NEW AGE CONSTRAINTS ON THE LATE MIOCENE ORINDA FORMATION IN THE SOUTHERN CONTRA COSTA BASIN

The southern portion of the Contra Costa Basin (CCB) is a narrow fault bounded Miocene transform basin that stretches from the hills of Hayward to San Jose, California. This portion of the basin is bounded to the west by the Hayward and Chabot faults and the Mission and Calaveras faults to the east. It resides in the core of a faulted syncline and overlies the marine Briones formation. It consists of fluvial and alluvial sandstone, shale, and conglomerate most commonly arranged in fining upward sequences. The Orinda formation is the basal unit of the nonmarine CCB, which is much more extensive to the north in the Berkeley Hills. Clasts consist of Franciscan chert, graywacke, greenstone, Great Valley Sequence sandstone, and varying amounts of reworked volcanic debris. Notably lacking are Franciscan metamorphic clasts which are common in the type section. Unlike other portions of the CCB, this area is rich in interbedded volcanic units. Volcanic units range from basalt to rhyolite and were deposited as ash flows and pyroclastic surges. New Ar/Ar age dating of the volcanics in this section yields an age of 10.84±0.01 Ma on sanidine near the base and 10.46±0.08 Ma on plagioclase near the top of the section.