Complete Bouguer gravity and general geology of the Cape San Martin, Bryson, Piedras Blancas, and San Simeon quadrangles, California

Abstract

Complete Bouguer gravity coverage of 390 stations and general geologic mapping were compiled for the Cape San Martin, Bryson, Piedras Blancas, and San Simeon quadrangles, California. These quadrangles constitute a 30-by 30-minute rectangle covering approximately 600 square miles of land area, most of which is in the rugged Santa Lucia Range. Two distinct basement units underlie the map area. In the northeast part, the granitic-metamorphic Salinian block constitutes the basement. The eugeosynclinal Franciscan Formation, however, underlies most of the map area and here constitutes all but a small part of the Santa Lucia Range. The Nacimiento fault is commonly believed to separate these major basement blocks. Overlying both basement units is a sequence of Cretaceous and Tertiary marine deposits and the nonmarine Paso Robles Formation. Detailed gravity data indicate that the unserpentinized core of the Burro Mountain ultramafic body has a subsurface volume no greater than 1 to 2 cubic kilometers and extends no deeper than 2,000 to 3,000 feet. Aeromagnetic data seem to preclude a large volume of subsurface serpentinite. The major features defined by regional gravity data include: (1) a rather even gradient of 3 milligals per mile in the entire southern half of the area which probably reflects deep structure of the continental margin, (2) a 10-milligal high coincident with the topographic mass of the Santa Lucia Range which suggests a density of over 2.8 grams per cubic centimeter for this mass, (3) a broad gravity low associated with Lockwood Valley which suggests that the valley is underlain by as much as 7,000 feet of low density sediments, and (4) a conspicuous gravity gradient of up to 20 milligals per mile which cuts diagonally across the entire Bryson quadrangle and represents a fault which vertically displaces the basement surface 5,000 to 10,000 feet. The most significant structural feature of the map area, the contact between the Franciscan and Salinian basement blocks, shows little or no gravity expression.