--> Abstract: Subsurface Geology, Neogene Tectonics, and Petroleum of the Southwestern Santa Clara Valley ("Silicon Valley"), California, by R. G. Stanley, R. C. Jachens, P. G. Lillis, R. J. McLaughlin, K. A. Kvenvolden, F. D. Hostettler, and L. B. Magoon; #90935 (1998).

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Abstract: Subsurface Geology, Neogene Tectonics, and Petroleum of the Southwestern Santa Clara Valley ("Silicon Valley"), California

STANLEY, R. G., R. C. JACHENS, P. G. LILLIS, R. J. McLAUGHLIN, K. A. KVENVOLDEN, F. D. HOSTETTLER, AND L. B. MAGOON, U.S. Geological Survey, Menlo Park, CA

The presence of a concealed, oil-bearing sedimentary basin beneath a densely populated part of the southern San Francisco Bay area is demonstrated by gravity anomalies, records from historical oil wells, outcrop geology, and new organic geochemical results. An isostatic gravity low that extends about 35 km from Los Gatos to Palo Alto can be modeled as an asymmetric, northwesttrending body of low-density strata, principally of Miocene and younger age, that rest on higher-density rocks of the Franciscan Complex. The thickest basin fill (about 3 km) occurs along the basin's steep southwestern margin, which may be controlled by buried, northeast-dipping normal faults that were active during an episode of Miocene sedimentation, volcanism, and crustal extension related to passage of the northwest-migrating Mendocino triple junction. During the Pliocene and Quaternary, the Miocene rocks and faults were deformed by movement along northeastvergent reverse and dextral(?) faults of the Berrocal, Shannon, and Monte Vista fault zones. This deformation was accompanied by uplift of the adjacent Santa Cruz Mountains and is attributed to compression caused by the combined effects of a local restraining bend in the San Andreas fault and regional transpression along the Pacific-North American plate boundary. About 20 exploratory wells have been drilled in the area since 1891; oil and gas were found in the Miocene Monterey Formation, but commercial production was never established. Oil from the Peck well in Los Gatos is highly biodegraded and contains biomarkers and carbon isotopes suggesting derivation from Monterey source rocks. Preliminary calculations suggest that roughly a billion barrels of oil may have been generated from the Monterey in the area between Los Gatos and Cupertino. Some of this oil may have migrated into anticlinal and fault traps along the complicated structural boundary between the Santa Clara Valley and Santa Cruz Mountains.

AAPG Search and Discovery Article #90935©1998 AAPG Pacific Section Meeting, Ventura, California