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1.
The Early Pleistocene is an important interval in the Quaternary period as a time not only of climatic and environmental change, but also of key events in human evolution. However, knowledge of this period in northwest Europe is hampered by the limited extent of deposits of this age. Westbury Cave in the Mendip Hills of Somerset preserves an understudied example of fossil-bearing Early Pleistocene sediments, with rare potential to inform our understanding of British Early Pleistocene stratigraphy and landscape evolution outside the East Anglian Crag Basin. This study identifies the processes responsible for deposition of the Early Pleistocene Siliceous Member in Westbury Cave, thereby aiding taphonomic and palaeoenvironmental interpretations of associated fossil assemblages. New excavations revealed over ten metres of Siliceous Member stratigraphy, dominated by fine-grained silts/clays with interbedded sands and gravels, interpreted as being deposited within a subterranean lake or flooded conduit with fluvial input. All sediments sampled were reversely magnetised and are assigned to the Matuyama Reversed Chron. Lithological analysis of gravel clasts revealed variable components of durable non-local and non-durable local clasts. Gravels containing the latter are interpreted as distal talus slope deposits, and those lacking non-durable lithologies as stream or flood deposits. However, it remains unclear from available data whether apparently non-local clasts were sourced from long distance or stem from a more local, now denuded catchment. Siliceous Member bio- and magnetostratigraphy suggest that deposition occurred late in the Early Pleistocene, a period apparently otherwise unrepresented in the UK.  相似文献   

2.
A small but stratigraphically significant exposure of Quaternary sandy sediments (Widdington Sands) was observed and recorded in the early 1970s in northwest Essex. These data are here re-examined and re-evaluated, yielding new insights into early proto-Thames aggradation following the marine recession of the Norwich Crag Formation (MIS 74–71, about 2 Ma). As the proto-Thames trajectory shifted south eastwards, a period of landscape stability ensued in the early Middle Pleistocene (MIS 19–13) during which the Valley Farm Soil was formed. This and the succeeding Barham Soil can be recognised in the stratigraphy, the second palaeosol heralding the arrival of glaciation in the Anglian Stage (MIS 12, 480–420 ka). These pedogenic signatures are enclosed within palaeokarstic features in the form of infilled sinkhole pipes. A large doline has functioned as a sediment trap preserving pre-truncation structures including reverse ring faults. These confirm basal support removal leading to upward migration of a dissolution cavity and roof collapse within the sinkhole pipe. The process and timing of subsidence can thus be defined more clearly than for similar features found in comparable Kesgrave aggradations of the Middle Thames. The likely glacitectonic origin of the planar sub-till surface is examined and discussed. Dating of Early Pleistocene fluvial activity is constrained by estimating the height of a former terrace surface whose elevation points to a correlation with the higher Stoke Row Member (MIS 64, 1.8 Ma), suggesting the oldest known proto-Thames activity within southern East Anglia.  相似文献   

3.
Joseph Bonaparte Gulf is a large embayment on the northwestern continental margin of Australia. It is approximately 300 km east‐west and 120 km north‐south with a broad continental shelf to seaward. Maximum width from the southernmost shore of Joseph Bonaparte Gulf to the edge of the continental shelf is 560 km. Several large rivers enter the gulf along its shores. The climate is monsoonal, sub‐humid, and cyclone‐prone during the December‐March wet season. A bedrock high (Sahul Rise) rims the shelf margin. The sediments within the gulf are carbonates to seaward, grading into clastics inshore. A seaward‐thinning wedge of highstand muds dominates the sediments of the inner shelf of Joseph Bonaparte Gulf. Mud banks up to 15m thick have developed inshore. Coarse‐grained sand ridges up to 15 m high are found off the mouth of the Ord River. These overlie an Upper Pleistocene transgressive lag of mixed carbonate and gravelly siliciclastic sand. Four drowned strandlines are present on the inner shelf at depths of 20, 25, 28 and 30 m below datum. These are interpreted as having formed during stillstands in the Late Pleistocene transgression. Older strandlines at great depths are inferred as having formed during the fall in sea‐level following the last highstand. For the most part the Upper Pleistocene‐Holocene marine sediments overlie an erosion surface cut into older Pleistocene sediments. Incised valleys cut into this erosion surface are up to 5 km wide and have a relief of at least 20 m. The largest valley is that cut by the Ord River. Upper Pleistocene sediments deposited in the incised valleys include interpreted lowstand fluvial gravels, early transgressive channel sands and floodplain silts, and late transgressive estuarine sands and gravels. Older Pleistocene sediments are inferred to have been deposited before and during the 120 ka highstand (isotope stage 5). They consist of sandy calcarenites deposited in high‐energy tide‐dominated shelf environments. Still older shelf and valley‐fill sediments underlie these. The contrast between the Holocene muddy clastic sediments and the sandy carbonates deposited by the 120 ka highstand suggests that either the climate was more arid in the past, with less fluvial transport, or that mud was more effectively trapped in estuaries, allowing development of carbonate depositional environments inshore.  相似文献   

4.
Quaternary sands and gravels form important, yet often highly heterogeneous economic deposits. Detailed 3-D analysis of the sedimentary structure and stratigraphy of these deposits allows for an accurate estimation of exploitable material. This paper presents a case study in SW Germany reconstructing the 3-D distribution of glacial sediments based on a high-resolution, process-orientated sedimentary facies classification and lithostratigraphy integrated within the geo-modelling package gOcad. Situated along the maximal ice-extent of the Rhine glacier during the last glaciation, the study area is characterised by a morphologically prominent terminal moraine and its associated sandur, which form the stratigraphically youngest sediments of a glacial basin, partially exposed in two gravel pits. These outcrops helped to reconstruct the complex sedimentary architecture of the northern part of the glacial basin. The regional analysis is based on core data, as well as geoelectric and geomagnetic surveys. All data were integrated into km-scale, high-resolution geological 3-D models, depicting the large-scale dynamics of multiple glacier advances and retreats during the Early to Late Pleistocene. These models allow quantifying the thickness and volume distribution of exploitable sandy gravel and help to evaluate the occurrence of non-exploitable interbedded diamicton horizons. The high quality and quantity of gravels particularly north of the terminal moraine classifies the investigated area as very prospective for raw materials exploitation in the future.  相似文献   

5.
Many bedrock-confined fjord valleys along the Norwegian coast contain thick accumulations of fine-grained sediments that were deposited during and after the last deglaciation. The deposits gradually emerged above sea level due to glacioisostatic uplift, and fjord marine sedimentation was gradually followed by shallow marine and fluvial processes. During emergence terraces and river-cut slopes were formed in the valleys. Subsequent leaching of salt ions from the pore water in the marine deposits by groundwater has led to the development of quick clay. The deposits are subject to river erosion and destructive landslides involving quick clay. Most slides are of prehistoric age. Others are known from modern observations as well as from historic records.Landforms such as distinct slide scars or the hummocky terrain of slide deposits may be strongly modified by secondary processes. In addition, deposits from the most liquid part of quick clay slides may have planar surfaces. Clay-slide deposits on a fluvial or deltaic terrace, therefore, are not always easily recognized from morphology, and only exposures may reveal their internal structures and allow them to be distinguished from overbank flood sediments. Detailed sedimentological work shows that slide deposits in such setting consist of distinct facies containing reworked marine sediments. We propose three facies successions of clay-slide deposits that form a continuum. The dominant components of these succession types are: slightly deformed blocks of laminated clay and silt (A), highly deformed clay and silt with gravel clasts (B) and massive to stratified clay and silt with scattered clasts (C). We suggest that in many cases a basal muddy diamicton is a characteristic, and possibly diagnostic feature. Processes and depositional models are interpreted from the different succession types. The results may be relevant for identifying clay-slide deposits elsewhere and may be useful during general mapping of fjord marine deposits and characterization of slide-prone areas as well as during identification of prehistoric slides.  相似文献   

6.
The Quaternary sediments of the West Sussex Coastal Plain have produced a wide range of floral, faunal and archaeological remains. These sediments consist of marine sands and gravels exhibiting transgressive and regressive trends which occur from present day sea-level to c. +43.0 m O.D. and are overlain by terrestrial silts and cold climate periglacial sediments. At the present day coastline, channel fill deposits occur below modern beach levels. New field observations, coupled with a re-investigation of old sites and literature, suggest that five discrete high stands of sea-level may be preserved in the area of the West Sussex Coastal Plain. Age estimates for these deposits suggest that they span large parts of the Middle and Upper Pleistocene (Oxygen Isotope Stages 13 to 5). Conformable relationships between many of the marine and terrestrial sediments suggest that the potential exists within the area to correlate the marine and terrestrial Quaternary stratigraphic records. In addition sediments associated with two of these high sea-level stands are associated with extensive buried landsurfaces covering large areas of the coastal plain. At some locations these intact landsurfaces are associated with evidence for human activity and represent stratigraphic and cultural resources of international importance.  相似文献   

7.
Two kinds of buried structures are described from Dzirżenin, north-east of Warsaw, where they occur within a glaciofluvial landform: (1) narrow till ridges, showing vertically oriented structures, excavated from stratified gravel and sands; and (2) a narrow vertical zone of massive gravelly/sandy material, involving vertically oriented lens-like layers composed of massive sand with pebbles, or of diamicton. The gravelly/sandy zone is also closely surrounded by stratified glaciofluvial sediments. In spite of their vertical position and internal deformation, the till ridges and gravelly/sandy zone show non-tectonic contacts with the surrounding, stratified, undisturbed sediments. The glaciofluvial sediments that occur immediately next to the structures under discussion are characterized by the occurrence of comparatively coarse material and interbeddings of diamicton, which wedge out away from these structures. The gravelly/sandy zone separates different kinds of water-laid deposits. The buried structures are interpreted as former debris-laden bands, thrust upwards within the frontal part of the ice sheet and then transformed into still-frozen debris ridges projecting over the already dead ice. Further melting of the decaying ice resulted in abundant glaciofluvial sedimentation, and the debris ridges also supplied material for the deposition of the neighbouring stratified deposits. One of the ridges separated different glaciofluvial environments. The glaciofluvial sediments completely buried the ice-cemented ridges, which were finally transformed by a melting-out process into the till ridges and the gravelly/sandy zone. The former are interpreted as having been transformed from upturned debris-laden bands with a high concentration of debris or from the bands composed of frozen-up sediment slabs. The gravelly/sandy zone is interpreted as having (most probably) been deposited from upturned bands characterized by a lesser concentration of debris.  相似文献   

8.
The Kachchh Mainland Fault (KMF) is a major E–W trending seismically active fault of the Kachchh palaeorift basin whose neotectonic evolution is not known. The present study deals with the eastern part of the KMF zone where the fault is morphologically expressed as steep north facing scarps and is divisible into five morphotectonic segments. The Quaternary sediments occurring in a narrow zone between the E–W trending KMF scarps and the flat Banni plain to the north are documented. The sediments show considerable heterogeneity vertically as well as laterally along the KMF zone. (The Quaternary sediments for a northward sloping and are exposed along the north flowing streams which also show rapid decrease in the depth of incision in the same direction.) The deposits, in general, comprise coarse as well as finer gravelly deposits, sands and aeolian and fluvial miliolites. The Quaternary sediments of the KMF zone show three major aggradation phases. The oldest phase includes the colluvio-fluvial sediments occurring below the miliolites. These deposits are strikingly coarse grained and show poor sorting and large angular clasts of Mesozoic rocks. The sedimentary characteristics indicate deposition, dominantly by debris flows and sediment gravity flows, as small coalescing alluvial fans in front of the scarps. These deposits suggest pre-miliolite neotectonic activity along the KMF. The second aggradation phase comprises aeolian miliolites and fluvially reworked miliolites that have been previously dated from middle to late Pleistocene. The youngest phase is the post-miliolite phase that includes all deposits younger than miliolite. These are represented by comparatively finer sandy gravels, gravelly sands and sand. The sediment characteristics suggest deposition in shallow braided stream channels under reduced level of neotectonic activity along the KMF during post-miliolite time evidenced by vertical dips of miliolites and tilting of gravels near the scarps. The tectonically controlled incision and dissection of the Quaternary deposits is the result of neotectonic activity that continues at present day. The overall nature, sedimentary characteristics and geomorphic setting of the sediments suggest that the KMF remained neotectonically active throughout the Quaternary period.  相似文献   

9.
Erratic clasts with a mass of up to 15 kg are described from preglacial shallow marine and coastal deposits (Wroxham Crag Formation) in northeast Norfolk. Detailed examination of their petrology has enabled them to be provenanced to northern Britain and southern Norway. Their clustered occurrence in coastal sediments in Norfolk is believed to be the product of ice-rafting from glacier incursions into the North Sea from eastern Scotland and southern Norway, and their subsequent grounding and melting within coastal areas of what is now north Norfolk. The precise timing of these restricted glaciations is difficult to determine. However, the relationship of the erratics to the biostratigraphic record and the first major expansion of ice into the North Sea suggest these events occurred during at least one glaciation between the late Early Pleistocene and early Middle Pleistocene (c. 1.1–0.6 Ma). In contrast to the late Middle (Anglian) and Late Pleistocene (Last Glacial Maximum) glaciations, where the North Sea was largely devoid of extensive marine conditions, the presence of far-travelled ice-rafted materials implies that earlier cold stage sea-levels were considerably higher.  相似文献   

10.
In this paper we present Quaternary stratigraphy of the area around Chennai based on archaeological findings on the ferricrete surface, geomorphological observations supplemented by radiocarbon dating. The coastal landscape around Chennai, Tamil Nadu, has preserved ferruginised boulder gravel deposits, ferricretes and fluvial deposits of varying thickness. The area studied is approximately 150 km east to west and 180 km north to south with a broad continental shelf towards the seaward. Several rivers enter the Bay of Bengal along its shores like the Koratallaiyar, Cooum and the Adyar. Precambrian charnockite and Upper Gondwana sandstone and shale bedrock rim the shelf margin. For the most part, the Upper Pleistocene-Holocene fluvial sediments overlie an erosion surface that has cut into older Pleistocene sediments and ferricrete surface. Incised valleys that cut into this erosion surface are up to 5–6 km wide and have a relief of at least 30 m. The largest valley is that cut by the Koratallaiyar River. Holocene sediments deposited in the incised valleys include fluvial gravels, early transgressive channel sands and floodplain silts. Older Pleistocene sediments are deposited before and during the 120-ka high stand (Marine isotope stage 5). They consist of ferricretes and ferricrete gravel formed in nearshore humid environments. Muddy and sandy clastic sediments dated to the ca. 5 ka highstand suggest that the climate was semi arid at this time with less fluvial transport. The coarsening up sequence indicates deposition by high intensity channel processes. Pedogenic mottled, clayey silt unit represents an important tectonic event when the channel was temporarily drained and sediment were sub aerially exposed. Uplift of the region has caused the local rivers to incise into the landscape, forming degradation terraces.  相似文献   

11.
Pleistocene sediments at Leet Hill, southern Norfolk are examined in terms of their sedimentary structures, palaeocurrent indicators, clast and heavy mineral lithology and litho- and morphostratigraphic position. Colour of the quartzite and vein-quartz clasts is used to differentiate the Bytham and the Kesgrave sands and gravels, with the Bytham sands and gravels having a significantly higher proportion of coloured material. The Kirby Cane sands and gravels are the lower sedimentary unit and were deposited by the Bytham river, which drained a catchment extending into central England. At Leet Hill, erosion of the Kesgrave Sands and Gravels by the Bytham river has given the Kirby Cane sands and gravels a distinctive lithological assemblage. Trace clast lithologies suggest that the Kesgrave Sands and Gravels in the region of Leet Hill were deposited in a coastal location with an input from northern sources as well as southern and Welsh sources diagnostic of the Thames catchment. The glaciofluvial Leet Hill Sands and Gravels were deposited by outwash from the Anglian Scandinavian ice sheet. Initially the flow direction of the outwash was determined by the Bytham river valley, but this changed to a southerly direction once the valley had been infilled. This paper provides the first indication of the location of the boundary (Early Pleistocene coastline) between the fluvial Kesgrave Sands and Gravels and the marine equivalent reworked by coastal processes, and demonstrates the way the pre-glacial relief initially controlled patterns of glaciofluvial sedimentation during the early part of the Anglian glaciation. Copyright © 1999 John Wiley & Sons, Ltd.  相似文献   

12.
The marine geology of Port Phillip is described in detail, based on data from seismic profiling, vibrocoring and grab sampling. Three major unconsolidated facies can be distinguished: sands and muddy sands peripheral to the present coastline, muds covering the major central region, and channel fills of muds and sands. The first two facies units result from an increase in wave sorting towards the coast, reworking of Tertiary and Quaternary sandstone outcrops around the coast, and a dominant mud supply from river sources into the central area. The distribution and thicknesses of the unconsolidated facies have been augmented by a shallow‐seismic program that reveals the thicknesses of the modern sediments overlying an older surface comprised of consolidated clays and sandy clays of Pleistocene or older age. In central Port Phillip, muds and sands up to 27 m‐thick have infilled Pleistocene channels cut into underlying consolidated units. Sediments immediately above the channel bases show characteristic seismic patterns of fluvial deposition. The presence of peat deposits together with gas phenomena in the water column suggest organic breakdown of channel‐fill deposits is releasing methane into the bay waters. Outside the channel areas, carbon‐14 dating indicates that the unconsolidated sediments largely post‐date the last glaciation sea‐level rise (<6500 a BP), with an early Holocene period of rapid deposition, similar to other Australian estuaries. Stratigraphic and depositional considerations suggest that the undated channel‐fill sequences correlate with the formation of cemented quartz‐carbonate aeolianite and barrier sands on the Nepean Peninsula at the southern end of Port Phillip. Previous thermoluminescence dating of the aeolianites suggests that channel‐fill sequences B, C and D may have been deposited as fluvial and estuarine infills over the period between 57 and 8 ka. The eroded surface on the underlying consolidated sediments is probably the same 118 ka age as a disconformity within the Nepean aeolianites. Further estuarine and aeolianite facies extend below the disconformity to 60 m below sea‐level, and may extend the Quaternary depositional record to ca 810 ka. Pliocene and older Tertiary units progressively subcrop below the Quaternary northwards up the bay.  相似文献   

13.
The Norwich Crag of north-eastern Suffolk is mainly composed of near-shore sands representing several sequences (cycles of transgression and regression). It is difficult to separate out the different sequences but the final sequence here is well known for the localised development of beds of flint gravel that have been interpreted as the in-situ remnants of prograding beaches. While a review of the evidence supports the involvement of this sedimentary environment in the overall processes, the evidence shows that virtually only gravels associated with rip-channels represent in-situ beach gravels and that thicker gravel beds are the infill of much larger channels. From consideration of the characteristics of the large channels it is concluded that these large channels were tidal-inlets between prograding barrier islands and that the gravels were derived from the adjacent up-drift beach faces of the barrier-islands.  相似文献   

14.
HARRISON  & YAIR 《Sedimentology》1998,45(3):507-518
The interdunal areas in the Nizzana linear sand dune field contain both sandy and silty sediments. A series of trenches was excavated across the interdunal corridor exposing stacked sequences of silty and sandy units which are locally restricted to palaeodepressions. The silty units contain fining upward sequences and are interpreted as overbank deposits from the Nahal Nizzana. Thermoluminescence dating and identification of buried palaeosols indicates that the silt and clay layers were deposited over a period of several thousands of years in the late Pleistocene. The sands between the silt layers have been fluvially reworked and are not primary aeolian deposits. The stacked sequences of fluvial deposits indicate that the palaeodepressions persisted in the landscape for a significant time attesting to long-term stability of the interdunal areas. It also suggests that the linear dunes themselves have not moved laterally during this time despite climatic changes and devegetation. Since the end of the late Pleistocene the Nahal Nizzana has downcut and overbank deposition no longer occurs within the interdunal corridors. The playa deposits today are positive relief features indicating that topographic inversion has occurred and that the interdunal areas are geomorphically active.  相似文献   

15.
Recent geological mapping on the Isle of Wight by the British Geological Survey has shown the ‘Plateau Gravel’ to be a mixture of fluvial, solifluction, pedogenic and marine deposits ranging from pre-Anglian to Holocene age. As part of the resurvey of the island, several new exposures of the ‘Plateau Gravel’ between Newport and Downend were examined. A working gravel pit on St George's Down, near Newport, revealed a succession of flint gravels with an inter-bedded sequence of laminated silts. An upper in situ succession of pre-Anglian fluvial gravels caps the plateau, but a second, probably younger suite of gravel-rich sediments is exposed in a quarry on a topographically lower spur. These overlie in situ Clay-with-flints resting on Upper Cretaceous Chalk. These lower sediments are well exposed and display a complex stratigraphy. They consist predominantly of flint gravel, but include a dipping succession of laminated silts and palaeosols preserved in a hollow or small channel feature, intercalated between two distinct soliflucted cold-stage gravel sheets. Palynological and pedological evidence analysis suggests that these laminated silts and sands were deposited under a temperate climate but with frequent episodes of disruption caused by mass-movement and possibly freeze-thaw. The age of these laminated sediments are not known with any certainty but are likely to date to a temperate interval within the Late Pleistocene. The top of the laminated unit is cut by a heavily cryoturbated horizon presumed to be of Devensian age.  相似文献   

16.
Sedimentary rocks of the Solomon Islands-Bougainville Arc are described in terms of nine widespread facies. Four facies associations are recognised by grouping facies which developed in broadly similar sedimentary environments.A marine pelagic association of Early Cretaceous to Miocene rocks comprises three facies. Facies Al: Early Cretaceous siliceous mudstone, found only on Malaita, is interpreted as deep marine siliceous ooze. Facies A2: Early Cretaceous to Eocene limestone with chert, overlies the siliceous mudstone facies, and is widespread in the central and eastern Solomons. It represents lithified calcareous ooze. Facies A3: Oligocene to Miocene calcisiltite with thin tuffaceous beds, overlies Facies A2 in most areas, and also occurs in the western Solomons. This represents similar, but less lithified calcareous ooze, and the deposits of periodic andesitic volcanism.An open marine detrital association of Oligocene to Recent age occurs throughout the Solomons. This comprises two facies. Facies B1 is variably calcareous siltstone, of hemipelagic origin; and Facies B2 consists of volcanogenic clastic deposits, laid down from submarine mass flows.A third association, of shallow marine carbonates, ranges in age from Late Oligocene to Recent. Facies C1 is biohermal limestone, and Facies C2 is biostromal calcarenite.The fourth association comprises areally restricted Pliocene to Recent paralic detrital deposits. Facies D1 includes nearshore clastic sediments, and Facies D2 comprises alluvial sands and gravels.Pre-Oligocene pelagic sediments were deposited contemporaneously with, and subsequent to, the extrusion of oceanic tholeiite. Island arc volcanism commenced along the length of the Solomons during the Oligocene, and greatly influenced sedimentation. Thick volcaniclastic sequences were deposited from submarine mass flows, and shallow marine carbonates accumulated locally. Fine grained graded tuffaceous beds within the marine pelagic association are interpreted as products of this volcanism, suggesting that the Santa Isabel-Malaita-Ulawa area, where these beds are prevalent, was relatively close to the main Solomons chain at this time. A subduction zone may have dipped towards the northeast beneath this volcanic chain. Pliocene to Pleistocene calcalkaline volcanism and tectonism resulted in the emergence of all large islands and led to deposition of clastic and carbonate facies in paralic, shallow and deep marine environments.  相似文献   

17.
The stratigraphy and sedimentology of the glacial deposits exposed along the coast of east Yorkshire are reviewed. Critical sections at Filey Brigg, Barmston and Skipsea are examined to reassess the stratigraphy of Devensian Dimlington Stadial glacial deposits in the light of recent developments in glacial sedimentology. Sedimentary and glaciotectonic structures studied in the field and by using scanning electron microscopy are emphasised. Two hypotheses are considered for the genesis of the interbedded diamictons and stratified sediments. The first involves the deposition of lodgement till and/or deformation till followed by meltout till, which was overridden to produce more deformation till, reflecting periods of ice stagnation punctuated by glacier thickening. The second hypothesis, which is favoured on the basis of field evidence and micromorphology, involves the vertical accretion of a deforming till layer associated with cavity/channel or tunnel valley fills, beneath active ice. At Barmston the upper part of the diamicton contains elongate pendant structures containing gravels, indicating that the diamicton was saturated and able to flow. The diamictons, therefore, represent a complex sequence of tills deposited and deformed by active ice during the Dimlington Stadial. Previously published stratigraphical schemes involving classifications of multiple tills in east Yorkshire should be simplified and it is more appropriate to assign these to a single formation, the Skipsea Till Formation. Rhythmic glaciolacustrine and proglacial glaciofluvial sediments overlie the tills at Barmston and Skipsea. These were deposited in sag basins during deglaciation as the tills settled and deformed under thickening sediment and as buried ice melted out. Extensive sands and gravels cap the succession and were deposited on a sandur during the later stages of deglaciation.  相似文献   

18.
This paper provides sedimentological and morphological data from an investigation of the Late Devensian glacigenic deposits along the Tyne valley, northeast England. The area lies in the central sector of the British-Irish Ice Sheet, with the lowlands influenced by both the Tyne Gap and Tweed-Cheviot ice streams. The sequences here provide insights into the existence of complex, multi-phase activity within the British-Irish Ice Sheet. Field mapping of the area reveals kamiform topography in the Tyne lowlands and lower South Tyne valley, whilst the mid Tyne is characterised by high-level sandur terraces. Inset below the glacial features are river terraces. The sedimentary sequence comprises diamicton overlain by gravel and sandy gravels; sands, muddy sands and gravels; laminated silty sands and muds; and well sorted sands and gravel. The depositional environments indicate ice-contact, subaqueous and terrestrial sedimentation, with supraglacial, proglacial, subaquatic and paraglacial landsystems. Following the onset of deglaciation, westward retreat of Tyne Gap ice resulted in land to the east and southeast of its margin becoming ice-free. Continued/renewed southward flow of ice along the North Sea coast formed a persistent barrier to sediment-charged meltwaters draining the Tyne Gap ice margin. The separation of these two ice masses allowed a glacial lake to develop in the lower Tyne fed by a large proglacial sandur system, which with ice marginal retreat subsequently merged with Glacial Lake Wear. The sediment sequences record the final waning of the Tyne Gap ice stream, and are contiguous with sediments that extend west through the Tyne Gap and into the Cumbrian lowlands.  相似文献   

19.
Tampa Bay, a large, microtidal, clastic-filled estuary incised into Tertiary carbonate strata, is the largest estuary on Florida’s west coast. A total of 250 surface sediment samples and 17 cores were collected in Tampa Bay in order to determine the patterns and controlling factors governing the recent infilling and modern sediment distribution, and to examine the results in terms of current models of estuarine sedimentation and development. Surficial sediments in Tampa Bay consist of three facies types, each occurring in a distinct zone: modern terrigenous clastic muds occurring in the upper bay and around the bay periphery; relict, reworked-fluvial, quartz-rich sands occupying the open portion of the middle bay; and modern carbonate-rich, marine-derived sands and gravels occupying the lower bay. Factors controlling sediment distribution include: sediment source and supply rate; bathymetry, which is a function of the antecedent topography; and the winnowing effect of wind-generated waves that prohibits modern accumulation in the shallow middle bay. These factors also play a major role in the recent infilling history of Tampa Bay, which has progressed in four stages during the Holocene sea-level rise. Recently developed models of estuarine sedimentation are based primarily on mesotidal to macrotidal estuaries in terrigenous clastic settings in which sedimentation patterns and infilling history are a result of the relative contribution of marine and fluvial processes. Tampa Bay differs in that it was originally incised into carbonate strata, and neither fluvial or marine processes are interpreted to be major contributors to modern sediment distribution. Tampa Bay, therefore, provides an example of an unusual estuary type, which should be considered in future modeling efforts. *** DIRECT SUPPORT *** A01BY083 00004  相似文献   

20.
Pleistocene sediments and soils exposed at Stebbing in central Essex, England are described, analysed and interpreted. The sand and gravel units above Eocene London Clay and Upper Pliocene Red Crag are shown to be a high level member of the Kesgrave Formation, with a surface immediately beneath that of the Westland Green Gravels, which are tentatively assigned to the Pre-Pastonian ‘a’ Stage of the British Quaternary succession. The rubified, argillic soil developed in the surface of these fluvial deposits is a composite of the Valley Farm and Barham Soils and displays micromorphological evidence of several phases of clay illuviation, gleying and clay coating disruption. Originally truncated and buried beneath Anglian gelifluction deposits, cover sand and till, the soil has been exhumed in most places by subsequent erosion. The full succession, however, is preserved within large gulls that formed by periglacial cambering prior to this erosion. More recent loess incorporation and pedogenesis have modified the exhumed soil and the materials within the gulls.  相似文献   

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