THE EOCENE MOONLIGHT FLORA, PLUMAS COUNTY, CALIFORNIA, AND THE CLARNO CONNECTION

The Moonlight fossil flora is named for its location north of Moonlight Valley and east of Moonlight Peak in Plumas County, California. It has been described as part of the “auriferous gravels,” Tertiary paleochannels of the Sierra Nevada, but is unique in several ways. Comparison with floras and sedimentary sections from the Eocene Clarno formation in Oregon may increase the contextual understanding of this flora.

Most of the paleochannels of the Sierra Nevada have a westward flow, with their source area and the paleodivide in Nevada. However the Jura River, or the Buckeye- Bear Hill channel, in which the Moonlight flora is preserved may have had a more southward flow. The sedimentary sections at Moonlight and the nearby Susanville locality of the same paleochannel are approximately 300 and 250 meters thick respectively. In comparison, sections from channels to the south rarely exceed thicknesses of 100 meters. This suggests that the Moonlight and Susanville sections may represent longer time intervals and / or a greater input of sediment. With a source area to the north, sediment could have been supplied in part by the Clarno arc volcanoes which produced thick sedimentary sections in eastern Oregon. At both Moonlight and Susanville, lenses of reworked volcanic material and altered pumice fragments are present. Dating may be possible using U/Th methods on zircons.

The majority of the specimens in the Moonlight flora collection come from the basal portion of the section in layers of sand and discontinuous lenses of clay. Detailed leaf architecture descriptions of 45 different angiosperm leaf morphotypes have been compiled. The genus Macginitiea is common in this and other Eocene floras including Chalk Bluffs, Clarno, Comstock, Republic and Green River. It provides a biostratigraphic constraint as it is not found in Oligocene or younger floras. Also present in the Moonlight flora are ferns, a palm, a Menispermaceae moon-seed, and the inflorescence Macginicarpa.

Future research will involve further comparison of the Moonlight flora with other Eocene floras and their depositional environments. Fossil floras hold valuable paleoclimatic data if the context of their age and elevation is understood.