Piedras Blancas and San Simeon Quadrangles California

GEOPHYSICAL FIELD INVESTIGATIONS
COMPLETE BOUGUER GRAVITY AND GENERAL GEOLOGT
OF THE CAPE SAN MARTIN, BRYSON, PIEDRAS BLANCAS,
AND SAN SIMEON QUADRANGLES, CALIFORNIA
By STEPHEN H. BURGH
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.
obtained for the Burro Mountain ultramafic body,
the regional coverage was collected as part of a broader
survey of the San Luis Obispo 1:250,000 Army M&p
Service gravity sheet. The gravity map is overprinted
on a generalized geologic map compiled from sever*!
sources.
The four quadrangles constituting the map area (fig.
1) form a 30- by 30-minute block covering approximately
600 square miles of land area, most of which is
in the rugged Santa Lucia Eange. The area extends
INTRODUCTION
This report presents and interprets a 2-mgal (milligal)
complete Bouguer gravity map of the Cape San
Martin, Bryson, Piedras Blancas, and San Simeon
quadrangles, California. Detailed gravity coverage was
FIGURE 1. Location of the Cape San Martin, Bryson,
Piedras Blancas, and San Simeon quadrangles, California.
1, area described by Burch and Durham (1970) ;
2, area described by Hanna, Burch, and Dibblee (1971).
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