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1.
Four phases of cross‐cutting tunnel valleys imaged on 3‐D seismic datasets are mapped within the Middle–Late Pleistocene succession of the central North Sea basin (Witch Ground area). In plan the tunnel valleys form complex anastomosing networks, with tributary valleys joining main valleys at high angles. The valleys have widths ranging from 250 to 2300 m, and base to shoulder relief varying between 30 and 155 m, with irregular long‐axis profiles characteristic of erosion by water driven by glaciostatic pressures. The youngest phase of tunnel valleys are smaller and have a thinner infill than the older generations. The fill of the larger valleys comprises three seismic facies, the lowermost of which has high amplitudes and is discontinuous. The middle facies consists of wedge‐shaped packages of low‐angle dipping reflectors and is overlain by a facies characterised by sub‐horizontal reflectors, which onlap the valley margins. The seismic character, and comparison with lithologies identified in other northwest European Pleistocene tunnel valleys both onshore and offshore, suggests that the lower two seismic facies are most likely sand and gravel‐dominated, while the uppermost facies consists of glaciolacustrine and marine muds. The 3‐D morphology of the valley margins combined with the geometry of the infill packages suggest that episodic discharge of subglacial meltwater was responsible for incising the valleys and depositing at least some of the infill. Proglacial glaciofluvial deposits are inferred to account for some of the fill overlying the subglacial deposits. Glaciolacustrine and marine muds filled remaining valley topography as the ice sheet retreated. The preserved valley margins are shown to be time‐transgressive erosion surfaces that record changes in geometry of the tunnel valley system as it evolved through time, implying that valleys associated with each ice‐sheet advance/retreat cycle were dynamic and probably long‐lived. Within the constraints of the existing stratigraphy the oldest tunnel valleys in the Witch Ground area of the central North Sea are most likely to be Marine Isotope Stage (MIS) 12 (Elsterian, ca. 470 ka) in age and the youngest pre‐MIS 5e (last interglacial, ca. 120 ka). If each tunnel valley phase was formed during the retreat of a major ice sheet then four glaciations with ice coverage of the central North Sea are recorded in the pre‐Weichselian, Middle–Late Pleistocene stratigraphy. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

2.
The lithostratigraphy of pre‐Late Weichselian sediments and OSL‐dating results from four localities in the Suupohja area of western Finland, adjacent to the centre of the former Scandinavian glaciations, are presented. The studied sections expose glacifluvial, quiet‐water, littoral and aeolian deposits overlain by Middle and/or Late Weichselian tills. Litho‐ and biostratigraphical results together with seven OSL age determinations on buried glacifluvial sediment at Rävåsen (94±15 ka) and on till‐covered littoral and aeolian sediments at Risåsen, Rävåsen, Jätinmäki and Kiviharju (79±10 to 54±8 ka), accompanied by previous datings and interpretations, suggest that the glacifluvial sediments at Risåsen were deposited at the end of the Saalian Stage (MIS 6) and those at Risåsen were deposited possibly in the Early Weichselian Substage (MIS 5d?). Palaeosol horizons and ice‐wedge casts together with the dated littoral and aeolian sediments between the Harrinkangas Formation (Saalian) and the overlying till(s) indicate that western Finland was ice‐free during most of the Weichselian time. Littoral deposits, dated to the Middle Weichselian (MIS 4–3), occur at altitudes of 50–90 m a.s.l., which indicates significant glacio‐isostatic depression. The depression resulted from expansion of the ice sheet in the west of Finland at that time.  相似文献   

3.
In North-West Europe no evidence has been found for glaciations older than the Elsterian. The Elsterian seems to consist of at least two separate ice advances with a minor interval in between. During the Elsterian in the area south of the Baltic Sea over 400 m deep channels were cut into the underlying substratum by a combination of glacial erosion and meltwater activity. The channels were active until the Late Elsterian and were subsequently filled with meltwater deposits and glaciolacustrine silt and clay. During the Saalian no comparative channels were being formed. Three different ice advances can be distinguished within the Saalian, the stratigraphical correlation of which is to be discussed. No convincing evidence was found for any interglacial within the Saalian sequence. There are good reasons to assume that the ice advances were only separated from each other by minor ice-free intervals.  相似文献   

4.
Alexanderson, H., Landvik, J. Y. & Ryen, H. T. 2010: Chronology and styles of glaciation in an inter‐fjord setting, northwestern Svalbard. Boreas, 10.1111/j.1502‐3885.2010.00175.x. ISSN 0300‐9483. A 30‐m‐thick sedimentary succession at Leinstranda on the southwestern coast of Brøggerhalvøya, northwestern Svalbard, spans the two last glacial–interglacial cycles and reveals information on glacial dynamics, sea‐level changes and the timing of these events. We investigated the deposits using standard stratigraphical and sedimentological techniques, together with ground‐penetrating radar, and established an absolute chronology based mainly on optically stimulated luminescence dating. We identified facies associations that represent depositional settings related to advancing, overriding and retreating glaciers, marine and littoral conditions and periglacial surfaces. The environmental changes show an approximate cyclicity and reflect glaciations followed by high sea levels and later regression. The luminescence chronology places sea‐level highstands at 185 ± 8 ka, 129 ± 10 ka, 99 ± 8 ka and 36 ± 3 ka. These ages constrain the timing of recorded glaciations at Leinstranda to prior to c. 190 ka, between c. 170 and c. 140 ka (Late Saalian) and between c. 120 ka and c. 110 ka (Early Weichselian). The glaciations include phases with glaciers from three different source areas. There is no positive evidence for either Middle or Late Weichselian glaciations covering the site, but there are hiatuses at those stratigraphic levels. A high bedrock ridge separates Leinstranda from the palaeo‐ice stream in Kongsfjorden, and the deposits at Leinstranda reflect ice‐dynamic conditions related to ice‐sheet evolution in an inter‐fjord area. The environmental information and the absolute chronology derived from our data allow for an improved correlation with the marine record, and for inferences to be made about the interaction between land, ocean and ice during the last glacial–interglacial cycles.  相似文献   

5.
The Fiskarheden quarry, situated in NW Dalarna, central Sweden, reveals thick coarse‐grained sediments of Scott type facies association representing a sandur deposited in an ice‐proximal proglacial environment. Optically Stimulated Luminescence (OSL) dating of the sandur sediments suggests a pre‐Last Glacial Maximum (LGM) age. Most acquired ages are pre‐Saalian (>200 ka) and we regard each of these ages to represent non/poorly bleached sediment except for one small‐aliquot OSL age of 98±6 ka. This age comes from the top surface of an arguably well‐bleached sand bed deposited on the lee‐side of a braid‐bar, putting the sandur build‐up into the Early Weichselian. Large‐scale glaciotectonic structures show an imbricate thrust fan involving both ductile and brittle deformation. The deformation was from the WNW, which largely coincides with the formative trend of the predominating streamlined terrain and Rogen moraine tracts surrounding Fiskarheden. It is suggested that the deformation of the sandur sediments took place when the advancing glacier approached and pushed its own proglacial outwash sediment, during an ice‐marginal oscillation either at the inception of one of the Early Weichselian glaciations in the area, or during a general ice retreat amid a deglacial phase. The Fiskarheden sandur deposits are covered by a subglacial traction till deposited from the NE/NNE. This direction corresponds with younger streamlined terrain flowsets cross‐cutting the older NNW–SSE system and probably represents deglaciation in the area following the LGM. This study will add to the understanding of the formation and deformation of Pleistocene sandur successions and their relationship to past ice‐sheet behaviour.  相似文献   

6.
Continuous high-resolution mass accumulation rates (MAR) and X-ray fluorescence (XRF) measurements from marine sediment records in the Bay of Biscay (NE Atlantic) have allowed the determination of the timing and the amplitude of the ‘Fleuve Manche’ (Channel River) discharges during glacial stages MIS 10, MIS 8, MIS 6 and MIS 4–2. These results have yielded detailed insight into the Middle and Late Pleistocene glaciations in Europe and the drainage network of the western and central European rivers over the last 350 kyr. This study provides clear evidence that the ‘Fleuve Manche’ connected the southern North Sea basin with the Bay of Biscay during each glacial period and reveals that ‘Fleuve Manche’ activity during the glaciations MIS 10 and MIS 8 was significantly less than during MIS 6 and MIS 2. We correlate the significant ‘Fleuve Manche’ activity, detected during MIS 6 and MIS 2, with the extensive Saalian (Drenthe Substage) and the Weichselian glaciations, respectively, confirming that the major Elsterian glaciation precedes the glacial MIS 10. In detail, massive ‘Fleuve Manche’ discharges occurred at ca 155 ka (mid-MIS 6) and during Termination I, while no significant discharges are found during Termination II. It is assumed that a substantial retreat of the European ice sheet at ca 155 kyr, followed by the formation of ice-free conditions between the British Isles and Scandinavia until Termination II, allowed meltwater to flow northwards through the North Sea basin during the second part of the MIS 6. We assume that this glacial pattern corresponds to the Warthe Substage glacial maximum, therefore indicating that the data presented here equates to the Drenthe and the Warthe glacial advances at ca 175–160 ka and ca 150–140 ka, respectively. Finally, the correlation of our records with ODP site 980 reveals that massive ‘Fleuve Manche’ discharges, related to partial or complete melting of the European ice masses, were synchronous with strong decreases in both the rate of deep-water formation and the strength of the Atlantic thermohaline circulation. ‘Fleuve Manche’ discharges over the last 350 kyr probably participated, with other meltwater sources, in the collapse of the thermohaline circulation by freshening the northern Atlantic surface water.  相似文献   

7.
Passchier, S., Laban, C., Mesdag, C.S. & Rijsdijk, K.F. 2010: Subglacial bed conditions during Late Pleistocene glaciations and their impact on ice dynamics in the southern North Sea. Boreas, Vol. 39, pp. 633–647. 10.1111/j.1502‐3885.2009.00138.x. ISSN 0300‐9483. Changes in subglacial bed conditions through multiple glaciations and their effect on ice dynamics are addressed through an analysis of glacigenic sequences in the Upper Pleistocene stratigraphy of the southern North Sea basin. During Elsterian (MIS 12) ice growth, till deposition was subdued when ice became stagnant over a permeable substrate of fluvial sediments, and meltwater infiltrated into the bed. Headward erosion during glacial retreat produced a dense network of glacial valleys up to several hundreds of metres deep. A Saalian (MIS 6) glacial advance phase resulted in the deposition of a sheet of stiff sandy tills and terminal moraines. Meltwater was at least partially evacuated through the till layer, resulting in the development of a rigid bed. During the later part of the Saalian glaciation, ice‐stream inception can be related to the development of a glacial lake to the north and west of the study area. The presence of meltwater channels incised into the floors of glacial troughs is indicative of high subglacial water pressures, which may have played a role in the onset of ice streaming. We speculate that streaming ice flow in the later part of the Saalian glaciation caused the relatively early deglaciation, as recorded in the Amsterdam Terminal borehole. These results suggest that changing subglacial bed conditions through glacial cycles could have a strong impact on ice dynamics and require consideration in ice‐sheet reconstructions.  相似文献   

8.
Lüthgens, C., Böse, M. & Preusser, F. 2011: Age of the Pomeranian ice‐marginal position in northeastern Germany determined by Optically Stimulated Luminescence (OSL) dating of glaciofluvial sediments. Boreas, 10.1111/j.1502‐3885.2011.00211.x. ISSN 0300‐9843 The Pomeranian ice margin is one of the most prominent ice‐marginal features of the Weichselian glaciation in northern Europe. Previous results of surface‐exposure dating (SED) of this ice margin disagree with established chronologies and ice retreat patterns, i.e. are much younger than previously expected. We crosscheck the age of the Pomeranian ice‐marginal position in northeastern Germany using single‐grain quartz Optically Stimulated Luminescence (OSL) dating of glaciofluvial sediments. OSL dating indicates an active ice margin between 20.1±1.6 ka and 19.4±2.4 ka forming outwash plains attributed to the Pomeranian ice‐marginal position. On the basis of these results, we suggest a critical reassessment of previous SED data available for the Pomeranian ice‐marginal position within their respective regional geomorphological contexts. From a process‐based point of view, SED ages derived from glacigenic boulders document the stabilization of the landscape after melting of dead ice and landscape transformation under periglacial conditions rather than the presence of an ice margin. SED indicates a first phase of boulder stabilization at around 16.4±0.7 ka, followed by landscape stabilization within the area attributed to the recessional Gerswalder subphase around 15.2±0.5 ka. A final phase of accumulation of glaciolacustrine and glaciofluvial sediments at around 14.7±1.0 ka documents the melting of buried dead ice at that time.  相似文献   

9.
In this paper, fluvial deposits of Middle Pleistocene age in the mountain‐foreland area of southern Poland (Eastern Sudetes and Western Carpathians) are studied in order to document the evolution of fluvial systems during the coldest stages of glacial periods when the Scandinavian Ice Sheet advanced far to the south. The focus is on fluvial response to climate change and glacial impact on river system behaviour. Also considered is the tectonic uplift of the mountain part of river catchments and its potential influence on the style of fluvial sedimentation in the fore‐mountain area. Three drainage basins that were active during the Elsterian and Saalian glaciations are investigated. Facies analyses are carried out on thick successions of braided river deposits covered with till or glaciolacustrine sediments, which result in a reconstruction of the fluvial activity synchronous with the ice‐sheet advance. The results suggest that fluvial activity declined prior to ice‐sheet advance into the fore‐mountain area. This climatically induced change is directly recorded in alluvial successions by upward‐decreasing bed thicknesses and grain sizes. River longitudinal profiles were shortened in front of the advancing ice sheet. The base level of the studied rivers, created by the ice‐sheet margin, rose in parallel with glacial advance. As a result, the successive reaches of rivers (degradational, transitional, aggradational) underwent shortening and moved upstream within the catchments. Moreover, tectonically induced local increases of river slopes may have influenced the depositional processes.  相似文献   

10.
During the last (MIS 2) and older glaciations of the North Sea, a North Sea Lobe (NSL) of the British-Irish Ice Sheet flowed onshore and terminated on the lowlands of eastern England, constructing inset sequences of either substantial ice-marginal deposits and tills or only a thin till veneer, indicative of complex and highly dynamic glaciological behaviour. The glaciation limit represented by the Marsh Tills and the Stickney and Horkstow Moraines in Lincolnshire is regarded as the maximum margin of the NSL during MIS 2 and was attained at ∼19.5 ka as determined by OSL dating of overridden lake sediments at Welton le Wold. A later ice marginal position is recorded by the Hogsthorpe-Killingholme Moraine belt, within which ice-walled lake plains indicate large scale ice stagnation rapidly followed ice advance at ∼18.4 ka based on dates from supraglacial lake deposits. The NSL advanced onshore in North Norfolk slightly earlier constructing a moraine ridge at Garrett Hill at ∼21.5ka. In addition to the large ice-dammed lakes in the Humber and Wash lowlands, we propose that an extensive Glacial Lake Lymn was dammed in the southern Lincolnshire Wolds by the NSL ice margin at the Stickney Moraine. Previous proposals that older glacier limits might be recorded in the region, lying between MIS 2 and MIS 12 deposits, are verified by our OSL dates on the Stiffkey moraine, which lies immediately outside the Garrett Hill moraine and appears to be of MIS 6 age.  相似文献   

11.
The Burhan Budai Shan in NE Tibet represents a key location for examining the variable influence of the mid‐latitude westerly and monsoonal circulations on late Quaternary glaciations in this sector of the Tibetan Plateau. Our study investigates the glacial history of mountains near Lake Donggi Cona (35°17′N, 98°33′E) using field mapping in combination with 10Be surface exposure dating and numerical reconstructions of former glacial equilibrium line altitudes (palaeo‐ELA). A set of 23 new exposure ages, collected from moraines in four glacial valleys, ranges from 45 to 190 ka, indicating ice expansion during the early and middle part of the last glacial cycle, and during the penultimate and possibly an earlier Mid‐Pleistocene glaciation. Ice advances reaching 12–15 km in length occurred at around 190–180 ka (≥MIS 6), between 140–100 ka (late MIS 6/MIS 5), and 90–65 ka (late MIS 5/early MIS 4), with a maximum ELA depression of 400–500 m below the estimated modern snowline. Exposure ages from the valley headwaters further indicate a small glaciation between c. 60–50 ka (late MIS 4/early MIS 3), which was essentially restricted to the cirque areas. Significantly, we find no evidence for any subsequent glaciation in the area during MIS 2 or the Holocene period. These results indicate a diminishing trend of glaciation in the region since at least MIS 4, and corroborate the case of a ‘missing LGM’ in the more interior parts of the northeastern Tibetan Plateau. The emerging pattern suggests that the most favourable conditions for glaciation during the Late Pleistocene correspond to periods of relatively moderate cooling combined with an intermediate or rising East Asian monsoon strength.  相似文献   

12.
Glacial landforms and sediments mapped in three presently unglaciated mountain massifs, the Nanhuta Shan, the Hsueh Shan and the Yushan, support the concept of repeated, multi-stage glaciations in the Taiwanese high mountain range during the late Pleistocene. New results from surface exposure dating using in situ produced cosmogenic 10Be measured in samples taken from erratic and moraine boulders in Nanhuta Shan at altitudes between 3100 and 3500 m are presented here. The results confirm independent and previously reported Optically Stimulated Luminescence (OSL) ages and 10Be exposure ages from glacial deposits in the same area and suggest a Lateglacial and early Holocene glaciation, the so called Nanhuta glacier advance with two substages at about 12–15 ka and 9.5 ka BP. The respective equilibrium line altitudes (ELA) were calculated at 3340 m and 3440 m with corresponding ELA depressions of 610 ± 100 m and 510 ± 100 m relative to the present day (theoretical) ELA, which is estimated to be at about 3950 ± 100 m in Taiwan. Large-scale erosional landforms indicate a much wider glacier extent during an earlier stage, which is not dated in Nanhuta Shan so far. Luminescence dating from near Hsueh Shan suggests an age of marine isotope stage (MIS) 4 for this stage.  相似文献   

13.
Several sections have been studied to understand the distribution and interrelation of basic genetic types of Quaternary deposits in the Uimon basin and adjacent area. The OSL date of 101 ± 10 ka from the glaciolacustrine terrace on the northeastern rim of the basin corresponds to the cool substage of MIS 5. The glaciolacustrine sediments of the northern rim are covered with widespread diamictic flows of the outburst draining of the ice-dammed lake. The diamicts are overlain by a subaerial complex of loesses and three fossil soils. From the OSL loess dates in the range of 43 to 49 ka we infer that the complex formed from the early MIS 3 through the Holocene. This is also supported by radiocarbon dates from alluvial deposits incised into the glaciolacustrine terrace and into megaflood sediments of the final draining of the last paleolake. The OSL dates in the range of 77 to 89 ka from alluvial sediments indicate that postglacial downcutting of the present-day Katun’ valley probably started just after MIS 5. The discrepancy between the beryllium dates from dropstones and Holocene TL-dates of the Katun’ valley floods ranging from 23 to 6 ka can be explained if the younger floods are related to the draining of moraine- and rock slide dammed rather than ice-dammed lakes. The younger floods, though being less voluminous than the glacial megafloods, were capable of producing giant ripple marks.  相似文献   

14.
晚第四纪MIS6以来柴达木盆地成盐作用对冰期气候的响应   总被引：2，自引：0，他引：2  
气候是控制柴达木盆地盐类沉积的主要因素之一,但是其作用机制尚待明确。作者以柴达木盆地察汗斯拉图盐湖的3个含盐剖面为研究对象,采用多接收电感耦合等离子质谱(MC-ICP-MS)铀系测年测定其沉积时代,并通过X射线粉晶衍射(XRD)分析测定其盐类矿物种类。铀系测年显示D18剖面石盐和芒硝层的沉积时代为13.1±2.0 ka BP～15.9±2.5 ka BP,其中芒硝沉积年代属于末次冰期MIS2晚期;MXK2剖面芒硝层的沉积时代分别为131.7±39.5 ka BP和158.3±10.8 ka BP,D12剖面芒硝层的沉积时代分别为166.6±20.2 ka BP和198.0±20.6 ka BP,可以对应于倒数第二次冰期MIS6。XRD分析确定了3个剖面的盐类矿物主要为芒硝、石盐和石膏。综合多个盐湖晚第四纪成盐数据,本文认为倒数第二次冰期MIS6和末次冰期MIS2是柴达木盆地晚第四纪重要的成盐期,冰期的冷干气候有利于石盐和芒硝等盐类沉积。柴达木盆地"冰期成盐"的根本原因,是由于冰期环境下盆地周边山体冰川规模的扩张以及干冷的冰期气候,共同造成了盐湖补给水量的减少。此外,晚第四纪MIS6和MIS2的冰期降温也是导致盆地中冷相盐类沉积的直接原因。  相似文献   

15.
The Corsica‐Sardinia block is a lithospheric fragment whose recent role in the geodynamics of the central‐Western Mediterranean basin is still enigmatic. The most recent regional structure in Sardinia is the Plio‐Pleistocene Campidano Basin, which is considered in a ‘post‐rift’ stage since the Middle Pleistocene. New structural and stratigraphic geological surveys along with luminescence ages provide evidence to support an ongoing tectonic activity since the Marine Isotopic Stage 7 (MIS7; ca. 220 ka) on the Sinis peninsula, the structural high that bounds the north‐western side of the Campidano Basin. In particular, this paper reveals for the first time the presence of N–S striking normal faults system offsetting late Pleistocene aeolianites (130 ± 12 ka; 82 ± 9 ka).  相似文献   

16.
Loess‐paleosol sequences are important terrestrial archives of palaeoenvironmental change. Such sequences are rich in pedogenic carbonate, the oxygen and carbon isotopic values of which can provide important palaeoenvironmental information. Although some studies have pioneered the use of O and C isotopes in loess‐paleosol sequences, they are not routinely used as palaeoclimate proxies. In this study we analysed the sedimentology, micromorphology, geochronology and isotopic geochemistry of a Middle Pleistocene loess‐paleosol section, located at Kärlich, Germany. The section studied correlates with the Elsterian glacial (MIS 12) and Holsteinian interglacial (MIS 11). Embedded tephra layers yielded 40Ar/39Ar ages of 466±3 ka, 447±1 ka and 361±3 ka. The sedimentology and micromorphology of the sequence record a shift from accretionary loess accumulation (MIS 12) to prolonged pedogenesis at a stable land surface (MIS 11). Soil carbonate δ18O values record an enrichment of ～3‰ during the accumulation of the loess, reaching peak values comparable with those found in the MIS 11 soil. The δ18O signal is interpreted as reflecting temperature, highlighting the potential of δ18O analysis of European loess soil carbonates as a means of reconstructing palaeotemperature history.  相似文献   

17.
The coastal cliff section at Kås Hoved in northern Denmark represents one of the largest exposures of marine interglacial deposits in Europe. High‐resolution analyses of sediments, foraminifera, ostracods, and stable isotopes (oxygen and carbon) in glacial‐interglacial marine sediments from this section, as well as from two adjacent boreholes, are the basis for an interpretation of marine environmental and climatic change through the Late Elsterian‐Holsteinian glacial‐interglacial cycle. The overlying glacial deposits show two ice advances during the Saalian and Weichselian glaciations. The assemblages in the initial glacier‐proximal part of the marine Late Elsterian succession reveal fluctuations in the inflow of sediment‐loaded meltwater to the area. This is followed by faunal indication of glacier‐distal, open marine conditions, coinciding with a gradual climatic change from arctic to subarctic environments. Continuous marine sedimentation during the glacial‐interglacial transition is presumably a result of a large‐scale isostatic subsidence caused by the preceding extended Elsterian glaciation. The similarity of the climatic signature of the interglacial Holsteinian and Holocene assemblages in this region indicates that the Atlantic Ocean circulation was similar during these two interglacials, whereas Eemian interglacial assemblages indicate a comparatively high water temperature associated with an enhanced North Atlantic Current. The foraminiferal zones are correlated with other Elsterian‐Holsteinian sites in Denmark, as well as those in the type area for the Holsteinian interglacial in northern Germany and the southern North Sea. Correlation of the NW European Holsteinian succession with the marine isotope stages MIS 7, 9 or 11 is still unresolved.  相似文献   

18.
In central and northern Sweden, glacial sediments and landforms, formed during Early and Middle Weichselian stadials and their transition into interstadials, are often preserved in spite of having been overridden by later glacial advances. This study presents an OSL‐dated glacial stratigraphy from Idre in west‐central Sweden, expanding the area in which Middle Weichselian ice‐free conditions have been identified. Three sedimentary units were identified, with the lowermost unit consisting of glaciolacustrine sand, deposited in a stagnant water‐body. Nine OSL samples gave ages ranging from 54 to 41 ka, suggesting deposition during a deglacial phase in MIS 3. Normal faults and silt veins, formed after deposition, indicate that the area was ice‐free for a prolonged period, enabling the melting of buried stagnant ice. Above an erosional unconformity is a sediment unit characterized by gravels and sands deposited in a proximal braided‐river environment. OSL ages range from 180 to 41 ka, indicating poor sediment bleaching during deposition. We thus consider them to give a maximum age of the sedimentation, indicating deposition at or after 41 ka. The uppermost unit consists of a stacked succession of subglacial traction tills and glaciotectonite beds, representing the Late Weichselian glaciation of the area, probably during the inception phase with a wet‐based glacier regime. At the last deglaciation of the area there was extensive meltwater erosion, eroding all sedimentary units and forming a landscape with terraces and channels, and erosional remnants of the uppermost diamict as free‐standing hummocks.  相似文献   

19.
At Kap Herschell, in the outer fjord zone of central northeast Greenland, exposed sections in a Late Pleistocene ice-cored moraine revealed four major stratigraphic units deposited during the complex Kap Herschell Stade . All contain fragmented and redeposited marine shells that most likely belong to an Eemian or Early Weichselian marine episode. The oldest unit consists of buried ground ice with folded and sheared debris bands. Isotopic analyses show that the slope of the regression line for δ2H vs. δ18O of the ice is about 8.5. which suggests correlation with the Global Meteoric Water Line (GMWL). Data strongly suggest that the ground ice at Kap Herschell is a remnant of a Late Pleistocene glacier. It was probably generated at low altitudes (< 1000 m) in the inner fjord region or in the nunatak zone. The ground ice is unconformably overlain by all younger stratigraphic units, the oldest of which is a diamicton probably deposited as ablation till from the ice. A complex unit composed of mainly glaciolacustrine deposits and subordinate beds of fluvial and deltaic origin overlies the till and ground ice. Luminescence dating of the lacustrine sediments indicates maximum ages younger than 43 ka BP, suggesting deposition during isotope stages 3 or 2. The glaciolacustrine deposits suffered strongly from glaciotectonic deformation, caused by renewed glacier advance through the fjord. It reached the inner shelf and led to deposition of a discordant till at Kap Herschell, most probably during the Late Weichselian.  相似文献   

20.
The Kleszczów Graben in central Poland was formed by late Oligocene to Middle Pleistocene extensional tectonics. During the Pleistocene it was infilled with a 200 m thick sequence of predominantly glacial sediments. Four distinct formations of Elsterian and Saalian age are identified, each containing 15–40 m of glaciolacustrine strata. The boundaries between formations are marked by erosional surfaces and, in part, by angular discordances caused by tectonism. Glaciolacustrine sedimentation was tectonically controlled: the thickness of the sequences in the graben are three to five times greater than outside the area of fault-controlled subsidence. Deposition in the proglacial lakes was controlled by differential subsidence rates within the basin: deep-lake facies (varved clays) were deposited in sub-basins with high subsidence rates and deltaic to shallow-water facies accumulated in areas of moderate subsidence or occasional uplift. These variations led to the development of a very complex, ‘mosaic’ of lateral facies relationships, suggesting that several sub-basins with differing subsidence rates were present. The Vertical successions show proximal-distal sequences typical of glacier-fed lakes that have limited contact with the ice sheet. However, gravity flow facies are very common, and occur both in the shallow- and deep-water deposits. These deposits are interpreted to have been formed adjacent to active fault scarps which bordered the lake basin. Although several distinct phases of glaciolacustrine sedimentation occurred during the history of trough infilling, the location of the areas of high subsidence varied through time.  相似文献   

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