THE RENOVA FORMATION: A RECORD OF PALEOLANDSCAPE, PALEODRAINAGE, AND POST-COMPRESSIONAL BASIN DEVELOPMENT ATOP THE CORDILLERAN OROGENIC WEDGE, SOUTHWESTERN MONTANA
Paleocurrent and compositional data from basin-margin alluvial fan and hillslope facies document radiating paleodispersal away from high-relief Paleogene highlands coincident with modern uplifts. Source rocks within the Paleogene highlands include the same Archean metamorphic; Proterozoic, Paleozoic, and Mesozoic sedimentary; and Mesozoic plutonic and volcanic rocks as exposed in modern uplifts. In contrast, paleocurrent and compositional data from highly polymictic fluvial conglomerates and quartzofeldspathic fluvial sandstones document the existence of a through-going, interbasinal drainage system in the Divide, North Beaverhead, Jefferson, and Three Forks Basins that likely had distal sources in the Cordilleran hinterland. Paleocurrent and compositional data from oligomictic conglomerates and quartzofeldspathic sandstones in the North Boulder and Harrison Basins document that the fluvial systems that occupied these basins were tributary to the ancestral Jefferson-Three Forks fluvial tract. Overall, the Paleogene drainage network closely mimicked that of the modern Missouri River headwater system.
The Renova Formation records the early stages of decay of the Cordilleran orogenic wedge in southwestern Montana. Paleotopography was markedly three-dimensional during Paleogene time, with relatively high-relief mountainous highlands and intermontane basins closely resembling modern geography. Initial deposition of the Renova Formation signaled a switch from fluvial incision and paleovalley development to fluvial aggradation and basin back-filling. Lateral facies distributions document the locations, shapes, and sizes of Paleogene basins. Parallelism to the structural and lithologic fabric of the orogenic wedge indicates a strong infrastructural control on basin development.