EOCENE ZIRCONS AND FOSSIL FLORAS FROM THE NORTHERN SIERRA NEVADA, CALIFORNIA

The Moonlight and Susanville floras are preserved in paleochannel deposits in the Northern Sierra Nevada. Though the localities have long been regarded as Eocene based on their floras, there was no radiometric age control. New detrital zircon data from these areas now confirms the Eocene age of the floras.

Zircons were dated using the new LA ICP MS multi-collector at UC Santa Barbara. Samples included a tuff near Moonlight, the fossiliferous sandstone that preserves the Moonlight flora, and a reworked tuff above the Susanville fossil flora locality.

The tuff near Moonlight yielded only late Eocene grains. The fossil bearing sandstone at Moonlight contained Eocene grains as well as Cretaceous, Paleozoic and Proterozoic age peaks. The Susanville sample had a broader spread of Eocene and Mesozoic ages but did not include any of Paleozoic or Proterozoic age.

Though the two localities are less than ten miles apart, these dates indicate that they may represent different drainages with distinct source areas. Possible source areas for these channels include northern California, northwestern Nevada and central Nevada. The Susanville locality may have had its source to the north while Moonlight was perhaps more influenced from sources to the east.

All sample sizes were between 40 and 70 grains. Further analysis may reveal additional age peaks. Based on the youngest zircon ages, the Moonlight flora is 34.8Ma +/- 0.6. The Susanville flora is 34Ma or older.