SEISMIC REFLECTION DATA FROM LAKE ELSINORE, SOUTHERN CA, DETAILS A NEW CLIMATE DRIVEN RECORD FOR HOLOCENE LAKE LEVEL VARIATIONS

Lake Elsinore, located 120 kilometers southeast of Los Angeles, California, is Southern California's largest natural lake. In December 2008, we collected over 75km of high-resolution seismic reflection data using a CHIRP single-channel seismic-reflection Edgetech Geostar and the Geopulse™ Boomer systems. Interpretations reveal several interesting seismic sequences over the past 10,000 calendar years before present (cy BP: age control is based on dated sediment cores tied to the seismic lines). Three distinct sediment packages were identified from their erosional boundaries. The lowest sequence (I) is subdivided into three subsequences (a,b,c) and is composed of reflectors that are wavy, show little horizontally continuity and low frequency. Overall, sequence I is interpreted to represent short-lived lake level regressions and transgressions. Only subsequence Ic, however, shows evidence for a sustained lake level regression in the form of prominent truncated stratal terminations at the upper boundary. This interval is interpreted as a lake level lowstand bracketed between ~ 3,000 and 1,800 cy BP. Sediment core data in the form of mudcracks, decreased average grain size, and high delta-18O(calcite) data from a range of depositional environments (i.e., littoral and profundal) within Lake Elsinore provide additional support for Sequence Ic as a sustained lowstand. Regionally, there is evidence for a similar interval of dry climate at Tulare Lake, the Mojave lakes, the Salton Sea, and possibly Dry Lake. The middle sequence (II) has extremely continuous, parallel, low frequency reflectors and is interpreted as a rise in base level and subsequent high lake level. The upper seismic sequence (III) shows weak, horizontal reflectors, which onlap onto the upper boundary of sequence II and is interpreted as a lake lowstand known in the historic record as the 1950's lowstand.