The ice/bed interface mosaic: deforming spots intervening with stable areas under the fringe of the Scandinavian Ice Sheet at Samplawa, Poland KAROL TYLMANN, JAN A. PIOTROWSKI AND WOJCIECH WYSOTA 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 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. Karol Tylmann (e-mail: karolgeo@doktorant.umk.pl) and Wojciech Wysota (e-mail: wysota@umk.pl), Faculty of Earth Sciences, Nicolaus Copernicus University, Lwowska 1, 87-100 Torun, Poland; Jan A. Piotrowski (e-mail: jan.piotrowski@geo.au.dk), Department of Geoscience, Aarhus University, Høegh-Guldbergs Gade 2, DK-8000 Aarhus C, Denmark; received 26th April 2012, accepted 14th August 2012. Since the recognition of strong rheological coupling between a glacier sole and the soft sediments below (Boulton 1986; Murray 1997), suggesting that the basal part of a glacier and the uppermost part of the deform- able bed below should be considered as a subglacial continuum, much research has been devoted to the role of bed deformation in glacier dynamics (Boulton & Hindmarsh 1987; Engelhardt & Kamb 1998), subgla- cial sediment advection (Alley 1991; Hart 1995) and the formation of certain landforms (King et al. 2009; Clark 2010). Earlier studies advocating thick, widespread subglacial deforming beds underneath both modern (Alley et al. 1986; Blankenship et al. 1986) and past (Clark 1994) ice sheets were followed by more con- servative estimates emphasizing the complexity of sub- glacial systems, with modes of ice movement switching in time and space between bed deformation and enhanced basal sliding (Iverson et al. 1995; Clarke 2005). In the conceptual model of an ice/bed mosaic (Piotrowski & Kraus 1997; Piotrowski et al. 2004), the bed is envisaged as a raster of (i) quasi-stable patches separated from the ice sole by a layer of pressurized water preventing stress transformation to the bed, or patches of well-drained deposits that are strong enough to withstand glacial shear stresses; and (ii) areas of soft, deforming sediment readily advecting in accord with ice movement. Alternating phases of basal ‘stick’ and ‘slip’ (Anandakrishnan & Alley 1994; Fischer & Clarke 1997; Hooke et al. 1997; Fischer et al. 1999; Iverson 2010) related to variations in pore-water content and/or pressure, shear stress magnitude and ice-flow velocity documented under modern glaciers support theoretical models of ‘sticky spots’ (Alley 1993; Knight 2002; Stokes et al. 2007). Furthermore, in situ measurements of till deformation dynamics beneath present-day gla- ciers (Iverson et al. 1995; Hart et al. 2011) document a highly non-linear behaviour characterized by frequent switches between more stable and more dynamic phases. Another factor contributing to the complexity of the ice/bed interface (IBI) may be a discontinuous subglacial permafrost, which, if present, enhances rheo- logical contrasts in the bed (Waller et al. 2009, 2011). Recent studies of sediments deposited and deformed by large Pleistocene ice sheets yield further support to the concept of co-existing stable and deforming patches transient in time and space (Lee & Phillips 2008; Lese- mann et al. 2010), but unequivocal evidence of selective subglacial deformation is notoriously difficult to provide. Here we present a geological record of subgla- cial processes under the last Scandinavian Ice Sheet as exposed in a gravel pit in northern Poland. Our aims are (i) to document the signature of contrasting proc- esses that operated at the IBI at this locality; (ii) to suggest the sequence of events that led to the formation of this geological record and relate these events to spe- cific glaciological conditions; and (iii) to propose a This ms is for the special issue on Subglacial Environments. DOI 10.1111/j.1502-3885.2012.00294.x © 2012 The Authors Boreas © 2012 The Boreas Collegium