ORIGIN OF PALEOGENE PALEOSOLS AND SANDSTONES EXPOSED IN THE SOUTHERN CALIFORNIA AREA

Paleogene paleosols, ranging up to 15 meters thick, occur in the Santa Monica Mountains and San Joaquin Hills of Southern California. Coarse grained, moderately sorted, subangular to subrounded arkoses and lithic arenites exhibiting large scale cross beds, channels lined with conglomerates, and wood fragments commonly enclose the paleosols and support a fluvial interpretation. Petrographic analysis of the sandstones and conglomerates suggests that they were derived from silicic plutonic and quartz-rich metamorphic rocks. Compositionally, the paleosols are dominated by mostly quartz and kaolinite, and conglomerate clasts composed of durable lithologies such as quartzites. Plutonic rock fragments are conspicuously absent in the paleosols. Intense weathering of the sandstones likely led to the kaolinization and/or dissolution of unstable minerals and rock fragments. Quartz grains also show evidence of dissolution. Paleosol rip-up clasts occur in the sandstone and were derived through the erosion of previously formed paleosols. Additionally, a quartz-rich sandstone overlying the paleosol in the San Joaquin Hills likely formed through the recycling of quartz grains derived from the paleosol. Our interpretation centers on the development of soils on exposed sand banks or bars of the Paleogene rivers during a warm climate which resulted in extreme chemical weathering of sediments. Relatively abundant wood fragments and coaly layers support a vegetated depositional environment. Shifting channels led to the erosion of previously formed paleosols.