LAUSR

The Lower Cretaceous Kootenai Formation in western Montana records the onset of foredeep sedimentation in the Idaho‐Montana retroforeland basin of the North American Cordillera. Contrasting models for sediment dispersal during Early Cretaceous time have been proposed. We use sandstone petrography, large‐n (n = 600) detrital zircon U‐Pb geochronology, and mixture modeling to determine the provenance of the Kootenai Formation. These new data combined with additional data from the literature suggest three discrete provenance signatures. Type I sandstones are quartz‐rich with U‐Pb age peaks at ca. 158, 238, 410, 600, and 1,035 Ma; they were sourced from exhumed Jurassic continental strata and transported northward by axial drainages. Type II sandstones are lithic‐rich with U‐Pb age peaks at ca. 154, 166, 1,840, 1,920, 2,080, and 2,700 Ma; they were deposited by transverse rivers that drained exhumed lower‐middle Paleozoic strata from the Sevier belt. Type III sandstones contain feldspars and minor volcanic lithics with U‐Pb age spectra that are dominated by ca. 112 and 162 Ma ages; they were transported by transverse drainages that connected the hinterland of the orogen with the western Montana foreland. Based on provenance analysis of the Kootenai Formation, subtle Early Cretaceous thick‐skinned deformation in southwestern Montana did not exhume basement nor did it partition the foreland into locally sourced depocenters. Sediment sources for the western Montana foreland are consistent with those to the north in Alberta and different than southern Idaho. These data also provide insight into the translational history of terranes in the hinterland of the orogen during this time.