MORPHOLOGICAL AND SEDIMENTARY SIGNATURES OF GLACIOHYDRAULIC SUPERCOOLING, ICELAND

Reconstruction of Quaternary glacial environments relies upon understanding of modern process-form relationships. Recently, meltwater has been advocated as an effective mechanism of entraining large volumes of sediment within glaciers. Sediment entrainment to temperate glaciers can occur by glaciohydraulically supercooled meltwater flowing under a spectrum of regimes, ranging from diurnal cycles to rainfall events and glacier outburst floods (jökulhlaups). Entrapment of fluvial sediment load by frazil ice has been observed at several glaciers in north America and Iceland. Sequences of young stratified ‘basal’ ice have also been attributed to supercooled accretion at distance from the margins of several glaciers. Little is known about the controls on, and detailed characteristics of, sedimentary structures within modern supercooled frazil ice vent units or young stratified ‘basal ice’ of supercooled origin. There is also a dearth of information about the preservation of supercooling-related sedimentary structures upon melt-out. This study presents: (1) detailed evidence of sedimentary structures with englacial sediment bodies associated with glaciohydraulic supercooling; and (2) evidence of sediment preservation upon melt-out from glaciers.

A number of supercooling-related englacial sedimentary bodies with Skeiðarárjökull and Skaftafellsjökull, Iceland, indicate the presence of primary fluvial bedforms associated with vertical flows from depth. Bedding angles within frozen fluvial sedimentary successions are commonly too high to be accounted for by normal fluvial sedimentation. High-angle sand volcano-like structures comprising material of up to boulder in size are found at former meltwater fountain locations and in englacial cavities. Sedimentary structures preserved within subglacial vents reveal temporal variations in sediment supply and meltwater flow strength during supercooled hydrological events. We present sedimentary models for fracture infill from coarse-grained sediment-rich supercooled flows. Although glaciohydraulic supercooling must have occurred within Quaternary ice masses, landforms and deposits attributed to this mechanism have yet to be identified. We present a model of sedimentary structures diagnostic of deposition under supercool conditions.