Abstract: Submarine-Slump Features Seaward of Icy Bay and Malaspina Glacier, Northeast Gulf of Alaska

Paul R. Carlson

The earthquake-prone and storm-wracked continental shelf of the northeastern Gulf of Alaska has bottom features that indicate mass movement of a large area of the seafloor. A total of 29 seismic lines was run across the 1,200 sq-km area, south of Icy Bay and the Malaspina Glacier, on five cruises between September 1974 and June 1976. The acoustic profiles show disrupted bedding and irregular topographic expression, characteristics commonly associated with submarine slides and slumps. The slump structures are in water depths of 70 to 150 m on a slope of less than 0.5° are about 0.5 km wide (front to back), have a relief of 2 to 5 m, and consist of low-strength, poorly sorted, clayey silt. The slump "blocks" show progressive failure due to lateral extension or stretchi g of a sedimentary unit at the base of the blocks. This unit lies at a depth in the Holocene sediment of 35 to 50 m. Slump features of similar size and shape are present off the mouth of the Copper River, Alaska. The Copper River slump blocks probably were created by the intense ground shaking that accompanied the 1964 Alaska earthquake. The slump features southwest of the Malaspina Glacier cannot be related to a specific earthquake. However, the presence of a fault on the south side of the Icy Bay structure, the numerous earthquakes near the mouth of Icy Bay, and the active seismicity of nearby Pamplona Ridge (three magnitude-6 shocks in 1970) indicate that prolonged ground shaking is common and of sufficient intensity to cause the mass movement.

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC