Glaciofluvial crevasse and conduit fills as indicators of supraglacial dewatering during a surge, Skeidararjokull, Iceland

This paper documents the glaciological structures associated with the surge of Skeidararjokull, Iceland, in 1991. These structures are interpreted as units of stratified ice, low-angle fractures, vertical and sub-vertical fra ctures (crevasse traces) and thrusts. The inferred thrusts are debris-rich and, unusually, have both down-glacier and up-glacier dips close to the ice margin. Sediment infills consist of either massive sand or horizontally st ratified sand units. The most significant debris-rich structures on the gla cier surface, however er. are supraglacial crevasse and conduit fills, whic h contain either massive or horizontally stratified silts, sands and granul e-gravels. These sediments infill both vertical fractures (relict crevasses ) and englacial conduits. At the stratigraphic base of these sediment fills there is evidence of syn-sedimentary deformation, suggesting that sediment ation occurred during crevasse closure and continued thereafter. We argue t hat these structures relate to an episode of supraglacial meltwater flow du ring the 1991 surge, caused by the build-up of subglacial water pressure in a linked-cavity system or some similar distributed drainage system beneath the glacier. The development of this high-level drainage route may have he lped regulate basal water pressures and therefore the active phase of the s urge. The idea that the supraglacial leakage of subglacial water may have p layed a role in terminating the surge is explored.