A Comparison of the Depositional Environment of the San Andres Formation in the Palo Duro Basin to Recent Evaporitic Environments

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Chapman, Jenny Burgen

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The safe storage of high-level radioactive waste in a geologic repository requires a detailed knowledge of the properties of the host rock and surrounding beds, and the continuity of these properties. One of the serious problems faced in the characterization of the deep formations under consideration is that sufficiently detailed descriptions cannot be obtained from the small number of widely spaced test holes available. The examination of modern analogs to the formation under consideration can provide important insights into geochemical characteristics and their degree of continuity.

The first step in identifying a natural analog is to adequately describe the formation of interest and to determine its environment of deposition. The formation under consideration in the Palo Duro Basin, the San Andres Formation, has been described elsewhere (Presley, 1979a & b; 1980a & b, 1981; Presley and Ramondetta, 1981; Ramondetta, 1981; Handford, 1981a & b; Handford and Wiggins, 1981; Bassett and Palmer, 1981; Bassett and Roedder, 1981; Budnik and Smith, 1982; Roedder, 1982). In the Palo Duro Basin, the San Andres Formation is an evaporite sequence containing halite, anhydrite, carbonates, and mudstones. The lithology of the San Andres and the stratigraphic sequence prior to San Andres deposition indicate that the formation was deposited at the end of a long-term shift from fan-delta, marine shelf, and deep-basin environments during the Pennsylvanian to shallow marine, brine pan, and evaporite conditions during Late Permian time. A modern analog environment should be located in a relatively shallow basin that has already been filled by marine sedimentation and is now a broad shelf undergoing long-term oceanic regression and slow subsidence.


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