Provenance of Lower Kootenai (Lower Cretaceous) fluvial sandstone bodies, Sandy Hollow/Big Hole River area, southwestern Montana

Terrigenous clastics of the Lower Kootenai Formation (Lower Cretaceous), display non-marine deposition as multiple fluvial sand bodies within a Sevier foreland basin, reflecting tectonic and sedimentologic dynamics in southwestern Montana. Late Cretaceous Laramide tectonic activity has exposed the Lower Kootenai along a number of strike ridges in the Big Hole River area, southwestern Montana. The base of the Lower Kootenai Formation is a conglomerate that rests unconformably upon the Upper Jurassic Morrison Formation. The Lower Kootenai comprises an alluvial sequence with deposits of medium to coarse grained, chert arenite, over-bank deposits of calcareous mudstone, caliche and calcrete paleosols and is capped by interspersed lacustrine limestone and dolomite units. Thicknesses of the alluvial sandstone bodies range from five to 35 feet (1.5 to 10.7 meters). Caliche rip-up clasts are prominent within conglomeratic units at the bases of many of the upper channel deposits. Sandstone bodies, presumed to be overbank splays, form lenses within a prominent mudstone interval that separates a lower and upper succession of alluvial sandstone within the lower clastics. Sedimentary structures include small to large-scale trough, festoon, and tabular cross strata. Trough measurements within the upper sandstone units indicate a west to east delivery direction. Major grain constituents consist of approximately 50 percent monocrystalline and polycrystalline quartz, 46 percent SRF's (mostly chert) and 4 percent feldspars. Chert grains range from medium to coarse sand and are sub-angular. Quartz grains occur as sub-rounded fine to medium sand. Grain constituents within sandstone bodies compare to Pennsylvanian-Permian rocks of the Phosphoria, Wood River, and Quadrant Formations.