Significance of the record provided by Pleistocene fluvial deposits and their included molluscan faunas for palaeoenvironmental reconstruction and stratigraphy: case studies from the English Midlands

Abstract

The vast majority of Pleistocene fluvial deposits in northwest Europe are of cold climate origin with the most important sediment type being coarse sands and gravels formed under nival regimes. Less Commonly, sands and silts of interglacial deposition are preserved in the lower levels of major aggradations or in channels preserved beneath them. Molluscan faunas are common in both types of deposit, although the fine-grained temperate sediments show the best preservation and the most diverse assemblages. This paper will discuss the types of molluscan faunas found in fluvial deposits and assess their suitability for both climatic reconstruction and stratigraphic studies. While it has been clear for many years that reliable local environmental reconstructions can be made using Mollusca they have been less used for regional climatic assessment. Work in England over the past thirty years shows that faunas obtained from fluvial sediments, particularly if they contain both freshwater and terrestrial species, can provide a basis for regional climatic reconstruction. Mollusca may also allow the separation from each other of interglacials and interstadials, and permit these episodes to be more precisely defined than is allowed by other means such as palynology. Molluscan faunas may offer potential for the identification of temperature episodes which may have the same climatic status as interglacials, but which are not recognised as such by other palaeontological evidence such as pollen. The use of molluscan faunas for biostratigraphy has been firmly rejected by previous authors. However, recent work concentrating on sites of similar facies within single catchments has shown that both presence and absence of individual species, particularly those now exotic to the British Isles, [e.g. Belgrandia marginata (Michaud) and Corbicula fluminalis (Müller)] can be a useful guide to relative age. Examples illustrating the significance of these faunas in the stratigraphic record are largely taken from the fluvial deposits of the major rivers of the English Midlands (Warwickshire Avon, Great Ouse, Nene), but reference is also made to other studies on the deposits of the Thames. The ages of the faunas described range from the Cromerian Interglacial to the Devensian Glacial stage and includes data from sites in integlacials identified in the stratigraphic record but so far not formally named.