LATE CRETACEOUS-CENOZOIC SEDIMENT DISPERSAL PATTERNS IN SOUTHWEST MONTANA

Southwest Montana preserves a rich sedimentary record of the tectonic history of the North America Cordillera. The purpose of this study is to reconstruct the paleogeography of southwest Montana during and following the transition from Sevier-Laramide compression to Paleogene-Neogene extension. We apply U-Pb detrital zircon (DZ) provenance analysis to sandstones from Late Cretaceous to Cenozoic strata deposited in foreland and extensional basins located between the Madison Valley and Crazy Mountains Basin. We collected 10 samples from the Late Cretaceous Livingston Formation and Sphinx Conglomerate, Late Cretaceous-early Cenozoic Hoppers and Fort Union Formations, and the Cenozoic Bozeman Group. Samples from the Livingston Formation and Sphinx Conglomerate in the Madison Range contain the same pre-Mesozoic age populations, but differ in their <250 Ma populations. The Sphinx Conglomerate has two <250 Ma age-probability peaks at 160 Ma and 90 Ma and lacks any grains <86 Ma, the latter of which are found in the underlying Livingston Formation. This result is surprising given that the Sphinx is constrained to be deposited 75-58 Ma, a period of abundant zircon generation in nearby volcanic centers. DZ spectra from Cenozoic formations show age populations with peaks of 12 Ma, 34 Ma, 53 Ma, and 76 Ma, as well as Proterozoic ages (1.3-2.4 Ga) and Archean ages (2.5-2.9 Ga). We interpret that zircons in our samples were derived from source regions to the west, namely the Pioneer and Boulder batholiths and Dillion Volcanics. Interestingly, sandstones from fluvial strata of the Late Cretaceous-early Cenozoic Hoppers and Fort Union Formations in the Crazy Mountains Basin have a youngest age-probability peak of ~75 Ma, which likely reflects provenance west of the modern Bozeman Pass drainage divide. This implies that the drainage divide developed after much of the strata that comprises the Crazy Mountains Basin was deposited, possibly during Paleogene time. Further investigation and integration of data from the Bridger Range fault system will seek to constrain the timing of uplift and drainage reorganization in the Bozeman-Livingston region during Basin and Range extension.