⁴⁰Ar/³⁹Ar dating and magnetostratigraphic correlation of the terrestrial Cretaceous-Paleogene boundary and Puercan Mammal Age, Hell Creek-Tullock formations, eastern Montana

The recovery and subsequent prolific radiation of mammals in the northern Western Interior of North America following the Cretaceous-Paleogene (K-P) boundary is well documented in rocks attributed to the Puercan Land Mammal Age. The most complete Puercan record is that of the Tullock Formation, which overlies the dinosaur-bearing Hell Creek Formation and consists of a stratigraphic series of channel and overbank deposits from which well-preserved Puercan faunas have been collected. These channel deposits are typically bracketed by widespread coal beds. The IrZ- and Z-coals mark the base of the Puercan at the K-P boundary. The IrZ-Coal is overlain sequentially by the Z-, HFZ-, Y-, W-, and U-coals. ⁴⁰Ar/³⁹Ar dating of single crystals of sanidine separated from bentonites in the IrZ-, Z-, HFZ-, W-, and U-coals has yielded high-precision ages with standard errors of 0.1%. Replicate analyses of single crystals of sanidine yield weighted mean ages for bentonites in the following coals: IrZ, 65.16 ± 0.04 Ma; Z, 65.01 ± 0.03 Ma; HFZ, 64.77 ± 0.06 Ma; W, 64.11 ± 0.02 Ma; and U, 63.90 ± 0.04 Ma. These ages provide detailed calibration of Puercan through earliest Torrejonian land mammal ages and provide crucial calibration for the nonmarine K-P boundary and the Early Paleogene portion of the geomagnetic polarity time scale. -from Authors