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Description
The variably thick, Pleistocene Bautista formation is scattered throughout the San Jacinto fault zone. Based on published data, the Bautista beds were derived from eastern sources, after termination of Pliocene movement on the Western Salton Detachment, and during the Pleistocene development of the San Jacinto fault zone. A principal goal of fault zone architectural studies is to understand fully what happens to rocks as they become incorporated into the damage zone lying adjacent to fault cores. Unfortunately, the petrology of sandstones within the Bautista formation has never been studied. To mitigate against this deficiency in data, I undertook a detailed study of sandstones in the Bautista formation at the Hog Lake locality. At the Hog Lake locality, a SW verging thrust fault appears to merge off the main trace of the San Jacinto fault. It places, high grade metamorphic rocks of the pre-mid Cretaceous Burnt Valley complex of the hanging wall block, over pebbly to cobbly sandstones of the Bautista formation in the footwall. Sixteen samples of sandstone were collected across the exposed outcrop belt of the Bautista and were thin sectioned and point counted. Each thin section was chemically stained to distinguish between plagioclase and K-feldspar. The Gazzi-Dickinson point-count method was used, and 300 points at a spacing of 1 mm were identified and tabulated. The results of this exercise indicate that sandstones at the Hog Lake locality are poorly to very poorly sorted and are composed of a framework population that is dominated by very angular to subround crystal and rock fragments. On the classification scheme of W.R. Dickinson, all samples plot in the feldspathic sandstone field. Not surprisingly, on the QFL provenance-discrimination diagram developed by W.R. Dickinson and students in 1983, samples spread about the boundary separating the basement uplift from transitional continental block fields. In thin section, plutonic micro-phaneritic rock fragments are significantly more abundant than are metamorphic aphanitic rock fragments. The latter commonly contain sillimanite ± K-feldspar. Plagioclase to total feldspar ratios when compared to standard ranges for different plutonic rock types indicate a predominantly granodioritic source. Tonalitic and high-grade metamorphic rocks contributed lesser amounts of sandsized detritus. Porosity was determined from grain and bulk density measurements. Outside the zone of damaged rock adjacent to the overthrust Burnt Valley Complex, porosity varies from ~15%-30%.
