Morphology and sedimentology of a giant supraglacial, ice-walled, jökulhlaup channel, Skeiðarárjökull, Iceland: implications for esker genesis

This paper examines the sedimentary infill of a spectacular, 500-m-long, 100-m-wide ice-walled supraglacial channel, excavated into the snout of Skeiðarárjökull, Iceland during the November 1996 jökulhlaup. The ice-walled channel developed in an area of the glacier, which was extensively fractured during the jökulhlaup. Sculpting of the ice-walled channel into the active snout of Skeiðarárjökull suggests that the presence of stagnating glacier ice 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 for meltwater during the November 1996 jökulhlaup. The geometry of the supraglacial ice-walled channel system acted as a major control on the morphology and sedimentology of jökulhlaup deposits, through the tremendous spatial variability of resultant flow conditions. Maximum calculated jökulhlaup powers and shear stresses for the supraglacial ice-walled channel reached 40,000 W m−2 and 5000 N m−2, respectively, with associated mean flow velocities between 7 and 11 m s−1. Within the main ice-walled channel, Ground Penetrating Radar and outcrop exposure provide evidence of an 8-m-thick progradational and aggradational gravel macroform succession. The supraglacial ice-walled channel system is therefore analogous to a bedrock-confined fluvial system. This study provides a new analogue for the interpretation of ice-contact glaciofluvial deposits associated with former ice margins in Iceland and other areas subject to high magnitude discharges. Former supraglacial ice-walled channels resulting from tunnel collapse and ice margin break-up during high magnitude jökulhlaups will be associated with extensive coarse-grained, heavily kettled proglacial outwash surfaces. It is clear that the relationship between the characteristics of former ice-walled channels labeled as eskers and the prevailing glaciological and hydrological conditions needs to be modified in light of our knowledge of a modern flood-related large-scale supraglacial channel and its sedimentary infill. Such re-evaluation may provide a valuable new insight into former ice margin positions, modes of glacier retreat, and the role of high magnitude floods within the sedimentary record of former proglacial areas. This study therefore improves our understanding of the meltwater magnitude and frequency regime of former glaciers.