THE RENOVA FORMATION AS A RECORD OF PALEOLANDSCAPE, PALEODRAINAGE, AND EARLY TERTIARY BASIN DEVELOPMENT UPON THE CORDILLERAN OROGENIC WEDGE, SW MONTANA

The Paleogene Renova Formation (~42-19 Ma) is the earliest post-Laramide record of sedimentation within the extensionally reactivated Helena thrust-belt salient of southwestern Montana. Although the Renova Formation consists primarily of fine-grained deposits, data derived from coarse facies in the Divide, northern Beaverhead, Jefferson, North Boulder, Harrison, and Three Forks basins delineate paleogeography and paleodispersal, helping to clarify basin origin. Paleocurrent and compositional data from alluvial fan facies document radiating dispersal away from high-relief paleo-uplifts coincident with modern uplifts. Source rocks within the paleo-uplifts include the same Archean metamorphic rocks; Proterozoic, Paleozoic and Mesozoic sedimentary rocks; and Mesozoic granitic plutons as exposed in the modern uplifts. In contrast, paleocurrent data from highly polymictic conglomerate and granitic sand-rich fluvial bodies located beyond the coarse alluvial facies document through-going trunk fluvial systems during the Eocene with a drainage pattern very similar to that of the modern Missouri-headwater system. Primary crystalline source rocks for the trunk fluvial network included the Idaho-Anaconda batholith complex and the Boulder and Tobacco Root batholiths.

Paleotopography was markedly three-dimensional during early Paleogene time with relatively high-relief uplifts and intermontane basins resembling modern geography. High-energy, transport-efficient, through-going trunk systems were established prior to Renova deposition as evidenced by paleovalley development, the caliber and composition of earliest fluvial deposits, and mass loss from above unroofed plutons and Archean/Proterozoic basement blocks to depocenters beyond the drainage complex. The paleobasins and drainage systems were both structurally and topographically restricted. They parallel the structural and lithologic fabric of the Cordilleran orogenic wedge, indicating strong infrastructural control on paleovalley erosion and later post-Laramide extension (Constenius, 1996). Deposition of the Renova Formation signaled a switch from fluvial incision to aggradation and basin (paleovalley) back filling with high-energy systems followed by lower energy fluvial, alluvial, and lacustrine systems.