CENTRAL NEVADA PROVENANCE FOR OLIGOCENE AND MIOCENE GRAVEL IN THE MONTANA ROCKIES

Distinctive pebbles that occur in Oligocene and Miocene river gravel in the Montana Rockies indicate source regions as far away as the central Nevada Roberts Mountain allochthon and Antler foreland basin. While similar lithologies occur in central Idaho, they were largely buried by Eocene volcanics and unavailable as a major source region. The Oligocene gravel occurs in the Bitterroot Valley, a Paleogene graben on the flank of the Bitterroot core complex. Paleocurrent indicators show northward flow. The Bitterroot gravel is tilted beneath an angular unconformity at the base of the Middle Miocene Sixmile Creek Formation. No exotic clasts occur within the Sixmile Creek Formation in the Bitterroot Valley, indicating that the central Nevada source region was cut off from the Bitterroot by the Middle Miocene. In SW Montana, however, the central Nevada lithologies occur in the Sixmile Creek Formation above the angular unconformity, but not in the Renova Formation below the unconformity. Faulting at about 17 Ma evidentally shunted the flow from the Bitterroot Valley into a NE-trending graben system in SW Montana. The central Nevada lithologies occur Middle and Upper Miocene fluvial gravel deposits as far north as Great Falls, where they cap terraces along the Missouri River that are concordant with the High Plains surface. The fluvial system that transported the central Nevada lithologies into Montana appears to have been cut off by faulting along the Snake River Plain at about 5 Ma, in response to propagation of the Yellowstone hot spot track.