­GEOCHRONOLOGY OF LATE ALBIAN-EARLY CENOMANIAN STRATA IN THE US WESTERN INTERIOR

Since the dating of Cretaceous bentonites in the Western Interior Basin (WIB) (Obradovich, 1993) and Japan (Obradovich et al., 2002) improvements in the ⁴⁰Ar/³⁹Ar method reflect shifts to astronomically-calibrated standard ages and multi-collector mass spectrometry. From 16 bentonites dated by Obradovich (1993) we present new multi-collector ⁴⁰Ar/³⁹Ar sanidine ages that reduce uncertainties from ±400 to ±50 ka for Albian-Cenomanian strata in the WIB. New ages include: 97.52 ± 0.05 Ma for the top of Mowry Shale, 98.17 ± 0.08 Ma near Casper, WY and 99.12 ± 0.14 Ma for the Arrow Creek bentonite, at the base of the Mowry-equivalent in MT. Ages of 99.49 ± 0.03 and 99.58 ± 0.08 Ma are from the upper and middle Newcastle Sandstone from the Black Hills, WY. The middle Newcastle Sandstone is indistinguishable in age from 99.62 ± 0.04 and 99.67 ± 0.13 Ma bentonites in the lower Shell Creek Shale in the Wind River Basin, and Greybull, WY. A lowest Shell Creek bentonite from the Soap Creek Dome, Bighorn Basin, MT, is 100.07 ± 0.04 Ma, close to the Albian-Cenomanian boundary age of 100.2 ± 0.2 Ma from Hokkaido, Japan. The Muddy Sandstone in central WY is dated from Maverick Springs Dome in the Wind River Basin at 101.23 ± 0.06 Ma, and is coeval with the uppermost Thermopolis Shale at 101.32 ± 0.05 Ma in the Bighorn Basin, WY. It is entirely uppermost Albian, but older than the Newcastle Sandstone by 1.5 Ma. The Vaughan and Taft Hill Members of the Blackleaf Fm. in the Sweetgrass Arch of north-central MT yield ages of 102.67 ± 0.04 Ma and 103.08 ± 0.07, respectively. New U-Pb zircon dates of Dakota Sandstone from Dinosaur Ridge, Denver CO are 103.92 ± 0.04 and 104.02 ± 0.04 Ma, thus older than both the Newcastle Sandstone and Muddy Sandstone of Wyoming. A new U-Pb zircon date from the Skull Creek Shale of 104.69 ± 0.05 Ma at Dinosaur Ridge is relatively close in age to the 104.87 ± 0.05 Ma ⁴⁰Ar/³⁹Ar age of the Skull Shale in the Black Hills, WY, but there are 5 Ma missing in the unconformity between the Skull Creek Shale of the Black Hills and the overlying Newcastle Sandstone. The lower Thermopolis Shale that is commonly correlated with the Skull Creek Shale in the Black Hills yields an age of 106.37 ± 0.06 Ma in the Bighorn Basin which is 1.5 Ma younger than the Skull Creek Shale. Sequences and contacts correlated by lithology require independent geochronology to verify that units above and below an unconformity are stratigraphically and temporally equivalent. There is much more time missing in unconformities than has been previously recognized in these important, petroleum-bearing reservoir strata.