submarine fan

submarine fan, accumulation of land-derived sediment on the deep seafloor; in configuration, a fan is like the section of a very low cone, with its apex at the lower mouth of a submarine canyon incised into a continental slope. Submarine canyons have steep courses with high walls and funnel occasional dense slurries of water and terrigenous sediment (turbidity currents) to the abyssal seafloor. Upon reaching the base of the submarine canyon, the sudden loss of gradient and confinement lessens the velocity of a turbidity current, and the suspended matter begins to drop out of suspension. Finer and finer particles are deposited as the turbidity current continues to decelerate down the fan. Thus, the sediments of a submarine fan consist largely of successive layers of sandy material, each of which grades upward into finer material. Submarine fan valleys, with low relief and natural levees, commonly occur on submarine fans, branching outward and downward into distributary channels, which serve to distribute the turbidity current sediment over the entire fan by migrating laterally in much the same way that the distributaries of a river delta do. Several fans may coalesce laterally, forming a continental rise.