Reef

A reef is a ridge or shoal of rock, coral, or similar relatively stable material lying beneath the surface of a natural body of water.

[1] Many reefs result from natural, abiotic (non-living) processes such as deposition of sand or wave erosion planning down rock outcrops.

Both mounds and reefs are considered to be varieties of organosedimentary buildups, which are sedimentary features, built by the interaction of organisms and their environment.

Corals and calcareous algae grow on top of one another, forming a three-dimensional framework that is modified in various ways by other organisms and inorganic processes.

Examples of biostromes formed by cyanobacteria occur in the Great Salt Lake in Utah, United States, and in Shark Bay on the coast of Western Australia.

Whereas, bryozoans and crinoids, common contributors to marine sediments during the Mississippian period, produce a different kind of mound.

[5][7] The Proterozoic Belt Supergroup contains evidence of possible microbial mat and dome structures similar to stromatolite and chicken reef complexes.

[clarification needed][5][8] Rocky reefs are underwater outcrops of rock projecting above the adjacent unconsolidated surface with varying relief.

Ancient reefs buried within stratigraphic sections are of considerable interest to geologists because they provide paleo-environmental information about the location in Earth's history.

In addition, reef structures within a sequence of sedimentary rocks provide a discontinuity which may serve as a trap or conduit for fossil fuels or mineralizing fluids to form petroleum or ore deposits.

[10] Corals, including some major extinct groups Rugosa and Tabulata, have been important reef builders through much of the Phanerozoic since the Ordovician Period.

During the Cambrian Period, the conical or tubular skeletons of Archaeocyatha, an extinct group of uncertain affinities (possibly sponges), built reefs.