VERTEBRATE PALEONTOLOGY AND GEOLOGY OF HIGH ELEVATION TERTIARY DEPOSITS IN THE GRAVELLY RANGE, SOUTHWESTERN MONTANA

Tertiary strata exposed in four high elevation areas in the south-central Gravelly Range yield significant assemblages of Late Eocene to Oligocene mammals. The thickest stratigraphic sections of Tertiary strata are in the Lion Mountain-Black Butte area. The Lion Mountain section age is based primarily on American Museum of Natural History collections; the lower part of this section is Duchesnean-Chadronian (39-33 Ma) and the uppermost beds are Whitneyan (32-31 Ma). Age of the basal part of the Black Butte section is Duchesnean-Chadronian based on Museum of Comparative Zoology collections. Recent collections that include Miohippus indicate a probable Orellan age for uppermost exposures. The Teepee Mountain section is notable for abundant brontothere remains and is probably Duchesnean-Chadronian (approx. 39-33 Ma). The Rapamys site is the oldest vertebrate locality and is late Uintan to early Duchesnean (42-38 Ma) based on recently recovered specimens of Rapamys, Protoreodon, and Lycophocyon.

The Tertiary strata in this part of the Gravelly Range include fluvial, aeolian, and tufa deposits that are most likely mainly associated with localized Oligocene volcanism. The Lion Mountain section is about 270 meters in thickness; the lower half of the section is largely aeolian, with fluvial units comprising much of the upper section. Based upon age data, the 140 meter Black Butte section correlates to the lower 50-70 meters of the Lion Mountain section. The basal 20 meters of the Black Butte section contain some fluvial features, but much of the remaining section is largely aeolian in origin. Paleosols and extensive burrowing also occur within the Black Butte section. Stratigraphic section thickness decreases rapidly away from the Black Butte-Lion Mountain area, with section thicknesses of about 20 meters for the largely aeolian Rapamys and Tepee Mountain sections. Tufa deposits are located along the west-central edge of the Gravelly Range where they are associated with previously mapped thrust faults. Leaf imprint assemblages of Eocene-Miocene age are contained within these tufas. Strata previously mapped as Upper Cretaceous-Paleocene Beaverhead Formation are now variously reassigned to the lower Cretaceous Kootenai Formation, southwestern Montana Cenozoic Sequence 2, and diverse Quaternary units.