Geological evidence for subglacial deformation of Pleistocene permafrost

Abstract

Recent work in modern and ancient glacial environments has demonstrated the ability of cold-based glaciers to interact with permafrost. Geological evidence for glacier–permafrost interactions is revealed in Arctic regions where permafrost has persisted since deglaciation. Whilst similar interactions probably occurred near the margins of former ice sheets in the mid-latitudes, this interpretation is rarely applied to unfrozen glacigenic sequences. This review considers the extent to which this alternative hypothesis can explain two key aspects of the glaciotectonic sequences of North Norfolk that have traditionally been attributed to the deformation of unfrozen sediment. The substantial thickness (>10m) of the pervasively deformed sequences and the preservation of stratified sand intraclasts within them are consistent with deformation at temperatures slightly below the pressure melting point (warm permafrost). Such deformation is also consistent with the pre-glacial environment, which was characterised by continuous permafrost. The hypothesis of deformation at sub-freezing temperatures should be considered more widely when interpreting glaciotectonically deformed, ice-marginal sequences in the mid-latitudes. The application of geological evidence to reconstruct basal thermal regimes beneath former glaciers would complement existing geomorphological inverse models and provide additional information to improve the parameterisation of subglacial processes in numerical ice-sheet models.