Till genesis at the bed of an Antarctic Peninsula palaeo-ice stream as indicated by micromorphological analysis

Sediment cores from cross-shelf troughs on the NE Antarctic Peninsula shelf recovered tills with variable shear strengths that represent different subglacial depositional regimes. In addition to detailed qualitative micromorphological descriptions, a quantitative method was applied, which revealed a higher abundance of boudins and intraclasts and a lower abundance of crushed and fractured grains in samples from the soft till compared with samples from the underlying stiff till. This is the first evidence of significant (micro-scale) differences between the two types of till and thus strengthens previous interpretations that were based primarily on shear strength. The differences between the soft and stiff till relate to a deforming continuum whereby the initial deposition of till as ice advanced across the shelf produced ductile structures before dewatering and compaction led to the formation of brittle structures such as crushed and fractured grains in the now stiff till. A change in ice-flow dynamics led to streaming flow and the deformation of the upper parts of the stiff till that was being reworked into a soft till. The soft till facilitated ice streaming, and progressive shearing led to the homogenization of the ice stream substrate, which was partially advected downstream. The resulting till thus contains poly-deformational structures, with deformation structures inherited from the stiff till being generally poorly preserved. Our micromorphological analysis of the soft till provides the first widespread sedimentological evidence of deformation across the palaeo-ice stream bed on the NE Antarctic Peninsula shelf.

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