U‐Pb and40Ar/39Ar Ages for a Tephra Lens in the Middle Jurassic Page Sandstone: First Direct Isotopic Dating of a Mesozoic Eolianite on the Colorado Plateau

Joint U‐Pb and40Ar/39Ar geochronology for pyroclastic zircon and biotite crystals from tephra lenses intercalated within eolianite of the Middle Jurassic Page Sandstone on the Colorado Plateau provides the first direct isotopic age for a Jurassic eolianite on the Colorado Plateau and indicates a chronostratigraphic age of 171.5–169.5 Ma for the Page Sandstone. With stated uncertainties in the age boundaries of global stratigraphic stages taken into account, the isotopic age of lower Bajocian is coordinate with the inferred biostratigaphic age of upper Bajocian from lateral correlation of Page Sandstone with equivalent marine strata. Comparative isotopic ages imply that the Page Sandstone was deposited during the same time frame as the Temple Cap Formation, inferred conventionally to be older, and that the J1 and J2 surfaces of the Colorado Plateau are therefore not reliable chronostratigraphic marker horizons.