two volcano-sedimentary sequences (high-grade Milanoa Group: maximum depositional age approximately 750 Ma; low grade Daraina Group: extrusive age = 720-740 Ma). These supracrustal rocks are intruded by another suite of arc-generated metamorphosed plutonic rocks, known as the Manambato Suite, 4 samples of which gave U-Pb SHRIMP ages between 705 and 718 Ma. Whole-rock geochemical data confirm the calc-alkaline, arc-related nature of the plutonic rocks. The volcanic rocks of the Daraina and Milanoa groups also

  16. Environmental Change in the Prelude to a Neoproterozoic Ice Age: Sulfur Isotope Evidence from the Shaler Supergroup, Northwest Territories, Canada

    NASA Astrophysics Data System (ADS)

    Kaufman, A. J.; Williams, B. P.; Johnston, D. T.; Farquhar, J.; Knoll, A. H.; Butterfield, N. J.; Rainbird, R.

    2006-05-01

    The >723 Ma Shaler Supergroup on Victoria Island in the Northwest Territories of Canada is unusual for marginal marine Neoproterozoic successions insofar as thickly bedded sulfate evaporites are preserved. This unique sedimentary window for the Neoproterozoic allows us to investigate isotopic changes in the oxidized sulfur reservoir in the run up to a potential ice age, which is evidenced by a strong negative δ13C excursion in Kilian Formation carbonates at the top of the interval. Similar carbon isotope anomalies are recorded in carbonates beneath glacial diamictites of broadly the same age from the Mackenzie Mountains in Canada and in the Otavi Group of northern Namibia. Stratigraphically coincident with the 8‰ drop in 13C abundances in the Kilian Formation is a 15‰ rise in 34S of bedded sulfates, to a maximum of near +30‰. Anti-correlated isotope trends of smaller magnitude are recorded in mixed carbonates and evaporites at the top of the underlying Minto Inlet Formation. Comparison of 33S and 34S abundances of Shaler sulfates with values modeled from the results of culture experiments suggests that both anaerobic sulfate reducers and aerobic sulfur disproportionators populated the shallow open ocean. Notably, the profound 34S enrichment in the Kilian Formation is not coupled with similar enrichments in 33S. This observation is consistent with a system primarily driven by an increase in pyrite burial. Given the presence of shallow marine evaporites it is unlikely that sulfate concentrations were limited at this time. We hypothesize that this flux was enhanced during sea level fall at the onset of an ensuing ice age by the spread of anoxic conditions in deep oceans populated by sulfate reducing bacteria.

  17. Laser Remote Optical Granulometry (LROG), a tool for the textural study of inaccessible outcrops: could this method help to study Martian sedimentary successions?

    NASA Astrophysics Data System (ADS)

    Sarocchi, Damiano; Bartali, Roberto; Norini, Gianluca; Nahmad-Molinari, Yuri

    2010-05-01

    We present a new tool for the textural study of inaccessible outcrops of pyroclastic and epiclastic deposits. The new method, called Laser Remote Optical Granulometry (LROG), is based on high resolution tele-photography and stereologic techniques. LROG consists on taking several pictures of the outcrop with a high resolution CCD camera coupled to a small aperture telescope that can be placed several tenths of meters away. The scale of the image is obtained projecting an equilateral triangle with known size on the outcrop by means of three laser beams. The LROG allows the measurement of clasts less than 1 mm in size from a distance of 80 to 100 meters, and can reach much better resolution when operated closer to the outcrop. Perspective distortion can be corrected with the equilateral triangle projected by the lasers. To get high resolution images and remove the effects of air turbulence, hundreds of frames of the same field are captured in rapid sequence and then stacked and averaged with image processing algorithms developed for astronomical imaging. The LROG was validated on the pyroclastic deposits of the Joya Honda maar (San Luis Potosi, Mexico). The LROG provided precise granulometric analysis and vertical granulometric profiles of this pyroclastic sequence, useful to recognize the eruptive history of the volcano. This method can be used for the analysis of any kind of sedimentary deposits in the granulometric range of clasts greater than fine sand. We are improving the LROG to obtain other useful textural information like clast shape and apparent fabric. This method, implemented on a robotic probe could be a promising tool to carry out detailed stratigraphic and sedimentological study of Martian sedimentary successions.

  18. Continental flood basalt weathering as a trigger for Neoproterozoic Snowball Earth

    NASA Astrophysics Data System (ADS)

    Cox, Grant M.; Halverson, Galen P.; Stevenson, Ross K.; Vokaty, Michelle; Poirier, André; Kunzmann, Marcus; Li, Zheng-Xiang; Denyszyn, Steven W.; Strauss, Justin V.; Macdonald, Francis A.

    2016-07-01

    Atmospheric CO2 levels and global climate are regulated on geological timescales by the silicate weathering feedback. However, this thermostat has failed multiple times in Earth's history, most spectacularly during the Cryogenian (c. 720-635 Ma) Snowball Earth episodes. The unique middle Neoproterozoic paleogeography of a rifting, low-latitude, supercontinent likely favored a globally cool climate due to the influence of the silicate weathering feedback and planetary albedo. Under these primed conditions, the emplacement and weathering of extensive continental flood basalt provinces may have provided the final trigger for runaway global glaciation. Weathering of continental flood basalts may have also contributed to the characteristically high carbon isotope ratios (δ13 C) of Neoproterozoic seawater due to their elevated P contents. In order to test these hypotheses, we have compiled new and previously published Neoproterozoic Nd isotope data from mudstones in northern Rodinia (North America, Australia, Svalbard, and South China) and Sr isotope data from carbonate rocks. The Nd isotope data are used to model the mafic detrital input into sedimentary basins in northern Rodinia. The results reveal a dominant contribution from continental flood basalt weathering during the ca. 130 m.y. preceding the onset of Cryogenian glaciation, followed by a precipitous decline afterwards. These data are mirrored by the Sr isotope record, which reflects the importance of chemical weathering of continental flood basalts on solute fluxes to the early-middle Neoproterozoic ocean, including a pulse of unradiogenic Sr input into the oceans just prior to the onset of Cyrogenian glaciation. Hence, our new data support the hypotheses that elevated rates of flood basalt weathering contributed to both the high average δ13 C of seawater in the Neoproterozoic and to the initiation of the first (Sturtian) Snowball Earth.

  19. Thermal evolution of Lower Paleozoic sedimentary successions from organic and inorganic studies: the case history of the Holy Cross Mountains (central Poland)

    NASA Astrophysics Data System (ADS)

    Trolese, Matteo; Stefano Celano, Antonio; Corrado, Sveva; Caricchi, Chiara; Schito, Andrea; Aldega, Luca

    2015-04-01

    The rapid increase in shale gas production in the USA has triggered a growing interest in unconventional resources in Eastern and Northern Europe. In this framework, the potential shale gas reserves in Poland are the most promising in Europe, extending from the Baltic Sea to the Ukraine border. In this area, the Baltic, Podlasie and Lublin basins have already become objective of shale gas exploration and the Holy Cross Mountains (HCM, Central Poland) represents the outcropping analog of the buried targeted Lower Paleozoic successions, providing a unique opportunity to study and assess source rock potential. In this work, we provide new thermal maturity data of Paleozoic rocks exposed in the HCM. A multi-method approach, coupling organic matter/graptolites (i.e., marine organoclasts) optical analysis and X-ray diffraction of clay-sized fraction of sediments, was applied to constrain the burial - thermal evolution of the sedimentary succession. The investigated area of the HCM includes two different tectonic blocks: the Łysogóry region to the North and the Kielce region to the South, separated by the Holy Cross Fault (HCF). lllite content in mixed layer illite-smectite determinations and vitrinite/graptolites reflectance measurements (Roeq%), performed on samples (Cambrian - Devonian) collected from both the regions, show a substantial difference between the two blocks in terms of thermal maturity and burial history. Roeq% values in the southern block range from 0.5% to 1.0%, with few exceptions, indicating early to mid-mature stage of hydrocarbon generation. Samples collected in the northern block show much higher values, mainly from 1.2% up to 1.7%, representative of the gas generation window. The I-S ordering type also shows relevant differences in the two blocks. In the southern block, mixed-layered clay minerals varies from R1 (short-range) to R3 (long-range), whereas R3 structures are recorded in the northern block. Vitrinite reflectance and mixed-layer I

  20. Learning to tell Neoproterozoic time

    NASA Technical Reports Server (NTRS)

    Knoll, A. H.

    2000-01-01

    In 1989, the International Commission on Stratigraphy established a Working Group on the Terminal Proterozoic Period. Nine years of intensive, multidisciplinary research by scientists from some two dozen countries have markedly improved the framework for the correlation and calibration of latest Proterozoic events. Three principal phenomena--the Marinoan ice age, Ediacaran animal diversification, and the beginning of the Cambrian Period--specify the limits and character of this interval, but chemostratigraphy and biostratigraphy based on single-celled microfossils (acritarchs), integrated with high-resolution radiometric dates, provide the temporal framework necessary to order and evaluate terminal Proterozoic tectonic, biogeochemical, climatic, and biological events. These data also provide a rational basis for choosing the Global Stratotype Section and Point (GSSP) that will define the beginning of this period. A comparable level of stratigraphic resolution may be achievable for the preceding Cryogenian Period, providing an opportunity to define this interval, as well, in chronostratigraphic terms--perhaps bounded at beginning and end by the onset of Sturtian glaciation and the decay of Marinoan ice sheets, respectively. Limited paleontological, isotopic, and radiometric data additionally suggest a real but more distant prospect of lower Neoproterozoic correlation and stratigraphic subdivision.

  1. Neoproterozoic evolution of the basement of the South-American platform

    NASA Astrophysics Data System (ADS)

    de Brito Neves, Benjamim Bley; Fuck, Reinhardt A.

    2013-11-01

    Neoproterozoic geologic and geotectonic processes were of utmost importance in forming and structuring the basement framework of the South-American platform. Two large domains with distinct evolutionary histories are identified with respect to the Neoproterozoic era: the northwest-west (Amazonian craton and surroundings) and the central-southeast (the extra-Amazonian domain). In the first domain, Neoproterozoic events occurred only locally and were of secondary significance, and the geologic events, processes, and structures of the pre-Neoproterozoic (and syn-Brasiliano) cratonic block were much more influential. In the second, the extra-Amazonian domain, the final evolution, structures and forms are assigned to events related to the development of a complex net of Neoproterozoic mobile belts. These in turn resulted in strong reworking of the older pre-Neoproterozoic basement. In this domain, four distinct structural provinces circumscribe or are separated by relatively small pre-Neoproterozoic cratonic nuclei, namely the Pampean, Tocantins, Borborema and Mantiqueira provinces. These extra-Amazonian provinces were formed by a complex framework of orogenic branching systems following a diversified post-Mesoproterozoic paleogeographic scenario. This scenario included many types of basement inliers as well as a diversified organization of accretionary and collisional orogens. The basement inliers date from the Archean to Mesoproterozoic periods and are different in nature. The escape tectonics that operated during the final consolidation stages of the provinces were important to and responsible for the final forms currently observed. These latest events, which occurred from the Late Ediacaran to the Early Ordovician, present serious obstacles to paleogeographic reconstructions. Two groups of orogenic collage systems are identified. The older system from the Tonian (>850 Ma) period is of restricted occurrence and is not fully understood due to strong reworking

  2. The Neoproterozoic oxygenation event: Environmental perturbations and biogeochemical cycling

    NASA Astrophysics Data System (ADS)

    Och, Lawrence M.; Shields-Zhou, Graham A.

    2012-01-01

    transition from dominantly pyrite burial to sulfate burial after the Neoproterozoic. Strong evidence for the oxygenation of the deep marine environment has emerged through elemental approaches over the past few years which were able to show significant increases in redox-sensitive trace-metal (notably Mo) enrichment in marine sediments not only during the GOE but even more pronounced during the inferred NOE. In addition to past studies involving Mo enrichment, which has been extended and further substantiated in the current review, we present new compilations of V and U concentrations in black shales throughout Earth history that confirm such a rise and further support the NOE. With regard to ocean ventilation, we also review other sedimentary redox indicators, such as iron speciation, molybdenum isotopes and the more ambiguous REE patterns. Although the timing and extent of the NOE remain the subjects of debate and speculation, we consider the record of redox-sensitive trace-metals and C and S contents in black shales to indicate delayed ocean ventilation later in the Cambrian on a global scale with regard to rising oxygen levels in the atmosphere which likely rose during the Late Neoproterozoic.

  3. Visualizing the sedimentary response through the orogenic cycle using multi-dimensional scaling

    NASA Astrophysics Data System (ADS)

    Spencer, C. J.; Kirkland, C.

    2015-12-01

    Changing patterns in detrital provenance through time have the ability to resolve salient features of an orogenic cycle. Such changes in the age spectrum of detrital minerals can be attributed to fluctuations in the geodynamic regime (e.g. opening of seaways, initiation of subduction and arc magmatism, and transition from subduction to collisional tectonics with arrival of exotic crustal material). These processes manifest themselves through a variety of sedimentary responses due to basin formation, transition from rift to drift sedimentation, or inversion and basement unroofing. This generally is charted by the presence of older detrital zircon populations during basement unroofing events and is followed by a successive younging in the detrital zircon age signature either through arrival of young island arc terranes or the progression of subduction magmatism along a continental margin. The sedimentary response to the aforementioned geodynamic environment can be visualized using a multi-dimensional scaling approach to detrital zircon age spectra. This statistical tool characterizes the "dissimilarity" of age spectra of the various sedimentary successions, but importantly also charts this measure through time. We present three case studies in which multi-dimensional scaling reveals additional useful information on the style of basin evolution within the orogenic cycle. The Albany-Fraser Orogeny in Western Australia and Grenville Orogeny (sensu stricto) in Laurentia demonstrate clear patterns in which detrital zircon age spectra become more dissimilar with time. In stark contrast, sedimentary successions from the Meso- to Neoproterozoic North Atlantic Region reveal no consistent pattern. Rather, the North Atlantic Region reflects a signature consistent with significant zircon age communication due to a distal position from an orogenic front, oblique translation of terranes, and complexity of the continental margin. This statistical approach provides a mechanism to

  4. Neoproterozoic Earth System change: Observations of the rock record

    NASA Astrophysics Data System (ADS)

    Prave, A. R.; Fallick, A. E.; Hoffmann, K. H.; Benn, D.

    2003-04-01

    Radically nonuniformitarian modes of Earth System behaviour have been hypothesized as hallmarks of Neoproterozoic Earth history. In particular, severe climatic extremes (worldwide glaciations marked by globally frozen oceans to ultra-greenhouses) are envisaged to have occurred at least twice; these inferred harsh climates are invoked as a potential driving mechanism for biospheric evolution. Such hypotheses are intellectually elegant and their vividness grabs public attention, but how strictly do they adhere to the observational facts of the rock record? Here we show examples of Neoproterozoic glacigenic successions that imply severe, but not catastrophic climate change. The first example is the Port Askaig Tillite of Scotland. This unit was deposited in low latitudes (<30^o) and records the older ("Sturtian") glacial episode. Importantly, it contains evidence for freeze-thaw cycles on a number of time scales and, when combined with similar features observed in glacial rocks elsewhere (from both the younger and older glacial episodes), indicates that low-latitude climate does not record a simple unidirectional, long-term refrigeration of Earth. The second example is δ13C datasets associated with the older and younger glacial episodes in Namibia and California, USA. When combined with detailed stratigraphic data, these show: (1) that presumed coeval carbonates that cap the glacigenic successions display a wide range in δ13C values but share similar long-term trends; and (2) carbonates units below the erosive base of the glacigenic rocks record a decline in δ13C (as noted by previous workers) but in several cases a recovery towards heavier values is preserved in the topmost carbonate strata. These data imply that biogeochemically mediated conditions were regionally variable, both before and in the immediate aftermath of Neoproterozoic glaciations. Thus, any attempts to infer or model the nature, magnitude and potential significance of Neoproterozoic climate change

  5. Integrated Geochemical-Petrographic Insights on Neoproterozoic Ocean Oxygenation

    NASA Astrophysics Data System (ADS)

    Hood, A.; Planavsky, N.; Wallace, M. W.; Wang, X.; Gueguen, B.

    2015-12-01

    Novel isotope systems have the potential to provide new insights into biogeochemical cycling in Earth's evolving oceans. However, much recent paleo-redox work has been done without extensive consideration of sample preservation or paleoenvironmental setting. Neoproterozoic reef complexes from South Australia provide a perfect setting to test geochemical redox proxies (e.g. uranium isotopes and trace metal chemistry) within a well-defined sedimentological and petrographic context. These reefs developed significant frameworks over ~1km of steep platform relief from the seafloor, and contain a variety of carbonate components including primary dolomite marine cements. Analysis of a variety of components within these reefs reveals significant variation in uranium isotope composition and trace metal chemistry between components, even within a single sample. Marine cements, which precipitated directly from seawater, have much lower contamination element concentrations (e.g. Al, Zr, Th) than depositional micrites, and appear to represent the best archive of ancient ocean conditions. These cements have high levels of Fe, Mn in shallow and deep reef facies (e.g. 2-3wt% Fe), but only Fe-oxide inclusions in peritidal settings. This distribution suggests ferruginous conditions under a surficial chemocline in this Neoproterozoic seawater. Uranium isotopes from pristine marine cements have relatively heavy values compared to modern seawater (median = -0.22 δ238U). These values are essentially unfractionated from riverine inputs, which we interpret as tracking extensive near quantitative low-T reduction of U(VI) to U(IV) by abundant soluble iron in seawater. Depositional components and late stage cements have a much lighter and more variable U isotope compositions (-0.71 to -0.08 δ238U). This work highlights the need for fundamental petrographic constraints on the preservation of depositional geochemical signatures in the future use and development of sedimentary redox proxies.

  6. World Map Showing Surface and Subsurface Distribution, and Lithologic Character of Middle and Late Neoproterozoic Rocks

    USGS Publications Warehouse

    Stewart, John H.

    2007-01-01

    INTRODUCTION The map was prepared to outline the basic information on where Neoproterozoic rocks are present in the World, and of the lithologic character of these rocks. The information provides a better understanding of major Neoproterozoic tectonic subdivisions useful in paleogeographic and plate tectonic reconstructions. The time frame of the map is within the middle and late Neoproterozoic from approximately 870 to 540 Ma and is after widespread Mesoproterozoic Grenville-age collisional events that are considered to have formed the hypothetical supercontinent of Rodinia. Much of the time represented by the map is interpreted to be during the fragmentation of Rodinia. The recognition of Neoproterozoic rocks is commonly difficult because of limited isotopic or paloeontological dating. Thus, some rocks shown on the map could be older or younger than the age indicated. However, at the scale of the map the the problem may be minor. Enough information seems to be available to indicate the general age of the rocks. Many of the successions contain diamictite deposits considered to be glaciogenic and dated as middle or late Neoproterozoic. These deposits thus show a rough correlation of middle and late Neoproterozoic rocks of the world. The map is a Richardson map projection, except for Antarctica which is a polar projection. The map was prepared from about 650 references, shown in the text linked below under 'Sources of Information', used to outline distribution patterns, determine rock types, and provide information on the regional and local geologic framework of the rocks. The focus of the references is on the geologic information needed to prepare the map. Other information, such as plate tectonic reconstructions or paleomagnetic studies is generally not included. The 'Sources of Information' lists references alphabetically for each of 14 regions. In brackets is a code for each area. These codes provide help in locating the specific regions in the references.

  7. The evolution of Neoproterozoic magmatism in Southernmost Brazil: shoshonitic, high-K tholeiitic and silica-saturated, sodic alkaline volcanism in post-collisional basins.

    PubMed

    Sommer, Carlos A; Lima, Evandro F; Nardi, Lauro V S; Liz, Joaquim D; Waichel, Breno L

    2006-09-01

    The Neoproterozoic shoshonitic and mildly alkaline bimodal volcanism of Southernmost Brazil is represented by rock assemblages associated to sedimentary successions, deposited in strike-slip basins formed at the post-collisional stages of the Brasilian/Pan-African orogenic cycle. The best-preserved volcano sedimentary associations occur in the Camaquã and Campo Alegre Basins, respectively in the Sul-riograndense and Catarinense Shields and are outside the main shear belts or overlying the unaffected basement areas. These basins are characterized by alternation of volcanic cycles and siliciclastic sedimentation developed dominantly on a continental setting under subaerial conditions. This volcanism and the coeval plutonism evolved from high-K tholeiitic and calc-alkaline to shoshonitic and ended with a silica-saturated sodic alkaline magmatism, and its evolution were developed during at least 60 Ma. The compositional variation and evolution of post-collisional magmatism in southern Brazil are interpreted as the result mainly of melting of a heterogeneous mantle source, which includes garnet-phlogopite-bearing peridotites, veined-peridotites with abundant hydrated phases, such as amphibole, apatite and phlogopite, and eventually with the addition of an asthenospheric component. The subduction-related metasomatic character of post-collisional magmatism mantle sources in southern Brazil is put in evidence by Nb-negative anomalies and isotope features typical of EM1 sources. PMID:16936944

  8. Physical volcanology, geochemistry and basin evolution of the Ediacaran volcano-sedimentary succession in the Bas Draâ inlier (Ouarzazate Supergroup, Western Anti-Atlas, Morocco)

    NASA Astrophysics Data System (ADS)

    Karaoui, Brahim; Breitkreuz, Christoph; Mahmoudi, Abdelkader; Youbi, Nasrrddine

    2014-11-01

    New geologic mapping, lithofacies and granulometric analysis, and geochemistry from the volcano-sedimentary successions of the central part of the Bas Draâ inlier, Western Anti-Atlas, constrain the Ediacaran Ouarzazate Supergroup evolution during the post-collisional stage of the Pan-African orogeny. Volcanosedimentary facies analysis is the key aspect of the present contribution. We distinguished sixteen terrestrial volcanosedimentary lithofacies in the Bas Draâ succession (BDS), which reaches a total thickness of 2000 m. BDS evolution can be grouped into four units (Aouinet Aït Oussa I to IV, AO I-AO IV). The earliest volcanic activity produced rhyolitic ignimbrite sheets (AO I), which had been considered as lava flows by previous workers, and which were presumably related to caldera system(s). During AO II, a complex of high-silica andesitic and rhyolitic lavas formed, punctuated by the explosive eruption of a high-temperature silica-rich magma leading to the formation of parataxitic ignimbrite. AO III consists of basalt and andesite lava fields and small explosive, in parts phreatomagmatic volcanic vents. It is dissected by fluvial systems depositing external non-volcanic and local volcanic debris. BDS evolution terminated with the formation of a large SiO2-rich lava dome complex (AO IV), accompanied by small basalt effusive event. Volcanosedimentary facies analysis infers that the BDS evolved in a continental extensional setting developing in a low topography under humid paleoclimatic conditions. Alteration textures are dominated by a piemontite-calcite-albite-quartz (+ iron oxides) assemblage. Chemical analysis of BDS volcanic and subvolcanic rocks belongs to high-k calc-alkaline and alkali-calcic to alkaline magmatic trend typical for a post-collision setting. Trace elements spidergrams show a pattern typical for subduction-related suites of orogenic belts. REE patterns show moderate enrichment in LREE relative to flat HREE, with strong negative Eu

  9. Sedimentary Rocks

    NASA Technical Reports Server (NTRS)

    2005-01-01

    6 November 2005 This Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image shows outcrops of sedimentary rocks in a crater located just north of the Sinus Meridiani region. Perhaps the crater was once the site of a martian lake.

    Location near: 2.9oN, 359.0oW Image width: width: 3 km (1.9 mi) Illumination from: lower left Season: Northern Autumn

  10. Characterization of depositional age and structure of sedimentary successions by U-Pb TIMS and LA-ICP-MS dating of volcanic horizons and detrital zircons: an example from the western Trondheim Nappe Complex, Scandinavian Caledonides

    NASA Astrophysics Data System (ADS)

    Gasser, Deta; Grenne, Tor; Corfu, Fernando; Eivind Augland, Lars

    2016-04-01

    Revealing the absolute depositional age of non-fossiliferous sedimentary successions represents a long-lasting challenge in Earth Sciences. Lacking age control hampers the correct interpretation of the temporal evolution of depositional systems, and, if deformed, of the architecture of fold-and-thrust belts. Dating of detrital zircons within clastic sedimentary successions has over the past decades become a popular method to approximate the absolute depositional age and to characterize the source areas of such rocks. If combined with other geochronological information, such as dating of contemporaneous volcanic horizons, a much better resolution of the stratigraphy and structure of non-fossiliferous sedimentary successions can be achieved. The western Trondheim nappe complex in the central Scandinavian Caledonides is a classical area in this respect. On top of Late Cambrian to Early Ordovician ophiolitic fragments, various volcanic, volcano-clastic and clastic successions tell a complex story of island-arc formation, ocean closure and continent collision. Several famous fossil horizons indicate deposition during the Middle to Upper Ordovician (ca. 470-445 Ma), but large areas lack an absolute age control and several contrasting stratigraphic schemes and structural interpretations have been presented in the past. In this contribution we present the results of LA-ICP-MS detrital U-Pb zircon dating of clastic horizons as well as U-Pb TIMS zircon dating of volcanic horizons and magmatic clasts in conglomerates in order to characterize the depositional age and structure of the western Trondheim nappe complex in more detail. Together with field observations, including way up criteria, the zircon data enable significant revisions of existing stratigraphic and structural models. At least four (volcano-)sedimentary successions can be distinguished above the ca. 480-485 Ma greenstones: (1) ca. 470-463 Ma shales, limestones and andesitic porphyrites (Hølonda and Fanabekken

  11. Geochronological Constraints on Neoproterozoic Glaciations, the first appearance of Metazoans, and the Cambrian Explosion.

    NASA Astrophysics Data System (ADS)

    Bowring, S.; Condon, D.; Ramezani, J.; Myrow, P.; Landing, E.

    2004-05-01

    Studies of Neoproterozoic climate fluctuations, plate reconstructions, biological evolution and their interrelationships have been hindered by a lack of high-precision geochronological constraints. The correlation and estimates of duration for Neoproterozoic glaciations has relied on physical/chemo-stratigraphy, and thermal subsidence models respectively. New geochronological constraints from Neoproterozoic successions worldwide have sharpened the debate as to the number, synchroneity, and duration of glacial episodes and the relationship, if any, between Metazoan evolution and global glaciation(s). Crucial to the debate are correct interpretation of geochronological data that range from U-Pb zircon studies of intercalated volcanic ash-beds, U-Pb detrital zircon studies, Re-Os from black shales, Rb-Sr from clay-rich rocks, U-Pb and Pb-Pb from carbonates and phosphates, and Lu-Hf from phosphates. Development of a highly resolved Neoproterozoic timescale will require integration and cross-calibration of multiple dating techniques and consideration of what is actually being recorded by each chronometer. A review of available geological and geochronological data indicate that there were at least three and perhaps as many as five periods of Neoproterozoic glacial deposition including rocks from United States (Idaho and Virginia), Newfoundland and the Northwest Territories of Canada, Namibia, and Oman. What must be evaluated is how the paleogeographic distribution of glaciated regions varied with time during the Neoproterozoic. Do Neoproterozoic glacial successions distributed worldwide record a small number of globally synchronous, long-lived glaciations, or numerous diachronous glacial epochs, or a combination of both? At present, the duration of only one glacial deposit, the ca 581 Ma Gaskiers Formation (Newfoundland), is known and it is on the order of 1 Ma, at odds with a long-lived global glaciation predicted by the snowball Earth hypothesis. Other major issues are

  12. Paleomagnetic constraints on the tectonic evolution of the Costa Rican subduction zone: New results from sedimentary successions of IODP drill sites from the Cocos Ridge

    NASA Astrophysics Data System (ADS)

    Li, Yong-Xiang; Zhao, Xixi; Jovane, Luigi; Petronotis, Katerina E.; Gong, Zheng; Xie, Siyi

    2015-12-01

    The near-flat subduction of the Cocos Ridge (CR) along the Middle American Trench (MAT) plays a pivotal role in governing the geodynamic evolution of the central American convergent margin. Elucidating the onset of its subduction is essential to understand the tectonic evolution and seismogenesis of the Costa Rican convergent margin, a typical erosive convergent margin and modern example of a flat-slab subduction. Initial subduction of the CR has been previously investigated by examining upper plate deformation that was inferred to have resulted from the initial CR subduction. However, little attention has been paid to the extensive sedimentary archives on the CR that could hold important clues to the initial CR subduction. Drilling on the CR during IODP Expedition 344 discovered a pronounced sedimentary hiatus at Site U1381. Here we present paleomagnetic and rock magnetic results of the Cenozoic sedimentary sequences at this site that bracket the hiatus between ca. 9.61 and 1.52 Ma. We also examine the areal extent, timing, and geologic significance of the hiatus by analyzing sedimentary records from five other ODP/IODP sites on CR and Cocos plate. The analyses show that the hiatus appears to be regional and the presence/absence of the sedimentary hiatus at different locations on CR implies a link to the onset of CR shallow subduction, as a result of either bottom current erosion or CR buckling upon its initial collision at the MAT. Records directly from CR thus provide a new window to unraveling the geodynamic evolution of the central American margin.

  13. Taphonomic and evolutionary changes across the Mesoproterozoic-Neoproterozoic transition

    NASA Technical Reports Server (NTRS)

    Knoll, A. H.; Sergeev, V. N.

    1995-01-01

    The principal biological distinction between Mesoproterozoic and Neoproterozoic is the abundance and diversity of eukaryotic fossils in the Neoproterozoic rocks, but the two eras also differ in the composition of preserved cyanobacterial assemblages. Evolving eukaryotes provide a partial explanation for observed differences in prokaryotic fossils, but the taphonomic and environmental influences of shifting carbonate depositional pattern are also important.

  14. Neoproterozoic Glacial Extremes: How Plausible is the

    NASA Astrophysics Data System (ADS)

    Peltier, W. R.

    2004-05-01

    The suggestion that the glaciation events of the Neoproterozoic could have been global in extent, so-called "snowball" glaciations, during which the oceans were entirely covered by sea ice and the continents by massive continental ice sheets, is an idea tha is recurrent in the geological and climate dynamics literature. It is an idea that haa both critics and defenders but concensus concerning it's plausiblity has yet to emerge. Previous work on this problem has led to the suggestion that a more likely scenario than the "hard snowball" is one in which open water continues to persist at the equator, thus enabling biological evolution into the Cambrian to proceed, perhaps stimulated by the transition from the cold conditions of the Neoproterozoic to the warm condition of the Cambrian, thus leading to the Cambrian "explosion of life". We will discuss recent extensions of our previous efforts to model the extreme climate of the Neoproterozoic, using both the University of Toronto Glacial Systems Model and the NCAR Community Climate System Model. With an appropriate choice for the albedo of sea ice, the former model conntinues to deliver hysteresis in the surface temperature vs. CO2 concentration space when solar luminosity is reduced by 6% below modern, and thus continues to suggest the existence of the previously hypothesized "CO2 attractor". We argue here that the system could be locked onto this attractor by the strong "out of equilibrium" effects of the carbon cycle recently discussed by Rothman et al. (PNAS, 2003). The open water solution is confirmed as the preferred mode of the system by the detailed CCSM integrations that we have performed.

  15. Strontium isotopic variations of Neoproterozoic seawater: implications for crustal evolution.

    PubMed

    Asmerom, Y; Jacobsen, S B; Knoll, A H; Butterfield, N J; Swett, K

    1991-01-01

    We report high precision Sr isotopic data on carbonates from the Neoproterozoic Shaler Group, Victoria Island, Northwest Territories, Canada. Lithostratigraphic correlations with the relatively well-dated Mackenzie Mountains Supergroup constrain Shaler deposition to approximately 770-880 Ma, a range corroborated by 723 +/- 3 Ma lavas that disconformably overlie Shaler carbonates and by Late Riphean microfossils within the section. Samples with low 87Rb/86Sr ratios (<0.01) were selected for Sr isotopic analysis. Delta 18O, Mn, Ca, Mg, and Sr data were used to recognize altered samples. The altered samples are characterized by high Mn/Sr (> or = 2) and variable delta 18O; most are dolomites. The data indicate that between ca. 790-850 Ma the 87Sr/86Sr ratio of seawater varied between 0.70676 and 0.70561. The samples show smooth and systematic variation, with the lowest 87Sr/86Sr value of 0.70561 at ca. 830 Ma. The low 87Sr/86Sr ratio of carbonates from the lower parts of our section is similar to a value reported for one sample from the Adrar of Mauritania (approximately 900 Ma), West African Craton. Isotopic ratios from the upper part of the Shaler section are identical to values from the lower part of the Neoproterozoic Akademikerbreen Group, Spitsbergen. Although a paucity of absolute age determinations hinders attempts at the precise correlation of Neoproterozoic successions, it is possible to draw a broad outline of the Sr isotopic composition of seawater for this period. Indeed, the Sr isotope data themselves provide a stratigraphic tool of considerable potential. Data from this study and the literature are used to construct a curve of the 87Sr/86Sr ratio of Neoproterozoic seawater. The new data reported in this study substantially improve the isotopic record of Sr in seawater for the period 790-850 Ma. The Sr isotope composition of seawater reflects primarily the balance between continental Sr input through river input and mantle input via hydrothermal

  16. The structure and stratigraphy of the sedimentary succession in the Swedish sector of the Baltic Basin: New insights from vintage 2D marine seismic data

    NASA Astrophysics Data System (ADS)

    Sopher, Daniel; Erlström, Mikael; Bell, Nicholas; Juhlin, Christopher

    2016-04-01

    We present five interpreted regional seismic profiles, describing the full sedimentary sequence across the Swedish sector of the Baltic Sea. The data for the study are part of an extensive and largely unpublished 2D seismic dataset acquired between 1970 and 1990 by the Swedish Oil Prospecting Company (OPAB). The Baltic Basin is an intracratonic basin located in northern Europe. Most of the Swedish sector of the basin constitutes the NW flank of a broad synclinal depression, the Baltic Basin. In the SW of the Swedish sector lies the Hanö Bay Basin, formed by subsidence associated with inversion of the Tornquist Zone during the Late Cretaceous. The geological history presented here is broadly consistent with previously published works. We observe an area between the Hanö Bay and the Baltic Basin where the Palaeozoic strata has been affected by transpression and subsequent inversion, associated with the Tornquist Zone during the late Carboniferous-Early Permian and Late Cretaceous, respectively. We propose that the Christiansø High was a structural low during the Late Jurassic, which was later inverted in the Late Cretaceous. We suggest that a fan shaped feature in the seismic data, adjacent to the Christiansø Fault within the Hanö Bay Basin, represents rapidly deposited, coarse-grained sediments eroded from the inverted Christiansø High during the Late Cretaceous. We identify a number of faults within the deeper part of the Baltic Basin, which we also interpret to be transpressional in nature, formed during the Caledonian Orogeny in the Late Silurian-Early Devonian. East of Gotland a number of sedimentary structures consisting of Silurian carbonate reefs and Ordovician carbonate mounds, as well as a large Quaternary glacial feature are observed. Finally, we use the seismic interpretation to infer the structural and stratigraphic history of the Baltic and Hanö Bay basins within the Swedish sector.

  17. Tectonics and sedimentation of the Lower and Middle Pleistocene mixed siliciclastic/bioclastic sedimentary successions of the Ionian Peloritani Mts (NE Sicily, Southern Italy): the onset of opening of the Messina Strait

    NASA Astrophysics Data System (ADS)

    Stefano, Agata; Longhitano, Sergio

    2009-03-01

    Biostratigraphic analyses carried out on siliciclastic/bioclastic deposits discontinuously cropping out along the Ionian flank of NE Sicily, indicate that they form two sedimentary events of Early and Middle Pleistocene, respec tively. Vertical facies successions, showing transgressive trends, suggest that sedimentation occurred within semi-enclosed marine embayments, where sublittoral coastal wedges developed on steep ramp-type shelves. Sediments accumulated in shoreface to offshore transitions along steep bottom profiles. This depositional scenario was strongly conditioned by the tectonic activity of the rift zone linking Western Calabria and Eastern Sicily. The effects of glacio-eustatism were also recognized. According to our reconstruction, the study area was controlled by a transfer fault system which affected the coastal margin producing major episodes of uplift and subsidence. Block-faulting was responsible for significant cannibalization and recycling of older deposits during the Middle Pleistocene. Such a tectonic setting can be considered the precursor scenario for the formation of the Messina Strait between Calabria and Sicily. This narrow, linear basin influences the hydrodynamic setting of sublittoral deposits along the Ionian coast of Sicily, giving rise to strong flood/ebb tidal currents. The uppermost part of the Middle Pleistocene succession recognized in the study area is indeed dominated by tide-influenced associations of sedimentary structures which most likely record the first stage of the opening of this `seaway' of the central Mediterranean Sea.

  18. Neoproterozoic fragmentation of the Scottish Sector of Laurentia - an ancient analogue for the Iberian and UK/Irish ocean-continent transition zones

    NASA Astrophysics Data System (ADS)

    Leslie, G.; Krabbendam, M.

    2009-04-01

    The Neoproterozoic Dalradian Supergroup of Scotland and Ireland is intensively deformed and metamorphosed by mid-Ordovician arc-accretion (c. 460 Ma) during the Caledonian Orogeny. Emplacement of an extensive suite of Siluro-Devonian Caledonian granitoids further complicates reading the sedimentary record. Nevertheless we can determine a history of stretching and break-up affecting the Neoproterozoic supercontinent of Rodinia and leading to creation of the Iapetus Ocean. Three key intervals of late-Neoproterozoic sediment accumulation are recognised - new geological mapping, isotopic datasets (Sr, O and C, U/Pb zircon, Sm/Nd WR), and sequence stratigraphical approaches are refining constraints on the lithostratigraphical architecture and basin evolution of the Dalradian Supergroup. Thick siliciclastic deposits accumulated (pre-800 Ma?) during an early stretching phase (distributed high angle faulting) that led to crustal thinning (low angle shearing). Three major limestone - pelite - quartzite depositional cycles succeeded these earlier siliciclastic deposits, recording episodic subsidence in an intracratonic but largely marine environment; the second cycle overlaps the late Precambrian (Cryogenian) glaciation and concludes with the distinctive Marinoan tillite succession (c. 635Ma). The last of the three cycles is terminated, in some parts of the Dalradian, by deposition of serpentinitic muds and conglomerates and volcaniclastic sediments; pods and lenses of both massive and serpentinised ultramafic rock also interrupt the sedimentary record at this level (thus possibly indicating mantle exhumation). In other areas, a major part of the ‘type' Dalradian succession is absent and we now recognise a major overstep unconformity at this level. From this level onwards across the Dalradian, rapid foundering of the margin, and the transition from rift- to drift-dominated processes, resulted in an overstepping accumulation of laterally and vertically variable

  19. Giant submarine landslide grooves in the Neoproterozoic/Lower Cambrian Phe Formation, northwest Himalaya: Mechanisms of formation and palaeogeographic implications

    NASA Astrophysics Data System (ADS)

    Draganits, E.; Schlaf, J.; Grasemann, B.; Argles, T.

    2008-04-01

    Giant groove casts have been found in the upper Proterozoic to Lower Cambrian Phe Formation (Haimanta Group), a siliciclastic sandstone/shale succession in the Tethyan Zone of the Higher Himalaya tectonic unit. The grooves are among the largest linear erosion structures related to submarine mass-movements observed in the geologic record. They are up to 4 m wide, about 0.2 m deep and can be traced for more than 35 m without changing their character. The grooves are straight, subparallel to cross-cutting striations with shallow semi-circular cross-sections and well-defined superimposed minor ridges and grooves. Groove casts exist on the soles of several sandstone beds within a 73 m thick logged section, commonly associated with flute casts. Their characteristics were compared with several other types of ancient and modern submarine linear erosion structures. A sand-rich, non-channelized basin floor depositional environment is inferred from the lithofacies, the combination of sedimentary structures, the lack of coarse-grained pebbly facies, the lateral continuity of beds, and the lack of channel structures. The grooves probably formed by laminar debris flows/concentrated density flows dragging blocks of already lithified sediment across the basin floor. When the bedding is structurally rotated back to horizontal, the groove casts show consistent North-South oriented palaeocurrent trends, with South-directed palaeocurrent directions indicated by flute casts. These palaeocurrent orientations contrast with previous palaeogeographic reconstructions of this area, which propose sediment delivery from the South. We therefore suggest a new "double provenance" model for the spatial relationship of late Proterozoic to Early Cambrian strata of the Himalaya, in which Lesser and Tethyan Himalayan age-equivalent sediment was deposited in a connected basin, where the former received detritus from the South, and the latter from a hitherto unknown source in the North. One possible

  20. Neoproterozoic-Cambrian stratigraphic framework of the Anti-Atlas and Ouzellagh promontory (High Atlas), Morocco

    NASA Astrophysics Data System (ADS)

    Álvaro, J. Javier; Benziane, Fouad; Thomas, Robert; Walsh, Gregory J.; Yazidi, Abdelaziz

    2014-10-01

    In the last two decades, great progress has been made in the geochronological, chrono- and chemostratigraphic control of the Neoproterozoic and Cambrian from the Anti-Atlas Ranges and the Ouzellagh promontory (High Atlas). As a result, the Neoproterozoic is lithostratigraphically subdivided into: (i) the Lkest-Taghdout Group (broadly interpreted at c. 800-690 Ma) representative of rift-to-passive margin conditions on the northern West African craton; (ii) the Iriri (c. 760-740 Ma), Bou Azzer (c. 762-697 Ma) and Saghro (c. 760?-610 Ma) groups, the overlying Anezi, Bou Salda, Dadès and Tiddiline formations localized in fault-grabens, and the Ouarzazate Supergroup (c. 615-548 Ma), which form a succession of volcanosedimentary complexes recording the onset of the Pan-African orogeny and its aftermath; and (iii) the Taroudant (the Ediacaran-Cambrian boundary lying in the Tifnout Member of the Adoudou Formation), Tata, Feijas Internes and Tabanite groups that have recorded development of the late Ediacaran-Cambrian Atlas Rift. Recent discussions of Moroccan strata to select new global GSSPs by the International Subcommissions on Ediacaran and Cambrian Stratigraphy have raised the stratigraphic interest in this region. A revised and updated stratigraphic framework is proposed here to assist the tasks of both subcommissions and to fuel future discussions focused on different geological aspects of the Neoproterozoic-Cambrian time span.

  1. Constructing a Neoproterozoic Seawater Strontium Isotope Curve

    NASA Astrophysics Data System (ADS)

    Zhou, Y.; Shields-Zhou, G. A.; Manning, C. J.; Thirlwall, M.; Thurow, J. W.; Zhu, M.; Ling, H.

    2014-12-01

    The strontium isotopic composition of seawater has varied throughout Earth history in response to the balance between Sr isotopic exchange with ocean crust and input of riverine Sr derived from continental weathering. Because of this, seawater 87Sr/86Sr highs are interpreted to reflect erosion events, related to mountain building, while 87Sr/86Sr lows are considered to result from low weathering rates or increased seafloor spreading. Seawater 87Sr/86Sr also responds to changes in the isotopic composition of material undergoing weathering. The largest ever increase in seawater 87Sr/86Sr took place sometime from approximately 900 Ma to 500 Ma, and was associated with a permanent step shift in baseline 87Sr/86Sr composition. The unprecedented size of this increase, its timing and causation remains unconstrained. This study attempts firstly to reconstruct global seawater 87Sr/86Sr trends through this increase, using well-preserved carbonate rock samples from the North China craton, calibrated against additional 87Sr/86Sr and δ13C data from Neoproterozoic samples collected from other sections around the world. Sample preparation techniques for bulk carbonate Sr isotope stratigraphy are being honed during the course of this study. Other stable isotope systems (δ13C and δ18O) and trace elements, including REE have been investigated on the same samples to identify pristine samples for Sr isotope analysis and help in interpretation. The newly obtained data from this study (mainly Huaibei group of Huaibei area), using the excellently preserved early marine calcite cements and some bulk rock samples, confirm that the carbonate strata across the Jiao-Liao-Xu-Huai stratigraphic realm of the North China Craton exhibit the moderately positive δ13C values and low 87Sr/86Sr values that are characteristic of the early Neoproterozoic (Tonian).The results help to recreate the global curve by linking negative excursions in the Shijia (Xuzhou) (Xiao et al., 2014, Precambr. Res., 246

  2. Late Neoproterozoic Paleogeography of the Western Part of Baltica- Constraints From Paleomagnetic and Tectonic Analysis.

    NASA Astrophysics Data System (ADS)

    Nawrocki, J.; Poprawa, P.; Boguckyj, A.

    2003-12-01

    Neoproterozoic break-up of supercontinent Rodinia resulted with development of several individual lithospheric plates, including Baltica. Global configuration of continents after break-up, in particular paleogeographic position of Baltica and its relation to Godwana, are the matter of recent disputes. According to Scandinavian paleomagnetic poles (c.f. Torsvik and Rehnstr”m, 2001) in the late Neoproterozoic and Cambrian time the Baltic plate occupied moderate paleolatitudes of southern hemisphere. Vendian paleomagnetic pole from the White Sea region (Popov et al., 2002) is, however, completely different. It implies moderate northern paleolatitudes of Baltica in the Vendian time and probably equatorial position of this plate in the Early Cambrian (c.f. Nawrocki, 2003). In the present contribution we discuss impact of recent paleomagnetic and tectonic analysis conducted for the western part of Baltica on the above mentioned issue. As a consequence of the break-up process the western and central part of Baltica became a site of intensive basic volcanism (Orsha-Volhyn Zone) and development of sedimentary basins related to rifting. Geochronology of volcanic processes at the western slope of Baltica is poorly controlled, except of its expiring, dated by U/Pb method as 551 Ma (Compston et al., 1995). However its geotectonic setting is geochemically documented as rift-related (Bialowolska et al., 2002). Subsequently during the late-most Neoproterozoic-Middle Ordovician a system of sedimentary basins developed, controlled by post-rift thermal sag mechanism. Regional pattern of their subsidence allows to reconstruct passive continental margin along western margin of Baltica (Peri-Tornquist Zone, corresponding to later Trans-European Suture Zone) (Poprawa et al., 1999). Preliminary paleomagnetic analysis of the Vendian basalts and underlying basaltic tuffs from the Volhynia (NW Ukraine) revealed paleomagnetic directions concordant with the data from the White Sea region

  3. Diachronism in the late Neoproterozoic-Cambrian arc-rift transition of North Gondwana: A comparison of Morocco and the Iberian Ossa-Morena Zone

    NASA Astrophysics Data System (ADS)

    Álvaro, J. Javier; Bellido, Félix; Gasquet, Dominique; Pereira, M. Francisco; Quesada, Cecilio; Sánchez-García, Teresa

    2014-10-01

    In the northwestern border of the West African craton (North Gondwana), a transition from late Neoproterozoic subduction/collision to Cambrian rift processes was recorded in the Anti-Atlas (Morocco) and in the Ossa-Morena Zone (Iberia). Cambrian rifting affected both Pan-African and Cadomian basements in a stepwise and diachronous way. Subsequently, both areas evolved into a syn-rift margin episodically punctuated by uplift and tilting that precluded Furongian sedimentation. A comparison of sedimentary, volcanic and geodynamic evolution is made in the late Neoproterozoic (Pan-African and Cadomian) belts and Cambrian rifts trying to solve the apparent diachronous (SW-NE-trending) propagation of an early Palaeozoic rifting regime that finally led to the opening of the Rheic Ocean.

  4. How juvenile is the Arabian Nubian Shield? Evidence from Nd isotopes and pre-Neoproterozoic inherited zircon in the Bi'r Umq suture zone, Saudi Arabia

    NASA Astrophysics Data System (ADS)

    Hargrove, U. S.; Stern, R. J.; Kimura, J.-I.; Manton, W. I.; Johnson, P. R.

    2006-12-01

    The Bi'r Umq suture zone (BUSZ) in western Saudi Arabia comprises Neoproterozoic oceanic-arc plutonic, volcanosedimentary, and ophiolitic rocks that record some of the earliest magmatic and tectonic events of the East African Orogen in the Arabian-Nubian Shield (ANS). New Nd isotopic analyses are combined with data on zircon inheritance and published isotopic studies to establish the case that pre-Neoproterozoic crust had a greater influence on the oceanic portion of the ANS than is appreciated. Highly positive initial ɛNd (+ 3.9 to + 8.5) and Nd model ages (560-830 Ma) that approximate crystallization ages (573-813 Ma) of BUSZ igneous rocks are comparable to upper crust in other parts of the ANS considered to be juvenile (mantle-derived) and to some xenoliths from the lower crust and lithospheric mantle. Overall, the data suggest that the 40-km-thick crust beneath much of the Arabian Shield is juvenile and that most of it was extracted from depleted mantle during the interval ˜ 740-830 Ma. Although much of the ANS is isotopically juvenile, some Neoproterozoic igneous rocks in the northern ANS contain zircon inherited from pre-Neoproterozoic sources. Samples from the BUSZ that show inheritance yield slightly lower initial ɛNd than contemporary samples that do not show inheritance, suggesting that some juvenile magmas assimilated older continental material. The age of that material is inferred to be largely early Neoproterozoic and Mesoproterozoic, with minor Paleoproterozoic and Archean components, based on U-Pb ages of inherited zircon and Nd model ages for ANS upper crustal rocks and xenoliths of the lower crust and mantle lithosphere. Inherited zircon may have been assimilated from terrigenous sediment shed from nearby passive margins, and transported fluvially or by glaciers, or by assimilation of cryptic early Neoproterozoic to Archean basement that underlies the "juvenile" core of the ANS. Zircon morphologies are consistent with both sedimentary origins

  5. Sedimentary Parameters Controlling Occurrence and Preservation of Microbial Mats in Siliciclastic Depositional Systems

    NASA Technical Reports Server (NTRS)

    Noffke, Nora; Knoll, Andrew H.

    2001-01-01

    Shallow-marine, siliciclastic depositional systems are governed by physical sedimentary processes. Mineral precipitation or penecontemporaneous cementation play minor roles. Today, coastal siliciclastic environments may be colonized by a variety of epibenthic, mat-forming cyanobacteria. Studies on microbial mats showed that they are not randomly distributed in modern tidal environments. Distribution and abundancy is mainly function of a particular sedimentary facies. Fine-grained sands composed of "clear" (translucent) quartz particles constitute preferred substrates for cyanobacteria. Mat-builders also favor sites characterized by moderate hydrodynamic flow regimes, which permit biomass enrichment and construction of mat fabrics without lethal burial of mat populations by fine sediments. A comparable facies relationship can be observed in ancient siliciclastic shelf successions from the terminal Neoproterozoic Nama Group, Namibia. Wrinkle structures that record microbial mats are present but sparsely distributed in mid- to inner shelf sandstones of the Nudaus Formation. The sporadic distribution of these structures reflects both the narrow ecological window that governs mat development and the distinctive taphonomic conditions needed to preserve the structures. These observations caution that statements about changing mat abundance across the Proterozoic-Cambrian boundary must be firmly rooted in paleoenvironmental and taphonomic analysis. Understanding the factors that influence the formation and preservation of microbial structures in siliciclastic regimes can facilitate exploration for biological signatures in Earth's oldest rocks. Moreover, insofar as these structures can be preserved on bedding surfaces and are not easily mimicked by physical processes, they constitute a set of biological markers that can be searched for on Mars by remotely controlled rovers.

  6. Diachronous evolution of volcano-sedimentary basins north of the Congo craton: Insights from U Pb ion microprobe dating of zircons from the Poli, Lom and Yaoundé Groups (Cameroon)

    NASA Astrophysics Data System (ADS)

    Toteu, Sadrack Félix; Penaye, Joseph; Deloule, Etienne; Van Schmus, William Randall; Tchameni, Rigobert

    2006-04-01

    Ion microprobe U-Pb dating of zircons from Neoproterozoic volcano-sedimentary sequences in Cameroon north of the Congo craton is presented. For the Poli basin, the depositional age is constrained between 700-665 Ma; detrital sources comprise ca. 920, 830, 780 and 736 Ma magmatic zircons. In the Lom basin, the depositional age is constrained between 613 and 600 Ma, and detrital sources include Archaean to Palaeoproterozoic, late Mesoproterozoic to early Neoproterozoic (1100-950 Ma), and Neoproterozoic (735, 644 and 613 Ma) zircons. The Yaoundé Group is probably younger than 625 Ma, and detrital sources include Palaeoproterozoic and Neoproterozoic zircons. The depositional age of the Mahan metavolcano-sedimentary sequence is post-820 Ma, and detrital sources include late Mesoproterozoic (1070 Ma) and early Neoproterozoic volcanic rocks (824 Ma). The following conclusions can be made from these data. (1) The three basins evolved during the Pan-African event but are significantly different in age and tectonic setting; the Poli is a pre- to syn-collisional basin developed upon, or in the vicinity of young magmatic arcs; the Lom basin is post-collisional and intracontinental and developed on old crust; the tectono-metamorphic evolution of the Yaoundé Group resulted from rapid tectonic burial and subsequent collision between the Congo craton and the Adamawa-Yade block. (2) Late Mesoproterozoic to early Neoproterozoic inheritance reflects the presence of magmatic event(s) of this age in west-central Africa.

  7. Neoproterozoic to Early Cambrian clastics sedimentation and stratigraphy in the Central and Southern Appalachians: An overview

    SciTech Connect

    Schwab, F.L. . Dept. of Geology)

    1993-03-01

    A clear understanding of paleogeography, tectonics, and sedimentary framework now exists for Neoproterozoic to Early Cambrian clastics in the Central and Southern Appalachians. It is based on well-constrained data on mineralogy, texture, and sedimentary structures and less precise information on age and regional variations in lithology and thickness. From 900 m.y. ago until 600 m.y. ago, tension along the eastern edge of North America produced a series of NE-SW basins (grabens and aulacogens ). These rift-related basins filled with thick, coarse, arkosic clastics (Mechum River Fm., Mt. Rogers Volc. Gp., Grandfather Mtn. Fm., portions of the Ocoee Series) mimicking the setting that later typified the Triassic of eastern North America. Coeval sequences exposed along the southeastern edge of the Blue Ridge in Va. and N.C. (Fauquier Fm., Lynchburg Gp., Ashe Fm.) define the hinge zone of a developing continental margin. Farther south in Tenn., Ga., and Ala., the picture is less clear. In latest Precambrian and Early Cambrian time, a passive Atlantic-type'' margin existed. This consisted of paired continental shelf and continental slope-rise areas (shallow water deposits of the Chilhowee Gp. and overlying muds and carbonates to the northwest; deep water clastics of the Evington Gp. and Alligator Back Fm. to the southeast). The cohesiveness of this framework argues against these tectonostratigraphic belts being considered terranes.

  8. Geochronology of Neoproterozoic Plutons and Sandstones in the Western Jiangnan Orogen: a Reappraisal of Amalgamation between Yangtze and Cathaysia Blocks in South China

    NASA Astrophysics Data System (ADS)

    Ma, X.; Yang, K.

    2015-12-01

    Widespread exposure of Meso-Neoproterozoic strata and abundant Neoproterozoic plutons occur along the Jiangnan orogenic belt, a tectonic suture between the composite Cathaysia and Yangtze Blocks in South China, with remarkable angular unconformities between the Xiajiang Group and the underlying Sibao. LA-ICP-MS U-Pb dating of the basement sedimentary sequences (e.g. Xiajiang Group and Sibao Group) and the relevant granitic pluton (Motianling pluton) provides new information about the pre-Cambrian evolution of the southeastern Yangtze block margin along the western Jiangnan orogen. The depositional age of Sibao Group located in the Southeast Guizhou Province can be constrained at 825-835 Ma by the youngest detrital zircon ages and crystallization age of the intrusive Motianling granitic pluton. The maximum depositional age of the Xiajiang Group is estimated to be ca. 795 Ma. Four main age populations are evident in Sibao Group: 0.83-1.0 Ga, 1.3-1.5 Ga, 1.6-1.8 Ga and 2.2-2.6 Ga. The distinguished age populations in the Xiajiang Group are identical to those in the Sibao Group but lack in ranges of 1.3-1.5 Ga. Local abundant (<0.9 Ga) granitoid outcrops, distant early-Neoproterozoic (0.9-1.0 Ga) components and pre-Neoproterozoic recycled materials from the interior of the Yangtze Block both contribute to the sedimentation of the Neoproterozoic basement sequences. We suggest that a rapid exhumation and uplift occurred after 820 Ma along the southeastern margin of Yangtze Block, and provided abundant detritus to the Neoproterozoic basin between the Yangtze and Cathaysia blocks, after which the assembly process gradually ceased till ca. 800 Ma rather than immediately paused or switched to extension after ca. 820 Ma. Regarding the location of the South China Block in the breakup the Rodinia, it is more reasonable to place Yangtze and Cathaysia Blocks on the western margin rather than the center of Rodinia supercontinent during the late-Neoproterozoic time.

  9. Replacement of benthic communities in two Neoproterozoic-Cambrian subtropical-to-temperate rift basins, High Atlas and Anti-Atlas, Morocco

    NASA Astrophysics Data System (ADS)

    Clausen, Sébastien; Álvaro, J. Javier; Zamora, Samuel

    2014-10-01

    The ‘Cambrian explosion’ is often introduced as a major shift in benthic marine communities with a coeval decline of microbial consortia related to the diversification of metazoans and development of bioturbation (‘Agronomic Revolution’). Successive community replacements have been reported along with ecosystem diversification and increase in guild complexity from Neoproterozoic to Cambrian times. This process is recorded worldwide but with regional diachroneities, some of them directly controlled by the geodynamic conditions of sedimentary basins. The southern High Atlas and Anti-Atlas of Morocco record development of two rifts, Tonian (?) - early Cryogenian and latest Ediacarian-Cambrian in age, separated by the onset of the Pan-African Orogeny. This tectonically controlled, regional geodynamic change played a primary control on pattern and timing of benthic ecosystem replacements. Benthic communities include microbial consortia, archaeocyathan-thromboid reefal complexes, chancelloriid-echinoderm-sponge meadows, and deeper offshore echinoderm-dominated communities. Microbial consortia appeared in deeper parts of the Tonian (?) - early Cryogenian fluvio-deltaic progradational rift sequences, lacustrine environments of the Ediacaran Volcanic Atlasic Chain (Ouarzazate Supergroup) and the Ediacaran-Cambrian boundary interval, characterized by the peritidal-dominated Tifnout Member (Adoudou Formation). They persisted and were largely significant until Cambrian Age 3, as previous restricted marine conditions precluded the immigration of shelly metazoans in the relatively shallow epeiric parts of the Cambrian Atlas Rift. Successive Cambrian benthic communities were replaced as a result of distinct hydrodynamic and substrate conditions, which allow identification of biotic (e.g., antagonistic relationships between microbial consortia and echinoderms, and taphonomic feedback patterns in chancelloriid-echinoderm-sponge meadows) and abiotic (e.g., rifting

  10. Detrital zircon geochronology of Neoproterozoic to Middle Cambrian miogeoclinal and platformal strata: Northwest Sonora, Mexico

    USGS Publications Warehouse

    Gross, E.L.; Stewart, John H.; Gehreis, G.E.

    2000-01-01

    Eighty-five detrital zircon grains from Mesoproterozoic and/or Neoproterozoic to Middle Cambrian sedimentary strata in northwest Sonora, Mexico, have been analyzed to determine source terranes and provide limiting depositional ages of the units. The zircon suites from the Mesoproterozoic and/or Neoproterozoic El Alamo Formation and El Aguila unit yield ages between 1.06 Ga and 2.67 Ga, with predominant ages of 1.1 to 1.2 Ga. Zircons from the Lower? and Middle Cambrian Bolsa Quartzite show age groups from 525 Ma to 1.63 Ga, with a dominant population of 1.1 to 1.2 Ga grains. Grains older than 1.2 Ga in the samples were most likely derived from basement terranes and ???1.4 Ga granitic bodies of the southwest U.S. and northwest Mexico. It is also possible that the sediments were transported from the south, although source rocks of the appropriate age are not presently exposed south of the study area in northern Mexico. Three possibilities for the dominant 1.1 to 1.2 Ga grains include derivation from: (I) exposures of the Grenville belt in southern North America, (2) local 1.1-1.2 Ga granite bodies, or (3) a southern source, such as the Oaxaca terrane, that was subsequently rifted away. Sampling of additional units in the western U.S. and northern Mexico may help resolve the ambiguity surrounding the source of the 1.1 to 1.2 Ga grains.

  11. A model for Iapetan rifting of Laurentia based on Neoproterozoic dikes and related rocks

    USGS Publications Warehouse

    Burton, William C.; Southworth, Scott

    2010-01-01

    Geologic evidence of the Neoproterozoic rifting of Laurentia during breakup of Rodinia is recorded in basement massifs of the cratonic margin by dike swarms, volcanic and plutonic rocks, and rift-related clastic sedimentary sequences. The spatial and temporal distribution of these geologic features varies both within and between the massifs but preserves evidence concerning the timing and nature of rifting. The most salient features include: (1) a rift-related magmatic event recorded in the French Broad massif and the southern and central Shenandoah massif that is distinctly older than that recorded in the northern Shenandoah massif and northward; (2) felsic volcanic centers at the north ends of both French Broad and Shenandoah massifs accompanied by dike swarms; (3) differences in volume between massifs of cover-sequence volcanic rocks and rift-related clastic rocks; and (4) WNW orientation of the Grenville dike swarm in contrast to the predominately NE orientation of other Neoproterozoic dikes. Previously proposed rifting mechanisms to explain these features include rift-transform and plume–triple-junction systems. The rift-transform system best explains features 1, 2, and 3, listed here, and we propose that it represents the dominant rifting mechanism for most of the Laurentian margin. To explain feature 4, as well as magmatic ages and geochemical trends in the Northern Appalachians, we propose that a plume–triple-junction system evolved into the rift-transform system. A ca. 600 Ma mantle plume centered east of the Sutton Mountains generated the radial dike swarm of the Adirondack massif and the Grenville dike swarm, and a collocated triple junction generated the northern part of the rift-transform system. An eastern branch of this system produced the Long Range dike swarm in Newfoundland, and a subsequent western branch produced the ca. 554 Ma Tibbit Hill volcanics and the ca. 550 Ma rift-related magmatism of Newfoundland.

  12. Neoproterozoic Glacial Strata of the Centralian Superbasin: New Insight From Subsurface Data in the Southern Georgina Basin, Australia

    NASA Astrophysics Data System (ADS)

    Verdel, C.; Willink, R. J.; Gurney, J.

    2014-12-01

    The Georgina Basin portion of the Centralian Suberbasin locally preserves extensive successions of Neoproterozoic sediments, including some of the thickest Cryogenian glacial deposits in the world. Surficial exposure of these units is poor, however, necessitating description and sampling of subsurface stratigraphic records. We have examined drillcore from boreholes in the southern part of the Georgina Basin that penetrate particularly thick accumulations of glaciogenic strata. One of these cores includes, in stratigraphic succession, ~500 meters of laminated diamictite, an overlying 150 meters of coarse-grained sandstone and conglomerate, and an upper 30 meter interval of carbonate that includes conspicuous pink dolostone. C isotope values of the carbonate interval are approximately -1‰ at its base, rise to values around 0‰ within the pink dolostone, then decline to -1 to -2% at the top of the cored interval. While it is currently unclear whether the carbonate is a Neoproterozoic cap or an unconformably overlying Cambrian unit, correlations based on regional seismic and well data suggest that the thick accumulation of diamictite is a well-preserved record of Neoproterozoic glaciation. We have obtained high-resolution visible and shortwave-infrared reflectance spectroscopy data from these cores with a HyLogger instrument. These data permit detailed mineralogical description of the glacial interval at a scale of ~1 cm and comprise a fully digital stratigraphic record.

  13. The importance of XRD analysis in provenance and palaeoenvironmental studies of the Piedras de Afilar Formation, Neoproterozoic of Uruguay

    NASA Astrophysics Data System (ADS)

    Pamoukaghlian, K.; Poiré, D. G.; Gaucher, C.; Uriz, N.; Cingolani, C.; Frigeiro, P.

    2009-04-01

    ímite norte del Terreno Piedra Alta (Uruguay). Importancia de la faja milonítica sinestral de Colonia. Actas XVI Congreso Argentino de Geología, de La Plata. Gaucher, C., Poiré, D.G., Finney, S.C., Valencia, V.a., Blanco, G., Pamoukaghlian, K., Gómez Peral, L. (2008). Detrital zircón ages of Neoproterozoic sedimentary successions in Uruguay and Argentina: Insights into the geological evolution of the Rio de la Plata Craton. Precambrian Research. Hartmann, L.A., Campal, N., Santos, J.O., Mc. Neughton, N.J., Schipilov, A., Lafon, J.M. (2001). Archean crust in the Rio de la Plata Craton, Uruguay - SHRIMP U-Pb zircon reconnaissance geochronology. Journal of South American Earth Science, 14, 557-570. Pamoukaghlian, K., Gaucher, C., Bossi, J., Sial, N., Poire, D.G. (2006). First C and O isotopic data for the Piedras de Afilar Formation, Tandilia Terrane, Uruguay: their bearing on correlation and age. Fifth South American Symposium on Isotope Geology, Punta del Este.

  14. Constraining the margins of Neoproterozoic ice masses: depositional signature, palaeoflow and glaciodynamics

    NASA Astrophysics Data System (ADS)

    Busfield, Marie; Le Heron, Daniel

    2016-04-01

    The scale and distribution of Neoproterozoic ice masses remains poorly understood. The classic Snowball Earth hypothesis argues for globally extensive ice sheets, separated by small ocean refugia, yet the positions of palaeo-ice sheet margins and the extent of these open water regions are unknown. Abundant evidence worldwide for multiple cycles of ice advance and recession is suggestive of much more dynamic mass balance changes than previously predicted. Sedimentological analysis enables an understanding of the changing ice margin position to be gained through time, in some cases allowing it to be mapped. Where the maximum extent of ice advance varies within a given study area, predictions can also be made on the morphology of the ice margin, and the underlying controls on this morphology e.g. basin configuration. This can be illustrated using examples from the Neoproterozoic Kingston Peak Formation in the Death Valley region of western USA. Throughout the Sperry Wash, northern Kingston Range and southern Kingston Range study sites the successions show evidence of multiple cycles of ice advance and retreat, but the extent of maximum ice advance is extremely variable, reaching ice-contact conditions at Sperry Wash but only ice-proximal settings in the most distal southern Kingston Range. The overall advance is also much more pronounced at Sperry Wash, from ice-distal to ice-contact settings, as compared to ice-distal to ice-proximal settings in the southern Kingston Range. Therefore, the position of the ice margin can be located at the Sperry Wash study site, where the more pronounced progradation is used to argue for topographically constrained ice, feeding the unconstrained shelf through the northern into the southern Kingston Range. This raises the question as to whether Neoproterozoic ice masses could be defined as topographically constrained ice caps, or larger ice sheets feeding topographically constrained outlet glaciers.

  15. Archaean Crustal Growth, Proterozoic Terrane Amalgamation and the Pan-African Orogeny, as Recorded in the NE African Sedimentary Record.

    NASA Astrophysics Data System (ADS)

    Najman, Y.; Fielding, L.; Millar, I.; Butterworth, P.; Andò, S.; Padoan, M.; Barfod, D. N.; Kneller, B. C.

    2015-12-01

    The cratons of Central Africa are formed of various blocks of Archaean and Palaeoproterozoic crust, flanked or truncated by Palaeoproterozoic to Mesoproterozoic orogenic belts. The geology of east Africa has largely been shaped by the events of the Pan-African Orogeny when east and west Gondwana collided to form 'Greater Gondwana' at the end of the Neoproterozoic. The Pan-African orogeny in NE Africa involved the collision of Archaean cratons and the Saharan Metacraton with the Arabian Nubian Shield, a terrane comprising Neoproterozoic juvenile oceanic island arcs. Phanerozoic cover sedimentary rocks, eroded from the Pan-African orogenies, blanket much of NE Africa. Detrital data from these Phanerozoic cover sedimentary rocks, and modern rivers draining both the cover the basement, provide a wealth of information on basement evolution, of particular relevance for regions where the basement itself is poorly exposed due to ancient or modern sedimentary cover. From samples collected in Uganda, Ethiopia, Sudan and Egypt, we provide combined U-Pb and Hf-isotope zircon, U-Pb rutile and Ar-Ar mica datasets, heavy mineral analyses, and bulk trace element data, from Archaean basement, Phanerozoic cover and modern river sediment from the Nile and its tributaries to document the evolution of the North African crust. The data document early crust-forming events in the Congo Craton and Sahara Metacraton, phased development of the Arabian Nubian Shield culminating in the Neoproterozoic assembly of Gondwana during the Pan African Orogeny, and the orogen's subsequent erosion, with deposition of voluminous Phanerozoic cover.

  16. Neoproterozoic-Early Paleozoic tectonic evolution of the western part of the Kyrgyz Ridge (Northern Tian Shan) caledonides

    NASA Astrophysics Data System (ADS)

    Degtyarev, K. E.; Ryazantsev, A. V.; Tretyakov, A. A.; Tolmacheva, T. Yu.; Yakubchuk, A. S.; Kotov, A. B.; Salnikova, E. B.; Kovach, V. P.

    2013-11-01

    The conducted comprehensive study of the western part of Kyrgyz Ridge provided new data on the structure, composition and age of Precambrian and Early Paleozoic stratified and igneous complexes. The main achievements of these studies are: (1) the establishment of a wide age spectrum, embracing the interval from the Neoproterozoic to the end of the Early Ordovician, for the clastic-carbonate units composing the cover of the Northern Tian Shan sialic massif; (2) the reconstruction and dating of Early and Late Cambrian ophiolite complexes formed in suprasubduction settings;(3) the discovery and dating of the Early-Middle Ordovician volcano-sedimentary complex of island-arc affinity; and (4) proof of the wide occurrence of Late Ordovician granitoids, some of which bear Cu-Au-Mo ores. The intricate thrust-and-fold structure of the western part of the Kyrgyz Ridge, formed in several stages from the Middle Cambrian (?) until the end of the Middle Ordovician, was scrutinized; the importance of the Early Ordovician stage was demonstrated. The intrusion of large batholiths in the early Late Ordovician accomplished the caledonide structural evolution. Formation of Neoproterozoic and Early Paleozoic caledonide complexes, which were possibly related to the protracted and entangled evolution of the active continental margin, ceased by the Late Ordovician.

  17. Late Neoproterozoic metamorphic assemblages along the Pan-African Hamisana Shear Zone, southeastern Egypt: Metamorphism, geochemistry and petrogenesis

    NASA Astrophysics Data System (ADS)

    Ali-Bik, Mohamed W.; Sadek, Mohamed F.; Ghabrial, Doris Sadek

    2014-11-01

    A variety of Late Neoproterozoic gneisses and amphibolites are distributed along the N-S trending Hamisana Shear Zone (HSZ), in southeastern Egypt. The HSZ originated after the accretion of the Arabian-Nubian Shield (ANS) and covers an area of about 1500 km2 in southeastern Egypt and northeastern Sudan. The architecture of the northern part of the HSZ is best explained as a tectono-stratigraphic column, in which allochthonous ophiolitic mélange was thrusted onto metamorphosed island-arc assemblages (gneisses and amphibolites). The latter rock units were generally subjected to two successive phases of amphibolite facies metamorphism, followed by a thermal phase and retrograde overprint. The early penetrative, low- to medium-pressure metamorphism (M1) was synchronous with D1-gneissosity and N-S trending lineation, demarcating the high strain HSZ. The mineral assemblages formed during the M1 phase include quartz + andesine + hornblende (I) + biotite (I) in hornblende-biotite gneiss, quartz + andesine + pargasitic hornblende (I) + ferroan pargasitic hornblende (I) + edenitic hornblende (I) in hornblende-schist, quartz + plagioclase + biotite + muscovite in psammopelitic gneiss, and diopside + tremolite + calcite + sphene ± garnet in calc-silicates, being characteristic for amphibolite facies with metamorphic conditions of 600 ± 50 °C and 5-6.5 kbar. The second metamorphic phase (M2) is related to the crystallization of biotite and/or hornblende in S2 foliation demarcating the NE-SW trending dextral shear deformation (D2). The calculated temperature for this M2 phase is about 592 °C. Subsequent thermal events are documented by growth of spinel and scapolite in calc-silicate rocks and of cordierite in psammopelitic gneiss in response to uplift, decomposition and heat provided by the nearby late-formed igneous intrusions. Finally, the rocks reached a temperature of about 530 °C during the cooling retrogressive stage. Based on geological, petrological and geochemical

  18. Late Paleoproterozoic-Neoproterozoic multi-rifting events in the North China Craton and their geological significance: A study advance and review

    NASA Astrophysics Data System (ADS)

    Zhai, Mingguo; Hu, Bo; Zhao, Taiping; Peng, Peng; Meng, Qingren

    2015-11-01

    An important Paleoproterozoic mobile belt event took place in the North China Craton (NCC), termed the Hutuo Movement. This event has been interpreted to represent cratonic reworking characterized by rifting-subduction-collision processes. The NCC then evolved into a stable platform or para-platform tectonic setting in Earth's middle age period more than ~ 1.0 Ga. Thick Late Paleoproterozoic-Neoproterozoic sedimentary sequences were extensively deposited on the early metamorphic basement. The major sedimentary basins include the Xiong'er aulacogen system in the south-central NCC, the Yan-Liao aulacogen system in the north-central NCC, the Northern marginal rift system in the northwestern NCC and the Eastern marginal rift system in the eastern NCC. The following four stages of magmatic activity are recognized in the Late Paleoproterozoic to Neoproterozoic interval: (1) ~ 1800 to 1780 Ma Xiong'er igneous province (XIP), (2) ~ 1720 to 1620 Ma anorogenic magmatic association, (3) ~ 1350 to 1320 Ma diabase sill swarms, and (4) ~ 900 Ma mafic dyke swarms. These four magmatic events suggest that the NCC was situated in an intra-plate setting for a long time from ~ 1.8 Ga to ~ 0.7 Ga or even younger, and the magmatic events were associated with multi-stage rifting activities. We document that the NCC was in a long-term extensional tectonic setting during Late Paleoproterozoic-Neoproterozoic era. The main ore deposits in this period are magmatic type iron deposits related to anorthosite-gabbro bodies, REE-Nb-Fe and Pb-Zn-Cu-Fe deposits related to Mesoproterozoic-Neoproterozoic rifts. Orogenic metal deposits are absent. There is no evidence indicating that the Grenville or other orogenic events affected the NCC. The reason for the absence of Grenvillian aged events in the NCC is probably because it was far from the edge of the Nuna supercontinent, if such a supercontinent did exist. There is another possibility that the Earth's middle age represented a particular tectonic

  19. Status of Early Paleozoic biostratigraphy of the Tethyan Himalayan successions

    NASA Astrophysics Data System (ADS)

    Parcha, Suraj Kumar; Pandey, Shivani

    2015-04-01

    The early Paleozoic successions of the Tethys Himalaya is exposed in the Spiti- Zanskar, Kashmir, Kumaun and in Garhwal regions. The most complete sequence described from the Tethys Himalayan region is exposed in the Spiti- Zanskar basin. The sedimentary succession of Spiti-Zanskar basin has a thick sequence of early Paleozoic age. The early Paleozoic rocks of these basins rest over the crystalline rock. The contact between underlying crystallines with the Paleozoic rocks has been interpreted unconformable/conformable and gradual/faulted by various workers. There is no definite record of faunal elements from the Neoproterozoic metasedimentary rocks of both the basins. Above the metasedimentary rocks of Neoproterozoic the fossiliferous Cambrian rock rests. The fossiliferous Cambrian sequence of Spiti- Zanskar basins are corresponding to one another as far as the distribution of fauna is concerned. The early Cambrian successions in both the basins have more or less identical ichnogenera. Whereas, the Middle Cambrian of Zanskar Basin is dominated by agnostid trilobites along with polymerid trilobites on the other hand in the Spiti Basin Pagetides along with polymerid trilobites dominates during this period with few agnostid. In the Kashmir Basin the early Cambrian is equally dominated by Ichnofossils and the Middle Cambrian is controlled similarly by trilobite fauna like that of Spiti- Zanskar basins. In the Kumaun-Garhwal region so far no detailed studies have been carried out however earlier studies and in recent years ichnofossils of early Cambrian age has been reported along with some fragmentary report of trilobites. But from the Ordovician and Silurian successions of Garhwal basin brachiopods have been reported. The Ordovician succession of Spiti basin indicates shallow water depositional cycle, whereas the Zanskar basin indicates the sub aerial fluvial and deltaic depositional environment. A gradational contact has been observed between the Ordovician and

  20. Gale Sedimentary Rocks

    NASA Technical Reports Server (NTRS)

    2003-01-01

    MGS MOC Release No. MOC2-439, 1 August 2003

    Gale Crater, located in the Aeolis region near 5.5oS, 222oW, contains a mound of layered sedimentary rock that stands higher than the rim of the crater. This giant mound suggests that the entire crater was not only once filled with sediment, it was also buried beneath sediment. This Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image shows some of the eroded remains of the sedimentary rock that once filled Gale Crater. The layers form terraces; wind has eroded the material to form the tapered, pointed yardang ridges seen here. The small circular feature in the lower right quarter of the picture is a mesa that was once a small meteor impact crater that was filled, buried, then exhumed from within the sedimentary rock layers exposed here. This image is illuminated from the left.

  1. Trace fossils and sedimentary facies from a Late Cambrian-Early Ordovician tide-dominated shelf (Santa Rosita Formation, northwest Argentina): Implications for ichnofacies models of shallow marine successions

    USGS Publications Warehouse

    Mangano, M.G.; Buatois, L.A.; Acenolaza, G.F.

    1996-01-01

    The Santa Rosita Formation is one the most widely distributed lower Paleozoic units of northwest Argentina. At the Quebrada del Salto Alto section, east of Purmamarca, Jujuy Province, it is represented by four sedimentary facies: thick-bedded planar cross-stratified quartzose sandstones (A), thin-bedded planar cross-stratified quartzose sandstones and mudstones (B), wave-rippled sandstones and bioturbated mudstones (C), and black and greenish gray shales (D). Paleocurrent data, sandstone architecture, and sedimentary structures from facies A and B indicate bipolar/bimodal paleoflows, suggesting the action of tidal currents. The succession is interpreted as that of a tide-dominated shelf, with only secondary influence of wave processes. Trace fossils are restricted to facies B and C. The Cruziana ichnocoenosis is preserved on the soles of thin-bedded planar cross-stratified quartzose sandstones (facies B). This ichnocoenosis consists of Conostichus isp., Cruziana omanica, C. semiplicata, C. cf. tortworthi, Cruziana isp. Helminthopsis abeli, Monomorphichnus bilinearis, M. multilineatus, Palaeophycus tubularis, Rusophycus carbonarius, R. latus, and R. isp. The occurrence of Cruziana semiplicata, C. omanica, C. cf. tortworthi, and Rusophycus latus supports a Late Cambrian-Tremadoc age. Slabbing of Cruziana shows complex interactions between biologic and sedimentologic processes, and suggests a predominance of exhumed traces, washed out and recast by tractive sand deposition. Sandstone soles are densely packed with biogenic structures and exhibit distinctive clusters of Rusophycus isp. that most likely represent trilobite nesting burrows. The Cruziana ichnocoenosis records the resident fauna of a protected, lower intertidal to subtidal interbar setting. The Skolithos ichnocoenosis is represented by high to low density vertical burrows of Skolithos linearis, which extend downwards to the quartzose sandstone soles of facies B and cross the Cruziana ichnocoenosis. The

  2. Zircon (U-Th)/He thermochronology of Neoproterozoic strata from the Mackenzie Mountains, Canada: Implications for the Phanerozoic exhumation and deformation history of the northern Canadian Cordillera

    NASA Astrophysics Data System (ADS)

    Powell, Jeremy; Schneider, David; Stockli, Daniel; Fallas, Karen

    2016-03-01

    Sedimentary strata of the Neoproterozoic Mackenzie Mountains Supergroup (MMSG) and Windermere Supergroup (WSG) occupy the cores of anticlines in the Mackenzie Mountains of the Canadian Cordilleran Foreland Belt. Stratigraphic and structural evidence suggest that these rocks have undergone several episodes of burial and unroofing relatively intact. We report single-grain detrital muscovite 40Ar/39Ar and zircon (U-Th)/He (ZHe) data from a suite of samples across the fold-thrust belt and the Neoproterozoic stratigraphic record. The strata have not reached high enough temperatures to reset the muscovite 40Ar/39Ar system, and instead our detrital muscovite data refine Tonian-Cryogenian depositional ages. Single-crystal ZHe dates range from 432 ± 35 to 46 ± 4 Ma, indicating that MMSG and WSG strata have not been heated sufficiently to fully reset the ZHe system. These factors make the Neoproterozoic strata an attractive natural laboratory to test the utility of the zircon radiation damage and annealing model on the quantification of thermal histories from detrital zircon populations that have accumulated radiation damage over long geologic timescales. Thermal modeling reveals that (1) a substantial sedimentary package was deposited following the Devonian and removed during Permo-Triassic cooling, and (2) the Cordilleran deformation front propagated through the study area from the Albian to the Paleocene, with a moderate increase in cooling rates between 75-67 Ma in the southwest and 60-55 Ma at the deformation front. Ultimately, relationships between radiation damage and helium diffusion kinetics in zircon explain substantial ZHe date dispersion and elucidate the temperature-time history of the northern Canadian Cordillera.

  3. Detrital zircon provenance of Silurian-Devonian and Triassic sedimentary rocks of the western Yangtze Block: Constraint for the location of South China in Gondwana supercontinent

    NASA Astrophysics Data System (ADS)

    Chen, Qiong; Sun, Min; Zhao, Guochun

    2016-04-01

    During Paleozoic even to early Mesozoic, South China, along with a series of Asia continental blocks, dispersed from the northern margin of Gondwana, drifting across the Tethys Ocean and accreting to the final assembly of Asia in Triassic, which also accepted sediments sourced from the adjoining segments of east Gondwana and itself successively. However, the exact location of South China within the east Gondwana and other Asia blocks is arguing and confusing. Detrital zircon U-Pb geochronology and Hf isotope data from Paleozoic sedimentary rocks and massive Triassic flysch sequences kept in the western margin of South China (Danba-Longmenshan regions) provide a record of the source from which they were derived, and thus being applied to constraining the distribution of basement block in paleogeographic reconstructions and the tectonic setting of the basin. Two Longmenshan Devonian sediments exhibit dominant Grenvillian ages (0.9-1.1 Ga), with mid-Neoproterozoic (730-850 Ma), Pan-African (500-680 Ma) and Neoarchean (2.4-2.5 Ga) age populations, indicative of a typical Gondwana-derived affinity, which is also recorded by the Danba Silurian sample and other Paleozoic sediments (Devonian-Cambrian) in the resting South China block, including the east Yangtze block and the Cathaysia. However, the similar age patterns are not observed in the Devonian sample of Danba region, which exhibits a different age pattern with only two significant age groups of Pan-African (440-600 Ma) and Neoproterozoic (660-994 Ma) with an apparent lack of older zircon grains (>1.0 Ga). The Triassic sandstone from Songpan-Ganze covering sequences shows a distinguished zircon age distribution with prominant mid-Neoproterozoic (649-843 Ma), mid-Paleoproterozoic (1724-1951 Ma) and subordinated Permian-Triassic (236-298 Ma), Paleozoic (375-530 Ma) ages, mainly derived by melting of old crust with few input of juvenile material. Considering provenance changes along with the temporal and variation in

  4. Sedimentary Rocks and Dunes

    NASA Technical Reports Server (NTRS)

    2004-01-01

    25 November 2004 This Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image shows buttes composed of light-toned, sedimentary rock exposed by erosion within a crater occurring immediately west of Schiaparelli Basin near 4.0oS, 347.9oW. Surrounding these buttes is a field of dark sand dunes and lighter-toned, very large windblown ripples. The sedimentary rocks might indicate that the crater interior was once the site of a lake. The image covers an area about 3 km (1.9 mi) wide. Sunlight illuminates the scene from the lower left.

  5. Sedimentary Rock Remnants

    NASA Technical Reports Server (NTRS)

    2005-01-01

    29 July 2005 This Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image shows knobs of remnant, wind-eroded, layered sedimentary rock that once completely covered the floor of a crater located west of the Sinus Meridiani region of Mars. Sedimentary rock outcrops are common throughout the Sinus Meridiani region and its surrounding cratered terrain.

    Location near: 2.2oN, 7.9oW Image width: width: 3 km (1.9 mi) Illumination from: lower left Season: Northern Autumn

  6. Sr isotopic characteristic of neoproterozoic carbonate sediments from the southern Yenisei Ridge

    NASA Astrophysics Data System (ADS)

    Vishnevskaya, I. A.; Kochnev, B. B.; Letnikova, E. F.; Kuznetsov, A. B.; Proshenkin, A. I.

    2012-04-01

    This paper presents the first Sr isotopic data for the Late Precambrian carbonate rocks of the southern Yenisei Ridge. Their geochemical study allowed estimation of the degree of secondary alterations and gave the possibility to reveal rocks with a less disturbed Rb-Sr isotopic system. The Sr isotopic data indicated Neoproterozoic sedimentation of the rocks about 1070-750 Ma ago. Sr and C isotopic data showed that carbonate rocks of the Sukhoi Pit, Tungusik, and Shirokino groups are Late Riphean and could be comparable with sedimentary sequences of three Precambrian key sections of the Northern Eurasia: the subsequent Derevnino, Burovaya, and Shorikha formations from the Turukhansk Uplift, the Lakhanda Group from the Uchur-Maya region, and the Karatav Group from the South Urals. All studied carbonate rocks are older than 750 Ma and, according to the International Stratigraphic Chart, accumulated prior to global glaciations in the Cryogenian. This is evident from sedimentological study indicating the absence of tillite horizons in the studied sections. δ13C values in the sections vary from +0.4 up to +5.3‰, which testifies to the absence of periods of great cold.

  7. Is the Neoproterozoic oxygen burst a supercontinent legacy?

    NASA Astrophysics Data System (ADS)

    Macouin, Melina; Roques, Damien; Rousse, Sonia; Ganne, Jerome; Denele, Yoann; Trindade, Ricardo

    2015-09-01

    The Neoproterozoic (1000-542 Myr ago) witnessed the dawn of Earth as we know it with modern-style plate tectonics, high levels of O2 in atmosphere and oceans and a thriving fauna. Yet, the processes leading to the fully oxygenation of the external envelopes, its exact timing and its link with the inner workings of the planet remain poorly understood. In some ways, it is a "chicken and egg" question: did the Neoproterozoic Oxygenation Event (NOE) cause life blooming, low-latitudes glaciations and perturbations in geochemical cycles or is it a consequence of these phenomena? Here, we suggest that the NOE may have been triggered by multi-million years oxic volcanic emissions along a protracted period at the end of the Neoproterozoic when continents were assembled in the Rodinia supercontinent. We report a very oxidized magma source at the upper mantle beneath a ring of subducting margins around Rodinia, and detail here the evidence at the margin of the Arabian shield. We investigate the 780 Ma Biotite and Pink granites and associated rocks of the Socotra Island with rock magnetic and petrographic methods. Magnetic susceptibility and isothermal remanent magnetization acquisitions show that, in these granites, both magnetite and hematite are present. Hematite subdivides magnetite grains into small grains. Magnetite and hematite are found to be primary, and formed at the early magmatic evolution of the granite at very high oxygen fugacity. Massive degassing of these oxidized magmas would reduce the sink for oxygen, and consequently contribute to its rise in the atmosphere with a net O2 flux of at least 2.25 x 107 Tmol. Our conceptual model provides a deep Earth link to the NOE and implies the oxygenation burst has occurred earlier than previously envisaged, paving the way for later changes in the outer envelopes of the planet epitomized on the extreme Neoproterozoic glaciations and the appearance of the first animals.

  8. The White Nile sedimentary system

    NASA Astrophysics Data System (ADS)

    Garzanti, Eduardo; Andò, Sergio; Padoan, Marta; Resentini, Alberto; Vezzoli, Giovanni; Villa, Igor

    2014-05-01

    The Nile River flows for ~6700 km from south of the Equator to finally reach the Mediterranean Sea at northern subtropical latitudes (Woodward et al. 2007). This is the longest sedimentological laboratory on Earth, a unique setting in which we are investigating changes in sediment composition associated with diverse chemical and physical processes, including weathering and hydraulic sorting. The present study focuses on the southern branch of the Nile across 20° of latitude, from hyperhumid Burundi and Rwanda highlands in central Africa to Khartoum, the capital city of Sudan at the southern edge of the Sahara. Our study of the Kagera basin emphasizes the importance of weathering in soils at the source rather than during stepwise transport, and shows that the transformation of parent rocks into quartzose sand may be completed in one sedimentary cycle (Garzanti et al. 2013a). Micas and heavy minerals, less effectively diluted by recycling than main framework components, offer the best key to identify the original source-rock imprint. The different behaviour of chemical indices such as the CIA (a truer indicator of weathering) and the WIP (markedly affected by quartz dilution) helps us to distinguish strongly weathered first-cycle versus polycyclic quartz sands (Garzanti et al. 2013b). Because sediment is efficiently trapped in East African Rift lakes, the composition of Nile sediments changes repeatedly northwards across Uganda. Downstream of both Lake Kyoga and Lake Albert, quartzose sands are progressively enriched in metamorphiclastic detritus supplied from tributaries draining amphibolite-facies basements. The evolution of White Nile sediments across South Sudan, a scarcely accessible region that suffered decades of civil war, was inferred from the available information (Shukri 1950), integrated by original petrographic, heavy-mineral and geochemical data (Padoan et al. 2011). Mineralogical and isotopic signatures of Bahr-el-Jebel and Sobat sediments, derived

  9. Global Neoproterozoic (Sturtian) post-glacial sulfide-sulfur isotope anomaly recognised in Namibia

    NASA Astrophysics Data System (ADS)

    Gorjan, Paul; Walter, Malcolm R.; Swart, Roger

    2003-02-01

    The Neoproterozoic Earth experienced at least two, probably global, glaciations. Each glaciation was superceded by deposition of a layer of carbonate ('cap-carbonate') that has a distinctive lithology and depletion in 13C ( δ13C carbonate ˜ -5‰). The ˜700 Ma Sturtian glaciation is followed by deposition of a cap-carbonate and post-glacial succession which contain bacterially produced sulfides extremely enriched in 34S (average δ34S sulfide ˜ +30‰) with maximum values up to +60‰. This level of 34S enrichment in sulfides is unique to the Sturtian post-glacial succession and recognised in Australia, Canada, and China. In the Neoproterozoic of the Nama Basin, Namibia, the Gobabis Member is the basal unit of the Court Formation, which overlies the glacial Blaubeker Formation. δ13C carbonate analyses from the Gobabis Member range from -5.2 to -2.2‰ (average = -3.7‰; n = 10). δ34S sulfide ranges from +16.1 to +61.1‰ (average = +37.6‰; n = 8). These results are consistent with a Sturtian age for the Blaubeker Formation and overlying Gobabis Member, which have previously been interpreted as Sturtian. The sulfur isotopic results are comparable with δ34S sulfide in Sturtian post-glacial units of Australia, Canada and China. This adds to the evidence for correlation of the Blaubeker Formation with Sturtian glaciations on other continents. The cause of such elevated δ34S sulfide is enigmatic. Geochemical evidence suggests the sulfide was not formed from low sulfate waters nor in euxinic conditions, which discounts any known modern analogue. 34S enrichment in sulfides is therefore postulated to be caused by enrichment of 34S in contemporaneous seawater ( δ34S sulfate up to +60‰?). The rise in seawater δ34S sulfate is considered to be the result of intense bacterial sulfate reduction in an anoxic ocean during the Sturtian glaciation.

  10. Ancient Sedimentary Rocks

    NASA Technical Reports Server (NTRS)

    2003-01-01

    MGS MOC Release No. MOC2-469, 31 August 2003

    The terraced area in this Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image is an outcropping of ancient, sedimentary rock. It occurs in a crater in western Arabia Terra near 10.8oN, 4.5oW. Sedimentary rocks provide a record of past environments on Mars. Field work will likely be required to begin to get a good understanding of the nature of the record these rocks contain. Their generally uniform thickness and repeated character suggests that deposition of fine sediment in this crater was episodic, if not cyclic. These rocks might be indicators of an ancient lake, or they might have been deposited from grains settling out of an earlier, thicker, martian atmosphere. This image covers an area 3 km (1.9 mi) across and is illuminated from the lower left.

  11. Schiaparelli's Sedimentary Rocks

    NASA Technical Reports Server (NTRS)

    2004-01-01

    9 October 2004 Schiaparelli Basin is a large, 470 kilometer (292 miles) impact crater located east of Sinus Meridiani. The basin might once have been the site of a large lake--that is, if the sedimentary rocks exposed on its northwestern floor were deposited in water. This Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image shows a 1.5 meter per pixel (5 ft per pixel) view of some of the light-toned, finely-bedded sedimentary rocks in northwestern Schiaparelli. The image is located near 1.0oS, 346.0oW, and covers an area about 3 km (1.9 mi) wide. Sunlight illuminates the scene from the left.

  12. Gale Sedimentary Rocks

    NASA Technical Reports Server (NTRS)

    2005-01-01

    15 April 2005 This Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image shows outcroppings of layered, sedimentary rock in eastern Gale Crater. North-central Gale Crater is the site of a mound that is more than several kilometers thick and largely composed of sedimentary rocks that record a complex history of deposition and erosion. At one time, Gale Crater might have been completely filled and buried beneath the martian surface.

    Location near: 4.9oS, 221.6oW Image width: 3 km (1.9 mi) Illumination from: upper left Season: Southern Winter

  13. Tithonium Chasma's Sedimentary Rocks

    NASA Technical Reports Server (NTRS)

    2003-01-01

    MGS MOC Release No. MOC2-565, 5 December 2003

    Exposures of light-toned, layered, sedimentary rocks are common in the deep troughs of the Valles Marineris system. This Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image shows an example from western Tithonium Chasma. The banding seen here is an eroded expression of layered rock. Sedimentary rocks can be composed of (1) the detritus of older, eroded and weathered rocks, (2) grains produced by explosive volcanism (tephra, also known as volcanic ash), or (3) minerals that were chemically precipitated out of a body of liquid such as water. These outcrops are located near 4.8oS, 89.7oW. The image covers an area 3 km (1.9 mi) wide and is illuminated from the lower left.

  14. Broken Sedimentary Rocks

    NASA Technical Reports Server (NTRS)

    2005-01-01

    18 May 2005 This Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image shows broken-up blocks of sedimentary rock in western Candor Chasma. There are several locations in western Candor that exhibit this pattern of broken rock. The manner in which these landforms were created is unknown; it is possible that there was a landslide or a meteoritic impact that broke up the materials. One attribute that is known: in some of these cases, it seems that the rock was broken and then buried by later sedimentary rocks, before later being exhumed so that they can be seen from orbit today.

    Location near: 6.9oS, 75.5oW Image width: 3 km (1.9 mi) Illumination from: upper left Season: Southern Winter

  15. Sedimentary Rock Layers

    NASA Technical Reports Server (NTRS)

    2003-01-01

    MGS MOC Release No. MOC2-348, 2 May 2003

    This Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image acquired in March 2003 shows dozens of repeated layers of sedimentary rock in a western Arabia Terra crater at 8oN, 7oW. Wind has sculpted the layered forms into hills somewhat elongated toward the lower left (southwest). The dark patches at the bottom (south) end of the image are drifts of windblown sand. These sedimentary rocks might indicate that the crater was once the site of a lake--or they may result from deposition by wind in a completely dry, desert environment. Either way, these rocks have something important to say about the geologic history of Mars. The area shown is about 3 km (1.9 mi) wide. Sunlight illuminates the scene from the left.

  16. Global sedimentary geology program

    SciTech Connect

    Ginsburg, R.N.; Clifton, H.E.; Weimer, R.J.

    1986-07-01

    The Society of Economic Paleontologists and Mineralogists, in collaboration with the International Association of Sedimentologists and the International Union of Geological Sciences Committee on Sedimentology, is developing a new international study under the provisional title of Global Sedimentary Geology Program (GSGP). Initially, three research themes are being considered: (1) event stratigraphy-the documentation of examples of mass extinctions, eustatic fluctuations in sea level, major episodes of volcanisms, and changes in ocean composition; (2) facies models in time and space-an expansion of the existing data base of examples of facies models (e.G., deltas, fluvial deposits, and submarine fans) and global-scale study of the persistence of facies at various times in geologic history; and (3) sedimentary indices of paleogeography and tectonics-the use of depositional facies and faunas in paleogeography and in assessing the timing, locus, and characteristics of tectonism. Plans are being developed to organize pilot projects in each of these themes.

  17. Terby Sedimentary Rocks

    NASA Technical Reports Server (NTRS)

    2004-01-01

    27 December 2003 This Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image shows layered sedimentary rock outcrops in Terby Crater, located near 27.7oS, 285.4oW. The layered sediments in Terby are several kilometers thick, attesting to a long history of deposition in this ancient basin. The picture covers an area 3 km (1.9 mi) wide. Sunlight illuminates the scene from the upper left.

  18. Eroded Sedimentary Rock

    NASA Technical Reports Server (NTRS)

    2003-01-01

    MGS MOC Release No. MOC2-372, 26 May 2003

    This high resolution Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image shows eroded, layered sedimentary rock exposures in an unnamed western Arabia Terra crater at 8oN, 7oW. The dark material is windblown sand; much of the erosion of these layers may have also been caused by wind. Sunlight illuminates the scene from the left.

  19. Iani Sedimentary Rocks

    NASA Technical Reports Server (NTRS)

    2005-01-01

    23 February 2005 This Mars Global Surveyor (MGS) Orbiter Camera (MOC) image shows light-toned sedimentary rocks exposed by erosion in the Iani Chaos region of Mars.

    Location near: 4.2oS, 18.7oW Image width: 1 km (0.6 mi) Illumination from: upper left Season: Southern Winter

  20. Melas Sedimentary Rocks

    NASA Technical Reports Server (NTRS)

    2004-01-01

    17 July 2004 This Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image shows layered, sedimentary rock outcrops in southwestern Melas Chasma, one of the troughs of the vast Valles Marineris system. Sunlight illuminates this scene from the upper left; it is located near 9.8oS, 76.0oW, and covers an area about 3 km (1.9 mi) wide.

  1. Possible Record of Neoproterozoic Ice Sheet Collapse: The Kapp Lyell Diamictite Sequence of southwest Spitsbergen, Svalbard

    NASA Astrophysics Data System (ADS)

    Bjornerud, M.

    2008-12-01

    appearance of ca. 500 m of unlayered, unsorted diamicite in beds 1-5 m thick, and some intervening graded layers of conglomerate to sandstone. The massive, unsorted "Heinrich"-type layers contain sand- to boulder-sized material and tend to be laterally discontinuous, while the graded beds are traceable for hundreds of meters. In the uppermost 1000 -1500 m of the sequence, such coarse graded beds become predominant, with only rare unsorted intervals. These beds are the highest preserved Neoproterozoic strata in the region. Overall, the transition from laminated, to unsorted, to graded diamictites may represent change from 1) a stable ice margin that released rare icebergs into a deep, quiet basin to 2) a collapsing ice sheet that unleashed flotillas of icebergs and large volumes of sediment to 3) submarine landslides that triggered turbidity flows from the rapidly deposited, gravitationally unstable sediments. No absolute ages are available for the Kapp Lyell sequence itself, but indirect evidence suggests that it records the ca. 635 Ma Marinoan stage of the "Snowball Earth" glaciations. In southernmost Spitsbergen, a unit correlative with the underlying Konglomeratfjellet Fm has yielded a metamorphic monazite age of 653 +/- 39 Ma (Majka et al 2007). Because there is no known stratigraphic discontinuity or metamorphic contrast between this older unit and the Kapp Lyell sequence, it seems most likely that the Kapp Lyell sequence and Konglomeratfjellet Fm. represent the Marinoan and Sturtian glaciations, respectively. If so, the sedimentary characteristics of the Kapp Lyell sequence suggest a catastrophic end to a Marinoan ice sheet, an ironic conclusion to be drawn from rocks named for the father of Uniformitarianism.

  2. Ancient Life at the Extremes: Molecular Fossils and Paleoenvironmental Contexts of a Neoproterozoic Hypersaline Setting

    NASA Astrophysics Data System (ADS)

    Schinteie, R.; Brocks, J. J.

    2011-12-01

    We present the first molecular investigation of the biotic composition and biogeochemistry of an evaporitic, hypersaline environment from the Neoproterozoic (~800 Ma). Through detailed analyses of both sedimentary textures and their lipid biomarkers, we provide the oldest evidence of organisms that could exist at extremely saline conditions. Such research is timely, since the discovery of evaporite deposits on Mars highlights the need to understand their capacities as biological archives. Samples for this study were derived from evaporitic sediments of the Neoproterozoic Bitter Springs Formation, Amadeus Basin, central Australia. Due to the broad shallow nature of the basin and a tenuous connection with the ocean, the water was characterized by elevated salinity levels during that time. As a result, thick (100 m to >2000 m) evaporite units were deposited. All samples for this study were derived from drill cores held at Geoscience Australia (Canberra) and at the Northern Territory Geological Survey at Alice Springs. We extracted biomarkers from evaporitic sediments composed of dolomite, anhydrite and/or halite. The dolomite layers commonly assume the shape of microbial mat-like formations that exhibit roll-up structures and tearing. Framboidal pyrite was commonly associated with the dolomite and together with other textural and stable isotopic data provide the oldest putative evidence for biologically-induced dolomite precipitation. Full scan gas chromatography-mass spectrometry (GCMS) of the saturate fractions of these evaporites revealed high ratios of mono-and dimethylalkanes relative to n-alkanes. Such a pattern is typical of Precambrian and Cambrian samples. These molecules, together with elevated relative concentrations of n-C17, provide evidence of a photoautotrophic community - especially cyanobacteria. A further outstanding characteristic of these samples are the presence of several pseudohomologous series of both regular (to C25) and irregular (to C40

  3. A true polar wander model for Neoproterozoic plate motions

    SciTech Connect

    Ripperdan, R.L. )

    1992-01-01

    Recent paleogeographic reconstructions for the interval 750--500 Ma (Neoproterozoic to Late Cambrian) require rapid rates of plate motion and/or rotation around an equatorial Euler pole to accommodate reconstructions for the Early Paleozoic. Motions of this magnitude appear to be very uncommon during the Phanerozoic. A model for plate motions based on the hypothesis that discrete intervals of rapid true polar wander (RTPW) occurred during the Neoproterozoic can account for the paleogeographic changes with minimum amounts of plate motion. The model uses the paleogeographic reconstructions of Hoffman (1991). The following constraints were applied during derivation of the model: (1) relative motions between major continental units were restricted to be combinations of great circle or small circle translations with Euler poles of rotation = spin axis; (2) maximum rates of relative translational plate motion were 0.2 m/yr. Based on these constraints, two separate sets of synthetic plate motion trajectories were determined. The sequence of events in both can be summarized as: (1) A rapid true polar wander event of ca 90[degree] rafting a supercontinent to the spin axis; (2) breakup of the polar supercontinent into two fragments, one with the Congo, West Africa, Amazonia, and Baltica cratons, the other with the Laurentia, East Gondwana, and Kalahari cratons; (3) great circle motion of the blocks towards the equator; (4) small circle motion leading to amalgamation of Gondwana and separation of Laurentia and Baltica. In alternative 1, rifting initiates between East Antarctica and Laurentia and one episode of RTPW is required. Alternative 2 requires two episodes of RTPW; and that rifting occurred first along the eastern margin and later along the western margin of Laurentia. Synthetic plate motion trajectories are compared to existing paleomagnetic and geological data, and implications of the model for paleoclimatic changes during the Neoproterozoic are discussed.

  4. Modelling the carbon cycle though Neoproterozoic Earth system changes

    NASA Astrophysics Data System (ADS)

    Bjerrum, C. J.; Canfield, D. E.

    2011-12-01

    The Neoproterozoic-Cambrian records major changes in geochemical proxies as a result of a profound reorganization of the Earth system. Extensive glaciations and the first oxygenation of the deep ocean with a shift from sulfidic/ferruginous conditions to more oxic conditions was accompanied by the radiation of the first animals. The reorganization was also recorded in enigmatic large-amplitude fluctuations in the isotopic composition of marine carbonate carbon (δ13CIC ), were only some are associated with major known glaciations. The carbon isotope events seem to grow in amplitude through the Neoproterozoic culminating in the Shuram anomaly - the largest in Earth history. The δ13CIC events are also accompanied by changes in the isotope composition of marine organic carbon (δ13COC), where the co-variation of δ13CIC and δ13COC seems to evolve from markedly positive relationship over a subdued δ13COC variation and an almost inverse pattern. There is limited understanding as to why or how the structure of these isotope events evolved over time and how these events may tie to the reorganization of the Earth system. We use our published quantitative model of the Shuram anomaly to explore carbon cycle dynamics during the Neoproterozoic. By changing in pre-event atmosphere-ocean chemistry we explore which factors contribute to the observed patterns of the large Neoproterozoic carbon isotope events. In particular, decreasing atmospheric CO2 and a slight increase of oxygen together with an increasing CO source from rising DOC concentrations results in progressively larger event amplitudes with changing co-variation between δ13CIC and δ13COC , culminating with the structure observed for the Shurum-Wonaka anomaly in the Ediacaran. In our model, the carbon isotope excursions were driven by methane from sediment-hosted clathrate hydrate deposits. Being a powerful greenhouse gas, methane increased temperature and melted icecaps. These combined to produce a negative 18O

  5. Neoproterozoic Glaciations and the Early Evolution of Animals

    NASA Astrophysics Data System (ADS)

    Narbonne, G. M.

    2004-05-01

    The intense climatic changes that characterized the Neoproterozoic world were marked by equally profound evolutionary changes that ultimately led to the Cambrian Explosion. Early and Middle Neoproterozoic oceans contained prokaryotes and diverse eukaryotic lineages, including crown-group red, green, and heterokont algae. The survival of diverse eukaryotic lineages through the Sturtian, Marinoan, and Gaskiers glaciations implies that, although these were among the most extreme glaciations Earth has ever experienced, sea ice was not as thick or pervasive as required by earlier "hard Snowball" models. Most molecular clocks predict the existence of animals well before 600 Ma and a few tantalizing hints have been found, but the oldest definite evidence of animal life are phosphatized eggs and embryos overlying Marinoan glacial deposits in China. The subsequent Late Neoproterozoic is characterized by the global occurrence of the Ediacara biota, an assemblage of cm- to m-scale fossils of soft-bodied organisms that probably represent a mixture of stem groups of modern phyla and "failed experiments" in evolution. The oldest Ediacaran fossils occur in eastern Newfoundland, and postdate the glacial diamictites and cap carbonate of the Gaskiers Formation (580 Ma) by only 5 million years, implying a causal relationship between the end of the Neoproterozoic glaciations and the proliferation of animal life. These fossils include architecturally complex fronds up to two metres long, implying either extremely rapid rates of evolution or a pre-glacial origin of the Ediacara biota. Fossils of the Mistaken Point biota (575-560 Ma) were completely sessile and show a similar fractal architecture that is difficult to relate to any existing life forms. Some of these taxa persisted into the White Sea biota (560-550 Ma), which also contains trace fossils and metameric fossils that confirm the evolution of mobile bilaterians. The youngest Ediacaran fossils (550-543 Ma) exhibit the first

  6. Evolution of Sedimentary Rocks

    NASA Astrophysics Data System (ADS)

    Veizer, J.; MacKenzie, F. T.

    2003-12-01

    For almost a century, it has been recognized that the present-day thickness and areal extent of Phanerozoic sedimentary strata increase progressively with decreasing geologic age. This pattern has been interpreted either as reflecting an increase in the rate of sedimentation toward the present (Barrell, 1917; Schuchert, 1931; Ronov, 1976) or as resulting from better preservation of the younger part of the geologic record ( Gilluly, 1949; Gregor, 1968; Garrels and Mackenzie, 1971a; Veizer and Jansen, 1979, 1985).Study of the rocks themselves led to similarly opposing conclusions. The observed secular (=age) variations in relative proportions of lithological types and in chemistry of sedimentary rocks (Daly, 1909; Vinogradov et al., 1952; Nanz, 1953; Engel, 1963; Strakhov, 1964, 1969; Ronov, 1964, 1982) were mostly given an evolutionary interpretation. An opposing, uniformitarian, approach was proposed by Garrels and Mackenzie (1971a). For most isotopes, the consensus favors deviations from the present-day steady state as the likely cause of secular trends.This chapter attempts to show that recycling and evolution are not opposing, but complementary, concepts. It will concentrate on the lithological and chemical attributes of sediments, but not deal with the evolution of sedimentary mineral deposits (Veizer et al., 1989) and of life ( Sepkoski, 1989), both well amenable to the outlined conceptual treatment. The chapter relies heavily on Veizer (1988a) for the sections dealing with general recycling concepts, on Veizer (2003) for the discussion of isotopic evolution of seawater, and on Morse and Mackenzie (1990) and Mackenzie and Morse (1992) for discussion of carbonate rock recycling and environmental attributes.

  7. Sedimentary Rocks in Ganges

    NASA Technical Reports Server (NTRS)

    2004-01-01

    13 November 2004 This Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image shows portions of two massifs composed of light-toned, sedimentary rock in Ganges Chasma, part of the Valles Marineris trough system. On the steeper slopes in this vista, dry talus shed from the outcrop has formed a series of dark fans. Surrounded by dark, windblown sand, these landforms are located near 8.6oS, 46.8oW. The image covers an area approximately 3 km (1.9 mi) across and sunlight illuminates the scene from the upper left.

  8. Sedimentary Rocks in Melas

    NASA Technical Reports Server (NTRS)

    2005-01-01

    This Mars Global Surveyor (MGS) Orbiter Camera (MOC) image shows a butte and several other landforms eroded into light-toned, layered, sedimentary rock in southern Melas Chasma. Melas is part of the vast Valles Marineris trough system.

    Location near: 11.8oS, 74.6oW Image width: 3.0 km (1.9 mi) Illumination from: lower left Season: Southern Spring

  9. Sedimentary Rock Layers

    NASA Technical Reports Server (NTRS)

    2004-01-01

    27 January 2004 This Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image shows layers of sedimentary rock in a crater in western Arabia Terra. Layered rock records the history of a place, but an orbiter image alone cannot tell the entire story. These materials record some past episodes of deposition of fine-grained material in an impact crater that is much larger than the image shown here. The picture is located near 3.4oN, 358.7oW, and covers an area 3 km (1.9 mi.) wide. Sunlight illuminates the scene from the lower left.

  10. Sedimentary Rock in Candor

    NASA Technical Reports Server (NTRS)

    2005-01-01

    11 February 2005 This Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image shows dozens of light- and a few dark-toned sedimentary rock layers exposed by faulting and erosion in western Candor Chasma, part of the vast Valles Marineris trough system.

    Location near: 6.5oS, 77.0oW Image width: 3.0 km (1.9 mi) Illumination from: upper left Season: Southern Autumn

  11. Melas Sedimentary Rocks

    NASA Technical Reports Server (NTRS)

    2004-01-01

    28 August 2004 Light-toned, layered, sedimentary rock outcrops are common within the vast martian Valles Marineris trough system. This Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image shows a recent example from southern Melas Chasma at 1.5 m/pixel (5 ft/pixel) resolution. The image is located near 11.3oS, 73.9oW, and covers an area about 1.8 km (1.1 mi) across. Sunlight illuminates the scene from the upper left.

  12. a Snowball's Chance in Death Valley: Re-Evaluation of the Number and Magnitude of Neoproterozoic Ice Ages

    NASA Astrophysics Data System (ADS)

    Kaufman, A. J.; Corsetti, F. A.; Marenco, P. J.

    2002-05-01

    The Neoproterozoic Ibex Formation, previously considered to represent a basinal facies of platform carbonates in the lower Noonday Dolomite, Great Basin, USA, is shown to rest on the eroded surface of the lower Noonday and older units. At the type section, the basal Ibex Formation consists of polymict conglomerate and laminated mudstone; the upper surface of the mudstone is pierced by large angular clasts of all underlying units, including distinctive lower Noonday tube stromatolites. A thin, finely laminated pink dolostone unit that records negative carbon isotope values caps the Ibex conglomerate. We interpret the erosional unconformity upon which the basal Ibex Formation is deposited to be glacioeustatic in origin, the basal conglomerate-pierced mudstone to be glaciogenic, and the overlying dolostone to be a classic cap carbonate. Above the cap dolostone marine transgression led to the deposition of deeper water ferruginous shale and limestone, which is overlain by dolostone as water depths again shallowed. These post-glacial Ibex carbonates also record negative carbon isotope values similar to upper Noonday lithofacies preserved on the platform. A notable oxidized paleosol occurs at the top of the upper Ibex dolostone immediately below a coarse sandstone correlative with the basal Johnnie Formation. Combined with the record of glacial sediments and cap carbonates from underlying units, in particular the Kingston Peak Formation, the Death Valley succession unambiguously records three discrete Neoproterozoic ice ages in a single continuous section. These new observations provide the lithological and geochemical proof that at least three, and potentially more, ice ages characterized Neoproterozoic time. As each sustained global glaciation represents a critical environmental hurdle, the number and the magnitude of discrete ice ages is an important constraint on the tempo of metazoan evolution.

  13. Evidence for Mojave-Sonora megashear-Systematic left-lateral offset of Neoproterozoic to Lower Jurassic strata and facies, western United States and northwestern Mexico

    USGS Publications Warehouse

    Stewart, John H.

    2005-01-01

    Major successions as well as individual units of Neoproterozoic to Lower Jurassic strata and facies appear to be systematically offset left laterally from eastern California and western Nevada in the western United States to Sonora, Mexico. This pattern is most evident in units such as the "Johnnie oolite," a 1- to 2-m-thick oolite of the Neoproterozoic Rainstorm Member of the Johnnie Formation in the western United States and of the Clemente Formation in Sonora. The pattern is also evident in the Lower Cambrian Zabriskie Quartzite of the western United States and the correlative Proveedora Quartzite in Sonora. Matching of isopach lines of the Zabriskie Quartzite and Proveedora Quartzite suggests ???700-800 km of left-lateral offset. The offset pattern is also apparent in the distribution of distinctive lithologic types, unconformities, and fossil assemblages in other rocks ranging in age from Neoproterozoic to Early Jurassic. In the western United States, the distribution of facies in Neoproterozoic and Paleozoic strata indicates that the Cordilleran miogeocline trends north-south. A north-south trend is also suggested in Sonora, and if so is compatible with offset of the miogeocline but not with the ideas that the miogeocline wrapped around the continental margin and trends east-west in Sonora. An imperfect stratigraphic match of supposed offset segments along the megashear is apparent. Some units, such as the "Johnnie oolite" and Zabriskie-Proveedora, show almost perfect correspondence, but other units are significantly different. The differences seem to indicate that the indigenous succession of the western United States and offset segments in Mexico were not precisely side by side before offset but were separated by an area-now buried, eroded, or destroyed-that contained strata of intermediate facies. ?? 2005 Geological Society of America.

  14. Schiaparelli Sedimentary Rocks

    NASA Technical Reports Server (NTRS)

    2003-01-01

    MGS MOC Release No. MOC2-403, 26 June 2003

    Some of the most important high resolution imaging results of the Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) experiment center on discoveries about the presence and nature of the sedimentary rock record on Mars. This old meteor impact crater in northwestern Schiaparelli Basin exhibits a spectacular view of layered, sedimentary rock. The 2.3 kilometer (1.4 miles) wide crater may have once been completely filled with sediment; the material was later eroded to its present form. Dozens of layers of similar thickness and physical properties are now expressed in a wedding cake-like stack in the middle of the crater. Sunlight illuminating the scene from the left shows that the circle, or mesa top, at the middle of the crater stands higher than the other stair-stepped layers. The uniform physical properties and bedding of these layers might indicate that they were originally deposited in a lake (it is possible that the crater was at the bottom of a much larger lake, filling Schiaparelli Basin); alternatively, the layers were deposited by settling out of the atmosphere in a dry environment. This picture was acquired on June 3, 2003, and is located near 0.9oS, 346.2oW.

  15. Selenium isotope evidence for progressive oxidation of the Neoproterozoic biosphere

    PubMed Central

    Pogge von Strandmann, Philip A. E.; Stüeken, Eva E.; Elliott, Tim; Poulton, Simon W.; Dehler, Carol M.; Canfield, Don E.; Catling, David C.

    2015-01-01

    Neoproterozoic (1,000–542 Myr ago) Earth experienced profound environmental change, including ‘snowball' glaciations, oxygenation and the appearance of animals. However, an integrated understanding of these events remains elusive, partly because proxies that track subtle oceanic or atmospheric redox trends are lacking. Here we utilize selenium (Se) isotopes as a tracer of Earth redox conditions. We find temporal trends towards lower δ82/76Se values in shales before and after all Neoproterozoic glaciations, which we interpret as incomplete reduction of Se oxyanions. Trends suggest that deep-ocean Se oxyanion concentrations increased because of progressive atmospheric and deep-ocean oxidation. Immediately after the Marinoan glaciation, higher δ82/76Se values superpose the general decline. This may indicate less oxic conditions with lower availability of oxyanions or increased bioproductivity along continental margins that captured heavy seawater δ82/76Se into buried organics. Overall, increased ocean oxidation and atmospheric O2 extended over at least 100 million years, setting the stage for early animal evolution. PMID:26679529

  16. Selenium isotope evidence for progressive oxidation of the Neoproterozoic biosphere.

    PubMed

    Pogge von Strandmann, Philip A E; Stüeken, Eva E; Elliott, Tim; Poulton, Simon W; Dehler, Carol M; Canfield, Don E; Catling, David C

    2015-01-01

    Neoproterozoic (1,000-542 Myr ago) Earth experienced profound environmental change, including 'snowball' glaciations, oxygenation and the appearance of animals. However, an integrated understanding of these events remains elusive, partly because proxies that track subtle oceanic or atmospheric redox trends are lacking. Here we utilize selenium (Se) isotopes as a tracer of Earth redox conditions. We find temporal trends towards lower δ(82/76)Se values in shales before and after all Neoproterozoic glaciations, which we interpret as incomplete reduction of Se oxyanions. Trends suggest that deep-ocean Se oxyanion concentrations increased because of progressive atmospheric and deep-ocean oxidation. Immediately after the Marinoan glaciation, higher δ(82/76)Se values superpose the general decline. This may indicate less oxic conditions with lower availability of oxyanions or increased bioproductivity along continental margins that captured heavy seawater δ(82/76)Se into buried organics. Overall, increased ocean oxidation and atmospheric O2 extended over at least 100 million years, setting the stage for early animal evolution. PMID:26679529

  17. Selenium isotope evidence for progressive oxidation of the Neoproterozoic biosphere

    NASA Astrophysics Data System (ADS)

    Pogge von Strandmann, Philip A. E.; Stüeken, Eva E.; Elliott, Tim; Poulton, Simon W.; Dehler, Carol M.; Canfield, Don E.; Catling, David C.

    2015-12-01

    Neoproterozoic (1,000-542 Myr ago) Earth experienced profound environmental change, including `snowball' glaciations, oxygenation and the appearance of animals. However, an integrated understanding of these events remains elusive, partly because proxies that track subtle oceanic or atmospheric redox trends are lacking. Here we utilize selenium (Se) isotopes as a tracer of Earth redox conditions. We find temporal trends towards lower δ82/76Se values in shales before and after all Neoproterozoic glaciations, which we interpret as incomplete reduction of Se oxyanions. Trends suggest that deep-ocean Se oxyanion concentrations increased because of progressive atmospheric and deep-ocean oxidation. Immediately after the Marinoan glaciation, higher δ82/76Se values superpose the general decline. This may indicate less oxic conditions with lower availability of oxyanions or increased bioproductivity along continental margins that captured heavy seawater δ82/76Se into buried organics. Overall, increased ocean oxidation and atmospheric O2 extended over at least 100 million years, setting the stage for early animal evolution.

  18. Biologically induced initiation of Neoproterozoic snowball-Earth events

    PubMed Central

    Tziperman, Eli; Halevy, Itay; Johnston, David T.; Knoll, Andrew H.; Schrag, Daniel P.

    2011-01-01

    The glaciations of the Neoproterozoic Era (1,000 to 542 MyBP) were preceded by dramatically light C isotopic excursions preserved in preglacial deposits. Standard explanations of these excursions involve remineralization of isotopically light organic matter and imply strong enhancement of atmospheric CO2 greenhouse gas concentration, apparently inconsistent with the glaciations that followed. We examine a scenario in which the isotopic signal, as well as the global glaciation, result from enhanced export of organic matter from the upper ocean into anoxic subsurface waters and sediments. The organic matter undergoes anoxic remineralization at depth via either sulfate- or iron-reducing bacteria. In both cases, this can lead to changes in carbonate alkalinity and dissolved inorganic pool that efficiently lower the atmospheric CO2 concentration, possibly plunging Earth into an ice age. This scenario predicts enhanced deposition of calcium carbonate, the formation of siderite, and an increase in ocean pH, all of which are consistent with recent observations. Late Neoproterozoic diversification of marine eukaryotes may have facilitated the episodic enhancement of export of organic matter from the upper ocean, by causing a greater proportion of organic matter to be partitioned as particulate aggregates that can sink more efficiently, via increased cell size, biomineralization or increased C∶N of eukaryotic phytoplankton. The scenario explains isotopic excursions that are correlated or uncorrelated with snowball initiation, and suggests that increasing atmospheric oxygen concentrations and a progressive oxygenation of the subsurface ocean helped to prevent snowball glaciation on the Phanerozoic Earth. PMID:21825156

  19. Triple oxygen isotope evidence for elevated CO2 levels after a Neoproterozoic glaciation.

    PubMed

    Bao, Huiming; Lyons, J R; Zhou, Chuanming

    2008-05-22

    Understanding the composition of the atmosphere over geological time is critical to understanding the history of the Earth system, as the atmosphere is closely linked to the lithosphere, hydrosphere and biosphere. Although much of the history of the lithosphere and hydrosphere is contained in rock and mineral records, corresponding information about the atmosphere is scarce and elusive owing to the lack of direct records. Geologists have used sedimentary minerals, fossils and geochemical models to place constraints on the concentrations of carbon dioxide, oxygen or methane in the past. Here we show that the triple oxygen isotope composition of sulphate from ancient evaporites and barites shows variable negative oxygen-17 isotope anomalies over the past 750 million years. We propose that these anomalies track those of atmospheric oxygen and in turn reflect the partial pressure of carbon dioxide (P(CO2)) in the past through a photochemical reaction network linking stratospheric ozone to carbon dioxide and to oxygen. Our results suggest that P(CO2) was much higher in the early Cambrian than in younger eras, agreeing with previous modelling results. We also find that the (17)O isotope anomalies of barites from Marinoan (approximately 635 million years ago) cap carbonates display a distinct negative spike (around -0.70 per thousand), suggesting that by the time barite was precipitating in the immediate aftermath of a Neoproterozoic global glaciation, the P(CO2) was at its highest level in the past 750 million years. Our finding is consistent with the 'snowball Earth' hypothesis and/or a massive methane release after the Marinoan glaciation. PMID:18497821

  20. Biostratigraphy of Cretaceous-Paleogene marine succession, foraminiferal changes across the K/T boundary, sequence stratigraphy and response to sedimentary cyclicity in the Haymana Basin (Central Anatolia, Turkey)

    NASA Astrophysics Data System (ADS)

    Amirov, Elnur

    2016-04-01

    The aim of this study is to establish the planktonic foraminiferal biozonation, to construct the sequence stratigraphical framework and to determine the foraminiferal response to sedimentary cyclicity in the sedimentary sequence spanning Upper Cretaceous-Paleocene in the Haymana basin (Central Anatolia, Turkey). In order to achieve this study, the stratigraphic section was measured from sedimentary sequence of the Haymana, Beyobası and Yeşilyurt formations. The sedimentary sequence is mainly characterized by flyschoidal sequence that is composed of alternating of siliciclastic and carbonate units. On the account of the detailed taxonomic study of planktonic foraminifers, the biostratigraphic framework was established for the Maastrichtian-Paleocene interval. The biozonation includes 7 zones; Pseudoguembelina hariaensis, Pα, P1, P2, P3, P4 and P5 zones. The Cretaceous-Paleogene (K/P) boundary was delineated between the samples HEA-105 and 106. In order to construct the sequence-stratigraphical framework, the A, B, C and D-type meter-scale cycles were identified. Based on the stacking patterns of them, six depositional sequences, six third and two second order cycles were determined. Third order cycles coincide with the Global Sea Level Change Curve. On the account of the conducted petrographic analysis sandstone, mudstone, marl, limestone and muddy-limestone lithofacies were recorded in the studied samples. In order to demonstrate the response of foraminifers to cyclicity, quantitative analysis has been carried out by counting the individuals of planktonic, benthonic foraminifers and ostracods. The best response to sedimentary cyclicity was revealed from planktonic foraminifers. The average abundance of planktonic foraminifers increases in the transgressive systems tract and decreases in the highstand systems tract. Foraminifera are the most abundant marine protozoa in the benthic, epipelagic and pelagic realm. Because of the complexity and diversity of habitats

  1. Faulted Sedimentary Rocks

    NASA Technical Reports Server (NTRS)

    2004-01-01

    27 June 2004 This Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image shows some of the layered, sedimentary rock outcrops that occur in a crater located at 8oN, 7oW, in western Arabia Terra. Dark layers and dark sand have enhanced the contrast of this scene. In the upper half of the image, one can see numerous lines that off-set the layers. These lines are faults along which the rocks have broken and moved. The regularity of layer thickness and erosional expression are taken as evidence that the crater in which these rocks occur might once have been a lake. The image covers an area about 1.9 km (1.2 mi) wide. Sunlight illuminates the scene from the lower left.

  2. Ladon Sedimentary Rocks

    NASA Technical Reports Server (NTRS)

    2005-01-01

    6 June 2005 This Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image shows light-toned, layered, sedimentary rocks exposed by the fluids that carved the Ladon Valles system in the Erythraeum region of Mars. These rocks are so ancient that their sediments were deposited, cemented to form rock, and then eroded by the water (or other liquid) that carved Ladon Valles, so far back in Martian history that such liquids could still flow on the planet's surface.

    Location near: 20.8oS, 30.0oW Image width: 3 km (1.9 mi Illumination from: upper left Season: Southern Spring

  3. Meridiani Sedimentary Rocks

    NASA Technical Reports Server (NTRS)

    2003-01-01

    MGS MOC Release No. MOC2-545, 15 November 2003

    Northern Sinus Meridiani is a region of vast exposures of layered, sedimentary rock. Buried within these layers are many filled impact craters. Erosion has re-exposed several formerly-buried craters in this Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image. Arrows 1 and 2 indicate craters that are still emerging from beneath layered material; arrow 3 indicates a crater that has been fully re-exposed. This image is located near 5.1oN, 2.7oW. The area shown is about 3 km (1.9 mi) wide and illuminated from the left/upper left.

  4. Sedimentary Rock Outcrops

    NASA Technical Reports Server (NTRS)

    2004-01-01

    16 August 2004 This Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image shows eroded layered rock outcrops in a crater north of Meridiani Planum near 2.7oN, 359.1oW. The dozens and dozens of sedimentary rock layers of repeated thickness and similar physical properties at this location suggest that they may have been deposited in a lacustrine (lake) setting. The crater in which these layers occur may once have been completely filled and buried, as is the case for many craters in the Sinus Meridiani region. This image covers an area about 3 km (1.9 mi) across; sunlight illuminates the scene from the left.

  5. Sedimentary Rock Near Coprates

    NASA Technical Reports Server (NTRS)

    2003-01-01

    MGS MOC Release No. MOC2-420, 13 July 2003

    This mosaic of two Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) narrow angle camera images, one from 2001, the other from 2003, shows light-toned, layered, sedimentary rock outcrops exposed on the floor of a trough that parallels Coprates Chasma in the Valles Marineris system. Layered rocks form the pages from which the history of a place can be read. It may be many years before the story is read, but or now at least we know where one of the books of martian history is found. This picture is located near 15.2oS, 60.1oW. Sunlight illuminates the scene from the left.

  6. Sedimentary condensation and authigenesis

    NASA Astrophysics Data System (ADS)

    Föllmi, Karl

    2016-04-01

    Most marine authigenic minerals form in sediments, which are subjected to condensation. Condensation processes lead to the formation of well individualized, extremely thin (< 1m) beds, which were accumulated during extremely long time periods (> 100ky), and which experienced authigenesis and the precipitation of glaucony, verdine, phosphate, iron and manganese oxyhydroxides, iron sulfide, carbonate and/or silica. They usually show complex internal stratigraphies, which result from an interplay of sediment accumulation, halts in sedimentation, sediment winnowing, erosion, reworking and bypass. They may include amalgamated faunas of different origin and age. Hardgrounds may be part of condensed beds and may embody strongly condensed beds by themselves. Sedimentary condensation is the result of a hydrodynamically active depositional regime, in which sediment accumulation, winnowing, erosion, reworking and bypass are processes, which alternate as a function of changes in the location and intensity of currents, and/or as the result of episodic high-energy events engendered by storms and gravity flow. Sedimentary condensation has been and still is a widespread phenomenon in past and present-day oceans. The present-day distribution of glaucony and verdine-rich sediments on shelves and upper slopes, phosphate-rich sediments and phosphorite on outer shelves and upper slopes, ferromanganese crusts on slopes, seamounts and submarine plateaus, and ferromanganese nodules on abyssal seafloors is a good indication of the importance of condensation processes today. In the past, we may add the occurrence of oolitic ironstone, carbonate hardgrounds, and eventually also silica layers in banded iron formations as indicators of the importance of condensation processes. Besides their economic value, condensed sediments are useful both as a carrier of geochemical proxies of paleoceanographic and paleoenvironmental change, as well as the product of episodes of paleoceanographic and

  7. Relationship between the Neoproterozoic snowball Earth and Cambrian explosion

    NASA Astrophysics Data System (ADS)

    Maruyama, S.; Yoshihara, A.; Isozaki, Y.

    2007-12-01

    Origin of snowball Earth has been debated in terms of greenhouse gas (e.g., Hoffman and Schrag), obliqueness of Earth's rotation axis (Williams, 1975), true polar wander (Evans, 2003), Galactic cosmic ray radiation (Shaviv and Veizer, 2003; Svensmark, 2006), or weakened geomagnetism (Maruyama and Yoshihara, 2003). A major difficulty for the greenhouse gas hypothesis is the on-off switch causing decrease and increase of appropriate amounts of CO2 by plume- and plate tectonics, and also in available amount of CO2 in atmosphere to be consistent with the observations. In contrast, the cosmic ray radiation models due to the star burst peaked at 2.5- 2.1 Ga and 1.4-0.8 Ga can explain on-off switch more easily than the greenhouse gas model. Cosmic ray radiations, however, must be modified by the geomagnetic intensity, fluctuating 150"% to < 10"% of the present-day level through geologic time. Our compilation suggests the idea of extensive glaciation appeared when the intensity decreased below 50% of the present-day value, as typically seen in the Neoproterozoic time. This proposes the idea of extensive cloudiness by increased cosmic rays in the Neoproterozoic to cause the snowball Earth. Time difference between the Neoproterozoic snowball Earth and Cambrian explosion is as large as 250 millions years, and this refuses their direct close-relationship. Role of frequent mass extinctions, i.e., 8 times during 100 m.y. from 585 Ma to 488 Ma, during the Ediacaran and Cambrian, has been proposed (Zhu et al., 2007). This frequency is one order of magnitude higher compared to that in the post-Ordovician time. Yet, the Cambrian explosion cannot be explained by mass extinction which replaced the vacant niches shortly after the mass extinction and never created a new animal with a new body plan. A new model proposed herein is derived from weakened geomagnetism and resultant extensive cosmic radiation to alter gene and genome for a long period over advancement of low magnetic

  8. Heterogeneity of Sedimentary Aquifers: effect on microbial dynamics at successive spatial scales as revealed by geophysical imaging: Final report to the Department of Energy on Award DE-FG02-9ER62478

    SciTech Connect

    Donald J. P. Swift

    2004-02-10

    This report describes the geological component of the interdisciplinary study of the experimental aquifer at Oyster, Virginia, by the NABIR program, Department of Energy (Natural and Accelerated Bioremediation Research), between 1997 and 2003, as conducted by the Sediment dynamics group of Old Dominion University. The Geological component of the Oyster study was designed to (1) predict patterns of physical heterogeneity in sedimentary aquifers that control groundwater flow by application of geological first principles, (2) determine the geophysical imaging signatures of these patterns, and (3) relate patterns of physical heterogeneity thus sampled to observed microbial populations. The geological study began in 1997 at the North Oyster site, but in 2002, moved to the South Oyster site.

  9. The Santa Terezinha-Campos Verdes emerald district, central Brazil: structural and Sm-Nd data to constrain the tectonic evolution of the Neoproterozoic Brası´lia belt

    NASA Astrophysics Data System (ADS)

    D'el-Rey Silva, Luiz José Homem; Barros Neto, Leonel de Souza

    2002-12-01

    Structural analysis coupled with Sm-Nd isotope data and a detailed description of the geology of the Santa Terezinha-Campos Verdes emerald district (Goiás State, Central Brazil) constrain the evolution of the Neoproterozoic Brası´lia belt. The area is composed of tectonic slices of Archean-Paleoproterozoic gneiss, a Meso-Neoproterozoic metavolcanic sedimentary sequence called the Santa Terezinha sequence, and crustal-derived intrusive rocks such as mylonitic (ortho)gneiss and a syntectonic porphyry granite. It underwent a Neoproterozoic greenschist facies polyphase ductile deformation (D 1-D 3). Structures indicate an event of rotational deformation along a typical frontal ramp dipping gently to the west (i.e. an event of simple shear with top to ESE relative regional movement due to a subhorizontal WNW-ESE compression). A Sm-Nd whole-rock isochron age of 577±77 Ma for the intrusive rocks constrains the timing of at least part of the deformation/metamorphism in the area. Primary and metamorphic planar structures (mainly D 1-D 2) strike SW-NE and dip at low to moderate angles to the NW in the northern part of the area. However, they gradually rotate to SSE in the central SE part, where the Peixe River synclinorium is developed. This synclinorium is also the nest of the D 2 sheath folds that control emerald ore shoots. The Santa Cruz dome is a basement-cored, major elliptic structure in the SW of the area. The Santa Terezinha sequence represents a back-arc basin that received input from the Neoproterozoic Goiás magmatic arc to the west and the São Francisco ancient continental margin to the east. The basal and upper sections of this sequence correlate, respectively, with other passive margin and back-arc sequences of the Brası´lia belt.

  10. Evidence of giant sulphur bacteria in Neoproterozoic phosphorites.

    PubMed

    Bailey, Jake V; Joye, Samantha B; Kalanetra, Karen M; Flood, Beverly E; Corsetti, Frank A

    2007-01-11

    In situ phosphatization and reductive cell division have recently been discovered within the vacuolate sulphur-oxidizing bacteria. Here we show that certain Neoproterozoic Doushantuo Formation (about 600 million years bp) microfossils, including structures previously interpreted as the oldest known metazoan eggs and embryos, can be interpreted as giant vacuolate sulphur bacteria. Sulphur bacteria of the genus Thiomargarita have sizes and morphologies similar to those of many Doushantuo microfossils, including symmetrical cell clusters that result from multiple stages of reductive division in three planes. We also propose that Doushantuo phosphorite precipitation was mediated by these bacteria, as shown in modern Thiomargarita-associated phosphogenic sites, thus providing the taphonomic conditions that preserved other fossils known from the Doushantuo Formation. PMID:17183268

  11. Neoproterozoic ice ages, boron isotopes, and ocean acidification

    NASA Astrophysics Data System (ADS)

    Kasemann, S. A.; Prave, A. R.; Fallick, A. E.; Hawkesworth, C. J.; Hoffmann, K.

    2010-12-01

    The Neoproterozoic Earth underwent at least two severe glaciations, each extending to low palaeomagnetic latitudes and punctuating warmer climates. In concert with the environmental changes, the rocks display large amplitude fluctuations in their stable isotopic composition. These fluctuations are stratigraphically systematic, occur in many sections worldwide and are interpreted as being globally significant1. Thus, the Neoproterozoic carbonates provide a unique geological and isotopic archive to improve our understanding of major non-anthropogenically influenced changes in Earth System behaviour. The two widespread older and younger Cryogenian glacial deposits (commonly referred to as the Sturtian and Marinoan, respectively) in Namibia are directly overlain by cap carbonates deposited under inferred periods of high atmospheric carbon dioxide concentrations. Oceanic uptake of carbon dioxide decreases ocean pH and here we present a record of Cryogenian inter-glacial ocean pH, based on boron isotopes in marine carbonates. Our data document characteristically different B isotope profiles of the two Cryogenian carbonate transects that are consistent with the presence of two panglacial climate states, but indicate that each had its own distinct environmental conditions. The Marinoan interglacial δ11B profiles are systematic and remarkably consistent, and they vary by up to 11‰. This yields a relative pH variation of up to 1.5 pH units, and implies a pH of 8.5 at the onset of cap carbonate deposition, followed by a decrease in pH to ~7 and then a return to pH ~8 for the upper part of the section. The transient ocean acidification excursion and the alkaline pH condition near the start and termination of the inferred greenhouse state suggests a rapid draw-down of CO2 initiated at the start of the deglaciation and supports inferences of a thick, global sea-ice shield with minimal air-sea gas exchange during glaciation. In contrast, largely constant B isotope values for

  12. Ganges Sedimentary Rocks

    NASA Technical Reports Server (NTRS)

    2004-01-01

    24 May 2004 Mariner 9 images acquired in 1972 first revealed a large, light-toned, layered mound in Ganges Chasma, part of the vast Valles Marineris trough system. This Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image shows a higher-resolution view of these rocks than was achieved by Mariner 9 or Viking, and higher than can be obtained by Mars Odyssey or Mars Express. The image, with a resolution of about 3.7 meters (12 feet) per pixel, shows eroded layered rock outcrops in Ganges Chasma. These rocks record a history of events that occurred either in Ganges Chasma, or in the rocks brought to the surface by the opening of Ganges Chasma. Either way, the story they might tell could be as fascinating and unprecedented as the story told by sedimentary rocks investigated this year in Meridiani Planum by the Opportunity Mars Exploration Rover ... no one knows. The image is located near 7.3oS, 48.8oW, and covers an area about 3 km (1.9 mi) across. The picture is illuminated by sunlight from the upper left.

  13. Evidence from detrital zircons for recycling of Mesoproterozoic and Neoproterozoic crust recorded in Paleozoic and Mesozoic sandstones of southern Libya

    NASA Astrophysics Data System (ADS)

    Meinhold, Guido; Morton, Andrew C.; Fanning, C. Mark; Frei, Dirk; Howard, James P.; Phillips, Richard J.; Strogen, Dominic; Whitham, Andrew G.

    2011-12-01

    The geodynamic history of the Precambrian basement in central North Africa as well as the age and provenance of its sedimentary cover sequence are still poorly constrained. Here we present first detrital zircon ages (obtained by LA-SF-ICP-MS and SHRIMP) from Paleozoic and Mesozoic sandstones of the eastern Murzuq Basin, southern Libya, which unconformably overlie the Saharan Metacraton. Establishing the age and provenance of these sandstones has important implications for our understanding of the evolution of northern Gondwana during the Paleozoic, especially for reconstructions of paleo-source areas and transport paths. Detrital zircons from the sandstones show mainly early Paleozoic to Neoarchean ages with four main age populations, at 2750-2500 Ma (8%), 2200-1750 Ma (16%), 1060-920 Ma (18%), and 720-530 Ma (39%). About 13% of all concordant grains yield ages of 1600-1000 Ma. In addition, there are 9 zircon grains (0.7% of all concordant grains) with ages of 3600-2800 Ma. The presence of a high number of ca. 1 Ga zircons is enigmatic and their origin is controversial. Besides direct sourcing from ca. 1 Ga igneous rocks in eastern Chad and ca. 1 Ga igneous rocks along the southeastern margins of the Congo and Tanzania cratons, recycling of Neoproterozoic sediments containing ca. 1 Ga zircons is another alternative hypothesis to explain the presence of ca. 1 Ga zircons in the Paleozoic sedimentary sequence of central North Africa. The ubiquitous occurrence of ca. 1 Ga zircons in Paleozoic sediments of southern Libya provides insights into the correlation and paleotectonic arrangement of Gondwana-derived terranes, present, for example, in the eastern Mediterranean and in southwestern Europe. Current paleotectonic models of dextral terrane transport along the northern Gondwana margin during the early Paleozoic may need to be revised.

  14. The Neoproterozoic and Paleozoic tectonostratigraphic evolution of Southwestern Mongolia and implications for crustal growth in Asia

    NASA Astrophysics Data System (ADS)

    Macdonald, F. A.; Bold, U.; Buchwaldt, R.; Smith, E. F.

    2013-12-01

    Neoproterozoic and Paleozoic strata on the Zavkhan Terrane of southwestern Mongolia contain unique geochemical and paleontological records that have become central to our understanding of this pivital era of Earth history. Here we present sedimentological, stratigraphic, structural, geochemical, and geochronological data that provide context for these records, and for the tectonic evolution of the Central Asian Orogenic Belt (CAOB). The CAOB is commonly cited as the largest region of Phanerozoic crustal growth on Earth, yet the tectonic evolution of this region is poorly constrained. In contrast to previous studies that depicted a back-arc basin setting for deposition of Neoproterozoic strata in Mongolia, we propose that the Zavkhan Terrane is a segment of a ribbon continent, which formed through collapse of an extensive ca. 811-802 Ma continental arc system on the Tarim margin of Rodinia, associated with the subduction of a mid-ocean ridge at around 800 Ma. Extension is recorded by emplacement of mafic dike swarms, normal faulting, growth fault deposition of more than 500m of cobble conglomerates and the eruption of ignimbrites in the Zavkhan Volcanics. As the Zavkhan Terrane detached, an additional succession of coarse siliciclastic strata was deposited in the Khasagtin suite, followed by passive margin sedimentation consisting of limestone and glacial deposits of the Tsagaan Olom Group on an isolated carbonate platform. Detrital zircon in the glacigenic Maikhan Ul Formation constrain its age to younger than 730 Ma. Chemostratigraphy in the overlying Tayshir Formation further suggests that the Maikhan Ul diamictite is correlative with the ca. 717-662 Ma Sturtian glacial epoch. By the time limestone of the overlying Tayshir Formation was deposited, arc volcanism had been inactive for over 100 Myrs. After rifting, the Proterozoic terranes of Mongolia formed an isolated ribbon continent that was mantled with passive margin carbonate platform sedimentation through

  15. Vestiges of an Iapetan rift basin in the New Jersey Highlands: Implfications for the Neoproterozoic Laurentian margin

    USGS Publications Warehouse

    Gates, A.E.; Volkert, R.A.

    2004-01-01

    Thin, discontinuous remnants of Neoproterozoic intracratonic rift-basin deposits of the Chestnut Hill Formation occur in the western New Jersey Highlands. These deposits form an important link between well-documented Iapetan rift-basins in both the northern and southern Appalachians. The close spatial relations of Chestnut Hill rocks to Paleozoic sedimentary rocks open the possibility that additional Iapetan rift-basins could be concealed beneath the rocks of the Valley and Ridge Province to the west indicating a much broader zone of rifting than has been previously proposed. The Chestnut Hill Formation is intermittently exposed along a 100 km-long band that extends northeast from Pennsylvania nearly to New York State. The lower part of the Chestnut Hill Formation is composed of interbedded lithic pebble- to boulder-conglomerate and feldspathic sandstone grading upward into interbedded phyllite, feldspathic and quartz sandstone, local paleosaprolite, quartz-pebble conglomerate, thin limestone lenses, volcanic, and volcaniclasic rocks, abundant bedded ironstone (hematite ore), and ultimately into diamictites that are interpreted as possible tilloids and containing rounded intra and extrabasinal clasts of the other lithologies. Extensive soft-sediment deformation, cross bedding, and clastic dikes are common in all but the lowest and upper facies. Banded hematite layers occur preferentially in fine-grained tuffs and tuffaceous sediments, but hematitization has affected most lithologies. Volcanic rocks consist of altered rhyolitic tuffs and lapilli tuffs that are interbedded with sediments. The Chestnut Hill Formation is interpreted to have been deposited in early alluvial, and later a complex of fluvial, lacustrine and deltaic environments. Provenance studies based upon petrographic and geochemical analysis of clastic rocks indicate that the sediments are predominantly immature and reflect derivation from local uplifted felsic basement sources in a rifted

  16. Neoproterozoic cap-dolostone deposition in stratified glacial meltwater plume

    NASA Astrophysics Data System (ADS)

    Liu, Chao; Wang, Zhengrong; Raub, Timothy D.; Macdonald, Francis A.; Evans, David A. D.

    2014-10-01

    Neoproterozoic cap carbonates host distinctive geochemical and sedimentological features that reflect prevailing conditions in the aftermath of Snowball Earth. Interpretation of these features has remained contentious, with hypotheses hinging upon timescale and synchronicity of deposition, and whether or not geochemical signatures of cap carbonates represent those of a well-mixed ocean. Here we present new high-resolution Sr and Mg isotope results from basal Ediacaran cap dolostones in South Australia and Mongolia. Least-altered Sr and Mg isotope compositions of carbonates are identified through a novel incremental leaching technique that monitors the purity of a carbonate sample and the effects of diagenesis. These data can be explained by the formation of these cap dolostones involving two chemically distinct solutions, a glacial meltwater plume enriched in radiogenic Sr, and a saline ocean residue with relatively lower 87Sr/86Sr ratios. Model simulations suggest that these water bodies remained dynamically stratified during part of cap-dolostone deposition, most likely lasting for ∼8 thousand years. Our results can potentially reconcile previous conflicts between timescales estimated from physical mixing models and paleomagnetic constraints. Geochemical data from cap carbonates used to interpret the nature of Snowball Earth and its aftermath should be recast in terms of a chemically distinct meltwater plume.

  17. Strontium isotopic variations of Neoproterozoic seawater - Implications for crustal evolution

    NASA Technical Reports Server (NTRS)

    Asmerom, Yemane; Jacobsen, Stein B.; Knoll, Andrew H.; Butterfield, Nicholas J.; Swett, Keene

    1991-01-01

    High-precision Sr isotopic data were obtained on carbonate samples from the Neoproterozoic Shaler Group, Victoria Island (Canada). Results indicate that, between ca. 790 and 850 Ma, the Sr-87/Sr-86 ratio of seawater varied betweeen 0.70676 and 0.70561, with the minimum value at about 830 Ma. A curve of the Sr-87/Sr-86 seawater ratio vs. age showed that the new data substantially improve the existing isotopic record of Sr in seawater for the period 790-850 Ma. The Sr isotopic system data were coupled with data for the Nd isotopic system to model changes in the seafloor spreading rates (hydrothermal flux) and the continental erosion for the period 500-900 Ma. Results indicate that hydrothermal flux reached a maximum value at ca. 830 Ma, while a maximum in erosion rate occurred at ca. 570 Ma. These peaks are considered to be related to the developments in the Pan-African and related orogenic events.

  18. Polyphase Neoproterozoic orogenesis within the east Africa- Antarctica orogenic belt in central and northern Madagascar

    USGS Publications Warehouse

    Key, R.M.; Pitfield, P.E.J.; Thomas, Ronald J.; Goodenough, K.M.; Waele, D.; Schofield, D.I.; Bauer, W.; Horstwood, M.S.A.; Styles, M.T.; Conrad, J.; Encarnacion, J.; Lidke, D.J.; O'connor, E. A.; Potter, C.; Smith, R.A.; Walsh, G.J.; Ralison, A.V.; Randriamananjara, T.; Rafahatelo, J.-M.; Rabarimanana, M.

    2011-01-01

    Our recent geological survey of the basement of central and northern Madagascar allowed us to re-evaluate the evolution of this part of the East Africa-Antarctica Orogen (EAAO). Five crustal domains are recognized, characterized by distinctive lithologies and histories of sedimentation, magmatism, deformation and metamorphism, and separated by tectonic and/or unconformable contacts. Four consist largely of Archaean metamorphic rocks (Antongil, Masora and Antananarivo Cratons, Tsaratanana Complex). The fifth (Bemarivo Belt) comprises Proterozoic meta-igneous rocks. The older rocks were intruded by plutonic suites at c. 1000 Ma, 820-760 Ma, 630-595 Ma and 560-520 Ma. The evolution of the four Archaean domains and their boundaries remains contentious, with two end-member interpretations evaluated: (1) all five crustal domains are separate tectonic elements, juxtaposed along Neoproterozoic sutures and (2) the four Archaean domains are segments of an older Archaean craton, which was sutured against the Bemarivo Belt in the Neoproterozoic. Rodinia fragmented during the early Neoproterozoic with intracratonic rifts that sometimes developed into oceanic basins. Subsequent Mid- Neoproterozoic collision of smaller cratonic blocks was followed by renewed extension and magmatism. The global 'Terminal Pan-African' event (560-490 Ma) finally stitched together the Mid-Neoproterozoic cratons to form Gondwana. ?? The Geological Society of London 2011.

  19. Neoproterozoic oceanic remnants in eastern Brazil: Further evidence and refutation of an exclusively ensialic evolution for the Araçuaí West Congo orogen

    NASA Astrophysics Data System (ADS)

    Pedrosa-Soares, Antônio Carlos; Vidal, Philippe; Leonardos, Othon Henry; Bley de Brito Neves, Benjamin

    1998-06-01

    The Araçuaí (eastern Brazil) and West Congo (southwestern Africa) belts are counterparts of the same Neoproterozoic orogen located between the São Francisco and Congo cratons. The Macaúbas Group represents a major passive margin sequence and is a key unit for interpreting the evolution of that orogen. The Salinas Formation is the distal rock assemblage of the Macaúbas Group and consists of a deep-sea sand-mud sequence, and a volcanic-sedimentary unit called the Ribeirão da Folha facies. The latter includes metamorphosed volcanic-exhalative sediments associated with ocean-floor basalts (amphibolites). The magmatic protoliths of these amphibolites crystallized at about 816 ± 72 Ma (Sm-Nd whole-rock isochron, ɛNd(t) =+3.8 ± 0.2). Regional metamorphism reached the amphibolite facies at about 630 Ma (Rb-Sr whole-rock isochron), when slabs of ultramafic rocks were tectonically emplaced over the Ribeirão da Folha facies. We consider this volcanic-sedimentary facies and the coeval slabs of ultramafic rocks to be remnants of a branch of the Adamastor-Brazilide ocean. The extensive occurrence of syntectonic to late tectonic calc-alkalic granitoids along the internal domain of the Araçuaí belt implies that a reasonably large amount of ocean crust was consumed, via an east-dipping subduction zone, during formation of the Araçuaí West Congo orogen.

  20. Martian sediments and sedimentary rocks

    NASA Technical Reports Server (NTRS)

    Markun, C. D.

    1988-01-01

    Martian sediments and sedimentary rocks, clastic and nonclastic, should represent a high priority target in any future return-sample mission. The discovery of such materials and their subsequent analysis in terrestrial laboratories, would greatly increase the understanding of the Martian paleoclimate. The formation of Martian clastic sedimentary rocks, under either present, low-pressure, xeric conditions or a postulated, high-pressure, hydric environment, depends upon the existence of a supply of particles, various cementing agents and depositional basins. A very high resolution (mm-cm range) photographic reconnaissance of these areas would produce a quantum jump in the understanding of Martian geological history. Sampling would be confined to more horizontal (recent) surfaces. Exploration techniques are suggested for various hypothetical Martian sedimentary rocks.

  1. Nd isotopes and the provenance of detrital sediments of the Neoproterozoic Brası´lia Belt, central Brazil

    NASA Astrophysics Data System (ADS)

    Pimentel, M. M.; Dardenne, M. A.; Fuck, R. A.; Viana, M. G.; Junges, S. L.; Fischel, D. P.; Seer, H. J.; Dantas, E. L.

    2001-11-01

    The Neoproterozoic Brası´lia Belt, in central Brazil, includes in its eastern part a thick pile of sediments deposited and deformed along the western margin of the São Francisco-Congo Craton. Several lithostratigraphic units are identified (the Araı´, Paranoá, Serra da Mesa, Araxá, Ibiá, Vazante, Canastra and Bambuı´ groups) and have been traditionally interpreted as part of a passive margin association (<1.2 Ga), with sediments being derived from Archaean or Paleoproterozoic continental sources to the north and east. Nd isotopic signatures of fine-grained detrital sediments of the several rock units of the belt were investigated in order to assess: (i) the nature and average crustal residence ages of the source areas, and (ii) the tectonic significance of the different sedimentary units in respect to the evolution of the Brası´lia Belt. TDM model ages of the ca. 1.2-0.9 Ga old Paranoá and Canastra rhythmites, shales and phyllites vary within the interval between 1.9 and 2.3 Ga, suggesting relatively uniform Paleoproterozoic continental sources within the São Francisco continent. The sediments of the detritic/carbonatic Vazante Group also display Paleoproterozoic model ages indicating, however, a distinct shift towards slightly younger TDM values (1.7-2.1 Ga). These three sequences are interpreted as the typical representatives of the passive margin sequence, with dominance of Paleoproterozoic sources. The Ibiá and Araxá groups show a bimodal distribution of model age values, with a set of samples displaying TDM values between 1.8 and 2.1 Ga (similar to the passive margin sequence), and another set with younger model ages, between ca. 1.0 and 1.3 Ga. Younger sources such as those represented by the Neoproterozoic Goiás Magmatic Arc (0.93-0.64 Ga) in the west, are required to explain the young model ages for these sediments. Immature sediments (feldspathic micaschists) within the magmatic arc, in fact, have TDM model ages mostly between 1.0 and 1

  2. Sedimentary Rocks of Aram Chaos

    NASA Technical Reports Server (NTRS)

    2004-01-01

    4 February 2004 Aram Chaos is a large meteor impact crater that was nearly filled with sediment. Over time, this sediment was hardened to form sedimentary rock. Today, much of the eastern half of the crater has exposures of light-toned sedimentary rock, such as the outcrops shown in this Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image. The picture is located near 2.0oN, 20.3oW, and covers an area 3 km (1.9 mi) wide. Sunlight illuminates the scene from the left.

  3. Biotic enhancement of weathering, atmospheric oxygen and carbon dioxide in the Neoproterozoic

    NASA Astrophysics Data System (ADS)

    Lenton, Timothy M.; Watson, Andrew J.

    2004-03-01

    It has been suggested that biological colonization of the land surface began in the Neoproterozoic 1000-544 million years ago (Ma). We hypothesize that this colonization involved selective weathering of P from rocks, as well as an amplification of overall weathering rates. We show that two recent models, despite differences in the feedback mechanisms represented, predict that an increase in the weathering flux of P to the ocean would have caused a rise in atmospheric O2 in the Neoproterozoic. This in turn may have provided a necessary condition for the evolution of animals with hard skeletons seen in the 'Cambrian explosion'. Increased weathering of silicate rocks would also have caused a decline in atmospheric CO2, which could have been a causal factor in the Neoproterozoic glaciations.

  4. Early Neoproterozoic Ocean Chemistry: Fe-S Systematics from the Chuar Group

    NASA Astrophysics Data System (ADS)

    Johnston, D. T.; Poulton, S. W.; Dehler, C. M.; Canfield, D. E.; Knoll, A. H.

    2008-12-01

    Recent work suggests that Fe-rich, Archean-like ocean conditions returned during the Neoproterozoic. This provocative finding raises a number of questions about the triggers for this transition and the geochemical and biological consequences. To better address these questions, we present a high-resolution geochemical record through the early Neoproterozoic Chuar Group. The Chuar Group is composed of more than 1500 meters of gently folded, unmetamorphosed inter-bedded fossiliferous shales and meter scale carbonate- sandstones. Focusing primarily on the shales, Fe-speciation and sulfur isotope data provide the backbone of the study. Fe-speciation data, in particular, allows for the story of Neoproterozoic ocean anoxia to be updated and extended. Interestingly, these data illustrate transitions between Fe-rich and S-rich anoxic conditions, and shed light on how these reversions behave and are initiated. Just as important as the local information gained from the Chuar sediments, these data further elucidate our understanding of the early Neoproterozoic world. Specifically, we gain information about one of the most influential and complex factors in Neoproterozoic geochemistry - the size and speciation of the marine sulfur reservoir. For instance, necessarily low levels of seawater sulfate 1) limit the prominence of sulfate reducing bacteria in remineralization reactions, 2) increase the relative iron efflux from hydrothermal systems, and 3) may have consequences for the marine methane hydrate budget. When paleontological and biomarker records are interpreted in light of these geochemical findings, questions ranging from the abundance and distribution of microfossils, to transitions in dominant primary producers, and potential inhibitors to eukaryotic evolution, are more fully understood. Together, these data contribute to an evolving picture of the Neoproterozoic world.

  5. Mid-Paleozoic arc granitoids in SW Japan with Neoproterozoic xenocrysts from South China: New zircon U-Pb ages by LA-ICP-MS

    NASA Astrophysics Data System (ADS)

    Aoki, Kazumasa; Isozaki, Yukio; Yamamoto, Atsushi; Sakata, Shuhei; Hirata, Takafumi

    2015-01-01

    The Kurosegawa belt in SW Japan preserves fragments of Early-Middle Paleozoic granitoids traditionally called the Mitaki igneous rocks and previously dated ca. 470-435 Ma by several isotope analyses such as Rb-Sr, K-Ar and U-Pb, together with high-grade metamorphic rocks and Silurian strata, in a narrow belt. However, the timing of the oldest arc-related plutonism in Japan is constrained by laser-ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb ages of zircon from Mitaki granitoids in 3 areas in SW Japan; i.e. Kyushu, Shikoku, and Kii Peninsula. Weighted mean U-Pb ages of these zircons are 431.8 ± 3.9 for a granodiorite from the Mt. Mitaki area in Shikoku, 444.1 ± 5.8 Ma for a granodiorite from Kuraoka area in central Kyushu, and 444.5 ± 7.6 Ma for a quartzdiorite from the Nabaenohana area in western Kii peninsula, respectively. These ages confirm that the Mitaki igneous rocks have ca. 445-435 Ma (late Ordovician to mid-Silurian) ages. They are some of the oldest subduction-related plutonic rocks in SW Japan. Particularly noteworthy is a "tonalite" from the Nabaenohana area, which has a unique spectrum of zircon U-Pb ages with distinct 3 clusters; ca. 700-500 Ma (Neoproterozoic-Cambrian), ca. 1350-830 Ma (Meo-Neoproterozoic), and ca. 3230-1560 Ma (Paleoarchean-Mesoproterozoic). Of the 44 dated zircon grains, the youngest (possibly xenocrystic) grains are ca. 500 Ma. Older zircon grains with ages >500 Ma range up to 3230 Ma and are interpreted as inherited xenocrysts in the "tonalite". The Mitaki igneous rocks are interpreted to have been derived by melting of post-500 Ma terrigenous sedimentary rocks that yielded a unique tonalitic S-type granitoid magma. From the presence of abundant 1350-700 Ma (Meso- to Neoproterozoic) zircon grains in the "tonalite" we conclude that during the Early Paleozoic, proto-Japan was located close to the Cathaysian margin of South China.

  6. Quantitative characterisation of sedimentary grains

    NASA Astrophysics Data System (ADS)

    Tunwal, Mohit; Mulchrone, Kieran F.; Meere, Patrick A.

    2016-04-01

    Analysis of sedimentary texture helps in determining the formation, transportation and deposition processes of sedimentary rocks. Grain size analysis is traditionally quantitative, whereas grain shape analysis is largely qualitative. A semi-automated approach to quantitatively analyse shape and size of sand sized sedimentary grains is presented. Grain boundaries are manually traced from thin section microphotographs in the case of lithified samples and are automatically identified in the case of loose sediments. Shape and size paramters can then be estimated using a software package written on the Mathematica platform. While automated methodology already exists for loose sediment analysis, the available techniques for the case of lithified samples are limited to cases of high definition thin section microphotographs showing clear contrast between framework grains and matrix. Along with the size of grain, shape parameters such as roundness, angularity, circularity, irregularity and fractal dimension are measured. A new grain shape parameter developed using Fourier descriptors has also been developed. To test this new approach theoretical examples were analysed and produce high quality results supporting the accuracy of the algorithm. Furthermore sandstone samples from known aeolian and fluvial environments from the Dingle Basin, County Kerry, Ireland were collected and analysed. Modern loose sediments from glacial till from County Cork, Ireland and aeolian sediments from Rajasthan, India have also been collected and analysed. A graphical summary of the data is presented and allows for quantitative distinction between samples extracted from different sedimentary environments.

  7. Estimating duration and intensity of Neoproterozoic snowball glaciations from Ir anomalies.

    PubMed

    Bodiselitsch, Bernd; Koeberl, Christian; Master, Sharad; Reimold, Wolf U

    2005-04-01

    The Neoproterozoic glaciations supposedly ended in a supergreenhouse environment, which led to rapid melting of the ice cover and precipitation of the so-called cap carbonates. If Earth was covered with ice, then extraterrestrial material would have accumulated on and within the ice and precipitated during rapid melting at the end of the glaciation. We found iridium (Ir) anomalies at the base of cap carbonates in three drill cores from the Eastern Congo craton. Our data confirm the presence of extended global Neoproterozoic glaciations and indicate that the duration of the Marinoan glacial episode was at least 3 million, and most likely 12 million, years. PMID:15821088

  8. Phanerozoic cycles of sedimentary carbon and sulfur.

    PubMed

    Garrels, R M; Lerman, A

    1981-08-01

    A reservoir model of a Recent steady-state sedimentary system in which the reduced sulfur and oxidized sulfur reservoirs were coupled with the oxidized carbon and reduced carbon reservoirs was constructed. The time curve of the sulfur isotope ratios of the sedimentary sulfate reservoir was used to drive the model back to the beginning of Cambrian time (600 million years ago), producing the reservoir sizes and isotope values and material fluxes of the carbon-sulfur system. The predicted values of carbon isotope ratios of the carbonate reservoir agree well with observed values, showing that the model is basically sound. Some general conclusions from this success are (i) material flux rates in the carbon-oxygen-sulfur system of the geologic past (averaged over tens of millions of years) lie within about a factor of 2 of Recent rates. (ii) The oxidation-reduction balances of Phanerozoic time were dominated by reciprocal relationships between carbon and sulfur compounds. (iii) The rate of production of atmospheric oxygen by storage in sediments of organic carbon of photosynthetic origin increased from the Cambrian Period to the Permian Period and declined somewhat from the Permian Period to the Present. (iv) The storage of oxygen in oxidized sulfur compounds kept pace (within the limits of the data) with oxygen production. (v) Transfer of oxygen from CO(2) to SO(4) from the Cambrian to the Permian Period was several times the Recent free oxygen content of the atmosphere. PMID:16593066

  9. Linking lithosphere deformation and sedimentary basin formation over multiple scales

    NASA Astrophysics Data System (ADS)

    Huismans, Ritske S.

    2014-05-01

    In the spirit of Peter Ziegler we are interested in and explore the relationships between tectonic deformation and sedimentary basin formation. Resolving the interaction and feedback between tectonic crust-lithosphere scale deformation and surface processes through erosion of elevated areas and formation of sedimentary basins over multiple scales has been a long-standing challenge. While forward process based models have been successful at showing that a feedback is expected between tectonic deformation and redistribution of mass at the earth's surface by erosion, transport, and deposition, demonstrating this coupling for natural systems has been an even greater challenge and is strongly debated. Observational constraints on crust-lithosphere deformation and surface processes are typically collected at highly varying spatial and temporal scales, while forward process based models are typically run at either very large lithosphere-mantle scale, or at the scale of the sedimentary basin making it difficult to investigate and explore the detailed interaction and feedback between these systems. Here I will report on recent advances in forward modelling linking crust-lithosphere deformation with surface processes over a large range of scales resolving tectonic plate scale deformation and sedimentary basin formation at stratigraphic scales. The forward numerical models indicate a linkage and interaction between the structural style of thick-skinned large-scale mountain belt and rift-passive margin formation, erosion-transport-deposition processes operating at the surface, and the thin-skinned deformation occurring in the associated sedimentary basins.

  10. Provenance and tectonic significance of the Palaeoproterozoic metasedimentary successions of central and nothern Madagascar

    USGS Publications Warehouse

    De Waele, B.; Thomas, Ronald J.; Macey, P.H.; Horstwood, M.S.A.; Tucker, R.D.; Pitfield, P.E.J.; Schofield, D.I.; Goodenough, K.M.; Bauer, W.; Key, R.M.; Potter, C.J.; Armstrong, R.A.; Miller, J.A.; Randriamananjara, T.; Ralison, V.; Rafahatelo, J.-M.; Rabarimanana, M.; Bejoma, M.

    2011-01-01

    New detrital zircon U–Pb age data obtained from various quartzite units of three spatially separated supracrustal packages in central and northern Madagascar, show that these units were deposited between 1.8 and 0.8 Ga and have similar aged provenances. The distribution of detrital zircon ages indicates an overwhelming contribution of sources with ages between 2.5 and 1.8 Ga. Possible source rocks with an age of 2.5 Ga are present in abundance in the crustal segments (Antananarivo, Antongil and Masora Domains) either side of a purported Neoproterozoic suture ("Betsimisaraka Suture Zone"). Recently, possible source rocks for the 1.8 Ga age peak have been recognised in southern Madagascar. All three supracrustal successions, as well as the Archaean blocks onto which they were emplaced, are intruded by mid-Neoproterozoic magmatic suites placing a minimum age on their deposition. The similarities in detrital pattern, maximum and minimum age of deposition in the three successions, lend some support to a model in which all of Madagascar's Archaean blocks form a coherent crustal entity (the Greater Dharwar Craton), rather than an amalgamate of disparate crustal blocks brought together only during Neoproterozoic convergence. However, potential source terranes exist outside Madagascar and on either side of the Neoproterozoic sutures, so that a model including a Neoproterozoic suture in Madagascar cannot be dispelled outright.

  11. Polygon/Cracked Sedimentary Rock

    NASA Technical Reports Server (NTRS)

    2004-01-01

    4 December 2004 Exposures of sedimentary rock are quite common on the surface of Mars. Less common, but found in many craters in the regions north and northwest of the giant basin, Hellas, are sedimentary rocks with distinct polygonal cracks in them. This Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image shows an example from the floor of an unnamed crater near 21.0oS, 311.9oW. Such cracks might have formed by desiccation as an ancient lake dried up, or they might be related to ground ice freeze/thaw cycles or some other stresses placed on the original sediment or the rock after it became lithified. The 300 meter scale bar is about 328 yards long. The scene is illuminated by sunlight from the upper left.

  12. Sedimentary Rocks of Aram Chaos

    NASA Technical Reports Server (NTRS)

    2004-01-01

    10 May 2004 This Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image shows outcroppings of light-toned, layered, sedimentary rock within Aram Chaos, an ancient, partly-filled impact crater located near 3.2oN, 19.9oW. This 1.5 meters (5 feet) per pixel picture is illuminated by sunlight from the left and covers an area about 3 km (1.9 mi) across.

  13. Deep inside a neoproterozoic intra-oceanic arc: growth, differentiation and exhumation of the Amalaoulaou complex (Gourma, Mali)

    NASA Astrophysics Data System (ADS)

    Berger, Julien; Caby, Renaud; Liégeois, Jean-Paul; Mercier, Jean-Claude C.; Demaiffe, Daniel

    2011-10-01

    We show here that the Amalaoulaou complex, in the Pan-African belt of West Africa (Gourma, Mali), corresponds to the lower and middle sections of a Neoproterozoic intra-oceanic arc. This complex records a 90-130-Ma-long evolution of magmatic inputs and differentiation above a subducting oceanic slab. Early c. 793 Ma-old metagabbros crystallised at lower crustal or uppermost mantle depths (25-30 km) and have geochemical characteristic of high-alumina basalts extracted from a depleted mantle source slightly enriched by slab-derived sedimentary components ((La/Sm)N < 1; ɛNd: +5.4-6.2; 87Sr/86Sr: 0.7027-0.7029). In response to crustal thickening, these mafic rocks were recrystallised into garnet-granulites (850-1,000°C; 10-12 kbar) and subject to local dehydration-melting reactions, forming trondhjemititic leucosomes with garnet-clinopyroxene-rutile residues. Slightly after the granulitic event, the arc root was subject to strong HT shearing during partial exhumation (detachment faults/rifting or thrusting), coeval with the emplacement of spinel- and garnet-pyroxenite dykes crystallised from a high-Mg andesitic parental magma. Quartz and hornblende-gabbros (700-660 Ma) with composition typical of hydrous volcanic rocks from mature arcs ((La/Sm)N: 0.9-1.8; ɛNd: +4.6 to +5.2; 87Sr/86Sr: 0.7028-0.7031) were subsequently emplaced at mid-arc crust levels (~15 km). Trace element and isotopic data indicate that magmas tapped a depleted mantle source significantly more enriched in oceanic sedimentary components (0.2%). Exhumation occurred either in two stages (700-660 and 623 Ma) or in one stage (623 Ma) with a final exhumation of the arc root along cold P-T path (550°C, 6-9 kbar; epidote-amphibolite and greenschist facies conditions) during the main Pan-African collision event (620-580 Ma). The composition of magmas forming the Cryogenian Amalaoulaou arc and the processes leading to intra-arc differentiation are strikingly comparable to those observed in the deep section

  14. The Jebel Ohier deposit—a newly discovered porphyry copper-gold system in the Neoproterozoic Arabian-Nubian Shield, Red Sea Hills, NE Sudan

    NASA Astrophysics Data System (ADS)

    Bierlein, F. P.; McKeag, S.; Reynolds, N.; Bargmann, C. J.; Bullen, W.; Murphy, F. C.; Al-Athbah, H.; Brauhart, C.; Potma, W.; Meffre, S.; McKnight, S.

    2016-08-01

    Ongoing exploration in the Red Sea Hills of NE Sudan has led to the identification of a large alteration-mineralization system within a relatively undeformed Neoproterozoic intrusive-extrusive succession centered on Jebel Ohier. The style of mineralization, presence of an extensive stockwork vein network within a zoned potassic-propylitic-argillic-advanced argillic-altered system, a mineralization assemblage comprising magnetite-pyrite-chalcopyrite-bornite (±gold, silver and tellurides), and the recurrence of fertile mafic to intermediate magmatism in a developing convergent plate setting all point to a porphyry copper-gold association, analogous to major porphyry Cu-Au-Mo deposits in Phanerozoic supra-subduction settings such as the SW Pacific. Preliminary U-Pb age dating yielded a maximum constraint of c. 730 Ma for the emplacement of the stockwork system into a significantly older ( c. 800 Ma) volcanic edifice. The mineralization formed prior to regional deformation and accretion of the host terrane to a stable continental margin at by c. 700 Ma, thus ensuring preservation of the deposit. The Jebel Ohier deposit is interpreted as a relatively well-preserved, rare example of a Neoproterozoic porphyry Cu-Au system and the first porphyry Cu-Au deposit to be identified in the Arabian-Nubian Shield.

  15. The Jebel Ohier deposit—a newly discovered porphyry copper-gold system in the Neoproterozoic Arabian-Nubian Shield, Red Sea Hills, NE Sudan

    NASA Astrophysics Data System (ADS)

    Bierlein, F. P.; McKeag, S.; Reynolds, N.; Bargmann, C. J.; Bullen, W.; Murphy, F. C.; Al-Athbah, H.; Brauhart, C.; Potma, W.; Meffre, S.; McKnight, S.

    2015-12-01

    Ongoing exploration in the Red Sea Hills of NE Sudan has led to the identification of a large alteration-mineralization system within a relatively undeformed Neoproterozoic intrusive-extrusive succession centered on Jebel Ohier. The style of mineralization, presence of an extensive stockwork vein network within a zoned potassic-propylitic-argillic-advanced argillic-altered system, a mineralization assemblage comprising magnetite-pyrite-chalcopyrite-bornite (±gold, silver and tellurides), and the recurrence of fertile mafic to intermediate magmatism in a developing convergent plate setting all point to a porphyry copper-gold association, analogous to major porphyry Cu-Au-Mo deposits in Phanerozoic supra-subduction settings such as the SW Pacific. Preliminary U-Pb age dating yielded a maximum constraint of c. 730 Ma for the emplacement of the stockwork system into a significantly older (c. 800 Ma) volcanic edifice. The mineralization formed prior to regional deformation and accretion of the host terrane to a stable continental margin at by c. 700 Ma, thus ensuring preservation of the deposit. The Jebel Ohier deposit is interpreted as a relatively well-preserved, rare example of a Neoproterozoic porphyry Cu-Au system and the first porphyry Cu-Au deposit to be identified in the Arabian-Nubian Shield.

  16. Siberian Origins of Neoproterozoic to Upper Triassic Rocks of Arctic Alaska

    NASA Astrophysics Data System (ADS)

    Clough, J. G.; Blodgett, R. B.

    2007-12-01

    found in the Canadian Arctic Islands Richly diverse Upper Triassic fauna (halobiid and monotid bivalves, brachiopods) are present in the both the Shublik Formation and Otuk Group. These show closer affinities with NE Siberia rather than to western or northern North America, suggesting close spatial relationships between Siberia and Arctic Alaska at least until Late Triassic time. Sedimentary provenance studies in eastern Brooks Range Precambrian rocks indicate age ranges that are dissimilar to Proterozoic detrital-zircon ages from clastic rocks of the northern Canadian Cordillera and Canadian Arctic Islands where a detrital source within the Grenville orogen is indicated. Paleocurrent directions for the Neoproterozoic Katakturuk Dolomite in the northeast Brooks Range and similar-age units in the adjacent Victoria Island and Amundsen Basin are in approximately 100 degree opposition for a counterclockwise rotational- restored Arctic Alaska. Upper Devonian clastics of northern Alaska are in 180 degree opposition to coeval units in the Canadian Arctic Islands when the Arctic Alaska plate is restored in the rotational model. Therefore, based on paleobiogeography, sediment provenance, stratigraphy and sedimentology, tectonic models for the opening of the Canada Basin must take into account that Triassic and older rocks in Arctic Alaska have Siberian origins or were deposited proximal to Siberia.

  17. Recognition and characterisation of high-grade ignimbrites from the Neoproterozoic rhyolitic volcanism in southernmost Brazil

    NASA Astrophysics Data System (ADS)

    Sommer, Carlos Augusto; Lima, Evandro Fernandes; Machado, Adriane; Rossetti, Lucas de Magalhães May; Pierosan, Ronaldo

    2013-11-01

    Neoproterozoic magmatism in southern Brazil is associated with translithospheric shear belts and strike-slip basins in a post-collisional setting related to the last stages of the Brasilian-Pan African Orogenic Cycle. It evolved from an association of high-K calc-alkaline, leucocratic-peraluminous and continental tholeiitic magmas, to an association with shoshonitic magmas and, eventually, to an association with magmas of the sodic mildly alkaline series. This magmatism varies from metaluminous to peralkaline and exhibits alkaline sodic affinity. A large volcanism is related to this alkaline sodic magmatism and is named the Acampamento Velho Formation. This unit was coeval with subaerial siliciclastic sedimentation in post-collisional basins preserved in the region. The Acampamento Velho Formation consists of pyroclastic and effusive volcanic deposits, which are mainly silicic, emplaced under subaerial conditions. The best exposures of this volcanism occur on the Ramada and Taquarembó plateaus, located southwest of Rio Grande do Sul in southernmost Brazil. The pyroclastic flow deposits are composed mainly of juvenile fragments such as pumices, shards and crystal fragments. Welding is very effective in these units. High-grade ignimbrites occur at the base and intermediate portions of the deposits and rheoignimbrites are observed at the top. The pre-eruptive temperature calculations, which were obtained at the saturation of zircon, revealed values between 870 °C and 978 °C for Taquarembó Plateau and 850 °C-946 °C for Ramada Plateau. The calculated viscosity values vary from 6.946 to 8.453 log η (Pas) for the rheoignimbrites and 7.818 to 10.588 log η (Pas) for the ignimbrites. Zr contents increase toward the top of the pyroclastic sequence, which indicates an increase in peralkalinity and determines the reduction in viscosity for clasts at the upper portions of the flows. The patterns of the structures of the ignimbrites and rheoignimbrites in the Taquaremb

  18. Sedimentary Rocks in Ladon Vallis

    NASA Technical Reports Server (NTRS)

    2004-01-01

    25 January 2004 This is a Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) picture of an outcrop of light-toned, layered, sedimentary rock exposed by erosion in Ladon Vallis. These rocks preserve clues to the martian past. However, like books in a library, one needs to go there and check them out if one wishes to read what the layers have to say. This November 2003 picture is located near 21.1oS, 29.8oW, and covers an area 3km (1.9 mi.) wide. Sunlight illuminates the scene from the left.

  19. Multisensor classification of sedimentary rocks

    NASA Technical Reports Server (NTRS)

    Evans, Diane

    1988-01-01

    A comparison is made between linear discriminant analysis and supervised classification results based on signatures from the Landsat TM, the Thermal Infrared Multispectral Scanner (TIMS), and airborne SAR, alone and combined into extended spectral signatures for seven sedimentary rock units exposed on the margin of the Wind River Basin, Wyoming. Results from a linear discriminant analysis showed that training-area classification accuracies based on the multisensor data were improved an average of 15 percent over TM alone, 24 percent over TIMS alone, and 46 percent over SAR alone, with similar improvement resulting when supervised multisensor classification maps were compared to supervised, individual sensor classification maps. When training area signatures were used to map spectrally similar materials in an adjacent area, the average classification accuracy improved 19 percent using the multisensor data over TM alone, 2 percent over TIMS alone, and 11 percent over SAR alone. It is concluded that certain sedimentary lithologies may be accurately mapped using a single sensor, but classification of a variety of rock types can be improved using multisensor data sets that are sensitive to different characteristics such as mineralogy and surface roughness.

  20. Inversion of Extensional Sedimentary Basins

    NASA Astrophysics Data System (ADS)

    Buiter, Susanne J. H.; Pfiffner, O. Adrian

    The evolution of extensional sedimentary basins is governed by the surrounding stress field and can, therefore, be expected to be highly sensitive to variations in these stresses. Important changes in basin geometry are to be expected in the case of an even short-lived reversal from extension to compression. We investigate the evolu- tion of fold and thrust structures which form in compression after extension, when basin forming processes have come to a complete stop. To this purpose, we use a two- dimensional, viscoplastic model and start our experiments from a pre-existing exten- sional geometry. We illustrate the sensitivity of the evolving structures to inherited extensional geometry, sedimentary and erosional processes, and material properties. One series of our model experiments involves the upper- to middle crust only in order to achieve a high detail in the basin area. We find that our results agree with examples from nature and analogue studies in, among others, the uplift and rotation of syn-rift sediments, the propagation of shear zones into the post-rift sediments and, in specific cases, the development of back-thrusts or basement short-cut faults. We test the out- come of these models by performing a second series of model simulations in which basins on a continental margin are inverted through their progressive approach of a subduction zone. These latter models are on the scale of the whole upper mantle.

  1. Geochemistry and zircon ages of mafic dikes in the South Qinling, central China: evidence for late Neoproterozoic continental rifting in the northern Yangtze block

    NASA Astrophysics Data System (ADS)

    Zhu, Xiyan; Chen, Fukun; Liu, Bingxiang; Zhang, He; Zhai, Mingguo

    2015-01-01

    Neoproterozoic volcanic-sedimentary sequences of the southern Qinling belt, central China, were intruded by voluminous mafic dikes. secondary ion mass spectrometry zircon U-Pb dating indicates that these dikes were emplaced at 650.8 ± 5.2 Ma, coeval with mafic rocks occurring at the northern margin of the Yangtze block. The dikes are characterized by enrichment of large ion lithophile elements, high Ti contents (up to 3.73 wt%) and Nb/Ta ratios between 14.5 and 19.6, suggesting a mantle source of oceanic island basalt affinity. Initial 87Sr/86Sr ratios show positive correlation with SiO2 contents and negative correlation with Zr/Nb ratios, implying that these rocks were affected by crustal contamination during the magma ascend and emplacement process. The dikes have initial ɛ Nd values of +0.2 to +3.3, low 206Pb/204Pb ratios of 16.96-17.45, and moderate 87Sr/86Sr ratios of 0.7043-0.7076, likely pointing to the involvement of an enriched mantle source. The mafic dikes and coeval mafic volcanic equivalents in the South Qinling and the northern Yangtze are hypothesized to be related with the prolonged breakup of the supercontinent Rodinia, suggesting that continental rifting lasted until ca. 650 Ma.

  2. Identification of a Sturtian cap carbonate in the Neoproterozoic Sete Lagoas carbonate platform, Bambuí Group, Brazil

    NASA Astrophysics Data System (ADS)

    Vieira, Lucieth Cruz; Trindade, Ricardo I. F.; Nogueira, Afonso C. R.; Ader, Magali

    2007-03-01

    A sedimentological and C-O isotopic study has been carried out in nine sections of the Sete Lagoas Formation at its classical outcropping area, in the southern tip of the São Francisco craton (central Brazil), with the objective of refining its stratigraphic position within the Neoproterozoic. At the study area, the Neoproterozoic Sete Lagoas Formation comprises two shallowing-upward megacycles, corresponding to more than 200 m in thickness. Each cycle is limited by a flooding surface amalgamated with a third-order sequence boundary. The first megacycle presents deep-platform deposits with abundance of crystal fans (aragonite pseudomorphs). These deposits are characterized by negative C-isotope values (-4.5‰). They grade upward to storm-wave and tide-influenced layers with δ13C values around 0‰. In the second megacycle, a new transgression drowned the platform, depositing a thick, mixed sub-storm wave-base succession. This megacycle comprises deposits of lime mudstone-pelite rhythmite, which grade to crystalline limestone rich in organic matter, both with unusually positive δ13C values (up to + 14‰). Regional correlation of Sete Lagoas deposits indicate that they rest atop glaciomarine rocks of the Macaúbas Group and basal strata show seafloor precipitates with negative δ13C values. Therefore, it is possible to characterize the Sete Lagoas carbonate as a cap carbonate sequence. The very high δ13C in the second megacycle together with geochronologic data suggest that this unit correlates better with post-Sturtian sequences. Some differences in the depositional record are observed between Sete Lagoas and the other post-Sturtian units previously described in North America, Australia, and Namibia. Those differences may in part be due to deposition in shallower settings of the Sete Lagoas carbonates, thus preserving a thick record of storm- and wave-influenced sedimentation not found elsewhere. Alternatively, they may also be attributed to diachronic

  3. Neoproterozoic Land Colonisation, Rising Oxygen, Global Cooling and the Cambrian Explosion

    NASA Astrophysics Data System (ADS)

    Lenton, T. M.; Watson, A. J.

    2004-12-01

    The Neoproterozoic (1000-542 Ma BP) was a time of severe glaciations and a major transition from microscopic to macroscopic life forms. We develop the hypothesis that a rise in atmospheric oxygen in the Neoproterozoic was driven by the biological colonization of the land surface. If early forms of photosynthetic land life selectively weathered continental rock in order to extract nutrients, this would have led to an increase in the flux of biologically available phosphorus to the ocean. We show that recent models for coupled biogeochemical cycles, despite differences in the feedback mechanisms represented, predict this would lead to a rise in atmospheric oxygen concentration, consistent with biological and geochemical evidence. Increased weathering of silicate rocks would also have caused a decline in atmospheric carbon dioxide, which could have been a causal factor in the Neoproterozoic glaciations. A rise in oxygen may have provided a necessary condition for the evolution of animals with hard skeletons seen in the Cambrian explosion. Furthermore, an increase in phosphorus supply to the ocean may have driven an increase in the phosphorus content of marine primary producers. This would have represented an increase in food quality for grazing animals, which have a high phosphorus requirement, and may thus have removed a further limitation on their evolutionary radiation.

  4. Biotic enhancement of weathering, atmospheric oxygen and carbon dioxide in the Neoproterozoic

    NASA Astrophysics Data System (ADS)

    Watson, A.; Lenton, T.

    2003-04-01

    The Neoproterozoic (1000-544Ma BP) was a time of severe glaciations and a major transition from microscopic to macroscopic life forms. Here we develop the hypothesis that a rise in atmospheric oxygen in the Neoproterozoic was driven by the biological colonization of the land surface. If early forms of photosynthetic land life selectively weathered continental rock in order to extract nutrients, this would have led to an increase in the flux of biologically available phosphorus to the ocean. We show that recent models for coupled biogeochemical cycles, despite differences in the feedback mechanisms represented, predict this would lead to a rise in atmospheric oxygen concentration, consistent with biological and geochemical evidence. A rise in oxygen may in turn have provided a necessary condition for the evolution of animals with hard skeletons seen in the Cambrian explosion. Increased weathering of silicate rocks would also have caused a decline in atmospheric carbon dioxide, which could have been a causal factor in the Neoproterozoic glaciations.

  5. Molar tooth structures in calcareous nodules, early Neoproterozoic Burovaya Formation, Turukhansk region, Siberia

    NASA Astrophysics Data System (ADS)

    Pope, Michael C.; Bartley, Julie K.; Knoll, Andrew H.; Petrov, Peter Yu.

    2003-05-01

    Molar tooth structures are abundant in large (1-2 m diameter) carbonate nodules within fine-grained, subtidal carbonates of the early Neoproterozoic (lower Upper Riphean) Burovaya Formation along the Sukhaya Tunguska River, Turukhansk Uplift, northwestern Siberia. Although molar tooth structures are regionally abundant in this unit, here they occur only within the nodules. Stable isotopic compositions of molar-tooth-filling dolomicrospar cements and of thinly bedded dolomicrite within and surrounding the nodules are indistinguishable from one another. The carbon isotopic compositions (mean δ13C=+2.8‰ PDB±0.4) reflect mean average oceanic surface water composition during their formation; the light oxygen isotopic compositions (mean δ18O=-6.4‰ PDB±2.2) are generally similar to those of other little-altered Meso- to Neoproterozoic limestones and dolostones. These molar tooth structures have no features that would support a tectonic origin; they more likely formed through bacterial processes. Carbonate cement filling of these voids occurred soon after their formation, but the mechanism responsible for this carbonate precipitation is currently uncertain. Local restriction of molar tooth structures to early diagenetic nodules suggests that penecontemporaneous lithification was required for the formation, or at least preservation, of these widespread Mesoproterozoic to Neoproterozoic features.

  6. Chemostratigraphy of stable chromium isotopes in cap carbonate sequences - tracing the aftermath of Earth's Neoproterozoic icehouse climates

    NASA Astrophysics Data System (ADS)

    Frei, R.; de Andrade Caxito, F.; Gaucher, C.

    2012-12-01

    The Neoproterozoic Era (1000-542 Ma) was a time of extreme climatic variation as recorded in sedimentary rocks of this age across the globe. Of special interest are often occurring associations of glacial deposits with warm climate carbonate platforms, features which are preferentially explained to have resulted from extreme icehouse-greenhouse fluctuations unprecedented in the Phanerozoic record. Despite local differences in the sedimentation regime (clastic, mixed or carbonate), these events are represented by glacial deposits of diverse nature, overlain by distinctive "cap carbonate" sequences. The chemostratigraphy (particularly of δ13C and 87Sr/86Sr signatures) of carbonate sequences has been invoked as a promising alternative tool for regional and global correlation., and these signatures provide proxies for seawater composition at the time of deposition, and may indirectly signalize climatic fluctuations on land. We studied a cap carbonate profile pertaining to the Bambuí Group (Sete Lagoas Formation; Correntina section; previously studied by Caxito et al., 2012)) in the north central part of the São Franciso basin in Brazil. This section lies atop Archean to Paleoproterozoic gneisses of the São Francisco craton basement. The section begins with two-metre thick massive to finely laminated pink dolostone which grade upward into a reddish to purple limestone rhythmite. δ53Cr values of the cap dolostone are within the range typical of magmatic inventory signatures (δ53Cr = 0.1 +/- 0.1 permil; Schoenberg et al., 2008). Our preliminary few first data from the sequence above the cap dolostones show magmatic values also for samples from within the first 20 metres of laminated limestones, which then tend to increase to δ53Cr values of ~+0.3 permil in the following ca. 100 metres of carbonates. Although our data set at this stage is sparse, we note a trend that δ53Cr values correlate with fluctuations of δ13C and δ18O values (Caxito et al., 2012). These

  7. Elastic Properties of Sedimentary Rocks

    NASA Astrophysics Data System (ADS)

    Melendez Martinez, Jaime

    Sedimentary rocks are an important research topic since such rocks are associated to sources of ground water as well as oil, gas, and mineral reservoirs. In this work, elastic and physical properties of a variety of sedimentary samples that include glacial sediments, carbonates, shales, one evaporite, and one argillite from a variety of locations are investigated. Assuming vertical transverse isotropy, ultrasonic compressional- and shear-waves (at 1 MHz central frequency) were measured as a function of confining pressure on all samples with the exception of glacial samples which were tested assuming isotropy. Tensile strength tests (Brazilian test) were also carried out on selected glacial samples and, in addition, static-train measurements were conducted on shales and argillite samples. Lithological and textural features of samples were obtained through thin section techniques, scanning electron microscopy images and micro-tomography images. X-ray diffraction and X-Ray fluorescence provided the mineralogical oxides content information. Porosity, density, and pore structure were studied by using a mercury intrusion porosimeter and a helium pycnometer. The wide range of porosities of the studied samples (ranging from a minimum of 1% for shales to a maximum 45% for some glacial sediments) influence the measured velocities since high porosity sample shows an noticeable velocity increment as confining pressure increases as a consequence of closure of microcracks and pores, unlike low porosity samples where increment is quasi-lineal. Implementation of Gassmann's relation to ultrasonic velocities obtained from glacial samples has negligible impact on them when assuming water saturated samples, which suggests that state of saturation it is no so important in defining such velocities and instead they are mainly frame-controlled. On the other hand, velocities measured on carbonate and evaporite samples show that samples are at best weak anisotropic, thus the intrinsic

  8. Simulation model of clastic sedimentary processes

    SciTech Connect

    Tetzlaff, D.M.

    1987-01-01

    This dissertation describes SEDSIM, a computer model that simulates erosion, transport, and deposition of clastic sediments by free-surface flow in natural environments. SEDSIM is deterministic and is applicable to sedimentary processes in rivers, deltas, continental shelves, submarine canyons, and turbidite fans. The model is used to perform experiments in clastic sedimentation. Computer experimentation is limited by computing power available, but is free from scaling problems associated with laboratory experiments. SEDSIM responds to information provided to it at the outset of a simulation experiment, including topography, subsurface configuration, physical parameters of fluid and sediment, and characteristics of sediment sources. Extensive computer graphics are incorporated in SEDSIM. The user can display the three-dimensional geometry of simulated deposits in the form of successions of contour maps, perspective diagrams, vector plots of current velocities, and vertical sections of any azimuth orientation. The sections show both sediment age and composition. SEDSIM works realistically with processes involving channel shifting and topographic changes. Example applications include simulation of an ancient submarine canyon carved into a Cretaceous sequence in the National Petroleum Reserve in Alaska, known mainly from seismic sections and a sequence of Tertiary age in the Golden Meadow oil field of Louisiana, known principally from well logs.

  9. Marine and Lacustrine Organic-rich Sedimentary Unit Time Markers: Implications from Rhenium-Osmium Geochronology

    NASA Astrophysics Data System (ADS)

    Selby, D.

    2011-12-01

    Geochronology is fundamental to understand the age, rates and durations of Earth processes. This concerned Arthur Holmes who, for much of his career, attempted to define a geological time scale. This is a topic still important to Earth Scientists today, specifically the chronostratigraphy of sedimentary rocks. Here I explore the Re-Os geochronology of marine and lacustrine sedimentary rocks and its application to yield absolute time constraints for stratigraphy. The past decade has seen the pioneering research of Re-Os organic-rich sedimentary rock geochronology blossom into a tool that can now to be used to accurately and precisely determine depositional ages of organic-rich rock units that have experienced up to low grade greenschist metamorphism. This direct dating of sedimentary rocks is critical where volcanic horizons are absent. As a result, this tool has been applied to timescale calibration, basin correlation, formation duration and the timing of key Earth events (e.g., Neoproterozoic glaciations). The application of Re-Os chronometer to the Devonian-Mississippian boundary contained within the Exshaw Formation, Canada, determined an age of 361.3 ± 2.4 Ma. This age is in accord with U-Pb dates of interbedded tuff horizons and also U-Pb zircon date for the type Devonian-Mississippian Hasselbachtal section, Germany. The agreement of the biostratigraphic and U-Pb constraints of the Exshaw Formation with the Re-Os date illustrated the potential of the Re-Os chronometer to yield age determinations for sedimentary packages, especially in the absence of interbedd tuff horizons and biozones. A Re-Os date for the proposed type section of the Oxfordian-Kimmeridgian boundary, Staffin Bay, Isle of Skye, U.K., gave an age of 154.1 ± 2.2 Ma. This Re-Os age presents a 45 % (1.8 Ma) improvement in precision for the basal Kimmeridgian. It also demonstrated that the duration of the Kimmeridgian is nominally 3.3 Ma and thus is 1.6 Ma shorter than previously indicated. In

  10. Organic carbon cycling as the keystone of Neoproterozoic climate evolution (Invited)

    NASA Astrophysics Data System (ADS)

    Pierrehumbert, R.

    2009-12-01

    Snowball glaciations are the most charismatic feature of the Neoproterozoic, but the central problem of Neoproterozoic climate evolution is the operation of the carbon cycle, as evidenced in the return of extreme 13C fluctuations after nearly a billion years of quiescence. A key organizing principle for the Neoproterozoic is the existence of a massive hypothetical organic carbon pool in the ocean, which is oxidized by the end of the Neoproterozoic (as laid out by Fike, et al. 2006 ). The carbon cycle couples to climate through its influence on atmospheric greenhouse gas content -- notably CO2 and CH4; accumulation of atmospheric O2 feeds back on this through atmospheric and oceanic chemistry. Evolution of oxygenic photosynthesis unquestionably occurred long before the dawn of the Neoproterozoic, so the Neoproterozoic climate and carbon turmoil is a product of the organic carbon storage dynamics rather than gross biological innovation. In this talk I will discuss how certain key components of the physical and geochemical system operate, though a comprehensive model accounting for all crucial aspects of the geological record is still lacking. Conversion of CO2 to O2 by photosynthesis and carbon burial, converts a greenhouse gas to a non-greenhouse gas,cooling the climate. Conversely, oxidation of an organic carbon pool either through respiration or sulfate reduction, releases CO2 and acts as a warming influence,also leading to a negative carbonate 13C excursion, as probably happened during the Shuram. If the Marinoan excursion has a similar mechanism we face the question of how such an event could initiate a glaciation, whereas the much stronger Shuram excursion did not. (The Gaskiers glaciation came long after the Shuram , and was not a Snowball). I will discuss how the climate response depends on the time scale of the exchange, with emphasis on buffering due to the response of silicate weathering. Insofar as the organic carbon pool existed at all, the key question

  11. Sedimentary facies in submarine canyons

    NASA Astrophysics Data System (ADS)

    Sumner, E.; Paull, C. K.; Gwiazda, R.; Anderson, K.; Lundsten, E. M.; McGann, M.

    2013-12-01

    Submarine canyons are the major conduits by which sediment, pollutants and nutrients are transported from the continental shelf out into the deep sea. The sedimentary facies within these canyons are remarkably poorly understood because it has proven difficult to accurately sample these heterogeneous and bathymetrically complex environments using traditional ship-based coring techniques. This study exploits a suite of over 100 precisely located vibracores collected using remotely operated vehicles in ten canyons along the northern Californian margin, enabling better understanding of the facies that exist within submarine canyons, their distribution, and the processes responsible for their formation. The dataset reveals three major facies types within the submarine canyons: extremely poorly sorted, coarse-grained sands and gravels with complex and indistinct internal grading patterns and abundant floating clasts; classical normally graded thin bedded turbidites; and a variety of fine-grained muddy deposits. Not all facies are observed within individual canyons, in particular coarse-grained deposits occur exclusively in canyons where the canyon head cuts up to the modern day beach, whereas finer grained deposits have a more complex distribution that relates to processes of sediment redistribution on the shelf. Pairs of cores collected within 30 meters elevation of one another reveal that the coarse-grained chaotic deposits are restricted to the basal canyon floor, with finer-grained deposits at higher elevations on the canyon walls. The remarkable heterogeneity of the facies within these sediment cores illustrate that distinctive processes operate locally within the canyon. In the authors' experience the canyon floor facies represent an unusual facies rarely observed in ancient outcrops, which potentially results from the poor preservation of ancient coarse-grained canyon deposits in the geological record.

  12. Coupled Changes in Sulfur and Carbon Isotopes Preceding the Sturtian Glaciation of the Neoproterozoic

    NASA Astrophysics Data System (ADS)

    Gouldey, J.; Hurtgen, M.

    2014-12-01

    The relationship between the carbon and sulfur cycles in the Neoproterozoic has proven to be complex. Unusually high δ13Ccarbonate persists throughout this period, however large negative excursions precede both the Sturtian (~710 Ma) and the Marinoan (635 Ma) glaciations, the mechanisms of which are still debated. Previous data shows that during the interglacial interval, sulfate concentrations were very low and abnormally enriched δ34Spyrite values inversely correlate with shifts in δ13Ccarbonate. However, very little carbonate-associated sulfate (CAS) could be extracted from these sections. To better understand the relationship between the global carbon and sulfur cycles during this tumultuous time, high-resolution coupled records of δ13Ccarbonate and δ34Ssulfate are needed. Here we present paired δ34Ssulfate and δ34Spyrite data from carbonates of the Coates Lake Group (Mackenzie Mountains, Canada), which represent deposition preceding the Sturtian glaciation, to further explore the connection between the Neoproterozoic sulfur and carbon cycles, and to gain insight into the possible mechanisms driving major perturbations in the carbon isotope record. Leading into the glaciation, δ13Ccarbonate increases from -9.5‰ to 8‰ over approximately 200 meters of section, then abruptly drops to ~2‰ over 30 meters. Both sulfate and pyrite isotopes track these changes, indicating that as organic carbon burial is steadily increasing and driving δ13Ccarbonate more positive, pyrite burial is increasing as well. Δ34S decreases as δ13Ccarbonate and δ34S increase, providing further evidence that more organic carbon and pyrite are being buried, leading to a decrease in oceanic sulfate concentrations. Δ34S then increases as δ13Ccarbonate decreases, indicating a positive flux to the sulfate reservoir prior to the Sturtian glaciation. This relationship between carbon and sulfur differs significantly from what is seen during the interglacial interval, and represents

  13. Molecular Fossils for Understanding Biodiversity During the Neoproterozoic-Cambrian Transition in China

    NASA Astrophysics Data System (ADS)

    Pi, Y.; Tuo, J.; McFadden, K.; Xiao, S.; Zhang, C. L.

    2005-12-01

    Neoproterozoic-Cambrian rocks in South China contain an extraordinary fossil record, including exceptionally well preserved animal embryos, acritarchs, and multi-cellular algae. The goal of this study was to evaluate the microbial diversity associated with these remarkably preserved fossil assemblages at the Neoproterozoic-Cambrian transition. Rock samples of 520-632 Ma in age were collected in the Yangtze Gorges area and southern Anhui Province, China. Samples were powdered and extracted for organic biomarkers. The content of bitumen A accounted for 4-16% of the rock material and most of it (49-79%) was asphaltenes. Saturated and aromatic hydrocarbons accounted for 2-6% and 1-3%, respectively. Analysis using GC-MS indicated the predominance of n-alkanes and less abundant isoprenoid alkanes in the saturated fractions. The n-alkanes were characterized by homologues dominated by C15-C17, which is consistent with the result of high thermal-evolution (Brocks et al., 2003. GCA 67:4321-4335). Hopanoids were present in less abundance and ranged from C29 to C32. A smaller amount of heavy-molecular-weight n-alkanes (C23-C39) was also detected, which indicated a source of high plants and must be contamination from younger organic matter. Still, patterns of variation can be detected among these samples. For example, the ratio of pristane to phytane was all greater than 1.0 except for one sample (JLW9.3) from Yangtze Gorges area. The results indicate that sample JLW9.3 might have been deposited in a reducing environment whereas the other samples might have been formed in relatively oxidative environments. The overall results, however, suggest that rock samples from the Neoproterozoic-Cambrian transition in China have gone through significant metamorphism; thus, understanding of microbial communities using molecular biomarkers in such altered rocks needs to be cautiously executed.

  14. Pressure-temperature evolution of Neoproterozoic metamorphism in the Welayati Formation (Kabul Block), Afghanistan

    NASA Astrophysics Data System (ADS)

    Collett, Stephen; Faryad, Shah Wali

    2015-11-01

    The Welayati Formation, consisting of alternating layers of mica-schist and quartzite with lenses of amphibolite, unconformably overlies the Neoarchean Sherdarwaza Formation of the Kabul Block that underwent Paleoproterozoic granulite-facies and Neoproterozoic amphibolite-facies metamorphic events. To analyze metamorphic history of the Welayati Formation and its relations to the underlying Sherdarwaza Formation, petrographic study and pressure-temperature (P-T) pseudosection modeling were applied to staurolite- and kyanite-bearing mica-schists, which crop out to the south of Kabul City. Prograde metamorphism, identified by inclusion trails and chemical zonation in garnet from the micaschists indicates that the rocks underwent burial from around 6.2 kbar at 525 °C to maximum pressure conditions of around 9.5 kbar at temperatures of around 650 °C. Decompression from peak pressures under isothermal or moderate heating conditions are indicated by formation of biotite and plagioclase porphyroblasts which cross-cut and overgrow the dominant foliation. The lack of sillimanite and/or andalusite suggests that cooling and further decompression occurred in the kyanite stability field. The results of this study indicate a single amphibolite-facies metamorphism that based on P-T conditions and age dating correlates well with the Neoproterozoic metamorphism in the underlying Sherdarwaza Formation. The rocks lack any paragenetic evidence for a preceding granulite-facies overprint or subsequent Paleozoic metamorphism. Owing to the position of the Kabul Block, within the India-Eurasia collision zone, partial replacement of the amphibolite-facies minerals in the micaschist could, in addition to retrogression of the Neoproterozoic metamorphism, relate to deformation associated with the Alpine orogeny.

  15. Co-evolution of Eukaryotes and Ocean and Atmosphere Oxygenation in the Neoproterozoic and Paleozoic Eras

    NASA Astrophysics Data System (ADS)

    Lenton, T. M.; Daines, S. J.; Mills, B.; Boyle, R. A.

    2014-12-01

    The nature, timing and cause(s) of the Earth's second oxygenation event are widely debated. It has been argued that there was a single pronounced rise in atmospheric oxygen toward present levels in the Late Neoproterozoic, which in turn triggered the evolution of animals. Here we suggest a more complex co-evolutionary scenario, with fluctuations in ocean and atmosphere oxygenation in the Late Neoproterozoic and Early Paleozoic caused partly by the evolution of animals, followed by a pronounced rise of atmospheric oxygen to present levels later in the Paleozoic caused by the rise of land plants. Current geochemical evidence suggests some parts of the deep oceans became oxygenated during the Ediacaran, but there was subsequent de-oxygenation of the ocean during the Cambrian that may have persisted into the Ordovician. Only later in the Paleozoic is there evidence for widespread oxygenation of the deep ocean, together with charcoal indicating atmospheric oxygen had approached present levels. The limited Neoproterozoic oxygenation of the ocean could be explained by the evolution of filter-feeding sponges removing oxygen demand from the water column and encouraging a shift from cyanobacteria to faster-sinking eukaryotic algae, which transferred oxygen demand to greater depths and into sediments. The resulting oxygenation of shelf bottom waters would have increased phosphorus removal from the ocean thus lowering global productivity and oxygen demand in a positive feedback loop encouraging ocean oxygenation [1]. The subsequent Cambrian de-oxygenation of the ocean could be explained by the evolution of bioturbating animals oxygenating the sediments and thus lowering the C/P burial ratio of organic matter, reducing organic carbon burial and lowering atmospheric oxygen [2]. The later rise of land plants, selectively weathering phosphorus from continental rocks and producing recalcitrant high C/P biomass, increased organic carbon burial and atmospheric oxygen, finally

  16. Archaean and Palaeoproterozoic gneisses reworked during a Neoproterozoic (Pan-African) high-grade event in the Mozambique belt of East Africa: Structural relationships and zircon ages from the Kidatu area, central Tanzania

    NASA Astrophysics Data System (ADS)

    Vogt, M.; Kröner, A.; Poller, U.; Sommer, H.; Muhongo, S.; Wingate, M. T. D.

    2006-06-01

    This study presents new zircon ages and Sm-Nd whole-rock isotopic compositions for high-grade gneisses from the Udzungwa Mountain area in the central part of the Mozambique belt, Tanzania. The study area comprises a succession of layered granulite-facies para- and orthogneisses, mostly retrograded to amphibolite-facies. The original intrusive contacts became obscured or severely modified during non-coaxial ductile deformation, and extensive shearing occurred during retrogression. Structures reflecting the early deformational history were mostly obscured when the rocks were transported into the lower crust as documented by severe flattening. Only the fragmented gneisses in the eastern part of the area testify to a brittle regime. Structures in narrow low strain zones that predate the currently observed layering are preserved in rootless isoclinal folds and boudins. Magmatic and detrital zircons from tonalitic to felsic orthogneisses and a metapelite sample were dated using the U-Pb and Pb-Pb evaporation methods and SHRIMP II. Cathodoluminiscence images reveal ubiquitous xenocrystic cores, rimmed by clear, unzoned overgrowth due to high-grade metamorphism. Discordant U-Pb data therefore reflect core-rim relationships, and it was not always possible to obtain precise crystallisation ages. The analyses reveal Neoarchaean, Palaeoproterozoic and Neoproterozoic protolith ages. Nd isotopic systematics yielded strongly negative ɛNd( t) -values and Neoarchaean to Palaeoproterozoic model ages, even for gneisses emplaced in the Neoproterozoic. The trace element distribution suggests upper crustal derivation of the gneisses. Therefore, our study provides evidence that recycling of older crust played a major role during the evolution of the Kidatu area. Neoarchaean rocks are interpreted to represent fragments of the Tanzania craton. Our results, together with those of earlier workers, lead to the conclusion that the central part of the Mozambique belt mainly consists of ancient

  17. Geology and metallogeny of the Ar Rayn terrane, eastern Arabian shield: Evolution of a Neoproterozoic continental-margin arc during assembly of Gondwana within the East African orogen

    USGS Publications Warehouse

    Doebrich, J.L.; Al-Jehani, A. M.; Siddiqui, A.A.; Hayes, T.S.; Wooden, J.L.; Johnson, P.R.

    2007-01-01

    The Neoproterozoic Ar Rayn terrane is exposed along the eastern margin of the Arabian shield. The terrane is bounded on the west by the Ad Dawadimi terrane across the Al Amar fault zone (AAF), and is nonconformably overlain on the east by Phanerozoic sedimentary rocks. The terrane is composed of a magmatic arc complex and syn- to post-orogenic intrusions. The layered rocks of the arc, the Al Amar group (>689 Ma to ???625 Ma), consist of tholeiitic to calc-alkaline basaltic to rhyolitic volcanic and volcaniclastic rocks with subordinate tuffaceous sedimentary rocks and carbonates, and are divided into an eastern and western sequence. Plutonic rocks of the terrane form three distinct lithogeochemical groups: (1) low-Al trondhjemite-tonalite-granodiorite (TTG) of arc affinity (632-616 Ma) in the western part of the terrane, (2) high-Al TTG/adakite of arc affinity (689-617 Ma) in the central and eastern part of the terrane, and (3) syn- to post-orogenic alkali granite (607-583 Ma). West-dipping subduction along a trench east of the terrane is inferred from high-Al TTG/adakite emplaced east of low-Al TTG. The Ar Rayn terrane contains significant resources in epithermal Au-Ag-Zn-Cu-barite, enigmatic stratiform volcanic-hosted Khnaiguiyah-type Zn-Cu-Fe-Mn, and orogenic Au vein deposits, and the potential for significant resources in Fe-oxide Cu-Au (IOCG), and porphyry Cu deposits. Khnaiguiyah-type deposits formed before or during early deformation of the Al Amar group eastern sequence. Epithermal and porphyry deposits formed proximal to volcanic centers in Al Amar group western sequence. IOCG deposits are largely structurally controlled and hosted by group-1 intrusions and Al Amar group volcanic rocks in the western part of the terrane. Orogenic gold veins are largely associated with north-striking faults, particularly in and near the AAF, and are presumably related to amalgamation of the Ar Rayn and Ad Dawadimi terranes. Geologic, structural, and metallogenic

  18. Late neoproterozoic igneous complexes of the western Baikal-Muya Belt: Formation stages

    NASA Astrophysics Data System (ADS)

    Fedotova, A. A.; Razumovskiy, A. A.; Khain, E. V.; Anosova, M. O.; Orlova, A. V.

    2014-07-01

    The paper presents new geological, geochemical, and isotopic data on igneous rocks from a thoroughly studied area in the western Baikal-Muya Belt, which is a representative segment of the Neoproterozoic framework of the Siberian Craton. Three rock associations are distinguished in the studied area: granulite-enderbite-charnockite and ultramafic-mafic complexes followed by the latest tonalite-plagiogranitegranite series corresponding to adakite in geochemical characteristics. Tonalites and granites intrude the metamorphic and gabbroic rocks of the Tonky Mys Point, as well as Slyudyanka and Kurlinka intrusions. The tonalites yielded a U-Pb zircon age of 595 ± 5 Ma. The geochronological and geological information indicate that no later than a few tens of Ma after granulite formation they were transferred to the upper lithosphere level. The Sm-Nd isotopic data show that juvenile material occurs in rocks of granitoid series (ɛNd(t) = 3.2-7.1). Ophiolites, island-arc series, eclogites, and molasse sequences have been reviewed as indicators of Neoproterozoic geodynamic settings that existed in the Baikal-Muya Belt. The implications of spatially associated granulites and ultramafic-mafic intrusions, as well as granitoids with adakitic geochemical characteristics for paleogeodynamic reconstructions of the western Baikal-Muya Belt, are discussed together with other structural elements of the Central Asian Belt adjoining the Siberian Platform in the south.

  19. A global transition to ferruginous conditions in the early Neoproterozoic oceans

    NASA Astrophysics Data System (ADS)

    Guilbaud, Romain; Poulton, Simon W.; Butterfield, Nicholas J.; Zhu, Maoyan; Shields-Zhou, Graham A.

    2015-06-01

    Eukaryotic life expanded during the Proterozoic eon, 2.5 to 0.542 billion years ago, against a background of fluctuating ocean chemistry. After about 1.8 billion years ago, the global ocean is thought to have been characterized by oxygenated surface waters, with anoxic and sulphidic waters in middle depths along productive continental margins and anoxic and iron-containing (ferruginous) deeper waters. The spatial extent of sulphidic waters probably varied through time, but this surface-to-deep redox structure is suggested to have persisted until the first Neoproterozoic glaciation about 717 million years ago. Here we report an analysis of ocean redox conditions throughout the Proterozoic using new and existing iron speciation and sulphur isotope data from multiple cores and outcrops. We find a global transition from sulphidic to ferruginous mid-depth waters in the earliest Neoproterozoic, coincident with the amalgamation of the supercontinent Rodinia at low latitudes. We suggest that ferruginous conditions were initiated by an increase in the oceanic influx of highly reactive iron relative to sulphate, driven by a change in weathering regime and the uptake of sulphate by extensive continental evaporites on Rodinia. We propose that this transition essentially detoxified ocean margin settings, allowing for expanded opportunities for eukaryote diversification following a prolonged evolutionary stasis before one billion years ago.

  20. The Neoproterozoic-Cambrian Paraguay Belt, central Brazil: Part I - New structural data and a new approach on the regional implications

    NASA Astrophysics Data System (ADS)

    Silva, Luiz José Homem D'el-Rey; Walde, Detlef Hans-Gerd; Saldanha, Davi Oliveira

    2016-04-01

    Together with the Araguaia and Brasília belts, the Paraguay belt forms in central Brazil, the Tocantins Province that is one of the largest orogens of western Gondwana. The Corumbá area occupies the site where the northern and southern parts of the Paraguay belt form, together with the Chiquitos-Tucavaca aulacogen (stretching E-W in the adjacent Bolivian territory) an R-R-R basin system opened-filled in the ~ 700/650-540 Ma interval within the Amazon-Rio Apa paleo-continent. The sedimentary (volcanic) rocks of the Jacadigo and Corumbá Groups found around the Corumbá city record part of the Neoproterozoic-Cambrian passive margin precursor of the Paraguay belt. Our pioneer structural analysis reveals that these rocks experienced progressive deformation (phases D1-D2-D3) and low-grade metamorphism during the Brasiliano Cycle (540-513 Ma). The crystalline basement was also involved, according to structural data and K-Ar ages in the literature. The paleo-passive margin was thickened during the D1-D2 deformation and was lately shortened (D3) in two orthogonal directions, SE-NW (D3P) and SW-NE (D3T). Developed co-axially and verging to NW, D1-D2-D3P structures record the closure of the basin precursor of the Paraguay belt, whereas D3T structures seem related to the inversion of the aulacogen. Although the tectonic transport to NW, as observed in the Corumbá area, matches the reported transport of Paraguay belt's supracrustal rocks towards the eastern margin of the Rio Apa block and Araguaia belt's rocks towards the Amazon craton, the transport direction is opposite in other parts of the Paraguay belt. Our comprehensive discussion of these facts brings to light profound regional implications.

  1. Mineralogy and geochemistry of Neoproterozoic siliceous manganese formations from Ntui-Betamba (Cameroon Pan-African Fold Belt): implications for mineral exploration

    NASA Astrophysics Data System (ADS)

    Timoleon, Ngnotue; Sylvestre, Ganno; Kouankap Nono, Gus Djibril; Paul, Nzenti Jean

    2015-07-01

    The siliceous manganese formations from Ntui-Betamba area have been studied with the aim to constrain their mineralogy, geochemistry, and genesis. Neoproterozoic manganese-bearing rocks at Ntui-Betamba include garnet-galaxite-bearing quartzite and galaxite-bearing quartzite that—besides a characteristic quartz, spessartine-garnet, and galaxite (MnAl2O4) assemblage—comprises also graphite and rutile. Galaxite crystals show chemical zoning from core to rim due to the change in composition from galaxite to cryptomelane. Bulk rock geochemistry reveals that the studied rocks are characterized by high SiO2 content (52.83 wt%). They are enriched in Fe2O3 and Al2O3 and display Mn/Fe ratio of 1.2. MnO concentrations are high in galaxite-bearing quartzite (16 wt%), and moderate (8.03 wt%) in garnet-galaxite-bearing quartzite. The transition trace metal (Co, Cu, and Ni) contents are lower, with Co and Cu markedly depleted relative to Ni, and variable contents of Zn. The REE pattern of garnet-galaxite-bearing quartzite shows Ce-positive anomaly similar to the hydrogenous sediment, but differs from the later with strong Eu-negative anomaly and the lack of weak positive anomaly in Yb, whereas the galaxite-bearing quartzite is characterized by the lack of Ce anomaly and the weak Eu-negative anomaly. Both rocks show HREE content close to that of hydrothermal sediment and exhibit flat HREE trend. The high contents of silica, iron, and aluminum, together with the Co/Zn ratios (~0.3) and the contrasting behavior of REE, suggest that the studied manganese-bearing rocks are of sedimentary origin probably derived from a mixed hydrothermal-hydrogenous source. The high content of Mn-bearing minerals in these rocks represents indicators of concealed or buried mineralization and could be used as exploration guide.

  2. The Mafic Lower Crust of Neoproterozoic age beneath Western Arabia: Implications for Understanding African Lower Crust

    NASA Astrophysics Data System (ADS)

    Stern, R. J.; Mooney, W. D.

    2011-12-01

    We review evidence that the lower crust of Arabia - and by implication, that beneath much of Africa was formed at the same time as the upper crust, rather than being a product of Cenozoic magmatic underplating. Arabia is a recent orphan of Africa, separated by opening of the Red Sea ~20 Ma, so our understanding of its lower crust provides insights into that of Africa. Arabian Shield (exposed in W. Arabia) is mostly Neoproterozoic (880-540 Ma) reflecting a 300-million year process of continental crustal growth due to amalgamated juvenile magmatic arcs welded together by granitoid intrusions that make up as much as 50% of the Shield's surface. Seismic refraction studies of SW Arabia (Mooney et al., 1985) reveal two layers, each ~20 km thick, separated by a well-defined Conrad discontinuity. The upper crust has average Vp ~6.3 km/sec whereas the lower crust has average Vp ~7.0 km/sec, corresponding to a granitic upper crust and gabbroic lower crust. Neogene (<30 ma) lava fields in Arabia (harrats) extend over 2500 km, from Yemen to Syria. Many of these lavas contain xenoliths, providing a remarkable glimpse of the lower-crustal and upper-mantle lithosphere beneath W. Arabia. Lower crustal xenoliths brought up in 8 harrats in Saudi Arabia, Jordan, and Syria are mostly 2-pyroxene granulites of igneous (gabbroic, anorthositic, and dioritic) origin. They contain plagioclase, orthopyroxene, and clinopyroxene, and a few contain garnet and rare amphibole and yield mineral-equilibrium temperatures of 700-900°C. Pyroxene-rich and plagioclase-rich suites have mean Al2O3 contents of 13% and 19%, respectively: otherwise the two groups have similar elemental compositions, with ~50% SiO2 and ~1% TiO2, with low K2O (<0.5%) and Na2O (1-3%). Both groups show tholeiitic affinities, unrelated to their alkali basalt hosts. Mean pyroxene-rich and plagioclase-rich suites show distinct mean MgO contents (11% vs. 7%), Mg# (67 vs. 55), and contents of compatible elements Ni (169 vs. 66 ppm

  3. Sedimentary basins in Ross Sea, Antarctica

    SciTech Connect

    Cooper, A.K.; Davey, F.J.

    1986-07-01

    The Ross Sea lies in the Pacific sector of the Antarctic continental margin. Three major sedimentary basins (from east to west, the Eastern, Central, and Victoria Land basins) lie beneath the broad, deep continental shelf of the Ross Sea. These north-south-trending basins occur in the extensionally deformed region between East and West Antarctica. Multichannel seismic reflection (MCS) surveys have been conducted over these basins since 1980 by West German, French, Japanese, and US expeditions. The MCS and previous geophysical surveys have shown that the three basins contain 5-6 km of sedimentary rock, possibly Late Cretaceous and younger. An additional 6-8 km of sedimentary and volcanic rock lies within the deeper parts of the Victoria Land basin. The basins are separated by uplifted and eroded basement ridges covered by thin sedimentary sections. Each basin has distinct characteristics, commonly related to its extensional origin. Petroleum hydrocarbons are unknown from the Ross Sea region, with the possible exception of ethane gas recovered by the Deep Sea Drilling Project. Previous model studies, based on estimated sediment thickness, assumed temperature gradients, and postulated seismostratigraphy, indicate that hydrocarbons could be generated at depths of 3.5-6km within the sedimentary section. However, this hypothesis cannot be verified without further geologic and geophysical data from the Ross Sea region.

  4. Two diamictites, two cap carbonates, two δ13C excursions, two rifts: The Neoproterozoic Kingston Peak Formation, Death Valley, California

    NASA Astrophysics Data System (ADS)

    Prave, A. R.

    1999-04-01

    Stratigraphic mapping of the Neoproterozoic glaciogenic Kingston Peak Formation (Death Valley, California) provides evidence for two temporally discrete extensional deformation episodes. These episodes are bracketed by the Sourdough Limestone and Noonday Dolomite, the facies characteristics and δ13C data (ranging between 2.15 and -2.56‰ and -1.88 and -4.86‰, respectively) of which make them equivalent to Sturtian and Varangian age cap carbonates, respectively. This constrains the two extensional episodes along the southwestern margin of Laurentia to ca. 700 Ma and ca. 600 Ma. These observations and data show that the field evidence for mid-Neoproterozoic breakup and the predictions from tectonic subsidence curves for a latest Neoproterozoic breakup are both correct. Thus, Neoproterozoic plate reconstructions must account for two discrete rift episodes separated by 100 m.y. or more. Confining rifting to within the Kingston Peak Formation thereby places the younger Proterozoic rocks of the southwestern Great Basin in the rift to drift tectonic phase.

  5. GCM Simulations of Neoproterozoic "Snowball Earth" Conditions: Implications for the Environmental Limits on Terrestrial Metazoans and Their Extraterrestrial Analogues

    NASA Technical Reports Server (NTRS)

    Sohl, L. E.; Chandler, M. A.

    2001-01-01

    The Neoproterozoic Snowball Earth intervals provide excellent opportunities to examine the environmental limits on terrestrial metazoans. A series of GCM simulations was run in order to quantify climatic conditions during these intervals. Additional information is contained in the original extended abstract.

  6. Sedimentary structures and stratal geometries at the foothills of Mount Sharp: their role in paleoenvironmental interpretation

    NASA Astrophysics Data System (ADS)

    Gupta, S.; Rubin, D. M.; Sumner, D. Y.; Grotzinger, J. P.; Lewis, K. W.; Stack, K.; Kah, L. C.; Banham, S.; Edgett, K. S.

    2015-12-01

    The Mars Science Laboratory Curiosity rover has been exploring sedimentary rocks at the foothills of Mount Sharp since August 2014. Robust interpretation of the paleoenvironmental contexts requires detailed facies analysis of these rocks including analysis and interpretation of sedimentary structures and sediment body geometries. Here, we describe some of the detailed sedimentary structures and sedimentary geometries observed by Curiosity between the Pahrump_Hills field site and its current location at Marias Pass. The Pahrump Hills sedimentary section comprises a succession dominated by finely laminated mudstones of the Murray formation that are interpreted to have been deposited in an ancient lake within Gale crater. Toward the top of the Pahump Hills succession, we observe the appearance of coarser-grained sandstones that are interstratified within the lacustrine mudstones. These sandstones that include Whale Rock and Newspaper Rock show lenticular geometries, and are pervasively cross-stratified. These features indicate that currents eroded shallow scours in the lake beds that were then infilled by deposition from migrating subaqueous dunes. The paleoenvironmental setting may represent either a gullied delta front setting or one in which lake level fall caused fluvial erosion and infilling of the shallow scours. Since leaving Pahrump_Hills, Curiosity has imaged extensive exposures of strata that are partly correlative with and stratigraphically overlie the uppermost part of the Pahrump section. Isolated cross-bedded sandstones and possible interstratified conglomerates beds occur within Murray formation mudstones. Capping sandstones with a likely variety of environmental contexts overlie mudstones. Where imaged in detail, sedimentary structures, such as trough-cross bedding and possible eolian pinstriping, provide constraints on plausible sedimentary processes and bounds on depositional setting.

  7. The Precambrian of Transangaria, Yenisei Ridge (Siberia): Neoproterozoic microcontinent, Grenville-age orogen, or reworked margin of the Siberian craton?

    NASA Astrophysics Data System (ADS)

    Kuzmichev, Alexander B.; Sklyarov, Eugene V.

    2016-01-01

    The Yenisei Ridge was traditionally perceived as an uplifted segment of the western Siberian craton affected by Neoproterozoic collision events. However, the suggestions for Archaean or Palaeoproterozoic ('Siberian') basement in Transangaria have not been confirmed by reliable geochronological data. A new view regards most of the Ridge, namely, its Transangarian segment, to be an exotic Neoproterozoic terrane that collided with Siberia in the late Neoproterozoic. This paper presents new U-Pb SHRIMP zircon ages demonstrating that Archaean rocks (2611 ± 12 Ma) actually exist in this territory. We also provide a review of published U-Pb zircon ages for igneous and metamorphic rocks of Transangaria together with our new age data. This geochronological dataset clarifies the geology of the Yenisei Ridge and leads to new conclusions, as follows. (1) It is likely that Transangaria was originally underlain by an Archaean-Palaeoproterozoic basement, similar to that of the Siberian craton. (2) Geochronological data do not confirm the idea of widespread "Greenvillian age" granitoides in Transangaria. (3) The Neoproterozoic evolution of the Yenisei Ridge segment of the Siberian craton margin includes the following events. (i) Collision of an unidentified terrane with the western margin (in recent coordinates) of the Siberian craton during 900-855 Ma. The colliding terrane is no longer present in the current structure. (ii) Dextral shearing during 830-800 Ma may have been caused by counter-clockwise rotation of the Siberian craton. (iii) Extensional conditions prevailed during 800-700 Ma. The Isakovka oceanic basin formed at this time interval. (iv) Thrusting of the Isakovka island arc and accretionary prism onto the Siberian margin occurred during the late Neoproterozoic (650-630 Ma) and caused high-pressure metamorphism.

  8. The evolution of the neoproterozoic São Gabriel juvenile terrane, southern Brazil based on SHRIMP and LA-ICP-MS U-Pb ages and ?18O data on detrital zircon

    NASA Astrophysics Data System (ADS)

    Lena, L. O.; Pimentel, M. M.; Philipp, R. P.; Armstrong, R. A.; Sato, K.

    2013-12-01

    The São Gabriel terrane is a segment of juvenile crust exposed in the western part of the Dom Feliciano Belt in the southern Mantiqueira Province, southern Brazil. In this study, SHRIMP and LA-ICPMS U-Pb geochronological data for 171 detrital zircons of the Cambaizinho Complex are used to investigate the tectonic evolution of this juvenile terrain. Eighty-one grains were later selected for SIMS δ18O analyses. Ages ranged from 840 to 660 Ma, with a strong concentration between ca. 750 and 700 Ma. The age spectrum of the detrital zircon grains from this meta-sedimentary succession suggests that the original sediments were derived from the erosion of the arc, most likely in a short-lived syn-orogentic basin. Th/U ratios and internal structures of the zircon grains reveal that they were mostly eroded from the arc magmatic rocks, without any relevant contribution from their metamorphic counterparts, or from any other older source. The δ18O values varied from 3.2 to 9.6‰, indicating the coeval crystallization of both unaltered, pristine mantle magmas alongside altered mantle magmas, and strongly contaminated continental crustal magmas generated in both continental and oceanic arc setting. Three periods in the progressive evolution of the terrane were recognized: Period I is represented by the installation of an island-arc subduction zone. The data set suggest that this period started at ca. 840 Ma and went on until 750 Ma. In this period δ18O values varied between 3.2 to 5.5 ‰, suggesting the crystallization of normal mantle-derived magmas, and juvenile magmas with assimilation of hydrothermally altered crust. Period II took place between ca. 750-690 Ma, which represents the peak of magmatic activity suggested by the large abundance of detrital zircons in the probability density diagrams. Here, δ18O values ranged from 4.0 to 9.4 ‰. The wide range of δ18O values reveals the coexistence of magmas with continental, mantle, and altered mantle isotopic signatures

  9. Geology, exploration status of Uruguay's sedimentary basins

    SciTech Connect

    Goso, C.; Santa Ana, H. de )

    1994-02-07

    This article attempts to present the geological characteristics and tectonic and sedimentary evolution of Uruguayan basins and the extent to which they have been explored. Uruguay is on the Atlantic coast of South America. The country covers about 318,000 sq km, including offshore and onshore territories corresponding to more than 65% of the various sedimentary basins. Four basins underlie the country: the Norte basin, the Santa Lucia basin, the offshore Punta del Este basin, and the offshore-onshore Pelotas-Merin basin. The Norte basin is a Paleozoic basin while the others are Mesozoic basins. Each basin has been explored to a different extent, as this paper explains.

  10. Stable isotopes of oxygen and carbon compositions in the Neoproterozoic of South Gabon (Schisto-Calcaire Subgroup, Nyanga Basin): Are cap carbonates and lithoherms recording a particular destabilization event after the Marinoan glaciation?

    NASA Astrophysics Data System (ADS)

    Préat, Alain; Prian, Jean-Pierre; Thiéblemont, Denis; Obame, Rolf Mabicka; Delpomdor, Franck

    2011-06-01

    Geologic evidence of tropical sea level glaciation in the Neoproterozoic remains a matter of debate in the Snowball Earth hypothesis. The Niari Tillite Formation and the cap carbonates record the late Neoproterozoic Marinoan glaciation in South Gabon. These cap carbonates are located at the base of the Schisto-Calcaire Subgroup a predominantly carbonate succession that rests with sharp contact on top of the Niari Tillite. Integrating sedimentological and stable isotope data, a consistent sequence of precipitation events is proposed, with strongly negative δ 13C values pointing to a particular event in the cap carbonates (average δ 13C value = -3.2‰ V-PDB) and in a further newly defined lithohermal unit (average δ 13C value = -4.6‰ V-PDB). Subsequent shallow evaporitive platform carbonates display carbon and oxygen isotopic compositions indicative of relatively unaltered seawater values. Strongly negative δ 18O values in the lithoherms and replacement of aragonite fans by equigranular calcite suggest flushing of meteoric water derived from glacial meltwater.

  11. Stratigraphic modeling of sedimentary basins

    SciTech Connect

    Aigner, T. ); Lawrence, D.T. )

    1990-11-01

    A two-dimensional stratigraphic forward model has been successfully applied and calibrated in clastic, carbonate, and mixed clastic/carbonate regimes. Primary input parameters are subsidence, sea level, volume of clastics, and carbonate growth potential. Program output includes sequence geometries, facies distribution lithology distribution, chronostratigraphic plots, burial history plots, thermal and maturity histories, and crossplots. The program may be used to predict reservoir distribution, to constrain interpretations of well and seismic data, to rapidly test exploration scenarios in frontier basins, and to evaluate the fundamental controls on observed basin stratigraphy. Applications to data sets from Main Pass (US Gulf Coast), Offshore Sarawak (Malaysia), Rub'al Khali basin (Oman), Paris basin (France), and Baltimore Canyon (US East Coast) demonstrate that the program can be used to simulate stratigraphy on a basin-wide scale as well as on the scale of individual prospects.

  12. Multiple rifting and alkaline magmatism in southern India during Paleoproterozoic and Neoproterozoic

    NASA Astrophysics Data System (ADS)

    Renjith, M. L.; Santosh, M.; Satyanarayanan, M.; Rao, D. V. Subba; Tang, Li

    2016-06-01

    The Southern Granulite Terrane (SGT) in India preserves the history of tectonothermal events ranging from Paleoarchean to latest Neoproterozoic-Cambrian. Here we investigate alkaline magmatism possibly associated with rifting events in Paleoproterozoic and Neoproterozoic based on petrological, geochemical and zircon U-Pb and Lu-Hf isotopic studies on the alkaline complexes of Korangani (KGAC) and Kambamettu (KAC) in the Madurai Block of SGT. The mica pyroxenite which represents the first intrusive phase at KGAC crystallized from a mildly alkaline hydrous magma derived from a metasomatized mantle. The younger shoshonitic syenite was emplaced at 2533 ± 16 Ma, carries mafic microgranular enclaves, and shows trace-elements ratios consistent with magma mixing trend, and zircon εHf(t) values display mixed positive and negative values - 2.6 to 3.6 suggesting the mixing of adakite-like felsic crustal melt and non-adakitic mantle derived melt. In KAC, four distinct magmatic intrusions are identified: i) quartz-monzonite (emplaced at 2498 ± 16 Ma), an ultrapotassic adakitic rock derived from a carbonated alkali-rich lower crustal source with negative zircons εHf(t) values in zircon (- 8.0 to - 0.8); Y/Nb (> 1.2) and Th/Ce (0.03-0.8) ratios; lower Ni (< 30 ppm) and Cr (< 14 ppm) contents; ii) phlogopite-rich pyroxenite, crystallized from an alkali-rich basaltic parental magma derived from carbonate metasomatized mantle; iii) mantle derived high Ba-Sr carbonatite (emplaced at 2470 ± 15 Ma); and iv) shoshonitic peralkaline syenite rock (emplaced at 608 ± 6 Ma) with strong adakitic signature, low MgO (< 1 wt.%), Ni (12-5 ppm) and Cr (49-35 ppm) contents and negative zircon εHf(t) values (- 30.3 to - 27.3) and trough of Zr-Hf in spidergrams suggesting a carbonated alkali-rich garnet-bearing crustal source. The geochemical features and petrogenetic considerations of the felsic shoshonitic-ultrapotassic adakite-like rocks (syenite, quartz monzonite), mica-pyroxenites and

  13. Orogen styles in the East African Orogen: A review of the Neoproterozoic to Cambrian tectonic evolution

    NASA Astrophysics Data System (ADS)

    Fritz, H.; Abdelsalam, M.; Ali, K. A.; Bingen, B.; Collins, A. S.; Fowler, A. R.; Ghebreab, W.; Hauzenberger, C. A.; Johnson, P. R.; Kusky, T. M.; Macey, P.; Muhongo, S.; Stern, R. J.; Viola, G.

    2013-10-01

    The East African Orogen, extending from southern Israel, Sinai and Jordan in the north to Mozambique and Madagascar in the south, is the world´s largest Neoproterozoic to Cambrian orogenic complex. It comprises a collage of individual oceanic domains and continental fragments between the Archean Sahara-Congo-Kalahari Cratons in the west and Neoproterozoic India in the east. Orogen consolidation was achieved during distinct phases of orogeny between ∼850 and 550 Ma. The northern part of the orogen, the Arabian-Nubian Shield, is predominantly juvenile Neoproterozoic crust that formed in and adjacent to the Mozambique Ocean. The ocean closed during a protracted period of island-arc and microcontinent accretion between ∼850 and 620 Ma. To the south of the Arabian Nubian Shield, the Eastern Granulite-Cabo Delgado Nappe Complex of southern Kenya, Tanzania and Mozambique was an extended crust that formed adjacent to theMozambique Ocean and experienced a ∼650-620 Ma granulite-facies metamorphism. Completion of the nappe assembly around 620 Ma is defined as the East African Orogeny and was related to closure of the Mozambique Ocean. Oceans persisted after 620 Ma between East Antarctica, India, southern parts of the Congo-Tanzania-Bangweulu Cratons and the Zimbabwe-Kalahari Craton. They closed during the ∼600-500 Ma Kuungan or Malagasy Orogeny, a tectonothermal event that affected large portions of southern Tanzania, Zambia, Malawi, Mozambique, Madagascar and Antarctica. The East African and Kuungan Orogenies were followed by phases of post-orogenic extension. Early ∼600-550 Ma extension is recorded in the Arabian-Nubian Shield and the Eastern Granulite-Cabo Delgado Nappe Complex. Later ∼550-480 Ma extension affected Mozambique and southern Madagascar. Both extension phases, although diachronous,are interpreted as the result of lithospheric delamination. Along the strike of the East African Orogen, different geodynamic settings resulted in the evolution of

  14. Orogen styles in the East African Orogen: A review of the Neoproterozoic to Cambrian tectonic evolution☆

    PubMed Central

    Fritz, H.; Abdelsalam, M.; Ali, K.A.; Bingen, B.; Collins, A.S.; Fowler, A.R.; Ghebreab, W.; Hauzenberger, C.A.; Johnson, P.R.; Kusky, T.M.; Macey, P.; Muhongo, S.; Stern, R.J.; Viola, G.

    2013-01-01

    The East African Orogen, extending from southern Israel, Sinai and Jordan in the north to Mozambique and Madagascar in the south, is the world́s largest Neoproterozoic to Cambrian orogenic complex. It comprises a collage of individual oceanic domains and continental fragments between the Archean Sahara–Congo–Kalahari Cratons in the west and Neoproterozoic India in the east. Orogen consolidation was achieved during distinct phases of orogeny between ∼850 and 550 Ma. The northern part of the orogen, the Arabian–Nubian Shield, is predominantly juvenile Neoproterozoic crust that formed in and adjacent to the Mozambique Ocean. The ocean closed during a protracted period of island-arc and microcontinent accretion between ∼850 and 620 Ma. To the south of the Arabian Nubian Shield, the Eastern Granulite–Cabo Delgado Nappe Complex of southern Kenya, Tanzania and Mozambique was an extended crust that formed adjacent to theMozambique Ocean and experienced a ∼650–620 Ma granulite-facies metamorphism. Completion of the nappe assembly around 620 Ma is defined as the East African Orogeny and was related to closure of the Mozambique Ocean. Oceans persisted after 620 Ma between East Antarctica, India, southern parts of the Congo–Tanzania–Bangweulu Cratons and the Zimbabwe–Kalahari Craton. They closed during the ∼600–500 Ma Kuungan or Malagasy Orogeny, a tectonothermal event that affected large portions of southern Tanzania, Zambia, Malawi, Mozambique, Madagascar and Antarctica. The East African and Kuungan Orogenies were followed by phases of post-orogenic extension. Early ∼600–550 Ma extension is recorded in the Arabian–Nubian Shield and the Eastern Granulite–Cabo Delgado Nappe Complex. Later ∼550–480 Ma extension affected Mozambique and southern Madagascar. Both extension phases, although diachronous,are interpreted as the result of lithospheric delamination. Along the strike of the East African Orogen, different geodynamic settings

  15. Detrital zircon geochronology of some neoproterozoic to triassic rocks in interior alaska

    USGS Publications Warehouse

    Bradley, D.C.; McClelland, W.C.; Wooden, J.L.; Till, A.B.; Roeske, S.M.; Miller, M.L.; Karl, S.M.; Abbott, J.G.

    2007-01-01

    We report 777 U-Pb SHRIMP detrital zircon ages from thirteen sandstones and metasandstones in interior Alaska. About sixty grains per sample were analyzed; typically, half to three-fourths of these were concordant within ?? 10%. Farewell terrane. Two quartzites were collected from Ruby quadrangle and a third from Taylor Mountains quadrangle. All three are interpreted to represent a low stratigraphic level in the Nixon Fork platform succession; the samples from Ruby quadrangle are probably late Neoproterozoic, and the sample from Taylor Mountains quadrangle is probably Cambrian in age. The youngest detrital zircon in any of the three is 851 Ma. The two Ruby quadrangle samples area almost identical: one has a major age cluster at 1980-2087 and minor age clusters at 944-974 and 1366-1383 Ma; the other has a major age cluster at 1993-2095 Ma and minor age clusters at 912-946 and 1366-1395 Ma. The Taylor Mountains sample shows one dominant peak at 1914-2057 Ma. Notably absent are zircons in the range 1800-1900 Ma, which are typical of North American sources. The detrital zircon populations are consistent with paleontological evidence for a peri- Siberian position of the Farewell terrane during the early Paleozoic. Mystic subterrane of the Farewell terrane. Three graywackes from flysch of the Mystic subterrane, Talkeetna quadrangle, were sampled with the expectation that all three were Pennsylvanian. Asample from Pingston Creek is Triassic (as revealed by an interbedded ash dated at ca. 223 Ma) and is dominated by age clusters of 341-359 and 1804-1866 Ma, both consistent with a sediment source in the Yukon-Tanana terrane. Minor age clusters at 848-869 and 1992-2018 Ma could have been sourced in the older part of the Farewell terrane. Still other minor age clusters at 432-461, 620-657, 1509-1536, and 1627-1653 Ma are not readily linked to sources that are now nearby. Asample from Surprise Glacier is mid-Mississippian or younger. Adominant age cluster at 1855-1883 and a

  16. Tectonic evolution of Kazakhstan and Tien Shan in Neoproterozoic and Early-Middle Paleozoic

    NASA Astrophysics Data System (ADS)

    Samygin, S. G.; Kheraskova, T. N.; Kurchavov, A. M.

    2015-05-01

    Geological information on Kazakhstan and the Tien Shan obtained up to the present time has been considered and integrated in order to demonstrate the main features of continental massifs, basins with oceanic crust, island arcs, marginal volcanic-plutonic belts, and transform fault zones differing in type and age. We ascertained the character and probable causes of their evolution and transformations resulting in the origination and development of mosaic structural assembly at margin of the Paleoasian ocean that existed from Neoproterozoic. The main stages of the geodynamic history of Paleozoides in Kazakhstan and Tien Shan are characterized, and a model of the probable course of regional tectonic events has been proposed. This model is illustrated by published paleomagnetic data and a series of paleotectonic reconstructions for time intervals 950-900, 850-800, 750-700, 650-630, 570-550, 530-515, 500-470, 460-440, and 390-380 Ma.

  17. Sedimentary Rocks and Methane - Southwest Arabia Terra

    NASA Technical Reports Server (NTRS)

    Allen, Carlton C.; Oehler, Dorothy Z.; Venechuk, Elizabeth M.

    2006-01-01

    We propose to land the Mars Science Laboratory in southwest Arabia Terra to study two key aspects of martian history the extensive record of sedimentary rocks and the continuing release of methane. The results of this exploration will directly address the MSL Scientific Objectives regarding biological potential, geology and geochemistry, and past habitability.

  18. Neoproterozoic-Early Paleozoic rifting of the craton margin in eastern Kentucky: Evidence from subsidence analysis

    SciTech Connect

    Goodman, P.T. . Dept. of Geological Sciences); Walker, D. )

    1992-01-01

    Analysis of subsidence along the craton margin in eastern Kentucky indicates a Neoproterozoic to Early through Middle Cambrian rifting event developing on a subsiding passive margin of the Laurentian craton to the Iapetus Ocean. Subsidence associated with rifting is confined to the Rome Trough; an internally broken half-graben within the Laurentian craton; the trough trends sub-parallel to the Appalachian orogenic belt. In cross section the through as an abrupt faulted margin on the carton side and a tapering, gentle extension toward the orogenic belt. The stratigraphic sequence within the Rome Trough and toward the orogen consists of Neoproterozoic or early Cambrian basal sands overlying Grenville basement, and succeeded by silts, shales and discontinuous carbonates of the Rome Fm. that are overlain by shales and carbonates of the Conesauga Fm. Stratigraphic relationships suggest that an out-of-sequence, inboard rift developed along the Laurentian margin adjacent to a drift-phase continental shelf represented by strata of the Blue Ridge and Valley and Ridge. Analysis of the subsidence history of this region reveals trends which support the notion that the subsidence history of this area cannot be accounted for by typical passive-margin development. The subsidence history of the area within the Rome Trough presents a pattern of high thermal subsidence and produces beta values greater than in areas nearer the craton margin. These data indicate that an inboard locus of anomalous crustal extension occurred in the area of the Rome Trough while the remainder of the cratonal margin underwent drift-phase subsidence, and that the timing and magnitude of this event is related to the development of the Iapetan margin.

  19. Formation of Neoproterozoic metamorphic complex during oblique convergence (Eastern Desert, Egypt)

    NASA Astrophysics Data System (ADS)

    Fritz, H.; Wallbrecher, E.; Khudeir, A. A.; Abu el Ela, F.; Dallmeyer, D. R.

    1996-10-01

    Major portions of the Pan-African Orogen in the Eastern Desert of Egypt were formed by island-arc accretion in the Neoproterozoic. These areas are characterized by their lack of major crustal thickening. Metamorphic core complexes occur parallel to the strike of the Eastern Desert Orogen. These domes exhibit polyphase metamorphism and deformation in contrast to the structurally overlying nappes which include ophiolitic melanges and island-arc volcanic rocks. These nappes show northwest directed, orogen-parallel thrusting in the internal parts and west to southwest directed imbrication in the external parts of the orogen. Structures related to exhumation of the metamorphic core complexes partition into different displacement paths localized within a crustal-scale wrench corridor of the Najd fault system. Northwest trending orogen-parallel, sinistral strike-slip faults define the western and eastern margins of the domes. North and south dipping low-angle normal faults developed along the northern and southern margins of the domes and form extensional bridges between them. {40Ar}/{39Ar} ages obtained from syntectonic muscovites within the shear zones gave Neoproterozoic ages of 595.9±0.5 and 588.2±0.3 Ma. The synchronous activity of strike-slip and normal faults suggests a regional east-west shortening which was accomodated by deep-level basal decollement beneath the metamorphic core complexes and a coeval northwest-southeast, orogen-parallel extension. This extension was accompanied by intramontane molasse sedimentation and emplacement of calc-alkaline plutons. Since the rapid exhumation of gneisses in the core complexes cannot be explained by thickening of the crust, the authors favour a model which calls for enhanced heat flow along the Najd fault system which would have enabled the formation of syn-extensional plutonism and triggered the exhumation of the metamorphic core complexes. Lateral buoyancy forces were concentrated within the Najd wrench corridor and

  20. Post-collisional subvolcanic rhyolites associated with the Neoproterozoic Pelotas Batholith, southern Brazil

    NASA Astrophysics Data System (ADS)

    Oliveira, Diego Skieresz de; Sommer, Carlos Augusto; Philipp, Ruy Paulo; Lima, Evandro Fernandes de; Basei, Miguel Ângelo Stipp

    2015-11-01

    Neoproterozoic volcanic and subvolcanic rhyolitic systems in southernmost Brazil are correlated with acid magmatism linked to different petrotectonic associations of the Sul-Rio-Grandense Shield. A portion of this volcanism in the Dom Feliciano Belt is associated with the Pelotas Batholith, which resulted from magmatic episodes associated with the Ediacaran post-collisional evolution of southern Brazil. Ana Dias Rhyolite is the main subvolcanic occurrence of this volcanism that took place in the Quitéria region, in the central part of Rio Grande do Sul State. The acid magmatism has been commonly associated with the most differentiated granite suite phases during the final stages of emplacement of the Pelotas Batholith. The Ana Dias Rhyolite is characterized as an intrusive body with rocks that present a porphyritic to seriated texture and a gradational variation to fine-grained equigranular rocks. New zircon U-Pb dating indicates crystallization age of 581.9 ± 1.9 Ma for the Ana Dias Rhyolite. Geochemistry data characterize the rhyolites as belonging to the alkaline series; they present a metaluminous to peraluminous character; elevated SiO2 and alkali concentrations, high FeOt/FeOt + MgO ratios and agpaitic index; and low Al2O3, CaO, and MgO contents. The Zr, Rb, Y, Nb, and Ga concentrations are moderate when compared with the relatively low Ba and Sr contents. These geochemistry characteristics are common in acid magmas with alkaline affinity. The behavior of certain trace elements and REE demonstrate enrichment in more incompatible elements, in addition to the negative anomaly of Ba, the slight enrichment in Ce relative to adjacent elements, as well as the enrichment in K2O and Rb relative to Nb, suggesting magmas derived from mantle sources enriched in incompatible elements with some crustal contamination. The chemical characteristics are similar to those of A-type granites associated with Neoproterozoic post-collision magmatism in the Sul

  1. Glacial trinity: Neoproterozoic Earth history within the British-Irish Caledonides

    NASA Astrophysics Data System (ADS)

    McCay, G. A.; Prave, A. R.; Alsop, G. I.; Fallick, A. E.

    2006-11-01

    Two distinct Neoproterozoic glacial episodes are known for the Dalradian Supergroup in the British-Irish Caledonides, the Port Askaig Formation and the Inishowen Loch na Cille ice-rafted debris (IRD) beds. Here we describe a third, intermediate between those two, the Stralinchy-Reelan formations, composed of diamictite and IRD. Developed directly above these rocks is the Cranford Limestone, which consists of a basal, 1 6-m-thick, tan-gray dolostone overlain by a variably developed, but as much as 340-m-thick, sequence of thin-bedded limestone and dolostone. This unit exhibits a C isotopic trend that begins negative in the basal dolostone, reaches a nadir of -7‰, and then rises to 0‰ 2‰. These characteristics match strikingly those of Marinoan-style cap carbonates. Consequently, we interpret the Port Askaig Formation, the Stralinchy-Reelan units, and the Inishowan Loch na Cille beds as equivalents of the ca. 700 Ma Sturtian, the 635 Ma Marinoan, and the ca. 580 Ma Gaskiers glacials, respectively. Two additional observations are noteworthy. Carbonate rocks below the Port Askaig Formation record a δ13C decline to -6‰ that implies that such downturns may occur in both pre-Sturtian and pre-Marinoan strata. In addition, the Bonahaven Dolomite is not a cap carbonate to the Port Askaig Formation, but exhibits a δ13C rise to 12‰, which we correlate with the inferred global Keele peak. These data further document the utility of Neoproterozoic glacial cap carbonate sequences in global correlations and denote the base of the Cranford Linestone as the Cryogenian-Ediacaran boundary.

  2. Physical Modelling of Sedimentary Basin

    SciTech Connect

    Yuen, David A.

    2003-04-24

    The main goals of the first three years have been achieved, i.e., the development of particle-based and continuum-based algorithms for cross-scaleup-scale analysis of complex fluid flows. The U. Minnesota team has focused on particle-based methods, wavelets (Rustad et al., 2001) and visualization and has had great success with the dissipative and fluid particle dynamics algorithms, as applied to colloidal, polymeric and biological systems, wavelet filtering and visualization endeavors. We have organized two sessions in nonlinear geophysics at the A.G.U. Fall Meeting (2000,2002), which have indeed synergetically stimulated the community and promoted cross-disciplinary efforts in the geosciences. The LANL team has succeeded with continuum-based algorithms, in particular, fractal interpolating functions (fif). These have been applied to 1-D flow and transport equations (Travis, 2000; 2002) as a proof of principle, providing solutions that capture dynamics at all scales. In addition, the fif representations can be integrated to provide sub-grid-scale homogenization, which can be used in more traditional finite difference or finite element solutions of porous flow and transport. Another useful tool for fluid flow problems is the ability to solve inverse problems, that is, given present-time observations of a fluid flow, what was the initial state of that fluid system? We have demonstrated this capability for a large-scale problem of 3-D flow in the Earth's crust (Bunge, Hagelberg & Travis, 2002). Use of the adjoint method for sensitivity analysis (Marchuk, 1995) to compute derivatives of models makes the large-scale inversion feasible in 4-D, , space and time. Further, a framework for simulating complex fluid flow in the Earth's crust has been implemented (Dutrow et al, 2001). The remaining task of the first three-year campaign is to extend the implementation of the fif formalism to our 2-D and 3-D computer codes, which is straightforward, but involved.

  3. 3D Inversion of Gravity Anomalies for the Interpretation of Sedimentary Basins using Variable Density Contrast

    NASA Astrophysics Data System (ADS)

    Ekinci, Yunus Levent; Ertekin, Can

    2015-04-01

    Concern about sedimentary basins is generally related to their genetic and economic significance. Analysis of sedimentary basins requires the acquisition of data through outcrop studies and subsurface investigations that encompass drilling and geophysics. These data are commonly analysed by computer-assisted techniques. One of these methods is based on analysing gravity anomalies to compute the depth of sedimentary basin-basement rock interface. Sedimentary basins produce negative gravity anomalies, because they have mostly lower densities than that of the surrounding basement rocks. Density variations in a sedimentary fill increase rapidly at shallower depths then gradually reach the density of surrounding basement rocks due to the geostatic pressure i.e. compaction. The decrease of the density contrast can be easily estimated by a quadratic function. Hence, if the densities are chosen properly and the regional background is removed correctly, the topographical relief of the sedimentary basin-basement rock interface might be estimated by the inversion of the gravity data using an exponential density-depth relation. Three dimensional forward modelling procedure can be carried out by introducing a Cartesian coordinate system, and placing vertical prisms just below observation points on the grid plane. Depth to the basement, namely depths to the bottom of the vertical prisms are adjusted in an iterative manner by minimizing the differences between measured and calculated residual gravity anomalies. In this study, we present a MATLAB-based inversion code for the interpretation of sedimentary basins by approximating the topographical relief of sedimentary basin-basement rock interfaces. For a given gridded residual gravity anomaly map, the procedure estimates the bottom depths of vertical prisms by considering some published formulas and assumptions. The utility of the developed inversion code was successfully tested on theoretically produced gridded gravity data set

  4. Hydrogeologic framework of sedimentary deposits in six structural basins, Yakima River basin, Washington

    USGS Publications Warehouse

    Jones, M.A.; Vaccaro, J.J.; Watkins, A.M.

    2006-01-01

    The hydrogeologic framework was delineated for the ground-water flow system of the sedimentary deposits in six structural basins in the Yakima River Basin, Washington. The six basins delineated, from north to south are: Roslyn, Kittitas, Selah, Yakima, Toppenish, and Benton. Extent and thicknesses of the hydrogeologic units and total basin sediment thickness were mapped for each basin. Interpretations were based on information from about 4,700 well records using geochemical, geophysical, geologist's or driller's logs, and from the surficial geology and previously constructed maps and well interpretations. The sedimentary deposits were thickest in the Kittitas Basin reaching a depth of greater than 2,000 ft, followed by successively thinner sedimentary deposits in the Selah basin with about 1,900 ft, Yakima Basin with about 1,800 ft, Toppenish Basin with about 1,200 ft, Benton basin with about 870 ft and Roslyn Basin with about 700 ft.

  5. Palynostratigraphy of the Erkovtsy field of brown coal (the Zeya-Bureya sedimentary basin)

    SciTech Connect

    Kezina, T.V.; Litvinenko, N.D.

    2007-08-15

    The Erkovtsy brown coal field in the northwestern Zeya-Bureya sedimentary basin (129-130{sup o}E, 46-47{sup o}N) is structurally confined to southern flank of the Mesozoic-Cenozoic Belogor'e depression. The verified stratigraphic scheme of the coalfield sedimentary sequence is substantiated by palynological data on core samples from 18 boreholes sampled in the course of detailed prospecting and by paleobotanical analysis of sections in the Yuzhnyi sector of the coalfield (data of 1998 by M.A. Akhmetiev and S.P. Manchester). Sections of the Erkovtsy, Arkhara-Boguchan, and Raichikha brown-coal mines are correlated. Stratigraphic subdivisions distinguished in the studied sedimentary succession are the middle and upper Tsagayan subformations (the latter incorporating the Kivda Beds), Raichikha, Mukhino, Buzuli, and Sazanka formations.

  6. Sedimentary structures from different proglacial glaciofluvial settings in Lithuania

    NASA Astrophysics Data System (ADS)

    Šinkūnė, E.; Šinkūnas, E.

    2012-04-01

    Proglacial glaciofluvial sedimentation during old continental glaciations created a wide range of landforms. However, proglacial deposit sequences are not always well expressed in form of typical landforms. In that case, and also when we are dealing with buried deposits, we face difficulties to recognise their origin or conditions of sedimentation. Sediment bedding structures vary considerably depending on the sedimentation conditions; consequently, they are most helpful in this case. The sediment bedding structures were studied in number of proglacial sediment sequences of different origin in Lithuania to gain the most characteristic complexes of sedimentary structures from particular conditions of sedimentation. In order to analyse the successions of bedforms created in different settings of proglacial glaciofluvial sedimentation the sedimentary models for particular sites in sandur, meltwater streamway and glaciofluvial delta were built. Trough, tabular and planar cross bedding and climbing-ripple cross-lamination is characteristic of proglacial glaciofluvial sediment sequences. However, distinct complexes of bedding structures were recognised for specific conditions of sedimentation in particular sites. This study was financed by the Research Council of Lithuania (No. LEK- 03/2010).

  7. NATURAL GAS RESOURCES IN DEEP SEDIMENTARY BASINS

    SciTech Connect

    Thaddeus S. Dyman; Troy Cook; Robert A. Crovelli; Allison A. Henry; Timothy C. Hester; Ronald C. Johnson; Michael D. Lewan; Vito F. Nuccio; James W. Schmoker; Dennis B. Riggin; Christopher J. Schenk

    2002-02-05

    From a geological perspective, deep natural gas resources are generally defined as resources occurring in reservoirs at or below 15,000 feet, whereas ultra-deep gas occurs below 25,000 feet. From an operational point of view, ''deep'' is often thought of in a relative sense based on the geologic and engineering knowledge of gas (and oil) resources in a particular area. Deep gas can be found in either conventionally-trapped or unconventional basin-center accumulations that are essentially large single fields having spatial dimensions often exceeding those of conventional fields. Exploration for deep conventional and unconventional basin-center natural gas resources deserves special attention because these resources are widespread and occur in diverse geologic environments. In 1995, the U.S. Geological Survey estimated that 939 TCF of technically recoverable natural gas remained to be discovered or was part of reserve appreciation from known fields in the onshore areas and State waters of the United. Of this USGS resource, nearly 114 trillion cubic feet (Tcf) of technically-recoverable gas remains to be discovered from deep sedimentary basins. Worldwide estimates of deep gas are also high. The U.S. Geological Survey World Petroleum Assessment 2000 Project recently estimated a world mean undiscovered conventional gas resource outside the U.S. of 844 Tcf below 4.5 km (about 15,000 feet). Less is known about the origins of deep gas than about the origins of gas at shallower depths because fewer wells have been drilled into the deeper portions of many basins. Some of the many factors contributing to the origin of deep gas include the thermal stability of methane, the role of water and non-hydrocarbon gases in natural gas generation, porosity loss with increasing thermal maturity, the kinetics of deep gas generation, thermal cracking of oil to gas, and source rock potential based on thermal maturity and kerogen type. Recent experimental simulations using laboratory

  8. Age, composition, and source of continental arc- and syn-collision granites of the Neoproterozoic Sergipano Belt, Southern Borborema Province, Brazil

    NASA Astrophysics Data System (ADS)

    Oliveira, Elson P.; Bueno, Juliana F.; McNaughton, Neal J.; Silva Filho, Adejardo F.; Nascimento, Rosemery S.; Donatti-Filho, José P.

    2015-03-01

    The Sergipano belt is the outcome of collision between the Pernambuco-Alagoas Domain (Massif) and the São Francisco Craton during Neoproterozoic assembly of West Gondwana. Although the understanding of the Sergipano belt evolution has improved significantly, the timing of emplacement, geochemistry and tectonic setting of granitic bodies in the belt is poorly known. We recognized two granite age groups: 630-618 Ma granites in the Canindé, Poço Redondo and Macururé domains, and 590-570 Ma granites in the Macururé metasedimentary domain. U-Pb SHRIMP zircon ages for granites of first age group indicated ages of 631 ± 4 Ma for the Sítios Novos granite, 623 ± 7 Ma for the Poço Redondo granite, 619 ± 3.3 Ma for the Lajedinho monzodiorite, and 618 ± 3 Ma for the Queimada Grande granodiorite. These granitoids are dominantly high-K calc-alkaline, magnesian, metaluminous, mafic enclave-rich (Queimada Grande and Lajedinho), or with abundant inherited zircon grains (Poço Redondo and Sitios Novos). Geochemical and isotope data allow us to propose that Sítios Novos and Poço Redondo granites are product of partial melting of Poço Redondo migmatites. Sr-Nd isotopes of the Queimada Grande granodiorite and Lajedinho monzodiorite suggest that their parental magma may have originated by mixing between a juvenile mafic source and a crustal component that could be the Poço Redondo migmatites or the Macururé metasediments. Other 630-618 Ma granites in the belt are the mafic enclave-rich Coronel João Sá granodiorite and the Camará tonalite in the Macururé sedimentary domain. These granites have similar geochemical and isotopic characteristics as the Lajedinho and Queimada Grande granitoids. We infer for the Camará tonalite and Coronel João Sá granodiorite that their parental magmas have had contributions from mafic lower crust and felsic upper crust, most probably from underthrust São Francisco Craton, or Pernambuco-Alagoas Domain. The younger 590-570 Ma granite

  9. New aspects of deformed cross-strata in fluvial sandstones: Examples from Neoproterozoic formations in northern Norway

    NASA Astrophysics Data System (ADS)

    Røe, Signe-Line; Hermansen, Marita

    2006-05-01

    Extensive (20-200 m long) exposures of tabular cross-sets in Neoproterozoic fluvial sandstone in Northern Norway demonstrate that deformed cross-strata, in the form of recumbently folded cross-strata with associated massive sand, are localized features passing in both up- and down-current direction into undeformed, concave-upward or sigmoidal cross-strata. The deformation occurs in down-current inclined, tangential wedge-shaped zones beneath reactivation surfaces, and less commonly as flat-topped lenticular zones. The localized nature of the sediment deformation is attributed to local liquefaction below the top of the bed in the case of the flat-topped lenses and at the dune front in the case of the more common tangential wedges. The position of the flat-topped lenses suggests deformation by the shear stress of high-velocity, suspension-laden currents. Although liquefaction of the dune front implies the action of gravity forces, it is argued that the fluvial currents were the main driving force at the instant of bed liquefaction. Post-folding gravitational shearing probably enhanced the deformation within the upper part of the wedges, with their long, flat-lying toeset resulting from redeposition of downslope-moving liquefied sand. The down-current alternation of deformed tangential wedges and undeformed cross-strata suggests that the mechanism that triggered the liquefaction of the dune lee side was related to the fluvial system itself and hence was of autokinetic origin. The tabular cross-sets have previously been interpreted as a product of the dune upper-stage plane-bed flow regime. In this flow context, it can be speculated that the liquefaction and deformation occurred when the flow conditions approached the plane-bed phase, probably inducing a highly differential turbulent pattern and pressure fluctuations sufficient to liquefy the fine/medium sand. The small flat-topped deformation lenses also suggest liquefaction by cyclic loading, whereas the solitary

  10. Tide-influenced sedimentary environments and facies

    SciTech Connect

    De Boer, P.L.; Van Gelder, A.; Nio, S.D.

    1988-01-01

    This volume contains examples of recent as well as fossil tide-influenced sedimentary facies. Studies of recent tidal processes and sediments provide an insight into the way in which tidal facies and sequences develop, and into the processes which are active. The studies performed on fossil rocks give information on one-to-one scale model experiments that have been executed by nature both relatively recently and in the distant past. In this work, the parallel presentation of papers on recent and fossil examples of tide-influenced sedimentary facies and environments follows the philosophy of comparative sedimentology, aiming at an understanding of both the past and the present, with the aim also, of forecasting future developments.

  11. Petroleum potential of the Libyan sedimentary basins

    SciTech Connect

    Hammuda, O.S.; Sbeta, A.M.

    1988-08-01

    Contrary to prevailing opinion, all Libyan sedimentary basins and the Al-Jabal Al-Akhdar platform contain prolific petroleum accumulations with very high prospectivity. A systematic review of the types of traps and pays in this central part of the southern Mediterranean province reveals great variability in reservoir and source rock characteristics. The reservoir rocks are of almost all geologic ages. The thick source rock sequences also vary in nature and organic content. The organic-rich facies have accumulated in intracratonic and passive margin basins or in marginal seas. Most of the oil discovered thus far in these basins is found in large structural traps. Future discoveries of stratigraphic traps or small structural traps will require intensified efforts and detailed studies using up-to-date multidisciplinary techniques in sedimentary tectonics, biostratigraphic facies analysis, and geochemical prospecting in order to develop a better understanding of these basins, thus improving their prospectivity.

  12. Biological Feedbacks as Cause and Demise of Neoproterozoic Icehouse: Astrobiological Prospects for Faster Evolution and Importance of Cold Conditions

    PubMed Central

    Janhunen, Pekka; Kaartokallio, Hermanni; Oksanen, Ilona; Lehto, Kirsi; Lehto, Harry

    2007-01-01

    Several severe glaciations occurred during the Neoproterozoic eon, and especially near its end in the Cryogenian period (630–850 Ma). While the glacial periods themselves were probably related to the continental positions being appropriate for glaciation, the general coldness of the Neoproterozoic and Cryogenian as a whole lacks specific explanation. The Cryogenian was immediately followed by the Ediacaran biota and Cambrian Metazoan, thus understanding the climate-biosphere interactions around the Cryogenian period is central to understanding the development of complex multicellular life in general. Here we present a feedback mechanism between growth of eukaryotic algal phytoplankton and climate which explains how the Earth system gradually entered the Cryogenian icehouse from the warm Mesoproterozoic greenhouse. The more abrupt termination of the Cryogenian is explained by the increase in gaseous carbon release caused by the more complex planktonic and benthic foodwebs and enhanced by a diversification of metazoan zooplankton and benthic animals. The increased ecosystem complexity caused a decrease in organic carbon burial rate, breaking the algal-climatic feedback loop of the earlier Neoproterozoic eon. Prior to the Neoproterozoic eon, eukaryotic evolution took place in a slow timescale regulated by interior cooling of the Earth and solar brightening. Evolution could have proceeded faster had these geophysical processes been faster. Thus, complex life could theoretically also be found around stars that are more massive than the Sun and have main sequence life shorter than 10 Ga. We also suggest that snow and glaciers are, in a statistical sense, important markers for conditions that may possibly promote the development of complex life on extrasolar planets. PMID:17299594

  13. Neoproterozoic Orogeny-Related Magmatic Event in the Northwestern Yangtze Block and Its Implications for the Unification of South China

    NASA Astrophysics Data System (ADS)

    Wafa, B.

    2014-12-01

    Neoproterozoic tectonic history of South China has long been an issue of debate, including its connection with the cycle of Rodinia. The South China block has been traditionally thought to have formed by Neoproterozoic welding between the Yangtze and Cathysian blocks along the Jiangnan Orogen. However, increasing evidence suggests that the Neoproterozoic unification of South China comprises a set of collision events. Here we report a newly recognized syn-collisional intrusive suite at the northwestern Yangtze block. The igneous suite is located at the Wangcang area, NE Sichuan province. It consists of tholeiitic- and alkali mafic rocks, and adakitic to calc-alkali felsic intrusions. They are dated at 8554Ma to 8675 Ma by U-Pb zircon, and show arc-related geochemical affinities with highly variable whole rock ɛNd (up to +7.78) and zircon ɛHf(t) values. The Wangcang suite is featured by high deformation and metamorphism of upper greenschist to amphibolitic facieses, which is contrast to the low grade ~820-750 Ma igneous suites in the same region. It suggests that the northwestern Yangtze block experienced an arc-continental orogenic event occurred during ~860-820 Ma. Our recent work has revealed that the Shennongjia terrain and the Yangtze continental nucleus was welded between ~1100-950Ma. These findings suggest that the unification of South China comprises a set of collision events. This work provides a new clue for our understanding of the connection between the Neoproterozoic evolution of South China and Rodinia cycle. This study was supported by the NSFC (Grants 41173048 and 41373037).

  14. Evaluating key parameters for the initiation of a Neoproterozoic Snowball Earth with a single Earth System Model of intermediate complexity

    NASA Astrophysics Data System (ADS)

    Spiegl, T. C.; Paeth, H.; Frimmel, H. E.

    2015-04-01

    Even after more than two decades of intense research the main drivers for a potential Neoproterozoic Snowball Earth continue to be discussed controversially. In this study we present results from 37 sensitivity experiments that were performed with the Planet Simulator (PlaSim), an Earth System Model of intermediate complexity. In contrast to previous studies, in which only a limited number of potential climate-controlling parameters were assessed with different climate models, we tested our presumed key parameters within one single model. This approach makes it easier to compare the influence of the various parameters on extreme climate change as postulated for the Neoproterozoic Era. Furthermore we compare the results obtained to most recent high complexity state-of-the-art approaches. This comparison helps to estimate, which internal model interactions and physics are crucial for a Snowball Earth simulation and hence should be included into a model that is capable of realistically simulating a Neoproterozoic climate. To this effect we carried out simulations that involved reduced solar irradiation, land-sea distributions, atmospheric CO2 concentrations, relief of the land surface and length of day. In addition, we focus on different land surface albedo values, which were most likely exceptionally low and similar to the Martian albedo, and obliquity changes between 23.5° and 80°. Our findings suggest that changes in land surface albedo are a strong climate driver that can compensate a much lower Neoproterozoic total solar irradiance if it is combined with shifts in obliquity or atmospheric CO2 levels. We also obtained a critical threshold for increased obliquities beyond which a Snowball Earth situation turns into an extreme greenhouse climate with almost absent cryosphere, and furthermore, obliquity values that lead to a tropical ice age with sea ice spreading from the equator to high latitudes.

  15. Biological feedbacks as cause and demise of the Neoproterozoic icehouse: astrobiological prospects for faster evolution and importance of cold conditions.

    PubMed

    Janhunen, Pekka; Kaartokallio, Hermanni; Oksanen, Ilona; Lehto, Kirsi; Lehto, Harry

    2007-01-01

    Several severe glaciations occurred during the Neoproterozoic eon, and especially near its end in the Cryogenian period (630-850 Ma). While the glacial periods themselves were probably related to the continental positions being appropriate for glaciation, the general coldness of the Neoproterozoic and Cryogenian as a whole lacks specific explanation. The Cryogenian was immediately followed by the Ediacaran biota and Cambrian Metazoan, thus understanding the climate-biosphere interactions around the Cryogenian period is central to understanding the development of complex multicellular life in general. Here we present a feedback mechanism between growth of eukaryotic algal phytoplankton and climate which explains how the Earth system gradually entered the Cryogenian icehouse from the warm Mesoproterozoic greenhouse. The more abrupt termination of the Cryogenian is explained by the increase in gaseous carbon release caused by the more complex planktonic and benthic foodwebs and enhanced by a diversification of metazoan zooplankton and benthic animals. The increased ecosystem complexity caused a decrease in organic carbon burial rate, breaking the algal-climatic feedback loop of the earlier Neoproterozoic eon. Prior to the Neoproterozoic eon, eukaryotic evolution took place in a slow timescale regulated by interior cooling of the Earth and solar brightening. Evolution could have proceeded faster had these geophysical processes been faster. Thus, complex life could theoretically also be found around stars that are more massive than the Sun and have main sequence life shorter than 10 Ga. We also suggest that snow and glaciers are, in a statistical sense, important markers for conditions that may possibly promote the development of complex life on extrasolar planets. PMID:17299594

  16. Sedimentary Facies Analysis Using AVIRIS Data: A Geophysical Inverse Problem

    NASA Technical Reports Server (NTRS)

    Boardmann, Joe W.; Goetz, Alexander F. H.

    1990-01-01

    AVIRIS data can be used to quantitatively analyze and map sedimentary lithofacies. The observed radiance spectra can be reduced to 'apparent reflectance' spectra by topographic and reflectance characterization of several field sites within the image. These apparent reflectance spectra correspond to the true reflectance at each pixel, multiplied by an unknown illumination factor (ranging in value from zero to one). The spatial abundance patterns of spectrally defined lithofacies and the unknown illumination factors can be simultaneously derived using constrained linear spectral unmixing methods. Estimates of the minimum uncertainty in the final results (due to noise, instrument resolutions, degree of illumination and mixing systematics) can be made by forward and inverse modeling. Specific facies studies in the Rattlesnake Hills region of Wyoming illustrate the successful application of these methods.

  17. Rapid lithification masks the Venus sedimentary cycle

    NASA Astrophysics Data System (ADS)

    Ghail, R.

    2015-10-01

    Venera lander data are usually assumed to indicate basaltic lavas but a significant fraction of the rock material must be volatiles, such as sulphur, implying at least strongly weathered basalts. The lander images most closely resemble sedimentary material, with layered strata (which may be pyroclastic in origin)that are sometimes broken into cobbles and fine grained sediment. The Magellan SAR was relatively insensitive to loose fine-grained material under Venus surface conditions but the reprocessed data reveal a range of weathering processes, particularly at higher elevations, and mass wasting of steep slopes. Mean wind speeds are strongly altitude dependent and are able to erode and transport material throughout the highland regions. In some areas, this material is deposited on adjacent plains where, under the extreme Venus surface conditions, lithification is an apparently rapid process. Thus the largely featureless plains may not be igneous at all but sedimentary in origin. The settling out and lithification of sedimentary material is consistent with observed crater degradation, in which low-lying crater floors are infilled first.

  18. The Guarguaraz Complex and the Neoproterozoic-Cambrian evolution of southwestern Gondwana: Geochemical signatures and geochronological constraints

    NASA Astrophysics Data System (ADS)

    López de Azarevich, Vanina L.; Escayola, Mónica; Azarevich, Miguel B.; Pimentel, Márcio M.; Tassinari, Colombo

    2009-12-01

    The Guarguaraz Complex, in western Argentina, comprises a metasedimentary assemblage, associated with mafic sills and ultramafic bodies intruded by basaltic dikes, which are interpreted as Ordovician dismembered ophiolites. Two kinds of dikes are recognized, a group associated with the metasediments and the other ophiolite-related. Both have N-MORB signatures, with ɛNd between +3.5 and +8.2, indicating a depleted source, and Grenville model ages between 0.99 and 1.62 Ga. A whole-rock Sm-Nd isochron yielded an age of 655 ± 76 Ma for these mafic rocks, which is compatible with cianobacteria and acritarchae recognized in the clastic metasedimentary platform sequences, that indicate a Neoproterozoic (Vendian)-Cambrian age of deposition. The Guarguaraz metasedimentary-ophiolitic complex represents, therefore, a remnant of an oceanic basin developed to the west of the Grenville-aged Cuyania terrane during the Neoproterozoic. The southernmost extension of these metasedimentary sequences in Cordón del Portillo might represent part of this platform and not fragments of the Chilenia terrane. An extensional event related to the fragmentation of Rodinia is represented by the mafic and ultramafic rocks. The Devonian docking of Chilenia emplaced remnants of ocean floor and slices of the Cuyania terrane (Las Yaretas Gneisses) in tectonic contact with the Neoproterozoic metasediments, marking the Devonian western border of Gondwana.

  19. Neoproterozoic/Lower Palaeozoic geodynamic evolution of Dronning Maud Land: integrating geology and geophysics

    NASA Astrophysics Data System (ADS)

    Jacobs, Joachim; Andreas, Läufer; Clark, Chris; Kleinhanns, Ilka; Elburg, Marlina; Ruppel, Antonia; Estrada, Solveig; Damaske, Detlef; Jokat, Wilfried; Riedel, Sven; Lucka, Nicole

    2013-04-01

    East Antarctica probably formed by amalgamation of a number of cratons along distinct Ediacaran mobile belts, including the ca. 600-500 Ma East African-Antarctic Orogen (EAAO) that dissects Dronning Maud Land (DML). New field-work during the international expeditions Geodynamic Evolution of East Antarctica (GEA) I + II in the austral summers 2010/11 and 2011/12, and first geochronological results from eastern DML reveal a complex tectonic architecture across the belt. In western DML, the EAAO reworks older Mesoproterozoic crust of the Maud Belt; the westernmost boundary of the mobile belt is characterized by a major dextral transpressional shear zone. In central DML, a major magnetic anomaly, the Forster anomaly, was interpreted as a cryptic suture of the EAAO (Riedel et al. 2012). The area where the Forster anomaly crosses the DML mountains is poorly investigated so far, but appears to coincide with a major strike slip shear zone in the southern Kurze Mts. and the occurrence of major Ediacaran granulite bodies. East of the Forster anomaly, the magnetic anomaly pattern changes significantly and typical Maud type crust is not present any longer. GEA II targeted a range of nunataks between Sør Rondane and central DML that had never been visited previously (from Blåklettane and Bergekongen in the E to Urna and Sørsteinen in the W). These nunataks are dominated by medium- to high-grade metasedimentary and metavolcanic rocks of possibly Neoproterozoic age, including abundant marble and graphite schists. Sør Rondane in eastern DML, is dominated by two distinct blocks separated by the dextral Main Shear Zone. The northwestern block is still part of the eastern EAAO, where new SHRIMP zircon data from metamorphic rims provide ages of ca. 560 Ma. The southeastern block is made up of a TTG terrane, which provides four new SHRIMP zircon dates between 990-980 Ma, interpreted as igneous crystallization ages (oceanic arc). The TTG terrane shows limited tectonic overprint and

  20. The Jormungand Global Climate State and Implications for the Neoproterozoic Snowball Paradox (Invited)

    NASA Astrophysics Data System (ADS)

    Abbot, D. S.; Voigt, A.; Koll, D.; Pierrehumbert, R. T.

    2010-12-01

    We present a previously undescribed global climate state, the Jormungand state, that is nearly ice-covered with a narrow (~10-15 degrees of latitude) strip of open ocean near the equator. This state is sustained by internal dynamics of the hydrological cycle and the cryosphere. There is a new bifurcation in global climate climate associated with the Jormungand state that leads to significant hysteresis. We investigate the Jormungand state in a coupled ocean-atmosphere GCM, in multiple atmospheric GCMs coupled to a mixed layer ocean run in an idealized configuration, and we make a simple modification to the Budyko-Sellers model so that it produces Jormungand states. We suggest that the Jormungand state may be a better model for the Neoproterozoic glaciations (~635 Ma and ~715 Ma) than either the hard Snowball or the Slushball models. A Jormungand state would have a large enough region of open ocean near the equator to explain the micropaleontological and molecular clock evidence that photosynthetic eukaryotes thrived both before and immediately after the Neoproterozoic episodes. Additionally, since there is significant hysteresis associated with the Jormungand state, it can explain the cap carbonate sequences, the oxygen isotopic evidence that suggests high CO2 values, and the various evidence that suggests lifetimes for the glaciations of 1 Myrs or more. Since there is not significant hysteresis associated with the Slushball model, the Slushball model cannot explain these observations. Finally, we note that although the Slushball and Jormungand models share the characteristic of open ocean in the tropics, the Jormungand state is produced by entirely different physics, is entered through a new bifurcation in global climate, and is associated with significant hysteresis. Bifurcation diagram of global climate in the CAM global climate model, run with no continents, a 50 m mixed layer with no ocean heat transport, an eccentricity of zero, and annually and diurnally

  1. Microanalyzes of remarkable microfossils of the Late Mesoproterozoic-Early Neoproterozoic

    NASA Astrophysics Data System (ADS)

    Cornet, Yohan; Beghin, Jérémie; Baludikay, Blaise; François, Camille; Storme, Jean-Yves; Compère, Philippe; Javaux, Emanuelle

    2016-04-01

    The Late Mesoproterozoic-Early Neoproterozoic is an important period to investigate the diversification of early eukaryotes [1]. Following the first appearance of red algae in the Late Mesoproterozoic, other (morphological or molecular) fossils of crown groups are recorded during the Early Neoproterozoic, including green algae, sponges, amoebozoa and possibly fungi. Other microfossils also includes unambiguous eukaryotes, including several distinctive forms for that time period, such as the acritarchs Cerebrosphaera buickii (˜820-720 Ma), Trachyhystrichosphaera aimika and T . botula (1100-720 Ma), and the multicellular eukaryotic problematicum taxon Jacutianema solubila (1100-?720 Ma). To further characterize the taxonomy of these microfossils and to test hypotheses about their possible relationships to crown groups, we combine analyzes of their morphology, wall ultrastructure and microchemistry, using optical microscopy, Scanning and Transmission Electron microscopy (SEM and TEM), as well as Raman and FTIR microspectroscopy respectively. Cerebrosphaera populations from the Svanbergfjellet formation, Spitsbergen, and from the Kanpa Formation, Officer Basin, Australia, include organic vesicles with dark and robust walls ornamented by cerebroid folds [2]. Our study shows the occurrence of complex tri- or bi-layered wall ultrastructures and a highly aromatic composition [3]. The genus Trachyhystrichosphaera includes various species characterized by the presence of a variable number of hollow heteromorphic processes [2]. Preliminary infrared microspectroscopy analyzes performed on two species, T. aimika and T. botula, from the 1.1 Ga Taoudeni Basin, Mauritania, and from the ˜1.1 - 0.8 Ga Mbuji-Mayi Supergroup, RDC, indicate a strong aliphatic and carbonyl composition of the wall biopolymer, with some differences linked to thermal maturity between the two locations. TEM is also performed to characterize the wall ultrastructure of these two species. Various morphotypes

  2. Dronning Maud Lands (East Antarctica) significance for Late Mesoproterozoic/Early Neoproterozoic supercontinent reconstructions

    NASA Astrophysics Data System (ADS)

    Jacobs, Joachim; Elburg, Marlina; Laeufer, Andreas; Kleinhanns, Ilka C.; Henjes-Kunst, Friedhelm; Estrada, Solveig; Ruppel, Antonia; Damaske, Detlef; Montero, Pilar; Bea, Fernando

    2015-04-01

    The recent study of a so far white spot on the geological map of Dronning Maud Land (DML) during the international GEA expeditions sheds new light on the significance of major tectono-metamorphic provinces of Dronning Maud Land, East Antarctica. The western part of eastern DML allows the characterization and ground-truthing of a large and mostly ice-covered area, that is geophysically distinct and which was previously interpreted as a potentially older cratonic block south of a Late Neoproterozoic/EarlyPaleozoic mobile belt, which is exposed in the Sør Rondane Mts. (SRM). SHRIMP/SIMS zircon analyses of 20 samples together with new geochemistry indicate that the exposed basement consists of a ca. 1000-900 Ma juvenile terrane that is very similar to the juvenile rocks of the SW-Terrane of the SRM, a characteristic gabbro-trondhjemite-tonalite-granite suite. However, in contrast to the southern part of the SW-Terrane, our study area shows intense crustal reworking at medium to high-grade conditions between ca. 630-520 Ma, associated with significant felsic melt production, including A-type granitoid magmatism. Therefore, the study area, and thereby the aeromagnetically distinct SE DML province does neither represent the foreland of a Late Neoproterozoic/EarlyPaleozoic mobile belt, nor a craton, as has previously been speculated. It more likely represents the westward continuation of Rayner-age crust (1000-900 Ma) that has undergone additional protracted LN/EP overprinting. We interpret the southern part of the only weakly overprinted SW-Terrane as a mega-boudin within a broad, rheologically weaker, NW-SE trending LN/EP mobile belt. Rayner-type crust likely continues further west, where it abuts along the SW-trending Forster Magnetic Anomaly. The latter is interpreted as a suture, which separates typical Grenville-age crust of the Maud Belt to the W from Rayner-age crust to the E. The study area has therefore clearly Indian affinities. Its juvenile character with a

  3. Intra-platform tectono-sedimentary response to geodynamic transition along the margin of the Tarim Basin, NW China

    NASA Astrophysics Data System (ADS)

    Gao, Zhiqian; Fan, Tailiang

    2014-12-01

    The Tarim Basin has experienced three tectonic evolutionary phases from the Cambrian to Ordovician: (1) Regional extension from the late Neoproterozoic to Mid-Early Cambrian, (2) Relatively weak regional compression from the Late Cambrian to Mid-Early Ordovician, and (3) Regional compression during the Late Ordovician. Intra-platform tectonic and sedimentary characteristics indicate a clear linkage to the tectonic evolution of the basin margin during early Paleozoic time. During the Cambrian, small intra-platform rift-related depressions formed during an extensional setting. During the Mid-Early Ordovician, a transition from extension to compression caused formation of the Tazhong and Tabei paleo-uplifts and major unconformities T74 (base of the Late Ordovician). The evolving paleo-geomorphology led to differentiation of sedimentary facies, and numerous intra-platform shoals formed during deposition of the Early Ordovician Yingshan Formation. During the Late Ordovician, regional compression began, which changed the platform margin slopes into four slopes that surrounded the three isolated island uplifts of Tabei, Tazhong, and Tangnan in the Late Ordovician. Simultaneously, the basin margin dynamic conditions also changed the relative sea level and filling pattern of the basin. In the Early and Middle Cambrian, the Tarim Basin mainly developed a progradational ramp-type platform due to relative sea level fall. From the Late Cambrian to Early Ordovician the relative sea level began to rise, resulting in an aggradational-retrograding rimmed margins-type platform. In the Late Ordovician, along with a further rise in relative sea level, the basin mainly developed isolated platform.

  4. The carbon-isotopic composition of Proterozoic carbonates: Riphean successions from northwestern Siberia (Anabar Massif, Turukhansk Uplift)

    NASA Technical Reports Server (NTRS)

    Knoll, A. H.; Kaufman, A. J.; Semikhatov, M. A.

    1995-01-01

    Thick carbonate-dominated successions in northwestern Siberia document secular variations in the C-isotopic composition of seawater through Mesoproterozoic and early Neoproterozoic (Early to early Late Riphean) time. Mesoproterozoic dolomites of the Billyakh Group, Anabar Massif, have delta 13C values that fall between 0 and -1.9 permil versus PDB, with values in the upper part of the succession (Yusmastakh Formation) consistently higher than those of the lower (Ust'-Il'ya and Kotuikan formations). Consistent with available biostratigraphic and radiometric data, delta 13C values for Billyakh carbonates compare closely with those characterizing early Mesoproterozoic carbonates (about 1600-1200 Ma) worldwide. In contrast, late Mesoproterozoic to early Neoproterozoic limestones and dolomites in the Turukhansk Uplift exhibit moderate levels of secular variation. Only the lowermost carbonates in the Turukhansk succession (Linok Formation) have delta 13C values that approximate Billyakh values. Higher in the Turukhansk succession, delta 13C values vary from -2.7 to +4.6 permil (with outliers as low as -5.0 permil interpreted as diagentically altered). Again, consistent with paleontological and radiometric data, these values compare well with isotopic values from 1200 to 850 Ma successions elsewhere. Five sections measured in different parts of the Turukhansk basin show nearly identical patterns of variation, confirming that carbonate delta 13C correlates primarily with time and not facies. The Siberian sections illustrate the potential of integrated biostratigraphic and chemostratigraphic data in the intra- and interbasinal correlation of Mesoproterozoic and early Neoproterozoic rocks.

  5. Detrital zircon record of the early Paleozoic meta-sedimentary rocks in Russian Altai: Implications on their provenance and the tectonic nature of the Altai-Mongolian terrane

    NASA Astrophysics Data System (ADS)

    Chen, Ming; Sun, Min; Cai, Keda; Buslov, Mikhail M.; Zhao, Guochun; Rubanova, Elena S.; Voytishek, Elena E.

    2015-09-01

    An integrated U-Pb and Hf-isotope study on detrital zircons from the early Paleozoic meta-sedimentary rocks along the Charysh-Terekta-Ulagan-Sayan suture zone in Russian Altai was conducted in order to trace their provenance and tectonic setting. Most of the zircons possess oscillatory zoning and high Th/U ratios (> 0.1), indicating their magmatic origin. The investigated samples yield similar zircon populations, i.e., dominant groups with late Neoproterozoic to early Paleozoic ages, followed by those from Mesoproterozoic to late Neoproterozoic and minor ones from Archean to middle Mesoproterozoic, indicating multiple tectono-thermal events in the source area. Comparison with surrounding tectonic units shows that the Tuva-Mongolian terrane and its adjacent island arcs possibly provided substantial materials to the sedimentary basin. These rocks show detrital zircon age patterns and Hf-isotope compositions similar to their counterparts in the Chinese Altai and Tseel terrane in western Mongolia, but different from those in the Gorny Altai terrane. Therefore, the investigated meta-sedimentary units possibly represented the northernmost segment of the Altai-Mongolian terrane. With combination of previous studies in the Chinese Altai and Tseel terrane, our data suggest that the Altai-Mongolian terrane possibly represents a coherent continental arc-accretionary prism system built upon the active margin of the western Mongolia during the Cambrian to Ordovician and thus does not support the micro-continent model with a passive margin. A compilation of U-Pb and Hf-isotope data of detrital zircons from the whole Altai-Mongolian terrane shows that the source area (i.e., the western Mongolia) underwent two most extensive magmatic activities at ca. 1.02-0.67 Ga and 0.67-0.43 Ga. These zircons possess both positive and negative εHf(t) values, suggesting significant crustal growth and reworking during the magmatic activities. Our study underlines a crucial role of Precambrian

  6. Sedimentary architecture of the Shaler outcrop, Gale Crater, Mars: paleoenvironmental and sediment transport implications

    NASA Astrophysics Data System (ADS)

    Gupta, S.; Edgar, L. A.; Rubin, D. M.; Lewis, K. W.; Kocurek, G.; Anderson, R. B.; Bell, J. F.; Dromart, G.; Edgett, K. S.; Grotzinger, J. P.; Hardgrove, C. J.; Kah, L. C.; Leveille, R. J.; Malin, M.; Mangold, N.; Milliken, R.; Minitti, M. E.; Muller, J.; Rice, M. S.; Rowland, S. K.; Schieber, J.; Stack, K.; Sumner, D. Y.; Team, M.

    2013-12-01

    Sedimentary rocks are archives of ancient depositional processes and environments on planetary surfaces. Reconstructing such processes and environments requires observations of sedimentary structures and architecture (the large-scale geometry and organisation of sedimentary bedsets). We report the analysis of the distinct Shaler outcrop, a prominent stratified unit located between the Bathurst Inlet outcrop and the floor of Yellowknife bay. The Shaler outcrop is an ~1 m thick stratal unit that spans approximately 30 m outcrop in length, and was examined by Curiosity on sols 120-121 and more recently on sols 309-324. Detailed stereo observations of the outcrop across most of its entire lateral extent were made using Navigation and Mast Cameras. These data permit detailed analysis of stratal geometries, distribution of sedimentary structures, and broad grain size trends. Overall the Shaler outcrop comprises a heterogeneous assemblage of interstratified platy sandstones separated by recessive, likely finer-grained beds. Coarser-grained beds are characterised by decimeter-scale trough cross-bedding. The north-eastern section of the outcrop shows greater abundance of interstratified sandstones and finer-grained beds. The southwestern section is characterised by darker bedsets that are likely coarser grained interstratified with finer-grained sandstones. The darker bedsets appear to comprise stacked trough-cross stratified bedsets. Finer-grained recessive intervals are not apparent in this section. The presence and scale of trough cross-stratification indicates that sediment was transported by the migration of sinuous crested dunes. Bedding geometries indicate sub-critical angles of climb. We examine the large-scale bedset architecture to evaluate the original depositional geometry of the Shaler sedimentary system, and consider its plausible depositional processes and paleoenvironmental setting. Finally, we consider its relationship to the sedimentary succession exposed

  7. Technical Note: n-Alkane lipid biomarkers in loess: post-sedimentary or syn-sedimentary?

    NASA Astrophysics Data System (ADS)

    Zech, M.; Kreutzer, S.; Goslar, T.; Meszner, S.; Krause, T.; Faust, D.; Fuchs, M.

    2012-07-01

    There is an ongoing discussion whether n-alkane biomarkers - and organic matter (OM) from loess in general - reflect a syn-sedimentary paleoenvironmental and paleoclimate signal or whether they are significantly a post-sedimentary feature contaminated by root-derived OM. We present first radiocarbon data for the n-alkane fraction of lipid extracts and for the first time luminescence ages for the Middle to Late Weichselian loess-paleosol sequence of Gleina in Saxony, Germany. Comparison of these biomarker ages with sedimentation ages as assessed by optically stimulated luminescence (OSL) dating shows that one n-alkane sample features a syn-sedimentary age (14C: 29.2 ± 1.4 kyr cal BP versus OSL: 27.3 ± 3.0 kyr). By contrast, the 14C ages derived from the other n-alkane samples are clearly younger (20.3 ± 0.7 kyr cal BP, 22.1 ± 0.7 kyr cal BP and 29.8 ± 1.4 kyr cal BP) than the corresponding OSL ages (26.6 ± 3.1 kyr, 32.0 ± 3.5 kyr and 45.6 ± 5.3 kyr). This finding suggests that a post-sedimentary n-alkane contamination presumably by roots has occurred. In order to estimate the post-sedimentary n-alkane contamination more quantitatively, we applied a 14C mass balance calculation based on the measured pMC (percent modern carbon) values, the calculated syn-sedimentary pMC values and pMC values suspected to reflect likely time points of post-sedimentary contamination (current, modern, 3 kyr, 6 kyr and 9 kyr). Accordingly, current and modern root-contamination would account for up to 7%, a 3 kyr old root-contamination for up to 10%, and an Early and Middle Holocene root-contamination for up to 20% of the total sedimentary n-alkane pool. We acknowledge and encourage that these first radiocarbon results need further confirmation both from other loess-paleosol sequences and for different biomarkers, e.g. carboxylic acids or alcohols as further lipid biomarkers.

  8. U-Pb detrital zircon analysis of pre-Timanian passive-margin successions and Caledonian nappes of North Norway

    NASA Astrophysics Data System (ADS)

    Zhang, Wen; Roberts, David; Pease, Victoria

    2014-05-01

    The Neoproterozoic passive-margin successions of the pre-Timanian margin, northern Norway, include the thick, deep-marine to deltaic, basinal Barents Sea Group and a fluvial to shallow-marine platformal domain to the south. To the west, different rock successions occur in the Lower, Middle and Upper Allochthons of the Norwegian Caledonides. Many detrital investigations of circum-Arctic terranes claim to recognize a Timanian 'fingerprint' (c. 610-560 Ma zircon ages from subduction-related granitoids generated during Timanian orogenesis), yet the detrital zircon U-Pb age spectrum of these sediments has not been fully assessed. Provenance analysis of pre-Timanian passive-margin formations and selected Caledonian nappe rocks is used to characterize their provenance. This will allow us to evaluate to what extent (if any) these passive-margin sediments have been recycled, to recognize them in younger sedimentary formations, and to possibly correlate the now widely distributed allochthonous fragments which occur throughout the circum-Arctic. Twelve samples were collected across four tectonic units. The principal results so far include: 1) A single sample (STP1) from the Late Ediacaran Stáhpogieddi Formation, Gaissa Nappe Complex (GNC), has a major peak at c. 550 Ma and is likely to represent deposition in the Timanian foreland basin. Another sample (BRE1) from the same region is much different with two major peaks at 2.8-2.7 Ga and 2.4 Ga whose significance remains to be determined. 2) Seven samples show classic Baltican affinity, including FUG1, GRN1 and GMS1 from parautochthonous/autochthonous formations in the Tanafjorden-Varangerfjorden Region (TVR), VEI1 and F-4 from formations lying unconformably upon in-situ Palaeoproterozoic- Archean metamorphic complexes, and LAN1 and IFJ1 from the Laksefjord Nappe Complex. Their provenance includes: i) age peaks at c. 2.8-2.7 Ga, indicating input from the northern Fennoscandian Shield which is dominated by Neoarchaean complexes

  9. Paleobiology of a Neoproterozoic tidal flat/lagoonal complex: the Draken Conglomerate Formation, Spitsbergen

    NASA Technical Reports Server (NTRS)

    Knoll, A. H.; Swett, K.; Mark, J.

    1991-01-01

    Carbonates and rare shales of the ca 700-800 Ma old Draken Conglomerate Formation, northeastern Spitsbergen, preserve a record of environmental variation within a Neoproterozoic tidal flat/lagoon complex. Forty-two microfossil taxa have been recognized in Draken rocks, and of these, 39 can be characterized in terms of their paleoenvironmental distributions along a gradient from the supratidal zone to permanently submerged lagoons. Supratidal to subtidal trends include: increasing microbenthic diversity, increasing abundance and diversity of included allochthonous (presumably planktonic) elements, decreasing sheath thickness of mat-building organisms (with significant taphonomic consequences), and an increasing sediment/fossil ratio in fossiliferous rocks. Five principal and several minor biofacies can be distinguished. The paleoecological resolution obtainable in the Draken Conglomerate Formation rivals that achieved for most Phanerozoic fossil deposits. It documents the complexity and diversity of Proterozoic coastal ecosystems and indicates that both environment and taphonomy need to be taken into explicit consideration in attempts to understand evolutionary trends in early fossil record. Three species, Coniunctiophycus majorinum, Myxococcoides distola, and M. chlorelloidea, are described as new; Siphonophycus robustum, Siphonophycus septatum, and Gorgonisphaeridium maximum are proposed as new combinations.

  10. Geochemistry of the Neoproterozoic metabasic rocks from the Negele area, southern Ethiopia: Tectonomagmatic implications

    NASA Astrophysics Data System (ADS)

    Yihunie, Tadesse; Adachi, Mamoru; Yamamoto, Koshi

    2006-03-01

    Neoproterozoic metabasic rocks along with metasediments and ultramafic rocks constitute the Kenticha and Bulbul lithotectonic domains in the Negele area. They occur as amphibolite and amphibole schist in the Kenticha, and amphibole schist and metabasalt in the Bulbul domains. These rocks are dominantly basaltic in composition and exhibit low-K tholeiitic characteristics. They are slightly enriched in large ion lithophile (LIL) and light rare earth (LRE) elements and depleted in high field strength (HFS) and heavy rare earth (HRE) elements. They exhibit chemical characteristics similar to back-arc basin and island-arc basalts, but include a few samples with slightly higher Y, Zr and Nb contents. Initial Sr isotopic ratios and ɛNd values for the Kenticha metabasic rocks range from 0.7048 to 0.7051 and from 4.7 to 9.6 whereas for the Bulbul metabasic rocks they range from 0.7032 to 0.7055 and from -0.1 to 5.5, respectively. The trace elements and Sr-Nd isotope compositions of samples from the Kenticha and Bulbul domains suggest similar, but isotopically heterogeneous magma sources. The magma is inferred to have derived from depleted source with a contribution from an enriched mantle source component.

  11. Concordant paleolatitudes for Neoproterozoic ophiolitic rocks of the Trinity Complex, Klamath Mountains, California

    USGS Publications Warehouse

    Mankinen, E.A.; Lindsley-Griffin, N.; Griffin, J.R.

    2002-01-01

    New paleomagnetic results from the eastern Klamath Mountains of northern California show that Neoproterozoic rocks of the Trinity ophiolitic complex and overlying Middle Devonian volcanic rocks are latitudinally concordant with cratonal North America. Combining paleomagnetic data with regional geologic and faunal evidence suggests that the Trinity Complex and related terranes of the eastern Klamath plate were linked in some fashion to the North American craton throughout that time, but that distance between them may have varied considerably. A possible model that is consistent with our paleomagnetic results and the geologic evidence is that the Trinity Complex formed and migrated parallel to paleolatitude in the basin between Laurasia and Australia-East Antarctica as the Rodinian supercontinent began to break up. It then continued to move parallel to paleolatitude at least through Middle Devonian time. Although the eastern Klamath plate served as a nucleus against which more western components of the Klamath Mountains province amalgamated, the Klamath superterrane was not accreted to North America until Early Cretaceous time.

  12. Possible organisms similar to Ediacaran forms from the Bhander Group, Vindhyan Supergroup, Late Neoproterozoic of India

    NASA Astrophysics Data System (ADS)

    De, Chirananda

    2003-01-01

    Fossil Medusoid genera resembling Ediacaria ( Sprigg 1947) and Hiemalora ( Fedonkin 1982) and having distinctive Ediacaran affinity have been discovered in a shale horizon occurring at the base of the Bhander Group, the uppermost unit of the Vindhyan Supergroup of central India. This is the first record of unequivocal occurrence of Ediacara-like fossils in a Proterozoic basin of the Peninsular India. This finding substantially extends the previously known biogeographic range of the Ediacaran elements to Peninsular India and further enhances their biostratigraphpic potential for correlation of the upper Vindhyans with some Ediacaran horizons of Canada, Australia, South Africa and Russian Platforms. With this extension, the representatives of the genus Ediacaria can be regarded as having a global distribution in places now occupying both lower and higher latitudes. The genus Hiemalora, which appeared to be endemic to the Russian block, also has wide biogeographic coverage. These fossils assign an Ediacaran (550-543 Ma) age for the host Lakheri Limestone and suggest that the Lakheri unit was deposited within 6 million years of the Precambrian-Cambrian boundary. They also support and refine the traditional view of the Late Neoproterozoic age for the lower Bhander Group. These fossils provide positive stratigraphic clues for locating the Precambiran-Cambrian boundary strata in the overlying Lakheri-Sirbu segment of the Vindhyan sequence. They also indicate a depositional environment typical of a muddy shallow shelf setting above storm wave-base.

  13. Co-evolution of eukaryotes and ocean oxygenation in the Neoproterozoic era

    NASA Astrophysics Data System (ADS)

    Lenton, Timothy M.; Boyle, Richard A.; Poulton, Simon W.; Shields-Zhou, Graham A.; Butterfield, Nicholas J.

    2014-04-01

    The Neoproterozoic era (about 1,000 to 542 million years ago) was a time of turbulent environmental change. Large fluctuations in the carbon cycle were associated with at least two severe -- possible Snowball Earth -- glaciations. There were also massive changes in the redox state of the oceans, culminating in the oxygenation of much of the deep oceans. Amid this environmental change, increasingly complex life forms evolved. The traditional view is that a rise in atmospheric oxygen concentrations led to the oxygenation of the ocean, thus triggering the evolution of animals. We argue instead that the evolution of increasingly complex eukaryotes, including the first animals, could have oxygenated the ocean without requiring an increase in atmospheric oxygen. We propose that large eukaryotic particles sank quickly through the water column and reduced the consumption of oxygen in the surface waters. Combined with the advent of benthic filter feeding, this shifted oxygen demand away from the surface to greater depths and into sediments, allowing oxygen to reach deeper waters. The decline in bottom-water anoxia would hinder the release of phosphorus from sediments, potentially triggering a potent positive feedback: phosphorus removal from the ocean reduced global productivity and ocean-wide oxygen demand, resulting in oxygenation of the deep ocean. That, in turn, would have further reinforced eukaryote evolution, phosphorus removal and ocean oxygenation.

  14. Calcified microbes in Neoproterozoic carbonates: implications for our understanding of the Proterozoic/Cambrian transition

    NASA Technical Reports Server (NTRS)

    Knoll, A. H.; Fairchild, I. J.; Swett, K.

    1993-01-01

    Tidal flat and lagoonal dolostones of the Neoproterozoic Draken Formation, Spitsbergen, exhibit excellent preservation of carbonate fabrics, including heavily calcified microfossils. The crust-forming cyanobacterium Polybessurus is preserved locally by carbonate precipitated on and within sheaths in mildly evaporitic upper intertidal to supratidal environments. In contrast, calcified filaments in columnar stromatolites reflect subtidal precipitation. Filament molds in dolomicrites independently document extremely early lithification. The presence of heavily calcified cyanobacteria in Draken and other Proterozoic carbonates constrains potential explanations for the widespread appearance of calcified microorganisms near the Proterozoic-Cambrian boundary. We propose that the rarity of Proterozoic examples principally reflects the abundance and wide distribution of carbonate crystals precipitated on the sea floor or in the water column. Cyanobacterial sheaths would have competed effectively as sites for carbonate nucleation and growth only where calcitic and/or aragonitic nuclei were absent. In this view, the Proterozoic-Cambrian expansion of calcified microfossils primarily reflects the emergence of skeletons as principal agents of carbonate deposition.

  15. Martian Sedimentary Basins and Central Mound Formation

    NASA Astrophysics Data System (ADS)

    Bennett, K. A.; Bell, J. F., III

    2014-12-01

    Central mounds on Mars are observed as sedimentary deposits within crater interiors, but the specific processes responsible for their formation and subsequent modification are still debated. The deposits are hypothesized to have been created by either subaerial or subaqueous processes through one of two general formation mechanisms. The prevailing hypothesis suggests that after their craters were formed, sediment filled the entire crater and was later eroded into the morphologies we observe today. Alternatively, the sediment could have been deposited as the features we observe today without any significant erosion contributing to their mound shape. We conducted a survey of central mounds that occur within craters larger than 25 km in diameter located between ± 60° latitude on Mars. We use mound locations, mound offsets within their host craters, and mound heights to address various mound formation hypotheses. The results of this survey support the hypothesis that mound sediment once filled the entire host crater and was later eroded into the features we observe today. We propose that large Martian impact craters act as simplistic sedimentary basins. These basins "catch" any sediment that is being transported through the region. Any geologic process that involves transport of material (airfall dust, explosive volcanism, impact ejecta, etc.) could have contributed to the growth of this sediment fill, although the dominant process could vary based on location. During this depositional phase, several processes (ice/frost, water, etc.) could have cemented the material; then, at some point, the environment changed from depositional to erosional, leading to the formation of isolated mounds of sediment within these craters. Our study reveals that most mounds are offset from the center of their host crater in the same direction as the regional winds. For example, the mounds in Arabia Terra are offset towards the western portion of their craters. This observation is

  16. Stochastic Particle Tracking in Fractured Sedimentary Formations

    NASA Astrophysics Data System (ADS)

    Willmann, Matthias; Kinzelbach, Wolfgang

    2014-05-01

    Particle tracking simulations are very useful tools to assess transport behavior in deep subsurface formations. Unfortunately, those formations are often fractured. And particle tracking accounting for fracture and matrix transport simultaneously are conceptually complex and difficult to implement. Major problems are that particles moved within the matrix might jump over fractures, and the unclear nature of exchange between fractures and matrix. Due to these difficulties transport simulations are most often reduced to pure fracture transport. The matrix contribution is either ignored or approximated by using a retention mechanism like matrix diffusion. In crystalline rocks this appears to be a reasonable assumption, but in sedimentary rocks should be studied without any a priori assumption on the type of matrix transport. For sedimentary rocks advective transport within the matrix is expected to influence strongly the overall transport behavior. We developed a stochastic particle tracking method that models transport explicitly in both fractures and matrix. Similar to most flow simulators we conceptualize transport as a superposition of two separate domains, the fracture and the matrix domain, which exchange particles between them. But in our case this exchange is possible at each position within the fractures and not only at the nodes of the fracture. We restrict ourselves to an orthogonal grid for the matrix and here we allow only that fractures lay on the sides of individual matrix cells. This leads to step-like fractures with an enlarged path length inside the fractures. But it also enables us to use the Pollock method without major modifications. Now we calculate the position where a particle leaves an individual matrix cell. At a cell face a check is performed whether a fracture is present. This avoids a time consuming search for fractures at each point of the particle path. The time a particle stays in a fracture before being released again to the matrix

  17. Sedimentary environments and Pleistocene chronology of the Botany Basin, N.S.W., Australia

    NASA Astrophysics Data System (ADS)

    Albani, A. D.

    1981-09-01

    The sedimentary sequence of the Pleistocene deposits of the Botany Basin were investigated using borehole samples and seismic data. A succession of environments from marine to terrestrial, separated by erosional surfaces, were recognized and, although absolute dating is not possible, a relative “minimum” chronology was established correlating erosional surfaces with sea-level fluctuations. Seismic surveys and borehole material from other sites indicate that the model presented here is applicable to other estuaries of N.S.W.

  18. Archean sedimentary systems and crustal evolution

    NASA Technical Reports Server (NTRS)

    Lowe, D. R.

    1985-01-01

    Current knowledge of preserved Archean sedimentary rocks suggests that they accumulated in at least three major depositional settings. These are represented generally by sedimentary units: (1) in early Archean, pre-3.0 Ga old greenstone belts, (2) on late Archean sialic cratons, and (3) in late Archean, post-3.0 Ga old greenstone belts. Research suggests that the Archean was characterized by at least two distinctive and largely diachronous styles of crustal evolution. Thick, stable early Archean simatic platforms, perhaps analogous to modern oceanic islands formed over hot spots, underwent a single cycle of cratonization to form stable continental blocks in the early Archean. Later formed Archean continents show a two stage evolution. The initial stage is reflected in the existence of older sialic material, perhaps representing incompletely cratonized areas or microcontinents of as yet unknown origin. During the second stage, late Archean greenstone belts, perhaps analogous to modern magmatic arcs or back arc basins, developed upon or adjacent to these older sialic blocks. The formation of this generation of Archean continents was largely complete by the end of the Archean. These results suggest that Archean greenstone belts may represent a considerable range of sedimentological and tectonic settings.

  19. Tunnel boring machine performance in sedimentary rock

    SciTech Connect

    Nelson, P.

    1983-01-01

    Full-face tunnel boring machine (TBM) performance during the excavation of six tunnels is considered in terms of utilization, penetration rate, and cutter wear. Construction records for over 75,000 ft (22,860m) of tunnel in sedimentary rock are analyzed, and the results are used to investigate factors affecting TBM performance. Machine utilization is strongly affected by site specific conditions, including geology, construction planning, and contractor practice. The relative importance of each of 21 downtime causes is discussed, and recommendations are made for modifications in excavation system design which could help to reduce delays. Effects of machine operation rate were investigated. The interrelationship among penetration, thrust, and rolling force is analyzed with a three-dimensional model which provides a rational basis for explaining variations in cutter forces and penetration rate as a function of rock type. The most useful rock index for estimating TBM performance in sedimentary rock is shown to be a combination of Schmidt Hammer rebound and abrasion hardness. Variation in cutter wear is considered as a function of position on the cutterhead and the rock type being excavated. Rolling distances for center cutters are less sensitive to rock type than for other positions. A fracture mechanics approach, of use in modeling the process chip formation, is proposed. The use of fracture material properties for empirical prediction of TBM performance is reported. Recommendations are made for future work, and observations and records required for future performance evaluations are summarized.

  20. The sedimentary basins of Tanzania - reviewed

    NASA Astrophysics Data System (ADS)

    Mbede, E. I.

    The sedimentary basins of Tanzania have been classified into four morphotectonic groups: the coastal basin, the Karoo rift basins, basins found within the present East African rift valley and the cratonic sag basins. Except for the cratonic sag basins, each of these basin group has been affected by rifting at one time or another. The geology of each basin is discussed, structural evolution is evaluated and the prospectivity is thence looked into. Coal is exploited at Songwe-Kiwira coalfield and is found in potentially economic quantities in other Karoo basins. Prospecting for hydrocarbon resources has been going on since the 50s. Gas has been discovered in Songosongo and Mnazi bay fields, uneconomical quantities of oil have also been reported in Songosongo. Being basically rift basins which have reached different stages of development, source rocks normally associated with Initial-rifting, synrifting as well as post-rifting processes are probably well developed. Reservoir rocks, traps and cap rocks are normally not rare in such tectonic environments. Thermal gradients associated with the rifting stage are normaly high to effect maturation of source rocks even at low sedimentary thicknesses. Studies done so far are still inconclusive, because while testing has mainly been focused on structural traps stratigraphic traps seems to be more promising.

  1. n-Alkane lipid biomarkers in loess: post-sedimentary or syn-sedimentary?

    NASA Astrophysics Data System (ADS)

    Zech, Michael; Kreutzer, Sebastian; Goslar, Tomasz; Meszner, Sascha; Krause, Tobias; Faust, Dominik; Fuchs, Markus

    2013-04-01

    There is an ongoing discussion whether n-alkane biomarkers - and organic matter (OM) from loess in general - reflect a syn-sedimentary paleoenvironmental and paleoclimate signal or whether they are significantly a post-sedimentary feature contaminated by root-derived OM (Zech et al., 2012, 2013; Wiesenberg and Gocke, 2013). We present first radiocarbon data for the n-alkane fraction of lipid extracts and for the first time luminescence ages for the Middle to Late Weichselian loess-paleosol sequence of Gleina in Saxony, Germany. Comparison of these biomarker ages with sedimentation ages as assessed by optically stimulated luminescence (OSL) dating shows that one n-alkane sample features a syn-sedimentary age (14C: 29.2 ± 1.4 kyr calBP versus OSL: 27.3 ± 3.0 kyr). By contrast, the 14C ages derived from the other n-alkane samples are clearly younger (20.3 ± 0.7 kyr calBP, 22.1 ± 0.7 kyr calBP and 29.8 ± 1.4 kyr calBP) than the corresponding OSL ages (26.6 ± 3.1 kyr, 32.0 ± 3.5 kyr and 45.6 ± 5.3 kyr). This finding suggests that a post-sedimentary n-alkane contamination presumably by roots has occurred. In order to estimate the post-sedimentary n-alkane contamination more quantitatively, we applied a 14C mass balance calculation based on the measured pMC (percent modern carbon) values, the calculated syn-sedimentary pMC values and pMC values suspected to reflect likely time points of post-sedimentary contamination (modern, last decades, 3 kyr, 6 kyr and 9 kyr). Accordingly, modern and last decadal root-contamination would account for up to 7%, a 3 kyr old root-contamination for up to 10%, and an Early and Middle Holocene root-contamination for up to 20% of the total sedimentary n-alkane pool. We acknowledge and encourage that these first radiocarbon results need further confirmation both from other loess-paleosol sequences and for different biomarkers, e.g. carboxylic acids or alcohols as further lipid biomarkers. Zech, M., Kreutzer, S., Goslar, T., Meszner, S

  2. Areal distribution of sedimentary facies determined from seismic facies analysis and models of modern depositional systems

    SciTech Connect

    Seramur, K.C.; Powell, R.D.; Carpenter, P.J.

    1988-02-01

    Seismic facies analysis was applied to 3.5-kHz single-channel analog reflection profiles of the sediment fill within Muir Inlet, Glacier Bay, southeast Alaska. Nine sedimentary facies have been interpreted from seven seismic facies identified on the profiles. The interpretations are based on reflection characteristics and structural features of the seismic facies. The following reflection characteristics and structural features are used: reflector spacing, amplitude and continuity of reflections, internal reflection configurations, attitude of reflection terminations at a facies boundary, body geometry of a facies, and the architectural associations of seismic facies within each basin. The depositional systems are reconstructed by determining the paleotopography, bedding patterns, sedimentary facies, and modes of deposition within the basin. Muir Inlet is a recently deglaciated fjord for which successive glacier terminus positions and consequent rates of glacial retreat are known. In this environment the depositional processes and sediment characteristics vary with distance from a glacier terminus, such that during a retreat a record of these variations is preserved in the aggrading sediment fill. Sedimentary facies within the basins of lower Muir Inlet are correlated with observed depositional processes near the present glacier terminus in the upper inlet. The areal distribution of sedimentary facies within the basins is interpreted using the seismic facies architecture and inferences from known sediment characteristics proximal to present glacier termini.

  3. Quantitative compositional analysis of sedimentary materials using thermal emission spectroscopy: 1. Application to sedimentary rocks

    NASA Astrophysics Data System (ADS)

    Thorpe, Michael T.; Rogers, A. Deanne; Bristow, Thomas F.; Pan, Cong

    2015-11-01

    Thermal emission spectroscopy is used to determine the mineralogy of sandstone and mudstone rocks as part of an investigation of linear spectral mixing between sedimentary constituent phases. With widespread occurrences of sedimentary rocks on the surface of Mars, critical examination of the accuracy associated with quantitative models of mineral abundances derived from thermal emission spectra of sedimentary materials is necessary. Although thermal emission spectroscopy has been previously proven to be a viable technique to obtain quantitative mineralogy from igneous and metamorphic materials, sedimentary rocks, with natural variation of composition, compaction, and grain size, have yet to be examined. In this work, we present an analysis of the thermal emission spectral (~270-1650 cm-1) characteristics of a suite of 13 sandstones and 14 mudstones. X-ray diffraction and traditional point counting procedures were all evaluated in comparison with thermal emission spectroscopy. Results from this work are consistent with previous thermal emission spectroscopy studies and indicate that bulk rock mineral abundances can be estimated within 11.2% for detrital grains (i.e., quartz and feldspars) and 14.8% for all other mineral phases present in both sandstones and mudstones, in comparison to common in situ techniques used for determining bulk rock composition. Clay-sized to fine silt-sized grained phase identification is less accurate, with differences from the known ranging from ~5 to 24% on average. Nevertheless, linear least squares modeling of thermal emission spectra is an advantageous technique for determining abundances of detrital grains and sedimentary matrix and for providing a rapid classification of clastic rocks.

  4. The Namuskluft and Dreigratberg sections in southern Namibia (Kalahari Craton, Gariep Belt): a geological history of Neoproterozoic rifting and recycling of cratonic crust during the dispersal of Rodinia until the amalgamation of Gondwana

    NASA Astrophysics Data System (ADS)

    Hofmann, Mandy; Linnemann, Ulf; Hoffmann, Karl-Heinz; Gerdes, Axel; Eckelmann, Katja; Gärtner, Andreas

    2014-07-01

    magmas generated from Mesoproterozoic crustal material show more juvenile ɛHf( t) signatures of +14, +8 to +4 with TDM model ages of 1.05-1.6 Ga. During Neoproterozoic deposition, only old cratonic crust with an inherited continental arc signature was available in the source area clearly demonstrated by Hf isotope composition of detrital zircon and geochemical bulk analysis of sedimentary rocks. The granodiorites of the Palaeoproterozoic basement underlying Namuskluft section are ca. 1.9 Ga old and show ɛHf( t) signatures of -3 to -5.5 with TDM model ages of 2.4-2.7 Ga. These basement rocks demonstrate the extreme uplift and deep erosion of the underlying Kalahari Craton at its western margin before general subsidence during Cryogenian and Ediacaran time. The sedimentary sequence of the two examined sections (Namuskluft and Dreigratberg) proposes the presence of a basin and an increasing subsidence at the SW part of the Kalahari Craton during the Cryogenian. Therefore, we propose the initial formation of an intra-cratonic sag basin during the Lower Cryogenian that evolved later to a rift basin at the cratonic margin due to increasing crustal tension and rifting together with the opening of the Adamastor Ocean. As the zircons of the sedimentary rocks filling this basin show neither rift-related U/Pb ages nor an exotic craton as a possible source area, the only plausible sedimentary transport direction providing the found U/Pb ages would be from the E or the SE, directly from the heart of the Kalahari Craton. Due to subsidence and ongoing sedimentation from E/SE directions, the rift-related magmatic rocks were simply covered by the input of old intra-cratonic material that explains the absence of Neoproterozoic zircon grains in our samples. The geochemical analyses show the erosion of a continental arc and related sedimentary rocks with an overall felsic provenance. The source area was a deeply eroded and incised magmatic arc that evolved on continental crust, without

  5. Geochemical constraints on the evolution of mafic and felsic rocks in the Bathani volcanic and volcano-sedimentary sequence of Chotanagpur Granite Gneiss Complex

    NASA Astrophysics Data System (ADS)

    Saikia, Ashima; Gogoi, Bibhuti; Ahmad, Mansoor; Ahmad, Talat

    2014-06-01

    The Bathani volcanic and volcano-sedimentary (BVS) sequence is a volcanic and volcano-sedimentary sequence, best exposed near Bathani village in Gaya district of Bihar. It is located in the northern fringe of the Chotanagpur Granite Gneiss Complex (CGGC). The volcano-sedimentary unit comprises of garnet-mica schist, rhyolite, tuff, banded iron formation (BIF) and chert bands with carbonate rocks as enclaves within the rhyolite and the differentiated volcanic sequence comprises of rhyolite, andesite, pillow basalt, massive basalt, tuff and mafic pyroclasts. Emplacement of diverse felsic and mafic rocks together testifies for a multi-stage and multi-source magmatism for the area. The presence of pillow basalt marks the eruption of these rocks in a subaqueous environment. Intermittent eruption of mafic and felsic magmas resulted in the formation of rhyolite, mafic pyroclasts, and tuff. Mixing and mingling of the felsic and mafic magmas resulted in the hybrid rock andesite. Granites are emplaced later, cross-cutting the volcanic sequence and are probably products of fractional crystallization of basaltic magma. The present work characterizes the geochemical characteristics of the magmatic rocks comprising of basalt, andesite, rhyolite, tuff, and granite of the area. Tholeiitic trend for basalt and calc-alkaline affinities of andesite, rhyolite and granite is consistent with their generation in an island arc, subduction related setting. The rocks of the BVS sequence probably mark the collision of the northern and southern Indian blocks during Proterozoic period. The explosive submarine volcanism may be related to culmination of the collision of the aforementioned blocks during the Neoproterozoic (1.0 Ga) as the Grenvillian metamorphism is well established in various parts of CGGC.

  6. "Sands of Pangea"-analysing the supercontinent formation and the Mesozoic sedimentary record using LA ICP MS U Pb zircon dating on samples from Germany-

    NASA Astrophysics Data System (ADS)

    Hofmann, Mandy; Linnemann, Ulf; Gerdes, Axel; Voigt, Thomas

    2013-04-01

    The Elbe Zone at the northern Bohemian Massif (part of the Central European Variscides, Saxony, Germany) contains important structures and outcrops that help to understand the final pulse of the Variscan Orogeny in Europe leading to the formation of supercontinent Pangea. We will present zircon U-Pb data from that area that allow the timing of the final stage of these movements and of related plutonic, volcano-sedimentary and tectonic processes. In addition, we will show U-Pb detrital zircon ages of sandstones from the Triassic, Jurassic and Cretatceous of Germany to draw conclusions about the sedimentary record and the source areas (provenance analyses) of these "sands of Pangea". We have analysed detrital zircon grains from the Buntsandstein (Lower Triassic) and the Keuper (Upper Triassic) regarding their U-Pb ages. These analyses indicate different zircon ages with a main peak at ca. 250 Ma to ca. 700 Ma. Distinct zircon grains of Meso- and Paleoproterozoic ages were found. In addition to the Triassic samples we analysed detrital zircon grains from the Middle Jurassic (Dogger) and the Cretaceous. The Cretaceous samples show similar ages as the Triassic ones: the main peak of zircon ages lies between ca. 240 Ma and 700 Ma. Also, there are a few isolated zircon grains with Meso- to Paleoproterozoic ages. A real change shows the Jurassic Sandstone, as the zircons of this sample have main ages at ca. 950 Ma to 1900 Ma. The zircon ages show, that the source areas for the Mesozoic sedimentary record changed clearly. We interpret the Paleozoic to Neoproterozoic ages of all samples as the influx of reworked local material, such as the Avalonian/Armorican basement units and the Variscan Basement. In our interpretation, the enormous amounts of Mesoproterozoic to upper Paleoproterozoic zircon ages in the Jurassic sample originated in the oceanic connection between Middle Europe and Baltica, as these specific zircon ages are typical for Baltica. This oceanic connection was

  7. Generation of continental crust in the northern part of the Borborema Province, northeastern Brazil, from Archaean to Neoproterozoic

    NASA Astrophysics Data System (ADS)

    de Souza, Zorano Sérgio; Kalsbeek, Feiko; Deng, Xiao-Dong; Frei, Robert; Kokfelt, Thomas Find; Dantas, Elton Luiz; Li, Jian-Wei; Pimentel, Márcio Martins; Galindo, Antonio Carlos

    2016-07-01

    (TZ). Early Neoproterozoic volcanism at 1091 Ma, and A-type plutonism, from 920 to 775 Ma, mark the intracontinental magmatism in the TZ. In the Seridó Domain, the Late Neoproterozoic registers several events of plutonism, at 600-593, 575-560, 548-533, 528-510, 495-450 Ma. These rocks cover ca. 15% of the area, while Neoproterozoic supracrustal rocks cover ca. 30%. The most important magmatic event is that at 575 Ma, consistent with the peak of widespread transpression and synchronous high temperature metamorphism. The Neoproterozoic rocks are mostly K-enriched alkaline or transitional to calc-alkaline. Inherited Archaean and Palaeoproterozoic zircons and Nd model ages, as well as moderate to strongly negative (-21 to -9) epsilon Nd, and persistent negative anomalies for Ta-Nb, Ti and P indicate significant crustal contributions in their genesis. While a convergent setting (subduction zone) could explain the Archaean and Palaeoproterozoic units, this is not so for the Neoproterozoic rocks which mimic the geochemical and isotopic features of the older sources. In the study area, the peak of juvenile accretion (mantle derived magmas) took place in the Archaean (3.4-2.7 Ga) and Palaeproterozoic (2.4-2.11 Ga), whereas crustal recycling predominated in the Neoproterozoic.

  8. Micropaleontology and chemostratigraphy of the Neoproterozoic Mbuji-Mayi Supergroup, Democratic Republic of Congo.

    NASA Astrophysics Data System (ADS)

    Baludikay, Blaise; Bekker, Andrey; Baudet, Daniel; Asael, Dan; Storme, Jean-Yves; Javaux, Emmanuelle

    2014-05-01

    The Mbuji-Mayi Supergroup, deposited between 1170 ± 22 Ma and ca. 800 Ma [1], outcrops in the eastern Oriental Kasai Province and western Katanga Province of the Democratic Republic of Congo. It is the youngest Precambrian unit of the Kasai block and was deposited in the SE-NW trending failed-rift Sankuru-Mbuji-Mayi-Lomami-Lovoy basin filled with siliciclastic and carbonate sediments. In the northern part of this basin (Oriental Kasai Province), the Mbuji-Mayi Supergroup rests unconformably upon the Archean Dibaya Granite Complex, but in the southern part (northeastern Katanga Province), it overlies the Mesoproterozoic Kibaran Supergroup. The Supergroup is divided into two groups: the lower, ~ 500-m thick siliciclastics-rich BI Group and the upper, ~ 1000-m thick carbonate-rich BII Group. Our own and previous sedimentological observations [2] indicate facies ranging from subtidal, low-energy stromatolitic environments to overlying intertidal to supratidal evaporitic settings of lagoon and sabkha. In order to characterize the diversity of microfossil assemblages, their paleobiology and paleoecology as well as redox conditions in their depositional setting, we have sampled three drill cores (KAFUKU 15, B13 KANSHI, and S70 LUBI) from the collections of the Royal Museum for Central Africa (RMAC). Our biostratigraphic and chemostratigraphic data also provide further constraints on the age of the Mbuji-Mayi Supergroup. Here we present preliminary data on microfossil diversity from the Kanshi drill core and carbon isotope chemostratigraphy for all three drill cores. The well-preserved and diverse assemblage of acritarchs and filamentous forms includes prokaryotes and eukaryotes, and is similar to other coeval assemblages described worldwide outside of Africa. The presence of the acanthomorph acritarch Trachyhystrichosphaera aimika is significant as it is indicative of the late Meso- to early Neoproterozoic age elsewhere, and is reported for the first time in Central

  9. An O-17 record of Neoproterozoic snowball Earth in Kimberley, Western Australia

    NASA Astrophysics Data System (ADS)

    Bao, H.; Chen, Z.; Zhou, C.

    2011-12-01

    Non-mass-dependently 17O-depleted signatures have recently been found in sulfate associated with the aftermath of Marinoan glacial meltdown at ca. 635 million years ago. The anomaly was proposed to be inherited from atmospheric O2 via oxidative weathering of sulfides. An extremely high pCO2 atmosphere could produce such an anomaly. The finding has become one of the strongest lines of evidence supporting a "hard" snowball Earth hypothesis. Further studies linking the Δ17O of barite to sedimentological-geological context in Marinoan South China confirm that the 17O-depleted sulfate was derived from oxidative weathering and the anomalous 17O signal was detectable at sites close to paleo-continental runoffs while diluted and often undetectable in the open oceans. On the other hand, host minerals or rocks for the Marinoan 17O anomalies have been limited to barite (South China and West Africa) and carbonate-associated sulfate (CAS) in limestone lenses within the diamictite (Svalbard). If the 17O depletion is indeed closely related to an extraordinary atmospheric condition associated with the great Marinoan meltdown, the signal should be global in its distribution. Kimberley region of Western Australia was close to a continent in late Neoproterozoic, according to Rodinia reconstruction and regional geology. A confirmation of this anomalous signal in Australia is a critical test to our hypothesis. We report here that the Δ17O of carbonate-associated sulfate reaches as low as -0.68% in the dolostones draping the Neoproterozoic Moonlight-Valley (MLV) diamictite, Texas/Mabel Downs, Kimberley, Western Australia. The magnitude of the 17O depletion gradually decreases (i.e. approaching normal) when going up towards the overlying Ranford sandstone. A positive correlation between the δ34S and the Δ17O exists, as does the case in South China and Svalbard. The CAS in the MLV cap dolostones in the Palm Spring section, however, does not bear distinct 17O depletion. Neither does

  10. Geochronological constraints on terminal Neoproterozoic events and the rise of Metazoan

    NASA Astrophysics Data System (ADS)

    Bowring, S.; Myrow, P.; Landing, E.; Ramezani, J.; Grotzinger, J.

    2003-04-01

    A full understanding of Neoproterozoic history has been plagued by a lack of precise geochronological constraints. In particular the correlation and duration of global or "Snowball" glaciations has relied on physical stratigraphy and chemostratigraphy which has given rise to much debate about the number, magnitude, and duration of glacial deposits. New U-Pb constraints on the age of the Gaskiers Formation glacial deposits in Newfoundland, an age for the oldest Ediacaran fossils in the same area, and the age and significance of the Cambrian/Precambrian boundary in Oman have important implications for global correlation and the timing of the rise of Metazoans. In the central and eastern parts of the Avalon Peninsula, southeastern Newfoundland, the oldest rocks are arc-related tuffs, agglomerates, and flows of the Harbour Main Group (>1.5 km thick) that have published dates from 606-630 Ma. These are overlain by approximately 7.5 km of marine siliciclastic rocks of the Conception Group. Over 2,300 m of deep-water deposits of the Mall Bay and Drook formations are separated by the regionally extensive glacial diamictite of the Gaskiers Formation (up to 300 m-thick). This unit is often described as a Varanger-age glaciomarine deposit and is locally overlain by a thin cap carbonate bed with a highly negative C isotopic signature. Thin (1-15 cm) silicic ash beds are found interlayered with turbidites just below and above the glacial deposits and the glacial deposits locally contain volcanic bombs, pyroclastic debris, and lavas. U-Pb geochronology of zircons separated from ash-beds below, within, and above the glacial deposits indicates that they are ca 580 Ma. This is considerably younger than previous estimates and calls into question many of the global correlations with similar rocks. An ash-bed within the overlying Drook formation is preserved in depositional contact with spectacular surfaces exposing Ediacaran fossils. Zircons separated from it yield an age of 575 Ma

  11. Is high obliquity a still plausible explanation for Neoproterozoic low-latitude glaciations ?

    NASA Astrophysics Data System (ADS)

    Levrard, B.; Laskar, J.

    2003-04-01

    The observations of enigmatic low-latitude glaciogenic sequences during the Sturtian (˜ 750-700 Ma) and the Varanger (˜ 620-570 Ma) glacial intervals of the Neopropterozo& uml;i c era remain the subject of controversial debates concerning possible scenarii. Of the many models that have been proposed to account for theses paradoxical features, the high-obliquity scenario invoked by G.E. Williams (1975) appears to be still largely considered. However, a such scenario requires a dissipative mechanism to bring back the Earth's obliquity from a value higher than 55o to the present value (˜ 23.5o) in less than 200 Ma. Williams (1993) suggested that core-mantle friction could have explained this substantial decrease. However, it was demonstrated by Néron de Surgy and Laskar (1997) and confirmed by Pais et al. (1999) that, not only it requires abnormal values of effective viscosity, but due to the conservation of the angular momentum, it is also largely conflicting with the paleorotation data. D.M Williams et al. (Nature, 1998) recently proposed that ``climate friction" could have produced this decrease in less than 100 Ma between ˜ 600 Ma and 500 Ma. We have revisited in details this mecanism (Levrard and Laskar, submitted to Geo. J. Int., 2002) for the Neoproterozoic glaciations. In response to periodic variations in the obliquity, the redistribution of ice/water mass and the isostatic adjusment to the surface loading affect the dynamical ellipticity of the Earth. Delayed responses in the mass redistribution may introduce a secular term in the obliquity evolution (Bills, 1994; Rubincam, 1995; Ito et al, 1995). We analyze the obliquity-oblateness feedback using non-linear response of ice sheets (Imbrie and Imbrie, 1980) to insolation forcing and layered models with Maxwell visco-elastic rheology. Possible increase in the non-linear response of ice sheets to insolation forcing and latitudinal changes in this forcing strongly limit the contribution of the obliquity

  12. Neoproterozoic, Paleozoic, and Mesozoic granitoid magmatism in the Qinling Orogen, China: Constraints on orogenic process

    NASA Astrophysics Data System (ADS)

    Wang, Xiaoxia; Wang, Tao; Zhang, Chengli

    2013-08-01

    The Qinling Orogen is one of the main orogenic belts in Asia and is characterized by multi-stage orogenic processes and the development of voluminous magmatic intrusions. The results of zircon U-Pb dating indicate that granitoid magmatism in the Qinling Orogen mainly occurred in four distinct periods: the Neoproterozoic (979-711 Ma), Paleozoic (507-400 Ma), and Early (252-185 Ma) and Late (158-100 Ma) Mesozoic. The Neoproterozoic granitic magmatism in the Qinling Orogen is represented by strongly deformed S-type granites emplaced at 979-911 Ma, weakly deformed I-type granites at 894-815 Ma, and A-type granites at 759-711 Ma. They can be interpreted as the products of respectively syn-collisional, post-collisional and extensional setting, in response to the assembly and breakup of the Rodinia supercontinent. The Paleozoic magmatism can be temporally classified into three stages of 507-470 Ma, 460-422 Ma and ˜415-400 Ma. They were genetically related to the subduction of the Shangdan Ocean and subsequent collision of the southern North China Block and the South Qinling Belt. The 507-470 Ma magmatism is spatially and temporally related to ultrahigh-pressure metamorphism in the studied area. The 460-422 Ma magmatism with an extensive development in the North Qinling Belt is characterized by I-type granitoids and originated from the lower crust with the involvement of mantle-derived magma in a collisional setting. The magmatism with the formation age of ˜415-400 Ma only occurred in the middle part of the North Qinling Belt and is dominated by I-type granitoid intrusions, and probably formed in the late-stage of a collisional setting. Early Mesozoic magmatism in the study area occurred between 252 and 185 Ma, with the cluster in 225-200 Ma. It took place predominantly in the western part of the South Qinling Belt. The 250-240 Ma I-type granitoids are of small volume and show high Sr/Y ratios, and may have been formed in a continental arc setting related to subduction

  13. Post-collisional potassic granitoids from the southern and northwestern parts of the Late Neoproterozoic East African Orogen: a review

    NASA Astrophysics Data System (ADS)

    Küster, Dirk; Harms, Ulrich

    1998-12-01

    Potassic metaluminous granitoids with enrichments of HFS elements constitute part of widespread post-collisional magmatism related to the Late Neoproterozoic Pan-African orogeny in northeastern Africa (Sudan, Ethiopia, Somalia) and Madagascar. The plutons were emplaced between 580 and 470 Ma and comprise both subsolvus and hypersolvus biotite-granite, biotite-hornblende-granite, quartz-monzonite and quartz-syenite. Pyroxene-bearing granitoids are subordinate. Basic dikes and enclaves of monzodioritic composition are locally associated with the granitoid plutons. Granitoids emplaced in pre-Neoproterozoic crust have Sr i-ratios between 0.7060 and 0.7236 and ɛNd( t) values between -15.8 and -5.6 while those emplaced in, or close to the contact with, juvenile Neoproterozoic crust have lower Sr i-ratios (0.7036-0.7075) and positive ɛNd( t) values (4.6). However, it is unlikely that the potassic granitoids represent products of crustal melting alone. The association with basic magmas derived from subduction-modified enriched mantle sources strongly suggests that the granitoids represent hybrid magmas produced by interaction and mixing of mantle and crust derived melts in the lower crust. The most intense period of this potassic granitoid magmatism occurred between 585 and 540 Ma, largely coeval with HT granulite facies metamorphism in Madagascar and with amphibolite facies retrogression in northeastern Africa (Somalia, Sudan). Granitoid magmatism and high-grade metamorphism are probably both related to post-collisional lithospheric thinning, magmatic underplating and crustal relaxation. However, the emplacement of potassic granites continued until about 470 Ma and implies several magmatic pulses associated with different phases of crustal uplift and cooling. The potassic metaluminous granites are temporally and spatially associated with post-collisional high-K calc-alkaline granites with which they share many petrographical, geochemical and isotopical similarities

  14. Neoproterozoic structural evolution of the NE-trending Ad-Damm Shear Zone, Arabian Shield, Saudi Arabia

    NASA Astrophysics Data System (ADS)

    Hamimi, Zakaria; El-Sawy, El-Sawy K.; El-Fakharani, Abdelhamid; Matsah, Mohamed; Shujoon, Abdulrahman; El-Shafei, Mohamed K.

    2014-11-01

    The Ad-Damm Shear Zone (AdSZ) is a major NE- (to NNE-) trending fault zone separating Jiddah and Asir tectonic terranes in the Neoproterozoic Juvenile Arabian Shield (AS). AdSZ is characterized by the development of dextral transcurrent shear-sense indicators and moderately to steeply NW plunging stretching lineations. It is mainly developed under high amphibolite-to greenschist-facies conditions and extends ∼380 km, with an average width ∼2-4 km, from the conspicuous Ruwah Fault Zone in the eastern shield to the Red Sea Coastal plain. It was believed to be one of the conjugate shears of the NW- to NNW-trending sinistral Najd Shear System. This assumption is, based on the noteworthy dextral shear criteria recorded within the 620 Ma mylonitic granite of No'man Complex. A total shear-zone strike length exceeding 117 km is carefully investigated during this study to reconstruct its structural evolution. Shear-sense indicators and other field observations including overprinting relations clearly demonstrate a complicated Neoproterozoic history of AdSZ, involving at least three phases of deformations (D1-D3). Both D1 and D2 phases were of contractional regime. During D1 phase a NW-SE compression led to the formation of NE-oriented low-angle thrusts and tight-overturned folds. D2 is represented by a NE-SW stress oriented that led to the development of an open folding. D3 is expressed by the NE-SW intensive dextral transcurrent brittle-ductile shearing. It is overprinting the early formed fabrics and played a significant role in the creation of AdSZ and the mega-scale related folds. Such deformation history reflects the same Neoproterozoic deformation regime recognized in the NE-trending shear zones in the Arabian-Nubian Shield (ANS).

  15. Neoproterozoic extension in the greater dharwar craton: A reevaluation of the "betsimisaraka suture" in madagascar

    USGS Publications Warehouse

    Tucker, R.D.; Roig, J.-Y.; Delor, C.; Amlin, Y.; Goncalves, P.; Rabarimanana, M.H.; Ralison, A.V.; Belcher, R.W.

    2011-01-01

    The Precambrian shield of Madagascar is reevaluated with recently compiled geological data and new U-Pb sensitive high-resolution ion microprobe (SHRIMP) geochronology. Two Archean domains are recognized: the eastern Antongil-Masora domain and the central Antananarivo domain, the latter with distinctive belts of metamafic gneiss and schist (Tsaratanana Complex). In the eastern domain, the period of early crust formation is extended to the Paleo-Mesoarchean (3.32-3.15 Ga) and a supracrustal sequence (Fenerivo Group), deposited at 3.18 Ga and metamorphosed at 2.55 Ga, is identified. In the central domain, a Neoarchean period of high-grade metamorphism and anatexis that affected both felsic (Betsiboka Suite) and mafic gneisses (Tsaratanana Complex) is documented. We propose, therefore, that the Antananarivo domain was amalgamated within the Greater Dharwar Craton (India + Madagascar) by a Neoarchean accretion event (2.55-2.48 Ga), involving emplacement of juvenile igneous rocks, high-grade metamorphism, and the juxtaposition of disparate belts of mafic gneiss and schist (metagreenstones). The concept of the "Betsimisaraka suture" is dispelled and the zone is redefined as a domain of Neoproterozoic metasedimentary (Manampotsy Group) and metaigneous rocks (Itsindro-Imorona Suite) formed during a period of continental extension and intrusive igneous activity between 840 and 760 Ma. Younger orogenic convergence (560-520 Ma) resulted in east-directed overthrusting throughout south Madagascar and steepening with local inversion of the domain in central Madagascar. Along part of its length, the Manampotsy Group covers the boundary between the eastern and central Archean domains and is overprinted by the Angavo-Ifanadiana high-strain zone that served as a zone of crustal weakness throughout Cretaceous to Recent times.

  16. Late Neoproterozoic Nuqara Dokhan Volcanics, Central Eastern Desert, Egypt: Geochemistery and petrogenesis

    NASA Astrophysics Data System (ADS)

    Hassan, Tharwat; Asran, Asran; Amron, Taha; Natflos, Theo

    2014-05-01

    The Nuqara volcanic is one of the northernmost outcrops of the Arabian-Nubian Shield Dokhan volcanics. The origin and tectonic setting of the late Neoproterozoic Dokhan volcanics (ca. 610-560 Ma) in the Egyptian Eastern Desert is highly debated. The debate concerns the tectonic setting where they formed during transition between convergent to extensional regime or after the East- and the West-Gondwana collision (~600Ma). In order to solve this problem, lavas from Nuqara area were studied geologically and geochemically. Nuqara Dokhan volcanics comprises two main rock suites: (a) an intermediate volcanic suite, consisting of basaltic andesite, andesite and their associated pyroclastics rocks; and (b) a felsic volcanic suite composed of dacite, rhyolite and ignimbrites. The two suites display well-defined major and trace element trends and continuum in composition with wide ranges in SiO2 (52-75.73%), CaO (9.19-0.22%), MgO (5.29-0.05%), Sr (1367-7.4 ppm), Zr (688.5-172.7 ppm), Cr (207-0.4 ppm), and Ni (94.3-0.2 ppm). The Nuqara Dokhan volcanics are characterized by strong enrichment in LILE relative to HFSE and affiliated to the calc-alkaline subducted - related magmatism. Geochemical Modeling displays that the evolution of these rocks was governed by fractional crystallization of plagioclase, amphiboles, pyroxene, magnetite and apatite in the intermediate varieties and plagioclase, amphibole, magnetite, apatite and zircon in the felsic varieties. The obtained mineral chemistry of these volcanics reveals: (a) Plagioclase range in composition from An55 to An40 in basaltic andesite and from An39 to An24 in andesite. (b) Alkali feldspars have sanidine composition. (c) Clinopyroxenes have augite composition. The low Al2O3 contents (1.94-5.588 wt %) indicate that clinopyroxene crystallized at low - pressure conditions. (d) Amphiboles have magnesio- hornblende composition.

  17. An emerging picture of Neoproterozoic ocean chemistry: Insights from the Chuar Group, Grand Canyon, USA

    NASA Astrophysics Data System (ADS)

    Johnston, David T.; Poulton, Simon W.; Dehler, Carol; Porter, Susannah; Husson, Jon; Canfield, Donald E.; Knoll, Andrew H.

    2010-02-01

    chemistry and life in the dynamic Neoproterozoic Earth system.

  18. The petrogenesis of late Neoproterozoic mafic dyke-like intrusion in south Sinai, Egypt

    NASA Astrophysics Data System (ADS)

    Azer, M. K.; Abu El-Ela, F. F.; Ren, M.

    2012-08-01

    New field, petrographical and geochemical studies are presented here for the late Neoproterozoic Rimm intrusion (˜15 km long) exposed in the southern Sinai Peninsula, Egypt in the northernmost Arabian-Nubian Shield (ANS). Field relations indicate that the Rimm intrusion is younger than the surrounding metamorphic rocks and calc-alkaline syn-tectonic granodiorite and it was not affected by regional metamorphism. The anorogenic peralkaline granite of Gebel Serbal crosscuts the Rimm intrusion. The Rimm intrusion is made up of several consanguineous rock types with gradational contacts. It is composed chiefly of pyroxene-hornblende gabbro, hornblende gabbro and minor quartz diorite. The chemical composition of the mafic minerals indicated that the studied rocks derived from calc-alkaline mafic magma. Geochemically, the studied rocks are characterized by enrichment in LILE relative to HFSE and LREE relative to HREE [(Ce/Yb)N = 4.50-6.36]. Quartz diorite display slightly concave HREE pattern and slightly negative Eu-anomaly [(Eu/Eu*)n = 0.91] which may be the result of fractionation of amphibole and plagioclase from the source melt, respectively. The Rimm intrusion evolved from mafic mantle magma into different type rocks by fractional crystallization with minor crustal contamination. The initial magma corresponds to pyroxene-hornblende gabbro and the crystallization of hornblende was caused by slight H2O increase in magma after crystallization of near-liquidus clinopyroxene and Ca-rich plagioclase. Amphiboles geobarometer indicate that the gabbroic rocks of the Rimm intrusion crystallized at pressures between 4.8 and 6.4 Kb, while quartz diorite crystallized at 1.3-2.1 Kb. Crystallization temperatures range between 800 and 926 °C for the gabbros and between 667 and 784 °C for the quartz diorite. The Rimm intrusion represents a post-orogenic phase formed during the crustal thinning and extension of the Arabian-Nubian Shield.

  19. Tectonic significance of Neoproterozoic magmatism of Nakora area, Malani igneous suite, Western Rajasthan, India

    NASA Astrophysics Data System (ADS)

    Kumar, Naresh; Vallinayagam, G.

    2014-05-01

    Three magmatic phases are distinguished in the Neoproterozoic Nakora Ring Complex (NRC) of Malani Igneous Suite (MIS), namely (a) Extrusive (b) Intrusive and (c) Dyke phase. Magmatism at NRC initiated with minor amount of (basic) basalt flows and followed by the extensive/voluminous acid (rhyolites-trachytes) flows. The ripple marks are observed at the Dadawari area of NRC in tuffaceous rhyolite flow which suggests the aqueous condition of flows deposition. The emplacement of the magma appears to have been controlled by a well defined NE-SW tectonic lineament and cut by radial pattern of dykes. These NE-SW tectonic lineaments are the linear zones of crustal weakness and high heat flow. The spheroidal and rapakivi structures in the Nakora acid volcanics indicate the relationship between genetic link and magma mixing. Basalt-trachyte-rhyolite association suggests that the large amount of heat is supplied to the crust from the magma chamber before the eruption. The field (elliptical/ring structures), mineralogical and geochemical characteristics of Nakora granites attest an alkaline character in their evolution and consistent with within plate tectonic setting. The emplacement of these granites and associated volcanics is controlled by ring structures, a manifestation of plume activity and cauldron subsidence, an evidence of extensional tectonic environment. NRC granites are the product of partial melting of rocks similar to banded gneiss from Kolar Schist Belt of India. The present investigations suggest that the magmatic suites of NRC rocks are derived from a crustal source and the required heat supplied from a mantle plume.

  20. Emplacement of magma in sedimentary basins

    NASA Astrophysics Data System (ADS)

    Malthe-Sorenssen, A.; Planke, S.

    2002-12-01

    Sheet-like intrusive complexes are commonly present in sedimentary basins on rifted volcanic margins. Such sill complexes have important impact on petroleum maturation, migration and trapping. We are currently completing an integrated seismic, field and theoretical study on the petroleum implications of sill intrusions. One aspect of this study has been to get new understanding of the magma emplacement processes based on integrated numerical modeling and geophysical/geological mapping activities. Extensive sill complexes have been identified and mapped in the NE Atlantic and Karoo basins based on seismic, borehole, remote sensing and field data. Early Tertiary intrusive complexes are present in the Voring and More basins offshore mid-Norway. Similar sill complexes are exposed onshore in Cretaceous to Permian age sedimentary sequences on the conjugate central-east Greenland margin. A voluminous Jurassic age intrusive complex is well exposed in the Permian to Jurassic Karoo basin as the erosionally strong dolerites form an impressive mountainous landscape in large parts of South Africa. The sheet intrusions are found at paleodepths of 0-6 km. Deep intrusions are generally long and smooth, whereas shallow intrusions are rough, transgressive and commonly saucer-shaped. Saucer-shaped intrusions are present in unstructured basin segments. The diameter of the saucers increases with depth. Structured basin segments are characterized by a variety of sill complex geometries. The intrusions generally mimic the basin structure. In nature, magma is emplaced in internally pressurized, planar cracks. The emplacement process is controlled by the local stress field and complex interactions of buoyancy forces, host rock resistance to fracture, elastic deformation of country rock, magma hydrostatic pressure and fluctuating magma pressure, magma viscosity and weight of overburden. We have developed a discrete element model to study the emplacement process. Results from the modeling

  1. Copper Deposits in Sedimentary and Volcanogenic Rocks

    USGS Publications Warehouse

    Tourtelot, Elizabeth B.; Vine, James David

    1976-01-01

    Copper deposits occur in sedimentary and volcanogenic rocks within a wide variety of geologic environments where there may be little or no evidence of hydrothermal alteration. Some deposits may be hypogene and have a deep-seated source for the ore fluids, but because of rapid cooling and dilution during syngenetic deposition on the ocean floor, the resulting deposits are not associated with hydrothermal alteration. Many of these deposits are formed at or near major tectonic features on the Earth's crust, including plate boundaries, rift valleys, and island arcs. The resulting ore bodies may be stratabound and either massive or disseminated. Other deposits form in rocks deposited in shallow-marine, deltaic, and nonmarine environments by the movement and reaction of interstratal brines whose metal content is derived from buried sedimentary and volcanic rocks. Some of the world's largest copper deposits were probably formed in this manner. This process we regard as diagenetic, but some would regard it as syngenetic, if the ore metals are derived from disseminated metal in the host-rock sequence, and others would regard the process as epigenetic, if there is demonstrable evidence of ore cutting across bedding. Because the oxidation associated with diagenetic red beds releases copper to ground-water solutions, red rocks and copper deposits are commonly associated. However, the ultimate size, shape, and mineral zoning of a deposit result from local conditions at the site of deposition - a logjam in fluvial channel sandstone may result in an irregular tabular body of limited size; a petroleum-water interface in an oil pool may result in a copper deposit limited by the size and shape of the petroleum reservoir; a persistent thin bed of black shale may result in a copper deposit the size and shape of that single bed. The process of supergene enrichment has been largely overlooked in descriptions of copper deposits in sedimentary rocks. However, supergene processes may be

  2. An exposed slab window margin: the eastern part of the Neoproterozoic Baikal-Muya belt, Russia

    NASA Astrophysics Data System (ADS)

    Fedotova, A.; Khain, E.; Razumovskiy, A.; Anosova, M.; Orlova, A.

    2012-04-01

    A series of dykes and small laccolith-type magmatic bodies of tonalite, trondhjemite, granodiorite and granites marks the final stage in the geological history of the Baikal-Muya belt in an adjacent area of the Siberian craton. These bodies intrude granulite-enderbite-charnokite complex, as well as troctolite Tonkiy Mys massif. These high-silica (SiO2N56%), high alumina (Al2O3>15%), pyroxene or and amphibole bearing granitoids, with Na2O>4%, high Sr (>400 ppm), low Y (<7 ppm), high Sr/Y(>35), low Yb (<1), and are geochemically similar to adakites. The same structural position was defined for granites showing K2O=4.5% and K2O/Na2O=1,4. The tonalites of this complex collected f yielded the U-Pb zircon age of 591±3 Ma (LA ICP-MS data). Previously obtained age estimate for the enderbites is 617±5 Ma [Amelin et al., 2000]. Three samples of enderbites and gneisse provided U-Pb zircon ages fallen into the time span between 603-620 Ma. The Slyudinskii massif of high-Ti gabbronorites spatially related to the enderbite-granulite complex was crystallized 618±61 Ma ago [Makrygina et al., 1993]. The published data obtained for the massif (585±22 Ma [Makrygina et al., 1993]) show that troctolite-gabbro massifs crystallized simultaneously or insignificantly later than granulite-enderbite complex at upper levels of the lithosphere. Rocks of these massifs and the gabbro-granulite-enderbite series formed at lower levels have been joined during a stage of tectonic activity and intruded by adakites 591±3 Ma ago. High tectonic activity also reflected in deposition of the coarse clastics occurred in a lower part of Kholodnenskaya suite and contemporary formations. The following events can be reconstructed for the eastern part of the Baikal-Muya belt. Formation of the heterogeneous accretional orogenic structure (tectonic collage), with ophiolites and remnants of earlier formed Neoproterozoic island arcs and related complexes [Izokh, 1998; Rytsk, 2001; Shatskii et al., 1996], was

  3. What Can Earth Paleoclimates Reveal About the Resiliency of Habitable States? An Example from the Neoproterozoic Snowball Earth

    NASA Astrophysics Data System (ADS)

    Sohl, L.

    2014-04-01

    The Neoproterozoic "Snowball Earth" glaciations (~750-635 Ma) have been a special focus for outer habitable zone investigations, owing in large part to a captivating and controversial hypothesis suggesting that Earth may have only narrowly escaped a runaway icehouse state on multiple occasions (a.k.a. "the hard snowball"; Hoffman and Schrag 2001). A review of climate simulations exploring snowball inception (Godderis et al. 2011) reveals that a broad range of models (EBMs, EMICs and AGCMs) tend to yield hard snowball solutions, whereas models with greater 3-D dynamic response capabilities (AOGCMs) typically do not, unless some of their climate feedback responses (e.g., wind-driven ocean circulation, cloud forcings) are disabled (Poulsen and Jacobs 2004). This finding raises the likelihood that models incorporating dynamic climate feedbacks are essential to understanding how much flexibility there may be in the definition of a planet's habitable zone boundaries for a given point in its history. In the first of a series of new Snowball Earth simulations, we use the NASA/GISS ModelE2 Global Climate Model - a 3-D coupled atmosphere/ocean model with dynamic sea ice response - to explore the impacts of wind-driven ocean circulation, clouds and deep ocean circulation on the sea ice front when solar luminosity and atmospheric carbon dioxide are reduced to Neoproterozoic levels (solar = 94%, CO2 = 40 ppmv). The simulation includes a realistic Neoproterozoic land mass distribution, which is concentrated at mid- to tropical latitudes. After 300 years, the sea ice front is established near 30 degrees latitude, and after 600 years it remains stable. As with earlier coupled model simulations we conclude that runaway glacial states would have been difficult to achieve during the Neoproterozoic, and would be more likely to have occurred during earlier times in Earth history when solar luminosity was less. Inclusion of dynamic climate feedback capabilities in habitable zone

  4. Sedimentary response to ocean gateway circulation changes

    NASA Astrophysics Data System (ADS)

    Heinze, Christoph; Crowley, Thomas J.

    1997-12-01

    Previous modeling studies suggested that changes in ocean gateways may have exerted a dramatic influence on the ocean circulation. In this pilot study we extend those results to examining the potential ramifications of circulation changes on the sedimentary record. A version of the Hamburg carbon cycle/sediment model is used in these sensitivity experiments. Results indicate that internal reorganization of the ocean circulation can potentially cause very large regional changes in lysocline depth (1500-3000 m) and opal deposition. These shifts are sometimes comparable in magnitude to those imposed by changes in external forcing (e.g., climate, sea level, and weathering). Comparisons of the model response with the geologic record indicate some significant levels of first-order agreement. This exercise suggests that opportunities now exist for physically based modeling of past sediment responses to circulation and climate changes.

  5. Incidences of polluting episodes in sedimentary media.

    PubMed

    Casas-Ruiz, M; Feria, F; Ligero, R A

    2009-09-01

    There exist diverse radioactivity sources in the environment coming from anthropogenic activities that alter the natural levels of radiation. The detailed study of the environmental impact of these sources is of great interest, because it provides knowledge for subsequent decontamination works and environmental control. In this work, it is analyzed the radioactive contamination caused by the radionuclide (226)Ra in sedimentary media under a liquid sheet. In this way, the dumping of the radionuclide in sediment columns has been studied in laboratory, determining how its penetration in depth develops along time and for different grain sizes. For this purpose, a migration model based on the numeric resolution of the diffusion equation has been devised. PMID:19359190

  6. Sedimentary processes and crustal cycling on Venus

    NASA Technical Reports Server (NTRS)

    Warner, J. L.

    1983-01-01

    Sediment exists on the Venus surface. It is observed in Venera images between outcrops and boulders of sedimentary rocks. Sediment is produced by pyroclastic volcanism and chemical weathering. Chemical weathering is driven by an enhanced activity of water and an elevated surface temperature. Sediment is transported by wind action and lithified by cementration and diagenesis. Cementation may be by carbonate or silica cement; diagenesis may be products of chemical weathering acting as cement, or by compaction and recrystallization of sediment into a texture with interlocking grains. Sediment may be transported from the top of sialic continents (such as Ishtar) to the modal plains where it is deposited, lithified, and integrated into thy local crust. As new layers are added, the bottom of the crust melts and is, in part, returned to the mantle. A steady-state chemical exchange might exist by this mechanism of crustal cycling that links atmosphere, continents, modal plains, and mantle.

  7. Remote sensing of some sedimentary rocks.

    NASA Technical Reports Server (NTRS)

    Brennan, P. A.; Lintz, J., Jr.

    1971-01-01

    Sedimentary rocks including varying sized clastics and carbonates were overflown by aircraft between 1966 and 1971 producing data in the ultraviolet to microwave regions of the electromagnetic spectrum. This paper reports that multispectral analysis increases the ease and rapidity of discrimination of rock types having subtle differences in physical characteristics, but fails to enhance and may degrade distinctions where physical characteristics are significantly different. Brief resumes of color and color IR photographic data are presented. Thermal infrared is found to be useful in the mapping of rock units, but limitations such as moisture variation, soil cover, and vegetation may exceed in one formation the distinction between differing lithologies. A brief review of previously published SLAR data is included for completeness. Remote sensing techniques should reduce field geological effort by as much as 50%.

  8. Sedimentary remagnetizations involving pyrrhotite and greigite (Invited)

    NASA Astrophysics Data System (ADS)

    Roberts, A. P.

    2013-12-01

    Late diagenetic growth of the ferrimagnetic iron sulfide minerals, monoclinic pyrrhotite and greigite, will give rise to remagnetizations of their host sediments. Sedimentary pyrrhotite formation has been the subject of much confusion in the paleomagnetic literature. Its growth is kinetically limited. Authigenic monoclinic pyrrhotite has, therefore, not been reported in modern sediments. If monoclinic pyrrhotite is present in modern sediments, it is likely to have a detrital origin, as has been demonstrated in marginal basins adjacent to denuding regional metamorphic orogenic belts. If authigenic monoclinic pyrrhotite occurs in sediments, it will carry a magnetization that was acquired much later than deposition. Identification of authigenic pyrrhotite should, therefore, lead an investigator to suspect that the sediment has been at least partially remagnetized. Iron sulfides have high electron backscatter and are easily investigated using electron microscopy. Microtextural observations of sedimentary pyrrhotite can often provide direct confirmation that the pyrrhotite formed during later diagenetic reactions (e.g., after sediment compaction). Greigite can also give rise to remagnetizations, but, unlike pyrrhotite, it can grow quickly, even within an anoxic water column before deposition. Multiple generations of greigite can also form even during early diagenesis. These possibilities make it more difficult to use electron microscopic observations to detect remagnetization mechanisms, although microtextural relationships with other minerals can still provide strong direct evidence for remagnetization. For both pyrrhotite and greigite, remagnetizations are generally detected using conventional paleomagnetic observations. These include detection of magnetic polarities that contradict those carried by coexisting detrital magnetic minerals and failure of paleomagnetic field tests (mainly the fold and reversal tests). An overview of the features associated with

  9. Permanganate diffusion and reaction in sedimentary rocks.

    PubMed

    Huang, Qiuyuan; Dong, Hailiang; Towne, Rachael M; Fischer, Timothy B; Schaefer, Charles E

    2014-04-01

    In situ chemical oxidation using permanganate has frequently been used to treat chlorinated solvents in fractured bedrock aquifers. However, in systems where matrix back-diffusion is an important process, the ability of the oxidant to migrate and treat target contaminants within the rock matrix will likely determine the overall effectiveness of this remedial approach. In this study, a series of diffusion experiments were performed to measure the permanganate diffusion and reaction in four different types of sedimentary rocks (dark gray mudstone, light gray mudstone, red sandstone, and tan sandstone). Results showed that, within the experimental time frame (~2 months), oxidant migration into the rock was limited to distances less than 500 μm. The observed diffusivities for permanganate into the rock matrices ranged from 5.3 × 10(-13) to 1.3 × 10(-11) cm(2)/s. These values were reasonably predicted by accounting for both the rock oxidant demand and the effective diffusivity of the rock. Various Mn minerals formed as surface coatings from reduction of permanganate coupled with oxidation of total organic carbon (TOC), and the nature of the formed Mn minerals was dependent upon the rock type. Post-treatment tracer testing showed that these Mn mineral coatings had a negligible impact on diffusion through the rock. Overall, our results showed that the extent of permanganate diffusion and reaction depended on rock properties, including porosity, mineralogy, and organic carbon. These results have important implications for our understanding of long-term organic contaminant remediation in sedimentary rocks using permanganate. PMID:24566296

  10. Modes of sedimentary basin formation in the north-eastern Black Sea

    NASA Astrophysics Data System (ADS)

    Stephenson, Randell; Starostenko, Vitaly; Sydorenko, Grygoriy; Yegorova, Tamara

    2016-04-01

    The Greater Caucasus and Black Sea sedimentary basins developed in a Mesozoic back-arc setting, the former older than the latter (Jurassic v. Cretaceous). Compressional shortening of the former and accompanying ongoing development of marginal basin depocentres in the north-eastern Black Sea - which is closely tied to the formation of the Crimea-Greater Caucasus orogen - is a Cenozoic phenomenon, starting in the Eocene and proceeding until the present day. Recently, the sedimentary basin/crust/lithosphere geometry of the study area has been characterised across a range of scales using regional seismic reflection profiling, long-offset refraction/wide-angle reflection profiling and local earthquake tomography. These provide a new integrated image of the present-day crustal structure and sedimentary basin architecture of the northern margin of the eastern Black Sea, north across the Azov Sea and provide evidence of the deeper expression of sedimentary basins and the processes controlling the geometry of their inversion during the Cenozoic. It is inferred that the Greater Caucasus paleo-Basin, lying stratigraphically below the Black Sea and younger sedimentary successions, extends further to the west than previously known. This basin has significant thickness in the area between the Azov and Black seas and probably forms the deeper core of the Crimea-Caucasus inversion zone. The Crimea-Greater Caucasus orogenic belt is the expression of "basin inversion" of the Jurassic Greater Caucasus paleo-Basin, the degree of inversion of which varies along strike. The Greater Caucasus foredeep basins - Indolo-Kuban and Sorokin-Tuapse troughs -represent syn-inversional marginal troughs to the main inversion zone. The Shatsky Ridge - the northern flank of the main East Black Sea Basin - may also be mainly a syn-inversional structure, underlain by a blind thrust zone expressed as a northward dipping zone of seismicity on the northern margin of the eastern Black Sea.

  11. Geoarchaeological investigations of a Mesolithic-Neolithic Sedimentary Sequence from Queens Sedgemoor, Somerset, UK

    NASA Astrophysics Data System (ADS)

    Hill, Tom; Whittaker, John; Brunning, Richard; Law, Matthew; Bell, Martin; Wilkinson, Keith

    2016-04-01

    A geoarchaeological investigation was undertaken at Queens Sedgemoor in Somerset, southwest England, as part of the English Heritage funded project 'the Mesolithic wetland/dryland edge in Somerset' (EH 6624). This project was designed to address the National Heritage Protection Plan (Topic 4G) associated with the assessment of the character and significance of sedimentary and wetland archaeology. As part of the project, a sediment core extracted from the site and has undergone high resolution radiocarbon dating, with subsequent detailed and directed micropalaeontological analyses (pollen, diatom, foraminifera, ostracoda, mollusca) focussing on the sedimentary sequence associated with the Mesolithic and early Neolithic periods. The presentation summarises the results of this multiproxy analyses and explains how it has assisted in understanding the landscape during a period of substantial prehistoric importance in southwest England. The sedimentary sequence dates back to the Mesolithic period (7.6ky BP) and the microfossil evidence indicates hydroseral succession has taken place, with the initial establishment of a freshwater lake, prior to undergoing terrestrialisation and the eventual development of a raised bog. Holocene sea-level change also influenced the sedimentary archive. Due to a rise in relative sea level c. 6.7ky BP, subsequent coastal inundation and estuarine sedimentation took also place and is hereby associated with the Lower Wentlooge Formation of the Somerset Levels. Although poor pollen preservation was encountered within the section specifically associated with the Mesolithic-Neolithic transition, a clear picture of landscape change is presented for the sedimentary archive, with evidence indicative of landscape modification by humans since the late Mesolithic.

  12. Sedimentary basins reconnaissance using the magnetic Tilt-Depth method

    USGS Publications Warehouse

    Salem, A.; Williams, S.; Samson, E.; Fairhead, D.; Ravat, D.; Blakely, R.J.

    2010-01-01

    We compute the depth to the top of magnetic basement using the Tilt-Depth method from the best available magnetic anomaly grids covering the continental USA and Australia. For the USA, the Tilt-Depth estimates were compared with sediment thicknesses based on drilling data and show a correlation of 0.86 between the datasets. If random data were used then the correlation value goes to virtually zero. There is little to no lateral offset of the depth of basinal features although there is a tendency for the Tilt-Depth results to be slightly shallower than the drill depths. We also applied the Tilt-Depth method to a local-scale, relatively high-resolution aeromagnetic survey over the Olympic Peninsula of Washington State. The Tilt-Depth method successfully identified a variety of important tectonic elements known from geological mapping. Of particular interest, the Tilt-Depth method illuminated deep (3km) contacts within the non-magnetic sedimentary core of the Olympic Mountains, where magnetic anomalies are subdued and low in amplitude. For Australia, the Tilt-Depth estimates also give a good correlation with known areas of shallow basement and sedimentary basins. Our estimates of basement depth are not restricted to regional analysis but work equally well at the micro scale (basin scale) with depth estimates agreeing well with drill hole and seismic data. We focus on the eastern Officer Basin as an example of basin scale studies and find a good level of agreement between previously-derived basin models. However, our study potentially reveals depocentres not previously mapped due to the sparse distribution of well data. This example thus shows the potential additional advantage of the method in geological interpretation. The success of this study suggests that the Tilt-Depth method is useful in estimating the depth to crystalline basement when appropriate quality aeromagnetic anomaly data are used (i.e. line spacing on the order of or less than the expected depth to

  13. Sedimentary basins reconnaissance using the magnetic Tilt-Depth method

    NASA Astrophysics Data System (ADS)

    Salem, Ahmed; Williams, Simon; Samson, Esuene; Fairhead, Derek; Ravat, Dhananjay; Blakely, Richard J.

    2010-09-01

    We compute the depth to the top of magnetic basement using the Tilt-Depth method from the best available magnetic anomaly grids covering the continental USA and Australia. For the USA, the Tilt-Depth estimates were compared with sediment thicknesses based on drilling data and show a correlation of 0.86 between the datasets. If random data were used then the correlation value goes to virtually zero. There is little to no lateral offset of the depth of basinal features although there is a tendency for the Tilt-Depth results to be slightly shallower than the drill depths. We also applied the Tilt-Depth method to a local-scale, relatively high-resolution aeromagnetic survey over the Olympic Peninsula of Washington State. The Tilt-Depth method successfully identified a variety of important tectonic elements known from geological mapping. Of particular interest, the Tilt-Depth method illuminated deep (3km) contacts within the non-magnetic sedimentary core of the Olympic Mountains, where magnetic anomalies are subdued and low in amplitude. For Australia, the Tilt-Depth estimates also give a good correlation with known areas of shallow basement and sedimentary basins. Our estimates of basement depth are not restricted to regional analysis but work equally well at the micro scale (basin scale) with depth estimates agreeing well with drill hole and seismic data. We focus on the eastern Officer Basin as an example of basin scale studies and find a good level of agreement between previously-derived basin models. However, our study potentially reveals depocentres not previously mapped due to the sparse distribution of well data. This example thus shows the potential additional advantage of the method in geological interpretation. The success of this study suggests that the Tilt-Depth method is useful in estimating the depth to crystalline basement when appropriate quality aeromagnetic anomaly data are used (i.e. line spacing on the order of or less than the expected depth to

  14. Neoproterozoic Chuar Group (˜800-742 Ma), Grand Canyon: a record of cyclic marine deposition during global cooling and supercontinent rifting

    NASA Astrophysics Data System (ADS)

    Dehler, Carol M.; Elrick, Maya; Karlstrom, Karl E.; Smith, Gary A.; Crossey, Laura J.; Timmons, J. Michael

    2001-06-01

    Chuar Group sediments were deposited in a marine cratonic basin synchronously with marine deposition in other basins of western North America and Australia. The top of the Chuar Group is 742 Ma; thus, this succession offers a potentially important record of global climatic and tectonic changes during the mid-Neoproterozoic — a critical time leading to possible global glaciation during supercontinent rifting. Chuar Group cycles and sequences may record glacioeustatic fluctuations during the climatic transition into the Sturtian Ice Age (˜750-700 Ma), as well as extension related to the dispersal of Rodinia. The mid-Neoproterozoic Chuar Group (1600 m thick) is predominantly composed of mudrock with subordinate meter-scale beds of dolomite and sandstone. Facies analysis leads to the interpretation of a wave- and tidal-influenced marine depositional system. Diagnostic marine features include: (1) marine fossils and high local pyrite content in the mudrock facies; (2) mudcracked mud-draped symmetric ripples and reverse flow indicators in the sandstone facies; (3) facies associations between all facies; and (4) no unequivocal terrestrial deposits. Chuar facies stack into ˜320 dolomite- and sandstone-capped meter-scale cycles (1-20 m thick) and non-cyclic intervals of uniform mudrock facies (20-150 m thick). Nearly all cycles have mudrock bases indicating subtidal water depths. Dolomite-capped cycles shallow to peritidal environments (peritidal cycles), some indicating subaerial exposure of varying degrees (exposure cycles). Sandstone-capped cycles shallow to peritidal environments with no evidence of prolonged exposure (peritidal cycles). Non-cyclic intervals represent the deepest water depths (˜10s of meters). Peritidal cycles and non-cyclic intervals dominate the lower and middle Chuar Group. In the upper Chuar Group, the percentage of exposure cycles increases, as does the thickness of individual cycles and non-cyclic intervals, and the degree of subaerial

  15. Clastic sedimentary rocks of the Michipicoten Volcanic-sedimentary belt, Wawa, Ontario

    NASA Technical Reports Server (NTRS)

    Ojakangas, R. W.

    1983-01-01

    The Wawa area, part of the Michipicoten greenstone belt, contains rock assemblages representative of volcanic sedimentary accumulations elsewhere on the shield. Three mafic to felsic metavolcanic sequences and cogenetic granitic rocks range in age from 2749 + or - 2Ma to 2696 + or - 2Ma. Metasedimentary rocks occur between the metavolcanic sequences. The total thickness of the supracrustal rocks may be 10,000 m. Most rocks have been metamorphosed under greenschist conditions. The belt has been studied earlier and is currently being remapped by Sage. The sedimentrologic work has been briefly summarized; two mainfacies associations of clastic sedimentary rocks are present - a Resedimented (Turbidite) Facies Association and a Nonmarine (Alluvial Fan Fluvial) Facies Association.

  16. Development of sedimentary cycles on the east Sahara craton since Silurian time (northwest Sudan/southwest Egypt)

    SciTech Connect

    Wycisk, P. )

    1988-08-01

    The sedimentary succession of southwest Egypt and northwest Sudan, formerly called the Nubia(n) Sandstone, has been subdivided into a number of formations. The predominantly fluvial sediments which characterize Silurian to Upper Cretaceous strata of this region were repeatedly interrupted by marine transgressions that rapidly progressed toward the south since Ordovician time. Thin, shallow marine sequences of different ages can be traced for more than 1,000 km within the studied area. The development of the sedimentary cycles will be pointed out by surface and subsurface data along a cross section from the southern Dakhla basin in the north to the Misaha trough and Abyad basin in the south.

  17. Removing the effects of metamorphism from the Neoproterozoic carbon isotope record: a case study on Islay, western Scotland

    NASA Astrophysics Data System (ADS)

    Skelton, Alasdair

    2016-04-01

    The Port Askaig Formation on Islay, western Scotland is the first discovered tillite (glacial sediment) of Neoproterozoic age. This formation is sandwiched between carbonate rocks which preserve an extreme negative carbon isotope excursion. This so called "Islay anomaly" has been correlated with other such anomalies worldwide and together with the tillites has been cited as evidence of major (worldwide) glaciation events. During subsequent mountain building, this carbonate-tillite- carbonate sequence has been folded, producing a major en-echelon anticlinal fold system. Folding was accompanied by metamorphism at greenschist facies conditions which was, in turn, accompanied by metamorphic fluid flow. Mapping of the δ18O and δ13C values of these carbonate rocks reveals that metamorphic fluids were channelled through the axial region of the anticlinal fold. The metamorphic fluid was found to have a highly negative δ13C value, which was found to be in equilibrium with metamorphosed graphitic mudstones beneath the carbonate-tillite-carbonate sequence. Devolatilisation of these mudstones is therefore a likely source of this metamorphic fluid. Removal of the effects of metamorphic fluid flow on δ13C values recorded by metamorphosed carbonate rocks on Islay allows us to re-evaluate the isotopic evidence used to reconstruct Neoproterozoic climate. We are able to show that extreme negative δ13C values can partly be attributed to metamorphic fluid flow.

  18. Geochemical Signatures of Neoproterozoic Granites and Granitoid-gneisses from Angavo Belt, Central Madagascar

    NASA Astrophysics Data System (ADS)

    Raharimahefa, T.

    2015-12-01

    The basement rocks of Madagascar record high-grade metamorphism, magmatism, and contractional and extensional structures that accompanied the collision of the eastern and western Gondwana segments of the supercontinent followed by its collapse. In the eastern central Madagascar, granitoids dominate the landscape and occur in a large area near and within the N-S trending highly strained zone known as the Angavo Shear Zone or Angavo belt. The area is a key in understanding the evolution of basement of Madagascar and the reconstruction of the Gondwana supercontinent. These granitoids range from layered to massive and previously published U-Pb zircon dating yielded three distinctive Neoproterozoic magmatisms at 770 Ma to 820 Ma, ca. 660Ma, and ca 550Ma. However, it was unknown whether these ages represent distinct magmatic pulses or reflect a continuous granitoid emplacement. This work contributes to the knowledge of the Malagasy basement rocks and to explore and discuss the origin and petrotectonic evolution of the granites and granitoid-gneisses from this part of the Madagascar. The rock samples are placed into three groups: group A and B for granitoid-gneisses and a third group for the granite layers. Group A and B are metaluminous and slightly peraluminous, respectively. All rocks have typical subduction zone calc-alkaline signatures. Group A is characterized by enrichment in large ion lithophile elements (LILE) but low U, LREE enrichment, depletion in the high field strength elements (HFSE). In contrast, Group B has REE patterns closely similar to Archean sediments. The granite layers show fractionated REE patterns in which HREE patterns show strong correlation with Zr abundances. Trends in major element variation diagrams and the enrichment of incompatible elements could be explained by simple fractional crystallization, while the overall geochemical signatures reflect either (1) melting of ancient crust or (2) crustal contamination of a more evolved magmas that

  19. Archean sedimentary styles and early crustal evolution

    NASA Technical Reports Server (NTRS)

    Lowe, D. R.

    1986-01-01

    The distinctions between and implications of early and late Archean sedimentary styles are presented. Early Archean greenstone belts, such as the Barberton of South Africa and those in the eastern Pilbar Block of Australia are characterized by fresh or slightly reworked pyroclastic debris, orthochemical sediments such as carbonates, evaporites, and silica, and biogenic deposits including cherts and stromatolitic units. Terrigenous deposits are rare, and it is suggested that early Archean sediments were deposited on shallow simatic platforms, with little or no components derived from sialic sources. In contrast, late Archean greenstone belts in the Canadian Shield and the Yilgarn Block of Australia contain coarse terrigenous clastic rocks including conglomerate, sandstone, and shale derived largely from sialic basement. Deposition appears to have taken place in deepwater, tectonically unstable environments. These observations are interpreted to indicate that the early Archean greenstone belts formed as anorogenic, shallow water, simatic platforms, with little or no underlying or adjacent continental crust, an environment similar to modern oceanic islands formed over hot spots.

  20. Microbial shaping of sedimentary wrinkle structures

    NASA Astrophysics Data System (ADS)

    Mariotti, G.; Pruss, S. B.; Perron, J. T.; Bosak, T.

    2014-10-01

    Wrinkle structures on sandy bed surfaces were present in some of the earliest sedimentary environments, but are rare in modern environments. These enigmatic millimetre- to centimetre-scale ridges or pits are particularly common in sediments that harbour trace fossils and imprints of early animals, and appeared in the aftermath of some large mass extinctions. Wrinkle structures have been interpreted as possible remnants of microbial mats, but the formation mechanism and associated palaeoenvironmental and palaeoecological implications of these structures remain debated. Here we show that microbial aggregates can form wrinkle structures on a bed of bare sand in wave tank experiments. Waves with a small orbital amplitude at the bed surface do not move sand grains directly. However, they move millimetre-size, light microbial fragments and thereby produce linear sand ridges and rounded scour pits at the wavelengths observed in nature within hours. We conclude that wrinkle structures are morphological biosignatures that form at the sediment-water interface in wave-dominated environments, and not beneath microbial mats as previously thought. During early animal evolution, grazing by eukaryotic organisms may have temporarily increased the abundance of microbial fragments and thus the production of wrinkle structures.

  1. Sedimentary organic molecules: Origins and information content

    NASA Technical Reports Server (NTRS)

    Hayes, J. M.; Freeman, K. H.

    1991-01-01

    To progress in the study of organic geochemistry, we must dissect the processes controlling the composition of sedimentary organic matter. Structurally, this has proven difficult. Individual biomarkers can often be recognized, but their contribution to total organic materials is small, and their presence does not imply that their biochemical cell mates have survived. We are finding, however, that a combination of structural and isotopic lines of evidence provides new information. A starting point is provided by the isotopic compositions of primary products (degradation products of chlorophylls, alkenones derived from coccoliths). We find strong evidence that the isotopic difference between primary carbonate and algal organic material can be interpreted in terms of the concentration of dissolved CO2. Moreover, the isotopic difference between primary and total organic carbon can be interpreted in terms of characteristic isotopic shifts imposed by secondary processes (responsive, for example, to O2 levels in the depositional environment. In favorable cases, isotopic compositions of a variety of secondary products can be interpreted in terms of flows of carbon, and, therefore, in terms of specific processes and environmental conditions within the depositional environment.

  2. Investigating Coccolithophorid Biology in the Sedimentary Laboratory

    NASA Astrophysics Data System (ADS)

    McClelland, H. L. O.; Barbarin, N.; Beaufort, L.; Hermoso, M.; Rickaby, R. E. M.

    2014-12-01

    Coccolithophores are the ocean's dominant calcifying phytoplankton; they play an important, but poorly understood, role in long-term biogeochemical climatic feedbacks. Calcite producing marine organisms are likely to calcify less in a future world where higher carbon dioxide concentrations will lead to ocean acidification (OA), but coccolithophores may be the exception. In coccolithophores calcification occurs in an intracellular vesicle, where the site of calcite precipitation is buffered from the external environment and is subject to a uniquely high degree of biological control. Culture manipulation experiments mimicking the effects of OA in the laboratory have yielded empirical evidence for phenotypic plasticity, competition and evolutionary adaptation in asexual populations. However, the extent to which these results are representative of natural populations, and of the response over timescales of greater than a few hundred generations, is unclear. Here we describe a new sediment-based proxy for the PIC:POC (particulate inorganic to particulate organic carbon ratio) of coccolithophore biomass, which is equivalent to the fractional energy contribution to calcification at constant pH, and a biologically meaningful measure of the organism's tendency to calcify. Employing the geological record as a laboratory, we apply this proxy to sedimentary material from the southern Pacific Ocean to investigate the integrated response of real ancient coccolithophore populations to environmental change over many thousands of years. Our results provide a new perspective on phenotypic change in real populations of coccolithophorid algae over long timescales.

  3. Seismic attenuation anisotropy in reservoir sedimentary rocks

    SciTech Connect

    Best, A.I.

    1994-12-31

    Seismic attenuation is a fundamental property of reservoir sedimentary rocks; it is strongly related to reservoir permeability. Knowledge of its variation with lithology, with burial depth, and with wave propagation direction is vital for understanding the attenuation mechanism. Given this information, realistic theoretical models may be constructed for predicting attenuation, and hence permeability, over a wide frequency range. Accurate ultrasonic attenuation measurements were made in the laboratory over a range of effective pressures on sandstone samples with different amounts of humic organic matter. The organic matter formed fine laminations along the bedding planes of the sandstones. The results show that the sandstones are highly attenuating at 5 MPa mainly because of the presence of grain contact microcracks giving rise to squirt flow; at 40 MPa, when most of the microcracks are closed, the clean sandstones are poorly attenuating, but the organic-rich sandstones remain highly attenuating. It is postulated that the compliant organic matter is responsible for causing squirt flow at high and at low pressures. The results also show that the maximum attenuation occurs when the particle motion of the propagating wave is perpendicular to the planes of the organic matter laminations. These results are consistent with the squirt flow theory of Akbar et al (1993) for compressional waves.

  4. New Constraints on Amazonian Versus West African Cratonic Source Components of the Peri-Gondwanan Sedimentary Basins

    NASA Astrophysics Data System (ADS)

    Schofield, D. I.; Horstwood, M. S.

    2009-05-01

    . This is juxtaposed against the eastern 'Eburnean' domain along a narrow high strain zone that has yielded metamorphic and syn-tectonic intrusive ages between of c.2.1 Ga. The 'Eburnean' domain itself comprises an extensive region of plutonic rocks, making up approximately 50% of the shield, ranging from ca. 2.1 to 2.0 Ga in age. Following the Palaeoproterozoic orogen, the Reguibat Shield remained relatively stable, with only an episode of rigid extension and emplacement of zircon-poor dykes which has been dated at ca. 1.7 to 1.5 Ga by the Rb-Sr technique. A ca. 1.2-1.0 Ga, Grenvillian age event is also recognised from the Mauritenide Belt, although new zircon ages date the main Neoproterozoic magmatic phase of the orogen at 0.77 to 0.63 Ga. These new data largely support the strongly bimodal detrital zircon record from the adjacent sedimentary basins. However, the newly recognised, extensive, Neoarchaean source area may have implications for the palaeogeography of terranes that have yielded detrital zircon peaks of a similar age. In particular Cadomia- Bohemia, which may have developed immediately outboard of the northern margin of the West African Cratron. Reference: Nance R.D. and nine others, 2008. Neoproterozoic-early Palaeozoic tectonostratigraphy and palaeogeography of the peri-Gondwanan terranes: Amazonian v. West African connections. In The boundaries of the West African Craton, Ennih, W. and Liégeois J-P. (eds), Geological Society, London, Special Publication 297, 345-383.

  5. The 780 Gunbarrel Magmatic Event: U-Pb Documentation of Widespread Mafic Magmatism Along the Neoproterozoic Western of Laurentia

    NASA Astrophysics Data System (ADS)

    Harlan, S. S.; Heaman, L.; Lecheminant, A. N.; Premo, W. R.

    2003-12-01

    Precise U-Pb baddeleyite dating of mafic igneous rocks provide evidence for a widespread and synchronous magmatic event that extended for >2400 km along the western margin of the Neoproterozoic Laurentian craton. The gabbros dated in this study include three mafic sheets that intrude crystalline basement rocks of the Northwest Territories (the Gunbarrel, Calder and Faber Lake gabbros), a mafic sill exposed in the Mackenzie Mountains (Concajou Canyon sill), a mafic dike in the Muskwa Ranges of British Columbia (Muncho Lake diabase), and three northwest-trending mafic dikes in the Tobacco Root (group B dikes) and Beartooth Mountains (Christmas Lake dike) of Montana and Wyoming. U-Pb baddeleyite analyses for eight intrusions from seven localities yield 207Pb/206Pb ages that range from 775 to 782 Ma, with most analyses less than 2% discordant. The 207Pb/206Pb dates from the eight mafic dikes and sheets are statistically indistinguishable and we have combined sixteen individual baddeleyite analyses to yield a composite U-Pb Concordia age of 780.3 +/- 1.4 Ma (95% confidence level). We term this 780 Ma event the Gunbarrel magmatic event based on exposures of the Gunbarrel gabbro found along Gunbarrel Inlet, Great Bear Lake, Northwest Territories. Other mafic rocks that may be part of the gunbarrel event include some of the voluminous mafic sills that intrude the Belt/Purcell Supergroups in the U.S. and Canada and the northwest-trending dike at Mount Moran in the Teton Mountains of Wyoming. Overall, this is the geographically largest mafic event yet identified along the Neoprotoerozoic western margin of Laurentia. The origin of the mafic magmatism of the Gunbarrel event is not clear, but it may be related to mantle plume activity or the upwelling of asthenosphere leading to crustal extension accompanying initial breakup of the supercontinent Rodinia and development of the proto-Pacific Ocean. The mafic magmatism of the Gunbarrel magmatic event at 780 Ma predates the

  6. Pre-Snowball Earth ecosystems, insights from nitrogen isotopes in the Neoproterozoic Kwagunt Formation of the Chuar Supergroup, Grand Canyon.

    NASA Astrophysics Data System (ADS)

    Junium, C. K.; Arthur, M. A.; Freeman, K. H.

    2008-12-01

    Reconstructing marine ecosystems prior to the Neoproterozoic Snowball Earth episodes is an important constraint to our understanding of these events and to the biogeochemical evolution of the Neoproterozoic. The mixed siliciclastic and carbonate sequence comprising the Chuar Supergroup, Grand Canyon, USA, encompasses the period in Earth history immediately prior to the first of the Snowball Earth episodes 740 million years ago and presents a unique opportunity explore this issue in relatively immature rocks. In the Phanerozoic, there is a consistent relationship between euxinic basins, widespread black shale deposition, and δ15N values below 0‰ that are indicative of nitrogen-fixation supported primary productivity. Fe-speciation data from black shales of the Walcott Member of the Kwagunt Formation, upper Chuar Supergroup suggest that the Chuar water column and the middle Neoproterozoic deep ocean was euxinic. Bulk δ15N values from the Walcott Member range from +1.7 to +4.7‰, which are not directly supportive of nitrogen fixation. Preliminary nitrogen isotope data from organic extracts suggest that organic nitrogen may be more significantly enriched than bulk isotopes suggest. These relatively 15N- enriched values contrast those from Phanerozoic episodes of black shale deposition. The Walcott primary producer community is interpreted to have utilized a nitrogen substrate that had an isotopic composition more like that of nitrate in modern, relatively oxygenated marine systems. On the basis of biomarker data many black shale sequences of the Phanerozoic (e.g. Devonian, Permo-Triassic and Cretaceous) record the presence of an active phototrophic sulfide oxidizer community. In contrast, our best efforts have yet to yield biomarker evidence of photic zone euxinia within the Walcott. We therefore suggest that the sulfidic chemocline during Walcott deposition was consistently below the photic zone. Oxygenic phototrophic primary producers utilized nitrate from the

  7. The Sedimentary Signature of Recent Tsunamis

    NASA Astrophysics Data System (ADS)

    Jaffe, B. E.; Peters, R. B.; Richmond, B. M.

    2011-12-01

    The 2011 Tohoku-Oki tsunami, which killed approximately 21,000 and at most locations in northeastern Japan inundated farther inland than historic tsunamis, underscores the need to use the geologic record for an accurate assessment of tsunami hazard. The ability to identify tsunami deposits, the primary geologic record of tsunamis, has greatly improved as teams of scientists have collected data on deposit and flow characteristics as soon as possible after recent tsunamis. Although sand and boulder transport and deposition were reported for the 1946 tsunami in Hawaii, detailed post-tsunami sedimentological surveys were not conducted until the early 1990s. Since then, documenting sedimentary deposits has been part of the scientific response to nearly all major tsunamis. In total, there have been detailed geologic surveys of more than a dozen tsunamis; these surveys have increased in scope and sophistication. Sandy tsunami deposits on coastal plains have common characteristics (Peters and Jaffe, 2010). They are typically deposited in sheets that drape pre-existing topography. Deposits are absent near the shoreline in a zone of bypass or erosion that increases in width with tsunami size and can extend more than 100 m inland. Deposits generally thin landward, typically are less than 30 cm thick near the shoreline, but in rare cases may be more than a meter thick locally in depressions and bends in rivers. Conversely deposits are thinner on local topographic highs, such as ridges. Deposits are comprised of layers that are interpreted as forming during half-wave cycles. Typically there are 1 to 4 layers, each with a sharp lower contact. Each layer's lower portion is often either massive or inversely graded and its top suspension graded, a form of normal grading created when sediment in suspension settles out of the water column as the flow speed decreases. In muddy environments, deposits commonly contain mud and soil rip-up clasts and mud often caps the deposits or

  8. Mid-Latitude Sedimentary Rock: Spallanzani Crater

    NASA Technical Reports Server (NTRS)

    2001-01-01

    [figure removed for brevity, see original site]

    Although most of the best examples of layered sedimentary rock seen on Mars are found at equatorial and sub-tropical latitudes, a few locations seen at mid- and high-latitudes suggest that layered rocks are probably more common than we can actually see from orbit. One extremely good example of these 'atypical' layered rock exposures is found in the 72 km-diameter (45 miles) crater, Spallanzani (58.4oS, 273.5oW). Located southeast of Hellas Planitia, the crater is named for the 18th Century Italian biologist, Lazzaro Spallanzani (1729-1799). Picture A presents a composite of the best Viking orbiter image (VO2-504B55) of the region with 4 pictures obtained June 1999 through January 2001 by the Mars Global Surveyor Mars Orbiter Camera (MOC). Each MOC narrow angle image is 3 km across. Taken in the MOC's 'survey mode,' all four images were acquired at roughly 12 meters (39 ft) per pixel. Picture B zooms-in on the portion of the composite image that includes the 4 MOC images (the 100%-size view is 20 m (66 ft) per pixel). Other craters in the region near Spallanzani show features--at Viking Orbiter scale--that are reminiscent of the layering seen in Spallanzani. Exactly what these layers are made of and how they came to be where we see them today are mysteries, but it is possible that they are similar to the materials seen in the many craters and chasms of the equatorial latitudes on Mars.

  9. Continental Growth and the Sedimentary Record

    NASA Astrophysics Data System (ADS)

    Dhuime, B.; Hawkesworth, C. J.; Robinson, R. A. J.; Cawood, P. A.

    2014-12-01

    Detrital sedimentary rocks provide average samples of the continental crust formed at different times and in different places. Some materials are more susceptible to erosion and/or to preservation bias than others, and one issue is to understand how the compositions of a range of source rocks are then recorded in the sediments. Here we considered two different approaches to model the growth of the continental crust: (i) The variation of Nd isotopes in continental shales with different deposition ages, which requires a correction of the bias induced by preferential erosion of younger rocks through an erosion parameter usually referred to as 'K'. The determination of K, and the extent to which it varies in different erosion systems, thus have fundamental implications for the models of continental growth based on radiogenic isotopes in continental sediments. (ii) The variations in U-Pb, Hf and O isotopes in detrital zircons, from 'modern' sediments sampled worldwide. In this approach, O isotopes are used to screen 'hybrid' Hf model ages (i.e. ages resulting from mixing processes of crustal material from different ages) from Hf model ages that represent actual crust formation ages. These two approaches independently suggest that the continental crust has been generated continuously, but with a marked decrease in the continental growth rate at ~3 Ga. The >4 Ga to ~3 Ga period is characterised by relatively high net rates of continental growth (~3.0 km3.a-1), which are similar to the rates at which new crust is generated, and destroyed, at the present time. Net growth rates are much lower since 3 Ga (~0.8 km3.a-1), which may be attributed to higher rates of destruction of continental crust. The inflexion in the continental growth curve at ~3 Ga indicates a change in the way the crust was generated and preserved. This change may be linked to onset of subduction-driven plate tectonics and discrete subduction zones.

  10. Identification of a late Quaternary alluvial-aeolian sedimentary sequence in the Sichuan Basin, China

    NASA Astrophysics Data System (ADS)

    Feng, Jin-Liang; Ju, Jian-Ting; Chen, Feng; Hu, Zhao-Guo; Zhao, Xiang; Gao, Shao-Peng

    2016-03-01

    The late Quaternary sedimentary sequence in the northwestern part of the Sichuan Basin consists of five lithological units and with increasing depth include the: Chengdu Clay; Brown Clay; Red Clay; Sandy Silt; and basal Muddy Gravel. The genesis, provenance and age of the sediments, as well as the possible presence of hiatuses within this sequence are debated. Measurements of grain-size, magnetic susceptibility, quartz content, quartz δ18O values, element composition, and Sr-Nd isotopic concentrations of samples from a typical sedimentary sequence in the area provides new insights into the genesis and history of the sequence. The new data confirm that the sediments in study site are alluvial-aeolian in origin, with basal alluvial deposits overlain by aeolian deposits. Like the uppermost Chengdu Clay, the underlying Brown Clay and Red Clay are aeolian in origin. In contrast, the Silty Sand, like the basal Muddy Gravel, is an alluvial deposit and not an aeolian deposit as previously thought. Moreover, the succession of the aeolian deposits very likely contains two significant sedimentary hiatuses. Sedimentological analysis demonstrates that the source materials for the aeolian deposits in the northwestern part of the Sichuan Basin and those on the eastern Tibetan Plateau are different. Furthermore, the loess deposits on the eastern Tibetan Plateau are derived from heterogeneous local sources.

  11. Late Mesozoic North African continental margin: Sedimentary sequences and subsidence history

    SciTech Connect

    Kuhnt, W.; Obert, D.

    1988-08-01

    Cretaceous facies types and subsidence history have been studied along two well outcropping and almost complete transversals through the Tellian units of the Mesozoic North African margin, the Western Rif (Morocco), and the Babors (Algeria). Sedimentologic observations and characteristic foraminiferal assemblages enabled estimates for Late Cretaceous paleobathymetries. Both palinspastic reconstruction and sedimentologic and biofacies analyses led to the following results. (1) The morphology and evolution of the Cretaceous North African margin, which in general represents a classic passive continental margin, were complicated by various factors such as Late Cretaceous compressional and lateral movements, the onset of (tectonically controlled ) diapirism, and the existence of intramarginal highs and basins. (2) The Cretaceous subsidence history of both areas can be divided into four stages which are accompanied by characteristic sedimentary formations: (I) distension and subsidence of the margin (Early Cretaceous); (II) a first compressional phase with uplift and slight metamorphism in the Albian/early Cenomanian which affected mainly the northerly paleogeographic zones, accompanied by first diapiric movements and resedimentation of Triassic saliferous material; (III) a Late Cretaceous stage of subsidence (Cenomanian-Santonian); and (IV) a second compressional phase starting with the Campanian and reflected by the formation of sedimentary klippes and olistostromes. (3) As a general trend, sedimentary basins deepened from south to north during Campanian/Maastrichtian time, giving rise to a characteristic succession of bathymetric zones which have been observed on both transversals.

  12. Areal distribution of sedimentary facies determined from seismic facies analysis and models of modern depositional systems

    SciTech Connect

    Seramur, K.C.; Powell, R.D.; Carpenter, P.J.

    1988-01-01

    Seismic facies analysis was applied to 3.5-kHz single-channel analog reflection profiles of the sediment fill within Muir Inlet, Glacier Bay, southeast Alaska. Nine sedimentary facies have been interpreted from seven seismic facies identified on the profiles. The interpretations are based on reflection characteristics and structural features of the seismic facies. The following reflection characteristics and structural features are used: reflector spacing, amplitude and continuity of reflections, internal reflection configurations, attitude of reflection terminations at a facies boundary, body geometry of a facies, and the architectural associations of seismic facies within each basin. The depositional systems are reconstructed by determining the paleotopography, bedding patterns, sedimentary facies, and modes of deposition within the basin. Muir Inlet is a recently deglaciated fjord for which successive glacier terminus positions and consequent rates of glacial retreat are known. In this environment the depositional processes and sediment characteristics vary with distance from a glacier terminus, such that during a retreat a record of these variations is preserved in the aggrading sediment fill. Sedimentary facies within the basins of lower Muir Inlet are correlated with observed depositional processes near the present glacier terminus in the upper inlet.

  13. Middle-Late Mesozoic sedimentary provenances of the Luxi and Jiaolai areas: Implications for tectonic evolution of the North China Block

    NASA Astrophysics Data System (ADS)

    Xu, Jianqiang; Li, Zhong

    2015-11-01

    Provenances of sedimentary rocks may provide important constraints on the tectonic evolution of the North China Block (NCB). Previous studies have demonstrated that the northern NCB (NNCB) and the Xing-Meng orogenic belt (XMOB) supplied massive detritus southward into the hinterland of the NCB during the Jurassic. In order to study the evolution of sedimentary provenance during the Middle-Late Mesozoic, U-Pb geochronology and Hf isotopic geochemistry of detrital zircon grains and chemical compositions of detrital garnets from sandstones in the Luxi and Jiaolai areas, eastern NCB, were analyzed in combination with published data on the Jurassic sandstones. The Late Paleozoic-Mesozoic (367-139 Ma) zircons in the lowermost Cretaceous Mengyin Formation samples from the Luxi area show εHf(t) values of -15.3 to -3.2 and +1.3 to +10.0, which are very similar to the results of analyses of the Jurassic formations. Further, the increased amount of Mesozoic zircons and granulite-derived garnets in the Mengyin Formation samples, compared to those in the Jurassic samples, indicates there was more detritus supply from the NNCB than from the XMOB. In the overlying Qingshan Formation samples, zircon grains do not exhibit Paleozoic ages, but most of them have Early Cretaceous ages and negative εHf(t) values, which are similar to the zircon grains extracted from the widespread Early Cretaceous igneous rocks in the NCB. This suggests that the provenance might have changed to a locally derived source. In contrast, the zircon population of the Early Cretaceous sandstones from the Jiaolai basin is dominated by grains of mid-Neoproterozoic age (700-900 Ma) which signifies contribution from the Sulu orogen. Moreover, the detrital garnet assemblages of sandstones in the Luxi area are not consistent with those from representative metamorphic rocks in the Sulu orogen. The above results seem to confirm that the Mesozoic sedimentary provenance of the Luxi area had no evident connection with

  14. Release of biodegradable dissolved organic matter from ancient sedimentary rocks

    NASA Astrophysics Data System (ADS)

    Schillawski, Sarah; Petsch, Steven

    2008-09-01

    Sedimentary rocks contain the largest mass of organic carbon on Earth, yet these reservoirs are not well integrated into modern carbon budgets. Here we describe the release of dissolved organic matter (DOM) from OM-rich sedimentary rocks under simulated weathering conditions. Results from column experiments demonstrate slow, sustained release of DOM from ancient sedimentary rocks under simulated weathering conditions. 1H-NMR analysis of shale-derived DOM reveals a highly aliphatic, carbohydrate-poor material distinct from other natural DOM pools. Shale-derived DOM is rapidly assimilated and biodegraded by aerobic heterotrophic bacteria. Consequently, no compositional signature of shale-derived DOM other than 14C-depletion is likely to persist in rivers or other surface reservoirs. Combined, these efforts show that dissolution provides a mechanism for the conversion of refractory kerogen into labile biomass, linking rock weathering with sedimentary OM oxidation and the delivery of aged OM to rivers and ocean margins.

  15. Precambrian shield and basement tectonics in sedimentary basin analysis

    SciTech Connect

    Touborg, J.F.

    1984-04-01

    This study focused on the use of (1) regional structural analysis of basement and Precambrian rocks surrounding a sedimentary basin, and (2) tracing basement structures into the sedimentary basin. The structural analysis of the Precambrian shield has a fundamental bearing on interpretation of overlying sedimentary cover rocks. This is expressed in the southern part of the Hudson's Bay basin and its southeastern arm, the Moose River basin. For instance, the rims of both basins are controlled by faults or graben structures. Approximately 13 major fault systems with strike lengths of 200-300 km (125-186 mi) or more can be traced from the exposed Precambrian shield into the basin in terms of lineament arrays and/or aeromagnetic and/or gravity signature. The data suggest reactivation of faults during basin sedimentation. This type of basement structural analysis in areas adjacent to sedimentary basins can provide a valuable interpretation base for subsequent seismic surveys and basin evaluation.

  16. High-pressure thermal aureoles around two Neoproterozoic synorogenic magmatic epidote-bearing granitoids, Northeastern Brazil

    NASA Astrophysics Data System (ADS)

    Caby, Renaud; Sial, Alcides N.; Ferreira, Valderez P.

    2009-02-01

    Unusual high-pressure inner thermal aureoles are described from the Minador and Angico Torto epidote-bearing tonalitic plutons that emplaced into greenschist-facies metasedimentary rocks of the Neoproterozoic Cachoeirinha-Salgueiro belt, northeastern Brazil. The foliated pelitic hornfelses display the mineral assemblage garnet, kyanite, staurolite, muscovite, biotite, plagioclase ± quartz. Rare fibrolite is only found very close to the contacts. Hornfelses display steep mineral lineations and steeply-dipping foliations concordant with magmatic contacts. Leucocratic veinlets containing quartz, oligoclase, garnet, kyanite, staurolite, rutile and ilmenite suggest that limited melting conditions were reached very close to magmatic contacts ( T ⩾ 650 °C, P around 8 kbar). These high-pressure hornfelses form a few meters thick, rigid envelopes around the two plutons. Contrary to known examples of kyanite-bearing hornfelses that recorded high-temperature decompression, the nearly isobaric cooling down to ca. 450 °C is constrained by 3.20-3.30 Si contents of retrogressive phengites from both inner hornfelses and ductilely-deformed tonalite at the pluton margins. Isograds and bathograds are, therefore, apparently telescoped due to HP/LT shearing, possibly caused by subsequent differential vertical movements affecting these two solidified plutons. The unusual depth of emplacement of these syn-kinematic calc-alkaline plutons is explained by a tentative geodynamic model involving a pre-620 Ma-subduction setting. Resumen Las aureolas internas que rodean dos plutones tonalíticos emplazados dentro de rocas cajas en facies esquistos verdes del Cinturón-plegado Cachoeirinha-Salgueiro al noreste de Brasil, contienen hornfelses pelíticos foliados con granate, kyanita, estaurolita, muscovita, biotita, plagioclasa ± cuarzo. Fibrolita es rara ó es encontrada solamente cerca de las zonas de contacto. Los hornfelses desarrollaron foliaciones concordantes con buzamiento fuerte

  17. Neoproterozoic geodynamic evolution of SW-Gondwana: a southern African perspective

    NASA Astrophysics Data System (ADS)

    Frimmel, H. E.; Basei, M. S.; Gaucher, C.

    2011-04-01

    Our current understanding of the tectonic history of the principal Pan-African orogenic belts in southwestern Africa, reaching from the West Congo Belt in the north to the Lufilian/Zambezi, Kaoko, Damara, Gariep and finally the Saldania Belt in the south, is briefly summarized. On that basis, possible links with tectono-stratigraphic units and major structures on the eastern side of the Río de la Plata Craton are suggested, and a revised geodynamic model for the amalgamation of SW-Gondwana is proposed. The Río de la Plata and Kalahari Cratons are considered to have become juxtaposed already by the end of the Mesoproterozoic. Early Neoproterozoic rifting led to the fragmentation of the northwestern (in today's coordinates) Kalahari Craton and the splitting off of several small cratonic blocks. The largest of these ex-Kalahari cratonic fragments is probably the Angola Block. Smaller fragments include the Luis Alves and Curitiba microplates in eastern Brazil, several basement inliers within the Damara Belt, and an elongate fragment off the western margin, named Arachania. The main suture between the Kalahari and the Congo-São Francisco Cratons is suspected to be hidden beneath younger cover between the West Congo Belt and the Lufilian/Zambezi Belts and probably continues westwards via the Cabo Frío Terrane into the Goiás magmatic arc along the Brasilia Belt. Many of the rift grabens that separated the various former Kalahari cratonic fragments did not evolve into oceanic basins, such as the Northern Nosib Rift in the Damara Belt and the Gariep rift basin. Following latest Cryogenian/early Ediacaran closure of the Brazilides Ocean between the Río de la Plata Craton and the westernmost fragment of the Kalahari Craton, the latter, Arachania, became the locus of a more than 1,000-km-long continental magmatic arc, the Cuchilla Dionisio-Pelotas Arc. A correspondingly long back-arc basin (Marmora Basin) on the eastern flank of that arc is recognized, remnants of which

  18. Origin of Neoproterozoic ophiolitic peridotites in south Eastern Desert, Egypt, constrained from primary mantle mineral chemistry

    NASA Astrophysics Data System (ADS)

    Khedr, Mohamed Zaki; Arai, Shoji

    2013-10-01

    The ophiolitic peridotites in the Wadi Arais area, south Eastern Desert of Egypt, represent a part of Neoproterozoic ophiolites of the Arabian-Nubian Shield (ANS). We found relics of fresh dunites enveloped by serpentinites that show abundances of bastite after orthopyroxene, reflecting harzburgite protoliths. The bulk-rock chemistry confirmed the harzburgites as the main protoliths. The primary mantle minerals such as orthopyroxene, olivine and chromian spinel in Arais serpentinites are still preserved. The orthopyroxene has high Mg# [=Mg/(Mg + Fe2+)], ~0.923 on average. It shows intra-grain chemical homogeneity and contains, on average, 2.28 wt.% A12O3, 0.88 wt.% Cr2O3 and 0.53 wt.% CaO, similar to primary orthopyroxenes in modern forearc peridotites. The olivine in harzburgites has lower Fo (93-94.5) than that in dunites (Fo94.3-Fo95.9). The Arais olivine is similar in NiO (0.47 wt.% on average) and MnO (0.08 wt.% on average) contents to the mantle olivine in primary peridotites. This olivine is high in Fo content, similar to Mg-rich olivines in ANS ophiolitic harzburgites, because of its residual origin. The chromian spinel, found in harzburgites, shows wide ranges of Cr#s [=Cr/(Cr + Al)], 0.46-0.81 and Mg#s, 0.34-0.67. The chromian spinel in dunites shows an intra-grain chemical homogeneity with high Cr#s (0.82-0.86). The chromian spinels in Arais peridotites are low in TiO2, 0.05 wt.% and YFe [= Fe3+/(Cr + Al + Fe3+)], ~0.06 on average. They are similar in chemistry to spinels in forearc peridotites. Their compositions associated with olivine’s Fo suggest that the harzburgites are refractory residues after high-degree partial melting (mainly ~25-30 % partial melting) and dunites are more depleted, similar to highly refractory peridotites recovered from forearcs. This is in accordance with the partial melting (>20 % melt) obtained by the whole-rock Al2O3 composition. The Arais peridotites have been possibly formed in a sub-arc setting (mantle wedge), where

  19. Transposition of structures in the Neoproterozoic Kaoko Belt (NW Namibia) and their absolute timing

    NASA Astrophysics Data System (ADS)

    Ulrich, Stanislav; Konopásek, Jiří; Jeřábek, Petr; Tajčmanová, Lucie

    2010-05-01

    The Neoproterozoic Kaoko Belt in Namibia is classical example of lower to middle crust transpressional orogeny developed between attenuated Congo Craton margin and the Coastal Terrane. The transpression has been described as a two phase event of early oblique thrusting followed by sinistral wrench shearing on the same foliation planes rotated into the subvertical orientations (e.g. Goscombe et al., 2003). Konopásek et al. (2005) argued that early fabric is not rotated, but intensely folded and the wrench stage operates on newly developed foliation parallel to axial planes of these folds. Three structural profiles across the Coastal Terrane, the Boundary Igneous Complex and the Orogen Core derived from the Congo Craton have been studied in order to assess mechanism of transpression and evaluate absolute timing of individual deformation events. The oldest known subhorizontal Si fabric occurs in the Coastal Terrane only, and is inherited from a pre-collisional HT-LP event dated at 650-630Ma. The S1 fabric occurs in all tectonic units, it is gently dipping to the W-SW and contains subhorizontal stretching lineation. Temperature as well as intensity of its development decreases westwards from penetrative granulite facies fabric in the Orogen Core to lower amphibolite facies axial plane cleavage in the Coastal Terrane. Associated kinematic criteria as S-C fabric in deformed granitoids of the Boundary Igneous Complex show very oblique, top-to-the-SE-oriented thrusting to sinistral shearing. Superimposed subvertical S2 fabric developed in axial planes of upright isoclinal folds, almost homogeneously reworking S1 fabric in the Orogen Core, whereas in the Boundary Igneous Complex and the Coastal Terrane, the S2 fabric is developed with increasing intensity from south to north. Temperature conditions of S2 development decrease westwards. Stretching lineations developed on S2 planes show the same orientation as those on the S1 planes and kinematic indicators associated with D

  20. Alteration of Sedimentary Clasts in Martian Meteorite Northwest Africa 7034

    NASA Technical Reports Server (NTRS)

    McCubbin, F. M.; Tartese, R.; Santos, A. R.; Domokos, G.; Muttik, N.; Szabo, T.; Vazquez, J.; Boyce, J. W.; Keller, L. P.; Jerolmack, D. J.; Anand, M.; Moser, D. E.; Delhaye, T.; Shearer, C. K.; Agee, C. B.

    2014-01-01

    The martian meteorite Northwest Africa (NWA) 7034 and pairings represent the first brecciated hand sample available for study from the martian surface [1]. Detailed investigations of NWA 7034 have revealed substantial lithologic diversity among the clasts [2-3], making NWA 7034 a polymict breccia. NWA 7034 consists of igneous clasts, impact-melt clasts, and "sedimentary" clasts represented by prior generations of brecciated material. In the present study we conduct a detailed textural and geochemical analysis of the sedimentary clasts.

  1. Sedimentary Geology Context and Challenges for Cyberinfrastructure Data Management

    NASA Astrophysics Data System (ADS)

    Chan, M. A.; Budd, D. A.

    2014-12-01

    A cyberinfrastructure data management system for sedimentary geology is crucial to multiple facets of interdisciplinary Earth science research, as sedimentary systems form the deep-time framework for many geoscience communities. The breadth and depth of the sedimentary field spans research on the processes that form, shape and affect the Earth's sedimentary crust and distribute resources such as hydrocarbons, coal, and water. The sedimentary record is used by Earth scientists to explore questions such as the continental crust evolution, dynamics of Earth's past climates and oceans, evolution of the biosphere, and the human interface with Earth surface processes. Major challenges to a data management system for sedimentary geology are the volume and diversity of field, analytical, and experimental data, along with many types of physical objects. Objects include rock samples, biological specimens, cores, and photographs. Field data runs the gamut from discrete location and spatial orientation to vertical records of bed thickness, textures, color, sedimentary structures, and grain types. Ex situ information can include geochemistry, mineralogy, petrophysics, chronologic, and paleobiologic data. All data types cover multiple order-of-magnitude scales, often requiring correlation of the multiple scales with varying degrees of resolution. The stratigraphic framework needs dimensional context with locality, time, space, and depth relationships. A significant challenge is that physical objects represent discrete values at specific points, but measured stratigraphic sections are continuous. In many cases, field data is not easily quantified, and determining uncertainty can be difficult. Despite many possible hurdles, the sedimentary community is anxious to embrace geoinformatic resources that can provide better tools to integrate the many data types, create better search capabilities, and equip our communities to conduct high-impact science at unprecedented levels.

  2. Sedimentary Rocks of the Buckeye Range, Horlick Mountains, Antarctica.

    PubMed

    Long, W E

    1962-04-27

    In the Buckeye Range of the Horlick Mountains, 4000 feet of sedimentary rocks nonconformably overlie a granitic basement and underlie a thick diabasic sill. The sedimentary section consists of Devonian sandstone and shale (Horlick formation), Carboniferous (?) tillite (Buckeye formation), Permian (?) platy and carbonaceous shale (Discovery Ridge formation), and Permian arkose, shale, and numerous coal beds (Mount Glossopteris formation). This apparently is the first report of a Paleozoic tillite in Antarctica. PMID:17745908

  3. Direct stable isotope porewater equilibration and identification of groundwater processes in heterogeneous sedimentary rock.

    PubMed

    David, Katarina; Timms, Wendy; Baker, Andy

    2015-12-15

    The off-axis integrated cavity output spectrometry (ICOS) method to analyse porewater isotopic composition has been successfully applied over the last decade in groundwater studies. This paper applies the off-axis ICOS method to analyse the porewater isotopic composition, attempts to use the isotopic shift in groundwater values along with simple geochemical mixing model to define the groundwater processes in the Sydney Basin, Australia. Complementary data included geophysical, hydrogeological, geochemical, and mineralogical investigations. Porewater from core samples were analysed for δ(18)O and δ(2)H from various sedimentary units in the Basin and compared to endpoint water members. Stable δ(18)O and δ(2)H values of porewaters in the Basin (-9.5 to 2.8‰ for δ(18)O and -41.9 to 7.9‰ for δ(2)H) covered a relatively narrow range in values. The variability in water isotopes reflects the variability of the input signal, which is the synoptic variability in isotopic composition of rainfall, and to a minor extent the subsequent evaporation. The porosity, bulk density and mineralogy data demonstrate the heterogeneity that adds the complexity to variations in the isotope profile with depth. The source of chloride in the sedimentary sequence was related to rock-water and cement/matrix-water interaction rather than to evaporation. The heterogeneous character of the sedimentary rock strata was supported by a change in pore pressures between units, density and variability in rock geochemical analyses obtained by using X-ray fluorescence (XRF) and X-ray power diffraction analyses. This research identified distinct hydrogeological zones in the Basin that were not previously defined by classic hydrogeological investigations. Isotopic signature of porewaters along the detailed vertical profile in combination with mineralogical, geochemical, geophysical and hydrogeological methods can provide useful information on groundwater movement in deep sedimentary environments. The

  4. A complex microbiota from snowball Earth times: Microfossils from the Neoproterozoic Kingston Peak Formation, Death Valley, USA

    PubMed Central

    Corsetti, Frank A.; Awramik, Stanley M.; Pierce, David

    2003-01-01

    A thin carbonate unit associated with a Sturtian-age (≈750–700 million years ago) glaciogenic diamictite of the Neoproterozoic Kingston Peak Formation, eastern California, contains microfossil evidence of a once-thriving prokaryotic and eukaryotic microbial community (preserved in chert and carbonate). Stratiform stromatolites, oncoids, and rare columnar stromatolites also occur. The microbial fossils, which include putative autotrophic and heterotrophic eukaryotes, are similar to those found in chert in the underlying preglacial units. They indicate that microbial life adapted to shallow-water carbonate environments did not suffer the significant extinction postulated for this phase of low-latitude glaciation and that trophic complexity survived through snowball Earth times. PMID:12682298

  5. Geochemistry of the Neoproterozoic metasediments of Malhaq and Um Zariq formations, Kid Metamorphic Complex, Sinai, Egypt: implications for source-area weathering, provenance, recycling, and depositional tectonic setting

    NASA Astrophysics Data System (ADS)

    El-Bialy, Mohammed Z.

    2013-04-01

    The Kid Metamorphic Complex of SE Sinai represents a thick volcano-sedimentary succession that underwent polyphase deformation and greenschist to upper amphibolite facies metamorphism in the NE part of the Arabian-Nubian Shield (ANS). The Malhaq and Um Zariq Formations, the target of this study, occupy roughly the northern half of this complex. The Malhaq Formation records several phases of Ediacaran sedimentation and volcanic activity (615-607 Ma), whereas Um Zariq Formation metasediments are relicts of an older sedimentary sequence (Cryogenian; 813±6 Ma). The Malhaq Formation comprises a series of dark gray structureless to schistose felsic to intermediate metavolcanics interbedded and intercalated with fine- to medium-grained foliated mica-rich phyllites and schists, while the Um Zariq Formation is a dominantly metasedimentary sequence, mainly represented by well-bedded metapelitic schists. Malhaq metasediments are enriched in SiO2, CaO and K2O and depleted in TiO2, Al2O3 and K2O relative to those of Um Zariq Formation. Aside from the relatively low Ni and Cr concentrations, compatible transition elements of these metasediments are comparable to average crustal contents. Except for marked Sr depletion, LILEs are around average continental crust values. Pronounced negative Nb-Ta anomalies in all samples, and general enrichment of Um Zariq samples in Th, U, Zr, Ti and Y relative to Malhaq ones are the main features of HFSEs. The REE patterns of all samples are parallel to sub-parallel LREE-enriched, with distinct negative Eu anomalies and weakly fractionated HREE segments. Geochemical investigations have revealed that the source rocks of Malhaq Formation metasediments underwent mild to moderate chemical weathering, whereas those of Um Zariq Formation have suffered severe chemical weathering. These metasediments are predominately derived from felsic to intermediate igneous sources, with a particular slight addition from recycled sedimentary source to the Malhaq

  6. Neoproterozoic to Paleozoic Geological Evolution of Mongolia: Constraints on Modes of "Crustal Growth" in the Central Asian Orogenic Belt

    NASA Astrophysics Data System (ADS)

    Macdonald, F. A.; Bold, U.; Smith, E.; Olin, P. H.; Crowley, J. L.; Schmitz, M. D.

    2012-12-01

    The Central Asian Orogenic Belt (CAOB) is widely considered the largest area of Phanerozoic juvenile crustal growth on Earth. However, the timing and nature of the orogenic events in the core of the CAOB in Mongolia has remained poorly constrained due to a dearth of detailed geological and geochronological studies. To bridge this gap and test models of crustal growth, here we refine the sequencing of geological events by focusing on the formation and destruction of Neoproterozoic and Paleozoic tectonic basins. Mongolia's basins record a complete Neoproterozoic to Cambrian Wilson cycle with rifting of the Mongolian continent at ca. 700 Ma, the development of a Cryogenian to Ediacaran thermally subsiding passive margin, an arc-continent collision at ca. 520 Ma, and a continent-arc-continent collision at ca. 500 Ma. During this collisional orogeny, that is the Cambrian Altaids, crustal growth occurred largely through the obduction of ophiolites. Rifting of the southern margin occurred during the Ordovician Period, with the development of a Silurian passive margin. Oblique northwest-dipping subduction was initiated during the Devonian and resulted in a transpressional accretionary orogen. The CAOB culminated with a continent-arc-continent collision and the accretion of the North China and Tarim Blocks in the latest Permian. The Devonian to early Permian accretionary orogen is associated not only with voluminous plutonism, but also, major translational structures oblique to the margin resulting in the appearance of many accreted terranes. These data are consistent with existing coarse Hf and Nd isotopic data, but also provide a framework for future detailed studies. Although our geological constraints suggest distinct periods of apparent crustal growth through either collisional or accretionary orogenies, net crustal growth after accounting for recycling is equivocal.

  7. Extreme events in the sedimentary record of maar Lake Pavin: Implications for natural hazards assessment in the French Massif Central

    NASA Astrophysics Data System (ADS)

    Chassiot, Léo; Chapron, Emmanuel; Di Giovanni, Christian; Albéric, Patrick; Lajeunesse, Patrick; Lehours, Anne-Catherine; Meybeck, Michel

    2016-06-01

    A set of sedimentary cores, high resolution swath bathymetry and subbottom profiler data provides new insights on sedimentary processes in meromictic maar Lake Pavin, France. Three sedimentary environments (i.e., littoral, plateau and basin) have been identified in the lake from sediment composition using bulk organic geochemistry and the analysis of hydroacoustic images. Various forms of rapidly deposited layers (RDLs) have been identified and radiocarbon dated. An up to date stratigraphy of sedimentary events matching coeval RDLs across the lake is presented and illustrates a wide range of natural hazards linked to Lake Pavin during the last 2000 years. In AD 600, a sudden lake outburst triggered a slump deposit along with a 9 m lake-level drop that drove shifts in sedimentary organic matter composition. Outside the lake, outburst flood deposits have been described downstream and provide sedimentary evidence for this event. The lake-level drop also favored the generation of gravity reworking processes, as shown by (1) a regional earthquake-triggered large slope failure on the plateau connected to a mass-wasting deposit in the basin dated to AD 1300, and (2) a succession of turbidites in AD 1825 and AD 1860 contemporaneous to two historic earthquakes, suggesting that this lake is sensitive to earthquakes with a minimum epicentral intensity of V. Finally, past observations of lake water color changes in AD 1783 and AD 1936, similar to reports in other meromictic lakes, match iron-rich deposits identified in maar lake sediments and suggest that Lake Pavin could have undergone limnic eruptions.

  8. Capitol Success.

    PubMed

    Sorrel, Amy Lynn

    2015-08-01

    This legislative session, medicine resolved to ensure physicians can give their patients the best care possible. The hard work paid off in significant victories that largely build on the Texas Medical Association's 2013 legislative successes. PMID:26263520

  9. Molybdenum (Mo) and Iron (Fe) Isotope Evidence of Tepla-Barrandian Black Shales Against Widespread Deep Ocean Oxygenation in the Late Neoproterozoic

    NASA Astrophysics Data System (ADS)

    Kurzweil, F.; Pasava, J.; Drost, K.; Wille, M.; Schoenberg, R.

    2014-12-01

    The late Neoproterozoic was a period of major environmental perturbations including tectonic reorganizations, biologic evolution and environmental oxygenation (Neoproterozoic oxygenation event). Authigenic enrichments in redox-sensitive elements such as Mo, V and U in late Neoproterozoic black shales prior to the appearance of the first metazoan fossils indicate that increasing oxygen levels in the atmosphere-hydrosphere system have facilitated the evolution and diversification of multi-cellular life. The isotopic composition of these elements is another tool to trace (possibly global) changes in the oceanic redox state. For example, significantly higher δ98Mo of seawater and black shales are expected, when the sink of isotopically light Mo in oxic deep marine settings increased. Accordingly, modern anoxic sediments in the Black Sea as well as the well oxygenated seawater show high δ98Mo of 2.3 ‰. However, Mesoproterozoic black shales show relatively low δ98Mo values up to 1.4 ‰. To test if the enrichment of redox-sensitive elements and metazoan evolution temporally correlate with an increase in seawater δ98Mo, we present Mo and Fe isotope data of slightly younger late Neoproterozoic black shales of the Tepla-Barrandian, Czech Republic. We observe a perfect correlation of Fe/Al ratios with δ56Fe that is best explained by mixing of detrital derived Fe with δ56Fe of ~0.1 ‰ and hydrothermal sourced Fe with δ56Fe of ~-0.7 ‰. Hydrothermally dominated samples with low δ56Fe are also enriched in heavy metals such as Ni, Cu and Zn as well as hydrothermally derived Mo (with δ98Mo of ~0.6 ‰). Samples with minor hydrothermal influence show authigenic Mo enrichment from seawater with a maximum δ98Mo of 1.2 ‰. This estimate indicates no significant increase in the seawater δ98Mo during the Neoproterozoic and after the evolution of metazoan life. Thus, our Mo isotope dataset provides no evidence for deep ocean oxygenation during the Neoproterozoic

  10. Geometry of the neoproterozoic and paleozoic rift margin of western Laurentia: Implications for mineral deposit settings

    USGS Publications Warehouse

    Lund, K.

    2008-01-01

    The U.S. and Canadian Cordilleran miogeocline evolved during several phases of Cryogenian-Devonian intracontinental rifting that formed the western mangin of Laurentia. Recent field and dating studies across central Idaho and northern Nevada result in identification of two segments of the rift margin. Resulting interpretations of rift geometry in the northern U.S. Cordillera are compatible with interpretations of northwest- striking asymmetric extensional segments subdivided by northeast-striking transform and transfer segments. The new interpretation permits integration of miogeoclinal segments along the length of the western North American Cordillera. For the U.S. Cordillera, miogeoclinal segments include the St. Mary-Moyie transform, eastern Washington- eastern Idaho upper-plate margin, Snake River transfer, Nevada-Utah lower-plate margin, and Mina transfer. The rift is orthogonal to most older basement domains, but the location of the transform-transfer zones suggests control of them by basement domain boundaries. The zigzag geometry of reentrants and promontories along the rift is paralleled by salients and recesses in younger thrust belts and by segmentation of younger extensional domains. Likewise, transform transfer zones localized subsequent transcurrent structures and igneous activity. Sediment-hosted mineral deposits trace the same zigzag geometry along the margin. Sedimentary exhalative (sedex) Zn-Pb-Ag ??Au and barite mineral deposits formed in continental-slope rocks during the Late Devonian-Mississippian and to a lesser degree, during the Cambrian-Early Ordovician. Such deposits formed during episodes of renewed extension along miogeoclinal segments. Carbonate-hosted Mississippi Valley- type (MVT) Zn-Pb deposits formed in structurally reactivated continental shelf rocks during the Late Devonian-Mississippian and Mesozoic due to reactivation of preexisting structures. The distribution and abundance of sedex and MVT deposits are controlled by the

  11. Tectono-sedimentary processes at hyper-extended rifted margins: the Alpine and Pyrenean analogues

    NASA Astrophysics Data System (ADS)

    Masini, E.; Manatschal, G.; Mohn, G.; Tugend, J.

    2012-04-01

    The discovery of hyper-extended crust at deep-water rifted margins challenged the way of interpreting rifting and continental breakup. Indeed, syn-rift sedimentary basins, also referred to as either "supra-detachment basins" or "hyper-extended sag basins" are observed over thin hyper-extended crust that tapers oceanwards and is laterally replaced by exhumed subcontinental mantle. Studies performed off- and on-shore have shown that hyper-extended domains are formed by a complex interplay of exhumation faults leading to characteristic basin architectures. Despite of numerous interpretations of seismic sections across passive margins, the stratigraphic architecture of hyper-extended rift domains and their tectono-sedimentary evolution remain little understood. In this presentation, we present field-data from two well-preserved fossil hyper-extended domains exposed in the Alps and Pyrenees analogues that show some insights into the tectono-sedimentary evolution of hyper-extended rifted margins. Despite these two examples show a different sedimentary evolution, they exemplify how detachment systems can control accommodation space and sedimentary architecture. In both cases, the syn-rift sedimentary record can be studied in 3D allowing to evolutionary steps of supra-detachment basins to be identified. The study of these two field examples enables to identify key architectural elements of supra-detachment basins. These are detachment breakaway block and extensional allochthons, both controlling the first-order architecture of the basins. The former is the consequence of two successive low-angle detachment faults separated by a delaminated upper crustal block, whereas the latter results from the delamination of hanging-wall blocks and its emplacement over exhumed footwall. Exhumation along active detachment systems implies a very efficient creation of new real estate seafloor and results in a very specific stratigraphic record that includes from base to top: 1) a syn

  12. On the occurrence of gold mineralization in the Pala Neoproterozoic formations, South-Western Chad

    NASA Astrophysics Data System (ADS)

    Tchameni, Rigobert; Doumnang, Jean Claude; Deudibaye, Marambaye; Branquet, Yannick

    2013-08-01

    The Pala region, in southwestern Chad, belongs to the northern part of the Central African Pan-African Fold Belt. It is made up of greenschist-facies schists and is characterized by bimodal, mainly mafic, magmatism. This schist unit named Goueigoudoum Series is intruded by pre- to post-tectonic plutonic rocks dated between 737 and 570 Ma and dykes of quartz. Gold is mined artisanally from alluvial deposits and primary chalcopyrite-pyrite-bearing quartz veins, brecciated and silicified zones and shear zones. The majority of the mineralized shear zones and some quartz veins generally trend N-S to NNE-SSW or NW-SE and are interpreted as extensional shear fractures related to regional NE-SW-trending sinistral strike-slip shear zones. The geological context of the Pala region clearly indicates hydrothermal fluids formed along active continental margins during collisional orogenesis, and subsequent associated fluid migration typically occurred during strike-slip events. Although the origin of fluids may be varied (magmatic, metamorphic or meteoric fluids, Proterozoic seawater, or sedimentary basin formation waters), the distribution of the mineralizations along the granitoid intrusions suggests that magmatism played a major role in the dynamics of the mineralizing fluids.

  13. Triple-oxygen and sulfur isotopic evidence for diagenetic overprinting of carbonate-associated sulfate in Neoproterozoic samples from a drill core

    NASA Astrophysics Data System (ADS)

    Peng, Y.; Wang, W.; Pratt, L. M.; Zhou, C.; Bao, H.; Hayles, J. A.

    2014-12-01

    Carbonate-associated sulfate (CAS) is used in many studies to reconstruct the isotopic composition of ancient seawater sulfate and to infer stages in the development of Earth's oxygenated atmosphere. CAS is acid extractable and commonly is referred to as structurally substituted sulfate in carbonate minerals. Several recent studies, however, have raised concerns about sulfate overprinting during early or late diagenesis, including contamination by modern secondary atmospheric sulfate (SAS) and by sulfide oxidation during laboratory processing. To test for overprinting and contamination, we studied the isotopic composition of sulfate in a bedded carbonate succession of the Neoproterozoic Lantian Formation, South China. Materials were obtained from a drilling core (635Ma- 551Ma). Water-leachable sulfate (WLS), acid-leachable sulfate (ALS, i. e. extracted CAS), and chromium-reducible sulfur (CRS) were sequentially extracted out and triple oxygen isotopic compositions of WLS and ALS were analyzed as well as sulfur isotope of WLS, ALS, and CRS. We also analyzed the oxygen isotope of sulfate resulting from pyrite oxidation at a condition similar to the extraction of WLS and ALS in the laboratory and the δ18O value is at ~ -1.4‰ (VSMOW). The slightly negative ∆17O values of all WLS and ALS indicates that the ALS was not contaminated by sulfate of modern SAS. The WLS from the first 24 hours with consistently negative values of δ18O (about -11.0‰) and low δ34S values (about +5‰) suggests that the WLS resulted from sulfide oxidation in water with very negative δ18O values, likely glacial melt-water in the distant past, which had likely soaked the whole stratigraphy of Lantian Formation for a long time. The WLS also comprised a significant fraction of ALS because both δ18O and δ34S of ALS have wide ranges, from -6.9 to +15.8‰, and +12.7 to +31.7‰, respectively. More importantly, there is a strong positive correlation between δ18O and δ34S of ALS. Our

  14. Neoproterozoic A-type granitoids of the central and southern Appalachians: Intraplate magmatism associated with episodic rifting of the Rodinian supercontinent

    USGS Publications Warehouse

    Tollo, R.P.; Aleinikoff, J.N.; Bartholomew, M.J.; Rankin, D.W.

    2004-01-01

    Emplacement of compositionally distinctive granitic plutons accompanied two pulses (765-680 and 620-550Ma) of crustal extension that affected the Rodinian craton at the present location of the central Appalachians during the Neoproterozoic. The dominantly metaluminous plutons display mineralogical and geochemical characteristics of A-type granites including high FeO t/MgO ratios, high abundances of Nb, Zr, Y, Ta, and REE (except Eu), and low concentrations of Sc, Ba, Sr, and Eu. These dike-like, sheet complexes occur throughout the Blue Ridge province of Virginia and North Carolina, and were emplaced at shallow levels in continental crust during active extension, forming locally multiple-intrusive plutons elongated perpendicular to the axis of extension. New U-Pb zircon ages obtained from the Polly Wright Cove (706??4Ma) and Suck Mountain (680??4Ma) plutons indicate that metaluminous magmas continued to be replenished near the end of the first pulse of rifting. The Suck Mountain body is presently the youngest known igneous body associated with earlier rifting. U-Pb zircon ages for the Pound Ridge Granite Gneiss (562??5Ma) and Yonkers Gneiss (563??2Ma) in the Manhattan prong of southeastern New York constitute the first evidence of plutonic felsic activity associated with the later period of rifting in the U.S. Appalachians, and suggest that similar melt-generation processes were operative during both intervals of crustal extension. Fractionation processes involving primary minerals were responsible for much of the compositional variation within individual plutons. Compositions of mapped lithologic units in a subset of plutons studied in detail define overlapping data arrays, indicating that, throughout the province, similar petrologic processes operated locally on magmas that became successively more chemically evolved. Limited variation in source-sensitive Y/Nb and Yb/Ta ratios is consistent with results of melting experiments and indicates that metaluminous

  15. Tephrostratigraphy of the last 170 ka in sedimentary successions from the Adriatic Sea

    NASA Astrophysics Data System (ADS)

    Calanchi, Natale; Dinelli, Enrico

    2008-10-01

    In this study are discussed new SEM-EDS analyses performed on glass shards from five cores collected in the Central Adriatic Sea and two cores recovered from the South Adriatic Sea. A total of 26 tephra layers have been characterized and compared with the geochemical features of terrestrial deposits and other tephra archives in the area (South Adriatic Sea and Lago Grande di Monticchio, Vulture volcano). The compositions are compatible with either a Campanian or a Roman provenance. The cores, located on the Central Adriatic inner and outer shelf, recorded tephra referred to explosive events described in the literature: AP3 (sub-Plinian activity of the Somma-Vesuvius, 2710 ± 60 14C years BP); Avellino eruption (Somma-Vesuvius, 3548 ± 129 14C years BP); Agnano Monte Spina (Phlegrean Fields, 4100 ± 400 years BP); Mercato eruption (Somma-Vesuvius, 8010 ± 35 14C years BP; Agnano Pomici Principali eruption (Phlegrean Fields, 10,320 ± 50 14C years BP); Neapolitan Yellow Tuff (Phlegrean Fields, 12,100 ± 170 14C years BP). Some of these layers were also observed in the South Adriatic core IN68-9 in addition to younger ( AP2, sub-Plinian eruption, Somma-Vesuvius, 3225 ± 140 14C years BP), and older layers ( Pomici di Base eruption, Somma-Vesuvius, 18,300 ± 150 14C years BP). Significant is the tephra record of core RF95-7 that, for the first time in the Adriatic Sea, reports the occurrence of tephra layers older than 60 ka: the well known Mediterranean tephra layers X2 (ca. 70 ka), W1 (ca. 140 ka) and V2 (Roman origin, ca. 170 ka) as well as other tephra layers attributed, on the basis of geochemistry and biostratigraphy, to explosive eruptions occurred at Vico (138 ± 2 and 151 ± 3 ka BP) and Ischia (147-140 ka BP). Previous tephra correlations performed on other cores in the Central Adriatic Sea were also critically revised according to new available data, and integrated with the results of this study for a correlation at a regional scale. The most important key beds for correlation are the "Agnano Monte Spina" and "Neapolitan Yellow Tuff" tephra layers, which occur in both Central and South Adriatic sea. Other tephra layers, younger than 15 ka, are relatively less frequent: the Y1 (Etna, 14,180 ± 260 14C years BP) and "Agnano Pomici Principali". In the more recent record, in the shelf cores north of Gargano Promontory, the AP3 tephra layer is also frequent.

  16. Magmatic versus sedimentary volcanism: similarities of two different geological phenomena

    NASA Astrophysics Data System (ADS)

    Mazzini, Adriano

    2015-04-01

    Sedimentary volcanoes (or more commonly called mud volcanoes) are geological phenomena that are present in sedimentary basins of passive and active margins. At these localities gas and water related to hydrocarbon diagenetic and catagenetic production generate overpressure facilitating the rise of mobile and ductily deformable materials that breach through the denser overlying rocks. The results are surface powerful manifestations of mud eruptions that strikingly resemble to those of magmatic volcanoes. Magmatic and sedimentary volcanoes share many other similarities. Initially both systems are essentially gas driven and the subsurface plumbing systems are characterized by intrusions and a complex system of fractures and conduits that bifurcate from a central feeder channel that manifest in the surface as numerous satellite seeps and vents. In both cases are inferred secondary shallower chambers where reactions take place. Comparable structural morphologies (e.g. conical, elongated, pie-shaped, multicrater, swap-like, caldera collapse, subsiding flanks, plateau-like) and/or alteration of the original shape are in both cases related to e.g. density and viscosity of the erupted solids, to the gas content, to the frequency of the eruptions, and to the action of meteoric factors (e.g. strong erosion by rain, wind, temperature changes etc. etc.). Like for magmatic volcanoes, the periodicity of the eruptive activity is related to the time required to charge the system and create new overpressure, as well as how the structure seals during periods of dormancy. Earthquakes are documented to be a powerful trigger capable to activate faults (often hosting magmatic and sedimentary volcanoes) and/or facilitating the breaching of the upper layers, and allowing the rise of deeper charged fluids. Finally, both systems significantly contribute as active source for CH4 (sedimentary) and CO2 (magmatic) resulting of great importance for global budget estimates of sensitive gasses. The

  17. Can We Determine Temperatures Associated with Critical Transitions During the Evolution of Metazoan life? Application of 'Clumped' Isotope Thermometry to the Neoproterozoic and Paleozoic

    NASA Astrophysics Data System (ADS)

    Defliese, W.; Gutierrez, M.; Flores, S.; Retallack, G.; Tripati, A.

    2015-12-01

    The evolution and development of metazoan life during the Neoproterozoic and Paleozoic was one of the largest and monumental events in Earth history. Conditions surrounding these events are uncertain, as there remain many questions about the types of environment transitions such as the development of multicellular life, evolution of hard shells, and the transitions of life to land took place in. While mass-47 clumped isotope signatures are prone to thermal resetting and diagenesis, it remains the best tool for reconstructing temperatures in uncertain regimes, and can be integrated along with traditional tools such as textural petrography and cathodoluminescence to screen for diagenetic alteration. In this context, we analyze suites of Neoproterozoic and Paleozoic sediments and brachiopods for clumped isotope temperatures, and combine with microscopy and stratigraphic data to infer diagenetic and burial histories of these rocks. Samples judged to be unaltered will be further analyzed for the conditions prevalent during critical transitions during the evolution of metazoan life.

  18. Cataloging Common Sedimentary and Deformation Features in Valles Marineris

    NASA Astrophysics Data System (ADS)

    Urso, A.; Okubo, C. H.

    2015-12-01

    The sedimentary deposits in the Valles Marineris region of Mars are investigated to build a catalog of sedimentary and deformational features. The occurrence of these features provides new and important constraints on the origins of these sedimentary deposits and of their broader geologic histories. Regional surveys and mapping of these features is warranted given the plethora of recently acquired observations by the Mars Reconnaissance Orbiter. Select sedimentary and deformational features were identified using High Resolution Imaging Science Experiment (HiRISE) observations and stereo pairs, along with Context camera images. Feature locations were cataloged using Java Mission-planning and Analysis for Remote Sensing (JMARS) the geospatial information system. Images acquired in and around Hebes, Ophir, Tithonium, Candor, Ius, Melas and Coprates Chasmata were the focus of this investigation. Mass wasting processes, soft-sediment deformation structures, and fan-like deposits are known to occur in abundance across the Valles Marineris region. For this reason, the features recorded in this investigation were landslides, contorted bedding, injectites, putative mud volcanoes, faults, folds, and fan-shaped deposits. Landslides, faults, and fan-shaped deposits were found to be common occurrences, while contorted bedding, injectites, putative mud volcanoes, and folds occur less frequently and in clusters. The placement and frequency of these features hint at past tectonic and depositional processes at work in Valles Marineris. This catalogue of sedimentary and deformational features in the Valles Marineris region of Mars is being used to define targets for future HiRISE observations.

  19. Folding and faulting of strain-hardening sedimentary rocks

    USGS Publications Warehouse

    Johnson, A.M.

    1980-01-01

    The question of whether single- or multi-layers of sedimentary rocks will fault or fold when subjected to layer-parallel shortening is investigated by means of the theory of elastic-plastic, strain-hardening materials, which should closely describe the properties of sedimentary rocks at high levels in the Earth's crust. The most attractive feature of the theory is that folding and faulting, intimately related in nature, are different responses of the same idealized material to different conditions. When single-layers of sedimentary rock behave much as strain-hardening materials they are unlikely to fold, rather they tend to fault, because contrasts in elasticity and strength properties of sedimentary rocks are low. Amplifications of folds in such materials are negligible whether contacts between layer and media are bonded or free to slip for single layers of dolomite, limestone, sandstone, or siltstone in media of shale. Multilayers of these same rocks fault rather than fold if contacts are bonded, but they fold readily if contacts between layers are frictionless, or have low yield strengths, for example due to high pore-water pressure. Faults may accompany the folds, occurring where compression is increased in cores of folds. Where there is predominant reverse faulting in sedimentary sequences, there probably were few structural units. ?? 1980.

  20. Project Success.

    ERIC Educational Resources Information Center

    Meredith, Larry D.

    Project Success consists of after-school, weekend, and summer educational programs geared toward minority and disadvantaged students to increase their numbers seeking postsecondary education from the Meadville, Pennsylvania area. The project is funded primarily through the Edinboro University of Pennsylvania, whose administration is committed to…

  1. Neoproterozoic Structural Evolution of the NE-trending 620-540 Ma Ad-Damm Shear Zone, Arabian Shield, Saudi Arabia

    NASA Astrophysics Data System (ADS)

    Hamimi, Z.; El-Sawy, E. K.; El-Fakharan, A. S.; Shujoon, A.; Matsah, M.; El-Shafei, M.

    2012-04-01

    Ad-Damm Shear Zone (ASZ) is a NE-trending fault zone separating Jeddah and Asir tectonostratigraphic terranes in the Neoproterozoic juvenile Arabian Shield. ASZ extends ~380 km, with an average width ~2-3 km, from the eye-catching Ruwah Fault Zone in the eastern shield to the Red Sea Coastal plain. It was believed to be one of the conjugate shears of the NW- to NNW- trending sinistral Najd Shear System based on noteworthy dextral shear criteria recorded within the 620 Ma sheared granites of Numan Complex, as well as right-lateral offsets within quartz veins and dikes transected by the shear zone. The present study is an integrated field-based structural analysis and remote sensing. We utilized the ASTER data for lithologic discrimination and automatic structural lineament extraction and analysis of the Neoproterozoic basement lithologies encountered along and within the vicinity of ASZ. Various false color composite images were generated and evaluated for lithological mapping and structural lineaments. The obtained map was analyzed using GIS techniques to interpret the behavior of the existing lineaments and their spatial distribution. Based on the results of the ASTER data, two significant areas; around Bir Ad-Damm and to the south of Wadi Numan, are selected for detailed field investigation. Shear-sense indicators and overprinting relations clearly show a complicated Neoproterozoic history of ASZ, involving at least three deformations: (1) an early attenuated NE-SW sinistral shearing; followed by (2) a SE-directed thrusting phase resulted in the formation SE-verging thrusts and associated thrust-related folds; and (3) late NE-SW intensive dextral transcurrent shearing played a significant role in the creation of mesoscopic shear-zone related folds, particularly in the area near Bir Ad-Damm. Such deformation history demonstrates the same episode of Neoproterozoic deformation exhibited in the NE-trending shear zones in the Arabian-Nubian Shield (ANS).

  2. Lithofacies, diagenetic spectra and sedimentary cycles of messinian (Late Miocene) evaporites in SE Spain

    NASA Astrophysics Data System (ADS)

    Michalzik, Dieter

    1996-11-01

    Messinian evaporites have been deposited within a number of connected basins that formed the so-called 'Betic Strait' in SE Spain during the Late Miocene. Pre-, syn- and postevaporitic sedimentary successions may well be compared with other strata of the same age surrounding the Mediterranean Sea. There is no evidence for a cataract-like connection of these basins with a strong unidirectional flow regime before and during the formation of Messinian evaporites as previously suggested. The most frequently occurring gypsum lithofacies presumably related to shallow-water depositional environments are crystalline selenitic gypsum ('grass-like gypsum'), laminated gypsum and cross-bedded gypsarenite. Only few real sabkha type chicken wire alabastrine gypsum have been identified. Evenly laminated gypsum together with graded gypsarenites (turbidites), gypsrudites (debrites) and slumps is restricted to basins or parts of basins with considerable paleoslopes. Evidence for diagenetic overprinting is abundant in all outcrop areas studied. This includes the formation of giant gypsum twin crystals, 'super cones' and alabastrine nodular gypsum. The formation of Messinian evaporites in SE Spain is thought to be controlled by a superimposed third-order sea-level lowstand. Smaller sedimentary units fall well into the range of fourth- to sixth-order cyclic and rhythmic successions. They are clearly related to basin dynamics or to autogenetic processes of the local facies zone (facies dynamics). None of the smaller-scale cycles (fourth and higher order), as observed in outcrops, are related to high-frequency eustasy.

  3. Petrology and Sm-Nd dating of the Genina Gharbia Alaskan-type complex (Egypt): Insights into deep levels of Neoproterozoic island arcs

    NASA Astrophysics Data System (ADS)

    Helmy, Hassan M.; Abd El-Rahman, Yasser M.; Yoshikawa, Masako; Shibata, Tomoyuki; Arai, Shoji; Tamura, Akihiro; Kagami, Hiroo

    2014-06-01

    The deep levels of Neoproterozoic island arcs are poorly known due to limited accessibility. The Genina Gharbia Alaskan-type complex (south Eastern Desert, Egypt) is the remains of a magma chamber that crystallized at the base (crust-mantle boundary) of a mature Neoproterozoic island arc. The rock assemblage comprises hornblende-bearing harzburgite, lherzolite, pyroxenite, norite and gabbro. All lithologies show cumulus texture with evidence of extensive cumulus mineral-melt interactions. Clinopyroxenes from all lithologies have similar rare earth element (REE) patterns with slight medium-rare earth element (MREE) enrichment. Hornblendes are slightly enriched in MREE and light rare earth elements (LREE). Island arc signatures are indicated by high contents of large ion-lithophile elements and low concentration of high field-strength elements. Positive initial εNd (+ 5.7 to + 7.0) and Nd model ages (963 ± 81 Ma) are consistent with the Genina Gharbia magma being extracted from a depleted mantle source. Modeling of estimated parental magma indicates 10% partial melting of a 90% depleted mantle source with a 10% (MORB + sediments)-derived fluid, commencing in the spinel stability field (< 85 km). Relative to Phanerozoic arcs, the Neoproterozoic arcs were more hydrous, had low oxidation states and probably lasted shorter time to build-up. The hydrous nature of the sub-Arabian-Nubian Shield mantle and the long-life of the arcs are among reasons responsible for the vast crustal growth during the Pan-African Orogeny throughout the Gondwana.

  4. Towering sponges in an Early Cambrian Lagerstätte: Disparity between nonbilaterian and bilaterian epifaunal tierers at the Neoproterozoic-Cambrian transition

    NASA Astrophysics Data System (ADS)

    Yuan, Xunlai; Xiao, Shuhai; Parsley, Ronald L.; Zhou, Chuanming; Chen, Zhe; Hu, Jie

    2002-04-01

    Epifaunal, suspension-feeding bilaterian animals in the Cambrian lived close to the sediment-water interface, and hence their ecological tiering levels were low (<10 cm). Here we report an Early Cambrian (Diandongian or probably Tommotian-Atdabanian) Lagerstätte from the Hetang Formation in Anhui Province, south China. The Hetang biota is characterized by high-tiering (to 50 cm) sponges and small (<0.5 cm) bilaterians (including orthothecid hyoliths and bivalved arthropods). Nonbilaterian suspension feeders (sponges, cnidarians, and archaeocyathids) as high-tiering animals and bilaterian suspension feeders as low-tiering animals also characterize other Neoproterozoic-Cambrian assemblages, such as the Ediacaran, Chengjiang, Burgess Shale, and Sinsk biotas. These data are consistent with medium- to high-tiering levels in Neoproterozoic-Cambrian epifaunal communities, but suggest that nonbilaterians achieved such tiering levels long before bilaterian suspension feeders did so in the Early Ordovician. The disparity between bilaterian and nonbilaterian tierers during the Neoproterozoic-Cambrian transition and the delayed appearance of high-tiering bilaterians demand phylogenetic and ecological explanations. The Cambrian substrate revolution may have triggered a cascade of ecological evolution, including the rise of bilaterian animals in high-tiering levels during the Ordovician radiation of the Paleozoic fauna.

  5. Neoproterozoic diamictite in the Eastern Desert of Egypt and Northern Saudi Arabia: evidence of ~750 Ma glaciation in the Arabian-Nubian Shield?

    NASA Astrophysics Data System (ADS)

    Ali, Kamal A.; Stern, Robert J.; Manton, William I.; Johnson, Peter R.; Mukherjee, Sumit K.

    2010-06-01

    The Neoproterozoic Atud diamictite in Wadi Kareim and Wadi Mobarak in the Eastern Desert of Egypt and the Nuwaybah formation in NW Saudi Arabia consist of poorly sorted, polymictic breccia, with clasts up to 1 m of granitoid, quartz porphyry, quartzite, basalt, greywacke, marble, arkose, and microconglomerate in fine-grained matrix. Stratigraphic relations indicate that the diamictite was deposited in a marine environment. Integrated field investigation, petrographic study and U-Pb SHRIMP zircon ages demonstrate that the Atud and Nuwaybah are correlative. The distribution of zircon ages indicate that ~750 Ma ages are dominant with a significant component of older materials, characterized by minor Mesoproterozoic and more abundant Paleoproterozoic and Neoarchean ages. Some matrix and metasedimentary clast zircons yield ages that are a few 10s of Ma younger than the age of the youngest clast (754 ± 15 Ma), suggesting Atud/Nuwaybah diamictite deposition ~750 Ma or slightly later, broadly consistent with being deposited during the Sturtian glaciation (740-660 Ma). The Paleoproterozoic and Neoarchean clasts have no source within the ensimatic Arabian-Nubian Shield. The distribution of the pre-Neoproterozoic ages are similar to the distribution of the pre-Neoproterozoic ages in Yemen and Saharan Metacraton, suggesting that these clasts have been transported hundreds of kilometers, maybe by ice-rafting. The Atud diamictite may represent important evidence for Cryogenian “Snowball Earth” in the Arabian-Nubian Shield.

  6. Eolian Dust and the Origin of Sedimentary Chert

    USGS Publications Warehouse

    Cecil, C. Blaine

    2004-01-01

    This paper proposes an alternative model for the primary source of silica contained in bedded sedimentary chert. The proposed model is derived from three principal observations as follows: (1) eolian processes in warm-arid climates produce copious amounts of highly reactive fine-grained quartz particles (dust), (2) eolian processes in warm-arid climates export enormous quantities of quartzose dust to marine environments, and (3) bedded sedimentary cherts generally occur in marine strata that were deposited in warm-arid paleoclimates where dust was a potential source of silica. An empirical integration of these observations suggests that eolian dust best explains both the primary and predominant source of silica for most bedded sedimentary cherts.

  7. Three-dimensional simulations of ground motions in sedimentary basins

    USGS Publications Warehouse

    Frankel, Arthur

    1993-01-01

    This report describes work being done at the U.S. Geological Survey on 3-D simulations of earthquake ground motions in sedimentary basins. The ultimate goal of this research is to predict strong ground motions in sedimentary basins for expected large earthquakes. This report emphasizes the inadequacy of using flat-layered models for synthesizing ground motions in sedimentary basins. 2-D and 3-D simulations have demonstrated how the slope of the alluvium-bedrock interface can trap S-waves in the basins, producing prolonged surface wave trains. These large surface waves are not generated in 1-D flat layered models, which underestimate the duration and peak amplitude of shaking. We present results of 3-D simulations for the San Bernardino and Santa Clara valleys, California, for earthquakes on the San Andreas fault.

  8. Some characteristics of Hirsizdere sedimentary magnesite deposits, Denizli, SW Turkey

    NASA Astrophysics Data System (ADS)

    Zedef, Veysel; Russell, Michael

    2016-04-01

    Approximately 8% of Turkey is covered by ultramafic rocks which host economically important deposits of magnesite, chromite and olivine. Magnesite deposits are of three types: (1) Massive or crystalline, (2) Cryptocrystalline and (3) Sedimentary. Cryptocrystalline and sedimentary type magnesite deposits are widespread all over Turkey although the massive type deposits are seemingly absent. In this study, we examined the sedimentary magnesite deposits of Hirsizdere, located in the province of Denizli, SW Turkey. The deposits formed as five beds within an ultramafic environment. The thickness of the magnesite beds can reach up to 4 meters and may be traced up to 3 km from west to east. The deposit comprises half a million tons of magnesite with some associated dolomite.

  9. Géochimie et cadre géodynamique du volcanisme néoprotérozoïque terminal (vendien) du Haut Atlas occidental, Maroc(Geochemical features and tectonic setting of late Neoproterozoic Vendian volcanism in the western High Atlas, Morocco)

    NASA Astrophysics Data System (ADS)

    Jouhari, A.; El-Archi, A.; Aarab, M.; El-Attari, A.; Ennih, N.; Laduron, D.

    2001-05-01

    Late Neoproterozoic Vendian volcanic and volcaniclastic rocks are widely distributed in the western High Atlas. They are located north of the Tizi n'Test Fault, separating the West African Craton from a northerly adjacent craton. These volcanic rocks overlie a semipelitic formation, which represents the equivalent of the Tidilline and Anzi Formations of the Anti-Atlas. The geochemical characteristics of these volcanic rocks suggest a calc-alkaline active margine environment associated with the post Pan-African tectonics. They differ from those of the Anti-Atlas by their lower content of K 2O. The later rock type was generated by a melting process of the crust subducted beneath the northern craton. A carbonate-shale unit, which contains examples of interstratified calc-alkaline dacite, overlies the volcanic succession, demonstrating that the volcanic activity continued sporadically until Early Cambrian times.

  10. Lithologic mapping in a sedimentary environment using multipolarization SAR images

    NASA Technical Reports Server (NTRS)

    Evans, D. L.; Schenck, L. R.

    1985-01-01

    Multipolarization Synthetic Aperture Radar (SAR) data from the NASA/JPL aircraft SAR were used in conjunction with LANDSAT Thematic Mapper (TM), Thermal Infrared Multispectral Scanner (TIMS), and Airborne Imaging Spectrometer (AIS) data as part of a three-year research program to evaluate the utility of remote sensing measurements for analysis of sedimentary basins. The purpose of this research effort is to construct stratigraphic columns, map variations in the lithology, geometry, and structure of sedimentary rocks in the Wind River/Bighorn Basin area, Wyoming, and to integrate remote sensing data with conventional rain models of basin formation and evolution.

  11. Excess europium content in Precambrian sedimentary rocks and continental evolution

    NASA Technical Reports Server (NTRS)

    Jakes, P.; Taylor, S. R.

    1974-01-01

    It is proposed that the europium excess in Precambrian sedimentary rocks, relative to those of younger age, is derived from volcanic rocks of ancient island arcs, which were the source materials for the sediments. Precambrian sedimentary rocks and present-day volcanic rocks of island arcs have similar REE patterns, total REE abundances, and excess Eu, relative to the North American shale composite. The present upper crustal REE pattern, as exemplified by that of sediments, is depleted in Eu, relative to chondrites. This depletion is considered to be a consequence of development of a granodioritic upper crust by partial melting in the lower crust, which selectively retains europium.

  12. Carbonate concretions: an ideal sedimentary host for microfossils.

    USGS Publications Warehouse

    Blome, C.D.; Albert, N.R.

    1985-01-01

    Enhanced preservation correlates with early diagenetic concretion formation at or near the sediment-water interface and with higher carbonate, organic material, and metallic cation content than in surrounding rocks. Early diagenetic growth is inferred by diverging sedimentary laminations and small-scale sedimentary structures in fossiliferous carbonate concretions. High initial concentration of microorganisms or fecal pellets may commonly be responsible for incipient carbonate-concretion growth. Excellent preservation is demonstrated by radiolarians and palynomorphs extracted from a carbonate concretion from the Middle Jurassic Shelikof Formation, S Alaska.-from Authors

  13. Anisotropy of thermoremanent magnetisation of Cryogenian glaciogenic and Ediacaran red beds, South Australia: Neoproterozoic apparent or true polar wander?

    NASA Astrophysics Data System (ADS)

    Schmidt, Phillip W.; Williams, George E.

    2013-11-01

    Determining the effects of compaction-related inclination shallowing of remanence directions is crucial for ascertaining the validity of low palaeolatitudes for Neoproterozoic red beds in South Australia that are central to the debate concerning low-latitude Proterozoic glaciation. The inclination correction (or flattening) factor, f, is defined as tan(ID)/tan(IF), where ID and IF are the inclinations of the measured detrital remanence and the ancient inducing field, respectively. The anisotropy can be estimated using anisotropy of magnetic susceptibility and the anisotropy of high-field isothermal remanence (hf-AIR). The elongation-inclination (E-I) method has also been used to infer inclination shallowing. We add the anisotropy of thermoremanent magnetisation (ATR) to these methods. For the late Cryogenian Elatina Formation arenites, which constitute the bulk of the Elatina data set, the inclination correction using f = 0.738 derived from ATR increases the palaeolatitude of the Elatina Formation from 6.5 ± 2.2° to 8.8 ± 3.2°, which confirms that the Elatina glaciation occurred near the palaeoequator. Inclination corrections for the Ediacaran argillaceous Brachina and Wonoka formations, using f = 0.35-0.38 derived from ATR, are significantly greater than for the more arenaceous Elatina Formation, which increases their palaeolatitudes from ~ 12° to ~ 30°. Carbonates from the basal Ediacaran Nuccaleena Formation yielded f = 0.8 from ATR, which represents only a small palaeolatitude correction from 19° to 23°. The anisotropy results imply that the characteristic remanent magnetisations carried by all these units were acquired early as depositional remanent magnetisations, essentially at the time of deposition. The shift of the palaeopoles from argillaceous units indicating significantly higher palaeolatitudes introduces a distinctive loop into the late Cryogenian-Ediacaran-Cambrian pole path for Australia. This loop shows similarities with the North American

  14. Beginning the Modern Regime of Subduction Tectonics in Neoproterozoic time: Inferences from Ophiolites of the Arabian-Nubian Shield

    NASA Astrophysics Data System (ADS)

    Stern, R.

    2003-04-01

    It is now clear that the motive force for plate tectonics is provided by the sinking of dense lithosphere in subduction zones. Correspondingly, the modern tectonic regime is more aptly called ``subduction tectonics" than plate tectonics, which only describes the way Earth's thermal boundary layer adjusts to subduction. The absence of subduction tectonics on Mars and Venus implies that special circumstances are required for subduction to occur on a silicate planet. This begs the question: When did Earth's oceanic lithosphere cool sufficiently for subduction to began? This must be inferred from indirect lines of evidence; the focus here is on the temporal distribution of ophiolites. Well-preserved ophiolites with ``supra-subduction zone" (SSZ) affinities are increasingly regarded as forming when subduction initiates as a result of lithospheric collapse (± a nudge to get it started), and the formation of ophiolitic lithosphere in evolving forearcs favors their emplacement and preservation. The question now is what percentage of ophiolites with ``supra-subduction zone" (SSZ) chemical signatures formed in forearcs during subduction initiation events? Most of the large, well-preserved ophiolites (e.g., Oman, Cyprus, California, Newfoundland) may have this origin. If so, the distribution in space and time of such ophiolites can be used to identify ``subduction initiation" events, which are important events in the evolution of plate tectonics. Such events first occurred at the end of the Archean (˜2.5Ga) and again in the Paleoproterozoic (˜1.8 Ga), but ophiolites become uncommon after this. Well-preserved ophiolites become abundant in Neoproterozoic time, at about 800±50 Ma. Ophiolites of this age are common and well-preserved in the Arabian-Nubian Shield (ANS) of Egypt, Sudan, Ethiopia, Eritrea, and Saudi Arabia. ANS ophiolites mostly contain spinels with high Cr#, indicating SSZ affinities. Limited trace element data on pillowed lavas supports this interpretation

  15. Identification of thick sedimentary plains north of Hellas

    NASA Astrophysics Data System (ADS)

    Salese, Francesco; Mangold, Nicolas; Ansan, Veronique; Carter, John; Ody, Anouck; Poulet, François; Ori, Gian Gabriele

    2016-04-01

    Understanding the origin and timing of intercrater plains is crucial to understand the Martian history in relation with endogenic and/or exogenic cycles. Intercrater plains north of Hellas basin on Mars are thought to have hosted different sedimentary environments during the Late Noachian/Early Hesperian, and they offer a well-preserved insight into the regional geological history of Mars. Our new geologic mapping of the intercrater plains north of Hellas Basin is based on the rich data set from MRO and Mars Express and provides new insights into the region's geological history. These findings appear to constrain the interpretation of the nature and age of intercrater plains in this region, although we acknowledge that for example the source of the sedimentary deposits must be subject to further analysis. The northern part of Hellas basin displays topographically flat area, which was characterized during the Late Noachian by sedimentary deposition and later, in the Late Hesperian, by fissural volcanism. The map and crater retention ages enable us to interpret the geologic history of the region. The stratigraphically lower unit is represented by crustal outcrops. Across most of the region, the sedimentary unit covers the basement and is eroded into mesas, erosional windows and perched by fresh craters. Intercrater plains' sedimentary deposits north of Hellas display horizontal light-toned layered rich in Fe/Mg-phyllosilicates and local crossbedding stratification. The Noachian sedimentary deposits of the intercrater plains north of Hellas are locally covered by Hesperian lava flows, showing that intercrater plains are sedimentary and volcanic in origin. We found different erosional (regional and local) surfaces, at HiRISE scale inside sediments due to local erosional windows and at CTX scale we found two important regional erosional surfaces. The oldest between crustal outcrops and sediments, which is likely Middle Noachian in age and the youngest between sediments

  16. Multiple sulphur and iron isotope composition of detrital pyrite in Archaean sedimentary rocks: A new tool for provenance analysis

    NASA Astrophysics Data System (ADS)

    Hofmann, Axel; Bekker, Andrey; Rouxel, Olivier; Rumble, Doug; Master, Sharad

    2009-09-01

    Multiple S ( δ34S and δ33S) and Fe ( δ56Fe) isotope analyses of rounded pyrite grains from 3.1 to 2.6 Ga conglomerates of southern Africa indicate their detrital origin, which supports anoxic surface conditions in the Archaean. Rounded pyrites from Meso- to Neoarchaean gold and uranium-bearing strata of South Africa are derived from both crustal and sedimentary sources, the latter being characterised by non-mass dependent fractionation of S isotopes ( Δ33S as negative as - 1.35‰) and large range of Fe isotope values ( δ56Fe between - 1.1 and 1.2‰). Most sediment-sourced pyrite grains are likely derived from sulphide nodules in marine organic matter-rich shales, sedimentary exhalites and volcanogenic massive sulphide deposits. Some sedimentary pyrite grains may have been derived from in situ sulphidised Fe-oxides, prior to their incorporation into the conglomerates, as indicated by unusually high positive δ56Fe values. Sedimentary sulphides without significant non-mass dependent fractionation of S isotopes were also present in the source of some conglomerates. The abundance in these rocks of detrital pyrite unstable in the oxygenated atmosphere may suggest factors other than high pO 2 as the cause for the absence of significant non-mass dependent fractionation processes in the 3.2-2.7 Ga atmosphere. Rounded pyrites from the c. 2.6 Ga conglomerates of the Belingwe greenstone belt in Zimbabwe have strongly fractionated δ34S, Δ33S and δ56Fe values, the source of which can be traced back to black shale-hosted massive sulphides in the underlying strata. The study demonstrates the utility of combined multiple S and Fe isotope analyses for provenance reconstruction of Archaean sedimentary successions.

  17. Late Proterozoic-Paleozoic evolution of the Arctic Alaska-Chukotka terrane based on U-Pb igneous and detrital zircon ages: Implications for Neoproterozoic paleogeographic reconstructions

    USGS Publications Warehouse

    Amato, J.M.; Toro, J.; Miller, E.L.; Gehrels, G.E.; Farmer, G.L.; Gottlieb, E.S.; Till, A.B.

    2009-01-01

    The Seward Peninsula of northwestern Alaska is part of the Arctic Alaska-Chukotka terrane, a crustal fragment exotic to western Laurentia with an uncertain origin and pre-Mesozoic evolution. U-Pb zircon geochronology on deformed igneous rocks reveals a previously unknown intermediate-felsic volcanic event at 870 Ma, coeval with rift-related magmatism associated with early breakup of eastern Rodinia. Orthogneiss bodies on Seward Peninsula yielded numerous 680 Ma U-Pb ages. The Arctic Alaska-Chukotka terrane has pre-Neoproterozoic basement based on Mesoproterozoic Nd model ages from both 870 Ma and 680 Ma igneous rocks, and detrital zircon ages between 2.0 and 1.0 Ga in overlying cover rocks. Small-volume magmatism occurred in Devonian time, based on U-Pb dating of granitic rocks. U-Pb dating of detrital zircons in 12 samples of metamorphosed Paleozoic siliciclastic cover rocks to this basement indicates that the dominant zircon age populations in the 934 zircons analyzed are found in the range 700-540 Ma, with prominent peaks at 720-660 Ma, 620-590 Ma, 560-510 Ma, 485 Ma, and 440-400 Ma. Devonian- and Pennsylvanian-age peaks are present in the samples with the youngest detrital zircons. These data show that the Seward Peninsula is exotic to western Laurentia because of the abundance of Neoproterozoic detrital zircons, which are rare or absent in Lower Paleozoic Cordilleran continental shelf rocks. Maximum depositional ages inferred from the youngest detrital age peaks include latest Proterozoic-Early Cambrian, Cambrian, Ordovician, Silurian, Devonian, and Pennsylvanian. These maximum depositional ages overlap with conodont ages reported from fossiliferous carbonate rocks on Seward Peninsula. The distinctive features of the Arctic Alaska-Chukotka terrane include Neoproterozoic felsic magmatic rocks intruding 2.0-1.1 Ga crust overlain by Paleozoic carbonate rocks and Paleozoic siliciclastic rocks with Neoproterozoic detrital zircons. The Neoproterozoic ages are

  18. Adjoint-tomography Inversion of the Small-scale Surface Sedimentary Structures: Key Methodological Aspects

    NASA Astrophysics Data System (ADS)

    Kubina, Filip; Moczo, Peter; Kristek, Jozef; Michlik, Filip

    2016-04-01

    Adjoint tomography has proven an irreplaceable useful tool in exploring Earth's structure in the regional and global scales. It has not been widely applied for improving models of local surface sedimentary structures (LSSS) in numerical predictions of earthquake ground motion (EGM). Anomalous earthquake motions and corresponding damage in earthquakes are often due to site effects in local surface sedimentary basins. Because majority of world population is located atop surface sedimentary basins, it is important to predict EGM at these sites during future earthquakes. A major lesson learned from dedicated international tests focused on numerical prediction of EGM in LSSS is that it is hard to reach better agreement between data and synthetics without an improved structural model. If earthquake records are available for sites atop a LSSS it is natural to consider them for improving the structural model. Computationally efficient adjoint tomography might be a proper tool. A seismic wavefield in LSSS is relatively very complex due to diffractions, conversions, interference and often also resonant phenomena. In shallow basins, the first arrivals are not suitable for inversion due to almost vertical incidence and thus insufficient vertical resolution. Later wavefield consists mostly of local surface waves often without separated wave groups. Consequently, computed kernels are complicated and not suitable for inversion without pre-processing. The spatial complexity of a kernel can be dramatic in a typical situation with relatively low number of sources (local earthquakes) and surface receivers. This complexity can be simplified by directionally-dependent smoothing and spatially-dependent normalization that condition reasonable convergence. A multiscale approach seems necessary given the usual difference between the available and true models. Interestingly, only a successive inversion of μ and λ elastic moduli, and different scale sequences lead to good results.

  19. Flexure and faulting of sedimentary host rocks during growth of igneous domes, Henry Mountains, Utah

    USGS Publications Warehouse

    Jackson, M.D.; Pollard, D.D.

    1990-01-01

    A sequence of sedimentary rocks about 4 km thick was bent, stretched and uplifted during the growth of three igneous domes in the southern Henry Mountains. Mount Holmes, Mount Ellsworth and Mount Hillers are all about 12 km in diameter, but the amplitudes of their domes are about 1.2, 1.85 and 3.0 km, respectively. These mountains record successive stages in the inflation of near-surface diorite intrusions that are probably laccolithic in origin. The host rocks deformed along networks of outcrop-scale faults, or deformation bands, marked by crushed grains, consolidation of the porous sandstone and small displacements of sedimentary beds. Zones of deformation bands oriented parallel to the beds and formation contacts subdivided the overburden into thin mechanical layers that slipped over one another during doming. Measurements of outcrop-scale fault populations at the three mountains reveal a network of faults that strikes at high angles to sedimentary beds which themselves strike tangentially about the domes. These faults have normal and reverse components of slip that accommodated bending and stretching strains within the strata. An early stage of this deformation is displayed at Mount Holmes, where states of stress computed from three fault samples correlate with the theoretical distribution of stresses resulting from bending of thin, circular, elastic plates. Field observations and analysis of frictional driving stresses acting on horizontal planes above an opening-mode dislocation, as well as the paleostress analysis of faulting, indicate that bedding-plane slip and layer flexure were important components of the early deformation. As the amplitude of doming increased, radial and circumferential stretching of the strata and rotation of the older faults in the steepening limbs of the domes increased the complexity of the fault patterns. Steeply-dipping, map-scale faults with dip-slip displacements indicate a late-stage jostling of major blocks over the central

  20. A Sedimentary Rock Classification Scheme for Introductory Geology.

    ERIC Educational Resources Information Center

    Davis, Larry Eugene; Eves, Robert Leo

    1986-01-01

    Presents a classification scheme for identifying sedimentary rocks in introductory geology laboratories. The key consists of an ordered sequence of tests to perform and observations to make which then suggests a rock name or directs the student to additional tests and/or observations. (ML)

  1. Using Aluminum Foil to Record Structures in Sedimentary Rock.

    ERIC Educational Resources Information Center

    Metz, Robert

    1982-01-01

    Aluminum foil can be used to make impressions of structures preserved in sedimentary rock. The impressions can be projected onto a screen, photographed, or a Plaster of Paris model can be made from them. Impressions of ripple marks, mudcracks, and raindrop impressions are provided in photographs illustrating the technique. (Author/JN)

  2. Sedimentary Geochemistry of Martian Samples from the Pathfinder Mission

    NASA Technical Reports Server (NTRS)

    McLennan, Scott M.

    2001-01-01

    The purpose of this research project was to evaluate the APXS data collected on soils and rocks at the Pathfinder site in terms of sedimentary geochemistry. Below are described the major findings of this research: (1) An influential model to explain the chemical variation among Pathfinder soils and rocks is a two component mixing model where rocks of fairly uniform composition mix with soil of uniform composition; (2) The very strong positive correlation between MgO and SO, points to a control by a MgSO4 mineral however, spectroscopic data continue to suggest that Fe-sulfates, notably schwertmannite and jarosite, may be important components; (3) In an attempt to better understand the causes of complexities in mixing relationships, the possible influence of sedimentary transport has been evaluated; (4) Another aspect of this research has been to examine the possibility of sedimentary silica being a significant phase on Mars; and (5) On Earth, the geochemistry of sedimentary rocks has been used to constrain the chemical composition of the continental crust and an important part of this research was to evaluate this approach for Mars.

  3. CUTS FOR MTR EXCAVATION ILLUSTRATE SEDIMENTARY MANTLE OF SOIL AND ...

    Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

    CUTS FOR MTR EXCAVATION ILLUSTRATE SEDIMENTARY MANTLE OF SOIL AND GRAVEL OVERLAYING LAVA ROCK FIFTY FEET BELOW. SAGEBRUSH HAS BEEN SCOURED FROM REST OF SITE. CAMERA PROBABLY FACES SOUTHWEST. INL NEGATIVE NO. 67. Unknown Photographer, 6/4/1950 - Idaho National Engineering Laboratory, Test Reactor Area, Materials & Engineering Test Reactors, Scoville, Butte County, ID

  4. Influence of sedimentary environments on mechanical properties of clastic rocks

    NASA Astrophysics Data System (ADS)

    Meng, Zhaoping; Zhang, Jincai; Peng, Suping

    2006-10-01

    The sedimentary environments are the intrinsic factor controlling the mechanical properties of clastic rocks. Examining the relationship between rock sedimentary environments and rock mechanical properties gives a better understanding of rock deformation and failure mechanisms. In this study, more than 55 samples in coal measures were taken from seven different lithologic formations in eastern China. Using the optical microscope the sedimentary characteristics, such as components of clastic rocks and sizes of clastic grains were quantitatively tested and analyzed. The corresponding mechanical parameters were tested using the servo-controlled testing system. Different lithologic attributes in the sedimentary rocks sampled different stress-strain behaviors and failure characteristics under different confining pressures, mainly due to different compositions and textures. Results demonstrate that clastic rocks have the linear best-fit for Mohr-Coulomb failure criterion. The elastic moduli in clastic rocks are highly dependent upon confining pressures, unlike hard rocks. The envelope lines of the mechanical properties versus the contents of quartz, detritus of the grain diameter of more than 0.03 mm, and grain size in clastic rocks are given. The compressive strength or elastic modulus and the grain diameter have a non-monotonic relation and demonstrate the “grain-diameter softening” effect.

  5. Examples of transient sounding from groundwater exploration in sedimentary aquifers

    USGS Publications Warehouse

    Fitterman, D.V.

    1987-01-01

    Examples of the use of transient electromagnetic soundings for three groundwater exploration problems in sedimentary aquifers are given. The examples include: 1) estimating depths to water table and bedrock in an alluvium-filled basin, 2) mapping a confined freshwater aquifer in bedrock sediments, and 3) locating a freshwater/saltwater interface in a glacial-outwash aquifer. -from Author

  6. Thermal imaging of sedimentary features on alluvial fans

    NASA Astrophysics Data System (ADS)

    Hardgrove, Craig; Moersch, Jeffrey; Whisner, Stephen

    2010-03-01

    Aerial thermal imaging is used to study grain-size distributions and induration on a wide variety of alluvial fans in the desert southwest of the United States. High-resolution aerial thermal images reveal evidence of sedimentary processes that rework and build alluvial fans, as preserved in the grain-size distributions and surface induration those processes leave behind. A catalog of constituent sedimentary features that can be identified using aerial thermal and visible imaging is provided. These features include clast-rich and clast-poor debris flows, incised channel deposits, headward-eroding gullies, sheetflood, lag surfaces, active/inactive lobes, distal sand-skirts and basin-related salt pans. Ground-based field observations of surface grain-size distributions, as well as morphologic, cross-cutting and topographic relationships were used to confirm the identifications of these feature types in remotely acquired thermal and visible images. Thermal images can also reveal trends in grain sizes between neighboring alluvial fans on a regional scale. Although inferences can be made using thermal images alone, the results from this study demonstrate that a more thorough geological interpretation of sedimentary features on an alluvial fan can be made using a combination of thermal and visible images. The results of this study have potential applications for Mars, where orbital thermal imaging might be used as a tool for evaluating constituent sedimentary processes on proposed alluvial fans.

  7. Late Neoproterozoic magmatism in South Qinling, Central China: Geochemistry, zircon U-Pb-Lu-Hf isotopes and tectonic implications

    NASA Astrophysics Data System (ADS)

    Wang, Ruirui; Xu, Zhiqin; Santosh, M.; Yao, Yuan; Gao, Li'e.; Liu, Chunhua

    2016-06-01

    The Neoproterozoic tectonic evolution of the northern margin of the Yangtze Block in South China remains debated. In this study, we present results from LA-ICP-MS zircon U-Pb geochronology on a suite of intermediate-felsic rocks in South Qinling, Central China which show a mean age of ca. 630 Ma. The zircon εHf(t) values of these rocks mostly range from + 0.44 to + 14.78. Geochemically, the granites and syenite show high total alkali contents, with enrichment in LREE, LILE (Rb, Ba, and K), and HFSE (Th, U, Nb, Ta, Zr, and Hf), and depletion in Sr, P, and Ti, similar to the features of A-type granites. The meta-diorite shows high Na2O, with depletion in Eu, Ti, and LILE (Sr, Rb, Ba, and K), and enrichment in HFSE (Th, U, Nb, Ta, Zr, and Hf). The geochemical features are consistent with formation of the intermediate-felsic suite through fractionation from underplated basaltic magma that originated from sub-continental lithospheric mantle metasomatized by asthenosphere-derived oceanic-island-basalt-like (OIB-like) melts, coupled with minor crustal contamination. We correlate the ca. 630 Ma magmatism with a back-arc rift setting that probably developed in relation to slab tearing during continued slab rollback.

  8. Pedogenic calcretes within fracture systems and beddings in Neoproterozoic limestones of the Irecê Basin, northeastern Brazil

    NASA Astrophysics Data System (ADS)

    Borges, S. V. F.; Balsamo, F.; Vieira, M. M.; Iacumin, P.; Srivastava, N. K.; Storti, F.; Bezerra, F. H. R.

    2016-07-01

    Calcretes or caliches are continental limestones developed by surficial weathering process that takes place mostly in arid and semi-arid regions. In the Irecê Basin, northeastern Brazil, in addition to the regular occurrence of pedogenic calcretes, a peculiar type of structurally controlled calcretes occurs on Neoproterozoic limestones. These peculiar calcretes developed near the surface and occur (1) between layers, (2) inside fractures and (3) within major thrust faults. Fieldwork on selected outcrops was integrated with petrographic, mineralogic, geochemical, density and mercury intrusion porosity analyses to constrain the environment of formation and their petrophysical properties. The results revealed that this type of calcrete is the product of multiepisodic events of dissolution and precipitation occurring during the wet and dry seasons in the region along fractures and beddings. Based on the petrophysical results, we suggest that these calcretes may have an important role in the migration of fluids through the impermeable host carbonate rock and that they act as a conduit for fluid flow, as revealed by their high porosity (mean value = 26%) and remarkable pore connectivity.

  9. Petrogenesis of the Neoproterozoic West Highland Granitic Gneiss, Scottish Caledonides: Cryptic mantle input to S-type granites?

    NASA Astrophysics Data System (ADS)

    Fowler, M.; Millar, I. L.; Strachan, R. A.; Fallick, A. E.

    2013-05-01

    The Neoproterozoic (c. 870 Ma) West Highland Granitic Gneiss, exposed in the Northern Highlands Terrane of Scotland, has elemental characteristics that are strikingly similar to those of the host Moine metasediments, which are thus consistent with an origin involving major Moine melting. Most of the constituent bodies have compositions significantly removed from minimum melts of pelites, and trace element constraints suggest variable but significant restite entrainment leading to less silicic bulk compositions with enhanced REE, Zr and Y. However, initial Nd and Hf isotope ratios are not coincident with contemporary Moine and imply a significant juvenile contribution. Close association with a regional suite of metabasites prompts consideration of mafic magma input, for which binary mixing models offer qualitative support. Quantitative difficulties with typical Moine metasediments are eased with radiogenic pelites or by partial melting of the mafic component. A possible alternative is currently unexposed Grenvillian felsic crust. Subsequent interaction of the granitic gneisses with meteoric water has significantly perturbed the oxygen and Sr isotope systems, the timing of which is equivocal but probably occurred during Caledonian events. The elemental characteristics of the West Highland Granitic Gneiss show many similarities with Scandinavian (rift-related?) granites of the same age, but since their geochemistry is largely inherited from the protolith it would be unwise to pursue palaeotectonic attribution on this basis. However, the probable incorporation of significant mantle-derived mafic magma of MORB-like affinity is consistent with an extensional setting.

  10. Hypothesized link between Neoproterozoic greening of the land surface and the establishment of an oxygen-rich atmosphere.

    PubMed

    Kump, Lee R

    2014-09-30

    Considerable geological, geochemical, paleontological, and isotopic evidence exists to support the hypothesis that the atmospheric oxygen level rose from an Archean baseline of essentially zero to modern values in two steps roughly 2.3 billion and 0.8-0.6 billion years ago (Ga). The first step in oxygen content, the Great Oxidation Event, was likely a threshold response to diminishing reductant input from Earth's interior. Here I provide an alternative to previous suggestions that the second step was the result of the establishment of the first terrestrial fungal-lichen ecosystems. The consumption of oxygen by aerobes respiring this new source of organic matter in soils would have necessitated an increase in the atmospheric oxygen content to compensate for the reduced delivery of oxygen to the weathering environment below the organic-rich upper soil layer. Support for this hypothesis comes from the observed spread toward more negative carbon isotope compositions in Neoproterozoic (1.0-0.542 Ga) and younger limestones altered under the influence of ground waters, and the positive correlation between the carbon isotope composition and oxygen content of modern ground waters in contact with limestones. Thus, the greening of the planet's land surfaces forced the atmospheric oxygen level to a new, higher equilibrium state. PMID:25225378

  11. Succession planning.

    PubMed

    Catanzaro, Thomas E

    2006-03-01

    This article provides the reader with an appreciation of the diverse elements that go into a buy-sell, affiliation, or merger situation for veterinary practices. In the changing market place of American veterinary medicine, old paradigms no longer hold comfort. The generational differences are briefly explored herein as well as the new economic realities. A few examples are offered to illustrate just how much variability exists in the current business of veterinary medicine and the subsequent practice transitions needed to enhance value. Functioning models are explored, as well as affiliation and merger options. Practice valuation is discussed in general terms, referencing the cutting-edge factors. The six-point summary provides almost all practices a solid operational base for daily operations and succession planning. PMID:16442447

  12. Depositional dynamics and self-organization in travertine sedimentary systems

    NASA Astrophysics Data System (ADS)

    Violante, C.; Marino, G.; Sammartino, S.

    2003-04-01

    Travertines are terrestrial sedimentary systems associated with flowing water oversaturated with respect to calcium carbonate. They form terraced wedge-shaped organogenic bodies, fan-shaped in plan view, with internal achitecture characterized by downslope elongated domal structures (mound), juxtaposed by onlap geometries. Internal features of travertine mounds includes both upward decrease (up to subhorizontal) and downhill increase (up to subvertical) of clinostratification angles suggesting progradational mechanisms. The basic components of travertine deposits are aquatic sessile plants and microbes, developing along water flows. Regardless their role in carbonate precipitation, organisms appear as living templates able to organize primary carbonate encrustations along their growth directions. This results in early-lithified skeletal sedimentary bodies with rapid upward growth. Travertine accumulation transforms original slopes into gently inclined flat areas (travertine terraces), limited downhill by steeper slopes, eventually evolving in subvertical escarpments. Both terraces and escarpments are depositional rather then erosional features, being geomorphic expression of very shallow lacustrine deposits and waterfall structures respectively. Modern to fossil comparison among travertine systems located in southern and central Italy suggest a sedimentary model based on continued feedback between processes and products, which increase the complexity of depositional system over time. Encrusting waters display chemical gradients along their flow, modulating shape and downhill development of resulting travertine deposits. Upward growth gradually decreases original slope angles, so that the water flow is laterally displaced toward adjacent areas of steeper slope, accounting for juxtaposition of travertine mounds. By means of continuous lateral shifting of encrustation process travertine deposition gradually transform original slopes in gently inclined flat areas

  13. Sedimentary manganese metallogenesis in response to the evolution of the Earth system

    NASA Astrophysics Data System (ADS)

    Roy, Supriya

    2006-08-01

    The concentration of manganese in solution and its precipitation in inorganic systems are primarily redox-controlled, guided by several Earth processes most of which were tectonically induced. The Early Archean atmosphere-hydrosphere system was extremely O 2-deficient. Thus, the very high mantle heat flux producing superplumes, severe outgassing and high-temperature hydrothermal activity introduced substantial Mn 2+ in anoxic oceans but prevented its precipitation. During the Late Archean, centered at ca. 2.75 Ga, the introduction of Photosystem II and decrease of the oxygen sinks led to a limited buildup of surface O 2-content locally, initiating modest deposition of manganese in shallow basin-margin oxygenated niches (e.g., deposits in India and Brazil). Rapid burial of organic matter, decline of reduced gases from a progressively oxygenated mantle and a net increase in photosynthetic oxygen marked the Archean-Proterozoic transition. Concurrently, a massive drawdown of atmospheric CO 2 owing to increased weathering rates on the tectonically expanded freeboard of the assembled supercontinents caused Paleoproterozoic glaciations (2.45-2.22 Ga). The spectacular sedimentary manganese deposits (at ca. 2.4 Ga) of Transvaal Supergroup, South Africa, were formed by oxidation of hydrothermally derived Mn 2+ transferred from a stratified ocean to the continental shelf by transgression. Episodes of increased burial rate of organic matter during ca. 2.4 and 2.06 Ga are correlatable to ocean stratification and further rise of oxygen in the atmosphere. Black shale-hosted Mn carbonate deposits in the Birimian sequence (ca. 2.3-2.0 Ga), West Africa, its equivalents in South America and those in the Francevillian sequence (ca. 2.2-2.1 Ga), Gabon are correlatable to this period. Tectonically forced doming-up, attenuation and substantial increase in freeboard areas prompted increased silicate weathering and atmospheric CO 2 drawdown causing glaciation on the Neoproterozoic Rodinia

  14. Carbonate factories: A conundrum in sedimentary geology

    NASA Astrophysics Data System (ADS)

    Pomar, L.; Hallock, P.

    2008-03-01

    by the end-Permian extinctions, excess photosynthesis by phytoplankton and microbial assemblages in surface waters, induced by moderately high CO 2 and temperature during the Early Mesozoic, supported proliferation of non-tissular metazoans (e.g., sponges) and heterotrophic bacteria at the sea floor. Metabolic activity by those microbes, especially sulfate reduction, resulted in abundant biologically-induced geochemical carbonate precipitation on and within the sea floor. For example, with the opening of Tethyan seaways during the Triassic, massive sponge/microbe boundstones (the benthic automicrite factory) formed steep, massive and thick progradational slopes and, locally, mud-mounds. As tectonic processes created shallow epicontinental seas, photosynthesis drove lime-mud precipitation in the illuminated zone of the water column. The resulting neritic lime-mud component of the shallow-water carbonate factory became predominant during the Jurassic, paralleling the increase in atmospheric pCO 2, while the decreasing importance of the benthic automicrite factory parallels the diversification of calcifying metazoans, phytoplankton and zooplankton. With atmospheric pCO 2 declining through the Cretaceous, the potential habitats for neritic lime-mud precipitation declined. At the same time, peak oceanic Ca 2+ concentrations promoted biotically-controlled calcification by the skeletal factory. With changes produced by extinctions and turnovers at the Cretaceous-Tertiary boundary, adaptations to decreasing Ca 2+ and pCO 2, coupled with increasing global temperature gradients (i.e., high-latitude and deep-water cooling), and strategies that efficiently linked photosynthesis and calcification, promoted successive changes of the dominant skeletal factory through the Cenozoic: larger benthic foraminifers (protist-protist symbiosis) during the Paleogene, red algae during the Miocene and modern coral reefs (metazoan-protist symbiosis) since Late Miocene.

  15. Influence of syn-sedimentary faults on orogenic structures in a collisional belt: Insights from the inner zone of the Northern Apennines (Italy)

    NASA Astrophysics Data System (ADS)

    Brogi, Andrea

    2016-05-01

    This paper discusses the possible influence of syn-sedimentary structures on the development of orogenic structures during positive tectonic inversion in the inner Northern Apennines (Italy). Examples from key areas located in southern Tuscany provided original cartographic, structural and kinematics data for Late Oligocene-Early Miocene thrusts, organized in duplex systems, verging in the opposite direction of the foreland propagation (back-thrusts), which affected the Late Triassic-Oligocene sedimentary succession of the Tuscan Domain, previously affected by pre-orogenic structures. These latter consist of mesoscopic-to cartographic-scale Jurassic syn-sedimentary normal faults and extensional structures, which gave rise to effective stratigraphic lateral variation and mechanical heterogeneities. Structural analysis of both syn-sedimentary faults and back-thrusts were therefore compared in order to discuss the possible role of the pre-existing anisotropies in influencing the evolution of the back-thrusts. As a result, it can be reasonably proposed that back-thrusts trajectories and stacking pattern were controlled by relevant syn-sedimentary normal faults; these latter were reactivated, in some cases, if properly oriented. Such an issue adds new inputs for discussing the potential role of structural inheritance during tectonic inversions, and helps to better understand the processes suitable for the development of back-thrusts in the inner zones of orogenic belts, as it is the case of the inner Northern Apennines.

  16. Neoproterozoic-middle Paleozoic tectono-magmatic evolution of the Gorny Altai terrane, northwest of the Central Asian Orogenic Belt: Constraints from detrital zircon U-Pb and Hf-isotope studies

    NASA Astrophysics Data System (ADS)

    Chen, Ming; Sun, Min; Buslov, Mikhail M.; Cai, Keda; Zhao, Guochun; Zheng, Jianping; Rubanova, Elena S.; Voytishek, Elena E.

    2015-09-01

    The Gorny Altai terrane (GA) is a key area in understanding the crustal evolution of the Central Asian Orogenic Belt (CAOB). This paper reports U-Pb and Hf-isotope data for detrital zircons from Cambrian to early Devonian sedimentary sequences to constrain their provenance, as well as the tectono-magmatic events and crustal growth in this region. Nearly all the detrital zircons are characterized by euhedral to subhedral morphology, high Th/U ratios (ca. 0.1-1.6) and typical oscillatory zoning, indicating a magmatic origin. The three samples from the Gorny Altai Group (middle Cambrian to early Ordovician) yield detrital zircon populations that are composed predominantly of 530-464 Ma grains, followed by a subordinate group of 641-549 Ma old. The Silurian and Devonian samples exhibit similar major zircon populations (555-456 Ma and 525-463 Ma, respectively), but a significant amount of additional 2431-772 Ma zircons occur in the early Devonian sample. Our results suggest that detritus from the nearby Kuznetsk-Altai intra-oceanic island arc served as a unitary source for the Cambrian-Silurian sedimentary sequences, but older detritus from other sources added to the early Devonian sequence. The low abundance of ca. 640-540 Ma detrital zircons may testify that this island arc was under a primitive stage in this period, when mafic volcanic rocks probably dominated. In contrast, the dominant population of ca. 530-470 Ma zircons may indicate an increased amount of granitic rocks in the source area, suggesting that the Kuznetsk-Altai island arc possibly evolved into a mature one in the Cambrian to early Ordovician. The ca. 530-470 Ma detrital zircons are almost exclusively featured by positive εHf(t) values and have two-stage Hf model ages of ca. 1.40-0.45 Ga, indicating that the precursor magmas were sourced predominantly from heterogeneous juvenile materials. We conclude that the late Neoproterozoic to early Paleozoic magmatism in the Kuznetsk-Altai arc made a

  17. Successful Lecturing

    PubMed Central

    Copeland, H Liesel; Longworth, David L; Hewson, Mariana G; Stoller, James K

    2000-01-01

    OBJECTIVE In a study conducted over 3 large symposia on intensive review of internal medicine, we previously assessed the features that were most important to course participants in evaluating the quality of a lecture. In this study, we attempt to validate these observations by assessing prospectively the extent to which ratings of specific lecture features would predict the overall evaluation of lectures. MEASUREMENTS AND MAIN RESULTS After each lecture, 143 to 355 course participants rated the overall lecture quality of 69 speakers involved in a large symposium on intensive review of internal medicine. In addition, 7 selected participants and the course directors rated specific lecture features and overall quality for each speaker. The relations among the variables were assessed through Pearson correlation coefficients and cluster analysis. Regression analysis was performed to determine which features would predict the overall lecture quality ratings. The features that most highly correlated with ratings of overall lecture quality were the speaker's abilities to identify key points (r = .797) and be engaging (r = .782), the lecture clarity (r = .754), and the slide comprehensibility (r = .691) and format (r = .660). The three lecture features of engaging the audience, lecture clarity, and using a case-based format were identified through regression as the strongest predictors of overall lecture quality ratings (R2= 0.67, P = 0.0001). CONCLUSIONS We have identified core lecture features that positively affect the success of the lecture. We believe our findings are useful for lecturers wanting to improve their effectiveness and for educators who design continuing medical education curricula. PMID:10886470

  18. Parametric Analysis of the Factors Controlling the Costs of Sedimentary Geothermal Systems - Preliminary Results (Poster)

    SciTech Connect

    Augustine, C.

    2013-10-01

    Parametric analysis of the factors controlling the costs of sedimentary geothermal systems was carried out using a modified version of the Geothermal Electricity Technology Evaluation Model (GETEM). The sedimentary system modeled assumed production from and injection into a single sedimentary formation.

  19. The origin of the paragenesis of nonferrous metals and naphtides in carbonate-type sedimentary basins

    SciTech Connect

    Gorzhevskiy, D.I.; Donets, A.I.

    1994-09-01

    In this article, we present evidence for a paragenesis of stratiform lead-zinc deposits localized in carbonate rocks and napthide in sedimentary basins of the carbonate type. We suggest that the cause of this paragenesis is the similar behaviors of lead, zinc, and organic matter during sedimentary-diagenetic and catagenetic stages in the transformation of argillaceous and carbonate rocks of sedimentary basins.

  20. Sedimentary and microbial record of the Middle/Late Ordovician phosphogenetic episode in the northern Holy Cross Mountains, Poland

    NASA Astrophysics Data System (ADS)

    Trela, Wiesław

    2008-01-01

    Numerous phosphorite beds and phosphatic nodules occur in the upper Middle and lower Upper Ordovician carbonate-shale succession of the Bukowiany Formation outcropping in the northern Holy Cross Mountains, central Poland. The vertical stacking pattern of this succession indicates that phosphatic and accompanying strata reflect a conformable sedimentary succession accumulated during a rise of relative sea level. The base of the Bukowiany Formation is marked by a conspicuous phosphorite horizon revealing a low net sediment accumulation rate reflecting a switch into a mesotrophic ecological system. This horizon was produced by reworking and redeposition of pristine phosphate sediment (e.g. by currents activity) during the late Darriwilian transgression. The overlying sedimentary record appears to reflect nucleation of the phosphate phase in the sediment-water interface and its subsequent burial by the accompanying sediment. The phosphatized tiny stromatolites and nodules preserved within the Bukowiany Formation indicate that benthic microbial communities played an important role in redistribution and concentration of phosphate during deposition of this succession.

  1. Neoproterozoic ophiolite and related high-grade rocks of the Baikal-Muya belt, Siberia: Geochronology and geodynamic implications

    NASA Astrophysics Data System (ADS)

    Kröner, A.; Fedotova, A. A.; Khain, E. V.; Razumovskiy, A. A.; Orlova, A. V.; Anosova, M. O.; Perelyaev, V. I.; Nekrasov, G. E.; Liu, D. Y.

    2015-11-01

    We report zircon for from ophiolitic and high-grade rocks of the Neoproterozoic Baikal-Muya belt of Siberia that occupies an arc-shaped area on the southeastern margin of the Siberian craton. It consists of arc-related plutonic, metavolcanic and metasedimentary rocks as well as fragmented ophiolites and high-grade metamorphic assemblages. Magmatic zircons from two plagiogranite dyke samples of the Mamakan ophiolite complex in the Sredne-Mamakan massif of the eastern Baikal-Muya belt yielded similar and concordant SHRIMP mean 206Pb/238U ages of 640.0 ± 4.1 and 650 ± 6 Ma, respectively, that reflect the time of dyke emplacement and from which we suggest an age of ca. 645 Ma as the most likely time of ophiolite formation. Enderbitic gneisses of the North Baikal area, in the western part of the Baikal-Muya belt, contain complex zircon populations that reflect variable recrystallization, Pb-loss and metamorphic overgrowth during granulite-facies metamorphism. LA-ICP-MS dating of these zircons yielded inconclusive results that led us to undertake a detailed study of cathodoluminescence images combined with U-Pb SHRIMP dating. Well-preserved magmatic domains in zircons from enderbite sample 2821 yielded concordant results with a mean 206Pb/238U age of 640 ± 5 Ma, slightly higher but broadly comparable with the data obtained by LA-ICP-MS. The zircon populations of two more enderbitic gneiss samples are more complex, and their LA-ICP-MS data constitute broad swaths along concordia between ca. 840 and 600 Ma, reflecting two end-member isotopic components, namely an igneous crystallization event at ca. 800 Ma and a Pb-loss and recrystallization event at ca. 600 Ma. SHRIMP analyses of magmatic zircon domains of these samples yielded concordant data with identical mean 206Pb/238U ages of 826 ± 7.5 Ma and 826 ± 8 Ma, respectively, whereas low-U metamorphic rims crystallized at 640 ± 7 Ma. Newly crystallized ball-round metamorphic zircons in one sample produced a mean 206Pb

  2. Chemical variations of mineral inclusions in Neoproterozoic high-Cr chromitites from Egypt: Evidence of fluids during chromitite genesis

    NASA Astrophysics Data System (ADS)

    Khedr, Mohamed Zaki; Arai, Shoji

    2016-01-01

    This paper details the mode of occurrence, petrography, and chemistry of mineral inclusions hosted in chromian spinels of the Neoproterozoic chromitites in the Southern Eastern Desert of Egypt. Neoproterozoic podiform chromitites from the Arais, Balamhindit, and Abu Dahr areas, in the Southern Eastern Desert, can be texturally and chemically classified into two main types: primary high-Al (spinel Cr# < 0.67) and high-Cr (spinel Cr# > 0.75) chromitites. The former, being free of primary-mineral inclusions, was crystallized mainly from the MORB-like tholeiitic melt generated during proto-forearc spreading at the initiation of subduction, whereas the latter was formed from boninitic melts resulting from the high-degree melting of the sub-arc depleted mantle in the presence of slab-derived fluids at a mature-arc stage. The primary mineral inclusions, such as Na- and K-phlogopites, pargasite-edenite and olivine with subordinate pyroxenes, millerite, and laurite, were trapped within the chromian spinel during the magmatic precipitation of the chromitites. The Abu Dahr chromitites are free of primary hydrous inclusions; on the other hand, Arais and Balamhindit high-Cr chromitites are enriched in Na- and K-phlogopites, respectively, as a result of a difference in the K/Na ratio of the magma responsible for chromitite crystallization at different mantle depths. This difference in the K/Na ratio can possibly be attributed to fractionation of the upward-migrating hydrous fluids/melts by the crystallization of K- or Na-rich minerals. The Balamhindit complex, where the chromitite showed K-phlogopite inclusions within the chromian spinel, was probably derived from a deeper part of the mantle than the other areas, where the chromitite shows inclusions of Na-rich hydrous phases. Both K- and Na-phlogopites were possibly formed from alkali-rich hydrous fluids/melts trapped within the chromian spinels during the chromitite formation at different mantle depths, where the K/Na ratio

  3. Mid-Neoproterozoic (ca. 830-800 Ma) metamorphic P-T paths link Tarim to the circum-Rodinia subduction-accretion system

    NASA Astrophysics Data System (ADS)

    Ge, Rongfeng; Zhu, Wenbin; Wilde, Simon A.

    2016-06-01

    Long-lived exterior accretionary orogeny shapes tectonothermal evolution of the peripheral building blocks of supercontinents and leads to considerable crustal growth. However, such accretionary orogeny has only been locally recognized for the Rodinia supercontinent. Here a suite of newly discovered mid-Neoproterozoic high-grade metamorphic rocks in the northern Tarim Craton, NW China, are used to test the exterior accretion hypothesis for Rodinia. These rocks occur as dark-colored mafic and calc-silicate boudins in impure marbles and mica schists. Geochemical data suggest a protolith of arc-related basalts metasomatized by Ca-rich fluids. Mineral assemblages, phase diagram modeling, and mineral compositions for a garnet pyroxenite and a garnet clinopyroxene gneiss reveal upper amphibolite to high-pressure granulite facies peak metamorphism (660-700°C, 11-12 kbar) following a counterclockwise P-T path, which is characterized by prograde burial and heating, followed by near-isothermal burial and retrograde exhumation and cooling. This P-T path is interpreted to have recorded crustal thickening of an earlier magmatic arc transformed to a fore arc by subduction erosion and subsequent burial along bent isotherms near the subduction channel. All studied samples record ca. 830-800 Ma metamorphic zircon U-Pb ages, which probably date the early exhumation and cooling according to Ti-in-zircon temperatures, zircon rare earth element patterns, and Hf isotopes. This is the first mid-Neoproterozoic P-T-t path in Tarim, and it provides metamorphic evidence for a mid-Neoproterozoic advancing-type accretionary orogeny, which is coeval with the initial breakup events of Rodinia and thus links Tarim to the circum-Rodinia accretion system, supporting the peripheral subduction model.

  4. Geochemistry of accreted metavolcanic rocks from the Neoproterozoic Gwna Group of Anglesey-Lleyn, NW Wales, U.K.: MORB and OIB in the Iapetus Ocean

    NASA Astrophysics Data System (ADS)

    Saito, Takuya; Uno, Masaoki; Sato, Tomohiko; Fujisaki, Wataru; Haraguchi, Satoru; Li, Yi-bing; Sawaki, Yusuke; Yamamoto, Shinji; Maruyama, Shigenori

    2015-11-01

    The Gwna Group in Anglesey-Lleyn, NW Wales, UK, is a Neoproterozoic accretionary complex that consists of basalt, bedded chert, red claystone, and trench turbidite that have been intercalated in coherent and incoherent mélanges that are considered typical Ocean plate stratigraphy (OPS). The sediments in the OPS can be useful for constraining the geological environment in the Iapetus Ocean. Most basalts in this area have undergone hydrothermal alteration, greenschist facies regional metamorphism, and surface oxidation. This indicates that immobile elements such as Al2O3 and TiO2, Rare Earth Elements (REE) and High Field Strength Elements (HSFE) are appropriate for discriminating the origin of the basalts in the Gwna Group. Most basalts showing light REE-enriched pattern in CI chondrite-normalized spider diagrams in within-plate basalt (WPB) fields, and some have flat patterns in spider diagrams in mid-oceanic ridge basalt (MORB) fields. In view of these relations, we conclude that the former erupted in an oceanic island. Oceanic island basalts (OIB) are common in Phanerozoic accretionary complexes, and this study presents the first evidence of OIB in a Neoproterozoic accretionary complex of the Gwna Group in Anglesey-Llyen and Llyen area. The OIB-like basalts are locally capped by red hematite-rich claystones. This indicates that a fully oxic pelagic condition was present around the oceanic island in the Iapetus Ocean in the Neoproterozoic, which is consistent with the redox condition estimated from contemporaneous shallow marine sediments. On the other hand, the presence of black mudstones on top of MORB-like meta-basalts suggests that deep-sea anoxia conditions were prevalent during the end-Proterozoic.

  5. SHRIMP U-Pb geochronology of Neoproterozoic Windermere Supergroup, central Idaho: Implications for rifting of western Laurentia and synchroneity of Sturtian glacial deposits

    USGS Publications Warehouse

    Lund, K.; Aleinikoff, J.N.; Evans, K.V.; Fanning, C.M.

    2003-01-01

    In central Idaho roof pendants, a northwest-trending belt of metamorphosed strata, correlative with the Windermere Supergroup, links northern and southern segments of the western Laurentia Neoproterozoic rift belt. Nine newly named formations within the Gospel Peaks sequence-A through Gospel Peaks sequence-D record Cryogenian preglacial, rift-glacial, and postglacial events as well as Neoproterozoic III glacial and rift events. The Edwardsburg Formation of Gospel Peaks sequence B includes interfingered bimodal rift-related volcanic and glaciogenic diamictite strata. Zircons from a rhyodacite flow in the lower Edwardsburg Formation and from a rhyolite flow at its top, dated by using the sensitive high-resolution ion microprobe (SHRIMP), yielded a weighted average of 685 ?? 7 Ma and 684 ?? 4 Ma. Reevaluation of geochronology and correlations indicates that Cryogenian rifting may have been (1) protracted between 780 and 685 Ma, (2) diachronous along the Cordillera, and/or (3) stepwise with a Cordilleran-wide event at ca. 685 Ma that initiated the formation of the Cordilleran miogeocline and set its geometry. Reevaluation of the Cryogenian glacial record indicates that (1) two associated ca. 685 Ma glacial intervals in the Edwardsburg Formation correlate with the Rapitan glaciation, (2) the Sturtian snowball Earth event must be reevaluated on the basis of revision of Rapitan glaciation from 750-700 Ma to ca. 685 Ma, and (3) there were older Cryogenian glaciations or Cryogenian glaciations were not globally synchronous. New dates and correlations significantly impact the number and synchroneity of possible snowball Earth events and the paleolatitudes of Cryogenian glaciations. Western Laurentian events at ca. 685 Ma particularily affect Neoproterozoic paleocontinental reconstructions by indicating diachronous and multi step breakup of supercontinent Rodinia.

  6. U-Pb zircon geochronology and Nd-Hf-O isotopic systematics of the Neoproterozoic Hadb adh Dayheen ring complex, Central Arabian Shield, Saudi Arabia

    NASA Astrophysics Data System (ADS)

    Ali, Kamal A.; Jeon, Heejin; Andresen, Arild; Li, Shuang-Qing; Harbi, Hesham M.; Hegner, Ernst

    2014-10-01

    A combined study of single zircon U-Pb dating, Hf-O zircon isotopic analyses and whole-rock Nd isotopic compositions was carried out to infer the magma sources of Neoproterozoic post-collisional A-type granitoids in Saudi Arabia. U-Pb zircon dating of magmatic zircons of two samples from the Hadb adh Dayheen ring complex yielded ages of 625 ± 11 Ma for a hornblende-biotite granite sample, and 613 ± 4 Ma for a monzogranite sample. The granitic rocks show initial εNd values of + 4.1 to + 5.3 and εHf of + 4.5 to + 8.4 that are lower than those of a model depleted mantle (εHf ~+ 14 and εNd ~+ 6.5) and consistent with melting of subduction-related crustal protoliths that were formed during the Neoproterozoic assembly of the Arabian-Nubian Shield (ANS). Crustal-model ages (Hf-tNC) of 0.81 to 1.1 Ga are inconsistent with depleted-mantle Nd model ages of 0.71 to 0.81 Ga and indicate that the post-collisional Hadb adh Dayheen granites were derived mostly from juvenile crust formed in Neoproterozoic time. Single zircons data show a wide range in δ18O values from + 3.2‰ to + 6.4‰, possibly indicating crystallization of zircon from magma derived from magmatic rocks altered by meteoric water in a magma chamber-caldera system.

  7. Isotope studies of dolomite formation under sedimentary conditions

    USGS Publications Warehouse

    Clayton, R.N.; Jones, B.F.

    1968-01-01

    Measurements of stable isotope abundances of the carbonate portion of the sediment in Deep Springs Lake, California, indicate the presence of at least three phases: a magnesian calcite, a primary sedimentary dolomite, and a detrital dolomite. The former two have isotopic compositions consistent with precipitation at isotopic equilibrium from waters of the lake area. The measured isotopic fractionation factor between sedimentary dolomite and its interstitial water is 1.0351, which is outside the range possible for calcite-water. This indicates that the dolomite has formed by direct crystallization from solution and not from a caloite precursor without further isotope exchange. Isotopic and X-ray evidence does not support the contention of Peterson et al. (1966) that Deep Springs Lake dolomite crystals grow by means of a calcite-like surface layer. ?? 1968.

  8. Heavy-mineral analysis of sedimentary rocks of northern Alaska

    USGS Publications Warehouse

    Morris, Robert Hamilton

    1952-01-01

    The Navy Oil Unit of the United States Geological Survey has been investigating the geology of Naval Petroleum Reserve No. 4, northern Alaska. As part of this program, heavy-mineral samples were prepared from cores of the test wells and core holes and studied to determine stratigraphic correlations. Using the following criteria: (1) presence of diagnostic minerals or mineral suites; (2) relative abundance of specific minerals; (3) degree of rounding of mineral grains; (4) distinction as to grain form; eight heavy-mineral zones have been recognized in Triassic, Jurassic, Cretaceous, and Quaternary sedimentary rocks. Correlations based on these zones are shown. Source areas and rocks are discussed in relation to geologic history and genesis of the Mesozoic and Quaternary sedimentary rocks.

  9. Superposed folding and associated fracturing influence hypogene karst development in Neoproterozoic carbonates, São Francisco Craton, Brazil

    NASA Astrophysics Data System (ADS)

    Ennes-Silva, Renata A.; Bezerra, Francisco H. R.; Nogueira, Francisco C. C.; Balsamo, Fabrizio; Klimchouk, Alexander; Cazarin, Caroline L.; Auler, Augusto S.

    2016-01-01

    Porosity and permeability along fractured zones in carbonates could be significantly enhanced by ascending fluid flow, resulting in hypogene karst development. This work presents a detailed structural analysis of the longest cave system in South America to investigate the relationship between patterns of karst conduits and regional deformation. Our study area encompasses the Toca da Boa Vista (TBV) and Toca da Barriguda (TBR) caves, which are ca. 107 km and 34 km long, respectively. This cave system occurs in Neoproterozoic carbonates of the Salitre Formation in the northern part of the São Francisco Craton, Brazil. The fold belts that are around and at the craton edges were deformed in a compressive setting during the Brasiliano orogeny between 750 and 540 Ma. Based on the integrated analysis of the folds and brittle deformation in the caves and in outcrops of the surrounding region, we show the following: (1) The caves occur in a tectonic transpressive corridor along a regional thrust belt; (2) major cave passages, at the middle storey of the system, considering both length and frequency, developed laterally along mainly (a) NE-SW to E-W and (b) N to S oriented anticline hinges; (3) conduits were formed by dissolutional enlargement of subvertical joints, which present a high concentration along anticline hinges due to folding of competent grainstone layers; (4) the first folding event F1 was previously documented in the region and corresponds with NW-SE- to N-S-trending compression, whereas the second event F2, documented for the first time in the present study, is related to E-W compression; and (5) both folding events occurred during the Brasiliano orogeny. We conclude that fluid flow and related dissolution pathways have a close relationship with regional deformation events, thus enhancing our ability to predict karst patterns in layered carbonates.

  10. Paleogeographic Evolution of the Late Neoproterozoic and Early Phanerozoic with New Paleomagnetic Constraints from West African Craton

    NASA Astrophysics Data System (ADS)

    Robert, B.; Besse, J.; Blein, O.; Greff-Lefftz, M.; Baudin, T.; Fernando, L.; Meslouh, S.; Belbadaoui, M.

    2015-12-01

    The paleogeographic evolution of the late Neoproterozoic and early Phanerozoic is dominated by the dispersion of Rodinia and the assembly of Gondwana. The timing of these two episodes is still highly debated, partly due to the low number of good quality paleomagnetic data. In order to better constrain the paleogeography for this epoch, we bring new paleomagnetic data on volcanic series from the West African Craton (WAC), which is a key block to understand the evolution of these two supercontinents. We have sampled well dated pyroclastic and lava flows from the groups of Ouarzazate (upper Ediacaran) and Taroudant (lower Cambrian) in the Anti-Atlas (Morocco). 500 samples from 105 sites were thermally demagnetized in laboratory. Our results highlight two major groups of directions, mainly carried by minerals of the titano-hematite family. Magnetite may also contribute sometimes to the magnetization. The first group displays a single polarity direction, with a shallow inclination and a south-east declination. This direction close to the expected direction derived from the Permo-Carboniferous segment of the Gondwana apparent polar wander path (APWP) is due to a remagnetization acquired during the Kiaman reversed polarity superchron (320-262Ma). The second group, observed in the Ouarzazate and Taroudant groups, consists of a dual polarity high inclination direction and may represent the characteristic magnetization. On the basis of geologic and paleomagnetic data from literature, we constructed an APWP for both WAC and Amazonia between 615 and 530Ma, assuming these two blocks were already accreted. We found a paleomagnetic solution in which Laurentia and WAC-Amazonia remained attached from ~615Ma up to the late Ediacaran, Laurentia remaining at low latitude during this period. Around ~550Ma, WAC-Amazonia separated from Laurentia and finally collided with the other Gondwanan blocks during the lower Cambrian, marking the final accretion of Gondwana.

  11. Neoproterozoic Cana Brava chrysotile deposit (Goiás, Brazil): Geology and geochemistry of chrysotile vein formation

    NASA Astrophysics Data System (ADS)

    Biondi, João Carlos

    2014-01-01

    The Cana Brava chrysotile asbestos deposit of Goiás, Brazil, contains approximately 150 Mt of ore with an average of 3.5 wt.% of cross-fiber chrysotile and lies in the differentiated, mafic-ultramafic Neoproterozoic Cana Brava complex. This complex was formed at approximately 0.79 Ga and metamorphosed at 0.77 to 0.76 and 0.63 Ga. The 0.77 to 0.76 Ga metamorphic event was a high-grade one that transformed the mafic and ultramafic rocks into meta-peridotites and meta-pyroxenites. The low-grade 0.63 Ga metamorphism allowed the formation of black, red and brown serpentinite, graphitic, magnesite-rich talc serpentinite, and rodingite, which became folded and foliated. At the end of the 0.63 Ga metamorphism, black serpentinites were oxidized to form red serpentinites, the main type of serpentinite that outcrops today at the Cana Brava mineralized region. Post-metamorphic fluids reactivated the process of serpentinization, thereby generating massive green serpentinite from the red. Green formed on the most fractured zones, and double red and green reaction rims formed on the sides of the veins located outside the green serpentinite zones. This process did not cause significant variation in the volume of the rocks and resulted in a strongly reducing system thanks to the loss of Fe2O3 and iron and the subsequent crystallization of magnetite within veinlets and altered rocks. Low angle shear, developed under brittle conditions, caused hydraulic fracturing and the generation of oversaturated, oxidizing fluids that crystallized the cross-fiber chrysotile inside open fractures. Very densely fractured zones with fractures filled with cross-fiber chrysotile constitute the ore that is mined at present.

  12. Jurassic sedimentary basins in the Central Asian orogenic belt

    SciTech Connect

    Bebeshev, I.I.

    1995-05-01

    The principal stages of development of Jurassic sedimentary basins (from their origin to the end of their existence) in the Central Asian orogenic belt are considered. The interrelations of the basins with the surrounding paleorises are investigated. Paleogeographic maps are compiled representing the evolution of paleolandscapes and revealing their interrelations in space and time for each stage. Areas with the highest prospects for coal are found.

  13. Preliminary catalog of the sedimentary basins of the United States

    USGS Publications Warehouse

    Coleman, James L., Jr.; Cahan, Steven M.

    2012-01-01

    One hundred forty-four sedimentary basins (or groups of basins) in the United States (both onshore and offshore) are identified, located, and briefly described as part of a Geographic Information System (GIS) data base in support of the Geologic Carbon Dioxide Sequestration National Assessment Project (Brennan and others, 2010). This catalog of basins is designed to provide a check list and basic geologic framework for compiling more detailed geologic and reservoir engineering data for this project and other future investigations.

  14. Organic sedimentary deposits in Titan's dry lakebeds: Probable evaporite

    USGS Publications Warehouse

    Barnes, J.W.; Bow, J.; Schwartz, J.; Brown, R.H.; Soderblom, J.M.; Hayes, A.G.; Vixie, G.; Le, Mouelic S.; Rodriguez, S.; Sotin, C.; Jaumann, R.; Stephan, K.; Soderblom, L.A.; Clark, R.N.; Buratti, B.J.; Baines, K.H.; Nicholson, P.D.

    2011-01-01

    We report the discovery of organic sedimentary deposits at the bottom of dry lakebeds near Titan's north pole in observations from the Cassini Visual and Infrared Mapping Spectrometer (VIMS). We show evidence that the deposits are evaporitic, making Titan just the third known planetary body with evaporitic processes after Earth and Mars, and is the first that uses a solvent other than water. ?? 2011 Elsevier Inc.

  15. Provenance and chemostratigraphy of the Neoproterozoic West Congolian Group in the Democratic Republic of Congo

    NASA Astrophysics Data System (ADS)

    Frimmel, Hartwig E.; Tack, Luc; Basei, Miguel S.; Nutman, Allen P.; Boven, Ariel

    2006-10-01

    Lithogeochemical, chemostratigraphic, Rb-Sr and Sm-Nd isotope as well as U-Pb single zircon age data from the principal stratigraphic units of the West Congolian Group (West Congo Supergroup) in the West Congo Belt, Democratic Republic of Congo, presented in this reconnaissance study provide new insights into regional stratigraphic correlation and sediment provenance. The oldest unit (Sansikwa Subgroup) is a continental rift basin fill that culminated in a marine glacial diamictite (Lower Mixtite Formation), which is correlated with the global 750-720 Ma Sturtian glaciation. This is followed by a post-glacial, marine carbonate-dominated succession (Haut Shiloango Subgroup), a second diamictite unit (Upper Mixtite Formation) for which a syn-Marinoan (636 Ma) age is favoured, a post-glacial carbonate succession (Schisto-Calcaire Subgroup) and finally molasse sediments that experienced orogenic deformation at c. 566 Ma, as indicated by new 40Ar- 39Ar data. The youngest unit (Inkisi Group) is post-orogenic, derived from the West Congo Belt, and thus not part of the same supersequence. The proposed regional to global correlation of the various units is in line with progressively younger detrital zircon age spectra up-section. Overall, these age spectra indicate a Neoarchaean basement source (Congo Craton) and a Palaeoproterozoic source from the known Kimezian Supergroup to the west of the belt, which corresponds in age with the Eburnean of southern Africa. In addition, we identified a hitherto unknown late Mesoproterozoic source to the west of the belt, the age of which is comparable to the Espinhaço Supergroup of the Araçuai Belt in Brazil.

  16. Paleoproterozoic mojaveprovince in northwestern Mexico? Isotopic and U-Pb zircon geochronologic studies of precambrian and Cambrian crystalline and sedimentary rocks, Caborca, Sonora

    USGS Publications Warehouse

    Lang, Farmer G.; Bowring, S.A.; Matzel, J.; Maldonado, G.E.; Fedo, C.; Wooden, J.

    2005-01-01

    Whole-rock Nd isotopic data and U-Pb zircon geochronology from Precambrian crystalline rocks in the Caborca area, northern Sonora, reveal that these rocks are most likely a segment of the Paleoproterozoic Mojave province. Supporting this conclusion are the observations that paragneiss from the ??? 1.75 Ga Bamori Complex has a 2.4 Ga Nd model age and contains detrital zircons ranging in age from Paleo- proterozoic (1.75 Ga) to Archean (3.2 Ga). Paragneisses with similar age and isotopic characteristics occur in the Mojave province in southern California. In addition, "A-type" granite exposed at the southern end of Cerro Rajon has ca 2.0 Ga Nd model age and a U-Pb zircon age of 1.71 Ga, which are similar to those of Paleoproterozoic granites in the Mojave province. Unlike the U.S. Mojave province, the Caborcan crust contains ca. 1.1 Ga granite (Aibo Granite), which our new Nd isotopic data suggest is largely the product of anatexis of the local Precambrian basement. Detrital zircons from Neoproterozoic to early Cambrian miogeoclinal arenites at Caborca show dominant populations ca. 1.7 Ga, ca. 1.4 Ga, and ca. 1.1 Ga, with subordinate Early Cambrian and Archean zircons. These zircons were likely derived predominately from North American crust to the east and northeast, and not from the underlying Caborcan basement. The general age and isotopic similarities between Mojave province basement and overlying miogeoclinal sedimentary rocks in Sonora and southern California is necessary, but not sufficient, proof of the hypothesis that Sonoran crust is allochthonous and was transported to its current position during the Mesozoic along the proposed Mojave-Sonora megashear. One viable alternative model is that the Caborcan Precambrian crust is an isolated, autochthonous segment of Mojave province crust that shares a similar, but not identical, Proterozoic geological history with Mojave province crust found in the southwest United States ?? 2005 Geological Society of America.

  17. Zircon U-Pb and Lu-Hf isotope study of the Neoproterozoic Haizhou Group in the Sulu orogen: Provenance and tectonic implications

    NASA Astrophysics Data System (ADS)

    Zhou, Jian-Bo; Wilde, Simon A.; Liu, Fu-Lai; Han, Jie

    2012-04-01

    The Neoproterozoic Haizhou Group crops out sporadically in the Sulu orogen in east-central China. It is divided into the Jinping and Yuntai formations and consists of quartzite, quartz schist, marble and graphite- and apatite-bearing sequences. Major and trace element data for quartz schist from the two formations indicate that these rocks have a greywacke protolith and have been deposited during strong tectonic activity. LA-ICPMS U-Pb dating of detrital zircon yields ages of 635 to 1074 Ma for three samples from the Jinping Formation and 611 to 943 Ma for two samples from the Yuntai Formation. More than 78% of the detrital zircons from the two formations have U-Pb ages grouped between 700 and 890 Ma, with two clusters peaking at 758 Ma and 828 Ma, respectively. This indicates that their provenance is magmatic rocks of Neoproterozoic age that have a tectonic affinity to the South China Block (SCB). A few older zircon populations with peak U-Pb ages at 943 and 1074 Ma are also present. A younger population shows peaks at 661 and 611 Ma. This suggests that deposition of the Haizhou Group was later than ~ 611 Ma rather than during the Mesoproterozoic as previously thought. Zircon Lu-Hf isotope data collected from the same U-Pb sites show negative ɛHf(t) values of - 22.8 to - 7.4 and Hf model ages of 2341 to 3100 Ma. This indicates that the Neoproterozoic magmatic rocks were derived from reworking of ancient Paleoproterozoic to Archean crust. The results support the contention that the Haizhou Group is similar to the Wulian Group at the northwestern edge of the Sulu orogen, both having a SCB affinity, but that the Penglai Group does not belong to the SCB because of the absence of Neoproterozoic ages. This lends support to the conclusion that the Triassic suture between the North China and South China blocks is located along the Baichihe-Yantai Fault, which lies north of the Wulian Complex and south of the Jiaobei Terrane; thus the Wulian-Yantai Fault is not the suture

  18. A deep-time perspective of land-ocean linkages in the sedimentary record.

    PubMed

    Romans, Brian W; Graham, Stephan A

    2013-01-01

    It is increasingly important to understand and predict how marine environments respond to changes in climate and sea level and to variability in sediment flux from rivers. The dynamics of these factors occur over several orders of temporal magnitude and, under favorable geologic conditions, contribute to long-lived sediment accumulation. Thus, stratigraphic successions along continental margins are archives of these environmental changes and can be used to reconstruct land-ocean linkages, which provide important context for shorter-term and future modifications to this critical zone. Here, we discuss an integrated approach to the analysis of deep-time sediment archives (>10(6) years) that considers the entire system, from eroding catchments where sediment is produced to subsiding basins where sediment accumulates. This holistic approach is presented within the framework of fundamental concepts about sedimentary-basin analysis and stratigraphic characterization through a combination of foundational literature and studies that represent the state of the art. PMID:22809187

  19. Successful Predictions

    NASA Astrophysics Data System (ADS)

    Pierrehumbert, R.

    2012-12-01

    In an observational science, it is not possible to test hypotheses through controlled laboratory experiments. One can test parts of the system in the lab (as is done routinely with infrared spectroscopy of greenhouse gases), but the collective behavior cannot be tested experimentally because a star or planet cannot be brought into the lab; it must, instead, itself be the lab. In the case of anthropogenic global warming, this is all too literally true, and the experiment would be quite exciting if it weren't for the unsettling fact that we and all our descendents for the forseeable future will have to continue making our home in the lab. There are nonetheless many routes though which the validity of a theory of the collective behavior can be determined. A convincing explanation must not be a"just-so" story, but must make additional predictions that can be verified against observations that were not originally used in formulating the theory. The field of Earth and planetary climate has racked up an impressive number of such predictions. I will also admit as "predictions" statements about things that happened in the past, provided that observations or proxies pinning down the past climate state were not available at the time the prediction was made. The basic prediction that burning of fossil fuels would lead to an increase of atmospheric CO2, and that this would in turn alter the Earth's energy balance so as to cause tropospheric warming, is one of the great successes of climate science. It began in the lineage of Fourier, Tyndall and Arrhenius, and was largely complete with the the radiative-convective modeling work of Manabe in the 1960's -- all well before the expected warming had progressed far enough to be observable. Similarly, long before the increase in atmospheric CO2 could be detected, Bolin formulated a carbon cycle model and used it to predict atmospheric CO2 out to the year 2000; the actual values come in at the high end of his predicted range, for

  20. Arctic coastal zone mapping: Evolution of sedimentary coasts in Greenland

    NASA Astrophysics Data System (ADS)

    Bendixen, M.; Kroon, A.

    2015-12-01

    Climate change threatens many of the coastal areas all over the world. In the Arctic, the warming happens at a rate which is three times faster than the global average increasing the pressure on the coast. Arctic coasts differ from coasts in lower latitude in terms of the natural conditions prevailing, i.e. sea-ice, permafrost, and thermal erosion. These factors are likely to change with an increasing temperature, and thereby the erodibility of the shores and the erosivity of the coastal processes are changing. The majority of studies on arctic coasts focus on tundra coasts. Here, there is a general increase of coastal erosion rates over the last decades. However, the arctic coastal areas of Greenland differ; they are often close to hard rock protrusions and are characterized by large differences in geomorphology, erodibility of sediments, and erosivity by coastal processes. Sedimentary coasts in Greenland are only sporadically investigated, and it is thus difficult to predict the impact of climate changes in these areas. With this work we focus on sedimentary coasts in Greenland and present shoreline analysis of two sedimentary coastal sites. We show how the position of the shoreline has changed since the 1930'ies and we address the responsible factors controlling this evolution. The hotspots of coastal change are all located near delta mouths and the detected changes are coupled to dominating process occurring here.

  1. Sedimentary transfers evolution and hydrological modifications in small sahelian watersheds

    NASA Astrophysics Data System (ADS)

    Le-Breton, E.; Mamadou, I.; Cosandey, C.; Bouzou Moussa, I.; Gautier, E.; Descroix, L.

    2009-04-01

    The area of Niamey (Niger) is composed of a mosaic of small endorheic watersheds. Since 1950, this region is undergoing both the consequences of an exceptional drought and a sharp increase of anthropogenic pressure. This is reflected in the disappearance of the tiger bush which covered naturally the crusted table-land and widespread planting of slopes. These changes generate a strong rise in runoff coefficients producing a change in local hydrology and an increase of sediment transfers. The aim of our study is to highlight the hydrological changes induced by these sedimentary transfers. For that, we monitored different areas on two small experimental watersheds during the AMMA experiment. From topographical, hydrometrical and sedimentary surveys, we put forward the environmental changes (water supplying changes of groundwater, segmentation of ponds, ponds mobility, deflection, etc.). Monitoring the progress of erosion/sedimentation forms allows the emphasis on increasing water flow and sediment in two main areas. The first one is formed by areas of sedimentary deposits growing at mid-slope, allowing the storage and infiltration of runoff. The second area is formed on the downstream ponds with modifications of water and sediment supplies witch change the pond's dynamic. Keywords: erosion; hydrology; environmental changes; anthropic pressure; Sahel; Niger; AMMA

  2. DOE workshop: Sedimentary systems, aqueous and organic geochemistry

    SciTech Connect

    Not Available

    1993-07-01

    A DOE workshop on sedimentary systems, aqueous and organic geochemistry was held July 15-16, 1993 at Lawrence Berkeley Laboratory. Papers were organized into several sections: Fundamental Properties, containing papers on the thermodynamics of brines, minerals and aqueous electrolyte solutions; Geochemical Transport, covering 3-D imaging of drill core samples, hydrothermal geochemistry, chemical interactions in hydrocarbon reservoirs, fluid flow model application, among others; Rock-Water Interactions, with presentations on stable isotope systematics of fluid/rock interaction, fluid flow and petotectonic evolution, grain boundary transport, sulfur incorporation, tracers in geologic reservoirs, geothermal controls on oil-reservoir evolution, and mineral hydrolysis kinetics; Organic Geochemistry covered new methods for constraining time of hydrocarbon migration, kinetic models of petroleum formation, mudstones in burial diagenesis, compound-specific carbon isotope analysis of petroleums, stability of natural gas, sulfur in sedimentary organic matter, organic geochemistry of deep ocean sediments, direct speciation of metal by optical spectroscopies; and lastly, Sedimentary Systems, covering sequence stratigraphy, seismic reflectors and diagenetic changes in carbonates, geochemistry and origin of regional dolomites, and evidence of large comet or asteroid impacts at extinction boundaries.

  3. Occurrence and origin of rhythmic sedimentary rocks on Mars

    NASA Astrophysics Data System (ADS)

    Lewis, Kevin W.; Aharonson, Oded

    2014-06-01

    Sedimentary rocks preserved on the surface of Mars represent a natural archive of past climate conditions. Although the details of their formation often remain poorly constrained, the recent detection of rhythmic bedding patterns in the Arabia Terra region suggests the influence of orbital variations on sedimentary deposition. Here we detail a number of new sites which exhibit quasiperiodic stratigraphic variations, demonstrating their occurrence throughout the equatorial region of the planet. We characterize these recorded signals as well as the local geomorphic context and structural attributes. Two cyclic units are identified within Gale crater, the landing site of the Mars Science Laboratory mission, enabling estimation of possible formation timescales for the geologic units that may be studied in situ by the rover. We find a general lack of fluvial features in connection with rhythmic geologic units, contrasting these sites with the aperiodic deltaic stratigraphy found at Eberswalde crater. Possible formation scenarios and their climatic implications are discussed for the diverse set of quasiperiodic sedimentary units. We propose multiple depositional pathways for recording cyclic climate changes, including repeated evaporitic precipitation from groundwater discharge in topographic lows as well as largely anhydrous accumulation of atmospheric dust for deposits outside of confined basins. The preservation of orbital signals in sediments distributed across a wide range of geographic settings suggests a pervasive influence on Martian climate conditions through time.

  4. Physical processes affecting the sedimentary environments of Long Island Sound

    USGS Publications Warehouse

    Signell, R.P.; Knebel, H. J.; List, J.H.; Farris, A.S.

    1997-01-01

    A modeling study was undertaken to simulate the bottom tidal-, wave-, and wind-driven currents in Long Island Sound in order to provide a general physical oceanographic framework for understanding the characteristics and distribution of seafloor sedimentary environments. Tidal currents are important in the funnel-shaped eastern part of the Sound, where a strong gradient of tidal-current speed was found. This current gradient parallels the general westward progression of sedimentary environments from erosion or non-deposition, through bedload transport and sediment sorting, to fine-grained deposition. Wave-driven currents, meanwhile, appear to be important along the shallow margins of the basin, explaining the occurrence of relatively coarse sediments in regions where tidal currents alone are not strong enough to move sediment. Finally, westerly wind events are shown to locally enhance bottom currents along the axial depression of the sound, providing a possible explanation for the relatively coarse sediments found in the depression despite tide- and wave-induced currents below the threshold of sediment movement. The strong correlation between the near-bottom current intensity based on the model results and the sediment response as indicated by the distribution of sedimentary environments provides a framework for predicting the long-term effects of anthropogenic activities.

  5. Sedimentary sequence evolution in a Foredeep basin: Eastern Venezuela

    SciTech Connect

    Bejarano, C.; Funes, D.; Sarzalho, S.; Audemard, F.; Flores, G.

    1996-08-01

    Well log-seismic sequence stratigraphy analysis in the Eastern Venezuela Foreland Basin leads to study of the evolution of sedimentary sequences onto the Cretaceous-Paleocene passive margin. This basin comprises two different foredeep sub-basins: The Guarico subbasin to the west, older, and the Maturin sub-basin to the east, younger. A foredeep switching between these two sub-basins is observed at 12.5 m.y. Seismic interpretation and well log sections across the study area show sedimentary sequences with transgressive sands and coastal onlaps to the east-southeast for the Guarico sub-basin, as well as truncations below the switching sequence (12.5 m.y.), and the Maturin sub-basin shows apparent coastal onlaps to the west-northwest, as well as a marine onlap (deeper water) in the west, where it starts to establish. Sequence stratigraphy analysis of these sequences with well logs allowed the study of the evolution of stratigraphic section from Paleocene to middle Miocene (68.0-12.0 m.y.). On the basis of well log patterns, the sequences were divided in regressive-transgressive-regressive sedimentary cycles caused by changes in relative sea level. Facies distributions were analyzed and the sequences were divided into simple sequences or sub- sequences of a greater frequencies than third order depositional sequences.

  6. Decoupled sedimentary records of combustion: Causes and implications

    NASA Astrophysics Data System (ADS)

    Hanke, Ulrich M.; Eglinton, Timothy I.; Braun, Ana L. L.; Reddy, Christopher M.; Wiedemeier, Daniel B.; Schmidt, Michael W. I.

    2016-05-01

    Pyrogenic carbon (PyC) is a collective term for carbon-rich residues comprised of a continuum of products arising from biomass burning and fossil-fuel combustion. PyC is ubiquitous in the environment where it can be transported by wind and water before being deposited in aquatic sediments. We compare results from four different methods used to trace PyC that were applied to a high-temporal resolution sedimentary record in order to constrain changes in PyC concentrations and fluxes over the past ~250 years. We find markedly discordant records for different PyC tracers, particularly during the preindustrial age, implying different origins and modes of supply of sedimentary PyC. In addition to providing new insights into the composition of sedimentary combustion products, this study reveals that elucidation of past combustion processes and development of accurate budgets of PyC production and deposition on local to regional scales requires careful consideration of both source characteristics and transport processes.

  7. Timing of maturation of a Neoproterozoic oceanic arc during Pan-African Orogeny: the Asmlil complex (Anti-Atlas, South Morocco)

    NASA Astrophysics Data System (ADS)

    Triantafyllou, Antoine; Berger, Julien; Baele, Jean-Marc; Bruguier, Olivier; Diot, Hervé; Ennih, Nasser; Plissart, Gaëlle; Monnier, Christophe; Watlet, Arnaud; Vandycke, Sara

    2016-04-01

    Many intra-oceanic paleo-arcs are exposed in the Pan-African belt surrounding the West African Craton. In the Moroccan Anti-Atlas, remnants of Intra-Oceanic Subduction Zone (IOSZ) are preserved in few erosional windows moulded along the Anti-Atlas Major fault. These complexes highlight a Neoproterozoic paleo-suture made of 760 My back-arc ophiolites thrusted to the south onto a dismembered band of oceanic arc relics. The Asmlil arc complex, located in the southern part of the Bou Azzer inlier, is made of (i) 755 to 745 My- intermediate banded gneiss interpreted as metavolcanic products of a juvenile oceanic arc. This latter has been intruded by (ii) medium-grained hornblende-gabbro and dioritic magmas, in turn intruded by (iii) medium- to coarse grained hornblenditic-granodioritic decametric intrusions under sub-magmatic HT conditions. Hornblende-gabbros are made of garnet + amphibole/cpx relics + epidote + rutile paragenesis. Calculated pseudosections yielded P ~ 11-12 kbar for T ranging between 600 and 720°C for garnet growth. Measured Zr-in-rutile thermometer gave slightly higher temperature ranging between 710-790°C. On the field, garnet-rich leucocratic veinlets suggest that moderate partial melting of the mafic rock or localized dehydration reactions took place under garnet-granulite conditions (>800°C for hydrated chemical system). New geochronological data on garnet-bearing leucogabbros constrain their emplacement at 700 ±7 My (U-Pb zircon with low Th/U < 0.3). Cooling age (< 700°C) of these HP-HT rocks yielded to a younger age of 654 ±7 My (U-Pb method on rutile). Geochemical data of each mafic and ultramafic facies (hornblende gabbro, garnet-bearing facies and hornblendite) show typical arc signature (marked by e.g. Nb-Ta anomaly, (La/Sm)N: 0.8-1.6 ; (Nb/La) < 0.46 ; high Nb/Ba ratio ; 0.4 < K2O < 2.1 wt%). Intrusive granodioritic magmas show depleted HREE trend similar to granitoids in the Kohistan paleo-arc. Melting modeling suggests they are

  8. Paleoenvironments in Meso-Neoproterozoic carbonates of the Mbuji-Mayi Supergroup (Democratic Republic of Congo) - Microfacies analysis combined with C-O-Sr isotopes, major-trace elements and REE + Y distributions

    NASA Astrophysics Data System (ADS)

    Delpomdor, F.; Blanpied, C.; Virgone, A.; Préat, A.

    2013-12-01

    The Meso- and Neoproterozoic Mbuji-Mayi Supergroup (1155 Ma to ca. 800 Ma) was deposited in the SE-NW trending siliciclastic-carbonate failed-rift in the Sankuru-Mbuji-Mayi-Lomami-Lovoy Basin. Drillcore- and outcrop-derived microfacies, isotope (C, O and Sr) compositions of carbonates and REE + Y distributions are integrated to unravel the paleoenvironmental and chemical conditions prevailing during deposition and alteration (or contamination) of the Mbuji-Mayi carbonates. The carbonate succession (BIe subgroup and BIIa to BIIe subgroups), composed of 11 microfacies (MF), records the evolution of a marine ramp submitted to evaporation, with basinal and low-energy outer-ramp environments (MF1-MF5), biohermal mid-ramp (MF6) and restricted tide-dominated lagoon inner-ramp (MF7-MF9) settings, overlain by lacustrine (MF10) and sabkha (MF11) environments. The ramp margin is characterized by thick stacks of stromatolitic bioherms. δ13C and δ18O relationships in the Mbuji-Mayi carbonates allow discrimination between meteoric (δ13C: -7.5‰ to +0.0‰, δ18O: -7.0‰ to -1.0‰) and burial lithifications (δ13C: -1.5‰ to +0.0‰, δ18O: -15.1‰ to -7.0‰), that overprinted a primary marine signal (δ13C: -1.5‰ to +2.0‰, δ18O: -3.0‰ to +0.5‰) partially preserved in the subgroups. Unaltered pristine signals are found in the Mbuji-Mayi carbonates with 87Sr/86Sr ratios (0.7065-0.7082) similar to those of the marine-preserved strontium signatures of the early Neoproterozoic oceans. The PAAS-normalized REE + Y distributions indicate that the BIe carbonates were altered by Fe-oxide-rich hydrothermal fluids. BIIb and BIIe carbonates exhibit uniform light REE depleted patterns suggesting inputs of detrital river material whereas a marine seawater, highlighted by the REE + Y distributions is preserved in BIIc and BIId carbonates. The pattern of carbon, oxygen and strontium isotopic variations in the Mbuji-Mayi carbonates reflects deposition and early diagenesis in

  9. Compound-specific nitrogen isotopes of equatorial Pacific sedimentary record

    NASA Astrophysics Data System (ADS)

    Sauthoff, W.; Ravelo, A. C.; Mccarthy, M. D.

    2014-12-01

    Compound specific nitrogen isotopic analysis of amino acids (δ15N-AA) is a technique that is widely used in regional ecology and food web studies, with newly expanding applications in organic geochemistry. However, its applicability to marine sediment has been minimally examined. This study is one of the first δ15N-AA applications into the paleorecord of marine sediment. We explore how δ15N-AA measurements provide insights into past changes in water column N cycling and N utilization, and into post-depositional processes that impact sedimentary N. This is possible because δ15N-AA investigates the molecular-level basis of the bulk sedimentary δ15N signal, revealing possible diagenetic alteration of sedimentary organic matter. Our goal was is to investigate the extent of alteration (vs. preservation) of individual sedimentary amino acid δ15N values from surface nitrate δ15N across a wide range of depositional environments. The δ15N of bulk sediment differs from that of the surface nitrate δ15N signal because of water column processes or more often because of alteration of the signal during initial sedimentation. To investigate this alteration we compare δ15N-AA to bulk δ15N measurements in a suite of equatorial Pacific core tops (378-4360 m below sea level) across contrasting oceanographic and sedimentary depositional conditions (e.g. high vs. low productivity, hypoxic vs. oxic bottom waters). To examine down core diagenetic alteration of the sediment record, we present δ15N-AA and bulk δ15N of selected deeper depths to observe 1) if diagenetic shift is coherently resolved by both types of measurements and 2) if select individual δ15N-AA values remain representative of the surface organic δ15N signal. We hypothesize that compound specific analysis (δ15N-AA) will provide a molecular level assessment of mechanism for diagenetic changes in bulk organic δ15N values while also preserving detailed information about planktonic ecosystem structure.

  10. Chlorine isotope behavior during prograde metamorphism of sedimentary rocks

    NASA Astrophysics Data System (ADS)

    Selverstone, Jane; Sharp, Zachary D.

    2015-05-01

    Chlorine stable isotope compositions of two sedimentary sequences and their metamorphic equivalents were measured in order to study fractionation effects during prograde metamorphism and devolatilization. Protoliths (n = 25) were collected from a 50 m section of Triassic fluvial and playa-lake strata and Jurassic (Liassic) marine black shales in a well-characterized quarry. Low greenschist to middle amphibolite facies equivalents (n > 80) were collected from the Glarus Alps, Urseren Zone, and Lucomagno region. Bulk δ37Cl values are constant within individual sedimentary layers, but vary from -2.0 to + 2.4 ‰ in Triassic rocks and from -3.0 to 0‰ in the black shales. Dolomitic and gypsiferous samples have positive δ37Cl values, but marls and shales are isotopically negative. Bulk Cl contents show only small declines during the earliest stages of metamorphism. Metamorphic equivalents of the Triassic and Liassic protoliths record the same overall ranges in δ37Cl as their protoliths. Samples with highly correlated bulk compositions but different metamorphic grade show no statistically significant difference in δ37Cl. These data lead to the following conclusions: (1) Terrestrial and marine sedimentary rocks display large primary heterogeneities in chlorine isotope composition. As a result, an unambiguous "sedimentary signature" does not exist in the chlorine stable isotope system. (2) No isotopic fractionation is discernable during metamorphic devolatilization, even at low temperatures. Alpine-style metamorphism thus has little to no effect on bulk chlorine isotopic compositions, despite significant devolatilization. (3) Cl is largely retained in the rocks during devolatilization, contrary to the normally assumed hydrophilic behavior of chlorine. Continuous release of mixed-volatile C-O-H fluids likely affected Cl partitioning between fluid and minerals and allowed chlorine to remain in the rocks. (4) There is no evidence for fluid communication across (meta)sedimentary

  11. Discussion on the sedimentary structure, geochemical characteristics and sedimentary environment of Ping Chau formation at Tung Ping Chau, Hong Kong.

    PubMed

    Wang, Lulin; Tian, Mingzhong; Wu, Fadong

    2015-07-01

    Ping Chau Formation has long been regarded as the youngest formation in Hong Kong since its emergence from Tung Ping Chau, an island on the northeast of Mirs Bay of New Territories. On the basis of field survey, the present study re-collates and re-stipulates typical sedimentary structure and sedimentary environment of Ping Chau Formation at Tung Ping Chau. Furthermore, with combination of geochemical laboratory methods for the first time a systematic analysis of the major and trace elements of Ping Chau Formation, and geochemical characteristics of rare earth elements were studied. The results showed that: Ping Chau Formation was formed in a passive continental margin structural environment; the study area belonged to transition facies ranging from brackish water to fresh water, with salinity increase from bottom to top on the profile showing a tendency of gradual salinization, The sedimentary environment of Ping Chau Formation had an anoxic reducing environment; The study concluded that Ping Chau Formation was formed in a reducing environment of brackish water or shore-shallow lake with low salinity. PMID:26387352

  12. The Role of the Sedimentary Regime in Shaping the Distribution of Subtidal Sandbank Environments and the Associated Meiofaunal Nematode Communities: An Example from the Southern North Sea

    PubMed Central

    Schratzberger, Michaela; Larcombe, Piers

    2014-01-01

    We combined sediment and faunal data to explore the role of the sedimentary regime in shaping the distribution of subtidal sandbank environments and the associated meiofaunal nematode communities at Broken Bank and Swarte Bank, in the southern North Sea. A variety of sediment transport processes occur in the area, differing in the frequency and magnitude of sediment mobility, and the continuum between erosion, translation and sediment accumulation. The seabed contained a variety of bedforms, including longitudinal furrows, and small to very large sandwaves. The bed sediments were dominated by fine and medium sands, with admixtures of silt and gravel. Based on sedimentary bedforms and grain size analysis, a total of 11 sedimentary facies were delineated, of which 8 were analysed in detail for their relationships with the meiofauna. The sedimentary facies fell clearly into groups of facies, respectively representing high, high-moderate and moderate, and episodic sediment mobility. For those sedimentary facies where daily movement of sediments and bedforms occurred (‘high’ sediment mobility), the resulting spatially homogeneous environments were dominated by an impoverished nematode community comprising small deposit feeders and large predators. Resistance to sediment movement and the ability to exploit alternative food sources were prominent functional features of the successful colonisers. Those facies characterised by relatively infrequent sediment mobility (‘episodic’ and ‘high-moderate and moderate’ sediment mobility) comprised a heterogeneous suite of benthic habitats, containing taxonomically and functionally diverse assemblages of nematodes of various sizes, feeding types and reproductive potential. Faunal distribution patterns here indicated trade-offs between the resistance to sediment movement, environmental tolerance and competitive abilities. Our focus on diverse assemblages of organisms with high turnover times, inhabiting highly dynamic

  13. Evaluation of the Re-Os Geochronometer in Organic-rich Mudrocks as a Method for Constraining the Absolute Ages of Neoproterozoic Glaciogenic Deposits

    NASA Astrophysics Data System (ADS)

    Kendall, B. S.; Creaser, R. A.; Ross, G. M.

    2002-12-01

    Absolute-age constraints on the Neoproterozoic glaciations are generally poor due to a paucity of suitable plutonic and volcanic igneous rocks that are temporally and spatially related to Neoproterozoic glaciogenic deposits and are amenable to radiometric dating methods. In this study, the Re-Os isotope systematics of dark gray, sulfidic slates from the Old Fort Point Formation (OFP) of the Windermere Supergroup (near Jasper, Alberta) were examined to test the ability of the Re-Os geochronometer to provide an absolute age constraint for a Neoproterozoic glaciogenic deposit. The OFP has been interpreted as the deep water expression of post-glacial sea level rise and therefore is comparable stratigraphically to cap carbonates that immediately overlie glaciogenic deposits worldwide. Despite the relatively low Re (6-16 ppb) and Os (0.07-0.14 ppb) concentrations and total organic contents (~ 0.5% TOC) of the slates compared to other organic-rich mudrocks used in previous Re-Os isotope studies, precise well-fitted Re-Os isochrons have been obtained with two different dissolution methods. An age of 620.8 +/- 8.1 Ma (MSWD = 0.9; initial 187Os/188Os = 0.68 +/- 0.06) is obtained using conventional aqua regia dissolution. Using a method designed to selectively dissolve organic matter alone, an age of 609.0 +/- 8.3 Ma (MSWD = 1.5; initial 187Os/188Os = 0.62 +/- 0.05) is obtained. These absolute age results are in accord with existing age constraints (e.g., stratigraphically younger Hamill Group with a U-Pb zircon age of 569 Ma). The well-defined Re-Os systematics of the OFP slates demonstrates for the first time that the Re-Os system is not disturbed in organic-rich sediments during lower greenschist (-chlorite) grade metamorphic conditions. The whole-rock analysis of each individual sample yields consistently higher initial 187Os/188Os isotope ratios than the corresponding organic matter analysis and suggests that a significant radiogenic detrital Os component is present

  14. Petrogenesis and geodynamic setting of Neoproterozoic and Late Paleozoic magmatism in the Manzhouli-Erguna area of Inner Mongolia, China: Geochronological, geochemical and Hf isotopic evidence

    NASA Astrophysics Data System (ADS)

    Gou, Jun; Sun, De-You; Ren, Yun-Sheng; Liu, Yong-Jiang; Zhang, Shu-Yi; Fu, Chang-Liang; Wang, Tian-Hao; Wu, Peng-Fei; Liu, Xiao-Ming

    2013-05-01

    U-Pb dating and Hf isotopic analyses of zircons from various granitoids, combined with major and trace element analyses, were undertaken to determine the petrogenesis and geodynamic setting of Neoproterozoic and Late Paleozoic magmatism in the Manzhouli-Erguna area of Inner Mongolia, China. The Neoproterozoic granitoids are mainly biotite monzogranites with zircon U-Pb ages of 894 ± 13 Ma and 880 ± 10 Ma, and they are characterised by enrichment in large ion lithophile elements (LILEs; e.g., Rb, Ba, K) and light rare earth elements (LREEs), depletion in high field strength elements (HFSEs; e.g., Nb, Ta, Ti) and heavy rare earth elements (HREEs). The Late Devonian granitoids are dominantly syenogranites and mylonitised syenogranites with zircon U-Pb ages of 360 ± 4 Ma, and they form a bimodal magmatic association with subordinate gabbroic rocks of the same age. The Late Devonian syenogranites have A-type characteristics including high total alkalis, Zr, Nb, Ce and Y contents, and high FeOt/MgO, Ga/Al and Rb/Sr ratios. The Carboniferous granitoids are mainly tonalites, granodiorites and monzogranites with U-Pb ages varying from 319 to 306 Ma, and they show very strong adakitic characteristics such as high La/Yb and Sr/Y ratios but low Y and Yb contents. The Late Permian granitoids are dominated by monzogranites and syenogranites with zircon U-Pb ages ranging between 257 and 251 Ma. Isotopically, the ɛHf(t) values of the Neoproterozoic granitoids range from +4.3 to +8.3, and the two-stage model ages (TDM2) from 1.2 to 1.5 Ga. The Late Devonian granitoids are less radiogenic [ɛHf(t) from +12.0 to +12.8 and TDM2 from 545 to 598 Ma] than the Carboniferous [ɛHf(t) from +6.8 to +9.5 and TDM2 from 722 to 894 Ma] and Late Permian granitoids [ɛHf(t) from +6.1 to +9.4 and TDM2 in the range of 680-895 Ma]. These data indicate (1) the Neoproterozoic granitoids may have been generated by melting of a juvenile crust extracted from the mantle during the Mesoproterozoic

  15. Mg Isotope variations of Marinoan Cap Carbonates: implications for the chemical evolution of Neoproterozoic Ocean after snowball earth

    NASA Astrophysics Data System (ADS)

    Liu, C.; Macdonald, F. A.; Raub, T.; Wang, Z.; Evans, D. A.

    2012-12-01

    We report Mg isotope profiles of two cap-carbonates: Nuccaleena formation from south Australia (mostly dolostones) and Tsagaan Oloom Formation from southwest Mongolia (including dolostones, aragonite crystal fans, and lime-mudstones). These data provide additional constraints on the chemical evolution of Neoproterozoic Oceans after the Marinoan deglaciation. An incremental leaching technique using ammonium acetate and various concentrations of acetic acid and hydrochloric acid was applied to separate metals in various forms from cap-carbonates (including surface adsorbed phases, calcite, dolomite and clay minerals). The leachates were then passed through chromatographic columns to extract pure Mg and Sr, which were then analyzed for their isotopic compositions by MC-ICP-MS (Neptune) at Yale University. Elemental ratios (Mg/Ca and Sr/Ca) in each leaching steps were also measured. Our results show that small variations of δ26MgDSM3 with leaching steps were observed in most dolostone samples when secondary calcite is absent. In contrast, large Mg isotope variations (up to 1.5 per mil) were shown in the leaching steps of limestone and crystal fans. The primary δ26MgDSM3 value of each sample was chosen from the leachate that has the lowest 87Sr/86Sr ratios. The δ26MgDSM3 value of Nuccaleena dolostone increases from -2.2‰ at the basal part of the section to -1.7‰ in the middle, and then turns back to -2.0‰ on the top, with a positive correlation between 26Mg/24Mg and 87Sr/86Sr ratios, implying that the high δ26MgDSM3 values may be caused by alteration or inherit from continental-derived fluids. In contrast, small δ26MgDSM3 variations in Tsagaan Oloom dolostones were exhibited in different leaching steps or cross the section (~-1.7‰), with high 87Sr/86Sr ratios (~0.7090), resembling cap dolostones from middle part of Nuccaleena dolostone, implying that they are formed in a similar environment. However, the δ26MgDSM3 value of upper lime-mudstones and crystal

  16. Late Neoproterozoic overprinting of the cassiterite and columbite-tantalite bearing pegmatites of the Gatumba area, Rwanda (Central Africa)

    NASA Astrophysics Data System (ADS)

    Dewaele, Stijn; Henjes-Kunst, Friedhelm; Melcher, Frank; Sitnikova, Maria; Burgess, Ray; Gerdes, Axel; Fernandez, Max Alonso; De Clercq, Friso; Muchez, Philippe; Lehmann, Bernd

    2011-08-01

    The Mesoproterozoic Kibara belt in Central Africa has recently been redefined and subdivided into the Karagwe-Ankole belt (KAB) and the Kibara belt (KIB), separated by Palaeoproterozoic (Rusizian) terranes. The KIB and KAB are characterised by the presence of numerous rare metal mineralised (Nb-Ta-Sn) pegmatites and Sn-W mineralised quartz veins that are related to the youngest granite generation, i.e. the G4-granites in Rwanda, which formed at 986 ± 10 Ma. The pegmatites of the Gatumba area (western Rwanda) have historically been mined for their columbite-tantalite and cassiterite mineralisation, but contain also beryl, apatite, spodumene, amblygonite, and rare phosphates. Columbite-tantalite formed during the crystallisation of the pegmatites, followed by intense alkali metasomatism, i.e. widespread growth of albite and white mica. The major part of the cassiterite mineralisation is, however, concentrated in zones associated with intense phyllic alteration. U-Pb ages of columbite-tantalite samples vary between ˜975 Ma and ˜930 Ma. The oldest ages (975 + 8.2/-8.3 Ma and 966 + 8.7/-8.6 Ma) overlap with previous reported Rb-Sr ages of the emplacement of the pegmatites (˜965 Ma) and are interpreted to reflect the crystallisation of the Nb-Ta mineralisation. The youngest ages (951 ± 15 Ma to 936 ± 14 Ma) are apparently related to variable degrees of resetting by (metasomatic) post-crystallisation processes. The resetting could either be due to recrystallisation of early Nb-Ta minerals or due to the disturbance of the U-Pb isotopic signature of the Nb-Ta minerals. The 40Ar- 39Ar spectra of muscovite samples associated with different steps in the paragenesis of the pegmatites show a spread of apparent ages between ˜940 Ma and ˜560 Ma that reflect Late Neoproterozoic tectonothermal events. One plateau age of 592.2 ± 0.8 Ma is interpreted to reflect far-field effects of the East African orogeny on the Karagwe-Ankole Belt.

  17. The lithospheric architecture of a Neoproterozoic collision zone in Southern Africa inferred from deep probing magnetotelluric data

    NASA Astrophysics Data System (ADS)

    Khoza, D. T.; Jones, A. G.; Muller, M. R.; Evans, R. L.; Miensopust, M. P.; Hamilton, M.; Garcia, X. A.; Cole, P.; Ngwisanyi, T.; Hutchins, D.; Fourie, S.; Jelsma, H.; Pettit, W.; Aravanis, T.; Webb, S. J.; Wasborg, J.

    2009-12-01

    Cratonic margins have played a major role in shaping the modern Southern African tectonic landscape, and central to our knowledge is information about the lithospheric-scale structures and geometries using geophysical methods. Understanding the evolution of the younger orogenic belts around Archean cratons using electrical resistivity data is one of the primary objectives of the Southern African Magnetotelluric Experiment (SAMTEX); the largest ever land-based magnetotelluric (MT) project. MT data were acquired along four profiles crossing the enigmatic Rehoboth Terrane, the Neo-Proterozoic Ghanzi-Chobe/Damara belts (collectively termed the Damara Mobile Belt, DMB) and the southern Angola craton. The extended Groom and Bailey distortion decomposition technique was applied to the MT data and analyses show significant depth and along-profile variations in geo-electric strike and dimensionality on all transects crossing these three tectonic units (i.e. Rehoboth Terrane, Angola craton and the DMB). Geo-electric strikes are generally parallel to the north-east trending tectonic fabric, as inferred from the magnetic data, but the significant strike variations with depth are expressions of heterogeneity in lithospheric structure. Electrical resistivity models derived from the data provide the first pseudo three-dimensional tectonic structure of the Damara orogen and adjacent terranes. Regional-scale resistivity models constructed from two-dimensional inversions of the MT data indicate significant variations in lithospheric resistivity structure along and across strike from the younger orogen to the older adjacent cratons. The Damara belt lithosphere, although generally more conductive and significantly thinner than adjacent Angola craton and Rehoboth terrane, exhibits upper resistive upper crustal features tentatively interpreted to be caused by igneous intrusions emplaced during Pan-African magmatic event. Upper crustal listric faults are imaged as conductive features and

  18. Syn- and Post-Accretionary Structures in the Neoproterozoic Central Allaqi-Heiani Suture Zone, Southeastern Egypt

    NASA Astrophysics Data System (ADS)

    Abdeen, M. M.; Abdelghaffar, A. A.

    2012-04-01

    The Allaqi-Heiani suture (AHS) is the western part of the main Allaqi-Heiani-Gerf-Onib-Sol Hamed-Yanbu suture and represents one of the Neoproterozoic, arc-arc sutures in the Arabian-Nubian Shield (ANS). It separates the ca. 750 Ma South Eastern Desert terrane in the north from the ca. 830-720 Ma Gabgaba terrane in the south. The AHS is a deformed belt of ophiolitic rocks, syn-tectonic granitoids and metasediments. The central AHS zone is divided into three structural domains. The western domain (Ι) is characterized by NNE low thrusts and SSW-vergent folds. The central domain (ΙΙ) includes upright tight to isoclinal NNW-SSE oriented folds and transpressional faults. The eastern domain (ΙΙΙ) shows NNW-SSE oriented open folds. Structural analysis indicates that the area has a poly-phase deformation history involving at least two events. Event D1 was an N-S to NNE-SSW regional shortening generating the SSW-verging folds and the NNE dipping thrusts. Event D2 was an ENE-WSW shortening producing NNW-SSE oriented folds in the central and eastern parts of the study area and reactivating older thrusts with oblique-slip reverse fault movement. The tectonic evolution of the area involves two episodes of collision: an early collision between the South Eastern Desert terrane and the Gabgaba terrane along the AHS after the consumption of a basin floored by oceanic crust above a north-dipping subduction zone; and a later collision between East- and West-Gondwanas at ca. 750-650 Ma, leading to the closure of the Mozambique Ocean. This collision deformed the AHS along N-S trending shortening zones and produced NW-SE and NE-SW oriented sinistral and dextral transpressional faults, respectively. The early collision episode is related to the terrane accretion during the early Pan-African orogen, while the later phase is related to a late Pan-African or Najd orogen.

  19. Pore water colloid properties in argillaceous sedimentary rocks.

    PubMed

    Degueldre, Claude; Cloet, Veerle

    2016-11-01

    The focus of this work is to evaluate the colloid nature, concentration and size distribution in the pore water of Opalinus Clay and other sedimentary host rocks identified for a potential radioactive waste repository in Switzerland. Because colloids could not be measured in representative undisturbed porewater of these host rocks, predictive modelling based on data from field and laboratory studies is applied. This approach allowed estimating the nature, concentration and size distributions of the colloids in the pore water of these host rocks. As a result of field campaigns, groundwater colloid concentrations are investigated on the basis of their size distribution quantified experimentally using single particle counting techniques. The colloid properties are estimated considering data gained from analogue hydrogeochemical systems ranging from mylonite features in crystalline fissures to sedimentary formations. The colloid concentrations were analysed as a function of the alkaline and alkaline earth element concentrations. Laboratory batch results on clay colloid generation from compacted pellets in quasi-stagnant water are also reported. Experiments with colloids in batch containers indicate that the size distribution of a colloidal suspension evolves toward a common particle size distribution independently of initial conditions. The final suspension size distribution was found to be a function of the attachment factor of the colloids. Finally, calculations were performed using a novel colloid distribution model based on colloid generation, aggregation and sedimentation rates to predict under in-situ conditions what makes colloid concentrations and size distributions batch- or fracture-size dependent. The data presented so far are compared with the field and laboratory data. The colloid occurrence, stability and mobility have been evaluated for the water of the considered potential host rocks. In the pore water of the considered sedimentary host rocks, the clay

  20. Subcritical water extraction of organic matter from sedimentary rocks.

    PubMed

    Luong, Duy; Sephton, Mark A; Watson, Jonathan S

    2015-06-16

    Subcritical water extraction of organic matter containing sedimentary rocks at 300°C and 1500 psi produces extracts comparable to conventional solvent extraction. Subcritical water extraction of previously solvent extracted samples confirms that high molecular weight organic matter (kerogen) degradation is not occurring and that only low molecular weight organic matter (free compounds) are being accessed in analogy to solvent extraction procedures. The sedimentary rocks chosen for extraction span the classic geochemical organic matter types. A type I organic matter-containing sedimentary rock produces n-alkanes and isoprenoidal hydrocarbons at 300°C and 1500 psi that indicate an algal source for the organic matter. Extraction of a rock containing type II organic matter at the same temperature and pressure produces aliphatic hydrocarbons but also aromatic compounds reflecting the increased contributions from terrestrial organic matter in this sample. A type III organic matter-containing sample produces a range of non-polar and polar compounds including polycyclic aromatic hydrocarbons and oxygenated aromatic compounds at 300°C and 1500 psi reflecting a dominantly terrestrial origin for the organic materials. Although extraction at 300°C and 1500 psi produces extracts that are comparable to solvent extraction, lower temperature steps display differences related to organic solubility. The type I organic matter produces no products below 300°C and 1500 psi, reflecting its dominantly aliphatic character, while type II and type III organic matter contribute some polar components to the lower temperature steps, reflecting the chemical heterogeneity of their organic inventory. The separation of polar and non-polar organic compounds by using different temperatures provides the potential for selective extraction that may obviate the need for subsequent preparative chromatography steps. Our results indicate that subcritical water extraction can act as a suitable

  1. Chemical composition of sedimentary rocks in California and Hawaii

    USGS Publications Warehouse

    Hill, Thelma P., (compiler)

    1981-01-01

    A compilation of published chemical analyses of sedimentary rocks of the United States was undertaken by the U.S. Geological Survey in 1952 to make available scattered data that are needed for a wide range of economic and scientific uses. About 20,000-25,000 chemical analyses of sedimentary rocks in the United States have been published. This report brings together 2,312 of these analyses from California and Hawaii. The samples are arranged by general lithologic characteristics and locality. Indexes of stratigraphy, rock name, commercial uses, and minor elements are provided. The sedimentary rocks are classified into groups and into categories according to the chemical analyses. The groups (A through F2) are defined by a system similar to that proposed by Brian Mason in 1952, in which the main parameters are the three major components of sedimentary rocks: (1) uncombined silica, (2) clay (R203 ? 3Si02 ? nH20), and (3) calcium-magnesium carbonate. The categories are based on the degree of admixture of these three major components with other components, such as sulfate, phos- phate, and iron oxide. Common-rock, mixed-rock, and special-rock categories apply to rocks consisting of 85 percent or more, 50-84 percent, and less than 49 percent, respectively, of the three major components combined. Maps show distribution of sample localities by States; triangular diagrams show the lithologic characteristics and classification groups. Cumulative-frequency curves of each constituent in each classification group of the common-rock and mixed-rock categories are also included. The numerous analyses may not adequately represent the geochemical nature of the rock types and formations of the region because of sampling bias. Maps showing distribution of sample localities indicate that many of the localities are in areas where, for economic or other reasons, special problems attracted interest. Most of the analyzed rocks tended to be fairly simple in composition - mainly mixtures of

  2. Beryllium-10 in Australasian tektites - Evidence for a sedimentary precursor

    NASA Technical Reports Server (NTRS)

    Pal, D. K.; Moniot, R. K.; Kruse, T. H.; Herzog, G. F.; Tuniz, C.

    1982-01-01

    Each of seven Australasian tektites contains about 100 micron atoms of beryllium-10 (half-life, 1.53 million years) per gram. Cosmic-ray bombardment of the australites cannot have produced the measured amounts of beryllium-10 either at the earth's surface or in space. The beryllium-10 contents of these australites are consistent with a sedimentary precursor that adsorbed from precipitation beryllium-10 produced in the atmosphere. The sediments must have spent several thousand years at the earth's surface within a few million years of the tektite-producing event.

  3. Beryllium-10 in australasian tektites: evidence for a sedimentary precursor.

    PubMed

    Pal, D K; Tuniz, C; Moniot, R K; Kruse, T H; Herzog, G F

    1982-11-19

    Each of seven Australasian tektites contains about 1 x l0(8) atoms of beryllium-10 (half-life, 1.53 x 10(6) years) per gram. Cosmic-ray bombardment of the australites cannot have produced the measured amounts of beryllium-10 either at the earth's surface or in space. The beryllium-10 contents of these australites are consistent with a sedimentary precursor that adsorbed from precipitation beryllium-10 produced in the atmosphere. The sediments must have spent several thousand years at the earth's surface within a few million years of the tektite-producing event. PMID:17771035

  4. Beryllium-10 in Australasian tektites: evidence for a sedimentary precursor

    SciTech Connect

    Pal, D.K.; Tuniz, C.; Moniot, R.K.; Kruse, T.H.; Herzog, G.F.

    1982-11-19

    Each of seven Australasian tektites contains about 1 x 10/sup 8/ atoms of beryllium-10 (half-life, 1.53 x 10/sup 6/ years) per gram. Cosmic-ray bombardment of the australites cannot have produced the measured amounts of beryllium-10 either at the earth's surface or in space. The beryllium-10 contents of these australites are consistent with a sedimentary precursor that adsorbed from precipitation beryllium-10 produced in the atmosphere. The sediments must have spent several thousand years at the earth's surface within a few million years of the tektite-producing event.

  5. The impact of sedimentary coatings on the diagenetic Nd flux

    NASA Astrophysics Data System (ADS)

    Abbott, April N.; Haley, Brian A.; McManus, James

    2016-09-01

    Because ocean circulation impacts global heat transport, understanding the relationship between deep ocean circulation and climate is important for predicting the ocean's role in climate change. A common approach to reconstruct ocean circulation patterns employs the neodymium isotope compositions of authigenic phases recovered from marine sediments. In this approach, mild chemical extractions of these phases is thought to yield information regarding the εNd of the bottom waters that are in contact with the underlying sediment package. However, recent pore fluid studies present evidence for neodymium cycling within the upper portions of the marine sediment package that drives a significant benthic flux of neodymium to the ocean. This internal sedimentary cycling has the potential to obfuscate any relationship between the neodymium signature recovered from the authigenic coating and the overlying neodymium signature of the seawater. For this manuscript, we present sedimentary leach results from three sites on the Oregon margin in the northeast Pacific Ocean. Our goal is to examine the potential mechanisms controlling the exchange of Nd between the sedimentary package and the overlying water column, as well as the relationship between the εNd composition of authigenic sedimentary coatings and that of the pore fluid. In our comparison of the neodymium concentrations and isotope compositions from the total sediment, sediment leachates, and pore fluid we find that the leachable components account for about half of the total solid-phase Nd, therefore representing a significant reservoir of reactive Nd within the sediment package. Based on these and other data, we propose that sediment diagenesis determines the εNd of the pore fluid, which in turn controls the εNd of the bottom water. Consistent with this notion, despite having 1 to 2 orders of magnitude greater Nd concentration than the bottom water, the pore fluid is still <0.001% of the total Nd reservoir in the

  6. In situ NMR analysis of fluids contained in sedimentary rock

    PubMed

    de Swiet TM; Tomaselli; Hurlimann; Pines

    1998-08-01

    Limitations of resolution and absorption in standard chemical spectroscopic techniques have made it difficult to study fluids in sedimentary rocks. In this paper, we show that a chemical characterization of pore fluids may be obtained in situ by magic angle spinning (MAS) nuclear magnetic resonance (NMR), which is normally used for solid samples. 1H MAS-NMR spectra of water and crude oil in Berea sandstone show sufficient chemical shift resolution for a straightforward determination of the oil/water ratio. Copyright 1998 Academic Press. PMID:9716484

  7. Ecosystem development following deglaciation: A new sedimentary record from Devils Lake, Wisconsin, USA

    NASA Astrophysics Data System (ADS)

    Williams, Joseph J.; McLauchlan, Kendra K.; Mueller, Joshua R.; Mellicant, Emily M.; Myrbo, Amy E.; Lascu, Ioan

    2015-10-01

    Processes and rates of ecosystem development can be reconstructed using lacustrine sedimentary sequences, but this approach often requires records that contain the start of primary succession. Most lakes in the upper Midwestern U.S. were formed by glaciers at the end of the last Ice Age approximately 11,700 cal yr BP. Devils Lake, Wisconsin is a rare example of a lake from this region whose sediments extend into the Pleistocene and may include the Last Glacial Maximum. Sediment magnetic, geochemical, pollen, and charcoal records were generated from a 10 m core whose basal sediments may be 28,000 years old. Together with a previously published pollen record, these proxies combine to reveal a history of long-term climatic, vegetative and geologic change during the late Pleistocene to Holocene. We identify six sedimentary units that indicate a series of consecutive events rather than a predictable trajectory of ecosystem development at the site. Productivity in the lake was low during the late Pleistocene and increased during the Holocene, as reflected by the sediment lithology, which shows a sudden shift from glacial vivianite-rich and organic-poor clastic-dominated sediments to Holocene diatomaceous sapropels. Several important processes initiated around 17,000 cal yr BP, including the onset of organic matter accumulation and fire in the terrestrial ecosystem. However, the post-glacial landscape was not devoid of vegetation because pollen assemblages indicate that terrestrial vegetation, likely a spruce tundra, survived near the site. A switch to a hardwood forest period during the Holocene also led to a change in the fire regime, with increased frequency of burning. Aquatic ecosystem productivity lagged terrestrial ecosystem productivity throughout the record. Nutrient cycling (as recorded by sedimentary δ15N) was variable but not directional, and appeared to be correlated with climate conditions early in the record, and terrestrial ecosystem processes later in

  8. Spatial Distribution and Sedimentary Facies of the 2007 Solomon Islands Tsunami Deposits

    NASA Astrophysics Data System (ADS)

    Nakamura, Y.; Nishimura, Y.; Woodward, S.

    2007-12-01

    We conducted a field survey of the extent of damage, crustal deformation, and onshore deposits caused by 2007 Solomon Islands tsunami in Ghizo and adjacent islands in the western Solomon Islands, from 13th to 18th April, 2007. Our survey team was comprised of six Japanese and one American researcher. Three of us, the authors, mainly investigated tsunami deposits in three villages (Titiana, Suva, and Pailongge) in southern Ghizo Island. One member of our team re-investigated the deposits in June 2007. The tsunami generated sheet-like deposits of coral beach sand on the flat plain in Titiana. Beside the sea coast, the tsunami wave eroded ground surfaces and formed small scarps at 30 m from the sea. Just interior of the scarps, tsunami deposits accumulated up to 9 cm in thickness. The thickness decreased with distance from the sea and was also affected by microtopography. No sandy tsunami deposits were observed on the inland area between 170 m and 210 m from the sea. The upper boundary of inundation was recognized at about 210 m from the sea because of accumulation of driftwood and floating debris. In Suva and Pailongge, the outline of sand-sheet distribution is the same as it in Titiana. The tsunami had a maximum thickness of 10 cm and two or three sand layers are separated by thin humic sand layers. These humic layers were likely supplied from hillslopes eroded by the tsunami and transported by return-flows. These successions of deposits suggest that tsunami waves inundated at least two times. This is consistent with the number of large waves told by eyewitnesses. In the Solomon Islands, the plentiful rainfall causes erosion and resedimentation of tsunami deposits. Furthermore, the sedimentary structures will be destroyed by chemical weathering in warm and moist environment, and bioturbation by plants, animals, and human activities. The sedimentary structures had been preserved till the end of June 2007, but had already been penetrated by plant roots and sandpipes

  9. Palynostratigraphy and sedimentary facies of Middle Miocene fluvial deposits of the Amazonas Basin, Brazil

    NASA Astrophysics Data System (ADS)

    Dino, Rodolfo; Soares, Emílio Alberto Amaral; Antonioli, Luzia; Riccomini, Claudio; Nogueira, Afonso César Rodrigues

    2012-03-01

    Palynostratigraphic and sedimentary facies analyses were made on sedimentary deposits from the left bank of the Solimões River, southwest of Manaus, State of Amazonas, Brazil. These provided the age-dating and subdivision of a post-Cretaceous stratigraphic succession in the Amazonas Basin. The Novo Remanso Formation is subdivided into upper and lower units, and delineated by discontinuous surfaces at its top and bottom. The formation consists primarily of sandstones and minor mudstones and conglomerates, reflecting fluvial channel, point bar and floodplain facies of a fluvial meandering paleosystem. Fairly well-preserved palynoflora was recovered from four palynologically productive samples collected in a local irregular concentration of gray clay deposits, rich in organic material and fossilized wood, at the top of the Novo Remanso Formation upper unit. The palynoflora is dominated by terrestrial spores and pollen grains, and is characterized by abundant angiosperm pollen grains ( Tricolpites, Grimsdalea, Perisyncolporites, Tricolporites and Malvacearumpollis). Trilete spores are almost as abundant as the angiosperm pollen, and are represented mainly by the genera Deltoidospora, Verrutriletes, and Hamulatisporis. Gymnosperm pollen is scarce. The presence of the index species Grimsdalea magnaclavata Germeraad et al. (1968) indicates that these deposits belong to the Middle Miocene homonymous palynozone (Lorente, 1986; Hoorn, 1993; Jaramillo et al., 2011). Sedimentological characteristics (poorly sorted, angular to sub-angular, fine to very-coarse quartz sands facies) are typical of the Novo Remanso Formation upper part. These are associated with a paleoflow to the NE-E and SE-E, and with an entirely lowland-derived palinofloristic content with no Andean ferns and gymnosperms representatives. All together, this suggests a cratonic origin for this Middle Miocene fluvial paleosystem, which was probably born in the Purus Arch eastern flank and areas surrounding the

  10. Experiments in a Deep Underground Science and Engineering Laboratory (DUSEL) Hosted in Sedimentary Rocks

    NASA Astrophysics Data System (ADS)

    Burbey, T. J.; Kimballton, M. O.; Science Team

    2004-12-01

    characterization are needed. A DUSEL hosted in sedimentary rocks provides opportunities to simulate the formation of hydrocarbon reservoirs in well-characterized rock at the kilometer scale. Such a facility can be used to study the factors that control generation, migration and trapping of aqueous and hydrocarbon fluids, while allowing greater understanding of the role of microbes in terminal electron accepting processes in fractured-rock environments. DUSEL will provide unique opportunities to assess and evaluate seismic reservoir characterization techniques, monitoring methods, and modeling approaches. Carefully monitored subsequent excavation into the reservoir will permit direct assessment of imaging success and evaluation of geochemical mechanisms, pathways and processes. Many of the metals and energy resources used by modern industrialized societies are formed as a result of fluid-rock interactions. An understanding of the physical and chemical factors that determine solubility, transport, and deposition is critical to development of successful exploration strategies for new resources. DUSEL will permit experiments aimed at perturbing the natural system, while allowing for measurements and sample collection under carefully controlled conditions to constrain key parameters that form copper, zinc, lead and other metal deposits. These experiments will provide clues to how host rocks affect fluid chemistries and the role that biological activity plays in the formation of metal sulfide deposits.

  11. Detrital zircon geochronology and Nd isotope geochemistry of an early Paleozoic succession in Korea:

    NASA Astrophysics Data System (ADS)

    Lee, Yong Il; Choi, Taejin; Lim, Hyoun Soo; Orihashi, Yuji

    2015-04-01

    This study reports the results of an analysis of U-Pb ages of detrital zircons and Nd isotope compositions from the well-established lower Paleozoic platform succession developed on the Precambrian gneiss and metasedimentary rocks in South Korea. The three stratigraphic units in the basal part of the succession are the Jangsan, Myeonsan, and Myobong Formations. The unfossiliferous Jangsan (white­to­pink quartz sandstone) and Myeonsan (dark-gray ilmenite-rich sandstone/shale) Formations are in fault contact and are generally considered to be coeval (Early Cambrian). Both formations are also generally considered to be conformably overlain by the dark­ gray, fossiliferous, fine-grained Myobong Formation (late Early-early Middle Cambrian). We here report U-Pb ages of detrital zircons and Nd isotopic data from the Jangsan, Myeonsan, and Myobong Formations. The Jangsan and Myeonsan Formations provide Archean-Paleoproterozoic U-Pb ages, but the former is characterized by Archean Sm-Nd model ages and the latter by late Paleoproterozoic Sm-Nd model ages, which is indicative of a significant change in provenance. This suggests that the Jangsan Formation predates the Myeonsan Formation. The Myobong Formation provides dominantly Meso- to Neoproterozoic U-Pb ages and Sm-Nd model ages that are slightly younger than those of the Myeonsan Formation. Contrary to the conventional wisdom, the combined evidence of unconformable contact and marked changes in zircon U-Pb ages and Nd isotopic compositions suggests that the Myobong Formation overlies the Jangsan and Myeonsan Formations unconformably. Considering the metamorphic age of the immediately underlying Precambrian basement metasediments (0.8 to 0.9 Ga), this stratigraphic relationship strongly suggests that the Jangsan Formation may be Neoproterozoic in age and that the Myeonsan Formation may be latest Neoproterozoic to Early Cambrian and calls for reevaluation of Precambrian-Paleozoic history of the Korean Peninsula. The

  12. Microbiological Profiles of Deep Terrestrial Sedimentary Rocks Revealed by an Aseptic Drilling Procedure

    NASA Astrophysics Data System (ADS)

    Suzuki, Y.; Suko, T.; Fukuda, A.; Kouduka, M.; Nanba, K.; Sakata, S.; Ito, K.

    2009-12-01

    Unlike the near-surface environments, it is difficult to determine the community structure and biogeochemical functions of microorganisms in the deep subsurface mainly due to accessibility without contamination and disturbance. In an inland fore-arc basin in central Japan, we applied a new drilling procedure using deoxygenated and/or filter-sterilized drilling fluid(s). Although DNA-stained and cultivable cell numbers and the contents of phospholipid fatty acids (PLFA) all indicated the presence of metabolically active microbial populations in sedimentary rocks at a depth range from 200 to 350 m, it was not successful to extract DNA from the drilled core samples. During drilling, drilling fluid used for drilling and coring in the borehole was collected from the borehole bottom and subjected to DNA extraction. Quantitative fluorogenic PCR revealed that bacterial DNA were detected in drilling fluid samples when drilling was performed for siltstone and silty sandstone layers with the limited flow of drilling fluid. Bacterial 16S rRNA gene sequences retrieved from the drilling fluid samples below a depth of 324 m were mostly related to Pseudomonas putida or Flavobacterium succinicans, while those related to other Pseudomonas spp. were predominant at depths of 298 and 299m. PLFA profiles of core samples from a depth range between 250 and 351 m showed the abundance of 16:0, 16:1ω7 and 18:1ω9 fatty acids, which are known as major cellular lipid components of Pseudomonas and Flavobacterium spp. From these results, it was suggested that the members of the genera Pseudomonas and F. succinicans might represent dominant microbial populations that inhabit the deep terrestrial sedimentary rocks in Central Japan. This study was supported by grants from the Nuclear and Industrial Safety Agency (NISA) and Japan Nuclear Energy Safety Organization (JNES).

  13. Seedling establishment in a dynamic sedimentary environment: a conceptual framework using mangroves

    PubMed Central

    Balke, Thorsten; Webb, Edward L; van den Elzen, Eva; Galli, Demis; Herman, Peter M J; Bouma, Tjeerd J

    2013-01-01

    1. Vegetated biogeomorphic systems (e.g. mangroves, salt marshes, dunes, riparian vegetation) have been intensively studied for the impact of the biota on sediment transport processes and the resulting self-organization of such landscapes. However, there is a lack of understanding of physical disturbance mechanisms that limit primary colonization in active sedimentary environments. 2. This study elucidates the effect of sediment disturbance during the seedling stage of pioneer vegetation, using mangroves as a model system. We performed mesocosm experiments that mimicked sediment disturbance as (i) accretion/burial of plants and (ii) erosion/excavation of plants of different magnitudes and temporal distribution in combination with water movement and inundation stress. 3. Cumulative sediment disturbance reduced seedling survival, with the faster-growing Avicennia alba showing less mortality than the slower-growing Sonneratia alba. The presence of the additional stressors (inundation and water movement) predominantly reduced the survival of S. alba. 4. Non-lethal accretion treatments increased shoot biomass of the seedlings, whereas non-lethal erosion treatments increased root biomass allocation. This morphological plasticity in combination with the abiotic disturbance history determined how much maximum erosion the seedlings were able to withstand. 5. Synthesis and applications. Seedling survival in dynamic sedimentary environments is determined by the frequency and magnitude of sediment accretion or erosion events, with non-lethal events causing feedbacks to seedling stability. Managers attempting restoration of mangroves, salt marshes, dunes and riparian vegetation should recognize sediment dynamics as a main bottleneck to primary colonization. The temporal distribution of erosion and accretion events has to be evaluated against the ability of the seedlings to outgrow or adjust to disturbances. Our results suggest that selecting fast-growing pioneer species and

  14. Tectonic and climatic control on geomorphological and sedimentary evolution of the Mercure basin, southern Apennines, Italy

    NASA Astrophysics Data System (ADS)

    Robustelli, Gaetano; Ermolli, Elda Russo; Petrosino, Paola; Jicha, Brian; Sardella, Raffaele; Donato, Paola

    2014-06-01

    The morpho-tectonic and sedimentary evolution of the Mercure intramontane basin (Calabria-Lucania boundary, southern Apennines) has been assessed through facies analysis, morphostratigraphy and geomorphological correlation with adjacent areas. The Mercure basin, one of the most active seismogenic zones of the southern Apennines, is a favorable area for reconstructing the main stages of landscape evolution of the axial zone because of its capability to record changes in base level during the Quaternary. In addition, the presence of both erosional and depositional Palaeosurfaces is a useful marker for reconstructing tectonic and morphogenetic events, and hence to detect the role played by tectonics and climate in its genesis, evolution and extinction. The present study identifies the key role of tectonics and denudation, combined with high-frequency floods, as mechanisms controlling alluvial sedimentation in the study area. During endorheic conditions, denudational processes driven by pulses of extensional deformation of the basin margin caused strong alluvial inputs that resulted in the development of alluvial fans. Alluvial facies are mainly characterized by turbulent, subaerial, hyperconcentrated flood flows deposited during the glacial, semi-arid conditions of MIS 14. The retrogradational stacking pattern of the alluvial system indicates decreasing rates of tectonic activity along with declining river gradients. The Mercure coalescing alluvial fans were inundated by lake transgression during MIS 13 in response to (i) abrupt tectonic subsidence at the basin margins and (ii) large decrease of coarse sediment supply due to the interplay among climate, tectonics and catchment size changes. In this regard, it is suggested that tectonic control on the drainage network along with climate and long-term slope evolution may have caused marked pulses in sediment supply, thus influencing the arrangement of facies associations in the sedimentary succession. In addition, the

  15. Magnetostratigraphy of the Middle-Upper Jurassic sedimentary sequences at Yanshiping, Qiangtang Basin, China

    NASA Astrophysics Data System (ADS)

    Song, Chunhui; Zeng, Yongyao; Yan, Maodu; Wu, Song; Fang, Xiaomin; Bao, Jing; Zan, Jinbo; Liu, Xifang

    2016-05-01

    A series of important geological events occurred in the Tibetan Plateau area during the Jurassic, such as the collision of the Lhasa and Qiangtang Terranes, the closure of the Meso-Tethyan Ocean, the opening of the Neo-Tethyan Ocean and the cessation of the mega-monsoon. The ˜3,000-m-thick Jurassic sedimentary sequence in the Qiangtang Basin on the central Tibetan Plateau, which is called the Yanshiping Group, recorded these geological events. However, the chronology of the sequence is surprisingly poorly constrained. Here, we perform a detailed palaeomagnetic analysis on the ˜1,060-m-thick middle and upper portions of the Yanshiping Group (the Xiali and Suowa Formations) in the Yanshiping section of the eastern Qiangtang Basin. Three bivalve zones at stratigraphic intervals of ˜40-140 m, 640-800 m and 940-1040 m are identified, which yield a Bathonian-Callovian age for the Lower Xiali Fm., a Callovian-Oxfordian age for the Lower Suowa Fm., and an Oxfordian-Kimmeridgian age for the Upper Suowa Fm. A total of 544 oriented palaeomagnetic samples were collected from the section. By combining thermal and alternating field demagnetizations, clear characteristic remanent magnetization (ChRM) directions are isolated for most of the samples. The robust ChRM directions pass fold and reversals tests, which support the primary nature of the ChRMs and yield a palaeopole at 76.8°N/297.2°E (dp = 2.2°, dm = 3.7°). A total of 27 normal and 26 reversed polarity zones were successfully recorded in the section. Combined with fossil age constraints, results suggest that the section is plausibly composed of a Callovian-Early Kimmeridgian age sedimentary sequence.

  16. Planktonic and sedimentary bacterial diversity of Lake Sayram in summer.

    PubMed

    Fang, Lei; Chen, Lei; Liu, Yuan; Tao, Wei; Zhang, Zhongzhe; Liu, Haiying; Tang, Yong

    2015-10-01

    Lake Sayram is an ancient cold water lake locating at a mountain basin in Xinjiang, China. The lake water is brackish, alkaline, unpolluted, and abundant in SO4(2-) and Mg(2+). The lacustrine ecosystem of Lake Sayram has been intensely investigated. However, profiles of the microbial communities in the lake remain largely unknown. In this study, taxonomic compositions of the planktonic and sedimentary bacterial communities in Lake Sayram were investigated using 16S rRNA metagenomics. The lacustrine bacterial communities were generally structured by environmental conditions, including the hydrological and physicochemical parameters. Proteobacteria was the dominating phylum. In the lake water, the genera Acinetobacter and Ilumatobacter held an absolute predominance, implying their metabolic significance. In the bottom sediment, biogeochemically significant bacteria and thermophilic or acidothermophilic extremophiles were recovered. In contrast to the planktonic bacteria, an appreciable portion of the sedimentary bacteria could not be classified into any known taxonomic unit, indicating the largely unknown bacteriosphere hiding in the bottom sediment of Lake Sayram. PMID:26242906

  17. Planktonic and sedimentary bacterial diversity of Lake Sayram in summer

    PubMed Central

    Fang, Lei; Chen, Lei; Liu, Yuan; Tao, Wei; Zhang, Zhongzhe; Liu, Haiying; Tang, Yong

    2015-01-01

    Lake Sayram is an ancient cold water lake locating at a mountain basin in Xinjiang, China. The lake water is brackish, alkaline, unpolluted, and abundant in SO42− and Mg2+. The lacustrine ecosystem of Lake Sayram has been intensely investigated. However, profiles of the microbial communities in the lake remain largely unknown. In this study, taxonomic compositions of the planktonic and sedimentary bacterial communities in Lake Sayram were investigated using 16S rRNA metagenomics. The lacustrine bacterial communities were generally structured by environmental conditions, including the hydrological and physicochemical parameters. Proteobacteria was the dominating phylum. In the lake water, the genera Acinetobacter and Ilumatobacter held an absolute predominance, implying their metabolic significance. In the bottom sediment, biogeochemically significant bacteria and thermophilic or acidothermophilic extremophiles were recovered. In contrast to the planktonic bacteria, an appreciable portion of the sedimentary bacteria could not be classified into any known taxonomic unit, indicating the largely unknown bacteriosphere hiding in the bottom sediment of Lake Sayram. PMID:26242906

  18. Sedimentary facies and ecosystems in the Balearic shelf

    SciTech Connect

    Fornos, J.J.; Pomar, L.; Rodriguez-Perea, A.

    1988-08-01

    Sediments of the Balearic platform are mainly carbonate with low percentages of terrigenous influx. These influences are located in littoral areas related to rushing streams. Main bioclastic components are red algae, mollusks, foraminifers, and bryozoans. Terrigenous components are mainly calcareous lithoclasts. Productive ecosystems are built by sea grasses of Posidonia oceanica and marl. Less important are coralline Vidalia volubilis, Cymodocea nodosa, Caulerpa prolifera, and several communities of photophile algae. Each of these ecosystems is the origin of their correlated biofacies. These biofacies are medium biogenic sands, algal sands and gravels, muddy sands, and coarse terrigenous sands. Sediment distribution is directly related to ecosystem distribution, which can be matched with depth and energy variables. Hydrodynamic circumstances are revealed by minor sedimentary structures, like wavy and current ripples, or by hectometric sand waves placed south of Minorca Island at a depth of 48 to 68 m. Sedimentary grains are suffering from erosion and boring processes, which are produced mainly by fungi but also by sponges, polychaetes, bryozoans, and mollusks. These bioerosions are sometimes accompanied by mechanical abrasion and less frequently by chemical corrosion, and this is the principal source of carbonate mud that is transferred to the outer shelf.

  19. Diffusive flux and pore anisotropy in sedimentary rocks.

    PubMed

    Schaefer, C E; Towne, R M; Lazouskaya, V; Bishop, M E; Dong, H

    2012-04-01

    Diffusion of dissolved contaminants into or from bedrock matrices can have a substantial impact on both the extent and longevity of dissolved contaminant plumes. For layered rocks, bedding orientation can have a significant impact on diffusion. A series of laboratory experiments was performed on minimally disturbed bedrock cores to measure the diffusive flux both parallel and normal to mineral bedding of four different anisotropic sedimentary rocks. Measured effective diffusion coefficients ranged from 4.9×10(-8) to 6.5×10(-7)cm(2)/s. Effective diffusion coefficients differed by as great as 10-folds when comparing diffusion normal versus parallel to bedding. Differences in the effective diffusion coefficients corresponded to differences in the "apparent" porosity in the orientation of diffusion (determined by determining the fraction of pore cross-sectional area measured using scanning electron microscopy), with the difference in apparent porosity between normal and parallel bedding orientations differing by greater than 2-folds for two of the rocks studied. Existing empirical models failed to provide accurate predictions of the effective diffusion coefficient in either bedding orientation for all four rock types studied, indicating that substantial uncertainty exists when attempting to predict diffusive flux through sedimentary rocks containing mineral bedding. A modified model based on the apparent porosity of the rocks provided a reasonable prediction of the experimental diffusion data. PMID:22326688

  20. Distribution of sedimentary mercury off Svalbard, European Arctic.

    PubMed

    Bełdowski, J; Miotk, M; Zaborska, A; Pempkowiak, J

    2015-03-01

    The European Arctic, including the Svalbard archipelago, receives mercury loads due to long range atmospheric transport, local contamination, melting of glaciers and as a result of bedrock weathering. Few studies have been devoted to the contamination history and sources of sedimentary mercury in the Svalbard area. This knowledge gap is addressed in this study. Concentrations of total mercury (10-80ng/g), fractions of mercury differing with affinity to the sediment matrix (88-97% refractory, 3-12% mobile), organic and methyl mercury (100-500pg/g) were measured in surface and subsurface sediments in the Spitsbergen fjords and in the Barents Sea off Svalbard. The atmospheric mercury signal can be observed in the Barents Sea, while in the Svalbard fjords it is strongly modified by supply of mercury from natural sources that may include weathering of rocks and glaciers melting, all modified by organic matter supply. Sedimentary methyl mercury concentrations seem to be dependent on environmental factors affecting mercury methylation rather than on location of sampling stations. PMID:25532769

  1. Geological study of sedimentary basins using SPOT data

    NASA Astrophysics Data System (ADS)

    Pisot, Nathalie; Xavier, Jean-Paul; Miegebielle, Véronique; Coquelet, Dominique; Leymarie, Pierre

    This paper illustrates the benefits of DTMs created from SPOT images for the exploration of sedimentary basins. We chose an example located in the Ebro sedimentary basin in Spain, characterized by good outcropping conditions and slight deformations. The data used consist of a pair of SPOT panchromatic images and a SPOT XS image. The wor