3 Pedogenic Processes in Thick Sand Deposits on a Marine Terrace, Central California

Pedological studies in thick sedimentary sequences are generally limited to the upper few meters. Field investigation of thick (~50 m) sand deposits on an emergent Pleistocene marine terrace in central California showed morphological differences between the solum at the surface and the deep regolith. Based on morphological and geochemical features, four units were identified within the regolith. Two zones of active pedogenesis occur within three of these units. The surficial unit is in Holocene sand deposits (mixed, thermic, Argic Xeropsamments), and has darkened A horizons, a slightly reddened subsoil, and incipient lamellae at the depth of wetting front in filtration. These lamellae have slightly more day and Fe oxides than the soil above. Mineral weathering is intense at the surface. The other zone of active pedogenesis is at the base of the regolith, where a lithologic discontinuity above the terrace plat form forms an aquitard, and throughflow occurs. Meteoric water percolates through thin regolith deposits above the shoreline angle, and at other locations on the terrace where sediment has been removed by erosion. Percolating water carries clay, organi