ABSTRACT: Strontium and Sulfur Isotope Study of Well-Preserved Permian Anhydrite, Palo Duro Basin, Texas

David A. Leary

The ^dgr³⁴S, ⁸⁷Sr/⁸⁶Sr ratio, and strontium concentrations for 50 well-preserved samples of Permian marine anhydrite have been determined. The samples were collected from two continuous cores drilled through cyclic Permian evaporites. The Department of Energy drilled the samples in its search for a permanent storage facility for high-level nuclear waste. Primary depositional fabrics (selenite pseudomorphs) and high strontium concentrations (average 1850 ppm), in association with published bromide and fluid inclusion data from associated halite, suggest primary seawater ⁸⁷Sr/⁸⁶Sr ratios may be recorded in many of the samples.

The general shape of the ⁸⁷Sr/⁸⁶Sr ratio curve through the Permian is in accord with previously published observations. However, the increased stratigraphic detail from this unique set of cores constrains the abrupt change in ⁸⁷Sr/⁸⁶Sr during the Permian at a precision previously available only in Deep Sea Drilling Project material. Interpretation of the more complex portions of the curve is limited by poor biostratigraphic control, the specter of provincial early diagenetic effects, and interpretation of the time significance of hiatal surfaces in cyclic strata. Age relationships are constrained by a K-Ar date on an interbedded volcanic ash in the Ochoan strata, and fusulinid age determinations of a well-documented regional transgression du ing the earliest Guadalupe. Sulfur isotopes yield typical Permian values of 12^pmil during the marine portion of the basin fill phase, and abruptly shift to 10^pmil in those cycles with a significant component of siliciclastic sediment.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990