Lithology, Microstructures, Fluid Inclusions, and Geochemistry of Rock Salt and of the Cap-Rock Contact in Oakwood Dome, East Texas: Significance for Nuclear Waste Storage

Abstract
Oakwood salt dome in Leon and Freestone Counties, Texas, has a core composed of a diapiric salt stock at a depth of 355 m. A vertical borehole in the center of the salt stock yielded 57.3 m of continuous rock-salt core overlain by 137 m of anhydrite-calcite cap rock. The lower 55.3 m of rock salt exhibits a strong, penetrative schistosity and parallel cleavage dipping at 30° to 40° and more than 60 variably dipping layers of disseminated anhydrite. Anhydrite constitutes 1.3 ± 0.7 percent of the rock-salt core. The upper 2 m of rock salt is unfoliated, comprising a lower 1.4-m interval of medium-grained granoblastic rock salt and an upper 0.6-m interval of coarse-grained granoblastic rock salt. An abrupt, cavity-free contact separates rock salt from laminated cap rock consisting of granoblastic-polygonal anhydrite virtually devoid of halite or pore space.Microstructures and concentration gradients of fluid inclusions suggest that the unfoliated rock salt at the crest of the salt stock was once strongly foliated, but that this fabric was destroyed by solid-state recrystallization. Downward movement of brine from the rock-salt - cap-rock contact was apparently accompanied by two recrystallization fronts. Dissolution of halite at the contact released disseminated anhydrite that presumably accumulated as sand on the floor of the dissolution cavity. Renewed rise of the salt stock closed the cavity, and the anhydrite sand was accreted against the base of the cap rock. Much, if not all, of the lamination in the 80 m of anhydrite cap rock may result from cycles of dissolution, recrystallization, and upward movement in the salt stock, followed by accretion of base of the cap rock. These processes, which are strongly influenced by fluids, act both to breach waste repositories and to geologically isolate them. Despite repeated attrition and uplift of the salt stock, the geologic system has the ability to offset, at least partially, these negative processes by self-sealing and recovery.
Authors
Owen R. Dix
Martin P. A. Jackson
Citation

Dix, O. R., and Jackson, M.P.A., 1982, Lithology, Microstructures, Fluid Inclusions, and Geochemistry of Rock Salt and of the Cap-Rock Contact in Oakwood Dome, East Texas: Significance for Nuclear Waste Storage: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 120, 59 p.

Code
RI120
ISSN
2475-367X
Number
120
Number of figures
37
Number of pages
59
Publisher
The University of Texas at Austin, Bureau of Economic Geology
Series
Report of Investigation
Year
1982