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1

Mountain Meadows Dacite: Oligocene Intrusive 

Complex That Welds Together the Los Angeles Basin, 

Northwestern Peninsular Ranges, and Central 

Transverse Ranges, California

By Thane H. McCulloh,1 Larry A. Beyer,2 and Ronald W. Morin3

Abstract

Dikes and irregular intrusive bodies of distinctive Oligocene biotite dacite and serially related hornblende latite and 

felsite occur widely in the central and eastern San Gabriel 

Mountains, southern California, and are related to the Telegraph Peak granodiorite pluton. Identical dacite is locally 

present beneath Middle Miocene Topanga Group Glendora 

Volcanics at the northeastern edge of the Los Angeles Basin, 

where it is termed Mountain Meadows Dacite. This study 

mapped the western and southwestern limits of the dacite 

distribution to understand the provenance of derived redeposited clasts, to perceive Neogene offsets on several large strikeslip faults, to test published palinspastic reconstructions, and 

to better understand the tectonic boundaries that separate 

contrasting pre-Tertiary rock terranes where the Peninsular 

Ranges meet the central and western Transverse Ranges and 

the Los Angeles Basin.

Transported and redeposited clasts of dacite-latite occur 

in deformed lower Miocene and lower middle Miocene sandy 

conglomerates (nonmarine, nearshore, and infrequent upper 

bathyal) close to the northern and northeastern margins of the 

Los Angeles Basin for a distance of nearly 60 km. Tie-lines 

between distinctive source suites and clast occurrences indicate that large tracts of the ancestral San Gabriel Mountains 

were elevated along range-bounding faults as early as 16-15 

Ma. The tie-lines prohibit very large strike-slip offsets on 

those faults. Transport of eroded dacite began south of the 

range as early as 18 Ma.

Published and unpublished data about rocks adjacent 

to the active Santa Monica-Hollywood-Raymond oblique 

reverse left-lateral fault indicate that cumulative left slip totals 

13-14 km and total offset postdates 7 Ma. This cumulative 

slip, with assembly of stratigraphic and paleogeographic data, 

invalidates prior estimates of 60 to 90 km of left slip on these 

faults beginning about 17-16 Ma. 

A new and different palinspastic reconstruction of a 

region southwest of the San Andreas Fault Zone is proposed. 

Our reconstruction incorporates 20° of clockwise rotation of 

tracts north of the Raymond Fault from the easternmost Santa 

Monica Mountains to the Vasquez Creek Fault (San Gabriel 

south branch). We interpret the Vasquez Creek Fault as a 

reverse and right-lateral tear fault. Right slip on the tear 

becomes reverse dip slip on the northeast-striking ClamshellSawpit fault complex, interpreted as an offset part of the 

Mount Lukens Fault. This explains the absence of evidence 

for lateral offset of the Glendora Volcanics and associated 

younger marine strata where those are broken farther east by 

the eastern Sierra Madre reverse fault system. About 34 km 

of right slip is suggested for all breaks of the San Gabriel 

fault system.

New paleogeographic maps of the Paleogene basin 

margin and of a Middle Miocene marine embayment and 

strandline derive in part from our palinspastic reconstruction. 

These appealingly simple maps fit well with data from the 

central Los Angeles Basin to the south and southwest.

Introduction

The southeastern end of the western Transverse Ranges, 

the central Transverse Ranges (San Gabriel Mountains), and 

the northeastern part of the Los Angeles Basin meet the northwestern tip of the Peninsular Ranges within a 4,000-km2

rectangular area of southern California (fig. 1). This area is 

subdivided three-dimensionally by a complex of more or less 

conspicuous intersecting or interlocking fault zones, many 

of which are currently active. Some of these zones record 

repeated movements beginning before 15 Ma. The resultant 

mosaic of fault blocks show complex and partly divergent 

histories (Powell, 1993, plate I). A petrologically distinctive 

and petrographically unique intrusive igneous complex of 

restricted areal extent, the 27.6-Ma Mountain Meadows 

Dacite (Shelton, 1955; Nourse and others, 1998; this report), 

straddles several major fault zones and thereby binds together 

key blocks of the mosaic. The disposition of this complex, 

 17136 Aberdeen Avenue, Dallas, Texas 75230-5407

2U.S. Geological Survey, MS 977, 345 Middlefield Road, Menlo Park, 

California 94025-3591 3Morin Biostratigraphic Studies, 2016 Wedgewood Lane, Carrollton, 

Texas 75006-1722

Introduction
2 Mountain Meadows Dacite: Oligocene Intrusive Complex

and of several connected Miocene sedimentary and extrusive 

formations, imposes strict limits on the range of movement 

histories for the mosaic. This paper (1) summarizes salient 

facts about the Mountain Meadows Dacite and some older 

and younger associated units, (2) relates those facts to histories of particular faults and fault blocks, (3) develops a 

new palinspastic reconstruction, and (4) considers structural 

consequences and regional paleogeographic and paleotectonic 

implications of the findings.

Acknowledgements

Most of the new work reported here was supported by 

the U. S. Geological Survey, albeit intermittently and over 

five decades. F. H. Olmstead introduced the Telegraph Peak 

granodiorite and its chilled dacitic border facies to the senior 

author in 1948. A. O. Woodford and J. S. Shelton were 

early guides. Los Angeles Basin subsurface geology became 

an appreciated resource especially through the work of J. 

E. Schoellhamer. Data, cores, and cuttings from key wells 

were crucial to our study. Particular thanks are extended 

to Leslie Klinchuch and John Urbach (Chevron USA, Inc.), 

Reon Moag (Union Oil Co. of California), Charles James 

(Shell Oil Co., Aera Energy LLC), and Russ Robinson (California Well Sample Repository) for crucial well samples. 

Publications of T. E. Wright, J. S. Hornafius, and E. Hauksson 

proved invaluable. R. M. Enrico and S. A. Root gave expert 

paleontological assistance and S. Cook key field assistance. 

V. A. Frizzell determined and generously provided important 

radiometric age values. R. E. Denison provided access to 

petrographic facilities. R. E. Stanley gave encouragement and 

help. Critical input was provided by R. E. Powell, both in 

Figure 1.—Principal geographic features and simplified fault map of a part of southern California southwest of the San Andreas Fault 

Zone. Outlines of our study area (fi gs. 2, 5, and 6) and of an area covering the Santa Monica-Hollywood-Raymond Hill Fault Zones (fi g. 

9) are indicated. Partial extents of basement rock domains 1 and 2 (fi g. 4) are also shown. EP-AH is the Elysian Park-Monterey ParkAlhambra structural high discussed in text.

0 5 10 15 20 25 30

KILOMETERS

PACIFIC OCEAN

Portion of north and northeast border of domain1 

basement rocks, the "Catalina" terrane (Howell

and others, 1985) (see fig. 4). 

Border of area 2 beneath Los Angeles Basin 

central syncline where basement rocks are 

inaccesible and unknown (see fig. 4). 

North

Fault, dashed where uncertain, dotted where buried.

 

Thrust fault, saw teeth on upper plate. 

Faults queried where doubtful.

PALOS VERDES FAULT

WHITTIER FAULT

CHINO

ELSINORE

CRISTIANITOS

LAS CIENEGAS FAULT

SAN JACINTO FAULT

ANACAPA or "DUME" FAU LT

FAULT

FAULT

FAULT

? ?

? ?

? ? ?

? ? ?

?

?

? ?

? ? ?

SAN GABRIEL NORTH BRANCH FAULT

FIGS. 2, 5, 6

FIG. 9

SANTA MONICA - HOLLYWOOD FAULT RAYMOND FAULT

SIERRA MADRE - DUARTE FAULT

FRAZIER

MTN.

EASTERN

VENTURA 

BASIN

SAN GABRIEL

FAULT

SAN ANDREAS FAULT

SAN FRANCISQUITO FAULT

SANTA MONICA

MALIBU COAST FAULT

SAN

FERNANDO

VALLEY

VERDUGO

"SW EDGE of BASEMENT

SANTA

SUSANA

FAULT

RIDGE"

MTNS. BENEDICT

CANYON

FAULT

SAN GABRIEL

LOS

ANGELES

BASIN

MOUNTAINS

SAN

GABRIEL

VALLEY

POMONA

VALLEY

SANTA ANA

MOUNTAINS

NORTHWEST

PENINSULAR

RANGE

SAN

BERNARDINO

MOUNTAINS

NEWPORT - INGLEWOOD ZONE

?

?

MOUNT

WILSON

34°30'

34°00'

118°30' 118°00' 117°30'

FAULT

? ? ? ?

SAN JOSE HILLS

PUENTE HILLS E.P -- A.H.

EXPLANATION

CUCAMONGA

FAULT
3

personal conversations and through his encyclopedic publications. Others who helped us include C. L. Powell and Zenon 

Valin (USGS), Karen Fortus and Steve Bear (U. S. Forest 

Service), and Rachel Wing (City of Monrovia, California, 

Parks Division). Helpful USGS reviews were provided by R. 

H. Campbell, D. M. Morton, and R. E. Powell.

Regional Structural Framework

Most faults considered in this study are exposed at the 

surface and have been mapped in greater or lesser detail 

(Jennings and Strand, 1969). A few are known only from 

drill hole information augmented partly by interpretations of 

gravity or seismic reflection data. Evidence of ground-water 

barriers and studies of earthquake sequences reveal a few 

additional faults and illuminate the state of stress connected 

with others. Faults we consider are shown on figure 2 and 

summarily described in table 1.

Structural evolution and lithostratigraphic development 

are interdependent in tectonically active depositional settings. 

Deciphering either the structural or sedimentary history consequently requires dual approaches. As a starting complement 

to the fault pattern, figure 2 also shows the present configuration of the basement-sedimentary rock interface. This configuration is based substantially on drill hole and outcrop data 

northeast of the Whittier Fault Zone and north of the ìMonterey Park faultî (Davis and others, 1989; Wright, 1991). 

Southwest of those structures the configuration is adapted 

mainly from interpretations of gravity data (McCulloh, 1960; 

Chai and Hinze, 1988; Langenheim and Jachens, 1996). For 

Figure 2.—Simplifi ed geologic map of the study area emphasizing surface traces of faults mentioned in the text. Also shown are outcrop areas 

of basement rocks older than Los Angeles Basin sedimentary cover and outcrops of Telegraph Peak granodiorite (Miller and Morton, 1977), 

plus a generalized confi guration of the base of the sedimentary cover modifi ed from McCulloh (1960), Chai and Hinze (1988), Wright (1991), and 

Langenheim and Jachens (1996). Fault abbreviations: BC, Benedict Canyon; C, Chino; C-S, Clamshell-Sawpit; CUC, Cucamonga; EC, Evey Canyon; 

EM, East Montebello; LC, Las Cienegas; ML, Mount Lukens; MP, Monterey Park; R, Raymond (or Raymond Hill); SA, San Andreas; SAC, San Antonio 

Canyon; SC, Stoddard Canyon; SG, San Gabriel; SG-N, San Gabriel north branch; SJ, San Jose; SM-D, Sierra Madre (includes “Duarte” fault); 

SM-H, Santa Monica-Hollywood; SS, Santa Susana (eastern extension); V, Verdugo (includes Eagle Rock and San Rafael); VC, Vasquez Creek 

(also called San Gabriel south branch); VT, Vincent Thrust; W, Whittier. Fault symbols as in fi gure 1.

Regional Structural Framework

Pre-sedimentary basement rock outcrops

SG

SG-N

VC

SS

V

BC

SM-H

R

MP

C-S

SM-D

SJ

C

VT

SA

SAC

SC

CUC

W

LC

ML

?

?

? ?

?

? ?

? ? ?

?

?

?

?

?

?

?

118°15' 118°00' 117°45'

34°00'

34°15'

0 5 10 15

Oligocene Telegraph Peak granodiorite Key drill holes (see table 4 and appendix 1)

EM

EC

Z

EXPLANATION

KILOMETERS

? 8

2

12

14 16

18

4

10

0

20

22

24

10

12

14

4

6

8

8

6

6

8

10

12

8

10

8

4

2

0

0

0

2

4

6

4

2

0

6

6

6

2

4

0

?

?

?

?

?

6 Structure contours on base of sedimentary cover

(in thousands of feet below sea level)

4

?

DD

F E K

A

C B

D

H

G

I

J

Q

CC

X

W

V

AA

R

T U

N

S

Z

Verdugo Mtns.

San

Rafael

Hills
4 Mountain Meadows Dacite: Oligocene Intrusive Complex

most of the area between the Whittier and Sierra MadreDuarte Fault Zones, the present deformed surface was a 

nearly level erosional surface before about 17 Ma. South 

of the Whittier Fault, and locally also north of the Santa 

MonicañHollywood Fault Zone, variable but modest thicknesses of late Cretaceous and Paleogene clastic strata separate 

an unmapped 17-Ma horizon from the basement interface. 

However, the structure contours in figure 2 are an approximation of all post-17 Ma differential structural rearrangements 

because subsidence and sedimentation have dominated much 

of the area since 17 Ma. North of the Verdugo and Sierra 

MadreñCucamonga Fault Zones, and partly northwest of the 

Santa MonicañRaymond Fault Zone, uplift and erosion dominated for all or most of the past 17 million years, and especially since about 4 Ma. Much of the Los Angeles Basin 

porous sedimentary fill (McCulloh, 1960) originated through 

Neogene to Holocene erosion of the structurally elevated and 

mountainous tracts to the north.

Basement Rock Units

Basement rocks that predate the Los Angeles Basin sedimentary cover are a diverse and heterogeneous metamorphic 

and plutonic assemblage. They range from Early Proterozoic 

amphibolite and orthogneiss to Early Cretaceous tonalites, 

granodiorites, and granitic rocks (see, for example, Schoellhamer and Woodford, 1951; Yerkes and others, 1965, p. 

A21-A24 and fig 5; Yeats, 1973; Ehlig, 1981; Sorensen, 1988; 

Wright, 1991, fig. 4; Powell, 1993, fig. 8). An overview 

of these complex assemblages is facilitated by subdividing 

the region along major tectonic boundaries into natural ìtectonostratigraphic terranesî (Howell and others, 1985, p. 4 and 

fig. 3). Characteristics and locations of the resultant six major 

areas of strongly contrasting basement rock assemblages are 

shown in figures 3 and 4, respectively (see also figure 1).

A profound tectonic boundary separates area 4 (the central San Gabriel Mountains, San Rafael Hills, and Verdugo 

Mountains) from area 3 (the San FernandoñSan Gabrielñ

Table 1. Characteristics of faults pertinent to this study, from published sources.

[Fault strike, dip, slip characteristics and movement histories refl ect the state of general understanding prior to this study and are based mainly on published 

literature, tempered in some cases by our knowledge and judgements. Sources of information are not comprehensive and may exclude some pertinent 

publications. Location of faults are shown in fi gure 2.]
5

Pomona Valleys plus Santa Monica MountainsñAlhambra 

highñPuente HillsñSanta Ana Mountains and northwestern 

Peninsular Ranges). Area 4 basement rocks range in age from 

older Proterozoic to early Late Cretaceous and consist of multiple distinctive and widespread intermediate metamorphic 

and plutonic units (Ehlig, 1981; Powell, 1993) related to 

the southwestern North American craton. Area 3 basement 

rocks are arc-related metasedimentary and metavolcanic units 

of Jurassic and early Cretaceous age intruded by Early Cretaceous to early Late Cretaceous calc-alkaline plutons like those 

of the western Peninsular Ranges batholith (Baird and others, 

1974, figs. 6-8; Silver and Chappell, 1988). Area 3 rocks older 

than late Jurassic are not known, and any connection to the 

North American craton is remote (but see Gastil, 1985, p. 

276).

The contrast between basement rocks of area 1 (ìCatalinaî of Howell and others, 1985, fig. 3) and area 3 (the eastern Santa Monica Mountains) is equally impressive. Area 1 

high-pressure glaucophane schist and related greenschists and 

saussurite gabbro are oceanic. Their initial metamorphism, 

dated as 115 Ma, late Jurassic (Grove and Bebout, 1995), took 

place at temperatures of 700-800°C and pressures appropriate 

for a depth of about 16 km. In area 3 at about the same time, 

argillaceous and tuffaceous sediments were being deposited 

(now the slates, phyllites, and schists of the Santa Monica 

Formation) and Cretaceous plutons emplaced. The nature 

and origin of the tectonic boundary between areas 1 and 3 

are matters of continuing study (Crouch and Suppe, 1993; 

Bohannon and Geist, 1998).

Basement rocks beneath the deeply downfolded Los 

Angeles Basin central synclinal trough have never been sampled. They may be tectonically denuded Catalina Schist 

(Crouch and Suppe, 1993), middle Miocene volcanic rocks 

intruded into the axial part of a rifted zone (Redin, 1991, fig. 

4), or a floor ìdeveloped on stretched and attenuated marginal 

rocksî or ìas a complex of volcanic rocks and sedimentsî 

 Table 1. Characteristics of faults pertinent to this study, from published sources—Continued.

Basement Rock Units
6 Mountain Meadows Dacite: Oligocene Intrusive Complex

Figure 3.—Basement rock domains and assemblages of the San Gabriel Mountains, the eastern part of the western Transverse Ranges, the Los Angeles end of the Peninsular Ranges. The nature of the boundaries between domains is also shown. Domain locations and boundaries are shown mainly on 

1. Each domain is a “tectonostratigraphic terrane” (Howell and others, 1985), a distinct tectonic entity. Note that the Mountain Meadows Dacite adomains 3 through 6.

"CATALINA", 

PALOS VERDES,

LA BASIN SW

LA BASIN

CENTRAL

SYNCLINE 

TROUGH

E. SANTA MONICA MTNS. & SAN 

FERNANDO VALLEY, SAN 

GABRIEL & POMONA-CHINO 

VALLEYS & PUENTE HILLS

MIO.

OLIG.

EOC.

PAL.

PALEOZOIC

K

J

TR

CENOZOIC

Uplift, coolingThermal overprint; high pressure

McCULLOH, 1960;REDIN, 1991, SCHNEIDER andothers; 1996; SHAW & SHEARER, 1999

Metamorphism ofblueschist blocks with garnet and epidote at high T & P. 

Newport-Inglewood fault zone

and other (?) discontinuities

Santa Monica fault zone and 

Las Cienegas fault

HOOTS, 1931; NEUERBERG, 1953; YERKES and others,1965; COLBURN, 1973; TAYLOR & SILVER, 1978, fig. 2; SORENSEN, 1985; SILVER & CHAPPELL, 1988

Santa Monica"Slate", and related rocks

Glendora Volcanics

?

?

?

?

? ? ? ?

?

XXXX

XXXX

NATURE OF FOUNDATIONOR BASEMENT ROCKSCONJECTURAL

?

? ?

XXXX

Verdugo-Raymond Hill-Sierra

Madre-Cucamonga faults

Santiago Pk.Volcanics &Bedford Cyn.Formation

"Granitoid"

Plutons

Cretaceous sed-imentary strata of the Santa Monica& Santa Ana Mtns.

12 3

* XXXXXXXXXXXXXXXX

GROVE & BEBOUT,1995; SCHOELLHAMER& WOODFORD, 1951; YEATS, 1973; WRIGHT,1991

Sespe Fm

LOGARITHMIC TIME SCALE IN Ma

10

20

50

100

10002000

CENTRAL SAN GABRIEL MTNS.

AND VERDUGO & SAN RAFAEL HILLS SE SAN GABRIEL MTNS

BAIRD and others, 1974, figs. 6 & 7; MILLER & MORTON, 1980; EHLIG,1981; POWELL, 1993, fig. 8 & appendix 1; BARTH and others, 1995

ALF, 1948; HSÜ, 1955; MORTO1975; MAY & WALKER, 1989;MATTI & MORTON, 1993

Mt. Josephinegranodioritic plutons

Mt. Lowe Grandodiorite

Placerita Fm. (metasediments) or Limerock Canyon amphibolitic rocks

Anorthosite-Syenitecomplex

Mendenhall Orthogneiss

*

Evey Canyon-San Antonio

Canyon fault zone

?

(Joseph and others, 1982)

*

"Major sinistral transformalong s. mtn. edge (May& Walker, 1989)

Pegmatite dike

Upper amphibolite faciemetasedimentary rocks

? ?

?

4 5 

Granulitic mylonite and gneiss of uncertain age

XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

Mountain Meadows Dacite Intrusives of Shelton (1955); Hsü and others (1963); Nourse and others, (1998); 

MESOZOIC PROTEROZOIC *
7

(Crowell, 1974). Alternatively, they might be downfaulted 

correlatives of the sequence that crops out in the Santa Ana 

Mountains (Schoellhamer and others, 1981).

Most basement rocks of the southeastern San Gabriel 

Mountains closely resemble those of the central part of that 

range. Important differences are the absence, east of the boundary faults, of Triassic Lowe granodiorite and metasyenitic rocks 

and proved representatives of the Proterozoic suite, and the 

presence along the southeast edge of the mountain range of 

a unique suite of synplutonic mylonites and related granulitic 

gneiss (Alf, 1948; Hs ̧, 1955; the ìCucamonga terraneî of 

May and Walker, 1989, figs. 2, 3). Broad similarity of much 

of this southeastern San Gabriel suite to parts of the eastern 

Peninsular Ranges prompted Matti and Morton (1993, p. 

124-125) to suggest that these rocks originated as part of 

the Peninsular Ranges and were structurally juxtaposed with 

the central San Gabriel Mountains. Late Cretaceous or early 

Paleogene ductile movements (May and Walker, 1989) or 

Neogene brittle fault displacements (Matti and Morton, 1993, 

p. 125) might have accompanied juxtaposition. Results of 

this study suggest that both late Cretaceous and Neogene 

tectonism may have contributed to the enigmatic assemblage.

The south- to southwest-dipping Vincent Thrust and 

genetically related overlying mylonites separate basement 

rocks of both the southeast and central San Gabriel Mountains 

from muscovite-albite-quartz schist and local greenschist of 

the Pelona Schist (Ehlig, 1981; Morton and Matti, 1993). The 

age of metamorphism of the Pelona Schist is thought to be 

≥65-60 Ma (Jacobson, 1990). U-Pb ages of ìdetrital zirconsî 

in the schist in the western San Gabriel region are interpreted 

to be about 90-80 Ma (Jacobson and others, 2000, figs. 2C, 

3C). Neither the Pelona Schist nor the associated mylonitic 

rocks of the Vincent Thrust are intruded by Cretaceous granitoid plutons, even though U-Pb isotope relations in zircons of 

some nearby plutons yield ages of 90-75 Ma (Silver, 1971) 

and 88-78 Ma (May and Walker, 1989, p. 1260). These facts, 

together with the late Turonian age (>89 Ma) of basal fossiliferous marine sedimentary rocks in both the Santa Monica and 

Santa Ana Mountains, indicate a profound regional tectonic 

significance for both the Vincent Thrust and the synplutonic 

mylonite north of the Cucamonga Fault separating areas 3 

and 5.

Two occurrences of basement rocks may not fit simply 

into our tectonostratigraphic terrane scheme (fig. 4). Gneissoid granite and tonalite outcrops north of the San Jose Fault 

at the eastern edge of the San Jose Hills (Shelton, 1955, p. 

54) yielded a questionable K/Ar (biotite) age of 96.4 Ma (F. 

K. Miller, written communication, 1995). The 0.70824 initial 

strontium isotope ratio (D. M. Morton, oral communication, 

1998) fits well with gneissoid rocks north of the frontal faults 

Figure 4.—Areal limits in study area of basement rock domains summarized in fi gure 3 and discussed in the text. See fi gure 1 for areal limit of 

domain 2 and partial extent of domain 1. Fault symbols as in fi gure 1; see fi gures 1 and 2 for fault labels.

Basement Rock Units

2

3

4

5

6

?

? ?

?

?

?

?

?

? ?

?

? ?

? ? ?

?

?

?

?

?

?

?

118°15' 118°00' 117°45'

34°00'

34°15'

0 5 10 15

KILOMETERS

Preferred boundary

?

?

?

? Alternate boundary

?
8 Mountain Meadows Dacite: Oligocene Intrusive Complex

of the San Gabriel Mountains and is a misfit with western 

Peninsular Ranges granitoid rocks (Taylor and Silver, 1978, 

fig. 2). This singular anomaly, in conjunction with distinctive 

associated magnetic susceptibility and aeromagnetic response 

characteristics (D. M. Morton, oral communication, 2000), 

suggests that the boundary between basement rocks of the San 

Gabriel Mountains and those of the western extension of the 

Peninsular Ranges may be hidden beneath the unconformity 

at the base of the middle Miocene Glendora Volcanics just 

south of the San Jose Fault (Nourse and others, 1998, p. 10-8). 

However, any boundary structure south of the questionably 

dated gneissoid rock must predate the eruption of the volcanics at about 16 Ma and almost certainly the deposition of 

about 17-million-year-old uppermost Sespe fluvial deposits.

The second possible exception to our basement rock subdivision is an occurrence of metagabbro or metadiorite in drill 

holes beneath the Las Cienegas oil field on the hanging wall 

of the north-dipping Las Cienegas Fault (see appendix 1, drill 

hole no. 19, the ìgneissic metadioriteî of Yerkes and others, 

1965, p. 424). Thorough studies of this rock unit (Yeats, 

1973, p. 134-135; Sorensen, 1985, p. 999; Sorensen, 1988, 

p. 1005-1012) indicate that it is ìalbite and oligoclase-epidote 

amphiboliteî derived from an ìarc-likeî metabasite, possibly 

related chemically to Jurassic Santa Cruz Island Schist 

and Jurassic Santa Monica Formation metapelitic rocks and 

metatuffs. This amphibolite is undated and appears poorly 

suited for determination of age or initial strontium ratio. Absent 

more definitive data, we surmise that the rock is most closely 

related to our extension of the Peninsular Ranges province.

The oldest rock unit common to all parts of the San 

Gabriel Mountains, the northern Los Angeles Basin, and the 

northwestern extension of the Peninsular Ranges province is 

Mountain Meadows Dacite and its relatives. It crosses most 

of the fundamental tectonic boundaries of our study area, 

including the Vincent Thrust, the Sierra Madre zone of faults, 

the north branch of the San Gabriel Fault, and the Vasquez 

Creek Fault.

Mountain Meadows Dacite

Innumerable dikes, sills, and irregular discordant bodies 

of biotite dacite and serially related latite are widespread in 

the eastern San Gabriel Mountains, where they intrude varied 

Cretaceous and older metamorphic and plutonic units, both 

north and south of the San Gabriel Fault (north branch) and 

east and west of the Evey CanyonñSan Antonio Fault Zone 

(fig. 2). South of the Sierra Madre-Cucamonga Fault Zone, 

valley alluvium or Cenozoic strata of the Los Angeles Basin 

succession largely cover both basement rocks and dacite. 

However, biotite dacite identical to the mountain occurrences 

crops out locally at the northeastern edge of the San Jose 

Hills, the type area for the Mountain Meadows Dacite (Woodford and others, 1946), and occurs in surrounding wells (Shelton, 1955) (fig. 5). It intrudes gneissoid granodiorite and 

granite, and subsurface protomylonitic rocks to the west. 

Andesitic flows and fragmental volcanic rocks of the Middle 

Miocene Glendora Volcanics (Shelton, 1955) unconformably 

overlie both the Mountain Meadows Dacite of the type area 

and the intruded granitoid rocks. All of these are overlain in 

turn by a thick to thin sequence of marine strata of the Los 

Angeles Basin succession that spans the age range from 15.6 

Ma (or more) to less than 8.7 Ma (West and Redin, 1991; 

Wright, 1991, fig. 5).

Mountain Meadows Dacite of the type area is a light 

gray to tan porphyry. Phenocrysts of euhedral biotite and 

rounded, partly resorbed quartz bipyramids are conspicuous 

and distinctive. Roughly equal volumes of normally zoned 

oligoclase and potassium feldspar prisms are subordinate. A 

finely crystalline groundmass of feldspar and quartz constitutes the bulk of the rock. Almost identical (but unweathered) 

rock was cored at a depth of about 1,220 m (4,000 ft) in drill 

hole I located about 3 km southwest of the outcrops (table 2, 

fig. 5). Biotite from that core gave a K/Ar age of 27.6 ± 0.4 

Ma (V. A. Frizzell, oral communication, 1982).

Substantial variations from typical dacite occur among 

innumerable small to large intrusive bodies in the San Gabriel 

Mountains. At one extreme are light gray to tan nearly aphanitic rocks, some flow-layered; another extreme is nonfoliated 

medium- to coarse-grained biotite granodiorite in large discordant stocks in the northeasternmost San Gabriel Mountains 

(Hs ̧ and others, 1963; Miller and Morton, 1977, p. 644-645). 

Marginal parts of the stocks consist in places of porphyry 

identical to Mountain Meadows Dacite. In the westernmost 

and northeastern parts of the dacite area, quartz-bearing rock 

grades into texturally very similar latite containing prismatic 

hornblende phenocrysts, with or without euhedral biotite but 

generally lacking quartz phenocrysts. Many small intrusive 

bodies in the mountains consist of fractured and tectonized 

rock partly altered hydrothermally. Partly or wholly chloritized biotite plus carbonate minerals, sericite, or laumontite 

(or leonhardite) commonly coat fractures or locally replace 

plagioclase.

Trains of microscopic fluid inclusions hosting minute 

mobile vapor bubbles, within partly resorbed quartz phenocrysts where present, are a distinctive characteristic of the 

most common and abundant dacite. Such fluid inclusions are 

extremely rare in other rocks of this region and constitute 

a helpful test of kinship in small clasts, some individual 

well cuttings, or even clastic grains in younger granule sandstones. Fluid inclusions were not found in quartz of the 

coarser and more evenly grained Telegraph Peak granodiorite. 

Most hornblende-bearing latite lacks quartz phenocrysts and 

therefore lacks such inclusions. Chemical data are unfortunately limited for the dacite (Shelton, 1955; Nourse and 

others, 1998) and are lacking for the latite.

An unequivocal Oligocene age of emplacement of Mountain Meadows Dacite and its genetic relatives rests on more 

than nine different radiometric determinations from seven 

widely separated locations (Hs ̧ and others, 1963; Morton, 

1973; Miller and Morton, 1977; V. A. Frizzell, oral communication, 1982; May and Walker, 1989, figs. 2, 8; Nourse 

and others, 1998; R. J. Fleck, oral communication, 2001). 

The crystallization age range that best fits all available data 

is from 27.6 Ma for dacite to 26.3 Ma for Telegraph Peak 

granodiorite. Dacite is clearly younger than subsurface Upper 
9

Cretaceous, Paleocene, and Eocene sedimentary rocks south 

of latitude 34° N. (McCulloh and others, 2000), but it is older 

than most or all of the Sespe Formation of this area. The 

older sedimentary units are not in direct contact with in-place 

dacite, either in outcrops or in any drill holes. The limits of 

the basin area known or interpreted to be underlain by strata 

older than the dacite are less than the limits of the Sespe 

Formation that are shown on figure 5 (compare with Wright, 

1991, fig. 4).

Published data only hint at the areal extent of the dacite 

intrusive bodies in the San Gabriel Mountains and in the 

valley region where the Los Angeles Basin succession conceals them (Bellemin, 1940; Shelton, 1955; Morton, 1973; 

Miller and Morton, 1977; Ehlig, 1981; Evans, 1982; Nourse 

and others, 1998, fig. 1). No published map shows their areal 

limits or full extent. Through selective field mapping, extensive field checks, examination of core samples, and careful 

literature review, we have roughly mapped the westerly limits 

of the principal regions where dacite or consanguineous latite 

occurs (fig. 5). Our mapped boundaries are only approximate 

limits because the abundance of individual intrusive bodies 

is highly uneven, though with a general decrease toward the 

west. In the valley region, where rare outcrops are augmented 

by a few encounters of dacite in drill holes, we define the edge 

of a subcircular dacite region (see Shelton, 1955, plate 1 and 

table 3). Here we assume that a singular and coherent large 

dacite body is defined by the observed occurrences, although 

a field of many smaller intrusions is equally possible. 

Mountain Meadows Dacite and its genetic relatives 

record an apparently singular igneous episode that predates 

Figure 5.—Areas in which scattered large to small dikes, sills, or irregular intrusive bodies of Mountain Meadows Dacite and consanguinous 

granodiorite and latite have been observed or mapped. Also shown are areas where subsurface fl uvial deposits of the Sespe Formation (and of 

locally underlying, less extensive marine Paleogene or Upper Cretaceous clastic strata) occur in scattered exploration drill holes (identifi ed in 

appendix 1). Faults symbols as in fi gure 1; see fi gure 2 for fault labels.

Mountain Meadows Dacite

?

? ?

? ?

?

? ?

?

? ? ?

?

?

? ?

?

x ?

limit of control x

?

AA

M

CC

W X

V EE L

Y

Z

BB

U

S

T

P

N

R

+

Subsurface extent of nonmarine 

Sespe Fm.

Areas of Mountain Meadows 

Dacite and related dacite 

x isolated occurrence

Outcrop

Subsurface

+ type locality

Oligocene Telegraph Peak granodiorite

?

?

? ?

?

? ?

? ? ?

?

?

?

?

?

?

?

118°15' 118°00' 117°45'

34°00'

34°15'

0 5 10 15

KILOMETERS

?

I

II

Interpreted edge of buried dacite

 intrusion or intrusive bodies from

 drill hole control

Mapped approximate boundary of

 outcrops with discontinuous dacite

 intrusive bodies

Buried edge of Sespe Formation

Y

V

EXPLANATION

I Key Dacite drill holes 

 (see table 2 and text)

Drill holes described

in appendix 1

Y
10 Mountain Meadows Dacite: Oligocene Intrusive Complex Table 2. Petroleum exploration drills holes discussed in the text and shown on fi gures 5, 6, or 10.
11

capture of the Monterey and Arguello Plates (Atwater and 

Stock, 1998, fig. 10), onset of transrotation of the western 

Transverse Ranges (Hornafius and others, 1986; Luyendyk, 

1990; Nicholson and others, 1994), and opening of Los Angeles Basin. This study investigates the spatial distribution 

of the intrusives and their relationships to both younger 

and older formations and to major associated structures to 

improve understanding of the structural evolution of of the 

region. 

Dacite Clasts in Superjacent Strata

Mountain Meadows Dacite of the San Jose Hills area 

was erosionally leveled after about 27 Ma, along with the 

Mesozoic biotite granodiorite and other granitoid rocks into 

which it was intruded. It is now effectively part of the 

deformed basement rock surface portrayed in figure 2.

Sedimentary Rocks Older Than Dacite

Predacite sedimentary rocks cover the basement rocks in 

a large subsurface area south of the Whittier Fault and beneath 

many square kilometers northeast of the fault (McCulloh and 

others, 2000). These subsurface strata include Upper Cretaceous (locally), Paleocene, and lower Eocene shallow marine 

to nonmarine clastic beds. Undated coarse fluvial strata of 

the Sespe Formation overlie the Paleogene units and underlie 

an even larger area (fig. 5). Marine interbeds of the Sespe 

gravels are unknown in this area; the northernmost known 

occurrences of such marine facies (Vaqueros Formation, about 

18 Ma or older) are outcrops in the northern Santa Ana 

Mountains, at least 9 km south of the area. The age range 

of the fluvial Sespe deposits is consequently known only 

through regional correlations and from the ages of underlying 

and overlying fossiliferous strata. In Santa Ana Mountains 

outcrops 14 to 22 km south of the area, terrestrial vertebrate 

fossils associated with Vaqueros marine fossils show that 

the youngest part of that section is about 18 Ma (Lucas 

and others, 1997, p. 3), substantially younger than the dacite 

emplacement age. The oldest part of the Santa Ana Mountains section may be late Eocene, considerably older than 27.6 

Ma.

Dacite Clasts in Pre-Topanga Group Deposits

Dacite clasts, angular to subrounded and from coarse 

sand to cobble size, occur in a core of nonfossiliferous sandy 

conglomerate of the ìSespe (?)î Formation (Daviess and 

Woodford, 1949) from 1,706 to 1,709 m (5,598-5,608 ft) from 

drill hole II (table 2, fig. 5). Clasts of biotite dacite and 

felsite (plus separate individual quartz grains with characteristic fluid inclusions) are Mountain Meadows Dacite products. 

Associated clasts include metasandstone, biotite-muscovite 

meta-arkose, massive meta-andesite containing altered amphibole(?) replaced by chlorite, carbonate, and epidote, plus 

altered plagioclase phenocrysts, granitoid detritus, and a singular subangular clast of metapelite that strongly resembles 

some specimens of Santa Monica Slate. No clasts of volcanic 

rocks from the Middle Miocene Topanga volcanics sequences 

(Shelton, 1955; Eaton, 1958) occur. We conclude that the 

dacite clasts are in young fluvial Sespe Formation. Subaerial 

exposure, erosion, and redeposition of nearby dacite prior 

to Middle Miocene volcanism is demonstrated. The nearest 

known dacite (subsurface) is about 13 km N. 55° E. from the 

well, along a path where postdepositional structural complications are minimal (fig. 5). This suggests a possible early 

Miocene southwestern paleoslope and drainage system. 

One outcrop of Sespe gravels just south of the study 

area contains clasts of Mountain Meadows Dacite. Subrounded grey dacite clasts as much as 13 cm long occur in 

north-dipping uppermost Sespe beds located just north of the 

southeastern part of the Whittier Fault Zone (33.8323° N., 

117.6754° W.). The most likely source for the clasts, based 

partly on associated clast lithologies, is about 22 to 25 km 

to the north-northwest, possibly the same paleotopographic 

high that fed clasts to the Whittier Hills well. This suggests 

a possible southeast-directed paleoslope and drainage system 

from an elevated region now beneath the northeastern Puente 

Hills.

Thin sections of coarse sandstones from conventional 

core recovered from 2,917 to 2,923 m (9,571-9,591 ft) in drill 

hole V (table 2, fig. 5) contain some quartz grains derived 

from Mountain Meadows Dacite, as indicated by their distinctive fluid inclusions with mobile gas bubbles. This is the 

only occurrence of dacite detritus in Sespe strata that we have 

found south of the Whittier Fault Zone. Accepting about 9 km 

of right slip on the Whittier Fault Zone (McCulloh and others, 

2000), the occurrence is consistent with a south-directed 

paleoslope and drainage system from the area beneath the San 

Jose Hills.

These occurrences of dacite clasts in Sespe strata, and 

their inferred derivation from exposed and eroding areas of 

dacite south of the Raymond, Sierra Madre, and Cucamonga 

Faults, are evidence that Paleogene and Sespe cover did not 

extend as far north as those faults in lower Miocene time. 

Rather, these observations add credence to the interpretation 

that the present northern edge of Sespe strata (eroded and 

subsurface) is close to the original edge of the depositional 

basin (McCulloh and others, 2000). This evidence invalidates 

previous interpretations of 60 to 90 km of post-Sespe sinistral 

slip on the whole Malibu Coast-Cucamonga Fault System 

(Yeats, 1968, p. 55; Yerkes and Campbell, 1971; Sage, 1973, 

1975; Truex, 1976; Campbell and Yerkes, 1976). The three 

dacite clast occurrences in Sespe strata also indicate that the 

paleogeography during deposition of the uppermost part of 

the Sespe Formation in southern California was more complicated than that suggested by Howard (2000).

Nature and Distribution of Topanga Group

Middle Miocene Glendora Volcanics overlie an erosional 

unconformity that transects dacite and all pre-Cenozoic founDacite Clasts in Superjacent Strata
12 Mountain Meadows Dacite: Oligocene Intrusive Complex

dation units in the San Jose Hills (figs. 1, 6). The Glendora 

Volcanics in this area (Shelton, 1955) are the basal unit of 

an assemblage called the Topanga Group of Wright (1991, 

fig. 5). Wrightís ìTopanga Groupî is not identical to the 

ìTopanga Groupî of Yerkes and Campbell (1979, p. E13) of 

the central Santa Monica Mountains. Regionally, Wrightís 

Topanga Group comprises basal shallow marine to locally 

nonmarine coarse clastic sedimentary strata overlain in some 

places by extrusive basalt, basaltic andesite, and rare silicic 

flows and fragmental ejecta (mainly marine but locally subaerial) (Shelton, 1955; Eaton, 1958). These are overlain in 

turn in the San Jose Hills by generally fine-grained open 

marine units of middle Miocene age that grade or interfinger 

laterally toward the east and north with coarse and massive 

conglomerates of eastern and central San Gabriel Mountains 

derivation. Glendora Volcanics have been dated radiometrically at 16.3 ± 1 and 15.9 ± 0.3 Ma (Nourse and others, 1998, 

p. 10-2). Overlying marine strata are ìRelizianî (17.4ñ16.5 

Ma) grading upward into ìLuisianî (16.5-14 Ma) (Blake, 

1991, fig. 6; West and Redin, 1991). We confirm published 

benthic foraminiferal stage assignments by using fossil calcareous nannoplankton (nannos) from drill hole III (fig. 6, table 

2). Nannos of CN zone 3 (18.3 to 15.6 Ma) occur in core from 

536 to 540 m (1,760-1,770 ft), identified by the co-occurrence 

Figure 6.—Distribution of rocks of the Topanga Group (Glendora Volcanics and subsurface Topanga Volcanics overlain by or interbedded with 

Luisian and locally Relizian marine strata ). Also shown is the approximate location of the uppermost Topanga shoreline as evidenced by the 

transition from marine to nonmarine deposits. Topanga deposits north of the transition are all nonmarine in the area between the Verdugo and 

Hollywood Faults. Drill hole and outcrop sites are identifi ed where we have found (1) clasts of Mountain Meadows Dacite (latite) in Topanga 

conglomerates and (2) calcareous nannofossils that have been studied (tables 2, 3, and appendix 2). Also shown are tielines between dacite clast 

sites and known (solid) or inferred (dashed) clast sources. Structure contours (unlabeled) as in fi gure 2. Fault symbols as in fi gure 1; see fi gure 2 

for fault labels. EP-AH is the Elysian Park-Monterey Park-Alhambra structural high discussed in text.

? ? ?

?

2

4

0

?

?

?

?

?

? ?

?

III

IV

I

II

limit of control

? ?

V

Localities c-l

Table 3

?

?

?

? ?

?

? ?

? ? ?

?

?

?

?

?

?

?

118°15' 118°00' 117°45'

34°00'

34°15'

0 5 10 15

KILOMETERS

? ?

?

?

? ? ?

?

0

With volcanics

Conjectural extension

now eroded

Eroded, covered by

Mohnian and possibly

younger rocks

Without volcanics

TOPANGA VOLCANIC ROCKS

TOPANGA SEDIMENTARY ROCKS

β α

Drill hole with Topanga volcanics mostly on

basement

Outcrop fossil localities (see text and

appendix 2)

Drill hole with post-Topanga on basement

Drill hole with upper Topanga on basement

Key drill holes (see table 2, or appendix

1 and text)

α

a

b

EXPLANATION

DD

F E K

A

C

H

X

W

V

AA

R

T

N U S

M

D

J

I B

O

Q

G

Topanga outcrops with dacite clasts (see

table 3 and text for localities a and b)

Tie lines from identified source(s) to dacite

clasts

?

Interpreted edge of Topanga volcanics

Uppermost Topanga shoreline

Truncated base of Luisian sequence

EP-AH

Lines broken where extrapolated along

trend beyond data control:
13

of Helicosphaera ampliaperta, Calcidiscus premacintyrei and 

Reticulofenestra lockeri. This flora is succeeded by another, 

CN zone 3-4 (>13.6 Ma), that includes Sphenolithus heteromorphus in core from 326-336 m (1,071-1,101 ft). The location of this drill hole is less than 4 km from the closest points 

of the latest Luisian (end Topanga) shoreline as mapped by 

Woodford and others (1946) and reproduced on figure 6. 

Details about the nannofloral compositions of each of the 

horizons we have studied are provided in a complete floral 

checklist (appendix 2). We choose to follow the locally well 

known age equivalence of the benthic foraminiferal zonation 

shown by Blake (1991, fig. 6) even though the time scale 

that was used (Berggren and others, 1985) has been upgraded 

(Berggren and others, 1995).

What we know and surmise about the distribution and 

relationships of units of the Topanga Group are summarized 

on figure 6. A mostly buried but essentially continuous blanket of Topanga Group volcanic rocks extends from the Whittier Fault northward and northeastward to outcrops in the 

northeastern San Jose Hills and foothills of the San Gabriel 

Mountains. Numerous dikes and irregular intrusive bodies of 

compositionally similar basaltic and andesitic volcanic rocks 

are widespread in the San Gabriel Mountains (Nourse and 

others, 1998). Comparable rocks occur also south of the 

Whittier Fault as subsurface flows that appear to be sandwiched between upper Topanga (ìLuisianî) marine strata 

(Gaede, 1957; Eaton, 1958). The extrusive volcanic blanket 

rests unconformably in different places on Relizian sedimentary rocks (Whittier Hills and just north of the Whittier Fault; 

Daviess and Woodford, 1949), on Sespe nonmarine gravels 

(eastern Puente Hills; Durham and Yerkes, 1964), on schistose metavolcanic basement rocks along part of the Whittier 

Fault hanging wall (Yerkes, 1972), on various other basement 

rocks beneath the San Jose Hills (Shelton, 1955), and on 

Mountain Meadows Dacite in and around that unitís type area. 

No Paleogene or Sespe strata extend beneath the blanket of 

Topanga Group volcanics to the identified southern boundary 

faults of the San Gabriel Mountains. Furthermore, the occurrence of Middle Miocene (Relizian) volcanics and marine 

sediments in the San Jose Hills and also in areas just to 

the north, across those boundary faults, prohibits any palinspastic reconstruction involving very large post-Lower Miocene strike-slip displacements on those boundary faults, as 

postulated by Sage (1973, 1975), Truex (1976), and Campbell 

and Yerkes (1976).

The Topanga Group and its Glendora Volcanics member 

are thickest (more than 1,220 m or 4,000 ft and 914 m or 

3,000 ft, respectively) a few kilometers southwest of their 

outcrops (Wright, 1991, fig. 5). Volcanics thin rapidly toward 

the west to an inferred edge beneath unconformable Luisian 

marine strata under the San Gabriel Valley. They thin rapidly 

also to the east because of postvolcanic erosion, and they 

thin to the south through a combination of original thinning 

and (probably) posteruption erosion. No volcanic rocks are 

known in the subsurface of the western San Gabriel Valley, or 

north of the Raymond-Hollywood Fault Zone east of the east 

end of the Santa Monica Mountains (Lamar, 1970). Volcanic 

outcrops north of the western Verdugo Mountains and in 

the easternmost Santa Monica Mountains are of the same 

approximate age (Eaton, 1958) but of separate eruptive fields.

Where the Topanga Group and Glendora Volcanics are 

thickest, there is no evidence of a hiatus or depositional 

change between the Relizian and Luisian parts of the postvolcanic section. Where the volcanics are absent farther west, 

no Relizian strata are known east of the Santa Monica Mountains. Instead, fine to coarse clastic marine strata yield 

benthic foraminifers that include Luisian guide species, the 

nanno Sphenolithus heteromorphus indicating CN zones 3-4 

(18.3-13.6 Ma), and fish remains. Luisian beds rest directly 

on basement rocks without intervening volcanics except in 

drill hole IV (fig. 6, table 2), which bottomed in altered volcanics beneath nonfossiliferous conglomeratic beds overlain 

by Luisian marine strata. The evidence suggests (but does not 

prove) that the Luisian sequence represents a regional marine 

transgressive episode following much more localized deposition of Relizian marine beds restricted mainly to the area of 

Glendora Volcanics volcanism and extending south into the 

eastern part of the main Los Angeles Basin embayment south 

of the Whittier Fault.

The paleogeography and paleooceanography during 

deposition of the uppermost Los Angeles Basin Topanga 

Group (about 15-14 Ma) are known only incompletely and 

locally (Woodford and others, 1946, fig. 11; Neuerburg, 

1953; Nagle and Parker, 1971, p. 270; Dibblee, 1989, p. 

218, 220). Major obstacles to comprehensive understanding 

include postdepositional erosion of extensive proximal areas, 

widespread structural disruption and distortion, deep burial of 

extensive areas of distal deposits, thick sections of extremely 

coarse proximal deposits, and very sparse fossil control in 

most outcrops. To extend and upgrade understanding of the 

basin shape, we took the following steps: (1) We mapped 

a line connecting the most proximal localities where marine 

fossils occur in outcrops and subsurface locations to approximate the location of the marine-nonmarine transition. (2) Key 

benthic foraminiferal assemblages were interpreted paleobathymetrically. (3) Nannos were studied quantitatively to 

gain understanding of the distribution of open marine and 

upwelling conditions in the late Topanga seaway. (4) Clasts 

derived from Mountain Meadows dacite-latite intrusives were 

identified in outcrops and well cores of Topanga Group conglomerates and tied to dacite source locations. Each step 

yielded positive results.

The early Middle Miocene marine incursion culminated 

with deposition of latest Luisian conglomerates, sandstones, 

and open marine siltstones of the northernmost Los Angeles 

Basin that extended as far northeast as the San Gabriel Mountain front east of the junction of the Raymond and Sierra 

Madre Faults. The Luisian sea flooded the southern margin 

of what is now the mountain range even while more elevated 

parts of the ancestral range were shedding coarse detritus 

into the basin margin. Coarse Topanga conglomerates (with 

some breccias) also occur south of the Verdugo Fault Zone 

and interfinger with marine bathyal Luisian sandy siltstones 

north of the Raymond Fault (Lamar, 1970, his ìYork Ave. 

faultî). Coarse conglomerates with locally derived breccias 

occur above Topanga volcanics in the easternmost Santa 

Dacite Clasts in Superjacent Strata
14 Mountain Meadows Dacite: Oligocene Intrusive Complex

Monica Mountains and were clearly derived from the north 

or northeast (Dibblee, 1989), but apparently in part also from 

southeast of the Benedict Canyon Fault (fig. 6).

Dacite Clasts in Topanga Group Conglomerates

Clasts of Mountain Meadows Dacite and latite occur in 

strata of the Topanga Group in five widely separated places of 

our map area. Dacite pebbles were also reported by Shelton 

(1955, p. 54) at two unspecified localities in basal members of 

the Glendora Volcanics that we assume are of local derivation 

in the dacite type area.

Outcrops of Upper Topanga ìBuzzard Peakî pebble to 

boulder conglomerate (Woodford and others, 1946) in the 

core of an anticline north of the western part of the San Jose 

Fault (locality a, fig. 6 and table 3) contain a wide range 

of basement and volcanic rock clasts, among which very 

dark to black flinty mylonite and light-colored subangular 

to subrounded clasts of dacite are minor but conspicuous 

Table 3. Outcrop localities where clasts of Mountain Meadows Dacite or related latite and felsite occur in conglomerates of the

Topanga and Sespe Formations (fi gs. 6, 10).
15

components. Nearby drillhole I (fig. 6) penetrated 645 m 

(2,115 ft) of Luisian and Relizian conglomerate plus 538 

m (1,765 ft) of Glendora Volcanics before reaching in-place 

Mountain Meadows Dacite at 1,183 m (3,880 ft) (Shelton, 

1955, table 3; West and Redin, 1991). The conglomerate 

interfingers in the subsurface to the southwest and north with 

fine-grained Luisian and Relizian marine sequences of the 

Los Angeles Basin succession. Subrounded clasts of Mountain Meadows-type dacite occur in conventional core samples 

from 617 to 618 m (2,025-2,028 ft) together with angular 

clasts of Glendora Volcanics and mylonitic rocks. These are 

from the inferred Relizian part of the section, presumably 

older than 16 Ma. All the data are consistent with clast 

derivation from the ìblack beltî mylonite (Alf, 1948), presently 22 km or more to the northeast (fig. 6) along the front 

of the southeastern San Gabriel Mountains. The clasts tie 

together the Relizian-Luisian depositional basin edge and the 

early-elevated ancestral San Gabriel Mountains (area 5, figs 

3, 4).

Outcrops of Upper Topanga conglomerate near the range 

front north of the San Gabriel Valley (locality b, fig. 6 

and table 3) are interbedded with Glendora Volcanics (Shelton, 1955, plate 1; Morton, 1973; Crook and others, 1987). 

Steeply dipping very coarse but well-stratified polymictic 

boulder to cobble sandy conglomerate and sandstone contain 

a subrounded boulder of calcitized dacite 30 cm in diameter. 

Associated with the rare dacite are angular to subrounded 

clasts of a wide range of the basement rock types from the 

mountains to the north plus infrequent clasts of basaltic or 

andesitic volcanic rocks like those of the Glendora Volcanics. 

Clast sizes and shapes, poor sorting, proximity to the most 

northerly break of the Sierra Madre Fault Zone at the mountain front, and clast compositional heterogeneity all suggest 

proximity to sources in a rapidly eroding terrain. Underlying 

beds (including both volcanic and clastic units) continue 

downdip to the south and dictate that the eroded sources 

were to the north. ìUpper middle Miocene foraminiferaî 

(presumably Luisian) are reported from the younger part of 

the sequence (Shelton, 1955, p. 67) and show that the shoreline lay still farther north at the time of deposition. The 

occurrence ties together the northern edge of the Los Angeles 

Basin at or before about 14 Ma and an uplifted and eroding 

part of the ancestral central San Gabriel Mountains (area 4, 

figs. 3, 4) within the area of mapped dacite intrusives.

Drill hole IV reached a depth of 2,394 m (7,854 ft) 

south of the Duarte branch of the Sierra Madre Fault Zone 

(fig. 6, table 2). It bottomed in altered basalt (Glendora Volcanics) beneath a thick sequence of generally poorly sorted 

sandstones with interbedded conglomerates and minor marine 

siltstones. Petrographic study of core samples from a depth of 

1,905-1,910 m (6,250-6,267 ft) identified clasts of Mountain 

Meadows-type dacite among others of both granitoid and 

volcanic derivation. The 13.7-Ma ìzonalî boundary given 

by the drill hole operator at about 1,689 m (5,540 ft) is 

in a steeply dipping (53 ̊ to 68 ̊) sequence of interbedded 

fine-grained sandstone and organic-rich siltstone (table 2). 

This interpretation is supported by the rarity of the nanno 

Reticulofenestra pseudoumbilica (>7 μm), which suggests a 

position close to the CN 4-5a zonal boundary based on the 

bioseries summary by Young (1998). Nannos and abundant 

fossil fish debris suggest a probable open marine environment 

with active upwelling.

When plotted on a graph of the Fisher alpha index 

(Murray, 1973), nanno diversity (1.1 to 1.6) in this drill hole 

(fig. 7A) compares well with other Los Angeles Basin nanno 

floras and with modern samples from the southern California 

Borderland (Winter, 1985). The average diversity of our Los 

Angeles Basin samples is greater than occurs today in both 

Belize back reef environments (Kling, 1975) and the south 

Atlantic offshore Namibia (Giraudeau and others, 1993). The 

abundance of Coccolithus pelagicus in our floras is probably 

an indicator of cold water upwelling and enhanced productivity. Such a relationship is supported in a ternary plot of the 

Los Angeles Basin species (fig. 7B) using Emiliania huxleyi

as a modern substitute for the dominant species in the Los 

Angeles Basin floras, Dictyococcites productus. The proportion of C. pelagicus in Los Angeles Basin samples is comparable to the proportion in samples from upwelling waters of 

Namibia (Giraudeau and others, 1993) but not in samples 

from southern California Borderland waters (Winter, 1985; 

Ziveri and others, 1995). Even the two Los Angeles Basin 

samples dominated by C. pelagicus are analogous to a modern 

sediment population from cold waters of the Greenland Sea 

(Samtleben and others, 1995), but the dominance may also be 

an artifact of dissolution enrichment (Berger, 1973; Roth and 

Coulbourn, 1982). The alternative view that the ecologic limitation of C. pelagicus in the middle Miocene (Bukry, 1981) 

was much broader than the 10 ̊C optimum limit (McIntyre 

and others, 1970) needs more study. In the upwelling 

off southwest Africa (Giraudeau, 1992) and western Iberia 

(Cachao and Moita, 2000), C. pelagicus is thriving where sea 

surface temperatures average higher than 16 ̊C. The samples 

from drill hole IV accumulated in sediment deposited probably at least 4-5 km from the paleoshoreline, on the basis of 

the distance from the mapped shoreline of similar floras in 

drill hole III (table 2, fig. 6).

Dacite clasts of drill hole IV occur in beds that can be no 

younger than 15.6 Ma. The basalt cored in this drill hole is 

the most northwesterly known Los Angeles Basin occurrence 

of Glendora Volcanics. The clasts tie the site to one part 

of the uplifted ancestral San Gabriel Mountains, while the 

basalt and open marine Topanga tie it to the Los Angeles 

Basin. The tie raises fundamental questions about amounts, 

directions, and timing of slip on the Sierra Madre Fault and 

other related faults such as the Vasquez Creek and San Gabriel 

(main strand).

Pebbles to cobbles of dacite and consanguineous latite 

occur at multiple sites in thick and very coarse Topanga 

conglomerates south of the Verdugo Fault Zone (Lamar, 1970) 

and north of the Raymond-Hollywood Fault Zone. The localities occur in a belt more than 2.4 km from east to west 

(sites c through l, table 3 and fig. 6). Accepting the validity 

of published structure sections (Lamar, 1970, plate 2), clastbearing outcrops range from very near the base to about 1,400 

m (4,600 ft) above the base of the Topanga sequence. The 

overwhelming majority (probably more than 99 percent) of 

Dacite Clasts in Superjacent Strata
16 Mountain Meadows Dacite: Oligocene Intrusive Complex

the associated clasts are granitoid (granite to granodiorite), 

although rare andesitic clasts occur, together with even rarer 

quartzite, gneiss, and diorite. The tan to light green dacitelatite clasts are porphyries mostly rich in biotite but generally 

without quartz phenocrysts. Many contain dark stubby euhedral hornblende prisms in addition to biotite euhedrae. Most 

clasts are well rounded to subrounded. No such clasts were 

found in sparse exposures west of the localities shown on 

figure 6.

Marine fish scales recovered from fine-grained interbeds 

in a conglomerate, breccia, and sandstone sequence from site 

α (fig. 6) were identified as upper Luisian (Lamar, 1970, 

fig. 9). A well-preserved upper Luisian benthic foraminiferal 

fauna from this locality suggests an upper bathyal depositional environment near a shelf edge (R. J. Enrico, written 

communication, 2000). Associated nanno floras are CN 3-4 

and indicate an age of 18.3-13.6 Ma.

A reconnaissance search for dacitic intrusive bodies in 

the southeastern San Rafael Hills (directly north of the 

Topanga conglomerate clasts) yielded none. Farther north 

and northwest, in an area about 2 km east and 2 km west 

from 118° 12.5í W. (fig. 5), widely spaced dikes of dacite 

porphyry contain conspicuous quartz phenocrysts and either 

lack or have only very subordinate hornblende. They are 

therefore unlike most of the dacite-latite clasts in the Topanga 

conglomerates south of the Verdugo Fault. However, still farther north, dikes of hornblende-biotite latite or dacite (Crook 

and others, 1987, plate 2.1) match closely the conglomerate 

clast lithologies and occur in a limited area southwest of 

the Vasquez Creek and Mount Lukens Faults but north of 

the eastward extension of the Santa Susana Fault (Figs. 2, 

5). Another important tie is thereby established between 

the eroding ancestral southwestern part of the central San 

Gabriel Mountains and a peripheral part of the subsiding 

northwestern Los Angeles Basin. This connection predates 14 

Ma (the younger bound of the Luisian ìstageî) and questionably postdates ca. 16.5 Ma. Paleogeographic and paleostructural implications of this finding, and the other occurrences 

of dacite clasts in superjacent strata, are outlined in the next 

major section.

Dacite Clasts in Post-Topanga Group Strata

A major unconformity or regressive discontinuity marks 

the top of the Topanga Group in most parts of the Los Angeles 

Basin, including much of the study area. Basal strata of 

the Puente Formation (or younger units) that rest above the 

unconformity generally are part of the Los Angeles Basin 

succession that contains benthic foraminifers of age 14.0 to 

13.5 Ma. This horizon is conspicuous because it follows 

a regressive discontinuity and precedes a regional marine 

transgression during which a widespread blanket of organicrich and generally fine-grained phosphatic and diatomaceous 

siltstones and mudstones, with more or less interbedded sandstone, was deposited. Virtually basinwide (Wright, 1991, 

fig. 10), this transgressive pulse culminated near 8.7-7.0 Ma, 

probably the maximum Neogene ocean incursion for this part 

of California (Woodford and others, 1946, fig. 11). 

Deformation plus uplift and erosion of the Elysian ParkMonterey Park-Alhambra structural high (EP-AH, figs. 1, 6) 

Figure 7.—Comparisons of middle Miocene Los Angeles Basin 

calcareous nannofossil floras with modern equivalents from selected 

marine settings. A, Crossplot of number of species versus number 

of individuals, showing “Fisher alpha indices” (Murray, 1973, p. 8, 

fi g. 4) for the Los Angeles Basin fossil fl oras compared with fl oras 

living in modern ocean settings of the southern California Borderland, 

Namibia upwelling regions, and a Belize back reef lagoon. Floras from 

offshore Iberia and the Greenland Sea could not be plotted because 

the abundances of all taxa from these areas were not reported. 

B, Ternary diagram comparing the Los Angeles Basin fl oras with 

floras living in open marine settings of the southern California 

Borderland, Greenland Sea, and Namibia upwelling regions to illustrate 

possible environmental implications of the Los Angeles Basin fossil 

assemblages. Ratios for offshore Iberian floras could not be calculated 

because the abundance of E. huxleyi was not recorded. For locations 

of Los Angeles Basin sites, see fi gure 6 and tables.

2

2

2

Los Angeles Basin

fossil floras

 Locality 

 Locality 

 Drill hole M

 Drill hole O 

 Drill hole IV

 Drill hole III

 Drill hole J

 Drill hole Q

Dictyococcites productus (fossil)

 Emiliania huxleyi (Holocene)

100% 100%

Coccolithus pelagicus other species

100%

Holocene floras

 California sediment trap

 California waters

 Greenland Sea sediment

 Namibia waters

A

B

Number of individuals

100 1000 10,000

0

5

10

15

20

25

Number of species

α = 1

α = 3

α = 6

α = 2

α = 4

α = 5

Belize

lagoon

Namibia

upwelling

Los Angeles Basin

middle Miocene

Southern California

Borderland

100,000

α = 7

α

β
17

before 13 Ma is evidenced by the unconformity that separates 

middle and upper Miocene marine strata of the Puente Formation from older rocks. Uplift led to localized erosional 

removal of all pre-Mohnian strata along the high as shown by 

figure 6. The gap attests to an early origin for the ìElysian 

Park Thrust and Fold Beltî (Shaw and Shearer, 1999), which 

continues active (Oskin and others, 2000) and is therefore of 

considerable current interest to seismologists.

Clasts derived from ìMountain Meadows Daciteî were 

recognized in some outcrops of deformed Los Angeles Basin 

strata long ago (Bellemin, 1940; Woodford and others, 1945; 

Woodford and others, 1946; Olmstead, 1950) but most of 

those occurrences are in Puente Formation or younger strata. 

Post-Topanga uplift, erosion, and redeposition after both secondary and primary transport make interpretation of these 

younger dacite clasts difficult and beyond the scope of this 

study. The stratigraphic relationships described in preceding 

sections are shown in fi gure 8, where stratigraphic positions 

of dacite sources and clasts in Los Angeles Basin Sespe and 

Topanga Group strata of Wright (1991) are related in a time 

framework to our interpretations of fault movements.

Upper Topanga Strandline Reconstruction

Upper Topanga strata in the Los Angeles Basin south of 

the Monterey Park Fault Zone generally are a fine-grained, 

offshore open-marine facies. Before this study, the transition 

from marine to nonmarine facies had been mapped only at 

the northeastern basin margin (Woodford and others, 1946, 

fig. 11). As indicated above, Upper Topanga strata are 

extremely thick and very coarse near the north and northeast 

basin margins. This, together with post-Topanga structural 

disruption, has discouraged definition and mapping of the 

marine-terrestrial transition. Because a transition or shoreline facies is important in our palinspastic reconstruction, we 

define a late Luisian strandline by (1) adopting and verifying 

the ìTopanga shorelineî of Woodford and others (1946, fig. 

11), (2) extending that line to the west using interpretation of 

nanno occurrences and assemblages from key wells, and (3) 

generating an equivalent facies boundary in the area of Topanga outcrops south of the Verdugo Fault by focusing on the 

distribution limits of marine fossil localities and paleoecological interpretations of fossil assemblages (Lamar, 1970, 

fig. 9; this report). The resulting approximate shoreline is 

shown on figure 6.

The approximate distribution of Miocene strata younger 

than the Topanga Group (< 14 Ma) is shown for the northeasternmost basin edge by Woodford and others (1946, fig. 

11), along with their best estimates of the locations of 

younger Miocene shorelines. Other published approximations are extremely generalized and unsupported by specific 

data (Schwartz and Colburn, 1987, fig. 3; Wright, 1987, fig. 

4).

Palinspastic Reconstruction

Our interpretation of the paleogeographic and paleostructural evolution of the study area uses (1) fault separations 

or offsets and movement histories of key major faults; (2) 

areal distribution of Mountain Meadows Dacite (and related) 

intrusive bodies; (3) locations of dacite clasts in Los Angeles 

Basin superjacent strata older than 14 Ma; (4) available subsurface geological and geophysical data that constrain timing 

and nature of fault movements or offsets, including published 

seismological data; and (5) published interpretations of paleomagnetic declinations that could be influential.

Santa Monica-Hollywood-Raymond Fault Zone

The Santa Monica-Hollywood-Raymond zone of 

faults (fig. 2) is central to our study. An oblique reverse leftlateral fault zone that records significant cumulative left slip 

(Wright, 1991, p. 56-63), it separates upthrown Santa Monica 

Mountains from the structurally depressed northwest part of 

the Plio-Pleistocene Los Angeles Basin depocenter and is 

seismically active (Real, 1987; Jones and others, 1990; Dolan 

and others, 1997, 2000). The trace of the part of the fault 

shown on our maps is based importantly on surface geologic 

Figure 8.—Graphic summary of the timing of Mountain Meadows 

Dacite intrusion, northeastern Los Angeles Basin sedimentary 

deposition, and deposition of dacite-latite clasts relative to fault 

movements and tectonic rotation of the entire western Transverse 

Ranges Province.

Palinspastic Reconstruction
18 Mountain Meadows Dacite: Oligocene Intrusive Complex

mapping (Weber and others, 1980; Crook and others, 1987), 

tempered by our interpretation of gravity anomalies (McCulloh, 

1957; Chapman and Chase, 1979).

Four lines of surface and subsurface evidence, the latter 

three of which are graphically summarized in figure 9, independently suggest that cumulative left separation is about 13-14 

km: (1) The reconstructed axis of the >7 Ma Tarzana submarine 

fan (Sullwold, 1960) is offset about 16 km (Redin, 1991, fig. 

9). However, this amount includes 2.4 km of left slip on the 

Benedict Canyon Fault (Hoots, 1931; Weber and others, 1980) 

that merges with the Santa Monica zone west of our area; it also 

makes no allowance for possible differential shortening of either 

adjacent block because of movements on reverse faults or folds. 

(2) The outcrop belt of Jurassic spotted (cordierite-bearing) 

Santa Monica Slate and associated schists (Hoots, 1931, plate 

16 and p. 88-89) is offset more than 10.2 km from identical 

rocks cored in drill hole E (fig. 9, table 4) on the Elysian Park 

high southeast of the zone (Schoellhamer and Woodford, 1951). 

(3) The intrusive contact between Cretaceous granodiorite and 

schistose Santa Monica Formation north of the zone is offset 

less than 22.5 km from a presumably correlative subsurface 

contact southeast of the zone (Dibblee, 1982, p. 111; but also 

see Wright, 1991, p. 62). (4) The junction of the Verdugo 

Fault (Luisian and older) with the Raymond Fault is offset more 

than 13 km from approximate alignment with the eastern extension of the Sierra Madre Fault, with no allowance for possible 

postseparation shortening through reverse slip on the latter fault. 

A cumulative left-lateral strike separation of 13-14 km for the 

eastern Santa Monica-Raymond Fault Zone seems an acceptable 

compromise in view of all constraints. 

An interpretation that 60 to 90 km of left slip accumulated 

on the Malibu Coast-Santa Monica-Hollywood-Raymond-Sierra 

Madre zone since onset in middle Miocene time is well 

entrenched. This interpretation depends on hypothetical extensions of Sespe fluvial strata (Truex, 1976), a hypothetical late 

Paleocene paleoshoreline and associated strata (Sage, 1973, 

1975), and a postulated but unsupported lower Miocene Vaqueros Formation paleoshoreline (Campbell and Yerkes, 1976) 

from the Santa Ana Mountains north to the San Gabriel Mountains frontal faults. These hypothetical extensions do not exist 

(McCulloh and others, 2000; this report). The 60 to 90 km of 

cumulative left slip is invalid. 

Evidence for timing and duration of movement on the 

Santa Monica-Hollywood-Raymond Fault Zone is straightforward. Erosion from the upthrown block north of the zone 

influenced basinal deposits as old as 3.7 Ma (Wright, 1991, 

p. 60, 62). The youngest turbidite sandstones of the Tarzana 

submarine fan were deposited at about 7.8 Ma (Pierce, 1956, 

table 1, fig. 6) and the fan axis is offset the same amount 

(within approximation limits) as basement rock contacts. Left 

slip must have begun later than 8-7 Ma, much later than the 

middle Miocene time suggested by Campbell and Yerkes (1976), 

and long after the 17.5 ± Ma start of clockwise rotation of the 

western Transverse Ranges (Luyendyk, 1990; Dickinson, 1996, 

118°30'

34°00'

118°15'118°00'

13.2 km

(fault offset)

14.2 km

("spotted slate")

(intrusive contact)

22.5 km

xxxxxx

xxxxx

2.4 km

EXPLANATION

Plutonic granitoid rocks 

 drill hole occurrence

?

? ?

? ?

?

?

? ?

?

?

0 5 10

Kilometers

BC

SM-H

V

R

MP

SM-D

A C B

D

F E

G H

I

J

K

xxxxx

E.P. - A.H.

xxxxxxxxxxxx

?

xxxxxx

Extension along trend of known

contacts between distinctive 

basement rock units

Phyllitic metasediments

 drill hole occurrence

Schist and cordierite slate

 drill hole occurrence

Figure 9.—Approximate outcrop and drill hole geology that supports estimate of 13-14 km of sinistral strike slip on the eastern Santa 

Monica-Hollywood-Raymond Fault System. Not shown is the offset axis of the Miocene Tarzana (submarine) fan, which testifies to the Neogene 

age of the offset and supports offset estimates of “6 to 8” miles (Wright, 1987, p. 24) or “10-15 km” (Redin, 1991, p. 250; Wright, 1991, p. 62) without 

considering the additive effect of 2.4 km of left strike slip contributed by the Benedict Canyon Fault (Hoots, 1931; Durrell, 1954). For details on 

labeled drill holes A through K, see table 4. Fault symbols as in fi gure 1; see fi gure 2 for fault labels.
19

fig. 10, table 2). However, left slip clearly began before 3.7 

Ma and continues today (Jones and others, 1990). Therefore, 

at a rotation rate of 5.8° per m.y., found by Luyendyk (1990, 

fig. 3) for the entire western Transverse Ranges block, a total 

clockwise rotation of this southeastern end of the block might 

be in the range between 20° and 40° (assuming that translation 

and rotation occur together). We return to this point later, but 

assume conservatively for now that translation and rotation in 

the study area occurred together and probably only since 4-3 

Ma.

San Gabriel-Sierra Madre Zone of Faults

We suggest that the 13-14 km of sinistral slip on the 

Raymond Fault effectively disappears at the junction with the 

Sierra Madre zone. Right slip on the Vasquez Creek branch 

of the San Gabriel-Sierra Madre zone, in part overprinted by 

reverse slip because of the merge with the Santa Susana break, 

translates partially to reverse dip slip on the south break of 

the Clamshell-Sawpit fault complex near the same junction. 

Left slip (with clockwise rotation) of the region north of the 

Raymond Fault and southwest of the Vasquez Creek Fault 

makes room (in plan view) for the isosceles-shaped block 

bounded by the Vasquez Creek-Sierra Madre, the south break 

of the Clamshell-Sawpit fault complex, and part of the San 

Gabriel North Branch zones. The Vasquez Creek Fault is, in 

effect, a tear fault; the south break Clamshell-Sawpit reverse 

fault is the same shear; and the segment of the Sierra Madre 

zone that connects the Vasquez Creek Fault and the south 

break Clamshell-Sawpit fault might better be viewed as the 

Vasquez Creek Fault, a connector between the Sierra Madre 

Table 4. Petroleum exploration drill holes that provide critical control for determination of total left slip on the Raymond Fault (fi gs. 9, 10) and partial 

control for confi guration of basement rocks (fi g. 2).

Palinspastic Reconstruction
20 Mountain Meadows Dacite: Oligocene Intrusive Complex

Figure 10.—Palinspastically reconstructed fault blocks, faults, Mountain Meadows Dacite distribution, and Topanga and Sespe Formation dacite clasts with lines) or inferred (dashed lines). The palinspastically rearranged latest Topanga shoreline is also shown. Fault symbols as in fi gure 1; see fi gure 2 for fault l

118°15' 118°00' 117°45'

118°00'

34°15'

34°15'

118°15'

118°00'

118°00'

118°15'

34°00'

117

1

?

?

? ?

?

?

?

?

? ?

?

X

Approximate margin and area of Mountain Meadows Dacite and related latite and felsite intrusives, queried where uncertain.

Outcrops of Telegraph Peak granodiorite Topanga Group of Wright (1991) clastic strata

arine

errestrial

ormer presence (inferred)

 Mountain Meadows Dacite and Topanga Group both presentUppermost Topanga (late Luisian) shorelineOutcrops where dacite clasts occur in Sespe and upper  Topanga strata (see table 3)Drill holes with dacite clasts or faunal evidence in  upper Topanga or older strata (see fig. 6 and table 2)Tie lines connecting dacite clast locality to identified or

 inferred (?) source (see fig. 6)Gap generated by palinspastic reconstruction, suggesting expression of present-day overthrustingOverlap generated by palinspastic reconstruction, suggesting possible expression of present-day  extensionOutcrop fossil localities (see text and appendix 2)

?

?

? ?

β

subsurface

surface

III

IV

I

II

0 5 10 15

KILOMETERS

V

SG

SG-N

VT

SA

C-S

VC

SG

SS

ML

BC

M-D

R

C-S

SG-N

S

SJ

W

W

LC

SM-H

EXPLANATION
21

and the Santa Susana reverse faults (although reverse dip slip 

east of the merge with the Santa Susana overprints right slip). 

The north break of the Clamshell-Sawpit fault complex is 

a reverse fault that is the offset eastern continuation of the 

Mount Lukens reverse fault, broken and dextrally offset by the 

Vasquez Creek-south break Clamshell-Sawpit tear and thrust. 

In this construct none of the left slip on the Raymond Fault 

passes onto the Sierra Madre Fault Zone east of the junction. 

Farther east the latter is characterized by reverse dip slip at 

least as far east as its junction with the Evey Canyon-San 

Figure 11.—Northern Los Angeles Basin strandline deposits for the uppermost Topanga Formation (late Luisian) and 

Paleogene basin-edge deposits (with inferred connections) before and after palinspastic reconstruction according to 

the model in this report. Fault symbols as in fi gure 1; see fi gure 2 for fault labels.

Palinspastic Reconstruction
22 Mountain Meadows Dacite: Oligocene Intrusive Complex

Antonio fault complex, with continuation on the Cucamonga 

Fault farther east. Oblique reverse right slip on the south 

break of the Clamshell-Sawpit fault complex hypothetically 

merges to the northeast with the eastern part of the San 

Gabriel North Branch Fault in our model. [ Note: Dibblee 

(1998) assigns the names North Sawpit fault and South 

Sawpit fault to what are here termed the north and south 

breaks of the Clamshell-Sawpit fault complex.]

Reconstruction

Our model (fig. 10) derives from (1) the 13-14 km of 

left slip on the eastern Santa Monica-Hollywood-Raymond 

Fault Zone, (2) the late onset (<7 and >3.7 Ma) and rapid rate 

of strike slip on the zone, (3) clockwise rotation of the region 

north of the zone during translation, (4) coordinated simultaneous right slip on the Vasquez Creek Fault (and San Gabriel 

Fault main strand), (5) correlation of the Mount Lukens Fault 

with the north branch of the Clamshell-Sawpit fault complex 

that is right-laterally displaced about 13 km by the Vasquez 

Creek Fault, and (6) about 9 km of right slip on the Whittier 

fault.

Prior to initial slip on the Vasquez Creek-Sawpit Canyon 

tear and thrust, the isosceles-shaped block was part of the 

region north of the San Gabriel (main) Fault. Palinspastic 

restoration to its pre-Vasquez Creek Fault position, shown 

approximately in figure 10, realigns the western edge of the 

field of dacite intrusives across the San Gabriel North Branch 

with the appendage south of the Mount Lukens Fault and 

southwest of the Vasquez Creek Fault. By itself, this realignment is support for the reconstruction. All upper Topanga 

dacite-latite clasts also remain tied to unique sources despite 

back-rotation of the eastern Santa Monica Mountains-Verdugo Mountains and backslip on the Raymond Fault. The 

proposed reconstruction also provides a credible reorganized 

alignment of the late Luisian shoreline (fig. 11). 

A substantial body of other observational facts is consistent with this reconstruction. The steeply dipping Vasquez 

Creek Fault merges with minimal change with the steeply dipping segment of the Sierra Madre Fault Zone west of its junction with the Raymond Fault (Morton, 1973, p. 16-17); we 

view the Vasquez Creek and western Sierra Madre as strands 

of the same fault. Conversely, the gently north-dipping Mount 

Lukens Fault (Smith, 1986) is like the moderately northwestdipping north break of the Clamshell-Sawpit fault complex 

(Morton, 1973; Dibblee, 1998); in our model the latter is the 

offset continuation of the former. 

The 1991 Sierra Madre earthquake showed that slip on 

the Clamshell-Sawpit zone is now reverse and nearly dip slip 

(Hauksson, 1994). Seismologic evidence indicates left slip 

on the Raymond Fault parallel to the fault strike (Jones and 

others, 1990). The approximate alignment of these faults in 

map view hints at affinity, but seismological evidence and our 

model suggest they are separate faults.

The previously puzzling absence of evidence for large 

dextral displacement along the south flank of the San Gabriel 

Mountains east of the Raymond Fault junction, especially 

because of the ìcontinuous distribution of the 15-m.y.-old 

Glendora Volcanics across the projected fault pathî (Powell 

and Weldon, 1992, p. 457), is now readily understood. Right 

slip on the Vasquez Creek-Sierra Madre tear fault converts 

substantially to dip slip on the (south) Clamshell-Sawpit zone 

and does not continue eastward beyond the junction with the 

Raymond Fault. 

Backslip of 22 km on the San Gabriel North Branch 

Fault is required to realign the mapped western edges 

of the field of dacite intrusives across the fault. This 

amount matches reasonably closely the ì20 kmî of 

right-lateral strike slip indicated by the ìnearly vertical 

contact between the Mendenhall Gneiss and Mount Lowe 

Granodioriteî(Dillon and Ehlig, 1993, p. 210-211, fig. 5). 

ìA right slip of 7 miles [11 km] would be a minimum 

displacement along the southern branch of the [San Gabriel 

or Vasquez Creek] faultî (Ehlig, 1975a, p. 14) on the basis 

of clast compositional uniqueness and possible clast-source 

ties observed in gravel beds in Mio-Pliocene marine sandstones in Gold Canyon (Oakshott, 1958, p. 77) along the 

San Gabriel Fault main break. This approaches our 12-13 

km value.

Kinematically the linkage of right slip on the Vasquez 

Creek-Sierra Madre tear fault with reverse dip slip on the 

Clamshell-Sawpit zone is analogous to the way that ìrightlateral strands of the San Jacinto Fault curve westward and 

southwestward into the [southeastern] San Gabriel Mountains and transform into east- to northeast-trending northdipping faults that have reverse left-slip displacementî (Matti 

and Morton, 1993, p. 114).

Our palinspastic reconstruction assumes simplistically 

that all fault blocks are rigid, all movements are confined 

to the major faults, and all movements are in a level plane. 

We recognize that these assumptions are only partly valid. 

Gaps between fault blocks of our map suggest regions of 

compressive overriding along boundaries. Overlaps along 

junctions suggest regions where tensional separation might 

have occurred since our presumed starting arrangement (fig. 

10). The poor fit between the isosceles-shaped block of 

figure 10 and its surroundings is in part because we have 

made no effort to incorporate internal deformations associated with insufficiently mapped second-order faults and 

likely differential tiltings.

The distribution of Oligocene dacite-latite intrusives in 

the southwestern San Gabriel Mountains, San Rafael Hills, 

and Verdugo Mountains suggests continuity across the eastern Santa Susana Fault Zone (Smith, 1986; Crook and others, 

1987) (fig. 5). In turn this suggests that cumulative strike 

separation along that fault zone is negligible, although dip 

separation might be appreciable.

Note that we have made no effort to reconstruct the 

southeastern San Gabriel Mountains, because our work 

on the principal bounding faults there (Cucamonga, Evey 

Canyon, San Antonio Canyon, Stoddard Canyon, Icehouse 

Canyon, and others) is insufficient to deal with the challenges. However, the present-day north-dipping reverse 

Cucamonga Fault probably was a south-dipping normal fault 

that had substantial topographic relief and was active as early 
23

as about 16 Ma. It likely continued west along the Sierra 

Madre Fault Zone trace to the present junction with the 

Raymond Fault, where it joined the subsequently separated 

and rotated Verdugo Fault.

We also leave unanswered the question of what rocks 

once filled the large blank area of our reconstruction east 

of the Clamshell-Sawpit fault complex and north of the 

Sierra Madre zone. Preliminary work suggests that outcrops 

between the San Jacinto and San Andreas Fault Zones eastsoutheast of our area may supply the answer.

Rotations from Paleomagnetic Measurements

Paleomagnetic declination data and interpretations indicate that the entire western Transverse Ranges province 

underwent clockwise rotation of 78° to 95° since early Miocene (Hornafius and others, 1986; Liddicoat, 1990). However, partial and fragmentary data suggest that rotation of the 

eastern Santa Monica Mountains, the northern San Fernando 

Valley and the eastern Ventura Basin was more limited, 

probably at lower rotational rates, and less regular than 

for the province as a whole (Hornafius and others, 1986, 

fig. 8 and table A; Liddicoat, 1992; Levi and Yeats, 1993). 

This eastern end of the province, close to the rotational 

hub, behaved in a fragmented fashion; different fault blocks 

rotated at different rates (or not at all) and for different 

lengths of time. Thus, for example, the young Saugus Formation (<2.3 Ma) of the eastern Ventura Basin north of the 

Santa Susana Fault records about 30° of clockwise rotation, 

whereas the same strata south of the fault did not rotate 

during the same time interval (Levi and Yeats, 1993, p. 

700-701). Similarly, dolomite beds of estimated age 11.5 

± 1.2 Ma in the eastern Santa Monica Mountains, west of 

the Benedict Canyon Fault, record only about 37.5° of 

rotation (Hornafius and others., 1986, fig. 8 and p. 1481). 

This suggests that the 20° of rotation we incorporate for the 

more easterly portion of the block north of the HollywoodRaymond block is acceptably consistent with those more 

westerly observations.

Paleomagnetic declination data pertinent to the San 

Gabriel Mountains sensu strictu come only from outside our 

map area, northwest of most of the basement rock outcrops, 

and northeast of the San Gabriel Fault main break. Two 

episodes of opposing rotation were interpreted for that subregion from less than ideally constrained data: 53° ± 32° 

of clockwise rotation between about 21 Ma and 15 Ma, followed by 16° ± 30° of apparent counterclockwise rotation 

since 11-10 Ma (Hornafius and others, 1986, fig. 7). These 

findings imply that the strike of the (future) northwestern 

San Gabriel Fault must have been nearly east-west until 

between 18 and 11 Ma. Our reconstruction assumes that the 

strike of the eastern San Gabriel north branch (which is now 

about east-west) has not rotated. This is consistent with 

the observational fragment that ìpaleomagnetic data collected from both formations [Glendora Volcanics and El 

Modena Volcanics of the eastern Los Angeles Basin] do not 

show a rotationî (J. C. Liddicoat in Earth Sciences Board, 

1988, p. 246). Cretaceous strata southeast of our area show 

negligible rotation (Fry and others, 1985). Our reconstruction assumes that evidence for rotations from the region 

northwest of the central San Gabriel Mountains does not 

pertain directly to the study area.

Discussion and Conclusions

We have mapped the areal extent, both surface and subsurface, of western limits of the late Oligocene Mountain 

Meadows Dacite intrusive complex, Telegraph Peak granodiorite, and consanguinous felsites and latites, where they 

occur in the San Gabriel Mountains and the adjacent northeastern Los Angeles Basin. This distinctive suite transects 

major tectonic boundaries and ties together the northern Los 

Angeles Basin, the central San Gabriel Mountains (both 

north and south of the San Gabriel Fault north branch), the 

southeastern San Gabriel Mountains (east of the San Antonio 

Fault), the northeastern San Gabriel Mountains (north of 

the Vincent Thrust), and the southwestern San Gabriel Mountains, San Rafael Hills, and Verdugo Mountains (southwest of 

the Mount Lukens and Vasquez Creek Fault Zones). 

A systematic search was made for dacite, felsite, or 

latite clasts redeposited in ≥14 Ma strata of the Los Angeles 

Basin succession (Topanga Group or older). Such clasts 

were transported and deposited at multiple sites during the 

times when Sespe beds and strata of the Topanga Group were 

accumulating along the northeast and northern Los Angeles 

Basin margin. Elevation of the ancestral San Gabriel Mountains with concomitant erosion began probably as early as 

16 Ma. Uplift occurred on faults (probably normal) that 

extended eastward from the western Verdugo Fault Zone, 

along the Sierra Madre zone east of its junction with the 

Raymond Fault to include the Cucamonga Fault Zone at 

the southeast end of the range. At the same time, bathyal 

deposits accumulated in the Los Angeles Basin bight or 

seaway beneath open marine waters that supported nutrientrich upwelling close to the fault-controlled shoreline.

Even earlier erosion and transport of Mountain Meadows Dacite, prior to eruption of Glendora Volcanics and 

probably as early as 18 Ma, deposited dacite clasts in fluvial 

deposits of the Sespe Formation now found as outcrops and 

subsurface remnants in the Whittier and Puente Hills. A 

lower Miocene eroding topographic high with southwest- to 

southeast-sloping fluviatile distributary systems is implied.

Even without palinspastic reconstruction, ties between 

dacite-latite sources and derived clasts in Topanga strata illuminate the petrologic connection of the complex junction of 

the Los Angeles Basin with the westernmost extension of the 

Peninsular Ranges and the San Gabriel Mountains part of the 

Transverse Ranges. Palinspastic adjustments reinforce the 

connection, improve understanding of the movement history 

of faults of the San Gabriel Fault System, and lead to a new 

and simple model of latest middle Miocene paleogeography.

Timing of major depositional and structural events that 

bridge across boundaries separating the Los Angeles Basin 

from the northwestern Peninsular Ranges Province and the 

Transverse Ranges are summarized by figure 12. All faults 

Discussion and Conclusions
24 Mountain Meadows Dacite: Oligocene Intrusive Complex

considered in our reconstruction are shown (fig. 10), along 

with timing of Neogene igneous events and provincial benthic foraminiferal zonal divisions (Blake, 1991).

Our palinspastic reconstruction furnishes a new gauge 

of the total right-lateral strike slip on the San Gabriel main 

strand (mostly northwest of our area), its North Branch, and its 

southern branch, the Vasquez Creek Fault. Right-lateral strike 

slip separation of 20-22 km on the San Gabriel North Branch 

Fault (east of the junction with the Vasquez Creek branch) is 

beyond dispute (Ehlig, 1975b, 1981; Dillon and Ehlig, 1993; 

this study). Right-lateral strike slip separation of the north 

break of the Clamshell-Sawpit zone from the Mount Lukens 

Fault by the Vasquez Creek Fault is about 13 km. These 

separations pass to the northwest onto the main San Gabriel 

Fault, suggesting a total there of about 34 km of right slip. 

This postulated total ignores possible differential shortening 

along one wall or the other of these several faults and the 

possibilities that some other branch of the San Gabriel zone 

absorbed slip, that minor left slip on northeast-striking faults 

occurred, or that associated vertical deformation affects our 

estimate. Our 34-km total should be compared with the aggregate of ì40 to 45î km supported by Powell (1993, p. 36), or 

ì60 kmî (Crowell, 1975; Ehlig and Crowell, 1982; May and 

others, 1993; Yeats and others, 1994, p. 1058-1060 and fig. 11; 

Dickinson, 1996, p. 25-26), or ì70 kmî (Ehlert, 1982).

The Canton Fault in the eastern Ventura Basin (or ìsouthwest edge of [buried] basement ridgeî fault of Yeats and 

others, 1994, fig. 3) was previously proposed as a candidate to 

accommodate some possibly missing right slip from the main 

San Gabriel Fault (Powell, 1993, fig. 14 and p. 40) (fig. 1). 

We see some reasons to correlate the Verdugo Fault of our 

area with this break. However, we see no evidence that the 

Verdugo Fault has undergone notable post-Relizian right slip. 

The ìbasement ridgeî break in the eastern Ventura Basin displays some drill hole characteristics that limit any strike slip 

to 1-2 km at most.

20 Ma 10 0 (km)

Type and period of displacement Net slip

n

r

L+r

n

n

n

n R+r

R+r

R+r

R+r

R+r

R

R

r

r

r

r

r

r

r

r ?

?

?

?

?

?

? ?

L

? 3-4 n

?

10-15 r

13-14 L

34 R

22 R

12 R

? 4

? 4

? 3 L

9 r

2-3 r

?

?

2-3 L

? 1 R

intrusive diabase

Glendora Volcanics

Saucesian

Relizian

Luisian

Delmontian

Repettian

Mohnian

lower upper

15 Ma start (assumed)

Verdugo block

IGNEOUS EPISODES

BENTHIC FORAMINIFERAL

"STAGES"

CLOCKWISE ROTATION

 (5.8°/Ma)

FAULTS

?

L+r

?

Verdugo

Mt. Lukens

Clamshell - Sawpit

Raymond

San Gabriel

San Gabriel - North Branch

Vasquez Creek

Sierra Madre

Cucamonga

San Antonio - Evey Canyon

Whittier

Las Cienegas

Monterey Park

 (+ "Elysian Park fold & thrust")

Santa Susana

Benedict Canyon

Chino

? ? r ?

Figure 12.—Chronology of Neogene fault movements, igneous events, and fault-block rotations consistent with the palinspastic reconstruction 

for this study. Comparison with prior results (table 1) is instructive. “R” and “L” indicate right-lateral and left-lateral strike slip, respectively; 

“n” and “r” indicate normal and reverse slip, respectively.
25

Minor right slip might have splayed from the Vasquez 

Creek Fault segment of the Sierra Madre zone onto the eastern 

segment. However, lack of substantial strike offset of both the 

Los Angeles Basin Glendora Volcanics and associated marine 

Puente Formation beds containing foraminifers indicative of 

an age between 13 and 8.7 Ma (Proctor and others., 1970, 

p. 7) permits only substantial reverse dip slip. Vertical movements associated with strike slip, for example, the ì * * * 

listric, east-southeast-dipping oblique-slip fault * * * î elucidated by combining reflection seismic and surface geological 

mapping in the northwestern Ridge Basin (May and others, 

1993), could contribute to the balance sheet. However, we 

believe future investigators will be challenged to reconcile 

the right-slip difference of 6-11 km between Powell (1993) 

and this report, and even more so the difference of 26-36 km 

between some earlier estimates and this report.

Many palinspastic reconstructions have sought to explain 

the structural, stratigraphic, and paleogeographic evolution 

and complexities in our study area. Thirteen deserve attention 

because they contribute relevant data, valuable or original 

concepts, ingenious approaches, or are important historical 

landmarks (Woodburne, 1975; Truex, 1976; Yeats, 1976; Hornafius and others, 1986; Wright, 1991, fig. 36; Powell and 

Weldon, 1992, fig. 4C; Crouch and Suppe, 1993, fig. 4; Dillon 

and Ehlig, 1993, fig. 5; Powell, 1993; Matti and Morton, 

1993, figs. 7A-K; Yeats and others, 1994; Fritsche, 1998, 

figs. 2, 3, 8; Ingersoll and Rumelhart, 1999). Two of these 

prior reconstructions (Powell and Weldon, 1992, or Powell, 

1993; Yeats and others, 1994) approach closest to a partial 

match with ours, even though neither completely overlaps 

our study area. Our maps of the areal distribution of Mountain Meadows Dacite sources and clasts and of the restricted 

extent of Sespe and older pre-Topanga sedimentary rocks can 

be used to test these or other reconstructions. 

A partly independent test of our reconstruction is how 

well it pieces together fragmentary and tectonically dismembered remnants of ancient depositional systems into 

coherent paleogeographic and paleoceanographic constructs. 

We have shown how backslip of the Raymond Fault (with 

accompanying back-rotation) reconnects the marine embayment southwest of the Verdugo Fault with the Luisian 

marine depositional area of the San Gabriel Valley and San 

Jose Hills. The Luisian shoreline crudely followed the fault 

scarp boundary that separated the eroding ancestral Verdugo 

Mountains and western and central San Gabriel Mountains 

from the broad deep seaway that extended northward from 

the main Los Angeles Basin and as far east as the paleoshoreline mapped through the eastern San Jose Hills by 

Woodford and others (1946) (fig. 6). Nannos that thrived 

in open marine and probably upwelling environments near 

the steeply sloping northern and northeastern basin edges 

are associated with upper bathyal benthic foraminifers (fig. 

6, drill hole IV, localities α and β). Distances to shore 

of perhaps at least 4-5 km are suggested by the presence 

of nannos. Upper bathyal benthic foraminifers suggest 

great local bathymetric relief, an interpretation consistent 

with associated coarse breccias and boulder conglomerates 

(Lamar, 1970; Dibblee, 1989).

Moving back in time to latest Paleocene (about 55 Ma), 

our reconstruction allows us to piece together a few remaining 

surface and subsurface fragments of the depositional edge of 

a sequence of interbedded shallow marine and fluviatile strata. 

A reconstructed partial depositional basin margin (McCulloh 

and others, 2000) was questionably extended westward from 

scattered subsurface control in the eastern Puente Hills to the 

Whittier Fault not far northwest of the intersection of 34° N. 

and 118° W. Thin remnants of the correlative sequence crop 

out locally in the eastern Santa Monica Mountains (Colburn 

and Novak, 1989, figs. 8, 9) and define a transition from 

nonmarine to interbedded marine and continental depositional 

settings. Our study provides the first basis for projecting 

a connection between these Santa Monica Mountains basin 

margin deposits and those that occur only in wells in the eastern Puente Hills. These two areas of fragmented basin margin 

remnants connect in a plausible manner in our reconstruction 

(fig. 11).

A comprehensive and thorough tectonic reconstruction 

for the southern California region (ca. 20 Ma) was proposed 

in an impressively inclusive and well documented study of 

the evolution of the entire San Andreas Fault System (Powell, 

1993). We have not determined in what ways our results 

augment or conflict with Powellís conclusions about the entire 

system. We instead relate our results to the opening of the 

Los Angeles Basin and the clockwise rotation of the western 

Transverse Ranges. Our conclusion of limited right slip on 

the San Gabriel Fault System, as an older subset of the San 

Andreas Fault System, ought to affect understanding of the 

entire region. Our results should also increase interest in 

the incorporation of fault-block domain rotation into future 

models of the entire system, particularly as new data illuminate the importance of three-dimensional block rotations (for 

example, Scotti and Nur, 1990).

The relatively late onset of left strike slip on the Raymond Fault and coupled clockwise rotation of the eastern 

Santa Monica-Verdugo block raise several important questions: (1) Where is the hub or rotation axis about which this 

easternmost part of the western Transverse Ranges turned? 

(2) Why was the onset of rotation so late relative to the 

much earlier beginning of rotation for the great bulk of the 

transrotating beam farther southwest (Hornafius and others, 

1986; Luyendyk, 1990; Nicholson and others, 1994; Dickinson, 1996)? (3) What connection and significance is there 

between the Los Angeles Basin opening and the late onset 

of transrotation of this easternmost part of the entire transrotational region? (4) Is there special temporal significance to the 

growing mass of evidence for blind thrusts and earthquakes 

on cryptic structures (Hauksson, 1990, p. 15369)? 

Our work leads toward answers to some of these questions and suggests promising routes toward answers to others. 

One of the answers may be in hand. Transrotation of the 

western Transverse Ranges beam has probably been progressive, from southwest to north, and segmented both areally and 

temporally, with the eastern segments of the beam the last to 

participate. The rotational hub of the youngest of the transrotating blocks almost certainly shifted as the several involved 

fault blocks moved (three dimensionally). One approximate 

Discussion and Conclusions
26 Mountain Meadows Dacite: Oligocene Intrusive Complex

pivot point, an artifact of the figure 10 reconstruction, is 

located at present day latitude 34.333° N. and longitude 

118.039° W., with a 5-km-diameter circle of uncertainty.

Our work implies much about vertical tectonic movements, but little has been stated explicitly because quantitative 

information is meager. The 1991 Sierra Madre earthquake 

(ML= 5.8) raised the southern part of the San Gabriel Mountains directly above the hypocenter by less than 1 cm (Hauksson, 1994, p. 1071) and was clearly associated with reverse 

dip slip on the Clamshell-Sawpit fault complex. The southern 

edge of the western San Gabriel Mountains (west of our 

study area) rose by as much as 2 meters during the 1971 San 

Fernando earthquake (ML=6.4), which ruptured the ground 

surface and occurred on the north-northeast-dipping Santa 

Susana thrust fault (Allen and others, 1975). Longer term 

uplift of the southern margin of the range is indicated by 

widely distributed late Tertiary and Quaternary marine strata 

in the westernmost part of our map area that are now folded, 

faulted, and uplifted to elevations well above present sea level. 

A special example is the Mio-Pliocene ìfossiliferous marineî 

pebbly sandstone south of the San Gabriel Fault Zone at the 

western edge of our study area (Oakshott, 1958, p. 77) that 

presently is about 670 to 790 m (2,200-2,600 ft) above sea 

level. These marine sandstones testify to the recency, duration, and amount of uplift of this part of the mountains. 

Other evidence of vertical movements is either less quantitative or less amenable to unique interpretion (Cummings 

and others, 1982). Future research will doubtless lead to 

improved understanding and better palinspastic models. A 

comprehensive study of the thermal history of the Mountain 

Meadows intrusive field and its different parts is one particularly inviting avenue for such research.

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32 Mountain Meadows Dacite: Oligocene Intrusive Complex APPENDIX 1 Petroleum exploration drill holes that furnish information about the presence or absence of Sespe or pre-Sespe sedimentary rocks (fi g. 5), the 

pre-sedimentary basement rocks (fi g. 2), and distribution of sedimentary and volcanic rocks of the Topanga Group (fi g. 6).
33

APPENDIX 1—Continued

Appendix
34 Mountain Meadows Dacite: Oligocene Intrusive Complex

APPENDIX 2

Calcareous nannofossils identifi ed in key outcrop and drill hole samples in the northernmost part of the Los Angeles Basin that help 

provide understanding of the Middle Miocene stratigraphy and paleoceanography.