PALEOGEOGRAPHIC AND TECTONIC IMPLICATIONS OF THE PALEOGENE PALEO-MISSOURI HEADWATER SYSTEM IN SOUTHWEST MONTANA

Recent work on the Renova Fm in SW Montana has documented a Paleogene (>50 Ma) intermontane basin system and drainage network similar in configuration to that of the modern system in a region that includes the Divide, Beaverhead, Jefferson, Gallatin, and Radersburg-Townsend intermontane basins. That region constitutes much of the modern and Paleogene, northward-flowing, Missouri River headwater drainage. In this study, facies, paleocurrent, compositional, and detrital zircon-age analyses have been expanded in order to evaluate the headwardmost paleodrainage system with regard to provenance, paleogeography, and tectonism. Although Paleogene erosion and deposition overlapped terminal Sevier-Laramide contraction and incipient Tertiary extension, this southern study area has been overprinted by Basin-and-Range (≤16 Ma) and Yellowstone hotspot trackway (≤6 Ma) uplift and extension.

A large-scale, cobble to boulder, Eocene fluvial conglomerate system in the Beaverhead basin demonstrates northward paleoflow and serves as evidence the major inlux of Proterozoic clasts from tributaries in the south and southwest. Other detrital components and zircon provenance data document minor to moderate sediment supply from exposures of Cretaceous plutons in the Pioneer Mountains, Archean basement, and early Paleogene volcanics. Fluvial conglomerates within the Frying Pan, Grasshopper, Medicine Lodge, and Horse Prairie basins substantiate primary sources of Proterozoic strata in the SWMT-central Idaho region as well as more proximal recycling from thrust sheets of Late Cretaceous/Paleocene Beaverhead Fm. This, and scattered exposures of basin-margin alluvial facies, indicate a mountainous regional topography with drainage into the paleo-Beaverhead basin most likely through a paleovalley in the Beaverhead Canyon region as well as southeastward from the southern end of the Pioneer Mountains. Fluvial paleoflow data for the headwater basins indicate axial orientations similar to those of the modern basins. Overall, Late Cretaceous to early Eocene fluvial systems are interpreted to have carved intermontane-scale paleovalleys with reliefs on the 100-1000 m scale into the Sevier orogenic wedge along zones of structural and stratigraphic weakness to form the paleo-Missouri River headwater system.