MAPPING THE SAN DIEGO FORMATION NEAR CHULA VISTA, CALIFORNIA

Deep monitoring wells have been helpful with mapping the San Diego Formation in the subsurface near Chula Vista, California. In particular, the contact between the lower marine part of the San Diego Formation and the underlying Otay Formation is well defined by changes in lithology and borehole geophysical logs. The contact occurs at the bottom of a coarse-grained basal sand about 10-feet thick, which is coincident with a cool deflection in a geothermal log for a well (SDOT) near San Diego Bay. This contact is exposed in outcrops east of the La Nacion Fault Zone, and occurs at depths of about 500 to 600 feet below land surface west of the fault zone. The inferred vertical offset across the fault zone is about 900 feet.