National Oceanic and
Atmospheric Administration
United States Department of Commerce


FY 1987

Geomorphology, sediment, and sedimentary processes

Hampton, M.A., P.R. Carlson, H.J. Lee, and R.A. Feely

In The Gulf of Alaska: Physical Environment and Biological Resources, D.W. Hood and S.T. Zimmerman (eds.), DOC/NOAA, DOI, 93–143 (1986)

The Gulf of Alaska continental margin, from Cross Sound in the east to Chirikof Island in the west, has been shaped directly and indirectly by the forces of ice, plate tectonics, and ocean currents. Grounded ice extended to the shelf break at least once during the Pleistocene epoch, covering most or all of the shelf and sculpting broad flat banks and elongated troughs. Glacial, glacial-marine, and glacial-fluvial sediment was deposited in nearly all areas as the ice advanced. As the climate warmed and the ice retreated, the region was inundated by the sea, giving rise to the present geologic environments. The high, youthful mountains to the north of the Gulf provide a plentiful source of sediment that is delivered to the coastline by a few large rivers and remnant glaciers. The major input of sediment occurs in the northeastern Gulf (Copper River, Alsek River, Bering Glacier, and Malaspina Glacier sources) and at the head of Cook Inlet (Knik, Matanuska, and Susitna River sources). Ocean currents in the northeastern Gulf carry the sediment predominantly to the west, depositing much of the load near the shore and in the troughs but delivering some sediment into Prince William Sound and Shelikof Strait. Large embayments in the eastern Gulf coastline accumulate thick, underconsolidated deposits of sediment delivered by local high-gradient streams and glaciers. The coarse sediment from the rivers at the head of Cook Inlet is deposited near the points of entry and, along with the relict glacial sediment in the remainder of the Inlet, is reworked by strong tidal currents. As a result, fields of large sand waves and other current-related bed forms have developed. The fine sediment from the rivers is transported south down the Inlet and is deposited as a progressively sorted sediment blanket throughout Shelikof Strait. The Kodiak Shelf receives little modern sediment, but ocean currents rework the relict glacial debris, leaving coarse-grained lag deposits on the shallow banks and winnowed, fine-grained sediment in the troughs. Collision between the North American and Pacific lithospheric plates generates strong tectonic forces throughout the region. Over long durations of geologic time, these forces cause changes in seafloor elevation that create deep sedimentary basins and uplifted banks and islands. In the short term, strong and frequent earthquakes trigger submarine sediment slides in the deposits of soft sediment on the northeastern Gulf shelf and along the entire upper continental slope.

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