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Abstract


Gulf Coast Association of Geological Societies Transactions
Vol. 3 (1953), Pages 147-148

ABSTRACT: The Mineralogy of the Salt and Cap Rock of Gulf Coast Salt Domes

Ralph E. Taylor (1)

ABSTRACT

Development of oil, salt, and sulphur deposits at Gulf Coast salt domes has continued to add to knowledge about these structures. Additional data of particular significance have resulted from deeper drilling near the margins of the basins in east Texas and southwestern Alabama, and production of salt from a number of additional domes both for the purposes of chemical manufacture and to provide storage for propane and butane. Data relating to the cap rock continue to be developed at the older sulphur mines and at recently opened new ones, as well as in exploratory drilling for oil and sulphur. It is the purpose of this paper to review the mineralogy of the salt and cap rock of Gulf Coast salt domes in the light of the new data.

The presence of salt has been proved at 234 domes which occur in the series of closely related basins extending from southwestern Alabama across Mississippi, Louisiana, and Texas to the Rio Grande River. Data from some 35 domes representing all of the basin areas show that the salt all has the same general mineralogical characteristics. Typically it is a bluish-gray, compact aggregate of large rounded to elongate grains of halite that contain varying amounts of small anhydrite grains. The anhydrite is concentrated in narrow dark bands that were complexly folded when the

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salt was intruded from depth to form the salt domes. Recent studies by Robert Balk at Grand Saline (Texas) and Jefferson Island (Louisiana) mines have brought out details of structure in the salt that are related to intrusion.

The water insoluble residues of the salt, although composed predominantly of sand-size crystals and cleavage fragments of anhydrite also include some 20 other minerals in the form of small crystals. The minerals of these residues are composed of the more abundant elements found in sea water, and are believed to have been precipatated when the salt crystallized from solution. The chemical and mineral composition of the salt in domes together with the banding or bedding, the inclusions of gas and liquid, and of deformed beds of anhydrite, sand, and shale prove that the salt originally was deposited as a normal bedded deposit.

Salt from deep wells in southwestern Alabama has horizontal bands or bedding planes that apparently are undeformed and has different insoluble residue than salt from domes. Argillaceous insoluble residue from this bedded salt contained Jurassic pollen. The striking difference between the bedded salt and the salt of the nearby McIntosh dome, which is typical of Gulf Coast salt domes, indicates that the two types of salt are not correlative and that the dome salt probably is the older. Similar undeformed bedded salt, that is unlike the salt in domes and that apparently also is of Jurassic age, has been encountered in east Texas and north Louisiana wells.

Typically the cap rock of salt domes consists of coarsely crystalline, horizontally banded anhydrite overlain by a zone of secondary, coarsely crystalline calcite. Gypsum, sulphur, pyrite, and other secondary minerals occur in varying abundance, particularly at the contact of the two zones and in the calcite zone. Detailed petrographic studies have shown that the anhydrite zone contains the same mineral suite as the water insoluble residue of the salt and that the calcite zone has formed as a result of alteration of the anhydrite zone. Further evidence that even very thick salt dome cap rocks can be formed by the accumulation of insoluble residue is being provided by the literally thousands of cubic yards of anhydrite sand that are being removed from brine wells at several domes including McIntosh (Alabama), Choctaw and West Hackberry (Louisiana), and Bryan Mound, Stratton Ridge and Palangana (Texas). More direct evidence of the manner of origin of cap rock has been obtained at the Lake Washington and Pelican Island domes in Louisiana where coring recently has revealed that fragments of bedded anhydrite included in the cap rock are of the same character as fragments occurring in the salt.

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ACKNOWLEDGMENTS AND ASSOCIATED FOOTNOTES

(1) Humble Oil and Refining Co., Houston, Texas

Copyright © 1999 by The Gulf Coast Association of Geological Societies