Age of the Cenomanian-Turonian boundary in the Western Interior of the United States

High precision ⁴⁰Ar/³⁹Ar laser-microprobe ages of individual sanidines,⁴⁰Ar/³⁹Ar plateau age spectra on bulk sanidine concentrates, U-Pb zircon ages, and zircon and apatite fission-track ages from three bentonites bracketing the Cenomanian-Turonian boundary in the Western Interior of the United States suggest an age for the boundary of 93.1 ± 0.3 (2σ. The lowermost bentonite comes from the Upper Cenomanian Sciponoceras gracile biozone, and gives a weighted mean laser-fusion single-crystal ⁴⁰Ar/³⁹Ar age of 93.50 ± 0.52 Ma (2σ, standard error of the mean, n = 14) for sanidine. The middle bentonite comes from the Upper Cenomanian Neocardioceras juddii biozone, accepted in both North America and Europe as the uppermost Cenomanian ammonite zone; it gives an average single-crystal ⁴⁰ /³⁹Ar age of 93.33 ± 0.50 Ma (n = 29), a bulk-sample ⁴⁰Ar/³⁹Ar plateau age of 93.09 ± 0.34 Ma (2σ) for sanidine, and concordant ²⁰⁶Pb/²³⁸U and ²⁰⁷ Pb/²³⁵U ages of 93.48 ± 0.32 Ma on zircon. The upper bentonite comes from near the base of the Turonian, immediately above the first occurrence of the basal Turonian bivalve Mytiloides and sanidines from it give an average single-crystal ⁴⁰Ar/³⁹Ar age of 93.46 ± 0.60 Ma (n = 12) and a bulk-sample ⁴⁰ Ar/³⁹Ar plateau age of 92.87 ± 0.34 Ma. The composition of these Cenomanian-Turonian bentonites from Colorado and Utah, the types of phenocrysts present, and the morphology of included zircons all indicate that the pre-alteration ash was rhyolitic and probably generated in a subduction setting involving a significant crustal component.