DETRITAL ZIRCON PROVENANCE OF QUARTZITE GASTROLITHS IN THE JURASSIC MORRISON FORMATION, WYOMING, USA

The Jurassic Morrison Formation of the Rocky Mountain and Colorado Plateau areas is famous for its dinosaur fossils. The Morrison 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 derived from the west and south. A striking feature of the Morrison is the occurrence of exotic pebble- and cobble- size durable clasts of quartize, chert and vein quartz that are referred to as exoliths. These exoliths occur widely dispersed in mudstone. We interpret these exoliths to be dinosaur gastroliths. Many of these quartzite gastroliths are red and pink in color, and are lithologically identical to Yavapai-Mazatzal (1.6-1.8 Ga) quartzites that occur in Wisconsin, Minnesota, Arizona, and New Mexico. We determined the provenance of these red quartzite gastroliths using detrital zircon U-Pb geochronology using LA-ICPMS methods at the University of Arizona Laserchron Center. Zircons were separated from four red quartzite gastrolith cobbles collected from the Morrison in the northeastern Bighorn Basin, Wyoming (n=32, 36, 42, 50). Two of the gastroliths have Yavapai and Penokean maximum depositional ages, and peak ages that are Penokean and Archean. We interpret that these two gastroliths were derived from Yavapai (Baraboo Interval) quartzites exposed more than 1000 km to the east. The other two gastroliths contain early Paleozoic and Grenville age zircons in addition to populations Paleoproterozoic and Archean grains. We interpret that these gastroliths were derived from Pennsylvanian strata that were exposed several hundred km to the west of the sampling locality. The gastrolith detrital zircon age spectra are statistically distinct from overlying and underlying Morrison Formation sandstones. We believe that these gastroliths were ingested by sauropods well beyond the current depositional extent of the Morrison in Minnesota and Wisconsin and then transported to Wyoming within the animals.