Paper No. 16-3
Presentation Time: 8:00 AM-6:00 PM
DETRITAL ZIRCON U-PB AGES FROM A FOSSILIFEROUS SIERRA NEVADAN MIOCENE PALEOCHANNEL
HODGES, Montana1, NEWBY, Tahlor2, BUSBY, Cathy3, SWARNER, Holli3, SHAPIRO, Russell4, FRANCEK, Greg5, DAILEY, Charles6 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 plethora of fauna and flora of unknown age was discovered in 2020 in fluvial sedimentary rocks of the Mokelumne River drainage, in the western Sierra Nevada foothills. These fossils are preserved in the Mokelumne paleochannel and show great biodiversity, including mastodon, gomphothere, camel, rhino, horse, rodent, tortoise, and abundant trees. Prior to this study, biochronology loosely constrained the age of the fossils as 15 to 5 Ma. This project sampled the stratigraphic section for detrital zircons U-Pb geochronology by LA-ICPMS to determine detrital zircon maximum depositional ages (MDA).
One sample was collected in sandstone below the fossils, a second from sandstones of the fossiliferous deposit, and a third at the base of volcanic debris flow deposits that cap the fossils. The Unmix routine was utilized to deconvolute the zircon suite of dates. The first two samples have MDAs of 8.23 ± 0.08 Ma, and the third has a MDA of 5.96 ± 0.09 Ma with an older population Unmix age at 7.24 ± 0.07 Ma. The age of the fossils is thus interpreted to be at most 8.23 ± 0.08 Ma but probably older than 7.24 ± 0.07 Ma. The detrital zircons were likely sourced from contemporaneous Ancestral Cascades arc volcanism that occurred up-paleochannel, near the present-day Sierra Nevada range crest.1
The population of 795 zircons also shows a profile of ages from Mesozoic Sierra metamorphic and plutonic rocks. This includes a rim-core pair with ages of 88.9 ± 0.8 Ma and 1680 ± 20 Ma, respectively, suggesting that some of the Sierran Cretaceous igneous melts were contaminated by Paleoproterozoic rocks.
1Swarner et al., 2023, GSA Cordilleran Section Abstract.
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