Final answer:
Sediments on abyssal plains are very fine-grained, accumulate gradually, and are primarily composed of clay and microscopic shells. Evaporites, while a sediment type, are not the main constituent of abyssal plain sediments. The accumulation rate of sediments on abyssal plains is slow, occurring over long periods.
Step-by-step explanation:
The abyssal plains, which are the vast, flat areas of the deep ocean floor, primarily collect sediment that is characterized by specific features. Sediments in these regions tend to be very fine-grained, composed predominantly of clay and the microscopic shells of sea organisms like plankton, as noted in descriptions of shale with plankton fossils and fine laminations. This composition can be understood by examining the depositional environments of the sediments.
In the marine environment, different types of sediments are deposited at increasing distances from the shore. Sand is deposited close to shore due to its larger grain size and higher density, which causes it to settle quickly. As the distance from the shore increases, the sediment particles become finer; silt is deposited further away, followed by clay. It is only in the farthest reaches of the ocean, and under specific conditions, that limestone, which consists mainly of the remains of sea creatures, can be deposited.
Contrary to the deposition of sand and finer particles, evaporites such as salt and some limestones form quite differently. They result when dissolved minerals are precipitated out of water due to evaporation. However, evaporites are not the main constituents of abyssal plain sediments because these plains are typically located in deeper waters where evaporation is not a significant factor for sedimentation.
Another important consideration is the rate at which these sediments accumulate. Sediment accumulation on abyssal plains is gradual, contrary to rapid accumulation. It takes a long time for the fine particles to settle through the vast water column until they reach the seafloor. Turbidite deposits can occur on the sea bottom in the case of significant underwater events like slope failures, indicating more rapid deposition, but these are not the usual conditions for the continuous sedimentation on abyssal plains.