Stratigraphic investigation of the North Westside Basin, San Francisco, California

T. M. McGuire and K. Grove
Department of Geosciences, San Francisco State University

The Plio-Pleistocene Merced Formation is a vital groundwater resource used by the city of San Francisco. The unit is bounded to the southwest by the San Andreas fault, dissected by the Serra fault and composed of over 1700 meters of uplifted, marginal marine sediments deposited in a tectonically active basin.

In this study we integrate geophysical data, well cuttings, drillers’ logs, sequence facies data, bedrock surface geometry, global sea level and structural data to create geologic cross sections and a subsurface model of the North Westside Groundwater Basin.

Our model indicates that relevant aquifer-forming sequences were deposited in a beach-barrier system in which the shoreline trended more to the northwest than the current shoreline. Behind this shoreline, emabayments formed and produced fine-grained sediments that now form a series of aquitards that restrict flow and compartmentalize the aquifer.

The Merced strata in this location are uplifted as much as 240 meters where the blind Serra thrust fault has produced a northeast-vergent monoclinal fold. Sub-parallel, and to the east of the Serra fault, there is a fault or fold that uplifts strata, but much less than does the Serra fault. The Serra fault, and this eastern fault or fold, deform aquifer-forming strata into a northwest-southeast trending, asymmetric syncline whose limbs open to the northwest. Based on our model, the stratigraphy and structure in this part of the basin form a northwestsoutheast- trending, compartmentalized, bowl-shaped aquifer that becomes less laterally restricted and more vertically homogenous to the north and northeast within the study area.

AAPG Search and Discover Article #90087 © 2008 AAPG/SEG Student Expo, Houston, Texas