Abstract: Mixed-Provenance Sands in the Upper Miocene Contra Costa Group, Upper San Leandro Watershed and Cull Canyon, Alameda-Contra Costa Counties, CA

A. V. Buising

A suite of stratigraphically located samples from the Contra Costa Group (CCG) were thin-sectioned, stained for K-spar and point-counted (400 points/sample) using both Gazzi-Dickinson (for sand bulk composition) and "Indiana" (for detailed rock fragment data) techniques. Results suggest that the CCG records mixing of sediment derived from Sierran and Coast Range sources, rather than being exclusively Coast Range-derived as previous workers have suggested. Sands from the lower Contra Costa Group (~215 m above the base of the unit) plot in the recycled orogen field of Q_mFL_t and Q_tFL plots; point-count data from the overlying 1200 m of the CCG show intercalation of recycled orogen- and arc-provenance sands in packets 50-300 m thick. Sand angu arity precludes significant grain recycling. Both arc and recycled orogen sands are closely associated with Coast Range-derived coarse clasts in outcrop; some arc sands also include sand grains of probable Coast Range derivation. Coast Range coarse clasts include: ?Franciscan sandstone and chert; white vein quartz; Claremont Shale; and mafic volcanic rocks. Coarse clasts of possible Sierran origin include black chert, argillite, mica schist, and some volcanic and metavolcanic clasts. Coast Range-derived sand-grade rock fragments include Franciscan blueschist, sandstones and metasandstones as well as distinctive Miocene basalts from the Berkeley Hills. Sand-grade rock fragments of likely Sierran derivation include abundant quartz-rich, quartzo-feldspathic, and pelitic metamorphic rocks. A c sands of the CCG record periods when the bulk of the sediment reaching the Contra Costa Basin was derived from the Sierra; overall sand composition reflects the volumetric dominance of Sierran detritus, while the presence of Coast Range-derived material attests to local Coast Range uplift during Contra Costa time. Recycled orogen sands, derived from Franciscan-Great Valley-Coast Range ophiolite source rocks west of the Hayward Fault trend, record times when Sierran detritus was diverted from reaching the Contra Costa Basin, either by structurally controlled changes in the evolving landscape or by autocyclic processes.

AAPG Search and Discovery Article #90958©1995 AAPG Pacific Section Meeting, San Francisco, California