OLIGOCENE-MIOCENE MEDICINE LODGE BEDS, NORTHEASTERN SNAKE RIVER PLAIN: : CORRELATIVE TO THE RENOVA AND SIXMILE CREEK FORMATIONS, MONTANA

Oligocene-Miocene Medicine Lodge Beds, northeastern Snake River Plain, Idaho: Correlative to the Renova and Sixmile Creek Formations, Montana

The Medicine Lodge beds are a gently tilted sequence of tuffaceous sandstones, ash beds, paleosols, lacustrine limestones, and conglomerates found in the Medicine Lodge Creek valley, in Clark County, Idaho, and in sparse locations to the southwest as far as the southern Lemhi Range. The Medicine Lodge beds rest on Paleozoic bedrock, and possibly also on Cretaceous-Paleocene Beaverhead Conglomerate. Late Miocene ash-flow tuffs (the 6.57 Ma Edie School, 6.19 Ma Blue Creek and 4.19 Ma Heise tuffs) and lacustrine limestones discontinuously armor the top of the section, affording exposures for some of the landslide-prone Medicine Lodge beds. The Medicine Lodge beds in the Medicine Lodge Creek area are up to 250 meters thick, and are intermittently exposed along the streambed and near Black Mountain, Deep Creek, Warm Springs Creek, Fritz Creek and Bannack Pass. Indian Creek Basalt, 5.23 Ma + 0.10 Ma, filled Medicine Lodge Creek canyon, and has since been dissected.

The lithologies of the Medicine Lodge beds bear a marked resemblance to the Renova and overlying Sixmile Creek formations of the Bozeman Group in Montana. Samples of the Medicine Lodge beds taken near the top of the section east of Black Mountain indicate ages of 33.2 + 1.1 Ma and 29.7 + 1.7 Ma. These ages are similar to those of the Renova Formation, which has been dated as Arikareean by fossil assemblages, fission-track and K/Ar methods. Lacustrine limestone is found conformably beneath the Edie School Tuff (6.57 Ma), which indicates a late Miocene age, similar to the Sixmile Creek Formation’s Timber Hill Basalt (6.0 + 0.1 Ma).

The rocks of the Renova and Sixmile Creek Formation are separated by an angular unconformity in Montana. No such unconformity has yet been found in the Medicine Lodge beds, which in general are tilted gently to the east or northeast in most of the Medicine Lodge, Idaho, area.