Silica‐Cemented Terrace Edges, Central California Coast

Abstract The morphologic development of soils on marine terraces is a function of terrace age and geomorphic evolution. In this study, interrelationships between terrace landscape evolution and pedogenic silica cementation were investigated. Soil morphology, micromorphology, 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 of soils at the terrace edges. Some of these 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 prominent in deeper horizons at the edge, but are absent from the interior. These depletions are formed through seasonal saturation and Fe reduction, 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 processes and landscape evolution. Contrasts between interior and edge soils suggest that caution is essential in using edge exposures as representative of soils under a geomorphic surface.