PROBABLE LATERITIC PALEOCENE PALEOSOLS OF SOUTHERN CALIFORNIA

One to ten meters thick Paleocene paleosols present in the Las Virgenes Sandstone, Santa Monica Mountains, and in the Silverado Formation of the Santa Ana Mountains and San Joaquin Hills likely mark a period of tropical to subtropical climate. The paleosols consist of white to mottled red and white kaolinitic zones containing free-floating etched and embayed quartz grains. Iron pisolites occur in paleosols in the Santa Monica and Santa Ana Mountains, while silicified pisolites occur in the Santa Monica Mountains and silicified wood fragments in the San Joaquin Hills. Locally occurring coal seams overlie or underlie the paleosols. The paleosols are interstratified with poorly to moderately sorted, coarse grained, subangular arkoses and litharenites that exhibit some fining upward successions and sedimentary structures consistent with deposition in a high energy fluvial to transitional marine depositional environments. Provenance studies indicate sandstone derivation from a rapidly uplifted and eroded largely plutonic highland. Intense weathering of deposited sediments during a period of relative stability in the depositional environment led to kaolinization of the feldspars, partial dissolution of the quartz grains and to iron pisolite formation through the mobilization and concentration of iron. Alteration and dissolution of framework grains resulted in increased concentration of silica in pore fluids and eventually to silicification of earlier formed iron pisolites and deposited wood fragments. The characteristics of Paleocene paleosols of southern California are similar to those of lateritic soils mostly characteristic of tropical to subtropical areas. Plant fossils in the Silverado Formation support a subtropical climate.