Provenance of the Lower Cretaceous Kootenai Formation in southwestern Montana: Implications for regional thrusting, foreland basin development and paleogeographic reconstructions

Abstract:

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 sediment dispersal patterns 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 for the region. Our new data, combined with additional data from the literature, suggests three type provenance signatures regionally. Type I sandstones are quartz-rich and have prominent age peaks at ca. 158, 238, 410, 600 and 1035 Ma; these sandstones are reflective of recycling of underlying Jurassic continental strata regionally 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 II 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 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 reveals that much of southwestern Montana was blanketed by Type II sandstone, suggesting that the foreland was not partitioned into subbasins with distinct intrabasinal sources. Rather, we find that only minor localized recycling of Jurassic continental strata in the distal foreland occurred during this time. Furthermore, the Lower Paleozoic thrust-belt derived source for the Type II sandstone requires partial unroofing of the thrust sheets prior to deposition of the Kootenai Formation. Lastly, consistent with tectonic models of previous workers, provenance of the Type III sandstone suggests that portions of the Intermontane superterrane may have been west of Idaho around 100 Ma.