Lower Permian Wolfcamp and Wichita carbonates and anhydrites in the Palo Duro Basin, Texas Panhandle, record a change from normal-marine to marine evaporite depositional environment. These strata also contain a widespread porous and permeable interval that currently comprises a deep-basin brine aquifer system. In this study, we combine lithologic, mineralogic, and petrographic evidence of depositional conditions with the isotopic compositions of carbon, oxygen, sulfur, and strontium to determine the geochemical conditions during deposition and diagenesis. This information is then combined with chemical and isotopic analyses of formation water and the geologic history of the region to interpret the history of pore fluids in a deep-basin ground-water system and to establish time constraints on changes in pore-water composition. Carbonate mudstone containing a normal-marine fauna was initially deposited from Early Permian seawater on an open, shallow shelf. The isotopic composition of carbon and oxygen indicates precipitation in equilibrium with Permian seawater for Wolfcamp carbonates distal from shelf margins. Samples from nearshore locations show depleted carbon and oxygen isotopic compositions that reflect the influxof terrestrial organic carbon and meteoric water. Depositional conditions gradually became more restricted, and seawater was concentrated by evaporation, resulting in deposition of penecontemporaneous, hypersaline dolomite and anhydrite in the overlying Wichita Group. Similarity in the sulfur and strontium isotopic compositions of Wolfcamp and Wichita anhydrite suggests that dolomitization of carbonate mudstone and displacive growth of anhydrite nodules in Wolfcamp strata occurred during reflux of Wichita evaporite brine. Application of the sulfur and strontium isotope age curves yields discordant ages for strata at the Wolfcamp-Wichita boundary. Carbonate mudstone (the primary Wolfcamp lithology) and bedded nodular anhydrite (the primary Wichita lithology) have strontium isotopic compositions that indicate a Leonardian age, whereas sulfur isotopic compositions suggest an older (Wolfcampian) time of precipitation. This discrepancy records either a previously unrecognized excursion in the sulfur isotope age curve or inadequate cross-calibration of the two isotope curves. Strontium and sulfur isotopic compositions and petrographic relations indicate that bedded nodular mosaic (Wichita) and unaltered replacive nodular (Wolfcamp) anhydrite precipitated from Early Permian seawater. Silicified nodular, coarsely crystalline nodular, fossil-replacing, and vein-filling anhydrites have sulfur isotopic compositions that record minor sulfate reduction prior to anhydrite precipitation, whereas replacive anhydrite cement contains isotopically depleted sulfur indicative of sulfide oxidation prior to anhydrite precipitation. The strontium isotopic composition of most anhydrite forms indicates an Early Permian marine origin. However, strontium in anhydrite veins is slightly radiogenic compared with all other anhydrite forms, indicate that radiogenic strontium released locally from detrital siliciclastic minerals or imported by river water was present during vein filling. The successive presence of four compositionally distinct pore fluids in Lower Permian strata of the Palo Duro Basin is interpreted from lithologic, rnineralogic, and petrographic evidence of depositional conditions, isotopic compositions of carbon, oxygen, and strontium in limestone and dolomite, limited data from fluid inclusions in sphalerite, and chemical and isotopic analyses of formation water. Permian seawater that initially filled pore spaces in Wolfcamp sediments was replaced by marine evaporite brine during deposition of overlying Wichita dolomite and anhydrite. Expulsion of warm, saline formation water from deeper in the basin through upper Wolfcamp strata is recorded in fluid inclusions in sphalerite. This change in pore-fluid composition occurred before or during the regional Tertiary uplift and tilting that led to the present hydrologic system. Meteoric recharge of at least the western third of the upper Wolfcamp aquifer occurred during the past 10 to 15 Ma, contemporaneous with deposition of the Neogene Ogallala Formation.
Fisher, R. S., and Posey, H. H., 1990, Deposition and Diagenesis in a Marine-to-Evaporite Sequence: Permian Upper Wolfcamp Formation and Lower Wichita Group, Palo Duro Basin, Texas Panhandle: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 195, 34 p