Large submarine sediment slides occur on the continental shelf and upper continental slope in the Gulf of Alaska. Detailed geological and geotechnical studies of five individual slides point out the significant controls on slope instability. For two slides on the upper continental shelf adjacent to the Kodiak shelf in the western Gulf, removal of slope support by faulting, augmented by seismic ground accelerations and possibly tectonic slope steepening, control sediment failure. In the northeastern Gulf, slides on the Copper River prodelta, in Kayak Trough, and off Icy Bay - Malaspina Glacier occur because the sediments are under consolidated due to high sedimentation rates, and because of seismic accelerations and wave loading. The presence of free interstitial gas may contribute to instability on the Copper River prodelta and in Kayak Trough. Wave loading should be important in water depths less than 150 m.
Marine geologic studies conducted by the U.S. Geological Survey on the continental shelf and upper continental slope in the Gulf of Alaska have revealed several areas of slope instability, indicated by the presence of submarine slides (Fig. 1). The slides were first identified in 3.5 kilohertz, uniboom, minisparker, and 30-160 kilojoule sparker seismic-reflection records during reconnaissance geo-environmental surveys of OCS lease-sale areas in the northeastern Gulf and on the Kodiak shelf. Succeeding detailed surveys defined the extent and morphology of some of the slides, and side-scanning sonar and underwater television observations were made to study small surface features. Cores and grab samples were collected to characterize sediment types.
Detailed study of the seismic-reflection records and the sediment samples showed a variety of geologic conditions responsible for the slope instability, typically differing from one area of slides to the next. During the 1977 field season, attention was focused on five individual slides that best represent the range of instability environments. Piston, gravity, and hydroplastic cores were collected in and adjacent to the slides. Geotechnical measurements from core samples, along with estimates of the magnitudes of geologic forces, have been used for an engineering analysis to test the conclusions made from the geologic data and to identify the significant environmental variables responsible for the instability. Insight into the mechanics of submarine sliding gained from this study contributes to the development of models to evaluate the potential for movement in areas of suspected instability.
Large slides have been identified on the upper continental slope adjacent to the Kodiak shelf. The two major slides chosen for detailed study are on the slope off middle Albatross Bank and off Portlock Bank (Fig. 1) (see also Hampton and Bouma, 1977). In the northeastern Gulf, slides occur in several major areas of the shelf, as well as on the upper continental slope. The slides studied in detail include those on the Copper River prodelta, in Kayak Trough, and in the offshore area adjacent to Icy Bay and the Malaspina glacier (Fig. 1) (see also Carlson and Molnia, 1977; Molnia, Carlson, and Bruns, 1977).
