UPPER CRETACEOUS PEAY BENTONITES (NORTH-CENTRAL WYOMING): PROVENANCE AND TECTONICS INTERPRETATION FROM ASH COMPOSITION

The Peay bentonites belong to the basal Frontier Formation (Bighorn Basin, north-central Wyoming), primarily outcrop in the Bighorn Basin, rest on an extensively bioturbated sandstone unit, the Peay Sandstone, and are generally the thinner bentonitic unit. Beds of very light gray to greenish gray bentonite are also abundant in the lower Frontier units between Kaycee and Mayoworth (Powder River Basin) and are very rarely as much as 3 m thick. Bentonite occurs within the interstratified shale, sandstone, and siltstone sequences of the lower Frontier unit throughout much of the southwestern Powder River Basin. The purpose of this study is to use field, geochemical, petrological, and zircon morphology data to establish a chemical fingerprint of the original ash composition and to provide a tectonic and provenance interpretation. Peay units show distinct mineralogy consisting of quartz, kaolinite, illite, smectite, mixed-layer clays, K-feldspar, plagioclase, chlorite, calcite, and dolomite. Fairly uniform concentrations of SiO_2, Na₂O_, CaO, MgO, Al₂O₃, Zr, Ti, Nb, and Ni have been noted in the Peay bentonites, such consistent concentrations of these elements in Peay bentonite samples imply that they have not been strongly modified by secondary processes. Peay bentonite samples show a consistent geochemical trend towards the alkali basalt field. On the basis of the similarity in gross stratigraphic development, feldspar compositions and trace element geochemistry, the source for the Peay bentonites is most likely located in the same general area. This is further supported by the presence of Ti-rich augite and the distinctive zircon morphology. The uniformity of composition and similarity in stratigraphic development of Peay bentonites over the region studied argues strongly against the contemporaneous involvement of several extrabasinal volcanoes. Evidence for active Cretaceous volcanism in the Idaho and in south-central Montana has been documented by Gill and Cobban, 1973. The Peay bentonites broadly correlate to the emplacement of batholiths in this region. Igneous rocks in northern Idaho and north-central Montana have yielded isotopic dates similar to those of the Peay bentonites (ca 90 Ma; Obradovich, 1993; Obradovich and Cobban 1975).

Funded by a PSC-CUNY Research Grant awarded to the author.