Abyssal fan

An abyssal fan, also called a deep-sea fan, submarine fan or underwater delta, is a large underwater accumulation of sediment laid down by turbidity currents. It can be thought of as the deep-ocean counterpart of an alluvial fan, and fans range in width from several kilometres to several thousand kilometres. The largest is the Bengal Fan, followed by the Indus Fan, and major fans also lie off the mouths of the Amazon, Congo and Mississippi. A fan begins when geological activity pushes sediment over the edge of a continental shelf and down the continental slope, setting off an underwater landslide. A dense slurry of mud and sand races toward the foot of the slope until the current slows and loses the power to carry its load, so the grains settle out and build the fan. The deposits form graded sequences of sand, silt and mud called turbidites, described by the Bouma sequence.