LOCATIONS AND AGES OF MIDDLE

TERTIARY VOLCANIC CENTERS IN

COASTAL CALIFORNIA

By Richard G. Stanley1, Douglas S. Wilson2, and Patricia A. McCrory1

Open-File Report 00-154

2000

This report is preliminary and has not been reviewed for conformity with U.S. Geological

Survey editorial standards or with the North American Stratigraphic Code. Any use of

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

endorsement by the U.S. Government.

U. S. DEPARTMENT OF THE INTERIOR

U. S. GEOLOGICAL SURVEY

1U. S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025

2Dept. of Geological Sciences, University of California, Santa Barbara, CA 93106
Locations and ages of middle Tertiary volcanic centers in coastal California

By Richard G. Stanley, Douglas S. Wilson, and Patricia A. McCrory

This report includes a map (fig. 1) and three tables (tables 1, 2, and 3) that

summarize available information on the locations and ages of late Oligocene to middle

Miocene volcanic centers in coastal California. This information, in turn, provides

supporting documentation for a planned formal publication by D.S. Wilson, P.A.

McCrory, and R.G. Stanley on the implications of volcanism in coastal California for the

tectonic history of western North America.

In this report, “coastal California” generally means the area west of the San

Andreas fault from Point Arena to the northwestern tip of Mexico (fig. 1). However, the

map and tables in this report also include a few localities that are east of the San

Andreas fault, including (1) the Neenach Volcanic Formation (fig. 1, locality NV), which

is thought to have been contiguous with the Pinnacles Volcanic Formation (fig. 1,

locality PV) prior to about 315 km of right–lateral displacement along the San Andreas

fault (Matthews, 1973a, 1973b); (2) volcanic rocks in the Diligencia Formation (fig. 1,

locality DF), which may have been adjacent to volcanic rocks of similar lithology and

age in the Plush Ranch Formation (fig. 1, locality PL) and the Vasquez Formation (fig. 1,

locality VF) prior to right–lateral displacements of about 210 km along the San Andreas

fault and 60 km along the San Gabriel fault (Crowell, 1962; Bohannon, 1975); and (3)

volcanic rocks in the Tecuya Formation (fig. 1, locality TE) and in the Tunis Creek area

(fig. 1, locality TU), which may have been adjacent to volcanic rocks in the northern

Gabilan Range (fig. 1, locality GR) prior to about 280–305 km of right–lateral

displacement along the San Andreas fault (Hill and Dibblee, 1953, p. 448–449; Dickinson

and others, 1972; Weigand and Thomas, 1990).

Tables 1, 2, and 3 include many, but not all, of the volcanic centers listed in

previous compilations by Turner (1968, 1970), Weigand (1982), Johnson and O’Neil

(1984), Fox and others (1985), Stanley (1987), Cole and Basu (1995), Dickinson (1997),

and Cole and Stanley (1998). Also included in tables 1–3 are several volcanic centers,

mainly in southern California, that were not listed in these older compilations.

Additionally, the tables contain many new isotopic age determinations contained in

recent publications by Luyendyk and others (1998), Nourse and others (1998), and

Stakes and others (1999), and a small number of unpublished, preliminary age

determinations by the U.S. Geological Survey.

The volcanic centers listed in this report are divided on the basis of age into three

groups—27–25 Ma (table 1), 25–20 Ma (table 2), and 20–10 Ma (table 3)—mainly to

facilitate discussion and the construction of paleogeographic maps (currently in

progress). In a few cases, some overlap between the three age groups is possible on the

basis of the reported determinations of age and their ranges of analytical error.

Nevertheless, it seems to us that the available age data are consistent with the

hypothesis that episodes of relatively intense volcanic activity occurred about 27–25 Ma,

24–22 Ma, and 18–15 Ma, and that these episodes were separated by intervals of

reduced volcanic activity about 25–24 Ma and 22–18 Ma. This hypothesis can be tested

further by obtaining new age data from the many occurrences of volcanic rocks in

coastal California that remain to be accurately dated using modern isotopic,

biostratigraphic, and paleomagnetic methods (see examples below).

Many well–known volcanic occurrences in coastal California and vicinity are

intentionally omitted from tables 1, 2, and 3. These omissions include the following:
• Volcanic rocks that are mostly or entirely younger than 10 Ma. Examples include

volcanic rocks along the Elsinore fault zone in Riverside County, which have

yielded K–Ar ages generally in the range of 6.7 to 11.9 Ma (Morton and

Morton, 1979a, b); volcanic rocks along the Calaveras fault zone in Santa

Clara County, which have yielded K–Ar ages generally in the range of 2.5 to

3.6 Ma (Nakata and others, 1993); and an andesite dike along the Silver Creek

fault zone in Santa Clara County that has yielded K–Ar ages of 9.3 to 10.5 Ma

(Nakata and others, 1993).

• Volcanic rocks that are mostly or entirely older than 10 Ma and that are inferred by

Stanley (1987), McLaughlin and others (1996), and Dickinson (1997) to be related to

northwestward migration of the Mendocino triple junction. Examples include the

Page Mill Basalt in Palo Alto, which has yielded K–Ar ages generally in the

range of 14 to 15 Ma (Turner; 1970, p. 106; Mark Mason, University of

California, Berkeley, oral commun., 1989); an unnamed dacitic tuff breccia

near Los Gatos, which has yielded a K–Ar age of 15.6 ± 0.4 Ma (McLaughlin

and others, 1991, p. 13; Nakata and others, 1993, p. 28); the Quien Sabe

Volcanics (Taliaferro, 1948; Leith, 1949) and related(?) volcanic rocks of Santa

Clara and San Benito Counties, which have yielded K–Ar ages generally in

the range of 7.4 to 12.3 Ma (Nakata and others, 1993); and numerous

occurrences of volcanic rocks in the Berkeley–East Bay Hills, northern San

Francisco Bay, and Clear Lake areas, which have yielded K–Ar ages ranging

from about .04 Ma to about 12.3 Ma (McLaughlin and others, 1996, and

references therein).

• Volcanic rocks whose ages are so poorly known that we could not confidently assign

them to one of the three age groupings in tables 1, 2, and 3. Examples include

unnamed basalt in an exploratory oil well near Point Reyes in Marin County

(Clark and others, 1984); unnamed volcanic rocks described as “augite

andesite” in the northern Santa Lucia Range (Trask, 1926, p. 145–146);

unnamed volcanic rocks in the Monterey Formation in the southern Salinas

Valley (Dibblee, 1971; Durham, 1974, p. 41; Graham, 1976, p. 121); many small

intrusions of diabase that cut Mesozoic and Cenozoic sedimentary strata in

northern Santa Barbara County (Vedder and others, 1995 and unpublished

data); several occurrences of basaltic and andesitic rocks in the vicinities of

Sunland, Pacoima, and the eastern Santa Susana Mountains in Los Angeles

County (Eaton, 1957); and numerous reported occurrences of volcanic rocks

in the California offshore (for example, Hoskins and Griffiths, 1971; Vedder

and others, 1974, 1981; Epstein and Nary, 1982; Crain and others, 1987, p. 416;

Ogle and others, 1987; McCulloch, 1987, 1989; Clark and others, 1991).

The locations of the volcanic centers in tables 1, 2, and 3 are intended for use in

constructing page–size paleogeographic maps at scales of 1:5,000,000 or thereabouts. In

most cases, information on the actual locations of ancient volcanic vents is unavailable,

and the specific locations given are based on (1) occurrences of associated intrusive

rocks, some of which may represent conduits that conveyed magma to the surface; (2)

visual approximations of the centers of the thickest accumulations of volcanic rocks, or

(3) visual approximations of the geographic centers of the most areally extensive

masses of volcanic rocks.

Most of the locations given in tables 1, 2, and 3 were measured from

1:24,000–scale geologic maps, but the locations of some (for example, the areally

extensive Pinnacles Volcanic Formation of Matthews, 1973b) were measured from
1:250,000–scale geologic maps. Latitudes and longitudes were determined from paper

copies of the maps using “The Coordinator,” a commercially–manufactured device

designed for this purpose, and converted to digital degrees using a pocket calculator.

The latitudes and longitudes are reported to the nearest .01 degrees, implying

accuracies of about ±1.1 km measured in a north–south direction and about ±0.9 km

measured in an east–west direction at latitude 36 degrees north.

ACKNOWLEDGEMENTS

We thank Thane McCulloh (consulting geologist, Dallas, TX) and Ron Cole

(Allegheny College, Meadville, PA) for reviewing earlier drafts of this report and

providing many helpful suggestions.
Figure 1. Map showing approximate locations of middle Tertiary volcanic centers in 

coastal California. See tables 1, 2, and 3 for explanation of letter symbols.

124 ̊

39 ̊

38 ̊

37 ̊

36 ̊

35 ̊

34 ̊

33 ̊

122 ̊

118 ̊

116 ̊

120 ̊

SG1

MR-IH

PC

WR

MM

CA

YM1

IB

HB MB

PB ZC

AN

GR

PV

SQ

LC

SI LM1 TU

NV

VF

PL

TE

DF

YM2

NC1

NC2

TB

CR

SR TM

PS HH

OF

LM2

CV

WH

WC ZV

PVP

AC

SM

SRI

SCR CP

IO

GV

EM

DO LO SJ

SG2

BF

SB BR

SN

SCL

STC

RB

JV

SAN ANDREAS FAULT

0 300 KILOMETERS

0 200 MILES

AGE OF VOLCANISM:

ABOUT 20-10 Ma

ABOUT 25-20 Ma

ABOUT 27-25 Ma

CALIFORNIA

Area shown

Pacific

 Ocean

Sacramento

Fresno

Bakersfield

CALIFORNIA

MEXICO

POINT ARENA

4

NEVADA

CALIFORNIA
5

Table 1. Some occurrences in coastal California of volcanic rocks thought to

represent volcanic centers that erupted approximately 27–25 Ma. See fig. 1 for

locations

Latitude

 ( ̊N)

Longitude

( ̊W)

Map

symbol

Occurrence Basis for age assignment

36.56 -121.94 CA Unnamed basalt of Carmel area,

Monterey County

K–Ar ages (Clark and others, 1984): 27.1

± 0.8 Ma and 27.0 ± 0.8,

whole–rock(?)

35.49 -120.83 YM1 Unnamed felsic volcanic rocks of

York Mountain area (older

sequence), San Luis Obispo

County

Field relations: volcanic rocks are

interbedded with and overlain by

sedimentary rocks of late Oligocene

age (Seiders, 1982; Stanley and

others, 1996a)

35.32 -120.73 MR–IH Morro Rock–Islay Hill complex

and Cambria Felsite of Ernst

and Hall (1974), San Luis

Obispo County

K–Ar ages (Turner, 1968): 27.1 ± 0.8 Ma*

on biotite, 25.6 ± 1.2 Ma* and 22.7 ±

0.9 Ma* on plagioclase, 27.2 ± 0.8

Ma* on sanidine

K–Ar age (Hall and others (1966): 24.1 ±

1.8 Ma* on plagioclase

K–Ar ages (Buckley, 1986): 28.0 ± 1.0

Ma and 26.0 ± 0.8 Ma on biotite, 25.1

± 0.8 Ma whole–rock

40Ar/39Ar single crystal laser fusion ages

(J. D. Obradovich, M.A. Mason, and

C.C. Swisher, cited by Cole and

Stanley, 1998): 26.5–27 Ma on

sanidine

35.32 -120.22 PC Unnamed basaltic andesite of

upper Pine Creek area, San

Luis Obispo County

K–Ar age (Vedder and others, 1991): 26.5

± 0.5 Ma on plagioclase

34.71 -119.20 WR Unnamed felsic intrusive rocks of

the Wagon Road Canyon

area, Ventura County,

(Vedder and others, 1973)

40Ar/39Ar single crystal laser fusion ages

(R.J. Fleck and R.G. Stanley, U.S.

Geological Survey, unpublished data):

25.2–25.0 Ma on sanidine

34.26 -117.60 SG1 Unnamed felsic intrusive rocks of

the eastern San Gabriel

Mountains, Los Angeles and

San Bernardino Counties

40Ar/39Ar ages (Nourse and others, 1998):

26.30 ± 0.04 Ma and 25.7 ± 0.2 Ma

on rhyolite dikes

U/Pb age (May and Walker, 1989): 25.6 ±

1 Ma on zircon from granodiorite

Rb/Sr ages (Hsu and others, 1963): 25 ±

15 Ma and 19 ± 15 Ma on biotite

from quartz monzonite

K–Ar ages (Hsu and others, 1963): 26 ± 3

Ma* and 17 ± 5 Ma* on biotite from

quartz monzonite

K–Ar ages (Miller and Morton, 1977):

19.1 ± 0.6 Ma*, 14.9 ± 0.4 Ma*, and

14.4 ± 0.4 Ma* on biotite from

granodiorite

[Note: the wide range of ages reported

from these rocks is puzzling. There

may have been multiple episodes of

intrusion; alternatively, some of the

reported ages may be erroneous]
6

34.10 -117.78 MM Mountain Meadows Dacite

Porphyry, Los Angeles

County

40Ar/39Ar ages (Nourse and others, 1998):

28.0–27.5 Ma on biotite

40Ar/39Ar single crystal laser fusion age

(L.A. Beyer, R.J. Fleck, T.H.

McCulloh, and R.G. Stanley, U.S.

Geological Survey, unpublished data):

27.3 ± 0.1 Ma on biotite

* Isotopic age from original reference has been corrected for changes in decay constants using the method

of Dalrymple (1979).
7

Table 2. Some occurrences in coastal California of volcanic rocks thought to

represent volcanic centers that erupted approximately 25–20 Ma. See fig. 1 for

locations

Latitude

( ̊N)

Longitude

( ̊W)

Map

symbol

Unit Basis for age assignment

38.86 -123.65 IB Iversen Basalt of Weaver (1944),

Mendocino County

K–Ar ages (Turner, 1970): 24.9 ± 1.3

Ma*, 23.4 ± 1.1 Ma*, and 23.2 ± 1.2

Ma* on plagioclase

37.47 -122.38 HB Unnamed basalt near Half Moon

Bay, San Mateo County

Field relations: basaltic rocks are

interbedded with sedimentary rocks of

late Oligocene and (or) early Miocene

age (Mack, 1959; Classen, 1959;

Stanley, 1987)

37.33 -122.25 MB Mindego Basalt and related

volcanic rocks, San Mateo

and Santa Cruz Counties

K–Ar age (Turner, 1970): 20.2 ± 1.2 Ma*

on plagioclase, interpreted by Turner

as minimum age owing to incipient

alteration of the dated plagioclase

Field relations: basaltic rocks are

interbedded with sedimentary rocks of

late Oligocene and (or) early Miocene

age (Cummings and others, 1962;

Stanley, 1987)

37.27 -122.41 PB Volcanic rocks (andesite?) in

Vaqueros(?) Formation of

Clark (1981), Pescadero

Beach, San Mateo County

K–Ar age (Taylor, 1988, 1990; Stanley,

1987): 22.0 ± 0.7 Ma on plagioclase

37.12 -122.02 ZC Basalt in Vaqueros Sandstone,

Zayante Creek, Santa Cruz

County

K–Ar age (Turner, 1970): 23.7 ± 0.7 Ma*

on plagioclase

37.11 -122.32 AN Volcanic rocks (andesite and

basalt) in Vaqueros(?)

Formation of Clark (1981),

Point Año Nuevo, San

Mateo County

Field relations: volcanic rocks apparently

overlie sedimentary rocks of late

Oligocene age and are overlain by

sedimentary rocks of early Miocene

age (Brabb and others, 1977; Clark,

1981; McDougall, 1983; Stanley,

1987)

36.82 -121.55 GR Unnamed volcanic rocks

(andesite, dacite, rhyolite,

basalt) of the northern

Gabilan Range, Monterey

and San Benito Counties

K–Ar age (Turner, 1968): 22.2 ± 0.7 Ma*

on biotite

K–Ar ages (Weigand and Thomas, 1990):

23.5 ± 1.4 Ma on biotite, 21.3 ± 1.3

Ma whole–rock on andesite

36.80 -122.00 SQ Unnamed basaltic andesite in

Soquel Canyon, Monterey

Bay

40Ar/39Ar incremental heating age (Stakes

and others, 1999): 23.7 ± 0.5 Ma on

plagioclase
8

36.47 -121.18 PV Pinnacles Volcanic Formation of

Matthews (1973b) (mainly

andesite, dacite, and

rhyolite), Monterey and San

Benito Counties

K–Ar ages (Turner, 1968): 22.1 ± 3.2 Ma*

on biotite, 24.1 ± 0.7 Ma* on

plagioclase, 24.5 ± 1.2 Ma* and 24.3

± 0.7 Ma* whole–rock on rhyolite

40Ar/39Ar incremental heating ages

(Weigand and Swisher, 1991;

Weigand, 1991, written commun.):

22.82 ± 0.16 Ma on plagioclase, 23.24

± 0.06 Ma on matrix from rhyolite

35.92 -120.50 LC Unnamed volcanic rocks (mainly

rhyolite) of Lang Canyon,

Monterey County

K–Ar age (D.L. Turner, cited by Sims,

1993): 23.8 ± 0.7 Ma whole–rock on

rhyolite

35.15 -120.47 LM1 Unnamed basaltic rocks near

Lopez Mountain (older

sequence), San Luis Obispo

County

Field relations: basaltic rocks are

interbedded with and overlain by

fossiliferous sedimentary rocks of

earliest Miocene age (McLean, 1995)

35.18 -120.21 SI Basalt in Simmler Formation,

Alamo Creek, San Luis

Obispo County (Vedder and

others, 1988)

K–Ar ages (Ballance and others, 1983):

23.4 ± 0.8 Ma and 22.9 ± 0.7 Ma

whole–rock

34.97 -118.79 TU Unnamed volcanic rocks (mainly

basalt, andesite, and dacite)

of Tunis Creek area, Kern

County (Dibblee, 1973c)

40Ar/39Ar incremental heating ages (Plescia

and others, 1994): 23.0 ± 1.0 Ma, 22.9

± 0.9 Ma, and 22.3 ± 0.7 Ma

whole–rock on basalt

34.93 -118.98 TE Volcanic rocks (basalt, andesite,

dacite, rhyolite) in Tecuya

Formation, Kern County

(Dibblee, 1973a, b, d, e;

Cole and DeCelles, 1991)

K–Ar ages (Turner, 1968, 1970): 25.2 ±

2.9 Ma*, 24.6 ± 7.2 Ma*, and 18.1 ±

7.6 Ma* on plagioclase; 22.9 ± 0.7

Ma*, 22.5 ± 0.7 Ma*, 22.4 ± 0.7 Ma*,

and 22.1 ± 0.6 Ma* on biotite

34.78 -119.09 PL Volcanic rocks (mainly basalt

and andesite) in Plush Ranch

Formation of Carman

(1964), Ventura County

K–Ar ages (Crowell, 1973): 20.1 ± 1.1

Ma* and 17.9 ± 3.7 Ma* on

plagioclase

K–Ar ages (Frizzell and Weigand, 1993):

26.5 ± 0.5 Ma, 25.3 ± 0.6 Ma, 24.2 ±

0.4 Ma, 23.9 ± 0.3 Ma, 23.1 ± 0.3 Ma,

20.9 ± 0.9 Ma, and 20.4 ± 0.9 Ma

whole–rock

[Note: the 20.9 Ma and 20.4 Ma ages

reported by Frizzell and Weigand

(1993) and the 20.1 Ma and 17.9 Ma

ages reported by Crowell (1973) fail a

critical value test and therefore are

considered unreliable by Frizzell and

Weigand (1993, p. 279)]
9

34.75 -118.55 NV Neenach Volcanic Formation of

Dibblee (1967) (mainly

andesite, dacite, and

rhyolite), Los Angeles

County

K–Ar minimum ages (D.L. Turner, cited

by Matthews, 1973a): 24.1 Ma*, 23.0

Ma*, 22.5 Ma*, 19.3 Ma*, and 19.0

Ma*

K–Ar ages (Weigand and Thomas (1990,

who say that these are “improbably

old”): 44 ± 1.3 Ma and 35 ± 1 Ma on

plagioclase

40Ar/39Ar incremental heating ages

(Weigand and Swisher, 1991;

Weigand, 1991, written commun.,

who says that only the plagioclase

dates are believed to represent the age

of eruption):, 23.55 ± 0.25 Ma and

21.30 ± 0.22 Ma on plagioclase, 13.34

± 0.16 Ma and 11.95 ± 0.24 Ma on

matrix from andesite, 14.28 ± 0.20

Ma on matrix from rhyolite

34.47 -118.29 VF Volcanic rocks (mainly basalt,

andesite, and dacite) in

Vasquez Formation, Los

Angeles County

K–Ar age (Woodburne, 1975): 20.7 ± 0.8

Ma* on plagioclase

K–Ar ages (Crowell, 1973): 25.6 ± 2.1

Ma* and 24.5 ± 0.8 Ma* on

plagioclase

K–Ar ages (Frizzell and Weigand, 1993):

23.6 ± 0.4 Ma, 25.4 ± 0.6 Ma, 20.6 ±

0.4 Ma, 19.6 ± 1.5 Ma, 17.2 ± 0.4 Ma,

17.0 ± 0.5 Ma, 15.2 ± 0.4 Ma, and

14.7 ± 0.4 Ma whole–rock; 37.5 ± 1.0

Ma on biotite

[Note: Frizzell and Weigand (1993, p.

279) accept only the 23.6 Ma age

reported by them and the 25.6 Ma and

24.5 Ma ages reported by Crowell

(1973); all other ages are rejected as

unreliable by Frizzell and Weigand

(1993, p. 279) because they are

inconsistent with biostratigraphic data

and (or) fail a critical value test]

33.55 -115.68 DF Volcanic rocks (mainly basalt

and andesite) in Diligencia

Formation, Riverside County

K–Ar age (Spittler, 1974): 19.1 ± 1.9 Ma*

on plagioclase

K–Ar ages (Crowell, 1973): 23.0 ± 2.9

Ma* and 20.6 ± 8.9 Ma* on

plagioclase

K–Ar ages (Frizzell and Weigand, 1993):

23.6 ± 0.5 Ma, 22.9 ± 0.5 Ma, 22.6 ±

0.3 Ma, 22.2 ± 0.5 Ma, 22.0 ± 0.5 Ma,

and 21.3 ± 0.6 Ma whole–rock

[Note: the 19.1 Ma age of Spittler (1974)

is rejected as unreliable because it is

inconsistent with biostratigraphic data

(Frizzell and Weigand, 1993, p. 280)]

* Isotopic age from original reference has been corrected for changes in decay constants using the method

of Dalrymple (1979).
10

Table 3. Some occurrences in coastal California of volcanic rocks thought to

represent volcanic centers that erupted approximately 20–10 Ma. See fig. 1 for

locations

Latitude

( ̊N)

Longitude

 ( ̊W)

Map

symbol

Unit Basis for age assignment

35.49 -120.83 YM2 Unnamed basaltic rocks of York

Mountain area (younger

sequence), San Luis Obispo

County

Field relations: basaltic rocks intrude

sedimentary rocks of early Miocene

age (Seiders, 1982; Stanley and

others, 1996a)

35.33 -120.09 NC1 Unnamed basaltic rocks of the

northwestern Caliente

Range, San Luis Obispo

County

Field relations: basaltic rocks overlie and

are interstratified with fossiliferous

sedimentary rocks of late early

Miocene age and are overlain by

sedimentary rocks of late early

Miocene to middle Miocene age

(Bartow, 1991)

35.26 -120.55 LM2 Unnamed volcanic rocks (basaltic

andesite and vitric tuff) near

Lopez Mountain (younger

sequence), San Luis Obispo

County

K–Ar age (McLean, 1994): 17.0 ± 0.5 Ma

on plagioclase from basalt

35.22 -119.95 NC2 Unnamed basaltic rocks of the

northwestern Caliente

Range, San Luis Obispo

County

Field relations: basaltic rocks overlie and

are interstratified with fossiliferous

sedimentary rocks of late early

Miocene age and are overlain by

sedimentary rocks of late early

Miocene to middle Miocene age

(Bartow, 1991)

35.08 -120.77 OF Obispo Formation and related

volcanic rocks (mainly

rhyolite and dacite, with

subordinate andesite and

basalt), San Luis Obispo and

Santa Barbara Counties

K–Ar ages (Turner, 1970): 16.9 ± 0.8

Ma*, 16.9 ± 1.2 Ma*, 16.7 ± 0.5 Ma*,

15.8 ± 0.5 Ma*, 15.7 ± 0.5 Ma*, 15.7

± 0.5 Ma*, and 15.7 ± 0.9 Ma* on

plagioclase

Field relations: volcanic rocks overlie

fossiliferous sedimentary rocks of

early Miocene (Saucesian) age and

are interbedded with and overlain by

fossiliferous sedimentary rocks of late

early Miocene to middle Miocene

(Relizian) age (Hall and Corbató,

1967; Hall, 1973; Cole and Stanley,

1998)

[Note: the location of the eruptive center

of the white tuff that constitutes the

bulk of the Obispo Formation is

unknown (Schneider and Fisher,

1996) but may have been in the

offshore and (or) Diablo Canyon area

(Cole and Stanley, 1998). Other,

subordinate eruptive centers (not

shown on fig. 1) also probably existed

but have not been documented]
11

35.00 -119.59 TB Triple Basalts of Eaton (1939),

Caliente Range, San Luis

Obispo County

K–Ar ages (Turner, 1970): 16.5 ± 1.3

Ma*, 14.8 ± 0.8 Ma*, and 14.6 ± 0.6

Ma* on plagioclase

Field relations: basalt flows are

interstratified with fossiliferous

nonmarine and marine strata of late

early Miocene to middle Miocene age

(Turner, 1970; Vedder, 1970)

34.89 -120.64 PS Trachybasalt sill in Point Sal

Formation near Point Sal,

Santa Barbara County

Field relations: sill intrudes fossiliferous

sedimentary rocks of late early

Miocene to middle Miocene

(Relizian) age (Woodring and

Bramlette, 1950; Stanley and others,

1996b; Cole and Stanley, 1998)

34.76 -119.98 CR Unnamed volcanic rocks (mainly

basalt) of Catway Road area,

Santa Barbara County

K–Ar age (Vedder and others, 1994): 18.8

± 1.5 Ma on plagioclase

K–Ar ages (Hall, 1981) 20.3 ± 1.6 Ma*

and 17.2 ± 2.5 Ma* on whole–rock(?)

Field relations: volcanic rocks overlie

fossiliferous sedimentary rocks of

early Miocene age, and are overlain

by fossiliferous sedimentary rocks of

late early Miocene to middle Miocene

age (Stanley and others, 1996a; Cole

and Stanley, 1998)

34.70 -119.88 HH Unnamed basalt near Hells Half

Acre, Santa Barbara County

K–Ar age (Fritsche and Thomas, 1990):

18.5 ± 2.0 Ma on whole–rock

34.61 -120.26 SR Unnamed basaltic andesite of

Santa Rosa Creek, Santa

Barbara County

K–Ar age (Turner, 1970): 17.4 ± 1.2 Ma*

on whole–rock

Field relations: basaltic andesite overlies

and is overlain by fossiliferous

sedimentary rocks of late early

Miocene age (Cole and Stanley, 1998)

34.58 -120.56 TM Tranquillon Volcanics of Dibblee

(1950), related volcanic

rocks in Lospe Formation of

central Santa Maria basin,

and related volcanic rocks in

lower part of Monterey

Formation near Santa

Barbara (mainly rhyolitic

and dacitic tuffs), Santa

Barbara County

40Ar/39Ar single crystal laser fusion ages

(Stanley and others, 1996a): 18.42 ±

0.06 Ma, 17.80 ± 0.05 Ma, and 17.70

± 0.02 Ma on sanidine; 17.39 ± 0.06

Ma on plagioclase

K–Ar ages (Turner, 1970): 16.5 ± 0.6 Ma*

and 17.2 ± 0.5 Ma* on plagioclase

Field relations: volcanic rocks are

interbedded with fossiliferous

sedimentary rocks of late early

Miocene (Saucesian) age (Dibblee,

1950, 1966; De Paolo and Finger,

1991; Stanley and others, 1992, 1994,

1996a; Cole and Stanley, 1994, 1998)

34.20 -117.75 SG2 Andesitic and basaltic intrusive

rocks of eastern San Gabriel

Mountains, Los Angeles

County

Correlated with Glendora Volcanics on the

basis of similarities in geochemical

composition (Nourse and others,

1998)

Field relations: andesite and basalt intrude

late Oligocene felsic intrusive rocks

and are offset by late Miocene and

younger right–lateral strike–slip faults

(Nourse and others, 1998)
12

34.17 -118.92 CV Conejo Volcanics of Yerkes and

Campbell (1979) (basalt,

andesite, and dacite),

Ventura County

K–Ar ages (Turner and Campbell, 1979):

16.0 ± 0.6 Ma*, 15.9 ± 0.8 Ma*, 15.2

± 0.4 Ma*, 15.0 ± 0.7 Ma*, 14.3 ± 0.4

Ma*, and 13.4 ± 0.9 Ma* on

plagioclase

K–Ar ages (Turner, 1968; Turner, 1970):

16.6 ± 2.4 Ma* and 14.5 ± 1.0 Ma*

on plagioclase

Field relations: volcanic rocks overlie

fossiliferous sedimentary rocks of late

early Miocene to middle Miocene

(Saucesian to Relizian) age, and

interfinger with and are overlain by

sedimentary rocks of middle Miocene

(Luisian) age (Turner and Campbell,

1979; Dibblee and Ehrenspeck, 1993)

34.13 -118.38 CP Extrusive and minor intrusive

rocks (basalt and andesite) in

Middle Topanga Formation

of Dibblee (1991a, b) near

Cahuenga Pass, eastern

Santa Monica Mountains,

Los Angeles County

Field relations: basaltic and andesitic

rocks are interstratified with and

intrude sedimentary rocks of late

early Miocene to middle Miocene

(Relizian and Luisian) age (Eaton,

1957)

34.10 -117.80 GV Glendora Volcanics (andesite and

subordinate basalt, dacite,

and rhyolite), Los Angeles

County

K–Ar ages (Nourse and others, 1998; P.W.

Weigand, written commun., 1998):

19.6 ± 1.1 Ma, 18.8 ± 1.2 Ma, 18.2 ±

1.1 Ma, and 13.2 ± 0.4 Ma

whole–rock on andesite and dacite

40Ar/39Ar ages (Nourse and others, 1998;

P.W. Weigand, written commun.,

1998): 16.3 ± 1 Ma and 15.9 ± 0.3 Ma

on plagioclase

Field relations: Glendora Volcanics

overlie late Oligocene dacite, and are

interstratified with fossiliferous

sedimentary rocks of late early

Miocene and middle Miocene age

(Shelton, 1955; T.H. McCulloh, oral

and written communs., 1998)

34.03 -119.77 SCR Santa Cruz Island Volcanics of

Nolf and Nolf (1969)

(mainly andesite, with

subordinate dacite and

basalt)

40Ar/39Ar incremental heating ages

(Luyendyk and others, 1998): 17.12 ±

0.12 Ma, 16.9 ± 0.2 Ma, and 16.33 ±

0.26 Ma whole–rock on andesite; 17.0

± 0.1 Ma and 16.9 ± 0.2 Ma on

plagioclase from andesite

K–Ar ages (Crowe and others, 1976): 19.9

± 0.9 Ma* on plagioclase from dike;

15.9 ± 0.7 Ma* whole–rock on dike

K–Ar ages (Turner, 1970): 16.5 ± 0.8 Ma*

and 16.1 ± 0.9 Ma* on plagioclase

from andesite

Field relations: volcanic rocks are

interbedded with and overlain by

fossiliferous sedimentary rocks of late

early Miocene to middle Miocene

(Relizian) age (Crowe and others,

1976)
13

34.03 -120.30 SM San Miguel Volcanics of Avila

and Weaver (1969), San

Miguel Island (basalt,

basaltic andesite, and dacite)

40Ar/39Ar incremental heating ages

(Luyendyk and others, 1998): 18.5 ±

1.0 Ma and 17.0 ± 0.8 Ma whole–rock

on basalt

Field relations: volcanic rocks overlie

fossiliferous sedimentary rocks of

early Miocene (Saucesian) age and

are overlain by fossiliferous

sedimentary rocks of late early

Miocene to middle Miocene

(Relizian) age (Weaver and Doerner,

1969)

[Note: the following published K–Ar ages

are rejected as too old because they

are contradicted by field relations and

40Ar/39Ar ages: 31.8 ± 3.5 Ma and

28.6 ± 3.2 Ma whole–rock on basalt

(Kamerling and Luyendyk, 1985);

25.0 ± 3.0 Ma* on plagioclase from

dacite (Crowe and others, 1976)]

34.02 -119.36 AC Unnamed volcanic rocks (basalt)

of Anacapa Island

40Ar/39Ar incremental heating ages

(Luyendyk and others, 1998): 16.28 ±

0.18 Ma and 16.15 ± 0.20 Ma on

plagioclase; 15.8 ± 0.2 Ma

whole–rock

34.00 -118.37 IO Unnamed intrusive and extrusive

rocks (basalt and andesite) in

subsurface, Inglewood oil

field, Los Angeles County

Field relations: extrusive rocks apparently

are interstratified with sedimentary

rocks of late early Miocene to middle

Miocene (Relizian to Luisian) age

(Eaton, 1958; Wright, 1987, 1991);

intrusive rocks cut sedimentary rocks

of middle Miocene (Luisian) age

(Wright, 1991, p. 98)

34.00 -118.80 ZV Zuma Volcanics of Yerkes and

Campbell (1979) (basalt and

andesite), Los Angeles

County

K–Ar age (Berry and others, 1976): 15.0 ±

1.0 Ma* on plagioclase

Field relations: volcanic rocks are

interstratified with sedimentary rocks

of early and middle Miocene age, and

are overlain by sedimentary rocks of

middle and late Miocene age (Yerkes

and Campbell, 1979)

33.98 -119.73 BF Inferred source of dacitic,

andesitic, and basaltic

volcaniclastic rocks and

minor lava flows in Blanca

Formation of Weaver and

others (1969) and Beechers

Bay Formation of Dibblee

and Ehrenspeck (1998),

Santa Rosa Island

40Ar/39Ar age (Weigand and others, 1998,

p. 39): 15.8 ± 0.08 Ma on plagioclase

in andesite clast

K–Ar ages (McLean and others, 1976):

14.9 ± 0.8 Ma* on basalt flow; 13.3 ±

1.2 Ma* on dacite clast

Field relations: volcaniclastic rocks are

interstratified with fossiliferous

sedimentary rocks of late early

Miocene and middle Miocene age

(McLean and others, 1976; Nuccio

and Wooley, 1998)
14

33.95 -117.89 WH Unnamed diabasic rocks (mainly

in subsurface) within and

northeast of Whittier fault

zone, Los Angeles and

Orange Counties

Field relations: diabase intrudes

fossiliferous sedimentary rocks of late

early Miocene to middle Miocene

(Relizian to early Mohnian) age

(Eaton, 1958; Durham and Yerkes,

1964; Yerkes, 1972; T.H. McCulloh,

oral and written communs., 1996);

clasts of similar diabase occur in early

Pliocene (Repettian) conglomerate

(Durham and Yerkes, 1964; Yerkes,

1972)

33.95 -120.10 SRI Santa Rosa Island Volcanics of

Dibblee and Ehrenspeck

(1998) (basalt and basaltic

andesite)

40Ar/39Ar incremental heating ages

(Luyendyk and others, 1998): 19.3 ±

1.1 Ma and 18.1 ± 0.34 Ma

whole–rock on diabase intrusions

Field relations: volcanic rocks are

interstratified with fossiliferous

sedimentary rocks of late early

Miocene (Saucesian to early Relizian)

age, and diabase intrudes sedimentary

strata as young as late early Miocene

(Dibblee and Ehrenspeck, 1998)

33.90 -117.98 WC Volcanic rocks (basalt and

andesite) in subsurface, West

Coyote oil field and vicinity,

Orange County

Field relations: volcanic rocks overlie and

may be interstratified with

sedimentary rocks of late early

Miocene to middle Miocene

(Relizian) age and are interstratified

with sedimentary rocks of middle

Miocene (Luisian) age (Yerkes and

others, 1965; Yerkes, 1972; Blake,

1991; Wright, 1991, p. 86–87)

33.87 -118.24 DO Unnamed volcanic rocks

(andesite and basalt) in

subsurface, Dominguez oil

field, Los Angeles County

Field relations: volcanic rocks overlie

sedimentary rocks of middle Miocene

(Luisian) age and are interstratified

with and overlain by sedimentary

rocks of middle Miocene (lower

Mohnian) age (Graves, 1954; Eaton,

1958; West and others, 1987, 1988;

T.H. McCulloh, oral and written

communs., 1996 and 2000)

33.82 -118.16 LO Unnamed volcanic rocks in

subsurface, Long Beach

Airport oil field, Los

Angeles County

Field relations: volcanic rocks are

interstratified with sedimentary rocks

that are possibly but not assuredly of

middle Miocene (Luisian) age

(Wright, 1991, p. 71; T.H. McCulloh,

oral and written communs., 2000)
15

33.78 -117.79 EM El Modeno Volcanics (basalt and

andesite), Orange County

40Ar/39Ar incremental heating ages

(Luyendyk and others, 1998): 11.2 ±

0.2 Ma and 10.7 ± 0.2 Ma,

whole–rock on basaltic andesite

K–Ar age (Turner, 1970, p. 109): 14.1 ±

1.6 Ma* on plagioclase from andesite

Field relations: volcanic rocks overlie and

are interstratified with fossiliferous

sedimentary rocks of middle Miocene

(Luisian) age and are overlain by

fossiliferous sedimentary rocks of

upper middle Miocene (lower

Mohnian) age (Yerkes, 1957; Smith,

1960; Schoellhamer and others, 1981;

T.H. McCulloh, written commun.,

1998)

33.75 -118.35 PVP Rhyolitic tuffs and basaltic and

andesitic rocks in Altamira

Shale Member of Monterey

Formation, Palos Verdes

Peninsula, Los Angeles

County

K–Ar age (Turner, 1970): 14.9 ± 1.1 Ma*

on plagioclase in Portuguese tuff bed

of Woodring and others (1946)

Field relations: tuffs and basaltic extrusive

rocks are interstratified with

fossiliferous sedimentary rocks of

middle Miocene (Relizian and

Luisian) age; dikes and sills of

andesite and basalt intrude

sedimentary rocks of middle Miocene

(Relizian and Luisian) age (Woodring

and others, 1946; Eaton, 1957; Yerkes

and others, 1965; Conrad and Ehlig,

1987; Blake, 1991)

[Note: Eaton (1957) believes that the

basaltic and andesitic rocks represent

an episode of igneous activity older

than the episode that produced the

rhyolitic tuffs]

33.60 -117.80 SJ Extrusive and intrusive rocks

(andesite and basalt) in the

San Joaquin Hills–Newport

Bay–Laguna Beach area,

Orange County

K–Ar age (Turner, 1970): 15.8 ± 1.3 Ma*

on plagioclase from andesite

Field relations: andesitic volcanic rocks

are interstratified with fossiliferous

sedimentary rocks of late early

Miocene to middle Miocene age

(Relizian to Luisian), whereas basaltic

and andesitic dikes and sills intrude

sedimentary rocks of late early

Miocene to middle Miocene (Relizian

to lower Mohnian) age (Vedder and

others, 1957; Eaton, 1957, 1958;

Yerkes and others, 1965; Blake,

1991)
16

33.46 -119.04 SB Santa Barbara Island Volcanics

of Weigand and others

(1998) (basalt and andesite)

40Ar/39Ar incremental heating ages

(Luyendyk and others, 1998): 15.5 ±

1.0 Ma whole–rock on basalt

K–Ar ages (Howell, 1976): 16.8 ± 2.0

Ma* and 14.8 ± 1.8 Ma* on

plagioclase from andesite

Field relations: volcanic rocks are

interstratified with fossiliferous

sedimentary rocks of late early

Miocene to middle Miocene (Relizian

to Luisian) age (Vedder and Howell,

1976)

33.38 -118.42 STC Unnamed volcanic and igneous

intrusive rocks (andesite,

dacite, basalt, and rhyolite)

of Santa Catalina Island

40Ar/39Ar incremental heating age

(Luyendyk and others, 1998): 17.2 ±

0.6 Ma whole–rock on basalt

K–Ar age (Forman, 1970): 19.6 ± 0.7 Ma*

on hornblende from quartz diorite

K–Ar age (Vedder and others, 1979): 19.0

± 0.6 Ma on biotite from granodiorite

K–Ar ages (Vedder and others, 1979):

14.5 ± 0.4 Ma and 14.2 ± 0.7 Ma on

plagioclase from dacite; 14.7 ± 0.3

Ma and 14.6 ± 0.3 Ma whole–rock on

dacite; 14.8 ± 0.3 Ma, 13.8 ± 0.3 Ma,

12.7 ± 1.1 Ma, and 12.4 ± 1.1 Ma

whole–rock on andesite; 13.2 ± 2.3

Ma and 13.0 ± 2.2 Ma on plagioclase

from andesite; 14.0 ± 2.7 Ma and 12.9

± 2.7 Ma whole–rock on basalt

K–Ar age (Hawkins and Divis, 1975): 15

Ma on biotite from dacite

Field relations: volcanic rocks appear to

be interstratified with fossiliferous

sedimentary rocks of middle and

late(?) Miocene age (Vedder and

Howell, 1976; Vedder and others,

1979)

33.36 -119.69 BR Unnamed rhyolite of Begg Rock Field relations: rhyolite of uncertain but

probable Miocene age is bounded by

seafloor outcrops of fossiliferous

sedimentary rocks of late early

Miocene to late(?) Miocene age

(Vedder and Howell, 1976)

33.22 -119.46 SN Basaltic and andesitic dikes of

San Nicolas Island

Field relations: dikes of uncertain but

probable Miocene age (Vedder and

Howell, 1976) intrude Eocene

sedimentary strata but not Pleistocene

deposits (Vedder and Norris, 1963)
17

32.92 -118.5 SCL Unnamed volcanic rocks

(andesite, dacite, rhyolite

and basalt) of San Clemente

Island

40Ar/39Ar incremental heating ages

(Luyendyk and others, 1998): 15.95 ±

0.15 Ma, 15.32 ± 0.04 Ma, 15.15 ±

0.15 Ma, and 14.48 ± 0.15 Ma

whole–rock on basaltic andesite

K–Ar ages (Turner, 1970): 15.4 ± 1 Ma*

on plagioclase from andesite; 13.6 ±

0.4 Ma* on plagioclase from rhyolite

K–Ar ages (Merifeld and others, 1971):

16.1 ± 0.8 Ma* and 15.9 ± 0.7 Ma*

whole–rock on andesite

Field relations: volcanic rocks are

interbedded with and overlain by

fossiliferous sedimentary rocks of late

early Miocene to middle Miocene

(Relizian to Luisian) age (Vedder and

Howell, 1976; Vedder and Moore,

1976)

32.75 -116.09 JV Volcanic rocks (basalt and

andesite) in

Jacumba–Vallecitos area,

San Diego and Imperial

Counties

K–Ar ages (Gastil and others, 1979): 20.5

± 2.0 Ma*, 19.2 ± 1.3 Ma*, and 16.4

± 1.0 Ma* whole–rock on andesite

and basalt; 19.0 ± 1.3 Ma* on

plagioclase from andesite; 19.1 ± 1.2

Ma* on hornblende from andesite

32.48 -117.10 RB Andesitic and basaltic volcanic

rocks of the Rosarito Beach

Formation of Minch (1967),

Tijuana–Ensenada area,

Mexico

40Ar/39Ar incremental heating ages

(Luyendyk and others, 1998): 16.2 ±

0.3 Ma, 16.14 ± 0.06 Ma, 15.9 ± 0.6

Ma, and 15.51 ± 0.14 Ma whole–rock

on basalt

K–Ar ages (Hawkins, 1970): 15.1 ± 1.3

Ma* and (or) 14.7 ± 2.6 whole–rock

on basalt

K–Ar age (Gastil and others, 1975): 16.5 ±

2.1 Ma on plagioclase from basaltic

andesite

Field relations: basaltic volcanic rocks are

interstratified with sparsely

fossiliferous sedimentary rocks of

Miocene age (Minch, 1967; Gastil

and others, 1975)

* Isotopic age from original reference has been corrected for changes in decay constants using the method

of Dalrymple (1979).
18

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