U-Pb zircon ages of volcanic ashes integrated with ammonite biostratigraphy, Fernie Formation (Jurassic), Western Canada, with implications for Cordilleran-Foreland basin connections and comments on the Jurassic time scale

Abstract The Jurassic system of the Western Canada Sedimentary Basin records the transition in its tectonic setting from a “passive” back-arc platformal basin to a foreland basin at the western margin of ancient North America. We report new U-Pb zircon ages from bentonite layers and from probable volcanic ash components of clastic detritus in other strata of the Fernie Formation, which encompasses most of the Jurassic in the western portions of the basin and which is now deformed in the Rocky Mountain fold-and-thrust belt. The bentonite ages come from the lower Nordegg Member (Pliensbachian) and an equivalent ash layer in the Lower Fernie phosphatic shale. Detrital zircon spectra from the Bathonian Gryphaea Bed silty limestone and the zircon ages from the mainly Oxfordian Green Beds glauconitic sandstone also are likely indicative of contemporaneous ash-falls. In addition, we review previously published U-Pb bentonite ages from the Fernie Formation and comment on the Jurassic time scale as represented on the International Chronostratigraphic Chart. We have compiled an updated local stratigraphic correlation chart against a time scale that incorporates ages for some of the Middle and Upper Jurassic stage boundaries, from the literature, that differ from those on the current standard charts. The presence of multiple volcanic ashes throughout the Jurassic system in the Western Canada Sedimentary Basin supports tectonostratigraphic models with relatively nearby western magmatic activity. The southeastern Omineca crystalline belt and Quesnellia terrane contain magmatic rocks with ages that could account for all of the Fernie ashes, and are closest to the depositional basin, but source terranes farther afield cannot be ruled out.