Age of the Cenomanian-Turonian boundary in the Western Interior of the United States
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Abstract
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.
Study Area
Publication type | Article |
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Publication Subtype | Journal Article |
Title | Age of the Cenomanian-Turonian boundary in the Western Interior of the United States |
Series title | Cretaceous Research |
DOI | 10.1006/cres.1995.1007 |
Volume | 16 |
Issue | 1 |
Year Published | 1995 |
Language | English |
Publisher | Elsevier |
Contributing office(s) | Eastern Geology and Paleoclimate Science Center |
Description | 21 p. |
First page | 109 |
Last page | 129 |
Country | United States |
State | Arizona, Colorado, New Mexico, Utah |
Other Geospatial | Western Interior of the United States |
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