Nearly 20 years of marine benthic studies along the rim of the Gulf of Mexico permits recognition of areal (environmental) facies, most of which have exact counterparts in the Gulf Coast Tertiary. These facies have been recognized on the basis of invertebrate faunal diversity, benthic community size and structure, geomorphology of sea bottom, and sedimentary characteristics.

Ultimate facies control is exhibited by prevailing climates around the Gulf Coast, ranging from tropical moist in the southeast and southwest to almost xerophytic (dry) in the west. Northern regions are cool-temperate in winter and subtropical in summer with average moisture conditions ranging from very wet for several years to prolonged droughts in following years--the most variable climate in the world.

Megafaunal assemblages in shallow waters consist of those found in saltwater and freshwater marshes, river estuaries, low to medium salinity enclosed bays (inter-reef), low and high salinity oyster or mollusk reefs, high salinity open bay centers; open bay sandy margins, inlets, and sand or clay open beaches. Open Gulf or deeper water assemblages are characteristics of shallow shelf (1-20 m), intermediate shelf (21-72 m), outer shelf (73-132 m), and upper slope (132-700 m). Detrital carbonate muds provide slight variations to these facies in areas on the south. Reef-forming organisms create micro-epifaunal habitats modified by wave energy and depth.

Faunal composition within each of these habitats is controlled by sediment type and by stability of other environmental factors. Unstable environments with wide ranges of ecologic variables and clay sediments produce low species diversity within small populations. Stable environments on sand-clay mixed bottoms produce high diversity within large populations. Principles used to define these habitats can be applied to Tertiary formations in mapping ancient environments.

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