T10 The Virtue of Fieldwork in Volcanology, Sedimentology, Structural Geology, and Tectonics I: A Session to Honor Cathy Busby, MGPV Distinguished Geological Career Award Recipient of 2020

Sunday, 9 October 2022
8:00 AM - 12:00 PM
207

GEOLOGY OF THE MOKELUMNE PALEOCHANNEL AT THE MIOCENE ZOO, SIERRA NEVADA FOOTHILLS, CALIFORNIA

Paper number: T10-7
Program: Technical Sessions
Time: 9:25 AM – 9:40 AM

Authors

Abstract

The Mokelumne paleochannel is one of a series of E-W oriented, Eocene to Late Miocene paleochannels that provide constraints on the topographic and tectonic evolution of the Sierra Nevada. One of the largest fossil troves in California, the “Miocene Zoo” was recently discovered in the lower reaches of Mokelumne paleochannel near Ione, California. The site is now the subject of intense paleontological study and includes camels with necks as long as giraffes, rhinos, a mastodon skull with 6’ tusks, hundreds of fossilized logs, and much more. New U-Pb detrital zircon geochronology indicates a maximum depositional age of 8.23 +- .08 Ma for the fluvial sandstones that contain the fossils. We focus on the geologic setting of the Miocene Zoo, by making digital map compilations and cross sections, a series of measured sections, and by characterizing tephras we discovered at the fossil site.

New compilation mapping shows that the Mokelumne paleochannel contains a series of deep erosional unconformities before it fans out down-paleoslope into a broad alluvial plain about 15 km east of the modern eastern edge of the California Central Valley. Unconformity 1, the base of the paleochannel where Eocene Ione Formation rests on Mesozoic basement, is ~70 m deep; unconformity 2, at the base of the Oligocene Valley Springs Formation, is ~110 m deep; and unconformity 3, at the base of the Miocene Mehrten Formation, is ~120 m deep. These unconformities are also present about 100 km up-paleochannel on the Sierra Crest at Ebbetts Pass1, suggesting regional tectonic control. A series of measured sections within the Mehrten Formation shows that the fossil site lies in the axis of a fluvial paleochannel, with overbank deposits preserved outside of the paleochannel. Tephras are also concentrated in the fluvial paleochannel axis, and they provided the silica that preserved the fossils. The tephras include fine-grained water-lain tuff, as well as two types of rounded pumice clasts, distinguished by petrography, geochemistry and 40Ar/39Ar geochronology.

1Busby et al. 2016, Geosphere v. 12, p. 135-175

Geological Society of America Abstracts with Programs. Vol. 54, No. 6

doi: 10.1130/abs/2022AM-382899

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