Studies of soil chronosequences on marine terraces facilitate the use of terraces for tectonic and paleoclimatic interpretation. However, many areas on the California coast have received substantial eolian deposits after pedogenesis began, so do not qualify as chronosequences. These areas are worthy of study because they are widespread coastal landscape features, and they enable us to interpret pedogenic and geomorphic processes in sandy regolith. The objective of this study was to use soil and deep regolith morphology and chemistry to interpret pedogenic and geomorphic evolution on a sequence of four marine terraces, San Luis Obispo County, California. The terraces are numbered I, 2, 3, and 4, from oldest to youngest. The eroded terrace platforms have been tentatively dated at 560, 420 or 480,320, and 120 ka, respectively. The sand deposits on each platform may be considerably younger. The amount of land surface dissection and the depth of stream incision increase with increasing terrace age and elevation. Morphological features suggest that the soils (Xeropsamments on Terrace 4, Haploxerolls on Terrace 3) on the two youngest terraces are well drained, and their morphological development is typical of soils in eolian sand deposits. Morphological and chemical features of the basal regolith contrast with those of the soils. Clay, Fe oxides, and opaline silica were deposited by groundwater flow above the bedrock platform. Above the shoreline angle, where the deep regolith receives additional groundwater from higher terraces; redoximorphic features have developed. Gray mottles are larger, more common, and more distinct in the basal regolith of progressively older terraces. Erosion has removed much of the overburden on Terraces 2 and I, and soils (Epiaquolls on Terrace 2, Epiaqualfs on Terrace I) have developed in what was once deep regolith. The landscape, soils, and deep regolith show an evolution of processes, whereby the path and direction of water movement through the regolith is controlled first by terrace morphology and stratigraphy, then by the development of pedogenic features in the soil and deep regolith, and by terrace dissection.


Environmental Sciences



URL: https://digitalcommons.calpoly.edu/nrm_fac/54