PALEOGENE POST-COMPRESSIONAL INTERMONTANE BASIN EVOLUTION ALONG THE FRONTAL CORDILLERAN FOLD-AND-THRUST BELT OF SOUTHWESTERN MONTANA

The Paleogene Renova Formation (~42-21 Ma) is the earliest record of sedimentation following compressional tectonism in the Sevier-Laramide overlap zone of southwestern Montana (SWMT). The variety and distribution of depositional facies that comprise the Renova Formation indicate that SWMT was topographically complex during Paleogene time. Paleocurrent and compositional data from basin-margin (alluvial fan) facies document radiating paleodispersal away from high-relief uplands into small intermontane basins. Paleotopographic highs were coincident with modern mountainous areas. Contrary to previous thought, the basins were not characterized by internally-drained lacustrine systems. Paleocurrent and compositional data from basin-interior facies reveal fluvial interconnectivity of nearly all of the small basins in SWMT, resulting in an integrated drainage network that ultimately flowed eastward into the foreland region. Overall, the distribution of Paleogene basins and intervening highlands closely resembled modern geography, and the Paleogene drainage network was strikingly similar to the modern Missouri River headwater system.

The Renova Formation provides a unique opportunity to investigate the evolution of the northern Rocky Mountain Cordillera following the cessation of Sevier and Laramide deformation. Late Cretaceous to Paleocene time was characterized by regional, kilometer-scale surface uplift and subtropical climatic conditions. This prompted deep fluvial incision into the orogenic wedge along zones of structural and stratigraphic weakness, removing at least 5 km of lithologic overburden and generating local relief ≥ 2 km. Although early Eocene extensional reactivation has been demonstrated in eastern Idaho and southwesternmost Montana, we contend that deep fluvial incision was of primary importance in generating topography and accommodation in much of the Sevier-Laramide overlap zone. The basin network established in the early Paleogene was then back-filled during Eocene to early Miocene time, coeval with climatic cooling and aridification.