PROVENANCE OF THE BASAL FLUVIAL CONGLOMERATE AND SANDSTONE OF THE MADISON VALLEY FORMATION IN THE MADISON INTERMONTANE BASIN, SOUTHWEST, MONTANA

Gravel constituents and sandstone from the basal fluvial facies of the middle Miocene Madison Valley Formation located above the mid-Tertiary unconformity in the Madison intermontane basin were studied to determine provenance and the probable paleodrainage system present. The Madison intermontane basin is part of an extensional basin complex developed in the middle Eocene. Petrologic data indicate particular source rocks ranging from Precambrian to Eocene in age with locations ranging from basin flanking uplifts to distal western sources in western Montana and eastern to central Idaho. Gravel constituents such as scoria, two-mica granite, pink quartzite and black chert are unique and can be used to indicate specific source areas. The scoria is likely eroded from the Absarokas, located in uplifts directly to the southeast of the basin while the two mica-granite source indicates either the Boulder Batholith, located to the east in the intermontane basin region but not in uplifts directly adjacent to the basin, or the Idaho Batholith, located outside of the intermontane basin region in central and eastern Idaho. The pink quartzite and black chert indicate the supergroup and Milligan formation, both located in central to eastern Idaho, as source areas. Paleocurrent data indicate a northward flowing system within the basin but provenance suggests that the overall drainage system was incorporating sediment from adjacent uplifts with larger trunk systems supplying sediment from southwestern areas in central Idaho and western to northwestern areas in western Montana and eastern Idaho. The data supports the hypothesized middle Miocene drainage system of a fluvial system flowing from northwest Montana and eastern Idaho into the intermontane basin region, south through the canyons of the Clark Fork River, and a second system flowing from central Idaho northeast through the Ruby and Beaverhead Grabens, into the intermontane basin region.