Skip to Next Section This paper presents breakthroughs in the chronostratigraphy of the heretofore poorly-constrained Yellow Cat Member of the Cedar Mountain Formation, which is an important record of terrestrial environments, ecosystems, and global change in the ancient North American Cordilleran foreland. Zircon populations from ten stratigraphic horizons in the Yellow Cat Member yield youngest single grain ages ranging from 142.5±2.7 Ma to 133.7±2.7 Ma (Berriasian–late Valanginian); those from one mudstone palaeosol yield a robust Concordia Age of 136.3±1.3 (Valanginian). Additionally, a new palynoflora-one of a few to be published from the Cedar Mountain Formation-is assigned to the middle Berriasian to early Hauterivian stages, based on the presence of Foraminisporis wonthaggiensis and Trilobosporites sp. cf. T. canadensis, and the absence of F. asymmetricus, Appendicisporites spp. and angiosperms. Furthermore, these chronostratigraphic data allow us to interpret part of the so-called "Weissert Event" C-isotope excursion (Valanginian) in a new C-isotope profile through a palaeosol-bearing alluvial succession in the Yellow Cat Member. This research extends a firm understanding of the formation further back into the Early Cretaceous than was the case previously and sets the stage for future advancements. Supplementary material at https://doi.org/10.6084/m9.figshare.c.4697138
