Styles of till deposition at the margin of the Last Glacial Maximum North Sea lobe of the British-Irish Ice Sheet: An assessment based on geochemical properties of glacigenic deposits in eastern England

摘要

The geochemical composition of Last Glacial Maximum (LGM) tills and glacitectonites in east Yorkshire and Lincolnshire was analysed to assess vertical changes in their geochemical signature with respect to the current regional stratigraphic division and generic models of till deposition. Results fail to precisely differentiate the traditional Basement, Skipsea and Withernsea till types. Instead, there is often more variability within these till types than between them. The geochemical signatures are also laterally discontinuous and are often repeated vertically at a number of sites, suggesting that the till sequences are composed of a number of lithologically distinct rafts, which have been tectonically stacked or elevated to higher levels in the sediment pile. At Dimlington the production of a glacitectonically folded and stacked moraine is proposed as a mechanism to explain the remarkably thick sequence of Withernsea Till and the repetitions in geochemical composition. We argue that the changes in till matrix geochemistry are related to changes in the dominance of ice lobes of different provenance, which reworked pre-existing Quaternary sediments, excavated local bedrock and progressively mixed these sediments with more far-travelled material from further north. The most significant implication of these findings is that the current LGM till stratigraphy of east Yorkshire and Lincolnshire is too simplified and therefore does not recognise the sedimentary and structural complexity produced by repeated onshore, possibly surging, flow by a dynamic North Sea lobe at the eastern margin of the British-Irish Ice Sheet.