A Late Jurassic to Early Cretaceous record of orogenic wedge evolution in the Western Interior basin, USA and Canada

[摘要] The Late Jurassic to Early Cretaceous fill of the Western Interior foreland basin is characterized using geochronological data in order to assess the stratigraphic expression of wedge-top geomorphology, as controlled by sediment cover and denudation. In northern Montana, USA, and Alberta, Canada, wedge-top deposits are poorly preserved; however, their former presence may be inferred from the detrital record in the foreland basin. We present new U/Pb detrital zircon data from nine samples collected near Great Falls, Montana, augmented with field data. The stratigraphy at Great Falls is characterized by Late Jurassic marine and nonmarine deposits, which are truncated by a basin-wide sub-Cretaceous unconformity. Aptian and lower Albian strata overlying the unconformity are dominated by nonmarine deposits, which transition up-section into a predominantly marine succession related to a major transgression of the Boreal Seaway in the Albian. Detrital zircon grains from Great Falls strata yield age spectra that can be subdivided into three groups using multidimensional scaling. Group 1 is characterized by diverse zircon populations, which are interpreted to record recycling of pre-Cordilleran sedimentary strata transported via foreland basin-axial river systems with headwaters in the southwestern United States. Group 2 is characterized by the dominance of Mesozoic detrital zircon grains, which are interpreted to record sediment dispersal by fluvial systems with headwaters in the Cordillera. Group 3 is intermediate between groups 1 and 2, based on its proportion of Mesozoic zircon grains. This group records a diversification of the provenance from one dominated by Cordilleran igneous rocks to include recycled sedimentary strata. New data are integrated with three other data sets from Montana and Alberta such that stratal thicknesses (a proxy for accommodation development) and provenance evolution can be compared across the basin. The detrital record in each area, which transitions from diverse provenance to predominantly Cordilleran through the entire stratigraphic section, can be linked to the burial of the pre-foreland strata in the wedge-top depozone. This record elucidates a period of evolution of the western margin of North America to a more Andean-type system with primary input to the basin from an active magmatic arc.

[学科分类] 地质学