DETRITAL ZIRCON U-PB PROVENANCE OF QUARTZITE COBBLES FROM THE EARLY PLIOCENE BULLHEAD ALLUVIUM: IMPLICATIONS FOR SEDIMENT SOURCES OF THE EARLY COLORADO RIVER

The lower Colorado River system evolved as a series of spillover lakes and, eventually, a through-flowing fluvial system that connected a large drainage network routing water and sediment from the southern Rockies and Colorado Plateau through the Grand Canyon and lower Colorado River corridor and ultimately to the proto-Gulf of California. The earliest sedimentary deposits that record this transition are the latest Miocene(?) to Pliocene Bouse formation, consisting of a basal lacustrine carbonate and overlying lacustrine siliciclastic facies, and the stratigraphically higher early Pliocene Bullhead Alluvium, which represents the entirely fluvial Colorado River system. During Bullhead deposition the paleo-Colorado River transported a bedload composed of coarse, lithologically diverse siliciclastic material, and notably large cobbles of well-rounded quartzite. Quartzite clasts in the Bullhead Alluvium have yet to be characterized and their provenance remains unknown. There are several potential sources for quartzite clasts, including pre-Grand Canyon alluvial and fluvial deposits (rim gravels), locally derived recycled quartzite gravels from Cretaceous or Paleocene deposits, or potentially eroded from within the Grand Canyon itself as it underwent a phase of incision between ca. 5 – 4 Ma. For example, the Mesoproterozoic Shinumo quartzite is exposed in the inner gorge of the Grand Canyon and represents a potential source lithology for the abundant clasts found in the Bullhead. To test hypotheses for the source(s) of quartzite clasts, we will use U-Pb detrital zircon geochronology on five clasts of well-rounded orthoquartzite cobbles, one cobble of meta-rhyolite, and one sample of fine to medium-sized sand from the type section of the Bullhead Alluvium. We will measure U-Pb DZ ages from the processed samples, and will present the new data set along with statistical comparisons to existing age data from orthoquartzite clasts and quartzite bedrock samples from other potential source lithologies in the Southwest United States. Our new data set will help place constraints on the source for the abundant quartzite clasts present in the Bullhead Alluvium and aid further studies seeking to link coarse material eroded during carving of the Grand Canyon to depositional sinks along the Colorado River system.