SEDIMENTARY ARCHITECTURE OF A SUPRAGLACIAL JÖKULHLAUP CHANNEL-FILL, SKEIÐARÁRJÖKULL, ICELAND

Interpretation of relict ice-contact sedimentary successions enables valuable reconstruction of former glaciers and their hydrological systems. The complex and often highly energetic nature of ice-contact depositional environments, make palaeohydrological reconstruction difficult. We examine the sedimentary infill of a spectacular, 500 m long, 100 m wide ice-walled supraglacial channel, deposited by discharges exceeding 20,000 m ³s^-1 during a glacier outburst flood or ‘jökulhlaup’ November 5-7, 1996. The ice-walled channel developed in an extensively fractured area of Skeiðarárjökull.

Sedimentary sections and Ground Penetrating Radar surveys (50 and 100 MHz) reveal well-defined, moderate angled (10-15°), high relief amplitude (15-20 m) upstream-dipping beds. Large-scale ~1-2 m thick cross-beds contain boulder-sized clasts. Beds are defined by laterally extensive alternately polymodal matrix- and clast-supported units. These poorly-sorted beds show few signs of clast imbrication or clustering.

Upstream-dipping beds are interpreted as the product of stoss-side aggradation and upstream bar migration. Absence of diagnostic fluvial imbrication and clustering, combined with knowledge of enormous stream powers (up to 40,000 Wm^-2) and associated sediment transport capacity, suggest rapid deposition from a flow with a highly concentrated bed-layer. Stoss-side deposition occurred beneath a large hydraulic jump, migrating upstream simultaneously with ice-walled channel head-ward erosion. Upstream-dipping beds were deposited during the late rising flow stage, within 17 hours, as the ice-walled channel enlarged.

Channel-scale stoss-side bars are useful indicators of rapid channel expansion during high magnitude meltwater events. The large-scale bars are deposited by highly concentrated bedload sheets within flood flows superficially exhibiting turbulent fluvial characteristics. Our study provides a new model for the interpretation of ice-contact glaciofluvial deposits associated with former ice margins subject to high magnitude discharges. Former ice-walled channels resulting from surface excavation, tunnel collapse and ice margin break-up during high magnitude jökulhlaups should be associated with extensive coarse-grained heavily kettled proglacial outwash surfaces.