STRAIN MAGNITUDE IN TILLS ASSOCIATED WITH THE VARIABLE SUBGLACIAL DRAINAGE CONDITIONS UNDER THE SCANDINAVIAN ICE SHEET

Subglacial water plays a dominant role in many glacial processes influencing till formation processes and mechanisms of ice sheet movement over the substrate. Widespread deformation of soft, water-saturated basal sediments beneath the Pleistocene ice sheets was a consequence of glacier stress exerted on the bed. However, fluctuations of subglacial water pressure around the hydraulic ice lifting level along the ice/bed interface caused cyclic ice sheet decoupling from the bed and enhanced basal sliding. Water between ice and bed was a barrier for shear stress transmission into the underlying sediments preventing their deformation.

A combination of sedimentary characteristics, grain-size distribution, till fabric and till micromorphology analyses was applied to investigate in detail the properties of the Last Glaciation till (formed under the Scandinavian Ice Sheet) at Kozlowo in northern Poland. The till was up to 6 m thick and consisted of a number of structurally diversified units which recorded variable subglacial drainage conditions, till formation processes and ice sheet movement over the bed by some combination of basal sliding and bed deformation.

Estimated microfabric and macrofabric S₁ eigenvalues together with micromorphological parameters indicate low shear strains in the order of 10-10². The highest strains were recorded in the units of high subglacial water pressure conditions where decoupling of the ice sheet from the bed was a frequent process along with basal sliding over the thin water layer.

This work was supported by the National Science Centre, Poland 2015/17/D/ST10/02143.