IMPLICATIONS OF NEW CHEMICAL CORRELATIONS OF TEPHRA FOR DISPLACEMENT ALONG THE RODGERS CREEK AND HAYWARD FAULTS, NORTHERN SAN FRANCISCO BAY REGION, CALIFORNIA

Widespread deposits of fluvial, estuarine and marine sediments in the Coast Ranges of central California accumulated in the rapidly evolving San Andreas transform system during the Neogene. These deposits were derived from similar source areas, formed in similar depositional settings, and are time-transgressive, resulting in sediments of different ages, in many cases segmented by faulting, being mapped as the same unit. Chemical correlations of tephra interbedded with the sediments have become an indispensable tool in unraveling stratigraphic relationships of these units. In the northern San Francisco Bay region there are several such sedimentary units associated with the Sonoma Volcanics, including the Petaluma, Huichica, and the Glen Ellen formations. The Petaluma Formation is fluvial and estuarine, with minor marine intervals near its base. Its upper part is interbedded with the Sonoma Volcanics, and contains the informally named, 6.26 Ma Roblar tuff. The fluvial Glen Ellen overlies and in places interfingers with the youngest (3.2-2.5 Ma) Sonoma Volcanics as does the older, fluvial Huichica Formation. The Roblar tuff is present in sediments within all the blocks offset by the San Andreas Fault System. The outcrop pattern of the tuff suggests that 308 km right lateral offset has taken place across the system as a whole with about 18 to as much as 68 km of displacement across the Rodgers Creek (RCF) and Hayward faults. Older maps show strata mapped as Petaluma on both sides of the RCF which would appear to indicate little or no displacement along the RCF. However, recently analyzed tephra from these sediments are correlative with tephra found in the Glen Ellen (~ 3.3 Ma) and the Huichica (~4.7 Ma) formations. Thus the sediments shown on some maps as Petaluma formation east of the RCF are much younger than the Roblar bearing Petaluma Formation directly across the RCF. These results support the reconstructions based on the Roblar tuff as well as correlations of volcanic rocks across the Rodgers Creek and Hayward faults indicating displacements ranging from 18 km to 68 km.