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California Division of Alines
Special Report 56

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UNIVERSITY Or CALIFORNIA
DAVIS

NOV % 1959
LldHARY

Digitized by the Internet Archive

in 2012 with funding from

University of California, Davis Libraries

http://archive.org/details/franciscancherti56gold

GEOLOGY AND ECONOMIC POSSIBILITIES
OF THE LIMESTONE AND DOLOMITE DEPOSITS
OF THE NORTHERN GABILAN RANGE, CALIFORNIA

By OLIVER E. BOWEN, JR. and CLIFFTON H. GRAY, JR.

Mining Geologists, California Division of Mines

Special Report 56

CALIFORNIA DIVISION OF MINES

FERRY BUILDING, SAN FRANCISCO, 1959

STATE OF CALIFORNIA

EDMUND G. BROWN, Governor

DEPARTMENT OF NATURAL RESOURCES

tDeWITT NELSON, Director
DIVISION OF MINES
GORDON B. OAKESHOTT, Chief

Special Report 56
Price 50£

CONTENTS Page

Abstract 5

Introduction n

General geology jq

Sur series jj

Quartz monzonite and granodiorite 13

Other granitic rocks 15

Vaqueros group and associated Tertiary rocks 15

Rock units mapped in detail in the Fremont Peak vicinity 16

Sequence and structure in the Fremont Peak rocks 19

Sur series section 19

Rocky Ridge fault zone 21

Field features useful in estimating the economic possibilities of the carbonate-rock masses— 21

Economic possibilities of limestone and dolomite deposits 23

Los Vergeles district 23

San Juan Canyon district 23

Barbee Ranch deposits 23

Bryan and Pearee-Twohy deposits 23

Flint-Steinbeck deposit 25

Harmony Hills deposit 25

Underwood deposits 25

Natividad district 25

Kaiser Aluminum and Chemical Company deposits 25

Porter dolomite deposit 28

Sugarloaf deposits 28

Other deposits 28

Fremont Peak district 28

Bardin Ranch deposits on Rocky Ridge 28

East Gabilan deposit 28

Fremont Peak deposits 31

Bird Creek district 31

A. S. and R. deposit 31

Bird Canyon Ledge 31

Middle Dam deposit 32

Power line deposits 32

Upper Bird Creek deposit 32

Cienega- Vineyard district 33

Garner-Harris deposits 33

Kaiser-Harris (Permanente) dolomite deposit 35

Martin Ranch deposits 35

McCray Ranch deposits 35

Palmtag-Harris deposits 36

Reeves North deposits 36

Westvaco (O'Hara Ranch) deposit 37

Hartnell district 37

Bluerock Mountain-Quail Creek deposits 37

Mount Hanlan-McPhails Peak district 37

Cowell-Thompson Creek deposit . 37

Hamilton, Harlan, Mayries, and McPhail deposits 38

Westphal Ranch deposits 38

Willow Creek district 39

Barite deposits west of Fremont Peak 39

References 40

(3)

Illustrations Page

Plate 1. Geologic map and structure section of Fremont Peak area, Monterey and San

Benito Counties, California In pocket

Figure 1. Index map of the northern Gabilan Range showing area mapped (shaded)

and topographic maps covering area 7

Figure 2. Regional geologic map of northern Gabilan Range 10

Figure 3. Generalized columnar section, northern Gabilan Range 11

Figure 4. Map showing location of limestone and dolomite deposits, Fremont Peak area 22

(Photos by Mary Hill)
Frontispiece. Fremont Peak from the west 6

Photo 1. Fremont Peak from the east 8

2. Rocky Ridge, Fremont Peak in background 9

3. Fremont Peak from the southeast 9

4. Panorama across Salinas Valley 12

5. Rolling late Tertiary surface south of Fremont Peak 14

6. Pinecate sandstone near Ideal cement plant 14

7. Vaqueros limestone conglomerate 15

8. Steep-dipping limestone beds, Fremont Peak road 16

9. Limestone outcrop, south slope of Fremont Peak 16

10. Detail of solution pits in limestone 16

11. Closeup of coarsely crystalline limestone 16

12. Large solution pits in cavernous limestone 17

13. Hackly weathered surface of siliceous limestone 17

14. Elephant-skin texture on dolomite, Sugarloaf 17

15. Etched joint pattern in dolomite, Sugarloaf 17

16. Banded assemblage of mixed carbonate rocks, Fremont Peak 18

17. Detail of banded carbonate rock 18

18. Quartzite hill east of park headquarters 19

19. Hackly, lichen-covered surfaces of replacement quartz, east of park headquarters 20

20. Ideal cement plant near San Juan Bautista 24

21. Ideal Cement Company quarries 24

22. Kaiser Aluminum and Chemical Co. plant (photo by Cat Pictures) 26

23. Quarries and dumps at Natividad 27

24. Quarry face and floor, Natividad 27

25. Granite dikes in dolomite, Natividad 28

26. Initial stage in exploration of dolomite deposit on Reeves Ranch 29

27. Closer view of drill rig shown in photo 26 29

28. Bouldery limestone outcrops, power line deposits 30

29. Westvaco operation near Vineyard winery 33

30. Westvaco 's crushing and sizing installation 34

31. Quarry benches in white dolomite, Westvaco 35

32. Closeup of quarry-run dolomite, Westvaco 36

33. Old lime kilns at Cowell-Thompson Creek deposit 38

34. Rocky Ridge, barite dumps 39

35. Closeup of barite veins in Rocky Ridge 40

(4)

ABSTRACT

Deposits of limestone and dolomite in the northern Gabilan Range have become
important sources of raw materials for San Francisco Bay area industries, supplying
one cement plant and three magnesia refractory and chemical plants. The deposits
are chiefly in the crystalline basement complex of the range, as pendants suspended
in granitic rocks and schist. Some of the pendants contain masses of limestone and
dolomite of commercial size and purity, whereas others contain mixed calcitic and
dolomitic rocks of no present economic importance. Criteria useful in recognizing calcitic
and dolomitic rocks in the Fremont Peak vicinity are grain relief, grain size and grain
color, as seen on weathered surfaces, and the disposition of solution pits. The more
siliceous varieties of carbonate rock are also likely to be magnesian.

The major rock units of interest in the area are the metamorphosed Sur series of
probable Paleozoic age, undivided granitic rocks of Jura-Cretaceous age, and the
Vaqueros group of lower Miocene age. Carbonate rocks of potential commercial impor-
tance are found in both the Sur series and the conglomerate portions of the Vaqueros
group. Quartz monzonite and granodiorite predominate among the granitic rocks.
Sur series rocks, in descending order of abundance, are quartz-mica schist, limestone,
dolomite, and quartzite. An upper and lower schist member and a middle carbonate-and-
schist member are recognized. Subunits mapped for economic purposes in the Fremont
Peak vicinity are: limestone; dolomite; limestone with granitic rock and schist intercala-
tions; mixed limestone and dolomite; siliceous limestone; limy, siliceous dolomite,- gray
replacement quartz; quartz-mica schist; pink quartzite; mixed schist and granitic rocks;
and quartz monzonite-granodiorite. Vein and replacement masses of barite are found in
the carbonate rocks west of Fremont Peak.

(5)

GEOLOGY AND ECONOMIC POSSIBILITIES OF THE LIMESTONE AND
DOLOMITE DEPOSITS OF THE NORTHERN GABILAN RANGE, CALIFORNIA

By OLIVER E. BOWEN, JR. and CLIFFTON H. GRAY, JR.

INTRODUCTION

Purpose of the Study. In recent years there has been
increasing- interest in both the limestone and dolomite
leposits of the northern Gabilan Range because of their
jroximity to markets and to heavy transportation. Up
;o the time of the present study no detailed work had
)een done on the carbonate rocks and only two parts of
;he northern Gabilan Range had been mapped in which
he carbonate rocks were even roughly delineated. So far
is the authors know, this study is the first attempt made
n this vicinity to separate limestone and dolomite masses
nto mapping units. The present study was instigated
vith three objectives: (1) for the purpose of gaining
itratigraphic information on the Sur series — the ancient
netasedimentary sequence which contains the carbonate
■ocks; (2) to find out possible geologic controls over the
listributions of limestone and dolomite deposits in the
Jabilan Range; (3) and to map in greater detail than
leretofore the carbonate-rock masses in the Fremont
*eak vicinity. The Fremont Peak vicinity was chosen
or study because there the Sur series appears to be more
learly intact, stratigraphically, and exposures are
letter there than in other parts of the Gabilan Range.
Phe detailed geologic map of the Fremont Peak vicinity
ras completed in the fall of 195-4 and rechecked in the
all of 1957. Work on the individual deposits extended
rom 1954 to late 1957. The data presented are complete
o January 1, 1958.

This paper describes in some detail the geology of 8
quare miles of terrain in the vicinity of Fremont Peak.
t also includes a discussion of the general geologic
eatures of the northern Gabilan Range, together with

generalized small-scale geologic map. Most of the
lasses of carbonate rock in the northern Gabilan Range
re discussed with regard to their geology and economic
tossibilities ; deposits of Los Vergeles, San Juan Canyon,
Jatividad, Fremont Peak, Bird Creek, Cienega-Vine-
ard, Hartnell, Mount Harlon-McPhails Peak, and Wil-
aw Creek districts are tabulated, described briefly, and
valuated.

Mapping was done on U. S. Department of Agriculture
holographs, series ABO-BUX, on a scale of 1:10,000.
'he geology was transferred from this to a composite
opographic base derived from the new Hollister and
Ian Juan Bautista 7^-minute sheets and the Gonzales
5-minute sheet of the U. S. Geological Survey.

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SANTA

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Salinas

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Figure 1. Index map of the northern Gahilan Range showing
area mapped (shaded), and topographic maps covering the area.

A total of 97 samples were collected by the authors for
analysis. Fifty-six of these were analyzed by Abbot A.
Hanks, Inc., of San Francisco, and the remainder by
L. A. Caetano of the San Juan Bautista plant of Ideal
Cement Company. In addition, the Ideal Cement Com-
pany made 444 analyses, done in former years on ma-
terial on the Bryan, Pearce-Twohy, Underwood, and
Barbee deposits, available to the authors. These 541
analyses are the bases upon which the probable economic
worth of the various deposits was determined.

Location and Accessibility. The area studied in detail
is a rectangular strip about 5 miles long and 14. miles
wide oriented along the crest of the Gabilan Range. Fre-
mont Peak, also called Gabilan Peak, the highest in the
vicinity at 3,171 feet, is the approximate center of the
area. Fremont Peak lies (\\ airline miles south of San
Juan Bautista or about 80 miles southeast of San Fran-
cisco. Most of the plot mapped is in Monterey County —
but the northern and eastern margins are in San Benito
County. The district is readily accessible to wheeled
vehicles by the paved Fremont Peak State Park road,
although present grades and curves are not favorable to
heavy trucking. The peak is about 11 miles by road from

(7)

CALIFORNIA DIVISION OF MINES

[Special Report 5

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Photo 1.

Fremont Peak from the east. The twin peaks are surmounted by partly silicified dolomitic limestone. A tongue of schist

occupies the saddle.

San Juan Bautista. Several dirt roads connect with the
Fremont Peak road. All of the area mapped except the
State Park is privately owned ranch land not open to
public entry; nor may mineral locations be made in the
park.

Physical Features. The Gabilan Range, one of the
prominent topographic features of the central Coast
Ranges, is essentially a horst or fault-block range made
up predominantly of pre-Tertiary crystalline rocks. The
deeply alluviated Salinas and San Juan-San Benito River
Valleys form, respectively, its west and east limits. Over
much of its length the range is bordered on the east by
the San Andreas fault zone. Rising abruptly from local
base level at an elevation of 200 feet, the mountains gain
heights exceeding 3,700 feet. The surface has been deeply
dissected along the flanks of the range and along north-
west-trending faults, but parts of a late Tertiary or early
Quaternary mature surface yield some gently rolling
laud along the range crest — such as the land southeast
of Fremont Peak.

As fairly homogeneous granitic rocks underlie much
of the range, the drainage pattern developed is largely
dendritic; but locally, trellis drainage has developed
along fault zones. The main drainage south of the ridge
surmounted by Fremont Peak is into Gabilan Creek;
the arc;) to the northwest is drained by its main tribu-
tary. Mud Creek. Both drain into Salinas Valley. Bird
Creek and its tributaries drain the northeast part of the
area and ultimately join the San Benito River.

The climate in the Gabilan Range is of Mediterranear
type, with summer fogs and winter rainfall, which aver
ages about 20 inches. Temperatures fall below freezing
for short periods during the winter months and reach
80 to 100 degrees in summer.

Previous Gedtogic Work. No previous work has cov-;
ered the Gabilan Range or the Fremont Peak area in
detail. Several published reports include the Gabilan
Range or parts thereof but do not deal specifically with
carbonate rocks of the Fremont Peak area. J. D. Whit-
ney, in 1865, gave the first geologic description of this
region in Volume I of the Geological Survey of Cali-
fornia. He described the rock types present but named
no formations. In 1888 George F. Becker commented on
some of the general features of the Gabilan Range and
was the first to call the carbonate rocks the "Gabilan
limestone". The first complete report published, which
covered part of the Fremont Peak vicinity, was by J. E.
Allen (1946) on the geology of San Juan Bautista quad-
rangle ; it includes a geologic map on a scale of 1 : 62,500.
Taliaferro (1948) published a geologic map of Hollister
quadrangle on the same scale. These two maps include
most of the area of the present study. Unpublished work
includes maps of the Bird Canyon and East Gabilan
limestone deposits made by Bailey and Robin Willis in
1925, and lists of chemical analyses of samples collected
at various times by Pacific Portland Cement Company.

Acknowledgments. The authors wish to express their
appreciation to John A. Wolfe, W. E. Brown, and Leon-

1591

LIMESTONE AND DOLOMITE, NORTHERN GABILAN RANGE

**te*

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Photo 2. Rocky Ridge terminated in the background by the twin peaks of Fremont Peak, observer facing east. White
dumps of the barite workings are prominent in the middle ground. The crest of Rocky Ridge is marked by a spine of dolomitic,
partly silicified, partly brecciated limestone. This is the edge of a steeply south-dipping, sheet-like mass projecting irregularly
downward into a sea of granitic rock and schist much like the roots of a row of teeth. A tunnel driven through the ridge
500 feet below the crest would in many places be entirely within granite and schist.

').,

lag 1 * *

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Photo 3 Fremont Peak as seen from the southeast. The two brush-accented belts of carbonate rock and the dividing
schist tongue that crosses the saddle can easily be seen. The foreground rocks are quartz replacements of dolomitic lime-
stone a gray cavernous quartzite. The silica was probably introduced at the time of intrusion of the granitic rocks.

CALIFORNIA DIVISION OF MINES

[Special Report 5(

A N A T I ON

River terrace
•9t.i de po s i l s.

Gravels and
e d sands.

| P j Pur i s i m a group
M\? I Volcanic rocKs.

TI2S

T I3S

SALI N AS

T I4S

T I5S

R 3 E

R 4 E R5E R6E

Figure 2. Regional geologic map of northern Gabilan Range.

R 7E

ard Caetano of Ideal Cement Company, and L. N. Bry-
ant, formerly of the Ideal Cement Company, whose
cooperation made this study possible. The willing co-
operation of the owners of the Reeves and Crowe-Harris
Ranches in allowing the writers access to their lands is
gratefully acknowledged. Howard Harris provided much
valuable information on the terrian and outcrops south-
cast of Fremont (Gabilan) Peak. The park rangers of
the State Division of Beaches and Parks at Fremont
Peak State Park helped the authors in many ways dur-
ing tlic course of field work.

T. C. Sersen, John A. Wolfe, W. A. Young, R. F.
Moran, Howard Harris, and Ivan Hall all read and
criticized the manuscript or parts thereof. Their assist-
ance is greatly appreciated.

The excellent photo coverage was the work of Mary
Hill and Elisabeth Egenhoff of the Division of Mines
editorial staff.

GENERAL GEOLOGY

Rocks of the northern Gabilan Rang'.' are predom-
inantly biotite quart/, monzonite and associated granitic
intrusives — most probably of Late Jurassic or Early Cre-
s age. Suspended in these granitic rocks are pend-
3 Of strongly metamorphosed sedimentary rocks of

probable Paleozoic age among which mica schist, lime-
stone, dolomite, and quartzite are the principal types.
Many of the metasedimentary pendants are aligned in
an easterly direction whereas the axis of the range
trends northwest. Along the northeast margin of the
range in the vicinity of San Juan Bautista is a succes-
sion of clastic marine sediments and andesitic volcanic
rocks of Tertiary age that has been strongly folded and
faulted, but almost everywhere this group of rocks is
in fault contact with the crystalline basement rocks
previously mentioned. Miocene ( ?) rhyolite, both intru-
sive and extrusive, is exposed in the southern part of the
Gabilan Range but is not found within the limits of this
study.

The metasediments have been correlated with the Sur
series of the Santa Lucia Mountains and Sierra de Salinas
by several previous authors, and evidence uncovered dur-
ing this investigation is not in conflict with this thesis.
The carbonate rocks of the sequence, even though they
appear at numerous horizons in the Sur sequence, were
lumped together and called "Gabilan limestone" by
Becker (1888) and Trask (1926). The schists and quartz-
ites have been lumped together as "Sur schist" by many
California geologists. Inasmuch as the names Gabilan
limestone and Sur schist have no real formational or

1959]

LIMESTONE AND DOLOMITE, NORTHERN GABILAN RANGE

Figure 3. Generalized columnar section, northern Gabilan
Range. In part after Allen, 1946, p. 18.

stratigraphic significance, they will not be used in this
study, and the less specific term "Sur series" will be
used for the entire metasedimentary sequence.

Sur Series

The Sur series of the Santa Lucia Mountains has been
described at length by Trask (1926) and Reiche (1937).
Reiehe in particular lists many petrographic data on
rocks of the series. Neither author, however, names a
type locality or describes a typical sequence in the
series. Because of great elongation and distortion of
the beds during metamorphism, identification of tops
and bottoms of beds has been rendered virtually im-
possible. Consequently, both Trask 's and Reiche 's esti-
mates of thickness of the series are based on an assumed
homoclinal attitude whereas they recognize the possibil-
ity of repetition by both folding and faulting. Trask
estimates the thickness of the series on Point Sur quad-
rangle to be in excess of 5,000 feet. Reiche recognizes a
possible thickness of 10,000 feet for the series in Lucia
quadrangle and describes the carbonate rocks as "nu-
merous layers — intercalated in the schist".

In the most nearly complete section in the vicinity
of Fremont Peak the Sur series has a thickness of at
least 8,000 feet, if a homocline dipping north is assumed
and leaves of intercalated granitic rock are disregarded.
Both "upper" and "lower" contacts are against granite
intrusions, so that much of the sequence is probably
missing. Three crudely defined members are recognized :
an upper schist about 1,400 feet thick; a middle car-
bonate member about 3,400 feet thick, and a lower schist
member at least 3,500 feet thick. This section is described
in greater detail under Sequence and Structure. The
schist members both contain injections of granitic ma-
terial and have both been granitized to some extent —
that is, the original clastic material has been replaced
by rock of granitic character. Lenses of schist are in the
carbonate members and lenses of carbonate are in schist,
indicating that the original sedimentary sequence was
heterogeneous. Sheets and irregular masses of quartz
monzonite also intrude the carbonate rock masses as well
as the schist.

In previous works concerning the Sur series and Gab-
ilan limestone, no mention has been made of the presence
of dolomite in the carbonate rock masses, and it may be
that dolomite is rare in the Sur series of the Santa Lucia
Mountains. However, dolomite is present in the carbon-
ate rocks of the Sierra de Salinas and is an exceedingly
common rock type in the northern Gabilan Range. Be-
cause the distribution of limestone and dolomite is of
prime importance to manufacturers of cement, magne-
sium chemicals, and magnesium refractories, as well as to
many other consuming industries, particular attention
has been paid in this study to dolomite and dolomite-
rich rocks.

Dolomites are widely distributed among the many
carbonate-rock masses and are not found at any par-
ticular horizon or horizons. Where presence of bedding
planes can be determined the dolomite invariably trans-
gresses the bedding planes and nowhere is confined to
distinct beds. Contacts between dolomite and calcite
rocks as a rule are complex, with feathery and finger-
like salients of dolomite penetrating the adjacent calcite
rocks. These features suggest thai the dolomtie has come

CALIFORNIA DIVISION OP MINES

[Special Report

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1959]

LIMESTONE AND DOLOMITE, NORTHERN GABILAN RANGE

in as replacement of limestone and is not an original
constituent of sedimentation.

Dolomite rock in the Fremont Peak vicinity is readily
distinguishable from the adjacent limestone (caleite
rock) on weathered surfaces because of its finer grain
size, cream to pale buff color, and the character and
arrangement of solution pits. These differences are dis-
cussed in greater detail in later paragraphs. Graphite
flakes and dust-like inclusions of organic matter are
common in caleite rocks and rare in dolomite.

Medium-grained and fine-grained quartzite in pale
pink to pale red hues is present as non-persistent, len-
ticular beds in mica schist. The beds range from a few
inches to 2 or 3 feet in thickness, and there are grada-
tional facies between the quartzite and quartz-mica
schist. The texture of the quartzite and its relationship
to adjacent schist beds suggest that it was laid down as
sandstone interbedded with clay shale.

Another type of quartzite, in large masses in the Sur
series of the Fremont Peak vicinity, is clearly of replace-
ment origin. This rock is light dull gray in hand speci-
men or on weathered surfaces, and has numerous vugs
lined with small quartz crystals. It is found in or adja-
cent to masses of carbonate rock, and commonly includes
small, relict patches of the limestone it has replaced.
Quartzite of this type forms most of the ridge top east
of the Fremont Peak Park headquarters and is also
found extensively in the Bird Canyon limestone deposit
and along Rocky Ridge west of Fremont Peak.

Vein and replacement masses of coarse-grained, light-
gray and white barite are also found along Rocky Ridge
west of Fremont Peak. These closely resemble the coars-
er-grained facies of limestone and are distinguished
chiefly by the much greater weight of the barite rock.
Barite has been mined from time to time in the Fremont
Peak vicinity.

The greatest bulk of rock in the Sur series is quartz-
mica schist and gneiss. The principal minerals are
quartz, biotite, and muscovite ; plagioclase, orthoclase,
andalusite or sillimanite, and garnet are present in
minor amounts. Biotite predominates in the dark bands
and quartz in the light bands. Lime silicate minerals
such as diopside, wollastonite, and tremolite have devel-
oped where the original shales were limy. The mica
schists are gradational into gneissoid and normal quartz
monzonite and granodiorite, and granitic rocks and their
aplitic satellites intimately invade the schist sequences in
many localities.

A mass of calc-silicate hornfels crops out over several
acres on the southeast slope of Fremont Peak. The rock
is dense, microcrystalline, and greenish black on fresh
surfaces, but weathers to a rusty brown. It occurs along
the border of a pendant of carbonate-rock, chiefly
dolomitic limestone, and has formed by contact meta-
morphism of such magnesian limestone. Under the
microscope the rock is a mosaic of crystals of tremolite,
wollastonite, diopside, magnetite, and green spinel, with
scattered clots of biotite and andalusite and a few
patches of late-formed chlorite and pyrite.

Garnet-epidote tactite showing a little scheelite is in
sparse, small patches west of the barite prospects; but
elsewhere garnet-bearing contact-metamorphic rocks are
absent, or developed only in very small patches.

Age and Correlation. No precise age-determining
features were found in any of the Sur series rocks, but
they clearly arc older than the -Jura-Cretaceous granitic
rocks which have intruded them (see under quartz, mon-
zonite and granodiorite). Indistinct forms suggestive of
cup corals were found at several places along the ridge
crest west of Fremont Peak and rocks suggestive of
erinoidal calcarenites were observed in the same general
vicinity. The series is similar to many metamorphosed
late Paleozoic and early Mesozoic successions in the
southern Sierra Nevada and Tehachapi Mountains.

Quartz Monzonite and Granodiorite

Lawson (1893) first applied the name "Santa Lucia
granite" to the plutonic intrusives in the vicinity of Car-
mel Bay. In todays nomenclature, Lawsons' "granite"
is a biotite granodiorite, inasmuch as the average com-
position is 30-35 percent quartz, 20-25 percent orthoclase,
35-45 percent oligoclase and 3-7 percent biotite. Trask
(1926) correlated the plutonic rocks of Point Sur quad-
rangle with those of Lawson and designated them col-
lectively as the "Santa Lucia quartz diorite. " Trask,
however, recognized facies ranging from quartz diorite
to granite. Reiche (1937) recognized both granite and
quartz diorite in the Lucia quadrangle but attached no
place names to them. Andrews (1936) adopted Lawson 's
name Santa Lucia "granite" for the group of plutonic
rocks in the southern Gabilan Range but recognized
numerous facies ranging from soda granite to quartz
diorite.

The granitic rocks of the northern Gabilan Range
(particularly those in the Fremont Peak vicinity) are
principally quartz monzonite and granodiorite, quartz
monzonite predominating. Biotite is the common acces-
sory mineral although hornblende is locally abundant in
melanocratic segregations. The quartz content is uni-
versally high in all varieties; most commonly oligoclase
forms 35-40 percent of the quartz monzonite, perthitic
microcline 30-35 percent, and biotite 1 to 3 percent. In
the granodioritic facies, oligoclase or oligoclase-andesine
increases at the expense of microcline and the biotite
commonly increases to 5 percent or more. The prevailing
texture is coarse hypidiomorphic granular without the
conspicuous phenocrysts of orthoclase that typify the
granitic rocks farther west at Carmel Bay and Point
Lobos. Where the quartz monzonite is in contact with
quartz-mica schist it commonly grades subtly into gneiss-
oid or schistose facies in which biotite-rich bands are
developed parallel to the regional planarity of the schists.
This suggests partial regional granitization of the meta-
sedimentary sequence. In many places, however, the con-
tacts between granitic rocks and metasediments are
distinctly intrusive, the intruding rock showing evidence
of considerable fluidity. Although the granitic rocks
probably are susceptible to subdivision into at least two
mappable units in the Fremont Peak area, such subdivi-
sion has been beyond the scope of this work.

Age and Correlation. The granitic rocks are un-
doubtedly traceable southeastward to the Pinnacles vicin-
ity described by Andrews (1936) and although sepa-
rated from the basement complex of the Sierra de Salinas
and Santa Lucia Mountains by alluvial fill of the Salinas
Valley, there seems little doubt that the metasediments

CALIFORNIA DIVISION OF MINES

[Special Report 5(

Photo 5. A view of one of the larger, gently rolling late Tertiary uplifted surfaces that forms the pasture land of the
Reeves Ranch ; observer facing southeast from Fremont Peak.

COTO <i. A pair of strike ridges in the weather-resistant Pinecate sandstone member of the Oligocene San Lorenzo group
as seen in the vicinity of the Ideal cement plant at San Juan Bautista.

1959]

LIMESTONE AND DOLOMITE, NORTHERN GABILAN RANGE

Photo

Reds of limestone conglomerate in the Vaqueros formation along the San Juan Grade ,'U miles by road south-
west of San Juan Bautista.

and granitic rocks of the northern Gabilan Range are
parts of the same sequence found in the Sierra de Salinas
ind Santa Lucia Mountains. Several age determinations
made upon granitic rock debris north and south of Mon-
terey Bay (Hutton, 1951; Curtis, et ah, 1956) have
shown them to be Middle to Late Cretaceous. Some of the
jranitic rocks of the Gabilan Range may be as old as
Upper Jurassic, but it is doubtful that any are older.

Other Granitic Rocks

Minor dikes and irregular intrusions of several other
types of granitic rocks cut the Sur series and the quartz
nonzonite in many places. Hornblende-biotite quartz
iiorite porphyry dikes cut dark hornfels and schist on
the south slope of Fremont Peak. In hand specimen the
porphyry shows euhedral hornblende crystals ranging
from I to 1 centimeter in longest dimension, together
frith smaller phenocrysts of biotite and feldspar. Most
)f the groundmass is indeterminate in hand specimen.
Under the microscope the rock is seen to be composed of
hornblende (40%), andesine (30%), quartz (15%), di-
jpside (15%), and biotite {1' ', ).

Pegmatite and aplite dikes a few inches to a foot or
two in thickness also cut the quartz monzonite-Sur series
;omplex. These are mineralogically simple, consisting
dmost entirely of quartz, feldspar, and biotite. Ortho-
:lase or microcline generally predominates over oligo-
dase and the quartz content varies widely from place
to place even in the same dike. Muscovite is irregularly
aid sparsely distributed or absent.

Vaqueros Group and Associated Tertiary Rocks

The only Tertiary units in the northern Gabilan
Range that contain carbonate rocks of possible economic
significance are members of the lower Miocene Vaqueros
group. These have been described by Allen (1946). The
Vaqueros group is overlain by Miocene andesitic vol-
canic rocks and underlain by sandstone and siltstone of
the Oligocene San Lorenzo group. Vaqueros rocks crop
out discontinuously in a complexly folded and faulted
area of about 30 square miles in a northwest-trending,
arcuate patch centering 1] miles southwest of San Juan
Bautista. They are well exposed along the San Juan
grade, which roughly bisects the area of outcrops. Con-
glomerates, composed predominantly of limestone peb-
bles, cobbles, and boulders derived from the Sur scries,
form much of the lower part of the Vaqueros strati-
graphic sequence. Conglomerate members reach a thick-
ness of 500 feet and the total thickness of the group
ranges up to 1,800 feet (Allen, 1946, p. 28). Conglom-
erate masses are lenticular to irregular in form, having
been laid torrent ially in stream channels. They are inter-
bedded and interfingered with sandstone, conglomerate,
and sedimentary breccia having only a small proportion
of carbonate debris.

Because of the proximity of the portland cement plant
at San Juan Bautista, the Vaqueros limestone conglom-
erate is of potential use in portland cement. Insufficient
sampling has been done by the operating company 1o
establish the worth of these deposits as a supplemental

CALIFORNIA DIVISION OF MINES

[Special Report 56

Photo 8. Steep-dipping beds of limestone along the Fremont
Teak road where it crosses Rocky Ridge just east of Fremont Peak.
The grass-covered depression between the limestone beds in the
center of the photo has developed by erosion of one of the less
resistant interleaved sheets of schist and granitic rock.

source of carbonate raw materials to extend the re-
serves of massive crystalline rocks in the plant vicinity.
Some of the conglomerate deposits lie within a mile of
the cement plant, but the extent to which these can be
used depends largely upon the proportion of dolomite
and magnesian limestone debris present (the MgO con-
tent in portland cement clinker must not exceed 5 per-
cent). As massive samples of mill-run proportions woidd
be necessary to adequately test the content of the con-
glomerates, such sampling and analytical work was not
attempted by the California Division of Mines. The
authors did note the presence of appreciable numbers of
dolomite and dolomitic limestone cobbles and pebbles in
several exposures of Vaqueros conglomerate near the
cement plant, so the magnesium content of the limestone
conglomerate would have to be carefully checked.

ROCK UNITS MAPPED IN DETAIL IN THE
FREMONT PEAK VICINITY

In preparing the geologic map of the Fremont Peak
vicinity, which accompanies this report, rock units were
chosen to bring out to best advantage the economic pos-
sibilities of the rocks of the area. Rock types that were
too intimately mixed for adequate representation on the
map scale adopted, or which were too intimately mixed
to allow selective mining of the constituent rock types,
were grouped together as a single unit.

Photo 9. A typical outcrop of limestone on the south slope of
Gabilan Peak showing the coarsely crystalline, complexly jointed,
partly brecciated character of the rock. Colors are white to blue-
gray.

Photo 10. Well-developed, sharp-edged, cup-like solution pits
in high-grade limestone. Compare these solution pits and the gen-
eral surface appearance with the dolomite surfaces shown in photos
14 and 15.

1 w->

Photo 11. Closeup of a coarsely crystalline limestone surface
showing the well-etched cleavage surfaces of the individual crystals.
Most of the dolomites of the Fremont Peak vicinity are noticeably
finer grained.

[959]

LIMESTONE AND DOLOMITE, NORTHERN GABILAN RANGE

; 7 . S^i

Photo 12. Large solution pits in cavernous limestone. In eon-
act with the limestone at the left side of the photo is a replace-
oent patch of dolomite which has no solution pits.

Photo 13. Typical hackly surfaces on siliceous limestone where
uartz and silicate minerals have developed in patches under influ-
nce of contact metamorphism.

Photo 14. Elephant-skin texture of weathered dolomite out-
rops on the southwest slope of Sugarloaf. Silver half-dollar gives
cale. Cup-shaped, sharp-edged solution pits are conspicuously
bsent as compared with the weathered limestone surfaces shown in
ihotos 10 and 12.

Limestone (Is)*. The material mapped as limestone
is uniform, high in calcium and low in magnesium, usa-
ble in portland cement, and in most cases, for numerous
other commercial purposes. Type samples were found to
contain less than 3 percent MgO and more than 94 per-
cent CaO. A number of samples ran as high as 98 percent
CaC0 3 . The iron oxide content of most samples falls
within the range 0.11 to 0.20 percent with a maximum
of about 0.50 percent Fe 2 3 . The limestone ranges from
medium-grained varieties having an average grain di-
ameter of about 1 mm to coarse-grained varieties aver-
aging more than 1 cm in diameter. Colors range from
dark blue-gray to white on fresh surfaces. No amorphous
or microcrystalline varieties were observed.

Photo 15. Etched joint pattern in medium to coarse-grained
dolomite on Sugarloaf, Natividad district.

Dolomite (do). The material mapped as dolomite
is uniform, of potential commercial quality, and has an
MgO content of more than 17 percent. In some places,
however, the iron content is too high for the rock to be
used for refractory purposes. Analyses show from 0.12
to 1.16 percent Fe 2 0-i content, with the mean at about
0.4 percent. Colors range from blue gray to creamy white
on fresh surfaces. No pure-white dolomite was observed
in the Fremont Peak vicinity. Rock having a high de-
gree of whiteness is present at Natividad, Monterey
County, and on the Crowe-Harris, Martin, Hamilton,
and intervening ranches on the southwest side of Cienega
Road in the Bird Creek- Vineyard district of San Benito
County. Dolomite in the Fremont Peak vicinity is fine
grained, in marked contrast to the medium and coarse-
grained limestones. It is also much finer grained than the
white dolomites of the Bird Canyon and Natividad dis-
tricts, and much of it will not disintegrate under cal-
cination.

Limestone and Dolomite (Isdo). The material mapped
as limestone and dolomite consists of limestone masses
having numerous replacement patches of dolomite. It is
by far the largest unit mapped insofar as areal extent is
concerned. The proportion of dolomite to limestone
varies from place to place but the authors doubt that

• Symbols "Is," "do," etc., which follow the unit names are used on
the accompanying geologic map.

CALIFORNIA DIVISION OP MINES

[Special Report 56

• !

Photo 1G. Banded assemblage of mixed carbonate rocks on the
south slope of Fremont Peak. The dolomite bands stand out in
relief from the limestone matrix.

any rock so mapped could be used in manufacture of
portland cement without extensive beneficiation or with-
out prohibitively expensive selective mining. However,
within this unit are small patches of good limestone and
others of good dolomite which could be quarried selec-
tively for an operation that does not require large ton-
nages or utilize massive mining procedures.

Siliceous Limestone with Minor Dolomite (ds). The
unit mapped as siliceous limestone with minor dolomite
includes various proportions of limestone and dolomite
plus replacement patches of silica in the form of quartz-
ite, and scattered veins and replacements of barite. The
rock is virtually unusable for any commercial purpose
except aggregate and fill unless it is crushed and the
principal mineral constituents calcite, dolomite, quartz i
and, locally, barite, are separated.

Limy Siliceous Dolomite (dosl). The unit mapped
as limy siliceous dolomite is chiefly blue-gray, medium-
grained calcitic dolomite containing considerable quartz
and miscellaneous silicate minerals formed by contact

I #

1". Detail of banded carbonate rocks on the south slope of Fremont Peak showing the cup-shaped solution pits that
develop on limestone and the seamed surface of relief on the dolomite bands. The outcrop is about six feet wide.

1959]

LIMESTONE AND DOLOMITE, NORTHERN GABILAN RANGE

+*'-

% v

Photo 18. A landscape just east of the Fremont Peak park headquarters. The rounded hill is underlain by gray,
vuggy quartzite formed by replacement of dolomitic limestone. Patches of gray replacement silica of this sort are found in
several places in the Fremont Peak district, but this mass, covering more than seven acres, is the largest.

metaraorphism. These rocks stand out boldly on weath-
ered surfaces, particularly where they are in contact
with limestone. Most samples analyzed contain more than
15 percent MgO and 9 percent Si0 2 . Possible commercial
usage would depend entirely upon the cost of separation
of calcite, dolomite, quartz, and silicate mineral grains,
and upon the percentage of waste residue.

Gray Quartz (q): The material mapped as gray
quartz is light gray, commonly vuggy quartz rock formed
by replacement of carbonate rock. The large mass which
extends east from Fremont Peak State Park boundary
onto Reeves Ranch might be usable as a source of silica.
It contains several hundred thousand tons of uniform
material, but no analyses are available.

Limestone with Granitic Rock and Schist Intercala-
tions (Isqs) : This unit consists of limestone and dolo-
mite interleaved with quartz-mica schist, quartz monzo-
nite and other granitic rocks. The proportion of car-
bonate rock to granitic rock is commonly small. Schist
and granitic rock are commonly present in roughly equal
proportions.

Quartz-Mica Schist (sch): The quartz-mica schist
unit consists of quartz-biotite-muscovite schist, commonly
containing a little feldspar and locally containing anda-
lusite, sillimanite, tremolite, and wollastonite, singly or
together; it is probably derived from clay shale.

Pink Quartzite (qp): The pink quartzite is a well-
bedded, slightly micaceous quartzite probably derived
from sandstone. Pinkish to vitreous hues prevail on fresh
surfaces. Weathered surfaces commonly are rusty brown.
It is generally associated with schist.

Schist and Granitic Rock (grs) : This unit contains
intimately mixed quartz-mica schist, pink quartzite, and
gneissoid to schistose quartz monzonite. It is commonly
cut by pegmatite and aplite dikes.

Granitic Rocks (gr) : The granitic rocks are largely
biotite quartz monzonite with local intrusions of biotite
granodiorite.

SEQUENCE AND STRUCTURE IN THE
FREMONT PEAK ROCKS

Sur Series Section

The Sur series section at Fremont Peak was selected
for study because it seemed the thickest, most nearly
intact sequence in the northern Gabilan Range. Detailed
mapping, however, revealed that the section was dis-
rupted in several places by granitic rocks which shoul-
dered aside the invaded metasediments, and that nowhere
could tops and bottoms of beds be determined with
certainty because of the amount of deformation and
degree of metamorphism to which the rocks had been
subjected. Inasmuch as overturning in any part of the
section cannot be proven or even suggested, and inas-
much a.s the layered rocks dip uniformly north at steep
angles in the neighborhood of 70 degrees, it may be
assumed that the older or lower part of the section lies
south of Fremont Peak and that the younger or upper
beds of the sequence lie to the north in a homoclinal suc-
cession. If these assumptions are accepted, the Sur series
sequence top to bottom (see section AA on the map) can
be described as follows :

CALIFORNIA DIVISION OP MINES
Sur series section at Fremont Peak.

[Special Report 56

Stratigraphic unit

Lithology

Thickness in feet

Subunit

Unit

Upper schist-

Middle carbonate schist -

Subunit 1 -

Subunit 2

Subunit 3

Subunit 4__

Subunit 5 . .

Subunit 6

Subunit 7

Subunit 8

Subunit 9

Subunit 10

Subunit 11_.

Subunit 12

Subunit 13

Subunit 14__

Lower schist .

Subunit 1

Subunit 2

Subunit 3

Brown quartz-mica schist with some interleaves of quartz monzonite.

White, coarse-crystalline limestone

Bluish medium-crystalline limestone

Black amphibolite

Brown quartz-mica schist

Gray limestone with cream-colored dolomite

Brown schist interbedded with pink quartzite

Gray limestone with cream-colored dolomite

Brown quartz-mica schist with quartzite

Gray limestone; no dolomite.. .

Gray limestone with cream-colored dolomite

Brown quartz-mica schist with interleaved quartzite and quartz monzonite

Cream-colored dolomite with relict patches of gray limestone

Gray limestone; no dolomite

Brown quartz-mica schist; limestone lenses

Brown quartz-mica schist

Gray limestone with cream-colored dolomite

Brown quartz-mica schist with interleaved quartz monzonite.

100
170
100
60
335
160
375
165
125
290
500
400
360
280

1,100

160

2,250

1,415
3,420

3,510

Total 8,345

J* 6

Mrs*; % frl>

; r (

,*• "■%

*>i

■ • -

\ H

• r V\\

%£m^

« -

.^•'

•^t..

a:#k

L .

- ^

Hackly, lichen-covered surfaces of replacement quartz cropping out a quarter of a mile east of Fremont Peak

park headquarters.

1959] LIMESTONE AND DOLOMITE, NORTHERN GABILAN RANGE 21

For comparative purposes the section across a large seated succession. (See accompanying cross section.) In

Sur series pendant west of Gabilan School is reproduced the present state of erosion the granitic rocks which have

from Allen (1946, p. 20) as follows: welled up under the Sur series have been exposed and

Fine-grained mica schist 200 ft. themselves deeply eroded. Many of the carbonate-rock

Limestone _. 250 ft. deposits cannot, therefore, be expected to carry down-
Limestone ~ I 300 ft ward more than a few hundreds of feet, whereas prior

Quartzite, pure, well-bedded 180 ft. to erosion they carried down many thousands of feet.

— The shallow nature of the carbonate-rock pendants crop-
Total _ 980 ft. pj ng out a i on g. Rocky Ridge west of Fremont Peak is

It is the belief of the authors that the thickness of wel1 shown in the lower tunnel of the ba.rite workings
units mapped as "schist and quartz monzonite" have not where the thickness of carbonate-rock has lessened ma-
been appreciably increased through invasion of granitic terially from what it is at ridge crest,
material, at least along the chosen line of cross section. Rocky Ridge Fault Zone
Much of the granitic rock within these units is schistose . .

or gneissoid and abnormally high in quartz and micas, A nar r °Z f ™ of brecciation follows the north slope

indicating that it has formed mainly by transformation of Rocky Ridge for more than a mile beginning at the

of aluminous sediment such as clay shale. The proba- road U P the eJ \ st face ° £ P "P° nt Peak - J h f bant f mm "

bility of thinning of the units by "compression during eralization and sporadic sihcification of the carbonate

folding is much greater than the probability of material roek follows \ his zo " e i; of brecciation. The zone apparently

thickening of any of the units through addition of gra- represents a line of faulting older than the invasion of

nitic material. Some of the carbonate units undoubtedly the granitic rock Granitic rock and schist probably have

have been thickened at some points and thinned at others J 10 * been involved in the brecciation, although accumu-

bv plastic flow during folding. latl0ns , °J talus m f k * he z ° ne n ! s « veral P la f s - The

* The following conclusions may be drawn from a study several short, northeast-trending faults mapped at the

of the stratigraphic succession in the Fremont Peak area: cre f ° f . Rock ^ R^ge probably are related to this line

1. The limestone and dolomite are not confined to any °! 1 fa ? t ^! s - The ma « nltude + of movement along this prob-
particular horizon or horizons in the Sur series sequence ; able ^ult cannot be ascertained as no significantly dif-

f, ,, -, n A + +• x,- ™„„i,. iLj„ „„ terent units have been brought into contact and the

there are no well-defined stratigraphic marker beds or . . , ^ ..

v • i • ,i topojjrap nc expression is not distinctive,

horizons anywhere m the sequence. F " ' F

2. The dolomite is present as replacement patches of „,„.,,„_„ ,,„_,.,., ,„, _„,.,.. AT , K .^ TL ,^

i •* -i.u- i v 4. i FIELD FEATURES USEFUL IN ESTIMATING THE

extreme irregularity within larger limestone masses and -*^„«....«. „^„„.„.. .,-..-„ ^- ,...-

, -x • t !.• £ *■ i • ECONOMIC POSSIBILITIES OF THE

nowhere is its presence indicative of any particular posi- „.„„«.,„„ „„„„ ..»„„.-„

.. . ,, \. , . * ^ v CARBONATE-ROCK MASSES
tion in the stratigraphic sequence.

3. Because of the extreme irregularity of distribution In the climate that prevails over the Gabilan Range
of dolomite in the carbonate-rock masses, presence of the carbonate rocks weather and erode far less readily
magnesium, the chief element controlling the commercial than the enclosing schist and granitic rocks. Hence they
utilization of the carbonate rocks, must be carefully stand out boldly in relief and may give a false impres-
checked by adequate sampling and chemical analysis of sion in estimating reserves. In areas where schist inter-
file samples. beds and granitic rocks may be numerous, accumulations

4. Most of the potentially commercial limestone and of talus and soil mantle may mask them. Presence of
dolomite deposits in the northern Gabilan Range are in granitic rock and schist in an apparently homogeneous
the central 3,000- to 4,000-foot thickness of the Sur mass of carbonate rock may generally be detected by
series, and are likely to be found enveloped in several presence of granitic and schist debris in the soil and by
thousand feet of mica schist. '& the distribution of erosion depressions worn into the

The metasedimentary rock units of 'the Fremont Peak softer rocks,

section appear to be "in conformable succession; and, As previously mentioned, limestone and dolomite in

although neither the top nor base of the series can be the Fremont Peak vicinity may be differentiated on

observed because of obliteration by granitic intrusions, weathered surfaces with considerable accuracy by dif-

the 8345 feet of beds present in the Fremont Peak section ferences in grain relief, grain size, and color. Limestones

probably represents only part of the original thickness of the Fremont Peak area are almost invariably coarser

of the sequence. As can'be seen from the accompanying grained and less dense than the replacing dolomite. On

map all three of the crudely defined units of the Sur se- weathered surfaces of mixed carbonate rock, dolomite

ries include both carbonate-rock and schist subunits. It is stands out in relief from matrix calcite because of its

also apparent that many of the subunits are lenticular greater resistance to weathering. So also do quartz and

and non-persistent, even in the small area mapped, and other silicate minerals, but these are detected by large

that the original succession was heterogenous. In addi- differences in hardness and texture between them and

tion the strong compressive deformation the metamor- the carbonate minerals. Dolomite most commonly weath-

phic rocks have been subjected to has undoubtedly caused ers pale buff or cream because of the slightly increased

thickening and thinning of many of the members, par- iron content of most dolomites. Limestones range from

tieularlv by means of plastic flow in the carbonate units. pure white through light gray to dark gray, or variega-

There are a number of structural features unrelated tions of these colors, and rarely weather buff or cream.

to the stratigraphic succession that should be mentioned. Pure white, medium-grained, crystalline dolomites were

The homoelfnal metasediments at Fremont Peak repre- observed only at the Natividad and Bird Creek Cienega

sent only the roots of a formerly vast, much deeper- districts. Coarse-grained dolomites were not observed.

CALIFORNIA DIVISION OF MINES

[Special Report 56

1959]

LIMESTONE AND DOLOMITE, NORTHERN GABILAN RANGE

Solution pits in dolomite generally are fewer and less
complex than those seen on weathered limestone surfaces
and they commonly develop in crisscrossing linear pat-
terns controlled by joints. Limestone surfaces are gener-
ally more universally and deeply pitted than dolomite
surfaces and lack the linear pit patterns. Unfortunately,
most of the readily apparent differences mentioned above
do not apply to freshly broken surfaces, so that they are
of little value to the quarryman. Chemical analysis or
staining with chemicals such as ferric chloride or cupric
nitrate are the most suitable methods of estimating cal-
cite-dolomite proportions in drill cores or freshly quar-
ried samples.

Where quartz or silicate minerals have been intro-
duced into the carbonate rocks by contact metamorphism
or have developed during metamorphism from original
impurities, they generally stand out in relief on weath-
ered surfaces of either limestone or dolomite. Conse-
quently, rocks having: prohibitively large proportions of
such minerals are easily detected on weathered surfaces.
Silicate-mineral impurities generally are also visible on
freshly broken surfaces of carbonate rocks because of
hardness and color differences.

An additional feature, probably applicable only to the
rocks of the Gabilan Peak vicinity, is that carbonate
rocks containing more than 2 or 3 percent silica generally
contain too great a proportion of magnesium to be use-
able in portland cement. As the siliceous minerals nor-
mally are detectable on weathered surfaces and com-
monly on broken surfaces too, their presence may be
used as a field guide in prospecting for cement rock.

ECONOMIC POSSIBILITIES OF LIMESTONE AND
DOLOMITE DEPOSITS

The following section of the report deals with most of
the deposits in the Gabilan Range north of the latitude
passing through Chualar which are sufficiently extensive
and reasonably accessible to be of economic interest. It
is arranged geographically north to south by districts
and alphabetically by deposit within each district. Be-
cause of time limitations, it has not been possible to
examine or describe each deposit in detail, and the fol-
lowing data are intended to indicate the general eco-
nomic possibilities of the deposits rather than to describe
them in detail.

Los Vergeles District (1) *

The geologic map of San Juan Bautista quadrangle
(Allen, 1946) shows a gently curving arc of limestone
pendants extending northwest across the San Juan Grade
(the county road connecting San Juan Bautista and
Salinas) between Mud Creek and Crazy Horse Canyons.
Most of the pendants are on Los Vergeles Ranch. One
of these pendants is well exposed in a small roadside
quarry on the west side of the road 3 miles northeast
of Lagunita reservoir. It is also well exposed on grazing
land east and west of the San Juan Grade.

The crystalline limestone is blue-gray to white on
fresh surfaces and generally weathers to a light blue-
gray. Rock textures range from medium coarse-grained
(cleavage surfaces averaging about 5 mm in longest
dimension) to fine-grained (grains averaging less than

* Numbers following the name of deposit or district correspond with
map locations on figure 4.

1 mm in longest dimension). Analyses of the rock are
not available, but a small tonnage, apparently taken from
the San Juan Grade quarry sometime during the period
1906-25 was found suitable for steel flux (Laizure, 1925,
p. 43). Much of the rock exposed in fields east of the
San Juan Grade shows replacement patches of silica and
dolomite. Exposures to the northeast of the Crazy Horse
Canyon road show even more extensive contamination
by masses of dolomite and silica. Any deposit developed
for commercial usages other than aggregate would have
to be carefully sampled and tested. In most places, the
selective mining that would be necessary to maintain
a uniform grade of rock probably would prove prohibi-
tively expensive. The largest lenses lie toward the
northern end of the arcuate belt of pendants adjacent
to Crazy Horse Canyon.

San Juan Canyon District

The San Juan Canyon district lies astride San Juan
Canyon 3 to 5 miles southeast of San Juan Bautista.
Deposits are nearly all limestone, no potentially com-
mercial dolomite deposits having been found in the vi-
cinity. The Ideal Cement Company plant at San Juan
Bautista is supplied from deposits of this district. Al-
though the deposits are not particularly large, the mag-
nesium problem, ever present in other districts, is not
critical in the San Juan Canyon district.

Barbee Ranch Deposits (2)

The Barbee Ranch deposits consist of several concen-
trations of fragmental carbonate rock astride Barbee
Canvon 1 mile south of the Ideal cement plant in the
N| SEi sec. 9, T. 13 S., R. 4 E., M.D. The mineral rights
are held by Ideal Cement Company. Some rock has been
taken from the vicinity in past years, probably for trial
purposes, but nothing has been taken in recent years.
Reserves are believed to be small. Blocks and boulders
of crystalline limestone mixed with soil and non-car-
bonate talus crop out in a. narrow belt crossing Barbee
Canyon in a southeasterly direction. According to Allen 's
map (1946) this material is part of the steep-dipping
lower Miocene Vaqueros formation. Typical samples col-
lected and analyzed by the cement company yielded the
following results :

CaO MgO AUO, Fe,0, K t O Na,0 SiOi

Sample 1 54.86 .4.'? 1.26 .40 NT).* N.I). 1.46

Sample 2 54.86 .52 .8!) .33 N.D. N.I). 1.06

Sample 3 51.02 .57 1.73 .75 .06 .07 5.10

* Not determined.

Bryan and Pearce-Twohy Deposits (3)

The Bryan and Pearce-Twohv properties lie in the
N^ sec. 24 (projected), T. 13 S., R. 4 E., and in the WJ
sec. 19 (projected), T. 13 S., R. 5 E., M.D., in what was
originally part of the Spanish grant Cienega del Gab-
ilan. They adjoin and cover elements of the same lime-
stone mass. Both properties are leased by Ideal Cement
Company; they constitute the principal current source
of supply for the San Juan Bautista cement plant.

The limestone mass is an elongate lens set on edge
between granite and quartz-mica schist walls. The mass
is slightly arcuate in plan, irregular in outline, and
oriented with its longest direction trending northwest.
It reaches a maximum width of 400 feet, an exposed
depth of 700 feet, and has been traced along its length

CALIFORNIA DIVISION OF MINES

[Special Report 56 J

*40

■ ■ ■*..-

Photo 20. Ideal Cement Company plant near San Juan Bautista, observer facing south. The plant utilizes
crystalline limestone and weathered granite and schist obtained locally, and clay shale hauled in from Chittenden
at the south end of Santa Clara Valley.

PHOTO 21. Underwood and Pearce-Twohy quarries of Ideal Cement Company three miles south of San Juan
ista as seen from the Fremont Peak road, observer facing northeast. The quarries are in white crystalline
one pendants, The pendants are enveloped in granitic rocks and schist and some granitic dike rocks penetrate
the limestone masses.

1959]

LIMESTONE AND DOLOMITE, NORTHERN GAB1LAN RANGE

for about 3,000 feet. Indicated reserves are roughly 20,-
000,000 tons, but part of this tonnage is not readily
recoverable because of unfavorable stripping ratios or
necessity for underground mining.

The average analysis obtained by taking the mean of
50 typical surface samples and 500 feet of diamond
drill cores (samples analyzed by the Ideal Cement Com-
pany laboratory) is as follows:

CaO 51.1

MgO 1.4

C0 2 41.7

Si0 2 4.1

Fe 2 3 )
AU0 3 J

1.7

Flint-Steinbeck Deposit (4)

This deposit is If airline miles west and slightly south
of the Pearce-Twohy deposit. The quarries are in sec.
23, T. 13 S., R. 4 E, M.D. and were driven into both
sides of the ridge that forms the southeast wall of Stein-
beck (Steinbach) Canyon. It was once a major source
of supply for the cement plant at San Juan Bautista,
but has been idle for many years.

Limestone occurs in a lono- thin lens, set on edge,
the edge trending N. 80° E. The sheetlike mass, tapered
at both ends, dips steeply south, has a traceable length
of nearly 3,000 feet and an average width of about 100
feet. It has been exposed to a maximum depth of 440
feet by erosion but probably continues below the level
of the adjacent canyon-bottoms. Wall rocks are granite
and quartz-mica schist. The limestone is similar in phys-
ical character and chemical content to the rock from
the Bryan and Pearce-Twohy properties. Considerable
limestone remains in the lens but recovery would be
expensive because of the attitude of the mass and be-
cause of the steepness of the topography. Much of the
remaining material would have to be mined underground
or else stripped at high cost.

Harmony Hills Deposit (5)

The Harmony Hills deposit is located at the north
end of the Crowe-Harris ranch close to the Fremont
Peak State Park road on part of the Cienega del Gabilan
land grant. It is near the center of sec. 20 (projected),
T. 13 S., R. 5 E., M.D. A dirt road crossing the deposit
connects with the Fremont Peak road.

White, coarsely crystalline limestone occurs in an
east-trending lenticular mass the exact outlines of which
are masked by soil and undergrowth. The mass is at
least 750 feet long and the thickest part of the lens
reaches 150 feet. Erosion has exposed the rock to a depth
of 60 to 80 feet. Probably reserves are well in excess
of 1,000,000 tons. A typical clean surface sample col-
lected by the authors yielded the following analysis
(Abbot A. Hanks, San Francisco, analysts) :

CaO H2.37

Mgo 2.eo

SiU 2 -32

Fe 2 O a -08

AU0 3 - 14

p s o s 01

Remainder 44.48 (chiefly CO s )

Underwood Deposits (6)

The Underwood deposits are a series of small, discon-
tinuous masses of limestone located near the San An-
dreas fault zone close to and on the northeast side of

the San Juan Canyon Road half a mile north of the
Pearce-Twohy deposit. The Underwood property is part
of the former Cienega del Gabilan land grant. It was
operated for many years as a source of cement rock by
the San Juan Portland Cement Company. The limestone
reserves have been largely depleted and the quarries
have been long idle.

The limestone masses arc within a triangular area
bordered on the north and east by faults and on the
west by San Juan Valley alluvium. The total reserves
probably never exceeded a million tons, and individual
masses ranged from a few thousand tons to several hun-
dred thousand tons. The masses are in granite and
quartz-mica schist wall rocks. The rock is similar in
character to that of the Bryan and Pearce-Twohy de-
posits.

Natividad District

The Natividad district includes those deposits within
the angle formed by the course of Gabilan Creek where
it turns southwest to debouch into Salinas Valley. Most
of the deposits lie between the town of Natividad, on
the south, and Sugarloaf Peak on the north. Dolomite
deposits greatly predominate over limestone deposits and
nearly all of the commercial production from the dis-
trict has been from dolomite pendants. The commercial
dolomite rock is snow white and medium crystalline, and
in chemical composition is near that of the pure mineral
dolomite. The best deposits lie south of Sugarloaf Peak,
as those of Sugarloaf Peak have been extensively con-
taminated by iron oxides. Elsewhere, the iron content
seldom exceeds 0.2 percent.

Kaiser Aluminum and Chemical Company Deposits (7)

Kaiser Aluminum and Chemical Company controls
most of the commercially desirable pendants of dolomite
near Natividad. These deposits supply two plants, one
at Natividad and one at Moss Landing. Opened in 1943,
the various quarries have furnished close to 3,000,000
tons of dolomite over the 14-year period.

Most of the dolomite has been quarried from a single
irregular mass aggregating almost three-quarters of a
square mile in plan. According to Allen (1946, p. 69),
the dolomite strikes N. 60-70° W. and dips 40-45° NE
but bedding is hard to identify in any of the quarry
faces. The mass has been exposed by erosion and quarry-
ing through a depth of 700 feet, but the lower surface
is very irregular and numerous dikes and sills of granitic
rock hinder quarrying and processing of the dolomite.
Waste material from the quarrying process sometimes
reaches 50 percent (Logan, 1947, p. 257). The rock is
uniform, snow-white, medium-crystalline material, the
roughly circular cleavage surfaces averaging 2 to 3 mm
in diameter. Virtually the only contaminating materials
come from granite intrusions and from stains coating the
joint surfaces. Because of the numerous altered granitic
intrusions and resultant poor recovery of diamond drill
cores, estimation of reserves is crude ; but the main mass
may contain as much as 100 million tons of dolomite of
relatively uniform commercial grade. A representative
analysis, furnished by the company is as follows :

CaO 31.7

MrO 20.5

SiOi 1.0

Fe 2 Oa 0.2

AUOa 0.2

C0 2 40.4

CALIFORNIA DIVISION OF MINES

[Special Report 56

1959]

LIMESTONE AND DOLOMITE, NORTHERN GABILAN RANGE

Photo 23. Quarries and waste dumps at the Xatividad plant of Kaiser Aluminum and Chemical Corporation. Granitic
intrusions into the dolomite cause a high proportion of waste, but the white crystalline dolomite is of unusually high purity
and total reserves are estimated to be about 100 million tons.

-^*£0'

Photo 24. A quarry face and i

oor at the Xatividad operation of Kaiser Aluminum and Chemical Corporation.

CALIFORNIA DIVISION OF MINES

[Special Report 56

*?S

, *?

4b * '"^6BJS-

*^^

* w

'-•• •,,.-*

f?*&Fi

. *-

■:.'4"-.

MgO

R 0:>

CO? & HoO

11.20

.004

.010

.06

.52

45.64

20.67

.003

.008

.05

.90

45.10

20.35

.003

.006

.05

.50

46.53

Photo 25. A newly blasted "boulder" of dolomite on the floor
of one of the Natividad quarries showing dikelets of granite peg-
matite cutting the dolomite. Granitic intrusions are the principal
detriment to quarry practice in this vicinity.

Porter Dolomite Deposit (7a)

Owned by James Porter, 701 Old Stage Road, Salinas,
California. Located 1£ miles south of the Kaiser holdings
and 1£ miles east of Natividad. The deposit is roughly
300 feet wide and 3,000 feet long, with an east elonga-
tion and nearly vertical dip. Granitic intrusions pene-
trate the mass in many places. Most of the rock is white,
medium crystalline dolomite similar to that in the Kaiser
deposits. Three adjacent 10-foot-long chip channel
samples cut perpendicular to the strike probably reflect
the chemistry of the deposit. Much of the deposit cor-
responds to samples 2 and 3.

Sample Fe SiOj AliO;i CaO

1 .10 1.52 .38 41.20

2 .40 .68 .30 33.00

3 .20 .84 .21 32.00

Sugar-loaf Deposits (8)

A broad belt of carbonate rock 600 to 700 feet
wide and over a mile long crosses Sugarloaf Peak in a
northeasterly direction. The southwest third of the mass
is mainly dolomite but toward the northeast this grades
into mixed dolomite-calcite rock. Northeast of the crest
the mass is poorly exposed because of thick brush and
soil mantle. Thus far, there has been no commercial
utilization of the Sugarloaf Peak rocks.

The dolomite is grayish-white, mottled with small
spots and clots of red iron oxide. Veinlets of iron oxide
and ferruginous silica cut the mass in numerous places.
Where the carbonate mass is poorly exposed east of the
peak, float consists of medium-grained and coarse-
grained blue-gray to white dolomitic limestone and
finer-grained off-white dolomite.

Because of the impure nature of the dolomite on the
southwest slope of Sugarloaf Peak and because of the
apparent mixture of calcite and dolomite rock east of the
peak, it is doubtful if satisfactory commercial deposits
can be developed in this vicinity.

Other Deposits

Several large pendants were mapped by Allen (1946)
east and northeast of Natividad, but he does not describe
them in the text and does not differentiate the limestone

and dolomite masses. Bethlehem Steel Company (for-
merly Pacific Coast Steel Co.) produced dolomite from a
small quarry half a mile north of the Kaiser Aluminum
and Chemical Company Quarries at various times between
1900 and 1940. Allen (1946, p. 67) mentions production
of limestone from the district about 1900 for use in
beet-sugar manufacturing, but the quarry is misidenti-
fied. The authors had no opportunity to examine deposits
other than those of Kaiser Aluminum and Chemical
Company and Sugarloaf Peak, but it is probable
that most of the others consist of mixed masses of cal-
cite and dolomite rock and that there are no other sub-
stantial deposits of commercial dolomite or limestone in
the district.

Fremont Peak District

As previously described, the Fremont Peak district
includes vast pendants of carbonate rock, but most of
these consist of intermingled masses of calcite and dolo-
mite rock that are not of present commercial interest.
Also, the remoteness of the district from utilities and
from rail and truck transportation reduces the chances
for exploitation of the deposits in the near future. Most
of the deposits lie on the Reeves and Bardin ranches,
parts of the Spanish grant Cienega del Gabilan. They
lie in an east-trending belt along the slopes of Rocky
Ridge and Fremont Peak, the belt dying out toward the
east in the Bird Creek drainage basin. None of the de-
posits in this district has been utilized commercially
thus far.

Bardin Ranch Deposits on Rocky Ridge (9)

Three small patches of limestone of potential com-
mercial grade are outlined on the geologic map which
accompanies this report, on the south slope of Rocky
Ridge about 1 mile west of Fremont Peak. Part of the
rock in these deposits is medium-grained, blue-gray
material suitable for general use where color is not im-
portant, and part is coarse-grained, pure-white material
suitable for whiting and white filler. None of the de-
posits has been developed or tested except for a few
samples taken by the authors but, judging from surface
exposures, more than a million tons of commercial ma-
terial might reasonably be developed in the three masses.
They are currently accessible by Gabilan Creek Canyon
via unimproved dirt road.

East Gabilan Deposit (10)

The East Gabilan deposit is the largest in the Fre-
mont Peak district and is situated on terrain favorable
to low-cost quarrying. It is on the Reeves Ranch just
west of the main ranch access road three-quarters of a
mile east and slightly south of Fremont Peak. The areal
extent is shown on the accompanying geologic map. The
mineral rights for this part of the Reeves Ranch are held
by Ideal Cement Company.

As seen in plan, the limestone mass averages about 300
feet in width and is approximately 2,800 feet long. The
beds, though crudely defined, strike N. 70-80° W. and
dip 65-80° N. Erosion has exposed limestone to a depth
of 140 feet below the highest outcrops and the mass has
been penetrated by several hundred feet of adits driven
for sampling purposes by predecessors to Ideal Cement
Company. Although overlain stratigraphically by dolo-
mite and dolomitic limestone, the stratigraphic sequence

1959]

LIMESTONE AND DOLOMITE, NORTHERN CiABILAN RANGE

Photo 26. A typical landscape on the Reeves Road west of Vineyard School showing initial stages in exploration of a
dolomite deposit by Westvaco Chemical Division of Food Machinery and Chemical Corporation. The brush-covered deposit
has been laid bare by a bulldozer cut and a diamond-drill rig is working.

'ir-v^ —

A closer view of the diamond drill rig shown in photo 2G.

• : *»''<£

\; -

* >iSw v

\~ s -J**^--

■<4b^-.-jC*.s,

J 21" ;

-^ LIT* 1 -

•' L

^•v**-

1959]

LIMESTONE AND DOLOMITE, NORTHERN GABILAN RANGE

Photo 28. (opposite). Bouldery limestone outcrops on oak
savannah at the power line deposits on the Reeves Ranch. These
deposits are among the largest undeveloped reserves in the Cienege-
Vineyard district.

dips too steeply for such material to cause an overburden
problem. Over most of the deposit there is no overburden
whatever. Estimated reserves calculated to a depth of
240 feet below the uppermost outcrops (a reasonable
recovery depth) total approximately 16,000,000 tons.
Substantial additional tonnage could be developed by
underground mining methods. The following analyses are
typical of the limestone. Samples 1-3 were done by Ideal
Cement Co. ; samples 5-7 by Abbot A. Hanks Inc.

Oxide #1 #2 #3 #4 #5 #6 #7

CaO 55.42 54.42 54.73 54.43 54.37 54.35 54.02

MgO .41 .92 .41 .53 .45 .48 .99

Si0 2 .24 .94 1.52 .86 .86 .92 .82

Fe,Oa .20 .14 .32 .11 .08 .11 .13

AbOa .10 .14 .34 .29 .26 .17 .13

K 2 .03 .02 .06 N.D. N.D. N.D. N.D.

Na»0 .04 .03 .02 N.D. N.D. N.D. N.D.

P,0 6 N.D.* N.D. N.D. .06 .03 .02 .02

Ignition loss ___ 43.72 43.53 43.18 43.72 43.95 43.95 43.89
(chiefly C0 2 )
* Not determined.

Fremont Peak Deposits (11)

Four substantial masses of limestone of probable com-
mercial grade are grouped together about a quarter of
a mile southeast of Fremont Peak. These appear on the
accompanying geologic map. The authors are not fa-
miliar with property boundaries in the vicinity, but the
limestone masses appear to be partly on the Bardin
Ranch and partly on Reeves'- ranch. Probable reserves
aggregate a million tons. The rock typically is light
blue-gray and ranges from medium to coarse crystalline.
The following analyses are representative of the rock.
Analyses are by L. A. Caetano, courtesy of Ideal Cement
Company.

Oxide Sample G-13

Sample G-14

Sample G-15

CaO 55.02

53.94

55.52

MgO .69

1.53

.46

SiOs 1.58

.42

.14

ALO3 .58

.08

.17

Fe^Oa .30

.08

.13

K2O .03

.01
.01

.01

Na a O .04

.04

Ignition loss 42.14

43.62

43.60

(chiefly C0 2 )

Bird Creek District

Included in this district are the deposits found on
the slopes of the Bird Creek drainage system. The dis-
trict adjoins the Fremont Peak district on the east, there
being no well-defined gap between them. Most of the
carbonate-rock pendants are limestone, and commercial
dolomite deposits have not thus far been found in the
district. The terrain is rugged and the vegetation is so
dense that not all of the carbonate-rock masses have
been explored. None of the deposits have been commer-
cially exploited thus far although several of them are
being held for future commercial use. The district falls
entirely within the boundaries of the Crowe-Harris,
Reeves, and Martin ranches.

A.S. and R. Deposit (12)

The American Smelting and Refining Company held
tin 1 A. S. and R. deposit for many years and at one
time planned to build rail connections to it. The deposit
is on the Crowe-Harris ranch at the confluence of Bird
Creek and North Canyons. Pure white, and blue-gray
and white variegated, coarsely crystalline limestone is
in a thin, lenticular, vertical-standing pendant within
granite and schist walls. It forms the crest of a ridge
trending N. 65-70° W. and has little or no overburden.
The limestone is exposed for a length of 1,800 feet and
an average width of 120 feet. A maximum of 200 feet
of depth is exposed above the level of Bird Creek but
the pendant continues downward for an undetermined
distance. Accessible reserves in the lens probably exceed
1,000,000 tons. A few small granitic dikes penetrate the
limestone but are not believed to be a serious problem in
quarrying. Most of the limestone is white rather than
colored. The following analyses on samples collected by
the authors and analyzed by Abbot A. Hanks, Inc., are
believed to be representative of the variations found in
the deposit.

Sample

a. p. 1.',

50.79

3.89

.36

.17

Oxide

CaO

MgO

SiOa

FesOa

A1,.0 3

P.Os

Remainder _.
(chiefly CO»)

Sample
G.P. 13
. 54.4S
_ 3.43
. 1.74
. .19
. .29
. .05
. 39.82

.03

44.51

Sample

G.P. 15

47.80

6.26

.58

.17

.27

.02

44.90

Sample

G.P. 16

52.24

.73

3.66

.23

.73

.04

42.37

53.18

55.30

55.50

55.45

55.50

37.99

2.13

0.43

0.36

0.54

0.35

14.83

0.25

0.30

0.35

0.2

0.1

0.30

0.10

0.35

0.25

0.20

44.13

43.90

43.96

44.10

43.25

46.77

95.

98.7

99.1

99.0

99.1

68.0

Seven other samples collected and analyzed by the
owner, Howard Harris, a trained chemist, yielded the
following results. The best of these probably indicate the
maximum quality of rock available in the deposit.
Oxide # 1

CaO 54.76

MgO 0.79

Si0 2 0.30

Fe,<> 3 0.25

AUOa

COo 44.69

CaCOa 97.7

Bird Canyon Ledge (13)

In 1925 the Old Mission Portland Cement Company,
one of the predecessors to Ideal Cement Company, ex-
plored the Bird Canyon Ledge deposit and acquired the
mineral rights on it. A geologic map was made by Bailey
and Robin Willis and some drilling was done with the
idea of adding to the cement-rock reserves for the plant
at San Juan Bautista. Ideal Cement Company still holds
the mineral rights, but the deposit remains undeveloped.
It is located 3| airline miles east of Fremont Peak on
the upper reaches of the main fork of Bird Creek at its
confluence with one of its unnamed tributaries. The de-
posit is accessible from the paved Fremont Peak road by
way of the Reeves ranch road and Bird Canyon jeep
trail, but is 14 miles by road from the cement plant at
San Juan Bautista. This distance could be materially
shortened by construction of a road from San Juan
Canyon up Bird Creek.

The Bird Canyon Ledge occupies the south slope of a
ridge trending N. 65° E. The north border of the prin-
cipal limestone mass lies close to the ridge crest. The
south border, which is more irregular, lies close to the

CALIFORNIA DIVISION OF MINES

[Special Report 56

canyon bottom. In plan the deposit is about 600 feet
long parallel to the ridge and 800 feet long perpen-
dicular to the ridge. From ridge crest to creek bottom,
a depth of 560 feet of limestone is exposed. The predom-
inating surface trace of well-developed joints in the
limestone strikes X. 55-60° E. and the joint surfaces dip
very steeply southeast, but it is doubtful if this repre-
sents bedding. Bonafide bedding was not observed in
I he main mass. The limestone is bounded on the north
and south by quartz-mica schist and granite; on the east
by granite; and on the west by a mass of gray replace-
ment silica. From exposures seen along the creek the
deposit bottoms in granite and silica and the chances of
it continuing down below the level of the creek bottom
are small. Roughly 10,000,000 tons of carbonate rock are
present in the main mass but there is some question as
to whether all of this is suitable for manufacture of
Portland cement.

The old Willis map shows the locations of six drill
holes, but the cement company was unable to find a rec-
ord of structure sections or analyses connected with this
drilling. Although the surface of the mass does not show
the presence of excessive dolomite there is sufficient var-
iation among the analyses in the table below to indicate
that the magnesium content of the mass as a wdiole may
be too high for use in portland cement.

Middle Dam Deposit (14)

The Middle Dam deposit is on the Crowe-Harris ranch
on the east slope of North Canyon a quarter of a mile
northwest of the A. S. and R. deposit. It is undeveloped.
A geologic map published by Taliaferro (1948) shows a
lenticular, arcuate, northwest-trending mass of lime-
stone nearly a mile long and reaching a width of 500
feet at the broadest part of the lens. However, the lime-
stone is not continuous at the surface within the area
shown on the map and contains numerous leaves of schist
and granodiorite. Individual masses of mineable lime-
stone probably do not exceed 200,000 tons each, although
several might be suitable for development. The terrain
is rugged.

The only available analysis was done on a typical
sample of the rock collected by Harris and Bowen in
lf>54, Abbot A. Hanks, Inc., analysts.

Remainder,
Sample no. SiO- Fe-Os Al-Os CaO MgO P2O5 chiefly CO-
OP^ .52 .08 .16 43.17 10.32 Trace 45.7,")

The analysis shows that at least part of the rock in the
lens is dolomitic, although some rock of lower magnesium
content might be expected in the deposit.

Power Line Deposits (15)

The Pow-er Line deposits are 9 airline miles southeast
of San Juan Bautista on the Reeves ranch at the head-
waters of the Bird Creek drainage. They lie on the divide
between Bird and Swamp Creeks. Mineral rights are
held by either Ideal Cement Company or Dr. Rollin
Reeves. The deposits are accessible by improved dirt road
via the Fremont Peak and Reeves Ranch roads.

Limestone crops out over rolling terrain suitable for
ordinary quarrying operations. Reserves total many mil-
lions of tons, but so far as the authors know have not
been explored or tested. On surface outcrop the limestone
is medium-coarse-grained, light blue-gray rock without
apparent large dolomite replacement masses. Although
somewhat remotely situated, the Power Line deposits
are sufficiently large to warrant exploration and testing
for possible use in portland cement or for general chem-
ical use.

Upper Bird Creek Deposit (16)

The Upper Bird Creek limestone mass is on the west
side of Bird Creek Canyon a quarter of a mile south of
the Bird Canyon Ledge, or nearly midway between the
Bird Canyon Ledge and Power Line deposits. It is on
the Reeves ranch, but mineral rights are probably held
by Ideal Cement Company.

In plan this deposit is shaped like a T-bone steak
with the longest axis trending northwest. It is about
1,600 feet long, 800 feet in maximum width, and has
been exposed to a depth of over 400 feet. Most of the
rock is medium-grained, blue-gray material suitable for

Analyses of samples from ihe Bird Canyon Ledge.

Sample number

CaO

MgO

Si0 2

Fe203

AI2O3

P2O5

Na 2

K2O

Ignition loss
(chiefly CO2)

G-21 51.20

G-22 50.00

G-23 35.60

G-24 41.70

G-25... 39.00

G-26 39.10

G-42 53.93

G-43 54.83

G-44 43.38

G-45 46.17

G-46 42.29

G-47 36.86

G-48 40.64

44.74

29.64

34.63

38.00

50.95

35.88

49.45

3.25

7.36

3.26

3.72

17.54

9.96

14.94

2.12

19.13

4.86

13.74

4.00

1.21

0.80

0.10

0.15

17.90

7.84

0.81

7.38

6.22

13.95

3.72

4.45

6.04

6.79

4.24

18.57

7.06

17.54

0.12

11.37

6.04

1.91

3.52

16.78

0.28

5.34

0.32

0.19
0.23
0.73
0.12
0.39
0.53
0.05
0.05
0.02
0.03
0.25
0.13
0.16
0.14
0.17
0.03
0.03
0.02
0.02
0.01

0.39
0.35
0.89
0.22
0.51
1.05
0.20
0.02
3.67
0.21
1.80
0.70
0.68
1.16
0.83
0.03
1.79
0.42
0.12
0.07

N. D*

0.14

0.05

37.88

N. D*

0.09

0.03

42.21

N. D*

0.05

34.27

N. D*

0.03

0.01

39.68

N. D*

0.03

0.01

34.66

N. D*

0.05

0.10

39.68

0.03

N. D.

43.78

0.27

N. D.

43.93

0.01

N. D.

34.87

0.01

N. D.

44.93

0.17

N.D.

N. D.

41.89

0.10

N. D.

44.54

0.14

N. D.

41.89

0.56

N. D.

42.37

0.10

N. D.

47.63

0.02

N. D.

42.70

0.07

N. D.

43.02

0.16

N. D.

46.91

0.01

N. D.

44.78

0.03

N. D.

one.

6 were analyzed by L. A. Caetano, Ideal Cement Company, San Juan Bautista. Samples G-42 through G-55 by
-San Francisco.

1959]

LIMESTONE AND DOLOMITE, NORTHERN GABILAN RANGE

Portland cement, but a dolomite streak crosses the north-
west end of the mass and the deposit would have to be
thoroughly tested by drilling to establish whether the
mass as a whole is sufficiently low in magnesium. Five to
10 million tons of limestone could probably be developed
in the Upper Bird Creek deposit. The following analyst's
done by L. A. Caetano, courtesy Ideal Cement, on type
samples collected by Gray are representative of most
of the deposit :

Ignition

Sample no.

SiOs

FerOa

Al .

CaO

Mi)0

K,0

Na :

loss

G-16

.38

.12

.18

55.4

.35

.03

.10

43.65

G-17

.50

.12

54.S

.54

.03

.04

43.6S

G-18

.88

.08

.10

50.2

4.38

.03

.05

44.18

G-19

1.28

.11

.13

54.0

.39

.05

.13

43.34

G-20

.98

.15

.17

54.2

.35

.05

43.26

Cienega- Vineyard District

The Cienega-Vineyard district adjoins the Bird Can-
yon district on the southeast. Most of the deposits lie
along the west side of Cienega Road between Cienega
Valley and Bird Creek 6 to 9 miles southeast of Hollister.
Both commercial dolomite and commercial limestone de-
posits are found in the district, as well as mixed deposits
of no present economic importance. The chief product
quarried at present is white crystalline dolomite. Both
medium-grained and coarse-grained deposits are avail-
able. The carbonate-rock deposits lie largely on the
Crowe-Harris, Martin, Beeves, and Hamilton ranches.

Garner- Harris Deposits (17)

The Garner-Harris deposits are a series of discontin-
uous masses of limestone and dolomite on the Garner-
Harris ranch three-quarters of a mile west of Vineyard
School. The property is adjacent to the Kaiser-Harris
dolomite deposit on the north. The property is owned
by Howard Harris, 7800 Cienega Road, Hollister.

The limestone bodies crop out at or near the crest of
an east-trending ridge. Their exact outline is obscured
by soil and vegetation. Much of the limestone is brec-
ciated, but not badly contaminated by impurities. Coarse-
grained grayish-white and medium-grained bluish-gray
rocks are the most common types. The masses are partly
developed by trenching and test pits which indicate that
several hundred thousand tons of rock could be selec-
tively mined. Schist interbeds and granitic intrusions
interfere with quarrying and some stripping of these ma-
terials would be necessary to fully exploit the masses of
limestone. The limestone and dolomite masses commonly
are separate and only a few masses of mixed rock were
conspicuously exposed. Four type samples collected by
Harris and Bowen in 1954 yielded the following analyses
(Abbot A. Hanks, Inc., analysts) :

Remainder,
Sample no. S1O2 FtiOi AI2O3 CaO MgO P1O5 chiefly COj

GP-17 1.44 .1!) .67 53.5 .69 .02 43.49

GP-18 1.76 .21 .33 54.2 .51 .01 42.08

GP-19 1.18 .34 .36 54.0 .46 .03 43.03

HU 1 .32 .11 .10 25.0 26.16 .02 48.29

Photo 29 General view of the Westvaco Chemical Division of Food Machinery and Chemical Corporation operation near

Vineyard, San Benito County.

1959]

LIMESTONE AND DOLOMITE, NORTHERN GABILAN RANGE

Photo 30. (opposite). Dolomite crushing and sizing installa-
tion at the Vineyard quarries of the Westvaco company.

Kaiser- Harris (Permanente) Dolomite Deposit (18)

Kaiser Aluminum and Chemical Company owns a
dolomite deposit near the corner common to the Martin,
Reeves, McCray and Garner-Harris ranches about 2
miles southwest of Vineyard Winery. It was purchased
from Cassie Crowe and Howard Harris in 1943 after an
extensive trenching and diamond-drilling program had
been completed by the company.

Coarse-grained, pure-white crystalline dolomite is ex-
posed in a series of bulldozer trenches cut into a len-
ticular pendant having irregular boundaries. The long
axis of the pendant, which is oriented northeast, is at
least 1,000 feet long and the width of the mass at its
broadest point exceeds 400 feet. The mass has been ex-
posed by erosion to a depth of 140 feet. Reserves total
several million tons.

An analysis of a typical sample, collected by Harris
and Bowen in 1954 and analyzed by Abbot A. Hanks,
Inc., yielded the following result :

Remainder,
Sample no. SiO- Fe.'Cb Ab0:< CaO MgO P.O.-. chiefly COa

G.P. 23 .14 .11 .17 30.80 22.13 Trace 47.63

Martin Ranch Deposits (19)

There are several limestone and dolomite deposits in
the southeast corner of the E. L. Martin ranch, which
corners on the Kaiser-Harris dolomite property 2 miles
southwest of Vineyard School. The largest dolomite

deposit was explored to some extent by Westvaco Min-
eral Products Division of Pood Machinery and Chemical
Corporation in 1956. Although reserves are not yet
proven, this pendant is both large and uniform and
ultimately will probably be developed for refractory dol-
omite.

Although the boundaries of the dolomite pendant are
obscured by soil cover and vegetation, the mass is at least
1,200 feet long and 200 feet wide. The long axis of the
deposit trends northwest over a slope favorable for
quarrying. Five or six bulldozer cuts spaced 50 to 100
feet apart have uncovered uniform, coarsely crystalline,
white dolomite. The mass lias been exposed by erosion
to a depth of at least 300 feet.

Remainder,

Sample

no.

SiOj

Fej0:i

AIlO:>

CaO

MgO

P.O.

chiefly CO-

M.I).

.2(i

.02

.00

2!).7.">

22.19

.02

46.70

M.D.

.is

.05

.04

20.r>7

22.35

.02

47.79

M.D.

.44

.01

.OS

20.71

22.13

.01

47.02

M.D.

.32

.0(1

.10

29.91

21.98

.03

47.00

McCray Ranch Deposits (20)

The McCray ranch borders the Kaiser dolomite prop-
erty on the south and parts of the carbonate-rock pend-
ant extend south into the McCray property. The McCray
ranch is owned by Dr. Rollin Reeves of Salinas.

Although not well exposed, the white dolomite of the
Kaiser deposit apparently extends onto the McCray
ranch. The lobes of limestone that extend onto the Mc-
Cray property are not extensive and presence of dolo-
mite of commercial grade is improbable. Much of the
limestone, however, appears to be of good quality as

Photo 31 (helow). Quarry benches in snow-white, crystalline dolomite at Westvaco's Vineyard quarries. Pendants of dolomite are
suspended in granitic rock and schist. Granitic intrusions offer some hindrance to quarry management but part of this cost is offset by
the broken character of the rock. The quarry is close to the San Andreas fault zone and the deposit was well broken up prior to the time
of quarrying.

■

f r

CALIFORNIA DIVISION OP MINES

[Special Report 56

Photo 32. Closeup of quarry-run crystalline dolomite at Westvaco's Vineyard quarries.

shown in the following analyses done on type samples
collected by Harris and Bowen in 1954 and analyzed by
Abbot A. Hanks, Inc. :

Remainder,
Sample no. SiOs Fe.O AI2O3 CaO MgO P2O5 chiefly CO ■

GP-20 .20 .15 .19 54.44 .66 .02 44.34

GP-21 .34 .09 .27 54.80 .54 .03 43.97

Palmtag-Harris Ranch Deposits (21)

Several lenses of limestone are present on the old
Palmtag property now owned by Howard Harris of Hol-
lister. These deposits are about half a mile southwest of
Vineyard School.

The principal mass of limestone occurs as an irregular
lens-like pendant, the long axis of which trends about
X. 70° W. It is roughly 2,000 feet long by 400 feet wide
and is exposed to a depth of over 120 feet. Several mil-
lion tons of carbonate rock are present, but the relative
abundance of calcitic and dolomitic strata in the mass
has not been adequately determined. The stratification,
although crudely developed, appears. to strike northwest
and to dip steeply northeast. Dolomitic strata appear to
be most abundant in the southwestern or lower part of
the section. The limestone is medium to coarsely crystal-
line and is chiefly blue-gray with faint banding.

The following table of analyses done on random
samples taken by Harris and Bowen in 1954 and 1957
give some idea of what range of chemical composition
is likely in the mass. The single dolomite sample HHP-10
was hand-picked from one thin band and is not represent-
ative of a significant volume of rock. Analyses are by
Abbot A. Hanks, Inc., of San Francisco.

Remainder

Sample no.

SiOa

Fe-03

AI2O3

CaO

MgO

P2O5

chiefly CO

HHP-1

1.22

.15

.48

53.72

.46

.02

43.95

HHP-2

.61

.15

.30

54.45

.69

.01

43.79

HHP3

.59

.11

.31

54.49

.75

.02

43.73

HHP-4

.25

.11

.26

54.67

.69

.07

43.95

HHP-5

6.59

.18

1.34

49.60

.60

.02

41.67

HHP-6

.84

.11

.44

54.49

.48

.03

43.61

HHP-7

.39

.29

.43

54.90

.25

.05

43.69

HHP-8

2.06

.29

.69

53.40

.45

.02

43.09

HHP-9

2.34

.22

.81

51.56

1.90

.11

43.06

HHP-10

___ .28

.09

30.51

21.44

.02

47.57

Reeves Northeast Deposits (22)

The Reeves Northeast deposits are in the northeast
corner of the Reeves ranch where it corners on the Mar-
tin, McCray, and Kaiser properties. The principal mass
is probably the northwest extension of the pendant
which crops out on the Kaiser and McCray properties,
but the boundaries of this mass have not been mapped.

Blue-gray, medium and coarse crystalline limestone
without overburden, crops out along the summit of a
northwest-trending ridge. The beds have a general east
strike and steep north dip. No patches of dolomite were
noticed in the exposures examined and no granitic intru-
sions were seen. Several million tons of limestone could
probably be developed. Only one analysis is available,
done on a type sample collected by Harris and Bowen
in 1954. The analysis is by Abbot A. Hanks, Inc., of
San Francisco :

Remainder,
Sample no. SiOs re . Al 0: CaO MgO P. -Or. chiefly CO2

G.P. 22 1.56 .17 .95 52.67 1.41 .03 44.21

1959]

LIMESTONE AND DOLOMITE, NORTHERN GABILAN RANGE

Westvaco (O'Hara Ranch) Deposit (23)

The quarries of the Westvaco deposit are a prominent
landmark in the hills just southwest of Vineyard Win-
ery. They were opened in 1947 by the present owners,
the Westvaco Mineral Products Division of Food Ma-
chinery and Chemical Corporation, to supply the chem-
ical plant at Newark. Since that time the quarries have
produced roughly a million tons of high-grade dolomite.
Prior to 1947 small tonnages of dolomite had been quar-
ried from time to time — some as early as 1915.

"White, medium crystalline dolomite occurs in a north-
west-elongated mass roughly oval in plan. The mass is
at least 1,800 feet long and 600 feet wide and has been
explored to a depth of nearly 200 feet. It is enveloped
in deeply weathered schist and granitic rock and granitic
intrusions penetrate the dolomite in several places. The
deposit is in or close to the San Andreas fault zone
and the dolomite has been thoroughly crushed through-
out the deposit. This lowers the cost of quarrying but
raises the proportion of waste material. Several million
tons of usable rock were proved. Further exploration
was being done during the summer of 1958. According
to the company the rock runs close to the theoretical
composition for dolomite — slightly over 21 percent MgO.
Iron oxide stains along the fracture surfaces are the only
visible impurity. Logan (1947, p. 278) lists an analysis
made by Smith-Emery Company from a sample col-
lected toward the north end of the mass from a quarry
then operated by A. E. Hamilton, which is probably
representative of the deposit :

SiOs

Abth

FtjOi

CaO

MgO

CO?

.17

.36

.11

31.00

21.23

.006

47.30

Hartnell District (24)

The Hartnell district, which adjoins the Natividad
district on the south end, lies 5 to 7 miles east of Salinas.
The geologic map of C. L. Herold, made in 1934 as part
of a graduate thesis at the University of California, de-
lineates a large number of limestone pendants scattered
over an area of 40 or 50 square miles. Most of these are
on the J. C. Bardin ranch and Barnes ranches.

Pendants in the Hartnell district that were examined
by the authors were found to consist either of inter-
mingled dolomite and calcite rocks or of carbonate rocks
intermingled with granitic rocks and with quartz-mica
schist. Although small masses of white limestone and
white dolomite of acceptable continuity and satisfactory
commercial quality probably can be developed in the
Hartnell district, it is doubtful if many of these exceed
200,000 tons; many would be much smaller. One small
quarry on the old Hartnell College property once yielded
limestone for construction purposes, but this has not
been operated recently.

Bluerock Mountain-Quail Creek Deposits (24a)

These are on the Barnes ranch (formerly the Norvel
and Silacei properties) on Old Stage Road 8 miles south-
east of Salinas and 3 to 6 miles northeast of Chnalar. The
Bluerock Mountain deposit lies on the west slopes of the
mountain on rugged topography. It is an oval mass
having a slight east elongation and underlies most of
the SW| sec. 25, T. 14S., R. 4E., M.D. It has not yet been
sampled or otherwise explored. Most of the mass appears

to be medium crystalline, blue-gray limestone. Reserves
probably aggregate many millions of tons.

The Quail Creek deposit was under development in
May, 1959, by Barnes Construction Company of San
Marino, California, as a source of roofing granules and
industrial limestone. A small quarry at the northwest
end of the deposit was worked as a source of material
for lime early in the 1900 's. The main mass is a sheet-
like pendant set on edge striking N. 65-75°W. and dip-
ping 22-45°SW. It forms a blanket (dip slope) on the
west slope of the ridge. The thickness of the sheet varies
from 120 feet in the center of the mass to less than 50
feet at the northwest end. The limestone is enveloped
in granitic rock and schist and granite dikes penetrate
the mass in numerous places. The northwest end of the
deposit consists of mixed limestone and dolomite but the
southeasterly two-thirds appears to be principally lime-
stone. Most of the deposit has been stripped of its thin
overburden of soil and caliche and has been thoroughly
explored by trenching, rotary drilling and diamond-
drilling. More than 2,000,000 tons of rock have been
blocked out, but because of granitic intrusions and dis-
coloration patches, recovery may run less than 50%. The
CaC0 3 content may be expected to vary from 96% to
less than 60%. The proportion of high-grade to marginal
and sub-marginal-grade rock has not been determined.

Mount Harlan-McPhails Peak District

Limestone masses of many different sizes are found in
the Mount Harlan vicinity west of Cienega Valley be-
tween Pescadero Canyon and McPhails Peak. A number
of these have long been held by commercial firms as
potential sources of commercial limestone, and ruins of
several lime kilns operated prior to 1910 are found in
the district. As the deposits are more remotely situated
from markets and from rail transportation than those
to the north and west, there has been less incentive to
develop them. Dolomite deposits of notable size have
thus far not been found. The pure white crystalline de-
posits are the most likely to be used in the near future.
None of the limestone deposits are believed to be large
enough to support a cement plant of comparable size to
most operating in California.

Cowell-Thompson Creek Deposit (25)

For more than 50 years the Henry Cowell Lime and
Cement Company held this deposit and it once supported
a bank of large lime kilns. Deposit and kilns are conspic-
uous landmarks on the north side of Thompson Creek
Canyon, 3 miles by road southwest of Cienega School.
The property is administered by the Henry Cowell Trust
Estate, 2 Market Street, San Francisco, California.

The deposit is a thin, sheet-like pendant, set on edge,
60 to 100 feet wide and nearly half a mile long. It crops
out over very steep topography, between granite and
schist wall rocks. The long axis of the mass trends almost
due west and the strata dip very steeply north. Granite
sheets and quartz-mica schist interbeds are found at nu-
merous places within the limestone. Because of the
thinness and attitude of the limestone mass, much of it
would have to be mined underground. Dolomite was not
observed in the deposit and most of the rock appears
to be medium to coarsely crystalline white or blue-gray
limestone low in impurities. Analyses are not available.

CALIFORNIA DIVISION OF MINES

[Special Report 56

Photo 33. Old lime kilns and quarry at the Cowell Thompson Creek deposit in the Mt. Harlan district about 16 miles

by road south of Hollister.

The pendant contains several millions tons of limestone
but selective mining would be necessary to maintain a
uniform grade of rock.

Hamilton, Harlan, Mayries, and McPhail Deposits (26)

The Hamilton, Harlan, Mayries, and McPhail are ad-
joining properties clustered together on and to the north-
cast of Mount Harlan. They are in sections 13, 14, 22,
23, and 24, T. 14 S., R. 6 E.,' M.D. The disposition of the
property boundaries is not known to the authors. A good
dirt road connects with the properties by way of the
Thompson Creek and Cienega Valley roads. A. E. Ham-
ilton, Box 621, Hollister, is reported to have done devel-
opment work at one of the deposits during 1956.

The most promising deposit, which consists of pure
white, coarsely crystalline limestone without visible im-
purities or serious discoloration, lies close to the bound-
ary common to the SW^ sec. 14 and the NW^ sec. 23.
The rock crops out on a hill, occupying 5 or 10 acres,
but the exact size and shape of the mass is not known
to the authors. A small quarry, from which a few thou-
sand tons of material has been removed, evidently sup-
plied the bank of kilns which lie close to the Harlan
Creek road northwest of the quarry. The extent of out-
crops suggests that there may be half a million to a
million tons of limestone in the mass, but it would have
to be selectively mined because of the prevalence of gra-
nitic intrusions. This deposit is attractive as a source of
high-calcium, pure white limestone for glass and for
ialty products.
■ Hamilton deposit, on the south slope of Mount
Harlan, is a lenticular, east-striking, south-dipping mass

about 1,000 by 3,000 feet as seen in plan. It may contain
as much as 20,000,000 tons of carbonate rock. The fol-
lowing mean analysis is the composite of 33 samples
taken at 5-foot intervals perpendicular to the strike of
the main part of the mass.

CaO '- 52.22

MgO 2.60

SiOs 0.75

RaOa 0.64

Loss on ignition 44.79

Logan (1947) lists an analysis of a sample taken from
the Hamilton property and analyzed by Smith-Emery
Company of Los Angeles :

SiOa 0.14

AlsOa 0.10

Fe 2 3 0.02

CaO 54.19

MgO 0.84

Loss on ignition 44.34

Calculated as CaCOa 96.72

There are other masses of white and blue-gray limestone
in the five sections listed above, but most of these are un-
developed and untested.

Westphal Ranch Deposits (27)

These deposits are on the south slopes of Mt. Olds 9
miles east of Chualar in sees. 10 and 11, T. 15 S., R. 5 E.,
M.D. They are owned by the Herald Ranch, a trust es-
tate, Herb G. Meyer, 16 San Pedro St., Salinas, Califor-
nia, manager. The beds are sinuous but have a general
east strike and a steep south dip. The carbonate rocks are

1959]

LIMESTONE AND DOLOMITE, NORTHERN GABILAN RANGE

Photo 34. The western portion of Roeky Ridge, observer facing west, showing white dumps of the barite workings. Be-
tween 1016 and 1920 about 2,700 tons were mined from veins in this vicinity that yielded $31,000.

interbeddecl with quartz-mica schist and quartzite, and
some skarn-rock has developed along granitic contacts.
None of the lenses exceed 200 feet in width and they are
scattered along more than a mile of strike length. The
rock is medium-to-coarse-crystalline and blue-gray to
nearly white. The chemical variations based upon nearly
100 surface samples are indicated in the following table :

CO and
MgO S P Mu RO, HO

.0-18.2 .01-. 003 .001-. 13 .02-. 06 .54-. 93 42-44

Fe SiO:. AfeOa CaO
.15-.30 .28-5.2 .07-. 63 32.9-53.8

Because of the heterogeneous nature of the deposit uni-
form rock can only be obtained in masses containing less
than 1,000,000 tons each; many are much smaller than
this.

Willow Creek District (28)

Wilson's (1943) geologic map of San Benito quad-
rangle shows two elongated masses of limestone in the
San Andreas fault zone just west of the county road
and the San Benito River between Willow Creek and
Jungle Inn. According to Logan (1947, p. 274), most
of the limestone is on the George Mendeley ranch in
the Wi sec. 27, T. 15 S., R. 7 E., M.D.

The limestone extends along the county road in a
northwesterly direction for over a mile and reaches a
thickness of 400 or 500 feet. The rock has been thor-
oughly brecciated in the San Andreas fault zone but in
general has been firmly re-cemented. It is medium-
grained, blue-gray or blue-gray and white rock. A single
composite sample taken across a 75-foot width of sec-
lion near the southern end of the main mass by C. A.
Logan in 1946 yielded the following results (Abbot A.
Hanks, Inc., of San Francisco, analysts) :

CaCOa 92.08

MgC0 3 2.99

SS :::::::::::::::::::::::::::::::} °-

Insolubles, chiefly SiOu 4.59

No development work has been done on the deposit but
the surface area underlain by it is large enough to attract
a cement company should the above analysis be repre-
sentative of the deposit.

Barite Deposits West of Fremont Peak (29)

Small deposits of barite have been found in the Fre-
mont Peak area extending westward from the peak along
the ridge for a distance of nearly 1 mile. An area ap-
proximately 800 feet long, beginning about 800 feet
west of the peak, has been the most extensively pros-
pected. Three northwest-trending adits have been driven
into the ridge and there are a number of shallow prospect
pits along the south side of the ridge. The eastern part of
the deposit lies within Fremont Peak State Park; the re-
mainder is on the Bardin ranch.

According to Bradley and Logan (1917, p. 624) out-
croppings on Fremont Peak were investigated as early
as 1864 by miners who believed they had discovered
silver ore, but who abruptly ceased prospecting when
they found no silver. Apparently there was no further
development until 1915 when L. II. Day and II. W. Un-
derwood of Hollister took an option on the property and
made a few shallow prospect cuts. Mr. Day reported
that two assays showed 98.6 percent and 99.7 percent
barium sulfate. In December 1915, Win. A. Farish Jr.
of San Francisco was reported to be opening the prop-
erty (Bradley and Logan, op. cit.). This venture marked

II)

CALIFORNIA DIVISION OF MINES

[Special Report 56

Photo 35. Closeup of white crystalline barite in veins and
replacement salients cutting gray dolomitic limestone. The veins
are developed along a zone of brecciation, probably an old fault
zone, which parallels the east side of the crest of Rocky Ridge. The
barite-bearing solutions which formed the veins probably were
related to the granitic intrusions. A little scheelite is also found
along the granitic rock-limestone contact below the barite workings
and farther toward the west.

the beginning of a 5-year period of operation with pro-
duction as follows :

Short tons
Year of barite * Value *

1916 225 $1,373.00

1917 1226 11,034.00

1918 100 1,500.00

1919 1000 15,000.00

1920 150 2,250.00

2701 $31,157.00

• Statistics compiled by C. J. Kundert, January 1956.

During this 5-year period the Fremont Peak area furn-
ished an average of approximately 25 percent of the
total recorded barite production in California and in
one year, 1918, 100 percent (Bradley, 1921, p. 116).

By 1920, the property was being operated by A. R.
Raskins, llollister, under a lease from Messrs. J. and H.
Bardin, Salinas (Boalieh, 1921, p. 156). The material
was hauled by truck to the railroad at San Juan Bau-
tista, a distance of about 12 miles. This was the last year
for which production was reported, and the property
has been inactive since.

The barite is crystalline, white to gray in color, and
ranges from fine to coarse grained. It occurs as pods or
discontinuous veins which pinch and widen from 2
inches or less up to several feet and are complexly
interwoven with silicified dolomitic limestone. The barite
zones cut across the apparent bedding of the carbonate
host rock, which in places is a very cavernous limestone
with a notable development of light brown colored drip-
stone.

The barite shows a characteristic banded structure.
Well-defined barite vein material replaces dolomitic
limestone and fades off into a replacement pattern in
the carbonate rock typical of epithermal deposits. Low-
temperature mineralization is also indicated by the ab-
ut' wollastonite and epidote.
principal mining centered in an area about
eet west of Fremont Peak. Here an upper adit,

20 2-59

stoped to the surface in places, was driven into the ridge
from the north side at about N. 30° W. and opens into
a large stope, open to the surface, on the south side of
the ridge. The stope more or less parallels the ridge and
is about 75 feet long, 25 feet wide, and of unknown depth.
There is also a lower adit driven in the same direction
about 150 feet, which probably was used to pull ore from
the stopes. This adit was accessible in August 1955,
but in need of some cleaning out. Below the stoped area
is a zone of vuggy vein silica, probably a replacement of
dolomitic limestone ; dolomitic limestone lies above.
Abundant barite and replacement silica, as well as dolo-
mitic limestone, were found on the dumps.

The deposits must have required selective hand mining
methods, due to the interwoven nature of the barite,
siliceous rock, and dolomitic limestone. Present market
requirements are such that ore must be very selectively
mined or ordinary mine-run material must be benefi-
eiated. However, as reserves exposed in the workings
are apparently small, neither course seems feasible.
These conditions, coupled with the fact that part of the
deposit lies within a state park, make it unlikely that
further production will be made from this deposit.

REFERENCES

Allen, John E., 1946, Geology of the San Juan Bautista quad-
rangle : California Div. Mines Bull. 133, 75 pp.

Andrews, Philip, 1936, Geology of the Pinnacles National Monu-
ment : Univ. California Dept. Geol. Sci. Bull., vol. 24, no. 1,
pp. 1-38.

Aubury, L. E., and others, 1906, Structural and industrial ma-
terials of California : California Min. Bur. Bull. 38, pp. 72-73.

Becker, George F., 1888, Geology of the quicksilver deposits of
the Pacific slope: U. S. Geol. Survey Mon. 13, p. 181.

Boalieh, E. S., 1921, Monterey County : California Div. Mines
Rept. 117, pp. 156-157.

Bowen, O. E. Jr., and Gray, C. H. Jr., 1958, Stratigraphy of
the Sur series at Fremont Peak, northern Gabilan Range, Califor-
nia [abstract] : Geol. Soc. America Bull., vol. 69, no. 12, pt. 2,
p. 1676.

Bradley, W. W., Barytes : California Div. Mines Bull. 90, p. 116.

Bradley, W. W., 1930, Barite in California: California Div.
Mines Rept. 26, pp. 45-57.

Bradley, W. W., and Logan, C. A., 1917, San Benito County:
California Div. Mines Rept. 15, pp. 616-673.

Curtis, G. H., Evernden, J. F., and Lipson, J., 1958, Age deter-
mination of some granitic rocks in California by the potassium-
argon method : California Div. Mines Special Rept. 54, 16 pp.

Hutton, C. O., 1951, Uranoan thorite and thorian monazite from
blacksand paystreaks, San Mateo County, California (abstract) :
Geol. Soc. America Bull., vol. 62, no. 12, pi. 2, pp. 1518-1519.

Lawson, A. C, 1893, The geology of Carmelo Bay, California :
Univ. California Dept. Geol. Sci. Bull., vol. 1, no. 1, pp. 1-59.

Laizure, C. McK, 1925, Monterey County : California Div.
Mines Rept. 21, p. 28.

Logan, C. A., 1947, Limestone in California : California Jour.
Mines and Geology, vol. 43, no. 3, pp. 177-351.

Reiche, Parry, 1937, Geology of the Lucia quadrangle, Cali-
fornia : Univ. California Dept. Geol. Sci. Bull., vol. 24, no. 7,
pp. 115-168.

Taliaferro, N. L., 1948, Geologic map of the Hollister quad-
rangle : California Div. Mines Bull. 143, pi. 1.

Trask, P. D., 1926, Geology of the Point Sur quadrangle, Cali-
fornia : Univ. California Dept. Geol. Sci. Bull., vol. 16, pp. 119-180.

Waring, C. A. and Bradley, W. W., 1919, Monterey County :
California Div. Mines Rept. 15, pp. 595-615.

Whitney, J. D., 1865, Report of progress and synopsis of the
field work from I860 to 1864-65, pt. 1, Geology of the Coast
Ranges, sec. Ill, The Gavilan Range: California Geol. Survey,
pp. 159-160.

Willis, Bailey, and Willis, Robin, 1925, Geologic map of the
Bird Canyon Ledge. From an unpublished private report.

Wilson, Ivan F., 1943, Geology of San Benito quadrangle, Cali-
fornia : California Div. Mines Rept. 39, pp. 183-270.

printed in California state printing office

DIVISION OF MINES

GORDON 8 OAKESHOTT, CHIEF

STATE OF CALIFORNIA
DEPARTMENT OF NATURAL RESOURCES

3500 -,

3000 -

(its ^^

—-l

Isdo

~T~~^- lsd0

23O0 -
ZOOO -

flr

"--

grs

Isdo

Isdo
/ / /

grs

■■ i i i ■. i

ISOO -

lOOO -

Contact, dashed where gradational
or approximate

Fault, dashed where approximate

Granitic rocks-, largely quartz monzonite
and gronodiorite

FEET ABOVE SEA LEVEL

SECTION A - A - A - A

500 1000 1500 2000 2500 1000

/so y

Dip

and

str
M

ke of beds or sen

/«,

Jointing

ine or prospect

stosity

SCALE IN FEET

r ;- Limestone with granitic rock and

' Isgs

schist intercalations

Siliceous limestone and minor dolomite

Limy siliceous dolomite

, ; •; s c h .' ;

Limestone and dolomite

Quartz mica schist

Schist and granitic rock

Pink quartzite

GEOLOGIC MAP AND STRUCTURE SECTION OF FREMONT PEAK AREA, MONTEREY AND SAN BENITO COUNTIES, CALIFORNIA

S**

/»

**'

- «.

' v A

VJ*B _ *«

■S-C

14.

J *

■

1.* ' '

1 i

1 ** Jf

[ .V

- ■ ■ . - -