CONTROLS ON THE SEDIMENTARY ARCHITECTURE OF A SINGLE EVENT ESKER: SKEIÐARÁRJÖKULL, ICELAND

Eskers have been used to infer the dynamics and palaeohydrology of large ice sheets during Quaternary glaciations. Although detailed work has been carried out concerning esker morphology and sedimentology, only a handful of depositional models exist. Furthermore, the timescale of deposition is commonly poorly constrained, due to a lack of suitable modern analogues. We aim to identify the controls on the large-scale sedimentary architecture of a large (>700 m long, < 30 m high) esker deposited during a single, high-magnitude jökulhlaup at Skeigarárjökull, Iceland. Over 3.8 km of ground-penetrating radar (GPR) line were collected (in 2006 & 2007) as grids on all workable sections of the esker and associated supraglacial ice-walled canyon-fill sediment. Six main radar facies were identified: 1) discontinuous, undular reflections, interpreted as trough-cross strata, associated with the development of migrating gravel dune forms; 2) sub-horizontal reflections (dip <10°), associated with deposition of upper stage gravel plane beds; 3) moderate to high-angle (>10°) inclined reflections, dipping upstream, interpreted as backset accretion associated with large-scale bedform development; 4) high-angle (>15°) inclined reflections, dipping downstream, interpreted as foreset accretion associated with large-scale bedform progradation; 5) point source reflectors, interpreted as zones of boulder clustering; and 6) zones of enhanced attenuation, bounded on the upper surface by a continuous reflection sub-parallel to the ground surface, interpreted as buried ice associated with the conduit/channel base. These data suggest: 1) the esker was generated englacially; 2) deposition was non-uniform, with the assemblage being composed of a series of truncated units and cut-and-fill structures. Our study suggests greater sedimentary complexity than previously anticipated for a single event; 3) dramatic vertical and proximal-distal variations in sedimentary architecture suggests that within-event evolution of conduit geometry is a major control. This study provides the first detailed insight into the large-scale sedimentary architecture of a single event jökulhlaup esker, which provides a modern analogue for esker deposition during Quaternary glaciations.