Abstract 51288 CLAY DUNES AT SODA LAKE, CARRIZO PLAIN, CALIFORNIA Presented by Rhodes, Dallas D.. Authors: Arrowsmith, J Ramon, Kilpatrick, Jeffrey P., Eigenbrode, Jennifer. Key words: clay-dunes; Carrizo-Plain; California; climate-change In Session 56 Quaternary Geology and Geomorphology (Posters) Tuesday, 27-Oct-98 AM in Room: Hall-E at 8:00 AM for 240 minutes. Abstract: Clay dunes (lunettes) occur downwind of deflated pans in hypersaline environments around the world. In San Luis Obispo County, California, active clay dunes exist in the area surrounding Soda Lake, the sink for runoff from the 1,230 km2 Carrizo Plain drainage, the only closed basin in the Southern Coast Ranges. Internal drainage of the Carrizo Plain began during Plio-Pleistocene time when tectonic deformation associated with the San Andreas Fault defeated a stream that once drained the valley. An originally fresh to brackish water lake probably persisted through much of the Pleistocene during which coastal California was wetter and cooler than now. Diminished Holocene precipitation and a higher evaporation rate led to shrinkage of the ancestral lake and associated increased salinity which set the stage for clay dune formation. The Soda Lake complex consists of two large basins and at least 130 smaller pans. Water levels in the basins rise and fall seasonally. Following exceptionally wet winters (typically El Nino years) the large North and South Basins never dry completely, although the water retreats toward the center of the basin leaving a salt crust up to 20 cm thick. Most of the large and small pans are fringed by clay dunes. The largest dune bounds the eastern and southern edges of the North Basin which has a surface area of ~10.5 km2. This dune is up to 470 m wide, 16.7 m high, and nearly 9.5 km in length. The southern portion of the dune is active, receiving sediment from the mud flat exposed between the dune and the salt pan. Most of the eastern (north-south) leg of the dune, which is lower and narrower, is currently inactive. Westward retreat of the shoreline exposed the former lake flat to colonization and stabilization by salt-resistant plants. For this eastern leg of the dune to have formed, the lake level must have been about 3 m higher than at present; tectonic warping of the basin may account for the abandonment of the former shoreline.