Origin and Diagenesis of Cap Rock, Gyp Hill and Oakwood Salt Domes, Texas

Petrographic and geochemical studies of caprock core from two salt domes, Gyp Hill in South Texas and Oakwood in East Texas, reveal the significantly different diagenetic histories of each dome. Cap rock on Gyp Hill is now forming within a shallow meteoric aquifer. In contrast, cap rock on Oakwood Dome formed principally during the geologic past within deep, saline aquifers in the East Texas Basin. Gyp Hill cap rock, which is 890 ft (271 m) thick, is composed of 490 ft (149 m) of anhydrite overlain by 400 ft (122 m) of gypsum. Uncemented anhydrite sandstone marks the salt/cap-rock interface. From 13 ft (4 m) above the interface to the top of the anhydrite, porosity is occluded by poikilotopic gypsum cement. Occurrence of gypsum cement indicates low-temperature, low-salinity conditions during caprock formation; that is, dome dissolution is occurring in a shallow meteoric aquifer. The overlying gypsum results from hydration of anhydrite by meteoric ground water. Oakwood cap rock, which is 450 ft (137 m) thick, is composed of 256 ft (78 m) of anhydrite overlain by 194 ft (59 m) of calcite. In contrast to Gyp Hill anhydrite, Oakwood anhydrite is entirely devoid of gypsum cement except at the interface between anhydrite and calcite. The anhydrite has been recrystallized into a moderately well developed granoblastic texture that is indicative of high-temperature and high-pressure conditions. Fluid inclusions in the salt at the salt/cap-rock interface represent waters from the last dissolution event. The delta 18O of water from a fluid inclusion is +/-5.4 ppm, indicating a deep-basin origin of the water. The anhydrite section is considered to have accumulated during salt dissolution under deep, high-temperature, saline conditions. Timing of the development of major rim synclines surrounding Oakwood Dome indicates that the anhydrite cap rock formed in Early Cretaceous time. The calcite section of the cap rock is composed of alternating layers of dark and light calcite. Petrographic, geochemical, and isotopic data on dark calcite indicate that the dark calcite is the product of calcium sulfate reduction by hydrocarbons in a saline, deep-basin fluid. Another deep-basin fluid, more enriched in Sr, Ba, Mg, and Mn, dissolved some of the dark calcite, which then reprecipitated as coarsely crystalline light calcite. The only effect of meteoric water on either the anhydrite or the calcite section of the Oakwood cap rock is the presence of gypsum in the calcite/anhydrite transition zone. Anhydrite cap rock beneath the salt overhang of Oakwood Dome (~6,000 ft deep) had an origin similar to that of the cap rock on top of the dome. Petrographic analyses of cap rock from Rayburn's and Vacherie Domes (Louisiana) and Cypress Creek and Richton Domes (Mississippi) further substantiate the two different types of cap rock. The cap rock from Rayburn's Dome is similar to that at Gyp Hill Dome, whereas the cap rock from Vacherie is similar to Oakwood Dome cap rock. The mineralogy and textures observed in cap rocks from Richton and CypressCreek Domes indicate that they are intermediate between Gyp Hill and Oakwood cap rocks. Petrographic and geochemical studies of cap rock are important in evaluating the hydrologic stabilities of salt domes being considered as repositories for high-level nuclear wastes. Anhydrite cap rocks such as those at Oakwood Dome contain recrystallized anhydrite, are devoid of gypsum, have a tight cap-rock/salt contact, and formed early in the geologic history of the basin. There is no evidence within the cap rock of recent salt dissolution by meteoric ground water. In contrast, cap rocks such as those at Gyp Hill contain unrecrystallized anhydrite, are cemented with gypsum, and have both uncemented anhydrite sands and a cavity at the salt/cap-rock contact. They also exhibit ample evidence of recent salt dissolution by meteoric water, a condition unacceptable in a salt dome being considered as a high-level nuclear waste repository.