Sedimentary processes, facies, and accumulation rates at Alaskan temperate tidewater glaciers
Cooper, Jonathan P.
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Sixteen sediment cores and twenty-nine grab samples from the west arm of the Glacier Bay fjord system, southeast Alaska, have been studied to describe lithofacies and then evaluate processes and rates of sediment transport and deposition. Transport and deposition of sediment in ice-distal (> 1.3 km) locations from temperate tidewater glaciers in the fjord system is accomplished by three main processes: (1) overflows/interflowst (2) sediment gravity flows: and (3) ice rafting. Bergstone mud comprises sediment deposited from overflow plumes and ice-rafted debris (IRD) (< 10% by weight). Proportions of such fine-grained (ave. Mz = 8.0 phi) deposits increase away from west arm glaciers and are masked near the glaciers (< 2.0 km) by turbidites and IRD. Laminae in ice distal cores are less common and thinner. The genesis, distribution, and thickness of individual sediment gravity-flow deposits in this setting are controlled by bottom topography. Turbidites decrease in number with increased distance from glacial source, and are identified by internal structures, gradational tops, and particle-size distributions that are unimodal to bimodal with strong sand (2.5 to 3.5 phi) and silt (4.5 phi) modes. Debrites occur rarely 3 to 5 km from glacial sources and result from redistribution of bergstone mud or unconsolidated older sediments on fjord walls. Debrites exhibit sharp upper and lower contacts, are thin (< 1.5 cm), have broad particle-size modes near 6.5 phi, can correlate among cores, can have coarse mean particle sizes, are poorly-sorted, and can be dark brown to dark gray in color. The proportion of IRD close to west arm glaciers (< 1 km) shows wide variation spatially (5 to 95% by weight). However, in distal areas, although in a lower proportion, IRD has a more uniform spatial distribution. IRD laminae in cores appear similar to debrites, but can have slightly graded upper contacts, poorer sorting, and do not correlate among cores. A pilot study, using Reid Inlet cores, determined sedimentation rates using lead-210 dating techniques. Rates (0.03 to 0.25 m/a) support average annual rates estimated using seismic profiles (0.3 m/a) and sediment traps (0.11 m/a).