Influence of the glacier bed lithology on the formation of a subglacial till sequence - ring-shear experiments as a tool for the classification of subglacial tills

A glaciogenic sediment sequence deposited by a local glacier in the Rhine c atchment area near Sargans (Eastern Swiss Alps) was investigated in order t o examine the shear behaviour of two different tills. In addition to ring-s hear experiments, examination of clast lithology, clay mineralogy and physi cal properties of all samples were determined. SEM-analyses of artificially produced deformation patterns in the till samples show only a very thin sh ear zones of 2-3 mum thickness, despite long shear distances of up to 145 m m. A clear relationship between local bedrock and the formation of a thin b asal layer of heavily deformed till can be demonstrated. This locally deriv ed till layer exhibits distinctly different shearing properties compared to the massive overlying till unit that is composed of more distantly transpo rted lithic material from the upper catchment area of the glacier. The diff erence in the angle of internal friction between the two tills is > 10 degr ees at residual shear strength, although the clay mineralogy of both tills is similar. It can further be demonstrated that after the deposition of a 1 0-15 cm thin till layer directly on bedrock, the flow dynamics of the glaci er tongue may have changed due to differences in glacier bed debris litholo gy. Differences in the shear behaviour of the sediments, allied to microsco pic analyses, point to an important environmental change from a deformation till to an overlying "non-deformed till". (C) 2001 Elsevier Science Ltd. A ll rights reserved.