Architectural analysis of a volcaniclastic jökulhlaup deposit, southern Iceland: sedimentary evidence for supercritical flow

The 1918 eruption of the glacially capped Katla volcano, southern Iceland, generated a violent jo¨kulhlaup, or glacial outburst flood, inundating a large area of My´rdalssandur, the proglacial outwash plain, where it deposited ca 1 km3 of volcaniclastic sediment. The character of the 1918 jo¨kulhlaup is contentious, having been variously categorized as a turbulent water flow, a hyperconcentrated flow or as a debris flow, based on localized outcrop analysis. In this study, outcrop-based architectural analyses of the 1918 deposits reveal the presence of lenticular and tabular bedsets associated with deposition from quasi-stationary antidunes and down-current migrating antidunes, and from regular based bedsets, associated with transient chuteand- pool bedforms, all of which are associated with turbulent, transcritical to supercritical water flow conditions. Antidune wavelengths range from 24 to 96 m, corresponding to flow velocities of 6 to 12 m sec)1 and average flow depths of 5 to 19 m. This range of calculated flow velocities is in good agreement with estimates made from eyewitness accounts. Architectural analysis of the 1918 jo¨kulhlaup deposits has led to an improved estimation of flow parameters and flow hydraulics associated with the 1918 jo¨kulhlaup that could not have been achieved through localized outcrop analysis. The observations presented here provide additional sedimentological and architectural criteria for the recognition of deposits associated with transcritical and supercritical water flow conditions. The physical scale of sedimentary architectures associated with the migration of bedforms is largely dependent on the magnitude of the formative flow events or processes; sedimentary analyses must therefore be undertaken at the appropriate physical scale if reliable interpretations, regarding modes of deposition and formative flow hydraulics, are to be made.