Rb-Sr Ages of Diagenesis of Mg-rich Clay in Permian Sediments, Palo Duro Basin, Texas Panhandle, U.S.A.

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ABSTRACT Mid- to upper Permian sedimentary rocks in the shallow (< 3 km), intracratonic Palo Duro Basin, Texas Panhandle, occur in multicyclic carbonate-evaporite-redbed sequences, interpreted to be hypersaline shelf depositional assemblages. Certain marker horizons can be traced continuously over the entire basin. Clay minerals include distinctive varieties of Mg-rich smectite, chlorite, or mixed-layer species resulting from diagenesis in an evaporitic environment. Discrete particle-size fractions of clay were separated from core recovered from the Tubb (Leonardian), San Andres Unit #4 (lower Guadalupian), upper Seven Rivers (upper Guadalupian), and Salado-Tansill (Guadalupian-Ochoan) Formations to determine the times of mineral genesis. Rb-Sr isotopic data, for aliquots of a clay sample that has been leached by diverse ion exchange reagents, plot as a linear array in a Rb-Sr isochron diagram. For ultra-fine clay (< 0.05 µm), interpreted to be diagenetic, the slope of the data array corresponds to the time of diagenesis. Ages of clay diagenesis in the four sampled stratigraphic units agree generally with ages of deposition according to the geologic time scale, signifying that diagenesis was penecontemporaneous with deposition. Analyses of coarser clay fractions (0.2-0.5 µm and 2-4 µm) indicate the presence of a detrital component. In most instances, initial 87Sr/86Sr in diagenetic clay is higher than the Sr isotope ratio measured directly in associated anhydrite. Introduction of radiogenic Sr from occasional stream influx or from concurrent diagenetic dissolution of silicate detritus could have elevated 87Sr/86Sr in the ambient brine that mediated clay diagenesis.