Magnetostratigraphy and U-Pb geochronology of the middle Eocene Bridger Formation (Wyoming, USA): Implications for the age and correlation of the Bridgerian/Uintan NALMA boundary and calibration of the Geomagnetic Polarity Time Scale

Abstract The rich fossil record of the central Rocky Mountains has been fundamental to constructing the terrestrial biostratigraphy and biochronology that forms the basis of the North American Land Mammal “Ages” (NALMAs). New integrated magnetostratigraphy and high-precision U-Pb geochronology (chemical abrasion–isotope dilution–thermal ionization mass spectrometry [CA-ID-TIMS] method) from the middle Eocene Bridger Formation of the Bridger Basin in southwestern Wyoming, USA, provide a robust chronostratigraphic framework for the Bridgerian to earliest Uintan NALMAs. Paleomagnetic analysis of samples from 90 sites in an ~560 m composite section spanning from Bridger B through Bridger E members indicates two paleomagnetic reversals, and the section is correlated to Chrons C22n, C21r, and C21n by comparison to the Geomagnetic Polarity Time Scale (GPTS). This correlation places the Bridgerian/Uintan boundary within Chron C21n during the Early Eocene Climatic Optimum and subsequent initial cooling. Based on this bio- and magnetostratigraphic correlation, we also correlate other key Bridgerian/Uintan boundary bearing sections from the Washakie Basin and East Fork Basin to the GPTS and determine that the boundary occurs everywhere in Chron C21n in western North America. Furthermore, the Bridgerian NALMA is now estimated to have lasted for ~5 million years. Integrating this magnetostratigraphy with existing and new U-Pb zircon geochronology from the interbedded Church Butte tuff, Henrys Fork tuff, and Sage Creek Mountain tuff in the Bridger Formation permits assessment of existing age calibration models for the early-middle Eocene interval of the GPTS. Evaluation of the calibration models identifies two previously published models that best reconcile the geochronological data from the Greater Green River Basin, improving calibration accuracy of the Geomagnetic Polarity Time Scale in the early to middle Eocene.