Patterns of deformation within a subglacial shear zone: Implications for palaeo-ice stream bed evolution

Abstract This paper presents macro and microsedimentological data recording subglacial deformation of a sediment succession at Przeszkoda in north-central Poland under the Vistula Ice Stream at the southern fringe of the Scandinavian Ice Sheet during the Weichselian glaciation. The sediment succession consists of (i) undeformed outwash sand, (ii) deformed outwash sand, (iii) bedded till, and (iv) massive till. These deposits were examined in a closely sampled vertical profile for kinematic structures, macrofabrics, grain shape characteristics, and micromorphology (detrital components, microstructures, clast microfabrics). The analyzed parameters suggest that the deposits were generated by a progressive homogenization and mixing of local material with far-travelled detritus successively released from the active ice sole. Grain crushing and abrasion characteristics lack any obvious trends in the vertical profile, indicating that the deformation was focused in a thin (cm-scale) layer which progressively migrated upwards parallel with sediment accretion. The closely spaced thin sections were collected in the deformed outwash sand, bedded till and massive till units. These revealed formation and evolution of stress-induced foliation in the glacier bed whereby three microfabric generations including S1 (earliest), S2 and S3 (latest) foliation were recognized. The geometry of these microfabrics is consistent with their formation in response to the development of subhorizontal Y-type shears, and up-ice dipping P-type and down-ice dipping R-type Riedel shears within the subglacial shear zone. The geometry and cross-cutting relationships between the microfabrics throughout the succession reflects changing deformation styles within a thin deforming bed that migrated time-transgressively up the profile during till accretion. Strain magnitude accumulated in the whole sediment succession was low, in the range of 101 to 102, suggesting that enhanced basal sliding was an important component contributing to fast flow of the ice stream.