PROVENANCE OF THE LOWER CRETACEOUS KOOTENAI FORMATION IN SOUTHWESTERN MONTANA: IMPLICATIONS FOR FORELAND BASIN SEDIMENTATION AND REGIONAL TECTONIC MODELS

The Lower Cretaceous Kootenai Formation in western Montana records the onset of foredeep sedimentation in the Idaho-Montana retroarc region of the North American Cordillera. Contrasting models for sedimentation during the early stages of basin development have been proposed. We use sandstone petrography, high-n detrital zircon U-Pb geochronology, and mixture modeling techniques to determine the provenance of the Kootenai Formation and test previous models. Our new data, combined with additional data from the literature, suggests three type provenance signatures regionally. Type I sandstones are lithic-rich and contain prominent detrital zircon age peaks at ca. 154, 166, 1840, 1920, 2080 and 2700 Ma; these sandstones were deposited by transverse rivers that drained exhumed Lower Paleozoic strata from the Sevier fold-thrust belt. Type II sandstones are quartz-rich and have prominent age peaks at ca. 158, 238, 410, 600 and 1035 Ma; these sandstones are reflective of regional recycling of underlying Jurassic continental strata mainly in an axial drainage, however, they also record minor local recycling related to early activity on blind thrusts and subtle basement uplift in the distal foreland. Type III sandstones contain feldspars and volcanic lithics with detrital zircon spectra that are dominated by ca. 112 and 162 Ma ages. These sandstones were transported to the basin by transverse drainages that connected Mesozoic plutons from the Omineca Belt with the western Montana foreland. Provenance analysis suggests that the foreland was not partitioned into subbasins with distinct intrabasinal sources; only minor localized recycling of Jurassic continental strata in the distal foreland occurred during this time. These data require partial unroofing of east-central Idaho thrust sheets prior to deposition of the Kootenai Formation. Lastly, provenance of the Type III sandstone suggests that portions of the Intermontane superterrane may have been west of Idaho around 100 Ma.