Abstract 51288 


Presented by Rhodes, Dallas D.. 
      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.