PALEOSOLS IN THE LOWER YELLOW CAT MEMBER OF THE CEDAR MOUNTAIN FORMATION IN EASTERN UTAH INDICATE THAT THE EARLIEST CRETACEOUS (BERRIASIAN) IN THE COLORADO PLATEAU REGION WAS EXCEPTIONALLY WET

The Jurassic-Cretaceous (J-K) unconformity spans 20–25 million years (my) along most of the Western Cordillera. A revised Berriasian age (145–142 Ma) for the base of the lower Yellow Cat Mbr. (YC) of the Cedar Mountain Fm. (CM) in eastern Utah is indicated by microfossils, chemostratigraphy, and U-Pb dating of paleosols, reducing the J-K unconformity to 5–7 my with local deposition beginning much earlier near the base of Cretaceous. Morrison “style” dinosaur faunas dominated by diverse sauropods (mostly diplodocids), stegosaurs, small to medium ornithopods, diverse large theropods (ceratosaurs, megalosaurids, and allosaurids), and uncommon coelurosaurs persisted for the last 10 my of the Jurassic. In contrast, the first 25 my of the Early Cretaceous had Wealden “style” dinosaur faunas dominated by a low diversity of turiasaur and titanosauriform sauropods, polacanthid ankylosaurs, large ornithopods, low diversity large megalosauroid spinosaurids (exclusive to Europe) and allosauroid carcharodontosaurid theropods, and an increased abundance of larger coelurosaurs documenting the J-K dinosaur extinction. The Berriasian paleosol sequence at the base of the YC is laterally equivalent to the Buckhorn Conglomerate Mbr. of CM and consists of basal ferruginous paleosols (gleysols) overlain by silicified peat (histosols), which are then overlain by calcareous paleosols (vertisols and aridosols). A kaolinite bed (oxisol) has been identified at the J-K unconformity locally and research on clay chemistry is underway. Thus, the earliest Cretaceous in Utah is characterized by a short interval of wet paleoclimates following those of the drier Late Jurassic and before the drier remainder of the Early Cretaceous. Potential drivers close to the J-K boundary for this climatic excursion and faunal turnover include a mega-shield volcano(s) at Shatsky Rise and associated flood basalts, and South Africa’s Morokweng impact. A massive impact into Precambrian continental crust would have different environmental effects than an impact into a coastal barrier reef system such as the Chicxulub impact. Quiescence between the Elko and Sevier orogenies might also explain a short-term climatic shift at the J-K faunal boundary.