Provenance of Volcanic Clasts within the Paleogene Rim Gravels, Colorado Plateau: Implication for Paleogeography and history of the western Grand Canyon
Jennifer Luscombe

Jennifer Luscombe
Advisor: Dr. Dave Kimbrough

Friday May 4th, 2018
CSL 422 – 9:30 to 10:15 am
watch Jennifer’s defense

The Rim gravels are a widespread, discontinuously exposed Paleogene fluvial unit located along the Mogollon Rim which is an erosional escarpment bounded by the Transition Zone of central Arizona on the southern edge of the Colorado Plateau. A western exposure of the Rim gravels is locally called the Music Mountain Formation. Paleocurrent indicators show that the Rim gravels were deposited by northward flowing drainages and prior work suggests proximal sediment sources for these deposits within the Transition Zone. The focus of this work is the provenance of volcanic clasts in Music Mountain Formation conglomerates. Previous work produced K-Ar ages from seventeen volcanic clasts that ranged from 54-120 Ma. Here we report new zircon U-Pb ages from twenty-one volcanic clasts collected from the Long Point area, Frazier Wells area, Peach Springs Canyon, and Hindu Canyon where clasts for the earlier K-Ar study were also collected. Twenty of the twenty-four clasts yield statistically indistinguishable ages that range narrowly around 162 ± 1.6 Ma. Detrital zircon U-Pb ages from a Music Mountain Formation sandstone collected 5 meters beneath the early Eocene freshwater limestone at Duff Brown Tank are dominated by Proterozoic Yavapai Mazatzal craton ages but ~10% of the grains yield Middle Jurassic ages that overlap the ages determined from the clasts. Grenville age zircon is conspicuously absent. Whole rock X-ray fluorescence major and trace element compositions of the volcanic clasts show that the clasts are uniformly high silica rhyolites characterized by elevated K2O concentrations reflecting potassium metasomatic processes that altered the volcanic rock. The new ages suggest a provenance for the volcanic component of the Music Mountain Formation from the Middle Jurassic arc that traverses across southeastern Arizona and southern Arizona extending into Sonora Mexico. This result requires reevaluation of the sediment dispersal patterns and paleogeography associated with the Rim gravels. Comparing these clasts’ whole rock chemistry, U-Pb zircon analyses, and physical descriptions, these clasts are determined to be sourced from an extra regional source to the south of the Transition Zone. Also, the similarity of the Music Mountain Formation detrital zircon U-Pb age distribution to analyses from Eocene strata north of the Grand Canyon, implicates sediment dispersal routes that suggest a northward flowing pre-Colorado River system and strengthens the argument for a “young” Grand Canyon.