Abstract: Submarine Slumps and Sediment Flows Off San Nicolas Island, Southern California
Michael E. Field, Samuel H. Clarke
An area of 150 sq km east of San Nicolas Island was surveyed in detail in April 1976 using a deep-tow geophysical vehicle to examine the nature and extent of gravity-controlled sediment transport. The deep-tow vehicle, developed by the Marine Physical Laboratory at Scripps, is equipped with up-, down-, side-looking, and bottom-penetrating (4 kHz) sonars, stereocameras, and television. Towed at a speed of approximately 2 knots and about 75 m above the seafloor, the system permits accurate delineation of bottom morphology.
The east slope of the San Nicolas platform lies 200 to 1,100 m below mean sea level and slopes at 6 to 8°. The seafloor is incised by numerous gullies and canyons exhibiting several hundred meters of relief; slopes of canyon walls locally exceed 20°. Sediments are transported from the island platform to the adjacent basin by three major gravity-driven processes: slumping, grain flow, and debris-flow/turbidity currents. Slump deposits, characterized by chaotic and discontinuous subbottom reflectors and hummocky topography, are common on the upper slope. These zones are generally more than 0.5 km across and up to 50 m thick; individual slump masses measure tens of meters on each side. Canyons are shaped both by erosion of the canyon floor and deposition on the adjacent slopes; on the upper canyon floor are well-sorted, coarse, gravelly sands which may be moved downcanyon by grain flow. Absence of slump deposits on the lower slope, combined with evidence of active erosion within and deposition between the canyons, suggests that sediments set in motion on the upper slope are transported by debris flows or turbidity currents.
AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC