More than half of the proved domes in the Gulf Coast area are known to have cap rock which is composed chiefly of anhydrite, calcite, gypsum, and locally celestite. A detailed study of cap rock from 39 salt domes as well as a thorough literature search on the subject indicates large lithologic and chemical variations in its character. The two most widely accepted theories of origin--(1) residual accumulation and secondary alteration of the less soluble minerals contained in the salt, and (2) precipitation in place--cannot satisfactorily account for these variations in cap rock and its relation to the surrounding environment.

It is proposed here that these variations may be explained by a "modified precipitation in place" theory in which most of the cap rock was precipitated from ascending solutions rich in calcium sulfate, and a smaller amount was contributed by dissolution of the upper salt surface which released the less soluble residue.

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Fig. 1. Location of salt domes included in study.

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