We've Moved!
Visit SDSU’s new digital collections website at https://digitalcollections.sdsu.edu
Description
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 Rim gravels were deposited by northward flowing drainages and prior work suggests proximal sediment sources 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, similarly to clast collection for the earlier K-Ar study. Twenty of the twenty-four clasts yield ages that range narrowly around 162 Ma. Detrital zircon U-Pb ages from a Music Mountain Formation sandstone collected 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 ages determined from clasts. Grenville age zircon is conspicuously absent. Whole rock X-ray fluorescence major and trace element compositions of volcanic clasts show that clasts are mostly dacite/rhyolite with high K2O concentrations. The new ages and chemistry suggest a provenance for the volcanic component of the Music Mountain Formation from the Middle Jurassic arc that traverses across southern Arizona extending into Sonora Mexico. This result suggests a larger drainage basin for Music Mountain deposits extending southwards of the Transition Zone. The similarity of detrital zircon U-Pb ages from Music Mountain Formation sandstone to detrital zircon ages from Oligocene aeolian Chuska dune deposits of eastern Colorado Plateau indicates dune sands were likely derived from fluvial Rim gravel deposits by aeolian deflation. Further, detrital zircon ages from the Laramide Uinta basin depocenter north of the Grand Canyon suggests a northward flowing pre-Colorado River Rim gravel sediment dispersal path may have traversed the region now occupied by the modern Grand Canyon.
