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Geology and surrounding recent marine sediments of Anacapa Island

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Geology and surrounding recent marine sediments of Anacapa Island

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Content GEOLOGY AND SURROUNDING RECENT MARINE SEDIMENTS
OF ANACAPA ISLAND
A Thesis
Presented to
the Faculty of the Graduate School
The University of Southern California
In Partial Fulfillment
of the Requirements for the Degree
Master of Science
(Geology)
David W. Scholl
January 1959
UMI Number: EP58482
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UMI EP58482
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789 East Eisenhower Parkway
P.O. Box 1346
Ann Arbor, Ml 48106- 1346
UNIVERSITY OF SOUTHERN CALIFORNIA
G R A D U A TE SCHOOL.
U N IV E R S IT Y PARK
LO S A N G E LE S 7
^ C, 0 /
This thesis, w ritten by
Dav.id—W1.1 liam--£c.lmLl_.........
under the guidance of h.is...Faculty Committee,
and approved by a ll its members, has been p re
sented to and accepted by the Faculty of the
Graduate School, in p a rtial fu lfillm en t of the
requirements fo r the degree of
......
Date..__
Faculty Committee
/ C o.
................. f C h a l Chairman
TABLE OF CONTENTS
CHAPTER
I. INTRODUCTION ..................
General ....................
Location and Early History . .
Climate ....................
Previous Investigations . • .
Bottom Sampling and Field Work
Acknowledgments ............
II. PHYSIOGRAPHY AND GEOMORPHOLOGY .
Island Topography ..........
General ..................
Drainage ..................
Sea Cliffs ................
Sea Caves ................
Elevated Marine Terraces • •
Effects of Wave Attack . . •
Bottom Topography ..........
Data Source ..............
General............ . . .
Shelf Break ..............
Shelf Slopes and Terraces
Minor Relief ..............
CHAPTER PAGE
III. GEOLOGY...................................... 31
General.................... 31
Stratigraphy of Anacapa Island ............ 32
General........................ 32
Miocene Series .......................... 33
Cone jo Volcanic ...................... 33
San Onofre Breccia.................... 36
Pleistocene Series ...................... 41
Marine Terrace Deposits ................ 41
Nonmarine Terrace Sands ................ 43
Insular Shelf Stratigraphy ................ 44
General.................................. 44
Miocene Series .......................... 4?
Conejo Volcanics ...................... 47
Monterey Shale........................ 47
Structure................................ . 49
IV. OCEANOGRAPHY................................ 51
Shelf Currents........................ 51
Longshore Currents ........................ 52
Water Character............................ 53
V. SHELF SEDIMENTS.............................. 54
Analyses.................................. 54
Texture.................................... 56
Median Diameter.......................... 56
iv
CHAPTER PAGE
Sorting Coefficients ...................... 60
Mineralogy . . ............................. 60
Light Minerals and Rocks Fragments ..... 60
Heavy Minerals............................ 63
Distribution of Heavy Minerals.......... 65
Distribution of Glaucophane............ 66
Transported Rock ............ 69
Calcium Carbonate..................... 70
Organic Matter....................... 75
Classification-Sediment Types .............. 77
Detrital Sediments ........................ 83
Gravelly Coarse to Medium Sand ...... 83
Medium to Fine Gray Sand .......... 83
Fine to Very Fine Light Green Sand .... 84
Silty Fine to Very Fine Light Green Sand . 84
Calcareous Sediments..................... 85
Gravelly Coarse to Medium Algal Sand ... 85
Gravelly Medium Bryozoan Sand ...... 85
Gravelly Medium Shell Sand ........ 86
Medium Shell S a n d 86 j
Fine to Silty Fine Foraminlferal Sand . . 86;
Sedimentary Environments ... .............. 87j
Eastern and Western Shelf Environment ... 87!
Northern and Southern Shelf Environment . . 89*
V
CHAPTER PAGE
Significance of the Anacapa Shelf Sediments . . 91
VI. GEOLOGIC HISTORY .............................. 94
BIBLIOGRAPHY ........................................ 98
APPENDIX I ........................................... 101
APPENDIX I I ........................................... 104
vi
LIST OF FIGURES
FIGURE PAGE
1.
Bathymetric Chart ......................
2.
Early Chart of Anacapa Island ..........
... 5
3.
Profiles of the Anacapa Shelf .......... ... 25
4. Geologic Map ..........................
Back
5.
Bottom Lithology ...................... ... 46
6. Isopleth of Median Diameter ............ ... 58
7.
Distribution of Heavy Minerals ........ ... 64
8. Distribution of Glaucophane ............ ... 68
9.
Distribution of Calcium Carbonate . . . .
10. Dominant Calcareous Forms ..............
11. Distribution of Organic Matter ........
12. Distribution of Sediment Types ........
13.
Cumulative Curves of Sediment Types . . .
LIST OF PLATES
PLATE PAGE
1. Anacapa Island ................................. 4
2. Eastern Anacapa Island ......................... 7
3. Gap between eastern and central Islands * . • • 12
4. Passage between central and western islands • . 13
5. North side of western islands................ 16
6. North side of central islands............... . 18
7. Sea caves.......................................19
8. Distant view of northern side of Anacapa Island. 22
9. East end of Anacapa Island.......................22
10. Outcrops of the San Onofre breccia on western
island ................................ 38
11. Conejo volcanics overlying the San Onofre
breccia on the western island................ 38
12. Outcrops of the San Onofre breccia on the northern
sea cliff on the central island.............. 40
13* San Onofre Breccia on the central island .... 40
viii
LIST OF TABLES
TABLE PAGE
1* Dimensional Parameters of Anacapa Island. ... 14
2. Gross Character of Shelf Sediments. ...... 57
3. Mineralogical Composition of Shelf Sediments. . 61
4. Average Character of Sediment Types ...... 80
ABSTRACT
Anacapa Island lies nearly twelve miles off the
southern California coast along latitude 34° 00* 30'* N.
It is the easternmost member of the four Santa Barbara
Islands and is also the closest to shore of the eight
California channel islands. The island is approximately
five miles in length but is only 1.05 square miles in
area. Two gaps divide the island into smaller eastern,
central, and western islands. The western island has a
summit elevation of 930 feet, and it is the highest and
largest of the three. Drainage is principally to the
north by intermittent resequent streams.
Prominent wave-cut terraces are present at
elevations of about 600 and 250 feet. The higher terrace
is present only on the western island, but the lower
terrace occurs on each of the islands and forms a summit
platform atop the central and eastern islands.
The insular Anacapa shelf includes about thirty-
six square miles of the surrounding shallow submerged sea
floor. North of the island the shelf is approximately
three miles wide, but the southern shelf is less than
half this width. To the west, the shelf area extends
beneath Anacapa Passage and merges with the shelf of
neighboring Santa Cruz Island. The shelf area east of
the island is poorly defined.
Declivities of the northern and the southern shelves
average about 1° 401 out to a depth of about 270 feet.
The outer edge of the southern shelf usually occurs at
this depth, but north of the island a two-mile wide
terrace with an overall gradient of 0° 5* extends to near
the shelf edge, which occurs at an average depth of 300
feet. Other flattenings of the shelf are present at
depths of 60, 100, 180, and 330 feet. The submarine
terraces are thought to be wave-cut features of Wisconsin
age.
Lower Middle Miocene andesitic lavas and pyro-
clastic rocks of the Conejo formation form the bulk of
Anacapa Island. Approximately 1700 feet of this forma
tion is exposed on the south side of the western island.
Near the base of the volcanic series are two interbeds of
San Onofre breccia, which are separated from each other
2
"by 100 to 150 feet of eruptive rocks. This formation crops
out on the western and central islands; each unit of the
formation has a maximum thickness of thirty five or forty
feet. Fossiliferous Lower Pleistocene terrace deposits
overlie the San Onofre breccia on the central island.
The marine deposits are overlain in turn by Upper
Pleistocene and Recent nonmarine terrace sands.
The geology of the submerged Insular shelf is
similar to the island proper except for outcrops of the
Monterey formation beneath Anacapa Passage. Most sub
marine outcrops occur on the shelf areas east and west of
the island, where a thin veneer of Recent and relict
Pleistocene sediments is present. A paucity of rock
bottom north of the island indicates a thick cover of
Recent sediment.
Anacapa is essentially a gentle north-dipping
(2-20°) sequence of volcanic rocks, and it is structurally
related to the other Santa Barbara Islands and to the
Santa Monica Mountains. The islands and the Santa Monica
Mountains form the structural province of Anacapia and
also the western portion of the Transverse Range
Physiographic Province.
Uhconsolid©,ted sediments overlying the insular
shelf are characterized by an average median diameter
of 0.360 mm, a sorting coefficient of 1.73, a carbonate
content of 45.5 per cent, and a 0.7 per cent organic
matter content. Calcium carbonate and organic matter
increase with distance from the island. Median diameters
decrease with increasing depth north and south of the
Island, but increase with distance to the east and west.
Detrital sediments or deposits which have carbonate
contents less than fifty per cent occur principally north
and south of the Island. They are coarsest to the south
because grains smaller than about 0.125 mm tend to by-pass
the narrow and rather steeply sloping southern shelf.
Calcareous sediment types are characteristic of the
eastern and western shelf areas where an environment of
nondetrital sedimentation prevails. Most of the calcare
ous matter is composed of mollusk fragments, Lithothamnium
debris, bryozoan zoaria, and echinoid spines and plates.
Foraminifera also contribute, but they are most abundant
in the sediments near the outer edges of the northern
and southern shelves.
GEOLOGY AND SURROUNDING REGENT MARINE SEDIMENTS
OP ANACAPA ISLAND
INTRODUCTION
General
Eight islands rise above submerged shelves off the
southern California coast. These islands fringe tectonic
basins of a drowned portion of the continent known as the
Continental Borderland (Shepard and Emery, 1941).
Geologists have been interested in these islands for
nearly eighty years and have published geologic maps for
most of them. Studies of the geology and sediments of
many Insular shelves have also been completed.
Until this study, Anacapa Island, one of the
smallest of these islands, had never been mapped
geologically and little was known of Its shelf sediments.
Anacapa is unique not only because it lies closer to the
mainland than the others, but because persistent wave
attack has destroyed so much of the island that complete
wave-leveling is foreseeable in the near geologic future.
Considering these facts, the island of Anacapa should
provide Interesting mapping and sediment studies. The
investigation should also contribute to the rapidly
expanding knowledge of the Continental Borderland.
2
Location and Early History
Anacapa Island lies nearly twelve miles off the
southern California coast along latitude 34° 001 30” N.,
and is the easternmost member of the four Santa Barbara
Islands (see Figure 1 and Plate 1). The island trends
east-west and Is approximately bounded by longitudes
119° 19* W. and 119° 27* W.
The Spanish explorer Cabrillo, first noted the
position of Anacapa during the winter of 1542 (Bancroft,
1890; Vol. I, p. 72). Subsequently another explorer,
Portola, named the island Las Mesitas (Spanish for The
Little Tables) in 1769* Later, in 1792, Captain Vancouver
charted the island under the Chumash Indian name,
Enneeapah, which probably means deception (Comm. Atkins,
U.S.C.G., personal communication). Repeated misspellings
evolved the present name, which came into use about the
middle of the 19th century. An early chart of Anacapa
Island issued In 1854 by the United States Coast Survey
is presented on figure 2. A sketch of the eastern end of
Anacapa by the famous J. A. Whistler is also Included on
this chart.
Anacapa was made part of Ventura County in 1873*
In 1938 this island with Santa Barbara Island was set
aside as the Channel Island National Monument. A 600,000
3
II9*2S’
00'
ANACAPA ISLAND
, LOS
ANGELES w
BATHYMETRIC CHART
STATUTE MILES
F ig u r e 1„ L o c a tio n a n d b a th y m e tr ic c h a r t o f A n a c a p a Is la n d 0 C ir c le s
in d ic a te p o s itio n s o f b o tto m s a m p le s (s e e a p p e n d ix ).
Plate 1. Anacapa Island
( Photograph by U.S. Dept,
of Agriculture)
f
<7^,
N o te
Anacapa Island is due. East o f Santa Cruz Island in
IapproxjLat. 3d O0' and Long. 119 23’ W . from Greenwich
Observatory.
Variation o f the Magnetic Needle ------------- i3 zl Iv
U.S.COAST Sen VET
A.D. BATHE Supdt.
S k e tc h o f
A .\ A ( A I'A ISLAND
I X
S A N T A B A R B A R A C H A N N E L
B y L ie u t T.B. STEVENS 1 7 . S.N, A ssist 1 ■ S.C.S.
1854
View of th e E a s te r n e x tr e m ity of A n a c a p a I s l a n d — fro m th e S o u th w ard
P r “3 by W B. M'MnrtrU
E ng? by J A. Whistler .T.Young & CA Knight
Figure 2. Early chart of Anacapa Island. Lower
sketch is by J. A. Whistler, who reportedly lost his posi
tion for including the sea gulls. (Robinson, 1955; p. IB).
6
candlepower lighthouse was constructed on the eastern tip
of the Island In 1932 by the United States Coast Guard.
The personnel operating this light are the only permanent
inhabitants of the island (see Plate 2).
Climate
Climatic data for Anacapa have been available since
the construction of the lighthouse in 1932. Duhkle (1950)
synthesized much of this information and cited an average
annual precipitation of 12.58 inches. On the basis of this
figure, and the vegetation on the island, Dunkle divided
Anacapa into arid and semi-humid maritime Mediterranean
climates. The semi-humid climate includes only the
western one-third of the island, which is much higher and
receives more precipitation than do the lower central and
eastern portions. Rainfall is concentrated in the winter
and early spring months. Anacapa is frequented by
morning and night summer fogs which must add appreciably
to the measured annual precipitation.
Records for the years 1948 through 1952 show a
mean annual temperature of 58.3° F. (Relchelderfer, 1948
through 1952). During this same period the coldest winter
months averaged about 14° F. lower than the warmest
summer months. Freezing temperatures were rare, and
summer highs only occasionally reached the mid-eighties.
Plate 2. Eastern Anacapa Island, United States Coast Guard lighthouse.
Elevation at base of lighthouse is 250 feet, (phote by U.S. Navy)
8
Crowell (1952) compiled the direction of wind motion
in the general vicinity of Anacapa for the years 1936
through 1938. His data show that nearly ninety per cent
of the winds blow from the north, northwest, and west-
northwest. Maximum winds probably occur during the fall
of the year when violent northeasterly winds reach the
island from the continent. Velocities exceeding fifty
knots were measured aboard the research ship Velero IV
during one of these storms in November 1957.
Previous Investigations
The sinking of the steamship Winfield Scott after
she struck Anacapa in the winter of 1853, aroused Interest
in the Island as a possible lighthouse site. Lieut.
Stevens* report on this site In 1854 contains the earliest
mention of the volcanic rooks which make up the bulk of
the Island. In 1890 Yates prepared a general description
of the island*s geology. This work was a standard
reference for Anacapa until a generalized columnar section
was published in 1952 (A.A.P.G-. correlation sections, 1952;
channel islands prepared by W. E. Kennett).
A number of investigators made regional Inferences
from Yates* report, though few visited the Island themselvesi
Notable among these are: Bremner (1932 and 1933), Reed
and Hollister (1936), Shepard and Emery (1941), Emery and
9
Shepard (1945), Bailey and Jahns (1954) and Clements (1955)#
Previous knowledge of the submarine geology and
shelf sediments is virtually limited to bottom notations.
These appear on the U. S. Coast and Geodetic Chart no. 5114
and Smooth Sheets nos. 5445 and 5446. Earlier charts
Issued in 1854 and 1856 also show numerous bottom
notations. Some generalizations of the Anacapa shelf
sediments were given by Trask (1931) and Revelle and
Shepard (1939). Uchupi (1954) prepared a sediment
distribution map of a portion of the southern shelf of
Santa Cruz Island which is continuous with the Anacapa
shelf. Several detailed profiles of the Anacapa shelf
have been constructed by Emery (1958).
Bottom Sampling and Field Work
A total of 98 bottom samples were taken aboard the
research ship, Velero IV, from the shallow sea floor
adjacent to Anacapa Island. Samples were collected with
the Hayworth or orange-peel grab during two cruises in the
fall of 1957, and one cruise in the spring of 1958. About
seventy of these samples are from the insular shelf, which
includes an area of about thirty-six square miles.
Dividing this area by the number of samples gives a
sampling density of 0.51, or about two samples for every
square mile of shelf area.
10
Six days were spent in mapping the geology of the
island. Most of this work was done during a three day
sojourn in November, 1957.
Ackno wle dgment s
The writer wishes to express his appreciation to
Drs. K. 0. Emery, R. H. Merrlam, and ¥. H. Easton, members
of his guidance committee, whose suggestions, comments,
and criticism were of great value to the author in the
conduct of the investigation and in the preparation of the
manuscript. Special thanks is due Dr. K. 0. Emery,
chairman of the writer1s committee, who gave freely of
much of his time to discuss problems which arose during
the course of the study.
Appreciation is also gratefully acknowledged to the
Allan Hancock Foundation for providing office and
laboratory space, and for the opportunity of using the
facilities of the research vessel, Velero IV. The writer
is very thankful for the aid given in collecting the
bottom samples by Dr. J. Barnard, Mr. M. Karim, and the
crew of the Velero IV.
Dr. 0. L. Bandy identified Foraminifera collected
from the island. Identification of megafossils was
corroborated by Dr. ¥. H. Easton, and petrologic assign
ments of the rocks were checked by Dr. R. H. Merriam.
PHYSIOGRAPHY AND GEOMORPHOLOGY
Island Topography
General
Anacapa is a gently northward dipping questa of
volcanic rocks which is slightly arcuate to the south
(see Plate 1). The length of the island measured along a
straight line from tip to tip barely exceeds five miles.
It Is 3700 feet wide at its maximum width, and has an
area of 1.05 square miles.
Two narrow gaps break the island Into three smaller
but distinct portions. These are usually referred to as
the eastern, central, and western Islands. The western
Island is the largest and highest of the three. A 612 foot-
wide gap separates the eastern and central islands.
Several stacks and shallow submerged reefs obstruct this
opening even during highest tides (see Plate 3).
Separating the central and western Islands is a passage
less than thirty feet across. Small boats can use this
passage at high water ( see Plate 4). Some Interesting
dimensional statistics of the islands are presented on
table 1.
Plate 3. Gap between the eastern and central islands at low tide, looking
south. Numerous stacks and shallow submerged rocks obstruct the passage, which is 612
feet wide. (Photo by U.S. Navy) H
tu
PASSAGE COLLAPSED ARCH
Plate 4. Passage between the central and western islands at low tide, looking
south. Debris from a collapsed arch blocks what will eventually be another passage.
The collapse has occurred since 1890. Vessel in left foreground is the research ship
Velero IV of the University of Southern California (see Plate 7) (Photo by U.S. Navy)
TABLE I
DIMENSIONAL PARAMETERS OF ANACAPA ISLAND
Western Central Eastern Anacapa
Island Island Island Island
Maximum length 11,679 ft.* 8892* 6264* 26,835 (5.08 miles)
Maximum width 3,717 ft.* 1479* 1479* 3,717 feet
Approximate length of
northern coastline 14,600 ft. 11,900 9600 36,100 (6.8 miles)
Approximate length of
southern coastline 14,400 ft. 12,000 8600 ' 35,100 (6.6 miles)
Area 0.67 sq. miles 0.220 0.16 1.050 sq. miles
Highest elevation 930 ft. 325 250 930 feet
*W.M. Johnson, 1855; p. 96
15
Drainage
Drainage is principally to the north hy inter-
mittant resequent streams flowing down the dip slopes of
tilted volcanic rocks. They are best developed on the
western island, poorly developed on the central island,
and practically non-existent on the eastern island. This
is due to the questa structure of Anacapa which is more
strongly expressed to the west, and because the central
and eastern islands have nearly flat summits whereas the
western island rises to a peak of 930 feet. Moreover, the
western island receives the greatest annual rainfall.
Obsequent drainage down the anti-dip southern slopes is
also better developed on this island, but it is not
prominent because of active landslldlng.
Resequent streams on the western island head near
its summit and cut rather deeply into two lower marine
terraces (see Plate 5). They do not maintain their
gradients to sea level, however, for upon reaching preci
pitous sea cliffs at the outer edge of the lower terrace
they form dry waterfalls which plunge nearly 200 feet to
the ocean. Only four rather small intermittent streams
draining the central island have reached the upper summit
platform by headward erosion. Drainage on this island is
practically restricted to high gradient stream channels
Plate 5. North side of western island. Two wave-cut terraces can be
distinguished at elevations of approximately 6 0 0 . . and 250 feet. The summit of the
island is at 930 feet, which is the highest point on Anacapa Island. Note the
numerous sea caves at the base of the precipitous coastal cliff. (Photo J>y U.S. Navy)
i —1
cn
17
developed on nearly vertical cliff faces (see Plate 6).
Sea Cliffs
Sea cliffs of spectacular heights surround Anacapa
Island. Along the southern coasts of the eastern and
central islands precipitous walls rise respectively to
elevations of 250 and 325 feet. The summit of Anacapa is
at an elevation of 930 feet atop the southern sea cliff
of the western island (see Plate 5).
Judging from numerous arcuate scars, landsliding
is very active on the cliff faces. Slides appear to he
most active on the higher southern or anti-dip coastal
cliffs. Mass wasting has completely destroyed several
marine terraces which formerly existed along the south
side of the western island. This is known because two
prominent marine terraces which are present on the
northern and western sides of the island are not present
along the sheer southern sea* cliffs (see Plate 5).
Sea Caves
More than one hundred sea caves have been noted
along the Anacapa shoreline (see Plates 5 and 7).
Invariably, they are fault-or joint-controlled. Many of
the caveb are quite large, and in the past several were
used by the Canallno Indians as temporary shelters
Plate 8. North side of the central island. Resequent streams on the eastern
one-half of the island have reached the upper summit terrace by headward erosion into
the San Onofre breccia (whitish outcrops) and overlying nonmarine terrace sediments.
Summit terrace slopes about 5° to the north, its average elevation is approximately
250 feet. (Photo by U.S. Navy) * •
00 .
Plate 7. Sea caves along the southern coast near the passage between the
western (left) and central islands. Approximately one-half mile of shoreline is
shown (Photo by U.S. Navy)
i —
co
(P. C. Orr, Santa Barbara Museum of Natural History,
personal communication).
Nearly all of the sea caves are flooded at high
tide, but many may be entered at slack water. Commonly
deep channels lead into the caves through a fronting
"storm terrace". These channels were probably formed by
the collapse of the outer portion of the caves with the
extension of the "storm terrace" behind Jihe cave entrance.
Cave formation appears to have been an important
process in creating the inter-island passages. Striking
evidence of this can be gained by standing atop the cave-
riddled eastern tip of the western island. During high
tides, the Impact of the surf entering these caverns can
be felt through nearly one hundred feet of overlying rock.
Yates, in 1890 (see Figure 2), Illustrated an arch
connecting this eastern tip with the rest of the western
island. Subsequently, the arch collapsed, and the fallen
debris blocks what will be eventually a third passageway
(see Plate 4).
Elevated Marine Terraces
Two prominent wave cut terraces, and possibly a
third, can be recognized on Anacapa Island. The highest
terrace is present only on the north and west sides of the
western island where it occurs at an elevation of about
21
six hundred feet. Streams have dissected much of this
platform hut it can he discerned as a flattening on the
between canyon ridges near the summit of the island (see
Plate 5)*
The second of the prominent terraces occurs at an
elevation of 250 feet and forms the flat summits of the
central and eastern islands (see Plates 1 and 6). It is
also well-developed on the north side of the western
island. A third terrace may he present at an elevation
between thirty and forty feet along both sides of the
central island, however, it is difficult to recognize
because of concealing landslide debris and talus (see
Plate 6).
Both of the higher terraces slope noticeably to the
north. A declivity of five degrees was measured on the 250
foot terrace atop the central island. However, at the
eastern and western tips of Anacapa the slope of this
terrace is not towards the north, but to the east and
west respectively. In fact, at the eastern end of the
island the 250 foot terrace shows a marked increase in
declivity which can be traced to within fifty feet of sea
level across the beveled summits of several outlying sea
stacks (see Figure 2 and Plates 2, 8, and 9). This is
interpreted to mean that the terrace slope is primarily a
wave-cut surface, and not a tilted or warped surface.
22
Plate 8, Distant view of the northern coast of
Anacapa Island. A marked slope increase of the summit
terrace at the eastern (left) end of the island can be
traced across several outlying stacks to within fifty feet
of sea level, (see Plate 2 and Figure 2).
Plate 9. Closer view of eastern tip of Anacapa
Island. The wave-cut summit surface can be traced eastward
across the tops cf the outlying stacks (see Plate 2 and
Figure 2).
j
That some north tilting has occurred Is probable because
submarine topography suggests that the surrounding Insular
shelf has been slightly depressed to the north since the
elevation of the Island terraces. This is discussed more
fully in a following section.
Effects of Wave Attack
The island of Anacapa has been greatly modified by
incessant wave attack. Evidence of this is clearly shown
by the precipitous sea cliffs, the between-island passages,
and the numerous outlying stacks which nearly encircle the
island.
Destruction of the island is principally effected
by the collapse of the sea cliffs. The mechanism of
collapse Involves the carving of a wave-cut terrace and
the excavation of caves along faults and zones of weakness.
Construction of these features oversteepen and undermine
the cliffs, whereupon, collapse results. Once the wave-
cut terrace is maturely developed, it becomes difficult
for waves to reach the receded sea cliffs, but channels
lead to the eaves and undermining continues without
Interruption. In this manner Anacapa was breached in two
places and a third passage will doubtless soon be opened.
The presence of elevated wave-cut terraces suggests
that the island is not teetonically stable, and thus, it
24
Is hazardous to postulate eventual wave-leveling; but it
is reasonable to expect that if present conditions
continue Anacapa may some day be charted as a bank or
shoal rather than as an island.
Bottom Topography
Data Source
Knowledge of the sea floor topography surrounding
Anacapa Island is primarily based on data taken from
United States Coast and Geodetic Survey Smooth Sheets
nos, 5445 and 5446, These sheets were used to construct
the general bathymetric chart shown on figure 1.
Additional information was obtained from fathogram traces
and several detailed bottom profiles constructed by an
edited acoustic sounding technique described by
Emery (1958). Emery published two such profiles of the
Anacapa shelf; one of these is included on figure 3.
General
The insular Anacapa shelf is defined as the
surrounding submerged platform which extends seaward from
the island to a marked basinward steepening of the sea
floor. By this definition the shelf includes about
thirty-six square miles of the encircling ocean bottom.
25
1 . EMERY C l « e ) PROFILE 47 _
2. EDITED A & IS T IC SOUNDINGS —
3-4U.S.C.G.S SMOOTH SHEETS NOS. 5 4 4 5 ^
5. FATHOGRAM TRACE
Figure 3» Profiles of the Anacapa shelf. SymbolsT end 1 bracket inner
and outer edges of terraces ( see Emery, 1958)•
26
The shelf is considered to be composed of separate
northern, southern, eastern, and western areas. The
eastern and western shelves comprise the entire shelf
region which lies respectively east and west of the island;
correspondingly, the other two areas are restricted to the
shallow submerged platforms fringing the island1s
southern and northern coasts.
Nearly fourteen square miles of shelf area border
the northern side of the island, but less than seven square
miles flanh the southern coast. This difference is due to
the greater width of the northern shelf, which at its
maximum is about three miles; the southern shelf averages
less than one-half this breadth. East of the island the
shelf is intermediate in seaward extent between its
northern and southern counterparts. To the west, the
shelf extends beneath Anacapa Passage (see Figure 1) and
merges with the eastern shelf of Santa Cruz Island. The
distance separating the two islands is four miles, but only
the eastern one-half of the sea floor beneath this shallow
passage is considered to be part of the Anacapa shelf.
Tectonic maps of the Continental Borderland show a
prominent fault at the base of the southern insular slope
(Shepard and Emery, 1941; p. 47; and Corey, 1954). This
fault can be inferred from the steepness of the slope,
which reaches nineteen degrees, and its straightness.
27
The proximity of this fault to the island is clearly
responsible for the narrow southern shelf. A fault may
also exist at the base of the northern insular slope, but
topographic evidence there is not entirely convincing.
Maximum gradients on this slope are about six degrees.
Initial vertical movements along both sides of the island
probably began in late Miocene time (Emery and Shepard,
1945). The cutting of the shelf on top of the uplifted
Anacapa block is generally attributed to the erosive
action of the numerous lowered Pleistocene sea levels.
Shelf Break
Gradients exceeding five degrees mark the outer
edge of the Anacapa shelf. The shelf edge, however, does
not oceur at uniform depths around the island. It is
deepest north of the island, where thirteen measurements
yield an average depth of three hundred feet. The break
in slope south of the island is shallower, occurring at
depths averaging 270 feet. A maximum difference of 102
feet was measured between the depths at the outer edges of
these two shelf areas. East of the island the shelf edge
is not well defined, but appears to occur at depths
near 270 feet. Figure 3 shows some typical profiles of
the Anacapa shelf.
It does not seem reasonable that an average
28
difference of thirty feet, and maximum of 102 feet, could
have been produced with the cutting of the outer edges of
the northern and southern shelves. More likely, it
reflects a slight north tilting (0° 4*) of the insular
platform.
Shelf Slopes and Terraces
Steep initial submarine slopes ranging from 2° 30*
to 5° flank the island. The overall declivity of the
shelf out to a depth of about 270 feet is 1° 40*. Except
for occasional terraces, the sea floor south of the island
slopes rather uniformly to the outer edge of the shelf.
However, the Inclination of the northern shelf is greatly
reduced upon reaching a depth of about 270 feet, which
usually occurs at a distance of about one mile from the
island. Beyond this depth, a broad terrace with an
overall gradient of 0° 5* extends to near the shelf edge.
A corresponding terrace is present on the southern shelf,
but it is poorly expressed, steeper, and occurs closer to
a depth of 240 feet.
Emery (1958) recognized three submarine terraces on
the southern shelf. The seaward edges of these terraces
are at depths of 100, 180, and 240 feet respectively. It
is thought that they are wave-cut features carved by lower
Wisconsin sea levels (Emery, 1958). North of the Island
29
the hundred-foot terrace is not well developed, hut it may
he present as a slight flattening of the shelf slope at a
depth of 120 feet (see Figure 3 and Profile 2), Profiles
constructed from smooth sheet data indicate a shallower
terrace near sixty feet; it also is present south and east
of the island. Both of the deeper shelf flattenings occur
on the northern shelf. The corresponding 240 foot terrace,
however, is fully thirty feet deeper. A 330 foot terrace
commonly forms the outer edge of the northern shelf
(see Figure 3 and Profiles 4 and 5).
The sea floor heneath Anacapa Passage is
essentially the westward continuation of the prominent 180
foot terrace. Examination of profiles reveals the passage
floor is inclined to the north (see Figure 3 and
Profile 5). The direction of this declivity, and the fact
that submerged terraces on the northern shelf are deeper
than corresponding flattenings south of the island, are
thought to support an earlier contention that the Anacapa
shelf has been slightly depressed to the north.
Minor Relief
The sea floor immediately adjacent to the island is
made irregular by stacks and shallow submerged outcrops.
Submerged knolls, probably representing former stacks, are
common out to depths of 120 feet. They usually rise about
30
ten or twenty feet above the bottom. Betifeen these
occasional projections, the sea floor tends to be rather
smooth. Bottom sampling has shown these smooth areas to
be tracts of unconsolidated Recent sediments.
Except for terraces, the entire southern shelf
beyond a depth of about 120 feet is more or less feature
less. Local relief greater than one fathom appears to be
rare even near the shelf edge where submarine outcrops are
known. At depths less than 270 feet the northern shelf
also tends to be featureless. However, on the prominent
270 foot terrace rounded knolls as much as three-tenths of
a mile in diameter rise twenty or thirty feet above the
bottom. Sediments collected from these submerged highs
indicate they are not relict sediment mounds, but are
irregularities in the bedrock of the shelf which have
been covered by a thin veneer of Recent deposits. The
lack of similar knolls on the southern shelf suggest either
a thicker blanket of Recent sediments there, or a more
uniformly cut shelf. It is thought that the latter is the
more probable, because submarine outcrops are widespread
over this shelf. Moreover, the southern shelf is on the
windward side of the island where stronger wave action
would be expected to produce a more even shelf.
GEOLOGY
General
The stratlgraphlc section exposed on Anacapa Island
Is very Incomplete in comparison with those of the other
Santa Barbara Islands. Because Anacapa Is structurally
related to the other Islands, it is best to summarize the
geology of the island chain briefly before discussing the
limited stratlgraphlc section exposed on Anacapa Island.
The rocks which crop out on the Santa Barbara
Islands are largely marine sandstones and shales of
Tertiary age. Thick sequences of Middle Miocene volcanic
rocks also occur on each of the islands. A maximum
Tertiary section of approximately 17>750 feet is exposed
on Santa Cruz Island. This section ranges in age from
Paleocene through Late Miocene (Kennett, 1952), Cretaceous
6utcrops have been reported from San Miguel and Santa Rosa
Islands. Pliocene rocks are conspicuously absent, but
marine and nonmarine Quaternary sediments occur atop wave-
cut terraces on all the islands (BremHer, 1932 and 1933,
and Kennett, 1952).
Granitic and metamorphic basement rocks are exposed
on Santa Cruz Island. These have been compared by
Bremner (1932) and others to similar basement rocks
32
cropping out nearly seventy miles to the east in the
Santa Monica Mountains, Occurrence of Franc is can-type
basement rocks both north and south of the Santa Barbara
Islands and the Santa Monica Mountains, has led Reed and
Hollister to consider them as belonging to a distinct
geologic province. Appropriately, they chose the term,
Anacapia, to designate this province (Reed and Hollister,
1936; p. 105).
The Santa Barbara Islands are structurally and
physiographically the westward continuation of the Santa
Monica Mountains, which form the western part of the
Transverse Range Physiographic Province on the mainland.
The Santa Monica Mountains are a large east-west trending
complex anticline which is cut obliquely by the coastline
between Point Dome and Point Hueneme (Bailey and
Jahns, 1954). Study of cross-sections through Santa Rosa
and Santa Cruz Islands show that the anticline continues
seaward as the submarine ridge above which the Santa
Barbara Islands rise.
Stratigraphy of Anacapa Island
General
Anacapa Island is composed principally of a series
of andeslte flows and pyroclastics. The occurrence of
33
two beds of the San Onofre breccia within the body of the
1 eruptive rocks suggests that the volcanic rocks are of
\ early Middle Miocene age. Quaternary marine and nonmarine
\ deposits occur atop elevated wave-cut terraces (see
Figure 4; geologic map, in back pocket).
Miocene Series
Conejo Volcanics (Lower Middle Miocene)
The name Conejo Volcanics was applied by
Taliaferro (1924) to the sequence of volcanic rocks and
Miocene sediments which crop out in the vicinity of the
Conejo Mountains at the western end of the Santa Monica
Mountains, Galifornia. Kennett (1952) extended the range
of this formation to the Santa Barbara Islands.
Black to dark-red vesicular and porphyritic basic
extrusive and pyroclastic rocks are the oldest and
principal rocks occurring on Anacapa Island. These rocks
consist of massive-to-thinly bedded lavas, autobreeclated
flows, lenses of lapilli tuffs and tuff breccias, and beds
of volcanic breccias and agglomerates. Ho attempt was
made to differentiate these units because of the difficul
ties of access to the precipitous cliffs surrounding the
island. The volcanic series strikes west or slightly
north of west and dips a maximum of about twenty degrees
to the north. A maximum thickness near 1700 feet is
34
exposed on the south side of the western island (see
Figure 4).
Petrographic studies of several flows reveal that
phenoerysts of labradorite make up at least thirty percent
of the rock. The feldspar occurs as euhedral laths up
to five mm in length. Phenoerysts of hypersthene and
augite also occur, but they are less common and rarely
exceed one mm. The matrix is typically composed of
minute microlites (less than 0.1 mm) of labradorite,
hypersthene, augite, and biotite set in an isotropic
groundmass. Pyrlte is abundant as disseminated grains.
Secondary opal and ehalacedony occur as vein and cavity
fillings.
Shelton (1955> P.57) described nearly identical
Middle Miocene rocks from the Glendora area east of the
Santa Monica Mountains, which, by means of chemical
analyses, were identified as andesites. Lacking similar
analyses of the Anacapa volcanics, the writer tentatively
classified them as andesites on the basis of Shelton's
descriptions of the Glendora volcanics.
Xenoliths of blocks and angular cobbles and
pebbles of metamorphic rocks are common where the
andesites overlie sedimentary breccias of the San Onofre
formation. A sharp baked or altered zone is present at
these contacts. Inclusions of fine-grained greenish
35
calcareous shales and pebbly shales are abundant In the
andesite flows beneath the sedimentary breccias. These
shales are often twisted into sinuous curves or are
rolled-up about a nucleus of andesite. The contortion
which the shales have undergone indicates that they were
in a plastic state at the moment of engulfment. Fossils
were not found, but it is probable they were marine
sediments picked up by submarine extrusions. Evidence of
subaqueous flows is found on the central island where
siltstones bearing marine fossils are overlain by
andesitic lavas (see Figure 4 fossil locality No. 1). In
contrast, subaerial extrusions are indicated where ande
sites overlie alluvial deposits of the San Onofre
formation.
Bremner (1933) and Kennett (1952), have compared
the volcanic rocks of Anacapa with those forming the
eastern tip of Santa Cruz Island. Kennett correlated
the eruptive rocks of both islands with the Middle
Miocene ConeJo volcanics of the Santa Monica Mountains.
The Conejo volcanics of Anacapa Island may be slightly
older than corresponding rocks on Santa Cruz Island.
This is inferred because the eruptive series on Santa
Cruz Island overlies the San Onofre breccia, whereas on
Anacapa the andesites crop out both above and below the
breccia. This contention, of course, assumes that the
36
San Onofre formation on both islands is of the same age.
Miocene fossils found on the central island were not
identifiable, but Helizian fossils have been collected
from the San Onofre formation on Santa Cruz Island
(Redwine, 1952). A similar early Middle Miocene age is
thus suggested for the Conejo volcanics on Anacapa Island.
San Onofre Breccia (Lower Middle Miocene)
Ellis (1919) defined coarse breccias of gametiferous
glaucophane schists and other schistose rocks as the San
Onofre breccia from exposures of the formation at San
Onofre Hill, San Diego County, California. A compre
hensive description of the formation was given by
Woodford (1925).
A coarse sedimentary breccia composed of metamorphic
rock fragments is exposed on the western and central
islands (see Figure 4). Blocks and angular cobbles and
pebbles of glaucophane schists, hornblende schists,
chlorite schists, and pink quartzites are the principal
constituents. Some of the rock fragments are estimated
to weigh over one hundred pounds. Interbedded with the
breccias are pink and light-green sandstones which are
composed of metamorphic minerals and rock fragments
common to the coarser breccia beds.
Woodford and Bailey (1928) have described a similar
37
outcrop of sedimentary breccia over thirty five miles east
of Anacapa in the Point Dume area. They identified this
exposure as the northwestern continuation of the San Onofre
breccia, a formation which is common to the southern
California coastal area. Outcrops of the San Onofre
breccia have also been recognized on neighboring Santa
Cruz Island (Bremner, 1932; p. 21). On the basis of
lithologic similarities with the Point Dume and Santa
Cruz Island outcrops, the metamorphic-sedimentary breccias
exposed on Anacapa Island are thought to be the San Onofre
formation. The occurrence of this formation of the island
adds the connecting exposure between Point Dume and
Santa Cruz Island.
Outcrops on the western island occur near its
eastern end along the base of the precipitous southern
sea cliffs (see Plates 10 and 11). Two distinct strata
crop out which are separated by 100 to 150 feet of
volcanic rocks. Eruptive rocks of the ConeJo formation
both underlie and overlie the breccia horizons. A
clearly defined baked or altered zone is present along the
upper contacts of each breccia unit. The upper stratum
has a maximum thickness of about thirty feet and is
traceable for nearly 4000 feet along the cliff faces. A
similar thickness is reached by the lower horizon, but it
is limited to an exposure of less than 500 feet (see
38
Plate 10, Outcrops of the San Onofre breccia (Tso)
along the southern coastal cliff of the western island.
The San Onofre formation crops out within the volcanic
series of the Conejo formation (Tcv).
Plate 11. Conejo volcanics (Tcv) overlying the
upper San Onofre breccia unit (Tso) on the western island.
Geologic hammer rests against a boulder of schist estimated
to weigh more than 100 pounds (arrow). Exposure is near
the western end of the outcrop belt shown on plate 10.
Figure 4). Both strata strike about N. 80 W. and dip
between fifteen and twenty degrees to the northeast.
Because of the configuration of the coast the western
limits of both outcrops are formed where the plane of the
dip extends below sea level. Talus conceals the eastern
edges of their outcrop belts, but projections of dip
planes indicate that they pass below sea level beneath
the talus.
Outcrops of the San Onofre formation on the central
island occur near its eastern end along the northern and
southern sea cliffs. Both units of the breccia crop out
on the north side of the island (see Plates 12 and 13)
where they are separated by 150 feet of volcanic rocks.
Lavas and pyroclastic rocks underlie the lower horizon but
the upper unit is overlain by marine and nonmarine terrace
deposits. Along the southern sea cliffs the lower stratum
is either very thin or absent, and the upper unit is
overlain by andesitic lavas or nonmarine terrace sands
(see Figure 4, cross-section A-A*). Poorly preserved
marine gastropods were collected from silty sandstones
exposed' at the top of the higher horizon (see Figure 4
Fossil locality No. 1). The breccias strike approximately
N. 90 E. and dip a few degrees to the north. Each unit
has a maximum thickness of. about thirty five feet,
however, they thin rapidly and are not traceable for more
40
J l. v ' -
Plate 12. Outcrops of the San Onofre breccia (Tso)
on the north side of the central island. The Conejo
volcanics (Tcv) underlie and overlie the lower breccia
unit, the upper breccia is overlain by Quaternary non
marine terrace sediments (Qtn) (see Plate 6).
Plate 13. Closer view of the formations cropping
out along the northern coastal cliff of the central
island. See Plate 12 for explanation of symbols.
41
than a few thousand feet along the cliff faces.
With the exception of the marine sandstone
mentioned above, the lithological and textural character
♦
of the breccia suggest it is of fluvial origin. A
southerly source for the metamorphic rocks is probable in
view of the convincing evidence cited by Woodford (1925)
and Woodford and Bailey (1928). On the Basis of Woodford's
work, Reed (1933, p. 293) designated this Tertiary land
area as the Franciscan metamorphic province of Catalina.
Fossils collected frogi the breccia were too poorly
preserved for identification, but characteristic Middle
Miocene micro-and megafossils occur in the San Onofre
breccia on Santa Cruz Island. Two units also crop out on
Santa Cruz Island, but only the upper horizon is
lithologically similar to the Anacapa breccias.
Redwine (1952) considered this upper unit to be of
Relizian age. ICLeinpell (1938, p. 119), and Reed and
Hollister (1936, p. 122) have made corroborating opinions
concerning the age of the mainland outcrops near Point
Dume. It is thought the San Onofre breccia on Anacapa
Island is of Relizian age, or early Middle Miocene.
Pleistocene Series
Marine Terrace Deposits (Lower Pleistocene)
A poorly consolidated fossiliferous sandstone less
than three feet thick crops out for a few hundred feet
atop the northern sea cliff of the central Island (see
Figure 4, fossil locality No. 2). The sandstone was
undoubtedly laid down contemporaneously with the cutting
of the 250 foot terrace atop the island (see Figure 4).
It unconformably overlies the San Onofre breccia from
which most of its clasts have been derived. Nonmarine
terrace sands lie in apparent conformity above the
sandstone, but an unconformity is suspected as the
nonmarine sediments locally rest directly on the San Onofre
breccia.
Mollusks collected from the sandstone indicate a
time correlation with the Lower Pleistocene San Pedro
formation of the Los Angeles Basin (Grant and Gale, 1931).
Characteristic forms are: Olivella blpllcata. Hvalina
jewettii. Conus californicus. Bittium rugaturn.
Schizothaerus nuttalli. Venerupls staminea, and Mytllus sp.
Common non-molluscan forms are: Dendraster sp. (possibly
D. dlegoensis or D. venturaensls). Balanus concavus. and
Terebratullna s p . Foraminifera also suggest a time
correlation with the San Pedro formation, in particular
with it shallow water biofacies (Dr. 0. L. Bandy, personal
communication). It is Dr. Bandyfs opinion, that the
occurrence of two dominant species, Elphidium fax, and
E. microgranulosum. indicate a warm water fauna probably
43
deposited during one of the early Pleistocene interglacial
ages. Marine terrace deposits of general Early Pleistocene
age have been mapped on the other Santa Barbara Islands
(Breinner, 1932 and 1933; and Kennett, 1952).
Nonmarine Terrace Sands (Upper Pleistocene. to Recent ?_)
Nonmarine terrace sediments are commonly absent
or very thin on the western and eastern islands; however,
a portion of the summit terrace atop the central island is
covered by a thick accumulation of fluvial sands and silty
sands (see Figure 4). Atop the northern sea cliff the
nonmarine sediments are about twenty five feet thick, and
overlie either the marine terrace deposits or the San
Onofre breccia (see Plates 12 and 13). The fluvial sands
at the southern edge of the summit terrace are only three
to four feet thick and rest on either the San Onofre
formation or the Conejo volcanics. This thinning to the
south, and the fact that the mineralogy of the sands Is
essentially identical with that of the San Onofre Breccia,
are Interpreted to mean that the underlying San Onofre
breccia once cropped out on a higher portion of the
island which formerly existed to the south. It is
probable then that the present summit terrace in the recent
geologic past was a fringing platform along the northern
side of a much larger and higher island. This relationship
44
exists on the western island and it is a reasonable
assumption that the eastern island, like the central
island, is merely a terraced remnant of a former larger
island. The San Onofre breccia did not crop out above the
terraces on the western and eastern islands, and thus these
surfaces never received a thick blanket of fluvial terrace
sediments.
Insular Shelf Stratigraphy
General
Hocks recovered from the Anacapa shelf were
classified as transported material, or as rock essentially
in place. In making the distinction, many of the critera
listed by Emery and Shepard (1945, p. 434) were used;
these are:
Rock Essentially in Place
1. Fresh fractures
2. Large size of individual rocks
3. Abundant rocks of similar lithology
4. General angularity of the rocks
5. Fragile or poorly consolidated rock
Transported Rock
1. Varied lithology
2. General roundness
3. Small size of rocks
Only submarine outcrops, or rocks essentially in
place, are considered here; transported debris is
45
discussed more completely in a later section.
Rocks more than two feet long and weighing over
eighty pounds were frequently recovered from the shelf.
Whenever large rocks were brought to the surface at any
one particular station they were invariable of one
lithology and many showed fresh fractures. It is apparent
that because large rock fragments often had encrusting
organisms on all sides that they were lying free on the
bottom and were not directly attached to bedrock. Boulders
and cobbles of siliceous shales frequently had pholad
borings on all sides. In one instance, a very cohesive
mud was sampled which contained abundant Miocene
Radiolaria and Diatoms. Irregular bottom topography near
shore also suggested the presence of bedrock; spot
samplings of many of these irregular areas confirmed the
occurrence of outcropping rock.
In all, fourteen submarine outcrops were located
(see Appendix II). The llthologic character of most of
these can readily be correlated with formations cropping
out on Santa Cruz or Anacapa Islands (see Figure 5). In
a few instances outcrops were located but because of great
resistance or poor exposure, rocks could not be obtained
with the sampling device used.
Rock bottom is most widespread over the western
shelf area beneath Anacapa Passage (see Figure 5).
46
S TA TU TE MILES
0
"300FT. B O T T O M LITHOLOGY
- c KNRCTcv}
NRCTcvJ
LEGEND
SUBMARINE OUTCROPS
T m T-MONTEREY s h a l e
Tc v -C O E J O VOLCANCS
NR-NO RECOVERY
119*30* 28' 26' 24' 20'
APPfLIXIMATE u m its o f r o c k
\ BOTTOM, OR ROCK BOTTOM
_ J BENEATH A T H N SEDIMENT COVER
TRANSPORTED ROCK
O MONTEREY SHALE
• CONEJO \O LC A M CS _
A METAMORPHIC ROCKS
■ GRANOOCWTE
18'
Figure 5. Submarine outcrops and transported rocks. See appendix
II Tor .further description of bottom lithology.
47
However, portions of the passage floor are covered by a thin
veneer of Recent and relict Pleistocene sediments. Exposed
and partially buried outcrops are also abundant on the
eastern and southern shelves, but are lacking on the
northern shelf except in shallow near shore areas. It will
be shown in subsequent sections that areas of rock bottom
are also regions of slow sedimentation and active currents.
Miocene Series
Conejo Volcanlcs (Lower Middle Miocene)
Outcrops of the Gonejo volcanics occur nearshore
along the northern and southern sides of the island. On
the eastern shelf, a nearly continuous outcrop belt of
volcanic rock is traceable beyond the shelf break. Other
exposures occur at or near the outer edge of the southern
shelf and over portions of the western shelf (see Figure 5).
Monterey Shale (Middle Ml+cene)
Blake (1855) applied the name Monterey shale to the
diatomaceous and siliceous shales exposed near the city of
Monterey, California. Outcrops of the formation on the
mainland immediately north of Anacapa have been described
by Dibblee (1950); Bremner (1932) discussed exposures of
the Monterey shales on adjacent Santa Cruz Island.
Oream-colored calcareous and siliceous shales
48
and gray mudstone crop out over much of the shelf area
beneath Anacapa Passage. The shales closely match
Bremnerfs (1932, p. 32) description of the Middle Miocene
Monterey formation which crops out near the eastern tip of
Santa Cruz Island. Diatoms and Radiolaria are abundant
in the mudstone and are similar to known Middle and Upper
Miocene forms (Dr. 0. L. Bandy, personal communication).
Shales of the Monterey formation stratigraphically
overlie the Conejo volcanics on Santa Cruz Island. They
are in fault contact near the eastern end of the island,
and both formations are exposed along the western side of
Anacapa Passage (Bremner, 1932, and personal communication
with W. E. Kennett; Phillips Oil Company, Los Angeles).
Beneath the passage, the shales crop out at least 180 feet
lower than the base of the 1700 feet of volcanic rocks
exposed on the western island. This is thought to be strong
evidence that the two formations are in fault contact on
the western Anacapa shelf. However, on Santa Cruz Island
exposure occurs where the shales of the Monterey formation
overlie the Conejo Volcanics or occur as interbedded
lenses within the volcanic sequence. Thus, it may not be
necessary to rely on faulting to explain the presence of
outcrops of the Monterey formation beneath the passage.
49
Structure
Anacapa is a gentle monocline or cuesta of north
dipping rocks. However, the dip is not uniform along its
length but tends to become steeper to the west. A maximum
of about twenty degrees occurs on the western island.
Dips on the central and eastern islands are usually less
than five degrees.
Faults are very common, but except for presumed
faulting beneath Anacapa Passage, no evidence was found of
large-scale movements. Most of the faults are high-angle
strike-slip fractures which trend transverse to the length
of the island. Sea caves are invariably formed where these
faults intersect the cliff faces. Fractures also occur
which approximately parallel the east-west trend of the
island. A fault of this type is present along the south
side of the central island and appears to have dip-or
oblique-slip movements (see Figure 4, cross-section A-A*).
Only the most prominent faults have been included on the
geologic map.
Anacapa is structurally the eastward continuation
of the northern side of Santa Cruz Island, which is
essentially a large east-west trending complex anticline
that is split along its axis by a major obilque-slip
fault (Bremner, 1932). The eastward extention of this
50
fracture can be traced along the base of the Insular slope
south of Anacapa Island.
OCEANOGRAPHY
Shelf Currents
Tidal currents in Anacapa Passage are probably the
strongest water motion affecting the Anacapa shelf.
Surface velocities near 0.8 knots, which were aided by
twenty knot winds, were measured in the passage during a
rising tide. The height of this tide was four feet.
Other currents affecting the shelf are controlled by
oceanographic conditions existing off the southern
California coast. These conditions are largely set up by
prevailing winds and may change considerably during the
year (Sverdrup et al., 1942; p. 724-727). During the fall
and winter months, the Davidson Current brings sluggish
northwest-flowing water past the island. Persistant
northwest winds of spring and early summer months
frequently cause intense upwelling south of Point
Conception which alters the currents about Anacapa Island.
At times of active upwelling, Anacapa lies transverse to
a counterclockwise eddy which brings water to the island
from the south and southeast. When upwelling is not
active, the southeastward-flowing California Current is
present off the island. Velocities of the above currents
are usually not more than a few tenths of a knot
(Sverdrup and Fleming, 1941).
52
In summary, it can "be stated that except for tidal
currents, the water motion over the Anacapa shelf is weak
and inconsistent. However, most of the water reaching the
island arrives from the south and southeast. It seems
reasonable then, that the eastern, southern and western
shelves would be most affected by these currents. This
condition may in part explain the abundance of submarine
outcrops on the above shelf regions.
Longshore Currents
Crowell (1950) and Emery (in press) compiled data on
surface wave propagation over the Anacapa shelf. Their
data show that most of the waves approach the island from
the west and northeast, in harmony with prevailing wind
motions. The north side of Anacapa is somewhat protected
from these waves by Point Conception and the other Santa
Barbara Islands, but waves reaching its southern coast
have only been refracted by the other islands into a more
westward propagation. Because Anacapa trends east-west,
these waves set up east-flowing longshore currents when
they reach the southern shore (see Plate 1). A similar
but weaker longshore drift probably exists along the
northern or lee side of the island.
53
Water Character
Surface water temperatures change with the seasons
and the presence or absence of upwelled water (Scrlpps
Institute of Oceanography, summary of various cruise
reports, 1937-1957). Temperatures frequently reach 18°C
during the late summer and are as low as 12.5°C. during
spring. Surface salinities vary from about 33.2 °/oo
to 33.7 °/oo. Highest values are usually associated with
coldest water as though in respose to upwelling.
SHELF SEDIMENTS
Analyses
Field examination of the unconsolidated sediments
collected from the Anacapa shelf indicates the existence
of a variety of sediments types• In order to determine
the character, distribution, and significance of these
types, investigations were made of their textural,
mineralogical, petrological, and chemical properties.
Textural parameters of sandy sediments were
determined with the Emery Settling Tube (Emery, 1938).
Finer-grained sediments were analyzed by the pipette method
(Krumbein and Pettijohn, 1938, p. 162). Samples having
important gravel, sand, and siltKJlay fractions were first
screened to separate the components. The latter two
fractions were analyzed according to the procedures noted
above; gravels were further divided into grade sizes with
additional screening.
Mineralogic determinations of sands and coarse silts
were facilitated by separating light and heavy specific
gravity minerals after treatment with dilute hydrochloric
acid. Acetylene tetrabromide, specific gravity 2.96
at 20°C., was used as the separating liquid. The light
mineral suite was treated with hydrofluoric acid and
55
stained with a solution of sodium cobaltinitrite. In this
manner quartz, sodic feldspars, and potash feldspars were
distinguished (Twehhofel and Tyler, 1941; p. 131)*
Optical methods were used to identify the heavy minerals.
Both hand-lens inspection and optical techniques were
employed in describing the lithologlc character of
transported rock.
Chemical analyses were made only for calcium
carbonate and organic carbon. Carbonate content was
measured by leaching a known weight of sediment with
dilute hydrochloric acid. Any weight loss after treatment
and filtration was calculated as percent calcium
carbonate. Duplicate analyses usually differed by less
than one percent, but the technique inherently tends to
give high results because some non-carbonate materials are
unavoidably lost during filtration. It is thought the
carbonate percentage given in this paper are two to five
percent too high. Organic carbon was determined by the
potassium dichromate reduction technique described by
Allison (1935). Organic carbon percentages were
multiplied by a factor of 1.7 to convert to total organic
matter (Emery and Rittenberg, 1952; p.78l). The Allison
method can be expected to give only semi-quantitative
data.
56
Texture
Median Diameter
Median diameters of the Anacapa shelf sediments
range from 2.0 mm. to 0.89 mm., and average 0.360 mm.
(see Tahle 2). Shelf sediments having median diameters
greater than 0.500 mm are largely restricted to the
western and eastern shelves (see Figure 6). It will he
shown subsequently that these areas are covered by
calcareous debris, and that their coarseness is principally
due to the presence of animal and plant fragments.
Outcrops however, are abundant on the western and eastern
shelves and residual gravels and coarse sands have
accumulated locally. The average median diameters of
these two shelf areas are respectively 0.714 mm and
0.401 mm (see Table 2). The apparent lack of fine-grained
sedimentation to the east and west is attributed to a
small volume of detrital clasts reaching these areas, and
to a by-passing of detrital clasts caused by sweeping
currents.
Shelf sediments north and south of the island
decrease in grain size with distance from shore. Near
shore, a narrow belt of medium sands encircles the island
and grades northward into wider zones of fine and
TABLE II
GROSS CHARACTER OF SHELF SEDIMENTS
Eastern Western Southern Northern Entire
Shelf Shelf Shelf Shelf Shelf
Average median diameter 0.401 mm 0.714 0.181 0.142 0.360
Average sorting coefficient 2*34 1.84 1.32 1.42 1.73
Average carbonate content 60*5 % 75.0 22.3 25.5 45.4
Dominant calcareous form shell then shell then shell and Foraminifera Shell then
byrozoan algae Foraminifera then shell Foraminifera
Average light mineral 30.8 % 18.7 67.5 67.5 46.3
content
Average heavy mineral 8.1 % 5.6 9.5 6.1 7.6
content
Average organic matter 0.6 % 0.7 0.7 0.9 0.7
content
Number of samples analyzed Id. 11. 16. 13. 50.
II9'30'
28' 26' 20' 2 4 ' 22'
S TA TU TE MILES
04'
ISOPLETH OF
MEDIAN DIAMETERS
-.250
3 4 '
00”
34'
00”
250
v m FIN E SAND
VEHr COARSE SAND
28' 2 6 ' ; 22'
Figure 6. Isopleth of median diameters. Control stations are shorn
on .figure 1.
VJt
59
very fine sands, but only to fine sands on the southern
shelf. It Is also traceable across the southern portion
of the eastern shelf. The very fine sands on the
northern shelf are largely restricted to the broad 270 foot
terrace (see Figure 6). An average median diameter
of 0.142 mm is associated with the sediments of this
shelf area. By-passing of fine-grained detrital clasts
over the rather steep continuous slopes of the southern
shelf probably accounts for its higher average median
diameter of 0.181 mm and the lack of the outer belt of
very fine sands.
It is thought that the general seaward reduction
in grain size is a reflection of detrital sedimentation
and the inability of shelf currents to transport coarser
detrital clasts into deep water. Reversals in this trend
occur in areas of submarine outcrops where residual
deposits have accumulated, and also near the shelf edge.
Similar increases in grain size at the outer shelf edge
have been found over much of the world (Shepard and
Wrath, 1937; p. 43). Such occurrence are thought by most
marine geologists to be either relict Pleistocene
sediments, winnowed shelf deposits, or residual accumu
lations. Relict and residual deposits seem to be most
common on the Anacapa shelf.
60
Sorting; Coefficients
Most of the northern and southern shelf sediments
have sorting coefficients below Trask’s well sorted limit
of 2,50. In general, the southern shelf sediments are
better sorted than those on the northern shelf. This is
perhaps a reflection of more effective currents south of
the island. Average coefficients are respectively 1.32
and 1.42. Some of the highly calcareous sediments east
and west of the island have coefficients greater than 4.00.
However, the average sediment of these areas is
characterized by coefficients less than 2.50 (see Table 2).
Mineralogy
Light Minerals and Rock Fragments
Detrital minerals and rock fragments with specific
gravities less than 2.96 dominate the non-carbonate
fractions of the shelf sediments (see Table 3). They also
constitute the major portion of the northern and southern
shelf sedimentB (see Tables 2 and 3). Labradorite is
singularly the most abundant light mineral, and may form
as much as fifty per cent of the acid-insoluble fraction.
Untwinned albite, sodic feldspars, quartz, biotite, opal
(sponge spicules), and glauconite make up the remainder
TABLE III
MINERALOGICAL COMPOSITION OF SHELF SEDIMENTS
Eastern Shelf Western Shelf Southern Shelf Northern Shelf
Average of Average of Average of Average of
10 samples 11 samples 16 samples 13 samples
(A) (B) (A) (B) (A) (B) (A) (B)
% in non- % in total
carb. fract. sample
Light Minerals
Labradorite 40.6 16.0 40.9 10.2 46.9 36.0 48.2 35.7
Untwinned albite
P P P P P P P P
Orthoclase 0.5 0.2 0.4 0.1 0.1 0.1 0.8 0.6
Quartz 3.5 1.4 4.9 1.2 3.1 2.5 2.8 2.1
Glauconite 2.3 0.9 2.4 0.6 1.4 1.1 1.7 1.2
Rock particles 31.2 12.3 25.8 6.4 35.7 27.4 35.5 26.3
Sponge spicules
(opal)
P P
0.4 0.1 0.5 0.4 2.1 1.5
Biotite
P P P P P P P P
Total 78.1 30.8 74.9 18.7 87.8 67.5 91.2 67.5
leavy Minerals
Augite 12.9 4.7 15.5 3.3 7.0 5.5 5.8 4.0
Hypersthene 7.9 2.9 7.0 1.5 4.0 3.1 2.3 1.6
TABLE III Cont.
Eastern Shelf Western Shelf Southern Shelf Northern Shelf
(A) (B) (A) (B) (A) (B) (A) (B)
Pyrite 0*2
P
0.2 0.1 0.4 0.3 0.3 0.2
Limonite 0.1
P
0.9 0.2 0.3 0.2 0.1 0.1
Rock particles 0.3 0.1
P P P P
0.1
P
Glaucophane 0.1
P P P
0.1 0.1
P P
Crossite
--- --- --- ---
P P P P
Epidote
P P P P P P P P
Hornblende
P P P P P P P P
Colorless Garnet
P P P P P P P P
Zircon
--- --- --- ---
P P P P
Ilraenite and
magnetite
P P P P P P P P
Total 21.9 8.1 25.1 5.6 12.2 9.5 8.8 6.1
Calcium Carbonate
----
60.5
----
75.0
----
22.3
----
25.5
Organic Matter
----
00.6
----
00.7
----
00.7
----
00.9
Sum of Totals 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0
o
to
63
of the light mineral suite.
G-lauconite is the only authigenic mineral present
in noticeable amounts. The average shelf sediment
contains about one per cent of the mineral. It is most
abundant beneath Anacapa Passage where the non-carbonate
residue averages 2.4 per cent glauconite. The mineral
occurs typically as fillings of cavities especially in
species of Oassldulina. Elphidlum. and the Miliolidae.
Echinold spines, biotite grains, and coprolites show
varying degrees of glauconitization.
Rock fragments of small granule and sand size also
form an important part of the shelf sediments (see
Table 3). Nearshore, and in areas of submarine outcrops,
they may constitute fifty per cent of the bottom sediment.
Particles of andesite are most common; fragments of
metamorphic rocks and orange-colored chalcedony also
occur, but they are never dominant.
Heavy Minerals
Minerals with specific gravities greater than 2.96
rarely comprise more than ten per cent of the shelf
sediments. They may, however, form up to fifty per cent
of the acid insoluble fraction (see Figure 7). Based on
fifty samples, the average shelf sediments has a heavy
mineral concentration of 7*6 per cent (see Table 2), which
64
26' Z t S 22' 20'
S TA TU TE MILES
I HEAVY MINERALS IN
INSOLUBLE RESIDUE
CONTOUR INTERVAL 10 °/o STARTING AT 5 °/o
04'
02"
34'
0 0 "
34*1
00'
5 8 "
2 6' 2 8' 22' 20'
Figure 7. Distribution of heavy minerals. Control stations are shown
on figure 1.
65
Is about twice the heavy mineral content of the shelf
sediments of Catalina Island (Shepard and Wrath, 1937;
p. 48). Catalina is the only other channel island whose
rocks have heavy minerals which are similar to those
occurring on Anacapa.
Augite and hypersthene are ubiquitous in the heavy
mineral suite, and commonly comprise over ninety per cent
of these minerals. Less abundant are pyrite, limonite,
hornblende, epidote, and glaucophane (see Table 3). It
seems certain that the augite and hypersthene are derived
from the Conejo volcanics, as these rocks have phenocrysts
of both minerals. G-laucophane and epidote are character
istic of the San Onofre breccia (Woodford, 1925), and
occur In abundance only near outcrops of the formation.
Distribution of Heavy Minerals
Studies of other shelf areas by Emery, Butcher,
G-ould, and Shepard (1952), and Emery, Gorsline, Uchupi,
and Terry (194-7), have shown that most heavy minerals tend
to concentrate in fine and very fine sands because of their
inherent small size. The distribution of heavy minerals
on the ‘ Anacapa shelf, however, is precisely the reverse
(see Figure 7); that is, they form a greater percentage
of the non-carbonate fractions of coarse and medium
grained sands then they do of fine and very fine shelf
sands. For this reason, heavy minerals percentages
decrease with distance north and south of the island, and
increase with distance to the east and west.
Explanation of the distribution pattern is found
in the large grain size of the augite and hypersthene
crystals, which, as previously stated, commonly make up
ninety or more per cent of the heavy mineral suite.
Within the sediments, these minerals are frequently
larger than 0.250 mm; as phenocrysts in the Gonejo
volcanics, they are often greater than 0.5 mm in length.
Thus the combined occurrence on the eastern and western
shelves of coarse-grained sediments and outcrops of the
Conejo formation readily accounts for heavy mineral
concentrations as high as forty five per cent in the
non-carbonate fractions of the shelf deposits. Local
high concentrations also occur on the shelf areas adjacent
to outcrops of the San Onofre breccia (see Figure 7).
Distribution of Glaucophane
A study of the distribution of glaucophane in the
shelf sediments has afforded valuable knowledge of the
net movements of detrltal clasts from the island to the
surrounding insular platform. Glaucophane is an easily
recognizable highly colored sodic-amphibole, which is
characteristic of the San Onofre breccia. This formation
67
crops out along the southern coast of the western island,
and along both sides of the central island (see Figure 4).
The distribution of the mineral was determined by
contouring on a 1.0 per cent interval the percentage that
glaucophane formed of the heavy mineral suite (see
Figure 8). Because most of the glaucophane grains are of
fine sand size (0.250-0.125 mm), their distribution more
directly reflects the net-movements of detrital clasts
occurring in this grade size.
The indication is rather strong that the net
transport of the finer detrital clasts is directed towards
the adjacent insular slopes and basins. South of the
western island, the trace of a decreasing concentration
pattern leads straight across the shelf and onto the
contiguous insular slope where a pronounced lateral
expansion of the contours occurs (see Figure 8).
G-laucophane (and therefore fine sand grains) is deposited
in the silty fine sands covering much of the slope; as
concentrations are as high as five per cent which are
about twice those in the coarser nearshore shelf sands.
The significance of the concentration of glaucophane in
the slope sediments is complicated by the presence of
small pebbles of glaucophane schist which may serve as
secondary sources for the mineral. Nevertheless, it is
clear that glaucophane is transported beyond the shelf
6B
II9'30' 2 S' 26' 24'
STA TU TE MILES
04'
X GLAUCOPHANE IN
HEAVY MINERAL SUITE
CONTOUR INTERNAL 1.0 Vo
02"
02'
34’
0 0 *
oof
5 6 "
28'
Figure 8. Distribution of glaucophane in heavy mineral suite* Control
stations are shown on figure 1.
edge and that the mineral tends to accumulate in the
slope sediments.
Well defined patterns of decreasing percentages
also extend seaward from outcrops on the central island
(see Figure 8). The trend is better developed on the
northern shelf where it can be traced to the outer edge
of the shelf, a distance of more than three miles. A
distinct lateral dispersion, or expansion, of the contours
occurs over the broad surface of the 270 foot terrace.
This is Interpreted to mean that detrital clasts falling
in the finer sand grades sizes do not readily by-pass
the northern shelf. Because a similar dispersion does not
occur south of the island, it is believed that clastic
detritus finer than about 0.125 mm is largely transported
beyond the shelf edge. This lends support to an earlier
contention based on the lack of very fine detrital sands
on the shelf south of the island (see Figure 6).
Transported Rock
Rounded pebble and cobbles of transported rock form
a minor but interesting portion of the bottom sediments.
They are widely scattered over the western, eastern, and
southern shelf areas but are rare on the northern shelf
except near its outer edge (see Figure 5). Most of the
transported rocks are fragments of the Cone jo volcanics,
70
but shales of the Monterey formation are abundant beneath
Anacapa Passage, and over the western portion of the
southern shelf. Metamorphie debris from the San Onofre
formation is locally abundant at and below the southern
shelf edge. A single well rounded two-inch pebble of
granodiorite was recoverd from atop the eastern shelf
break (see Figure 5). The nearest known outcrop of
granitic rocks is about twenty mileB to the west on
Santa Cruz Island (Bremner, 1932).
Emplacement of the transported rocks through
rafting (Emery, 1955) has undoubtedly occurred, especially
considering the position of the granodiorite pebble.
However, the restriction of much of the conveyed debris to
areas of submarine outcrops, or to the shelf edge, suggests
that they are relict or lag deposits of a former lower
sea level. Considering that they rest on submerged
terraces cut during the Wisconsin, then they must be of
this general age. Uchupi (1954) described Pleistocene
transported shales and volcanic rocks from the adjacent
southern Santa Cruz Island shelf. Lag deposits which
may have been present on the northern Anacapa shelf
have been buried by Recent sedimentation.
Calcium Carbonate
Organic calcareous debris is a major constituent
71
of the northern and southern shelf sediments, and is the
dominant component of the shelf deposits east and west of
the island (see Figure 9). The average carbonate content
of these shelf areas ranges from 25.5 to 75.0 per cent
(see Table 2). Based on fifty samples, a carbonate
content of 45.4 per cent is associated with the average
shelf sediment. Studies of other shelf areas show that
sediment having the same carbonate content and median dia
meter as the average Anacapa shelf deposit, are associated
with shelf knolls and submerged offshore banks (Emery,
G-orsllne, Uchupi, and Terry, 1957; p. 113).
In the Shoal nearshore areas shell or mollusk
fragments are the dominant calcareous forms in the shelf
sediments but they rarely constitute more than ten per cent
of the deposits (see Figures 9 and 10). In a few
Instances accumulation of shell and bryozoan detritus
causes percentages to rise above thirty per cent and to
become as high as seventy per cent. Similar high
carbonate areas were found on the southern shelf of
Santa Cruz Island (Uchupi, 1954).
The carbonate content north and south of the
island Increases steadily beyond a depth of about 150 feet.
At these greater depths, Foraminifera are the dominant
calcareous forms except near the outer edge of the
northern shelf where coarse-grained shell debris is
72
2 6 ' II9 " 3 0 ' 22'
S TA TU TE MILES
CALCIUM CARBONATE
CONTOUR INTERVAL 20 STARTING AT W Vo
02'
X7CP 34'
0 0 *
2 8 ' 2 4 ' 22' 2 6 '
Figure 9* Distribution of calcium carbonate. Control stations are
shown on figure 1.
73
119 *30' 26'
STATUTE MN-ES
300 r r
DOMINANT X A L CAREOUS FORMS
021
3 4 *
o e r
34*
0 0 "
2 0' 26* 22* 26'
Figure 10. Distribution of dominant organic calcareous forms, Control
stations are shown on figure 1.
locally abundant. Percentages of about fifty occur over
the outer portion of the northern shelf. These values
are fifteen to twenty per cent higher than those for the
corresponding area south of the island due to a greater
accumulation of foraminiferal tests on the broad 270—foot
terrace north of the island. A comparable but more
distinct relationship has been found between opposite
shelves off Catalina Island (Shepard and Wrath, 1937).
The shelf sediments east and west of Anacapa are
characterized by carbonate contents exceeding fifty per
cent. Shell and bryozoan fragments are the principal
components of the eastern shelf deposits, and make up
60.5 per cent of the average sediment (see Table 2).
Beneath Anacapa Passage shell detritus is most common, but
sediments containing living calcareous algae are abundant
over the central portion of the passage floor.
Dr. E. Y. Dawson, algalogist at the University of
Southern California, has identified the algae as
belonging to the genous LIthethamnium. It is not uncommon
for algal colonies to form more than ninety per cent of
the bottom sediment, however, the average carbonate content
for the western shelf is 75.0 per cent. As was brought
out previously, it is thought the calcareous sediments
east and west of the islands are a reflection of sweeping
shelf currents which prevent the accumulation of detrital
75
clasts.
Organic Matter
Particulate organic matter in the shelf sediments
increases from about 0.5 per cent near shore, to more
than 1.5 per cent at the edge of the northern shelf (see
Figure 11). This seaward Increase is in harmony with a
decreasing sediment grain size which also occur with
distance from shore. Concentration of organic detritus
in fine-grained sediments has been well documented by
Trask (1937) and Emery and Rittenberg (1952, p.778). In
agreement with this relationship the percentages near the
edge of the southern shelf are only about one-half those
over the corresponding area of the northern shelf (see
Figure 11). The average for the latter area is 0.9
per cent, which is 0.2 per cent higher than the average
organic content of the southern shelf (see Table 2).
Coarse-grained sediments on the eastern shelf
contain only about 0.6 per cent organic matter, in
agreement with the general absence of detrital sediments
in this area. However, the coarser algal deposits on the
opposite western shelf have organic contents greater
than 1.0 per cent (see Figure 11). Living tissue in
these algaliferous sediments is thought to account for
their high organic contents. It was not possible to
STATUTE MILES
7. ORGANIC MATTER
CONTOUR INTERVAL 0 . 5 %
Figure 11. Distribution of organic matter* Control stations are
shown on figure 1 as listed in appendix I.
77
remove embedded tissue before making the analyses.
Based on fifty samples, the average shelf sediment
contains 0.7 P©** cent organic matter, Trask*s (1937, p.429)
world-wide average of 2.50 per cent for nearshore sediments
is considerably greater than this value. It is, however,
close to the organic content associated with the shelf
sediments south of Santa Cruz Island, which range
between 0.85 and 1.53 par cent (Uchupi, 1954; p. 49).
Data compiled by Emery, Gorsline, Uchupi, and Terry
(1958, p. 113), show that sediment having the same median
diameter and organic matter content as the average
Anacapa shelf deposit are characteristic of shelf knolls
off the southern California and Lower California coasts.
Considering the active upwelling which is known
to occur over the Anacapa shelf, it is apparent that
little of the organic matter produced in the overlying
water accumulates in the bottom sediments. Oxidizing
bottom environments, slow sedimentation rates, and
sweeping currents probably account for the low degree of
preservation.
Classificatlon-Sediment Types
Shelf sediments are classified here as calcareous j
and detrital deposits. Calcareous deposits are
distinguished from detrital accumulations by having
carbonate contents greater than fifty per cent. Because
sand is the dominant grade class of these sediments, they
can be further subdivided into distinct sediment types
based on their median grain size and on secondary grade
classes which make up more than ten per cent of the
deposit. For example, a detrital sediment having a
median diameter of 0.150 mm, and a silt-clay content of
twelve per cent, would be classified as a silty fine
sand. Clay size grains are considered to be part of the
silt content as they rarely form more than a few per cent
of the bottom sediments. Color is also used to further
distinguish detrital sediments. Calcareous deposits are
classified in a similar manner, except that the dominant
calcareous organism is used to further differentiate the
sediment types.
Nine sediment types are described, four of which
belong to the detrital group. The distribution of the
types is shown on figure 12, their textural, mineraloglcal,
and chemical properties are listed on table 4; and their
average cumulative curves are presented on figure 13.
Cumulative curve of each sediment type was prepared by
averaging the combined grain size distribution of all the
sediments samples of that type.
11 9 30
S TA TU TE MILES
0 I
SEDIMENT TYPES " 300 F T
shew ,.
LEG E N
GRAVELLY C H S . TO M ED. SAND
MED lO FINE GRAY SAND
FINE TO VERY-FINE LIGHT-GRK SANO
SILTY-FINE TO VERY-FINE LIGHT-GRN SAND
GRAVELLY CES. TO MED. ALGAL SAND
GRAVELLY MED. SHELL SAND
GRAVELLY MED. IRY020AN SAND
MED SHjLL SAND
FIN E ID SILTY VERY-FINE FORAM SAND
M9‘3 u
Figure 12. Distribution of sediment types* Control stations are shown
TABLE IV
AVERAGE CHARACTER OF SEDIMENT TYPES
Median Diameter Sorting
(Md.) Coefficient
(So.)
CaCO. Organic
Matter
DETRITAL SEDIMENTS
Gravelly medium to 0.505 mm
coarse sand
Medium to fine gray sand 0.185 mm
Fine to very fine light 0.145 mm
green sand
Silty fine to very fine 0.091 mm
light green sand
CALCAREOUS SEDIMENTS
Gravelly coarse to medium 1.700 mm
algal sand
Gravelly medium bryozoan 0.460 mm
sand
1.70
1.51
1.47
1.16
2.80
2.74
43.2 % 0.27 %
16.1 % 0.48 %
29.9 % 0.91 %
14.2 % 1.15 ° / o
85.6 % 1.30 %
68.9 % 0.40 %
Heavy
Minerals
8.3 %
8.7 %
6.8 %
3.5 %
7.0 %
11.0 %
TABLE IV cont.
Median Diameter Sorting CaCO- Organic Heavy
(Md.) Coefficient Matter Minerals
(So.)
CALCAHEOUS SEDIMENTS
Gravelly medium shell sand 0.425 mm 1.91 70.7 % 0.67 % 7.3 %
Medium shell sand 0.330 mm 1.51 72.7 % 0.77 % 4.3
%
Fine to very fine 0.175 mm 1.56 54.7 % 1.08
%
9.0
%
foraminiferal sand
oo
H*
7S
s i l t y f in e t o v e r y - fin e lig h t- c a e cm i
FINE TO VERY-FINE LIGHT— GREEN SANO
3
III
2
<
-I
FINE TO SILTY FINE FORA MINI FERAL SANO
i
3
u
GRAVELLY COARSE T O MEDIUM ALGAL
|0 S MM. 10 O.S MM. Oil OOS MM. 041 0 4 0 0 MM. M M
F ig u r e 13 0 A v e r a g e c u m u la tiv e c u rv e s o f th e s e d im e n t ty p e s
83
Detrital Sediments
J
Detrital sediments occur primarily on the northern
and southern shelves. They are the most widespread of the
shelf deposits, and cover approximately fifty seven per
cent of the insular platform, or about twenty square miles.
Gravelly Coarse to Medium Sand
Gravelly sands and coarse sands are associated
with submarine outcrops on the eastern, southern, and
northern shelves (see Figure 12). They also occur on the
southern insular slope. These sediments have an average
median diameter of 0.505 mm and are the coarsest and most
poorly sorted of the detrital types (see Table 4). Low
organic contents of 0.27 per cent accompany the deposits.
Although most of the gravelly debris consists of rock
fragments, shell and bryozoan clasts are abundant and
carbonate contents average 43*2 per cent.
Medium to Fine Gray Sand
An encircling belt of gray sand lies immediately
adjacent to the island and also extends over a portion of
the eastern shelf. The dark color is due to an abundance
of rock particles, which are chiefly of volcanic ■
lithology. Average median diameter and sorting j
coefficient are respectively 0.185 mm and 1.51 mm.
Calcareous debris consists mostly of shell fragments and
averages 16.1 per cent. An average organic matter
content of 0.48 per cent is characteristic of this sand.
Fine to Very Fine Light G-reen Sand
Seaward of the nearshore gray sand is a broad
tract of greenish finer-grained sand which extends to the
shelf edge both north and south of the island. It
overlies about twenty two per cent of the shelf area, or
about eight square miles, and is the most extensive
deposit occurring of the insular platform. A median
grain size of 0.145 mm, and a sorting coefficient of 1.47
is associated with this sediment type. Carbonate
averages close to thirty per cents shell debris is common,
but foraminiferal tests are the dominant calcareous
forms. Organic matter is rather high, and averages 0.9
per cent.
Siltv Fine to Very Fine Light Green Sand
Sediment having a median diameter of 0.091 mm, and
a sorting value of 1.16, occupies the central portion
of the northern shelf at depths greater than 270 feet.
It represents the finest and most well sorted sediment
occurring on the Anacapa shelf. Organic matter is
concentrated in this silty sand and averages 1.15 P©** cent.
85
However, calcium carbonate averages only 14.2 per cent,
the lowest of all the shelf sediments.
Calcareous Sediments
Sediments of the calcareous group cover forty three
per cent of the shelf area, or about sixteen square miles.
They are principally restricted to the shelf areas east
and west of the island, but they also occur as isolated
patches on the northern and southern shelves.
G-ravelly Coarse to Medium Algal Sand
Shelf sediment composed of Lithothamnium debris
blankets the outer portion of the shelf area beneath
Anacapa Passage. It has an average median diameter
of 1.70 mm, a sorting value of 2.80, and is the coarsest
and most poorly sorted shelf deposit. Calcium carbonate
and organic matter are most concentrated in this sediment
type, and respectively averages 85.6 and 1.3 per cent.
G-ravelly Medium Bryozoan Sand
Submerged rocky areas east and south of the island
are locally covered by a thin veneer of calcareous
sediments containing abundant bryozoan zoaria. An
average median grain size of 0.460 mm, a sorting
coefficient of 2.74, and a mean carbonate content of 68.9
per cent, serve to distinguish this sediment type.
86
Organic matter is not a prominent constituent, averaging
only 0.4 per cent.
Gravelly Medium Shell Sand
Gravelly shell sand is widely distributed over the
western and eastern shelves. A small patch also occurs
near the outer edge of the northern shelf. Calcareous
debris may form more than ninety per cent of the sediment;
however, the average is 70.7 per cent. A median diameter
of 0.425 mm, and a sorting coefficient of 1.19 are
associated with this sediment. Organic matter averages
0.67 per cent.
Medium Shell Sand
Contiguous with the gravelly shell sand are
embracing tracts of median shell sand. Both sediment
types have essentially the same carbonate and organic
matter contents, and can be considered as facies of a
bottom sediment composed principally of mollusk fragments.
The median shell sand is distinguished by an average
median diameter of 0.330 mm, and sorting value of 1.51.
It is the most widespread of the calcareous sediments,
and covers approximately fifteen per cent of the shelf
area, which corresponds to about 5.5 square miles.
Fine to Silty Fine Foraminiferal Sand
Seaward of the western island are small areas of
87
fine-grained foraminiferal sand which blanket the outer
edges of the northern and southern shelves. This sediment
is characterized by a median grain size of 0.175 mm,
a sorting coefficient of 1.56. It is the finest of the
calcareous types; which accounts for its high average
organic matter content of 1.08 per cent.
Sedimentary Environments
Two distinct sedimentary environments are indicated
by the distribution of the sediment types. The first of
these milieus dominates the eastern and western shelf
areas, and is one of little or no detrital sedimentation.
In marked contrast, the second sedimentary environment is
one of detrital deposition and prevails north and south
of the island.
Eastern and Western Shelf Environment
Shelf sediments which occur east and west of the
island are coarse-grained and highly calcareous due to
the accumulation of shell fragments, Lithothamnium debris,
bryozoan zoaria, and echinoid spines and plates (see
Table 2, Figures 6, 9, 10, and 12). Glauconite, an
authigenic mineral characteristic of areas of slow or
Insignificant detrital deposition, is most abundant in
these sediments (see Table 3)*
88
Coarse and medium-grained detrital sands also occur
on the eastern shelf, but they are associated with sub
marine outcrops and probably represent residual and
relict deposits. It is possible that during the rise of
sea level that accompanied the final retreat of the
Wisconsin ice fields these sediments were carried onto
the eastern shelf by longshore currents. The present
longshore transport is primarily to the east, but it
could not possibly distribute coarse detrital clasts
beyond the most nearshore regions of the eastern shelf.
The sedimentary environment of the eastern and
western shelves is not conducive to the deposition nor
the preservation of organic detritus. Shelf sediments east
of the island have the lowest (0.6 per cent) average
organic content of the four major shelf areas (see
Table 2). Unusually high percentages occur on the
western shelf, but as explained earlier this may be due
to the presence of living plant tissue in the bottom
sediment.
A combination of several conditions is responsible
for the absence of detrital sediments on the western and
eastern shelves. These are: (1) both areas lie seaward
of the tapering ends of the island and consequently little I
sediment is directly shed to them, (2) both shelf areas !
are exposed to prominent currents which sweep across the
Anacapa shelf from the northwest or the southeast; these
currents are probably strong enough to prevent the
deposition of the finer (silt-clay) detrital clasts,
(3) tidal currents, in particular over the western shelf,
are probably swift enought to prevent the accumulation of
most detrital clasts. It could be argued that upwelling
and strong current motion over these two shelf areas supply
them with nutrient-rich waters which promote a high
organic productivity, and thus detrital sedimentation is
merely outstripped rather than prevented. However, rock
bottom is exposed over much of the shelf area east and
west of the island (see Figure 5) and most of the
calcareous debris shows clear signs of abrasion and
solution. It is apparent then that the Recent sediment
blanket is thin on these two shelf areas and that
sedimentation is slow and primarily restricted to the
accumulation of calcareous animal and plant fragments.
Northern and Southern Shelf Environment
North and south of the island the shelf deposits
are coarse to very fine sands which consist mostly of low
specific gravity minerals and rock fragments (see Table 2).
A consistant seaward decrease in grain size is interpreted
to mean these clasts are largely derived from the island.
Reversals in this trend occur but they can be correlated
90
with known submarine outcrops or with localized accumu
lations of coarse calcareous sediments.
Studies of the distribution of glaucophane indicate
that detrital clasts finer than about 0,250 mm are
transported toward the outer part of the shelf, but that
clasts finer than 0,125 mm tend to by-pass the shelf area
and be deposited on flanking insular slopes and in
adjacent basins. The occurrence of silty very fine sands
on the broad 270-foot terrace north of the island suggest
that by-passing is not as important for this shelf as it
may be for the southern shelf, which lacks both the outer
terrace and the very fine sands. Shelf currents are not
able to transport clasts larger than about 0.500 mm beyond
the nearshore areas.
It is evident from an increasing carbonate content,
that detrital sedimentation becomes less prevalent with
distance from shore (see Figure 9). The increasing
calcareous matter is chiefly contributed by foraminiferal
tests, Hear the outer edge of the shelf areas north and
south of the western island, foraminiferal tests and
shell fragments contribute more than fifty per cent of
the bottom sediment (see Figure 11). Organic matter also
increases with distance from shore apparently in response
to the seaward decreasing sediment grain size. Deposits
on the northern shelf, which are the finest on the
91
insular platform, have the highest average organic matter
content (see Table 2, and Figure 11),
Active cliff erosion is thought to contribute the
greatest volume of detrital clasts to the northern and
southern shelf areas. Olasts supplied by stream erosion
must also add to the shelf deposits, but the sediments do
not noticeably differ adjacent to stream mouths. The rate
of sedimentation is probably highest north of the island,
and the Recent sediment blanket is thickest on the
northern shelf. This is interpreted from the paucity of
rock bottom and the rarity of lag or relict deposits on
the northern shelf.
Significance of the Anacapa Shelf Sediments
The geologic significance of Recent shelf sediments
and Recent deposits on shallow submerged banks and ridges
has been amply discussed by Emery, Butcher, Gould, and
Shepard (1952), Holtzman (1952), Emery (1952), Uchupi
(1954), and Emery, Gorsline, Uchupi, and Terry (1957),
thus, a lengthly treatment of the general significance of
the insular Anacapa sediments is not presented here.
However, the present study has brought out an interesting
relationship between shelf deposits and shelf topography
which does warrant additional consideration.
It has been demonstrated that the principal
92
difference between the detrital sediments north and south
of the island is the presence of silty very fine sands
on the 270-foot terrace forming the outer two miles of
the northern shelf. Similar deposits and the outer shelf
platform are lacking south of the island. The implication
is that shelf currents are not able to transport grains
of sand size across the gently sloping outer northern
shelf (0° 5*)> "but that particles finer than about 0.125 mm
are carried across the more or less continuously sloping
steeper (1° 40H) southern shelf. Because shelf currents
may be stronger south of the island, it is difficult to
determine if the by-passing is promoted by steeper shelf
slopes or is caused by stronger currents acting along
these slopes.
It is interesting to note that the insular sediments
of Catalina Island show a similar relationship to shelf
slopes. That is, they are coarser and thinner where the
shelf declivity is steepest (Shepard and Wrath, 1937;
Figures 1 and 2), and finer and probably thicker where
the insular platform is broad and gently sloping
(McG-lasson, 1957; P. 27). The currents affecting the
Catalina shelf are poorly understood (McG-lasson, 1957;
p. 22) and, as is true of the Anacapa shelf, it is not
known whether by-passing is a direct result of steeper
slopes or of stronger currents sweeping these slopes.
In the writer’s opinion, the relative shelf slopes
north and south of Anacapa Island are probably more
responsible for the character of their overlying detrital
sediments than are shelf currents. The shelf currents
themselves are weak (about 0.2 knots) and the difference
in velocity between the two shelf areas is probably less
than 0.1 knot. In a general sense, this contention impliesj
i
that shelf topography may have a more significant control
over the distribution of detrital sediments than is
commonly thought. For an interesting discussion of this
problem the reader is referred to a recent paper by
Emery and Terry (1956) who have described the sediments
on a steep submarine slope off southern California.
GEOLOGIC HISTORY
The geologic history of the exposed rocks on
Anacapa Island "began in the early part of the Middle
Miocene with the accumulation of andesitic lavas and pyro-
clastic rocks of the Conejo formation. Judging from the
formations cropping out on adjacent Santa Cruz Island,
older Miocene rocks were buried by the initial volcanism.
Shortly following the opening of the eruptive episode,
alluvial fans composed of metamorphie rock debris were
deposited over cooled extrusive and pyroelastic rocks.
At least twice, the deposition of the San Onofre breccia
was interrupted by volcanic outpours. Accumulation of
the breccias did not resume following the second effluence.
Evidence previously cited suggests that the volcanic rocks
were deposited both subaerially and subaqueously.
There is no Pliocene record on Anacapa, or on any
of the Santa Barbara Islands. This fact has led many
geologist to believe the island chain was above sea level
during the Pliocene (Bremner, 1932; p. 32; and Reed, 1933;
p. 252). Regional studies show that a major folding
Affected the islands in the Middle Pliocene (Bailey and
Jahns, 1954; p. 91). Tilting of the Miocene rocks on
Anacapa probably occurred during this episode of
folding.
95
Submergence of the Island took place in Early
Pleistocene time, and was rather quickly followed by at
least two intermittent stages of uplift. Sandstones of
general Early Pleistocene age were deposited on a marine
terrace cut subsequent to the second uplift. The
character of the microfauna in this deposit suggests that
it accumulated beneath an interglacial sea of Early
Pleistocene age. The marine sandstones have since been
uplifted to an elevation of about 200 feet. It is not
known when the uplift took place, but it must have
occurred before the beginning of Wisconsin glaciation.
This is known because submarine terraces of Wisconsin age
have been identified on the surrounding shelf which have
not been affected by movements of this magnitude
(Emery, 1958). It might be argued that the sandstones
were deposited beneath an interglacial sea level which
stood 200 feet higher than the present ocean surface; in
this case, little or no uplift is indicated. In the
general vicinity of the present central island, the marine
sandstones were covered by a wedge of Late Pleistocene
nonmarine sediments spreading north from a higher portion
of the island which has probably been destroyed within
Recent time. Recent sediments are represented by talus
and landslide debris, and by most of the sediments
covering the surrounding Insular shelf.
Since the beginning of Recent time incessant wave
attack by a more or less steadly rising ocean surface has
separated Anacapa from neighboring Santa Cruz Island, and
has carved the present coastal configuration. Regional
studies and considerations of relict vegetation and
vertebrate fossils collected on the other Santa Barbara
Island suggest that Anacapa may not have been separated
from the mainland until after Middle Pleistocene time
(Clements, 1955). Within Recent time the island and
the surrounding shelf appears to have been gently tilted
to the north.
The construction of the insular Anacapa shelf began
in the Early Pleistocene concomitant with the first
glacially lowered sea level. Shelf cutting continued
throughout the Pleistocene and the effects of fluctuating
Wisconsin sea levels is attested by three and possibly
four submerged wave-cut terraces (Emery, 1958).
Marine sedimentation on the insular shelf
undoubtedly took place during each of the interglacial
ages. Much of this material was probably stripped from
the shelf platform during periods of continental
glaciation. Since the end of the Wisconsin Recent detrital
sediments have accumulated north and south of the island
whereas largely non-detri&al calcareous deposits have
accumulated to the east and west. ,The shelf sediments
97
were laid down in part over relict Late Pleistocene
deposits which had been widely scattered over the shelf
area by fluctuating Wisconsin sea levels.
The Recent sediments are thickest north of the
island as indicated by the paucity of relict sediments
and exposed bedrock which are common on the other shelf
areas. The thinner blanket of calcareous deposits east
and west of the island is undoubtedly due to swift
currentb which have prevented the accumulation of
detrital clasts. However, on the southern shelf where
strong currents are not active, it is most probable that
the detrital sediment cover here is thin because steep
submarine slopes (1° 40!) have allowed clasts finer than
about 0.250 mm to by-pass the shelf area. In comparison
to the other shelf areas, the northern shelf is broad,
relatively flat, and is not affected by strong currents.
For these reasons detrital sedimentation has been highest
north of Anacapa Island during Recent time.
93
BIBLIOGRAPHY
Allison, L. E., 1935, Organic carbon by reduction of
chromic acid: Soil Science, Vol. 40, pp. 311-320.
Bailey, T. L., and Jahns, R. H., 1954, Geology of the
Transverse Range Province, southern California:
Calif. Div. Mines, Bull. 170, ch. II, pp. 83-106.
Bancroft, H. H., 1890, History of California: The History
Company, San Francisco, Vol. I, 744 p.
Blake, ¥. P., 1855, Notice of remarkable strata con
taining the remains of Infusoria and Plythalmia in
the Tertiary formation of Monterey, California:
Phila. Acad. Nat. Science, Proc., Vol. 7, pp. 328-331.
Bremner, Carl, St. J., 1932, Geology of Santa Cruz Island,
Santa Barbara County, California: Santa Barbara Mus.
Nat. History, Occasional Papers, No. 1, 33p.
Bremner, Carl, St. J., 1933, Geology of San Miguel Island,
Santa Barbara County, California: Santa Barbara Mus.
Nat. History, Occasional Papers, No. 2, 23 p.
Clements, T., 1955, The Pleistocene history of the channel
island region, southern California: Essays in the
Natural Sciences in Honor of Captain Allan Hancock,
Univ. So. California Press, pp. 311-323.
Corey, ¥. H., 1954, Tertiary basins of southern California:
Calif. Div. Mines, Bull. 170, ch. Ill, pp. 73-83.
Crowell, J. C., 1950, Submarine canyons bordering central
and southern California: Jour. Geology, Vol. 60,
pp. 58-83.
Dibblee, T. ¥., 1950, Geology of southwestern Santa
Barbara County, California: Calif. Divi. Mines,
Bull. 150, 95 p.
Dunkle, M. B., 1950, Plant ecology of the channel islands
of California: Univ. Southern California Press,
Allan Hancock Pacific expeditions, Vol. 13, pp. 247-
386.
Ellis, A. J., 1919, Geology and ground waters of the
western part of San Diego County, California: U.S.
Geol. Survey, ¥ater Supply Paper 446, 321 p.
Emery, K. 0., 1938, Rapid method of mechanical analysis
of sands: Jour. Sed. Petrology, Vol. 8, pp. 105-111.
Emery, K. 0., and Shepard, F. P., 1945, Lithology of the
sea floor off southern California: Geol. Soc.
America, Vol. 56, pp. 431-478.
Emery, K. 0., 1952, Continental shelf sediments of
southern California: Geol. Soc. America, Vol. 63,
pp. 1108-1952.
99
Emery, K. 0,, Butcher, W. S., G-ould, H. R., and Shepard,
F. P., 1952, Submarine geology off San Diego,
California: Jour, Geology, Vol. 6, pp. 511-548.
Emery, K. 0., and Rittenberg, S. C., Early diagenesis of
California basin sediments in relation to origin of
oil: Amer. Assoc. Petrol. Geologists, Vol. 36,
PP. 735-806.
Emery, K. 0., 1955, Transportation of rocks by driftwood:
Jour. Sed. Petrology, Vol. 25, pp. 51-57.
Emery, K. 0., and Terry, R. D., 1956, A submarine slope of
southern California: Jour. Geology, Vol. 64,
pp. 271-280
Emery, K. 0., Gorsline, D. S., Uchupi, E., and Terry, R. D.,
1957, Sediments of three bays of Baja California:
Sebastian Viscaino, San Cristobal, and Todos Santos:
Jour. Sed. Petrology, Vol. 27, PP. 95-115.
Emery, K. 0., 1958, Shallow submerged marine terraces of
southern California: Geol. Soc. America, Vol. 69,
pp. 39-60.
Grant, U. S., and Gale, H. R., 1931, Pliocene and
Pleistocene Mollusca of California: San Diego Soc.
Nat, History, Mem. Vol. 1.
Holzman, J. F., 1952, Submarine geology of Cortes and
Tanner Banks: Jour. Sed. Petrology, Vol. 22,
pp. 97-118.
Johnson, W. M., 1855, Report of the Superintendent of the
United States Coast Survey: Annual report for the
year 1855, P. 96.
Eennett, W. E,, 1952, Cenozoic correlation section,
western Ventura Basin, from Point Conception to
Ventura and channel islands: Amer. Assoc. Petrol.
Geologists, sheet 2., channel islands.
Kleinpell, R. M., 1938, Miocene stratigraphy of California:
Amer. Assoc. Petrol. Geologist, Tulsa, Oklahoma,
540 p.
Krumbein, ¥. C., and Pettijohn, F, J., 1938, Manual of
sedimentary petrography: Appelton-Century-Croft, Inc.
New York, 549 p.
McGlasson, R. H., 1957, Foraminiferal biofacies around
Santa Catalina Island: Unpublished Master*s Thesis,
University of Southern California.
Reed, R. D., 1933, Geology of California: Amer. Assoc.
Petrol. Geologists, Tulsa, Oklahoma, 355 P.
Reed, R. D., and Hollister, J. S., 1936, Structural
evolution of southern California: Amer. Assoc. Petrol,.
Geologists, Tulsa, Oklahoma, 157 p.
Redwine, L. E., 1952, Cenozoic correlation section, western
Ventura Basin, from Point Conception to Ventura and
channel islands: Amer. Assoc. Petrol. Geologists,
sheet 2, notes on channel islands.
100
Reichelderfer, F. W., 1948-1952, Climatological data,
annual summary for California, United States Weather
Bureau, Department of Commerce.
Revelle, R., and Shepard, F. P., 1939, Sediments off the
California coast: Recent Marine Sediments, Amer.
Assoc, Petrol, G-eologists, Tulsa Oklahoma, pp.
245-282,
Robinson, W. W., 1955, The story of Ventura County: Title
Insurance and Trust Company, Los Angeles, 48 p.
Shelton, J, S., 1955, Glendora volcanic rocks, Los Angeles
Basin, California: Geol. Soc. America, Vol. 66,
PP. 45-90.
Shepard, P. P., and Emery, K. 0., 1941, Submarine
topography off the California coast: Canyons and
tectonic interpretation: Geol. Soc. America, Special
Paper No. 31, 171 P.
Stevens, Lieut. T. H., 1854, Report of the Superintendent
of the United States Coast Survey: Annual report for
the year, 1854, p. 218-219.
Sverdrup, H, U., and Fleming, R. H., 1941, The waters off
the coast of southern California, March to July,
1927: Bull. Scripps Inst, of Oceanography, Vol. 4.
pp. 261-378.
Sverdrup, H. U., Johnson, M. W., and Fleming, R. H., 1942,
The Oceans: Prentice-Hall Inc., New Jersey, 1087 p.
Taliaferro, N. L., 1924, Notes on the geology of Ventura
County, California: Amer. Assoc. Petrol. Geologist,
Vol. 8, pp. 789-810.
Trask, P. D., 1931, Sedimentation in the channel islands
region: Econ. Geologists, Vol. 26, pp. 24-43.
_______ 1939, Organic content of Recent marine sediments:
Recent Marine Sediments, Amer. Assoc. Petrol.
Geologist, Tulsa, Oklahoma, pp. 428-453.
Twenhofel, W. H. and Tyler, S. A., 1941, Methods of study
of sediments: McGraw-Hill Company, Inc., New York
and London, 183 p.
Uchupi, E., 1954, Submarine geology of Santa Rosa-Cortes
Ridge: Unpublished Master1s Thesis, University of
Southern California, 72 p.
Woodford, A. 0., 1925, The San Onofre Breccia; its nature
and origin: Univ. Calif. Publ. Dept. Geol. Sciences,
Vol. 15 PP. 159-280.
Woodford, A. 0., and Bailey, T. L., 1928, Northwestern
continuation of the San Onofre Breccia: Univ. Califs
Publ. Dept. Geol. Sciences, Vol. 17, PP. 187-191.
Yates, L. G., 1890, Stray notes on the geology of the
channel islands: Annual reports of the California
State Mineralogist, No. 9, PP. 171-174.
101
APPENDIX I
PARAMETERS OF RECENT SEDIMENTS
, . . ~~J~"
Station Latitude Longitude Md. So. Organic CaC03
Number North West mm Matter
5342 34o00'16''
119°25 35" 0.139 1.29 0.43 13.7
5343 33°59'48"
25
36" 0.170 1.39 --- 48.9
5344
59'25" 25
36" 0.140 1.33 0.79
44.2
5345
59'02"
25
36"
_ _ _ _ _ --------------- — —
53.3
5346
59'03" 25 03"
_ _ _ _ _ — — — —
44.4
5347 34°00'03" 25 03" 0.530 1.87 --- 46.1
5348 33°59'38"
25 03" 0.177 1.50 0.64 40.4
5349 59'20"
25 03" 0.144 1.35 0.73 22.7
5380 59'33" 23 21" (see Appendix II)
5381 34°00'06" 24 20"
0.465 1.91 ---
81.1
5382 00'04" 24 48" (see Appendix II)
5383
33059<57"
25
22" 0.195 1.85 1.20 34.5
5384 34°00'll"
25
32" 0.110 1.29 0.66 23.9
5385 00’23" 25 56" (see Appendix II)
5386 00’28" 26
29"
0.270 1.46 0.58 61.6
- 5387
00’02" 26
47"
(see Appendix II)
5388
33°59'37" 27 05" 0.322 1.33 0.58 87.7
5389
59’12" 27 25" 0.425 1.32 0.70
88.8
5390 59'38" 27
28" 0.490 1.52 0.62 93.9
'5391
34 00' 24" 28 52" (see Appendix II)
5422 00'34" 28 27" 4.000 2.00 1.30 92.1
5423 00*43" 27
30" 0.660 3.85 1.20 75.1
5424
00'37"
26 43" 0.350 1.37 0.42 69.0
5425
00' 54" 26 52" 0.590 1.37 --- 46.9
-5426 01’07" 27
50" (see Appendix II)
5427
01'20" 28 30" 0.465 2.82 1.20 79.1
'5428
01'35" 27 31"
(see Appendix II)
5429
02'08" 28 04" 0.290 1.58 0.56 58.1
5430 02'42" 27 47" 0.333 1.47 0.60
61.8
5431 02*15" 27 23" 0.410 1.59 --- 82.9
5432 01'42" 26 48" 0.255 1.51 0.90
62.8
5433
01'36" 26 24" (see Appendix II)
5434 02*25" 26
27"
0.120 1.37 0.86 23.9
5435 03'15" 25 35" 0.125 1.82 1.50 51.4
5436 03’43"
26 40" 0.110 1.42 1.20 39.6
5438
5439
5440
5441
5442
5443
5444
5445
5446
5447
5448
5449
5450
5451
5452
5453
5454
5455
5456
5457
5458
5459
5460
5461
5462
5463
5464
5465
5466
5467
5468
5469
5470
5471
5472
5473
5474
5475
APPENDIX I cont.
Latitude Longitude Md. So. Organic
North West nun Matter
34°04 281 1 119°26 47" 0.083
1.26 2.30
04 18”
25
11" 0.101
1.45
1.80
03 15"
24 18" 0.125 1.79
----
02 20"
25
16"
0.089 1.18 1.10
01 06"
25 43"
0.221 1.40 0.67
01 10"
25 15" 0.159
1.38
----
00 54" 24
57"
0.130 1.30 0.60
01 40" 24 32" 0.093
1.26 1.20
00
55"
24
19"
0.140
1.53
0.81
00 32" 24 02" 0.119 1.23
0.61
00 44"
23 25" (see Appendix II)
02 08" 24
09" 0.117 1.49 1.27
02 IT"
23
22"
0.123 1.36
— —
02 46"
23
22" 0.111 1.26 1.25
03
22"
23 22"
0.435 1.52
----
03 49" 23
22" 0.036 1.94 3.98
03
32" 21 52"
0.089
1.42 2.00
02
39" 23 03" 0.190 1.31
----
01
51"
22 59" 0.120
1.23
----
01 48" 22 12"
0.165 1.90 0.64
00 52" 22
55"
0.152 1.34
— —
01
05"
22 21" 0.300 1.30
0.37
01
05"
21 29" (see Appendix II)
01 46" 21 14"
0.335
1.66 0.46
02 31" 21 00"
0.335 1.59 0.87
02 14"
19 51" 0.490
3.75
0.60
03 13" 19
22" 0.330
1.77
1.22
02 46"
17 58" (see Appendix II)
02 06"
19
08" (see Appendix II)
01 44"
19 49" 0.700 3.26 0.80
01 14" 20 38" 0.370 3.16
0.27
00 53"
20
59" 0.440 4.00 0.40
01
35" 19 05" 0.260
1.59
0.46
01 06"
19
54" 0.390 1.22 0.25
00
45" 20
35"
0.201
1.23 0.69
00 23" 20 64" 0.225
1.30 0.70
33° 59
50" 21
09" 0.189 1.38 0.48
34°00 02" 21 16" 0.145 1.18 0.64
00 37" 21 38" 0.340 1.36 0 • 66
5476
5477
5478
5479
5480
5481
5482
5483
5484
5485
5486
5487
5488
5489
5490
5491
5492
5493
5494
5495
5677
5678
5679
5680
APPENDIX I cont.
Latitude Longitude Md. So. Organic
North West mm Matter
34°00*38” 119°22*
03”
0.180 1.48
mm mm mm mm
00* 35”
22*27” 0.152 1.30 0.43
00* 07, f
22*
15” 0.131
1.40 1.01
33°59*36”
22*00” (see Appendix II)
59*12” 21*
47 »
0.210 1.81
59* 04” 22*32” 1.318 1.94
59* 35” 22*42”
0.135 1.29 0.70
59*55”
22*50” 0.150
1.37
1.22
34°00*l6” 22*59” 0.171 1.34 0.38
00*06”
23*
28”
0.315
1.28 0.27
33 59*49" 23*
26” 0.140
1.25
---------
59*28”
23*57”
0.161 1.41 0. ?6
58*57” 23*
11” 0.150 1.72 1.43
58*48”
23*49” 0.126
1.51
59*32” 24*40”
0.179 1.33
- - - -
58*49” 24*37” 0.132 1.38 1.40
58*41” 26*
27” 0.315 1.77
1.60
59*11"
26*20”
0.199 1.31 1.09
59*34” 26*12”
0.215 1.24 0.6?
34°00*00” 26*04” 0.152 1.44 1.00
3337’ 53” 29*
40” (see Appendix
XI)
34°01*56” 28*45” (see Appendix
II)
03*20”
1$'
30” (see Appendix
II)
04*35” 15*
42” (see Appendix II)
104
APPENDIX II
SUBMERGED OUTCROPS
Station
Number
Lithology Formation Age
5380 Amygdaloidal andesite ConeJo Early Middle Miocene
5382 No recovery ConeJo 7 7
5385 No recovery ConeJo 7 7
5387
Siliceous calcareous
tan silty shale
Monterey Middle Miocene
5391
n
Monterey Middle Miocene
5426 Diatomaceous mudstone Monterey Middle Miocene
5428 Calcareous silty shale Monterey Middle Miocene
5433
Porphyritic andesite ConeJo Early Middle Miocene
5447
Tuff ConeJo Early Middle Miocene
5459
Porphyritic andesite ConeJo Early Middle Miocene;
5464
» l
ConeJo Early Middle Miocene
5465
No recovery ConeJo 7 7
5479
Andesite ConeJo Early Middle Miocene
5677
Andesite 7 (basalt) ConeJo Early Middle Miocene
5678 Andesite 7 (basalt) ConeJo Early Middle Miocene
5679 Only rounded pebbles of 7 7
i
i
andesite and siliceous
shale were recovered.
Rock bottom present,
but not samples.
105
APPENDIX II oont.
SUBMERGED OUTCROPS
Station L i t h o l o g y Formation Age
Number
5680 Poorly indurated mud- ? Late Pleistocene 1
stone containing
possible late Pleisto
cene Foraminifera
(Dr. 0. L. Bandy
personal communication)
26' I 24' 119° 2 £ _ 4
GEOLOGIC MAP OF ANACAPA ISLAND
1 1 / 2 0 1
STATUTE MILES
CONTOUR INTERVAL 500 FEET
GEOLOfly- -BY-
D.W. SCHOLL
1959
LEGEND
LANDSLIDE AND TALUS
DEBRIS
NONMARINE TERRACE
SANDS
•CJtW
MARINE TERRACE DEPOSITS
CONEJO VOLCANICS
W IT H XNTERBEDDED LENSES
OF THE SAN ONOFRE BRECCIA C Tso 0 .
CONTACTS
W E LL DEFINED
INFE R R E D
TALUS AND LANDSLIDE
FAULTS
W ELL DEFINED
INFERRED
5YMBSL5
STRIKE AND DIP
ELEVATION
FOSSIL LO C A LITY
>1 J'!I >
VERTICAL AND HORIZONTAL
SCALES X 2
24' 119° 22'
26'

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Asset Metadata

Creator Scholl, David William (author)

Core Title Geology and surrounding recent marine sediments of Anacapa Island

Contributor Digitized by ProQuest (provenance)

Degree Master of Science

Degree Program Geology

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag Marine Geology,OAI-PMH Harvest

Language English

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c30-103050

Unique identifier UC11224597

Identifier usctheses-c30-103050 (legacy record id)

Legacy Identifier EP58482.pdf

Dmrecord 103050

Document Type Thesis

Rights Scholl, David William

Type texts

Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the au...

Repository Name University of Southern California Digital Library

Repository Location USC Digital Library, University of Southern California, University Park Campus, Los Angeles, California 90089, USA