QUATERNARY DEPOSITS OF THE SANTA CLARA VALLEY, CALIFORNIA

The Santa Clara Valley is underlain by 300-600 m of flat-lying alluvial deposits that accumulated during the past 1 Ma as the basin subsided. Estuarine influence during sea-level highstands did not penetrate far south of the present San Francisco Bay. Analysis is supported by 8 new 300-m wells with geophysical logs and cores, 28 km of high-resolution seismic reflection profiles, geophysical logs from preexisting wells, and drillers logs.

The base of the Quaternary section is a basin-wide unconformity that extends across a central basement high and the flanking Neogene Cupertino and Evergreen basins. This surface has local relief up to 200 m and is offset by the Silver Creek fault, but otherwise is regionally flat, based on reconstruction from wells, seismic profiles, and gravity inversion. Seismic reflection profiles and inter-well correlation indicate that sedimentary layering is relatively flat and horizontally continuous.

No age-diagnostic fossils have been found, but paleomagnetic study indicates that the 780-ka Bruhnes/Matuyama boundary lies at a depth of about 300 m. No gravels have been recognized in the subsurface with compositions similar to the various distinctive Plio-Quaternary gravels exposed around the basin margin, even in wells that reach bedrock, indicating that deposits older than about 1 Ma may be absent above the unconformity. Gravel in wells as far east as Coyote Creek contains abundant Franciscan greenstone derived from the southwest, indicating that the present pattern of large alluvial fans emerging from the southwest and smaller ones from the northeast separated by northwest-flowing axial drainage persisted as the section accumulated.

Detailed stratigraphy developed from geophysical logs and carefully described core defines 8 fining-upward sequences in the valley axis where erosive bases to each sequence are probable. Correlation of the sequence boundaries with the marine oxygen-isotope record back to stage 18 defines a regular subsidence curve of about 0.4 mm/yr. The base of the Holocene sequence, cored in 4 wells, is constrained between C14 ages of 12,400 and 32,850 CY. Picks of the base in geotechnical borings (Clahan and others, 2002) supports contouring that surface, and ongoing study of well data should permit contouring of older selected sequence boundaries.