Powell, Ross D.
M.S. (Master of Science)
Department of Geology
Drift; Sediments (Geology)--Analysis; Glaciers
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.
Heiny, Janet S., "Sediment characteristics of rapidly retreating valley glaciers" (1983). Graduate Research Theses & Dissertations. 4844.
xiii, 243 pages
Northern Illinois University
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