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Sample records for african shelf margin

  1. Mesozoic evolution of northeast African shelf margin, Libya and Egypt

    SciTech Connect

    Aadland, R.K.; Schamel, S.

    1989-03-01

    The present tectonic features of the northeast African shelf margin between the Nile delta and the Gulf of Sirte are products of (1) precursory late Paleozoic basement arches, (2) early Mesozoic rifting and plate separation, and (3) Late Cretaceous structural inversion. The 250 km-wide and highly differentiated Mesozoic passive margin in the Western Desert region of Egypt is developed above a broad northwest-trending Late Carboniferous basement arch. In northeastern Libya, in contrast, the passive margin is restricted to just the northernmost Cyrenaica platform, where subsidence was extremely rapid in the Jurassic and Early Cretaceous. The boundary between the Western Desert basin and the Cyrenaica platform is controlled by the western flank of the basement arch. In the middle Cretaceous (100-90 Ma), subsidence accelerated over large areas of the Western desert, further enhancing a pattern of east-west-trending subbasins. This phase of rapid subsidence was abruptly ended about 80 Ma by the onset of structural inversion that uplifted the northern Cyrenaica shelf margin and further differentiated the Western Desert subbasin along a northeasterly trend.

  2. Mesozoic evolution of the northeast African shelf margin, Libya and Egypt

    SciTech Connect

    Aadland, R.K.; Schamel, S.

    1988-08-01

    The present tectonic features of the northeast African shelf margin between the Nile delta and the Gulf of Sirte are products of (1) precursory late Paleozoic basement arches, (2) early Mesozoic rifting and plate separation, and (3) Late Cretaceous structural inversion. Isopach and structural maps, cross sections, and sediment accumulation (geohistory) curves constructed from 89 wells in the Western Desert and 27 wells in northeastern Libya depict the structural and stratigraphic development of the northeast African shelf margin.

  3. Southern African continental margin: Dynamic processes of a transform margin

    NASA Astrophysics Data System (ADS)

    Parsiegla, N.; Stankiewicz, J.; Gohl, K.; Ryberg, T.; Uenzelmann-Neben, G.

    2009-03-01

    Dynamic processes at sheared margins associated with the formation of sedimentary basins and marginal ridges are poorly understood. The southern African margin provides an excellent opportunity to investigate the deep crustal structure of a transform margin and to characterize processes acting at these margins by studying the Agulhas-Falkland Fracture Zone, the Outeniqua Basin, and the Diaz Marginal Ridge. To do this, we present the results of the combined seismic land-sea experiments of the Agulhas-Karoo Geoscience Transect. Detailed velocity-depth models show crustal thicknesses varying from ˜42 km beneath the Cape Fold Belt to ˜28 km beneath the shelf. The Agulhas-Falkland Fracture Zone is embedded in a 50 km wide transitional zone between continental and oceanic crust. The oceanic crust farther south exhibits relatively low average crustal velocities (˜6.0 km/s), which can possibly be attributed to transform-ridge intersection processes and the thermal effects of the adjacent continental crust during its formation. Crustal stretching factors derived from the velocity-depth models imply that extension in the Outeniqua Basin acted on regional as well as more local scales. We highlight evidence for two episodes of crustal stretching. The first, with a stretching factor β of 1.6, is interpreted to have influenced the entire Outeniqua Basin. The stresses possibly originated from the beginning breakup between Africa and Antarctica (˜169-155 Ma). The second episode can be associated with a transtensional component of the shear motion along the Agulhas-Falkland Transform from ˜136 Ma. This episode caused additional crustal stretching with β = 1.3 and is established to only have affected the southern parts of the basin. Crustal velocities directly beneath the Outeniqua Basin are consistent with the interpretation of Cape Supergroup rocks underlying most parts of the basin and the Diaz Marginal Ridge. We propose that the formation of this ridge can be either

  4. Patterns of late Quaternary shelf-margin sedimentation, southwest Louisiana

    SciTech Connect

    Suter, J.R.; Berryhill, H.L.

    1986-09-01

    Late Quaternary extension of the continental shelf in the northern Gulf of Mexico has been largely accomplished by deposition at the shelf margin during sea level lowstands. The distribution and geometry of facies suggest that delta progradation during sea level fall and lowstand is a principal process for shelf accretion. Along the shelf margin of southwest Louisiana, sets of deltaic deposits corresponding to the last two lowstands of sea level have been mapped from high-resolution seismic profiles. Individual deltas extend farther than 5000 m/sup 2/ and are more than 160 m thick. Diapirism has had a controlling effect on sedimentation patterns of the shelf-margin deltas throughout their depositional histories. Shelf-margin deltas have also been the loci for the transfer of large volumes of sediment from the shelf to the upper slope by mass transport, with buried submarine troughs formed by retrogressive shelf-edge failure in association with major streams acting as conduits for sediment movement. In southwest Louisiana, mass transport deposits follow depressions formed by salt diapirism rather than creating broad aprons on the slope.

  5. Evolution of Devonian carbonate-shelf margin, Nevada

    USGS Publications Warehouse

    Morrow, J.R.; Sandberg, C.A.

    2008-01-01

    The north-trending, 550-km-long Nevada segment of the Devonian carbonate-shelf margin, which fringed western North America, evidences the complex interaction of paleotectonics, eustasy, biotic changes, and bolide impact-related influences. Margin reconstruction is complicated by mid-Paleozoic to Paleogene compressional tectonics and younger extensional and strike-slip faulting. Reports published during the past three decades identify 12 important events that influenced development of shelf-margin settings; in chronological order, these are: (1) Early Devonian inheritance of Silurian stable shelf inargin, (2) formation of Early to early Middle 'Devonian shelf-margin basins, (3) propradation of later Middle Devonian shelf margin, (4) late Middle Devonian Taghanic ondap and continuing long-term Frasnian transgression, (5) initiation of latest Middle Devonian to early Frasnian proto-Antler orogenic forebulge, (6) mid-Frasnian Alamo Impact, (7) accelerated development of proto-Antler forebulge and backbulge Pilot basin, (8) global late Frasnian sentichatovae sea-level rise, (9) end-Frasnian sea-level fluctuations and ensuing mass extinction, (10) long-term Famennian regression and continept-wide erosion, (11) late Famennian emergence: of Ahtler orogenic highlands, and (12) end-Devonian eustatic sea-level fall. Although of considerable value for understanding facies relationships and geometries, existing standard carbonate platform-margin models developed for passive settings else-where do not adequately describe the diverse depositional and, structural settings along the Nevada Devonian platform margin. Recent structural and geochemical studies suggest that the Early to Middle Devonian-shelf-margin basins may have been fault-bound and controlled by inherited Precambrian structure. Subsequently, the migrating latest Middle to Late Devonian Antler orogenic forebulge exerted a dominant control on shelf-margin position, morphology, and sedimentation. ??Geological Society of

  6. Statistics of Stacked Strata on Experimental Shelf Margins

    NASA Astrophysics Data System (ADS)

    Fernandes, A. M.; Straub, K. M.

    2015-12-01

    Continental margin deposits provide the most complete record on Earth of paleo-landscapes, but these records are complex and difficult to interpret. To a seismic geomorphologist or stratigrapher, mapped surfaces often present a static diachronous record of these landscapes through time. We present data that capture the dynamics of experimental shelf-margin landscapes at high-temporal resolution and define internal hierarchies within stacked channelized and weakly channelized deposits from the shelf to the slope. Motivated by observations from acoustically-imaged continental margins offshore Brunei and in the Gulf of Mexico, we use physical experiments to quantify stratal patterns of sub-aqueous slope channels and lobes that are linked to delta-top channels. The data presented here are from an experiment that was run for 26 hours of experimental run time. Overhead photographs and topographic scans captured flow dynamics and surface aggradation/degradation every ten minutes. Currents rich in sediment built a delta that prograded to the shelf-edge. These currents were designed to plunge at the shoreline and travel as turbidity currents beyond the delta and onto the continental slope. Pseudo-subsidence was imposed by a slight base-level rise that generated accommodation space and promoted the construction of stratigraphy on the delta-top. Compensational stacking is a term that is frequently applied to deposits that tend to fill in topographic lows in channelized and weakly channelized systems. The compensation index, a metric used to quantify the strength of compensation, is used here to characterize deposits at different temporal scales on the experimental landscape. The compensation timescale is the characteristic time at which the accumulated deposits begins to match the shape of basin-wide subsidence rates (uniform for these experiments). We will use the compensation indices along strike transects across the delta, proximal slope and distal slope to evaluate the

  7. Shelf-margin sedimentation in Wilcox group, south-central Louisiana

    SciTech Connect

    Lowry, P.; Lemoine, R.C.; Moslow, T.F.

    1986-09-01

    Two well-defined trends of Wilcox oil and gas production occur in Louisiana: an updip dip-oriented trend and a downdip strike-oriented trend. The downdip trend delineates an uppermost Wilcox shelf margin and includes recent discoveries at Lockhart Crossing field and the long-established Fordoche field. This Wilcox shelf margin is adjacent to carbonate reef trends that formed a stable shelf margin throughout central Louisiana during the early Cretaceous. Shelf-margin locations remained essentially unchanged at least through the late Paleocene, by which time Wilcox clastics began to prograde out to and beyond the established margin. The depositional features of this progradational episode reflect the unstable nature of the shelf margin and a high wave-energy environment. Shelf-margin processes were responsible for the formation of a regional syndepositional normal fault system, the incision and subsequent infilling of a major submarine canyon system, and the development of stacked paralic sequences. The submarine canyon system is manifest as a mudstone-filled channel in St. Landry Parish where it reaches a maximum thickness of 1000 ft and is up to 12 mi wide. Lower to middle shoreface facies of the stacked paralic sequences constitute the primary reservoir sandstones for both Lockhart Crossing and Fordoche fields. The major producing intervals of both fields belong to two discrete sandstone bodies that are at least 30-40 mi long, 6-7 mi wide, and 30-40 ft thick, and are analogous to late Quaternary Gulf Coast shelf-margin deltas. The co-occurrence of these laterally continuous thick sandstones, favorable structural trapping mechanisms, and previous performance suggest a high potential for future exploration along this and other Gulf Coast Tertiary shelf-margin trends.

  8. Carbonate shelf edge off southern Australia: A prograding open-platform margin

    NASA Astrophysics Data System (ADS)

    James, Noel P.; von der Borch, Christopher C.

    1991-10-01

    The southern continental margin of Australia is an extensive shelf that has been a site of cool-water carbonate deposition since Eocene time. The platform has no rim and is swept by high-energy waves and swells throughout the year. The shelf is deep (40 to 100 m) and typified by bryozoan-rich sediments. The shelf margin is a gentle incline that becomes progressively steeper seaward, except where it laps down onto offshore terraces. The edge of the Eucla Platform in the Great Australian Bight is used to illustrate that the margin is a series of extensive prograding clinoforms. Progradation is interpreted to be the result of off-shelf sediment transport and in-place carbonate production by actively growing deep-water bryozoa and sponges. This area is a potential model for ancient high-energy platform margins during geologic periods when large skeletal reef-building metazoans were scarce.

  9. Geometry, lateral variability, and preservation of downlapped regressive shelf deposits, eastern Tyrrhenian Margin, Italy

    SciTech Connect

    Field, M.E. ); Trincardi, F. )

    1990-05-01

    The shelf of the eastern Tyrrhenian margin changes substantially in width, shelf-break depth, and sea-floor steepness over relatively short distances, largely due to marked lateral changes in geologic structure. Remnants of late Pleistocene prograded coastal deposits are locally preserved on the middle and outer parts of this complex shelf. Through the authors studies of these prograded deposits they recognize two major controls on the distribution, lateral extent, thickness, and preservation potential. First, prograded (downlapped) deposits formed only where the physiographic shelf break was deeper than the lowstand shoreline, thus providing accommodation space for the lowstand deposits. Second, the proximity and relative size of sediment sources and the local coastal dispersal system influenced the geometry of the deposit. Mid-shelf and shelf-margin bodies composed of seaward-steepening downlapping reflectors were deposited as thin-to-thick continuous prograding sheets over an irregular eroded shelf surface and onto the shelf edge during the last fall and lowstand of sea level. A dearth of sediment at the end of lowstand conditions led to a switch from deposition to erosion. During sea level rise, shoreface erosion produced a major marine erosional (ravinement) surface landward of the 120-m isobath, and much, and in many places all, of the downlapping deposit was removed. Preservation of downlapping deposits is largely a function of their thickness. Thick continuous deposits are common on the shelf edge, whereas on the mid-shelf only thin remnants are preserved locally where depressions or morphologic steps were present in the shelf surface.

  10. Investigating the marginal ice zone on the Newfoundland Shelf

    NASA Astrophysics Data System (ADS)

    Smith, Peter C.; Tang, C. L.; MacPherson, J. Ian; McKenna, Richard F.

    From the ice and current data collected over the Newfoundland Shelf by the second Canadian Atlantic Storms Program (CASP II), it is evident that ice motion is affected by wind-generated ocean current. This points to the importance of coupled ice-ocean response to wind forcing in the study of shortterm ice motion and operational ice forecasting. The mutual influence of ice and the ocean can also be seen in the water properties.To study the mature stages of explosive cyclogenesis in east coast winter storms and to investigate their influence on the circulation and sea ice properties on the Newfoundland continental shelf and Grand Banks, CASP II was conducted by scientists from the Bedford Institute of Oceanography (BIO), the Atmospheric Environment Service (AES), the National Research Council (NRC), and many universities, private companies, and other government agencies.

  11. Active processes on a mixed clastic carbonate Brazilian shelf margin: Importance for hydrocarbon exploration in turbidites

    SciTech Connect

    Cainelli, C. )

    1991-03-01

    The search for subtle hydrocarbon accumulations in turbidite systems requires additional approaches for more successful exploration, particularly when direct recognition on seismic lines is difficult. This includes the determination and understanding of processes controlling sand distribution on the shelf and the mapping of sand pathways from the shelf to the slop/basin that can guide efforts to look for more favorable sites for turbidite sandstone deposition. The approach can be exemplified in the Sergipe-Alagoas basin, on the Brazillian Atlantic passive margin. The section analyzed is the Piacabucu Formation, a thick seaward prograding wedge composed of coastal sandstones and shelf edge carbonates on a narrow shelf and slope-basin shales with turbidite lenses. Waves and currents control the redistribution of sediments transported to the shelf by rivers. More wave energy is expended in ten hours in the San Francisco delta than in an entire year in the Mississippi delta. Such environment precludes deposition of mud on the shelf, but it stimulates the development of shelf edge carbonates. Rimed carbonates along the shelf break serve as a barrier for downslope movements of coarse-grained sediment, where turbidites are oil targets. The search for gaps in the carbonate barrier which can tap the behind-barrier sands is critical for sand-rich turbidite development. It is believed that canyons create these gaps and act as active turbidity current routes.

  12. Sequence stratigraphy and continental margin development of the northwestern shelf of the South China Sea

    SciTech Connect

    Chen, P.P.H. ); Zhi Yuong Chen; Qi Min Zhang )

    1993-05-01

    Based on the sequence stratigraphic analyses of the Tertiary in the northwestern continental margin of the South China Sea, this paper summarizes the regional tectonics and their influences on the basin developments and discusses the systems tract distributions through the continental margin growth. The study area is underlain by two Tertiary basins separated by a major basin boundary fault across which two distinctly different basin structural styles developed. The Qiongdongnan basin shows characteristic rift structures and is closely related to the opening of the South China Sea. The Yinggehai basin is filled with thick Neogene monoclines overlying basement without apparent rift structures. The Yinggehai's genesis has been linked to the tectonism along the Red River suture zone. The margin development is characterized by depositional environments of sediment influx constantly exceeding accommodation space. Under these environments, the highstand deltas frequently developed near shelf edges, where vertically stacked shallow water sands of both highstand deltas and lowstand wedges/deltas could produce favorable exploration targets. High rates of sedimentation in the study area allow the identification of fourth-order cyclicity in the Quaternary. Regional sequence stratigraphic correlations suggest that many sequence boundaries appear to be synchronous, but not all boundaries exist in all basins along the eastern and southern continental margins of mainland China. The margin evolved through the repeated development of lowstand fans, lowstand wedges, transgressive systems tracts, and highstand deltas. Prior to the Pliocene, the shelf was sourced both from Vietnam and Hainan Island and, as a result, two shelf margins developed. The southwestern margin stopped growing in the late Miocene, and the northern margin continued to prograde with possible additional sediment inputs through the Red River system in the Quaternary. 29 refs., 22 figs.

  13. Across-shelf sediment transport since the Last Glacial Maximum, southern California margin

    USGS Publications Warehouse

    Sommerfield, C.K.; Lee, H.J.

    2004-01-01

    Correlation of continental shelf-slope stratigraphy in Santa Monica Bay (southern California) with Ocean Drilling Program records for nearby slope-basin sites has illuminated the timing and scale of terrigenous sediment dispersal on margin since the Last Glacial Maximum (LGM). Marine flooding surfaces preserved in a transgressive sequence on the Santa Monica Shelf provide a key link between base-level elevation and sediment transport across shelf. Sediment-accumulation rates at slope-basin sites were maximal ca. 15-10 ka, well after the LGM, decreased during the 12-9 ka transition from fluvial-estuarine to fully marine conditions on the shelf, and decelerated throughout the Holocene to 30%-75% of their values at the LGM. The deceleration is interpreted to manifest a landward shift in the margin depocenter with the onset of transgressive sedimentation beginning when sea level surmounted the shelf edge ca. 13 ka, as predicted by sequence-stratigraphic models. However, the records make clear that factors other than base level modulated slope-basin accumulation rates during the deglaciation. ?? 2004 Geological Society of America.

  14. Lower and middle Guadalupian shelf carbonates, eastern margin of Central Basin platform, Permian basin, west Texas

    SciTech Connect

    Ward, R.F.; Chalcraft, R.G.

    1988-01-01

    Lower and middle Guadalupian shelf carbonates serve as the reservoir for a nearly continuous band of oil fields extending 100 mi along the eastern margin of the Central Basin platform of west Texas. Approximately 5 billion bbl of oil have been produced from stratigraphic-structural traps within the Upper Permian (Gaudalupian Series) dolomites of the San Andrea and Grayburg Formations in Upton, Crane, Ector, Pecos, and Andrews Counties, Texas. The San Andrea and Grayburg Formations are cyclical shallowing-upward carbonate sequences of open shelf through sabkha facies whose depositional strike parallels the eastern margin of the Central Basin platform. Porosity and permeability of reservoir rock are governed by diagenetic processes such as dolomitization, anhydrite porosity occlusion, leaching, silicification, and authigenic clay formation. Self sediments are primarily burrowed wackestones and packstones that locally contain pelletal, skeletal, and ooid grainstones. Typical subtidal shelf sediments are capped by algal-laminated dolomite, nodular anhydritic dolomite, and bedded anhydrite. The fauna is normally sparse and dominated by foraminifera and algae. Less common faunal components include pelecypods, crinoids, sponges, Bryozoa, brachiopods, gastropods, and coral that are associated with the development of small scattered patch reefs. Lowering the sea level during the early Guadalpian initiated basinward progradation of San Andres carbonate facies with hydrocarbon reservoirs best developed in shallow self fusulinid wackestones to packstone and oolitic grainstone. Reservoir dolomites of the Grayburg formation are present east of San Andres fields with optimal reservoir properties occurring near the San Andreas outer shelf margin.

  15. Sequence stratigraphy and composition of late quaternary shelf-margin deltas, Northern Gulf of Mexico

    SciTech Connect

    Morton, R.A.; Suter, J.R.

    1996-04-01

    High-resolution seismic profiles and foundation borings from the northwestern Gulf of Mexico record the physical attributes and depositional histories of several late Quaternary sequences that were deposited by wave-modified, river-dominated shelf-margin deltas during successive periods of lowered sea level. Each progressively younger deltaic sequence is thinner and exhibits a systematic decrease in the abundance and concentration of sand, which is attributed to a shift in the axes of trunk streams and greater structural influence through time. Our study shows that (1) contemporaneous structural deformation controlled the thickness of each sequence, the oblique directions of delta progradation, the axes of major fluvial channels, and the geometries of delta lobes at the shelf margin; (2) sedimentation was rapid in response to rapid eustatic fluctuations and structural influence; (3) boundaries of these high-frequency sequences are the correlative conformities of updip fluvial incision and coincide with downlap surfaces at the shelf margin; (4) the downlap surfaces are not true surfaces, but zones of parallel reflections that become progressively higher and younger in the direction of progradation; (5) the downlap zones are composed of marine muds that do not contain the high concentrations of shell debris expected in condensed sections; (6) possible paleosols capping the two oldest sequences are regressive surfaces of subaerial exposure that were preserved during transgressions; and (7) no incised valleys or submarine canyons breach the paleoshelf margin, even though incised drainages were present updip and sea level curves indicate several periods of rapid fall.

  16. Multiple uplift phases inferred from the Southwest African Atlantic margin

    NASA Astrophysics Data System (ADS)

    Scheck-Wenderoth, Magdalena; Cacace, Mauro; Dressel, Ingo

    2015-04-01

    The South Atlantic basins offshore Namibia and South Africa stored more than 10 km thick sedimentary successions that are separated by major unconformities into several sequences. These sedimentary units rest on a thinned continental crust of a magmatic passive margin. Using a 3D forward modelling approach considering flexural compensation of a rheologically differentiated lithosphere in response to sedimentary loading after stretching on one hand and the thermal feed-back between cooling of the stretched lithosphere and insulating sediments on the other hand we derive quantitative estimates on how vertical movements have influenced the margin after stretching. The approach combines the consideration of observations on sediment configuration as well as on crustal thickness (ß-factor) with the process of lithosphere thinning and subsequent thermal re-equilibration. These estimates are conservative estimates as they are based on the preserved sediments only whereas eroded sediments are not considered. Nevertheless, the approach considers thermo-mechanical coupling in 3D and both initial conditions as well as sedimentary history are constrained by observations. Specific effects include the delayed thermal re-equilibration of the thinned lithosphere due to deposition of insulating sediments and the related thermal feedback on lithosphere rheology and therefore on the flexural response to sediment loading. Our results indicate that in addition to predominantly continuous subsidence also phases of uplift have affected the southwestern African margin during the syn-rift and post-rift evolution. The spatio-temporal variation of vertical movements is controlled by the amount of initial thinning of the lithosphere, the variation of rheological characteristics (lithology and temperature) but also by the distribution of sediment supply (loading and thermal insulation).

  17. Cenozoic drainage evolution of the West African transform marginal upwarp

    NASA Astrophysics Data System (ADS)

    Grimaud, Jean-Louis; Chardon, Dominique; Beauvais, Anicet; Rouby, Delphine

    2013-04-01

    We explore the large-scale relief and drainage evolution of the West African marginal upwarp by a spatial analysis of lateritic relict landscapes recording successive incision stages of a low relief, Early Cenozoic bauxitic envelope topography called the African surface. Four generations of stepped ironduricrust-capped paleolandsurfaces have been formed and abandoned on the slopes of interfluves below bauxitic relicts. Incision chronology is constrained by stratigraphic dating of the bauxites and Ar-Ar geochronology of Mn oxy-hydroxides produced in the weathering mantle of each paleolandsurface from the type locality of Tambao, in Northern Burkina Faso [1]. The Bauxites of the African surface result from intense rock chemical weathering that ended in the Middle Eocene (ca. 45 Ma). The so-called Intermediate paleolandsurface developed until the Oligocene-Miocene transition (ca. 24 Ma). Three generations of pediment (glacis from the French literature) emplaced afterwards. The so-called High glacis was shaped and weathered until ca. 11 Ma. The Middle Glacis settled by the end of the Pliocene (ca. 7-6 Ma) and the Low Glacis, which is mostly connected to the modern base level, dates from the end of the Pliocene. The regional study reasonably assumes the broad synchronicity of the lateritic levels at the scale of West Africa. We have produced elevation maps of the first three erosion levels corresponding to the topography of the marginal upwarp at ca. 45, 24 and 11 Ma. They show the successive positions of the main drainage divides and thus drainage reorganisation since the Eocene. The elevation of paleolandsurface relicts along the main drains allowed reconstructing paleo-river long profiles at ca. 45, 24, 11 and 6 Ma to be compared with the modern long profiles. The modern drainage of West Africa was established before the Oligocene-Miocene transition as a consequence of the inland growth of coastal catchments that have cut through the Eocene marginal upwarp. At this

  18. Shelf margin bioherms and associated facies in the Lower Permian Hueco Group (Late Wolfcampian), Hueco Mountains, West Texas

    SciTech Connect

    Wahlman, G.P.; Tasker, D.R.; St. John, J.W.; Werle, K.J. )

    1992-01-01

    Late Wolfcampian phylloid algal/Tubiphytes biohermal complexes are exposed in three erosional oilers lying about 3 miles west of and parallel to the main Hueco Mountains in far West Texas. The biohermal complexes are located paleogeographically along the shelf margin between the Diablo Platform and Orogrande Basin. Based on fusulinids the shelf margin buildups correlate with well-bedded shelf carbonates of the type Hueco Group in the main Hueco Mountains. The phylloid algal/Tubiphytes shelf margin bioherms contain an upward shallowing facies succession, which, in ascending order, consists of: (1) phylloid algal wackestone-bafflestone, (2) phylloid algal-fusulinid bafflestone-packstone, and (3) Tubiphytes boundstone and Tubiphytes-fusulinid-phylloid algal packstone-grainstone. The crest of the southernmost outlier has a different type of bioherm that consists of nodular boundstones composed of calcisponges, encrusting bryozoans and laminar red algae. The shelf margin complexes prograded over slope facies of dark-gray cherty limestones, which generally lack skeletal fossils, but contain common ichnofossils in upper slope beds. Overlapping tongues and channels of lithoclastic-skeletal packstones and grainstones extend seaward from the phylloid algal/Tubiphytes bioherms into the dark-gray slope facies. Proximal backreef facies consist of mainly skeletal-peloidal packstones and wackestones. The Hueco Mountains outlier exposures are important because: (1) they confirm a Late Wolfcampian shelf margin with distinct topographic relief in the southern Orogrande Basin, and (2) they provide an easily accessible field laboratory where Wolfcampian shelf-to-basin facies relationships and shelf margin bioherms can be studied. Wolfcampian bioherms represent a significant stage in the evolutionary history of Late Paleozoic reef communities and form important petroleum reservoirs in the adjacent Permian Basin.

  19. Lower Permian facies of the Palo Duro Basin, Texas: depositional systems, shelf-margin evolution, paleogeography, and petroleum potential

    SciTech Connect

    Handford, C.R.

    1980-01-01

    A Palo geological study suggests that potential hydrocarbon reservoirs occur in shelf-margin carbonates, delta-front sandstones, and fan-delta arkoses. Zones of porous (greater than 10 percent) dolomite are concentrated near shelf margins and have configurations similar to productive Lower Permian shelf-margin trends in New Mexico. Delta-front sandstones (log-computed porosity of 18 to 25 percent) are similar to producing deltaic sandstones of Morris Buie-Blaco Fields in North-Central Texas. Porous (18 percent) fan-delta sandstones along the south flank of the Amarillo Uplift may form reservoirs similiar to that of the Mobeetie Field on the north side of the Amarillo Uplife in Wheeler County, Texas. Potential hydrocarbon source beds occur in slope and basinal environments. Total organic carbon generally ranges from 1 to 2.3 percent by weight and averages 0.589 percent by weight.

  20. Depositional patterns and shelf-margin styles of a lowstand slope fan systems tract, offshore Louisiana

    SciTech Connect

    Bowen, B.E. ); Pacht, J. ); Shaffer, B.L.S.

    1990-05-01

    A complete spectrum of facies from deep-water fans to up-dip deltaics and fluvial sediments occurs in the Pliocene-Pleistocene lowstand fan systems tracts of the Gulf of Mexico. Seismic data, well logs, and biostratigraphy have been used to delineate and map the upper Pliocene slope fan systems tract, commonly referred to as the Lenticulina I interval. This tract contains sands that are major hydrocarbon producers in the southern part of the Louisiana shelf. Both a ramp margin and a growth fault modified ramp margin can be recognized in this unit. Large submarine fans with well-developed channel levee complexes occur basinward of the growth fault margin. The sands in these sediments typically are thin-bedded and highly lenticular. On the upside of the growth faults, the slope fan sediments can be chronostratigraphically correlated to deltaics by using the encasing condensed sections that have been defined by biostratigraphy. The sands in the deltaics are coarsening-upward cycles, occasionally blocky, are generally thicker bedded than the downdip fan deposits. Updip, the semicontinuous to continuous reflectors of the deltaic sediments are replaced by highly discontinuous reflectors with variable amplitude characteristic of upper delta plain or fluvial deposits. A different set of facies is characteristic of the ramp margin. Extensive slope fan sediments are rarely developed and the updip deltaic wedge tends to be thicker and is characterized by slumping along the seaward margin. The deltaics occur in an area that extends about 100 mi along strike and 75 mi in the dip direction.

  1. Seismic analysis of clinoform depositional sequences and shelf-margin trajectories in Lower Cretaceous (Albian) strata, Alaska North Slope

    USGS Publications Warehouse

    Houseknecht, D.W.; Bird, K.J.; Schenk, C.J.

    2009-01-01

    Lower Cretaceous strata beneath the Alaska North Slope include clinoform depositional sequences that filled the western Colville foreland basin and overstepped the Beaufort rift shoulder. Analysis of Albian clinoform sequences with two-dimensional (2D) seismic data resulted in the recognition of seismic facies inferred to represent lowstand, transgressive and highstand systems tracts. These are stacked to produce shelf-margin trajectories that appear in low-resolution seismic data to alternate between aggradational and progradational. Higher-resolution seismic data reveal shelf-margin trajectories that are more complex, particularly in net-aggradational areas, where three patterns commonly are observed: (1) a negative (downward) step across the sequence boundary followed by mostly aggradation in the lowstand systems tract (LST), (2) a positive (upward) step across the sequence boundary followed by mostly progradation in the LST and (3) an upward backstep across a mass-failure d??collement. These different shelf-margin trajectories are interpreted as (1) fall of relative sea level below the shelf edge, (2) fall of relative sea level to above the shelf edge and (3) mass-failure removal of shelf-margin sediment. Lowstand shelf margins mapped using these criteria are oriented north-south in the foreland basin, indicating longitudinal filling from west to east. The shelf margins turn westward in the north, where the clinoform depositional system overstepped the rift shoulder, and turn eastward in the south, suggesting progradation of depositional systems from the ancestral Brooks Range into the foredeep. Lowstand shelf-margin orientations are consistently perpendicular to clinoform-foreset-dip directions. Although the Albian clinoform sequences of the Alaska North Slope are generally similar in stratal geometry to clinoform sequences elsewhere, they are significantly thicker. Clinoform-sequence thickness ranges from 600-1000 m in the north to 1700-2000 m in the south

  2. Norwegian margin outer shelf cracking: a consequence of climate-induced gas hydrate dissociation?

    NASA Astrophysics Data System (ADS)

    Mienert, J.; Vanneste, M.; Haflidason, H.; Bünz, S.

    2010-10-01

    A series of en echelon cracks run nearly parallel to the outer shelf edge of the mid-Norwegian margin. The features can be followed in a ~60-km-long and ~5-km-wide zone in which up to 10-m-deep cracks developed in the seabed at 400-550 m water depth. The time of the seabed cracking has been dated to 7350 14C years BP (8180 cal years BP), which corresponds with the main Storegga Slide event (8100 ± 250 cal. years BP). Reflection seismic data suggest that the cracks do not appear to result from deep-seated faults, but it cannot be ruled out completely that tension crevices were created in relation to past movements on the headwall of the Storegga slide. The cracking zone corresponds well to the zone where the base of the hydrate stability zone (BHSZ) outcrops. Evidence of fluid release in the BHSZ outcrop zone comes from an extensive pockmark field. We suggest that post-glacial ocean warming triggered the dissociation of gas hydrates while the interplay between dissociation, overpressure, and sediment fracturing on the outer shelf remains to be understood.

  3. Seasonal Trends in Sedimentation on the Fly River Margin: Shelf, Channel, and Open Clinoform

    NASA Astrophysics Data System (ADS)

    Crockett, J. S.; Nittrouer, C. A.; Ogston, A. S.

    2004-12-01

    Recent multibeam mapping and sedimentary studies on the Fly River margin, Papua New Guinea, have revealed across-shelf heterogeneity that likely impacts sediment-transport processes on the margin. Several ancient river valleys with up to 50 m of relief have been discovered and their sedimentary history provides insights to the mechanisms of channel infilling and sedimentation on this margin. Umuda channel, which is located adjacent to the Northern Entrance of the Fly River, exhibits the greatest extent of infilling, and will be a focus of this paper. Kasten ( ˜2 m) and box ( ˜50 cm) cores were collected during the energetic Trade-wind season (June to November), the relatively quiescent Monsoon season (December to March), and the Transition from Monsoon to Trade-wind conditions (April/May). 210Pb, x-radiograph, and grain-size analysis reveal the patterns of sedimentation within Umuda channel, and on the open clinoform to the north. The surface 150 cm of sediment in Umuda channel has relatively uniform excess 210Pb activity that abruptly decreases to background levels below ˜150 cm. These data suggest that the surface 150 cm were deposited relatively quickly within the last ˜100 y. Over the course of sampling seasons, a decrease of ˜10 cm was observed in the elevation of the seabed within Umuda channel. During the Trade-wind season, seabed elevation was at a maximum. Surface sediments were removed between the Trade-wind and Monsoon seasons, and again between the Monsoon and Transition seasons. This pattern is similar on the open clinoform to the north. We hypothesize that this represents a seasonal transfer of sediment from shallow to deeper water.

  4. Lithosphere structure and subsidence evolution of the conjugate S-African and Argentine margins

    NASA Astrophysics Data System (ADS)

    Dressel, Ingo; Scheck-Wenderoth, Magdalena; Cacace, Mauro; Götze, Hans-Jürgen; Franke, Dieter

    2016-04-01

    The bathymetric evolution of the South Atlantic passive continental margins is a matter of debate. Though it is commonly accepted that passive margins experience thermal subsidence as a result of lithospheric cooling as well as load induced subsidence in response to sediment deposition it is disputed if the South Atlantic passive margins were affected by additional processes affecting the subsidence history after continental breakup. We present a subsidence analysis along the SW African margin and offshore Argentina and restore paleobathymetries to assess the subsidence evolution of the margin. These results are discussed with respect to mechanisms behind margin evolution. Therefore, we use available information about the lithosphere-scale present-day structural configuration of these margins as a starting point for the subsidence analysis. A multi 1D backward modelling method is applied to separate individual subsidence components such as the thermal- as well as the load induced subsidence and to restore paleobathymetries for the conjugate margins. The comparison of the restored paleobathymetries shows that the conjugate margins evolve differently: Continuous subsidence is obtained offshore Argentina whereas the subsidence history of the SW African margin is interrupted by phases of uplift. This differing results for both margins correlate also with different structural configurations of the subcrustal mantle. In the light of these results we discuss possible implications for uplift mechanisms.

  5. Large Illinoisan shelf-margin channel in the South Timbalier and Ewing Bank areas, offshore Gulf of Mexico

    SciTech Connect

    Ruggiero, R.W.; Bowman, P. )

    1991-03-01

    During an Illinoisan lowstand, a major channel became entrenched on the shelf and deposited deltaic sediment of the shelf-slope break in the South Timbalier and Ewing Bank areas. This shelf-edge delta overloaded the margin, creating a major slope failure. This 'channel,' or slump canyon, which was originally deep and narrow, evolved through retrogradational wasting of the shelf into a broader feature during back-filling and abandonment. Mapping with integrated seismic and well data indicates a feature that is 6 mi wide and has 2300 ft of relief, from the base of the channel to the highest margin. Gravel lag fills the deepest portion of the scour and constitutes an excellent, though restricted, gas reservoir. The limits of the reservoir are defined by mapping of a seismic amplitude anomaly. The balance of the back-fill is comprised of thinly bedded silstone and sandstone sequences that generally thin and fine upward; each is capped by marine shales. The fill geometry is primarily nested or stacked channels. Positive topographic features on the outer shelf tower above the channel floor, either as uneroded remnants or as part of channel margin slump mounds. Sand-prone sediment onlapped these highs, thereby creating numerous stratigraphic traps. As sea level rose, the locus of deposition within the channel shifted progressively shoreward, and the fill was capped by a clean, transgressive shale. Further work is necessary to ascertain the location of the submarine fan far down-dip of this shelf-edge channel. Laterally extensive reservoirs can be expected in more distal depositional environments.

  6. Shelf basin exchange along the Siberian continental margin: Modification of Atlantic Water and Lower Halocline Water

    NASA Astrophysics Data System (ADS)

    Bauch, Dorothea; Cherniavskaia, Ekaterina; Timokhov, Leonid

    2016-09-01

    Salinity and stable oxygen isotope (δ18O) evidence shows a modification of Atlantic Water in the Arctic Ocean by a mixture of sea-ice meltwater and meteoric waters along the Barents Sea continental margin. On average no further influence of meteoric waters is detectable within the core of the Atlantic Water east of the Kara Sea as indicated by constant δ18O, while salinity further decreases along the Siberian continental slope. Lower Halocline Waters (LHW) may be divided into different types by Principal Component Analysis. All LHW types show the addition of river water and an influence of sea-ice formation to a varying extent. The geographical distribution of LHW types suggest that the high salinity type of LHW forms in the Barents and Kara seas, while other LHW types are formed either in the northwestern Laptev Sea or from southeastern Kara Sea waters that enter the northwestern Laptev Sea through Vilkitsky Strait. No further modification of LHW is seen in the eastern Laptev Sea but the distribution of LHW-types suggest a bifurcation of LHW at this location, possibly with one branch continuing along the continental margin and a second branch along the Lomonosov Ridge. We see no pronounced distinction between onshore and offshore LHW types, as the LHW components that are found within the halocline over the basin also show a narrow bottom-bound distribution at the continental slope that is consistent with a shelf boundary current as well as a jet of water entering the western Laptev Sea from the Kara Sea through Vilkitsky Strait.

  7. Sea level and geostrophic current control on carbonate shelf-slope depositional sequences and erosional patterns, south Florida platform margin

    SciTech Connect

    Locker, S.D.; Hine, A.C. ); Shinn, E.A. )

    1991-03-01

    High-resolution seismic reflection profiles across the shelf-slope margin between the Dry Tortugas and Key West, Florida, indicate that sea-level fluctuations and the eastward flowing Florida Current are major controls on late Quaternary sequence stratigraphy. The study area, a transition zone between the open south Florida shelf and the lower Florida Keys island/reef system, is typified by a shallow shelf with reef margin adjacent to a deeper lower-shelf/slope. The lower-shelf/slope is composed of stacked or prograding sequences that downlap and pinchout on the Pourtales Terrace. Strike oriented stratigraphic sections exhibit many sea-level controlled features such as lowstand erosion, transgressive unconformities, and highstand system tracts. Lowstand reefs, notches, or barriers are observed as deep as 150 m below present sea level. Depositional styles change along-slope from west to east. The western portion of the study area is characterized by thick, low-amplitude, prograding sequences related to abundant supply of sediment through off-shelf transport during high sea-levels as well as along-slope reworking by Florida current. Part of this section has been severely eroded by along-slope currents producing localized cut-and-fill structures and widespread erosional unconformities. To the east, a thinner section of high-amplitude reflections is common seaward of the lower Florida Keys reef tract system. This study provides a new evidence of how strong geostrophic boundary currents along with fluctuating sea levels have interacted to control depositional sequences on a carbonate slope in the Florida/Bahamas platform complex.

  8. Cross-shelf Water Exchange Between the Kuroshio and the Northern Marginal Seas of China

    NASA Astrophysics Data System (ADS)

    Yuan, D.

    2012-04-01

    Satellite ocean color and sea surface temperature data and satellite-tracked surface drifter trajectories are used to study the shelf circulation in the Yellow and East China Seas. The ocean color images show a significant turbid water plume extending in the southeast direction from the Subei coasts of China to the shelf edge southwest of Cheju during fall-winter, suggesting significant cross-shelf current in the Yellow Sea/East China Sea in winter. The turbid plume grows in fall, reaches its maximum expansion and intensity in winter-spring, and subsides in late spring. Cold sea surface temperature is associated with the cross-shelf currents, suggesting its nearshore origin in winter. The cross-shelf current is also expressed explicitly by the latest surface drifter trajectories released in the Yellow and East China Sea and tracked by the satellite. The dynamics of the cross-shelf current are found to be induced by the divergence of the wind-driven currents over the middle shelf in winter. A southward sea level gradient is set up by the northerly winds through the propagation of topographic Rossby waves, which forces a broad cross-shelf current in the Yellow and East China Seas in winter. Satellite images also suggest that the East China Sea shelf is frequented with the Cross-shelf Penetrating Fronts (CPF), which are direct and large-distance excursions of coastal mean fronts across the shelf for distances usually exceeding 50 km. The CPFs bring nearshore waters to the middle-to-outer shelf of the East China Sea, representing an important path of material transport across the shelf.

  9. Quantifying deposition from the very muddy Waipaoa River on the Poverty shelf and margin re-entrant, New Zealand

    NASA Astrophysics Data System (ADS)

    Orpin, A.; Carter, L.; Lewis, K.; Kuehl, S.; Alexander, C.

    2003-04-01

    The East Coast margin is characterised by high terrigenous sediment flux, dramatic effects of land-use changes, and by complex sediment-tectonic interactions on a steep and unstable continental slope. An estimated 18 cubic km of sediment has been deposited in an actively subsiding mid-shelf basin and outer shelf apron since 18 ka BP, with a maximum thickness of approximately 45 m. The shelf is boarded along its seaward edge by two growing anticlines, but a significant component of the sediment leaks through a 13 km-wide gap between the anticlines and cascades into a large structural re-entrant that is heavily incised by the Poverty submarine canyon system. 210Pb mass accumulation profiles indicate that the modern post-settlement sedimentation rate of around 0.9 cm/yr on the outer shelf is double that recorded at the mid-shelf. Hence, the modern sediment accumulation is inconsistent with the post-glacial sediment thicknesses, which show the largest volume has accumulated on the mid-shelf. This may suggest an increasing frequency of Waipaoa-derived hyperpycnal flows with the ability to transport sediment seawards, or a change in the storage pattern within Poverty Bay. Accumulation rates on the slope are an order of magnitude less, around 0.1 cm/yr. Cores and multibeam images suggest that Poverty Canyon is inactive as a modern sediment pathway. The mouth and floor of the canyon are composed of stiff Pleistocene mud, overlain by a thin drape of unconsolidated mud. The seabed at the canyon mouth is highly reflective, deeply scoured, and lacks a fan. These features indicate little or no sediment flux. Accepting near-full capture of riverine-derived sediment on the shelf and slope, accumulation rates indicate that the modern (post-settlement) sediment yield from the Waipaoa River is almost an order of magnitude higher than the average for the Holocene. This is broadly compatible with accelerated rates of landscape erosion measured onshore. A revised sediment budget

  10. Geomorphic characterization of four shelf-sourced submarine canyons along the U.S. Mid-Atlantic continental margin

    USGS Publications Warehouse

    Obelcz, Jeffrey; Brothers, Daniel S.; Chaytor, Jason D.; ten Brink, Uri S.; Ross, Steve W.; Brooke, Sandra

    2013-01-01

    Shelf-sourced submarine canyons are common features of continental margins and are fundamental to deep-sea sedimentary systems. Despite their geomorphic and geologic significance, relatively few passive margin shelf-breaching canyons worldwide have been mapped using modern geophysical methods. Between 2007 and 2012 a series of geophysical surveys was conducted across four major canyons of the US Mid-Atlantic margin: Wilmington, Baltimore, Washington, and Norfolk canyons. More than 5700 km2 of high-resolution multibeam bathymetry and 890 line-km of sub-bottom CHIRP profiles were collected along the outer shelf and uppermost slope (depths of 80-1200 m). The data allowed us to compare and contrast the fine-scale morphology of each canyon system. The canyons have marked differences in the morphology and orientation of canyon heads, steepness and density of sidewall gullies, and the character of the continental shelf surrounding canyon rims. Down-canyon axial profiles for Washington, Baltimore and Wilmington canyons have linear shapes, and each canyon thalweg exhibits morphological evidence for recent, relatively small-scale sediment transport. For example, Washington Canyon displays extremely steep wall gradients and contains ~100 m wide, 5–10 m deep, v-shaped incisions down the canyon axis, suggesting modern or recent sediment transport. In contrast, the convex axial thalweg profile, the absence of thalweg incision, and evidence for sediment infilling at the canyon head, suggest that depositional processes strongly influence Norfolk Canyon during the current sea-level high-stand. The north walls of Wilmington, Washington and Norfolk canyons are steeper than the south walls due to differential erosion, though the underlying cause for this asymmetry is not clear. Furthermore, we speculate that most of the geomorphic features observed within the canyons (e.g., terraces, tributary canyons, gullies, and hanging valleys) were formed during the Pleistocene, and show only

  11. Cenozoic Source-to-Sink of the African margin of the Equatorial Atlantic

    NASA Astrophysics Data System (ADS)

    Rouby, Delphine; Chardon, Dominique; Huyghe, Damien; Guillocheau, François; Robin, Cecile; Loparev, Artiom; Ye, Jing; Dall'Asta, Massimo; Grimaud, Jean-Louis

    2016-04-01

    The objective of the Transform Source to Sink Project (TS2P) is to link the dynamics of the erosion of the West African Craton to the offshore sedimentary basins of the African margin of the Equatorial Atlantic at geological time scales. This margin, alternating transform and oblique segments from Guinea to Nigeria, shows a strong structural variability in the margin width, continental geology and relief, drainage networks and subsidence/accumulation patterns. We analyzed this system combining onshore geology and geomorphology as well as offshore sub-surface data. Mapping and regional correlation of dated lateritic paleo-landscape remnants allows us to reconstruct two physiographic configurations of West Africa during the Cenozoic. We corrected those reconstitutions from flexural isostasy related to the subsequent erosion. These geometries show that the present-day drainage organization stabilized by at least 29 Myrs ago (probably by 34 Myr) revealing the antiquity of the Senegambia, Niger and Volta catchments toward the Atlantic as well as of the marginal upwarp currently forming a continental divide. The drainage rearrangement that lead to this drainage organization was primarily enhanced by the topographic growth of the Hoggar swell and caused a major stratigraphic turnover along the Equatorial margin of West Africa. Elevation differences between paleo-landscape remnants give access to the spatial and temporal distribution of denudation for 3 time-increments since 45 Myrs. From this, we estimate the volumes of sediments and associated lithologies exported by the West African Craton toward different segments of the margin, taking into account the type of eroded bedrock and the successive drainage reorganizations. We compare these data to Cenozoic accumulation histories in the basins and discuss their stratigraphic expression according to the type of margin segment they are preserved in.

  12. Mesozoic Source-to-Sink of the African margin of the Equatorial Atlantic

    NASA Astrophysics Data System (ADS)

    ye, jing; Chardon, Dominique; rouby, delphine; Guillocheau, François; Robin, Cecile; Loparev, Artiom; Huyghe, damien; Dall'Asta, Massimo; Brown, Roderick; wildman, mark; webster, david

    2016-04-01

    The objective of the Transform Source to Sink Project (TS2P) is to link the dynamics of the erosion of the West African Craton to the offshore sedimentary basins of the African margin of the Equatorial Atlantic at geological time scales. This margin, alternating transform and oblique segments from Guinea to Nigeria, shows a strong structural variability in the margin width, continental geology and relief, drainage networks and subsidence/accumulation patterns. We analyzed this system combining onshore geology and geomorphology as well as offshore sub-surface data. We produced paleogeographic maps at the scale of West Africa spanning the continental domain and offshore basins since 200 Ma. Mapping spatial and temporal distribution of domains either in erosion (sources) or in accumulation (sinks) document the impact of the successive rifting of Central and Equatorial Atlantic on the physiography of the area. We use low temperature thermochronology dating along three transects perpendicular to the margin (Guinea, Ivory Coast and Benin) to determine periods and domains of denudation in that framework. We compare these data to the Mesozoic accumulation histories in passive margin basins and discuss their stratigraphic expression according to the type of margin segment they are preserved in. Syn-rift architectures (Early Cretaceous) are largely impacted by transform faults that define sub-basins with contrasted width of crustal necking zone (narrower in transform segments than in oblique/normal segments). During the Late Cretaceous post-rift, sedimentary wedges record a transgression along the all margin. Proximal parts of the sedimentary wedge are preserved in basins developing on segments with wide crustal necking zone while they were eroded away in basins developing on narrow segments. As a difference, the Cenozoic wedge is everywhere preserved across the whole width of the margin.

  13. Shelf-to-basin resedimented carbonates of the southern margin of the Jurassic central high Atlas trough, Morocco

    SciTech Connect

    Hazlett, B.H.; Warme, J.E.

    1988-08-01

    The Central High Atlas Mountains occupy the site of an Early to Middle Jurassic east-west-trending seaway known as the Central High Atlas trough. Late Triassic-Early Jurassic continental rifting, combined with a transtensional structural regime, formed a system of pull-apart basins comprising the trough. A thick sequence of carbonate shelf-to-basin-plain deposits filled the trough and were later uplifted and exposed during the Oligocene Alpine orogeny. Stratigraphic analysis of 50 km/sup 2/ located along the trough's southern shelf margin reveals a 1,200 m-thick wedge of slope and basin-plain deposits. These deposits are divided into four lithostratigraphic units: (1) pelagic mudstones, (2) channel deposits composed of grainstones and packstones interbedded with marls and mudstones, (3) turbidite deposits composed of grainstones, packstones, and wackestones cyclically interbedded with marls and mudstones, and (4) cyclically interbedded marls and mudstones. This laterally continuous thick wedge of resedimented deposits suggests that a line source of platform-margin sediments fed coalescing base-of-slope aprons. These aprons probably accumulated in an actively subsiding half-graben parallel to the shelf margin. While tectonics played the major role in basin evolution, sea level fluctuations and climate influenced the influx of carbonate and terrigenous sediments. Rapid cessation of mid-Sinemurian shallow-water platform deposition and regionally transgressive Domerian-Toarcian marls indicate eustatic sea level rises affected basin sedimentation. Periodic climate changes, perhaps related to the Milankovitch effect, may have led to systematic variations in carbonate sediment supply, resulting in cyclic sedimentation.

  14. iSIMM (Integrated Seismic Imaging and Modelling of Margins): Seismic Acquisition on the Faroes Shelf, Hatton Bank and adjacent Continental Margins

    NASA Astrophysics Data System (ADS)

    White, B.; Kusznir, N.; Christie, P.; Roberts, A. M.; Lunnon, Z.; Roberts, A. W.; Hurst, N.; Smith, L.; Parkin, C.; Surendra, A.; Davies, A.

    2002-12-01

    The iSIMM project is using state-of-the art seismic techniques with long-offset and wide-angle data, to image the crust formed on volcanic continental margins in parallel with developing and testing new quantitative models of rifted margin formation, incorporating heterogeneous stretching, the effects of melt generation and emplacement and varying thermal anomalies in the mantle. During June-July 2002, we used RRS Discovery to acquire wide angle and normal incidence seismic data on the Faroes Shelf and adjacent continental margin, Hatton-Rockall Basin, Hatton Bank and the adjacent oceanic crust using OBS and MCS. In August 2002, WesternGeco's Topaz used three single-sensor, Q-Marine streamers, 12km plus two 4km, to overshoot the wide-angle profiles on the Faroes Shelf and adjacent continental margin. In the Faroes region we deployed 85 4-component ocean bottom seismometers (OBS) and 5 vertical arrays along a 350km-long profile extending from the Faroes-Shetland Channel across the Faroes Shelf and continental margin into the oceanic crust of the Norwegian Sea. The entire profile was shot twice. First with a 6,300 cubic inch airgun array towed at 20 m depth and tuned to enhance the initial peak output pressure pulse. Second, with the airguns reconfigured to enhance the low-frequency bubble waveform, producing a source rich in low frequency energy (centred on 10-12 Hz), and with a broad bandwidth. Shots were spaced at either 75 m or 100 m, giving shot repetition rates in excess of 60 secs, thus avoiding contaminating with wrap-round energy from the previous shot. The Q-Marine acquisition used a 48-gun, 10,170 cu. in. airgun array, also tuned to enhance the low-frequency bubble signature, shooting at 50m/20s intervals and recorded on individual sensors for optimal grouping. The streamer configuration provides swath coverage at shorter offsets, while the long offsets record diving waves and wide-angle reflections. Shot-by-shot source signature recording will facilitate

  15. Meso-Cenozoic Source-to-Sink analysis of the African margin of the Equatorial Atlantic

    NASA Astrophysics Data System (ADS)

    Chardon, Dominique; Rouby, Delphine; Huyghe, Damien; Ye, Jing; Guillocheau, François; Robin, Cécile; Dall'Asta, Massimo; Brown, Roderick; Webster, David

    2015-04-01

    The Transform Source to Sink Project (TS2P) objective is to link the evolution of the offshore sedimentary basins of the African margin of the Equatorial Atlantic and their source areas on the West African Craton. The margin consists in alternating transform and oblique margin portions from Guinea, in the West, to Nigeria, in the East. Such a longitudinal structural variability is associated with variation in the margin width, continental geology and relief, drainage networks and subsidence/accumulation patterns that we analyzed using offshore seismic data and onshore geology and geomorphology. We compare syn- to post rift offshore geometry and long-term stratigraphic history of each of the margin segments. Transform faults appear to play a major role in shaping Early Cretaceous syn-rift basin architectures. Immediate post-rift Late Cretaceous sedimentary wedges record a transgression and are affected by the reactivation of some of transform faults. We produced A new type of inland paleogeographic maps for key periods since the end of the Triassic, allowing delineation of intracratonic basins having accumulated material issued from erosion of the marginal upwarps that have grown since break-up along the margin. We use offshore and onshore basin analysis to estimate sediment accumulation and integrate it in a source-to-sink analysis where Mesozoic onshore denudation will be estimated by low-temperature thermochronology. Cenozoic erosion and drainage history of the continental domain have been reconstructed from the spatial analysis of dated and regionally correlated geomorphic markers. The stationary drainage configuration of the onshore domain since 30 Ma offers the opportunity to correlate the detailed onshore morphoclimatic record based on the sequence of lateritic paleolandsurfaces to offshore stratigraphy, eustasy and global climatic proxies since the Oligocene. Within this framework, we simulate quantitative solute / solid erosional fluxes based on the

  16. Collapse and flow of lowstand shelf-margin deposits: An example from the eastern Tyrrhenian Sea, Italy

    USGS Publications Warehouse

    Trincardi, F.; Field, M.E.

    1992-01-01

    The upper slope of the eastern Tyrrhenian Sea margin has a complex morphology shaped by Quaternary tectonism and by sedimentation phases controlled by changing sea levels. Sediment slides of widely varying size and shape are common in Quaternary deposits of the upper slope, particularly where gradients are steep. Our study of a large sediment failure in lowstand prograded coastal deposits west of Cape Licosa indicates that the nature of shelf-margin deposition is an additional important control on failure. The failure zone has a mobilization surface showing in-situ deformation in the sediment above it; an upper failure surface; a head scarp; and a zone of ponded sediment debris downslope from the exposed surface of failure. The basal mobilization surface is roughly parallel to the seafloor and coincident with a major downlap surface. The failed section is less that 20 m thick and local in extent, but deformation on the basal mobilization surface extends outside the immediate area of the failure. Directly downslope of the slide scarp are internally stratified mounds that show no evidence of deformation or movement. Most of the prograded deposit experienced in-situ deformation that evolved into the collapse of part of the sediment pile above the mobilization surface. A portion of the mobilized sediment flowed a few kilometers basinward and accumulated at the base of a slope-parallel ridge. Sediment failure occurred on the Licosa shelf margin following a major pulse of coastal sedimentation along the lowstand shoreline. On many continental margins, sea-level lowering is thought to be an important cause of failure unconsolidated sediment deposited during previous high-stand conditions. The Licosa slide demonstrates that sea-level fall has another, equally important but indirect, role in sediment failure. As sea level falls and reaches its lowstand position, streams are at their peak efficiency and a coarsening-upward clastic coastal wedge is rapidly emplaced at

  17. Shallow-mantle Recycling and Anomalous, Voluminous Volcanism along the Northern and Northwestern African Continental Margin

    NASA Astrophysics Data System (ADS)

    Bryce, J. G.; Blichert-Toft, J.; Graham, D. W.; Miller, S. A.

    2015-12-01

    Mantle-derived volcanism on Earth's surface is generally associated with magma generation as a consequence of volatile addition to suprasubduction zone mantle or in response to decompression melting at diverging plates or in thermochemical anomalies thought to originate deep in the convecting mantle. Many of the hotspots surrounding the northern and northwestern African margin are thought to originate from decompression melting due to upwellings from deep thermochemical anomalies. Similar compositions of lavas erupted in Sicily in the Hyblean Plateau and Mount Etna, Europe's largest most active volcano, have been attributed to contributions from subduction zone enrichments. Considering high-MgO lavas from the northern to northwestern African-Mediterranean margins in the context of recent petrologic models we find the strong majority of the lavas in this region are predominantly alkaline and bear geochemical signatures consistent with derivation from fusible lithologies (volatilized peridotite and/or pyroxenite) [1]. Such results are consistent with implications from recent experimental results that suggest that the mobilization of hydrous, carbonate-rich melts commonly occurs during subduction zone processing [2]. Accordingly, we argue many products generally considered "hot spot" volcanism in this region largely result from partial melting of easily fusible pyroxene-rich and carbonated mantle domains that are relics of shallow-level recycling of volatile-rich melts and/or lithosphere shed during plate boundary processes along the African margin. Long-lived volcanism near continental margins subsequently develops as a consequence of convective anomalies associated with unique tectonic arrangements (oversteepened slabs or slab windows) [3] or, alternatively, as manifestations of convective tectonic anomalies beneath thin lithosphere juxtaposed next to thicker, more stable continental margins [4]. [1] Herzberg and Asimow, 2008; [2] Poli, 2015; [3] Schellart, 2010; [4

  18. 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.

  19. Benthic remineralisation rates in shelf and slope sediments of the northern Benguela upwelling margin

    NASA Astrophysics Data System (ADS)

    Neumann, Andreas; Lahajnar, Niko; Emeis, Kay-Christian

    2016-02-01

    The Benguela Upwelling System off Namibia is a region of intensive plankton production. Remineralisation of this biomass frequently causes the formation of an oxygen minimum zone. A part of the organic matter is further deposited on the broad shelf in form of an extensive mudbelt with high TOC concentrations. During February 2011 we retrieved sediment samples from shelf and slope sediment along the Namibian coast to establish fluxes of nutrients, oxygen, and N2 on the basis of pore water concentrations. In mudbelt sediment, fluxes were estimated as high as 8 mmol NH4+ m-2 d-1 and 0.9 mmol PO43 - m-2 d-1, which is probably attributable to the activity of large sulphur bacteria. Especially phosphate is mobilised from sediment overlain by oxygen deficient bottom water when and where bottom water oxygen concentrations fall below 50 μmol l-1. In comparison to nutrient transport by Southern Atlantic Central Water flowing onto the Namibian shelf, benthic nutrient fluxes of the mudbelt contribute less than 5% to the nutrient budget of the shelf.

  20. Nutrient distributions, transports, and budgets on the inner margin of a river-dominated continental shelf

    EPA Science Inventory

    Physical and biogeochemical processes determining the distribution and fate of nutrients delivered by the Mississippi and Atchafalaya rivers to the inner (<50 m depth) Louisiana continental shelf (LCS) were examined using a three-dimensional hydrodynamic model of the LCS and obse...

  1. The great West African Tertiary coastal uplift: Fact or fiction? A perspective from the Angolan divergent margin

    NASA Astrophysics Data System (ADS)

    Jackson, M. P. A.; Hudec, M. R.; Hegarty, K. A.

    2005-12-01

    We explore exhumation in the coastal Kwanza Basin by combining analyses of Tertiary hiatuses and apatite fission tracks. Planktonic biozones show five major hiatuses in the Oligo-Miocene and Plio-Pleistocene. Between gaps, Oligo-Miocene strata accumulated under marine conditions. A marine setting refutes the idea of a massively raised coastal plateau in the mid-Tertiary. Marine conditions continued until ˜5 Ma. Fission track data suggest three thermal events: ˜150 Ma, during rifting and volcanism; ˜100-70 Ma, during shortening and volcanism; and ˜20-10 Ma, during exhumation. Tertiary uplift was spatially highly variable. For the Kwanza Basin, we infer that Tertiary uplift on the West African margin is indeed a fact but that estimates of uplift timing and size are unreliable when extrapolated to adjoining areas. Massive uplift (2000-4000 m) of the Precambrian craton had little structural effect in the outer basin. Instead, minor uplifts on the shelf drove late Tertiary deformation on the slope.

  2. South Harmony Church field, Southwest Louisiana - Further insights on uppermost Wilcox shelf-margin trend

    SciTech Connect

    Belvedere, P.G.

    1988-01-01

    The uppermost Wilcox Group (Eocene) in the South Harmony Church field is a 2,000-ft thick sequence of single and multistory sandstones and shales. Cores of three separate intervals from four different wells display sedimentologic and lithologic features characteristic of inner shelf and shoreface deposition, representing an offshore bar complex. Typical lithofacies assemblages within an interval include, from top to bottom, (1) bioturbated, muddy sandstone--inner shelf, (2) less bioturbated, arenaceous sandstone--mostly inner shelf, (3) structureless to faintly horizontally laminated sandstone--mostly lower shoreface, and (4) horizontal to cross-laminated, and commonly deformed sandstone with shale and siltstone interbeds--lower surface. Sandstones are fine to very fine-grained, poorly to moderately well-sorted sublitharenites. Optimum reservoir quality is observed in the bioturbated, clean sandstones of the self-shoreface transition zone, where porosity ranges from 5 to 23% and permeability from less than 0.01 to 31 md. Porosity is mostly secondary and attributed to the leaching of labile constituents.

  3. Submarine landslides along the Malacca Strait-Mergui Basin shelf margin: Insights from sequence-stratigraphic analysis

    NASA Astrophysics Data System (ADS)

    Lin, Yu-Nung Nina; Sieh, Kerry; Stock, Joann

    2010-12-01

    The enormously destructive tsunami of December 2004, caused by sudden motion of the Sunda megathrust beneath the Indian Ocean, raised concerns about tectonically induced tsunami worldwide. Submarine landslides may also trigger dangerous tsunami. However, the potential and repeat time for such events is in most places poorly known due to inadequate exploration of the sea floor and age constraints. The high sediment flux and tectonic subsidence rate of the Malacca Strait-Mergui Basin shelf margin NE of northernmost Sumatra provide a favorable environment to generate and preserve submarine landslides. From ten seismic reflection profiles acquired in 2006, we identify three sediment packages that exhibit sliding characteristics such as headscarps, distorted beds and debris-toe structures. We assign lowstand marine isotope stages to the paleo-shoreline indicators observed in the profiles. We then determine the ages of these submarine landslides as 20-30 ka, 342-364 ka and 435-480 ka by the paleo-shoreline indicators that bound the top and bottom of the slide bodies. This sequence-stratigraphic approach shows that these events occurred near times of sea-level lowstands, which implies that a large amount of direct sediment influx during glacial periods is an essential precondition for basin-margin submarine landsliding. Spatiotemporal variations of sediment input due to lobe switching or Asian monsoon intensity changes also control basin-margin instability. Because we are currently at a highstand stage, and sediment flux to the continental margin is relatively small, so the chance of having a repeat submarine landslide and landslide tsunami along this basin-margin is low.

  4. Data based 3D modelling of the southwest African continental margin

    NASA Astrophysics Data System (ADS)

    Freymark, J.; Sippel, J.; Scheck-Wenderoth, M.; Götze, H.-J.; Reichert, C.

    2012-04-01

    The volcanic passive continental margin of southwest Africa was formed in consequence of rifting and continental break-up of Gondwana in the Late Mesozoic. Our study focusses on an area extending from the Walvis Ridge in the north to the Agulhas Falkland Fracture Zone in the south including some important petroliferous sedimentary basins such as the Walvis Basin, the Luderitz Basin, and the Orange Basin. Due to decades of industrial exploration and scientific research, some of these areas reveal a large pool of structural and geophysical data. Thus, much is known about the individual tectonic and depositional histories of several subdomains of the area. The goal of our study is to understand the margin in its entirety. We present a 3D model of the present-day configuration of the southwest African continental margin. This model integrates well information, seismic reflection and refraction data, a previously published 3D structural model (Maystrenko et al., 2011), as well as freely available global data sets on the crustal structure (e.g. crust2.0 of Bassin, Laske & Masters, 2000). To extrapolate local information on crustal thickness (respectively the depth of the Moho) across the whole margin, we perform 3D gravity modelling using the software IGMAS+ (Götze & Schmidt, 2010; Schmidt et al., 2011). As parts of the first results, we show margin-wide depth and thickness distributions of a Palaeozoic to Cenozoic sedimentary layer and a Paleoproterozoic to Mesozoic crystalline crustal layer.

  5. Subsidence, extension and thermal history of the West African margin in Senegal

    NASA Astrophysics Data System (ADS)

    Brun, Marie Véronique Latil; Lucazeau, Francis

    1988-10-01

    The subsidence of the Atlantic margin in Senegal clearly shows two rapid stages related to the formation of (1) the Central Atlantic during the early Jurassic (around 200 Ma), and (2) the Equatorial Atlantic during the Cretaceous (100 Ma). A simple model of extension is used to interpret the subsidence history and to derive the thermal evolution of this basin. The present-day gravity, bathymetry, bottom hole temperatures (BHT) in oil exploration boreholes and heat flow density are used to control the validity of the model. Two cross sections from the outcropping basement to oceanic crust are used, one in Casamance and the other one at the south to latitude of Dakar. The model can fully explain the first-order subsidence history as well as the present-day observations, and therefore can provide valuable information about the thermal evolution of sediments and about the structure of the continental crust along the margin. Comparisons with the opposite margin in North America (Blake Plateau and Carolina trough) indicate a rather different evolution (the North American margin did not undergo the second stage of rifting) and a different crustal structure (crustal thinning is less important on the African margin).

  6. Processing and interpretation of data from a sonobouy refraction seismic section in the East China Sea shelf marginal ridge

    NASA Astrophysics Data System (ADS)

    Lou, Qing-Yuan; Yu, Pu-Zhi; Chen, Zuo-Lin

    1998-03-01

    Refraction seismic data obtained from a sonobuoy (SB7) in the East China Sea shelf marginal ridge was processed and interpreted. Intercept times and layer velocities were used to set up a one-dimension depth-velocity model based on which a two-dimenssion depth-velocity crustal model was established through two-dimension ray-tracing. The computation results showed that the upper crustal depth-velocity model beneath lineSB7 is composed of four layers with primary wave velocities of 1.80 2.20, 2.85 2.90, 3.24 3.34 and 4.55 6.00 m/s respectively. Neighbouring reflection seismic data and data on the East China Sea (ECS) regional geology were used to study the relationship between the velocities and their formation ages and also the contact relationship between the velocity boundaries.

  7. Bimodal pattern of seismicity detected at the ocean margin of an Antarctic ice shelf

    NASA Astrophysics Data System (ADS)

    Lombardi, Denis; Benoit, Lionel; Camelbeeck, Thierry; Martin, Olivier; Meynard, Christophe; Thom, Christian

    2016-06-01

    In Antarctica, locally grounded ice, such as ice rises bordering floating ice shelves, plays a major role in the ice mass-balance as it stabilizes the ice sheet flow from the hinterland. When in direct contact with the ocean, the ice rise buttressing effect may be altered in response of changing ocean forcing. To investigate this vulnerable zone, four sites near the boundary of an ice shelf with an ice rise promontory in Dronning Maud Land, East-Antarctica were monitored for a month in early 2014 with new instruments that include both seismic and GPS sensors. Our study indicated that this transition zone experiences periodic seismic activity resulting from surface crevassing during oceanic tide-induced flexure of the ice shelf. The most significant finding is the observation of apparent fortnightly tide-modulated low frequency, long duration seismic events at the seaward front of the ice rise promontory. A basal origin of these events is postulated with the ocean water surge at each new spring tide triggering basal crevassing or basal slip on a local bedrock asperity. Detection and monitoring of such seismicity may help identifying ice rise zones vulnerable to intensified ocean forcing.

  8. Bimodal pattern of seismicity detected at the ocean margin of an Antarctic ice shelf

    NASA Astrophysics Data System (ADS)

    Lombardi, Denis; Benoit, Lionel; Camelbeeck, Thierry; Martin, Olivier; Meynard, Christophe; Thom, Christian

    2016-08-01

    In Antarctica, locally grounded ice, such as ice rises bordering floating ice shelves, plays a major role in the ice mass balance as it stabilizes the ice sheet flow from the hinterland. When in direct contact with the ocean, the ice rise buttressing effect may be altered in response of changing ocean forcing. To investigate this vulnerable zone, four sites near the boundary of an ice shelf with an ice rise promontory in Dronning Maud Land, East-Antarctica were monitored for a month in early 2014 with new instruments that include both seismic and GPS sensors. Our study indicated that this transition zone experiences periodic seismic activity resulting from surface crevassing during oceanic tide-induced flexure of the ice shelf. The most significant finding is the observation of apparent fortnightly tide-modulated low-frequency, long-duration seismic events at the seaward front of the ice rise promontory. A basal origin of these events is postulated with the ocean water surge at each new spring tide triggering basal crevassing or basal slip on a local bedrock asperity. Detection and monitoring of such seismicity may help identifying ice rise zones vulnerable to intensified ocean forcing.

  9. Methane Seepage From the Arctic Shelf; 20 Years of Research on the Beaufort Sea Margin

    NASA Astrophysics Data System (ADS)

    Lorenson, T. D.; Paull, C. K.; Collett, T. S.; Dallimore, S. R.

    2008-12-01

    The U. S. Geological Survey has lead or played major roles in several efforts over the past 20 years to find geochemical evidence for gas hydrate dissociation on the Beaufort Sea shelf, a region of complex and varied geologic features that include: 1) several river deltas entering the Arctic Ocean, the largest of which is the Mackenzie River, 2) submerged continental shelf underlain by permafrost, 3) known petroleum systems of northern Alaska and the Mackenzie River Delta - Canada, 4), submerged pingo-like features (PLF's ) and, 5) pockmark fields. The results of these studies show that gas hydrate is present and that methane source can be both microbial and thermogenic. In light of our rapidly changing climate, the instability and potential methane release from Arctic gas hydrate deposits are reemerging as pivotal uncertainties. On the Alaskan Beaufort Shelf in water depths or about 10 m or less, methane concentrations in seawater are elevated relative to atmosphere. This methane likely originates from microbial degradation of organic matter deposited by rivers or coastal currents, and may be associated with organics in destabilized from recently thawed submerged shelf permafrost. In deeper water, north and west of the Prudhoe Bay area, some exceptionally high bottom water methane concentrations were measured with carbon isotopic signatures very similar (about -46 to -48‰) to gas hydrate sampled from the Mount Elbert 01 gas hydrate test well drilled in 2007 in the same area. This methane is presumably associated with the Prudhoe Bay gas hydrate and petroleum system, and likely from either gas hydrate dissociation or simple gas migration. Gas venting in and around the Mackenzie River delta is associated with offshore Pingo-like features (PLF's) and pockmarks. These PLF's resemble onshore pingos, but with an unknown origin. The region is underlain by an active petroleum system, submerged shelf permaforst, and gas hydrate. Methane concentrations are elevated in

  10. Shelf morphology as an indicator of sedimentary regimes: A synthesis from a mixed siliciclastic-carbonate shelf on the eastern Brazilian margin

    NASA Astrophysics Data System (ADS)

    Bastos, Alex C.; Quaresma, Valéria S.; Marangoni, Mariana B.; D'Agostini, Danielle P.; Bourguignon, Silvia N.; Cetto, Paulo H.; Silva, Alex E.; Amado Filho, Gilberto M.; Moura, Rodrigo L.; Collins, Michael

    2015-11-01

    Modern shelf morphology is the result of the interplay between short and long term sedimentary processes. The relation between rates of sediment supply/carbonate growth and accommodation space creation will not only control coastal transgression and regression, but will also define the shelf sedimentary regimes acting to shape the seabed. Herein, shelf morphology and sedimentology are investigated in order to discuss how these characteristics can be representative of distinct sedimentary regimes. The study area is the eastern Brazilian shelf where coastal transgression and regression coexist with the most important coral reef system of the South Atlantic. A compilation of existing published and unpublished data was carried out in order to produce morphological and faciological maps and compare the mapped features with high-resolution seismic and sonographic data. The results show three major regions or morphological compartments: Abrolhos Shelf, Doce River Shelf and the Paleovalleys Shelf. In terms of shelf sedimentary domain, rhodolith beds predominate over the outer shelf along the entire area, coralline reefs are present along the northern Abrolhos inner shelf and a significant terrigenous mud deposit is observed associated to the Doce River adjacent inner shelf beds. The rest of the shelf is composed by bioclastic or terrigenous mud sand and gravel. Terrigenous sedimentation is always restricted to the shoreface or inner shelf shallower areas and carbonate sands and gravels are predominant elsewhere. The Abrolhos shelf shows two distinct sectors; the northern area is a typical mixed sediment environment that has a supply regime along the coast/shoreface, mainly due to longshore transport and a carbonate regime along the inner and outer shelf. The southern shelf morphology and sedimentation are controlled by the antecedent topography and is typically a accommodation regime shelf with associated rhodolith beds. The Doce river shelf is a supply regime environment

  11. Inherited segmentation of the Iberian-African margins and tectonic reconstruction of a diffuse plate boundary.

    NASA Astrophysics Data System (ADS)

    Fernàndez, Manel; Torne, Montserrat; Vergés, Jaume; Casciello, Emilio

    2016-04-01

    Diffuse plate-boundary regions are characterized by non-well defined contacts between tectonic plates thus making difficult their reconstruction through time. The Western Mediterranean is one of these regions, where the convergence between the African and Iberian plates since Late Cretaceous resulted in the Betic-Rif arcuate orogen, the Gulf of Cadiz imbricate wedge, and the Alboran back-arc basin. Whereas the Iberia-Africa plate boundary is well defined west to the Gorringe Bank and along the Gloria Fault, it becomes much more diffuse eastwards with seismicity spreading over both the south-Iberian and north-African margins. Gravity data, when filtered for short wavelengths, show conspicuous positive Bouguer anomalies associated with the Gorringe Bank, the Gulf of Cadiz High and the Ronda/Beni-Bousera peridotitic massifs reflecting an inherited Jurassic margin segmentation. The subsequent Alpine convergence between Africa and Iberia reactivated these domains, producing crustal-scale thrusting in the Atlantic segments and eventually subduction in the proto-Mediterranean segments. The Jurassic segmentation of the Iberia-Africa margins substantiates the double-polarity subduction model proposed for the region characterized by a change from SE-dipping polarity in the Gorringe, Gulf of Cadiz and Betic-Rif domains, to NW-dipping polarity in the proto-Algerian domain. Therefore, the Algerian and Tyrrhenian basins in the east and the Alboran basin in the west are the result of SSE-E and NW-W retreating slabs of oceanic and/or hyper-extended Tethyan domains, respectively.

  12. Mid-Permian shelf margin erosion surfaces, western escarpment, Guadalupe Mountains, Texas

    SciTech Connect

    Franseen, E.K.; Pray, L.C.; Fekete, T.E.

    1987-05-01

    Three basin-sloping erosion surfaces, interpreted as submarine seismic sequence boundaries, are spectacularly exposed along the Guadalupe Mountains' western escarpment. Each surface has about 300 m of basin margin relief over 2 to 3 km and displays 30 to 80 m of local channeling and/or listric-shaped relief. The upper (post-Grayburg) and lower (post-Victorio Peak) surfaces truncate predominantly flat-bedded, shoaling-upward bank (or low-angle ramp) dolomite strata. The middle surface (post-Cutoff) locally cuts through the deep marine drape of 300 m relief of the 75-m thick Cutoff formation. The post-Grayburg surface is listric-shaped and truncates about 140 m of Grayburg in a basinward distance of 1.5 km. This post-Grayburg erosion surface marks a major change in Permian carbonate sedimentation from earlier bank strata to the steep foreslopes of 400 + relief of the Goat Seep-Capitan reef complex. Erosional retreat of the Grayburg and Victorio Peak bank margin may have been 100 m to 1 km or more. Evidence of submarine erosion includes deep marine facies directly overlying the erosion surfaces, the regional improbability of hundreds of meters of sea level lowering, and the lack of recognition of subaerial or coastal features. The erosion processes remain uncertain. Retrograde slumping is favored for the listric-shaped post-Grayburg surface. Bottom-hugging density currents were likely important for the lower two surfaces and perhaps for all three surfaces. Bare and eroded slopes are increasingly being recognized in modern submarine environments. They are likely more abundant and important in the geologic record than has been recognized to date.

  13. Quaternary hyperpycnal fans on the shelf of an active margin basin, Northern Santa Barbara Channel, California

    NASA Astrophysics Data System (ADS)

    Steel, E.; Simms, A.; Warrick, J. A.

    2013-12-01

    The small mountainous rivers draining the Transverse Ranges of southern California are known to form sediment-rich hyperpycnal plumes on the adjacent shelf during flooding events. Six fans were recently identified using sonar and lidar data in the northern Santa Barbara Channel and represent a unique opportunity to sample hyperpycnal deposits that have not been reworked or remobilized by other sedimentary processes. The two largest of these fans are those located directly offshore of Refugio and Tajiguas Creeks and are found in 20m to 70m water depths. We conducted shallow seismic surveys to image the morphology and internal architecture of the two fans. Internal reflectors define three seismic packages within the fans and isopach maps of these packages are presented. Geometries of the seismic reflections are interpreted to represent a shift from erosion of material over the most proximal fan locations to deposition of sediment with little or no erosion in distal portions. In several locations, dipping reflections can be clearly seen beneath an unconformity. Where present, this unconformity is interpreted to represent the base of the Holocene section overlying deformed Neogene strata. We report the results of a coring campaign on one of these fans designed to characterize the grain size, grain shape, and facies trends of hyperpycnal deposits. Additionally, petrology of core samples was compared to modern river samples to determine source regions. By analyzing sedimentation patterns and structures found in the fans of the Santa Barbara Channel, we hope to identify features that can be used to distinguish hyperpycnal deposition from other density-driven flows.

  14. Analysis of Submarine Landslides and Canyons along the U.S. Atlantic Margin Using Extended Continental Shelf Mapping Data

    NASA Astrophysics Data System (ADS)

    Chaytor, J. D.; Brothers, D. S.; Ten Brink, U. S.; Hoy, S. K.; Baxter, C.; Andrews, B.

    2013-12-01

    U.S. Geological Survey (USGS) studies of the U.S. Atlantic continental slope and rise aim to understand the: 1) the role of submarine landslides in tsunami generation, and 2) the linkages between margin morphology and sedimentary processes, particularly in and around submarine canyon systems. Data from U.S. Extended Continental Shelf (ECS) and numerous subsequent mapping surveys have facilitated the identification and characterization of submarine landslides and related features in fine detail over an unprecedented spatial extent. Ongoing analysis of USGS collected piston cores, sub-bottom and multichannel seismic (MCS) reflection profiles, and an extensive suite of legacy MCS data from two landslides, the Southern New England landslide zone and the Currituck Landslide, suggest that the most recent major landslide events are pre-Holocene, but that failures were complex and most likely multi-phase, at times resulting in extensive overlapping debris deposits. Piston core records plus visual observations of the seafloor from recent TowCam deployments and NOAA Ship Okeanos Explorer ROV dives reveal ongoing development of colluvial wedge-style debris aprons at the base of scarps within these landslides, showing that these regions continue to evolve long after the initial failure events. Multibeam bathymetry data and MCS profiles along the upper slope reveal evidence for vertical fluid migration and possible seabed gas expulsion. These observations underscore the need to reevaluate the sources of pore fluid overpressure in slope sediments and their role in landslide generation. ECS and more recent multibeam mapping have provided the opportunity to investigate the full extent of submarine canyon morphology and evolution from Cape Hatteras up to the US-Canadian EEZ, which has led to better understanding of the important role of antecedent margin physiography on their development. Six submarine canyon systems along the margin (Veatch, Hydrographer, Hudson, Wilmington

  15. Shelf architectures of an isolated Late Cretaceous carbonate platform margin, Galala Mountains (Eastern Desert, Egypt)

    NASA Astrophysics Data System (ADS)

    Scheibner, C.; Marzouk, A. M.; Kuss, J.

    2001-12-01

    An asymmetrical carbonate platform margin to basin transect has been investigated in the Upper Campanian-Maastrichtian succession of the Galala Mountains, northern Egypt. Identification of systems tracts and their lateral correlation was possible in slope sections only, whereas the monotonous chalk-marl alternations of the basinal sections could not be subdivided with respect to sequence stratigraphic terminology. The platform asymmetry is expressed by varying large-scale depositional architectures exhibiting a rimmed platform with a sigmoidal slope curvature in south-easterly dip-sections and a ramp with a linear slope curvature in south-westerly dip-sections. The rimmed platform is subdivided into a gentle upper slope and a steep lower slope. The platform formed as a result of the initial topography that was controlled by the tectonic uplift of the Northern Galala/Wadi Araba Syrian Arc structure. The calculated angles of the steep lower slope of the rimmed part range from 5 to 8°, whereas the ramp part has an angle of less than 0.1°.

  16. Multichannel seismic depth sections and interval velocities over outer continental shelf and upper continental slope between Cape Hatteras and Cape Cod: rifted margins

    USGS Publications Warehouse

    Grow, John A.; Mattick, Robert E.; Schlee, John S.

    1979-01-01

    Six computer-generated seismic depth sections over the outer continental shelf and upper slope reveal that subhorizontal Lower Cretaceous reflectors continue 20 to 30 km seaward of the present shelf edge. Extensive erosion on the continental slope has occurred primarily during the Tertiary, causing major unconformities and retreat of the shelf edge to its present position. The precise age and number of erosional events is not established, but at least one major erosional event is thought to be Oligocene and related to a marine regression in response to a worldwide eustatic lowering of sea level. Velocities derived from the multichannel data reveal distinctive ranges and lateral trends as functions of sediment age, depth of burial, and distance from the coastline. Seismic units beneath the shelf and slope of inferred Tertiary age range from 1.7 to 2.7 km/sec, increasing with age and depth of burial. Units interpreted as Upper Cretaceous rocks beneath the shelf range from 2.3 to 3.6 km/sec and show a distinct lateral increase across the shelf followed by a decrease beneath the present continental slope. Inferred Lower Cretaceous and Upper Jurassic rocks beneath the shelf increase from 3.7 to 4.8 km/sec from nearshore to offshore and indicate a change in facies from clastic units below the inner shelf to carbonate units beneath the outer shelf and upper continental slope. Both reflection and refraction data suggest that thin, high-velocity limestone units (5.0 km/sec) are present within the Lower Cretaceous and Upper Jurassic units beneath the outermost shelf edge, but that these change lithology or pinch out before reaching the middle shelf. Although lateral changes in velocity across the shelf and local velocity inversions appear, the interval velocities along the length of the margin show excellent continuity between Cape Hatteras and Cape Cod. The high-velocity horizons within the Lower Cretaceous and Upper Jurassic shelf-edge complex indicate the presence of a

  17. Using pressure transient analysis to improve well performance and optimize field development in compartmentalized shelf margin deltaic reservoirs

    SciTech Connect

    Badgett, K.L.; Crawford, G.E.; Mills, W.H.

    1996-12-31

    BP Exploration`s Gulf of Mexico group developed procedures to conduct effective well tests on conventional production wells and employed them during the development drilling phase of the Mississippi Canyon 109 (MC109) field. Bottomhole pressure data were recorded during the initial few weeks of production. Typically, a 48 hour pressure buildup survey (surface shut-in) was obtained near the end of data acquisition. Data from these tests were analyzed for completion efficiency, reservoir flow capacity, reservoir heterogeneities, and drainage area. Initially wells were gravel packed for sand control, until buildup interpretations indicated skins greater than 20. Frac packing technology was then employed, and an immediate improvement was observed with skins dropping into the teens. Over a period of time frac packs were optimized using the test derived skins as a metric. Analysis of pressure data also played an important role in identifying reservoir compartmentalization. The two major reservoir horizons at MC 109 are interpreted as shelf margin deltas. However, each of these has distinctly different compartmentalization issues. The continuous character of the G Sand made it easier to define the depositional system and investigate reservoir compartmentalization issues using a combination of well log, 3D seismic, static pressure trends, and fluid information. In the more distal deltaic reservoirs of the J Sand however, complications with seismic amplitudes and a less reliable tie between wireline and seismic data required the use of pressure transient analysis to efficiently exploit the reservoir.

  18. Abbot Ice Shelf, structure of the Amundsen Sea continental margin and the southern boundary of the Bellingshausen Plate seaward of West Antarctica

    PubMed Central

    Cochran, James R; Tinto, Kirsty J; Bell, Robin E

    2015-01-01

    Inversion of NASA Operation IceBridge airborne gravity over the Abbot Ice Shelf in West Antarctica for subice bathymetry defines an extensional terrain made up of east-west trending rift basins formed during the early stages of Antarctica/Zealandia rifting. Extension is minor, as rifting jumped north of Thurston Island early in the rifting process. The Amundsen Sea Embayment continental shelf west of the rifted terrain is underlain by a deeper, more extensive sedimentary basin also formed during rifting between Antarctica and Zealandia. A well-defined boundary zone separates the mildly extended Abbot extensional terrain from the deeper Amundsen Embayment shelf basin. The shelf basin has an extension factor, β, of 1.5–1.7 with 80–100 km of extension occurring across an area now 250 km wide. Following this extension, rifting centered north of the present shelf edge and proceeded to continental rupture. Since then, the Amundsen Embayment continental shelf appears to have been tectonically quiescent and shaped by subsidence, sedimentation, and the advance and retreat of the West Antarctic Ice Sheet. The Bellingshausen Plate was located seaward of the Amundsen Sea margin prior to incorporation into the Antarctic Plate at about 62 Ma. During the latter part of its independent existence, Bellingshausen plate motion had a clockwise rotational component relative to Antarctica producing convergence across the north-south trending Bellingshausen Gravity Anomaly structure at 94°W and compressive deformation on the continental slope between 94°W and 102°W. Farther west, the relative motion was extensional along an east-west trending zone occupied by the Marie Byrd Seamounts. Key Points: Abbot Ice Shelf is underlain by E-W rift basins created at ∼90 Ma Amundsen shelf shaped by subsidence, sedimentation, and passage of the ice sheet Bellingshausen plate boundary is located near the base of continental slope and rise PMID:26709352

  19. High-resolution modelling of ocean-shelf exchange: assessment of a 1/60th NEMO configuration of the Atlantic margin (AMM60)

    NASA Astrophysics Data System (ADS)

    Guihou, Karen; Harle, James; Holt, Jason; O'Dea, Enda; Polton, Jeffrey

    2015-04-01

    The North West European shelf-break is a barrier to the transport and exchange between the open ocean and the coastal waters. The tides, the complex bathymetry, the winds, the slope current, mesoscale processes and internal tides play a key role in the variability of the shelf-sea exchanges. A better understanding of these various processes is a first step towards a better quantification of the seasonal, inter-annual and climate variability of the European continental shelf seas. The FASTNEt (Fluxes Across Sloping Topography of the North East Atlantic) project is a Directed Research Grant funded by the Natural Environment Research Council. It is a collaboration between SAMS, NOC, PML, and the universities of Bangor, Liverpool and Plymouth and aims at elucidate the key processes controlling seasonal, inter-annual and regional variations in ocean-shelf exchange, through a combined strategy of observations and modelling. In this context, a new 1/60th degree resolution (~1.8km) NEMO configuration has been developed: AMM60 extends on the same domain as the UK Met Office operational Atlantic Margin Model (~7km), from 40°N to 64°N, and 20°W to 10°E, enveloping the whole North West European Atlantic margin. With 51 sigma levels, it reproduces high-resolution processes such as internal tides, slope current meanders and eddies, as well along the slope as on the shelf. The capability of AMM60 to reproduce these processes has been assessed through strict validation with observations. The tidal cycle is well reproduced, as well as the seasonal variability of stratification on the shelf. The spatio-temporal variability of high-resolution structures, such as internal tides, is compared against measurements conducted in the frame of the FASTNEt project. This work allows us to quantify the spatio-temporal variability of the ocean-shelf exchanges at several scales and aims to highlight the importance of fine scale processes in controlling the wide area across-shelf budgets.

  20. Crustal structure of the southern margin of the African continent: Results from geophysical experiments

    NASA Astrophysics Data System (ADS)

    Stankiewicz, J.; Parsiegla, N.; Ryberg, T.; Gohl, K.; Weckmann, U.; Trumbull, R.; Weber, M.

    2008-10-01

    A number of geophysical onshore and offshore experiments were carried out along a profile across the southern margin of the African Plate in the framework of the Inkaba yeAfrica project. Refraction seismic experiments show that Moho depth decreases rapidly from over 40 km inland to around 30 km at the present coast before gently thinning out toward the Agulhas-Falkland Fracture Zone, which marks the transition zone between the continental and oceanic crust. In the region of the abruptly decreasing Moho depth, in the vicinity of the boundary between the Namaqua-Natal Mobile Belt and the Cape Fold Belt, lower crustal P-wave velocities up to 7.4 km/s are observed. This is interpreted as metabasic lithologies of Precambrian age in the Namaqua-Natal Mobile Belt, or mafic intrusions added to the base of the crust by younger magmatism. The velocity model for the upper crust has excellent resolution and is consistent with the known geological record. A joint interpretation of the velocity model with an electrical conductivity model, obtained from magnetotelluric studies, makes it possible to correlate a high-velocity anomaly north of the center of the Beattie magnetic anomaly with a highly resistive body.

  1. Abbot Ice Shelf, structure of the Amundsen Sea continental margin and the southern boundary of the Bellingshausen Plate seaward of West Antarctica

    NASA Astrophysics Data System (ADS)

    Cochran, James R.; Tinto, Kirsty J.; Bell, Robin E.

    2015-05-01

    Inversion of NASA Operation IceBridge airborne gravity over the Abbot Ice Shelf in West Antarctica for subice bathymetry defines an extensional terrain made up of east-west trending rift basins formed during the early stages of Antarctica/Zealandia rifting. Extension is minor, as rifting jumped north of Thurston Island early in the rifting process. The Amundsen Sea Embayment continental shelf west of the rifted terrain is underlain by a deeper, more extensive sedimentary basin also formed during rifting between Antarctica and Zealandia. A well-defined boundary zone separates the mildly extended Abbot extensional terrain from the deeper Amundsen Embayment shelf basin. The shelf basin has an extension factor, β, of 1.5-1.7 with 80-100 km of extension occurring across an area now 250 km wide. Following this extension, rifting centered north of the present shelf edge and proceeded to continental rupture. Since then, the Amundsen Embayment continental shelf appears to have been tectonically quiescent and shaped by subsidence, sedimentation, and the advance and retreat of the West Antarctic Ice Sheet. The Bellingshausen Plate was located seaward of the Amundsen Sea margin prior to incorporation into the Antarctic Plate at about 62 Ma. During the latter part of its independent existence, Bellingshausen plate motion had a clockwise rotational component relative to Antarctica producing convergence across the north-south trending Bellingshausen Gravity Anomaly structure at 94°W and compressive deformation on the continental slope between 94°W and 102°W. Farther west, the relative motion was extensional along an east-west trending zone occupied by the Marie Byrd Seamounts. The copyright line for this article was changed on 5 JUN 2015 after original online publication.

  2. Generic analyses for evaluation of low Charpy upper-shelf energy effects on safety margins against fracture of reactor pressure vessel materials

    SciTech Connect

    Dickson, T.L.

    1993-07-01

    Appendix G to 10 CFR Part 50 requires that reactor pressure vessel beltline material maintain Charpy upper-shelf energies of no less than 50 ft-lb during the plant operating life, unless it is demonstrated in a manner approved by the Nuclear Regulatory Commission (NRC), that lower values of Charpy upper-shelf energy provide margins of safety against fracture equivalent to those in Appendix G to Section XI of the ASME Code. Analyses based on acceptance criteria and analysis methods adopted in the ASME Code Case N-512 are described herein. Additional information on material properties was provided by the NRC, Office of Nuclear Regulatory Research, Materials Engineering Branch. These cases, specified by the NRC, represent generic applications to boiling water reactor and pressurized water reactor vessels. This report is designated as HSST Report No. 140.

  3. Crustal Structure of the Southern Margin of the African Continent: Results from Geophysical Experiments.

    NASA Astrophysics Data System (ADS)

    Stankiewicz, J.; Parsiegla, N.; Ryberg, T.; Gohl, K.; Weckmann, U.; Trumbull, R.; Weber, M.

    2009-04-01

    A number of geophysical onshore and offshore experiments were carried out along a profile across the southern margin of the African Plate in the framework of the Inkaba yeAfrica project. Refraction seismic experiments show that Moho depth decreases rapidly from over 40 km inland to around 30 km at the present coast, before gently thinning out towards the Agulhas-Falkland Fracture Zone, which marks the transition zone between continental and oceanic crust. In the region of the abruptly decreasing Moho depth, in the vicinity of the boundary between the Namaqua-Natal Mobile Belt and the Cape Fold Belt, lower crustal P-wave velocities up to 7.4 km/s are observed. This is interpreted as metabasic lithologies of Precambrian age in the Namaqua-Natal Mobile Belt, or mafic intrusions added to the base of the crust by younger magmatism. The velocity model for the upper crust has excellent resolution. It is consistent with the known geological record, and also reveals a number of new features. These include a high velocity anomaly north of the centre of the Beattie Magnetic Anomaly. A joint interpretation of the velocity model with an electrical conductivity model, obtained from magnetotelluric studies, makes it possible to correlate this feature with a highly resistive body. Furthermore, a synclinal low velocity feature was identified in the Mesoproterozoic basement beneath the front of the Cape Fold Belt, south of the above mentioned feature. The northern edge of this feature correlates with the second magnetic body necessary to account for the BMA's signature.

  4. The Golden Lady: The Storied Life of a Multilingual Teacher and Author of Supplemental Reading Materials in a Marginalized South African Language

    ERIC Educational Resources Information Center

    Horan, Deborah A.; Sailors, Misty; Martinez, Miriam; Skerrett, Allison; Makalela, Leketi

    2012-01-01

    Personal narratives can be powerful venues for understanding human experiences. In this paper, we tell the story of Lutanyani, a Black South African multilingual teacher and author of supplemental reading materials in a marginalized South African language. Through various word images, we convey the role of language, in particular written language,…

  5. Basin Dynamics and Sedimentary Infilling of Miocene Sandstone Reservoir Systems In Eastern Tunisian African Margin

    NASA Astrophysics Data System (ADS)

    Bédir, Mourad; Khomsi, Sami

    2015-04-01

    Most of hydrocarbon accumulations and aquifers within the Cap Bon, Gulf of Hammamet and Sahel basins in eastern tunisian foreland are reservoired within the Upper Miocene Birsa and Saouaf sandstones and shales Formations. In the gulf of Hammamet, these sandstones constitutes oil and gas fields and are exploited on anticline highs and described as varying from shoreface to shallow marine and typically exhibit excellent reservoir quality of 30% to 35% porosity and good permeability from 500 to 1100 md. In addition, the fracturing of faults enhanced the reservoir quality potential. In contrary, the same hydrocarbon reservoirs are important hydrogeologic ones in the Cap Bon and Sahel basins with huge amount of hundred millions of cubic meters of water only partially exploited. Integrated wire line logging correlations, seismic sequence stratigraphic, tectonics and outcrop geologic analogue studies had permitted to highlight the basin structuring and sedimentary environments of sequence deposits infilling of the reservoir distribution between high platforms to subsiding graben and syncline basins bounded by deep-seated transtensive and transpressive flower faults. Seven third order sequence deposits limited by downlap prograding and onlap/toplap aggrading/retrograding system tracts extend along the eastern margin around the three basins by facies and thickness variances. System tracts exhibit around high horst and graben a channelized and levee infillings extending from 100 meters to more than a kilometer of width. They present a stacked single story and multistory channels types showing space lateral and vertical migrations along NE-SW, E-W and N-S directions. Paleogeographic depositional reservoir fair maps distribution highlight deltaic horst domain with floodplain and incised valley of fluvial amalgamed and braided sandstones distributary channels that occupy the high folded horsts. Whereas folded horst-graben and syncline borders domain of Shelf prodelta are

  6. Sources of terrestrially-derived organic carbon in lower Mississippi River and Louisiana shelf sediments: Implications for differential sedimentation and transport at the coastal margin

    USGS Publications Warehouse

    Bianchi, T.S.; Mitra, Siddhartha; McKee, B.A.

    2002-01-01

    In this study, we examined the temporal and spatial variability of terrestrial organic carbon sources in lower Mississippi River and Louisiana shelf sediments (during 11 cruises over a 22-month period) to further understand the sorting dynamics and selective transport of vascular plant materials within the primary dispersal system of the river. Bulk ??13C values in lower river sediments ranged from -21.90??? to -24.64??? (mean=-23.20??1.09???), these values were generally more depleted than those found in shelf sediments (-22.5??? to -21.2???). The ??8 (??8 = sum of vanillyl, syringyl and cinnamyl phenols produced from the oxidation of 100 mg of organic carbon) values in the lower river ranged from 0.71 to 3.74 (mean = 1.78??0.23). While there was no significant relationship between ??8 and river discharge (p>0.05), the highest value occurred during peak discharge in April 1999-which corresponded to the highest observed C/N value of 17.41. The ??8 values on the shelf ranged from 0.68 to 1.36 (mean = 0.54??0.30) and were significantly lower (p <0.05) than the average value for lower river sediments. The range of S/V (syringyl/vanillyl) and C/V (cinnamyl/vanillyl) ratios on the shelf, 0.11 to 0.95 and 0.01 to 0.08, respectively, were similar to that found in the lower river. These low C/V ratios are indicative a mixture of woody and non-woody carbon sources. Recent work by Goni et al. [Nature 389 (1997) 275; Geochim. Cosmochim. Acta 62 (1998) 3055], which did not include sampling transects within the primary dispersal system of the Mississippi River, showed a non-woody vascular plant signature on the Louisiana shelf. This suggests that riverine-derived woody tissues preferentially settle out of the water column, in the lower river and inner shelf, prior to the selective dispersal of C3 versus C4 non-woody materials in other regions the shelf and slope. This works further demonstrates the importance of differential settlement of particles, sampling location within the

  7. African Studies in a Canadian Academy: A Tool for Liberation or Marginalization?

    ERIC Educational Resources Information Center

    Fredua-Kwarteng, Eric

    2006-01-01

    This research uses critical race theory (CRT) as a conceptual perspective to study and analyze the experiences of ten students of African descent who enrolled in several African studies courses or related courses in an Ontarian university. The students, two females and eight males were interviewed between June and August 2005, using…

  8. Moving beyond the Margins: An Exploration of Low Performing African American Male College Students

    ERIC Educational Resources Information Center

    Jackson, Ronald C.

    2011-01-01

    Data have shown that African American male college students are being outperformed. Compared to all other populations by ethnicity and gender, African American males most often fare the worst in terms of persistence, performance, and completion. The impetus of this study was to explore the motivation of those that have low academic performance and…

  9. Sedimentary response to Milankovitch-type climatic oscillations and formation of sediment undulations: evidence from a shallow-shelf setting at Gela Basin on the Sicilian continental margin

    NASA Astrophysics Data System (ADS)

    Kuhlmann, Jannis; Asioli, Alessandra; Trincardi, Fabio; Klügel, Andreas; Huhn, Katrin

    2015-01-01

    A multi-proxy chronological framework along with sequence-stratigraphic interpretations unveils composite Milankovitch cyclicity in the sedimentary records of the Last Glacial-Interglacial cycle at NE Gela Basin on the Sicilian continental margin. Chronostratigraphic data (including foraminifera-based eco-biostratigraphy and δ18O records, tephrochronological markers and 14C AMS radiometric datings) was derived from the shallow-shelf drill sites GeoB14403 (54.6 m recovery) and GeoB14414 (27.5 m), collected with both gravity and drilled MeBo cores in 193 m and 146 m water depth, respectively. The recovered intervals record Marine Isotope Stages and Substages (MIS) from MIS 5 to MIS 1, thus comprising major stratigraphic parts of the progradational deposits that form the last 100-ka depositional sequence. Calibration of shelf sedimentary units with borehole stratigraphies indicates the impact of higher-frequency (20-ka) sea level cycles punctuating this 100-ka cycle. This becomes most evident in the alternation of thick interstadial highstand (HST) wedges and thinner glacial forced-regression (FSST) units mirroring seaward shifts in coastal progradation. Albeit their relatively short-lived depositional phase, these subordinate HST units form the bulk of the 100-ka depositional sequence. Two mechanisms are proposed that likely account for enhanced sediment accumulation ratios (SAR) of up to 200 cm/ka during these intervals: (1) intensified activity of deep and intermediate Levantine Intermediate Water (LIW) associated to the drowning of Mediterranean shelves, and (2) amplified sediment flux along the flooded shelf in response to hyperpycnal plumes that generate through extreme precipitation events during overall arid conditions. Equally, the latter mechanism is thought to be at the origin of undulated features resolved in the acoustic records of MIS 5 Interstadials, which bear a striking resemblance to modern equivalents forming on late-Holocene prodeltas of other

  10. Geology and biology of the "Sticky Grounds", shelf-margin carbonate mounds, and mesophotic ecosystem in the eastern Gulf of Mexico

    NASA Astrophysics Data System (ADS)

    Locker, Stanley D.; Reed, John K.; Farrington, Stephanie; Harter, Stacey; Hine, Albert C.; Dunn, Shane

    2016-08-01

    Shelf-margin carbonate mounds in water depths of 116-135 m in the eastern Gulf of Mexico along the central west Florida shelf were investigated using swath bathymetry, side-scan sonar, sub-bottom imaging, rock dredging, and submersible dives. These enigmatic structures, known to fisherman as the "Sticky Grounds", trend along slope, are 5-15 m in relief with base diameters of 5-30 m, and suggest widespread potential for mesophotic reef habitat along the west Florida outer continental shelf. Possible origins are sea-level lowstand coral patch reefs, oyster reefs, or perhaps more recent post-lowstand biohermal development. Rock dredging recovered bioeroded carbonate-rock facies comprised of bored and cemented bioclastics. Rock sample components included calcified worm tubes, pelagic sediment, and oysters normally restricted to brackish nearshore areas. Several reef sites were surveyed at the Sticky Grounds during a cruise in August 2010 with the R/V Seward Johnson using the Johnson-Sea-Link II submersible to ground truth the swath-sonar maps and to quantify and characterize the benthic habitats, benthic macrofauna, fish populations, and coral/sponge cover. This study characterizes for the first time this mesophotic reef ecosystem and associated fish populations, and analyzes the interrelationships of the fish assemblages, benthic habitats and invertebrate biota. These highly eroded rock mounds provide extensive hard-bottom habitat for reef invertebrate species as well as essential fish habitat for reef fish and commercially/recreationally important fish species. The extent and significance of associated living resources with these bottom types is particularly important in light of the 2010 Deepwater Horizon oil spill in the northeastern Gulf and the proximity of the Loop Current. Mapping the distribution of these mesophotic-depth ecosystems is important for quantifying essential fish habitat and describing benthic resources. These activities can improve ecosystem

  11. Comparative analysis of post-breakup basin evolution along the South-American and South-African margins, southern Atlantic

    NASA Astrophysics Data System (ADS)

    Strozyk, F.; Back, S.; Kukla, P. A.

    2012-04-01

    Recently, considerable attempts have been made to compare the sedimentary basin evolution and the associated tectonic framework on both sides of the South-Atlantic. However, yet there are still unresolved questions concerning the tectono-sedimentary styles of margin basin evolution that markedly differ from north to south. Amongst the most striking observations is that multiple phases of uplift and subsidence are recorded after the break-up of the southern South Atlantic margin segment on both sides of the Florianopolis-Walvis Ridge volcanic complex, features that are regarded as atypical when compared to published examples of other post-breakup margin successions. Adding to the heterogeneity of the system, the northernmost segment of the South Atlantic rift and salt basins is also characterized by a pronounced asymmetry, with the Brazilian margin now comprising narrower and deeper rift basins with less salt than the Congo-Gabon conjugate margin. This project deals with a large-scale comparison of this very different post-breakup tectono-stratigraphic development of the southern and northern South American and African continental margins that both record thick post-rift sedimentary successions. To gain detail of the basin margin evolution, we focus on a regional comparison between the post-breakup records archived in the large offshore southern Brazil basins (Pelotas, Santos, Campos) and the post-breakup continental margin successions of offshore Namibia (e.g. Orange Basin) and southern Angola (e.g. Kwanza Basin). A tectonic-stratigraphic comparison of representative geological transects provides a comprehensive basin-to-basin documentation of key factors influencing margin development which include the subsidence development through time, the sediment (in-)flux and storage patterns and the respective type of basin fill (e.g. salt vs. non-salt systems; carbonate-rich vs. clastics-dominated systems). Data from the salt-prone areas offshore South America and southern

  12. Temporal relationship between Holocene human occupation and vegetation change along the northwestern margin of the Central African rainforest

    NASA Astrophysics Data System (ADS)

    Lézine, Anne-Marie; Holl, Augustin F.-C.; Lebamba, Judicaël; Vincens, Annie; Assi-Khaudjis, Chimène; Février, Louis; Sultan, Émmanuelle

    2013-07-01

    This paper discusses the relationship between patterns of human settlements and environmental change during the Holocene along the northwestern margins of the equatorial rain forest of central Africa. Palaeoenvironmental data from high-resolution sediment cores from lacustrine deposits, plant macro-remains from forest soils, and archaeological data are harnessed to discuss the differential impact of climate and/or humans on the central African rain forest. It is shown that climate change impacted the rain forest well before the widespread expansion of human settlements all over the study area.

  13. Depositional architecture and evolution of the Late Miocene slope channel-fan-system in the northeastern shelf-margin of South China Sea

    NASA Astrophysics Data System (ADS)

    Jiang, Jing; Lin, Changsong; Zhang, Zhongtao; Tian, Hongxun; Tao, Ze; Liu, Hanyao

    2016-04-01

    The Upper Miocene in the Pearl River Mouth Basin of northwestern shelf-margin of South China Sea Basin contains a series of slope channel - fan systems. Their depositional architecture and evolution are documented in this investigation based on an integrated analysis of cores, logs, and seismic data. Four depositional-palaeogeomorphological elements have been identified in the slope channel-fan systems as follows: broad, shallow and unconfined or partly confined outer-shelf to shelf-break channels; deeply incised and confined unidirectionally migrating slope channels; broad or U-shaped, unconfined erosional-depositional channels; frontal splays-lobes and nonchannelized sheets. The slope channels are mostly oriented NW-SE, which migrated unidirectionally northeastwards and intensively eroded almost the whole shelf-slope zone. The channel infillings are mainly mudstones, interbedded with siltstones. They might be formed by gravity flow erosion as bypassing channels. They were filled with limited gravity flow sediments at the base and mostly filled with lateral accretionary packages of bottom current deposits. At the end of the channels, a series of small-scale slope fans developed and coalesced into fan aprons along the base of the slope. The unconfined erosional-depositional channels at the upper parts of the fan-apron-systems display compound infill patterns, and commonly have concave erosional bases and convex tops. The frontal splays-lobes representing middle to distal deposits of fan-apron-systems have flat-mounded or gull-wing geometries, and the internal architectures include bidirectional downlap, progradation, and chaotic infillings. The distal nonchannelized turbidite sheets are characterized by thin-bedded, parallel to sub-parallel sheet-like geometries. Three major unconformities or obvious erosional surfaces in the channel-fan systems of the Upper Miocene are recognized, and indicate the falling of sea-level. The depositional architecture of sequences

  14. Plate Kinematic model of the NW Indian Ocean and derived regional stress history of the East African Margin

    NASA Astrophysics Data System (ADS)

    Tuck-Martin, Amy; Adam, Jürgen; Eagles, Graeme

    2015-04-01

    Starting with the break up of Gondwana, the northwest Indian Ocean and its continental margins in Madagascar, East Africa and western India formed by divergence of the African and Indian plates and were shaped by a complicated sequence of plate boundary relocations, ridge propagation events, and the independent movement of the Seychelles microplate. As a result, attempts to reconcile the different plate-tectonic components and processes into a coherent kinematic model have so far been unsatisfactory. A new high-resolution plate kinematic model has been produced in an attempt to solve these problems, using seafloor spreading data and rotation parameters generated by a mixture of visual fitting of magnetic isochron data and iterative joint inversion of magnetic isochron and fracture zone data. Using plate motion vectors and plate boundary geometries derived from this model, the first-order regional stress pattern was modelled for distinct phases of margin formation. The stress pattern is correlated with the tectono-stratigraphic history of related sedimentary basins. The plate kinematic model identifies three phases of spreading, from the Jurassic to the Paleogene, which resulted in the formation of three main oceanic basins. Prior to these phases, intracontinental 'Karoo' rifting episodes in the late Carboniferous to late Triassic had failed to break up Gondwana, but initiated the formation of sedimentary basins along the East African and West Madagascan margins. At the start of the first phase of spreading (183 to 133 Ma) predominantly NW - SE extension caused continental rifting that separated Madagascar/India/Antarctica from Africa. Maximum horizontal stresses trended perpendicular to the local plate-kinematic vector, and parallel to the rift axes. During and after continental break-up and subsequent spreading, the regional stress regime changed drastically. The extensional stress regime became restricted to the active spreading ridges that in turn adopted trends

  15. How Does the Morphology of Shelf-Margin Deltas Relate to Sea-Level Elevation? Clues From the Modern Mississippi River Delta

    NASA Astrophysics Data System (ADS)

    Ghoshal, S.; Bart, P. J.

    2005-12-01

    Scientists have attempted to precisely determine eustatic changes using various techniques. One of these is the sequence stratigraphic technique which relies on vertical changes in the position of coastal onlap (Vail et al, 1977). In subsequent years, several authors have recognized many limitations in this approach (Christie-Blick et al., 1990). Our ongoing study focuses on the possibility of using the morphology of modern and ancient shelf-margin deltas as sea-level indicators to determine past sea-level elevations. The Mississippi River Delta provides an excellent opportunity to study the precise relationship between shelf-edge delta morphology and sea-level elevation. A morphological analysis of the modern Mississippi River Delta suggests that there may be a direct relationship between the sea-level and the offlap break features of the delta. The offlap break is defined at the intersection of topset and foreset at the location of a marked increase in slope. At this point, the bathymetric curvature (i.e. second derivative of sea-floor) is highest. This is analogous to O'Grady and Syvitski (2002) definition of shelf break offshore Greenland. More than a hundred bathymetric cross sections were made across the modern Balize lobe of the Mississippi River Delta using Rivertools software. The geomorphic analysis showed that there were two distinct morphologic types of the delta. In one type, there is a sharp transition from the topset to the foreset, whereas in a second type, the topset-foreset transition is gradual. It was observed that sharp offlap breaks characterize the eastern part of the delta at water depths of 1 ± 0.3 m. The southern and southwestern parts of the delta are characterized by gradual offlap breaks at water depths of 4 ± 2 m.

  16. Geology and biology of the "Sticky Grounds," shelf-margin carbonate mounds, and mesophotic ecosystem in the eastern Gulf of Mexico

    USGS Publications Warehouse

    Locker, Stanley D.; Reed, John K.; Farrington, Stephanie; Harter, Stacey; Hine, Albert C.; Dunn, Shane

    2016-01-01

    Shelf-margin carbonate mounds in water depths of 116–135 m in the eastern Gulf of Mexico along the central west Florida shelf were investigated using swath bathymetry, side-scan sonar, sub-bottom imaging, rock dredging, and submersible dives. These enigmatic structures, known to fisherman as the “Sticky Grounds”, trend along slope, are 5–15 m in relief with base diameters of 5–30 m, and suggest widespread potential for mesophotic reef habitat along the west Florida outer continental shelf. Possible origins are sea-level lowstand coral patch reefs, oyster reefs, or perhaps more recent post-lowstand biohermal development. Rock dredging recovered bioeroded carbonate-rock facies comprised of bored and cemented bioclastics. Rock sample components included calcified worm tubes, pelagic sediment, and oysters normally restricted to brackish nearshore areas. Several reef sites were surveyed at the Sticky Grounds during a cruise in August 2010 with the R/V Seward Johnson using the Johnson-Sea-Link II submersible to ground truth the swath-sonar maps and to quantify and characterize the benthic habitats, benthic macrofauna, fish populations, and coral/sponge cover. This study characterizes for the first time this mesophotic reef ecosystem and associated fish populations, and analyzes the interrelationships of the fish assemblages, benthic habitats and invertebrate biota. These highly eroded rock mounds provide extensive hard-bottom habitat for reef invertebrate species as well as essential fish habitat for reef fish and commercially/recreationally important fish species. The extent and significance of associated living resources with these bottom types is particularly important in light of the 2010 Deepwater Horizon oil spill in the northeastern Gulf and the proximity of the Loop Current. Mapping the distribution of these mesophotic-depth ecosystems is important for quantifying essential fish habitat and describing benthic resources. These activities can improve

  17. Biodiversity of the Deep-Sea Continental Margin Bordering the Gulf of Maine (NW Atlantic): Relationships among Sub-Regions and to Shelf Systems

    PubMed Central

    Kelly, Noreen E.; Shea, Elizabeth K.; Metaxas, Anna; Haedrich, Richard L.; Auster, Peter J.

    2010-01-01

    Background In contrast to the well-studied continental shelf region of the Gulf of Maine, fundamental questions regarding the diversity, distribution, and abundance of species living in deep-sea habitats along the adjacent continental margin remain unanswered. Lack of such knowledge precludes a greater understanding of the Gulf of Maine ecosystem and limits development of alternatives for conservation and management. Methodology/Principal Findings We use data from the published literature, unpublished studies, museum records and online sources, to: (1) assess the current state of knowledge of species diversity in the deep-sea habitats adjacent to the Gulf of Maine (39–43°N, 63–71°W, 150–3000 m depth); (2) compare patterns of taxonomic diversity and distribution of megafaunal and macrofaunal species among six distinct sub-regions and to the continental shelf; and (3) estimate the amount of unknown diversity in the region. Known diversity for the deep-sea region is 1,671 species; most are narrowly distributed and known to occur within only one sub-region. The number of species varies by sub-region and is directly related to sampling effort occurring within each. Fishes, corals, decapod crustaceans, molluscs, and echinoderms are relatively well known, while most other taxonomic groups are poorly known. Taxonomic diversity decreases with increasing distance from the continental shelf and with changes in benthic topography. Low similarity in faunal composition suggests the deep-sea region harbours faunal communities distinct from those of the continental shelf. Non-parametric estimators of species richness suggest a minimum of 50% of the deep-sea species inventory remains to be discovered. Conclusions/Significance The current state of knowledge of biodiversity in this deep-sea region is rudimentary. Our ability to answer questions is hampered by a lack of sufficient data for many taxonomic groups, which is constrained by sampling biases, life

  18. The tectono-stratigraphic evolution of the mid-Norway shelf in an Atlantic margin context, and its implications for prospectivity

    SciTech Connect

    Dore, A.G.; Birkeland, O.; Lundin, E.

    1995-08-01

    Source rock, reservoir rock and trap formation along the Atlantic margin between offshore Ireland and the northern part of the Mid-Norwegian shelf can be related to repeating tectonic motifs. Interaction of N-S and NE-SW fault and basin trends and NW-SE transfer lineaments is particularly significant. At least some of this structural grain can be related to basement anisotrophy dating back to the late Caledonian or earlier. Using tectonic maps, facies maps and profiles we demonstrate the establishment and exploitation of these structural elements in response to successive extensional episodes will varying stress fields, magmatic episodes and transtension/transpression. The following key tectonic episodes are selected for discussion: Late Caledonian establishment of basement grain; Carboniferous - Triassic extension; Late Jurassic rifting with a dominant E-W extension vector; Cretaceous extension and magmatism with a dominant NW-SE extension vector; Latest Cretaceous-Eocene tectonism, magmatism and break-up; Cenozoic (primarily Oligocene-Miocene) compression/inversion; and Late Cenozoic uplift. The analysis is described with special reference to the new exploration areas offshore Mid-Norway - the More Basin, Voring Basin and Lofoten area. The potential for play analogies and transfer of exploration models along the Atlantic margin trend is particularly emphasised.

  19. Topography, substratum and benthic macrofaunal relationships on a tropical mesophotic shelf margin, central Great Barrier Reef, Australia

    NASA Astrophysics Data System (ADS)

    Bridge, T. C. L.; Done, T. J.; Beaman, R. J.; Friedman, A.; Williams, S. B.; Pizarro, O.; Webster, J. M.

    2011-03-01

    Habitats and ecological communities occurring in the mesophotic region of the central Great Barrier Reef (GBR), Australia, were investigated using autonomous underwater vehicle (AUV) from 51 to 145 m. High-resolution multibeam bathymetry of the outer-shelf at Hydrographers Passage in the central GBR revealed submerged linear reefs with tops at 50, 55, 80, 90, 100 and 130 m separated by flat, sandy inter-reefal areas punctuated by limestone pinnacles. Cluster analysis of AUV images yielded five distinct site groups based on their benthic macrofauna, with rugosity and the presence of limestone reef identified as the most significant abiotic factors explaining the distribution of macrofaunal communities. Reef-associated macrofaunal communities occurred in three distinct depth zones: (1) a shallow (<60 m) community dominated by photosynthetic taxa, notably scleractinian corals, zooxanthellate octocorals and photosynthetic sponges; (2) a transitional community (60-75 m) comprising both zooxanthellate taxa and azooxanthellate taxa (notably gorgonians and antipatharians); and (3) an entirely azooxanthellate community (>75 m). The effects of depth and microhabitat topography on irradiance most likely play a critical role in controlling vertical zonation on reef substrates. The lower depth limits of zooxanthellate corals are significantly shallower than that observed in many other mesophotic coral ecosystems. This may be a result of resuspension of sediments from the sand sheets by strong currents and/or a consequence of cold water upwelling.

  20. Principles of Geological Mapping of Marine Sediments (with Special Reference to the African Continental Margin). Unesco Reports in Marine Science No. 37.

    ERIC Educational Resources Information Center

    Lisitzin, Alexandre P.

    Designed to serve as a complement to the Unesco Technical Papers in Marine Science, this report concentrates on theoretical and practical problems of geological mapping of the sea floor. An introduction is given to geological mapping procedures at continental margins as well as some practical recommendations taking as an example the African region…

  1. The habitat of petroleum in the Brazilian marginal and west African basins: A biological marker investigation

    SciTech Connect

    Mello, M.R.; Soldan, A.L. ); Maxwell, J.R. ); Figueira, J. )

    1990-05-01

    A geochemical and biological marker investigation of a variety of oils from offshore Brazil and west Africa, ranging in age from Lower Cretaceous to Tertiary, has been done, with the following aims: (1) assessing the depositional environment of source rocks, (2) correlating the reservoired oils, (3) comparing the Brazilian oils with their west African counterparts. The approach was based in stable isotope data; bulk, elemental, and hydrous pyrolysis results; and molecular studies involving quantitative geological marker investigations of alkanes using GC-MS and GC-MS-MS. The results reveal similarities between groups of oils from each side of the Atlantic and suggest an origin from source rocks deposited in five types of depositional environment: lacustrine fresh water, lacustrine saline water, marine evaporitic/carbonate, restricted marine anoxic, and marine deltaic. In west Africa, the Upper Cretaceous marine anoxic succession (Cenomanian-Santonian) appears to be a major oil producer, but in Brazil it is generally immature. The Brazilian offshore oils have arisen mainly from the pre-salt sequence, whereas the African oils show a balance between origins from the pre-salt and marine sequences. The integration of the geochemical and geological data indicate that new frontiers of hydrocarbon exploration in the west African basins must consider the Tertiary reservoirs in the offshore area of Niger Delta, the reservoirs of the rift sequences in the shallow-water areas of south Gabon, Congo, and Cuanza basins, and the reservoirs from the drift sequences (post-salt) in the deep-water areas of Gabon, Congo Cabinda, and Cuanza basins.

  2. Thermal history from both sides of the South Atlantic passive margin - A comparison: Argentinean pampa vs. South African escarpement.

    NASA Astrophysics Data System (ADS)

    Kollenz, Sebastian; Glasmacher, Ulrich A.

    2014-05-01

    The eastern Argentina South Atlantic passive continental margin is distinguished by a very flat topography. Out of the so called Pampean flat two mountain ranges are arising. These mountain ranges, the Sierras Australes and the Sierras Septentrionales, are located in the State of Buenos Aires south of the capital Buenos Aires. In existing literature the Sierras Australes are correlated with the South African cape fold belt (Torsvik 2009; Lopez Gamundi & Rossello 1998). Existing thermochronological data shows different post-breakup cooling histories for both areas and different AFT-ages. Published thermochronological ages (e.g. Raab et al. 2002, 2005, Gallagher et al et al. 1998)from the south African escarpement vary around 150 and 100 Ma (Gallagher et al. 1998). Only some spots in the eastern part of South Africa towards the pacific margin show older ages of 250 Ma and older than 350 Ma (Gallagher et al. 1998). New thermochronological data (AHe, AFT and ZHe) from the Sierras Australes indicate a different cooling history by revealing a range of varying ages due to younger tectonic activity. By comparing the data sets from both areas it is getting clear that the post-rift evolution of both continents is differing very strong. Gallagher, K., Brown, R. and Johnson, C. 1998. Fission track analysis and its application to geological problems. Annual review of Earth and Planetary Science, 26, 519-572. Lopez Gamundi, O.R., Rossello, E.A. (1998): Basin fill evolution and paleotectonic patterns along the Samfrau geosyncline: the Sauce Grande basin-Ventana foldbelt (Argentina) and Karoo basin-Cape foldbelt (South Africa) revisited. Geol Rundsch 86 :819-834. Raab, M.J., Brown, R.W., Gallagher, K., Carter, A. and Webber, K. 2002. late Cretaceous reactivation of major crustal shear zones in northern Namibia: constraints from apatite fission track analysis. Tectonophysics. 349, 75-92. Raab, M.J., Brown, R.W., Gallagher, K., Webber, K. and Gleadow, A.J.W. 2005. denudational and

  3. Sediment movement and dispersal patterns on the Grand Banks continental shelf and slope were tied to the dynamics of the Laurentide ice-sheet margin

    NASA Astrophysics Data System (ADS)

    Rashid, H.; MacKillop, K.; Piper, D.; Vermooten, M.; Higgins, J.; Marche, B.; Langer, K.; Brockway, B.; Spicer, H. E.; Webb, M. D.; Fournier, E.

    2015-12-01

    The expansion and contraction of the late Pleistocene Laurentide ice-sheet (LIS) was the crucial determining factor for the geomorphic features and shelf and slope sediment mobility on the eastern Canadian continental margin, with abundant mass-transport deposits (MTDs) seaward of ice margins on the upper slope. Here, we report for the first time sediment failure and mass-transport deposits from the central Grand Banks slope in the Salar and Carson petroleum basins. High-resolution seismic profiles and multibeam bathymetry show numerous sediment failure scarps in 500-1600 m water depth. There is no evidence for an ice margin on the upper slope younger than MIS 6. Centimeter-scale X-ray fluorescence analysis (XRF), grain size, and oxygen isotope data from piston cores constrain sediment processes over the past 46 ka. Geotechnical measurements including Atterberg limit tests, vane shear measurements and triaxial and multi-stage isotropic consolidation tests allowed us to assess the instability on the continental margin. Cores with continuous undisturbed stratigraphy in contourite silty muds show normal downcore increase in bulk density and undrained peak shear strength. Heinrich (H) layers are identifiable by a marked increase in the bulk density, high Ca (ppm), increase in iceberg-rafted debris and lighter δ18O in the polar planktonic foram Neogloboquadrina pachyderma (sinistral): with a few C-14 dates they provide a robust chronology. There is no evidence for significant supply of sediment from the Grand Banks at the last-glacial maximum. Mass-transport deposits (MTD) are marked by variability in the bulk density, undrained shear strength and little variation in bulk density or Ca (ppm) values. The MTD are older than 46 ka on the central Grand Banks slope, whereas younger MTDs are present in southern Flemish Pass. Factor of safety calculations suggest the slope is statically stable up to gradients of 10°, but more intervals of silty mud may fail during earthquake

  4. Southeast African records reveal a coherent shift from high- to low-latitude forcing mechanisms along the east African margin across last glacial-interglacial transition

    NASA Astrophysics Data System (ADS)

    Chevalier, Manuel; Chase, Brian M.

    2015-10-01

    Late Quaternary climate variability in the southern African subtropics is still only poorly resolved, with significant complexity and apparent contradictions in the regional dataset. To more effectively interpret and synthesize key regional records, we reanalysed the data from 13 pollen sequences from the summer rainfall zone of South Africa spanning the last 45,000 years, obtaining directly comparable quantitative reconstructions of mean annual temperature and summer rainfall. Temperature reconstructions from across the region provide consistent results, with all sites reflecting trends observed in southwest Indian Ocean sea-surface temperatures in the adjacent Mozambique Channel. Precipitation reconstructions are more heterogeneous, with two distinct subregions being identified. In the northeast, long-term trends in precipitation are determined by sea-surface and continental temperature trends, revealing a positive relationship between temperature and rainfall. This long-term pattern appears to be primarily driven by high northern latitude mechanisms, with direct local insolation being subordinate. Their relative impact reversed during terminal glacial period/early Holocene, at which time direct insolation forcing became the main driver of rainfall variability. Further south, in central South Africa, precipitation variability appears also to be influenced by the latitudinal position of the Southern Hemisphere westerlies, which combine with tropical flow to create tropical-temperate trough, advecting moisture into the interior. In this region, periods of maximum precipitation coincide with periods of elevated SSTs and equatorward expansions of the westerly storm track. This study allows for a fully constrained understanding of climate dynamics along the eastern African margin for the last 45,000 years, linking dynamics to drivers and describing how the climate systems evolved across the last glacial-interglacial transition.

  5. X-ray synchrotron diffraction study of natural gas hydrates from African margin

    NASA Astrophysics Data System (ADS)

    Bourry, Christophe; Charlou, Jean-Luc; Donval, Jean-Pierre; Brunelli, Michela; Focsa, Cristian; Chazallon, Bertrand

    2007-11-01

    Natural gas hydrates recovered from the Congo-Angola basin and Nigerian margins are analyzed by synchrotron X-ray powder diffraction. Biogenic methane is the most abundant gas trapped in the samples and others minor components (CO2, H2S) are co-clathrated in a type I cubic lattice structure. The refinement for the type I structure gives lattice parameters of a = 11.8646 (39) Å and a = 11.8619 (23) Å for specimens from Congo-Angola and Nigerian margins respectively at 90 K. These values, intermediate between the lattice constant of less pure methane specimens and pure artificial methane hydrates, indicate that lattice constants can be affected by the presence of encaged CO2, H2S and other gas molecules, even in small amounts. Thermal expansion is also presented for Congo-Angola hydrate in the temperature range 90-200 K. The coefficients are comparable with values reported for synthetic hydrates at low temperature and tend to approach thermal expansion of ice at higher temperature.

  6. Seismic stratigraphy, facies architecture, and reservoir character of a pleistocene shelf-margin delta complex, Eugene Island Block 330 field, offshore Louisiana

    SciTech Connect

    Hart, B.S.; Sibley, D.M.; Flemings, P.B.

    1997-03-01

    The GA interval of the Eugene Island Block 330 field is the deposit of late Pleistocene ({approximately}0.8 Ma) shelf-margin lowstand delta complex. We integrated three-dimensional (3-D) seismic, wireline log, core, and cuttings data to examine the delta`s internal architecture and to reconstruct its depositional history. This interval displays a complex vertical and lateral interfingering of channel, clinoform, and base-of-slope failure deposits over short distances (a few kilometers), and is the product of delta lobe progradation structural development (growth faults, rollover anticline) and relative sea level change. We then integrated our sequence stratigraphic interpretation with production data. Hydrocarbon accumulations in the interval are primarily associated with updip facies (delta mouth bar, delta front) beneath the flooding surface at the top of the interval, and not the sequence boundary at the base of the interval. Maps of seismic amplitudes associated with the top of the GA interval show patchy (mouth bar deposits) and curvilinear (interdistributary delta front) trends that indicate reservoir heterogeneities associated with depositional features. There is a good qualitative relationship between seismic amplitude and production characteristics, with the best production being from high-amplitude areas that sit high on the structure.

  7. Cenozoic denudation rates of the West African marginal upwarp recorded by lateritic paleotopographies

    NASA Astrophysics Data System (ADS)

    Beauvais, Anicet; Chardon, Dominique

    2013-04-01

    Quantifying long-term erosion of tropical shields is crucial to constraining the role of lateritic regolith covers as prominent sinks and sources of CO2 and sediments in the context of long-term Cenozoic climate change. It is also a key to understanding long-term landform evolution processes operating over most of the continental surface, particularly passive margins, and their control onto the sediment routing system. We study the surface evolution of West Africa over three erosion periods (~ 45-24, ~ 24-11 and ~ 11-0 Ma) recorded by relicts of 3 sub-continental scale lateritic paleolandsurfaces whose age is bracketed by 39Ar/40Ar dating of lateritic K-Mn oxides [1]. Denudation depths and rates compiled from 380 field stations show that despite heterogeneities confined to early-inherited reliefs, the sub-region underwent low and homogeneous denudation (~ 2-20 m Ma-1) over most of its surface whatever the considered time interval. This homogeneity is further documented by a worldwide compilation of cratonic denudation rates, over long-term, intermediate and modern Cenozoic time scales (100 - 107 yr). These results allow defining a steady-state cratonic denudation regime that is weathering-limited i.e. controlled by the thickness of the (lateritic) regolith available for stripping. Steady-state cratonic denudation regimes are enabled by maintained compartmentalization of the base levels between river knick points controlled by relief inheritance. Under such regimes, lowering of base levels and their fossilization are primarily imposed by long-term eustatic sea level fall and climate rather than by epeirogeny. The results suggest that Cenozoic post-rift vertical mobility of marginal upwarps in the tropical belt was unable to modify slow, weathering-controlled, steady state denudation regimes. The potentially complex expression of steady-state cratonic denudation regimes in clastic sedimentary fluxes remains to be investigated. [1] Beauvais et al., Journal of

  8. Post-rift uplift, paleorelief and sedimentary fluxes: the case example of the African margin of the South Atlantic

    NASA Astrophysics Data System (ADS)

    Guillocheau, F.; Dauteuil, O.

    2012-04-01

    volume of eroded sediments. This can explain abnormal stratigraphic response along the African South Atlantic passive margins, such as thin clayey basin floor fans at time of uplift and erosion of weathering profiles. Keywords: Africa, Cenozoic, Siliciclastic sediment fluxes, Tectonics, Climate

  9. New Seep Sites along the West-African Passive Margin Identified from Seismo-Acoustic Data

    NASA Astrophysics Data System (ADS)

    Fekete, N.; Spiess, V.; Caparachin, C.; Ding, F.; Gehrmann, R.; Riepshoff, H.; Trampe, A.; Foucher, J.-P.

    2009-04-01

    Recent and ongoing venting activity is documented offshore West Africa by large pockmarks, as has been observed by previous marine expeditions along the continental margin (e.g. Meteor Cruises M47/3 and M56). In summer 2008, an interdisciplinary campaign was carried out in cooperation between the MARUM Center for Marine Environmental Sciences in Bremen and IFREMER, Brest. The main scientific goal of the cruise was to assess the influence of different geological settings on the nature of venting and related features. Multi-frequency seismo-acoustic tools including swath bathymetry, sediment echosounder, flare imagery, and high-resolution multichannel seismics were utilized within the scope of geophysical studies for investigating the distribution of seep structures and associated subsurface feeder systems. Observations confirm a widespread occurrence of pockmarks along the continental margins of Gabon, Congo and Angola in deep water. Spatial surveys have further shown that venting-related features are present on different scales, particularly with sizes of tens of meters in diameter and topographical expressions on the meter scale. While these structures seem to be related to relatively shallow gas reservoirs, larger ones reveal roots to gas reservoirs in several hundred meters below the seafloor. At some sites, gas flares of a few hundred to over a thousand meters height could be identified within the water column. Comparing target areas north and south of the Congo Canyon, it has become evident that different driving forces and sedimentary and tectonic boundary conditions may be responsible for fluid venting and its distribution. While in the north, a thick sediment cover restricts seepage to selected zones of weakness and higher permeability, salt diapirism in the south is massively fracturing overlying sediments, creating numerous promising morphological features at the seafloor. However, only a few active vents could be found in the area of salt diapirism

  10. Deep crustal structure of the North-West African margin from combined wide-angle and reflection seismic data (MIRROR seismic survey)

    NASA Astrophysics Data System (ADS)

    Biari, Y.; Klingelhoefer, F.; Sahabi, M.; Aslanian, D.; Schnurle, P.; Berglar, K.; Moulin, M.; Mehdi, K.; Graindorge, D.; Evain, M.; Benabdellouahed, M.; Reichert, C.

    2015-08-01

    The structure of the Moroccan and Nova Scotia conjugate rifted margins is of key importance for understanding the Mesozoic break-up and evolution of the northern central Atlantic Ocean basin. Seven combined multichannel reflection (MCS) and wide-angle seismic (OBS) data profiles were acquired along the Atlantic Moroccan margin between the latitudes of 31.5° and 33° N during the MIRROR seismic survey in 2011, in order to image the transition from continental to oceanic crust, to study the variation in crustal structure, and to characterize the crust under the West African Coast Magnetic Anomaly (WACMA). The data were modeled using a forward modeling approach. The final models image crustal thinning from 36 km thickness below the continent to approximately 8 km in the oceanic domain. A 100 km wide zone characterized by rough basement topography and high seismic velocities up to 7.4 km/s in the lower crust is observed westward of the West African Coast Magnetic Anomaly. No basin underlain by continental crust has been imaged in this region, as has been identified north of our study area. Comparison to the conjugate Nova Scotian margin shows a similar continental crustal thickness and layer geometry, and the existence of exhumed and serpentinized upper mantle material on the Canadian side only. The oceanic crustal thickness is lower on the Canadian margin.

  11. Comparative analysis of the Late Cretaceous to Recent post-breakup basin evolution of the South-American and South-African margin of the southern Atlantic

    NASA Astrophysics Data System (ADS)

    Kukla, Peter; Back, Stefan

    2010-05-01

    Recently, considerable attempts have been made to compare the sedimentary basin evolution and the associated tectonic framework on both sides of the South-Atlantic (e.g. Mohriak et al., 2008, and references therein). Yet there are still unresolved questions. Amongst the most striking observations is that multiple phases of volcanism, uplift and subsidence are recorded after the break-up of the southern South Atlantic margin segment on both sides of the Florianopolis - Walvis Ridge volcanic complex, features that are regarded as atypical when compared to published examples of other post-breakup continental margin successions. However, the tectono-sedimentary and magmatic styles markedly differ from south to north across the volcanic complex. In seismic reflection data, voluminous extrusives are manifested by the occurrence of large wedges of seaward dipping reflector sequences south of the volcanic complex, whilst large volumes of Cretaceous mafic alkaline rocks only occur north of the Florianopolis - Walvis Ridge complex. It can be expected that these differences are of a broad importance for the understanding of both break-up and post-breakup processes. This presentation focuses on a comparison of the post-breakup stratigraphic development of the South American and South African continental margins that both record thick post-rift sedimentary successions. Basins along the southern African margin are much narrower in comparison to their South American counterparts, constituting a pronounced margin asymmetry across the Atlantic. Adding to the heterogeneity of the system, the northernmost segment of the South Atlantic rift and salt basins is also characterized by a pronounced asymmetry, with the Brazilian margin now comprising narrower and deeper rift basins with less salt than the Congo-Gabon conjugate margin. In general, it seems that in the salt-prone areas both offshore South America and southern Africa, salt-related tectonics are amongst the key parameters

  12. Recognition and interpretation of depositional units in a late Neogene progradational shelf margin complex, Taranaki Basin, New Zealand: foraminiferal data compared with seismic facies and wireline logs

    NASA Astrophysics Data System (ADS)

    Scott, G. H.; King, P. R.; Crundwell, M. P.

    2004-02-01

    the Milankovitch precessional band, is recorded by the γ-ray log. This sedimentary pattern continued in lobe #1, which was deposited higher on the slope. Assemblages are species-poor. The change from progradation foresets to aggradational topsets forms a major boundary on reflection profiles. The composition of assemblages does not change at the boundary but species abundances are more evenly distributed in the topset faunas. Although seismic reflection imagery primarily defines depositional units in the Giant Foresets, the position and character of some bounding reflectors is equivocal and biofacies classifications and summary signatures provide useful, complementary, information. Foraminiferal biofacies data are multivariate and reflect characters of the bottom environment and overlying water masses. Diverse analyses are required to decode the environmental signals. Despite significant costs in their acquisition, biofacies data appear to be a rich source of information for studies of prograding sequences at shelf margins.

  13. Kinematics of a growth fault/raft system on the West African margin using 3-D restoration

    NASA Astrophysics Data System (ADS)

    Rouby, Delphine; Raillard, Stéphane; Guillocheau, François; Bouroullec, Renaud; Nalpas, Thierry

    2002-04-01

    The ability to quantify the movement history associated with growth structures is crucial in the understanding of fundamental processes such as the growth of folds or faults in 3-D. In this paper, we present an application of an original approach to restore in 3-D a listric growth fault system resulting from gravity-induced extension located on the West African margin. Our goal is to establish the 3-D structural framework and kinematics of the study area. We construct a 3-D geometrical model of the fault system (from 3-D seismic data), then restore six stratigraphic surfaces and reconstruct the 3-D geometry of the system at six incremental steps of its history. The evolution of the growth fault/raft system corresponds to the progressive separation of two rafts by regional extension, resulting in the development of an intervening basin located between them that evolved in three main stages: (1) the rise of an evaporite wall, (2) the development of a symmetric basin as the elevation of the diapir is reduced and buried, and (3) the development of asymmetric basins related to two systems of listric faults (the main fault F1 and the graben located between the rollovers and the lower raft). Important features of the growth fault/raft system could only be observed in 3-D and with increments of deformation restored. The rollover anticline (associated with the listric fault F1) is composed of two sub-units separated by an E-W oriented transverse graben indicating that the displacement field was divergent in map view. The rollover units are located within the overlap area of two fault systems and displays a 'mock-turtle' anticline structure. The seaward translation of the lower raft is associated with two successive vertical axis rotations in the opposite sense (clockwise then counter-clockwise by about 10°). This results from the fact that the two main fault systems developed successively. Fault system F1 formed during the Upper Albian, and the graben during the Cenomanian

  14. Crustal structure variations along the NW-African continental margin: A comparison of new and existing models from wide-angle and reflection seismic data

    NASA Astrophysics Data System (ADS)

    Klingelhoefer, Frauke; Biari, Youssef; Sahabi, Mohamed; Aslanian, Daniel; Schnabel, Michael; Matias, Luis; Benabdellouahed, Massinissa; Funck, Thomas; Gutscher, Marc-André; Reichert, Christian; Austin, James A.

    2016-04-01

    Deep seismic data represent a key to understand the geometry and mechanism of continental rifting. The passive continental margin of NW-Africa is one of the oldest on earth, formed during the Upper Triassic-Lower Liassic rifting of the central Atlantic Ocean over 200 Ma. We present new and existing wide-angle and reflection seismic data from four study regions along the margin located in the south offshore DAKHLA, on the central continental margin offshore Safi, in the northern Moroccan salt basin, and in the Gulf of Cadiz. The thickness of unthinned continental crust decreases from 36 km in the North to about 27 km in the South. Crustal thinning takes place over a region of 150 km in the north and only 70 km in the south. The North Moroccan Basin is underlain by highly thinned continental crust of only 6-8 km thickness. The ocean-continent transition zone shows a variable width between 40 and 70 km and is characterized by seismic velocities in between those of typical oceanic and thinned continental crust. The neighbouring oceanic crust is characterized by a thickness of 7-8 km along the complete margin. Relatively high velocities of up to 7.5 km/s have been imaged between magnetic anomalies S1 and M25, and are probably related to changes in the spreading velocities at the time of the Kimmeridgian/Tithonian plate reorganization. Volcanic activity seems to be mostly confined to the region next to the Canary Islands, and is thus not related to the initial opening of the ocean, which was associated to only weak volcanism. Comparison with the conjugate margin off Nova Scotia shows comparable continental crustal structures, but 2-3 km thinner oceanic crust on the American side than on the African margin.

  15. Submarine allochthonous salt sheets: Gravity-driven deformation of North African Cretaceous passive margin in Tunisia - Bled Dogra case study and nearby salt structures

    NASA Astrophysics Data System (ADS)

    Masrouhi, Amara; Bellier, Olivier; Ben Youssef, Mohamed; Koyi, Hemin

    2014-09-01

    We used structural, stratigraphic and sedimentologic data, together with a comparison of nearby structures and a Bouguer gravity map, to evaluate the evolution of the Bled Dogra salt structure (northern Tunisia) during the Cretaceous. Triassic salt sheets are recognized in the northwestern region of the Tunisian Atlas. These salt sheets are the result of Cretaceous thick and/or thin-skinned extension along the south Tethyan margin. The Bled Dogra salt structure is one of these submarine allochthonous salt sheets, which was emplaced during the Early Cretaceous. The geologic framework, during this period, produces conditions for a predominantly gravity-driven deformation: extension has produced space for the salt to rise; vigorous differential sedimentation created differential loading that resulted in the emplacement and extrusion of a large volume of Triassic salt and formation of large submarine salt sheets. Geologic field data suggest an interlayered Triassic salt sheet within Albian sequences. Salt was extruded at the sea floor during the Early-Middle Albian and was initially buried by Middle-Late Albian strata. The Coniacian corresponds to a second transgressive cover onto the salt sheet after the gliding of the first salt cover (Late Albian-Turonian). In addition, this northwest Tunisian area exposes evidences for salt flow and abundant slump features at the base of a northward facing submarine slope, which was probably dominant from the Early Cretaceous to Santonian. Two gravity deformation processes are recognized: gravity gliding and gravity spreading. Acting concurrently, these two processes appear indistinguishable in this geologic context. Like the present-day salt-involved passive margins - such as the northern Gulf of Mexico, the Atlantic margin of Morocco, the Brazilian Santos basin, the Angola margin, Cadiz in western Iberia, and the Red Sea - the North African Cretaceous passive margin in Tunisia provides evidences that deformation in a passive-margin

  16. Geochemical compositional differences of the supramicron plankton-dominated fraction in two regimes of the Marginal Ice Zone (MIZ) of the outer East Siberian Arctic Shelf

    NASA Astrophysics Data System (ADS)

    Panova, E.; Tesi, T.; Pearce, C.; Salvado, J. A.; Karlsson, E. S.; Krusa, M.; Semiletov, I. P.; Gustafsson, O.

    2015-12-01

    The >10 um fraction of surface water in ice-covered and ice-free MIZ regimes of the East Siberian Arctic Shelf (ESAS) was investigated to improve understanding of its source and composition as well as to provide end-member constraints on the planktonic contribution to the underlying sedimentary organic matter. Samples were collected during the SWERUS-C3 2014 expedition in outer shelf open waters (Laptev Sea) and ice-covered conditions (East Siberian Sea). Our analyses indicate a contrasting composition in the two regimes. The stable carbon isotopic composition (δ13C) is more enriched in the ice-covered eastern ESAS where the radiocarbon age is also slightly depleted compared to the modern atmospheric value. By contrast, the western ESAS shows opposite trends with more depleted δ13C values but modern 14C ages (Fmod >1). Because the influence of terrigenous organic carbon in these samples is negligible (as documented at molecular level by lignin biomarkers), we interpret these compositional differences as a result of the HCO3- uptake by sea ice algae during carbon fixation due to the restricted access to CO2. This is consistent with the lipid biomarker IP25, unique for specific sea ice diatoms, which was found in relatively high concentration in the eastern shelf. Preliminary identifications of the phytoplankton taxa show also that open waters are characterized by a mixture of dinoflagellates, silicoflagellates and diatoms whereas the ice-covered region is mainly dominated by diatoms. In addition, while the distribution of nitrates in surface waters is fairly homogenous over the shelf, the nitrogen stable isotopic composition (δ15N) is more depleted in the western than in the eastern shelf, likely mirroring the difference in phytoplankton taxa. Our results indicate that, as the sea ice reduces due to Arctic warming, phytoplankton assemblages will progressively adjust to the different climate conditions which will affect both trophic chain and the biogeochemistry

  17. A facies distribution model controlled by a tectonically inherited sea bottom topography in the carbonate rimmed shelf of the Upper Tithonian-Valanginian Southern Tethyan continental margin (NW Sicily, Italy)

    NASA Astrophysics Data System (ADS)

    Basilone, Luca; Sulli, Attilio

    2016-08-01

    The Upper Tithonian-Valanginian shallow-water carbonates outcropping in the Palermo Mts (NW Sicily) consist of several facies associations reflecting different depositional environments of a carbonate rimmed shelf, pertaining to the Southern Tethyan continental margin. The reconstructed depositional model, based on the sedimentological features, cyclic facies arrangement and biota distribution, shows that a wide protected lagoon, dominated by algae, molluscs and scattered patch reefs, was bordered landward by a tidal flat, where stromatolitic algal mats were cyclically subaerial exposed, and seaward by a marine sand belt and reef complex. Oolitic packstone-grainstone lithofacies, cyclically subjected to subaerial exposure, suggests the occurrence of a barrier island, located nearly to the lagoonal carbonate shoreline, allowing the development of narrow embayments with restricted circulation. In the outer platform, the oolitic lithofacies of the marine sand belt pass landward into the protected lagoon, where washover oolite sands occur, and seaward into a high-energy zone (back-reef apron) gradually merging in the reef complex. In the latter, coral framestone occupied the inner sector (reef flat), while the facies association dominated by boundstone with Ellipsactinia sp. developed in the outer sectors (reef wall), adjacent to the fore-reef and upper slope environments. Stratigraphic evidence, associated with the recognized facies associations, helped to reconstruct the geo-tectonic setting of the carbonate platform, where the distribution of the depositional facies along the shelf and their extension were influenced by the tectonically-inherited sea bottom topography. In a regime of extensional tectonics, localized and thin succession of high-energy prograding oolite sand belt depositional facies occupied structural highs (footwall uplift), while the largely diffused and thick low energy aggrading peritidal-to-lagoonal depositional facies developed in subsiding

  18. Changes in opal fluxes along the northwest African margin during the last glacial period; linking high and low latitude patterns of productivity

    NASA Astrophysics Data System (ADS)

    Bradtmiller, L. I.; Galgay, M.; McGee, D.; Kinsley, C. W.; Anderson, R. F.

    2014-12-01

    Recent studies have proposed competing hypotheses to explain increased opal fluxes in high and low latitudes during the most recent deglaciation. Anderson et al. (2009) rely on increased wind-driven upwelling in the Southern Ocean to explain the increased availability of Si in both the Southern Ocean and tropical thermoclines, leading to increased opal fluxes in both regions coincident with the deglacial rise in CO2. Meckler et al. (2013) suggest that a decrease in the presence of North Atlantic intermediate water (GNAIW) during the deglaciation allowed Si-rich southern-sourced waters to fill the tropical Atlantic leading to increased opal burial. We attempt to distinguish between these two mechanisms by reconstructing opal fluxes and fluxes of windblown dust over the past ~65ka at four sites along the northwest African margin. The records include the deglaciation, including Heinrich Event 1 (H1) and the Younger Dryas (YD), as well as several earlier Heinrich events. We find that opal and dust fluxes increase simultaneously during the deglaciation, and more highly resolved cores record H1 and the YD as distinct peaks within the deglaciation. Furthermore, opal and dust fluxes scale approximately linearly with one another during these events. We observe opal peaks associated with most Heinrich Events through H6. Finally, we observe a strong similarity between patterns of opal flux in the Southern Ocean and along the African Margin. This suggests that the pattern of diatom productivity and opal flux along the African Margin reflects a combination of changes in wind strength due to shifting temperature gradients, and changes in the export of silica-rich water from the Southern Ocean, both as a result of the global scale climate changes associated with Heinrich Events. Anderson, R. F., S. Ali, L. I. Bradtmiller, S. H. H. Nielsen, M. Q. Fleisher, B. E. Anderson and L. H. Burckle. Wind-Driven Upwelling in the Southern Ocean and the Deglacial Rise in Atmospheric CO2

  19. High-resolution seismic stratigraphy of the late Neogene of the central sector of the Colombian Pacific continental shelf: A seismic expression of an active continental margin

    NASA Astrophysics Data System (ADS)

    Martínez, Jaime Orlando; López Ramos, Eduardo

    2011-02-01

    The sedimentary prism of the central Pacific continental shelf of Colombia was affected by regional folding and faulting, and probably later mud diapirism, from the Late Miocene to the Holocene. Interpretation of high-resolution seismic lines (2 s/dt) revealed that the prism consists of 13 high-resolution seismic units, that can be separated into 5 seismic groups. Deposition of the prism and the associated stacking pattern, are probably the response to variable uplift and subsidence in a fore-arc basin that underwent important tectonic events by the end of the Miocene. Throughout the Pliocene, the continental shelf sedimentation was affected by the growing of a dome structure probable due to mud diapirism. This fact caused peripheral faults both normal and reverse that controlled the distribution of some of the seismic units. During the Late Pleistocene (Wisconsin stage?) a eustatic sea level fall caused the shoreline to advance about 50 km westward of its present position. Because of this eustatic sea level change, a strong fluvial dissection took place and is interpreted as the probable extension of the San Juan River to the south of the present day river mouth. Within this framework it is believed that the Malaga and Buenaventura Bays were the passageways of branches of the old drainage system of the San Juan River. The inner branch circulated through the present Buenaventura Bay and runs southward leaving the mark of an apparent valley identified in the seismic information in the eastern sector of the study area. This old fluvial valley and its filling material located in the present day inner continental shelf front of Buenaventura are postulated as important targets to find placer minerals such as gold and platinum.

  20. Terrigenous organic matter signals in submarine canyons along the southwestern Gulf of Lion margin during Dense Shelf Water Cascading and quiescent conditions

    NASA Astrophysics Data System (ADS)

    Pasqual, Catalina; Tesi, Tomasso; Sanchez-Vidal, Anna; Goñi, Miguel; Calafat, Antoni; Lee, Cindy; Canals, Miquel

    2010-05-01

    Previous projects in the Gulf of Lion have analyzed the path of terrigenous compounds in the Rhone deltaic system, the continental shelf and the canyon heads. In this study we present results from the HERMES project, focused on the GoL slope to further assess the particulate exchange with the interior ocean. Experimental design consisted in nine sediment traps deployed along the Lacaze-Duthiers and Cap de Creus submarine canyons (from the canyon heads to the canyon mouth) and the southern open slope during 1 year. Sediment trap materials were analyzed by CuO oxidation to investigate spatial and temporal variability in the yields and compositional characteristics of lignin-derived phenols. These organic biomarkers are uniquely synthesized by vascular plants, most of which inhabit the aerial ecosystems, and have been used extensively to trace the contribution and source of terrigenous organic matter in many environments. Sediment trap data indicate that the Dense Shelf Water Cascading event that took place in winter 2006 dominates overall particle fluxes in both canyons. During the deployment, fluxes of biogeochemical parameters were positively correlated with the total mass flux. Lignin fluxes were also highly correlated with mass fluxes, but the ratio of lignin to lithogenic material and lignin composition varied significantly indicating significant changes in the composition and nature of land-derived materials mobilized along both canyons. For example, during the DSWC period land-derived material is not significantly enriched in lignins and main changes in lignin composition are the enrichment in vanillyn phenols respect to syringyl and cinamyl phenols in the middle Cap de Creus canyon and upper open slope samples. Lithogenic-normalized lignin contents were higher during late spring and summer at all stations, when overall particle fluxes are relatively low. During these periods, lignin compositions were characterized by elevated cinnamyl to vanillyl phenol

  1. Plio-Quaternary paleostresses in the Atlantic passive margin of the Moroccan Meseta: Influence of the Central Rif escape tectonics related to Eurasian-African plate convergence

    NASA Astrophysics Data System (ADS)

    Chabli, Ahmed; Chalouan, Ahmed; Akil, Mostapha; Galindo-Zaldívar, Jesús; Ruano, Patricia; Sanz de Galdeano, Carlos; López-Garrido, Angel Carlos; Marín-Lechado, Carlos; Pedrera, Antonio

    2014-07-01

    The Atlantic Moroccan Meseta margin is affected by far field recent tectonic stresses. The basement belongs to the variscan orogen and was deformed by hercynian folding and metamorphism followed by a post-Permian erosional stage, producing the flat paleorelief of the region. Tabular Mesozoic and Mio-Plio-Quaternary deposits locally cover the Meseta, which has undergone recent uplift, while north of Rabat the subsidence continues in the Gharb basin, constituting the foreland basin of the Rif Cordillera. The Plio-Quaternary sedimentary cover of the Moroccan Meseta, mainly formed by aeolian and marine terraces deposits, is affected by brittle deformations (joints and small-scale faults) that evidence that this region - considered up to date as stable - is affected by the far field stresses. Striated faults are recognized in the oldest Plio-Quaternary deposits and show strike-slip and normal kinematics, while joints affect up to the most recent sediments. Paleostress may be sorted into extensional, only affecting Rabat sector, and three main compressive groups deforming whole the region: (1) ENE-WSW to ESE-WNW compression; (2) NNW-SSE to NE-SW compression and (3) NNE-SSW compression. These stresses can be attributed mainly to the NW-SE oriented Eurasian-African plate convergence in the western Mediterranean and the escape toward the SW of the Rif Cordillera. Local paleostress deviations may be related to basement fault reactivation. These new results reveal the tectonic instability during Plio-Quaternary of the Moroccan Meseta margin in contrast to the standard passive margins, generally considered stable.

  2. Role of sea-level change in deep water deposition along a carbonate shelf margin, Early and Middle Permian, Delaware Basin: implications for reservoir characterization

    NASA Astrophysics Data System (ADS)

    Li, Shunli; Yu, Xinghe; Li, Shengli; Giles, Katherine A.

    2015-04-01

    The architecture and sedimentary characteristics of deep water deposition can reflect influences of sea-level change on depositional processes on the shelf edge, slope, and basin floor. Outcrops of the northern slope and basin floor of the Delaware Basin in west Texas are progressively exposed due to canyon incision and road cutting. The outcrops in the Delaware Basin were measured to characterize gravity flow deposits in deep water of the basin. Subsurface data from the East Ford and Red Tank fields in the central and northeastern Delaware Basin were used to study reservoir architectures and properties. Depositional models of deep water gravity flows at different stages of sea-level change were constructed on the basis of outcrop and subsurface data. In the falling-stage system tracts, sandy debris with collapses of reef carbonates are deposited on the slope, and high-density turbidites on the slope toe and basin floor. In the low-stand system tracts, deep water fans that consist of mixed sand/mud facies on the basin floor are comprised of high- to low-density turbidites. In the transgression and high-stand system tracts, channel-levee systems and elongate lobes of mud-rich calciturbidite deposits formed as a result of sea level rise and scarcity of sandy sediment supply. For the reservoir architecture, the fan-like debris and high-density turbidites show high net-to-gross ratio of 62 %, which indicates the sandiest reservoirs for hydrocarbon accumulation. Lobe-like deep water fans with net-to-gross ratio of 57 % facilitate the formation of high quality sandy reservoirs. The channel-levee systems with muddy calciturbidites have low net-to-gross ratio of 30 %.

  3. Quaternary sedimentary processes on the northwestern African continental margin - An integrated study using side-scan sonar, high-resolution profiling, and core data

    SciTech Connect

    Masson, D.G.; Huggett, Q.J.; Weaver, P.P.E. ); Kidd, R.B. ); Gardner, J.V. )

    1991-08-01

    Side-scan sonar data, cores, and high-resolution profiles have been used to produce an integrated model of sedimentation for the continental margin west of the Canary Islands. Long-range side-scan sonar (GLORIA) data and a grid of 3.5-kHz profiles, covering some 200,000 km{sup 2} allow a regional appraisal of sedimentation. More detailed studies of selected areas have been undertaken using a new 30 kHz deep-towed side-scan sonar (TOBI) developed by the U.K. Institute of Oceanographic Sciences. Sediment cores have been used both to calibrate acoustic facies identified on sonographs and for detailed stratigraphic studies. The most recent significant sedimentation event in the area is to Saharan Sediment Slide, which carried material from the upper continental slope off West Africa to the edge of the Madeira Abyssal Plain, a distance of some 1000 km. The authors data shows the downslope evolution of the debris flow. Near the Canaries, it is a 20-m-thick deposit rafting coherent blocks of more than 1 km diameter; side-scan records show a strong flow-parallel fabric on a scale of tens of meters. On the lower slope, the debris flow thins to a few meters, the flow fabric disappears, and the rafted blocks decrease to meters in diameter. Side-scan data from the lower slope show that the Saharan Slide buries an older landscape of turbidity current channels, typically 1 km wide and 50 m deep. Evidence from the Madeiran Abyssal Plain indicates a history of large but infrequent turbidity currents, the emplacement of which is related to the effects of sea level changes on the northwest African margin.

  4. Pre-collisional geodynamic context of the southern margin of the Pan-African fold belt in Cameroon

    NASA Astrophysics Data System (ADS)

    Nkoumbou, C.; Barbey, P.; Yonta-Ngouné, C.; Paquette, J. L.; Villiéras, F.

    2014-11-01

    We reassess the geodynamic context close to the Congo craton during the pre-collisional period of the Pan-African orogeny from whole-rock major and trace element compositions and isotopic data obtained in the westward extension of the Yaounde series (Boumnyebel area, Cameroon). The series consists of metasediments (micaschists, minor calc-silicate rocks and marbles) and meta-igneous rocks (hornblende gneisses, amphibolites, metagabbros, pyroxenites and talcschists) recrystallized under high-pressure conditions. Chemically, the micaschists correspond to shales and greywackes similar to the Yaounde high-grade gneisses. 87Sr/86Sr initial ratios (0.7084-0.7134), moderately negative εNd(620 Ma) values (-5.75 to -7.81), Nd model ages (1.66 < TDM < 1.74 Ga) and radiometric ages point to the conclusion that the Yaounde basin was filled with siliciclastic sediments derived from both reworked older continental crust (Palaeoproterozoic to Archaean in age) and Neoproterozoic juvenile volcanogenic material. This occurred in the same time span (625-1100 Ma) as the deposition of the Lower Dja, Yokadouma, Nola and Mintom series (Tonian-Cryogenian). Dolomitic marble associated with mafic/ultramafic rocks and characterized by high Cr (854-1371 ppm) and Ni (517-875 ppm) contents, are considered to result from chemical precipitation in relation with submarine magmatic activity. Talcschists (orthopyroxenitic to harzburgitic in composition) show primitive-mantle-normalized multi-element patterns with significant negative Nb-Ta anomalies, and slopes similar to that of average metasomatically altered lithospheric mantle. These rocks could be mantle slices involved in the collision tectonics. Amphibolites show the compositions of island-arc basalts with systematic negative Nb-Ta anomalies, 87Sr/86Sr initial ratios mostly <0.7047 and positive εNd(620 Ma) values (+1.41 to +6.58). They are considered to be the expression of incipient oceanisation to the north of the Congo craton during the

  5. The Brazilian continental margin

    NASA Astrophysics Data System (ADS)

    Martins, L. R.; Coutinho, P. N.

    1981-04-01

    The Brazilian continental margin, with its interesting morphology, structure and sediments, has become better known only during the last two decades. Six physiographical provinces can be recognized at the continental margin and the adjacent coast: (1) Cabo Orange-Parnaiba delta; (2) Parnaiba delta-Cabo Sa˜o Roque; (3) Cabo Sa˜o Roque-Belmonte; (4) Belmonte-Cabo Frio; (5) Cabo Frio-Cabo Santa Marta; and (6) Cabo Santa Marta-Chui. The shelf is rather wide near the Amazon Mouth, becoming narrower eastwards, continuing very narrow along the northeastern and eastern coast, and becoming wider again in the south towards the Plate River. Prominent morphological features along the margin are the Amazon cone, the marginal plateaus off northeastern Brazil, the Sa˜o Francisco cone and canyon, the Abrolhos Bank, and the deep-sea plateaus of Pernambuco and Sa˜o Paulo. On the shelf proper a number of relief elements exist, such as sand waves east of the Amazon, submarine terraces at various places, and irregularities of structural origin. The shelf break is rather smooth in the far north and south, more abrupt in the remainder. Surface sediments of the Brazilian shelf show five distinct facies types: littoral quartz sands, mud, transition sand-mud, coralline algae, and biodetrital. The terrigenous elastic fractions dominate off the Amazon and in southern Brazil; between these areas they occupy a very narrow strip near the coast. The carbonate facies, predominantly composed of calcareous algae, is abundant between the Parnaiba delta and Cabo Frio; to the south this facies is more biodetrital and restricted to the outer shelf. Economically important on the Brazilian continental margin besides oil, are sands and gravels, carbonate deposits, evaporites and some subsurface coal. Other possible mineral resources could be phosphate, heavy minerals and clays for ceramics.

  6. 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

    characteristics of the Ar Rayn terrane are analogous to the Andean continental margin of Chile, with opposite subduction polarity. The Ar Rayn terrane represents a continental margin arc that lay above a west-dipping subduction zone along a continental block represented by the Afif composite terrane. The concentration of epithermal, porphyry Cu and IOCG mineral systems, of central arc affiliation, along the AAF suggests that the AAF is not an ophiolitic suture zone, but originated as a major intra-arc fault that localized magmatism and mineralization. West-directed oblique subduction and ultimate collision with a land mass from the east (East Gondwana?) resulted in major transcurrent displacement along the AAF, bringing the eastern part of the arc terrane to its present exposed position, juxtaposed across the AAF against a back-arc basin assemblage represented by the Abt schist of the Ad Dawadimi terrane. Our findings indicate that arc formation and accretionary processes in the Arabian shield were still ongoing into the latest Neoproterozoic (Ediacaran), to about 620-600 Ma, and lead us to conclude that evolution of the Ar Rayn terrane (arc formation, accretion, syn- to postorogenic plutonism) defines a final stage of assembly of the Gondwana supercontinent along the northeastern margin of the East African orogen. ?? 2007 Elsevier B.V. All rights reserved.

  7. The Role of Plumes in Breakup Processes - Traces Found in the Deep Crustal Structure at the Intersection of Walvis Ridge with the African Continental Margin

    NASA Astrophysics Data System (ADS)

    Fromm, T.; Jokat, W.; Behrmann, J. H.; Ryberg, T.; Weber, M. H.

    2014-12-01

    Large igneous provinces (LIP) are often found in close temporal and spatial proximity with continental breakups, supporting the model, that an arriving mantle plume produces large amounts of melt and has a massive influence on the breakup process. The South Atlantic is a classical example, with flood basalts on both adjacent continents and a paired age progressing ridge system connecting them with the current hotspot location at Tristan da Cunha. To estimate the influence of the plume on the preexisting continental crust, a large-scale geophysical experiment was conducted in 2011 at the intersection of Walvis Ridge with the African continent. We present four P-wave velocity models derived from seismic refraction data. One extends 430 km along the ridge crest and continues onshore to a total length of 730 km, while the other three crossing the ridge perpendicular: one (480 km long) far offshore in the oceanic regime, one (600 km) close to shelf break and the last one (400 km) onshore. Crustal velocities beneath Walvis Ridge range between 5.5 km/s and 7.0 km/s, which are typical velocities for oceanic crust. The crustal thickness, however, is approximately three times larger than of normal oceanic crust: 17 km in the western part increasing to 22 km towards the continent. The continent ocean transition is characterized by 30 km thick crust with a high velocity body (HVB) in the lower crust and seismic velocities up to 7.5 km/s. The western extend of the HVB is to a similar distance from shore as for HVBs observed south of Walvis Ridge. In contrast, the eastern boundary lies well within the continental domain, at the 40 km thick crust of the Kaoko fold belt. Here, the variation of seismic velocities indicates that hot material intruded the continental crust during the initial rifting stage. However, beyond this relatively sharp boundary (40 km wide), the remaining continental crust seems unaffected by intrusions and the root of the Kaoko belt is no eroded. The cross

  8. Offshore Benin, a classic passive margin

    SciTech Connect

    Mathalone, J.M.P. )

    1991-03-01

    Offshore Benin comprises a narrow east-west continental shelf, some 30 km wide. A sharp shelf break running parallel to the coast borders the shelf, whereupon water depths rapidly increase to over 7000 ft. The area lies within the Dahomey Embayment, one of a series of Cretaceous and younger basins lining the coast of Africa that owe their inception to the Late Mesozoic break-up of the Gondwanaland Continent. The basin extends some 100 km inland, but sedimentary section is thin onshore compared to a maximum of 20,000 ft of sediment offshore. Initial sedimentation in this basin was of Neocomian alluvial and lacustrine clastics. These were deposited in east-west-trending narrow half-grabens associated with the initial break up of the South American and African continents. They are covered unconformably by more extensive Albian and Cenomanian transgressive clastics and shallow marine Turonian sandstones which are the main reservoir at Seme, Benin's only oilfield. The Senonian section offshore comprises passive margin deep sea clastic sediments prograding southwards. Very large proximal deep sea channels up to 2500 ft thick are developed in this interval. These channels are associated with excellent petroleum source rocks, averaging 4-5% oil-prone organic carbon, and form the main exploration target in the area when configured in a trap morphology. Seismic data quality is excellent in the region allowing detailed examination of the relationships between the rifted section and later units. In addition, these data illustrate clearly both internal and external morphology of the Senonian proximal deep sea channels.

  9. End-Member modelling and quantification of terrigenous flux rates to the NW African continental margin during the late Pleistocene to Holocene

    NASA Astrophysics Data System (ADS)

    Just, Janna; von Dobeneck, Tilo; Bickert, Torsten; Frederichs, Thomas

    2010-05-01

    The region off Gambia is an interesting study area because of its geographical location in between the ITCZ summer and winter position. We study a sediment core spanning the last 60 kyrs off the Gambia river mouth (W Africa) to identify different sources of the terrigenous sediment components exported to the continental margin. Our aim is the quantification of terrigenous flux rates of fluvial and aeolian load, respectively to improve our understanding of palaeoclimatic conditions and climatic changes in the Sahel and Sahara. It is known that in western Africa arid conditions prevailed during glacials and North Atlantic Heinrich Events. After the Last Glacial Maximum (LGM) a humid climate dominated northern Africa between 5-12 kyrs BP, known as the African Humid Period (AHP). These climatic changes have already been documented in magnetic, chemical, mineralogical and sedimentological proxies, respectively. However, these investigations were mainly carried out in qualitative approaches and lack an integrated multi-proxy validation. We apply a multi-proxy approach using XRF-element data and environmental magnetic parameter analysis on 5 cm interval samples of sediment core GeoB13602-1 (13°32.71 N; 17°50.96 W). Carbonate and biogenic opal content were analysed to estimate the total terrigenous fraction. Environmental magnetic parameters including ARM, IRM, HIRM, SIRM and frequency-dependent susceptibility allow the estimation of magnetic minerals, e.g. magnetite, hematite and goethite. Ratios of these parameters reflect grainsizes of the magnetic minerals which are indicative of transport mechanisms. We performed an End-Member (EM) analysis of IRM acquisition curves, decomposing the bulk sample into different components which represent individual sediment sources. Our approach is to include chemical, sedimentological and magnetic parameters in this EM model to reconstruct the composition as well as the transport pathways of the sediments. Based on an age model

  10. Divergent/passive margin basins

    SciTech Connect

    Edwards, J.D. ); Santogrossi, P.A. )

    1989-01-01

    This book discusses the detailed geology of the four divergent margin basins and establishes a set of analog scenarios which can be used for future petroleum exploration. The divergent margin basins are the Campos basin of Brazil, the Gabon basin, the Niger delta, and the basins of the northwest shelf of Australia. These four petroleum basins present a wide range of stratigraphic sequences and structural styles that represent the diverse evolution of this large and important class of world petroleum basins.

  11. Post Rift Evolution of the Indian Margin of Southern Africa

    NASA Astrophysics Data System (ADS)

    Baby, Guillaume; Guillocheau, François; Robin, Cécile; Dall'asta, Massimo

    2016-04-01

    The objective of this study is to discuss the evolution of the South African Plateau along the Indian margin of Southern Africa. Since the classical works of A. du Toit and L.C. King and the improvement of thermochronological methods and numerical models, the question of the uplift of South African Plateau was highly debated with numerous scenarios: early Cretaceous at time of rifting (Van der Beek et al., J.Geophys.Res., 2002), late Cretaceous (Braun et al., Solid Earth, 2014), late Cenozoic (Burke & Gunnell, Geol.Soc.of America, 2008). Limited attention has been paid on the constraints provided by the offshore stratigraphic record of the surrounding margins. The objective of our study is to integrate onshore and offshore data (seismic profiles and industrial wells) to (1) analyse the infill of the whole margin (21°S to 31°S) from its hinterland to the distal deep water basin, (2) to constrain and quantify the vertical movements. We discuss the impact on accommodation and sediments partitioning, and their significance on South African Plateau uplift history. 1. Sedimentary basins of the Indian margin of Southern Africa are related to the break-up of Gondwana during late Jurassic, resulting in rifts and flexural basins. First marine incursions started during early Cretaceous times (oldest marine outcropping sediments are of Barremian age ~128 Ma). The region developed as a normal continental shelf at the Aptian-Albian transition (~113 Ma). 2. The Cretaceous geological history of the basins is characterized by differential uplift and subsidence of the basement, controlled by structures inherited from break up. As example, major early Cretaceous depocenters of the margin are located on the north of Save-Limpopo uplift (Forster, Paleogography, Paleoclimatology, Paleoecology, 1975) showing an eastward drainage pattern, maybe related to a proto Limpopo drainage. Those observations suggest that the escarpment bordering the Bushveld depression is an old relief inherited

  12. Evolution of Cambrian-Ordovician carbonate shelf, United States Appalachians

    SciTech Connect

    Read, J.F.

    1985-02-01

    Cross sections and isopach maps (palinspastic) of the Cambrian-Ordovician continental shelf, US Appalachians, show that thickness and facies trends are controlled by the Adirondack, New Jersey, and Virginia highs and depocenters in Tennessee, Pennsylvania, and by the Rome trough. Carbonate sedimentation was initiated with drowning of Early Cambrian clastics, deposition of carbonate ramp and rimmed shelf facies followed by drowning, then regional regression and deposition of Early to Middle Cambrian red beds and platform margin rimmed shelf facies. During subsequent regional transgression, the Conasauga intrashelf shale basin formed, bounded toward the shelf edge and along depositional strike by Middle to Upper Cambrian oolitic ramp facies and cyclic peritidal carbonates. Intrashelf basin filling and regional regression caused progradation of Late Cambrian cyclic carbonates and clastics across the shelf. By this time, the margin had a relief of 2.5 km. During the Early Ordovician, incipient drowning of the shelf formed subtidal carbonates and bioherms that passed up into cyclic carbonate as sea level oscillations decreased in magnitude. Numerous unconformities interrupt this sequence in the northern Appalachians. The earlier high relief rimmed shelf was converted into a ramp, owing to uplift in the basin, heralding approaching collision. Subsidence rates on the margin were low (4 cm/1000 yr) and typical of a mature passive margin. Shelf sedimentation in the southern Appalachians ceased with arc-continent collision and development of the Knox unconformity, which dies out into the Pennsylvania depocenter. Major exploration targets are in the Late Cambrian-Early Ordovician Knox Group.

  13. Tectonic structure of the Arctic shelf

    NASA Astrophysics Data System (ADS)

    Bogdanov, N.; Khain, V.

    2003-04-01

    The Arctic shelf of Eurasia occupies over half (51 percent) of the Arctic Ocean area. The Shelf is in general shallow-water except its western part (the Barents Sea). The sea depth within the shelf rarely reaches 200 m. In fact, everywhere, it is separated from structures of the ocean central part by a high-angle continental escarpment with depth differences over 1000 m. In the west up to the New Siberian Islands, major tectonic elements of the shelf floor structure are N-S oriented, i.e. are found nearly at a right angle to the shelf margin. East of the New Siberian Islands, just like in passive margins of oceans, structures of the floor are parallel to the shoreline and shelf margin. The analysis of the geological evolution of the Arctic shelf in Eurasia indicates that modern structures are stipulated by the geodynamics of its formation. In the west within the Barents Sea, the Arctic shelf was formed after the closing of the Yapetus Ocean, displacement of Scandinavian sheets, and intrusion of Lower Devonian collisional granitoids. In that epoch, the Svalbard microplate with its Grenville basement joined the East-European craton, in addition to roots of Scandinavian Caledonides. On the east in the Pechora Sea region, the Timan-Pechora microplate had joined the craton still earlier, in the Vendian. In the Lower Devonian, the boundary of the East European plate was traced in the shelf along the axis of the East Barents trough. Bordering the latter on the east the Novaya Zemlya-Pai Hoi belt, just like the Uralian, isolates the East European plate from a complicated collage of blocks and microplates, tha makes up the base of the West Siberian basin. They are inherited by Triassic troughs, which were filled in the Jurassic and Cretaceous with sand-shale oil-bearing sedimentary sequences. The northern continuation of the Siberian plate is Taymyr with its folded and collisional belts and with the Kara plate with Grenville basement, confined to the plate from the north

  14. Crustal structure variations along the NW-African continental margin: a comparison of new and existing models from wide angle and reflection seismic data

    NASA Astrophysics Data System (ADS)

    Biari, Y.; Klingelhoefer, F.; Sahabi, M.; Aslanian, D.; Philippe, S.; Louden, K. E.; Berglar, K.; Moulin, M.; Mehdi, K.; Graindorge, D.; Evain, M.; Benabellouahed, M.; Reichert, C. J.

    2014-12-01

    Deep seismic data represent a key to understand the geometry and mechanism of continental rifting. The passive continental margin of NW-Africa is one of the oldest on earth, formed during the Upper Triassic-Lower Liassic rifting of the central Atlantic Ocean over 200 Ma. We present new and existing wide-angle and reflection seismic data from three study regions along the margin located in the North Moroccan salt basin, on the central continental margin offshore Safi and in the south, offshore Dakhla. In each of the study areas several combined wide-angle and reflection seismic profiles perpendicular and parallel to the margin have been acquired and forward modelled using comparable methods. The thickness of unthinned continental crust decreases from 36 km in the North to about 27 km in the South. In the North Moroccan Basin continental crust thins from originally 36 km to about 8 km in a 150 km wide zone. The basin itself is underlain by highly thinned continental crust. Offshore safi thinning of the continental crust is confined to a 130 km wide zone with no neighboring sedimentary basin underlain by continental crust. In both areas the zone of crustal thinning is characterised by the presence of large blocks and abundant salt diapirs. In the south crustal thinning is more rapid in a zone of 90 km and asymmetric with the upper crust thinning more closely to the continent than the lower crust, probably due to depth-dependent stretching and the presence of the precambrian Reguibat Ridge on land. Oceanic crust is characterised by a thickness of 7-8 km along the complete margin. Relatively high velocities of up to 7.5 km/s have been imaged between magnetic anomalies S1 and M25, and are probably related to changes in the spreading velocities at the time of the Kimmeridgian/Tithonian plate reorganisation. Volcanic activity seems to be confined to the region next to the Canary Islands, and is thus not related to the initial opening of the oceanic, which was related to no

  15. Petroleum geology of Cote d`Ivoir (Abidjan margin)

    SciTech Connect

    Reymond, A.

    1995-08-01

    The Cote d`Ivoire sedimentary basin is part of a typical transform passive margin developed along the West African coast from Liberia to Ghana. It straddles the coastline and the sedimentary section expands dramatically South of the East-West trending Lagune Fault, with up to 10,000 metres of sediments from Aptian to Present in age. Albo-Aptian rift series, mainly continental clastics without evaporites, have accumulated progressively in a tilted semi-graben. The drift stage marks a widespread marine sedimentation organized in progradational sequences which blanket the Albian block-fault topography of the continental break-up. Reservoirs are mainly clastics and present in the section from Middle Albian to Maastrichtian. Sand bodies are associated with identified submarine fans, infill structures or channelized units deposited in a shelf or outer-shelf environment. Trapping opportunities are due to block-faulting in the rift section, or gravity tectonics in the drift section, often combined with sand pinch-outs to constitute mixed structural-stratigraphic traps. Thick top-seal units formed by marine shales are widespread. Source-rocks have been shown to belong mainly to the rift series and they have been characterized in terms of geochemistry and maturation timing. An efficient simulation model has been used to recontruct the expulsion, migration and trapping of hydrocarbons along a selected North-South cross-section and to better define the Petroleum Systems.

  16. A Thin-Plate (Shallow Shelf) Treatment of Viscoelastic Ice-Shelf Flexure with Applications

    NASA Astrophysics Data System (ADS)

    MacAyeal, D. R.; Sergienko, O. V.; Banwell, A. F.; Rosier, S. H. R.; Gudmundsson, G. H.

    2014-12-01

    We derive a Shallow Shelf style treatment of the viscoelastic flexural deformation of a floating ice shelf and apply it to various problems relevant to ice-shelf response to sudden changes of loads (e.g., draining supra glacial lakes, iceberg calving, basal crevassing). Our analysis is based on the assumption that total deformation is the simple sum of elastic and viscous (or power-law creep) deformations (i.e., akin to a Maxwell model having a spring and dashpot in series). The key simplification in our analysis is the assumption that strain and its time derivative vary linearly as a function of depth through the thin ice shelf, and are zero at the neutral plane half the distance between the surface and base. We develop an analytic solution for idealized geometry and loading functions, and compare to a hierarchy of numerical treatments, including comparisons with full-Stokes solutions of viscoelastic flexure using an advanced finite-element package. Our work is applicable to numerous problems in ice-shelf and ice-tongue glaciology where impulsive loads or impulsive changes in geometry are involved. Examples of application include hydrostatic rebound in the aftermath of sudden surface lake drainage on Larsen B Ice Shelf immediately prior to its disintegration, ice-shelf and iceberg margin response to sudden edge-on-edge collisions between the two (push-mound formation), impulsive changes to geometry associated with ice-shelf calving, ice-front geometry modification by melting, necking phenomena, and basal crevassing.

  17. Ice-shelf - ocean interactions at Fimbul Ice Shelf, Antarctica from oxygen isotope ratio measurements

    NASA Astrophysics Data System (ADS)

    Price, M. R.; Heywood, K. J.; Nicholls, K. W.

    2008-03-01

    Melt water from the floating ice shelves at the margins of the southeastern Weddell Sea makes a significant contribution to the fresh water budget of the region. In February 2005 a multi-institution team conducted an oceanographic campaign at Fimbul Ice Shelf on the Greenwich Meridian as part of the Autosub Under Ice programme. This included a mission of the autonomous submarine Autosub 25 km into the cavity beneath Fimbul Ice Shelf, and a number of ship-based hydrographic sections on the continental shelf and adjacent to the ice shelf front. The measurements reveal two significant sources of glacial melt water at Fimbul Ice Shelf: the main cavity under the ice shelf and an ice tongue, Trolltunga, that protrudes from the main ice front and out over the continental slope into deep water. Glacial melt water is concentrated in a 200 m thick Ice Shelf Water (ISW) layer below the base of the ice shelf at 150-200 m, with a maximum glacial melt concentration of up to 1.16%. Some glacial melt is found throughout the water column, and much of this is from sources other than Fimbul Ice Shelf. However, at least 0.2% of the water in the ISW layer cannot be accounted for by other processes and must have been contributed by the ice shelf. Just downstream of Fimbul Ice Shelf we observe locally created ISW mixing out across the continental slope. The ISW formed here is much less dense than that formed in the southwest Weddell Sea, and will ultimately contribute a freshening (and reduction in δ18O) to the upper 100-150 m of the water column in the southeast Weddell Sea.

  18. Ice-shelf - ocean interactions at Fimbul Ice Shelf, Antarctica from oxygen isotope ratio measurements

    NASA Astrophysics Data System (ADS)

    Price, M. R.; Heywood, K. J.; Nicholls, K. W.

    2007-09-01

    Melt water from the floating ice shelves at the margins of the southeastern Weddell Sea makes a significant contribution to the fresh water budget of the region. In February 2005 a multi-institution team conducted an oceanographic campaign at Fimbul Ice Shelf on the Greenwich Meridian as part of the Autosub Under Ice programme. This included a mission of the autonomous submarine Autosub 25 km into the cavity beneath Fimbul Ice Shelf, and a number of ship-based hydrographic sections on the continental shelf and adjacent to the ice shelf front. The measurements reveal two significant sources of glacial melt water at Fimbul Ice Shelf: the main cavity under the ice shelf and an ice tongue that protrudes from the main ice front and out over the continental slope into deep water. Glacial melt water is concentrated in a 200 m thick Ice Shelf Water (ISW) layer below the base of the ice shelf at 150-200 m, with a maximum glacial melt concentration of up to 1.16%. Some glacial melt is found throughout the water column, and much of this is from sources other than Fimbul Ice Shelf. However, at least 0.2% of the water in the ISW layer cannot be accounted for by other processes and must have been contributed by the ice shelf. Just downstream of Fimbul Ice Shelf we observe locally created ISW mixing out across the continental slope. The ISW formed here is much less dense than that formed in the southwest Weddell Sea, and will ultimately contribute a freshening (and reduction in δ18O) to the upper 100-150 m of the water column in the southeast Weddell Sea.

  19. East Africa continental margins

    SciTech Connect

    Bosellini, A.

    1986-01-01

    New well data from Somalia, together with the history of sea-floor spreading in the Indian Ocean derived from magnetic anomalies, show that the East African margins from latitude 15/sup 0/S into the Gulf of Aden comprise four distinct segments that formed successively by the southward drift of Madagascar from Somalia during the Middle to Late Jurassic and Early Cretaceous, by the northeastward drift of India along the Owen Transform during the Late Cretaceous and Paleocene, and by the opening of the Gulf of Aden during the Neogene.

  20. Influence of subaqueous shelf angle on coastal plain-shelf-slope deposits resulting from a rise or fall in base-level

    SciTech Connect

    Wood, L.J.; Ethridge, F.G.; Schumm, S.A. )

    1991-03-01

    Extensive research in the past decade concerning the effects of base-level fluctuations on coastal plain-shelf-slope systems along passive margins has failed to properly assess the influence of the subaqueous shelf angle on the development, character, and preservation of the resulting deposits. A series of experiments were performed in a 4 m by 7 m flume to examine the effect that differing shelf angles have on a simulated coastal plain-shelf-slope system undergoing a cycle of base-level rise and fall. Results of the experiments indicate that the angle of the shelf affects (1) the amount of sediment available for deposition, (2) the timing of the influx of drainage basin sediment into the lower portions of the fluvial system, and (3) the width to depth ratio and sinuosity of fluvial systems that develop on the shelf. Base-level fall over a steep shelf results in deep, narrow, straight fluvial channels on the shelf and fine-grained, thick shelf-margin deltas. Depositional systems show high sedimentation rates, but a low ratio of coarse-grained to fine-grained sediment. Multiple fluvial incisions on the shelf are rapidly abandoned for a single incised valley. In contrast, gentle shelf angles result in shallow, wide, meandering fluvial channels on the shelf and coarser-grained, thinner shelf-margin deltas. Depositional systems have a lower sedimentation rate, but a high ratio of coarse-grained to fine-grained sediment. Multiple fluvial incisions on the shelf are active for a longer period of time. During subsequent base-level rise, deposits have a low potential for preservation owing to their thin nature and the slower rate at which transgression occurs over the shelf.

  1. Evolution of Cambrian-Ordovician carbonate shelf, US Appalachians

    SciTech Connect

    Read, J.F.

    1985-01-01

    Newly compiled cross sections and isopach maps of the Cambro-Ordovician continental shelf, U.S. Appalachians shows that thickness and facies trends are controlled by the Adirondack, New Jersey and Virginia and Alabama arches, and depocenters in Tennessee, Pennsylvania and the Rome Trough. Carbonate sedimentation was initiated with drowning of Early Cambrian clastics, deposition of carbonate ramp facies followed by drowning, regional regression and deposition of Early to Middle Cambrian red beds and platform margin rimmed shelf lime sands and reefs. During subsequent regional transgression the Conasauga intrashelf shale basin formed, bounded toward the shelf edge and along depositional strike by Middle to Upper Cambrian oolitic ramp facies and cyclic peritidal carbonates. During Middle Cambrian rifting, the Rome Trough was filled by thick clastics and carbonates. Intrashelf basin filling and regional regression caused progradation of Late Cambrian cyclic carbonates and clastics across the shelf. By this time, the margin had a relief of 2.5 kms. During the Early Ordovician, incipient drowning of the shelf formed subtidal carbonates and bioherms that passed up into cyclic carbonates which grade seaward into lime sands and reefs. Numerous unconformities interrupt this sequence in the Northern Appalachians. Early dolomitization patterns were controlled by regional highs. Subsidence rates on the margin were low (4 cm/1000 yrs) and typical of a mature passive margin. Shelf sedimentation in the Southern Appalachians ceased with arc-continent collision and development of the Knox unconformity, which dies out into the Pennsylvania depocenter.

  2. East African Rift Valley, Kenya

    NASA Technical Reports Server (NTRS)

    1990-01-01

    This rare, cloud free view of the East African Rift Valley, Kenya (1.5N, 35.5E) shows a clear view of the Turkwell River Valley, an offshoot of the African REift System. The East African Rift is part of a vast plate fracture which extends from southern Turkey, through the Red Sea, East Africa and into Mozambique. Dark green patches of forests are seen along the rift margin and tea plantations occupy the cooler higher ground.

  3. Downslope Eulerian mean flow associated with high-frequency current fluctuations observed on the outer continental shelf and upper slope along the northeastern United States continental margin: implications for sediment transport

    USGS Publications Warehouse

    Butman, B.

    1988-01-01

    Eulerian current measurements made 5-7 m above bottom at six stations along the United States east coast continental margin show a net downslope flow of 1-5 cm s-1. Although the scalar current speed decreases with water depth and toward the bottom, fluctuations in the cross-isobath flow were stronger and increasingly asymmetric near the bottom. Maximum downslope flow exceeded maximum upslope flow by a factor of two to three. The strength of the low-passed downslope flow was proportional to the upslope Reynolds flux of density as well as to the amplitude of the current fluctuations that have periods shorter than 30 h. These flow characteristics may be caused by differential vertical mixing in the bottom boundary layer where a stratified fluid flows upslope (unstable) and downslope (stable). The asymmetry in current strength clearly favors net downslope transport of sediments that move as bedload. ?? 1988.

  4. Pathways of shelf water export from the Hatteras shelf and slope

    NASA Astrophysics Data System (ADS)

    Churchill, James H.; Gawarkiewicz, Glen G.

    2012-08-01

    It has long been recognized that a massive flow of Middle Atlantic Bight (MAB) shelf water is exported to the deep ocean in the region near Cape Hatteras, North Carolina. We examine the details of this export using data from an extensive array of 26 moorings, deployed over the shelf and slope between Cape Hatteras and the Chesapeake Bay mouth (from 35° 27‧ to 36° 40‧ N) as part of the U.S. Department of Energy's Ocean Margins Program. Our analysis indicates that the flow of the MAB shelf-edge frontal jet, which typically extends over the MAB slope, falls victim to export over the length of the mooring array, essentially vanishing by the southern extreme of the array. By contrast, the flow of MAB shelf water entering the study region over the inner and middle shelf (to roughly the 40-m isobath) tends to experience very little loss over the extent of the OMP array. Based on our findings and those of previous studies, we hypothesize that this inner and middle shelf flow is diverted seaward upon encountering the Hatteras Front, which separates MAB and South Atlantic Bight shelf waters. Some fraction of this flow appears to return to the OMP array, moving northeastward over the upper slope en route to the deep ocean. Our analysis also suggests that the export of MAB shelf water is enhanced as the Gulf Stream approaches the shelf-edge near Diamond Shoals, a process we deem to be a high priority for future study.

  5. Shelf-fed turbidite system model and its application to the Oligocene deposits of the Campos Basin, Brazil

    SciTech Connect

    Peres, W.E. )

    1993-01-01

    Despite the large number of models involving the genesis and sedimentary facies of deep-water sandstones, none of these models adequately explain the origin and evolution of the extremely clean, widespread (over 6000 km[sup 2]), predominantly massive, thick (over 150 m), blanket-like sandstones deposited in the deep-water environment of the Campos Basin during the Oligocene. Consequently, to explain this sandstone, the author proposes a shelf-fed turbidite system model, which is strongly based on the Campos Basin data set. The basic framework necessary for the development of a shelf-fed turbidite system includes (1) deposition of a large volume of clastics during the buildup of a shelf-sand-rich unit, which later constitutes the main source of sediment for the system, (2) localized tectonic pulses that modify the outer shelf declivity and trigger mass flows; and (3) a relative fall of sea level, which causes the subaqueous exposure of the shelf sediments to reworking in a shallow, high-energy marine environment. These three basic elements are equally important for shelf-fed turbidity system development, but relative sea level position controls the development of the progradational, aggradational, and retrogradational depositional phases within the system. Submarine canyons commonly are scoured during all three phases on the outer shelf and lower slope environments. The shelf-fed turbidite system model may apply to other sedimentary basins, principally to those of the Atlantic-continental margins that have a thick evaporite sublayer. Halokinesis can provide the necessary room for the shelf sedimentary-unit buildup, the tectonic pulses that trigger the flows, and even localized relative sea level oscillations that can accelerate or abort any one of the depositional phases of the system. 25 refs., 26 figs.

  6. Cenozoic evolution of the Antarctic Peninsula continental margin

    SciTech Connect

    Anderson, J.B. )

    1990-05-01

    Cenozoic evolution of the Antarctic Peninsula continental margin has involved a series of ridge (Aluk Ridge)-trench collisions between the Pacific and Antarctic plates. Subduction occurred episodically between segments of the Pacific plate that are bounded by major fracture zones. The age of ridge-trench collisions decreases from south to north along the margin. The very northern part of the margin, between the Hero and Shackleton fracture zones, has the last surviving Aluk-Antarctic spreading ridge segments and the only remaining trench topography. The sedimentary cover on the northern margin is relatively thin generally less than 1.5 km, thus providing a unique setting in which to examine margin evolution using high resolution seismic methods. Over 5,000 km of high resolution (water gun) seismic profiles were acquired from the Antarctic Peninsula margin during four cruises to the region. The margin is divided into discrete fracture-zone-bounded segments; each segment displays different styles of development. Highly tectonized active margin sequences have been buried beneath a seaward-thickening sediment wedge that represents the passive stage of margin development Ice caps, which have existed in the Antarctic Peninsula region since at least the late Oligocene, have advanced onto the continental shelf on numerous occasions, eroding hundreds of meters into the shelf and depositing a thick sequence of deposits characterized by till tongues and glacial troughs. Glacial erosion has been the main factor responsible for overdeepening of the shelf; isostasy is of secondary importance. As the shelf was lowered by glacial erosion, it was able to accommodate thicker and more unstable marine ice sheets. The shelf also became a vast reservoir for cold, saline shelf water, one of the key ingredients of Antarctic bottom water.

  7. Thermostable Shelf Life Study

    NASA Technical Reports Server (NTRS)

    Perchonok, M. H.; Antonini, D. K.

    2008-01-01

    The objective of this project is to determine the shelf life end-point of various food items by means of actual measurement or mathematical projection. The primary goal of the Advanced Food Technology Project in these long duration exploratory missions is to provide the crew with a palatable, nutritious and safe food system while minimizing volume, mass, and waste. The Mars missions could be as long as 2.5 years with the potential of the food being positioned prior to the crew arrival. Therefore, it is anticipated that foods that are used during the Mars missions will require a 5 year shelf life. Shelf life criteria are safety, nutrition, and acceptability. Any of these criteria can be the limiting factor in determining the food's shelf life. Due to the heat sterilization process used for the thermostabilized food items, safety will be preserved as long as the integrity of the package is maintained. Nutrition and acceptability will change over time. Since the food can be the sole source of nutrition to the crew, a significant loss in nutrition may determine when the shelf life endpoint has occurred. Shelf life can be defined when the food item is no longer acceptable. Acceptability can be defined in terms of appearance, flavor, texture, or aroma. Results from shelf life studies of the thermostabilized food items suggest that the shelf life of the foods range from 0 months to 8 years, depending on formulation.

  8. Thermostabilized Shelf Life Study

    NASA Technical Reports Server (NTRS)

    Perchonok, Michele H.; Catauro, Patricia M.

    2009-01-01

    The objective of this project is to determine the shelf life end-point of various food items by means of actual measurement or mathematical projection. The primary goal of the Advanced Food Technology Project in these long duration exploratory missions is to provide the crew with a palatable, nutritious and safe food system while minimizing volume, mass, and waste. The Mars missions could be as long as 2.5 years with the potential of the food being positioned prior to the crew arrival. Therefore, it is anticipated that foods that are used during the Mars missions will require a 5 year shelf life. Shelf life criteria are safety, nutrition, and acceptability. Any of these criteria can be the limiting factor in determining the food's shelf life. Due to the heat sterilization process used for the thermostabilized food items, safety will be preserved as long as the integrity of the package is maintained. Nutrition and acceptability will change over time. Since the food can be the sole source of nutrition to the crew, a significant loss in nutrition may determine when the shelf life endpoint has occurred. Shelf life can be defined when the food item is no longer acceptable. Acceptability can be defined in terms of appearance, flavor, texture, or aroma. Results from shelf life studies of the thermostabilized food items suggest that the shelf life of the foods range from 0 months to 8 years, depending on formulation.

  9. Amery Ice Shelf

    Atmospheric Science Data Center

    2013-04-16

    article title:  Amery Ice Shelf's "Loose Tooth" Gets Looser     View Larger Image The Amery Ice Shelf is an important dynamic system responsible for draining about 16% of ... glaciers. Mass loss from the system occurs through basal melting and iceberg calving. These images from the Multi-angle Imaging ...

  10. Cross-Shelf Exchange.

    PubMed

    Brink, K H

    2016-01-01

    Cross-shelf exchange dominates the pathways and rates by which nutrients, biota, and materials on the continental shelf are delivered and removed. This follows because cross-shelf gradients of most properties are usually far greater than those in the alongshore direction. The resulting transports are limited by Earth's rotation, which inhibits flow from crossing isobaths. Thus, cross-shelf flows are generally weak compared with alongshore flows, and this leads to interesting observational issues. Cross-shelf flows are enabled by turbulent mixing processes, nonlinear processes (such as momentum advection), and time dependence. Thus, there is a wide range of possible effects that can allow these critical transports, and different natural settings are often governed by different combinations of processes. This review discusses examples of representative transport mechanisms and explores possible observational and theoretical paths to future progress. PMID:26747520

  11. Modeling the dynamics of continental shelf carbon.

    PubMed

    Hofmann, Eileen E; Cahill, Bronwyn; Fennel, Katja; Friedrichs, Marjorie A M; Hyde, Kimberly; Lee, Cindy; Mannino, Antonio; Najjar, Raymond G; O'Reilly, John E; Wilkin, John; Xue, Jianhong

    2011-01-01

    Continental margin systems are important contributors to global nutrient and carbon budgets. Effort is needed to quantify this contribution and how it will be modified under changing patterns of climate and land use. Coupled models will be used to provide projections of future states of continental margin systems. Thus, it is appropriate to consider the limitations that impede the development of realistic models. Here, we provide an overview of the current state of modeling carbon cycling on continental margins as well as the processes and issues that provide the next challenges to such models. Our overview is done within the context of a coupled circulation-biogeochemical model developed for the northeastern North American continental shelf region. Particular choices of forcing and initial fields and process parameterizations are used to illustrate the consequences for simulated distributions, as revealed by comparisons to observations using quantitative statistical metrics. PMID:21329200

  12. Ross Ice Shelf

    Atmospheric Science Data Center

    2013-04-16

    ... funded by the National Science Foundation, several penguin colonies near the Ross Ice Shelf, Antarctica have not been able to ... affected include one of Antarctica's most populous Adelie penguin colonies, as well as a small colony of Emperor penguins. These ...

  13. Lower Cretaceous barrier reef and outer shelf facies, Sligo Formation, south Texas

    SciTech Connect

    Kirkland, B.L.; Lighty, R.G.; Rezak, R.; Tieh, T.T.

    1987-09-01

    Along the south Texas margin, a vast carbonate-shelf complex with an extensive barrier-reef system and abundant shallow-lagoon and skeletal-shoal deposits existed during the Aptian to Albian. The Sligo Formation represents more than 609.6 m (2000 ft) of deposition along this margin. Facies types along the shelf edge were quantitatively delineated by cluster analysis of detailed point-count data from 90 thin sections of whole cores from five wells. In addition, studies of 42.6 m (140 ft) of core slabs and thin sections of well cuttings from four other wells were used to establish a regional depositional model. Along the Sligo shelf edge, three major facies occur: reef or reef rubble (two subfacies), back reef (three subfacies), and lagoonal (two subfacies). Reef facies are dominated by caprinids and also contain solenoporid algae, stromatoporoids, and an assortment of corals. Behind the reef, a spectrum of extensive back-reef deposits interfinger with shallow (< 5 m), lagoonal sediments. Farther behind the shelf-margin reef complex, along the outer shelf, benthic foraminifera, peloids, and ooids were deposited in high-energy shoals, and are interbedded with low-energy lagoonal sediments. The two types of buildups probably existed along the Sligo shelf margin and the equivalent Cupido shelf margin to the south: (1) wave-resistant coral-caprinid-stromatoporid barrier reefs (adjacent to restricted lagoonal facies), and (2) low-lying rudist banks (adjacent to diverse, washed lagoonal facies).

  14. North Sinai-Levant rift-transform continental margin

    SciTech Connect

    Ressetar, R.; Schamel, S.; Travis, C.J.

    1985-01-01

    The passive continental margin of northern Egypt and the Levant coast formed during the Early mesozoic as the relatively small Anatolia plate broke away from northern Africa. The oceanic basin of the eastern Mediterranean and the unusual right-angle bend in the North Sinai-Levant shelf margin are both products of plate separation along a rift-transform fracture system, the south arm of Tethys. The north-south trending Levant transform margin is considerably narrower than the east-west trending rift margin of northern Egypt. Both exhibit similar facies and depositional histories through the mid-Tertiary. Analysis of subsurface data and published reports of the regional stratigraphy point to a three-stage tectonic evolution of this passive margin. The Triassic through mid-Cretaceous was marked by crustal breakup followed by rapid rotational subsidence of the shelf margins about hinge lines located just south and east of the present shorelines. Reef carbonates localized on the shelf edge separated a deep marine basin to the north from a deltaic-shallow marine platform to the south and east. In the Late Cretaceous-Early Tertiary, inversion of earlier formed half-grabens produced broad anticlinal upwarps of the Syrian Arc on the shelf margin that locally influenced facies patterns. The episode of inversion corresponds with the onset of northward subduction of the Africa plate beneath southern Asia. Beginning in the Oligocene and continuing to the present, there has been renewed subsidence of the North Sinai shelf margin beneath thick, outward building clastic wedges. The source of this large volume of sediment is the updomed and erosionally stripped margins of the Suez-Red Sea Rift and the redirected Nile River.

  15. The northern Egyptian continental margin

    NASA Astrophysics Data System (ADS)

    Badawy, Ahmed; Mohamed, Gad; Omar, Khaled; Farid, Walid

    2015-01-01

    Africa displays a variety of continental margin structures, tectonics and sedimentary records. The northern Egyptian continental margin represents the NE portion of the North African passive continental margin. Economically, this region is of great importance as a very rich and productive hydrocarbon zone in Egypt. Moreover, it is characterized by remarkable tectonic setting accompanied by active tectonic processes from the old Tethys to recent Mediterranean. In this article, seismicity of the northern Egyptian continental margin has been re-evaluated for more than 100-years and the source parameters of three recent earthquakes (October 2012, January 2013 and July 2013) have been estimated. Moment tensor inversions of 19th October 2012 and 17th January 2013 earthquakes reveal normal faulting mechanism with strike-slip component having seismic moment of 3.5E16 N m and 4.3E15 N m respectively. The operation of the Egyptian National Seismic Network (ENSN) since the end of 1997 has significantly enhanced the old picture of earthquake activity across northern Egyptian continental margin whereas; the record-ability (annual rate) has changed from 2-events/year to 54-event/year before and after ENSN respectively. The spatial distribution of earthquakes foci indicated that the activity tends to cluster at three zones: Mediterranean Ridge (MR), Nile Cone (NC) and Eratosthenes Seamount (ERS). However, two seismic gaps are reported along Levant Basin (LEV) and Herodotus Basin (HER).

  16. Carbonate comparison of west Florida continental margin with margins of eastern United States

    SciTech Connect

    Doyle, L.J.

    1986-05-01

    Temperate carbonate margins may have as many similarities to clastic margins as to other carbonate systems. An example is the west Florida continental margin north of Florida Bay, a vast area of more than 150,000 km/sup 2/. The facies of this area differ from those of other Holocene carbonates, such as the Bahama Banks, the Great Barrier Reef, and the Caribbean and Pacific bioherms. The west Florida margin is analogous to the predominantly clastic southeastern US in both physiology and sedimentary processes. The shelf facies is a veneer of carbonate sand, primarily molluscan shell fragments, with low sedimentation rates. It is similar to the southeastern US sand veneer with the clastic component removed. Like the US system, the west Florida shelf has a ridge and swale topography replete with sedimentary structures, such as sand waves, with a series of drainage systems incised into its surface at lower stands of sea level. On the outer edge, it is commonly bounded by outcrops with considerable positive relief. The upper slope of the west Florida margin is a calcilutite, a Holocene chalk deposit accumulating at rates of tens of centimeters/1000 years, comparable to the clastic lutite depositional rates of the eastern US continental slope, and two orders of magnitude higher than deep-sea oozes of similar composition. These relatively high rates are probably caused by fines pumped from and across the coarser shelf-sand sheets in both systems.

  17. Interaction of tectonic and depositional processes that control the evolution of the Iberian Gulf of Cadiz margin

    USGS Publications Warehouse

    Maldonado, A.; Nelson, C.H.

    1999-01-01

    This study provides an integrated view of the growth patterns and factors that controlled the evolution of the Gulf of Cadiz continental margin based on studies of the tectonic, sedimentologic and oceanographic history of the area. Seven sedimentary regimes are identified, but there are more extensive descriptions of the late Cenozoic regimes because of the larger data base. The regimes of the Mesozoic passive margin include carbonate platforms, which become mixed calcareous-terrigenous deposits during the Late Cretaceous-early Tertiary. The Oligocene and Early Miocene terrigenous regimes developed, in contrast, over the active and transcurrent margins near the African-Iberian plate boundary. The top of the Gulf of Cadiz olistostrome, emplaced in the Late Miocene, is used as a key horizon to define the 'post-orogenic' depositional regimes. The Late Miocene progradational margin regime is characterized by a large terrigenous sediment supply to the margin and coincides with the closing of the Miocene Atlantic-Mediterranean gateways. The terrigenous drift depositional regime of the Early Pliocene resulted from the occurrence of high eustatic sea level and the characteristics of the Mediterranean outflow currents that developed after the opening of the Strait of Gibraltar. The Late Pliocene and Quaternary regimes are dominated by sequences of deposits related to cycles of high and low sea levels. Deposition of shelf-margin deltas and slope wedges correlate with regressive and low sea level regimes caused by eustasy and subsidence. During the highstand regimes of the Holocene, inner shelf prograding deltas and deep-water sediment drifts were developed under the influence of the Atlantic inflow and Mediterranean outflow currents, respectively. A modern human cultural regime began 2000 years ago with the Roman occupation of Iberia; human cultural effects on sedimentary regimes may have equalled natural factors such as climate change. Interplay of tectonic and

  18. Uncovering the glacial history of the Irish continental shelf (Invited)

    NASA Astrophysics Data System (ADS)

    Dunlop, P.; Benetti, S.; OCofaigh, C.

    2013-12-01

    In 1999 the Irish Government initiated a €32 million survey of its territorial waters known as the Irish National Seabed Survey (INSS). The INSS is amongst the largest marine mapping programmes ever undertaken anywhere in the world and provides high-resolution multibeam, backscatter and seismic data of the seabed around Ireland. These data have been used to provide the first clear evidence for extensive glaciation of the continental shelf west and northwest of Ireland. Streamlined drumlins on the mid to outer shelf record former offshore-directed ice flow towards the shelf edge and show that the ice sheet was grounded in a zone of confluence where ice flowing onto the shelf from northwest Ireland merged with ice flowing across the Malin Shelf from southwest Scotland. The major glacial features on the shelf are well developed nested arcuate moraine systems that mark the position of the ice sheet margin and confirm that the former British Irish Ice Sheet was grounded as far as the shelf edge around 100 km offshore of west Donegal at the last glacial maximum. Distal to the moraines, on the outermost shelf, prominent zones of iceberg plough marks give way to the Barra/Donegal fan and a well developed system of gullies and canyons which incise the continental slope. Since 2008 several scientific cruises have retrieved cores from the shelf and slope to help build a more detailed understanding of glacial events in this region. This presentation will provide an overview of the glacial history of the Irish shelf and will discuss ongoing research programmes that are building on the initial research findings to produce a better understanding of the nature and timing of ice sheet events in this region.

  19. Larsen Ice Shelf, Antarctica

    NASA Technical Reports Server (NTRS)

    2002-01-01

    Warmer surface temperatures over just a few months in the Antarctic can splinter an ice shelf and prime it for a major collapse, NASA and university scientists report in the latest issue of the Journal of Glaciology. Using satellite images of tell-tale melt water on the ice surface and a sophisticated computer simulation of the motions and forces within an ice shelf, the scientists demonstrated that added pressure from surface water filling crevasses can crack the ice entirely through. The process can be expected to become more widespread if Antarctic summer temperatures increase. This true-color image from Landsat 7, acquired on February 21, 2000, shows pools of melt water on the surface of the Larsen Ice Shelf, and drifting icebergs that have split from the shelf. The upper image is an overview of the shelf's edge, while the lower image is displayed at full resolution of 30 meters (98 feet) per pixel. The labeled pond in the lower image measures roughly 1.6 by 1.6 km (1.0 x 1.0 miles). Full text of Press Release More Images and Animations Image courtesy Landsat 7 Science Team and NASA GSFC

  20. Ross ice shelf vibrations

    NASA Astrophysics Data System (ADS)

    Bromirski, P. D.; Diez, A.; Gerstoft, P.; Stephen, R. A.; Bolmer, T.; Wiens, D. A.; Aster, R. C.; Nyblade, A.

    2015-09-01

    Broadband seismic stations were deployed across the Ross Ice Shelf (RIS) in November 2014 to study ocean gravity wave-induced vibrations. Initial data from three stations 100 km from the RIS front and within 10 km of each other show both dispersed infragravity (IG) wave and ocean swell-generated signals resulting from waves that originate in the North Pacific. Spectral levels from 0.001 to 10 Hz have the highest accelerations in the IG band (0.0025-0.03 Hz). Polarization analyses indicate complex frequency-dependent particle motions, with energy in several frequency bands having distinctly different propagation characteristics. The dominant IG band signals exhibit predominantly horizontal propagation from the north. Particle motion analyses indicate retrograde elliptical particle motions in the IG band, consistent with these signals propagating as Rayleigh-Lamb (flexural) waves in the ice shelf/water cavity system that are excited by ocean wave interactions nearer the shelf front.

  1. African American Women in Higher Education

    ERIC Educational Resources Information Center

    Zamani, Eboni M.

    2003-01-01

    African American women hold a unique position as members of two groups that have been treated in a peripheral manner by postsecondary education (Moses, 1989). Membership in both marginalized groups often makes African American women invisible in colleges and universities. Given the complex intersection of race and gender, more attention should be…

  2. Larsen B Ice Shelf

    Atmospheric Science Data Center

    2013-04-16

    ... ice shelf and the rough crevasses of glaciers appear orange. In contrast to the spectral composite, which provides information on ... surfaces appear brighter on their illuminated faces, the orange color in the multi-angle composite suggests a macroscopically rough ice ...

  3. High pressure flexible pipes for marginal oilfield development

    SciTech Connect

    Griffiths, A.D.

    1985-01-01

    The major fields in the North Sea are currently in production and the average size of prospective development fields will decrease with time. The smaller or 'marginal' field development technology will represent a major factor in the future of oil and gas development in the North West European Continental Shelf area. Worldwide the number of exploitable marginal discoveries to date is believed to exceed 800. In the North West European Continental Shelf it is forecast by Hoare Govett that more than 45 marginal developments will be underway before 1990 in this area. These will be chosen from a known potential of 200 discoveries. The sub-sea completion of marginal oilfield production facilities is simplified by the use of high pressure flexible pipes. Short pipes are used to ''Tie-In'' rigid pipelines to sub-sea structures and continuous length pipes are used increasingly as a replacement for rigid, steel, pipelines.

  4. Upper Jurassic and Lower Cretaceous facies relationships in a passive margin basin, western North Atlantic

    SciTech Connect

    Prather, B.E.

    1988-02-01

    Correlation of facies from hydrocarbon-bearing continental and transitional marine sandstones to time-equivalent high-energy shelf-margin carbonates provide insight into hydrocarbon habitats of the Baltimore Canyon basin. These facies occur within a thick (> 10,000 ft) prograded wedge of shelf sediments in this passive margin basin. Wells drilled to test structural closures in shallow-water (< 600 ft) areas of Baltimore Canyon penetrate clastic facies which are time-equivalent to the downdip carbonate facies tested in deep-water wells. Numerous hydrocarbon shows, including a noncommercial gas and gas-condensate accumulation, occur with sandstone units that were deposited in prograding continental/fluvial and transitional marine environments located updip of the Oxfordian/Kimmeridgian carbonate shelf edge. The continental and transitional facies are overlain by a fine-grained deltaic complex which forms a regionally extensive top seal unit. The deltaic complex was deposited during aggradation of the Kimmeridgian through Berriasian shelf-margin carbonates penetrated by the deep-water wells. Deep-water wells (> 5000 ft) drilled off the continental shelf edge to test large structural closures along the downdip termination of the Upper Jurassic/Lower Cretaceous carbonate shelf edge encountered no significant hydrocarbon shows. Reservoir rocks in these wells consist of (1) oolite grainstone which was deposited within a shoal-water complex located at the Aptian shelf edge, and (2) coral-stromatoporoid grainstone and boundstone which formed an aggraded shelf-margin complex located at the Kimmeridgian through Berriasian shelf edge. Structural closures with reservoir and top seals are present in both updip and downdip trends. The absence of hydrocarbon shows in downdip carbonate reservoirs suggests a lack of source rocks available to charge objectives at the shelf margin.

  5. Holocene sediment dynamics on a cool-water carbonate shelf: Otway, southeastern Australia

    SciTech Connect

    Boreen, T.D.; James, N.P. )

    1993-07-01

    The Otway Shelf is covered by cool waters and veneered by bryozoan-dominated carbonate sediments. Radiocarbon dating and stratigraphy of shelf vibracores and slope gravity cores document late Pleistocene/Holocene deposition. Shelf sediments of the late Pleistocene high-stand are rare, either never having been deposited or having been removed during the following sea-level fall. During the subsequent lowstand the shelf was exposed, facies shifted basinward, and beach/dune complexes were constructed near the shelf edge. The deep shelf was characterized by nondeposition and hardground formation, and the shelf margin became locally erosional. Upper-slope bryozoan/sponge assemblages continued to grow actively, and lower-slope foraminifera and nannofossil ooze was increasingly enriched in hemipelagic terrigenous mud swept off the wide shelf. Coarse shelf debris and lowstand dune sands were erosively reworked and transported onto the upper slope and redistributed to deep-slope aprons during early transgression. The late Quaternary shelf record resembles that of flat-topped, warm-water platforms with Holocene sediment overlying Pleistocene/Tertiary limestone, but for different reasons. The slow growth potential, uniform profile of sediment production and distribution, and inability of constituent organisms to construct rigid frameworks favor maintenance of a shallow ramp profile and makes the cool-water carbonate system an excellent modern analog for interpretation of many ancient ramp successions.

  6. Heat flow of the Norwegian continental shelf

    NASA Astrophysics Data System (ADS)

    Pascal, Christophe

    2015-04-01

    Terrestrial heat flow influences a large collection of geological processes. Its determination is a requirement to assess the economic potential of deep sedimentary basins. Published heat flow calculations from e.g. major oil provinces are however seldom. Robust heat flow determinations in drillholes require logging of undisturbed temperatures and intensive sampling of core material for petrophysical measurements. Temperature logging in exploration drillholes is traditionally conducted during drill breaks or shortly after drilling, resulting in temperatures severely disturbed by mud circulation and coring is restricted to selected intervals. Alternatively, test temperatures, information from electric logs and lithological descriptions of drill cuttings can be used to overcome these limitations. The present contribution introduces new heat flow determinations based on 63 exploration drillholes from the Norwegian North Sea, the Mid Norway Margin and the Barents Shelf. Our analyses are based on released DST temperatures, precise lithological descriptions of drill cuttings, previously measured rock matrix thermal conductivities and established porosity laws. Our results suggest median heat flow values of 64 mW/m2, 65 mW/m2 and 72 mW/m2 for the North Sea, the Mid Norway Margin (mainly the Trøndelag Platform) and the SW Barents Shelf respectively. The Barents Shelf shows significantly high heat flow, suggesting lateral transfer of heat from the mantle of the adjacent young ocean. In detail, heat flow increases by ~ 10 mW/m2 from the southern Norwegian North Sea towards the Mid Norway Margin. This result appears to be in very good agreement with seismic tomographic studies suggesting northward thinning of the underlying mantle lithosphere. Our results together with published marine heat flow data from the Mid Norway Margin suggest a gradual decrease in heat flow levels from both the North Sea and the Trøndelag Platform towards the centres of the deep Møre and V

  7. The Continental Margins of the Western North Atlantic.

    ERIC Educational Resources Information Center

    Schlee, John S.; And Others

    1979-01-01

    Presents an interpretation of geological and geophysical data, which provides a summary of the structural and sedimentary history of the United States Atlantic Margin. The importance of an understanding of the development of the outer continental shelf to future hydrocarbon exploration is detailed. (BT)

  8. Sequence stratigraphy of an Oligocene carbonate shelf, Central Kalimantan, Indonesia

    SciTech Connect

    Saller, A.; Armin, R. ); Ichram, L.O. ); Glenn-Sullivan, C. )

    1991-03-01

    Interpretations of Oligocene shelfal limestones from Central Kalimantan, Indonesia, suggest caution in predicting sea-level lowstands from seismic reflector patterns or published sea-level curves. Three major depositional sequences, each 200-400 m thick, were delineated in outcrops and seismic lines: late Eocene to early Oligocene (34-38 Ma), middle Oligocene (29.7-32 Ma), and early late Oligocene (28-29.7 Ma). The lowest sequence is mainly shale with tin sandstones and limestones (large-foram wackestone). The middle and upper sequences are carbonate with transgressive systems tracts (TSTs) overlain by highstand systems tracts (HSTs). TSTs contain large-foram wackestone-packstones and coral wackestone-packstones. HSTs are characterized by (1) shale and carbonate debris flows deposited on the lower slope, (2) argillaceous large-foram wackestones on the upper slope, (3) discontinuous coral wackestones and boundstones on the shelf margin, (4) bioclastic packstones and grainstones on backreef flats and shelf-margin shoals, and (5) branching-coral and foraminiferal wackestones in the lagoon. Bases of sequences are characterized by transgression and onlap. Deepending and/or drowning of the carbonate shelf occurred at the top of the middle and upper sequences. Basinal strata that apparently onlap the middle and upper carbonate shelf margins might be misinterpreted as lowstand deposits, although regional studies indicate they are prodelta sediments baselapping against the shelf. Shallowing the subaerial exposure of the carbonates might be expected during the large mid-Oligocene (29.5-30 Ma) sea-level drop of Haq et al. (1987), instead of the observed deepening and local drowning.

  9. Comparative evolution of Pennsylvanian platform margins in Oklahoma and north-central Texas

    SciTech Connect

    Cleaves, A.W.; Puckette, J.O. )

    1991-08-01

    Pennsylvanian evolution of the Midland basin's eastern shelf and the northern shelves of the Anadarko and Arkoma basins demonstrates a strongly contrasting pattern with regard to the facies composition and stability of the shelf margin. For the Midland basin a carbonate ramp system developed adjacent to the Eastern shelf during the early Desmoinesian but received no coarse-grained clastic sediment until after the central Fort Worth basin was completely filled by Ouachita orogenic debris in the late Desmoinesian. At that time, a distinct north-south hingeline formed between the shelf and incipient Midland basin that allowed for subsequent vertical accretion of a Missourian-age double bank system. Due to the absence of active deltaic depocenters across the southern two-thirds of the shelf, the Missourian shelf margin did not prograde basinward nor did a submarine fan system develop adjacent to this reciprocal bank complex. Later, during the Virgilian, a single shelf-edge bank and submarine fan complex prograded the shelf edge westward. The shelf edges for the Anadarko and Arkoma basins demonstrate a significantly different pattern. Only during the late Desmoinesian (Marmaton Group) did a shelf-edge bank develop in association with shelf-slope reciprocal sedimentation. For the Anadarko basin, widespread submarine fans, fed from a northeasterly cratonic source, are first seen with Red Fork deposition. Post-Tonkawa cyclic sedimentation prograded the shelf edge southward and gave rise to a more carbonate-dominated shelf sequence. In virtually all instances the regressive submarine fan units indicate eustatic lowstands of sea level.

  10. Marginalization and health geomatics.

    PubMed

    Alexander, Gregory L; Kinman, Edward L; Miller, Louise C; Patrick, Timothy B

    2003-01-01

    Marginalized groups have been defined as groups that have been peripheralized from the center of society. Increasing nursing knowledge of marginalized groups and the dynamics of population diversity will enable nurses to better recognize shifting health patterns, plan for utilization of health services, and determine ethnic and cultural differences that exist in marginalized populations. The authors of this article review theoretical models responsible for defining the concept marginalization, describe geographical information systems as a recommended tool to evaluate marginalized groups, and provide a case study utilizing tools and maps as a means of assessing marginal situations. PMID:14643736

  11. Geology of continental shelf, Onslow Bay, North Carolina, as revealed by submarine outcrops ( USA).

    USGS Publications Warehouse

    Blackwelder, B. W.; Macintyre, Ian G.; Pilkey, O.H.

    1982-01-01

    Lithologic and stratigraphic data from rocks dredged from the continental shelf off Onslow Bay, North Carolina, provide surface control for seismic studies of the southeastern United States continental margin and help to explain the distribution of potentially economic phosphate-rich sediments on this shelf. Outcropping Miocene rocks in this area indicate that the region has long been a positive geologic feature and has received relatively little Pliocene and Pleistocene sedimentation. -from Authors

  12. Permian of Norwegian-Greenland sea margins: future exploration target

    SciTech Connect

    Surlyk, F.; Hurst, J.M.; Piasecki, S.; Rolle, F.; Stemmerik, L.; Thomsen, E.; Wrang, P.

    1984-09-01

    Oil and gas exploration in the northern North Sea and the southern Norwegian shelf has mainy been concentrated on Jurassic and younger reservoirs with Late Jurassic black shale source rocks. New onshore investigations in Jameson Land, central East Greenland, suggest that the Permian of the Norwegian-Greenland Sea margins contains relatively thick sequences of potential oil source rocks interbedded with carbonate reefs. The East Greenland, Upper Permian marine basin is exposed over a length of 400 km (250 mi) from Jameson Land in the south to Wollaston Forland in the north, parallel with the continental margin. The Upper Permian black shale is relatively thick, widely distributed, has a high organic carbon content, and a favorable kerogen type. Consequently, the possibilities for a Permian play in the northern part of the Norwegian shelf and along parts of the Norwegian-Greenland Sea margins are worth evaluating.

  13. Cross-shelf subtidal variability in San Pedro Bay during summer, 2001

    USGS Publications Warehouse

    Hamilton, P.; Noble, M.A.; Largier, J.; Rosenfeld, L.K.; Robertson, G.

    2006-01-01

    A total of 16 moorings were deployed across the San Pedro shelf, one of the two wider embayments in the Southern California Bight, from near the surfzone to the upper-slope. On the middle and outer shelf in the summer of 2001, the currents flowed strongly equatorward at the surface and had large vertical shears through the well-stratified water column. This equatorward flow differs from predominantly poleward flow found in previous studies of the coastal margin further west. In deeper water, near the shelf break, the shears were such that near-bottom flows were poleward and incorporated into the upper parts of the Southern California Undercurrent over the slope. Mid-shelf current fluctuations, with periods of 10-25 days, along with upwelling over the shelf, were not related to local winds, but were significantly correlated with the large-scale alongshore pressure gradient. Shorter period (???7-10 days) inner shelf alongshore currents, however, were significantly correlated with the alongshore wind at the shelf break. A CEOF analysis gives two significant modes, with the first mode dominant over the outer and middle shelf. The wind-forced second mode connects the inner shelf to the poleward undercurrent over the slope such that increases in the poleward flow over the slope are correlated with increases in the equatorward current inshore of the 15 m isobath.

  14. Eel River margin source-to-sink sediment budgets: revisited

    USGS Publications Warehouse

    Warrick, Jonathan A.

    2014-01-01

    The Eel River coastal margin has been used as a representative source-to-sink sediment dispersal system owing to its steep, high-sediment yield river and the formation of sedimentary strata on its continental shelf. One finding of previous studies is that the adjacent continental shelf retains only ~25% of the Eel River fine-grained sediment (less than 63 μm) discharged over time scales of both individual floods and the 20th century, thus suggesting that the Eel shelf trapping-efficiency is uniquely lower than other similar systems. Here I provide data and analyses showing that sediment discharge relationships in the Eel River have varied strongly with time and include substantial decreases in suspended-sediment concentrations during the latter 20th century. Including these trends in margin-wide sediment budgets, I show that previous Eel River sediment discharge rates were overestimated by a factor of two. Thus, revised sediment budgets shown here reveal that the Eel shelf retained ~50% of the discharged river fine-grained suspended sediment during intensively sampled events of 1995–97 and over the 20th century. In light of this, hypotheses about high rates of sediment export away from the primary shelf depocenter should be reevaluated.

  15. Heat Flow of the Norwegian Continental Shelf

    NASA Astrophysics Data System (ADS)

    Pascal, C.

    2015-12-01

    Terrestrial heat flow determination is of prime interest for oil industry because it impacts directly maturation histories and economic potential of oil fields. Published systematic heat flow determinations from major oil provinces are however seldom. Robust heat flow determinations in drillholes require logging of undisturbed temperatures and intensive sampling of core material for petrophysical measurements. Temperature logging in exploration drillholes is traditionally conducted during drill breaks or shortly after drilling, resulting in temperatures severely disturbed by mud circulation and coring is restricted to selected intervals. Alternatively, test temperatures, information from electric logs and lithological descriptions of drill cuttings can be used to overcome these limitations. The present contribution introduces new heat flow determinations based on 63 exploration drillholes from the Norwegian North Sea, the Mid Norway Margin and the Barents Shelf. Our analyses are based on released DST temperatures, precise lithological descriptions of drill cuttings, previously measured rock matrix thermal conductivities and established porosity laws. For the sake of comparison, we carefully review previous heat flow studies carried out both onshore and offshore Norway. Our results suggest median heat flow values of 64 mW/m2, 65 mW/m2 and 72 mW/m2 for the North Sea, the Mid Norway Margin (mainly the Trøndelag Platform) and the SW Barents Shelf respectively. In detail, heat flow increases by ~ 10 mW/m2 from the southern Norwegian North Sea towards the Mid Norway Margin. This result appears to be in very good agreement with seismic tomographic studies suggesting northward thinning of the underlying mantle lithosphere. Our results together with published marine heat flow data from the Mid Norway Margin suggest a gradual decrease in heat flow levels from both the North Sea and the Trøndelag Platform towards the centres of the deep Møre and Vøring basins. This latter

  16. A geophysical study of the northern Svalbard continental margin

    NASA Astrophysics Data System (ADS)

    Geissler, Wolfram H.; Jokat, Wilfried

    2004-07-01

    In the summer of 1999, the first systematic seismic profiles were acquired across the northern Svalbard continental margin east of 15°E. Approximately 1470 km of multi-channel seismic reflection data as well as sonobuoy wide-angle data were collected up to 82°N. With few exceptions the signals imaged the whole sedimentary cover down to the acoustic basement. The uppermost sedimentary deposits of the inner shelf yield P-wave velocities of 2 km s-1 and higher, indicating erosion and compaction due to a former ice load. The inner shelf east of Hinlopen Strait has only a thin veneer of over-consolidated sediments above the acoustic basement. Beneath the outer shelf, up to 3.5 km of sedimentary deposits cover the down-faulted acoustic basement. The continental slope is heavily eroded due to bottom current activity and slumping. At about 30°E the morphology of the continental slope has a smooth appearance. Shelf progradation only in the vicinity of glacial troughs crossing the shelf (associated with submarine fans) indicates main sediment transport by ice streams during former glacial periods. The maximum sedimentary thickness in the Sophia Basin is more than 9 km, and the Nansen Basin has a sediment thickness of 4.5 km close to the margin. Gravity modelling along the seismic profiles was performed to constrain the position of the continent-ocean transition. Existing sedimentary thickness and structural maps were extended over the area investigated. The new data provide no evidence for the presence of former extensive subaerial volcanic sequences (seaward-dipping reflectors), which would have been emplaced during the break-up along the margin. Thus, we consider this part of the margin as non-volcanic.

  17. Systematic mapping of the Spanish continental margin

    NASA Astrophysics Data System (ADS)

    Acosta, Juan; Muñoz, Araceli; Uchupi, Elazar

    2012-07-01

    For economic, environmental, recreational, military, and political reasons it is critical for coastal states to have up-to-date information on their marine margins. Spain began to acquire such data 17 years ago. From 1995 to the present, the Spanish Oceanographic Institute (IEO), a research organization of the state, has carried out a systematic geological and geophysical study of the Spanish margins. Among these projects are (1) the hydrographic and oceanographic study of the Spanish Exclusive Economic Zone (EEZ) that was implemented by the Navy Hydrographic Institute (IHM); (2) the Espace Project, a study of the Spanish continental shelf; and (3) the Capesme Project, which created fisheries maps of the Mediterranean Sea. The latter two projects were carried out in collaboration with the Secretariat General of the Sea (SGM).

  18. Investigating the Flow Dynamics at Ice Shelf Calving Fronts

    NASA Astrophysics Data System (ADS)

    Wearing, Martin; Hindmarsh, Richard; Worster, Grae

    2015-04-01

    Ice-shelf calving-rates and the buttressing ice shelves provide to grounded ice are both difficult to model and quantify. An increased understanding of the mechanics of this process is imperative in determining the dynamics of marine ice sheets and consequently predicting their future extent, thickness and discharge. Alley et al. (2008) proposed an empirically derived calving law, relating the calving rate to the strain rate at the calving front. However, Hindmarsh (2012) showed that a similar relationship could be deduced by considering the viscous flow of the ice shelf. We investigate the relationship between the ice shelf flow field and the strain rate field in the area close to the calving front. Analysis is undertaken of ice surface velocity data for a range of Antarctic ice shelves (data from Rignot et al., 2011) and an inferred strain rate field produced from that data. These geophysical results are compared with a simple mathematical model for laterally confined ice shelf flow. Correlations are calculated between the same variables as Alley et al. but using a new and larger data compilation, which gives a greater degree of scatter. Good agreement is observed between the expected theoretical scaling and geophysical data for the flow of ice near the calving front in the case of laterally confined ice shelves. This lateral confinement ensures flow is aligned in the along-shelf direction and resistance to flow is provided by near stationary ice in the grounded margins. In other cases, the velocity is greater than predicted, which we attribute on a case-by-case basis to marginal weakening or the presence of ice tongues. We develop statistical methodologies for identifying these outliers.

  19. Larval Transport on the Atlantic Continental Shelf of North America: a Review

    NASA Astrophysics Data System (ADS)

    Epifanio, C. E.; Garvine, R. W.

    2001-01-01

    This review considers transport of larval fish and crustaceans on the continental shelf. Previous reviews have contained only limited treatments of the physical processes involved. The present paper provides a physical background that is considerably more comprehensive. It includes a discussion of three principal forcing agents: (1) wind stress; (2) tides propagating from the deep ocean; and (3) differences in density associated with the buoyant outflow of estuaries, surface heat flux, or the interaction of coastal and oceanic water masses at the seaward margin of the shelf. The authors discuss the effects of these forcing agents on transport of larvae in the Middle Atlantic and South Atlantic Bights along the east coast of North America. The discussion concentrates on three species (blue crab, menhaden, bluefish) that have been the subject of a very recent multi-disciplinary study. Taken as a whole, the reproductive activities of these three species span the entire year and utilize the entire shelf, from the most seaward margin to the estuarine nursery. The blue crab is representative of species affected by physical processes occurring during summer and early autumn on the inner and mid-shelf. Menhaden are impacted by processes occurring in winter on the outer and mid-shelf. Bluefish are influenced primarily by processes occurring during early spring at the outer shelf margin near the western boundary current. The authors conclude that alongshore wind stress and density differences, i.e. buoyancy-driven flow, are the primary agents of larval transport in the region. Circulation associated with the western boundary current is only important at the shelf margin and tidally driven processes are generally inconsequential.

  20. North African geology: exploration matrix for potential major hydrocarbon discoveries

    SciTech Connect

    Kanes, W.H.; O'Connor, T.E.

    1985-02-01

    Based on results and models presented previously, it is possible to consider an exploration matrix that examines the 5 basic exploration parameters: source, reservoir, timing, structure, and seal. This matrix indicates that even those basins that have had marginal exploration successes, including the Paleozoic megabasin and downfaulted Triassic grabens of Morocco, the Cyrenaican platform of Libya, and the Tunisia-Sicily shelf, have untested plays. The exploration matrix also suggests these high-risk areas could change significantly, if one of the 5 basic matrix parameters is upgraded or if adjustments in political or financial risk are made. The Sirte basin and the Gulf of Suez, 2 of the more intensely explored areas, also present attractive matrix prospects, particularly with deeper Nubian beds or with the very shallow Tertiary sections. The Ghadames basin of Libya and Tunisia shows some potential, but its evaluation responds strongly to stratigraphic and external nongeologic matrix variations based on degree of risk exposure to be assumed. Of greatest risk in the matrix are the very deep Moroccan Paleozoic clastic plays and the Jurassic of Sinai. However, recent discoveries may upgrade these untested frontier areas. Based on the matrix generated by the data presented at a North African Petroleum Geology symposium, significant hydrocarbon accumulations are yet to be found. The remaining questions are: where in the matrix does each individual company wish to place its exploration capital and how much should be the risk exposure.

  1. Export of terrigenous organic carbon along submarine canyons driven by dense shelf water cascading

    NASA Astrophysics Data System (ADS)

    Tesi, T.; Puig, P.; Goni, M.; Canals, M.; Langone, L.; Palanques, A.; Miserocchi, S.; Heussner, S.; Trincardi, F.; Calafat, A.; Turchetto, M.; Fabres, J.; Durrieu de Madron, X.; Boldrin, A.

    2008-12-01

    At current highstand in sea level, shelves are considered major sites of terrigenous organic carbon (OCterr) accumulation with relatively little connectivity to the ocean interior. In recent years, the process of dense water cascading from the continental shelf, which occurs in numerous places around the world, has been suggested as carrier for OCterr to the deep ocean. The land-locked Mediterranean Sea is characterized by intense and recurrent cascades of dense shelf water. In winter, cold and dry winds cause the formation of dense water over the shelf that may overflow it and travel down to the outer margin and basin. Moored instruments were deployed in the canyons of the Gulf of Lion (France-Spain) and the Adriatic Sea (Italy) to intercept particulate material escaping the shelf and to investigate hydrodynamic and physical properties of the water column. Surface sediments along the shelves were also sampled to evaluate their contribution to the particle fluxes. The relative fractions of autochthonous and advected OC in sediment trap samples were investigated using biogeochemical proxies including alkaline CuO oxidation products (lignin phenols, dicarboxylic acids, and fatty acids), radiocarbon measurements (Ä14C), and elemental and carbon stable isotope (ä13C) compositions. Lignin-derived CuO products were a powerful biogeochemical tool that allowed us to identify the provenance of the material from the continental margin and to assess the amount of OCterr transferred across the slope in both Mediterranean regions. The results indicate that the composition of OC escaping the shelf through submarine canyons depends on the geomorphological setting. At the present sea level stage, cascading on a broad shelf limits the transport of OCterr, promoting instead the down-slope export of material accumulated in the mid- and outer-shelf. In contrast, cascade events on narrow shelves lead to the efficient export of OCterr from shallower regions of the margin along with

  2. Numerical modelling and data assimilation of the Larsen B ice shelf, Antarctic Peninsula.

    PubMed

    Vieli, Andreas; Payne, Antony J; Du, Zhijun; Shepherd, Andrew

    2006-07-15

    In this study, the flow and rheology of pre-collapse Larsen B ice shelf are investigated by using a combination of flow modelling and data assimilation. Observed shelf velocities from satellite interferometry are used to constrain an ice shelf model by using a data assimilation technique based on the control method. In particular, the ice rheology field and the velocities at the inland shelf boundary are simultaneously optimized to get a modelled flow and stress field that is consistent with the observed flow. The application to the Larsen B ice shelf shows that a strong weakening of the ice in the shear zones, mostly along the margins, is necessary to fit the observed shelf flow. This pattern of bands with weak ice is a very robust feature of the inversion, whereas the ice rheology within the main shelf body is found to be not well constrained. This suggests that these weak zones play a major role in the control of the flow of the Larsen B ice shelf and may be the key to understanding the observed pre-collapse thinning and acceleration of Larsen B. Regarding the sensitivity of the stress field to rheology, the consistency of the model with the observed flow seems crucial for any further analysis such as the application of fracture mechanics or perturbation model experiments. PMID:16782611

  3. Evolving Toward the Next Antarctic Ice Shelf Disintegration: Recent Ice Velocity, Climate, and Ocean Observations of the Larsen B Ice Shelf Remnants

    NASA Astrophysics Data System (ADS)

    Scambos, T. A.; Shuman, C. A.; Truffer, M.; Pettit, E. C.; Huber, B. A.; Haran, T. M.; Ross, R.; Domack, E. W.

    2013-12-01

    Ice shelf / ice tongue disintegrations and break-ups have a major effect on glacier mass balance, and nowhere has this been more evident than in the northern sections of the Larsen Ice Shelf in the Antarctic Peninsula. Ice flux in this region surged 2- to 6-fold after the 1995 and 2002 ice shelf disintegration events, driven by a group of processes based on the presence of extensive surface melt lakes. However, precursor changes in the ice shelves beginning more than a decade before the events have been identified. A new assessment of these provides insight on the earliest causes of ice shelf change. Among the precursor changes are an increase in meltwater lake extent, structural changes in the ice shelf shear margins, grounding line changes, and pre-breakup acceleration of the ice shelves and feeder glaciers. In the aftermath of the 2002 disintegration of the Larsen B, the two large remnant ice shelves at Seal Nunataks (~400 km2) and Scar Inlet (~2400 km2) have also evolved in these ways. These changes have been measured by a combination of in situ automated observation systems (AMIGOS: see Scambos et al., 2013, J. Glaciol.) and remote sensing as part of the Larsen Ice Shelf System, Antarctica (LARISSA) NSF project and NASA Cryosphere Program funding. Ice flow speed on the central Scar Inlet ice shelf has increased 60% between 2002 and 2012 (425 to 675 m/yr), and by 20% (540 to 660 m/yr) just above the grounding line of Flask Glacier, a tributary. Elevation change data from ICESat altimetry and ASTER stereo images show evidence of grounding line movement for Flask between 2003 and 2008, and for Crane Glacier prior to the 2002 break-up. In late 2002, and again in late 2012, major new rifts have formed on the southern portion of the Scar Inlet shelf, and the northwestern shear zone has rapidly evolved. The ice speed increase and the new rifts are inferred to be due to significant structural changes in the ice shelf shear margin on its northern side (concentration of

  4. North African petroleum geology: regional structure and stratigraphic overview of a hydrocarbon-rich cratonic area

    SciTech Connect

    O'Connor, T.E.; Kanes, W.H.

    1985-02-01

    North Africa, including Sinai, contains some of the most important hydrocarbon-producing basins in the world. The North African Symposium is devoted to examining the exploration potential of the North African margin in light of the most recent and promising exploration discoveries. The geologic variety of the region is extraordinary and can challenge any exploration philosophy. Of primary interest are the Sirte basin of Libya, which has produced several billion barrels of oil, and the Gulf of Suez, a narrow, evaporite-capped trough with five fields that will produce more than 5 billion bbl. Both are extensional basins with minimal lateral movement and with good source rocks in direct proximity to reservoirs. Structural models of these basins give firm leads for future exploration. More difficult to evaluate are the Tethyan realm basins of the northern Sinai, and the Western Desert of Egypt, the Cyrenaican Platform of Libya, and the Tunisia-Sicily shelf area, where there are only limited subsurface data. These basins are extensional in origin also, but have been influenced by lateral tectonics. Favorable reservoirs exist, but source rocks have been a problem locally. Structural models with strong stratigraphic response offer several favorable play concepts. The Paleozoic Ghadames basin in Libya, Tunisia, and Algeria has the least complex structural history, and production appears to be limited to small structures. A series of stratigraphic models indicates additional areas with exploration potential. The Paleozoic megabasin of Morocco, with its downfaulted Triassic grabens, remains an untested but attractive area.

  5. The geodynamics of the Levant margin

    NASA Astrophysics Data System (ADS)

    Ben-Avraham, Z.

    2006-12-01

    The Levant continental margin, offshore Israel, Lebanon and Syria, is usually defined as a passive margin that was formed through rifting processes. During the formation two major continental fragments are assumed to separate from the northern edge of the Afro-Arabian plate to form the Levant basin: the Tauride and Eratosthenes blocks. Today an oceanic crust and, in places, a very thin continental crust are present between the Levant margin and Eratosthenes seamount. The margin can be divided into two distinct provinces that are separated by the Carmel Structure, which extends from seawards to the northwest across the continental shelf and slope. The preservation of segmentation, both in the shallow and in the deep structure, insinuates that the two segments were formed through different continental breakup processes, which continue to dictate the style of sediment accumulation. The nature and development of the continental margin offshore Israel were the subject of numerous studies, which suggest that the southern Levant segment (south of the Carmel Structure) was formed through continental rifting processes. In contrast, the northern segment, from the Carmel structure northwards and offshore southern Lebanon, was hardly studied before. Recent studies however indicate that the northern segment shows a strong similarity to classical transform margins in the world. In view of the new classification of the northern Levant margin a modified scenario is suggested for: (a) the initial stages in which the Levant margin was formed; and (b) the present day structural differences between the two segments of the margin. At present, the northern Levant continental margin is being reactivated by transpressional faulting of the marine continuation of the Carmel fault which bends northward at the base of the continental slope due to the rheological discontinuity in this region. This fault system coincides with the sharp continental-oceanic crustal transition, and acts as an

  6. Mesozoic and Cenozoic structural trends under southern Bering Sea Shelf

    SciTech Connect

    Marlow, M.S.; Cooper, A.K.

    1980-12-01

    Mesozoic rocks exposed near the tip of the Alaska Peninsula form an antiformal structure that flanks the southern side of Bristol Bay basin and that can be traced with geophysical data about 700 km offshore to the vicinity of the Pribilof Islands. Upper Jurassic sandstone and Upper Cretaceous mudstone dredged from the top and flanks of this structure near the islands confirm that Mesozoic rocks extend from the Alaska Peninsula to the Bering sea margin. The southern part of the Bering Sea Shelf is underlain by several large structural basins. These filled basins encompass an offshore area of about 31,000 sq km. Reflection profiles show that the surface of the offshore antiformal structures is an angular unconformity overlain by Cenozoic beds. The downdip trace of the unconformity in Bristol Bay basin is underlain by reflectors paralleling the contact, a relation suggesting that the basin and perhaps other shelf basins may be underlain by ancient Mesozoic depocenters. The bulk of the thick sections in these basins is, however, thought to be mainly Cenozoic in age. Strata in the basins are cut by high-angle growth faults. The faults commonly offset the seafloor, which implies that basin subsidence and filling continue to the present. Shallow-water diatomaceous mudstone of Eocene and Oligocene age dredged from the continental slope near the Pribilof Islands indicates that collapse of the margin and outer shelf basins began by at least early Tertiary time. In Mesozoic time, the Bering margin between Siberia and the Alaska Peninsula (Beringian margin) may have been a zone of either oblique underthrusting or transform motion between the North American and Pacific lithosphere (Kula plate.). This motion may have rifted the edge of the North American plate, resulting in the formation of a series of elongate basins and ridges paralleling the plate edge.

  7. Natural constraints on exploring Antarctica's continental margin, existing geophysical and geological data basis, and proposed drilling program

    SciTech Connect

    Anderson, J.B.

    1987-05-01

    There have been a number of multichannel seismic reflection and seismic refraction surveys of the Antarctic continental shelf. While glacial erosion has left acoustic basement exposed on portions of the inner shelf, thick sedimentary sequences occur on the passive margin of east Antarctica. The thickness and age of these strata vary due to different breakup histories of the margin. Several sedimentary basins have been identified. Most are rift basins formed during the early stages of Antarctica's separation from other Gondwana continents and plateaus. The west Antarctic continental shelf is extensive, being approximately twice the size of the Gulf of Mexico shelf. It has been poorly surveyed to date, owing mainly to its perennial sea ice cover. Gradual subduction of the spreading center from south to north along the margin resulted in old active margin sequences being buried beneath passive margin sequences. The latter should increase in thickness from north to south along the margin although no data bear this out. Hydrocarbon potential on the northern portion of the west Antarctic margin is considered low due to a probable lack of reservoir rocks. Establishment of ice sheets on Antarctica caused destruction of land vegetation and greatly restricted siliciclastic sand-producing environments. So only sedimentary basins which contain pre-early Miocene deposits have good hydrocarbon prospectivity. The Antarctic continental shelf is the deepest in the world, averaging 500 m and in places being more than a kilometer deep. The shelf has been left rugged by glacial erosion and is therefore prone to sediment mass movement. Widespread sediment gravity flow deposits attest to this. The shelf is covered with sea ice most of the year and in a few areas throughout the year. Icebergs, drift freely in the deep waters of the shelf; drift speeds of 1 to 2.5 km/year are not uncommon.

  8. [Marginalization and health. Introduction].

    PubMed

    Yunes, J

    1992-06-01

    The relationship between marginalization and health is clear. In Mexico, for example, life expectancy is 53 years for the poorest population sectors and 20 years more for the wealthiest. Infant mortality in poor Colombian families is twice that of wealthier families, and one-third of developing countries the rural population is only half as likely as the urban to have access to health services. Women in the Southern hemisphere are 12 times likelier than those in the Northern to die of maternal causes. The most important step in arriving at a solution to the inequity may be to analyze in depth the relationship between marginality and health. Marginality may be defined as the lack of participation of individuals or groups in certain key phases of societal life, such as production, consumption, or political decision making. Marginality came to be viewed as a social problem only with recognition of the rights of all individuals to participate in available social goods. Marginality is always relative, and marginal groups exist because central groups determine the criteria for inclusion in the marginal and central groups. Marginality thus always refers to a concrete society at a specific historical moment. Marginal groups may be of various types. At present, marginal groups include women, rural populations, people with AIDS or mental illness or certain other health conditions, refugees, ethnic or religious groups, homosexuals, and the poor, who are the largest group of marginal persons in the world. Even in developed countries, 100-200 million persons live below the poverty line. Latin America is struggling to emerge from its marginal status in the world. The economic crisis of the 1980s increased poverty in the region, and 40% are not considered impoverished. Latin America is a clear example of the relationship between marginality and health. Its epidemiologic profile is intimately related to nutrition, availability of potable water, housing, and environmental

  9. Late quaternary sequence stratigraphy, South Florida margin

    SciTech Connect

    Locker, S.D.; Hine, A.C.

    1995-12-01

    Late Quaternary sea-level change and the Florida Current have combined to produce a progradational shelf-slope margin along the western portion of the south Florida Platform facing the Straits of Florida. Analysis of high resolution seismic reflection profiles suggest at least eight 5th order late Quaternary sequences downlap onto the Pourtales Terrace at 250 m water depth. Along most of the south Florida margin, this Late Quaternary section is very thin, and only where significant accumulations occur can the stratigraphic patterns produced by sea-level change be clearly observed. Recognition of systems tracts and their boundaries from high-resolution seismic data is important for prediction of sedimentary facies and stratigraphic development of margins. Many south Florida seismic boundaries can be fit to the Exxon sequence stratigraphy model. Others appear to reflect the added effect of bottom-current erosion that complicates the signal produced by sea-level change. Overall, the sea-level signal appears to dominate the stratigraphic record, especially from the 2-dimensional perspective of dip-oriented seismic profiles. However, the 3-dimensional geometry of deposits are strongly influenced by along slope accumulation patterns controlled by the Florida Current. This study provides new insight on the importance of both geostrophic boundary currents and sea-level change in controlling stratigraphic development of a carbonate platform margin. Similar anomalously thick slope deposits in ancient sequences may indicate similar controls on accumulation and could lend to predictions of related paleo-platform configurations.

  10. Coordination: Southeast continental shelf studies

    SciTech Connect

    Menzel, D.W.

    1989-01-26

    The objective of this investigation is to obtain model descriptions of the flow modifications in the Southeast Atlantic continental shelf due to Gulf Stream fluctuations and topographic effects. 2 refs., 4 figs.

  11. Shelf-Stable Food Safety

    MedlinePlus

    ... process of packing meat and poultry in glass bottles, corking them, and submerging them in boiling water. ... fsis.usda.gov. [ Top of Page ] Are any egg products shelf stable? Pasteurized, dried egg products can ...

  12. Speedup and fracturing of George VI Ice Shelf, Antarctic Peninsula

    NASA Astrophysics Data System (ADS)

    Holt, T. O.; Glasser, N. F.; Quincey, D. J.; Siegfried, M. R.

    2013-01-01

    George VI Ice Shelf (GVIIS) is located on the Antarctic Peninsula, a region where several ice shelves have undergone rapid breakup in response to atmospheric and oceanic warming. We use a combination of optical (Landsat), radar (ERS 1/2 SAR) and laser altimetry (GLAS) datasets to examine the response of GVIIS to environmental change and to offer an assessment on its future stability. The spatial and structural changes of GVIIS (ca. 1973 to ca. 2010) are mapped and surface velocities are calculated at different time periods (InSAR and optical feature tracking from 1989 to 2009) to document changes in the ice shelf's flow regime. Surface elevation changes are recorded between 2003 and 2008 using repeat track ICESat acquisitions. We note an increase in fracture extent and distribution at the south ice front, ice-shelf acceleration towards both the north and south ice fronts and spatially varied negative surface elevation change throughout, with greater variations observed towards the central and southern regions of the ice shelf. We propose that whilst GVIIS is in no imminent danger of collapse, it is vulnerable to on-going atmospheric and oceanic warming and is more susceptible to breakup along its southern margin in ice preconditioned for further retreat.

  13. Speedup and fracturing of George VI Ice Shelf, Antarctic Peninsula

    NASA Astrophysics Data System (ADS)

    Holt, T. O.; Glasser, N. F.; Quincey, D. J.; Siegfried, M. R.

    2013-05-01

    George VI Ice Shelf (GVIIS) is located on the Antarctic Peninsula, a region where several ice shelves have undergone rapid breakup in response to atmospheric and oceanic warming. We use a combination of optical (Landsat), radar (ERS 1/2 SAR) and laser altimetry (GLAS) datasets to examine the response of GVIIS to environmental change and to offer an assessment on its future stability. The spatial and structural changes of GVIIS (ca. 1973 to ca. 2010) are mapped and surface velocities are calculated at different time periods (InSAR and optical feature tracking from 1989 to 2009) to document changes in the ice shelf's flow regime. Surface elevation changes are recorded between 2003 and 2008 using repeat track ICESat acquisitions. We note an increase in fracture extent and distribution at the south ice front, ice-shelf acceleration towards both the north and south ice fronts and spatially varied negative surface elevation change throughout, with greater variations observed towards the central and southern regions of the ice shelf. We propose that whilst GVIIS is in no imminent danger of collapse, it is vulnerable to ongoing atmospheric and oceanic warming and is more susceptible to breakup along its southern margin in ice preconditioned for further retreat.

  14. A review of the geology and petroleum possibilities of the continental margins of India

    SciTech Connect

    Sahay, B.

    1984-05-01

    Continental margins which flank western and eastern limits of India are broadly divided into West and East Coast. The width of the Western Continental Shelf varies from 300 km near Bombay to approximately 80 kms near Porbandar, narrowing further towards south to 60 kms near Cochin. The Eastern Continental Shelf which extends from Cape Comorin to Calcutta, has the width varying from 2.5 km, about 150 kms south of Madras to 210 kms upto Ganges river delta, near Calcutta.

  15. Geology and petroleum potential of Shumagin continental margin, western Gulf of Alaska

    SciTech Connect

    Bruns, T.R.; Von Huene, R.; Culotta, R.D.; Lewis, S.D.; Ladd, J.W.

    1986-07-01

    Interpretations of multichannel seismic reflection data indicate that the Shumagin continental margin seaward of the Border Ranges fault is underlain by two major seismic sequences, separated by an erosional unconformity beneath the shelf and by the time-correlative conformity seaward. Rocks above the unconformity are late Miocene and younger. Rocks below the unconformity can be as young as middle Miocene beneath the outer shelf and slope, seaward of a paleoshelf break. However, beneath the shelf they are primarily Late Cretaceous turbidites of the Shumagin Formation and Paleocene granodiorite. Late Miocene and younger structures of the Shumagin margin include Shumagin, Sanak, and Unimak basins and Unimak Ridge, a midslope structural high. Strata in Sanak and Unimak basins were deposited on a subsiding outer shelf and slope, and trapped behind Unimak Ridge and its now-buried structural continuation. Sanak and Unimak basins are in part bounded by northwest-trending extensional faults that parallel both the early Tertiary Beringian margin and a transverse tectonic boundary that segments the fore-arc. These faults may have developed during collapse and extension along the southeastward continuation of the old Beringian margin, analogous to the processes that created the Bering Shelf basins. The most promising areas of the Shumagin margin for petroleum potential are Sanak, and Unimak basins, which contain strata 8 and 4.5 km thick, respectively, and beneath the outer shelf and slope. Paleogene source rocks like those on the adjacent Alaska Peninsula may be preserved offshore, seaward of the inferred paleoshelf break. Reservoir rocks might have formed from granitic-rich erosional products derived during Oligocene and Miocene erosion of the shelf plutons.

  16. The Impact of Different CD4 Cell-Count Monitoring and Switching Strategies on Mortality in HIV-Infected African Adults on Antiretroviral Therapy: An Application of Dynamic Marginal Structural Models.

    PubMed

    Ford, Deborah; Robins, James M; Petersen, Maya L; Gibb, Diana M; Gilks, Charles F; Mugyenyi, Peter; Grosskurth, Heiner; Hakim, James; Katabira, Elly; Babiker, Abdel G; Walker, A Sarah

    2015-10-01

    In Africa, antiretroviral therapy (ART) is delivered with limited laboratory monitoring, often none. In 2003-2004, investigators in the Development of Antiretroviral Therapy in Africa (DART) Trial randomized persons initiating ART in Uganda and Zimbabwe to either laboratory and clinical monitoring (LCM) or clinically driven monitoring (CDM). CD4 cell counts were measured every 12 weeks in both groups but were only returned to treating clinicians for management in the LCM group. Follow-up continued through 2008. In observational analyses, dynamic marginal structural models on pooled randomized groups were used to estimate survival under different monitoring-frequency and clinical/immunological switching strategies. Assumptions included no direct effect of randomized group on mortality or confounders and no unmeasured confounders which influenced treatment switch and mortality or treatment switch and time-dependent covariates. After 48 weeks of first-line ART, 2,946 individuals contributed 11,351 person-years of follow-up, 625 switches, and 179 deaths. The estimated survival probability after a further 240 weeks for post-48-week switch at the first CD4 cell count less than 100 cells/mm(3) or non-Candida World Health Organization stage 4 event (with CD4 count <250) was 0.96 (95% confidence interval (CI): 0.94, 0.97) with 12-weekly CD4 testing, 0.96 (95% CI: 0.95, 0.97) with 24-weekly CD4 testing, 0.95 (95% CI: 0.93, 0.96) with a single CD4 test at 48 weeks (baseline), and 0.92 (95% CI: 0.91, 0.94) with no CD4 testing. Comparing randomized groups by 48-week CD4 count, the mortality risk associated with CDM versus LCM was greater in persons with CD4 counts of <100 (hazard ratio = 2.4, 95% CI: 1.3, 4.3) than in those with CD4 counts of ≥100 (hazard ratio = 1.1, 95% CI: 0.8, 1.7; interaction P = 0.04). These findings support a benefit from identifying patients immunologically failing first-line ART at 48 weeks. PMID:26316598

  17. The Impact of Different CD4 Cell-Count Monitoring and Switching Strategies on Mortality in HIV-Infected African Adults on Antiretroviral Therapy: An Application of Dynamic Marginal Structural Models

    PubMed Central

    Ford, Deborah; Robins, James M.; Petersen, Maya L.; Gibb, Diana M.; Gilks, Charles F.; Mugyenyi, Peter; Grosskurth, Heiner; Hakim, James; Katabira, Elly; Babiker, Abdel G.; Walker, A. Sarah

    2015-01-01

    In Africa, antiretroviral therapy (ART) is delivered with limited laboratory monitoring, often none. In 2003–2004, investigators in the Development of Antiretroviral Therapy in Africa (DART) Trial randomized persons initiating ART in Uganda and Zimbabwe to either laboratory and clinical monitoring (LCM) or clinically driven monitoring (CDM). CD4 cell counts were measured every 12 weeks in both groups but were only returned to treating clinicians for management in the LCM group. Follow-up continued through 2008. In observational analyses, dynamic marginal structural models on pooled randomized groups were used to estimate survival under different monitoring-frequency and clinical/immunological switching strategies. Assumptions included no direct effect of randomized group on mortality or confounders and no unmeasured confounders which influenced treatment switch and mortality or treatment switch and time-dependent covariates. After 48 weeks of first-line ART, 2,946 individuals contributed 11,351 person-years of follow-up, 625 switches, and 179 deaths. The estimated survival probability after a further 240 weeks for post-48-week switch at the first CD4 cell count less than 100 cells/mm3 or non-Candida World Health Organization stage 4 event (with CD4 count <250) was 0.96 (95% confidence interval (CI): 0.94, 0.97) with 12-weekly CD4 testing, 0.96 (95% CI: 0.95, 0.97) with 24-weekly CD4 testing, 0.95 (95% CI: 0.93, 0.96) with a single CD4 test at 48 weeks (baseline), and 0.92 (95% CI: 0.91, 0.94) with no CD4 testing. Comparing randomized groups by 48-week CD4 count, the mortality risk associated with CDM versus LCM was greater in persons with CD4 counts of <100 (hazard ratio = 2.4, 95% CI: 1.3, 4.3) than in those with CD4 counts of ≥100 (hazard ratio = 1.1, 95% CI: 0.8, 1.7; interaction P = 0.04). These findings support a benefit from identifying patients immunologically failing first-line ART at 48 weeks. PMID:26316598

  18. Retreat of northern margins of George VI and Wilkins Ice Shelves, Antarctic Peninsula

    USGS Publications Warehouse

    Lucchitta, B.K.; Rosanova, C.E.

    1998-01-01

    The George VI and Wilkins Ice Shelves are considered at risk of disintegration due to a regional atmospheric warming trend on the Antarctic Peninsula. Retreat of the northern margin of the George VI Ice Shelf has been observed previously, but the Wilkins Ice Shelf was thought to be stable. We investigated the positions of the northern fronts of these shelves from the literature and looked for changes on 1974 Landsat and 1992 and 1995 European remote-sensing satellite (ERS) synthetic aperture radar images. Our investigation shows that the northern George VI Ice Shelf lost a total of 906 km2 between 1974 and 1992, and an additional 87 km2 by 1995. The northern margin of the Wilkins Ice Shelf lost 796 km2 between 1990 and 1992, and another 564 km2 between 1992 and 1995. Armadas of tabular icebergs were visible in front of this shelf in the ERS images. These two ice shelves mark the southernmost documented conspicuous retreat of ice-shelf margins.

  19. Surficial sediments on the western Canadian continental shelf

    NASA Astrophysics Data System (ADS)

    Bornhold, Brian D.; Barrie, J. Vaughn

    1991-08-01

    The active continental margin off western Canada is characterized in the south by convergence between the Explorer and Juan de Fuca Plates and the America Plate, and in the north by transform fault motion between the Pacific and America Plates along the Queen Charlotte Fault. Except in some of the deepest troughs and basins shelf sediments are dominated by immature lithic arenites reflecting this tectonic setting. The Vancouver Island shelf is from 5 to 75 km wide and displays complex topography on the inner shelf and a relatively featureless mid- and outer shelf. An exception is the area off southwestern Vancouver Island where large basins bounded by morainal deposits extend more than two-thirds of the distance across the shelf. The shelf edge varies from 180 to 225 m depth and is indented by numerous canyons. Nearshore sediments consist mainly of gravels and boulders and become finer offshore such that muds are slowly accumulating in depths greater than 100 m. Off northwestern Vancouver Island calcareous sediments are abundant with carbonate values often exceeding 75%. High wave and current energies and efficient sediment trapping in coastal fiords have resulted in low rates of accumulation. Olive, glauconitic, Holocene muds and muddy sands are generally less than 0.3 m thick and accumulate only on the outer shelf over an extensive stiff, gray, glaciomarine sandy mud. Queen Charlotte Sound exhibits three broad, shallow, glacially scoured troughs, filled mainly with clayey silts and fine sands and separated by sand and gravel covered banks. Hecate Strait, between the Queen Charlotte Islands and the mainland, consists of a prominent southward-opening trough along the east side of the Strait bounded by Dogfish and Laskeek Banks on the west adjacent to Graham and Moresby Islands, respectively. The trough below 200 m is filled by silts. The banks are covered by discontinuous sands and gravels of variable calcareous content. The bank edges often display megaripples and

  20. Ocean margins workshop

    SciTech Connect

    1990-12-31

    The Department of Energy (DOE) is announcing the refocusing of its marine research program to emphasize the study of ocean margins and their role in modulating, controlling, and driving Global Change phenomena. This is a proposal to conduct a workshop that will establish priorities and an implementation plan for a new research initiative by the Department of Energy on the ocean margins. The workshop will be attended by about 70 scientists who specialize in ocean margin research. The workshop will be held in the Norfolk, Virginia area in late June 1990.

  1. Organizing marginalized workers.

    PubMed

    Taylor, A K

    1999-01-01

    Figures from the U.S. Department of Labor show that low-wage or marginalized workers are more likely to be injured on the job and suffer more work-related medical conditions than better-paid workers. Despite an increasingly hostile organizing climate, market globalization, and corporate downsizing, significant progress has been made in organizing marginalized workers. A multifaceted, comprehensive organizing strategy, incorporating union-building strategies that include (but are not limited to) safety and health, must be used by unions to successfully organize marginalized workers and obtain the first contract. PMID:10378982

  2. Sedimentation of shelf sandstones in Queen Formation, McFarland and Means fields, central basin platform of Permian basin

    SciTech Connect

    Malicse, A.; Mazzullo, J.; Holley, C.; Mazzullo, S.J.

    1988-01-01

    The Queen Formation is a sequence of carbonates, evaporites, and sandstones of Permian (Guadalupian) age that is found across the subsurface of the Central Basin platform of the Permian basin. The formation is a major hydrocarbon reservoir in this region, and its primary reservoir facies are porous shelf sandstones and dolomites. Cores and well logs from McFarland and Means fields (on the northwest margin of the Central Basin platform) were examined to determine the sedimentary history of the shelf sandstones.

  3. Recent carbonate slope development on southwest Florida continental margin

    SciTech Connect

    Brooks, G.R.; Holmes, C.W.

    1987-05-01

    The southwest Florida continental slope bordering the Florida Strait contains a thick sequence of seaward-prograding sediments. Sediments consist principally of a mixture of shallow water and pelagic carbonate sands and muds deposited rapidly on the upper slope. Sedimentary patterns are interpreted to be a function of high-frequency sea level fluctuations. Most vigorous off-shelf transport and highest sedimentation rates (exceeding 2.5 m/1000 years) occur during early transgressions and late regressions when water depths on the shelf are shallow. During sea level highstands, off-shelf transport is less vigorous and sedimentation rates decrease. During sea level lowstands, no off-shelf transport takes place and erosion of the previously deposited sequence occurs as a result of an increase in erosional capacity of the Florida Current. The presence of at least nine such sequences, all with similar characteristics, indicates that these processes have been occurring since at least the late Pleistocene in response to high-frequency glacial fluctuations. The location of the southwest Florida slope between the rimmed Bahama platform and the nonrimmed remainder of the west Florida margin, as well as similarities with ancient carbonate slope deposits formed during periods when shelf-edge reef-forming organisms were lacking, suggest that depositional patterns on the southwest Florida slope may be indicative of a transition between rimmed and nonrimmed carbonate platform environments. The southwest Florida slope may provide a valuable modern analog for identifying similar transitional environments in the geologic record.

  4. Marginalization of Vocational Psychology.

    ERIC Educational Resources Information Center

    Tinsley, Howard E. A.

    2001-01-01

    Although vocational psychology has diverse theoretical models and an empirical tradition, it is marginalized within counseling psychology. Its vitality is weakened by those who take a dabbler, pundit, or booster approach to scholarship. (Contains 46 references.) (SK)

  5. African Aesthetics

    ERIC Educational Resources Information Center

    Abiodun, Rowland

    2001-01-01

    No single traditional discipline can adequately supply answers to the many unresolved questions in African art history. Because of the aesthetic, cultural, historical, and, not infrequently, political biases, already built into the conception and development of Western art history, the discipline of art history as defined and practiced in the West…

  6. Provenance and distribution of clay minerals in the sediments of the western continental shelf and slope of India

    NASA Astrophysics Data System (ADS)

    Purnachandra Rao, V.; Ramalingeswara Rao, B.

    1995-12-01

    The distribution of clay minerals from 156 surficial sediments of the western continental margin of India, ranging from 17 to 2000 m water depth, indicate that there are three principal sources of sediments. The illite and chlorite-rich assemblage derived from the Indus (Indus Province) is predominant in the continental margin sediments to the north of the Gulf of Kachchh. An assemblage of smectite with minor kaolinite, illite and chlorite, mostly derived from the Deccan Trap basalts (Deccan Trap Province), occurs all along the inner shelf from Saurashtra to Goa. Illite, however, dominates smectite in the outer shelf of Saurashtra and on the continental slope from Saurashtra to Goa. Some samples on the outer shelf of the Gulf of Cambay-Goa show trace contents of all clay minerals, while others from the same region show the dominance of smectite over illite. A smectite and kaolinite-rich assemblage with minor illite, chlorite and gibbsite derived from the Gneissic Province occurs both on the shelf and slope between Goa and Cochin. It appears that the Indus derived sediments are transported onto the continental slope and, to a lesser extent, the outer shelf of western India by a southerly surface current and admix with clays transported from the hinterland. The influence of the Indus borne sediments on the continental slope decreases from north to south and cross shelf transport processes dominate in the southwestern continental margin between Goa and Cochin.

  7. Shelf response to intense offshore wind

    NASA Astrophysics Data System (ADS)

    Grifoll, Manel; Aretxabaleta, Alfredo L.; Espino, Manuel

    2015-09-01

    Cross and along-shelf winds drive cross-shelf transport that promotes the exchange of tracers and nutrients to the open sea. The shelf response to cross-shelf winds is studied in the north shelf of the Ebro Delta (Mediterranean Sea), where those winds are prevalent and intense. Offshore winds in the region exhibit strong intensities (wind stress larger than 0.8 Pa) during winter and fall. The monthly average flow observed in a 1 year current meter record at 43.5 m was polarized following the isobaths with the along-shelf variability being larger than the cross-shelf. Prevalent southwestward along-shelf flow was induced by the three-dimensional regional response to cross-shelf winds and the coastal constraint. Seaward near-surface velocities occurred predominantly during offshore wind events. During intense wind periods, the surface cross-shelf water transport exceeded the net along-shelf transport. During typically stratified seasons, the intense cross-shelf winds resulted in a well-defined two-layer flow and were more effective at driving offshore transport than during unstratified conditions. While transfer coefficients between wind and currents were generally around 1%, higher cross-shelf transfer coefficients were observed in the near-inertial band. The regional extent of the resulting surface cold water during energetic cross-shelf winds events was concentrated around the region of the wind jet. Cross-shelf transport due to along-shelf winds was only effective during northeast wind events. During along-shelf wind conditions, the transport was estimated to be between 10 and 50% of the theoretical Ekman transport.

  8. Sequence stratigraphy on an early Cretaceous passive margin, Exmouth Plateau

    SciTech Connect

    Boyd, R.; Gorur, N.; Ito, M.; O'Brien, D.; Wilkens, R.; Tang, C.

    1989-03-01

    Permian-Jurassic rifting of northwestern Australia resulted in the development of a passive continental margin flanking the northeastern Indian Ocean. On this margin the relatively thin synrift to postrift sedimentary sequence of southern Exmouth Plateau was drilled during ODP Leg 122. A sequence-stratigraphy analysis of the complete Mesozoic-Cenozoic sedimentary succession at Sites 762 and 763 was derived from a synthesis of seismic stratigraphy, wireline logs, lithostratigraphy, biostratigraphy, and magnetostratigraphy. Results indicate that during breakup, the southern Exmouth Plateau was a transform margin with an extensional component. Between the Tithonian and Valanginian, a thick clastic wedge prograded from the transform margin south of Site 763 northwestward toward Site 762 and onto subsiding continental crust. Southern clastic supply decreased into the Aptian-Cenomanian, and cyclic deposition of deep-water mudstones continued during subsidence of the earlier shelf margin wedge. Between the Albian and Cenomanian, deposition gradually became dominated by pelagic carbonates. Two regional unconformities mark the Cenomanian/Turonian and Cretaceous/Tertiary boundaries. Each was an erosional event, succeeded by renewed pelagic carbonate deposition that began in the distal northern basin and onlapped progressively toward the topographic high, which persisted into the Tertiary along the southern margin. The entire Jurassic to Holocene record at the southern Exmouth Plateau ODP sites is less than 1500 m thick and represents a classic rift to mature ocean passive-margin succession.

  9. Community structure and benthic habitats across the George V Shelf, East Antarctica: Trends through space and time

    NASA Astrophysics Data System (ADS)

    Post, Alexandra L.; Beaman, Robin J.; O'Brien, Philip E.; Eléaume, Marc; Riddle, Martin J.

    2011-03-01

    Physical and biological characteristics of benthic communities are analysed from underwater video footage collected across the George V Shelf during the 2007/2008 CEAMARC voyage. Benthic habitats are strongly structured by physical processes operating over a range of temporal and spatial scales. Iceberg scouring recurs over timescales of years to centuries along shallower parts of the shelf, creating communities in various stages of maturity and recolonisation. Upwelling of modified circumpolar deep water (MCDW) onto the outer shelf and cross-shelf flow of high-salinity shelf water (HSSW) create spatial contrasts in nutrient and sediment supply, which are largely reflected in the distribution of deposit- and filter-feeding communities. Long-term cycles in the advance and retreat of icesheets (over millennial scales) and subsequent focussing of sediments in troughs such as the Mertz Drift create patches of consolidated and soft sediments, which also provide distinct habitats for colonisation by different biota. These physical processes of iceberg scouring, current regimes and depositional environments, in addition to water depth, are important factors in the structure of benthic communities across the George V Shelf. The modern shelf communities mapped in this study largely represent colonisation over the past 8000-12,000 years, following retreat of the icesheet and glaciers at the end of the last glaciation. Recolonisation on this shelf may have occurred from two sources: deep-sea environments and possible shelf refugia on the Mertz and Adélie Banks. However, any open-shelf area would have been subject to intense iceberg scouring. Understanding the timescales over which shelf communities have evolved and the physical factors which shape them will allow better prediction of the distribution of Antarctic shelf communities and their vulnerability to change. This knowledge can aid better management regimes for the Antarctic margin.

  10. Tectonic signatures on active margins

    NASA Astrophysics Data System (ADS)

    Hogarth, Leah Jolynn

    High-resolution Compressed High-Intensity Radar Pulse (CHIRP) surveys offshore of La Jolla in southern California and the Eel River in northern California provide the opportunity to investigate the role of tectonics in the formation of stratigraphic architecture and margin morphology. Both study sites are characterized by shore-parallel tectonic deformation, which is largely observed in the structure of the prominent angular unconformity interpreted as the transgressive surface. Based on stratal geometry and acoustic character, we identify three sedimentary sequences offshore of La Jolla: an acoustically laminated estuarine unit deposited during early transgression, an infilling or "healing-phase" unit formed during the transgression, and an upper transparent unit. The estuarine unit is confined to the canyon edges in what may have been embayments during the last sea-level rise. The healing-phase unit appears to infill rough areas on the transgressive surface that may be related to relict fault structures. The upper transparent unit is largely controlled by long-wavelength tectonic deformation due to the Rose Canyon Fault. This unit is also characterized by a mid-shelf (˜40 m water depth) thickness high, which is likely a result of hydrodynamic forces and sediment grain size. On the Eel margin, we observe three distinct facies: a seaward-thinning unit truncated by the transgressive surface, a healing-phase unit confined to the edges of a broad structural high, and a highly laminated upper unit. The seaward-thinning wedge of sediment below the transgressive surface is marked by a number of channels that we interpret as distributary channels based on their morphology. Regional divergence of the sequence boundary and transgressive surface with up to ˜8 m of sediment preserved across the interfluves suggests the formation of subaerial accommodation during the lowstand. The healing-phase, much like that in southern California, appears to infill rough areas in the

  11. Modern cool-water siliciclastic/carbonate sediments, lacepede shelf South Australia

    SciTech Connect

    Bone, Y.; Gostin, V. ); James, N.P. ); Von der Borch, C.C. )

    1991-03-01

    The Lacepeded Shelf is a 130 km {times} 100 km open embayment along the southern passive continental margin of Australia. The shelf includes the mouth of the River Murray - Australia's largest drainage system- the extensive arcuate Coorong strand, the 50-70 m deep and flat plateau of the shelf proper, the nonrimmed shelf break, and the upper slope to depths of 200 m. The shelf bathymetry is locally interrupted by seafloor highs, reflecting the underlying rugged terrain of deformed Precambrian and early Paleozoic bedrock in the west and Tertiary limestones and Quaternary dunes in the east. The late Pleistocene/Holocene sediment blanket is formed by discrete sedimentary facies. Quartz sands cover a significant cross-shelf zone opposite the river mouth, with current generated offsets. High-resolution seismic profiles reveal buried lowstand channels. The mid-shelf is an area of conspicuously coarse-grained, mud-free loose sediments composed of variable amounts of bryozoans and bivalves. The shelf break and upper slop bryozoan sands are similar, apart from species differences, from 40 to 100 m, with an increase in mud below 100 m. The seafloor highs are sites of prolific bryozoan, calcareous algae, sponge, and bivalve growth, the skeletons of which are shed onto the shelf. Bryozoan distribution is moderated by water depth and substrate type. Most forms are low-Mg calcite to high-Mg calcite, but two major groups are aragonitic. Distribution of these different mineralogical types is important for later diagenesis. Both terrigenous clastic and carbonate sediments are a mixture of relict and modern components, depending upon location, and reflect Holocene glacio-eustatic sea-level changes.

  12. Coordination: southeast continental shelf studies. Progress report

    SciTech Connect

    Menzel, D.W.

    1981-02-01

    The objectives are to identify important physical, chemical and biological processes which affect the transfer of materials on the southeast continental shelf, determine important parameters which govern observed temporal and spatial varibility on the continental shelf, determine the extent and modes of coupling between events at the shelf break and nearshore, and determine physical, chemical and biological exchange rates on the inner shelf. Progress in meeting these research objectives is presented. (ACR)

  13. Early Miocene sequence development across the New Jersey margin

    USGS Publications Warehouse

    Monteverde, D.H.; Mountain, Gregory S.; Miller, K.G.

    2008-01-01

    Sequence stratigraphy provides an understanding of the interplay between eustasy, sediment supply and accommodation in the sedimentary construction of passive margins. We used this approach to follow the early to middle Miocene growth of the New Jersey margin and analyse the connection between relative changes of sea level and variable sediment supply. Eleven candidate sequence boundaries were traced in high-resolution multi-channel seismic profiles across the inner margin and matched to geophysical log signatures and lithologic changes in ODP Leg 150X onshore coreholes. Chronologies at these drill sites were then used to assign ages to the intervening seismic sequences. We conclude that the regional and global correlation of early Miocene sequences suggests a dominant role of global sea-level change but margin progradation was controlled by localized sediment contribution and that local conditions played a large role in sequence formation and preservation. Lowstand deposits were regionally restricted and their locations point to both single and multiple sediment sources. The distribution of highstand deposits, by contrast, documents redistribution by along shelf currents. We find no evidence that sea level fell below the elevation of the clinoform rollover, and the existence of extensive lowstand deposits seaward of this inflection point indicates efficient cross-shelf sediment transport mechanisms despite the apparent lack of well-developed fluvial drainage. ?? 2008 The Authors. Journal compilation ?? 2008 Blackwell Publishing.

  14. Geology and petroleum potential of Adelie Coast margin, east Antarctica

    SciTech Connect

    Wanneson, J.

    1987-05-01

    The few rock outcrops on Adelie Coast-Wilkes Land consist mainly of Precambrian plutonic rocks and metasediments. On the continental margin, several multichannel seismic surveys, including the 1982 IFP survey, reveal the presence of a thick sedimentary basin, especially beneath the outer continental shelf and upper slope, where it may exceed 6000 m. Thin basin results from the creation and evolution of a continental margin, initiated some 100 Ma from the separation of Australia and Antarctica. Beneath the outer shelf, which is 400-500 m deep, the sedimentary series consist of four units separated by three major unconformities: (1) a predrift unit including a Precambrian basement, possible Paleozoic and early Mesozoic sediments, and a Mesozoic syn-rift sequence; (2) an upper Eocene to Oligocene unit in a shallow marine environment; and (3) a Neogene glacial prograding unit. The predrift and early postrift units are considered to be a promising target with reference to other passive margins, although no major hydrocarbon accumulation has been discovered as yet on the Australian conjugate margin.

  15. West Florida Shelf: A natural laboratory for the study of ocean acidificiation

    USGS Publications Warehouse

    Hallock, Pamela; Robbins, Lisa L.; Larson, Rebekka A.; Beck, Tanya; Schwing, Patrick; Martinez-Colon, Michael; Gooch, Brad

    2010-01-01

    southwest Florida shelf is a rimmed carbonate margin where organisms produce virtually all of the substrate; it also exhibits a greater sediment thickness as compared to the west Florida shelf (Enos, 1977). Temperature, which is usually associated with latitude, plays a major role in locations of foramol versus chlorozoan assemblages, but other factors beyond latitude influence temperature on the west and southwest Florida shelves. The potential of cooler, deep-water upwelling and transport over the bottom waters of the shelf may have a significant role in the species assemblage at the sediment/water interface and ultimately on location of foramol versus chlorozoan production. Deep water transported onto and over the shelf may also have environmental ramifications beyond temperature by bringing in water of different chemistry.

  16. Mapping Subsea Permafrost, Relict Methane Hydrate, and Gas Migration: New Cross-Shelf Multichannel Seismic Surveys on the Central US Beaufort Shelf

    NASA Astrophysics Data System (ADS)

    Ruppel, C. D.; Hart, P. E.; Moore, E.; Worley, C.; Brothers, L.

    2012-12-01

    In August 2012, the USGS Gas Hydrates Project, with support from DOE's Methane Hydrates R&D Program, conducted the first research-oriented multichannel seismic survey in 35 years across the Alaskan Beaufort Sea continental shelf. Our Central Beaufort margin study area stretches from Camden Bay on the west to Harrison Bay on the east and lies offshore some of the North Slope's most important petroleum systems. The new MCS data were collected in the eastern part of the Alaskan passive margin terrane, near the transition zone to the compressional Canning Mackenzie Deformed Margin described by Houseknecht and Bird. The Central Beaufort shelf was mostly exposed subaerially during Late Pleistocene time, leading to the formation of continuous permafrost and associated gas hydrates at depths greater than ~225 m. As Holocene sea level rise inundated the present-day shelf, the now-subsea permafrost began to thaw and associated gas hydrates would have begun to dissociate. The new surveys constitute the shelf component of site survey activities for Integrated Ocean Drilling pre-proposal 797, which outlines a multiplatform drilling program at 9 sites from the innermost shelf to the upper continental slope of the Alaskan Beaufort margin. The proposed drilling program will elucidate Late Pleistocene to contemporary climate history by accessing sediments currently or formerly hosting subsea permafrost and permafrost-associated methane hydrates on the shelf and sediments in which gas hydrate dynamics are driven by warming of impinging intermediate waters on the upper continental slope. Using a 24-channel digital streamer and a 2 kJ sparker source, the new MCS surveys provided up to several hundred meters of subseafloor penetration and were complemented by 4-24 kHz Chirp surveys for the shallowmost section, high frequency water column imaging for gas plumes, and Swathplus bathymetric mapping at water depths less than 60 to 80 m. The new MCS data, which in part reoccupy 30-year

  17. Development of an Upper Cambrian rimmed shelf along the Mississippi Valley Graben, Reelfoot Rift, and the southeastern Ozarks, southern Missouri

    SciTech Connect

    Palmer, J.R. . Dept. of Natural Resources)

    1993-03-01

    The paleogeographic distribution of intrashelf basin shales and limestones in the Bonneterre (Dresbachian) and Davis (Franconian) Formations, and associated data, suggests that rimmed shelf conditions separated the central Missouri part of the shelf from the Mississippi Valley Graben (MVG) of the Reelfoot Rift to the southeast. Middle Dresbachian rocks of the intrashelf basin area, predominantly limestones, indicate a discontinuous carbonate shelf rim. The succeeding widespread shales of the Franconian intrashelf basin indicate that a continuous rim had developed. The margin of the shelf rim is preserved in part of the 4,700-ft-thick Upper Cambrian succession along the northwest margin of the Mississippi Valley Graben (MVG) of the Reelfoot Rift. Equivalent rocks within the MVG are dominantly dark shales (1,600+ ft thick). The Amoco Spence test well penetrated the rim succession on the northwest margin of the MVG. At least 11 large-scale transgressive-regressive (T-R) carbonate cycles (120--600 ft thick) have been defined in this well; 7 cycles are equivalent to the Bonneterre and Davis Formations. These latter cycles have shaly limestone, or limestone at the base, and grade upward to dolostone or coarsely crystalline dolostone, which are interpreted to be shallowing-upward, bank margin-peritidal carbonates. Northwest of the rim margin and on the southeast side of the St. Francois Mountains, rocks equivalent to the rim succession consist of a series of ramp to platform cycles made up of dolostones and local limestones. Many carbonate shelves on passive margins are rimmed. The development of this Upper Cambrian rimmed shelf along the northwest margin of the MVG suggests that failed rifts can also localized such rims.

  18. Organic carbon production, mineralization and preservation on the Peruvian margin

    NASA Astrophysics Data System (ADS)

    Dale, A. W.; Sommer, S.; Lomnitz, U.; Montes, I.; Treude, T.; Gier, J.; Hensen, C.; Dengler, M.; Stolpovsky, K.; Bryant, L. D.; Wallmann, K.

    2014-09-01

    Carbon cycling in Peruvian margin sediments (11° S and 12° S) was examined at 16 stations from 74 m on the inner shelf down to 1024 m water depth by means of in situ flux measurements, sedimentary geochemistry and modeling. Bottom water oxygen was below detection limit down to ca. 400 m and increased to 53 μM at the deepest station. Sediment accumulation rates and benthic dissolved inorganic carbon fluxes decreased rapidly with water depth. Particulate organic carbon (POC) content was lowest on the inner shelf and at the deep oxygenated stations (< 5%) and highest between 200 and 400 m in the oxygen minimum zone (OMZ, 15-20%). The organic carbon burial efficiency (CBE) was unexpectedly low on the inner shelf (< 20%) when compared to a global database, for reasons which may be linked to the frequent ventilation of the shelf by oceanographic anomalies. CBE at the deeper oxygenated sites was much higher than expected (max. 81%). Elsewhere, CBEs were mostly above the range expected for sediments underlying normal oxic bottom waters, with an average of 51 and 58% for the 11° S and 12° S transects, respectively. Organic carbon rain rates calculated from the benthic fluxes alluded to a very efficient mineralization of organic matter in the water column, with a Martin curve exponent typical of normal oxic waters (0.88 ± 0.09). Yet, mean POC burial rates were 2-5 times higher than the global average for continental margins. The observations at the Peruvian margin suggest that a lack of oxygen does not affect the degradation of organic matter in the water column but promotes the preservation of organic matter in marine sediments.

  19. Shelf Reading as a Collaborative Service Model

    ERIC Educational Resources Information Center

    Brown, Kevin N.; Kaspar, Wendi Arant

    2006-01-01

    Shelf reading the stacks is very often not seen as scholarly work in library circles and is therefore overlooked. However, there is a very real frustration of a patron who cannot find the material they need. There are very few studies that provide a working model for shelf reading. The authors suggest a collaborative shelf reading model based on…

  20. Middle Cambrian to Late Ordovician evolution of the Appalachian margin: Foundering of a passive margin to form a subduction zone and volcanic arc

    SciTech Connect

    Washington, P.A. , Southern Pines, NC )

    1994-03-01

    From late Middle Cambrian to early Late Ordovician time, the Appalachian passive margin experienced a series of orogenic events culminating in the Taconic orogeny. Most of these events are generally viewed as enigmatic and isolated, but they can be viewed as a coherent tectonic sequence of events. The early stages involved broad uplifts and localized extension, especially of internal shelf and adjacent continental interiors. Later stages involved increased subsidence rates of the outer shelf, resulting in retreat of the outer margin of the carbonate platform.The beginning of volcanic activity coincides with, or immediately follows, the rapid subsidence. Onset of compressional orogenesis is often temporally separated from the initial rapid subsidence. These events can be integrated into a tectonic model in which the passive margin is converted into an active Andean margin. Early uplift and extension events represented the surface expression of the beginning of deep-seated downward mantle convection. Subsequent rapid subsidence events represented the mechanical failure of the lithosphere as the convection reaches maturity. Failure of the lithosphere resulted in a subduction zone that quickly created arc volcanism. The compressive Taconic orogenesis occurred when the arc was thrust back onto the shelf margin as the subduction zone migrated continentward in response to progressively channeled convective flow.

  1. Marginal energy prices report

    SciTech Connect

    Chaitkin, Stuart; Biermayer, Peter; Bretz, Sarah; Brown, Steve; Constantine, Sachu; Fisher, Diane; Hakim, Sajid; Liew, Lucy; Lutz, Jim; Marnay, Chris; McMahon, James E.; Moezzi, Mithra; Osborn, Julie; Rawner, Esther; Roberson, Judy; Rosenquist, Greg; Ryan, Nancy; Turiel, Isaac; Wiel, Stephen

    1999-06-24

    This report responds to a recommendation from the Department of Energy's (DOE) Advisory Committee on Appliance Energy Efficiency Standards. It presents the derivation of estimated consumer marginal energy prices for the commercial and residential sectors for use in the life-cycle cost (LCC) analyses for four of the high priority appliances' energy efficiency standards rule makings --clothes washers, water heaters,fluorescent lamp ballasts, and central airconditioners/heat pumps. Marginal prices as discussed here are those prices consumers pay (or save) for their last units of energy used (or saved). Marginal prices reflect a change in a consumer's bill (that might be associated with new energy efficiency standards) divided by the corresponding change in the amount of energy the consumer used.

  2. Structure of the North American Atlantic Continental Margin.

    USGS Publications Warehouse

    Schlee, J.S.; Klitgord, K.K.

    1986-01-01

    Off E N America, where the structure of the continental margin is essentially constructional, seismic profiles have approximated geologic cross sections up to 10-15km below the sea floor and revealed major structural and stratigraphic features that have regional hydrocarbon potential. These features include a) a block-faulted basement hinge zone; b) a deep, broad, rifted basement filled with clastic sediment and salt; and c) a buried paleoshelf-edge complex that has many forms. The mapping of seismostratigraphic units over the continental shelf, slope, and rise has shown that the margin's developmental state included infilling of a rifted margin, buildup of a carbonate platform, and construction of an onlapping continental-rise wedge that was accompanied by erosion of the slope. -from Authors

  3. Forward modeling of late pleistocene shelf-edge deltas offshore Louisiana

    SciTech Connect

    Nissen, S.E.; Combes, J.M.; Scott, R.W.

    1995-12-31

    The University of South Carolina`s SEDPAK program has been used to construct a two-dimensional forward stratigraphic model of the Louisiana shelf edge from 168 ka to the present. With eustasy as the only time-varying parameter, the model effectively reproduces the geometries of major sequence stratigraphic surfaces, general lithofacies distributions, and paleobathymetric trends. Modeling also confirms that shelf margin delta sand bodies were deposited during sea level lowstands. A more accurate model could be obtained by slight variations in sediment supply and subsidence rates through time.

  4. Evidence for a former large ice sheet in the Orville Coast- Ronne Ice Shelf area, Antarctica.

    USGS Publications Warehouse

    Carrara, P.

    1981-01-01

    The Orville Coast area of the Antarctic Peninsula was extensively glacierized in the past. Striations, polished rock surfaces, and erratics on nunatak summits indicate that this area was covered by a broad regional ice sheet whose grounded ice margin was on the continental shelf, in the present-day Ronne Ice Shelf area. If the glacial history of Antarctica has been controlled by eustatic sea-level changes, the destruction of this ice sheet would have been contemporaneous with that of the Ross Sea ice sheet due to the world-wide rise of eustatic sea-level at the end of the Wisconsin glaciation. -Author

  5. Obesity and African Americans

    MedlinePlus

    ... Data > Minority Population Profiles > Black/African American > Obesity Obesity and African Americans African American women have the ... ss6304.pdf [PDF | 3.38MB] HEALTH IMPACT OF OBESITY More than 80 percent of people with type ...

  6. Effects of basin type on coastal plain-shelf-slope systems during base-level fluctuations: An experimental approach

    SciTech Connect

    Wood, L.J. )

    1991-03-01

    Sequence stratigraphic models emphasize the importance of basin type on the reactions of coastal plain-shelf-slope systems to base-level changes. A series of experiments were performed in a 4.5 m by 7 m flume to examine the effects of a passive margin basin with a shelf/slope break versus a ramp margin basin on coastal plain-shelf-slope deposits that result from base-level fluctuations. Results indicate that basin type has a strong influence on the erosional features and deposits that develop in response to base-level fluctuations. Fluctuations that occur along a shelf/slope break margin result in well-defined, deeply incised valleys, which develop early in lowstand time and have low width:depth ratios. Rivers may incise into and deposit over outer-middle shelf deposits of the previous highstand. Late lowstand deposits are coarser grained than early lowstand deposits and include fine- and coarse-grained slope and basin floor fans. During subsequent base-level rise early transgressive deposits are confined to incised valleys. Fluctuations along a ramp margin result in shallow, wide incised valleys with high width:depth ratios, which develop late in lowstand time. Incision occurs into shelf deltaic deposits and these are overlain by valley fill deposits. Deposits of the lowstand systems tract do not coarsen upward significantly and contain only sand-rich, small, thin delta front fan deposits. During subsequent base-level rise transgressive deposits are not confined to incised valleys. Transgressive deposition within the valleys occurs over a short time interval and is followed closely by flooding of the adjacent shelf.

  7. A first approximation of sand distribution and provenance on the Canterbury Shelf, New Zealand - implications for across shelf vs. along shelf transport based on sediment cores recovered during IODP Expedition 317

    NASA Astrophysics Data System (ADS)

    Bender, C.; Marsaglia, K. M.; Browne, G.; Carson, D.; Jaeger, J. M.; Kemp, D.; Lever, H.; McHugh, C. M.; Murakoshi, N.; Richaud, M.; Tanabe, S.; Uramoto, G.; Fulthorpe, C.; Hoyanagi, K.; Blum, P.; Shipboard Scientific Party, E.

    2010-12-01

    Expedition 317 of the Integrated Ocean Drilling Program drilled a transect of four sites across the Canterbury Margin of South Island, New Zealand all of which encountered sandy sediment. Seismic correlation suggests that from west (shallow water) to east (deeper water), Sites U1353 and 1354 sampled only shelf facies, Site U1351 sampled shelf to slope facies, and Site U1352 sampled only slope to bathyal facies. Sand was present in the cores as discrete beds as well as dispersed material in muddy lithologies. Sandy beds locally occurred within well-defined facies motifs in the Quaternary section. Recovery ranged from good to poor in the thicker sand units. The dispersed sand was likely a product of mixing through bioturbation. The arrangement of lithologies encountered at Sites U1353, U1354 and the upper part of U1351 is characteristic of a dip-fed, eustatically-influenced shelf. Large onshore river systems emanating from the Southern Alps would seem the most likely delivery mechanisms of sandy sediment to the shelf. These rivers drain mostly Torlesse Terrane rocks, sedimentary to metasedimentary lithologies of Mesozoic age that produce lithic and quartz-rich sand. However, the shelf facies contain some clastics with high mica, quartz and feldspar contents that are more characteristic of schist detritus found in rivers draining the Otago Schist to the south. Thus there has been significant along shelf transport of sandy sediment. In a simple model, the proportion of Torlesse-sourced vs. schist-sourced sand should provide an approximation of the relative influence of current/wave and fluvial processes in constructing the Canterbury shelf sequences. This has implications for calculating sediment budgets for the basin.

  8. On the Evolution of Glaciated Continental Margins

    NASA Astrophysics Data System (ADS)

    Sverre Laberg, Jan; Rydningen, Tom Arne; Safronova, Polina A.; Forwick, Matthias

    2016-04-01

    Glaciated continental margins, continental margins where a grounded ice sheet repeatedly has been at or near the shelf break, are found at both northern and southern high-latitudes. Their evolution are in several aspects different from their low-latitude counterparts where eustatic sea-level variations possess a fundamental control on their evolution and where fluvial systems provide the main sediment input. From studies of the Norwegian - Barents Sea - Svalbard and NE Greenland continental margins we propose the following factors as the main control on the evolution of glaciated continental margins: 1) Pre-glacial relief controlling the accommodation space, 2) Ice sheet glaciology including the location of fast-flowing ice streams where source area morphology exerts a fundamental control, 3) Composition of the glacigenic sediments where the clay content in previous studies have been found to be important, and 4) Sea-level controlled both by eustacy and isostacy. From three case studies, 1) the western Barents Sea, 2) part of the North Norwegian (Troms), and 3) the Mid-Norwegian margin, the influence on these factors for the sea-floor morphology, sedimentary processes of the continental slope - deep sea and continental margin architecture are discussed. The pre-glacial relief of the mid-Norwegian and Troms margins relates to the onset of rifting and plate break-up from the early Cenozoic while for the SW Barents Sea, plate shear was followed by rifting. A wide zone of extended continental crust occurs offshore mid-Norway while this zone is much narrower offshore Troms leading to a more pronounced pre-glacial relief. Regarding sediment delivery and ice sheet glaciology the western Barents Sea exemplifies very high sediment input corresponding to an estimated average erosion of the source area of ~0.4 mm/yr (SW Barents Sea), much of which is related to subglacial erosion of Mesozoic - Cenozoic sedimentary rocks from large paleo-ice streams. The mid-Norwegian margin

  9. Sandstream on the northeast Brazilian shelf

    NASA Astrophysics Data System (ADS)

    Vianna, Marcio L.; Solewicz, Reynaldo; Cabral, Alexandre P.; Testa, Viviane

    1991-06-01

    upstream trough are presented, and species composition of bryozoa observed as epibionts on live gastropod shells are analysed. This sandstream shows some similarities to one described from the southeast African continental shelf, which is also under the influence of unidirectional currents.

  10. Depositional architecture and evolution of inner shelf to shelf edge delta systems since the Late Oliocene and their respone to the tectonic and sea level change, Pear River Mouth Basin, northern South China Sea

    NASA Astrophysics Data System (ADS)

    Lin, Changsong; Zhang, Zhongtao; liu, Jingyan; Jiang, Jing

    2016-04-01

    The Pear River Mouth Basin is located in the northern continent margin of the South China Sea. Since the Late Oligocene, the long-term active fluvial systems (Paleo-Zhujiang) from the western basin margin bebouched into the northern continental margin of the South China Sea and formed widespread deltaic deposits in various depositional geomorphologies and tectonic settings. Based of integral analysys of abundant seismic, well logging and drilling core data, Depositional architecture and evolution of these delta systems and their respone to the tectonic and sea level change are documented in the study. There are two basic types of the delta systems which have been recognized: inner shelf delta deposited in shallow water enviroments and the outer shelf or shelf-edge delta systems occurred in deep water settings. The paleowater depths of these delta systems are around 30 to 80m (inner shelf delta) and 400-1000m (shelf-edge delta) estimated from the thickness (decompaction) of the delta front sequences. The study shows that the inner shelf delta systems are characterized by relatively thin delta forests (20-40m), numereous stacked distributary channel fills, relative coarse river mouth bar deposits and thin distal delta front or distal bar and prodelta deposits. In contrast, the outer shelf or shelf edge delta systems are characteristic of thick (300-800m) and steep (4-60) of deltaic clinoforms, which commonly display in 3D seismic profiles as "S" shape reflection. Large scale soft-sediment deformation structures, slump or debris flow deposits consisting mainly of soft-sediment deformed beds, blocks of sandstones and siltstones or mudstones widely developed in the delta front deposits. The shelf edge delta systems are typically associated with sandy turbidite fan deposits along the prodelta slopes, which may shift basinwards as the progradation of the delta systems. The delta systems underwent several regional cycles of evolution from inner shelf deltas to shelf edge

  11. Computer simulation of shelf and stream profile geomorphic evolution resulting from eustasy and uplift

    SciTech Connect

    Johnson, R.M. )

    1993-04-01

    A two-dimensional computer simulation of shelf and stream profile evolution with sea level oscillation has been developed to illustrate the interplay of coastal and fluvial processes on uplifting continental margins. The shelf evolution portion of the simulation is based on the erosional model of Trenhaile (1989). The rate of high tide cliff erosion decreases as abrasion platform gradient decreases the sea cliff height increases. The rate of subtidal erosion decreases as the subtidal sea floor gradient decreases. Values are specified for annual wave energy, energy required to erode a cliff notch 1 meter deep, nominal low tidal erosion rate, and rate of removal of cliff debris. The values were chosen arbitrarily to yield a geomorphic evolution consistent with the present coast of northern California, where flights of uplifted marine terraces are common. The stream profile evolution simulation interfaces in real time with the shelf simulation. The stream profile consists of uniformly spaced cells, each representing the median height of a profile segment. The stream simulation results show that stream response to sea level change on an uplifting coast is dependent on the profile gradient near the stream mouth, relative to the shelf gradient. Small streams with steep gradients aggrade onto the emergent shelf during sea level fall and incise at the mountain front during sea level rise. Large streams with low gradients incise the emergent shelf during sea level fall and aggrade in their valleys during sea level rise.

  12. Quantification of Changes for the Milne Ice Shelf, Nunavut, Canada, 1950 -- 2009

    NASA Astrophysics Data System (ADS)

    Mortimer, Colleen Adel

    This study presents a comprehensive overview of the current state of the Milne Ice Shelf and how it has changed over the last 59 years. The 205 +/-1 km2 ice shelf experienced a 28% (82 +/-0.8 km 2) reduction in area between 1950 -- 2009, and a 20% (2.5 +/-0.9km 3 water equivalent (w.e.)) reduction in volume between 1981 -- 2008/2009, suggesting a long-term state of negative mass balance. Comparison of mean annual specific mass balances (up to -0.34 m w.e. yr-1) with surface mass balance measurements for the nearby Ward Hunt Ice Shelf suggest that basal melt is a key contributor to total ice shelf thinning. The development and expansion of new and existing surface cracks, as well as ice-marginal and epishelf lake development, indicate significant ice shelf weakening. Over the next few decades it is likely that the Milne Ice Shelf will continue to deteriorate.

  13. Highstand shelf fans: The role of buoyancy reversal in the deposition of a new type of shelf sand body

    USGS Publications Warehouse

    Steel, Elisabeth; Simms, Alexander R.; Warrick, Jonathan; Yokoyama, Yusuke

    2016-01-01

    Although sea-level highstands are typically associated with sediment-starved continental shelves, high sea level does not hinder major river floods. Turbidity currents generated by plunging of sediment-laden rivers at the fluvial-marine interface, known as hyperpycnal flows, allow for cross-shelf transport of suspended sand beyond the coastline. Hyperpycnal flows in southern California have deposited six subaqueous fans on the shelf of the northern Santa Barbara Channel in the Holocene. Using eight cores and nine grab samples, we describe the deposits, age, and stratigraphic architecture of two fans in the Santa Barbara Channel. Fan lobes have up to 3 m of relief and are composed of multiple hyperpycnite beds ∼5 cm to 40 cm thick. Deposit architecture and geometry suggest the hyperpycnal flows became positively buoyant and lifted off the seabed, resulting in well-sorted, structureless, elongate sand lobes. Contrary to conventional sequence stratigraphic models, the presence of these features on the continental shelf suggests that active-margin shelves may locally develop high-quality reservoir sand bodies during sea-level highstands, and that such shelves need not be solely the site of sediment bypass. These deposits may provide a Quaternary analogue to many well-sorted sand bodies in the rock record that are interpreted as turbidites but lack typical Bouma-type features.

  14. Paleogeography and evolution of the Ordovician/Silurian (Whiterockian-Llandoverian) continental margin in central Nevada

    SciTech Connect

    Britt, L.W. )

    1991-02-01

    In central Nevada, stratigraphic successions of Whiterockian-Llandoverian lithofacies, transitional with autochthonous platform/shelf carbonates to the east, occur in isolated windows in outer slope to basinal lithotopes of the Roberts Mountains allochthon. Petrologic, chronostratigraphic and lithostratigraphic, and paleontologic comparison of those successions with platform/shelf facies to the east is integral for reconstruction of Ordovician-Silurian platform margin paleogeography and pre-Antler genesis of the western North American continental margin. Numerous facies changes and/or stratigraphic omissions in central Nevada can be related to sea level fluctuation and aggradation/progradation of the carbonate platform to the east, and not to a postulated, offshore geanticline (i.e., the Toiyabe Ridge). Stratigraphic omission of the Eureka Quartzite above Pogonip equivalents in transitional successions of the Toquima Range and the presence of correlative quartzite in outer slope/basinal parautochthonous facies of the Toiyabe Range suggest development of a possible bypass-margin during the Middle Ordovician. Deposition of Late Ordovician platform margin dolostones (Ely Springs Dolostone) and upper ramp limestones (Hanson Creek Formation and Martin Ridge strata) followed Late Ordovician transgression that drowned the margin and reestablished the carbonate factory. Glacioeustatic drawdown of Late Ordovician-earliest Silurian seas due to the Gondwanan glacial fluctuation can be recognized in strata along the platform margin and upper ramp. Rapid, Early Silurian transgression produced dark-gray carbonates and may have induced marginal flexure and regional, massive slope failure in central Nevada, generating stratigraphic hiatuses west of the platform margin.

  15. Deep continental margin reflectors

    USGS Publications Warehouse

    Ewing, J.; Heirtzler, J.; Purdy, M.; Klitgord, Kim D.

    1985-01-01

    In contrast to the rarity of such observations a decade ago, seismic reflecting and refracting horizons are now being observed to Moho depths under continental shelves in a number of places. These observations provide knowledge of the entire crustal thickness from the shoreline to the oceanic crust on passive margins and supplement Consortium for Continental Reflection Profiling (COCORP)-type measurements on land.

  16. Predicting service life margins

    NASA Technical Reports Server (NTRS)

    Egan, G. F.

    1971-01-01

    Margins are developed for equipment susceptible to malfunction due to excessive time or operation cycles, and for identifying limited life equipment so monitoring and replacing is accomplished before hardware failure. Method applies to hardware where design service is established and where reasonable expected usage prediction is made.

  17. Hourly marginal emissions tool

    EPA Science Inventory

    The hourly marginal emissions tool is an excel workbook that estimates the hourly NOx, SO2 and CO2 emission reductions of energy efficiency and renewable energy policies and programs in the electric power sector. It will be based on EPA's proposed "Road map for Incorporating ene...

  18. Marginalization and School Nursing

    ERIC Educational Resources Information Center

    Smith, Julia Ann

    2004-01-01

    The concept of marginalization was first analyzed by nursing researchers Hall, Stevens, and Meleis. Although nursing literature frequently refers to this concept when addressing "at risk" groups such as the homeless, gays and lesbians, and those infected with HIV/AIDS, the concept can also be applied to nursing. Analysis of current school nursing…

  19. Contribution Margin Budgeting.

    ERIC Educational Resources Information Center

    Tambrino, Paul A.

    2001-01-01

    Describes Iowa Valley Community College District's Contribution Margin Budgeting (CMB) program, successfully implemented to stave off bankruptcy. In this program, each responsibility center receives credit for all income generated and is charged for all expenditures, and each must build its own reserve against revenue shortfalls and unanticipated…

  20. Marginality and Triangle Inequality

    NASA Astrophysics Data System (ADS)

    Nánásiová, O. L̆ga; Valášková, L̆ubica

    2010-12-01

    In this paper we study conditions for the existence of a 3-dimensional s-map on a quantum logic under assumption that marginal s-maps are known. We show that the existence of such a 3-dimensional s-map depends on the triangle inequality of d-map, which on a Boolean algebra represents a measure of symmetric difference.

  1. Volcanic passive margins

    NASA Astrophysics Data System (ADS)

    Geoffroy, Laurent

    2005-12-01

    Compared to non-volcanic ones, volcanic passive margins mark continental break-up over a hotter mantle, probably subject to small-scale convection. They present distinctive genetic and structural features. High-rate extension of the lithosphere is associated with catastrophic mantle melting responsible for the accretion of a thick igneous crust. Distinctive structural features of volcanic margins are syn-magmatic and continentward-dipping crustal faults accommodating the seaward flexure of the igneous crust. Volcanic margins present along-axis a magmatic and tectonic segmentation with wavelength similar to adjacent slow-spreading ridges. Their 3D organisation suggests a connection between loci of mantle melting at depths and zones of strain concentration within the lithosphere. Break-up would start and propagate from localized thermally-softened lithospheric zones. These 'soft points' could be localized over small-scale convection cells found at the bottom of the lithosphere, where adiabatic mantle melting would specifically occur. The particular structure of the brittle crust at volcanic passive margins could be interpreted by active and sudden oceanward flow of both the unstable hot mantle and the ductile part of the lithosphere during the break-up stage. To cite this article: L. Geoffroy, C. R. Geoscience 337 (2005).

  2. Contrasting evolutionary patterns of Lower Permian shelf and basinal facies, Midland basin, Texas

    SciTech Connect

    Mazzullo, S.J.; Reid, A.M.

    1987-05-01

    The evolution of carbonate and siliciclastic shelf-to-basin depositional systems of the Lower Permian in the Midland basin was influenced by eustatic fluctuations, changing shelf-margin biota, and concurrent tectonism. The development of these systems from Wolfcampian to Leonardian time (28-m.y. duration) involved seven distinct phases that are recognized seismically as third and lesser order cycles. These phases are (1) highstand progradational shelf carbonate packages, separated by low-stand deltaic and basinal shales, deposited during relatively long-term eustatic cycles (early Wolfcampian); the component carbonate systems evolved from ramps to distally steepened ramps associated with nonframe-building algal reefs and grainstones, with little resedimented foreshelf detritus; (2) dominantly carbonate deposition during the middle and early late Wolfcampian, with construction of offlapping (but laterally juxtaposed) progradational shelves with steep platform margins deposited during a lengthy period of stillstand and/or slow submergence; dolomitized platform-margin facies are composed of marine-cemented, sponge-algal reefs and grainstones, with resedimented carbonate megabreccia to micrite channels and lobes in the contiguous shale basin; (3) shelf emergence and erosion during a major late middle(.) to late Wolfcampian lowstand contemporaneous with basinwide tectonism, with mass wastage into the basin of the terminal Wolfcampian platform-margin carbonate section; (4) regional transgression and black shale deposition followed by a repeat of Phase 2 type systems in the latest Wolfcampian to earliest Leonardian but under arid conditions; rapid vertical platform margin accretion by dolomitized, marine-cemented, sponge-algal-coral reefs and grainstones, and deposition of thick foreshelf megabreccia wedges, aprons, channels, and lobes;

  3. Manganese flux from continental margin sediments in a transect through the oxygen minimum

    SciTech Connect

    Johnson, K.S. Monterey Bay Aquarium Research Inst., Pacific Grove, CA ); Berelson, W.M.; Iams, H.D.; Kilgore, T.E. ); Coale, K.H.; Coley, T.L.; Elrod, V.A.; Fairey, W.R.; Nowicki, J.L. )

    1992-08-28

    The flux of manganese from continental margin sediments to the ocean was measured with a free-vehicle, benthic flux chamber in a transect across the continental shelf and upper slope of the California margin. The highest fluxes were observed on the shallow continental shelf. Manganese flux decreased linearly with bottom water oxygen concentration, and the lowest flux occurred in the oxygen minimum zone (at a depth of 600 to 1,000 meters). Although the flux of manganese from continental shelf sediments can account for the elevated concentrations observed in shallow, coastal waters, the flux from sediments that intersect the oxygen minimum cannot produce the subsurface concentration maximum of dissolved manganese that is observed in the Pacific Ocean.

  4. Depositional patterns of kerogen, Atlantic Margin, North America

    SciTech Connect

    Armentrout, J.M.

    1985-02-01

    Geochemical and biostratigraphic data from offshore wells along the Atlantic margin of North America define a depositional history dominated by coastal-plain and shallow-shelf facies containing degraded and residual continent-derived kerogen. Exceptions to this generalization are 4 depositional facies containing hydrogen-rich amorphous kerogen assemblages. The rocks containing hydrogen-rich amorphous kerogen assemblages are: (1) Upper Jurassic inner-shelf facies probably deposited in algal-rich lagoonlike environments, (2) Lower Cretaceous nonmarine coaly facies, probably deposited in algal-rich swamplike environments, (3) middle Cretaceous abyssal-plain facies probably deposited by turbidity currents that originated on an algal-rich slope, and (4) Miocene outer-shelf to upper-slope facies probably deposited under algal-rich upwelling systems. Correlations of these facies to seismic packages allows for extrapolation of probable organic facies distribution throughout the continental margin. Such modeling of organic facies distributions in conjunction with plate-tectonic and ocean-circulation models permits refinement of strategies for hydrocarbon exploration.

  5. Variations in carbonate shelf cycles in response to Appalachian tectonism

    SciTech Connect

    Algeo, T.J.

    1986-05-01

    Shelf facies strata of the Upper Mississippian Bangor Limestone in northwest Georgia and southeast Tennessee comprise asymmetric regressive cycles that are similar to shallowing-upward cycles described in many ancient and modern shallow marine carbonate sequences. Typical Bangor cycles consist of a lower 0.6-m transgressive hemicycle of poorly sorted intraclast-oolite grainstones, and an upper 15-m regressive hemicycle that grades vertically from open-marine fossil wackestone and packstone through barrier-bar oolite grainstone, to burrowed lagoonal wackestone and laminated fenestral tidal-flat mudstone and dolostone. Lateral variations in the number, thickness, and facies composition of cycles were controlled by the position of each Bangor section relative to the Mississippian shoreline and shelf margin, and by localized shelf downwarping in response to Appalachian foreland basin evolution. To the northeast, at Monteagle, Tennessee, evaporitic tidal flats flanked the low-lying Nashville dome. There, laminated fenestral mudstone and dolostone dominate a thin (58-m) Bangor section, with only one major marine transgression reaching this area. At Raccoon Mountain, Tennessee, in the midshelf area, syndepositional downwarping of the Raccoon Mountain trough controlled sedimentation and deposited a thick (120-m) Bangor section containing seven cycles of highly variable thickness and facies composition. To the southeast, at Pigeon Mountain, Georgia, the outer shelf was increasingly influenced by foreland basin sedimentation during the late Bangor. There, the lower part of a thin (52-m) Bangor section contains two normal regressive cycles, but abundant thin shale laminae and frequent facies shifts in the upper 15 m document increasing clastic influx and tectonic instability in source areas to the southeast.

  6. Evolution of Northeast Atlantic magmatic continental margins

    NASA Astrophysics Data System (ADS)

    England, Richard; Cornwell, David; Ramsden, Alice

    2014-05-01

    from fissures which are now observed as the ODRS. The oceanward dip on the ODRS is predominantly the result of post-eruption differential subsidence, as opposed to syn-eruption extension. The timing of intrusion of the lower crustal sill complexes remains unclear but they are most likely to have been emplaced as the supply of magma increased, which implies they are a late stage addition. The structure of the Main Ethiopian rift appears to have been influenced by the pre-existing basement structure at an early stage in the rift process, defining the geometry of the rift and providing a control on the later magmatic phase and modification of the crust. This early influence of existing structure is less clear on the NE Atlantic margins and in the UK and Irish sectors it is difficult to link substantial along strike variations in the properties of the margin to variations in basement structures which can be traced across the continental shelf. It is possible that such variations are completely overprinted by magmatic additions to the crust to the point at which they no longer influence the break-up mechanism.

  7. High-resolution seismic stratigraphy of North Carolina continental margin, Cape Fear Terrace: sea level cyclicity, Paleobathymetry, and Gulf Stream dynamics

    SciTech Connect

    Matteucci, T.D.; Hine, A.C.; Snyder, S.W.; Riggs, S.

    1985-02-01

    A high-resolution seismic stratigraphic study of the Cape Fear Terrace (outer continental shelf off North Carolina) combined with biolithostratigraphic data has yielded a chronostratigraphic framework of the Quaternary sequences that comprise this portion of the North American continental margin. The Cape Fear Terrace is an anomalous, point-source, prograding, shelf-margin feature that has experienced positive relief through much of the Quaternary. This upbuilding or outbuilding followed a period of active, early Pliocene, submarine erosion in which the ancestral Gulf stream cut an erosional path beneath the presence shelf margin. The terrace was originally built up during a relative lowstand of sea level with the construction of a shelf-edge deltaic feature. Severe modification of this delta front occurred during a relative highstand of sea level as the Gulf Stream began to impinge upon the margin. The anomalously thick accumulation of shelf-edge sediments acted as a barrier to flow, inducing complex flow patterns of the Gulf Stream. Excavation of these sediments yielded a terrace feature with preferential erosion on the upstream side. Subsequent deposition in the terrace region may have resulted during fairly highstands of sea level, as evidenced by the presence of active seaward-prograding sand waves in the terrace region today. Once this shelf-edge bathymetric irregularity (the terrace) had been established, the Gulf Stream acted as a dynamic force inducing cellular flow structures within the shelf environment, which enabled sediments to be transported seaward along the paleo-shoals complex.

  8. OESbathy version 1.0: a method for reconstructing ocean bathymetry with generalized continental shelf-slope-rise structures

    NASA Astrophysics Data System (ADS)

    Goswami, A.; Olson, P. L.; Hinnov, L. A.; Gnanadesikan, A.

    2015-09-01

    We present a method for reconstructing global ocean bathymetry that combines a standard plate cooling model for the oceanic lithosphere based on the age of the oceanic crust, global oceanic sediment thicknesses, plus generalized shelf-slope-rise structures calibrated at modern active and passive continental margins. Our motivation is to develop a methodology for reconstructing ocean bathymetry in the geologic past that includes heterogeneous continental margins in addition to abyssal ocean floor. First, the plate cooling model is applied to maps of ocean crustal age to calculate depth to basement. To the depth to basement we add an isostatically adjusted, multicomponent sediment layer constrained by sediment thickness in the modern oceans and marginal seas. A three-parameter continental shelf-slope-rise structure completes the bathymetry reconstruction, extending from the ocean crust to the coastlines. Parameters of the shelf-slope-rise structures at active and passive margins are determined from modern ocean bathymetry at locations where a complete history of seafloor spreading is preserved. This includes the coastal regions of the North, South, and central Atlantic, the Southern Ocean between Australia and Antarctica, and the Pacific Ocean off the west coast of South America. The final products are global maps at 0.1° × 0.1° resolution of depth to basement, ocean bathymetry with an isostatically adjusted multicomponent sediment layer, and ocean bathymetry with reconstructed continental shelf-slope-rise structures. Our reconstructed bathymetry agrees with the measured ETOPO1 bathymetry at most passive margins, including the east coast of North America, north coast of the Arabian Sea, and northeast and southeast coasts of South America. There is disagreement at margins with anomalous continental shelf-slope-rise structures, such as around the Arctic Ocean, the Falkland Islands, and Indonesia.

  9. Mixed carbonate-siliciclastic sequence stratigraphy of a Paleogene transition zone continental shelf, southeastern USA

    NASA Astrophysics Data System (ADS)

    Coffey, Brian P.; Fred Read, J.

    2004-04-01

    The sequence stratigraphy and facies of the Paleogene in the subsurface of the Albemarle Basin, North Carolina was defined using 1600 thin sections of plastic impregnated well cuttings from 24 wells, wireline logs, biostratigraphic data, and seismic data. The facies formed in the transition zone between warm subtropical and temperate conditions on a swell-wave dominated, open shelf exposed to major boundary current activity. The shelf has a distinctive seismic profile consisting of a shallow inner shelf, inner-shelf break, deep shelf (depths in excess of 200 m), and the continental slope. The inner shelf was characterized by distinctive quartz sand and sandy mollusk facies inshore, passing seaward into a broad, wave-swept abrasional shelf, and then into storm-influenced bryozoan-echinoderm limestones to depths of several tens of meters. Argillaceous lime mud (marl) deposition was widespread across the deep shelf, extending onto the inner-shelf during major highstands. Sediment thickness trends were controlled by greater differential subsidence of crustal blocks within the Albemarle Basin, which considerably modified but did not obliterate the effects of eustatic sea level changes in this passive margin setting. Five supersequences were identified on seismic and in wells, each consisting of multiple regionally identifiable sequences. The Paleocene supersequence is dominated by widespread marl deposition, reflecting shelf flooding into the Late Paleocene thermal maximum. This warming corresponds with widespread inner-shelf skeletal carbonate deposition from the Late Paleocene through the Middle Eocene. The two Eocene supersequences identified are dominated by bryozoan-echinoderm-rich carbonates that formed a seismically definable sediment buildup 50 km wide by 100 m thick across the deepest inner-shelf during the Lower to early Middle Eocene. Middle to Upper Eocene supersequence highstand sequences indicate increased progradation and greater mixing of shelf

  10. Morphology of turbidite systems within an active continental margin (the Palomares Margin, western Mediterranean)

    NASA Astrophysics Data System (ADS)

    Perez-Hernandez, S.; Comas, M. C.; Escutia, C.

    2014-08-01

    The Palomares Margin, an NNE-SSW segment of the South Iberian Margin located between the Alboran and the Algerian-Balearic basins, is dissected by two major submarine canyon systems: the Gata (in the South) and the Alías-Almanzora (in the North). New swath bathymetry, side-scan sonar images, accompanied by 5 kHz and TOPAS subbottom profiles, allow us to recognize these canyons as Mediterranean examples of medium-sized turbidite systems developed in a tectonically active margin. The Gata Turbidite System is confined between residual basement seamounts and exhibits incised braided channels that feed a discrete deep-sea fan, which points to a dominantly coarse-grained turbiditic system. The Alías-Almanzora Turbidite System, larger and less confined, is a good example of nested turbiditic system within the canyon. Concentric sediment waves characterize the Alías-Almanzora deep-sea fan, and the size and acoustic character of these bedforms suggest a fine-grained turbidite system. Both canyons are deeply entrenched on a narrow continental shelf and terminate at the base of the continental slope as channels that feed deep sea fans. While the Alías-Almanzora Turbidite System is the offshore continuation of seasonal rivers, the Gata Turbidite System is exclusively formed by headward erosion along the continental slope. In both cases, left-lateral transpressive deformation influences their location, longitudinal profiles, incision at the upper sections, and canyon bending associated with specific fault segments.

  11. Strategies for managing margins.

    PubMed

    2012-08-01

    Potential Medicare and Medicaid reimbursement cuts have made it critical for home health agencies to manage their gross and net operating profit margins. Agencies need to develop tools to analyze their margins and make sure they are following best practices. Try as you may, your agency might still face the question, "Why am I not meeting my budget?" Get some answers in this session from David Berman and Andrea L. Devoti. Berman is a principal at Simione Healthcare Consultants in Hamden, CT, where he is responsible for merchant acquisitions, business valuation due diligence, and oversight of the financial monitor benchmarking tool besides serving as interim chief financial officer. Devoti is chairman of the NAHC board and President & CEO of Neighborhood Health Visiting Nurse Association in West Chester PA. PMID:23074756

  12. Africans in America.

    ERIC Educational Resources Information Center

    Hart, Ayanna; Spangler, Earl

    This book introduces African-American history and culture to children. The first Africans in America came from many different regions and cultures, but became united in this country by being black, African, and slaves. Once in America, Africans began a long struggle for freedom which still continues. Slavery, the Civil War, emancipation, and the…

  13. African Outreach Workshop 1974.

    ERIC Educational Resources Information Center

    Schmidt, Nancy J.

    This report discusses the 1974 African Outreach Workshop planned and coordinated by the African Studies Program at the University of Illinois at Urbana-Champaign. Its major aim was to assist teachers in developing curriculum units on African using materials available in their local community. A second aim was for the African Studies Program to…

  14. Reactivation of Precambrian faults on the southwestern continental margin of India: Evidence from gravity anomalies

    NASA Astrophysics Data System (ADS)

    Subrahmanyam, V.; Ramana, M. V.; Rao, D. Gopala

    1993-03-01

    Gravimetric and bathymetric studies on the southwestern continental margin of India confirm the extension of onshore NW-SE-, NNW-SSE-, N-S-, NE-SW-, ENE-WSW- and E-W-trending lineaments of Precambrian age over a considerable distance into the offshore region. The bight in the bathymetry off Coondapoor the offsets of Bessas de Pedro bank and the Cora Divh Island of the Laccadive group, the Prathap Ridge, and the inferred mid-shelf basement ridge suggest block movements on the southwestern continental margin. The physiographic expression on the Prathap Ridge (around 14° 20' N and 72° 50' E) is unaffected by some of the ENE-WSW lineaments, which probably indicates that these lineaments predate, the evolution of the topographic expression. As seafloor spreading advanced with respect to the Carlsberg Ridge, some of the ENE-WSW and NE-SW lineaments on the western continental margin appear to have been reactivated, and block movements took place. The presence of a basement ridge in the mid-shelf and the shelf margin basement high (Prathap Ridge) west of the slope resembles the structural style of a passive continental margin.

  15. Petrophysical models of high velocity lower crust on the South Atlantic rifted margins: whence the asymmetry?

    NASA Astrophysics Data System (ADS)

    Trumbull, Robert B.; Franke, Dieter; Bauer, Klaus; Sobolev, Stephan V.

    2015-04-01

    Lower crustal bodies with high seismic velocity (Vp > 7km/s) underlie seaward-dipping reflector wedges on both margins of the South Atlantic, as on many other volcanic rifted margins worldwide. A comprehensive geophysical study of the South Atlantic margins by Becker et al. (Solid Earth, 5: 1011-1026, 2014) showed a strong asymmetry in the development of high-velocity lower crust (HVLC), with about 4 times larger volumes of HVLC on the African margin. That study also found interesting variations in the vertical position of HVLC relative to seaward-dipping reflectors which question a simple intrusive vs. extrusive relationship between these lower- and upper crustal features. The asymmetry of HVLC volumes on the conjugate margins is paradoxically exactly the opposite to that of surface lavas in the Paraná-Etendeka flood basalt province, which are much more voluminous on the South American margin. This contribution highlights the asymmetric features of magma distribution on the South Atlantic margins and explores their geodynamic significance. Petrophysical models of the HVLC are presented in the context of mantle melt generation, based on thickness-velocity (H-Vp) relations. These suggest that the greater volumes and average Vp values of HVLC on the African margin are due to active upwelling and high temperature, whereas passive upwelling under a thick lithospheric lid suppressed magma generation on the South American margin. The contrast in mantle upwelling rate and lithospheric thickness on the two margins predictably causes differential uplift, and this may help explain the greater accomodation space for surface lavas on the South American side although melt generation was strongest under the African margin.

  16. Rift basins of ocean-continent convergent margins

    SciTech Connect

    Forsythe, R.D.; Newcomb, K.R.

    1986-05-01

    Modern and ancient circum-Pacific convergent margins contain many examples of forearc basins where subsidence, occurring simultaneously with subduction of oceanic lithosphere, is controlled by rifting transverse to the margin. The elongate axes of these deep and narrow basins jut obliquely from the plate margin into the interior of the forearc. Similar to aulacogens, faulting and related subsidence appear greatest at their seaward limits and decreases inland. Examples from eastern Pacific forearcs suggest that localized rifting accommodates margin-parallel extension of forearc blocks that are kinetically linked to motions along major margin-parallel strike-slip fault systems. The most prominent examples of modern forearc rift basins are the Sanak and East Sanak basins of the western Alaska Peninsula subduction zone. In this region, the continental shelf is being rifted apart by a series of northwest- and northeast-trending faults. Basement-activated normal faults bounding the basins have listric geometries. Seismostratigraphic relationships within the basins indicate the protracted, synsedimentary, and active nature of faulting and basin subsidence. Along the Peru-Chile trench, two prominent rifted basins also occur: the Gulf of Guayaquil and the Gulf of Penas-Taitao basin of southern Chile. There, margin-parallel rifting controls subsidence in localized basins at the southern terminus to margin-parallel dextral fault systems. These and other examples suggest that strike-slip motion and transverse rifting of forearcs is a common phenomenon inadequately described by existing two-dimensional models of forearcs. Margin-parallel motions of forearc blocks can be related not only to oblique plate convergence, but also to the geometric and compositional nature of the overriding and subducted plates.

  17. Passive margin formation, Timor Sea, Australia

    SciTech Connect

    Hillis, R.R. )

    1990-06-01

    Recent ODP data show that sea-floor spreading began in the Argo Abyssal Plain in the earliest Cretaceous, and not the Callovian-Oxfordian as had previously been believed. These data are now consistent with the Callovian-Valanginian rifting observed on seismic records over the adjacent continental shelf (Vulcan subbasin, western Timor Sea). Tectonic subsidence plots have been constructed for well, extrapolated well, and significant off-well (seismically based) locations in the Vulcan subbasin and adjacent highs. The fully corrected plots show relatively little tectonic subsidence during the Callovian-Valanginian rift phase, even in the depocenter of the Swan Graben, where the Callovian-Valanginian interval reaches its maximum thickness. This is atypical for a passive margin basin. Assuming an extensional origin for the margin, the absence of tectonic subsidence is considered to indicate that continental rifting in the area was wet (accompanied by major volcanic activity). Recent studies have shown that extensive volcanism may occur where rift zones cut through regions of anomalously hot mantle (100-200{degree}C above normal). The addition to the crust of igneous material, the density of which has been modified by adiabatic decompression, inhibits syn-rift subsidence. A wet rifting model also has implications for the origin of the nearby marginal plateaux such as the Scott Plateau. Their relatively thick crust and lack of subsidence may be due to igneous underplating associated with wet rifting. As such the plateaux may be regarded as transitional between oceanic and continental crust. The post-Valanginian Cretaceous subsidence of the Vulcan subbasin and adjacent areas is consistent with typical post-rift thermal subsidence, the predicted exponentially decaying subsidence history for a wet rift being indistinguishable from that of a dry rift.

  18. No reff-rimmed margins to the Permian carbonate platforms of Thailand

    NASA Astrophysics Data System (ADS)

    Dawson, Orapin; Baird, Angus; Bosence, Dan

    In the central Thailand platform, marginal buildups (Early Permian) are formed mainly by fossiliferous grainstones with boundstones occurring only as small biostrome. Boundstones have a dominant skeletal element of encrusting Archaelithoporella and Tubiphytes with calcisponges and abundant syndepositional marine cements. This boundstone texture is similar to that of many other well documented Permian examples, such as El Capitan, U.S.A. and Trogkofel, Austria. In the Ratburi area of Peninsular Thailand, small bioherms (Middle Permian) are interbedded within grainstone shoals. Boundstone textures are matrix rich, with bryozoa, the main skeletal element and Tubiphytes, the main binding element. The Peninsular Thailand buildups formed on platform interior ridges and not in a shelf marginal position. These small biostromal and biohermal buildups in central and Peninsular Thailand did not form massive shelf-margin reef bodies as have been described from Permian platforms in western U.S.A. and southern China.

  19. Rapid and widespread dispersal of flood sediment on the northern California margin

    USGS Publications Warehouse

    Wheatcroft, R.A.; Sommerfield, C.K.; Drake, D.E.; Borgeld, J.C.; Nittrouer, C.A.

    1997-01-01

    The dispersal of flood sediment from small river systems is a poorly studied, yet potentially important aspect of active continental-margin sedimentation. In January 1995, during a flood with a 30 yr return period, the Eel River (northern California) delivered an estimated 25 ?? 3 ?? 106 t (metric tons) of tine-grained (<62 ??m) sediment to the ocean. The flood formed a distinct layer on the sea bed that was centered on the 70 m isobath, extended for 30 km along shelf and 8 km across shelf, and was as thick as 8.5 cm, but contained only 6 ?? 106 t of sediment. Thus, 75% of the flood-derived sediment did not form a recount/able deposit, but was instead rapidly and widely dispersed over the continental margin. Stratigraphic models of, and compilations of sediment flux to, active continental margins need to take the dispersive nature of small river systems into account.

  20. Tectonic model of the Bering shelf in Mesozoic and Cenozoic

    NASA Astrophysics Data System (ADS)

    Krylov, K. A.

    2001-12-01

    of Kankaran Island Arc magmatism, which stepped seaward with time, in conjunction with roll-back of the subducted slab. Extension of previously deformed and accreted crust occurred in the Brooks Range, Central Alaska, and offshore Bering Shelf as well during the interval 115-90 Ma. At this time, continued of subduction along the continental margin of Asia resulted in the formation of the Okhotsk-Chukchi volcanic belt, which developed in a weakly extensional tectonic regime. The formation of several island arcs all occurred in Late Cretaceous in Koryak as a broad belt of plutons were emplaced in Alaska. The Eocene was an important time, when plate motions changed again and subsequent formation of the Aleutian Arc. Sinking of the subducted slab at this time resulted in continued extension within the Bering Shelf and the beginning of formation of deep sedimentary basins. The combination of extension with right-lateral strike-slip faulting in Alaska resulted in the formation of a right-slip boundary along the edge of the Bering Shelf, with transpressional pull-apart basins developed along the shelf edge as final consequence of these motions. In conclusion, the geodynamic history of the BSA was dictated by a series of events and complex processes related to subduction of oceanic lithosphere along the Pacific plate boundary. In particular, under the active North American continental margin, two similar events involved the southern migration of subduction zones, together with variable extension of crust

  1. Farallon slab detachment and deformation of the Magdalena Shelf, southern Baja California

    USGS Publications Warehouse

    Brothers, Daniel S.; Harding, Alistair J.; Gonzalez-Fernandez, Antonio; Holbrook, W.S. Steven; Kent, Graham M.; Driscoll, Neal W.; Fletcher, John M.; Lizarralde, Daniel; Umhoefer, Paul J.; Axen, Gary

    2012-01-01

    Subduction of the Farallon plate beneath northwestern Mexico stalled by ~12 Ma when the Pacific-Farallon spreading-ridge approached the subduction zone. Coupling between remnant slab and the overriding North American plate played an important role in the capture of the Baja California (BC) microplate by the Pacific Plate. Active-source seismic reflection and wide-angle seismic refraction profiles across southwestern BC (~24.5°N) are used to image the extent of remnant slab and study its impact on the overriding plate. We infer that the hot, buoyant slab detached ~40 km landward of the fossil trench. Isostatic rebound following slab detachment uplifted the margin and exposed the Magdalena Shelf to wave-base erosion. Subsequent cooling, subsidence and transtensional opening along the shelf (starting ~8 Ma) starved the fossil trench of terrigenous sediment input. Slab detachment and the resultant rebound of the margin provide a mechanism for rapid uplift and exhumation of forearc subduction complexes.

  2. Coordination: southeast continental shelf studies. Progress report

    SciTech Connect

    Menzel, D.W.

    1980-03-01

    The GABEX I experiment is designed to provide synoptic coverage of a series of Gulf Stream wave-like disturbances, the effect of these on the circulation of the entire shelf, and on biological and chemical processes. This study was initiated in February 1980 when current meter arrays were deployed. These meters will be removed in July 1980. In April three ships will simultaneously study the effects of Gulf Stream disturbances on the hydrography, chemistry, and biology of the shelf. One vessel will track a specific wave-like disturbance and provide synoptic coverage of the shelf area. The second vessel will determine the effect of shelf break processes on adjacent shelf water; and the third will study trace metal distributions in and outside of disturbances. Research progress is reported in continental shelf studies, nearshore and estuarine studies (diffusion of freshwater out of nearshore zone), tidal currents and material transport, and mixing of inlet plumes.

  3. Is Submarine Groundwater Discharge a Gas Hydrate Formation Mechanism on the Circum-Arctic Shelf?

    NASA Astrophysics Data System (ADS)

    Frederick, J. M.; Buffett, B. A.

    2015-12-01

    Methane hydrate is an ice-like solid that can sequester large quantities of methane gas in marine sediments along most continental margins where thermodynamic conditions permit its formation. Along the circum-Arctic shelf, relict permafrost-associated methane hydrate deposits formed when non-glaciated portions of the shelf experienced subaerial exposure during ocean transgressions. Gas hydrate stability and the permeability of circum-Arctic shelf sediments to gas migration is closely linked with relict submarine permafrost. Heat flow observations on the Alaskan North Slope and Canadian Beaufort Shelf suggest the movement of groundwater offshore, but direct observations of groundwater flow do not exist. Submarine discharge, an offshore flow of fresh, terrestrial groundwater, can affect the temperature and salinity field in shelf sediments, and may be an important factor in submarine permafrost and gas hydrate evolution on the Arctic continental shelf. Submarine groundwater discharge may also enhance the transport of organic matter for methanogenesis within marine sediments. Because it is buoyancy-driven, the velocity field contains regions with a vertical (upward) component as groundwater flows offshore. This combination of factors makes submarine groundwater discharge a potential mechanism controlling permafrost-associated gas hydrate evolution on the Arctic continental shelf. In this study, we quantitatively investigate the feasibility of submarine groundwater discharge as a control on permafrost-associated gas hydrate formation on the Arctic continental shelf, using the Canadian Beaufort Shelf as an example. We have developed a shelf-scale, two-dimensional numerical model based on the finite volume method for two-phase flow of pore fluid and methane gas within Arctic shelf sediments. The model tracks the evolution of the pressure, temperature, salinity, methane gas, methane hydrate, and permafrost fields given imposed boundary conditions, with latent heat of

  4. Rheology of the Ronne Ice Shelf, Antarctica, Inferred from Satellite Radar Interferometry Data using an Inverse Control Method

    NASA Technical Reports Server (NTRS)

    Larour, E.; Rignot, E.; Joughin, I.; Aubry, D.

    2005-01-01

    The Antarctic Ice Sheet is surrounded by large floating ice shelves that spread under their own weight into the ocean. Ice shelf rigidity depends on ice temperature and fabrics, and is influenced by ice flow and the delicate balance between bottom and surface accumulation. Here, we use an inverse control method to infer the rigidity of the Ronne Ice Shelf that best matches observations of ice velocity from satellite radar interferometry. Ice rigidity, or flow law parameter B, is shown to vary between 300 and 900 kPa a(sup 1/3). Ice is softer along the side margins due to frictional heating, and harder along the outflow of large glaciers, which advect cold continental ice. Melting at the bottom surface of the ice shelf increases its rigidity, while freezing decreases it. Accurate numerical modelling of ice shelf flow must account for this spatial variability in mechanical characteristics.

  5. Breakup of Pack Ice, Antarctic Ice Shelf

    NASA Technical Reports Server (NTRS)

    1991-01-01

    Breakup of Pack Ice along the periphery of the Antarctic Ice Shelf (53.5S, 3.0E) produced this mosaic of ice floes off the Antarctic Ice Shelf. Strong offshore winds, probably associated with strong katabatic downdrafts from the interior of the continent, are seen peeling off the edges of the ice shelf into long filamets of sea ice, icebergs, bergy bits and growlers to flow northward into the South Atlantic Ocean. 53.5S, 3.0E

  6. Coral reef complexes at an atypical windward platform margin: Late Quaternary, southeast Florida

    USGS Publications Warehouse

    Lidz, B.H.

    2004-01-01

    Major coral reef complexes rim many modern and ancient carbonate platforms. Their role in margin evolution is not fully understood, particularly when they border a margin atypical of the classic model. Classic windward margins are steeply inclined. The windward margin of southeast Florida is distinct with a very low-gradient slope and a shelf edge ringed with 30-m-high Quaternary outlier reefs on a shallow upper-slope terrace. A newly developed synthesis of temporally well-constrained geologic events is used with surface and subsurface seismic-reflection contours to construct morphogenetic models of four discontinuous reef-complex sequences. The models show uneven subsurface topography, upward and landward buildups, and a previously unreported, rapid, Holocene progradation. The terms backstepped reef-complex margin, backfilled prograded margin, and coalesced reef-complex margin are proposed for sections exhibiting suitable signatures in the stratigraphic record. The models have significant implications for interpretation of ancient analogues. The Florida record chronicles four kinds of geologic events. (1) Thirteen transgressions high enough for marine deposition occurred between ca. 325 ka and the present. Six gave rise to stratigraphically successive coral reef complexes between ca. 185 and ca. 77.8 ka. The seventh reef ecosystem is Holocene. (2) Two primary coral reef architectures built the outer shelf and margin, producing respective ridge-and-swale and reef-and-trough geometries of very different scales. (3) Massive outlier reefs developed on an upper-slope terrace between ca. 106.5 and ca. 80 ka and are inferred to contain corals that would date to highstands at ca. 140 and 125 ka. (4) Sea level remained below elevation of the shelf between ca. 77.8 and ca. 9.6 ka. ?? 2004 Geological Society of America.

  7. Geomorphic characterization of the U.S. Atlantic continental margin

    USGS Publications Warehouse

    Brothers, Daniel S.; ten Brink, Uri S.; Andrews, Brian D.; Chaytor, Jason D.

    2013-01-01

    The increasing volume of multibeam bathymetry data collected along continental margins is providing new opportunities to study the feedbacks between sedimentary and oceanographic processes and seafloor morphology. Attempts to develop simple guidelines that describe the relationships between form and process often overlook the importance of inherited physiography in slope depositional systems. Here, we use multibeam bathymetry data and seismic reflection profiles spanning the U.S. Atlantic outer continental shelf, slope and rise from Cape Hatteras to New England to quantify the broad-scale, across-margin morphological variation. Morphometric analyses suggest the margin can be divided into four basic categories that roughly align with Quaternary sedimentary provinces. Within each category, Quaternary sedimentary processes exerted heavy modification of submarine canyons, landslide complexes and the broad-scale morphology of the continental rise, but they appear to have preserved much of the pre-Quaternary, across-margin shape of the continental slope. Without detailed constraints on the substrate structure, first-order morphological categorization the U.S. Atlantic margin does not provide a reliable framework for predicting relationships between form and process.

  8. Hydrocarbon traps within passive-margin evolution of Louisiana

    SciTech Connect

    Lavoie, D. ); Lowrie, A.

    1993-09-01

    The evolutionary dynamics of the Louisiana continental margin as applied to the Neogene to present are sufficiently well understood that we present a preliminary model. The external components influencing the geologic evolution are sediment input (amount, type, and transport mechanisms) and sea level oscillations (periodicity and range). The internal dynamics are subsidence (rate, total amount, and location), salt tectonics (type and rate of motion), and sediment deposition (amount, type and mechanisms). The model presented is restricted geographically to the offshore region, from the shelf to the Sigsbee Escarpment, and temporally during the Neogene, the past 20 m.y. The notion that tectonic periodicity controls the evolutionary dynamics is integral to the model. The general loci of maximal deposition and tectonics are dictated by Milankovitch fourth-order cycles ranging from 1 x 10[sup 4] to 1 x 10 [sup 5] yr. superimposed on third-order cycles of up to 1 to 2 x 10[sup 6]yr. This model suggests a highly energetic phase in overall continental margin evolution during which the Sigsbee salt wedge migrated past an arbitrary fixed reference point, changing the physiography from lower slope to shelf. The energetic phase, which lasts between 2 and 4 m.y., separated two much longer phases are the drift phase, characterized by sedimentation along lower continental rises and abyssal plains, and a depositional phase, generally minor, and erosion along the shelf, coastal plain, and interior basins. This latter phase is characterized by regional subsidence and [open quotes]catch-up[close quotes] deposition as equilibrium along the continent is maintained. We also discuss hydrocarbon traps and their ephemeral nature with the overall continental margin.

  9. Morphology and evolution of salt/mini-basin systems: Lower shelf and upper slope, central offshore Louisiana

    SciTech Connect

    Sumner, H.S.; Robinson, B.A.; Dirks, W.K.; Holliday, J.C. )

    1991-03-01

    A complex array of deformed salt bodies and localized salt withdrawal depocenters termed 'mini-basins' dominate the subsurface geology of the central Louisiana lower shelf and upper slope. Individual salt bodies climb basinward as horizontal sills supplied by vertical to inclined 'feeders.' Each level of salt remobilization and emplacement is overlain by an association of structural features that occurs repetitively in space and time owing to sediment loading caused by progradation of Miocene to Recent shelf margins. Mini-basins are generally circular to elliptical (in map view) and contain thick sequences of sediment ponded by salt withdrawal. Mini-basins on the slope typically have a simple symmetric to asymmetric internal structure and are often almost entirely bounded by large, bathymetrically high salt bodies. Salt displaced out of slope mini-basins is loaded by the advancing shelf margin, initiating formation of shelf mini-basins. Shelf mini-basins generally have well-defined fault/piercement salt boundaries and a strongly asymmetric to the south internal structure. Salt displaced downdip by shelf mini-basin loading forms major shallow-seated sill-like bodies that intrude upper slope strata. Subsequent reactivation of these allochthonous salt sills often results in a secondary level of structural features that detach on a salt withdrawal surface ('Roho'). Salt/mini-basin systems exert a profound influence on hydrocarbon distribution. Current production occurs mainly in traps associated with shelf mini-basins. Additional production occurs in traps found above reactived salt sills ('Rohos') on the shelf. Industry exploration is presently focused on identification of traps in the slope mini-basin setting.

  10. 26 CFR 1.638-1 - Continental Shelf areas.

    Code of Federal Regulations, 2012 CFR

    2012-04-01

    ... 26 Internal Revenue 7 2012-04-01 2012-04-01 false Continental Shelf areas. 1.638-1 Section 1.638-1...) INCOME TAXES (CONTINUED) Continental Shelf Areas § 1.638-1 Continental Shelf areas. (a) General rule. For.... The terms Continental Shelf of the United States and Continental Shelf of a possession of the...

  11. 26 CFR 1.638-1 - Continental Shelf areas.

    Code of Federal Regulations, 2011 CFR

    2011-04-01

    ... 26 Internal Revenue 7 2011-04-01 2009-04-01 true Continental Shelf areas. 1.638-1 Section 1.638-1...) INCOME TAXES (CONTINUED) Continental Shelf Areas § 1.638-1 Continental Shelf areas. (a) General rule. For.... The terms Continental Shelf of the United States and Continental Shelf of a possession of the...

  12. 26 CFR 1.638-1 - Continental Shelf areas.

    Code of Federal Regulations, 2013 CFR

    2013-04-01

    ... 26 Internal Revenue 7 2013-04-01 2013-04-01 false Continental Shelf areas. 1.638-1 Section 1.638-1...) INCOME TAXES (CONTINUED) Continental Shelf Areas § 1.638-1 Continental Shelf areas. (a) General rule. For.... The terms Continental Shelf of the United States and Continental Shelf of a possession of the...

  13. 26 CFR 1.638-1 - Continental Shelf areas.

    Code of Federal Regulations, 2010 CFR

    2010-04-01

    ... 26 Internal Revenue 7 2010-04-01 2010-04-01 true Continental Shelf areas. 1.638-1 Section 1.638-1...) INCOME TAXES (CONTINUED) Continental Shelf Areas § 1.638-1 Continental Shelf areas. (a) General rule. For.... The terms Continental Shelf of the United States and Continental Shelf of a possession of the...

  14. 26 CFR 1.638-1 - Continental Shelf areas.

    Code of Federal Regulations, 2014 CFR

    2014-04-01

    ... 26 Internal Revenue 7 2014-04-01 2013-04-01 true Continental Shelf areas. 1.638-1 Section 1.638-1...) INCOME TAXES (CONTINUED) Continental Shelf Areas § 1.638-1 Continental Shelf areas. (a) General rule. For.... The terms Continental Shelf of the United States and Continental Shelf of a possession of the...

  15. The Cadiz margin study off Spain: An introduction

    USGS Publications Warehouse

    Nelson, C.H.; Maldonado, A.

    1999-01-01

    The Cadiz continental margin of the northeastern Gulf of Cadiz off Spain was selected for a multidisciplinary project because of the interplay of complex tectonic history between the Iberian and African plates, sediment supply from multiple sources, and unique Mediterranean Gateway inflow and outflow currents. The nature of this complex margin, particularly during the last 5 million years, was investigated with emphasis on tectonic history, stratigraphic sequences, marine circulation, contourite depositional facies, geotechnical properties, geologic hazards, and human influences such as dispersal of river contaminants. This study provides an integrated view of the tectonic, sediment supply and oceanographic factors that control depositional processes and growth patterns of the Cadiz and similar modem and ancient continental margins.

  16. OESbathy version 1.0: a method for reconstructing ocean bathymetry with realistic continental shelf-slope-rise structures

    NASA Astrophysics Data System (ADS)

    Goswami, A.; Olson, P. L.; Hinnov, L. A.; Gnanadesikan, A.

    2015-04-01

    We present a method for reconstructing global ocean bathymetry that uses a plate cooling model for the oceanic lithosphere, the age distribution of the oceanic crust, global oceanic sediment thicknesses, plus shelf-slope-rise structures calibrated at modern active and passive continental margins. Our motivation is to reconstruct realistic ocean bathymetry based on parameterized relationships of present-day variables that can be applied to global oceans in the geologic past, and to isolate locations where anomalous processes such as mantle convection may affect bathymetry. Parameters of the plate cooling model are combined with ocean crustal age to calculate depth-to-basement. To the depth-to-basement we add an isostatically adjusted, multicomponent sediment layer, constrained by sediment thickness in the modern oceans and marginal seas. A continental shelf-slope-rise structure completes the bathymetry reconstruction, extending from the ocean crust to the coastlines. Shelf-slope-rise structures at active and passive margins are parameterized using modern ocean bathymetry at locations where a complete history of seafloor spreading is preserved. This includes the coastal regions of the North, South, and Central Atlantic Ocean, the Southern Ocean between Australia and Antarctica, and the Pacific Ocean off the west coast of South America. The final products are global maps at 0.1° × 0.1° resolution of depth-to-basement, ocean bathymetry with an isostatically adjusted, multicomponent sediment layer, and ocean bathymetry with reconstructed continental shelf-slope-rise structures. Our reconstructed bathymetry agrees with the measured ETOPO1 bathymetry at most passive margins, including the east coast of North America, north coast of the Arabian Sea, and northeast and southeast coasts of South America. There is disagreement at margins with anomalous continental shelf-slope-rise structures, such as around the Arctic Ocean, the Falkland Islands, and Indonesia.

  17. The crustal structure of the southern Argentine margin

    NASA Astrophysics Data System (ADS)

    Becker, Katharina; Franke, Dieter; Schnabel, Michael; Schreckenberger, Bernd; Heyde, Ingo; Krawczyk, Charlotte M.

    2012-06-01

    Multichannel reflection seismic profiles, combined with gravimetric and magnetic data provide insight into the crustal structure of the southernmost Argentine margin, at the transition from a rifted to a transform margin and outline the extent of the North Falkland Graben. Based on these data, we establish a regional stratigraphic model for the post-rift sediments, comprising six marker horizons with a new formation in the Barremian/Lower Cretaceous. Our observations support that a N-S trending subsidiary branch of the North Falkland Graben continues along the continental shelf and slope to the Argentine basin. During the rift phase, a wide shelf area was affected by the E-W extension, subsequently forming the North Falkland Graben and the subsidiary branch along which finally breakup occurred. We propose the division of the margin in two segments: a N-S trending rifted margin and an E-W trending transform margin. This is further underpinned by crustal scale gravity modelling. Three different tectono-dynamic processes shaped the study area. (1) The Triassic/Early Jurassic extensional phase resulting in the formation of the North Falkland Graben and additional narrower rift grabens ended synchronously with the breakup of the South Atlantic in the early Valanginian. (2) Extensional phase related to the opening of the South Atlantic. (3) The transform margin was active in the study area from about Hauterivian times and activity lasted until late Cretaceous/early Cenozoic. Both, the rifted margin and the transform margin are magma-poor. Very limited structures may have a volcanic origin but are suggested to be post-rift. The oceanic crust was found to be unusually thin, indicating a deficit in magma supply during formation. These findings in combination with the proposed breakup age in the early Valanginian that considerably predates the formation of the Paraná-Etendeka continental flood basalt provinces in Brazil and Namibia question the influence of the Tristan da

  18. African American Women Principals: Heeding the Call to Serve as Conduits for Transforming Urban School Communities

    ERIC Educational Resources Information Center

    Newcomb, Whitney Sherman; Niemeyer, Arielle

    2015-01-01

    African American women leaders are often found in urban schools that have been exhausted of resources and lack support. However, due to their disproportionate representation in urban schools, African American women principals have become adept at uniting and engaging stakeholders in marginalized school settings into action. The intent for this…

  19. Superstar or Scholar? African American Male Youths' Perceptions of Opportunity in a Time of Change

    ERIC Educational Resources Information Center

    Conchas, Gilberto Q.; Lin, Alex R.; Oseguera, Leticia; Drake, Sean J.

    2015-01-01

    Through a Multiple Marginality Framework, this exploratory case study highlights how African American male youth in an urban high school setting perceive the opportunity structure during the historic election of the first African American President. Youth optimism generated by Obama's election gives students a sense of hope despite the persistent…

  20. Amphetamine margin in sports

    SciTech Connect

    Laties, V.G.; Weiss, B.

    1981-10-01

    The amphetamines can enhance athletic performance. That much seem clear from the literature, some of which is reviewed here. Increases in endurance have been demonstrated in both humans and rats. Smith and Beecher, 20 years ago, showed improvement of running, swimming, and weight throwing in highly trained athletes. Laboratory analogs of such performances have also been used and similar enhancement demonstrated. The amount of change induced by the amphetamines is usually small, of the order of a few percent. Nevertheless, since a fraction of a percent improvement can make the difference between fame and oblivion, the margin conferred by these drugs can be quite important.

  1. Structural Margins Assessment Approach

    NASA Technical Reports Server (NTRS)

    Ryan, Robert S.

    1988-01-01

    A general approach to the structural design and verification used to determine the structural margins of the space vehicle elements under Marshall Space Flight Center (MSFC) management is described. The Space Shuttle results and organization will be used as illustrations for techniques discussed. Given also are: (1) the system analyses performed or to be performed by, and (2) element analyses performed by MSFC and its contractors. Analysis approaches and their verification will be addressed. The Shuttle procedures are general in nature and apply to other than Shuttle space vehicles.

  2. Linking margin morphology to sedimentary processes along the US East Coast passive continental margin

    NASA Astrophysics Data System (ADS)

    Brothers, D. S.; ten Brink, U. S.; Andrews, B.; Twichell, D.

    2010-12-01

    The morphology of the US East Coast continental slope and rise has a surprising amount of along-margin variation. Multibeam bathymetry datasets that cover the slope and rise from Cape Hatteras to Georges Bank provide a unique opportunity to analyze both first-order and higher-order morphologies, including submarine canyons, landslides, slumps and sedimentary bedforms. Using the morphological characterization coupled with seismic and core data, we hope to better understand how ancient and modern sedimentary processes control the shape of the margin. As a first step, the margin bathymetry was subdivided into 20 shelf-perpendicular regions from which several statistical parameters were analyzed. Within each region, the slope gradient was computed separately for down-slope and across-slope aspect directions. Distribution curves in each region for down- and across-slope gradients and seafloor roughness as functions of depth were grouped according to their statistical similarities. Four basic groups emerge and each approximately corresponds to known regions of Quaternary glacial, fluvial, current-controlled and gravity-driven sedimentary transport. In the second part of the study, published lithologic and chronostratigraphic frameworks of this margin were used to examine the relationship between seafloor morphology and the underlying geology. Along the upper continental rise, thick Quaternary deposits appear to have a strong influence on the short- and long-wavelength variation in rise topography, revealing a complex interplay between down-slope and along-slope sediment transport. Despite the close correlation between continental slope morphology and Quaternary environmental conditions, initial results suggest that the underlying, older, stratigraphy also plays a primary role. Along the continental slope, Quaternary processes appear to control the relief of slope-confined canyons and other short-wavelength (<5 km) topography, but the first order morphology of the slope

  3. A Combined MCS and Refraction Study of the Convergent Nicaraguan Margin

    NASA Astrophysics Data System (ADS)

    Berhorst, A.; Flueh, E. R.; McIntosh, K.; Ranero, C. R.; Walther, C. H.; Dole, J.

    2001-12-01

    The presented seismic profile is part of an extensive MCS and wide-angle survey. The seismic measurements were carried out using a 6 km long streamer, 14 ocean bottom hydrophones (OBH) and 12 landstations recording marine airgun shots. One important objective of this study is the imaging of the subduction zone from the trench to the coast and the determination of the margin P-wave velocity structure. The image of the upper seismogenic plate boundary zone beneath the shelf should improve the knowledge of the seismogenic character and the evolution of this convergent margin. The profile was acquired during a cruise with the R/V Maurice Ewing as part of the MARGINS program. The purpose of this seismic investigation was to improve the scarce knowledge about the structure of the Nicaraguan margin, which is located between the intensively studied margins of Guatemala to the north and Costa Rica to the south.

  4. The multifaceted West Greenland passive margin

    NASA Astrophysics Data System (ADS)

    Breuer, Sonja; Damm, Volkmar; Block, Martin; Schreckenberger, Bernd; Heyde, Ingo; Nelson, Catherine; Kouwe, Wim

    2013-04-01

    The Baffin Bay located between Greenland and Canada, is the northward extension of the Labrador Sea. The Davis Strait High separates these two marine basins. The evolution of these basins is closely linked, and is as well affiliated to the opening of the North Atlantic Ocean. The opening history started in the Cretaceous with the formation of several terrestrial rift basins with a block-faulted, metamorphic Precambrian basement. The further opening of the Baffin Bay coincides with the volcanic activity (60.9-52.5 Ma) along the West Greenland margin (Storey et al., 1998). The subsequent seafloor spreading in the Baffin Bay is linked to the Labrador Sea by the Ungava Fault Zone (UFZ), which is the most prominent transform fault in this region. Two main problems are still unsolved: 1) There are clear indications for normal seafloor spreading in the Baffin Bay like the seaward dipping reflectors (SDRs) on the Canadian side (Skaarup et al., 2006) and on the Greenland side based on our data. On the other hand, associated magnetic spreading anomalies are not yet discovered in the Baffin Bay or are not formed. These findings may either point to slow or ultraslow spreading or underlying strongly extended continental crust and/or serpentinised mantle. 2) The Greenlandic margin is much wider than the Canadian. In addition, a breakup unconformity can only be traced on the Greenland side and is not reported for the Canadian side. Which process causes this asymmetric margin and differences in shelf width? Is it a result of asymmetric spreading or connected to volcanic activity during breakup processes? In summer 2008, a marine geoscientific expedition (MSM09/03) was conducted with the research vessel "Maria S. Merian" in the Davis Strait and southern Baffin Bay. Approximately 1800 km of multichannel reflection seismic data were acquired. To supplement the database, a subsequent marine geoscientific expedition ARK-XXV/3 with RV POLARSTERN in summer 2010 was conducted. In our

  5. Preliminary summary of the 1976 Atlantic Margin Coring Project of the U.S. Geological Survey

    USGS Publications Warehouse

    Hathaway, John Cummins; Schlee, J.J.; Poag, C.W.; Valentine, P.C.; Weed, E.G.A.; Bothner, Michael H.; Kohout, F.A.; Manheim, F. T.; Schloam, R.; Miller, R.E.; Schultz, D.M.

    1976-01-01

    The U.S. Geological Survey Atlantic Margin Coring Project, 1976, a 60-day expedition to obtain core samples by drilling beneath the floor of the Continental Shelf and Slope of the eastern United States, was carried out in July, August, and September 1976 aboard D/V GLOMAR CONCEPTION. The coring penetrated as much as 310 meters below the sea floor at 19 sites along the continental margin from Georgia to Georges Bank off New England in water depths ranging from 20 to 300 meters; 1,020 meters of material were recovered in 380 cores, ranging in age from Late Cretaceous to Holocene. One of the major findings was the discovery of relatively fresh water (salinities less than 3 parts per thousand) extending beneath the Continental Shelf as much as 60 nautical miles seaward from the New Jersey coast. Water of about 1 part per thousand salinity was found beneath the shelf more than 7 nautical miles off Ocean City, Maryland and Barnegat Inlet, New Jersey. Analyses for light hydrocarbons in the cores show the highest concentrations (as much as 412,000 ppm) at sites in water depth greater than 200 meters (the shelf-slope break), principally in Pleistocene sediments, although methane concentrations greater than 400,000 ppm also were found in Miocene sediments at one site near the shelf edge. (Woodard-USGS)

  6. New High-Resolution Mapping of Submarine Canyons in the Mid-Atlantic Continental Margin

    NASA Astrophysics Data System (ADS)

    ten Brink, U. S.; Chaytor, J. D.; Brothers, D. S.; Twichell, D. C.; Ross, S. W.; Brooke, S.

    2011-12-01

    During June 2011, a BOEMRE/NOAA/USGS -funded multibeam bathymetry survey mapped the upper reaches (<900-m depths) of the Norfolk, Washington, and Baltimore canyons. Combined with existing multibeam bathymetry of the continental slope and rise, the new data provide a detailed view of the sedimentary processes that shaped the mid-Atlantic margin. The shelf-breaching canyon heads are surrounded by two terraces at depths of 95-100 m and 115-125 m in the Norfolk and Washington canyons and at depths of 115-125 m and 135-145 m in the Baltimore canyon. These terraces may represent paleo-shorelines formed during sea level stillstands. The canyon thalwegs within the shelf appear to be filled with sand in accord with old core results. The gradient of their thalweg profile is variable and relatively low across the shelf, slope, and upper rise, in contrast to the concave gradient of most non-shelf breaching canyons in the region. A few of the non-shelf breaching canyons in the mid-Atlantic margin also have relatively low and variable gradients suggesting that they once breached the shelf but are now completely filled. The seaward extensions of the Norfolk, Washington, and Baltimore canyons onto the continental rise are characterized by channels bordered by 100-200 m high levees. In places, these channels meander tightly. The extensions of other canyons onto the rise are either defined by subtle, linear depressions or cannot be traced. Channel-capture by adjacent canyons and channel abandonment originate in the lower slope and were prompted by either landslides or levee breaching. These observations indicate dynamic outer shelf deltas fed by large rivers, which were active at the last glacial maximum (LGM). The river channels on the shelf have been progressively filled during the Holocene. The clear expression of levied channels on the continental rise that extend from shelf-breaching canyons suggests that these canyons were the last ones to deliver turbidity flows to the rise

  7. A new bathymetry of the Northeast Greenland continental shelf: Constraints on glacial and other processes

    NASA Astrophysics Data System (ADS)

    Arndt, Jan Erik; Jokat, Wilfried; Dorschel, Boris; Myklebust, Reidun; Dowdeswell, Julian A.; Evans, Jeffrey

    2015-10-01

    A new digital bathymetric model (DBM) for the Northeast Greenland (NEG) continental shelf (74°N-81°N) is presented. The DBM has a grid cell size of 250 m × 250 m and incorporates bathymetric data from 30 multibeam cruises, more than 20 single-beam cruises and first reflector depths from industrial seismic lines. The new DBM substantially improves the bathymetry compared to older models. The DBM not only allows a better delineation of previously known seafloor morphology but, in addition, reveals the presence of previously unmapped morphological features including glacially derived troughs, fjords, grounding-zone wedges, and lateral moraines. These submarine landforms are used to infer the past extent and ice-flow dynamics of the Greenland Ice Sheet during the last full-glacial period of the Quaternary and subsequent ice retreat across the continental shelf. The DBM reveals cross-shelf bathymetric troughs that may enable the inflow of warm Atlantic water masses across the shelf, driving enhanced basal melting of the marine-terminating outlet glaciers draining the ice sheet to the coast in Northeast Greenland. Knolls, sinks, and hummocky seafloor on the middle shelf are also suggested to be related to salt diapirism. North-south-orientated elongate depressions are identified that probably relate to ice-marginal processes in combination with erosion caused by the East Greenland Current. A single guyot-like peak has been discovered and is interpreted to have been produced during a volcanic event approximately 55 Ma ago.

  8. Sedimentary processes on the Bengal continental shelf as revealed by clay-size mineralogy

    NASA Astrophysics Data System (ADS)

    Segall, M. P.; Kuehl, S. A.

    1992-04-01

    Shelf sediment dispersal seaward of the Ganges-Brahmaputra river system is examined during January/February (low discharge) through analysis of sediment cores and surface grab samples for clay-size mineralogy. Illite is the dominant mineral in surficial sediments landward of the 50-m isobath, comprising >40% of the clay-size assemblage. Kaolinite concentrations of surficial sediments generally are greater than 20% throughout the area; however, very high abundances occur off Bhola Island (25-33%) and the Chittagong coast (34-39%), probably as a result of extensive shoreline erosion of cultivated areas enriched in kaolinite. Smectite and illite/smectite concentrations, reflecting input from distributaries of the Ganges, are enriched on the western inner shelf and along the western margin of the "Swatch of No Ground", a major submarine canyon west of the present river mouth. Although surficial chlorite abundances on the inner shelf in water depths <20m generally are low to negligible (<5%), mid-shelf abundances range from 5 to 13% with the highest concentrations near the head of the submarine canyon. These high concentrations probably reflect sediment dispersal to the mid-shelf region during the high-discharge season (May-October) when chlorite-enriched sediments from the nearby Himalayas are discharged by the river. During low discharge, chlorite-poor sediments are concentrated on the inner shelf and do not dilute the "relict" chlorite signal which is preserved offshore. Downcore zones of high chlorite concentrations east of the submarine canyon indicate that at least part of the high-discharge signal is preserved in the sedimentary record. Chlorite is depleted west of the canyon, suggesting that the "Swatch of No Ground" presently interrupts the westward along-shelf transport of riverine sediments.

  9. Humans, Tectonics and Climate, Changing S2S Systems over Time: Waipaoa River Margin Example

    NASA Astrophysics Data System (ADS)

    Kuehl, S. A.; Alexander, C. R.; Corbett, D. R.; Harris, C. K.; Ogston, A. S.; Orpin, A. R.; Walsh, J. P.

    2013-12-01

    Recent interdisciplinary studies of the Waipaoa River margin, New Zealand North Island, provide a clear picture of human and natural signal propagation and preservation in Holocene and contemporary sedimentary sequences of the continental shelf and slope. This active margin setting presents both extraordinary high sediment yields and high sediment accommodation which are controlled, in part, by tectonic uplift and deformation. Unlike many passive margins, the resulting sediment deposits on the Waipaoa shelf and slope provide a high-resolution record of changing climate and landscape since the Last Glacial Maximum (LGM). Giant piston cores collected using the Marion Dufresne reveal definitive textural and carbon isotopic downcore trends that clearly reflect the transgressive and regressive phases of relative sea-level since the LGM. Moreover, A distinct coarsening in sediment texture around 3ka reflects the intensification of the El Nino Southern Oscillation in the western Pacific Ocean at that time, either through increased landsliding caused by enhanced precipitation, or by higher significant wave heights and resuspension that may have accompanied increased cyclonic storm activity. A distinct signature of fine sediment in the upper sections of the Marion Dufresne cores is thought to herald the Anthropocene period, reflecting deforestation and an increase both in sediment load and the release of fine sediment from the catchment from runaway gully formation. Sediment budgets indicate that whereas the continental shelf off the Waipaoa was a very efficient sediment trap during much of the Holocene, the Anthropocene is marked by the dominance of off- and along-shelf sediment escape. Contemporary sediment transport studies, seabed observations and modeling suggest a strong disconnect with average Holocene dispersal patterns, with much sediment currently escaping along the shelf to the north. It appears that this dramatic shift in sediment dispersal on the Waipaoa margin

  10. Benthic faunal assemblages and carbon supply along the continental shelf/shelf break-slope off Cape Hatteras, North Carolina

    NASA Astrophysics Data System (ADS)

    Aller, J. Y.; Aller, R. C.; Green, M. A.

    Patterns of benthic faunal abundances, biomass, and productivity were examined in the continental shelf-break/upper-slope and mid-slope region of the Ocean Margins Program study area off Cape Hatteras, NC in July 1994, and July and August 1996. Macrofaunal abundances were comparable to or slightly higher than other shelf-slope locales in the North Atlantic. Similar to previous studies in the region, there were no clear depth (75-900 m) or latitudinal (36°20'N-35°25') trends. Sta. S300 in 300 m had greatest abundances (539,000±38,400 m -2) for individuals >0.3 mm, more than 3 times higher than the average for all stations. Annelids of all sizes dominated numerically, equaling >80% of all macrofauna regardless of size. The majority of infauna were found in the upper 5 cm, but direct visual observations and geochemical evidence from other studies imply a deep-burrowing benthos. Meiofauna (excluding benthic foraminifera) were twice as abundant at shelf-break/upper-slope stations than mid-slope stations, while foraminifera were more abundant at deeper stations. Meiofaunal-sized polychaetes and nematodes were found to at least 7-8 cm below the sediment surface. Bacterial inventories at shelf-break/upper-slope depths were high relative to other shelf regions, but declined precipitously deeper than 500 m. Relative biomass patterns were similar for all stations, highest for macrobenthos and lowest for bacteria. Although densities were high, the contribution of nematodes to benthic biomass was <1%. Macrofaunal biomass averaged 54±47 g C m -2 and ranged from 6 g C m -2 at station N455 to 188 g C m -2 (>0.3 mm) at station S300, while metazoan meiofauna contributed from 0.6 g C m -2 at station N-274 to 11 g C m -2 at M76, averaging 2.2±2.4 g C m -2. Bacterial biomass over the upper 10 cm was ˜4 times higher at shelf-break/upper-slope stations than mid-slope stations, averaging 1.05±1.14 g C m -2 and ranging from 5 g C m -2 at M76 to 0.12 g C m -2 at MLB-679Rb. Benthic

  11. 75 FR 61512 - Outer Continental Shelf Official Protraction Diagrams

    Federal Register 2010, 2011, 2012, 2013, 2014

    2010-10-05

    ... Bureau of Ocean Energy Management, Regulation and Enforcement Outer Continental Shelf Official... Outer Continental Shelf Official Protraction Diagrams (OPDs) located within Atlantic Ocean areas, with... informational purposes only. Outer Continental Shelf Official Protraction Diagrams in the North Atlantic,...

  12. Modelling Glacier Retreat after Ice Shelf Collapse

    NASA Astrophysics Data System (ADS)

    De Rydt, J.; Gudmundsson, G. H.; Rott, H.; Bamber, J. L.

    2014-12-01

    Satellite measurements have shown the consistent and ongoing speed-up and retreat of glaciers that were once buttressed by the collapsed Larsen B ice shelf. Understanding the response of grounded ice to ice shelf collapse is a prerequisite to future predictions of sea level rise as other ice shelfs such as Scar Inlet or the Larsen Ice Shelf further weaken due to changing atmospheric and ocean conditions.We present model results for a number of sensitivity experiments that aim to simulate the response of glaciers to the collapse of Larsen B. For this purpose we use a state of the art shallow shelf model with grounding line resolving capabilities. The model is initialized to observed pre-2002 conditions with the ice shelf in place, and transient runs are done that study the response to a weakening and removal of the ice shelf. Results are compared to a novel dataset of observed ice velocities, which provides the most comprehensive overview of dynamical changes after the collapse to-date. In addition, we investigate glacier response to the future collapse of Scar Inlet, a remnant of the Larsen B ice shelf which has been suggested to show signs of weakening in recent years. Results will also be used to inform a future Antartic Peninsula-wide modelling study.

  13. Marginally Stable Nuclear Burning

    NASA Technical Reports Server (NTRS)

    Strohmayer, Tod E.; Altamirano, D.

    2012-01-01

    Thermonuclear X-ray bursts result from unstable nuclear burning of the material accreted on neutron stars in some low mass X-ray binaries (LMXBs). Theory predicts that close to the boundary of stability oscillatory burning can occur. This marginally stable regime has so far been identified in only a small number of sources. We present Rossi X-ray Timing Explorer (RXTE) observations of the bursting, high- inclination LMXB 4U 1323-619 that reveal for the first time in this source the signature of marginally stable burning. The source was observed during two successive RXTE orbits for approximately 5 ksec beginning at 10:14:01 UTC on March 28, 2011. Significant mHz quasi- periodic oscillations (QPO) at a frequency of 8.1 mHz are detected for approximately 1600 s from the beginning of the observation until the occurrence of a thermonuclear X-ray burst at 10:42:22 UTC. The mHz oscillations are not detected following the X-ray burst. The average fractional rms amplitude of the mHz QPOs is 6.4% (3 - 20 keV), and the amplitude increases to about 8% below 10 keV.This phenomenology is strikingly similar to that seen in the LMXB 4U 1636-53. Indeed, the frequency of the mHz QPOs in 4U 1323-619 prior to the X-ray burst is very similar to the transition frequency between mHz QPO and bursts found in 4U 1636-53 by Altamirano et al. (2008). These results strongly suggest that the observed QPOs in 4U 1323-619 are, like those in 4U 1636-53, due to marginally stable nuclear burning. We also explore the dependence of the energy spectrum on the oscillation phase, and we place the present observations within the context of the spectral evolution of the accretion-powered flux from the source.

  14. 7Be as a tracer of flood sedimentation on the northern California continental margin

    USGS Publications Warehouse

    Sommerfield, C. K.; Nittrouer, C. A.; Alexander, C. R.

    1999-01-01

    Sediment inventories of the cosmogenic radionuclide 7Be (t1/2=53 d) were measured on the Eel River shelf and slope (northern California continental margin) to investigate sedimentation processes associated with coastal river flooding. Seabed coring shortly after major riverflow events in 1995 and 1997 documented a shelf-wide flood deposit, and subsequent radionuclide studies determined 7Be to be a powerful tracer of fine-grained river sediment. In addition, distinctive signatures of 234Th and 210Pb were observed in oceanic flood deposits and provided additional information regarding depositional processes. During the 1995–1997 monitoring period, 7Be was present (2–35 dpm cm-2) in shelf and slope sediments only after periods of high rainfall and river runoff during the winter months. It is suggested that fluvial input was the primary source of 7Be in shelf sediments after the floods. 7Be sediment inventories and sediment-trap fluxes determined after the 1997 flood revealed that fine-grained fluvial sediments were rapidly (within one month) broadcast over the continental margin, to the 500 m isobath. Dispersal was apparently facilitated by energetic storm waves, which resuspended and redistributed some fraction of the suspended load residing on the shelf prior to accretion as flood deposits. These observations illustrate that floods are an important sedimentary process for modern environments of the Eel shelf and slope, and perhaps for other fluviomarine sedimentary systems of the northern California continental margin. Ratios of the 210Pb sediment-accumulation rate (100 yr average) to the 7Be deposition rate (1–2 month average) for shelf sites illustrate the episodic nature of shelf sedimentation, and suggest that a minimum of 3–30 depositional events complete the most recent stratigraphic record. This observation is consistent with the magnetude and frequency of fluvial sediment input, as Eel River floods with return periods of 3–33 yr (3% of the time of

  15. Structural and dynamic changes of Wilkins Ice Shelf, Antarctic Peninsula

    NASA Astrophysics Data System (ADS)

    Rankl, Melanie; Helm, Veit; Braun, Matthias

    2013-04-01

    Wilkins Ice Shelf (WIS) has shown considerable ice front retreat since 1990. This retreat includes various break-up events, such as recently in 2008 (Feb: 425 km², May: 160 km², Jul: 1220 km²) and in 2009 (790 km²). The break-up events took place under contrasting surface conditions, which indicates potentially different mechanisms for break-up. WIS shows quite specific peculiarities like a high amount of ice rises, highly variable ice thicknesses across the ice shelf, tributary glaciers draining into inlets as well as only limited nourishing by direct inflow from tributary glaciers. The present study aims to better understand the dynamics and mechanisms leading to disintegration and break-up of WIS. We hence investigate satellite data to reveal changes of glaciological structures like fractures and shear margins, the position of the grounding line, changes of frontal positions and ice surface velocities. Very few in situ measurements are available at WIS, which emphasizes the use of satellite data. Especially Synthetic Aperture Radar (SAR) data show high potential for glaciological purposes. We use SAR data (ALOS PALSAR, TerraSAR-X, TanDEM-X, ERS-1/2) in order to calculate surface velocities of the ice shelf and its tributaries at different times using SAR offset tracking procedures. The combined use of TanDEM-X InSAR surface elevations and IceSAT, CryoSat and NASA Ice Bridge ATM data enables the estimation of ice thickness assuming a constant ice density. First results show surface velocities before and after the break-up events in 2008 and 2009 as well as changing flow velocities of tributary glaciers. The combination of InSAR surface elevation and altimeter data allows for a comprehensive estimation of ice thickness across WIS. Both data sets can be used for subsequent ice dynamic modeling and fracture mechanics.

  16. Structure of the continental shelf west of San Francisco, California

    USGS Publications Warehouse

    Cooper, Alan K.

    1973-01-01

    The continental shelf west of San Francisco, California is divided into two structural provinces along the offshore extension of the Seal Cove fault, the Farallon and Golden Gate platforms. The Farallon platform lies west of the Seal Cove fault and is characterized by a thick sequence of Tertiary sediments with a maximum thickness of 3.5 km. These sediments are bounded at the edge of the continental shelf by a ridge of Cretaceous granitic rock which crops out at the Farallon Islands. The granitic basement is vertically displaced along the eastern flank of the granitic ridge and along two high angle reverse faults located near Point Reyes. The Point Reyes fault trends east-west and has possibly offset the granitic basement 1500 meters since late Pliocene time. Two northwest trending anticlines lie south of the Point Reyes fault. The eastern boundary of the Tertiary basin is delineated by a structural high composed of deformed basin sediments. This structural high is truncated at the Seal Cove fault. On the Golden Gate platform, the San Andreas fault zone is marked by a near surface graben, showing a maximum vertical displacement of 20 meters, and by a wider linear depression observed to a depth of 700 meters. The Pilarcitos fault continues offshore and joins the San Andreas fault south of Bolinas. The development of the present Tertiary basin under the continental shelf commenced in late Miocene time with. the initial uplift of the Farallon ridge and the compression of the eastern basin margin. Strike-slip faulting on the seal Cove fault in Pliocene time truncated middle Tertiary sediments at the fault and thus formed a distinct structural boundary between the Farallon and Golden Gate platforms. Surface extensional features observed within the San Andreas fault zone are suggestive of strike-slip faulting in late Tertiary time. The compressional high angle reverse faulting at Point Reyes occurred during the late Pliocene orogenic period of the California Coast Ranges.

  17. On instability and mixing on the UK Continental Shelf

    NASA Astrophysics Data System (ADS)

    Liu, Zhiyu

    2016-06-01

    The stability of stratified flows at locations in the Clyde, Irish and Celtic Seas on the UK Continental Shelf is examined. Flows are averaged over periods of 12-30 min in each hour, corresponding to the times taken to obtain reliable estimates of the rate of dissipation of turbulent kinetic energy per unit mass, ε. The Taylor-Goldstein equation is solved to find the maximum growth rate of small disturbances to these averaged flows, and the critical gradient Richardson number, Ric. The proportion of unstable periods where the minimum gradient Richardson number, Rimin, is less than Ric is about 35%. Cases are found in which Ric < 0.25; 37% of the flows with Rimin < 0.25 are stable, and Ric < 0.24 in 68% of the periods where Rimin < 0.25. Marginal conditions with 0.8 < Rimin/Ric < 1.2 occur in 30% of the periods examined. The mean dissipation rate at the level where the fastest growing disturbance has its maximum amplitude is examined to assess whether the turbulence there is isotropic and how it relates to the wave-turbulence boundary. It is concluded that there is a background level of dissipation that is augmented by instability; instability of the averaged flow does not account for all the turbulence observed in mid-water. The effects of a horizontal separation of the measurements of shear and buoyancy are considered. The available data do not support the hypothesis that the turbulent flows observed on the UK shelf adjust rapidly to conditions that are close to being marginal, or that flows in a particular location and period of time in one sea have stability characteristics that are very similar to those in another.

  18. Controls of tectonics and sediment source locations on along-strike variations in transgressive deposits on the northern California margin

    USGS Publications Warehouse

    Spinelli, G.A.; Field, M.E.

    2003-01-01

    We identify two surfaces in the shallow subsurface on the Eel River margin offshore northern California, a lowstand erosion surface, likely formed during the last glacial maximum, and an overlying surface likely formed during the most recent transgression of the shoreline. The lowstand erosion surface, which extends from the inner shelf to near the shelfbreak and from the Eel River to Trinidad Head (???80 km), truncates underlying strata on the shelf. Above the surface, inferred transgressive coastal and estuarine sedimentary units separate it from the transgressive surface on the shelf. Early in the transgression, Eel River sediment was likely both transported down the Eel Canyon and dispersed on the slope, allowing transgressive coastal sediment from the smaller Mad River to accumulate in a recognizable deposit on the shelf. The location of coastal Mad River sediment accumulation was controlled by the location of the paleo-Mad River. Throughout the remainder of the transgression, dispersed sediment from the Eel River accumulated an average of 20 m of onlapping shelf deposits. The distribution and thickness of these transgressive marine units was strongly modified by northwest-southeast trending folds. Thick sediment packages accumulated over structural lows in the lowstand surface. The thinnest sediment accumulations (0-10 m) were deposited over structural highs along faults and uplifting anticlines. The Eel margin, an active margin with steep, high sediment-load streams, has developed a thick transgressive systems tract. On this margin sediment accumulates as rapidly as the processes of uplift and downwarp locally create and destroy accommodation space. Sequence stratigraphic models of tectonically active margins should account for variations in accommodation space along margins as well as across them. ?? 2003 Elsevier Science B.V. All rights reserved.

  19. Sedimentary architecture of the Amundsen Sea Embayment shelf, West Antarctica, from pre-glacial to glacial processes

    NASA Astrophysics Data System (ADS)

    Gohl, Karsten; Uenzelmann-Neben, Gabriele; Hillenbrand, Claus-Dieter; Larter, Robert; Nitsche, Frank

    2013-04-01

    Studies of the sedimentary architecture and characteristics of Antarctic shelves provide clues of past ice sheet advance-retreat cycles and help improve constraints for paleo-ice dynamic models since early glacial periods. A first seismostratigraphic analysis of the Amundsen Sea Embayment shelf and slope of West Antarctica reveals insights into the structural architecture of the continental margin and shows stages of sediment deposition, erosion and transport history from pre-glacial times to early glaciation and to the most recent glacial periods. The shelf geometry consists of a large pre- and syn-rift basin in the middle shelf region between outcropping basement of the inner shelf and basement ridges and highs beneath the outer shelf. A middle shelf sub-basin exists which may have formed as a result of motion along an early West Antarctic Rift System branch. At least 4 km of pre-glacial strata has been eroded from the present inner shelf and coastal hinterland by ice sheet advances since the onset of glaciation. Some of the eroded sediments were deposited as a progradational wedge extending the outer shelf by 25 to 65 km oceanward of the pre-glacial shelf-break. Comparing the observed seismic characteristics with those of other Antarctic shelf sequences, we assign an Early Cretaceous age for bottom sedimentary unit ASS-1, a Late Cretaceous to Oligocene age for unit ASS-2, an Early to Mid-Miocene age for unit ASS-3, a Mid-Miocene age for unit ASS-4, a Late Miocene to Early Pliocene age for unit ASS-5, and a Pliocene to Pleistocene age for the top unit ASS-6. The survival of buried grounding zone wedges in the upper part of unit ASS-5 of the outer shelf is consistent with the onset of a long warming phase and a retreated ice sheet in the early Pliocene as observed for the Ross Sea shelf and reconstructed from paleo-ice sheet models. Our data also reveal that the paleo-ice flow paths of the central Pine Island Trough system have remained stationary across the

  20. Black African Traditional Mathematics

    ERIC Educational Resources Information Center

    Zaslavsky, Claudia

    1970-01-01

    Discusses the traditional number systems and the origin of the number names used by several African peoples living south of the Sahara. Also included are limitations in African mathematical development, and possible topics for research. (RP)

  1. Integration of high-resolution seismic with core data delineates sequence stratigraphy of a shelf-edge delta complex

    SciTech Connect

    Combes, J.M.; Nissen, S.E.; Scott, R.W.

    1995-12-31

    Correlation of high resolution seismic and corehole data sets obtained offshore Louisiana by a cooperative consortium of Louisiana State University and ten petroleum industry partners has resulted in a detailed sequence stratigraphic interpretation of a Late Pleistocene shelf margin delta system. High resolution a Late Pleistocene shelf margin delta system. High resolution stratal geometries have been interpreted within this framework of genetically related facies and key sequence surfaces have been identified both on the high resolution seismic lines and in the core data. Regional expressions of chronostratigraphically identified sequence-bounding unconformities and transgressive ravinement surfaces emphasize the importance of these surfaces in determining stratigraphic relationships. Several key conclusions resulted from this study: (1) The optimum location for interpretation of sequence surfaces is within or near the locus of maximum deposition. (2) At a distance from a depocenter the characteristic features of sequence surfaces lose seismic resolution and minor, subtle variations in the reflection character are the only seismic indicators of major boundaries. (3) Shelf edge deltaic deposits are known to contain important hydrocarbon reservoirs and this latest Pleistocene system provides an excellent model for older Cenozoic systems. (4) Potential deep sea fan reservoirs may accumulate seaward of shelf margin deltas during both falling and rising sea level stages depending upon local sedimentological conditions.

  2. Measuring Changes in the Vicinity of the Seal Nunataks Ice Shelf Remnant from Imagery and Altimetry

    NASA Astrophysics Data System (ADS)

    Shuman, C. A.; Berthier, E.; Scambos, T.

    2015-12-01

    Analysis of repeated imagery and ICESat laser altimetry has enabled the ongoing losses from the northern Larsen ice shelf remnant to be assessed in detail. The remnant, the Seal Nunataks ice shelf (SNIS), has four ICESat tracks that cross it as well as adjacent tracks that cross Robertson Island (RI) and its remaining tributary, Rogosh Glacier (RG), on the Antarctic Peninsula. The altimetry data from ICESat (2003-2009) shows that elevation losses increase from west to east across the SNIS. Ice elevation differences suggest mean ice shelf thinning rates of up to 1.6 m a-1 and reveal processes impacting the remaining shelf ice as well. Limited altimetry data across RG suggests elevation losses of almost 1 m a-1 inland from the grounding zone with smaller losses further up the evolving tributary. Farther east, asymmetric elevation changes across RI suggest the magnitude of regional climate impacts vary distinctly depending on slope aspect. Imagery analysis using Landsat 7 and ASTER images from 2001-2013 shows that ice area losses continued on the shelf remnant following the Larsen A break up in 1995 as well as after the Larsen B break up in 2002. The largest losses (~350 km2) occurred on the north side of the remnant in late 2004 into 2005 with smaller losses along the remaining margins. Despite a slight regional cooling recently and more persistent sea ice since early 2008 as seen in MODIS imagery, the SNIS is still losing ice along its margins and appears to be retreating past its pinning nunataks. In contrast to SNIS, RI has experienced minor ice area losses that suggest most of its ice is grounded and thus less directly impacted by ocean interactions. Combining these remote sensing data sets provides additional insights about ongoing ice loss processes in this part of the Antarctic Peninsula.

  3. African American Homeschooling and the Quest for a Quality Education

    ERIC Educational Resources Information Center

    Mazama, Ama; Lundy, Garvey

    2015-01-01

    Academic interest in homeschooling has increased over the last decade, as what was once perceived as a marginal development, has, in fact, turned into a significant and growing phenomenon. There has been, in recent years, a noticeable surge in African American involvement in the homeschooling movement as well. However, there continues to be a…

  4. Exploring Artistic Practice in Global Communities of the African Diaspora

    ERIC Educational Resources Information Center

    Ellis, Auburn E.

    2014-01-01

    In 2012 an African Centered single case study was conducted in the United States. The problem is as follows: K-12 practitioners in urban areas are faced with unique circumstances while serving marginalized students in urban areas. As a response to this issue, the purpose of this study was to identify and describe curricula used in three African…

  5. Constructing Academic Inadequacy: African American Athletes' Stories of Schooling.

    ERIC Educational Resources Information Center

    Benson, Kirsten F.

    2000-01-01

    This qualitative study interviewed eight academically "at risk" African American athletes at a southeastern university with a major revenue-producing football program. Analysis suggested that the athletes' marginal academic performance was constructed in a system of interrelated practices engaged in by all the significant members of the academic…

  6. The African Connection

    ERIC Educational Resources Information Center

    Oguntoyinbo, Lekan

    2012-01-01

    From student and faculty exchanges to joint research projects, U.S. universities maintain a broad spectrum of collaborative relationships with African universities. It's unclear how many U.S. colleges and universities have partnerships with African universities. The African Studies Association, an organization of scholars, doesn't keep that kind…

  7. Neodymium isotope constraints on provenance, dispersal, and climate-driven supply of Zambezi sediments along the Mozambique Margin during the past ˜45,000 years

    NASA Astrophysics Data System (ADS)

    van der Lubbe, H. J. L.; Frank, Martin; Tjallingii, Rik; Schneider, Ralph R.

    2016-01-01

    Marine sediments deposited off the Zambezi River that drains a considerable part of the southeast African continent provide continuous records of the continental climatic and environmental conditions. Here we present time series of neodymium (Nd) isotope signatures of the detrital sediment fraction during the past ˜45,000 years, to reconstruct climate-driven changes in the provenance of clays deposited along the Mozambique Margin. Coherent with the surface current regime, the Nd isotope distribution in surface sediments reveals mixing of the alongshore flowing Zambezi suspension load with sediments supplied by smaller rivers located further north. To reconstruct past changes in sediment provenances, Nd isotope signatures of clays that are not significantly fractionated during weathering processes have been obtained from core 64PE304-80, which was recovered just north of the Zambezi mouth at 1329 m water depth. Distinctly unradiogenic clay signatures (ɛNd values <-14.2) are found during the Last Glacial Maximum, Heinrich Stadial 1, and Younger Dryas. In contrast, the Nd isotope record shows higher, more radiogenic isotope signatures during Marine Isotope Stage 3 and between ˜15 and ˜5 ka BP, the latter coinciding with the timing of the northern hemisphere African Humid Period. The clay-sized sediment fraction with the least radiogenic Nd isotope signatures was deposited during the Holocene, when the adjacent Mozambique Shelf became completely flooded. In general, the contribution of the distinctly unradiogenic Zambezi suspension load has followed the intensity of precession-forced monsoonal precipitation and enhanced during periods of increased southern hemisphere insolation and high-latitude northern hemispheric climate variability.

  8. Demographic marginalization, social integration, and adolescents' educational success.

    PubMed

    Benner, Aprile D; Wang, Yijie

    2014-10-01

    Links between schools' demographic composition and students' achievement have been a major policy interest for decades. Using a racially/ethnically diverse sample from the National Longitudinal Study of Adolescent Health (N = 6,302; 54% females; 53% White, 21% African American, 15% Latino, 8% Asian American, 2% other race/ethnicity), we examined the associations between demographic marginalization, students' later social integration (loneliness at school, school attachment), and educational performance and attainment. Adolescents who were socioeconomically marginalized at school [i.e., having <15% same-socioeconomic status (SES) peers] had lower cumulative grade point averages across high school and lower educational attainment. A similar disadvantage was observed among students who were both socioeconomically and racially/ethnically marginalized at school (i.e., having <15% same-SES peers and <15% same-racial/ethnic peers). Indirect effects were also observed, such that demographic marginalization was linked to poorer school attachment, and poorer school attachment, in turn, was related to poorer academic performance. These results highlight the educational barriers associated with demographic marginalization and suggest potential targets for future intervention efforts. PMID:25034249

  9. Reconstruction of the East Africa and Antarctica continental margins

    NASA Astrophysics Data System (ADS)

    Nguyen, Luan C.; Hall, Stuart A.; Bird, Dale E.; Ball, Philip J.

    2016-06-01

    The Early Jurassic separation of Antarctica from Africa plays an important role in our understanding of the dispersal of Gondwana and Pangea. Previous reconstruction models contain overlaps and gaps in the restored margins that reflect difficulties in accurately delineating the continent-ocean-boundary (COB) and determining the amount and distribution of extended continental crust. This study focuses on the evolution of the African margin adjacent to the Mozambique Basin and the conjugate Antarctic margin near the Riiser-Larsen Sea. Satellite-derived gravity data have been used to trace the orientations and landward limits of fracture zones. A 3-D gravity inversion has produced a crustal thickness model that reliably quantifies the extent and amount of stretched crust. Crustal thicknesses together with fracture zone terminations reveal COBs that are significantly closer to the African and Antarctic coasts than previously recognized. Correlation of fracture zone azimuths and identified COBs suggests Antarctica began drifting away from Africa at approximately 171 Ma in a roughly SSE direction. An areal-balancing method has been used to restore the crust to a uniform prerift thickness so as to perform a nonrigid reconstruction for both nonvolcanic and volcanic margins. Both margins reveal a trend of increasing extension from east to west. Our results suggest Africa underwent extension of 60-120 km, while Antarctic crust was stretched by 105-180 km. Various models tested to determine the direction of extension during rifting suggest that Antarctica moved away from Africa in a WNW-ESE direction during the period between 184 and 171 Ma prior to the onset of seafloor spreading.

  10. Cruise report; RV Coastal Surveyor Cruise C1-99; multibeam mapping of the Long Beach, California continental shelf; April 12 through May 19, 1999

    USGS Publications Warehouse

    Gardner, James V.; Hughes-Clarke, John E.; Mayer, Larry A.

    1999-01-01

    The greater Los Angeles area of California is home to more than 10 million people. This large population puts increased pressure on the adjacent offshore continental shelf and margin with activities such as ocean disposal for dredged spoils, explosive disposal, waste-water outfall, and commercial fishing. The increased utilization of the shelf and margin in this area has generated accelerated multi-disciplinary research efforts in all aspects of the environment of the coastal zone. Prior to 1996 there were no highly accurate base maps of the continental shelf and slope upon which the research activities could be located and monitored. In 1996, the United States Geological Survey (USGS) Pacific Seafloor Mapping Project began to address this problem by mapping the Santa Monica shelf and margin (Fig. 1) using a state-of-the-art, high-resolution multibeam sonar system (Gardner, et al., 1996; 1999). Additional seafloor mapping in 1998 provided coverage of the continental margin from south of Newport to the proximal San Pedro Basin northwest of Palos Verdes Peninsula (Gardner, et al., 1998) (Fig. 1). The mapping of the seafloor in the greater Los Angeles continental shelf and margin was completed with a 30-day mapping of the Long Beach shelf in April and May 1999, the subject of this report. The objective of Cruise C-1-99-SC was to completely map the broad continental shelf from the eastern end of the Palos Verdes Peninsula to the narrow shelf south of Newport Beach, from the break in slope at about 120-m isobath to the inner shelf at about the 10-m isobath. Mapping the Long Beach shelf was jointly funded by the U.S. Geological Survey and the County of Orange (CA) Sanitation District and was conducted under a Cooperative Agreement with the Ocean Mapping Group from the University of New Brunswick (OMG/UNB). The OMG/UNB contracted with C&C Technologies, Inc. of Lafayette, LA for use of the RV Coastal Surveyor and the latest evolution of high-resolution multibeam sonars, a

  11. Ivory Coast-Ghana margin: model of a transform margin

    SciTech Connect

    Mascle, J.; Blarez, E.

    1987-05-01

    The authors present a marine study of the eastern Ivory Coast-Ghana continental margins which they consider one of the most spectacular extinct transform margins. This margin has been created during Early-Lower Cretaceous time and has not been submitted to any major geodynamic reactivation since its fabric. Based on this example, they propose to consider during the evolution of the transform margin four main and successive stages. Shearing contact is first active between two probably thick continental crusts and then between progressively thinning continental crusts. This leads to the creation of specific geological structures such as pull-apart graben, elongated fault lineaments, major fault scarps, shear folds, and marginal ridges. After the final continental breakup, a hot center (the mid-oceanic ridge axis) is progressively drifting along the newly created margin. The contact between two lithospheres of different nature should necessarily induce, by thermal exchanges, vertical crustal readjustments. Finally, the transform margin remains directly adjacent to a hot but cooling oceanic lithosphere; its subsidence behavior should then progressively be comparable to the thermal subsidence of classic rifted margins.

  12. Crustal structure from the Faroes Shelf to the Norwegian Basin

    NASA Astrophysics Data System (ADS)

    Roberts, A. W.; White, R. S.; Kusznir, N. J.; Christie, P.; Roberts, A. M.; Isimm Team

    2003-04-01

    We show the crustal structure along a 400km seismic profile extending across a prime example of a volcanically rifted margin, from the Faroes shelf across the continent-ocean boundary northeast of the Faroe Islands, and 100km into oceanic crust of the Norwegian Sea formed immediately after continental break-up. 85 4-component OBS were used for the survey, giving wide-angle arrivals visible to beyond 120km offset. The survey was complemented by a 12 km Q-streamer profile along the same line. Integration of the normal incidence through wide-angle arrivals for the OBS and streamer data allow us to make a constrained velocity model through the active crust and into the upper mantle. We used a large airgun source comprising 14 guns with a total volume of 6,360 cu. in. towed at 20m depth. The resulting output was dominated by low frequencies (peak at 9Hz) to allow improved imaging through the basalts. A thickened oceanic crust is found, indicative of high temperatures caused by the Iceland mantle plume, and the presence of clear seaward dipping reflectors is evidence of extrusive lavas. Underplating is also inferred on the margin from the high seismic velocities in the lower crust. Academia and industry seek to understand magmatic margin evolution for its impact on deep water hydrocarbon prospecting. The NE Atlantic has been chosen as our research area because of its accessibility, wealth of related data and current exploration on the Atlantic margin. The iSIMM programme's long term goals are to characterise volcanically rifted margins and to develop theoretical models of the formation and subsidence of rifted margins. iSIMM investigators are: R.S. White (1), N.J. Kusznir (2), P.A.F. Christie (3), A.M. Roberts (4), N. Hurst (2), Z.C. Lunnon (1,3), C.J. Parkin (1), A.W. Roberts (1), L.K. Smith (1), R. Spitzer (1), V. Tymms (2), A. Davies (1), A. Surendra (1), with funding from NERC, Agip UK, BP, Amerada Hess Ltd., Anadarko, Conoco, Phillips, Shell, Statoil, and WesternGeco.

  13. Transgressive systems tract development and incised-valley fills within a quaternary estuary-shelf system: Virginia inner shelf, USA

    USGS Publications Warehouse

    Foyle, A.M.; Oertel, G.F.

    1997-01-01

    High-frequency Quaternary glacioeustasy resulted in the incision of six moderate- to high-relief fluvial erosion surfaces beneath the Virginia inner shelf and coastal zone along the updip edges of the Atlantic continental margin. Fluvial valleys up to 5 km wide, with up to 37 m of relief and thalweg depths of up to 72 m below modern mean sea level, cut through underlying Pleistocene and Mio-Pliocene strata in response to drops in baselevel on the order of 100 m. Fluvially incised valleys were significantly modified during subsequent marine transgressions as fluvial drainage basins evolved into estuarine embayments (ancestral generations of the Chesapeake Bay). Complex incised-valley fill successions are bounded by, or contain, up to four stacked erosional surfaces (basal fluvial erosion surface, bay ravinement, tidal ravinement, and ebb-flood channel-base diastem) in vertical succession. These surfaces, combined with the transgressive oceanic ravinement that generally caps incised-valley fills, control the lateral and vertical development of intervening seismic facies (depositional systems). Transgressive stratigraphy characterizes the Quaternary section beneath the Virginia inner shelf where six depositional sequences (Sequences I-VI) are identified. Depositional sequences consist primarily of estuarine depositional systems (subjacent to the transgressive oceanic ravinement) and shoreface-shelf depositional systems; highstand systems tract coastal systems are thinly developed. The Quaternary section can be broadly subdivided into two parts. The upper part contains sequences consisting predominantly of inner shelf facies, whereas sequences in the lower part of the section consist predominantly of estuarine facies. Three styles of sequence preservation are identified. Style 1, represented by Sequences VI and V, is characterized by large estuarine systems (ancestral generations of the Chesapeake Bay) that are up to 40 m thick, have hemicylindrical wedge geometries

  14. Rift basins in western margin of India and their hydrocarbon prospects with special reference to Kutch basin

    SciTech Connect

    Biswas, S.K.

    1982-10-01

    The western continental margin of India can be classed as a divergent or passive margin. The western continental shelf is an extensive carbonate bank (Bombay offshore basin) passing into clastic sediments on the north and south. Three craton-margin embayed basins-Kutch, Cambay, and Narmada- in the northern part of the shelf, are filled predominantly with clastic sediments. These basins occupy grabens bounded by faults diverging seaward. The grabens were formed by three rift systems along major Precambrian tectonic trends. The rifting developed sequentially from north to south around the Saurashtra horst. Kutch basin was formed in the Early Jurassic, followed by Cambay basin in Early Cretaceous time, and the Narmada in the Late Cretaceous. It appears that these rifting events occurred at successive stages during the northward migration of the Indian plate after its break from Gondwanaland in Late Triassic or Early Jurassic. It is inferred that these rift basins opened up successively as a result of the counterclockwise drift of the Indian craton. Bombay offshore and Cambay are two major oil-producing basins in the western margin. These basins are characterized by high geothermal gradients attributed to the shallowness of the mantle in this region. Oil has not been found in KUtch basin, which is mainly an onshore Mesozoic basin. The basin basin depocenter shifted offshore at the northwestern part of the continental shelf where the shelf is wide.

  15. Exchanges between the open Black Sea and its North West shelf

    NASA Astrophysics Data System (ADS)

    Shapiro, Georgy; Wobus, Fred; Zhou, Feng

    2014-05-01

    offshore flow over a large section of the shelf break. Due to the short duration of strong wind effects (4-7 days) the horizontal extent of cross-shelf-break exchanges is limited to the outer shelf. The effect of Ekman drift is confined to the upper layers. In contrast, eddies and meanders penetrate deep down to the bottom, but they are restricted laterally. During the strong wind events of April 15 - 22 and July 1 - 4, some 0.66×1012 and 0.44×1012 m3of water were removed from the northwestern shelf respectively. In comparison, the single long-lived Sevastopol Eddy generated a much larger offshore transfer of 2.84×1012 m3 over the period April 23 to June 30, which is equivalent to 102% of the volume of northwestern shelf waters. This result is consistent with the data obtained from satellite derived information (Shapiro et al, 2010). The open Black Sea is generally warmer and more saline than the northwest shelf. Hence the exchanges contribute to the increase in both salinity and temperature of shelf waters. Over the study period, salt exchanges increased the average density of the shelf waters by 0.67 kg m-3 and reduced the density contrast between the shelf and deep sea, while lateral heat exchanges reduced the density of the shelf waters by 0.16 kg m-3 and thus enhanced density contrast across the shelf break. This study was supported by the EU (via PERSEUS grant FP7-OCEAN-2011-287600 and MyOcean SPA.2011.1.5-01 grant 283367), Ministry of Science and Technology of China (Grant 2011CB409803), the Natural Science Foundation of China (Grant 41276031), Zhejiang Association for International Exchange of Personnel, and the University of Plymouth Marine Institute Innovation Fund. References Huthnance, J. M., 1995. Circulation, exchange and water masses at the ocean margin: the role of physical processes at the shelf edge, Prog Oceanogr, 35(4), 353-431, Ivanov L.I., Besiktepe S., Ozsoy E., 1997. In: E.Ozsoy and A.Mikaelyan (eds). Sensitivity to change: Black Sea , Baltic Sea

  16. Adolescent Substance Use: The Role of Demographic Marginalization and Socioemotional Distress

    ERIC Educational Resources Information Center

    Benner, Aprile D.; Wang, Yijie

    2015-01-01

    We investigated the links between racial/ethnic marginalization (i.e., having few same-race/ethnic peers at school) and adolescents' socioemotional distress and subsequent initiation of substance use (alcohol and marijuana) and substance use levels. Data from 7,731 adolescents (52% female; 55% White, 21% African American, 16% Latino, 8% Asian…

  17. Colorado Basin Structure and Rifting, Argentine passive margin

    NASA Astrophysics Data System (ADS)

    Autin, Julia; Scheck-Wenderoth, Magdalena; Loegering, Markus; Anka, Zahie; Vallejo, Eduardo; Rodriguez, Jorge; Marchal, Denis; Reichert, Christian; di Primio, Rolando

    2010-05-01

    The Argentine margin presents a strong segmentation with considerable strike-slip movements along the fracture zones. We focus on the volcanic segment (between the Salado and Colorado transfer zones), which is characterized by seaward dipping reflectors (SDR) all along the ocean-continent transition [e.g. Franke et al., 2006; Gladczenko et al., 1997; Hinz et al., 1999]. The segment is structured by E-W trending basins, which differs from the South African margin basins and cannot be explained by classical models of rifting. Thus the study of the relationship between the basins and the Argentine margin itself will allow the understanding of their contemporary development. Moreover the comparison of the conjugate margins suggests a particular evolution of rifting and break-up. We firstly focus on the Colorado Basin, which is thought to be the conjugate of the well studied Orange Basin [Hirsch et al., 2009] at the South African margin [e.g. Franke et al., 2006]. This work presents results of a combined approach using seismic interpretation and structural, isostatic and thermal modelling highlighting the structure of the crust. The seismic interpretation shows two rift-related discordances: one intra syn-rift and the break-up unconformity. The overlying sediments of the sag phase are less deformed (no sedimentary wedges) and accumulated before the generation of oceanic crust. The axis of the Colorado Basin trends E-W in the western part, where the deepest pre-rift series are preserved. In contrast, the basin axis turns to a NW-SE direction in its eastern part, where mainly post-rift sediments accumulated. The most distal part reaches the margin slope and opens into the oceanic basin. The general basin direction is almost orthogonal to the present-day margin trend. The most frequent hypothesis explaining this geometry is that the Colorado Basin is an aborted rift resulting from a previous RRR triple junction [e.g. Franke et al., 2002]. The structural interpretation

  18. Surficial bioturbation and rapid benthic remineralization in the Cape Hatteras shelf/slope region. Final report

    SciTech Connect

    Robert C. Aller; Josephine Y. Aller; C. Lee; J. Kirk Cochran

    1999-03-17

    This is a final report for the DOE of grant DE-FG02-92ER61464 ''Surficial bioturbation and rapid benthic remineralization in the Cape Hatteras shelf slope region''. Over the past 6 years we have participated in a multidisciplinary field study called the Ocean margins Program (OMP) to examine the importance of continental margins in the global carbon cycle. Specifically, we have focused on the southern portion of the Mid-Atlantic Bight between Cape Hatteras and Chesapeake Bay where a large flux of freshwater and organic carbon enters the North Atlantic Ocean. Additionally, during the first stage of this project, we developed the use of CM-a distributions in sediments as a quantitative indicator of benthic C flux and remineralization rates. The primary objective of our research group has been to understand mechanisms and quantify biogeochemical processes in the seabed that affect cycling, flux, and storage of carbon on the ocean margin of the Mid-Atlantic Bight.

  19. The marginalization of hormesis.

    PubMed

    Calabrese, E J; Baldwin, L A

    2000-01-01

    Despite the substantial development and publication of highly reproducible toxicological data, the concept of hormetic dose-response relationships was never integrated into the mainstream of toxicological thought. Review of the historical foundations of the interpretation of the bioassay and assessment of competitive theories of dose-response relationships lead to the conclusion that multiple factors contributed to the marginalization of hormesis during the middle and subsequent decades of the 20th century. These factors include: (a) the close-association of hormesis with homeopathy lead to the hostility of modern medicine toward homeopathy thereby creating a guilt by association framework, and the carry-over influence of that hostility in the judgements of medically-based pharmacologists/ toxicologists toward hormesis; (b) the emphasis of high dose effects linked with a lack of appreciation of the significance of the implications of low dose stimulatory effects; (c) the lack of an evolutionary-based mechanism(s) to account for hormetic effects; and (d) the lack of appropriate scientific advocates to counter aggressive and intellectually powerful critics of the hormetic perspective. PMID:10745293

  20. The marginalization of hormesis.

    PubMed

    Calabrese, E J; Baldwin, L A

    1999-01-01

    Despite the substantial development and publication of highly reproducible toxicological data, the concept of hormetic dose-response relationships was never integrated into the mainstream of toxicological thought. Review of the historical foundations of the interpretation of the bioassay and assessment of competitive theories of dose-response relationships lead to the conclusion that multiple factors contributed to the marginalization of hormesis during the middle and subsequent decades of the 20th Century. These factors include the following: (a) the close association of hormesis with homeopathy, which led to the hostility of modern medicine toward homeopathy, thereby creating a guilt-by-association framework, and the carryover influence of that hostility toward hormesis in the judgements of medically based pharmacologists/toxicologists; (b) the emphasis of high-dose effects linked with a lack of appreciation of the significance of the implications of low-dose stimulatory effects; (c) the lack of an evolution-based mechanism(s) to account for hormetic effects; and (d) lack of appropriate scientific advocates to counter aggressive and intellectually powerful critics of the hormetic perspective. PMID:10207983

  1. Geology of New England passive margin

    SciTech Connect

    Austin, J.A. Jr.; Uchupi, E.; Shaughnessy, D.R. III; Ballard, R.D.

    1980-01-01

    The New England continental margin began to develop in the Middle Triassic, when rifting of Precambrian/Paleozoic terrane produced a complex arrangement of horsts and grabens. During the Late Triassic-Early Jurassic, these grabens were filled with terrigenous clastics, volcanics, and evaporites. When plate separation took place and seafloor spreading began approximately 195 to 190 m.y.B.P., the newly formed continental edge was uplifted and eroded, truncating preexisting rift structures. As North America began to drift away from Africa, subsidence occurred along a series of normal faults now beneath the outer continental shelf. This hinge zone may represent the boundary between continental crust and a transitional zone of continental and oceanic crustal fragments. Atop the faulted and subsiding crustal platform, thick sediments were deposited. The lower part of the drift sequence is an evaporite-carbonate unit of Early-Middle Jurassic age, and the upper part is a clastic wedge of Middle Jurassic to Cenozoic age. More than 80% of these sediments are Jurassic. Their total thickness may be as much as 13 km beneath the southeastern part of Georges Bank.

  2. Ice sheet margins and ice shelves

    NASA Technical Reports Server (NTRS)

    Thomas, R. H.

    1984-01-01

    The effect of climate warming on the size of ice sheet margins in polar regions is considered. Particular attention is given to the possibility of a rapid response to warming on the order of tens to hundreds of years. It is found that the early response of the polar regions to climate warming would be an increase in the area of summer melt on the ice sheets and ice shelves. For sufficiently large warming (5-10C) the delayed effects would include the breakup of the ice shelves by an increase in ice drainage rates, particularly from the ice sheets. On the basis of published data for periodic changes in the thickness and melting rates of the marine ice sheets and fjord glaciers in Greenland and Antarctica, it is shown that the rate of retreat (or advance) of an ice sheet is primarily determined by: bedrock topography; the basal conditions of the grounded ice sheet; and the ice shelf condition downstream of the grounding line. A program of satellite and ground measurements to monitor the state of ice sheet equilibrium is recommended.

  3. Aeromagnetic and gravity investigations of the Coastal Area and Continental Shelf of Liberia, West Africa, and their relation to continental drift

    USGS Publications Warehouse

    Behrendt, John C.; Wotorson, Cletus S.

    1970-01-01

    An aeromagnetic survey has shown the existence of several basins in which magnetic basement depths are greater than 5 km on the continental shelf off Liberia. Magnetic diabase of 176 to 192 m.y. (Jurassic) in age intruding the Paleozoic (?) rocks and overlain by younger rocks onshore requires the distinction between “magnetic basement” and “basement.” Several lines of evidence suggest that the Paleozoic(?) rocks are less than 1 km thick; this implies that the diabase does not introduce a large error in depth-to-basement estimates. The dikes or their extrusive equivalents are traceable, on the basis of the magnetic data, beneath the younger sedimentary rock in the basins to the edge of the continental slope. The magnetic data also delineate a second zone of diabase dikes 90 km inland, parallel to the coast, which cross the entire country. The intrusion of the younger dikes probably coincides with rifting at the beginning of the separation of Africa and South America, and the associated magnetic anomaly zones appear to be parallel with and continuous into the anomaly bands in the Atlantic. A major northeast-trending break in the magnetic fabric intersects the coast near 9° W. and is associated with Eburnean age rocks (about 2000 m.y.) to the southeast as contrasted with Liberian-age rocks (about 2700 m.y.) to the northwest. Change in magnetic fabric direction inland from northeast to northwest in the coastal area allows recognition of a boundary between the Liberian-age rocks inland and Pan-African-age (about 550 m.y.) rocks in the coastal area northwest of about 9° 20'W. Sets of north-northwest-and west-northwest—trending faults of 1 to 2 km vertical displacement cut the Cretaceous sedimentary rocks onshore and can be traced into the offshore basins. Vertical displacements of several kilometers in the magnetic basement underlying the continental shelf suggest a pattern of block faulting all along the coast and continental shelf. Negative Bouguer

  4. Marginal deformation of crustal plates as key to crustal motion, crustal spirals, and the driving force

    SciTech Connect

    Wood, B.G.M.

    1986-07-01

    Present plate tectonic models concentrate on compressive- and extensive-type plate margins, often incorporating shear margins as a subtype of compressive margins. However, if a single moving plate is considered, it becomes apparent that the leading edge is a compressive margin, the trailing edge is an extensive margin, and the lateral edges are shear margins. Conversely, if a plate's margin can be recognized by identifying areas of subduction (compression), rifting (extension), and strike slip and buckle folding (shearing), then not only can a plate be identified but its motion can also be inferred. The Pacific plate provides an excellent example. It is bounded by subduction trenches along its west-northwest margin, extension rifts along its east-southeast margin, and shear and buckle-fold complexes along its south-southwest and north-northeast margins. A west-northwest motion is inferred. As other major plates are examined, two striking features are revealed. A west-northwest to northwest motion is consistently identified, and the plates line up end to end forming a northwest-spiraling segmented band encircling the globe. The lateral margins of this band form the well-known Tethyan shear system. The plates comprising this band are of varying sizes and composition, and the extent of deformation along a plate margin is directly related to plate size. The Pacific and Eurasian plates dominate in size and marginal deformation. The tail of this north-spiraling ribbon of crustal plates is deformed in the Southern Hemisphere, most notably in the area of the African, Indian, and Australian plates. Each southern plate has a strong north component of motion as well as a counterclockwise spiraling action. The plates appear to have rotated in response to drag along the southern margin of the Pacific and Eurasian plates.

  5. Inorganic geochemistry of surface sediments of the Ebro shelf and slope, northwestern Mediterranean

    USGS Publications Warehouse

    Gardner, J.V.; Dean, W.E.; Alonso, B.

    1990-01-01

    Distributions of major, minor, and trace elements in surface sediment of the continental shelf and upper slope of the northeastern Spanish continental margin reflect the influences of discharge from the Ebro River and changes in eustatic sea levels. Multivariate factor analysis of sediment geochemistry was used to identify five groupings of samples (factors) on the shelf and slope. The first factor is an aluminosilicate factor that represents detrital clastic material. The second factor is a highly variable amount of excess SiO2 and probably represents a quartz residuum originating from winnowing of relict detrital sediments. A carbonate factor (Factor 3) has no positive correlation with other geochemical parameters but is associated with the sand-size fraction. The carbonate in these sediments consists of a mixture of biogenic calcite and angular to subangular detrital grains. Organic carbon is associated with the aluminosilicate factor (Factor 1) but also factors out by itself (Factor 4); this suggests that there may be two sources of organic matter, terrestrial and marine. The fifth factor comprises upper slope sediments that contain high concentrations of manganese. The most likely explanation for these high manganese concentrations is precipitation of Mn oxyhydroxides at the interface between Mn-rich, oxygen-deficient, intermediate waters and oxygenated surface waters. During eustatic low sea levels of the glacial Pleistocene, the Ebro Delta built across the outer continental shelf and deposited sediment with fairly high contents of organic carbon and continental components. The period of marine transgression from eustatic low (glacial) to eustatic high (interglacial) sea levels was characterized by erosion of the outer shelf delta and surficial shelf sediments and the transport of sediment across the slope within numerous canyons. Once eustatic high sea level was reached, delta progradation resumed on the inner shelf. Today, coarse-grained sediment (silt and

  6. Petroleum possibilities in continental margin off central Chile

    SciTech Connect

    Gonzalez, E.

    1986-07-01

    The continental margin off central Chile, from Valparaiso to Valdivia, encompassing an area of 100,000 km/sup 2/, has been the target of exploratory activity by Empresa Nacional del Petroleo since 1970. Exploratory drilling began in 1972. By August 1984, total exploratory efforts had resulted in drilling 14 offshore wells and acquiring 12,130 km of seismic reflection lines. A biogenic gas accumulation was discovered in the F well. Because these attempts to find oil were unsuccessful and because drilling costs have escalated, exploratory activities have been curtailed. Forearc basins off central Chile are characterized by low geothermal gradient and a sedimentary filling of Cretaceous and Tertiary strata. Tertiary sequences are characterized by low organic carbon content, immature humic-type organic matter, and a biogenic gas potential. Cretaceous sequences are characterized by higher organic carbon content, good reservoir rocks, and fair to good source rocks. The organic matter is sapropelic, with vitrinite and liptinites, and is favorable for oil and gas generation. Seismic and well data suggest that Mesozoic and Cenozoic sedimentary rock sequences filling the basins (more than 4000 m thick at the shelf edge) extend 40-70 km beyond the present shelf edge. Mesozoic rocks deposited on the slope may generate petroleum and gas that could migrate upslope and accumulate in traps associated with the faulted basement highs and graben-type depressions existing at the shelf edge. This geologic setting favors the development of large petroleum accumulations along the shelf edge and graben on the sedimentary basins off central Chile.

  7. Margin Architecture and Sediment Flux as Controls on Submarine Fan Development: Tectonic-Climate Interactions in the Gulf of Alaska

    NASA Astrophysics Data System (ADS)

    Gulick, S. P. S.; Montelli, A.; Swartz, J. M.; Morey, S.; Jaeger, J. M.; Mix, A. C.; Reece, R.; Somchat, K.; Wagner, P. F.; Worthington, L. L.

    2015-12-01

    The oblique collision of the Yakutat microplate into southeast Alaska generates the St. Elias Mountains, a coastal orogen with significant moisture from the Gulf of Alaska resulting in large, temperate glacial systems that expand to and eventually cross the continental shelf during glacial maxima. We present an overview of the evolution of sediment routing on this margin from integration of seismic images, updated age models and core-log-seismic correlations from IODP Expedition 341 drilling sites, and mapping efforts from shelf, slope, and fan. We focus on the three dominant glacial systems during the climatically important intensification of Northern Hemisphere glaciation at the Plio-Pleistocene transition and the further intensification of glaciation since the mid-Pleistocene transition. Along strike, sediment delivery to deepwater from the three glacial systems varied according to Pleistocene shelf accommodation space. The Alsek crossed a narrower shelf with a bedrock high near the shelf edge; the Malaspina-Hubbard system crossed an undeformed, ~1 km deep shelf; the Bering-Bagley system crossed a several km deep shelf deforming as an active fold and thrust belt. The Malaspina and Bering catchments exhibit high exhumation rates onshore due to the Yakutat collision and upon reaching the shelf edge these glaciers generate trough mouth fans (TMFs) on the adjacent continental slope but only after first filling the available accommodation with glacigenic sediment and lowering the slope gradient through progradation. The Alsek crosses the shelf earliest but never with sufficient sediment flux to generate a TMF. An east-west transition in adjacent deepwater submarine channels that feed and generate the Surveyor Fan suggests that shelf accommodation and sediment flux are primary controls on sediment routing from orogen to submarine fan. Both of these parameters are in turn a function of initial tectonic architecture and ongoing orogen dynamics.

  8. Deep Structures of The Angola Margin

    NASA Astrophysics Data System (ADS)

    Moulin, M.; Contrucci, I.; Olivet, J.-L.; Aslanian, D.; Géli, L.; Sibuet, J.-C.

    1 Ifremer Centre de Brest, DRO/Géosciences Marines, B.P. 70, 29280 Plouzané cedex (France) mmoulin@ifremer.fr/Fax : 33 2 98 22 45 49 2 Université de Bretagne Occidentale, Institut Universitaire Europeen de la Mer, Place Nicolas Copernic, 29280 Plouzane (France) 3 Total Fina Elf, DGEP/GSR/PN -GEOLOGIE, 2,place de la Coupole-La Defense 6, 92078 Paris la Defense Cedex Deep reflection and refraction seismic data were collected in April 2000 on the West African margin, offshore Angola, within the framework of the Zaiango Joint Project, conducted by Ifremer and Total Fina Elf Production. Vertical multichannel reflection seismic data generated by a « single-bubble » air gun array array (Avedik et al., 1993) were recorded on a 4.5 km long, digital streamer, while refraction and wide angle reflection seismic data were acquired on OBSs (Ocean Bottom Seismometers). Despite the complexity of the margin (5 s TWT of sediment, salt tectonics), the combination of seismic reflection and refraction methods results in an image and a velocity model of the ground structures below the Aptian salt layer. Three large seismic units appear in the reflection seismic section from the deep part on the margin under the base of salt. The upper seismic unit is layered with reflectors parallel to the base of the salt ; it represents unstructured sediments, filling a basin. The middle unit is seismically transparent. The lower unit is characterized by highly energetic reflectors. According to the OBS refraction data, these two units correspond to the continental crust and the base of the high energetic unit corresponds to the Moho. The margin appears to be divided in 3 domains, from east to west : i) a domain with an unthinned, 30 km thick, continental crust ; ii) a domain located between the hinge line and the foot of the continental slope, where the crust thins sharply, from 30 km to less than 7 km, this domain is underlain by an anormal layer with velocities comprising between 7,2 and 7

  9. Origin and extent of fresh paleowaters on the Atlantic continental shelf, USA

    USGS Publications Warehouse

    Cohen, D.; Person, M.; Wang, P.; Gable, C.W.; Hutchinson, D.; Marksamer, A.; Dugan, B.; Kooi, H.; Groen, K.; Lizarralde, D.; Evans, R.L.; Day-Lewis, F. D.; Lane, J.W., Jr.

    2010-01-01

    While the existence of relatively fresh groundwater sequestered within permeable, porous sediments beneath the Atlantic continental shelf of North and South America has been known for some time, these waters have never been assessed as a potential resource. This fresh water was likely emplaced during Pleistocene sea-level low stands when the shelf was exposed to meteoric recharge and by elevated recharge in areas overrun by the Laurentide ice sheet at high latitudes. To test this hypothesis, we present results from a high-resolution paleohydrologic model of groundwater flow, heat and solute transport, ice sheet loading, and sea level fluctuations for the continental shelf from New Jersey to Maine over the last 2 million years. Our analysis suggests that the presence of fresh to brackish water within shallow Miocene sands more than 100 km offshore of New Jersey was facilitated by discharge of submarine springs along Baltimore and Hudson Canyons where these shallow aquifers crop out. Recharge rates four times modern levels were computed for portions of New England's continental shelf that were overrun by the Laurentide ice sheet during the last glacial maximum. We estimate the volume of emplaced Pleistocene continental shelf fresh water (less than 1 ppt) to be 1300 km3 in New England. We also present estimates of continental shelf fresh water resources for the U.S. Atlantic eastern seaboard (104 km3) and passive margins globally (3 ?? 105 km3). The simulation results support the hypothesis that offshore fresh water is a potentially valuable, albeit nonrenewable resource for coastal megacities faced with growing water shortages. ?? 2009 National Ground Water Association.

  10. Origin and extent of fresh paleowaters on the Atlantic continental shelf, USA.

    PubMed

    Cohen, Denis; Person, Mark; Wang, Peng; Gable, Carl W; Hutchinson, Deborah; Marksamer, Andee; Dugan, Brandon; Kooi, Henk; Groen, Koos; Lizarralde, Daniel; Evans, Robert L; Day-Lewis, Frederick D; Lane, John W

    2010-01-01

    While the existence of relatively fresh groundwater sequestered within permeable, porous sediments beneath the Atlantic continental shelf of North and South America has been known for some time, these waters have never been assessed as a potential resource. This fresh water was likely emplaced during Pleistocene sea-level low stands when the shelf was exposed to meteoric recharge and by elevated recharge in areas overrun by the Laurentide ice sheet at high latitudes. To test this hypothesis, we present results from a high-resolution paleohydrologic model of groundwater flow, heat and solute transport, ice sheet loading, and sea level fluctuations for the continental shelf from New Jersey to Maine over the last 2 million years. Our analysis suggests that the presence of fresh to brackish water within shallow Miocene sands more than 100 km offshore of New Jersey was facilitated by discharge of submarine springs along Baltimore and Hudson Canyons where these shallow aquifers crop out. Recharge rates four times modern levels were computed for portions of New England's continental shelf that were overrun by the Laurentide ice sheet during the last glacial maximum. We estimate the volume of emplaced Pleistocene continental shelf fresh water (less than 1 ppt) to be 1300 km(3) in New England. We also present estimates of continental shelf fresh water resources for the U.S. Atlantic eastern seaboard (10(4) km(3)) and passive margins globally (3 x 10(5) km(3)). The simulation results support the hypothesis that offshore fresh water is a potentially valuable, albeit nonrenewable resource for coastal megacities faced with growing water shortages. PMID:19754848

  11. Diversity of the benthic macrofauna off northern Namibia from the shelf to the deep sea

    NASA Astrophysics Data System (ADS)

    Eisenbarth, Simone; Zettler, Michael L.

    2016-03-01

    In late summer 2011, shortly after an upwelling event, 17 stations ranging from 30 to 2513 m water depth have been sampled at 20° south in the northern part of the Benguela Current Large Marine Ecosystem (BCLME) for the investigation of the benthic macrofauna. Sediments of this area are dominated by silt. At the time of sampling, oxygen conditions on the shelf were poor (between 0.42 and 0.68 ml l- 1) but not hypoxic. Below 400 m, however, concentrations rose steadily up to 5.28 ml l- 1. Macrozoobenthic communities along this depth gradient are described, revealing among others the community structure for the continental margin area and the deep sea off northern Namibia for the first time. Cluster analysis revealed 5 different communities along the depth gradient with three shelf communities, one continental margin community and one deep-sea community. All in all, 314 different taxa were found with polychaetes being the most abundant group. Diversity index (Shannon) was lowest for the shallow water community with 2.21 and highest for the deep-sea community with 4.79, showing a clear trend with increasing water depth. Species richness, however, reached its maximum with 187 taxa along the continental margin between 400 and 1300 m water depth. Dominant species for each community are named with the two Cumacea, Iphinoeafricana and Upselaspis caparti, being characteristic for the shallow water community. On the shelf, we found surprisingly high biomass values (23-123 g m- 2), mainly caused by polychaetes, the bivalve Sinupharus galatheae and the gastropod Nassarius vinctus. In terms of composition, the remaining communities were dominated by polychaetes with members of the Paraonidae dominating along the continental margin where we also found surprisingly high abundances of the bivalves Pecten sp. and Dosinia sp. Spionid polychaetes and some representatives of the genus Paraonis were the most common organisms for the deep-sea community.

  12. Exchanges between the open Black Sea and its North West shelf

    NASA Astrophysics Data System (ADS)

    Shapiro, Georgy; Wobus, Fred; Zhou, Feng

    2014-05-01

    offshore flow over a large section of the shelf break. Due to the short duration of strong wind effects (4-7 days) the horizontal extent of cross-shelf-break exchanges is limited to the outer shelf. The effect of Ekman drift is confined to the upper layers. In contrast, eddies and meanders penetrate deep down to the bottom, but they are restricted laterally. During the strong wind events of April 15 - 22 and July 1 - 4, some 0.66×1012 and 0.44×1012 m3of water were removed from the northwestern shelf respectively. In comparison, the single long-lived Sevastopol Eddy generated a much larger offshore transfer of 2.84×1012 m3 over the period April 23 to June 30, which is equivalent to 102% of the volume of northwestern shelf waters. This result is consistent with the data obtained from satellite derived information (Shapiro et al, 2010). The open Black Sea is generally warmer and more saline than the northwest shelf. Hence the exchanges contribute to the increase in both salinity and temperature of shelf waters. Over the study period, salt exchanges increased the average density of the shelf waters by 0.67 kg m-3 and reduced the density contrast between the shelf and deep sea, while lateral heat exchanges reduced the density of the shelf waters by 0.16 kg m-3 and thus enhanced density contrast across the shelf break. This study was supported by the EU (via PERSEUS grant FP7-OCEAN-2011-287600 and MyOcean SPA.2011.1.5-01 grant 283367), Ministry of Science and Technology of China (Grant 2011CB409803), the Natural Science Foundation of China (Grant 41276031), Zhejiang Association for International Exchange of Personnel, and the University of Plymouth Marine Institute Innovation Fund. References Huthnance, J. M., 1995. Circulation, exchange and water masses at the ocean margin: the role of physical processes at the shelf edge, Prog Oceanogr, 35(4), 353-431, Ivanov L.I., Besiktepe S., Ozsoy E., 1997. In: E.Ozsoy and A.Mikaelyan (eds). Sensitivity to change: Black Sea , Baltic Sea

  13. 49 CFR 192.10 - Outer continental shelf pipelines.

    Code of Federal Regulations, 2013 CFR

    2013-10-01

    ... 49 Transportation 3 2013-10-01 2013-10-01 false Outer continental shelf pipelines. 192.10 Section... shelf pipelines. Operators of transportation pipelines on the Outer Continental Shelf (as defined in the Outer Continental Shelf Lands Act; 43 U.S.C. 1331) must identify on all their respective pipelines...

  14. 49 CFR 192.10 - Outer continental shelf pipelines.

    Code of Federal Regulations, 2011 CFR

    2011-10-01

    ... 49 Transportation 3 2011-10-01 2011-10-01 false Outer continental shelf pipelines. 192.10 Section... shelf pipelines. Operators of transportation pipelines on the Outer Continental Shelf (as defined in the Outer Continental Shelf Lands Act; 43 U.S.C. 1331) must identify on all their respective pipelines...

  15. 49 CFR 192.10 - Outer continental shelf pipelines.

    Code of Federal Regulations, 2014 CFR

    2014-10-01

    ... 49 Transportation 3 2014-10-01 2014-10-01 false Outer continental shelf pipelines. 192.10 Section... shelf pipelines. Operators of transportation pipelines on the Outer Continental Shelf (as defined in the Outer Continental Shelf Lands Act; 43 U.S.C. 1331) must identify on all their respective pipelines...

  16. 49 CFR 192.10 - Outer continental shelf pipelines.

    Code of Federal Regulations, 2012 CFR

    2012-10-01

    ... 49 Transportation 3 2012-10-01 2012-10-01 false Outer continental shelf pipelines. 192.10 Section... shelf pipelines. Operators of transportation pipelines on the Outer Continental Shelf (as defined in the Outer Continental Shelf Lands Act; 43 U.S.C. 1331) must identify on all their respective pipelines...

  17. 49 CFR 192.10 - Outer continental shelf pipelines.

    Code of Federal Regulations, 2010 CFR

    2010-10-01

    ... 49 Transportation 3 2010-10-01 2010-10-01 false Outer continental shelf pipelines. 192.10 Section... shelf pipelines. Operators of transportation pipelines on the Outer Continental Shelf (as defined in the Outer Continental Shelf Lands Act; 43 U.S.C. 1331) must identify on all their respective pipelines...

  18. Shelf-Life Prediction of Chilled Foods

    NASA Astrophysics Data System (ADS)

    Gudmundsson, Gudmundur; Kristbergsson, Kristberg

    All foods have a finite shelf life. Even foods, which mature with time, will in the end deteriorate, although their life span can exceed 100 years. Definitions of shelf life of food products differ. Some stress the suitability of the product for consump¬tion, others for how long the product can be sold. The Institute of Food Science and Technology emphasizes safety in its definition of shelf life: "The period of time under defined conditions of storage, after manufacture or packing, for which a food product will remain safe and be fit for use" ( http://www.ifst.org ). This definition does not describe what makes a food product "safe" or "fit" for use, but one can say all factors which restrict the shelf life of a food product either affect safety or quality or both.

  19. Iceberg B-15, Ross Ice Shelf, Antarctica

    NASA Technical Reports Server (NTRS)

    2002-01-01

    Iceberg B-15 broke from the Ross Ice Shelf in Antarctica in late March. Among the largest ever observed, the new iceberg is approximately 170 miles long x 25 miles wide. Its 4,250 square-mile area is nearly as large as the state of Connecticut. The iceberg was formed from glacial ice moving off the Antarctic continent and calved along pre-existing cracks in the Ross Ice Shelf near Roosevelt Island. The calving of the iceberg essentially moves the northern boundary of the ice shelf about 25 miles to the south, a loss that would normally take the ice shelf as long as 50-100 years to replace. This infrared image was acquired by the DMSP (Defense Meteorological Satellite Program) F-13 satellite on April 13, 2000. For more images see Antarctic Meteorological Research Center Image courtesy of the University of Wisconsin - Madison, Space Science and Engineering Center, Antarctic Meteorological Research Center

  20. Morphology and Dynamics of Subaqueous Dunes in a Tideless Outer Shelf (murcia Continental Shelf, Western Mediterranean Sea)

    NASA Astrophysics Data System (ADS)

    Durán, R.; Rivera, J.; Guillén, J.; de Cárdenas, E.; Muñoz, A.; Acosta, J.

    2014-12-01

    rates towards the southwest (migration rates < 1 m yr-1). Bedform asymmetry and migration are consistent with the general circulation of the margin, characterized by a predominant geostrophic current flowing south-westward along the shelf break.

  1. New Ulm field: an example of Cretaceous shelf-slope instability in east Texas

    SciTech Connect

    Pinero, E.

    1984-04-01

    The New Ulm field in Austin County, Texas, is an example of the structural and stratigraphic complexity above the Cretaceous Edwards shelf margin of east Texas. Deep wells and improved seismic data provide documentation of structural patterns and deepwater facies not previously considered in the Gulf Coast reservoir play modeling. Study of the data implies the Late Cretaceous to Eocene section was deposited along a shelf-slope break. Late Cretaceous, pre-Midway sedimentation was affected by structurally induced slope instability, and consequent gravity faulting and slumping resulted in an irregular sea-floor surface. Paleocene Midway sands were carried onto this surface by storm-generated density currents where the uneven topography caused deposition in constructional channels. Continued deposition of the fluvio-deltaic Wilcox on this surface caused faulting and folding by differential compaction. The folds are minor and the faults small and steep, not like the typical large growth faults of the Gulf Coast. Upper Wilcox sediments were progressively less disturbed as the region stabilized. New Ulm field production includes gas from the Midway Formation and oil and gas from the Wilcox Group. Midway reservoirs are stratigraphic, consisting of fluvio-deltaic sandstones within faulted anticlines. This study adds evidence to data describing shelf-slope geology along the Edwards margin. The setting can be a new type of hydrocarbon play in the Gulf Coast.

  2. Middle and upper jurassic depositional environments at outer shelf and slope of Baltimore Canyon Trough

    SciTech Connect

    Gamboa, L.A.; Stoffa, P.L.; Truchan, M.

    1985-04-01

    New CDP data acquired in the Baltimore Canyon Trough during project LASE made it possible to map a continuous Jurassic sedimentary sequence from the continental margin to the abyssal plain without interruption by basement structures. Intense carbonate sedimentation is inferred at the outer shelf during the Middle and Late Jurassic. Carbonate sedimentation probably started during the Middle Jurassic with a platform that prograded seaward with the development of ramps. By the Late Jurassic, a major reef complex had developed at the outer continental shelf. The onset of reef growth can be tentatively dated as 138 Ma by using the J1 reflector dated by the Deep Sea Drilling Project. A well-developed reef-talus deposit can be identified overlying the interface that generates the J1 reflector. A detailed analysis of semblancederived interval velocities in the reef-talus sequence indicates a compressional velocity of 4.3-4.5 km/sec (14,100-14,800 ft/sec) for that interval, which was part of a major barrier reef along the United States eastern margin. After the reef formed, the deep oceanic basin was mostly starved from shelf-derived sediments until the reef died and was buried by clastic sediments. By correlation of our seismic data and COST well information, that in the Baltimore Canyon Trough this reef had terminated by about the end of the Jurassic Period.

  3. Mud transportation on a steep shelf, Rio de La Plata shelf, Puerto Rico

    USGS Publications Warehouse

    Grove, K.A.; Pilkey, O.H.; Trumbull, J.V.A.

    1982-01-01

    Hurricanes David (August 29-30, 1979) and Frederick (September 2-5, 1979) caused major flooding of the Rio de La Plata in northern Puerto Rico. A thin mud layer was deposited across the narrow insular shelf adjacent to the river mouth. Within 5 months, fair-weather shelf-winnowing processes moved the mud layer entirely from the shelf, 0.5 to 2 km to the shelf break at the 50-m contour and beyond. The process of mud movement is termed 'mud hopping.' ?? 1982 A. M. Dowden, Inc.

  4. Modified, Packaged Tortillas Have Long Shelf Life

    NASA Technical Reports Server (NTRS)

    Bourland, Charles; Glaus-Late, Kimberly

    1995-01-01

    Tortillas made from modified recipe and sealed in low-pressure nitrogen in foil pouches in effort to increase their shelf life at room temperature. Preliminary tests show that shelf life of these tortillas at least five months; in contrast, commercial tortillas last only few days. Part of water in recipe replaced with glycerin. Particularly necessary to avoid Clostridium botulinum, which grows in anaerobic environments and produces deadly toxin that causes botulism.

  5. Freshwater peat on the continental shelf

    USGS Publications Warehouse

    Emery, K.O.; Wigley, R.L.; Bartlett, A.S.; Rubin, M.; Barghoorn, E.S.

    1967-01-01

    Freshwater peats from the continental shelf off northeastern United States contain the same general pollen sequence as peats from ponds that are above sea level and that are of comparable radiocarbon ages. These peats indicate that during glacial times of low sea level terrestrial vegetation covered the region that is now the continental shelf in an unbroken extension from the adjacent land areas to the north and west.

  6. Freshwater peat on the continental shelf.

    PubMed

    Emery, K O; Wigley, R L; Bartlett, A S; Rubin, M; Barghoorn, E S

    1967-12-01

    Freshwater peats from the continental shelf off northeastern United States contain the same general pollen sequence as peats from ponds that are above sea level and that are of comparable radiocarbon ages. These peats indicate that during glacial times of low sea level terrestrial vegetation covered the region that is now the continental shelf in an unbroken extension from the adjacent land areas to the north and west. PMID:17801856

  7. Cenozoic prograding sequences of the Antarctic continental margin: a record of glacio-eustatic and tectonic events

    USGS Publications Warehouse

    Cooper, A. K.; Barrett, P.J.; Hinz, K.; Traube, V.; Letichenkov, G.; Stagg, H.M.J.

    1991-01-01

    Sedimentary sections up to 6-14 km thick lie beneath many areas of the Antarctic continental margin. The upper parts of the sections contain up to 6 km of Cenozoic glacial and possibly non-glacial sequences that have prograded the continental shelf up to 85 km. We describe the Cenozoic sequences using two general categories based on their acoustic geometries. Type IA sequences, which account for most prograding of the Antarctic continental shelf, have complex sigmoidal geometries and some acoustic characteristics atypical of low-latitude margins, such as troughs and mounds lying parallel and normal to the shelf edge and high velocities (2.0-2.6 km/s) for flat layers within 150 m of the seafloor. Type IIA sequences, which principally aggrade the paleoshelf, lie beneath type IA sequences and have mostly simple geometries and gently dipping reflections. The prograding sequences are commonly located near the seaward edges of major Mesozoic and older margin structures. Relatively rapid Cenozoic subsidence has occured due to the probable rifting in the Ross Sea, thermal subsidence in the Antarctic Peninsula, and isostatic crustal flexure in Wilkes Land. In Prydz Bay and the Weddell Sea, prograding sequences cover Mesozoic basins that have undergone little apparent Cenozoic tectonism. Grounded ice sheets are viewed by us, and others, as the principal mechanism for depositing the Antarctic prograding sequences. During the initial advance of grounded ice the continental shelf is flexurally overdeepened, the inner shelf is heavily eroded, and gently dipping glacial strata are deposited on the shelf (i.e type IIA sequences). The overdeepened shelf profile is preserved (a) during glacial times, by grounded ice sheets episodically crossing the shelf, eroding sediments from onshore and inner shelf areas, and depositing sediments at the front of the ice sheet as outer shelf topset-banks and continental slope foreset-aprons (i.e. type IA sequences), and (b) during interglacial

  8. Recent sedimentary study of the shelf of the Basque country

    NASA Astrophysics Data System (ADS)

    Jouanneau, J.-M.; Weber, O.; Champilou, N.; Cirac, P.; Muxika, I.; Borja, A.; Pascual, A.; Rodríguez-Lázaro, J.; Donard, O.

    2008-07-01

    The Northern Iberian margin of the Spanish Basque country (provinces of Gipuzkoa and Viscaia) is characterized by a narrow continental platform, which receives inputs of riverine particulate matter from the numerous riverine systems located within the Basque country. This particulate matter is subsequently deposited within the Bay of Biscay, and Gouf de Capbreton [Frouin, R., Fiuza, A.F.G., Ambar, I., Boyd, T.J., 1990. Observations of a poleward surface current off the coasts of Portugal and Spain during winter. Journal of Geophysical Research 95 (C1), 679-691]. The main goal of this study is to establish a map of the surface sediment distribution of the Basque continental shelf and more specifically to map the muddy patch located at the eastern side of that continental shelf. Three oceanographic cruises were conducted in 2003 and 2004. From these campaigns 340 surface samples, 12 short cores and 3 gravity cores have been collected over the mid and outer shelf from depths ranging between 50 m and 150 m deep. 3 seismic profiles were obtained across the shelf mud patch using a Sparker device. Sediment grain-size analyses were performed by the classical physical method of sieving and use of settling columns. The POC (Particular Organic Carbon) amounts in sediment and water samples were determined using the Strickland and Parsons' method [Strickland, J.D.H., Parsons, T.R., 1972. Determination of particulate carbon. In : A practical handbook of seawater analysis. Fisheries ResearchBoard of Canada, Ottawa, pp. 207-211] as adapted by Etcheber [Etcheber, H., 1981. Comparaison des diverses méthodes d'évaluation des teneurs en matières en suspension et en carbone organique particulaire des eaux marines du plateau continental aquitain. Journal de Recherche Océanographique VI (2), 37-42]. Radioisotopic measurements ( 210Pb exc) were made using a semi-planar germanium detector coupled to a multichannel analyser. Radiographical analysis was performed with an X-ray equipment

  9. Lomonosov Ridge as a Natural Component of Continental Margin

    NASA Astrophysics Data System (ADS)

    Poselov, V.; Kaminsky, V. D.; Butsenko, V. V.; Grikurov, G. E.

    2010-12-01

    In geodynamic context, Lomonosov Ridge is interpreted as a rifted passive margin framing the Eurasian oceanic basin. At the same time its near-Siberian segment is intimately associated with the Russian Arctic shelf, as evidenced by morphological data and the results of “Trans-Arctic 1992” and “Arctic-2007” geotransect studies. Coring and ACEX data demonstrated the presence in the uppermost geological section of the ridge of Late Cretaceous through Cenozoic sediments and Jurassic-Cretaceous sedimentary rocks; the latter may belong to deeper levels of sedimentary cover, or may represent the Mesozoic folded basement. Coarse bottom debris contains also the fragments of Riphean-Paleozoic rocks probably derived from the local bedrock source. Structure of sedimentary cover is imaged by continuous seismic observations from the shelf of East Siberian Sea along the length of Lomonosov Ridge to 85 N. In the upper part of the section there are two sedimentary sequences separated by a regional unconformity; their seismic velocities are 2.4-3.1 km/s in the upper sequence and 3.4-4.0 km/s in the lower one, and the total thickness reaches ~ 8 km in the deepest part of New Siberian Basin. Both these sequences and the unconformity are traced from Lomonosov Ridge into Amundsen Basin on seismic reflection sections obtained by drifting ice stations North Pole 2479 and 2480. The low-velocity sediments are underlain by a metasedimetary sequence with velocities decreasing from 4.7-4.9 km/s on the shelf to 4.4-4.9 km/s beneath continental slope and 4.2-4.8 km/s on Lomonosov Ridge. The thickness of metasedimentary sequence is about 7 km on the shelf, up to 3.5 km under continental slope, and strongly variable (1-5 km) on Lomonosov Ridge. The upper layer of consolidated crust is 8-9 km thick on the shelf with velocities 6.1-6.2 km/s; on Lomonosov Ridge both its thickness and velocities increase to 10 km and 6.0-6.4 km/s, respectively. In the lower crust the velocities do not exceed 6

  10. Shelf export of particulates/transport in continental margin waters. Final report

    SciTech Connect

    Pietrafesa, L.J.

    1995-07-01

    During the present funding period, research activities at NCSU have been directed towards: publishing the results of SEEP-I; publishing further results from NCSU`s South Atlantic Bight studies; designing and constructing four cages which house the 3 NCSU and 1 BNL RD-Acoustic Doppler Current Profilers used successfully in SEEP-II, calibrating all current meters, transmissometers, thermister chains and conductivity pressure and temperature sensors for SEEP-II phases 2 and 3; determining the temporal and spatial scales of physical processes observed during phase 1 of SEEP-II in preparation for finalizing the mooring positions and sampling intervals for SEEP-II; shipping all NCSU gear to the URI and ODU; and successful deployment of NCSU SEEP-II, phases 1 and 2 moorings.

  11. Paleochannels of the Delaware River on the mid-Atlantic shelf

    SciTech Connect

    Krantz, D.E. . Coll. of Marine Studies); McGeary, S.E.; Madsen, J.A. . Dept. of Geology); Gayes, P.T. . Dept. of Geology and Marine Science)

    1993-03-01

    Shallow seismic data recently collected from the inner and middle continental shelf off Delaware reveal what appears to be a relatively complete stratigraphic record of middle to late Pleistocene sea-level events. At least four separate paleochannels of the Delaware River that have been identified and traced across the inner and middle shelf can be projected to the major submarine canyons of the continental slope. These same channels can be traced landward through the modern Delaware Bay to the fluvial portion of the Delaware River; two of these paleochannels trend underneath the peninsula of Cape May. The relative ages of the channels have been inferred from superposition and cross-cutting relationships. Subsurface reflectors associated with these paleochannels can be traced laterally to define the shallow flanks of estuaries formed as the valley was flooded during a subsequent transgression. In two cases, the depositional margins identified in the seismic profiles coincide with distinct geomorphic boundaries on the seafloor, delimiting allostratigraphic units interpreted as Pleistocene estuaries that are preserved on the modern shelf. The complementary highstands for this system are preserved on the emergent coastal plain as shoreline complexes and paralic sequences. These data reveal a much more intricate preserved record than previously realized, and change substantially the current models for the Quaternary development of the shelf.

  12. Cetacean high-use habitats of the northeast United States continental shelf

    SciTech Connect

    Kenney, R.D.; Winn, H.E.

    1986-04-01

    Results of the Cetacean and Turtle Assessment Program previously demonstrated at a qualitative level that specific areas of the continental shelf waters off the northeastern US coast consistently showed high-density utilization by several cetacean species. They have quantified, on a multispecies basis and with adjustment for level of survey effort, the intensity of habitat use by whales and dolphins, and defined areas of especially high-intensity utilization. The results demonstrate that the area off the northeast US, which is used most intensively as cetacean habitat, is the western margin of the Gulf of Maine, from the Great South Channel to Stellwagen Bank and Jeffreys Ledge. Secondary high-use areas include the continental shelf edge and the region around the eastern end of Georges Bank. High-use areas for piseivorous cetaceans are concentrated mainly in the western Gulf of Maine and secondarily at mid-shelf east of the Chesapeake region, for planktivores in the western Gulf of Maine and the southwestern and eastern portions of Georges Bank, and for teuthivores in the western Gulf of Maine and the southwestern and eastern portions of Georges Bank, and for teuthivores along the edge of the shelf. In general, habitat use by cetaceans is highest in spring and summer, and lowest in fall and winter.

  13. Ocean-atmosphere-wave characterisation of a wind jet (Ebro shelf, NW Mediterranean Sea)

    NASA Astrophysics Data System (ADS)

    Grifoll, Manel; Navarro, Jorge; Pallares, Elena; Ràfols, Laura; Espino, Manuel; Palomares, Ana

    2016-06-01

    In this contribution the wind jet dynamics in the northern margin of the Ebro River shelf (NW Mediterranean Sea) are investigated using coupled numerical models. The study area is characterised by persistent and energetic offshore winds during autumn and winter. During these seasons, a seaward wind jet usually develops in a ˜ 50 km wide band offshore. The COAWST (Coupled Ocean-Atmosphere-Wave-Sediment Transport) modelling system was implemented in the region with a set of downscaling meshes to obtain high-resolution meteo-oceanographic outputs. Wind, waves and water currents were compared with in situ observations and remote-sensing-derived products with an acceptable level of agreement. Focused on an intense offshore wind event, the modelled wind jet appears in a limited area offshore with strong spatial variability. The wave pattern during the wind jet is characterised by the development of bimodal directional spectra, and the ocean circulation tends to present well-defined two-layer flow in the shallower region (i.e. inner shelf). The outer shelf tends to be dominated by mesoscale dynamics such as the slope current. Due to the limited fetch length, ocean surface roughness considering sea state (wave-atmosphere coupling) modifies to a small extent the wind and significant wave height under severe cross-shelf wind events. However, the coupling effect in the wind resource assessment may be relevant due to the cubic relation between the wind intensity and power.

  14. Basin evolution at the SW Barents Sea margin and its conjugate off NE Greenland

    NASA Astrophysics Data System (ADS)

    Faleide, Jan Inge; Wong, Po Wan; Helge Gabrielsen, Roy; Tsikalas, Filippos; Blaich, Olav A.; Planke, Sverre; Myklebust, Reidun

    2015-04-01

    marginal high. The northern Sørvestsnaget Basin and Vestbakken Volcanic Province formed in a pull-apart setting related to a releasing bend in the margin. These sedimentary basins subsided rapidly and received large amount of erosional products from the uplifted Barents Shelf, in particular the Stappen High in the north (surrounding Bjørnøya). Several of the marginal basins experienced reactivation by contraction/inversion, and finally they were buried by a thick westward prograding wedge of Plio-Pleistocene glacial sediments derived from the uplifted Barents Shelf.

  15. Manufacturing Marginality among Women and Latinos in Neoliberal America.

    PubMed

    Massey, Douglas S

    2014-01-01

    Intersectionality is the study of how categorical distinctions made on the basis of race, class, and gender interact to generate inequality, and this concept has become a primary lens by which scholars have come to model social stratification in the United States. In addition to the historically powerful interaction between race and class, gender interactions have become increasingly powerful in exacerbating class inequalities while the growing exclusion of foreigners on the basis of legal status has progressively marginalized Latinos in U.S. society. As a result, poor whites and immigrant-origin Latinos have increasingly joined African Americans at the bottom of American society to form a new, expanded underclass. PMID:25309007

  16. Manufacturing Marginality among Women and Latinos in Neoliberal America

    PubMed Central

    Massey, Douglas S.

    2014-01-01

    Intersectionality is the study of how categorical distinctions made on the basis of race, class, and gender interact to generate inequality, and this concept has become a primary lens by which scholars have come to model social stratification in the United States. In addition to the historically powerful interaction between race and class, gender interactions have become increasingly powerful in exacerbating class inequalities while the growing exclusion of foreigners on the basis of legal status has progressively marginalized Latinos in U.S. society. As a result, poor whites and immigrant-origin Latinos have increasingly joined African Americans at the bottom of American society to form a new, expanded underclass. PMID:25309007

  17. 41 CFR 101-27.205 - Shelf-life codes.

    Code of Federal Regulations, 2010 CFR

    2010-07-01

    ... 41 Public Contracts and Property Management 2 2010-07-01 2010-07-01 true Shelf-life codes. 101-27...-Management of Shelf-Life Materials § 101-27.205 Shelf-life codes. Shelf-life items shall be identified by use of a one-digit code to provide for uniform coding of shelf-life materials by all agencies. (a)...

  18. Acidalia Planitia Channel Margin

    NASA Technical Reports Server (NTRS)

    2005-01-01

    [figure removed for brevity, see original site]

    The THEMIS VIS camera is capable of capturing color images of the Martian surface using five different color filters. In this mode of operation, the spatial resolution and coverage of the image must be reduced to accommodate the additional data volume produced from using multiple filters. To make a color image, three of the five filter images (each in grayscale) are selected. Each is contrast enhanced and then converted to a red, green, or blue intensity image. These three images are then combined to produce a full color, single image. Because the THEMIS color filters don't span the full range of colors seen by the human eye, a color THEMIS image does not represent true color. Also, because each single-filter image is contrast enhanced before inclusion in the three-color image, the apparent color variation of the scene is exaggerated. Nevertheless, the color variation that does appear is representative of some change in color, however subtle, in the actual scene. Note that the long edges of THEMIS color images typically contain color artifacts that do not represent surface variation.

    This false color image shows craters and a channel margin, in the region of southern Acidalia Planitia where Tiu and Ares Valles empty into the planitia. This image was collected during the Northern Spring season.

    Image information: VIS instrument. Latitude 23.8, Longitude 327.5 East (32.5 West). 37 meter/pixel resolution.

    Note: this THEMIS visual image has not been radiometrically nor geometrically calibrated for this preliminary release. An empirical correction has been performed to remove instrumental effects. A linear shift has been applied in the cross-track and down-track direction to approximate spacecraft and planetary motion. Fully calibrated and geometrically projected images will be released through the Planetary Data System in accordance with Project policies at a later time.

    NASA's Jet Propulsion

  19. African American Female Professors' Strategies for Successful Attainment of Tenure and Promotion at Predominately White Institutions: It Can Happen

    ERIC Educational Resources Information Center

    Jones, Brandolyn; Hwang, Eunjin; Bustamante, Rebecca M.

    2015-01-01

    In their pursuit of tenure and promotion, African American female faculty members continue to prevail over workplace adversities such as ridicule, marginalization, alienation, isolation, and lack of information. In this descriptive phenomenological study, the lived experiences of five African American female professors who successfully navigated…

  20. Post-Rift Compressional Deformation on the Passive Margin of a young Mediterranean Backarc Basin (Eastern Sardinian Margin, Tyrrhenian Sea)

    NASA Astrophysics Data System (ADS)

    Chanier, F.; Gaullier, V.; Maillard, A.; Thinon, I.; Sage, F.; Lymer, G.; Vendeville, B.; Giresse, P.; Bassetti, M. A.; Lofi, J.

    2014-12-01

    Compressional deformation has been reported on many passive margins, mostly attributed to thin-skinned tectonics in response to gravity gliding or spreading from viscous layers (overpressured shales, salt décollement). However some of the reported structures are obviously related to regional stress and also affect the basement, not only the upper sedimentary cover. Such deformation has been documented and discussed in the last decade mainly from the northern Atlantic margins (Doré et al., 2008 ; Pereira et al., 2011, & ref. herein). The compressional structures on passive margins have been notably considered as linked to tectonomagmatic and active asthenospheric upwelling, post-breakup compression and compactional stresses. The western margin of the Tyrrhenian Sea (Central Mediterranean) is a passive margin formed during the late Miocene opening of a back-arc basin in relation with the roll-back and retreat of the Ionian subducting lithosphere (African Plate). From our new data, we can show evidence for compressional features that developed in the Pliocene, shortly after the main rifting period on the western Tyrrhenian Sea (Middle to Late Miocene) and beginning of oceanic spreading (Earliest Pliocene). We could describe such structures across the inner margin onshore, from field analysis, as well as offshore, from newly acquired seismic data (METYSS 1 & 3; Gaullier et al. 2014). The characters and distribution of such compressional deformation, occurring very shortly after the onset of oceanic spreading in the deep basin (earliest Pliocene), allow us to discuss the possible interactions between breakup processes and inversion episodes on passive margins. Doré A.G., Lundin E.R., Kusznir N.J., & Pascal C., 2008. Potential mechanisms for the genesis of Cenozoic domal structures on the NE Atlantic margin: Pros and cons and some new ideas. Geol. Soc. London Spec. Pub., 306, 1-26. Gaullier V., Chanier F., et al., 2014. Salt tectonics and crustal tectonics along the

  1. Late Pleistocene stratigraphy of a carbonate platform margin, Exumas, Bahamas

    NASA Astrophysics Data System (ADS)

    Aalto, K. R.; Dill, Robert F.

    1996-05-01

    Detailed field studies of the southern Exuma Cays on the eastern margin of the Great Bahama Bank show a complex history of late Pleistocene island construction. Pleistocene rocks include island core eolianites, overlain at island margins by fossil patch reefs and reef sands, which in turn are overlain by, and/or grade laterally into, talus breccia cones derived from the erosion of island core eolianite at paleo-seacliffs situated at approximately 5-6 m above present mean high tide. Laminated pedogenic calcrete widely caps Pleistocene rocks. Minor zones of penetrative subsurface calcretization, developed in association with root growth, occur along permeable horizons, including: contacts between talus units or crossbed sets, along tension joints, and (possibly) at the Pleistocene reef-eolianite contact. Among Pleistocene eolianite samples studied in thin-section, the relative proportions of ooids-intraclasts+grapestones-skeletal grains-peloids are approximately 48:39:6:7. Marginal to the Exuma Sound and on the Brigantine Cays, a greater proportion of ooids have peloidal nuclei and cortices with numerous laminae, which may reflect ooid derivation from shelf margin and broad platform interior regions that were characterized by high wave energy during ooid formation. Between these two areas, ooids are more commonly superficial and have cortices with few laminae and nuclei composed of subrounded micrite or pelmicrite intraclasts. Such ooid nuclei are most likely derived from storm erosion of partially cemented seafloor muds. Some skeletal-rich eolianite in this region may reflect local sediment input from platform margin reefs, or may be part of an older(?) stratigraphic unit.

  2. Stratigraphic framework and petroleum potential of Northeastern Baltimore Canyon Trough, Mid-Atlantic Outer Continental Shelf

    SciTech Connect

    Libby-French, J.

    1984-01-01

    Geologic data from 29 wells in the Baltimore Canyon Trough were released to the public as of 1982, and provide sufficient information for establishing Upper Jurassic through Cenozoic rock-stratigraphic units. The oldest rocks penetrated by exploratory wells are of Late Jurassic age and are correlative to the Scotian Shelf Mohawk, Mic Mac, and Abenaki Formations. The Mohawk(.) sandstone and Mic Mac shale equivalents in the Baltimore Canyon Trough represent lower delta plain to predominantly prodelta environments, and the Abenaki-equivalent limestone represents a shelfmargin carbonate buildup. A destructional phase of the delta is represented by the Naskapi equivalents a calcareous shale. Stratigraphic traps may be present in the Baltimore Canyon Trough Upper Jurassic and Lower Cretaceous deltaic sandstones and shelf-margin carbonates. The deltaic units contain channel and distributary-mouth-bar sandstones, which may be potential reservoirs. The Abenaki stratigraphic-reef trend provides another potential target.

  3. Map of Distribution of Bottom Sediments on the Continental Shelf, Gulf of Alaska

    USGS Publications Warehouse

    Evans, Kevin R.; Carlson, Paul R.; Hampton, Monty A.; Marlow, Michael S.; Barnes, Peter W.

    2000-01-01

    Introduction The U.S. Geological Survey has a long history of exploring marine geology in the Gulf of Alaska. As part of a cooperative program with other federal and state agencies, the USGS is investigating the relations between ocean-floor geology and benthic marine biohabitats. This bottom sediment map, compiled from published literature will help marine biologists develop an understanding of sea-floor geology in relation to various biological habitats. The pattern of sea-floor sedimentation and bottom morphology in the Gulf of Alaska reflects a complex interplay of regional tectonism, glacial advances and retreats, oceanic and tidal currents, waves, storms, eustatic change, and gravity-driven processes. This map, based on numerous cruises during the period of 1970-1996, shows distribution of bottom sediments in areas of study on the continental shelf. The samples were collected with piston, box, and gravity corers, and grab samplers. The interpretations of sediment distribution are the products of sediment size analyses combined with interpretations of high-resolution seismic reflection profiles. The sea floor was separated into several areas as follows: Cook Inlet -- Hazards studies in this embayment emphasized sediment distribution, sediment dynamics, bedforms, shallow faults, and seafloor stability. Migrating mega-sandwaves, driven by strong tidal currents, influence seabed habitats and stability of the seafloor, especially near pipelines and drilling platforms. The coarseness of the bottom sediment reinforces the influence of the strong tidal currents on the seafloor habitats. Kodiak Shelf -- Tectonic framework studies demonstrate the development of an accretionary wedge as the Pacific Plate underthrusts the Alaskan landmass. Seismic data across the accretionary wedge reveal anomalies indicative of fluid/gas vent sites in this segment of the continental margin. Geologic hazards research shows that movement along numerous shallow faults poses a risk to sea

  4. Reconstruction of the East Africa and Antarctica continental margins

    NASA Astrophysics Data System (ADS)

    Nguyen, L. C.; Hall, S. A.; Ball, P.; Bird, D. E.

    2015-12-01

    The Early Jurassic separation of Antarctica from Africa plays an important role in our understanding of the dispersal of Gondwana. Previously proposed reconstruction models often contain overlaps and gaps in the restored margins that reflect difficulties in accurately delineating the continent-ocean boundary (COB) and determining the amount and distribution of extended continental crust. This study focuses on the evolution of the African margin adjacent to the Mozambique Basin and the conjugate margin of Antarctica near the Riiser Larsen Sea. New satellite-derived gravity data have been used to trace the orientations and landward limits of fracture zones in the study area. A 3-D gravity inversion has produced a crustal thickness model that reliably quantifies the extent and amount of stretched crust. Information on crustal thickness along with the identification of fracture zones reveal the COBs that are located significantly closer to the coasts of Africa and Antarctica than previously recognized. Correlation of both fracture zone azimuths and the identified COBs over the conjugate margins suggest Antarctica began drifting away from Africa at approximately 171 Ma in a roughly SSE direction. Of several scenarios examined, the Beira High is most likely oceanic and may be a conjugate feature of the southern Astrid Ridge. An areal-balancing method that involves restoring the crust to a uniform pre-rift thickness has been used to perform the non-rigid reconstruction for both non-volcanic and volcanic margin with magmatic underplating. Based on the results, Africa underwent extension of 65-105 km while Antarctic crust was stretched by 90-190 km. Both margins reveal a trend of increasing extension from east to west. Various models tested to determine the direction of extension during rifting suggest that Antarctica underwent a counter-clockwise rotation with respect to Africa between 186-171 Ma prior to the onset of seafloor spreading.

  5. African and non-African admixture components in African Americans and an African Caribbean population.

    PubMed

    Murray, Tanda; Beaty, Terri H; Mathias, Rasika A; Rafaels, Nicholas; Grant, Audrey Virginia; Faruque, Mezbah U; Watson, Harold R; Ruczinski, Ingo; Dunston, Georgia M; Barnes, Kathleen C

    2010-09-01

    Admixture is a potential source of confounding in genetic association studies, so it becomes important to detect and estimate admixture in a sample of unrelated individuals. Populations of African descent in the US and the Caribbean share similar historical backgrounds but the distributions of African admixture may differ. We selected 416 ancestry informative markers (AIMs) to estimate and compare admixture proportions using STRUCTURE in 906 unrelated African Americans (AAs) and 294 Barbadians (ACs) from a study of asthma. This analysis showed AAs on average were 72.5% African, 19.6% European and 8% Asian, while ACs were 77.4% African, 15.9% European, and 6.7% Asian which were significantly different. A principal components analysis based on these AIMs yielded one primary eigenvector that explained 54.04% of the variation and captured a gradient from West African to European admixture. This principal component was highly correlated with African vs. European ancestry as estimated by STRUCTURE (r(2)=0.992, r(2)=0.912, respectively). To investigate other African contributions to African American and Barbadian admixture, we performed PCA on approximately 14,000 (14k) genome-wide SNPs in AAs, ACs, Yorubans, Luhya and Maasai African groups, and estimated genetic distances (F(ST)). We found AAs and ACs were closest genetically (F(ST)=0.008), and both were closer to the Yorubans than the other East African populations. In our sample of individuals of African descent, approximately 400 well-defined AIMs were just as good for detecting substructure as approximately 14,000 random SNPs drawn from a genome-wide panel of markers. PMID:20717976

  6. The African superswell

    NASA Technical Reports Server (NTRS)

    Nyblade, Andrew A.; Robinson, Scott W.

    1994-01-01

    Maps of residual bathymetry in the ocean basins around the African continent reveal a broad bathymetric swell in the southeastern Atlantic Ocean with an amplitude of about 500 m. We propose that this region of anomalously shallow bathymetry, together with the contiguous eastern and southern African plateaus, form a superswell which we refer to as the African superswell. The origin of the African superswell is uncertain. However, rifting and volcanism in eastern Africa, as well as heat flow measurements in southern Africa and the southeastern Atlantic Ocean, suggest that the superswell may be attributed, at least in part, to heating of the lithosphere.

  7. Geometries of hyperextended continental crust in northeastern continental brazilian margin: insights from potential field and seismic interpretation

    NASA Astrophysics Data System (ADS)

    Magalhães, José; Barbosa, José; Ribeiro, Vanessa; Oliveira, Jefferson; Filho, Osvaldo; Buarque, Bruno

    2016-04-01

    The study region encompasses a set of three basins located at Northeast Brazilian continental margin: Pernambuco (south sector), Paraíba and Natal platform (north sector). These basins were formed during the last stage of separation between South America and African plates during Cretaceous. The continental breakup in these regions occurred probably during the Middle-Upper Albian (~102 m.y). The adjacent basement rocks belong to Borborema Province (BP), which was formed due a complex superposition between Pre-Cambrian orogenic cycles. The structural framework of BP is dominated by large shear zones that divided this province in three main tectonic domains: South, Central and North. The Pernambuco Basin is located in the South Domain and the Paraíba and Natal platform basins are related to the Central Domain. The tectonic and magmatic evolution of the Pernambuco Basin was influenced by oblique rifting (~ 35° to rift axis) and a thermal anomaly probably caused by the Santa Helena hotspot. The north sector represents a continental shelf characterized by basement high with a narrow platform and an abrupt shelf break on transition to the abyssal plain. The continental platform break of this sector was parallel to the rift axis. In this way, we present a regional structural interpretation of these sectors of Brazilian rifted margin based on interpretation and 2D forward modeling of potential field and 2D seismic data. The magnetic maps (Reduction to magnetic pole and Analytic signal) revealed the influence of an alternating pattern of large narrow magnetic and non-magnetic lineaments, oriented NE-SW, E-W and NW-SE. In the Pernambuco Basin these lineaments (NE-SW and E-W) are related to shear zones in the hyperextended basement which is interpreted as a continuation of the granitic-gneissic and metasedimentary rocks of the South Domain of BP. The Paraíba and Natal platform basins show a slight change in the orientation of structures trending E-W (shear zones in

  8. Steel Industry Marginal Opportunity Analysis

    SciTech Connect

    none,

    2005-09-01

    The Steel Industry Marginal Opportunity Analysis (PDF 347 KB) identifies opportunities for developing advanced technologies and estimates both the necessary funding and the potential payoff. This analysis determines what portion of the energy bandwidth can be captured through the adoption of state-of-the-art technology and practices. R&D opportunities for addressing the remainder of the bandwidth are characterized and plotted on a marginal opportunity curve.

  9. Terrestrial organic carbon contributions to sediments on the Washington margin

    SciTech Connect

    Prahl, F.G.; Sparrow, M.A.; Eversmeyer, B. ); Ertel, J.R. ); Goni, M.A. )

    1994-07-01

    Elemental and stable carbon isotopic compositions and biomarker concentrations were determined in sediments from the Columbia River basin and the Washington margin in order to evaluate geochemical approaches for quantifying terrestrial organic matter in marine sediments. The biomarkers include: an homologous series of long-chain n-alkanes derived from the surface waxes of higher plants; phenolic and hydroxyalkanoic compounds produced by CuO oxidation of two major vascular plant biopolymers, lignin and cutin. All marine sediments, including samples collected from the most remote sites in Cascadia Basin, showed organic geochemical evidence for the presence of terrestrial organic carbon. Using endmember values for the various biomarkers determined empirically by two independent means, the authors estimate that the terrestrial contribution to the Washington margin is [approximately] 60% for shelf sediments, [approximately] 30% for slope sediments, and decreases further to [le] 15% in basin sediments. Results from the same geochemical measurements made with depth in gravity core 6705-7 from Cascadia Seachannel suggest that this approach to assess terrestrial organic carbon contributions to contemporary deposits on the Washington margin can be applied to the study of sediments depositing in this region since the last glacial period.

  10. Latest Quaternary outer shelf and slope deposits, northern Gulf of Mexico, USA: Industry research consortium

    SciTech Connect

    Winn, R.D. Jr. )

    1991-03-01

    A consortium of oil companies is undertaking a multiyear shallow coring and high-resolution seismic investigation of the outer shelf-slope deposits of the northern Gulf of Mexico. The program's objective is to reconstruct the depositional, seismic stratigraphic, biostratigraphic, isotopic, and organic geochemical record of the latest Quaternary of a portion of the Gulf margin. Results will serve as an analog to deeper hydrocarbon reservoirs and to help in understanding sedimentation with glacial-interglacial sea-level changes. Nearly continuous cores up to 850 ft long from Main Pass Blocks 303, 242, 288 and Viosca Knoll Block 774 are being described and analyzed. Main Pass 303 core samples Holocene to Oxygen Isotope Stage 6 deposits and as old or older strata are expected to have been penetrated in the other boreholes. Three or four sea level cycles are represented. Seismic acquisition is planned for late 1990. The outer shelf and slope of the northern Gulf is an area of active growth faulting, high sedimentation rate, diapirism, and slumping. Recent work, including this study, shows that near-surface stratigraphy consists of coarse clastics alternating with carbonate-rich mud. Sands and gravels are inferred to have been deposited mostly during lowstands and rising sea level in deltas and valley-fill systems. Delta clinoforms on high-resolution seismic may represent shelf-margin wedges, prograding wedges, or highstand systems tracts. Highstand deposits are characterized largely by thin, continuous intervals of bioturbated muds containing abundant diagenetic carbonate and pyrite. Precise dating and correlation of outer shelf-upper slope facies to magnitude and direction of sea level change is intended during the project.

  11. Seismic observations of sea swell on the floating Ross Ice Shelf, Antarctica

    NASA Astrophysics Data System (ADS)

    Cathles, L. M.; Okal, Emile A.; Macayeal, Douglas R.

    2009-06-01

    A seismometer operating on the floating Ross Ice Shelf near its seaward ice front (Nascent Iceberg) for 340 days (out of 730 days) during the 2004, 2005, and 2006 Antarctic field seasons recorded the arrival of 93 distantly sourced ocean swell events displaying frequency dispersion characteristic of surface gravity waves propagating on deep water. Comparison of swell event dispersion with the NOAA Wave Watch III (NWW3) ocean wave model analysis reveals that 83 of these events were linked to specific storms located in the Pacific, Southern, and Indian oceans. Nearly all major storms in the NWW3 analysis of the Pacific Ocean were linked to signals observed at the Nascent site during the period of seismometer operation. Swell-induced motion of the Ross Ice Shelf is found to increase by several orders of magnitude over the time period that sea ice surrounding Antarctica decreases from its maximum extent (October) to its minimum extent (February). The amplitude of vertical vibration of the ice shelf in the frequency band between 0.025 and 0.14 Hz varies between tens of micrometers to millimeters as sea ice decays to its minimum seasonal extent. This suggests that climate influence on sea ice extent may indirectly modulate swell energy incident on the calving margins of the Antarctic Ice Sheet. The largest swell signals observed on the Ross Ice Shelf come from storms in the tropical Pacific and Gulf of Alaska. These remote events emphasize how the iceberg calving margin of Antarctica is connected to environmental conditions well beyond Antarctica.

  12. Delineation of Late Quaternary depositional sequences by high-resolution seismic stratigraphy, Louisiana continental shelf

    SciTech Connect

    Suter, J.R.; Berryhill, H.L. Jr.; Penland, S.

    1987-05-01

    Interpretations of over 20,000 line km of single-channel, high-resolution seismic reflection profiles, coupled with nearshore vibracores and logs of industrial platform borings, provide the data base for determining the history and stratigraphy of late Quaternary sea level fluctuations on the Louisiana continental shelf. Regional unconformities, formed by subaerial exposure of the shelf during glacio-eustatic sea level withdrawals and modified by shoreface erosion during ensuing transgression, serve as markers to identify the boundaries of depositional sequences. Unconformities are recognizable on seismic profiles by high-amplitude reflectors as well as discordant relationships between reflectors. Within the upper Quaternary section, six depositional sequences have been recognized. Five of these are related to glacio-eustatic sea level fluctuations, involving sea level fall close to, or beyond, the margin of the continental shelf. Three of these fluctuations culminated in the deposition of shelf margin delta sequences. Extensive fluvial channeling characterizes the regressive phase of these sequences. Transgressive phases are marked by infilling of fluvial channels, flood-plain aggradation, truncation, or deposition of sand sheets, depending upon sediment supply and rate of sea level rise. Sequences 4 and 5 are correlated with the late Wisconsinan glacial stage and Holocene transgression. The upper portion of sequence 5 consists of an early Holocene Mississippi delta complex. Abandonment and transgression of this delta are responsible for the formation of sequence 6. Although these deposits cover a smaller area, this demonstrates that deltaic processes can produce sequences similar to those driven by glacially controlled sea level changes.

  13. Arctic and Antarctic submarine gullies—A comparison of high latitude continental margins

    NASA Astrophysics Data System (ADS)

    Gales, J. A.; Forwick, M.; Laberg, J. S.; Vorren, T. O.; Larter, R. D.; Graham, A. G. C.; Baeten, N. J.; Amundsen, H. B.

    2013-11-01

    Submarine gullies are common features of high latitude continental slopes and, over the last decade, have been shown to play a key role in continental margin evolution, submarine erosion, downslope sediment transport, slope deposits, and the architecture of petroleum reservoirs. However, the processes that form these gullies, the timescales over which they develop, and the environmental controls influencing their morphology remain poorly constrained. We present the first systematic and comparative analysis between Arctic and Antarctic gullies with the aim of identifying differences in slope character, from which we infer differences in processes operating in these environments. Quantitative analysis of multibeam echosounder data along 2441 km of the continental shelf and upper slope and morphometric signatures of over 1450 gullies show that six geomorphically distinct gully types exist on high latitude continental margins. We identify distinct differences between Arctic and Antarctic gully morphologies. In the Arctic data sets, deep relief (> 30 m gully incision depth at 50 m below the shelf edge) and shelf-incising gullies are lacking. These differences have implications for the timescales over which the gullies were formed and for the magnitude of the flows that formed them. We consider two hypotheses for these differences: (1) some Antarctic gullies developed through several glacial cycles; and (2) larger Antarctic gullies were formed since the Last Glacial Maximum as a result of erosive flows (i.e., sediment-laden subglacial meltwater) being more abundant on parts of the Antarctic margin over longer timescales. A second difference is that unique gully signatures are observed on Arctic and on Antarctic margins. Environmental controls, such as the oceanographic regime and geotechnical differences, may lead to particular styles of gully erosion observed on Arctic and Antarctic margins.

  14. Assessment of primary production and optical variability in shelf and slope waters near Cape Hatteras, North Carolina. Final project report

    SciTech Connect

    Redalje, Donald G.; Lohrenz, Stevern E.

    2001-02-12

    In this project we determined primary production and optical variability in the shelf and slope waters off of Cape Hatteras, N.C. These processes were addressed in conjunction with other Ocean Margins Program investigators, during the Spring Transition period and during Summer. We found that there were significant differences in measured parameters between Spring and Summer, enabling us to develop seasonally specific carbon production and ecosystem models as well as seasonal and regional algorithm improvements for use in remote sensing applications.

  15. Export and Cycling of Continental Shelf Carbon: A Modeling Study

    NASA Astrophysics Data System (ADS)

    Siedlecki, S.; Archer, D.; Mahadevan, A.

    2004-12-01

    Continental margins play a significant role in the production and burial of organic carbon in the ocean, but these areas are poorly resolved in global circulation models. In this study, a high-resolution three-dimensional, nonhydrostatic idealized coastal model of the eastern United States after Mahadevan and Archer, 2000, 1998, was modified to simulate organic carbon production and export off the shelf. The model assumes a periodic north and south boundary, solid offshore and bottom boundaries, and a shelf-break density front determined by bathymetry. The model uses a free surface and a sigma grid in the vertical. We are in the process of formulating a carbon and nutrient component for this model. The model is initialized with a vertical nutrient profile taken from the open Atlantic Ocean. Mesoscale wind-driven circulation and vertical diffusion bring nutrients to the euphotic zone. Primary production is based on light availability and nutrient concentration. The particles advect with the flow and sink with a specified velocity. Remineralization is first-order in carbon concentration, and produces ammonia. Ammonia is slowly reoxidized to nitrate in subsurface waters, and used for recycled production in the euphotic zone. We are searching for a model of the production, sinking, and interconversion of multiple types of particles, which predicts the observed trends in f-ratio from coastal to pelagic ecosystems. The model is sensitive to sinking velocity, remineralization rate, vertical diffusivity, the uptake rate of nitrate, the uptake rate of ammonia, and the oxidation rate of ammonia to nitrate. Using the steady state solution of the one-dimensional model to initialize the three-dimensional model, we study the effect of vertical and horizontal advection and three-dimensional oceanographic processes on the distribution and export of carbon from the coastal system. We will compare the sensitivities of a box-budget, a one-dimensional diffusional, and the full 3-D

  16. Glider monitoring of shelf suspended particle dynamics and transport during storm and flooding conditions

    NASA Astrophysics Data System (ADS)

    Bourrin, François; Many, Gaël; Durrieu de Madron, Xavier; Martín, Jacobo; Puig, Pere; Houpert, Loic; Testor, Pierre; Kunesch, Stéphane; Mahiouz, Karim; Béguery, Laurent

    2015-10-01

    Transfers of particulate matter on continental margins primarily occur during energetic events. As part of the CASCADE (CAscading, Storm, Convection, Advection and Downwelling Events) experiment, a glider equipped with optical sensors was deployed in the coastal area of the Gulf of Lions, NW Mediterranean in March 2011 to assess the spatio-temporal variability of hydrology, suspended particles properties and fluxes during energetic conditions. This deployment complemented a larger observational effort, a part of the MOOSE (Mediterranean Ocean Observing System of the Environment) network, composed of a coastal benthic station, a surface buoy and moorings on the continental slope. This set of observations permitted to measure the impact of three consecutive storms and a flood event across the entire continental shelf. Glider data showed that the sediment resuspension and transport observed at the coastal station during the largest storm (Hs>4 m) was effective down to a water depth of 80 m. The mid-shelf mud belt, located between 40 and 90 m depth, appears as the zone where the along-shelf flux of suspended sediment is maximum. Besides, the across-shelf flux of suspended sediment converges towards the outer limit of the mid-shelf mud belt, where deposition of suspended particles probably occurs and contributes to the nourishment of this area. Hydrological structures, suspended particles transport and properties changed drastically during stormy periods and the following flood event. Prior to the storms, the shelf waters were weakly stratified due in particular to the presence of cold dense water on the inner- and mid-shelf. The storms rapidly swept away this dense water, as well as the resuspended sediments, along the shelf and towards a downstream submarine canyon. The buoyant river plumes that spread along the shelf after the flooding period provoked a restratification of the water column on the inner- and mid-shelf. The analysis of glider's optical data at

  17. The sandy inner shelf as a repository for muddy sediment: an example from Northern California

    NASA Astrophysics Data System (ADS)

    Crockett, J. S.; Nittrouer, C. A.

    2004-01-01

    To understand the sedimentary history of a siliciclastic, tectonically active inner-shelf environment, 50 vibracores were collected in water depths ranging from 20 to 55 m on the Eel margin, Northern California. Sediments exhibit changes in grain size along- and across-shelf, as well as vertically. The mean grain size of the surface sand fraction is 2.2-3.6 φ (217-82.5 μm), with 7-40% of the sediment finer than 4 φ (64 μm). The along-shelf distribution of sand is controlled predominantly by proximity to the sediment source and by the prevailing oceanic conditions (e.g., waves and currents). Fine sand dominates within ˜10 km of the Eel River mouth, which is consistent with sediment settling velocities out of the Eel River plume. Coarser sediment is found farther (>10 km) north of the Eel River mouth, and may have a source other than the modern Eel River. Cores tend to demonstrate upward fining (except cores collected at ˜30 m) and other vertical grain-size variations, which preserve the record of floods on the Eel margin. Muddy layers, which are assumed to record large floods, are found interbedded with sand. Two types of muddy layers are observed. "Distinct" mud layers have >70% mud and are identifiable through visual observation and a decrease in bulk density. "Diffuse" mud layers contain only 5-15% mud due to reworking caused by wave activity during or after deposition. Diffuse mud layers are not visually obvious, and poor sorting increases their bulk density. Recent flood layers have detectable 210Pb and 137Cs, and the deepest detectable 137Cs is 120 cm below the surface. Assuming that the mud layers result from major floods (e.g., 1964), and using 210Pb and 137Cs radioisotopes to establish chronological control, the accumulation rate of sand on the inner shelf since 1964 ranges from 1.3 to 3.3 cm/yr. Approximately 6-13% of the fine-grained sediment discharged by the Eel River over the past 36 yr is accumulating interspersed with the inner-shelf sand. An

  18. Assessment of the petroleum, coal, and geothermal resources of the Economic Community of West African States (ECOWAS) region

    USGS Publications Warehouse

    Mattick, R. E., (compiler)

    1982-01-01

    Approximately 85 percent of the land area of the ECOWAS (Economic Community of West African States) region is covered by basement rocks (igneous and highly metamorphosed rocks) or relatively thin layers of Paleozoic, Upper Precambrian, and 'Continental Intercalaire? sedimentary rocks. These areas have little or no petroleum potential. Areas of the ECOWAS region that have potential for petroleum production or potential for increased petroleum production include the narrow belt of sedimentary rocks that stretches along the continental margin from Mauritania to Nigeria and the Niger Delta and the Benue depression. The Senegal Basin, located on the continental margin of Mauritania, Senegal, Gambia, Guinea Bissau, and Guinea, has been intensely explored by the oil industry and most of the larger structures onshore and on the shelf probably have been tested by drilling with little or no resulting commercial production. Unless basic ideas pertaining to the petroleum geology of the Senegal Basin are revised, future discoveries are expected to be limited to small fields overlooked by industry at a time when petroleum prices were low. On the continental shelf of Sierra Leone and the continental shelf of northeast and central Liberia, the sedimentary rocks are relatively thin, and industry has shown little interest in the area. On the continental rise of these countries, however, the sedimentary section, deposited in a complex fault-block system, increases in thickness. A renewal of industry interest in this deep-water area will probably follow further development of deep-water production technology. A recent oil discovery on the continental slope off the Ivory Coast is expected to spur further exploration offshore of southeastern Liberia, Ivory Coast, Ghana, Togo, and Benin. This relatively unexplored area in the Gulf of Guinea has good possibilities .for the discovery of giant oil fields. Nigeria's oil development from the Niger Delta may have peaked, as 13 of 14 giant oil

  19. Assessing the Fate of Terrigenous Dissolved Organic Carbon in River-influenced Ocean Margins

    NASA Astrophysics Data System (ADS)

    Fichot, C. G.; Benner, R. H.

    2014-12-01

    The mineralization of terrigenous dissolved organic carbon (tDOC) discharged by rivers can impact nutrient and trace metal cycling, biological productivity, net ecosystem metabolism, and air-sea CO2 exchange in ocean margins. The extreme heterogeneity of river-influenced ocean margins represents a major challenge for quantitative assessments of tDOC transformations and thereby obscures the role of tDOC in biogeochemical cycles. Here, we demonstrate how a lignin-based optical proxy for tDOC and a shelf-wide mass balance approach can be used to quantitatively assess the fate of tDOC discharged on a river-influenced ocean margin. Such a mass balance was carried out in the northern Gulf of Mexico and revealed that ~40% of the tDOC discharged by Mississippi-Atchafalaya River System between March 2009 and March 2010 was mineralized to CO2 on the Louisiana shelf, with two thirds of the mineralization taking place in the mixed layer. A strong seasonality in tDOC mineralization was observed, with mineralization rates severalfold higher during summer than during winter. Independent assessments of specific mineralization processes indicated biomineralization accounted for ~94% of the tDOC mineralization on an annual basis, and suggested that photochemical transformations of tDOC enhanced biomineralization by ~50% in the mixed layer. Direct photomineralization accounted for a relatively small fraction (~6%) of the tDOC mineralization on an annual basis. This quantitative assessment provides direct evidence confirming ocean margins are major sinks of the tDOC discharged by rivers, and indicates that tDOC mineralization rates in the shelf mixed layer are sufficiently large to influence whether the Louisiana shelf is a net sink or source of atmospheric CO2. In this presentation, we also demonstrate how the remote sensing of ocean color can be used to trace tDOC and its cross-shelf export and further advance our understanding of the fate of tDOC in ocean margins.

  20. New Data From the Southern Levantine Continental Margin and outer Nile Cone - the GEMME Project

    NASA Astrophysics Data System (ADS)

    Huebscher, C. P.; Ben-Avraham, Z.; Dehghani, A.; Gohl, K.; Paetzold, J.

    2002-12-01

    In order to investigate the tectonic and sedimentary setting of the southern Levantine continental margin as well as the Late Quaternary paleoceanography of the outer Nile Cone a geophysical and geological survey - the GEMME project - was carried out in the eastern Mediterranean between February and March 2002. The German research vessel METEOR operated for 5 weeks in the territorial waters of Israel and Egypt. The experiments included refraction and reflection seismics, gravity and magnetics, swath sounding and subbottom profiling, and sediment sampling with gravity and multi-corer. We found a strong correlation between salt tectonic features and the position of channel levee complexes evolved on the outer Nile Cone, which is different to other submarine fans without a mobile layer beneath where the location of the complexes is an autocyclic process. The high-resolution stratigraphy of the Post-Messinian sediment prism reflects the interplay between sediment input, transport mechanisms, uplift and subsidence, halokinetics, and changes of sea level and climate. We found a wide abundance of gassy clastic sediments above the basal Pliocene unconformity and relation between chemoherms, faulting, and gas/fluid migration. The dominant disturbances along the continental slope off Israel are most likely triggered by salt (gravity) tectonic. The position of the landward termination of the Messinian evaporites is located beneath the present shelf in the north but beneath the lower continental slope of bottomset in the south. The salt tectonic is most likely responsible for the along-strike variation of the margin morphology. The Pelusium line reflects the salt tectonic and is presumably not a basement tectonic feature. We found evidence of active tectonic off Haifa Bay in the prolongation of the Carmel rift and it related branches. The paleoclimate history of the Nile deposits will be studied from the extensive sediment core collection. Four different sediment profiles were

  1. New Exploration of Kerguelen Plateau Margins

    NASA Astrophysics Data System (ADS)

    Vially, R.; Roest, W. R.; Loubrieu, B.; Courreges, E.; Lecomte, J.; Patriat, M.; Pierre, D.; Schaming, M.; Schmitz, J.

    2008-12-01

    France ratified the United Nations Convention on the Law of the Sea in 1996, and has since undertaken an ambitious program of bathymetric and seismic data acquisition (EXTRAPLAC Program) to support claims for the extension of the legal continental shelf, in accordance with Article 76 of this convention. For this purpose, three oceanographic surveys took place on board of the R/V Marion Dufresne II on the Kerguelen Plateau, in Southern Indian Ocean: MD137-Kergueplac1 (February 2004), MD150-Kergueplac2 (October 2005) and MD165-Kergueplac3 (January 2008), operated by the French Polar Institute. Thus, more than 20 000 km of multibeam bathymetric, magnetic and gravimetric profiles, and almost 6 000 km of seismic profiles where acquired during a total of 62 days of survey in the study area. Ifremer's "rapid seismic" system was used, comprised of 4 guns and a 24 trace digital streamer, operated at speeds up to 10 knots. In addition to its use for the Extraplac Program, the data set issued from these surveys gives the opportunity to improve our knowledge of the structure of the Kerguelen Plateau and more particularly of its complex margins. In this poster, we will show the high resolution bathymetry (200 m) data set, that allows us to specify the irregular morphology of the sea floor in the north Kerguelen Plateau, characterised by ridges and volcanoes chains, radial to the plateau, that intersect the oceanic basin on the NE edge of the Kerguelen Plateau. We will also show magnetic and gravity data, which help us to understand the setting up of the oceanic plateau and the kinematics reconstructions. The seismic profiles show that the acoustic basement of the plateau is not much tectonised, and displays a very smooth texture, clearly contrasting it from typical oceanic basement. Both along the edge of the plateau as in the abyssal plain, sediments have variable thicknesses. The sediments on the margin of the plateau are up to 1200 meters thick and display irregular

  2. Sediment dynamics and post-glacial evolution of the continental shelf around the Blanes submarine canyon head (NW Mediterranean)

    NASA Astrophysics Data System (ADS)

    Durán, Ruth; Canals, Miquel; Lastras, Galderic; Micallef, Aaron; Amblas, David; Pedrosa-Pàmies, Rut; Sanz, José Luis

    2013-11-01

    head rim constitute a source of coarse sediment. High-energy processes, namely river floods and coastal storms, are the main controls over the river-shelf-canyon sediment exchange. River floods increase the delivery of terrigenous particles to the coastal system. Storms, mainly from the east, remobilize the sediment temporarily accumulated on the shelf towards the canyon head, so that the finer fractions are preferentially removed and a coarse lag is normally left on the shelf floor. Exceptionally, very strong storms also remove the coarse fractions from the shelf drive them into the canyon. Processes like dense shelf water cascading, which is much more intense in canyons to the north of BC, and the Northern Current also contribute to the transport of suspended sediment from far distant northern sources. During the last post-glacial transgression the BC had a strong influence on the evolution of the inner continental margin, as it interrupted the shelf sediment dispersal system by isolating the shelves to its north and south, named La Planassa and Barcelona shelves, respectively. The detailed study of the geomorphology and uppermost sediment cover of the continental shelf surrounding the Blanes submarine canyon yields insight into the past and present shelf sediment dynamics and the shelf-to-canyon sediment exchanges. The continental shelf near the canyon head consists of mosaic where erosional, or non-depositional, and depositional zones coexist. East of the canyon and offshore Tossa de Mar, the modern sediment deposition is mostly confined to the inner and middle shelf, whilst most of the La Planassa shelf is sediment depleted with numerous relict morphosedimentary features cropping out. Rocky outcrops, narrow ridges and relict coarse sand deposits suggesting erosion or non-deposition of fine sediments in modern times occupy the middle and outer shelf floor east and northeast of the canyon head. In contrast, north and west of the canyon head, the middle and outer

  3. Morphotectonics and sedimentation in convergent margin basins: An example from juxtaposed marginal sea basin and foreland basin, Northern South China Sea

    NASA Astrophysics Data System (ADS)

    Yu, Ho-Shing; Huang, Zehn-Yin

    2009-03-01

    arc-continent collision in the southern Taiwan. The slope profile of the passive Chinese margin is characterized by a sigmoidal curvature, representing a typical primary depositional setting of a passive margin. Slope canyons occur mainly on the upper slope and cut the sea floor, resulting in irregular topography and representing effects of erosion. Being under the initial arc-continent condition, the offshore SW Taiwan has experienced the transition from a rifted margin to an overthrust belt and becomes a foreland basin, which is acting as a catch basin for orogenic debris derived from SW Taiwan. Arguments from tectonics and sedimentation suggest that the Koping shelf-slope region is considered to be an immature foreland basin rather than an accretionary wedge. More specifically, the Kaoping shelf-slope can be regarded as a depositional wedge top where exists an overlap area between rapid sedimentation and active deformation of the Kaoping shelf-slope sediments in the external orogenic wedge of southern Taiwan.

  4. Sedimentological cross section of Cambro-Ordovician carbonate shelf (Knox group, Conassauga Formation) in central Alabama: facies, diagenesis, potential reservoirs

    SciTech Connect

    Sternbach, L.R.

    1984-04-01

    Cambro-Ordovician thrust-imbricated carbonates in central Alabama are the focus of renewed exploration interest. Samples from east-west-trending core holes within the surface-most thrust plates reconstruct the carbonate shelf and shelf-edge facies before deformation. The Upper Cambrian shelf margin now is in the subsurface of Talledega County; coeval dolostones in the western part of the state represent the former shelf interior. Rock analogs to former environments include the following. (1) Barrier shoals (Conasauga Formation) - dark colored, partially dolomitized ooid and skeletal grainstones. (2) Submerged back-barrier and offshelf dolomitized sediments (lower Knox Group) - western belt: finely crystalline algal thrombolites, fenestral dolopelmicrites, rippled beds; eastern belt: finely laminated dolostones, slope-derived pebbles and graded beds. (3) Tidal flats (upper Knox Group) - light-colored, crystalline dolostones, dolomitized pellet grainstones, algal laminites, pseudomorphs after sulfates and early diagenetic chertification. (4) Former emergent shelf -(Knox unconformity)-pelmicrite, skeletal wackestones, erosional chert pebble conglomerate. Multiple possibilities for hydrocarbon reservoirs appear throughout the sequence. Vuggy and intercrystalline dolostone porosity is primarily in the lower Knox formations. Primary interparticle pores are retained in lower Knox algal buildups. Breccia porosity occurs in the strata below the Knox unconformity through solution of the underlying Knox Group. Fractures in the subsurface are believed to enhance permeability in all porosity types.

  5. Sedimentological cross section of Cambro-Ordovician carbonate shelf (Knox group, Conasauga Formation) in central Alabama: facies, diagenesis, potential reservoirs

    SciTech Connect

    Sternbach, L.R.

    1984-04-01

    Cambro-Ordovician thrust-imbricated carbonates in central Alabama are the focus of renewed exploration interest. Samples from east-west-trending core holes within the surface-most thrust plates reconstruct the carbonate shelf and shelf-edge facies before deformation. The Upper Cambrian shelf margin now is in the subsurface of Talledega County; coeval dolostones in the western part of the state represent the former shelf interior. Rock analogs to former environments include the following. (1) Barrier shoals (Conasauga Formation) - dark colored, partially dolomitized ooid and skeletal grainstones. (2) Submerged back-barrier and offshelf dolomitized sediments (lower Knox Group) - western belt: finely crystalline algal thrombolites, fenestral dolopelmicrites, rippled beds; eastern belt: finely laminated dolostones, slope-derived pebbles and graded beds. (3) Tidal flats (upper Knox Group) - light-colored, crystalline dolostones, dolomitized pellet grainstones, algal laminites, pseudomorphs after sulfates and early diagenetic chertification. (4) Former emergent shelf -(Knox unconformity)-pelmicrite, skeletal wackestones, erosional chert pebble conglomerate. Multiple possibilities for hydrocarbon reservoirs appear throughout the sequence. Vuggy and intercrystalline dolostone porosity is primarily in the lower Knox formations. Primary interparticle pores are retained in lower Knox algal buildups. Breccia porosity occurs in the strata below the Knox unconformity through solution of the underlying Knox Group. Fractures in the subsurface are believed to enhance permeability in all porosity types.

  6. Marine geological and geophysical records of the last British-Irish Ice Sheet on the continental shelf west of Ireland

    NASA Astrophysics Data System (ADS)

    O'Cofaigh, Colm; Callard, S. Louise; Benetti, Sara; Chiverell, Richard C.; Saher, Margot; van Landeghem, Katrien; Livingstone, Stephen J.; Scourse, James; Clark, Chris D.

    2015-04-01

    The record of glaciation on the continental shelf west of Ireland has, until recently, been relatively poorly studied. The UK NERC funded project BRITICE-CHRONO collected marine geophysical data in the form of multibeam swath bathymetry and sub-bottom profiles supplemented by over 50 vibro- and piston cores across the continental shelf west of Ireland during cruise JC106 of the RRS James Cook in 2014. Across the western Irish shelf, offshore of counties Galway and Clare, a series of large arcuate moraines record the former presence of a grounded ice sheet on the shelf. However, geophysical data from further to the west across the Porcupine Bank show a series of ridges and wedge-shaped sedimentary features whose form is consistent with an origin as moraines and/or grounding-zone wedges. Sediment cores from several of these landforms recovered stiff, massive diamictons containing reworked shells that are interpreted as subglacial tills. Cores from the eastern Porcupine Bank recovered laminated muds with cold-water glacimarine foraminifera, in some cases overlying till. Collectively the geophysical and sedimentary data imply the presence of grounded ice across the northern Porcupine Bank and thus much further west on the Irish margin than has previously been considered. This ice underwent retreat in a glacimarine setting. The large 'Olex Moraine' on the western Irish shelf is thus interpreted as recessional feature. Work is currently underway to dates these features and to obtain a retreat chronology for this sector of the last British-Irish Ice Sheet.

  7. 16 Extraordinary African Americans.

    ERIC Educational Resources Information Center

    Lobb, Nancy

    This collection for children tells the stories of 16 African Americans who helped make America what it is today. African Americans can take pride in the heritage of these contributors to society. Biographies are given for the following: (1) Sojourner Truth, preacher and abolitionist; (2) Frederick Douglass, abolitionist; (3) Harriet Tubman, leader…

  8. African Studies Computer Resources.

    ERIC Educational Resources Information Center

    Kuntz, Patricia S.

    African studies computer resources that are readily available in the United States with linkages to Africa are described, highlighting those most directly corresponding to African content. Africanists can use the following four fundamental computer systems: (1) Internet/Bitnet; (2) Fidonet; (3) Usenet; and (4) dial-up bulletin board services. The…

  9. African Literature as Celebration.

    ERIC Educational Resources Information Center

    Achebe, Chinua

    1989-01-01

    Describes the Igbo tradition of "Mbari," a communal creative enterprise that celebrates the world and the life lived in it through art. Contrasts the cooperative, social dimension of pre-colonial African culture with the exclusion and denial of European colonialism, and sees new African literature again celebrating human presence and dignity. (AF)

  10. Educating African American Males

    ERIC Educational Resources Information Center

    Bell, Edward E.

    2010-01-01

    Background: Schools across America spend money, invest in programs, and sponsor workshops, offer teacher incentives, raise accountability standards, and even evoke the name of Obama in efforts to raise the academic achievement of African American males. Incarceration and college retention rates point to a dismal plight for many African American…

  11. Africans Away from Home.

    ERIC Educational Resources Information Center

    Clarke, John Henrik

    Africans who were brought across the Atlantic as slaves never fully adjusted to slavery or accepted its inevitability. Resistance began on board the slave ships, where many jumped overboard or committed suicide. African slaves in South America led the first revolts against tyranny in the New World. The first slave revolt in the Caribbean occurred…

  12. Keeping African Masks Real

    ERIC Educational Resources Information Center

    Waddington, Susan

    2012-01-01

    Art is a good place to learn about our multicultural planet, and African masks are prized throughout the world as powerfully expressive artistic images. Unfortunately, multicultural education, especially for young children, can perpetuate stereotypes. Masks taken out of context lose their meaning and the term "African masks" suggests that there is…

  13. Ice-shelf melting around Antarctica

    NASA Astrophysics Data System (ADS)

    Rignot, E.; Jacobs, S.

    2008-12-01

    The traditional view on the mass balance of Antarctic ice shelves is that they loose mass principally from iceberg calving with bottom melting a much lower contributing factor. Because ice shelves are now known to play a fundamental role in ice sheet evolution, it is important to re-evaluate their wastage processes from a circumpolar perspective using a combination of remote sensing techniques. We present area average rates deduced from grounding line discharge, snow accumulation, firn depth correction and ice shelf topography. We find that ice shelf melting accounts for roughly half of ice-shelf ablation, with a total melt water production of 1027 Gt/yr. The attrition fraction due to in-situ melting varies from 9 to 90 percent around Antarctica. High melt producers include the Ronne, Ross, Getz, Totten, Amery, George VI, Pine Island, Abbot, Dotson/Crosson, Shackleton, Thwaites and Moscow University Ice Shelves. Low producers include the Larsen C, Princess Astrid and Ragnhild coast, Fimbul, Brunt and Filchner. Correlation between melt water production and grounding line discharge is low (R2 = 0.65). Correlation with thermal ocean forcing from the ocean are highest in the northern parts of West Antarctica where regressions yield R2 of 0.93-0.97. Melt rates in the Amundsen Sea exhibit a quadratic sensitivity to thermal ocean forcing. We conclude that ice shelf melting plays a dominant role in ice shelf mass balance, with a potential to change rapidly in response to altered ocean heat transport onto the Antarctic continental shelf.

  14. Feedbacks between ice and ocean dynamics at the West Antarctic Filchner-Ronne Ice Shelf in future global warming scenarios

    NASA Astrophysics Data System (ADS)

    Goeller, Sebastian; Timmermann, Ralph

    2016-04-01

    The ice flow at the margins of the West Antarctic Ice Sheet is moderated by large ice shelves. Their buttressing effect substantially controls the mass balance of the WAIS and thus its contribution to sea level rise. The stability of these ice shelves results from the balance of mass gain by accumulation and ice flow from the adjacent ice sheet and mass loss by calving and basal melting due to the ocean heat flux. Recent results of ocean circulation models indicate that warm circumpolar water of the Southern Ocean may override the submarine slope front of the Antarctic Continent and boost basal ice shelf melting. In particular, ocean simulations for several of the IPCC's future climate scenarios demonstrate the redirection of a warm coastal current into the Filchner Trough and underneath the Filchner-Ronne Ice Shelf within the next decades. In this study, we couple the finite elements ocean circulation model FESOM and the three-dimensional thermomechanical ice flow model RIMBAY to investigate the complex interactions between ocean and ice dynamics at the Filchner-Ronne Ice Shelf. We focus on the impact of a changing ice shelf cavity on ocean dynamics as well as the feedback of the resulting sub-shelf melting rates on the ice shelf geometry and implications for the dynamics of the adjacent marine-based Westantarctic Ice Sheet. Our simulations reveal the high sensitivity of grounding line migration to ice-ocean interactions within the Filchner-Ronne Ice Shelf and emphasize the importance of coupled model studies for realistic assessments of the Antarctic mass balance in future global warming scenarios.

  15. Tidal Modulation of Ice-shelf Flow: a Viscous Model of the Ross Ice Shelf

    NASA Technical Reports Server (NTRS)

    Brunt, Kelly M.; MacAyeal, Douglas R.

    2014-01-01

    Three stations near the calving front of the Ross Ice Shelf, Antarctica, recorded GPS data through a full spring-neap tidal cycle in November 2005. The data revealed a diurnal horizontal motion that varied both along and transverse to the long-term average velocity direction, similar to tidal signals observed in other ice shelves and ice streams. Based on its periodicity, it was hypothesized that the signal represents a flow response of the Ross Ice Shelf to the diurnal tides of the Ross Sea. To assess the influence of the tide on the ice-shelf motion, two hypotheses were developed. The first addressed the direct response of the ice shelf to tidal forcing, such as forces due to sea-surface slopes or forces due to sub-ice-shelf currents. The second involved the indirect response of ice-shelf flow to the tidal signals observed in the ice streams that source the ice shelf. A finite-element model, based on viscous creep flow, was developed to test these hypotheses, but succeeded only in falsifying both hypotheses, i.e. showing that direct tidal effects produce too small a response, and indirect tidal effects produce a response that is not smooth in time. This nullification suggests that a combination of viscous and elastic deformation is required to explain the observations.

  16. Shelfal sediment transport by undercurrents forces turbidity current activity during high sea level, Chile continental margin

    NASA Astrophysics Data System (ADS)

    Bernhardt, Anne; Hebbeln, Dierk; Regenberg, Marcus; Lückge, Andreas; Strecker, Manfred. R.

    2016-04-01

    Understanding the links between terrigenous sediment supply and marine transport and depositional processes along tectonically active margins is essential to decipher turbidite successions as potential archives of climatic and seismic forcings and to comprehend timing and quantity of marine clastic deposition. Sequence stratigraphic models predict coarse-grained terrigenous sediment delivery to deep-marine sites mainly during sea-level fall and lowstand. Marine clastic deposition during periods of transgression and highstand has been attributed to the continued geomorphic connectivity between terrestrial sediment sources and marine sinks (e.g., rivers connected to submarine canyons) often facilitated by narrow shelves, high sediment supply causing delta migration to the shelf edge, and/or abrupt increases in sediment supply due to climatic variability or catastrophic events. To decipher the controls on Holocene highstand turbidite deposition, we analyzed twelve sediment cores of spatially disparate, coeval Holocene turbidite systems along the Chile margin (29-40°S) with changing climatic and geomorphic characteristics but uniform changes of sea level. Intraslope basins in north-central Chile (29-33°S) offshore a narrow to absent shelf record a shut-off of turbidite activity during the Holocene. In contrast, core sites in south-central Chile (36-40°S) offshore a wide continental shelf have repeatedly experienced turbidite deposition during sea-level highstand conditions, even though most of the depocenters are not connected via canyons to sediment sources. The interplay of stable high sediment supply related to strong onshore precipitation in combination with a wide shelf, over which undercurrents move sediment towards the shelf edge, appears to control Holocene turbidite sedimentation and sediment export to the deep sea.

  17. Glacier surge after ice shelf collapse.

    PubMed

    De Angelis, Hernán; Skvarca, Pedro

    2003-03-01

    The possibility that the West Antarctic Ice Sheet will collapse as a consequence of ice shelf disintegration has been debated for many years. This matter is of concern because such an event would imply a sudden increase in sea level. Evidence is presented here showing drastic dynamic perturbations on former tributary glaciers that fed sections of the Larsen Ice Shelf on the Antarctic Peninsula before its collapse in 1995. Satellite images and airborne surveys allowed unambiguous identification of active surging phases of Boydell, Sjögren, Edgeworth, Bombardier, and Drygalski glaciers. This discovery calls for a reconsideration of former hypotheses about the stabilizing role of ice shelves. PMID:12624263

  18. Larsen ice shelf has progressively thinned.

    PubMed

    Shepherd, Andrew; Wingham, Duncan; Payne, Tony; Skvarca, Pedro

    2003-10-31

    The retreat and collapse of Antarctic Peninsula ice shelves in tandem with a regional atmospheric warming has fueled speculation as to how these events may be related. Satellite radar altimeter measurements show that between 1992 and 2001 the Larsen Ice Shelf lowered by up to 0.27 +/- 0.11 meters per year. The lowering is explained by increased summer melt-water and the loss of basal ice through melting. Enhanced ocean-driven melting may provide a simple link between regional climate warming and the successive disintegration of sections of the Larsen Ice Shelf. PMID:14593176

  19. Diabetes in African Americans

    PubMed Central

    Marshall, M

    2005-01-01

    African Americans have a high risk for type 2 diabetes. Genetic traits, the prevalence of obesity, and insulin resistance all contribute to the risk of diabetes in the African American community. African Americans have a high rate of diabetic complications, because of poor glycaemic control and racial disparities in health care in the USA. African Americans with diabetes may have an atypical presentation that simulates type 1 diabetes, but then their subsequent clinical course is typical of type 2 diabetes. Culturally sensitive strategies, structured disease management protocols, and the assistance of nurses, diabetic educators, and other health care professionals are effective in improving the outcome of diabetes in the African American community. PMID:16344294

  20. Atlantic marginal basins of Africa

    SciTech Connect

    Moore, G.T.

    1988-02-01

    The over 10,000-km long Atlantic margin of Africa is divisible into thirty basins or segments of the margin that collectively contain over 18.6 x 10/sup 6/ km/sup 3/ of syn-breakup and post-breakup sediments. Twenty of these basins contain a sufficiently thick volume of sediments to be considered prospects. These basins lie, at least partially, within the 200 m isobath. The distribution of source rocks is broad enough to give potential to each of these basins. The sedimentation patterns, tectonics, and timing of events differ from basin to basin and are related directly to the margin's complex history. Two spreading modes exist: rift and transform. Rifting dates from Late Triassic-Early Jurassic in the northwest to Early Cretaceous south of the Niger Delta. A complex transform fault system separated these two margins. Deep-water communication between the two basins became established in the middle Cretaceous. This Mesozoic-Cenozoic cycle of rifting and seafloor spreading has segmented the margin and where observable, basins tend to be bounded by these segments.

  1. Seafloor morphology of the Montenegro/N. Albania Continental Margin (Adriatic Sea-Central Mediterranean)

    NASA Astrophysics Data System (ADS)

    Del Bianco, Fabrizio; Gasperini, Luca; Giglio, Federico; Bortoluzzi, Giovanni; Kljajic, Zoran; Ravaioli, Mariangela

    2014-12-01

    High-resolution multibeam morpho-bathymetric maps and a dense grid of seismic reflection profiles show relict and palimpsest geomorphologic features along the Montenegro/Northern Albanian Continental Margin. This sector of the Eastern Adriatic shelf, at the external front of the Dinarides Chain, is characterized by highly variable seafloor patterns and depositional styles, and shows a peculiar alternation of large-scale troughs and ridges, probably caused by tectonic compressive deformations. These tectonically controlled morphologies are overprinted by the result of sedimentary processes, such as progradation at river outflows, erosion, and reworking of sediments by longshore currents, as well as gravity-driven process caused by sediment loading and seismic shaking. Physiographic domains along this shelf-slope margin include (i) an inner and an outer shelf, separated by two major topographic highs, the Kotor and the Bar ridges; (ii) a drowned lobate delta formed during the last phase of sea level fall, likely fed by the Buna/Bojana drainage basin; and (iii) a continental slope affected by gravity-driven faulting and mass-wasting processes. Seafloor reflectivity maps, ground-truthed by grain-size analysis of bottom sediments, reveal that fine-grained deposits accumulate in the inner shelf, while other sectors appear starved. The effects of the last sea-level rise is testified by the presence of seabed forms diagnostic of erosion or depositional processes, such us large dunes, sediment ridges and sediment waves, which were studied to infer the effect of bottom currents under the present-day oceanographic regime and in the recent past. This paper presents a first description of geomorphologic features observed along the Montenegro/Northern Albanian Continental Margin, in the context of Late Quaternary sea-level changes.

  2. Ice-sheet retreat from the continental shelf offshore of Northwest Ireland following the last glacial maximum: sedimentary facies and initial chronology

    NASA Astrophysics Data System (ADS)

    Weilbach, Kasper; O'Cofaigh, Colm; Lloyd, Jerry; Benetti, Sara; Dunlop, Paul; Howe, John; Purcell, Catriona

    2015-04-01

    The glacial history of North-West Ireland and the adjoining continental shelf have been debated for over a century. The traditional reconstruction of a British-Irish Ice Sheet (BIIS) in this region was based predominantly on terrestrial evidence and showed an ice sheet that did not extend beyond the present coastline of Britain and Ireland. This traditional reconstruction of a relatively restricted ice sheet has been replaced in the last decade by the reconstruction of a more dynamic ice sheet that, during the Last Glacial Maximum (LGM), flowed onto the continental shelf and extended to the NW-Irish shelf edge. High resolution swath bathymetry and sub bottom profiler data along with sedimentological, micropalaeontological and geochronological investigations of sediment cores from the shelf offshore of NW Ireland are being used to reconstruct the timing, extent and the nature of retreat of the BIIS from the shelf following the LGM. A total of twenty seven vibro-cores were collected during two research cruises on the NW-Irish shelf in 2008 and 2014 on board the Irish and UK research vessels the Celtic Explorer and RRS James Cook The cores were collected in two east-west orientated transects across a series of arcuate recessional moraines from the shelf edge to Donegal Bay. These moraines record progressive stillstands of a lobate ice sheet margin during its retreat from the shelf edge, although to date, there has been a lack of direct dating control to constrain the timing and rate of ice retreat across the shelf. Sedimentary descriptions of core facies and physical properties, combined with taxonomic analysis of foraminifera will be presented along with radiocarbon dates. This forms the first detailed reconstruction of glacigenic sedimentation, depositional environments and the timing of ice sheet retreat across the shelf offshore of NW Ireland. The project is part of a larger EU funded research programme GLANAM ('Glaciated North Atlantic Margins') which is

  3. African bees to control African elephants

    NASA Astrophysics Data System (ADS)

    Vollrath, Fritz; Douglas-Hamilton, Iain

    2002-11-01

    Numbers of elephants have declined in Africa and Asia over the past 30 years while numbers of humans have increased, both substantially. Friction between these two keystone species is reaching levels which are worryingly high from an ecological as well as a political viewpoint. Ways and means must be found to keep the two apart, at least in areas sensitive to each species' survival. The aggressive African bee might be one such method. Here we demonstrate that African bees deter elephants from damaging the vegetation and trees which house their hives. We argue that bees can be employed profitably to protect not only selected trees, but also selected areas, from elephant damage.

  4. The Algerian Margin: an Example of a Reactivation in Compression of a Complex Cenozoic Passive Margin

    NASA Astrophysics Data System (ADS)

    Domzig, A.; Deverchere, J.; Yelles, K.; Govers, R.; Wortel, R.; Petit, C.; Cataneo, A.; Kherroubi, A.; Teams, M.

    2007-12-01

    The Western Mediterranean underwent a complex Cenozoic history involving subduction of the Tethys Ocean as well as subduction roll-back and associated opening of back-arc basins. During the Oligo-Miocene, the subduction roll-back to the south led to the collision of the Kabylies into the African plate, but subduction continued towards west, causing the Alboran slab to migrate towards the Gibraltar Arc. Northern Africa is at the southern border of this system and is therefore a major study area in the context of slow convergent plates to study the reactivation in compression of a Cenozoic passive margin but also the records of past geodynamic processes. This work aims to characterize the multi-scale structure of the offshore Algerian margin, based on the MARADJA'03 and MARADJA2/SAMRA'05 cruises data (multibeam bathymetry, seismic-reflection, side-scan sonar, backscattering, CHIRP, gravimetry). Tectonic (geomorphology, folds, faults) records reveal large recent and active structures as well as the geological inheritance of the margin. In western Algeria, slab roll-back is likely to have been accompanied by lithospheric tearing (STEP fault) as it has been modelled at a regional scale (Govers and Wortel, 2005): we provide first evidence for the presence of such structure(s) offshore Algeria. The geodynamical conditions have now changed, and we are facing new types of structures. Two main tectonic styles are identified: reverse to the centre and east; and strike-slip to the west. In Central Algeria, the compressional structures are active blind thrusts (Plio-Quaternary) verging to the north (opposite to pre-existing features) expressed as asymmetrical folds, sub-perpendicular to the convergence direction and often en echelon. These faults may all trigger M=6-7.5 earthquakes (e.g. Khair al Din fault near Algiers). Among them, the fault associated with the 2003 Boumerdes event (Mw=6.8) would continue to the surface by flats and ramps creating piggy-back basins or

  5. African hot spot volcanism: small-scale convection in the upper mantle beneath cratons.

    PubMed

    King, S D; Ritsema, J

    2000-11-10

    Numerical models demonstrate that small-scale convection develops in the upper mantle beneath the transition of thick cratonic lithosphere and thin oceanic lithosphere. These models explain the location and geochemical characteristics of intraplate volcanos on the African and South American plates. They also explain the presence of relatively high seismic shear wave velocities (cold downwellings) in the mantle transition zone beneath the western margin of African cratons and the eastern margin of South American cratons. Small-scale, edge-driven convection is an alternative to plumes for explaining intraplate African and South American hot spot volcanism, and small-scale convection is consistent with mantle downwellings beneath the African and South American lithosphere. PMID:11073447

  6. Continental Margins of the Arctic Ocean: Implications for Law of the Sea

    NASA Astrophysics Data System (ADS)

    Mosher, David

    2016-04-01

    A coastal State must define the outer edge of its continental margin in order to be entitled to extend the outer limits of its continental shelf beyond 200 M, according to article 76 of the UN Convention on the Law of the Sea. The article prescribes the methods with which to make this definition and includes such metrics as water depth, seafloor gradient and thickness of sediment. Note the distinction between the "outer edge of the continental margin", which is the extent of the margin after application of the formula of article 76, and the "outer limit of the continental shelf", which is the limit after constraint criteria of article 76 are applied. For a relatively small ocean basin, the Arctic Ocean reveals a plethora of continental margin types reflecting both its complex tectonic origins and its diverse sedimentation history. These factors play important roles in determining the extended continental shelves of Arctic coastal States. This study highlights the critical factors that might determine the outer edge of continental margins in the Arctic Ocean as prescribed by article 76. Norway is the only Arctic coastal State that has had recommendations rendered by the Commission on the Limits of the Continental Shelf (CLCS). Russia and Denmark (Greenland) have made submissions to the CLCS to support their extended continental shelves in the Arctic and are awaiting recommendations. Canada has yet to make its submission and the US has not yet ratified the Convention. The various criteria that each coastal State has utilized or potentially can utilize to determine the outer edge of the continental margin are considered. Important criteria in the Arctic include, 1) morphological continuity of undersea features, such as the various ridges and spurs, with the landmass, 2) the tectonic origins and geologic affinities with the adjacent land masses of the margins and various ridges, 3) sedimentary processes, particularly along continental slopes, and 4) thickness and

  7. Continental Margins and the Law of the Sea - an `Arranged Marriage' with Huge Research Potential

    NASA Astrophysics Data System (ADS)

    Parson, L.

    2005-12-01

    The United Nations Convention on the Law of the Sea (UNCLOS) requires coastal states intending to secure sovereignty over continental shelf territory extending beyond 200 nautical miles to submit geological/geophysical data, along with their analysis and synthesis of the relevant continental margin in support of their claim. These submissions are scrutinised and assessed by a UN Commission of experts who decide if the claim is justified, and thereby ultimately allowing the exploitation of non-living resources into this extended maritime space. The amount of data required to support the case will vary from margin to margin, depending on the local geological evolution, but typically will involve the running of new, dedicated marine surveys, mostly bathymetric and seismic. Key geological/geophysical issues revolve around proof of `naturalness' of the prolongation of land mass (cue - wide-angle seismics, deep drilling and sampling programmes) and shelf and slope morphology and sediment section thickness (cue - swath bathymetry and multichannel seismics programmes). These surveys, probably primarily funded by government agencies anxious not to lose out on the `land grab', will generate datasets which will inevitably boost not only the research effort leading to increased understanding of margin evolution in academic terms, but also contribute to wider applied aspects of the work such as those leading to refinement of deepwater hydrocarbon resource potential. It is conservatively estimated that in the region of fifty coastal states world-wide have a significant potential for claiming continental shelf beyond 200 nautical miles, and that the total area available as extended shelf could easily exceed 7 million square kilometres. However, while for the vast majority of these states a UNCLOS deadline of 2009 exists for submitting a claim - to date only four have done so (Russia, Brazil, Australia and Ireland). It is therefore predictable, if not inevitable, that within the

  8. Certifying nonlocality from separable marginals

    NASA Astrophysics Data System (ADS)

    Vértesi, Tamás; Laskowski, Wiesław; Pál, Károly F.

    2014-01-01

    Imagine three parties, Alice, Bob, and Charlie, who share a state of three qubits such that all two-party reduced states A-B, A-C, and B-C are separable. Suppose that they have information only about these marginals but not about the global state. According to recent results, there exists an example for a set of three separable two-party reduced states that is only compatible with an entangled global state. In this paper, we show a stronger result by exhibiting separable two-party reduced states A-B, A-C, and B-C, such that any global state compatible with these marginals is nonlocal. Hence, we obtain that nonlocality of multipartite states can be certified from information only about separable marginals.

  9. Pleistocene ice streaming and marine-margin breakup revealed by multibeam bathymetry data: The Minch, NW Scotland

    NASA Astrophysics Data System (ADS)

    Bradwell, Tom; Stoker, Martyn

    2013-04-01

    Extensive dynamically driven breakup and rapid ice loss is currently ongoing at tidewater margins of the Greenland and Antarctica Ice Sheets, yet few good analogues from the palaeo-record exist. Using ca. 55,000 km2 of echosounder bathymetry data from the continental shelf around NW Scotland we have mapped submarine glacial landforms relating to an ice sheet that covered much of the continental shelf during the Late Pleistocene and had extensive tidewater margins. Focusing on new multibeam bathymetry from the inner part of the shelf (The Minch), we present seabed geomorphological evidence showing breakup of a large marine portion of a palaeo-ice stream within the British-Irish Ice Sheet. Clearly defined, well preserved glacial lineations, elongate bedforms and seabed drumlins indicate former fast flow of a grounded palaeo-ice stream in a northerly direction in The Minch. In addition, the absence of moraines and grounding-line features deposited during ice sheet retreat and the abundance of large overprinted iceberg scours collectively indicate rapid marine-margin breakup by flotation and thinning. We suggest that this marine-margin breakup event was probably driven by unstable ice sheet retreat into shoreward deepening water and was inextricably linked with the abrupt demise of The Minch palaeo-ice stream. Importantly, this new evidence indicates that potentially large areas of the ice sheet margin were floating at times during British-Irish Ice Sheet retreat on the continental shelf. Ongoing work is seeking to date the timing of ice sheet breakup and ice stream demise in northern Scotland.

  10. Late Quaternary sedimentation on the North Aegean continental margin, Greece

    SciTech Connect

    Piper, D.J.W. ); Perissoratis, C. )

    1991-01-01

    The late Quaternary seismic stratigraphy of the North Aegean continental shelf and adjacent basins has been interpreted from boomer and 3.5-kHz seismic profiles. Ages derived from shallow cores and offshore wells, and relative offsets on small synsedimentary faults, provide chronological control. Sea level history inferred from seismic stratigraphy correlates with the global eustatic sea level record based on oxygen isotopic curves. The present depth of the delta plain formed on the outer shelf during the late stage 6 lowstand provides a dated and originally horizontal marker for estimating rates of tectonic subsidence. Gross distribution of sediment facies is similar in both tectonically stable and active areas. The shell break formed by delta progradation, but is marked by faults in most places because of the accommodation provided by graben subsidence rates of 0.3-1.5 mm/yr. Standard sequence stratigraphic analysis can be applied to these sediments deposited during high-amplitude Quaternary sea level oscillations. High rates of subsidence result in the preservation an unusually complete record of sea level change. Major lowstand progradation is dependent on the duration, rather than the magnitude, of sea level lowstand. The long glaciations in isotopic stages 6, 12, 16, and 22 resulted in the most prominent seaward progradation on the margin. Sandy lowstand turbidite deposits formed only when there was rapid fall in sea level; otherwise sand was trapped on delta tops and silty muds were deposited in deep water.

  11. Sequence stratigraphy simulations of carbonate, clastics, and mixed basin margins

    SciTech Connect

    Kendall, C.G.St.C.; Moore, P.; Birdwell, B.A.; Rouchie, L.; Cannon, R. ); Biswas, G. ); Bezdek, J. )

    1991-03-01

    Clastics, carbonates, and their mixtures have different depositional and post-depositional behavior that produces the different margin characteristics seen in seismic sequences. Carbonates undergo early cementation while maintaining higher angles of repose, while clays and sands accumulate at lower-angle slopes whose inclination is proportional to the grain size and post-depositional cohesive behavior. In higher energy regimes, waves or currents winnow less cohesive finer material that is transported downdip to from slope sediments rimming the basin. Simulations of mixed carbonate-clastic sediment accumulation, tectonism, and eustasy for settings in the Permian basin of west Texas and New Mexico show that sharp differentiation of clastics from carbonates is a product of higher angles of repose that carbonates maintain and the higher rates of clastic input at lowstands in sea level. In contrast, simulation of mixed grain-size margins like the Exmouth Plateau of Western Australia, the Baltimore Canyon, and the Gulf Coast Tertiary indicate that muds are winnowed preferentially from shelf-margin crests but accumulate on slopes, while sands accumulate on higher energy shelves. When they bypass at lowstands in sea level, they accumulate in the near slope basin but not on the slope. Simulation of pure carbonate systems like that of the Bahamian platform suggests that progradation is greatest in areas of low wave and current energy while backstepping and cliffed margins occur in high energy settings. The ability to accurately simulate mixed carbonate-clastic slopes is a key to development of exploration and production models of these systems.

  12. Neotectonism along the Atlantic passive continental margin: A review

    NASA Astrophysics Data System (ADS)

    Gardner, Thomas W.

    1989-09-01

    An extensive body of geologic data including the modern state of stress, historical seismicity, surface and subsurface stratigraphy, numerical models of crustal deformation, surficial geomorphic systems, and historical precise leveling and tidal gauge records constrain the style and rate of neotectonic deformation for the Appalachians and Atlantic passive continental margin. There are two major styles of neotectonism in the eastern United States. The northeastern United States is dominated by isostatic uplift and northward migration of peripheral bulge collapse in response to deglaciation. This locally rapid, but decreasing rate of deformation is superimposed upon slower, long-term deformation along the Atlantic margin. Most of the long-term, continental margin deformation is attributed to lithospheric flexuring in response to sediment loading in sedimentary basins (especially the Baltimore Canyon Trough and Carolina Trough), isostatic deformation in response to continental denudation and water loading of the shelf, and stress from far-field plate tectonic sources. Significant deformational features include an uplift anomaly near Cape Fear, N.C.; northward and southward tilting of the Coastal Plain into the Salisbury and Southeast Georgia Embayments respectively; seaward tilting of the Coastal Plain/Piedmont, and a complex pattern of postglacial uplift and later subsidence in the northeast. Estimates of vertical crustal velocities for similar locations vary over several orders of magnitude. Measurement interval bias and systematic leveling errors may account for some of the discrepancies. Evidence for periodic deformation in the eastern United States in substantial and it is possible that historic data indicate a period of accelerated deformation along the Atlantic continental margin.

  13. Post-breakup Basin Evolution along the South-Atlantic Margins

    NASA Astrophysics Data System (ADS)

    Strozyk, Frank; Back, Stefan; Kukla, Peter

    2014-05-01

    The post-breakup tectono-stratigraphic evolution of large offshore basins along the South American and African continental margins record strongly varying post-rift sedimentary successions. The northernmost segment of the South Atlantic rift and salt basins is characterized by a pronounced asymmetry, with the Brazilian margin comprising narrower and deeper rift basins with less salt in comparison to the Congo-Gabon conjugate margin. Another important observation is that multiple phases of uplift and subsidence are recorded after the break-up of the southern South Atlantic on both sides of the Florianopolis-Walvis Ridge volcanic complex, features that are regarded as atypical when compared to published examples of other post-breakup margin successions. A regional comparison based on tectonic-stratigraphic analysis of selected seismic transects between the large basins offshore southern Brazil (Espirito Santo Basin, Campos Basin, Santos Basin, Pelotas Basin) and southwest Africa (Lower Congo Basin, Kwanza Basin, Namibe Basin, Walvis Basin) provides a comprehensive basin-to-basin documentation of the key geological parameters controlling ocean and continental margin development. This comparison includes the margin configuration, subsidence development through time, sediment influx and storage patterns, type of basin fill (e.g. salt vs. non-salt systems; carbonate-rich vs. clastics-dominated systems) and finally major tectonic and magmatic events. Data from the salt basins indicate that salt-related tectonic deformation is amongst the prime controls for the non-uniform post-rift margin development. The diversity in the stratigraphic architecture of the conjugate margins offshore southern Brazil, Namibia and Angola reflects variations in the interplay of a number of controlling factors, of which the most important are (a) the structural configuration of each margin segment at the time of break-up, (b) the post break-up subsidence history of the respective margin segment

  14. 41 CFR 101-27.205 - Shelf-life codes.

    Code of Federal Regulations, 2014 CFR

    2014-07-01

    ... of a one-digit code to provide for uniform coding of shelf-life materials by all agencies. (a) The... essential items, and medical items with a shelf life greater than 60 months. Agencies shall...

  15. 41 CFR 101-27.205 - Shelf-life codes.

    Code of Federal Regulations, 2013 CFR

    2013-07-01

    ... of a one-digit code to provide for uniform coding of shelf-life materials by all agencies. (a) The... essential items, and medical items with a shelf life greater than 60 months. Agencies shall...

  16. 41 CFR 101-27.205 - Shelf-life codes.

    Code of Federal Regulations, 2011 CFR

    2011-07-01

    ... of a one-digit code to provide for uniform coding of shelf-life materials by all agencies. (a) The... essential items, and medical items with a shelf life greater than 60 months. Agencies shall...

  17. 41 CFR 101-27.205 - Shelf-life codes.

    Code of Federal Regulations, 2012 CFR

    2012-07-01

    ... of a one-digit code to provide for uniform coding of shelf-life materials by all agencies. (a) The... essential items, and medical items with a shelf life greater than 60 months. Agencies shall...

  18. Sedimentology of the NW margin of the Arabian plate and the SW NE trending Nahr El-Kabir half-graben in northern Syria during the latest Cretaceous and Cenozoic

    NASA Astrophysics Data System (ADS)

    Hardenberg, Mathew F.; Robertson, Alastair H. F.

    2007-10-01

    The Late Cretaceous (Maastrichtian) to Recent sedimentary record of NW Syria documents the palaeoenvironments and regional tectonic setting during a time when the Neotethys ocean to the north was closing, culminating in regional collision by mid-Miocene time. This detailed sedimentological study builds on a biostratigraphical framework established by Russian workers. During the mid-Maastrichtian the Baer-Bassit ophiolite and related melange were emplaced southwards from Neotethys onto the northern margin of the Arabian (African) plate, resulting in flexurally controlled deepening to the south. After short-lived emergence and erosion of emplaced units a renewed marine transgression ushered in shelf-depth, open-marine carbonate deposition during the Palaeocene-early Eocene. This deposition was characterized by generally westward-channelised mass flow from shallow-marine carbonate banks to the east, probably influenced by storm conditions. Flexurally controlled uplift and erosion took place during late Eocene-Oligocene, related to final closure of Neotethys. Shelf-depth carbonate deposition resumed during the Aquitanian (early Miocene), influenced by tectonic subsidence. During the Burdigalian, a SW-NE trending, narrow (up to 15 km wide), linear (˜ 60 km long) basin developed in the vicinity of the present-day Nahr-El Kabir river valley. This basin is attributed to extension or transtension along the Nahr El-Kabir Fault. This important lineament runs onshore from the southern limit of the Cyprus arc (Tartous Ridge), through NW Syria. It is then offset by the Dead Sea Transform in the Ghab Valley, before merging with the South Anatolian Transform Fault in Turkey further northeast. The basin in NW Syria developed into the asymmetrical Nahr El-Kabir half-graben during the mid-Miocene with uplift of the northern margin and rapid fault-controlled subsidence of the basin axis. Large volumes of terrigenous clastic and shallow-marine bioclastic material were redeposited from

  19. 17 CFR 41.45 - Required margin.

    Code of Federal Regulations, 2011 CFR

    2011-04-01

    ... 17 Commodity and Securities Exchanges 1 2011-04-01 2011-04-01 false Required margin. 41.45 Section... PRODUCTS Customer Accounts and Margin Requirements § 41.45 Required margin. (a) Applicability. Each security futures intermediary shall determine the required margin for the security futures and...

  20. 17 CFR 41.45 - Required margin.

    Code of Federal Regulations, 2010 CFR

    2010-04-01

    ... 17 Commodity and Securities Exchanges 1 2010-04-01 2010-04-01 false Required margin. 41.45 Section... PRODUCTS Customer Accounts and Margin Requirements § 41.45 Required margin. (a) Applicability. Each security futures intermediary shall determine the required margin for the security futures and...

  1. 17 CFR 242.403 - Required margin.

    Code of Federal Regulations, 2011 CFR

    2011-04-01

    ... 17 Commodity and Securities Exchanges 3 2011-04-01 2011-04-01 false Required margin. 242.403...) REGULATIONS M, SHO, ATS, AC, AND NMS AND CUSTOMER MARGIN REQUIREMENTS FOR SECURITY FUTURES Customer Margin Requirements for Security Futures § 242.403 Required margin. (a) Applicability. Each security...

  2. 12 CFR 220.4 - Margin account.

    Code of Federal Regulations, 2011 CFR

    2011-01-01

    ... 12 Banks and Banking 3 2011-01-01 2011-01-01 false Margin account. 220.4 Section 220.4 Banks and... BROKERS AND DEALERS (REGULATION T) § 220.4 Margin account. (a) Margin transactions. (1) All transactions not specifically authorized for inclusion in another account shall be recorded in the margin...

  3. Amundsen Sea sector ice shelf thickness, melt rates, and inland response from annual high-resolution DEM mosaics

    NASA Astrophysics Data System (ADS)

    Shean, D. E.; Joughin, I. R.; Smith, B. E.; Alexandrov, O.; Moratto, Z.; Porter, C. C.; Morin, P. J.

    2014-12-01

    Significant grounding line retreat, acceleration, and thinning have occurred along the Amundsen Sea sector of West Antarctica in recent decades. These changes are driven primarily by ice-ocean interaction beneath ice shelves, but existing observations of the spatial distribution, timing, and magnitude of ice shelf melt are limited. Using the NASA Ames Stereo Pipeline, we generated digital elevation models (DEMs) with ~2 m posting from all ~450 available WorldView-1/2 along-track stereopairs for the Amundsen Sea sector. A novel iterative closest point algorithm was used to coregister DEMs to filtered Operation IceBridge ATM/LVIS data and ICESat-1 GLAS data, offering optimal sub-meter horizontal/vertical accuracy. The corrected DEMs were used to produce annual mosaics for the entire ~500x700 km region with focused, sub-annual products for ice shelves and grounding zones. These mosaics provide spatially-continuous measurements of ice shelf topography with unprecedented detail. Using these data, we derive estimates of ice shelf thickness for regions in hydrostatic equilibrium and map networks of sub-shelf melt channels for the Pine Island (PIG), Thwaites, Crosson, and Dotson ice shelves. We also document the break-up of the Thwaites ice shelf and PIG rift evolution leading up to the 2013 calving event. Eulerian difference maps document 2010-2014 thinning over fast-flowing ice streams and adjacent grounded ice. These data reveal the greatest thinning rates over the Smith Glacier ice plain and slopes beyond the margins of the fast-flowing PIG trunk. Difference maps also highlight the filling of at least two subglacial lakes ~30 km upstream of the PIG grounding line in 2011. Lagrangian difference maps reveal the spatial distribution of ice shelf thinning, which can primarily be attributed to basal melt. Preliminary results show focused ice shelf thinning within troughs and large basal channels, especially along the western margin of the Dotson ice shelf. These new data

  4. Seabed corrugations beneath an Antarctic ice shelf revealed by autonomous underwater vehicle survey: Origin and implications for the history of Pine Island Glacier

    NASA Astrophysics Data System (ADS)

    Graham, Alastair G. C.; Dutrieux, Pierre; Vaughan, David G.; Nitsche, Frank O.; Gyllencreutz, Richard; Greenwood, Sarah L.; Larter, Robert D.; Jenkins, Adrian

    2013-09-01

    Ice shelves are critical features in the debate about West Antarctic ice sheet change and sea level rise, both because they limit ice discharge and because they are sensitive to change in the surrounding ocean. The Pine Island Glacier ice shelf has been thinning rapidly since at least the early 1990s, which has caused its trunk to accelerate and retreat. Although the ice shelf front has remained stable for the past six decades, past periods of ice shelf collapse have been inferred from relict seabed "corrugations" (corrugated ridges), preserved 340 km from the glacier in Pine Island Trough. Here we present high-resolution bathymetry gathered by an autonomous underwater vehicle operating beneath an Antarctic ice shelf, which provides evidence of long-term change in Pine Island Glacier. Corrugations and ploughmarks on a sub-ice shelf ridge that was a former grounding line closely resemble those observed offshore, interpreted previously as the result of iceberg grounding. The same interpretation here would indicate a significantly reduced ice shelf extent within the last 11 kyr, implying Holocene glacier retreat beyond present limits, or a past tidewater glacier regime different from today. The alternative, that corrugations were not formed in open water, would question ice shelf collapse events interpreted from the geological record, revealing detail of another bed-shaping process occurring at glacier margins. We assess hypotheses for corrugation formation and suggest periodic grounding of ice shelf keels during glacier unpinning as a viable origin. This interpretation requires neither loss of the ice shelf nor glacier retreat and is consistent with a "stable" grounding-line configuration throughout the Holocene.

  5. New insights into slide processes and seafloor geology revealed by side-scan imagery of the massive Hinlopen Slide, Arctic Ocean margin

    NASA Astrophysics Data System (ADS)

    Hogan, Kelly A.; Dowdeswell, Julian A.; Mienert, Jürgen

    2013-08-01

    The submarine Hinlopen Slide, located along the Arctic Ocean margin, is one of the largest known mass movements on Earth. The slide scar has several unusual morphometric characteristics, including headwalls up to 1,500 m high and spectacularly large, steep-sided rafted megablocks. The slide processes and continental margin properties that produced these features are not well known. A new high-resolution TOBI (towed ocean bottom instrument) side-scan sonar dataset reveals information about the detailed seafloor morphology and, therefore, slide dynamics during the final stages of sliding. First, the headwall area was efficiently and almost completely evacuated of slide debris, which is unusual for large submarine slides. Second, features relating to the propagation of extension to the shelf behind the headwall are absent, suggesting "strong" cohesive shelf material here or that a very stable shelf configuration was reached, possibly defined by NE-SW-trending faults. Third, there is little evidence for the translation of shelf material, again uncommon for submarine slides. Taken together with the occurrence of massive megablocks in the slide debris, Hinlopen Slide is distinct because of the juxtaposition of apparently "stronger" shelf material that has remained intact (headwalls, megablocks), and "weaker" shelf material that disaggregated fully during slope failure. Nevertheless, there is sonograph evidence of variable post-slide disintegration of the megablocks. Contrary to previous interpretations, this suggests that the blocks comprise sedimentary lithologies that are prone to failure, a key aspect awaiting confirmation.

  6. Circulation and melting beneath the ross ice shelf.

    PubMed

    Jacobs, S S; Gordon, A L; Ardai, J L

    1979-02-01

    Thermohaline observations in the water column beneath the Ross Ice Shelf and along its terminal face show significant vertical stratification, active horizontal circulation, and net melting at the ice shelf base. Heat is supplied by seawater that moves southward beneath the ice shelf from a central warm core and from a western region of high salinity. The near-freezing Ice Shelf Water produced flows northward into the Ross Sea. PMID:17734137

  7. Climate science: A great Arctic ice shelf

    NASA Astrophysics Data System (ADS)

    Domack, Eugene

    2016-02-01

    Newly mapped features on the floor of the Arctic Ocean suggest that the Arctic basin was once covered by a one-kilometre-thick, flowing ice shelf derived from large ice sheets in eastern Siberia, Arctic Canada and the Barents Sea.

  8. Carbon isotopic composition of Amazon shelf sediments

    SciTech Connect

    Showers, W.J.; Angle, D.G.; Nittrouer, C.A.; Demaster, D.J.

    1985-02-01

    The distribution of carbon isotopes in Amazon shelf sediment is controlled by the same processes that are forming the modern subaqueous delta. The terrestrial (-27 to -25 per thousand) isotopic carbon signal observed in surficial sediments near the river mouth extends over 400 km northwest along the shelf. Terrestrial carbon is associated with areas of rapid sediment accumulation (topset and foreset regions). A sharp boundary between terrestrial (-27 to -25 per thousand) and marine (-23 to -22 per thousand) isotopic carbon values in surficial sediments is associated with a change in depositional conditions (foreset to bottomset regions) and a decrease in sediment accumulation rate. POC water-column isotopic values (-27 per thousand) near the river mouth are similar to the underlying surficial-sediment TOC isotopic values, but POC water-column samples collected 20 km off the river mouth have marine carbon isotopic values (-22 to -19 per thousand) and differ from the underlying surficial-sediment TOC isotopic values. These water column observations are related to variations in turbidity and productivity. Down-core isotopic variation is only observed in cores taken in areas of lower sediment accumulation rates. These observations indicate that the organic carbon in Amazon shelf sediment is dominantly terrestrial in composition, and the location of deposition of this carbon is controlled by modern processes of sediment accumulation. The modern Amazon shelf is similar to large clinoform shale deposits of the Cretaceous in North America. Thus, the stratigraphic setting may help predict the isotopic variations of carbon in ancient deposits.

  9. Coordination: Southeast Continental Shelf studies. Progress report

    SciTech Connect

    Menzel, D.W.

    1981-02-01

    An overview of the Oceanograhic Program of Skidaway Institute of Oceanograhy is presented. Included are the current five year plan for studies of the Southeast Continental Shelf, a summary of research accomplishments, proposed research for 1981-1982, current status of the Savannah Navigational Light Tower, and a list of publications. (ACR)

  10. Elephant teeth from the atlantic continental shelf

    USGS Publications Warehouse

    Whitmore, F.C., Jr.; Emery, K.O.; Cooke, H.B.S.; Swift, D.J.P.

    1967-01-01

    Teeth of mastodons and mastodons have been recovered by fishermen from at least 40 sites on the continental shelf as deep as 120 meters. Also present are submerged shorelines, peat deposits, lagoonal shells, and relict sands. Evidently elephants and other large mammals ranged this region during the glacial stage of low sea level of the last 25.000 years.

  11. Ocean circulation on the North Australian Shelf

    NASA Astrophysics Data System (ADS)

    Schiller, Andreas

    2011-07-01

    The ocean circulation on Australia's Northern Shelf is dominated by the Monsoon and influenced by large-scale interannual variability. These driving forces exert an ocean circulation that influences the deep Timor Sea Passage of the Indonesian Throughflow, the circulation on the Timor and Arafura Shelves and, further downstream, the Leeuwin Current. Seasonal maxima of northeastward (southwestward) volume transports on the shelf are almost symmetric and exceed 10 6 m 3/s in February (June). The associated seasonal cycle of vertical upwelling from June to August south of 8.5°S and between 124°E and 137.5°E exceeds 1.5×10 6 m 3/s across 40 m depth. During El Niño events, combined anomalies from the seasonal means of high regional wind stresses and low inter-ocean pressure gradients double the northeastward volume transport on the North Australian Shelf to 1.5×10 6 m 3/s which accounts for 20% of the total depth-integrated transport across 124°E and reduce the total transport of the Indonesian Throughflow. Variability of heat content on the shelf is largely determined by Pacific and Indian Ocean equatorial wind stress anomalies with some contribution from local wind stress forcing.

  12. Submarine slumps, slides, and flows dominate sculpting of Beringian Margin, Alaska

    SciTech Connect

    Carlson, P.R.; Karl, H.A.; Edwards, B.D.; Gardner, J.V.; Hall, R. )

    1990-06-01

    The 1,400 km long Beringian margin is characterized by several very large submarine canyons and by a large oceanic plateau at the southern end. GLORIA sidescan-sonar imagery provides a perspective of this margin that is unattainable with conventional acoustic profiles. The broad coverage of GLORIA images emphasizes that, of all the sedimentary processes affecting this vast margin, mass movement is clearly the dominant shaping process. Styles of failure include mud and debris flows, slumps, and massive block slides, some covering areas greater than 1,500 km{sup 2}. GLORIA imagery and seismic-reflection profiles show evidence for a wide variety of slides and slumps in the canyons of the northern margin, Navarin and Pervenets. The 100 km long shelf edge between these two canyons is characterized by a series of scalloped slide scars and incipient scars associated with blocks of sedimentary material, 1 to 2 km across. One of the largest single slide masses is a huge block tens of kilometers wide that occurs on the rise in the central part of the margin beyond the mouth of Zhemchug Canyon. Sliding of this block may have initiated the incision of the world's largest submarine canyon. The removal of this block accelerated headward erosion by retrograde failure until Zhemchug Canyon was cut back to a fault parallel to the shelf edge. Mass movement along the southern margin is widespread at the edges of Umnak Plateau. One mass failure, well-defined by GLORIA, is about 30 km wide and 55 km long. This and other slides along the plateau are associated with diapiric-like structures, suggesting relatively recent tectonism.

  13. Structure and hydrocarbon potential of sedimentary basins of the far east marginal seas

    SciTech Connect

    Gnibidenko, H. ); Kononov, V. )

    1990-05-01

    Crustal structural of the Bering, Okhotsk Japan, East China, and South China marginal seas consists of continental plates and deep sea basins that are the elements of the lowermost order in the structure of transition zone from the Asia continent to the Pacific Ocean. Two stages are recognized in the crustal evolution of the northwest Pacific transition zone: (1) geosyncline development that began in the pre-Paleozoic and continues to the present within island arcs; and (2) quasiplatform stage that began in the late Cretaceous and continues to the Holocene within shelf plates. The continental margins of the Far East seas consist of Cenozoic terrigenous cover and pre-Cenozoic basement composed of geosyncline rock association. Normal faults control graben features in the basement and develop rift systems. Paleogene subcontinental formations make up the lowermost section of the cover. A major Oligocene-Holocene sequence (marine formations) overlies and smooths rough topography of the basement and creates giant sedimentary basins. Sediment thickness of the basins attains 10 km. Tectonic evolution of the marginal seas implies the shelf plates to be young platforms and deep-sea basins are believed to be parts of the Pacific thalassocraton fenced by island arcs. The tectonic criterion enables us to differ and grade the provinces according to a hydrocarbon potential. Nearly 100 promising sedimentary basins are presently known in the Bering, Okhotst Japan, East China, and South China seas. About ten basins have been identified as hydrocarbon resources. Deep-sea basins also look promising for hydrocarbons. All the economically significant hydrocarbon accumulations in the Far East marginal seas are attributed to the Cenozoic sediment cover. Major resources are concentrated in the Miocene and Pliocene terrigenous sequences composed of progradation facies within the shelf plates.

  14. 49 CFR 195.9 - Outer continental shelf pipelines.

    Code of Federal Regulations, 2011 CFR

    2011-10-01

    ... 49 Transportation 3 2011-10-01 2011-10-01 false Outer continental shelf pipelines. 195.9 Section... HAZARDOUS LIQUIDS BY PIPELINE General § 195.9 Outer continental shelf pipelines. Operators of transportation pipelines on the Outer Continental Shelf must identify on all their respective pipelines the specific...

  15. 75 FR 1076 - Outer Continental Shelf Civil Penalties

    Federal Register 2010, 2011, 2012, 2013, 2014

    2010-01-08

    ... Minerals Management Service Outer Continental Shelf Civil Penalties AGENCY: Minerals Management Service.... SUMMARY: The Outer Continental Shelf Lands Act requires the MMS to review the maximum daily civil penalty assessment for violations of regulations governing oil and gas operations in the Outer Continental Shelf...

  16. 49 CFR 195.9 - Outer continental shelf pipelines.

    Code of Federal Regulations, 2014 CFR

    2014-10-01

    ... 49 Transportation 3 2014-10-01 2014-10-01 false Outer continental shelf pipelines. 195.9 Section... HAZARDOUS LIQUIDS BY PIPELINE General § 195.9 Outer continental shelf pipelines. Operators of transportation pipelines on the Outer Continental Shelf must identify on all their respective pipelines the specific...

  17. 49 CFR 195.9 - Outer continental shelf pipelines.

    Code of Federal Regulations, 2012 CFR

    2012-10-01

    ... 49 Transportation 3 2012-10-01 2012-10-01 false Outer continental shelf pipelines. 195.9 Section... HAZARDOUS LIQUIDS BY PIPELINE General § 195.9 Outer continental shelf pipelines. Operators of transportation pipelines on the Outer Continental Shelf must identify on all their respective pipelines the specific...

  18. 49 CFR 195.9 - Outer continental shelf pipelines.

    Code of Federal Regulations, 2013 CFR

    2013-10-01

    ... 49 Transportation 3 2013-10-01 2013-10-01 false Outer continental shelf pipelines. 195.9 Section... HAZARDOUS LIQUIDS BY PIPELINE General § 195.9 Outer continental shelf pipelines. Operators of transportation pipelines on the Outer Continental Shelf must identify on all their respective pipelines the specific...

  19. 49 CFR 195.9 - Outer continental shelf pipelines.

    Code of Federal Regulations, 2010 CFR

    2010-10-01

    ... 49 Transportation 3 2010-10-01 2010-10-01 false Outer continental shelf pipelines. 195.9 Section... HAZARDOUS LIQUIDS BY PIPELINE General § 195.9 Outer continental shelf pipelines. Operators of transportation pipelines on the Outer Continental Shelf must identify on all their respective pipelines the specific...

  20. Cretaceous shelf-sea chalk deposits

    SciTech Connect

    Hattin, D.E.

    1988-01-01

    The word ''chalk'' is linked etymologically to the Cretaceous, but chalky facies neither dominate that system nor are confined to it. As used commonly, the term ''chalk'' refers to a variety of marine limestone that is white to light gray very fine grained, soft and friable, porous, and composed predominantly of calcitic skeletal remains, especially those derived from coccolithophores. No simple definition suffices to embrace all Cretaceous chalks, which include sandy, marly, shelly, phospatic, glauconitic, dolomitic, pyritic and organic-rich lithotypes. Most of the world's exposed Cretaceous chalk deposits were formed at shelf depths rather than in the deep sea. Cretaceous shelf-sea chalks are developed most extensively in northern Europe, the U.S. Gulf Coastal Plain and Western Interior, and the Middle East, with lesser occurrences alo in Australia. Most Cretaceous shelf-sea chalks formed in the temperature zones, and in relatively deep water. Cretaceous chalks deposited on well-oxygenated sea floors are bioturbated and massive where deficient in terrigenous detritus, or bioturbated and rhythmically interbedded with argillaceous units where influx of terrigenous detritus varied systematically with climate changes. Accumulation of sufficient pelagic mud to form vast deposits of Cretaceous shelf-sea chalk required (1) sustained high productivity of calareous plankton, (2) extensive development of stable shelf and continental platform environments, (3) highstands of seal level, (4) deficiency of aragonitic skeletal material in chalk-forming sediments, and (5) low rates of terrigenous detrital influx. These conditions were met at different times in different places, even within the same general region.

  1. Widespread methane leakage from the sea floor on the northern US Atlantic margin

    NASA Astrophysics Data System (ADS)

    Skarke, A.; Ruppel, C.; Kodis, M.; Brothers, D.; Lobecker, E.

    2014-09-01

    Methane emissions from the sea floor affect methane inputs into the atmosphere, ocean acidification and de-oxygenation, the distribution of chemosynthetic communities and energy resources. Global methane flux from seabed cold seeps has only been estimated for continental shelves, at 8 to 65 Tg CH4 yr-1, yet other parts of marine continental margins are also emitting methane. The US Atlantic margin has not been considered an area of widespread seepage, with only three methane seeps recognized seaward of the shelf break. However, massive upper-slope seepage related to gas hydrate degradation has been predicted for the southern part of this margin, even though this process has previously only been recognized in the Arctic. Here we use multibeam water-column backscatter data that cover 94,000 km2 of sea floor to identify about 570 gas plumes at water depths between 50 and 1,700 m between Cape Hatteras and Georges Bank on the northern US Atlantic passive margin. About 440 seeps originate at water depths that bracket the updip limit for methane hydrate stability. Contemporary upper-slope seepage there may be triggered by ongoing warming of intermediate waters, but authigenic carbonates observed imply that emissions have continued for more than 1,000 years at some seeps. Extrapolating the upper-slope seep density on this margin to the global passive margin system, we suggest that tens of thousands of seeps could be discoverable.

  2. Black versus Black: The Relationship among African, African American, and African Caribbean Persons.

    ERIC Educational Resources Information Center

    Jackson, Jennifer V.; Cothran, Mary E.

    2003-01-01

    Surveyed people of African descent regarding relationships among African, African-American, and African-Caribbean persons, focusing on contact and friendship, travel to countries of the diaspora, cross-cultural communication, thoughts and stereotypes, and education. Most respondents had contacts with the other groups, but groups had preconceived…

  3. Cross-shelf transport of freshwater in the New Jersey shelf

    NASA Astrophysics Data System (ADS)

    Castelao, R. M.; Glenn, S.; Schofield, O.; Chant, R.; Kohut, J.

    2007-05-01

    The New Jersey Shelf Observing System, which includes real-time access to the international constellation of ocean satellites, a nested HF CODAR radar network, cabled moorings and a fleet of Webb Sloccum gliders, is used to investigate the freshwater content over the New Jersey shelf. Repeated hydrographic surveys about 100 km south of the Hudson River mouth were conducted using gliders during spring and summer 2006. Observations reveal a strong seasonal cycle in the surface salinity. Buoyant water is restricted to being close to the coast during spring, but spans the entire shelf width during summer. During late July and August, freshwater lenses with large density anomaly are found up to 100 km from the coast. The cross-shelf transport of the freshwater is inconsistent with a model based on Ekman dynamics [Lentz, 2004], suggesting that other processes are responsible for the rapid cross-shelf transport. Surface velocity maps derived from HF radar, satellite imagery and drifter trajectories revealed the existence of a jet directed offshore and to the south, from near the river mouth toward the study region. This provides a direct pathway for transporting freshwater and any biogeochemical material it contains (including phytoplankton, dissolved organic and non-algal particulate matter) across the shelf. The highest frequency of observation of the freshwater lenses offshore occurs when the jet transport is large, and the river discharge is relatively high. The transport in the jet is correlated with upwelling winds on scales of a few days.

  4. "The Heart of Alien Conquest": Constructing Modern-Global Identity and Marginality through Language Arts in Malawian Classrooms.

    ERIC Educational Resources Information Center

    Sharra, Steve L.

    The metanarrative of modernity as constructed by language arts teaching in Malawian classrooms constructs an identity among Malawians that denies the multiplicity of influences, including the participation of Malawian and other African societies, in the narrative of the creation of school knowledge. It also perpetuates the marginality of…

  5. Numerical simulations of groundwater flow at New Jersey Shallow Shelf

    NASA Astrophysics Data System (ADS)

    Fehr, Annick; Patterson, Fabian; Lofi, Johanna; Reiche, Sönke

    2016-04-01

    During IODP Expedition 313, three boreholes were drilled in the so-called New Jersey transect. Hydrochemical studies revealed the groundwater situation as more complex than expected, characterized by several sharp boundaries between fresh and saline groundwater. Two conflicting hypotheses regarding the nature of these freshwater reservoirs are currently debated. One hypothesis is that these reservoirs are connected with onshore aquifers and continuously recharged by seaward-flowing groundwater. The second hypothesis is that fresh groundwater was emplaced during the last glacial period. In addition to the petrophysical properties measured during IODP 313 expedition, Nuclear Magnetic Resonance (NMR) measurements were performed on samples from boreholes M0027, M0028 and M0029 in order to deduce porosities and permeabilities. These results are compared with data from alternative laboratory measurements and with petrophysical properties inferred from downhole logging data. We incorporate these results into a 2D numerical model that reflects the shelf architecture as known from drillings and seismic data to perform submarine groundwater flow simulations. In order to account for uncertainties related to the spatial distribution of physical properties, such as porosity and permeability, systematic variation of input parameters was performed during simulation runs. The target is to test the two conflicting hypotheses of fresh groundwater emplacements offshore New Jersey and to improve the understanding of fluid flow processes at marine passive margins.

  6. The Northwestern Atlantic Moroccan Margin From Deep Multichannel Seismic Reflection

    NASA Astrophysics Data System (ADS)

    Malod, J. A.; Réhault, J. P.; Sahabi, M.; Géli, L.; Matias, L.; Zitellini, N.; Sismar Group

    The NW Atlantic Moroccan margin, a conjugate of the Nova Scotia margin, is one of the oldest passive margins of the world. Continental break up occurred in the early Jurassic and the deep margin is characterized by a large salt basin. The SISMAR cruise (9 April to 4 May 2001) acquired 3667 km of 360 channel seismic reflection profiles. In addition, refraction data were recorded by means of 48 OBH/OBS deployments. Simultaneously, some of the marine profiles were extended onshore with 16 portable seismic land stations. WNW-ESE profiles 4 and 5 off El Jadida show a good section of the margin. The crustal thinning in this region is fairly abrupt. These profiles image the crust above a strong seismic reflector at about 12 s.twt., interpreted as the Moho. The crust exhibits several different characteristics from the continent towards the ocean.: - highly diffractive with a thickness larger than 25 km beneath the shelf. - stratified at a deep level and topped by few "tilted blocks" with a diffractive acoustic facies and for which 2 hypotheses are proposed: either continental crust tilted during the rifting or large landslides of crustal and sedimentary material slid down later. Liassic evapor- ites are present but seem less thick than to the south. - layered with seaward dipping reflectors: this type of crust correlates with the magnetic anomaly S1 and corresponds to the continent-ocean transition. - diffractive with an oceanic character. Oceanwards, the crust becomes more typically oceanic, but shows internal reflectors that may be re- lated to compressional reactivation during the Tertiary attested by large scale inverted basins. Our results allow us to discuss the nature and location of the continent-ocean transition at a regional scale and the rifting to spreading evolution of the very ma- ture continental margin off El Jadida. This provide us with some constraints for the initial reconstruction between Africa, North America and Iberia. Moreover, these re- sults help

  7. Middle Jurassic to early Cretaceous igneous rocks along eastern North American continental margin

    SciTech Connect

    Jansa, L.F.; Pe-Piper, G.

    1988-03-01

    Late Middle Jurassic and Early Cretaceous mafic dikes, sills, flows, and local volcaniclastic sediments are intercalated within continental shelf sediments from the Baltimore Canyon Trough northward to the Grand Banks of Newfoundland. The igneous rocks on the eastern North American margin are mainly alkali basalts of intraplate affinity. The late Middle Jurassic igneous activity was of short duration, at about 140 Ma, and was restricted to Georges Bank where it led to construction of several volcanic cones. The main period of igneous activity was concentrated at about 120 Ma in the Aptian/Berremian. The activity consists of dike swarms in Baltimore Canyon, occasional dikes on the Scotian Shelf, and the growth of stratovolcanoes on the Scotian Shelf and Grand Banks. Younger dikes (approx. 95 Ma) also are present on the Grand Banks. With regard to oil exploration on the continental margin, care must be taken to properly identify igneous and volcaniclastic rocks on mechanical logs, drill cuttings, and cores. Reflection seismic profiles can be used to map the areal extent of sills, flows, and low-angle dikes, which commonly show distinctive seismic responses. However, steeply dipping dikes generally produce little, if any, seismic response. Isotopic-age determinations of igneous rocks, combined with biostratigraphic-age determinations of adjacent strata, are invaluable for stratigraphic correlation, establishing chronology of seismic sequences, and analysis of basin sedimentation and tectonic history. 9 figures, 2 tables.

  8. Stratigraphy of Atlantic coastal margin of United States north of Cape Hatteras; brief survey

    USGS Publications Warehouse

    Perry, W.J.; Minard, J.P.; Weed, E.G.A.; Robbins, E.I.; Rhodehamel, E.C.

    1975-01-01

    A synthesis of studies of sea-floor outcrops of the sedimentary wedge beneath the northeastern United States continental shelf and slope and a reassessment of coastal plain Mesozoic stratigraphy, particularly of the coastal margin, provide insight for estimating the oil and gas potential and provide geologic control for marine seismic investigations of the Atlantic continental margin. The oldest strata known to crop out on the continental slope are late Campanian in age. The Cretaceous-Tertiary contact along the slope ranges from a water depth of 0.6 to 1.5 km south of Georges Bank to 1.8 km in Hudson Canyon. Few samples are available from Tertiary and Late Cretaceous outcrops along the slope. Sediments of the Potomac Group, chiefly of Early Cretaceous age, constitute a major deltaic sequence in the emerged coastal plain. This thick sequence lies under coastal Virginia, Maryland, Delaware, southeastern New Jersey, and the adjacent continental shelf. Marine sands associated with this deltaic sequence may be present seaward under the outer continental shelf. South of the Norfolk arch, under coastal North Carolina, carbonate rocks interfinger with Lower Cretaceous clastic strata. From all available data, Mesozoic correlations in coastal wells between coastal Virginia and Long Island have been revised. The Upper-Lower Cretaceous boundary is placed at the transition between Albian and Cenomanian floras. Potential hydrocarbon source beds are present along the coast in the subsurface sediments of Cretaceous age. Potential reservoir sandstones are abundant in this sequence.

  9. Greenhouse to Icehouse Antarctic Paleoclimate and Ice History from George V Land and Adélie Land Shelf Sediments

    NASA Astrophysics Data System (ADS)

    Williams, T.; Escutia, C.; De Santis, L.; O'Brien, P.; Pekar, S. F.; Brinkhuis, H.; Domack, E. W.

    2013-12-01

    Along the George V and Adélie Land continental shelf of East Antarctica, shallowly-buried strata contain a record of Antarctica's climate and ice history from the lush forests of the Eocene greenhouse to the dynamic ice sheet margins of the Neogene. Short piston cores and dredges have recovered Early Cretaceous and Eocene organic-rich sediment at the seabed, and in 2010, IODP Expedition 318 recovered earliest Oligocene and early Pliocene subglacial and proglacial diamictites. However, challenging ice and drilling conditions from the JOIDES Resolution on the shelf resulted in poor core recovery and sites had to be abandoned before the stratigraphic targets could be reached. Therefore, in a new IODP drilling proposal submitted earlier this year, we propose to use the MeBo sea bed drill for improved core recovery and easier access to the shelf, and drill a stratigraphic transect of shallow (~80m) holes. To investigate the evolution of the Antarctic ice sheet in this sector, we target strata above and below regional erosional and downlap surfaces to date and characterize major episodes of ice sheet advance and retreat. These direct records of ice extent on the shelf can be set in the context of Southern Ocean records of temperature, ice-rafted debris (IRD) and latitudinal fluctuations of the opal belt, and hence we can relate ice sheet evolution to paleoclimate conditions. Targets include possible late Eocene precursor glaciations, the Eocene/Oligocene boundary erosion surface, Oligocene and Miocene ice extents, and ice margin fluctuations in the Pliocene. At the Cretaceous and Eocene proposed sites, marine and terrestrial temperature proxies and palynological records will provide information on high-latitude paleoenvironments and pole-equator temperature gradients. Here we present existing data from the area and the proposed new drill sites. The ice and climate history of the George V and Adélie Land margin can provide warm-world scenarios to help understand ice

  10. Genesis and emplacement of oil in the San Andres Formation, Northern Shelf of the Midland Basin, Texas. Report of Investigations No. 116

    SciTech Connect

    Ramondetta, P.J.

    1982-01-01

    San Andres oil constitutes more than 80 percent of the total production from the Northern Shelf of the Midland Basin, Texas. The San Andres and Clear Fork carbonate rocks of the Northern Shelf contain sufficient amounts of lipid-rich organic material to rank them as potential petroleum source beds. Organic maturation of these rocks as revealed from vitrinite reflectance and kerogen color, however, is not sufficient to have initiated categenesis. Therefore, oil within Northern Shelf reservoirs was derived mostly from other sources. San Andres oils have a common source, as evidenced by their remarkably uniform composition, which is revealed in liquid and gas chromatography. Wolfcampian basinal clastics and dark argillaceous limestones of the northern Midland Basin are the most likely source rocks for this oil. Vertical expulsion of basinal oil through fractures into overlying shelf and shelf-margin carbonates has occurred along the Lower Permian Abo Reef trend. The trapping mechanism in the Northern Shelf is a combination of structural and facies control. Good reservoir conditions exist in San Andres strata that are draped and subsequently fractured over the subjacent shelf-margin buttress. Late Cretaceous uplift in New Mexico exposed Permian strata, iniating a west-to-east flow of relatively fresh ground water. Passage of this meteoric water through San Andres and Clear Fork reservoirs caused downdip degradation and flushing of the oil. As a result of this ground-water movement, oil/water contacts tilt downdip 0.3/sup 0/ to 0.5/sup 0/, and oil production is slightly offset downdip from local structural highs. The San Andres and Clear Fork oil was degraded by anaerobic sulfate-reducing bacteria, which resulted in an enrichment of sulfur and light aromatics and a slight depletion of saturated hydrocarbons. This biodegradation progressively increases updip, as evidenced by higher sulfur contents and lower API gravity. 28 figures, 7 tables.

  11. Benthic foraminifers on the continental shelf and upper slope, Russian River area, northern California ( USA).

    USGS Publications Warehouse

    Quinterno, P.J.; Gardner, J.V.

    1987-01-01

    We analyzed benthic foraminifers from 71 surface samples collected from the sea floor of the continental margin. One hundred and six different taxa were identified, and Q-mode factor analysis was used to identify assemblages. Six foraminiferal assemblage factors explain 94% of the variation in the data matrix. The Inner Shelf Assemblage is characterized by Trichohyalus ornatissima, Rotalia columbiensis, Cassidulina limbata, Cibicides fletcheri, Elphidiella hannai and Elphidium sp. 1 and occupies water depths less than 50 m. The Middle Shelf Assemblage is characterized by Nonionella basispinata, Elphidium excavatum and Florilus labradoricus and occupies water depths between 50 and 90 m. A Middle Shelf to Upper Bathyal Assemblage is characterized by Uvigerina juncea, Globobulimina spp. and Nonionella basispinata and occupies depths between about 90 and 450 m. Two overlapping assemblages make up the Upper Middle Bathyal Assemblage and are most abundant between water depths of 500 and 1300 m. They are associated with low- oxygen conditions. The Mid-Bathyal Assemblage is dominated by Uvigerina proboscidea and occurs on the slope at water depths ranging from 1200 to 2500 m. -from Authors

  12. Shelf exchange driven by coastal-trapped waves in an Arctic fjord/trough system

    NASA Astrophysics Data System (ADS)

    Inall, Mark; Nilsen, Frank; Cottier, Finlo; Daae, Ragnhild

    2015-04-01

    Ubiquitous and dominant in mid-latitude coastal exchange processes, tides play a relatively minor role in many Arctic fjord and shelf system. Here we show that the class of low frequency (sub-inertial) waves known as coastal-trapped waves (CTWs) play an analogous role to tides in one west Svalbard fjord, and by extension more widely along the west Svalbard and east Greenland margins, where similar conditions prevail. We show that CTWs generated by weather systems passing across the sloping topography of the shelf break propagate into the fjord, steered by the topography of a cross shelf trough. The CTWs have characteristic periods of approximately two days, set by the passage time of weather systems. Phase speeds and wavelengths vary seasonally by a factor of two, according to stratification. Typical winter (summer) values are c = 0.25 ms-1 (0.5 cms-1) and λ = 40 km (84 km). CTW-induced flow velocities in excess of 0.3 ms-1 at 100 m water depth are recorded. Scaled CTW model results indicate that CTW-induced flows may exceed 0.5 ms-1 at the top of the slope. A range of consequences for this dominant mode of sub-inertial variability in Arctic fjord/ocean exchange is discussed, including: generalised geostrophic adjustment, horizontal advection and mixing, resuspension, higher order Stoke's drift effects, and diapycnal mixing via Richardson number instability.

  13. Evolution to decay of upwelling and associated biogeochemistry over the southeastern Arabian Sea shelf

    NASA Astrophysics Data System (ADS)

    Gupta, G. V. M.; Sudheesh, V.; Sudharma, K. V.; Saravanane, N.; Dhanya, V.; Dhanya, K. R.; Lakshmi, G.; Sudhakar, M.; Naqvi, S. W. A.

    2016-01-01

    Observations along 10 shelf transects in 2012 near 10°N in the southeastern Arabian Sea revealed the usual warm oligotrophic conditions during the winter monsoon and upwelling of oxygen-deficient, nutrient-rich cool water during the summer monsoon (SM). By changing an oligotrophic to a nutrient-replete condition, the upwelling is the major process that regulates the biogeochemistry of this shelf. Its onset is perceptible at 100 m depth between January and March. The upwelling reaches the surface layer in May and intensifies during June-July but withdraws completely and abruptly by October. Despite the nutrient injection, the primary production during SM, integrated for euphotic zone, is comparable to that during the preceding spring intermonsoon (SIM). Again, as usual, the high oxygen demand coupled with low concentration in the upwelled subsurface waters causes severe oxygen depletion below the shallow pycnocline. The oxygen concentrations/saturations of 2012 on the midshelf are similar from those of mid-1958 to early 1960, except for marginally higher values during the peak upwelling period due to relatively weak upwelling in 2012. This implies little anthropogenic influence on coastal hypoxia unlike many other coastal regions. In 2012, the inner shelf system shifted from net autotrophy in SIM to net heterotrophy in SM but on an annual basis it was net autotrophic (gross primary production to community respiration ratio, GPP/R:1.11 ± 0.84) as organic production exceeded consumption.

  14. Asymmetry and polarity of the South Atlantic conjugated margins related to the presence of cratons: a numerical study

    NASA Astrophysics Data System (ADS)

    Andrés-Martínez, Miguel; Pérez-Gussinyé, Marta; de Monserrat Navarro, Albert; Morgan, Jason P.

    2015-04-01

    Tectonic asymmetry of conjugated passive margins, where one margin is much narrower than the conjugate one, is commonly observed at many passive margins world-wide. Conjugate margin asymmetry has been suggested to be a consequence of lateral changes in rheology, composition, temperature gradient or geometries of the crust and lithosphere. Here we use the South Atlantic margins (from Camamu/Gabon to North Santos/South Kwanza) as a natural laboratory to understand conjugate margin asymmetry. Along this margin sector the polarity of the asymmetry changes. To the North, the Brazilian margin developed in the strong Sao Francisco craton, and this constitutes the narrow side of the conjugate pair. To the South, the Brazilian margin developed in the Ribeira fold belt, and the margin is wide. The opposite is true for the African side. We have thus numerically analysed how the relative distance between the initial location of extension and the craton influences the symmetry/asymmetry and polarity of the conjugate margin system. Our numerical model is 2D visco-elasto-plastic and has a free surface, strain weakening and shear heating. The initial set-up includes a cratonic domain, a mobile belt and a transition area between both. We have run tests with different rheologies, thickness of the lithosphere, and weak seeds at different distances from the craton. Results show asymmetric conjugated margins, where the narrower margin is generally the closest to the craton. Our models also allow us to study how the polarity is controlled by the distance between the initial weakness and the craton, and help to understand how the presence of cratonic domains affects the final architecture of the conjugated margins.

  15. Convective Removal of Continental Margin Lithosphere at the Edges of Subducting Oceanic Plates

    NASA Astrophysics Data System (ADS)

    Levander, A.; Bezada, M. J.; Palomeras, I.; Masy, J.; Humphreys, E.; Niu, F.

    2013-12-01

    Although oceanic lithosphere is continuously recycled to the deeper mantle by subduction, the rates and manner in which different types of continental lithospheric mantle are recycled is unclear. Cratonic mantle can be chemically reworked and essentially decratonized, although the frequency of decratonization is unclear. Lithospheric mantle under or adjacent to orogenic belts can be lost to the deeper mantle by convective downwellings and delamination phenomena. Here we describe how subduction related processes at the edges of oceanic plates adjacent to passive continental margins removes the mantle lithosphere from beneath the margin and from the continental interior. This appears to be a widespread means of recycling non-cratonic continental mantle. Lithospheric removal requires the edge of a subducting oceanic plate to be at a relatively high angle to an adjacent passive continental margin. From Rayleigh wave and body wave tomography, and receiver function images from the BOLIVAR and PICASSO experiments, we infer large-scale removal of continental margin lithospheric mantle from beneath 1) the northern South American plate margin due to Atlantic subduction, and 2) the Iberian and North African margins due to Alboran plate subduction. In both cases lithospheric mantle appears to have been removed several hundred kilometers inland from the subduction zones. This type of ';plate-edge' tectonics either accompanies or pre-conditions continental margins for orogenic activity by thinning and weakening the lithosphere. These processes show the importance of relatively small convective structures, i.e. small subducting plates, in formation of orogenic belts.

  16. Astronomy for African development

    NASA Astrophysics Data System (ADS)

    Govender, Kevindran

    2011-06-01

    In recent years there have been a number of efforts across Africa to develop the field of astronomy as well as to reap benefit from astronomy for African people. This presentation will discuss the case of the SALT (Southern African Large Telescope) Collateral Benefits Programme (SCBP) which was set up to ensure societal benefit from astronomy. With African society as the target, the SCBP has embarked on various projects from school level education to public understanding of science to socio-economic development, the latter mainly being felt in the rural communities surrounding the South African Astronomical Observatory (home to SALT). A development plan for ``Astronomy in Africa'' will also be discussed. This plan has been drawn up with input from all over Africa and themed ``Astronomy for Education''. The Africa case stands as a good example for the IYA cornerstone project ``Developing Astronomy Globally'' which focuses on developing regions.

  17. African American Suicide

    MedlinePlus

    ... accounted for 83.8% of Caucasian elderly suicides. • Firearms were the predominant method of suicide among African ... per 100,000 annually. Source: Centers for Disease Control and Prevention. National Vital Statistics System. Mortality Data. ...

  18. Simulating Sediment Delivery to and Accumulation on the Poverty Shelf, New Zealand

    NASA Astrophysics Data System (ADS)

    Kettner, A. J.; Gomez, B.; Syvitski, J. P.; Hutton, E. W.

    2006-12-01

    The 2203 km2 Waipaoa and 312 km2 Waimata river basins which lie within the north-central sector of the tectonically active East Coast Continental Margin, North Island, New Zealand, annually deliver ~16 Mt of suspended sediment to Poverty Bay and the adjacent Poverty Shelf. This region has experienced four magnitude >7 earthquakes and been blanketed by ash from one of the most violent and explosive volcanic eruptions known in the past 2.5 kyr. Natural fires and severe storms also periodically disturbed the native vegetation prior to the arrival of Polynesian settlers in the thirteenth century A.D., but their activities had little effect on erosion rates and the very high contemporary rate of sediment discharge is a product of deforestation, which occurred after European colonists arrived in the nineteenth century A.D. We used an integrated suite of numerical models run with daily time steps to examine the effect these events had on basin sediment yield and the sedimentary record preserved on the Poverty Shelf, which represents the major repository of the fluvial sediment discharged throughout the past 8 kyr. We used HydroTrend, which is a numerical model that creates synthetic river discharge and sediment load time series over centuries or millennia as a function of climate and basin morphology, to reconstruct the suspended sediment discharge of the Waipaoa and Waimata rivers during the past 3 kyr. Our simulations suggest that, under the indigenous forest cover, a 10% increase in precipitation is associated with a 5% increase in suspended sediment discharge, and that the disturbance to the vegetation cover by ashfall during the Taupo eruption caused an 80% increase in suspended sediment discharge. The piecemeal disturbances that occurred after Polynesian arrival and the earliest European clearances had a minimal impact on suspended sediment discharge, but the subsequent clearances in the headwaters caused suspended sediment discharge to increase by 850%. The

  19. Linking Late Pleistocene alpine glacial erosion and continental margin sedimentation: Insights from 40Ar/39Ar dating of silt-sized sediment, Canterbury Basin, New Zealand

    NASA Astrophysics Data System (ADS)

    Villaseñor, Tania; Jaeger, John M.; Foster, David A.

    2016-01-01

    Quaternary climatic and eustatic cycles in mid-latitude regions have led to more extensive alpine glaciations and continental shelf progradation, respectively. However, the glacial influence on sediment fluxes to the ocean creating continental margin strata is poorly documented. This contribution analyzes the provenance of fine sediment accumulating on the continental shelf during the Late Pleistocene to evaluate the influence of glacial cycles on sediment erosion and routing to the continental shelf. Taking advantage of the contrasting bedrock ages exposed across the Southern Alps, New Zealand, we perform 40Ar/39Ar incremental heating on the bulk silt-size sediment from three drill sites of IODP Expedition 317, Canterbury Basin, New Zealand. The results suggest that a large proportion of sediment accumulating on the continental shelf results from erosion within the Main Divide fault zone of the Southern Alps. Sediment 40Ar/39Ar age fluctuations over this time period suggest that bedrock with various 40Ar/39Ar cooling ages has been differentially eroded in the upper Waitaki River catchment and mixed in the Waitaki-Canterbury sediment-routing system. Across-shelf variations in sediment 40Ar/39Ar age reflect changing modes of sediment dispersal on the continental shelf. Fluvial material, likely derived from the main drainage divide zone, preferentially accumulates in the middle continental shelf, whereas material representing erosion of older bedrock (Torlesse Terrane), located lower in the drainage basin, is dispersed uniformly across the shelf. The age signature of the muddy sediment accumulating on the continental shelf reflects Late Pleistocene landscape evolution of the Southern Alps and its influence on sediment dispersal to the continental shelf.

  20. The Northeast Greenland Shelf - Evidence of the existence of a pronounced salt-province

    NASA Astrophysics Data System (ADS)

    Schmitz, T.; Jokat, W.

    2003-04-01

    The Northeast Greenland shelf (NEGS) is the part of the continental margin of east Greenland located between the Jan Mayen Fracture Zone at about 72°N in the south and the Spitzbergen Fracture Zone at 81°N in the north. The eastern boundary, at the shelf edge, is the approximate position of the boundary between continental and oceanic crust and the western boundary is the coastline of Greenland. The shelf has a N-S orientation, is about 1000 km long, and between 125 km (southern part) and 380 km (at 78°N) wide. Based on present data the NEGS can be subdivided into a southern part influenced by Tertiary tectonism and volcanism (approx. 72°N to 75°N) and a northern, nonvolcanic, part (approx. 75°N to 81°N). Today the sedimentary history, stratigraphy, structure and origin of the basement below the sedimentary shelf south of 74°N are reasonable known, but only sparse information exists about the northern part of the shelf. Until 1990 there weren't any seismic lines north of 74°N, and all interpretations of stratigraphy and basin structures of the northern part of the NEGS were based on aeromagnetic data. During the last decade, the first seismic lines were shot over the northern part of the shelf to give more detailed information about sediment thickness, stratigraphy, and the structure of the sedimentary shelf. The area under investigation lies on the nonvolcanic northern part of the shelf between 78°30'N and 81°N. The sea floor topography indicates some submarine banks with water depth as shallow as 30 m, which are separated by valleys up to 500 m deep. These valleys were formed through erosion processes caused by cyclic movements of big grounded glacier tongues during the last ice-ages with a maximum expansion during the Wisconsin-Weichselian glaciation. During two scientific expeditions with the German research icebreaker Polarstern in 1997 and 1999, more than 1100 km of multichannel seismic data were collected. The cruise tracks during seismic

  1. Regional seismic stratigraphy and controls on the Quaternary evolution of the Cape Hatteras region of the Atlantic passive margin, USA

    USGS Publications Warehouse

    Mallinson, D.J.; Culver, S.J.; Riggs, S.R.; Thieler, E.R.; Foster, D.; Wehmiller, J.; Farrell, K.M.; Pierson, J.

    2010-01-01

    Seismic and core data, combined with amino acid racemization and strontium-isotope age data, enable the definition of the Quaternary stratigraphic framework and recognition of geologic controls on the development of the modern coastal system of North Carolina, U.S.A. Seven regionally continuous high amplitude reflections are defined which bound six seismic stratigraphic units consisting of multiple regionally discontinuous depositional sequences and parasequence sets, and enable an understanding of the evolution of this margin. Data reveal the progressive eastward progradation and aggradation of the Quaternary shelf. The early Pleistocene inner shelf occurs at a depth of ca. 20-40 m beneath the western part of the modern estuarine system (Pamlico Sound). A mid- to outer shelf lowstand terrace (also early Pleistocene) with shelf sand ridge deposits comprising parasequence sets within a transgressive systems tract, occurs at a deeper level (ca. 45-70 m) beneath the modern barrier island system (the Outer Banks) and northern Pamlico Sound. Seismic and foraminiferal paleoenvironmental data from cores indicate the occurrence of lowstand strandplain shoreline deposits on the early to middle Pleistocene shelf. Middle to late Pleistocene deposits occur above a prominent unconformity and marine flooding surface that truncates underlying units, and contain numerous filled fluvial valleys that are incised into the early and middle Pleistocene deposits. The stratigraphic framework suggests margin progradation and aggradation modified by an increase in the magnitude of sea-level fluctuations during the middle to late Pleistocene, expressed as falling stage, lowstand, transgressive and highstand systems tracts. Thick stratigraphic sequences occur within the middle Pleistocene section, suggesting the occurrence of high capacity fluvial point sources debouching into the area from the west and north. Furthermore, the antecedent topography plays a significant role in the evolution

  2. Stratigraphical links between Miocene Alpine Foreland basin and Gulf of Lion Passive Margin during lowstands

    NASA Astrophysics Data System (ADS)

    Rubino, Jean-Loup; Gorini, Christian; Leroux, Estelle; Aslanian, Daniel; Rabineau, Marina; Parize, Olivier; Besson, David

    2015-04-01

    Miocene peri-alpine foreland basin is connected toward the south with the Gulf of Lion passive margin and is predominantly filled by marine shallow water molassic deposits ranging from lower Miocene to Pliocene in age. Nine to ten depositional sequences are recorded and partly preserved in this basin and can be traced into the post rift part of the Gulf of Lion. One of the most surprising feature of the stratigraphic infill is the total lack of lowstand deposits within the foreland basin ; All superimposed sequences only includes transgressive and highstand System Tracts separated by erosional sequence boundaries and the development of incised valley networks filled by tidal deposits during transgression; Besson et al. 2005. It means that the entire foreland basin in SE France is exposed during lowstand periods without any preservation of fluvial deposits. By place few forced regression wedges are preserved at the transition between the foreland and the passive margin, close to the present day coastline. To date no real lowstand wedges have never been reported in the offshore of the Gulf of Lion. A reinterpretation of the best old vintage 2D dip seismic profiles along the passive margin validates the idea that the foreland basin is entirely exposed as well as the proximal part of the passive margin; first because some incised valleys can be occasionally picked on the shelf and second mainly because well defined superimposed or juxtaposed prograding lowstand wedges with nicely defined clinoforms onlapping the sequence boundaries can be recognized on the distal part of the shelf from the Burdigalian to the Messinian. Their ages being constrains by the Calmar well calibration. Unfortunately, they can't be continuously mapped all along the shelf break because of the strong erosion related to the Messinian Unconformity and the associated huge sea level fall.So we have to explain why during the lowstands, exceptionally long fluvial valley networks (more than 300km) can

  3. Moisture and shelf life in sugar confections.

    PubMed

    Ergun, R; Lietha, R; Hartel, R W

    2010-02-01

    From hardening of marshmallow to graining of hard candies, moisture plays a critical role in determining the quality and shelf life of sugar-based confections. Water is important during the manufacturing of confections, is an important factor in governing texture, and is often the limiting parameter during storage that controls shelf life. Thus, an understanding of water relations in confections is critical to controlling quality. Water content, which is controlled during candy manufacturing through an understanding of boiling point elevation, is one of the most important parameters that governs the texture of candies. For example, the texture of caramel progresses from soft and runny to hard and brittle as the moisture content decreases. However, knowledge of water content by itself is insufficient to controlling stability and shelf life. Understanding water activity, or the ratio of vapor pressures, is necessary to control shelf life. A difference in water activity, either between candy and air or between two domains within the candy, is the driving force for moisture migration in confections. When the difference in water activity is large, moisture migration is rapid, although the rate of moisture migration depends on the nature of resistances to water diffusion. Barrier packaging films protect the candy from air whereas edible films inhibit moisture migration between different moisture domains within a confection. More recently, the concept of glass transition, or the polymer science approach, has supplemented water activity as a critical parameter related to candy stability. Confections with low moisture content, such as hard candy, cotton candy, and some caramels and toffees, may contain sugars in the amorphous or glassy state. As long as these products remain below their glass transition temperature, they remain stable for very long times. However, certain glassy sugars tend to be hygroscopic, rapidly picking up moisture from the air, which causes

  4. The Tweeting Ice Shelf: geophysics and outreach

    NASA Astrophysics Data System (ADS)

    Van Liefferinge, Brice; Berger, Sophie; Drews, Reinhard; Pattyn, Frank

    2015-04-01

    Over the last decade the Antarctic and Greenland ice sheets have contributed about one third of the annual sea level rise (Hanna et al., 2013). However, it remains difficult to reconcile global mass balance estimates obtained from different satellite-based methods. A typical approach is to balance the mass input from atmospheric modelling with the outgoing mass flux at the ice-sheet boundary (Shepherd et al., 2012). The flux calculations at the boundary rely on satellite-derived surface velocities, which are currently only available as snapshots in time, and which need ground truth for validation. Here, we report on continuous, year-round measurements that aim at improving the input-output method in several aspects and carefully map the flow speed allowing for detecting seasonal variability. For this purpose, we set up in December 2014 three stand-alone single-frequency GPSes on the Roi Baudouin ice shelf (East Antarctica). The GPSes are installed across a surface depression (typical for large ice-shelf channels), where subglacial melting is expected. This setup allows us to investigate how these channels behave, i.e., if they become wider, whether or not they enhance the ice flow, and, in combination with an installed phase-sensitive radar, what amount of melting occurs below the channels in contact with the ocean. The GPS data are transmitted on a daily basis. Ice-shelf velocity is derived from the raw hourly location following the methods described in den Ouden et al. (2010), Dunse et al. (2012), and Ahlstrøm et al. (2013). However, a reference station has not been used for the correction. Basic processing involves outliers removal, smoothing, time-series analysis and comparison with tidal models. The project comes alongside an outreach event: on a weekly basis, the ice shelf 'tweets' its position, motion and relays other information with respect to the project. The GPS systems can be followed on Twitter via @TweetinIceShelf as well as the Tweeting Ice Shelf

  5. Variability of South Atlantic Central Water in the last century based on stable isotopes and benthic foraminifera of southeast Brazilian continental shelf.

    NASA Astrophysics Data System (ADS)

    Barbosa, C. F.; De Faria, G. R.; Albuquerque, A. L.

    2014-12-01

    In order to better understand the changes in productivity and water masses that permeate the bottom of the continental shelf of southeast Brazilian margin influenced by upwelling, three box-cores were collected in a bathymetric transect. Cores were analyzed for assemblage composition of benthic foraminifera and isotopes. 55 samples were analyzed and 227 benthic foraminifera were identified, but only 18 of them showed relative frequencies that contributed to the understanding of both oceanographic dynamics and productivity conditions of the area. There was dominance of Globocassidulina subglobosa, which is considered an indicator of areas with pulses of phytodetritus. Statistical analysis separated samples into different groups according to their location, indicating environmental factors on the continental shelf. Innershore core showed the highest flux of benthic foraminifera and the offshore one showed the major diversity and variation of foraminifera assemblages over time. Mid shelf cores showed little variation of the assemblages and indicate a stable environment. The δ13C values of Uvigerina peregrina indicated more degraded organic matter in the center of the shelf. The mean composition of Cibicides spp. δ13C is, on average, 0.25 ‰ depleted related to the values of δ13CDIC. Such values associated with high frequency of phytodetritus species indicated the formation of a phytodetritus layer at the bottom of the continental shelf of Cabo Frio. The paleotemperature calculated from oxygen isotopes pointed the permanence of SACW at the shelf bottom over the last few centuries.

  6. Shock margin testing of a one-axis MEMS accelerometer.

    SciTech Connect

    Parson, Ted Blair; Tanner, Danelle Mary; Buchheit, Thomas Edward

    2008-07-01

    Shock testing was performed on a selected commercial-off-the-shelf - MicroElectroMechanical System (COTS-MEMS) accelerometer to determine the margin between the published absolute maximum rating for shock and the 'measured' level where failures are observed. The purpose of this testing is to provide baseline data for isolating failure mechanisms under shock and environmental loading in a representative device used or under consideration for use within systems and assemblies of the DOD/DOE weapons complex. The specific device chosen for this study was the AD22280 model of the ADXL78 MEMS Accelerometer manufactured by Analog Devices Inc. This study focuses only on the shock loading response of the device and provides the necessary data for adding influence of environmental exposure to the reliability of this class of devices. The published absolute maximum rating for acceleration in any axis was 4000 G for this device powered or unpowered. Results from this study showed first failures at 8000 G indicating a margin of error of two. Higher shock level testing indicated that an in-plane, but off-axis acceleration was more damaging than one in the sense direction.

  7. Global distribution of naturally occurring marine hypoxia on continental margins

    NASA Astrophysics Data System (ADS)

    Helly, John J.; Levin, Lisa A.

    2004-09-01

    Hypoxia in the ocean influences biogeochemical cycling of elements, the distribution of marine species and the economic well being of many coastal countries. Previous delineations of hypoxic environments focus on those in enclosed seas where hypoxia may be exacerbated by anthropogenically induced eutrophication. Permanently hypoxic water masses in the open ocean, referred to as oxygen minimum zones, impinge on a much larger seafloor surface area along continental margins of the eastern Pacific, Indian and western Atlantic Oceans. We provide the first global quantification of naturally hypoxic continental margin floor by determining upper and lower oxygen minimum zone depth boundaries from hydrographic data and computing the area between the isobaths using seafloor topography. This approach reveals that there are over one million km 2 of permanently hypoxic shelf and bathyal sea floor, where dissolved oxygen is <0.5 ml l -1; over half (59%) occurs in the northern Indian Ocean. We also document strong variation in the intensity, vertical position and thickness of the OMZ as a function of latitude in the eastern Pacific Ocean and as a function of longitude in the northern Indian Ocean. Seafloor OMZs are regions of low biodiversity and are inhospitable to most commercially valuable marine resources, but support a fascinating array of protozoan and metazoan adaptations to hypoxic conditions.

  8. Subinertial and tidal currents on the Abrolhos Bank shelf

    NASA Astrophysics Data System (ADS)

    Castro, Belmiro M.; Dottori, Marcelo; Pereira, Adriene F.

    2013-11-01

    Analysis of simultaneous current measurements in the middle (point PA1) and outer (point PA2) Abrolhos Bank shelf showed different characteristics and forcing mechanisms between the two shelf regions. The mean currents at both positions point generally southwestward but are weak due to the high time variability; at PA1, the mean along-shelf currents are almost one order of magnitude larger than the mean cross-shelf currents, while at PA2, both mean current components exhibit the same order of magnitude. At both points, the tidal currents are more important in the cross-shelf direction than they are in the along-shelf direction. The subinertial currents are mainly barotropic and along-shelf at PA1, while at PA2, the baroclinic first mode dominates the subinertial variability, especially in the cross-shelf direction, though the currents are also more energetic in the along-shelf direction. Correlations are significant between the along-shelf subinertial currents and synoptic winds at PA1, showing a local response (almost in phase) to the wind stress. At PA2, the correlations between winds and currents are generally not significant, showing that the subinertial current variability near the shelf break is forced by other mechanisms; the mesoscale variability of the Brazil Current is most likely important at this position. A simple frictional balance model allows a dynamical interpretation of the significant correlations between winds and currents at PA1.

  9. Paleogeographic evolution of early deep-water Gulf of Mexico and margins, Jurassic to middle Cretaceous (Comanchean)

    SciTech Connect

    Winker, C.D.; Buffler, R.T.

    1988-03-01

    The paleobathymetric configuration of the early Gulf of Mexico is inferred from (1) Cretaceous carbonate shelf margins interpreted from seismic profiles and other stratigraphic data; (2) distribution of Jurassic and Cretaceous platform and basinal facies; and (3) hindcasting of subsidence history in the central basin. Substantial paleogeographic ambiguity results from uncertainty about (1) kinematics and timing of Late Triassic to Jurassic extensional opening of the Gulf basin, which probably involved major strike-slip faulting, (2) the magnitude of subsequent compressive deformation on the western and southern basin margins, and (3) possible accretion of allochthonous terranes. 18 figures, 3 tables.

  10. Extension of Greenland Ice Sheet outlets to the shelf edge bordering Baffin Bay during the last glacial cycle

    NASA Astrophysics Data System (ADS)

    O'Cofaigh, C.; Dowdeswell, J. A.; Jennings, A. E.; Kilfeather, A. A.; Hogan, K.; Andrews, J. T.

    2010-12-01

    Despite much research seeking to understand recent dynamic changes to the Greenland Ice Sheet our understanding of the longer-term history of the ice sheet remains surprisingly poor for many sectors. This is particularly the case on the central west Greenland continental margin bordering Baffin Bay. In this region, several major fast flowing outlets, including Jakobshavns Isbrae, drain the ice sheet today. Marine geological and geophysical records recently acquired from the continental shelf and slope offshore of central west Greenland provide new evidence for the extension of ice sheet outlets to the shelf edge during the last glaciation. Bathymetric troughs extend from the mouths of Umanak Fjord and Disko Bugt to the shelf edge. Streamlined subglacial bedforms imaged on geophysical records and subglacial tills recovered in cores from these troughs record the flow of grounded ice sheet outlets along these troughs to the shelf edge during the last glaciation. Major submarine fans deposited at the mouths of the troughs record associated glacigenic sediment delivery to the continental slope. Cores and geophysical records from the fans show significant differences between them in terms of the nature of sediment delivery and slope morphology, with examples of both low gradient debris-flow dominated fans and much steeper channelized fans. These differences imply marked spatial variations in the nature of continental slope sedimentation in front of fast flowing ice sheet outlets. Over thirty AMS radiocarbon dates provide new chronological control on the timing and rate of glacigenic sediment delivery to the continental slope and the timing of subsequent ice sheet retreat from the shelf edge. These new landform, sediment and chronological records from the central west Greenland shelf and slope will be presented and the wider implications for Greenland Ice Sheet history at and following the LGM discussed.

  11. Architectures of the Moroccan continental shelf of the Alboran Sea: insights from high-resolution bathymetry and seismic data.

    NASA Astrophysics Data System (ADS)

    Lafosse, Manfred; Gorini, Christian; Leroy, Pascal; d'Acremont, Elia; Rabineau, Marina; Ercilla, Gemma; Alonso, Belén; Ammar, Abdellah

    2016-04-01

    The MARLBORO and the SARAS oceanographic surveys have explored the continental shelf in the vicinity of the transtensive Nekor basin (South Alboran Sea, Western Mediterranean) and over three submarine highs located at several tens of kilometers from the shelf. Those surveys have produced high-resolution (≤29m²/pixel) bathymetry maps. Simultaneously, seismic SPARKER and TOPAS profiles were recorded. To quantify and understand Quaternary vertical motions of this tectonically active area, we searched for morphological and sedimentary paleobathymetric or paleo-elevations markers. Shelf-edge wedges associated marine terraces and paleo-shorelines have been identified on the bathymetry and on seismic cross-sections. These features reflect the trends of long term accommodation variations. Along the Moroccan continental shelf the lateral changes of shelf-edges geometries and the spatial distribution of marine landforms (sedimentary marine terraces, sediment wave fields, marine incisions) reflect the interaction between sea level changes and spatial variations of subsidence rates. Positions of paleo-shorelines identified in the studied area have been correlated with the relative sea-level curve (Rohling et al., 2014). Several still stands or slow stands periods have been recognized between -130-125m, -100-110m and -85-80m. The astronomical forcing controls the architecture of Mediterranean continental shelves. Marine landforms distribution also reveals the way sea level changed since the LGM. The comparison with observations on other western Mediterranean margins (e.g. the Gulf of Lion, the Ionian-Calabrian shelf) allowed a first order access to vertical motion rates.

  12. Quantitative morphologic analysis of the Gulf of Alaska Yakutat margin: evidence for recent trough mouth fan growth

    NASA Astrophysics Data System (ADS)

    Swartz, J. M.; Gulick, S. P. S.; Goff, J. A.

    2014-12-01

    Glaciated continental shelves are host to numerous morphologic features that help understand past glacier dynamics. Southeastern Alaska is home to the St. Elias Mountains, an active orogen being impacted by temperate marine glaciers. During glacial periods ice streams advance across the continental shelf, carving shelf-crossing troughs that reach the shelf edge. We use high-resolution multibeam data to develop the relationship between two troughs, the Yakutat and Alsek Sea Valleys, and associated continental slope morphology. The shelf and slope geomorphology can be divided into statistical groupings that relate to the relative balance of erosion and deposition. Our analysis indicates that only the Yakutat system has been able to build an incipient trough-mouth fan. The extreme sediment supply from this region was able to overwhelm the steep initial topography of the transform margin, while farther to the east sediment slope-bypass dominates. This analysis provides an extreme end member to existing studies of temperate glaciation along continental margins. The unique interplay between rapid uplift due to ongoing collision and the massive erosion caused by temperate glaciers in a coastal system with extremely high precipitation provides for sedimentary flux far greater than other systems and thus allows for formation of a trough mouth fan in spite of a tectonically generated steep slope.

  13. Structural design/margin assessment

    NASA Technical Reports Server (NTRS)

    Ryan, R. S.

    1993-01-01

    Determining structural design inputs and the structural margins following design completion is one of the major activities in space exploration. The end result is a statement of these margins as stability, safety factors on ultimate and yield stresses, fracture limits (fracture control), fatigue lifetime, reuse criteria, operational criteria and procedures, stability factors, deflections, clearance, handling criteria, etc. The process is normally called a load cycle and is time consuming, very complex, and involves much more than structures. The key to successful structural design is the proper implementation of the process. It depends on many factors: leadership and management of the process, adequate analysis and testing tools, data basing, communications, people skills, and training. This process and the various factors involved are discussed.

  14. Seismicity and Attenuation of the Continental Margin Off New York and Southern/ New England from OBS Data

    NASA Astrophysics Data System (ADS)

    Ten Brink, U. S.; Flores, C. H.; McGuire, J. J.; Collins, J. A.

    2014-12-01

    Earthquake data recorded during two ocean-bottom-seismometer (OBS) deployments on the continental slope off New York and Southern New England are used to evaluate micro-seismicity and attenuation of the continental margin. The deployments were partly motivated by the detection by land seismometers of several ML2.5-3.8 continental margin earthquakes in this region during the previous four years. The data come from a five-month-long cluster deployment (1 km radius) of eleven short-period OBS in July 2012 at a water depth of ~835 m and a subsequent nine-month deployment of a network of five short-period OBS spaced 50 km apart at water depths between 900-2400 m. The land-based seismic network did not identify seismic activity on the margin during the two deployments. The OBS network located only one earthquake of ML~1 near the shelf edge, suggesting that seismic activity of the margin is probably successfully monitored by land seismometers. A successful land-based detection of earthquakes on the margin may indicate that low seismic attenuation extends from the Eastern U.S. to the continental shelf and slope. To better quantify the attenuation of the margin we derive the peak spectral attenuation of 6-8 ML2.8-4.1 earthquakes recorded by the OBS during the two deployments. These earthquakes were located on land close to shore and offshore between New Jersey and Nova Scotia, and propagation paths are almost entirely within the continental margin. Onshore earthquakes of ML ≤2.6 were not detected by the OBS. Information about the seismicity and attenuation of the margin will be used to evaluate the probability of earthquake-generated landslides.

  15. [Marginality, ethnic groups and health].

    PubMed

    Corretger, J M; Fortuny, C; Botet, F; Valls, O

    1992-06-01

    Main marginated ethnic groups in Span are to be found among gypsies and 3rd world immigrants. The first group include about 250,000 persons and the second group more tan half a million people. Their origins and their being past of the less fortunate social layers made them a group of health risk. Pediatric pathologies are those favored by socio-economic shortcomings as well as hygienic-sanitary deficiencies. Imported pediatric pathologies have a small incident. PMID:1636945

  16. Ice-shelf melting around Antarctica.

    PubMed

    Rignot, E; Jacobs, S; Mouginot, J; Scheuchl, B

    2013-07-19

    We compare the volume flux divergence of Antarctic ice shelves in 2007 and 2008 with 1979 to 2010 surface accumulation and 2003 to 2008 thinning to determine their rates of melting and mass balance. Basal melt of 1325 ± 235 gigatons per year (Gt/year) exceeds a calving flux of 1089 ± 139 Gt/year, making ice-shelf melting the largest ablation process in Antarctica. The giant cold-cavity Ross, Filchner, and Ronne ice shelves covering two-thirds of the total ice-shelf area account for only 15% of net melting. Half of the meltwater comes from 10 small, warm-cavity Southeast Pacific ice shelves occupying 8% of the area. A similar high melt/area ratio is found for six East Antarctic ice shelves, implying undocumented strong ocean thermal forcing on their deep grounding lines. PMID:23765278

  17. Submesoscale activity over the Argentinian shelf

    NASA Astrophysics Data System (ADS)

    Capet, X.; Campos, E. J.; Paiva, A. M.

    2008-08-01

    Submesoscale activity over the Argentinian shelf is investigated by means of high resolution primitive equation numerical solutions. These reveal energetic turbulent activity (visually similar to the one occasionally seen in satellite images) at scales O(5 km) in fall and winter that is linked to mixed layer baroclinic instability. The air-sea heat flux responsible for (i) deepening the upper ocean boundary layer (at these seasons) and (ii) maintaining a cross-shelf background density gradient is the key environmental parameter controlling submesoscale activity. Implications of submesoscale turbulence are investigated. Its mixing efficiency estimated by computing a diffusivity coefficient is above 30 m2 s-1 away from the shallowest regions. Aggregation of surface buoyant material by submesoscale currents occurs within hours and is presumably important to the ecosystem.

  18. Ice-Shelf Melting Around Antarctica

    NASA Astrophysics Data System (ADS)

    Rignot, E.; Jacobs, S.; Mouginot, J.; Scheuchl, B.

    2013-07-01

    We compare the volume flux divergence of Antarctic ice shelves in 2007 and 2008 with 1979 to 2010 surface accumulation and 2003 to 2008 thinning to determine their rates of melting and mass balance. Basal melt of 1325 ± 235 gigatons per year (Gt/year) exceeds a calving flux of 1089 ± 139 Gt/year, making ice-shelf melting the largest ablation process in Antarctica. The giant cold-cavity Ross, Filchner, and Ronne ice shelves covering two-thirds of the total ice-shelf area account for only 15% of net melting. Half of the meltwater comes from 10 small, warm-cavity Southeast Pacific ice shelves occupying 8% of the area. A similar high melt/area ratio is found for six East Antarctic ice shelves, implying undocumented strong ocean thermal forcing on their deep grounding lines.

  19. Breakup of the Larsen Ice Shelf, Antarctica

    NASA Technical Reports Server (NTRS)

    2002-01-01

    Recent Moderate-resolution Imaging Spectroradiometer (MODIS) satellite imagery analyzed at the University of Colorado's National Snow and Ice Data Center revealed that the northern section of the Larsen B ice shelf, a large floating ice mass on the eastern side of the Antarctic Peninsula, has shattered and separated from the continent. This particular image was taken on March 5, 2002. The shattered ice formed a plume of thousands of icebergs adrift in the Weddell Sea. A total of about 3,250 square kilometers of shelf area disintegrated in a 35-day period beginning on January 31, 2002. Over the last five years, the shelf has lost a total of 5,700 square kilometers and is now about 40 percent the size of its previous minimum stable extent. Ice shelves are thick plates of ice, fed by glaciers, that float on the ocean around much of Antarctica. The Larsen B shelf was about 220 meters thick. Based on studies of ice flow and sediment thickness beneath the ice shelf, scientists believe that it existed for at least 400 years prior to this event and likely existed since the end of the last major glaciation 12,000 years ago. For reference, the area lost in this most recent event dwarfs Rhode Island (2,717 square kilometers) in size. In terms of volume, the amount of ice released in this short time is 720 billion tons--enough ice for about 12 trillion 10-kilogram bags. This is the largest single event in a series of retreats by ice shelves along the peninsula over the last 30 years. The retreats are attributed to a strong climate warming in the region. The rate of warming is approximately 0.5 degrees Celsius per decade, and the trend has been present since at least the late 1940s. Overall in the peninsula, the extent of seven ice shelves has declined by a total of about 13,500 square kilometers since 1974. This value excludes areas that would be expected to calve under stable conditions. Ted Scambos, a researcher with the National Snow and Ice Data Center (NSIDC) at

  20. Four-decade record of pervasive grounding line retreat along the Bellingshausen margin of West Antarctica

    NASA Astrophysics Data System (ADS)

    Christie, Frazer D. W.; Bingham, Robert G.; Gourmelen, Noel; Tett, Simon F. B.; Muto, Atsuhiro

    2016-06-01

    Changes to the grounding line, where grounded ice starts to float, can be used as a remotely sensed measure of ice-sheet susceptibility to ocean-forced dynamic thinning. Constraining this susceptibility is vital for predicting Antarctica's contribution to rising sea levels. We use Landsat imagery to monitor grounding line movement over four decades along the Bellingshausen margin of West Antarctica, an area little monitored despite potential for future ice losses. We show that ~65% of the grounding line retreated from 1990 to 2015, with pervasive and accelerating retreat in regions of fast ice flow and/or thinning ice shelves. Venable Ice Shelf confounds expectations in that, despite extensive thinning, its grounding line has undergone negligible retreat. We present evidence that the ice shelf is currently pinned to a sub-ice topographic high which, if breached, could facilitate ice retreat into a significant inland basin, analogous to nearby Pine Island Glacier.

  1. Geochemical evidence of mantle reservoir evolution during progressive rifting along the western Afar margin

    NASA Astrophysics Data System (ADS)

    Rooney, Tyrone O.; Mohr, Paul; Dosso, Laure; Hall, Chris

    2013-02-01

    The Afar triple junction, where the Red Sea, Gulf of Aden and African Rift System extension zones converge, is a pivotal domain for the study of continental-to-oceanic rift evolution. The western margin of Afar forms the southernmost sector of the western margin of the Red Sea rift where that margin enters the Ethiopian flood basalt province. Tectonism and volcanism at the triple junction had commenced by ˜31 Ma with crustal fissuring, diking and voluminous eruption of the Ethiopian-Yemen flood basalt pile. The dikes which fed the Oligocene-Quaternary lava sequence covering the western Afar rift margin provide an opportunity to probe the geochemical reservoirs associated with the evolution of a still active continental margin. 40Ar/39Ar geochronology reveals that the western Afar margin dikes span the entire history of rift evolution from the initial Oligocene flood basalt event to the development of focused zones of intrusion in rift marginal basins. Major element, trace element and isotopic (Sr-Nd-Pb-Hf) data demonstrate temporal geochemical heterogeneities resulting from variable contributions from the Afar plume, depleted asthenospheric mantle, and African lithosphere. The various dikes erupted between 31 Ma and 22 Ma all share isotopic signatures attesting to a contribution from the Afar plume, indicating this initial period in the evolution of the Afar margin was one of magma-assisted weakening of the lithosphere. From 22 Ma to 12 Ma, however, diffuse diking during continued evolution of the rift margin facilitated ascent of magmas in which depleted mantle and lithospheric sources predominated, though contributions from the Afar plume persisted. After 10 Ma, magmatic intrusion migrated eastwards towards the Afar rift floor, with an increasing fraction of the magmas derived from depleted mantle with less of a lithospheric signature. The dikes of the western Afar margin reveal that magma generation processes during the evolution of this continental rift margin

  2. Submarine geomorphology of the Celtic Sea - new observations and hypotheses for the glaciation of a mid-latitude continental shelf

    NASA Astrophysics Data System (ADS)

    Praeg, Daniel; McCarron, Stephen; Dove, Dayton; Cofaigh, Colm Ó.; Monteys, Xavier; Coxon, Peter; Accettella, Daniella; Cova, Andrea; Facchin, Lorenzo; Romeo, Roberto; Scott, Gill

    2015-04-01

    The southern limit of glaciation of the European continental margin lies in the Celtic Sea, where the full extent and dynamics of the British-Irish Ice Sheet (BIIS) remain in question. This is in part because the broad continental shelf contains no obvious glacial geomorphological features, but is dominated by a system of shelf-crossing sediment ridges, up to 60 m high, 10 km wide and 300 km long, traditionally interpreted as moribund palaeo-tidal sand banks. Ice sheet extent has been constrained by samples of subglacial and glacimarine sediments recovered (in the 1970s) between the ridges, and in places on their flanks, used to propose a tidewater ice margin that advanced to a grounding line on the mid-shelf, overriding a precursor ridge system. New information on the glaciation of the Celtic Sea is available from geophysical and core data acquired during Italian- and Irish-led campaigns in 2009, 2012, and 2014, both from the mid- and outer shelf. On the mid-shelf, multibeam seabed imagery of a 25 x 100 km area reveal a distinctive rectilinear network of en echelon ridge segments giving way laterally and longitudinally to transverse ribs. Seismic correlation to glacigenic sediments previously cored on a ridge flank (at core site 49/-09/44) indicates the ribs to be composed in part of glacimarine sediments, above a till reflection that can be traced across the ridge crest. No change in seabed morphology is observed across the proposed grounding line. On the outer shelf, new cores of glacigenic sediments were obtained from the flank of a shelf-crossing ridge, and provide evidence of ice sheet advance to the shelf edge, 150 km beyond the proposed grounding line. The cores from outer Cockburn Bank contain facies interpreted to record subglacial deformation and glacimarine deposition from turbid meltwater plumes during withdrawal of a tidewater ice sheet margin from the shelf edge by 24,265 ± 195 cal BP. These sediments are inferred to form part of a sheet of

  3. The sequestration sink of soot black carbon in the Northern European Shelf sediments

    NASA Astrophysics Data System (ADS)

    SáNchez-GarcíA, Laura; Cato, Ingemar; Gustafsson, Örjan

    2012-03-01

    To test the hypothesis that ocean margin sediments are a key final repository in the large-scale biogeospheric cycling of soot black carbon (soot-BC), an extensive survey was conducted along the ˜2,000 km stretch of the Swedish Continental Shelf (SCS). The soot-BC content in the 120 spatially distributed SCS sediments was 0.180.130.26% dw (median with interquartile ranges), corresponding to ˜5% of total organic carbon. Using side-scan sonar constraints to estimate the areal fraction of postglacial clay sediments that are accumulation bottoms (15% of SCS), the soot-BC inventory in the SCS mixed surface sediment was estimated at ˜4,000 Gg. Combining this with radiochronological constraints on sediment mass accumulation fluxes, the soot-BC sink on the SCS was ˜300 Gg/yr, which yielded an area-extrapolated estimate for the Northern European Shelf (NES) of ˜1,100 Gg/yr. This sediment soot-BC sink is ˜50 times larger than the river discharge fluxes of soot-BC to these coastal waters, however, of similar magnitude as estimates of atmospheric soot-BC emission from the upwind European continent. While large uncertainties remain regarding the large-scale to global BC cycle, this study combines with two previous investigations to suggest that continental shelf sediments are a major final repository of atmospheric soot-BC. Future progress on the soot-BC cycle and how it interacts with the full carbon cycle is likely to benefit from14C determinations of the sedimentary soot-BC and similar extensive studies of coastal sediment in complementary regimes such as off heavily soot-BC-producing areas in S and E Asia and on the large pan-Arctic shelf.

  4. Key Largo Limestone revisited: Pleistocene shelf-edge facies, Florida Keys, USA

    USGS Publications Warehouse

    Gray, Multer H.; Gischler, E.; Lundberg, J.; Simmons, K.R.; Shinn, E.A.

    2002-01-01

    New dates and analysis of 12 deep and 57 shallow cores allow a more detailed interpretation of the Pleistocene shelf edge of the Florida Platform as found in various facies of the Key Largo Limestone beneath the Florida Keys. In this study a three-phase evolution of the Quaternary units (Q1-Q5) of the Key Largo is presented with new subdivision of the Q5. (1) In the first phase, the Q1 and Q2 (perhaps deposited during oxygen-isotope stage 11) deep-water quartz-rich environment evolved into a shallow carbonate phase. (2) Subsequently, a Q3 (presumably corresponding to oxygen-isotope stage 9) flourishing reef and productive high-platform sediment phase developed. (3) Finally, a Q4 and Q5 (corresponding to oxygen-isotope stages 7 and 5) stabilization phase occured with reefs and leeward productive lagoons, followed by lower sea levels presenting a sequence of younger (isotope substages 5c, 5a) shelf-margin wedges, sediment veneers and outlier reefs. The Key Largo Limestone provides an accessible model of a carbonate shelf edge with fluctuating water depth, bordering a deep seaward basin for a period of at least 300 ka. During this time, at least four onlaps/offlaps, often separated by periods of karst development with associated diagenetic alterations, took place. The story presented by this limestone not only allows a better understanding of the history of south Florida but also aids in the interpretation of similar persistent shelf-edge sites bordering deep basins in other areas.

  5. Structural framework, stratigraphy, and evolution of Brazilian marginal basins

    SciTech Connect

    Ojeda, H.A.O.

    1982-06-01

    The structural framework of the Brazilian continental margin is basically composed of eight structural types: antithetic tilted step-fault blocks, synthetic untilted step-fault blocks, structural inversion axes, hinges with compensation grabens, homoclinal structures, growth faults with rollovers, diapirs, and igneous structures. The antithetic tilted and synthetic untilted step-fault blocks are considered as synchronous, complementary structural systems, separated by an inversion axis. Two evaporitic cycles (Paripueira and Ibura) were differentiated in the Sergipe-Alagoas type basin and tentatively correlated to the evaporitic section of other Brazilian marginal basis. Four phases are considered in the evolution of the Brazilian marginal basins: pre-rift, rift, transitional, and drift. During the pre-rift phase (Late Jurassic-Early Cretaceous), continental sediments were deposited in peripheral intracratonic basins. In the rift phase (Early Cretaceous), the breakup of the continental crust of the Gondwana continent gave rise to a central graben and rift valleys where lacustrine sediments were deposited. The transitional phase (Aptian) developed under relative tectonic stability, when evaporitic and clastic lacustrine sequences were being deposited. In the drift phase (Albian to Holocene), a regionl homoclinal structure developed, consisting of two distinct sedimentary sequences, a lower clastic-carbonate and an upper clastic. From the Albian to the Holocene Epoch, structures associated to plastic displacement of salt or shale developed in many Brazilian marginal basins. Two phases of major igneous activity occurred: one in the Early Cretaceous associated with the rift phase of the Gondwana continent, and the other in the Tertiary during the migration phase of the South American and African plates.

  6. Tectonic structure and evolution of the Atlantic continental margin

    SciTech Connect

    Klitgord, K.D.; Schouten, H.; Hutchinson, D.R.

    1985-01-01

    The Atlantic continental margin developed across the boundary between continental and oceanic crust as rifting and then sea-floor spreading broke apart and separated the North American and African plates, forming the Atlantic Ocean Basin. Continental rifting began in Late Triassic with reactivation of Paleozoic thrust faults as normal faults and with extension across a broad zone of subparallel rift basins. Extension became localized in Early to Middle Jurassic along the zone that now underlies the large marginal basins, and other rift zones, such as the Newark, Hartford, and Fundy basins, were abandoned. Rifting and crustal stretching between the two continents gave way to sea-floor spreading Middle Jurassic and the formation of oceanic crust. This tectonic evolution resulted in formation of distinctive structural features. The marginal basins are underlain by a thinner crust and contain a variety of fault-controlled structures, including half-grabens, seaward- and landward-tilted blocks, faults that die out within the crust, and faults that penetrate the entire crust. This variable structure probably resulted from the late Triassic-Early Jurassic pattern of normal, listric, and antithetic faults that evolved from the Paleozoic thrust fault geometry. The boundary between marginal basins and oceanic crust is marked approximately by the East Coast Magnetic Anomaly (ECMA). A major basement fault is located in the Baltimore Canyon trough at the landward edge of the ECMA and a zone of seaward dipping reflectors is found just seaward of the ECMA off Georges Bank. The fracture zone pattern in Mesozoic oceanic crust can be traced landward to the ECMA.

  7. Iceberg scouring on the Norwegian continental shelf

    SciTech Connect

    Lien, R.

    1983-05-01

    This paper is a condensed version of parts of a Dr. ing. thesis to be presented during 1983. The first part of the paper deals with the regional distribution of iceberg scouring on the Norwegian continental shelf, and some general aspects related to it. The second part deals with iceberg scouring as a local phenomenon and its relation to the sea floor topography, sediment distribution, and geological and geotechnical properties of the sediments.

  8. Anaerobic methane oxidation on the Amazon shelf

    SciTech Connect

    Blair, N.E.; Aller, R.C.

    1995-09-01

    Anaerobic methane oxidation on the Amazon shelf is strongly controlled by dynamic physical sedimentation processes. Rapidly accumulating, physically reworked deltaic sediments characteristic of much of the shelf typically support what appear to be low rates of steady state anaerobic methane oxidation at depths of 5-8 m below the sediment-water interface. Methane oxidation in these cases is responsible for < {approximately}10% of the {Sigma}CO{sub 2} inventory in the oxidation zone and is limited largely by the steady-state diffusive flux of methane into the overlying sulfate reduction zone. In contrast, a large area of the shelf has been extensively eroded, reexposing once deeply buried (>10 m) methane-charged sediment directly to seawater. In this nonsteady-state situation, methane is a major source of recently produced {Sigma}CO{sub 2} and an important reductant for sulfate. These observations suggest that authigenic sedimentary carbonates derived from anaerobic methane oxidation may sometimes reflect physically enhanced nonsteady-state exposure of methane to sulfate in otherwise biogeochemically unreactive deposits. The concentration profiles of CH{sub 4}, SO{sub 4}{sup =}, and {Sigma}CO{sub 2} in the eroded deposit were reproduced by a coupled reaction-transport model. This area of the shelf was reexposed to seawater approximately 5-10 years ago based on the model results and the assumption that the erosion of the deposit occurred as a single event that has now ceased. The necessary second order rate constant for anaerobic methane oxidation was {le}0.1 mM{sup -1} d{sup -1}.

  9. Nature of decadal-scale sediment accumulation on the western shelf of the Mississippi River delta

    NASA Astrophysics Data System (ADS)

    Reide Corbett, D.; McKee, Brent; Allison, Mead

    2006-11-01

    Sediment delivered to coastal systems by rivers (15×10 9 tons) plays a key role in the global carbon and nutrient cycles, as deltas and continental shelves are considered to be the main repositories of organic matter in marine sediments. The Mississippi River, delivering more than 60% of the total dissolved and suspended materials from the conterminous US, dominates coastal and margin processes in the northern Gulf of Mexico. Draining approximately 41% of the conterminous US, the Mississippi and Atchafalaya river system deliver approximately 2×10 8 tons of suspended matter to the northern Gulf shelf each year. Unlike previous work, this study provides a comprehensive evaluation of sediment accumulation covering majority of the shelf (<150 m water depth) west of the Mississippi Delta from 92 cores collected throughout the last 15 years. This provides a unique and invaluable data set of the spatial and modern temporal variations of the sediment accumulation in this dynamic coastal environment. Three types of 210Pb profiles were observed from short cores (15-45 cm) collected on the shelf. Proximal to Southwest Pass in 30-100 m water depths, non-steady-state profiles were observed indicating rapid accumulation. Sediment accumulation rates in this area are typically >2.5 cm yr -1 (>1.8 g cm -2 yr -1). Kasten cores (˜200 cm in length) collected near Southwest Pass also indicate rapid deposition (>4 cm yr -1; >3 g cm -2 yr -1) on a longer timescale than that captured in the box cores. Near shore (<20 m), profiles are dominated by sediments reworked by waves and currents with no accumulation (the exception is an area just south of Barataria Bay where accumulation occurs). The remainder of the shelf (distal of Southwest Pass) is dominated by steady-state accumulation beneath a ˜10-cm thick mixed layer. Sediment accumulation rates for the distal shelf are typically <0.7 cm yr -1 (<0.5 g cm -2 yr -1). A preliminary sediment budget based on the distribution of 210Pb

  10. The role of an Arctic ice shelf in the climate of the MIS 6 glacial maximum (140 ka)

    NASA Astrophysics Data System (ADS)

    Colleoni, F.; Krinner, G.; Jakobsson, M.

    2010-12-01

    During the last decade, Arctic icebreaker and nuclear submarine expeditions have revealed large-scale Pleistocene glacial erosion on the Lomonosov Ridge, Chukchi Borderland and along the Northern Alaskan margin indicating that the glacial Arctic Ocean hosted large Antarctic-style ice shelves. Dating of sediment cores indicates that the most extensive and deepest ice grounding occurred during Marine Isotope Stage (MIS) 6. The precise extents of Pleistocene ice shelves in the Arctic Ocean are unknown but seem comparable to present existing Antarctic ice shelves. How would an Antarctic-style ice shelf in the MIS 6 Arctic Ocean influence the Northern Hemisphere climate? Could it have impacted on the surface mass balance (SMB) of the MIS 6 Eurasian ice sheet and contributed to its large southward extent? We use an Atmospheric General Circulation Model (AGCM) to investigate the climatic impacts of both a limited MIS 6 ice shelf covering portions of the Canada Basin and a fully ice shelf covered Arctic Ocean. The AGCM results show that both ice shelves cause a temperature cooling of about 3 °C over the Arctic Ocean mainly due to the combined effect of ice elevation and isolation from the underlying ocean heat fluxes stopping the snow cover from melting during summer. The calculated SMB of the ice shelves are positive. The ice front horizontal velocity of the Canada Basin ice shelf is estimated to ≈ 1 km yr -1 which is comparable to the recent measurements of the Ross ice shelf, Antarctica. The existence of a large continuous ice shelf covering the entire Arctic Ocean would imply a mean annual velocity of icebergs of ≈12 km yr -1 through the Fram Strait. Our modeling results show that both ice shelf configurations could be viable under the MIS 6 climatic conditions. However, the cooling caused by these ice shelves only affects the Arctic margins of the continental ice sheets and is not strong enough to significantly influence the surface mass balance of the entire MIS

  11. Sub-ice shelf circulation and basal melting of the Fimbul Ice Shelf

    NASA Astrophysics Data System (ADS)

    Nost, Ole Anders

    2010-05-01

    The Fimbul Ice Shelf is the largest of the ice shelves in Dronning Maud Land. Due to a narrow and some places non-existent continental shelf, the ice shelves in Dronning Maud Land are situated close to the Warm Deep Water. The Antarctic Slope Front separates the Warm Deep Water from the ice shelves and complicated exchange processes working across this front controls the melting of the Fimbul Ice Shelf and the other ice shelves in Dronning Maud Land. Here we will present analysis of unique data from the Dronning Maud Land coastal zone, as well as preliminary results from the 2009/2010 field work on the Fimbul Ice Shelf. In 2008 eight elephant seals equipped with CTD data loggers collected hydrographic data in the Dronning Maud Land coastal zone from February through October. Analysis of these data shows that overturning of the Antarctic Slope Front is the main process exchanging heat into the ice shelf cavities. This overturning together with an onshore surface Ekman flow leads to a seasonal cycle in the salinity of the coastal water masses, while glacial melting sea ice formation has little influence. During the 2009/2010 field season on the Fimbul Ice Shelf glaciological and oceanographic data were collected. We will show preliminary results of ice flow, ice thickness and basal melting measured using stake nets and phase sensitive radar. Oceanographic data were collected through three hot water drilled access holes in the ice shelf. These data show a water column with temperatures close to the surface freezing point over most of the water column. Relatively warmer water was observed near the bottom on one of the CTD stations. Maximum observed temperature is -1.57 °C. We compare the sub iceshelf hydrography with the hydrography observed by the elephant seals near the ice front in an attempt to reveal the sub ice shelf circulation. We also compare estimated melt rates from the oceanographic data with melt rates estimated with the phase sensitive radar and stake

  12. 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.

  13. Trophic state of benthic deep-sea ecosystems from two different continental margins off Iberia

    NASA Astrophysics Data System (ADS)

    Dell'Anno, A.; Pusceddu, A.; Corinaldesi, C.; Canals, M.; Heussner, S.; Thomsen, L.; Danovaro, R.

    2013-05-01

    The bioavailability of organic matter in benthic deep-sea ecosystems, commonly used to define their trophic state, can greatly influence key ecological processes such as biomass production and nutrient cycling. Here, we assess the trophic state of deep-sea sediments from open slopes and canyons of the Catalan (NW Mediterranean) and Portuguese (NE Atlantic) continental margins, offshore east and west Iberia, respectively, by using a biomimetic approach based on enzymatic digestion of protein and carbohydrate pools. Patterns of sediment trophic state were analyzed in relation to increasing water depth, including repeated samplings over a 3 yr period in the Catalan margin. Two out of the three sampling periods occurred a few months after dense shelf water cascading events. The benthic deep-sea ecosystems investigated in this study were characterized by high amounts of bioavailable organic matter when compared to other deep-sea sediments. Bioavailable organic matter and its nutritional value were significantly higher in the Portuguese margin than in the Catalan margin, thus reflecting differences in primary productivity of surface waters reported for the two regions. Similarly, sediments of the Catalan margin were characterized by significantly higher food quantity and quality in spring, when the phytoplankton bloom occurs in surface waters, than in summer and autumn. Differences in the benthic trophic state of canyons against open slopes were more evident in the Portuguese than in the Catalan margin. In both continental margins, bioavailable organic C concentrations did not vary or increase with increasing water depth. Overall, our findings suggest that the intensity of primary production processes along with the lateral transfer of organic particles, even amplified by episodic events, can have a role in controlling the quantity and distribution of bioavailable organic detritus and its nutritional value along these continental margin ecosystems.

  14. Along-shelf current variability on the Catalan inner-shelf (NW Mediterranean)

    USGS Publications Warehouse

    Grifoll, Manel; Aretxabaleta, Alfredo L.; Espino, Manuel; Warner, John C.

    2012-01-01

    We examine the circulation over the inner shelf of the Catalan Sea using observations of currents obtained from three ADCPs within the inner-shelf (24 and 50 m depth) during March-April 2011. The along-shelf current fluctuations during that period are mainly controlled by the local wind stress on short time scales and by remote pressure gradients on synoptic time scales. Different forcing mechanisms are involved in the along-shelf momentum balance. During storm conditions, wind stress, sea level gradients and the non-linear terms dominate the balance. During weak wind conditions, the momentum balance is controlled by the pressure gradient, while during periods of moderate wind in the presence of considerable stratification, the balance is established between the Coriolis and wind stress terms. Vertical variations of velocity are affected by the strong observed density gradient. The increased vertical shear is accompanied by the development of stratified conditions due to local heating when the wind is not able to counteract (and destroy) stratification. The occasional influence of the Besòs river plume is observed in time scales of hours to days in a limited area in front of Barcelona. The area affected by the plume depends on the vertical extend of the fresher layer, the fast river discharge peak, and the relaxation of cross-shore velocities after northeast storm events. This contribution provides a first interpretation of the inner-shelf dynamics in the Catalan Sea.

  15. Evolution of the central Catalan margin of the Valencia trough (western Mediterranean)

    NASA Astrophysics Data System (ADS)

    Bartrina, M. T.; Cabrera, L.; Jurado, M. J.; Guimerà, J.; Roca, E.

    1992-03-01

    The evolution of the central Catalan margin was conditioned by major fault activity and thermal processes generated by crustal thinning. The continental margin structure is characterised by major NE-SW-striking, southeast-dipping basement faults which have given rise to a graben, half-graben and horst system. These faults acted essentially as normal faults. No major Neogene strike-slip activity has affected the continental margin, although minor episodic variations from the dominant extensional regime to strike-slip tectonics cannot be discounted. Two major structural evolutionary stages took place at the margin, the transition from one stage to the other being gradual: (1) rifting stage (Late Oligocene? to early Burdigalian) related to the opening of the Valencia trough and from which originated both the Barcelona and Vallès-Penedès half-grabens; (2) thermal subsidence stage (late Burdigalian to present) generated by the subsequent crustal cooling and thinning. Fault activity during this stage was essentially concentrated in the major half-graben bounding faults. The sedimentary evolution of the central sector of the Catalan margin was controlled by the tectonic and crustal evolution as well as by the successive sea-level and paleoclimatic changes which affected the region. Drastic changes in the depositional systems took place during the transition from the rifting stage into the thermal subsidence stage. During Aquitanian-Langhian time—characterized by an overall trend of pulsating encroachment of marine conditions over the continental margin—a noticeable diversity of depositional systems (alluvial, lacustrine, coastal evaporitic, fan delta-bay-shelf, carbonate platform systems) developed. From the latest Serravallian up to the present, the depositional framework was less varied due to a general lowering of sea-level and to the final onlapping of most of older structural highs, which are now buried below the Miocene sediments. Two progradational

  16. West Indian Ocean variability and East African fish catch.

    PubMed

    Jury, M; McClanahan, T; Maina, J

    2010-08-01

    We describe marine climate variability off the east coast of Africa in the context of fish catch statistics for Tanzania and Kenya. The time series exhibits quasi-decadal cycles over the period 1964-2007. Fish catch is up when sea surface temperature (SST) and atmospheric humidity are below normal in the tropical West Indian Ocean. This pattern relates to an ocean Rossby wave in one phase of its east-west oscillation. Coastal-scale analyses indicate that northward currents and uplift on the shelf edge enhance productivity of East African shelf waters. Some of the changes are regulated by the south equatorial current that swings northward from Madagascar. The weather is drier and a salty layer develops in high catch years. While the large-scale West Indian Ocean has some impact on East African fish catch, coastal dynamics play a more significant role. Climatic changes are reviewed using 200 years of past and projected data. The observed warming trend continues to increase such that predicted SST may reach 30 degrees C by 2100 while SW monsoon winds gradually increase, according to a coupled general circulation model simulation with a gradual doubling of CO(2). PMID:20471674

  17. Unique Views of a Shattered Ice Shelf

    NASA Technical Reports Server (NTRS)

    2002-01-01

    Both single and multi-angle views of the breakup of the northern section of the Larsen B ice shelf are shown in this image pair from the Multi-angle Imaging SpectroRadiometer. The Larsen B ice shelf collapsed and broke away from the Antarctic Peninsula during February and March, 2002 -- a progression observed by Terra's Moderate-resolution Imaging SpectroRadiometer (MODIS) and analyzed at the University of Colorado's National Snow and Ice Data Center. The collapse is thought to have been accelerated by warm summer temperatures which caused meltwater to fill crevasses along the landward side of the Larsen shelf, leading to intensified pressures within the sheet