SEDIMENTOLOGIC EVIDENCE FOR EARLY PALEOGENE BASIN DEVELOPMENT ATOP THE CORDILLERAN OROGENIC WEDGE IN THE VICINITY OF THE BOULDER BATHOLITH, SOUTHWEST MONTANA

The Renova Formation (~42-19 Ma) is the earliest post-Laramide record of Tertiary sedimentation within intermontane basins surrounding the Highland Range-Boulder batholith complex. Data from coarse facies in the North Boulder, Jefferson, northern Beaverhead, and Divide basins are used to identify paleogeography, paleodispersal, and basin origin. In combination with paleocurrent data, proximal to medial alluvial fan facies including landslide blocks, talus, debris flow, sheet-wash, and fan channel deposits document radiating dispersal away from high-relief source areas coincident with the modern McCarthy Mountain, Highland Range-Boulder batholith, Bull Mountain, and Doherty-Devil's Fence uplifts. Clast composition also marks derivation from adjacent Precambrian and Phanerozoic rocks within the uplifts, including abundant granitic debris from the Boulder batholith. In contrast, paleocurrent data from finer-grained fluvial sandstone bodies located beyond the coarse alluvial facies and near the axial zones of modern intermontane basins (Divide, Jefferson, North Boulder) indicate the existence of Paleogene trunk fluvial systems with drainage patterns similar to those of modern systems. Evidence for syndepositional faulting was not observed and the estimated minimum for maximum paleorelief is 600 m.

Early Tertiary paleotopography in the vicinity of the Boulder batholith was markedly three-dimensional, with paleobasins and adjacent paleouplifts closely mimicking modern topography. The Boulder batholith and associated Archean/Proterozoic basement blocks, as well as other nearby Late Cretaceous plutons, were substantially unroofed. Overall, the Paleogene basins parallel the structural and lithologic fabric of the Laramide orogenic wedge, suggesting strong infrastructural control on pre-mid-Eocene paleovalley erosion rather than an extensional tectonic origin for the basins.