Sediment that consists mostly of microscopic shells and hard parts of organisms is called biogenic sediment.

Here's a breakdown:

* Biogenic: Means "originating from living organisms."

* Sediment: Loose, solid material that is broken down from rocks, minerals, and organic matter.

Types of Biogenic Sediment:

* Calcareous ooze: Made primarily from the calcium carbonate shells of foraminifera, coccolithophores, and other tiny marine organisms.

* Siliceous ooze: Made primarily from the silica shells of diatoms and radiolarians.

* Diatomaceous earth: A type of siliceous ooze that is often mined for its absorbent properties.

Biogenic sediment is very common in marine environments, especially in areas where there is a high abundance of plankton and other microscopic organisms.