Abstract: San Leandro Basin: A Filled Sedimentary Basin Beneath South San Francisco Bay

M. S. Marlow, R. J. Anima, P. R. Carlson, J. R. Childs, P. E. Hart, R. C. Jachens, D. M. Mann, R. W. Sliter

High-resolution seismic-reflection data show a filled sedimentary basin east of San Bruno Shoal in south San Francisco Bay. This basin is informally named San Leandro basin for the nearby city. Data were collected and digitally recorded using a 20 cu. in. airgun source and a 24-channel streamer with 6.25 in spacing between acoustic receivers. The seismic-reflection profiles reveal a major eastward-dipping erosional unconformity that is about 40 in deep (subbottom) near San Bruno Shoal, and reaches a maximum observed depth of 500 in about 6 km to the east. The age of the unconformity is unknown but may be late Cenozoic when these strata were subaerial. Except for possible fold and fault disruption beneath the shoal, the reflections overlying the unconformity are flat-lying whereas the reflections beneath the unconformity dip steeply to the east. The east-dipping reflections may be from folded Cenozoic(?) strata that were thrust to the west. The flat-lying section fills a shallow basin outlined by an elliptical residual gravity low that is centered east of the survey profiles and extends from Oakland International Airport on the northwest to nearly the Coyote Hills on the southeast. Although the flat-lying section thickens toward the center of the gravity low, we infer that the low is caused mainly by the deeper sedimentary section based on the spatial correlation between the east dipping reflections and the gravity anomaly. At the eastern end of the profiles, the east-dipping reflections extend to a depth of 800 m subbottom. The center of the gravity anom ly is about 2 km east of the end of the profile. Projecting the east-dipping reflections to the center of the gravity low suggests that the total section of flat-lying and dipping reflections in the basin may exceed 1000 m. We will present new reflection profiles collected in 1994 that are now being processed.

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