CONTROLS ON THE MORPHOLOGY AND SEDIMENTOLOGY OF A JOKULHLAUP ESKER, SKEIDARARJOKULL, ICELAND

Quaternary eskers are used to infer large-scale meltwater movements and ice sheet dynamics. Despite the presence of many large esker systems, often extending for 100s of km, they have very few modern analogues. Sedimentary evidence suggests that long eskers are formed synchronously within subglacial, or occasionally englacial, conduits. Although a number of distinctive sedimentary architectures have been attributed to fluvial deposition within dynamic ice conduits, there is uncertainty over depositional timescale and esker evolution style. We discuss the controls on esker development by examining an esker complex where depositional timescale and hydraulic processes are well constrained.

Two large englacial eskers at Skeidararjokull, Iceland ascend directly towards a supraglacial ice-walled canyon, excavated within less than 17 hours during the November 1996 jokulhlaup by a peak discharge of 25-30,000 m³s^-1. The largest esker ridge has cross-sectional areas ranging from 100 to 1000 m² and displays an undulating multiple crested surface morphology. Esker ridges also coalesce with a rectilinear pattern of sediment-filled fractures.

Main esker ridge sedimentary architecture is dominated by a 20 m thick set of 15-20° back set beds composed of boulder size sediment. These beds support a series of climbing gravel dunes and terminate in large-scale foreset beds. In cross-section, the esker displays an anticlinal structure. Deposits range from bimodal cobble-sand and cobble-silt units to polymodal matrix-supported units displaying signs of boulder imbrication and clustering. Inter-fingering of esker ridge and fracture-fill sediments demonstrates simultaneous deposition.

Esker ridge sedimentation evolved rapidly (1-2 hours) from a complex network of hydrofractures, to a pattern of localised conduit expansion and macroform growth. Irregular esker morphology is explained by variability of mechanical tunnel enlargement processes and the presence of primary fluvial bedforms within newly developed cavities. It is clear that substantial eskers can form under non-equilibrium conditions during jokulhlaups as long as there is a suitable supply of readily entrained sediment. Our modern analogue may assist those interpreting the meltwater magnitude and frequency regimes of ancient eskers.