Sediment characteristics of rapidly retreating valley glaciers
Heiny, Janet S.
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Rapidly retreating (0.5-1.0 kmy⁻¹) temperate valley glaciers leave a thin, distinctive record of glacigenic sediment adjacent to tidewater fronts. Most glacial debris originates locally from freeze-on of pre-deposited sediment and metasomatized, weakened bedrock. Supraglacial debris is coarse-textured and angular, and originates from rockfall onto the glacier surface. Englacial debris occurs as layers, balls and tubes, and dispersed. Layers have variable thickness (2-50 cm), and contain sandy debris interlaminated with small volumes of ice (< 5%). Debris tubes and balls may form in various ways: 1) attenuation of englacial debris layers by flow near the ice margin and around bedrock obstructions, 2) differential basal freeze-ori over irregular substrates, and 3) meltwater transport of fine-grained debris between ice crystals and deposition in ice voids. Dispersed debris occurs in low concentrations (< 25%) in thick ice layers (0.5-2.0 m) as blebs (1-2 mm) or individual grains of sand and silt, originating from basal freeze-on or englacial transport from the accumulation zone. Basal debris has < 5% ice in thick stratified to massive layers (to 1.5 m), originating by freeze-on of sediment, or plucking of bedrock. Sediment originates from debris melt-out and retains most original characteristics with a slight loss of coarse grains and decrease in mean grain-size. Sediment gravity flows occur at high slope angles and water saturation, further reducing the mean grain- size and textural proportions. Marginal rockfall till is deposited by debris sliding off the ice front, undergoing gravity sorting and size grading. Short-headed braided meltwater streams deposit deltaic sediments at discharges up to 602 cfs (17 cms) and transport suspended sediment in concentrations up to 23,000 mgL⁻¹. Glacigenic sediments of the terminus region are composed of till complexes interbedded with sediment gravity flows and in association with glaciofluvial sediments.