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ABSTRACT: Replacement of Evaporites with the Mid-Permian (Leonardian-Guadalupian) Park City Formation, Bighorn Basin, Wyoming

Dana S. Ulmer, Peter A. Scholle

The Park City Formation consists of cyclic subtidal to supratidal carbonates controlled by glacio-eustatic sea level fluctuations and localized tectonic uplift. Subsurface cores show significant preserved interstitial evaporite, but on outcrop this unit has extensive silica and calcite replacement of former gypsum and anhydrite crystals and nodules.

These replacements appear to be a multistage phenomenon. Field and petrographic evidence (matted fabrics in nodules; evaporite inclusions) indicate that silicification involved direct replacement of evaporites and probably occurred during early stages of burial. Calcitization, however, appears to be a much later phenomenon and involved precipitation of coarse crystals within evaporite molds. The replacement calcites have a wide range of isotopic values (oxygen -6.04 to -25.02fi, average -18.00^pmil; carbon 2.82 to -25.26, average -7.01^pmil; all values relative to PDB). The calcites are typically free of evaporite remnants but are laden with hydrocarbon inclusions.

The light carbon and oxygen isotopic values and the presence of oil inclusions within the calcites support replacement during late diagenesis, generally following hydrocarbon emplacement. The extremely broad isotopic range indicates that calcitization occurred during a long period of progressive uplift and increased circulation of groundwater associated with middle Tertiary block faulting of the Bighorn basin. Rainfall-derived meteoric fluids (^sim12%) migrated into these porous carbonates at probable depths of a few kilometers, mixing with preexisting hydrocarbon-rich pore fluids and precipitating calcites in evaporite molds. Thus, significant porosity change has taken place in Park City carbonate strata during basin uplift, and the isotopic variations in the calcitized evaporites pr vide a history of water and hydrocarbon migration through these units.

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