QUATERNARY STRATIGRAPHY AND SOIL DEVELOPMENT, SAN CLEMENTE ISLAND, CALIFORNIA

Marine terraces are the most prominent geomorphic surfaces on San Clemente Island. Eight well developed terraces and two less distinct terraces have been mapped on the northern end of the island. The 1st, 2nd and 5th terraces have fossil mollusks which can be used for relative age assignments when amino acid ratios are compared. The 2nd terrace also has hydrocoralline material which has been dated using U-series techniques at 127 ka. This date, the amino acid ratios and an assumed rate of isoleucine epimerization derived from deep-sea foraminifera studies can be used to assign absolute ages to the 1st and 5th terraces; these are 60-107 ka and 345-475 ka, respectively. These terraces thus correlate with high sea level stands in the oxygen isotope record and with terraces near San Diego and on Barbados and New Guinea. The dates and estimates of sea level derived from other areas yield uplift rates on San Clemente Island of 0.19 to 0.26 m/1,000 yr, similar to other locations in southern California with strike-slip tectonics.;Evidence from filed observations, dust traps and mineralogy suggest that all soils on San Clemente Island have been influenced by airborne dustfall to one degree or another. Satellite imagery and field observations indicate that dust is blown offshore from the Mojave Desert during Santa Ana wind conditions.;The combination of eolian additions and sea spray influx results in rapid soil profile development. Typic Natrixeralfs can develop on andesitic alluvial fan deposits in less than 3,000 yr; these soils are also found on surfaces with ages of 60-107 ka and 127 ka, but are beginning to show vertic characteristics. On surfaces about 200 ka or older, Typic Chromoxererts are dominant. These observations suggest that Natrixeralfs develop into Chromoxererts after sufficient smectite clay has accumulated.;The dominant minerals in the clay fraction of both the Natrixeralfs and the Chromoxererts are smectite and mica. Mica is apparently of eolian origin and smectite accumulates by weathering of plagioclase and pyroxenes derived from the andesitic parent material and alteration of mica under neutral to akaline conditions. Decreasing internal drainage in Vertisols over time creates conditions especially favorable for the formation of trioctahedral smectite, and with time and deeper in the profiles, mica peaks on X-ray diffractograms become progressively broader and smectite peaks become progressively sharper.;Radiocarbon dating of fossil land snails in a paleosol underlying a cemented calcareous dune deposit suggests major dune deposition shortly after ca. 21,000 yr BP. Well-developed soils on dunes farther inland suggest major dune formation much earlier. All dunes probably developed during sea level minima, when calcareous shelf sands were freshly exposed.;Soils on gently sloping andesite bedrock exhibit differences related to slope position. Crest and shoulder soils are Lithic Haploxeralfs; the toeslope soil is a proto-Pelloxerert with higher clay content. The dominant processes appear to be controlled by movement of material downslope by overland flow and movement of basic cations in solution downslope by throughflow. These data suggest that the steady state condition exhibited by Vertisols on marine terraces ranging in age from about 200 ka to greater than 1,000 ka may cease if uplift of the island block continues, which would result in downcutting, stream integration and better drainage of the marine terraces.