STRATIGRAPHY AND SEDIMENTOLOGY OF THE BLACK LION CONGLOMERATE, EASTERN PIONEER MOUNTAINS, BEAVERHEAD COUNTY, SOUTHWESTERN MONTANA

The purpose of this study was to investigate the age and depositional setting of the Black Lion Conglomerate in the Eastern Pioneer Mountains of southwest Montana. Located at Section 5, T3S, R11W, Beaverhead County, Montana, the Grace Lake field site is the primary focus of this study. Other field sites include Hecla, Black Lion Lake, Black Lion Creek, and Sheep Mountain. Previous studies suggested the Black Lion Conglomerate is either Cambrian or Proterozoic in age. The base of the Black Lion Conglomerate is not exposed at any location. The Black Lion Conglomerate is at least 120m (394ft) thick.In this study, the sedimentology, stratigraphic architecture, and provenance of the Black Lion Conglomerate were investigated through lithofacies descriptions, paleocurrent data, clast counts, thin section point counts, and compositional analysis using pXRF, Raman Microscopy, and Scanning Electron Microscopy – Energy Dispersive X-ray analysis. Six types of lithofacies composing the Black Lion Conglomerate were identified (Gms, Gp, Sh, Sp, Ss, and St) and are interpreted to be braided stream deposits. Paleocurrents show a dominant west-northwest flow direction with scatter typical of fluvial systems. Clast constituents included quartzite, quartz sandstone, red siltstone, and red quartz grains. Mineral constituents consist of monocrystalline and polycrystalline quartz grains, potassium and plagioclase feldspars, zircon, titanium rich magnetite, and rutile. The grain composition showed the Black Lion Conglomerate to be a subarkose. The stratigraphic columns from Sheep Mountain and Grace Lake showed no clear trends in clast composition for the Black Lion Conglomerate. Sediment was likely derived from Proterozoic and Archean basement rocks, possibly similar to rocks exposed in the present Highland and Tobacco Root mountains and are from transitional continental crust.