Subglacial processes at the fringe of the last Scandinavian Ice Sheet in central Poland derived from macro‐ and micro‐sedimentological proxies

Abstract

Geological information contained in tills is important for deciphering processes that operated under past ice sheets and helps to constrain the fundamentals of glacial erosion, transport and deposition. Despite the progress in understanding glacial systems, interpretation of ancient tills and related deposits remains fragmentary, mostly because of the inaccessibility of ice–beds under present glaciers as modern analogues of the ancient systems. Here we report on the properties of a glacial sediment succession including various till facies and subglacial meltwater deposits from a site in central Poland close to the outermost extent of the Scandinavian Ice Sheet during the last glaciation. We use outcrop‐scale and micromorphological proxies to interpret the processes of till formation and deformation, and to infer the ice movement mechanisms. A combination of macro‐scale sedimentological and micromorphological data (till fabric signatures, grain‐size patterns, glaciotectonic structures, features of channelized and distributed subglacial drainage) indicates time‐transgressive succession of ice‐marginal deposition, subglacial deposition and deformation, pressurized meltwater flows at the ice–bed interface, and material release from stagnant ice. The investigated tills are to a large extent hybrid products of multiple superposed processes involving lodgement, deformation, and material cannibalization that mostly fall into the category of subglacial traction tills. Ample field and micromorphological evidence of basal decoupling suggests that the basal water pressure fluctuated around the ice flotation point. Ice movement was by a combination of enhanced basal sliding and very shallow (millimetre‐thick) bed deformation. Drainage of basal meltwater was facilitated by channelized flows in conduits subsequently filled by glacifluvial outwash. Our data indicate a mosaic of (sub)glacial processes that generated a complex deposit bearing evidence of overlapping accretion and deformation modulated by hydrological conditions at the ice–bed interface.