SYNCHRONOUS GEOMORPHIC RESPONSE TO HOLOCENE CLIMATE CHANGE IN THE CARRIZO PLAIN, CALIFORNIA

Paleoseismological and neotectonic studies of the San Andreas Fault in the Carrizo Plain, California provide an impetus to characterize landscape responses to Holocene climate change. Five independently dated and studied elements of the Carrizo Plain's geomorphic system display synchronous change following the end of Mid Holocene Thermal Maximum (~4,500 cal BP). Wallace Creek, small channels a few hundred meters southeast of Wallace Creek, and Phelan Creeks-about 1.6 km to the southeast (John D. Sims, unpublished data) have been extensively studied for paleoseismology by trench investigations and their history of channel abandonment and incision is well constrained with 14C dates. The fluvial systems drain to closed Soda Lake basin. Analysis of isotopic data from lake sediment cores provided a history of relative water depth. Mapping and thermal luminescence dating of the final geomorphic element--clay dunes developed along the edge of Soda Lake--further contribute to this record of landscape response to climate change.

By 3.3+/-0.2 ka (TL date), Soda Lake had become hypersaline and a 9.5-km long clay dune had grown along its eastern and southern limits. With the cooler and wetter climate that followed, the average lake level began to rise and reached a high stand (2.9-2.6 ka). During this same period, channel incision occurred at Wallace Creek (after 3.7 ka), the small channels nearby, and the Phelan Creeks (3.5-2.5 ka). With higher lake levels, the clay dune system was stabilized and underwent a period of erosion and soil formation. The average level of Soda Lake dropped precipitously at ~2.1 ka and then remained relatively low. This interval was characterized by stability in the fluvial systems. A short-lived slightly higher lake level occurred at the same time that incision occurs in the small channels (1.35 ka). The other drainages were stable. The average lake level remained low until the modern level was achieved at 0.8 ka. Incision occurred at Phelan Creeks about 0.5 ka. Most of the dune complex is now eroding. Fluvial, aeolian, and lacustrine processes of this closed system show synchronous response to climate change as recorded by channel incisions and dune stability during high lake level and channel stability and dune formation during low lake level.