Basin Plains and Aprons off Southern California

Abstract

Precision echo soundings made across the submarine basins off southern California show that parts of their floors are remarkably smooth and horizontal. Fringing these flat areas are larger areas which have gentle slopes and extend to the basin sides. Where submarine canyons reach from near the shore to the basins, the upper parts of the gently sloping plains take the form of subsea fans. Physiographic and sedimentary evidence indicate that turbidity currents flow from the canyon mouths and deposit most of their loads of sediment on the subsea fans and on the aprons beyond them. Thus the subsea fans are akin to alluvial fans on land. Only a small amount of sediment from turbidity currents reaches the nearly horizontal basin plains and the far sides of the basins. As the basins become filled with sediments from turbidity currents, from suspension in the whole water column, and from marginal mass-movements, the sediments spill from basin to basin in a manner analogous to the prograding of alluvial deposits in the block-faulted desert region of the western United States.