Biogenic Rocks

sedimentary rocks consisting of the remains of animal and vegetable organisms or products of the vital activity of such organisms. The organisms have the ability to concentrate certain compounds, forming skeletons or tissues that are preserved in fossil form. A distinction may be made among carbonate, siliceous, and some phosphate biogenic rocks, as well as coals.

Biogenic carbonate rocks (limestones) consist of the shells of foraminifers, corals, bryozoans, brachiopods, mollusks, algae, and other organisms. The reef limestones that make up atolls and barrier reefs, as-well as writing chalk, are unique representatives of this group. The biogenic siliceous rocks include diatomite, spongolite, and radiolarite. Diatomites consist of the opaline frustules of diatoms and the spicules of siliceous sponges and radiolarians. Spongolites are rocks that usually contain more than 50 percent siliceous sponge spicules. Their cement is siliceous and consists of round opal bodies, or it is clayey and slightly calcareous and frequently includes secondary chalcedony. Radiolarites are siliceous rocks that consist of more than 50 percent radiolarian skeletons; in modern oceans they form radiolarian ooze. Radiolarites also include individual spicules of sponges, occasional shells of diatoms and coccolithophores, and opal and clay particles. Upon recrystallization, radiolarites become jaspers.

Biogenic phosphate rocks are not widespread. They include coquinas composed of the phosphate shells of Silurian brachiopods (Obolidae); accumulations of the bones of fossil vertebrates found in deposits of various ages; and guano, the decomposition products of bird droppings that usually accumulate in layers on islands with dry climates.