Paper No. 14-7
Presentation Time: 10:20 AM
BIOCHRONOLOGY AND GEOCHRONOLOGY OF THE MIOCENE ZOO IN THE MOKELUMNE PALEOCHANNEL, WESTERN SIERRA NEVADA FOOTHILLS
HODGES, Montana1, NEWBY, Tahlor2, SWARNER, Holli3, BUSBY, Cathy3, SHAPIRO, Russell4, FRANCEK, Greg5, DAILEY, Charles6, NIES, Sean4, COBAIN, Bradley4 and HODGES, Christopher7, (1)Geological Sciences & Engineering, University of Nevada, Reno, 1664 N. Virginia Street, Reno, NV 89557, (2)Department of Geosciences, University of Nevada Reno, 1664 N Virginia Street, Reno, NV 89557, (3)Earth and Planetary Sciences, University of California Davis, Davis, CA 95616, (4)Earth and Environmental Sciences, California State University Chico, 400 W. 1st Street, Chico, CA 95929, (5)Mokelumne Resource Patrol Unit, East Bay Municipal Utility District, 15083 Camanche PKWY, South Valley Springs, CA 95252, (6)Sierra College, 5100 Sierra College Blvd, Rocklin, CA 95677, (7)California State University Sacramento, 6000 J St, Sacramento, CA 95819
A fossil site in sedimentary rocks on the lower western slope of the Sierra Nevada was discovered recently (2020) and has captured a great deal of media attention. These fossils include gomphothere, mastodon, camel, horse, tortoise, peccary, bird, fish, rodent, canine, dwarf tapirs, and hundreds of tree, root cast, limb, and leaf. We report on the geologic setting of the fossil site, and new age constraints from detrital zircon U-Pb geochronology using LA-ICPMS analysis to determine maximum depositional ages.
Our compilation mapping and new recordings shows that the fossil site lies in the axis of the lower reaches of the Cenozoic Mokelumne paleochannel1. This paleochannel is carved into Mesozoic basement rocks, and its fill consists of the following formations, each separated by a deep erosional unconformity that records re-incision events: (1) sedimentary rocks of the Eocene Ione Formation; (2) silicic pyroclastic rocks of the Oligoene-Early Miocene Valley Springs Formation; and (3) andesitic volcanic-lithic sandstones and volcanic debris flow deposits of the Miocene-Early Pliocene Mehrten Formation. The fossils were determined to lie in the Mehrten Formation, and biochronology loosely constrained the age of the fossils as 15 to 5 Ma. The detrital zircons have a maximum depositional age of 8.23 ± 0.08 Ma in the beds containing the fossils. These zircons were likely sourced by coeval silicic eruptions up-paleochannel in the Leviathan Peak volcanic center of the Ancestral Cascades arc2.
1Swarner et al., 2022, GSA Annual Meeting Abstract.
2Swarner et al., 2023, GSA Cordilleran Section Abstract.
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