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Volcanic Rocks of the

Santa Maria Province, California

Age and Tectonic Inferences from a Condensed!?) 

Succession of Upper Cretaceous, Paleocene, and Eocene 

Strata, Big Pine Mountain Area, Santa Barbara County, 

California

Bulletin 1995-R, S

Geophysical section offshore Santa Maria basin

Geologic section onshore Santa Maria basin

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Volcanic Rocks of the

Santa Maria Province, California

By RONALD B. COLE and RICHARD G. STANLEY

Age and Tectonic Inferences from a Condensed!?) 

Succession of Upper Cretaceous, Paleocene, and 

Eocene Strata, Big Pine Mountain Area, Santa 

Barbara County, California

By JOHN G. VEDDER, RICHARD G. STANLEY, HUGH 

McLEAN, MARY LOU COTTON, MARK V. FILEWICZ, and 

DAVIDR. YORK

Chapters R and S are issued as a single volume 

and are not available separately

U.S. GEOLOGICAL SURVEY BULLETIN 1995

EVOLUTION OF SEDIMENTARY BASINS/ONSHORE OIL AND GAS INVESTIGATIONSSANTA MARIA PROVINCE

Edited by Margaret A. Keller
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Thomas J. Casadevall, Acting Director

Any use of trade, product, or firm names

in this publication is for descriptive purposes only

and does not imply endorsement by the U.S. Government

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1998

For sale by

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Library of Congress Cataloging in Publication Data

Volcanic rocks of the Santa Maria Province, California/by Ronald B. Cole 

and Richard G. Stanley. Age and tectonic inferences from a condensed!?) 

succession of Upper Cretaceous, Paleocene, and Eocene strata, Big Pine 

Mountain area, Santa Barbara County, California/by John G. Vedder...[et al.].

p. cm. U.S. Geological Survey bulletin ; 1995)

(Evolution of sedimentary basins/onshore oil and gas investigations Santa 

Maria Province; ch. R, S)

"Chapters R and S are issued as a single volume and are not available 

separately."

Includes bibliographical references.

Supt. of Docs, no.: I 19.3:1995-R, S.

1. Rocks, Igneous California Santa Maria Basin. 2. Geology, Stratigraphic Tertiary. 3. Geology California Santa Maria Basin. 4. Santa Maria 

Basin (Calif.) 5. Mudstone California Big Pine Mountain Region. I. Stanley, 

Richard G. II. Vedder, John G. III. Title. IV. Title: Age and tectonic inferences 

from a condensed(?) succession of Upper Cretaceous, Paleocene, and Eocene 

strata, Big Pine Mountain area, Santa

Barbara County, California. V. Series. VI. Series: Evolution of sedimentary 

basins/onshore oil and gas investigations Santa Maria Province ; ch. R, S.

QE75.B9 no. 1995-R, S 

[QE461)

557.3s dc21 

[552'.2'097949]

97-48362 

CIP
Chapter R

Volcanic Rocks of the

Santa Maria Province, California

By RONALD B. COLE and RICHARD G. STANLEY

U.S. GEOLOGICAL SURVEY BULLETIN 1995

EVOLUTION OF SEDIMENTARY BASINS/ONSHORE OIL AND GAS INVESTIGATIONSSANTA MARIA PROVINCE

Edited by Margaret A. Keller
CONTENTS

Abstract Rl

Introduction Rl

Acknowledgements R3

Stratigraphy, ages, and descriptions of volcanic rocks R3

Tranquillon Volcanics of Dibblee (1950) R3

Unnamed basaltic andesite of Santa Rosa Creek R6

Unnamed lower Miocene tuffs of the Santa Barbara coast R7

Tuff deposits in the Lospe Formation near Point Sal R7

Sill near Point Sal RIO

Obispo Formation and related rocks Rll

Unnamed volcanic rocks of the Catway Road area R13

Unnamed volcanic rocks of the Lopez Mountain area R14

Morro Rock-Islay Hill complex R14

Cambria Felsite R14

Unnamed volcanic rocks of upper Pine Creek R14

Subsurface volcanic rocks (onshore and offshore) R15

Other occurrences of volcanic rocks R15 

Geochemistry of volcanic rocks R16 

Volcanic source areas and paleogeography R19 

Relations of volcanism to regional tectonic events R22 

Conclusions R24 

References cited R25 

Appendixes 

1. Data, interpretations, and assumptions used to construct the stratigraphic chart for 

middle Tertiary volcanic units in the Santa Maria province (fig. 2) R29

2. Compilation of available major element and selected trace element data for 

volcanic rocks of the Santa Maria area R31

3. Subsurface data used to construct lower Miocene volcanic thickness map of figure 15 R34

FIGURES

1. Map of the Santa Maria province R2

2. Stratigraphic chart for middle Tertiary volcanic units in the Santa Maria province R4

3. Schematic stratigraphic column of the Tranquillon Volcanics of Dibblee (1950) near 

Tranquillon Mountain R6

4. Photographs of the Tranquillon Volcanics of Dibblee (1950) in its type area near 

Tranquillon Mountain R6

5. Unnamed basaltic andesite near Santa Rosa Creek in an outcrop about 25 km east 

of Tranquillon Mountain R7

6. Schematic composite stratigraphic section of the Lospe Formation in its type area 

near Point Sal R8

7. Photographs of tuff deposits in the Lospe Formation in its type area near Point 

Sal. R9

8. Composite stratigraphy of subaqueous tuffs in the upper member of the Lospe 

Formation that were deposited as reworked ash from high-density turbidity 

flows RIO
9. Photomicrograph under cross-polarized light of trachybasalt sill that intrudes the 

Point Sal Formation near Point Sal Rll

10. Representative strati graphic columns of the Obispo Formation drawn from original 

field data of this study combined with data reported by Schneider and Fisher 

(1996) Rll

11. Photographs of the Obispo Formation R12

12. Catway volcanics in the Catway Road area R13

13. Photomicrograph of unnamed basalt at Lopez Mountain under cross-polarized 

light R14

14. Photographs of intrusion at Morro Rock R15

15. Thickness map of lower Miocene volcanic rocks in part of the Santa Maria 

province R16

16. Silica (SiO2) versus total alkalis (Na2O + K2O) for Santa Maria province volcanic 

rocks showing lithologic classifications of LeBas and others (1986) R17

17. Silica (SiO2) versus titanium (TiO2), magnesium (MgO), sodium (Na2O), and 

potassium (K2O) for volcanic units in the Santa Maria province R18

18. Samarium (Sm) versus hafnium (Hf) variation diagram for selected 20-16 Ma 

volcanic units in the Santa Maria province R19

19. Lanthanum (La) versus samarium (Sm), ytterbium (Yb), and niobium (Nb) for 

selected 20-16 Ma volcanic units in the Santa Maria province R20

20. Early Miocene paleogeographic reconstruction for the Santa Maria province and 

simplified model of block rotations and zones of crustal extension R23

TABLE

1. Compilation of available radiometric ages for volcanic rocks in and around the 

Santa Maria province R5

Volcanic Rocks of the Santa Maria Province, California

By Ronald B. Cole1 and Richard G. Stanley

ABSTRACT

Volcanic and intrusive rocks within the Santa Maria 

province of central California comprise two age groups 

(28-23 Ma and 20-1 6 Ma) that record at least two separate episodes of crustal extension or transtension. These 

episodes occurred during a transition from a convergent 

to a transform plate boundary in western California as 

segments of the Farallon-Pacific spreading ridge intersected the continental margin in the vicinity of the Santa 

Maria province. The source for the volcanic and intrusive rocks included basaltic upper-mantle-derived magmas (similar to mid-ocean ridge basalt magmas) that 

melted and assimilated silicic crustal rocks to form a bimodal igneous suite.

Santa Maria province volcanic units include subaerial 

and subaqueous acidic pyroclastic flow deposits, pyroclastic fallout deposits, basaltic breccias, volcaniclastic 

turbidites, and various basaltic flows. Felsic and mafic 

intrusive rocks are also present within the Santa Maria 

province. Based on a combination of surface and subsurface data, the thickness of lower Miocene volcanic rocks 

ranges from zero in much of the central Santa Maria province (including the Santa Maria basin) to more than 4,900 

feet (1,490 meters) in places along the margins. Major 

eruptive centers identified on the basis of volcanic fades, thickness of volcanic units, paleoflow data, and 

paleobathymetry were most likely present in the northern and southern parts of the Santa Maria province along 

the Santa Maria River and Santa Ynez River Faults, respectively. These eruptive centers probably coincided with 

zones of extension that resulted from clockwise rotation 

of the western Transverse Ranges and slivering of the Santa 

Maria basin by northwest-trending faults. Eruptive centers for Santa Maria volcanism may also have been along 

the Hosgri Fault Zone in the present offshore Santa Maria 

basin, where reported lower Miocene volcanic rocks in 

the subsurface locally exceed 900 feet (274 meters) in 

thickness.

INTRODUCTION

The Santa Maria province of central California contains several middle Tertiary volcanic units that provide

'Department of Geology, Allegheny College, Meadville, PA 16335. 

Manuscript approved for publication January 13, 1998

regional and local stratigraphic marker horizons in 

petroliferous sedimentary basins, including the onshore and 

offshore Santa Maria basins (fig. 1). Volcanism and coeval basin development in the Santa Maria province were 

coincident with a transition from convergent to transform 

plate boundaries along western California as segments of 

the Farallon-Pacific spreading ridge intersected North 

America (Atwater, 1970, 1989). This paper summarizes 

and combines existing data with new information on the 

stratigraphy, distribution, depositional histories, geochemistry, and tectonic implications of Santa Maria province 

volcanic units. This information is important for understanding the relations between middle Tertiary volcanic 

events, basin development, and tectonism in west-central 

California.

Volcanic rocks have long been recognized and mapped 

within the Santa Maria province (for example, Woodring 

and Bramlette, 1950; Dibblee, 1950, 1966, 1988a,b, 1989; 

Hall and Corbato, 1967; Vedder and others, 1967, 1994; 

Hall, 1973, 1978, 1982; Ernst and Hall, 1974; Hall and 

others, 1979; McLean, 1994). Within this area more than a 

dozen separate volcanic units have been identified, with 

exposures that range from small outcrops of tens to hundreds of square meters to regional outcrop belts extending 

for tens of kilometers (fig. 1). The volcanic units, some of 

which are not yet formally named, include a wide array of 

mafic and felsic lavas and pyroclastic deposits that are 

interbedded with nonmarine and marine sedimentary rocks. 

The volcanic rocks overall preserve a history of diverse 

eruption styles and deposition around several eruptive centers within the Santa Maria province. Hall and others 

(1966), Fisher (1977), and Surdam and Hall (1984) discuss aspects of the Obispo Formation, one of the larger 

volcanic units in the region, which includes a diverse assemblage of mafic to felsic pyroclastic deposits and lavas. 

Recently in another paper of this bulletin, Schneider and 

Fisher (1996) interpret thick pyroclastic units of the Obispo 

Formation as submarine deposits of remobilized ash. The 

work of Robyn (1979, 1980) represents the first focused 

analysis of felsic pyroclastic rocks at Tranquillon Mountain, which we believe is in the vicinity of an important 

eruptive center that existed in the southern portion of the 

Santa Maria province. Cole and Stanley (1994) interpret 

felsic tuffs in the Lospe Formation as the deposits of pyroclastic flows and subaqueous sediment gravity flows of

Volcanic Rocks of the Santa Maria Province, California R1
reworked ash that was probably derived from eruptions in 

the vicinity of Tranquillon Mountain. In addition, several 

smaller, less well exposed volcanic units within the Santa 

Maria province have been described elsewhere (for example, Ballance and others, 1983; Vedder and others, 

1991; McLean, 1994; Hornafius, 1994). This paper provides the first coherent summary of information on Santa 

Maria province volcanic rocks that allows for new inter121° 00' 120° 30' 120° 00'

35° 30'

35° 00'

34° 30'

Offshore

Santa Maria

basin

El Capitan .. .. beach Naples

20 KILOMETERS Sanfa Barbara coastal area

Figure 1 . Map of the Santa Maria province showing outcrop locations of middle Tertiary igneous rocks (Obispo Formation 

represented by vertical lines; other units shaded black), major faults, the onshore Santa Maria basin (stippled), and the 

offshore Santa Maria basin. Abbreviations: CB, Catway volcanics; CF, Cambria Felsite; LT, tuffs in the Lospe Formation; 

MR-IH, Morro Rock-lslay Hill complex; SRB, basaltic andesite near Santa Rosa Creek; TB, sill at Point Sal. Map 

compiled from Jennings (1958, 1959) and Pacific Gas and Electric Company (1988).

R2 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
pretations of a regional history of volcanism in the Santa 

Maria province.

Although volcanism was integral in the geologic history of the Santa Maria province, previous work that addressed the origin and tectonic significance of volcanic 

rocks in the Santa Maria province is sparse. Dickinson and 

Snyder (1979) proposed one of the first regional tectonic 

models in which volcanism that occurred in the Santa Maria 

province was attributed to a northwest-migrating 

asthenospheric slab window that formed in conjunction 

with the Mendocino Triple Junction. In a different interpretation, Hall (198la) proposed that some of the Santa 

Maria province volcanic rocks were erupted into a continental marine environment along a leaky transform fault 

that was associated with the intersection of the FarallonPacific spreading ridge with western California. Since this 

earlier work, we have gained more and better stratigraphic 

and radiometric age control as well as geochemical data 

on these volcanic rocks. With these new data, which are 

described in this paper, we are able to offer alternative tectonic models to explain the origin of Santa Maria volcanism in relation to ridge-trench interactions (Cole and Basu, 

1992, 1995; McCrory and others, 1995) and the ensuing 

transtension and block rotations that occurred during development of the Santa Maria basin (Hornafius and other, 

1986; Stanley and others, 1996).

ACKNOWLEDGMENTS

This research was supported in part by grants from 

the American Chemical Society-Petroleum Research Fund 

and the Geological Society of America. We thank J.G. 

Vedder, K.J. Bird, Hugh McLean, and M.A. Keller for their 

informative reviews. We also benefited from discussions 

with J.G. Vedder, Hugh McLean, K.J. Bird, R.V. Fisher, 

M.A. Keller, A.R. Basu, J.L. Schneider, and P.W Weigand. 

We thank the U.S. Air Force for access to outcrops located 

on Vandenberg Air Force Base and Maurice Twitchell for 

allowing access to private land. N.A. Frambes provided 

assistance in manuscript preparation.

STRATIGRAPHY, AGES, AND DESCRIPTIONS 

OF VOLCANIC UNITS

Volcanic rocks in the Santa Maria province range in 

age from late Oligocene to early Miocene (fig. 2, table 1, 

app. 1). On the basis of available radiometric dates there 

appear to be two age groups of Santa Maria volcanic rocks. 

An older group exhibits radiometric ages ranging from 

28.0+1.0 to 22.7+0.9 Ma (29.0 to 21.9 Ma including the 

range of reported errors; table 1); we refer to the age range 

of this group as about 28 to 23 Ma. A younger group exhibits radiometric ages ranging from 20.3+1.6 to 15.7+0.9

Ma (21.9 to 14.7 Ma including the range of reported errors; table 1); we refer to the age range of this group as 

about 20 to 16 Ma. Within these two groups are several 

major volcanic units, which are described below in terms 

of local stratigraphy, ages, thicknesses, petrographic characteristics, and possible origins.

Tranquillon Volcanics of Dibblee (1950)

Volcanic rocks on and near Tranquillon Mountain were 

mapped and described by Dibblee (1950) as "1200 feet of 

rhyolite, agglomerate, and ash exposed on Tranquillon 

Mountain ridge and vicinity." Dibblee (1950) designated 

these rocks the Tranquillon Volcanics and proposed a type 

section along a ridge between Tranquillon Mountain and 

Point Arguello (fig. 1). Based on measured sections and 

detailed stratigraphic analyses by Robyn (1980), combined 

with our field investigation, these rocks consist of up to 

150 m of nonwelded and welded rhyolitic tuff and lapilli 

tuff and minor basalt (figs. 3, 4). The tuff and lapilli tuff 

most likely formed by combinations of pyroclastic flows 

and pyroclastic fallout (Robyn, 1980; Cole and others, 

199la). Detailed petrographic characteristics of the type 

Tranquillon volcanic rocks are given by Robyn (1979, 

1980). In general these deposits consist of 60-90 percent 

glass shards, 15-40 percent pumice, 5-15 percent crystals, 

and less than 5 percent sedimentary lithic fragments. Vitric 

components are in places diagenetically altered to analcime and clinoptilelite and also have devitrified to potassium feldspar and quartz (cristobalite) (Robyn, 1980). The 

welded-tuff intervals are defined by a eutaxitic texture 

where pumice grains and other glass shards are flattened 

and aligned parallel to bedding (fig. 4A). These flattened 

pumice grains and glass shards are commonly bent and 

deformed around crystals. Crystal components include 

mostly euhedral to subhedral quartz and sanidine with 

minor amounts of albite (less than 5 percent). Many crystals show smooth and embayed boundaries.

In the vicinity of Tranquillon Mountain, the 

Tranquillon Volcanics unconformably overlie the Rincon 

Shale (Dibblee, 1950) and are unconformably overlain by 

the Monterey Formation (fig. 2; Dibblee, 1950, 1988b). 

Near Tranquillon Mountain the lower 30 cm of the 

Tranquillon Volcanics is a pebble-granule conglomerate 

composed mainly of dolomite clasts and abundant robust, 

shallow marine invertebrate fossils. The dolomite clasts 

are probably recycled dolomite concretions derived from 

the underlying Rincon Shale (Stanley and others, 1996). 

The fossils include Turritella ocoyana topangensis 

Merriam, Turritella temblorensis Wiedey, and Spondylus 

perrini Wiedey, all of which are representative of the uppermost "Vaqueros" or "Temblor" molluscan "Stages" 

(early Miocene) (J.G. Vedder, oral commun., 1987; Stanley 

and others, 1996). A single sample of welded tuff from

Volcanic Rocks of the Santa Maria Province, California R3
I

AGE 

(Ma) STAGE1 Morro Rock- Islay Hill area San Luis Obispo- Cambria area Lopez Mountain Point Sal area Figueroa Mountain Santa Rosa Creek Tranquillon Mountain Santa Barbara coastal area Offshore Santa Maria basin9

16-

18-

20-22-24-

26-28-

Luisian

Relizian

Saucesian

Zemorrian

Monterey 

Formation

Monterey 

Formation

Monterey 

Formation

Monterey 

Formation

Monterey 

Formation

Monterey 

Formation

Monterey 

Formation

Rincon Shale

Vaqueros Formation

Unnamed 

sandstone and conglomerate5

Unnamed marine

sandstone 

and mudstone

Rincon Shale Rincon Shale Rincon Shale

Vaqueros 

Formation

Vaqueros 

Formation

Vaqueros 

Formation

Sespe 

Formation

1 Modified from Kleinpell (1938,1980).

2Lospe Formation of Hall (1974), Hall and Prior (1975), and Hall and others (1979); Lospe Formation(?) of Ernst and Hall (1974). 3Cambria Felsite of Ernst and Hall (1974). 4Unnamed volcanic rocks of McLean (1994, map units Tvt and Tvf). 

5Unnamed sandstone and conglomerate of McLean (1994, map unitTsc).6Type Lospe Formation (Wissler and Dreyer, 1943, p.237); consists of sedimentary and minor volcanic rocks discussed in text. 7Tranquillon Volcanics of Dibblee (1950).

8Called by various informal names including "Bentonite Bed" (Kleinpell and Weaver, 1963, p. 11), Tranquillon bentonite (Hornafius, 1994, p. 52), 

Tranquillon tuff (Hornafius, 1994, p. 52), and Summerland rhyolite tuff (Turner, 1970, p. 118). 

9AfterMcCulloch(1989).

Figure 2. Stratigraphic chart for middle Tertiary volcanic units (gray shaded areas) in the Santa Maria province. Vertical lines represent missing time along unconformities. 

Data, interpretations, and assumptions used in constructing this chart are given in appendix 1.
Table 1 . Compilation of available radiometric ages for volcanic rocks in and around the Santa Maria province.

ST

Volcanic unitTranquillon Volcanics of Dibblee (1950)

Unnamed basaltic andesite of Santa Rosa CreekUnnamed lower Miocene tuffs of the Santa Barbara coast:

Summerland rhyolite tuff (informal name)

Tuff near base of Monterey Formation at Naples

Tuff deposits in the Lospe Formation near Point Sal

Obispo Formation

Unnamed volcanic rocks of the Catway Road areaUnnamed volcanic rocks of the Lopez Mountain area

Morro Rock-Islay Hill complex

Cambria Felsite

Unnamed volcanic rocks of upper Pine CreekUnnamed basalt at Hell's Half Acre (about 9 km southeast

of Figueroa Mountain)

Basalt in the lower part of the Simmler Formation about 30

km northeast of Nipomo

Age (Ma) 1

17.80 + 0.05

17.4+1.217.2 + 0.5

16.5 + 0.6

18.42 + 0.06

17.70 + 0.0217.39 + 0.06

15.7±0.9to 16.9+1.2

17.2 + 2.5

20.3 ±1.6

18.8+ 1.517.0 + 0.5

27.1 + 0.8

25.6+1.222.7 + 0.927.2 + 0.828.0+1.026.0 + 0.825.1 ±0.8

27 to 26.5

26.6 ±0.5

18.5 ±2.0

23.4 ±0.822.9 ±0.7

Method

40Ar/39Ar

40K/°Ar40K/°Ar

... do 

40Ar/39Ar40Ar/39Ar

... do 

40K/

wAr

40K/°Ar

... do 

do 

40K/

wAr

40K/40Ar

... do 

... do 

... do 

... do ... do ... do 

40Ar/39Ar

40K/40Ar40K/°Ar40K/°Ar

... do 

Mineral

sanidine

whole rock

plagioclase

... do 

sanidinesanidine

plagioclaseplagioclase

??

plagioclaseplagioclase

biotite

plagioclaseplagioclase

sanidine

biotite

biotite

whole rock

?

plagioclase

whole rock

whole rock

... do 

References

Stanley and others (1996)

Turner (1970)Turner (1970)

..... do

Stanley and others (1996)

Stanley and others (1996)

.......... do

Turner (1970)

Hall (198 la)

..... do - Vedder and others (1994)

McLean (1994)

Turner (1968)

..... do

..... do..... do

Buckley (1986)

..... do..... do

unpublished data; J.D. Obradovich, oral

commun. (1994); M.E. Tennyson

and M.A. Keller, oral commun.(1994); M.A. Mason and C.C.S wisher, oral commun. (1990)

Vedder and others (1991)Fritsche and Thomas (1990)

Ballance and others (1983)

............. do -

] K/Ar ages from Turner (1968, 1970) and Hall (1981a) corrected using methods described in Dalrymple (1979).
near the top of Tranquillon Mountain yielded an 40Ar/39Ar 

age of 17.80+0.05 Ma on sanidine (Stanley and others, 

1996). These ages are consistent with other biostratigraphic 

data that constrain the Tranquillon volcanic rocks as early 

Miocene (Dibblee, 1950). Based on differences between 

the Tranquillon volcanic rocks and tuffs in the Obispo Formation in major element chemistry, stratigraphy, and depositional settings, Robyn (1979, 1980) suggested that these

EPOCH

Miocene

UNIT

^fl> -n 

§^ Tranquillon Volcanics 3 

§u

o ro

C JC cE w

LITHOLOGY

 

- " / \ - x 

'^'^ 

'1 '" /-I '

N *- N x 

~ S , ~ /

'^x'"'^ 

'^ 1 x ,/^ 1

_ ~

~ ' / ~\~ '

'- x'"'^

'- o '"- '

N x ^ N x

~ / , - /

' -vV -

'-xl -_r-x'

II

^~~C:~

- / / - - /

/ ^ N / ; ^

°°o°c ° °°o'

 _

THICKNESS (IN METERS)

11-27

4-28

5-16

8-15

4-9

1-5

0.3

DESCRIPTION

White to pink and yellow 

nonwelded tuff

White to pink-white and 

yellow-white welded 

tuff and lapilli tuff

White to white-green 

nonwelded tuff

Gray to brown welded 

pumice lapilli tuff

Black welded vitric tuff

White to yellow bentonite

Pebble-granule conglomerate 

, with clasts of dolomite and 

\ robust fossils (see text for 

\ details)

Figure 3. Schematic stratigraphic column of the Tranquillon 

Volcanics of Dibblee (1950) near Tranquillon Mountain. 

Modified from Robyn (1980).

Figure 4. Photographs of the Tranquillon Volcanics of Dibblee 

(1950) in its type area near Tranquillon Mountain. A, 

Photomicrograph of welded rhyolite lapilli tuff. Note eutaxitic 

texture where glass shards are deformed around crystal 

fragments. Abbreviations: Q, quartz; AP, altered pumice; S, 

sedimentary lithic fragment. Field of view is 6 mm across. B, 

Massive spheroidal "bomb" bed (SB; fallout deposit) overlain 

by a welded lapilli tuff (WT; pyroclastic flow deposit). Contact 

is shown as dashed line. Hammer is 27 cm long.

two volcanic units are separate and were erupted from separate sources on opposite sides of the Santa Maria basin. We 

agree with Robyn (1979, 1980) and discuss possible volcanic 

sources and paleogeography more fully in a later section.

Unnamed Basaltic Andesite of Santa Rosa Creek

Basaltic rocks mapped as Tranquillon Volcanic Formation by Dibblee (1988a) crop out about 25 km east of 

Tranquillon Mountain near Santa Rosa Creek (fig. 1; 

Dibblee, 1950, 1988a). This deposit consists of basaltic 

andesite breccia of unknown thickness that stratigraphically 

overlies the Rincon Shale and underlies the Monterey Formation (Dibblee, 1950, 1988a; figs. 2, 5). The unit includes 

blocks of aphanitic basaltic andesite that consist of very 

fine grained plagioclase laths in pilotaxitic texture. Minor 

amounts of brown glass and iron-titanium oxides are 

present between the plagioclase laths. Some small blocks 

of mudstone, perhaps derived from the underlying Rincon 

Shale, are incorporated into the basaltic andesite breccia. 

Mudstone along with sand- to pebble-sized basaltic andesite fragments form a matrix between the coarser blocks. A 

radiometric date for this basalt from Turner (1970), corR6 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
rected with the methods of Dalrymple (1979), is 17.4+1.2 

Ma, which is about the same age as the rhyolitic volcanic 

rocks at Tranquillon Mountain.

The blocky and angular nature of basaltic andesite 

clasts in this deposit as well as the close-fitting fabric of 

the clasts suggest that this volcanic unit may have been a 

subaqueous lava flow that was rapidly quenched and shattered by water. This origin is consistent with the marine 

depositional setting implied by the nonvolcanic marine 

deposits that underlie and overlie the basaltic andesite breccia. It is likely that these breccias were derived from a 

nearby source because of the close-fitting clast fabric, 

which suggests minimal, if any, postdepositional transport.

Unnamed Lower Miocene Tuffs of the Santa 

Barbara Coast

A thin tuff bed was mapped by Dibblee (1966) within 

the lowest part of the Monterey Formation along the coast 

near Santa Barbara (fig. 1), directly above the contact between the Monterey and the overlying Rincon Shale (fig. 

2). Biostratigraphic data from the Rincon Shale and the 

Monterey Formation (Dibblee, 1950, 1966; Kleinpell and 

Weaver, 1963; Turner, 1970; DePaolo and Finger, 1991; 

Stanley and others, 1992b, 1994) constrain the age of this 

tuff to early Miocene (upper part of the Saucesian Stage of 

Kleinpell, 1938). This tuff, locally and informally known 

as the Tranquillon bentonite and (or) Tranquillon tuff 

(Hornafius, 1994, p. 52) and the Summerland rhyolite tuff 

(Turner, 1970, p. 118), is an important stratigraphic marker 

because it appears to occur at about the same stratigraphic 

level as the Tranquillon Volcanics of Dibblee (1950). At 

one site near the town of Summerland (about 75 km east 

of Point Conception) this tuff is 1.8 m thick and yields 

radiometric dates of 16.5±0.6 and 17.2+0.5 Ma (Turner, 

1970, corrected using methods of Dalrymple, 1979). A 

sample of altered tuff from near the base of the Monterey 

Formation in a sea-cliff exposure near Naples yielded a 

single crystal laser fusion 40Ar/39Ar age of 18.42+0.06 

Ma (Stanley and others, 1996). At a site about 40 km 

west of Santa Barbara, a tuff in the same stratigraphic 

position is described by Stanley and others (1992b) as 

a 70-cm-thick weathered, soft, poorly exposed, apparently felsic tuff.

Hornafius (1994) also describes two other related sets of 

tuff beds within strata of the Monterey Formation that are assigned to the Upper Saucesian Stage of Kleinpell (1938). One 

tuff, sample NB440 of group lo (Hornafius, 1994, p. 52), includes a dacite vitric tuff about 440 ft (134 m) above the base 

of the Monterey Formation at Naples (fig. 1). This tuff is chemically similar to acidic tuffs of the Obispo Formation (Hornafius, 

1994; app. 2). Another set, group 2 (Hornafius, 1994, p. 52, 

57), includes three separate tuff beds at about 151 ft (46 m), 

288 ft (87.8 m), and 400 ft (121.9 m) above the base of the

Monterey Formation at Naples and a tuff bed at about 250 

ft (76.2 m) above the base of the Monterey Formation at 

El Capitan Beach. The group 2 tuffs include intermediate 

compositions (basaltic andesite-andesite) and dacite 

(Hornafius, 1994, table Ib; app. 2).

The absence of tuff beds in the Rincon Shale beneath 

the Tranquillon tuff at El Capitan Beach led Hornafius 

(1994) to suggest that the Tranquillon tuff represents the 

oldest Miocene volcanic ash in the Santa Barbara coastal 

area. The Tranquillon tuff is also the thickest lower Miocene ash deposit in the area, reaching up to 60 ft (18.3 m) 

offshore near Gaviota and thinning eastward to about 5 ft 

(1.5 m) in the offshore near Santa Barbara (Hornafius, 

1994). This thickness variation may represent thinning 

away from a volcanic source that could have been located 

toward the west at Tranquillon Mountain, as suggested by 

Dibblee (1950) and Hornafius (1994). Overall, the lower 

Miocene tuff layers of the Santa Barbara coast generally 

decrease in thickness with increasing stratigraphic height 

within the Monterey Formation (Hornafius, 1994, p. 45). 

This pattern could reflect a decrease in the volume of ash 

due to waning explosive volcanism through lower Miocene 

time in the Santa Maria province. The origin of thin Middle 

Miocene (Luisian) ash beds in the Monterey Formation in 

this area is uncertain, but they could have been the ash 

fallout deposits from volcanic sources outside of the Santa 

Maria province (Hornafius, 1994).

Tuff Deposits in the Lospe Formation Near 

Point Sal

The Lospe Formation near Point Sal is an 800-m-thick 

interval of nonmarine to shallow marine sedimentary rocks

Figure 5. Unnamed basaltic andesite near Santa Rosa Creek in 

an outcrop about 25 km east of Tranquillon Mountain. Dark 

clasts are blocks of aphanitic basaltic andesite. Light clasts are 

blocks of mudstone, probably derived from the underlying 

Rincon Shale. Matrix material between clasts consists of 

mudstone and sand- to pebble-sized basaltic andesite fragments.

Volcanic Rocks of the Santa Maria Province, California R7
with interbedded volcanic rocks (Woodring and Bramlette, 

1950; Stanley and others, 1990, 1991; Johnson and Stanley, 

1994). The Lospe Formation records initial subsidence of 

the onshore Santa Maria basin (Stanley and others, 1992a, 

1996; McCrory and others, 1995). The lower member of 

the Lospe Formation is about 210 m thick and consists of 

alluvial fan and fan-delta deposits (Johnson and Stanley,

Q 

O

o:

LU 

CL

TERTIARY

I

o 

o

CL 

LU

Miocene

JURASSIC

UNIT

Point Sal 

Formation

c 

o

"fo

E 

o

LL

<D 

Q. 

W 

O

Upper member

Lower member

Point Sal 

Ophiolite

LITHOLOGY

~ ~ ' ~ ' ~ ~ ' '

"vWWW^

 '.~ .~ ~

-I-. I:. I7.-

~ ~ '

v: V' '('"

~ ~ ~ ~ 

l^ ^- 

^V^'^'v''

 - - 

-".'' / ''

.._..__.._

ifriiifrli-rT:

_. . . . . ._

: V/^r^

'-.-. =" '. <=."-_'- =

-; >"- ; 5."-j.;-;.'r

CLV'T-'-Q-'T'-C;,''^"''

 Cj-'r'-ci-'r'-ci-'^

cy <r''- .' r'-c ,-V1

'_; <= ',.- => -, =

-'^^CJ.'^Vv' .'

 > -<T ' -=-_) ' ~^ ' _,-'-. '

'. 

THICKNESS (IN METERS)

25-30

15-30

200- 

600

ISO210

?

DESCRIPTION

Marine deposits

Trachybasalt sill

Marine deposits

Shallow marine 

deposits

Lacustrine 

deposits

Fan delta and 

alluvial fan 

deposits

(of Hopson and 

Frano, 1977)

EXPLANATION___

Mostly shale, siltstone, ' ,'.'' '' Rhyolite and 

and fine sandstone dacite tuff

Mostly sandstone and ~~^-^~ Unconformity 

conglomerate

Figure 6. Schematic composite stratigraphic section of the Lospe 

Formation in its type area near Point Sal. Generalized from 

Stanley and others (1991, 1996).

1994), whereas the upper member consists of about 600 m 

of interbedded turbidite sandstone and mudstone that were 

deposited in a lake (Stanley and others, 1990). Interbedded 

with the deposits of both members are several rhyolite and 

dacite tuffs (figs. 6, 7). These tuff deposits include 15-cm 

to 6-m-thick intervals in the alluvial-fan and fan-delta deposits of the lower member of the Lospe Formation, and 

up to 20-m-thick deposits in the lacustrine deposits of the 

upper member. The nonvolcanic deposits of the Lospe Formation provide a basis for interpreting depositional environments of the interbedded tuffs; those tuffs in the lower 

member of the Lospe Formation are most probably subaerial, whereas those in the upper member are most likely 

subaqueous lacustrine deposits.

The tuffs within the lower, conglomeratic member of 

the Lospe Formation are variable in composition and 

include vitric, lithic, and crystal tuffs. Several thin- to 

medium-bedded (15-30 cm thick) lithic-rich tuffs are 

interbedded with streamflow conglomerate and sandstone and may represent reworked volcaniclastic deposits (fig. 1A). Thicker tuff intervals range from 1.5 to 6 

meters thick and consist mostly of vitric shards with 

few lithic fragments derived from the Jurassic Point Sal 

ophiolite of Hopson and Frano (1977) (fig. IB). These 

thicker tuffs are massive with inverse grading at the 

bases of beds and exhibit alignment of elongate clasts 

parallel to bedding. These tuffs were most likely deposited by subaerial pyroclastic flows (Cole and others, 199la; Cole and Stanley, 1994).

Most tuff deposits in the upper member of the 

Lospe Formation are vitric-rich and consist of 95-98 

percent vitric shards (cuspate fragments and pumice) 

and 2-5 percent crystals (quartz and plagioclase) (fig. 

1C) (Cole and Stanley, 1994). These tuff deposits exhibit repetitive and predictable stratigraphic and sedimentologic characteristics that are illustrated by a 

composite unit shown in figure 8. The tuffs in the upper member can be attributed to deposition of reworked 

ash in the form of high-density turbidity flows (fig. 8) 

(Cole and others, 1991a; Cole and Stanley, 1994). Individual turbidite intervals range from 2 to 12m thick 

and are amalgamated to form mappable tuff horizons 

that are up to 20 m thick (fig. ID).

One tuff in the upper member of the Lospe Formation 

consists of a 65-cm-thick cross-bedded crystal- and vitrictuff bed (figs. IE, IF). Restored paleoflow direction based 

on the dip direction of large-scale foresets is toward the 

northeast, suggesting flow from the southwest. This crossbedded tuff is interbedded with lacustrine mudstone and 

presumably represents subaqueous reworking by unidirectional currents. Very limited exposure precludes a detailed 

interpretation for the origin of this tuff bed .

Single-crystal 40Ar/39Ar analyses of sanidine from 

two nonwelded tuffs in the Lospe Formation yield ages 

of 17.70+0.02 and 17.39+0.06 Ma (Stanley and others,

R8 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
Figure 7. Photographs of tuff deposits in the Lospe Formation in its type area near Point Sal. A, Thin reworked vitric-lithic tuff (T) 

interbedded with alluvial-fan deposits (AF) in the lower member of the Lospe Formation. B, Subaerial pyroclastic flow deposit of the 

lower member of the Lospe Formation. Lapilli-sized clasts are fragments of Point Sal ophiolite. C, Photomicrograph of subaqueous 

vitric tuff in the upper member of the Lospe Formation under plane-polarized light. Note the cuspate and bladed shapes of vitric 

shards; field of view is 3 mm across. D, Subaqueous vitric tuff interbedded with shale and fine-grained sandstone of the upper 

member of the Lospe Formation. Tuff unit is 20 m thick. Arrows point to intervals of the tuff deposit that are planar or thin bedded. 

E, Cross-bedded vitric-crystal tuff in the upper member of the Lospe Formation exposed at Point Sal. F, Photomicrograph of vitriccrystal tuff in the upper member of the Lospe Formation under cross-polarized light. Crystals of subrounded to subangular plagioclase 

are aligned with their long axes parallel to bedding. The plagioclase grains are set in a finer grained matrix of vitric shards (dark 

grains). Field of view is 3 mm across.

Volcanic Rocks of the Santa Maria Province, California R9
1996). These dates are for tuff samples from 30 m and 

210 m above the base of the Lospe Formation at North 

Beach, respectively. These ages plus the presence of early 

Miocene palynomorphs and benthic foraminifers in the 

upper member of the Lospe Formation indicate a late early 

Miocene age for the volcanism that was the source of the 

tuffs in the Lospe Formation. Locations of possible eruptive sources for these tuffs include the Tranquillon Mountain area to the south, regions now located in the offshore 

Santa Maria basin to the south and west, and possibly regions north of Point San Luis (fig. 1). A more detailed discussion of potential eruptive centers and the potential 

source(s) of volcaniclastic rocks in the Santa Maria province is given in a later section.

Sill Near Point Sal

A trachybasalt sill is present within the lower part 

of the Point Sal Formation in the vicinity of Point Sal 

(fig. 1) (Dibblee, 1989). This intrusive unit is about 25- 

30 m thick and consists of plagioclase, pyroxene, and 

altered olivine (iddingsite) (fig. 9). Parts of the unit are 

coarsely crystalline and can be considered a diabase. 

Portions of the matrix material between crystals exhibit 

axiolitic and fibrous textures. This sill intrudes the 

lower part of the Point Sal Formation, which contains 

benthic foraminifers of the lower Miocene Relizian 

Stage (Woodring and Bramlette, 1950); therefore, the 

sill is of early Miocene age or younger (fig. 2). This

Nonvolcanic shale, siltstone, 

and fine sandstone

Thin to medium beds of tuff with 

sharp basal contacts, pumice-rich 

tops, diffuse planar laminae, 

dewatering structures, and 

laminated mudstone drapes

Lacustrine turbidite deposits

Small-volume, high-density 

volcaniclastic turbidity and liquefied 

flow deposits; combined tractive and 

suspension sedimentation processes

Massive vitric tuff with outsized 

intraclastic blocks and alignment 

of elongate grains parallel to 

bedding

Large-volume, high-density volcaniclastic 

turbidity flow; grain-support mechanisms 

included matrix buoyancy and turbulence; 

deposited primarily as suspended 

sediment load

Plane-bedded vitric tuff; inverse 

grading; clast imbrication; 

grades up into massive tuff

Traction carpet deposits of overlying 

massive tuff; tractive sedimentation 

with grains in dispersive equivalence

Nonvolcanic, matrix-supported 

conglomerate and breccia

Subaqueous debris flow deposits; 

possibly triggered by seismic event at 

the onset of volcanic activity

Nonvolcanic shale, siltstone, 

and fine sandstone

Lacustrine turbidite deposits

~'-~ Shale and siltstone

EXPLANATION

Laminated mudstone Sandstone

Conglomerate 

and breccia

Rhyolite and 

dacite tuff

Figure 8. Composite stratigraphy of subaqueous tuffs in the upper member of the Lospe Formation that were deposited 

as reworked ash from high-density turbidity flows. Modified from Cole and Stanley (1994).

R10 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
basaltic intrusive represents a magmatic pulse that was 

younger than that which formed the tuffs in the Lospe Formation, also found in the region of Point Sal.

Obispo Formation and Related Rocks

The Obispo Formation is a widespread and thick volcanic unit that outcrops in a northwest-southeast-trending zone 

from north of Estero Bay to near Santa Maria (fig. 1). The 

Obispo Formation consists of up to 1,500 m of rhyolite, dacite, 

and andesite tuff and lapilli tuff; basalt and andesite lava flows 

and intrusives; basaltic hyaloclastite and breccias; 

volcaniclastic sandstones; and tuffaceous shales (Hall and 

Corbato, 1967; Fisher, 1977; Hall and others, 1979; Hall, 

1981b; Surdam and Hall, 1984; Schneider and Fisher, 1996; 

fig. 10). The Obispo Formation conformably overlies the 

Rincon Shale and is overlain conformably by the Monterey 

Formation (fig. 2; Hall and Corbato, 1967). Radiometric dates 

obtained from acidic tuffs in the lower part of the Obispo Formation range from 15.7±0.9 to 16.9±1.2 Ma (Turner, 1970, 

corrected with methods of Dalrymple, 1979).

Nonwelded, white acidic vitric tuff and pumice lapilli 

tuff, some of the most common lithologies in the Obispo Formation (figs. 11A-D), were deposited mostly as high-density 

sediment gravity flows and turbidity flows of remobilized ash 

in a marine setting (Fisher, 1977; Schneider and Fisher, 1996). 

Schneider and Fisher (1996) estimate the volume of these deposits to be between 250 and 450 km3 . These deposits can be 

subdivided into thick, massive, poorly sorted intervals and wellbedded intervals (Schneider and Fisher, 1996; fig. 10). Massive tuff and lapilli-tuff intervals are well exposed in the 

seacliffs at Shell Beach and were likely deposited as mass flows 

of remobilized pyroclastic detritus (Schneider and Fisher, 

1996). Several types of soft-sediment deformation, including 

asymmetric slump folds, load casts, and folded rip-up clasts, 

are present along the base of the upper massive tuff unit at 

Shell Beach (fig. 11A). Imbrication of rip-up clasts, slump 

fold vergence, and soft-sediment glide plane orientations reveal an east to northeast direction of paleoflow for this unit 

(Schneider and Fisher, 1996). Well-bedded acidic tuff intervals are exposed in areas east of Shell Beach in the vicinity of 

San Luis Obispo (along Highway 101) and Twitchell Dam 

and were most likely deposited as turbidites of reworked ash 

(fig. 1 ID) (Fisher, 1977; Schneider and Fisher, 1996). These 

intervals contain normally graded beds, ripple laminations, rare 

flute casts, burrowed horizons, and interbedded shales. On the 

basis of flute casts, ripple foresets, and pebble imbrication (total 

n=46), Schneider and Fisher (1996) report east to northeast 

paleoflow for the well-bedded tuff interval at Twitchell Dam. 

The well-bedded tuff intervals may represent distal, lateral 

equivalents to the massive tuff intervals (fig. 10); collectively 

these deposits could have originated from subaerial pyroclastic flows that entered the sea, from submarine-erupted pyroclastic flows that mixed with water, or as large slumps from

Figure 9. Photomicrograph under cross-polarized light of 

trachybasalt sill that intrudes the Point Sal Formation near Point 

Sal. Subophitic texture with plagioclase laths partially enclosed 

by clinopyroxene. Some alteration of plagioclase is present in the 

lower center of the photomicrograph. Field of view is 6 mm across.

West

Diablo

Canyon

area

Shell

Beach

area

East

Twitchell 

Dam area

EXPLANATION

Massive 

vitric tuff

Thin-bedded 

vitric tuff

Thin-bedded basalt 

hyaloclastite

Basaltic 

intrusives

Figure 10. Representative stratigraphic columns of the Obispo 

Formation drawn from original field data of this study combined 

with data reported by Schneider and Fisher (1996). Intrusive 

basaltic rocks are more prevalent in the western exposures of 

the Obispo Formation (Surdam and Hall, 1984; Schneider and 

Fisher, 1996). These intrusive and related extrusive basalts were 

most likely reworked to form the basaltic hyaloclastite turbidites 

at Shell Beach. Reworked acidic tuff beds near Twitchell Dam 

were probably deposited from turbidites, which may also have 

been derived from the west (Schneider and Fisher, 1996).

Volcanic Rocks of the Santa Maria Province, California R11
E F

Figure 11. Photographs of the Obispo Formation. A, Massive, light-colored vitric-pumice lapilli tuff at Shell Beach with large, dark-colored, softsediment deformed shale blocks. The shale blocks were most likely eroded from the underlying Rincon Shale by subaqueous pyroclastic flows 

from which the tuffs were deposited. B, Massive to planar-bedded pumice-lapilli tuff at Shell Beach. These are most likely subaqueous pyroclastic 

flow deposits. C, Photomicrograph of vitric lapilli tuff at Shell Beach under plane-polarized light. Note the bladed vitric shards and the pumice 

lapilli in the upper center of the photomicrograph. Field of view is 6 mm across. D, Thin-bedded vitric tuff near Twitchell Dam. These most likely 

are turbidite deposits of vitric ash that was reworked from the west. E, Photomicrograph of basaltic hyaloclastite turbidite deposits at Shell Beach 

under cross-polarized light. Dark blocky grains are basalt glass; matrix is calcite cement. A few subangular to subrounded plagioclase grains are 

present. Field of view is 6 mm across. F, Outcrop of basalt (B; dark intervals) interbedded with the lower part of the Monterey Formation (light 

intervals) along the north side of Highway 166 about 4.5 km east of Santa Maria. C, Pillow basalt with spheroidal weathering interbedded with 

the lower part of the Monterey Formation along Highway 166. Clipboard shown for scale is 14 in. long. H, Photomicrograph of basalt from 

outcrop in figure 11 C under crossed-polarized light. This rock exhibits intersertal and subophitic textures where plagioclase laths are partially 

contained within dark, iron-rich glass and clinopyroxene. Field of view is 3 mm across.

R12 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
Figure 11. Continued

the subaqueous flanks of a silicic volcano (Schneider and 

Fisher, 1996).

Welded acidic tuff in the Obispo Formation is reported 

by Schneider and Fisher (1996) as a 5- to 10-m-thick and 4- 

km-wide interval that is interbedded with volcaniclastic sedimentary rocks, breccias, and basaltic lava flows in the Nipomo 

area. The bounding marine sedimentary deposits imply subaqueous deposition of the welded tuff with emplacement temperatures estimated from thermal remanant magnetism to have 

been in the 450-550 °C range (Schneider and Fisher, 1996). 

Anisotropy of magnetic susceptibility reveals a possible northeast-southwest (020-035°) paleoflow lineation in this unit 

(Schneider and Fisher, 1996).

Interbedded with the acidic tuffs in the Shell Beach and 

Nipomo areas are beds of basaltic hyaloclastite and basaltic 

turbidite sandstones (figs. 10, 1 IE) (Schneider and Fisher, 

1996). Aphanitic basalt flows, pillow basalts, and possible sills 

within the upper part of the Obispo Formation (Hall, 1978) 

are exposed in roadcuts along Highway 166 in the Twitchell 

Dam area (figs. 1, I IF, HGj. These basalt units exhibit

subophitic, hyaloophitic, and intersertal textures with 

clinopyroxene and laths of plagioclase in a dark matrix of ironrich glass (fig. 1 IH).

Unnamed Volcanic Rocks of the Catway Road 

Area

Basalt is present in the vicinity of Catway Road in a 3- 

km2 area north of Figueroa Mountain in the San Rafael Mountains (fig. 1). Some outcrops display pillow structures (fig. 

12A), but basaltic dikes, tuffs, and breccias also are present. 

Modal mineralogy includes olivine, clinopyroxene, and plagioclase where some of the plagioclase is present as coarser 

grained phenocrysts (fig. 12Z?). This basalt unit, informally 

referred to as the Catway volcanics, overlies unnamed sandstone, shale, and minor basaltic tuff and breccia that locally 

contain Turritella ocoyana Conrad van, suggesting that these 

rocks fall within the early Miocene Saucesian and(or) Relizian 

Stages (J.G. Vedder, oral commun., 1993; fig. 2). Overlying

v.

A B

Figure 12. Catway volcanics in the Catway Road area. A, Outcrop of pillow basalt. B, Photomicrograph under cross-polarized light 

showing intersertal and hyalopilitic texture where phenocrysts of plagioclase are contained in a finer groundmass of olivine, 

clinopyroxene, and plagioclase laths. Some plagioclase phenocrysts are zoned. Field of view is 3 mm across.

Volcanic Rocks of the Santa Maria Province, California R13
the Catway volcanics is the Monterey Formation, which is 

biostratigraphically dated as Relizian and (or) Luisian in this 

area (J.G. Vedder, oral commun., 1993; fig. 2). The contact 

between the Catway volcanics and Monterey Formation is not 

exposed, but it may be erosional because interbeds of sandstone in the Monterey Formation contain basalt fragments. 

Published radiometric ages for the Catway volcanics include 

17.2+2.5 Ma and 20.3+1.6 Ma (Hall, 198la, corrected with 

methods of Dalrymple, 1979) and 18.8+1.5 Ma (Vedder and 

others, 1994). Stratigraphic position of these samples is unknown, owing to poor exposures and complicated structure. 

Hall (1981b) originally mapped this interval as part of the 

Temblor Formation. We elect to use an informal designation 

and unnamed status for these rocks because the name "Temblor" is more commonly applied only to units on the northeast side of the San Andreas fault.

Unnamed Volcanic Rocks of the Lopez Mountain 

Area

Near Lopez Mountain in the Santa Lucia Range (fig. 1), 

a poorly exposed and unnamed sequence of light-gray vitric 

tuff and basaltic andesite rests unconformably on Cretaceous 

and Jurassic sedimentary rocks. The tuff is well bedded and 

appears to grade upward into shales of the Miocene Monterey 

Formation, which in this area yields microfossil assemblages 

ranging in age from Saucesian to Mohnian (McLean, 1994). 

The tuff is crystal poor and could not be dated, but a sample of 

interbedded basaltic rock yielded a K-Ar age on plagioclase 

of 17.0+.0.5 Ma (McLean, 1994). The basaltic flows are approximately 25 m thick, and the exposed thickness of the tuff 

is about 20 m (McLean, 1994). The basalt exhibits ophitic to 

subophitic textures in which plagioclase laths are enclosed by

Figure 13. Photomicrograph of unnamed basalt at Lopez 

Mountain under cross-polarized light. Plagioclase laths are 

partially to completely enclosed by clinopyroxene to form a 

subophitic to ophitic texture. Olivine, partially altered to clay 

minerals, is present in the center of the photomicrograph. Field 

of view is 3 mm across.

clinopyroxene (fig. 13). Altered olivine is present and appears 

to be exclusive of the ophitic minerals, suggesting that the 

olivine may have had a separate crystallization history.

Morro Rock-lslay Hill Complex

The informally named Morro Rock-lslay Hill complex 

of Ernst and Hall (1974) is a 25- km-long chain of dacite, 

trachyte, and rhyolite volcanic necks, plugs, lava domes, and 

dikes that extends southeast from Morro Bay (figs. 1, 14A) 

(Ernst and Hall, 1974; Buckley, 1986). These rocks intrude 

melange and metamorphic rocks of the Franciscan Complex, 

and occur along a linear trend of about N. 55° W. This alignment implies that intrusion was fault controlled (Greenhaus 

and Cox, 1979). Radiometric dates for these intrusive rocks 

range from 22.7+D.9 to 28+1.0 Ma (fig. 2, table 1; Turner, 1968; 

Buckley, 1986). Detailed petrographic descriptions for the Morro 

Rock-lslay Hill complex are given by Ernst and Hall (1974) and 

Buckley (1986). In general, these rocks consist mostly of glassy 

matrix and plagioclase phenocrysts with minor amounts of quartz, 

biotite, hornblende, and iron oxides (fig. 14B). One potential 

extrusive equivalent to the Morro Rock-lslay Hill complex is 

the Cambria Felsite.

Cambria Felsite

The Cambria Felsite was first named and described by Ernst 

and Hall (1974). This unit consists of about 115 m of rhyolitedacite crystalline felsite and poorly exposed tuff in the region 

north of Estero Bay (fig. 1; Ernst and Hall, 1974). The Cambria 

Felsite unconformably overlies Jurassic and Cretaceous rocks 

of the Franciscan Complex and is unconformably overlain by 

late Oligocene nonmarine sedimentary rocks of the Lospe(?) 

Formation of Ernst and Hall (1974) (fig. 2). Mineralogically the 

Cambria Felsite consists of plagioclase, quartz, biotite, and iron 

oxides in a fine glassy matrix. Unpublished 40Ar/39Ar isotopic 

data from the Cambria Felsite suggest an age of about 27 to 26.5 

Ma (J.D. Obradovich, oral commun., 1994; M.E. Tennyson and 

M.A. Keller, oral commun., 1994; M.A. Mason and C.C. Swisher, 

oral commun., 1990).

Unnamed Volcanic Rocks of Upper Pine Creek

Several small patches of unnamed basaltic andesite were 

mapped by Hall and Corbato (1967) and Vedder and others (1991) 

in the upper Pine Creek area (fig. 1). The outcrops are very poor, 

and it is unclear whether the basaltic andesites are intrusive or 

extrusive. Hall and Corbato (1967) originally assigned these rocks 

to the Obispo Formation but a K-Ar age on plagioclase of 

26.6±0.5 Ma (Vedder and others, 1991) implies that this unnamed 

basaltic andesite unit is older than rocks of the Obispo Formation. Thin sections show that these rocks consist of lath-shaped 

phenocrysts of plagioclase in a groundmass of strongly flowR14 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
oriented (trachytic) plagioclase microlites, with subordinate 

amounts of augite and opaque oxides. The volcanic rocks of upper 

Pine Creek are about the same age as the Morro Rock-Islay Hill 

complex but do not fall along the same structural trend. They 

may instead represent a localized volcanic center that is genetically related to the Sur-Nacimiento Fault Zone (Vedder and others, 1991).

Subsurface Volcanic Rocks (Onshore and Offshore)

The Santa Maria province contains large oil and gas reserves and has been explored extensively with well and seismic data (Hall, 1982; Ogle, 1984; Grain and others, 1985; 

Fischer, 1987; McCulloch, 1987; Dunham and others, 1991; 

Mclntosh and others, 1991; Meltzer and Levander, 1991). By 

combining this published information with unpublished wellFigure 14. Photographs of intrusion at Morro Rock. A, Morro 

Rock at Morro Bay. This outcrop may be the remnant of a 

volcanic plug. B, Photomicrograph of Morro Rock under crosspolarized light. Porphyritic texture with rounded quartz, 

euhedral and zoned plagioclase, and euhedral to subhedral 

biotite in a glassy groundmass. Some plagioclase phenocrysts 

are clumped together and form a glomeroporphyritic texture. 

Field of view is 3 mm across.

log data (K.J. Bird, oral commun., 1991) and surface data, a 

regional thickness map for volcanic rocks in the Santa Maria 

province has been constructed (fig. 15). The sources of data 

for this map are listed in appendix 3. Because available data 

from wells is very sparse, figure 15 shows only limited onshore trends and information only for the southern portion of 

the offshore Santa Maria basin. These results are therefore 

tentative and should be treated as preliminary.

The precise age of volcanic rocks in the subsurface Santa 

Maria province is poorly constrained. Subsurface volcanic 

rocks onshore are found overlying the upper Oligocene part 

of the Vaqueros Formation or the lower Miocene Rincon Shale, 

or are interbedded with the lower Miocene Lospe Formation, 

and are overlain by the middle Miocene Monterey Formation 

(Hall, 1982; Tennyson and others, 1991; Stanley and others, 

1996). Examination of exploratory well logs along with available seismic interpretations show that the subsurface volcanic 

rocks in the southern offshore Santa Maria basin 

unconformably overlie Jurassic and (or) Cretaceous rocks and 

are overlain by either the lower Miocene Point Sal Formation 

or the lower and middle Miocene Monterey Formation (Grain 

and others, 1985; Meltzer and Levander, 1991; K.J. Bird, oral 

commun., 1991). Therefore, the offshore subsurface volcanic 

rocks have a potential age range between Cretaceous and lower 

Miocene. Because these stratigraphic relations are similar to 

the stratigraphic relations of exposed and radiometrically dated 

onshore Santa Maria province 20-16 Ma volcanic rocks, we tentatively refer to the subsurface volcanic rocks as lower Miocene.

In general, the thickness patterns of lower Miocene volcanic rocks follow the major structural trends within the Santa 

Maria province (fig. 15). For example, linear zones of thick 

volcanic deposits are parallel to the Santa Ynez River and Los 

Osos Faults and are present along the Hosgri Fault (fig. 15). 

Available seismic data suggest that the offshore volcanic rocks 

thicken across several small, north west-trending normal faults 

that were active during early Miocene time (McCulloch, 1987; 

Mclntosh and others, 1991; Meltzer and Levander, 1991; K.J. 

Bird, oral commun., 1991).

Other Occurrences of Volcanic Rocks

Several other occurrences of volcanic rocks are not shown 

on figure 2 because their stratigraphic relations are poorly 

known or because they occur just outside the Santa Maria province. About 15 km southeast of Lopez Mountain (fig. 1), recent mapping by Hugh McLean (oral commun., 1995) has 

revealed small patches of basaltic rocks that interfinger with 

and are overlain by shallow-marine sandstone of the lower 

Miocene Vaqueros Formation, which in this area contains the 

marine gastropod Turritella inezana and is overlain by Rincon 

Shale containing calcareous nannofossils of the earliest Miocene CN1 and (or) CN2 zones. Small undated basaltic dikes 

that intrude Cretaceous and lower(?) Miocene strata occur at 

scattered localities in the San Rafael Mountains east, northVolcanic Rocks of the Santa Maria Province, California R15
east, and north of Figueroa Mountain and along the Sisquoc 

and Cuyama Rivers east of Santa Maria (Vedder and others, 1967; Hall and Corbato, 1967; J.G. Vedder, oral 

commun., 1995). At Hell's Half Acre, about 9 km southeast of Figueroa Mountain and east of the edge of the map 

in figure 1, Fritsche and Thomas (1990) found a lens of 

basalt that yielded a whole-rock potassium-argon age of 

18.5+2.0 Ma (table 1). About 3 km northeast of the SurNacimiento fault and 25 km northeast of Nipomo (fig. 1), 

a basalt flow in the lower part of the Oligocene and lower 

Miocene Simmler Formation yielded whole-rock potassium-argon ages of 23.4+0.8 and 22.9+0.7 Ma (table 1; 

Ballance and others, 1983).

GEOCHEMISTRY OF VOLCANIC ROCKS

Several studies provide information on the geochemical 

characteristics of some of the volcanic rocks in the Santa Maria 

province (Ernst and Hall, 1974;Robyn, 1980;Weigand, 1982; 

Buckley, 1986; Cole and others, 1991b; Cole and Basu, 1992, 

1995; Hornafius, 1994; app. 2). Available data on major element chemistry show that the Santa Maria province volcanic 

rocks generally comprise a bimodal suite with basalt and basaltic andesite as a basic end-member and rhyolite, dacite, and 

trachyte forming an acidic end-member (fig. 16). Intermediate compositions (for example, andesites) are notably sparse 

from the available sample set.

121° 00' 120° DO00'

34° 30'

20 KILOMETERS 

I

Figure 15. Thickness map of lower Miocene volcanic rocks in part of the Santa Maria province. Gray shading 

represents zones where volcanic rock thickness is 500 ft (1 52 m) or more. Dashed line represents a contour of 

zero thickness for volcanic rocks. Numbered points represent wells or outcrops where the thickness of volcanic 

units was available. Data for these points are listed in appendix 3. Outcrop patterns of volcanic rocks are as 

shown in figure 1. Abbreviations: DC, Diablo Canyon area; SB, Shell Beach; SMRF, Santa Maria River Fault; 

SNF, Sur-Nacimiento Fault; EHF, East Huasna Fault; WHF, West Huasna Fault; LHF, Lions Head Fault; SYRF, 

Santa Ynez River Fault; PF, Pacifico Fault; SYF, Santa Ynez Fault; HF, Hosgri Fault; LOF, Los Osos Fault; PCF, 

Pezzoni-Casmalia Fault; SLBF, Santa Lucia Bank Fault.

R16 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
Variations of some major oxides with respect to SiO2 are 

shown in figure 17. No apparent geochemical differences exist between the 20-16 and 28-23 Ma volcanic suites. The coherent trends among these elements suggest that the suites of 

Santa Maria province volcanic rocks may have had a similar 

petrogenetic history. The trend of decreasing MgO and TiO2 

with increasing SiO2 may have been a result of fractional crystallization of olivine, pyroxene, and ilmenite (Wilson, 1989). 

The increase of K2O with increasing SiO2 could possibly be 

attributed to assimilation of high-K crust by basaltic magmas 

(Watson, 1982). In addition, the Na2O contents of a suite of 

rocks produced by crustal contamination may be expected to 

remain relatively constant with respect to K^O (Watson, 1982). 

Although there is considerable variation in Na2O content 

among just the acidic volcanic rocks, the average Na2O content among the combined basic and acidic end-members is a 

narrow range between 3 and 5 weight percent (fig. 17). These 

variations suggest that combinations of fractional crystallization and crustal assimilation were important during the petrogenetic history of the Santa Maria province volcanic rocks.

Available Nd- and Sr-isotopic and trace element data support the variations of major element chemistry and indicate 

that the bimodal suite of Santa Maria volcanic rocks evolved 

by combinations of assimilation and fractional crystallization 

(Cole and Basu, 1992, 1995). Initial ratios of 87Sr/86Sr range 

between 0.702575 and 0.711308, and initial eNd values range 

between +9.3 and -3.2 (Cole and Basu, 1992, 1995). These 

data indicate that the source of basaltic magma for Santa Maria 

volcanism included suboceanic mantle similar to the source 

of mid-ocean ridge basalt. This basaltic source underwent some 

fractionation and also melted local enriched crustal rocks to 

form acidic magmas, resulting in the bimodal suite of Santa 

Maria province volcanic rocks (Cole and Basu, 1992, 1995). 

The presence of modal olivine, clinopyroxene, and plagioclase 

in Santa Maria province basalt samples indicates that these 

rocks are tholeiitic basalt, which is consistent with derivation 

from mid-ocean ridge type magma.

Limited trace element data are available and have been 

compiled for some of the Santa Maria province volcanic rocks 

(app. 2). Because these data are from different sources in which

LU 

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A Tranquillon Volcanics of Dibblee (1950)

 Unnamed volcanics of Santa Rosa Creek

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D Sill at Point Sal

 Obispo Formation

O Catway basalt unit

a Unnamed volcanic rocks of the Lopez Mountain area

a Morro Rock-lslay Hill complex

Cambria Felsite

Unnamed volcanic rocks of upper Pine Creek

A 

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40 50 60 70 

SiO2, IN WEIGHT PERCENT

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Figure 16. Silica (SiO2 ) versus total alkalis (Na2O+ KO) for Santa Maria province volcanic rocks showing lithologic 

classifications of LeBas and others (1986). Data as shown in appendix 2 are compiled from Ernst and Hall (1974), 

Robyn (1980), Weigand (1982), Buckley (1986), Cole and Basu (1992, 1995), and Hornafius (1994).

Volcanic Rocks of the Santa Maria Province, California R17
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Figure 18. Samarium (Sm) versus hafnium (Hf) variation diagram for selected 20-16 Ma volcanic 

units in the Santa Maria province. Data as shown in appendix 2 are compiled from Hornafius 

(1994) and Cole and Basu (1995), and include new data for tuffs in the Lospe Formation. Symbols 

of volcanic units are the same as in figure 16. Dashed lines are best-fit linear regression lines for 

subsets A and B of the 20-16 Ma volcanic rocks, as labelled. The equation for each line is also 

shown. Note that subsets A and B are defined by different correlation lines that may represent 

eruption of each subset from separate magmatic suites.

measurements were made by different methods, rigorous quantitative petrogenetic modeling is not warranted. Nevertheless, 

simple qualitative observations of the compiled trace element 

data appear to be useful for recognizing possible correlations 

(comagmatic sets) among the 20-16 Ma acidic volcanic rocks. 

The 28-23 Ma volcanic rocks are not included in this discussion 

because very little trace element data are available for these rocks. 

Two chemically distinct subsets among the 20-16 Ma 

acidic volcanic rocks are defined by comparing concentrations 

of incompatible elements (figs. 18, 19). Subset A includes the 

Tranquillon Volcanics of Dibblee (1950), tuffs in the Lospe 

Formation, and tuffs of the Santa Barbara coast; subset B includes rocks of the Obispo Formation. The variation of these 

subsets with respect to Srn and Hf are displayed in figure 18, 

where subset B is defined by a less steep correlation line. Subsets A and B are also distinguished by the variation of Nb and 

La (La/Nb = 3.03 and 2.0 for subsets A and B, respectively) 

and on the basis of their enrichment of light rare earth elements (LREE), where subset A exhibits greater LREE enrichment (for subset A, La/Yb = 12.14 and La/Sm = 5.52, but for 

subset B, La/Yb = 5.82 and La/Sm = 4.15; see fig. 19). Because Sm-Hf and La-Nb are incompatible element pairs that 

have similar bulk partition coefficients, the ratio of these element pairs is not expected to vary significantly during the 

course of fractional crystallization or partial melting of mantlederived magmas (Bougault and others, 1980; Sun and

^Figure 17. Silica (SiO2) versus titanium (TiO,), magnesium 

(MgO), sodium (Na2O), and potassium (K2O) for volcanic units 

in the Santa Maria province. Data compiled from the same 

sources as in figure 16. Symbols of volcanic units are the same 

as in figure 16.

McDonough, 1989; Rollinson, 1993). These ratios would, 

however, be expected to vary as a result of magma mixing or 

crustal contamination. The variation of Hf/Sm and La/Nb between subsets A and B might therefore characterize a different contamination and (or) mixing history of suboceanic 

mantle-derived magmas (for example, Cole and Basu, 1995) 

that erupted separately to form the two subsets of volcanic 

rocks. The difference in LREE enrichment between subsets A 

and B could also be attributed to different contamination histories of suboceanic mantle-derived magma. Based upon these 

observations, we suggest that the volcanic rocks of subsets A 

and B were erupted from different volcanic centers within the 

Santa Maria province. In other words, we suggest that these 

geochemical variations reveal that the Tranquillon Volcanics 

of Dibblee (1950), tuffs in the Lospe Formation, and tuffs of 

the Santa Barbara coast were comagmatic and formed from 

one eruptive region, whereas the Obispo Formation volcanic 

rocks represent a separate comagmatic suite. As noted by 

Hornafius (1994), tuff samples NB440 and NB399 of the Santa 

Barbara coast appear to be chemically similar to the Obispo 

Formation and lie within subset B (fig. 19); these two tuffs 

could represent fallout deposits that were derived from the 

Obispo eruptive source.

VOLCANIC SOURCE AREAS AND 

PALEOGEOGRAPHY

Available information on the distribution, thickness, 

stratigraphy, facies, and geochemical characteristics of the 

Santa Maria province volcanic rocks allow correlation of 

widely separated volcanic units and identification of possible

Volcanic Rocks of the Santa Maria Province, California R19
source areas for volcanism. This information can be combined 

with existing data on middle Tertiary sedimentary basin development and fault activity to test paleogeographic models 

and to relate episodes of volcanism to tectonic events.

Two episodes of Santa Maria province volcanism are recognized on the basis of the ages of the volcanic units. An older 

group of rocks ranges between about 28 and 23 Ma and includes the Morro Rock-Islay Hill complex, the Cambria Pel-

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La, IN PARTS PER MILLION

Figure 19. Lanthanum (La) versus samarium (Sm), ytterbium (Yb), and niobium (Nb) for selected 

20-16 Ma volcanic units in the Santa Maria province. Data compiled from same sources as in 

figure 18. Symbols of volcanic units are the same as in figure 16. Gray dotted lines separate 

subsets A and B on the basis of their relative enrichment of La. Samples of subset A consistently 

show greater La/Nb, La/Yb, and La/Sm ratios than samples of subset B. In addition, samples of 

subset A appear to lie along different correlation lines than samples of subset B, especially for 

La-Yb and La-Sm. Samples NB440 and NB399 are from two unnamed tuffs of the Santa Barbara 

coast that appear to be chemically similar to the Obispo Formation and lie within subset B; all 

of the other unnamed tuffs of the Santa Barbara coast appear to be magmatically related to 

subset A. Sample 1 A3, a tuff in the Lospe Formation near Point Sal, displays an anomalously 

low La concentration; the significance of this sample is uncertain.

R20 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
site, and the unnamed volcanic rocks at upper Pine Creek. A 

younger group of rocks ranges between about 20 and 16 Ma 

and includes the Tranquillon Volcanics of Dibblee (1950), 

unnamed basaltic andesite of Santa Rosa Creek, Obispo Formation, tuffs in the Lospe Formation, unnamed lower Miocene 

tuffs of the Santa Barbara coast, the Catway volcanics, and 

the Lopez Mountain volcanic rocks. The poor age control on 

volcanic rocks in the offshore Santa Maria province does not 

allow them to be placed in the younger or older volcanic suite 

so we tentatively group them as lower Miocene.

The first episode of Santa Maria province volcanism (28- 

23 Ma) included the emplacement of the Morro Rock-Islay 

Hill complex and the Cambria Felsite. Based on the similar 

stratigraphic positions and the similar geochemical affinities 

of these two units, Ernst and Hall (1974) suggested that the 

Cambria Felsite was the extrusive equivalent of the Morro 

Rock-Islay Hill complex. The Cambria Felsite generally is 

more enriched in FC,O and SiO2 and less enriched in MgO 

than the intrusive rocks of the Morro Rock-Islay Hill complex (figs. 16, 17). This contrast in chemistry could indicate 

that the Cambria Felsite formed when high silica, K-rich crustal 

components were melted and assimilated by a magma that is 

partially preserved as the Morro Rock-Islay Hill intrusive 

rocks. Further field studies of the Cambria Felsite, including 

facies analysis and detailed thickness distributions, are required 

to determine the location(s) of the eruptive source area(s). Irrespective of the eruptive centers, the Morro Rock-Islay Hill 

complex and the Cambria Felsite range from somewhat older 

to about the same age as several other volcanic units and sedimentary basins that formed in central and southern California 

during the late Oligocene and early Miocene (Stanley, 1987,1988; 

Atwater, 1989; Cole and Basu, 1995). These volcanic rocks were 

most likely erupted during regional extension or transtension 

along the continental margin during the initial transition from a 

subduction to a strike-slip plate boundary between the Pacific 

and North American plates (Atwater, 1970,1989; Stanley, 1987, 

1988; Severinghaus and Atwater, 1990).

Possible volcanic source areas for the second episode of 

middle Tertiary volcanisrn in the Santa Maria province (20-16 

Ma) can be inferred by using thicknesses of volcanic units (assuming that volcanic units thicken toward their source), 

volcaniclastic facies, paleoflow data, relative abundance of intrusive rocks, and geochemical parameters. The Tranquillon 

Volcanics of Dibblee (1950) at Tranquillon Mountain include 

thick, welded lapilli tuffs that were most likely deposited from 

pyroclastic flows, and thick nonwelded tuffs and bomb-beds that 

probably represent fallout deposits (figs. 3, 4). These facies are 

typical of relatively proximal pyroclastic deposits (Smith, 1960; 

Fisher and Schmincke, 1984; Cas and Wright, 1987). In addition, the unconformity at the base of these volcanic rocks suggests that the region of Tranquillon Mountain had undergone 

uplift, possibly related to thermal doming near a volcanic source 

(Robyn, 1980). Volcaniclastic deposits that were distal equivalents to the type Tranquillon rocks would most likely be thinner 

and nonwelded. Such deposits may include the tuffs that are

interbedded in the Lospe Formation about 30 km north of 

Tranquillon Mountain (figs. 1, 6; Cole and others, 199Ib) and 

most of the unnamed lower Miocene tuffs of the Santa Barbara coast. This hypothesis is consistent with available trace 

element data, which reveal that the rocks of these three areas 

most likely had a similar magmatic history (figs. 18, 19). In 

this hypothesis, the tuffs of the Lospe Formation would have 

been shed northward (in present and early Miocene time) by 

pyroclastic flows and subaqueous sediment gravity flows of 

reworked ash (Cole and Stanley, 1994). Northward-directed 

flow is corroborated by crossbeds in one tuff bed in the Lospe 

Formation. Sparse well data (for example, index numbers 23, 

18, 19, and 12 on fig. 15 and in app. 3) between Tranquillon 

Mountain and the tuffs of the Lospe Formation suggest that 

there are volcanic rocks in the subsurface between the two 

units, which thin from south to north. This is consistent with 

the hypothesis that the tuffs in the Lospe Formation were derived from sources to the south. Most of the thin tuffs and 

bentonites exposed along the Santa Barbara coast, including 

the thickest and stratigraphically lowest tuff (for example, the 

Tranquillon tuff) probably represent deposits from small pyroclastic flows or airborne ash clouds that drifted southward 

and eastward away from the eruptive center near Tranquillon 

Mountain.

The source(s) for volcaniclastic rocks of the Obispo Formation was (were) probably different than the source of the 

volcaniclastic units to the south (for example, tuffs in the Lospe 

Formation and the Tranquillon volcanic rocks). Sparse well 

data (for example, index numbers 9 and 11) show an absence 

of subsurface volcanic rocks in the area between outcrops of 

the Obispo Formation and the tuffs in the Lospe Formation 

(fig. 15). The Obispo Formation as well as the unnamed volcanics 

of Santa Rosa Creek also display a different trace element composition than tuffs in the Lospe Formation and the Tranquillon 

Volcanics of Dibblee (1950) which suggests a different magmatic and eruptive history. Several lines of evidence indicate 

that an eruptive source area for the Obispo Formation was located in the offshore and (or) Diablo Canyon area west of the 

present Obispo Formation outcrop belt (Surdam and Hall, 1984; 

R.V. Fisher, oral commun., 1990; Schneider and Fisher, 1996; 

fig. 1). First, paleoflow data from tuffs in the Obispo Formation 

indicate transport from west to east (Surdam and Hall, 1984; 

Schneider and Fisher, 1996). Second, the thickest and more proximal pyroclastic facies of the Obispo Formation is exposed in the 

west (Shell Beach area) with thinner and more distal facies present 

eastward (Twitchell Dam area; Schneider and Fisher, 1996). 

Third, the concentration of mapped intrusive rocks is greatest at 

the northwestern end of the Obispo Formation exposures (Surdam 

and Hall, 1984; R.V. Fisher, oral commun., 1990). Finally, water 

depths during deposition of the Obispo Formation, based on faunal evidence, were more shallow to the west (50 m) and deeper 

to the east (>500 m; Surdam and Hall, 1984). Collectively, these 

data suggest that a topographically high volcanic source region 

was probably located west to northwest of the Obispo Formation outcrops in the present offshore Santa Maria basin. Based

Volcanic Rocks of the Santa Maria Province, California R21
upon a similarity in chemical composition with the Obispo Formation, it is possible that one or two of the unnamed tuffs of the 

Santa Barbara coast (for example, samples NB440 and NB399 

of Hornafius, 1994; fig. 19) were formed from ash that drifted 

southward from an Obispo eruptive source.

The thickest preserved lower Miocene volcanic rocks in 

the Santa Maria province reach about 4,900 ft (1,479 m) along 

the Santa Maria River Fault (index number 17 on fig. 15). These 

thick subsurface volcanic rocks could represent either proximity 

to an eruptive center or ponding in a topographic low area along 

the south side of the fault. The source of these volcanic rocks 

could have been the eruption center that is inferred in the Diablo 

Canyon area and (or) offshore areas to the northwest. This hypothesis is consistent with the Santa Maria River Fault being 

active during early Miocene volcanism and bounding a structural depression to the south where thick intervals of volcanic 

rocks accumulated.

Eruptive centers for some Santa Maria province volcanic 

rocks could also have been situated along and (or) west of the 

Hosgri Fault in the present offshore Santa Maria basin, where 

there are thick accumulations of lower Miocene volcanic rocks 

(fig. 15). Such a volcanic center(s) could have been a source for 

the Obispo Formation, the tuffs in the Lospe Formation, the type 

Tranquillon volcanic rocks, and (or) the Summerland tuff unit. 

Available seismic data for the offshore Santa Maria basin show 

several examples where lower Miocene volcanic and nonvolcanic 

rocks thicken across steep-dipping faults, but the overlying 

Pliocene rocks are not crosscut by these faults (McCulloch, 1987; 

Mclntosh and others, 1991; Meltzer and Levander, 1991; K.J. 

Bird, oral commun., 1991). This includes data for the San 

Gregorio-Hosgri Fault, which may have been a southwest-dipping normal fault during the early Miocene (McCulloch, 1987; 

Mclntosh and others, 1991). These data suggest that the offshore region was undergoing an episode of synvolcanic and 

syndepositional extension during early Miocene time. Eruptive centers may have been concentrated along these faults in 

this region of extension.

Eruptive sources for the several 20 to 16 Ma Santa Maria 

province basalt flows and basaltic breccias are not as readily 

discerned as the acidic pyroclastic eruptive sources. The basaltic flows probably were not as far travelled as the acidic 

volcaniclastic deposits and therefore may be relatively close 

to their eruptive sources.

To better understand the 20-16 Ma episode of volcanism in the Santa Maria province, these volcanic rocks have 

been restored to their approximate early Miocene positions 

(fig. 20) using paleogeographic reconstructions based upon 

paleomagnetic studies (Hornafius, 1985; Hornafius and 

others, 1986; Luyendyk, 1991). These studies suggest that 

the present east-west-trending western Transverse Ranges 

were previously oriented north-south during the early Miocene, prior to 95° of post-early Miocene clockwise rotation. During rotation right-lateral slip, with displacements 

generally less than 10-20 km, probably occurred along several northwest-trending faults in the Santa Maria basin,

including the Lions Head and Santa Maria River Faults 

(Hornafius and others, 1986). These faults formed the 

boundaries between individual crustal blocks and probably 

had some component of extension as well as strike-slip 

(Hornafius, 1985; Luyendyk, 1991).

The models discussed above predict that zones of extension within the Santa Maria province would have been 

developed between individual rotating and nonrotating 

blocks in an overall transtensional regime. The thickest accumulations of onshore Santa Maria volcanic rocks and 

the postulated eruptive centers for these rocks seem to be 

coincident with those regions in which the greatest extension is predicted by these models (fig. 20). For example, 

the predicted source of the Obispo Formation is near the 

northwest corner of the Santa Maria province and coincides with a zone of predicted crustal extension between 

the Hosgri fault and the northwest-trending Santa Maria 

River and Los Osos Faults. In addition, the Tranquillon 

Volcanics are thickest along trend with the Santa Ynez River 

Fault, which may have been adjacent to several triangular 

zones of extension during block rotations (Hornafius, 1985; 

Luyendyk, 1991; fig. 20).

RELATIONS OF VOLCANISM TO REGIONAL 

TECTONIC EVENTS

The bimodal geochemical character of volcanic rocks in 

the Santa Maria province is consistent with the regional trend 

of Tertiary bimodal volcanisrn that occurred in western California as Sierra Nevada arc magmatism ended and near-trench 

bimodal volcanism began (Christiansen and Lipman, 1972). 

This change in the nature of volcanism in western California 

most likely occurred in response to the transition from a convergent to a strike-slip plate boundary when segments of the 

Farallon-Pacific spreading ridge intersected western North 

America in the vicinity of southern California (Atwater, 1970; 

1989; Christiansen and Lipman, 1972; Snyder and others, 

1976; McCrory and others, 1995). We propose that the combination of oblique-slip or transtension between the Pacific 

and North American plates and the source of heat and upper 

mantle magmas supplied to the base of the continental margin 

by subducted segments of the Farallon-Pacific spreading ridge, 

resulted in the episodes of Santa Maria province volcanism. 

In this model, the Farallon-Pacific spreading-ridge segments 

provided basaltic magmas for Santa Maria volcanism, and the 

transtension along the continental margin caused zones of 

crustal thinning and faulting that provided conduits for magmas to reach the surface. Isotopic and geochemical data support this hypothesis because several Santa Maria province 

basalt samples retain mid-ocean ridge basalt signatures (Cole 

and Basu, 1992, 1995).

If this model is accurate, then the two episodes of volcanism imply two episodes of Farallon-Pacific spreading-ridge collision with western North America. Tectonic reconstructions by

R22 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
McCrory and others (1995) reveal that a large segment of the 

Farallon-Pacific ridge stopped spreading by 24 Ma, and that this 

segment intersected the area between Point Arguello to near San 

Francisco, which embraces the area of 28-23 Ma volcanic rocks 

of central and southern California. A second ridge segment, the 

Pacific-Monterey segment, stopped spreading at about 19 Ma in

the vicinity of the central Santa Maria province (McCrory and 

others, 1995). Spatial and temporal correlations between spreading-ridge segments along the continental margin and 

palinspastically restored Santa Maria province volcanic rocks 

(as well as other similar-aged volcanic rocks in southern and 

central California; Stanley, 1987; Cole and Basu, 1995) are conBEFORE ROTATION

Areas of 

greatest 

extension

No scale 

DURING ROTATION

Figure 20. A, Early Miocene paleogeographic reconstruction for the Santa Maria province; restorations of Hornafius

(1985), Hornafius and others (1986), and Luyendyk (1991). Volcanic thickness trends were adjusted from the presentday thickness map shown in figure 15. Representative thickness data points are shown for cross-reference with figure 15. 

Volcanic rocks in the offshore region are shown with 60 km of right-lateral displacement removed from the Hosgri Fault; 

this represents a median estimate for Hosgri Fault displacement. Total offset along the Hosgri Fault may have been as 

little as 5 km (Hamilton, 1982) or as much as 115 km (Graham and Dickinson, 1978a,b) or more (Graham and Peabody, 

1981; Bachman and Abbott, 1988). Dashed-dotted lines represent restored modern coastline. Abbreviations: CB, Catway 

volcanics; LT, tuffs in the Lospe Formation; SRB, basalt near Santa Rosa Creek; TQ, Tranquillon Mountain; LOF, Los Osos 

Fault; SMRF, Santa Maria River Fault; PCF, Pezzoni-Casmalia Fault; LHF, Lions Head Fault; HF, Hosgri Fault; SYRF, Santa 

Ynez River Fault; SLBF, Santa Lucia Bank Fault; DC, Diablo Canyon area; SB, Shell Beach. B, Simplified model of block 

rotations and zones of crustal extension (cross-hatched) predicted in models of Hornafius (1985), Hornafius and others

(1986), and Luyendyk (1991).

Volcanic Rocks of the Santa Maria Province, California R23
sistent with the model that volcanism occurred in response to 

spreading ridge-trench interactions. This model is also consistent with the slab window hypothesis in which upwelling asthenosphere fills a gap beneath western California following the 

demise of Farallon-Pacific spreading-ridge segments (Dickinson 

and Snyder, 1979; Severinghaus and Atwater, 1990).

Previous tectonic models have been suggested for Santa 

Maria province volcanism. In some respects our model is similar to a model proposed by Hall (198la) that suggested that volcanism occurred along a leaky-transform fault that was a 

precursor to the San Andreas fault. The similarity between the 

two models is that each implies a mantle-derived magma source 

for volcanism. Our model, however, includes specific hypotheses that relate Santa Maria province volcanism to spreading 

ridge-trench interactions and discrete episodes of crustal extension and (or) transtension. Our model also differs from some 

previous models that suggest that volcanism in the Santa Maria 

province was a response to subduction-related processes (Crouch, 

1978; Weigand, 1982). Isotopic and geochemical data reveal a 

bimodal magma source for Santa Maria province volcanic rocks 

where the basic end-member magmas were derived from upper 

mantle melts (similar to mid-ocean ridge basalt magma) as opposed to a melted subducted oceanic slab (Cole and Basu, 1992, 

1995). In addition, based on radiometric ages for volcanic rocks 

described in this paper and paleomagnetic studies (for example, 

Hornafius and others, 1986; Luyendyk, 1991), we can infer that 

many of the Santa Maria province volcanic rocks were erupted 

in a transtensional regime during episodes of crustal block rotations when segments of the Farallon-Pacific spreading ridge 

were intersecting western California.

CONCLUSIONS

The volcanic rocks of the Santa Maria province form 

important stratigraphic markers that are useful for surface and 

subsurface correlations and paleogeographic reconstructions 

in this petroleum-rich region. The Santa Maria volcanic units 

have distinctive lithologies that occupy discrete stratigraphic 

intervals. These volcanic units are, in some cases, laterally 

persistent and mappable for several to tens of kilometers.

Two separate age groups of volcanic rocks can be defined in the Santa Maria province on the basis of radiometric 

and biostratigraphic data. An older group of volcanic rocks 

that includes the Morro Rock-Islay Hill complex, the Cambria 

Felsite, and unnamed volcanic rocks of upper Pine Creek 

ranges from about 28 to 23 Ma. A younger group of volcanic 

rocks that includes the Obispo Formation, Tranquillon 

Volcanics of Dibblee (1950), tuffs in the Lospe Formation, 

and several other thin acidic tuffs and basaltic units ranges in 

age from about 20 to 16 Ma.

The vicinity of eruptive centers for Santa Maria volcanic 

units are determined by the thickness of volcanic units, volcanic facies, paleoflow data, the abundance of intrusive rocks, 

and geochemical data. Based upon their similar ages, compositional similarity, and close proximity, the Cambria Felsite 

may be the extrusive equivalent of the Morro Rock-Islay Hill 

complex. This relation is not well defined and can be tested 

by more detailed facies analysis of the Cambria Felsite.

Among the younger volcanic units, the Tranquillon 

Volcanics of Dibblee (1950) most likely represent proximal remnants of the southern Santa Maria province volcanic centers. This 

volcanic source may have shed ash as pyroclastic flows northward into the Santa Maria basin to form the tuffs within the Lospe 

Formation; this same eruptive source may also have formed ash 

clouds or small pyroclastic flows that transported ash to the south 

and east to form most of the unnamed lower Miocene tuffs of 

the Santa Barbara coast (for example, the Tranquillon tuff and 

the Summerland tuff). Collectively, these volcanic units 

(Tranquillon Volcanics of Dibblee (1950), tuffs in the Lospe 

Formation, and unnamed lower Miocene tuffs) are distinguished 

from volcanic rocks of the northern Santa Maria province by 

lower Hf/Sm, higher La/Nb, and greater light rare earth element 

enrichment (higher La/Yb and La/Sm). The Obispo Formation 

volcanic rocks probably were erupted from a source area in the 

northern part of the Santa Maria province. The source for these 

rocks is not well established, but paleoflow data and other evidence suggest that an eruptive center was present west (offshore) 

of the present Obispo outcrop belt. This eruptive center formed 

welded and nonwelded pyroclastic flow deposits (composed of 

cuspate glass shards, pumice, pyrogenic crystals, and minor lithic 

grains), some of which were remobilized as eastward-directed 

submarine sediment gravity flows. Ash clouds from the Obispo 

eruptive region may also have drifted southward to form a few 

of the unnamed tuffs of the Santa Barbara coast (for example, 

tuff samples NB440 and NB399 of Hornafius, 1994; fig. 19 

and app. 2).

Volcanic source areas may also have existed in the present 

southern half of the offshore Santa Maria basin. Lower Miocene strata, including volcanic rocks, thicken across several 

faults in the offshore region (Grain and others, 1985; Mclntosh and others, 1991; Meltzer and Levander, 1991). These 

faults apparently were growth faults with a normal sense of 

displacement in a region of extension where volcanic eruptions may have been centered. The volcanic rocks in the southern half of the offshore Santa Maria basin may have 

accumulated in depocenters that were formed as downdropped 

blocks bounded by the normal faults.

At least some of the eruptive centers for Santa Maria 

volcanism may have been localized by zones of crustal extension between rotating crustal blocks as predicted by rotation 

models of the western Transverse Ranges (Hornafius, 1985; 

Hornafius and others, 1986; Luyendyk, 1991). When plotted 

on paleogeographic reconstructions that take into account paleomagnetic rotations, the volcanic eruptive centers and distribution of thick volcanic rocks coincide with crustal gaps 

that are predicted by the paleomagnetic rotation models.

Volcanic episodes in the Santa Maria province can be 

related to regional tectonic events. During middle Tertiary time, 

several segments of the Farallon-Pacific spreading ridge inR24 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
tersected the North American continental margin in the vicinity of the Santa Maria province. As these spreading-ridge segments reached the continental margin, an oblique-slip plate 

boundary was formed between the Pacific and the North American plates (Atwater, 1970, 1989). The oblique-slip motion resulted in transtension along the preexisting subduction zone 

(Atwater, 1989; Luyendyk, 1991). During the same time, upper mantle material was juxtaposed beneath the continental 

margin from the subducted spreading-ridge segments. The 

volcanic rocks of the Santa Maria province were derived from 

basaltic magmas of the upper mantle that reached the surface 

during separate episodes of transtension along the obliqueslip plate boundary (Cole andBasu, 1992,1995). Early phases 

of oblique slip resulted in an episode of regional transtension 

that occurred in central and southern California, during which 

time the 28-23 Ma Santa Maria volcanic rocks were emplaced 

(for example, the Morro Rock-Islay Hill complex, the Cambria 

Felsite, and unnamed volcanic rocks of upper Pine Creek). 

Subsequent oblique slip resulted in rotations of crustal blocks 

that included the western Transverse Ranges. Younger Santa 

Maria volcanic rocks (for example, Tranquillon volcanic units, 

the Obispo Formation) may have erupted from volcanic centers in zones of extension or transtension between the rotating and nonrotating crustal blocks.

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R.B., and Obradovich, J.D., 1991, Age, correlation, and origin

Volcanic Rocks of the Santa Maria Province, California R27
of the type Lospe Formation (lower Miocene), Santa Maria basin, central California: American Association of Petroleum Geologists Bulletin, v. 75, p. 382.

Stanley, R.G., Valin, Z.C., and Pawlewicz, M.J., 1992b, Rock-eval 

pyrolysis and vitrinite reflectance results from outcrop samples 

of the Rincon Shale (lower Miocene) collected at the Tajiguas 

Landfill, Santa Barbara County, California: U.S. Geological Survey Open-File Report 92-571, 27 p.

Sun, S.S., and McDonough, W.F., 1989, Chemical and isotopic systematics of oceanic basalts: implications of the earth and mantle 

evolution: Earth and Planetary Science Letters, v. 35, p. 429- 

448.

Surdam, R.C., and Hall, C.A., 1984, Diagenesis of the Miocene 

Obispo Formation, Coast Range, California, in Stratigraphic, 

tectonic, thermal, and diagenetic histories of the Monterey Formation, Pismo and Huasna basin, California: SEPM Guidebook 

no. 2, p. 8-20.

Tennyson, M.E., Keller, M.A., Filewicz, M.V., and Cotton-Thornton, 

M.L., 1991, Contrasts in early Miocene subsidence history across 

Oceanic-West Huasna fault system, northern Santa Maria province, California [abs.]: American Association of Petroleum Geologists Bulletin, v. 75, no. 2, p. 383.

Turner, D.L., 1968, Potassium-argon dates concerning the Tertiary 

foraminiferal time scale and San Andreas fault displacement: 

Ph.D. dissertation, Berkeley, University of California, 99 p.

 1970, Potassium-argon dating of Pacific Coast Miocene foraminiferal stages: Geological Society of America Special Paper 

124, p. 91-129.

Turner, D.L., Surdam, R.C., and Hall, C.A., Jr., 1970, The Obispo 

Formation and associated volcanic rocks in the central California Coast Ranges K-Ar ages and biochronologic significance 

[abs.]: Geological Society of America Abstracts With Programs, 

v. 2, p. 155.

Vedder, J.G., Gower, H.D., Clifton, H.E., and Durham, D.L., 1967, 

Reconnaissance geologic map of the central San Rafael Mountains and vicinity, Santa Barbara County, California: U.S. Geological Survey Miscellaneous Field Investigations Map 1-487, 

scale 1:48,000.

Vedder, J.G., McLean, H., and Stanley, R.G., 1994, New 1:24,000- 

scale geologic maps show Stratigraphic and structural relations 

that require reinterpretation of Cretaceous and Cenozoic tectonic 

events in the Sierra Madre-San Rafael Mountains area, California [abs.]: Geological Society of America Abstracts With Programs, v. 26, no. 2, p. 100-101.

Vedder, J.G., McLean, H., Stanley, R.G., and Wiley, T.J., 1991, Paleogeographic implications of an erosional remnant of Paleogene 

rocks southwest of the Sur-Nacimiento fault zone, southern Coast 

Ranges, California: Geological Society of America Bulletin, v. 

103, p. 941-952.

Watson, E.B., 1982, Basalt contamination by continental crust: some 

experiments and results: Contributions to Mineralogy and Petrology, v. 80, p. 73-87.

Weigand, P.W., 1982, Middle Cenozoic volcanism of the western 

Transverse Ranges, in Fife, D.L., and Minch, J.A., eds., Geology and mineral wealth of the California Transverse Ranges: 

Santa Ana, Calif., South Coast Geological Society, p. 170-188.

Wilson, M., 1989, Igneous petrogenesis: London, Unwin Hyman, 

466 p.

Wissler, S.G., and Dreyer, F.E., 1943, Correlation of the oil fields of 

the Santa Maria district, in Jenkins, O.P., ed., Geologic formations and economic development of the oil and gas fields of California: California Division of Mines Bulletin 118, p. 235-238.

Woodring, W.P., and Bramlette, M.N., 1950, Geology and paleontology of the Santa Maria district, California: U.S. Geological Survey Professional Paper 222, 185 p.

R28 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
APPENDIX 1. DATA, INTERPRETATIONS, AND 

ASSUMPTIONS USED TO CONSTRUCT THE 

STRATIGRAPHIC CHART FOR MIDDLE TERTIARY 

VOLCANIC UNITS IN THE SANTA MARIA 

PROVINCE (FIG. 2).

The age of the boundary between the Miocene and the 

Oligocene is about 23.8 Ma (Cande and Kent, 1992, p. 13,933). 

The boundary between the middle Miocene and early Miocene 

corresponds to the top of chron CSCn.ln (Cande and Kent, 

1992, p. 13,938) which has an age of about 16.014 Ma (Cande 

and Kent, 1995, p. 6,094). The boundary between the late Oligocene and early Oligocene falls between the base of chron 

ClOn.ln and the top of chron C10n.2n (Cande and Kent, 1992, 

p. 13,938) which have ages of about 28.512 Ma and 28.578 

Ma, respectively (Cande and Kent, 1995, p. 6,094).

In southern California, the boundary between the Luisian 

and Relizian Stages of Kleinpell (1938,1980) occurs at about 

the same stratigraphic level as the boundary between the middle 

Miocene and early Miocene (Bartow, 1982). The age of the 

boundary between the Relizian and Saucesian Stages of 

Kleinpell (1938, 1980) is uncertain but must be younger than 

17.4 Ma (Stanley and others, 1996) and may be about 17 Ma 

(Bartow, 1982). The age of the boundary between the Saucesian 

and Zemorrian Stages of Kleinpell (1938, 1980) is uncertain 

but may be slightly younger than the boundary between the 

Miocene and the Oligocene (Ingle, 1980; Bartow, 1982). New 

biostratigraphic work suggests that the Zemorrian Stage may 

not be recognizable in coastal southern California owing to 

problems with Kleinpell's original definition of the stage (M.L. 

Cotton, M.V. Filewicz, and D.R. York, Unocal Corp., oral and 

written communs., 1995).

Igneous rocks of the Morro Rock-Islay Hill complex of 

Ernst and Hall (1974) intrude the Jurassic and Cretaceous 

Franciscan Complex and have yielded radiometric ages of 

22.7+0.9 Ma to 28.0+1.0 Ma (ages from Turner, 1968, and 

Turner and others, 1970, corrected for changes in decay constants using the method of Dalrymple, 1979; additional ages 

from Buckley, 1986; see table 1).

In the San Luis Obispo-Cambria area, volcanic rocks of 

the Obispo Formation have yielded radiometric ages ranging 

from 15.7±0.9 Ma to 16.9±1.2 Ma (ages from Turner, 1970, 

corrected for changes in decay constants using the method of 

Dalrymple, 1979; see table 1). Biostratigraphic data indicate 

that the Obispo Formation is overlain by Relizian strata variously assigned to the Monterey and Point Sal Formations (Hall 

and Corbato, 1967, p. 570; Turner, 1970, p. 103; Hall, 1973, 

1978; Hall and others, 1979). These biostratigraphic data 

strongly suggest that the top of the Obispo is of Relizian age 

and therefore somewhat older than 16.0 Ma, a conclusion that 

is consistent with the reported estimates of analytical precision of the youngest reported radiometric ages from the Obispo. 

According to Hall and Corbato (1967, p. 570) and Hall (1973,

p. 3), molluscan fossils from the Obispo suggest an age of 

Saucesian to Relizian, implying that the base of the Obispo 

may be older than 17.0 Ma. On our stratigraphic chart (fig. 2), 

the age of the base of the Obispo is queried and placed at 

about 18.1 Ma, which is the maximum age allowed by the 

estimate of analytical precision of the oldest reported radiometric age from the Obispo. The Obispo conformably overlies the Rincon Shale (of Hall and others, 1979) which in this 

area contains Saucesian benthic foraminifers as well as calcareous nannofossils of early Miocene zones CN1 and (or) 

CN2 (Tennyson and others, 1991; M.E. Tennyson, oral 

commun., 1992). The Vaqueros Formation in this area contains molluscan fossils of the "Vaqueros Stage" of Addicott 

(1972) that have yielded 87Sr/86Sr ratios consistent with ages 

of late Oligocene to early Miocene (Tennyson and others, 1991; 

M.A. Keller and M.E. Tennyson, oral communs., 1992,1996). 

Nonmarine sedimentary rocks in this area (mapped as Lospe 

Formation by Hall, 1974; Ernst and Hall, 1974; Hall and Prior, 

1975; and Hall and others, 1979) have not been directly dated 

but are of late Oligocene age on the basis of their stratigraphic 

position between the overlying Vaqueros Formation and the 

underlying Cambria Felsite of Ernst and Hall (1974). The age 

of the Cambria Felsite is about 26.5 Ma to about 27 Ma on the 

basis of unpublished radiometric ages (M.A. Mason and C.C. 

Swisher, oral commun., 1990; J.D. Obradovich, oral commun., 

1994; M.E. Tennyson and M.A. Keller, oral commun., 1994; 

see table 1).

Near Lopez Mountain, unnamed volcanic rocks (map 

units Tvt and Tvf of McLean, 1994) apparently overlie Mesozoic sedimentary rocks and are in turn overlain by the Monterey 

Formation, which in this area contains calcareous nannofossils 

of middle Miocene zone CN4 and assemblages of benthic foraminifers ranging in age from Saucesian(?) and Relizian to late 

Mohnian (McLean, 1994). A basaltic interbed in tuff yielded 

a radiometric age of 17.0±0.5 Ma (map unit Tvt of McLean, 

1994; see table 1). These data are consistent with the hypothesis that the unnamed volcanic rocks near Lopez Mountain 

are about the same age as the Obispo Formation in the nearby 

San Luis Obispo-Cambria area (fig. 2). In the same general 

area, but not in contact with the volcanic rocks, are patches of 

unnamed sandstone and conglomerate to which McLean (1994, 

map unit Tsc) assigned an age of Miocene and (or) Oligocene.

In the Point Sal area, biostratigraphic data show that the 

lower part of the Monterey Formation is of Luisian and Relizian 

age (Woodring and Bramlette, 1950, p. 100) and that the Point 

Sal Formation is of Relizian and latest Saucesian age (Stanley 

and others, 1996, p. M12-M13). The boundary between the 

Monterey and Point Sal Formations must therefore occur 

within the Relizian Stage, but its precise age is uncertain. The 

ages of the top and bottom of the Lospe Formation are about 

17 and 18(?) Ma, respectively, on the basis of biostratigraphic 

data and radiometric ages on tuffs (Stanley and others, 1996; 

see table 1).

On Figueroa Mountain, the Monterey Formation is of 

Relizian and (or) Luisian age on the basis of biostratigraphic

Volcanic Rocks of the Santa Maria Province, California R29
data (J.G. Vedder, oral commun., 1993, cited by Stanley and 

others, 1996, p. M27) and therefore may be correlative with 

parts of the Monterey Formation in the Lopez Mountain and 

San Luis Obispo-Cambria areas. The age range of the informally-named Catway volcanics is poorly constrained by three 

radiometric ages that exhibit large reported estimates of analytical precision (table 1). The age of the lower boundary of 

the volcanic sequence is tentatively placed at 20.3 Ma on the 

basis of the oldest potassium-argon age reported by Hall (1981, 

p. 1,017) corrected for changes in decay constants using the 

method of Dalrymple (1979). The Catway volcanics rest conformably on an unnamed sequence of sandstone and mudstone 

that contains molluscan fossils of Saucesian and (or) Relizian 

age (J.G. Vedder, oral commun., 1993, cited by Stanley and 

others, 1996, p. M27).

Along Santa Rosa Creek, the age of the unnamed basaltic rocks is poorly constrained by a single radiometric age of 

17.4+1.2 Ma (from Turner, 1970, corrected for changes in decay constants using the method of Dalrymple, 1979; see table 

1). The minimum and maximum ages allowed by this date 

(16.2-18.6 Ma) were used to depict the age range of the unnamed basaltic rocks on the stratigraphic chart (fig. 2). The 

unnamed basaltic rocks are overlain by the Monterey Formation, the lower part of which is of Luisian and Relizian age 

(Dibblee, 1988a). In turn, the unnamed basaltic rocks overlie 

the Rincon Shale and Vaqueros Formation, which we provisionally correlate with units of the same names in the nearby 

Santa Barbara coastal area (see discussion below). In the Santa 

Rosa Creek area, geologic map relations depicted by Dibblee 

(1988a) indicate that the Vaqueros rests unconformably on 

marine sedimentary rocks of late and (or) middle Eocene age 

(Howard, 1995, p. H27).

On Tranquillon Mountain, the Tranquillon Volcanics of 

Dibblee (1950) have yielded a radiometric age of 17.80+0.05 

Ma (Stanley and others, 1996, p. Mil; see table 1). The 

Tranquillon Volcanics are overlain in apparent angular 

unconformity by the Monterey Formation; the amount of time 

represented by this unconformity is uncertain but probably 

not large, because a sample from the lower part of the Monterey 

yielded early and middle Miocene diatoms and Relizian(?) 

foraminifers (Dunham and Blake, 1987, p. 31). Thickness 

trends, available paleocurrents, and similarities in age, petrography, and major and trace element composition indicate that 

the Tranquillon Volcanics are at least partly correlative with 

an unnamed tuff at the base of the Monterey Formation along 

the Santa Barbara coast (Cole and others, 199la, b; Stanley 

and others, 1991, 1992a, 1996; Hornafius, 1994); a sample of 

this tuff from near Naples yielded a radiometric age of 

18.42+0.06 Ma (Stanley and others, 1996, p. Mil; see table 

1). The Tranquillon Volcanics on Tranquillon Mountain rest 

unconformably on lower Miocene (Saucesian) strata of the 

Rincon Shale (Stanley and others, 1996, p. M25); the amount 

of time represented by this unconformity is unknown.

In the Santa Barbara coastal area, the unnamed tuff within 

the lowest part of the Monterey Formation has long been known

to be of late Saucesian age because the tuff conformably overlies fossiliferous upper Saucesian strata of the Rincon Shale 

and is conformably overlain by fossiliferous upper Saucesian 

strata of the Monterey Formation (Dibblee, 1950, p. 34; 

Kleinpell and Weaver, 1963, p. 11-12; Dibblee, 1966, p. 50; 

Turner, 1970, p. 105; DePaolo and Finger, 1991; Stanley and 

others, 1992b, 1994, 1996). As mentioned above, a sample of 

the unnamed basal Monterey tuff from near Naples yielded a 

radiometric age of about 18.4 Ma (table 1). Also at Naples, 

the unnamed tuff occurs below Monterey strata that yielded 

mean 87Sr/86Sr values of 0.70865±0.00001, which correspond 

to planktic foraminiferal zone N6 and calcareous nannofossil 

zone CN2 and falls within the middle of geomagnetic chron 

C5Dn (DePaolo and Finger, 1991) dated at about 17.3-17.6 

Ma (Cande and Kent, 1995). Samples of tuff from the base of 

the Monterey Formation near Summerland, about 9 km east 

of Santa Barbara, yielded radiometric ages of 16.5+0.6 Ma 

and 17.2+0.5 Ma (ages from Turner, 1970, corrected for 

changes in decay constants using the method of Dalrymple, 

1979; see table 1). However, compared to the biostratigraphic 

and strontium isotopic data from Naples, the younger of these 

two radiometric ages is too young, even within the reported 

range of analytical precision, unless the tuff at Summerland is 

at a higher stratigraphic level than the tuff at Naples. The age 

of the base of the Rincon Shale in the Santa Barbara coastal 

area is about 23.3 Ma on the basis of evidence from calcareous nannofossils (Stanley and others, 1994) and calculated rates 

of rock accumulation (R.G. Stanley, unpublished data). The 

base of the Vaqueros Formation in the Santa Barbara coastal 

area is thought to be an unconformity (Howard, 1995, p. H5); 

strontium isotopic analyses of fossil shells suggest that the 

age of the lowest part of the Vaqueros is about 24+1 Ma 

(Rigsby, 1994). According to Howard (1995, p. H27), upper 

Oligocene strata within the upper part of the Sespe Formation 

rest unconformably on upper Eocene strata within the lower 

part of the Sespe, and lower Oligocene strata are absent.

In the offshore Santa Maria basin, unnamed volcanic 

rocks rest unconformably on Mesozoic and Paleogene strata 

and are in turn overlain by the Monterey Formation 

(McCulloch, 1989, p. 446). Little is known about the age of 

the offshore volcanic rocks, but they are at least partly of early 

Miocene (Saucesian) age, are overlain by Relizian sedimentary strata, and have been correlated with the onshore 

Tranquillon Volcanics by Grain and others (1985, p. 540).

R30 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
APPENDIX 2. 

COMPILATION OF 

AVAILABLE MAJOR 

ELEMENT AND 

SELECTED TRACE 

ELEMENT DATA FOR 

VOLCANIC ROCKS 

OF THE SANTA 

MARIA AREA

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Volcanic Rocks of the Santa Maria Province, California R31
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R32 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
ooooooooooo

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Volcanic Rocks of the Santa Maria Province, California R33
APPENDIX 3. SUBSURFACE DATA USED TO 

CONSTRUCT LOWER MIOCENE VOLCANIC 

THICKNESS MAP OF FIGURE 15

Index No.

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

Well

Getty "Montadoro" 1

C.C Townsend "Townsend-Gunter" 1

Pismo Oil "Mello" 1

Holly Oil "Meherin" 1

Homestake Prod. "Tar Spring" 1

N.B. Hunt 1

Chevron "Porter" 1

Gulf "Porter" 1

Stansbury Webb "Bosse" 1

Union "Vernon Wineman" 2

Golden Bear Oil "Moretti" 4-1

Union "Jesus Maria" 2

Texaco "La Brea" 1

J.I. Anderson "Elliott" 1

Texaco "Miller" 1

Texaco "Sorenson" 1

Union "Hansen" 1

Prairie Oil & Gas 1

W.H. Taylor "Packard" 1

Getty "Leonsis" 1

Point Conception COST well (OCS-CAL

78-164)

Rothschild Oil "Sacremento J.M." 2

Texaco "Intex-Lagomarsino" 1

Anza Pacific "Spanne" 1

Amerada Hess "Sudden" 1

Gulf "Sudden" 1

Gulf "Sudden" 3

Standard "Sudden" 1

Conoco 322-1

Mobil 321-1

Chevron 3 18-1

Chevron 3 18-2

Chevron 450-3

Chevron 3 16-2

Chevron 452-2

Chevron 3 17-1

Chevron 446-2

Chevron 443-1

Arco 444- 1

Union 44 1-2

Exxon 440-2

Exxon 440-1

Arco 437-1

Total depth 

(feet)

6,146

3,286

1,505

4,072

3,535

10,010

9,690

2,939

2,670

3,665

3,070

1,664

4,519

2,455

5,078

3,004

8,168

5,638

5,587

4,949

9

4,151

4,696

6,241

1,884

2,009

2,284

2,861

9,343

11,463

96,43

8,100

8,881

11,416

7,485

8,956

6,733

6,952

8,067

5,430

5,837

5,525

6,609

Lower Miocene 

volcanic thickness 

(feet)

1,270

1,645

>198

670

926

1,500

2,200

850

0

1,900

0

<100

1,227

82

462

375

4,900

204

193

890

0

270

624

50

700

300

>400

>210

0

0

0

0

0

0

220

0

170

>400

10

0

0

0

70

R34 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
Index No.

44

45

46

47

48

49

50

51

52

53

54

55

56

57

58

59

60

61

62

63

64

65

66

67

68

69

70

Well

Texaco 496-1

Pennzoil 427-1

Arco 425-1

Exxon 41 1-1

Phillips 406-1

Reading & Bates 4 16-1

Sun Exploration 415-2

Reading & Bates 4 15-1

Oxy 409-3

Oxy 409-6

Phillips 408-1

Phillips 402-1

Phillips 403-1

Getty 395-1

Standard-Humble Oceano 1 060-1

Phillips 396-1

Phillips 397-2

Phillips 397-1

Texaco 394-1

Sun 422-1

Phillips 426-1

Shell 435-1

Getty 424-1

Arco 430-1

Chevron 450-1

Getty 449-1

Conoco 320-1

Total depth 

(feet)

6,120

6,156

3,681

5,303

5,755

5,961

6,309

9,916

9,465

7,917

6,895

8,687

6,897

7,746

8,020

7,702

8,055

3,665

5,475

?

8,073

6,901

4,010

5,772

11,950

8,071

9,234

Lower Miocene 

volcanic thickness 

(feet)

0

0

>230

480

440

100

160

233

320

0

>210

117

336

680

>600

440

>525

300

920

220

0

0

150

0

550

0

0

Volcanic Rocks of the Santa Maria Province, California R35

Chapter S

Age and Tectonic Inferences from a Condensed(?) 

Succession of Upper Cretaceous, Paleocene, and 

Eocene Strata, Big Pine Mountain Area, Santa 

Barbara County, California

By JOHN G. VEDDER, RICHARD G. STANLEY, HUGH 

McLEAN, MARY LOU COTTON, MARK V. FILEWICZ, and 

DAVID R. YORK

U.S. GEOLOGICAL SURVEY BULLETIN 1995-S

EVOLUTION OF SEDIMENTARY BASINS/ONSHORE OIL AND GAS INVESTIGATIONSSANTA MARIA PROVINCE

Edited by Margaret A. Keller

Age and Tectonic Inferences from a Condensed(?) 

Succession of Upper Cretaceous, Paleocene, and Eocene 

Strata, Big Pine Mountain Area, Santa Barbara County, 

California

Eyjohn G. Vedder1 , Richard G. Stanley1 , Hugh McLean1 , Mary Lou Cotton2, Mark V. Filewicz3 , 

and David R. Vork4

Abstract

A 350-m-thick mudstone unit near the top of Big Pine 

Mountain spans the Cretaceous-Tertiary boundary. This 

mudstone unit is underlain by more than 1,650 m of Upper Cretaceous submarine-fan deposits and overlain by 

as much as 1,000 m of lower and middle(?) Eocene subsea-fan deposits. Biostratigraphic zonation based upon assemblages of calcareous nannofossils, palynomorphs, and 

foraminifers indicates that the mudstone unit includes beds 

of latest Cretaceous, late Paleocene, and early Eocene age 

spanning an interval of about 22 m.y. Physical stratigraphic 

evidence for hiatuses is lacking. Paleobathymetric indicators suggest that most of the mudstone was deposited at 

upper to middle bathyal depths. A technically generated 

barrier to sediment transport probably a fault-bounded 

ridge is the most likely primary cause for the prolonged 

slow rate of deposition, although eustatic high sea level may 

have been a secondary control. Presumably, this barrier lay 

northeast of the present trace of the Sur-Nacimiento fault 

zone, yet no known vestiges are preserved. Post-middle(?) 

Eocene, pre-middle Miocene strike-slip tectonism may have 

shifted the site of the barrier ridge to a position far from its 

origin, or erosion may have destroyed the ridge before such 

a shift. Alternatively, post-middle(?) Eocene erosion may have 

obliterated the ridge after an episode of major displacement 

along the Sur-Nacimiento fault zone. Uplift of the ridge may 

be attributable to strike-slip tectonism that resulted from oblique subduction near the end of Cretaceous time.

A 15-km-long belt of mudstone beds along the south side 

of the Big Pine fault is nearly the same age as the 350-m-thick 

mudstone sequence on Big Pine Mountain but differs in that it 

is as much as 800 m thick and contains thin limestone zones 

of late Paleocene age. This mudstone belt presumably

'U.S. Geological Survey, Menlo Park, CA 94025. 

Consultant, 244 Hermosa Drive, Bakersfield, CA 93305. 

3Unocal Frontier Exploration, Far East Group, P.O. Box 4570, Houston, TX 77210.

"Unocal Corporation, P.O. Box 4551, Houston, TX 77210.

Approved for publication April 22, 1997 

Edited by Scan M. Stone

represents part of the same basin that is manifested by the 

condensed(?) succession on Big Pine Mountain. Beds within 

the limestone zones consist mainly of allochemical detritus that 

was transported downslope from algal-bank deposits and are 

the same age as the type Sierra Blanca Limestone. Directional 

features measured at a few places in the mudstone belt suggest 

sediment transport from opposite sources and imply deposition in a borderland-like basin. Regional distribution of partly 

coeval deep-water mudstone units, including the Anita Shale 

of Kelley (1943), indicates that basins containing bathyal, 

hemipelagic sediments were extensive during late Paleocene 

and early Eocene time. Deep-water environments, slow deposition rates, and limited short-lived shallow-water conditions 

may have been controlled by borderland-like topography that 

developed in response to an episode of widespread strike-slip 

tectonism.

INTRODUCTION 

Purpose

Most published geologic maps of the Big Pine Mountain area (figs. LA, IB) in eastern Santa Barbara County show 

Upper Cretaceous strata unconformably overlain by Eocene 

strata (Gower and others, 1966; Vedder and others, 1967; and 

Vedder and Brown, 1968; Dibblee, 1973). One exception is a 

generalized map (Comstock, 1975, fig. 2), which depicts narrow belts of Paleocene strata at Big Pine Mountain and along 

the south side of the Big Pine fault. In an attempt to reconcile 

this mapping difference, samples were collected from the 

upper and lower parts of a mudstone unit that hitherto had 

produced inconclusive or conflicting biostratigraphic evidence 

of age. Because these samples yielded microfossil assemblages of Late Cretaceous age from the lower part of the

Figure 1 (following pages). A, Location of study area (shaded box) 

and geographic features mentioned in text; faults shown as heavy lines, 

dashed where approximately located. B, Generalized distribution of 

rocks and geologic structures noted in text; approximate locations of 

stratigraphic columns shown in figure 3 are numbered and areas of 

geologic maps shown in figures 4 and 6 are outlined.

Age and Tectonic Inferences, Upper Cretaceous, Paleocene, and Eocene Strata, Big Pine Mountain Area, California S1
If) «0-

S2 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
120' 119°45' 119°30'

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EXPLANATION

Miocene rocks Locally include older and younger rocks

Eocene rocks Locally include Paleocene rocks northeast of 

Sur-Nacimiento fault zone

Lower Eocene, Paleocene, and Upper Cretaceous rocks

Upper Cretaceous rocks

Lower Cretaceous rocks Locally include Upper Jurassic rocksFranciscan ComplexFaultContact

2 Approximate location of stratigraphic columns shown in figure 3

Chiefly Miocene and younger rocks

15 KILOMETERS

- 34°45'

34°30'
mudstone unit and assemblages of late Paleocene and early 

Eocene age from the upper part, it became evident that the 

relatively thin succession of mudstone, even if incomplete, 

represents a very slow rate of deposition. To help resolve the 

age boundaries and duration of sedimentation, additional 

samples were collected from throughout the mudstone sequence at Big Pine Mountain.

Mudstone beds that closely resemble those at Big Pine 

Mountain extend for 15 km along the south side of the Big 

Pine fault between Coche Creek and Alamar Canyon (figs. 

IA, IB). These mudstone beds were selectively sampled in 

the vicinity of Bluff Campground to determine whether or 

not they correlate with the mudstone succession at Big Pine 

Mountain. Although the biostratigraphic analyses from both 

units are incomplete and do not explicitly resolve the problems of age and correlation, the preliminary results together 

with the lithostratigraphic evidence seem to be sufficient to 

shape some inferences on the effects of tectonism on sedimentation throughout the area. Partly coeval and somewhat 

similar stratal sequences elsewhere in the Santa Maria-Santa 

Barbara region are briefly compared in order to clarify age 

differences and stratigraphic discontinuities.

Acknowledgments

Some of the biostratigraphic information used in this report was furnished by individuals from several different organizations, especially throughout the reconnaissance phase 

of the study. During this early work (1965-68), P.J. Smith 

and R.L. Pierce, U.S. Geological Survey (USGS), and J.W. 

Ruth, Standard Oil Company of California, provided age assignments for foraminiferal assemblages; and D.L. Jones, 

USGS, identified the Cretaceous mollusks. More recently 

collected mollusks were identified (1977-83) by D.L. Jones 

and J.W. Miller, USGS, and L.R. Saul, Los Angeles County 

Museum of Natural History. Later (1978-84), calcareousnannofossil, palynological, and foraminiferal data from 

supplementary samples were supplied by David Bukry, N.O. 

Frederiksen, R.E. Arnal, and P.J. Quinterno, USGS. During 

the last phase of the work (1992-93), W.V. Sliter and K.A. 

McDougall, USGS, added updated zonations for the foraminiferal assemblages collected before 1984, and David Bukry 

reported calcareous-nannofossil ages from newly collected 

samples. A.A. Almgren, consulting geologist and paleontologist, contributed important information on localities and age 

assignments from his own studies. W.P Elder, USGS, revised 

some of the molluscan taxonomy and age ranges for the Late 

Cretaceous mollusks.

Permission by the U.S. Forest Service to use their cabin 

at Bluff Campground facilitated the fieldwork, and their continuing support is much appreciated. Information contributed 

by S.C. Comstock, Mobil Oil Corp., and D.G. Howell, USGS, 

was helpful in interpreting depositional environments. Comments and suggestions by J.A. Bartow, USGS, and W.V. Sliter

on a preliminary version of the manuscript helped to improve 

the final report.

Previously Published Work

Fairbanks (1894) was the first to recognize the general 

structural and stratigraphic elements of the San Rafael Mountains on the basis of traverses he made east and northwest of 

Big Pine Mountain. Nelson (1925) and Keenan (1932) mapped 

and described parts of the large expanse of Cretaceous and 

Tertiary strata south of the Big Pine fault. Reed and Hollister 

(1936, pi. VI-A through pi. VII-B) photographed outcrops 

along the newly constructed Big Pine Road and apparently 

made the first published reference to the presence of Cretaceous beds at Big Pine Mountain. Dibblee (1966) mapped 

unnamed Upper Cretaceous sandstone and shale units west 

of Big Pine Mountain between Cachuma Mountain and Mission Pine Basin, as well as in the lower Indian Creek-Agua 

Caliente Canyon area. Gower and others (1966) and Vedder 

and others (1967) mapped the San Rafael Primitive Area (now 

San Rafael Wilderness) and recorded the presence of Upper 

Cretaceous and Eocene fossils in the vicinity of Big Pine 

Mountain. Mapping east of Big Pine Mountain by Vedder and 

others 1973) revealed details of Upper Cretaceous and Eocene 

stratigraphy in the upper Mono Creek area. Comstock (1975) 

was the first to present evidence for the occurrence of Paleocene fossils in the Big Pine Mountain area. On the basis of 

preliminary biostratigraphic data, Vedder and others (1991) 

mentioned the mudstone beds at Big Pine Mountain and 

briefly noted that the sequence ranges in age from Late Cretaceous to early Eocene. In a regional study of Paleogene limestone, Whidden and others (1995a) concluded that the Sierra 

Blanca Limestone represents both Paleocene and Eocene shallow-water deposition on locally uplifted and tilted Cretaceous 

forearc strata.

GEOLOGIC SETTING 

Regional Stratigraphy

Exposed strata in the vicinity of Big Pine Mountain constitute the upper part of a thick late Mesozoic and early Cenozoic sedimentary cover on regionally extensive rocks that were 

designated the Stanley Mountain terrane (fig. 2) by Blake and 

others (1982). This terrane is characterized by accreted Middle 

and Late Jurassic oceanic-arc ophiolite and depositionally 

overlying Late Jurassic and Early Cretaceous forearc-basin 

accumulations, which generally consist of thin basal tuffaceous chert and succeeding thick, terrigenous flyschlike 

deposits (Howell and others, 1987; Robertson, 1989). Late 

Cretaceous and Paleogene sedimentation was largely on submarine fans that developed during the unroofing of mid-Cretaceous granitoid plutons in neighboring continental terranes

S4 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
to the east (Page, 1970; Howell and others, 1977; Lee-Wong 

and Howell, 1977; Vedder and others, 1983). Regional tectonic events that influenced these late-stage forearc depositional systems included oblique subduction, terrane accretion, 

and likely strike-slip deformation (Page and Engebretson, 

1984; Nilsen, 1987).

In the central San Rafael Mountains north of the Big 

Pine fault (fig. IB), unnamed Upper Cretaceous strata, chiefly 

sandstone, mudstone, and conglomerate turbidites, are more 

than 3,400 m thick (fig. 3, cols. 1, 2, and 3) and represent a 

remnant of a widespread submarine-fan system (Howell and 

others, 1977; Vedder and others, 1983). Farther southeast, in 

the lower Mono Creek-Agua Caliente Canyon area south of 

the Munson Creek fault, a correlative sequence may be 

more than 5,000 m thick and is overlain by Paleogene 

marine strata nearly 3,000 m thick (Page and others, 1951; 

Dibblee, 1966; Vedder and others, 1973, 1983; MacKinnon, 

1978).

A distinctive white limestone generally less than 70 m 

thick (fig. 3, col. 5) forms a narrow belt along the south limb 

of the Loma Pelona syncline and extends from East Fork Santa 

Cruz Creek southeastward to a point about 3 km west of the 

confluence of Alamar Canyon and Mono Creek (figs. IA, IB). 

Named the Sierra Blanca Limestone by Nelson (1925), this 

limestone consists largely of calcareous detritus derived by 

downslope transport from algal bank deposits (Keenan, 1932; 

Walker, 1950; Johnson, 1968; Comstock, 1975, 1976; Sliter 

and others, 1994; Whidden and others, 1995a). New bios122° 120° 118°

36C

34°

100 200 KILOMETERS

Figure 2. Tectonostratigraphic terranes of part of southwestern 

California. Modified from Blake and others (1982). Shaded area 

shows extent of Stanley Mountain terrane; stipple pattern depicts 

coastline.

tratigraphic evidence indicates that the age of the Sierra Blanca 

Limestone in the area of the type section (Vedder, 1972) is 

late Paleocene on the basis of planktonic foraminifers (W.V. 

Sliter, written commun., 1993) rather than early Eocene. In 

the vicinity of its type locality at Indian Creek, the Sierra 

Blanca lies unconformably on Upper Cretaceous beds and is 

overlain by lower and middle Eocene strata that commonly 

are assigned to the Juncal Formation. Northeast of Sierra 

Blanca Mountain these Eocene strata consist largely of sandstone and mudstone turbidites nearly 1,000 m thick. In the 

southeastern San Rafael Mountains east of Agua Caliente 

Canyon, the Juncal Formation is as much as 1.600 m thick 

(Thompson, 1988).

Middle and upper Eocene marine strata, as well as the 

unconformably overlying upper Eocene(?) to lower Miocene(?) redbeds, are present east and southeast of the study 

area. Miocene marine strata, including a basal transgressive 

sandstone and overlying deep-marine shale, are unconformable on all of the older rocks throughout the region (Dibblee, 

1966; Dickinson and Lowe, 1966; Vedder and others, 1967, 

1973; Fritsche and Shmitka, 1978).

Regional Structure

The northwest-trending Sur-Nacimiento fault zone (fig. 

IB) presumably is the surface expression of the boundary between the Stanley Mountain and Salinia terranes and is the 

primary structural feature in the Sierra Madre-San Rafael 

Mountains area. This fault zone truncates the Eocene strata 

that flank the northeastern side of Big Pine Mountain. Although the history, nature of movement, and amount of offset 

along the Sur-Nacimiento fault zone are poorly understood, 

major events along it have been episodic (Vedder and Brown, 

1968; Page, 1970; Dibblee, 1976; Yaldezian and others, 1983; 

Vedder and others, 1983, 1991; Dickinson, 1983). The easttrending Big Pine fault, which truncates the Upper Cretaceous 

and Paleogene strata along the south side of Big Pine Mountain, is a strike-slip fault, along which there has been about 

14 km of post-Oligocene left-lateral separation (Hill and 

Dibblee, 1953; Crowell, 1962; Carman, 1964; Vedder and 

others, 1973). Its early history is unknown, although Carman 

(1964) speculated that the fault may have been active near 

the end of the Oligocene, and Stewart and Crowell (1992) 

pointed out that dip slip occurred along the eastern part during Oligocene-early Miocene time.

Primary folds that deform rocks in the study area include the post-middle Miocene Hurricane Deck syncline and 

the post-early Eocene Mission Pine anticline, which is truncated along its south limb by the Big Pine fault, The Loma 

Pelona syncline is a complex post-middle Miocene structure 

that trends eastward to join the Hildreth syncline north of the 

Hildreth and Munson Creek faults (fig. IB). All of these folds 

are asymmetric, show slight to moderate southward vergence, 

and are oblique to the major faults (Vedder and others, 1967,

Age and Tectonic Inferences, Upper Cretaceous, Paleocene, and Eocene Strata, Big Pine Mountain Area, California S5
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S6 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
structure sections B-B' and C-C"; Vedder and others, 1973, 

structure sections A-A' and C-C').

Paleomagnetic evidence for tectonic rotation is lacking 

for structural blocks in the central San Rafael Mountains. It 

is likely, however, that late Cenozoic rotations farther south 

and east (Luyendyk, 1991; Whidden and others, 1995b) affected parts of the study area. If future work demonstrates 

such rotations, sediment-transport directions described herein 

must be revised accordingly.

STRATIGRAPHIC RELATIONS AT BIG PINE 

MOUNTAIN

The 350-m-thick condensed(?) succession of Upper Cretaceous to lower Eocene mudstone near the top of Big Pine 

Mountain records a prolonged episode of greatly diminished 

coarse-clastic sedimentation, even though the preserved rocks 

may not represent continuous deposition (figs. 4,5). This relatively thin sequence of mudstone beds overlies regionally extensive Upper Cretaceous strata that are more than 3,000 m 

thick and underlies a limited tract of lower and middle(?) 

Eocene strata at least 1,000 m thick north of the Big Pine 

fault (fig. 3). Brief descriptions of the underlying and overlying thick stratal units together with details of the relatively 

thin mudstone unit provide a frame of reference for comparison with similar coeval rock units south of the Big Pine fault.

Bates and Jackson (1987) defined a condensed succession as "a relatively thin but uninterrupted stratigraphic succession of considerable time duration, in which the deposits 

accumulated very slowly; it is generally represented by a timeequivalent thick succession elsewhere in the same sedimentary basin or region." The word "condensed" is queried herein 

because the continuity of the described succession is uncertain.

Subjacent Upper Cretaceous Strata

West of Big Pine Mountain and north of the Big Pine 

fault, a 1,650-m-thick sequence of interbedded Upper Cretaceous sandstone, mudstone, and subordinate conglomerate 

forms the high-standing ridge that links Big Pine Mountain 

and San Rafael Mountain (fig. I A; fig. 3, col. 3). These lenticular submarine-fan strata continue northwestward from the 

San Rafael Mountain-McKinley Mountain area to the north 

side of Manzana Creek and from there, extend across the

Figure 3. Generalized composite stratigraphic columns of Upper 

Cretaceous and Paleogene sedimentary rocks at selected sites in 

Sierra Madre and San Rafael Mountains. Approximate locations 

of columns shown on figure 1 B. Map-unit symbols on columns 

3-5 refer to map units on figures 4 and 6; those on columns 1 

and 2 refer to rock units mapped and described by Vedder and 

others (1967); and those on column 6 refer to rock units mapped 

and described by Vedder and others (1973).

Sisquoc River into the Sierra Madre Mountains southwest of 

the Sur-Nacimiento fault zone (fig. 1A). Nonmarine and shallow-marine conglomerate and sandstone beds are intertongued 

with the submarine-fan deposits along the southwestern flank 

of the Sierra Madre (fig. 3, col. 2), where they form a narrow, 

32-km-long, fault-disrupted belt between South Fork La Brea 

Creek and Sweetwater Canyon (Vedder and others, 1967, 

1977).

Upper Cretaceous beds that are stratigraphically below 

the thick succession exposed between Big Pine Mountain and 

the San Rafael Mountain-McKinley Mountain area are present 

along the south side of Manzana Creek between Manzana 

Camp and Dry Creek, where they consist of an additional 

1,150 m of submarine-fan deposits (fig. 3, col. 1). The entire 

Upper Cretaceous submarine-fan system from Big Pine Mountain to Dry Creek is composed largely of mid-fan and inner 

fan-channel deposits in which sediment transport generally 

varied from northwestward to southwestward (Comstock, 

1975, 1976; Howell and others, 1977). Exceptions are in the 

McKinley Mountain and upper Coche Creek areas, where 

Comstock (1976) noted northward transport directions based 

upon a limited number of observations. The near-shore deposits in the Sisquoc River-La Brea Creek belt show sediment-transport directions that range from westward to 

southward (Vedder and others, 1977). In combination, these 

transport directions, if not tectonically rotated, imply a sediment source that lay northeast of the Sur-Nacimiento fault 

zone. The exact location of this source, however, has not been 

identified.

Although the thick sequence of Upper Cretaceous strata 

north of the Big Pine fault is sparsely fossiliferous, age-diagnostic foraminifers and mollusks are present in sufficient numbers and at enough places to permit a generalized 

biostratigraphic zonation. Several taxa of ammonites representative of the Cenomanian and Turonian Stages have been 

reported from the lower part of the sequence along a 2-km 

stretch of Manzana Creek near Nira Campground (Vedder 

and others, 1967; Vedder and Elder, unpub. data, 1994). Fossils are rare in stratigraphically higher beds along the south 

limb of Hurricane Deck syncline north of Manzana Creek, 

but those found suggest the presence of the Campanian and 

(or) Maastrichtian Stages. North of Hurricane Deck syncline, 

mollusks, including ammonites, from sites along the Sisquoc 

River between Mine and Sweetwater Canyons are indicative 

of the upper Campanian and (or) lower Maastrichtian Stages 

(Vedder and others, 1967,1977). Structural complications and 

covering Miocene rocks, however, hamper direct correlation 

of Upper Cretaceous strata along the Sisquoc River with those 

on the south limb of Hurricane Deck syncline.

In areas northwest and west of Big Pine Mountain, most 

sites sampled for foraminifers yielded sparse assemblages diagnostic only of a Cretaceous, undifferentiated, age. One exception, a sample from the ridge about 3.5 km east of San 

Rafael Mountain (Vedder and others, 1967, Ml3), contains 

an assemblage of benthic foraminifers that is assigned to the

Age and Tectonic Inferences, Upper Cretaceous, Paleocene, and Eocene Strata, Big Pine Mountain Area, California S7
TKu Teu 119°40'

(South edge partly overlaps 

north edge of figure 6)

119°40'

2 KILOMETERS

Figure 4. Generalized geologic map of Big Pine Mountain area. Location shown on figure 1 B. Shaded area indicates 

distribution of condensed(?) succession (TKu). Modified from Vedder and others (1967) and J.G. Vedder, Hugh McLean, 

and R.G. Stanley (unpub. mapping, 1990-93).

S8 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
EXPLANATION 

Surficial deposits (Quaternary)

Sedimentary rocks (Eocene and Paleocene?) Chiefly sandstone, 

northeast of Sur-Nacimiento fault zone

Sedimentary rocks (middle and lower Eocene) Chiefly sandstone, 

southwest of Sur-Nacimiento fault zone

Sedimentary rocks (lower Eocene, Paleocene, and Upper Cretaceous) 

Condensed(?) succession, chiefly mudstone

Sedimentary rocks (Upper Cretaceous) Chiefly interbedded sandstone 

and mudstone

Contact Includes well defined, poorly defined, and inferred

Fault Dotted where concealed; dashed where uncertain. Arrows 

indicate direction of movement

 1 Sync line Approximately located

Strike and dip of beds

Inclined Representative of numerous readings, both measured and 

estimated

Overturned

Line of measured section Includes sample sites V-689 to V-692 and 

V-628 to V-651 on figure 5 and table 1

Sandstone lentil Shown in unit TKu between Big Pine Mountain and 

Big Pine fault: mapping incomplete

x v-527 Fossil locality Shown on figure 5 or noted in table 1 or in text

upper Campanian or lower Maastrichtian (R.L. Pierce, written commun., 1967). Calcareous nannofossils are exceptionally rare and poorly preserved, and samples generally are 

barren; one site about 1.0 km northeast of Nira Campground 

produced several taxa that range from Coniacian to 

Maastrichtian. Palynomorphs from a sample locality about 

1.5 km south of Big Pine Mountain (fig. 4, V-556) have a 

likely range of Santonian to Danian (N.O. Frederiksen, written commun., 1985); the same locality contains poorly preserved Late Cretaceous calcareous nannofossils (David Bukry, 

written commun., 1992).

Despite the absence of well-founded evidence for the 

occurrence of the Coniacian and Santonian Stages in the central San Rafael Mountains north of the Big Pine fault, it seems 

reasonable to expect that these stages are represented in the 

thick Upper Cretaceous stratal successions north of Manzana 

Creek and along the Sisquoc River.

Condensed(?) Succession 

Stratigraphy

The mudstone sequence that is the primary target of this 

study extends from near the confluence of Big Pine Canyon and 

the Sisquoc River southeastward across the western and southern flanks of Big Pine Mountain (fig. 4). At its northern end, the

unit is truncated by the Sur-Nacimiento fault zone at the mouth 

of Judell Canyon; at its southern end, it is cut by the Big Pine 

fault 2.0 km east of Bluff Campground. Thinning in both directions from Big Pine Mountain is suggested by the mapped relations. However, placement of the lower contact is uncertain along 

the western side of Big Pine Canyon, and dips may be steeper 

than indicated in the southernmost exposures near the Big Pine 

fault. Consequently, the thinning may only be apparent. Most of 

the northeast-trending faults that cut the mudstone have vertical 

displacements of less than 15 m. Incomplete exposures and deeply 

weathered zones in parts of the section measured at Big Pine 

Mountain (figs. 4, 5) hindered collection of equally spaced 

samples suitable for biostratigraphic analysis. Stratigraphic intervals between the samples taken are approximate because bedding inclinations cannot be accurately determined in the highly 

fractured, indistinctly layered mudstone.

The base of the measured section (fig. 5) is marked by an 

abrupt sandstone-mudstone contact. Very coarse to coarse-grained 

sandstone beds directly beneath the contact are laterally persistent, thick-bedded, normally graded turbidites; these resemble 

lithofacies B submarine-fan deposits described by Ricci-Lucchi 

(1975). The upper surface of the uppermost sandstone is imprinted with numerous crossing trails and burrows, most of which 

are horizontal, meandering and straight back-filled feeding traces 

about 1 to 2 cm wide (IPlanolites). Although the lowermost part 

of the overlying mudstone sequence consists of a 2- to 3-m-thick 

zone of silty and sandy mudstone with thin interbeds of normally graded fine- and medium-grained sandstone (lithofacies 

D and E beds of Ricci-Lucchi, 1975), silty mudstone characteristic of lithofacies G (Ricci-Lucchi, 1975) is the dominant rock 

type throughout the remainder of the condensed(?) succession. 

Cursory microscopic examination reveals that most samples are 

composed of poorly sorted micaceous clayey siltstone and silty 

claystone and that beds in the lower part tend to be siltier and 

more micaceous. Rare, irregular grains of glauconite are present 

about 150 m above the base. Bedding is indistinct, and fracture 

varies from splintery or hackly to conchoidal. Fresh exposures 

are dark gray to olive gray; weathered ones, brownish gray to 

light-olive gray. Zones of randomly scattered ellipsoidal calcareous concretions are conspicuous in the lower and middle parts 

of the measured section. Sand-filled burrows are common in the 

lower part, and very small, tubular, clay-rich traces 

(IHelminthoida sp.) give the mudstone a mottled appearance in 

many beds throughout the entire section. Common plant fragments and sparse fish remains are disseminated as dark particles 

in much of the mudstone.

Benthic foraminifers suggest that the lower 270 m of 

the measured section was deposited at bathyal depths in a 

basin environment and that the uppermost 50 m was deposited at upper to middle bathyal depths in slope and (or) basin 

environments. Beds near the top of the mudstone sequence in 

Big Pine Canyon include benthic species that range into lower 

bathyal depths (table 1).

About 70 m above the base of the measured section, a 

ledge-forming sandstone lentil less than 1 m thick interrupts

Age and Tectonic Inferences, Upper Cretaceous, Paleocene, and Eocene Strata, Big Pine Mountain Area, California S9
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-9-

LATE CRETACEOUS

Molluscan 

stage 

or age

"Capay"

Late 

CampanianMaastrichtianBenthic 

foraminifer 

age or zone

C(ofLaiming)- 

CtopseudoC"

CCPaleogenePaleogeneCretaceous -

PaleogeneCretaceous v 

PaleogeneCretaceous - 

PaleogeneCretaceous '

PaleogeneCretaceous -

(ofGoudkoff)-

Late 

Cretaceous '

Cretaceous(?)-

Late 

Cretaceous »

Probable > 

Cretaceous -

D-E 

(of Goudkoff) -

D2-E -^

Planktonic 

foraminifer 

zone

P9- 

P11(?) 

P9 

P7-P9- 

P8 

P7 

P7 

Calcareous 

nannofossil 

zone

CP11-

CP11- 

CP11-

CP10 -

Probable 

CP9

CP9

EXPLANATION FEET MET

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udstone 

indstone 

alcareous mudstone

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-50

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-20 

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Palynomorph 

age

Eocene - 

Eocene -

Late 

Paleocene - 

Late 

Paleocene? -

Probable 

Paleogene -

MaastrichtianMaastrichtianSantonian 

Maastricht ian 

Late 

Cretaceous(?)-

Maastrichtian- Santonian - ^"

Lithology

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Fig

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ag( 

fro 

Sej 

on

Sample 

number

5/19/92B 

 F-4VRM1-59

 981-25-5(H)

 V-651 

 V-557 

^-V-650 

 981-25-3(H)

 V-649 

 V-648

 V-647

 V-646 

 1084-1 6-1 (M) ^- V-645 

 1 084-1 6-2(M) 

 V-643, V-644

_ V-642 

 V-641 

 V-640

 V-639 

 V-638 

 V-637

 V-636 

 V-635 

 V-634 

 V-633

 V-524a 

 V-632

^- V-631 

 V-630 

 V-629

 V-527 

 65CB75 ^- V-692

 V-691 

\V-525 

>V-526,V-690 

^V-689

| ISEGMENTC| SEGMENT SEGMENTS AD

Remarks

-About 260 m upsection from F-3 VRM1-59; 

equivalent to P9-P10.

-F18 of Vedder and others (1967); M3542, USGS, Menlo 

Park, California, Cenozoic megafossil collection.

Approximately 1,000 m of strata, chiefly sandstone 

between V-651 and Sur-Nacimiento fault zone.

- Projected from about 3.7 km northwest of section; 

5 cm below sandstone contact at that place. 

Top measured section about 8 m above V-651; 

stratigraphic position approximate.

-Projected from about 3.7 km northwest of section.

Section between V-646 and V-647 poorly exposed.

-Stratigraphic position approximate; R3368, USGS, 

Reston, Virginia, palynology archive. 

-Stratigraphic position approximate; R3373, USGS, 

Reston, Virginia, palynology archive.

^Comstock's (1975) locality 2 may project near here. 

Section between V-639 and V-640 incompletely exposed.

-Approximate position V-524 and M43 (Vedder and others, 

1967). 

-Stratigraphic position approximate.

- Highest mapped sandstone lentil in measured section.

Section between V-692 and V-628 poorly exposed and 

deeply weathered; thickness uncertain.

Projected from about 130 m west-southwest of 

-section. 

-Projected from about 225 m south of section.

--Stratigraphic position approximate. 

V-526 = M7589, USGS, Menlo Park, California, Mesozoic 

megafossil collection.

Base measured section 2 m below V-689; about 

1650 m downsection to Big Pine fault.

ure 5. Measured section of condensed(?) succession o( 

per Cretaceous, Paleocene, and lower Eocene mudstone 

ap unit TKu, fig. 4) at Big Pine Mountain. Zonation anc 

3 assignments are explained in table 1; molluscan data 

m W.P. Elder and J.G. Vedder (unpub. data, 1991). 

;;ments A-D refer to divisions of measured section shown

figure 4.

S10 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
Table 1. Age-diagnostic taxa, zonation, and paleobathymetry of microfossils from Big Pine Mountain area

[Localities shown on figures 4 and (or) 5. Arranged stratigraphically from top to bottom. Assignments by M.L. Cotton, M.V. Filewicz, and D.R. York unless noted as 

follows: R.L.P., R.L. Pierce; J.W.R., J.W. Ruth; R.E.A., R.E. Arnal; D.B., David Bukry; N.O.F., N.O. Frederiksen; P.J.S., P.J. Smith; P.J.Q., P.J. Quinterno; K.M., K.A. 

McDougall. Zonation schemes used: dinoflagellates, Williams and Bujak (1985); calcareous nannofossils, Okada and Bukry (1980); planktonic foraminifers, Blow 

(1969, as emended by Toumarkine and Luterbacher, 1985); benthic foraminifers, Goudkoff (1945), Laiming (1939, as emended by Almgren and others, 1988). Age 

boundaries from Haq and others (1987). Paleobathymetry based upon classifications by Hedgpeth (1957) and Ingle (1975)]

Field No. Age-diagnostic and associated taxa Age or zone Paleobathymetry Remarks

981-25-5(H)

981-25-4(H)

981-25-2(H)

981-25-3(H)

GP33

981-25-HH)

V-651

V-557

Calcareous nannofossils: Coccolithus crassus, Zygrablithus

bijugdtus, Discoaster barbadiensis, D. lodoensis, Discoasteroides 

kuepperi, Helicosphaera seminulum, Pontosphaera plana

Benthic foraminifers: assemblage

Planktonic foraminifers: assemblage

Calcareous nannofossils: Coccolithus grandis, C. crassus,

Discoasteroides kuepperi, Sphenolithus radians, Zygrablithus 

bijugatus

Benthic foraminifers: assemblage

Planktonic foraminifers: assemblage

Calcareous nannofossils: Chiasmolithus grandis, Coccolithus 

crassus, C. magnicrassus, Discoaster lodoensis, Discoasteroides 

kuepperi

Benthic foraminifers: assemblage

Planktonic foraminifers: assemblage

Calcareous nannofossils: Zygrablithus bijugatus, Tribrachiatus

orthostylus 

Benthic foraminifers: assemblage

Planktonic foraminifers: assemblage

Benthic and planktonic foraminifers: assemblage

Calcareous nannofossils: Coccolithus crassus, Discoaster lodoensis, 

Braarudosphaera sp., Micrantholithus sp., Pontosphaera sp., 

Zygrablithus sp.

Benthic foraminifers: assemblage

Planktonic foraminifers: assemblage

Calcareous nannofossils: Sphenolithus radians, Zygrablithus 

bijugatus, Discoaster lodoensis, Helicosphaera lophota, 

Coccolithus crassus, Rhabdosphaera morionum, R. tennis, 

Helicosphaera seminulum, Discoaster binodosus

Benthic foraminifers: Bulimina macilenta, B. callahani, Anomalina 

dorri aragonensis, Marginulina asperuliformis

Planktonic foraminifers: Globorotalia bullbrooki, G. griffinae, G. 

caucasica

Calcareous nannofossils: Coccolithus crassus, Discoaster lodoensis,

Discoasteroides kuepperi 

Benthic foraminifers: assemblage

Planktonic foraminifers: assemblage

CPU (Discoaster

lodoensis Zone)

(D.B.) 

Probable Laiming

pseudo-C

(R.E.A., K.M.) 

P9 toPll (R.E.A.)

CPU (Discoaster 

lodoensis Zone) 

(D.B.)

Probable Laiming 

C to pseudo-C 

(R.E.A., K.M.)

P8 to P10 (K.M.)

CPU (Discoaster 

lodoensis 

Zone) (D.B.)

Probable Laiming 

C to pseudo-C 

(R.E.A., K.M.)

P8 (K.M.)

CP10 (D.B.)

Probable Laiming 

C to pseudo-C 

(R.E.A., K.M.)

P8 to P9 (K.M.)

Eocene (P.J.S.)

CPU (Discoaster 

lodoensis Zone)

Laiming C(?) to 

pseudo-C(?) 

(R.E.A., K.M.)

P8 to P9 (K.M.)

CPU

Laiming C to 

pseudo-C

P9

CPU (Discoaster 

lodoensis Zone)

Probable Laiming 

C to pseudo-C 

(R.E.A., K.M.)

P7 to P9 (K.M.)

> 2,000 m (K.M.)

> 2,000 m (K.M.)

> 2,000 m (K.M.)

> 2,000 m (K.M.

Upper to middle 

bathyal

Middle(?) bathyal 

(P.J.Q.)

Sample collected 5 cm below contact 

with overlying sandstone unit about 

3.7 km northwest of measured 

section at Big Pine Mountain.

Approximately 10 m below 981-25- 

5(H).

Sample collected from upper part of 

the mudstone unit about 4.2 km 

northwest of measured section. 

Presumably a few meters above 

stratigraphic position of 981 -25- 

3(H).

Approximately 10 m below 981-25- 

4(H). Tribrachiatus orthostylus is 

sparse and possibly redeposited; 

Coccolithus crassus is absent.

Relative stratigraphic position 

uncertain, probably close to 981-

25-2(H).

Approximately 10 m below 981-25- 

2(H).

Includes Eocene dinoflagellates 

(N.O.F.) and trace fossil 

THelminthoida sp.

Age and Tectonic Inferences, Upper Cretaceous, Paleocene, and Eocene Strata, Big Pine Mountain Area, California S11
Table 1. Age-diagnostic taxa, zonation, and paleobathymetry of microfossils from Big Pine Mountain area Continued

Field No. Age-diagnostic and associated taxa Age or zone Paleobathymetry Remarks

V-650

V-649

V-648

V-647

V-646 

1084-16-1(M;

V-645 

1084-16-2(M;

V-644 

V-643 

V-642

V-641 

V-640

V-639 

V-638

Calcareous nannofossils: Sphenolithus radians, Zygrablithus

bijugatus, Discoaster lodoensis, Helicosphaera lophota,

Coccolithus crassus, Tribrachiatus orthostylus 

Benthic foraminifers: Bulimina macilenta, B. callahani, Anomalina

dorri aragonensis, Marginulina asperuliformis 

Planktonic foraminifers: Globorotalia caucasica, G. broedermannl,

G. subbotinae, G. aragonensis, G. soldadoensis

Palynomorphs: Wetzeliella articulata

Calcareous nannofossils: Tribrachiatus orthostylus, Sphenolithus

radians, S. anarrhopus, Chiasmolithus grandis, Discoaster mirus,

D. minimus, D. barbadiensis, Rhabdosphaera crebra, Toweius sp. 

Benthic foraminifers: Bulimina macilenta, B. callahani, Anomalina

dorri aragonensis 

Planktonic foraminifers: Globorotalia subbotinae, G. aragonensis, G.

soldadoensis

Calcareous nannofossils: Tribrachiatus orthostylus, Sphenolithus 

radians, Chiasmolithus grandis, Lophodolithus nascens, 

Transversopontis pulcher, Coccolithus cribellum, Toweius sp., 

Markalius sp.

Benthic foraminifers: Bulimina macilenta, B. callahani, Anomalina 

dorri aragonensis

Planktonic foraminifers: Globorotalia subbotinae, G. aragonensis, G. 

soldadoensis

Palynomorphs: assemblage (not zoned) 

Calcareous nannofossils: barren

Benthic foraminifers: Recurvoides spp., Silicosigmoilina sp., 

Saccamina sp.

(Report pending)

Palynomorphs: AlangiopolUs cribellata type, Betulaepollinites sp., 

Paraalnipollinites confusus, probable Ulmipollinites tricostatus

Palynomorphs: assemblage not zoned

Benthic foraminifers: Recurvoides spp., Silicosigmoilina sp., 

Saccamina sp.

Palynomorphs: "Much the same flora as in R3368," Field number 

1084-16-1(M)

Calcareous nannofossils: barren

do.

Benthic foraminifers: (same as in V-645)

Palynomorphs: Areoligera sp.

Benthic foraminifers: (same as in V-645)

Palynomorphs: ICyclonepheleum sp. ? Glaphyrocysta sp. 

Benthic foraminifers: (same as in V-645)

(Report pending)

Palynomorphs: Isabelidinium cretaceum, Dinogymnium acuminatum 

Benthic foraminifers: (same as in V-645)

CPU

Laiming C 

P8

Eocene 

Probable CP9

Laiming C 

P7

Probable CP9

Laiming C 

P7

Cretaceous to 

Paleogene

Late Paleocene 

(N.O.F.)

Cretaceous to 

Paleogene

Probable late 

Paleocene 

(N.O.F.)

Cretaceous to 

Paleogene

Late Cretaceous 

to Paleogene

Cretaceous to 

Paleogene

Probable 

Paleogene

Cretaceous to 

Paleogene

Maastrichtian

Cretaceous to

Paleogene

Upper to middle 

balhyal

Upper to middle 

bathyal

Upper to middle 

bathyal

Bathyal

Bathyal

Bathyal

Bathyal

Bathyal

Bathyal

Discoaster diastypus absent.

Rare specimens of Micula

staurophora reworked from Upper' 

Cretaceous strata; Discoaster 

diastypus absent.

Assigned to R3368, USGS, Reston, 

Va., palynology archive.

Assigned to R3373, USGS, Reston, 

Va., palynology archive.

Comstock's (1975) locality 2 (late 

Paleocene) may project 

stratigraphically between here and 

V-642.

Rare specimens of both taxa.

S12 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
Table 1. Age-diagnostic taxa, zonation, and paleobathymetry of microfossils from Big Pine Mountain area Continued

Field No.

V-637

V-636

V-635

GP46,

V-524,

V-634

V-633

V-524a

V-632

V-631

V-630

V-629

V-628

V-527

65CB75

V-692

V-691

V-525

V-690

Age-diagnostic and associated taxa

(Report pending)

(Report pending)

Palynomorphs: Dinogymnium acuminatum

Benthic foraminifers: Cribrostomoides cretacea, Silicosigmoilina

califomica, Dorothia bulletta, Trochammina sp. cf. T.

ribstonensis, Bathysiphon sp., Gaudryina sp. cf. G. flliformis,

Cibicides sp. cf. C. coonensis (R.L.P., GP46)

Palynomorphs: Proteacidites thalmanni

Palynomorphs: habelidium cretaceum, Dinogymnium acuminatum

Benthic foraminifers: assemblage not zoned (R.E.A.)

(Report pending)

Palynomorphs: Proteacidites thalmanni

Benthic foraminifers: Recurvoides spp., Silicosigmoilina sp.,

Saccammina sp.

Benthic foraminifers: (same as in V-631)

do.

Palynomorphs: Dinogymnium acuminatum

Benthic foraminifers: (same as in V-631)

Benthic foraminifers: assemblage (not zoned)

do.

(Report pending)

(Report pending)

Benthic and planktonic foraminifers: assemblage (not zoned)

(Report pending)

Age or zone

Santonian to

Maastrichtian

Late(?)

Cretaceous,

possibly

Goudkoff C and

(or) Dl

(R.L.P.); Late

Cretaceous

(R.E.A., V-524)

Late Cretaceous

to Paleogene

Maastrichtian

Late Cretaceous

(R.E.A.)

Possible Late(?)

Cretaceous

Cretaceous to

Paleogene

do.

do.

Santonian to

Maastrichtian

Cretaceous to

Paleogene

Late Cretaceous

(R.E.A.)

Probable

Cretaceous

(J.W.R.)

Late Campanian

or Maastrichtian

Goudkoff D2 to

E (R.E.A.)

Paleobathymetry

Bathyal

Bathyal

do.

do.

do.

Remarks

GP46 = M43 of Vedder and others

(1967). All three samples from

approximately same site.

Rare specimens.

Common specimens of /. cretaceum.

Common specimens of P. thalmanni.

Abundance of Recurvoides spp.

suggests Cretaceous rather than

Paleogene age.

Do.

Do.

Do.

Stratigraphic position uncertain.

Collected about 130 m WSW of V628.

Stratigraphic position uncertain.

Collected about 225 m S of V-628.

Contains calcareous benthic,

planktonic, and arenaceous

foraminifera (R.E.A.).

Approximately same Stratigraphic

position as megafossil locality V526 Pachydiscus ootacodensisl

(M7589, USGS, Menlo Park,

Calif., Mesozoic megafossil

collection).

Age and Tectonic Inferences, Upper Cretaceous, Paleocene, and Eocene Strata, Big Pine Mountain Area, California S13
Table 1. Age-diagnostic taxa, zonation, and paleobathymetry of microfossils from Big Pine Mountain area Continued

Field No.

V-689

1084-16-1(H)

Age-diagnostic and associated taxa

Benthic foraminifers: Eponides sp. cf. E. bandyi, Gyroidina 

quadrata, Gaudryina navarroensis crassiformis, Bulimina spinata

Palynomorphs: Proteacidites spp. and assemblage

Age or zone

Goudkoff D2 to E

Santonian to 

Danian (N.O.F.)

Paleobathymetry

Bathyal

Remarks

Stratigraphic position uncertain; 

within 10 m of base. About 175 m 

north of measured section.

Assigned to R3370, USGS, Reston, 

Va., palynology archive.

the mudstone sequence. This lentil consists of medium- to 

coarse-grained micaceous quartzofeldspathic sandstone in 

which bedding is normally graded, indistinct, or absent. Other 

lenticular sandstone beds are present in the mudstone along 

Big Pine Road less than 1 km southeast of the measured section, where they are as much as 5 m thick. All of these lenticular sandstone beds are interpreted to be channel-fill 

deposits. Farther southeast near the Big Pine fault, a zone of 

sandstone beds within the mudstone unit is estimated to be 

about 15m thick.

Although covered by colluvium and landslides at Big 

Pine Mountain, the upper contact of the condensed(?) succession is sharply defined along the sheer eastern wall of lower 

Big Pine Canyon near its juncture with the Sisquoc River. On 

the cliff face, mudstone beds (lithofacies G) are depositionally 

overlain without discordance by resistant sandstone beds 

(lithofacies C and B of Ricci-Lucchi, 1975) that constitute 

the basal part of the superjacent sequence. Where examined, 

the contact is characterized by flame structures, small load 

casts, and an absence of large-scale erosional features.

Zonation and Age

Paleontologic evidence indicates that the condensed(?) 

succession includes strata that range in age from Late Cretaceous through most of the early Eocene. The assigned ages 

are based upon zonations that are derived chiefly from assemblages of calcareous nannofossils, palynomorphs, and 

foraminifers supplemented by rare mollusks in the lower part. 

Biostratigraphic subdivision of the microfossil assemblages 

is based upon the following schemes: 

Calcareous nannofossils: CP zones, Okada and Bukry

(1980);

Palynomorphs: age or stage, Williams and Bujak (1985); 

Planktonic foraminifers: P zones, Blow (1969), as emended

by Toumarkine and Luterbacher (1985); and 

Benthic foraminifers: H-C zones, Goudkoff (1945), as 

emended by Almgren (1986) for Upper Cretaceous assemblages; E-A zones, Laiming (1939), as emended by 

Almgren and others (1988) for Paleogene assemblages.

Ages of geochronologic units and boundary ages of 

chronostratigraphic units follow the usage of Haq and others

(1987) with the exception of the early Eocene-middle Eocene 

boundary, which, in California, is more easily defined by the 

CPll/CP12a indicators. Preliminary results from the biostratigraphic analyses are summarized in tabular form (table 

1), and the zones represented are shown on the Stratigraphic 

column (fig. 5).

Fossils from the lowermost 30 m of the measured section (fig. 5) include foraminifers and mollusks indicative of 

the Campanian and (or) Maastrichtian Stages (table 1, V689, V-525, and V-526). Identifiable calcareous nannofossils 

and palynomorphs are absent in samples from the same 30- 

m-thick interval. Above an overlying deeply weathered, partly 

covered interval about 40 m thick, palynomorph assemblages 

that range from Santonian to Maastrichtian occur through 80 

m of section; two of these assemblages (V-524a, V-638) are 

restricted to the Maastrichtian Stage. Foraminifers from the 

same 80-m-thick interval are long-ranging agglutinated taxa 

that were assigned either a Cretaceous age or a Cretaceous to 

Paleogene age. The Paleocene foraminiferal assemblage listed 

by Comstock (1975, locality 2; 1976, locality BP-4m) from 

a single site on Big Pine Mountain, when projected into the 

measured section, falls at least 60 m above the 80-m-thick 

interval (near V-642). Thus, the Cretaceous-Tertiary boundary may lie between 160 m and 200 m from the base (between V-638 and V-641). Late Paleocene palynomorphs are 

present in samples about 238 m and 250 m above the base 

(1084-16-2(M), 1084-16-1(M)). The lowest Stratigraphic 

occurrence of early Eocene taxa is in a sample from approximately 300 m above the base (V-648), in which Zone CP9 

calcareous nannofossils and Zone P7 planktonic foraminifers 

occur together. Younger early Eocene assemblages represent 

Zones CP10 and CPU and Zones P8 and P9 in the uppermost 30 m of section (V-650 to V-651). A sample (981-25- 

5(H)) from 5 cm below the upper contact in Big Pine Canyon 

contains microfossils indicating Zone CPU and Zone P9(?)-

If the duration of sedimentation is interpreted to have lasted 

from the end of early Maastrichtian through Zone CP1 1, as suggested by the preliminary biostratigraphic data in table 1, the 

time interval represented by the condensed(?) succession may 

have been 21.5 m.y. (Haq and others, 1987, time scale). Because 

there is no definitive fossil evidence for the presence or absence 

of the uppermost Maastrichtian Stage and all of the Danian Stage 

in the sequence, deposition may or may not have been continuS14 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
ous. If deposition was interrupted by an hiatus or erosion, a physical record of such an event is not apparent, and the sample suite 

is not biostratigraphically complete enough to be conclusive. If, 

indeed, the absence of uppermost Maastrichtian and Danian beds 

can be verified, approximately 8 m.y. of time are unaccounted 

for in the depositional record. Alternatively, these 

chronostratigraphic units may be present but are not recognizable biostratigraphically for several reasons, among which are 

the following:

1) Benthic foraminifers from open-ocean, deep-water 

forearc settings tend to have longer stratigraphic ranges 

than those from shallower water restricted environments 

(Almgren and others, 1988);

2) Exceptionally slow sedimentation rates and macro-benthic 

predation reduces microfaunal preservation (R.G. Douglas 

and D.S. Gorsline, written commun., 1993);

3) Diagenetic dissolution selectively destroys extremely 

small, age-diagnostic foraminifers;

4) Post-depositional deep weathering dissolves most calcareous foraminifers and nannofossils; and

5) Insufficient sampling and widely spaced sampling intervals hinder precise zonation.

In summary, nearly 22 m.y. of Late Cretaceous and early 

Tertiary time may have elapsed during deposition of the mudstone in the measured section at Big Pine Mountain; yet the position of the Cretaceous-Tertiary boundary in this section has 

not been established on either biostratigraphic or lithostratigraphic 

discontinuities. Evidence for the presence or absence of the 

Danian Stage is equivocal, and a definite Paleocene-Eocene 

boundary cannot be determined from the samples examined. 

Despite the lack of documentation for positioning these two 

boundaries and the Danian Stage in the measured section, the 

duration of time represented by the condensed(?) succession 

seems incontestable, even though unrecognized hiatuses may 

break the apparent continuity of deposition. Because stratal sequences that show uninterrupted sedimentation across the Cretaceous-Tertiary boundary have not been identified with certainty 

elsewhere in southern California, the assertion that deposition 

was continuous at Big Pine Mountain is likely to be challenged.

Superjacent Lower and Middle(?) Eocene Strata

The cliff-forming unit that overlies the condensed(?) succession is composed mainly of submarine-fan sandstone 

(lithofacies B). In the area between the top of Big Pine Mountain and the topographic saddle north of the former site of Alamar 

Guard Station (fig. 4), at least 1,000 m of strata are included in 

this sandstone unit; 3 to 4 km farther southeast, the unit may be 

1,200 m or more thick. Along its entire northeastern margin, this 

sandstone unit is truncated by the Sur-Nacimiento fault zone.

The sandstone in this unit is quartzofeldspathic, poorly 

sorted, and largely medium to very coarse grained. Thin, nonpersistent zones of granules and small pebbles are interspersed

in some beds, and pebble-cobble conglomerate (lithofacies A of 

Ricci-Lucchi, 1975) is interbedded at a few places. Bedding in 

the sandstone generally is thick to massive and locally is normally graded. Some beds display dish structure; others commonly are bioturbated. Relatively thin zones of mudstone and 

thin-bedded sandstone characteristic of lithofacies D and (or) E 

are sporadically present throughout the dominantly sandstone 

unit. Siltstone and very fine to fine-grained sandstone partings 

commonly delineate bedding.

Sediment-transport directions within this unit have been 

measured only along Big Pine Road, and the influence of postdepositional tectonic rotation is unknown. Bidirectional 

paleocurrent features, chiefly parting lineations, indicate either 

northeastward or southwestward flow. Rare groove and flute casts, 

together with fairly common, preferentially oriented flame structures suggest southward and westward flow directions. A sediment source northeast of the Sur-Nacimiento fault zone is 

inferred.

Biostratigraphic data from this unit are sparse. A small assemblage of poorly preserved mollusks collected about 120 m 

above the base of this unit at Big Pine Mountain suggests an age 

of early Eocene ("Capay" Stage). A larger assemblage of mollusks from granule-pebble conglomerate about 335 m above the 

base (fig. 5) may be early or middle Eocene in age ("Capay" or 

"Domengine" Stage). Although these assemblages were provisionally assigned a middle Eocene age by Vedder and others 

(1967, localities F18, F19), revisions of the West Coast Eocene 

stages (Squires, 1988) require the change to older Eocene ages. 

Comstock (1975,1976) reported early Eocene foraminifers from 

a locality (8, BP 3m) 2.0 km east-southeast of Big Pine Mountain. If projected along strike, his locality apparently falls 

stratigraphically slightly below the higher of the two mollusk 

localities. Foraminifers from a 15-cm-thick mudstone zone (5/ 

19/92B) about 260 m stratigraphically above the upper mollusk 

locality include benthic species that range from early into middle 

Eocene (equivalent to Zone P9-P10) and that suggest a depth of 

2,000 m or more (K.A. McDougall, written commun., 1993).

Conclusions

The condensed(?) succession of mudstone at Big Pine 

Mountain is indicative of very slow, possibly continuous, deepsea hemipelagic sedimentation. The lower contact represents an 

abrupt shutdown of rapid, coarse-clastic sedimentation, and the 

upper contact indicates a sudden renewal of relatively high rates 

of coarse-clastic deposition. Although slow deposition rates occurred in other Paleogene formations in the southern Coast 

Ranges, such as the Lodo Formation and Anita Shale of Kelley 

(1943), no other mudstone unit in this region is known to have 

endured as long or to have spanned the Cretaceous-Tertiary 

boundary. Other formations in the region may incorporate both 

upper Maastrichtian and Danian beds (Saul, 1983, 1986), but 

these rock units consist largely of relatively rapidly deposited sandstone and conglomerate.

Age and Tectonic Inferences, Upper Cretaceous, Paleocene, and Eocene Strata, Big Pine Mountain Area, California S15
119°40' (North edge partly overlaps south edge of figure 4)

34°40' 34°40'

119°40'

2 KILOMETERS

I

Figure 6. Generalized geologic map of area south of Big Pine fault. Location shown on figure 1 B, Patterns indicate 

distribution of limestone units and adjacent Cretaceous to Eocene strata. Modified from Vedder and others (1 967) and 

}.G. Vedder, R.D. Brown, \r., T.W. Dibblee, \r.f Hugh McLean, and R.G. Stanley (unpub. mapping, 1965-93).

CORRELATIVE SEQUENCES SOUTH OF BIG 

PINE FAULT

Grapevine Creek-Bluff Campground-Alamar 

Canyon Area

South of Big Pine Mountain and flanking the south side 

of the Big Pine fault is a 12-km-long belt of strata, chiefly 

mudstone, that extends eastward along the north limb of Loma 

Pelona syncline from the vicinity of Grapevine Creek to

Alamar Canyon (figs. IA and 6). Beds within this belt, the 

base of which is not exposed, contain microfossils that are 

Late Cretaceous, late Paleocene, and early Eocene in age 

(tables 2-4). As much as 800 m of strata are exposed in 

the segment of this belt that extends westward from Bluff 

Campground to East Fork Santa Cruz Creek. Discontinuous, thin zones of bioclastic limestone that were assigned 

to the Sierra Blanca Limestone by Gower and others (1966), 

Vedder and others (1967), and Comstock (1975, 1976) are 

present in the middle part (fig. 7 A, B). In contrast to the

S16 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
20

EXPLANATION 

Surficial deposits (Quaternary) 

Sedimentary rocks (Miocene)

Sedimentary rocks (middle and lower Eocene) Chiefly sandstone; 

commonly assigned to Juncal Formation and Matilija(?) Sandstone

Sierra Blanca Limestone (Paleocene) Includes strata for which name 

was proposed (Nelson, 1925) and type locality and type section in 

Indian Creek (Keenan, 1932; Vedder, 1972)

Sedimentary rocks (Paleocene) Mudstone and limestone, interbedded; 

contains thin limestone zones; called Sierra Blanca Limestone by 

some workers

Sedimentary rocks (lower Eocene to Upper Cretaceous) Chiefly 

mudstone; commonly called Juncal Formation

Sedimentary rocks (Upper Cretaceous) Chiefly mudstone; 

stratigraphically equivalent to upper part of type Mono Shale of 

Nelson (1925)

Sedimentary rocks (Upper Cretaceous) Chiefly sandstone; includes 

strata assigned to Indian Conglomerate and Mono Shale by Nelson 

(1925) and Keenan (1932)

Contact Includes well defined, poorly defined, and inferred

Fault Dotted where concealed; dashed where uncertain. Arrows 

indicate direction of movement

Strike and dip of beds

Inclined Representative of numerous readings, both measured and 

estimated

Overturned Queried where uncertain 

Vertical

Syncline Approximately located 

Overturned syncline Approximately located

Calcareous sandstone bed Shown in upper part of unit TKm; mapping 

incomplete

Fossil locality Shown on figure 7 or noted in tables 2-4 or in text

type area of the Sierra Blanca on the south limb of Loma 

Pelona syncline, there is no evidence for an unconformity 

at the base of the limestone zones (fig. 8). Where examined in the Bluff Campground area, the limestone commonly exhibits normally graded bedding, Bouma 

sequences, sharp erosional bases, and gradational tops; 

these features suggest turbidity-current deposition. The enclosing mudstone beds contain upper to middle and middle 

to lower bathyal assemblages of benthic foraminifers. Farther west, in the Grapevine Creek-East Fork Santa Cruz 

Creek area (fig. 1A), bioclastic limestone breccia probably 

represents downslope-transported algal-bank rubble 

(Vedder and others, 1994) rather than in situ bank deposits 

as interpreted by Comstock (1975,1976). Stratigraphically 

above the limestone zones, beds of reddish and greenish 

foraminiferal mudstone are locally present (fig. IB, C). 

These variegated mudstone beds are similar to the "Poppin

-fshale" of local usage in the Anita Shale of Kelley (1943). 

Non-persistent, largely calcareous sandstone turbidites 1 

to 10 m thick are interspersed in the mudstone upsection 

from the limestone zones (fig. 6). In the upper Indian Creek 

area, some of these calcareous quartzofeldspathic sandstone 

beds were mapped as Sierra Blanca Limestone by earlier 

workers. The mudstone belt is overlain gradationally by 

more than 1,000 m of lower and middle Eocene strata, 

chiefly sandstone (fig. 6; fig. IB), in which southward to 

westward sediment transport is suggested by measurements 

along Big Pine Road.

The stratigraphically lowest sample (V-693) from the 

mudstone belt was collected close to the trace of the Big 

Pine fault in a tributary of Indian Creek about 1.5 km east 

of Bluff Campground and contains calcareous nannofossils 

that range from Santonian into Maastrichtian (fig. 7C, table 

4). Zones CP5 into CP10 and CP11(?) and Zones P6 into 

P8-P9 are represented in samples from higher in the sequence (fig. 7; tables 2-4). The limestone zones near Bluff 

Campground are bracketed by mudstone beds that are assigned to Zones CP7 and CP8. Foraminifers from the Sierra Blanca Limestone at Big Pine Road (fig. 6) are 

assigned to Zone P4 or older and represent an open-marine, slope(?) setting (W.V. Sliter, written commun., 1993). 

Calcareous nannofossils from poorly exposed reddish and 

greenish mudstone beds directly overlying the Sierra 

Blanca where it crosses Big Pine Road (V-727, fig. 6) are 

assigned to Zone CP10 (David Bukry, written commun., 

1993), and Zone CP8 and CP9 beds are absent.

The easternmost outcrops of the mudstone belt that 

parallels the south side of the Big Pine fault extend across 

Alamar Canyon and are truncated by the fault in a tributary about 1.0 km east of Dutch Oven Campground (fig. 

1A) (Vedder and others, 1973, unit Ks?). Comstock (1975, 

1976) reported late Paleocene foraminifers from two sites 

east of the area shown on figure 6. Algal limestone beds 

apparently are absent in Alamar Canyon.

Coche Creek Area

Mudstone beds exposed along a 4-km stretch of Coche 

Creek between the North and South Branches of the Big 

Pine fault (figs. \A, \E) are similar to and probably correlative with those in the mudstone belt that extends between Grapevine Creek and Alamar Canyon. Samples 

collected from the banks of Coche Creek contain deepwater (>2,000 m) foraminifer assemblages that probably 

are early Tertiary in age (K.A. McDougall, written 

commun, 1993). Comstock (1975, 1976) reported late Paleocene foraminifers from a tributary to Coche Creek adjacent to the trace of the Big Pine fault. Algal limestone 

beds are not known to be present in this mudstone sequence.

Beds within the mudstone belt that parallels the south 

side of the Big Pine fault are coeval with those in the conAge and Tectonic Inferences, Upper Cretaceous, Paleocene, and Eocene Strata, Big Pine Mountain Area, California S17
B

Big Pine Road and vicinity, south of Bluff Campground

§.

5'

FEET METERS 

1000-1-300

-200

500-200- 

100-

EXPLANATION

-r-Zri Hq Mudstone

-..:.:!:.':.:.:.:.:..-J Sandstone

-100

-50 I I I I I

Fault

Upper East Fork Santa Cruz Creek 

1 .5 to 2.5 km west of Bluff Campground

I

LJJ z.

01

-?-

UJ

g

1

-?-

£

1

Benthic foraminifer zone

E 

Planktonic foraminifer 

zone

P4 

Calcareous nannofossil age or zone

Late

Pateocene

CP8-

Late- PaleoCP7/

CP7 

CP6-7-

CP5-

'c

Q. to

TKm

Tpl

TKm

o

5

irir^_-HIE-Crtl

 _r~^r~ 

Ir^j-^rl. ~-T~-^~-^r

- J- ' _ ~^~-^~-

- KHZ-IIril ZNot exposed in canyon; mudstone

on ad- joining Yrinpc 1 lU^CO

:-z-z-:

CO «=

^ 592-20-1 (M)

r-592-20-2(M) -592-20-3(M)

-591-20-5(M)

-591 -20-1 (M)

-591-20-2(M)

-591-20-3(M)

-591-20-4(M)

Big Pine fault

1 

&

LJJ

Z 

§

LJJ LJJB

Benthic foraminifer zone

C 

C? 

C C? 

Cor pseudoC

/ » __ E 

Planktonic foraminifer 

zone

P9 

P9 

P&-P10-

P7 P7 

P6 

Calcareous nannofossil age or zone

CP11 

CP 

10/11

CP10-

Earty Eocene<?)

Early 

Eocene

CP9(?)-

CP9 

Late-- 

Paleocene

CP8- CP8- 

CP7-

«

Q. CO

Te

TKm

Tpl 

TKm

o 

1 ----- J.ll.'-l'.l

: . -

- - - -

fTvr-VT:

 - ^

...............irnlr-ilrrl:

 -^^TJL

r.: . . .:.-

 -:::: :' : '-f. .-. : :

fL^_T5-;.4

T; T^_ .TTJ.VT

 _. . 

^-3--:-:

_ ._

Ei1^:^^^:~^.

o>SK

Si

V-661 

.-V-743 '^v-eeo

GP48 

V-742

_ V-659 

GP47

V-711

Discontinuous 

 exposures, landslides

V-741V-710

^V-709 

V-6525/20/91J

-V-658 -V^O^V-BM

.X^V-740 

^^V-707B^657, V-707 K~ V-739 

V5/20/91H 

X5/20/91G 

5/20/91A-D, AAA80-1

Big Pine fault

Upper Indian Creek area 1.5 to 1 .8 km east and southeast of Bluff Campground

I

LJJLJJ

LU§

2

I'H

Jft

CJ

Q.C3O

Benthic foraminifer 

zone

C C 

C 

Planktonic foraminifer 

zone

P8-P9-

P8-P9-

P8 

P9 

Calcareous nannofossil age or zone

\,

CP11(?)CP11(?)

CP10-^

CPKV11- CP9 

Late Creta--

ceous

1

Q. 

(0

Te

TKM

1

^i^is^i^.

 ^7 .-

'=Trrr~r.=^^:r.

i1^^/:rl-EI-E:;;r--^_r!'^

 z_"T_~i;.-- -E-i=-^-:-2 ^-z_-^.--~'"NbT'~

exposed in canyon; mudstone

on adjoining ridges.

Projected limestone

=rr=iT=:-

!!

V-697V-696

V-695

^-5/20/91 K\V-738 

NV-694

_V-693

Big Pine fault

Figure 7. Generalized composite stratigraphic columns of Upper Cretaceous and Paleogene rocks south of Big Pine fault (map units TKm, Tpl, 

and lower part of Te, fig. 6). Approximate locations of columns are shown on figure 6. Zonation and age assignments are explained in tables 2-4.
Table 2. Age-diagnostic taxa, zonation, and paleobathymetry of microfossils from upper East Fork Santa Cruz Creek

[Localities shown on figures 6 and (or) 1A. Arranged stratigraphically from top to bottom. See table 1 for sources of data]

Field No.

592-20- 1(M)

592-20-2(M)

592-20-3(M)

591-20-5CM)

59 1-20- 1(M)

591-20-2(M)

591-20-3(M)

Age-diagnostic and associated taxa

Calcareous nannofossils: Coccolithus pelagicus, Neochiastozygus 

junctus 

Benthic foraminifers: large assemblage (>45 taxa) (K.M.)

Calcareous nannofossils: Discoaster falcatus, D. multiradiatus, 

Ellipsolithus macellus, Fasciculithus typaniformis, 

Neochiastozygus distentus, Zygrablithus bijugatus 

Benthic foraminifers: large assemblage (>75 taxa) (K.M.)

Calcareous nannofossils: Coccolithus pelagicus, Fasciculithus 

tympaniformis(l), Zygrablithus sigmoides(l)

Benthic foraminifers: large assemblage (>75 taxa) (K.M.)

Calcareous nannofossils: Toweius sp. Coccolithus pelagicus, 

Fasciculithus tympaniformis. Discoaster nobilis, D. delicatus 

Benthic foraminifers: (Report pending)

Calcareous nannofossils: Toweius sp., Coccolithus pelagicus, 

Fasciculithus tympaniformis, Discoaster nobilis, D. delicatus, D. 

mohleri, Chiasmolithus consuetus

Benthic foraminifers: (Report pending)

Calcareous nannofossils: Coccolithus pelagicus, Fasciculithus 

tympaniformis, Chiasmolithus consuetus, Zygodiscus sp., Ericsonia 

robusta, Heliolithus reideli, Neochiastozygus chiastus, Prinsius 

bisulcus

Benthic foraminifers: Bulimina arkadelphiana, B. macilenta, Dorothia 

oxycona, Silicosigmoilina californica 

Planktonic foraminifers: Globorotalia psuedomenardii, Globigerina 

triloculinoides

Calcareous nannofossils: Coccolithus pelagicus, Toweius sp., 

Fasciculithus tympaniformis, Heliolithus kleinpelli, Chiasmolithus 

consuetus, C. bidens, Zygrablithus sp., Scapholithus fossilis 

Benthic foraminifers: (Report pending)

Age or zone

Late Paleocene 

(D.B.) 

Late Paleocene 

(K.M.)

CP8 (D.B.)

Late Paleocene

(K.M.)

Late 

Paleocene(?) 

(D.B.) 

Late Paleocene

(K.M.)

CP7

CP7

CP6-CP7

Laiming E 

P4

CP5

Paleobathymetry

Upper bathyal 

(K.M.)

>2,000 m

>2,000 m

Middle to lower 

bathyal

Remarks

Sample contains abundant 

calcareous debris. 

Includes planktonic taxa (K.M.).

Abundant flora.

Includes displaced upper bathyal 

and neritic indicators.

Contains reworked specimens of 

Cretaceous Micula decussata.

Includes displaced upper bathyal 

and neritic indicators.

Specimens rare and poorly 

preserved.

Specimens rare and poorly 

preserved.

Specimens rare and poorly 

preserved.

Specimens rare and poorly 

preserved.

Loma Pelona syncline Te

Big Pine fault

Figure 8. Schematic cross section, Loma Pelona syncline, vicinity of Big Pine Road. 

See figure 6 for symbol explanation. Units Tsb and Tpl represent Sierra Blanca 

Limestone and coeval limestone lenses. Dashed line in unit Te represents highest 

stratigraphic occurrence of beds similar to "Poppin shale" of local usage. T, toward; 

A, away. Small faults not shown.

Age and Tectonic Inferences, Upper Cretaceous, Paleocene, and Eocene Strata, Big Pine Mountain Area, California S19
Table 3. Age-diagnostic taxa, zonation, and paleobathymetry of microfossils from Big Pine Road south of Bluff Campground

[Localities shown on figures 6 and (or) 7B. Arranged stratigraphically from top to bottom. See table 1 for sources of data]

Field No. Age-diagnostic and associated taxa Age or zone Paleobathymetry Remarks

V-661

V-743

V-660

GP48

V-742

V-659

GP47

V-711

V-741

Calcareous nannofossils: Sphenolithus radians. Chiasmolithus

grandis, Coccolithus crassus, Discoaster lodoensis, Helicosphaera 

lophota, Rhabdosphaera tennis, Discolithina distincta, 

Chipfragmolithus acanthodes, Reticulofenestra samudorovi, 

Neochiastozygus distentus

Benthic foraminifers: Bulimina macilenta, B. callahani, Anomalina 

dorri aragonensis

Planktonic foraminifers: Globorotalia caucasica, G. spinulosa, 

Globigerina inaequispira

Calcareous nannofossils: Coccolithus crassus, Helicosphaera lophota

(D.B.) 

Benthic foraminifers: Karreriella conversa, Silicosigmoilina

californica, Cibicidoides subspiratus, Pullenia eocenica (K.M.)

Benthic foraminifers: Bulimina macilenta, B. callahani, Anomalina

dorri aragonensis 

Planktonic foraminifers: Globorotalia caucasica, G. spinulosa,

Globigerina inaequispira

Benthic foraminifers: Vaginulinopsis asperuliformis, Nodosaria 

latejugata, Rhabdamina eocenica, Tritaxilina colei, 

Haplophragmoides eggeri, Verneulina triangulata, 

Plectofrondicularia kerni, Parrella culter midwayana (R.L.P.)

Planktonic foraminifers: Globigerina yeguaensis, Globorotalia 

aragonensis, G. spinoinflata (R.L.P.)

Calcareous nannofossils: Coccolithus crassus, Discoaster lodoensis, 

Discoasteroides kuepperi, Helicosphaera lophota, Toweius 

magnicrassus (D.B.)

Benthic foraminifers: large assemblage (>30 taxa) (K.M.)

Calcareous nannofossils: Toweius sp., Chiasmolithus crassus,

Discoasteroides kuepperi, Tribrachiatus orthostylus, Sphenolithus 

radians, S. anarrhopus, Zygrhablithus bijugatus, Discoaster 

lodoensis, D. barbadiensis, D. diastypus, Helicosphaera seminulum

Benthic foraminifers: Bulimina macilenta, B. callahani, Anomalina 

dorri aragonensis

Planktonic foraminifers: Globorotalia caucasica, G. spinulosa, 

Globigerina inaequispira

Benthic foraminifers: Martinottiella sp. cf. M. petrosa, Rhabdammina 

eocenica, Haplophragmoides eggeri, Eggerella sp., Dorothia 

principiensis, Tritaxalina colei, Clavulinoides californicus, 

Allomorphina conica, Plectofrondicularia kerni, Amphimorphina 

ignota, Cibicidoides sp. cf. C. venezuelanus, Cibicides 

martinezensis C. pachecoensis, Asterigina crassaformis 

umbilicatula, Trifarina advena californica, Hastigerina micro, 

Dentalina basiplanata, Lenticulina theta, Pleurostomella sp. cf. P. 

alternans

Planktonic foraminifers: Globigerina cf. G. yeguaensis, Globorotalia 

aragonensis, G. spinuloinflata (R.L.P.)

Calcareous nannofossils: Zygrablithus bijugatus, Chiasmolithus 

grandis, C. magnicrassus, Discoaster lodoensis, Discoasteroides 

kuepperi, Helicosphaera seminulum, Lophodolithus reniformis, 

Sphenolithus radians, Tribrachiatus orthostylus (D.B.)

Benthic foraminifers: large assemblage (>75 taxa) (K.M.)

Calcareous nannofossils: Zygrablithus bijugatus, Toweius

magnicrassus, Tribrachiatus orthostylus (D.B.) 

Benthic foraminifers: Bulimina macilenta, Cibicidoides venezuelana,

Spiroplectammina richardi (K.M.)

Probable CP12a

Laiming C to

pseudo-C 

P9

CPU (D.B.)

Early to middle 

Eocene (K.M.)

Laiming C to

pseudo-C 

P9

Early or early 

middle Eocene 

(R.L.P.)

P9 toPll

CPU (D.B.)

Early to middle 

Eocene (K.M.)

CPU

Laiming C to

pseudo-C 

P9

Laiming C

P9 to PI 1

CP10/CP11 (D.B.)

Early Eocene

(K.M.)

CP10 (D.B.)

Upper to middle 

bathyal

Lower bathyal

Upper to middle 

bathyal

>2,000 m (K.M.)

Upper to middle 

bathyal

>2,000 m (K.M.)

Middle to lower 

bathyal

Specimens frequent, poorly

preserved. Subzone tentative in 

absence of Discoaster 

sublodoensis. Stratigraphic 

position uncertain.

Benthic assemblage coeval with P9 

(K.M).

M51 of Vedder and others (1967). 

Benthic assemblage equivalent to 

P8 to early P9 (K.M.).

Benthic assemblage coeval with P8 

to P9 (K.M.).

M47 of Vedder and others (1967). 

Benthic assemblage equivalent to 

P8 to early P9 (K.M.).

Specimens common.

Includes abundant planktonic taxa. 

Benthic assemblage equivalent to 

P9 (K.M.).

Benthic assemblage coeval with P7 

to P14 (K.M.).

S20 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
Table 3. Age-diagnostic taxa, zonation, and paleobathymetry of microfossils from Big Pine Road south of Bluff Campground Continued

Field No. Age-diagnostic and associated taxa Age or zone Paleobathymetry Remarks

V-710

V-709

V-652

5/20/9 U

V-658

V-708

V-653

V-654 

V-655 

V-656

V-740 

V-707B

Calcareous nannofossils: Tribrachiatus orthostylus (large and small) 

(D.B.)

Benthic foraminifers: unidentifiable (K.M.)

Calcareous nannofossils: Chiasmolithus grandis, Coccolithus 

formosus, Discoaster salisburgensis, Sphenolithus radians, 

Tribrachiatus orthostylus, Zygrablithus bijugatus, Braarudosphaera 

bigelowii (D.B.)

Benthic foraminifers: large assemblage (>50 taxa) (K.M.)

Benthic foraminifers: Anomalina garzaensis, Buliminella grata 

Planktonic foraminifers: Globigerina eocaena, G. linaperta

Calcareous nannofossils: Zygrhablithus bijugatus, Sphenolithus 

radians, Tribrachiatus orthostylus, Discoasteroides kuepperi, 

Toweius sp., Discoaster barbadiensis, D. minimus, D. falcatus, 

Transversopontis pulcher, Lophodolitus mascens, Ellipsolithus 

macellus

Benthic foraminifers: (Report pending)

Calcareous nannofossils: Tribrachiatus orthostylus, Sphenolithus 

radians, S. anarrhopus, Chiasmolithus grandis, Toweius sp., 

Discoaster delicatus, D. mediosus, Ellipsolithus macellus, Ericsonia 

subpertusa, Neochiastozygus chiastus

Benthic foraminifers: Bulimina macilenta, B. callahani, Anomalina 

dorri aragonensis

Planktonic foraminifers: Globorotalia formosa formosa, G. formosa 

gracilis, G. subbotinae

Calcareous nannofossils: Neochiastozygus junctus, Coccolithus 

pelagicus, Chiasmolithus califomicusl, Micrantholithus aequalis

Benthic foraminifers: Anomalinoides welleri, Bulimina midwayensis, 

B. callahani, Cibicidoides dayi, C. hvpalis, Osangularia 

velascoensis, Alabamina wilcoxensis (K.M.)

Calcareous nannofossils: Toweius sp., Discoaster multiradiatus, 

Ellipsolithus macellus, Fasciculithus tympaniformis, Zygrhablithus 

bijugatus

Benthic foraminifers: Anomalina garzaensis, Buliminella grata

Planktonic foraminifers: Globigerina eocaena, G. linaperta 

Benthic foraminifers: assemblage; age-diagnostic taxa absent 

do.

Benthic foraminifers: Anomalina garzaensis, A. dorri aragonensis,

Bulimina corrugata 

Planktonic foraminifers: Globorotalia quetra

Calcareous nannofossils: Discoaster multiradiatus, Fasciculithus

schaubii, Neochiastozygus distentus 

Benthic foraminifers: assemblage of >20 taxa, difficult to identify

(K.M.)

Calcareous nannofossils: Coccolithus pelagicus, Fasciculithus 

tympan ifo rmis ?

Benthic foraminifers: Anomalinoides rubiginosus, Bulimina callahani 

(K.M.)

Early Eocene(?) 

(D.B.)

Early Eocene 

(D.B.)

Early to middle 

Eocene, 

probable early 

(K.M.)

Eocene

do.

CP9

Probable CP9

Laiming C 

P7

Late Paleocene 

(D.B.)

Late Paleocene 

(K.M.)

CP8

Eocene 

do.

Laiming C to

pseudo-C 

P6 to P9

CP8 (D.B.)

Late Paleocene 

(K.M.)

Late

Paleocene(?) 

(D.B.)

Late Paleocene 

(K.M.)

2,000 to 4,000 m 

(K.M.)

Upper to middle 

bathyal

Upper to middle 

bathyal

Upper middle 

bathyal (K.M.)

Upper to middle 

bathyal

Bathyal 

Bathyal

Upper to middle 

bathyal

Middle bathyal

Very sparse flora.

Casts and molds only (K.M.). 

Abundant flora.

Includes displaced neritic indicators. 

Benthic assemblage equivalent to 

P8 to P10 (K.M.).

Approximately the same stratigraphic 

position as V-709.

Contains reworked specimens of Late 

Cretaceous Micula staurophora.

Species frequent to rare, poorly 

preserved. CP9 tentative in the 

absence of Discoaster diastypus.

Sparse, poorly preserved flora.

Limestone debris in mudstone

turbidite 3 m upsection. 

Includes displaced neritic indicators.

Benthic assemblage equivalent to

P5 to P6a (K.M.).

Contains reworked specimens of Late 

Cretaceous Micula staurophora.

Upper middle 

bathyal (K.M.)

Sampled beds may have been 

displaced by landslide.

Specimens poorly preserved.

Assemblage coeval with P4 to P6 

(K.M.).

Very sparse specimens, abundant 

calcareous debris. About 50 cm 

upsection from limestone lens.

Includes displaced neritic indicators 

(K.M.).

Age and Tectonic Inferences, Upper Cretaceous, Paleocene, and Eocene Strata, Big Pine Mountain Area, California S21
Table 3. Age-diagnostic taxa, zonation, and paleobathymetry of microfossils from Big Pine Road south of Bluff Campground Continued

Field No. Age-diagnostic and associated taxa Age or zone Paleobathymetry Remarks

V-707

V-657

V-739

5/20/91H

5/20/91G

AAA80-1

Calcareous nannofossils: Coccolithus pelagicus, Discoaster nobilis, 

Heliolithus sp., Fasciculithus sp. cf. F. involutus

Benthic foraminifers: Anomalinoides welleri, Bulimina cacumenta, B. 

callahani, B. midwayensis, Cibicidoides dayi, Osangularia 

velascoensis, Alabamina wilcoxensis (K.M.)

Calcareous nannofossils: Toweius sp., Discoaster multiradiatus, 

Ellipsolithus macellus, Fasciculithus tympaniformis, F. involutus, F. 

schaubi, Helicolithus kleinpelli, Zygodiscus plectopons

Benthic foraminifers: Anomalina garzaensis, A. dorri aragonensis, 

Bulimina corrugata

Planktonic foraminifers: Globorotalia aequa

Benthic foraminifers: assemblage of >20 identifiable taxa (K.M.)

Calcareous nannofossils: Toweius sp., Coccolithus pelagicus, 

Fasciculithus tympaniformis, Discoaster nobilis, D. delicatus, D. 

mohleri, Chiasmolithus consuetus

Calcareous nannofossils: Toweius sp., Coccolithus pelagicus, 

Fasciculithus tympaniformis, Discoaster nobilis, D. delicatus

Benthic foraminifers: Bathysiphon sp. cf. B. vitta, B. varians, 

Trochammina sp. cf. T. boehmi, Silicosigmoilina californica, 

Spiroplectammina gryzbowskii

densed(?) succession at Big Pine Mountain. This belt differs only in that it locally contains thin zones of limestone 

and variegated mudstone and is more than twice as thick, even 

though the lower part is truncated by the Big Pine fault.

PARTLY CORRELATIVE SEQUENCES, 

SOUTHEASTERN SAN RAFAEL MOUNTAINS

Upper Mono Creek-Upper Agua Caliente 

Canyon Area

About 12 km southeast of Big Pine Mountain on the south 

limb of Loma Pelona syncline near the juncture of Alamar 

Canyon and Mono Creek, lower Eocene mudstone at the 

base of the Juncal Formation rests without discordance on 

unnamed Upper Cretaceous mudstone and thin-bedded 

sandstone (Vedder and others, 1973), and the Sierra Blanca 

Limestone is absent (fig. 3, col. 6). In the bed of Mono 

Creek about 1.0 km east-northeast of The Narrows, the base 

of the Juncal is marked by a 0- to 10-cm-thick layer of 

poorly sorted, very fine to very coarse grained sandstone 

that includes abundant subangular pebbles of mudstone and 

rare rounded pebbles of limestone. A 10-m-thick sequence

Late Paleocene 

(D.B.)

Late Paleocene 

(K.M.)

CP8

Laiming C 

P6

Late Paleocene 

(K.M.)

CP7 

CP7

Laiming E(?)

Upper middle 

bathyal (K.M.)

Upper to middle 

bathyal

Middle bathyal

Abundant calcareous debris. 

Specimens sparse and poorly 

preserved. One reworked 

specimen of Cretaceous Micula 

decussata. About 40 cm 

downsection from limestone lens 

10 cm thick.

Includes many displaced upper 

bathyal and neritic indicators 

(K.M.).

Specimens poorly preserved.

Assemblage coeval with P4 to P6 

(K.M.).

Specimens rare, poorly preserved.

Specimens rare, poorly preserved. 

Contains reworked specimens of 

Late Cretaceous Micula 

staurophora.

Sample collected and analyzed by 

A.A. Almgren.

of reddish and greenish silty clay stone beds directly above 

the base contains Zone CPU calcareous nannofossils, together with Zone P8 to lower P9 and pseudo-C Zone foraminifers (Bukry and others, 1977; R.L. Pierce, written 

commun., 1968; K.A. McDougall, written commun., 1993). 

Early to middle Eocene nannofossil and foraminiferal assemblages are present in the overlying 475 m of 

interbedded sandstone and mudstone in the Juncal Formation along this segment of Mono Creek.

The thin belt of reddish and greenish silty claystone 

in the lower part of the Juncal Formation is traceable for 

more than 10 km from the southeastern side of Sierra 

Blanca Mountain, where it overlies the Sierra Blanca Limestone, to the Munson Creek fault in Agua Caliente Canyon 

(figs. I A, B). The limestone, however, lenses out about 3 

km southeast of Sierra Blanca Mountain, and Paleocene 

mudstone beds similar to those along the south side of the 

Big Pine fault are not known to be present between Alamar 

Canyon and Agua Caliente Canyon. A foraminiferal assemblage from about 75 m below the base of the Juncal 

northeast of The Narrows in Mono Creek is assigned to 

Zones upper E to lower D-2 (late Campanian to early 

Maastrichtian) (R.L. Pierce, written commun., 1968; W.V. 

Sliter, written commun., 1992). Although there is no measurable discordance across the contact, a 15- to 20-m.y.

S22 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
Table 4. Age-diagnostic taxa, zonation, and paleobathymetry of microfossils from upper Indian Creek and vicinity

[Localities shown in figures 6 and (or) 1C. Arranged stratigraphically from top to bottom. See table 1 for sources of data]

Field No. Age-diagnostic and associated taxa Age or zone Paleobathymetry Remarks

V-697

V-696

V-695

5/20/9 IK

V-706A

V-737

V-738

V-694

V-693

Calcareous nannofossils: Discoaster lodoensis, Coccolithus crassus, 

C. pelagicus, C. formosus, C. gammation, Chiasmolithus grandis, 

Zygrhablithus bijugatus, Sphenolithus radians, Discoaster kuepperi

Benthic foraminifers: Anomalina dorri, Asterigina crassaformis, 

Marginulina asperuliformis

Planktonic foraminifers: Globorotalia aragonensis, G. caucasica, G. 

broedermanni, G. quetra

Calcareous nannofossils: Discoaster lodoensis, Coccolithus crassus, 

C. pelagicus, C. formosus, C. grammation, Chiasmolithus grandis, 

Zygrhablithus bijugatus, Sphenolithus radians, Discoaster kuepperi

Benthic foraminifers: Anomalina dorri, Asterigina crassaformis, 

Marginulina asperuliformis

Planktonic foraminifers: Globorotalia aragonensis, G. caucasica, G. 

broedermanni, G. quetra

Calcareous nannofossils: Discoaster lodoensis, Coccolithus crassus, 

C. pelagicus, C. formosus, C. grammation, Chiasmolithus grandis, 

Zygrhablithus bijugatus, Sphenolithus radians, Discoaster kuepperi

Benthic foraminifers: Anomalina dorri, Asterigina crassaformis, 

Marginulina asperuliformis

Planktonic foraminifers: Globorotalia caucasica, G. aragonensis, G. 

subbotinae

Calcareous nannofossils: Coccolithus pelagicus, C. eopelagicus, C. 

formosus, C. grandis, Tribrachiatus orthostylus, Discoaster 

lodoensis, Zygrhablithus bijugatus, Sphenolithus radians, 

Rhabdosphaera rudis

Benthic foraminifers: Asterigerina crassaformis, Vulvulina curta

Planktonic foraminifers: Globorotalia sp. cf. G. subbotinae, G. 

aragonensis

Calcareous nannofossils: Chiasmolithus grandis, Coccolithus

magnicrassus, Discoasteroides kuepperi, Tribrachiatus orthostylus 

Benthic foraminifers: large assemblage (>70 taxa) (K.M.)

Calcareous nannofossils: Chiasmolithus grandis, Discoaster

barbadiensis, Rhabdosphaera perlonga, Tribrachiatus orthostylus 

(D.B.)

Benthic foraminifers: large assemblage (>60 taxa) (K.M.)

Calcareous nannofossils: Chiasmolithus grandis, Discoaster

lodoensis, Cophodolithus nascens (D.B.) 

Benthic foraminifers: large assemblage (>45 taxa) (K.M.)

Calcareous nannofossils: Zygrhablithus bijugatus, Sphenolithus 

radians, Tribrachiatus orthostylus, Discoaster kuepperi, D. 

barbadiensis, Toweius sp.

Calcareous nannofossils: Micula staurophora

Probable CPU

Laiming C 

P8toP9 

Probable CP11

Laiming C 

P8 toP9 

Probable CPU

Laiming C 

P8

CP10

Laiming C 

P8

CP10/CP11 (D.B.)

Early Eocene 

(K.M.)

Early Eocene 

(D.B.)

Early Eocene 

(K.M.)

CP10 to CP12

(D.B.) 

Late early Eocene

(K.M.)

CP9

Coniacian to 

Maastrichtian

Upper to middle 

bathyal

Upper to middle 

bathyal

Upper to middle 

bathyal

Upper to middle 

bathyal

Lower bathyal 

(K.M.)

>2,000 m (K.M.)

Lower bathyal 

(K.M.)

Specimens rare, poorly preserved.

Zone assignment questionable

owing to uncertainty of first

occurrence of C. crassus. 

Sample from reddish and greenish

mudstone zone similar to "Poppin

shale" of local usage.

Specimens rare, poorly preserved.

Zone assignment questionable

owing to uncertainty of first

occurrence of C. crassus. 

Sample from reddish and greenish

mudstone zone similar to "Poppin

shale" of local usage.

Co-occurrence of G. subbotinae and 

G. caucasica suggests assignment 

to P8 rather than P8 to P9 as in 

V-696 and V-697.

Specimens frequent, poorly

preserved. Stratigraphic position 

uncertain.

Specimens abundant, overgrown.

Includes planktonic taxa. Benthic 

assemblage equivalent to early 

P9 (K.M.).

Includes abundant planktonic

foraminifers. Benthic assemblage 

equivalent to P8 to P9 (K.M.).

Poor flora.

Includes abundant planktonic

foraminifers. Benthic assemblage 

equivalent to early P9 (K.M.). 

Relative Stratigraphic position 

uncertain.

Specimens frequent, poorly 

preserved.

Specimens rare, poorly preserved; 

includes unidentifiable placolith 

fragments.

Age and Tectonic Inferences, Upper Cretaceous, Paleocene, and Eocene Strata, Big Pine Mountain Area, California S23
interval of nondeposition and (or) erosion is indicated by 

the biostratigraphic evidence; and the lowermost part of 

the Juncal in Mono Creek north of the Hildreth and Munson 

Creek faults correlates with only the uppermost part of the 

condensed(?) succession at Big Pine Mountain.

Lower Mono Creek-Santa Ynez River Area

About 20 to 25 km south of Big Pine Mountain near 

the Santa Ynez River, Campanian and older strata are disconformably overlain by shale and sandstone of the Juncal 

Formation, and here the Sierra Blanca Limestone has limited distribution (Page and others, 1951; Dibblee, 1966). 

According to Thompson (1988), the lowermost shale beds 

in the Juncal east of lower Agua Caliente Canyon contain 

Zone CPU calcareous nannofossils and Zone P8 planktonic foraminifers, which suggest that a 20-m.y. hiatus 

separates the Campanian strata and the base of the Juncal 

Formation in that area, and beds coeval with most of the 

condensed(?) succession are absent. Moreover, Thompson 

(1988) reported redeposited Late Cretaceous and late Paleocene calcareous nannofossils in his study area, which 

suggests that strata of these ages were being eroded nearby 

during deposition of the Juncal.

Near the confluence of Mono Creek and the Santa 

Ynez River, the Sierra Blanca Limestone unconformably 

overlies Upper Jurassic to Lower Cretaceous strata; about 

2.5 to 3.5 km to the southeast near the mouth of Agua 

Caliente Canyon, the limestone is unconformable on Upper Cretaceous strata. The Sierra Blanca Limestone in these 

two areas is early Eocene in age (Sliter and others, 1994; 

Whidden and others, 1995a).

PARTLY CORRELATIVE SEQUENCES, 

SOUTHWESTERN SAN RAFAEL MOUNTAINS

Red rock Canyon-Oso Canyon area

In the Redrock-Oso Canyon area about 20 km southwest of Big Pine Mountain (fig. 1A), lower Eocene strata 

on the south side of the Little Pine fault unconformably 

overlie the Franciscan Complex and were mapped as the 

Sierra Blanca Limestone and Juncal Formation by Dibblee 

(1966). Although biostratigraphic data from these units are 

incomplete, preliminary results suggest an age range of 

early to middle Eocene. Schussler (1981) reported longranging Zone P4 to P10 foraminifers from glauconitic 

mudstone in the fore-slope-talus limestone facies and Zone 

P9 and P10 assemblages from the lower part of the Juncal. 

Samples of limestone collected by K.J. Whidden in Oso 

Canyon contain Zone P7 to P8 and P9 to P10 planktonic 

foraminifers (W.V. Sliter, written commun., 1993; Whidden 

and others, 1995a). Accordingly, this sequence includes

beds that are the same age as the uppermost part of the 

condensed(?) succession at Big Pine Mountain.

Lazaro Canyon-Cachuma Creek area

About 22 km west-southwest of Big Pine Mountain, 

Franciscan Complex rocks are depositionally overlain by a 

small tract of lower Eocene strata along the ridge that separates Lazaro Canyon and Cachuma Creek (figs. 1A, IB). These 

strata were mapped as the Meganos Formation by Nelson 

(1925), who found orbitoid foraminifers, echinoids, and mollusks in limestone at the base. The same beds were mapped 

as Sierra Blanca Limestone and Juncal Formation by both 

Dibblee (1966) and Hall (1981), who showed them in a faultbounded block within the Little Pine fault zone. Vedder and 

others (1991) reported orbitoid foraminifers from the limestone and early or middle Eocene planktonic foraminifers from 

mudstone beds estimated to be about 45 to 75 m upsection 

from the limestone. The age of these mudstone beds is here 

revised to latest early and earliest middle Eocene on the basis 

of recently collected samples that contain Zone CPU or 

CP12a and CP12a to CP12b calcareous nannofossils. Thus, 

the highest sampled mudstone beds near Lazaro Canyon are 

slightly younger than the uppermost beds in the condensed(?) 

sequence at Big Pine Mountain and are partly correlative with 

beds that may be as much as 1,800 m upsection from the 

Paleocene limestone zones in the Bluff Campground area 

south of the Big Pine fault.

COMPARABLE SEQUENCES IN ADJACENT 

AREAS

Upper Pine Creek Area

Near upper Pine Creek about 30 km north-northeast of 

Santa Maria (fig. 1A inset), a limited exposure of lower Eocene 

mudstone on the southwest side of the Sur-Nacimiento fault 

zone apparently is unconformable on Upper Cretaceous beds 

(Vedder and others, 1991). Samples from the mudstone, which 

is less than 25 m thick, contain Zone CP9 calcareous 

nannofossils, Zone P6 to P8 planktonic foraminifers, and Zone 

C to D bathyal benthic foraminifers. The presence of these 

zones indicates a direct correlation of this mudstone unit with 

strata in the upper part (fig. 5 and table 1, V-648 and V-649, 

Zones CP9, P7, C) of the condensed(?) succession at Big Pine 

Mountain.

Western Santa Ynez Mountains

Much of the Anita Shale of Kelley (1943) in the western Santa Ynez Mountains and adjacent offshore areas has 

lithogenetic and biostratigraphic aspects that are similar

S24 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
to those of the condensed(?) succession at Big Pine Mountain. Descriptions by several workers (Kelley, 1943; Dibblee, 

1950; Weaver, 1965; and Gibson, 1976) indicate that the Anita 

in its type area is about 300 m thick and consists largely of 

gray clayey mudstone and siltstone beds, some of which are 

tinted red and green. Limestone nodules, glauconitic zones, 

and thin sandstone beds are subordinate constituents that are 

only locally present. According to A.A. Almgren (written 

commun., 1993), limestone beds about 15 cm thick are present 

in the lower part in the type area. Biostratigraphic review and 

emendation of earlier work by Almgren and others (1988) 

showed that the Anita Shale includes benthic foraminifer Zones 

E, D, C, and pseudo-C and calcareous nannofossil Zones CP6 

to CP13 and therefore ranges in age from late Paleocene into 

middle Eocene. Slow sedimentation rates in lower bathyal 

depths dominated the overall depositional environment 

(Almgren and others, 1988). About 15 km northeast of the 

type area, a 20-m thick limestone unconformably underlies 

the Anita and is assigned a late Paleocene age (Gibson, 1976; 

Whidden and others, 1995a).

Although microfossil assemblages in the Anita Shale are 

more diverse and better preserved than those in the condensed^) succession, an unqualified correlation can be made 

between the lower part of the Anita and the upper part of the 

condensed(?) succession. Similarly, the belt of mudstone that 

borders the south side of the Big Pine fault closely matches 

the Anita, both biostratigraphically and lithologically.

ORIGIN OF THE CONDENSED(?) SUCCESSION 

AT BIG PINE MOUNTAIN

Diminished sedimentation that characterizes condensed 

sequences commonly is attributed to the effects of eustatically 

controlled high-standing sea level or to deposition in mid-ocean 

environments. Less commonly, slow sedimentation is ascribed 

to blockage or diversion of the influx of terrestrial material. 

To explain the origin of the condensed(?) succession at Big 

Pine Mountain, several means of shutting off coarse clastic 

sedimentation need to be considered. Leading causes for condensed successions in general include the following:

1) Eustatic high stand of sea level,

2) Formation of tectonically generated barriers,

3) Planation or subsidence of the source terrane,

4) Diversion of sediment by changes in coastal morphology, or

5) Combinations of the above.

Global eustatic sea level curves show a generally high 

stand punctuated by short-term fluctuations during late 

Campanian through early Eocene time (Haq and others, 1987). 

The same curves indicate that the long-term oscillations during the same time interval were relatively subdued and that 

several major condensed sequences developed worldwide. The

origin of the sequence at Big Pine Mountain, however, may 

be attributable to causes other than protracted high sea level 

or stacking of sediment-starved units. Among such causes, 

likely contributors may have been barriers to sedimentation, 

diminishment of sediment sources, and redirected sedimentdispersal patterns. Despite the fact that evidence is incomplete 

for the size and configuration of Upper Cretaceous to lower 

Eocene basins in the San Rafael Mountains region, 

paleobathymetric data (table 1) imply that deposition in the 

basins took place mainly at mid-bathyal depths. Uncertainties 

about timing of tectonic events and undetermined completeness of the sedimentary record hinder confident interpretation 

of the history of the condensed(?) succession. Particularly frustrating for making such interpretations are the gaps in the 

biochronologic record, which may or may not signify episodes 

of nondeposition, erosion, or biota impoverishment.

Precursory Events

On a regional scale, sediment dispersal that preceded 

deposition of the condensed(?) succession at Big Pine Mountain generally was westward and southward in a very large 

forearc submarine-fan system (Howell and others, 1977; 

Vedder and others, 1983), chiefly in bathyal environments. A 

noteworthy exception to the widespread, deep-water conditions are the paralic facies manifested in the narrow, 32-kmlong belt that parallels the southwest side of the Sur-Nacimiento 

fault zone between South Fork La Brea Creek and S weetwater 

Canyon (Vedder and others, 1977; 1983). Clast compositions 

in both the deep- and shallow-water environments suggest 

derivation from partly unroofed mid-Cretaceous batholithic 

rocks in the western Cordillera (Lee-Wong and Howell, 1977; 

Vedder and others, 1983). The relatively rapid rate of deposition of these submarine-fan and paralic strata contrasts sharply 

with the ensuing slow rate indicated by the condensed(?) succession at Big Pine Mountain (fig. 9) and the mudstone belt 

along the south side of the Big Pine fault.

Tectonic events that controlled these Late Cretaceous 

depositional systems are obscure. Suturing of the Stanley 

Mountain and Salinia Terranes presumably was underway or 

complete by early Maastrichtian time (Vedder and others, 1983; 

Howell and others, 1987). Local structures that may have been 

associated with such an event, however, have not been identified. Indeed, even the kinds of structures that may have developed as a result of this suturing are uncertain since the 

orientation of the travel paths of these terranes and the distances traveled are debatable. Because there is no evidence 

for a regional fold-and-thrust belt, orthogonal approach and 

collision are doubtful. Low-oblique or subparallel travel paths 

and development of transpressional and (or) transtensional 

structures upon conjoinment are in better accord with dextraloblique plate convergence and accompanying strike-slip tectonics that were operative near the end of Cretaceous time 

(Page and Engebretson, 1984; Stewart and Crowell, 1992).

Age and Tectonic Inferences, Upper Cretaceous, Paleocene, and Eocene Strata, Big Pine Mountain Area, California S25
Contributory Factors

The prolonged period of diminished sedimentation at Big 

Pine Mountain may have resulted from a single event or a 

combination of events. Likely causative circumstances include 

uplift of an intervening submarine barrier ridge in concert with 

planation or subsidence of the source terrane. Ancillary factors may have involved long-term sediment diversion by 

changes in the submarine-fan dispersal system without a barrier ridge or tectonic extension and consequent formation of 

borderland-like topography. Although global sea level was 

relatively high between 75 and 52 Ma, this long-term high 

stand may have had little additional influence on the suddenly 

reduced influx of sediment during that time. Processes accompanying terrane accretion, plate-motion change, and subduction-angle variation may have contributed to the aforementioned causes, but discussion of the effects of plate reorientation and terrane conjoinment are beyond the scope of this paper.

Evidence is lacking not only for the size and shape of the 

supposed barrier ridge but also its geographic position. Any 

such site to the north or northeast presumably was translocated by post-middle(?) Eocene strike-slip movement on the 

Sur-Nacimiento fault zone. However, the history, direction, 

and amount of slip along this fault zone are uncertain.

Late Paleocene uplift just before and during deposition 

of the Sierra Blanca Limestone in the Grapevine Creek-upper 

Indian Creek area is indicated by northward downslope transport of allochemical detritus derived from algal-bank deposits 

that lay directly to the south but are now completely eroded. 

An unconformity at the base of lower Eocene limestone near

r 10

uT O 

-Z

3

5

z

-5

- 1

-0

-i- Sur-Nacimiento fault zone

 Cretaceous-Tertiary 

boundary(?)

FEET METERS

2000-

1000-

-600

-400

-200

0--0

-L- Big Pine fault

TIME, 

SUBEPOCH

OR 

STAGE

THICKNESS

Figure 9. Diagram comparing time of deposition and thickness of strata in vicinity of Big 

Pine Mountain. Shaded segments represent the condensed(?) succession of mudstone (map 

unitTKu, fig. 4; fig. 5).

S26 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
the Santa Ynez River east of Gibraltar Reservoir substantiates 

a later phase of local uplift (Johnson, 1968; Schroeter, 1972, 

Comstock, 1976; Whidden and others, 1995a). Additional evidence for local uplift is the presence of abundant Franciscan 

detritus in the lower Eocene limestone in the Redrock Canyon-Oso Canyon area (Schussler, 1981) and Lazaro CanyonCachuma Creek area (Vedder and others, 1983), where the 

limestone was deposited directly on the Franciscan Complex. 

Because these uplift events began after initial development of 

the condensed(?) succession at Big Pine Mountain, they may 

have had little effect on reduced sedimentation there. Reed 

and Hollister's (1936) San Rafael uplift, a hypothetical Paleogene structural high to the west and south, may have affected 

depositional patterns in late early and middle Eocene time 

(Thompson, 1988), but this feature may not have culminated 

until Oligocene time (Vedder and others, 1991). Late Cretaceous, Paleocene, and Eocene events together with their effects and evidence for them are summarized in figure 10.

In the San Rafael Mountains, Late Cretaceous and early 

Paleogene shorelines, insular platforms, and submarine ridges 

are represented only by small remnants or are not preserved. 

Consequently, a borderland-like configuration of basins and 

ridges cannot be fully ascertained. Highly generalized, regional 

paleogeographic reconstructions (Howell, 1975; Nilsen and 

Clarke, 1975; Nilsen, 1987), however, indicate that Paleocene 

and Eocene borderland features were present. Although faultbounded ridges of this age have not been identified, precursors of the Little Pine and Camuesa faults (fig. IB) may have 

created such a ridge. It is significant that pre-middle Eocene 

folding occurred in areas north of the Santa Ynez fault (Page 

and others, 1951). Such local upwarping and downwarping 

rather than faulting would better explain a basin-and-ridge 

setting and would be comparable to the larger anticlinal welts 

that form the Santa Rosa-Cortes Ridge west of Santa Cruz 

and San Nicolas Basins on the modern southern California 

borderland. Moreover, oblique subduction, regional wrenching, and disassembly of the Late Cretaceous forearc basin 

(Vedder and others, 1983; Nilsen, 1987) were conducive to 

development of borderland-like morphology. Barriers to sedimentation are commonplace on the modern borderland 

(Gorsline, 1990). A similar setting (fig. 11), therefore, is considered plausible for the depositional environments in the San 

Rafael Mountains region during latest Cretaceous and earliest 

Tertiary time.

Subsequent Events

The lower and middle(?) Eocene submarine-fan deposits 

that overlie the condensed(?) succession at Big Pine Mountain signify renewed rapid sedimentation (fig. 10). The largely 

coarse-clastic sequence that lies north and east of Big Pine 

Mountain is truncated by the Sur-Nacimiento and Big Pine 

fault zones, which converge to restrict the lateral extent and 

vertical succession of these strata. As a result, interpretations

of depositional environments within this tract are hampered 

by incomplete distributional and stratigraphic data. If not tectonically rotated, the few directional features examined suggest westward and southward paleocurrent flow. Because the 

age of the upper part of this fault-truncated sequence is not 

known, duration of sedimentation is questionable (fig. 9). At 

least the lower 600 m of the 1,200-m-thick section was deposited in late early and early middle(?) Eocene time, and the 

entire sequence may represent a span of time as short as 5.0 

m.y. Even though waning tectonism and regional lowering of 

relief may have begun early in the Eocene (Howell and others, 1987), the sandstone unit that lies north and east of Big 

Pine Mountain presumably was derived from a relict highstanding area northeast of the Sur-Nacimiento fault zone. According to Haq and others (1987), eustatic sea level dropped 

abruptly between 49.5 Ma and 49.0 Ma. This event may have 

been partly responsible for the renewed episode of rapid sedimentation, but the global occurrence of such a low stand has 

been challenged by Miall (1993).

South of the Big Pine fault, subsea-fan deposits are present 

in the lower part of a regionally extensive remnant of an Eocene 

forearc basin (Nilsen and Clarke, 1975; Howell, 1975; Thompson, 1988). Within this basin remnant, lower and middle 

Eocene strata are preserved eastward from East Fork Santa 

Cruz Creek to Mono Creek and beyond. These strata consist 

mainly of sandstone and subordinate interbedded mudstone 

that conformably overlie the Upper Cretaceous to lower Eocene 

mudstone belt that lies along the south side of the fault. 

Throughout the southeastern San Rafael Mountains, the largely 

coarse-clastic lower and middle Eocene beds are customarily 

assigned to the Juncal Formation (Page and others, 1951; 

Dibblee, 1966; Vedder, 1972; Vedder and others, 1973).

Post-Oligocene rotations of small crustal blocks in the 

western Transverse Ranges (Hornafius and others, 1986; 

Luyendyk, 1991) also may have occurred in the parts of the 

San Rafael Mountains in which the condensed(?) succession 

and mudstone belt are present. Recently acquired paleomagnetic data from an area north of the Santa Ynez fault just east 

of figures 1A and IB suggest a clockwise rotation of 70° to 

90° (Whidden and others, 1993, 1995b). Because the extent, 

direction, and amount of late Cenozoic rotations are incompletely known, the original orientation of sediment flow in 

older strata is uncertain. As measured now, regional sedimenttransport directions during the Eocene were largely southwestward and westward (Howell, 1975; Nilsen, 1987). Locally, 

however, transport may have been northward (Comstock, 1975, 

fig. 7; J.G. Vedder, Hugh McLean, and R.G. Stanley, unpub. 

data, 1984,1992-93). Although based upon a limited number 

of measurements, chiefly in sandstone turbidites, this local 

northward transport is supportive evidence for the presence of 

short-lived, actively eroding submarine ridges or banks farther south. Biostratigraphic analyses of samples from outcrops 

along Big Pine Road on the north limb of Loma Pelona syncline (figs. 6, IB) suggest that the approximately 1,500-mthick chiefly sandstone sequence there has a short age range

Age and Tectonic Inferences, Upper Cretaceous, Paleocene, and Eocene Strata, Big Pine Mountain Area, California S27
Time (Ma) Event Effects and evidence Record at Big Pine Mountain

Lithostratigraphy Biostratigraphy

Eocene

Paleocene

Middle

 52.0-

Early

Late

 62.3

Early

-66.4-

Late Cretaceous

Ca. 75.0-

Continuing regional uplift and 

erosion of granitoid plutons 

north and east of study area.

Prevailing bathyal deposition of predominantly coarse-clastic submarine-fan sequences northeast of Big Pine Mountain 

and along trough of Loma Pelona syncline. 

Sandstone quartzofeldspathic.

Renewed regional uplift and 

erosion of granitoid plutons 

north and east of study area. 

Widespread uplift to south 

and west (incipient San 

Rafael uplift?).

Abrupt change from slow bathyal-abyssal 

sedimentation to accelerated rates in submarine-fan complex. Sharp contact between mudstone sequence and quartzofeldspathic sandstone/conglomerate sequence 

at Big Pine Mountain. Similar, less abrupt 

change from mudstone to sandstone along 

north limb of Loma Pelona syncline.

Unconformity at base of "Poppin shale" of 

local usage on south limb of Loma Pelona 

syncline; continuous deposition on north 

limb. Unconformity at base of Juncal Formation throughout southeastern San Rafael Mountains. Franciscan detritus in Eocene 

limestone south and west of study area.

Dwindling growth of submarine barrier ridge north or 

east of study area. Local sea 

floor upwarps south of 

Loma Pelona syncline.

Prevailing slow, deep-water deposition of 

hemipelagic mudstone at Big Pine Mountain. Unconformity at base of Sierra Blanca Limestone on south limb of Loma Pelona syncline; continuous deposition of hemipelagic mudstone and interbedded limestone on north limb. Algal-bank development on structural highs south of Loma 

Pelona syncline accompanied by northward 

downslope transport of algal detritus. 

Evolving borderland-like sea floor morphology.

Culmination of growth of submarine barrier ridge accompanied by curtailment of 

sedimentation to the south 

and west.

Very slow sedimentation rates or hiatus(?). 

Biostratigraphic record absent. Lithostratigraphic evidence suggests continued hemipelagic deposition in sediment-starved basin at Big Pine Mountain.

Incipient uplift of faultbounded(?) submarine ridge 

between eroding plutons 

and northern part of study 

area to form growing barrier 

to sedimentation.

Continuing uplift and erosional unroofing of granitoid 

plutons north and east of 

study area. Persisting regional forearc basin remaining from Cenomanian-Campanian precursor.

Diminishing sediment supply. Sharp contact 

between sandstone turbidites and massive 

mudstone at Big Pine Mountain.

Rapid bathyal sedimentation in regional coalescing submarine-fan system. Widespread deposition of quartzofeldspathic 

sandstone and granitoid/siliceous metavolcanic pebble-boulder conglomerate.

Figure 10. Chronology of events and geologic evidence for Late Cretaceous, Paleocene, and Eocene tectonism in San Rafael Mountains. 

Estimated ages of boundaries from Berggren and others (1985). Named topographic and structural features used only as geographic 

reference points and should not be construed as being present at time designated. If future work demonstrates late Cenozoic tectonic 

rotations, sediment transport directions must be revised accordingly.

S28 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
Quaternary

Late Paleocene and early Eocene

Figure 11. Schematic diagrams of tectonic barriers to sedimentation. A, Quaternary, California Continental Borderland 

(modified from Gorsline, 1990); B, Paleocene and early Eocene borderland, San Rafael Mountains area.

Age and Tectonic Inferences, Upper Cretaceous, Paleocene, and Eocene Strata, Big Pine Mountain Area, California S29
within the late early to earliest middle Eocene (CP10/11 into 

CP12a) (table 2), perhaps as short as 3.0 m.y.

Maria-Santa Barbara region was more extensive and complex 

in early Paleogene time than heretofore supposed.

CONCLUSIONS

Available biostratigraphic data indicate that the 350-mthick mudstone unit near the crest of Big Pine Mountain possibly is a condensed succession that ranges in age from latest 

Cretaceous (late Campanian or Maastrichtian) into earliest 

Tertiary (early Eocene). Elapsed time of accumulation may 

have been as long as 22 m.y. Even though the position of time 

boundaries within the succession are poorly constrained and 

the presence or absence of the Danian Stage has not been determined, physical evidence for stratigraphic discontinuities 

are not apparent. Pre-late Paleocene and early Eocene erosional events in neighboring areas, however, cannot be disregarded; for these events may have influenced depositional 

continuity in the mudstone succession at Big Pine Mountain. 

Ostensibly, the exceptionally long duration of sedimentation 

in bathyal depths suggests that this possible condensed succession resulted from the tectonic development of a barrier 

ridge (or system of ridges) rather than from a long-term stand 

of eustatic high sea level. Perplexing uncertainties about the 

origin, site, and fate of such a barrier remain. Strike-slip tectonism accompanying oblique subduction and terrane accretion near the end of the Cretaceous, when the Stanley Mountain 

terrane presumably was juxtaposed against the Salinia terrane, 

may have been partly responsible for inception of the barrier. 

Barring large amounts of late Cenozoic tectonic rotation, 

southwestward sediment-transport directions in the enclosing strata imply that the intervening ridge lay to the northeast. Movement along the ancestral Sur-Nacimiento fault near 

the end of the early Eocene may have translocated the barrier 

so that it no longer effectively blocked rapid sedimentation.

The belt of mudstone that flanks the south side of the Big 

Pine fault is the same age as the condensed(?) succession at 

Big Pine Mountain. Despite the presence of limestone lenses, 

greater thickness, and indication of local northward sediment 

transport, the mudstone belt south of the fault is believed to be 

a lateral equivalent of the condensed(?) succession on Big Pine 

Mountain but represents a shallower facies on the flank of the 

same sediment-starved basin. Restoration of about 14 km of 

left-lateral strike slip along the Big Pine fault would place the 

direct counterpart of the condensed(?) succession east of upper Mono Creek, where the mudstone beds presumably are 

concealed beneath middle Eocene and younger strata. Because 

at least two different high-standing areas were contributing 

detritus to the basin south of the present site of the fault during the late Paleocene and early to middle Eocene, a borderland-like paleogeographic setting seems to be a reasonable 

inference. If the late Paleocene to middle Eocene Anita Shale 

in the western Santa Ynez Mountains is considered to be a 

separate basin entity, and if the San Rafael high began to develop at the same time, borderland topography in the Santa

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Age and Tectonic Inferences, Upper Cretaceous, Paleocene, and Eocene Strata, Big Pine Mountain Area, California S33

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well as collections of short papers related to a specific topic.

Water-Supply Papers are comprehensive reports that present significant interpretive results of hydrologic investigations of wide interest 

to professional geologists, hydrologists, and engineers. The series covers investigations in all phases of hydrology, including hydrology, availability of water, quality of water, and use of water.

Circulars present administrative information or important scientific information of wide popular interest in a format designed for distribution at no cost to the public. Information is usually of short-term interest.

Techniques of Water-Resources Investigations Reports are papers of an interpretive nature made available to the public outside the 

formal USGS publications series. Copies are reproduced on request 

unlike formal USGS publications, and they are also available for public 

inspection at depositories indicated in USGS catalogs.

Open-File Reports include unpublished manuscript reports, maps, 

and other material that are made available for public consultation at 

depositories. They are a nonpermanent form of publication that may be 

cited in other publications as sources of information.

Maps

Geologic Quadrangle Maps are multicolor geologic maps on topographic bases in 7 1/2- or 15-minute quadrangle formats (scales mainly 

1:24,000 or 1:62,500), showing bedrock, surficial, or engineering geology. Maps generally include brief texts; some maps include structure 

and columnar sections only.

Geophysical Investigations Maps are on topographic or planimetric bases at various scales, showing the results of surveys using geophysical techniques, such as gravity, magnetic, seismic, or radioactivity, which reflect subsurface structures that are of economic or geologic 

significance. Many maps include correlations with the geology.

Miscellaneous Investigations Series Maps are on planimetric or 

topographic bases of regular and irregular areas at various scales; they 

present a wide variety of format and subject matter. The series also 

includes 7 1/2-minute quadrangle photogeologic maps on planimetric 

bases that show geology as interpreted from aerial photographs. The 

series also includes maps of Mars and the Moon.

Coal Investigations Maps are geologic maps on topographic or 

planimetric bases at various scales, showing bedrock or surficial geology, stratigraphy, and structural relations in certain coal-resource areas.

Oil and Gas Investigations Charts show stratigraphic information for certain oil and gas fields and other areas having petroleum potential.

Miscellaneous Field Studies Maps are multicolor or black-andwhite maps on topographic or planimetric bases of quadrangle or irregular areas at various scales. Pre-1971 maps show bedrock geology 

in relation to specific mining or mineral-deposit problems; post-1971 

maps are primarily black-and-white maps on various subjects, such as 

environmental studies or wilderness mineral investigations.

Hydrologic Investigations Atlases are multicolored or black-andwhite maps on topographic or planimetric bases presenting a wide range 

of geohydrologic data of both regular and irregular areas; the principal 

scale is 1:24,000, and regional studies are at 1:250,000 scale or smaller.

Catalogs

Permanent catalogs, as well as some others, giving comprehensive listings of U.S. Geological Survey publications are available under 

the conditions indicated below from USGS Map Distribution, Box 

25286, Building 810, Denver Federal Center, Denver, CO 80225. (See 

latest Price and Availability List.)

"Publications of the Geological Survey, 1879-1961" may be purchased by mail and over the counter in paperback book form and as a 

set of microfiche.

"Publications of the Geological Survey, 1962-1970" may be purchased by mail and over the counter in paperback book form and as a 

set of microfiche.

"Publications of the US. Geological Survey, 1971-1981" may 

be purchased by mail and over the counter in paperback book form 

(two volumes, publications listing and index) and as a set of microfiche.

Supplements for 1982, 1983, 1984, 1985, 1986, and subsequent 

years since the last permanent catalog may be purchased by mail and 

over the counter in paperback book form.

State catalogs, "List of U.S. Geological Survey Geologic and 

Water-Supply Reports and Maps For (State)," maybe purchased by mail 

and over the counter in paperback booklet form only.

"Price and Availability List of U.S. Geological Survey Publications," issued annually, is available free of charge in paperback booklet 

form only.

Selected copies of a monthly catalog "New Publications of the 

U.S. Geological Survey" is available free of charge by mail or may be 

obtained over the counter in paperback booklet form only. Those wishing a free subscription to the monthly catalog "New Publications of the 

U.S. Geological Survey" should write to the U.S. Geological Survey, 

582 National Center, Reston, VA 22092.

Note. Prices of Government publications listed in older catalogs, 

announcements, and publications may be incorrect. Therefore, the prices 

charged may differ from the prices in catalogs, announcements, and 

publications.