ALLUVIAL FACIES OF THE EOCENE RED HILL MAP UNIT OF THE RENOVA FORMATION, SOUTHWEST MONTANA - PALEOTOPOGRAPHIC AND EXTENSIONAL TECTONIC SIGNIFICANCE

The Eocene Red Hill map unit of southwest Montana is one of the oldest known Tertiary sedimentary deposits (Early Chadronian) in the extensionally faulted Helena Salient of the Cordilleran thrust belt. Outcrops of the Red Hill are concentrated along the southern margin of the structural salient, just north of the Southwest Montana Transverse Zone, where they unconformably overlie deformed Paleozoic and Precambrian bedrock and mark initial sedimentation within extension-related intermontane basins.

The Red Hill is characterized by red-brown kaolinitic mudstones and poorly sorted breccias containing Paleozoic clasts suspended in a sandy calcareous matrix. Detailed study of the unit in the North Boulder and Three Forks basins revealed lateral and vertical associations between several facies including debris flow breccias, mud flows, alluvial sheet flood conglomerates/breccias, entrenched runoff channels, landslide blocks, fluvial sandstone channels and splays, and floodplain paleosols. Clast and clay mineral composition within the mass flow facies indicate that adjacent basin-bounding ridges were Paleozoic carbonate-dominated and had most likely undergone long-term chemical weathering. Thin section petrography indicates that all of the sandstone channels were sourced by volcanic rocks whereas some of the channels had a granitic source. Distinct petrographic differences between channel bodies as well as directionally complex paleocurrent data indicate multiple source areas and a lack of detrital mixing.

The facies assemblage indicates that the Red Hill unit was deposited in alluvial fan environments proximal to basin-margin uplifts. The coarseness and angularity of the debris flow deposits and landslide blocks indicate a rugged paleotopography was present along this structurally complex sector of Montana during the middle to late Eocene. The fluvial channels and associated splays may represent fan-surface distributary channels or may have been associated with complex paleo-trunk systems that flowed longitudinally along the axis of the North Boulder basin and interfingered with distal fan deposits. In either case, fluvial sandstone composition reflects erosion of the Late Cretaceous Elkhorn Mountains Volcanics and unroofing of the nearby Boulder Batholith.