摘要:

AbstractThaw modification is the general process whereby frost‐fissure wedges are modified during thaw, and by which frost‐fissure pseudomorphs may develop. Specific processes of thaw modification are inferred from ice‐wedge pseudomorphs, composite‐wedge pseudomorphs and deformed sand wedges in the Pleistocene Mackenzie Delta: i.e. thermal erosion, collapse, subsidence, refreezing, loading, buoyancy, spreading, folding and shearing.Thaw modification is believed to result in selective preservation of pseudomorphs and wedges. Sand wedges are more likely to be preserved than are ice‐wedge pseudomorphs or compositewedge pseudomorphs, because only those sand wedges that penetrate massive ice or icy sediments are prone to thaw modification. Furthermore, whereas ice wedges preferentially develop in ice‐rich, fine‐grained sediments (thaw‐sensitive), their pseudomorphs appear to be selectively preserved in ice‐poor, coarse‐grained sedi

ISSN:0267-8179    

DOI:10.1002/jqs.3390080302

出版商:John Wiley&Sons, Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractRadiocarbon dates from two sites in the Andes (Ecuador and Peru) confirm that glaciers culminated a readvance after 11 000 yr BP. A moraine stage, equivalent in altitude and position relative to existing glaciers, is present in most glacierized ranges, but its age is equivocal. Broadly limiting dates from Colombia and Peru suggest that the stage may be Late‐glacial, as it is younger than 12100 yr BP, but formed before the early Holocene; in southern Chile a comparable moraine stage is older than ca. 9100 yr BP. Andean glaciers appear to have advanced at least twice during the Late‐glacial interval.Glacier reconstruction from these moraine limits suggests depression of the equilibrium line altitude by at least 300–400 m in the northern and north‐central Andes, and possibly less than this farther south.Late‐glacial climatic change occurred globally and possibly reflects North Atlantic temperature and circulation changes forced by deglaciation of the northern ice sheets, migrations north and south of the Atlantic Polar Front, and the switching off and on of a ‘dust pump’ in low

ISSN:0267-8179    

DOI:10.1002/jqs.3390080303

出版商:John Wiley&Sons, Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractA section in a gravel quarry at Somersham, Cambridgeshire, has revealed evidence for a lake, named Lake Sparks, in Fenland during the Late Devensian substage of the Pleistocene. Varved sediments were deposited in this lake over a minimum period of ca. 65 yr. The varved clays contain red diamicton clasts, interpreted as dump, delivered to the area by icebergs or floes from the ice‐front in the Wash that deposited the Hunstanton Till. The lake is therefore considered a result of impounding by the Late Devensian ice advance on the east coast. A small number of pale varves have a characteristic structure indicating increased calcite deposition in the summer. They are interpreted as a result of cooler summers with reduced gelifluction from the surrounding Jurassic (Ampthill) Clay. Such gelifluction introduced a mudflow into the varved sequence at the southern end of the section. Pollen analysis confirms the derivation of the clays from the surrounding Ampthill Clay.The varved clays are succeeded by fluviatile sediments related to a delta building into the lake from the north. The delta sediments show periodic influx of sand into the lake interrupted by quiet periods with the development ofCharameadows. A thin spread of fluviatile gravels succeed the delta sediments, indicating the development of a braided river plain as the lake drained on the melting of the Late Devensian ice. This was followed by permafrost development, with the formation of thin thermal contraction cracks and coversand deposition. Later, degradation of the permafrost was associated with the formation of diapirs and a solifluction mantle, and incision of the fluviatile and lacustrine sediments took place. Flandrian peat and marl later filled the valley so formed.A radiocarbon date of 18310 yr BP fromSalixleaves in a drift mud at the top of channel sands preceding lake sediment, in a neighbouring section, confirms the relation of the lake to the Late Devensian ice advance.The significance of the Late Devensian sediments at Somersham lies in the information they give on the timing and variety of processes related to drainage and ice movement in the period before, during and after the ice advance to the Wash. A period of low deposition rate in the lake was followed by rapid delta sedimentation and lake drainage, with implications for climatic chang

ISSN:0267-8179    

DOI:10.1002/jqs.3390080304

出版商:John Wiley&Sons, Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractThe mode of deglaciation of the last Scottish ice sheet is assessed from evidence provided by geomorphological mapping and sedimentology. Ice‐marginal deposits in the Dee valley have a distinctive morphological expression and a characteristically varied sedimentology that strongly resembles those from subpolar glaciers. The deposits tend to occur in certain topographic situations which can be accounted for by compression of ice near the margin and formation of an ice‐cored supraglacial land system. A series of recessional stages of the ice‐front can be mapped demonstrating that active retreat occurred. However, additional evidence shows there was probably a thin marginal zone of stagnant ice. Recessional stages are inferred to be stillstands that are considered to be topographically controlled rather than related to climate. Development of the supraglacial land system during deglaciation suggests that the ice sheet had a polythermal basal regime with a cold‐based margin. This implies that deglaciation took place in northeast Scotland while the climate remained cold, probably due to a precipitation deficit, which agrees well with chronostratigraph

ISSN:0267-8179    

DOI:10.1002/jqs.3390080305

出版商:John Wiley&Sons, Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractAn extensive stratigraphic section at Cape Alfred Ernest on the Wootton Peninsula, northwest Ellesmere Island contains six lithofacies which appear to record two glacial phases separated by an organic layer. (1) A lower massive gravel records a pre‐ice advance outwash phase; (2) massive fine‐grained sediments record a period of non‐glacial marine deposition when sea‐level was higher than present; (3) a massive diamicton records the advance of ice across the site; (4) intermediate stratified beds record supraglacial and proglacial outwash, and include an organic layer; (5) massive diamicton grading down‐valley to stratified diamicton and then massive, sheared diamicton, overlain by laminated fine‐grained sediments with dropstones, recording the last (late Wisconsinan) glaciation; (6) upward‐coarsening sands and gravels record proglacial outwash and grade to raised marine deltas.Radiocarbon dates of 39270 ± 640 and>51000 yr BP were obtained on samples from the organic layer by accelerator mass spectrometry and conventional radiocarbon dating, respectively. Palaeoecological data suggest that the organics accumulated in a wet sedge meadow environment when the climate was warmer than present. Stratigraphic considerations suggest that the organic layer represents an interglacial interval which, if valid, indicates that the site constitutes the northernmost interglacial stratigraphy in the Canadian Arctic. Alternatively, the organic layer may date to Plio‐P

ISSN:0267-8179    

DOI:10.1002/jqs.3390080306

出版商:John Wiley&Sons, Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractLate‐glacial lake sediments containing the Laacher See Tephra (LST, 11000 yr BP) have been analysed for pollen and diatoms at two sites (Hirschenmoor, Rotmeer) in the montane belt of the Black Forest (southern Germany) in order to detect the possible impact of this major volcanic eruption on terrestrial and aquatic biota.The pollen assemblages at both sites show a minor increase in grasses and sedges following the tephra deposition, whereas the pollen of aquatic macrophytes do not show any consistent pattern. Partial redundancy analysis and Monte Carlo permutation tests suggest, however, that the LST had no statistically significant effect either on the terrestrial or aquatic pollen assemblages at either site when the effects of time and climatic change are allowed for statistically.The diatom assemblages at both sites changed after the deposition of the LST and diatominferred pH estimates suggest a short but non‐significant excursion towards lower values. Multivariate analyses of the diatom data indicate a statistically significant impact of the deposition of the LST and associated changes in sediment lithology on the diatom assemblages when the effects of climatic change and time are allowed for statistically. However, owing to the interaction between tephra and lithology it is not possible to conclude if the diatom changes were caused by the deposition of the LST or by lithological chan

ISSN:0267-8179    

DOI:10.1002/jqs.3390080307

出版商:John Wiley&Sons, Ltd    

年代:1993

数据来源: WILEY

ISSN:0267-8179    

DOI:10.1002/jqs.3390080308

出版商:John Wiley&Sons, Ltd    

年代:1993

数据来源: WILEY

ISSN:0267-8179    

DOI:10.1002/jqs.3390080301

出版商:John Wiley&Sons, Ltd    

年代:1993

数据来源: WILEY