U-PB AGES OF DETRITAL ZIRCONS FROM LOWER JURASSIC NUGGET SANDSTONE OF NORTHERN UTAH

Previous analyses of Permian and Jurassic eolian sandstones on the Colorado Plateau of southwest Laurentia have yielded U-Pb ages that fall into six populations of widely different ages from Paleozoic to Archean. Further analyses conducted on Jurassic eolian sandstones, including the Nugget Sandstone, have yielded similar zircon age populations. The Lower Jurassic Nugget Sandstone of northern Utah is quartz-rich eolian sandstone equivalent in age to the Glen Canyon Group farther south. The sample was collected near Thistle, Utah from the upper Nugget Sandstone, a white eolian sandstone with massive cross-bedding that overlies redbeds and underlies the Twin Creek Limestone. U-Pb ages were determined for 100 individual detrital zircon grains using a LA-MC-ICPMS (Laser-Ablation Multi-Collector Inductively Coupled Plasma Mass Spectrometer) with a beam diameter of 35 microns. Analyses that were >20% discordant or had >10% uncertainty were removed from consideration. An age-probability plot of the remainder (n=85) displays the following age populations: 240-540 Ma (Paleozoic), n=21; 580-780 Ma (Pan-African), n=12; 860-1260 Ma with a peak near 1080 Ma (Grenvillean), n=29; 1300-1520 Ma (Mesoproterozoic), n=6; 1560-1860 (late Paleoproterozoic), n=10; >1860 Ma (early Paleoproterozoic and Archean), n=7. The main provenance of the Nugget Sandstone is interpreted to have been the Appalachian orogen. Nearly three-quarters of the zircons plotted fall within three age populations (Paleozoic, Pan-African, Grenville) derived from bedrock of ages known to be widespread within or bordering the Appalachian orogen but rare or absent elsewhere in North America. This inference coincides with previously published conclusions regarding the provenance of Permian and Jurassic eolian sandstones on the Colorado Plateau. The age probability plot for the Nugget Sandstone is closely analogous to plots for the Wingate Sandstone and Navajo Sandstone of the Glen Canyon Group at North Wash in southern Utah. Paucity of Mesoproterozoic and late Proterozoic grains (n=16 combined) indicates that Precambrian sources within the Yavapai-Mazatzal age belt of the nearby Ancestral Rocky Mountains province contributed only minor detritus to the Nugget Sandstone.