PETROGRAPHY OF THE NEROLY FORMATION (MIOCENE), DIABLO RANGE, SAN JOAQUIN COUNTY, CALIFORNIA

The Neroly Formation, the upper unit of the San Pablo Group, crops out along the eastern slopes of the Diablo Range on the west side of the San Joaquin Valley in central California. In the study area, the Neroly consists of interbedded blue-grey sandstones, light-brown mudstones, and granule-sized conglomerates. Some of the sandstones exhibit cross-bedding. Previous studies concluded that Neroly sediments were derived from the northern and central Sierra Nevada, and deposited during the Miocene Epoch in fluvial and shallow marine environments along the margins of a bay within a tectonically active setting to the east of modern San Francisco Bay. Samples of sandstone and mudstone were collected from three localities west of Tracy, California, and studied using XRD and petrographic microscopic analysis. XRD analysis indicates the presence of plagioclase and cristobalite in the fine-grained samples. Based on petrographic point-count analysis the Neroly sandstones are volcanic litharenites. Subrounded clasts of plagioclase-rich andesite were the most abundant sand sized grains (79%). Individual mineral grains included quartz (17%) and trace amounts of plagioclase, clinopyroxene and microcline. Michel-Lévy analysis of plagioclase feldspars indicated Andesine as the dominant series member. This study confirms an andesitic volcanic source for the Neroly sediments. Analysis of lithic fragments suggest provenance from multiple volcanic flows.