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1.
The Pleistocene deposits in the area of the village of Beckford, in the Carrant Valley, Worcestershire are described. A radio-carbon date of 27,650±250 years B.P. has been obtained from plant material in the terrace deposits in the valley. Detailed studies have been made of the terrace sediments and of the remains of Mollusca and Coleoptera in fossiliferous layers. Frost structures are described and their stratigraphy considered. The nature of the local environment and regional climate in the final stages of the Upton Warren Interstadial period is reconstructed in detail. The terrace appears to have aggraded in a periglacial environment, dominated initially by solifluction processes. Extensive solifluction may have led to local diversions of drainage. In time, the climate deteriorated, becoming cooler and somewhat less continental with the onset of the main Devensian glaciation. This latter stage was characterised by extensive ice-wedge growth, aeolian activity and, subsequently, by river incision. 相似文献
2.
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
3.
The lowest 17-km long reach of the Huerva River valley, down to its confluence with the Ebro River in Zaragoza city, flows across salt-bearing evaporites of the Ebro Tertiary Basin (NE Spain). Upstream, the horizontally lying Miocene evaporites are interfingered with non-soluble distal alluvial fan facies (shales and sandstones). The proportion of soluble facies in the Huerva River valley increases in a downstream direction towards the basin depocenter. On the basis of the type and magnitude of the paleosubsidence features, the valley has been divided into four reaches. Along reach I, undeformed terrace deposits less than 4 m thick rest on insoluble detrital bedrock. In reaches II and III, dissolution at the alluvium–bedrock boundary has generated local thickening, deformation and paleocollapse structures, which only affect the alluvial mantle. In reach IV, terrace deposits thicken to over 60 m resulting from a large-scale synsedimentary subsidence. In this sector, subsidence locally affects to both the alluvium and the underlying bedrock. This indicates that dissolution acts at the rockhead beneath the alluvial cover (alluvial karst) and within the evaporitic substratum (interstratal karst). The development of an intraevaporitic karst in reach IV is attributed to gypsum and salt dissolution. Irregular terrace gravel bodies (gravel pockets) embedded in a fine-grained matrix associated with paleocollapse structures have been interpreted as liquefaction–fluidization structures resulting from ground acceleration and suction induced by catastrophic collapses. Subsidence is currently active in the region affecting areas with a thin alluvial cover in reaches III and IV. The low subsidence activity in most of Zaragoza city is explained by the presence of thickened (around 50 m) and indurated alluvial deposits. In the surrounding area, numerous buildings in Cadrete and Santa Fe villages have been severely damaged by subsidence. Natural and human-induced subsidence favours the development of slope movements in the gypsum scarp overlooking Cadrete village. Several transport routes including the Imperial Canal (irrigation canal) and the recently completed Madrid–Barcelona high-speed railway are affected by human-induced sinkholes. The paleocollapse structures exposed in the trenches of this railway and a ring road under construction point to hazardous locations underlain by cavities and collapse structures where special protection measures should be applied. Rigid structures are recommended beneath the high-speed railway with sufficient strength to span the larger sinkholes with no deformation. Electronic monitoring devices linked to a warning system can detect subtle subsidence-induced deformations in carriageways or railways. This research demonstrates that the study of the paleokarst helps to understand the processes involved in the present-day subsidence phenomena and their general spatial distribution. 相似文献
4.
Mud infiltrate structures (MIS) are a class of clastic sedimentary structures that have formed by high-angle gravitational sedimentation in vadose cavernous limestones beneath the Miocene Hawthorn Group. Observations of MIS in caves beneath the Pelham Escarpment, southwest Georgia, suggest three main types occurring either beneath, within, or contiguous to subvertical feeder pipes in the ceiling: (1) rubbly, conical masses; (2) polished, striated, cylindrical masses; and, (3) mud-flow, -drape, and-splash forms. MIS abundance suggests gravitative deposits are a major component to the total volume of cave mud. The muds apparently do not include soil-mantle materials; rather, their sources are poorly cemented clay-rich layers within the overlying Hawthorn Group. In cases where an individual MIS volume is large in proportion to the clastic-cover thickness, but no surface depression is found, the MIS location hypothetically predicts a directly overlying site of potential cover-collapse sinkhole development. Smaller-volume and/or shallower MIS predict the location of potential cover-subsidence sinkholes. The MIS forms suggest plastic and fluid behavior of the sediments occurs during displacements. The capacity for subsurface voids to accept mobile sediments depends on several factors that influence sinkhole development: (1) void size and interconnectedness; (2) spacing and size of subvertical solution pipes which can act as MIS feeders; (3) depth beneath the sediment source of any main horizontal cave development; (4) thickness of the cover sediments; and (5) the presence and depth of any breakout domes. The Hawthorn is present over more than 50 percent of the area underlain by the Floridan Aquifer system and is the system's major upper confining unit. MIS-forming processes are likely to be widely distributed within this stratigraphic setting, promoting ground subsidence and subsurface porosity obliteration. Given the existence of feeder pipes, MIS also are expected to form within the phreatic zone. 相似文献
5.
《Journal of South American Earth Sciences》2005,18(3-4):355-369
The Catalão I carbonatite complex, central Brazil consists of ultramafic silicate rocks with subordinate carbonatite and associated phoscorite, nelsonite, and monazitite. In the Lagoa Seca area, lacustrine sediments discordantly overlie a 15-m thick unit of horizontally layered alkaline rocks that consist of a basal apatitite/nelsonite overlain by monazitite. The unit contains cylindrical to conic pipes filled with breccia, limited at the top by a discordance and at the bottom by phoscorites and carbonatites. X-ray diffraction and microprobe studies show that the pipes are filled dominantly by gorceixite and ilmenite, with subordinate apatite, calcite, pyrochlore, baryte, anatase, vivianite, and quartz and rare perovskite. This assemblage has possible primary phases and common alteration products of late-stage phoscorite-series rocks, such as carbonate nelsonites. The lower and intermediate portions of some pipes are fine grained, with cross- and coarsening-upward bedding. These structures are typical of diluted particulate flows (e.g. surges), which suggests that magma fragmentation occurred inside the chamber. The rocks and structures described here seem to represent an extreme case in which surge-like deposits formed within a conduit or even inside the magma chamber, implying that surge processes may develop at higher-than-atmospheric pressures. 相似文献
6.
John R. Graham 《Sedimentary Geology》1981,30(3):195-211
A succession of about 300 m of fluvial sediments from the Lower Carboniferous of northwest Ireland is described and interpreted. A lower, mainly red, formation contains fluvial channel deposits dominated by flat laminated sandstone. These are separated by interbedded sandstones and mudrocks with local caliche horizons and abundant mudcracks interpreted as levee and flood basin deposits. An upper, mainly non-red, formation contains fluvial channel deposits with common trough cross-stratification and epsilon cross-stratification also separated by interbedded sandstones and mudrocks. Evidence of desiccation is less common in the uppermost beds which pass transitionally upwards into marine sediments.The change in fluvial channel style is interpreted as due to increasing sinuosity and permanence of flow which may have been partly temporally and partly spatially controlled. The predominance of coarse sediments is thought to be largely controlled by limited subsidence. The Lower Carboniferous transgression was the major overall control of alluviation. 相似文献
7.
眉山阶地第四系剖面地处番禺断隆区的边缘,毗邻狮子洋断陷区,是珠江三角洲第一次海侵所到达的区域,附近的断裂包括文冲断裂、化龙—黄阁断裂、新会—市桥断裂等三角洲内典型断裂。发育于网纹红土基座上的堆积阶地,自下而上大致分为杂色砂层、白色砂层、淤泥质层3层,构成一个完整的从动水环境到静水环境的沉积旋回。野外调查及14C测年和光释光（OSL）测年结果表明,眉山第四系剖面年龄约在70～30 ka B.P.之间,相当于深海O同位素4—2阶段。结合前人的研究资料,认为阶地面与沉降区的下旋回原本在同一高度,但自晚更新世以来,在断块差异升降中被错开,垂直距离在20 m以上,直观地指示了断隆区与断陷区的运动特征。 相似文献
8.
This paper analyses the causes of the catastrophic failure of an earth dam that took place on 21 January 2001 during the first filling test in the NE of Spain (Altorricón village, Huesca Province). The San Juan reservoir, with a capacity of 850,000 m3, was built in 1999 on gypsiferous mantled pediment deposits overlying Tertiary dispersive clay sediments. The basin of the reservoir was excavated in the alluvial cover and Tertiary bedrock. An earth dam was constructed on the pediment surface along the perimeter of the artificial basin. The dam has a core of compacted clay material derived from the excavation, which is indented 1 m in the Tertiary shales of the bedrock, cutting the highly pervious alluvial mantle. Field observations, analysis of the basin and dam materials and eyewitnesses accounts have helped to infer the processes involved in the failure of the earth dam. These processes include (1) subsidence and ravelling (suffosion) processes induced by the dissolution of the 4-m-thick detrital cover with a gypsum content of around 40%; (2) piping processes affecting the embankments and core of the dam built with dispersive clays that have high exchangeable sodium percentage (ESP) and show active pipes; and (3) water circulation through biogenic burrows in the pediment deposits. This case study demonstrates the frequently hidden limitations that evaporite sediments and dispersive clay materials pose to the construction of dams. It also shows that highly permeable surficial deposits should be stripped before the construction of the dam structure. 相似文献
9.
D.P. Sen 《Sedimentary Geology》1981,30(4):291-310
During the Early Tertiary epoch a basin of deposition covered almost all the present Himalayan foot hill zone. Large quantities of detrital sediments were transported from the northern source area to the depositional basin. The rivers carrying the sediment load formed large delta complexes. In the area studied at least two such delta complexes, one around Kalka and the other around Nahan, were formed; of these the one around Kalka contains the greatest volume of sediments.
The recognition of the deltas is based on: (a) greater abundance of stream deposits than deposits of other environments; (b) lithologic criteria, particularly sedimentary structures, repetitive lithologic associations; (c) the general coarsening-upward nature of the sediments; (d) strong unimodal current direction; (e) the typical lenticular lithogenetic model, common in recent deltas.
The rocks of the Nahan Group are divisible into three formations at Kalka and two formations at Nahan where central formation did not develop. Himalayan tectonism has largely controlled both delta formation and stratigraphy.
The differential rates of sedimentation greatly influenced the rates of subsidence within the Nahan basin. The sediments of the Nahan Group are roughly bounded between two major phases of Himalayan uplift, namely Eocene and middle Miocene tectonic movements. The effects of intermittent tectonic pulses are well depicted in rocks of the Nahan Group. The intensity of the tectonism that controlled both the source-area rise and basin subsidence during deposition of the Nahan Group of sediments was nonuniform, both in vertical and lateral directions. Because of a generally higher rate of deposition compared to the rate of subsidence, the rocks of the Nahan group represent a normal regressive basin-filling sedimentation, forming thick detrital deposits due to progradation of younger deltas beyond the distal end of older deltas. 相似文献
10.
Onshore tsunami deposits may consist of inflow and backflow deposits. Grain sizes can range from clay to boulders of several metres in diameter. Grain‐size distributions reflect the mode of deposition and may be used to explore the hydrodynamic conditions of transport. The absence of unique sedimentary features identifying tsunami deposits makes it difficult in some cases to distinguish inflow from backflow deposits. On Isla Mocha off central Chile, the 27 February 2010 tsunami left behind inflow and backflow deposits of highly variable character. Tsunami inflow entrained sands, gravels and boulders in the upper shoreface, beach, and along coastal terraces. Boulders of up to 12 t were transported up to 300 m inland and 13 m above sea‐level. Thin veneers of coarse sand were found up to the maximum runup at 600 m inland and 19 m above sea‐level. Backflow re‐mobilized most of the sands and gravels deposited during inflow. The orientation of erosional structures indicates that significant volumes of sediment were entrained also during backflow. A major feature of the backflow deposits are widespread prograding fans of coarse sediment developed downcurrent of terrace steps. Fan sediments are mostly structureless but include cross‐bedding, imbrication and ripples, indicating deposition from bedload traction currents. The sediments are poorly sorted, grain sizes range between medium to coarse sand to gravel and pebbles. An assessment of the backflow transport conditions of this mixed material suggests that bedload transport at Rouse numbers >2·5 was achieved by supercritical flows, whereas deposition occurred when currents had decelerated sufficiently on the low‐gradient lower coastal plain. The sedimentary record of the February 2010 tsunami at Isla Mocha consists of backflow deposits to more than 90%. Due to the lack of sedimentary structures, many previous studies of modern tsunami sediments found that most of the detritus was deposited during inflow. This study demonstrates that an uncritical use of this assumption may lead to erroneous interpretations of palaeotsunami magnitudes and sedimentary processes if unknowingly applied to backflow deposits. 相似文献
11.
James L. Best 《地学学报》1989,1(2):203-208
The presence of fluidization pipes within pyroclastic sediments has been proposed as a criterion for differentiating these volcaniclastic deposits from those of epiclastic mass flows. Observations of pipe structures within low-temperature epiclastic mudflows produced by the 1971 eruption of Volcan Hudson, Southern Chile, demonstrate that this assumption is invalid and that the presence of pipes may only be utilized to interpret the precise mechanics of the fluidization process itself. 相似文献
12.
P. H. Walker 《Australian Journal of Earth Sciences》2013,60(2):683-696
Extensive terrace and flood plain deposits occur along the Lower Macleay River. A sequence of terraces from oldest to youngest was named: Madron, Corangula, Mungay, Mooneba, Belgrave and Macleay deposits (contemporary). Basal sediments in the Mooneba terrace were dated by radiocarbon analysis at 3,280 ± 55 years; basal sediments of the Mungay terrace were dated at 6,425 ± 105 years. The Madron and Corangula terraces are considered very much older than the Mungay. The flood plain consists of two early cycles of aggradation buried under 23m of estuarine sediment, which in turn is overlain by up to 6m of alluvium. The estuarine sediments were dated at 8,530 ± 200 years at elevation —4m relative to mean sea level. The base of the overlying Smithtown alluvium was dated at 3,295 ± 95 years. A general chronology is presented for the Lower Macleay valley, and a sequence of terrace soils is discussed. 相似文献
13.
<正>The Huang Shui River,a main tributary of the Yellow River,crosses a series of tectonically subsided and uplifted areas that show different patterns of terrace formation.The distribution of fluvial terrace of the Huang Shui River is studied through topographic and sedimentologic terrace mapping.Three terraces in the Haiyan Basin,four terraces in the Huangyuan Basin,19 terraces in the Xi'ning Basin(the four high terraces may belong to another river),nine terraces in the Ping'an Basin, five terraces in the Ledu Basin and 12 terraces in the Minhe Basin are recognized.Sedimentology research shows that the geomorphologic and sedimentological pattern of the Huang Shui River,which is located at the margin of Tibet,are different from that of the rivers at other regions.The formation process of the terrace is more complicated at the Huang Shui catchment:both accumulation terrace and erosion terrace were formed in each basin and accumulation terraces were developed in some basins when erosion terraces were formed in other basins,indicating fluvial aggradation may occur in some basins simultaneously with river incision in other basins.A conceptual model of the formation process of these two kinds of fluvial terraces at Huang Shui catchment is brought forward in this paper.First,the equilibrium state of the river is broken because of climatic change and/or tectonic movement,and the river incises in all basins in the whole catchment until reaching a new equilibrium state.Then,the downstream basin subsides quickly and the equilibrium state is broken again,and the river incises at upstream basins while the river accumulates at the subsidence basin quickly until approaching a new equilibrium state again.Finally,the river incises in the whole catchment because of climatic change and/or tectonic movement and the accumulation terrace is formed at the subsidence basin while the erosion terrace is formed at other basins.The existence of the accumulation terrace implied the tectonic subsidence in the sub-basins in Huang Shui catchment.These tectonic subsidence movements gradually developed from the downstream Minhe Basin to the upstream Huangyuan Basin.Dating the terrace sequence has potential to uncover the relationship between the subsidence in the catchment and the regional tectonic at the northeastern Tibetan Plateau. 相似文献
14.
K.B. Lewis 《Earth》1974,10(1):37-71
The continental terrace, defined as the sediment and rock underlying the coastal plain, the continental shelf and continental slope, has been extensively explored during the last decade using seismic techniques. Some facts and theories about its structure and formation are reviewed.The development of the continental terrace is considered in three main stages: first, the formation of the major discontinuity between continent and ocean basin; second, the deposition of sedimentary strata; and third, the Quaternary modification of the continental terrace to form the present topography.The major discontinuity may be formed either by rifting of a continent or by accretion of a large mass of contorted continental terrace and continental rise sediments to a pre-existing edge of a continent. Both types of discontinuity may be either ancient or presently forming in an orogenic area. Sedimentary strata are built generally upwards and outwards but at most places development is modified either by basement structure, by current and slump erosion or by growth of organic and diapiric structures.During the Quaternary Era the terrace has been extensively modified as a result of large fluctuations of sea level. The present wide continental shelf and deep shelf break are at most places the result of wave planation and offshore deposition during the last low sea level and during the subsequent rise of sea level. The final process at many places has been formation of a Holocene coastal plain and deposition of a prism of sediment on the inner continental shelf.Most continental terraces show evidence of continental flexure, which is the seaward tilting of land and seabed with a zero isobase, a line of no vertical movement, close to the shore. The reasons for continental flexure are discussed. The relative positions of the shore line, the zero isobase and the boundary between erosion and deposition are controlled by rates of tilting, by rates of sedimentary processes and by eustatic movements of sea level.From an assessment of the general processes operating today, some conclusions are reached about the changes in form and topography of continental terraces of the past. 相似文献
15.
A detailed analysis of depositional history of Miocene sediments and various effects which are governed for creation of accommodation
space as well as the processes of sedimentation inherent to the depositional system at that period is described in this work.
The early Miocene clastic sediments are deposited in prograding environment where sediment supply exceeds the accommodation
space available. The accommodation space created due to basin subsidence and source area upliftment due to local and regional
tectonic activity in the basin. In the early Miocene time, the Assam shelf major transgression occurred and several minor
transgression followed. There was wide spread deposition of the fluvial Tipam sandstones. In Miocene time due to thrust loading
and flexure subsidence, accommodation space was created for deposition of the sediments. The Tipam Sandstone is deposited
by cyclic deposition of fining upward sequence in a fluvial to brackish water environment of braided river processes. The
mechanism of braided rivers is also discussed in which it laterally expanded, leaving sheet like or wedge — shaped deposits
of channel and bar complexes preserving only minor amounts of flood plain material. 相似文献
16.
The halite-bearing Barbastro Formation crops out in the core of the Barbastro Anticline (Ebro Tertiary Basin). This anticline
is traversed perpendicularly by some of the most important Pyrenean drainages such as the Cinca and Noguera-Ribagorzana Rivers.
The terrace sequences of these fluvial systems have been used as markers to identify and assess dissolution-induced subsidence
and salt tectonics. In the limbs of the anticline, terrace deposits underlain by detrital bedrock do not show any evidence
of deformation and have a consistent thickness of less than 10 m. The deposits of certain terrace levels of the Noguera-Ribagorzana
River and its tributary, the Lo Reguer Creek, are locally thickened filling basins generated by dissolution-induced synsedimentary
subsidence up to several kilometers long and more than 100 m deep. Conversely, terraces of the Cinca River do not show anomalously
high thicknesses, but local uplifts related to differential upward flow of the halite-bearing bedrock. Locally, a minimum
uplift rate of 0.3 mm/year has been estimated from a 64-ka terrace tilted away from the valley. The subsidence hazards occur
chiefly in areas where the ground receives artificial water recharge. Serviceability of some canals has been notoriously affected
by evaporite karstification. The problem has been mitigated to acceptable levels by grouting. Numerous buildings of Ivars
de Noguera are severely damaged by dissolution subsidence, and possibly, by hydrocompaction of gypsiferous silts. The pipe
network has been replaced to ameliorate the subsidence risk. In the Cinca River valley, cavities with a total volume of about
180,500 m3 have been created by solution mining at depths greater than 500 m. No investigation methods are applied in the brine field
to monitor the distribution and evolution of artificial voids. Substantial increase in salinity of the Cinca River is another
evidence of subjacent evaporite dissolution. 相似文献
17.
18.
G. Endlicher 《Mineralium Deposita》1976,11(3):329-351
The fluorspar deposits of Arlos and Villabona in Western Asturias/Northern Spain occur as strata-bound mineralizations mainly in early Permotriassic sediments, and in upper parts of this formation, which is unconformably overlying the Hercynian folded palaeozoic basement. The red bed-like lithofacies developes from marine carbonate sediments to deposits of continental character. Sedimentary and diagenetic features point out clearly the syngenesis of the strata-bound mineralizations. These contain besides fluorspar predominately calcite, quartz (cherts) and vestigial iron and coppersulphides. Discordant veins comprise a synsedimentary phase of mineralisation in Permian fractures (palaeochannels) and a phase of mobilization (hydatogenic rearrangement) caused by alpide germanotyp block tectonics. Syngenetic fracture deformation of certain stratigraphical horizons, which are partly mineralized, are referred to Postvariscian movements belonging to the ceasing orogenesis. Thus is further indicated, that the mineralizations are principally Permian in age; however, those in conglomerates at higher stratigraphical levels could be Lower Triassic in age. The lacking hints at plain hydrothermal activities and the close relation between mineralizations and intensified weathering, increased rearrangement and frequent changes of facies suggest an exogenic-synsedimentary origin. The distribution of fluorine in certain palaeozoic sediments and the geochemical behaviour of fluorine within sedimentary circulation support this theory. 相似文献
19.
《Quaternary Science Reviews》2003,22(10-13):1223-1229
The southwestern coastal plain of Taiwan is heavily urbanised, with a population of more than 6 million. The area is known to be subsiding, and the resulting basin is filled with thick fluvial deposits, of at least 200 m thickness. In the past century, three large earthquakes have occurred in the area; two caused surface ruptures, and resulted in large property loss and more than 2000 casualties. There is an urgent need for an understanding of the dynamics and recurrence intervals of this neotectonic activity, but little is known of the chronology of the late Pleistocene deposits. Recently more than 20 cores longer than 250 m were taken from the coastal plain as part of a large hydrogeological investigation, and basic data on lithology, hydrogeology and palaeobiology were collected. The base of these cores is beyond the 14C age limit, and so the application of luminescence dating to these sediments has been investigated.Optically stimulated luminescence methods have been applied to quartz sand-sized grains extracted from 29 samples. Dosimetry based on gamma spectrometry is also compared with ICP-MS and XRF analyses. In the age range up to ∼40 ka, radiocarbon ages are compared with the luminescence results, to give confidence that the initial bleaching of these sediments was sufficient. The luminescence ages are then discussed and differential rates of basin subsidence are deduced. It is clear from these data that the study area is tectonically active, and it may be that regions of similar subsidence rate correlate with identifiable geological structures. 相似文献
20.
Vance T. Holliday John F. Hoffecker Paul Goldberg Richard I. Macphail Steven L. Forman Mikhail Anikovich Andrei Sinitsyn 《Geoarchaeology》2007,22(2):181-228
The Kostenki–Borshchevo localities include 26 Upper Paleolithic sites on the first and second terraces along the west bank of the Don River, near Voronezh on the central East European Plain. Geoarchaeological research from 2001 through 2004 focused on sites Kostenki 1, 12, and 14, with additional work at Kostenki 11 and 16, and Borshchevo 5. The strata are grouped into three units (bottom up): Unit 1, > 50 ka, consists of coarse alluvium (representing upper terrace 2 deposits) and colluvium, overlain by fine‐grained sediments. Unit 2 includes archaeological horizons sealed within two sets of thin lenses of silt, carbonate, chalk fragments, and organic‐rich soils (termed the Lower Humic Bed and Upper Humic Bed) dating 50–30 ka. Separating the humic beds is a volcanic ash lens identified as the Campanian Ignimbrite Y5 tephra, dated elsewhere by Ar/Ar to ca. 40 ka. The humic beds appear to result from the complex interplay of soil formation, spring deposition, slope action, and other processes. Several horizons buried in the lower part of Unit 2 contain Upper Paleolithic assemblages. The springs and seeps, which are still present in the area today, emanated from the bedrock valley wall. Their presence may account for the unusually high concentration of Upper Paleolithic sites in this part of the central East European Plain. Unit 3, < 30 ka, contains redeposited loess with a buried soil (Gmelin Soil) overlain by a primary full‐glacial loess with an associated Chernozem (Mollisol), forming the surface of the second terrace. © 2007 Wiley Periodicals, Inc. 相似文献