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

B. J. Kowallis·Eric H. Christiansen·Alan L. Deino·Michael J. Kunk·L. Heaman
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Abstract

High precision 40 Ar/ 39 Ar laser-microprobe ages of individual sanidines, 40 Ar/ 39 Ar 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 40 Ar/ 39 Ar 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 / 39 Ar age of 93.33 ± 0.50 Ma ( n = 29), a bulk-sample 40 Ar/ 39 Ar plateau age of 93.09 ± 0.34 Ma (2σ) for sanidine, and concordant 206 Pb/ 238 U and 207 Pb/ 235 U 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 40 Ar/ 39 Ar age of 93.46 ± 0.60 Ma ( n = 12) and a bulk-sample 40 Ar/ 39 Ar 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.

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Cretaceous ResearchArticle
90° N90° S · 180° W ← longitude → 180° E
Source-reported bounding extent: 31.332239° to 42.001567° latitude; -114.813613° to -102.04224° longitude. This indicates report coverage, not an exact sampling location. View area on OpenStreetMap.

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B. J. Kowallis, Eric H. Christiansen, Alan L. Deino, Michael J. Kunk, L. Heaman. 1995. Age of the Cenomanian-Turonian boundary in the Western Interior of the United States. https://doi.org/10.1006/cres.1995.1007

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