GPR investigations of the sedimentary architecture of jökulhlaup eskers : Skeidarájökull, Iceland and Bering glacier, Alaska
Burke, Matthew John
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Eskers are ridges of stratified glaciofluvial material deposited in englacial, subglacial or supraglacial channels and ice-walled canyons. Eskers have been used to infer the dynamics and palaeo-hydrology of large ice sheets, despite observations of palaeo-esker sedimentary architecture lacking rigorous constraints on depositional timescale. This research aims to identify the hydrological, glaciological and sedimentary controls on the sedimentary architecture of single event outburst flood (jokulhlaup) eskers at SkeiOarârjOkull and Bering Glacier. These eskers formed during monitored outbursts, providing time constraints on the depositional events, thereby making the eskers ideal analogues for palaeo-eskers. GPR data was collected using a pulseEKKO Pro 1100 system at SkeiOarârjOkull and Bering Glacier during field seasons in 2006 and 2007. At SkeiOarãrjOkull grids of 100 MHz GPR lines were collected on the glacier surface and 200 MHz GPR grids were collected on all workable sections of an esker and ice-walled canyon fill generated by a jokulhlaup in November 1996. At Bering Glacier 200 MHz GPR grids were collected on workable sections of an ice-walled canyon fill and esker generated during outbursts in July-August and October 1994, respectively. Examination of the GPR data has allowed development of site-specific models for esker and ice-walled canyon fill deposition, providing the first detailed insight into the sedimentary architecture of single event jokulhlaup eskers. These models show that single high-magnitude jOkulhlaups can generate eskers with complexities previously unexpected for single events. The englacial position of an esker is controlled by the presence of structural weaknesses within the ice and the jokulhlaup release mechanism. Esker sedimentary architecture, on the other hand, is controlled by a complex interaction between hydrological, glaciological and sedimentary factors. The most fundamental control on jokulhlaup esker sedimentary architecture is conduit geometry, which determines the type of macroform from which the esker is composed. Thus, eskers deposited during jOkulhlaups should be made up of both wide ridges as composite macroforms in areas of conduit expansion and narrow ridges composed of upper-stage plane beds in constricted conduits. The smaller scale sedimentary features, which include antidunes and boulder clusters, as well as the frequency of erosional structures, are controlled by interactions between the flow conditions and sediment supply. The eskers generated during the 1996 SkeiOarãrhlaup and 1994 Bering Glacier outburst flood demonstrate sedimentary architectures that are similar to those identified in many palaeo-eskers described from previous studies. The research suggests that jOkulhlaups may have had a greater role in palaeo-esker sedimentary architecture than previously hypothesised.