Mineral Resources of the Pinnacles Wilderness 

Contiguous Wilderness Study Area, Monterey 

and San Benito Counties, California

U.S. GEOLOGICAL SURVEY BULLETIN 1705-C

' VK-..

Chapter C

Mineral Resources of the 

Pinnacles Wilderness 

Contiguous Wilderness Study Area, 

Monterey and San Benito Counties, California

By STEVE LUDINGTON and KAREN GRAY 

U.S. Geological Survey

LUCIA KUIZON 

U.S. Bureau of Mines

U.S. GEOLOGICAL SURVEY BULLETIN 1705

MINERAL RESOURCES OF WILDERNESS STUDY AREAS: 

SOUTH-CENTRAL CALIFORNIA
DEPARTMENT OF THE INTERIOR 

DONALD PAUL MODEL, Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1987

For sale by the

Books and Open-File Reports Section

U.S. Geological Survey

Federal Center, Box 25425

Denver, CO 80225

Library of Congress Cataloging-in-Publication Data

Ludington, Steve.

Mineral resources of the Pinnacles Wilderness Contiguous

Wilderness Study Area, Monterey and San Benito Counties,

California.

U.S. Geological Survey Bulletin 1705-C

Bibliography

Supt. of Docs. No.: I 19.3:1705-C

1. Mines and mineral resources California Pinnacles

Wilderness Contiguous Wilderness. 2. Pinnacles Wilderness

Contiguous Wilderness (Calif.) I. Gray, Karen. II. Kuizon,

Lucia. III. Title. IV. Series.

QE75.B9 No. 1705-C 557.3s 87-600298 

[553'.09794'76J
STUDIES RELATED TO WILDERNESS

Bureau of Land Management Wilderness Study Areas

The Federal Land Policy and Management Act (Public Law 94-579, October 21, 

1976) requires the U.S. Geological Survey and the U.S. Bureau of Mines to conduct 

mineral surveys on certain areas to determine the mineral values, if any, that may be 

present. Results must be made available to the public and be submitted to the President 

and the Congress. This report presents the results of a mineral survey of part of the 

Pinnacles Wilderness Contiguous Wilderness Study Area (CA-040-303), Monterey and San 

Benito Counties, California.

CONTENTS

Summary Cl

Abstract 1

Character and setting 1

Identified resources and resource potential 1 

Introduction 1

Location, access, and geography 2

Previous and present investigations 2

Geologic setting 3 

Appraisal of identified resources 3

Mineral-related activity 3

Identified resources 3 

Assessment of mineral resource potential 5

Geology 5

Geochemistry 5

Mineral and energy resources 6 

References cited 6 

Appendixes

Definition of levels of mineral resource potential and certainty 

of assessment 11

Resource/reserve classification 12

Geologic time chart 13

FIGURES

1. Index map showing location of the Pinnacles Wilderness Contiguous Wilderness 

Study Area, Monterey and San Benito Counties, California C2

2. Map showing mineral resource potential and generalized geology of the

Pinnacles Wilderness Contiguous Wilderness Study Area, Monterey and San Benito 

Counties, California 4

MINERAL RESOURCES OF WILDERNESS STUDY AREAS: 

SOUTH-CENTRAL CALIFORNIA

Mineral Resources of the 

Pinnacles Wilderness 

Contiguous Wilderness Study Area, 

Monterey and San Benito Counties, California

By Steve Ludington and Karen Gray 

U.S. Geological Survey

Lucia Kuizon

U.S. Bureau of Mines

SUMMARY 

Abstract

In 1984 and 1985, at the request of the Bureau 

of Land Management, the U.S. Geological Survey and 

the U.S. Bureau of Mines conducted surveys to assess 

the mineral resources (known) and mineral resource 

potential (undiscovered) of 2,ZOO acres of the 5,838- 

acre Pinnacles Wilderness Contiguous Wilderness Study 

Area (CA-040-303) in the southern Gabilan Range near 

Hollister, Calif.. In this report, the area studied is 

referred to as "the wilderness study area", or simply 

"the study area." No active mines, prospects, or 

identified mineral or energy resources were identified 

in the study area. Two areas were determined to have 

low potential for mineral resources. One of these 

areas has low potential for gold, and silver resources. 

The other has low potential for diatomite and oil and 

gas resources.

Character and Setting

The study area consists of five parcels with a 

total area of 2,200 acres located about 30 mi south of 

Hollister, Calif, (fig. 1). These parcels are adjacent to 

and contiguous with the Pinnacles National 

Monument. The study area is located in the southern 

Gabilan Range, part of the Coast Ranges physiographic 

province. The oldest rocks in the study area are 

Cretaceous (144-66 million years before present, Ma) 

granitic rocks, which are cut by a graben (downdropped fault block) that preserves Miocene (24-5 Ma)

volcanic rocks (fig. 2) (see appendix for geologic time 

chart). Overlying the volcanics are Miocene marine 

and continental sedimentary rocks, including units 

formed by erosion of the older igneous rocks. 

Although there has been some mining activity in the 

area in the past, there is no present mining or 

exploration activity.

Identified Resources and Mineral Resource Potential

There are no active mines, prospects, or 

identified mineral or energy resources in the study 

area. Two areas have mineral or energy resource 

potential (fig. 2). That part of the study area that 

overlies the source area of the volcanic rocks, 

represented by a swarm of rhyolitic dikes, has low 

potential for gold and silver in hot-spring deposits. 

Those areas underlain by Miocene sedimentary rocks 

have low potential for diatomite in shale deposits and 

oil and gas resources.

INTRODUCTION

This mineral resource study is a joint effort by 

the U.S. Geological Survey and the U.S. Bureau of 

Mines. The history and philosophy of such joint 

mineral surveys of U.S. Bureau of Land Management 

Wilderness Study Areas were discussed by Beikman and 

others (1983). Mineral assessment methodology and 

terminology were discussed by Goudarzi (1984). 

Identified resources are classified according to the

C1
121*30' 121'15'

PINNACLES WILDERNESS

CONTIGUOUS

WILDERNESS STUDY AREA 

(CA-040-303)

Figure 1. Index map showing location of the Pinnades Wilderness Contiguous Wilderness Study Area, Monterey and San Benito Counties, 

California.

system described by U.S Bureau of Mines and U.S. 

Geological Survey (1980). See appendixes for the 

definition of levels of mineral resource potential, 

certainty of assessment, and classification of 

identified resources. Studies by the U.S. Geological 

Survey are designed to provide a reasonable scientific 

basis for assessing the potential for undiscovered 

mineral resources by determining geologic units and 

structures, possible environments of mineral 

deposition, presence of geochemical and geophysical 

anomalies, and applicable ore-deposit models. The 

U.S. Bureau of Mines evaluates identified resources at 

individual mines and known mineralized areas by 

collecting data on current and past mining activities 

and through field examination of mines, prospects, 

claims, and mineralized areas.

Location, Access, and Geography

The Pinnacles Wilderness Contiguous Wilderness 

Study Area (CA-040-303) is located in west-central 

San Benito County and east-central Monterey County, 

about 30 mi south of Hollister, Calif, (fig. 1). At the 

request of the Bureau of Land Management, five 

parcels totalling 2,200 acres adjacent to the Pinnacles 

National Monument were studied. In this report, the 

area studied is referred to as "the study area."

The study area is in the southern Gabilan 

Range, which is part of the Coast Ranges 

physiographic province. Access on the north is via the 

county-maintained gravel road (Gloria Road) in

Bickmore Canyon that connects Gonzales with 

California Highway 25 and by private dirt roads and 

fire trails from the Gloria Road and California 

Highway 25. On the east and west, access is via 

California Highways 25 and 146 and private dirt roads 

that lead from them. Access from the south is poor; 

the nearest road is a county-maintained gravel road in 

the Topo Valley, 3 mi south of the study area.

Previous and Present Investigations

The geology of the Pinnacles National 

Monument and vicinity was first discussed by 

Fairbanks (1894). Andrews (1936) published the first 

detailed geologic map and description of the area. 

Subsequent geologic studies include Wilson (1943), 

Hinds (1952), Jennings and Strand (1959), Dibblee 

(1971, 1975), Ross (1972a, b), and Matthews (1976a, 

b). Descriptions of mines and prospects in or near the 

area were reported by Crawford (1894), Bradley and 

Logan (1919), Boalich (1922), Andrews (1936), Wilson 

(1943), Eric (1948), Butler and others (1962), Hart

(1966), and Smith and others (1971). Geohydrology of 

the area was studied by Evenson (1962) and Akers

(1967).

U.S. Bureau of Mines personnel examined the 

study area in May 1984 (Kuizon, 1985). Nine rock 

samples and five alluvium samples were collected. 

Rock samples were fire assayed for gold and silver, 

and analyzed spectrographically for 40 elements at the

C2
Bureau of Mines' Reno Research Center laboratory. 

Alluvium samples were concentrated and checked for 

gold and other heavy metals. Diatomite samples were 

evaluated with a petrographic microscope. Detailed 

information is available from the U.S. Bureau of 

Mines, Western Field Operations Center, E. 360 Third 

Avenue, Spokane, WA 99202.

U.S. Geological Survey personnel examined the 

study area in May of 1985. Existing geologic maps 

were field checked and 40 rock samples were 

collected. Twenty-eight samples were analyzed using 

energy-dispersive X-ray fluorescence methods for 

manganese, iron, copper, zinc, gallium, arsenic, 

rubidium, strontium, yttrium, zirconium, niobium, 

molybdenum, tin, barium, lanthanum, and cerium. 

Slabs of rock samples were cut in order to study their 

textures.

Geologic Setting

The study area is underlain predominantly by 

Cretaceous igneous rocks. Cutting this basement is a 

graben where a series of chiefly rhyolitic rocks 

(Pinnacles volcanic field) are preserved. The Pinnacles 

and Chalone Creek faults form the west and east 

boundaries, respectively, of the graben. Small 

erosional remnants of the volcanic rocks outside the 

graben are found in the northern and southern parts of 

the study area. A northwest-trending swarm of 

rhyolite dikes, presumably the feeders to the volcanic 

field, is present in the northwestern and southeastern 

parts of the area. Tertiary sedimentary rocks, younger 

than the volcanic rocks, are found in the northern part 

of the area and include coarse, continental sediments 

and the marine Miocene Monterey Formation. 

Younger gravels overlie rocks in the southern part of 

the study area.

Acknowledgments

Harry W. Campbell, mining engineer, assisted in 

work done by the U.S. Bureau of Mines. The 

coordination with landowners, including arrangements 

for access, was aided by the U.S. Bureau of Land 

Management and the National Park Service; this was 

greatly appreciated.

We thank personnel of the Hollister office of 

the Bureau of Land Management and the Pinnacles 

National Monument staff of the National Park Service 

for their cooperation. Several private landowners 

provided access to parts of the study area.

Defiance mines (fig. 2) in what is now the Pinnacles 

National Monument. Although the mine was not 

examined, flows and dikes sampled in this study did not 

contain detectable gold or any other significant metal 

values. A quartz pegmatite lens contained 0.03 oz/ton 

gold. No other pegmatite occurrences were found.

The Copper Mountain mine is located near the 

west boundary of Pinnacles National Monument. 

Workings consist of two 10-ft adits and two open cuts 

(Bradley and Logan, 1919). Secondary copper minerals 

and limonite were observed in a shear zone in altered 

granitic rock. No mineral production has been 

recorded. No similar deposits were found in the study 

area.

A uranium anomaly was noted in the Pinnacles 

National Monument by Butler and others (1962) in 

lapilli tuff and flow-banded rhyolite. An airborne 

gamma-ray spectrometer and magnetometer survey 

for the Department of Energy did not reveal any 

radioactive anomalies in the National Monument or 

vicinity (High Life Helicopters, Inc./QEB, Inc., 1981). 

A hydrogeochemical and stream-sediment 

reconnaissance by Cook (1981) did not indicate any 

anomalous mineral occurrences in the vicinity. During 

a reconnaissance survey with a hand-held 

scintillometer in 1984, no radioactive anomalies were 

found.

Parts of the northern and eastern parts of the 

study area were under oil and gas leases in the 1950's. 

These areas are underlain by the upper Miocene 

Monterey Formation, which is a favorable host for 

oil. A 3,239-ft wildcat well, drilled in 1951 at the 

north edge of the study area, was dry (Dolman, 1951). 

In 1984, oil and gas leases were filed east of the 

Pinnacles National Monument and the study area. The 

Bitterwater oil field lies 7 mi to the east.

Diatomaceous and siliceous shales are present 

in the northern part of the study area. Examination 

and sampling indicate that the occurrences are 

composed of interbedded, finely laminated tuffaceous 

shale, siltstone, sandstone, and ash-fall tuff, but 

contain few diatoms.

Numerous prospects were noted by Andrews 

(1936) west and southwest of the Pinnacles National 

Monument. The workings located in marble and 

tactile pendants of the Gabilan Limestone in granitic 

intrusive rock. Minor occurrences of tungsten, copper, 

and molybdenum were observed there (Laizure, 1926; 

Hart, 1966). These pendants do not crop out within the 

study area.

APPRAISAL OF IDENTIFIED RESOURCES

By Lucia Kuizon 

U.S. Bureau of Mines

Mineral-related Activity

Rhyolite containing disseminated gold was 

reported by Crawford (1894) at the Chalone and

Identified Resources

No active mines, prospects, or mineral 

resources were identified in the study area. Alluvium 

samples were collected in the eastern and southern 

parts of the study area. Two of the samples contained 

gold values of $0.01 and $0.36/yd 3 calculated for a 

gold price of $400/oz. A trace of native copper was 

observed in three of the samples. Surface sampling of 

other probable mineralized areas in the study area did 

not indicate significant mineral concentrations.

C3
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\ « '' i I >W\ I t.U|l|ini-

[ ^4 UlP^'M M°^nB in

,' /"»\ M ,^ X )J^ * " n"wJ ^/t » \ \\\\.h; / ^M

APPROXIMATE

BOUNDARY OF

PINNACLES

WILDERNESS

CONTIGUOUS

WILDERNESS STUDY

/REA

Figure 2. Mineral resource potential and generalized geology of the Pinnacles Wilderness Contiguous Wilderness 

Study Area, Monterey and San Benito Counties, California. Geology modified from Dibblee (1971, 1975).

C4
Au

Ag

EXPLANATION

Area with low mineral resource potential-See appendix 

for definition of mineral resource potential and 

certainty of assessment

Commodities

Gold 

Silver

[ ] Deposit types

Qa

QTp

TVS

Tm

Tsg

Tpb

~

Tpv

Tpr

Ki

Hot-spring precious-metal 

Diatomaceous shale

Geologic map units

Alluvium (Quaternary)

Paso Robles Formation (Quaternary and Tertiary)

Volcaniclastic sediments (Miocene)

Monterey Formation (Miocene)

Sandstone and conglomerate (Miocene)

Breccia of Pinnacles Formation (Miocene)

Rhyolite dikes (Miocene)

Vent breccia (Miocene)

Rhyolite of Pinnacles Formation (Miocene)

Intrusive rocks (Cretaceous)

Contact

Fault-Dotted where concealed

Strike and dip of beds

Dry hole

Drainage

Figure 2. Continued.

ASSESSMENT OF MINERAL RESOURCE POTENTIAL

By Steve Ludington and Karen Gray 

U.S. Geological Survey

Geology

The oldest rocks in the study area are felsic 

intrusive rocks, primarily of Cretaceous age, that 

enclose some small pendants and inclusions of 

metamorphic rock. They were described by Ross 

(1972a, b).

Overlying the Cretaceous igneous rocks is a 

series of volcanic rocks, assigned by Matthews (1976a, 

b) to the Pinnacles Volcanic Formation (Pinnacles 

Formation of Andrews, 1936). These rocks comprise a 

homoclinal, west-dipping section in a graben, bounded 

on the west by the Pinnacles fault and, on the east, by 

the Chalone Creek fault (fig. 2). Rocks from the 

Pinnacles Volcanic Formation were dated, using 

potassium-argon methods by Turner (1968), at about 

23.5 Ma, or Early Miocene in age. Although Matthews 

(1976a) presents a more complex subdivision, these 

rocks may be divided into two major units, a rhyolite 

unit and a breccia unit.

The rhyolite unit, which is the oldest volcanic 

unit, consists of a series of primarily rhyolitic flows

and intrusions. These flows and intrusions issued from 

a series of northwest-trending fissures that are 

represented by a swarm of rhyolitic dikes outside the 

graben. In a few areas, notably on South Chalone Peak 

(fig. 2) in the southern part of the study area, complex 

vent breccias are present, indicating the source of the 

youngest eruptions of this unit. In the central part of 

the study area, a small volume of more mafic rocks, 

dacite and andesite, is present. A few of the dikes are 

also dacitic in composition.

Overlying the flows and intrusions is the breccia 

unit, a thick section that consists primarily of rhyolitic 

breccias. These breccias form the spectacular 

pinnacles, which are the chief scenic attraction of the 

Pinnacles National Monument. The clasts in the 

breccia consist primarily of lithic fragments of the 

underlying rhyolitic flows and intrusions. The matrix 

is fine-grained volcanic material. Matthews (1976a) 

observed marine fossils, graded bedding, and 

stratification and concluded that this unit formed in a 

submarine environment as the result of mass slumping 

and turbidity currents on the flanks of an erupting 

vent.

Overlying the volcanic rocks is a sequence 

consisting primarily of continental arkose and 

conglomerate composed of fragments of the underlying 

granitic basement and debris from the Pinnacles 

Formation. These rocks were deformed into several 

open folds in the northeastern part of the area, 

northeast of the Chalone Creek fault. Beds there dip 

generally to the northeast. They were assigned a 

Middle Miocene age by Wilson (1943) and Dibblee 

(1971, 1975). Intercalated with these rocks are 

siliceous shales assigned to the Miocene Monterey 

Formation by Dibblee (1975), who also mapped a 

sedimentary rhyolitic breccia unit at the north end of 

the study area. The sedimentary breccia is composed 

almost entirely of rhyolitic debris from the Pinnacles 

Formation and represents the same depositional 

environment as the arkose and conglomerate, but at a 

time when the Pinnacles Formation was the primary 

source of the sediments.

A few terraces in the southern part of the area 

are covered with terrestrial gravels of late Pliocene or 

early Pleistocene age that Dibblee (1971) assigned to 

the Paso Robles Formation. The bottoms of many of 

the ephemeral stream valleys in the area are filled 

with river sediment of Quaternary age.

Geochemistry

Results of energy-dispersive X-ray fluorescence 

analysis of selected rock samples from the Pinnacles 

volcanic field have been reported in Ludington and 

Gray (1986).

The results show the Pinnacles rocks to be a 

typical calc-alkaline volcanic series. The rhyolites are 

relatively low silica (see Matthews, 1976a) and are not 

strongly enriched in alkalis or in incompatible trace 

elements such as rubidium or niobium. A few samples 

show perturbation of barium and strontium values, 

suggesting both leaching and enrichment. This is 

probably a result of vapor-phase alteration and does 

not indicate widespread hydrothermal alteration. The

C5
rock suite is enriched in arsenic. Several samples of 

intrusive and extrusive rhyolite have arsenic values 

greater than the 15 ppm detection limit. Thus, the 

rocks are a potential source for gold and silver 

deposits, which are characterized by high arsenic.

A sample of mineralized rock from the Copper 

Mountain mine, within the Pinnacles National 

Monument, shows a chemical pattern typical of 

epithermal gold and silver deposits related to calcalkaline igneous rocks. It is enriched in copper, lead, 

zinc, and arsenic.

Mineral and Energy Resources

The Pinnacles volcanic field is an appropriate 

setting for hot-spring gold-silver deposits (Berger, 

1985). These are found in the shallow part of young 

rhyolitic volcanic centers of calc-alkaline affinity that 

are related to subduction zones or transform faults. 

They are characterized geochemically by the 

association of arsenic and antimony with gold and 

silver. The geochemical signature of the samples 

analyzed (Ludington and Gray, 1986) is permissive for 

this type of deposit, and the calc-alkaline nature of 

the Pinnacles rhyolites is also appropriate.

Nevertheless, we assign only a low potential for 

gold and silver resources in hot-spring deposits in the 

Pinnacles volcanic field. Exposures are relatively 

good, and no significant areas of hydrothermal 

alteration that might indicate mineralization were 

observed. The level of certainty is C; available 

information gives a good indication of the mineral 

resource potential. Detailed geologic mapping, 

perhaps at a scale of 1:12,000 and lithogeochemical 

sampling would be necessary to raise the certainty 

level to D.

Other deposit types commonly found in rhyolitic 

rocks include volcanogenic uranium, volcanogenic 

beryllium, and rhyolite-hosted tin deposits. These 

models were considered and found to be inapplicable to 

the study area; they are found with high-silica alkali 

rhyolites, not the calc-alkaline type to which the 

Pinnacles rhyolites belong.

There is a low potential for diatomite resources 

in shale beds, with a certainty level of D. The known 

occurrences are well exposed, and the probability that 

concealed deposits exist is low.

The potential for oil and gas resources is 

similarly low, with certainty level D. Though the 

Monterey Formation is a favorable host for petroleum, 

the dry well drilled at the edge of the study area 

suggests a low potential for energy resources in this 

small area.

REFERENCES CITED

Akers, J.P., 1967, The geohydrology of Pinnacles 

National Monument, California: U.S. 

Geological Survey Open-File Report 67-2, 13 p.

Andrews, Philip, 1936, Geology of the Pinnacles 

National Monument: California University 

Department of Geology, Science Bulletin, v. 24, 

p. 1-38.

Beikman, H.M., Hinkle, M.E., Frieders, Twila, Marcus, 

S.M., and Edward, J.R., 1983, Mineral surveys 

by the Geological Survey and the Bureau of 

Mines, Bureau of Land Management Wilderness 

Study Areas: U.S. Geological Survey Circular 

901, 28 p.

Berger, B.R., 1985, Geologic-geochemical features of 

hot-spring precious-metal deposits, in Tooker, 

E.W., ed., Geologic characteristics of sedimentand volcanic-hosted disseminated gold deposits- 

-search for an occurrence model: U.S. 

Geological Survey Bulletin 1646, p. 47-54.

Boalich, E.S., 1922, San Benito County: California 

State Mining Bureau 18th Report, p. 211-220.

Bradley, W.W., and Logan, C.A., 1919, San Benito 

County: California State Mining Bureau 15th 

Report, p. 616-673.

Butler, A.P., Jr., Finch, W.I., and Twenhofel, W.S., 

1962, Epigenetic uranium deposits in the United 

States: U.S. Geological Survey Mineral 

Investigations Resource Map MR-21, scale 

1:3,168,000, 42 p.

Cook, J.R., 1981, Santa Cruz 1° by 2° NTMS Area, 

California hydrogeochemical and streamsediment reconnaissance: U.S. Department of 

Energy Report GJBX-314 (81), 18 p.

Crawford, J.J., 1894, Gold-San Benito County: 

California State Mining Bureau, 12th report of 

the state mineralogist, p. 227-228.

Dibblee, T.W., Jr., 1971, Geologic map of the 

Greenfield quadrangle, California: U.S. 

Geological Survey Open-File Report 71-38, 

scale 1:62,500.

__ 1975 Geologic map of the San Benito quadrangle, 

California: U.S. Geological Survey Open-File 

Report 75-394, scale 1:62,500.

Dolman, S.G., 1951, District No. 3 wildcat wells, in 

Summary of Operations, California Oil Fields: 

Division of Oil and Gas, 37th Annual Report, v. 

37, no. 2, 157 p.

Eric, J.H., 1948, Tabulation of copper deposits of 

California: California Division of Mines 

Bulletin 144, p. 197-357.

Evenson, R.E., 1962, Ground-water reconnaissance at 

Pinnacles National Monument, California: U.S. 

Geological Survey Water-supply Paper 147 5-K, 

p. 375-382.

Fairbanks, H.W., 1894, Geology of Northern Ventura, 

Santa Barbara, San Luis Obispo, Monterey, and 

San Benito Counties: California State Mining 

Bureau, 12th Report of the State Mineralogist, 

p. 493-526.

Goudarzi, G.H.. 1984, Guide to preparation of mineral 

survey reports on public lands: U.S. Geological 

Survey Open-File Report 84-787, 51 p.

Hart, E.W., 1966, Mines and mineral resources of 

Monterey County, California: California 

Division of Mines and Geology County Report 5, 

142 p.

High Life Helicopters Inc./QEB Inc., 1981, Airborne 

gamma ray spectrometer and magnetometer 

survey, Santa Cruz quadrangle: U.S. 

Department of Energy Report GJBX-50 (81).

Hinds, N.E.A., 1952, Evolution of the California 

landscape: California Division of Mines Bulletin 

158, p. 180-181.

C6
Jennings, C.W., and Strand, R.G., 1959, Geologic map 

of California - Santa Cruz sheet: California 

Division of Mines Map, scale 1:250,000.

Kuizon, Lucia, 1985, Mineral resources of the 

Pinnacles Wilderness Contiguous Wilderness 

Study Area (BLM No. CA-040- 303), San Benito 

and Monterey Counties, California: U.S. Bureau 

of Mines Mineral Land Assessment Open-File 

Report 29- 85, 12 p.

Laizure, C. M., 1926, San Francisco Field Division, San 

Benito County, 22nd Report of the State 

Mineralogist: California State Mining Bureau, 

p. 217-247.

Ludington, Steve, and Gray, Karen, 1986, 

Petrochemical studies of the Pinnacles 

Wilderness Contiguous Wilderness Study Area, 

Monterey and San Benito Counties, California: 

U.S. Geological Survey Open-File Report 87- 

135, 8 p.

Matthews, Vincent in, 1976a, Geology of the Pinnacles 

Volcanic Formation and the Neenach Volcanic 

Formation and their bearing on the San Andreas 

Fault problem: Santa Cruz, Calif., University 

of California, unpublished Ph.D. Dissertation, 

213 p.

__ 1976b, Correlation of the Pinnacles and Neenach 

Volcanic Formations and their bearing on the 

San Andreas fault problem: American 

Association of Petroleum Geologists Bulletin, v.

60, no. 12, p. 2128-2141.

Ross, D.C., 1972a, Geologic map of the pre-Cenozoic 

basement rocks, Gabilan Range, Monterey and 

San Benito Counties, California: U.S. 

Geological Survey Miscellaneous Field Studies 

Map MF-357, scale 1:125,000.

__ 1972b, Petrographic and chemical reconnaissance 

study of some granitic and gneissic rocks near 

the San Andreas fault from Bodega Head to 

Cajon Pass, California: U.S. Geological Survey 

Professional Paper 698, 92 p.

Smith, M.B., Engler, V.L., Lee, D.I., Horn, K.J., and 

Wayland, R.G., 1971, Reported occurrences of 

selected minerals in the central third of 

California: U.S. Geological Survey Mineral 

Investigations Resource Map MR-48, scale 

1:500,000.

Turner, D.L., 1968, Potassium-argon dates concerning 

the Tertiary Foramaniferal time scale and San 

Andreas Fault displacement: Berkeley, Calif., 

University of California, unpublished Ph.D. 

Dissertation.

U.S. Bureau of Mines and U.S. Geological Survey, 1980, 

Principles of a resource/reserve classification 

for minerals: U.S. Geological Survey Ciruclar 

831, 5 p.

Wilson, LF., 1943, Geology of the San Benito 

quadrangle, California: California Journal of 

Mines and Geology, v. 39, no. 2, p. 183-270.

C7

APPENDIXES

DEFINITION OF LEVELS OF MINERAL RESOURCE POTENTIAL 

AND CERTAINTY OF ASSESSMENT

Definitions of Mineral Resource Potential

LOW mineral resource potential is assigned to areas where geologic, geochemical, and geophysical 

characteristics define a geologic environment in which the existence of resources is permissive. This 

broad category embraces areas with dispersed but insignificantly mineralized rock as well as areas with 

few or no indications of having been mineralized.

MODERATE mineral resource potential is assigned to areas where geologic, geochemical, and geophysical 

characteristics indicate a geologic environment favorable for resource occurrence, where interpretations 

of data indicate reasonable likelihood of resource accumulation, and (or) where an application of 

mineral-deposit models indicates favorable ground for the specified type(s) of deposits.

HIGH mineral resource potential is assigned to areas where geologic, geochemical, and geophysical 

characteristics indicate a geologic environment favorable for resource occurence, where interpretations of 

data indicate a high degree of likelihood for resource accumulation, where data supports mineral-deposit 

models indicating presence of resources, and where evidence indicates that mineral concentration has 

taken place. Assignment of high resource potential to an area requires some positive knowledge that 

mineral-forming processes have been active in at least part of the area.

UNKNOWN mineral resource potential is assigned to areas where information is inadequate to assign low, 

moderate, or high levels of resource potential.

NO mineral resource potential is a category reserved for a specific type of resource in a well-defined area.

Levels of Certainty

t

U/A

UNKNOWN

POTENTIAL

H/B

HIGH POTENTIAL

M/B

MODERATE POTENTIAL

L/B

LOW

POTENTIAL

H/C

HIGH POTENTIAL

M/C 

MODERATE POTENTIAL

L/C

LOW

POTENTIAL

H/D

HIGH POTENTIAL

M/D 

MODERATE POTENTIAL

L/D

LOW POTENTIAL

N/D

NO POTENTIAL

B C 

LEVEL OF CERTAINTY

A. Available information is not adequate for determination of the level of mineral resource potential.

B. Available information suggests the level of mineral resource potential.

C. Available information gives a good indication of the level of mineral resource potential.

D. Available information clearly defines the level of mineral resource potential.

Abstracted with minor modifications from:

Taylor, R. B., and Steven, T. A., 1983, Definition of mineral resource potential: Economic Geology,

v. 78, no. 6, p. 1268-1270. 

Taylor, R. B., Stoneman, R. J., and Marsh, S. P., 1984, An assessment of the mineral resource potential

of the San Isabel National Forest, south-central Colorado: U.S. Geological Survey Bulletin 1638, p.

40-42. 

Goudarzi, G. H., compiler, 1984, Guide to preparation of mineral survey reports on public lands: U.S.

Geological Survey Open-File Report 84-0787, p. 7, 8.

C11
RESOURCE/RESERVE CLASSIFICATION

ECONOMIC

MARGINALLY 

ECONOMIC

SUBECONOMIC

IDENTIFIED RESOURCES

Demonstrated

Measured Indicated

I

I 

Reserves

I 

i

i

Mar 

Rest

I

ginal 

jrves

I 

Demonstrated 

Subeconomic 

Resources

Inferred

Inferred 

Reserves

Inferred 

Marginal 

Reserves

Inferred 

Subeconomic 

Resources

UNDISCOVERED RESOURCES

Probability Range

Hypothetical Speculative

Major elements of mineral resource classification, excluding reserve base and inferred reserve base. Modified from U.S. Bureau of Mines 

and U.S. Geological Survey, 1980, Principles of a resource/reserve classification for minerals: U.S. Geological Survey Circular 831, p. 5.

C12
GEOLOGIC TIME CHART 

Terms and boundary ages used by the U.S. Geological Survey in this report

EON

Phanerozoic

Proterozoic

Archean 

pre- Ar

ERA

Cenozoic

Mesozoic

Paleozoic

Late Proterozoic

Middle Proterozoic

Early Proterozoic

Late Archean

Middle Archean

Early Archean 

:hean 2

PERIOD

Quaternary

Tertiary

Neogene 

Subperiod

Paleogene 

Subperiod

Cretaceous

Jurassic

Triassic

Permian

Carboniferous 

Periods

Pennsylvanian

Mississippian

Devonian

Silurian

Ordovician

Cambrian

 -(3800?)- -

EPOCH

Holocene

Pleistocene

Pliocene

Miocene

Oligocene

Eocene

Pafeocene

Late 

Early

Late 

Middle 

Early

Late 

Middle 

Early

Late 

Early

Late 

Middle 

Early

Late 

Early

Late 

Middle 

Early

Late 

Middle 

Early

Late 

Middle 

Early

Late 

Middle 

Early

 -

AGE ESTIMATES 

OF BOUNDARIES 

(in Ma)

- 0.010

- 1.7

u

/ k ji

 QQ

UU 

_ fifi

- 96

_ 1QQ

one

- -240

_ OOft

f*f*/\ - ~330 

- ifin oow

- 410

toy

- 500

_ ~O/U ~~- K~l A1

ouv

_ 

 

iftnn IDUU 

_ ic/in ZOUU

- 3000

O*rUU 

 AKItf 

'Rocks older than 570 Ma also called Precambrian, a time term without specific rank. 

2 Informal time term without specific rank.

C13