MIDDLE MIOCENE THROUGH PLIOCENE SEDIMENTATION AND TECTONICS IN MONTANA: A RECORD OF THE OUTBREAK AND PASSAGE OF THE YELLOWSTONE HOTSPOT

The Neogene Sixmile Creek Formation fills many grabens south of the Lewis and Clark Tectonic Zone in western Montana, preserving a stratigraphic record of the outbreak and passage of the Yellowstone hotspot from ~17 Ma to ~2.5 Ma. The grabens were formed on a broad dome centered on the hotspot outbreak area in southwest Idaho and southeast Oregon, producing an angular unconformity as the underlying Renova Formation and older rocks were tilted, eroded, and buried by graben fill of the Sixmile Creek Formation. Where these grabens were connected to drainages from the hotspot bulge, the Sixmile Creek Formation consists of three spatially and temporally interleaved lithostratigraphic members. From oldest to youngest, they are the Sweetwater Creek, Anderson Ranch, and Big Hole River members, and are best exposed in the Ruby and Beaverhead grabens in southwest Montana. Early in the evolution of the grabens, debris and fluvial flows surged down from horst blocks and accumulated in the grabens. They captured runoff from the hotspot dome, resulting in thick deposits of interbedded fluvially deposited silicic tephra and far-traveled river gravel. As the hotspot track propagated northeastward along the eastern Snake River Plain, the northeasterly gradient increased, resulting in more gravel input and increasingly larger clast sizes. Around 4 Ma, the middle Miocene grabens were crosscut by northwest-trending Pliocene grabens as crustal stresses changed with the northeasterly passage of volcanic centers along the hotspot track. Some drainages were diverted into the new grabens, creating the present drainage system in southwest Montana.