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

摘要

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 (δ18O 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 (SM) composed of frozen diamict enriched with heavy stable isotopes overlain by laminated ice facies (SL) 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.