DETRITAL ZIRCON U-PB GEOCHRONOLOGY OF CRETACEOUS STRATA IN THE SOUTHWESTERN MONTANA FORELAND BASIN PROVIDES NEW TEMPORAL CONTROL FOR THE ONSET OF LARAMIDE DEFORMATION (Invited Presentation)

The Laramide orogeny in the western U.S. is commonly attributed to flat-slab subduction of oceanic lithosphere beneath North America. Though most models predict an inboard progression of deformation after ~80 Ma, the spatial extent of shallow subduction has been debated: some workers favor a broad expanse whereas others prefer a spatially restricted region. Both predict an inboard progression of deformation but the former would likely produce a broad, margin-parallel region of deformation and exhumation that migrated eastward, whereas the latter has been inferred to occur in a SW-NE corridor that stretches from California to South Dakota. Our new data from Lower-Upper Cretaceous strata in southwestern Montana help to constrain the evolution of the northernmost segment of the Laramide province. Detrital zircon U-Pb geochronology from fluvial sandstone and lacustrine carbonate suggest late Aptian–early Albian deposition for most of the Kootenai Formation. Sediment sources were Jurassic continental strata delivered via axial fluvial systems, lower-middle Paleozoic strata derived from the fold-thrust belt (FTB) to the west, and Jurassic hinterland plutons. The lowermost Albian Blackleaf Formation records a period of tectonic quiescence with a continuation of FTB sources to the western basin, but a shift to eastern-derived, Appalachian sources in the eastern basin. The upper Blackleaf Formation signals renewed tectonism with unroofing of both deeper, primarily Cambrian, sources and the introduction of sediment from Triassic-Upper Paleozoic strata derived from east-central Idaho. The Cenomanian to Santonian Frontier Formation and Coniacian-Maastrichtian Beaverhead Group archive continued recycling of Paleozoic-Cretaceous strata from the FTB, novel influx of sediment from the Belt Supergroup, and the first definitive evidence for unroofing of the Blacktail-Snowcrest basement uplift. Erosion of upper Paleozoic strata from this basement arch requires that it was exhuming prior to ~88 Ma in order to remove the Mesozoic overburden. Our new timing constraints for the initiation of basement-involved deformation in southwestern Montana thus require revision of the existing models for Laramide deformation or an alternative control on the transition from a thin‐ to a thick‐skinned structural style.