Sedimentological record of subglacial conditions and ice sheet dynamics of the Vistula Ice Stream (north-central Poland) during the Last Glaciation

摘要

Deposits of the Vistula Ice Stream draining the Scandinavian Ice Sheet during the Last Glaciation were investigated at four field sites in north-central Poland using micro- and macroscale features. The study reveals several till units with specific structural, textural and lithological characteristics. The individual till units are either macro-scopically massive or bedded, and the contacts between the units are either sharp or transitional. The nature of the contacts with the underlying sediments, ductile deformation structures, largely undeformed clayey dasts, tectonic lamination, thin horizontal stringers of sorted sediments, ploughing marks, boulder pavements, and striated upper surfaces of pebbles in the till indicate both bed deformation and enhanced basal sliding under high subglacial water pressure conditions. It is suggested that the till is a hybrid deposit generated by some combination of lodgement, deformation and ploughing punctuated by periods of basal decoupling. The depth of deformation at any point in time was thinner (up to several decimetres) than the maximum till thickness (c. 2.5 m). The ice sliding velocity estimations indicate velocities of less than 100 to over 2000 m yr~(-1), which suggests an unstable and highly dynamic ice lobe, consistent with spatial variability of till characteristics. Sand wedges in the deposits beneath the till and the nature of the till/bed interface indicate permafrost under the advancing ice sheet. We suggest that under the increasing ice thickness, a layer of thawed, water-saturated sediment formed on top of the still-frozen ground due to inefficient drainage, and contributed to ice streaming by promoting pervasive deformation and basal sliding.