U-PB AGES OF DETRITAL ZIRCONS IN MIDDLE TO UPPER JURASSIC EOLIANITES OF THE COLORADO PLATEAU

U-Pb ages were determined for detrital zircon grains (~100 per sample) in six samples of Middle to Upper Jurassic eolianites from the Colorado Plateau: Page Sandstone interfingering with Carmel Formation; younger Entrada Sandstone at North Wash in Utah, near Moab in eastern Utah, and near Gallup in northern New Mexico; and still younger Bluff Sandstone in southern Utah and Zuni Sandstone near Gallup. Zircon grains were dated by laser ablation ICP-MS using a beam diameter of 35-50 microns. After analysis, 4%-15% of the grains from each sample were removed from consideration because of either >20% age discordance or poor analytical precision. Approximately 80%-90% of the detrital zircon grains are older than 300 Ma, and include age populations similar to those in Early Jurassic ergs of the Colorado Plateau. By analogy, grains >300 Ma represent sediment derived primarily from the Appalachian orogen and secondarily from interior Laurentia. Nevertheless, 10%-20% of the detrital zircons (<300 Ma) were evidently derived from post-Carboniferous magmatic arcs lying south or west of the Colorado Plateau. Page Sandstone and Entrada Sandstone from west of the Colorado River contain only 1%-2% of the arc-derived grains, and are not amenable to close analysis. Entrada Sandstone from farther east near Moab and Gallup contains 7%-25% of the arc-derived grains, and approximately two-thirds in each case are 240-300 Ma, suggesting ultimate derivation from the East Mexico Permian-Triassic arc developed along the flank of Gondwanan Mexico after closure of the Ouachita suture belt with Laurentia. Jurassic paleorivers flowing northward from Mexico may have transported East Mexico arc detritus into the continental interior where prevailing paleowinds blowing toward the southwest could then have delivered sediment to the Entrada erg. Younger Bluff Sandstone and Zuni Sandstone of probable Oxfordian age contain 11%-14% arc-derived grains, and more than two-thirds in each case are 160-240 Ma, suggesting derivation from the continental-margin arc of the Cordilleran orogen. Prevailing paleowinds blowing from the west by Oxfordian time may have influenced eastward transport of sandy Cordilleran arc detritus to the Colorado Plateau.