Surface Sedimentary Units of Northern Gulf of Alaska Continental Shelf

Abstract

Four major sedimentary units are present on the seafloor of the continental shelf in the northern Gulf of Alaska. These units, defined on the basis of seismic and sedimentologic data, are: (1) Holocene sediments, (2) Holocene end moraines, (3) Quaternary glacial marine sediments, and (4) Tertiary and Pleistocene lithified deposits. A wedge of Holocene fine sand to clayey silt covers most of the inner shelf, reaching maximum thicknesses of about 350 m seaward of the Copper River and about 200 m seaward of Icy Bay. Holocene end moraines are present at the mouth of Icy Bay, south of Bering Glacier, and at the mouth of Yakutat Bay. Quaternary glacial marine sediments lie in a narrow arc that borders on the north and west side of Tarr Bank and in a large arc 20 km or more offshore that parallels the shoreline between Kayak Island and Yakutat Bay. Tertiary or Pleistocene stratified sedimentary rocks, which in profile commonly are folded, faulted, and truncated, crop out on Tarr Bank, offshore of Montague Island, and in several localities southeast and southwest of Cape Yakataga. The lack of Holocene cover on Tarr Bank and Middleton, Kayak, and Montague Island platforms may be due to the scouring action of swift bottom currents and large storm waves. West of Kayak Island the Copper River is the primary source of Holocene sediment. East of Kayak Island the major sediment sources are streams draining the larger ice fields, notably the Malaspina and Bering Glaciers. Transport of bottom and suspended sediment is predominantly to the west. If deglaciation of the shelf was completed by 10,000 years B.P., maximum rates of accumulation of Holocene sediment on the inner shelf may be as high as 10 to 35 m per 1,000 years.