DETRITAL ZIRCONS AS A METHOD FOR CONSTRAINING THE AGE OF EOCENE FOSSIL FLORAS, NORTHERN SIERRA NEVADA, CALIFORNIA, USA

LOVELOCK, Elizabeth Clare, National Park Service, John Day Fossil Beds National Monument, 32651 Hwy 19, Kimberly, OR 97848, TIFFNEY, Bruce H., Department of Earth Science and College of Creative Studies, University of California, Santa Barbara, CA 93106 and KYLANDER-CLARK, Andrew, Department of Earth Science, University of California, Santa Barbara, CA 93106, liz_lovelock@nps.gov

The Moonlight and Susanville floras in the northern Sierra Nevada have well preserved angiosperm leaf fossils which can be used as paleoclimate indicators. Using leaf margin analysis, a mean annual temperature of 20°C (+/- 2°C) was estimated from 45 morphotypes from the Moonlight flora. Also present at Moonlight are fossil fruits, ferns, and a palm. Conifers have been collected at Susanville but not Moonlight. The age of the Moonlight and Susanville floras has long been regarded as Eocene, an interpretation supported by the presence of the genus Macginitiea. U/Pb dating of detrital zircons at both localities was undertaken using laser-ablation ICP-MS at UCSB to refine this age estimate.

A tuff layer stratigraphically below the main fossil-bearing layer at Moonlight is dominated by 34.8Ma (+/- 0.7my) zircons. A detrital sample from the fossil bearing sandstone at Moonlight contains Eocene, Cretaceous, Paleozoic and Proterozoic zircons. Based on the youngest peak age in the detrital sandstone sample, the maximum depositional age of the fossil deposit at Moonlight is ~35Ma. This is much closer to the Eocene / Oligocene boundary than was previously thought based on comparison with the Chalk Bluffs and Clarno floras.

At Susanville, a reworked tuff stratigraphically above the flora has its largest and youngest detrital zircon age peak ca. 35Ma, with the second largest age peak ca. 100Ma matching the age of the underlying granite. Thus the Susanville flora could be 35Ma or older.

These first attempts at radiometric dates for Moonlight at Susanville provide a context for future work on Cenozoic terrestrial environmental change in the northern Sierra Nevada. A geochemical analysis of the tuff at Moonlight will provide data for comparison with other ~35Ma tuffs in the western North America and may aid in locating a source area for the volcanic material. The detrital zircon data from Moonlight and Susanville, along with detrital zircon analyses from other channels, may hold clues to the drainage patterns of the “auriferous gravels” paleochannels.

Session No. 288

T122. Cenozoic Terrestrial Environmental Change in North America: From Paleosols to Paleobotany

Wednesday, 3 November 2010: 1:30 PM-5:30 PM

Room 607 (Colorado Convention Center)

Geological Society of America Abstracts with Programs. Vol. 42, No. 5, p.672

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