A lagoon is a shallow area of saltwater that has been separated by a sand bank or a reef. As the sand or coral build up the salt water becomes trapped behind the "wall" that is formed and forms a shallow "pond" area that is called a lagoon.

sandbanks or reefs along shallow coastal waters

Tidal flats are sand or mud areas found in many estuarine areas and typically lack recognizable plant life. They are neither terrestrial nor aquatic but fall somewhere in between (ecotone) and are harsh, unpredictable environments.

The sediments found are mud deposits from other rivers.

Many reefs result from abiotic processes—deposition of sand, wave erosion planning down rock outcrops, and other natural processes.

The sediments found in a reef is a slab of rock mixed with sand which makes up its composition.

The continental shelf is an undersea extension of a continent which can stretch for many miles out to sea in some cases

The continental shelves are covered by terrigenous sediments

The continental slope is a submarine geological feature which connects the continental shelf to the abyssal plain,

Terrigeneous sediments

Abyssal plains are flat or very gently sloping areas of the deep ocean basin floor.

fine-grained sediments