Paleogene postcompressional intermontane basin evolution along the frontal Cordilleran fold-and-thrust belt of southwestern Montana

Abstract

The Paleogene Renova Formation is the earliest record of postcompressional sedimentation within and adjacent to the Helena Salient of the Cordilleran fold-and-thrust belt in southwestern Montana. Paleocurrent and compositional data from basin-margin facies document radiating paleodispersal away from high-relief (≥2 km) highlands coincident with modern mountainous areas. Source rocks within the paleohighlands included the same Archean metamorphic; Proterozoic, Paleozoic, and Mesozoic sedimentary; and Mesozoic plutonic and vo l canic rocks as exposed in modern uplifts. Paleocurrent and compositional data from trunk fluvial conglomerates and sandstones document the existence of an interbasinal drainage system that connected the Three Forks, western Gallatin, and Townsend Basins with headwaters farther to the west and southwest near the present-day Montana-Idaho border. Overall, the distribution of Paleogene mountainous areas and basins closely resembled modern geography, and the Paleogene drainage network was strikingly similar to the modern Missouri River headwater system. The Renova Formation records the early stages of decay of the Cordilleran orogenic belt, including the evolution of a complex intermontane basin network in southwestern Montana. High-energy Late Cretaceous to early Eocene fluvial systems carved deep, large-scale paleovalleys into the orogenic wedge along zones of structural and stratigraphic weakness. At least a 5 km thickness of overburden was removed during this time. Incision was temporally correlative with early Cenozoic regional uplift and subtropical climatic conditions. Subsequent deposition of the Renova Formation was temporally correlative with the cessation of uplift, the initiation of crustal extension, and climatic cooling. However, extension is not interpreted to have played a major role in earliest basin development.