Abyssal fans, also known as deep-sea fans, underwater deltas, and submarine fans, are underwater geological structures associated with large-scale sediment deposition and formed by turbidity currents. They can be thought of as an underwater version of alluvial fans and can vary dramatically in size, with widths from several kilometres to several thousands of kilometres The largest is the Bengal Fan, followed by the Indus Fan, but major fans are also found at the outlet of the Amazon, Congo, Mississippi and elsewhere.
Abyssal (or submarine) fans are formed from turbidity currents.
Turbidity currents start when something, for example an earthquake (or just the inherent instability of newly deposited sediments), triggers sediments to be pushed over the edge of the continental shelf and down the continental slope, creating a submarine landslide. A dense slurry of muds and sands accelerates towards the foot of the slope until the gradient levels off and the turbidity current slows. The slowing current has a reduced ability to transport sediments and deposition of the coarser grains begins, creating a submarine fan. The current continues to slow down as it moves towards the continental rise until it reaches the level bottom of the ocean. This final result is a series of graded sediments of sand, silt and mud and these are known as turbidites, as described by the Bouma sequence.
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