In southwestern Montana (SWMT), Paleogene strata that are preserved in the Sage Creek basin were deposited during the transition from late-stage Sevier-Laramide compressional tectonism to extensional reactivation of the northern Rocky Mountain region. Similar to correlative deposits across the Cordillera, Sage Creek basin strata host diverse paleoclimate and paleoenvironmental proxies such as mammal fossil assemblages, paleobotanical fossil assemblages, and thick paleosol successions. As such, Sage Creek basin strata have been the focus of recent paleoclimate and paleoenvironmental studies, which have yielded results that indicate significant tectonic, climatic, and topographic evolution of the northern Cordillera during Paleogene time.
This study revisits a succession of Paleocene to Oligocene terrestrial deposits in the Sage Creek basin of SWMT that have recently been at the forefront of Cenozoic paleoclimate studies in the northern Rocky Mountains. We present detailed physical stratigraphic trends, cobble and detrital zircon provenance data, and detrital zircon–based age constraints for the Paleocene to Oligocene basin fill that reveal the evolution of depositional systems, sediment routing, and approximate rates of deposition in the basin. The new data are compared with regional trends in structural deformation and volcanism to interpret a detailed depositional and tectonic history of the SWMT region. Specifically, the Paleogene strata record (1) the final pulse of Sevier-Laramide deformation in the region; (2) widespread erosional exhumation of the Sevier fold-thrust belt and Laramide intraforeland uplifts during Paleocene and early Eocene time; (3) middle Eocene onset of localized, bimodal volcanism and volcaniclastic sedimentation associated with gravitational collapse of the Cordilleran complex; and (4) middle Eocene to Oligocene extensional reactivation of the Sevier fold-thrust belt and Laramide intraforeland province. Constraints on the timing, duration, and sedimentary response to such events provide a more complete understanding of the tectonic and topographic evolution of SWMT, which provides an essential framework for interpreting the suites of terrestrial paleoclimate and paleoenvironmental data in the northern Cordillera.