Conglomerates of the Upper Middle Eocene to 

Lower Miocene Sespe Formation along the 

Santa Ynez Fault Implications for the 

Geologic History of the Eastern Santa Maria 

Basin Area, California

Reconnaissance Bulk-Rock and

Clay Mineralogies of Argillaceous Great Valley

and Franciscan Strata, Santa Maria

Basin Province, California

Geophysical section offshore Santa Maria basin

Geologic section onshore Santa Maria basin

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Conglomerates of the Upper Middle Eocene to 

Lower Miocene Sespe Formation along the 

Santa Ynez Fault Implications for the 

Geologic History of the Eastern Santa Maria 

Basin Area, California

By JEFFREY L. HOWARD

Reconnaissance Bulk-Rock and

Clay Mineralogies of Argillaceous Great Valley

and Franciscan Strata, Santa Maria

Basin Province, California

By RICHARD M. POLLASTRO, HUGH MCLEAN, and LAURA 

L ZINK

Chapters H and I 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 

Gordon P. Eaton, 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

Text and illustrations edited by James W. Hendley II

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1995

For sale by

U.S. Geological Survey

Information Services

Box 25286, Federal Center

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Library of Congress Cataloging in Publication Data

Howard, Jeffrey L.

Conglomerates of the Upper Middle Eocene to Lower Miocene Sespe 

Formation along the Santa Ynez Fault: implications for the geologic history of 

the eastern Santa Maria Basin area, California / by Jeffrey L. Howard. 

Reconnaissance bulk-rock and clay mineralogies of argillaceous Great Valley 

and Franciscan strata, Santa Maria Basin Province, California / by Richard M. 

Pollastro, Hugh McLean, and Laura L. Zink.

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

(Evolution of sedimentary basins/onshore oil and gas investigations 

Santa Maria Province ; ch. H, I)

"Chapters H and I are issued as a single volume and are not available 

separately."

Includes bibliographical references.

Supt. of Docs, no.: I 19.3:1995-H, I

1. Sedimentation and deposition California Santa Maria Basin. 

2. Conglomerate California Santa Maria Basin. 3. Geology, Stratigraphic 

Cenozoic. 4. Sespe Formation (Calif.) 5. Argillite California Santa Maria 

Basin. 6. Mineralogy California Santa Maria Basin. 7. Santa Maria Basin 

(Calif.) I. McLean, Hugh, 1939. II. Zink, Laura L. III. Pollastro, Richard M. 

Reconnaissance bulk-rock and clay mineralogies of argillaceous Great Valley 

and Franciscan strata, Santa Maria Basin Province, California. IV. Title. 

V. Title: Reconnaissance bulk-rock and clay mineralogies of argillaceous Great 

Valley and Franciscan strata, Santa Maria Basin Province, California. VI. Series. 

VII. Series: Evolution of sedimentary basins/onshore oil and gas 

investigations Santa Maria Province; ch. H, I. 

QE75.B9 no. 1995-H-l 

[QE571]

557.3 s dc20 94-39069 

[552'.5'09794] CIP
Chapter H

Conglomerates of the Upper Middle Eocene to 

Lower Miocene Sespe Formation along the 

Santa Ynez Fault Implications for the 

Geologic History of the Eastern Santa Maria 

Basin Area, California

By JEFFREY L. HOWARD

U.S. GEOLOGICAL SURVEY BULLETIN 1995

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

Edited by Margaret A. Keller
CONTENTS

Abstract HI

Introduction HI

Previous work H2

Methods of study H7

Sedimentology of the Sespe Formation H9

Stratigraphy and paleoenvironments H9

Paleocurrent analysis H14

Conglomerate clast assemblages H14

Metavolcanic clast varieties H19

Conglomerate clast morphology H19 

Geologic history of the study area H20

Sespe Conglomerate provenance H20

Depositional history H26

Tectonic history H29 

Conclusions H30 

References H31 

Appendix: List of conglomerate clasts from the Sespe Formation H37

FIGURES

1. Map showing location of study area in eastern Santa Maria basin and Santa Ynez 

Mountains H2

2. Map of study area showing geographic distribution of Sespe Formation and locations of measured sectionsH3

3. Correlation chart for Santa Maria basin, Santa Ynez Mountains, and Topatopa 

Mountains, Calif. H4

4. Stratigraphic nomenclature used previously in Santa Ynez Mountains, Calif. H4

5. Schematic Stratigraphic sections of Sespe Formation in study area Hll

6. Lithologic correlations of Sespe Formation in study area H12

7. Schematic Stratigraphic sections of Sespe Formation in southern Santa Ynez Mountains showing lithologic correlations H12

8. Facies relations between Sespe Formation and laterally equivalent marine units in 

the central and western Santa Ynez Mountains, Calif. H13

9. Ratios and maximum granitoid/gneiss clast size to maximum quartzite clast size 

versus distance west of the San Gabriel Fault H19

10. Inferred sediment dispersal directions in study area H24

11. Facies relations between Sespe Formation and laterally equivalent units in Santa 

Ynez-Topatopa Mountains, Calif. H27

TABLES

1. Lithofacies of the Sespe Formation in eastern Santa Maria basin and vicinity H10

2. Paleocurrent data for lithofacies C of the Sespe Formation in eastern Santa Maria 

basin HIS

3. Paleocurrent data for Sespe Formation in Santa Ynez Mountains HIS

4. Percentages of clasts in Sespe conglomerates of the eastern Santa Maria basin 

area H16

IV Contents
5. Maximum observed clast size in Sespe conglomerates of the eastern Santa Maria 

basin area H17

6. Upsection changes in Sespe conglomerate clast suites HIS

7. Analysis of metavolcanic clast suites in Cretaceous and Paleogene conglomerates 

by percentage H20

8. Tests for statistically significant differences in metavolcanic clast suites of Sespe 

and pre-Sespe conglomerates H21

9. Morphologic indices of clasts in Sespe and pre-Sespe conglomerates H22

10. Calculated t-values for tests of statistically significant differences in clast shapes 

of Sespe conglomerates H22

11. Calculated t-values for tests of statistically significant differences in clast shapes of 

Sespe and pre-Sespe conglomerates H23

Contents

Conglomerates of the Upper Middle Eocene to 

Lower Miocene Sespe Formation along the 

Santa Ynez Fault Implications for the Geologic History 

of the Eastern Santa Maria Basin Area, California

By Jeffrey L. Howard1

Abstract

The depositional history of the Sespe Formation was studied using sedimentary facies analysis, clast counts, paleocurrent and clast morphological measurements, and petrographic 

methods. Sedimentation is interpreted to have occurred 

mainly as part of two depositional sequences in a coastalbraid-plain forearc-basin setting. Both sequences are present in 

the Santa Ynez Mountains, whereas only the upper sequence is 

recognized in the eastern Santa Maria basin. The lower 

sequence is part of a late middle to late Eocene sedimentary 

offlap attributed to a high input of terrigenous sediment 

derived mostly from the Mojave Desert region. This sequence 

is capped by an intraformational erosional unconformity; 

much or all of the lower Oligocene section is inferred to be 

missing. The upper sequence is part of a late Oligocene to 

early Miocene sedimentary onlap and includes sediment 

derived from both Franciscan Complex and Mojave Desert 

sources. It probably represents basin backfilling in response to 

eustatic sea level rise. The intraformational erosional unconformity and associated upsection change to a Franciscan provenance were not necessarily created entirely by tectonism 

(Ynezan orogeny), as previously supposed. Regional relations 

suggest that the effects of a middle Oligocene eustatic sea level 

drop were also important. Eustatic effects may account for an 

upsection paleohydrological change from braided to meandering fluvial deposition in the upper depositional sequence. The 

observed sediment dispersal pattern is inconsistent with active 

faulting during sedimentation; the Ynezan orogeny can be 

explained by anticlinal warping of the San Rafael uplift. More 

recent horizontal offsets along the Santa Ynez and Little Pine 

Faults are suggested by the juxtaposition of contrasting lithofacies and sediment-source mismatches, but the sense, timing, 

and magnitude of such movements cannot be evaluated further without additional subsurface information.

Associate Professor, Department of Geology, Wayne State University, Detroit, MI 48202.

Manuscript approved for publication September 26, 1994.

INTRODUCTION

The Santa Maria structural basin in southwestern California is a wedge-shaped region bounded on the northeast 

by the Little Pine Fault and on the south by the Santa Ynez 

and Lompoc Faults (fig. 1), therefore, it is much larger than 

the physiographic basin of the Santa Maria vicinity and 

includes areas which might be considered parts of either the 

San Rafael Mountains in the Coast Ranges Province or the 

Santa Ynez Mountains in the western Transverse Ranges 

Province. Although there is disagreement over the particular tectonic regime responsible, stratigraphic relations show 

that the Santa Maria basin formed, along with others like 

the Los Angeles basin, during middle Miocene time as a 

result of profound subsidence, faulting, and volcanism 

(Crowell, 1987). Several earlier diastrophic events also 

affected the Santa Maria basin; however, because this area 

was apparently a structural high during early Cenozoic time 

(Reed and Hollister, 1936; Dibblee, 1950, 1966), this "prebasin" history is poorly preserved. A more extensive sedimentary record of these events is found in the nearby Santa 

Ynez Mountains (fig. 1). Nonmarine conglomerates of the 

upper middle Eocene to lower Miocene Sespe Formation 

crop out extensively in this area (fig. 2) and are of particular interest because they are well suited for paleogeographic 

studies. Unfortunately, a clearer understanding of the depositional history and tectonic implications of these deposits 

is clouded by numerous discrepancies in the literature 

regarding stratigraphic nomenclature, age and facies relations, and provenance. For example, it is uncertain how and 

when the Santa Ynez, Lompoc, and Little Pine Faults originated, and their displacement history is poorly defined.

The purpose of this investigation was to gather stratigraphic and sedimentologic data necessary to clarify the 

geologic history of the Sespe Formation. The area studied 

includes the eastern Santa Maria basin and the central 

and eastern Santa Ynez Mountains (figs. 1 and 2). The

Conglomerates of the Sespe Formation along the Santa Ynez Fault Implications for the Geologic History of Santa Maria Basin H1
conglomeratic parts of six stratigraphic sections of the 

Sespe Formation were studied intensively by using sedimentary facies analysis, paleocurrent measurements, clast 

counts, and petrographic methods. The intervening terrane 

between the measured sections was also studied in reconnaissance. A major aim of the study was to differentiate 

between first-cycle and multicycle clasts; hence, quartzite 

and metavolcanic clasts in conglomerates of the Sespe Formation were compared with those in older conglomerates 

on the basis of shape and lithology. This paper integrates 

data on the stratigraphy, paleontology, paleoenvironments, 

petrology, and provenance of the Sespe Formation conglomerates and discusses the implications of these deposits 

for the depositional and tectonic history of the eastern 

Santa Maria basin area.

Acknowledgments. I am especially grateful to A.G. 

Sylvester for introducing me to the tectonic problems of the 

Santa Ynez Fault and to M.A. Keller for encouraging me to 

write this paper. Thanks to A.G. Sylvester, J.R. Boles, and

R.V. Fisher for their assistance with the parts of this study 

done as a Ph.D. dissertation at the University of California, 

Santa Barbara. Thanks also to P.L. Abbott, M.A. Keller, 

R.G. Stanley, and S.W. Starratt for reviewing this manuscript, and to C.A. Rigsby, E.B. Lander, D.W. Weaver, and 

T.W. Dibblee Jr. for providing additional useful information. Special thanks to Warren Hamilton for taking the time 

to examine all of my thin sections of granitoid clasts from 

the Sespe Formation. Sandra Walter assisted with the clast 

shape measurements and thin sections, and Elaine Shelton 

helped prepare the manuscript. Partial funding by UNOCAL is also very gratefully acknowledged.

PREVIOUS WORK

The Sespe Formation was named (Watts, 1897; Eldridge 

and Arnold, 1907) for a type section along lower Sespe Creek 

about 6 miles north of Fillmore, Calif. (Keroher and others,

120° 119°

Area of figures 2 and 10

PACIFIC OCEAN

Figure 1. Map showing location of study area (area of figs. 2 

and 10) in eastern Santa Maria basin and Santa Ynez Mountains. Location names (cities indicated by open circles): SM, 

Santa Maria; L, Lompoc; SB, Santa Barbara; V, Ventura; F, 

Fillmore; S, Simi; SCI, Santa Cruz Island. Fault names: SYF, 

Santa Ynez Fault; LF, Lompoc Fault; LPF, Little Pine Fault; NF, 

Nacimiento Fault; BPF, Big Pine Fault; SAF, San Andreas Fault; 

GF, Garlock Fault; SGF, San Gabriel Fault; MCF, Malibu Coast 

Fault. Sample localities: Espada Fm. Manzana Creek (MC);

Jalama Fm. Romero Saddle (RS), Rose Canyon (OC); San 

Francisquito Fm. Fish Creek (FC); Cozy Dell Shale west 

Camino Cielo (WC); Sespe Fm. Glen Annie Canyon (GA), east 

Camino Cielo (EC), San Marcos Pass State Highway 154 (SMP), 

Gibraltar Road (GR), Montecito (MT), Lake Casitas (LC), Sespe 

Creek (TL). Other localities: Canada del Cojo (CC); Arroyo 

Bulito (AB); Canada del Aqua Caliente (CA); Gaviota Canyon 

(GC); Alisal Creek (AC); Canada de la Posa (CP); Canada del 

Refugio (RC); Canada del Capitan (CT); Bartlett Canyon (BC).

H2 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
1966). The type location is in the Topatopa Mountains about 

40 km east of the study area (fig. 1, locality TL). This rock 

unit was subsequently mapped throughout the Ventura basin 

region (Kew, 1919, 1924), in the Santa Ana (English, 1926) 

and Santa Monica (Hoots, 1931) Mountains, and in the Santa 

Maria basin (Dibblee, 1950, 1966). The Sespe Formation 

was defined as the sequence of sparsely fossiliferous, nonmarine clastic sedimentary rocks lying between fossiliferous 

marine Eocene and marine Miocene strata. It includes a 

conspicuous sequence of red beds in some areas but is generally composed of gray or brown strata. Relevant stratigraphic nomenclature2 is given in figure 3.

The position of the lower contact of the Sespe Formation in the Santa Ynez-Topatopa Mountains has been 

defined inconsistently because of disagreement over the 

stratigraphic affinity of a sequence of interbedded gray or 

brown sandstone and red mudrock physically transitional 

between the Coldwater Sandstone and the Sespe Formation. Watts' (1897) term Sespe brownstone formation

Note that important changes in the boundaries between certain 

chronostratigraphic and biochronologic units have resulted in significant 

modifications in the age assignments of some rock units from those 

found in the older literature. The Refugian Stage, once considered to be 

partly Oligocene, is now regarded as exclusively late Eocene; the Zemorrian Stage, once considered to include much of the Miocene, is now regarded as mostly Oligocene (Bartow, 1991). The Whitneyan, Orellan, and 

Chadronian stage boundaries have been changed following Swisher and 

Prothero (1990) and Prothero and Swisher (1992). The Eocene-Oligocene 

boundary age has been changed from 36.6 Ma to 33.6 Ma following 

Berggren and others (1992), Prothero and Swisher (1992), and Cande and 

Kent (1992).

obviously referred to the overlying sequence of red beds, 

but Eldridge and Arnold (1907) included these "transitional beds"3 as part of their Sespe Formation. Kew 

(1924) criticized this terminology on the basis that Watts' 

original definition had priority and consequently placed 

the lower contact at the base of the red bed sequence. 

Eschner (1969) drew the lower contact in the type area at 

the top of the uppermost strata containing marine fossils, 

thereby excluding most of the "transitional beds" from the 

Sespe Formation. Similar discrepancies are found in the 

literature pertinent to the parts of the Santa Ynez Mountains covered by this investigation (fig. 4). Thus, the "transitional beds" are equivalent to Lian's (1952) Hay Hill 

unit, which he later referred to informally as an unnamed 

lower member of the Sespe Formation (Lian, 1954). Dibblee (1966) also mapped these strata as the basal part of 

the Sespe Formation but did not discuss them as a separate 

unit. McCracken (1972) and Lander (1983), however, 

regarded the "transitional beds" as the uppermost part of 

the Coldwater Sandstone.

In the type area, the upper contact of the Sespe Formation is difficult to define because the uppermost part of 

the formation interdigitates with or grades into the overlying Vaqueros Formation. Eschner (1969) placed the lower 

contact of the Vaqueros Formation at the base of the lowest stratum containing marine mollusks. In the Santa Ynez 

Mountains and in the eastern Santa Maria basin, where the 

upper contact of the Sespe Formation contact is usually

3Not to be confused with the "transition stage" shown in figure 3.

34°30' 

PACIFIC OCEAN

Figure 2. Map of study area showing geographic distribution 

of Sespe Formation (enclosed stippled areas) and locations 

of measured sections. Franciscan Complex indicated by box 

pattern. Location names (cities indicated by open circles): G, 

Goleta; SB, Santa Barbara; C, Carpinteria. Fault names: SYF, 

Santa Ynez Fault; LF, Lompoc Fault; LPF, Little Pine Fault; 

APF, Arroyo Parida Fault; MF, Munson Creek Fault. Localities

discussed in text: Bartlett Canyon (BC); San Pedro Canyon 

(SP); San Jose Canyon (SJ); Old San Marcos Pass Road (OS); 

Tunnel Road (TR); Sycamore Canyon (SC); Toro Canyon (TC); 

Rincon Creek (RC); Superior Ridge (SR); Ventura River (VR); 

Ojai (OJ). Measured sections: 1, Loma Alta; 2, Oso Canyon; 

3, Redrock Camp; 4, Camino Cielo; 5, San Marcos Pass 

Highway 154; 6, Lake Casitas.

Conglomerates of the Sespe Formation along the Santa Ynez Fault Implications for the Geologic History of Santa Maria Basin H3
Age 

(Ma)

CO CO

NALMA4'6'6 PCFS7

Western 

Santa 

Maria

basin12

Eastern

Santa

Maria 

basin1 Vuh<ry

Western 

Santa 

Ynez

Mts15' 16

Central 

Santa 

Ynez

s study

Eastern 

Santa 

Ynez

Topatopa

Mts (type

location) 18'2

Hemphillian "Jacalitos"

Clarendonian Mohinian

20-

30-

40-

50-

60-

LU

z

LU 

CD 

O 

UJ

'Margaritan'

UJ O 

O

Barstovian

Hemingfordian

RelizlarT? - 

 ?-

Saucesian

"Temblor"

Marine

and 

volcanic

rocks

Marine 

rocks

Marine

and 

volcanic

rocks

Marine 

rocks

Marine 

rocks

Marine 

rocks

K-Ar13 -*

Lospe

Fm

Arikareean "Vaqueros"

Zemorrian

Whitneyan

Orellan "Unnamed1 

 9

Chadronian Refugian 'Gay/iota"

Duchesnean

Uintan

"Tejon"

Narizian N

Bridgerian >

Wasatchian

 ? - 

Ulatisian

"Transition" 

'Domengine'

Clarkforkian

Tiffanian

"Penutian" 

Bulitian

Ynezian

"Capay"

9 __ "Meganos" 

9 

"Martinez"

Torrejonian 

Puercan Cheneyan Unnamed

Vaqueros Fm

'aqueros Fm Vaqueros Fm Vaqueros Fm Vaqueros Fm

Sespe 

Fm

Alegria 

Fm

Gaviota 

Fm

SYLF57

Sespe 

Fm

Sacate 

Fm

Coldwater 

Ss

Marine 

rocks

Sespe 

Fm

Coldwater 

Ss

Sespe 

Fm

Coldwater 

Ss

Marine 

rocks

Marine 

rocks

Marine 

rocks

Marine 

rocks

CRETACEOUS SYSTEM Marine 

rocks

Marine 

rocks

Marine 

rocks

Marine 

rocks

Marine 

rocks

Marine 

rocks

Figure 3. Correlation chart for Santa Maria basin, Santa Ynez 

Mountains and Topatopa Mountains, Calif. Modified from 

Weaver and others (1944) and Bishop and Davis (1984). 

NALMA, North American land mammal ages; e, early; I, 

late. PCFS, Pacific coast foraminiferal stages; PCMS, California molluscan stages and zones. T. u. u. Z., Turritella 

uvasana uvasana Zone; T. v. Z., Turritella variata Zone. Subseries: L, lower; M, middle; U, upper. K-Ar isotopic-age date 

(black bar). Inferred age range of land vertebrate faunas 

(ruled bar). SYLF, Santa Ynez local fauna, References: 1, 

Berggren and others (1985); 2, Berggren and others (1992);

3, Cande and Kent (1992); 4, Woodburne (1987); 5, Swisher 

and Prothero (1990); 6, Prothero and Swisher (1992); 7, Bartow (1991); 8, Clark and Vokes (1936); 9, Addicott (1970, 

1973); 10, Weaver and Kleinpell (1963); 11, Givens and 

Kennedy (1979); 12, Woodring and Bramlette (1950); 13, 

Stanley and others (1991); 14, Dibblee (1966); 15, Dibblee 

(1950); 16, Kleinpell and Weaver (1963); 17, Lander (1983); 

18, Bailey (1947); 19, Vedder (1972); 20, Hornaday (1970); 

21, Eschner (1969). Note that biochronologic units are used 

in place of land mammal stages because chronostratigraphic 

units are not yet fully defined.

Lian (1952)

Vaqueros 

Sandstone

Sespe 

Formation

Calvary 

Conglomerate 

unit

Hay Hill 

unit

Coldwater 

Sandstone

Dibblee (1966)

Vaqueros 

Sandstone

Sespe 

Formation

Unnamed 

map unit

"Coldwater" 

Sandstone

McCracken (1972)

Vaqueros 

Formation

Sespe 

Formation

Coldwater 

Sandstone

Lander (1983)

Vaqueros 

Formation

Sespe 

Formation

Calvary 

Conglomerate 

unit

"Coldwater" 

Sandstone

Th s report

Vaqueros 

Formation

Sespe Formation

Facies 

D

Facies 

C,F,G

Facies 

A,B,E

Coldwater 

Sandstone

Figure 4. Stratigraphic nomenclature used previously in 

Santa Ynez Mountains, Calif. For facies descriptions see lithofacies in table 1.

H4 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
sharp, it seems to have been drawn consistently at the top 

of the uppermost red stratum and (or) at the base of the 

lowest stratum containing marine macrofossils (Kew, 

1924; Bailey, 1947; Dibblee, 1950, 1966; Lian, 1952, 

1954; Kleinpell and Weaver, 1963; Edwards, 1971; 

McCracken, 1972).

The middle part of the Sespe Formation in the Simi 

area (fig. 1) has yielded terrestrial vertebrate faunas of late 

Uintan ("Uinta C") and Duchesnean ages (Stock, 1932, 

1948; Durham and others, 1954; Golz, 1976; Golz and 

Lillegraven, 1977; Kelley, 1990). Both faunas were formerly considered late Eocene (see Stock, 1932, 1948), but 

now they are considered to be late middle and late Eocene 

in age (fig. 3) (Woodburne, 1987; Swisher and Prothero, 

1990; Prothero and Swisher, 1992). Oligocene faunas of 

Whitneyan and Arikareean ages are also present upsection 

in the Simi area (Stock, 1948; Savage and others, 1954; 

Lander, 1983), and an ash bed in the upper part of the 

Sespe Formation (South Mountain area, fig. 1) has been 

dated as 27.8±0.28 Ma (late Oligocene) by the K-Ar 

method (Mason and Swisher, 1988). The uppermost part 

of the Sespe Formation interfingers with the marine lower 

Miocene Vaqueros Formation in the Big Mountain area 

north of Simi (fig. 1) (Taylor, 1983) and in the Santa Monica (Yerkes and Campbell, 1979) and Santa Ana Mountains (Schoellhamer and others, 1981) (not shown in fig. 

1). The Vaqueros Formation contains mollusks assignable 

to the Turritella inezana inezana Zone of Loel and Corey 

(1932) and a foraminiferal fauna indicative of a latest 

Zemorrian or early Saucesian age (Sonneman, 1956; 

Edwards, 1971; Yerkes and Campbell, 1979; Blundell, 

1981; Schoellhamer and others, 1981; Blake, 1983). This 

evidence indicates that complete sections of the Sespe 

Formation range in age from late middle Eocene to early 

Miocene.

The Sespe Formation overlies the type Coldwater 

Sandstone in the Topatopa Mountains east of the study area 

(fig. 1), where Kew (1924) and Eschner (1969) described 

the contact between them as unconformable. However, if 

the contact is drawn instead at the base of the "transitional 

beds," it is conformable (Eldridge and Arnold, 1907). The 

Coldwater Sandstone contains few fossils in this area, but 

the presence of Turritella uvasana uvasana Conrad (Kew, 

1924) assigns it to the basal part of the "Tejon stage" of 

Clark and Yokes (1936). The age of these fossils, and those 

of the Uintan vertebrates in the Sespe Formation of the 

Simi area just to the south, suggests that the lower part of 

the Sespe Formation in the type area is of late middle 

Eocene age (fig. 3). In the eastern Santa Ynez Mountains 

near the Lake Casitas locality (fig. 1), Bailey (1947) 

reported T. uvasana uvasana Conrad from the upper part of 

the Coldwater Sandstone, but Blaisdell (1953) considered 

the T. uvasana uvasana Zone to lie in the underlying Cozy 

Dell Shale. A middle Eocene age is consistent with the 

presence of a "transition stage" molluscan fauna in the

Coldwater Sandstone (Givens, 1974) and the presence of 

Uintan vertebrates in the lower part of the Sespe Formation 

(Lindsay, 1968) near Pine Mountain just to the north. In the 

central Santa Ynez Mountains near the San Marcos Pass 

locality studied (fig. 1), the Coldwater Sandstone is characterized by a younger molluscan fauna assigned to the Turritella schencki delaguerrae Zone, and basal Sespe strata 

grade laterally westward into the Refugian (late Eocene) 

Gaviota Formation (Weaver and Kleinpell, 1963; Dibblee, 

1966). The above relations indicate that the ColdwaterSespe contact decreases in age westward across the Santa 

Ynez-Topatopa Mountains. In the western Santa Ynez 

Mountains west of the study area, the Coldwater Sandstone 

grades laterally westward and pinches out into contemporaneous strata of the Narizian Sacate Formation (Weaver and 

Kleinpell, 1963; Hornaday and Phillips, 1972; O'Brien, 

1973), and the Sespe Formation grades laterally westward 

into the Alegria Formation (Bailey, 1947; Weaver and 

Kleinpell, 1963; Dibblee, 1966) of Refugian and Zemorrian 

age (Kleinpell and Weaver, 1963; Weaver and Kleinpell, 

1963; Hornaday and Phillips, 1972; O'Brien, 1973).

The Sespe Formation is overlain by the Vaqueros Formation throughout the Santa Ynez Mountains. The sharp 

contact on the south side of this range (Dibblee, 1950, 

1966; Weaver and Kleinpell, 1963) has been interpreted as 

an erosional disconformity created by the onlap of the 

Vaqueros Formation (Edwards, 1971; Rigsby, 1989). The 

Sespe-Vaqueros contact is unconformable in the Santa 

Ynez Mountains as far east as the Ojai area (fig. 2) near 

Lake Casitas but gradational and conformable in the 

Topatopa Mountains to the east (Loel and Corey, 1932; 

Bailey, 1947; Eschner, 1969; Edwards, 1971). Molluscan 

macrofossils and foraminifers indicate that the lower part 

of the Vaqueros Formation is of Zemorrian (late Oligocene) age in the Santa Ynez Mountains and near Pine 

Mountain; the Zemorrian-Saucesian boundary falls in the 

overlying marine section (Weaver and Kleinpell, 1963; 

Dibblee, 1966; Edwards, 1971; Squires and Fritsche, 

1978). In the Topatopa Mountains, the Sespe and Vaqueros 

Formations are conformable or interfinger, and both upper 

and lower zones of the "Vaqueros stage" are present, suggesting that the Zemorrian-Saucesian boundary falls 

within the Vaqueros Formation (Loel and Corey, 1932; 

Eschner, 1969; Edwards, 1971). An early Miocene age for 

the uppermost Sespe Formation in this area is consistent 

with that found farther south as noted above. Thus, the 

Sespe-Vaqueros contact apparently decreases in age eastward across the Santa Ynez-Topatopa Mountains.

In contrast to data from complete sections of the Sespe 

Formation, the above data show that only the upper Eocene 

to Oligocene part of the formation is present in the areas of 

the Santa Ynez Mountains covered by this study. Some 

c Dnfusion has resulted from the fact that the lower contact 

of the Sespe Formation was defined inconsistently (fig. 4). 

For example, Lander (1983) reported that the fossil remains

Conglomerates of the Sespe Formation along the Santa Ynez Fault Implications for the Geologic History of Santa Maria Basin H5
of an oreodont (Sespia nitida Leidy) of presumed Oligocene age were found in "basal" strata of the Sespe Formation that were thought to grade laterally into marine Eocene 

rocks. This problem appears to have been resolved and is 

discussed below. There are other discrepancies in the literature regarding the age relations between the upper part of 

the Sespe Formation and the Vaqueros Formation. In particular, the correlation chart of Bishop and Davis (1984) suggests that deposition of the Vaqueros Formation in the 

Santa Ynez Mountains began about 31 Ma. This appears to 

be inconsistent with the Arikareean (Oligocene) age of Sespia nitida Leidy, which is found in the upper part of the 

underlying Sespe Formation (fig. 3) in both the central and 

western Santa Ynez Mountains (Weaver and Kleinpell, 

1963; Lander, 1983). This stratigraphic problem will also 

be discussed below.

The Sespe Formation in the eastern Santa Maria basin 

is unfossiliferous and its age is poorly constrained because 

the lower contact is unconformable (Dibblee, 1966; Schussler, 1981). In the Redrock Camp area (fig. 2), it is overlain conformably by the Vaqueros Formation which 

contains Zemorrian mollusks (Dibblee, 1966). An Sr isotopic age of 20±2 Ma for the Vaqueros Formation in this 

general vicinity has been reported (Rigsby, 1989); however, the reliability of this date is questionable (Rigsby, 

oral commun., 1993). In the Oso Canyon and Loma Alta 

areas (fig. 2), the Sespe Formation is overlain by the early 

or middle Miocene "Temblor" sandstone of Dibblee 

(1966). A bentonite bed in the "Temblor" sandstone of this 

general vicinity correlates with the Tranquillon volcanics 

farther west (Dibblee, 1966), isotopically dated at 

16.5+0.6 to 17.5+1.2 Ma4 (Turner, 1970). Dibblee (1966) 

describes the Sespe-'Temblor" contact at Loma Alta and 

Oso Canyon as gradational and conformable and suggests 

that the Sespe Formation at these localities may be a nonmarine facies of the Vaqueros Formation. However, his 

cross section shows the "Temblor" sandstone unconformably overlapping the Rincon, Vaqueros, and Sespe Formations near Oso Canyon. The Sespe Formation in the 

eastern Santa Maria basin is lithologically similar to the 

Lospe Formation mapped in the Santa Maria basin farther 

west (Woodring and Bramlette, 1950; Dibblee, 1950). Isotopic and paleontologic age control there indicates that the 

type Lospe Formation is entirely of early Miocene age 

(Stanley and others, 1991). This evidence, and the fact that 

Sespe-like strata are conformable beneath the "Temblor" 

sandstone in some places, suggests that the Sespe Formation may be much younger in the eastern Santa Maria 

basin than in the Santa Ynez Mountains, at least locally.

Cartwright (1928) and Gianella (1928) made early 

sedimentologic studies and inferred that the Franciscan 

Complex was the source of certain heavy minerals and

conglomerate clasts in the Sespe Formation of the Ojai 

area (fig. 2). Reed (1929, 1933) also recognized a Franciscan component but noted that the bulk of the detritus composing the Sespe Formation in the Santa Ynez-Topatopa 

Mountains was of granitic provenance and probably 

derived from the Mojave Desert region. He criticized earlier interpretations (for example, Kew, 1924; Reinhart, 

1928) that the Sespe Formation was composed principally 

of alluvial fan deposits. He concluded that some alluvial 

fan sedimentation did occur locally in the eastern Santa 

Maria basin but that the Sespe Formation was deposited 

mainly as the delta of a very large river. The interpretation 

of Bailey (1947) essentially concurs with that of Reed 

(1929) except that he favors a nearby source for the granitic detritus. Dibblee's (1950, 1966) conclusions also 

agree with those of Reed (1929), and although he does not 

say this in his published reports, he considers the Mojave 

Desert to have been the source for certain types of nonFranciscan clasts (Dibblee, oral commun., 1986). Flemal 

(1966) resurrected the alluvial fan model of Sespe deposition. Later interpretations by McCracken (1972), Bohannon (1976), Anderson (1980), Black (1982), and Nilsen 

(1984, 1987) all tend to emphasize the alluvial fan aspects 

of the conglomeratic strata composing the lower and middle parts of the Sespe Formation. In the alluvial fan scenario, Sespe conglomerate clasts are considered to be 

locally derived. This implies that where there is a mixture 

of Franciscan-type clasts with other types such as granite, 

the former are viewed as being first cycle and the latter as 

being recycled from older conglomeratic strata. Despite 

the discrepancies regarding the alluvial fan versus fluvialdeltaic origin of the rudaceous strata, there seems to be 

general agreement in the literature that the arenaceous 

strata composing the upper part of the Sespe Formation 

were deposited by meandering streams.

Stratigraphic relations show that during the early 

Cenozoic, the region corresponding to the Santa Maria 

basin was a structural high, which was called the "San 

Rafael uplift" by Reed and Hollister (1936). The upsection 

appearance of Franciscan detritus in the Sespe Formation 

of the Santa Ynez Mountains was interpreted to be the 

result of tectonism ("Ynezan orogeny"5 of Dibblee, 1950), 

which affected the San Rafael uplift during late Eocene 

and Oligocene time (Reed and Hollister, 1936; Dibblee, 

1950, 1966, 1977; Weaver and Kleinpell, 1963). The Ynezan orogeny is usually attributed to domal uplift in the 

older literature; however, more recent models often show, 

or at least imply, that Sespe conglomerates were deposited 

as alluvial fans in a rift basin bounded on the north by an 

active Santa Ynez Fault (McCracken, 1972; Bohannon, 

1976; Anderson, 1980; Black, 1982; Nilsen, 1984, 1987). 

In contrast, Namson (1987) inferred from structural evi4K-Ar isotopic ages corrected using the critical tables of Dalrymple

(1979).

5Dibblee (1966,1987) refers to this later as the "Ynezian orogeny,' 

however, his original term "Ynezan" is given precedence in this report.

H6 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
dence and Sespe conglomerate deposition that the Santa 

Ynez Fault originated as a south-dipping reverse fault during Oligocene time. Early indications were that the San 

Andreas transform fault system originated between 38 and 

29 Ma (Atwater, 1970); hence, it has been suspected that 

the Ynezan orogeny was the result of plate tectonic interactions (Nilsen, 1984, 1987; Glazner and Loomis, 1984).

Reverse or thrust faulting has been taking place on 

the major faults of the study area since the Pliocene or 

Pleistocene (Dibblee, 1950, 1966, 1987; Yerkes and Lee, 

1979; Namson and Davis, 1990; Hill, 1990). However, the 

presence of offset streams and canyons and the geometric 

orientation of fold axes suggest that a left-lateral component of displacement also has occurred recently on the 

Santa Ynez Fault (Page and others, 1951; Dibblee, 1950, 

1966). The Santa Ynez Fault is believed to splay westward 

into another left-lateral strand (Santa Ynez River Fault of 

Sylvester and Darrow, 1979) called the Lompoc Fault in 

this paper (fig. 1). McCracken (1972) noted that although 

Franciscan detritus is present in the Sespe Formation as 

far east as the Ojai area, the easternmost outcrops of Franciscan source rocks are far to the west (fig. 2). Thus, he 

suggested that about 35 km of left-lateral separation 

occurred on the Santa Ynez Fault after the Oligocene. 

McCulloh (1981) has indicated that a laumonite isograd 

also appears to be offset along this same fault in a leftlateral sense by 37 km. Other estimates of the amount of 

offset are contradictory (Sylvester and Darrow, 1979).

Plate tectonic models and the recognition of possible 

large offsets on faults of the San Andreas system have 

fueled much additional speculation and debate over the 

possibility of right-lateral offsets on faults of the Coast 

Ranges, such as the Little Pine Fault, as well as on the 

Santa Ynez Fault. There is some evidence that the Lompoc-Santa Ynez Fault joins the Hosgri Fault offshore and 

that large-scale right-lateral separation has occurred on 

this fault system (Schmitka, 1973; Hall, 1978; Dickinson, 

1983). It also has been suggested that the Hosgri-LompocSanta Ynez Fault once joined the San Gabriel Fault to the 

east and was therefore a through-going element of the San 

Andreas Fault system (Hall, 1978, 1981). Other workers, 

however, find evidence for only limited offset on the Hosgri Fault (Sedlock and Hamilton, 1991), and previous 

mapping shows that the Santa Ynez Fault dies out both to 

the east and west (Dibblee, 1966, 1987), with negligible 

horizontal separation at the eastern termination in the 

Topatopa Mountains (Gordon, 1978).

Paleomagnetic declinations in Miocene rocks of the 

Santa Ynez Mountains appear to be deflected clockwise by 

about 90° in contrast with those in the Santa Maria basin, 

which are essentially undeflected (Hornafius, 1985; Hornafius and others, 1986; Luyendyk and Hornafius, 1987). 

This contrast in paleomagnetic declinations has been 

explained by a model in which the entire western Transverse Ranges province was rotated tectonically as a single

rigid block (Luyendyk and others, 1980, 1985). Paleomagnetic data from the Sespe Formation in the Santa Ynez 

Mountains support this interpretation (Liddicoat, 1990). 

All of the above studies also documented anomalously flat 

paleomagnetic inclinations. These inclinations were originally thought to indicate that the rotated single rigid block 

had also been tectonically transported northward a great 

distance (Crouch, 1979). In current "suspect terrane" models, however, the Santa Maria basin and Santa YnezTopatopa Mountains are part of the Santa Lucia-Orocopia 

allochthon, which was accreted to southern California near 

the beginning of the Eocene (Champion and others, 1984, 

1986; Howell and others, 1987). The inclination data from 

Miocene rocks are thought to lack the precision necessary 

to document large-scale northward transport (Luyendyk 

and others, 1985; Nicholson and others, 1992).

METHODS OF STUDY

Lithofacies descriptions and paleoenvironmental interpretations in this study are based on characterization of six 

measured sections (fig. 2; localities 1 through 6). Sedimentary rocks were assigned colors according to the Munsell system and classified using the nomenclature of Folk 

(1974) and Dunham (1962). Other sedimentologic features 

were described using the terminology of Ingram (1954), 

Alien (1963), Campbell (1967), and Reineck and Singh 

(1973). Paleocurrent measurements were corrected stereographically for tectonic tilt about a single horizontal axis 

and analyzed statistically using Tukey's chi-square test 

(Middleton, 1965, 1967). Using this method, statistically 

significant differences indicate nonrandom distributions of 

paleocurrent vectors that is, a preferred orientation for a 

vector mean. Statistically significant differences are indicated where the calculated %2 value exceeds the tabulated 

value. Use of the terms "alluvial fan," "fan delta," "braid 

delta," and "braided river" follows that of McPherson and 

others (1988); "depositional sequence" is used in the sense 

of Vail and others (1977).

The proportions of different clast types composing 

conglomerates were established by counting all clasts 3 cm 

or larger of a given lithology present in a one square meter 

area of outcrop. Thus, the values are reported as number 

percentages (Howard, 1993). In counting, the term "metavolcanic" was used collectively for any predominantly aphanitic metaigneous rock, and "granitoid" was used for any 

species of leucocratic plutonic rock. All crystalline rocks 

containing visible chlorite, epidote, or garnet were termed 

"gneiss," regardless of whether foliation was present. A 

total of 31 conglomerate clasts from the Sespe Formation 

(appendix), along with selected clasts from older rock units, 

were examined in thin sections stained for K-feldspar. 

Modal analysis of granitoid clasts is based on counts of 100 

points per slide. The varietal study of metavolcanic clasts is

Conglomerates of the Sespe Formation along the Santa Ynez Fault Implications for the Geologic History of Santa Maria Basin H7
based on samples of 50 to 150 specimens collected from a 

2-m area of outcrop. Mafic clast types, which are commonly partially weathered, were excluded and only the 

hard silicified clast types were used. Silicified clasts were 

classified macroscopically as pyroclastic, porphyritic, and 

aphanitic (felsite) alter sawing open each specimen. Green 

clasts are 5Y or greener; red clasts are 2.5 YR or redder; 

brown clasts have hues of 5 YR, 10 YR., or 2.5 Y; and gray 

clasts have a chroma of 2 or less. Those clasts classified as 

andesite were required to have a chroma of 2 or less and no 

quartz or K-feldspar phenocrysts. Statistically significant 

differences in the proportions of metavolcanic clast varieties were assessed by the method of Dixon and Massey 

(1957). Thus, confidence intervals were calculated to test 

the null hypothesis that two proportions (pi and /?2) are the 

same according to the relation:

Pl-p2±t Jp l (l-pl )/nl +p2 (l-p2)/n2

where t is the tabulated value of Student's ^-distribution 

for ot/2 level of significance and n1+«2-2 degrees of freedom. If such an interval covers zero, the hypothesis is 

accepted; otherwise, it is rejected (Howard, 1993).

Clast shape measurements were made using only 

quartzite and metavolcanic clasts; thus the effects of 

weathering in outcrop or during temporary alluvial storage 

during deposition (Bradley, 1970) were minimized. At 

each sampling station, a set of 20 to 40 clasts with intermediate-axis diameters of 32 to 128 mm was obtained by 

choosing only clasts lacking an obvious planar fabric. The 

lower size limit of 32 mm was implemented because clasts 

smaller than this were not always common. Because of the 

natural size distribution of the conglomerates, the typical 

sample contained roughly twice as many clasts in the -5(|) 

to -6(|) (32 to 64 mm) grades as in the -6(|) to -7(|) (64 to 

128 mm) grades. Clasts were sampled intact (using a rock 

hammer and a chisel) by removing all of a desired type 

from a 1-m area of outcrop (enlarged slightly as necessary). Submarine fan samples were collected from a single 

thick, massive, or internally disorganized bed classified as 

facies A 1.1 using the system of Pickering and others 

(1986), except for that in the Cozy Dell Shale which 

belongs to facies A 1.2. In order to avoid operator bias during measuring, clast samples from different sites were 

assigned randomized lab numbers and combined. Clast 

shape distributions were characterized on the basis of 

oblate-prolate index (OPI) and modified Wentworth roundness (/?w) using the methods of Dobkins and Folk (1970). 

Thus, 7?w=r/R, where r is the radius of the sharpest developed corner and R is the radius of the largest inscribed 

circle. OPI is defined as:

where L, / and S are the long, intermediate and short axes, 

respectively. Axial ratios were calculated according to 

Zingg (1935) and measured using the long (L), intermediate (/), and short (5) axes as defined by Krumbein (1941). 

Maximum projection sphericity (\|/p) was calculated 

according to the relation:

(Sneed and Folk, 1958). All clasts were broken open prior 

to disposal to confirm hand specimen identifications. 

Using the chi-square test, selected samples were analyzed 

for "goodness of fit" to a normal distribution (see Davis, 

1986, for further explanation). Few of these samples were 

found to have shape index distributions that differed significantly from normality (Howard, 1992). Hence, the 

variances and means of the shape indices were analyzed 

using the F-test and Mest, respectively, to test for statistical significance (Davis, 1986). The ^-statistic was calculated as:

t =

where xl and x2 are the means of two samples and Se is 

the standard error (see Davis, 1986, for further explanation). If the calculated t exceeds the tabulated value, the 

hypothesis x^ =x2 is rejected and a statistically significant 

difference is identified. The associated F-statistic was calculated as:

10 S/L,

2 2 where s^ and s2 are the variances associated with means

jCj and x2 . If the calculated value of F exceeds the tabulated value, the hypothesis that the variances are the same 

is rejected and the Mest cannot be applied. The clast shape 

terminology used in this paper follows that of Barrett 

(1980); thus, "shape" refers collectively to all aspects of 

external morphology, "roundness" is a two-dimensional 

measure of the sharpness of particle corners, and "form" 

refers to the overall three-dimensional morphology defined 

in terms of the relative lengths of particle axes.

Older conglomerate rock units studied (fig. 1) are all 

of submarine fan origin and include the Jurassic and Cretaceous Espada Formation (McLean and others, 1977; 

MacKinnon, 1978; Nelson, 1979) in the southern Coast 

Ranges (locality MC), the Cretaceous Jalama Formation 

(Rust, 1966; Walker 1975; Krause, 1986) in the Santa 

Ynez-Topatopa Mountains (localities OC and RS), the 

Cretaceous and Paleocene San Francisquito Formation

H8 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
(Sage, 1973; Kooser, 1982) near the San Gabriel Mountains (locality FC), and the Eocene Cozy Dell Shale 

(Weaver and Kleinpell, 1963) in the Santa Ynez Mountains (locality WC). With the exception of the Cozy Dell 

Shale, all of the conglomerates sampled are clast supported and lack mud matrix.

SEDIMENTOLOGY OF THE SESPE FORMATION 

Stratigraphy and Paleoenvironments

Seven lithofacies are recognized in the Sespe Formation of the study area (table 1). Their stratigraphic distribution in the six measured sections studied is shown 

schematically in figure 5, except for lithofacies B and E, 

which are either too thin to show or not present in these 

particular sections. Lithofacies A and C correspond to the 

Hay Hill and Calvary units of Lian (1952), respectively. 

Lithofacies B, D, E, F, and G have not been previously 

designated.

Lithofacies A is characterized by features found in 

modern braided stream environments (Miall, 1977), but lateral westward gradation into fossiliferous deltaic deposits 

of the Gaviota Formation (O'Brien, 1973) suggests deposition as a braid delta. Lithofacies B was observed only in the 

San Pedro Canyon area (fig. 2) just west of San Marcos 

Pass where it is interstratified with lithofacies A. These 

strata resemble wave-worked conglomerates elsewhere 

(Clifton, 1973; Nemec and Steel, 1984), and similar deposits in the Los Angeles basin area contain clasts with both 

beach and fluvial shape characteristics suggesting deposition as river mouth bars (Howard, 1992). Lithofacies E is 

also only found interstratified with lithofacies A and crops 

out along Old San Marcos Pass Road (fig. 2). It includes 

crevasse-splay deposits suggesting deposition in a deltaic 

floodplain or interdistributary lake (Coleman and Prior, 

1982). Lithofacies C makes up the bulk of the Sespe Formation in the study area (fig. 5), except in the Camino 

Cielo area (fig. 2, section 4). It consists of repetitious 

sequences of normally graded conglomeratic sandstone and 

lithic-rich sandstone. These strata resemble those deposited 

episodically by modern braided rivers during waning flood 

stage (Nemec and Steel, 1984). The matrix-supported conglomerate of lithofacies G and the coarse boulder conglomerate of lithofacies F found in the Camino Cielo section 

resemble strata of debris flow and proximal braided-fluvial 

origin, respectively. Such deposits are typically found in 

modern alluvial fans (Bull, 1977; Nilsen, 1982). The upper 

part of the Sespe Formation is composed of lithofacies D, 

except in the Loma Alta section. The association of 

upward-fining sandstone-siltstone point-bar and levee 

deposits with overbank mudrock and crevasse-splay deposits in this facies indicates that it was deposited by meandering streams (Alien, 1965; Ray, 1976; Cant, 1982).

Lithologic correlations of the six measured sections 

(fig. 5) are shown in figure 6. The sections studied are 

much thinner north of the Santa Ynez Fault and range 

from only 35 m at Loma Alta to 50 m in Oso Canyon and 

to 100 m at Redrock Camp. South of the Santa Ynez 

Fault, the Sespe Formation ranges in thickness from about 

400 m at Camino Cielo to 1,100 and 1,700 m, respectively, at the San Marcos Pass (State Highway 154) and 

Lake Casitas localities. Only in the southern part of the 

Santa Ynez Mountains is it possible to continuously trace 

each lithofacies from one place to another. The other sections have been physically isolated from each other by 

erosion; hence, the correlations shown in figure 6 are 

based on lithologic similarities. Lithofacies F and G in the 

Camino Cielo section are correlated with lithofacies C 

elsewhere on the basis of similarities in conglomerate clast 

assemblages (discussed below).

Lithofacies A is a distinctive sequence of coarsegrained pinkish-gray sandstone and interbedded red mudrock, which is transitional between typical strata of the 

Coldwater Sandstone below and red beds of the Sespe 

Formation above. Lithofacies A sandstones are much 

coarser grained and tend to be more quartz rich than those 

of the Coldwater Sandstone. Arenites in the latter are usually buff colored, medium to fine grained, and very feldspathic; they are obviously of granitic provenance and 

occasionally contain small pebbles of fine-grained granite. 

Some red mudstone interbeds are present in the upper part 

of the Coldwater Sandstone below the "transitional beds" 

of lithofacies A. I have traced lithofacies A continuously 

from the San Marcos Pass area to Toro Canyon (fig. 7), 

and Lian (1952) reported that this lithofacies (his Hay Hill 

unit) eventually pinches out farther to the east near Rincon 

Creek (fig. 2). Dickinson and Leventhal (1987, 1988) 

described a somewhat similar uranium-bearing facies in 

the Superior Ridge area (fig. 2) just west of Lake Casitas; 

however, these outcrops were not visited during this study. 

Lithofacies A is definitely absent in the sections examined 

at Lake Casitas (figs. 5 and 6) and along the Ventura River 

near Ojai. A sequence of "transitional beds" that includes 

some red mudstone is present in the Ventura River section 

(fig. 2), but this is unlike lithofacies A lithologically. 

These "transitional beds" have been referred to as the 

upper part of the Coldwater Sandstone by previous workers (for example, Weaver and Kleinpell, 1963; Moser and 

Frizzell, 1982) who described the Coldwater-Sespe contact 

as interfingering in this area. The Ventura River section 

appears to resemble more closely the "transitional beds" 

of the type section of Sespe Formation, but a thorough 

investigation of this correlation problem was beyond the 

scope of this study.

As discussed earlier, the "transitional beds" of lithofacies A were called Coldwater Sandstone by McCracken 

(1972) despite the fact that they were mapped previously as 

the Sespe Formation by Lian (1952, 1954) and Dibblee

Conglomerates of the Sespe Formation along the Santa Ynez Fault Implications for the Geologic History of Santa Maria Basin H9
Table 1. Lithofacies of the Sespe Formation in the eastern Santa Maria basin and vicinity

Lithofacies Depositional mechanism Paleoenvironment Description

3-

o

A: Trough crossbedded pebbly 

sandstone and 

sandstone

Mainly flash flood, partly 

tidal deposition

Distal braided fluvial and 

delta distributary channel

Medium to very coarse grained, thick-be

light-gray arkose and pebbly arkose. Lar

festoon and channel-fill crossbedding ab

graded, thin-bedded sandstone. Minor c

or gray shale. Unfossiliferous.

8J

Q.

B: Massive sheet 

conglomerate

C: Graded sandstone/conglomerate

D: Upward-fining 

sandstone/ 

mudstone

E: Calcareous 

claystone

Wave worked

Flash flood

Lateral accretion of point 

bar/levee deposits. Vertical 

accretion of overbank 

crevasse-splay deposits

Lacustrine delta fill

River-mouth bar or 

beachface

Distal braided fluvial

Meandering fluvial

Interdistributary lake

Sheet-like, massive to crudely stratified, 

densely packed pebble-cobble conglome

or lenticular, well-sorted, medium to coa

bedding segregation and lateral continuity

Stacked sheet-like bodies composed of in

green, or brown, massive, densely packe

lenticular pebble-cobble conglomerate an

conglomeratic arkose and feldspathic lith

paleochannels. Minor mudstone. Very 

Repeated upward-fining sequences of re

arkose and feldspathic litharenite and silt

with mainly red, massive to weakly fissil

scattered sandstone and massive limeston

scale trough crosbedding; parting lineatio

common. Very rare terrestrial vertebrate 

Massive to weakly fissile, dark reddish-b

claystone with scattered light gray calca

interbeds. Unfossiliferous.

F: Boulder

conglomerate

G: Pebbly 

mudstone

Flash flood

Debris flow

Proximal braided fluvial 

or alluvial fan

Proximal braided fluvial or 

alluvial fan

Interbedded massive or crudely stratified 

very thick bedded.very poorly sorted, cl

brown bouldery pebble-cobble conglome

lenticular reddish-brown siltstone and pe

Interbedded massive and normally grade

lenticular, reddish- or greenish-brown, m

and mudstone. Locally cobbly and bould
n

o

<§.

o 

n

I

s

SL

o

era

 o

Section 1 

Loma Alta

Section 2 

Oso Canyon

Section 3 

Redrock Camp

Section 4 

Camino Cielo

Section 5 

San Marcos Pass Highway 154

Section 6 

Lake Casitas

Figure 5. Schematic stratigraphic sections (not to scale) of Sespe Formation in study area. See figure 2 for section locations; lit

described in table 1. Paleocurrent data are selected from tables 2 and 3. Pie diagrams indicate relative proportions of conglo

indicate number of paleocurrent measurements within a directional interval; modern magnetic north is at top of diagram, and 

circles in number of paleocurrent measurements.
1

Loma Alta

Temblor 

Formation

Sespe 

Formation

Matilija 

Sandstone l

Oso Canyon Bedrock Camp Camino Cielo 

Vaqueros Formation

San Marcos Pass 

Highway

Lake Casitas

Vaqueros 

Formation

ySespe 

Formation

Coldwater 

Sandstone

Figure 6. Lithologic correlations of Sespe Formation in study area (not drawn to scale). Lithologic symbols 

shown in figure 5. See figure 2 for section locations and table 1 for lithofacies description.

Camino Cielo Syncline Camino Cielo Syncline San Marcos Pass Old San Marcos Tunnel Road Toro Canyon 

(east side) (west side) Highway (section 5) Pass Road

vSespe 

Formation

Coldwater 

Sandstone

Figure 7. Schematic stratigraphic sections (not to scale) of Sespe Formation in southern Santa Ynez Mountains showing 

lithologic correlations. See figures 1 and 2 for site locations; lithofacies are described in table 1.

H12 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
WEST

GC 

l

CP 

I

RC 

I

SITE LOCATIONS

CT 

I

EAST

BC 

I

SMP 

I__

Alegria Formation (lower part) Gaviota Formation i^VV. Sespe Formaton (lower part)

j Kilometers Sacate Formation and Coldwater Sandstone

Figure 8. Facies relations between Sespe Formation and laterally equivalent marine units in the central and western Santa 

Ynez Mountains, Calif. Modified from Hornaday and Phillips (1972), as discussed in text. See figure 1 for site locations: GC, 

Gaviota Canyon; CP, Canada de la Posa; RC, Canada del Refugio; CT, Canada del Capitan; BC, Bartlett Canyon; SMP, San 

Marcos Pass State Highway 154.

Explanation

Covered section

°o°o Conglomerate 

I'ogol

Sandstone

Conglomeratic sandstone 

Trough crossbedded sandstone 

Mudrock 

Rip-up clasts

Planar Crossbedding

- Inferred contact

Lens-shaped body 

I I Unconformity

Figure 7. Continued.

(1966). Further justifications for including lithofacies A as 

part of the Sespe Formation are the facts that (1) strata of 

similar lithology are found elsewhere in the Sespe Formation (Howard, 1987, 1989) but have not been recognized in 

the underlying Coldwater Sandstone (Dibblee, 1966; Vedder, 1972; McCracken 1972) and (2) these strata grade laterally westward into the Gaviota Formation, which overlies

and is younger than the Coldwater Sandstone (Weaver and 

Kleinpell, 1963; Dibblee, 1966). Defining the lower contact 

at the base of lithofacies A includes many strata in the 

Sespe Formation that are not red but excludes those red 

beds found in the upper part of the Coldwater Sandstone. 

This is perhaps a violation of the priority of Watts' (1897) 

original usage of the unit as the Sespe brownstone. However, stratigraphers have agreed that priority should not prevent a more exact lithologic definition if the original 

definition is not everywhere applicable and if a change in 

the terminology does not require a new geographic term to 

be established (North American Commission on Stratigraphic Nomenclature, 1983). The presence or absence of 

red beds is an invalid criterion for a formal definition of the 

lower contact of the Sespe Formation because red beds are 

not a characteristic feature of the Sespe Formation in all 

locations where it has been mapped. Additional studies of 

the "transitional beds" at the type locality are needed to 

determine what lithologic criteria should be used to formally define the Sespe Formation.

In the Santa Ynez Mountains west of San Marcos Pass 

(fig. 1), the Sespe Formation grades laterally westward into 

the marine Gaviota and Alegria Formations (fig. 8). Nonmarine strata of the basal part of the Sespe Formation can 

be walked bed by bed across Bartlett Canyon into laterally 

equivalent marine rocks of the Gaviota Formation (Weaver 

and Kleinpell, 1963; Dibblee, 1950, 1966; Weaver, oral 

commun., 1985). West of Canada de la Posa (fig. 1), progressively higher beds of the Sespe Formation grade laterally westward into the marine Alegria Formation until the 

Sespe Formation pinches out a short distance west of Gaviota Canyon (Dibblee, 1950; Weaver and Kleinpell, 1963). 

In the western Santa Ynez Mountains west of Arroyo Bulito 

(fig. 1), the Vaqueros Formation rests unconformably on the 

Alegria Formation. From this point, the Vaqueros Formation overlaps the Alegria and Sespe Formations eastward

Conglomerates of the Sespe Formation along the Santa Ynez Fault Implications for the Geologic History of Santa Maria Basin H13
across the Santa Ynez Mountains. Thus, lateral facies 

changes involving marine and nonmarine strata and age 

data already discussed show that the upper Eocene to lower 

Miocene section of the Santa Ynez Mountains is a sedimentary offlap-onlap sequence. These relations have been 

known for many years, but it usually has been assumed that 

sedimentation was more or less continuous.

However, during the recuration of the California Institute of Technology collection of vertebrate fossils from the 

Sespe Formation by the Los Angeles County Museum, it 

was discovered that several specimens had been found in 

the Santa Ynez Mountains which were not described in the 

literature (Lander, oral commun., 1986). Hence, Lander 

(1983) reported that a fossil oreodont (Sespia nitida 

Leidy) was found at two locations. One locality is in the 

upper part of the Sespe Formation (lithofacies D of this 

report) at Sycamore Canyon north of Santa Barbara (fig. 

2) and the other is at the base of Lian's Calvary unit (lithofacies C of this report) along Maria Ygnacio Creek (fig. 

1, locality SMP). The latter specimen was apparently collected by D.W. Weaver and D.E. Savage (Weaver, oral 

commun., 1985) along San Marcos Pass highway (Lander, 

oral commun., 1986). This oreodont species is generally 

thought to be of Arikareean (Oligocene) age, and isotopic 

dates from rocks elsewhere in the western United States 

suggest an age of 26 to 30 Ma (Lander, 1983; Swisher and 

Prothero, 1990). These data indicate that at San Marcos 

Pass the oreodont-bearing red beds of lithofacies C are of 

late Oligocene age and that these red beds lie unconformably on pinkish-gray strata of lithofacies A that grade laterally westward into the Gaviota Formation of late Eocene 

age (fig. 8). The unconformity separating these two units 

(figs. 5 and 6) is, therefore, a significant hiatus representing much or all of early Oligocene time.

As noted earlier, confusion over the position of the 

boundary between the Coldwater Sandstone and Sespe Formation has resulted in errors in the interpretation of age 

relations. Thus, by drawing the lower boundary of the Sespe 

Formation at the base of the red bed interval instead of at the 

base of the "transitional beds," Lander (1983) misinterpreted 

the oreodont-bearing red beds to be of late Eocene age 

because "basal Sespe strata" had been reported to grade 

laterally westward into the Gaviota Formation of Refugian 

age. A reconnaissance of San Jose, San Pedro, and Bartlett 

Canyons (fig. 2) during this study showed that it is only the 

"transitional beds" (lithofacies A) that grade laterally westward into the Gaviota Formation. Strata directly above the 

"transitional beds" in these canyons are physically correlative with the red bed conglomerates farther east at San Marcos Pass that contain the late Oligocene oreodont remains.

Sespia nitida Leidy also has been discovered in the 

Alegria Formation farther west in Gaviota Canyon (fig. 1). 

Weaver and Kleinpell (1963) indicated that the fossil was 

found in a lower Zemorrian time-rock (chronostratigraphic) interval, presumably corresponding with members D through F of Dibblee (1950). Lander (1983), 

however, apparently miscorrelated the oreodont-bearing 

beds with members A and B of Refugian (late Eocene) age. 

Dibblee (1950) and Weaver and Kleinpell (1963) noted that 

the lower members of the Alegria Formation (up through the 

basal part of member D) contain a Refugian molluscan fauna 

identical to that in the Gaviota Formation. Lander (1983) 

recognized further that planktonic foraminifers belonging to 

zone PI7 and nannoplankton belonging to zones NP 19 and 

20 assign a late Eocene age to Alegria strata just below 

member C. These data suggest that the same hiatus found at 

San Marcos Pass within the Sespe Formation is also present 

within the Alegria Formation at Gaviota Canyon. If so, some 

Alegria strata of late Oligocene age must lie unconformably 

on other Alegria strata of late Eocene age (fig. 8). This 

modified view of lateral facies relations is interesting 

because it suggests that the Sespe Formation and its marine 

lateral equivalents were deposited rapidly during two distinct episodes rather than as one long continuous event spanning all of Oligocene time as previously supposed.

Paleocurrent Analysis

Paleocurrent data for braided-stream deposits of the 

Sespe Formation north of the Santa Ynez Fault are shown in 

table 2 and figure 5. The modes (class intervals containing 

the most observations) and calculated vector means suggest 

that paleocurrents flowed primarily toward the west and 

south, except at Loma Alta where paleoflow was directed 

northward. The chi-square test indicates that one vector 

mean from the lower part of the Sespe Formation at Redrock 

Camp is not significant, a determination which suggests that 

no preferred orientation is present. Nevertheless, the data 

from Oso Canyon and Redrock Camp seem to show an 

upsection shift from west to south and perhaps a change in 

paleoslope direction over time. South of the Santa Ynez 

Fault, paleocurrent modes and vector means (table 3; fig. 5) 

are directed mainly toward the west and southwest, but a 

southward component of transport is also present. The data 

for lithofacies A at the Camino Cielo locality, and for lithofacies C at the San Marcos Pass highway locality fail the chisquare test for statistical significance. This failure suggests 

that a preferred orientation is not present; hence, these data 

should be regarded as inconclusive. At the San Marcos Pass 

highway locality, clasts are very prolate, making imbrication 

difficult to find, but there is a well defined east-west-trending 

paleochannel cut on top of lithofacies A. Similar features are 

common at the Lake Casitas locality.

Conglomerate Clast Assemblages

The clast types characterizing Sespe conglomerates in 

the study area are classified into three assemblages (table 4) 

that are interpreted as being genetically distinct with regard 

to provenance. Assemblage 1 includes two varieties of chert

H14 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
Table 2. Paleocurrent data for lithofacies C of the Sespe Formation in the eastern Santa Maria basin

[All data are for pebble imbrication relative to modern magnetic north and corrected for bedding tilt, x2, test statistic; a, level of significance. See figure 2 

for site locations]

Site location

Parameter

Arithmetic mean(°)

Standard deviation(°)

Vector mean(°)

X (calculated)

Modal class(°)

No. of measurements

Loma

Alta

24. 1

20.7

27.2

3 21.3

0-29

12

Oso Canvon

Lower

254.6

50.1

256.4

27.7

240-269

7

Upper

135.1

22.2

137.1

3 24.5

150-179

15

Lower

286.5

32.5

288.2

311.6

240-269

8

Redrock Camp

Lower

233.6

61.6

229.0

1.8

240-269

8

Upper

184.2

46.3

187.5

3 10.9

150-179

20

Refers to stratigraphic position.

Significant difference (a=0.05; tabulated x 2 =5.99).

"5 fy

Highly significant difference (cc=0.01; tabulated x =9.21).

Table 3. Paleocurrent data for the Sespe Formation in the Santa Ynez Mountains

[All data are for pebble imbrication relative to modern magnetic north and corrected for bedding tilt, x2, test statistic; a, level of significance. See Figure 

2 for site locations]

Site locations

Camino Cielo San Marcos Pass Highway Lake Casitas

Parameter East 1 (F)2 West (A) West(F) Lower3 (A) Lower (C) Middle (C) Upper (C) Lower (C) Middle (C) Upper (C)

Arithmetic mean (°)........

Standard deviation (").... . Vector mean (°)..............

Modal interval (°)............

No. of measurements......

179 5

.......38.3

,....182.2

...... 48.5

....150-179

........6

272.7

21.2

275.0

5.9

270-299

3

279.1

40.3

283.8

59.6

270-299

8

277.7

29.6

280.5

5 11.6

240-269

7

284.7

98.7

31Q 2

5 10.5

330-359

14

261.3

41.8

250.1

5.5

240-269

7

255.0

83.0

285.0

59.8

270-299

10

252.5

43.5

251.5

5 13.1

210-239

10

271.4

36.0

272.3

5 18.3

240-269

14

214.4

19.4

217.2

532.6

210-239

18

Indicates east or west limb of syncline (see fig. 2). 

 Indicates lithofacies type (see table 1). 

1 Indicates stratigraphic position. 

\ Significant difference (oc=0.05; tabulated % 2=5.99). 

' Highly significant difference (a=0.01; tabulated X =9.21).

clasts (not differentiated in table 4). The most abundant type 

ranges in size from granule to boulder and is typically red, 

green, or gray, angular, and very thin bedded or laminated. 

The other type is black, well rounded, pebble sized, and rare. 

The lithic-arenite clasts resemble graywacke in hand specimen but contain less than 15 percent matrix. They apparently 

lack detrital K-feldspar, although only one clast was studied 

in thin section. The clasts counted as metabasalt are mainly 

aphanitic, but some of vesicular or diabasic texture are also 

present. Two of the three examined in thin section have 

basaltic texture and contain the greenschist-facies minerals 

chlorite, albite, epidote, and actinolite. The other specimen 

has ophitic texture and is of basaltic composition. The typical

clasts of ultramafic rock are partially serpentinized and found 

in association with jasper-calcite rock, glaucophane schist or 

chlorite schist, and vein quartz. A clast of leucogabbro examined in thin section contains uralitic hornblende and plagioclase with the composition of andesine-labradorite (An35_ 

55). Other gabbro-like hand specimens are apparently of 

dioritic composition, and some are quartz or garnet bearing. 

Assemblage 2 includes several lithologic varieties of 

feldspathic sandstone and limestone, none of which were 

examined in thin section. The sandstone clasts are mainly 

buff-colored arkosic arenite containing biotite and (or) muscovite and, locally, whole or broken molluscan fossils. 

Clasts of subarkosic arenite and quartz arenite are also

Conglomerates of the Sespe Formation along the Santa Ynez Fault Implications for the Geologic History of Santa Maria Basin HIS
Table 4. Percentages of clasts in Sespe conglomerates of the eastern Santa Maria basin area 

[Tr, trace amount <1.0 percent; facies are lithofacies in table 1. See figure 2 for site locations]

Clast type

Loma 

Alta

Site Location

Oso Redrock 

Canyon Camp

Camino Cielo San Marcos Pass Highway Lake Casita

Facies A Facies F Facies G Facies A Facies C Facies C Facies C Facies C Facies C

Assemblage 1

Chert............................

Lithic arenite........ .......

Ultramafic....................

Leucogabbro................

Glaucophane schist .....

Vesicular basalt .........

Quartz diorite...............

Vein quartz.................

.......2

......98

.......0

......tr

......0

...... tr

......tr

.......0

......0

...... tr

46

29

3

0

tr

1

9

0

0

tr

3

96

1

0

0

0

tr

0

0

tr

0

0

0

0

0

0

0

3

0

0

2

0

6

tr

76

0

0

0

2

0

10

0

13

0

77

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

32

4

tr

tr

0

0

3

0

0

0

10

1

0

0

0

0

0

0

0

0

4

1

0

tr

0

0

0

0

0

0

45

25

0

tr

0

0

tr

0

0

0

8

9

0

tr

0

0

1

0

0

0

Assemblage 2

Arkosic arenite........

Limestone..................

..........0

.........tr

2

1

tr

tr

3

0

0

0

0

0

10

1

9

12

1

2

tr

2

4

2

4

8

Assemblage 3

Quartzite......................

Felsic metaporphyry....

Felsic metatuff .............

Metaandesite.......... ......

Granitoid......................

Gneiss...........................

Sample size..................

......0

......0

......0

......0

......0

......0

....122

9

tr

0

tr

tr

tr

186

tr

tr

tr

tr

tr

tr

117

45

28

3

0

18

tr

122

0

0

0

6

0

143

0

0

0

0

0

0

944

50

22

7

0

10

0

184

27

4

tr

0

2

6

404

42

23

11

0

3

7

296

33

18

4

tr

10

28

227

4

5

tr

tr

1

14

113

17

16

3

tr

10

27

162

included. The limestone clasts are mainly grayish white, 

very fine grained with a saccharoidal texture, massive, and 

unfossiliferous. One specimen was found that was partially 

dolomitized and thinly laminated with black chert. The other 

common clast type counted as limestone is medium- to 

coarse-grained calcarenite containing sand- and granulesized lithic fragments of red and green chert and jasper.

The quartzite clasts in assemblage 3 are mainly gray 

and brown; however, distinctive maroon and black varieties are also common, along with a rare ochre-colored type. 

They range collectively from granular to vitreous and porcelaneous and from very fine grained to pebbly. Only one 

specimen was examined in thin section. This is a black, 

partially recrystallized specularitic variety that contains 

irregularly distributed clots of chlorite. The granitoid clasts 

in assemblage 3 vary widely in grain size and are typically 

biotite bearing and of midcrustal origin; granulitic types 

were not observed. One garnet-bearing lithology counted

as gneiss in the field was found in thin section to have a 

granitoid texture; thus of the nine clasts examined in thin 

section, four are biotite granite, three are garnet-biotite 

granite, and two are alkali-feldspar granites. Biotite in 

these samples is usually partially chloritized and plagioclase is commonly sericitized. Microcline and striped 

perthite are also common constituents. Most of the gneiss 

clasts appear to be of granitic composition and of greenschist-facies or lower amphibolite-facies grade. Some 

specimens are apparently of granodioritic or dioritic composition. One of the few samples studied in thin section is 

plagioclase rich and contains a strongly developed granophyric texture. One clast of silicified ultramylonite was 

found in lithofacies A at the Camino Cielo locality. Some 

of the garnet-bearing clasts are massive or weakly foliated 

and thus impossible to differentiate from granitoid clasts.

The silicified metavolcanic clasts of assemblage 3 are 

mainly of felsic composition, but andesitic varieties are also

H16 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
Table 5. Maximum observed clast size (centimeters) in Sespe conglomerates of the eastern Santa Maria basin area 

[See figure 2 for site locations. , not found]

Clast type

Loma 

Alta

Site location

Oso 

Canyon

Redrock 

Camp

Camino Cielo Lake Casitas

Facies A Facies F

San Marcos Pass Highway _____________ 

Facies G Facies A Facies C Facies C

Assemblage 1

Chert..............................

Lithic arenite.................

Leucogabbro.................

Glaucophane schist ......

Vesicular basalt ...........

Vein quartz...................

........60

........18

........10

........12

........90

.........3

30

20

3

17

8

13

3

26

25

3

...

...

4

...

3

7

20

10

38

5

13.5

5

3

8

_._

...

_._

23

4

18

5

30

_._

18

6

5

___

30

___

...

Assemblage 2

Arkosic arenite..........

Limestone...................

........... 15

............3 20

75

5

10 6

4

38

13

30

30

Assemblage 3

Quartzite................. .............. 

Felsic metaporphyry ............. 

Felsic metatuff ...................... - 

Metaandesite..... .................... 

Granitoid2 ............................. ---

Includes metadiabase. 

2 Includes pegmatite.

40

5

15

20

16

10

5

4

1

40

6

25

28

15

18

8

22 35

15

6

10 5

60

23

15

18

14

35

25

32

18

34

45

present. Both clast types contain secondary minerals suggesting a greenschist-facies grade of metamorphism. The 

felsic types range from those with obvious pyroclastic texture (volcanic breccia, welded tuff, lapilli-ash tuff) to porphyritic and aphanitic types. The porphyritic types are most 

common and typically contain a quartz- and K-feldspar-rich 

microgranular groundmass. Two porphyritic varieties are 

easily differentiated one type containing quartz, sanidine, 

and plagioclase phenocrysts and the other only feldspar phenocrysts. Plagioclase phenocrysts are typically altered to 

sericite, and mafics are sparse having been replaced by 

sphene, epidote, or piemontite. The felsic types appear to be 

mainly of rhyolitic composition and vary in color with 

groundmass composition. Those clasts containing abundant 

epidote are green; red clasts are hematitic; dark-greenishgray clasts are chloride; and brown clasts are goethitic or 

have heavily albitized plagioclase as a major groundmass 

component. The metaandesite clasts are dark colored (low 

chroma), contain abundant mafic phenocrysts, and have 

lath-shaped plagioclase phenocrysts that are usually saussuritized or albitized. Felted (pilotaxitic) texture, which is rare 

in the felsic types, is common in the metaandesitic clasts.

Assemblage 1 predominates north of the Santa Ynez 

Fault (fig. 5; table 4). Thus, Sespe conglomerate at Loma 

Alta and Redrock Camp is greenish gray owing to the 

abundance of lithic arenite clasts, but that at Oso Canyon 

is redder owing to the abundance of chert clasts of this 

color. The largest clasts observed (table 5) are boulders of 

red chert and jasper-calcite rock at Loma Alta and large 

boulders of oyster-bearing arkosic arenite at Redrock

Conglomerates of the Sespe Formation along the Santa Ynez Fault Implications for the Geologic History of Santa Maria Basin HI 7
Table 6. Upsection changes in Sespe conglomerate clast suites 

[See figure 2 for site locations. Tr, trace amount]

Site location

San Marcos Pass Highway Camino Cielo

Clast type Fades A (I) 1 Fades A (2) Fades A (3) Fades A (4) Facies A Fades F Facies F Facies G

Assemblage 1

Chert.............................

Metabasalt....................

Leucogabbro................

Glaucophane schist......

Jasper-calcite rock......

Vesicular basalt ...........

Quartz diorite...............

.......6

......0

......0

......0

......0

......0

.......0

.......0

.......0

4

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

3

3

2

0

0

0

0

0

0

0

0

0

0

0

0

0

3

0

2

0

6

tr

76

0

0

0

2

5

0

4

0

85

0

0

0

5

10

0

13

0

77

0

0

0

0

Assemblage 2

Arkosic arenite........

Limestone.................

.........22

..........0

1

0

10

1

3

3

3

0

0

0

0

0

0

0

Assemblage 3

Quartzite.. ....... .............

Felsic metaporphyry....

Felsic metatuff .............

Metaandesite................

Granitoid ......................

Gneiss...........................

.....49

......5

......4

......0

.....14

......0

59

7

16

0

12

1

50

22

7

0

10

0

59

2

11

4

7

3

45

28

3

0

18

tr

8

0

0

0

6

0

1

tr

0

0

0

0

0

0

0

0

0

0

Sample size...................... 122 158 184 201 122 143 169 244

Numbers in parentheses indicate ascending stratigraphic order.

Camp. Cobbles and small boulders of assemblage 3 are 

occasionally found at Redrock Camp and Oso Canyon, but 

no clasts of this assemblage are present at Loma Alta.

Localities south of the Santa Ynez Fault (Camino 

Cielo, San Marcos Pass, Lake Casitas) have a much more 

complex distribution of clasts, although those of assemblage 3 generally predominate (fig. 5; table 4). Lithofacies 

A at Camino Cielo and San Marcos Pass contains mainly 

assemblage 3 clasts. However, a few clast types from 

assemblages 1 and 2 appear and increase in abundance 

upsection in lithofacies A at San Marcos Pass (table 6). 

Boulders of oyster-bearing arkosic arenite 45 cm in diameter also are present in lithofacies A at San Jose Canyon 

(fig. 2). The conspicuous intraformational unconformity

between lithofacies A and C along San Marcos Pass highway and between lithofacies A and F at Camino Cielo (fig. 

5) corresponds with an upsection provenance change. At 

the San Marcos Pass highway locality, red beds above the 

unconformity contain abundant clasts of red chert, but at 

Camino Cielo boulder-sized leucogabbro and diorite clasts 

abruptly appear above the erosion surface (tables 4 

through 6). Clasts of leucogabbro or diorite are not found 

anywhere at San Marcos Pass, but some clasts of these 

types up to 6 cm in size are present in basal beds of lithofacies C in San Jose and San Pedro Canyons to the west 

(fig. 2). In the Lake Casitas section and near Ojai (fig. 2), 

lithofacies A is absent and interbedded conglomerate 

lenses in the basal part of the Sespe Formation (lithofacies

HI 8 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
D) are composed almost entirely of assemblage 1 

(Howard, 1989). Upsection, conglomerates of lithofacies C 

are rich in either assemblage 1 or 3, and alternate from 

bed to bed (fig. 5; table 4). In the San Marcos Pass and 

Lake Casitas areas (table 5) and elsewhere in the central 

and eastern Santa Ynez Mountains, some clasts of assemblage 3 range up to boulder size, but the average clast is 

generally less than 10 cm in size. Figure 9 shows that 

there is a systematic westward decrease in the maximum 

sizes of granitoid and gneiss clasts across the Santa YnezTopatopa Mountains. The difference in the size ratio of 

softer granitoid/gneiss to harder quartzite lithologic types 

of clasts in figure 9 presumably reflects the effects of abrasion with increasing transport distance.

Metavolcanic Clast Varieties

Silicified metavolcanic rocks are common as clasts in 

conglomerates of Cretaceous and Paleogene age in and 

around the study area. Because they are exceptionally 

durable and resistant to chemical weathering, it is theoretically possible for such clasts to have been recycled during 

deposition of the Sespe Formation. The metavolcanic clast 

suites of the Jurassic and Cretaceous Espada Formation in 

the San Rafael Mountains and the Cretaceous and Paleocene San Francisquito Formation near the San Gabriel

i i i i i i i i i i i

20 40 60 80 100 120 

DISTANCE, IN KILOMETERS

Figure 9. Ratios of maximum granite/gneiss clast size to 

maximum quartzite clast size versus distance west of the San 

Gabriel Fault. Granitoid-quartzite ratio (solid line); Gneissquartzite ratio (dashed line). See figure 1 for site locations: 

Sespe Creek (dot), Lake Casitas (square), Montecito (triangle), 

Gibraltar Road (diamond), San Marcos Pass State Highway 

154 (circle), Glen Annie Canyon (open square). Data from 

McCracken, 1972.

Mountains (fig. 1, localities MC and FC) are compared in 

table 7 with clast suites of Sespe conglomerate. Espada 

conglomerate is characterized by large quantities of 

metaandesite and dark gray metarhyolitic porphyry. Both 

varieties are uncommon in Sespe conglomerate at Camino 

Cielo (locality EC) and San Marcos Pass (locality SMP) in 

the central Santa Ynez Mountains. The Sespe suite 

includes abundant brown, goethitic metavolcanic clasts, 

which form only a minor component of the Espada clast 

suite. When the proportions (p\ and p2) of these varieties 

in Espada and Sespe sample pairs are compared, the differences are statistically significant at the 95 percent level 

(table 8). San Francisquito conglomerate contains more 

clasts of metaandesite and pyroclastic rocks and less of 

brown metaporphyry than Sespe conglomerate near Lake 

Casitas (locality LC) in the eastern Santa Ynez Mountains 

(table 7). The differences in these proportions are also statistically significant (table 8). These data suggest that metavolcanic clasts in Sespe Formation conglomerates at 

Camino Cielo and San Marcos Pass were not recycled 

from the Espada Formation. Similarly, the San Francisquito Formation does not appear to have supplied Sespe 

conglomerates at Lake Casitas with such clasts.

Conglomerate Clast Morphology

In table 9, clast shape data for rock units at the locations shown in figure 1 are summarized. For quartzite 

clasts, the values of /?w (roundness) are greater at the San 

Marcos Pass (SMP) and Gibraltar Road (GR) Sespe conglomerate localities than at the Lake Casitas locality (LC) 

farther east. The values of S/L and 5/7 are lower at the San 

Marcos Pass locality than at the Gibraltar Road and Lake 

Casitas localities. The results of t-tests when mean values 

at these locations are compared show which of the differences are statistically significant (table 10). These data 

suggest that Sespe quartzite clasts become rounder and 

flatter westward across the Santa Ynez Mountains. In the 

case of metavolcanic clasts, the same apparently holds for 

roundness; however, the other shape indices are highly 

variable, perhaps because of the effects of texture on 

shape (Drake, 1970; Howard, 1992).

Table 9 also shows that the values of the IIL index are 

generally greater and OPI values lower for the pre-Sespe 

submarine fan deposits studied (localities MC, OC, RS, 

FC, and WC) than for braided-stream deposits of the 

Sespe Formation (localities LC, GR, and SMP). In the 

eastern Santa Ynez Mountains, IIL and S/L values are significantly greater and \|/p (see definitions in methods section) and OPI values are lower (table 11) for Jalama 

conglomerate at Rose Canyon (OC) than for Sespe conglomerate at Lake Casitas (LC) when quartzite clasts are 

compared. However, the comparison of \j/p using quartzite 

clasts failed the F-test; thus, the f-test result should be

Conglomerates of the Sespe Formation along the Santa Ynez Fault Implications for the Geologic History of Santa Maria Basin H19
Table 7. Analysis of metavolcanic clast suites in Cretaceous and Paleogene conglomerates by percentage

[See figure 1 for site locations: MC, Manzana Creek; FC, Fish Creek; EC, east Camino Cielo; SMP, San Marcos Pass State Highway 154; 

LC, Lake Casitas. R, red; G, green; B, brown; D, gray; q, quartz phenocrysts; f, feldspar phenocrysts. Letters in parentheses indicate facies 

types (see table 1)]

Espada San Francisquito 

Formation Formation

Clast variety MC FC EC(A)

Sespe 

Formation

SMP(C) LC(C)

Andesitic suite

Porphyry and pyroclastic.....................37

Pyroclastic:

Welded tuff.........................................5

Lapilli-ash tuff...................................3

Volcanic breccia................................0

Porphyry:

Rqf............................................................O

Rf..............................................................O

GqL....................................................... 12

Gf....,........................................................7

Bqf............................................................7

Bf...............................................................7

Dqf.........................................................20

Felsite:

R..........................................,....................0

G................................................................O

B................................................................2

D................................................................O

Sample size...............................................67

37

Rhyolitic suite

5

22

0

0

0

4

5

4

23

0

0

0

0

0

74

11

4

0

2

3

13

7

26

26

3

0

0

0

0

113

29

0

0

2

0

7

6

19

26

0

0

0

6

0

135

0

3

2

11

3

40

0

0

0

27

0

89

regarded as inconclusive. San Francisquito conglomerate 

near the San Gabriel Fault (FC) also differs significantly 

from Sespe conglomerate at Lake Casitas (LC) in terms of 

IIL and OPI for the quartzite clasts. In the central Santa 

Ynez Mountains, S/L, S/I and \|/p values for metavolcanic 

clasts in Sespe conglomerate at Gibraltar Road (GR) are 

significantly lower (tables 9 and 11) than those in the 

Espada Formation (MC) and Cozy Dell Shale (WC). 

Quartzite and metavolcanic clasts in Sespe conglomerate 

at Gibraltar Road are also significantly rounder than those 

in the nearby Jalama Formation at Romero Saddle (RS). 

Farther west at San Marcos Pass, IIL values for quartzite 

and metavolcanic clasts in Sespe conglomerate (SM) are 

significantly lower than those in Espada (MC), Jalama 

(RS), and Cozy Dell (WC) conglomerates. All of these 

comparisons passed the F-test, with the exception of that 

using IIL values and quartzite clasts in Sespe and Cozy

Dell conglomerates, indicating that statistical comparisons 

using the f-test are valid. These relations suggest that 

quartzite and (or) metavolcanic clasts from the San Francisquito and Jalama Formations were not recycled to form 

conglomerates of the Sespe Formation at Lake Casitas. 

These same clast types in the Sespe Formation at San 

Marcos Pass and Gibraltar Road also do not appear to 

have been derived from nearby sources in the Jalama, 

Espada, or Cozy Dell rock units.

GEOLOGIC HISTORY OF THE STUDY AREA 

Sespe Conglomerate Provenance

The rock types found as clasts in assemblage 1 

(tables 4 through 6) generally crop out today as bedrock of

H20 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
Table 8. Tests for statistically significant differences in metavolcanic clast suites of Sespe and pre-Sespe conglomerates

[See figure 1 for site locations: MC, Manzana Creek (Espada Formation); FC, Fish Creek (San Francisquito Formation); EC, east Camino 

Cielo (Sespe Formation); SMP, San Marcos Pass State Highway 154 (Sespe Formation); LC, Lake Casitas (Sespe Formation)]

Sample pairs Confidence intervals (pj - P2) Hypothesis

Pyroclastic rocks

MC versus EC............................................. ....-0.022<pi -p2<0.162

MC versus SMP...............................................0.110<pi - P2<0.310

FC versus LC....................................................0.085<pi - p2<0.314

Andesite

MC versus EC.................................................. 0.254<pi - p2<0.486

MC versus SMP...............................................0.221<pi - p2<0.459

FC versus LC................................................,...0.189<pi - p2<0.431

Red felsite and porphyry

MC versus EC.................................................. 0.00 l<pi - p2<0.090

MC versus SMP..............................................-0.0004<pi - P2<0.044

FC versus LC...................................................-0.0005<pi - P2<0.065

Green felsite and porphyry

MC versus EC.................................................-0.109<pi -p2<0.129

MC versus SMP..............................................-0.050<pi - p2<0.170

FC versus LC...................................................-0.056<pi -p2<0.135

Brown felsite and porphyry

MC versus EC..................................................0.233<pi - P2<0.487

MC versus SMP...............................................0.228<pi - p2<0.472

FC versus LC....................................................0.29 l<pi - p2<0.569

Gray felsite and porphyry

MC versus EC.................................................. 0.069<pi -p2<0.271

MC versus SMP...............................................0.104<pi - p2<0.296

FC versus LC

Accept 

Reject 

Reject

Reject 

Reject 

Reject

Reject 

Accept 

Accept

Accept 

Accept 

Accept

Reject 

Reject 

Reject

Reject 

Reject 

Accept

Conglomerates of the Sespe Formation along the Santa Ynez Fault Implications for the Geologic History of Santa Maria Basin H21
Table 9. Morphologic indices of clasts in Sespe and pre-Sespe conglomerates

[Modified from Howard, 1992. See figure 1 for sample locations: MC, Manzana Creek (lower section); OC, Rose Canyon; RS, Romero Saddle; FC, Fish 

Creek; WC, West Camino Cielo; LC, Lake Casitas (midsection); GR, Gibraltar Road (midsection); SMP, San Marcos Pass State Highway 154 (midsection). M, metavolcanic; Q, quartzite. Morphologic indices: S/L, short/long axes; I/L, intermediate/long axes; 5/7, short/intermediate axes; R^, modified 

Wentworth roundness; \|/p maximum projection sphericity; OPI, oblate-prolate index; n, sample size; part, indicates that data applies to only part of the 

formation]

Moroholoeic indices

Rock unit Age Map symbol

(part).

Formation (part). 

Cozy Dell Shale...... Eocene ...... .....WC ........

Sespe Formation Oligocene ...... LC .........

(part). 

Sespe Formation Oligocene ...... GR .........

(part). 

Sespe Formation Oligocene ...... SMP.......

(part).

Clast type

..........M

..........Q

M 

..........Q

M 

..........Q

M 

..........Q

M 

..........Q

M 

..........Q

M 

..........Q

M

S/L

0.55

0.61 

0.54

0.57 

0.52

0.59 

0.53

0.59 

0.57

0.56 

0.54

0.61 

0.48

0.50 

0.54

I/L

0.78

0.81 

0.74

0.80 

0.77

0.81 

0.78

0.80 

0.78

0.73 

0.73

0.78 

0.79

0.72 

0.52

S/I

0.71

0.76 

0.71

0.72 

0.70

0.74 

0.68

0.74 

0.74

0.76 

0.73

0.78 

0.62

0.70 

0.76

/?w

0.68

0.54 

0.61

0.55 

0.52

0.54 

0.53

0.63 

0.61

0.55 

0.54

0.70 

0.65

0.64 

0.64

¥p

0.73

0.66 

0.74

0.72 

0.71

0.76 

0.72

0.77 

0.75

0.76 

0.73

0.78 

0.68

0.70 

0.73

OPI

-0.54

-0.42 

-0.66

-0.47 

-0.01

-0.24 

-0.68

-0.47 

0.19

2.12 

1.48

1.05 

-1.20

1.16 

0.42

n

66

20 

14

27 

16

22 

20

24 

19

27 

16

20 

15

23 

23

Table 10. Calculated f-values for tests of statistically significant differences in clast shapes of Sespe conglomerates (see text for 

further explanation)

[See figure 1 for locations: GR, Gibraltar Road; SMP, San Marcos Pass State Highway 154; LC, Lake Casitas. M, metavolcanic; Q, quartzite. Morphologic indices: S/L, short/long axes; I/L, intermediate/long axes; 5/7, short/intermediate axes; R^, modified Wentworth roundness; \|/P) maximum projection 

sphericity; OPI, oblate-prolate index]

Moroholoeic indices

Sample pairs Clast type

GR versus SMP.......... ...... .........Q

M 

LC versus GR.... ............. ...........Q

M 

LC versus SMP................ ...... .Q

M

S/L

32.99 

U.76

-1.43 

1.41

22.11 

0

I/L

1.41 

36.15

-1.46 

-1.52

0.25 

35.55

S/I

1.91 

3-3.34

-0.56

22.25

1.57 

-0.66

/?w

1.45 

0.2

3-4.28 

J -1.99

2-2.54 

l-l.70

¥p

32.96 

^l.SS

-0.81 

1.60

32.83 

0

OPI

-0.07 

-1.15

1.0

h.so

0.69 

0.82

Probably significant difference (a=0.10).

Significant difference (a=0.05). 

3Highly significant difference (a=0.01).

H22 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
Table 11. Calculated f-values for tests of statistically significant differences in clast shapes of Sespe and pre-Sespe conglomerates 

(see text for further explanation)

[See figure 1 for locations: GR, Gibraltar Road; SMP, San Marcos Pass State Highway 154; LC, Lake Casitas. M, metavolcanic; Q, quartzite. Morphologic indices: SIL, short/long axes; IIL, intermediate/long axes; SII, short/intermediate axes; Rw modified Wentworth roundness; \|/P5 maximum Projection 

sphericity; OPI, oblate-prolate index]

Sample pairs Clast type S/L

Morphologic indices

S/I Rw OPI

Eastern Santa Ynez Mountains

LC versus OC..............

LC versus FC..............

..............Q

M

..............Q

M

! 1.69

0.0

-1.04

0.23

2-2.18

-0.14

22.59

-1.43

0.0

0.41

0.58

1.07

0.28

-1.11

0.35

0.19

3.43.03

-0.04

0

0

2-2.01

1.52

22.03

0.57

Central Santa Ynez Mountains

GR versus MC..............

GR versus RS ...............

GR versus WC .............

SMP versus MC.. ..........

SMP versus RS.............

SMP versus WC. ..........

...........M

........,..Q

M 

............Q

M

...........M

............Q

M 

............Q

M

2-2.12

1.1

-1.31 

0.51

2-2.42 

-0.35

2-2.33

0.62 

3-2.78

-0.84

0.30

0.59

0.51 

-0.70

0.24 

3-8.95

2-2.0

3-6.61

2-4-2.18

3 -6.73

2_245

1.47

-1.63 

1.05

2-2.42 

1 59

-0.47

1.33 

-0.98

0.45

-0.72

33.43

22.66 

1.57

1.01 

-1.04

22.10

22.18 

0.23

0.63

2-2.23

h.92

-1.11 

0.33

2-2.30 

0

-0.72

0.71 

2-2.64

-0.68

-0.59

1.37

-0.74 

1.52

-0.82 

1.02

1.15

0.31 

1.18

0.16

Probably significant difference (<x=0.10). 

Significant difference (<x=0.05). 

Highly significant difference (<x=0.01). 

Did not pass F-test.

the Franciscan Complex (fig. 2), and the Franciscan component of Sespe conglomerate is easily recognized (Dibblee, 1966; McCracken, 1972; Anderson, 1980; Nilsen, 

1984). The Franciscan Complex is largely melange composed of a strongly deformed mass of sandstone and mudrock with large ophiolitic fragments scattered throughout 

(Page, 1981). Within the sedimentary sequence are lenticles of thin-bedded red and green chert and occasional 

conglomerate lenses containing well-rounded clasts of 

black chert and mafic metavolcanic rocks (Dibblee, 1966). 

The mafic masses are often strongly sheared and composed of metabasalt and ultramafic rocks. The metabasalts 

are more or less metamorphosed and usually aphanitic, 

although vesicular and amygdaloidal textures may be 

present (Bailey and others, 1964). The ultramafic rocks are 

usually serpentinized and sometimes altered to glaucophane schist or chlorite schist and jasper-calcite rock. 

Unlike the overlying Upper Cretaceous and Paleogene 

strata, Franciscan sandstones lack detrital K-feldspar 

(Dickinson and others, 1982), and although they are commonly called "graywacke," those in the Santa Maria basin 

vicinity typically contain less than 15 percent matrix 

(McLean, 1991).

Thus, Sespe conglomerates in the study area north of 

the Santa Ynez Fault (figs. 5 and 10) were probably derived 

mainly from nearby Franciscan sources. The Franciscan 

Complex lies unconformably beneath Miocene strata of the 

Santa Maria basin (Dibblee, 1950, 1966; Hall, 1978; 

McLean, 1991) and could have been eroded to form Sespe 

conglomerate at Loma Alta (fig. 10, locality 1). Franciscan 

bedrock north of the Little Pine Fault could have supplied 

Sespe conglomerate in Oso Canyon (fig. 10, locality 2)

Conglomerates of the Sespe Formation along the Santa Ynez Fault Implications for the Geologic History of Santa Maria Basin H23
with clasts; however, suitable source rocks do not crop out 

today northeast of Redrock Camp (fig. 10, locality 3). 

Clasts of assemblage 1 south of the Santa Ynez Fault (figs. 

5 and 10) were also derived from Franciscan sources to the 

north because no sources are present to the east. Southdirected paleocurrent vectors documented in table 3 and 

elsewhere (McCracken, 1972; Black, 1982; Howard, 1989) 

support this interpretation. The leucogabbro and diorite 

clasts found at Camino Cielo (fig. 10, locality 4) also probably are of Franciscan origin on the basis of their size, 

shape, abundance, and association with other clasts which 

are clearly of Franciscan origin. Leucogabbro and diorite 

bedrock do not crop out in the study area today, but quartzbearing diorite and gabbro are found elsewhere within the 

Franciscan Complex (Bailey and others, 1964). Rocks of 

similar lithology are also found as part of the Coast Range 

ophiolite in the Santa Maria basin (Hopson and others, 

1981) and as the Willows Plutonic Complex on Santa Cruz

EXPLANATION

Franciscan basement exposed

Sedimentary cover; no Franciscan exposed

Sespe conglomerates of Franciscan provenance

^:££.-; Sespe conglomerates of Franciscan and Mojave Desert provenance

Sespe conglomerates containing gabbro clasts

Paleo current vector

hI -V-.« £ '& . : . . } » -: . 

1 vW«.Q*7i. : : . : > .':' '. . .

Figure 10. Inferred sediment dispersal directions (large 

arrows) in study area. Clasts of Franciscan provenance were 

derived from north, where no Franciscan sources are 

exposed today. Gabbro clasts were derived from nearby 

sources to northeast, where no source is exposed today. 

Clasts of Mojave Desert provenance were derived from the

east. Observed paleocurrent vectors (small arrows) are based 

on data in Tables 2 and 3. Dashed line marks eastern limit 

of gabbro clasts. Fault names: SYF, Santa Ynez Fault; LF, 

Lompoc Fault; LPF, Little Pine Fault. Measured sections: 1, 

Loma Alta; 2, Oso Canyon; 3, Redrock Camp; 4, Camino 

Cielo; 5, San Marcos Pass Highway 154; 6, Lake Casitas.

H24 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
Island (Sorensen, 1985) (fig. 1). They most likely represent 

late stage differentiates and could conceivably be found as 

intrusions anywhere within Franciscan melange. Somewhat 

similar gabbro clasts are present in conglomerates of the 

Cozy Dell Shale in the Santa Ynez Mountains (fig. 1, locality WC); however, a thin section showed these to be olivine 

bearing and more mafic than the leucogabbro clasts in 

Sespe conglomerates.

The arkosic sandstone clast types in assemblage 2 

(tables 4 through 6) have been attributed previously to 

unspecified sources in the Upper Cretaceous to Paleogene 

sedimentary sequence of the region (McCracken, 1972; 

Anderson, 1980). The underlying Coldwater Sandstone, 

which is characteristically micaceous arkosic arenite with 

abundant oyster reefs in the upper part (Dibblee, 1966; 

McCracken, 1972; O'Brien, 1973), is the most probable 

source. Weaver and Kleinpell (1963) noted that molluscan 

fossils in these sandstone clasts are similar to those in the 

underlying Gaviota Formation and Coldwater Sandstone. 

Sespe conglomerates unconformably overlap progressively 

older Coldwater strata northward from the south side to 

the north side of the Santa Ynez Mountains (Dibblee, 

1966), and large boulders of oyster-bearing arkosic arenite 

and intact fossil fragments are found in the Redrock Camp 

section. Smaller clasts of this type are also common in the 

lower part of lithofacies C between San Marcos Pass and 

Lake Casitas (table 5).

MeCracken (1972) suggested that the limestone clasts 

in assemblage 2 were derived from the Sierra Blanca 

Limestone, which crops out just north of the Santa Ynez 

Fault at the base of the Paleogene sequence (Dibblee, 

1966). Several lithofacies have been identified in the 

Sierra Blanca Limestone including sedimentary breccia, 

algal-laminated lime mudstone, coral- or foraminifer-rich 

lime mudstone, and calcarenite containing red and green 

chert pebbles of Franciscan provenance (Walker, 1950; 

Page and others, 1951; Dibblee, 1966; Schroeter, 1972). 

Although the calcarenite lithology of the Sierra Blanca 

Limestone does strongly resemble some clasts found in 

Sespe conglomerate, a source in the Sierra Blanca Limestone is unlikely because the other lithofacies are not represented. The typical Sespe limestone clasts are massive 

and unfossiliferous. A much more probable source of the 

limestone clasts is the upper part of the Coldwater Sandstone, which contains massive calcareous nodules and 

highly calcareous oyster reefs as well as calcarenite with 

lithics of Franciscan provenance. Massive calcareous concretions are found in the Franciscan Complex in the study 

area (Dibblee, 1966; Bailey and others, 1964; Schussler, 

1981) and apparently furnished Sespe conglomerate at 

Loma Alta with limestone clasts.

As noted above, there is disagreement in the literature 

regarding the provenance of the suite of rocks composing 

assemblage 3. In the alluvial fan depositional model for the 

Sespe Formation, these clasts are regarded as reworked

from local rock units, whereas in the fluvial-deltaic model, 

they are first cycle and derived from more distant sources. 

Recycling of material from conglomeratic deposits should 

result in an abundance of sedimentary clasts, and although 

this is observed locally in the Pine Mountain area, the rarity 

of such clasts in the Los Angeles basin area implies a firstcycle origin (Howard, 1989; Howard and Lowry, 1994). 

Thus, with the possible exception of certain varieties of 

granitoid and gneiss also present in the coastal batholithic 

belt of southern California, first-cycle assemblage 3-type 

clasts must have come from sources at least as far east as 

the Mojave Desert region because no such bedrock presently crops out west of the San Andreas Fault (Woodford 

and others, 1968; Woodford and Gander, 1980). The closest 

possible source for the metavolcanic clasts is the 

Sidewinder volcanic series of Bowen (1954) in the western 

Mojave Desert near Victorville, California; however, specific sources for certain red metavolcanic-clast types also 

have been identified in northwestern Sonora, Mexico 

(Abbott and Smith, 1978, 1989). The presence of two-mica, 

garnet-bearing, and alkali-feldspar granite clasts in Sespe 

conglomerates suggests a provenance in the eastern Mojave 

Desert region (Howard, 1987), and certain types of quartzite clasts have been matched petrographically with bedrock 

sources in central Arizona (Howard and Lowry, 1994). 

These data suggest an Eocene-Oligocene paleogeography 

in which the Mojave-Sonora Desert region was an alluvial 

plain with long braided rivers draining westward from an 

uplifted hinterland in Arizona and northern Mexico.

Given the south- and west-directed Sespe paleocurrent vectors in the Santa Ynez Mountains (table 3; fig. 5) 

and assuming that recycling has taken place, possible conglomeratic source rocks include the Espada, Jalama, San 

Francisquito, and Cozy Dell rock units (fig. 1). However, 

tables 7 and 8 show that metavolcanic clast varieties in 

these rock units are significantly different from those in 

Sespe conglomerate. The morphologic indices of quartzite 

and metavolcanic clasts in the pre-Sespe conglomerates of 

submarine fan origin studied are also generally different 

from those in fluvial Sespe conglomerates (tables 9 and 

11). The contrasts in clast shape cannot be attributed to 

abrasion during reworking because no differences in 

roundness are observed. It also seems unlikely that such 

clasts were reworked upsection in the Santa Ynez Mountains from sources in the underlying Jalama and Cozy Dell 

rock units because the Sespe Formation is part of a conformable stratigraphic succession. Given these data, sediment recycling was not the predominant mechanism that 

provided Sespe conglomerate with such clasts. A firstcycle, eastern provenance for assemblage 3 in the Santa 

Ynez-Topatopa Mountains is also suggested by the eastward coarsening of granitoid and gneiss clasts (fig. 9) and 

by the magmatic arc provenance of associated Sespe sandstones (McCracken, 1972; Howard, 1987). A west-directed 

sediment dispersal pattern is further suggested by the

Conglomerates of the Sespe Formation along the Santa Ynez Fault Implications for the Geologic History of Santa Maria Basin H25
Westward increase in the proportions of flat quartzite and 

metavolcanic clasts (tables 9 and 10). Modern and ancient 

braided-stream deposits have been found to show a downcurrent increase in flat clasts owing to selective sorting by 

shape (Bluck, 1964, 1965; Bradley and others, 1972).

Sespe conglomerate in the Santa Ynez Mountains, 

therefore, appears to have a complex but largely first-cycle 

provenance (fig. 10). A Franciscan component was contributed from sources to the north, some clasts of assemblage 2 were eroded from the underlying upper part of the 

Coldwater Sandstone, and both suites were mixed with a 

large volume of assemblage 3 clasts derived from sources 

east of the San Andreas Fault, possibly including distant 

parts of the Mojave Desert region. Reworking of assemblage 3-type clasts probably did occur locally within the 

Sespe Formation. This is seen most clearly in the Camino 

Cielo section (fig. 5), where clasts of assemblage 3 were 

obviously cannibalized from lithofacies A below (table 6). 

The same mechanism may account for the small component of assemblage 3 clasts found locally at other sites 

north of the Santa Ynez Fault (fig. 5), although recycling 

of clasts from older conglomerates cannot be ruled out 

entirely.

Depositional History

Facies relations in the Santa Ynez Mountains illustrated in figure 8 and provenance interpretations discussed 

above suggest that the Sespe Formation was deposited as 

part of two depositional sequences separated by an intraformational erosional unconformity. The lower sequence (lithofacies A, B, and E) is a late Eocene phase of braided-river/ 

braid-delta deposition in an alluvial coastal-plain setting. 

The sediments deposited were probably of first-cycle origin 

and derived mainly from sources in the Mojave Desert 

region east of the San Andreas Fault. However, the northward thinning of the Gaviota Formation (Dibblee, 1950, 

1966; O'Brien, 1973) and the upsection appearance of 

clasts of Franciscan Complex and Coldwater Sandstone 

provenance in lithofacies A (table 6) suggest that a paleotopographic high ("San Rafael uplift") in the Santa Maria 

basin became exposed to subaerial erosion during late 

Eocene time. This paleogeography was even more strongly 

defined when the upper sequence (lithofacies C, D, F, and 

G) was deposited during late Oligocene time. In the area of 

the present Santa Ynez Mountains, first-cycle sediment of 

Mojave Desert provenance was deposited on an alluvial 

plain, probably by a long west-flowing braided river. However, in the area of the present Santa Maria basin, another 

alluvial plain was drained by much shorter south- or southwest-flowing braided tributary streams that carried considerable detritus of Franciscan origin. This sediment dispersal 

pattern is inferred from the facts that in the eastern and 

central Santa Ynez Mountains paleocurrent vectors are both

south and west directed and that clast suites were derived 

from both Franciscan and Mojave sources (in the Lake 

Casitas section these two clast suites alternate from bed to 

bed (table 4), but elsewhere they are intermixed). The upper 

depositional sequence was deposited after a significant hiatus and grades upsection from braided to meandering fluvial sedimentation, suggesting some paleohydrological 

response to changing tectonic or climatic conditions.

In the eastern Santa Maria basin, only the upper depositional sequence is present, as shown by the unconformable northward overlap and Franciscan provenance of 

Sespe conglomerates. The upsection change from braidedto meandering-stream deposition in the Oso Canyon and 

Redrock Camp sections suggests a correlation with the 

upper depositional sequence found in the Santa Ynez 

Mountains to the south. This correlation implies that the 

upsection shift from west- to south-directed paleocurrent 

vectors noted above was due to a gradual change in paleoslope direction during the late Oligocene. The significance 

of the north-directed vector mean at Loma Alta is unclear. 

If this vector represents deposition contemporaneous with 

those sections described above, it implies that a complex 

paleotopography was present. On the other hand, if Sespe 

conglomerate at Loma Alta is equivalent to the Lospe Formation of early Miocene age, this vector may represent a 

drainage reversal associated with a third, possibly unrelated, regional depositional sequence.

In figure 11,1 have integrated sedimentary facies relations in the study area and in the Santa Ynez Mountains 

farther to the west (fig. 8) with those in the Topatopa 

Mountains and the Los Angeles basin area to the east. 

Although the exact sequence stratigraphy is unknown 

because of poor internal age control, the reconstruction 

suggests that the lower part of the Sespe Formation and 

laterally equivalent marine strata (fig. 11, time lines Tl 

and T2) represent progradational (regressive) sedimentation from the late middle to late Eocene. This interpretation is supported by the fact that conglomerates in the 

lower part of the Sespe Formation of the Simi Valley area, 

which lie stratigraphically below rocks containing late 

Uintan fossil vertebrates, were probably deposited in a 

beach setting (Howard, 1992) and therefore define the 

location of a paleoshoreline of late middle Eocene age. As 

discussed above, the Coldwater-Sespe contact decreases in 

age westward across the Santa Ynez-Topatopa Mountains, 

and another paleoshoreline of late Eocene age is defined 

by the westward gradation of fluvial Sespe strata into deltaic Gaviota and Alegria strata west of San Marcos Pass. 

The westernmost extent of this offlap sequence is uncertain because it is apparently truncated by the inferred erosional unconformity within the Alegria Formation.

The basis for showing the intraformational erosional 

unconformity within the Sespe Formation is the conspicuous erosion surface at the San Marcos Pass highway locality (fig. 8), where late Oligocene oreodont-bearing red

H26 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
beds lie directly on a thin sequence of lighter colored late 

Eocene strata of lithofacies A. An unknown but substantial 

amount of relief must have been developed on lithofacies 

A in this particular area because nearby, along Old San 

Marcos Pass Road, lithofacies D rests directly on a much 

thicker section of lithofacies A (fig. 7). The same unconformity appears to be present to the north at the Camino 

Cielo locality, where lithofacies F overlies lithofacies A; 

lithofacies F unconformably overlaps progressively older 

rock units northward from this location (fig. 5). The 

unconformity is also interpreted as extending westward to 

Gaviota Canyon, but the amount of relief in the intervening area has not been assessed; therefore, the erosion surface is shown schematically in figures 8 and 11. The 

unconformity probably extends eastward to Rincon Creek 

where lithofacies A was eroded away completely, and it 

apparently corresponds with the Coldwater-Sespe contact 

(in other words, the "transitional beds"-Sespe contact) in 

the Lake Casitas (figs. 5 and 6) and Ventura River sections. The latter situation is inferred from the fact that the 

lowest beds at the base of the thick sequence of Sespe red 

beds at Lake Casitas and near Ojai are composed almost 

entirely of assemblage 1 clasts. The same unconformity is 

shown extending eastward beyond the study area and into 

the Topatopa Mountains on the basis of (1) sedimentologic 

evidence for two depositional sequences in the Sespe Formation of the Los Angeles basin area (Howard, 1989; 

Howard and Lowry, 1994), (2) magnetostratigraphic data 

from a section of the Sespe Formation in the Simi Valley 

area that suggest that much or all of the lower Oligocene 

section is missing (Prothero and others, 1992), and (3) 

studies in areas adjacent to the Transverse Ranges that

show the presence of a regional Oligocene unconformity 

(Davis and Lagoe, 1988).

Stratigraphically above the unconformity, the reconstruction shows the upper part of the Sespe Formation and 

laterally equivalent marine strata (fig. 11, time lines T3 and 

T4) as composing a retrogradational (transgressive) episode 

of sedimentation that occurred during the late Oligocene 

and early Miocene. The lower part of the upper depositional sequence of the Sespe Formation is interpreted as 

grading laterally westward into the upper part of the Alegria Formation, and the intraformational unconformity is 

shown extending westward into the Alegria Formation, as 

inferred from relations in the Gaviota Canyon area (fig. 8). 

In the westernmost Santa Ynez Mountains, the unconformity is inferred to lie at the base of the Vaqueros Formation, 

which rests with slight (<15°) angular discordance on Alegria strata of late Eocene age west of Cojo Canyon (fig. 1). 

The Vaqueros Formation in this area is an upward-deepening sequence that includes coarse proximal fan-delta deposits of Franciscan provenance in its lower part and distal fandelta and strandline deposits in its upper part (Rigsby, 

1989). Associated paleocurrent vectors are east-directed 

indicating a paleotopographic high at the west end of the 

Santa Ynez Mountains. These coarse fan-delta deposits are 

inferred to be time-stratigraphically equivalent with the deltaic Alegria Formation and in turn with the Franciscan-rich 

braided fluvial Sespe Formation conglomerates, which lie 

above the intraformational unconformity farther east (fig. 

11). This interpretation resolves the apparent stratigraphic 

discrepancy between the 30 to 26 Ma age of the Sespe 

Formation (based on the oreodont Sespia sp.) and the older 

ages of 31 to 29 Ma reported for the Vaqueros Formation

Sespe Fm. (upper part) 

 

_i_ _T-I 

Figure 11. Facies relations between the Sespe Formation and 

laterally equivalent units in Santa Ynez-Topatopa Mountains, 

Calif. Modified from Hornaday and Phillips (1972), as discussed in text. See figure 1 for site locations: CC, Canada del 

Cojo; GC, Gaviota Canyon; RC, Canada del Refugio; CT, Canada del Capitan; BC, Bartlett Canyon; SMP, San Marcos Pass 

State Highway 154; LC, Lake Casitas; TL, Sespe Creek. Time

lines T1 through T4 illustrate offlap-onlap relations. PCFS, 

Pacific coast foraminiferal stages; NALMA, North American 

land mammal ages. L, lower; M, middle; U, upper; e, early; I, 

late. References for chronologic framework: 1, Berggren and 

others (1992); 2, Cande and Kent (1992); 3, Bartow (1991); 4, 

Haq and others (1988); 5, Woodburne (1987); 6, Swisher and 

Prothero (1990); 7, Prothero and Swisher (1992).

Conglomerates of the Sespe Formation along the Santa Ynez Fault Implications for the Geologic History of Santa Maria Basin H27
(Bishop and Davis, 1984; Rigsby, 1989) at the west end of 

the Santa Ynez Mountains. The average age of the Vaqueros Formation in the western and central Santa Ynez Mountains farther to the east appears to be about 26 Ma (Rigsby, 

1989), an age which is consistent with the time-transgressive onlap of the shallow marine facies of the Vaqueros 

Formation. This reconstruction is further supported by the 

facts that (1) the Vaqueros Formation is locally conformable on the Alegria Formation between Arroyo Bulito and 

Canada del Agua Caliente (fig. 1) in the western Santa 

Ynez Mountains (Weaver and Kleinpell, 1963), (2) in the 

northern part of the western Santa Ynez Mountains (Alisal 

Creek area, fig. 1), the Sespe Formation is unconformable 

on Gaviota and older strata but grades conformably upward 

into the Vaqueros Formation (Dibblee, 1950), and (3) the 

uppermost part of the Sespe Formation near Gaviota Canyon is overlapped by the Vaqueros Formation instead of 

grading laterally into the Alegria Formation (Dibblee, 

1966).

The irregularity of the erosion surface below the 

upper sequence and the presence of exposed Franciscan 

Complex both to the north and west of the Santa Ynez 

Mountains suggest that a complex paleotopography was 

present during deposition of the upper depositional 

sequence of the Sespe Formation. These deposits evidently 

reflect basin backfilling following a major episode of baselevel lowering; however, in the Santa Maria basin and 

other areas nearest the Franciscan sources, these deposits 

are, nevertheless, relatively thinly and evenly bedded, suggesting deposition on a rather broad alluvial plain. The 

upper depositional sequence probably represents aggradation below and around paleotopographic highs of exposed 

Franciscan Complex bedrock. Although there are alluvialfan-like deposits locally, Sespe sedimentation probably did 

not involve discrete fan-shaped geomorphic features.

The upsection gradation from braided- to meanderingstream deposits is virtually a basinwide feature (Howard, 

1987, 1989); thus the upper depositional sequence of the 

Sespe Formation appears to cover a significant geographic 

area. The widespread geographic and stratigraphic distribution of Sespia sp. in the upper depositional sequence 

and the age of the overlying Vaqueros Formation suggest 

further that sedimentation occurred rather rapidly (from 

approximately 30 to 26 Ma) in the Santa Ynez Mountains. 

Farther east in the Topatopa Mountains and Los Angeles 

basin area, sedimentation persisted into early Miocene 

time as shown by a paleoshoreline defined by interfingering between the Sespe and Vaqueros Formations. A characteristic feature in the study area is the presence of 

abundant Franciscan detritus, but in the Topatopa Mountains and Los Angeles basin area there is an abundance of 

granitic detritus in the upper depositional sequence of the 

Sespe Formation (Howard, 1989).

The two depositional sequences or cycles in the Sespe 

Formation appear to be preserved most completely near

Simi (fig. 1) in the vicinity of the Los Angeles basin. Each 

cycle there is a couplet of braided and meandering fluvial 

deposits (Howard, 1989; Howard and Lowry, 1994). The 

presence of grazing, leaf-eating, and arboreal types of land 

vertebrate fossils (Savage and others, 1954) and the 

remains of palm tree fronds and fern (?) spores (Protzman, 

1960) shows that vegetation was abundant when the 

meandering stream facies were being deposited. Flemal 

(1966), McCracken (1972), and Peterson and Abbott 

(1979), however, cited sedimentolqgic evidence in favor of 

semiarid or drier conditions during deposition of the 

braided stream facies. Thus, climatic conditions may have 

fluctuated markedly as the Sespe Formation was deposited. Frederiksen (1991) recognized four pulses of climatic 

deterioration during the middle Eocene to earliest Oligocene. This deterioration of global climate may have 

peaked during the middle Oligocene, as shown by profound sea level drop (fig. 11).

The actual time span represented by the lower depositional sequence in the Sespe Formation is poorly constrained; thus, correlation with the global sea level curve 

(fig. 11) is highly speculative. May and Warme (1987) presented evidence that eustasy played a significant role during 

Paleogene sedimentation in southern California, despite a 

tectonically active environment. However, Miall (1992) 

recently questioned the validity of such correlations with 

the eustatic curve. Figure 11 also suggests that the late middle to late Eocene progradational episode of Sespe sedimentation spanned several such postulated episodes of 

global sea level change. The outbuilding of the Sespe 

coastal plain, therefore, probably reflects high terrigenous 

sediment influx from the Mojave Desert and is unrelated to 

eustasy. Uplift in the hinterland of the Sespe fluvial system 

caused by the Laramide orogeny may have been a contributing factor as shown by reset K-Ar ages in the Sonora 

Desert region (Abbott and Smith, 1989). Frakes and Kemp 

(1972) suggested that seasonal monsoon conditions may 

have existed in the southwestern United States during the 

Eocene. This also may have contributed to the development 

of large west-flowing river systems despite the fact that 

conditions may have been arid in coastal California.

The age and geographic extent of the intraformational 

unconformity and the upper depositional sequence in the 

Sespe Formation are also poorly defined. The presence of 

the oreodont Sespia nitida Leidy suggests that sedimentation resumed about 30 Ma, and if the unconformity is 

truly a basinwide feature, which remains to be proven, 

base-level fall may have resulted from the very drastic 

eustatic sea level drop that occurred at about this same 

time (Haq and others, 1988). Eustatic climatic control of 

the upper depositional sequence is suggested by the fact 

that deposition coincides with a period of rising or globally high sea level (fig. 11) and by the upsection change 

from braided to meandering deposition. A similar alteration in river morphology is recorded in the upper PleistoH28 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
cene to lower Holocene sedimentary sequences associated 

with many of the world's rivers; this alteration in morphology is thought to have developed in response to climatic change following the last major episode of 

glaciation (Schumm, 1968; Jackson, 1978; Baker, 1978). 

The coarse Sespe braided stream alluvium was possibly 

deposited initially under conditions of fading glacial aridity during the early stages of rising sea level, and then 

meandering stream sediments were deposited in response 

to more humid conditions, rising sea level, and decreasing 

river gradient.

Tectonic History

An angular unconformity is present locally beneath 

the Sespe Formation north of the Santa Ynez Fault. Below 

the unconformity in the eastern Santa Maria basin, the 

youngest rock unit deformed is apparently the Coldwater 

Sandstone (Dibblee, 1966); however, 28 km to the west in 

the northern part of the western Santa Ynez Mountains 

(Alisal Creek area, fig. 1), the Refugian Gaviota Formation is the youngest unit folded, and the Sespe Formation 

lies below Zemorrian strata of the Vaqueros Formation 

(Dibblee, 1950; Weaver and Kleinpell, 1963). Thus, the 

episode of base-level drop, which caused both erosion of 

the San Rafael uplift and the upsection appearance of 

abundant Franciscan detritus in the Sespe Formation, has 

been attributed to tectonism during the late Eocene and 

Oligocene Ynezan orogeny (Dibblee, 1950). The apparent 

absence of lower Oligocene rocks in the Santa Ynez 

Mountains may be exclusively the result of nondeposition 

owing to regional uplift during the Ynezan orogeny. However, the absence of an angular unconformity in the Santa 

Ynez-Topatopa Mountains south of the Santa Ynez Fault 

and in the Los Angeles basin area suggests that the Ynezan deformation was a local event. The apparent absence 

of the lower Oligocene section over such a wide geographic area probably indicates that eustasy contributed 

significantly to the episode of regional base-level lowering 

that occurred during deposition of the Sespe Formation.

Given the sediment dispersal pattern observed in the 

Sespe Formation, it seems unlikely that the Ynezan orogeny was caused by extensional deformation and that the 

Santa Ynez Fault was an active normal fault during Sespe 

deposition. By analogy with alluvial fans formed along 

active normal faults in modern settings (Bull, 1977; 

Nilsen, 1982), syntectonic Sespe conglomerates would be 

markedly coarser and thicker on the downthrown side of 

such a fault, and paleotransport indicators should define a 

paleoslope dipping in a direction perpendicular to the fault 

trace. However, the fan-like deposits at Camino Cielo neither thicken nor coarsen northward, and the absence of 

leucogabbro clasts at San Marcos Pass (fig. 10) implies a 

paleoslope dipping toward the southwest that is, in a

direction oblique to the trend of the Santa Ynez Fault. Furthermore, the folding or anticlinal warping of the Santa 

Maria basin suggests that the Ynezan orogeny reflects contractional rather than extensional strain. It is possible that 

reverse dip-slip separation was actively occurring on the 

Santa Ynez Fault during the Ynezan orogeny as postulated 

by Namson (1987). In his model, the Santa Ynez Fault 

developed as a backthrust, while the San Rafael uplift was 

rising vertically along a south-verging thrust fault. However, his reconstruction seems to require that the Santa 

Ynez Mountains tectonic block (on the hanging wall of 

the Santa Ynez Fault) be structurally and topographically 

higher than the San Rafael uplift. This is at odds with the 

late Oligocene paleogeographic reconstruction presented in 

this paper and with the unconformable northward overlap 

of Sespe Formation conglomerates. In terms of timing, it 

appears that the Santa Ynez Fault could have developed 

by Namson's (1987) proposed mechanism long after the 

end of Sespe deposition.

If any of the major faults in the study area were vertically active during Sespe sedimentation, relief must have 

been much lower than at present. Franciscan clasts 2 to 4 

m in size are common in modern streams of the study 

area, but even at Loma Alta where the coarsest Sespe conglomerate was observed, Franciscan clasts are less than 1 

m in size. As noted above, the Sespe Formation is conformable beneath the late early Miocene "Temblor" sandstone (Dibblee, 1966) at this locality and therefore may be 

correlative with the Lospe Formation of early Miocene 

age. If so, the Sespe Formation conglomerates in this particular case may reflect greater topographic relief created 

by the Lompocan orogeny, an early Miocene event which 

followed the Ynezan episode of deformation (Dibblee, 

1950). This would account for the apparent drainage 

reversal and the coarse nature of the detritus at the Loma 

Alta locality.

McCracken (1972) noted that Franciscan detritus is 

found in Sespe Formation conglomerates south of the Santa 

Ynez Fault as far east as Ojai, yet the present exposures of 

Franciscan bedrock on the north side of the fault are much 

farther west (fig. 2). This implies that approximately 35 km 

of left-lateral separation has occurred along the Santa Ynez 

Fault since the Oligocene. Horizontal offset along the Santa 

Ynez Fault is also suggested by the fact that no bedrock 

source is present northeast of the proximal, leucogabbrobearing conglomerate at Camino Cielo (fig. 10) and by the 

juxtaposition of this conglomerate facies with Sespe conglomerate at Redrock Camp, which has a significantly different assemblage of clasts. Lateral displacement on the 

Little Pine Fault also may be indicated by the absence of a 

suitable Franciscan bedrock source northeast of the 

Redrock Camp section. Crustal shortening has no doubt 

contributed to the apparent juxtaposition of contrasting lithofacies, and it is possible that Franciscan basement sources 

once exposed are now buried beneath hanging wall rocks of

Conglomerates of the Sespe Formation along the Santa Ynez Fault Implications for the Geologic History of Santa Maria Basin H29
the Santa Ynez and Little Pine Faults. Thus, the sense, 

magnitude, and timing of such lateral separations (if they 

exist) cannot be determined without additional subsurface 

information. Such data still may not provide a conclusive 

answer because it is also possible that fault slivers of Franciscan basement once present may have been eroded away 

entirely. Dibblee (1987) pointed out that severe constraints 

are placed on the amount of possible displacement by the 

negligible offset observed at the eastern termination of the 

Santa Ynez Fault.

Previous authors have concluded that the paleogeography of the western Transverse Ranges is explained better 

by using the tectonic rotation model of Luyendyk and others (1980, 1985), than by using sedimentologic data alone. 

For example, it has been suggested that paleocurrent vectors derived from Cretaceous and Paleogene deep-sea fan 

deposits in the Santa Ynez Mountains (Krause, 1986; 

Thompson, 1988) and on the Channel Islands (Kamerling 

and Luyendyk, 1985) appear to have been rotated. However, it should be noted that sediment transport is typically 

transverse in submarine fans; hence, lateral facies relations 

are perhaps more reliable indicators of regional paleoslope. 

In the Eocene section of the Santa Ynez Mountains where 

facies relations are reasonably well defined (for example, 

Van de Kamp and others, 1974; Thompson, 1988), they 

indicate an east-to-west transition from shallow to deep 

marine facies, a facies transition which one would expect in 

a basin that has not been rotated. The same pattern is evident in figures 8 and 11. A similar argument that has been 

made in favor of the rotation model is that deposition of the 

Sespe Formation as alluvial fans in a rift basin is explained 

better if the basin were originally oriented north to south 

(Luyendyk and others, 1985). The results of this study, 

however, suggest that the Sespe Formation was deposited 

in a forearc basin setting and not in a rift basin. These 

observations do not disprove the Luyendyk and others 

(1980,1985) rotation model; they only argue that tectonic 

rotation is not necessary to explain the sedimentologic data. 

On the basis of this rotation model, it has been postulated 

that the forearc basin sediments of the Santa Ynez Mountains were deposited in a trough which was originally oriented north to south (Hornafius and others, 1986). This 

basin geometry is reasonable and applies equally well for 

the Sespe Formation, but because sedimentologic data lack 

an absolute frame of reference, they cannot be used to 

either prove or disprove the rotation model.

In early plate tectonic models, the East Pacific Rise 

was thought to have collided with a subduction zone in 

southern California sometime between 38 and 29 Ma 

(Atwater, 1970; Atwater and Molnar, 1973). Consequently, 

the upper Eocene part of the Sespe Formation has been 

interpreted as the final phase of forearc basin sedimentation, and rifting was presumed to be the dominant factor 

that controlled deposition during the Oligocene (Nilsen, 

1984, 1987). It is now recognized that regional extension

began later (about 25 Ma) and then peaked during the early 

Miocene (from 23 to 18 Ma) (Spencer and Reynolds, 1989; 

Dokka and Glazner, 1982; Bartley and Glazner, 1991). This 

erogenic activity, which included detachment faulting in 

the Mojave Desert region, is now thought to have preceded 

strike-slip movements associated with the San Andreas system, and subduction-related volcanic and forearc basin sedimentation are believed to have continued into the early 

Miocene in coastal southern California (Tennyson, 1989; 

Cole and Basu, 1992). If these interpretations are correct, 

then the Ynezan orogeny was unrelated to the major change 

of tectonic regimes associated with ridge-trench collision as 

previously thought. These data imply that large-scale 

strike-slip movements are unlikely to have occurred during 

Sespe deposition; indeed, all of the major faults in the study 

area may in fact greatly postdate Sespe deposition.

CONCLUSIONS

The results of this study support the conclusion drawn 

long ago by Reed (1929) that the Sespe Formation conglomerates of the Santa Ynez-Topatopa Mountains were 

deposited mainly as part of the delta of a very large river 

and not as alluvial fans. Evidence has been presented here 

that suggests the Sespe Formation has a complex but 

mainly first-cycle provenance, including both a Mojave 

Desert suite of conglomerate clasts derived from distant 

sources and a Franciscan component derived from nearby 

sources. The former suite of clasts was probably derived 

in part from very distant sources to the east. This suggests 

an Eocene-Oligocene paleogeography in which the 

Mojave Desert region was a broad alluvial plain with long 

braided rivers draining westward from a hinterland farther 

east. The coastal part of this braid plain was bordered by a 

piedmont area developed around a Franciscan source terrane. Thus, the Franciscan clast assemblage is associated 

with alluvial-fan-like deposits in the eastern Santa Maria 

basin, but lateral facies relations define a relatively broad 

braid plain aggrading below and around paleotopographic 

highs of exposed Franciscan Complex bedrock; Sespe sedimentation probably did not involve discrete fan-shaped 

geomorphic features in this area.

A major outcome of this study is the recognition of 

an intraformational erosional unconformity of possible 

regional extent within the Sespe and Alegria Formations. 

This implies that these rock units were deposited as two 

discrete depositional sequences rather than as one long 

continuous event. The relations between sequence stratigraphy, tectonism, and eustasy remain to be fully determined; however, regional considerations suggest that 

climatic and eustatic effects were much more important 

than previously thought. The apparent absence of the 

lower Oligocene section probably reflects a combination 

of tectonism (Ynezan orogeny) and eustasy locally in the

H30 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
vicinity of the Santa Maria basin; elsewhere, it may be 

exclusively the result of a middle Oligocene global sea 

level drop. Additional studies are needed to better define 

this unconformity and to evaluate its geologic significance. 

The most important area to investigate further is probably 

the western Santa Ynez Mountains, where it may be possible to combine a careful study of the marine-nonmarine 

lateral facies relations with detailed paleontological and 

magnetostratigraphic studies of the Alegria Formation.

Another significant result of this study is the conclusion that the overall pattern of sediment dispersion suggests an absence of active faulting in the region studied at 

least until the end of Sespe-Vaqueros deposition during the 

early Miocene. Deformation associated with the Ynezan 

orogeny can be explained simply by anticlinal warping or 

broad folding of the San Rafael uplift. Thus, the alluvialfan/rift-basin scenario often found in the literature is 

unsupported by the results of this study. Sedimentation is 

interpreted to have occurred in a forearc-basin setting, an 

interpretation which is consistent with accumulating 

regional evidence indicating that subduction persisted 

along the continental margin in southern California until 

the early Miocene. Horizontal offsets along the Santa 

Ynez and Little Pine Faults after Sespe deposition are suggested by geologic relations in the study area, but the juxtaposition of contrasting lithofacies is probably partly the 

effect of crustal shortening. Sediment-source mismatches 

also are present but are difficult to evaluate without additional information on the subsurface distribution of Franciscan bedrock. Lateral offsets have been predicted by and 

incorporated into models involving large-scale tectonic 

rotations; however, the sense, timing, and magnitude of 

such movements are still unconstrained by independent 

geologic evidence.

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H36 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
Appendix. List of conglomerate clasts from the Sespe Formation studied in thin section 

[See figures 1 and 2 for sample locations. S. Y. Mts., Santa Ynez Mountains; metavolc., metavolcanic]

Sample number Lithofacies Sample location Lithology

17-1 ................................................ C...................... Sespe Creek, Topatopa Mts.................................................... biotite-granite

17-2 ................................................ C...................... Sespe Creek, Topatopa Mts.................................................... alkali feldspar-granite

16-1 ................................................ C...................... Lake Casitas, Santa Ynez Mts................................................ pebbly sandstone

16-2 ................................................ C...................... Lake Casitas, Santa Ynez Mts................................................biotite-granite

16-3 ................................................ C...................... Lake Casitas, Santa Ynez Mts................................................ garnet-biotite-granite

16-4 ................................................ C...................... Lake Casitas, Santa Ynez Mts................................................ garnet-biotite-granite

16-5 ................................................C......................Lake Casitas, Santa Ynez Mts................................................ biotite-granite

15-2 ................................................ A......................Toro Canyon, Santa Ynez Mts...............................................biotite-granite

15-3 ................................................ A......................Toro Canyon, Santa Ynez Mts...............................................biotite-granite

13-2 ................................................ A...................... Old San Marcos Pass Rd., S. Y. Mts...................................... biotite-granitic gneiss

13-3 ................................................ A......................Old San Marcos Pass Rd.,S.Y. Mts...................................... alkali feldspar-granite

1-2 .................................................. A......................CaminoCielo, Santa Ynez Mts..............................................red felsicmetavolc.

1-3 ..................................................A......................CaminoCielo, Santa Ynez Mts.............................................. felsicmetatuff

14 .................................................. A......................CaminoCielo, Santa Ynez Mts..............................................brown felsic metavolc.

1-5 .................................................. A......................CaminoCielo, Santa Ynez Mts.............................................. brown felsicmetavolc.

1-6 .................................................. A......................Camino Cielo, Santa Ynez Mts..............................................dioritic gneiss

1-7 .................................................. A......................Camino Cielo, Santa Ynez Mts..............................................green felsicmetavolc.

1-9 .................................................. A......................Camino Cielo, Santa Ynez Mts..............................................black quartzite

1-10 .................................................F ......................Camino Cielo, Santa Ynez Mts..............................................leucogabbro

1-12 ................................................ A......................Camino Cielo, Santa Ynez Mts..............................................green felsicmetavolc.

1-13 ................................................ A......................Camino Cielo, Santa Ynez Mts..............................................red felsic metavolc.

1-14 ................................................ A......................Camino Cielo, Santa Ynez Mts..............................................green felsicmetavolc.

1-15 ................................................A......................Camino Cielo, Santa Ynez Mts..............................................brown felsic metavolc.

1-16 ................................................A......................Camino Cielo, Santa Ynez Mts..............................................metaandesite

1-17 ................................................A......................Camino Cielo, Santa Ynez Mts..............................................metaandesite

1-18 ................................................ A......................Camino Cielo, Santa Ynez Mts..............................................metaandesite

1-19 .................................................F .....................Camino Cielo, Santa Ynez Mts..............................................metabasalt

1-20 .................................................F .....................Camino Cielo, Santa Ynez Mts..............................................metabasalt

1-21 .................................................F .....................Camino Cielo, Santa Ynez Mts..............................................metabasalt

20-1 ................................................C......................Redrock Camp, Santa Maria basin .........................................lithicarenite

20-1 ................................................C......................Redrock Camp, Santa Maria basin .........................................leucogabbro

Conglomerates of the Sespe Formation along the Santa Ynez Fault Implications for the Geologic History of Santa Maria Basin H37

Chapter I

Reconnaissance Bulk-Rock and

Clay Mineralogies of Argillaceous Great Valley

and Franciscan Strata, Santa Maria

Basin Province, California

By RICHARD M. POLLASTRO, HUGH MCLEAN, and LAURA 

L ZINK

U.S. GEOLOGICAL SURVEY BULLETIN 1995

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

Edited by Margaret A. Keller
CONTENTS

Abstract II 

Introduction II

Acknowledgments 12 

Methods 12 

Results 13

X-ray powder diffraction 13 

Bulk-rock mineralogy 13 

Clay mineralogy 14

Observations from SEM analysis 18 

Discussion 19

Mixed-layer chlorite/smectite (corrensite) 111 

Summary 111 

References cited 112

FIGURES

1-5. X-ray powder diffraction profiles of bulk rock and clay fractions of:

1. Outcrop and well-core samples from upper part of Great Valley 

sequence 15

2. Well-core samples from upper part of Great Valley sequence 15

3. Outcrop and well-core samples from lower part of Great Valley 

sequence 15

4. Well-core samples from Franciscan assemblage 16

5. Well-core samples from Franciscan assemblage containing abundant 

corrensite 16

6. Ternary plot of bulk-rock quartz/feldspar/total clay contents of samples from 

Franciscan assemblage and upper and lower parts of Great Valley sequence 16

7. Ternary plot of clay-mineral contents in <2 \im fraction of samples from Franciscan assemblage and upper and lower parts of Great Valley sequence 17 

8-10. Scanning electron micrographs showing:

8. Detrital textures in argillaceous samples of well core from Franciscan 

assemblage 18

9. Authigenic clay textures in argillaceous samples of well core from upper 

Great Valley sequence and Franciscan assemblage 19

10. Chloritic clay from melange matrix of Franciscan assemblage 110

TABLES

1. Semiquantitative X-ray powder diffraction analyses (in weight percent) of bulk 

rock and <2 jtim clay minerals in argillaceous samples of the Great Valley sequence and Franciscan assemblage from outcrop, Santa Maria basin, Calif. 13

2. Semiquantitative X-ray powder diffraction analyses (in weight percent) of bulk 

rock and <2 jtim clay minerals in argillaceous samples of the Great Valley sequence and Franciscan assemblage from well core, Santa Maria basin, Calif. 14

3. Standardless energy-dispersive elemental analysis of chloritic sample of Franciscan 

assemblage from outcrop on Highway 46 110
Reconnaissance Bulk-Rock and Clay Mineralogies of 

Argillaceous Great Valley and Franciscan Strata, 

Santa Maria Basin Province, California

By Richard M. Pollastro, Hugh McLean, and Laura L. Zink

Abstract

X-ray powder diffraction (XRD) analyses of samples 

from the Santa Maria basin province, Calif., of argillaceous 

rocks from the lower part of the Great Valley sequence 

(Upper Jurassic to lower Upper Cretaceous) and melange of 

the Franciscan assemblage are similar in general bulk-mineral composition, consisting mostly of quartz, feldspar, and 

clay minerals. Although quartz-feldspar-clay (QFC) contents 

of these argillaceous rocks generally overlap and have 

almost identical mean QFC compositions, they differ somewhat in the amount of potassium feldspar. The bulk-mineral 

relations of the argillaceous rocks are thus similar to those 

reported for interbedded sandstones.

Although the clay contents of Great Valley sequence 

and Franciscan assemblage argillites are quite similar in the 

Santa Maria basin, the clay mineral assemblages are somewhat different. Argillites in the lower and upper parts of the 

Great Valley sequence contain a similar, mostly illitic, claymineral suite of discrete illite, mixed-layer illite/smectite (I/S), 

and iron-rich chlorite, with rarer kaolinite and mixed-layer 

chlorite/smectite (C/S). In contrast, C/S is a common and 

abundant constituent of argillaceous samples from the Franciscan assemblage. Textural analysis suggests that some clay 

was changed or neoformed during burial diagenesis. The 

main difference in clay mineralogy where Great Valley argillites are more illitic and Franciscan argillites more chloritic 

in Santa Maria basin samples, however, probably reflects different primary source materials. A much greater contribution 

of mafic igneous detritus, probably altered ophiolite, was 

recognized for the argillaceous beds and melange matrix of 

the Franciscan assemblage in the basin.

INTRODUCTION

Upper Jurassic to Upper Cretaceous submarine fan 

deposits composed of sandstone, shale, and conglomerate 

called the Great Valley sequence by Bailey and others 

(1964), and more recently called the Great Valley Group

Manuscript approved for publication September 26, 1994.

by Ingersoll (1990), are widely exposed in the southern 

Coast Ranges of California and in the basement of the 

Santa Maria basin (8MB). Petrographic and provenance 

studies of Great Valley sequence sandstones exposed along 

the west side of the Sacramento and San Joaquin Valleys 

(Ojakangas, 1968; Dickinson and Rich, 1972; Ingersoll, 

1978) and of rocks of correlative age in the Coast Ranges 

that include basement rocks of the SMB (Gilbert and 

Dickinson, 1970; MacKinnon, 1978; Nelson, 1979; Gray, 

1980; Toyne, 1987) generally recognize a number of sandstone petrofacies that reflect the progressive unroofing of a 

late Mesozoic volcanic/plutonic complex of batholithic 

proportions.

Sandstones of the lower part of the Great Valley 

sequence (lower Great Valley sequence petrofacies of 

McLean, 1991) in and adjacent to the Santa Maria basin 

range in age from Tithonian to approximately Cenomanian. Sandstones in the lower part of the Great Valley 

sequence are characterized by abundant chert and mafic 

volcanic rock fragments, subordinate quartz, plagioclase, 

and trace amounts of potassium feldspar (K-feldspar). 

Interbedded conglomerate is generally composed of wellrounded, dark-colored chert pebbles and mafic volcanic 

rock fragments (Seiders, 1983). Conversely, sandstones in 

the upper part of the Great Valley sequence (upper Great 

Valley sequence petrofacies of McLean, 1991) range in 

age from approximately Turonian to Maastrichtian and are 

characterized by abundant quartz, plagioclase, and K-feldspar (Lee-Wong and Howell, 1977; Dickinson and others, 

1982). Rock fragments include an assortment of granitic, 

intermediate volcanic, and schistose metamorphic rock 

fragments. Upper Cretaceous conglomerate populations 

vary widely, although intermediate volcanic porphyries, 

quartzite, and granitoid clasts predominate (Howell and 

others, 1977).

Other lithotectonic elements of Mesozoic age in the 

SMB and vicinity include melange of the Franciscan 

assemblage (also called Franciscan Complex by some 

workers) and blocks of ophiolite that locally are conformably overlain by the lower part of the Great Valley

Bulk-Rock and Clay Mineralogies of Great Valley and Franciscan Strata, Santa Maria Basin Province, California 11
sequence (Hsii, 1968; Hopson and Frano, 1977). Franciscan assemblage melange contains a diverse assortment of 

chert, greenstone, sandstone, metagraywacke, and subordinate quantities of schist (including glaucophane schist) 

dispersed in sheared argillaceous matrix. The ophiolite 

suite includes a variety of ultramafic rocks, pillow lavas, 

and chert that locally (Point Sal and Stanley Mountain) 

grade upward into the lower part of the Great Valley 

sequence.

McLean (1991) found that "knockers" composed of 

sandstone and metagraywacke in Franciscan assemblage 

melange around the margin of the Santa Maria basin uniformly lack K-feldspar and that their quartz-feldspar-lithic 

(QFL) detrital modes tend to overlap those of the sandstones of the upper and lower parts of the Great Valley 

sequence. The absence of K-feldspar noted by McLean 

(1991) is an important characteristic of Franciscan sandstones and serves as a valuable petrographic tool in differentiating sandstones of the lower and upper parts of the 

Great Valley sequence from those of the Franciscan 

assemblage. Furthermore, textural and mineralogical similarities suggest that Franciscan sandstones may represent 

variably metamorphosed Upper Jurassic to Upper Cretaceous Great Valley sequence rocks that were initially 

deposited on an active accretionary continental margin and 

subsequently incorporated into melange.

Clay minerals are particularly useful in understanding 

both the depositional and diagenetic environments of sedimentary rocks and basins (Chamley, 1989). Many clay 

minerals are sensitive to the often changing geologic environment and are commonly good indicators of particular 

physical, chemical, or thermal conditions. Other clay minerals, however, are relatively stable under a wide range or 

variety of conditions, often with little or no change after 

sedimentary recycling. Of particular interest and importance are the mixed-layer clay minerals because they are 

commonly products of burial-diagenetic and hydrothermal 

reactions (Dunoyer de Segonzac, 1970; Hoffman and 

Hower, 1979). The most common mixed-layer clay minerals are illite/smectite (I/S) and chlorite/smectite (C/S); 

however, C/S is much less common and much less understood than I/S (Reynolds, 1988; Schiffman and Fridleifsson, 1991). For example, C/S is a common product of the 

alteration of basic igneous rocks, particularly ophiolite 

complexes (Evarts and Schiffman, 1983; Bevins and others, 1991). Thus, the presence of mafic-rock material in 

the various petrofacies may determine the occurrence of 

C/S in sandstones and(or) adjacent shales.

Several studies have addressed the petrologic aspects 

of sandstone and conglomerates of the upper and lower 

Great Valley and Franciscan petrofacies. Few studies, 

however, have addressed the composition of argillaceous 

interbeds within these units. We present here a reconnaissance study of the mineralogic composition of shale, argillaceous interbeds, and melange matrix in the upper and

lower Great Valley sequence and Franciscan sandstone 

knockers using X-ray powder diffraction (XRD). In particular, the clay-mineral assemblages of these argillaceous 

materials were studied to aid in distinguishing rocks of the 

lower and upper parts of the Great Valley sequence and 

Franciscan assemblage and their provenance.

Acknowledgments

We thank Isabelle K. Brownfield and Gary Skipp for 

preparing and mounting the clay specimens for XRD analysis. Special thanks are extended to Tom Finn for the 

preparation of bulk-rock specimens and running the bulk 

and clay XRD profiles. Larry Knauer kindly assisted our 

sampling program at the California Well Sample Repository located in Bakersfield, California. Reon Moag 

assisted our sampling program at the Unocal core repository in Brea, California. We thank Gene Whitney and 

Bruce Bohor for their critical reviews of the manuscript.

METHODS

Bulk-rock and <2 jLim clay mineralogies were determined by XRD on 20 shale samples from outcrops at various sites around the SMB's margin and 23 shale samples 

from cores of exploratory wells in the SMB. Cores were 

obtained from the repositories of Unocal Corporation 

(Brea, California) and the State of California (California 

State University, Bakersfield). Shale samples are grouped 

according to the three corresponding sandstone petrofacies, as defined by McLean (1991), and represent the 

upper and lower parts of the Great Valley sequence and 

the Franciscan assemblage. Samples were scrubbed to 

remove surficial contaminants, dried, and then ground to 

<35 mesh. Each sample was then split with a Jones splitter 

into two portions for (1) whole-rock XRD and (2) carbonate dissolution and clay mineral analysis. Carbonate was 

dissolved in IN HC1 and the residue filtered, washed, 

dried, and weighed (Pollastro, 1977). The weight percents 

of the acid-insoluble residue and (or) acid-soluble carbonate were then determined.

Qualitative identification and semiquantitative estimates of the minerals in bulk-rock samples were made 

using XRD analysis of randomly oriented powders that 

were ground to a maximum grain size of 44 (im (<325 

mesh) and packed into aluminum specimen holders from 

the back side. A fine, random texture was imparted onto the 

surface to be irradiated in order to further disrupt any preferred orientation created while mounting the sample (see 

Schultz, 1978). Semiquantitative weight-percent values for 

total clay (phyllosilicates) and other minerals or mineral 

groups were calculated by comparison with several prepared mixtures of minerals having similar XRD character12 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
Table 1. Semiquantitative X-ray powder diffraction analyses of bulk-rock and <2 |j,m clay minerals in argillaceous samples 

from outcrop, Santa Maria basin, Calif.

[Data reported in weight percent. Carb., carbonate as calcite unless suffixed by D, dolomite; plag, plagioclase; K-spar, potassium feldspar; I/S, 

mixed-layer illite/smectite; chl, chlorite; kaol, kaolinite; C/S, corrensite; leaders (--), none detected; tr, trace. Suffix P in "other" category is pyrite]

Bulk-rock

Sample No. Location Quartz Clay Carb Plag K-spar Other

<2 um clav

Illite I/S Chl Kaol C/S

Upper part of the Great Valley sequence

L87-3-1 - 

987-2-1 

987-2-2 

581-20-2B 

586-7-3 - 

M377-17-11-

Cuyama Gorge 15 75 

Highway 46- 18 60 

Highway 46 16 65 

Carrie Creek - 18 56 

Hi Mountain saddle 24 53 

Salsipeudes Creek 

tr

7

12

12

22

15

3

10

6

4

45

12

12

8

11

25

24 31

52 36

32 56

89

77

45

3

12

12 18

Lower part of the Great Valley sequence

M585-17-1A586-7-1 

586-7-2 

587-14-1 -

L87-6 -

L87-7 - 

L87-11 

581-20-1B987-2-4 

Rinconada Creek 

Rinconada Creek- 1 6 

Hi Mountain Saddle - 25

West Bald Mountain 25

Canada Honda 

Canada Honda 

Canada Honda 

Dear Trail Mine 

Toro Formation -

17

-- 12

-- 12

25

21

57 

65

66

77

62 3

76

68

61

27 

10

9

6

15

11

7

14

~

1

2

._

IP

4

51 

6

57

21

12

13

36

14

44 

35 

30

71

50

55

55

41

5 

40 

13

4

38

22

4

45

4

10

5

19

~

Franciscan assemblage

987-2-3- 

987-2-5- 

987-2-6- 

987-2-7- 

L87-2gHwy 46 

Vista Point 

melange matrix 

Highway 1, Gorda 

Cuyama Gorge 

17

13

16

21

14

72

55

71

71

66

1

1 

4D

30

9

4

18

6 20 27

37 28 20

17 12 32

54 1 45

51 24 25

47

9

39

istics using the procedures outlined by Schultz (1964) and 

Hoffman (1976), as modified by Pollastro (1985). The semiquantitative relative weight percent values calculated for 

total carbonate minerals by XRD were then compared with 

those determined using chemical dissolution.

Oriented clay aggregates of the <2-)Lim and <0.25-|0,m 

(equivalent spherical diameter) fractions were prepared 

using a modified filter-membrane peel technique (Pollastro, 

1982), similar to that described by Drever (1973). Semiquantitative XRD analysis of the clay-sized fractions were 

made using the method of Schultz (1964), as modified by 

Pollastro (1985). Ratios and ordering of I/S and C/S clay 

were determined on oriented, ethylene glycol-saturated 

specimens of both the <2-)Lim and <0.25-(im fractions using 

the methods of Reynolds and Hower (1970) and Moore and 

Reynolds (1989).

Selected samples from outcrops and well cores were 

also prepared for reconnaissance textural and petrologic 

analysis using the scanning electron microscope (SEM) to 

aid in interpreting detrital versus authigenic origin of individual mineral phases.

RESULTS

X-ray Powder Diffraction

Bulk-Rock Mineralogy

Semiquantitative bulk-rock XRD analysis of samples 

from the lower and upper parts of the Great Valley 

sequence and the Franciscan assemblage are listed and 

grouped in tables 1 and 2. Selected XRD profiles of shale 

from each of these three groups are also shown in figures 

1 through 5. The bulk-rock XRD data are also simplified 

into three components, quartz, feldspar, and clay (QFC), 

which are normalized and plotted in the ternary diagram 

of figure 6; QFC compositions for each petrofacies are 

also shown by different symbols. Samples containing <5 

weight percent carbonate were normalized to 100 percent 

after exclusion of the carbonate; three core samples having 

>5 weight percent carbonate are not included in figure 6.

Generally, QFC contents of the argillaceous rocks 

from the three petrofacies plot within the same compositional area of figure 6; all sample groups also have a similar

Bulk-Rock and Clay Mineralogies of Great Valley and Franciscan Strata, Sanfa Maria Basin Province, California 13
Table 2. Semiquantitative X-ray powder diffraction analyses of bulk-rock and <2 |J,m clay minerals in argillaceous samples 

from well-core samples, Santa Maria basin, Calif.

[Data reported in weight percent. Depth in feet. Carb, calcite; plag, plagioclase; K-spar, potassium feldspar; I/S, mixed-layer illite/smectite; chl, 

chlorite; kaol, kaolinite; CVS, corrensite; leaders (--), none detected; tr, trace. Suffixes in "other" category are P, pyrite and G, gypsum]

Bulk-rock

Well Name Depth Quartz Clay Carb Plag K-spar Other

<2 urn clav

Illite I/S Chl Kaol C/S

Upper part of the Great Valley sequence

Union Pezzoni 1 

Union Pezzoni 1 

Union Moretti 1 

Standard Sudden 1 - 

Continental Leroy 5-

Union Stinson 1- 

Texaco Petan 2 

1,270 

1,986-1,997

2,370

2 598

2,630-2,638 

2,951-2,966 

3,207-3,222 

3,211-3,219

5,126 

1,818-1,831

19

23

19

16

19

14

19

21

9

21

74

68

56

64

67

76

60

66

78

51

1

15

4

1

2

4

9

11

20

9

10

16

11

8

23

1 

3P, 3G

12

15

9

8

16

35

21

40

59

6

45 17

58 27

58 33

62 22

51 28

45 20

56 23

30 30

35 3

5 35

26

54

Lower part of the Great Valley sequence

Murphey Sinclair 74-6 1,093 10 76

Welch Janeway 1 ------ 5,694 18 55

Shell Barret 1 - 1,452 24 51

Amerada Tognazzi - 8,602-8,618

12 2 

3 23 1 

6 19 

 no bulk sample35 32 33

17 17 45

20 45 35

21 33 46

21

Franciscan assemblage

Superior Silva CH1 

Union W. Bradley 1 

Superior Beck CH1 - 

Union Wineman 2 

Texaco Fulwider 1 

\X//a1/-»1*i Tdn^ \i/ox/ 1 VV C1C11 JallCWay 1 --- - 

Union Phelan 1 

Chief Larson Brothers

Ericson Dart 1 - 

916-922

2,792

4,400

1,717-1,721

3

SC(\ O £_ C O

,ojO-3,ojo 

3,658-3,665

3,458

5,922

3,591-3,600

2,784

3,200

16

18

18

14

17

21

24

23

19

17

16

69

55

66

40

a Q DO

71

57

59

66

75

66

1

1

5

1

9

_

3

14

25

15

36

12

4

17

10

5

12

__

2

..

..

3

3

2

1

2 3P

3

IP

25

28

24

3

6

2

2

17

15

19

10

25

32

33

~

30

6

7

26

34

48

18

50

40

43

45

40

15

44

29

35

21 12

43

-

52

24

77

48

28

16

29

compositional range. Mean QFC compositions for the three 

groups are similar, having about 65 weight percent total 

clay (as phyllosilicates), 20 weight percent quartz, and 15 

weight percent total feldspar, and thus can not be distinguished from one another on this basis (fig. 6). Type and 

amount of feldspar and phyllosilicates (tables 1 and 2), 

however, show some differences among the three groups.

Shale interbeds from the upper part of the Great Valley sequence contain the greatest amount of K-feldspar, 

having a mean of >3 weight percent; average K-feldspar 

contents for samples of shale from the lower part of the 

Great Valley sequence and the Franciscan assemblage are 

0.8 and 1.2 weight percent, respectively. Similarly, petrographic studies by McLean (1991) found that sandstones 

of the upper part of the Great Valley sequence contain the 

greatest amount of K-feldspar by volume. Moreover, 

McLean (1991) found that the average volume percent Kfeldspar was higher in the upper Great Valley sandstones

from outcrop (about 10 volume percent) than in well-core 

(about 7 volume percent) samples. Similarly, K-feldspar 

contents in upper Great Valley argillaceous rocks of this 

study average about 6 weight percent in samples from outcrop and <2 weight percent in those from well cores.

Carbonate is low to absent in most samples, averaging 

<2 weight percent of the bulk rock; it is present mostly as 

calcite in core samples (tables 1, 2; figs. 1, 2, 4). Plagioclase is more abundant than K-feldspar. Coarse micas 

(muscovite and(or) biotite; figures 1A, 25) and chlorite 

(figs. 1 through 5) are also commonly present.

Clay Mineralogy

Qualitative and semiquantitative clay mineralogies of 

the <2-|Lim fraction are listed in tables 1 and 2. The clay 

data are also combined and plotted on the ternary diagram

14 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
<2 pm Bulk rock

30 26 22 18 14 10 8 2

<2 pr

30 26 22 32 28 24 20 1!

DEGREES 26

32 28 24 20 16 12 8 4

Figure 1. X-ray powder diffraction profiles of random-powder bulk rock (right) and oriented glycol-saturated <2 |j,m 

fractions (left) of argillaceous samples from upper part of 

Great Valley sequence. A, Outcrop sample L87-3-1 from 

Cuyama Gorge. B, Core sample from Standard Sudden well 

at depth of 2,599 ft. CuKa radiation. Q, quartz; CY, total 

clay (combined clay peaks); M, coarse mica; C, chlorite; PI, 

plagioclase; I, illite; I/S, illite/smectite.

<2 pm Bulk rock

30 26 22 18 14 10 6 36 32 28 12 8 4

B

30 26 22 18 14 10 6 2 36 32 28 24 20 16 12 8 4

DEGREES 28 DEGREES 29

Figure 2. X-ray powder diffraction profiles of random-powder 

bulk rock (right) and oriented glycol-saturated <2 |j,m fractions 

(left) of argillaceous samples from core from upper part of Great 

Valley sequence. A, Union Moretti 1 well at 2,370 ft. B, Texaco 

Petan 2 well at 1,818 to 1,831 ft. CuKa radiation. Q, quartz; CY, 

total clay; M, coarse mica; C, chlorite; PI, plagioclase; I, illite; I/ 

S, illite/smectite; Ca, calcite; C/S, chlorite/smectite (corrensite).

of figure 7. There is considerable variation in clay mineralogy for each of the three petrofacies from both outcrop 

and core samples; few diagnostic relations, however, are 

immediately evident from the data.

The clay minerals in these rocks are mainly discrete 

illite and chlorite, I/S, kaolinite, and C/S (mostly as corrensite). All samples contain illite and chlorite with varying 

amounts of I/S. All discrete chlorites have stronger (higher 

relative intensity) 002 and 004 basal reflections than 001, 

003, and 005 reflections, indicating that they are iron rich 

(Brindley, 1961; Moore and Reynolds, 1989). The ratio and 

ordering of I/S varies from sample to sample. For example, 

upper Great Valley core samples shown in figures IB and 

2A and the Franciscan core sample shown in figure 4A contain mainly smectitic I/S (RO I/S with 50 to 80 percent 

smectite layers), whereas the lower Great Valley core sample 

shown in figure 3B contains illitic, ordered I/S (Rl I/S with 

70 to 75 percent illite layers). Additionally, a heterogeneous 

assemblage of I/S is commonly present within a single sample (fig. 4B).

Figure 7 compares the clay mineralogy of the <2-jum 

fraction for each sample and petrofacies. The categories 

forming the points of the ternary diagram of figure 7 were 

selected on the basis of mineralogical associations determined from XRD and textural observations from reconnaissance SEM analysis. On average, th,e clay mineralogy

B

10 6 2 36 32 28 24 20 18 12

Bulk rock

30 26 22 18 14 10 6 2

DEGREES 20

36 32 28 24 20 16 12

DEGREES 28

Figure 3. X-ray powder diffraction profiles of random-powder 

bulk rock (right) and oriented glycol-saturated <2 |0.m fractions 

(left) of argillaceous samples from lower part of Great Valley 

sequence. A, Outcrop sample 987-2-4 on Highway 46 (Toro? 

Formation). B, Core sample from Shell Barret 1 well at 1,452 to 

1,464 ft. CuKa radiation. Q, quartz; CY, total clay; M, coarse 

mica; C, chlorite; PI, plagioclase; I, illite; I/S, illite/smectite.

Bulk-Rock and Clay Mineralogies of Great Valley and Franciscan Strata, Santa Maria Basin Province, California 15
<2 ti

30 26 22 18 14 10 6 2

B

<2 \am

36 32 2 B

30 26 22 18 14 10 6 2

DEGREES 2U

36 32 23 24 20 16 12

DEGREES 29

<2 ^n Bulk rock

22 18 14 10 6 2

DEGREES 20

36 32 28 24 20 16 12 8 4

DEGREES 28

Figure 4. X-ray powder diffraction profiles of random-powder bulk rock (right) and oriented glycol-saturated <2 |j,m 

fractions (left) of argillaceous samples from well cores in 

Franciscan assemblage. A, Chief Larson Brothers well at 

2,784 to 2,804 ft. B, Superior Silva CH1 well at 916 to 922 

ft. CuKa radiation. Q, quartz; CY, total clay; M, coarse mica; 

C, chlorite; PI, plagioclase; I, illite; I/S, illite/smectite.

% Upper part of the ~

Great Valley sequence ** Mean

 Lower part of the Q Mean 

Great Valley sequence

Franciscan assemblage A Mean

100

Figure 5. X-ray powder diffraction profiles of random-powder bulk rock (right) and oriented glycol-saturated <2 |j,m 

fractions (left) of argillaceous samples from a well core in 

Franciscan assemblage containing abundant chlorite/smectite 

(C/S) as corrensite. A,Union Wineman 2 at 3,658 to 3,665 ft. 

B, Texaco Fulwider 1 well at 3,458 ft. CuKa radiation. Q, 

quartz; CY, total clay; M, coarse mica; C, chlorite; F, plagioclase and potassium feldspars; PI, plagioclase; I, illite; Ca, 

calcite.

EXPLANATION

Figure 6. Ternary diagram 

showing bulk-rock quartz/ 

feldspar/total-clay contents 

(in weight percent) of argillaceous outcrop and well-core 

samples from Franciscan 

assemblage and upper and 

lower parts of Great Valley 

sequence. Data normalized 

on carbonate-free basis from 

tables 1 and 2. Mean values 

for each group designated 

by open symbols.

so

100

Feldspar "" Total Clay 

16 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
of upper Great Valley and lower Great Valley shale is similar, and they have an overall average composition of 

about 55 relative weight percent illite plus chlorite and 45 

weight percent I/S; kaolinite and C/S are less abundant. As 

mentioned previously, the I/S amount and ordering type 

varies greatly among samples (figs. 1, 4, and 7). Lower 

Great Valley shale samples commonly contain more illite 

and chlorite than those from upper Great Valley shale 

samples; upper Great Valley shale samples generally contain more I/S. Additionally, I/S in lower Great Valley argillites is commonly more illitic than that in samples from 

the upper part of the Great Valley sequence. An example 

of this relation is shown in the samples from the Hi 

Mountain Saddle outcrop (table 1). Note that upper Great 

Valley sample 586-7-3 contains 77 relative weight percent 

I/S as compared to 30 weight percent I/S in lower Great 

Valley sample 586-7-2. This relation is also shown by the 

abundance of solid circles in the upper portion of figure 7. 

Moreover, those samples with the least amount of I/S 

commonly contain ordered I/S (fig. 7).

Although some samples of argillite and shale from the 

Franciscan assemblage have a clay-mineral suite similar to 

those from the Great Valley sequence, more commonly the

clay minerals in Franciscan assemblage samples are markedly different. This difference is shown by the individual 

sample and mean clay compositions plotted on figure 7. 

The primary difference in clay mineralogy in argillitic samples from the Franciscan assemblage compared to those 

from Great Valley sequence shales is the common presence 

and abundance of C/S (tables 1 and 2; fig. 5). An extreme 

case is demonstrated by the Franciscan core sample from 

the Union Wineman 2 well at depths of 3,658 to 3,665 ft 

(table 2; fig. 5A). This shale sample consists of 71 weight 

percent total clay with C/S (as corrensite) constituting 77 

weight percent of the clay in the <2-jim fraction. Also, 

Franciscan samples are generally more chloritic, whereas 

Great Valley samples are more illitic. On weighted average, 

the <2-|Lim fraction of Franciscan shales is composed of 

about 58 percent chloritic clays (chlorite plus C/S), 41 percent illitic clays (illite plus I/S), and 1 percent kaolinite, as 

compared to the weighted average for shale samples from 

the Great Valley sequence, which average about 31 percent 

chloritic clays, 67 percent illitic clays, and 2 percent kaolinite. From this standpoint, the mean clay-mineral composition of shales from the upper and lower petrofacies of the 

Great Valley are nearly identical.

Illite/Smectite 

100

EXPLANATION 

Upper part of the ^ 

£( Great Valley sequence 1 mean

Lower part of the 

Great Valley sequence

A A Franciscan assemblage 3 mean

Illite

Chlorite

100 

Chlorite/Smectite

+ 

Kaolinite

Figure 7. Ternary diagram showing clay-mineral contents (in weight percent) in <2 |im 

fraction of argillaceous outcrop and well-core samples from the Franciscan assemblage and 

upper and lower parts of Great Valley sequence. Data compiled from tables 1 and 2. Solid 

symbols indicate samples containing random interstratified illite/smectite (I/S), whereas 

open symbols are those samples containing only ordered I/S and (or) chlorite/smectite (C/ 

S). Mean values for each group are designated by numbered asterisks. Arrows indicate 

general direction of increasing C/S with decreasing I/S.

Bulk-Rock and Clay Mineralogies of Great Valley and Franciscan Strata, Santa Maria Basin Province, California 17
Observations from SEM Analysis

SEM analysis of freshly broken surfaces of shale and 

argillite samples from the Great Valley sequence and Franciscan assemblage show both detrital and authigenic textures on and oblique to bedding planes. These 

observations apply to samples from both outcrop and wellcore samples. For example, scanning electron micrographs 

of core samples of the Franciscan assemblage from the 

Union Wineman 2 well at depths ranging from 3,650 to 

3,658 ft are shown in figures 8 and 9. Samples sometimes 

appear blocky or massive, especially when viewed perpendicular to bedding (fig. 8A). Layered and microgranular 

clay fabrics containing individual flakes and plates of clay 

are commonly observed (figs. 8 and 9), particularly when 

viewed parallel or oblique to bedding.

Of particular interest are textural characteristics indicative of origin for corrensite and I/S. The fine-grained 

nature of most samples, the dense, compacted, blocky-tobedded and granular fabrics, and the anhedral, ragged, 

platy clay habit (fig. 8) suggest a detrital origin for most 

of the clay minerals in these argillaceous samples. Coarser, 

nonclay, detrital mineral grains are also commonly dispersed throughout most samples (figs. SB, D).

Crystal habits indicative of authigenic clay-mineral 

formation were also commonly observed along bedding 

planes, microfractures, and in micropores. For example, 

some authigenic growth of I/S is evidenced by the presence 

of honeycomb- and flamelike structures on I/S substrates 

and along edges of I/S having a detrital cornflake habit (fig. 

9A to C) in core samples from the upper part of the Great 

Valley sequence. Samples containing both I/S and correns200 (Am 2 jim

10|im

Figure 8. Scanning electron micrographs of mostly detrital 

textures in argillaceous rocks from the Franciscan assemblage. 

A, Blocky or massive texture shown perpendicular to bedding 

in core sample from Union Wineman 2 well at 3,650 ft. B, 

Ragged platy to granular heterogeneous fabric of chloritic clay

10 jim

in core sample from Superior Beckett CH 1 well at 1 ,717 ft. C, 

Thin platy semibedded fabric of chloritic clay in Union Wineman 2 well at 3,650 ft. D, Flaky heterogeneous clay fabric with 

dispersed coarse mineral grains (arrows). Note authigenic clay 

coating on coarse grain at left.

18 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
ite also contained authigenic lath-, fiber-, and step-like 

overgrowths (fig. 9D). These overgrowth features are typical of authigenic-clay formation by burial diagenesis (Pollastro, 1985) and hydrothermal processes (Inoue and others, 

1987). Coarser mineral grains and cements are also commonly observed in samples; these are sometimes partly 

altered or replaced by authigenic clay (fig. 8Z)).

The standardless, normalized, energy-dispersive chemistry of the corrensite- and chlorite-rich Franciscan melange 

matrix outcrop sample 987-2-3 (table 1) and a scanning 

electron micrograph of the area of this sample analyzed are 

shown in table 3 and figure 10, respectively. These data 

support the interpretation from XRD analyses that chlorites 

are iron enriched (relative to magnesium).

DISCUSSION

Petrographic studies of the detrital modes in sandstones from the upper and lower parts of the Great Valley 

sequence found that upper Great Valley sandstones contain 

greater amounts of detrital quartz, K-feldspar, and biotite 

than sandstones from the lower part of the Great Valley 

sequence (Dickinson and others, 1982; Gray, 1980; 

McLean, 1991). Although detrital modes of individual 

lithic clasts or grains can not be determined by XRD analysis, bulk XRD compositions of argillites and shale determined on the bases of QFC contents are similar for all three 

groups shown in figure 6. XRD data does, however, show 

greater mean K-feldspar content in argillites designated as

10 urn

4 urn

Figure 9. Scanning electron micrographs of authigenic clay 

textures in argillaceous rocks from upper Great Valley 

sequence (A through Q and Franciscan assemblage (D). A, 

B, Honeycomb, flamelike, and wrinkled-sheet overgrowths 

and coatings (arrows) in illite/smectite-rich microporous core

sample from Union Moretti 1 well at 2,370 ft. C, Honeycomb- and flamelike textures in illite/smectite-rich core sample from Union Pezzoni 1 well at 1,986 ft. D, Lath-, fiber-, 

and steplike overgrowths in core from Union Wineman 2 

well at 3,650 ft.

Bulk-Rock and Clay Mineralogies of Great Valley and Franciscan Strata, Santa Maria Basin Province, California 19
representing the upper part of the Great Valley sequence, 

particularly in those from outcrop.

It is important to note that there is a wide range in diagenetic histories for the samples studied because (1) the diversity in location of outcrops and cores in the 8MB, (2) local 

variations in basin tectonics and burial histories, and (3) variation in present and (or) maximum burial depth of well-core 

samples. Thus, any interpretation of detrital clay mineralogy 

is complicated by the likely modification, transformation, or 

destruction of the original detrital assemblage (particularly 

clay minerals) and (or) the neoformation of coarse-grained 

cements and clay minerals as a consequence of burial.

The mean clay mineral compositions of the <2-jam 

fraction of argillites from the upper and lower parts of the 

Great Valley sequence are similar, with only slight differences in the amount and ordering type of I/S. Moreover, 

the lack of clay mineral data from interbedded sandstones

Table 3. Standardless energy-dispersive analysis normalized to 100 percent for area of sample 987-2-3 shown 

in figure 10

[Sample 967-2-3 listed in table 1]

Oxide compound Weight percent

Ai 2q, 

SiO2

K2O ----- 

FeO -

CaO ~

MgO -----

Total 

10.0

45.0

.5

30.5

2.4

11.9

100.0

4|um

Figure 10. Scanning electron micrograph of chloritic clay 

minerals from melange matrix of Franciscan assemblage 

from outcrop on Highway 46 (sample 987-2-3, table 1). Corresponding Standardless energy-dispersive analysis normalized to 100 percent is listed in table 3.

and their association with the hydrothermally metamorphosed Point Sal ophiolite (Hopson and Frano, 1977) 

caused further problems in distinguishing between detrital 

and authigenic assemblages and interpreting the diagenetic 

grade of the chloritic clay minerals.

Illite and chlorite are present in all samples. Interbedded sandstones contain coarse white micas, biotite, and 

chlorite as primary grains or within rock fragments 

(McLean, 1991). Thus, much of the illite and chlorite may 

be recycled detrital material from older eroded sedimentary 

and(or) metamorphic rocks. Illite commonly is highly 

ordered (fig. 2A), as defined by the ratio of XRD peak 

height and width (Kubler, 1968), indicating that some illite 

may have formed at high (>200°C) temperatures, an indication which is consistent with high-diagenetic/low-metamorphic grades. In some samples, highly expandable I/S 

coexists with illite of high crystallinity. These coexisting 

phases indicate a wide contrast in thermal histories (Hoffman and Hower, 1979) and strongly suggest a recycled origin for the well-ordered illite in many of the samples 

(Pollastro, 1990; 1993). Moreover, textural observations 

from SEM also suggest a detrital origin for most of the clay. 

Some discrete clay-sized chlorite may have formed 

from the alteration of basaltic or ophiolitic rocks and rock 

fragments (Schiffman and Fridleifsson, 1991) prior to deposition. Mafic volcanic rock fragments are particularly 

common in Franciscan graywackes and melange (Dickinson and others, 1982), as well as in some sandstones from 

the lower part of the Great Valley sequence (MacKinnon, 

1978). Also, some authigenic chlorite and illite may have 

formed from diagenetic clay-mineral reactions in those 

samples that have undergone burial to depths >4,000 m 

and (or) temperatures in excess of 200°C (Hower, 1981).

I/S is found in all samples in varying amounts, often 

varying in illite/smectite ratio and ordering types. These 

differences in I/S are commonly due to variations in (1) 

detrital source areas and materials, (2) burial histories, and 

(3) diagenetic origins of I/S (Hoffman and Hower, 1979; 

Rettke, 1981; Pollastro, 1990; 1993). Upper Great Valley 

argillites tend to contain a greater amount of I/S with 

higher smectite content, whereas argillites from the lower 

part of the Great Valley sequence have a higher illite content and more illitic I/S. The general relation shown in figure 7, where samples containing the least amount of I/S 

contain ordered I/S, suggests that illitization has occurred 

in these rocks through forming illite layers in I/S and 

some discrete illite, in part, by cannibalization of earlier 

smectite or I/S (Pollastro, 1985). The coexistence of C/S 

(mostly corrensite) with illitic ordered I/S provides dual 

clay geothermometers suggesting paleoburial temperatures 

in excess of 100°C (Hoffman and Hower, 1979). Additionally, figure 7 shows that there are progressively decreasing 

amounts of I/S, along with illite and chlorite, with increasing amounts of C/S (plus kaolinite), an observation which 

suggests that some C/S may form at the expense of I/S.

110 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
However, these relations are difficult to test because the 

formation of C/S may be strongly dependent on precursor 

source materials and not on postdepositional conditions, as 

will be discussed below.

Mixed-Layer Chlorite/Smectite (Corrensite)

C/S, identified here mostly as corrensite, is common 

in shale samples from the Franciscan assemblage; it is also 

present in some shale samples from the Great Valley 

sequence. Some XRD profiles containing C/S, however, 

may also be interpreted as mixed-layer chlorite/corrensite 

(Shau and others, 1990). Although corrensite occurs in 

diverse geologic settings (Reynolds, 1988), it is often a 

product of the hydrothermal alteration of mafic rocks (Furbish, 1975; Kristmannsdottir, 1975, 1979; Brigatti and 

Poppi, 1984; Evarts and Schiffman, 1983; Bettison and 

Schiffman, 1988; Bettison-Varga and others, 1991; Shau 

and others, 1990; Schiffman and Fridleifsson, 1991). Corrensite is also commonly formed in burial-diagenetic settings (Hoffman and Hower, 1979; Reynolds, 1988).

Roberson (1988), Bettison and Schiffman (1988), and 

Bettison-Varga and others (1991) have identified and 

described corrensite in hydrothermally altered basaltic 

rocks of the Jurassic Point Sal and Coast Range ophiolites 

in California. Similarly, Evarts and Schiffman (1983) have 

described the formation of smectite, random-ordered (RO) 

C/S, corrensite, and discrete chlorite clay minerals from 

the hydrothermal metamorphism of the Del Puerto ophiolite (Evarts, 1977) in central California. Evarts and Schiffman (1983) attributed the formation of RO C/S, corrensite, 

and chlorite to the interaction between the ophiolitic rocks 

and heated seawater shortly after formation and emplacement of the ophiolite in the submarine environment. They 

concluded that any effects of diagenetic or burial metamorphic overprint on submarine hydrothermal metamorphism of the Del Puerto ophiolite was negligible. A 

detailed petrographic, XRD, and chemical study by Bettison and Schiffman (1988) revealed that phyllosilicates 

(smectite, C/S, and chlorite) of the Point Sal ophiolite are 

similar to the composition and zonations of those from the 

Del Puerto remnant studied by Evarts and Schiffman 

(1983). Moreover, oxygen-isotope analyses suggest that 

phyllosilicates were formed during the initial alteration of 

Point Sal volcanic rocks when water/rock ratios and pCO2 

were high.

The origin of the corrensite, as well as some smectite 

and chlorite, is therefore strongly dependent upon the 

amount of detrital input and degree of alteration of volcanogenic and ophiolitic material deposited within these 

argillaceous sediments. McLean (1991) reported that sandstones in the lower part of the Great Valley sequence contain rock fragments consisting mainly of aphanitic and 

basaltic andesite in an altered glassy groundmass of semiopaque clay minerals. Although no XRD analyses were 

performed on the clay minerals in these sandstones, textural descriptions suggest that some clays may have 

formed by diagenetic processes. Moreover, SEM analysis 

has shown that some authigenic clay formed in these rocks 

as overgrowths on precursor clays and by the alteration or 

replacement of coarser mineral grains as well. However, 

unless extensive diagenetic modification has occurred, significant contributions of fine-grained altered ophiolitic 

material may be necessary to explain the abundance of 

corrensite in samples such as that found in the Union 

Wineman 2 well at depths of 3,658 to 3,665 ft (table 2: 

fig. 5A).

McLean (1991) also reported the highest (3.5 volume 

percent) epidote content in sandstones from well samples 

of the lower part of the Great Valley sequence. Argillaceous samples from the lower part of the Great Valley 

sequence in this study also have the highest chlorite content (40 weight percent) (table 2; fig. 7). High chlorite 

content is consistent with an early "epidote zone" grade of 

metamorphism (Evarts and Schiffman, 1983) and may be 

indicative of the original igneous detritus. The data of our 

study, however, can only suggest origins for specific clay 

minerals through relations such as those mentioned above. 

Detailed studies of both sandstone and shale integrating 

SEM, XRD, transmission electron microscopy, and electron microprobe analyses, similar to those performed by 

Shau and others (1990) and Bettison-Varga and others 

(1991), are necessary before the origins of specific clay 

minerals in the argillaceous rocks of the SMB that we 

studied can be confidently determined.

SUMMARY

XRD mineralogy of argillaceous samples from outcrop and well-core samples of the Franciscan assemblage 

and the upper and lower parts of the Great Valley 

sequence in the SMB shows that the main constituents are 

quartz, feldspar, and clay minerals with much lesser 

amounts of carbonates, pyrite, and gypsum. Although the 

total quartz-feldspar-clay contents of these three rock 

groups overlap and all three have almost identical mean 

compositions, they differ somewhat in the amount of Kfeldspar and in clay mineralogy. Similarly, McLean (1991) 

found that the quartz-feldspar-lithic detrital modes for 

sandstones from these three rock groups in the SMB overlapped and that variations in the amount of K-feldspar 

served as a petrographic tool for differentiating rocks in 

the lower and upper parts of the Great Valley sequence 

from those of the Franciscan assemblage.

The three rock groups in this study contain discrete 

illite and chlorite, as well as I/S; kaolinite occurs less 

commonly in several samples. Most discrete chlorite is 

iron rich. C/S (mainly corrensite) is abundant in most of

Bulk-Rock and Clay Mineralogies of Great Valley and Franciscan Strata, Santa Maria Basin Province, California 111
the Franciscan samples but also is present in some argillites of the Great Valley sequence. The ratio and ordering 

of I/S varies from sample to sample and is sometimes heterogeneous within a single sample, suggesting a variety of 

origins. The diversity in location of outcrops and cores, 

core-sample depths, and local variations in tectonic and 

burial histories in the 8MB among the samples studied, 

however, complicates any interpretation of either detrital 

or diagenetic origins for these clay minerals.

Argillaceous rocks in the upper and lower parts of the 

Great Valley sequence are composed of a similar mostly 

illitic clay-mineral suite consisting mostly of discrete illite, 

I/S, and chlorite; upper Great Valley samples, however, are 

commonly richer in I/S with a higher smectite content. In 

contrast, samples from the Franciscan assemblage commonly contain more chloritic clays. In particular, the common presence and abundance of C/S as corrensite in these 

argillaceous rocks indicates a different source material than 

for similar rocks from the Great Valley sequence. Although 

C/S is commonly a product of burial diagenesis, it is also 

ubiquitous in hydrothermally altered basaltic rocks and has 

been reported as a common constituent of the Point Sal and 

Del Puerto ophiolites in the western part of southern California. Thus, C/S may have been derived from hydrothermally altered detritus and (or) the burial diagenesis of basic 

igneous rocks. Similarly, the presence and abundance of 

smectitic and chloritic clay minerals in these argillaceous 

rocks are dependent on the amount and degree of alteration 

of original detrital volcanogenic material.

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Bulk-Rock and Clay Mineralogies of Great Valley and Franciscan Strata, Santa Maria Basin Province, California 113

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