Controls on englacial sediment deposition during the November 1996 Joekulhlaup, Skeidararjoekull, Iceland

This paper presents sedimentary evidence for rapid englacial debris entrainment during joekulhlaups. Previous studies of joekulhlaup sedimentology have focused predominantly on proglacial impact, rather than depositional processes within glaciers. However, observations of supraglacial floodwater outbursts suggest that englacial sediment emplacement is possible during joekulhlaups. The November 1996 joekulhlaup from Skeidararjoekull, Iceland presented one of the first opportunities to examine englacial flood deposits in relation to former supraglacial outlets. Using observations from Skeidararjoekull, this paper identifies and explains controls on the deposition of englacial flood sediments and presents a qualitative model for englacial joekulhlaup deposition. Englacial joekulhlaup deposits were contained within complex networks of upglacier-dipping fractures. Simultaneous englacial deposition of fines and boulder-sized sediment demonstrates that englacial fracture discharge had a high transport capacity. Fracture geometry was an important control on the architecture of englacial joekulhlaup deposits. The occurrence of pervasively frozen flood deposits within Skeidararjoekull is attributed to freeze-on by glaciohydraulic supercooling. Floodwater, flowing subglacially or through upglacier-dipping fractures, would have supercooled as it was raised to the surface faster than its pressure-melting point could increase as glaciostatic pressure decreased. Evidence for floodwater contact with the glacier bed is supported by the ubiquitous occurrence of sheared diamict rip-ups and intra-clasts of basal ice within joekulhlaup fractures, deposited englacially some 200-350 m above the bed of Skeidararjoekull. Evidence for fluidal supercooled sediment accretion is apparent within stratified sands, deposited englacially at exceptionally high angles of rest in the absence of post-depositional disturbance. Such primary sediment structures cannot be explained unless sediment is progressively accreted to opposing fracture walls. Ice retreat from areas of former supraglacial outbursts revealed distinct ridges characterized by localized upwellings of sediment-rich floodwater. These deposits are an important addition to current models of englacial sedimentation and demonstrate the potential for post-joekulhlaup landform development.