Holocene Slump on Continental Shelf Off Malaspina Glacier, Gulf of Alaska

Abstract

Bottom features on the earthquake- and storm-prone continental shelf of the northeastern Gulf of Alaska indicate mass movement of about 1,080 sq km of the seafloor. A total of 29 high-resolution seismic reflection lines was run across the area south of Icy Bay and the Malaspina Glacier on five cruises between September 1974 and June 1976. The reflection profiles show disrupted bedding and irregular topography, characteristics 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 long (front to back), have a relief of 2 to 5 m, and consist of low-strength, poorly sorted, clayey silt. The base of individual slump blocks lies at a depth in the Holocene sediment of 35 to 50 m. A the landward edge of the slump area, side-scan sonar records show small irregular, curvilinear features approximately parallel with the contours. These features are interpreted as incipient scarps and suggest a headward growth of the slump mass. The most likely triggering mechanism for this mass movement is one or more of the numerous earthquakes that have occurred in this highly seismic region. The slump features cannot be related to a specific earthquake; however, the presence of numerous faults on the continental shelf and adjacent coastal area plus the abundance of historic earthquakes in the region indicate that prolonged ground shaking is common and of sufficient intensity to cause the mass movement of the unstable sediment.