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<oai_dc:dc xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:oai_dc="http://www.openarchives.org/OAI/2.0/oai_dc/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.openarchives.org/OAI/2.0/oai_dc/ http://www.openarchives.org/OAI/2.0/oai_dc.xsd">
  <dc:contributor>Eric H. Christiansen</dc:contributor>
  <dc:contributor>Alan L. Deino</dc:contributor>
  <dc:contributor>Michael J. Kunk</dc:contributor>
  <dc:contributor>L. Heaman</dc:contributor>
  <dc:creator>B. J. Kowallis</dc:creator>
  <dc:date>1995</dc:date>
  <dc:description>&lt;p&gt;&lt;span&gt;High precision&amp;nbsp;&lt;/span&gt;&lt;sup&gt;40&lt;/sup&gt;&lt;span&gt;Ar/&lt;/span&gt;&lt;sup&gt;39&lt;/sup&gt;&lt;span&gt;Ar laser-microprobe ages of individual sanidines,&lt;/span&gt;&lt;sup&gt;40&lt;/sup&gt;&lt;span&gt;Ar/&lt;/span&gt;&lt;sup&gt;39&lt;/sup&gt;&lt;span&gt;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&amp;nbsp;&lt;/span&gt;&lt;i&gt;Sciponoceras gracile&lt;/i&gt;&lt;span&gt;&amp;nbsp;biozone, and gives a weighted mean laser-fusion single-crystal&amp;nbsp;&lt;/span&gt;&lt;sup&gt;40&lt;/sup&gt;&lt;span&gt;Ar/&lt;/span&gt;&lt;sup&gt;39&lt;/sup&gt;&lt;span&gt;Ar age of 93.50 ± 0.52 Ma (2σ, standard error of the mean,&amp;nbsp;&lt;/span&gt;&lt;i&gt;n&lt;/i&gt;&lt;span&gt;&amp;nbsp;= 14) for sanidine. The middle bentonite comes from the Upper Cenomanian&amp;nbsp;&lt;/span&gt;&lt;i&gt;Neocardioceras juddii&lt;/i&gt;&lt;span&gt;&amp;nbsp;biozone, accepted in both North America and Europe as the uppermost Cenomanian ammonite zone; it gives an average single-crystal&amp;nbsp;&lt;/span&gt;&lt;sup&gt;40&lt;/sup&gt;&lt;span&gt;&amp;nbsp;/&lt;/span&gt;&lt;sup&gt;39&lt;/sup&gt;&lt;span&gt;Ar age of 93.33 ± 0.50 Ma (&lt;/span&gt;&lt;i&gt;n&lt;/i&gt;&lt;span&gt;&amp;nbsp;= 29), a bulk-sample&amp;nbsp;&lt;/span&gt;&lt;sup&gt;40&lt;/sup&gt;&lt;span&gt;Ar/&lt;/span&gt;&lt;sup&gt;39&lt;/sup&gt;&lt;span&gt;Ar plateau age of 93.09 ± 0.34 Ma (2σ) for sanidine, and concordant&amp;nbsp;&lt;/span&gt;&lt;sup&gt;206&lt;/sup&gt;&lt;span&gt;Pb/&lt;/span&gt;&lt;sup&gt;238&lt;/sup&gt;&lt;span&gt;U and&amp;nbsp;&lt;/span&gt;&lt;sup&gt;207&lt;/sup&gt;&lt;span&gt;&amp;nbsp;Pb/&lt;/span&gt;&lt;sup&gt;235&lt;/sup&gt;&lt;span&gt;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&amp;nbsp;&lt;/span&gt;&lt;i&gt;Mytiloides&lt;/i&gt;&lt;span&gt;&amp;nbsp;and sanidines from it give an average single-crystal&amp;nbsp;&lt;/span&gt;&lt;sup&gt;40&lt;/sup&gt;&lt;span&gt;Ar/&lt;/span&gt;&lt;sup&gt;39&lt;/sup&gt;&lt;span&gt;Ar age of 93.46 ± 0.60 Ma (&lt;/span&gt;&lt;i&gt;n&lt;/i&gt;&lt;span&gt;&amp;nbsp;= 12) and a bulk-sample&amp;nbsp;&lt;/span&gt;&lt;sup&gt;40&lt;/sup&gt;&lt;span&gt;&amp;nbsp;Ar/&lt;/span&gt;&lt;sup&gt;39&lt;/sup&gt;&lt;span&gt;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.&lt;/span&gt;&lt;/p&gt;</dc:description>
  <dc:format>application/pdf</dc:format>
  <dc:identifier>10.1006/cres.1995.1007</dc:identifier>
  <dc:language>en</dc:language>
  <dc:publisher>Elsevier</dc:publisher>
  <dc:title>Age of the Cenomanian-Turonian boundary in the Western Interior of the United States</dc:title>
  <dc:type>article</dc:type>
</oai_dc:dc>