PROVENANCE OF PALEOGENE RIM GRAVELS VOLCANIC CLASTS INDICATE AN EXTRAREGIONAL SOURCE: IMPLICATIONS FOR PALEOGEOGRAPHIC RECONSTRUCTION OF SOUTHWESTERN ARIZONA

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 ten volcanic clasts collected from the Long Point area where clasts for the earlier K-Ar study were also collected. Nine of the ten clasts yield statistically indistinguishable ages that range narrowly around 161±2 Ma. A tenth clast yield an age of 152 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 K₂O concentrations. The compositions are suggestive of potassium metasomatic processes that likely 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.