BURIED IN LAHAR: DETRITAL ZIRCON U-PB MIOCENE AGE CONSTRAINTS OF THE MOKELUMNE PALEOCHANNEL

On the western slope of the Sierra Nevada, the Mokelumne paleochannel is known for Cenozoic volcanic deposits including the Miocene Mehrten Formation. Within the paleochannel, a rich fauna and flora fossil stratum of unknown age was recently identified by Greg Francek (East Bay Municipal Utility District) near Ione, California. The vast diversity of mammals was especially unique to gaps of paleontological history in the Sierra Nevada foothills and it is instrumental to determine stratigraphic distribution and geochronologic ages. In this project we established a maximum depositional age of 8.23 +- .08 Ma for the fossils by U-Pb LA-ICP-MS dating of detrital zircons from fluvial sandstones. We also infer a likely minimum depositional age of 5.96 +- .09 Ma from a volcanic debris flow that overlies the sandstones. This is the first report of detrital zircons from the Mokelumne paleochannel. A total of 795 U-Pb measurements from zircon crystals collected at three locations captured ages from the early Miocene to the late Paleoproterozoic with significant Miocene, Cretaceous, and Jurassic populations. No rim-core structures were observed in the zircons of Miocene age. However, rim-core zircons appear to contain the oldest ages. One rim-core zircon, which was sampled 6 times, has a rim of 89 +-1 Ma and a core of 1680 +-20 Ma. The source of these rim-core zircons has not been identified.