Sediment deformation beneath glaciers and its coupling to the subglacial hydraulic system

The extent and style of shear deformation in sediments beneath modern glaciers and the geological evidence for such deformation in deposited sediments are reviewed. New evidence is presented from beneath a modern glacier of the spatial and temporal patterns of water pressure fluctuation and of time dependent patterns of deformation in sediments. It is concluded that in most experimental sites beneath soft-bedded modern glaciers, deformation is a significant or major contributor to glacier movement and the resultant discharge of till is large enough to make sediment deformation a major till forming process. Particular modes of deformation facilitate incorporation of underlying material into the till, whilst the capacity of a deforming till to absorb strain can protect the underlying strata from deformation, leading to the commonly found relationship where till overlies other strata with a sharp planar interface. It is argued that the almost ubiquitous occurrence of drumlins on the beds of former ice sheets is a reflection of the widespread occurrence of sediment deformation beneath them, with important implications for the coupling of ice sheet flow and bed properties. It is argued that the mechanical behaviour of the subglacial system is not simply determined by till properties but largely controlled by the subglacial water pressure regime determined by the nature of subglacial drainage. Results of field experiments show how the nature of the basal hydraulic system can play a vital role in controlling the coupling between the glacier and till deformation processes. They show that rapid glacier advances can produce undrained loading of sediments, that effective pressure may increase either upwards or downwards in a till according to the direction of drainage and that interstitial water pressures in subglacial sediments can show large and rapid variations, producing strong variations in the rate and distribution of strain and in the partitioning of basal movement between sliding and deformation. (C) 2001 Elsevier Science Ltd and INQUA. All rights reserved.