Till formation under a soft-bedded palaeo-ice stream of the Scandinavian Ice Sheet, constrained using qualitative and quantitative microstructural analyses

This study combines micro- and macroscale studies, laboratory experiments and quantitative analyses to decipher processes of till formation under a palaeo-ice stream and the nature of subglacial sediment deformation. Till micromorphology (grain lineations, grain stacks, turbate structures, crushed grains, intraclasts and domains), grain-size and till fabric data are used to investigate a basal till generated by the Vistula Ice Stream of the Scandinavian Ice Sheet during the last glaciation in north-central Poland. A comparison of microstructures from the in situ basal till and laboratory-sheared till experiments show statistical relationships between the number of grain lineations and grain stacks; and between the number of grain lineations and turbate structures. Microstructures in the in situ till document both brittle and ductile styles of deformation, possibly due to fluctuating basal water pressures beneath the ice stream. No systematic vertical and lateral trends are detected in the parameters investigated in the in situ till, which suggests a subglacial mosaic of relatively stable and unstable areas. This situation can be explained by an unscaled space-transgressive model of subglacial till formation whereby at any given point in time different processes operated in different places under the ice sheet, possibly related to the distance from the ice margin and water pressure at the ice–bed interface. A new quantitative measure reflecting the relationship between the number of grain lineations and grain stacks may be helpful in discriminating between pervasive and non-pervasive deformation and constraining the degree of stress heterogeneity within a deformed bed. Independent strain magnitude estimations revealed by a quantitative analysis of micro- and macro-particle data show low cumulative strain in the ice-stream till in the order of 10–10².