--> Abstract: Distribution of Cenozoic Deposits in the Southern San Francisco Bay Trough from Gravity and Magnetic Anomalies, by R. C. Jachens, A. Griscom, and C. W. Roberts; #90958 (1995).

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Abstract: Distribution of Cenozoic Deposits in the Southern San Francisco Bay Trough from Gravity and Magnetic Anomalies

R. C. Jachens, Andrew Griscom, C. W. Roberts

Analyses of gravity and magnetic anomalies help define the 3-D distribution of Cenozoic, possibly water-bearing, deposits in the southern San Francisco Bay trough. The upper crust south of the Coyote Hills is composed of Mesozoic basement rocks of the Franciscan Complex, Great Valley sequence, and Coast Range ophiolite which are overlain by Cenozoic deposits generally <300 m thick but reaching much greater thicknesses in two major basins. A 28 mGal gravity low centered over Campbell reflects an asymmetric basin that extends ~35 km along the base of the Santa Cruz Mountains from Palo Alto to a few km SE of Los Gatos. The basin is deepest (~2 km) on the SW near Saratoga, has a steep, probably fault controlled SW margin, and gradually shallows to the NE over a distance of ~10 km. Farther NE along the base of the Diablo Range, a major elongate gravity low (as large as 40 mGal) delineates a deep basin bounded by the Silver Creek Fault on the SW and by the Madrone Springs and Calaveras Faults on the NE. This basin extends >50 km from the intersection of the Silver Creek and Calaveras Faults to Fremont. The gravity and magnetic anomalies suggest that the southern half of the Silver Creek Fault dips SW, away from the basin but that the northern concealed part is nearly vertical. Parts of the faults bounding the NE margin may also dip steeply away from the basin. The Cenozoic fill in this basin may in places exceed 3-4 km on the basis of the gravity anomaly and seismic tomography by A. Michael. This estimate is uncertain because, above most of this volume of Cenozoic deposits, rocks of the Great Valley sequence and Coast Range ophiolite are exposed or inferred from magnetic anomalies to lie at shallow depth, indicating structural complexity.

AAPG Search and Discovery Article #90958©1995 AAPG Pacific Section Meeting, San Francisco, California