DETRITAL ZIRCON PROVENANCE OF LOWER CRETACEOUS CONGLOMERATE BEDS DEPOSITED IN THE WESTERN INTERIOR FORELAND BASIN, MONTANA, USA, AND ALBERTA, CANADA

Lower Cretaceous deposits in western North America are characterized by a relatively thin, but widespread, interval of conglomerate beds. Although recognized and well-documented in numerous areas, the tectonic and basinal significance of these units remains unclear. Provenance investigations have shed light on some aspects of these deposits, however standard methods are hindered by the fact that the conglomerate beds are largely composed of chert and quartzite. We sampled Lower Cretaceous conglomerate beds over a north-south distance of more than 500 km and at multiple sites in the state of Montana and the province of Alberta in an effort to better understand the nature of these deposits. Samples were collected from the Cadomin Conglomerate in Alberta and from the Pryor Conglomerate in Montana. All samples from Alberta contain large populations of detrital zircons with ca. 1800 Ma ages and populations of detrital zircons with ages between 150 to 120 Ma. These populations are interpreted to have been derived from the erosion of Cretaceous arc-related rocks and Proterozic through Lower Paleozoic strata that were exposed in thrust sheets in the Cordilleran Orogen. Little variation exists between the samples collected throughout Alberta, although samples from southern Alberta contain more detrital zircons of Cretaceous-age relative to those from central Alberta. In contrast, the sample of the Pryor Conglomerate from Montana is characterized by a significant population of detrital zircons with ages of ca. 1000 Ma and numerous detrital zircons with ages between ca. 200 and 600 Ma. The significant differences between the populations from conglomerate beds in Alberta and Montana can be ascribed to numerous factors, although differing provenances is the most likely cause. Age populations of detrital zircons suggest maximum depositional ages of ca. 120 Ma for the majority of these units. The large and small variations between sampling locations suggest detrital zircon geochronology may be useful for defining and characterizing Lower Cretaceous conglomerate units in western North America.