General Geology, Petroleum 

Appraisal, and Nature of 

Environmental Hazards

Eastern Pacific Shelf 

Latitude 28° to 38°

North

GEOLOGICAL SURVEY CIRCULAR 786
General Geology, Petroleum Appraisal, and 

Nature of Environmental Hazards 

Eastern Pacific Shelf 

Latitude 28° to 38° North

By D. G. Howell, D. S. McCulloch, and J. G. Vedder

GEOLOGICAL SURVEY CIRCULAR 786

1978
United States Department of the Interior

CECIL D. ANDRUS, Secretary

Geological Survey

H. William Menard, Director

Free on application to Branch of Distribution, U.S. Geological Survey, 1200 South Eads Street, Arlington, VA 22202
CONTENTS

Page

Introduction .............................. 1

Setting .............................. 1

Plate tectonic processes. ..................... 1

Environmental hazards ....................... 1

Central California coast ........................ 3

Geography ............................. 3

Geology .............................. 3

Petroleum appraisal ........................ 7

Environmental hazards, outer Santa Cruz basin ........... 11

Transverse Ranges province ....................... 12

Geography ............................. 12

Geology .............................. 12

Petroleum appraisal ........................ 17

Environmental hazards ....................... 17

California Continental Borderland. ................... 18

Geography ............................. 18

Geology .............................. 18

Petroleum appraisal ........................ 24

Acknowledgments. ............................ 26

References ............................... 26

ILLUSTRATIONS

Page 

Figure 1; Physiographic index map for region discussed in text ..... 2

2. Map showing physiographic provinces of California and Baja

California, Mexico ...................... 3

3. Sketch maps showing interaction of Pacific, Farallon, and North

American plates during Neogene time. ............. 4

4-7. Maps showing:

4. Major Quaternary faults of California. ............ 6

5. Estimated seismically generated bedrock acceleration, western

United States. ........................ 7

6. Generalized geologic structures, Point Conception to Point

Reyes. ............................ 8

7. Basement relations along Sur-Nacimento and San Andreas fault

zones. ............................ 10

8. Generalized stratigraphic column for outer Santa Cruz basin. . 10

9. Geologic cross section through outer Santa Cruz basin. .... 11

10. Stratigraphic column, Santa Maria basin. ........... 12

11. Geologic cross section through Santa Maria basin ....... 13

12. Map showing selected physiographic features, Santa Barbara

Channel region ........................ 14

13. Geologic cross section through Santa Barbara Channel and

vicinity ........................... 15

14. Generalized stratigraphic columns, Santa Barbara Channel

region ............................ 16

15. Map showing boundaries of proposed DCS Lease Sale 48 and

geologic terranes, California Continental Borderland ..... 19

16. Stratigraphic columns, California Continental Borderland ... 20

17. Generalized geologic sections, California Continental

Borderland and Los Angeles Basin ............... 22

18. Tectonic map showing major structural trends and basins,

California Continental Borderland. .............. 23

19. Map showing location of major faults and earthquake epicenters,

California Continental Borderland. .............. 25
General Geology, Petroleum Appraisal, and Nature of

Environmental Hazards, Eastern Pacific Shelf

Latitude 28° to 38° North

By D. G. Howell, D. S. McCulloch, and J. G. Vedder

INTRODUCTION

SETTING

Additional petroleum exploration is 

anticipated within the Federal waters of the 

California offshore region. To assist in 

evaluating the impact of petroleum exploration 

and development, the geology and environmental 

setting of the Outer Continental Shelf (OCS) 

are reviewed for the region from the latitude 

of San Francisco Bay (38° N.) to the approximate latitude of Cedros Island (28° N.), Baja 

California, Mexico.

This area is nearly 2,000 kilometers long; 

within it, the shelf width varies from as 

little as 10 km south of Point Sur to as much 

as 280 km off San Diego (fig. 1). All or part 

of three physiographic provinces lie within 

the area. From north to south, these provinces 

are the central California shelf, the western 

Transverse Ranges, and the California 

Continental Borderland (fig. 2). The geologic 

history, tectonics, stratigraphy, and petroleum 

potential of each of these provinces are different. In addition, the origin of the anomalous west-trending Transverse Ranges is poorly 

understood. Consequently, the provinces are 

treated separately; and the general geology, 1 

petroleum potential, and environmental hazards 

are outlined for each.

PLATE TECTONIC PROCESSES

Plate tectonic processes have largely 

governed the geologic development of the three 

provinces. The west margin of California 

appears to have been a zone of oblique plate 

convergence throughout most of the late Mesozoic 

and early Cenozoic Eras. Atwater and Molnar 

(1973) conclude that subduction occurred along 

the entire western U.S. continental margin 

before 29 m.y. ago, after which an increasing 

segment of the margin was subjected to rightlateral translational shear; thus, in Oligocene 

time the tectonic setting for this area underwent a profound transition from a convergent 

to a sliding margin. At present, more than 90 

percent of the known petroleum accumulations 

west of the San Andreas fault in California 

occur in strata deposited in areally restricted 

Neogene basins that formed during this major 

tectonic reorganization of western California

1 The stratigraphic nomenclature used in this 

report is from many sources and may not entirely 

conform to U.S. Geological Survey usage.

(fig. 3} . Figure 4 shows most of the major 

late Quaternary faults that are believed to be 

a tectonic expression of the Pacific-North 

American plate motions.

ENVIRONMENTAL HAZARDS

Much of the study area is a zone of strong 

ground shaking due to earthquakes (fig. 5}. 

The areas where the highest ground accelerations 

would be expected are in California along the 

San Andreas fault and in the Owens Valley east 

of the Sierra Nevada. Maximum values within 

these areas represent accelerations of about 

0.8 £. Although acceleration does not describe 

all characteristics of ground motion, it is 

frequently used in establishing design criteria 

for earthquake-resistant structures.

Instability of the seafloor, whether 

resulting from seismic activity or sedimentary 

processes, is the principal hazard to emplacement of platforms and pipelines offshore. Hazards related directly to seismic activity 

include ground shaking, fault rupture, generation of tsunamis, and earthquake-induced failures such as liquefaction and slumping. Faults 

showing displacement either of the seafloor or 

of young (< 11,000 years) sediment and faults 

associated with historical earthquakes are considered active and therefore potentially hazardous to development. Instability of the seafloor 

also can result from dynamic forces, such as 

wave surge, and static forces, such as gravity 

acting independently of seismic activity. Some 

areas of the seafloor are prone to mass movement 

(slumps, slides) or other forms of sediment 

transport (flows, creep, or current scour). Oil 

and gas seeps, though not inherently hazardous, 

may indicate the presence of fractured reservoir 

and cap rocks and shallow overpressured gas 

pockets that can pose danger to drilling operations.

Tsunamis (seismic sea waves) are large 

oceanic waves generated by earthquakes, submarine volcanic eruptions, and large submarine 

landslides. The waves are formed in groups and 

have great wave length and a long period. In 

deep water, wave heights (crest to trough) may 

be a few meters or less, wave lengths a hundred 

kilometers or more, and velocities greater than 

750 km/hour. As a tsunami enters shallow water, 

wave velocity diminishes and height increases. 

At places, such waves are known to have crested 

at heights of more than 30 m and to have struck 

with devastating force.
I24 C I20 C 116°

34° -

30° -

Figure 1. Physiographic index map for region discussed in text.

2
Figure 2. Physiographic provinces of California and 

Baja California, Mexico.

CENTRAL CALIFORNIA COAST 

GEOGRAPHY

Offshore from central California south of 

lat 38° N. is a 500-km-long shelf, 100 km wide 

at the south end, west of Point Arguello, that 

narrows northward to 30 km wide west of Santa 

Cruz. Underlying the gentle relief of this 

shelf are three major structural highs, the 

FaralIon-Pigeon Point high, the Outer Santa 

Cruz high, and the Santa Lucia high (fig. 6). 

Cutting across Monterey Bay is a major east-west 

trending submarine canyon. On the shelf south 

of San Francisco there are two sediment-filled 

troughs, the Outer Santa Cruz and the Santa 

Maria basins (figs. 3, 6a, and b).

GEOLOGY

Beneath the seafloor, major structural 

highs and basin axes trend generally northwest 

(fig. 6a, b). At most places the basins are 

bounded on the east by major nearshore faults 

with a shoreward side up sense of vertical 

displacement. Between these faults and the 

shore, the structures generally have a more

westerly trend. The basement terrane of the 

shelf and the adjoining Coast Ranges province 

is composed of allochthonous blocks with spatial 

relations that reflect the complex kinematics 

of a dynamic plate margin (fig. 7). Sedimentary 

rock units of Late Jurassic to Holocene age are 

present in the Coast Ranges province, but discussion of rocks in the offshore region is 

limited to the strata of the Cenozoic basins 

where petroleum exploration may be expected.

Outer Santa Cruz basin. This relatively 

shallow late Tertiary basin, which is approximately 25 km by 100 km in extent, trends northwest across the shelf, and extends onto the 

adjacent continental slope. It is bounded on 

the west by the Outer Cruz high and on the 

northeast by the Pigeon Point high. Hoskins 

and Griffiths (1971) have suggested that this 

basin is underlain by granitic rocks, based on 

unstated geophysical evidence and on the proximity of granitic rocks to the east. Alternatively, McCulloch and others (1977) have suggested that the Pigeon Point high composes the 

western edge of the Salinian block, and that 

the Outer Santa Cruz basin may be underlain by 

the Franciscan assemblage.

Two exploratory wells in the basin bottomed 

in Upper Cretaceous strata, primarily sandstone; 

however, the distribution of the Cretaceous and 

early Tertiary (Oligocene?) rocks is not known 

(Hoskins and Griffiths, 1971) (figs. 8 and 9). 

Deformation and erosion followed both the 

Cretaceous and Oligocene(?) episodes of deposition. Fine-grained clastic rocks and volcanic 

strata are interbedded in the early Neogene 

strata. Overlying these rocks is a sequence of 

strata composed chiefly of cherty shale of middle Miocene age. Uplift of the structural highs 

by late Miocene time resulted in basin-filling 

deposition of terrigenous detritus.

The structural axis of the basin and the 

Outer Santa Cruz high plunge to the northwest, 

beyond the edge of the continental shelf. The 

sediment thickens downslope, but appears to be 

blocked along the base of the slope by a discontinuous volcanic ridge along which the 

Mulberry, Guide, and Pioneer Seamounts form 

prominent topographic highs. Beneath an early 

late Miocene unconformity, the rocks are gently 

folded; and the faults are generally high-angle 

reverse with a vergence to the west. The southwestern basin margin does not appear to be fault 

controlled, for the early Tertiary sediment is 

upturned along the flank of the Outer Santa Cruz 

high. The northeastern margin is controlled by 

down-to-basin faulting that displaces rocks at 

least as old as the late Miocene unconformity.

Santa Maria basin offshore. This basin 

measures approximately 40 km by 230 km and is 

elongate parallel to the coast. The northwest 

end of the offshore Santa Maria basin continues 

onto the continental slope. It is bounded on 

the northeast by Franciscan basement rocks that 

have been elevated along major coastal faults,
EXPLANATION 

|;': ';';i;| Pacific plate 

Uli Faro lion plate 

| | North American plate 

v v Subduct ion zone

Pacific plate motion 

relative to the North 

American plate

Marine basins

Technically "soft" margin of 

Pacific - North American plates

Oceanic ridge 

Transform fault

Figure 3. Schematic model of the interaction of Pacific-Farallon and North American plates for six Neogene 

time intervals, showing the time of initial development, location, and general shape of Neogene basins that 

formed in the tectonically "soft" or pliant zone between the Pacific and North American plates. The develop-

124' 

I

I22e 120'

 L__ L_____|_ 42 o

40 80 Miles

0 50 100 Kilometers

Figure 4. Major Quaternary faults of California (modified from Jennings and others, 1975)
Figure 5. Contours showing estimated seismically 

generated bedrock accelerations for which there is a 

constant 90 percent probability that the acceleration 

will not be exceeded in 50 years. Numbers indicate 

horizontal acceleration in hard rock expressed as a 

percentage of g, the Earth's gravity. The numbers 

within each contour are the maximum expected accleration within the 60-percent contour along the San 

Andreas and Garlock faults in California is 80 percent of g (from Algermissen and Perkins, 1977).

and on the southwest by the Santa Lucia high, 

an elevated basement block (fig. 6b). A 1972 

expedition of the USNS Bartlett recovered from 

dredge hauls over this bank a variety of rock 

types that included metasediments, altered mafic 

igneous rocks, and a large number of granitic 

fragments (R. E. von Huene, oral commun., 1978). 

Hoskins and Griffiths (1971) suggested the basement of the Santa Lucia high may be granitic 

because of the structural style of the high and 

adjoining basin as well as the presence of coarse 

arkosic sandstone of possible Late Cretaceous 

and Eocene age. If the basement is granitic, it 

would necessitate considerable revision of existing tectonic reconstructions of this area which 

now limit granitic basement to the Salinian block. 

It is more likely that the basement for the entire 

offshore area of Santa Maria consists of the

Franciscan assemblage and strata equivalent to 

the Great Valley sequence. Both groups of rock 

are exposed along the adjoining mainland coast, 

and included in the Franciscan assemblage are 

indurated massive arkosic sandstone beds of 

possible Late Cretaceous age. Where similar 

rocks crop out near Cambria and Point San Luis, 

they typically are folded isoclinally and lie 

above melange {Howell and others, 1977).

Available information concerning the character and distribution of post-Cretaceous rocks 

in the basin consists of one borehole and inferred seismic correlations of acoustic units 

with onshore lithologies. Based on these 

limited data a generalized stratigraphic column 

for the entire basin and a cross section of the 

basin have been constructed (figs. 10 and 11).

Paleogene rocks (marine mudstone, silty 

shale, sandstone) are present on the Santa 

Lucia high. These strata are truncated by an 

early Tertiary unconformity, and their distribution in the basin is thought to be limited to 

erosional remnants. The lowest Neogene rocks 

may be volcaniclastic and may contain a few 

thin igneous flow units,. The overlying, acoustically thin-bedded, wedge-shaped unit (Tmx) 

may correlate with tuff in the Humble Oceano 

No. 1 test well, an equivalent to the onshore 

Obispo Formation, which is early and middle 

Miocene in age (Crawford, 1971). This unit is 

verlain by a bedded unit (seafloor samples 

from the Santa Lucia high indicate that this 

unit is a foraminiferal siliceous shale of 

middle Miocene age. The thin-bedded, highly 

reflective acoustic character of this unit, 

together with the presence of chert in dredge 

hauls and dart cores from seafloor exposures 

suggest that it is in part equivalent to the 

Monterey Formation. The uppermost unit, 

principally of Pliocene and Pleistocene deposits, rests with local unconformity on the 

thin-bedded unit. The upper unit probably 

includes a minor amount of upper Miocene 

strata in the lower part.

PETROLEUM APPRAISAL

The area offshore from central California 

has been explored by the petroleum industry 

and geophysical companies during at least the 

last two decades. For example, Hoskins and 

Griffiths (1971) indicate that the Shell Oil 

Company completed an extensive network of 

shallow and deep penetration seismic reflection 

profiles over the entire shelf and collected 

seafloor samples by dart core and shallow 

borings. Some industry exploration was done 

in anticipation of and following the May 14, 

1963, lease sale that included the offshore 

Santa Maria and Outer Santa Cruz basins; and 

ultimately, three exploratory wells were 

drilled. Most targets appear to have been 

structural rather than stratigraphic traps in 

these two basins.
I23 ( I22<

38° -

37°

E XPLANATION

Fault, dashed where approximately 

located or inferred

<^V_^ 

Volcanic seamount -*- -+-

Anticlin* Synclina 

Major Structural Axes

o M<4.5 

A M> 4.5

Earthquake epicenters, 

showing magnitude (M)

20 Miles

10 20 Kilometers

Guide \ 

Seamount

Figure 6. Generalized geologic structures. Point Conception to Point Reyes. Sources include: Hoskins and 

Griffiths (1971), Greene and others (1971), Earth Science Associates (1974), Jennings (1975), and unpublished
I22 C 121'

36°

35 (

10 20 Miles 

J

SB 0 10 20Kilometers

data 

1976

from H. C. Wagner, E. A. Silver, D. S. McCulloch, and H. G. Greene. Epicenter locations from 1812 to 

from Gawthrop (1975, and written commun., 1977).
LATE MESOZOIC

Mzcl CONTINENTAL ACCRETIONARY 

COMPLEX AGE

OUTER SANTA CRUZ BASIN

DESCRI PTI ON

Figure 7. Basement relations along Sur-Nacinttento 

and San Andreas fault zones (partly based on Figure 

95 of King, 1959). Localities: B = Bakersfield; 

FI = Farallon Islands; LA - Los Angeles; PA = Point 

Arena; SF = San Francisco. (Modified from Page, 

1970) .

Production from fields in the onshore 

Salinian block adjacent to the Outer Santa 

Cruz basin has been low. The principal onshore fields (La Honda, Half Moon Bay, Oil 

Creek) had produced a cumulative total of 

only 1.3 million barrels of oil, and 300 

million cubic feet of gas by December 1975. 

Most production is from Miocene and Pliocene 

strata. The nearest significant production is 

from San Ardo field, nearly 200 km southeast 

of Santa Cruz, where oil and gas are recovered 

from coarse sandstone zones at the base and in 

the middle of the Miocene Monterey Formation. 

Cumulative production from San Ardo through 

December 1975 was 0.29 billion barrels of oil 

and 71 billion cubic feet of gas (California 

Division of Oil and Gas, 1960, 1975}. Probable 

source beds in the Outer Santa Cruz basin are 

tar-impregnated middle Miocene cherty shale 

that occurs throughout the basin. Pre-Miocene 

reservoir beds have not been drilled, but 

Hoskins and Griffiths (1971) state that the 

data are insufficient to conclude tha't such 

reservoir rocks are absent. Cretaceous rocks 

below the lower Miocene erosional unconformity 

are highly deformed, dense, and indurated. 

Minor oil shows are present in upper Miocene 

and Pliocene rocks, but these rocks generally 

are fine grained and of poor reservoir quality.

Nearly 75 percent of the oil (0.5 billion 

bbls) in the Santa Maria basin onshore is produced from reservoirs of middle Miocene fracQuot.

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.j Marine sandstone, shallow water 

jBosIn edge unconformity

Morine clay, silt and minor

very fine sand

Lower Pliocene deposits absent from

some areos

Marine cloystone and siltstone, 

minor sandstone

Morine sandstone, shallow water

Unconformity

Marine shole, cherty, commonly

finely laminated, abundant tar

Morine sandstone, shallow woter

Basin edge unconformity

v v v v> Morine shole with volcanic interbeds

 f

T..r...v..ry Marine sandstone, shallow woter

rKS^Pt Unconformity

;';'-. V;-.i:'.:'l Marine sandstone

7^~>H U n con f a r m i t y

'^^aTsi Morine sandstone, locally brecciated, 

\v:?n chloritired; some shale

_-^' _ ", "^7 |

v^f^^ Basement unknown

Figure 8. Generalized stratigraphic column for the 

Outer Santa Cruz basin. The stratigraphic nomenclature, modified from Hoskins and Griffiths (1971), 

does not necessarily follow the usage of the U.S. 

Geological Survey.

tured shale and Pliocene sandstone. Recoveries 

per well in the Santa Maria fields vary greatly. 

Average recoveries range from about 10,000 to 

50,000 barrels per acre but have been achieved 

only by dense well spacing and a long production history. Many fields in this basin have 

reservoir characteristics that pose difficult 

economic and technical problems; if similar 

reservoirs are found offshore, they may prove 

too uneconomic to exploit.

Reservoir beds are not known to be present 

offshore, but it is anticipated that fractured 

shale in the Miocene Monterey Formation is a 

potential objective. This presupposes that the 

rock types and diagenetic history of the 

Monterey Formation onshore and offshore are 

similar. Pliocene sandstone reservoirs onshore 

are limited to the northeast part of the basin 

and are not expected to extend far offshore.

Miocene source rocks similar to those 

onshore should be present within the offshore 

Santa Maria basin. However, seismic reflection 

data indicate that the offshore upper Tertiary 

section is thin. The total volume of Miocene 

or younger rocks in the basin is approximately 

7,500 km3 . There are only two relatively small 

areas where burial is as great as 3,000 m, a 

depth believed necessary for a thermal regime 

sufficiently high to generate hydrocarbons from 

these source rocks. However, a higher than

10
A'

SOI Okm

Sea level

2000- 

4000-

6000- 

meters

OUTER SANTA CRUZ HIGH

Short lint

CRET. to EOC.

SHALLOW 

GRANITE

- Sea level

-2000

-4000

-6000 

meters

EXPLANATION

HOLD = Holocene 

UP - upper Pliocene strata 

LP = lower Pliocene strata 

UM - upper Miocene strata 

MM - middle Miocene strata 

LM = lower Miocene strata 

EOC = Eocene strata 

CRET = Cretaceous strata

Figure 9. Geologic cross section through Outer Santa Cruz basin (from Hoskins and Griffiths, 1971). 

of section indicated on figure 6a.

Location

normal geothermal gradient exists onshore and 

may be present offshore as well, perhaps providing a thermal regime adequate for petroleum 

generation at depths less than 3,000 m. In 

addition, Claypool (in Taylor, 1976) has presented evidence that petroleum generation can 

occur at lower than normal temperatures in 

rocks similar to those in this Miocene section. 

Estimates of the undiscovered recoverable 

oil and gas resources for the two offshore 

basins are calculated by E. W. Scott (McCulloch 

and others, 1977} :

95 percent 5 percent Statistical 

probability probability mean

OIL 

(billion of barrels)

GAS

(trillions of cubic feet)

.44

These estimates include both Federal and State 

acreage at water depths ranging from 0 to 2,500 

m. The development of these estimates is described in Miller and others (1975).

ENVIRONMENTAL HAZARDS 

OUTER SANTA CRUZ BASIN

Seismioity and faulting. Seismically 

active faults displace Holocene deposits on the 

seafloor in Monterey Bay, at the south end of 

the Outer Santa Cruz basin (Greene and others, 

1973). These faults strike obliquely northwest 

toward, and terminate against, the Seismically 

active San Gregorio-Palo Colorado fault. First 

motions indicate that the Monterey Bay faults 

and the San Gregorio-Palo Colorado fault are

moving with right-lateral displacement. Faults 

also displace Holocene deposits on the seafloor 

in the Gulf of the Farallones west of San 

Francisco. Several earthquake epicenters have 

been located farther offshore along the margins 

of the Outer Santa Cruz and Farallon-Pigeon 

Point highs, but their relation to mapped faults 

is not known.

Active faults have been mapped in offshore 

Santa Maria basin and on the adjoining mainland 

(Jennings, 1975; Wagner, 1974; Earth Science 

Associates, 1974; McCulloch and others, 1977). 

The principal offshore faults (West Santa Lucia 

Bank, East Santa Lucia Bank, and Hosgri faults) 

all appear to be associated with seismic activity or to have segments along which Holocene 

deposits are offset (fig. 6). The East Santa 

Lucia Bank fault forms a major shoreward-facing 

seafloor escarpment that locally exhibits as 

much as 40 m of relief. The Hosgri fault forms 

a zone 3 to 5 km wide along the northeast edge 

of the area, between San Simeon Point and Point 

Sal. A branch of the Hosgri fault may connect 

with the onshore San Simeon fault zone which 

shows evidence of Quaternary movement (Hall, 

1975) . Seismic profiles indicate that the 

Hosgri fault shows evidence of Quaternary movement and that a zone of thrust faults in the 

south end of the basin may be active. This zone 

lies along the basin axis, and is approximately 

20 km wide and 50 km long. Sediment at least 

as young as Pliocene has been displaced by 

thrusting; and first-motion studies of seismic 

events in this zone indicate north-striking 

thrusts with a vergence to the west (Gawthrop, 

1975).

Studies of plate motion in the eastern 

Pacific indicate that horizontal displacement 

between the North American and Pacific plates

11
o «,

AGE Strati gra phic Units

Fox en Mudstone

Si sq u o c Formation

Monterey Formation

0 bispo For m a t ion 

Lospe Formation

Great

Valley

sequence

Coast Range ophiolite 

Franciscan assemblage

Figure 10. Stratigraphic column, Santa Maria basin. 

Inferred from seismic data, IBS log (Standard Oil 

Company of California OCEANO No. 1 well), and nearby 

land exposures (modified from Woodring and Bramlette, 

1950, and Hoskins and Griffiths, 1971). Stratigraphic 

nomenclature does not necessarily follow the usage of 

the U.S. Geological Survey.

is about 5.5 cm/yr (Atwater, 1970). Displacement on the San Andreas fault amounts to only 

about 2.5 cm/yr; the balance of this plate 

motion possibly is accommodated by faulting and 

folding to the east and west of the San Andreas 

fault, including the area which encompasses the 

offshore Santa Maria basin. Epicenters of about 

550 instrumentally recorded earthquakes for the 

periods 1934-69 and 1969-74 were catalogued and, 

where necessary, relocated on maps by Gawthrop 

(1975). About 100 of these earthquakes are in 

the offshore area, and of these approximately 

10 percent have magnitudes greater than 4.5 (fig. 

6).

Seafloov -instability. There are insufficient data from the Outer Santa Cruz basin to 

establish the presence or absence of slumps or 

slides. Seismic profiles suggest slumps and 

slides in the northern part of the offshore 

Santa Maria basin, where large discrete blocks

of Neogene rocks are recorded as relatively 

coherent reflectors that are both bounded laterally and underlain by contorted zones. Shallow slumps superimposed on these blocks have 

not been dissected by erosion, and depressions 

in the surface of the slumps do not contain 

ponded sediment, implying that these features 

are very young. The shallow failures are caused 

by seaward thrusting associated with high-angle 

reverse faults along the northeast edge of the 

basin. Similar deep-seated and shallow failures 

may be present in the area of active thrusting 

in the southern part of the basin.

Tsunamis. From the first observation of 

a tsunami in San Francisco in 1812 to 1967, the 

greatest reported change in water elevation was 

4.6 m at Half Moon Bay on October 18, 1859 (lida 

and others, 1967). A magnitude 7.4 earthquake 

(April 11, 1946) in the eastern Aleutian Islands 

produced tsunamis of 3.5 m at Santa Cruz and in 

Half Moon Bay, but the amplitudes were considerably smaller where measured along adjacent 

parts of the coast. The Alaska earthquake of 

March 1964 (M 8.5) generated a highly destructive tsunami at Crescent City but had a minor 

effect farther south, producing only a 1.3 m 

rise in water at San Francisco (lida and others, 

1967; Wiegel, 1970).

TRANSVERSE RANGES PROVINCE 

GEOGRAPHY

The Transverse Ranges province of southern 

California encompasses a diverse geologic terrane having a predominantly east-west structural 

grain. The western part of the province (fig. 

12) includes the Santa Barbara Channel, the 

Santa Ynez Mountains, and the northern group of 

the Channel Islands (San Miguel, Santa Rosa, 

Santa Cruz, and Anacapa). The Ventura structural 

basin is an emergent, former eastward continuation 

of the channel. The western part of the Santa 

Barbara Channel is constructed by the large 

Pescado submarine fan (fig. 12) that merges westward with the Arguello Plateau.

GEOLOGY

Strata in the Santa Barbara Channel region 

range in age from Late Jurassic to Holocene. 

The upper Mesozoic section is at least 7,000 m 

thick and the thickness of the Cenozoic section 

may reach 10,000 m in the northeast part of the 

channel (fig. 13) .

Although the basement rock beneath the 

Santa Barbara Channel is not known, several 

contrasting basement terranes locally are exposed along its margins (fig. 14). Rocks of 

the Franciscan assemblage crop out within and 

north of the Santa Ynez fault zone, and Catalina 

Schist occurs southeast and south of the Malibu 

Coast fault. Beneath the northeast part of 

Santa Cruz Island, Franciscan (?) pillow basalt 

with low-grade zeolite contiguous to lawsonitebearing rock was cored below a Tertiary section

12
Quoternary and upper Pliocene sediments

[rnT|

Middle Miocene rocks 

[Tmx[

Miocene felsic tuffs and related rocks 

(Obispo (?) Formation)

Due to vertical exaggeration on 

the seismic profile (6:1) all faults 

steeper than about 40° are 

shown as vertical.

In developing "true" depths for this 1:1 cross-section, v

assumed to increase with depth at 5000 ft/sec (ISOO

where « is the depth to the reflection horizon below 

The water column is assumed to have a constant ve

ft/sec (1440 m/sec).

tliJ

Lower Tertiary rocks. Nearshore are volcaniclastic 

rocks and thin flows; offshore, unspecified

[TKJ] 

Jurassic and (or) Cretaceous to Eocene (?) rocks

KILOMETERS

Figure 11. Geologic cross section through Santa Maria basin (McCulloch and H. C. Wagner, from unpublished U.S. 

Location of section indicated on figure 6b.
Figure 12. Selected physiographic features of the Santa Barbara Channel region.

in Union Oil Co. Gherini No. 1 well (Howell, 

McLean, and Vedder, 1976). Catalina Schist 

forms part of a small ridge just south of Santa 

Rosa Island. The basement complex exposed on 

the south side of Santa Cruz Island is inferred 

to be part of a dismembered ophiolite (Hill, 

1976). The basement of the Santa Monica 

Mountains is composed of acidic plutonic rocks 

and a pre-intrusive metamorphic complex comprising argillite, lithic wacke, and metavolcanic 

rocks equivalent to the Mariposa Formation of 

the Sierran Foothills region (Jones and Irwin, 

1975) .

Cretaceous and early Tertiary strata are 

principally thick sub-sea fan sandstone and 

siltstone beds. A regional marine regression 

occurred during late Eocene and Oligocene time, 

during which nonmarine clastic deposition advanced westward and marine deposition persisted 

only in the westernmost parts of the channel. 

In early Miocene time, the channel again deepened, and feldspathic sand and mud accumulated 

in thickest portions at the site of the present 

northern shoreline. Middle Miocene volcanic 

centers lay along the margins both northwest 

and south of the present channel area 

(Tranquillon Volcanics of Dibbles, 1950, and 

Santa Cruz Island Volcanics of Nolf and Nolf, 

1969, respectively). Post-middle Miocene sedimentation is represented principally by organicrich mudstone and local accumulations of subsea 

sandstone turbidites.

Holocene deposits cover the seafloor 

throughout most of the deeper parts of the 

Santa Barbara Channel but are absent locally 

from Mugu and Hueneme Submarine Canyons and are

absent from or very thin on large parts of the 

shallow shelf adjoining the northern Channel 

Islands and the mainland. Bottom sediments in 

the central deep are predominantly hemipelagic 

silt and clay. Rates of sediment erosion and 

deposition vary greatly from place to place 

within the channel. Sediment discharged by 

the Santa Clara River during major floods has 

formed sediment layers more than 15 cm thick on 

the inner part of the Oxnard Shelf; this sediment is eroded by wave and current action and 

redeposited over wide areas of the shelf and 

basin. The average rate of deposition in the 

deep central basin of the Santa Barbara Channel 

is about 2 mm per year (Fleischer, 1972; 

Hulsemann and Emery, 1961).

The major structures in the Santa Barbara 

Channel region are east-west oriented folds and 

left-slip faults, for example, the Santa Ynez 

fault, Santa Ynez Mountains, Rincon trend, and 

Montalvo trend. This structural grain may be 

superimposed on a northwestward trend in older, 

buried structures. The total structural relief 

on basement rocks in the channel region may be 

as much as 18,500 m.

The structural evolution of the Santa 

Barbara Channel region is complex and difficult 

to reconstruct, partly because of the lack of 

knowledge concerning the distribution of the 

basement rocks. Recent episodes of deformation 

can be related to (1) regional extension (normal 

faulting) during late middle Miocene to late 

Pliocene time between the uplifted blocks that 

now form the north and south boundaries of the 

western Transverse Ranges; (2) left-lateral slip 

at the southern boundary of the province; (3)

14
DIAGRAMMATIC STRUCTURE SECTIONS

Sandy 

Point I Sonto Barbara Basin

Molino

Hondo Off8hore 

Offshore area aas "«la

Sonto 

Ynez 

fault 

| Q | QUATERNARY DEPOSITS, UNDIVIDED

[Opl [ LOWER PLEISTOCENE AND UPPER PLIOCENE STRATA

[Tpu | UPPER PLIOCENE MARINE STRATA

[ Tpi | LOWER PLIOCENE MARINE STRATA

[~Tm 1 MIOCENE MARINE STRATA, UNDIVIDED

|Tmu| UPPER MIOCENE MARINE STRATA

Tmm MIDDLE MIOCENE MARINE STRATA

] Tmv I MIOCENE VOLCANIC ROCKS

ITIDI 1 LOWER MIOCENE MARINE STRATA

[TO 1 OLIGOCENE STRATA, UNDIVIDED

OLIGOCENE M

IromJ

H 

OLIGOCENE C

Miocene 

EOCENE AND 

CRETACEOUS 

Figure 13. Geologic cross section through the Santa Barbara Channel and vicinity. Location of sections indic

from draft Environmental Impact Statements Oil and gas development in the Santa Barbara Channel outer contine

75-35, 1975.)
SANTA YNEZ UNIT 8 VICINITY N.E. COAST, SANTA CRUZ I.

DESCRIPTION

« Sisquoc Formation

) > Monterey Formation

v Bentonlte

fe Rincon Shale

 5 Vaqueros Formation

o Alegria, Gaviota

and Sespe Formations

Sacate Formation 

Cozy Dell Shale 

Matilija Sandstone

Anita (?) Shale

Unconf.

Jalama Formation

\ N Espada Formation

Fault (?)

Franciscan assemblage

tu in 2 o: * tu o IAGE fa

TERTIARY 0 

Holo. and' 

_Plip;X 

|T> 1 § * I Us M!S

Oliao.

Age 

undet.

MESOZOIC

o 

*>

I

ISO

I

o-

=Si=lL

 « * * r

^ « t A

f A V *

t * <e 

jpTXK ^°/&^»o

 :-:njJ::\-;-::

~ff^~'"'~~~^

Y. . _ £J

y-/< <s*'>

^.y>s/W

4

DESCRIPTION 

Monterey Formation

«, Santa Cruz Island 

" Volcanics of Nolf 

t and Nolf(l969) 

,= San Onafre Breccia

JO 

V)

Franciscan 

assemblage

Lithologies 

inferred in part

Lithologic Symbols

-j/:^ Shate/Siltstone

Breccia

Volcanic rock /

Intrusive igneous rock

Sandstone / 

Conglomerate

Tuff / Chert

Blueschist and 

melange

Thicknesses 

are maximum

Figure 14. Generalized stratigraphic columns, Santa Barbara Channel region

and others (1976), does not necessarily follow the usage of the U.S. Geological Survey

Nomenclature, modified from Vedder

regional compression during late Pliocene to 

Holocene time between the same blocks, resulting 

in reverse dip-slip and significant north-south 

shortening by folding; and (4) possible, Neogene 

clockwise rotation locally of at least 75° based 

on paleomagnetic measurements in volcanic rocks 

(Kamerling and Luyendyk, 1977).

Subsidence of the Ventura basin probably 

accompanied regional Miocene to Pliocene northsouth crustal extension by normal faulting combined with large left-lateral displacement on 

the Malibu Coast fault to the south (Crowell, 

1976) The subsequent deformation, which may 

have begun as early as late Pliocene time, continues today and reflects regional north-south

compression; reverse faults associated with the 

deformation dip both inward from the outer margins of the province (the Santa Ynez and Santa 

Monica faults) and outward, away from the center 

of the thick accumulation of late Cenozoic 

sediments (the Oak Ridge and San Cayetano-Red 

Mountain faults) (Yeats, 1976). The effects 

of compressional stress may diminish toward 

the west (by giving way to right-lateral shear 

effects), as suggested by the northwestward 

bend of structural trends west of Point 

Conception and San Miguel Island and by the 

change from east-west to northwest trends of 

contours of Bouguer gravity anomalies in the 

central to western part of the channel.

16
PETROLEUM APPRAISAL

Petroleum exploitation in the Santa Barbara 

Channel and adjoining area has concentrated on 

Oligocene and younger strata. Production data 

are summarized below:

Eastern onshore 

(Ventura Basin)

Santa Barbara 

Channel (offshore)

Lower Miocene rocks 

Middle Miocene rocks 

Upper Miocene or

1 o

303 

6

65

1

253

3

Five oil fields (Dos Cuadras Offshore, 

Carpinteria Offshore, Rincon, San Miguelito, 

and Ventura) along the Rincon anticlinal trend 

form a nearly continuous giant field. Structures are believed to extend westward from the 

Dos Cuadras Offshore field into the Federal 

Ecological Preserve and the Federal Buffer Zone, 

in which petroleum development is prohibited 

(fig. 12). Although no single structure has 

been demonstrated to extend westward from the 

Rincon trend through the gap south of the Coal 

Oil Point-Capitan part of the coast, the 5-Mile 

trend lies along the western projection of the 

Rincon trend and may include a general westward 

continuation of the same structural zone.

The only OCS leaseholds presently producing 

in the region are on the Dos Cuadras and 

Carpinteria Offshore fields. Both fields are 

on the Rincon anticlinal trend and most of the 

production to date has been from Pliocene 

strata. Four potentially commercial fields 

have been delineated in the Santa Ynez Unit, 

and the unit operator is now proceeding with 

development. Three potential oil fields have 

been reported in the southeastern part of the 

channel region, and another potential oil field 

has been reported along the Pitas Point anticlinal trend, south of the Dos Cuadras Offshore 

and Carpinteria Offshore fields, and a potential 

commercial oil field also has been discovered 

north of San Miguel Island.

Within the Santa Barbara Channel region, 

the largest continuous unleased areas are: (1) 

the central deep, chiefly the area enclosed by 

the minus 500-m depth contour; (2) the lower 

part of the Pescado Fan, north of the western 

sill of the Santa Barbara Basin and west of the 

central deep; (3) the parts of the Channel 

Islands slope and Channel Islands platform that 

lie north of the west half of Santa Cruz Island 

and eastern and central parts of the Santa Rosa 

Island; (4) the Hueneme-Mugu slope and fan; and 

(5) the Federal Ecological Preserve and Federal 

Buffer Zone. There are also a few smaller areas, 

ranging from one to about four parcels in size, 

west of Ventura and northeast of Santa Cruz 

Island.

Little is known about the central deep. The 

very thick Pleistocene and Holocene deposits 

there are generally flat lying, and the thickness and structure of potential oil-bearing

strata below them are unknown. Projection from 

adjacent areas of known structural trends suggests that the upper Miocene to upper Pliocene 

strata of the Ventura basin are likely to be 

thinner on the south side of the Santa Barbara 

Channel than on the north. The geology of the 

lower part of the Pescado Fan is uncertain. Fan 

deposits conceal the structure and stratigraphic 

relations of older strata on available acoustic 

profiles. On the shelf and upper slope north 

of Santa Rosa and Santa Cruz Islands Miocene 

to upper Plicoene strata are represented by 

thin, discontinuous, and possibly shallower 

water deposits. Anticlinal structures are 

present, but exploratory drilling to date has 

not located significant oil or gas reserves.

Nearly 80 percent of oil and gas production 

from Santa Barbara Channel oil fields has come 

from Pliocene strata. However, in the giant 

Santa Ynez Unit, significant quantities of oil 

and gas occur only in Miocene and older strata 

(Monterey, Vaqueros, Sespe, Alegria and Sacate 

Formations and the Matilija Sandstone) .

ENVIRONMENTAL HAZARDS

Seismicity and faulting. Twenty four 

earthquakes of local magnitude 6 (Richter Scale) 

or larger have occurred in southern California 

during the past 60 years. Six of these are of 

special interest because they either (1) originated in the Channel region, (2) originated in 

the same structural province (western Transverse 

Ranges) and are associated with displacements 

and ground motions similar to those expected 

from earthquakes originating in the Channel 

region, or (3) originated in adjacent structural 

provinces and had sufficient intensity to cause 

damage in the Channel region. The seismic 

activity implies contemporary fault movement at 

depth. The region also shows geologic evidence 

of Holocene fault displacement and geodetic 

evidence of contemporary differential vertical 

movement. In the vicinity of Ventura vertical 

uplift over the last 100,000 years, measured 

from deformed movement terraces, is calculated 

to average as much as 7.7 m/1,000 years (K. R. 

Lajoie, oral commun., 1978).

Seafloor instability. Seafloor instability 

generally is associated with unconsolidated 

Holocene sediments where there are slope gradients of 1.0° or more. A veneer 1 to 5 m thick 

of late Quaternary sediments of clay, silt, 

sand, and gravel, is known to cover the entire 

slope from Point Conception to Ventura (Fischer, 

1976). These deposits thicken locally, such as 

in buried terrestial stream channels. Areas of 

thick Holocene deposits such as the Oxnard Plain, 

Oxnard Shelf, central deep, and Pescado Fan are 

likely sites for unstable seafloor conditions.

Submarine landslides have been mapped in 

the Santa Ynez Unit and on the upper part of 

the Hueneme-Mugu slope (Duncan and others, 1971). 

Deposits showing varying degrees of disturbed 

bedding are known to occur along Goleta, Oxnard,

17
and Channel Islands slopes. Additional study 

is required to appraise and analyze fully seafloor instability in the Santa Barbara Channel 

region.

Tsunamis. The east-trending embayment of 

the Santa Barbara Channel strongly mitigates 

the effects of seismic sea waves generated in 

the northern Pacific Ocean area, and the Channel 

Islands of the southern California borderland 

offer a sheltering effect to the populated area 

along the north and east coast of the Santa 

Barbara Channel. Maximum run-up for 100-year 

and 500-year tsunamis generated outside the 

immediate channel area is estimated to be 3.2 m 

and 6.6 m, respectively (Houston and Garcia, 

1974). The height of the largest tsunami that 

could be generated within the channel is not 

known. In general, seismic sea waves resulting 

from strike-slip faulting probably result in 

limited destructive effect; however, obliqueslip and dip-slip faults possibly could generate major sea waves.

Hydrocarbon seeps. Natural hydrocarbon 

seeps are common in the Santa Barbara Channel 

region. Although seeps are not inherently 

hazardous, they may provide clues to potentially 

hazardous conditions associated with fractured 

reservoir rock and shallow gas pockets. Fischer 

and Stevenson (1973) mapped 1,250 active gas or 

oil seeps within a 18-km2 area off Coal Oil 

Point. Seeps are primarily restricted to the 

north margin of the channel where active folding 

and faulting are inferred to be the major contributing factors.

CALIFORNIA CONTINENTAL BORDERLAND 

GEOGRAPHY

For the purpose of this report, the northern limit of the California Continental 

Borderland is arbitrarily placed at the south 

margin of the northern Channel Islands. However, 

it is noteworthy that pre-Miocene structures on 

these islands are similar to those of the borderland, whereas the geomorphology and postMiocene geologic features are more analogous to 

those of the Transverse Ranges province. The 

California Continental Borderland extends southward as far as Cedros Island, and westward to 

the base of the Patton Escarpment.

The borderland encompasses a large terrane 

with local relief comparable to the east scarp 

of the Sierra Nevada. It contains 19 major 

topographic basins that range from 35 to 58 km 

long and 10 to 40 km wide and that have depths 

of 400 to more than 2,000 m. These basins are 

bounded by mainland, island, and bank slopes 

that typically are steep to gently rounded, 

upward-convex curves and are cut by numerous 

submarine canyons. Distances to these basins 

from sources of sediment on the mainland range 

from 0 to over 200 km. South of lat 31° N., 

the borderland is about half as wide as it is

to the north, and it lacks the regularly spaced 

northwest-trending basins and ridges. The dominant feature in the southern borderland is a 

large composite basin that extends northward 

from Vizcaino Bay to about lat 31° N., where 

it separates into several smaller basins.

GEOLOGY

Northern part of California Continental 

Borderland. North of lat 31° N. there are three 

broad, northwest-trending terranes that possibly 

are fault bounded and allochthonous to one another. An inner borderland terrane (terrane III, 

fig. 15) lies east of a line that extends approximately along the west margin of San Clemente 

Island and thence to the eastern part of the 

Santa Cruz Island fault. This line coincides 

roughly with part of the pre-Pliocene East 

Santa Cruz Basin fault system postulated by 

Howell and others (1974). The east boundary 

of terrane III onshore is the Newport-Inglewood 

fault zone; southeast of Newport Beach the 

position of the boundary is uncertain. Terrane 

III comprises a high pressure-low temperature 

basement assemblage of schistose rocks (Catalina 

Schist) that is intruded and overlain by Miocene 

plutonic and volcanic rocks (fig. 16). Overlying and buttressing against these metamorphic 

and igneous units are late Cenozoic clastic 

rocks, most of which are middle Miocene and 

younger. The only known pre-Miocene strata in 

terrane III are Paleogene or Cretaceous siltstone, sandstone, and conglomerate blocks that 

are enclosed in a Miocene dike swarm at the 

southeast end of Santa Catalina Island (Vedder 

and Howell, 1977) .

The central borderland terrane (terrane II, 

fig. 15) extends from terrane III westward to a 

northwest-trending lineament that approximately 

parallels the west slope of the Santa RosaCortes Ridge from the west flank of Garrett 

Ridge to the edge of the platform southwest of 

San Miguel Island. Except for the mafic plutonic rocks and greenstone exposed on Santa Cruz 

Island, crystalline basement in this area is 

unknown. Cretaceous to Eocene strata make up 

thick sections and are widespread; locally, they 

are overlain by marine and nonmarine Oligocene 

sedimentary rocks, Miocene volcanic and volcaniclastic rocks, and correlative and younger 

clastic rocks (figs. 16 and 17).

The outer borderland terrane (terrane I, 

fig. 15) extends from terrane II westward to 

the base of the Patton Escarpment. Basement 

rocks in this area are composed of schistose 

rocks, mafic volcanic rocks, zeolite-bearing 

arenite and argillite, and ultramafic blocks. 

These rocks are somewhat similar to the Coastal 

Belt facies of the Franciscan assemblage in 

northern California. Late Oligocene and younger 

marine clastic and volcanic rocks fill sedimentary basins and drape slopes in this basement 

terrane (fig. 16) 0

The juxtaposition of these terranes implies 

a geologic history that presumably includes

18
.Santo Barbara

i N. ** "» l

___-' \,, v^x V

; .-- ! > v

"V '#'>

vs \ N ~?*\'T

IZOe

Figure 15. Boundaries of proposed DCS Lease Sale 48. The three-mile line is indicated by a solid line along the 

the islands. The 200-meter and 750-meter depth curves are shown by'short dashed lines. Areas within heavy solid 

numerals are geologic terranes. East boundary of terrane I is dashed and queried where its location is uncertain
THIRTY MILE BANK STRATIGRAPHIC TESTOCS-CAL 75-70 NO. I CENTRAL BLAKE 

QUAT.

TERT.

Hoi, and PMtt.

Plionni

Mio.

%

i

MESOZOIC ?

50

120

o

§

?

=. 7=_'

 T==ZrT7-'

^'."^1

 ^.f^y\

ffSffS\ Catalina Schist

Lithologies ond thicknesses 

inferred In port

NORTHERN PATTON RIDGE

CRETACEOUS

 =^=^=\ Unconformity 

L: :.-_vy,y*Fault ?

Espado ? Fm, 

Mesozoic' Franciscan (?)

NORTHERN SANTA ROSA-CORTES RIDGE

3) 

<r

OH 

AGE EZ

QUAT. Hol.ond Pltltt.

Oligocene

170

TERTIARY

Mio.

L.- 

ond 

M

Oligocene

Eocene

Paleoc

C R E T.

Bne

i

O

i?

9o

ID

in

27O

O 

O 

ID

=jf^lf

"i-i -TJS_1 . 1

- .= rr.rJ

rr. T: = -l

pjrr^r.S

SAN NICOLAS BASIN

QUAT.

-9-f 

>

(E

<

1-

<r

Ul

tHolocene

to

Plioc

Mio.

ine

D

S

J

Oligocene

and (or)

Eocene

ond (or)

Poleowrwp)

CR ETACEOUS

a

a

a 

 s

O

O

ID

O

NO

O

»

NO

O

in

- ' ?

. *

Thicknesses inferred 

Lithologies unknown

NORTHEAST BANK

Pliocene 

_9_9.

Miocene

TERT. Miocene

MESOZOIC ( ? )

SAN CLEMENTE 

£

UJ Mio. M.toU

MESOZOICf?)

O

9

 

FORTYMILE BAN

L

infe

DALL BANK

lUJ

KMiocene

Oligocene

Eocene

(0 

O

UJ

0

1-

UJ

ac.

o

3

-i

370

60

6

ID 

00

o

ID

i!

0

£
ifl 0) 

(D 3 3 a 

13

ft Q. O3

ss

» n 01 > o> o f" hj 

0) H. M ^1 H 

H- tn ^< 

M) I O in HI 

hj O O 

H- Z H 

01 O Q 

 THICK. 0 (METERS) 

THICK. 

" (METERS) 

H- (B 3 - n> 

rt iQ 

tu rt ft M y 

H- O 

g) O Hi 

hj 3 ft 

Q. (t 3" 

(D 01 (t 

hj 01 M (t G 

o> 01 . 3 CO 

a o< . S o 

o> a 

rt W 

H- Hi C 

iQ hj hj 

hj O <; !3 3

o THICK. = = (METERS) 

rt

o

o co 
EF

San Pedro Basin Palos Verdes Hills

Newport Inglewood 

fault zone r°

-3000

-6000

-9000 

meters

San Clemente 

Tanner Bank San Nicolas Basin Island platform

Pii*is^^-^^

" ''" : '-' :: ' :: - : i:'::j-;:!: !:!:j:;:;:5;i!^

Northeast 

Bank

Cortes 

Bank

30 Kilometers

EXPLANATION

h-.1 : ;!-J Quaternary and 

l,v.. : '"l Pliocene rocks

|\\y| Mlocens rocks 

Volcanic rocks

CzD Middle Miocene and older 

sedimentary rocks

Cretaceous, Eocene, and 

Otigocene rocks

Basement rocks

Figure 17. Generalized geologic sections through the California Continental Borderland and Los Angeles basin. 

Location of sections indicated On figure 15 (Vedder and others, 1974, and unpublished data).

episodes of subduction and transform faulting. 

We believe that the late Cenozoic geologic evolution of the borderland resulted from tectonic 

instability of the continental margin along the 

boundary between the Pacific and North American 

plates. A network of ridge-and-basin features 

developed as a result of right-lateral shear, 

which may have begun along the plate boundary 

about 30 m.y. ago (Atwater, 1970).

The structure of basins and banks in the 

eastern part of the borderland is complex; folds 

near the mainland coast along the seaward extension of the Newport-Inglewood fault zone are 

comparatively small and steep-flanked, and 

faults occur both as zones of in echelon breaks 

and as single traces. Most of the major folds 

and faults are oriented northwest (fig. 18). 

Some large anticlinal structures such as 

Coronado Bank are broad and nearly symmetrical, 

but little is known about their development. 

Faults that can be dated by stratigraphic 

analysis range in age from middle Miocene to 

Quaternary and have both strike-slip and dipslip separations. Most small folds probably 

are post-Miocene to pre-late Pleistocene in 

age.

Faults in the western part of the borderland are more diverse in age and displacement. 

Although the dominant structural trend is northwest, there are two conspicuous east-west fault 

zones; one zone is located in the vicinity of 

the northern Channel Islands, and the other is 

located south and east of San Nicolas Island. 

Strike-slip separation is evident on some 

faults, such as the San Clemente fault, and 

most show some amount of normal offset. Highangle reverse faults such as the curved fault 

along the saddle between Santa Cruz and San 

Nicolas Basins, also are common. Some offsets 

are possibly pre-Pliocene, such as the postulated East Santa Cruz Basin fault system of 

Howell and others (1974), but more commonly they 

are Pliocene and younger. Pre-middle Miocene 

thrust faults are inferred in the basement 

rocks of Santa Catalina Island (Platt, 1975).

Large anticlines generally trend westnorthwest at angles oblique to the major fault 

zones and at places appear to be arranged in 

echelon (fig. 18). Topographic highs reflect 

anticlinal structures in many places, and broad 

downwarped structural lows form both Santa Cruz 

and San Nicolas Basins. Many anticlines, for

22
121

Minor anticline 

 + Syncline 

T steeP fault' bar on

32C

Figure 18. Tectonic map showing major structural trends and basins of the California Continental Borderland 

area (Junger, 1976).

example, those that underlie Tanner Bank and 

the San Nicolas Island platform, are very large, 

nearly symmetrical, and have gentle dips on 

their flanks. Numerous small folds are superimposed on major upwarps such as the Santa RosaCortes Ridge, but these seem to die out basinward. Some anticlines deform sediments as young 

as Pleistocene, such as in the central San 

Nicolas Basin, and others probably are as old 

as early Miocene, such as the main anticline on 

northwestern Santa Rosa-Cortes Ridge, in which 

Miocene strata truncate Paleogene strata on 

both limbs. An episode of local deformation is 

suggested by an unconformity between the middle 

and late Miocene sequences on the flanks of the 

central part of Santa Rosa-Cortes Ridge.

Southern part of California Continental 

Borderland (Baja California). Geologic data 

from the borderland off Baja California are 

insufficient to determine the southern extent 

of the geologic terranes that characterize the 

borderland farther north. The bathymetry of 

this region (Krause, 1965) indicates that the 

continental borderland narrows considerably 

south of lat 31° N. Rock types similar to 

those in the northern part have been dredged

from ridge tops off Baja California (Doyle and 

Bandy, 1972); these include silicic and mafic 

plutonic rocks, metasedimentary rocks, phosphorite, and late Cenozoic sedimentary rocks.

Cretaceous and younger rocks on the mainland coast of Baja California are similar to 

units of the same age in the Peninsular Ranges 

north of the Mexican border. Moreover, complex 

geologic relations in the Mesozoic rocks exposed 

on Cedros Island and the neighboring Vizcaino 

Peninsula resemble those inferred for rocks of 

similar age in western California (Jones, Blake, 

and Rangin, 1976).

A major thrust, analogous to the "Coast 

Range thrust" of California, separates a low_er 

plate consisting of tectonized high pressurelow temperature rocks from a relatively undisturbed upper-plate terrane. The superjacent 

rocks include mafic plutonic and volcanic rocks 

that are thought to be part of an ophiolite, 

Late Jurassic to Eocene sedimentary rocks depositionally overlying the ophiolite, and an 

andesitic volcanic suite (possibly a remnant 

Jurassic arc-complex) that is intruded locally 

by a 140-m.y.-old tonalite pluton. Clastic 

Neogene rocks overlie an unconformity that 

probably cuts all of these pre-Oligocene rocks.

23
At present, these Neogene rocks occur in isolated basins, suggesting that the episode of 

right shear and crustal extension that disrupted 

the southern California borderland also affected 

the southern part of the borderland (Blake and 

others, 1978).

The offshore extent of the Mesozoic and 

early Cenozoic structural relations is not 

known. However, the structure of the borderland 

west of Baja California^ resembles the area to 

the north, and the north- to northwest-trending 

pattern of folds and faults probably developed 

in late Cenozoic time. The right-lateral shear 

presumably commenced after the southward migration of the triple junction separating the 

Farallon, Pacific, and North American plates 

(fig. 3) .

PETROLEUM APPRAISAL

Appraisals of the petroleum potential of 

the southern California borderland have been 

prepared recently by Vedder and others (1974, 

1976) and Taylor (1976) and have been supplemented by information from a stratigraphic test 

well OCS-CAL 75-70 No. 1 on Cortes Bank (Paul 

and others, 1976).

Estimate undiscovered recoverable oil 

resources, southern California borderland, 

[From Taylor, 1975. Values in billions 

of barrels of oil.]

Depth 95 percent 

(meters) probability

Inner basin area

Terrane III, offshore 

Terrane IV, offshore 

Outer basin area

0-200

> 200

> 200

0.4

.2

0

5 percent 

probability

2.0

2.0

1.6

Organic matter in Tertiary rock samples from 

the ridges is thermally immature; and recent 

exploratory drilling at several areas in terrane 

II has produced unfavorable results confirming 

the above low estimates for the outer basins.

An appraisal of the petroleum potential of 

the U.S. part of the California Continental 

Borderland and adjoining Santa Barbara Channel 

indicates the following amounts of oil and gas 

resources.*

95 percent 5 percent Statistical 

probability probability mean

Oil (billions

of barrels) 1.9 5.8

Gas (trillions

of cubic feet) 2.2 6.3

3.6

3.9

1 These figures, compiled in September 1976, 

represent that part of the DCS area between 

water depths of 0 to 2,500 m with the exception 

of the eastern basins where only the area with 

water depths greater than 200 m has been considered in the estimate. In this region most 

of the water-covered area above the 200-m depth 

is State owned, and much of that outside the 3- 

mile limit is either excluded from the sale or

ENVIRONMENTAL HAZARDS

Seismieity and faulting. within the 

southern California borderland only a few 

published studies deal with identifying conditions that could be hazardous to petroleum 

exploration and production (Vedder and others, 

1969; 1976; Ziony and others, 1974; Field, 

Clarke, and Greene, 1977). Published studies 

are restricted in general to the Mugu-Santa 

Monica, San Pedro, and Newport-San Diego 

shelves, parts of the Santa Rosa-Cortes Ridge, 

and the Tanner-Cortes Banks area. Geologic 

phenomena investigated in these areas include 

faults, seismicity, seafloor instability, sediment erosion, and hydrocarbon seeps.

The borderland is cut by numerous faults 

(fig. 19), many of which are identified as 

active because they either cut the seafloor, 

cut young sediment (< 11,000 years old), or 

can be correlated with historic seismic events. 

Earthquakes of magnitude 4.5 or greater have 

been frequent on the San Pedro shelf and in 

the vicinity of the Santa Cruz-Catalina Ridge; 

locations of these epicenters are shown in 

figure 19 (Hileman, Alien, and Nordquist, 1973; 

Teng and Henyey, 1975). Because Tanner and 

Cortes Banks lie beyond the limits of the seismographic network, there are no reliable epicenter data.

Four major fault zones transect the inner 

basins and banks area; these are the Palos 

Verdes, Malibu Coast, Newport-Inglewood, and 

Rose Canyon fault zones (fig. 19). Many faults 

associated with these zones may be active (Ziony 

and others, 1974; Jennings, 1975). Most of the 

active faults in the Mugu-Santa Monica shelf 

area are short and discontinuous; the Malibu 

Coast fault is the largest and most likely to 

generate a major earthquake. In the San Pedro 

shelf area, the Palos Verdes fault is more than 

64 km long and locally offsets the seafloor. 

Earthquake epicenters along its trace verify 

its continuing activity. Many other faults 

that extend upward to the seafloor must be considered environmentally hazardous, as they cut 

beds of Holocene age (Greene and others, 1975, 

plate 13).

The longest Quaternary fault mapped in 

the inner basins and banks area is the San 

Clemente fault; the northwestern segment is at 

least 80 km long and the southeastern segment 

more than 24 km long (Jennings, 1975). Several 

earthquakes have been reported in the vicinity 

of this fault; one of these, recorded near the 

southeast extension of the fault in 1941, had 

a magnitude of 5.9 to 6.0 (Lamar and others, 

1973). In addition, irany smaller faults cut 

the area, some of which appear active (figs.

is already under lease. State acreage, Federal 

reserves and reservations, and Federal leases 

granted at previous sales are included in the 

rest of the general sale area as assessed, but 

data are not available to make an assessment 

of these restricted or untested tracts.

24
121' 120° 119* 

34'

33*

32° ~

EXPLANATION

Quaternary offset

' San Nicolas Island ' ' . .' . '' .

 pre-Quaternary offset 

....... age not specified

. earthquake epicenter - greater 

than 4.5 Richter magnitude

120° 119' 118" 

Figure 19. Location of major faults and earthquake epicenters in the California Continental Borderland. Abbrev

Rose Canyon fault; ni = Newport-Inglewood fault zone; pv = Palos Verdes fault; sc = San Clemente fault; sm - San

Coast fault; mg = McGrath fault; or = Oak Ridge thrust; pp = Pitas Point fault; sci = Santa Cruz Island fault; s

escb = east Santa Cruz Basin fault (from Field, Clarke, and Greene, 1977).
18 and 19). The northern Santa Rosa-Cortes 

Ridge also seems to be tectonically unstable, 

having had at least 20 earthquakes ranging in 

magnitude from 2.5 to 4.5 during the period 

1932-1973 (Greene and others, 1975). Faults 

are numerous, but are most common along the 

ridge crest where relatively small apparent 

vertical separations are characteristic (Greene 

and others, 1975, plate 5). Beneath the flanks 

of the ridge, faults are less numerous but have 

greater apparent vertical separations than 

those on the ridge top. Seafloor offsets above 

displaced seismic reflectors suggest that some 

faults are active. In the Tanner-Cortes Banks 

area, faults are concentrated along the northern 

flank of Cortes Bank and in the structural saddle between the two banks (Greene and others, 

1975, plate 2). Many of these faults displace 

either Holocene sediment or the seafloor.

Seafloor instability. Detailed highresolution surveys have not covered the entire 

borderland, and for the areas surveyed, they do 

not extend basinward beyond the upper slopes of 

ridges. Areas known to be prone to submarine 

sliding occur along the mainland shelf and slope 

where the thickness of unconsolidated Quaternary 

deposits varies greatly (Field, Clarke, and 

Greene, 1977) .

The location of slide-related scarps and 

thicknesses of slump deposits suggest that most 

surface failures occurred at the shelf (or 

ridge) edge or on the upper slope. The relative 

importance of earthquakes, gravitational loading, and dynamic loading from storm waves in 

triggering these and other seafloor failures 

in the borderland is not known (Field, Clarke, 

and Greene, 1977).

Distribution of sediment types on the 

northern part of the Santa Rosa-Cortes Ridge 

(clastic sand on the edges, foraminiferal sand 

in the center) suggests that bottom currents 

may be strong on the perimeter and of lesser 

strength in the center. Both the sparse sediment cover on the ridge top due to the isolation from sediment sources and the abundance 

of rocky outcrops devoid of sediment suggest 

the influence of strong current activity 

(Greene and others, 1975, plates 8 and 9). On 

Tanner and Cortes Banks, strong current activity 

is suggested by areas of exposed bedrock and by 

the thinness of the sediment cover over much of 

the nearby area (Greene and others, 1975, plates 

1, 2, and 4). The low silt and clay content, 

relatively good sorting, and coarseness of banktop sediments also suggest current action, although the coarseness is partly a reflection of 

the abundant supply of coarse biogenic debris.

Tsunamis. Locally generated tsunamis have 

been recorded along the coast between Point 

Mugu and the Mexican border; however, these are 

few in number and have not caused major damage. 

One tsunami was noted in 1879 at Santa Monica, 

and two others were reported in 1925 (uncertain) 

and 1933 at Long Beach (lida and others, 1967).

The 1933 seismic sea wave resulted from the 

March 10, 1933 Long Beach earthquake. Although 

tsunami damage in this area has been minimal, 

the region is seismically active, and inundation 

along the coastal lowlands could result from 

tsunamis.

Hydrocarbon seeps. Known hydrocarbon seeps 

in the borderland are located on the Santa 

Monica shelf. Oil and gas seeps are present in 

the northern part of Santa Monica Bay along the 

probable extension of the Palos Verdes fault, 

and offshore between Point Vicente and Point 

Fermin (Wilkinson, 1971; Greene and others, 

1975; Henyey and others, 1975). Wilkinson 

(1971) shows two oil seeps and one gas seep in 

the San Pedro shelf area.

Oil and gas seeps have not been detected 

in recent surveys of the northern Santa RosaCortes Ridge and Tanner and Cortes Banks, nor 

have they been reported in literature on this 

region. However, the presence of hydrocarbons 

in sediment samples and the large number of 

faults in these areas suggest that surface seeps 

and subsurface gas-charged sediments may be 

present.

ACKNOWLEDGMENTS

The information and the inferred geologic 

framework for much the offshore area discussed 

in this paper is still in a preliminary state. 

We have benefited greatly by discussions with 

numerous U.S. Geological Survey geologists whose 

current research involves particular topics or 

specific areas within the Pacific OCS area. 

Initial drafts of this manuscript were greatly 

improved as a result of reviews by H. G. Greene, 

S. H. Clarke, Jr., Richard Willingham, G. L. 

Dolton, and K. A. Yenne.

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29 fr U.S. GOVERNMENT PRINTING OFFICE: 1976 689-035/28