Abyssal Fans, also known as deep-sea fans, underwater deltas, and submarine fans, are underwater structures that look like deltas formed at the end of many large rivers, such as the Nile or Mississippi Rivers. Abyssal fans are also thought of as an underwater version of alluvial fans.
Abyssal (or submarine) fans are formed due to turbidites. Turbidites are essentially gravity-driven underwater avalanches. As sediment is deposited on the continental slope, the steepest part of the ocean, it is prone to sliding down onto the continental rise due to gravity. Once the weight of the sediment accumulating gets to be too much, the pile of sediment will slide down all at once falling down onto the continental rise. After thousands of years of turbidite deposition on the rise, a fan forms towards the top of the continental rise. This fan is similar to an alluvial fan found on land near mountains and rivers. The abyssal fan has characteristics of a standard turbidite. The Bouma Sequence is used to describe the nature and sediment patterns of a turbidite.