Deformation mechanisms and kinematics of a soft sedimentary bed beneath the Scandinavian Ice Sheet, north-central Poland, revealed by magnetic fabrics

摘要

Characterising deformable substrates of modern and past ice sheets is a key factor in understanding ice mass behaviour and dynamics. This is challenging when field-scale signatures of deformation in soft sedimentary beds areweakly developed or absent. To overcome these difficulties, in this study, we combinemicro- and macroscale studies and quantitative analyses to decipher the nature of subglacial sediment deformation under a Pleistocene fast-flowing (streaming) ice sheet. We investigated a complex sedimentary succession at Nieszawa in north-central Poland. A multi-proxy approach involving till micromorphology in thin sections, ripple-cross lamination orientation, till fabric, clast microfabric and the anisotropy of magnetic susceptibility (AMS) data in a closely sampled vertical profile throughout the fluvial sand and tillwas used to reveal subglacial bed deformationmechanismsand kinematics. This records subglacial deformation of the sedimentary succession at Nieszawa under low basal shear stresses, where simple shear was the dominant bed deformation mechanism. Intergranular advection, decoupling at the ice-bed interface related to high subglacialwater pressure, clast ploughing, and basal sliding also took place. The main finding of this study is that AMS reveals the shear strain development and ice flow direction in the structurally and texturally complex deposits underweak coupling conditions at the ice-bed interface beneath a fast flowing/streaming ice sheet, where strain and kinematic indicators other than AMS fabrics are vague or absent. (C) 2021 Elsevier B.V. All rights reserved.