USING TEPHROCHRONOLOGY, 40AR/39AR, OSTRACODS, AND DIATOMS TO DATE AND RECONSTRUCT THE ENVIRONMENT OF AN EARLY PLEISTOCENE TIME-SLICE OF WAUCOBA LAKE, INYO COUNTY, CALIFORNIA, USA

Twenty-seven samples were collected from the Waucoba Lake beds in Duchess canyon and at the Ryser Tuff Mine in Inyo County, California (37.17341°, -118.20468°). Our purposes were to: 1) characterize, identify, and date volcanic glass using electron microprobe; 2) identify the microfossils (ostracods and diatoms) present using scanning electron microscope in order to refine the geochronology and biostratigraphy of the sites; 3) temporally constrain and reconstruct shifts in the paleoenvironment. Emphasis was placed on a time-slice between ~2.90 and ~2.00 Ma. The updated chronologic framework and paleoenvironmental interpretations are also supported by chemostratigraphic correlations and ⁴⁰Ar/³⁹Ar dates.

In Duchess canyon, the basal unit was identified as the <2.89 to >2.22 Ma tephra layers of the Badlands. Overlying beds were identified as the 2.22 to 2.13 Ma tephra layers of Blind Spring Valley. The eruptive source area for these tephra is near Glass Mountain. The presence of the age-diagnostic diatom Tertiarius pygmaeus in the beds bracketing the tephra layers of Blind Spring Valley further constrains the early Pleistocene age determination. These “fingerprinted” units were then chemostratigraphically correlated to ⁴⁰Ar/³⁹Ar dated tephra at the Ryser Tuff Mine.

At Duchess canyon, sample LMJ-WAC16-6 overlies LMJ-WAC16-5 the uppermost bed of the Tuff of Blind Spring Valley. LMJ-WAC-6 was primarily sampled for common to abundant and relatively well-preserved ostracods. Species of Limnocytheridae and Candonidae showed the highest percent abundances. A comparative increase in the relative abundance of Limnocythere sanctipatricii and the decrease in Cytherissa lacustris shortly after deposition of the tephra layers of Blind Spring Valley suggest that the lake water became more saline, shallower, and conditions became oligotrophic. Pulses between fresh and brackish water have been attributed to the opening and closing of the basin due to tectonic activity.

Future research will include study of additional fossiliferous layers in the Waucoba dry Lake, and further refinement of geochronology. Duchess canyon has been well examined; but other areas such a Ryser Tuff Mine can indicate whether hydrologic oscillations continued throughout the Quaternary.