Debris entrainment by basal freeze‐on and thrusting during the 1995–1998 surge of Kuannersuit Glacier on Disko Island, west Greenland

Abstract

Kuannersuit Glacier, a valley glacier on Disko Island in west Greenland, experienced a major surge from 1995 to 1998 where the glacier advanced 10·5 km and produced a ~65 m thick stacked sequence of debris-rich basal ice and meteoric glacier ice. The aim of this study is to describe the tectonic evolution of large englacial thrusts and the processes of basal ice formation using a multiproxy approach including structural glaciology, stable isotope composition (δ 18 O and δD), sedimentology and ground-penetrating radar. We argue that the major debris layers that can be traced in the terminal zone represent englacial thrusts that were formed early during the surge. Thrust overthrow was at least 200-300 m and this lead to a 30 m thick repetition of basal ice at the ice margin. It is assumed that the englacial thrusting was initiated at the transition between warm ice from the interior and the cold snout. The basal debris-rich ice was mainly formed after the thrusting phase. Two sub-facies of stratifi ed basal ice have been identifi ed; a lower massive ice facies (S M ) composed of frozen diamict enriched with heavy stable isotopes overlain by laminated ice facies (S L ) consisting of millimetre thick lamina of alternating debris-poor and debris-rich ice. We interpret the stratifi ed basal ice as a continuum formed mainly by freeze-on processes and localized regelation. First laminated basal ice is formed and as meltwater is depleted more sediment is entrained and fi nally the glacier freezes to the base and massive diamict is frozen-on. The increased ability to entrain sediments may partly be associated with higher basal freezing rates enhanced by loss of frictional heat from cessation of fast fl ow and conductive cooling through a thin heavily crevassed ice during the fi nal phase of the glacier surge. The dispersed basal ice facies (D) was mainly formed by secondary processes where fi ne-grained sediment is mobilized in the vein system of ice. Our results have important implications for understanding the signifi cance of basal ice formation and englacial thrusting beneath fast-fl owing glaciers and it provides new information about the development of landforms during a glacier surge.

Debris entrainment and landform genesis during tidewater glacier surges

“…There is evidence for the folding and stacking of basal ice resulting in overall thickening and the formation of complex facies architecture [e.g., Knight, 1989;Sharp et al, 1994;Waller et al, 2000;Larsen et al, 2010]. For the NW section this is clearest where solid stratified subfacies ice, interpreted as frozen-on subglacial till (and therefore representing stratigraphically the lowermost basal ice unit), has been elevated on top of both the banded subfacies and the englacial facies ice (Figure 2a).…”

“…Two main mechanisms have been proposed for the origin of upglacier-dipping structures: (1) debris-bearing thrust faults originating from the bed in a zone of longitudinal compression [e.g., Hambrey and Dowdeswell, 1997;Glasser et al, 1998;Lønne, 2006;Larsen et al, 2010] and (2) the deformation of structures initially formed as vertical crevasse squeezes, which are then reoriented in a down-ice direction during quiescent phase ice flow. This forms a favorably inclined slip plane along which thrust-style displacement (TSD) can occur during a subsequent surge Woodward et al, 2002;Rea and Evans, 2011].…”

“…Debris-rich basal ice is sometimes exposed at the margins of surge-type glaciers [e.g., Sharp et al, 1994;Bennett et al, 2004;Larsen et al, 2010]. The properties and character of this debris-rich ice can provide an insight into processes occurring at the glacier bed, particularly in relation to sediment entrainment [e.g., Lawson, 1979;Sugden et al, 1987;Alley et al, 1997], and therefore offer great potential for investigating basal conditions at surge-type glaciers.…”

“…Of these, debris-rich ice facies are typically thought to have formed due to the entrainment of subglacial material via four main processes: (1) en masse freeze-on [e.g., Weertman, 1961], (2) regelation around bedrock obstacles [e.g., Kamb and LaChapelle, 1964], (3) regelation into the bed [e.g., Iverson, 2000], and (4) glacihydraulic supercooling [e.g., Cook et al, 2010]. Existing work on basal ice sequences at surge-type glaciers has highlighted the importance of tectonic deformation in their formation, typically resulting in the thickening of units through faulting and folding [Sharp et al, 1994;Larsen et al, 2010].…”

“…In the case of polythermal surge-type glaciers, rapid freeze-on occurs upon surge termination as a cold wave propagates downward through the newly thinned and heavily crevassed glacier [cf. Alley et al, 1997;Larsen et al, 2010].…”