More than 300 diapiric structures formed by the intrusion of relatively pure salt are known in Alabama, Mississippi, Louisiana, Arkansas, and Texas. In form, they are rod-like, domal, anticlinal, and ridge-like; they rise vertically or nearly so; and they expand or contract with depth. Some reflect growth by a succession of differently positioned, local uplifts, as well as shifts in the locus of principal growth. Many are capped by residual masses of anhydrite, altered in varying degrees to gypsum, sulphur, and calcite.

Modern theory postulates growth resulting from density differences between the salt and surrounding sediments through (1) upthrusting (upward movement of salt through sediments in response to gravitational inequilibrium), or (2) downbuilding (maintenance of an essentially static level by the salt while the surrounding sediments subside). Model studies suggest that variations in overburden and faulting are primary motivators of growth.

The "parent" bed from which the salt came exists at depths which range from less than 10,000 to approximately 30,000 feet and is judged to have been as much as 5,000 feet thick. It may have covered as much as 150,000-200,000 square miles and may have had a volume of 50,000-100,000 cubic miles. The presence of large amounts of calcium sulphates peripheral to the Gulf of Mexico basin suggests that the salt is a precipitate from brines concentrated in the Gulf basin or in partially restricted marginal basins.

Surrounding sediments are arched adjacent to or over the salt masses. They may thin against or over the salt to more than half their normal thickness. Normal faults frequently disrupt them; reverse faults are extremely rare. Grabens, occasional horsts, multiple offsets in single or different directions, and radial, tangential, or peripheral faults often combine to form complex patterns.

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