Silica-Cemented Terrace Edges, Central California Coast

counties (Torrent et al., 1980), and in soils formed in late The morphologic development of soils on marine terraces is a function of terrace age and geomorphic evolution. In this study, inter­ relationships between terrace landscape evolution and pedogenic sil­ ica cementation were investigated. Soil morphology, micromorphol­ ogy, selective dissolution, penetration resistance, and hydraulic conductivity were used to determine the nature and extent of the cementation and evaluate the effect of cementation on soil properties. Soils on the interior part of the terrace (coarse-loamy, mixed, thermic Typic Epiaquolls) contain plinthite, continuous throughout the entire terrace, cemented to the extent that it is only slowly permeable. A seasonally perched water table develops above the plinthite, and water seeps out ofsoils at the terrace edges. Some ofthese soils are cemented by opaline silica, in both channels and interstitial voids. The silica cementation extends inward into the terrace soils only 4 m, but at the terrace edge is strong enough to result in block fall as the predominant mechanism of scarp retreat. Redoximorphic Fe depletions are promi­ nent in deeper horizons at the edge, but are absent from the interior. These depletions are formed through seasonal saturation and Fe re­ duction, by water seeping laterally through preferential flow paths in and adjacent to shrinkage cracks above the regolith-bedrock contact. Contrasting morphology and chemistry between terrace interior and edge both result from, and cause, the interaction of pedogenic pro­ cesses and landscape evolution. Contrasts between interior and edge soils suggest that caution is essential in using edge exposures as repre­ sentative of soils under a geomorphic surface. I N ARID AND SEMIARID climates, silica cementation in some soils results in hard aggregates (durinodes) or horizons (duripans) that do not slake in water. In arid regions, silica cementation in soils is generally accompa­ nied by calcite (Chadwick et al., 1987; Boettinger and Southard, 1990), whereas in semiarid climates, silica ce­ mentation is often associated with Fe oxides (Torrent et al., 1980). Duripans are found in very old soils on intrusive igneous rocks and the sediments derived from them, or in young soils that contain rapidly weathering, siliceous volcanic ash (Flach et al., 1969) or siliceous loess (Blank and Fosberg, 1991 ). Silcretes are similar to duripans in that they are cemented with silica as a result of low-temperature, surface or near-surface pro­ cesses, but silcretes also include silica-cemented geo­ logic material, as well as soils (Bates and Jackson, 1984; Oilier, 1991 ), and are not associated with either calcite or Fe oxides (Oilier, 1991). Once formed, both silcretes and duripans influence water movement through rego­ lith (Torrent et al., 1980; Oilier, 1991 ), and ultimately affect landscape evolution (Selby, 1982; Oilier, 1991 ). On the California coast, duripans have been described in soils on marine terraces in San Diego and Orange L.E. Moody, Soil Science Dep .. California Polytechnic State Univ., San Luis Obispo, CA 93407: and R.C. Graham. Dep. of Soil and Environmental Sciences, Univ. of California, Riverside, CA 92521­ 0424. Contribution of the Dep. of Soil and Environmental Sciences, Univ. of California, Riverside. Received 2 l Jan. 1997. *Corresponding author ([email protected]). Published in Soil Sci. Soc. Am. J. 61:17231729 (1997). to mid-Pleistocene coastal sand dunes in Santa Barbara County (D.L. Johnson et al., 1988, unpublished data). Both of these sites have a semiarid climate. Otherwise, most studies of marine terrace soils on the Pacific Coast of North America do not mention silica cementation (e.g., Orme, 1973; Muhs, 1982; Merritts et al. , 1991; Bockheim et al., 1992), but silica cementation was ob­ served in soils at south-facing edges of a dissected ma­ rine terrace on the California coast (Moody and Gra­ ham, 1995). In this study, our objectives were to: (i) determine the nature and extent of silica cementation in the marine terrace soils, and (ii) measure its effect on soil properties. MATERIALS AND METHODS