More than 300 diapiric structures formed by the intrusion of relatively pure salt are known in Alabama, Mississippi, Louisiana, Arkansas, Texas, Nuevo Leon, Veracruz, Tabasco, and Cuba. In form, the structures are rod-like, domal, anticlinal, and ridge-like. They rise vertically, or nearly so, and increase or decrease with height. Many are capped by residual masses of anhydrite, altered in varied degrees to gypsum, sulfur, and calcite.
Modern theory postulates growth resulting from density differences between the salt and surrounding sediments (1) by upward movement of the salt through the overlying sediments in response to gravitational inequilibrium, or (2) by salt structures remaining at an essentially constant level while the surrounding sediments of sedimentary rocks moved downward around them as deposition progressed. Model studies suggest that variations in overburden and faulting are primary causes of the initiation of salt movement.
The probable source of the salt in Gulf Coast salt domes is the Louann Salt. It may have been as much as 5,000 ft thick and have had an original volume of 200,000 cu mi.
Sediments enclosing salt stocks have varied structural configurations. The strata may be arched, they may be ruptured and pierced by the salt, they may be complexly faulted, or they may be deformed by various combinations of faulting and folding.
All the salt structures in the Gulf of Mexico basin probably are of similar genesis.
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“Diapir” and “diapirism” come from the Greek diapeirein, which means “to pierce.” Diapirism sensu lato is a process by which earth materials from deeper levels have pierced, or appear to have pierced, shallower materials; it is divided into magmatic intrusion and diapirism sensu stricto on the basis of the temperature at which piercement occurs. Diapirs s.s. are composed of evaporites, argillaceous sediments, coal, peat, ice, serpentine, or other earth materials which have the critical characteristics of low equivalent viscosity and low density. These materials range in age from Precambrian to Recent. Diapirs are found in all parts of the world except the shield areas. They have many forms, ranging from smoothly rounded pillows to complexly injected laminae, are either connected with or disconnected from the “mother” bed, and are present either at the surface, where they form distinctive features, or at considerable depth. Diapirs have well-developed internal structures indicative of an origin by flow. Strata around a diapir may be strongly affected structurally and/or stratigraphically by the diapir, or they may be unaffected. Field and model studies indicate that diapirs have developed as a result of horizontal compression, gravitational instability, or both. Diapiric structures of various types contain large quantities of oil and gas, sulfur, salt, and potash and are important for underground storage and nuclear testing.