Reconnaissance geologic map of Pauma Valley Area, Pala and Boucher Hill Quadrangles, San Diego County, California: Supplement 1 from "A brief study of the geology and ground water conditions in the Pauma Valley area, San Diego County, California" (Thesis)

Pauma Valley is located in a widening of the San Luis Rey River Valley six miles southeast of Pala in northern San Diego County, California. The area mapped covers approximately 25 square miles. Groundwater investigation was confined to an area of about 15 square miles in the San Luis Rey River Valley and in tributary valleys. Rocks of the Southern California batholith of Cretaceous age and Triassic meta-sediments are exposed throughout the area except along the floors of the stream valleys and alluvial fans, where Quaternary alluvial sediments have been deposited. The crystalline rock types present include gabbro, tonalite, and granodiorite. Pegmatite dikes occur in the gabbros and less commonly in the other igneous rocks. The Elsinore fault zone crosses the northeastern portion of the area. The exposed rocks are distorted in an area as much as one mile wide along the fault zone. The Agua Tibia Mountains were raised about 4000 feet along the fault zone during Quaternary time. Here the Quaternary movement along the Elsinore fault zone was largely of a reverse nature, with some strike-slip motion. Three alluvial fan deposits are present in the San Luis Rey River Valley. One of these, the Agua Tibia fan, was deposited so rapidly that the river was dammed and a lake formed upstream in the river valley. Ground-water is produced chiefly from the flood plain deposits of the San Luis Rey River. The deposits of the Rincon alluvial fan have yielded water in satisfactory amounts, but the Pauma Creek alluvial fan materials should be capable of more than their present groundwater production. The deposits of the Agua Tibia alluvial fan are poorly sorted and relatively impermeable, and do not appear to constitute an attractive aquifer. Ground-water in significant quantity may be moving laterally along the Elsinore fault zone. Locally, the ground-water table was lowered as much as 13 feet in the San Luis Rey River Valley during the period 1940-1950. However, the water supply in the lower parts of the river valley will remain adequate unless ground-water production is increased markedly