A PETROGRAPHIC ANALYSIS OF A PALEOSOL IN THE LAS VIRGENES SANDSTONE, SANTA MONICA MOUNTAINS

A paleosol in the Paleogene Las Virgenes Sandstone is exposed in Solstice Canyon, Santa Monica Mountains. It is approximately 8 meters thick and is divided into four zones. The lowermost Zone 1 is highly weathered sandstone. Zone 2 is a mottled interval, and Zone 3 is distinguished by pisolites. Zone 4 caps the paleosol and is characterized by silica replacement of pisolites and associated kaolinitic matrix. A petrographic analysis of the paleosol reveals that it contains mostly quartz and a matrix of kaolinite with lesser amounts of highly altered feldspars and biotite. Monocrystalline quartz displaying dissolution features such as etching and embayments dominates all zones. Polycrystalline quartz is less common probably due to disaggregation. Most feldspars and biotite are altered to kaolinite and the kaolinite matrix likely resulted from this alteration. Some feldspars display dissolution voids. Zone 1 consists of 59% matrix 35% quartz, 5% feldspar, and 1% biotite. Zone 1 detrital grains are coarse to very coarse, sub-angular, and float in the matrix. Zone 2 averages about 50% quartz, 43% matrix, 6% biotite, and 1% feldspar. Angular, matrix-supported, and granule-size grains dominate this zone. Zone 3 contains 60% matrix, 30% quartz, and 10% pisolites. Pisolites in Zone 3 range up to 6 mm in diameter and quartz grains in their cores average 0.7 mm in size. Zone 4 consists of 43% quartz, 32% matrix and 25% pisolites. Pisolites in this zone range up to 3cm in diameter and some contain very coarse grained quartz in the cores. Partially dissolved quartz grains in the cores of some pisolites indicate that they formed after the alteration of the original detrital components. Partially dissolved quartz, kaolinitized grains, and pisolites are commonly associated with soils formed in tropical to subtropical environments. The paleosol studied in the Santa Monica Mountains likely formed in a similar environment.