PRECISE 40AR/39AR BENTONITE AGE FRAMEWORK OF LATE ALBIAN-EARLY CENOMANIAN NORTH AMERICAN WESTERN INTERIOR CRETACEOUS STRATA AND SEQUENCE STRATIGRAPHIC CORRELATION

Since John Obradovich published his widely cited Cretaceous bentonite geochronology between 1993 and 2002, improvements in ⁴⁰Ar/³⁹Ar geochronology have included a shift to astronomically-calibrated ages for sanidine standards and development of a new generation of high-sensitivity multi-collector mass spectrometers. These improvements have brought the ⁴⁰Ar/³⁹Ar chronometer into close agreement with the best ID-TIMS U-Pb zircon dates and astrochronologic age models for Cretaceous strata. Cretaceous bentonite ages obtained by Obradovich had average uncertainties of ±400 kyr, allowing discrimination of stratigraphic events only at one million-year resolution. We have now dated sanidine in many of these bentonites using a 5-collector Noblesse mass spectrometer (Jicha et al., 2016, Chem. Geol.) and reduced analytical uncertainties by an order of magnitude to less than ±50 kyr in most samples.

New results from the US Western Interior include: 97.52 Ma for the top of Mowry from a new locality collected by Cobban and Merewether near Kaycee, WY, and 99.49 and 99.58 Ma for the upper and middle Newcastle Sandstone from the Black Hills, WY. The middle Newcastle is indistinguishable in age from a 99.62 Ma bentonite in the lower Shell Creek Shale Member in the Wind River Basin. A lower middle Shell Creek bentonite from the Soap Creek Dome area, northern Bighorn Basin, MT, is 100.07 Ma in age, close to the recalculated Albian/Cenomanian boundary age of 100.2 Ma±0.2 Ma of Obradovich and Matsumoto from Hokkaido. The age of the Newcastle is 0.6 Ma younger than the 101.23 Ma Muddy Sandstone at Maverick Springs Dome, Fremont County, WY. Uppermost Thermopolis Shale beneath the Muddy near Greybull, WY is slightly older, yet analytically distinct at 101.36 Ma. Farther northwest in the Sweetgrass Arch area, MT, type Vaughan Member (102.11 Ma) of the Blackleaf Formation is a million years younger than the underlying Taft Hill Member dated at 103.08 Ma. An age of 104.87 Ma for Skull Creek Shale in the northern Black Hills, WY differs from the upper Lower Thermopolis Shale at Greybull, WY in the Bighorn Basin age of 106.37 Ma. Correlation surprises revealed by high-precision ⁴⁰Ar/³⁹Ar dating place significant new constraints on sequence stratigraphic interpretation of these important petroleum source and reservoir sedimentary rocks.