Abstract

Western Montana and eastern Idaho intermontane basins received Cenozoic continental sediments and volcanics exceeding 4880 m (16,000 ft) thick, which unconformably overlie pre-basin rocks. Pre-late Eocene basin delineation resulted from late Laramide intra-arc extension. Developing basins were ‘imperfect’ and often interconnected. Rapid subsidence resulted in internal drainage and development of intermittent lakes. Progressively greater aridity is seen during the Tertiary. Mapping reveals four significant stratigraphic subdivisions separated by major unconformities: Pre-Renova fanglomerates and local volcanics; Renova fine-grained, ash-rich fluvial and lacustrine strata; Sixmile Creek ash-rich coarse clastics and siltstones; and Quaternary tills, outwash, lake silts, loess, and alluvial gravels.

Eocene igneous activity produced intrusions and extensive andesitic volcanics. Local volcanism subsided by late Eocene, but still-active centers shifted from andesites to basalts and rhyolites. Despite loss of local vents, abundant ash accumulated through Miocene, the Cascade volcanic belt is the apparent source. Late early Miocene tectonics tilted older Tertiary strata into range-front listric faults and modern appearing basins were produced. A change to a wet climate in the late Early Miocene resulted in erosion of a considerable volume of older Tertiary strata from the basins.

Fossils occur throughout the Cenozoic section. Mammalian local faunas are important in correlations and demonstrate provinciality due to geographic isolation as well as migration to and from the Pacific Region and the Great Plains.

Petroleum potential exists in the deeper basins where organic-rich Renova strata have generated liquid and gaseous hydrocarbons.