California Division of Mines and Geology Bulletin 202 Geology of the Point Reyes Peninsula, Marin County, California

The gravity field in the Point Reyes Peninsula has been described by Jones (1963), Clement (1965), Chapman (1966), and Chapman and Bishop (1968).

Clement's Bouguer gravity map on a scale of 1:125,000 covers the entire Point Reyes Peninsula. The San Andreas fault zone has no characteristic gravity anomaly associated with it, apart from effects due to contrasting densities of the surface rocks on either side.

Chapman and Bishop's Bouguer gravity map, scale 1:250,000, is superimposed on the San Francisco sheet of the Geologic Map of California. The area covered is limited on the north by latitude 38° N. which passes through Point Reyes. The scale permits the inclusion of much data obtained in the Pacific Ocean. The authors point out that "a steep gravity gradient south of Point Reyes may represent an east-trending fault that bounds the granitic rocks of Point Reyes on the south". This conclusion is supported by the geomorphology of the area.

A broad negative anomaly with an amplitude of about 50 milligals extends to the southeast from Point Reyes into the La Honda Basin. It could be caused by a prism of sedimentary rocks about 2.5 miles thick with an assumed density contrast of 0.3 grams per cubic centimeter (g/cm³), according to Ginscom (1966).

The available magnetic data for northern California adds little to the information obtained from the gravity data.

According to Griscom (1966), "the frequent occurrence of magnetic highs over the San Andreas fault suggests that serpentinite may be more common in the fault at shallow depth than is indicated by geologic mapping". The frequent occurrence of serpentine and related rocks in the San Andreas fault zone of the Point Reyes Peninsula was noted during the present study, but no magnetic data have been published covering this area.

The U.S. Geological Survey published a map (GP—483) entitled "Natural gamma aeroradioactivity map of parts of the San Francisco region, California," by Kenneth G. Brooks (1965). This natural aeroradioactivity is distinguished from cosmic radiation and radiation from radionuclides in the air. It comes from the upper few inches of the ground and is highest over igneous rocks or soils derived from igneous rocks and lowest over marsh areas due to the masking effect of water.

The radioactivity measured in the Point Reyes Peninsula was moderately low, varying from 400 to 500 counts per second (cps) in the southern part of the San Andreas fault zone near Bolinas to 500 to 700 cps over the granitic rocks of Tomales Point. Only the fault zone and Tomales Point were covered in the survey. These figures can be compared with readings of 800 to 1000 cps over Mount Diablo, and 300 to 500 cps over the San Francisco Peninsula.

Conclusions derived from the geophysical work briefly are as follows:

(1) The San Andreas fault zone has no characteristic gravity anomaly associated with it.

(2) An east-west trending fault in the ocean south of Point Reyes is indicated.

(3) The continuation of the Point Reyes sedimentary basin into the La Honda basin is confirmed.

(4) The occurrence of serpentinite may be more common in the San Andreas fault zone than has been recognized from geologic mapping.