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

Abstract

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 2–10 m wide and 20–60 cm 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.

The role of sediment supply in esker formation and ice tunnel evolution

“…Given the ice here was resting atop a fine-grained and probably saturated diamicton (Evans et al, 2008) or bedrock, this pressurized diamicton would have been squeezed into the low pressure ice tunnel, deforming any pre-existing sand and gravel and resulting in an esker with a deformed diamicton core, roughly conformable to the ridge (stage 3, Fig. 18c) (Stalker, 1960;Tylmann et al, 2013). If there is no deformable sediment beneath an ice tunnel, then in areas of very low sediment supply there would be no geomorphic signature of the ice tunnel, unless flows eroded into the substrate forming a tunnel channel or canal (cf.…”

Basal till and subglacial conditions at the base of the Upper Odra ice lobe (southern Poland) during the Odranian (Saalian) Glaciation

“…In a later model, soft subglacial beds were pre sented as a mo saic of de form ing and sta ble spots; the in ten sity of de for ma tion var ied in both time and space . Con cepts of spa tially var ied glacier bed char ac ter is tics have been de scribed also by other authors (see Knight, 2002;Van der Meer et al, 2003;Larsen et al, , 2007Stokes et al, 2007;Narloch et al, 2012Narloch et al, , 2013Tylmann et al, 2013). The dif fer ences in as sess ment of the role and scale of subglacial de for ma tion are largely due to the dif ferent rhe o log i cal mod els for the be hav ior of basal till, i.e.…”

Ductile and brittle styles of subglacial sediment deformation: An example from western Ireland

“…Here, ice flow vectors from bedrock striae, glacial bedforms and clast fabrics show consistent westward ice flow but with local variability related to bedrock topography (Synge, 1968;Hanvey, 1988;Smith and Knight, 2011). This evidence suggests relatively thin ice, complex patterns of ice interaction with its bed, and a 'checkerboard'-like substrate pattern in which bedrock is locally exposed in some places and sediments in others (e.g., Knight, 2002;Piotrowski et al, 2004;Tylmann et al, 2013). This setting provides the context for examining the coastal sediment exposure at Roonah Point, which is b 200 m long and 3-6 m high.…”