Late and Middle Pleistocene deposits at Somersham, Cambridgeshire, UK: a model for reconstructing fluvial/estuarine depositional environments

Gravel quarries at Somersham, Cambridgeshire, have yielded evidence for a P leistocene channel of the R. Great Ouse, containing temperate stage sedimen ts between cold stage sediments. In the earlier cold stage, fluviatile grav els and floodplain loessic sediment accumulated. In the later cold stage a further series of gravel units and floodplain sediments were deposited, tog ether with lake sediments. The lake sediments are associated with Lake Spar ks, dammed by Late Devensian ice in the Wash at ca. 18.5 ka BP. The lake se diments overlie gravels with a radiocarbon date from an organic horizon ind icating a Middle Devensian age. Clast lithological analyses from the earlie r and later gravels suggest that reworking of gravels has occurred within a relatively stable catchment. The petrography of the earlier cold stage loe ssic sediment and temperate stage fine sediment indicates an Anglian affini ty, which conflicts with the biostratigraphic interpretation. Pollen and ma croscopic plant remains from sediments of both cold stages and from the tem perate stage indicate, respectively, assemblages with a typical full-glacia l aspect with a rich flora of shrubs and open ground herbs(including an ass emblage at ca. 18 ka), and temperate freshwater and marine-influenced organ ic sediments. On the basis of pollen analysis these are ascribed to substag es Ip II and III of the Ipswichian Stage(O.I.S. 5e), with a Pinus-Quercus-C orylus biozone in the former and a biozone with Cai pinus in the latter. Ma rine-influenced sediments, at - 3.7 to - 0.3 m OD, indicate transgression i n Ip II and regression in Ip III. Molluscan assemblages from the temperate stage and the later cold stage are described; two are from the Late Devensian, at a time near the maximum ext ension of ice into the Wash. Foraminifer and ostracod faunas are described from post-Ipswichian sediments and may be reworked. Radiocarbon dates confi rm the age of the later gravel suite as Devensian and a calibration of the measurements is given. Amino acid ratios from Corbicula fluminalis valves f rom temperate stage sediments are reported, with measurements from differen t parts of the valve; the results tend to support an Ipswichian age. TL mea surements of the earlier cold stage loessic sediment and associated sand in dicate a pre-Ipswichian age for the sediments. The earlier cold stage is co rrelated with the pre-Ipswichian cold stage, the Wolstonian of Mitchell et al.(1973); problems with this correlation are discussed. Various periglacial phenomena, including thermal contraction networks and c racks, diapirs, involutions and coversand are associated with the Devensian sequence. The complex environmental history, based on stratigraphy and pal aeontology, is described, and related to other nearby sites in southern Fen land. (C) 1999 Elsevier Science Ltd. All rights reserved.