Terrestrial glacial sedimentation on the eastern margin of the Irish Sea basin: Thurstaston, Wirral

Abstract

This paper provides the first comprehensive description and interpretation of Pleistocene glacigenic deposits exposed in a cliff section at Thurstaston on the Wirral Peninsula, NW England. The section occupies a strategic position where the Irish Sea ice sheet impinged on the English Midlands and North Wales coast. The section comprises six lithofacies: diamicton, gravel, sand, laminites, mud and cobble pavements. The diamicton lithofacies can be divided into an upper, clast-poor sandy diamicton and multiple units of a lower, clast-rich sandy diamicton. Both diamictons exhibit cobble pavements. The two diamicton lithofacies are distinguished on the basis of textural composition, clast lithology, clast shape, clast surface features and clast macrofabrics. Between the diamicton lithofacies are interbeds of sands and gravels of variable thickness. Minor mud and laminites also occur in close association with the sands and gravels. Both the diamictons are interpreted as basal, deformation tills with the interbeds of the sand, gravel and mud lithofacies as indicators of subglacial meltwater flow and ponding. The cobble pavements are interpreted as the result of clasts sinking within, or to the base of, the deforming layer. The sedimentary succession at Thurstaston is best explained by the advance and subsequent recession of a single terrestrially based ice sheet during the Late Devensian. There is no evidence at Thurstaston to suggest a glaciomarine origin for the Late Devensian deglaciation sediments on this margin of the Irish Sea basin. The evidence at Thurstaston points to constructional deformation with the net accretion of till and interbeds caused by the upward migration of the deforming layer base.