Controls on englacial sediment deposition during the November 1996 jökulhlaup, Skeiđarárjökull, Iceland

Abstract

This paper presents sedimentary evidence for rapid englacial debris entrainment during jökulhlaups. Previous studies of jökulhlaup 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 jökulhlaups. The November 1996 jökulhlaup from Skeidarárjökull, Iceland presented one of the first opportunities to examine englacial flood deposits in relation to former supraglacial outlets. Using observations from Skeidarárjökull, this paper identifies and explains controls on the deposition of englacial flood sediments and presents a qualitative model for englacial jökulhlaup deposition. Englacial jökulhlaup 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 jökulhlaup deposits. The occurrence of pervasively frozen flood deposits within Skeidarárjökull 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 jökulhlaup fractures, deposited englacially some 200-350 m above the bed of Skeidarárjökull. 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-jökulhlaup landform development.

Icelandic jökulhlaup impacts: Implications for ice-sheet hydrology, sediment transfer and geomorphology

“…(b) Sedimentation also occurs within englacial voids dissected by hydrofractures. The presence of coarse-grained sediment also indicates that hydrofracture networks had to be sufficiently dilated to allow transit through the glacier (Roberts et al, 2001). (c) Typical upward-fanning or dmushroom' structures are seen within shallower ice near the Sú la vents on the western margin of Skeiðarársandur.…”

“…Although Waller et al (2001) suggested that localized ice collapse above large up-glacier dipping fractures resulted in normal faulting; Roberts et al (2001) reported that ice removal occurred instead in a zone of diffuse fractures that stem from a main or darterialT fracture (Fig. 4a-d).…”

“…During the November 1996 jö kulhlaup, arcuate icesurface depressions or dembayments' developed higher on the surface of Skeiðarárjö kull at intervals along ice flow transverse fractures, created by the mechanical removal of fragments of glacier ice, already heavily fractured by high water pressure during the November 1996 jö kulhlaup (Roberts et al, 2001;Waller et al, 2001). Although Waller et al (2001) suggested that localized ice collapse above large up-glacier dipping fractures resulted in normal faulting; Roberts et al (2001) reported that ice removal occurred instead in a zone of diffuse fractures that stem from a main or darterialT fracture (Fig.…”

“…Given the absence of an alternative source of fine-grained sediment, the initial sediment-laden appearance of supraglacial discharge is attributed to the purging of fine-grained diamicton from the glacier bed at the front of the flood wave. Englacial jö kulhlaup deposits, emplaced during the November 1996 jö kulhlaup, contain many rip-up clasts comprising sheared glacial diamicton and stratified glaciofluvial sediment (Roberts et al, 2001;Waller et al, 2001) (Fig. 4b).…”

“…Englacial jökulhlaup deposits at Skeiðarárjö kull, although having a low preservation potential, provide an intriguing insight into fluvial sedimentary processes within pressurised fracture networks. Englacial sedimentation is thought to be enhanced by highly supercooled meltwater that allows sediment accretion to vertical ice walls (Roberts et al, 2001. Sediment emplaced in photographs (a), (b) and (d) would have been at least 50 m below the surface of Skeiðarárjö kull in November 1996.…”