Morphology and sedimentology of a giant supraglacial, ice-walled, jokulhlaup channel, Skei d ararjokull, Iceland: implications for esker genesis

This paper examines the sedimentary infill of a spectacular, 500-m-long, 10 0-m-wide ice-walled supraglacial channel, excavated into the snout of Skeio ararjokull, Iceland during the November 1996 jokulhlaup, The ice-walled cha nnel developed in an area of the glacier, which was extensively fractured d uring the jokulhlaup. Sculpting of the ice-walled channel into the active s nout of Skeioararjokull suggests that the presence of stagnating glacier ic e is not a prerequisite for the development of ice-walled channels. The ice -walled channel occupied an inter-lobate location, which acted as a focus f or meltwater during the November 1996 jokulhlaup, The geometry of the supra glacial ice-walled channel system acted as a major control on the morpholog y and sedimentology of jokulhlaup deposits, through the tremendous spatial variability of resultant flow conditions. Maximum calculated jokulhlaup pow ers and shear stresses for the supraglacial ice-walled channel reached 40,0 00 W m(-2) and 5000 N m(-2), respectively, with associated mean flow veloci ties between 7 and 11 m s(-1) Within the main ice-walled channel, Ground penetrating Radar and outcrop ex posure provide evidence of an similar to 8-m-thick progradational and aggra dational gravel macroform succession. The supraglacial ice-walled channel s ystem is therefore analogous to a bedrock-confined fluvial system. This stu dy provides a new analogue for the interpretation of ice-contact glaciofluv ial deposits associated with former ice margins in Iceland and other areas subject to high magnitude discharges. Former supraglacial ice-walled channe ls resulting from tunnel collapse and ice margin break-up during high magni tude jokulhlaups will be associated with extensive coarse-grained, heavily kettled proglacial outwash surfaces. It is clear that the relationship betw een the characteristics of former ice-walled channels labeled as eskers and the prevailing,glaciological and hydrological conditions needs to be modif ied in light of our knowledge of a modem flood-related large-scale supragla cial channel and its sedimentary infill, Such re-evaluation may provide a v aluable new insight into former ice margin positions, modes of glacier retr eat, and the role of high magnitude floods within the sedimentary record of former proglacial areas. This study therefore improves our understanding o f the meltwater magnitude and frequency regime of former glaciers. (C) 2001 Elsevier Science B.V. All rights reserved.