ORIGIN OF GYPSUM IN DEEP CARBONATE ROCKS—PRIMARY GYPSUM VS. REHYDRATED ANHYDRITE

It is generally recognized that gypsum, which forms penecontemporanously in carbonate sediments, is altered to anhydrite during burial. Some workers have suggested that gypsum dehydration is complete at burial depths of 914 m (3,000 ft). However, gypsum has been documented at depths as great as 3,962 m (13,000 ft).

Two examples of deeply buried gypsum illustrate contrasting origins. Gypsum in the Permian San Andreas Formation in the Permian Basin of West Texas at depths of 1,828 m (6,000 ft) occur as anhydrite nodules with an outer rind of gypsum, suggesting anhydrite was rehydrated to gypsum by interaction with burial fluids. In contrast, gypsum in the carbonate beds of the Cretaceous Ferry Lake Anhydrite of the Mississippi Salt Basin at 3,962 m (13,000 ft) occur as gypsum nodules with a rind of anhydrite, suggesting the gypsum is primary and in the process of dehydration to anhydrite.

These contrasting origins are likely a function of evaporite and carbonate stratigraphy. In the Permian Basin a thick section of carbonate rocks is overlain by a thick section of evaporites. In the Mississippi Salt Basin carbonates are interbedded with multiple evaporite beds, which inhibit burial fluid migration.