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Ice-proximal sediment accumulation rates in a temperate glacial fjord, southeastern Alaska

By
Ellen A. Cowan
Ellen A. Cowan
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Ross D. Powell
Ross D. Powell
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Published:
January 01, 1991

Sediment traps and echo sounder profiles collected between 1984 and 1987 were used to determine the rate and style of sediment infilling of a fjord basin with a temperate tidewater glacier at its head. McBride Glacier retreated rapidly (0.25 km/yr, 1984–1986; 0.05 km/yr, 1986–1987) enlarging a 90- to 100-m-deep basin adjacent to the terminus. Highly turbid meltwater is discharged subglacially at the base of the water column and rises to the surface forming turbid overflow plumes. Sedimentation rates from suspension settling decrease exponentially with distance from the subglacial stream; from 24 g/cm2/day at the terminus to less than 1 g/cm2/day, 1.3 km away.

The total annual volume of sediment deposition within the ice-proximal basin of McBride Inlet was 3.6 × 106 m3 between 1984 and 1985, 3.0 × 106 m3 between 1985 and 1986, and 1.7 × 106 m3 between 1986 and 1987. This sediment is contributed by suspension deposition from meltwater plumes, bedload transported by the subglacial stream, direct meltout of glacial debris, ice rafting, and side-entry sources. Rapid glacier flow and large amounts of meltwater and precipitation, a result of the temperate climate, enhance the production and delivery of sediment to the fjord.

In McBride Inlet, meltwater discharge is the main ice-proximal sediment source. During 1984–1986, 67 percent of the total volume was deposited by suspension settling alone. Sediment infills the irregular bathymetry of the predepositional basin, producing a smooth, flat basin floor. Within this basin, sediment accumulation rates are as high as 13 m/yr, 300 m from the glacier.

Major sediment sources are episodic, including sediment gravity flows, and daily and seasonal variations occur in meltwater discharge. Within temperate fjords a thick sequence of glacial marine sediment rapidly accumulates, filling proximal basins with interlaminated sand/silt and mud.

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GSA Special Papers

Glacial marine sedimentation; Paleoclimatic significance

John B. Anderson
John B. Anderson
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Gail M. Ashley
Gail M. Ashley
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Geological Society of America
Volume
261
ISBN print:
9780813722610
Publication date:
January 01, 1991

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