Lithostratigraphic Record of North Soda Lake, Carrizo Plain, San Luis Obispo County, California: Implications for a Long-lived Pleistocene Lake

Randall Stephenson¹, Rob Negrini¹, Dallas Rhodes², Adam Herrera³, Staci Loewy¹, Nick Duerr⁴, and Gaby Noriega^5
1Department of Physics and Geology, California State University, Bakersfield, 9001 Stockdale Hwy., Bakersfield, CA 93311
²Dept. of Geosciences, Georgia Southern University, Statesboro, GA
³South High School, Bakersfield, CA
⁴Stockdale High School, Bakersfield, CA
⁵Dept. Environmental Health, Science, and Policy, Univ. California, Irvine, CA

Age control and initial paleoenvironmental analyses of a 40m sediment core obtained from Carrizo Plain suggests Soda Lake in the Carrizo Plain has been the site of lacustrine deposition for at least ~300,000 years. The core was collected from near the eastern shoreline of present-day Soda Lake. The uniform nature of the lithology suggests that the rate of sedimentation has been consistent. The lithology consists almost entirely of clay and evaporate minerals, such as gypsum. Color variations, magnetic susceptibility values, and SEM observations of the presence or absence of magnetite are the only initial indicators of lithologic variability and suggest that the depositional environment varied somewhat through time between deeper water, reducing conditions and vice-versa. The detrial component of sedimentation was likely restricted to a somewhat constant rain of either airborne or stream-transported clay-size particles with very little influx from high energy currents. Furthermore, the ubiquitous presence of en situ evaporite grains indicates that saline or hypersaline conditions have been dominant throughout the life of the lake. Radiocarbon dating of charcoal material found in the core corresponding to a depth of ≈1m provides a maximum age of 17.74 ± 0.33 14C kyr for this material. A second date on seed pods from 1.8 m lower in core yields an age of 19.09 ± 0.08 14C kyr. Both of these dates are from an upper reduced zone that may have corresponded to deep lake conditions. The age of this deep-lake zone suggests that it was formed during the most recent glacial maximum interval similar in timing to the highstands from other lakes in western North America. A deeper reduced zone at ~18 m would then correspond to the penultimate glacial maximum at ~160 kyr ago. This suggests an overall sedimentation rate of ~1 cm/100 yr and that the bottom of the core may be as old as 300,000 years or more.

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