Provenance of Lower Kootenai (Cretaceous) Sandstone Bodies, Sandy Hollow / Big Hole River Area, Southwestern Montana
Terrigenous clastics of the Lower Kootenai Formation (Cretaceous), display non-marine deposition of multiple fluvial sand bodies within the 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 of 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 5 to 35 feet. Caliche rip-up clasts are prominent within conglomeratic units at the bases of many of the channel deposits. Sandstone bodies, presumed to be crevasse 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 troughs, festoon bedding, and tabular cross strata. Trough measurements within the upper sandstone units indicate a west to east delivery direction. Major grain constituents consist of approximately 48 percent chert, 43 percent monocrystalline and polycrystalline quartz, and 9 percent sedimentary rock fragments. Chert grains range from medium to coarse sand and are subangular. Quartz grains occur as angular medium sand. The abundant chert grains probably originated from a westerly uplifted source, presumably the Permian Phosphoria Formation, uplifted by a pulse of the Sevier orogeny in Early Cretaceous time.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009