DETRITAL ZIRCON GEOCHRONOLOGY OF QUARTZITE GASTROLITHS IN THE JURASSIC MORRISON FORMATION, NORTHEAST UTAH

The Jurassic Morrison Formation of the Rocky Mountain and Colorado Plateau regions is famous for its dinosaur fossils. The Morrison Formation (150-157 Ma) is comprised mostly of sandstone and mudstone that was deposited in a terrestrial deposition system that included fluvial, paludal, and lacustrine environments. Paleocurrent data indicates that Morrison sediment was transported to the north and east. Within the Morrison Formation, we find exotic pebble- and cobble- size durable clasts of quartzite, chert and vein quartz weathering out of the mudstone paleosols. We interpret these exotic clasts as gastroliths, carried within the gastric mills of dinosaurs. For this study, we collected red and pink quartzites near Vernal, UT and Flaming Gorge Reservoir, UT. We determined the provenance of these red quartzite gastroliths using detrital zircon U-Pb geochronology and LA-ICPMS methods at the University of Arizona Laserchron Center. Five individual gastroliths were analyzed. Three of these (n=22, 25 and 90) are dominated by Mesoproterozoic zircons, and are interpreted to have been derived from quartzites of the Uinta Mountain Group. The fourth gastrolith (n=21) includes a population of early Paleozoic zircons, which indicates that it is likely Carboniferous in age. These three red quartzite gastroliths are different in age from those in the Bighorn Basin of Wyoming, which were derived from Yavapai “Baraboo Interval” quartzites of the Great Lakes region. Both the Bighorn Basin and Uinta Mountain areas contain gastroliths of Carboniferous age. Our data indicates that Neoproterozoic quartzites, perhaps of the Uinta Mountain Group, must have been exposed somewhere in the region during the deposition of the Morrison. We believe that these gastroliths were ingested by dinosaurs and then transported to northeastern Utah within the animals.