What kinds of sediment make up abyssal plains?

Abyssal plains are primarily composed of two distinct types of sediment that accumulate over time to smooth out the seafloor's irregular topography.1

• Turbidites: These are land-derived sediments transported to the deep sea by turbidity currents—dense, sediment-laden slurries that flow down continental slopes. Turbidites consist of horizontal beds of silt, sand, and occasionally gravel, often exhibiting a graded structure where grain size decreases from the bottom to the top of a layer. These deposits can comprise 2 to 90 percent of the sediment on an abyssal plain.12
• Fine-grained Pelagic Deposits: Between turbidity current episodes, the plains receive fine-grained clay and the microscopic skeletal remains of marine organisms (biogenic constituents) that settle slowly through the water column. These materials accumulate at very slow rates, typically ranging from a millimeter to several centimeters per 1,000 years.12

In some areas, these fine-grained sediments may also be supplied by slowly flowing, diffusely turbid bottom waters.1