U-PB AGES OF DETRITAL ZIRCONS IN LOWER JURASSIC EOLIANITES OF THE COLORADO PLATEAU AND ADJACENT AREAS

U-Pb ages of detrital zircons from Lower Jurassic eolian sandstones (180-200 Ma) of the Colorado Plateau were obtained by laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS). Units analyzed include the Navajo Sandstone and Wingate Sandstone of the Glen Canyon Group at North Wash, Nugget Sandstone equivalent to the Glen Canyon Group near Thistle, and Aztec Sandstone equivalent to Navajo Sandstone at Whitney Pockets. For each sample, one hundred individual zircon grains were analyzed, but analyses with >20% discordance or >10% uncertainty were removed from consideration. ²⁰⁶Pb/²³⁸U ages (for grains <1.0 Ga) and ²⁰⁶Pb/²⁰⁷Pb ages (for grains >1.0 Ga) were plotted on an age probability diagram of frequency versus age. All four samples contain six similar zircon age populations: Paleozoic (240-540 Ma), Pan-African (580-780 Ma), Grenvillean (860-1260 Ma), Mesoproterozoic (1300-1520 Ma), Late Paleoproterozoic (1560-1860), and Early Paleoproterozoic and Archean (>1860 Ma combined). The principal ultimate provenance of the Glen Canyon Group and its equivalents to the northwest (Nugget) and southwest (Aztec) is interpreted to have been the Appalachian orogen. Approximately 75% of the grains for each sample exhibit Paleozoic, Pan-African, or Grenvillean ages, with the most prominent age spike at 1.0 Ga – 1.2 Ga (Grenvillean). Our sample of Navajo Sandstone contains a relatively large population of 1850-2150 Ma Early Paleoproterozoic grains (n=15), provisionally from northwest Laurentia, but such grains are rare, one or two per sample, in samples of Wingate, Nugget, and Aztec Sandstones. All samples lack significant populations of Mesoproterozoic and Late Paleoproterozoic grains, suggesting that nearby basement sources of detritus, including the Ancestral Rocky Mountains, contributed little to the Jurassic eolian sands. To transport large volumes of sandy detritus from the Appalachian margin of the continent to the western interior during Early Jurassic time, a system of east-west flowing paleorivers is postulated, with headwaters in the incipient Atlantic rift belt. Paleowinds blowing toward southerly azimuths then transported sand into the Early Jurassic ergs of the Colorado Plateau.