Microstructures in thin sections of several kinds of till

Abstract

At Obórki in central Poland we examined a 3 m thick Late Weichselian basal till deposited by the Vistula Ice Stream. A multi-proxy approach involving a study of grain size, grain roundness, crushing index, petrographic composition, till fabric and till micromorphology in a closely sampled vertical profile was applied to decipher till formation and ice movement mechanisms. The till consists of a succession of four texturally and structurally different units generated by a combination of processes related to effective stresses exerted on the bed by active ice. Deformation, intergranular advection, clast ploughing and localized basal decoupling played important roles during till formation. Micromorphological analysis reveals that the most common S-matrix microstructures are grain lineations, grain stacks, turbate structures, till pellets and crushed grains. Microshears dominate whereas plasmic fabric is not well developed. We use grain roundness, crushing indices and microshear geometry to determine subglacial strain magnitude in the till matrix. Microfabric and macrofabric S1 eigenvalues together with IL indexes indicate low shear strains in the order of 10–102. Strain estimates vary depending on the particle sizes used suggesting a plastic mode of till deformation. All proxies studied suggest complex and temporally variable subglacial conditions under the marginal part of the Vistula Ice Stream, probably caused by changes in till rheology related to fluctuating porewater pressures. Fast ice flow was facilitated by a combination of bed deformation and enhanced basal sliding on a thin water layer. Our results may also be representative for other terrestrially-based Pleistocene ice streams moving over soft beds.