The Influence of Marine bottom Communities on the Depositional Environment of Sediments

Abstract

Physical forces and topography control the depositional environments of shelf and shallow-water sediments. However, certain bottom communities are capable of modifying water circulation and bottom conditions sufficiently to produce recognizably different sediments than those deposited without the organisms. The organisms responsible for such modifications may or may not be sediment contributors, and their sphere of influence can be limited to their habitat or can extend well beyond it. The biotic modifications of the physical environment constitute a grading spectrum of increasing influence, ranging from the trapping and binding of individual sediment particles by organic films, such as mats of blue-green algae, to the quiet-water lagoonal conditions produced by barrier reefs miles beyond their growth limits. The recognition of the influence of the organisms is based on anomalous structures, textures, or faunas of the sediments that cannot readily be explained solely by variations in physical factors or topography. Stabilization of the sediment by the community is also required if its influence is to be preserved. The best-known fossil examples of community influence on depositional environment are algal stromatolites and organic reefs; other examples are expected throughout the geologic past.