AGE OF THE CENOMANIAN-TURONIAN BOUNDARY IN THE WESTERN INTERIOR OF THE UNITED-STATES

High precision Ar-40/Ar-39 laser-microprobe ages of individual sanidin es, Ar-40/Ar-39 plateau age spectra on bulk sanidine concentrates, U-P b zircon ages, and zircon and apatite fission-track ages from three be ntonites bracketing the Cenomanian-Turonian boundary in the Western In terior of the United States suggest an age for the boundary of 93.1 +/ - 0.3 (2 sigma). The lowermost bentonite comes from the Upper Cenomani an Sciponoceras gracile biozone, and gives a weighted mean laser-fusio n single-crystal Ar-40/Ar-39 age of 93.50 +/- 0.52 Ma (2 sigma, standa rd 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 zon e; it gives an average single-crystal Ar-40/Ar-39 age of 93.33 +/- 0.5 0 Ma (n = 29), a bulk-sample Ar-40/Ar-39 plateau age of 93.09 +/- 0.34 Ma (2 sigma) for sanidine, and concordant Pb-206/U-238 and Pb-207/U-2 35 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 a n average single-crystal Ar-40/Ar-39 age of 93.46 +/- 0.60 Ma (n = 12) and a bulk-sample Ar-40/Ar-39 plateau age of 92.87 +/- 0.34 Ma. The c omposition of these Cenomanian-Turonian bentonites from Colorado and U tah, the types of phenocrysts present, and the morphology of included zircons all indicate that the pre-alteration ash was rhyolitic and pro bably generated in a subduction setting involving a significant crusta l component.