REFINED GEOCHRONOLOGIC FRAMEWORK OF THE EARLY CRETACEOUS NONMARINE WESTERN INTERIOR BASIN

The Early to mid-Cretaceous (145 – 93.9 Ma) marks a period of dynamic shifts in tectonism, depositional environment, climate, and biota across the mid-North American continent. Despite its importance for understanding biotic and geologic transitions, many Early to mid-Cretaceous nonmarine deposits of the Western Interior Basin (WIB) lack adequate age constraints needed for correlation. To establish a more refined geochronologic framework for the Early to mid-Cretaceous deposits of the WIB, we integrate new and existing detrital zircon U-Pb chemical abrasion-thermal ionization mass spectrometry (CA-TIMS) dates and chemostratigraphic data with Bayesian age modeling of deposits that span western Texas to southwestern Montana: the Yucca Formation (YF), the Blackleaf Formation (BLF), the Cedar Mountain Formation (CMF), the Kootenai Formation (KF), and the Cloverly Formation (CLF). CA-TIMS dates imply the basal Buckhorn Conglomerate (CMF) was deposited during the Berriasian to Valanginian (~145 - 139.8 Ma), possibly correlating with the basal conglomerate of the KF, Pryor Conglomerate (CLF), and Shutup Conglomerate (YF). Age modeling indicates the Yellow Cat Member (CMF) is Valanginian to Barremian or early Aptian (~139.8 – 121.7 Ma), suggesting contemporaneous deposition with the lower clastic sandstone of the KF. New CA-TIMS dates from the upper YF (118.05 ± 0.62 to 115.94 ± 0.07 Ma) suggest that it was deposited during the Aptian (125 – 113 Ma) at approximately the same time as the Ruby Ranch Member of the CMF and the lower calcareous member of the KF. During the Albian (113 – ~103 Ma), CA-TIMS dates from the CLF indicate rapid deposition between 111.6 ± 0.06 Ma and 109.69 ± 0.07 Ma, contemporaneous with the upper sandstone and upper calcareous members of the KF and the Glen Rose Formation overlying the YF. For the Late Albian to Cenomanian (~103 – 93.9 Ma), we report new lower and upper bounding constraints for the Vaughn Member of the BLF of 100.61 ± 0.07 Ma to 100.12 ± 0.07 Ma, placing its timing of deposition between the upper conglomerate of the Short Canyon member (103.08 ± 0.08 Ma) and the Mussentuchit Member (99.89 – 98.88 Ma) of the CMF. This refined geochronologic framework advances our understanding of depositional timing and correlations across the WIB during the Early to mid-Cretaceous.