Evidence for a Possible Tropical to Subtropical Climate in Southern California During the Paleocene: Laterite Paleosols of the Silverado Formation, San Joaquin Hills, CA
S. M. Leyva and P. C. Ramirez
Department of Geological Sciences, California State Univ, Los Angeles, Los Angeles, 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 nonmarine sediments in the Silverado Formation exposed in San Joaquin Hills. These claybeds are identified as kaolinitic and lateritic paleosol 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 promotes 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.
AAPG Search and Discovery Article #90904©2001 AAPG Pacific Section Meeting, Universal City, California