PETROGRAPHY AND PROVENANCE OF KOOTENAI (LOWER CRETACEOUS) FLUVIAL SANDSTONE UNITS IN SOUTHWESTERN MONTANA

The Kootenai Formation (Lower Cretaceous) of southwestern Montana is an alluvial sequence with interspersed lacustrine limestone and dolomite units. Late Cretaceous folding and thrusting involved the Kootenai and associated units forming multiple strike ridges and allowing multiple cross-sectional views of the channel and overbank deposits. Alluvial sandstone bodies of the Kootenai Formation in the southern part of Madison County adjacent to the Big Hole River range from two to 12 meters in thickness and are arrayed in multilateral patterns along outcrops perpendicular to depositional strike. Alluvial intervals include well-defined channel deposits of medium-grained chert arenite and overbank deposits of calcareous mudstone. Small to large-scale sets of trough and tabular cross strata occur to the exclusion most other sedimentary structures. Clasts of calcareous mudstone and occasional limestone are prominent in conglomerate units near the base of channels. Foresets within cross-stratified sets dip to the east. Dip azimuths display low dispersion within individual channel systems indicating deposition by east-flowing streams with low sinuosity. Framework grains in sandstone units are composed of approximately 52 percent chert, 43 percent monocrystalline and polycrystalline quartz, and five percent sedimentary rock fragments other than chert. Chert fragments are subangular and range from medium to coarse in grain size. Quartz grains are in the medium grain size range and are angular. The grain population reflects the early stages of the Sevier orogeny in southwestern Montana.