CALIFORNIA STATE UNIVERSITY, NORTHRIDGE

QUATERNARY EVOLUTION ~ AND SEISMIC STRATIGRAPHY OF THE

SAN PEDRO MARGIN, SOUTHERN CALIFORNIA

A thesis submitted in partial satisfaction of the requirements for the degree of Master of Science in

Geology

By

Juhani Henrique Rudat

January, 1980 The Thesis of Juhani Henrique Rudat is approved: ~

Dr./Martin L. Stout

California State University, Northridge

ii ACKNOWLEDGEMENTS

This author is indebted to Dr. Peter J. Fischer for his help,

endless patience, and numerous ideas and suggestions, not to mention his invaluable experience in marine sciences. The numerous students

of the CSUN Geoscience Department, whose enthusiasm and cooperation

continues to be a source of support, provided assistance in data

collecting and drafting. In particular, thanks to Calvin Lee who,

from the earliest trials and tribulations, spent many hours with the

author, including stints on various offshore platforms and assorted vessels.

Funding for this project was obtained from Shell Oil Company,

Houston, Texas, and from NOAA Sea Grants No. 04-5-158-20 and 04-6-158-

44021.

The author is grateful to Scripps Institute of Oceanography and

Dr. F. P. Shepard for providing boat time on the M/V GIANNA, and to

Mr. Arne Junger for providing reproducible copies of R/V KELEZ data, as well as valuable discussion of the local seismic stratigraphy.

Through the efforts of Mr. D. Clarke of the California State Lands

Division, the author obtained microfilms of all R/V KELEZ data.

This thesis is dedicated to my wife, Jane, for her support and understanding.

iii CONTENTS

CHAPTER PAGE

ACKNOWLEDGEMENTS iii

TABLE OF CONTENTS iv

ABSTRACT vi

1. INTRODUCTION 1

General Statement and Previous Work 1

Physiographic Setting 7

Geologic ?etting 8

2. METHODS AND PROCEDURES 12

California State University, Northridge (CSUN) Surveys 12

Instrumentation 12

Navigation 13

"Beta Platform Sites" Survey 13

USGS (R/V Kelez) Survey 13

Data Reduction and Basis of Interpretation 14

3. MESOZOIC TO MID-PLEISTOCENE SEISMIC STRATIGRAPHY 18

Basement Rocks 21

Miocene Rocks 27

Pliocene Rocks 31

Pleistocene Rocks 37

4. SEISMIC-STRATIGRAPHIC MODEL, WILMINGTON GRABEN LATE PLEISTOCENE AND SHELF HOLOCENE UNITS 46

Introduction 46

Seismic-Stratigraphic Model 47

Late Quaternary Chronology 57

iv CHAPTER PAGE

Late Pleistocene to Holocene Seismic Stratigraphy 67

LP IV 67

LP III 68

LP II 71

LP I 75

Holocene 83

5. STRUCTURE 94

Faults 94

Folds 101

6. EVOLUTION OF THE SAN PEDRO BASIN MARGIN 105

Pre-Quaternary Events 105

Qua ternary Evolution 108

Early Pleistocene 108

Middle Pleistocene (Oxygen-Isotope Stages 11 to 7) . 109

Pleistocene and Holocene (Oxygen-Isotope States 6 to 1) 110

San Gabriel Submarine Canyon 116

7. SUMMARY AND CONCLUSIONS 123

REFERENCES CITED 128

LIST OF FIGURES 134

LIST OF TABLES 136

LIST OF PLATES 137

v ABSTRACT

QUATERNARY EVOLUTION

AND SEISMIC STRATIGRAPHY OF THE

SAN PEDRO MARGIN, SOUTHERN CALIFORNIA

by

Juhani Henrique Rudat

Master of Science in Geology

'~he San Pedro basin margin is a relatively wide platform located southeast of the Palos Verdes Peninsula and the Los Angeles-Long Beach

Harbor. A departure from the normally narrow Southern California inner basin margins, the configuration of this area is the result of the Neogene evolution of two tectonic elements: the Palos Verdes uplift and the Wilmington graben. The interrelationships of tectonic events, sedimentation dominated by the Los Angeles, San Gabriel, and

Santa Ana Rivers, and glacioeustatic sea level fluctuations, are critical factors in explaining the evolution of this area;

Exposed and near-surface ~ocks on the outer San Pedro shelf (Palos

Verdes uplift) include basement rocks (Catalina Schist?), folded Miocene

(Monterey Formation) and Pliocene (Repetto Formation) strata (Mohnian

vi to Repettian benthic stages), Pleistocene San Pedro Formation(?) and

slope deposits. These are partly covered by Holocene sediments. A

thicker post-Miocene stratigraphic section in the Wilmington graben

is inferred from oil fields onshore. The interpreted seismic reflection

data reveals the presence, near the edges of the graben, of middle

Pleistocene (?) deposits ("unnamed upper Pleistocene deposits" of

Poland and others, 1956) and the San Pedro (?) Formation:j

tthe upper Quaternary section in the Wilmington graben is based on '---- a seismic-stratigraphic model of an upper Pleistocene slope unit (LP I),

and the Holocene shelf sediments. Analogous older units (LP II-IV) are

defined by unconformities and paleo-shelf breaks:, They are termed

seismic-stratigraphic sequences and as such have chronostratigraphic

significance. ('fhese units are believed to reflect glacioeustatic sea

level fluctuations~ and are provisionally correlated with stages in

the marine oxygen-isotope curve of Shackleton and Opdyke (1973).

Periods of incision and aggradation of the Newport-Inglewood zone

fluvial gaps are correlated with isotope stages 1 through 5, and with

units LP III through Holocene·~, The lowest (first) marine terrace of the

Palos Verdes Hills is correlated with unit LP II. LP III correlates

with the lowest terrace in the town of San Pedro, and the fourth and

possibly the third and second terraces of the Palos Verdes Hills.

C!he high-angle reverse and right-lateral Palos Verdes fault

separates the Palos Verdes uplift from the Wilmington graben. Slightly

oblique-trending folds are suggestive of convergent dextral shear along

the faultj Its late Quaternary seismic activity is shown by offsets of

units LP IV through LP I. Activity appears to have diminished since

vii the deposition of LP III (possibly starting about 140,000 years B.P.), the youngest horizon commonly disrupted on all splays. However, some seismic activity has continued until the present, as indicated by epicentral data and numerous offsets of the basal Holocene surface and the seafloor. [~n area off the Palos Verdes Peninsula is markedly bowed over the fault zone. The shoreward edge of the Wilmington graben is defined by another active fault zone, portions of which also offset the base of the Holocene interval. Abrupt upward tilting of LP IV and older strata, pinch out of LP III and LP II, and areas of no Holocene sediments on the upthrown side further delineate this fault;;

viii Chapter 1

INTRODUCTION

GENERAL STATEMENT AND PREVIOUS WORK

The last decade has seen a revolution in knowledge about the geo­ logy of continental margins. In California, the use of seismic reflec­ tion techniques from the initial work of Moore and Shumway (1959) and

Moore (1960) to efforts by universities and the U.S. Geological Survey, as well as the extensive work by the private industry sector, has brought the continental margins to the forefront in current geologic research. It has become increasingly evident that the submerged por­ tions of the continents will play a vital role in man's search for energy and resources.

The region underlying San Pedro Bay (The San Pedro Basin Margin

Figure 1), has been the focus of a great deal of academic research and economic interest. A progressive development in geophysical application to marine geology is chronicled by studies of this basin margin, particularly the shelf area. Moore (1954) contributed the first report on the area, including a discussion of surface sediments and foraminif­ eral and lithologic correlation of in-situ rock samples. The only geophysical data used for this study were fathograms. In his assess­ ment of the structure of the continental borderland, Moore (1960, 1969) utilized newer seismic systems, which produced subbottom penetration.

The evaluation of an area immediately seaward of Balsa Chica Beach

(Figure 2) for a proposed nuclear power plant involved a detailed survey of local structure and stratigraphy (Bechtel, 1967). Part of this project included several drillholes and numerous arcer seismic-

1 Figure 1

Index map 3

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.. II 1: lit II .. 0 Dl 'D _, 1: co oC

w a:: "'w > "'z <( ....a:: Figure 2

Location map

SH-Signal Hill, RH-Reservoir Hill, All-Alamitos Heights, LH-Landing Hill, BCM-Bolsa Chica Mesa, HEM-Huntington Beach Mesa, NM-Newport Mesa ·:-- .....

~ .. ' t '"'·----.. ..,.,.,,,1/lfllltl•ll,. 118 1 !} .,~-···/~ \i .... ""\I\IU~ ....,.,,hl'lolll/.t9~ ~ \ \":. \~··-. SH ,...... \ ''fiiNII,IH ,,,,, r··, ~ ,.~ ~ \ "••• RH "-. ,w""··· t~"' ~ or\ ~ NM i' •, (/'...... ,.J \ ., (...... ~ ...... , ... ,...... l l 1; \ ~-. f ...... ,.,••'"' / / HBM \ ~\ i \ •. 0 ~- BCM i j ~- ~ ··~ \':~.... LONG { ! ~ ' \\ \ BEACH \.,.,,.,."'+•/ { \ '.,\-:.,.~~~~ i '·~·.... 8 1 NEWPORT 01 \~~ m-t-~n:-j " •• ch; ~ \ ·, \ (BEACH 0 \ ·- 1-·-.J Stotoe ih HUNTINGTON \ '•,,,, , .... ~' ,e...... ,,,,,,, ...... ,., BEACH.... ~ ,i '.. \ "'"'•"' BECHTEL CORP. ~ • "BOLSA ISLAND" ,--.c SURVEY AREA • •

(f' ~· __,.,.,o ~.,o

---~sa, ~ ----- X

. ..,..,,·"' \ q•o•• <;,.~ '\

0 10km

33.30' FIGURE 2 6

reflection profiles. Junger and Wagner (1977) published a series of maps of the San Pedro and Santa Monica basins and surroundings based on

several types of seismic profiling data, some of which provide rela­

tively more resolution and quality than systems used in previous studies of this area. Some of their work was incorporated in a USGS open-file report (Greene and others, 1975) discussing environmental aspects of selected continental borderland areas; also in a symposium volume

(Howell, ed., 1976) on the geologic history of the borderland as a whole. Most recently, Nardin and Henyey (1978) published a study of the Palos Verdes uplift based on air-gun and magnetometer records, and bedrock samples.

This study considers the seismic stratigraphy, structure and

Quaternary evolution of the San Pedro margin. Using high-resolution seismic-reflection data in conjunction with bore-holes and jet arid dart cores, the distribution and thickness of Holocene unconsolidated sediments, the shallow structure and limits of geologic units below the Holocene overburden are shown. @'he study area is located off southeastern Los Angeles and southwestern Orange Counties (Figures 1 and 2). It is immediately offshore from the Los Angeles-Long Beach

Harbor, the landward boundary being the federal harbor breakwater and the Sunset Beach-Huntington Beach shoreline. The seaward boundary is the shelf break which ranges in depth from 50 m to 200 m. The eastern limit is the area off the Santa Ana River, and the western extent, the

Palos Verdes Peninsula and adjacent shelf segment, south of Pt. Fermin;

As part of a former leading plate-edge, the Southern California continental margin is generally a narrow submerged platform, in con­ trast with the broader east coast continental shelf. The unusual width 7

of the San Pedro margin, actually a basin shelf and slope, and the pre-

sence of two submarine canyons incised on its outer edge, arouses curi-

osity regarding the evolution of this area. The present morphologic

configuration of this margin can be explained by the interplay between

tectonic events, sedimentation by the Los Angeles, San Gabriel and

Santa Ana Rivers, and glacioeustatic sea-level fluctuations~

In addition to these academic considerations,/the San Pedro ~- margin has economic importance. The Federal Government's sale of petroleum leases in several areas of the borderland (BLM Sale No. 35,

1975) received the heaviest bidding in the San Pedro Bay area.').,.,., Among

these was Shell Oil Company's OCS Tracts 35-261 and 35-262, in the vicinity of the San Gabriel submarine canyon. In preparation for siting platforms in this area, the author was involved in a geophysical and geotechnical study of these tracts (Figure 2). The study was in 2 conjunction with Mesa , Inc. and the Marine Studies Office of the

California State University, Northridge Geoscience Department. The ample resources available to industry provided an excellent, well navigated, dense grid of seismic-reflection data. Also, bore-holes for lithologic correlation (Figure 2), as well as paleontologic ages and paleoenvironmental information were utilized (Fischer and others,

1977).

PHYSIOGRAPHIC SETTING

(ihe San Pedro margin is a coastal terrace which is part of the

California Continental Borderland Province (Shepard and Emery, 1941).

The on-land counterpart of this geomorphic province is the Peninsular 8

Ranges Province. Both are characterized by structurally controlled,

mostly northwest trending ridges and alluviated valleys or basins0 A

prominent feature of the landward portion is the large lowland called

the Los Angeles basin,. which extends from Santa Monica southeastward

along the coast to Newport Bay, and inland up to the Transverse Ranges

Province (Figure 1). (this seaward sloping plain is interrupted by two

areas of relief in the vicinity of the study area. The first of these

is a series of en-echelon anticlinal folds and topographic highs associ­

ated with the Newport-Inglewood zone of deformation. This deformation

produced Signal Hill, Reservoir Hill, Landing Hill, Balsa Chica Mesa,

Huntington Beach Mesa and Newport Mesa (Figure 2) which are important because they have controlled the access of drainage across the Newport­

Inglewood zone to the ocean (Poland and others, 1956). The Palos

Verdes Hills represent the greatest relief in the area. Rising to

400 m, these hills have been incised by 13 levels of marine terraces

(Woodring and others, 1946).

GEOLOGIC SETTING

Geologically, ~he San Pedro basin margin constitutes the southwest­ ern extent of the Los Angeles basin~ This is true as far south as the

"San Joaquin structural high," the marine extension of the anticline which underlies the San Joaquin Hills (Western Geophysical Corp.,

1972) (Figure 1). At its depocenter the Los Angeles basin contains as much as 10,000 m of sediments (Yerkes and others, 1965). These strata are mostly marine in origin and range in age from late Cretaceous to

Recent. Some mid-Miocene igneous intrusive rocks and volcanic flows are present. Due to varying histories of uplift and subsidence, as 9

well as contemporaneous folding and faulting and local erosion, the

stratigraphy is characterized by numerous facies changes, variable

thicknesses, and many local unconformities. {Quaternary alluvium is

mostly fluvial, derived from the Los Angeles, San Gabriel and Santa Ana

Rivers. The sedimentary accumulation of the Los Angeles basin is

probably underlain by metamorphic and igneous basement rocks of Precam-

brian to early late Cretaceous age (Yerkes and others, 1965). These

rocks have been divided into a western blueschist facies complex and an

eastern granitic complex (Woodford, 1925; Yerkes and others, 1965;

Hill, 1971; Yeats, 1973). Movement along this boundary is presumed to

have given rise to the Newport-Inglewood zone of deformation.

Basement rocks of the California Continental Borderland presumably

belong to the western blueschist complex. Exposures of this assemblage,

locally termed Catalina Schist by Schoellhamer and Woodford (1951) ;! at ·" the Palos Verdes Hills and on Catalina Island, samples from submarine

dredges and cores (Vedder and others, 1974), and clasts in the San

Onofre Breccia (Stuart 1975) substantiate this assumption. (Overlying

basement in the offshore basins are upper Cretaceous to Recent sedi- ments. Miocene rocks constitute most of the ridges, with a thin veneer

of Pleistocene to Recent cover (Vedder and others, 1974)J

crhe regional structure of both the Peninsular and Continental

Borderland provinces is characterized by northwest-trending faults. ~ This structural grain dies out or is truncated on the north by the

east-west grain of the Transverse Ranges. Within the Los Angeles basin, the Newport-Inglewood zone is the predominant structural feature.

Based on active seismicity, this zone was reported to extend from

Beverly Hills southePstward to a point offshore of Laguna Beach)(Jahns WEST NEWPORT~ ,,, l,'i ~;­ ·~~

,,

San Pedro Bay

Santa Monica Bay

0 5 KM SCALE Reverse fault ~ Normal fault ~ Strike-slip fault ;:' High-cmgle fault Plunging anticline 0~0 0 Non-plunging syncline ,.,, ' 0~ ~,-, ~tO ~0 ~tO I,_, ' Oil-field limits (Modified from Yeats,l973)

24 10

and others, 1971). Fischer and others (1979), on the basis of reflec­

tion profiles, demonstrate that ~he fault is active further southeast,

off San Onofre Bluff~ Another report (Barrows, 1974) extends the fault

all the way to the San Diego area, much as the interpretation of Emery

(1960). As stated previously, ~pis zone is comprised of cross-trending

en-echelon anticlinal folds and discontinuous faults, typical of wrench­

style deformation (Harding, 1973) with right-lateral (Barrows, 1974),

and cumulative strike-slip displacement about 3 kilometers according to

Yeats (1973).

Seaward of the Newport-Inglewood zone.is the Wilmington anticline

(Figure 1). This fold plunges southeastward, terminating 3.5 km off­

shore in the Long Beach Harbor area, and traps one of the largest oil deposits in the Los Angeles basin (Mayuga, 1970).] Farther southeast, following the same approximate trend is the Huntington Beach anticline, also the site of an economic oil reservoir (Hazenbush and Allen, 1958).

The topographic features of the borderland led early workers to envision numerous faults bounding the basins. Emery (1960), in particu­ lar, proposed such a picture with the aid of seismicity, gravity and magnetic surveys. Moore (1969) substantially revised Emery's conception with the addition of seismic-reflection data which confirms some of the more obvious topographic expressions as faults, and disproves others which apparently are folds. Furthermore, Vedder and others (1974) suggest that many faults thought to be throughgoing are, instead, short discontinuous segments, particularly in post-Miocene rocks. ~gional northwest-trending faults and cross-trending folds and faults appear to illustrate the structural style (Junger, 1976). To explain the present physiographic configuration of the borderland, both extensional tecton- 11 ics (Yeats, 1968) and large, purely strike-slip motions (Howell and others, 1974) have been proposed. Junger (1976) suggests that the cross-trending folds and faults are related to convergent wrench fault- ing, much as Wilcox and others (1973) had demonstrated theoretically, and Harding (1973) had suggested for the Newport-Inglewood zone.

Junger (1976) envisions deepseated miniplates bounded by northwest- trending right-lateral, shear zones associated with a broad transform fault zone~ Using clay-cake experiments, he suggests this origin for ../ the many associated minor, post-Miocene faults on the ridges, and the relatively undisturbed basins, as well as the compressional cross- trending features. Chapter 2

METHODS AND PROCEDURES

CALIFORNIA STATE UNIVERSITY, NORTHRIDGE (CSUN) SURVEYS

Instrumentation

Field work for this investigation by the writer consisted of gath­

ering 3.5 kHz continuous high-resolution seismic reflection profiles.

The electronic components used are from an Edo Western Corp. Model 515

system. Specifications are listed in Table 1. The 3.5 kHz transducer

was mounted on a CSUN constructed sled which was towed behind the

vessel. Depending on boat speed (5-7 knots) and angle of diving plane,

as well as water in the ballast tanks, this device averaged 5 to 10

meters of operating depth and 30 meters distance from the ship. During

the latter stages of data collection, a 3.5-7.5 kHz transducer (Edo

Model 616 system) mounted on a towed "fish" was used. This configura­

tion increases operating stability because of its hydrodynamic qualities

·(pitch and roll stability of 1-2°), and it allows a constant depth

(approximately 10 meters depending on cable length) at variable speeds.

The upper "bubble" pulse is confined to about 1 meter, and good resolu­

tion (0.3- 0.5 meters) with typical penetration of 25 meters is achieved.

Collection of profiles was done aboard the Southern California

Ocean Studies Consortium's R/V NAUTILUS during the summer of 1973 and

fall of 1975, and aboard Scripps Institute of Oceanography's M/V GIANNA

during the summer of 1975. Eight days were spent at sea.

12 13

Navigation

A total of 640 seismic km were traversed (Plate 1) with an average spacing of 1,500 meters between lines. Tracklines were positioned by the use of triangulation from two to three shore-based alidade stations, radar ranges and bearings, and dead reckoning in conjunction with bathy­ metry. Shore stations and on-board personnel were coordinated by citi­ zens-band radio, calling for simultaneous fixes at two-three minute intervals. Each fix was noted on the recordings and plotted on base maps. This technique was compared with "Mini-Rangefinder" positioning during surveys off San Diego County (Henry, 1976), and comparable accu­ racy (±30 meters) was maintained when visual contact was good. On tra­ verses too far offshore for ideal shore-based triangulation (more than

10 km) the positioning accuracy is less (±300 meters to 1 km).

"BETA PLATFORM SITES" SURVEY

Seismic data for the geophysical and geotechnical evaluation of

Shell Oil Company's Beta platform sites were gathered by General Ocean­ ographics, Inc. of San Diego, California aboard the R/V SEAMARK from

March 21, 1977 through March 29, 1977 (Fischer and others, 1977)

(Plate 1). Geophysical systems included sparker, Uniboom, side-scan sonar, a Sonia II 3.0 kHz tuned-transducer, and magnetometer (see Table 1 for specifications). Navigation was provided by Navigation Services,

Inc. Mini-Ranger with accuracy of ±10 meters.

USGS (R/V KELEZ) SURVEY

A total of 340 seismic kilometers of Uniboom data from the USGS

KELEZ cruise, Leg 2 (Wagner, 1975), were incorporated for this investi- 14 gation (Plate 1). Table 1 summarizes pertinent instrument specifica­ tions. Line spacing is typically 2 km, and data quality is generally excellent, with average penetration of 50 m (60 milliseconds, two-way time). Navigation was based on Shoran fixes using an on-board computer, with position accuracy of ±10m (Greene and others, 1975).

DATA REDUCTION AND BASIS OF INTERPRETATION

The basis of interpretation of seismic profiles is reviewed by several authors (Moore, 1969; Vedder and others, 1974; Greene and others,

1975; Dobrin, 1976). Briefly, records show surfaces which reflect the seismic pulse generated by the system. These surfaces (water-bottom interface, bedding planes, fault planes) represent a change in acoustic impedance. Thicknesses, displacements, water depth, and depth of pene­ tration are dependent on sound velocities in the propagating medium.

Records are compressed horizontally; as a result, the vertical dimension

(expressed as travel time of sound) is exaggerated as much as 10 to 15 times, depending on boat_ speed and recorder paper feed. Because of this exaggeration, very steep slopes (greater than 15°) are hard to discern.

Tight folds may be interpreted as fualts, and faults themselves are re­ cognized only by their offsets or by marked changes in echo character, including diffractions, dip changes separated by disturbed zones, and by change in seafloor surface. Crossings of fold axes and fault traces are plotted and connected by topographic expression and conformity with overall tectonic grain and strain patterns. Horizons are identified by extension from known on-land exposures and stratigraphic relationships

(particularly unconformities), echo character, and correlation with core holes, geotechnical soil borings, and bottom samples. 15

True thicknesses of particular units can be derived from reflection

profiles if sound velocities through that unit are known. These can be

ascertained by measurement, preduction of the interval velocity, or by

measurement or prediction of a sediment surface velocity and a velocity

gradient (Hamilton, 1974). This data is not available for determining

unconsolidated sediment thicknesses in the San Pedro area. However,

Hamilton suggests that in the absence of this information an interval velocity can be assumed based on the average sediment type in question,

and tabulated laboratory values. For sandy-silt (5.1 phi), Hamilton

cites an average laboratory velocity of 1664 m/sec. Ashley (1974) cal­

culated 1655 m/sec as an average seismic velocity for Southern California

shelf unconsolidated sediments, from Hamilton's tables. By comparing

actual thicknesses from drill holes to corresponding thicknesses shown

on sparker records, Bechtel Corporation (1967) produced a velocity of

1730 m/sec for approximately the upper 75 meters of unconsolidated sedi­ ments off Balsa Chica State Park (Figure 2). Taking all these figures

into consideration with realization that further determination of sedi­ ment velocities in the area will alter isopach maps, this author utilized an average velocity of 1700 m/sec. Table 1

Specifications and settings for geophysical systems 17

Table 1

SPECIFICATIONS AND SETTINGS FOR GEOPHYSICAL SYSTEMS

CSUN SURVEYS

Edo Western Model-515 System

Transceiver: Model: 248D Power: 2000 watts (increased to 8000 watts with a Model 465A power amplifier)

Recorder: Model: 333C (dry paper) Sweep: 1/4 sec (1/2 and 1 second occasionally)

Transducer: Model: 515 (and 616) Frequency: 3.5 kHz Pulse Length: 0.2-0.5 milliseconds Beam Width: 45° (conical)

"BETA PLATFORM" SURVEYS

Sparker: General Oceanographics, Inc. Power: 0-18 k Joules Filter: 75-150 kHz

Uniboom: E.G.G. Power: 600 Joules Filter: 500-1500 Hz

Side-Scan Sonar: Klein Frequency: 100 kHz Scan: 150 meters Towing Distance: Variable to 370 meters

Sonia II: Fugro, Inc. (Tuned Transducer) Frequency: 3 kHz

USGS (R/V KELEZ) SURVEY

Uniboom: E.G.G. Power: 600-600 Joules Sweep: 1/4 sec Firing Rate: 1/4 sec Filter: 650 to 2000 Hz Chapter 3

MESOZOIC TO MID-PLEISTOCENE SEISMIC STRATIGRAPHY

~he distribution and relationships of stratigraphic units on the

San Pedro margin can be separated into two general structurally con­

trolled sections: the Palos Verdes uplift and the Wilmington graben

(Junger and Wagner, 1977) (Figure 3)~] The result is two generalized

stratigraphic columns, with the Palos Verdes uplift column lacking a

thick Pleistocene section (Figure 4). !Along the offshore Palos Verdes

uplift the stratigraphy of seismic units can be ascertained from off­

shore projection of units mapped on the emergent Palos Verdes Hills by

Woodring and other~ (1946), by correlation with published dredge-haul

data (Moore, 1954; Vedder and others, 1974), jet and dart cores (Nardin

and Henyey, 1978), and by comparison with reconnaissance maps by Greene

and others (1976) and Junger and Wagner (1977).

an the Wilmington graben, the stratigraphy is inferred from infor­ mation provided by published reports of oil fields of the adjacent

coastal zone, in particular the Wilmington oil field (Muyaga, 1970;

Truex, 1974, 1978; Wissler, 1943; Hazenbush and Allen, 1958)~ Onshore

Quaternary stratigraphy is summarized in detail by Poland and others

(1956) from coastal water wells. Just shoreward of the Wilmington

graben these units are penetrated by bore-holes of Bechtel Corp. (1967)

(Figure 2). Further offshore, west of the San Gabriel submarine canyon,

Shell Oil Co.'s "Beta Platform Survey" bore-holes 261-1, 261-2 (Figure

2) (Fischer and others, 1977) were used to correlate seismic units from

R/V KELEZ (USGS) Uniboom acoustic profiles, Shell Oil Company Uniboom

and Sonia II profiles, and CSUN tuned-transducer, 3.5 kHz records.

18 Figure 3

Physiographic and structural setting 20

M

0 co

. CXl

... "' ... "'0 "'C'l 0 ...... c: en ~

a: w In 1- w ::J C) 0 z 0 ... ~ ~ IX «.,~" ~() ~ .._,"l" ...... 4 w In 0~ ()~ IX (., 04 w E o: > ,"l" 0 In z 4~ < cP IX ------==< z ==::!: ..... ' (.,~

0 21

This chapter will deal with the seismic stratigraphy of Mesozoic

to lower to mid-Pleistocene (San Pedro formation) units throughout the

shelf, as observed with the available data. The next chapter will

cover the middle to late Pleistocene seismic stratigraphy in the

Wilmington graben and Holocene sediments on the shelf, and basin slope.

Investigation of the late Quaternary stratigraphy requires the initial development of a seismic-stratigraphic model, and warrants a separate

chapter. Figure 4 and Table 2 summarize the stratigraphy of the area as investigated by numerous authors. For lithologies, thicknesses and contacts, the reader is referred to these two· illustrations. It should be noted that work on planktic foraminiferal zonation and radiometric dating suggests that redefining temporal boundaries of Neogene Epochs may be necessary (Nardin and Henyey, 1978). As a result of these zonations, the lower part of the Pico Formation is considered Pleisto­ cene and the base of the Pleistocene is well below the Lomita-Timms

Point complex. Also, the upper part of the Monterey Shale is considered

Pliocene. However, in conformity with past usage, the writer still refers to the Monterey Shale as (chiefly) Miocene, the Pico Formation as (chiefly) Pliocene, and Timms Point Silt as lower Pleistocene.

Additionally, Boellstorf~ (1978; see Chapter 4) indicates that the

Pleistocene Epoch started 2.5 million years B.P.

BASEMENT ROCKS

(]asement rocks of Late Jurassic to Late Cretaceous age are exposed along the core of an anticline in the Palos Verdes Hills (Woodring and others, 1946). These authors suggested the name Catalina Schist for these rock~and similar deposits on Santa Catalina Island, and Figure 4

Stratigraphic correlation chart 23

f''cI ·' '.<---··

l 0 CAliF. LONG BEACH TO YEARS 0 PAlOS VERDES WilMINGTON B.P. BENTHIC NEWPORT BEACH UPliFT GRABEN

PALOS VERDES SAND ~ TERRACE AND LP I2: TO lP I w ~~t' z y.t' COASTAL SANDS w / u / SAN PEDRO FM. SAN PEDRO FM. SAN PEDRO FM. 0 / ...... V) w___. 2 a.. -;:;::- PICO FM. 0." ._' Q) c: Ol 3 0 w w 3: """0 z z c: w / 0 / 0 u / L. N REPETTO FM. Q) 4 Ol 0 c: u ___. => a.. MONTEREY SH. I- .::::!. --' 0 MAlAGA ::::> w N MUDSTONE <( ?--? z u... VAlMONTE 0 0 ' DIATOMITE PUENTE FM. N PUENTE FM. V) z ALTAMIRA w 10 LU SHAlE 0 I- --' u 0:: w TOPANGA ::::> TOPANGA ~ \.'ll's't' > <( LL. 15 V) 0 0 w --' <( :E 20 a_ <( z z ::::> 30 OliGOCENE TO 60 PAlEOCENE u lATE CRETACEOUS 0 CATAliNA CATAliNA CATAliNA N 0 TO LATE JURASSIC SCHIST SCHIST SCHIST V) 90 w BASEME.NT ROCKS

FIGURE 4 24

Footnotes from Figure 4

1 This study and Woodring and others (1946) Nardin and Henyey (1978) Junger and Wagner (1977)

2 This study and Modified from Truex (1974) (Wilmington Oil Field) Mayuga (1970)

3 This study and Bechtel (1967), Balsa Island Site (Figure 2) Poland and others (1956), Coastal Water Wells Allen and Hazenbush (1957), Sunset Beach Oil Field Wissler (1943)

4 Nardin and Henyey (1978)

5 Note that time scale is progressively compressed

6 See Table 3 for detailed LPIV to Holocene stratigraphy

7 Table 2 summarizes lithologies, thicknesses, and other information regarding these units. Table 2

Lithologies, thicknesses and pertinent information about San Pedro area formations !

UNIT AREA. LITHOLOGY SOURCE COMMENTS

Lowest terrace! ~arine coarse grained sand 5 m Woodring and Locally rests on lower Pleistocene and San Pedro and gravel with silty sand, others (1946) lfiocene rocks, and Bitt, Thick nonmarine seds. cap this formation and merge into upper part of older allu­ vium of L.A: ·basin See Table for descript~on of correla­ tive strata on San Pedro shelf - LP IV to LP I. Palos Verdea Sand Off Bolaa Basal aandy gravel to con- 50 Ill Bechtel (1967) Conformable with San Pedro'Fm below. I Chica State glomerate grading to silt, DH10l@-62m Anomalous thickness suggests that this Beach sand and gravel in .varying (base) unit includes older deposits. ro ortions. Coaatal .15 m. Poland and ·Alao caps hills and 111esaa along Newport­ water wells Ingle!'ood zone, sand and gravel in'va ro ortiona Palos Verdes Marine cleanly washed, 35 m Wond ring and Up to 5 m of marine and 30 .~ of nonmarine (P, V, ~ Hills poorly aorted, stratified others (1946), sediments. higher sand and gravel with some Yerkes and Terrace and terraces silt and rubble. others (1965) nonmarine rubble, gravel, unnamed coaa tal and sand ca terraces. depoe its I Coastal Silt, clay, sand, and some ,2lO•m Poland and Found in Wilmington area, part of Long water wells gravel of fluvial origin . . others (1956) Beach plain, under Sunset Gap, and beneath Huntington Beach Mesa. In Bolaa Island, drill holes (Bechtel, 1967), strata na~ed Palos Verdes Sand may be correlative vith these sediments. Divided into three members: I 185m IWoodring and Members may be facies in. the San•Pedro Fm. (1) San Pedro Sand - cross- (90 m) others (1946) Nonetheless San Pedro Sand is always In bedded sand with gravel upper part. silty sand and silt. P.V. Hilla 1(2) Timms Pt. Silt - Silt (40 m) and silty sand. I San Pedro Fm I I (3) Lomita Marl - cal- (85 m) careous sand

Off Bolaa Poorly sorted, coarse sand, 270 II! Bechtel (1967) Base of "A" horizon on Figure Con- Chica State and gravel. drill holes @ formable with ·P. V. Sand above. Beach -44 to -72 m (top) -244 to -305 m

Coastal Silt, clav, and sand and 270 Ill Poland and Correlative but thicker and more hetero~te­ water wella gravel. others (1956) neoua than on P.V. Hills. Off Newport Poorly sorted coarse 180 m Lee (1977) Strong anguar unconf~rmity at baae.· Beach strained sand and stravel. Soft,.massive, alauconitic 50 m Woodrinj! and · Greatly abbreviated on the hills, compared P.V. Hips foraminiferal siltstone others (1946) with 1,200-1,500 m in L.A. Basin. Diaconformable vith underlying Malaga Mud­ atone Coastal Alternating beds of fine Wilmington issler (1943) In ~limington oil field Repetto and oil fields to coarse, loose to compact, 350 m Puente Fm below are conformable (Hayuga, Repetto Fill occasionaly pebbly, poorly Seal Beach (1970). cemented aand and sandy 830 m In Huntington Bch. field Repetto and - ___ ,r ___ ... , ... (A 11 and clay a tone a. l:)eacn; 6i'Om In Sunset Beach they are {Hazenbush and Allen, 1~ 5 ...... I off West NewPort Beach 1L~~ Alternating beds of fine 1200 m Junger and San Pedro Wagner (1977) basin and to coarse, loose to compact, Wilmington occasionally pebbly, pooriy graben cemented sand and sandy micaceous shales, siltstones and clava tones. - Unconformablv overlies Repetto Fm In Wilminston Wissler (1943) Fine to coarse, occasionally Wilmington, Sunset Bea~h, anci. Huntington 360 m Coastal pebbly, gray sand with olive Beach Oil Fields (Mayuga, 1970; Hazenbush gray and olive brown mostly Seal Beach and Allen, 1958; Allen and Hazenbuah, 1957) oil fields 830 m Pico F!!J soft, massive, claystone and Absent in P.V. Hilla. siltstone. Huntington Besch: -c;1o m Wagner and Junger Woodring and On the San Pedro shelf Divided into three members: {1975) estimate a thickness of 1300 m. others ~1946) Malaga Mudstone - Radiolarian 12ZO m P.V. Hilla mudstone Valmonte Diatomite - Diatomit• , diatomacroua shale, and diato Monterey maceoua mudstone. Shale Altamira Shale - (divided intc " 3 a ub-unita) • Silty, sandy, cherty, porcelaneous, phos~. pha tic, and chert and limestone. ! Present in Wilmington, Sunset Beach, and 2,000 Ill Wissler (1943) Coutal Organi~ ahale, aand and achiat Huntington Beach oil fields (iee Pica Fa. .t 1 fieldl bearing rongtomerates at the for sources). base. Platy shales, silty Submarine-fan

~~

N 0\ 27

Schoellhamer and Woodford (1951) formalized this name. These rocks may be correlative with the Franciscan assemblage (Reed, 1933).

QJnderlying the San Pedro margin, Western Geophysical Company

(1972) mapped an acoustic basement surface, which may be the top of

Catalina Schist;) at depths of 1 to 2.4 seconds (two-way reflection ·-~...t travel-time) from sea level. This surface corresponds to the base of sediments 'with interval velocities of 3,660 meters per second compared with velocities of 4,570 meters per second below this unit. {~ore-hole data indicate that off the Santa Ana River, basement is encountered

2,730 m below the seafloof) (Lee, 1977).

An area immediately ~eaward of Pt. Fermin, Palos Verdes Peninsula, is underlain by an acoustically dense and opaque seismic unit which the author presumes to be Catalina Schist or volcanic rock~) (Plate 2). It locally crops out on the seafloor and elsewhere is covered by a thin veneer of unconsolidated sediments (Plate 3). This unit was also identified as Catalina Schist by Junger and Wagner (1977). Figure 5 shows possible buried Pleistocene marine terraces cut into these base- ment rocks.

MIOCENE ROCKS

~verlying Catalina Schist on the Palos Verdes Hills is the Monterey

Shal~(Woodring and others, 1946). This formation is divided into three members: the Altamira Shale, the Valmonte Diatomite, and the

Malaga Mudstone.

Dredge hauls by Moore (1954) and oil industry jet and dart cores, as shown in Nardin and Henyey (1978) (Plate 2), indicate thaturocks of the Monterey Shale crop out on the outer San Pedro shelf, southwest of Figure 5

CSUN 3.5 kHz Line 5088- Catalina Schist (?) and buried marine terraces 29

LINE 5088

(FIX MARK) NE 6 5 4 sw

BASEMENT CATALINA SCHIST(?) (Bx) MULTIPLE

~0 ms.= 8.5 m. ~150m. ~

FIGURE 5 30

the Palos Verdes fault~ Nardin and Henyey assigned California Benthic stage names to these samples, based on lithologic and, where possible, peleontologic parameters. Recent work on planktic foraminiferal zona­ tion shows that benthic stages are time transgressive, and in particular the Delmontian stage transgresses the Miocene/Pliocene boundary. Thus, the upper portion of the Monterey Shale assigned to this stage, may be

Pliocene (Berggren and Van Couvering, 1974; Boellstorff and Steineck,

1975). Based on the cores, Plate 2 shows Mohnian rocks immediately south of Pt. Fermin separated to the southeast by Repettian rocks.

These Repettian rocks are in a structural saddle just west of the head of San Pedro sea valley, followed by Mohnian and then Delmontian out­ crops north of this valley. (Most of the outer shelf, to the southeast, is Repettian based on a majority of silty-shale samplesJ

The writer mapped the distribution of several seismic units on the

Palos Verdes uplift (Plate 3). Where they crop out on the seafloor, these units were correlated with the dart-core data, and the contacts between them were inferred from offshore extension of unconformities described on land. Seismic reflection data were USGS R/V KELEZ Uniboom, and CSUN 3.5 kHz tuned-transducer profiles. The data show a folded, rhythmically bedded unit as shown by highly reflective, moderately spaced (roughly 10m, 20 msec. ave. two-way reflection time), parallel reflectors with intermittent higher-amplitude reflectors (Figure 7).

This ~eismic signature is suggestive of the Monterey Shale and is mapped throughout most of the Palos Verdes uplift~ A contact with the

Repetto Formation is indicated by the dart-cores in the central-outer shelf is not readily apparent. Junger and Wagner (1977) .(profile m-m 1 ) infer this folded unit as Monterey Shale entirely. It should be pointed 31

out, however, that ~he Monterey (Puente) and Repetto contact, though unconformable in many areas, is disconformable on the Palos Verdes ' Hills, with no marked discordance'~) Thus the contact may not be easily identifiable on seismic-reflection records.

dn the vicinity of the Palos Verdes fault zone, this folded seismic unit is unconformably in contact with lower Pleistocene deposits (Fig- ure 6) and also commonly in fault contact with younger units, as the

Palos Verdes fault has juxtaposed it against upper Pleistocene strata

(Figure 6). Along the basin slope south of the Palos Verdes uplift, the folded unit is unconformably overlain by thin Pleistocene sediments~

PLIOCENE ROCKS

@ocks of Pliocene age in the study area have been assigned to the

Repetto and Pica formations (Woodring and others, 1946; Wissler, 1943)~

Though the name Repetto is not formalized, it was used by Junger and

Wagner (1977) for their study of the San Pedro basin, and in conformity with petroleum geologists (Wissler, 1943), it is used here.

@n the Palos Verdes Hills the Repetto Siltstone has an exposed thickness of 46 m compared with 1200 to 1500 m in the Los Angeles basi~

(Woodring and others, 1946). This greatly abbreviated section, which lies disconformably on the Malaga Mudstone, may have at one time covered most of the hills. Presently, however, (fleistocene rocks lie directly on Miocene strata in many areas. Deposition in the central Los Angeles basin continued throughout the late Miocene into Pliocene time; however, local unconformities resulted from uplift around the basin periphery during Pliocene time (Junger and Wagner, 1977; Yerkes and others,

1965). The complete absence of upper Pliocene (Pica formation) strata 32

in the Palos Verdes uplift suggests that the strongest period of local

deformation occurred after deposition of Repetto strata (Woodring and

others, 1946; Conrey, 1967~~~

Coarser-grain size, abundance of glauconite grains and foraminifera,

and scarcity of radiolaria and diatoms are used as diagnostic indicators

by Woodring and others (1946) to differentiate the Repetto Formation

from the Malaga Mudstone. However, these authors warn that in outcrops of small extent these differences may not be readily apparent. ~rincipal­

ly, biofacies, radiolarian content, and massive lithology from the dart

cores were used by Nardin and Henyey (1978) to map Repettian rocks

along the outer San Pedro shelf, surrounding the anticlinal core (Palos

Verdes anticlinorium) of Delmontian and Mohnian strata. However, as predicted by Woodring and others, the recognition of the Repettian rocks on the San Pedro shelf is uncertain~ The writer mapped a seismic­

stratigraphic unit(2n the shelf edge and slope from the San Pedro Sea

Valley to the Palos Verdes fault in the vicinity of the San Gabriel

submarine canyon (Plate 2);) This unit is characterized by a lack of distinct internal reflectors, @uggesting massive lithology of Repetto

Siltston~J Throughout the area, it is relatively undeformed and rests unconformably on folded Neogene rocks. Emery and Shepard (1945), Moore

(1954), and Jennings (1962) report izepetto strata on the north flank of the San Pedro basin~ Junger and Wagner (1977) suggest that(this uncle­ formed unit represents the earliest post-Miocene deposit in the San

Pedro basin prior to steepening of the basin flanks) According to the dart-core data however (Plate 2) the undeformed unit in question is interpreted as Pleistocene slope deposits, and Repetto strata must have been involved in folding during the strongest period of local deformation. Figure 6

USGS Uniboom line 453- Palos Verdes fault zone t 34

,?!? -·- •.

LINE 453

sw (TIME FIX) (TIME FIX) NE

PALOS VERDES PALOS VERDES UPLIFT ,_.... - FAULT ZONE SCARP (0.75 m.l ..... __ I IP II ~~ '-LJ..!'i\1-~ - r--- LP~ Ill .:S"I::::=-1 LP IV(?) ~ 'o 1 LPI 11~ 'LPI IV- ~ i ~~ M -M "MULTIPLE I10 m s. =8.5 m. -150m. L,___,J

FIGURE 6 Figure 7

CSUN 3.5 kHz line 5048- Rhythmically bedded Monterey Formation 7 36

------+------·-----·------. •

LINE5048

(FIX MARK) SW 11 NE

SEAFLOOR

Tm

Tm F MULTIPLE Io ms. =8.5 m. -150 "m. M- L.___J

FIGURE 7 37

(truex (1974, 1978) shows that the initial folding of the Wilmington

anticline began during late Miocene tim~and Pliocene over Miocene

erosional unconformities have been recognized off west Newport Beach,

Huntington Beach oil field and Sunset Beach oil field (Lee, 1977; Allen

and Hazenbush, 1957; Hazenbush and Allen, 1958). Taking these facts

into consideration, it appears that~olding in the region was probably

initiated during Monterey time and continued while Repetto sediments were deposited. As shown by Mayuga (1970) and Truex (1978), folding of

the Wilmington anticline ceased by middle Pica (Late Pliocene) time.

The surface separating folded rocks and the undeformed unit on the

outer San Pedro basin shelf may be partially correlative with the

surface between middle Pico and Repetto formations in the Wilmington

oil field (Truex, 1978). However evidence of continuing emergence of

the Palos Verdes uplift indicates that deformation of this block has

continued, or was renewed sometime in the Pleistocene Epoc~J(see Struc­

ture chapter).

PLEISTOCENE ROCKS

crfie lower portion of the Pleistocene section studied in the Palos

Verdes Hills by Woodring and others (1946) is represented by the San

Pedro Formation which unconformably overlies the Repetto Formation, or, more commonly, the Monterey Shale.' The San Pedro Formation is divided

into three members: the basal Lomita Marl, followed by the Timms Point

Silt, and the San Pedro Sanqj

In shelf areas of San Pedro Bay, Santa Monica Bay and offshore

Oxnard Plain, Wagner and Junger (1977) considered the base of a cross­ bedded shallow seismic unit (San Pedro Formation) to be the base of the 38

Pleistocene. In this study, ~trata inferred to be members of the

lower to mid-Pleistocene San Pedro Formation are represented by several

seismic units mapped seaward of the Palos Verdes fault (Plate 2)~

Near the L. A. Harbor breakwater, USGS line 441 (Figure 8) shows an

angular unconformity between presumably lower Pleistocene (Lomita Marl

?) well bedded 2-7 m-thick strata which grade into a massive unit

(Timms Point Silt?), and folded Neogene rocks. The upper part of the massive unit shows some signs of cross-bedding (San Pedro Sand?).

By extending the unconformity below the seafloor multiple on Figure 8, an apparent thickness of 90 m can be estimated for the lower thinly bedded mamber. This thickness, based on an arbitrary sonic velocity of 1700 m/sec, agrees favorably with the maximum thickness for the

Lomita Marl as it is exposed on the Palos Verdes Hills (Woodring and others, 1946). On the hills, it occurs in marked discordance with

Miocene diatomaceous shale and diatomite on the south limb of the

Gaffey syncline which is along strike from this subsea locality.

Further east, USGS line 451 shows the Miocene strata in contact with a fairly massive unit (Timms Point Silt ?) with no marked discordance evident. On CSUN 3.5 kHz profiles, these contacts were identified chiefly by changes in dips attributed to the angular unconformity described.

\the San Pedro Formation is assigned to the Lower Pleistocene

Epoch by Woodring and others (1946)) based on fauna which is more modern than that of the Coast Range Pliocene formations. Recently, amino acid racemization has been observed in calcarious organisms

(Wehmiller, 1971), and was used to date various units along the Pacific Figure 8

USGS Uniboom line 441- Angular unconformity between presumably lower Pleistocene strata and folded Neogene rocks 40

~~ ::;.:;•. :-=::- -.. ;....·:::;::.~~~~~-:;;:¢~~7"'!:'::".;.~.,_:;;..~-;:;:;_ ... ~-:;;:..:;::--~-~-- -..-...-. - ..... --R-- --... - !_ I ";

LINE 441

(FIX MARK) sw 40 NE

PEDRO---­ FM . ?)

-150m. I1o m .s.- a5 m. ._____,

FIGUR E 8 41 coast (Wehmiller and others, 1977). Wehmiller and others sampled the

San Pedro Sand and Timms Point Silt members of the San Pedro Formation.

Since the temperature history of a sample strongly influences the

absolute age which can be obtained through amino acid racemization,

oxygen isotope measurements were made on samples from equivalent

faunal zones. Based on the mollusks Saxidomus, Protothaca and Chione,

Wehmiller and others (1977) obtained ages of 350,000 to 450,000 and

one million (±200,000) Y.B.P. for the San Pedro Sand and the Timms

Point Silt, respectively.

Butzer (1974) recently proposed the following chronologie division of the Pleistocene: lower (1.8 million to 700,000 years B.P.), middle

(700,000 to 130,000 years B.P.), and upper (130,000 to 10,000 years

B.P.). These divisions, adopted by Wehmiller and others, are also used by the writer. Thus, the Timms Point silt is assigned to the

Lower Pleistocene, though uncertainties of its thermal history allow a possible Early-Middle Pleistocene assignment (Wehmiller and others,

1977). The San Pedro Sand assignment to the Middle Pleistocene is more certain. Amino acid dates from Tegula, Epilucina and Polinices mollusks by the same authors produced an age of 450,000 (±100,000) years for the twelfth or highest terrace of the Palos Verdes Hills.

This age, correlative with the San Pedro Formation, makes this terrace older than presumed by Woodring and others, (1946) who considered it

Late Pleistocene. By their own admission, however, it is improbable that terrace deposits of lower Pleistocene age could be distinguished from those of upper Pleistocene age on faunal grounds. Thus, ~pe San

Pedro Formation appears to have, been deposited as one or more of the high Palos Verdes Hills terraces were cut~ 42

(S'eparating the San Pedro Sand from the Late Pleistocene Palos

Verdes Sand in the Palos Verdes Hills area is an unconformity)recognized as early as 1903 by Ralph Arnold in his description and nomenclature of Pliocene and Pleistocene strata of the San Pedro waterfront. hiatus is represented by deposits laid down on the upper 12 terraces of the Palos Verdes Hills, and presumably by unnamed upper Pleistocene deposits of Poland and others (1956) (see Table 2 and Figure 4). This period is generally recognized as one of intense tectonic deformation

(Pasadenan phase of Cascadian Orogeny)~

Just offshore of Huntington Beach, R/V KELLEZ Uniboom line number

455 shows a cross-bedded unit (Figure 9 and Plates 2, 5) unconformably underlying unit LP IV (see discussion of LP IV to LP I units, next chapter) . This unit is tentatively correlated with the "unnamed upper

Pleistocene" (mid-Pleistocene in this report) deposits of Poiand and others (1956). Valentine (1959) reports the presence of this unit on- shore in this vicinity.

Further west,C§echtel (1967) drill-hold data (Figure 2) indicate a conformable contact between the San Pedro Formation and the Late

Pleistocene Palos Verdes Sand; and similar conditions of deposition are suggested for the time span represented by both formations~

'.The fourth terrace of the western Palos Verdes Hills was dated~~) "- using amino acid racemization, as 120,000-140,000 Y.B.P. (Wehmiller and others, 1977). ~his date indicates equivalence with the lowest terrace and the Palos Verdes Sand in the town of San Pedro :J The

350,000 to 450,000 Y.B.P. date already cited for the highest (12th) terrace places ~he intervening platforms in the Middle Pleistocene~ Figure 9

USGS Uniboom line 455- Shoreward edge of the Wilmington graben, off Huntington Beach ( 44

LINE 455

(FIX MARK) sw 10 NE

_/SEAFLOOR

~ L. L~ II M / -M .....------==/ Qpl / -- MULTIPLE - / /------Qp2 (SAN PEDRO FM?)

--150m. JJo ms.-8.5 m. I

FIGURE 9 45

This age assignment would be suggested also for the unnamed "upper"

Pleistocene deposits of the nearby coastal area which lie stratigraphi­ cally below an unconformity at the base of the Palos Verdes Sand and above an unconformity over the San Pedro Sand. Amino acid dates also indicate that three major terraces between sea level and 90 m in the

San Joaquin Hills and Newport Beach represent the same time period as the twelve terraces on the Palos Verdes Hills (Wehmiller and others,

1977).

~h~ upper Pleistocene deposits of the Palos Verdes Hills are found on the lowest terrace in the town of San Pedro, and were desig­ nated the Palos Verdes Sand by Woodring and others) (1946). Upper

Pleistocene units are also found capping the low hills and mesas along the Newport-Inglewood structural zone (Poland and others, 1956).

In bore holes drilled by the Bechtel Corporation (1967) (Figure 2), a sandy gravel to conglomerate found 60 m below the seafloor was inter­ preted to be the basal conglomerate of the Palos Verdes Sand. As mentioned previously, Bechtel (1967) indicated a conformable contact with the underlying San Pedro Formation. The lower portion of the "Palos

Verdes Sand" in this location may be older than deposits of the lowest

Palos Verdes terrace, and the upper portion yielded a very recent date

(Table 4), suggesting a misuse of Palos Verdes Sand nomenclature.

(Jn the Wilmington graben, several late Pleistocene units were mapped by this author. Shown on Plate 2, these units designated LP

(Late Pleistocene) IV to LP I by Fischer and others (1977) are believed to be correlative with the Palos Verdes Sand and parts of Poland and others (1956) unnamed "upper" Pleistocene deposit~~:) The following chapter discusses the seismic stratigraphy of these units. Chapter 4

SEISMIC-STRATIGRAPHIC MODEL, WILMINGTON GRABEN LATE PLEISTOCENE AND SHELF HOLOCENE UNITS

INTRODUCTION

In order to assess geologic conditions in the area of Shell Oil

Company's "Beta Prospect" on the San Pedro basin margin, the writer was involved in a detailed study of the area (Fischer and others, 1977)

(Figure 2). A detailed geophysical survey was conducted for this investigation, which included a dense grid (approximately 80 m x 80 m) of Sonia II high-resolution seismic-reflection profiles and slightly wider spaced Uniboom and sparker data. In addition, three core-holes

(soil borings) were utilized. From this information stratigraphic, lithologic, and paleontologic interpretations have been formulated, and serve as a basis for a seismic-stratigraphic model of the late Quater- nary evolution of the San Pedro basin margin.

Shell Oil Company core-hole (CH) 261-1 was drilled to a depth of

141 meters below the seafloor on the outer San Pedro shelf, west of the San Gabriel submarine canyon. Core-hole 261-2 is located 107 meters southwest of 261-1 and only penetrated 19 m of section. Core hole 262-1 was drilled to-152 m on the basin slope, 1.4 km southwest of

CH 261-1.

(tares 261-1 and 261-2, both on the shelf, penetrated approximately

13m and 11 m of unconsolidated Holocene sediments, respectively.

These flat-lying sediments containing gastropods, molluscs, and a mid- neritic foraminiferal fauna of Holocene age (Bergen, 1977 in Fischer,

1977) unconformably overlie a sequence of sands and gravels~

46 47

~ands and gravels below the Holocene sediments are interpreted to

be fluvial channel deposits~ This is based on lithology and their form

/ and distribution as interpreted from the acoustic profiles (Figure 10

and lOa). They are barren of microfossils, are believed to be late

Pleistocene, and are designated LP II. The basal Holocene unconformity

can be traced on seismic reflection profiles southeastward from the

shelf toward CH 262-1, on the basin slope. The unconformity becomes

heavily channeled, tilts southeastward abruptly beyond a paleo-shelf

break, and deepens to 25 m below the seafloor at CH 262-1. However,

faunal evidence from CH 262-1 (Bergen 1977b in Fischer and others,

1977), including benthic foraminifera and coiling ratios of the planktic

Neogloboquadrina pachyderma, places(}he/' Holocene/Pleistocene boundary at

least 15 m below the seafloor, well above this surface. Thus, an

intervening unit of slope sediments, designated LP I, must have accumu-

lated beyond the postulated paleo-shelf break after LP II and before

Holocene time. Core-hole 262-1 and seismic-reflection data indicate

that deposition on the slope was essentially continuous throughout this

LP !-Holocene period) Paleo-bathymetry during this time was outer

neritic (Bergen, 1977 in Fischer and others, 1977).

SEISMIC-STRATIGRAPHIC MODEL

tthese relationships indicate that sea level was at a low-stand

during the initial deposition of LP I. The shoreline was near the

paleo-shelf break when the LP I channels were cut. This was followed

by a transgression which first beveled the Holocene shelf surface, and

later resulted in deposition of the flat-lying Holocene sediment~

(Figure 12). On the basis of core-hole, Uniboom, and sparker data from Figure 10

Diagrammatic section along Shell Oil Company Uniboom line 103

Figure 11

Diagrammatic section along Shell Oil Company Uniboom line 210

(Fischer and others, 1977) z 0 0 >­ w z a:: <( ::> u C) w -I z I u.. w a: I a: <( m ::E I <( m I I C) ::> VI I I I I 0 0 -o -!£ I I I I I I r/ I ,I 0 -!! l/)/1 //

0 I ~I I ~ I / l I ~ I ..? I I

0 ""' (J -~ "1"11::r:: ""' I I I 1'\~ I ~'/ / / ~I I 1 ~ I I .... ~ I I I I _,0 I I It::! I II _..,0 /J~~" <"-·... -~ I <'-• ~ "'J ·~ ! / 0 I l:l -~ Q. 1'- .. .;j) I "'- ~ 0 -::t j I ~ '9)0/ !~ l:l I ~ ~ ~ I ., U) w 0 w J , m 0:: z 0 w 0 > N -on ~) f ~ ~ 8 U) 1- I I I { 0 -I -I ::> <( J Q ~ LJ._ ""~ i 2 it a..

E Figure lOa

Shell Oil Company Uniboom line 103

Figure 11a

Shell Oil Company Uniboom line 210 51

FIGURE lOa

~ - - -- ; . . I · ; I ' 1 . 1- . .

1 ! ·¥ :-; W¥% ·5tBS3!1ffi·©?i*HihiM1

FIGURE lla Figure 12

LPI/Holocene conceptual depositional model 53

OXYGEN-ISOTOPE STAGE 2 (18-20,000 Years B.P.)

Debouching. of sediments. F"_IU\flal channels cur. it ~helf-break. "WISCONSINAN" High secliment influx. / LOW-STAND

LP II

OXYGEN-ISOTOPE STAGE 1

(@-5,000 Years B.P.) I SEA LEVEL AFTER FAST (POST -"WISCONSINAN'~ RISE

LP 1 channels filled 1 Sea transgresses channels andcuis base Holocene filled sheif unconformTty·:· Deposition of Holocene -unit.

FIGURE 12 54

the Beta Platform study, LP II and two older units (LP III and IV) can be defined. On the Uniboom seismic-reflection data (Figures 10, lOa and 11, lla) units LP II to IV are defined by basal unconformaties.

~e base of LP II is an unconformity over a flat surface which increases in dip abruptly. These surfaces are interpreted as paleo-shelf and paleo-basin slope~ The Uniboom profiles show channeling below the paleo-shelf, much as those below the Holocene unconformity 0 Slope gullies analogous to those in LP I are suggested by high amplitude reflections (channel-fill) along the LP II paleo-slope. The sparker profiles reveal at least two older (LP III and IV) unconformities, but channeling cannot be resolved with this system. Table 3 summarizes the lithology of these units as sampled in the core-holes.

This author used CSUN and USGS data (see CH II, Methods and Proce- dures) to(map,_ the extent of LP I-IV on the San Pedro basin margin

(Plate 2). Their distribution and lithologic and seismic character- istics suggest that they are each genetically analogous to the Holocene-

LP I sequence, and represent progradational shelf-slope units. These units appear to be net offlapping sequences, presumably laid down by a

\ series of regressions and transgressions which extended over the shelf.

These oscillations resulted in progradation south and southeastward from nearshore toward paleo-shelf breaks) Each of these units can be designated a seismic-stratigraphic "depositional sequence," and are analogues of the depositional sequences of Mitchum and others (1977).

By definition, a seismic-stratigraphic depositional sequence is composed of a conformable succession of genetically related strata and is bounded at its top and bottom by unconformities or their correlative conformaties. Table 3

LP IV to Holocene stratigraphic section 1,000 AGE YRS (CALIF, BENTHIC 018/016 THICKNESS WITHIN B.P. STAGE) STAGE UNIT/DESCRIPTION STUDY AREA (from _l_FIG, 13 _i_FIG. 4) (FIG. 13 _tfrom CH261-1,2; 262-1, FIG. 2) CH261-l. 262-ll SEISMIC CORRELATION AND COMMENTS

0 HOLOCENE Basal silty, very fine to fine 0-55m Baoal angular unconformity on outer grained olive gray sand grading shelf. 1Weak basal unconformity on (HALL IAN) to silt and silty clay and then inner shelf correlated with Bechtel to sandy silt or very fine sand, (1967) drill-hole data. Conformable with Pleietocene strata on basin slope, Base at- 15m on CH262-l. Fifty-five meter thickness includes canyon deposita.

-10-11--j r"'""""'""' UNCONF. 2 LP I - Basal 3m-sand which 0-30m Basal channeled unconformity, top is constitutes part of conformable with overlying Holocene channel fill in slope- sediments. Only present on basin gullies. Grades to silt slope. Base at - 25m in CH261-2. -2~ r•cououo• .,,,. UNCONF ..,,,.,,,,,,.,,.,., 3-4 LP II - Base ia clay interval at 0-40m Basal channeled unconformity. 50m in CH261-l. Sand and Sand and gravel probably constitute gravel probably constitute fill. Correlative with the loweot fill in basal channels. (1st) P.V, Hilla Terrace, Silt with thin conglomer- atea extend upward to 25m and another aeries of sand and conglomeratic channel fill to 13m on CH261-l. The correlative interval i:i"' in CH262-l ia silt and g silty clay. E-o -3<>-----j ~ UNCONF. "'H "',..., 5-6 LP III - Base ia gravel bed about 0-60m Basal channeled unconformity. Baoe "" ~ 2m thick at - !!Om in at- 110m in CH261-l. Channels prob· "'E-o CH261-J. Another gravel ably filled with sand and gravel are :i bed with clayey silt and evident within the unit on acoustic sand matrix is recognized data. Correlative with lowest at about -70m in CH261-l terrace in San Pedro and the 4th (and possibly 3rd and 2nd) P.V. . Hilla Terrace . -14~ I 7-8 r"•'"'"LP IV - •• Silty"'" UNCONF. clay to clayey silt. 6-3lm Only upper portion of this unit was penetrated, Total post San Pedro section not penetrated ia J00-375m.

'0\::-

\J1 0\ 57 f .

Mitchum and others relate these sequences to global cycles of relative sea level changes. These units have chronometric significance as they were deposited during a given interval of geologic time limited by the ages of the sequence boundaries where they are conformities.

LATE QUATERNARY CHRONOLOGY

The evaluation of the glacial/interglacial fluctuations of the

Pleistocene epoch has generated intense debate and confusion. Although

Lyell did not originally define the Pleistocene Epoch on the basis of glaciation, it is generally associated with continental glaciation.

The debate as to the absolute ages and global correlation of glacial advances and retreats becomes increasingly confused and complex as the body of knowledge increases. Butzer (1974) presents an excellent summary of these considerations and concludes that Pleistocene stratig­ raphic boundaries must be tied to chronometric horizons. For North

America, Boellstroff (1978) demonstrates that the application of glacial­ stage terms in widely separated localities has resulted in grossly overlapping time spans for most of the glacial stages.

The oxygen isotopic record of deep sea cores provides an excellent chronologie division of the Pleistocene epoch. Emiliani (1955) and

Shackleton and Opdyke (1973) used 01 8/016 isotope abundance ratios as an indicator of ice volumes and sea levels during the Pleistocene Epoch

(Figure 13). ~y using magnetic stratigraphy and average sedimentation rates, they were able to define 22 oxygen isotope stages which represent alternating high and low Northern Hemisphere ice volumes:) Butzer's chronologie division of the Pleistocene places the late/middle boundary at the base of the last "Eemian" interglacial, as defined by Shackleton Figure 13

Marine 0 18 /016 isotope curve and correlations (modified from Wehmiller and others, 1977) LP I channeling. Final trenching of Newport-Inglewood gaps.

Slight aggradation Newport-Inglewood gaps.

Coiling ratios of Neogloboguadrina pachiderma change-shoaling begins. Initial trenching of Newport-Inglewood gaps.

paleobathymetry CH 262-1

0 0 0 ~

~ co-­ 0 cc LP I)LP 10 LP 111) 1.04-.-.-. ? ? ? UPPER de.ooo MIDDLE PLEISTOCENE 700,000 LOWER PLEISTOCENE PLEISTOCENE YRS. YRS. ~~------J Lowest {1st) 2nd-3rd P. V. 5th-12th P.V. Hills terrace _..,. .,. ______Timms Point Silt. Palos Verdes (P.V.) Hills terrace. San Pedro Sand Hills terrace. ------...... Palos Verdes Hills \. Palos Verdes Sand 1 2th terrace. 4th P.V. Hills terrace 1st terrace in San Pedro FIGURE 13 60

and Opdyke's oxygen-isotope stage 5e (125-130,000 years B.P.). The

middle/early boundary is placed at 700,000 ±years B.P., coincident

with the Brunhes-Matuyama geomagnetic reversal. Fission-track dating

of ash-bearing silts below the "classic Nebraskan" till, near Afton,

Iowa, yielded a date of 2.2 x 106 years (Boellstorff, 1978). A glacial

till below this ash is correlated with the first Neogene cold cycle

recorded in the Gulf of Mexico. A similar date (2.5 x 106 years) was

obtained from the stratotype for the Plio/Pleistocene boundary, the

Vrica Ash .at Crotone, Italy, which occurs between definite Pliocene

and definite Pleistocene sediments (Boellstorff, 1978). Thus, the

early Pleistocene Epoch spans the period from 700,000 to approximately

2.5 million years B.P., and(~laciation probably occurred not only

throughout the Pleistocene, but well into the Pliocene Epoch (in the

Northern Hemisphere)~ ./ As previously stated, the San Pedro and Palos Verdes Sands have been dated by amino acid racemization in the general onshore vicinity of this study area (Wehmiller and others, 1977). (The date cited for

the San Pedro Sand - 350,000-450,000 years B.P. - provides a lower age limit for the LP IV and younger units. Stratigraphically, LP IV over- lies the San Pedro Sand. This correlation is supported biostratigraphi- cally as LP I to LP IV are assigned to the Hallian California benthic stage as is the Palos Verdes San~) (Bergen, 1977 in Fischer and others,

1977). The cited amino acid date for the Palos Verdes Sand at the type locality is 120-140,000 years B.P., and provides a time frame within which LP I to IV can be placed. However, the Palos Verdes Sand has yielded a wide range of ages (Table 4) and a misuse of Palos Verdes nomenclature is evident. Wehmiller and others correlate the San Pedro 61

Table 4

Age-dates of San Pedro area units and marine terraces ·-- ,, ABS, AGE METHOD AUTHOR AND OJa/016 UNIT/TERRACE (Y.B.P.) AND SOURCE LOCATION STAGE COMMENTS

Palo~)Verdea 23,300 (!2,200-2,600) CJi• (wood) Bechtel (1967) · (P.V. Sand DHI02@ - 99 1 ~.v. Sand 32,700 (t2,000) Cl't (shells) Bechtel (1967) Reworked . OHIO!@ - 52' P.V. Sand (1) 69,000 (!7,000) Uranium series Szabo & Vedder (1971) Lowu t terrace closed ays tem Ml477 Laguna Beach & (shells) Newoort Beach P.V. Sand (?) 86,000 (!9,000) Uranium series Szabo & Rosholt (1969) Type local! ty Lowest San Pedro open s~~tem 1. 6 km SW of M5908 terrace (~hells P.V. Sand 91,000 (!15,000) ? Yerkes & Wentworth (1965) Lowest P.V. Hills Lowest P.V. terrace tt!rrace P.V. Sand 120,000-140,,000 Saxidomus Wehmiller and others Early stage 5 Corr. with Pt. Lama Lowe•t San Pedro (mollusk) (1977) M5908 & M2074 Nestor terrace & 4th P.V, terrace Amino acid terrace further W. Tvoe locali tv 4th P.V. Hilla 120,000-140,000 Tegula, Epilucina Early a tage 5 Based on ,similarities terrace (mollusks) with Newport Mesa & Amino acid Corona D.M. Terraces higher Wehmiller and others 7 Carr. with 3 terr. between than 4th (P.V. (1977) sea level and 90 m in San Hilla\ Joaquin and Newport Beach San Pedro Sand 350,000-450,000 Tegula, Epilucina, Wehmiller and others 9-11 (12th-highest (tlOO,OOO) Polinices (molloaks) (1977) P.V. Hills ter- Amino acid race\ 12th-(higheat) 3().0,000 Uranium series Szabo & Vedder (1971) P.V. Hilla ter- closed system race Timms Pt. 1,000,000 (t200,000) Saxidomus, Protothaca Wehmiller and others Silt ~nione, Xmino aero 11977\ Malaga Mudstone 3,500,000 Fission-track Boells torf an'd Age disputed by Bandy 4,500,000 Upper and Lower Steineck (1975) (1972) and Wehmiller and Malaoa Ash others l1977) Altamira Shale 14,500,000 K-Ar• Turner (1970) (tl 100 000\ Portu•uese Tuff

l - ,j

0"1 N 63

Sand onshore with stages 9-11 of the o18;o 16 curve, and the Palos

Verdes Sand with early stage 5 (Figure 13). Based on suggested correla- tions with the oxygen-isotope curve, (1P IV,is older than the Palos

Verdes Sand (Figure 13) and LP I and II are younger.

The latest Pleistocene (LP I)-Holocene depositional cycle provides an upper age limit for the late Quaternary chronology of the San Pedro basin margin0 A correlation of the Holocene-LP I sequence with oxygen- isotopic stages 1 and 2 is based upon:

(1) the Climap project report of a warming trend in deep sea cores that began 18,000 to 20,000 years ago (Mcintyre and others, 1976);

(2) Microfaunal evidence from core hole 262-1 (Figure 2)

(3) Bloom and others' (1974) estimate of a low sea level stand of -125±5m at this time (end Wisconsin). A paleo-shore­ line, near the position of the LP I paleo-shelf break, which marks the up-slope termination of the LP I channels would result from this low-stand (Figure 14); and

(4) onshore, C14 dating of landslides in the Palos Verdes and San Juan Capistrano areas, which have yielded ages of 16,000 to 20,000 years B.P. (Stout, 1977).

~liding, according to Stout, correlates with widespread erosion during the maximum low-stand as a result of lowered base levels and a wetter climate. The increased sediment load and the debouching of streams at the top of the paleo-slope would explain the LP-1 erosional channels. As the climate warmed and sea level began its rise to present- day levels, silts of LP-I age filled the channels and coastal streams aggraded. This regressive-transgressive cycle is the conceptual model for the older Pleistocene units (LP II to IV)~

In order to develop a chronology for the late-Quaternary evolution of the San Pedro basin margin the writer proposes a correlation of the Figure 14

LPI Paleochannels and paleoshelf-break within the Shell Oil Company "Beta Platform Sites" survey area 65

eLONG BEACH

\ ·.. '\ FIG. AREA NEWPORT BEACH

.. l· •••

0

Paleo Shelf- break

o'b 0 .....(:, '

Slope Gully

I \ I / / I

~oo' L__...! 1~0 m

t Gully AJ(is Q Platform Site(s) FIGURE 14 66

Holocene/LP I to LP IV units with the 01s/016 oxygen-isotope curve.

The correlation is based on the lower age limit of the San Pedro Sand,

correlated with stages 9-11 by Wehmiller and others (1977). The upper

age limit discussed places of Holocene/LP I within stage 1 and 2,

respectively. Within these constraints, correlation of LP II-IV with

the oxygen-isotope curve is speculative. If the regressive/transgres­

sive genetic model (Figure 12) for these units is correct, each of them was deposited during a cold/warm cycle of the curve. However, the varying degree of resolution of eustatic events reflected by the stratig­ raphy is an uncertainty. Non-uniform rates of sedimentation and subsi­

dence in the Wilmington graben, as well as varying amounts of erosion, may result in a more complex correlation. The following is a provi­

sional time sequence which will require refinements by further age dating. The ages presented are modified from those of Shackleton and

Opdyke (1973) for the respective oxygen-isotope stages (Figure 13).

Unit 01s/016 Stage Age (K Years B.P.)

(1) (a) Holocene 1 0-10

(b) LP I 2 10-20

(2) LP II 3-4 20-30

(3) LP III 5-6 32-140

(4) LP IV 7-8 140-230 (?)

Since LP IV overlies the San Pedro Sand, it is correlated with the next younger major cold/warm cycle (stages 7-8). LP III is correlated similarly. LP II is thinner than LP III and is believed to have been deposited during the brief stages 4-3 cycle. By this time the Newport­

Inglewood hills and mesas were well developed and a period of incision 67 of the fluvial gaps between them (Poland and others, 1956) may have provided enough sediments to deposit the unit during this relatively short cycle~~

LATE PLEISTOCENE TO HOLOCENE SEISMIC STRATIGRAPHY

LP IV

(Qver the San Pedro shelf:)outside the Beta Platform area, this unit is recorded on R/V Kelez Uniboom lines. On this data, (£p IV appears below a widely correlated reflector that marks the unconformity at the base of LP III} LP IV displays very little internal seismic character. Crhe Palos Verdes fault truncates this unit along its south- western mapped extent (Figure 6) where it is juxtaposed against lower

Pleistocene (San Pedro Formation?) and Monterey Formation rocks) Along the northeastern portion of the map, the base of this unit is a very high-amplitude reflector, believed to be an unconformity over middle

Pleistocene strata, (Poland and Piper's, 1956, "unnamed upper Pleisto- cene deposits," QP, in Figure 9), or over the lower Pleistocene San c Pedro Formation. ~n this area, LP IV is overlain by Holocene sediments and locally is exposed on the seafloor (Figure 9, Plate 2). The Balsa

Island report (Bechtel, 1957) states that upper Pleistocene vertebrate remains have been found embedded in the seafloor in this vicinity pre- sumably in unit LP IV. Plate 3, (LP I-Holocene Isopach) shows these outcrops partially delineated by 0 meter contours off Seal Beach, sea- ward of the Balsa Island drill-holes, and off Huntington Beach. ~ela- tive upward motion on faults cutting principally Pliocene beds along the northern edge of the Wilmington graben results in the exposure of J 68

LP IV in this area, as well as for pinching out of younger late Pleisto-

cene units. Seaward, LP IV is progressively overlain by LP III, and II

(Figure 15, Plate 2) and beyond the modern shelf break north of the

Palos Verdes Fault, by LP III, II, and I. Generally, LP IV is thin

along the northeast portion of the shelf, it thickens south and eastward,

and thins against the Palos Verdes fault:) /""""'-< ...... ~

LP III

~hree seismic stratigraphic facies within LP III]were mapped by

Fischer and others (1977) in the CH 261-1 vicinity (Figure 2), from

seismic character and geographic distribution of apparent gravels:

(£acies I - Conglomerate filled channels; Facies II - Sand filled

channels; and Facies III - Silt-sand filled channels. Generally, these

facies appear to produce progressively lower amplitude reflections in a

southeasterly direction. This suggests thinning of the gravels and

decrease in grain size, ·consistent with a probable overall facies

change from fluvial/non-marine, mid-neritic, to outer neritic/upper

bathyal environments;_) from CH 261-1 to 262-1 (Bergen, 1977a and b in

Fischer and others, 1977). LP III in the latter core hole is character-

ized chiefly by silt.

lon the San Pedro basin shelf LP III is a flat, relatively thin unit which increases in dip abruptly at a paleo-shelf break, north of

. '"\ the Palos Verdes fault (Plate 2)./ Near the northern branch of the San / Gabriel canyon this paleo-shelf break is under the modern shelf break

(Figure 16, Plate 5), whereas between the two branches it is more gradual and considerably shoreward. On the shelf,(the southwestern limit of LP III is the Palos Verdes fault~·\ Figure 6 shows offsets J Figure 15

USGS Uniboom line 455 LPIV-III-II-Holocene superposition and LPII marine terrace 70

LINE 455

sw (TIME FIX) NE

SE~LOOR

HOLOCEN£ ~ //_./~ LP Ill . LP IV --LP II M --~IMULTIPLE

--150m. I1o m..s.- 8.5 m. L.___j

FIGURE 15 71

~long the fault on the base of this unit, which is an unconformity over '-- LP IV.) This unconformity forms a relatively flat surface on the shelf with a very slight southeasterly dip and few undulations. (Generally,

LP III thickens to an average of 13 m northeastward, away from the

Palos Verdes Fault. The unit thins again at the northeastern edge of the Wilmington graben,'where its base tilts upward and Holocene beds partially truncate it (Figure 15, Plate 5). Off Huntington Beach and near the Bolsa Island site, LP III is partially exposed on the seafloor

(see Plate 3, 0 Holocene patches). This unit is also relatively thinner where it is overlain by LP II (Plate 5), indicating erosion after deposition of LP III by a transgressive pulse at the beginning of LP II time~ However, this second order thinning is superceded by overall thickening in a southeasterly direction, from the Los Angeles-Long

Beach Harbor breakwater where it pinches out, to the San Gabriel sub- marine canyon. In the vicinity of the canyon, LP III displays distinct internal acoustic character in the form of discontinuous high-amplitude reflectors inclined at apparent dip angles slightly steeper than the base of LP II. These reflectors are alternately comprised of three, and more commonly two distinct "legs" and are associated with Facies I and II (conglomerate and sand-filled channels) of the Beta Platform investigation. With Kelez Uniboom data, these reflectors can be mapped to an area near shore, off Huntington Beach, suggesting a possible fluvial source from the present land surface, perhaps an ancestral

Santa Ana River.

LP II

~ased on seismic data the base of LP II is an unconformity over 72

LP III, locally channeled, with sand and gravel probably constituting part of channel fi~V (Figure 10 and lOa). In mapping these features

Fischer and others (1977) show three linear bands 90 m wide, west and east of CH 261-1. Apparently these bands of high-amplitude reflectors are slope gullies analogous to those mapped by this author on the base of the Holocene-LP I unit.

(flate 2 shows the extent of LP II on the San Pedro basin shelf.

The limits of this unit are similar to those of LP III, but slightly seaward. LP II is a relatively thin unit over the shelf, and is under- lain by an unconformity. It is relatively flat, and increases sharply in dip at a paleo-shelf break in the San Gabriel submarine canyon area

(Figure 11 and 11a). Between the two canyon branches the maximum measurable thickness of LP II is 25 m. However, near the head of the eastern branch, LP II has been truncated by the basal LP I unconformity where both LP I and LP III mantle the walls (Figure 16). This may indicate stronger activity by that branch of the canyon after LP II deposition, probably coinciding with the channeling event at the base of LP I (see discussion of LP I). It should be noted that seismic data from the Beta Platform Survey indicates that~he western canyon branch truncates LP III to LP I units along its western wall, indicating that erosion continued until recently~] Lack of equivalent density of data along the eastern branch precludes a comparative determination of activity in that branch.

{!he Palos Verdes fault truncates LP II along the unit's southwest- ern mapped extent, as is the case for LP III (Figure 6). LP II thins near the fault and thickens to an average of 12 m away from it~ Along / 73

Figure 16

USGS Uniboom line 455 Truncation of LPII along the San Gabriel submarine canyon ? 74

--- ::L .,

LINE 455

(FIX MARK) sw 30 (TIME FIX) NE

SEAFLOOR

LP II M

...... 150m. ~0 ms. -8.5 m. L_____.!

FIGURE 16 75

" '

~he northeastern edge of the Wilmington graben, the basal LP II uncon­

formity forms a possible terrace, 4.8 kilometers off Huntington Beach

shoreline (Figure 15). Well developed, tightly spaced crossbeds within

LP II suggest a nearshore environment of deposition, perhaps prograding

terrace deposits. Holocene, flat-lying beds overlie these sediments

and truncation by that transgression is clearly evident. Further west,

LP II simply pinches out and the internal character of the unit is less

evident. Such is the case for the northwestern limit of the unit, ex­

cept for occasional steeply inclined reflectors. These may represent a

littoral environment but may also be indicative of channeling]

LP I

\:,ihis unit is restricted to the basin-slope northeast of the Palos

Verdes fault in the San Gabriel submarine canyon areaj and was encoun­

tered only inCH 262-2 (Figure 17, 11 and 11a).

Figures 14 and 18 are a paleochannel-shelf break map and a Holocene/­

LP I isopach map based on Shell Oil Company Sonia and Uniboom data.

They show the extent of these features in the Beta Platform Survey Area

(see Figure 2). A change in the character and thickness of the isopach unit is evident at approximately the 100 m (300 foot) isobath, where

channels come to an abrupt end at a paleo-shelf break. At this break,

LP I ends, and shoreward, a basal Holocene unconformity overlies LP II

channel deposits on a peleo-shelf surface.

(Qn the slope, are a series of anastomosing channels or gullies,

typically 300 m apart and 500 m wide. Wall relief is 15 to 25 m and fill-thickness is from 25 to 30 m. The gullies are cut progressively higher in the section in a northeasterly direction, away from the Figure 17

USGS Uniboom line 464 Wave-cut notches (terraces) on the base of LPI 77

LINE 464

(FIX MARK) NW (TIME FIX) (TIME MARK) 60 SE ~------~--~------~----~--~~--~

HOLOCENE SEAFLOOR LP I ...... _TERRACE(?) LP II

M \ MULTIPLE M

0 m.s. =aS m. -150m. I1 L....-.-....J

FIGURE 17 78

Figure 18

Holocene/LPI isopach map within the Shell Oil Co. "Beta Platform Sites" survey area 79

,,------\ ' ' .''

; ' FIGURE 18 Figure 19

Shell Oil Co. Sonia II line 134 Progressively younger LPI channels northeastward from the Palos Verdes uplift 7 81

I : .-· ... ,' ~~ .-...... ·-·- ' I .f ·! ~ . .. _, _ 1: I 1 .. ' 1 .., i i. '

·_,: :~ :. ;...... ; - I - ~ t - ,~· : : . '7"_- -<;,. ~ : : :._ ~ ; ! ·: ~.: -;·

I -.:·v ·_,· :' ·: ~-, · . .- • >.- . r: :::~~~·'ii::_ :.y:~' :\'· 3~ ~ ~:./: ~:: : .,· .; ~.) I ...... "· l . I I . . -- . . .- j; .. -~: -:;£:r::z- :i7f\:t~:s~t:~tttai4ttt~?<;:::·5¥:§!!tt :0-:!

LINE SONIA . 11 134

(FIX MARK) W-SW 125 120 115 110 105 100 E-NE

SAN GABRIEL SUBMARINE UPLIF

-150m. l._.__._J F

FIGURE 19 82

fault, indicating an influence on their development by the emerging

Palos Verdes uplift (Figure 19). Furthermore, the major channels,

which have distinct bathymetric expression, appear to be controlled by

splays of the fault (Fischer and others, 1977). Seismic data (Figure 19)

suggests that most of channel-fill encountered in CH 262-1 above the

basal sand is silt.

Between the two branches of the San Gabriel submarine canyon R/V

Kelez data indicate similar characteristics for LP I including channel­

ing on the basin slope and termination at or near a paleo shelf-break

(Plate 2). Two notches into LP II are possible marine terraces, the

shoreward one coinciding closely with the actual limit of LP I, along a

postulated ancient shoreline trend (Figure 17). East of the eastern

branch of the canyon, both Kelez Uniboom and CSUN 3.5 kHz data display

similar channeling on the paleo-slope at the base of LP I. Further

eastward, approaching the Newport submarine canyon, new data recently

acquired by McClelland Engineers, Inc. for the U.S.G.S. Conservation

Division (1979) in preparation for BLM lease-sale No. 49, also reveals

a slope in which channeling appears to be equivalent in age.

Data from the Beta Platform study suggests a relatively late phase

of pre-Holocene channeling (Fischer and others, 1977) (Figures 10 and

lOa). This phase is expressed as early channel-fill of the San Gabriel

submarine canyon. On the .shelf northwest of the limit of LP I the base of this fill clearly truncates units LP II and LP III, and Holocene materials unconformibly overlie it. Further down-slope the modern San

Gabriel submarine canyon truncates LP I channels and this early canyon fill is missing. These circumstances make it difficult to establish 83

relative age relationships between LP IA and LP I. However, it is probable that during the vigorous LP I channeling event the San Gabriel canyon was also highly active. LP IA canyon fill and LP I channel-fill may thus be age equivalents. During this time the submarine canyon may have been located slightly westward of its present position. Subsequent movement on the Palos Verdes block would then influence eastward migra- tion, as has been discussed for LP I channels.

Holocene

/Holocene sediments of the Long Beach to Newport Beach coastal zone '.,~ have been laid down primarily by the Los Angeles, San Gabriel and Santa

Ana Rivers~ The cones of these rivers form the Downey Plain, which re- ~-r"' presents an accumulation of sediments over an ancestral coastal plain, deformed during the late Pleistocene, and cut by two major trenches

(Piper and others, 1956). (~hese sediments consist of sand, gravel, silt and clay (Piper and others, 1956)~) These authors have divided the deposits into upper and lower divisions. Q:'he lower division consists of coarse sand and gravel found in entrenched channels, and typified by two main tongues,: the Talbert and Gaspur water-bearing zones. The

Talbert zone was laid down by the ancestral Santa Ana River and extends from Santa Ana Canyon to the ocean through the Santa Ana Gap, and Balsa

Gap, where it is very thin. The Gaspur zone was laid down by an ancient

San Gabriel River, from Whittier Narrows to the ocean through Dominguez

Gap (Figure 22).

The upper division of Holocene onshore sediments consists of fine sand, silt, and clay being deposited in present channels of the Los 84

Angeles, San Gabriel, and Santa Ana Rivers in alternating scour and fill cycles. Generally, these streams are aggrading (Piper and others,

1956).

The lithology of offshore Holocene sediments is derived from information from Shell Oil Company's core holes 261 and 262 (see intro­ ductory paragraph of this chapter) and from the Bechtel (1967) drill­ holes (Figure 2). In the vicinity of the Shell Oil "Beta Prospect" the sediments consist of a basal silty very fine to fine grained olive gray sand grading to silt and silty clay and then to sandy silt or very fine sand. Holocene sediments in the Bolsa Island site (Bechtel, 1967) consist of a.basal very fine to coarse grained, gray to dark gray micaceous sand and silty sand with scattered pebbly lenses, grading upward to a buff gray sand with scattered rounded pebbles and shell fragments. These sands are overlain by a dark gray silty sand which is very fine to coarse grained with abundant quartz and scattered mica.

~ isopach map (Plate 3) of latest Pleistocene/Holocene unconsoli­ dated sediments was constructed to show their thickness and distribution on the San Pedro basin margin. The thickness is controlled by the un­ named fault zone of Junger and Wagner (1977), the Wilmington graben, the Palos Verdes fault zone and the Palos Verdes uplift. Over most of the shelf north of the Palos Verdes fault, isopachs show the thickness of sediments above a subtle, nearly horizontal erosional surface seen on the acoustic records (Figure 15). This surface was cut during the

"Wisconsin" glacial and subsequent transgression (Figure 11), and the age of these sediments is assumed to be primarily Holocene) Near the shelf edge in the Beta Platform survey area core-holes 261-1 and 261-2 85

yielded benthic foraminifera of Holocene age (Bergen, 1977 in Fischer

and others, 1977).

(in the San Gabriel submarine canyon area south of the LP I limit

line, where deposition appears to have been continuous from late Pleisto-

cene into the Holocene, the isopachs indicate thickness of Holocene and

LP I sediments~

South of the Palos Verdes fault, thicknesses shown are of unconsoli-

dated sediments which on acoustic data appear as a relatively transparent

layer with weak parallel reflections above a denser zone of inclined

and folded Neogene bedrock (Figure 20). These sediments are assumed to

be principally Holocene.

Examination of the latest Pleistocene-Holocene isopach map reveals

several trends. {jelatively thick areas on the shelf shoreward of the ~

Palos Verdes fault are commonly associated with evidence of Holocene

channeling. A relatively thick trend extends southwest from the eastern

Bolsa Chica State Beach area between two zones of 0 Holocene sediments.

This trend, with a maximum· of 15 meters,. correlates with the onshore 1 drainage. The Santa Ana River is believed to have cut this gap during

the Holocene Epoch and portions of the Talbert water-bearing gravels

fill it (Piper and others, 1956). A major thick trend extends from the

eastern head of the San Gabriel submarine canyon. Near the canyon,

sediments attain a thickness of greater than 55 meters in a large chan- nel or filled canyon head. This channel clearly truncates base Holocene

and younger sediments, indicating that it had been active quite recently

(Figure 21). It can be correlated with the onshore Santa Ana Gap (Fig-

ure 22) which was also cut by an ancestral Santa Ana River and is also

floored by portions of the Talbert water-bearing sediment body~ Near Figure 20

CSUN 3.5 kHz line 5048 Holocene sediments above Neogene bedrock of the Palos Verdes uplift ? 87 /

------+------.. -- ~ - - :> ------~------~ ------i·- -

! •

LINE 5048

(FIX MARK) SW 11 NE

SEAFLOOR

Tm

Tm F MULTIPLE .Io ~- =8.5 m. -150 ·m. M- L.___J

FIGURE 20 Figure 21

USGS Uniboom line 466 Recently filled San Gabriel submarine canyon head 89

LINE 466

NW (TIME FIX) SE

SEAFLOOR FILLED CANYON HEAD CHANNEL

./

M / - -- MULTIP.LE

-150m. L---...J ,.

FIGURE 21 Figure 22

Fluvial gaps of the Newport-Inglewood zone. SH-Signal Hill~ RH-Reservoir Hill, AH-Alamitos Heights, LH-Landing Hill, BCM-Bolsa Chica Mesa, HEM-Huntington Beach Mesa, NM-Newport Mesa . :::• ...... ,.~ 118' ':{ ,~~~~~~~,,,,,1,1' 1 ''·'•,,._ \ i SH ...... ,.,.,,,,"~'""'"'"'''' Ill \~ '..~,,, ~,\ ~ ;~.. \''•--,,N,,,. RH ",,.• ALAMITOS ;7\ \ NM ., .. .,.... .,,,,...,,...,,._,,.,lltlflllfiiHIIUUUNIUUWII~~~·\ll,l , ,... ./") ~~ i~\ .,., ~ · ·. ~ GAP •·•••••·• -'BOLSA / HBM \ . ! BCM l I \ SANTA ~\ t LONG \_ .J GAP l \ ANA .·.- ,,,,,,,,,,,"., ~ ~ -\\ BEACH GAP ...... ,.~ so, •• c ~ \~ hie• ' \ \ \ 81 010 8o}0~ HUNTINGTO!'I \ '-·-::4,.,.,, ':..~".,,,,.,.,,., .. ,.....,~,~.. ACH J • !:'_<...,,_.,.,,,.. • • 20...-----:::

.,o·

::::::::------_~5o, ~ >oom 250m X

-.l.,,.'i \. 1>'0 ., ..- v• '"\

0

FIGURE 22 ...... ,.

1..0...... the Long Beach Harbor breakwater a weak 10 meter-thick trend may be associated with an ancestral San Gabriel River, which cut the Dominguez

Gap (Figure 22) and deposited the Gaspur water-bearing sediment body.

An additional deposit may be attributable to fault activity. (~P to 20 meters of Holocene sediments are located between the Palos Verdes fault zone and a smaller fault to the north, deposition taking place in a small graben between the two faults.

The San Gabriel submarine canyon and surrounding slopes are sites of significant accumulation of late Quaternary sediments. Plate 3 shows that the modern canyon contains an average of 25 meters of Halo- cene fill throughout its channels. The intercanyon slopes are blanketed with up to 45 meters of Holocene/LP I sediments. The thickest deposits on the slopes form in LP I channels whereas 7 to 10 m of Holocene sediments cover the interchannel ridges (Fischer and others, 1977).

Southwest of the Palos Verdes fault zone, areas up to 20 m in thickness are primarily structurally controlled, occupying lows along 0 synclinal axes of the underlying Neogene bedrock. Conversely, bedrock exposure near the shelf break coincides with the anticlinally folded edge of the shelf.

There are several areas on the San Pedro basin shelf where no

Holocene sediments are present. Shoreward of the Wilmington graben, /t large areas bounded by 0 isopachs (Plate 3) are on the upthrown side of

Junger and Wagner's (1977) unnamed fault. Thus, these areas suggest recent structural growth along this fault in addition to the absence of any steady sediment contribution by modern rivers in the vicinity~ 93

qhe largest area of 0 Holocene sediments is an exposed Miocene and

Pliocene bedrock high just seaward of the Palos Verdes fault zone.

This high coincides with the crest of an anticlinorium which seems to have formed due to compressive components of stress along the Palos

Verdes fault. Motion along the fault, and structural growth of the anticlinorium, appears to be exposing this area to scour by long period swell, which can resuspend fine sand at these depths (30-40 m) (Gorsline and Grant, 1972). The absence of any significant overburden and the anticlinal nature of this area explains the high incidence of possible hydrocarbon seeps which were mapped from CSUN 3.5 kHz data (Plate 2).

Off Pt. Fermin, the presence of an acousticaly opaque unit results in thinning and local absence of Holocene sediments. This unit, be­ lieved to be Catalina Schist basement, has been incised by two possible marine terraces, as shwon in Figure 5. Northeast of this area, rela­ tively thick deposits are believed to overlie another marine terrace. Chapter 5

STRUCTURE

qypical of the Peninsular Ranges geomorphic province and the

Southern California Continental Borderland, the structural grain of the study area is characterized by northwest trends. The Palos Verdes uplift and the Wilmington graben are the principal structural blocks

(Figure 3). These blocks are controlled by the Palos Verdes fault and an unnamed fault zone to the northeast: Although outside of the study area, the Newport-Inglewood zone is also a major structural feature~? -J

Faults

(!he most significant fault on the San Pedro shelf is the Palos 11 Verdes fault, which bounds the Palos Verdes uplift to the southwest.

Wells drilled along the northern border of the Palos Verdes Hills en- countered Miocene rocks overlying Pliocene rocks, suggesting a south- ward dipping reverse faul~~ No apparent surface rupture is reported onshore; however, the presence of the emergent Palos Verdes block, with numerous elevated and deformed marine terraces, as we~l as the location of several epicenters (Figure 23) attests to its existence. (gertical -..}f. !'-...! separation of 2,000 m along this fault, northeastern side down, is re-' ported (Greene and others, 1975). Based on lithologic changes in

Tertiary rocks and on basement surface disruption, significant compo- nents of strike-slip movement have been suggested (Yerkes and others,

1965). This fault has been mapped on the Santa Monica shelf, where it dies out against the Dume fault at the base of the Santa Monica Moun- tains (Junger and Wagner, 1976; Nardin and Henyey, 1978)~ @outheast

94 - - ...... - ...

Figure 23

Major earthquake epicenters in the San Pedro margin vicinity

(from Fischer and others, in prep.) • • • + •••••• 10' 20' . . . . 118° 33°50 ...... J ...... ,.....,. . . . . ,.~ Long Beach• . .·. .·. . ~)' 0 l"i... • • • '-~ -N-~ ~-- ~·.·.·. '1-~ (1'.1$: 00 ~ . . . ~v. • • • • '<.)' ~ ·. ·.·. · .~~ ......

·~ >c 0

-~--~~~-f'0 10km SOUNDINGS IN FEET MAGNITUDE • !5.0- !59

~ 4 0-4.9

0 3.5- 3 9 FIGURE 23

\0 0'\ 97

of the San Pedro shelf the fault eventually straddles the Lasuen Knoll

(Greene and others, 1975; Junger and Wagner, 1977) and two main splays

continue southeastward~!

Based on 3. 5 kHz and Uniboom data of this study, (the Palos Verdes

fault is mapped as a zone of numerous discontinuous en-echelon traces

suggestive of wrench-tectonics, similar to the Newport-Inglewood zone

(Harding, 1973) (Plate 4). These traces trend about N40°W near the

Palos Verdes Hills and swing to about N30°W from the central shelf

southeastward. On seismic profiles, this fault is recognized by dis- placement of late Pleistocene and Holocene units; as a juxtaposition of weak parallel reflections in a semi-transparent unit (late Quaternary

sediments) against stronger, inclined and folded reflections (Neogene

or Pleistocene units); as a disruption of the seafloor including bowing

and scarps (Figures 24 and 6). The seismic data indicate that activity

on this fault has been greatest near the Palos Verdes Hills, where a

topographic bulge is delineated (Plate 4) and several seafloor scarps of 0.75 to 1.5 mare present. Southeastward, the youngest disrupted surface is the basal Holocene surface, and near the San Gabriel sub- marine canyon the youngest consistently disrupted horizon on all traces is the basal LP III surface, with the exception of a perminent 0.75 m seafloor scarp on line #453 (Figure 6). Apparent vertical displacement of the base of the Holocene and LP II units is 1.5 to 2m. The maximum measured throw is 17 mat the base of unit LP III near the San Gabriel submarine canyon (line 453). There is a trend of younger horizons being disrupted progressively shoreward. This, in conjunction with a k progressive shoreward migration of LP I channels (which follow fault Figure 24

CSUN 3.5 kHz line 5026 Evidence of Palos Verdes fault 99

LINE 5026

(FIX MARK) sw 13 NE

PALOS VERDES FAULT

BOWED SEAFLOOR SEAFLOOR

-150m. MULTIPLE L___l }o m.s.- 8.5 m.- M ---~.3------M ---

FIGURE 24 100 splays, Figure 19), suggests that activity of the Palos Verdes fault zone has migrated northeastward in time. The fault was active through- out late Pleistocene and Holocene time up to the present. Units LP III to LP I consistently thin toward the fault zone, and LP IV to II are commonly truncated by a fault trace within the zone. Additionally, structural growth is evident on the upthrown side as it is devoid of any recent sediments from near the Palos Verdes Hi~ls to the Beta

Platform survey area (Plate 3). It appears that there was a period of increased activity during the deposition of LP III along its faulted edge, which is shown as a concealed contact on Plate 2~ This may account for the anomalous shoreward limit of the unit in this area.

(~ortheast of the Palos Verdes fault zone, a down-dropped graben contains up to 700 m of Quaternary sediments. This block, called the

Wilmington graben by Junger and Wagner (1977), is bounded by an unnamed fault zone along its shoreward edge.") This fault zone was first mapped _j' by Junger and Wagner (1977) and is shown on Plate 2, where it is modified from their map. They point out that most faults in this zone are not clear on the seismic profiles, and most of the splays die out at the top of lower Pliocene (Repetto) strata. To the northwest within the zone, this writer mapped several discontinuous traces which offset base

LP IV to base Holocene strata. The fault is most active in this area.

Generally though, many shallow conditions reflect activity in this zone. Zero Holocene patches begin just shoreward of the fault zone

(Plate 3), and the limits of LP II and LP III coincide very closely with it. As shown on cross-section A-A', LP IV and older units become exposed on the seafloor just shoreward of the zone. 101

A fault trace off Pt. Fermin may have controlled the formation of a large marine terrace (Figure 5). This fault forms a small graben in conjunction with a smaller fault farther west-southwest. Conversely, southwest of this feature a small elevated basement block is found.

Another important fault is the high-angle reverse San Gabriel sub- marine canyon fault zone which was mapped from numerous seismic lines in the Beta Platform Survey (Fischer and others, 1977), and whose northwestward extension was delineated in the present study, as it follows the trend of a partially filled submarine gully just west of the modern canyon.

Folds

IThe outer San Pedro shelf and basin margin southwest of the Palos

Verdes fault is essentially an anticlinorium. The crest of this anti- form forms the Tertiary bedrock outcrop on the seafloor and the south- western limb forms the basin slope to the adjacent San Pedro basin.

Numerous minor folds were mapped on the bedrock high on the outer San

Pedro shelf (Plate 2)·:-) Due to their abundance, size, and similarity, _j correlation of individual folds from line to line was difficult. Only those folds which could be extended with some confidence are shown.

Symmetry characteristics, size, grouping, and trends established on-. shore (Woodring and others, 1946) were used as correlation criteria.

Trends near the.Palos Verdes Hills are based on the best control and correlation is believed very good.

(Generally, the folding is gentle and not complex, though folds are "· often grouped tightly in long series of anticlines and synclines (Fig- ure 25):] This style seems particularly typical of the Monterey Shale. Figure 25

CSUN 3.5 kHz line 5048 Folded Neogene bedrock of the Palos Verdes uplift 103

LINE 5048

FIGURE 25 104

Incompetent behavior, close folding, and complicated minor structural features are emphasized by Bramlette (1946) in describing the fine grained, rhythmically bedded rocks of the Monterey Shale.

Gthere are two principal fold trends on the San Pedro basin shelf.

Near and semiparallel to the Palos Verdes fault zone, numerous minor folds trend N20°W to N25°W. Further seaward, near the shelf break, better defined and larger folds trend N35 0 W to N40 0 W, oblique to the fault. The San Pedro sea valley follows the trend of one of these syn- clines. Junger (1977) suggests that compressive features trending obliquely to the Palos Verdes fault are evidence that this fault has experienced right lateral strike-slip motion resulting in wrench-style deformation. The present study leads to the conclusion that near the

Palos Verdes fault zone the folds are essentially parallel to it and become only slightly oblique with increasing distance seaward. Neither the sharp change from parallel to strongly oblique (Nardin and Henyey,

1978), nor a uniformly oblique configuration (Junger, 1978) seem evident

(Plate 2) J Chapter 6

EVOLUTION OF THE SAN PEDRO BASIN MARGIN

(the present configuration of the San Pedro shelf can be explained

in terms of an interplay of tectonics and sedimentation. The outer

shelf (Palos Verdes uplift) may have begun to develop initially in con­

junction with relatively early tectonic events associated with the

formation of the borderland basins, and continued to form in response

to further plate adjustments. The inner shelf subsequently prograded

as sedimentation and concomitant sinking of the Wilmington graben

produced a thick post-Miocene sequence of strata~

PRE-QUATERNARY EVENTS

The plate-tectonics framework established by Atwater (1970), and

subsequent work summarized in Blake and others (1978), attributes the

(formation of Neogene basins off California to the impingement of a

spreading ridge system against western North America, and subsequent

interactions. Particularly, the progressive formation of a broad

transform boundary between-the North American and Pacific plates (the

San Andreas fault system in its broadest sense) as two triple junctions moved northward and southward is regarded as the "underlying" factor controlling the modern tectonic setting of the western United States.

The ongoing debate between the pure extensional (Yeats and others,

1974) and the oblique (strike-slip) extension proponents (Howell and others, 1974) is not yet resolved0 However, certain facts have emerged from recent work (Blake a·nd others, 1978; Henyey and Nardin, 1978). @_ change in relative motion between the North American and Pacific plates,

105 106

10m Y.B.P., to a more westerly direction (Blake and others, 1968) is thought to have produced a component of extension required to form the borderland basins. Subsequently, a slight divergence between plate motions and the orientation of the tectonically soft boundary between the plates initiated a degree of convergence along the established strike-slip faults (Henyey and Nardin, 1978). This latter change, thought to have occurred between 12 and 3m Y.B.P., produced a different style of deformation: from crustal dilation and block faulting over a wide region -- creating the basins -- to folding restricted to narrow zones and post-Miocene discontinuous faulting (Henyey and Nardin, 1978;

Junger, 1976):;)

~he Palos Verdes uplift began developing in mid-Miocene time during the initial development of the borderland) (P. J. Fischer, personal com- _i" munication). However, in accordance with theoretical experiments by

Wilcox and others (1973), as suggested generally by Crowell (1974) and

Junger (1976), and as proposed for the Santa Monica and San Pedro basin shelves by Henyey and Nardin (1978), @he Palos Verdes uplift is inter- preted to have been shaped primarily as a result of convergent dextral shear"0 Junger (1978) notes that the seaward edge of the uplift is pri- marily a folded flank, with little evidence of major modern faulting.

~~~- Data from the present study confirm this. \I,he Neogene rocks of the up- lift were probably most intensely folded after deposition of lower Plio- cene rocks. However the deformation either continued uninterrupted or was renewed during Pleistocene time, and is presently active.

The Palos Verdes fault may have been developed approximately 10 to

15m Y.B.P. (Blake and others, 1978). The fault is throughgoing from 107 basement rocks (Yerkes and others, 1965), and has a surface expression indicative of wrenching similar to other faults (Wilcox and others, 1973;

Harding, 1973). The apparent absence of large strike-slip displacements along this fault (Junger, 1976) requires the motion to have been con- vergent in order to produce the cross trending folds. This type of motion is attributed to divergent trends between the fault and regional shea.f) (Henyey and Nardin, 1978). @s is the case in folding, then, convergence appears to have also been initiated on the shelf during late

Miocene or early Pliocene time. It has been established in this study that the Palos Verdes fault zone migrated eastward (shoreward) through time; thus, the locus of shear moved similarly. The decreasing angle of folds progressively nearer to the fault is suggestive of their develop-

- ment later during the overall deformation~ This behavior is predicted by Wilcox and others' (1973) clay-cake experiments.

(fnitially, the Palos Verdes uplift probably was a submarine sill which separated the Los Angeles basin from the San Pedro and Santa Monica basins (Conrey, 1967; Henyey and Nardin, 1978). Throughout the Pliocene

Epoch, it acted as a barrier to sedimentation. Turbidity currents spread coarse sediments into the Los Angeles basin, and entered the Santa Monica and San Pedro basins from the north (Conrey, 1967; Junger, 1976) and from the south (P. J. Fischer, personal communication). Subaerial exposure of the uplift was followed by partial resubmergence. The greatly abbrevi- ated Pliocene section of the Palos Verdes uplift, and overlying Pleisto- cene marine San Pedro Formation, lead to this conclusio~~

\Activity along some of the faults of the shoreward Wilmington graben edge had apparently ceased by the end of Repetto time~ A dip-flank of 108

younger sediments is reported by Junger and Wagner (1977) and is docu- mented by the writer along a portion of 'the northern edge (Plate 5,

Cross-Section A-A'). However, other faults along the northwestern part of this zone remained active beyond this time (Plate 2).

QUATERNARY EVOLUTION

Early Pleistocene (By Early Pleistocene time the Los Angeles basin was essentially ~ filled and in the Wilmington graben progradation southwestward and southeastward of lower San Pedro Formation deltaic deposits had begup)

(Junger, 1977). A shoreline is thought to have oscillated across the present-day Downey plain, but never oceanward of the Newport-Inglewood zone, which paralleled it (Poland and others, 1956). Q?urther inland, a broad coastal plain was traversed by streams (ancestral Los Angeles and San Gabriel Rivers) which deposited San Pedro sediments in areas that must have been subsiding, judging by the thickness of material and narrow range of sea-level change. Barrier beaches with large lagoons behind them characterized the coastline, which was subject to strong longshore currents. The Palos Verdes Hills were a nearshore island, but as in the Pliocene, they did not contribute significantly to local sedimentatio~(Conrey, 1967; Poland and others, 1956).

Landward of the Newport-Inglewood zone in Orange County, hetero- geneous lower Pleistocene deposits, lacking coarse sediments, were laid down in a landward succession of lagoonal, flood plain, alluvial cone environments of the ancestral Santa Ana River system (Poland and others,

1956). 109

Middle Pleistocene (Oxygen-Isotope Stages 11 to 7)

(Jhe Mid-Pleistocene San Pedro Sand was deposited roughly between

150,000 and 300,000 years B. P. (Wehmiller and others, 1977). Deposition

was in very shallow to moderate water depth~(Woodring and others,

1946) during 9-11 of the deep sea oxygen-isotope record (Figure 13).

(Puring this time, the highest (12th) Palos Verdes Hills terrace was

cut, and the latest phase of local uplift (Pasadenan phase, Cascadian

Orogeny) began. This resulted in cutting of perhaps yet another Palos

Verdes terrace level during Middle Pleistocene time.

As indicated by amino acid dates, (Wehmiller and others, 1977) the

Pasadenan orogenic phase apparently continued throughout the middle

Pleistocene at a constant rate. This gave rise to the remainder of the

terraces except the lowest in San Pedro, and the fourth through first

ones on the Palos Verdes Hille) Local unconformities throughout the

coastal area as well as three terraces in the San Joaquin Hills also

represent this time period (Wehmiller and others, 1977). qn the coastal

zone, initial deformation associated with the Newport-Inglewood fault

apparently resulted in local highs which underwent some erosio~(Poland

and others, 1956). At the same time a large channel of the ancestral

Santa Ana River was cut into the area which forms the northern end of

Newport Bay.

(While the outer San Pedro shelf was probably positive throughout

the Middle-Pleistocene, the Wilmington graben was apparently still sub­

siding. Unit LP IV may have been deposited during the latter stages of

this period, as a transgressive-regressive glacioeustatic cycle of

stages 8 and 7 (Figure 13) swept across the San Pedro shelf, specula­

tively between about 230,000 and 140,000 years B.P~ 110

Late Pleistocene and Holocene (Oxygen-Isotope Stages 6 to 1)

r~- ~fter LP IV deposition, sea-level at a low-stand during stage 6 formed a shoreline which may have barely surrounded the Palos Verdes uplift, and was located close to the paleo-shelf break at the base of

LP III. During this time, while channels were being cut into the top of the LP IV unit, earliest LP III sediments were being deposited on the slope just beyond the shelf break. These sediments, not differenti- ated in this case, are presumably related to the LP III shelf deposits as the subdivided LP I unit is to the Holocene sediments~

Subaerial exposure of the Palos Verdes to Newport Beach coastal area during the Middle Pleistocene was followed by extensive submergence and deposition of the Palos Verdes sand, marking the beginning of Late-

Pleistocene time. (~n interglacial high-stand associated with stage 5 and correlative with maximum plaeobathymetry indicated by data from

Beta Platform coreholes, resulted in sea-level apparently 6 m higher than present sea-leve1) (Shackleton and Opdyke, 1973, Bloom and others,

1974). IThe transgression leading to this high-stand cut a regional surface surrounding the Palos Verdes Hills (lowest San Pedro terrace,

4th Palos Verdes Hills terrac~also on the flanks of the Santa Monica

Mountains, and on the lowest emergent terraces of Newport Mesa to

Laguna Beach. On most of the coastal area including the modern Newport-

Inglewood zone, this surface is uniformly about 10 m below the ground surface (Poland and others, 1956, Wehmiller and others, 1977). It formed a relatively flat plane across the Newport-Inglewood ·trend, which subsequently underwent the strongest deformation. 111

~he Palos Verdes Sand, dated at 120,000 to 140,000 (Wehmiller and others, 1977) to 23,000 years B.P. (Bechtel, 1967) (Table 4) was deposited during the stage 5 interglacial high-stand and the ensuing general re- gression through the stage 3 temporary interruption. In the Wilmington graben the unconformity at the base of unit LP III is tentatively corre- lated with the surface cut by the stage 5 transgression. A basal LP III gravel and sand was laid down as the sea advanced to a shoreline just beyond the Newport-Inglewood zone. Before regressing, the sea may have cut one or two more terrace levels in the Palos Verdes Hill~ (third and second terraces).

(bsr" sea level lowered sporadically after the stage 5 high-stand deformation of the Palos Verdes marine planation surface under the New- port-Inglewood zone formed the coastal hills and mesas. In response to a lowering base level, the San Gabriel and Santa Ana Rivers incised this surface and formed the gaps between the hill~) (Figure 22) (Poland and others, 1956). Trenching appears to have occurred in three stages. Dur- ing the first stage, while sea level was still relatively high, the gaps were cut down to the level of the present Downey Plain before incision continued inland. The next stage of trenching continued until a common base-level was reached in at least Bolsa and Santa Ana gaps (Poland and others, 1956). This temporary still-stand, estimated at-25m is herein correlated with isotope stage 4. An ancestral Santa Ana River, in addition to cutting Santa Ana gap, may have deposited LP III gravels which are present in the Beta Platform area (Fischer and others, 1977) and along a zone extending across the shelf toward the gap.

Most of the deformation of the Newport-Inglewood zone was completed prior to the second stage of trenching (Poland and others, 1956). The 112

;:;.' [Palos Verdes fault, possibly experiencing the greatest late-Quaternary

activity throughout this time, maintained the outer shelf at or near

sea level while the Wilmington graben continued subsiding. Subsidence was greatest along the Palos Verdes Fault as LP III is thickest over an

axis nearer the fault. Conversely, much of the shoreward edge of the

graben may have simply been an inactive dip-flank~ Within the survey

area, the Holocene transgression eroded any remnants of LP III beyond

this edge and beyond the mapped LP III extent to the northwest.

~uring the stage 4 low-stand, while LP III channels were being eroded on the shelf, the earliest LP II deposits were accumulating be- yond a paleo-shelf break at the base of LP II, nearly coincident with a probable shoreline5 These deposits, locally constitute gravels and

sands along slope gullies analogous to the LP I gullies (Figure 14).

~ period of aggradation evident in Balsa and Santa Ana gaps (Poland "' and others, 1956) is correlated with a transgressive pulse on the shelf, leading to isotope stage 3. This advance of the sea cut the basal LP II unconformity and established a shoreline near the shoreward edge of the

Wilmington graben, as suggested by a probable terrace off Huntington

Beach (Figure 15) and presumably the first exposed terrace of the Palos

Verdes Hill~_;) Elsewhere, any evidence of a shoreline was eroded by the

Holocene transgression. (A~ shelf sediments of unit LP II were being deposited, the Palos Verdes fault remained active as LP II is truncated by the uplift. LP II thickens slightly in the graben, and probably never extended significantly shoreward of its mapped limits. It is relatively thin, and the short-lived stage 3 high-stand soon retreated to the lowest late Pleistocene ("Wisconsinan") level of isotope stage ~~ 113

Continuing deformation of the Newport-Inglewood zone during this

time, lowering sea level, and probable increased precipitation resulted

in a final stage of transection of the local gaps. According to Poland

and others (1956) the streams from Whittier Narrows (ancestral San

Gabirel River) passed to the Dominguez gap, whereas further east a

stream was discharged from Santa Ana Canyon (ancestral Santa Ana River).

Antecedent channels of this system cut Newport Canyon, Balsa gap, Santa

Ana gap, and Alamitos gap (Figure 22). Of those east of Signal Hill,

only Santa Ana and Dominguez gap appear to have kept pace with the

rapidly decreasing base level. Apparently these courses offered the

best grade and thus the other gaps were beheaded.

The available data in the study area do not conclusively establish

a link between these gaps and_the San Gabriel submarine canyon. Never­

theless, enough signs of local channeling on the shelf (Plate 2), in

addition to trends revealed in the Holocene-LP I Isopach Map (Plate 3), make this probability likely. Furthermore, the presence of numerous

LP I gullies (Figure 14 and Plate 3), and evidence that the San Gabriel

canyon underwent channelized erosion presumably throughout this period,

suggests that significant amounts.of sediments reached the shelf-edge by some means other than sheet flow. A low-lying coastal plane with relatively shallow intermittent braided channels, rather than well defined courses is envisioned for both the ancestral San Gabriel River/­

Dominguez gap and the Santa Ana River/Santa Ana gap~Bolsa gap systems, as they spread broadly onto the San Pedro shelf. Of the two drainages, an association between the San Gabriel submarine canyon and the Santa

Ana system is most readily apparent. Bechtel (1967) data, just offshore 114

of Balsa gap, did not detect distinct fluvial evidence. However, that study did not rule out the possibility that off this minor gap the channel-fill may not have been sufficiently coarse to be differentiated from surrounding material. A trend in the Holocene Isopach Map suggests that a weak channel extended seaward. Also, an anomalously young date

(Table 4) was obtained from wood recovered from the drill holes, when compared with the relative age implied by the seismic stratigraphy of the locality (Plate 2). This wood may have been deposited in the channel extending seaward from Bolsa gap.

Q!hile these channels were being formed on the shelf, the shoreline was located near the modern shelf-break, and between the branches of the

San Gabriel submarine canyon a well developed buried terrace marks a portion of the shoreline's extent. Elsewhere it is approximated by the shoreward limit of LP I, the modern shelf-break around the Palos Verdes uplift, and the deepest of buried submerged terraces mapped off Point

Fermin (Plate 2)~

Shoreward migration, in time, of LP I gullies, and fault truncation of LP II and Holocene strata attest to the activity of the Palos Verdes fault throughout this time •. Qlcross the shelf, most sediment transport was probably confined into the subsiding Wilmington graben, and down the basin slopes and San Gabriel canyon into the Catalina trough~ Data from

McClelland Engineers for the BLM Lease Sale No. 49 (USGS, 1979) reveals the presence of a submarine fan at the base of the canyon.

11,000 Y.B.P., and the ensuing transgression cut the basal Holocene un­ conformity, planed off the outer San Pedro basin shelf (Palos Verdes up­ lift), and deposited a basal sand. As base level rose, local coastal 115

streams aggraded and the coarse Talbert and Gaspur water-bearing zones were deposited. Overlying these gravels are finer Holocene sediments) correlative with the shelf modern mud blanket of Curray (1965).

The Los Angeles River didn't drain into San Pedro Bay by way of

Dominguez gap until it was diverted from Ballona gap during a flood in

1825. This flood also diverted the Santa Ana River from Balsa gap to

Alamitos and Sunset gaps (Poland and others, 1956). How the Santa Ana

River returned to its present position is not clear. It may have occur­ red during the 1867-68 flood when the San Gabriel River, which flowed directly into San Pedro Bay, altered its course into Alamitos Bay. By

1861, the Newport Beach barrier bar had been built to nearly its modern size. In 1920, the present artificial channel was constructed (Felix,

1969).

The depletion of sediments entering the longshore flow, as the coastal rivers underwent flood control, coupled with the relatively emergent nature of the Seal Beach to Newport Beach nearshore area had resulted in a loss of littoral sand. This effect is further illustrated by the area of 0 Holocene sediments where LP IV and older material are exposed on the seafloor (Plate 3).

(§tructural activity of the Palos Verdes uplift has continued into

Holocene time up to the present. In the Palos Verdes Hills, Woodring and others (1946) document deformation of alluvium at the crest of

Gaffey anticline, and a gentle seaward dip of the lowest terrace in San

Pedro. Recent deformation is the strongest north of the hills. Mapping of Riccio and Mills (1977) confirms Holocene and possibly modern faulting in the area:] These authors demonstrate Holocene movement on a growth 116

fault which cuts the fourth terrace in the Bluff Cove area along the

western flank of the hills.

(Qn~ the San Pedro shelf the Palos Verdes fault offsets base Holocene,

Holocene sediments, and the seafloor. Bulging of the seafloor along the

fault near the Palos Verdes Peninsula further suggests very recent

activity. Finally, earthquake epicenters presumably associated with the

Palos Verdes fault onshore and many located throughout the Palos Verdes

uplift reflect present activity/(Figure 23). ~

SAN GABRIEL SUBMARINE CANYON

The San Gabriel submarine canyon has been referred to as a minor

slump scar associated with fault scarps (Gorsline and Grant, 1972).

Alternatively, it has also been associated with a speculated synclinor-

ium separating the Wilmington and San Pedro Bay (Palos Verdes Uplift)

anticlinal structures (Nardin and Henyey, 1978). While a fault has

obviously influenced development of at least the western branch of the

canyon (see Plate 2), one is hard pressed to find much evidence of any

slumping, except for one good example off the eastern wall of the east-

ern branch (Figure 26). There, a bend in the canyon may have resulted

in undercutting of the wall. Generally, slope stability of the western

branch walls is controlled by the regional southeast dip of the late-

Quaternary beds through which they cut, and by the general southeast

axial trend of the canyon (Fischer and others, 1977). These relation-

ships produce daylighted beds and out-of-slope dip components only where

the canyon swings westward across the regional dip. In one such area, detailed investigation (Fischer and others, 1977) produced no unequivocal

evidence of slumping or sliding. Along the eastern branch of the canyon Figure 26

CSUN 3.5 kHz line 5003 Slumping off San Gabriel submarine canyon wall 118

I i -I I -l I

LINE 5003

(FIX MARK) NE 3 2 sw

SAN GABRIEl SUBMARINE CANYON

_) \. SLUMP DEPOSIT - - I =:t:Jo m s. = 8.5 m. -150m. l._____j

FIGURE 26 Figure 27

CSUN 3.5 kHz line 5006 LPI and recent channeling in and near the San Gabriel submarine canyon 120

. ------· ------

------· ------:~ · --

LINE 5006

(FIX MARK) NE 9 8 sw

SEAFLOOR VERY RECENT CHANNEL FILL

SUBMARINE CANYON

VERY RECENT

}om.$. =8.5 m. -150m. l___j

FIGURE 27 121 the dip of late Quaternary beds is more south to southwest, and an east­ erly swing of the axial trend produced out-of-slope dip components. In this instance (Figure 27), dipping late Quaternary strata appear to have been eroded by early (LP I time) canyon activity and Holocene sediments drape the canyon wall with no signs of mass movement. The large sub­ marine gully on Figure 27 shows evidence of more than one period of cut and fill. This evidence, in addition to bifurcation of the main course of both canyon branches, and the presence of a submarine fan at the end would be better explained by channelized erosion rather than slumping.

It thus seems reasonable to assume that, as in LP I gullies and analogous older channels, erosion chiefly in the form of density flows, in addition to some minor slope failure, were involved in forming the San Gabriel submarine canyon. As is the case for most such canyons off the Southern

California coast (Fischer and Lee, 1974) this one was probably formed along zones of weakness, principally faults. The notion that the San

Gabriel canyon follows the trend of a synclinorium is not supported by sparker data of McClelland Engineers (USGS, 1979). ~he Wilmington graben may represent a synclinal trend at depth~but any influence on the canyon, is much more indirect than is the case for the San Pedro sea valley.

(Jhe age of initial carving of the· San Gabriel submarine canyon is speculative. The Redondo and Newport submarine canyons may have origi­ nated around 700,000 y.B.P. (Nardin and Henyey, 1978, Felix, 1969) when the Los Angeles basin had filled, and significant amounts of sediments began to reach the area by way of the ancestral Santa Ana·River. The

San Gabriel canyon however, not situated as close to sources of detritus, 122 I , probably did not develop until after deltaic deposits of the San Pedro

Formation had filled the subsiding Wilmington graben to about the area of present shelf-break.

The last active canyon-erosive period was during the stage 2

("Wisconsinan") glacial, evidenced by carving of LP I channels and portions of the canyon now filled with LP IA sediments (Figure 10), and further suggested by pinch out of LP II sediments between LP I and

LP III against the eastern branch of the canyon. This period of chan- neling continued into the Holocene Epoch in the canyon, as indicated by

Figure 21 which clearly shows the Holocene sediments truncated by a very recently filled canyon head and by recent channels~ Figure 27 also ,,- shows very recent Holocene channels and a gully filled with correlative deposits which overlie older LP I fill. Moreover, the modern Newport submarine canyon is reported to have been cut after early Holocene time

(Felix, 1969) judging by the presence of gravels attributable to the

Talbert body at a -72 m depth.

~hanneling of the San Gabriel canyon system must have continued past LP I time, during the initial Holocene fast rise in sea level, up to 6,000 - 5,000 y.B.P. Sedimentation and aggradation eventualLy caught up as the locus of accumulation moved up from the submarine fan and ultimately reached the shelf. Chapter 7

S~UffiY AND CONCLUSIONS

Several conclusions regarding the morphology and evolution of the

San Pedro basin margin can be derived from subdivision of the upper

Quaternary section of the inner basin margin, the distribution of bed- rock units of the outer margin, and the structural and stratigraphic relationships of these two blocks.

A departure from the typical narrow Southern California inner basin margin, (the,_ San Pedro margin owes its' width to an interplay be- tween tectonic events and sedimentation. This effect resulted in the formation of two distinct tectonic elements, separated by the Palos

Verdes fault: the Palos Verdes uplift and the Wilmington graben, a·

Neogene to modern depocenter, bounded shoreward by an unnamed fault zone (Junger and Wagner, 1977).

The Palos Verdes uplift, including the Palos Verdes Hills and a portion of the Santa Monica shelf, represents an emerging block. This is evidenced by elevated marine terraces on the Palos Verdes Hills.

On the San Pedro basin margin structural growth is indicated by seafloor exposure of Neogene and basement rocks, thinning and truncation of units LP IV to LP I, and active seismicity along the trend of the Palos

Verdes fault. The uplift was formed initially during mid(?)-Miocene block faulting related to the formation of the borderland, and was subsequently folded due to convergent dextral shear (Nardin and Henyey,

1978, Junger and Wagner, 1977). Minor folds, which are superimposed on an anticlinorium, are slightly oblique to the Palos Verdes fault and reflect this convergence:)~ The San Pedro sea valley follows,the trend

123 124

of one of the minor folds. ([he modern shelf-edge, though controlled

by faulting at depth (P. J. Fischer, 1979, personal communication),

owes its present relief to folding rather than to the dip component of

faults.

Dart-core data indicate that near-surface bedrock along the crest

of the Palos Verdes uplift is Mohnian in age near the Palos Verdes

Peninsu1~3 These rocks are surrounded by Delmontian and Repettian-age beds. On seismic-reflection profiles these ~trata are folded and overlain unconformably along ~he shelf-edge by undeformed Pleistocene

(Wheelerian) strata. This suggests that folding began in late Miocene time, and continued at least until early Pliocene time. However, whereas no folding younger than middle Pica time is evident from the

Wilmington anticline, deformation of the Palos Verdes uplift has con­ tinued until the present.

The Palos Verdes fault has produced wrench, style deformation.

Numerous short en-echelon trances characterize the shallow expression of the fault, whose active splays have migrated shoreward through time.

Within the Wilmington graben the distribution of late Pleistocene units

LP IV through LP I provides a record of the late Quaternary activity of the fault. The youngest horizon disrupted on all splays is LP III.

Since deposition of that unit, which probably began about 140,000 years

B.P. (based on tentative correlation with oxygen isotope curve,

Figure 13), activity appears to have diminished, as younger horizons are offset on fewer splays. However, offsets of LP II to modern hori­ zons, as well as earthquake epicenters, are indicative of continuing tectonic activity. Along the fault zone activity is concentrated near 125

the Palos Verdes Peninsula, where bowing of the seafloor and several seafloor scarps have been recognized.

The northeastern flank of the Wilmington graben is marked by the

"unnamed" fault zone of Junger and Wagner (1977). Some traces which disrupt Holocene strata have been mapped along this zone, and several earthquake epicenters are recorded along it. Additionally, LP IV and older units abruptly tilt up'ward and LP III and LP II pinch-out in this area. Shoreward of the fault zone relative upward motion and lack of modern sediment influx from coastal rivers are reflected by several areas devoid of Holocene sediments. The Wilmington graben, however, had been the depocenter of a thick sediment prism from the Los Angeles,

San Gabriel and Santa Ana Rivers throughout late Quaternary time. The graben subsided throughout late Pleistocene time, as indicated by thickenings of LP III and LP II. Subsidence was probably initiated after the deposition of Repettian strata (P. J. Fischer, 1979, personal communication) and was greatest near the Palos Verdes fault.

Based on the Holocene/LP I seismic-stratigraphic model and amino­ acid age-dates of correlative units onshore and micropaleotologic information, the LP II through LP IV units are believed to have been deposited in the Wilmington graben as sea-level underwent a series of glacioeustatic fluctuations. These units would probably have been stripped by each ensuing transgression were it not for the subsidence of the graben] Butzer (1974) suggests that with age-date control such litho-stratigraphic sequences can be tied to the marine oxygen-isotope stages, and provide a chronomentric correlation with glacial events.

The writer has tentatively correlated the LP IV to LP I/Holocene units 126

with oxygen-isotope stages 8 through 1. CU»it LP II is correlated with

the lowest marine terrace in the town of San Pedro and the fourth, and possibly third and second terraces in the Palos Verdes Hills (Figure 13).

Both sequences are apparently correlative with the Palos Verdes Sand.

As the Wilmington graben filled, the sediments prograded south and south-eastward behind the Palos Verdes uplift. This progradation is evidenced by the net offlapping sequence of units LP IV to.LP I.

Channels from the Santa Ana River system probably reached the area of present shelf-break during the low-stand of marine oxygen isotope stage 2 ("Wisconsinan"). During this low-stand, a shoreline was at a position marked by a paleo-shelf break coincident with the up-slope termination of numerous filled slope gullies of unit LP I. Debouching of fluvial channels from the shelf, laden with sediments due to a relatively wet post-glacial climate and increased mass wasting, provided erosive power to carve the LP I gullies. The San Gabriel submarine canyon underwent greatest recent channelized erosion during this time, and remained active well into the Holocene Epoc~) A similar model for the late Pleistocene-Holocene evolution of the Texas outer continental shelf and slope was recently proposed by Sidner and others (1978). Five cool/warm fluctuations determined from foraminifers were correlated by these authors with a generalized oxygen-isotope curve in order to place paleo-climatic and sedimentary events into a time framework. (§utbuild­ ing or progradation occurred during regressions and low-stands, and upbuilding accompanied transgressions and high-stands~

Whereas large-scale marine seismic~stratigraphic depositional sequences are difficult to associate with onshore hinterland sequences 127

(Mitchum and others, 1977), the smaller scale analogues of the present study are tentatively correlated with alternating cut and fill cycles in the Newport-Inglewood zone fluvial gaps. This study and the work of

Sidner and others (1978) demonstrate the feasibility of understanding the effects of Quaternary, third-order cycles of sea-level change, by applying seismic-stratigraphic principles to high-resolution seismic- reflection data. These principles have been used to define second-order changes (super-cycles) using deep seismic (CDP) data (Vail and others,

1977).

~ith the addition of reliable age-dates (Cl4• amino acid) and

01s/016 paleotemperature data, the chronometric significance of the

LP I/Holocene to LP IV units can be used to assess the rate of vertical movement and the history of recent activity of the Palos Verdes fault.

Similar information can be applied to investigate the activity of the

San Gabriel submarine canyon in terms of erosion and deposition~

This study proposes a seismic-stratigraphic model for the effect of late Quaternary glacioeustatic fluctuations on continental margins.

The model should be applicable in areas of temperate climate where sedimentation rates are high and late Quaternary uplift is minimal. 128

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FIGURES

FIGURE PAGE

1. Index map 3

2. Location map 5

3. Physiographic and structural setting 20

4. Stratigraphic correlation chart 23

5. CSUN 3.5 kHz Line 5088 - Catalina Schist(?) and buried marine terraces 29

6. USGS Uniboom line 453 - Palos Verdes fault zone 34

7. CSUN 3.5 kHz line 5048- Rhythmically bedded Honterey formation 36

8. USGS Uniboom line 441 - Angular unconformity between presumably lower Pleistocene strata and folded Neogene rocks 40

9, USGS Uniboom line 455 - Shoreward edge of the Wilmington graben, off Huntington Beach 44

10. Diagrammatic section along Shell Oil Co. Uniboom line 103 49

11. Diagrammatic section along Shell Oil Co. Uniboom line 210 49 lOa. Shell Oil Co. Uniboom line 103 50 lla. Shell Oil Co. Uniboom line 210 50

12. LPI/Holocene conceptual depositional model 53

13. Marine 01a/016 isotope curve and correlations 59

14. LPI Paleochannels and paleoshelf-break within the Shell Oil Co. "Beta Platform Sites" survey area 65

15. USGS Uniboom line 455 - LPIV-III-II-Holocene superposition and LPII marine terrace 70

16. USGS Uniboom line 455 - Truncation of LPII along the San Gabriel submarine canyon 74

17. USGS Uniboom line 464- Wave-cut notches (terraces) on the base of LP I 77 135

FIGURE PAGE

18. Holocene/LPI isopach map within the Shell Oil Co. "Beta Platform Sites" survey area 79

19. Shell Oil Co. Sonia II Line 134 - Progressively younger LPI channels northeastward from the Palos Verdes up- lift 81

20. CSUN 3.5 kHz line 5048 - Holocene sediments above Neogene bedrock of the Palos Verdes uplift 87

21. USGS Uniboom line 466- Recently filled San Gabriel submarine canyon head 89

22. Fluvial gaps of the Newport-Inglewood zone 91

23. Major earthquake epicenters in the San Pedro margin vicinity 96

24. CSUN 3.5 kHz line 5026 Evidence of Palos Verdes fault 99

25. CSUN 3.5 kHz line 5048 - Folded Neogene bedrock of the Palos Verdes uplift 103

26. CSUN 3.5 kHz line 5003 - Slumping off San Gabriel sub­ marine canyon wall 118

27. CSUN 3.5 kHz line 5006- LPI and recent channeling in and near the San Gabriel submarine canyon 120 136 r ,

TABLES

TABLE PAGE

1 Specifications and settings for geophysical systems 17

2 Lithologies, thicknesses and pertinent information about San Pedro area formations 26

3 LP IV to Holocene stratigraphic section 56

4 Age-dates of San Pedro area units and marine terraces 62 137

PLATES

PLATE BACK POCKET

1 Trackline map

2 Geologic map

3 Latest Pleistocene (LP I) Holocene isopach

4 Surface and near surface breaks of the Palos Verdes fault zone

5 Cross section A-Al

6 Cross section B-Bl