A PETROLOGIC STUDY OF THE BRIDGER CANYON SECTION OF THE SWIFT FORMATION, BOZEMAN MONTANA

The Late Jurassic Swift Formation is one of the sedimentary formations deposited in the partitioned Sundance Sea in Montana. The Swift is a calcite cemented, very poorly sorted chert-arenite, deposited in a marine environment as evidenced by the presence of glauconite and fossils of marine fauna. This petrologic study of one section, Bridger Canyon north of Bozeman included point counting of framework grains (Quartz, Feldspar, and Lithics) and a separate point count for lithic fragments. The collected data was applied to grain size analysis and statistics, lithologic classification, provenance characterization and determination of tectonic setting. Results of measurements of 50 grains per slide provide average grain size data. The number of grains counted per slide varied from 58 grains to 218 grains, with an average of 136 grains per slide; with low counts resulting from extensive alteration of some samples. Types of alteration include dolomitization calcitization and of grains, hematite coatings, quartz overgrowths, and pressure solution. Average grain size in samples ranges from 0.25mm-0.54mm, (medium sand). Types of grains counted include mono- and polycrystalline quartz, feldspar, chert, chalcedony, carbonates identified as extraclasts, and mudrock. The grain assemblages indicate both clastic and chemical sedimentary source areas. Based on QFL point counting data, the tectonic setting of the source area is a recycled orogen.