Suitability of salt domes in the East Texas Basin for nuclear waste isolation: final summary of geologic and hydrogeologic research (1978 to 1983)
This report summarizes results of the East Texas Waste Isolation (ETWI) program from January 1, 1978, to March 30, 1983. Having an extensive data base, the study comprised 33 different lines of research by 67 scientists and research assistants. The program included both basinwide and site-specific (mainly around Oakwood Dome) studies that used surface and subsurface data. Mesozoic opening of the Gulf of Mexico accompanied thermal processes that controlled sedimentation during filling of the East Texas Basin. The basin contains as much as 7,000 m (23,000 ft) of shallow-marine and continental sediments overlying the Louann Salt. Deformation in the basin resulted from subsidence of its floor and gravitational flow of salt and from salt loss in the subsurface and at the surface. Regional studies have defined the present-day distribution of salt structures and the growth history of domes. The East Texas Basin is divided into four provinces on the basis of the shape of salt structures. Five forces that make salt flow operated from near surface to the deepest parts of the basin to form these structures. Salt flow began in pre-Gilmer (Late Jurassic) time with the growth of salt pillows. Three groups of diapirs can be differentiated on the basis of age and distribution. The growing salt structures affected topography, thereby influencing depositional facies and resulting in deposition of low-permeability facies around the salt stocks. Rates of dome growth declined exponentially with time to rates of less than 0.6 m/104 yr. Geomorphic evidence does not preclude Quaternary uplift over Oakwood Dome, but its southern flank may have subsided. All regional fault systems in the basin appear to be related to slow gravitational creep of salt downward toward the basin center or upward into salt pillows and diapirs. Nevertheless, at least eight probable earthquakes were recorded near the southern margin of the basin in 1981 and 1982, and their probable focus, the Mount Enterprise fault, is poorly understood. The following conclusions were drawn from geologic study of a core drilled from Oakwood Dome: Salt core from the dome is more than 98 percent pure halite (anhydrite is the only other mineral present). The rock salt displays evidence of two distinct periods of recrystallization. Geometric analysis and strain analysis suggest that the crest of the dome was truncated, probably by ground-water dissolution, during the formation of anhydrite cap rock. Diapiric rise of salt formed a tight contact between salt and cap rock. The cap rock formed in a deep saline environment and appears to be a low-permeability barrier to dome dissolution.