A 40AR/39AR AND U-PB TIME SCALE FOR THE CRETACEOUS WESTERN INTERIOR BASIN, NORTH AMERICA

Improvements in analytical procedures in parallel with intercalibration of ⁴⁰Ar/³⁹Ar and U-Pb methods and laboratories, spurred since 2003 by the EarthTime geochronology community initiative, have led to ± 2s uncertainties on the order of 50 to 100 ka, or better for Cretaceous ash beds. Assembled here are fifty-seven ⁴⁰Ar/³⁹Ar ages and sixteen ²³⁸U-²⁰⁶Pb ages of volcanic ash beds in strata of the Western Interior Basin of North America determined during the last 15 years since these improvements have been made. These age determinations span from 108 Ma in the middle Albian to 66 Ma in the latest Maastrichtian. Five of the ⁴⁰Ar/³⁹Ar ages from Campanian and Maastrichtian strata are newly reported here, whereas the remainder are from the literature. Building on the pioneering work of John Obradovich and Bill Cobban, where possible these age determinations are tied to ammonite and inoceramid biostratigraphy. For most ash beds the temporal uncertainties, unlike earlier time scales for the WIB, are much shorter than the duration of fossil biozones. Proposed ages for stage boundaries based on this review of the radioisotopic ages include:

• Maastrichtian-Danian 66.02 ± 0.08 Ma
• Campanian-Maastrichtian 72.20 ± 0.20 Ma
• Santonian-Campanian 84.19 ± 0.38 Ma
• Coniacian-Santonian 86.49 ± 0.44 Ma
• Turonian-Coniacian 89.75 ± 0.38 Ma
• Cenomanian-Turonian 93.95 ± 0.05 Ma
• Albian-Cenomanian 100 ± 0.40 Ma

Five bentonites that occur within the Vascoceras diartianum, Prionocylus macombi, Scaphites preventricosus, Scaphites depressus, and Desmoscaphites bassleri ammonite zones, dated using both ⁴⁰Ar/³⁹Ar and U-Pb methods, yield ages in agreement to within 150 ka and form the backbone of the WIB time scale. In parallel, improvements in the taxonomy of ammonites, inoceramids, and foraminifera, and recent field work, is better establishing the biostratigraphic framework for these age determinations. These efforts each contribute to the progressive refinement of the chronostratigraphic framework of the WIB, and enhance its utility for global correlation.