A PEBBLE-COBBLE DEPOSIT IN
MONTEREY BAY, CALIFORNIA

by

Michael John Ma I one

LIBHABY

HAVAL I 'TE SCHOOL

I -^40

United States
Naval Postgraduate School

THESIS

A PEBBLE-COBBLE DEPOSIT

IN MONTEREY BAY, CALIFORNIA

by

Michael John Malone

June 1970

Tku documznt hat be.&n appAjovzd ^ok pub tic kz-
Iqa&c and 6alc; JUU dis&iibution u> untatuXzd.

T135206

A Pebble-Cobble Deposit in Monterey Bay, California

by

Michael John Malone

Lieutenant (Junior Grade) , United States Navy

B.S., United States Naval Academy, 1969

Submitted in partial fulfillment of the
requirements for the degree of

MASTER OF SCIENCE IN OCEANOGRAPHY

from the

NAVAL POSTGRADUATE SCHOOL
June 1970

■IBHAEY

[AVAL POSTGRADUATE SCHOOlf

[ONTEREY, CALIF. 93940

ABSTRACT

A deposit of pebbles and cobbles was discovered in approximately
60 fm of water on the continental shelf in Monterey Bay, California.
Samples were taken in the area and the extent of the deposit was deter-
mined. The material was characterized as well rounded, moderately
well sorted coarse pebbles derived mainly from the granites of the Santa
Lucia Formation with lesser amounts of alluvium from the Salinas Drain-
age Basin which flows into Monterey Bay. It was established that the
deposit probably represents a marine terrace of Pleistocene Age, indi-
cating a relative lowering of sea level of about 330 to 360 ft. This
appears to be the first reported evidence of submerged marine terraces
in Northern California.

TABLE OF CONTENTS

I. INTRODUCTION 9

A. PURPOSE 9

B. PREVIOUS WORK 9

II. DESCRIPTION OF AREA 12

A. SUBMARINE TOPOGRAPHY 12

B. GEOLOGIC SKETCH OF SURROUNDING AREA 12

1. Coastal Geology --: 12

2. Salinas Drainage Basin 14

III. PROCEDURE 18

A. COLLECTION OF DATA 18

1. Bathymetric Profiles 18

2. Bottom Samples 20

B. ANALYSIS OF DATA 20

1. Roundness and Shape 20

2. Size Analysis 22

3. Lithology 24

IV. RESULTS 25

A. BATHYMETRIC SURVEY 25

B. BOTTOM SAMPLES 27

1. Roundness and Shape 28

2. Size 28

3. Lithology 34

3

C. DISTRIBUTION Or THE PEBBLE-COBBLE DEPOSIT--- 35

V. CONCLUSIONS 38

A. ORIGIN OF PEBBLE-COBBLE DEPOSIT 38

1. Time of Deposition 38

2. Source Area 39

3. Movement Into the Area 40

B. CONDITIONS OF DEPOSITION 40

C. SUMMARY 42

D. FUTURE WORK 42

REFERENCES CITED 44

INITIAL DISTRIBUTION LIST 46

FORM DD 1473 47

LIST OF TABLES
TABLE TITLE PAGE

I. Stratigraphy of Surrounding Area 17

II. Statistical Parameters of Pebble-Cobble Deposit 33

III. Results of Petrographic Analysis of Selected Rocks 36

LIST OF ILLUSTRATIONS

FIGURE TITLE PAGE

1. Monterey Bay Area 10

2. Coastal Geology 13

3. Generalized Geology of Salinas Drainage Basin 16

4. Tracks of Bathymetric Profiles 19

5. Sample Locations 21

6. Bathymetric Profiles 26

7. Roundness and Sphericity vs . Particle Diameter 29

8. Cumulative Percent of Samples 31

9. Cumulative Percent of Combined Samples 32
10. Present Distribution of Pebble-Cobble Deposit 37

ACKNOWLEDGEMENTS

The author wishes to express his sincere appreciation to all
those individuals and groups whose generous assistance aided in the
completion of this report. Special thanks are due to my advisor,
Professor Robert S. Andrews, for his patience, guidance, and time,
especially the petrographic analysis; to my wife, Pat, who typed
several drafts of this report and supplied invaluable moral support;
to the technicians of the Department of Oceanography, especially
Wendall Ayers , who helped in so many ways; and to Gary Greene,
U. S. Geological Survey, Marine Geology Division, Menlo Park,
California, for discussions concerning seismic and bathymetric pro-
files in Monterey Bay and their interpretation.

I. INTRODUCTION

A. PURPOSE

The purpose of this study was to investigate the occurrence of
pebbles and cobbles discovered in significant quantities (J. Geary,
personal communication) on the outer edge of the continental shelf in
southern Monterey Bay, California (Fig. 1). Since this portion of the
continental shelf is characterized generally by green sand and silt,
the occurrence of these large particles, collectively referred to as the
pebble-cobble deposit for the remainder of this report, can be considered
anomalous. The approach of the study was to attempt to establish the
possibility that this deposit represents an ancient marine terrace.
Bathymetric profiles were recorded, bottom samples were collected, and
the texture, lithology, and distribution of the deposit were determined.

B. PREVIOUS WORK

Galliher (193?) studied the general nature of the sedimentary cover
on the continental shelf in Monterey Bay and reported a wide band of
gravel (particle diameters of 2-10 mm) with a few pebbles (particle
diameters greater than 10 mm) over the area of the present study. How-
ever, the largest pebbles according to histograms included in his report
were less than 14 mm in size, corresponding to a maximum size of
approximately -4 0 and comparing in size to only the smaller material

Depths in fathoms

SEASIDE

EREW

PRESS POINT
\^CARMEL

Figure 1

Adapted by Dorman (1968)
Monterey Bay Area

10

reported in the present study. Galliher concluded that the coarse
material represented a reworked alluvial deposit from the Salinas Drain-
age Basin.

Monteath (1965) reported on the depositional environments of the
bottom sediments on the continental shelf in southern Monterey Bay.
His results differed significantly from Galliher's in that he found that
the wide band of gravel did not exist, but rather that the area contained
silty sand with a few pebbles.

Yancey (1968) described the sediment cover in Monterey Bay,
primarily the northern half, and reported sediment types aligned in
bands approximately parallel to the submarine contours. The outermost
band, on the edge of the continental shelf, consisted predominantly of
coarse sand, including some cobbles. Yancey concluded that this
band is a "relict deposit" of Pleistocene Age and based his estimate
of the age on two factors. The first was the discovery, in a sample
containing coarse sand, of shallow water gastropods which could have
been found during the Pleistocene, and a cold water Pleistocene gastro-
pod. The second factor was his assumption that coarse sediments are
not being accumulated in this area under present conditions.

This paper apparently piesents the first report of a significant
pebble-cobble deposit in this area and it is the first report en the
nature of the material comprising the deposit.

11

II. DESCRIPTION OF AREA

A. SUBMARINE TOPOGRAPHY

The dominant topographical feature in Monterey Bay is the
Monterey Submarine Canyon, described in detail by Martin and Emery
(1967). The canyon extends to within 100 yards of the shoreline at
Moss Landing and divides the continental shelf in the bay into two
halves. The canyon is adjacent to the area where the pebble-cobble
deposit is located. In this area the transition between the continental
shelf and the continental slope, represented by the sides of the canyon,
is quite abrupt and the slope of the canyon approaches 45° immediately
to the west of the area studied. The shelf-slope transition, which was
used to define the western edge of the area studied, occurs at depths
ranging from about 90 m at the northern end to about 135 m at the
southern end. The general bathymetry of the continental shelf in
Monterey Bay is quite smooth and gently sloping with some rock out-
cropings or reefs occurring .

B. GEOLOGIC SKETCH OF SURROUNDING AREA
1 . Coastal Geology

The southern half of Monterey Boy is bordered on the south
by the Monterey Peninsula and on the east by sand dunes extending
from the city of Seaside to Flkhorn Slough (Fig. 2). The Santa Lucia
Formation of Cretaceous A ;e forms the northern and western end of the

12

Sand Dunes

Alluvium

River Terrace Deposits

Monterey Shale

A A

Santa Lucia Granodic: ite MONTEREY

Adapted by Monteath (1965)

Figure 2. Coastal Geology

13

peninsula. This formation is exposed continuously along the shoreline
from Cypress Point to Monterey, and also outcrops offshore to the north
and west of Point Pinos . The formation contains granitic rocks which
occur throughout the Santa Lucia and Gabilan Mountains and ranges in
composition from quartz diorite to granodiorite and granite, including
various porphyritic and pegmatitic facies (Hart, 1966).

The Monterey Formation of Middle and Upper Miocene Age
overlies the Santa Lucia Formation on the peninsula. It is basically
a siliceous shale with porcellanite and chert. The Monterey Formation
outcrops almost continuously throughout the entire Santa Lucia Range
and, although it does not actually occur along the shoreline in Monterey
Bay, it does outcrop in the bay (Dorman, 1968).

From Monterey to Moss Landing, the shoreline is backed
by unconsolidated Quaternary deposits. Recent sand dunes fringe the
shoreline, and areas of older sand dunes are found in the Fort Ord area.
Gravel, sand, silt, and clay of the stream channel and flood plain
deposits are also widely distributed. Another distinctive unit forming
the low hills bordering the lowlands of the bay is the Aromas Red Sands,
consisting typically of coarse red sands and some "gravel" and clay
(Hart, 1966).

2 . Salinas Drainage Basin

The geology of the Salinas Drainage Basin can be character-
ized by its complexity. Basement rocks of the basin are divided into
two mutually exclusive provinces by the San Andreas and Sur-Nacimiento

14

Fault zones (Page, 1966) and the rocks found range in age from Paleozoic
to Recent. The geology is summarized in Fig. 3 and Table 1 based on
reports by Bailey, Irwin, and Jones (1964), Hart (1966), and Page (1966).

15

Adapted from Yancey (1968) and Page (1970)

Figure 3. Generalized Geology of the Salinas Drainage Basin

16

Geologic Age

Recent

o

M
O

O

W

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o

M

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3

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U

<D

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Pleistocene

Pliocene

Miocene

Oligocene

Eocene

Paleocene

flta
1 01

Formation

Dime and beach sand
er dune sand

I

I Alluvial sand, gravel, and clay

I

I Stream terrace and fan deposits

Aromas Red Sands

Paso Robles

Mtchgoin, L'tchgoin-Jacalitos,
Pancho Rico

Santa Margarita

Monterey

Sandholci, Temblor, Vaqueros

Berry Conglomerate

The Rocks Sai dstone, Lucia Shale,
Junipero Sandstone

Carmelo

Asuncion
Santa Lucia

Franciscan

Sur

Adapted from Hart (1°66)

Table 1. Stratigraphy of Surrounding Area

17

III. PROCEDURE

A. COLLECTION OF DATA

The primary vessel used for the collection of data for this study
was U.S.N.S. BARTLETT (T-AGOR-13). This ship was used on two
occasions: November 3 to November 5, 1969, for the bathymetric survey
and collection of bottom samples, and April 27 to April 29, 1970, also
for the collection of bottom samples. The Naval Postgraduate School's
63 -ft research boat was used for additional bottom sampling.
1 . Bathymetric Profiles

Bathymetric profiles were recorded in the area (Fig. 4)
using a Precision Graphic Recorder. Eighteen profiles were taken
between the 50-fm and 100-fm contours, normal to the contours, and
spaced one-half mile apart. Additionally, two long profiles were
recorded to align the short profiles and to show features parallel to the
short profiles.

The purpose of the bathymetric survey was to determine
bottom topography of the area in greater detail than is possible with
available navigational charts. The track spacing, although not close
enough to reveal all details of bathymetry, was considered sufficient
to reveal all major features and significant trend of bathymetry with
dimensions greater than one-half mile.

18

45' N
00' W

Bathymetry Profiles

40' N

55' W
1000 2000 3000 4000 5000

— i 1 It 1 x

Scale, in Yards

10-fm contour interval
l-fm contour interval

Figure 4. Tracks of Bathymetric Profiles

19

2 . Bottom Samples

Twenty-one dredge samples (Fig. 5) were collected with a
12-inch diameter, 4 -ft long pipe dredge. Two factors were considered
in sampling; collecting a large enough quantity of pebbles and cobbles
with which to effectively work, and determining, as well as possible,
the distribution of the pebble-cobble deposit. Thus, samples were
first collected in the immediate area of the initial discovery (Fig. 5)
and then around the area to determine the extent of pebble-cobble deposit.

Dredge hauls were conducted by towing the dredge for
approximately 0.5 mile at a speed of from 1 to 2 knots with from 35 0 to
400 mof cable out. This provided a scope of from 3:1 to 4:1. This
dredging distance was reduced to about 0.3 mile after some experience
with the equipment.

In order to better define the distribution of the pebble-
cobble deposit, grab samples were collected in the region of the dredge
hauls.

B. ANALYSIS OF DATA

1. Roundness and Shape

During the collection of samples it was observed that the
pebbles and cobbles in the samples appeared quite rounded and spheri-
cal. Therefore, a single sample, D-5, which was considered quite
representative of all samples was selected for roundness and shape

20

45' N
00' W

DM-9

Location of
nitial Discovery

10-ftn contour interval
l-fm contour interval

Figure 5. Sample Location:.

21

analysis. This was done to get a quantitative measure of the round-
ness and shape of the particles, and the method of analysis followed
that discussed by Krumbein (1941).

The roundness of a particle is defined by Krumbein as a
measure of curvature of the corners and edges expressed as a ratio to
the average curvature of the particle as a whole, where, for practical
purposes, the average curvature is expressed in terms of the inscribed
circle drawn on a projection of the particles in a plane. The procedure
for determining roundness was to compare the particle with a set of
visual images of known roundness, included in Krumbein's paper, and
then to assign a value of roundness to the particle.

The shape of a particle is fundamentally the ratio of its
surface area to its volume. For practical purposes, the shape is
expressed as the ratio of the volume of the particle to the volume of its
circumscribing sphere. The cube root of this ratio is known as the
sphericity and is used as the quantitative measure of shape. The
determination of sphericity involved the measurement of three mutually
perpendicular diameters; a long axis, intermediate axis, and short axis
Using a plot of the sphericity vs. the ratios of the intermediate to the
long axis and the short to the intermediate axis (Krumbein, 1941) ,
sphericity could be read directly to an accuracy of 0.02.
2 . Size Analysis

The size distribution of the samples was determined at
half-phi intervals. For sample number D-5, the sample analyzed for

22

shape and roundness, the intermediate axis length was used as the
particle diameter. Emery (1955) showed that the intermediate diameter
is a valid criterion for size of a particle. For the other samples con-
taining pebbles and cobbles, the author analyzed the size of the
particles with a set of square holes cut in metal plates. The decision
to use square holes was arbitrary but considered the fact that sieves
or screens used for analysis of finer material are composed of square
holes. These holes corresponded to particle diameters from -3 0 to
-7.5 0, inclusive, in half-phi increments. The sizing plates greatly
facilitated the measurement of particle diameter and made it possible to
measure a large number of particles in a short period of time.

The frequency distribution of the samples was determined
in two ways. For the samples collected in November 1969, the number
of particles in each half-phi increment was multiplied by the cube of
the geometric mean of the limiting diameters of the size class to deter-
mine a volume frequency. For the samples collected in April 1970, the
actual weight of the particles in each size interval was measured to
determine a weight frequency. Emery (1955) showed that the results
of either method of analysis are similar and that either may be used.

Weight and volume cumulative frequencies were plotted
and statistical parameters computed. This was done for each individual
sample containing more than 50 particles, and total volume and weight
curves and statistics were compiled from a consideration of all pebbles
and cobbles from every dredge haul containing any coarse material.

23

The weight and volume curves were combined by converting volume to

3
weight by assuming a constant rock density of 2.67 gm/cm for the

samples .

3. Lithology

The general lithological character of the samples was deter-
mined by breaking each rock except the smallest ones to expose a
fresh surface and grouping the rocks according to types based on a
macroscopic analysis. A petrographic analysis was done on 15 thin
sections that represented examples of all rock types found, placing
emphasis on the more unusual rocks and those difficult to define by
macroscopic techniques.

24

IV. RESULTS

A. BATHYMETRIC SURVEY

The bathymetric survey (Fig. 4 and 6) in general showed the
gently sloping nature of the continental shelf and the abrupt transition
from the shelf to the slope of the canyon adjacent to the area studied.
However, several interesting features were noted. In the southeastern
corner of the area there is an area of positive relief. This knoll is indi-
cated on navigation charts, but this survey showed it to be consider-
ably greater in extent and magnitude of relief than is indicated on these
charts. Additio ally, a dredge haul made over the knoll with the
fathometer operating showed that the magnitude of the knoll's vertical
relief is greater (as much as 10 fm) than shown in the two profiles
crossing it in this survey. Also, on the landward side of the knoll,
there is a slight linear depression paralleling the knoll.

Several profiles in the northern half of the area showed a feature
with positive relief adjacent to the shelf-slope transition. This feature
is especially apparent on profile G-G1 (Fig. 6) and suggests a low
ridge at the edge of the continental shelf. Because the depth of the
ridge is not constant, it is obscured in Fig. 4. There is a linear bathy-
metric feature of negative relief extending in from the canyon and passing
to the south of the knoll toward Monterey Harbor. Results of seismic
work currently being analyzed by the U.S. Geological Survey, Marine

25

Figure 6. Bathymetric Profiles

(Reference level 50 fm)

26

Geology Division, Menlo Park, California, (G . Greene, personal
communication^ indicate that this linear depression represents the
surficial evidence of a buried channel, with some evidence that a fault
is associated with the channel. The channel could represent an ancient
drainage system for the northern half of the Monterey Peninsula, and
faulting evidence substantiates Martin's and Emery's (1967) inference
of the seaward projection of the Tularcitos Fault. Additionally, the
profiles in the southern part of the area showed much irregularity of the
bottom west of Point Pinos.

B. BOTTOM SAMPLES

Ten of the dredge hauls recovered material from the pebble-cobble
deposit. One dredge haul in the area of the knoll recovered freshly
broken segments of a fairly well cemented pebble conglomerate. This
conglomerate, along with more of the material recovered from the knoll
(J. Geary, personal communication) appeared to be similar in composi-
tion to the material comprising the pebble-cobble deposit. The grab
samples, with a maximum penetration of 5 inches, recovered no pebbles
or cobbles. However, it might be noted that the initial discovery of
the deposit was made with a Shipek grab which has a maximum penetra-
tion of 4 inches .

Only the pebbles and cobbles were analyzed in this study; the
finer material was disposed of, except for a small sample from each
dredge which was saved for later analysis. Some of the pebbles

27

recovered had marine growths, brachiopods and coral, on them and
appeared free of the matrix material on one side. These were apparently
surficial particles. However, most of the particles had no growth on
them and had the matrix material adhering to all sides, indicating that
the majority of the material was buried.

1 . Roundness and Shape

The roundness of the particles analyzed had values ranging
from 0.35 to 0.95 with an average of 0.68. The sphericity of the
particles had values ranging from 0.47 to 0.99 with an average of 0.74.
Based on a scale value of 1.0 indicating perfect sphericity and round-
ness, these results indicate that the particles had a high degree of
sphericity and roundness.

A plot of sphericity vs. roundness was made and no relation-
ship between them was observed. However, the average values of
sphericity and roundness were computed for the particles in each half-
phi increment of particle diameter. These values were plotted against
particle diameter and a slight but definite increase in sphericity and
roundness with size was noted (Fig„ 7).

2. Size

During initial sampling it was observed that there was a
sharp break rather than a gradual transition between the coarse particles
that are the subject of this report and the matrix material of fine sand
and silt. This break was found to be at -3 0, a value used as the

28

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PARTICLE DIAMETER, 0

Figure 7. Roundness and Sphericity vs. Particle Diameter

29

lower limit of particle diameter considered in this study. The largest
particle recovered was slightly greater than -7 0 .

In ignoring the fine portion of the samples and treating only
the coarse fraction, the present investigator was aware of the fact that
cumulative percent curves and statistical parameters derived from the
coarse fraction would be completely different from those that would be
determined from a treatment of the complete sample. However, if one
recognizes that the presence of two unique types of deposits in the
same sample, in this case the pebble-cobble deposit and the matrix
material, implies two different sedimentation processes and/or sources,
then the separate treatment of the coarse fraction should be expected
to reveal information of the conditions of its deposition.

The cumulative frequency curves plotted on arithmetic
probability paper are shown in Fig . 8 and 9 and those statistical
parameters computed, Inman median, mean, and sorting, Folk and
Ward mean and sorting, and Trask sorting, are shown in Table 2.
Individual samples are identified by sample numbers. In Fig. 8 and 9,
the combination of all samples analyzed by volume is "Total V," a
volume cumulative frequency curve, and those analyzed by weight is
"Total M," a weight cumulative frequency c. :ve. The total of all the
samples, combined by converting volume frequencies to weight is
identified as "Total" and is a weight cumulative frequency curve.
Observed data points are connected by solid lines and extrapolations
used to determine percentiles are represented by dashed lines. It can

30

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PARTICLE DIAMETER, 0
Figure 8. Cumulative Percent of Samples

-3.0

31

O TOTAL V

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PARTICLE DIAMETER, 0

Figure 9. Cumulative Percent of Combined Samples

32

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be seen that the samples can be generally characterized as well to
moderately well sorted coarse pebbles (mean size of about -5.5 0) .
Samples DM -9 and DM-10 contained smaller pebbles and were not
as well sorted. Although the effect of adding samples with different
mean sizes is to increase the sorting coefficient, the total of all
samples considered as one yields moderately well sorted pebbles.
3 . Lithology

The results of the petrographic analysis are shown in
Table 3. Using these results as a guide, the samples were estimated
to be 70% derived from the Santa Lucia Formation and 30% miscellaneous
rock types representing a cross section of those rock types found in the
Salinas Basin, including rocks from the Franciscan Formation. The
occurrence of these rock types correlates with Galliher's (1932) results
which showed the presence of Franciscan chert, granite, basalt, and
Monterey shale, although Galliher did not indicate the dominance of
the granite rock types derived from the Santa Lucia Formation.

Some interesting characteristics were noted on several of
the rocks. Rock No. 243, the chert breccia, showed an interesting
planar orientation of the fragments. The fossiliferous greywacke ,
Rock No. 353, has a very distinctive shape and appears to represent
a flute or load cast as pictured by Krumbein and Sloss (1963, p. 131).

Specific gravity measurements on ten samples of chert
showed that five had values ranging from 2.38 to 2.47 and the other
five had values ranging from 2.58 to 2.67. Goldman (1959) reported

34

Rock No

# 58

# 70

# 85

#106

#173

#241

Rock Type
Dacite Porphyry
Granodiorite
Rhyolite
Granite Gneiss
Dacite Porphyry

Well-Cemented Chert Breccia (angular
fragments of chert, jasper, and
recrystallized quartz; oriented
somewhat along a plane)
#243 Granite Gneiss (identical to #106)

#247 Micaceous Granodiorite

#2C8 Granite

#271 Granite

#309 Rhyolite

#318 Granite

#338 Interbedded Ferruginous Chert and Slate

#352 Dacite

#353 Fossiliferous Greywacke (silty sand in

texture)

Rock #241 appears to have been derived from other
than the immediate area of Monterey Bay and may represent
a Franciscan source.

(Petrographic analysis by R. S. Andrews)

Table 3. Results of Petrographic Analysis of Selected Rocks

35

studies of Franciscan and Monterey chert which showed values of
specific gravity ranging from 1.80 to 2.43 for Monterey chert and 2.6
to 2.8 for Franciscan chert. A comparison of these results with those
of the present study indicates the presence of both types of chert.

The presence of Franciscan rocks and other rocks character-
istic of the Salinas Basin and not of the Monterey Bay area indicates
that the remaining 20% of the rocks, those not associated with the
Santa Lucia or Monterey Formations, must have their source in the
Salinas River Basin alluvial or shallow-water marine deposits.

C. DISTRIBUTION OF THE PEBBLE-COBBLE DEPOSIT

Based on results of sampling, the pebble-cobble deposit was
determined to form a band approximately 2.5 miles long and 0.6 mile
wide and in a depth ranging from 55 to 59 fm (Fig. 10). Examination of
Fig. 6 and 10 shows that the distribution was approximately that of the
ridge observed in the discussion of the bathymetry. This distribution
corresponds to the distribution described by Galliher (1932) and Yancey
(1968); that is, a band c' coarse material aligned approximately parallel
to the depth contours .

36

40' N

55' W

1000 2000 3000 4000 5000
— i 1 _j 1 , J

Scale, in Yards

10-fm contour interval

• l-fm contour interval

Figure 10. Present Distribution of Pebble-Cobble Deposit

37

V. CONCLUSIONS

A. ORIGIN OF PEBBLE-COBBLE DEPOSIT

An analysis of the previously listed results enables one to make
a reasonable estimate of when the material was deposited, where it
came from, the means by which it was transported to its present loca-
tion, and the conditions under which it was deposited.

1 . Time of Deposition

To make a reasonable estimate of when the pebbles and
cobbles were deposited, several factors must be considered. The first
is Yancey's (1968) discovery, in coarse sand in the northern part of the
area studied in this report, of cold and shallow water fauna which were
estimated to be of Pleistocene Age. The second factor is Yancey's
statement, which the present author supports, that material this coarse
is not being deposited under present conditions. This is primarily
because the dynamic forces required to move the particles do not exist
in the present depth of water on the outer edge of the continental shelf,
since a current of more than 2.50 m/sec (4 knots) is the theoretical
threshold velocity for material 32 mm (-5 0) in size (Shepard, 1963,
p. 129). Also, as previously mentioned, the majority of the particles
were not surficial but were buried und a cover of presently accumulating
sediments. The fact that all of the particles are not yet buried does not
mean that they could not presently be in the process of being covered.
A consideration of all of these factors does indicate that the

38

individual pebbles and cobbles were deposited during the Pleistocene
Age when sea level was greatly lowered.
2 . Source Area

The results of the lithological analysis show that the
primary original source of the pebble-cobble deposit is the Santa Lucia
Formation. At a time of lowered sea level, the Santa Lucia would have
extended almost to the southern limit of the area (Martin and Emery,
1967). Additionally, the southern bathymetric profiles (Fig. 6) show
irregular topography that is hypothesized to be granitic sea cliffs
at times of lower sea level. These sea cliffs would have provided a
ready supply of granitic rocks to nearby areas. This in turn supports
the concept of deposition at a time of lowered sea level because the
closest source of granitic rocks at present is Point Pinos, 5 miles to
the south of the study area. Those rocks not derived from the Santa
Lucia Formation would have come from alluvial or shallow-water marine
deposits of the Salinas River Basin. It might be pointed out that, during
Mid-Pleistocene, the Pajaro and Salinas Rivers joined slightly below
the present head of the canyon and drained down the canyon (Martin
and Emery, 1967). Thus, deposits from this time could also have
included debris from the Pajaro River. However, this does not affect
the geological background since the geology of the Pajaro Basin is
merely a continuation of that of the Salinas basin, with the exception
that more of the Franciscan is present.

39

The pebble conglomerate discussed previously must also be
considered as a possible source of the pebble-cobble deposit due to
its nearby location and the fact that it appears to contain all the rock
types found in the deposit. The linear depression landward of the knoll
may represent a drainage scour channel at a time of lowered sea level.
3 . Movement Into the Area

Two possibilities were considered for motion of the material
into the area. If the knoll was the immediate source, the fine matrix of
the conglomerate could have been eroded away, leaving the pebbles and
cobbles as a lag deposit. Longshore movement under control of wave
action and subaerial stream flow is a means by which the material could
have been transported to its present location from more distant sources.
This transport could have moved material directly from the base of sea
cliffs and from Salinas Basin deposits or could have been working
simultaneously with erosion of the knoll. Steers and Smith (1956) and
Kidson, Carr, and Smith (1958) used radioactive tagging of particles to
study motion of pebbles in the nearshore region. They showed that
pebbles are moved in depths up to 25 ft when only moderate waves are
running and the effect could be expected to be greater in heavy seas .
Once again, conditions of lowered sea level would be required to find
the distribution of material in this study.

B. CONDITIONS OF DEPOSITION

Three factors were considered in determining the depositional
conditions of the pebble deposit* Roundness is a function of abrasion,

40

high roundness being associated with much abrasion. Abrasion can occur
over long distances of transport or may be caused by wave action. The
high degree of roundness of the samples must be due mainly to wave
action since 80% of the material was transported only a short distance.

Analysis of the cumulative percent curves and the statistical
parameters showed the most significant factors. The cumulative fre-
quency curves (Fig. 8 and 9) are approximately straight lines, indica-
ting a symmetrical distribution of grain sizes from which one may infer
that the given samples were deposited in a condition of equilibrium with
wave forces at a given time (Emery, 1955). Emery also prepared a
table summarizing all available information, including 'esults of his
own study, on Trask sorting coefficients of coarse gravels (greater
than 10mm) for beaches, streams, and alluvial fans. Results of Emery's
table show the following ranges of these coefficients: marine beaches
ranged from 1. 13 to 2. 14 with a median of 1.25; streams ranged from
1.34 to 5.49 with a median of 3.18; and alluvial fans ranged from 2.50
to 8.95 with a median of 5.33. Most of these values were determined
from samples selectively collected to include only coarse material
which did not generally characterize the entire beach. This sampling
technique is analogous to that of the present author. A comparison of
these results with the results of the present study (Table 2), showing
a range of Trask sorting of from 1.16 to 1.39, shows strong evidence
that the pebble deposit at one time formed a marine beach. It was not
necessary that this material form the actual beach; it would have been

41

possible for it to exist in the immediate nearshore area. Schupp (1953)
reported that the material forming cobble beach cusps had a primary
source area in a band immediately offshore in 20 to 30 ft of water and
migrated between the beach and the source area under varying wave
conditions .

C. SUMMARY

The pebble-cobble deposit in this study is probably a relict marine
beach of Pleistocene Age. The beach would represent a relative lowering
of sea level of about 330 to 360 ft. The author was unable to find any
reference to submerged marine terraces in the immediate area but this
lowering can be correlated with other marine terraces around the world
and possibly off Southern California (Emery, 1958). The primary source
of the material was marine sea cliffs of the Santa Lucia Formation, with
lesser amounts of debris from the Salinas Drainage Basin.

D. FUTURE WORK

It can be seen that further work in the area is desirable. The
analysis of the samples should be extended to include the matrix material.
This analysis should be carried out along the continental shelf in Monterey
Bay and an attempt should be made to relate the coarse and matrix
materials.

The knoll in the southeastern corner of the area should be completely
studied, including high and low resolution seismic reflection profiling,
and its exact relationship to the pebble-cobble deposit determined.

42

The other positive-relief features in the study area may also yield
some of the pebble conglomerate.

43

REFERENCES CITED

Bailey, E. H. , Irwin, W. P., and Jones, D. L. , 1964, Franciscan
and related rocks and their significance in the geology of
western California: Calif. Div. Mines and Geology Bull. , 183,
177 p.

Dorman, C. E. , 1968, The southern Monterey Bay littoral cell: a

preliminary budget study: Unpub. M.S. Thesis, Naval Postgraduate
School, Monterey, California, 234 p.

Emery, K. O. , 1955, Grain size of marine beach gravels: Jour. Geology
v. 63, p. 39-49.

1958, Shallow submerged marine terraces of southern California:

Geol. Soc . America Bull. , v. 69, p. 39-60.

Galliher, E. W. , 1932, Sediments of Monterey Bay, California: State
Mineralogist of Calif . Rept. 28, v. 28, p. 42-71.

Goldman, H. R. , 1959, Franciscan chert in California concrete aggre-
gates; Calif, Div. Mines Spec. Rept. 55, 28 p.

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

California: Calif. Div. Mines and Geology County Rept. 5, 142 p.

Kidson, C, Carr, A. R. , and Smith, D. B. , 1958, Further experiments
using radioactive methods to detect the movement of shingle over
the sea bed and alongshore: Geogr. Jour., v. 124, p. 210-218.

Krumbein, W. C, 1941, Measurement and geological significance of

shape and roundness of sedimentary particles: Jour. Sed. Petrology,
v. 11, p. 64-72.

Krumbein, W. C.,andSloss, L. L. , 1963, Stratigraphy and Sedimentation:
San Francisco, W. E. Freeman and Co. , 660 p.

Martin, B. D. and Emery, D. O. , 1967, Geology of Monterey Canyon,
California: Am. Assoc. Petroleum Geologists Bull., v. 51,
p. 2281-2304.

Monteath, G. M. , 1965, Environmental analysis of the sediments of
southern Monterey Bay, California: Unpub. M.S. Thesis, Naval
Postgraduate School, Monterey, Calif, 61 p.

44

Page, B. M. , 1966, Geology of the coast ranges of California: Calif.
Div. Mines and Geology Bull. 190, p. 255-276.

1970, Sur -Nacimiento fault zone of California: continental

margin tectonics: Geol. Soc. America Bull., v. 81, p. 667-690.

Schupp, R. D. , 1953, A study of the cobble beach cusps along Santa
Monica Bay, California: Unpub. M.S. Thesis, Univ. Southern
Calif., Los Angeles, Calif. 131 p.

Shepard, F. P. , 1963, Submarine Geology: New York, Harper and Row,
537 p.

Steers, J. A. , and Smith, D. B. , 1956, Detection of movement of

pebbles on the sea floor by radioactive methods: Geogr. Jour. ,
v. 122, p. 343-345.

Yancey, T. E. , 1968, Recent sediments of Monterey Bay, California:
Univ. California Hydraulic Engineering Laboratory Tech. Rept.
HEL-2-18, 145 p.

45

INITIAL DISTRIBUTION LIST

No. Copies

1. Defense Documentation Center 2
Cameron Station

Alexandria, Virginia 22314

2. Library, Code 0212 2
Naval Postgraduate School

Monterey, California 93940

3. Department of Oceanography, Code 58 3
Naval Postgraduate School

Monterey, California 9394 0

4. Professor Robert S. Andrews, Code 58Ad 3
Department of Oceanography

Naval Postgraduate School
Monterey, California 93 940

5. Professor Jack E. Geary, Code 58 02 1
Department of Oceanography

Naval Postgraduate School
Monterey, California 93940

6. Professor Warren C. Thompson, Code 58Th 1
Department of Oceanography

Naval Postgraduate School
Monterey, California 93940

7 . Gary Greene 1
U.S. Geological Survey

Marine Geology Unit

345 Middlefield Rd

Menlo Park, California 94025

8. Pat Wilde 1
IMR 438 Hearst Mining Bldg

Univ. California
Berkeley, California 94720

9. Michael J. Ma lone 1
RR #1

Currie , Minnesota 56123

46

Unclassified

Security Classification

DOCUMENT CONTROL DATA -R&D

{Security classification of title, body of abstract nnd indexing annotation must be entered when the overall report Is classified)

T CKICINATINO »CTIVITY (('M(i()f«le«lllhorJ in. REPORT SECURITY CLASSIFICATION

Naval Postgraduate School
Monterey, California

2b. GROUP

3 REPOR T TITLE

A Pebble-Cobble Deposit in Monterey Bay, California

4 DESCRIPTIVE NOTES (Type of report and,inclus ive deles)

Master's Thesis; June 1970

5. Au THORIS I (First name, middle initial, last name)

Michael J. Malone

5 REPOR T D A TE

June 1970

8«. CONTRACT OR GRANT NO.

b. PROJEC T NO.

7... TOTAL NO. OF t'AGES

47

7b. NO. OF REFS

18

9a. ORIGINATOR'S Rl.PORT NUM6EHIS)

9b. OTHER REPORT NO(S) (Any other numbers that may be assigned
this report)

10 DISTRIBUTION STATEMENT

This document has been approved for public release and sale; its distribution
is unlimited .

II. SUPPLEMENTARY NOTES

12. SPONSORING MILITARY ACTIVITY

Naval Postgraduate School
Monterey, California

13 ABSTRACT

A deposit of pebbles and cobbles was discovered in approximately 60 fm of
water on the continental shelf in Monterey Bay, California. Samples were taken in
the area and the extent of the deposit was determined. The material was character-
ized as well rounded, moderately well sorted coarse pebbles derived mainly from
the granites of the Santa Lucia Formation with lesser amounts of alluvium from the
Salinas Drainage Basin which flows into Monterey Bay. It was established that
the deposit probably represents a marine terrace of Pleistocene Age, indicating
a relative lowering of sea level of about 330 to 360 ft. This appears to be the
first reported evidence of submerged marine terraces in Northern California.

DD,?ol"..1473 ,PAGE "

FORM

I NOV »8

S/N 0)01 -807-681 1

47

Unclassified

Security Cltsii.. cation

A-31408

Unclassified

Security Classification

KEY WO ROS

Marine terraces
Pebble deposits
Sediments
Monterey Bay

iD ,F,T..1473 <

N 0101-807-0821

BACK

ROLL W T

ROLE W T

ROLC W T

e..

48

Unclassified

Security Classification

A- 31 409

Thesis

M27843 Ma lone

c.l A pebble-cobble

deposit in Monterey
Bay, California.

12 )092

thesM27843

A pebble-cobble deposit in Monterey Bay.

3 2768 002 04236 8

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