MICROFAUNAL SUCCESSION AND STRATIGRAPHY OF THE RENOVA FORMATION AT MCCARTY'S MOUNTAIN AND MANTLE RANCH, SOUTHWEST MONTANA

The biostratigraphy of the Eocene and Oligocene Renova Formation (Duchesnean and Chadronian, Bozeman Group) at McCarty’s Mountain and Mantle Ranch, Beaverhead Basin, southwestern Montana was surveyed to determine the structure and changes of paleofaunal communities and the timing and nature of sediment deposition. I focused on the microfaunal fossils excavated from anthill matrix and surface collected at McCarty’s Mountain and Mantle Ranch, which have remained virtually unstudied in the one hundred years that paleontologists have known of these fossil localities. The lack of study on the microfauna of this region has resulted in a superficial understanding of the ecosystems.

The Renova Fm. is deformed by gentle folds and faults and comprises thicknesses of 200 meters at McCarty’s Mountain, 120 meters at Mantle Ranch (main), and 70 meters at Mantle Ranch east. The formation is composed of channel-fill conglomerates, plagioclase arkoses, and complexly interbedded flood plain silty, tuffaceous sandstones and limestone paleosols. The Renova Fm. is locally capped by an angular unconformity with the Sixmile Creek Formation and onlaps Cretaceous granites and sandstones. The Renova Fm. was deposited by a high sinuosity stream system that flowed from southwest to northeast across the three localities in the late Eocene and early Oligocene Epochs. Deposition by a high sinuosity stream is indicated by the proportions of paleosols and floodplain sediments to channel-fill conglomerates and current direction is signified by sediment provenance.

The excavated anthills examined contain both rock material and tiny fossil teeth that are pulled to the surface by local ant populations. Individual anthills can be correlated with the successive beds of the formation, due to the average 25º dip, and can be used as markers for changes in dental morphology and community make-up. Correlations can be made for similar fauna types and for the Duchesnean and Chadronian fossil ages throughout North America.