Sedimentary Environment

Seaward of the continental shelves, continental slopes incline more steeply, so relict and modern sediments form deposits called deep-sea fans. These are similar to alluvial fans, but generally consist of sand- to clay-sized particles with little or no gravel. Deep-sea fans form the continental rise, a continuous apron of sediment at the base of the continental slope.

Even farther from land, the monotonous abyssal plains begin. Here mostly clay-sized sediment forms sheets up to 0.6 mi (1 km) thick. These deposits, composed of sediments that settle through the water column from shallow depths, thin to a feather edge at the oceanic ridges where new sea floor forms. Abyssal sediments are generally a mixture of three grain types: carbonate muds and siliceous muds of biogenic (organic) origin, and red clays of terrigenous origin. Carbonate-rich muds generally accumulate in water depths of less than 2-2.5 mi (3-4 km); at deeper depths, colder water and higher pressures combine to dissolve the carbonate. Siliceous muds occur where abundant nutrients in surface waters support high rates of biogenic silica (SiO₂) production. Red clays, transported from the land by winds and stream flow, predominate where quantities of carbonate and siliceous muds are insufficient to dilute these fine-grained terrigenous deposits.