The Geology of Las Trampas and Rocky Ridges, Alameda and Contra Costa Counties

The shallow-marine San Pablo Group in the hills east of San Francisco Bay, California, was deposited while the Pacific-North American plate margin was evolving from a convergent boundary to a transform margin at this latitude in middle to late Miocene time. Field research in the East Bay Hills has helped to confirm and refine our understanding of the depositional basin that formed within the evolving transform boundary. This research has also improved our understanding of the tectonics that subsequently deformed the basin fill. This study included detailed measurement of stratigraphic sections at scales of 1:1,200 and 1:120 and geologic mapping at a scale of 1: 12,000. The stratigraphic section in the study area suggests a depositional basin evolving through time from an open-marine environment to a fluvial environment. The Monterey Group of middle Miocene sedimentary rocks, which conformably underlies the San Pablo Group, probably represents open-marine shelf deposition. They are supplanted upsection by an apparently continuous shallowing-upward sequence of San Pablo Group rocks that may represent in turn, shoreface, estuarine and tidal flat deposition. Sedimentological and paleontological evidence suggests that the San Pablo Group was deposited in a protected marine embayment sheltered from the wind and wave effects of the open ocean possibly by a highland in the approximate location of the modern San Francisco Bay. The San Pablo Group rocks are overlain by fluvial deposits of the Contra Costa Group which were deposited after continued shallowing of the basin left the area emergent. The transition from open-shelfal conditions (Monterey Group) to a restricted, shallowing basin (San Pablo Group) is interpreted as the result of local tectonism driven by the evolving transform system. There was a net fall in global sea level throughout the Miocene (Haq et al. 1987), but the thick stratigraphic section of over 4,000 feet (1,220 m) of shallow-water deposits precludes shoaling simply due to falling sea level. The basin must have been subsiding to facilitate accumulation of the thick sequence of shallow-water deposits. Apparently, high sediment influx outpaced the rate of subsidence resulting in a net shallowing of the basin by infilling. San Pablo Group rocks -5 miles (8 km) east of the study area, in the foothills on the west side of Mount Diablo, may be correlative with the rocks in the study area. No attempt has been made in this study to palinspastically correct for crustal contraction in the area or for dextral strike-slip on the Calaveras fault, which lies between the two locations. Palinspastic paleogeographic reconstructions of the San Pablo depositional basin by Buising and Walker (1995) suggested a narrow elongate embayment trending roughly northeast. Narrow, elongate basins separated by local highlands have been documented in the southern California borderland where they formed within the evolving San Andreas transform (Crowell 1974), which suggests that the San Pablo basin may have had a similar origin. Geologic mapping at a scale of 1:12,000 has revealed several previously unmapped structures, which may improve our understanding of the structural evolution of the East Bay Hills. Brittle kinematic indicators found on previously unmapped faults on Rocky Ridge suggest dextral oblique-reverse separation toward - 025°. A previously unmapped fault, which daylights on the southwest slope of Rocky Ridge (herein named the "Rocky Ridge fault"), is interpreted as a southwest-vergent reverse fault, which may be part of a palm tree structure that includes the Bollinger fault. A previously unmapped structure on Las Trampas Ridge, interpreted as an antiformal syncline, suggests that the Las Trampas fault may be a more significant structure than previously recognized. The fault appears to separate the antiformal syncline from the upright Las Trampas anticline, suggesting that a large amount of intervening structure has been removed by fault displacement to juxtapose the two dissimilar structural domains. Detailed mapping in the study area has also clarified the stratigraphic position of a tuff bed within the Upper Briones Member in the Corduroy Hills syncline. The tuff has been dated as 11.72 Ma by chemical correlation with the Rainier Mesa tuff (Perkins et al. 1998), which suggests that deposition of the San Pablo Group may actually have begun in the middle Miocene, instead of the late Miocene as interpreted by previous researchers (Ham 1958, Wagner 1978). Analysis of the structures mapped suggests that the deformation of Las Trampas and Rocky Ridges may have resulted from a single evolving episode of transpression within the East Bay Hills block. The structures in the East Bay Hills must be late Miocene or younger in age because Contra Costa Group rocks deposited about 6.5 Ma are involved in the folding (Liniecki-Laporte and Andersen 1988). The structural grain of both ridges trends -325°, suggesting an initial period of purely contractile deformation directed -055°, essentially normal to the Hayward fault. This is interpreted as the result of the contractile component of transpression, driven from the west, acting alone within the East Bay Hills block because the shear component was accommodated by strike-slip on the Hayward fault. The original 325° structural grain appears to have been overprinted by a later period of deformation which produced several faults on Las Trampas Ridge that cut the earlier-formed structure. These faults have been compared with the strain ellipse model of Wilcox et al. (1973) that predicts the orientation of subsidiary structures that result from dextral strike-slip on a nearby master fault. By aligning the strain ellipse model with the strike of the nearby Calaveras fault as the master fault, all of the faults on Las Trampas Ridge that cut the 325° structural grain can be explained as subsidiary structures driven by dextral strike-slip on the Calaveras fault. The brittle kinematic indicators of dextral oblique-reverse separation, found on Rocky Ridge, may indicate overprinting of the original contractile structures by dextral shear, driven from the east, acting in concert with a fault-normal component of contraction. The interpretation of the faults on Las Trampas Ridge as subsidiary structures driven by the Calaveras fault strongly suggests that they may be active faults because the Calaveras fault is known to be an active strand of the San Andreas fault zone. The findings of this work indicate that San Pablo Group rocks in the East Bay Hills were deposited in a tectonically active environment probably driven by the coeval migration of the Mendocino triple junction past the latitude of the study area at about 12 Ma. The basin fill was subsequently deformed sometime after about 6 Main an evolving transpressional environment that initially resulted in pure contractile strain normal to the strike of the Hayward fault. The original structures that resulted from the purely contractile deformation were then apparently overprinted by a sequence of faulting driven by dextral strain on the Calaveras fault trend. It is uncertain whether these later structures preceded or followed rupture on the Calaveras fault, but their association with this active major fault implies that they have a potential for modern movement.