Till kinematics in the Stargard drumlin field, NW Poland constrained by microstructural proxies

Abstract

ABSTRACTDeciphering the relationships between streamlined subglacial landforms and deposits therein helps to constrain the dynamics of past ice sheets. Here we present microstructural analyses of till from the largest drumlin field in the North European Lowland generated by a major palaeo‐ice stream of the Scandinavian Ice Sheet. Data derived from thin sections and X‐ray microtomography (μCT) reveal multiple microstructures including microshears, circular structures, grain stacks and crushed grains. Long axes of micro‐clasts measured in thin sections are directionally clustered and have low, down‐ice‐oriented dip angles. Some samples reveal subordinate clustering oriented at high angles to the main direction. The μCT data show low clustering strengths of clasts and no systematic fabric pattern in the entire set of samples. There is no correspondence between the clast orientations and the directions of drumlin long axes, suggesting that the till is antecedent in relation to the drumlin‐shaping process. The observations indicate spatially and temporally variable, shallow‐seated brittle and ductile deformation controlled by porewater pressure fluctuations and generated time‐transgressively parallel to till accretion. Consistently with previous studies, the data suggest that the Stargard drumlins are erosional remnants of a pre‐existing terrain shaped by material removal from the inter‐drumlin areas.