Rosso, A.; Vertino, A.; Di Geronimo, I.; Sanfilippo, R.; Sciuto, F.; Di Geronimo, R.; Violanti, D.; Corselli, C.; Taviani, M.; Mastrototaro, F.; Tursi, A.
Thanatofacies and the skeletonized components of the living facies, from which they originate have been studied from the Santa Maria di Leuca (SML) deep-water coral mound province. Faunal analysis was carried out by means of bottom sampling and underwater video observations, taking into account all benthic taxonomic groups, mostly corals, molluscs, serpulids, bryozoans, ostracods, foraminifers and barnacles, which permitted recognition of six different facies. These thanatofacies are easily distinguishable and appear to be largely corresponding and overlapping with related living facies. Some occur in mound areas, others in the intermound ones. They are as follows: the Framework-building Coral facies (FC), characterised by colonial corals, mostly Madrepora oculata; the Coral Rubble facies (CR), with proximal and distal aspects, characterised by large- to small-sized and densely to loosely packed coral fragments; the Solitary Coral facies (SC), dominated by different species depending on the availability and dimensions of hard exploitable surfaces; the Gryphus and Isidella facies (GI) in relatively coarse-grained bottoms; the Mollusc Mud facies (MM) and the Foraminifer Mud facies (FM) in comparably homogeneous silty bottoms. Facies distribution and spatial variability are discussed, in relation to hydrology and sea-floor topography. Furthermore, the SML facies are compared with living facies from the present-day Mediterranean and Pleistocene sediments of the same area. Data on bioclastic assemblages can serve for comparison with other recent aphotic, non-tropical carbonates.
Swift, D. J.; Fan, S.; Niedoroda, A. W.; Reed, C.; Borgeld, J. C.; Crockett, J. S.
Seismic records and cores from ONR's STRATAFORM program indicate that the Holocene deposits on the northern California shelf consist of a succession of back-stepping, storm-generated event beds, deposited as sediment undergoes cross-shelf dispersal from intermittently flooding river mouths. The beds are modified to varying degrees by secondary processes (gravity transport, bioturbation). Box core observations show that there is "mud line" on the shelf surface at approximately the 45 m isobath. Long cores show that within the 3-dimensional sediment body, nearshore sand beds intertongue with offshore mud beds beneath this line. However, numerical simulations suggest a more complex relationship. Instead of intertonguing, most event beds begin as sand beds in the nearshore sand deposit, pass through an interbedded zone, and enter the offshore mud deposit as mud beds. Event stratification is difficult to discern both seaward and landward of the transitional zone, mainly because the Cutoff Percentage has been exceeded in these areas (percent thickness of an upward-fining bed which must be preserved to observe grain size contrast). There are thus three facies bodies present, an Amalgamated Sand Facies on the inner shelf (sand beds on sand beds), an Interbedded Sand and Mud Facies on the central shelf, and an offshore Laminated or Bioturbated Mud Facies. Several other parameters are useful for defining these facies. The degree of condensation (extent to which each bed has cannibalized its predecessor) can be measured by the Reworking Ratio (ratio of mean annual resuspension depth to deposition per event). This value decreases seaward across the shelf to a minimum in the Interbedded Facies in response to decreasing wave energy flux into the sea floor. It then increases seaward across the outer shelf, as the decrease in available sediment becomes more important. The standard deviation of bed thickness is (in part) a measure of variation in storm intensity, and is a
Farquharson, Louise; Anthony, Katey Walter; Bigelow, Nancy; Edwards, Mary; Grosse, Guido
Thermokarst lakes develop as a result of the thaw and collapse of ice-rich, permanently frozen ground (permafrost). Of particular sedimentological importance are thermokarst lakes forming in late Pleistocene icy silt (yedoma), which dramatically alter the land surface by lowering surface elevation and redistributing upland sediment into lower basins. Our study provides the first description of yedoma thermokarst lake sedimentology based on the cross-basin sampling of an existing lake. We present lake sediment facies descriptions based on data from sediment cores from two thermokarst lakes of medium depth, Claudi and Jaeger (informal names), which formed in previously non thermokarst-affected upland yedoma on the northern Seward Peninsula, Alaska. We identify four prominent facies using sedimentological, biogeochemical, and macrofossil indicators: a massive silt lacking aquatic macrofossils and other aquatic indicators situated below a sub-lacustrine unconformity (Facies 1); two basal deposits: interbedded organic silt and chaotic silt (Facies 2-3); and a silt-rich mud (Facies 4). Facies 1 is interpreted as yedoma that has thawed during lake formation. Facies 3 formed adjacent to the margin due to thaw and collapse events from the lake shore. Material from Facies 3 was reworked by wave action to form Facies 2 in a medium energy margin environment. Facies 4 formed in a lower energy environment toward the lake basin center. This facies classification and description should enhance our ability (i) to interpret the spatial and temporal development of lakes and (ii) to reconstruct long-term patterns of landscape change.
Nelson, C.H.; Lee, H.L. ); Twichell, D.C.; Schwab, W.C. ); Kenyon, N.H. )
Cores from a Mississippi outer-fan depositional lobe demonstrate that sublobes at the distal edge contain a complex local network of channelized-turbidite beds of graded sand and debris-flow beds of chaotic silt. Off-lobe basin plains lack siliciclastic coarse-grained beds. The basin-plain mud facies exhibit low acoustic backscatter on SeaMARC IA sidescan sonar images, whereas high acoustic backscatter is characteristic of the lobe sand and silt facies. The depth of the first sand-silt layer correlates with relative backscatter intensity and stratigraphic age of the distal sublobes (i.e., shallowest sand = highest backscatter and youngest sublobe). The high proportion (> 50%) of chaotic silt compared to graded sand in the distal, outer-fan sublobes may be related to the unstable, muddy, canyon-wall source areas of the extensive Mississippi delta-fed basin slope. A predominance of chaotic silt in cores or outcrops from outer-fan lobes thus may predict similar settings for ancient fans.
Malmon, Daniel V.; Howard, Keith A.; House, P. Kyle; Lundstrom, Scott C.; Pearthree, Philip A.; Sarna-Wojcicki, Andrei M.; Wan, Elmira; Wahl, David B.
The Chemehuevi Formation forms a conspicuous, widespread, and correlative set of nonmarine sediments lining the valleys of the Colorado River and several of its larger tributaries in the Basin and Range geologic province. These sediments have been examined by geologists since J. S. Newberry visited the region in 1857 and are widely cited in the geologic literature; however their origin remains unresolved and their stratigraphic context has been confused by inconsistent nomenclature and by conflicting interpretations of their origin. This is one of the most prominent stratigraphic units along the river below the Grand Canyon, and the formation records an important event or set of events in the history of the Colorado River. Here we summarize what is known about these deposits throughout their range, present new stratigraphic, sedimentologic, topographic, and tephrochronologic data, and formally define them as a lithostratigraphic unit. The Chemehuevi Formation consists primarily of a bluff-forming mud facies, consisting of gypsum-bearing, horizontally bedded sand, silt, and clay, and a slope-forming sand facies containing poorly bedded, well sorted, quartz rich sand and scattered gravel. The sedimentary characteristics and fossil assemblages of the two facies types suggest that they were deposited in flood plain and channel environments, respectively. In addition to these two primary facies, we identify three other mappable facies in the formation: a thick-bedded rhythmite facies, now drowned by Lake Mead; a valley-margin facies containing abundant locally derived sediment; and several tributary facies consisting of mixed fluvial and lacustrine deposits in the lower parts of major tributary valleys. Observations from the subsurface and at outcrops near the elevation of the modern flood plain suggest that the formation also contains a regional basal gravel member. Surveys of numerous outcrops using high-precision GPS demonstrate that although the sand facies commonly
Nelson, C.H.; Baraza, J.; Maldonado, A.; Rodero, J.; Escutia, C.; Barber, J.H., Jr.
The late Quaternary pattern of sedimentary facies on the Spanish Gulf of Cadiz continental shelf results from an interaction between a number of controlling factors that are dominated by the Atlantic inflow currents flowing southeastward across the Cadiz shelf toward the Strait of Gibraltar. An inner shelf shoreface sand facies formed by shoaling waves is modified by the inflow currents to form a belt of sand dunes at 10-20 m that extends deeper and obliquely down paleo-valleys as a result of southward down-valley flow. A mid-shelf Holocene mud facies progrades offshore from river mouth sources, but Atlantic inflow currents cause extensive progradation along shelf toward the southeast. Increased inflow current speeds near the Strait of Gibraltar and the strong Mediterranean outflow currents there result in lack of mud deposition and development of a reworked transgressive sand dune facies across the entire southernmost shelf. At the outer shelf edge and underlying the mid-shelf mud and inner shelf sand facies is a late Pleistocene to Holocene transgressive sand sheet formed by the eustatic shoreline advance. The late Quaternary pattern of contourite deposits on the Spanish Gulf of Cadiz continental slope results from an interaction between linear diapiric ridges that are oblique to slope contours and the Mediterranean outflow current flowing northwestward parallel to the slope contours and down valleys between the ridges. Coincident with the northwestward decrease in outflow current speeds from the Strait there is the following northwestward gradation of contourite sediment facies: (1) upper slope sand to silt bed facies, (2) sand dune facies on the upstream mid-slope terrace, (3) large mud wave facies on the lower slope, (4) sediment drift facies banked against the diapiric ridges, and (5) valley facies between the ridges. The southeastern sediment drift facies closest to Gibraltar contains medium-fine sand beds interbedded with mud. The adjacent valley floor
Sloss, Craig R.; Jones, Brian G.; McClennen, Charles E.; de Carli, John; Price, David M.
The geomorphological evolution of the Holocene wave-dominated barrier estuary at Burrill Lake on the New South Wales coast, Australia, has been investigated using a combination of seismic stratigraphy and lithostratigraphic analysis of vibracores collected from the back-barrier estuarine environment. A combination of radiocarbon and aspartic acid racemisation-derived ages obtained on Holocene fossil molluscs, and the thermoluminescent signal in remnant the Last Interglacial barrier provides the chronological framework for this investigation. Results from this paper show that the barrier estuary occupies a relatively narrow (< 1.5 km wide) and shallow (< 40 m deep) incised bedrock valley formed during sea-level lowstands. Late Pleistocene sedimentary successions and remnants of the Last Interglacial barrier have been preserved within the incised valley axis and at the mouth of the incised valley. These sediments, deposited during the Last Interglacial sea-level highstand, were partially removed during the last glacial maximum. Overlying the antecedent Late Pleistocene landsurface is a near basin-wide basal marine sand deposited in response to rising sea level associated with the most recent post-glacial marine transgression, which inundated the shallow incised valley ca. 7800 years ago. More open marine conditions, with a diverse assemblage of estuarine and marine mollusc species, persisted until ca. 4500 years ago when the emerging Holocene barrier resulted in the development of a low-energy back-barrier lagoonal environment. A Late Holocene 1-2 m regression of sea level ca. 3000 years ago further restricted oceanic circulation, increased the rate of fluvial bay-head delta progradation and the extension of the back-barrier central basin mud facies. The model of barrier estuary evolution developed for Burrill Lake is consistent with recent research conducted in Lake Illawarra and St. Georges Basin and can be applied to other estuaries that have formed in relatively
Chang, Tae Soo; Ha, Hun Jun; Chun, Seung Soo
The Heuksan mud belt (hereafter HMB) is 20~50 km wide, ~200 km long, and ~50 m thick, having accumulated in the course of the Holocene transgression on the tide-dominated epicontinental shelf southwest of Korea. The internal architecture of the HMB is characterized by offshore prograding clinoforms. Of particular interest are the depositional processes responsible for this anomalously thick mud accumulation within a relatively short period of time. Tidal currents are important in the dispersal of mud in the HMB, although these alone cannot explain such an enormous mud deposit. In order to understand the formative processes of the HMB, a detailed sedimentary facies analysis, including high-resolution grain-size measurements, has been conducted on more than 30 short cores and three long drill cores recovered from the mud belt. Five major mud facies were identified. Of these, mud sequences showing a thickening-thinning trend of alternating silt and clay laminae suggestive of a tidal origin occur dominantly at inner to mid shelf locations. By contrast, internally structureless muds with sharp bases and no bioturbation, which are interpreted of representing fluid-mud deposits, are widespread at mid to outer shelf locations. Wave-generated mud ripples and storm beds on the inner shelf suggest that storm waves in winter resuspend previously deposited mud to form near-bed fluid-mud suspensions with resulting gravity-driven mud transport across the low-gradient outer shelf. This previously not recognized process is probably a major factor controlling depositional processes on the giant mud belt, enabling rapid accumulation and offshore progradation even during transgression, i.e., at times of sea-level rise.
Shevenell, A.; Ishman, S. E.; Domack, E. W.; Leventer, A.; Rosenheim, B. E.; Vadman, K. J.
Marine-terminating ice regulates the mass balance and stability of Antarctica's ice sheets and, ultimately, global sea level. However, predictions of future ice sheet response to climate change are limited by short observational time-series and the complexity of associated forcings and feedbacks. Here, we present new chronologic and multiproxy data from 7 marine sediment records from the outer to inner shelf down the axis of the Anvers Island Trough and into Palmer Deep that better constrain the timing and rate of ice retreat and improve understanding of the mechanisms driving deglaciation of this hypothesized paleo-ice stream. The retreat chronology is based on 16 CaCO3 (foraminifer and mollusk) and 45 ramped pyrolysis (acid-insoluble organic matter) AMS 14C dates. Radiocarbon dates from the laminated diatom ooze and mud facies immediately above the glacial diamicton indicate that deglaciation of the mid-Anvers Island Trough post-dated the Palmer Deep deglaciation by ~2000 uncorrected 14C years and that collapse of the mid-shelf system was rapid, if not instantaneous. The laminated facies, present throughout the suite of cores, is consistent with the calving bay reentrant model for deglaciation. However, this facies is limited (~15 couplets) on the mid-shelf and expanded (~127 couplets) on the inner-shelf, suggesting regional differences in calving bay duration. Planktic and benthic foraminifer isotopes, together with foraminifer and diatom assemblages, and biogeochemical data provide evidence for the presence of warmer nutrient-rich modified Circumpolar Deep Water throughout the Anvers Island Trough during deglaciation. The observed retreat complexity may relate to a hypothesized mid-shelf ice-dome system and requires additional geological and geophysical data to understand ice retreat patterns south and west of Palmer Deep. Such data will provide important new constraints for ice sheet models.
Li, Shunli; Yu, Xinghe; Li, Shengli; Giles, Katherine A.
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 %.
Herrera, D. A.; Ortega, B.; Caballero, M.; Lozano, S.; Pi, T.; Brown, E. T.
Chalco lake is located SE of the outskirts of Mexico City, at the central part of the Trans Mexican Volcanic Belt. Previous studies show the importance of this lacustrine sequence as an archive of paleoenvironmental and paleoclimatic changes. A set of five cores up to 122 m depth were drilled in the basin, in order to analyze the sedimentary record and to extent the previous knowledge of past environmental changes in central Mexico. As an initial step, in this work we present the identification and classification of sedimentary facies. Preliminary paleomagnetism analyses recognize the possible record of the Blake Event (ca. 120 kyr BP), and suggest that the sequence might span the last 240 kyr. In this case, variations in sedimentary facies could reflect the conditions of the MIS 1-7. The facies are mostly diatom ooze, carbonate mud, organic rich silt and volcaniclastic, both massive and laminated, and massive dark gray to reddish brown silt. From 1 to 8 m depth dominates the organic rich silt facies, which correlates with the MIS 1. Intercalations of reddish brown and grayish brown silt facies, between 8 to 60 m depth, indicate changes occurred during MIS 2 to 5d. Between 60-75 m depth the sequence is characterized by dark grayish silty clay facies, which possibly coincide with the MIS 5e. At 79 m depth (ca. 130 kyr BP) we found struvite (MgNH4PO4.6H2O), which may be related to dry conditions. The laminated diatom ooze facies dominates between 90 to 122 m depth and indicates rhythmic changes in the sediment deposition of the basin. The volcaniclastic facies is represented by lapilli and ash deposits in more than 100 individual tephra layers of both mafic and felsic composition. Some of them correspond to main volcanic eruptions, as the Upper Toluca Pumice (13,500 cal yr BP), from the Nevado de Toluca volcano and the Pómez con Andesita (17,700 cal yr BP) from the Popocatépetl volcano. The carbonate mud facies is composed of calcite and siderite, with frequent
Ryang, Woo Hun; Kang, Sol-Ip
The marco-tide, open-coast Gochang beach, located on the southwestern coast of Korea, was studied in terms of four season variations in surface sediment and sedimentary environment. During the four seasons of winter (February), spring (May), summer (August), and fall (November) in 2014 year, surface sediments of total 252 sites were sampled across three survey lines, consisting of 21 sites at 30 m intervals in each transverse line to the coast, respectively. The Gochang beach comprises the Dongho, Kwangseungri, and Myeongsasipri beaches from north to south. The pocket-type Dongho beach is mainly composed of very fine sands to very coarse sands, and the ratio of fine sand is the largest. The average of grain size is the coarsest in the winter. The spatial distribution of surface sediments shows a coast-parallel trend of fine and medium sands during the four seasons. During the winter, the upper tidal flat was dominated by medium sand, while the lower tidal flat was dominated by find sand. The surface sediments of the Kwangseungri beach are mainly composed of fine-grained sands, and the mean grain size is the coarsest in winter. Grain-size distribution shows a uni-mode pattern in the four seasons. Mud facies partly exist in spring and summer seasons, whereas it is rarely shown in autumn and winter. The spatial distribution of surface sediments shows a coast-parallel trend of fine to coarse sand during the four seasons. The Myeongsasipri beach is mainly composed of very fine sands to very coarse sands, and the ratio of fine sand is the largest. Grain-size distribution shows a weak bi-modal trend in the autumn and a uni-mode pattern in the spring, summer and winter. The mean grain size of the winter is the coarsest among those of four seasons. The spatial distribution of four seasons also shows a coast-parallel trend. During the four seasons of 2014 year in the Gochang beach, overall distribution of the grain sizes represents a fining trend from upper to lower tidal
Margalef, Olga; Cañellas-Boltà, Núria; Pla-Rabes, Sergi; Giralt, Santiago; Pueyo, Juan Jose; Joosten, Hans; Rull, Valentí; Buchaca, Teresa; Hernández, Armand; Valero-Garcés, Blas L.; Moreno, Ana; Sáez, Alberto
The Rano Aroi mire on Easter Island (also known as Rapa Nui; 27°09‧S, 109°27‧W, 430 m above sea level) provides a unique non-marine record in the central South Pacific Ocean for reconstructing Late Pleistocene environmental changes. The results of a multiproxy study on two cores from the center and margin of the Rano Aroi mire, including peat stratigraphy, facies analysis, elemental and isotope geochemistry on bulk organic matter, X-ray fluorescence (XRF) core scanning and macrofossil analysis, were used to infer past water levels and vegetation changes. The chronology was based on 18 14C AMS dates for the upper 8.7 m. The extrapolated age for the base of the sequence is 70 kyr, which implies that this record is the oldest paleolimnological record on Easter Island. The recovered Rano Aroi sequence consists of a radicel peat formed primarily from the remains of sedges, grasses and Polygonaceae that have accumulated since Marine Isotopic Stage (MIS) 4 (70 kyr BP) to the present. From 60 to 40 kyr BP (MIS 3), high precipitation/runoff events were recorded as organic mud facies with lighter δ13C, low C/N values and high Ti content, indicating higher detritic input to the mire. A gradual shift in δ13C bulk organic matter from - 14% to - 26%, recorded between 50 and 45 cal kyr BP, suggests a progressive change in local peat-forming vegetation from C4 to C3 plant types. Post-depositional Ca and Fe enrichment during sub-aerial peat exposure and very low sedimentation rates indicate lower water tables during Late MIS 3 (39-31 cal kyr BP). During MIS 2 (27.8-19 cal kyr BP), peat production rates were very low, most likely due to cold temperatures, as reconstructed from other Easter Island records during the Last Glacial Maximum (LGM). Geochemical and macrofossil evidence shows that peat accumulation reactivates at approximately 17.5 cal kyr BP, reaching the highest accumulation rates at 14 cal kyr BP. Peat accretion decreased from 5.0 to 2.5 cal kyr BP, coinciding
Bastos, Alex Cardoso; Costa Moscon, Daphnne Moraes; Carmo, Dannilo; Neto, José Antonio Baptista; da Silva Quaresma, Valéria
Sediment dynamics in wave-dominated coastal embayments are generally controlled by seasonal meteorological conditions, storms having a particularly strong influence. In the present study, such hydrodynamic processes and associated deposits have been investigated in a coastal embayment located along the southeast coast of Brazil, i.e. Espírito Santo Bay, in the winter (June/July) of 2008. The bay has undergone a series of human interventions that have altered the local hydrodynamic processes and, consequently, the sediment transport patterns. Facies distribution and sediment dynamics were examined by acoustic seabed mapping, sediment and core sampling, hydrodynamic measurements and sand transport modelling. The results show that sediment distribution can be described in terms of nearshore and offshore zones. The offshore bay sector is predominantly composed of "palimpsest" lithoclastic medium-coarse sands deposited in the course of the early Holocene transgression that peaked about 5,000 years ago. In the inner bay or nearshore zone (up to depths of 4-8 m), these older transgressive deposits are today overlain by a thin (up to 30-cm-thick) and partly patchy blanket of younger regressive fine sand/muddy fine sands. Both coarse- and fine-grained facies are being reworked during high-energy events (Hs>1.5 m) when fine sediment is resuspended, weak tide-induced drift currents causing the sand patches to be displaced. The coarser sediment, by contrast, is mobilized as bedload to produce wave ripples with spacings of up to 1.2 m. These processes lead to a sharp spatial delimitation between a fine sand/mud facies and a rippled coarse sand facies. The fine sand patches have a relief of about 20-30 cm and reveal a typical internal tempestite depositional sequence. Fair-weather wave-induced sediment transport (Hs<1 m), supported by weak tidal currents, seems to only affect the fine sediment facies. Sediment dynamics in Espírito Santo Bay is thus essentially controlled by
Switzer, A. D.; Yu, F.; Chen, B.; Zheng, Z.; Wang, D.
The Holocene evolution of the Qing'ao embayment, Nan'ao Island, southern China, is primarily the result of the interaction of tectonic activity, climate variation and changes in relative sea level. Characterizing the evolutionary history of the relatively small Qing'ao embayment during the Holocene will help improve our understanding of the driving mechanisms of coastal evolution in the area. To reconstruct the Holocene evolution history we analyzed the grain size, loss on ignition (LOI) and carbonate content of modern and core samples. Modern environmental analogs were examined in surface samples ranging from the coastal sand dunes through to offshore. The results of these modern samples suggest that dune sand (mean size of ~2.33Phi) are slightly finer than beach sand (mean size of 2.13Phi), and nearshore sediment is much coarser than offshore sediment (mean size of 5.90Phi). This modern analogs were then applied to 8 percussion cores from the Qing'ao embayment. A chronological framework obtained from 11 radiocarbon samples suggests that the embayment started to accept deposition since early Holocene, ~8500 cal. yr. BP. Three main phases of Holocene evolution were identified. A basin wide shell-rich sand sheet forms the basal Holocene facies and overlies clay rich presumably Pleistocene sediments or bedrock. This facies records an initial sedimentation phase associated with the early Holocene transgression into the embayment (~8500-6000 cal. yr. BP). The basal facies grades upward to a mixed sandy-mud facies which includes lagoonal clayey-silts, flood tide delta sands and records an estuarine phase lasting from ~6000-1000 cal. yr. BP that appears coincident with falling regional sea levels. Coincident with the estuarine phase is a period of coastal dune building recorded as yet undated massive sands that are found in the upper fill. Toward the end of the estuarine phase it is apparent that dune migration has restricted the lagoon entrance and that this was
Efflorescent crusts at the Eagle Borax spring in Death Valley, California, contain an array of rare Mg and K borate minerals, several of which are only known from one or two other localities. The Mg- and/or K-bearing borates include aristarainite, hydroboracite, kaliborite, mcallisterite, pinnoite, rivadavite, and santite. Ulexite and probertite also occur in the area, although their distribution is different from that of the Mg and K borates. Other evaporite minerals in the spring vicinity include halite, thenardite, eugsterite, gypsum-anhydrite, hexahydrite, and bloedite. Whereas the first five of these minerals are found throughout Death Valley, the last two Mg sulfates are more restricted in occurrence and are indicative of Mg-enriched ground water. Mineral associations observed at the Eagle Borax spring, and at many other borate deposits worldwide, can be explained by the chemical fractionation of borate-precipitating waters during the course of evaporative concentration. The Mg sulfate and Mg borate minerals in the Eagle Borax efflorescent crusts point to the fractionation of Ca by the operation of a chemical divide involving Ca carbonate and Na-Ca borate precipitation in the subsurface sediments. At many other borate mining localities, the occurrence of ulexite in both Na borate (borax-kernite) and Ca borate (ulexite-colemanite) deposits similarly reflects ulexite's coprecipitation with Ca carbonate at an early concentration stage. Such ulexite may perhaps be converted to colemanite by later reaction with the coexisting Ca carbonate - the latter providing the additional Ca2+ ions needed for the conversion. Mg and Ca-Mg borates are the expected late-stage concentration products of waters forming ulexite-colemanite deposits and are therefore most likely to occur in the marginal zones or nearby mud facies of ulexite-colemanite orebodies. Under some circumstances, Mg and Ca-Mg borates might provide a useful prospecting guide for ulexite-colemanite deposits
Ryang, Woo Hun; Kang, Sol Ip
The Gochang Beach, located on the southwestern coast of Korea, was studied in terms of four season variations of surface sediment and sedimentary environment. The Gochang Beach consists of the Dongho, Kwangseungri, Myeongsasipri beaches from north to south. During the four seasons of spring (May), summer (August), and fall (November), and winter (February), surface sediments of 135 sites were sampled across nine survey lines (15 sites in each survey line), respectively. The pocket-type Dongho Beach is mainly composed of fine to coarse sands, and the ratio of fine sand is the largest. The average of grain size is the coarsest in the summer. The spatial distribution of surface sediments shows a coast-parallel band of fine and medium sands during three seasons of spring, fall, and winter, whereas medium sands dominated in the northern part of the study area during the summer. These results suggest that a tide is more effective than a wave in the surface sediments of the Dongho Beach during the summer. The surface sediments of the Kwangseungri Beach are mainly composed of fine-grained sands, and the mean grain size is the coarsest in winter. Mud facies partly exists in summer, whereas it is nearly absent in winter. The spatial distribution of surface sediments shows a coast-parallel band of fine and medium sands during spring, fall, and winter. In the northern part, the study area is dominated by fine sands during summer, whereas by coarse sands during winter. These results are interpreted that tide is more effective than wave on the surface sediment distribution of the Kwangseungri Beach during summer season. The open-coast Myeongsasipri Beach is mainly composed of fine to medium sand, the distribution of which shows a coast-parallel trend. Grain-size distribution shows a bi-modal trend in the summer and winter and a uni-mode in the spring and fall. Grain size of the winter is the coarsest among those of four seasons. During the winter, the upper tidal flat was
Hernández, Armand; Doolittle, Sara; Bao, Roberto; Trigo, Ricardo M.; Rubio-Inglés, Maria J.; Sánchez-López, Guiomar; Vázquez-Loureiro, David; Gonçalves, Vitor; Raposeiro, Pedro M.; Marques, Helena S.; Sáez, Alberto; Giralt, Santiago
four main climatic (and NAO) stages since 600 cal years BP have been established. The first stage (1350-1460 AD) correspond to dark-brown to black mud facies with high values in lake productivity and moderately shallow waters, which however represent the deepest condition during the studied interval. Mineralogical composition and high organic matter content also suggest a humid climate with abundant precipitations that might be related to a predominantly NAO- phase. The second stage, spanning between 1460 and 1800 AD, is represented by a similar facies presented by the previous phase. This stage is however characterized by a transitional period from a wet to more arid climate, probably related to a change in NAO conditions (from NAO- to NAO+), with lower values of lake productivity and lake level than the previous stage. From 1800 AD until 1930 AD (third stage) banded brown to pale-brown silty and muddy facies were deposited. During this stage the lowest lake water table and productivity in the whole sequence were reached suggesting a predominantly NAO+ phase in concordance with instrumental NAO records. However, heavy rainfall catastrophic events are recorded in the sequence as very coarse (gravely) alluvial intervals that may be related with intense NAO- negative winters or alternatively to autumn Tropical Storms that can reach the archipelago. Finally, the uppermost interval of the sequence is composed by brown to ochre massive mud. A lake level rise and a progressive increase in the productivity suggest a relatively humid fourth stage from AD 1930 until present, a period characterized with more NAO- values.