NEW EVDIENCE FOR THE AGE AND EXTENT OF LAKE DEPOSITS IN THE CARRIZO PLAIN, SAN LUIS OBISPO COUNTY, CALIFORNIA

RHODES, Dallas D., Department of Geology and Geography, Georgia Southern University, Statesboro, GA 30460, DRhodes@GeorgiaSouthern.edu

NEGRINI, Robert, Department of Geology, California State University, Bakersfield, 9001 Stockdale Highway, Bakersfield, CA 93311

ARROWSMITH, J Ramon, Department of Geological Sciences, Arizona State University, Tempe, AZ 85287

NORIEGA, Gabriela, Department of Environmental Analysis and Design, University of California, Irvine, CA 92697

The Carrizo Plain is the only closed basin in Californiaπs southern Coast Ranges and records both climate and tectonic activity. The Plio-Pleistocene Paso Robles formation that underlies the basin indicates drainage from the Carrizo once reached Monterey Bay through the Salinas River. Alluvial fans spreading into the Carrizo from the Temblor Range are cut by the San Andreas Fault. Warping of the Plain associated with the fault and tens of meters relative uplift on the southwestern side of the basin may have severed the connection to the Salinas. Extensive clay dune remnants on the northeastern and eastern sides of the main basin and a deeply embayed southwestern shoreline are also consistent with southwest-directed tectonic tilting of the lake floor (about 1 m/km) during the Holocene.

Luminescence and radiocarbon dates obtained during previous investigations of the lacustrine and clay dune deposits accumulated in the basin have indicated a maximum age of about 10 ka for the lake. These dates and the absence of identifiable strand lines led to the conclusion that drainage from the Carrizo remained open until approximately the end of the Younger Dryas.

Two exploratory core sites were sampled in May 2005. The first site is on the dry lake flat of Soda Lake's northern basin. This site was chosen to test the thickness of the lacustrine sediment by reaching coarse-grained alluvial deposits. The sediment was cored and augured to a depth of ~14 m (the limit for the equipment) without penetrating the lacustrine clay. The second core was taken in the swale between the outermost clay dune and the next lower dune, both of which fringed the retreating lake. Approximately 6 m of essentially featureless clay-rich sediment were retrieved. The top of the outer dune ridge is ~8 m above the core site and the next dune is ~4 m higher than the swale, so 10-14 m of clay dune sediment accumulated here. Taken together, the material retrieved from the cores is interpreted as evidence that the "proto-Soda Lake" that occupied the Carrizo Plain was much larger than had been suspected previously, that external drainage was probably lost during the Pleistocene (which has implications for the rate of deformation), and (on the basis on an OSL date obtained from the top of the outermost dune) that the lake became saline enough for clay dunes to form before 16.7 ka ago.