摘要:

AbstractMapping along a transect from the southeastern margin of the South Patagonian Ice‐field in Torres del Paine National Park (Chile) to the limits of fresh moraines of the last glacial cycle indentified eight glacier advances. The four younger ones have been dated by dendrochronology, tephrochronology and radiocarbon dating. Although the bases of 10 m deep bogs were sampled, close limiting radiocarbon dates were not obtained because bog formation in this rain‐shadow area appears not to have commenced until ca.12000 yr ago. The outermost Little Ice Age moraine formed during the seventeenth century and three inner ones were deposited around ad 1805, 1845 and after 1890. Densely vegetated older moraines contiguous with Little Ice Age deposits are possibly of late Holocene age. Tephra from the eruption of Reclus volcano at ca. 11 880 yr BP was incorporated by a readvance that deposited large multiple moraines 10–16 km from the modern ice‐front; the oldest basal peat found inside the moraine has been dated to ca. 9200 yr BP. These bracketing dates indicate that some eastern outlet glaciers of the ice‐field advanced at a time when some western tidewater outlet glaciers terminated inside their modern limits. This questions the view of J. H. Mercer and other that Patagonian glaciers did not readvance during the late‐glacial interval. A stadial event also occurred when the glaciers were some 18–20 km from their modern positions and is closely dated to ca. 11880 yr BP because Reclus pumice flushed down‐glacier forms thick upper beds in outwash deltas deposited in proglacial lakes.The four older moraines pre‐date the late‐glacial eruption of Reclus but are not dated closely. Comparison of their spatial extent with well‐dated moraines in the Chilean Lakes Region suggests that they may mark advances culminating at ca. 14000 yr BP, ca. 20

ISSN:0267-8179    

DOI:10.1002/jqs.3390100302

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractOn North Harris and southeast Lewis a weathering limit separates glacially‐moulded bedrock on low ground from frost‐shattered bedrock and blockfields on high plateaux. Analysis of the depths of horizontal stress‐release joints demonstrates significant contrasts in bedrock weathering above and below this boundary, and the survival of gibbsite only in soils above the weathering limit indicates that it represents the upper limit of Late Devensian glacial erosion. The weathering limit declines regularly in altitude on either side of the former ice shed, and is therefore interpreted as a periglacial trimline defining the upper limit of a locally‐nourished ice mass at its maximum extent, rather than a former thermal boundary between protective cold‐based and erosive warm‐based ice. Calculated basal shear stress values are consistent with this interpretation. The configuration of the trimline indicates that at the last glacial maximum the area supported an ice cap that achieved a maximum altitude of ca. 700 m above present sea level and declined in altitude to the west‐northwest and east‐southeast at an average gradient of ca. 20 m km−1. Extrapolation of the dimensions of this ice cap suggests that it terminated ca. 7–10 km west of the present coast of Harris, and was confluent with mainland ice a short distance east of th

ISSN:0267-8179    

DOI:10.1002/jqs.3390100303

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractThe Saalian sequence of Belchatów, central Poland, is exceptionally thick and complete. Five tills, two from the older Saalian (Odranian, Drenthe) and three from the younger Saalian (Wartanian, Warthe), which are separated by the fluvial Chojny Formation, have been identified. The Saalian sequence at Bełchatów is underlain by Holsteinian and overlain by Eemian sequences, both palaeobotanically analysed. The Chojny Formation contains deposits of meandering (lower member) and braided (upper member) rivers, with occasional aeolian deposits. The lower member of the formation contains numerous organic layers. Pollen analysis indicates temperate (sub‐boreal) to cold (sub‐arctic) climatic conditions, with coniferous to mixed forests in the optimum phase of the interstadial. The interstadial floras of the Chojny Formation are interpreted as representing the intra‐Saalian Pilica Interstadial. Profiles from Belchatów are designated as the stratotype profiles of this substage. The data from Bełchatów show clearly that reforestation occurred between the major advances of the Saalian ice sheet, although only interstadial rank, not interglacial, may be inferred from the

ISSN:0267-8179    

DOI:10.1002/jqs.3390100304

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractThe 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.

ISSN:0267-8179    

DOI:10.1002/jqs.3390100305

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractPrevious stratigraphical investigations of Romney Marsh have tended to be local in scale, and this has hindered efforts to establish a unifying stratigraphical framework for this area. This paper addresses this problem, by describing the results of a 12‐km transect across Romney Marsh, linking previously studied back‐ and fore‐marsh sites (Horsemarsh Sewer and Broomhill respectively), and presenting additional pollen, diatom and radiocarbon data from an intermediate, mid‐marsh site (Brookland). One main organic unit is recorded across much of Romney Marsh, although its age, altitude and composition varies. Microfossil and radiocarbon data from Brookland and elsewhere on Romney Marsh show that this organic unit accumulated under a general removal and return of marine conditions that took place between ca. 5100 and 2000 yr BP.A recently proposed model of barrier development is used to investigate the history of back‐barrier sedimentation in Romney and Walland Marshes. This model suggests a three‐phase life‐history for gravel barriers, which consists of initiation, stability and breakdown. Although there are problems in relating back‐barrier deposits directly to barrier dynamics, nevertheless the Romney Marsh data do, for the most part, agree with the expected number and sequence of sea‐level tendencies predicted by this model. The back‐barrier stratigraphical data suggest that initiation, stability and breakdown of the Dungeness foreland occurred between ca. 6000 and 5000 yr BP, 5000 and 2000 yr BP and 2000 yr BP and pr

ISSN:0267-8179    

DOI:10.1002/jqs.3390100306

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractOrganic sediments in a gravel quarry at Block Fen, Cambridgeshire, form a sheet dividing lower from upper gravels. Analyses of pollen, macroscopic plant remains and molluscs from these organic sediments are presented. They indicate the presence of temperate freshwater and slightly brackish fine floodplain sediments, which, on the basis of the palaeobotany, are correlated with the temperate Ipswichian Stage. The freshwater sediments, ascribed to Ipswichian substage IIb, occur at ca. −3 m OD. Marine‐influenced tidal sediments, ascribed to Ipswichian substage III, occur at ca. −6 m OD. No evidence was found for the presence of more than one temperate stage in the sequence. The lower gravels are then correlated with the cold Wolstonian Stage and the upper gravels with the cold Devensian Stage. In contrast to the woodland environments indicated by the palaeobotany of the Ipswichian organic sediments, post‐Ipswichian pollen diagrams and macroscopic plant remains in the upper suite of sands and gravels indicate open tree‐less vegetation typical of the cold Devensian Stage. They also contain a typical cold‐stage mollusc fauna. The sediments containing these floras and faunas are associated with thermal contraction cracks, indicating the presence of permafrost. The final sand and gravel aggradation in the Devensian forms the Block Fen Terrace, near 0 m OD. The evidence indicates that it is younger than the lacustrine sediments resulting from the blocking of the Fenland at the Wash by Late Devensian ice at ca. 18.5 ka BP.The sequence at Block Fen is related to nearby Ipswichian and Devensian sediments at Chatteris, March, Wimblington and Mepal, and to deposits at Wretton on the east margin of Fenland. The correlation permits an outline reconstruction of the history of the valley carrying the River Great Ouse between the Isle of Ely and the Chatteris and March ‘islands’ from the time of a gravel aggradation before the Ipswichian to the Flandrian. The reconstruction shows the time and level of the Ipswichian marine incursion into the Middle Level of Fenland and the extent of aggradation and erosion i

ISSN:0267-8179    

DOI:10.1002/jqs.3390100307

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

ISSN:0267-8179    

DOI:10.1002/jqs.3390100308

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

ISSN:0267-8179    

DOI:10.1002/jqs.3390100301

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY