摘要:

AbstractThe Transtrand Mountains, western Sweden, are characterised by an abundance of glaciofluvial meltwater channels. Crossing lateral channels and channel systems overprinted by fluting occur at several localities. Drainage channel systems from three older deglaciations are interpreted to have been preserved despite overriding by later ice sheets. Deep sections, machine excavated into the summit surface of the Hundfjället Mountain, revealed the presence of two lower diamictons into which two older generations of lateral channels were cut, and subsequently buried by a Late Weichselian basal till. Buried frost‐sorted structures, and frost‐fractured bedrock below the lowermost till, indicate the repeated occurrence of periglacial condil conditions. An organic layer between the two older units yielded an infinite radiocarbon age of>45000 yr BP. The pollen content of this layer indicates a local mountain heath flora dominated by ericaceous dwarf shrubs and graminids. The climate phase is interstadial or the beginning or end of an interglacial. The different diamictons also display different pollen spectra. During the last deglaciation the ice flow was from the north‐northeast. The second youngest system of drainage channels corresponds to ice flow from approximately west‐northwest. There are still older traces of separate deglaciation events from the east and north. A frozen bed is interpreted to have been the standard basal regime for the last ice sheets to cover the Transtrand M

ISSN:0267-8179    

DOI:10.1002/jqs.3390070102

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY

摘要:

AbstractThis paper describes a complex sequence of glacigenic sediments occupying a faultbounded depression at Aberdaron Bay, western Lleyn. The sequence offers an insight into sedimentary environments during deglaciation of the Irish Sea Basin. A lower stratified diamict association (LDA) containing contorted units of fine sand/silt and displaying strong, consistent clast fabrics, is overlain by an upper diamict association (UDA) with weaker fabrics and extensive sand and gravel layers and channel fills. Certain characteristics of the sequence can be explained by a glacimarine depositional model, but there are several problems. In particular, the geometry of the sediments is difficult to explain without recourse to the melting of buried ice. An alternative model that overcomes these problems involves the decay of a terrestrial glacier containing reworked marine sediments. Supraglacial sediment flows released during decay of thinner ice covering the surrounding hills (UDA) would have rapidly buried a thick stagnant ice mass in the Aberdaron depression, facilitating slow melting and release of basal meltout till (LDA). A model is presented that accounts for the stagnation andin situdecay of a debris‐rich Irish Sea glacier, and which could explain many of the deposits and landforms surrounding the Irish Sea Basin without recourse to high relative sea‐lev

ISSN:0267-8179    

DOI:10.1002/jqs.3390070103

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY

摘要:

AbstractIn this study the sedimentology, micromorphology and structure of four deformation es in end moraines or near‐end moraine settings are presented. Micromorphology has proven be a powerful tool in the interpretation of the sediments.The four sites are related to three of the four major glaciations known from this part of Patagonia: Nahuel Huapi, Anfiteatro and Pichileufú. All four sites are characterised by glaciolacustrine vironments in which sedimentation occurred at least partly on and/or against dead ice. With the exception of one section in the San Martin de los Andes area, where deformation occurred active ice‐push, all major disturbances (faults and folds) are the result of dead ice collapse. Comparison with micromorphological observations on basal tills shows that the sediments all lack clear subglacial imp

ISSN:0267-8179    

DOI:10.1002/jqs.3390070104

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY

摘要:

AbstractThe late Quaternary history of the middle Caquetá River area in Colombia, northwestern Amazonia is described, based on observations of river bank sections, radiocarbon dates and palynological analyses of organic layers in floodplain and low terrace sediments of the Caquetá River. It is shown that the Late Pleistocene and Holocene climatic changes that took place in the Andean Cordilleras, were related to the depositional and erosional history of the Caquetá River in the Colombian Amazonian lowlands.The low terrace sediments consist of sandy and gravelly deposits covered by clays that sometimes contain lenses of peaty material. From these organic low terrace sediments, seven finite radiocarbon dates were obtained of Middle Pleniglacial age, between 56 000 and 30 000 yr BP. The coarse textured basal deposits of the low terrace apparently stem from the early part of the Middle Pleniglaciai period, during which the effective rainfall in the Andes was relatively high and the Andean glaciers had a considerable extension. Palynological data from silty sediments with organic remains at one site, show an interval when drier and more open types of vegetation on poor soils must have covered a larger area than today, but Amazonian forest was still the dominating type of vegetation. This interval might correspond to one of the Middle Pleniglacial savanna intervals from eastern Amazonia (Carajas).No organic sediments from the Upper Pleniglacial period were found and hence radiocarbon dates were not obtained. In the Andes this period had a very cold climate with low effective rainfall and in the east Amazonian Carajas area it is characterised by the relative extension of open savanna vegetation. The river run‐off and sediment transport must have been much lower than in the Middle Pleniglacial and the Caquetá River cut itself down in its own sediments.Two Late‐glacial radiocarbon datings obtained at one site (ca. 12 500 yr BP) indicate the existence of a Late‐glacial sedimentation phase, separated from the Holocene sequence by a minor erosional phase.Organic layers in the Holocene floodplain sediments yielded 28 radiocarbon dates between 10 000 and 355 yr BP. Holocene sedimentation started with the rapid deposition of (sandy) clay possibly in a partly permanently inundated Caquetá valley. During the major part of the Holocene (silty) clays were deposited, with a dominant seasonal inunda

ISSN:0267-8179    

DOI:10.1002/jqs.3390070105

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY

摘要:

AbstractThe sequence of Late‐glacial and Holocene alluvial sedimentation in the middle Caquetá River Basin of Colombian Amazonia is described, based on the study of the sediments and palynology of several river bank sections and on 30 radiocarbon dates.An early Late‐glacial sedimentation cycle is recognised, followed by a minor late Late‐glacial erosion phase.The Holocene valley fill consists of grey clays (often present in the lower part of the sections) deposited in open water and silty clays often with faint yellow mottling, deposited under a regime of seasonal flooding. The base of the Holocene sections is formed by sands, where exposed. In two places the transition of sand to open‐water grey clay was dated around 10 000 yr BP and there is a suggestion that open water may have been more common at the beginning of the Holocene than later, when sedimentation by seasonal flooding became important. In many places much of the earlier Holocene sediments may have been removed by erosion and replaced by younger sediments, by a process of lateral aggradation. A considerable part of the present valley fill is younger than ca. 3500 yr. However, in several places older Holocene sediments are found, apparently only little affected by later erosion, lying below younger varzea silty clays.During the Holocene more organic sediments were formed in periods with reduced river discharge, related to drier climates in the Andes and possibly in Amazonia. These dry periods, deduced from data in the Caquetá River area, correspond well with dry phases in other parts of northwestern South America (e.g. between approximately 2700‐1900 yr BP and approximately 3200‐3800 yr BP). Rates of average net sedimentation, calculated from dated sections that apparently lack major hiatuses caused by erosion, were high in the lower Holocene, low during the middle Holocene and increase again in the upper Holocene. Levee deposits became coarser and the high river level of the Caquetá increased during the late Holocene. These phenomena may be explained by the increasing influence of man on the vegetation cover in the Andean headwater areas and possibly also in the Amazonian catchment area of the

ISSN:0267-8179    

DOI:10.1002/jqs.3390070106

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY

摘要:

AbstractThe lithostratigraphy and biostratigraphy of two sites (Allt Odhar and Dalcharn) in north‐central Scotland are described, where pollen spectra of temperate affinity have been obtained from organic deposits that underlie till. The pollen record from Allt Odhar, in association with evidence from plant macrofossils and Coleoptera, shows the expansion of birch woodland and its eventual replacement by open grassland under a climatic regime slightly cooler than that prevailing in the northern highlands of Scotland at the present day. The organic sediments accumulated during an Early Devensian interstadial episode, which has been dated by the uranium series disequilibrium method to ca. 106 ka BP. Evidence for one and possibly two Devensian glaciations may be preserved at the site. The pollen record from Dalcharn, by contrast, reflects the middle and later stages of an interglacial cycle with the transition from pine forest to grassland. The overlying till sequence contains evidence of at least two separate glacial episodes. The age of the warm stage cannot be established precisely on present evidence, but there are indications that it may predate the last (Ipswichian) interglacial. These are the first sites from the mainland of Scotland to provide evidence of wooded conditions during interstadial and interglacial episodes of the Middle/Late Pleistocen

ISSN:0267-8179    

DOI:10.1002/jqs.3390070107

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY

ISSN:0267-8179    

DOI:10.1002/jqs.3390070101

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY