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Richard C. Preece Simon A. Parfitt G. Russell Coope Kirsty E. H. Penkman Philippe Ponel John E. Whittaker 《第四纪科学杂志》2009,24(6):557-580
Considerable debate surrounds the age of the Middle Pleistocene glacial succession in East Anglia following some recent stratigraphical reinterpretations. Resolution of the stratigraphy here is important since it not only concerns the glacial history of the region but also has a bearing on our understanding of the earliest human occupation of north‐western Europe. The orthodox consensus that all the tills were emplaced during the Anglian (Marine Isotope Stage (MIS) 12) has recently been challenged by a view assigning each major till to a different glacial stage, before, during and after MIS 12. Between Trimingham and Sidestrand on the north Norfolk coast, datable organic sediments occur immediately below and above the glacial succession. The oldest glacial deposit (Happisburgh Till) directly overlies the ‘Sidestrand Unio‐bed’, here defined as the Sidestrand Hall Member of the Cromer Forest‐bed Formation. Dating of these sediments therefore has a bearing on the maximum age of the glacial sequence. This paper reviews the palaeobotany and describes the faunal assemblages recovered from the Sidestrand Unio‐bed, which accumulated in a fluvial environment in a fully temperate climate with regional deciduous woodland. There are indications from the ostracods for weakly brackish conditions. Significant differences are apparent between the Sidestrand assemblages and those from West Runton, the type site of the Cromerian Stage. These differences do not result from contrasting facies or taphonomy but reflect warmer palaeotemperatures at Sidestrand and a much younger age. This conclusion is suggested by the higher proportion of thermophiles at Sidestrand and the occurrence of a water vole with unrooted molars (Arvicola) rather than its ancestor Mimomys savini with rooted molars. Amino acid racemisation data also indicate that Sidestrand is significantly younger than West Runton. These data further highlight the stratigraphical complexity of the ‘Cromerian Complex’ and support the conventional view that the Happisburgh Till was emplaced during the Anglian rather than the recently advanced view that it dates from MIS 16. Moreover, new evidence from the Trimingham lake bed (Sidestrand Cliff Formation) above the youngest glacial outwash sediments (Briton's Lane Formation) indicates that they also accumulated during a Middle Pleistocene interglacial – probably MIS 11. All of this evidence is consistent with a short chronology placing the glacial deposits within MIS 12, rather than invoking multiple episodes of glaciation envisaged in the ‘new glacial stratigraphy’ during MIS 16, 12, 10 and 6. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
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Simon A. Parfitt G. Russell Coope Sylvia M. Peglar John E. Whittaker 《Proceedings of the Geologists' Association. Geologists' Association》2010,121(1):55-52
At three sites along the North Norfolk coast, freshwater sediments containing arctic plants have been reported to occur immediately beneath the glacial deposits. The sections at Mundesley and Beeston are no longer visible but a small exposure of this ‘Arctic Fresh-water Bed’ has recently been rediscovered at Ostend near Bacton. The fossils from this deposit are distinctly high boreal/arctic in character, indicating a small, possibly ephemeral, pool in a generally open landscape supporting some dwarf shrubs and conifers. Noteworthy records include three beetle species (Pterostichus brevicornis, P. middendorffi and Helophorus obscurellus) that today have their closest occurrences on either the Kanin or Kola peninsulas in arctic Russia. A Mutual Climatic Range reconstruction on the limited beetle fauna suggests that the mean temperatures of the warmest and coldest months were between 9 °C and 11 °C and between −36 °C and −10 °C, respectively. Other noteworthy occurrences include the aquatic pulmonate gastropod Gyraulus rossmaessleri (second and earliest British record), the ostracod Amplocypris tonnensis (earliest British record), ground squirrel (Spermophilus sp.) and the first record of arctic lemming (Dicrostonyx sp.) from the Cromer Forest-bed Formation (CF-bF). The ‘Arctic Fresh-water Bed’ has been assigned to the Bacton Member of the CF-bF and formed during the early Anglian, correlated with Marine Isotope Stage (MIS) 12 of the deep sea record. The Ostend assemblage is therefore important in providing a glimpse of conditions that occurred during a major cold stage in the early Middle Pleistocene. 相似文献
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Dynamic two‐phase interaction of soil can be modelled by a displacement‐based, two‐phase formulation. The finite element method together with a semi‐implicit Euler–Cromer time‐stepping scheme renders a discrete equation that can be solved by recursion. By experience, it is found that the CFL stability condition for undrained wave propagation is not sufficient for the considered two‐phase formulation to be numerically stable at low values of permeability. Because the stability analysis of the two‐phase formulation is onerous, an analysis is performed on a simplified two‐phase formulation that is derived by assuming an incompressible pore fluid. The deformation of saturated porous media is now captured in a single, second‐order partial differential equation, where the energy dissipation associated with the flow of the fluid relative to the soil skeleton is represented by a damping term. The paper focuses on the different options to discretize the damping term and its effect on the stability criterion. Based on the eigenvalue analyses of a single element, it is observed that in addition to the CFL stability condition, the influence of the permeability must be included. This paper introduces a permeability‐dependent stability criterion. The findings are illustrated and validated with an example for the dynamic response of a sand deposit. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
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Jonathan R. Lee Emrys Phillips Stephen J. Booth James Rose Hannah M. Jordan Steven M. Pawley Martin Warren Russell S. Lawley 《Proceedings of the Geologists' Association. Geologists' Association》2013
Recent investigations from modern environments demonstrate that many terminal moraines do not simply record a single glacial maximum, but instead reveal a complex oscillatory pattern of ice-marginal behaviour including polyphase retreat. Within this study, we examine the geomorphology, geology and internal structure of a terminal moraine complex – the ‘Cromer Ridge’ in north Norfolk to reconstruct patterns of ice-marginal behaviour. Previously, this landform was interpreted as the limit of a southern extension of the British Ice Sheet during a Middle Pleistocene glaciation. Evidence presented here reveals a more complicated pattern of ice-marginal behaviour with the ‘Cromer Ridge’ reinterpreted as a ‘complex’ comprising several ridge elements. We propose that the maximum ice extent lay further to the south, with the size and morphology of the largest ridge element (the ‘Cromer Ridge’ as previously defined) a facet of thrust-stacking at an ice-marginal still-stand. We recognise multiple oscillations of the ice-front recorded against a twelve-stage model for the decay of the southern margins of a fast-flowing lobe of North Sea ice. Changes in ice-marginal dynamics are identified by the superimposition and lateral and vertical evolution of glacitectonic styles. Differences between these various states, and switches between ‘shallow’ and ‘deep’ thin-skinned glacitectonics, are strongly influenced by sub-marginal and proglacial water availability. Examination of the evidence for the morphostratigraphic proposals for the glacitectonic assemblage, within the context of the above interpretation, suggests that many of the ‘glacigenic landforms’ are erosional and a MIS 12 age of formation is favoured although several anomalies remain to be explained. 相似文献
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Adrian M. Lister 《第四纪科学杂志》1993,8(2):95-108
Deer species provide a valuable biostratigraphical tool through Cromer Forest-bed times, due particularly to species turnover between the Early and early Middle Pleistocene. This study is based on the reidentification of 348 fossil antlers. The provenance of most large mammal fossils from the CF-bF, collected over 150 years, was recorded only by the nearest coastal village. None the less, analysis of the cervid taxa by these ‘localities’ reveals interesting patterns. The fauna of the West Runton Freshwater Bed is of early Middle Pleistocene complexion, and that from the foreshore at East Runton is of Early Pleistocene. Pure or nearly pure early Middle Pleistocene assemblages also occur at Kessingland-Pakefield and at Trimingham. At Overstrand, Sidestrand, Mundesley, Bacton and Happisburgh, there is a mixture of Early and early Middle Pleistocene elements. Analysis of Savin's data shows that fossils of earlier species were generally found further down the beach than those of later ones. Late nineteenth century geological surveys, made when bone collecting was at its peak, give additional information about fossil horizons, which in several cases can be related to modern stratigraphical units. Most Early Pleistocene large-mammal bones came from Pastonian conglomerates, in contrast to small-mammal and molluscan assemblages mostly extracted from Pre-Pastonian Crag. However, the diversity of Early Pleistocene CF-bF cervid species in comparison with continental faunas, and their pattern of distribution between sites, suggests they may span more than one chronostratigraphic stage. Early Middle Pleistocene assemblages came from strata now referred to the Cromerian, and the differing proportions of taxa between sites provide limited evidence of time-transgression. 相似文献
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