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ABSTRACT: Sediment Instability in the Northeastern Gulf of Alaska: A Retrospective

Bruce F. Molnia

Marine geological and geophysical investigations conducted by the USGS between 1974 and 1982 revealed several dozen areas of seafloor sediment instability in the northeastern Gulf of Alaska. Failures were identified in sediment in fiords, deltas, the shallow shelf, sea valleys, and the outer shelf edge. A variety of types of failures were observed, most of which occurred on very gentle slopes in rapidly accumulating, rock-flour-rich, high-water-content sediment. Sediment failures were studied in detail in the Copper River delta, Kayak Trough, Bering Trough, the continental shelf offshore of Malaspina Glacier, and the Alsek River prodelta; failed and disturbed sediments were found to be of Holocene age. Failure types include in situ collapse features and various types of g avity flows, especially slumps and slides. The dimensions of individual failures vary significantly. A large slump on the continental shelf offshore of the Malaspina Glacier encompasses an area of about 1080 km2, and a complex area of gravity flows and collapse features on the Alsek River prodelta covers only about 200 km2. The largest single failure feature identified is a submarine slide, 18 km long by 15 km wide by 115 m thick, that is located in Kayak Trough.

The primary causative agents for sediment failure are storm-wave and earthquake loadings; in water deeper than about 75 m, failures are generally the result of earthquake loadings, and, in shallower depths, storm-wave loadings are the most likely causative agent. Earthquake forces also play another causative role in northeastern Gulf of Alaska sediment failures. In 1958, an earthquake caused more than 106 m of rock material to fall into Lituya Bay. The resulting splash wave generated many massive submarine sediment failures that shoaled parts of Lituya Bay by about 80 m; four similar but smaller events had been documented in Lituya Bay.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990