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Size
Sediment may be classified by the size of each individual grain. From smallest to largest, these are: clay, silt, sand, granule, pebble, cobble and boulder. However, it is more common to classify sediment according to its source.
Biogenous Sediment
Biogenous sediment is made up of particles from dead organisms. The most common organisms found in these sediments are coral, mollusk shells, and microscopic planktonic shells. In the deep sea, the majority of biogenous sediment is from dead plankton, particularly that of diatoms, foraminiferans, radiolarians and coccolithophores. Diatoms and radiolarians have silica shells; formaniferans and coccolithophores have calcium carbonate shells. If the sediment has more than 30 percent silica, it is called siliceous ooze. If it has more than 30 percent calcium carbonate, it is called calcareous ooze.
Hydrogenous Sediment
Some sediments form directly from chemical processes in seawater. Common hydrogenous sediments include manganese nodules, phosphorites and carbonates. In shallower areas, such as on continental shelves and near islands, rock salt, calcium salts and sulfates may settle on the ocean floor.
Terrigenous Sediment
Continental rocks and particles may be blown from the land by wind, or carried by water or ice, to settle on the seafloor as sediment. Mud is a terrigenous sediment, made of clay and silt. The color of the clay provides clues as to its chemical makeup. Red clay, for example, is rich in iron.
Cosmogenous Sediment
Cosmogenous sediment originated in the atmosphere or deep space. Particles from comets or other bodies land in the ocean and settle on the ocean floor. These are the rarest kinds of sediments.