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Description
Near Hog Lake NE of Anza, California, the Pleistocene Bautista formation contains a vertebrate fauna consisting of horse, tapir, camel, antelope, ground sloth, and rabbit (Frick, 1921). This faunal assemblage is generally indicative of a Pleistocene climate that was cooler and moister than that existing today. Based on published paleocurrent and petrological studies, material making up the Bautista was derived mainly from eastern plutonic and metamorphic sources. These and other data indicate that the Bautista formation were deposited after Pliocene movement on the Western Salton Detachment had ceased, and as the San Jacinto fault zone developed. The clay mineralogy making up the < 2 μm sized fractions of sandstones in the Bautista formation is dominated by smectite, kaolinite, and illite, with a lesser component of mixed-layer illite/smectite. Given the feldspathic composition, poorly sorted, and generally angular nature of sand sized detritus in Bautista sandstones, it is likely that debris flows sampled during this study did not travel far. Hence, the clay mineralogy may reflect the overall Pleistocene weathering pattern within a moderately wet climate as sediment traversed the source to sink depositional system. Molar A, CN, and K data derived from chemical analysis of Bautista sandstones spread from the composition of unweathered granodiorite toward the field of unweathered high-grade metamorphic rocks analyzed from the Burnt Valley complex. Principal component 1 describes 99% of the variance in the Bautista sandstone data about the compositional linear trend. These results suggest that sandstones of the Bautista Formation were derived largely from granodiorite and variable, but generally lesser, amounts of high-grade metamorphic rock.
