EVIDENCE FOR A POSSIBLE TROPICAL TO SUBTROPICAL CLIMATE IN SOUTHERN CALIFORNIA DURING THE PALEOCENE: LATERITE PALEOSOLS OF THE SILVERADO FORMATION, SAN JOAQUIN HILLS, CA

The Paleocene Silverado Formation represents the base of the Tertiary sequence in the Santa Ana Mountains and San Joaquin Hills. A series of kaolinitic claybeds marks the transition from marine to non-marine sediments in the Silverado Formation exposed in San Joaquin Hills. These claybeds are identified as kaolinitic and lateritic paleosols developed in arkosic sandstones, and indicate a change in climate from arid to tropical occurred during the middle to late Paleocene.

Formation of tropical residual soils require the following: physical and chemical weathering, the leaching of insoluble materials and accumulation of insoluble residues, and the movement of fine particles downward. High precipitation rates found in humid tropical environments provide a nearly constant influx of water, and all minerals except quartz are leached out. The remaining Al and Si ions precipitate out as aluminum hydroxides, and further alter to kaolinite/gibbsite. Iron and aluminum oxides tend to remain in situ; hematite forms where the soil is subject to seasonal dry periods. When ferrallitic soils develop in quartz-rich rocks, kaolinite forms in subsurface horizons. Poor drainage within this kaolinitic horizon produces red and white mottling. Watertable fluctuations promote the development of a ferricrete crust.

The lowermost paleosol comprises four distinct beds and is best exposed in the northern part of the field area. These beds are, in stratigraphic order: a complete laterite sequence; a red and white mottled kaolinite horizon; a kaolinite horizon; and a pallid horizon. All units contain quartz in a kaolinitic matrix. These quartz grains are dissolved, etched and pitted; primarily monocrystalline; and disaggregation of polycrystalline quartz occurs along crystal boundaries. Hematite staining of the kaolinite matrix produces red to lavender mottles. Paleosol 1 grades laterally to the south into a saprolite. Mono- and polycrystalline quartz grains are partially dissolved, etched, and angular. Potassium feldspars are highly dissolved and are altering to kaolinite. Kaolinite rip-up clasts, poorly defined Fe-rich mottling and weak relict bedding are also present.