Geological Society of America

14 Paleontologic Advances in the Cordilleran Region

Thursday, 18 May 2023
8:00 AM - 12:00 PM
Whitney Peak Ballroom 3

BIOCHRONOLOGY AND GEOCHRONOLOGY OF THE MIOCENE ZOO IN THE MOKELUMNE PALEOCHANNEL, WESTERN SIERRA NEVADA FOOTHILLS

Paper number: 14-9
Program: Theme Sessions
Time: 10:20 AM – 10:40 AM

Authors

Montana Hodges Presenting Author
University of Nevada, Reno, Reno, NV
Christopher Hodges
California State University Sacramento, Sacramento, CA
Tahlor Newby
University of Nevada Reno, Reno, NV
Holli Swarner
University of California Davis, Davis, CA
Cathy Busby
University of California Davis, Davis, CA
Russell Shapiro
California State University Chico, Chico, CA
Greg Francek
East Bay Municipal Utility District, South Valley Springs, CA
Charles Dailey
Sierra College, Rocklin, CA
Sean Nies
California State University Chico, Chico, CA
Bradley Cobain
California State University Chico, Chico, CA

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 55, No. 4

doi: 10.1130/abs/2023CD-387595

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