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

摘要

The Paleogene Renova Formation is thenearliest record of postcompressional sedimentationnwithin and adjacent to the HelenanSalient of the Cordilleran fold-and-thrustnbelt in southwestern Montana. Paleocurrentnand compositional data from basin-marginnfacies document radiating paleodispersalnaway from high-relief ( 2 km) highlandsncoincident with modern mountainous areas.nSource rocks within the paleohighlandsnincluded the same Archean metamorphic;nProterozoic, Paleozoic, and Mesozoic sedimentary;nand Mesozoic plutonic and volcanicnrocks as exposed in modern uplifts. Paleocurrentnand compositional data from trunknfl uvial conglomerates and sandstones documentnthe existence of an interbasinal drainagensystem that connected the Three Forks,nwestern Gallatin, and Townsend Basins withnheadwaters farther to the west and southwestnnear the present-day Montana-Idaho border.nOverall, the distribution of Paleogene mountainousnareas and basins closely resemblednmodern geography, and the Paleogene drainagennetwork was strikingly similar to thenmodern Missouri River headwater system.nThe Renova Formation records the earlynstages of decay of the Cordilleran orogenicnbelt, including the evolution of a complexnintermontane basin network in southwesternnMontana. High-energy Late Cretaceousnto early Eocene fl uvial systems carved deep,nlarge-scale paleovalleys into the orogenicnwedge along zones of structural and stratigraphicnweakness. At least a 5 km thicknessnof overburden was removed duringnthis time. Incision was temporally correlativenwith early Cenozoic regional uplift andnsubtropical climatic conditions. Subsequentndeposition of the Renova Formation was temporally correlative with the cessation ofnuplift, the initiation of crustal extension, andnclimatic cooling. However, extension is not interpretednto have played a major role in earliestnbasin development.