Petrography of 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 inter-bedded blue gray sandstones, light brown mudstones, and granule-sized conglomerates. Some of the sandstones exhibit cross-bedding. Previous researchers concluded that Neroly sediments were deposited during the Miocene Epoch in shallow marine, shoreline and continental environments along the margins of a bay within a tectonically active setting to the east of modern San Francisco Bay. The source of the Neroly sediments is reported to have been volcanic rocks from the northern and central portions of the Sierra Nevada range. The purpose of this study was to develop a greater understanding of the provenance of the Neroly sediments. Samples of sandstone and mudstone were collected from three localities west of Tracy, California, and studied using X-ray Diffraction and petrographic microscopic analysis. X-ray diffraction analysis showed the presence of albite and cristobalite in the fine-grained samples. Petrographic sandstone analysis indicates that most of the mineral grains are angular plagioclase, K-rich feldspars, quartz, and clinopyroxene. Subrounded aphanitic and porphyritic rock fragments are also present. Phenocrysts in the porphyritic volcanics include plagioclase and pyroxene. Preliminary analysis of feldspar extinction angles reveals feldspars with medium to high anorthite content. This suite of clastic grains is consistent with intermediate to low silica volcanic source rock for the Neroly