TESTING CORRELATIONS AND REFINING FRAMEWORKS FOR PALEOBIOLOGICAL RECONSTRUCTIONS IN THE UPPER CRETACEOUS TWO MEDICINE AND JUDITH RIVER FORMATIONS, MONTANA, USING NEW CA-TIMS U-PB GEOCHRONOLOGY

The Campanian Two Medicine (TMF) and Judith River (JRF) formations of Montana are world-renowned for their rich and historic dinosaur records. The first dinosaur fossils described from a scientific perspective in North America were collected from the type area of the JRF in 1855, and amazing insights into dinosaurian nesting behavior, embryos, and babies have been gleaned from the exposures of the TMF since the 1970s. Erosion associated with the Sweetgrass arch in northwestern Montana has complicated the correlation between the two formations. Fortuitously, the abundance of interbedded bentonites suitable for radioisotopic dating has allowed for correlation across the Two Medicine–Judith River clastic tongue, as well as with more distant formations in the American Southwest and Canada. U-Pb analyses (CA-TIMS method) of zircon recovered from key bentonite beds with significantly improved age precision and accuracy provide a high-resolution temporal framework for testing hypotheses that relate to the stratigraphic evolution of the Western Interior Basin, as well as paleobiological reconstructions. New ²⁰⁶Pb/²³⁸U dates with 2σ internal uncertainties of ± 30 kyr or better from key bentonites in the JRF section confirm recent ⁴⁰Ar/³⁹Ar geochronology from these same bentonites, adding confidence to the absolute age framework.

New high-precision U-Pb geochronology places a through-going stratigraphic discontinuity (the mid-Judith discontinuity) that bisects the JRF in its type area at ca. 76.33 Ma, which also marks the onset of Bearpaw transgression in central Montana. Another bentonite date of ca. 75.22 Ma from the base of the overlying Bearpaw Formation in central Montana provides a highly refined correlation with the upper Kaiparowits Formation in southern Utah. U-Pb analyses currently in progress on the TMF bentonites will afford an opportunity to test whether the mid-Judith discontinuity in central Montana is expressed in upland alluvial facies of the TMF in northwestern Montana. The results of this collaborative effort will provide highly refined correlations of strata and fauna over a wide range of paleo-latitudes and across the Western Interior Basin during the apparent zenith of dinosaur diversity.