A reconnaissance study of the San Diego, La Jolla, and western one-thirds of the El Cajon and Jamul 15-minute quadrangles was conducted to determine the depth to basement, using gravity meter, available well data, and surface geology. A Worden gravimeter was used to occupy 368 stations with ½-mile spacing; drift, latitude, and elevation corrections were made; basement-sediment density contrasts range from 0.3 to 0.5 mgals.

Geologic units and gravity contours trend north-northwest as do the Peninsular ranges. Anomalies over areas underlain by batholithic rocks range from -6 to -26 mgals. Irregularities and small closures occur along the belt of dense (±2.85 gm./cc.) Santiago Peak

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volcanic rocks. At the south end of Diego Bay, a gravity anomaly of -36 mgals. and well data indicate the presence of a sedimentary basin ±6,000 feet deep. A +4 mgal. anomaly at Point Loma and near-zero anomalies at La Jolla reflect a positive westerly gradient.

At the surface, Santiago Peak volcanic rocks, a discontinuously exposed belt of Upper Jurassic and Lower Cretaceous(?) meta-volcanic and meta-volcaniclastic rocks, roughly separate mid-Cretaceous batholithic rocks at the northeast from Upper Cretaceous, Eocene, and Pliocene clastic sedimentary rocks at the southwest. The Campanian-Maestrichtian Rosario Formation crops out at La Jolla and Point Loma. Most surface exposures of undifferentiated Eocene rocks are north of Mission Valley. At the south, the Pliocene San Diego Formation overlaps the Eocene.

An irregular basement surface (batholith and older) dips west; it is elevated slightly under Point Loma and flattened under La Jolla. The Rosario Formation reaches a maximum thickness of ±4,000 feet at La Jolla and Point Loma. Undifferentiated Eocene sedimentary rocks attain a maximum thickness of ±2,500 feet south of San Diego Bay where they are overlain by more than 2,000 feet of the San Diego Formation.

Four distinct post-batholith structural blocks are delimited by an east-west Mission Valley hinge line and the north-south-trending Rose Canyon fault. The northeast stable block (Kearny mesa) received mostly Eocene sediments. The northwest block (La Jolla) and southwest block (Point Loma), separated by synclinal Mission Bay, received mostly Upper Cretaceous and Eocene sediments, and later were uplifted, faulted, and tilted. The southeast block (San Diego mesa) probably subsided continuously, receiving more than 2,000 feet of Upper Cretaceous, Eocene, and Pliocene sediments.

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