The ice/bed interface mosaic: deforming spots intervening with stable areas under the fringe of the Scandinavian Ice Sheet at Sampawa, Poland

Tylmann, K., Piotrowski, J. A. & Wysota, W. 2013 (April): The ice/bed interface mosaic: deforming spots intervening with stable areas under the fringe of the Scandinavian Ice Sheet at Samplawa, Poland. Boreas, Vol. 42, pp. 428-441. 10.1111/j.1502-3885.2012.00294.x. ISSN 0300-9483. The glacial sediment succession exposed close to the southern margin of the Late Weichselian Scandinavian Ice Sheet in Poland reveals a mosaic consisting of isolated patches of heavily deformed deposits separated by areas lacking any visible evidence of deformation. In the studied outcrop, the subglacial deforming spots composed of outwash deposits intercalated with till stringers are about 210m wide and 2060cm thick. They rest on outwash sediments and are covered by a basal till. Based on structural and textural characteristics, the deforming spots are interpreted as previous R-channels filled with meltwater deposits. Lack of deformation in outwash sediment immediately beneath the deforming spots and in the intervening areas between the channels suggests that the ice-bed was frozen and the deformation of the channel infill was facilitated by high pore-water pressure arising because water drainage into the bed was impeded by permafrost. Channel infill deposits and the till immediately above were coevally deformed to a strain of less than 9. This study documents the possible co-existence of deforming and stable areas under an ice sheet, generated by spatially varying thermal and hydrological conditions affecting sediment rheology.