Big Tujunga

Canyon 

Gold Canyon beds

Red Box Gap 

Vasquez CreekFault 

Post-Miocene Right Separation on the San 

Gabriel and Vasquez Creek Faults, with 

Supporting Chronostratigraphy, Western 

San Gabriel Mountains, California

U.S. Department of the Interior

U.S. Geological Survey

Professional Paper 1759
FRONT COVER

View southeastward down Gold Canyon and (in distance) up part of Big Tujunga Canyon in the southwestern San 

Gabriel Mountains, southern California. Both canyons parallel the strike of the San Gabriel Fault Zone, which cuts 

their lower northeast (left) walls. At the bottom of Big Tujunga Canyon, the fault zone bifurcates—the north branch 

strikes to the east through Red Box Gap, and the south branch, the Vasquez Creek Fault, strikes to the southeast. In 

the bottom of Gold Canyon are outcrops of fossiliferous nearshore shallow marine sandstone of earliest Pliocene 

age, designated the Gold Canyon beds, overlain on the northeast (left) by steeply dipping Pliocene-Pleistocene 

fanglomerate. Both units are truncated by the fault zone at the base of the steep slope to the left. Distinctive clasts 

within the Gold Canyon beds, which can be matched to a clast source unit on the other side of the San GabrielVasquez Creek Fault Zone, imply about 12 km of post-Miocene right separation on the zone. See figures 1 and 2 for 

location. USGS photograph by L.A. Beyer.
Post-Miocene Right Separation on the San 

Gabriel and Vasquez Creek Faults, with 

Supporting Chronostratigraphy, Western 

San Gabriel Mountains, California

Professional Paper 1759

By Larry A. Beyer, Thane H. McCulloh, Rodger E. Denison, Ronald W. Morin, Roy J. 

Enrico, John A. Barron, and Robert J. Fleck

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2009

Any use of trade, product, or firm names is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

Although this report is in the public domain, permission must be secured from the individual 

copyright owners to reproduce any copyrighted material contained within this report.

Suggested citation: 

Beyer, L.A., McCulloh, T.H., Denison, R.E., Morin, R.W., Enrico, R.J., Barron, J.A., and Fleck, R.J., 2009, Post-Miocene 

right separation on the San Gabriel and Vasquez Creek Faults, with supporting chronostratigraphy, western San 

Gabriel Mountains, California: U.S. Geological Survey Professional Paper 1759, 44 p.

Cataloging-in-publication data are on file with the Library of Congress (http://www.loc.gov/)

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This report and any updates to it are available online 

at: http://pubs.usgs.gov/pp/1759/
iii

Contents

Abstract...........................................................................................................................................................1

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

Geologic Setting.............................................................................................................................................3

Pre-Neogene Rocks ......................................................................................................................................3

Neogene Stratigraphy and Chronostratigraphic Findings......................................................................3

Volcanic Rocks of the Topanga Group ..............................................................................................3

Modelo Formation.................................................................................................................................3

Towsley Formation................................................................................................................................8

Pico Formation.......................................................................................................................................8

Saugus Formation.................................................................................................................................8

Chronostratigraphy: Ancilliary Paleogeographic and Strucutral Implications ..........................9

Gold Canyon Beds........................................................................................................................................10

Possible Correlative Strata ...............................................................................................................10

Right Separation on the Vasquez Creek Fault.........................................................................................14

Probable Source of Mount Lowe Intrusive Suite Clasts in the Gold Canyon Beds: 

Constraint on Vasquez Creek Fault Displacement ...........................................................14

Saugus Formation Clasts, Probable Cross-Fault Sources, and Relevant Perspectives .........16

Local Tectonic Block Rotation.................................................................................................16

Clast Size, Bed-Forming Process, and Distribution of Mount Lowe Clasts .....................17

Fault Truncation of Clast Lithofacies Boundary....................................................................17

Relative Abundances of Clast Types ......................................................................................17

Summary......................................................................................................................................19

Conclusions...................................................................................................................................................19

Acknowledgments.......................................................................................................................................22

References ...................................................................................................................................................22

Appendixes

1. Description of Saugus Formation Clasts .................................................................................30

2. Isotopic Age Determinations Using 87Sr/86Sr Ratio ...............................................................34

3. Identification, Age, and Paleoenvironment of Calcareous Nannofossils..........................38

4. Identification, Age, and Paleoenvironment of Foraminifers and Occurrences 

of Fossil Oyster Spat .............................................................................................................40

5. Identification and Zonation of Diatoms ...................................................................................42

6. Age of Basalt by 40Ar/39Ar Incremental-Heating Analysis.....................................................43
iv

Figures

 1. Principal geographic features and simplified fault map of San Gabriel 

Mountains and surrounding parts of southern California ................................................2 

 2. Geologic maps of the southwestern San Gabriel Mountains and adjacent 

area to the west, California ...................................................................................................4

 3. Columnar sections of middle Miocene through middle Pleistocene strata 

showing inferred correlations from west of San Fernando Pass to mouth 

of Big Tujunga Canyon............................................................................................................6

 4. Chronostratigraphic correlation diagram of middle Miocene through middle 

Pleistocene formations of the Merrick Syncline-Gold Canyon and San 

Fernando Pass areas..............................................................................................................7

 5. Local geologic map of Gold Canyon beds and upper Gold Canyon....................................9

 6. Photographs of macrofossiliferous sandstone and polymitic brecciaconglomerate lens of the Gold Canyon beds....................................................................11

 7. Simplified geologic map of the central and western San Gabriel Mountains 

emphasizing outcrops of Mount Lowe intrusive suite, anorthosite-syenite 

complex, and Paleoproterozoic gneiss suite, and faults................................................15

 8. Map showing “Merrick Syncline domain” local tectonic block and hypothetical 

points of block rotation about vertical axes .....................................................................17

 9. Photographs of Saugus Formation fanglomerate in upper Gold Canyon ........................18

10. Maps showing distribution of maximum clast sizes and bed-forming processes 

of the Saugus Formation. ................................................................................................. 20 

11. 40Ar/39Ar age spectrum and isochron diagrams for Topanga Group basalt 

sample from near mouth of Big Tujunga Canyon.............................................................43

Tables

1. Estimates of amount and timing of right separation on the Vasquez Creek Fault 

from published sources and this study..............................................................................12

2. Maximum size, shape, and relative abundance of Saugus clasts derived from the 

Mount Lowe intrusive suite and anorthosite-syenite complex, at 63 localities 

from Pacoima Wash to Gold Canyon... ..............................................................................30

3. Analytical results and derived 87Sr/86Sr ages of Neogene fossil samples at 15

 localities along the southwestern margin of the San Gabriel Mountains and 

in the San Fernando Pass area...........................................................................................34

4. Abundance and CN zone range of identified calcareous nannofossils in Modelo 

Formation samples at three localities from Little Tujunga Canyon to near mouth 

of Big Tujunga Canyon..........................................................................................................38 

5. Abundance, zone range, and suggested paleoenvironment of identified 

foraminifers in Modelo Formation samples at four localities from Little Tujunga 

Canyon to near mouth of Big Tujunga Canyon .................................................................40

6. Abundance and zone range of identified diatoms in Modelo Formation samples

 from two localities near mouth of Big Tujunga Canyon................................................42

7. Results of 40Ar/39Ar age analysis of Topanga Group basalt sample from locality 

near mouth of Big Tujunga Canyon ....................................................................................44
Post-Miocene Right Separation on the San Gabriel and Vasquez Creek Faults, 

with Supporting Chronostratigraphy, Western San Gabriel Mountains, California

Abstract

The right lateral San Gabriel Fault Zone in southern 

California extends from the northwestern corner of the Ridge 

Basin southeastward to the eastern end of the San Gabriel 

Mountains. It bifurcates to the southeast in the northwestern 

San Gabriel Mountains. The northern and older branch curves 

eastward in the range interior. The southern younger branch, 

the Vasquez Creek Fault, curves southeastward to merge with 

the Sierra Madre Fault Zone, which separates the San Gabriel 

Mountains from the northern Los Angeles Basin margin. 

An isolated exposure of partly macrofossiliferous nearshore 

shallow-marine sandstone, designated the Gold Canyon beds, 

is part of the southwest wall of the fault zone 5.5 km northwest 

of the bifurcation. These beds contain multiple subordinate 

breccia-conglomerate lenses and are overlain unconformably 

by folded Pliocene-Pleistocene Saugus Formation fanglomerate. The San Gabriel Fault Zone cuts both units.

Marine macrofossils from the Gold Canyon beds give an 

age of 5.2±0.3 Ma by 87Sr/86Sr analyses. Magnetic polarity 

stratigraphy dates deposition of the overlying Saugus Formation to between 2.6 Ma and 0.78 Ma (Levi and Yeats, 2003). 

Distinctive metaplutonic rocks of the Mount Lowe intrusive 

suite (of Morton and Miller, 2003; Barth and Ehlig, 1988) in 

the San Gabriel Range are the source of certain clasts in both 

the Gold Canyon beds and Saugus Formation. Angular clasts 

of nondurable Paleocene sandstone also occur in the Gold 

Canyon beds. The large size and angularity of some of the 

largest of both clast types in breccia-conglomerate lenses of 

the beds suggest landslides or debris flows from steep terrain. Sources of Mount Lowe clasts, originally to the north or 

northeast, are now displaced southeastward by faulting and 

are located between the San Gabriel and Vasquez Creek faults, 

indicating as much as 12±2 km of post-Miocene Vasquez 

Creek Fault right separation, in accord with some prior 

By Larry A. Beyer1 , Thane H. McCulloh2 , Rodger E. Denison3 , Ronald W. Morin4 , Roy J. Enrico5 , John A. 

Barron1, and Robert J. Fleck1

estimates (Ehlig, 1975b; McCulloh and others, 2001). PostMiocene right slip thus transferred onto the Vasquez Creek 

Fault southeast of the bifurcation. The right separation on the 

Vasquez Creek Fault adds to the generally accepted 22-23 km 

of middle-late Miocene right separation established for the San 

Gabriel Fault east of the bifurcation (Ehlig, 1981), resulting in 

total right separation of 34-35 km northwest of the bifurcation.

Clast sizes and lithologies in Saugus Formation deformed 

alluvial fan deposits in the Gold and Little Tujunga Canyons 

area indicate that alluvial stream flow was from the north 

or north-northeast. The alluvial fan complex is beheaded at 

the San Gabriel Fault Zone, and no correlative deposits have 

been found north of the fault zone. Likely sources of several 

distinctive clast types are east of the bifurcation and north of 

the Vasquez Creek Fault. Combining these data with right slip 

caused by the 34°±6° of clockwise local block rotation (Levi 

and Yeats, 2003) suggests that post-Saugus Formation (<2.6 to 

0.78 Ma) right separation along the fault zone is 4±2 km.

The fossils, lithology, and age of the Gold Canyon beds 

correlate with the basal Pico Formation (Oakeshott, 1958). 

The beds presumably connected southward or southwestward 

to a more open marine setting. A search for correlative strata 

to the south and southwest found that some strata previously 

mapped as Towsley Formation correlate with the Modelo 

Formation. Oyster spat in some Modelo Formation beds are 

the first recorded fossil occurrences and are especially remarkable because of associations with Miocene bathyal benthic 

foraminifers, planktonic calcareous nannofossils, and diatoms. 

Topanga Group basalt resting on basement rocks between 

Little and Big Tujunga Canyons gives an age of 16.14±0.05 

Ma from 40Ar/39Ar analysis. Improved understanding of the 

upper Miocene stratigraphy indicates large early movement on 

the eastern Santa Susana Fault at about 7-6 Ma. 

Introduction

The San Gabriel Fault Zone, a major zone of right slip in 

southern California, extends for >128 km from the northwestern corner of the Ridge Basin southeastward to the eastern end 

of the San Gabriel Mountains (Ehlig, 1975b; Morton, 1975; 

Dibblee, 1982; Powell, 1993, p. 33-34; Nourse, 2002; Crowell, 

1

 U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025

2

 7136 Aberdeen Ave., Dallas, TX 75230

3

 Department of Geosciences, University of Texas at Dallas, P. O. Box 830688, 

 Richardson, Texas 75083

4

 Morin Biostratigraphic Studies, 2016 Wedgewood Lane, Carrollton, TX 75006

5

 11137 Nile River Ct., Rancho Cordova, CA 95670
2 Post-Miocene Right Separation on the San Gabriel and Vasquez Creek Faults

2003, p. 172; Yeats and Stitt, 2003) (fig. 1). The fault zone 

branches to the southeast in the western San Gabriel Mountains. The north branch, the San Gabriel Fault, with 22-23 km 

of right separation (Ehlig, 1968, 1981; Powell, 1993, p. 34; 

Nourse, 2002), curves eastward from the bifurcation. The south 

branch, the Vasquez Creek Fault, curves southeastward through 

the mountains as a right-reverse-slip fault; it merges with the 

north-dipping reverse Sierra Madre Fault Zone along the southern mountain edge (Dibblee, 1968; Powell and others, 1983; 

Smith, 1986; Crook and others, 1987) (fig. 1). Slip histories of 

the San Gabriel and Vasquez Creek Faults are part of the structural evolution of the northern Los Angeles basin region (see, 

for example, McCulloh and others, 2001; Yeats, 2004).

The Gold Canyon beds are a sequence of partly macrofossiliferous marine sandstone with breccia-conglomerate 

lenses, exposed over a small isolated area 5.5 km along the 

San Gabriel Fault Zone northwest of its bifurcation. The beds 

crop out only in Gold Canyon, a minor southeast-draining 

tributary to lower Big Tujunga Canyon (fig. 2A). These strata, 

whose approximate age has previously been estimated as late 

Miocene or Pliocene (for example, Hill, 1930; Oakeshott, 

1958), lie on the south fault block and are cut by the fault zone 

(Hill, 1930; Oakeshott, 1958; Weber, 1982; this report). 

Ehlig (1975b, p. 14) recognized clasts from the marginal 

zone of the Mount Lowe intrusive suite (Morton and Miller, 

2003) in the Gold Canyon beds and noted their “closest possible source area...on the north side of the southern branch of 

the [San Gabriel] fault”; he concluded “a right slip of 7 miles 

[11 km] would be a minimum displacement along the southern 

branch of the fault.” Our discovery of angular, boulder-size 

Mount Lowe clasts in multiple polymictic breccia-conglomerate lenses reinforces Ehlig’s clast evidence. Ehlig (1975b, p. 

14) also states that “lower Pleistocene alluvial sediments of the 

Saugus Formation are faulted and intensely deformed where 

they occur along the south side of the San Gabriel Fault Zone 

in Little Tujunga Canyon, but there is little evidence for major 

0 5 10 15 20 25 30

KILOMETERS

North

EASTERN

VENTURA

BASIN

SANTA MONICA

SAN

FERNANDO

VALLEY

MTNS.

SAN

GABRIEL

LOS ANGELES

BASIN

MOUNTAINS

SAN

GABRIEL

VALLEY

POMONA

VALLEY ?

34 ̊30'

34 ̊00'

118 ̊30' 118 ̊00'

SAN JOSE HILLS

SAFZ

SAFZ

FM

SG

RIDGE

BASIN

SFQ

C

CC

SS

SS

MC

SOLEDAD BASIN

SG

V

SMo

H

R

ER

PA

SM

CM

EC

SM

ML

SF

Pacific Ocean

EXPLANATION

Fault, dashed where uncertain, dotted

where buried.

Thrust fault, saw teeth on upper plate.

Fig. 7

Fig. 2A

bifurcation

Faults queried where doubtful.

?

VC

C-S

? ?

?

? ?

? ? ?

Figure 1. Principal geographic features and simplified fault map of San Gabriel Mountains and surrounding parts of southern 

California. The San Gabriel Fault bifurcation and areas of figures 2A and 7 are also shown. Fault labels: C, Canton, CM, 

Cucamonga, C-S, Clamshell-Sawpit, EC, Evey Canyon, ER, Eagle Rock, MC, Malibu Coast, ML, Mount Lukens, R, Raymond, SAFZ, 

San Andreas Fault Zone, SF, San Fernando, SFQ, San Francisquito, SG, San Gabriel, SM, Sierra Madre-Duarte, SMo, Santa 

Monica, H, Hollywood, SS, Santa Susana, VC, Vasquez Creek, V, Verdugo. Geographic labels: FM, Frazier Mountain area, CC, 

Castaic, PA, Pasadena.
Neogene Stratigraphy and Chronostratigraphic Findings 3

strike-slip faulting.” However, smaller and better rounded 

Mount Lowe clasts of the same kind that Ehlig recognized in 

the Gold Canyon beds also occur in unconformably overlying 

Saugus Formation fanglomerate in Gold and Little Tujunga 

Canyons. We reemploy Ehlig’s reasoning with other data to 

postulate some right separation on the San Gabriel-Vasquez 

Creek Fault Zone since Saugus Formation deposition. 

Geologic Setting

Paleocene to Holocene sedimentary units are exposed 

near the Gold Canyon beds and to the southwest (fig. 2A). 

These units occur above the seismogenic north-dipping eastern 

Santa Susana and San Fernando reverse fault zones (Wentworth and Yerkes, 1971, p. 13; U. S. Geological Survey, 1971, 

p. 57; Yeats, 1987, p. 155 and fig. 9-1) or western range-front 

breaks of the Sierra Madre Fault Zone (Oakeshott, 1958, p. 92; 

Crook and others, 1987, p. 42). Coarse clastic Neogene units 

(Oakeshott, 1958, p. 69-85) were derived from San Gabriel 

Mountains basement rock suites (Jennings, 1977; Ehlig, 

1975a; Powell, 1993, appendix 1; Morton and Miller, 2003, 

p. 146-147; Yerkes and Campbell, 2005, p. 30-33). Neogene 

strata range from boulder conglomerate to siltstone, with 

subordinate foraminiferal and diatomaceous beds. These strata 

present a ~16-m.y. record of complex uplifts of the mountains 

and alternations of marine and nonmarine clastic deposition. 

Sedimentation began in shelf to lower slope marine settings 

close to a rocky shoreline and concludes on Holocene alluvial 

fans. 

The San Gabriel Fault Zone north of the Gold Canyon 

beds consists of three major strands, the Dillon, De Mille, and 

Watt Faults (Hill, 1930, p. 146-148; Oakeshott, 1958; Weber, 

1982, plates 1A and 1B; Lee and Schwarcz, 1996). The folded 

Watt Fault, the oldest strand, cuts the Saugus Formation and 

Gold Canyon beds (fig. 2A). The De Mille Fault cuts the Watt 

and Dillon Faults and trends to the east-southeast past the 

bifurcation as the Vasquez Creek Fault (Weber, 1982; Dibblee, 

1991b, 2002a). North-south shortening after Saugus Formation 

deposition is evidenced by the northwest-trending Merrick 

Syncline and by the Buck Canyon, Lopez, and Sunland reverse 

faults—breaks that dip north- to northwest and separate basement from overridden sedimentary units (Hill, 1930; Oakeshott, 1958; Kahle, 1975; Ehlig, 1975b) (fig. 2A). 

Pre-Neogene Rocks

The San Gabriel Mountains expose basement rocks ranging from Paleoproterozoic gneisses to Cretaceous granodiorites (Jennings, 1977; Ehlig, 1975a; Powell, 1993, appendix 

1). Three of these basement rock suites, relevant here, are 

described in appendix 1.

Paleocene Martinez Formation (of Oakeshott, 1958) 

marine sandstone with thin siltstone interbeds and wellcemented cobble conglomerate containing well-rounded 

durable clasts are the oldest strata exposed near the Gold Canyon beds. These strata crop out within the San Gabriel Fault 

Zone in a 12-km-long series of slices extending northwestward from Gold Canyon (Oakeshott, 1958, p. 57-58; Dibblee, 

1991a, 1991b) (fig. 2A). If these strata once extended east of 

their present eastern limit, evidence for such an extension is 

lacking.

Neogene Stratigraphy and 

Chronostratigraphic Findings

No sedimentary or volcanic strata older than 16 Ma are 

present in the southwestern San Gabriel Mountains south 

of the San Gabriel Fault Zone and east of the west-dipping 

normal Whitney Fault (Oakeshott, 1958, p. 44 and plate 1; 

Huftile and Yeats, 1996, fig. 5, cross section D; Yeats and Stitt, 

2003, fig. 3) (fig. 2A). Neogene formations show abrupt large 

changes in thickness and gross lithology. Few are fossiliferous, and microfossils, where present, are generally the most 

age-diagnostic forms. Fossil localities referred to here and in 

the appendixes appear in figures 2A and 2B. Measurements 

of 87Sr/86Sr were made on macrofossil shell, oyster spat, and 

fish debris and phosphatic material from a coprolite collected 

from units possibly correlative with the Gold Canyon beds or 

other relevant units (appendix 2). Calcareous nannofossils, 

benthic foraminifers, and diatoms were identified and support 

most isotopic determinations (appendixes 3, 4, and 5). Abrupt 

regional thickness changes are portrayed in figure 3. Figure 4 

emphasizes time to focus on recognized hiatuses in the depositional record. Numeric ages assigned to key stratal units in 

these figures are detailed in tables 3-7. 

Volcanic Rocks of the Topanga Group

Basaltic andesite flow breccia and basalt flows of middle 

Miocene age, at least 90 m thick, rest unconformably on basement about 4 km southwest of the Gold Canyon beds (Hill, 

1930; Oakeshott, 1958; Barrows and others, 1974; Dibblee, 

1991b). The age of agglomeratic basaltic andesite (locality D, 

fig. 2B), probably autobrecciated during flow, is 16.14±0.05 

Ma by 40Ar/39Ar analysis (appendix 6), augmenting data from 

presumably correlative 14.5±1 Ma basalt 25 km to the west 

(Turner, 1970; Weigand and Savage, 1993, p, 97, 99). Originally assigned to the middle Miocene Topanga Formation (of 

Shelton, 1955, and Oakeshott, 1958), these rocks correlate 

with part of the Conejo Volcanics Formation of the Topanga 

Group (Yerkes and Campbell, 1979).

Modelo Formation

Strata correlative with the Modelo Formation are exposed 

eastward from Pacoima Wash along the south flank of the 
4 Post-Miocene Right Separation on the San Gabriel and Vasquez Creek Faults

Figure 2. Geology of the southwestern San Gabriel Mountains and adjacent area to the west, California. Geology north of the San Gabriel Fault Zone and 

south of the Sierra Madre, San Fernando, and Santa Susana fault zones is not shown. Shown in A or B are fossil localities C, and E through X (appendixes 

2 through 5), 40Ar/39Ar-dated basalt locality D (appendix 6), columnar sections C1 through C9 (fig. 3), drill hole localities S and V (fig. 3), and paleomagnetic 

traverses of Levi and Yeats (1993, 2003). Fault labels: BC, Buck Canyon, D, Dillon, DM, De Mille, L, Lopez, S, Sunland, SF, San Fernando, SG, San Gabriel, SM, 

Sierra Madre, SS, Santa Susanna, VC, Vasquez Creek, W, Watt, WH, Whitney. Geographic labels: BTC, Big Tujunga Canyon, LTC, Little Tujunga Canyon, 
Neogene Stratigraphy and Chronostratigraphic Findings 5

PW, Pacoima Wash, SFP, San Fernando Pass. A, Generalized geologic map of the southwestern margin of the San Gabriel Mountains and eastern Santa 

Susana Mountains adapted from Hill (1930), Winterer and Durham (1962), Barrows and others (1974), Weber (1982), Crook and others (1987), and Dibblee 

(1991a, 1991b). Inset frames show areas of figures 2B and 3. B, Geologic map of area between Pacoima Wash and Big Tujunga Canyon with revised Modelo 

Formation-Towsley Formation contact (adapted and partly simplified from Hill, 1930; Oakeshott, 1958; Barrows and others, 1974; Crook and others, 1987; and 

Dibblee, 1991a, 1991b). Geology along and near the Sunland Fault is from Dibblee (1991b), based on our mapping and field checks.
Geologic C

K 5.5 ± 0.4 Ma * 6.4 ± 0.3 Ma 5.9 ± 0.4 Ma 2.7 ± 0.5 Ma 6.3 ± 0.3 Ma 5.7±0.2 Ma, PH “2.3 Ma” (Levi and Yeats, 1993) “2.6 Ma” (Levi a

FOR9.3 ± 0.5 

Pacoima Fm. Saugus Fm. Saugus Fm.

Topanga Gp ?

Saugus Fm.

Saugus

Fm.

Saugus Fm

Paleogene

Locality V

SEA LEVEL S

datum for

stratigraphic columns

Pico Fm.

Pico Fm. ??

Pico Fm.

Towsley

Modelo

Fm.

Fm.

Union Oil Co. of California“Needham” No. 11951, TD 4,008 feet

Towsley

Fm.

congl. Model

Modelo shale

Fm

X

U

P

N

L

M

K

magnetostratigraphy J 

(Levi and Yeats, 1993)

SANTA SUSANA FAULT

Stratigraphic Thickness

Sun Oil Co.“Stetson Sombrero” No. 11950, TD 12,027 feet

Locality S

CNDIAFOROSFDPH

c

angular unconformi

terminated within fowhere noted

interfingers with or lithologic facies cha

dfoofifi

C1

C2

C3 C4 C5

C6

C7

C8

6.43 km N54°W 4.37 km N86°W 4.33 km N58°W 6.95 km N58.5°W 3.04 km 5.79 km 

1.37 km 1.26 km

N74.5°W N42°E

?

?

?

?

? ? ?

?

Fm.

N97.5°W

meters feet

truncated by fault

lo

50004000300020001000

0

15001200

900600300

0

NW 

Figure 3. Columnar sections of middle Miocene through middle Pleistocene strata showing inferred correlations from west of San Fernando Pass Tujunga Canyon. Locations are shown in figure 2A. Thicknesses are measured from geologic maps of Oakeshott (1958), Winterer and Durham (19621991b, 1992), Levi and Yeats (1993), and Wright (2001), tempered by our work. Most age determinations and ranges are based on 87Sr/86Sr isotope a

macrofossil shell material (appendix 2). Underlined exceptions are fish debris (locality M), calcareous nannofossils (locality K), and calcareous nandiatoms (locality C) (fig. 2B). Ages are labeled with letters that correspond to fossil localities shown in figures 2 and 6, and discussed in the text an
San Fernando Pass Area Merrick Syncline - Gold Canyon Area

PACOIMA FM

PICO FM

MODELO FM

MODELO FM

diatomscalc nannoscalc nannosoyster spat fossil phosphate

Topanga Gp basalt

K/Ar

(Turner, 1970;

Weigand and

Savage, 1993)

?

?

X

N

A-B

U

M+

++

++

+

P L

K

E

D

F C

G

C*

*

*

°

°

?

? ? ? ?

?

?

?

?

?

?

?

?

?

?

?

? ? ?

?

?

? ?

? ?

SAUGUS FM

SAUGUS FM

PICO FM

Sunshine Ranch

Member ?

TOWSLEY FM

TOWSLEY FM

SANTA SUSANA FAULT

++

E

PolarityDiatom zones and subzones

PLEIST. PLIOCENE

LATE

CN15

CN14CN13CN12CN11CN10CN9CN5CN8

 5

 6

 7

 8

 9

10

11

12

 4

 3

 2

 1

LATE

C5ACC5ABC5AA

C5A

C5

C4A

C4

C3BC3A

C3

C2A

C1C2

ab

c

aba

cc

b

c

dadba

b

CN4CN3

MIOCENE

MIDDLE

Time (Ma)

Epoch

Chrons

Calcareous nannoplankton zones and subzones

15

16

C5AD

C5B

C5C

EARLY

CN7

CN6

ab

14

13

California

benthic

foraminiferal

stages

Relizian

Luisian

Mohnian

Delmontian

Repettian

Venturian

Wheelerian

Hallian

E M

Extension

(Nl Flti)

Compressive folds & faults

?

? ? ?

Tran

N. koizumii -N. kamtschatica

D. hustedtii

Thalassiosira

osetrupii

Thalassiosira

antiqua

Nitzschia

reinholdii

Denticulopsis hustedtiiD. lauta

Denticulopsis 

lauta

b

b

a

a

c

b

d

b

a

a

N. seminae

A. oculatus

N. koizumii

P. curvirostris

C.sawamurae

Figure 4. Chronostratigraphic correlation diagram of middle Miocene through middle Pleistocene formations of the Merrick Syncline-Gold Canyon and areas. Diagonal striping indicates missing section. Wavy lines denote unconformities or discontinuities. Vertical lines with terminating brackets are 87Sr/8

macrofossil shells, fish debris or oyster spat and one 40Ar/39Ar basalt age. Vertical rectangles are age ranges of calcareous nannoplankton, and diatoms. 

localities discussed in text or shown in figures 2 and 5, and the appendixes. Reference to Sunshine Ranch Member of Winterer and Durham (1958) at site the possibility that those marine fossils might represent the basal marine member of the Saugus Formation (see also Oakeshott, 1958, p. 82-83). Regional twork and the published record. Sources of reference columns are Lourens and others (2004), Barron and Gladenkov, (1995), Barron and Isaacs (2001, fig. (1980), with minor adjustments to the benthic foraminiferal stage boundaries. Normal polarity in the magnetic polarity column is shown in black, reversed 
8 Post-Miocene Right Separation on the San Gabriel and Vasquez Creek Faults

Merrick Syncline and also locally on its northeast flank, where 

they rest unconformably on Topanga Group lavas (Oakeshott, 

1958; Dibblee, 1991a, 1991b) (fig. 2B). The type Modelo 

Formation is located in the Santa Monica Mountains about 

30 km southwest of Big Tujunga Canyon (Hoots, 1931, p. 

102-115; Winterer and Durham, 1962, p. 286-287; Blake, 

1991, fig. 5). It ranges in age from the Luisian through part 

of the Delmontian benthic foraminiferal stages (BFS) (15-

14 Ma to 7.7-6.5 Ma, Barron and Isaacs, 2001, p. 394). In 

the study area, porcelaneous, diatomaceous, and calcareous 

siltstone and shale occur with subordinate interbedded 

arkosic sandstone; pebbly sandstone and cobble to boulder 

conglomerate dominate locally (Oakeshott, 1975, p. 24; 

Kahle, 1975, p. 120). 

We revise the top of the Modelo Formation along the 

south flank of the Merrick Syncline to be within the Towsley 

Formation of others (Barrows and others, 1974; Dibblee, 

1991a, 1991b). Somewhat lenticular, massively bedded cobble 

to boulder conglomerate units, previously assigned to the 

lower part of the Towsley Formation (for example, “Ttog” of 

Dibblee, 1991a, 1991b), are submarine-canyon or proximal 

submarine-fan deposits, based on benthic foraminifers from 

thin, fine-grained, calcareous siltstone interbeds and lenses 

(localities K and J, fig. 2B, and appendix 4). They are faunally 

equivalent to older parts of the type Modelo Formation. Strata 

we correlate with the Towsley Formation occur unconformably 

above these conglomerate beds and consist of more friable, 

massive to thinly laminated and bioturbated siltstone, with 

interbedded sandstone and weakly cemented conglomerate. 

The sandy siltstone beds and lenses, and thin to thick beds of 

sandy conglomerate (Oakeshott, 1958, p. 76), are consistent 

faunally and lithologically with marine deposition in outer 

to inner shelf environments. Near Pacoima Wash our revised 

Modelo Formation strata are up to 2,000 m thick, but they thin 

over a short distance to the east (Wright, 2001, fig. 5). 

Towsley Formation

The type Towsley Formation (Winterer and Durham, 

1958, 1962) contains microfossils of the Delmontian BFS 

(7.7-6.5 Ma to 5.2 Ma, Barron and Isaacs, 2001, p. 394). 

Basin-margin marine strata assigned to the Towsley Formation 

buttress against or onlap the basement complex. These strata 

occur partly as isolated erosional remnants atop the upper 

plate of the Santa Susana Fault Zone in the foothills of the 

westernmost San Gabriel Mountains southeast of San Fernando Pass (Winterer and Durham, 1962, p. 291-292 and fig. 

52; Dibblee, 1991a) (fig. 2A). South of the Santa Susana Fault 

Zone and farther east between Pacoima Wash and Big Tujunga 

Canyon, an eastward thinning sequence of partly shallowmarine sandstone and conglomerate with sandy siltstone and 

silty claystone is late Miocene to earliest Pliocene in age. 

Mapped as “Repetto formation” (Oakeshott, 1958, p. 75-78), 

“Towsley and/or Pico Formations undifferentiated” (Barrows and others, 1974, plate 2), and “Towsley (?) Formation” 

(Dibblee, 1991a, 1991b), parts of this sequence as described 

above are reinterpreted here as older Modelo Formation (fig. 

2B). The thickness of the revised Towsley Formation in Lopez 

Canyon is 650 m (reinterpreted from Oakeshott, 1958, plate 3, 

sec. BB’, and Dibblee, 1991a). 

Pico Formation

Marine siltstone, sandstone, and conglomerate of the 

Pico Formation (“Fernando Formation” of Yeats and others, 

1994, p. 1055) are early to earliest late Pliocene age. These 

strata interfinger along a diachronous contact with underlying 

siltstone and fine-grained sandstone of the Towsley Formation west of San Fernando Pass (Winterer and Durham, 1962, 

p. 308; Squires and others, 2006) (fig. 2A). Correlative strata 

have been mapped just southeast of San Fernando Pass within 

the Santa Susana Fault Zone (Winterer and Durham, 1962, 

p. 311 and plate 44) and reported to the south in the northern 

San Fernando Valley subsurface (Oakeshott, 1958; Tsutsumi 

and Yeats, 1999). They have not previously been recognized 

in outcrops between Pacoima Wash and Big Tujunga Canyon. 

We assign the isolated Gold Canyon beds to the Pico Formation (see below).

Saugus Formation

Alluvial and fluvial strata of the Saugus Formation (Kew, 

1924; Winterer and Durham, 1962, p. 317-320; Oakeshott, 

1958, p. 83-85; fig. 2B) are light gray to buff, moderately consolidated, poorly sorted, coarse- to medium-grained arkosic 

sandstone interbedded with polymictic pebble to boulder 

conglomeratic sandstone and conglomerate. Individual beds 

are lenticular. Conspicuous and locally thick beds of reddish sandy to clayey siltstone to fine-grained sandstone are 

abundant in the lower part of the section in the Little Tujunga 

Canyon area (Oakeshott, 1958, p. 84; this study) and may 

be paleosol or overbank deposits. Conglomerate is coarser 

grained and more abundant east than west of Little Tujunga 

Canyon (Oakeshott, 1958, p. 84). Approximately 3,700 m of 

Saugus Formation fanglomerate was penetrated by a drill hole 

west of Pacoima Wash (Oakeshott, 1958; Tsutsumi and Yeats, 

1999, fig. 4, section FF’; Wright, 2001, fig. 2) (locality W, fig. 

2A). The fanglomerate sequence thins southeasterly to 2,000 m 

at Lopez Canyon, 1,300 m just west of Little Tujunga Canyon 

(Levi and Yeats, 2003, p. 15-5), 600 m 3 km east of Little 

Tujunga Canyon, and is absent east of Big Tujunga Canyon 

(Oakeshott, 1958, p. 83-84; Crook and others, 1987) (fig. 2B).

The late Pliocene to middle Pleistocene age of the Saugus Formation is based on fragmentary terrestrial vertebrate 

fossils (Winterer and Durham, 1962, p. 322; Saul, 1975, p. 

55). A local age estimate of the Merrick Syncline section is 

based on analyses of magnetic polarity traverses made about 

13 km west (“Van Norman Lake domain”) and about 23 km 

west-northwest (“Transmission Line” or “Magic Mountain 

domain”) (Levi and Yeats, 1993, 2003) (fig. 2A). Polarity 

reversal patterns and correlations, a dated ash, and inferred 
Neogene Stratigraphy and Chronostratigraphic Findings 9

sedimentation rates date deposition of both sequences to 

between 2.3 and 0.5 Ma. Saugus Formation magnetic polarity 

stratigraphy in Little Tujunga Canyon “suggests deposition 

during the Matuyama chron, 0.78-2.60 Ma” (Levi and Yeats, 

2003) (fig. 2B), but the data do not record the Matuyama chron 

bounding magnetic reversals. We consider the chron to represent bounding time limits only. 

Chronostratigraphy: Ancillary Paleogeographic 

and Structural Implications

A significant hiatus with local evidence of erosion 

separates 16.14±0.05 Ma Topanga Group volcanic rocks from 

overlying littoral marine strata of the basal Modelo Formation on the northeast flank of the Merrick Syncline east of 

Little Tujunga Canyon (localities D, G, and E, fig. 2B). Partly 

pebbly mollusk-bearing Modelo Formation sandstone and 

siltstone grade upward into foraminiferal and locally diatomaceous, thinly bedded calcareous clayey siltstone and shale. 

The siltstone and shale are probably older than 11.5 Ma near 

their base (appendix 5). The original full easterly extent of the 

basal Modelo Formation beds is unclear, but surface mapping 

(Oakeshott, 1958; Dibblee, 1991a, 1991b) and an isopach 

map (Wright, 2001, fig. 5) suggest that the eastern limit of a 

west-opening mid-Miocene marine embayment may have been 

at long 118 ̊17.5’W, about 6 km west-southwest of the San 

Gabriel Fault bifurcation. 

The disconformable base of the Towsley Formation along 

the south flank of the Merrick Syncline correlates with an 

unconformity separating fossiliferous basal Towsley Formation strata from basement rocks north of the Santa Susana 

Fault Zone in the area 4 km northwest of Pacoima Wash. 

Stratigraphically, localities L and M correlate roughly with 

locality P near the base of the Towsley Formation (fig. 2A

and 2B, and appendix 2, table 3). The onset of large reverse 

slip on the Santa Susana Fault Zone of this area (Yeats, 1979, 

2001) is thus dated at 7-6 Ma (during the Delmontian BFS). 

Probably more than 2 km of Modelo Formation strata were 

eroded from the upthrown north block of the Santa Susana 

Fault Zone before deposition of the basal Towsley Formation beds at 6 Ma. The base of the Modelo Formation south 

of the Santa Susana Fault Zone, west of Pacoima Wash, is 

estimated to be about 6 km below sea level (Wright, 2001, fig. 

5). Santa Susana Fault growth was continuing at 5.2 Ma when 

littoral marine Gold Canyon beds were deposited, suggesting 

that uplift of the structurally high north block was episodic 

and subsidence of the south block dominated. Fault growth 

continued through the Pleistocene, as indicated by the present 

700-m elevation of the Gold Canyon beds and by the 3.7 km 

118°17’30”W 118°17’00”W

N

0 100 200 300 400 METERS Contour interval is 40 feet (12.2 m)

Tpgc

Qoa

QTs

QTs

Tmz

Watt

De Mille Fault

bc

B A

ls

bc

?

?

?

?

?

?

?

? ? ?

?

bc

bc

Fault

40

30

12

45

45

55

57

60

65

Landslide deposits (Holocene)

Older alluvial terrace, unconsolidated and dissected (Pleistocene

to Holocene?)

Saugus Formation (late Pliocene

to early Pleistocene

Gold Canyon beds of Pico 

Formation (Pliocene)

Martinez Formation (Oakeshott, 

1958) (Paleocene)

Plutonic and metamorphic rocks

(Proterozoic to Mesozoic)

ls

Geologic Units

Qoa

QTs

Tpgc

Tmz

bc

45

?

Base from U.S. Geological Survey Sunland quadrangle, 7.5-minute series, 1995

34°

19'

 N

Explanation

Contact, dashed where 

approximately located, 

dotted where buried or 

location undertain.

Fault, dashed where 

approximately located, 

dotted where buried or 

location uncertain.

Strike and dip of stratified 

units

 horizontal vertical

Outcrop of brecciaconglomerate in Gold 

Canyon beds.

Macrofossil locality 

(Appendix 2).

Figure 5. Local geologic map of Gold Canyon beds incorporating our observations (modified from Hill, 1930; Oakeshott, 1958; and 

Weber, 1982). Macrofossil localities A and B (appendix 2) and observed exposures of breccia-conglomerate in the Gold Canyon 

beds are also shown.
10 Post-Miocene Right Separation on the San Gabriel and Vasquez Creek Faults

of Saugus Formation beds penetrated by the Sunray “StetsonSombrero” No. 1 drill hole (Oakeshott, 1958, p. 122; locality 

S, fig. 2A). The 1971 MW 6.7 Sylmar earthquake, whose epicenter is 6 km north of the southwest end of the Buck Canyon 

Fault (fig. 2A), indicates that both the south and north blocks 

continue to rise differentially (Allen and others, 1975; Savage 

and others, 1975; Tsutsumi and Yeats, 1999). 

Gold Canyon Beds 

The Gold Canyon beds are exposed in an isolated area, 

100 m by 400 m, south of the Watt Fault in upper Gold Canyon. They are the easternmost known exposures of fossiliferous marine strata adjacent to the San Gabriel and Vasquez 

Creek Faults within the San Gabriel Mountains (fig. 5). They 

are mostly north dipping; some very poorly stratified brecciaconglomerate lenses may be south dipping, but such attitudes 

are questionable. The base of the sequence is unexposed, but 

might be an unconformity or fault that dips steeply north and 

strikes roughly N. 70 ̊ W. The sequence is overlapped to the 

west by the base of the Saugus Formation. The junction of the 

Watt Fault and the younger De Mille Fault to the northeast is 

obscured by colluvium and older alluvium. These faults cut 

off the easternmost Gold Canyon beds and Saugus Formation 

fanglomerate units (fig. 5). 

The estimated thickness of exposed Gold Canyon beds 

is less than 100 m. The dominant rock type is massive, light 

gray to light gray-brown medium- to fine-grained arkose, 

partly fractured and oyster bearing (fig. 6A). Parts are pebbly or conglomeratic, with polished well-rounded granitoid 

and gneissic cobbles. Some gneiss cobbles contain rutilated 

quartz, suggestive of Paleoproterozoic Mendenhall Gneiss 

(Oakeshott, 1958; appendix 1). Subordinate rock types are (1) 

fractured, dark gray-brown arkose containing rutilated quartz 

grains, (2) poorly sorted polymictic cobble to boulder brecciaconglomerate lenses with green-brown (partly reddish) arkose 

to arkosic grus matrices containing rutilated quartz grains (fig. 

6B), and (3) thin-bedded, dark gray biotitic arkose. 

Multiple horizons of lenticular breccia-conglomerate 

contain angular to rounded clasts derived from the Mount 

Lowe intrusive suite. Some angular to subangular Mount 

Lowe clasts as large as 1 m are matrix supported. Smaller 

clasts are subrounded to rounded (fig. 6B). Subangular to 

subrounded gneissose clasts in these lenses have granoblastic textures and rutilated quartz (appendix 1). Dark colored 

and nondurable subangular clasts as large as boulder size 

are biotite epidote hornblende arkose (one containing an 

altered foraminifer shell), derived from the distinctive but 

fault-separated Paleocene Martinez Formation (Oakeshott, 

1958) marine strata (fig. 5). No clasts from the “anorthositesyenite complex” (Powell, 1993, p. 81; also appendix 1) were 

found. Sources of the large and mostly angular clasts include 

the Mount Lowe intrusive suite, exposed today north of the 

Vasquez Creek Fault 12 km to the southeast, and the Paleocene Martinez Formation (Oakeshott, 1958) exposed today 

only to the north and northwest between strands of the San 

Gabriel Fault Zone (figs. 2A, 5).

Although macrofossils are abundant in parts of the Gold 

Canyon beds, age-diagnostic fossils are lacking. More than 

90 percent of fossil oyster shells examined in the uppermost 

(and relatively fossiliferous) part are ≥12.5 cm long and thick 

shelled, with pearly-lustered laminae. However, most intact 

shells are pervasively fractured. Many separate clusters of 

multiple shells occur. These oysters may be Crassostrea sp. 

(C.L. Powell, II, oral commun., 2004). Less common are separate aggregations of shell fragments with rare indeterminate 

gastropod and pectinid fragments. One 6-cm-long poorly preserved gastropod, possibly Kelletia sp., an echinoid fragment, 

bits of two different crab dactyles (moveable fingers) (C. L 

Powell, II, oral commun., 2004), and one 4-cm shell fragment 

of Lyropecten (?) sp. (J. G. Vedder, oral commun., 2008) were 

found. The nondiverse oyster-dominated fauna suggests a shallow, nearshore marine environment, possibly with restricted 

circulation and brackish water. The absence of shells in growth 

positions and the loose aggregates of smaller broken shell 

fragments indicate some transport before deposition (fig. 6A). 

No microfossils were found.

Our age for deposition of the Gold Canyon beds is 

5.2±0.3 Ma (earliest Pliocene), based on 87Sr/86Sr analyses of 

multiple fossil carbonate samples from five selected oysters 

and one pecten fragment (appendix 2, table 3) (localities A 

and B, fig. 5). This age, close to the Miocene-Pliocene boundary (Barron and Isaacs, 2001, p. 394), is correlative with either 

youngest Towsley Formation or oldest Pico Formation. The 

shallow-marine depositional environment and fauna suggest 

correlation with the basal part of the Pico Formation (Winterer 

and Durham, 1962). Prior age assignments of the Gold Canyon beds range between questionable latest Miocene and early 

Pliocene (Hill, 1930; Oakeshott, 1958. p. 77; Ehlig, 1975b; 

Weber, 1982, p. 65; Dibblee, 1991b).

Possible Correlative Strata

Our chronostratigraphic observations south and southwest of the Gold Canyon beds suggest strata possibly correlative with the Gold Canyon beds. The revised Towsley 

Formation south of the Santa Susana Fault thins eastward 

from Pacoima Wash to Little Tujunga Canyon by erosional 

truncation at the base of the overlying Saugus Formation. The 

base of the Towsley Formation is dated at about 6 Ma (localities L, M, and P, fig. 2A and 2B, and appendix 2, table 3) and 

is above a disconformity or correlative unconformity. Where 

not eroded, the top is a slight erosional angular unconformity 

with a minimum age of about 3 Ma (locality N, fig. 2B, and 

appendix 2, table 3). The macrofossiliferous shallow-marine 

sandstone of this youngest part of the Towsley Formation, just 

east of Pacoima Wash, might correlate with oldest Pico Formation. The upper half of the section assigned to the Towsley 

Formation just east of Pacoima Wash might include unidentified 5.2 Ma strata correlative with the Gold Canyon beds, 

judging from fossil ages at localities M and N (fig. 2B). 
 Gold Canyon Beds 11

A

B

Figure 6. Rock types in the Gold Canyon beds. A, Rain-dampened fossiliferous sandstone of the Gold Canyon 

beds showing some intact but fractured oyster shells (white arrowheads) plus abundant oyster shell fragments 

and associated very rare gastropods, bivalves, and crab fragments. Hammer is 32 cm long. B, Polymictic brecciaconglomerate lens of the Gold Canyon beds containing two prominent angular to subangular boulders (lower left 

and upper right) from the marginal facies of the Mount Lowe intrusive suite (appendix 1). These boulders, together 

with multiple smaller angular to subrounded Mount Lowe clasts, dominate the clast population at this outcrop. 
12 Post-Miocene Right Separation on the San Gabriel and Vasquez Creek Faults

Author 

(publication 

date)

Estimate 

of Vasquez 

Creek Fault 

right separation 

Estimate of 

time interval of slipEvidence of offset Remarks

Oakeshott 

(1958, 

1975)

unspecified unspecified

 "Right-lateral strike-slip movement 

on the San Gabriel fault has probably 

been on the order of 4 miles [~6 km] in 

Quaternary time, judging by displacement of older faults and sediments that 

lie on the crystalline rocks" (Oakeshott, 

1975, p. 27)

Observations of apparent right offset that range from 1 

1/2 to 4 miles [~2 to ~6 km] were observed westward 

from the bifurcation of the San Gabriel Fault to about 

long 118°30 ́W (Oakeshott, 1958, p. 91-92). Displacements on the Vasquez Creek Fault and northwesternmost 

San Gabriel Fault are not discussed.

Ehlig 

(1975b) 11 to 32 km

after 

depostion 

of "gravel 

lens" in 

Pliocene 

marine 

sandstone 

in Gold 

Canyon

"Gravel lens" within sliver of fossiliferous Pliocene marine sandstone along 

the south side of the San Gabriel Fault 

in Gold Canyon (here the Gold Canyon 

beds) contains pebbles and cobbles of 

Mt. Lowe intrusive suite. "The closest possible source area is on the north 

side of the southern branch of the fault 

[Vasquez Creek Fault], 7 miles [~11 km] 

to the east" and "furthest possible source 

area is more than 20 miles [~32 km] to 

the east" (Ehlig, 1975b, p. 14).

No clasts of anorthosite or Mendenhall Gneiss were 

observed in the gravel lens (Ehlig, 1975b, p. 14). Ehlig's 

range of estimated right separation on the Vasquez Creek 

Fault added to his generally accepted estimate of 22 km 

of right separation on the San Gabriel Fault east of its 

bifurcation results in a range of 33 to 54 km of right separation on the San Gabriel Fault west of its bifurcation.

Weber 

(1982, 

1988a)

unspecified unspecified

No measures are specified of total 

offset eastward from San Gabriel Fault 

crossing of upper Pacoima Canyon, but 

multiple observations of small offsets 

of streams and older alluvial deposits 

suggest apparent right, left or reverse 

motion on various strands of the fault 

zone (Weber, 1982, p. 38-47). 

"At least in late Quaternary time, the segment of the [San 

Gabriel] fault from Castaic to Pasadena, including the 

South branch [Vasquez Creek Fault], has consisted of 

a north-dipping reverse fault with a component of right 

slip..." (Weber, 1988a, p. IV-68). "Total right-lateral offset of Precambrian gneiss and anorthosite along the San 

Gabriel fault [nortwestward from the westernmost San 

Gabriel Mountains] is not more than 30 km, and is more 

likely about 5 km" (Weber, 1988b, p. 236).

Matti & 

Morton 

(1993)

22 km

Right separation of 22 km is hypothesized on the Vasquez Creek Fault (their 

"south branch" of the San Gabriel Fault) 

that transfers eastward along the south 

margin of the San Gabriel Mountains to 

offset the northeast end of the Raymond 

Fault about 22 km from a hypothetical 

connection to the southwest end of the 

Evey Canyon Fault (Matti and Morton, 

1993, p. 125-127, Fig. 6E). 

Reconstruction accepts 22 km of right separation (Ehlig, 

1968) on the San Gabriel Fault east of its bifurcation (their 

"north branch" of the San Gabriel Fault) following slip 

on the Vasquez Creek-Sierra Madre faults. Thus the San 

Gabriel Fault west of its bifurcation displays 22+22=44 km 

of right separation. Powell and Weldon (1992, p. 457) believe projection of "large dextral displacement [eastward] 

along the south flank of the San Gabriel Mountains is not 

supported by paleogeologic patterns and is disallowed by 

the continuous distribution of the 15 m.y.-old Glendora 

Volcanics across the projected fault path."

Powell 

(1993)

5 km

3-4 km 

from 6 or 

5 to 4 Ma; 

1-2 km 

from 4 or 3 

Ma to present (Powell, 1993, 

p. 43-44)

Proposes that 5 km of right separation 

on the Vasquez Creek Fault is "consistent with the distribution of Cretaceous 

quartz diorite and monzogranite...and realigns the Mount Lukens and ClamshellSawpit faults. Conversely, the distribution of crystalline rock units along the 

Vasquez Creek fault...seems to preclude 

large lateral displacement on that fault. 

Restoration of any displacement greater 

than a few kilometers results in a misalignment and apparent repetition of the 

quartz diorite line in the southern San 

Gabriel Mountains" (Powell, 1993, p. 

37; also see Powell and others, 1983). 

Review of published work led Powell (1993, p. 36-38) 

to conclude that there is no "compelling paleogeologic 

reconstructions" or "unambiguous piercing points" that 

justify more than the 40 to 45 km of total right separation 

on the northwesternmost San Gabriel Fault. His 40-45 km 

of right separation seems primarily based on the apparent 

right offset by the San Gabriel Fault of the San Francisquito Fault from the Frazier Mountain-Mount Pinos 

stretch of the San Andreas Fault. This is required by his 

reconstructions (Powell, 1993; also see Powell and Weldon, 1992; Weldon and others, 1993, p. 187-188). Powell 

(1993, p. 40, 43-44, Fig. 14) assigns the earliest 13 km of 

right separation "to the Canton fault and its hypothetical 

extension." The next 21-23 km is assigned to the San 

Gabriel Fault and its stretch east of its bifurcation in the 

San Gabriel Mountains (after Ehlig, 1968). The youngest 

5 km is assigned to the Vasquez Creek Fault.

Table 1. Estimates of amount and timing of right separation on the Vasquez Creek Fault, together with evidence for offset magnitude, 

from published sources and this study.
 Gold Canyon Beds 13

Author 

(publication 

date)

Estimate 

of Vasquez 

Creek Fault 

right separation 

Estimate of 

time interval of slipEvidence of offset Remarks

McCulloh 

and 

others 

(2001)

12-13 km

≤ 7 Ma 

(possibly ≤

4.5 Ma)

Latest middle Miocene palinspastic 

reconstruction of 12-13 km of right 

separation on the Vasquez Creek Fault 

plausibly realigns disrupted western 

edge of field of unique Oligocene 

dacitic intrusive bodies (McCulloh and 

ohthers, 2001, p. 22-23, Figs. 6 and 

10). Distributions of unique dacitic 

clasts in middle Miocene conglomerates south of the mountains are favorably restored to locations that correlate 

well to dacite clast source areas, as 

are dismembered Paleogene basinedge deposits and the middle Miocene 

strandline (McCulloh and others, 2000; 

2001, Fig. 11). Reconstruction reconnects the Mount Lukens Fault with the 

north break of the Clamshell-Sawpit 

Fault Zone. 

Reconstruction requires that (1) right separation on the 

Vasquez Creek Fault passed via the Sierra Madre Fault 

Zone to the north-dipping south break of the ClamshellSawpit Fault Zone where at least part of the displacement 

was converted to dip-slip and (2) 22 km of right separation occurred earlier on the San Gabriel Fault east of its 

bifurcation. Left separation on the Santa Monica-Hollywood-Raymond Fault Zone of 13-14 km coordinates with 

clockwise rotation of the block north of these faults and 

south of the Vasquez Creek Fault (McCulloh and others, 

2001, p. 17-23, Figs. 9 and 10).

Nourse 

(2002)

15 km 12-9 Ma 

Proposes 37 km of total right separation on the easternmost part of the San 

Gabriel Fault. Reconstruction transfers 

westward right separation of 22 km 

to the San Gabriel Fault and 15 km to 

a connected Clamshell-Sawpit-Sierra 

Madre-Vasquez Creek fault system 

(Nourse, 2002, p. 180-182, Figs. 6, 

7,and 8). 

Nourse adds 5 km of late Miocene right separation on 

the northwest San Gabriel Fault, absorbed by extension or transtension in the Los Angeles Basin, to concur 

with Powell's (1993) 42 km of total right separation 

there (15+22+5=42 km). Nourse concurs with the Matti 

and Morton (1993) hypothesis that displacement on the 

Clamshell-Sawpit-Sierra Madre-Vasquez Creek fault 

system preceded (12-9 Ma) movement on the San Gabriel 

Fault east of its bifurcation (9-5 Ma). 

Crowell 

(2003)

22-23 km ca. 12 to 

ca. 5 Ma

Crowell (2003, p. 174) concludes that 

45 km of right separation passed into 

the western San Gabriel Mountains 

from the northwest portion of the San 

Gabriel Fault. He accepts 22-23 km 

of right separation on the San Gabriel 

Fault east of its bifurcation (Ehlig, 

1968) and assigns the remaining 22-23 

km of right separation to the Vasquez 

Creek Fault. 

Crowell (2003, p. 157,195-196) assigns 80 km of right 

separation to the northwesternmost San Gabriel Fault. 

Following the hypothesis of Powell (1993; also see Stitt 

and Yeats, 1982), he apportions 35 km (instead of Powell's 13 km) of the earliest separation (from ca. 16-11 Ma) 

to the Canton Fault. 

this 

paper

4 ± 2 km ≤ 2.6 to

≤ 0.8 Ma 

The most proximal facies of the Saugus 

Formation is near the head of Gold 

Canyon where the distribution of clast 

size implies transverse sediment transport across the San Gabriel Fault Zone. 

The mapped boundary of Saugus Formation with and without Mount Lowe 

intrusive suite clasts against the San 

Gabriel Fault Zone west of Gold Canyon defines separately sourced parts 

of a beheaded alluvial fan complex. 

These facts together with a hypothetical link between fault right slip and 

local tectonic block rotation, coupled 

with mean fault slip rate, imply that 

post-Saugus Formation right separation 

on the San Gabriel and Vasquez Creekf 

faults might be about 4 ± 2 km.

Saugus clast size distribution in upper Gold Canyon 

and areas to the west and southwest suggests sediment 

transport toward the west-southwest. Added counterclockwise backrotation of 34° (Levi and Yeats, 2003) 

indicates original sediment transport direction was 

more orthogonal than parallel to the San Gabriel Fault 

Zone. Largest Mount Lowe intrusive suite clasts in Gold 

Canyon suggest transport distances compatible with their 

transport from sources in an ancestral upper Big Tujunga 

Canyon drainage system. 

Table 1.—Continued.
14 Post-Miocene Right Separation on the San Gabriel and Vasquez Creek Faults

The basal Saugus Formation unconformity along the 

south limb of the Merrick Syncline cuts out all Towsley 

Formation strata immediately east of Little Tujunga Canyon. 

Farther east the eroded top of the Modelo Formation is unconformably beneath Saugus Formation fanglomerate (fig. 2B). 

Towsley Formation strata have not been mapped on the north 

limb of the Merrick Syncline, but might be hidden there by the 

Sunland reverse fault (Oakeshott, 1958; Barrows and others, 

1974; Dibblee, 1991b) (fig. 2B). The erosional unconformity at 

the base of the Saugus Formation, clearly evident on the south 

limb of the syncline, probably extends northeastward to the 

faulted north limb where only older Modelo Formation strata 

and Topanga Group lavas crop out north of the Sunland Fault. 

Strata correlative to the Gold Canyon beds are missing there.

Right Separation on the Vasquez Creek 

Fault 

Published estimates of Vasquez Creek Fault right separation range between 5 and 32 km (table 1). Estimates of the 

time of onset of slip range from 12 to 4.5 Ma. Estimates of 

the cessation of slip range from 9 Ma to present. Only two 

published studies use offset features along or near the Vasquez 

Creek Fault, and these give right separation estimates of 5 km 

(Powell, 1993) and 12-13 km (McCulloh and others, 2001). 

The age of the Gold Canyon beds, their clast composition, and 

clasts in the Saugus Formation clarify post-Miocene right separation on the De Mille-Vasquez Creek Faults. Mount Lowe 

intrusive suite clasts in both the Gold Canyon beds and Saugus 

Formation have mapped source outcrops (now displaced to 

the east) located north of the De Mille-Vasquez Creek Faults 

(fig. 7). Clasts of anorthosite-leucogabbro and other Proterozoic basement types in Saugus Formation fanglomerate beds, 

together with their source outcrops (fig. 7), are not definitive 

but are compatible with our findings. 

Probable Source of Mount Lowe Intrusive Suite 

Clasts in the Gold Canyon Beds: Constraints on 

Vasquez Creek Fault Displacement

Mount Lowe boulders in the Gold Canyon beds are the 

main basis for attempts to interpret Vasquez Creek Fault offset 

and corroborate or modify Ehlig’s (1975b, p. 14) suggestion of 

“minimum displacement” of “7 miles” [11 km] on the “south 

branch.” The angularity and size of the largest Mount Lowe 

boulders in the Gold Canyon beds (location 1 on fig. 7) make 

long-distance transport unlikely except by landslide or debris 

flow. Because the trends of the Vasquez Creek-Sierra Madre 

Faults and the south boundary of the Mount Lowe intrusive 

suite are nearly parallel north of Pasadena (fig. 7), multiple 

hypothetical restoration points are possible. The crossing of 

the Arroyo Seco by the Vasquez Creek Fault (location 2 on fig. 

7), the point at which the fault presently is closest to presentday Mount Lowe outcrops, is taken conservatively as a hypothetical minimum restoration point for the Gold Canyon beds 

containing Mount Lowe boulders. (Areas east of Arroyo Seco 

also offer short transport distances for Mount Lowe debris 

but would require even more right separation on the Vasquez 

Creek-Sierra Madre Faults.) Fault displacement from location 

1 to location 2 (fig. 7) would entail 12±2 km of right separation on the DeMille-Vasquez Creek Faults after 5.2 Ma, absent 

hypothetical fault-parallel clast transport. 

Drainage systems during deposition of the Gold Canyon 

beds can only be inferred. Fault-parallel sediment transport by 

debris flows in river channels is hypothetically possible and 

might require less than Ehlig’s (1975b) and our estimates of 

fault offset. However, no evidence was found for fault-parallel 

clast transport that would shorten our 12±2 km right separation estimate. The angularity of the larger Mount Lowe boulders and the presence of angular nondurable Martinez Formation (Oakeshott, 1958) arkose clasts are inconsistent with the 

increased clast travel distance. The largest angular Mount 

Author 

(publication 

date)

Estimate 

of Vasquez 

Creek Fault 

right separation 

Estimate of 

time interval of slipEvidence of offset Remarks

this 

paper

12 ± 2 Km ≤ 5.2 

 0.3 Ma

Multiple outcrops of breccia-conglomeratic 

lenses in the ca. 5.2 Ma-old Gold Canyon 

beds contain Mount Lowe intrusive suite 

clasts, including angular to subangular 

boulders that indicate short transport 

distances. Nearest cross-fault source for 

these large clasts are Mount Lowe intrusive 

suite outcrops north of the Vasquez Creek 

Fault in the drainage of Arroyo Seco and 

adjacent canyons to the east (strengthening the observations and corroborating the 

conclusions of Ehlig, 1975b, p. 14).

Use of modern drainages for ancestral sediment routes is 

risky, especially when the effects on drainages of rapid 

Quaternary uplift along the south mountain front, including some reverse motion on the San Gabriel and Vasquez 

Creek faults, are unmeasured.

Table 1. Estimates of amount and timing of right separation on the Vasquez Creek Fault, together with evidence for offset magnitude, 

from published sources and this study—Continued.
4

4

4

6

5

3

1

2

0 1 2 3 4 5 6 7 8 9 1

KILOMETERS

LTC

GC

RB

Clear

Arroyo

Seco

Ck.

MW

PC

Big

Big

Tujunga

Tujunga

Canyon

Upper

Canyon

118°15‘ W

34°22.5’ N

34°15’

Explanation

Geographic Features

Geologic Units

Pacoima Canyon washLittle Tujunga CanyonGold CanyonRed Box GapMount Wilson

PC

LTC

GC

RB

MW

*

3

Qs Post-Saugus Formation deposits (early Pleistocene to Holocene)

Saugus Formation (late Pliocene to early Pleistocene)Towsley and Modeleo formations, undifferentiated (late Miocene to early Pliocene)Martinez Formation (Oakeshott, 1958) (Paleocene)Mount Lowe intrusive suite (Morton and Miller, 2003), interior phase (Ehlig, 1981) (Triassic)Mount Lowe intrusive suite (Morton and Miller, 2003), marginal phases (Ehlig, 1981) (Triassic)Predominantly plutonic rocks (Mesozoic), undifferentiated; locally includes metasedimentary (Paleozoic) or gneissic (Paleoproterozoic) rocks; south of the San Gabriel fault and east of about Longitude 118° 2.5’ locally consists of up to 50% Mount Lowe intrusive suite inclusions (Morton and Miller, 2003, p. 142, 145).Anorthosite-Syenite Complex (Mesoproterozoic)Gneiss suites; includes Mendenhall Gneiss (Paleoproterozoic)

Contact, accuracy ranges from well located to approximately located.Fault, saw teeth on upper plate of thrust faults.Location discussed in text.Boundaries of upper Big Tujunga Canyon and Arroyo Seco drainages.Sedimentary rocks of Soledad Basin are not shown. 

SG

DM

SF

SM ML

VC

VC

SG

SM

Qs

Qs

Qs

Sunland

*

Pasadena

Figure 7. Simplified geologic map of the central and western San Gabriel Mountains emphasizing mapped outcrops of Mount Lowe intrusive suite, acomplex, and Paleoproterozoic gneiss suite, together with selected faults (adapted from Oakeshott (1958), Carter and Silver (1972), Morton (1973), Ehlig Powell and others (1983), Crook and others (1987), Dibblee (1989, 2002a, 2002b), Nourse (2002), and Morton and Miller (2003)). Distinctive Mount Lowe cbeds and Saugus Formation are derived from the marginal phases of the intrusive suite. White-on-black circled numbers are key locations discussed Tujunga Canyon and Arroyo Seco drainage areas, Clear Creek, and selected geographic features are also shown. Fault labels are as in figures 1 and 2
16 Post-Miocene Right Separation on the San Gabriel and Vasquez Creek Faults

Lowe clasts, less rounded than comparably sized Mount Lowe 

boulders in Arroyo Seco gravels at the Vasquez Creek Fault 

crossing, likely eroded from steep slopes or cliffs. The Martinez Formation clasts imply that Paleocene rocks between the 

Watt and De Mille Faults were proximal to the Gold Canyon 

beds during their deposition. They also show that clast transport was from north to south for the larger angular clasts in the 

breccia-conglomerate lenses of the Gold Canyon beds. 

The poorly stratified and ill-sorted breccia-conglomerate 

lenses with both large angular and smaller rounded clasts are 

interpreted as landslide or debris flow deposits that entrained 

some traction-transported clasts in channels during movement from steep terrain to a nearshore marine shelf or estuary. 

Debris flows from short peripheral drainages along the south 

margin of the San Gabriel Mountains (for example, Scott, 

1971) might be modern analogs. 

Clasts of anorthosite and related gabbroic rocks were 

not found in the Gold Canyon beds (Ehlig, 1975b, p. 14; this 

study). The anorthosite-syenite complex was shedding debris 

before, during, and after deposition of the Gold Canyon beds, 

judging from the scattered occurrences of anorthosite clasts in 

conglomeratic beds of both the Modelo and Saugus Formations between Pacoima Wash and Big Tujunga Canyon, and 

in the Towsley Formation west of San Fernando Pass (Oakeshott, 1958, p. 70, 76, 84; Merifield, 1958, 11, 14, 20; Winterer and Durham, 1962, p. 290). The absence of such clasts 

in the breccia-conglomerate lenses, sequences that represent 

very short time spans, may be coincidental or because coeval 

source drainages, while tapping Mount Lowe intrusive suite 

terrain, did not extend far enough northward to reach the 

anorthosite-syenite complex. Rutilated quartz in clasts and as 

matrix grains of the Gold Canyon beds are interpreted as possibly derived from the older Mendenhall Gneiss (Oakeshott, 

1958; appendix 1). 

Saugus Formation Clasts, Probable Cross-Fault 

Sources, and Relevant Perspectives

We interpret post-Saugus Formation right separation on 

the De Mille-Vasquez Creek Faults to be about 4 km. This 

interpretation is based on (1) a scheme of local tectonic block 

rotation based on paleomagnetic declinations, (2) mapped 

distribution of maximum clast sizes, dominant bed-forming 

process, and Mount Lowe intrusive suite clasts in Saugus Formation beds, (3) fault truncation of a clast lithofacies boundary 

within the Saugus Formation, and (4) relative abundances of 

Mount Lowe and anorthosite-syenite complex clasts and types 

of anorthosite-syenite clasts. 

Local Tectonic Block Rotation

The hypothetical role of post-Saugus Formation 

local block rotation in the Little Tujunga Canyon area is 

one independent line of evidence. Three paleomagnetic 

“domains” (structural blocks) with horizontal dimensions on 

the order of 10-20 km occur in hanging walls of the Santa 

Susana and San Fernando Fault Zones southwest of, but 

adjoining, the San Gabriel Fault Zone; two of the three blocks 

record large clockwise rotations (Levi and Yeats, 2003, 

p. 15-10 and fig. 9). Regional late Cenozoic dextral shear 

and associated rotational torques favored clockwise block 

rotation (Levi and others, 2005, p. 401). “The magnitudes 

and sense of domain rotations depend on factors including 

their sizes, shapes, and interactions along boundaries with 

neighboring domains and adjacent structural elements” 

(Levi and Yeats, 2003, p. 15-10). The “Merrick Syncline 

domain” is bounded on the west and east by the Pacoima 

Wash (“Pacoima segment boundary”) and lower Big Tujunga 

Canyon (“Sunland segment boundary”), respectively, and 

on the north and south by the San Gabriel and San Fernando 

or Verdugo Faults, respectively; it has rotated 34 ̊±6 ̊ 

clockwise (Levi and Yeats, 2003, p. 15-8 and fig. 11). Studies 

showed “no stratigraphic, time-dependent pattern of the 

Matuyama declinations, indicating that the rotations did not 

commence until the youngest Matuyama strata [of the Saugus 

Formation] were deposited” or after 2.6 to 0.78 Ma (Levi and 

others, 2005, p. 404). Abrupt changes in the trends of folds 

and of the San Fernando Fault Zone at the Pacoima segment 

boundary led Levi and Yeats (2003, p. 15-12 and fig. 11) to 

select a vertical-axis pivot point there. They calculated 6 km 

of left separation on the Sunland segment boundary in Big 

Tujunga Canyon. They rejected, as we do, a pivot axis at the 

Verdugo Fault.

Our hypothesis interprets the 34° of clockwise rotation 

of the Saugus Formation in the Merrick Syncline block as 

associated with rotational torque caused by dextral shear 

on the De Mille Fault. Strike separation along both the San 

Fernando and Verdugo Faults after 13.6 Ma (end of youngest 

Topanga Group deposition south of the Verdugo-Eagle Rock 

Faults) has been shown to be negligible (McCulloh and 

others, 2001, p. 22 and figs. 5 and 6). Three hypothetical 

vertical pivot-axis points illustrate the possible right 

separations on the combined De Mille-Vasquez Creek Faults 

related to the block rotation. The first two pivot axes (P1 

and P2 on fig. 8) are at contrasting locations in the Pacoima 

segment boundary used by Levi and Yeats (2003). They 

give 2.1 km and 4.3 km of right separation on the De Mille 

Fault. The third pivot axis on the south flank of the Merrick 

Syncline just west of Little Tujunga Canyon, roughly in 

the center of the block and closest to the paleomagnetic 

declination measurements, gives 3.4 km of right separation 

(point P3 on fig. 8). Calculations assume that all dextral 

shearing between the rotated block and the presumably 

nonrotated region northeast of the San Gabriel Fault Zone 

is concentrated along the De Mille Fault and that all torque 

coupling manifests as block rotation. The measured 34° of 

clockwise rotation of the Merrick Syncline block is thus 

consistent hypothetically with as much as several kilometers 

of right separation on the combined De Mille-Vasquez Creek 

Faults since 2.6 to 0.78 Ma. Imperfect torque coupling to the 

rotating block would mean even more right separation. 
 Right Separation on the Vasquez Creek Fault 17

118°25ʹ W 118°20ʹ

34°

19ʹ

 N

34°

17ʹ

SAN

GABRIEL

FAULT

ZONE

SAN

FERNANDO

FAULT

LOPEZ

FAULT

SUNLAND

FAULT

4.3 km*

2.1 km* 3.1 km*

P2

P1

P3

PACOIMA

S.B.

SUNLAND

S.B.

N

0 1 2

KILOMETERS

3 4 5

MS

MS

34°

34°

34°

upper Gold

Canyon

*Right slip needed for 34° of clockwise block rotation

 Saugus Formation

SB = segment boundary

MS = Merrick Syncline

Figure 8. The “Merrick Syncline 

domain” local tectonic block 

between Pacoima Wash and Big 

Tujunga Canyon that is bounded 

by the “Sunland and Pacoima 

segment boundaries,” the San 

Gabriel Fault Zone, and probably 

the San Fernando Fault Zone 

(adapted and modified from Levi 

and Yeats, 2003, fig. 11). Thirtyfour degrees of clockwise block 

rotation about hypothetical 

vertical pivot point P1, P2, or P3 

correspond to 2.1, 4.3, or 3.1 km 

of right-slip on the San Gabriel 

Fault Zone, respectively, if all 

block rotation is due to perfectly 

coupled rotational torque 

generated by De Mille Fault right 

slip.

Clast Size, Bed-Forming Process, and 

Distribution of Mount Lowe Clasts

Saugus Formation fanglomerate coarsens to the north and 

east. Locally dominant interbeds of reddish fine- to coarsegrained sandstone occur throughout the area. In the area east 

of Little Tujunga Canyon, some clasts are as much as 1.2 m 

in diameter and bedding “becomes almost indistinguishable” 

(Oakeshott, 1958, p. 84). Clasts exceed 1.7 m in size in upper 

Gold Canyon, where debris-flow units are coarsest and most 

abundant (fig. 9). Fanglomerate beds near the head of Gold 

Canyon are the most proximal facies of the relatively large 

alluvial fan system bearing Mount Lowe clasts. Figure 10 

shows maximum size of Mount Lowe clasts and of all clasts, 

together with dominant bed-forming process (debris-flow or 

fluvial-flow). (See also appendix 1, table 2). These inconclusive data hint that sediment transport direction was to the 

west-southwest or southwest, roughly 45° or more from the 

trend of the De Mille Fault. The overall smaller size of clasts 

in the southwestern part of the fan complex (bearing Mount 

Lowe clasts) is consistent with a general west-southwesterly 

sediment transport direction. This suggests a beheaded alluvial 

fan complex that originally crossed the fault and was fed from 

the east or northeast. No confirmation of such evidence at 

other more western sites close to the fault is possible because 

of limited access and lack of exposures of coarse-grained 

facies. Restoration of 34° of post-Saugus clockwise block 

rotation, as deduced from magnetic declination measurements 

made only about 7 km to the west-southwest (Levi and Yeats, 

2003), suggests that the trend of sediment transport in upper 

Gold Canyon may have been more perpendicular than parallel 

to the De Mille Fault trend (fig. 10). 

Fault Truncation of Clast Lithofacies Boundary

An approximate clast lithofacies boundary between 

Saugus Formation outcrops with and without Mount Lowe 

intrusive suite clasts is shown in figure 10B. This boundary 

separates two differently sourced parts of a beheaded alluvial 

fan complex. Mount Lowe clasts occur at Saugus Formation 

locality 16 but are absent at locality 51, both sites adjacent 

to the Watt Fault. Location 3 (figs. 10 and 7) is the midpoint 

between these localities. A possible trend of the boundary southward away from the fault is problematic because 

of Saugus Formation depositional facies changes, possibly 

influenced by the paleotopography during Saugus deposition. 

The boundary might trend southwestward before curving more 

westwardly on the distal part of the fan complex. This clast 

lithofacies change along the trend of the adjacent Watt and De 

Mille Faults also supports a sediment transport direction that is 

more perpendicular than parallel to the De Mille Fault. 

Relative Abundances of Clast Types

Saugus Formation clasts derived from the anorthositesyenite complex are more widely distributed, and more 

abundant, than more durable but better rounded Mount Lowe 

intrusive suite clasts where both occur together (appendix 1, 

table 2). At a few places, anorthosite clasts are absent; at a few 
18 Post-Miocene Right Separation on the San Gabriel and Vasquez Creek Faults

A

B

Figure 9. Outcrops of Saugus Formation rocks. A, Fanglomerate exposed on the north wall of upper Gold Canyon about 250 m upstream 

from outcrops of Gold Canyon beds. B, Polymictic boulder bed exposed further upstream at locality 5 (fig. 10 and appendix 1, table 2). 

Anorthosite and marginal facies Mount Lowe intrusive suite clasts as large as 46 cm are common but are not the dominant or largest clast 

types. Hammer in white circle is 32 cm long. 
 Conclusions 19

others they constitute the entire clast population (Oakeshott, 

1958, p. 84, 105; Weber, 1982, plate 2, locality 125; appendix 

1, table 2). Intermittent flooding from different source areas 

can explain such differences. The predominance of anorthosite and leucogabbro clasts suggests that the principal source 

region was the south-central and eastern parts of the complex, 

where such rock types are most abundant (Carter, 1982, p. 10) 

(fig. 7). A small angular cobble composed of magnetite and 

hornblende from a northeastern Saugus Formation outcrop 

(fig. 10A, locality 9) is probably from one of many “titaniferous magnetite bodies” in the south-central and eastern part 

of the complex (Oakeshott, 1946, p. 245 and plate XXV; also 

“fgb”unit, Dibblee, 2002a, 2002b). All evidence is compatible 

with sources of the anorthosite clasts north or northeast of the 

San Gabriel Fault. Principal mapped Mount Lowe intrusive 

suite outcrops are far to the northeast (area of locations 4 on 

fig. 7). A notable exception is Mount Lowe rocks (without 

anorthosite) exposed in a much smaller area south of the San 

Gabriel Fault Zone east of its bifurcation, in the drainage area 

of the Clear Creek tributary of Big Tujunga Canyon (location 

5 on fig. 7). 

Summary

The large variety of rock types and degrees of rounding 

of most clasts in the very coarse-grained Saugus Formation 

facies (fig. 9) indicates a variety of source areas at various 

distances. Interbedded debris-flow and fluviatile beds indicate 

transportation and deposition by a river or large stream system 

with strong seasonal flow fluctuations. This relatively large 

drainage system was similar to the present upper Big Tujunga 

drainage basin north of the San Gabriel Fault Zone (fig. 7). 

The arithmetic mean of the largest dimension of the largest 

five subrounded to rounded Mount Lowe clasts in the Saugus 

Formation, all located in the upper Gold Canyon-Gold Canyon 

Saddle area, is 54 cm. Decrease of Mount Lowe clast size 

versus distance downstream from source outcrops was determined in modern gravels in Arroyo Seco (Krumbein, 1942, p. 

1380-1382). We supplemented those data with observations of 

subangular to subrounded Mount Lowe boulders up to 1 m in 

size in Arroyo Seco gravels at the Vasquez Creek Fault crossing. Assuming the stream gradient of the present upper Big 

Tujunga Canyon, which is less than half that of Arroyo Seco 

above its intersection with the Vasquez Creek Fault, these data 

suggest that the 54-cm largest Mount Lowe clasts in the Saugus Formation in upper Gold Canyon were transported 16 km 

or more from their source outcrops. This in turn suggests that 

Saugus Formation Mount Lowe clasts might have eroded from 

the extensive source outcrops located north of the San Gabriel 

Fault in the eastern part of the upper Big Tujunga Canyon 

drainage system (area of locations 4 on fig. 7).

Quantifying post-Saugus Formation right slip on the De 

Mille-Vasquez Creek Faults is problematic because no trustworthy cross-fault tie is established. Saugus Formation is not 

found on the block between the Watt and Dillon Faults (Hill, 

1930, p. 148; Oakeshott, 1958, p. 93) (fig. 2A). We are unaware 

of any perched remnant outcrops of Saugus Formation north of 

either the San Gabriel or Vasquez Creek Faults to the north and 

east. No Mount Lowe debris is found in south-directed tributaries to the 5-km-long Big Tujunga Canyon segment southeast 

of Gold Canyon. It is not known how far eastward the Saugus 

Formation strata south of the De Mille Fault in Gold Canyon 

originally extended. Parts of the easternmost Saugus Formation 

have been eroded, because the partly stratified beds, although 

very coarse grained and containing debris-flow deposits, also 

contain fine-grained fluviatile beds and are not proximal fan 

head deposits (fig. 9). The fan is now beheaded and eroded. 

Similarly, the original extent of the beheaded coalesced fan 

complex implied by the truncated boundary separating Saugus 

Formation with and without Mount Lowe clasts is unknown. 

Nevertheless, the cross-fault sources of Mount Lowe clasts are 

interpreted as being closer to the Little Tujunga Canyon-Gold 

Canyon area during Saugus Formation deposition than they are 

today. This is based on the more perpendicular than parallel 

transport direction of Saugus sediments across the Watt and 

De Mille Faults (after back-rotation), together with the location along the Watt Fault of the boundary of Saugus Formation 

with and without Mount Lowe clasts (location 3 on fig. 10). 

However, lacking correlatable piercing points, we cannot rule 

out some unknown amount of intervening fault-parallel sediment transport and therefore cannot offer conclusive measures 

of post-Saugus Formation right slip.

The hypothetical 2.1-4.3 km right separation based on 

post-Saugus Formation block rotation (fig. 8) is similar to a 

4±2 km estimate derived from a mean slip rate of 2.3 mm/yr. 

(The mean slip rate is based on fault displacement beginning 

at the ca. 5.2±0.3 Ma age of the Gold Canyon beds, our postbeds right separation estimate of 12±2 km, and the 2.6 to 0.8 

Ma estimated age of the Saugus Formation.) Lastly, the westernmost confirmed Mount Lowe intrusive suite clasts in the 

Saugus Formation adjacent to the Watt Fault are about 5.5 

km (measured along the De Mille Fault) from the present 

Big Tujunga Canyon (locations 3 to 6 on fig. 7). Although 

none of these numbers lead to measures of right separation, 

the multiple lines of circumstantial evidence are consistent 

with 4 ±2 km as a reasonable estimate of post-Saugus Formation right separation on the combined De Mille-Vasquez 

Creek Faults. 

Conclusions 

Strands of the San Gabriel Fault Zone cut both the Gold 

Canyon beds and unconformably overlying Saugus Formation fanglomerate in upper Gold Canyon. Evidence from these 

rock units leads to several inferences concerning the onset age 

and amount of post-Miocene right separation on the combined 

De Mille-Vasquez Creek Faults: (1) Dating of macrofossil 

shell carbonates from the Gold Canyon beds using 87Sr/86Sr 

indicates marine deposition at 5.2±0.3 Ma. This maximum 

age for early right slip on the De Mille-Vasquez Creek faults 

refines the independently estimated “<7 and >3.7 Ma” onset 
20 Post-Miocene Right Separation on the San Gabriel and Vasquez Creek Faults

age (McCulloh and others, 2001, p. 22, 24). (2) Large angular 

Mount Lowe intrusive suite boulders in the Gold Canyon beds, 

eroded from cross-fault sources now separated from the beds 

by faults, indicate right separation of as much as 12±2 km on 

the De Mille-Vasquez Creek Faults. This conclusion is similar 

to an 11-km estimate (Ehlig, 1975b, p. 14) and a 12-13-km 

estimate based on multiple independent facts and separate 

reasoning (McCulloh and others, 2001). (3) At the San Gabriel 

Fault bifurcation, the hypothesized right separation on the 

Vasquez Creek Fault joins 22-23 km of middle-late Miocene 

right separation (Ehlig, 1968) of the San Gabriel Fault. Right 

separation of as much as 34-35 km west of the bifurcation 

passes northwestward along the fault beyond the San Gabriel 

Mountains. (4) Saugus Formation clast sizes and lithologic 

D

W

B DM

3

Mount Lowe clasts

Largest Clast Size (cm)

< 12

≥12 to < 25

≥ 25 to < 50

≥ 50 to < 100

≥ 100

Clast type not observed

118 ̊ 22.5' W 118 ̊ 20' 118 ̊ 17.5'

34 ̊

20'

 N

1 3 2

5 4

7

68

12

11 13 10

9

14

15 16

17

18 19

23 22

24

21 25

51

53 52

55 54

56

57

58 62

64

60

59

63 61

49

48 47 46

44 45

43

42

41

40

39

38

36

37

35

34

33

32

31

30

29

28

26

27

20

50

N

0 1 2 3

KILOMETERS

BTC

D

W

DM

LTC

S

PW

BC

?

A

3

Upper

Gold Canyon

MS

L

Saugus Formation

Numbered localities where Saugus Formation

clasts examined (Appendix 1, Table 2)

118 ̊ 22.5' W 118 ̊ 20' 118 ̊ 17.5'

34 ̊

20'

 N

34 ̊

17.5'

34 ̊

17.5'

Figure 10. Distribution of maximum clast sizes and bed-forming processes of the Saugus Formation. Fault and geographic labels are 

as in figures 1 and 2. A, Simplified geologic map showing major faults and most of the mapped Saugus Formation between Pacoima 

Wash and upper Gold Canyon, with numbered localities where maximum sizes of Saugus Formation clasts were measured (appendix 

1, table 2). Older and younger bordering units (see fig. 2) are not differentiated here. B, Distribution of largest Mount Lowe clasts and 

sites where Mount Lowe clasts are absent. C, Distribution of largest clasts of all rock types. D, Distribution of dominant bed-forming 

process. White-on-black circle 3 (shown in A, B, D, and fig. 7) is the approximate location of the boundary between Saugus Formation 

outcrops adjacent to the Watt Fault with and without Mount Lowe intrusive suite clasts, chosen midway between localities 16 and 51.
 Conclusions 21

Figure 10.—Continued. 

118 ̊ 22.5' W 118 ̊ 20' 118 ̊ 17.5'

F D F

DF

D

F D

D

D D

10

F

D

F

F

F F F F

F

F F

F=D

D D D

D

F

D

D

F F

F

F

F F

F

F F F

F

F

F

F

D

F

F

F

F

F

F

F

F

F

F

F=D

F

F

F

F

F

F

N

0 1 2 3

KILOMETERS

BTC

D

W

DM

LTC

S

PW

BC

?

D

3

Upper

Gold Canyon

MS

L

Saugus Formation

Localities where dominant bed-forming

process noted (Appendix 1, Table 2)

Bed-Forming Process

D Debris flow (includes D>F, D>>F)

F Fluvial (includes F>D, F>>D)

34 ̊

20'

 N

D

W

C DM

All clast types

Largest Clast Size (cm)

< 12

≥12 to < 25

≥ 25 to < 50

≥ 50 to < 100

≥ 100

118 ̊ 22.5' W 118 ̊ 20' 118 ̊ 17.5'

34 ̊

20'

 N

34 ̊

17.5'

34 ̊

17.5'

proportions in the Gold and Little Tujunga Canyons indicate 

cross-fault clast sources to the north and northeast, sediment 

transport more nearly transverse than parallel to the present 

De Mille Fault strike, and transport distances 16 km or more 

for Mount Lowe clasts. These interpretations, combined with 

a fault-coupled local block-rotation hypothesis and a right-slip 

rate estimate of 2.3 mm/yr on the De Mille-Vasquez Creek 

Faults since 5.2 Ma, suggests that post-Saugus Formation 

(< 2.6 to 0.78 Ma) right separation is 4±2 km on the De MilleVasquez Creek Faults. 

The age of 5.2±0.3 Ma (earliest Pliocene) for the Gold 

Canyon beds indicates that they are equivalent to either youngest Towsley Formation or oldest Pico Formation. The depositional environment and fauna of the beds suggest kinship with 

the basal part of the Pico Formation. 

Establishing the age of the Gold Canyon beds and correlating them to more completely studied sections in regions 

to the south and west of the western San Gabriel Mountains 

required a comprehensive regional stratigraphic and geochronologic framework. This study produced several added results: 

(1) Large and abrupt changes in stratal thicknesses (or disconformity gaps) of this basin-margin setting are documented. (2) 

This is the first published account of fossil oyster spat, associated with deep marine benthic foraminifers and planktonic calcareous nannofossils and diatoms, in this area; they are found 

at two stratigraphically and geographically separated Modelo 
22 Post-Miocene Right Separation on the San Gabriel and Vasquez Creek Faults

Formation exposures. This unusual assemblage indicates an 

exceptional nearshore, but deep-water, upwelling marine setting. (3) Connection from the shallow marine Gold Canyon 

beds to more open marine settings probably extended south or 

southwest from Gold Canyon. Presumably correlative beds, 

lithologically dissimilar and largely nonfossiliferous, are just 

east of Pacoima Wash. Facies differences and both erosion and 

burial, probably before and also after deposition of the Saugus 

Formation, obscured or destroyed direct evidence of the 5.2-

Ma tidal connection, the existence of which is suggested today 

only by the presence of the isolated Gold Canyon beds. 

Acknowledgments

We thank D.M. Morton, J.C. Matti, and the U.S. Geological Survey Bradley Scholar Program for material support. We 

are especially grateful to the University of Texas at Dallas for 

crucial support; N.R. Miller provided ICP analyses of fossil 

carbonate samples. We are grateful to J.G. Vedder and C.L. 

Powell, II, who examined several molluscan assemblages and 

discussed their living environments. R.E. Powell, A.P. Barth, 

B.A. Carter, J.A. Nourse, and R.L. Squires helpfully responded 

to questions about their work in the region. J.D. Stock discussed alluvial processes with the senior author. Harry Filkorn 

provided lists of archived invertebrate fossils from the area. G.J. 

Saucedo furnished us with unpublished geologic maps of two 

areas. Steve Bear and John Keeler (Angeles National Forest, 

U.S. Forest Service) provided crucial access to the field area. 

The Wildlife Way Station and the Middle Ranch allowed entry 

to their lands. Sterling Klappell (Los Angeles County Public 

Works Department) provided Big Tujunga Canyon stream flow 

information and control that facilitated fieldwork. Two reviews 

each by R.E. Powell and D.M. Morton provided critical guidance, and reviews by P. Stone, J.T. Tinsley, and J.G. Vedder, 

were important. Zenon Valin and Peter Triezenberg prepared 

the figures. Nardia Beyer kindly accompanied the senior author 

during eight field days in Gold Canyon and surrounding areas.

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San Gabriel Fault Zone, Los Angeles and Ventura Counties, 

California: California Division of Mines and Geology 

Open-File Report 82-2 LA, 157 p., 3 plates. 

Weber, F.H., Jr., 1988a, Reinterpretation of structural elements 

and displacement history along the San Gabriel Fault between 

the San Andreas Fault and the westernmost San Gabriel 

Mountains, Los Angeles, Kern, and Ventura Counties, 

California, Part IV, Evaluation of activity of the San Gabriel 

Fault Zone, Los Angeles and Ventura Counties, California: 

Final Technical Report, Contract Number USGS 14-08-0001-

21928, Modification 2, State of California, Department of 

Conservation, submitted October 15, 1998, 115 p.
 References 27

Weber, F.H., Jr., 1988b, Geologic relationships along the 

San Gabriel Fault between Hardluck Canyon and Castaic: 

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and geologic evolution: Geological Society of America 

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nannofossil biostratigraphy: Chapman & Hall, British Micropalaeontology Society Series, p. 225-265.

Appendixes
30 Post-Miocene Right Separation on the San Gabriel and Vasquez Creek Faults

Map 

symbol

Mount Lowe 

intrusive 

suite (ML)

Anorthositesyenite 

complex 

(AN)

All 

clast 

types

Relative 

abundance 

of ML and 

AN

Remarks

Westward from upper Gold Canyon and east and south of the Lopez Fault

1 ≥5? 

SR

≥13? 

SR ≥48? AN>ML? Sandstone, coarse-grained, pebbles-cobbles, rare boulders. Limited outcrop, poor 

exposure.

2 51* 

SR

33 

SR 90 AN>ML

Conglomerate, poorly stratified, cobbles-boulders, with sandstone interbeds, massive, coarse-grained. Sequence overlies Gold Canyon beds and is stratigraphically 

lower than localities 3 through 5.

3 36 

SR

48 

SR 100 AN>ML Sandstone, massive, coarse-grained, and conglomerate, cobbles-boulders, beds ≥ 2 

m thick. 

4 25 

SR

30 

R-SR 107 AN>ML Sandstone, massive, coarse-grained, with conglomerate lenses, massive, boulders, 

and scarce red-tinted paleosol horizons.

5 46*# 

R-SR

46 

SR-SA 86 AN=ML Conglomerate, boulder, ,≥ 2 m thick (see fig. 9), and sandstone, coarse-grained, massive. Scarce red-tinted paleosol horizons. 

Table 2. Maximum size, shape, and relative abundance of clasts in the Saugus Formation that are identifiable as derived from the Mount 

Lowe intrusive suite and anorthosite-syenite complex at 63 Saugus Formation exposures between Pacoima Wash and Gold Canyon. 

[Included are maximum dimension of all clast types and summary lithologic description at each locality. Clast sizes and localities are shown in figure 10. AN 

= anorthosite-syenite complex clasts, ML = Mount Lowe intrusive complex clasts, N = none observed; R = rounded, SR = subrounded, SA = subangular, A = 

angular, NM = no measurements; * = Mount Lowe intrusive suite clasts with microcline phenocrysts; # = Mount Lowe intrusive suite clasts with garnet phenocrysts; ? = limited exposure or number of clasts: larger clasts possible or ML/AN ratio uncertain.]

Maximum clast dimension 

(cm) and shape

Appendix 1. Description of Saugus 

Formation Clasts

Three suites of San Gabriel Mountain basement rocks vary 

in importance as sources of clasts in the Saugus Formation. The 

Late Triassic Mount Lowe intrusive suite is of primary importance and is predominantly a monzodioritic to granodioritic 

metaplutonic complex with distinctive marginal zone facies 

(Miller, 1934; Ehlig, 1975a, 1981; Joseph and others, 1982; 

Powell and others, 1983; Barth and Ehlig, 1988; Barth and others, 1990; Nourse, 2002). A Mesoproterozoic anorthosite-syenite 

complex is composed of distinctive anorthosite and leucogabbro plus lesser syenite, monzodiorite, gabbro, and “ultramafite” 

(Oakeshott, 1958; Crowell and Walker, 1962; Carter and Silver, 

1972; Carter, 1982, p. 1). A Paleoproterozoic metasedimentary 

and metaigneous gneiss suite (Silver and others, 1963) includes 

the Mendenhall Gneiss (Oakeshott, 1958), distinctive retrograded 

quartzofeldspathic granulites, some containing rutile-bearing 

clear or distinctively colored lavender or blue quartz, (Higgs, 

1954; Oakeshott, 1958; Crowell and Walker, 1962; Barth and 

others, 1995; A.P. Barth, oral commun., 2005), and mafic granulite. Amphibolite facies layered and augen gneiss, and some 

aluminous schist (Silver and others, 1963; Ehlig, 1975a, 1981; 

Powell and others, 1983; Barth and others, 1995, 2001; Nourse, 

2002) are among other Paleoproterozoic gneissic rock types. 

Outcrop areas of these suites are shown in figure 7. 

Table 2 records maximum length and degree of rounding 

of distinctive clasts of Mount Lowe intrusive suite lithologies 

and anorthosite or leucogabbro clasts at 63 localities numbered 

from east to west in figure 10. The maximum dimension of all 

clasts is also tabulated. Massive dioritic to granitic plutonites 

and associated rock types dominate clast populations at most 

Saugus Formation localities. Multiple types of gneissoid and less 

common schistose clasts also occur. Most of the rare andesitic, 

felsitic, and sedimentary clasts are probably recycled from 

Paleocene conglomerates. Clasts sourced from the metaplutonic 

Mount Lowe intrusive suite and the anorthosite-syenite complex 

range between common and absent amidst the usually dominant 

other rock types. 

Multiple petrofacies of the marginal zone of the Mount 

Lowe intrusive suite are present as clasts. Represented are the 

hornblende facies (abundant hornblende metacrysts and accessory sphene notable), hornblende-K-feldspar facies (ovoid feldspar and widely spaced black hornblende metacrysts notable), 

and hornblende-K-feldspar-garnet facies (large K-feldspar and 

garnet metacrysts, and biotite notable) (Ehlig, 1975a, Smith, 

1986, p. 9; Barth and Ehlig, 1988, p. 193-194). Clasts from the 

hornblende-K-feldspar facies might be the most abundant. Clasts 

of the Paleoproterozoic gneiss suite are widespread in the study 

area but may be most abundant in the northwest sector. Massive 

plutonite clasts may be most abundant in the Gold Canyon-Gold 

Canyon Saddle area. Saugus Formation exposures on some areas 

of private property are not accessible.
 Appendixes 31

Map 

symbol

Mount Lowe 

intrusive 

suite (ML)

Anorthositesyenite 

complex 

(AN)

All 

clast 

types

Relative 

abundance 

of ML and 

AN

 Remarks

Maximum clast dimension 

(cm) and shape

6 25* 

SR

36 

SR >36 AN>ML Sandstone, massive, medium- to coarse-grained, red-tinted paleosol horizons, and pebble-boulder lenses. 

7 41* 

SR

41 

SR 112 AN>ML Conglomerate, massive, boulders, with sandstone lenses, coarse-grained. 

8 48* 

R-SR

53 

R-SR 168 ML>AN Conglomerate, massive, boulder, and sandstone lenses, thin, coarse-grain. Sequence rests depositionally on basement rocks. 

9 30*# 

R-SR

71 

R-SR 71 AN>>ML

Sandstone, coarse-grained, pebbles, conglomerate, cobbles-boulders, and minor 

red-tinted paleosol horizons. Rare magnetite-hornblende clasts from anorthositesyenite complex. 

10 25* 

SR

30 

SR 46 AN>ML Sandstone, coarse-grained, with pebble to boulder lenses. 

11 30* 

SR

30 

R-SR 81 AN>>ML Conglomerate, massive, small boulders, and conglomeratic sandstone, coarsegrained.

12 48* 

R-SR

38 

R-SR 109 AN>>ML Conglomerate, poorly stratified, boulders, with sandstone lenses. 

13 28* 

R

60 

SR-SA 66 AN>ML Conglomerate, massive, boulders, with rare thin sandstone lenses. 

14 76* 

R-SR

58 

SR-SA 76 AN>ML Conglomerate, massive, boulders, with single sandstone lens, medium-grained, current laminations. 

15 

NM NM NM AN≥ML "...almost wholly anorthosite; very little gabbro-norite and Mendenhall Gneiss" (Weber, 1982, plate 2, locality 125).

16 23*# 

R-SR

51 

R-SA 61 AN>ML

Sandstone, poorly stratified, coarse-grained, pebble-, and cobble-rich lenses, rare 

boulders; some lenses rich in Mount Lowe intrusive suite clasts, others in anorthosite clasts. 

17 N ≥4? 

A ≥4? AN>ML? Sandstone, massive, coarse-grained, horizons of angular anorthosite pebbles. 

18 N ≥2? 

A ≥2? AN>ML? Sandstone, masssive, coarse-grained, with red-tinted paleosol horizons, common 

anorthosite granules and small pebbles. 

19 ≥7?# 

SA

≥9? 

SR ≥28? AN>ML? Sandstone, massive, coarse-grained, with pebble- and small cobble-rich lenses, rare 

boulders; several red-tinted paleosol horizons. 

20 32# 

R-SR

25 

R-SR 51 AN>ML Sandstone, poorly stratified, conglomeratic, mostly pebbles and cobbles, few boulders. 

21 N ≥3? 

SA-A 30 AN Sandstone, massive, coarse-grained, with pebble- and cobble-rich lenses, rare boulders, and thin red-tinted paleosol horizons.

22 N 25 

SR 25 AN

Sandstone, poorly stratified, coarse-grained, with pebble-cobble lenses and reddish 

paleosol horizons. Most clasts are anorthosite and most are much smaller than the 

largest measured.

23 N 23 

SA 23 AN Sandstone, massive, fine- to medium-grained, red-tinted, paleosol horizons, interbedded with sandstone, massive, coarse-grained, pebbles-cobbles. 

24 N 11 

SR-SA 19 AN Sandstone, massive, coarse-grained, pebble- and cobble-rich lenses, and sandstone, ≤ 1.5 m thick, fine- to medium-grained, red-tinted paleosol horizons. 

25 N 10 

SR-SA 23 AN Sandstone, stratified (≤3 m thick), silty, fine- to medium-grained, red-tinted paleosol horizons, with interbedded sandstone, coarse-grained, pebbles and cobbles. 

26 ≥12* 

R-SR NM ≥12 Sandstone, massive, coarse-grained, with pebble- and cobble-rich lenses. 

Table 2. Maximum size, shape, and relative abundance of clasts in the Saugus Formation that are identifiable as derived from the Mount 

Lowe intrusive suite and anorthosite-syenite complex at 63 Saugus Formation exposures between Pacoima Wash and Gold Canyon—

Continued.
32 Post-Miocene Right Separation on the San Gabriel and Vasquez Creek Faults

Map 

symbol

Mount Lowe 

intrusive 

suite (ML)

Anorthositesyenite 

complex 

(AN)

All 

clast 

types

Relative 

abundance 

of ML and 

AN

 Remarks

Maximum clast dimension 

(cm) and shape

Table 2. Maximum size, shape, and relative abundance of clasts in the Saugus Formation that are identifiable as derived from the Mount 

Lowe intrusive suite and anorthosite-syenite complex at 63 Saugus Formation exposures between Pacoima Wash and Gold Canyon—

Continued.

27 N N 4 Sandstone, massive, poorly sorted, scarce pebbles, overlain by sandstone, fine- to 

medium-grained, red-tinted paleosol horizons. 

28 8 

SR

15 

SR-SA 28 AN>ML

Sandstone, massive, coarse-grained, pebbles, with conglomerate lens, boulders, 

overlying prominent 7.5-m thick sandstone, fine- to medium-grained, red-tinted 

paleosol horizons.

29 36* 

R

33 

R-SR 36 AN>ML Sandstone, partly massive, partly cross-bedded, medium- to coarse-grained, pebbles, and conglomerate, pebbles to boulders. Several cut-and-fill channels.

30 25* 

SR

41 

SR 41 AN>ML Sandstone, massive, coarse-grained, pebbles, and conglomerate, pebbles to boulders. 

31 25 

SR

30 

SR 36 AN>ML Sandstone,conglomeratic, with conglomerate lens, irregular, pebbles to boulders.

32 23 

R

36 

SR

36 

SR AN>ML Sandstone, massive, coarse-grained, pebbles, with sandstone lenses, conglomeratic, cobbles to boulders, and several red-tinted paleosol horizons.

33 10 

SR

18 

SR 30 AN>ML Sandstone, poorly stratified, coarse-grained, with conglomerate lenses, pebbles to rare boulders. 

34 36 

SA

5 

R-SR 36 ML>>AN Sandstone, poorly stratified, medium- to coarse-grained, and sandstone, conglomeratic, cobbles to scarce boulders. 

35 46* 

SR

61 

SR 61 AN>ML Sandstone, poorly stratified,coarse-grained, with conglomerate lens, boulders.

36 18 

SA N 28 ML Sandstone, massive, coarse-grained, with pebble lenses and cut by pebble to cobble conglomerate-filled channels. Rare boulders. 

37 10 

SR

13 

SR 20 AN>ML Sandstone, poorly stratified, coarse-grained, with cobble lenses. 

38 8 

SR

8 

SR 23 AN≈ML Sandstone, poorly stratified, coarse-grained, with cobble lenses. 

39 ≥8* 

SR N ≥17 ML Sandstone, poorly stratified, medium- to coarse-grained, with pebble-rich lenses, some cobbles. 

40 23 

SR N 23 ML Sandstone, poorly stratified, medium- to coarse-grained, with pebble-rich lenses, some cobbles.

41 38* 

R-SR N 38 ML Conglomerate, cobbles, boulders, with rare thin sandstone lenses.

42 33 

SR

7 

SR-SA 33 AN>ML Sandstone, poorly stratified, medium- to coarse-grained, with numerous pebble lenses, some cobbles, rare boulders. 

43 ≥6* 

SR

≥12 

SR ≥12 ML>AN Sandstone, poorly stratified, medium- to coarse-grained, pebbles, uncommon cobbles, rare small boulders.

44 13*# 

SR

10 

R 30 ML>AN Sandstone, moderately stratified, medium- to coarse-grained, pebbles, uncommon cobbles, rare boulders. 

45 48* 

R-SR

23 

SR 48 ML>AN Conglomerate, poorly stratified, sandy, pebbles, cobbles, rare boulders.

46 33 

R

18 

SR 33 AN>ML Sandstone, poorly stratified, coarse-grained, pebbles, rare cobbles and boulders. overlying >10-m-thick sandstone sequence, fine-grained, silty, red-tinted, paleosol horizons.
 Appendixes 33

47 15 

R-SR

20 

SR >20 AN>ML Sandstone, poorly stratified, coarse-grained, pebbles, with cobble-rich lenses. 

48 ≥9# 

SR

≥10 

SR-SA >10? AN>ML

Sandstone, poorly stratified, coarse-grained, with pebble- and cobble-rich lenses. 

Unweathered Saugus Formation cobble of reddish (10R 6/2) porphyritic andesite 

collected near this locality. 

49 ≥5# 12 

SR 15 AN Sandstone, partly to poorly stratified, coarse-grained, pebbles, cobbles.

50 ≥12 

R-SR

≥6 

SR ≥12 ML≥AN? Sandstone, poorly stratified, medium- to coarse-grained, pebbles, with cobble-rich 

lenses.

North and West of Lopez Fault

51 N N 36

Sandstone, poorly stratified, conglomeratic, angular to subangular pebbles, cobbles, 

and rare boulders, with sandstone lenses, coarse-grained, and minor red-tinted 

paleosol horizons.

52 N N 10 Sandstone, masssive, very poorly sorted, with scarce angular to subangular pebbles 

and cobbles.

53 N N 10 Sandstone, massive, very poorly sorted, with angular to subangular pebbles and 

scarce cobbles.

54 N N 25 Sandstone, poorly stratified, very poorly sorted, with abundant mostly angular to 

subangular pebbles and cobbles. 

55 N N 9 Sandstone, very poorly stratified, very poorly sorted, with abundant angular to 

subangular pebbles and cobbles. 

56 N N 7

Sandstone, massive to poorly bedded, medium- to coarse-grained, with scarce 

trains and thin lenses of subrounded to subangular pebbles-cobbles and red-tinted 

paleosol horizons.

57 N N 8 Sandstone, massive to poorly bedded, poorly sorted, mostly angular pebbles and 

rare cobbles.

58 N N 7 Sandstone, poorly bedded, poorly sorted, mostly angular pebbles, scarce cobbles.

59 N 35 

SR ≥35 AN Sandstone, conglomeratic, subangular to subrounded pebbles to boulders. 

60 N N >10

Sandstone, silty to medium grained, red-tinted, well-formed paleosols, with 

sandstone interbeds, poorly sorted, mostly angular to subangular pebbles, scarce 

cobbles. 

61 N 20 

SR ≥20 AN Sandstone, conglomeratic, mostly angular to subangular pebbles-cobbles.

62 N N 5 Sandstone, silty to medium grained, red-tinted paleosol horizons, with sandstone 

lenses, coarse-grained, pebbles.

63 N 20 

SR ≥20 AN Sandstone, conglomeratic, mostly angular to subangular pebbles-cobbles. 

64 N N 5 Sandstone, coarse grained, mostly subangular to angular pebbles, and sandstone, 

silty to medium-grained, red-tinted paleosol horizons.

Map 

symbol

Mount Lowe 

intrusive 

suite (ML)

Anorthositesyenite 

complex 

(AN)

All 

clast 

types

Relative 

abundance 

of ML and 

AN

 Remarks

Maximum clast dimension 

(cm) and shape

Table 2. Maximum size, shape, and relative abundance of clasts in the Saugus Formation that are identifiable as derived from the Mount 

Lowe intrusive suite and anorthosite-syenite complex at 63 Saugus Formation exposures between Pacoima Wash and Gold Canyon—

Continued.
34 Post-Miocene Right Separation on the San Gabriel and Vasquez Creek Faults

Appendix 2. Isotopic Age 

Determinations Using 87Sr/86Sr Ratio 

The 87Sr/86Sr ratio in seawater has varied systematically 

through geologic time (Burke and others, 1982). Calciumbearing minerals precipitated from seawater record this 

ratio. If unchanged during postprecipitation diagenesis, the 

ratio can be used for age determination by comparing it to 

a table of its variation in seawater over time. Identifying 

samples that have retained the original seawater ratio 

can be a challenge. The Neogene is a favorable time 

period in which to apply strontium isotope stratigraphy 

because seawater shows a rapid monotonic rise in 87Sr/86Sr, 

especially during the earlier Neogene. 

Originally low-Mg calcite shells are the most 

likely to retain the original 87Sr/86Sr ratio. In this study, 

translucent foliated oyster and pecten shells and, at some 

sites, barnacles that retain original internal structure were 

preferentially collected. During diagenesis, Mn and Fe 

increase and Ca and Sr decrease (for example, Brand and 

Veizer, 1980). Carbonates low in Fe and Mn and with 

Map 

symbol

Latitude (N) 

Longitude (W) 

(degrees)

87Sr/ 86Sr ∆sw Age (Ma) Fe 

(ppm)

Mn 

(ppm)

Sr 

(ppm) Field Fauna Quality Trace Agree

AB

 34.31490 

-118.28353

0.709029±12 -14.4 5.3 ± 0.5 132 70 1054 - + + + +

0.709033±10 -14.0 5.2 ± 0.4 103 141 1760 + + + + + 

0.709033±13 -14.0 5.2 ± 0.6 36 73 1259 + + + + + 

0.709054±17 -11.9 4.1 ± 1.7 116 88 1558 + + + + + 

 34.31482 

-118.28261

0.709051±14 -12.2 4.4 ± 1.7 68 54 877 - + + + + 

0.709057±14 -11.6 3.8 ± 1.3 16 24 1082 + + + + + 

C 34.27398 

-118.30159

0.708974±16 -29.9 6.4 ± 0.4 393 1704 832 + + + - -

0.708857±13 -32.2 11.2 ± 0.7 3005 953 3282 + + + + + 

F 34.29076 

-118.32632

0.708915±15 -25.8 9.2 ± 1.4 733 254 805 + + + + +

0.708900±14 -27.3 9.7 ± 0.8 654 995 647 + + + - + 

0.708908±16 -26.5 9.4 ± 1.1 213 229 1561 - - - + + 

0.708922±17 -25.1 8.8 ± 1.4 160 188 961 + + + + + 

0.708891±17 -28.2 9.9 ± 0.8 1029 4035 558 + + + - + 

G 34.29819 

-118.33673

0.708837±14 -33.6 12.1 ± 1.1 58 50 847 + + + + ± 

0.708876±11 -29.7 10.4 ± 0.5 503 813 455 + + + - ± 

H 34.30424 

-118.33842 0.709091±14 -8.2 1.9 ± 0.4 106 365 2479 - - - + - 

K 34.29272 

-118.36713

0.708916±14 -25.7 9.1 ± 1.4 364 32 814 + + + + + 

0.708907±14 -26.6 9.5 ± 0.9 135 27 547 + + + + + 

L 34.29324 

-118.36715

0.708995±10 -17.8 6.0 ± 0.2 104 336 850 - + - - -

0.708974±14 -19.9 6.4 ± 0.4 81 146 1660 - - - + + 

0.708962±12 -21.1 6.8 ± 0.4 129 62 1023 + + + + + 

0.708971±10 -20.2 6.5 ± 0.3 390 690 750 + + + - + 

[Locations are shown in figures 2-5. Ages are assigned from the look-up table of McArthur and Howarth (2004). Fe, Mn, and Sr abundances are by ICP analysis. Analytical 

procedure and assessment of age reliability are discussed in Appendix 2. ∆sw = (87Sr/86Srunknown - 87Sr/86Srmodern seawater) x 105. NBS987 = 0.710240; modern seawater = 0.709173. 

Qualitative judgement factors shown by plus or minus signs are based on “field” occurrence situation, fossil “quality” or suitability, “trace” element consideration in Sr analysis, and overall agreement among all analytical information. Weighted averaging of 87Sr/86Sr ratios and apparent ages in bold numerals are used to calculate “best estimate of 

age”; number of separate isotopic measurements are in parentheses; uncertainties partly governed by judgement factors in some cases. BFS = benthic foraminiferal stage.]

Table 3. Analytical results and derived 87Sr/86Sr ages of fossil samples (marine shell carbonate plus fish debris and collophane) from the 

Gold Canyon beds and from the late Miocene Modelo and late Miocene to early Pliocene Towsley Formations at 15 selected sites along the 

southwest margin of the San Gabriel Mountains and the San Fernando Pass area. 
 Appendixes 35

Best estimate of age

(87Sr/ 86Sr) 

(Ma) 

Fossil material used Remarks

0.709032 ± 2 (3)

5.2 ± 0.3

0.709043 ± 12 (6)

4.2 ± 1.3 

Oyster, grey, translucent, foliated fragments

Isotopic ratios, from a suite of excellent samples with favorable trace elements, equivalent to basal Pliocene seawater.

Pectinid, pinkish, foliated fragements

Oyster, white to pale pinkish, foliated fragments

Oyster, thin, white, translucent, foliated fragments

Oyster, thin, white, translucent, foliated fragments

Oyster, white, translucent, foliated fragments

anomalous ages

Oyster spat Modelo Fm. In silty diatomaceous and foraminiferal claystone (see Appendixes 3-5). Inconsistency between Mn/Sr ratios and apparent ages indicates 

altered Sr. 

Fish debris (scales and bones).

0.708919 ± 5 (2) 

9.0 ± 0.6

0.708911 ± 9 (5) 

9.3 ± 0.7 

Oyster, thin, tan, foliated fragments Correlative with late Mohnian BFS part of Modelo Formation (Barron 

and Isaacs, 2001, p. 294). Mapped as Monterey Fm. by Dibblee (1991b). 

Consistent results from samples of variable quality suggests a reliable age, 

but divergence from age based on calcareous nannofossils and diatoms, from 

nearby overlying strata (locality U), indicates this age is apparently young by 

about 1 Ma or more. One or both age calibrations may be in error. Disparity 

does not affect our structural interpretation.

Pectinid, grey to tan foliated fragments

Bivalve, white, chalky

Oyster, white, tranlucent, foliated fragments

Oyster, pink, blocky, fibrous fragments

0.708837 ± 14 (1)

12.1 ± 1.1

Pectinid, white, foliated fragments Equivalent to early Mohnian BFS part of Modelo Fm. on basis of Sr 

isotopic age. Mapped as Monterey Fm. by Dibblee (1991b). Better sample 

yields preferred age. 

Oyster, pinkish, blocky, fibrous fragments

anomalous age Bivalve, thin, white, chalky fragments Monterey (?) Formation of Dibblee (1991b). Undesirable fossil type with 

altered shells. 

0.708912 ± 14 (2)

9.3 ± 1.1 

Oyster spat Correlative with oldest part of late Mohnian BFS and therefore Modelo 

Formation (see appendixes 3 and 4). 

Oyster spat

0.708976 ± 13 (4) 

6.3 ± 0.3 

Pectinid, white to pinkish, partly opaque, foliated 

fragments Lower Delmontian BFS equivalent and therefore correlative of Towsley 

Formation. Consistent results from samples of variable quality indicates a 

reliable age.

Bivalve, white chalky fragments

Pectinid, large ribbed, grey, foliated shell

Pectinid, thin, gray, foliated shell

high Sr/Mn ratios are most likely to have retained original 

seawater ratios (Denison and others, 1994). Trace element 

composition of shell material is used to determine which 

samples are potentially reliable. As guidelines, samples 

having <300 ppm Mn, <3,000 ppm Fe, and Sr/Mn >2 are 

considered to have favorable trace elements (table 3). Less 

favorable shell species were analyzed from some sites, 

either because no better shell material was found or the 

extent of diagenetic exchange was sought.

Where possible, multiple shell fractions were collected 

and prepared for each locality. Crushed and washed shells 

were hand picked for analysis. A numeric age was assigned 

to each analyzed sample using the Look-up Table of 

McArthur and Howarth (2004). That table is based on curve 

construction using LOWESS fitting of isotopic analyses and 

stratigraphic assignments (Howarth and McArthur, 1997; 

McArthur and others, 2001). The Neogene part of the curve 

and Look-up Table is based largely on data from Deep Sea 

Drilling Project and Ocean Drilling Program sites. More 

details of strontium separation and isotopic analysis are 

described in McCulloh and others (2002).

The precision of the numeric age assigned to 

individual isotopic analyses in table 3 depends on the 

measurement error of the isotope ratio, the error band in 

the reference seawater curve, and the slope of the reference 

curve. The overall reliability of each analyzed sample is 

judged from the clarity of field relations, fossil quality 

(“fauna” column), trace-element content, and the agreement 

among analyses from the locality (table 3). Ultimately the 

isotopic agreement from shell material judged most reliable 

is the sole criterion of a retained ratio and therefore a 

meaningful age.

Table 3. Analytical results and derived 87Sr/86Sr ages of fossil samples (marine shell carbonate plus fish debris and collophane) from the 

Gold Canyon beds and fom the late Miocene Modelo and late Miocene to early Pliocene Towsley Formations at 15 selected sites along the 

southwest margin of the San Gabriel Mountains and the San Fernando Pass area—Continued.
36 Post-Miocene Right Separation on the San Gabriel and Vasquez Creek Faults

Map 

symbol

Latitude (N) 

Longitude (W) 

(degrees)

87Sr/ 86Sr ∆sw Age (Ma) Fe 

(ppm)

Mn 

(ppm)

Sr 

(ppm) Field Fauna Quality Trace Agree

M 34.29348 

-118.40030

0.709011±7 -16.2 5.7 ± 0.2 2326 873 2074 + ? ? - + 

0.709016±15 -15.7 5.6 ± 0.5 1323 804 1620 + ? ? - + 

N 34.30132 

-118.40858

0.709068±17 -10.5 2.7 ± 0.8 328 125 911 - + + + +

0.709071±13 -10.1 2.5 ± 0.6 92 41 805 - + + + + 

0.708989±17 -18.4 6.1 ± 0.3 718 241 910 - + + + - 

0.709077±16 -9.6 2.3 ± 0.6 163 58 890 - + + + + 

0.709055±14 -11.8 4.0 ± 1.5 27 107 785 - + + + + 

0.709052±15 -12.1 4.3 ± 1.7 62 31 987 - + + + + 

0.709027±13 -14.6 5.3 ± 0.5 21 10 1941 - - - + + 

0.709060±8 -11.3 3.4 ± 0.8 14 5 2193 - - - + + 

P

 34.33443 

-118.45564

0.709015±11 -15.8 5.7 ± 0.4 1108 73 797 + + + + +

 34.33705 

-118.45602

0.708991±15 -18.2 6.1 ± 0.3 45 12 992 + + + + + 

R 34.33207 

-118.45632 

0.708933±13 -24.0 8.2 ± 1.0 1950 611 519 - - - - - 

0.708881±14 -29.2 10.2 ± 0.6 151 74 1140 + + - + - 

T 34.32783 

-118.48837

0.708775±18 -39.8 15.2 ± 0.7 288 200 778 - + + + ±

0.707727±17 -144.6 anomalous 458 270 485 - + - - - 

0.708762±19 -41.1 15.4 ± 0.5 316 122 707 - + + + ± 

0.708663±19 -51.0 17.0 ± 0.4 210 125 554 - + + + - 

U 34.35936 

-118.49896

0.708966±9 -20.7 6.7 ± 0.3 211 69 1100 + + + + + 

0.708982±9 -19.7 6.2 ± 0.2 531 180 553 + - + + + 

W 34.34866 

-118.52919

0.708882±9 -29.1 10.2 ± 0.4 48 66 1590 - - + + ±

0.708848±15 -32.5 11.4 ± 0.8 81 37 2640 - - + + ± 

0.708742±10 -43.1 15.7 ± 0.3 98 484 806 - - - - - 

X 34.36063 

-118.55739

0.709010±9 -16.3 5.8 ± 0.3 170 45 810 + + + + +

0.709025±15 -14.8 5.4 ± 0.6 259 59 821 + + + + + 

0.709027±14 -14.6 5.3 ± 0.5 254 72 802 + + + + + 

Table 3. Analytical results and derived 87Sr/86Sr ages of fossil samples (marine shell carbonate plus fish debris and collophane) from the 

Gold Canyon beds and from the late Miocene Modelo and late Miocene to early Pliocene Towsley Formations at 15 selected sites along the 

southwest margin of the San Gabriel Mountains and the San Fernando Pass area—Continued.
 Appendixes 37

Best estimate of age

(87Sr/ 86Sr) 

(Ma) 

Fossil material used Remarks

0.709014 ± 1.0 (2)

5.7 ± 0.2 

Fish bone and phosphatic matrix from coprolite Towsley Formation equivalent. Experience with Sr isotopic dating of phos

phatic material is limited, but late Delmontian BFS is reasonable. 

Phosphatic matrix from same coprolite

0.709065 ± 11 (5)

2.7 ± 0.5

0.709059 ± 16 (7) 

2.9 ± 0.6

Pectinid fragments, handpicked

Mapped as an uppermost part of the Towsley Formation of Dibblee 

(1991a), it is possible these strata correlate to the Sunshine Ranch Member 

of the Saugus Formation (Winterer and Durham, 1962, plate 44, and text). 

Five pectinids with the most favorable trace elements yield consistent 

results.

Pectinid, deeply ribbed, foliated fragment

Pectinid. foliated fragment

Pectinid, foliated fragement

Pectinid, foliated fragment

Pectinid. foliated fragment

Bivalve (?), vitreous exterior with dense, brown 

interior

Bivalve (?), dense, brown fragment

0.709003 ± 17 (2) 

5.9 ± 0.4

Barnacle, resinous translucent fragments Basal Towsley Formation resting on basement. Correlative of middle Delmontian BFS based on Sr isotopic age. 

Pectinid, white to pinkish, translucent, foliated 

fragments

anomalous ages Mollusk, thin, white , partly chalky fragments Basal Towsley Formation. Inconsistency between Mn/Sr ratios and appar

ent ages indicates altered Sr (see Appendix 2). 

Barnacle, pinkish, dense fragments

anomalous ages

Pectinid, white to pinkish, ribbed, foliated fragments

From vertical strata beneath the north-dipping Santa Susana Fault. Strongly 

anomalous result for barnacle suggests all ages are anomalous. 

Barnacle, pink, unstructured fragments

Oyster, mostly foliated

Oyster, mostly foliated

0.708974 ± 11 (2)

 6.4 ± 0.3 

Pectinid, tan, translucent, foliated fragment Towsley Formation in Elsmere Canyon. Correlative with Delmontian BFS. 

Bivalve, brownish, resinous, translucent fragment

anomalous ages

Bivalve, thin, pinkish, vitreous fragments Pico Formation of Winterer and Durham (1962, plate 44) closely correlative with 5 Ma horizon. First two apparent ages judged unreliable due to 

aragonitic shells or occurrence in possibly recylced fossil-bearing clasts? 

Third apparent age unreliable because of Mn/Sr ratio. 

Gastropod, vitreous, some with chalky patina

Mollusk, uncertain origin, grey foliated fragments

0.709021 ± 9 (3)

5.5 ± 0.4 

Oyster, mostly translucent and foliated fragments

Top of Pico Formation of Winterer and Durham (1962, plate 44). Correlative with the late Delmontian BFS. 

Oyster, translucent, foliated fragments

Oyster, transparent, foliated fragments

Table 3. Analytical results and derived 87Sr/86Sr ages of fossil samples (marine shell carbonate plus fish debris and collophane) from the 

Gold Canyon beds and fom the late Miocene Modelo and late Miocene to early Pliocene Towsley Formations at 15 selected sites along the 

southwest margin of the San Gabriel Mountains and the San Fernando Pass area—Continued.
38 Post-Miocene Right Separation on the San Gabriel and Vasquez Creek Faults

Appendix 3. Identification, Age, and 

Paleoenvironment of Calcareous 

Nannofossils

Calcareous nannofossils were recovered from 

geographically and stratigraphically separated outcrops 

south and southeast of the Merrick Syncline (localities K 

and C on fig. 2B) and on its northeast flank (locality E on 

fig. 2B). These floras provide information about stratigraphic 

position, age range, and correlation to more fossiliferous 

and better studied sections, as well as giving hints about the 

paleooceanographic conditions when these phytoplankton 

communities lived.

Nannofossil assemblages from localities K and C allow 

assignment to the CN 7b-CN 8a zones (Okada and Bukry, 

1980; Young, 1998) that straddle the lower-upper Mohnian 

BFS boundary (Blake, 1991) and indicate an age between 9.8 

and 8.8 Ma (Fornaciari, 2000, table 6; Backman and Raffi, 

1997, table 1; respectively) (table 4). This age range for 

locality K brackets the 9.3±1.1 Ma age determined by 87Sr/86Sr 

analyses (appendix 2, table 3, appendix 4). For locality C, a 

9.8 to 8.8 Ma age range indicated by calcareous nannofossils 

is consistent with the Mohnian BFS assignment based on 

benthic foraminifers (appendix 4, table 5) and with the 9.5 to 

9.2 Ma assignment from diatom data (appendix 5, table 6), but 

it is younger than the 11.2 ±0.7 Ma age from 87Sr/86Sr analyses 

(appendix 2, table 3).

All samples from locality E yield nannofossils that are 

calcareous nannofossil zone CN 5a or younger (Okada and 

Bukry, 1980; Theodoridis, 1984; Young, 1998) (table 4). 

Assemblages in samples E-1 and E-1B are restricted to CN 5 

(Okada and Bukry, 1980; Young, 1998), but E-2 and E-4 may 

be as young as CN 7 (Okada and Bukry, 1980; Perch-Nielsen, 

1985; Young, 1998). This evidence places all four samples in 

the lower Mohnian BFS (Blake, 1991) and indicates a maximum age of 13.53 Ma and a minimum age of 10.9 Ma (E-1 

and E-1B) to 9.67 Ma (E-2 and E-4) (Lourens and others, 

2004, table A2.2). These results are consistent with, but better 

constrained by, the 12.3 to 11.5 Ma assignment of sample E-2 

based on diatom data (appendix 5, table 6). 

Phytoplankton from locality C are typical of well-mixed 

high nutrient upwelling based on abundant diatoms and rare 

nannofossils (Margalef, 1978) and on the high abundance of 

Reticulofenestra pseudoumbilicus (>5 μm) (Flores and others, 

2005). All sites yield rare warm-water, oligotrophic species 

of Discoaster and Sphenolithus and one or more abundant 

mesotrophic and/or eutrophic species: Reticulofenestra pseudoumbilicus (<5 μm), Reticulofenestra pseudoumbilicus (>5 

μm), and Coccolithus pelagicus (Flores and others, 2005). 

These assemblages suggest various phases of upwelling, even 

though abundant oyster spat at localities K and C (appendix 4) 

indicate deposition very close to the paleoshoreline.

Table 4. Calcareous nannofossils identified in Modelo Formation outcrop samples at three localities from Little Tujunga Canyon to near 

the mouth of Big Tujunga Canyon.

[See figure 2B for localities. Correlation of California benthic foraminiferal stages to Okada & Bukry (1980) calcareous nannofossil zones are after Blake 

(1991). Nannofossil ranges are from Young (1998) except for the ranges of Helicosphaera minuta and Syracosphaera fragilis, which are based on Perch-Nielson 

(1985) and Theodoridis (1984), respectively. The “Taxa abundance by locality” section reports the actual number of each species counted per sample. Below this 

section are statistics and comments about each sample. These statistics include “Nannofossils per mm2” which normalizes the abundance of all fossils per slide 

to show the wide range of their abundance. VR = very rare.] 

Age-diagnostic and

associated taxa

California benthic foraminiferal stages Taxa abundance

by locality

Luisian

Mohnian DelLower Upper montian Map symbol

CN Zones

E-1 E-1B E-2 E-4 K C

4 5a 5b 6 7a 7b 8a 8b 9a 9b

 Braarudosphaera bigelowii VR 4

 Calcidiscus leptoporus VR 8 VR 4 10 11

 C. tropicus 3 3 3

 Coccolithus miopelagicus (>14 μm) < X X VR

 C. pelagicus pelagicus 2 49 1 48 243 91

 Coronocyclus nitescens VR

 Cyclicargolithus floridanus < X IN1 VR

 Dictyococcites antarcticus 14 16 4 65 17

 D. productus 89 744 341 81 21 24

 Discoaster exilis < X X X X X 1 VR 3
 Appendixes 39

1

 Either the first or last occurrence of the species is within the zone.

Table 4. Calcareous nannofossils identified in Modelo Formation outcrop samples at three localities from Little Tujunga Canyon to near 

the mouth of Big Tujunga Canyon—Continued.

Age-diagnostic and

associated taxa

California benthic foraminiferal stages Taxa abundance

by locality

Luisian

Mohnian DelLower Upper montian Map symbol

CN Zones

E-1 E-1B E-2 E-4 K C

4 5a 5b 6 7a 7b 8a 8b 9a 9b

 D. sp. cf. D. exilis VR 5 3

 D. intercalaris 1

 D. spp. VR VR 11 1

 D. variabilis 1

 Helicosphaera carteri VR VR 1 11 1

 H. intermedia VR VR

 H. minuta X X X X X X 7 10

 Lithostromation perdurum 6 1 VR 1

 Minylitha convallis X X X IN1 6 1

 placoliths (unidentifiable species) 13 VR VR 17 33 61

 Pontosphaera discopora VR VR

 P. multipora VR

 Reticulofenestra minuta 12 6,231 2,397 24 2 4

 R. minutula 13 991 623 201 27 39

 R. pseudoumbilica (5-7 μm) < X X X X X X > 53 544 141 207 71 212

 R. pseudoumbilica (>7 μm) IN1 X X X X X > 117 37 1 46 68 200

 Rhabdosphaera pannonica 5

 R. sp. 4 VR

 Sphenolithus neoabies 4 6 1 19 8 7

 Syracosphaera fragilis X X X X X X X X 6

 Triorbis sp. VR VR

 Triquetrorhabdulus sp. cf. T. 

rugosus 

3 1

 T. sp. cf. T. rioi VR VR VR VR

 Umbilicosphaera sibogae foliosa 1

 General Preservation (P-Poor, M-Moderate, G-Good, E-Excellent) M E E G M M

 Nannofossils counted 317 8,636 3,536 729 521 678

 Nannofossils per mm2 15 8,636 3,536 209 2 3

 Identifiable nannofossils counted (excludes unidentifiable species) 304 8,636 3,536 712 477 616

 Species identified 8 12 14 9 14 16

 Fisher alpha index 1.5 1.9 1.4 1.5 2.7 3.0

 CN zones represented 5a-5b 5a-5b 5-7 5-7 7b-8a 7b-8a

 diatom fragments per mm2 1 33 376 170 1 400
40 Post-Miocene Right Separation on the San Gabriel and Vasquez Creek Faults

Taxa

Taxa abundance by locality 

(map symbol; N latitude, W longitude, decimal degrees) Benthic 

Foraminiferal Stage 

BFS or age range 

(Finger, 1990)

Paleoenvironment 

(Finger,1990) 

K

 34.29272 

-118.36713

J

 34.29049 

-118.36678

C

 34.27398 

-118.30159

E

 34.28663 

-118.31965 

E-1 E-2 E-3

Benthic Foraminifera

Ammobaculites? sp. F marsh

Bolivina advena R Saucesian-Holocene u. bathyal; O2

minimum

B. churchi C F R Saucesian-Mohnian

B. foraminata F R F R Saucesian-Pliocene u. mid. bathyal; 

O2 minimum

B. cf. B. hughesi VR

B. imbricata VR Saucesian-Holocene u. mid. bathyal

B. seminuda C u. mid. bathyal; 

O2 minimum

B. sinuata VR Mohnian-Pliocene u. bathyal

B. cf. B. spissa F

B. wissleri F Mohnian

Bulimina cf. B. ovata R

Buliminella curta C C Zemorrian-Pliocene outer shelf; O2

minimum

Buliminella subfusiformis C C Zemorrian-Pliocene u. bathyal; O2

minimum

Buliminella spp. F

Galliherina (Bulimina) uvigerinaformis C F Mohnian-Delmontian u. mid. bathyal

Table 5. Abundance, zone ranges, and suggested paleoenvironments of foraminifers from four Modelo Formation outcrop localities 

in the area from Little Tujunga Canyon to the mouth of Big Tujunga Canyon, California.

[See figure 2B for sample localities. VR = very rare, R = rare, F = few, C = common, A = abundant, Flood = very numerous. Includes abundances of fish 

debris and oyster spat.]

Appendix 4. Identification, Age, and 

Paleoenvironment of Foraminifers and 

Occurrences of Fossil Oyster Spat

Foraminifers (benthic and planktonic) were identified 

from stratigraphically separate outcrop localities K, J, and C 

on the south flank, and locality E on the northeast flank, of 

the Merrick Syncline (fig 2B). Fossil species identified are 

shown in table 5, together with zonal assignments (Blake, 

1991), age ranges for each species (Barron and Isaacs, 2001, 

Finger, 1990), and paleoenvironments (Finger, 1990). The 

unexpected discovery of fossil oyster spat associated with 

deep-water benthic foraminifers, an association apparently 

not previously described in the literature, suggests close 

proximity of shallow coastal marine waters to upper bathyal 

environments. Oyster spat from locality K were analyzed for 

87Sr/86Sr age (appendix 2). 

On the Merrick Syncline south flank, the 

stratigraphically highest sample (locality K) cannot be 

younger than 5.1 Ma (Delmontian BFS), based on the 

presence of Uvigerina hannai, but it probably is older 

(Mohnian BFS). The age range of the next stratigraphically 

lower sample (locality J) is 13.5 to 7.1 Ma (Mohnian 

BFS) based on the co-occurrence of Gyroidina rosaformis

and Bolivina sinuata. The age of the stratigraphically 

lowest sample (locality C) is also 13.5 to 7.1 Ma based 

on the occurrence of Galliherina uvigerinaformis and 

its stratigraphic position below locality J. The locality C 

sample probably is no younger than 9.4 Ma. Traditionally, 

G. uvigerinaformis has been restricted to the early Mohnian 

BFS (13.5 to 9.4 Ma), but Finger (1990) extended its range 

to include all of the Mohnian BFS and Delmontian BFS 

(13.5 to 5.1 Ma). On the Merrick Syncline northeast flank, 

the age of samples from locality E is Mohnian based on 

the co-occurrence of Galliherina uvigerinaformis, Bolivina 

churchi, and B. wissleri.
 Appendixes 41

Table 5. Abundance, zone ranges, and suggested paleoenvironments of foraminifers from four Modelo Formation outcrop localities 

in the area from Little Tujunga Canyon to the mouth of Big Tujunga Canyon, California—Continued.

1

 Unwashed material from E-1, E-2, and E-3 contain numerous thin-shelled foraminifers on bedding planes that probably are Praeglobobulimina sp.

Taxa

Taxa abundance by locality 

(map symbol; N latitude, W longitude, decimal degrees) Benthic 

Foraminiferal Stage 

BFS or age range 

(Finger, 1990)

Paleoenvironment 

(Finger,1990) 

K

 34.29272 

-118.36713

J

 34.29049 

-118.36678

C

 34.27398 

-118.30159

E

 34.28663 

-118.31965 

E-1 E-2 E-3

Gyroidina rosaformis R Zemorrian-Mohnian u. mid. bathyal

Hansenica cf. H. rotundimargo R

Holmanella (Planulina) baggi VR Saucesian-Pliocene outer shelf

Praeglobobulimina ? sp. VR C1 C1 C1 u. mid. bathyal; 

O2 minimum

Uvigerina hannai R Saucesian-Delmontian u. mid. bathyal

U. hootsi VR Saucesian-Pliocene u. mid. bathyal

U. cf. U. subperigrina R u. mid. bathyal; 

O2 minimum 

U. sp. (broken) VR

Planktic Foraminifera

Globigerina bulloides F F F R Zemorrian-Plio/Holocene temperate waters

G. sp. F

Other

fish debris F A A A R

oyster spat A Flood tidal

Concurrent age ranges of microfossils (diatoms, 

calcareous nannofossils, and foraminifers) and a strontium 

isotope age indicate a latest early Mohnian BFS to early late 

Mohnian BFS age range (9.8 to 8.8 Ma) for all three southflank samples. The stratigraphically highest sample (locality 

K) is no younger than early late Mohnian BFS (8.8 Ma), 

and the age of the stratigraphically lowest sample (locality 

C) is no younger than earliest late Mohnian BFS (9.2 Ma) 

and no older than latest early Mohnian BFS (9.9 Ma). The 

age of the middle sample (locality J) may be at or near the 

boundary between late and early Mohnian BFS (9.4 Ma). 

Concurrent age ranges of diatoms and calcareous nannofossils 

from sample E on the northeast flank indicate a middle early 

Mohnian BFS age range (12.3 to 11.5 Ma).

Benthic foraminifers from all samples indicate an 

upper bathyal to upper middle bathyal depositional depth, 

probably in or near the O2 minimum zone. The presence 

of pelagic open-ocean microfossils (planktic foraminifers, 

calcareous nannofossils, diatoms, and fish debris) in 

samples from localities C and E indicates open-ocean and 

possible upwelling conditions. The presence of calcareous 

nannofossils and fish debris in the locality K sample also 

indicates open-ocean conditions. The large number of oyster 

spat associated with deep open-water microfossils and fish 

debris in samples C and K suggest close proximity of a 

shallow coastal environment. The lack of shallow-water 

foraminifers suggests that the oyster spat came from the 

overlying water column rather than by downslope sediment 

transportation.

Spat is a term used here to include all the stages of shelled 

pelagic oyster larvae before attachment to a substrate. Because 

precise biological names for the various pelagic stages are 

based on soft-part development rather than shell size, the use 

of various stage names for fossil shell material is not possible. 

Spat shell length in modern oysters increases with stage 

development but shows much variation between oyster species. 

Modern spat usually range from 60 μ to 300 μ in length. Fossil 

spat from both localities C and K range in length from 55 μ

to 100 μ. Modern oyster larvae develop in 10-25 days, during 

which time water currents can transport them as far as 8 km 

from spawning grounds (Lorio and Malone 1994). The pelagic 

larvae have a limited ability for vertical movement, but cooling 

water conditions and turbidity can delay settling for as much as 

ten days (Lorio and Malone, 1994). 
42 Post-Miocene Right Separation on the San Gabriel and Vasquez Creek Faults

Locality ........................................... C E

Zone, subzone ............................... D. hustedtii-D. lauta subzone d D. hustedtii-D. lauta subzone c 

Assemblage age range (Ma) ..... 9.5-9.2 12.3-11.5

Species Abundance Taxon age range 

(Ma)

Actinocyclus ingens rare common 18.4 - 7.7

Azpeitia cf. nodulifera rare 13.3 - 0

Azpeitia cf. vetustissima rare rare ~14 - 0

Denticulopsis dimorpha rare 9.9 - 9.2

D. praekatayamae rare 9.5 - 8.6

D. lauta rare rare 15.1 - 9

D. praedimorpha very rare (reworked) abundant 12.4 - 11.5

D. simonsenii abundant abundant 13.1 - 8.6

Hemidiscus cuneiformis rare 11.5 - 0 

Nitzschia praereinholdii rare 12.2 - 7.6

Proboscia barboi rare rare 12.3 - 0.3

Thalassiosira brunii rare 12.2 - 8.9

T. cf. flexuosa rare 13.6 - 9.0

Table 6. Diatom species and abundance from two Modelo Formation outcrop samples.

[See figure 2B for localities.]

Appendix 5. Identification and Zonation 

of Diatoms

Well-preserved diatom assemblages were encountered in 

samples from locality C near the mouth of Big Tujunga Canyon 

and from locality E on the northeast flank of the Merrick 

Syncline (fig. 2B). The locality C assemblage is correlated with 

the upper part of Subzone d of the Denticulopsis hustedtii-D. 

lauta Zone (9.5-9.2 Ma), and the locality E assemblage is correlated with the lower part of Subzone c of the Denticulopsis 

hustedtii-D. lauta Zone (12.3-11.5 Ma) (Barron, 1981, 1986a) 

(table 6). The locality C assemblage also contains the silicoflagellate Distephanus pseudofibula, that ranges from ~9.5 to 

7.8 Ma. According to Barron and Isaacs (2001) and Barron 

(2003), the locality C assemblage is correlative with earliest 

late Mohnian BFS and the locality E assemblage is correlative 

with early Mohnian BFS, the latter being older than all other 

diatom-bearing San Fernando Valley assemblages studied 

by Barron, which do not range below Subzone d of the D. 

hustedii-D. lauta Zone. Interestingly, the locality E assemblage 

immediately predates a compressed interval within the middle 

and upper parts of Subzone c of the D. hustedtii-D. lauta Zone 

that occurs in numerous coastal sections of southern California 

and in offshore DSDP sections. Barron (1986b) cites papers by 

Hoskins and Griffiths (1971) and Crouch (1979) that relate this 

compressed interval to uplift of the southern California continental borderland. It also appears that this compressed interval 

coincides with the final phase in tectonic rotation of the Santa 

Ynez Range (Hornafius and others, 1986).
 Appendixes 43

Figure 11. 40Ar/39Ar age-spectrum 

(A) and isochron (B) diagrams for 

the Topanga Group basalt sample 

(locality D, fig. 2B). The statistically 

uniform ages obtained from the 

625°C through 1,150°C steps defines 

an age plateau representative of the 

age of the basalt. 

6

8

10

12

14

16

18

0 0.2 0.4 0.6 0.8 1.0

Cumulative 39Ar Fraction

Age (Ma)

Plateau age = 16.14±0.05 Ma

MSWD = 1.4

Includes 91.6% of the 39Ar

Akens Canyon Plagioclase

A

B

0

1,000

2,000

3,000

4,000

5,000

6,000

0 400 800 1,200 1,600 2,000

39Ar/ 36Ar

40Ar/ 36Ar

Age = 16.19±0.18 Ma

Initial 40Ar/ 36Ar =289.7±5.8

MSWD = 1.01

Appendix 6. Age of Basalt by 40Ar/39Ar 

Incremental-Heating Analysis

A Topanga Group volcanic sample from locality D (fig. 

2B), dated by 40Ar/39Ar analysis, is a partly altered, porphyritic basaltic andesite or basalt with fine-grained groundmass. 

Phenocrysts of altered olivine (largely opaque), clinopyroxene, and plagioclase are contained in a partly altered matrix 

of plagioclase microlites, clinopyroxene, and magnetite, with 

hematite, iddingsite, chlorite, and minor glass, some altered to 

clays. Plagioclase phenocrysts range from <500 μm to more 

than 1 mm in size, some with corroded and embayed margins. 

Reacted rims appear to be entirely products of magmatic 

reaction with the melt, and no significant alteration of these 

phenocrysts is noted. These phenocrysts are zoned and range 

in composition from about An30 to An60. The mean K2O 

content of the plagioclase derived from Ar-Ar analysis is 0.28 

percent. The minimum dimension of groundmass plagioclase 

laths averages about 10 μm, with no visible alteration. 

Plagioclase from the sample was analyzed by the 40Ar/39Ar (Ar-Ar) dating technique (Merrihue and Turner, 

1966). Separated 0.25 to 0.5 mm phenocrysts were irradiated 

with fast neutrons for 16 hours at about 1 MW power in the 

U.S. Geological Survey (USGS) TRIGA Reactor Facility in 

Denver, Colorado. USGS standard sanidine from the Taylor 

Creek Rhyolite (TCR-2), with an age of 27.87 Ma, was used 

as the flux monitor. This age is standardized to the Menlo 

Park intralaboratory standard biotite, SB-3, and to interlaboratory standard hornblende, MMhb1 (Samson and Alexander, 
44 Post-Miocene Right Separation on the San Gabriel and Vasquez Creek Faults

Table 7.—Results of Ar-Ar analyses of plagioclase from basaltic andesite of the Topanga Group.

[See locality D on figure 2B. Sample designation—Akens Canyon; irradiated in the USGS TRIGA reactor, Denver, CO, as USGS Menlo Park irradiation 

number IRR228-51; the measure of neutron flux is J = 0.003603159. ArK, potassium-derived argon; Ar*, radiogenic. MSWD is mean square of weighted 

deviates, after McIntyre and others, 1966.] 

1987) with an average age of 513.9 Ma. Decay and abundance 

constants for K and Ar isotopes are those recommended by 

Steiger and Jager (1977).

Argon was released by incremental heating of the sample 

from 550° to 1,300°C. Analytical errors in 40Ar/39Ar ages are 

reported at the 1s level. The 40Ar/39Ar plateau age identified in 

the age spectrum diagram (fig. 11) represents more than 50 percent of the 39Ar released and is defined as the weighted mean 

age of contiguous gas fractions for which no difference in age 

can be detected at the 95-percent level of confidence (Fleck and 

others, 1977). Analytical results are summarized in table 7. The 

40Ar/39Ar incremental-heating data define a “plateau-type” age 

spectrum with an age of 16.14±0.05 Ma that represents the age 

of eruption and cooling of the lava (fig. 11A). An isochron diagram of the plateau-age steps yields an age of 16.19±0.18 Ma 

and MSWD of 1.01 (fig 11B), confirming the plateau age and 

the agreement between estimated analytical error and the dispersion of data about the isochron (McIntyre and others, 1966). 

 

Step (°C) % 39ArK % 40Ar* 40Ar/39Ar 37Ar/39Ar 36Ar/39Ar K/Ca Cl/K Age (Ma)

550 1.06 4.91 34.1136 15.7293 0.1141 0.033 0.1123 10.976 ± 2.470

625 4.46 29.74 8.0452 17.3708 0.0239 0.030 0.0211 15.665 ± 0.567

700 9.54 75.57 3.2456 18.3893 0.0077 0.028 0.0058 16.069 ± 0.157

775 13.25 86.21 2.8239 18.5167 0.0064 0.028 0.0019 15.951 ± 0.117

850 17.76 92.44 2.6928 19.7914 0.0061 0.026 0.0020 16.322 ± 0.109

925 18.84 91.69 2.6997 20.3716 0.0063 0.025 0.0041 16.238 ± 0.105

1000 13.51 89.22 2.7645 18.7637 0.0061 0.028 0.0061 16.162 ± 0.115

1075 8.14 50.96 4.7582 12.8595 0.0114 0.040 0.2699 15.829 ± 0.205

1150 6.14 57.32 4.2978 10.7282 0.0091 0.049 0.0125 16.055 ± 0.225

1300 7.30 46.49 5.5459 13.2960 0.0137 0.039 0.0063 16.828 ± 0.225

Weighted Mean Plateau age (Ma) 16.14 ± 0.05

Isochron age (Ma) (MSWD=1.01) 16.19 ± 0.18

Total gas age (Ma) 16.106 ± 0.058

 

Produced in the Western Region, Menlo Park, California

Manuscript approved for publication, November 3, 2008

Text edited by Peter H. Stauffer

Layout and Design by Jeanne S. DiLeo

Beyer and others—Post-Miocene Right Separation on the San Gabriel and Vasquez Creek Faults, with Supporting Chronostratigraphy—Professional Paper 1759