A PLIO-PLEISTOCENE RECORD OF LACUSTRINE OSTRACODES FROM BUTTE VALLEY, CALIFORNIA: FAUNAL RESPONSES TO TECTONIC AND CLIMATIC CHANGE

A 102 meter sediment record from Butte Valley California in the southern Cascades was recovered in 1991 by the USGS as part of their Global Change Project. Preliminary data were published in USGS open file reports, but work on the microfossils then ceased (Adams et al., in USGS OFR 94-593 and Carter, USGS OFR 94-641). Here we return to the core samples and examine in detail the ostracode fauna. Roberts et al. (1996) conducted a magnetostratigraphic study using XRD and mineral magnetic methods, showing that the core penetrated the Gauss/Matuyama boundary and contains a 3 million year record. The paleolake formed in a tectonically active region of the southern Cascades. As the Cascades rose, the lake was affected by the growing rain shadow. Pleistocene glaciers formed in the region, and glacial outwash was carried into the valley. In late Pleistocene time, the Klamath River Gorge developed, which had a drawdown effect on the regional water table (Adams et al., 1994).

Fifteen species are present in the Butte Valley sediment core, forming three major assemblages also illustrated in constrained cluster analysis (farthest neighbor, squared chord distance). The lowest assemblage, from 90.343 to 77.343 m (ca 3 my to 2.75 my) is distinguished by Tuberocypris sp. (extinct), where it is abundant and strongly correlated with Cytherissa lacustris and Cypria sp. This late Pliocene assemblage indicates a stable, permanent freshwater lake. C. lacustris is present in the Butte Valley core in syngamic form, with shell characteristics comparable to C. lacustris baikalensis. Cytherissa makes its last appearance during the Olduvai Sub-Chron, circa 2 mya. A second assemblage (90.343 to 77.343 m – ca 2.75 my to 1.1 my) shows the appearances of Limnocythere sappaensis and L. ceriotuberosa, indicating rising concentrations of bicarbonate dominated saline water. These conditions end near the base of the Jaramillo Sub-Chron. The third assemblage (77.353 to 19.413 – ca. 1.1 my to 400 kya, during the Mid Pleistocene transition) shows considerable species variability, ranging from L. platyforma, indicative of freshening events, to halophylic L. sappaensis and L. ceriotuberosa. Overall the lake record trends toward greater salinity near the top of the core; conditions favored by the then dominant L. sappaensis and L. ceriotuberosa.