Abstract: Seafloor Instability on Pacific Continental Margins

M. E. Field, S. H. Clarke, Jr., H. G. Greene, B. F. Molnia, P. R. Carlson, M. A. Hampton, A. H. Bouma

The expanded search for petroleum resources on continental margins of the Pacific has created a need for better understanding of the geologic processes that may affect exploration and development. Delineation of potential and existing areas of downslope mass movement and measurement rates are of particular importance in establishing guidelines for safe emplacement of offshore facilities. Problems that may result from mass movement of sediment include platform and pipeline failure, oil spills, tsunami generation, and encroachment of seawater into submarine aquifers.

High-resolution seismic reflection profiling, side-scan sonography, and underwater photography of shelves, slopes, and canyons, ranging in steepness from < 1° to > 20°, have been used to identify submarine sediment slides, slumps, and flows. Initiating agents of these mass movements appear to be earthquakes, storm waves, and gravitational loading; local factors, such as gas saturation, also control movement. End members of sediment flows initiated as slumps or slides normally are the turbidite sequences that form a major component of modern basin fill.

Seafloor instability has been studied by the U.S. Geological Survey for several years in two United States offshore petroleum provinces--southern California and the Gulf of Alaska. These studies demonstrate a relation between seafloor instability and local tectonism, climate, sediment sources, and wave environment. In the southern California continental borderland, a tectonically formed basin-and-ridge margin, slides and flows transport significant volumes of sediment to the lower slopes and basins. Slumps range in size from 100 sq m to as much as 20 sq km and are present at all levels on the slope. In the Gulf of Alaska, also a tectonically active area, fine-grained, low-strength sediment of glacial origin accumulates rapidly on the eastern shelf. These deposits fail as wide thin slu ps on the shelf and large rotational slumps on the upper slope. In the western Gulf of Alaska, depositional rates are lower than in the east but large slumps and thin debris slides are common on continental slopes in seismically active zones.

AAPG Search and Discovery Article #90962©1978 AAPG 2nd Circum-Pacific Energy and Minerals Resource Conference, Honolulu, Hawaii