THE GEOLOGIC AND TECTONIC SIGNIFICANCE OF PLEISTOCENE GRAVELS AND RELATED DEPOSITS, SOUTHEAST BAY AREA FOOTHILLS, CALIFORNIA

Two distinct lithofacies of the Pleistocene Santa Clara Formation are located in the southeast Bay Area foothills from Fremont to San Jose. These lithofacies reveal clues as to the tectonic and sedimentary history of the region during the Pleistocene.

In the Fremont area, the Irvington lithofacies of the Santa Clara Formation was deposited in braided streams. The gravel of the Irvington lithofacies is similar to gravel in the coeval Upper Livermore Gravels, which is exposed 23 km east of the Irvington outcrop. Both units are composed primarily of Franciscan clasts from the central Diablo Range, with subordinate material from Tertiary sedimentary units. Franciscan clasts are mainly graywacke, chert, and vein quartz, and they include high-grade metamorphic clasts such as blueschist. Limited paleocurrent data from the Irvington lithofacies suggest a flow direction from east to west.

In the Milpitas and east San Jose foothills farther south, gravel clasts in a lithofacies of the Santa Clara Formation exposed along Piedmont Creek are mainly derived from the Miocene Claremont chert and the Cretaceous Berryessa sandstone. These gravels were also deposited in braided streams. This lithofacies is interbedded with a claystone unit to the north, in the Avalon area of Fremont that is exposed between the two gravel units. The claystone contains many gastropods and bivalves, and it is interpreted as a lacustrine deposit. The contact between the Irvington deposits and the lacustrine facies is not exposed, so the relationship between these two units is unclear.

The gravels represent fluvial systems with distinct source areas east of the Santa Clara Valley. The Irvington deposits contain sediment from a source in the Diablo Range east of the Calaveras fault, whereas the lithofacies of Piedmont Creek has a more local source in the area between the Calaveras and Hayward faults. The contrasting lithofacies may be coeval deposits that differ because they formed in different locations along the margin of the valley, or their difference could record evolution of the foothills east of the valley through time. In either case, the uplift of the area between the Calaveras and Hayward faults evidently occurred after early Pleistocene deposition of the Irvington lithofacies.