Abstract
Subglacial obstacles occurring in the path of advancing ice sheets generally generate higher longitudinal compression and higher frictional drag than a flat substrate. However, in the case of a soft sediment substratum, they can have a very different effect on ice sheet behaviour. This study concerns a substrate composed of very fine-grained sediments with low permeability. The relationship between subglacial obstacles and the overriding Scandinavian Ice Sheet was studied in an area of southern Poland where a small intervalley Neogene clay ridge (40m high) was present. Based on sedimentological and structural analysis of subglacial till and gravelly–sandy sediments, the basal depositional processes and subglacial conditions and their influence on ice sheet behaviour were analysed.The till and related deposits within the ridge reflecting high water pressure conditions and lack of glacitectonic deformations indicate that the clay ridge did not generate much resistance against the advancing ice sheet, but instead favoured basal slip: the impermeable substratum weakened the ice/bed coupling and promoted ice detachment from the substratum. Gravelly sandy inclusions at the till/clay contact indicate that during the first stage of ice sheet overriding, a canal drainage system developed at the ice/substrate interface. Varied geometry, size and location of inclusions of sorted sediments suggest periodic instability of the canal system, which could lead to its transformation from initially uniform to being composed of conduits of different sizes. During later stages of ice sheet overriding, a traction till was deposited and occasional drainage through a water film was sufficient to evacuate basal meltwater. The resulting change in the character of subglacial drainage was probably related to variations in water pressure gradient during progressive ice sheet advance.
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