BAYESIAN AGE-DEPTH MODELING IMPROVES THE CHRONOSTRATIGRAPHIC FRAMEWORK OF THE MID-CENOZOIC JOHN DAY FORMATION, JOHN DAY FOSSIL BEDS NATIONAL MONUMENT, EASTERN AND CENTRAL OREGON

Bayesian age-depth models merge geochronologic and stratigraphic data to constrain the timing and tempo of paleobiological, paleoclimate, and biogeochemical proxy records. We apply a deep time Bayesian age-depth model to a critical archive of Late-Eocene to Early-Miocene faunal and floral evolution and paleoclimate change, the John Day Formation of the John Day Fossil Beds National Monument, eastern and central Oregon. Our model integrates 12 new high-precision U-Pb CA-IDTIMS zircon dates for ash beds within the John Day Formation, 10 of which are from a 400-meter section of the Oligocene Turtle Cove Member. We use this new chronology to 1) infer ~4 million years of uniform depositional processes and sediment accumulation rates with no significant hiatuses; 2) resolve the timing of all biological events and zonations in the formation; and 3) calibrate the rates of regional environmental change from closed canopy rainforests of the Eocene to open savannah grasslands of the Oligocene and early Miocene. Our Bayesian age-depth model establishes a new high-fidelity continuous chronostratigraphic framework for the world-class mammalian faunal record of the John Day Formation, updating legacy 40Ar/39Ar dates and magnetostratigraphy, and reinforcing its status as a key reference section for the Arikareean North American Land Mammal Age assemblage.