U-Pb zircon geochronology and chronostratigraphy of the Eocene–Miocene John Day Formation of central and eastern Oregon

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Abstract The John Day Formation of central and eastern Oregon, USA, contains a rich record of late Eocene to early Miocene faunal and floral evolution, climate and environmental change, and landscape evolution, and accordingly, it has been studied for over a century to better understand ancient terrestrial ecosystems of North America. Further progress in leveraging the John Day Formation rock archive to study these systems requires an updated chronostratigraphic framework with a foundation of modern high-precision radioisotope geochronology. We present a comprehensive dataset of U-Pb zircon dates measured by chemical abrasion–isotope dilution–thermal ionization mass spectrometry that yielded 23 new highly precise eruption and depositional ages for volcanic beds in the John Day Formation stratigraphy, in the environs of John Day Fossil Beds National Monument. The new ages significantly refine the overall timing of the John Day Formation and reveal new spatiotemporal correlations among strata of the Sheep Rock, Painted Hills, and Clarno Units. We integrated our dataset with composite stratigraphic information through Bayesian age modeling to establish probabilistic posterior age constraints for all formal and informal lithostratigraphic divisions of the eastern facies of the John Day Formation and durations of previously identified long-lived hiatuses. The results robustly link the John Day Formation stratigraphy to the geologic and paleomagnetic time scales and global proxy records, offering new opportunities for investigating terrestrial records of the Eocene–Miocene Earth system in the Pacific Northwest and testing diachroneity of faunal assemblages that define North American Land Mammal Ages.

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