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The AAPG/Datapages Combined Publications Database

GCAGS Transactions


Gulf Coast Association of Geological Societies Transactions
Vol. 30 (1980), Pages 333-339

Caprock Formation and Diagenesis, Gyp Hill Salt Dome, South Texas) (1)

Shirley P. Dutton, Charles W. Kreitler (2)


Caprock from Gyp Hill salt dome, Brooks County, in South Texas, formed by salt dome dissolution that left a residuum of Previous HitanhydriteNext Hit sand. This Previous HitanhydriteNext Hit sand was subsequently cemented by gypsum and at a later time altered to gypsum by fresh meteoric ground water. The caprock consist of up to 90 m (300 ft) of gypsum at the surface and up to 273 (300 to 895 ft) of gypsum cemented Previous HitanhydriteNext Hit above the salt. Samples from the salt contain 13 to 42 percent disseminated Previous HitanhydriteNext Hit crystals and <1.0 percent dolomite rhombs in halite. The caprock-salt boundary is marked by a cavity several feet high. Salt dissolution has concentrated the insoluble material into an Previous HitanhydriteNext Hit sandstone with 20 percent porosity at the base of the caprock. Caprock porosity is largely occluded within 6 m (20 ft) above salt by poikilotopic gypsum cement and crushed anyhdrite laths (presumably from the overburden pressure of the caprock). A transition zone occurs between 90 and 120 m (300 and 400 ft) below the surface where Previous HitanhydriteNext Hit is being completely hydrated to gypsum. Above this zone, the caprock is entirely gypsum and indicates flushing by fresh meteoric ground water. Through the total thickness, Previous HitanhydriteNext Hit is in disequilibrium, as evidenced by the gypsum cement and embayed Previous HitanhydriteTop laths.

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