The Corcoran Clay at Great Depths in the Southern San Joaquin Valley, CA

Elizabeth Powers¹, Robert Negrini¹, Peter Wigand², Manuel Palacios-Fest³, and Franklin Foit^4
1 Department of Physics & Geology, California State University, Bakersfield, 9001 Stockdale Hwy., Bakersfield, CA 93311
²Great Basin and Mojave Paleoenvironmental Research and Consulting, 2210 Seneca Drive, Reno, NV 89506-9128
³Terra Nostra Earth Sciences Research, 6312 N. Barcelona Lane #606, Tucson, AZ 85704
⁴Washington State University Geochronology Lab, PO Box 642812, Webster 1052, Pullman, WA 99164-2812

The age and depositional environment of Pleistocene-age sediments in the southern San Joaquin Valley are at present poorly constrained. This study provides such constraints based on lithostratigraphy, paleoenvironmental analyses and preliminary tephrochronology on 102 grab samples collected from drill cuttings near the depocenter of Buena Vista lake. Preliminary results of major element geochemistry of glass shards collected from a grab sample at 720 ft below ground surface suggest that this volcanic glass layer is the 774 ± 2.8 ka Bishop tephra from the Mammoth caldera. The tephra resides in a ~75-100 meter-thick clay layer that, if the age is correct, would correlate with the Corcoran Clay found further north in the San Joaquin Valley. Pollen and ostracode paleontology suggest the presence of several extreme climatic events above the tephra including the suggested presence of fir growing near the shore of Buena Vista Lake. The frequency and duration of these events are consistent with similar events found in other lake basins of western North America, but only after ~700,000 years ago. Thus, the paleontology is consistent with the preliminary volcanic ash correlation and a projected sediment age of ~ 1 million years at a depth of 1 km below the Buena Vista Lake surface. Previous projections of this clay layer down to depths greater than 1300 m at the southern end of the Maricopa subbasin suggest an average sedimentation and/or subsidence rate of more than 1 mm/yr for the past million years.

AAPG Search and Discovery Article #90076©2008 AAPG Pacific Section, Bakersfield, California