U.S. Department of the Interior 

U.S. Geological Survey

Paleogeography of the Western Transverse Range Province, 

California: New Evidence From the Late Oligocene and Early 

Miocene Vaqueros Formation

Stratigraphy of the Fine-Grained Facies of the Sisquoc 

Formation, Santa Maria Basin, California 

Paleoceanographic and Tectonic Implications

The Sisquoc Formation-Foxen Mudstone Boundary in the 

Santa Maria Basin, California: Sedimentary Response to the 

New Tectonic Regime

Bulletin 1995-T, U, V

Geophysical section offshore Santa Maria basin

Geologic section onshore Santa Maria basin

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STENNIS SPACE CENTER, Mississippi-Bldg. 3101
Paleogeography of the Western Transverse 

Range Province, California: New Evidence 

From the Late Oligocene and Early Miocene 

Vaqueros Formation

By CATHERINE A. RIGSBY

Stratigraphy of the Fine-Grained Facies of the Sisquoc 

Formation, Santa Maria Basin, California 

Paleoceanographic and Tectonic Implications

By PEDRO C. RAMIREZ and ROBERT E. GARRISON

The Sisquoc Formation-Foxen Mudstone Boundary 

in the Santa Maria Basin, California: Sedimentary 

Response to the New Tectonic Regime

By RICHARD J. BEHL and JAMES C. INGLE, Jr.

Chapters T, U and V are issued as a single volume 

and are not available separately

U.S. GEOLOGICAL SURVEY BULLETIN 1995

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

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

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Thomas J. Casadevall, Acting Director

Any use of trade, product, or firm names

in this publication is for descriptive purposes only

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

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1998

For sale by

U.S. Geological Survey

Information Services

Box 25286

Denver Federal Center

Denver, CO 80225

Library of Congress Cataloging in Publication Data

Rigsby, Catherine A.

Paleogeography of the western Transverse Range Province, California: new 

evidence from the Late Oligocene and Early Miocene Vaqueros Formation / by 

Catherine A. Rigsby. Stratigraphy of the fine-grained facies of the Sisquoc 

Formation, Santa Maria Basin, California : paleoeanographic and tectonic 

implications / by Pedro C. Ramirez and Robert E. Garrison. The Sisquoc 

Formation-Foxen Mudstone boundary in the Santa Maria Basin, California: 

sedimentary response to the new tectonic regime / by Richard J. Behl and 

James C. Ingle, Jr.

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

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

Maria Province; ch. T, U, V)

"Chapters T, U and V are issued as a single volume and are not available 

separately."

Includes bibliographical references.

Supt. of Docs, no.: I 19.3:1995-1, U, V

1. Paleogeography California, Southern. 2. Geology, Stratigraphic 

Tertiary. 3. Geology California Santa Maria Basin. 4. Vaqueros Formation 

(Calif.) 5. Sisquoc Formation (Calif.) I. Garrison, Robert E. II. Ingle, James C. 

III. Ramirez, Pedro C. Stratigraphy of the fine-grained facies of the Sisquoc 

Formation, Santa Maria Basin, California. IV. Behl, Richard J. Sisquoc 

Formation-Foxen Mudstone boundary in the Santa Maria Basin, California. V. 

Title. VI. Title: Stratigraphy of the fine-grained facies of the Sisquoc Formation, 

Santa Maria Basin, California. VII. Title: Sisquoc Formation-Foxen Mudstone 

boundary in the Santa Maria Basin, California. VIII. Series. IX. Series: Evolution 

of sedimentary basins/onshore oil and gas investigations Santa Maria 

Province ; ch. T, U, V. 

QE75.B9 no. 1995-T, U, V 

[QE501.4} 

557.3 s dc21

[551.7'8'09794] 98-4760

CIP
Chapter T

Paleogeography of the Western Transverse 

Range Province, California: New Evidence 

From the Late Oligocene and Early Miocene 

Vaqueros Formation

By CATHERINE A. RIGSBY

U.S. GEOLOGICAL SURVEY BULLETIN 1995-T

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

Edited by Margaret A. Keller
CONTENTS

Abstract Tl 

Introduction Tl 

Acknowledgments Tl 

Tectonic setting T2 

Stratigraphy T3

Lithostratigraphy T3

Chronostratigraphy T4 

Lithofacies T8

Fan-delta conglomerates T8

Shoreline sandstones and conglomerates T8

Inner-shelf sandstones T8

Deltaic sandstones and siltstones T9

Gibraltar Reservoir area T9 

Paleocurrents Til 

Provenance Til

Petrofacies 1 Til

Petrofacies2 T13 

Discussion T14

Regional paleogeography T14

Stratigraphic implications T14

Tectonic implications T14 

References cited T16

FIGURES

1. Map showing extent of Vaqueros Formation in the western Transverse Ranges 

study area T2

2. Palinspastic reconstruction of southern California during early Tertiary time T3

3. Facies relations between the Vaqueros Formation and laterally equivilent units in 

the western Transverse Ranges province T4

4. Hypothetical Chronostratigraphy of the Vaqueros Formation T7

5. Present-day geographic extent of Vaqueros Formation lithofacies tracts in the 

western Transverse Ranges and in the offshore T9

6. Generalized, diagrammatic measured sections of the Vaqueros Formation in the 

coastal area outcrops T10

7. Vaqueros Formation strata of the deltaic fades tract in the Gibraltar Reservoir area T12

8. Sandstone QFLt and QtFL triangular diagrams showing two Vaqueros Formation petrofades T13

9. Palinspastic paleogeographic map of the western Transverse Ranges province during deposition of the 

late Oligocene and early Miocene Vaqueros Formation T15

TABLES

1. Strontium isotope data and apparent equivalent ages for samples from the Vaqueros, Rincon, 

and Alegria Formations in the study area T5

2. Apparent age of the Vaqueros Formation in various parts of the western Transverse Ranges T6
Paleogeography of the Western Transverse Range Province, 

California: New Evidence From the Late Oligocene and 

Early Miocene Vaqueros Formation

By Catherine A. Rigsby1

Abstract

Outcrops of the Vaqueros Formation in the westernmost 

Transverse Ranges, and Vaqueros hydrocarbon reservoir strata 

in the adjacent Santa Barbara Channel, reveal dual sediment 

sources and a complex late Oligocene/early Miocene shoreline geometry. Shelf deposits in the central Santa Ynez Mountains separate a gently sloping, west- and southwest-facing 

mainland shoreline to the east (eastern Santa Ynez Mountains 

and Santa Monica Mountains; all depositional coordinates are 

prerotation) from a steep, east- and south-facing shoreline to 

the west (westernmost Santa Ynez Mountains). The western 

and central regions are dominated by lithic-rich arenites and 

by Franciscan-clast conglomerates (petrofacies one) derived 

from an emergent portion of an early Tertiary trench-slope 

break. The eastern region is dominated by lithic-poor 

quartzofeldspathic sandstones (petrofacies two) derived from 

older (Upper Cretaceous and (or) Paleogene) sandstones on 

the mainland to the east. The two lithofacies interfinger in the 

central region.

During the late Oligocene, sediment was shed onto the 

Vaqueros Formation shelf from both the mainland to the east 

and the partially emergent trench-slope break to the west. 

Later, during the early Miocene, transgression proceeded 

across the region, drowning the westernmost shoreline. Strontium isotopic analyses suggest apparent ages for Vaqueros 

strata that get younger in the paleo-landward directions. These 

strata record deposition in a complex forearc basin within an 

obliquely convergent continental margin. The geometry of 

this basin was the product of early to middle Tertiary plate 

interactions.

INTRODUCTION

The Vaqueros Formation in the western Transverse 

Ranges occupies a key position, both temporally and spatially,

'Department of Geology, East Carolina University, Greenville, NC 27858. 

Manuscript approved for publication December 12, 1997.

in the development of the southern California transcurrent continental margin. The Vaqueros strata also record the onset of 

eustatic sea level rise in a tectonically active marginal basin 

(Howard, 1988, 1995; Rigsby 1988b, 1994a). An appreciation of the Vaqueros sedimentary record is important in understanding the tectonic history of this complex area of 

southern California.

This paper details the stratigraphy of the Vaqueros Formation in the western Transverse Ranges (fig. 1) and presents 

a paleogeographic reconstruction of the study area during late 

Oligocene/early Miocene time. This reconstruction uses petrologic and lithofacies data from the Vaqueros Formation and 

is presented as a starting point for future, more detailed investigations, which should include age-equivalent strata to the 

north of the study area as well as detailed (and as yet proprietary) subsurface data from the adjacent Santa Barbara Channel. The data and interpretations based on the Vaqueros 

Formation and presented in this report provide new information and insights into the evaluation of tectonic models, based 

on paleomagnetic data, for the Tertiary history of southern 

California. Without sedimentological data, these models cannot be fully evaluated or tested.

ACKNOWLEDGMENTS

This project was made possible by grants from the 

University of California's University-Wide Energy Research 

Group (1987/88 and 1988/89), from Mobil, and from 

UNOCAL. The strontium isotope analyses were performed 

by R.E. Denison at the Mobil Research Labs in Dallas. J.G. 

Vedder (U.S. Geological Survey, Menlo Park, California) 

identified many of the megafossils and provided instruction 

and advice that helped me identify specimens in the field. 

I thank the management of Exxon Company, U.S.A. and 

Shell Exploration and Production Company for access to 

cores, cuttings samples, wireline logs, and other data, and

Paleography of the Western Transverse Range Province, California T1
for initial versions of some of the figures. I benefited from 

conversations regarding subsurface stratigraphy with M.L. 

Kolb (ARCO), who also facilitated access to ARCO well 

data, and with G.A. Miles and D.O. Schwartz. The 

manuscript benefited from reviews by R.E. Garrison, Eli 

Silver, L. Laporte, Hugh McLean, S.Y. Johnson, Scott Starratt 

and Margaret A. Keller.

TECTONIC SETTING

Paleomagnetic declinations in rocks of the western Transverse Ranges suggest that a structural block bound on the north 

by the Santa Ynez fault, on the east by the San Gabriel fault, 

and on the south by the Malibu Coast fault (fig. 1) was tectonically rotated (approximately 90°) from an original northsouth orientation during middle Miocene time (Luyendyk and

others, 1980,1985). This rotation occurred during deposition 

of the Monterey Formation and corresponds in age to a regional disconformity within the Monterey (Luyendyk and others, 1985). The lack of an angular unconformity at this position 

in the stratigraphic record suggests that the rotation was not 

accompanied by tectonic compression within the rotating 

block. Further, the lack of throughgoing strike-slip faults, along 

with a systematic decrease in magnetic declination with decreasing age, suggests that the western Transverse Ranges were 

rotated as a single, coherent crustal block (Luyendyk and others, 1985). This rotation is attributed to the northwest-southeast-oriented shear couple that has existed between the Pacific 

and North American plates since the Oligocene (Cox, 1980; 

Luyendyk and others, 1985).

On the basis of paleomagnetic and stratigraphic data from 

the western Transverse Ranges, Hornafius (1985) suggested 

that there was no significant change in paleogeography beUUIUHUK MAK:

VAQUEROS FORMATION 

WESTERN SANTA YNEZ

Point Conception

Figure 1 . Map showing extent of Vaqueros Formation in the western Transverse Ranges study area. Outcrop pattern modified after 

Dibblee (1950). Abbreviations: SLO, San Luis Obispo; SM, Santa Maria; SB, Santa Barbara; LA, Los Angeles.

T2 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
tween early Tertiary and Miocene time. In his model (fig. 2), 

the Santa Ynez fault marks the hingeline of the early Tertiary 

forearc basin, the Santa Maria basin is an emergent segment 

of the Oligocene trench-slope break, and the area now occupied by the southern Santa Ynez Range is situated on the 

continentward margin of that trench-slope break and on the 

oceanward side of the forearc basin (Hornafms, 1985; Rigsby, 

1989a,b). Paleogeographic interpretations based on stratigraphic, petrologic, and depositional data from Vaqueros Formation strata in the western Transverse Ranges are presented 

here as a partial test of this model.

STRATIGRAPHY

Lithostratigraphy

The Vaqueros Formation is part of a regional late Oligocene and early Miocene deepening-upward sequence that

EXPLANATION

T T T T Paleoshoreline

(teeth on seaward side)

Forearc basin 

depositional axis

A

A 0 30 00 

> |_|_l I 1 I I

kilometers

FARALLON 

PLATE

(Modified after Homafius, 1985)

Figure 2. Palinspastic reconstruction of southern California during early Tertiary time (modified after Hornafius, 1985). Abbreviations: 

SLO, San Luis Obispo; SM, Santa Maria; SB, Santa Barbara; V, Ventura; LA, Los Angeles; SD, San Diego; SCI, Santa Cruz Island; SRI, 

Santa Rosa Island; SMI, San Miguel Island; SNI, San Nicolas Island; F, fault.

Paleography of the Western Transverse Range Province, California T3
correlates with a second-order eustatic sea level rise (fig. 3). 

Sea level lowstand is represented by the upper member of the 

Sespe Formation which forms a widespread nonmarine base 

to this transgressive sequence (Howard, 1988, 1995). With 

the onset of sea level rise, the Sespe continental deposits were 

onlapped by nearshore and shelf sediments of the Vaqueros 

Formation. The Vaqueros, in turn, is overlain by shelf and 

slope strata of the Rincon Formation, which correlates with 

the highest stands of sea level during early Miocene time. The 

nature of the contacts between the Vaqueros Formation and these 

adjacent units and a detailed knowledge of local stratigraphy are 

important for a regional stratigraphic and sedimentologic understanding.

The contact between the Vaqueros Formation and underlying formations is complex. The Sespe Formation grades westward into paralic and shelf facies of the Alegria Formation 

(Dibblee, 1950; Howard, 1988). In the eastern part of the study 

area for example, east of Lauro Canyon (fig. 1) and still farther east in the Santa Monica Mountains (Edwards, 1971; Lane, 

1987) the Vaqueros lies conformably and, locally, gradationally on the Sespe Formation. In the central part of the study 

area, at Canada de la Pila and Gaviota Gorge (fig. 1), for example, and in off shore wells at Hondo field (Miles and Rigsby, 

1990; Rigsby and Schwartz, 1990) the Vaqueros sharply and 

conformably overlies paralic facies of the Alegria Formation. 

Farther west, the contact with the Alegria Formation becomes 

unconformable. The unconformity becomes progressively more 

angular westward, attaining a maximum angularity of 10°. In 

the northern part of the study area, at Gibraltar Reservoir, the 

basal Vaqueros contact is poorly exposed, but Vaqueros strata 

appear to be conformable with the underlying Sespe. Viewed 

regionally, the basal surface of the Vaqueros Formation progresses 

from an angular unconformity in the west to a transgressive surface a surface cut by wave and current activity in a marine 

environment during or subsequent to a marine transgression, not 

a subaerial erosion surface (Nummendal and Swift, 1987) in 

the central and offshore regions, to a conformable surface in the 

east and the north (Rigsby, 1988b).

The contact between the Vaqueros Formation and overlying units is most commonly a sharp boundary. In the westernmost 

part of the study area (at Rodeo Canyon (fig.l), for example), 

this contact marks the change from fine-grained sandstone and 

siltstone of the Vaqueros to silty mudstone of the Rincon Formation. In outcrops along the central coastal region, the contact is 

between coarse-grained sandstone (at Canada de la Pila) and 

conglomerate (at Gaviota Gorge) of the Vaqueros and siltstone 

and mudstone of the Rincon. At the Hondo and Capitan fields 

(see fig. 5), the Rincon-Vaqueros contact is either sharp or gradational over a short vertical distance (Miles and Rigsby, 1990; 

Rigsby and Schwartz, 1990). In these oil fields, the contact marks 

the transition between sandstone and siltstone of the Vaqueros 

and siltstone and shale of the Rincon.

Only in the Gibraltar reservoir area (fig. 1) is the upper 

contact of the Vaqueros Formation more complex. In this area, 

massive amalgamated sandstones of the Vaqueros, with thick 

beds of intervening siltstones, grade upward into the thinner sandstone beds of the Rincon Formation (Dibblee, 1973), which contain thin siltstone interbeds. The sandstone beds become thinner 

and finer upward across the contact. No mudstones are present 

at this locality. This gradational contact is present throughout 

most of the Gibraltar Reservoir area. The Rincon thins northward and eastward, however, and pinches out entirely near the 

northern boundary of the study area. At Devil's Canyon the 

northernmost outcrop in this study the Rincon is not present. 

The Vaqueros Formation at Devil's Canyon is overlain by the 

fossiliferous, shallow-marine, lower Miocene Temblor Forma-

_^_l Sespe Fm. (upper part) 

Missing section

Figure 3. Facies relations between the Vaqueros Formation and laterally equivilent units in the western Transverse Ranges province. 

Modified after Howard (1995) to show decrease in age of the base of the Vaqueros Formation to the west, as discussed in text. See 

figures 1 and 2 for locations. Abbreviations: CC, Cementerio Canyon; GC, Gaviota Gorge; RC, Refugio Canyon; SB, Santa 

Barbara; V, Ventura; time lines T1 through T4 illustrate Howard's offlap-onlap relationships; PCFC, Pacific coast foraminiferal 

stages; NALMA; North American land mammal ages. L, lower; M, middle; U, upper; e, early; I, late. References for chronological 

framework: 1, Berggren and others (1992); 2, Cande and Kent (1992); 3, Bartow (1991); 4, Haq and others (1987); 5, Woodburne 

(1987); 6, Swisher and Prothero (1990); 7, Prothero and Swisher (1992).

T4 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
TABLE I. Strontium isotope data and apparent equivalent ages for samples from the Vaqueros (Tv), Rincon (Tr), Temblor (Mt), 

area.

[Sample locations marked (c) are from the coastal portion of the study area the Hollister Ranch and central coastal areas. S

northern portion of the study area the Gibraltar Reservoir area. Ratios marked * are normalized to NBS 986 = 0.71014. R

987 = 0.70914. Check marks (u) in the last column indicate that the sample is thought to have provided a reliable app

discussion.]

?

Iere 

3 o

1

n

1

ft

i

H

3

1

$

70

1

 o

I

3

n 

-J°

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2L

Sample #

HR-Bl-Tv

H R-C6-Ta

H R-C2BTv

H R-AC8BTv

HR-COY-Tv

B ULTTO 1-

Tv

P R-23-Tv

SY3 -Tv

SY4-Tv

SY7 -Tv

SY-Tr

SY9-Mt

LG1 -Tv

Location

Bulito Road (c)

Bulito Road (c)

Cementario

Canyon (c)

Cementario

Canyon (c)

Cementario

Canyon (c)

Alegria/Cuarta

Cyn. (c)

Coyote Canyon (c)

Bulito Canyon (c)

Paradise Road (n)

Paradise Road (n)

Paradise Road (n)

Paradise Road (n)

Paradise Road (n)

Paradise Road (n)

Paradise Road (n)

Paradise Road (n)

Paradise Road (n)

Brief description of sample 

location

From center of small outcrop of poorly

sorted fossil conglomerate

As above

Several feet below Vaqueros contact

As above

Uppermost Vaqueros lithofacies in this

outcrop

From the conglomeratic portion of a Tv

fining upward lithofacies (2nd lithofacies

from top of TV outcrop)

From near center of small outcrop

Middle of exposure

Just below channelized facies, in thinly

bedded sandstone and siltstone

Near base of Vaqueros

As above

At load structures near top of outcrop

At Tv/Tr contact

As above

As above

From Temblor "algal" bed

Near top Vaqueros

87 Sr/86Sr

0.708156 ±

28*

0.708512 ±

27*

0.707772 ±

30*

0.707936 ±

20*

0.708367 ±

14*

0.708804 ±

26*

0.708307 ±

31*

0.708404 ±

8*

0.708880 ±

22*

0.70858 ± 3**

0.70821 ±

3**

0.70891 ±

2**

0.70836 ±

4**

0.70848 ±

2**

0.70880 ±

2**

0.70875 ±

2**

0.70945 ±

3**

Percent 

residue

0.60

0.60

0.50

0.20

2.10

0.00

0.00

0.10

2.10

0.90

1.00

-

0.60

0.00

7.30

1.70

0.70

Sample typ

Fibrous oyster

Bivalve fragments

Bivalve fragments

Bivalve fragments

Bivalve fragments

Mixed mollusks

Bivalve fragments

Oyster fragments

Oyster fragments

Pearly oyster fragme

Fibrous, dense oyster 

Chalky bone fragme

Chalky pecten fragm

Pearly oyster fragme

Fibrous oyster fragm

Dense carbonate

Spar fragments
TABLE 2. Apparent age of the Vaqueros Formation in various parts of the western Transverse Ranges as reported by this and 

other studies.

[Only the three samples considered "reliable" (see table 1 and discussion in text) are included in the apparent age list for this 

study. Ages marked (b) are from the base of the Vaqueros; ages marked (t) are from the top of the Vaqueros.]

Dickinson and others (1987) 

(Total Vaqueros Fm. age estimate)

This study 

(Age of base or top of Vaqueros Fm.)

Hollister Ranch area

Gaviota Pass area 

Santa Barbara area 

Ventura area 

Gibraltar Reservoir area

27-25 Ma

26-24 Ma

26-23.7 Ma

24±1 Ma (b)

23±1 Ma (b), 19±1 Ma (t)

tion (also called Vaqueros by some workers, such as Dibblee, 

1973). The Temblor Formation is not present elsewhere in the 

study area.

Chronostratigraphy

Although rarely cited as a true chronostratigraphic stage, 

the "Vaqueros Stage" of Addicott (1977) is useful in that it 

denotes a group of megafossils distinctive to the Vaqueros 

Formation. A wide range of these Vaqueros Stage fauna was 

identified from Vaqueros outcrops on Vandenberg Air Force 

Base, on Hollister Ranch, and in the Gibraltar Reservoir area. 

Species present in the western and northern outcrop areas include the following: Lwopecten magnolia (a good Vaqueros 

Stage index fossil), Turritella inezana, "Trophon". Ostrea 

eldridgei? or subtititan?. Neverita reclusiana. Dosinia sp., 

Tivela inezana. and Saxidomus sp. or Periglypta sp. No identifiable forms are present in the central or eastern coastal outcrops, although the Canada de la Playa section contains 

abundant coarse sand-sized fossil fragments.

In addition to fossil evidence, this study compared data 

obtained from strontium isotopic analysis (table 1) of samples 

from outcrops in the Santa Barbara coastal region and in the 

Gibraltar Reservoir region to the seawater strontium curve of 

DePaolo (1986; DePaolo and Ingram, 1985) to ascertain the 

chronostratigraphy of the Vaqueros Formation in the study area. 

High quality, datable material is scarce in these areas and, as 

indicated in table 1, samples of various type and quality were 

analyzed. In the best cases, large unaltered shell fragments 

were available and were collected and analyzed. In the worst 

cases, small slightly altered shell fragments were the only 

material available and, as such, were collected and analyzed. 

As a result of this variation in sample type and quality, analyses yielded a wide range of apparent ages for the strata.

Fibrous oyster samples, such as those from Bulito Road 

and Paradise Road (table 1), yielded the best dates. Barnacle 

fragments also yielded usable ages. At Little Sespe Creek, for 

example, barnacles and oysters from the same unit yielded

identical ages. Bivalve fragments are of questionable reliability as age indicators. This unreliability may be the result of 

changes in the marine 87Sr/86Sr signal brought about by freshwater flux either after deposition or in the original marginal marine environment (Bryant and others, 1995).

Other sample types are generally unreliable. The 

mixed mollusk fragments from Alegria/Cuarta Canyon (although they appeared fresh in hand sample) yielded anomalously young apparent ages. The bivalve fragments from 

Coyote Canyon and the oyster fragments from Bulito Canyon 

also yield low ages. As expected, samples that were noticeably weathered in the field yielded consistently anomalous 

data. At Paradise Road (Gibraltar Reservoir), for example, 

only two of the samples yielded believable apparent ages a 

Vaqueros Formation sample at 23±1 Ma and a Rincon sample 

19±1 Ma. The other ages were anomalously low and (in some 

cases) inverted, implying that the marine 86Sr/87Sr signal has 

been affected..

Although the data presented in table 1 show apparent 

ages that are widely scattered and that in some cases may not 

be reproducible, careful examination suggests that at least 

3 of the 17 samples yielded reasonable data: one of the 2 

Bulito Road samples and 2 of the 9 Paradise Road samples. 

The apparent ages of these samples allow a reasonable 

reconstruction of Vaqueros Formation chronostratigraphy.

The COSUNA Project chart for southern California 

(Bishop and Davis, 1984) indicates that the Vaqueros Formation in the central Santa Ynez Mountains ranges from 31 to 

23.1 Ma (top of Zemorrian Stage). Within the error of this 

method, the strontium data presented here suggest that the 

Vaqueros strata in the Hollister Ranch and Paradise Road 

(Gibraltar Reservoir) areas were deposited 18 to 25 Ma. As 

reported by other workers, based on provincial foraminiferal 

stages in the overlying Rincon Formation (table 2), Vaqueros 

strata in the Gaviota Pass, Santa Barbara, and Ventura areas 

are slightly older (23.7-27 Ma; total Vaqueros Formation age 

range). Previous workers have suggested that the Vaqueros 

"youngs" landward to the east (fig. 3; Howard, 1995) and 

the north (T.H. McCulloh, oral commun., 1987). These stronT6 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
tium and foraminiferal data do not contradict these landwardyounging hypotheses; instead when viewed in the context 

of the early Tertiary paleogeography they suggest that landward younging took place over a very complex late Oligocene

and early Miocene topography. This complexity is outlined 

in detail in the discussions of Vaqueros lithofacies that follow 

and has been alluded to by previous workers (Hornafius, 1985; 

Rigsby, 1989a, 1994a,b).

Shoreline sandstones 

and conglomerates

Paleoshorehne

(teeth on seaward side)

Figure 4. Hypothetical chronostratigraphy of the Vaqueros Formation. The hypothetical time lines (T1 is older, T2 is younger) are based 

on the strontium isotope ages reported in this study, on the Vaqueros age ranges reported by Dickinson and others (1987), and on 

geological evidence (such as the positions of paleoshorelines) presented in this paper and in work by Howard (1988), Hornafius (1985), 

and others. As discussed in the text, this hypothesis suggests that the Vaqueros Formation decreases in age in all landward directions. 

Base map is the early Tertiary palinspastic reconstruction of southern California (Hornafius, 1985). See figure 2 for abbreviations.

Paleography of the Western Transverse Range Province, California T7
When hypothetical time lines based on the data in 

presented table 2 and on geological data, such as the positions 

of paleoshorelines are plotted on the palinspastic reconstruction of early Tertiary southern California (fig. 4), a pattern of landward younging does emerge. In this hypothesis, 

the older Vaqueros Formation strata (Tl on fig. 4) are approximately latest Oligocene to earliest Miocene and the younger 

Vaqueros Formation strata (T2 on fig. 4) are approximately 

early Miocene. Further, the time lines parallel the complex 

topography suggested by the paleomagnetic models. This 

chronstratigraphic reconstruction implies a time-transgressive 

nature for the Vaqueros Formation in this area. The Vaqueros 

transgression probably happened rapidly and at about the same 

time (late Oligocene) throughout the region and in adjacent 

areas (Stanley and others, 1992; Lagoe, 1988; Rigsby, 1989a; 

Keller and others, in press).

LITHOFACIES

Interpretation of lateral and vertical facies changes based 

on outcrop observations and on analysis of subsurface data 

(sparse public-domain well logs and cuttings as well as proprietary data from the Hondo, Capitan, and Elwood fields) 

suggests that the Vaqueros Formation in the study area may be 

divided into four major lithofacies tracts for the purpose of 

paleogeographic reconstruction. A facies tract is defined here 

as an area in which a single depositional system is predominant. The four Vaqueros depositional systems are summarized below. This summary is based primarily on previous 

work (Rigsby, 1988a,b, 1989a, 1994b). The present-day geographic extent of each depositional system is illustrated in figure 5 and selected, generalized measured sections are presented 

in figure 6. All directions used in this discussion are based on 

present-day geographic orientations (that is, relative to modern coordinates).

Fan-Delta Conglomerates

The Vaqueros Formation fan-delta conglomerates, described in detail by Rigsby (1989a, 1994b), are exposed exclusively in the westernmost Santa Ynez Mountains. They 

consist of coarse-grained, matrix-supported, variably fossiliferous conglomerate overlain by extensively bioturbated, locally crossbedded, coarse- to fine-grained sandstone (Sudden 

Canyon and Cementerio Canyon in fig. 6). Fan-delta plain, 

fan-delta front, and fan-delta slope units are present in a consistently deepening-upward sequence that documents the effect of late Oligocene and early Miocene sea level rise. Pebble 

imbrications throughout the sequence and channel orientations 

and crossbedding in the fan-delta front units record paleoflow 

to the east and southeast (Rigsby, 1989a, 1994b). No well 

data were available for examination in the area offshore of 

this facies tract.

Shoreline Sandstones and Conglomerates

Shoreline sequences (fig. 5; Alegria/Cuarta Canyon in 

fig. 6) present in the central coastal portion of the study area 

 on Hollister Ranch (fig. 1) consist of crossbedded, sharpbased beds of sandstone and conglomerate overlain by surfzone sandy conglomerate and, finally, by inversely graded, 

planar-laminated, coarse-grained sandstone (Rigsby, 1988b, 

1989a,b). These strata represent deposition in a prograding 

shoreface-beach environment similar to that described by 

Hunter and others (1979). Paleocurrent indicators in the surf 

zone strata record a southeast-directed paleoslope.

Similar rocks occur in Vaqueros Formation outcrops studied by Edwards (1971) and Lane (1987) in the Santa Monica 

Mountains, where paleocurrent indicators record northwest 

paleoslope directions.

Cuttings and electric logs from wells offshore of the central coastal portion of the study area indicate that the Vaqueros Formation in the offshore is thin and discontinuous (Rigsby, 

1989a). Examination of public domain well data reveals that 

the Vaqueros is present only in the Texaco Anita Block (fig. 

5). The Texaco Anita CHA and the Texaco Anita #9 wells 

(fig. 1) encountered approximately 60 feet of coarse-grained 

sandstone overlain by fine-grained sandstone and siltstone of 

the Rincon Formation and underlain by Alegria Formation 

sandstones. In wells to the west, such as the Texaco Jade #21 

(fig. 1), and to the east, such as the Arco Ames #3 (fig. 1), the 

Rincon Shale directly overlies the Alegria Formation with no 

intervening Vaqueros strata.

Inner-Shelf Sandstones

Outcrops along the eastern coastal portion of the study 

area are exemplified by the stratigraphic sequence at Canada 

de la Pila [referred to as Canada de la Playa in Rigsby (1988a,b) 

and as Quemado Canyon on fig. 6]. These strata consist of 

coarse-grained to granular sandstone that exhibit medium- to 

large-scale trough and planar crossbedding, and recording bidirectional paleoflows (Rigsby, 1988b, 1989a). Interpreted as 

inner shelf (tidal?) sand wave deposits, these units are conformable, but in sharp contact, with interbedded deltaic and 

nonmarine strata of the Alegria and Sespe Formations below 

and with shelf and slope strata of the Rincon Formation above.

Inner-shelf sandstones are identifiable in the subsurface 

in both the Hondo and Capitan fields (fig. 5). In the Hondo 

field the Vaqueros Formation consists of massive, bioturbated, 

medium-grained sandstone with few preserved inorganic sedimentary structures (Miles and Rigsby, 1990). In the Capitan 

field, the Vaqueros consists of medium- to coarse-grained, 

bioturbated, locally crossbedded and laminated sandstone 

(Rigsby and Schwartz, 1990). The lack of wave-formed sedimentary structures and the extensive bioturbation in Vaqueros 

strata in both areas suggest that these strata were deposited in

T8 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
GIBRALTAR 

RESERVOIR 

AREA

Point 

Arguello

EXPLANATION

___ FACIES 

l+^l FAN-DELTA 

NEARSHORE 

(23 INNER-SHELF 

&8 DELTAIC

MEAN PALEOCURRENT 

DIRECTION

KILOMETERS

Figure 5. Present-day geographic extent of Vaqueros Formation lithofacies tracts in the western Transverse Ranges and 

in the offshore (modified after Rigsby and Schwartz, 1990). Paleocurrent directions are estimated from qualitative field 

examination of Vaqueros outcrops.

an inner-shelf environment where coarse-grained material was 

available, but where depositional energy was low enough to 

allow extensive faunal development.

In general, the inner-shelf facies of the Vaqueros Formation (as recorded in outcrops, as well as in the Capitan and 

Hondo fields) fines, thins, and becomes more pervasively 

bioturbated in the present-day offshore direction. The preservation of laminations and crossbedding and the lack of significant amounts of fine-grained material in the Vaqueros 

sandstone in the Capitan field indicate that the sandstones in this field 

occupied a more proximal position than that of the Vaqueros sandstones in the Hondo field. Furthermore, the intensity of the 

bioturbation and the occurrence of fine-grained sandstone and 

claystone at the tops of some Vaqueros intervals in the Capitan field 

suggest that the Vaqueros Formation in the Capitan field strata occupied a more distal position than the Vaqueros outcrops along the 

eastern coastal portion of the study area.

The outcrops and the nature of the Vaqueros Formation 

strata in the subsurface suggest that the inner-shelf sandstone 

environment may trend parallel to the paleoshoreline as a series of discontinuous, lens-shaped, medium- to coarse-grained 

sandstone bodies as is common in the deposits of shelf strata 

in the Cretaceous of the western interior of North America

(Tillman and Martinsen, 1984). These sandstone bodies may be 

separated by areas of thinner bedded, finer grained sandstone and 

siltstone that may be difficult to distinguish from the Alegria Formation in both wireline logs and cores.

Deltaic Sandstones and Siltstones

The Vaqueros Formation in the Gibraltar Reservoir area 

(Rigsby, 1989a), as well as in parts of the Santa Monica Mountains to the east (Edwards, 1971; Lane, 1987), consists of deltaic sandstone and siltstone.

Gibraltar Reservoir Area

In the Gibraltar Reservoir area the deltaic facies is a 700- 

m-thick (maximum thickness; the lowermost part of the sequence contains an unknown thickness of fault-repeated 

section) coarsening- and thickening-upward sequence of sandstone and interbedded siltstone. The lower portion of the sequence is poorly exposed along the tops of canyon walls in 

Arroyo Burro (fig. 1). It consists of fine- to medium-grained, 

buff to yellow, extensively bioturbated and locally crossbedded

Paleography of the Western Transverse Range Province, California T9
w

SUDDEN CANYON CEMENTERIO

ALEGRIA/CUARTA O

Ta

Figure 6. Generalized, diagrammatic measured sections of the Vaqueros Formation in the coastal area outcrops.
sandstone interbedded with muddy siltstone. The siltstone in 

this part of the section occur as grass-covered slopes separating isolated sandstone outcrops.

The middle 200 m of the sequence consists of finingupward packets of medium- to coarse-grained, bioturbated and 

locally horizontally laminated and penecontemporaneously deformed (fig. 7A) sandstone interbedded with planar-laminated 

and bioturbated siltstone and mudstone. The fining-upward 

intervals coarsen upward into an uppermost 200-m-thick unit 

that consists of amalgamated channels of medium- to coarsegrained sandstone (fig. 7B).

Detailed paleocurrent studies were not possible because 

of the scarcity of sedimentary structures. However, the axes 

of large-scale (3-6 meter thick and tens of meters wide) channels in the upper unit generally trend northeast-southwest, 

suggesting a southwest-facing paleoslope.

These strata are interpreted as a progradational delta sequence based on the coarsening- and thickening-upward nature of the sequence, the lateral continuity of the sandstone 

beds in the lower and middle units, the uniformity of orientation of channel axes in the upper unit, the abundance of 

bioturbation in both mudstone and the sandstone, and the high 

mudstone/sandstone ratio (Coleman and Prior, 1980). Strata 

of the deltaic facies lie conformably between nonmarine sandstone and shale of the Sespe Formation below and shelf and 

slope turbidites of the Rincon Shale above, making a deltaic 

interpretation stratigraphically reasonable. It is possible sea 

level rise resulted in the abrupt submergence of the prograding 

delta at the end of Vaqueros Formation deposition. The inaccessibility of the steep cliff outcrops, combined with the lack 

of outcrops through large sections of the lower portion, prevent more detailed sedimentological interpretations.

PALEOCURRENTS

Paleocurrent data obtained from Vaqueros Formation 

outcrops are summarized in figure 5. From west to east 

(present-day orientations) along the coastal outcrop strip, Vaqueros paleocurrent directions rotate from generally southeast 

directed to south directed to bidirectional with northeastsouthwest paleoflow indicators. Farther to the east, work by 

Edwards (1971) and by Lane (1987) indicates southwesterly 

flow directions in shoreline and inner shelf sandstones of the 

Vaqueros. The Vaqueros outcrops in the Gibraltar Reservoir 

area suggest south- to southwest-directed paleocurrents. 

Paleocurrent indicators in Sespe Formation strata to the east 

of the study area (in the Santa Monica Mountains and near 

Ventura) that are age equivalent to the Vaqueros record westerly flow directions, as do indicators in the older Alegria Formation and Sespe strata that underlie the Vaqueros throughout 

the central coastal regions of the study area (Howard, 1988).

The diversity of paleoflow directions, when viewed in 

conjunction with the facies tracts discussed above and the sedimentary petrology discussed below, suggests a complex physiography during late Oligocene and early Miocene time in the 

western Transverse Range province.

PROVENANCE

Differences in sandstone and conglomerate composition 

between Vaqueros Formation strata west of Refugio Canyon 

and Vaqueros strata east of Refugio Canyon (and the underlying Alegria Formation sandstones) serve to reinforce interpretations of stratigraphic and paleocurrent relationships. The 

nature of these differences implies regional sedimentological 

and tectonic significance.

Representative samples from Vaqueros Formation sandstones and conglomerates throughout the study area and from 

underlying Alegria Formation sandstones were examined for 

variations in grain and clast composition. Conglomerate clasts 

were identified via visual examination in the field by examining freshly broken clast surfaces with the aid of a hand lens. 

Visual estimates of dominant clast types and percentages were 

made at each outcrop locality. Approximately 100 thin sections from Alegria and Vaqueros sandstones were examined 

to measure variations in framework mineralogy. Representative thin sections from each lithofacies and locality were point 

counted using the Gazzi-Dickinson point-counting method 

(Ingersoll and others, 1984).

Data on the framework mineralogy of point counted sandstones were normalized and are displayed in two ternary diagrams (fig. 8) emphasizing source area and compositional 

maturity. Both the QFLt and the QtFL plot show a significant 

difference between the composition of Vaqueros Formation 

sandstones west of Refugio Canyon and the other sandstones 

(Rigsby, 1988a). This difference, combined with field estimates of conglomerate composition and with hand lens and 

binocular microscope examination of limited well cuttings, 

allow the definition of two petrofacies: a coarse-grained, lithicrich petrofacies (petrofacies 1) and a sandy, micaceous, 

quartzofeldspathic petrofacies (petrofacies 2).

Petrofacies 1

Petrofacies 1 consists of coarse-to medium-grained lithicrich sandstone and pebble- to boulder-conglomerate that outcrops in the southern Santa Ynez Mountains west of Refugio 

Canyon and is present in well cuttings in the area directly offshore and to the west of Canada de la Pila. As indicated on 

the ternary diagrams, the sandstones in this area (the fan-delta 

and nearshore strata and the westernmost portion of the innershelf strata) are more lithic rich than those east of Refugio 

Canyon (the deltaic strata) and Vaqueros Formation strata in 

the Santa Monica Mountains; they are also more lithic rich 

than sandstones of the underlying Alegria Formation. Total 

lithics in the western sandstones average 36 percent. Predominant grain types are chert and graywacke fragments (81 perPaleography of the Western Transverse Range Province, California T11
Figure 7. Vaqueros Formation strata of the deltaic facies tract in the Gibraltar Reservoir area. 

T12 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
cent). Conglomerate clast compositions in this petrofacies 

mirror those of the lithic grains. The dominant clast types are 

buff to brown, well-indurated graywacke and red, black, and 

green (locally radiolarian-bearing) chert. Subordinate amounts 

of dark gray to black, fine-grained volcanic and metavolcanic 

clasts are also present.

The abundance of radiolarian-bearing chert and of 

graywacke, as well as the occurrence of volcanics and 

metavolcanics, suggest a Franciscan provenance for this 

petrofacies. Extensive outcrops of Franciscan assemblage 

rocks (also called the Fransican Complex by some workers) 

around the perimeter of the Santa Maria basin to the north 

(the paleo-upslope direction) is consistent with a Franciscan 

provenance. Neither of these criteria, however, unequivocally 

confirms such a provenance. The Jurassic Coast Range 

ophiolite (Hopson and others, 1981), which outcrops locally 

within and around the Santa Maria basin (although not directly in line with measured paleocurrents), also contains radiolarian chert and, as such, may have been a source for the 

chert in this petrofacies. Likewise, the Upper Jurrasic and 

Lower Cretaceous Espada Formation (Dibblee, 1950), which 

outcrops locally in the Santa Ynez Mountains may have contributed fragments of lithic-rich wacke as well as chert reworked from beds of chert-pebble conglomerate. Hence, cherts 

could be derived from the Cretaceous Franciscan melange, 

from the Point Sal ophiolite (Hopson and Franco, 1977) and 

correlative units, or from the Espada Formation chert-pebble 

conglomerates.

Petrofacies 2

Petrofacies 2 consists of fine-grained quartzofeldspathic 

sandstone of the Vaqueros Formation that outcrops east of

Refugio Canyon and in the northern portion of the study 

area (in the deltaic strata), and that occurs in offshore wells 

east of Refugio Canyon and at Hondo and Capitan fields (in 

inner-shelf strata). This sandstone is identical in framework 

grain composition to the underlying sandstones of the Alegria 

Formation. Unlike petrofacies 1, sandstones of petrofacies 

2 average only 25 percent lithic fragments. These lithics are 

evenly divided between volcanic (primarily microlitic) and 

sedimentary rock fragments. If rock fragments containing 

sand-sized mineral grains (and thus counted as the mineral 

grain) are included in the total rock fragment count for all 

samples, the difference in lithic content between the two 

petrofacies becomes even greater. In addition, Vaqueros 

sandstones east of Refugio Canyon and in the Gibraltar 

Reservoir area commonly contain granitic rock fragments that 

are not present in sandstones west of Refugio Canyon.

Conglomerate is rare in these eastern strata. Where it 

does occur (predominantly as thin lags in channel bases), 

the clasts are dominantly locally dervied, fine-grained 

quartzofeldspathic sandstone, mudstone (usually rip-ups), 

and rare fine-grained volcanics.

The quartzofeldspathic composition of these sandstones and the occurrence of granitic lithics suggest a granitic source for this petrofacies. Granitic rocks crop out 

extensively in the southern Sierra Nevada to the east and 

are the dominant source rocks for many older Tertiary units 

(Edwards, 1971; Lane, 1987; Howard, 1988). The dominant southwest and west paleocurrent directions recorded 

in the deltaic strata near Gibraltar Reservoir suggest a similar source for the Vaqueros Formation in that area. The 

variation in framework grain composition within the 

petrofacies 2 inner shelf strata, especially as documented 

for Vaqueros strata by Rigsby and Schwartz (1990), suggests that the Vaqueros inner shelf strata were derived, at

0 EXPLANATION

Vaqueros Fm. sandstones 

east of Refugio Canyon 

(this study)

Vaqueros Fm. sandstones 

Santa Monica Mountains

Vaqueros Fm. sandstones 

west of Refugio Canyon

Ot

Vaqueros Fm. sandstones, 

eastern Santa Ynez Range

Lt

Figure 8. Sandstone QFLt and QtFL triangular diagrams showing two Vaqueros Formation petrofacies.

Paleography of the Western Transverse Range Province, California T13
least in part, from the reworking of older shelf material. If 

this is indeed the case, the lateral variability of framework 

grain composition seen in petrofacies 2 strata may be 

controlled primarily by local variations in the rate of sediment supply that is, by the amount of "dilution" of relict 

sediments by new, extrabasinal sediments (Rigsby and 

Schwartz, 1990).

DISCUSSION

Regional Paleogeography

East-directed paleocurrents in nearshore deposits in the 

westernmost Santa Ynez Mountains along with dual 

petrofacies in Vaqueros Formation sandstones and conglomerates indicate that the geographic and geologic setting of 

Vaqueros deposition was complex. A world-wide sea level 

lowstand during the late Oligocene (fig. 3) resulted in the 

widespread deposition of fluvial-alluvial Sespe Formation 

strata (Howard, 1988, 1995) and in the initiation of deposition of the shallow-marine Vaqueros Formation. As sea level 

subsequently rose, strata of the Vaqueros Formation transgressed over the former fluvial-alluvial depositional surface. Facies differences in the Vaqueros reflect variations 

in local topographic relief of that surface.

A model for the paleogeography of the western Transverse Ranges province during Vaqueros Formation deposition is depicted in figure 9. The base map for this 

reconstruction is the palinspastic map of southern California proposed by Hornafius (1985). This map incorporates 

all of the paleomagnetic data for the region and has been 

used as a paleogeographic base map by Howard (1988) in 

work on the Sespe Formation and by Lane (1987) in work 

on the Vaqueros Formation in the Santa Monica Mountains. 

The distribution of Vaqueros sedimentary environments in 

the western Transverse Ranges and data from the previous 

studies referenced above require a paleogeographic model 

that includes both continentward and oceanward sides of 

the forearc basin. Hence, the data presented here serve as 

geologic confirmation of the paleomagnetically derived tectonic reconstruction used as the base map in figure 9.

All of the paleocurrents from this study, and from the 

studies by Lane (1987) and by Howard (1988), with 

the exception of paleocurrents from the Gibraltar Reservoir area, were rotated 90° counterclockwise as required 

by the paleomagnetic data. Rotation is unnecessary for 

the Gibraltar Reservoir paleocurrents because the area is 

north of the boundary of tectonic rotation (north of the Santa 

Ynez fault) and has experienced no significant rotation 

(Hornafius, 1985).

The Vaqueros Formation outcrops located south (modern position) of the Santa Ynez fault are all situated on the 

oceanward side of the Oligocene and early Miocene forearc 

basin. The fan-delta and shoreface-beach deposits delineate the Vaqueros shoreline position along the offshore

emergent high. Rotated paleocurrent directions indicate a 

source area to the west and an east-facing paleoslope, 

confirming the existence of the emergent high to the west. 

The nature of the uplifted landmass is unknown, but 

Hornafius (1985) suggested that it may have been similar 

to islands formed by emergent portions of the trench-slope 

break near northern Sumatra. The position of the western 

trench-facing shoreline in figure 9 is based on his model.

The inner-shelf deposits are, in part, petrographically 

similar to petrofacies 1 and exhibit rotated paleocurrents with 

northeast-southwest orientations. As sand wave deposits 

(whether or not tidally influenced), these strata suggest that 

the shoreline was even farther to the west in this area within 

what is now Santa Maria basin.

Deepwater Vaqueros Formation deposits in the western 

Santa Monica Mountains (represented by the westernmost open 

arrow near the Santa Monica Mountains in fig. 9) were situated on the oceanward side of the forearc basin but closer to 

the basin axis than fan-delta, nearshore, and inner-shelf facies 

tracts (facies tracts 1, 2, and 3). These deeper water deposits 

record east-directed paleocurrents (fig. 9) and were probably 

derived from the same landmass as facies tracts 1, 2, and 3 

(Lane, 1987).

The deltaic deposits in the Gibraltar Reservoir area delineate a position for the mainland shoreline in the northern 

part of the study area that is approximately 100 km southwest 

of the shoreline modeled by Hornafius (1985) on the basis of 

regional stratigraphic relationships. The southwest-directed 

paleoslopes recorded in the Gibraltar Reservoir strata do, however, confirm the orientation of his predicted shoreline. Because of the thick, prograding nature of the delta deposits, the 

shoreline on the paleogeographic map is placed at a nonspecific point within the outcrop belt. The placement of the southern continuation of the mainland shoreline is based on work 

by Lane (1987), which in turn is based on the eastward pinchout' 

of shallow-marine Vaqueros Formation deposits and on the 

interfingering relations with nonmarine strata of the Sespe 

Formation.

Stratigraphic Implications

The paleogeographic scheme illustrated in figure 9 explains the variations in the nature of the Vaqueros Formation 

lower contact. Nearshore deposits, which conformably overlie the Sespe Formation in the eastern (present-day orientation) Santa Monica Mountains, were situated on the mainland and represent a west-facing shoreline (east-flowing 

paleocurrents); shelf deposits (Rigsby, 1988a, 1989a,b) in the 

central part of the study area accumulated on the oceanward 

side of the forearc basin; and fan-delta and shoreface-beach 

sequences in the western study area were deposited on the 

eroded edge of a locally emergent accretionary wedge. The 

regional variations in the nature of the contact are the result of 

late Oligocene to early Miocene eustatic sea level rise, regional 

tectonic setting, and contemporaneous tectonic activity assoT14 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
LATE OLIGOCENE & EARLY MIOCENE 

PALEOGEOGRAPHY OF SOUTHERN CALIFORNIA

Paleocurrents |»*+* + 1 Fan delta conglomerates

Shoreline sandstones 

anc" conglomerates

Inner shelf sandstones

Deltaic sandstones and 

siltstones

Paleoshoreline

(teeth on seaward side)

160

(Modified alter Hornafius, 1985)

Figure 9. Palinspastic paleogeographic map of the western Transverse Ranges province during deposition of the late Oligocene 

and early Miocene Vaqueros Formation.

Paleography of the Western Transverse Range Province, California T15
elated with the final stages of subduction of the Farallon plate 

beneath North America (Hornafius, 1985; Rigsby, 1989a).

Tectonic Implications

Any sedimentological or structural model for the history 

of the western Transverse Ranges must consider the implications of the approach and ultimate subduction of the East Pacific Rise beneath the North America continental margin at a 

location very near the latitude of the Transverse Ranges province (Atwater, 1970; Atwater and Molnar, 1973; Dickinson 

andSnyder, 1979a,b; Engebretson and others, 1985). Although 

the exact latitude of the initial contact between the Rise and 

the North American plate is not well constrained, various workers (Christensen and Lipman, 1972; At water and Molnar, 1973; 

Dickinson and Snyder, 1979a,b; Engebretson and others, 1985; 

Molnar and Stock, 1985) have suggested that the first contact 

took place at a latitude somewhere between the Transverse 

Ranges and northern Baja California. Many of the geologic 

complexities in the western Transverse Ranges may be accounted for, however, if the East Pacific Rise first made contact at the latitude of the Transverse Ranges. The East Pacific 

Rise was close to the continental margin by about 38 Ma and 

first achieved contact about 29 Ma (Atwater, 1970; Engebretson 

and others, 1985). As a topographic consequence of ridge 

subduction, DeLong and Fox (1977) predict a gradual shoaling culminating in the subaerial exposure of the forearc basin 

followed by gradual basin submergence. The late Eocene and 

Oligocene sedimentary sequence in the western Transverse 

Ranges records a similar sequence of events (Howard, 1988, 

1995): sedimentary offlap in the lower part of the Sespe Formation records a late Eocene gradual shoaling, a basin-wide 

early Oligocene unconformity in the Sespe records a period 

of subaerial exposure, and backfilling of the Sespe basin 

records the gradual submergence of the forearc basin.

Hornafius (1985) suggested the existence of an emergent 

section of the early Tertiary trench-slope break in the location 

of the present-day Santa Maria basin. The Vaqueros Formation fan-delta facies in the westernmost Santa Ynez Mountains (Rigsby, 1994b) confirms the existence of such a high 

within the forearc basin as late as 24+1 Ma. The existence of 

shelf deposits in conformable contact above the underlying 

fan-delta slope deposits (Rigsby, 1989a,b) and the drowning 

of the shelf-sand wave deposits with slope deposits of the 

Rincon Formation (at Canada de la Pila) is a record of the 

rapid submergence of this high and, thus, of the complete submergence of the early Tertiary forearc basin system.

As noted by Howard (1988, 1995), although the SespeVaqueros-Rincon series of depositional events is remarkably 

similar to that predicted as a consequence of ridgetransform subduction, the timing coincides with several globally-recognized eustatic sea level drops (and subsequent rises; 

fig. 3); hence, these depositional events may not have been 

solely the result of plate tectonics. Indeed, the deepeningupward fan-delta sequence in the Hollister Ranch area (Rigsby, 

1988b; 1989a,b; 1994a) suggests that although initial deposition was in response to uplift of the pal eo-trench-slope break, 

sea level rise also may have played an important role in deposition. However, the existence of nearly contemporaneous 

shallowing- and deepening-upward sequences and of 

paleobathymetric subsidence that exceeded eustatic sea level 

fluctuation (Pinkerton and Rigsby, 1991) indicate that the 

eustatic sea level changes in the area were overprinted by local and regional tectonics.

The Vaqueros Formation paleogeography as presented here is a reflection of a complex, tectonically influenced physiography. The overprint of sea level fluctuations 

on that physiography is recorded in both the large-scale, regional, deepening-upward sequence (from nonmarine Sespe 

Formation deposits through deep-marine Monterey Formation 

deposits) and in the remarkable variation in vertical sequences 

found within the facies of the Vaqueros Formation.

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T18 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
Chapter U

Stratigraphy of the Fine-Grained Facies of the Sisquoc 

Formation, Santa Maria Basin, California 

Paleoceanographic and Tectonic Implications

By PEDRO C. RAMIREZ and ROBERT E. GARRISON

U.S. GEOLOGICAL SURVEY BULLETIN 1995-U

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

Edited by Margaret A. Keller
CONTENTS

Abstract Ul

Introduction Ul

Lithologies, composition, sedimentary structures and depositional environments U2

Stratigraphy U4

Paleoceanographic and tectonic controls on Sisquoc sedimentation U9

Summary U13

References Cited U13

FIGURES

1. Index map showing the fault-bounded Santa Maria basin and surface sections 

studied U2

2. Typical exposure of the laminated Sisquoc formation in the Pedernales area U3

3. Mean abundances of detritus and biogenic silica in both the Monterey and Sisquoc 

Formations, A, and Mean accumulation rates of silica and detritus in the Monterey 

and Sisquoc Formations, B. U4

4. Diatom zones and species ranges used for this study U5

5. Key to lithologic symbols used in figures 6, 7, 10, and 11 U6

6. Chronostratigraphic diagram and characteristic lithologies of the sections 

examined U6

7. Correlation chart of the sections examined U7

8. Nodular phosphatic conglomerate marking the contact between the Monterey and 

Sisquoc Formations, Mussel Rock section U8

9. Sedimentation rates for the measured sections U9

10. Eustatic sea level curve plotted with sections examined Ull

11. Chronostratigraphic diagram of the Lompoc Hills, Lompoc Quarry and S weeney 

Road sections U12
Stratigraphy of the Fine-Grained Facies of the Sisquoc 

Formation, Santa Maria Basin, California 

Paleoceanographic and Tectonic Implications

By Pedro C. Ramirez 1 and Robert E. Garrison"

Abstract

A detailed stratigraphic analysis of five measured sections (Casmalia, Mussel Rock, Sweeney Road, Lompoc 

Hills, and Pedernales) and of two sections (Lompoc Quarry 

and Harris Grade) previously studied of the fine-grained, 

mostly slope-deposited upper Miocene and lower Pliocene 

Sisquoc Formation provide the framework for correlating 

lithologic changes with paleoceanographic and tectonic 

events. In the Santa Maria basin, high-resolution diatom 

biostratigraphy indicates that the Sisquoc Formation examined in the measured sections, which predominantly 

consists of alternating laminated and massive diatomaceous lithologies and lesser amounts of phosphatic and 

chert-rich conglomerates, was deposited between approximately 6.0 and 4.8 Ma. In some areas of the basin (for 

example, Mussel Rock and the Lompoc Quarry), an erosional surface overlain by a phosphatic conglomerate 

marks the contact between the Sisquoc and the underlying Monterey Formation. The basal Sisquoc conglomerate in the Mussel Rock section may be related to a 6.3 Ma 

sea level drop and probably represents the erosion of the 

basin margin. The deposition of diatomaceous sediment 

continued uninterrupted in deeper areas (Casmalia, 

Sweeney Road, Pedernales, Lompoc Hills, and Harris 

Grade) of the basin. Prolific diatom sedimentation in the 

Sisquoc Formation in the Santa Maria basin is probably 

related to this sea level drop, as biologic productivity is 

hypothesized to have increased owing to intensified oceanic circulation and coastal upwelling. Furthermore, the 

flux of detritus (mostly clays) increased as more sediment 

was transported across the exposed narrow borderland 

shelves and into the basin during this sea level drop. Superimposed on major sea level changes and productivity 

variations were smaller scale variations in sea level and 

(or) productivity, generating the alternating massive and 

laminated units in the Sisquoc.

Opal-CT and quartz chert clasts in conglomerates and 

phosphatic conglomerates, which occur in the upper part

1 Department of Geological Sciences, California State University, 

Los Angeles, CA 90032.

2 Department of Marine Sciences, University of California, Santa 

Cruz, CA 95064.

Manuscript approved for publication December 4, 1997.

of the Sisquoc in the Mussel Rock section, probably resulted from rapid uplift of basin margins. Compositional 

variations in the clasts of these conglomerates record the 

de-roofing of Monterey-type lithologies (opal-CT and 

quartz cherts) to levels that were probably not attainable 

by sea level drops alone. Additionally, these upper Sisquoc 

conglomerates were deposited from 5.3 to 5.0 Ma, or 

during a rise in eustatic sea level, thereby supporting tectonic activity.

Relatively high detrital contents in the Sisquoc may also 

be related to erosion associated with this episode of tectonic activity in the Santa Maria basin. This interpretation is 

in accord with a number of previously published studies 

that indicate that the late Miocene and early Pliocene was a 

tectonically active period in the California borderland. The 

stratigraphy and sedimentological characteristics of the 

Sisquoc Formation, therefore, are the product of the complex interaction of paleoceanography and tectonic activity.

INTRODUCTION

This study refines the late Miocene and early Pliocene 

stratigraphy of the Santa Maria basin and through correlations across the basin provides the framework for documenting through time changes in oceanographic and 

tectonic conditions. Additionally, this refined stratigraphy, 

which suggests that the Sisquoc Formation studied is late 

Miocene and early Pliocene in age (see discussion in section below on Stratigraphy, Ramirez (1990), and Barren 

and Ramirez (1992)), sets the foundation for future studies of upper Miocene and lower Pliocene rocks in the Santa 

Maria basin. Stratigraphic correlations for this study are 

based on diatom biostratigraphy. Barron (1976, 1981, 

1986), Barron and Ramirez (1992), Dumont (1984, 1986, 

1989), and Dumont and Barron (1995) provide the biostratigraphic foundation for this study.

Canfield (1939), Woodring, Bramlette, and Lohman 

(1943), and Woodring and Bramlette (1950) established 

much of the original stratigraphic framework for the Santa 

Maria area. These studies reveal that the Sisquoc represents a portion of the Neogene succession infilling the Santa

Stratigraphy of the Fine-Grained Facies of the Sisquoc Formation, Santa Maria Basin, California U1
Maria basin. Initial basin subsidence probably began in 

the early Miocene with the deposition of the Lospe Formation (Stanley and others, 1996) and basin-infilling culminated with the deposition of the Plio-Pleistocene Careaga 

Sandstone. The Sisquoc reflects the commencement of a 

late Miocene basin-infilling phase. The underlying 

Monterey Formation records maximum basin subsidence, 

whereas the overlying Foxen Formation generally represents the continuation of basin-infilling, following initial 

but local tectonic subsidence and paleobathymetric deepening (Behl and Ingle, this volume).

Sisquoc Formation exposures occur along the central 

California coastal area and, more specifically, throughout 

the Santa Maria and Santa Barbara basins. In the Santa 

Maria basin (fig. 1), the Sisquoc consists of a more proximal, loosely consolidated sandstone facies and a deepmarine predominantly mudstone facies. Originally, the 

Sisquoc was named for its sandstone facies, which is exposed in the eastern Santa Maria basin (Porter, 1932). However, Porter also determined that fine-grained rocks in the 

basin, which were previously referred to as the Monterey 

Formation, were, in part, correlative with the sandstone 

and, therefore, part of the Sisquoc Formation. Woodring, 

Bramlette, and Lohman (1943) and Woodring and 

Bramlette (1950) felt that the more extensively exposed 

fine-grained facies, which is the focus of this study, was 

more representative of the Sisquoc and relegated the sandstone to member rank (Tinequaic Sandstone Member). 

These authors also noted lithologic variations in the fineFigure 1 . Index map depicting the fault-bounded Santa Maria basin 

and measured surface sections (darkened circles), as well as other 

sections (darkened rectangles) studied. Also shown is a portion of the 

Santa Barbara basin. SMRF, Santa Maria River Fault; FCF, Foxen Canyon 

Fault; LPF, Little Pine Fault; SF, Solvang Fault; SYF, Santa Ynez Fault; 

and HF, Hondo Fault (faults depicted are from Hall, (1978).

grained facies of the Sisquoc. Woodring, Bramlette, and 

Lohman (1943) and Woodring and Bramlette (1950) identified a typically dark, relatively dense, massive to indistinctly laminated and relatively diatom-poor mudstone and 

designated it the Todos Santos Claystone Member.

Recent efforts have been made to sedimentologically 

characterize the Sisquoc Formation and to determine its 

depositional environment. Woodring and Bramlette (1950) 

described the Sisquoc and speculated that it was deposited 

at moderate water depths and below active wave base. Ingle 

(1981b) determined that the Sisquoc in the Santa Ynez 

Mountains was probably deposited at upper bathyal depths. 

Isaacs (1980,1983, 1985) calculated silica and detritus accumulation rates in the Monterey and Sisquoc Formations 

and concluded that the Sisquoc was deposited in a marine 

environment where the accumulation rates of silica and 

detritus exceeded those of the Monterey. Henderson (1983) 

and Henderson and Ramirez (1990) indicate that Sisquoc 

deposition probably occurred on a slope. Ramirez (1987, 

1990), on the basis of detailed sedimentologic analysis, 

also concluded that much of the Sisquoc was deposited on 

the slope. This report represents an extension of the aforementioned studies.

Five surface sections (Mussel Rock, Pedernales, 

Casmalia, Sweeney Road, and Lompoc Hills) containing 

the fine-grained Sisquoc and some of the underlying 

Monterey Formation were measured and examined in detail for this study (fig. 1). Data from the Lompoc Quarry 

and Harris grade sections, which were studied by Barron 

(1975), Dumont (1986), and Barron and Ramirez (1992) 

are also incorporated in this study. The freshly washed, 

coastal exposures of the Mussel Rock and Pedernales sections reveal well-preserved, small-scale sedimentary structures and a variety of trace fossils, both of which aid the 

interpretations of depositional processes and environments 

(fig. 2). In contrast, intense weathering of the inland 

Casmalia, Sweeney Road, and Lompoc Hills sections results in bleached outcrops and the masking of sedimentary 

structures, as well as any subtle lithologic variations that 

might occur in the rocks. Consequently, the sedimentary 

structures and lithologic variations in the two coastal sections served as guides for the identification of structures 

and lithologic variations in the noncoastal sections.

LITHOLOGIES, COMPOSITION, 

SEDIMENTARY STRUCTURES, AND 

DEPOSITIONAL ENVIRONMENTS

The distal facies of the Sisquoc Formation, like the 

underlying siliceous facies of the Monterey Formation, consists largely of varying proportions of diatoms and clays 

and lesser amounts of silt, sand, and sponge spicules, as 

well as scarce foraminifers. Variations in the amounts of 

diatoms and clays largely control the variable lithologies

U2 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
in the Sisquoc. These lithologies range from a nearly pure 

mudstone to diatomite, with diatomaceous mudstone dominating in the measured sections. Diagenetic alteration of 

diatomaceous strata brought about by increased temperatures and pressures associated with burial (Isaacs, 1980, 

198la) produced siliceous mudstone, porcellanite, and 

minor chert (see Isaacs (1981b) and Ramirez (1990) for 

definitions of these lithologic terms). Strata modified by 

silica diagenesis are generally restricted to the Monterey 

and the basal Sisquoc in the sections examined. Also present 

in the Sisquoc Formation are relatively minor amounts of 

stratiform and nodular dolomite and some sedimentologically 

important phosphatic and chert-rich conglomerates.

A conspicuous feature of the Sisquoc and the upper 

Monterey Formations in the Santa Maria basin is the alternation of massive and laminated beds. Generally, the 

Sisquoc contains a greater proportion of massive to laminated units compared to the Monterey (Ramirez, 1990). 

Massive units in the Sisquoc, which vary from two to tens 

of meters thick, are not entirely structureless but usually 

exhibit a bioturbated fabric, which in places consists of 

excellently preserved trace fossils. The two to tens of 

meters thick laminated units in the Sisquoc Formation consist mostly of regularly alternating planar light-colored, 1 

to 2 mm thick, diatom-enriched layers and thicker (up to 2 

cm), darker layers usually composed of clay and lesser 

amounts of silt and diatoms.

Utilizing a variety of analytical methods, Ramirez 

(1990) showed that, compositionally, the Sisquoc is enriched in detritus (mostly clays) relative to biogenic silica 

(mostly diatoms) (fig. 3A). Comparatively, the underlying 

Monterey contains more silica and less detritus than the 

Sisquoc. Possible interpretations for these compositional 

trends from the Monterey into the Sisquoc include increased detritus entering the Santa Maria basin during 

Sisquoc deposition, decreased flux of silica during the 

deposition of the Sisquoc, or a combination of the two. 

Another possibility is that silica sedimentation increased 

during Sisquoc deposition but was masked by detritus accumulation that also accelerated during this time (fig. 3A). 

Ramirez (1990) calculated rates of silica and detritus accumulation in the Sisquoc and found that accumulation 

rates of detritus generally exceeded those of silica during 

the deposition of the Sisquoc (fig. 35). Figure 3B also 

shows that rates of silica and detritus accumulation in the 

Sisquoc exceed those of the underlying Monterey Formation in the Santa Maria basin. Isaacs (1983, 1984, 1985) 

reached a similar conclusion in her study of accumulation 

rates in the Monterey and Sisquoc Formations in the Santa 

Barbara coastal area. Therefore, lithological differences 

between the Monterey and the Sisquoc in the central California coastal area reflect changes in the rates of silica and 

detritus accumulation. As documented by Ingle (198la, 

198Ib), Isaacs (1983, 1984), Barron (1986), and Ramirez

Figure 2. Closeup view of typical Sisquoc Formation exposure in the Pedernales area. Light-colored laminae 

are composed almost entirely of diatoms and darker-colored ones are composed mostly of clays with some 

silt and diatoms. Note the small-scale intraformational breccia. Marker at base of the photograph is 

approximately 1 cm wide.

Stratigraphy of the Fine-Grained Facies of the Sisquoc Formation, Santa Maria Basin, California L8
(1990), the abundance of diatoms in the uppermost portions of the Monterey Formation and in the Sisquoc Formation reflect deposition beneath a highly productive late 

Miocene and early Pliocene upwelling system.

The absence or scarcity of environmentally diagnostic benthonic taxa, such as foraminifers or mollusks, and 

the fine-grained nature of the strata make it difficult to 

accurately determine the depositional settings of this facies of the Sisquoc. However, based on the presence of 

small-scale sedimentary structures, such as slump folds, 

intraformational breccias (see fig. 2), and angular 

discontinuities, Ramirez (1990) concluded that the depo100

(/) 

O 

a.

5 

O 

O

80

60

40

20

56

41

80

W 70 UJ u

< 60

oc . 

z^- 50 il 40

ii 30

O 20 

O

< 10

Sisquoc Monterey 

Formation Formation

B

36

| \ DETRITUS

HI SILICA

Sisquoc 

Formation

Monterey 

Formation

Figure 3. (A) Mean abundances of detritus and biogenic silica in 

both the Monterey and Sisquoc Formations in the Santa Maria 

basin. A total of 1 78 samples from the Sisquoc Formation and 

27 from the Monterey Formation were analyzed for major oxides; 

this analysis was then used to calculate mean abundances of 

detritus, silica, calcite, and dolomite using conversion factors 

developed by Isaacs (1980). Calcite, dolomite, and, apatite are 

relatively minor constituents of these rocks, and their abundances 

are not presented here. See Isaacs (1980) and Ramirez (1990) 

for additional information on the analytical techniques employed. 

Numbers in individual bars in graph represent relative percents. 

(B) A comparison of mean accumulation rates of silica and 

detritus in the Monterey and Sisquoc Formations of the Santa 

Maria basin. Isaacs (1985) and Ramirez (1990) present 

discussions of the methods used to calculate accumulation rates 

in fine-grained siliceous strata. Numbers in individual bars in 

graph represent relative accumulation rates. From Ramirez, 1990.

sition of much of the fine-grained facies of the Sisquoc 

occurred in a slope to upper slope-shelf-edge environment. 

Henderson (1983), also concluded that Sisquoc deposition 

probably occurred in a slope environment. The sedimentary structures mentioned likely reflect instability and sediment reworking within the slope environment.

STRATIGRAPHY

To better understand how sedimentologic changes occurred through time and to determine whether these 

changes relate to known late Miocene and early Pliocene 

oceanographic events, we developed a detailed biostratigraphic framework for the late Miocene rocks discussed 

in this study. Because foraminifers and molluscs in outcrop are for the most part either absent, scarce, and (or) 

poorly preserved in the fine-grained Sisquoc, diatoms became the logical choice for dating Sisquoc lithologies. 

Recent refinements of diatom zonations for the central 

California coastal area yield age resolutions of 250,000 to 

450,000 years for the latest Miocene and about 250,000 

for the earliest Pliocene (see Dumont, 1986; Dumont, 1989; 

Ramirez, 1990; and Barron and Ramirez, 1992). Figure 4 

shows the central California diatom zonation used for upper Miocene and Pliocene strata in the Santa Maria basin. 

The age resolution plus the datum points provided by the 

late Miocene and early Pliocene diatom zones of central 

California allow accurate and detailed biostratigraphic 

correlations of Sisquoc strata and refined calculations of 

bulk and component accumulation rates. The diatom biostratigraphy upon which much of this work is based is found 

in Barron and Ramirez (1992) and Dumont and Barron 

(1995), who use Berggren, Kent, and Van Couvering's 

(1985) geologic time scale.

Figure 5 is the key to lithologic symbols used in figures 6 and 1. Figure 6 is a chronostratigraphic diagram of 

our measured sections, and figure 1 shows the results of 

our correlations. Note that in the Lompoc Hills, Pedernales, 

and Sweeney Road sections the Monterey grades into the 

Sisquoc, while in the Mussel Rock section an erosional 

surface exists between the two formations. The contact 

between the Monterey and Sisquoc Formations has been 

the source of considerable discussion (see review of criteria used in distinguishing the Monterey from the Sisquoc 

in Ramirez, 1990, and also in Dumont and Barron, 1995). 

In the Mussel Rock section, a phosphatic conglomerate 

overlies the erosional surface present and clearly marks 

the formational boundary between the Monterey and the 

Sisquoc (fig. 8). Diatoms extracted from the uppermost 

Monterey and the lowermost Sisquoc in the Mussel Rock 

section indicate that, according to Dumont's (1986) and 

Dumont and Barren's (1995) proposed diatom zonations, 

the top of the Monterey coincides with the top of the Rouxia 

californica zone and the base of the Thalassiosira

U4 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
miocenica-Nitzschia miocenica zone. The boundary between these two diatom zones falls at about 6.0 Ma 

(Dumont, 1986; Dumont and Barren, 1995). Similarly, at 

the Lompoc Quarry (see fig. 1 for location), where a phosphatic conglomerate also overlies an erosional contact between the Monterey and the Sisquoc, the top of the 

Monterey is dated at about 6.0 Ma (Barren and Ramirez, 

1992). Based on extrapolated age-thickness plots, the 

Monterey-Sisquoc contact is also dated at about 6.0 Ma in 

the Casmalia section (Ramirez, 1990). Although an 

unconformity is absent, a lithologic change is present at 

the Monterey-Sisquoc contact in the Casmalia section. This 

change, which is visible in outcrop and which is confirmed 

by whole rock chemical analysis, is marked by laminated 

porcellanite of the Monterey Formation passing upward 

into a dense, dark, massive siliceous mudstone (Woodring,

Bramlette, and Lohman's (1943) Todos Santos Claystone 

Member of the Sisquoc Formation) that is enriched in detritus and depleted in silica relative to the Monterey Formation.

The above discussion indicates that the contact between the Monterey and the Sisquoc at Mussel Rock, the 

Lompoc Quarry, and in the Casmalia section falls at about 

6.0 Ma. In other sections, however, there is neither a sharp 

physical break nor a lithologic contrast between the two 

units (for example, Pedernales, Lompoc Hills, and Sweeney 

Road sections; fig. 6). Therefore, for the purpose of discussion and comparative analysis, in these sections at 

Lompoc Hills, Sweeney Road, and Pedernales, where the 

Monterey grades into the Sisquoc, we have chosen arbitrarily to locate the formational contact biostratigraphically 

at the boundary between the Rouxia californica and the

4.0-

45-

5.0-

 5.5-

-6.0-

 75-

8.0-

PALEOMAGNETIC 

EPOCHS

(0

i I

j= Magnetic c 

o polarity <

4A

DIATOM 

ZONATION

Barron (1986) 

Dumont (1986)

Thalassiosim

oestrupii

L praeoestrupii

T. hyalinopsis

T. miocenica/ 

N. miocenica

Rouxia 

californica

Thalassiosira 

antiqua

Denticulopsis 

hustedtii

7.6

Figure 4. Diatom zones and species ranges used in this study. Also depicted are the key 

diatom taxa and their ranges. Slightly modified from Barren and Ramirez (1992).

Stratigraphy of the Fine-Grained Facies of the Sisquoc Formation, Santa Maria Basin, California U5
EXPLANATION

LAMINATED DIATOMACEOUS MUDSTONE

MASSIVE TO DISCONTINUOUSLY LAMINATED 

DIATOMACEOUS MUDSTONE

MASSIVE SANDY/SILTY MUDSTONE 

PORCELLANITE

INTERBEDDED DIATOMACEOUS MUDSTONE 

AND PORCELLANITE

SILICEOUS MUDSTONE

CONGLOMERATE

NODULAR AND/OR PELLETAL PHOSPHORITE

SAND AND GRAVEL

STRATIFORM DOLOMITE

NODULAR DOLOMITE

GRADATIONAL TO UNCONFORMABLE 

FORMATION CONTACT

Figure 5. Key to lithologic symbols used in figures 6, 7, 10, and 11.

5.0 

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6.5 

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MUSSEL ROCK I _ t^>_.

PEDERNALES

LOMPOC HILLS

Figure 6. Chronostratigraphic diagram and characteristic lithologies of sections examined in this study (for section locations, see fig. 

1). Age dates from Dumont (1986) were used to construct the Sweeney Road column. Lithologic symbols are shown in figure 5.

U6 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
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Thalassiosira miocenica-Nitzschia miocenica diatom 

zones. This boundary correlates with the known age of the 

contact in those sections where it is unequivocal. While 

not a proper basis for defining formation contacts, the biostratigraphy provides an anchoring point for the late Miocene-early Pliocene stratigraphy which in turn forms the 

basis for our speculations on the changing oceanographic 

and tectonic conditions in the basin.

As with the Monterey-Sisquoc contact, preliminary 

work appears to indicate that the stratigraphic relation between the top of the Sisquoc and the base of the overlying 

Foxen Mudstone is also complex (see Behl and Ingle, this 

volume). The Casmalia and Harris Grade sections are the 

only sections studied that extend from the Sisquoc into the 

overlying Foxen Formation. In the Casmalia section, on 

the basis of extrapolated age-thickness plots, the SisquocFigure 8. Vertically dipping nodular, clast-supported phosphatic 

conglomerate marking the contact in the Mussel Rock section, 

between the underlying laminated diatomaceous mudstone of 

the upper part of the Monterey Formation to the right and the 

overlying massive, silty mudstones of the Sisquoc Formation. 

Note that this phosphatic conglomerate is graded, suggesting 

that it may have been deposited by turbidity or other bottom 

currents. Hammer shown for scale.

Foxen contact, which is covered, is dated at about 4.8 Ma. 

Barren and Ramirez (1992) also estimate that at Harris 

Grade the Sisquoc-Foxen contact lies at about 3.8 Ma. Behl 

and Ingle (this volume), relying on the work of Dumont 

and Barren (1995), indicate that the Sisquoc-Foxen contact in the Santa Maria basin may be as young as 3.8 Ma. 

Furthermore, Barren (1992) and Behl and Ingle (this volume) believe that the change from Sisquoc to Foxen deposition is probably related to a shift in relative motion 

between the Pacific and North American Plates at about 

that time. The incongruous ages for the Sisquoc-Foxen contact mentioned above suggest a diachronous formational 

contact. Published California Division of Oil and Gas 

(1974) maps show a gradational to unconformable SisquocFoxen contact; in the Casmalia area, subsurface data indicate that the contact is erosional. Perhaps the Sisquoc was 

eroded to deeper stratigraphic levels in the Casmalia area 

than in the Harris Grade area, thereby accounting for the 

older age of the Sisquoc near Casmalia. In the Santa Maria 

basin, therefore, it appears that the Sisquoc may extend 

from about 6.0 to about 3.8 Ma.

The termination of silica sedimentation in the Santa 

Maria basin does not correlate with the end of Sisquoc 

deposition. Behl and Ingle (this volume) report that siliceous mudstones ranging from 30 to 100 m thick occur 

locally at the base of the Foxen Formation. Woodring and 

Bramlette (1950) also observed local occurrences of siliceous lithologies at the base of the Foxen Formation.

Our detailed section measurements and correlations 

also reveal important features which illuminate sedimentation patterns in the Santa Maria basin and which allow 

us to speculate on the causes responsible for these sedimentation patterns. Conglomerates were deposited in the 

Mussel Rock area during Sisquoc accumulation, while concurrently, other portions (Pedernales, Casmalia, Sweeney 

Road) of the basin received the hemipelagic sediments most 

characteristic of the fine-grained facies of the Sisquoc (see 

figs. 6 and 7). Compositional differences that occur 

upsection in the clasts of the conglomerates in the Sisquoc 

Formation of the Mussel Rock section provide clues to tectonic activity in the Santa Maria basin. The lowermost 

major phosphatic conglomerate marking the MontereySisquoc boundary in the Mussel Rock section gives way 

upsection to conglomerates consisting of intermixed clasts 

of phosphate nodules, diatomaceous mudstone, dolomite, 

and minor chert. These conglomerates subsequently give 

way further upsection to conglomerates composed almost 

entirely of clasts of opal-CT and quartz chert.

Our section correlations also reveal a temporal and 

spatial relationship between the massive and laminated 

intervals of the Sisquoc Formation. Apparently, laminated 

units were deposited in some areas of the basin while contemporaneous massive units were deposited in other basin 

localities. For example, the base of the Sisquoc in the 

Sweeney Road area is mostly laminated while the base of

U8 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
the Sisquoc in the Casmalia section is massive. Moreover, 

the Sisquoc in the Pedernales section regularly alternates 

between massive and laminated units, whereas the Mussel 

Rock section is almost entirely massive. Associated with 

these laminated and massive units are distinct trace-fossil 

assemblages that hold important implications for 

paleoxygen levels in the Santa Maria basin (see Savrda 

and Bottjer, 1986, 1987a, 1987b, and Ramirez, 1990). 

Across the Monterey-Sisquoc contact in the Mussel Rock 

section, the relatively scarce trace-fossil assemblages 

change from one dominated by Planolites and Chondrites 

in the Monterey to a more abundant, diverse and largerbodied trace fossil assemblage (Chondrites, Planolites, 

Thalassinoides, and other unidentified trace fossils) in the 

Sisquoc (Ramirez, 1990). This change in trace fossil assemblages signifies an abrupt and sustained increase in 

oxygen levels and, as discussed in the succeeding section, 

probably results from changing tectonic and oceanographic 

conditions.

In contrast, the more regular alternation of laminated 

and massive intervals in the Sisquoc Formation at 

Pedernales records a change from mostly Chondrites trace 

fossils in laminated intervals to a combination of Chondrites, Planolites, and Thalassinoides in the massive intervals. These regular alternations in trace fossils, record 

(unlike the Mussel Rock section) a sustained period of regularly changing oxygen levels.

The duration or age span of the laminated and massive units in the Sisquoc is also variable. In the Pedernales 

section, the duration of a massive unit is a few thousand 

years, whereas in the Casmalia section, which records only 

a slightly higher sedimentation rate than the Pedernales 

section (fig. 9), the basal massive unit (including the covered zone that occurs within this massive interval) spans 

about 300,000 years. Moreover, in the Mussel Rock section, where sedimentation rates are the lowest of any section examined (fig. 9), the Sisquoc is completely massive 

over a stratigraphic interval representing about one million years. These types of spatial and temporal variations 

have not been documented in the underlying Monterey 

Formation. In fact, our study indicates that in comparison 

to the Monterey Formation, the onset of Sisquoc deposition was marked by more repeated oscillations in the oxygen minimum zone as evidenced by the interbedded 

massive and laminated units in the formation. Isaacs 

(1981c) also noted that the proportion of massive to laminated rocks increase in the Sisquoc of the Santa Barbara 

basin, in comparison to the underlying Monterey.

PALEOCEANOGRAPHIC AND TECTONIC 

CONTROLS ON SISQUOC SEDIMENTATION

The abrupt appearance of phosphatic conglomerates 

marking the Monterey-Sisquoc contact in the Mussel Rock

and Lompoc Quarry areas and the presence of a silty unit 

containing phosphatic nodules and pebbles from the 

Franciscan Complex and Monterey Formation at the base 

of the Sisquoc in the southern Santa Maria basin (Woodring 

and Bramlette, 1950) suggest fundamental changes in sedimentation in the late Miocene Santa Maria basin. Furthermore, the relatively abrupt change to detritus-enriched 

rocks occurring between the upper part of the Monterey 

Formation and the Sisquoc Formation in the Mussel Rock 

and Casmalia sections combined with the overall compositional differences existing between the Monterey and the 

Sisquoc probably result, as mentioned, from accelerated 

rates of silica and detritus accumulation during Sisquoc 

deposition. The aforementioned sedimentologic changes 

together with the highly variable occurrence of laminated 

and massive units in the Sisquoc, as discussed below, are 

tied to shifting oceanographic and tectonic conditions.

The onset of Sisquoc deposition at about 6.0 Ma is 

probably related to an eustatic sea level lowstand at about 

6.3 Ma (Haq, Hardenbol, and Vail, 1987). Important events 

closely associated with this sea level drop include the 

growth of the Antarctic ice cap (Shackleton and Kennett, 

1975; Mayewski, 1975; Mercer, 1976) and increased silica 

production between 6 to 5 Ma in the eastern margins of 

the Pacific (see Isaacs, 1983, 1984, 1985; Barren and 

Baldauf, 1990; Ramirez, 1990). In the central equatorial 

Pacific, decrease in silica production occurs at about 6.0 

Ma, or nearly coincident with the 6.3 Ma sea level drop, 

and may represent a shift in the silica sink from the central 

Pacific to the eastern Pacific (Leinen, 1979). Accelerated

LU

rr 

z ^

g >, ii

LU

5

O 

LU

600

500

400

300

200

100

0

216

580

Sisquoc 

Formation

j i Monterey 

I__I Formation

CASMALIA SWEENEY ROAD 

MUSSEL ROCK PEDERNALES

Figure 9. Sedimentation rates (uncorrected for compaction) for 

the Mussel Rock, Casmalia, Pedernales, and Sweeney Road 

sections. Note that only in the Mussel Rock section, where more 

than one biostratigraphic datum point occurs in the Monterey 

Formation in the measured section, were sedimentation rates 

calculated for the Monterey Formation. Shown above each bar 

is the average sedimentation rate in m/m.y.

Stratigraphy of the Fine-Grained Facies of the Sisquoc Formation, Santa Maria Basin, California U9
rates of silica accumulation in the Sisquoc relative to the 

underlying Monterey Formation probably reflect the increased silica production in the eastern margins of the Pacific coincident with the 6.3 Ma sea level drop. The 

Miocene growth of the Antarctic ice cap probably resulted 

in the lowering of sea level and, as suggested by Brewster 

(1980) and Ingle (198la), in intensified ocean circulation 

through increased latitudinal thermal gradients. Intensified 

oceanic circulation led to invigorated upwelling on the 

Pacific margins and subsequently to increased silica production and accumulation. Barren and Baldauf (1990) note 

that increased opal accumulation rates in the Sisquoc are 

related to steepened biologic productivity gradients resulting from regional changes in the patterns of opal accumulation. These variations in biologic productivity and in the 

patterns of opal accumulation are associated with a cooling event beginning at about 6.3 Ma.

More recently, Dumont and Barron (1995) note the 

significance of the 5.5 Ma sea level drop of Haq, Hardenbol, 

and Vail (1987) in influencing diatom sedimentation during the deposition of the Sisquoc. These authors suggest 

that intensified upwelling and increased primary productivity in the California borderland are associated with this 

sea level drop. Therefore, accelerated rates of silica accumulation at about 6.0 Ma do appear to suggest that the late 

Miocene sea level changes considerably modified sedimentation patterns in the California continental borderland.

Lowered sea levels during the late Miocene could also 

account, in part, for the increased accumulation rates of 

detritus in the Sisquoc as greater amounts of fine-grained 

sediment were transported across the exposed narrow 

shelves of the borderland and deposited in basin slope and 

floor environments during the lowstands. Additionally, the 

deposition of local phosphatic conglomerates at the base 

of the Sisquoc combined with the deposition of silty and 

sandy mudstones and phosphatic and chert-rich conglomerates in the uppermost part of the Sisquoc in the Mussel 

Rock section probably resulted, in part, from increased 

erosion on the basin margin due to lowered sea level. The 

conglomerates and mudstones in the Mussel Rock section 

may have been funneled through slope-incised channels 

developed during a period of lowered sea level (Ramirez, 

1990). The lack of sands in the Mussel Rock section is 

likely related to the probable upper slope to shelf-edge 

depositional setting of the Sisquoc in this area (Ramirez, 

1990). Sands were probably transported to deeper water 

environments. Garrison and Ramirez (1989) noted that conglomerates and breccias occur at the base of the Sisquoc 

in exposures along the Santa Maria and Santa Barbara 

coastal areas. They suggest that these conglomerates and 

breccias are in part attributable to a period of lowered sea 

level.

Superimposed on the major sea level changes influencing the sedimentologic character of the fill in the late 

Miocene-early Pliocene Santa Maria basin, were smallerscale shifts in oceanographic conditions. Small-scale fluctuations in the position of the oxygen-minimum zone 

(OMZ) are hypothesized to have generated the interbedded 

massive and laminated units of the Sisquoc. These smallscale fluctuations of the OMZ may have resulted from localized changes in sea level (Govean, 1980; Govean and 

Garrison, 1981) or from productivity variation in the water column, as suggested by Schrader and Baumgartner 

(1983) in their study of massive and laminated units in the 

Gulf of California. According to these authors, during times 

of increased biologic productivity, increased quantities of 

oxygen-consuming organic matter descend through the 

water column, leading to reduced oxygen levels, expansion of the OMZ, and the deposition of laminated sediments. In contrast, decreased productivity in the water 

column leads to a weakened oxygen minimum zone, increased oxygen-availability, and the proliferation of substrate-churning organisms, and, consequently, to the 

generation of massive intervals.

Shifting paleoceanographic conditions were not the 

only factors influencing the composition and fabric of the 

Sisquoc Formation. The increased detrital component of 

the Sisquoc relative to the underlying Monterey Formation can also be attributed, in part, to a period of late Miocene and Pliocene tectonism in and around the Santa 

Maria basin. Several lines of evidence support this notion. 

First, although the deposition of the most prominent basal 

phosphatic conglomeratic unit in the Mussel Rock section 

may be related to a sea level drop, subsequently deposited 

phosphatic and chert-rich conglomeratic units of the 

Sisquoc were deposited mostly during a rise in sea level 

(fig. 10). The majority of the conglomerates at Mussel Rock 

were deposited from about 5.3 to 4.8 Ma. A sea level drop 

occurred at about 5.5 Ma and a subsequent rise in sea level 

commenced at about 5.3 Ma (Haq and others, 1987). The 

sea level rise beginning at about 5.3 Ma correlates with a 

period of maximum conglomerate deposition in the Mussel Rock area of the Santa Maria basin (fig. 10), indicating 

that there may have been significant relief on the margins 

of the basin. The presence of significant relief on the basin 

margin is also supported by common elongate, subrounded 

conglomerate clasts with pholad borings. These clasts suggest relatively rapid erosion, limited transport, and temporary deposition in a nearshore environment prior to their 

redeposition to a deeper marine environment. In addition, 

upsection compositional changes in the clasts of the conglomerates in the Sisquoc of the Mussel Rock section indicate considerable erosion. The change from phosphatic 

clasts at the base of the Sisquoc in the Mussel Rock section to clasts of dolomite, diatomaceous mudstone, and 

phosphate in the middle portion of the section and finally 

to clasts of opal-CT and quartz cherts in the top part of the 

section reflects unroofing, to progressively deeper depths, 

of Monterey-type siliceous lithologies. This upsection compositional variation in the clast composition may be interU10 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
OQ 

3 o

_ 

I

5.0-6.0-

6.5-

Age

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SWEENEY ROAD

CASMALIA

MUSSEL ROCK

»^o \^<^»

PEDERNALES

«--^ \^o

Figure 10. Eustatic sea level curve from Haq, Hardenbol, and Vail (1987) plotted with the sections examined in this study. Shaded areas correspond to periods of relatively lower sea level. Note that in the Mussel Rock section the uppermost thick chert-rich conglomerate correlates with rising sea level. The onset of 

deposition of the massive to discontinuously laminated intervals that occur in the uppermost portions of the Casmalia and Sweeney Road sections correlate with 

lower sea level. Lithologic symbols are shown in figure 5.
preted to reflect, first, the erosion of shelf-deposited phosphatic nodules followed by the erosion of diatomaceous 

mudstone and dolomites. With deeper erosion, opal-CT and 

quartz chert lithologies were exposed and eroded. The opalA/opal-CT diagenetic boundary occurs at a burial depth of 

about 750 m and the opal-CT/quartz change at a burial 

depth of about 1,300 m in parts of the Santa Maria basin 

(Pisciotto, 1978). In the Santa Barbara basin, Isaacs (198 la) 

estimated that the opal-A/opal-CT and opal-CT/quartz 

boundaries occurred at burial depths of about 500 and 1,550 

m respectively. Lowered sea level by itself could not account for the depth of erosion necessary to expose the opal 

CT and quartz lithologies, indicating that tectonic uplift 

of parts of the basin occurred.

The deposition of laminated intervals in some areas 

of the basin and correlative massive intervals in other regions of the basin seems difficult to reconcile, considering 

that the laminated and massive units were deposited in 

nearly identical depositional environments. Tectonic uplift, especially in the Mussel Rock area, along with changing sea level probably combined to generate the spatial 

and temporal variations in the bedding types of the Sisquoc

Formation. Tectonic activity on the margins may have uplifted portions of the basin above the OMZ so that subsequently deposited sediments were churned by burrowing 

infauna leading to the production of thicker massive units. 

Large- and small-scale changes in sea level led to shifts in 

the position of the OMZ and consequently, to the production of more regularly alternating massive and laminated 

sediments along its margin.

Other evidence suggestive of tectonic activity in the 

Santa Maria basin during the late Miocene is the absence 

of the basal Sisquoc in the Lompoc Quarry area and its 

presence in nearby sections (fig. 11; see fig. 1 for location). Based on a conservative estimate of sedimentation 

rates, Ramirez (1990) suggested that approximately 500 

m of Sisquoc section are missing in the Lompoc Quarry. 

Sea level changes alone could not account for such localized erosion, and adjacent sections in similar depositional 

environments should have also been affected by the erosional affects attributable to a sea level lowering. Therefore, tectonism was probably responsible for the uplift and 

contemporaneous erosion concentrated in the Lompoc 

Quarry area.

5.0-

6.0-

6.5-

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CD

c

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g

LT

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LOMPOC HILLS

SWEENEY ROAD

LOMPOC QUARRY

West East

Figure 11.Chronostratigraphic diagram showing the east to west alignment of the Lompoc Hills, Lompoc Quarry, and Sweeney Road 

sections. The absence of the basal Sisquoc Formation in the Lompoc Quarry section and its presence in the Lompoc Hills and 

Sweeney Road sections suggests the presence of short-lived topographic highs, which probably resulted from tectonic activity in the 

Santa Maria basin. The Lompoc Quarry section is highly generalized and incorporates data from Barren (1975) and Dumont (1986). 

Lithologic symbols are shown in figure 5.

U12 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
This suggestion of middle Neogene uplift is supported 

by a number of studies indicating a late Miocene to 

Pliocene phase of tectonic activity in the Santa Maria basin and the central California continental borderland in 

general. Subsidence history plots for several borderland 

basins depict a late Miocene to early Pliocene phase of 

tectonic uplift (Dickinson and others, 1987). Furthermore, 

Hornafius (1985), Luyendyk and Hornafius (1987), and 

Luyendyk (1988) suggested a late Cenozoic (middle Miocene to Pliocene) phase of rotation of the Transverse 

Ranges, which border the southern margin of the Santa 

Maria basin. Any episode of rotation of the Transverse 

Range would probably have also impacted the Santa Maria 

basin. McCulloch and Lewis (1988) reported that strikeparallel folds in the offshore central California margin 

probably developed between approximately 5 to 3 Ma as a 

result of an episode of compressional motion between the 

Pacific and North American Plates. Porter (1932), 

Woodring and Bramlette (1950), and Crawford (1971), citing the thinning of late Miocene and Pliocene units on anticlinal ridges, suggested that many of the anticlinal ridges 

in the Santa Maria basin probably formed in the late Miocene or early Pliocene. Moreover, Porter (1932), Dibblee 

(1966), and Ingle (1981b) suggested that uplift of the Santa 

Ynez Range, which bounds the southern portion of the 

Santa Maria basin, may have occurred in the early Pliocene, 

which is coincident with the onset of deposition of the 

Sisquoc Formation. Additionally, Shepard and Emery 

(1941) and Teng and Gorsline (1989) indicated that much 

of the physiography of the present day borderland developed during late Miocene time, implying that late Miocene 

tectonic activity was prevalent during deposition of the 

Sisquoc. Barren (1992) also discusses the tectonic controls on the Pliocene diatom record of California.

SUMMARY

Stratigraphic and sedimentologic information indicates that both tectonism and oceanographic changes 

played a major role in shaping the sedimentological trends 

exhibited by the late Miocene and early Pliocene Sisquoc 

Formation in the Santa Maria basin. A phosphatic conglomerate marking the Monterey-Sisquoc contact in the Mussel Rock section is probably related to a eustatic sea level 

change at about 6.3 Ma. The increased accumulation of 

diatoms in the Sisquoc is probably related to changes in 

circulation and coastal upwelling patterns in the late Miocene ocean, resulting from polar cooling that also caused 

the growth of the Antarctic ice cap and led to an eustatic 

sea level drop. Alternating massive and laminated units 

were generated through small-scale variations in productivity and (or) sea level changes. Conglomerates in the 

upper part of the Sisquoc were deposited during a sea level 

rise and record evidence of significant erosion associated

with tectonic uplift of the basin margin. This evidence for 

late Miocene and early Pliocene uplift is in accord with a 

number of studies indicating an episode of tectonic activity at this time in the Santa Maria basin and in the California borderland in general.

REFERENCES CITED

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1335-1360

Stratigraphy of the Fine-Grained Facies of the Sisquoc Formation, Santa Maria Basin, California U15

Chapter V

The Sisquoc Formation-Foxen Mudstone Boundary 

in the Santa Maria Basin, California: Sedimentary 

Response to the New Tectonic Regime

By RICHARD J. BEHL and JAMES C. INGLE, Jr.

U.S. GEOLOGICAL SURVEY BULLETIN 1995

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

Edited by Margaret A. Keller
A

35°00'

34°30'

SANTA YNEZ MOUNTAIN.

120°30'

Figure 1 . Map of the onshore Santa Maria 

basin and proximal portions of the 

offshore basin. A, Locations of major 

roads, towns, outcrops, and important 

surface sections of the Pliocene Foxen 

Mudstone. Foxen outcrop distribution 

from Jennings (1959). B, Structures, oil 

fields (shaded regions), and oil wells 

(dots) mentioned in this report. 

Abbreviations: OF = Orcutt field, CCF = 

Careaga Canyon field. Section B-B1 , 

reproduced in fig. 4, is from Namson and 

Davis(1990).

SANTA YNEZ MOUNTAINS

V2 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
trend of basin shoaling (Canfield, 1939; Woodring and others, 1943; Woodring and Bramlette, 1950; Natland, 1957; 

Ramirez, 1990; McCrory and others, 1995), between the slope 

to shelf-edge deposits of the Sisquoc Formation (Ramirez, 

1990) and the littoral deposits of the Careaga Sandstone 

(McCrory and others, 1995).

Scope of study

A distinctive lithofacies of opal-CT chert, porcelanite, 

siliceous mudrocks, or dolostone has been penetrated in the 

lowermost part of the Foxen Mudstone by many oil wells 

drilled in the Santa Maria Valley and the Los Alamos-San 

Antonio Valley synclines, and by some wells in the offshore 

Santa Maria basin (fig. 1). The rocks of this interval (hereafter referred to as the "hemipelagic Foxen" or "basinal lowermost Foxen," for convenience) are dissimilar from the rest of 

the siliciclastic-dominated Foxen and the underlying 

hemipelagic, diatomaceous Sisquoc Formation, and they differ also from known Foxen exposures in surface sections. This 

study investigates the environmental and tectonic significance 

of this unique facies. Foraminiferal assemblages and structural relationships in the subsurface suggest that localized 

subsidence in synclinal sections resulted from compressive

Ma Series Stage Formation

o

5-

10-

15-

20-

Pleis.

CD

c

CD 

0

o

E

Miocene

Q_

=>

|

CD 

Q. 

Q.

=)

CD 

T3 

T3

5

CD 

0

Hallian ,

/Wheelerian

^ Venturian

Repettian

Delmontian 

Mohnian

Luisian

Relizian^

Saucesian

OrcuttSand ,

Paso Robles Fm./

Careaga Ss.

Foxen Mdst.

Sisquoc Fm.

Monterey 

Fm.

Point Sal Fm.

Obispo Fm.

Lospe Fm.

Figure 2. Neogene stratigraphy of the onshore Santa Maria basin 

(after Namson and Davis, 1990).

tectonic deformation during deposition near the start of Foxen 

sedimentation. In this study, we propose that these lithologic and faunal changes at the Sisquoc-Foxen boundary 

record the sedimentologic and environmental response to 

thrust-related tectonic load subsidence during the local 

initiation of regional transpressive tectonics, about 3.8- 

3.4 Ma, rather than that of simple progradation and shoaling of sedimentary facies.

METHODS 

Material

The data for this study are chiefly derived from petrologic and microfaunal analysis of well cuttings and cores. 

Geophysical well logs (resistivity, spontaneous potential and 

gamma ray) were used for supplemental stratigraphic analysis and lateral facies identification. Surface exposures of the 

Sisquoc Formation and Foxen Mudstone at Harris Grade 

in the Purisima Hills and at Shuman Cut in the Casmalia 

Hills (fig. 1A) were examined for comparison with subsurface sections.

Drill Cuttings Versus Outcrop Data

Data collected from the analysis of drill cuttings differ 

from data collected at stratigraphic sections in outcrop. Cuttings are collected from the mud circulation system of a drilling rig and are therefore not directly sampled from a precisely 

known stratigraphic interval, as is possible with core or outcrop. Continuous attention is required while drilling to accurately and consistently determine the depth of the cuttings' 

origin. This process involves monitoring penetration rate, pump 

output, and the true annular volume of the borehole. All of 

these values diverge from theoretical or optimal estimates in 

an irregular manner and must be frequently determined by 

empirical methods. Most of the cuttings examined for this study 

are composite samples collected at 10-30 ft intervals by one 

of the authors (RJB) and colleagues at Epoch Well Logging, 

Inc., and the assigned depth of samples are probably accurate 

to ±5-10 ft (2-3 m).

Two potential problems that arise with use of drill cuttings (or mud log) data are the accurate assignment of lithologic percentages and sample contamination. As cuttings are 

produced by the grinding or chipping away of rock by a rotating drill bit, the cuttings of well-indurated lithologies are overrepresented in the samples compared with poorly consolidated 

mudrocks that are crushed into fine-grained suspensions in 

the mud system. Lithologic contamination of the samples from 

uncased strata above the drill bit occurs as fragments cave in 

or are knocked off by the rotating drill string and mix with the 

fresh cuttings on their way to the surface. If drilling conditions are good, this is not a serious problem and the contamiThe Sisquoc Formation-Foxen Mudstone Boundary in the Santa Maria Basin, California: Sedimentary Response to the New Tectonic Regime V3
nation, or "slough," can be identified or removed from the 

samples. In poor conditions, however, unwanted slough can 

form a substantial portion of the cuttings samples and prevent 

the reliable quantitative assessment of in situ lithologic percentages at any specific depth. During drilling related to this 

study, conditions were generally good and contamination from 

cavings were minimal.

Foraminiferal Analysis and Paleobathymetry

Pioneering studies by Natland (1933, 1957), Crouch 

(1952), and Bandy (1953) early demonstrated the usefulness 

of benthic foraminifers for interpretation of paleoenvironments 

and water depths (that is, paleobathymetry) in the evolving 

Neogene basins of coastal California. More recent studies have 

utilized quantitative distributional patterns among Holocene 

foraminifera and associated data on water masses, substrate, 

and other environmental parameters to further enhance 

paleoceanographic, depositional, and paleobathymetric analyses (Douglas and Heitman, 1979; Ingle, 1980; Douglas, 1981; 

Blake, 1985). Concepts and techniques presented by these latter 

authors were employed to analyze foraminiferal faunas and 

interpret paleoenvironments and paleobathymetric trends 

within the Neogene sequence of the Santa Maria basin, including the Foxen Mudstone. Benthic foraminifers previously 

reported from Neogene units, as well as those analyzed during this study, were assigned to individual biofacies (for example, littoral biofacies, inner shelf biofacies, etc.) following

the paleoenvironmental classification and biofacies definitions 

and compositions of Ingle (1980, 1985).

Benthic foraminifers were originally common to abundant in the Neogene marine deposits of the Santa Maria 

basin, as shown by the contents of early diagenetic concretions. Weathering and other diagenetic processes have 

affected the preservation of foraminifers in every 

lithofacies, however, and are responsible for the complete 

destruction of foraminiferal tests in some horizons. Calcareous foraminifera] tests are commonly leached from 

surface exposures of the Foxen Mudstone, whereas wellpreserved faunas are often recovered from subsurface 

samples of the more siliciclastic facies of this formation.

Nine core samples from the Standard Los Flores No. 

1 well in the Cat Canyon fields (fig. IB) yielded well-preserved foraminiferal faunas from the upper part of the 

Sisquoc Formation and lower part of the Foxen Mudstone 

(Ingle, 1985). Samples were disaggregated in kerosene, 

washed on a 250-mesh screen (0.062-mm openings), dried, 

and quantitatively analyzed following standardized procedures reviewed by Ingle (1967). Separate counts were made 

of planktonic and benthic species. Relative abundances of 

selected benthic species and cumulative percentages of 

benthic biofacies were then plotted stratigraphically and used 

to interpret variations in water depth and paleoenvironments 

during deposition of Sisquoc and Foxen sediments in the Cat 

Canyon area (fig. 3).

Depth

(feet below

rig floor)

1,000-

2,000 -

3,000 -

4,000-

5,000 -

CO

g

o 

CO

o

D)

CO

CO

-9-

Biofacies 

(percent)

Selected species 

(percent)

100 0

Paleobathymetry 

(meters)

500

Depth

(meters below 

rig floor)

1000 \

- 500m

- 1000m

_ 1500m

* Core Samples

Figure 3. Quantitative distribution of benthic foraminiferal biofacies, variation in relative abundance of selected benthic foraminifers, 

and estimated paleobathymetry in the upper part of the Sisquoc Formation and lower part of the Foxen Mudstone as analyzed in 

core samples from the Standard Los Flores No. 1 well from the Cat Canyon fields, Santa Maria basin, California. Modified from 

Ingle (1985).

V4 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
Nomenclature and Significance of Siliceous 

Rocks

This paper follows the field terminology for siliceous 

rocks of Bramlette (1946) and Isaacs (1981). Chert is the 

nearly pure siliceous rock that is hard, dense, fractures 

conchoidally, and has a vitreous or waxy luster. Porcelanite 

is the impure siliceous rock that is less hard, less dense, 

and less vitreous than chert, and has a matte texture similar to that of unglazed porcelain. Disregarding carbonate 

content, porcelanite typically contains 60-85 percent silica 

(Isaacs, 1981), whereas chert contains >85 percent; most 

vitreous cherts have between 95 and 99 percent silica (Behl 

and Garrison, 1994). Classification of a diagenetic rock as 

chert or porcelanite is independent of the polymorph of 

silica present, so that either rock can be composed of opalCT or quartz-phase silica. For the stratigraphic sections 

discussed herein, X-ray diffraction indicates that only opalCT phase authigenic silica is present in the upper part of 

the Sisquoc Formation and the Foxen Mudstone.

Nearly all Phanerozoic chert and porcelanite formed 

by the diagenesis of biogenic silica, which was originally 

deposited as radiolarian tests, diatom frustules, or sponge 

spicules (Garrison, 1974). With sufficient time or burial, 

chert and porcelanite can form by one or two dissolution/ 

reprecipitation steps. First, biosilica (opal-A) transforms 

to metastable opal-CT (cristobalite-tridymite), and second, 

opal-CT converts to diagenetic quartz (Murata and Larson, 

1975). The purest diatomaceous deposits tend to undergo 

diagenesis from opal-A to opal-CT at much shallower 

burial depths than more detrital-rich deposits (Isaacs, 1981; 

Behl and Garrison, 1994). The Neogene deposits of much 

of the Pacific Rim are characterized by their abundant diatomaceous sediments or their diagenetic descendants 

chert, porcelanite, and siliceous mudstone (Ingle, 1981).

STRATIGRAPHY OF THE FOXEN MUDSTONE 

Vertical and Lateral Distribution

In the onshore Santa Maria basin, the lower to upper 

Pliocene Foxen Mudstone overlies the upper Miocene and 

lower Pliocene Sisquoc Formation, and is overlain by the 

upper Pliocene Careaga Sandstone (Canfield, 1939; 

Woodring and others, 1943; Woodring and Bramlette, 1950) 

(fig. 2). Grain size and percent terrigenous material generally increase upsection from the Miocene Monterey Formation (which underlies the Sisquoc Formation) through 

the youngest sediments (for example, the upper Pliocene 

to lower Pleistocene Paso Robles Formation). These sedimentologic trends are associated with gradual shoaling 

(Canfield, 1939; Woodring and others, 1943; Woodring and 

Bramlette, 1950; Ramirez, 1990; McCrory and others, 

1995) and decreased flux of diatom frustules (Barron, 1986,

1992). The upsection increase in detritus dilutes the biogenic component and complicates the application of microfossil biostratigraphy and the construction of an 

integrated chronostratigraphy (Barron, 1992). In spite of 

the latter problem, it is clear that Foxen deposition occurred 

during some part of the late-early [so-called middle(?)] to 

late Pliocene (Canfield, 1939; Woodring and others, 1943). 

Foxen strata display extreme lateral variation in thickness 

and lithofacies in a northeast-southwest cross section across 

the central Santa Maria basin (fig. 4). Subsurface synclinal troughs accumulated up to 1,000 m of Foxen sediments 

(Canfield, 1939), whereas anticlinal ridges (Pliocene 

paleobathymetric highs) completely lack Foxen deposits 

or contain presumably correlative condensed, sandy phosphatic sections only a few meters thick that are usually 

included in the overlying Careaga Sandstone (Canfield, 

1939; Woodring and others, 1943; Woodring and 

Bramlette, 1950). Exposures of Foxen strata in the Santa 

Maria basin are very limited (fig. 1 A). The thickest surface sections, located on the northern slopes of the 

Purisima and Casmalia Hills, are less than 300 m thick. 

The Foxen thickens toward the axes of two paleotroughs 

[the modern Santa Maria Valley and San Antonio-Los 

Alamos synclines (fig. IB)], and seaward, to the west or 

northwest (Canfield, 1939; Woodring and others, 1943; 

Woodring and Bramlette, 1950). The Foxen may also be 

present in the subsurface of the Santa Rita syncline to the 

south, but we are unaware of any oil well penetrations to 

confirm this conjecture (fig. 4).

The geometry and distribution of the Foxen Mudstone 

(or its lithostratigraphic equivalents) are less well known 

for the offshore Santa Maria basin than is the case for the 

onshore basin, which has a much simpler structural architecture. This is, in part, because many fewer wells have 

been drilled and, in part, due to the different structural fabric (NNW. offshore versus WNW. onshore) and many 

smaller fault blocks than onshore (Sorlein, 1994). Late 

Neogene tectonic deformation and the consequent local accumulation and geometries of post- and syndeformational 

sedimentary units also have slightly different histories and 

timings between the onshore and offshore Santa Maria 

basins (McCrory and others, 1995). As in the onshore basin, offshore deposits of the Foxen Mudstone or its approximate equivalents tend to thin over modern structural 

highs and thicken into structural depressions (Clark and 

others, 1991; Sorlein, 1994; McCrory and others, 1995). 

Because most of the published studies of the offshore are 

based on seismic stratigraphy, which relies upon stratal 

discontinuities or unconformities to distinguish between 

stratigraphic packages, the precise thickness and distribution of the lithostratigraphically equivalent Foxen Mudstone are not well known. Although this study focuses on 

the onshore Santa Maria basin, electric logs, mud logs, and 

well cuttings from a number of offshore wells were examined to establish that the lowermost hemipelagic Foxen

The Sisquoc Formation-Foxen Mudstone Boundary in the Santa Maria Basin, California: Sedimentary Response to the New Tectonic Regime V5
Mudstone directly overlies the Sisquoc Formation in at least 

a few sites in the offshore basin (described in a following 

section).

Age of the Foxen Mudstone

Woodring and others (1943) and Woodring and 

Bramlette (1950) assigned the Foxen Mudstone to the 

middle(?) and upper Pliocene on the basis of benthic foraminifers and molluscan fossils. The Foxen is considered 

to be age-equivalent to Repettian Stage (lower Pliocene) 

deep-water sediments in the offshore Santa Maria basin 

(Dunham and others, 1991; McCrory and others, 1995), 

but its provincial foraminiferal fauna cannot be strictly 

assigned to the Repettian stage due to the absence of defining marker species (M.L. Cotton-Thornton, 1992, oral 

commun.). In any case, Foxen strata are younger than the 

underlying strata of the Delmontian Stage in the upper part 

of the Sisquoc Formation (Barron and Baldauf, 1986), and 

these Delmontian floras and faunas are not younger than 

4.0 Ma (Warren, 1980; see discussion in Barron and 

Baldauf, 1986). Diatom assemblages from the lower part 

of the Foxen at Harris Grade are early Pliocene in age 

(Barron and Baldauf, 1986) and the conformable contact 

with the underlying Sisquoc Formation here is extrapolated to 3.8 Ma (Barron, 1992).

In the offshore Santa Maria basin, the base of Foxen 

Mudstone-equivalent(?) strata is placed at the early-late 

Pliocene unconformity, estimated at 3.4 Ma (Clark and others, 1991). The total likely age range (4.0-3.4 Ma) for the 

lower contact of the Foxen Mudstone reflects different methHarris <$rade 

section

B Santa Rita 

sw Valley

ods of determination as well as the time-transgressive nature 

of the Sisquoc Formation-Foxen Mudstone boundary (described in a following section). Also of chronologic significance is a warm-temperature planktonic foraminiferal biofacies 

containing unusual abundances of Globorotalia crassaformis 

and G. puncticulata present in the lower part of the Foxen 

(above the basal facies) of the Standard Los Flores No. 1 well 

(Ingle, 1985). This distinctive fauna can be correlated with a 

well-established middle(?) Pliocene warm climatic episode 

recorded in deep-sea and onshore sequences along the Pacific 

Coast of North America (Ingle, 1977) that occurred between 

approximately 4.0 and 3.2 Ma (Keller, 1979).

Dumont and Barron (1995) estimated that the uppermost 

Sisquoc Formation in the Shuman Cut section of the Casmalia 

Hills was deposited at approximately 5.0 Ma, apparently placing the formational contact considerably older than the other 

estimates reviewed here. The age of the top of the Sisquoc 

Formation is probably too old for initial Foxen Mudstone deposition, as a significant gap in sedimentation is indicated by an 

angular unconformity or by a bored, phosphatic hardground 

at the top of the Sisquoc in all exposed or cored contacts in the 

Casmalia Hills or adjacent Santa Maria Valley oilfield 

(Canfield, 1939; Woodring and Bramlette, 1950). The contact 

between the Sisquoc Formation and Foxen Mudstone is not 

exposed in the Shuman Cut section, and up to 1.2 m.y. may be 

missing in an undetected unconformity (Dumont and Barron, 

1995).

The contact between the Foxen Mudstone and the 

overlying Careaga Sandstone has received little attention 

and is poorly constrained by biostratigraphy. Woodring and 

Bramlette (1950) consider the macrofaunas of the upper 

part of the Foxen and lower part of the Careaga (Cebada

San Antonio -

Los Alamos

syncline

Figure 4. Regional cross section B-B' of Namson and Davis (1990) modified to show thickness of the Foxen Mudstone 

based on information in Woodring and Bramlette (1950), AAPG (1959), and Crawford (1971). Note that the Foxen (Tf) 

thickens in synclines and thins or disappears on anticlines. Heavy lines are faults; dotted lines are arbitrary structural 

markers. Abbreviations: KJf, Franciscan Complex and Jurassic Coast Range Ophiolite; Tl, lower Miocene Lospe and Obispo 

Formations; Tsr, middle to upper Miocene Point Sal and Monterey Formations; Ts, upper Miocene to lower Pliocene Sisquoc 

Formation; Tf, Pliocene Foxen Mudstone; QTs, upper Pliocene Careaga Sandstone, Pliocene and Pleistocene Paso Robles 

Formation, and Pleistocene Orcutt Sandstone.

V6 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
Member) to be similar and assigned them both to the late 

Pliocene. The upsection change from sandy mudstones of 

the Foxen to the sand and gravel facies of the Careaga may 

be the result of the global eustatic sea level fall at 3.0 Ma 

and subsequent sea level oscillations (Haq and others, 1988; 

cf. fig. 6 of this paper).

In summary, available biostratigraphic and 

lithostratigraphic evidence from the Sisquoc Formation, Foxen 

Mudstone, and Careaga Sandstone indicates that Foxen sediments were deposited during the early to late Pliocene, probably initiating no earlier than 4.0 Ma, but likely between 3.8 

and 3.4 Ma, and continuing to approximately 3.0 Ma. This 

age range for the Foxen Mudstone may not be consistently 

applicable in distal offshore locations due to the time-transgressive nature of the Foxen's upper and lower boundaries 

(Canfield, 1939) and slightly different deformational histories of small depocenters and adjacent local positive structures (Clark and others, 1991; Sorlein, 1994; McCrory and 

others, 1995).

Lithology of the Foxen Mudstone

Surface Observations

Two stratigraphic sections of the Foxen Mudstone were 

described by Woodring and Bramlette (1950) at Harris Grade 

and Shuman Cut (fig. 1A). The Harris Grade section is exposed in roadcuts of the old California Highway 1 on the north 

slope of the Purisima Hills between Lompoc and Santa Maria. 

At this location, the Foxen Mudstone conformably overlies 

the Sisquoc Formation and is approximately 244 m thick 

(Woodring and Bramlette, 1950). It consists chiefly of poorly 

exposed, weathered mudstone and siltstone with widely spaced 

dolostone beds and nodules. The mudrocks are notably diatomaceous in the lower part and contain increasing amounts of 

sponge spicules upsection. Dolostone horizons are not always 

clearly exposed but can be inferred at many levels by accumulations of dolostone fragments and nodules in the weathered, 

disaggregated mudstone. Eighteen kilometers to the northwest, 

the Shuman Cut section is exposed in a Southern Pacific Railroad cut adjacent to the Airox quarry on the north slope of the 

Casmalia Hills (Casmalia section of Dumont and Barron, 

1995). The Foxen Mudstone here is about 175 m thick and is 

generally much coarser grained than in the Harris Grade section, consisting of interbedded sandstone, siltstone, mudstone, 

and dolostone. Sandstone beds are generally poorly sorted and 

silty to coarse grained; many contain abundant bioclastic debris, including foraminifer, echinoderm, mollusk, and barnacle 

fragments (see measured section in Woodring and Bramlette, 

1950). Smear slides of the mudrocks show abundant sponge 

spicules, locally composing up to 50 percent of the siltstone 

beds (by volume). Carbonate beds in both surface sections of 

the Foxen are chiefly pure, dense, microsucrosic dolomite and 

are more common than in the underlying, generally calcitic

Sisquoc Formation. The contact with the underlying Sisquoc 

Formation is not exposed at the Shuman Cut section. At nearby 

locations in similar positions on the north limb of the Casmalia 

anticline, Woodring and Bramlette (1950) described a discontinuous basal conglomerate containing chert and porcelaneous 

shale clasts from the Monterey Formation and phosphatic nodules that pipe down into borings or fractures in the underlying 

Sisquoc Formation.

Subsurface Observations

In the subsurface of the onshore and offshore Santa Maria 

basins, the rocks of the Sisquoc Formation and Foxen Mudstone are easily distinguished, and the depth of the formational 

contact can be clearly identified in core or in cuttings (fig. 5). 

The Foxen is chiefly composed of olive-brown to olive-green, 

interbedded mudstone, siltstone, and fine-grained sandstone 

(Canfield, 1939; Woodring and Bramlette, 1950). The mudstone of the lowermost Foxen is dark grayish- to yellowishbrown, and when drilled in deeper parts of the basin it is well 

indurated and forms hard, angular drill cuttings. Upsection, it 

becomes increasingly sandy, uncemented, and grades in color 

from brown to olive green. The underlying, upper part of the 

Sisquoc is quite distinct from the Foxen; it is a more homogeneous deposit, composed mostly of light-gray, massive to 

thickly laminated, silty diatomaceous claystone or silty clayey 

diatomite. In most wells, the upper part of the Sisquoc is so 

poorly indurated that, when drilled, >80 percent is crushed 

into clay-size particles and dispersed in the circulating drilling mud. This contrasts sharply with the better preservation of 

the coarser grained, more indurated Foxen sediments.

Hemipelagic Strata of the Lowermost Foxen Mudstone

Canfield (1939) noted that in the deeper parts of the Santa 

Maria Valley field, the lower part of the Foxen Mudstone consists of "a hard chocolate-brown somewhat siliceous siltstone." 

Aside from that observation, the entire Foxen has been considered a siliciclastic lithologic unit (Woodring and others, 

1943; Woodring and Bramlette, 1950;Natland, 1957;Namson 

and Davis, 1990; Dunham and others, 1991). Our examination of drill cuttings from wells in the onshore and offshore 

Santa Maria basin, however, finds a distinctive siliceous and 

dolomitic lithofacies present at many locations. Because cores 

are not available from this interval, and this lithofacies does 

not crop out at the surface, the facies is known and described 

solely from drill cuttings. Opal-CT phase siliceous mudstone, 

porcelanite, chert, and dolostone occur at the base of the Foxen 

Mudstone in the subsurface of the Careaga Canyon field and 

beneath the San Antonio/Los Alamos Valley of the onshore 

Santa Maria basin (fig. 1). Dolostone and less abundant diatomaceous or opal-CT phase mudstone or porcelanite also occur in age-equivalent (Repettian to indeterminate Pliocene)

The Sisquoc Formation-Foxen Mudstone Boundary in the Santa Maria Basin, California: Sedimentary Response to the New Tectonic Regime V7
sediments in the Lion Rock and Santa Maria Units (wells P0395 No. 1, P-0396 No. 1, and P-0425 No. 2) of the offshore 

Santa Maria basin. As this study focuses on the onshore Santa 

Maria basin, we have only confirmed the existence and have 

determined the lithologic character of the basal hemipelagic 

Foxen facies in the offshore basin. It may, in fact, be more 

widespread in the offshore than indicated by the three wells 

discussed here.

The hemipelagic (that is, siliceous and dolomitic) interval of the basal Foxen Mudstone varies from 30 m to nearly 

100 m in thickness, extending upward from the Sisquoc Formation-Foxen Mudstone contact. Although chiefly composed 

of diatomaceous or siliceous mudstone, purer diagenetic 

porcelanite and chert make up 5 to 30 percent of the cuttings 

sampled from each 10 or 30 ft (~3 or 9 m) interval; dolostone 

and limestone contribute another 5 to 20 percent. Mudstone 

cuttings are variably siliceous, slightly phosphatic, and contain scattered, poorly preserved foraminifers. The dark grayish-brown to yellowish-brown mudstone grades 

compositionally and texturally to porcelanite and chert. The 

chert and porcelanite are composed of opal-CT phase silica 

and are distinguished from mudrocks by their resinous luster, 

smooth to slightly matte texture, conchoidal fracture, and reddish-brown color. All rock types locally display faint, thin (<1 

mm thick) laminations; the drill cuttings are too small to exhibit larger sedimentary structures.

The dolostone (rather than limestone) cuttings are 

stratigraphically associated with the most siliceous intervals 

and are the dominant nonsiliciclastic lithology in the basal 

hemipelagic Foxen Mudstone in the offshore Santa Maria basin (wells P-0395 No. 1, P-0396 No. 1, and P-0425 No. 2). 

The dolostone is composed of pure, dense, crystalline dolomite, similar to the carbonates observed in surface exposures 

of the Foxen at Harris Grade and Shuman Cut.

The Sisquoc Formation-Foxen Mudstone Boundary

In the Santa Maria Valley field the lowermost Foxen 

Mudstone strata overlie the Sisquoc Formation with a slight 

angular unconformity, which defines a transgressive surface 

that becomes younger in the landward direction to the east 

and north (Canfield, 1939). In shallower, more proximal parts 

of the field, a thin, discontinuous basal conglomerate and 

thicker, cross-bedded sandy siltstone (the Foxen "tar sand") 

mark the formational contact (Canfield, 1939). In outcrop exposures on the northern slope of the Casmalia Hills, Woodring 

and Bramlette (1950) described a basal Foxen conglomerate 

containing phosphatic nodules, chert, and porcelanite clasts 

from the Monterey Formation that overlies and pipes down 

into the underlying, bored or fractured Sisquoc Formation. 

Thus, throughout much of the Santa Maria basin, the basal 

Foxen is marked either by a coarse-grained facies in proximal 

or ridge flank locations, or by hemipelagic (siliceous/dolomitic), detritus-poor strata in distal, synclinal locations; these

two facies may represent different relative bathymetric and 

structural positions in the early Pliocene basin. Very thin layers (<1 cm) of sand (observed in cuttings and accidental cores 

while removing metal "junk" from wells) are locally associated with the bottom of the basal siliceous strata. These may 

be distal lithostratigraphic equivalents to the proximal Foxen 

"tar sand." Wherever it is observable in core or outcrop, the 

Sisquoc Formation-Foxen Mudstone contact is marked by an 

unconformity, with the only known exception being the Harris Grade section (Barron and Baldauf, 1986; Dumont and 

Barron, 1995).

The Sisquoc Formation-Foxen Mudstone contact is easily recognized in the subsurface by geophysical well logs (fig. 

5). The Sisquoc is characterized by a nearly invariant "shale 

baseline" in the spontaneous potential and deep induction logs, 

whereas gamma-ray logs show limited activity due to minor 

compositional variation. The lower part of the Foxen is identified by sharply increased and more variable gamma-ray and 

induction logs, and a negative shift with more activity in spontaneous potential logs.

In the central onshore basin, Foxen Mudstone sediments 

are more organic-rich than in the underlying upper part of the 

Sisquoc Formation (fig. 6). In the Union Hobbs No. 22 well 

in the Orcutt field, Isaacs and others (1990) report analyses 

indicating that the weight percent of total organic carbon 

(TOC) of bulk cuttings increases from 0.7-4.4 percent (1.6 

percent average) in the Sisquoc to 8.1-17.2 percent (11.2 

percent average) in the Foxen Mudstone. In the Careaga 

Canyon field to the south, we measured a similar, but 

smaller, increase in organic content across the formational 

boundary in the Coastal Hunter Careaga No. 3 well. Cuttings from the uppermost 30 m (100 ft) of the Sisquoc Formation contain 1.1-1.3 percent TOC (1.2 percent average), 

whereas TOC of the basal Foxen siliceous mudstone, 

porcelanite, and dolostone is three times greater (3.3-4.1 

percent, 3.7 percent average). The lower TOC values from 

the siliceous basal Foxen in the Coastal Hunter Careaga No. 

3 reflect the typically inverse relationship between biogenic 

silica and organic content in the Neogene rocks of California (Isaacs, 1984). Differences in average TOC between 

the two wells are also likely influenced by their depositional and structural settings in that the Hobbs No. 22 well 

is located on a structural high that largely developed since 

deposition of the lower Foxen, whereas the Hunter Careaga 

No. 3 well is located on the southern flanks of the structure in what was originally a thick, synclinal section 

(Namson and Davis, 1990).

Biofacies and Paleoenvironments

Five of the core samples analyzed from the Standard Los 

Flores No. 1 well span the upper part of the Sisquoc Formation and the lower part of the Foxen Mudstone. Although we 

have no samples from directly above or below the formational

V8 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
Drill cuttings 

(volume percent

______

~T ______

A _ _ _ _ _

AZ -----

AAZ ---- 

AZ 

ZZ 

I I

-=-

-

3,000 

3,500-

4,000-

4,500 _

Spontaneous potential 

(mV)

0 200

-

-

-

Foxen Mudi

Sisquoc Forn

Gamma-ray 

(API units)

0 200

>tone "5"

ia"°"f

£

>

Deep induction 

(ohm-m)

0.20 2000 j !

I -

i

I ;

3,000

D 

CD

_ 3,500 J 

CDi

CQ"

o" 

- 4,000 °

3

CD 

_ 4,500

[ 1 Brown ffss//e mudstone

PH Gray diatomaceous silty claystone

I | Dispersed clay- and silt-size fraction

\T] White to gray impure limestone

pzT| Brown sucrosic dolostone

[A] Brown porcelanite and chert

Figure 5. Typical mud log lithostratigraphic column and electric log response across the Sisquoc FormationFoxen Mudstone boundary in thick, synclinal sections. Figure based on the Coastal Hunter Careaga No. 3 

well and other wells in the Careaga Canyon field (north limb of the San Antonio-Los Alamos syncline).

Water

TOC depth 

Series Fm. Lith. (wt percent) (m ) Coastal onlap Eustatic sea level

Ma P so 1000 0.5 o +100 so om Ma

2-

q -

o

4-

5-

CD

C

CD

O

.0

E

Q.

Q. 3

ZT

5

_g

Miocene

CD

CO C 

O) O

CO C

C/)

C ^

X ^3 £ ^

C

0-2 o ^

13 CO

co E

WO

. . 

~-~.-~.

A"AA

- --TV

-/^V- 

- --TV

-TV -

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- --TV

% 

{

1

i i 1

i

I

j 

Sta

J

\L

\

'

V

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i 1

-2

-4

-5

50 1000 0-5

Diatomaceous silty claystone 

Porcelanite, chert, and dolostone 

Mudstone

0 +100 50 Om

Sandy siltstone 

Sandstone

Figure 6. Late Neogene correlation figure for synclinal sections of the onshore Santa Maria 

basin. Chronologic relations of formations (Fm.), general lithology (Lith.), total organic carbon 

(TOC), water depth, coastal onlap, and eustatic sea level curves (Haq and others, 1988). 

Note that TOC and water depth are from different wells, but both are from synclinal sections. 

Foraminiferal paleobathymetry is from the Standard Los Flores No. 1 well and from 

reconsideration of faunal data from Woodring and Bramlette (1950). Dimensions of shaded 

rectangles in the water depth column represent uncertainty in paleobathymetry and in age of 

samples. TOC is of cuttings from the Coastal Hunter Careaga No. 3 well. Age of the Foxen 

Mudstone (3.8-3.0 Ma) is estimated by this study.

The Sisquoc Formation-Foxen Mudstone Boundary in the Santa Maria Basin, California: Sedimentary Response to the New Tectonic Regime V9
boundary, contrasting foraminiferal biofacies indicate a distinct change in paleobathymetry and paleoenvironment between deposition of the upper part of the Sisquoc and the 

lowermost Foxen (fig. 3).

Benthic foraminifers reported from the Sisquoc Formation by Woodring and Bramlette (1950), Natland (1957), and 

Ingle (1985) indicate that the lower and middle portions of 

this unit were deposited in upper middle bathyal (500-1,500 

m) to upper bathyal (150-500 m) slope environments. Upper 

bathyal biofacies containing high abundances of 

Globobulimina pacifica also characterize the upper Sisquoc 

sediments analyzed from the Los Flores No. 1 well. However, 

these latter faunas are replaced by dominantly shelf-edge and 

shelf biofacies in the youngest Sisquoc samples, indicating 

that water depths shoaled from -500 m to -100 m during deposition of the uppermost Sisquoc (fig. 3). In contrast, an upper 

bathyal biofacies characterized by high abundances of the striate-costate species Uvigerina foxenensis is present in basal 

Foxen sediments, indicating that water depth abruptly increased 

to -500 m coincident with, or shortly after, the initiation of 

Foxen Mudstone deposition in the Cat Canyon area. Similar 

patterns of contrasting benthic biofacies and lithofacies characterize the Sisquoc Formation-Foxen Mudstone boundary 

elsewhere in the Santa Maria basin (Woodring and Bramlette, 

1950) and clearly point to an episode of rapid deepening to 

bathyal water depths (-500-600 m) during early Foxen deposition (McCrory and others, 1995). Subsequent Foxen deposition involved gradual shoaling as shown by the widespread 

appearance of shelf biofacies followed by deposition of coarse 

Careaga sediments containing littoral foraminifers and 

megafaunas that marked the termination of marine deposition in 

the onshore Santa Maria basin (Canfield, 1939; Woodring and 

Bramlette, 1950; Natland, 1957; McCrory and others, 1995).

CONTROLS OF SEDIMENTATION

The lithologic contrast between the Sisquoc Formation 

and the Foxen Mudstone has previously been interpreted to 

reflect the upward-shallowing from slope to outer-shelf depositional environments (Natland, 1957; Ramirez, 1990). The 

data presented in this paper suggest that instead of simple shoaling of a prograding clastic wedge, deposition of Sisquoc and 

Foxen sediments took place over a sea floor with increasing 

relief and lateral variation in environment (Canfield, 1939; 

Woodring and others, 1943; Woodring and Bramlette, 1950; 

Ramirez, 1990). This increasingly complex bathymetry probably developed as the Santa Maria basin and the rest of the 

Neogene borderland were deformed in response to a major 

change in relative plate motion from transform to transpression 

across the California segment of the Pacific-North American 

plate boundary. This change in plate vectors was originally 

proposed to have occurred at about 3.9-3.4 Ma (Harbert and 

Cox, 1989; Harbert, 1991) but recently has been shifted back 

to -5-6 Ma (Cande and others, 1992; D. Wilson, 1994, oral

commun.). Evidence for extreme lateral changes in facies and 

stratigraphic thickness in the lower part of the Foxen combined with sharply increased water depths at some locations 

indicate that tectonic deformation accelerated abruptly in the 

onshore Santa Maria basin near the beginning of Foxen deposition at approximately 3.8 Ma.

Sedimentation in marginal marine settings is primarily 

regulated by the interaction of sea level, oceanography, and 

tectonism (for example, see Doyle and Pilkey, 1979; Barron, 

1986). The rate of siliciclastic sedimentation or erosion, for 

example, depends chiefly on topographic/bathymetric gradients and base levels controlled by local tectonics and relative 

sea level. The rate, kind, and preservation of biogenous, 

hemipelagic sedimentation depends on the characteristics of 

the overlying water column. Whereas the production of diatomaceous sediments offshore of California is directly related 

to the upwelling of nutrient-rich intermediate waters, the preservation of organic matter and early diagenesis of sediments 

are influenced additionally by bottom-water composition. In 

the Pliocene Santa Maria basin, depositional trends in the 

Sisquoc Formation and Foxen Mudstone were apparently influenced to a greater degree by tectonism than by eustatic or 

large-scale paleoceanographic events (Ramirez, 1990; Barron, 

1992; Ramirez and Garrison, this volume). In the following 

sections we address the paleoceanography, sea level record, 

and tectonic setting of the lowermost Foxen.

Paleoceanography and Eustatic Sea Level

Deposition of the lower part of the Foxen Mudstone above 

the hemipelagic facies can be correlated to a well-dated oceanographic/climatic event, thus providing some constraint to the 

timing and setting of the basal Foxen siliceous facies. Planktonic foraminiferal biofacies at Deep Sea Drilling Project 

(DSDP) Site 173 off of northern California indicate a warming of surface waters at about 4 Ma (Keller, 1979), and both 

planktonic assemblages and oxygen isotopes indicate a return 

to colder surface waters at about 3.2 Ma (Shackleton and 

Opdyke, 1977; Keller, 1979). This warm interval is part of a 

widespread mid-latitude climatic event documented across the 

Pacific Ocean (Keller, 1979; Kennett, 1986). Although foraminifera cannot be extracted from the siliceous rocks of the 

basal Foxen, overlying strata of the lower Foxen, some 400 m 

above the formational boundary in the Los Flores No. 1 well, 

contain the distinctive planktonic foraminiferal biofacies of 

the middle Pliocene warm event, indicating that the basal Foxen 

strata were deposited prior to the termination of the warm event 

at 3.2 Ma.

The episode of middle Pliocene warming also corresponds 

to a longer interval of decreased production and (or) poor preservation of diatoms (4.8-2.6 Ma) at a number of DSDP sites 

offshore of California (Sites 173,467, and 469) (Barron, 1992). 

Because upwelling and productivity along the California margin are largely driven by the strength of the cool, southwardV10 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
flowing California Current (Sverdrup and others, 1942; Hickey, 

1979; Gorsline and Teng, 1989), Barron (1992) argues that 

the poor diatom record reflects high-latitude warming and the 

weakening or narrowing of the eastern boundary current. 

Therefore, although diatom sedimentation continued until the 

late Pliocene in a few locations (for example, the Santa Maria 

basin; Barron, 1992), general conditions along the Pliocene 

California margin were not conducive for widespread production and accumulation of biosiliceous sediments.

The record of Pliocene eustatic sea level (as indicated by 

seismic sequence stratigraphy) shows a series of sea level 

lowerings and lesser sea level rises during an overall regressive trend from an early Pliocene highstand (Haq and others, 

1988; fig. 6). The Pliocene highstand lasted over 1.2 m.y., from 

~5 Ma to 3.8 Ma, and included a minor sea level dip at 4.2 

Ma. The highstand was terminated by a sharp fall at 3.8 Ma, 

followed by others at 3.0 Ma and 2.4 Ma (Haq and others, 

1988). The overall trend reflects the growth of the Antarctic 

ice sheet and the establishment of northern hemisphere continental glaciation after 2.6-2.4 Ma (Kennett, 1986). As the entire 

Foxen Mudstone was probably deposited between 3.8 Ma and 

3.0 Ma (fig. 6), the siliceous strata of the basal Foxen were 

likely deposited entirely during the early Pliocene lowstand 

(3.8-3.4 Ma) proposed by Haq and others (1988). Although it 

is possible that Foxen deposition began as early as 4.0 Ma 

(see previous discussion of age) in the midst of the early 

Pliocene highstand, the abrupt and distinct changes in lithology (for example, color, TOC, grain size, and composition) 

suggest that the depositional environment changed just as 

markedly, such as across a major drop in eustatic sea level.

Tectonics

The Neogene Santa Maria basin and the California borderland developed within a zone of transform deformation 

between the Pacific and North American plates (Atwater, 1970, 

1989; Blake and others, 1978). Initial tectonic subsidence in 

the Santa Maria basin took place within a broad strike-slip to 

transtensional plate margin between about 17.5 Ma and 6.0 

Ma (Blake and others, 1978; Atwater, 1989; McCrory and others, 1995). Regional tectonic stresses changed in the latest 

Miocene and early Pliocene (Ingle, 1980), possibly in response 

to the eastward jump of the plate boundary from the base of 

the continental slope to the San Andreas Fault in southern and 

Baja California at 5.5 Ma (Sedlock and Hamilton, 1991); this 

new configuration created a restraining margin geometry and 

initiated a component of compressive shear in many southern 

and central California basins (Atwater, 1989). The resulting 

deformation and localized uplift contributed to high terrigenous sedimentation rates during deposition of the Sisquoc 

Formation (Ramirez, 1990; Barron, 1992; Ramirez and Garrison, this volume). In the late Miocene to early Pliocene, relative motion between the Pacific and North American plates 

along the California coast rotated 14° clockwise, evolving from

chiefly transform to transpressive motion (Harbert and Cox, 

1989; Harbert, 1991). This transpressional regime continues 

to the present day in the Santa Maria basin (Feigl and others, 

1990).

Although originally dated as early Pliocene (-3.9-3.4 

Ma; Harbert and Cox, 1989; Harbert, 1991), the tectonic reorganization is now thought to have initiated as early as 6-5 Ma 

(Cande and others, 1992; Wilson, 1993). In fact, the onset of 

crustal shortening at different locations across the central California margin during the late Neogene was markedly 

diachronous (McCrory and others, 1995; C. Sorlein, 1995, oral 

commun.). Yet, throughout the California borderland, lateral 

variations in formational thickness and facies provide compelling evidence for the Pliocene deformational phase (Ingle, 

1980). Canfield (1939), Woodring and others (1943), and 

Woodring and Bramlette (1950) proposed that the modern 

anticlines of the Santa Maria basin began to develop during 

Foxen deposition, controlling the distribution of the thick synclinal deposits and the thin or absent anticlinal sections. This 

hypothesis is further substantiated by balanced structural crosssections across the onshore Santa Maria basin which also suggest that regional compression commenced during deposition 

of the Foxen [estimated by Namson and Davis (1990) as 4.0- 

2.0 Ma]. Pliocene deformation is also demonstrated by a prominent, widespread early to late Pliocene angular unconformity 

in the offshore Santa Maria basin, placed at the boundary between the Sisquoc Formation and Foxen-equivalent strata, 

(Page and others, 1979; Grain and others, 1987; Clark and 

others, 1991). Clark and others (1991) placed the unconformity 

and the base of the "Foxen" at -3.4 Ma. The distinctly dolomitic and siliceous hemipelagic Foxen facies directly overlies 

the Sisquoc Formation in at least three wells from the offshore Santa Maria basin (P-0395 No. 1, P-0396 No. 1, and P0425 No. 2; fig. 1).

Namson and Davis (1990) suggest that much of the surface and shallow deformation across the onshore Santa Maria 

basin reflects thrusting along a deep, blind, north-vergent, lowangle detachment surface the Purisima-Solomon Thrust 

somewhat modified by shallower, high-angle normal and 

reverse faults (figs. 4, 7). The Lompoc-Purisima anticlinal 

trend is interpreted as a fault-bend fold that overlies the deep 

ramp of the Purisima-Solomon Thrust and the CasmaliaOrcutt anticlinal trend as a fault-propagation fold at the shallow, leading edge of the blind thrust. In this scenario, southward 

tilting and subsidence of the Santa Maria Valley syncline resulted from downdropping and rotation of hanging wall blocks 

on the north flank of the Casmalia-Orcutt anticline, and by 

tectonic loading of the footwall (foreland). The San AntonioLos Alamos syncline is located between the ramps underlying 

the two anticlinal trends and is shaped by the converging limbs 

of the Casmalia-Orcutt anticline to the north and the LompocPurisima anticline to the south (fig. 7). As the hanging wall 

strata in the San Antonio-Los Alamos syncline have been transported past the Lompoc-Purisima ramp, their fault-related 

motion has been entirely horizontal (fig. 7). In the Santa Maria

The Sisquoc Formation-Foxen Mudstone Boundary in the Santa Maria Basin, California: Sedimentary Response to the New Tectonic Regime V11
basin, the uplifted, structurally thickened anticlines are closely 

spaced with a wavelength of about 15 km. Locations such as 

the San Antonio-Los Alamos and Santa Maria Valley synclines, 

which were not actively growing upward during compressional 

shortening but were adjacent to structurally thickening sections, would likely experience isostatic subsidence in response 

to the increased tectonic load rigidly supported across the basin (fig. 7).

INTERPRETATION

We propose that the Sisquoc Formation-Foxen Mudstone 

boundary coincides with the onset or acceleration of crustal 

shortening and the development of marked bathymetric relief 

in the Santa Maria basin. Although growth of anticlinal ridges 

and synclinal troughs may have begun during Sisquoc deposition in the latest Miocene and earliest Pliocene (Ramirez, 1990; 

Sedlock and Hamilton, 1991) in concert with the initiation of 

a transpressive relationship between the Pacific and North 

American Plates, topographic relief increased dramatically 

with the local acceleration of compressional thrust faulting in 

the late early Pliocene (3.8-3.4 Ma) along new and reactivated faults striking to the west and northwest from the southeastern corner of the basin (Namson and Davis, 1990).

Canfield (1939) showed that the base of the Foxen Mudstone in the synclinal axis of the Santa Maria Valley field is a 

transgressive surface, yet the eustatic sea-level curve of Haq

and others (1988) shows a sea level fall and lowstand from 3.8 

Ma to 3.4 Ma during deposition of the lower part of the Foxen. 

Concurrently, benthic foraminiferal biofacies [for example, 

Standard Los Flores No. 1 well (fig. 3) and in Woodring and 

Bramlette (1950)] document a significant increase in water 

depth coincident with deposition of basal Foxen sediments 

(fig. 6). These data show that water depth increased from outer 

neritic (100-150 m) during deposition of the upper part of the 

Sisquoc Formation to upper bathyal depths (-500 m) during 

deposition of the lower part of the Foxen, before the global 

eustatic sea level rise at 3.4 Ma. We infer that local transgression and increased water depth resulted from subsidence of 

synclinal areas that outpaced the eustatic sea level fall. Exaggerated relief or bathymetric gradients are suggested by the 

coarsening-upward trend from the fine-grained upper part of 

the Sisquoc into the silty sand and conglomerates found in the 

basal Foxen in some portions of the Santa Maria Valley Field 

(Canfield, 1939; Woodring and Bramlette, 1950). At the same 

time that thick Foxen deposits accumulated in subsiding 

troughs, sediments were winnowed from anticlinal highs and 

little or no sediment accumulated there (Woodring and others, 

1943; Woodring and Bramlette, 1950). Accelerated subsidence of the Santa Maria Valley and San Antonio-Los Alamos 

synclines was likely due to rigid isostatic compensation for 

the thrust-related structural thickening throughout the Santa 

Maria basin; the synclines plummeted while the LompocPurisima and Casmalia-Orcutt anticlines continued to be uplifted over their respective subsurface thrust ramps.

Regions of 

tectonic uplift

Regions of tectonic 

load subsidence

sw

Lompoc-Purisima 

anticline

YVY

Orcutt anticline

San AntonioLos Alamos 

syncline

Santa Maria

Valley 

( syncline

Point San Luis 

anticline NE

10km

Point San Luis Thrust

10km

Figure 7. Structural model of section B-B' (figs. 1, 4) of compressional structures of the onshore Santa Maria basin. 

Modified from Namson and Davis (1990) to show regions of thrust ramp-related uplift and regions of tectonic load 

subsidence. Arrows with balls show vectors of blocks with thrust-related uplift and those with purely horizontal motion. 

Blocks with strictly horizontal fault motion would be expected to display tectonic load subsidence by increased 

paleobathymetry until filled by rapid sedimentation.

V12 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
The siliceous component of the hemipelagic facies of the 

basal Foxen Mudstone could express increased diatom productivity or decreased dilution of pelagic sediments by terrigenous siliciclastics, or both. Planktonic biofacies and the 

oxygen-isotope record indicate that surface waters of the late 

early Pliocene California margin were too warm, and the California Current too weak, to reflect vigorous upwelling and 

diatom productivity and preservation (Shackleton and Opdyke, 

1977; Keller, 1979; Kennett, 1986; Barron, 1992). It is more 

likely that the local transgression induced by tectonic load 

subsidence of the late early Pliocene Santa Maria Valley and 

San Antonio-Los Alamos troughs sequestered terrigenous 

detritus near the shoreline, resulting in the slow accumulation 

of relatively pure, hemipelagic, siliceous Foxen sediments in 

the newly subsiding troughs. Thus, the basal Foxen cherts and 

porcelanites primarily record a tectonic signal the tectonic 

load subsidence of localized depositional troughs due to compressional thrust faulting. In this sense, the tectonic and relative sea level control of sedimentation in the basal Foxen is 

quite similar to the factors responsible for the deposition of 

the detritus-poor, biosilica-rich Miocene Monterey Formation 

(Isaacs, 1984), but for a much briefer interval (-0.5 m.y. for 

the Foxen, as compared to -12 m.y. for the Monterey)

The presence of numerous dolostone horizons within the 

lowermost Foxen Mudstone similarly attests to siliciclastic 

starvation and slow sedimentation rates. Within organic-rich 

sediments, dolomite precipitation may be initiated or accelerated during pauses in sedimentation when sulfate and methane can continuously diffuse to the high-alkalinity zone of 

anoxic methane oxidation (Middelburg and others, 1990). 

Accordingly, many dolostone horizons in hemipelagic sedimentary sequences are representative of condensed sedimentation and parasequence boundaries (Grimm, 1992; L. White 

and R. Garrison, 1992, oral commun.). Therefore, the presence of numerous dolostone beds and nodular horizons in 

the lowermost Foxen suggests siliciclastic sediment starvation and decreased sedimentation rates during the local 

transgression caused by rapid tectonic subsidence of depositional troughs.

Comparison between sediments from the upper part of 

the Sisquoc Formation and the lowermost hemipelagic Foxen 

Mudstone indicates that the formational contact marks an environmental change from oxygenated to oxygen-deficient 

waters. In the subsurface, the diatomaceous siltstones and 

claystones of the upper part of the Sisquoc are light colored 

and massive to weakly banded, whereas siliceous rocks of the 

lowermost Foxen are dark colored and thinly laminated. Furthermore, the Foxen mudrocks are slightly phosphatic, whereas 

strata from the upper part of the Sisquoc are not. Upper Sisquoc 

sediments from the Coastal Hunter Careaga No. 3 well have 

low total organic carbon (1.1-1.3 percent), probably reflecting active bioturbation under oxygenated waters, whereas the 

dark-brown color, laminations, phosphate, and higher TOC 

(3.3-4.1 percent) of the lowermost Foxen sediments all suggest deposition within oxygen-deficient conditions (fig. 6).

Faunal evidence gives minor substantiation to the hypothesis of low-oxygen conditions during deposition of the lowermost Foxen Mudstone. No megafossils have been found from 

the lower part of the Foxen, whereas the upper part of the 

Sisquoc Formation contains abundant molluscan body fossils 

and molds (Woodring and others, 1943). This evidence suggests that larger invertebrates common in the shelf-depth upper Sisquoc sediments were excluded from lower Foxen 

sediments as synclinal troughs deepened enough (-500 m) to 

intercept oxygen-depleted upper intermediate waters.

SUMMARY

Plate tectonic reorganization in the late Miocene to early 

Pliocene shifted the Californian Pacific-North American Plate 

margin from a transform to transpressive tectonic regime. 

Crustal shortening within the onshore Santa Maria basin initiated or accelerated at 3.8-3.4 Ma, producing a dynamic relief 

of high-standing bathymetric ridges separated by low-lying 

synclinal troughs during deposition of the Pliocene Foxen 

Mudstone. A distinct change in sedimentation patterns across 

the Sisquoc Formation-Foxen Mudstone boundary was principally controlled by this tectonism. Rapid tectonic subsidence of the Santa Maria Valley and San Antonio-Los Alamos 

synclines is locally marked by angular unconformities in the 

subsurface, by paleobathymetric deepening from outer neritic 

(100-150 m) to lower upper bathyal environments (-500 m), 

and by the accumulation of biosiliceous and dolomitic sediments in the synclinal troughs. Biostratigraphic data from the 

lower part of the Foxen in the Standard Los Flores No. 1 well 

unequivocally demonstrate that water deepened to 500-600 

m before the global eustatic sea level rise at 3.4 Ma, indicating that synclinal subsidence was tectonically driven. The purer 

siliceous strata of the basal Foxen apparently resulted from a 

decrease in terrigenous input due to local transgression during a period of global eustatic sea level fall, rather than from 

an increase in diatom productivity and accumulation. Thick 

sequences of lower Foxen sediments accumulated only in subsiding synclinal troughs, which were initially semi-isolated 

and probably held oxygen-deficient waters. This environment 

is reflected in the dark-brown color, higher TOC, laminations, 

phosphate, dolomite, and lack of megafauna in the hemipelagic 

facies of the basal Foxen.

ACKNOWLEDGMENTS

RJB would like to thank Douglas Milham and Lewis 

Sutfin of Epoch and Horizon Well Logging, Inc., for their contributions in identifying, correlating, and recognizing the stratigraphic significance of the basal Foxen Mudstone porcelanites 

in the Santa Maria basin. We would like to thank the U.S. 

Geological Survey and the Minerals Management Service for 

providing access to samples, well data, and analytical faciliThe Sisquoc Formation-Foxen Mudstone Boundary in the Santa Maria Basin, California: Sedimentary Response to the New Tectonic Regime V13
Warren, A.D., 1980, Calcareous nannoplankton biostratigraphy of Woodring, W.P., and Bramlette, M.N., 1950, Geology and paleonCenozoic marine stages in California, in Kleinpell, R.M., The tology of the Santa Maria district, California: U.S. Geological

Miocene stratigraphy of California revisited: American Asso- Survey Professional Paper 222, 185 p.

ciation of Petroleum Geologists Studies in Geology No. 11, p. Woodring, W.P., Bramlette, M.N., and Lohman, K.E., 1943, Stratig97-122. raphy and paleontology of the Santa Maria district, California:

Wilson, D.S., 1993, Confirmation of the astronomical calibration American Association of Petroleum Geologists Bulletin, v. 27,

of the polarity time scale from rates of sea floor spreading: p. 1335-1360. 

Nature, v. 364, p. 788-790.

V16 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province

*U.S. GOVERNMENT PRINTING OFFICE: 1998 - 673-048 / 20104 REGION NO. 8


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