Abstract

Since 1974, the Department of Energy has been studying bedded salt deposits in southeastern New Mexico as a possible location for disposing of defense-generated transuranic and low-level radioactive wastes. This gravity survey is part of the geologic site characterization. The gravity survey of the Waste Isolation Pilot Plant delineates structural features near and at the proposed site. The gravity field was found to be dominated by lateral density variations within relatively flat-lying strata. The pattern includes elongate negative anomalies about one-half mGal in amplitude. Boreholes in the anomalies encountered normal stratigraphy and no unusual geologic structures. However, borehole densilogs showed lower densities and uphole velocity surveys showed lower acoustic velocities than are measured outside of the anomalies. The low densities adequately account for the observed gravity anomalies. The regional stratigraphy contains water-soluble minerals (halite, polyhalite, anhydrite-gypsum, carbonates). Much of this material has dissolved and the region is a karstland. At the site, dissolution is slowly affecting the Rustler Formation overlying the main salt-bearing units. The low rock densities, associated with the negative gravity anomalies, are due to alteration near solution conduits within the Rustler Formation. This interpretation is supported by (1) partial coincidence between the negative gravity anomalies and closed topographic depressions (alluvial dolines); (2) greater anhydrite-to-gypsum conversion detected in boreholes within the anomalies; and (3) solution conduits encountered in one of the boreholes.--Modified journal abstract.

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