Abstract: Development and Distribution of Lateritic Paleosols in the Paleogene Silverado Formation, Northern Peninsular Ranges, CA

LEYVA, S., California State University, Los Angeles, Los Angeles, CA; P. RAMIREZ, California State University, Los Angeles, Los Angeles, CA

We have identified two distinct paleosols exposed in the middle part of the Paleogene Silvarado Formation in the San Joaquin Hills. These paleosols, which were originally mapped as a single clay bed, occur in texturally immature arkoses and lithic arenites. Recent mapping of the lowermost paleosol has extended this unit an additional 1.5 miles to the south from the previously mapped location.

The lowermost paleosol (Paleosol 1) is comprise of four distinct laterite and kaolinitic beds and is best exposed in the northern part of the field area. These beds are, in stratigraphic order: a complete laterite sequence capped by a hematite-stained clayey sandstone, an incomplete laterite sequence comprised of a mottled zone and iron crust, followed by a kaoline layer and finally the lowermost portion of a laterite sequence (the pallid zone). In all units, dissolved, etched and pitted monocrystalline quartz grains float in a kaolinitic matrix. Disaggregation of polycrystalline quartz along crystal boundaries is evident. Within the laterite, hematite staining of the kaolinite matrix produces a mottled red to lavender effect.Paleosol 1 grades laterally to the south into a saprolite, comprised of partially dissolved, etched, angular, and predominantly monocrystalline quartz grains that float in a kaolinite-rich matrix. Kaolinite rip-up clasts, poorly defined Fe-rich mottling and weak relict bedding are also present. The second paleosol (Paleosol 2), located 30 feet stratigraphically above Paleosol 1, is not as well exposed and is traceable a few hundred feet before it is covered by alluvium and colluvium.

High precipitation rates associated with subtropical and temperate climates resulted in intense weathering and alteration of the arkose. Iron nodules, mottled red and white kaolinite zones, kaolinite-rich intervals and partially dissolved quartz grains, present in both paleosols, are consistent with laterite development. Feldspars and other labile constituents were altered to kaolinite, while quartz grains were partially etched and dissolved as intense leaching occurred.

AAPG Search and Discovery Article #90911©2000 AAPG Pacific Section and Western Region Society of Petroleum Engineers, Long Beach, California