U-PB ZIRCON AND 40AR/39AR BIOTITE AGES FOR TEPHRA WITHIN THE EOLIAN MIDDLE JURASSIC PAGE SANDSTONE AT LAKE POWELL (UTAH–ARIZONA)

Lenses of juvenile bentonitic tephra intercalated within eolian Page Sandstone at Lake Powell contain euhedral zircon and biotite ash crystals that provide the first direct isotopic ages for a Jurassic eolianite on the Colorado Plateau. A tephra lens near the base of the Page Sandstone (Harris Wash Member) in Gregory Bay yields a U-Pb age of 171.5±1.9 Ma (2ó) from LA-MC-ICP-MS analysis of 49 nearly isochronous zircon crystals, and a 40Ar/39Ar age of 171.09±0.49 Ma (2ó) based on replicate step-heating analyses of biotite. The latter age is relative to FC-2 sanidine at 28.201 Ma and a total 40K decay constant of 5.463 e–10/a. Mutual overlap of the two isotopic ages at 2ó provides confidence in the analytical results. A supplementary tephra sample from a similar stratigraphic horizon in nearby Face Canyon yields a nearly identical U-Pb age of 171.6±2.0 Ma (28 nearly isochronous zircon crystals), and a second supplementary tephra sample from near the top of the Page Sandstone (Leche-e Member) in Face Canyon yields a biotite 40Ar/39Ar age of 169.5±0.61 Ma. The indicated time span of 171.5-169.5 Ma for Page deposition is lower Bajocian from the GTS 2004 time scale, and thus slightly older than the upper Bajocian age inferred from marine faunas collected ~250 km north of Lake Powell in the Twin Creek Limestone, laterally equivalent to the Carmel Formation with which Page Sandstone interfingers. However, age uncertainties (at 1ó) for stage boundaries of the GTS 2004 time scale allow Page deposition to be either Aalenian or upper Bajocian instead of lower Bajocian. Comparison of the Page isotopic ages with previously published 40Ar/39Ar sanidine ages and newly determined U-Pb zircon ages for ash beds in the Carmel and Temple Cap Formations at Gunlock Reservoir ~200 km west of Lake Powell indicates that Temple Cap and Page sedimentation occupied the same time frame. The two formations both directly overlie Navajo Sandstone, and nowhere occur in the same stratigraphic succession because Temple Cap Formation does not extend beyond Johnson Canyon (~12 km east of Kanab UT) whereas no tongues of Page Sandstone extend as far west as Johnson Canyon. The available isotopic ages imply that the J1 surface (below Temple Cap) and the J2 surface (below Page), as currently defined to be of contrasting ages, are not reliable chronostratigraphic marker horizons.