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Cowan, E.A., 1988

Sediment transport and deposition in a temperate glacial fjord, Glacier Bay, Alaska

Bibliographic Reference

Cowan, E.A., 1988, Sediment transport and deposition in a temperate glacial fjord, Glacier Bay, Alaska: DeKalb, Illinois, Northern Illinois University, Ph.D. dissertation, 432 p., illust. (some color), maps.


A subglacial meltwater stream at the tidewater terminus of McBride Glacier, Glacier Bay, southeast Alaska, discharges large volumes of sediment to the base of the fjord water column. During four consecutive meltwater seasons (1984-1987), water samples, CTD profiles, sediment traps and echosound profiles were collected within 2-km-long McBride Inlet to quantify ice-proximal suspended sediment transport, deposition and oceanic controls on fjord circulation.

Less dense, sediment-laden water rises buoyantly from the submarine efflux to produce an upwelling near the glacier with surface sediment concentrations up to 1.4 g L-1. Mixing within the plume increases salinity up to 12‰ and produces widely dispersed, homogeneous sediment concentrations and flocculated particles. A thick, brackish surface layer is advected downfjord as hypopycnal flow, with highest suspended sediment concentrations (approximately 800 mg L-1) transported 4 to 6 m beneath the surface.

The highest vertical suspended sediment flux is initiated at slack low tide because of tidal control on current velocity of the surface layer. Sand and coarse silt settle as single grains; finer particles settle more slowly as high concentration, turbid layers. Turbid layers are observed to settle at an average rate of 5 m h-1 in transmissivity and water sample profiles. Cyclically interlaminated sand or silt with mud is deposited on the fjord floor by this process. These deposits are termed cyclopsams (sandy, thinly bedded to thickly laminated) and cyclopels (muddy, thinly laminated). Sediment traps confirm that these couplets are deposited at a rate of two or three per day and are controlled by tidal and discharge dynamics.

Sediment accumulation rates are as high as 20 m a-1 at the terminus. Sedimentation rates decrease exponentially from the terminus: from 24 g cm-2 day-1 to less than 1 g cm-2 day-1 over a horizontal transport distance of 1.2 km.

Major lithofacies from these meltwater discharges are cyclopsams and cyclopels, bergstone mud (marine outwash and ice-rafted sediment), and homogeneous mud in this temperate, ice-proximal glacimarine setting.

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