Cap rock from Gyp Hill salt dome, Brooks County, south Texas, was formed by salt dome dissolution that left a residuum of anhydrite sand, which was subsequently cemented by gypsum and at a later time altered to gypsum by fresh meteoric groundwater. The cap rock consists of gypsum at the surface (0 to 90 m) and gypsum-cemented anhydrite above the salt (90 to 273 m). Samples from the salt contain 13 to 42% disseminated anhydrite crystals and < 1.0% dolomite rhombs in halite. The cap-rock-salt boundary is marked by a cavity several meters high. Salt dissolution has concentrated the insoluble material into an anhydrite sandstone with 20% porosity at the base of the cap rock. Cap rock porosity is largely occluded within 6 m above salt by poikilotopic gypsum cement and crush d anhydrite laths (presumably from the overburden pressure of the cap rock). A transition zone occurs between 90 and 120 m below the surface where anhydrite is being completely hydrated to gypsum. Above this zone, the cap rock is entirely gypsum and indicates flushing by fresh meteoric groundwater. Through the total thickness, anhydrite is in disequilibrium, as evidenced by the gypsum cement and embayed anhydrite laths.

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