SEDIMENTARY RECORD AND DETRITAL ZIRCON PROVENANCE OF THE EOCENE-OLIGOCENE RENOVA FORMATION, BITTERROOT VALLEY, WESTERN MONTANA

The Bitterroot Valley south of Missoula, Montana contains gravels and sands, which are interpreted to be equivalent to the Eocene-Oligocene Renova Formation (58-24 Ma). The Renova Formation is locally capped by an angular unconformity. Above the angular unconformity lies the Sixmile Creek Formation, which is a large debris flow deposit containing cobbles and boulders. This unconformity is widespread across the northern Basin and Range and represents a fundamental tectonic reorganization associated in part with initiation of the Yellowstone hot spot. The unconformity postdates tilted 20 Ma Renova beds and predates the 16 Ma Barstovian Lower Sixmile Creek Formation. Clast counts at a quarry site near Stevensville, Montana have revealed the presence of locally derived lithologies as well as distinctive central Idaho clasts, such as the metamorphosed Devonian Milligen chert, Proterozoic quartzites, and Idaho Batholith granitoids. This clast distribution may indicate Middle Miocene faulting associated with the migration of the Yellowstone hotspot track, which could have captured the Paleogene Bitterroot River, resulting in the modern Salmon River drainage in Idaho. Sedimentary structures that occur within the formation include trough cross-beds, indicating a migratory river system, and imbrication of clasts indicating paleocurrent flows to the north, similar to the modern Bitterroot drainage. Zircons have been separated from the Renova gravels and sands for detrital U-Pb LA-ICP-MS analysis. These new data will allow for age constraints on the geometry of the paleodrainage during deposition of the Renova Formation prior to the outbreak of the Yellowstone hotspot. Understanding the source of the clasts will allow for interpretation of the Paleogene Bitterroot River prior to the Miocene disturbance.