Neogene Geohistory Analysis of 

Santa Maria Basin, California, and 

Its Relationship to Transfer of 

Central California to the Pacific Plate

Diatom Biochronology of the 

Sisquoc Formation in the Santa Maria Basin, 

California, and Its Paleoceanographic and 

Tectonic Implications

Geophysical section offshore Santa Maria basin

^^^^;W^^v^ -/: / - -' ' -"tt-A/.- < .-.vf M I. --« :^^^K .;.-si/-/issilm*z

Geologic section onshore Santa Maria basin

U.S. GEOLOGICAL SURVEY BULLETIN 1995-J, K
AVAILABILITY OF BOOKS AND MAPS OF THE U.S. GEOLOGICAL SURVEY

Instructions on ordering publications of the U.S. Geological Survey, along with prices of the last offerings, are given in the currentyear issues of the monthly catalog "New Publications of the U.S. Geological Survey." Prices of available U.S. Geological Survey 

publications released prior to the current year are listed in the most recent annual "Price and Availability List." Publications that are 

listed in various U.S. Geological Survey catalogs (see back inside cover) but not listed in the most recent annual "Price and Availability 

List" are no longer available.

Reports released through the NTIS may be obtained by writing to the National Technical Information Service, U.S. Department of 

Commerce, Springfield, VA 22161; please include NTIS report number with inquiry.

Order U.S. Geological Survey publications by mail or over the counter from the offices given below.

BY MAIL OVER THE COUNTER

Books

Professional Papers, Bulletins, Water-Supply Papers, Techniques of Water-Resources Investigations, Circulars, publications 

of general interest (such as leaflets, pamphlets, booklets), single 

copies of Earthquakes & Volcanoes, Preliminary Determination of 

Epicenters, and some miscellaneous reports, including some of 

the foregoing series that have gone out of print at the Superintendent of Documents, are obtainable by mail from

U.S. Geological Survey, Information Services

Box 25286, Federal Center

Denver, CO 80225

Subscriptions to periodicals (Earthquakes & Volcanoes and 

Preliminary Determination of Epicenters) can be obtained ONLY 

from the

Superintendent of Documents

Government Printing Office

Washington, DC 20402

(Check or money order must be payable to Superintendent of 

Documents.)

Maps

For maps, address mail orders to

U.S. Geological Survey, Map Distribution

Box 25286, Bldg. 810, Federal Center

Denver, CO 80225

Residents of Alaska may order maps from

U.S. Geological Survey, Earth Science Information Center

101 Twelfth Ave., Box 12

Fairbanks, AK 99701

Books and Maps

Books and maps of the U.S. Geological Survey are available 

over the counter at the following U.S. Geological Survey offices, 

all of which are authorized agents of the Superintendent of Documents.

 ANCHORAGE, Alaska-^230 University Dr., Rm. 101

 LAKEWOOD, Colorado Federal Center, Bldg. 810

 MENLO PARK, California Bldg. 3, Rm. 3128, 345 Middlefield Rd.

 RESTON, Virginia National Center, Rm. 1C402, 12201 

Sunrise Valley Dr.

 SALT LAKE CITY, Utah Federal Bldg., Rm. 8105, 125 

South State St.

 SPOKANE, Washington U.S. Post Office Bldg., Rm. 135, 

W. 904 Riverside Ave.

 WASHINGTON, D.C. Main Interior Bldg., Rm. 2650, 18th 

and C Sts., NW

Maps Only

Maps may be purchased over the counter at the U.S. Geological Survey offices:

 FAIRBANKS, Alaska New Federal Building, 101 Twelfth 

Ave.

 ROLL A, Missouri 1400 Independence Rd.

 STENNIS SPACE CENTER, Mississippi Bldg. 3101
Neogene Geohistory Analysis of 

Santa Maria Basin, California, and 

Its Relationship to Transfer of 

Central California to the Pacific Plate

By PATRICIA A. McCRORY, DOUGLAS S. WILSON, 

JAMES C. INGLE, Jr., and RICHARD G. STANLEY

Diatom Biochronology of the 

Sisquoc Formation in the Santa Maria Basin, 

California, and Its Paleoceanographic and 

Tectonic Implications

By MICHAEL P. DUMONT and JOHN A. BARRON

Chapters J and K are issued as a single volume 

and are not available separately

U.S. GEOLOGICAL SURVEY BULLETIN 1995

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

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

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director

Any use of trade, product, or firm names

in this publication is for descriptive purposes only

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

Text and illustrations edited by James W. Hendley II

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1995

For sale by

U.S. Geological Survey

Information Services

Box 25286, Federal Center

Denver, CO 80225

Library of Congress Cataloging in Publication Data

Neogene geohistory analysis of Santa Maria Basin, California, and its 

relationship to transfer of central California to the Pacific Plate / by 

Patricia A. McCrory ... [et al.]. Diatom biochronology of the Sisquoc Formation 

in the Santa Maria Basin, California, and its paleoceanographic and tectonic 

implications / by Michael P. Dumont and John A. Barron.

p. cm. (Evolution of sedimentary basins/onshore oil and gas 

investigations Santa Maria Province ; ch. J-K) (U.S. Geological Survey 

bulletin; 1995)

"Chapters J and K are issued as a single volume and are not available 

separately."

Includes bibliographical references.

Supt. of Docs, no.: I 19.3:1995-J, K

1. Geology, Stratigraphic Neogene. 2. Geology California Santa Maria 

Basin. 3. Plate tectonics California Santa Maria Basin. 4. Diatoms, Fossil 

California Santa Maria Basin. 5. Paleontology, Stratigraphic. 

6. Paleontology Miocene. 7. Paleontology Pliocene. 8. Sisquoc Formation 

(Calif.) 9. Santa Maria Basin (Calif.) I. McCrory, Patricia Alison. 

II. Dumont, Michael P. Diatom biochronology of the Sisquoc Formation in the 

Santa Maria Basin, California, and its paleoceanographic and tectonic 

implications. III. Barron, John A. IV. Series. V. Series: U.S. Geological Survey 

bulletin ; 1995. 

QE75.B9 no. 1995-J-K 

[QE693.5]

557.3 s dc20 95-19019 

[551.7'86'09794] CIP
Chapter J

Neogene Geohistory Analysis of

Santa Maria Basin, California, and

Its Relationship to Transfer of Central California

to the Pacific Plate

By PATRICIA A. McCRORY, DOUGLAS S. WILSON, 

JAMES C. INGLE, Jr., and RICHARD G. STANLEY

U.S. GEOLOGICAL SURVEY BULLETIN 1995

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

Edited by Margaret A. Keller
CONTENTS

Abstract Jl 

Introduction Jl

The southern Coast Ranges J3

Santa Maria basin lithostratigraphy J5

Acknowledgments J5 

Late Cenozoic tectonism in the Santa Maria area J5

Style, timing, and magnitude of late Cenozoic tectonism J6 

Santa Maria basin study sites J7

Point Arguello area (offshore) J8

Point Sal area (onshore) J12

Central basin area (onshore) J14

Eastern basin area (onshore) J16

Tectonic events recorded at study sites J17 

Late Cenozoic plate tectonic setting of central California J18

Reconstructed positions of central California J21

Reconstructed positions of the Farallon microplates J24 

Discussion J29 

Summary J31 

References cited J31 

Appendix Data used in backstrip programs J35

FIGURES

LA. Map showing present location of major tectonic features and normally magnetized 

magnetic anomalies on oceanic crust of central California continental margin and 

adjacent offshore area J2

IB. Location map of Santa Maria province and vicinity showing distribution of Neogene sedimentary rocks, major faults, and anticlinal fold axes J3

2. General lithostratigraphy for onshore Santa Maria basin J4

3. Late Cenozoic time scale for northeast Pacific Ocean J8

4A. Lithostratigraphy and paleobathymetry for Cost Well (OCS-CAL 78-164-1) J9

45. Age-depth plot for Cost well (OCS-CAL 78-164-1) showing inflection points indicating changes in rate of sediment accumulation J10

4C. Backstrip plot of the Cost well (OCS-CAL 78-164-1) showing isostatic loading 

and tectonic components of vertical movement J10

5A. Lithostratigraphy and paleobathymetry for Texaco Nautilus well (OCS-P 0496- 

1) Jll

5B. Age-depth plot for Texaco Nautilus well (OCS-P 0496-1) showing inflection 

points indicating changes in rate of sediment accumulation Jll

5C. Backstrip plot of Texaco Nautilus well (OCS-P 0496-1) showing isostatic loading 

and tectonic components of vertical movement J12

6A. Lithostratigraphy and paleobathymetry for Point Sal composite section. J13

6B. Age-thickness plot for Point Sal composite section showing inflection points indicating changes in rate of sediment accumulation J14

6C. Backstrip plot for Point Sal composite section showing isostatic loading and tectonic components of vertical movement J14

IV Contents
7A. Lithostratigraphy and paleobathymetry for Union Newlove No. 51 well (Orcutt

Field) J15 

IB. Age-depth plot for Union Newlove No. 51 well (Orcutt Field) showing inflection

points indicating changes in rate of sediment accumulation J16 

1C. Backstrip plot for the Union Newlove No. 51 well (Orcutt Field) showing isostatic loading and tectonic components of vertical movement J16 

8A. Lithostratigraphy and paleobathymetry for the Tidewater Davis No. 1 well (Zaca

Field) J17 

8J5. Age-depth plot for Tidewater Davis No. 1 well (Zaca Field) showing inflection

points indicating changes in rate of sediment accumulation J18 

8C. Backstrip plot for Tidewater Davis No. 1 well (Zaca Field) showing isostatic loading and components of vertical movement J18 

9A. Reconstruction of the Pacific, Farallon, and North America Plates with coastal

California block fixed, 33.0 Ma J19 

9B. Reconstruction of Pacific Plate, Monterey Microplate, Arguello Microplate, and

North America Plate with coastal California block fixed, 30.0 Ma J20 

9C. Reconstruction of Pacific Plate, Monterey Microplate, Arguello Microplate, and

North America Plate with coastal California block fixed, 24.0 Ma J21 

9D. Reconstruction of Pacific Plate, Monterey Microplate, Arguello Microplate, and

North America Plate with coastal California block fixed, 19.7 Ma J22 

9E. Reconstruction of Pacific Plate, Arguello Microplate, and North America Plate

with coastal California block fixed, 17.0 Ma J23 

9F. Reconstruction of the Pacific and North America Plates with coastal California

block fixed, 15.0 Ma J24 

9G. Reconstruction of the Pacific and North America Plates with coastal California

block fixed, 10.0 Ma J25 

9H. Reconstruction of the Pacific and North America Plates with coastal California

block fixed, 5.0 Ma J26 

91. Modern configuration of Pacific Plate, relict Monterey Microplate, and North

America Plate J27

10. Simplified summary diagram of figure 9 showing change in plate boundaries 

through time and transfer of central California from the North America Plate to 

Pacific Plate J28

11. Plate vectors in velocity space at 35° N., 122° W. with coastal California 

fixed J29

TABLES

1. Porosity-depth formulas used to decompact lithologies in backstrip program J8

2. Model for relative motion of western North America microplates J30

Contents

Neogene Geohistory Analysis of Santa Maria Basin, 

California, and Its Relationship to Transfer of 

Central California to the Pacific Plate

By Patricia A. McCrory, Douglas S. Wilson1 , James C. Ingle, Jr.2 , and Richard G. Stanley

Abstract

The central California continental margin records a 

complex response to changing plate motions during the 

Cenozoic. The record of Cenozoic tectonism preserved in 

Santa Maria basin strata, integrated with plate reconstructions, provides insights into the evolution of the central 

California continental margin during its transition from a 

convergent to a transform plate boundary and its transfer 

from the North America to the Pacific Plate. The continuation of subduction of the Monterey Microplate in the Miocene has significant implications for the evolution of the San 

Andreas transform boundary. In particular, this boundary 

originated as two separate segments. The first segment 

formed south of the Pioneer Fracture Zone about 30 Ma, 

probably near central California. The second formed between the Morro and Murray Fracture Zones about 25 Ma. 

The capture of the Monterey Microplate by the Pacific Plate 

about 19 Ma, terminating subduction, abruptly linked these 

transform boundaries to form a much more extensive 

Pacific-North America boundary.

Backstrip analyses of Santa Maria basin strata suggest 

that interactions between adjacent oceanic plates and the 

continental margin dominated the tectonic evolution of 

central California. The onset of late early Miocene subsidence and volcanism is likely linked to the transition from 

subduction to strike-slip tectonics, resulting from cessation 

of Monterey ridge spreading. Miocene subsidence occurred 

in two stages. An initial rapid phase (about 18 to 16 Ma) is 

attributed to extreme local extension of the continental 

crust, associated with the beginning of western Transverse 

Range rotation and triggered by Monterey Microplate capture. A subsequent slower phase (about 16 to 7 Ma) is 

attributed to thermal subsidence, associated with cooling of 

underplated young oceanic lithosphere. Transfer of the 

Monterey Microplate remnant to the Pacific Plate when 

spreading ceased would have created a wide Pacific-North 

America subhorizontal boundary zone at depth and perhaps 

allowed distributed shear between the partially subducted

University of California, Santa Barbara, CA 93106. 

2Stanford University, Stanford, CA 94305.

Manuscript approved for publication April 3, 1995.

oceanic lithosphere and overlying relict accretionary complex, creating a mechanism for in situ rotation of overlying 

fault-bounded blocks. Tectonic uplift of the Point Sal and 

Point Arguello areas in late Miocene time may have been 

related to regional development of a fold and thrust belt in 

response to reorientation of the Pacific Plate velocity vector 

about 5.5 Ma, as well as rotation of the western Transverse 

Ranges in a constraining geometry. Since about 3 Ma, 

motion along the plate boundary has been partitioned 

between strike-slip along the San Andreas Fault system and 

compression along reverse faults and associated folds both 

west and east of the San Andreas Fault system.

INTRODUCTION

The California continental margin records a complex 

response to changing plate motions in the Cenozoic. During 

the Cenozoic, as segments of the Pacific-Farallon spreading 

ridge approached the Farallon-North America subduction 

margin, spreading ceased and a newly formed Pacific-North 

America plate boundary elongated across the former subduction margin. At least two relict Farallon-derived microplates, in central California and Baja California, were 

stranded underneath the margin when spreading ceased on 

the associated ridge segments. Both microplate fragments 

were captured by the Pacific Plate, creating transient, broad 

Pacific-North America transform boundaries in each area.

The central California continental margin has been the 

focus of numerous geological and geophysical studies 

because of its crucial record of the tectonic evolution of the 

continental margin of North America, the petroleum accumulations within sedimentary basins along the margin, and the 

siting of energy facilities along the coast. However, the 

details of tectonic evolution are obscured by the overprinting 

of sequential phases of tectonic deformation, the uplift and 

erosion of Mesozoic and early Paleogene marine strata during 

the Oligocene, the lack or poor quality of surface exposures 

in many areas, the lack of known piercing points on major 

faults, and until recently, the lack of detailed geophysical 

imaging of deep structural features offshore and onshore.

Neogene Geohistory Analysis of Santa Maria Basin, California, and Its Relationship to Transfer of Central California to the Pacific Plate J1
Santa Maria province lies within central California at 

the southernmost end of the southern Coast Ranges and 

for the purposes of this report is bounded on the west by

the Santa Lucia Escarpment, on the south by the Santa 

Ynez Fault, and on the northeast by the Sur-Nacimiento 

Fault Zone (fig. 1). This province contains several filled

40°

38°

36°

34° -

32°

30°

128° 126° 124° 122° 120° 118°

Figure 1A Map showing present location of major tectonic features and normally magnetized magnetic 

anomalies (areas with patterns of crosses) on oceanic 

crust (see fig. 9 for age of numbered isochrons) of central 

California continental margin and adjacent offshore 

area. Solid lines are active transform faults; dashed lines

are fracture zones. Paleo-spreading ridge shown by 

dashed double lines; paleo-subduction zone shown by 

barbed dashed line. Offshore continental crust has lighter 

shading; onshore continental crust has darker shading. 

FZ, fracture zone. Box indicates area of figure 1 B. Modified from Lonsdale, 1991; Fernandez and Hey, 1991.

J2 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
sedimentary basins both offshore and onshore, including 

the Sur, Santa Lucia, Santa Maria, and Pismo-Huasna 

basins. Cenozoic strata within these basins contain a 

record of the timing and distribution of tectonic deformation along the evolving plate boundary.

This report uses the record of vertical tectonic and 

isostatic movement preserved in offshore and onshore 

Santa Maria basin strata, coupled with both recent published accounts of regional folding and faulting histories 

(Namson and Davis, 1990; Sedlock and Hamilton, 1991; 

Clark and others, 1991) and recent plate reconstructions 

(Atwater, 1989; Lonsdale, 1991; Fernandez and Hey, 

1991), to develop insights into the interplay between plate 

kinematics and continental margin deformation styles and 

rates. Because this complex region has several major unresolved tectonic questions, our tectonic reconstruction must 

be considered preliminary. More accurate reconstructions 

will require better constrained regional rotation histories 

and fault displacement histories and quantification of 

crustal shortening, as well as better constrained histories 

of microplate location along the continental margin.

The Southern Coast Ranges

Basement rocks of the southern Coast Ranges include 

subducted and uplifted oceanic sedimentary and volcanic 

rocks of the Mesozoic Franciscan Complex (Page, 1981), 

island-arc rocks of the Jurassic Coast Range ophiolite 

(Hopson and Frano, 1977) and overlying Upper Jurassic to

36°

35° _

34°

PISMO-HUASNA 

BASING

SANTA BARBARA-VENTURA 

BASING

122

Figure 16. Location map of Santa Maria province and vicinity showing distribution of Neogene sedimentary rocks, 

major faults, and anticlinal fold axes (modified from Jennings, 1977; Steritz, 1986). Onshore area has shading;

119°

onshore and offshore Neogene sedimentary rocks denoted 

by diagonal lines. A, Cost well; B, Texaco Nautilus well; C, 

North Beach site; D, Mussel Rock site; E, Union Los Nietos 

well; F, Union Newlove well; C, Tidewater Davis well.

Neogene Geohistory Analysis of Santa Maria Basin, California, and Its Relationship to Transfer of Central California to the Pacific Plate J3
Paleogene sedimentary rocks resembling forearc strata of 

the Great Valley sequence (fig. 2), and Cretaceous continental-arc plutons that intrude older metasedimentary 

rocks (Ross, 1984; Mattinson and James, 1985). These 

basement rocks are assigned to the San Simeon, Stanley 

Mountain, and Salinia tectonostratigraphic terranes, 

respectively, which were juxtaposed as early as the Late 

Cretaceous and accreted to North America during Paleocene or Eocene time (Vedder and others, 1983; Howell 

and others, 1987). Inferred late Cenozoic strike-slip displacement juxtaposed younger marine sedimentary rocks 

of the Patton terrane to the west of the San Simeon terrane 

(Page and others, 1979a, 1979b; McCulloch, 1987).

In much of the southern Coast Ranges, Mesozoic and 

Paleogene rocks are unconformably overlain by upper Oligocene and lower Miocene strata, including nonmarine

clastic rocks, rhyodacitic and basaltic volcanic rocks (26- 

22 Ma; Ernst and Hall, 1974), neritic sandstone, and bathyal fine-grained clastic rocks. In early Miocene time, 

extension and rapid subsidence occurred in the Sur, Santa 

Lucia, Santa Maria, and Pismo-Huasna basins (McCulloch, 1987; Tennyson, 1989; Tennyson and others, 1991). 

The Santa Maria basin (for the purposes of this report, 

"Santa Maria basin" will denote both the offshore and 

onshore components of the basin) contains as much as 

4.5 km of Miocene and younger strata (Woodring and 

Bramlette, 1950) on top of Point Sal ophiolite, Franciscan 

Complex, and Great Valley sequence basement rocks 

(McLean, 1991). Basin fill includes late early Miocene 

(18-16 Ma) bimodal volcanic rocks (Canfield, 1939; Dibblee, 1950, 1966; Woodring and Bramlette, 1950; McLean, 

1991) with oceanic-ridge affinities (Cole and Basu, 1992).

LITHOLOGY

20 _

40 _

60 _

80

100 _

120 _

140

160 _

180 _

Figure 2. General lithostratigraphy for onshore Santa Maria basin (modified 

from Woodring and Bramlette, 1950; Stanley and others, 1991).

J4 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
Santa Maria Basin Lithostratigraphy

The oldest Neogene fill within onshore Santa Maria 

basin consists of lower Miocene nonmarine conglomerate, 

sandstone, and minor tuff of the Lospe Formation (Stanley 

and others, 1991) (fig. 2). The lower Miocene Point Sal 

Formation (Stanley and others, 1990) overlies Lospe strata 

or basement rocks (Namson and Davis, 1990). The Point 

Sal Formation consists of interbedded mudstone and bathyal sandstone and records rapid subsidence and initiation 

of deep-marine conditions within the basin. The widespread middle and upper Miocene Monterey Formation 

overlies the Point Sal Formation. The Monterey Formation 

consists of massive to well-laminated biosiliceous rocks, 

dolomite, and minor bathyal sandstone. The middle Miocene to Pliocene Sisquoc Formation conformably and 

locally unconformably overlies the Monterey Formation 

(Woodring and Bramlette, 1950). The Sisquoc Formation 

is composed of massive to weakly laminated biosiliceous 

rocks, siltstone, and bathyal sandstone. The Sisquoc Formation is conformably and unconformably overlain by 

marine siltstone and fine-grained sandstone of the Pliocene 

Foxen Mudstone. The Sisquoc Formation and Foxen Mudstone are thin or absent along the basin margins and anticlinal trends but are as much as 2 km thick in the deep,

EXPLANATION

Conglomerate and sandstone

Sandstone

Sandstone and mudstone

Siltstone and mudstone

Claystone

Siliceous and calcareous mudstone

Calcareous and phosphatic mudstone

pPIj Siliceous mudstone and siltstone

(- . .-I Siliceous and calcareous mudstone

I' ' -I and sandstone or siltstone

gr^Q S Volcaniclastic conglomerate and sandstone

L__Jvj Melange

[^+ + * [ Ophiolite

+++++ Tuff

_____ Hiatus or condensed interval 

A/WWW Angular unconformity 

Figure 2. Continued.

central part of the Santa Maria basin (Namson and Davis, 

1990).

The upper Pliocene Careaga Sandstone is a coarsegrained neritic sandstone deposited during basin shoaling. 

In the onshore portion of Santa Maria basin, the Careaga 

Sandstone commonly overlies the Foxen Mudstone in 

angular unconformity along anticlinal trends. Nonmarine 

gravel and alluvial sand of the Pliocene and Pleistocene 

part of the Paso Robles Formation and Pleistocene Orcutt 

Sand conformably and unconformably overlie the Careaga 

Sandstone. Widespread angular discordances, basin shoaling, and deposition of nonmarine coarse-grained units 

resulted from Pliocene and Quaternary uplift and erosion. 

This phase of uplift and erosion is attributed to a period of 

crustal shortening (Namson and Davis, 1990).

Acknowledgments

We thank Robert Paul of the Minerals Management 

Service for the release of unpublished lithologic and biostratigraphic data from offshore wells. We thank Scott 

Drewry of the Minerals Management Service for discussions regarding the foraminifers found in the well strata. 

We thank Mary Lou Cotton and Robert Arends of Unocal 

for biostratigraphic data from offshore wells. We thank 

Richard Behl, University of California, Santa Cruz, and 

Christopher Sorlien, University of California, Santa Barbara, for discussions regarding late Neogene subsurface 

stratigraphy in the Santa Maria basin.

LATE CENOZOIC TECTONISM IN THE 

SANTA MARIA AREA

The tectonic evolution of Santa Maria province is 

complex, and many details of structures and timing of 

deformation are difficult to resolve, owing to overprinting 

of differing tectonic phases, reversals of bathymetric 

relief, lack of age control, and inability to image deep 

structures. The lack of Paleogene strata except for isolated 

remnants (Vedder and others, 1991) hampers attempts at 

reconstruction in Santa Maria basin. In addition, when 

Baja California peninsula is restored to its pre-5.5 Ma 

position relative to mainland Mexico (by closure of the 

Gulf of California by 310 km) and the western Transverse 

Ranges are back rotated 90°, the Point Arguello area 

(fig. 1) lies at the latitude of northernmost Mexico (31.5°

N.).

Basins within Santa Maria province record multiple 

phases of tectonism: (1) An episode of late Oligocene to 

earliest Miocene subsidence (Tennyson, 1989; Tennyson 

and others, 1991), which is missing in Santa Maria basin 

proper, (2) a late early Miocene phase of normal faulting 

and rapid subsidence during inferred transtension (Stanley

Neogene Geohistory Analysis of Santa Maria Basin, California, and Its Relationship to Transfer of Central California to the Pacific Plate J5
and others, 1992), (3) a middle Miocene phase of slow 

subsidence, (4) a late Miocene episode of bathymetric 

inversion, (5) a subsequent episode of reverse faulting and 

folding during inferred transpression, and (6) a poorly 

resolved history of strike-slip faulting during clockwise 

rotation of the adjacent western Transverse Ranges 

throughout changing stress regimes. For most of Santa 

Maria province, detailed fault-slip data are not available; 

thus, although surface and shallow subsurface fault orientations are known in many cases, the exact direction and 

amount of slip, amount of extension or shortening, and the 

fault orientations at depth are not known. Also, some 

faults display evidence of reactivation with differing slip 

directions, and some faults display evidence of merging 

into a midcrustal detachment zone (Crouch and others, 

1984; McCulloch, 1987; Sedlock and Hamilton, 1991; 

Mclntosh and others, 1991; Clark and others, 1991). 

Regional syntheses of available geological and geophysical data and more recent geochronology provide constraints summarized below.

Style, Timing, and Magnitude of 

Late Cenozoic Tectonism

The major faults in the southern Coast Ranges and 

directly offshore trend northwestward (fig. 15). Regional 

mapping has documented few piercing points on these 

faults; thus, their displacement histories are poorly constrained. No displacement data are available for the Santa 

Lucia Bank Fault offshore, but its long, linear trace and its 

lateral offset of Arguello submarine canyon suggest strikeslip displacement (McCulloch, 1987).

The Hosgri Fault Zone, which separates northwesttrending basement structures in onshore Santa Maria basin 

from north-trending Neogene growth structures in offshore 

Santa Maria basin (McCulloch, 1987), consists of several 

northeast-dipping segments that merge with or terminate 

against west-trending structures near the northwestern end 

of the Transverse Ranges (Steritz, 1986). The northern 

segments of this fault system (San Gregorio and San Simeon segments) have had 150 km of dextral slip since the 

early Paleocene (Clark and others, 1984; James and Mattison, 1985). As much as 115 km of slip on the San Gregorio-San Simeon segments may have occurred since the 

early Miocene (Graham, 1978; Graham and Dickinson, 

1978). However, recent reevaluation of the southern Hosgri segment suggests that it has been primarily a reversefault system in the late Cenozoic (Neogene and younger) 

with no more than 5 km post-Oligocene dextral slip 

(Crouch and others, 1984; Clark and others, 1991; Sedlock 

and Hamilton, 1991). Regardless, the Hosgri segment juxtaposes basement rocks with different Neogene structural 

histories (McCulloch, 1987). A mechanism for this juxtaposition needs to be resolved.

Post-early Miocene dextral slip of 5 to 8 km has been 

documented on the Oceanic-West Huasna fault using offset Miocene lavas (McLean, 1993). The Sur-Nacimiento 

Fault Zone has had about 90 km post-Eocene strike-slip 

displacement but no more than 6 km of post-Miocene displacement (Vedder and others, 1991). Post-early Miocene 

dextral slip on the Rinconada Fault is about 43 km in the 

Salinas Valley area (Graham, 1978). The Russell Fault, in 

the Cuyama basin area, had about 25 km of dextral slip 

during early Miocene time (23-19 Ma), but only a few 

kilometers of slip since that time (Yeats and others, 1989).

In summary, estimates of cumulative post-Oligocene 

dextral slip in the southern Coast Ranges and offshore 

(excluding the San Andreas Fault) range from as little as 85 

km to as much as 195 km (Sedlock and Hamilton, 1991; 

Vedder and others, 1991). Namson and Davis (1990) found 

no evidence in the southern Coast Ranges for significant 

strike-slip displacement during the late Pliocene and Quaternary and suggest that lateral slip in the Santa Maria province has been minimal (<l-2 km) since early Pliocene time. 

However, estimating cumulative lateral displacement from 

sparse data has an additional uncertainty that results from 

extrapolating displacement rates derived from offset piercing 

points on one portion or segment of a fault to a complete 

segment or fault system (see Bilham and Bodin, 1992).

Santa Maria basin strata are cut by numerous growth 

faults at the surface and in the shallow subsurface that trend 

subparallel to folds (Namson and Davis, 1990). Because 

these faults are generally high angle and are commonly 

associated with changes in thickness of Neogene strata and 

are commonly buried by younger strata, they are inferred to 

be dip-slip faults active during Neogene deposition (Namson 

and Davis, 1990). A local angular unconformity (about 8 to 

6 Ma; Clark and others, 1991) between the Monterey and 

Sisquoc Formations may result from this Neogene phase of 

dip-slip faulting that was widespread in central and southern 

California (Namson and Davis, 1990).

The lower Miocene Lospe and Point Sal Formations are 

thicker over the crest of the Orcutt Anticline in onshore Santa 

Maria basin than on its limbs, suggesting that the axial area 

of the present fold was a depocenter during deposition of 

these strata. Conversely, upper middle and upper Miocene 

Monterey and upper Miocene and Pliocene Sisquoc Formations thin gradually from the fold limbs to the crest, suggesting that the present crest was a structural high during the 

late middle and late Miocene (Namson and Davis, 1990). 

The bathymetric inversion indicated by these observations 

occurred before folding of the anticline (Namson and Davis, 

1990) and may have been synchronous with the phase of 

Neogene dip-slip faulting discussed above.

The Santa Maria basin and vicinity have undergone 

considerable Pliocene and Quaternary shortening and uplift 

resulting in extensive folds, reverse faults, and angular 

unconformities (Namson and Davis, 1990). In the onshore 

part of Santa Maria basin, significant folding and reverse

J6 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
faulting began in Pliocene time (about 4 to 2 Ma) and has 

continued into the Quaternary (Woodring and Bramlette, 

1950; Namson and Davis, 1990). In offshore Santa Maria 

basin, the main period of folding and reverse faulting 

occurred in the early Pliocene (5.3-3.4 Ma) and continued 

with decreased intensity into the late Pliocene and Quaternary 

(Crouch and others, 1984; Clark and others, 1991; Miller 

and others, 1992). In the Santa Lucia basin, west of the Santa 

Lucia Bank Fault, significant compression has continued into 

the Quaternary (McCulloch, 1987; Mclntosh and others, 

1991).

In summary, a Pliocene and younger phase of folding 

and reverse faulting in the southern part of Santa Maria 

province (Namson and Davis, 1990; Clark and others, 

1991) resulted in about 30 km of cumulative northeastdirected shortening across the 200-km-wide area of the 

southern Coast Ranges and offshore Santa Maria basin. 

This estimate of shortening is based on modeling folds at 

the surface as thrust faults at depth. The presence of subsurface thrust faults needs to be verified by deep seismic 

imaging.

Paleomagnetic studies suggest that the southern Coast 

Ranges have undergone little or no rotation during Neogene and Quaternary time, in contrast to the western 

Transverse Ranges to the south (Hornafius and others, 

1986; Luyendyk, 1991). However, available data are few, 

so this interpretation must be considered preliminary. Late 

Cenozoic clockwise rotations of up to 70° in the northern 

part of the Santa Maria province (Greenhaus and Cox, 

1979) are attributed to local accommodation of distributed 

dextral shear (Luyendyk, 1991).

The record of Cenozoic tectonism in Santa Maria 

province is incomplete; data from the early Cenozoic are 

sparse and the overprinting of subsequent stress regimes 

hampers reconstruction of the central California continental margin. The locations and displacements of the major 

faults active during early Miocene basin formation are 

poorly resolved. The amount of early Miocene crustal 

extension is unknown. A widespread middle Miocene hiatus is attributed to both tectonic and paleoceanographic 

causes (Barron, 1986). Estimates of lateral offset along 

possible strike-slip faults vary by more than 100 percent. 

Nonetheless, the available estimates of crustal shortening, 

lateral offset, and block rotation allow the following preliminary attempt at margin reconstruction. This reconstruction can be revised as additional quantitative geologic and 

geophysical data are acquired.

SANTA MARIA BASIN STUDY SITES

The offshore and onshore sites chosen for this study 

form two transects, one that parallels the structural trend 

in the southern part of Santa Maria province and the 

other that crosses the structural trend. Lithostratigraphic

and biostratigraphic data from these sites are used to estimate the timing and rate of vertical movement by using 

backstripping techniques (Sleekier and Watts, 1978; van 

Hinte, 1978). Backstripping reconstructs a burial history 

through time by systematically calculating the diagenetic 

effects of sediment loading. Discrepancies between paleobathymetry estimated from microfossils and paleobathymetry calculated from decompacting and unloading the 

strata through time are attributed to vertical movement of 

the basement.

Neogene and younger strata at the sites have been 

decompacted using porosity-depth formulas for specific 

lithologies (table 1) by using a computer program developed at the University of Arizona (Dickinson and others, 

1987) and modified at Stanford University (Roger Bloch, 

unpub. data, 1989). This computer program also calculates 

isostatic loading through time using a one-dimensional 

Airy model and lithologic densities calculated during 

decompaction. The isostatic loading component subtracted 

from the total basin uplift/subsidence curve yields a residual tectonic component of vertical movement. This tectonic component may include contributions resulting from 

crustal thinning, thermal decay, and flexural loading.

Backstrip modeling requires estimates of paleobathymetry through time, estimates of the age of the various 

stratigraphic units and duration of hiatuses, and estimates 

of proportions of sand, mud, biosiliceous, and micritic 

components in the units (see appendix). Benthic foraminifers with restricted paleoenvironmental ranges are used as 

proxies for paleobathymetry following the procedures and 

assignments of Ingle (1980). Uncertainties in water depth, 

deduced from microfossils, are the main source of error in 

backstrip analysis. Therefore, water depth is plotted within 

an envelope of estimated range in water depth.

Sea level has fluctuated periodically over a range of 

about 200 m during the time interval evaluated at sites in 

this study (Haq and others, 1987). However, our backstrip 

analyses do not attempt a correction to the modern sea 

level datum because the timing and magnitude of sea level 

variation are not well constrained at the study sites.

The principle source of age control at the sites is the 

benthic foraminiferal stages of Kleinpell (1938, 1980) 

(fig. 3), a source that is not ideal, as these organisms have 

restricted environmental ranges. Key indicator species may 

appear in a section late or disappear from a section prematurely owing to a change in environment (in other words, 

water-mass character), rather than true first or last appearance. For this reason, curves in the age-depth plots in this 

report are shown within an envelope representing each 

stage. Although the stages are shown as not overlapping in 

age, some stages may in fact overlap in age, owing to the 

reasons stated above. Independent age control from planktonic microfossil groups, such as diatoms, or from isotopic 

dating of volcanic material is needed to better constrain 

the ages determined from benthic foraminifers.

Neogene Geohistory Analysis of Santa Maria Basin, California, and Its Relationship to Transfer of Central California to the Pacific Plate J7
Table 1. Porosity-depth formulas used to decompact lithologies in backstrip program

[<)>, porosity; Z, burial depth; NA, not applicable. Mudstone, sandstone 

(undifferentiated), and limestone (undiffetentiated) formulas are from 

Dickinson and others (1987). Micrite formula is derived from Bond and 

Komitz (1984). Siliceous shale formula is from Ingle (1985b)]

quotes are here considered to be offshore biostratigraphic 

equivalents of named onshore units (for example, "Foxen" 

is the offshore equivalent of the Foxen Mudstone). This 

site subsequently began to shoal and had shoaled to middle bathyal depths by the end of "Foxen" deposition in 

late Pliocene time. No paleobathymetric data are available

Lithology Formula Burial depth

Mudstone

Sandstone

Limestone

Micrite

Siliceous Shale

<HX6/(1=0.001*Z)

<(»=0.5/(1+0.0005*Z)

<|>=0.5/(1.0+0.6*Z)

<|>=0.75-(7.0*0.001*Z) 

<|>=0.4/exp(0.5*(Z-500.0)/lOOO.O)

<|>=0.85/(1+0.003*Z) 

<|>=0.10

NA

NA

NA

0-500 m 

>500m

0-2500 m 

>2500m

The loss of sediment porosity with depth is strongly 

dependent on sediment composition (see Dickinson and 

others, 1987); thus, stratigraphic units are separated into 

five lithologies (mudstone/shale, undifferentiated sandstone, undifferentiated limestone, biosiliceous sediment, 

and micritic carbonate), each with a different porositydepth function derived from empirical studies. Silica phase 

changes during burial diagenesis further complicate the 

reconstruction of the burial histories of biosiliceous sediments, such as the Monterey Formation, which have high 

primary porosities. We use a simple porosity-depth curve 

formula developed by Ingle (1985b) and Garrison (1985) 

from data of Isaacs and others (1983) to estimate porosity 

changes in biosiliceous units. However, the silica phase 

changes are temperature and composition dependent, and 

neither parameter was reconstructed for this study. Thus, 

refinement of these formulas (table 1) awaits empirical 

data from wells in the Santa Maria area.

Point Arguello Area (Offshore)

The Cost well (OCS-CAL 78-164-1) (fig. IB) is 

located at the southernmost end of offshore Santa Maria 

basin, just north of the "Amberjack High" (a basement 

high considered to be the offshore boundary between the 

Coast Ranges and the Transverse Ranges) in 435 m of 

water. At this site, about 2,520 m of Neogene and younger 

strata unconformably overlie Great Valley sequence rocks 

(fig. 4). The basal interbedded sandstone and conglomerate 

mark initial subsidence from neritic (150-15 m) to middle 

bathyal (2,000-500 m) depths in early Miocene (?) time 

(Cook, 1979). The site gradually deepened during Monterey, Sisquoc, and early "Foxen" deposition and then 

abruptly subsided to lower bathyal (4,000-2,000 m) depths 

during late "Foxen" deposition. Units enclosed within

STAGES/ DIATOM 

AGE MA SUBSTAGES ZONES 

n

to c

1 8

Q.

Pliocene

Miocene

o>

Hi

LU

0)

Q)

 a

i

CO 

LU

- 2- 

- 4-

- 8- 

-10-

r-12-

 1C

- IDH

-18- 

on

Venturian

Repettian

Delmontian

Mohnian

o>

J3

 n 

CO 

LU

Luisian 

Relizian

Saucesian

O. seminae

Rhizosolenia 

curvirostris

Actinocyclus 

oculatus

D. seminae 

var. fossilis

D. seminae var. 

fossilis - 

D. kamtschatica

Thalassiosira 

oestrupii

11 *

s b

Denticulopsis 

hustedtii

d

 2 §.

||

a

«> b

| a

Actinocyclus 

ingens

Figure 3. Late Cenozoic time scale for 

northeast Pacific Ocean (modified from 

Armentrout and others, 1984; Garrison, 

1985; Barron, 1989). Stages/substages 

are benthic provincial foraminiferal 

stages of Kleinpell (1938, 1980) and 

are considered somewhat latitudinally 

diachronous, as ranges of foraminifers 

are restricted both by environmental 

and oceanographic conditions.

J8 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
for the upper 172 m of the Cost well, but apparently the 

site continued to shoal through this interval. A hiatus or 

interval of extremely slow sediment accumulation (about 

14 to 9 Ma) is inferred during Monterey deposition from

the unusually sparse accumulation of sediment during 

early Mohnian time (fig. 4A and E).

Microfossil data do not allow quantification of initial 

site subsidence; however, backstrip analysis indicates slow

Sw w

< m t

H3 Z

CO CO 3 LITHOLOGY PALEOBATHYMETRY

200

400

600

800

1000

1200 -

DC

1400

COI

1600 m

LU

1800 |

t

LU 

O

2000

2200

2400

2600

0 1000 2000 3000 

WATER DEPTH. IN METERS

Figure 4A Lithostratigraphy and paleobathymetry for Cost Well (OCSCAL 78-164-1). Shaded areas represent range in paleobathymetric 

estimates inferred from benthic foraminifers. Lithostratigraphic data 

modified from (Cook, 1979) and Isaacs and others (1989). Determination of stage/substage boundaries (see fig. 3) is based on biostratigraphic data from Unocal (unpub. report, 1990). Paleobathymetric 

estimates are from Unocal (unpub. report, 1990). Units enclosed 

within quotes are considered to be offshore stratigraphic equivalents 

of named onshore units. See figure 2 for explanation of patterns and 

symbols; 1/, unnamed rocks.

Neogene Ceohistory Analysis of Santa Maria Basin, California, and Its Relationship to Transfer of Central California to the Pacific Plate J9
subsidence from about 18? to 6.5 Ma (fig. 4Q. At 6.5 Ma, 

a short, rapid subsidence event is inferred from a change 

in paleobathymetry (at 846 m depth in the well) from middle bathyal to lower bathyal water depths, followed by 

slow subsidence until about 2.5 Ma. At 2.5 Ma, the site 

began to shoal owing both to sediment fill and tectonic 

uplift. Uplift continued into the Pleistocene and is probably still occurring as the upper 172 m of Quaternary sediment (compacted thickness) at the top of the well spans a 

1,000-m decrease in water depth (from 1,500 to 500 m), 

suggesting the possibility of missing section, as well as 

tectonic uplift.

The Texaco Nautilus well (OCS-P 0496-1), 27 km to 

the northwest of the Cost well, is also located in offshore 

Santa Maria basin (fig. 1) but in a different subbasin north 

of a basement high. This well is in 375 m of water and 

penetrates about 1,120 m of Neogene and younger strata 

unconformably overlying early Cretaceous sedimentary 

rocks (fig. 5). The basal sandstone and conglomerate are 

overlain by volcanic and volcaniclastic rocks. At this site, 

the initial deepening from neritic to lower bathyal depths 

is obscured by a hiatus separating volcaniclastic rocks and 

Monterey strata (fig. 5A and B) This hiatus or interval of 

extremely slow sediment accumulation is estimated to

cc 

o

LL 1000

2000

3000

I i i i I I i

20 16 12 8 4 

AGE, IN MILLIONS OF YEARS BEFORE PRESENT

Figure 4B. Age-depth plot for Cost well (OCS-CAL 78-164-1) showing inflection points 

indicating changes in rate of sediment accumulation. Steeper slopes indicate rapid rates of 

sediment accumulation; flatter slopes indicate slow rates of sediment accumulation. Solid 

line, interpolated rate of sediment accumulation; dashed line, extrapolated rate; wavy line, 

hiatus or condensed interval. Shaded areas represent benthic foraminiferal stages (see fig. 3).

2000

§ 4000 

O

6000

20 15 10

AGE. IN MILLIONS OF YEARS BEFORE PRESENT

Figure 4C Backstrip plot of Cost well (OCS-CAL 78-164-1) showing isostatic loading and 

tectonic components of vertical movement. Upper curve represents paleobathymetry 

inferred from benthic foraminifers (Unocal, unpub. report, 1990). Lower curve represents 

total (decompacted) movement of Great Valley sequence rocks. Middle curve represents 

residual tectonic movement after sediment and water loading are removed from lower 

curve. Shaded area on each curve represents range in paleobathymetric estimates. Positive 

slope indicates uplift; negative slope indicates subsidence; cumulative sediment thickness 

represented by vertical distance between basement (lower) and paleobathymetry (upper)

curves.

J10 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
1000

2000

3000

LITHOLOGY PALEOBATHYMETRY

200

_ 400

600

_ 800

_ 1000

_ 1200

_ 1400

PERCENT

0 1000 2000 3000 

WATER DEPTH, IN METERS

Figure 5A. Lithostratigraphy and paleobathymetry for Texaco Nautilus 

well (OCS-P 0496-1). Shaded areas represent range in paleobathymetric estimates inferred from benthic foraminifers. Lithostratigraphic 

data from Texaco (unpub. mudlog and report, 1986). Determination 

of stage/substage boundaries (see fig. 3) is based on biostratigraphic 

data from Unocal (unpub. mudlog and report, 1986). Paleobathymetric estimates are from this report using faunal data from Texaco 

(unpub. report, 1986). Units enclosed within quotes are considered to 

be offshore stratigraphic equivalents of named onshore units. See figure 2 for explanation of patterns and symbols; 1/, unnamed rocks.

20 16 12 8

AGE, IN MILLIONS OF YEARS BEFORE PRESENT

Figure 5B. Age-depth plot for Texaco Nautilus well (OCS-P 0496-1) showing inflection 

points indicating changes in rate of sediment accumulation. Steeper slopes indicate rapid 

rates of sediment accumulation; flatter slopes indicate slow rates of sediment accumulation. Solid line, interpolated rate of sediment accumulation; dashed line, extrapolated 

rate; wavy line, hiatus or condensed interval. S, strontium-isotope datum; V, K/Ar datum. 

Shaded areas represent ranges in benthic foraminiferal stages (see fig. 3).

Neogene Geohistory Analysis of Santa Maria Basin, California, and Its Relationship to Transfer of Central California to the Pacific Plate J11
have spanned 17.5 to 9.5 Ma, using age control from benthic foraminifers (indicating a lack of Relizian or Luisan 

(fig. 3) fauna) and siliceous microfossils, as well as strontium isotopic dating of dolomitic cement and K/Ar dating 

of volcanic rocks (Texaco, unpub. report, 1986).

Monterey and lower Sisquoc strata record a rapid 

shoaling to upper bathyal water depths. The site then 

deepened abruptly to lower bathyal water depths during 

late Sisquoc and "Foxen" time. The upper 200 m of post- 

"Foxen" strata were not logged at this site. These post- 

"Foxen" strata likely record shoaling from lower bathyal 

to present upper bathyal water depths. However, the 

details of this shoaling are not known.

Initial subsidence to marine bathyal conditions 

occurred during a middle Miocene hiatus; therefore, the 

details of subsidence are unknown. The site remained at 

lower bathyal depths during Monterey deposition (8.8-6.5 

Ma) but underwent rapid tectonic uplift to upper bathyal 

depths during Sisquoc deposition (6.5-5.0 Ma). A subsequent phase of rapid bathymetric deepening followed (5-4 

Ma). The site shoaled from 3 Ma to present. The more 

than 1,500-m decrease in water depth across the upper 200 

m of strata (compacted thickness) seems extreme, suggesting the possibility of missing upper Pliocene and Pleistocene section as well as tectonic uplift. Low apparent 

sediment accumulation rates (fig. 5B) from 3 Ma to 

present support this possibility.

The late Pliocene uplift was synchronous at the Cost 

and Nautilus well sites. However, during the late Miocene 

uplift event at the Nautilus site, the Cost site was undergoing renewed subsidence. If the timing of both tectonic 

events (about 6 Ma) is indeed synchronous, the stress 

regime in the latest Miocene must allow adjacent uplift

and subsidence. A possible explanation for this pattern of 

deformation is presented below.

Point Sal Area (Onshore)

The Point Sal site is a composite of two adjacent surface 

sections and a subsurface well section (the North Beach 

section, the Mussel Rock section, and Union Los Nietos 

(Leroy A-2) well 9 (fig. IB) section), which together represent about 2,800 m of Neogene and younger strata (fig. 6/4). 

The quality of this composite section is affected by the need 

to extrapolate the ages of the tops and bottoms of each 

subsection; nonetheless, this data set has the best age control 

and paleobathymetry of the sites studied and thus represents 

the best constrained site. In particular, the timing of initial 

subsidence from neritic to marine bathyal depths is well 

constrained by isotopically dated tuff units in the Lospe 

Formation (Stanley and others, 1991) and by benthic foraminifers in the Point Sal Formation.

The basal part of the Lospe Formation unconformably 

overlies an isolated block of Point Sal ophiolite (Hopson 

and Frano, 1977), just southeast of Point Sal. Upper Lospe 

and lower Point Sal strata, exposed at North Beach, record 

abrupt deepening from neritic to upper bathyal depths 

about 17.0 Ma. Upper Point Sal and lower Monterey 

strata, exposed at the Mussel Rock section, recorded gradual deepening to middle bathyal depths recorded (fig. 6). 

The lower part of the Monterey Formation contains a 

well-constrained hiatus or condensed interval (from 15.5 

to 14 Ma; Ingle, 1985a), which occurred 2 m.y. after the 

Mussel Rock section reached its maximum water depth. 

This relationship contrasts with the Nautilus well site to

2000

4000

6000

20 15 10 5 

AGE, IN MILLIONS OF YEARS BEFORE PRESENT

Figure 5C Backstrip plot of Texaco Nautilus well (OCS-P 0496-1) showing isostatic loading and tectonic components of vertical movement. Upper curve represents paleobathymetry inferred from benthic foraminifers (this report using faunal data from Texaco unpub. 

report, 1986). Lower curve represents total (decompacted) movement of Franciscan Complex rocks. Middle curve represents residual tectonic movement after sediment and water 

loading are removed from lower curve. Shaded area on each curve represents range in 

paleobathymetric estimates. Positive slope indicates uplift; negative slope indicates subsidence; cumulative sediment thickness represented by vertical distance between basement 

(lower) and paleobathymetry (upper) curves.

J12 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
LITHOLOGY PALEOBATHYMETRY

~\lf

- 200

0 1000 2000 3000 

WATER DEPTH, IN METERS

Figure 6A Lithostratigraphy and paleobathymetry for Point Sal composite section. Shaded areas represent range in paleobathymetric estimates inferred from benthic foraminifers. Determination of 

stage/substage boundaries (see fig. 3) is based on biostratigraphic data 

from Ingle (1985a), Garrison (1985), and Stanley and others (1990). 

Lithostratigraphic data and paleobathymetric estimates for North 

Beach surface section are from Stanley and others (1990). Biostratigraphic data for North Beach surface section is from Stanley and others (1990). Lithostratigraphic and biostratigraphic data, and 

paleobathymetric estimates for Mussel Rock surface section and 

Union Los Nietos well subsurface section are from Ingle (1985a) and 

Garrison (1985). See figure 2 for explanation of patterns and symbols.

Neogene Geohistory Analysis of Santa Maria Basin, Califorr ia, and Its Relationship to Transfer of Central California to the Pacific Plate |13
the southwest, where subsidence occurred within the missing interval. The composite site shoaled gradually during 

deposition of the Monterey and Sisquoc Formations, and 

Foxen Mudstone, except for a minor deepening in earliest 

Foxen lime marked in the Los Nietos well by a biosiliceous unit in the otherwise clastic Foxen Mudstone (Behl 

and Ingle, unpub. data, 1993). Deposition of the upper 

Pliocene Careaga Sandstone marked the end of marine 

conditions.

Backstrip analysis indicates rapid subsidence from 

nonmarine to middle bathyal depths between 17.7 and 

15.5 Ma, a 1.5-m.y. hiatus from 15.5 to 14 Ma during 

slow net subsidence, continued slow subsidence until 

about 7 Ma. The Point Sal site underwent rapid tectonic 

uplift from 7 to 6 Ma, followed by tectonic quiescence 

from 6 to 4 Ma. A pulse of rapid subsidence occurred

about 4 to 3 Ma, followed by gradual shoaling (in part due 

to uplift) that led to nonmarine conditions. In summary, 

this site records a rapid subsidence event about 17 Ma, a 

long period of quiescence, an uplift event about 6.5 Ma, 

and another quiescent period interrupted by a subsidence 

event about 3.5 Ma.

Central Basin Area (Onshore)

Our central onshore Santa Maria basin site, Union 

Newlove No. 51 well, is located 23 km southeast of the 

Point Sal site (fig. IB). This site is on the south flank of 

the Casmalia-Orcutt anticlinal trend and thus was located 

in a different subbasin than the Point Sal site during late 

Neogene crustal shortening. The well did not penetrate

3000

2000

1000

20 15 10 5 

AGE, IN MILLIONS OF YEARS BEFORE PRESENT

Figure 6B. Age-thickness plot for Point Sal composite section showing inflection points indicating changes in rate of sediment accumulation. Steeper slopes indicate rapid rates of sediment accumulation; flatter slopes indicate slow rates of sediment accumulation. Solid line, 

interpolated rate of sediment accumulation; wavy line, hiatus or condensed interval. A, 

Ar40/Ar39 datum; B, benthic foraminiferal datum; D, diatom datum; P, planktonic foraminiferal 

datum.

o -

2000

4000

6000

15 10 5 C 

AGE, IN MILLIONS OF YEARS OF YEARS BEFORE PRESENT

Figure 6C. Backstrip plot for Point Sal composite section showing isostatic loading and 

tectonic components of vertical movement. Upper curve represents paleobathymetry 

inferred from benthic foraminifers (Ingle, 1985a; Garrison, 1985; Stanley and others, 1990). 

Lower curve represents total (decompacted) movement of Point Sal ophiolite (basement). 

Middle curve represents residual tectonic movement after sediment and water loading are 

removed from lower curve. Shaded area on each curve represents range in paleobathymetric estimates. Positive slope indicates uplift; negative slope indicates subsidence; cumulative sediment thickness represented by vertical distance between basement (lower) and 

paleobathymetry (upper) curves.

J14 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
basement rocks, so the basement type is unknown. The 

Miocene strata at this site are quite thin as compared with 

the Point Sal site (approximately 1,200 m of Point Sal and 

Monterey Formations at the Point Sal site compared with 

650 m at this site). The Newlove site may have been isolated from the main loci of deposition during early and 

middle Miocene deposition.

This central Santa Maria basin site abruptly deepened 

in the early Miocene, as marked by the shift from nonmarine deposition of the Lospe Formation to middle bathyal 

deposition of the Point Sal Formation (fig. 1A). Water 

depths shoaled gradually during deposition of the upper 

part of the Point Sal and Monterey Formations, with an 

apparent hiatus or condensed section of weakly constrained duration marking the boundary between deposition of the Monterey and Sisquoc Formations. A decrease 

from middle to upper bathyal (500-150 m) water depths 

accompanied the hiatus or interval of slow sediment accumulation. The basin continued to shoal gradually during 

Sisquoc time. An angular unconformity and an abrupt 

change from upper bathyal to inner neritic water depths 

mark the boundary between the Sisquoc Formation and

the overlying Careaga Sandstone. The time interval represented by this unconformity is unknown.

Age control from benthic foraminifers roughly constrains the period of apparent slow sediment accumulation 

(fig. IB) from 14 to 9 Ma. Backstrip analysis (fig. 1C) 

indicates rapid subsidence from about 17.5 to 16 Ma, followed by tectonic quiescence from about 16 to 6 Ma. From 

6 Ma to present this site shoaled tectonically as it filled with 

a prograded sequence of nearshore and paralic sediment. Age 

control is somewhat poor at this site, resulting in a smoothed 

backstrip curve, which may hide detail, such as shorter duration hiatuses and abrupt bathymetric changes documented 

elsewhere. For example, the Standard Los Flores No. 1 well, 

8 km east of this site, records a short time interval of significant deepening between deposition of the upper part of the 

Sisquoc Formation and lower part of the Foxen Mudstone, 

marked by biosiliceous strata (Behl and Ingle, unpub. data, 

1993). This subsurface biosiliceous unit is found along synclinal trends elsewhere in Santa Maria basin. Behl and Ingle 

(unpub. data, 1993) attribute this regional deepening to early 

Pliocene subsidence in synclinal axes resulting from folding 

and regional tectonic compression.

LITHOLOGY PALEOBATHYMETRY

1

200

400

600

800

1000

1200

0 1000 2000 3000 

WATER DEPTH, IN METERS

Figure 7A. Lithostratigraphy and paleobathymetry for Union 

Newlove No. 51 well (Orcutt Field). Shaded areas represent range 

in paleobathymetric estimates inferred from benthic foraminifers. 

Data and stage/substage boundary (see fig. 3) determinations from 

Ingle (1985a), Garrison (1985), and Lagoe (1987). See figure 2 for 

explanation of patterns and symbols.

Neogene Geohistory Analysis of Santa Maria Basin, California, and Its Relationship to Transfer of Central California to the Pacific Plate J15
Eastern Basin Area (Onshore)

Our easternmost site, the Tidewater Davis No. 1 well, 

is located 25 km southeast of the Newlove site, in the same 

late Neogene subbasin, on top of Franciscan basement. The 

Tidewater site has 2,750 m of Neogene and younger strata; 

however, this stratigraphic sequence does not contain the 

Lospe Formation at its base. Thus, this site is missing a 

record of early nonmarine conditions and subsequent subsidence to marine bathyal conditions. Nonetheless, Neogene 

strata at the Tidewater site record a shoaling history similar 

to the Newlove shoaling history (fig. &4) stable middle

bathyal conditions during deposition of the Point Sal and 

Monterey Formations from about 17 (?) to 6.5 Ma, followed 

by gradual shoaling to the present. A condensed interval from 

about 14 to 8 Ma and formation of an angular unconformity 

about 3 Ma interrupt this sequence.

Age control at the Tidewater site is poor; thus it is 

premature to attempt to extract tectonic details from the 

backstrip analysis. However, it appears that this and perhaps other sites more distant from the continental margin 

(and inferred major basin-defining faults) had a more subdued response to regional tectonic events than sites to the 

west, even though the timing appears synchronous.

1000

2000

3000

20 16 12 8 4 

AGE, IN MILLIONS OF YEARS BEFORE PRESENT

Figure 7B. Age-depth plot for Union Newlove No. 51 well (Orcutt Field) showing inflection points indicating changes in rate of sediment accumulation. Steeper slopes indicate 

rapid rates of sediment accumulation; flatter slopes indicate slow rates of sediment accumulation. Solid line, interpolated rate of sediment accumulation; dashed line, extrapolated rate; wavy line, hiatus or condensed interval. Shaded areas represent benthic 

foraminiferal stages (see fig. 3).

o -

2000

4000

6000 I I I I L.

20 15 10 5 

AGE, IN MILLIONS OF YEARS BEFORE PRESENT

Figure 7C Backstrip plot for Union Newlove No. 51 well (Orcutt Field) showing isostatic 

loading and tectonic components of vertical movement. Upper curve represents paleobathymetry inferred from benthic foraminifers (Ingle, 1985a; Garrison, 1985; Lagoe, 1987). 

Lower curve represents total (decompacted) movement of Franciscan Complex rocks. Middle curve represents residual tectonic movement after sediment and water loading are 

removed from lower curve. Shaded area on each curve represents range in paleobathymetric estimates. Positive slope indicates uplift; negative slope indicates subsidence; cumulative sediment thickness represented by vertical distance between basement (lower) and 

paleobathymetry (upper) curves.

J16 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
Tectonic Events Recorded at Study Sites

The sequence of tectonic events documented in the 

Santa Maria area by others (see Namson and Davis, 1990; 

Sedlock and Hamilton, 1991; Clark and others, 1991) is 

reflected in vertical tectonic movement that is recorded at 

our study sites. In particular, all sites record an early Miocene subsidence event during inferred transtension and a 

middle Miocene interruption in sediment accumulation or 

a condensed interval. The sites with more detailed geohistories also record a complicated and disparate late Miocene episode (about 6.5 Ma) of tectonic deepening or

shoaling depending on location and an early Pliocene 

(about 3.5 Ma) deepening during inferred transpression. 

All sites record late Pliocene to Holocene uplift.

Two sites document initially abrupt early Miocene 

subsidence that later slowed. Rapid initial subsidence may 

have also occurred at the other sites, but the data are inadequate to resolve this possibility. It is also difficult to 

resolve whether late Miocene compression began throughout the province at the same time because of the limited 

availability of biostratigraphic data.

In summary, backstrip techniques are useful in constraining rates, timing, and magnitude of vertical movement.

o co

LITHOLOGY

Sg

15

PALEOBATHYMETRY

200

400

600

800

1000

1200

1400

1600

1800

2000

_ 2200

0__._--"1000 2000 3000

'WATER DEPTH, IN METERS

Figure 8A. Lithostratigraphy and paleobathymetry for Tidewater 

Davis No. 1 well (Zaca Field). Shaded areas represent range in 

paleobathymetric estimates^inferred from benthic foraminifers. 

Data and stage/substage boundary (see fig. 3) determinations from 

Ingle (1985a) and Garrison (1985). See figure 2 for explanation of 

patterns and symbols.

Neogene Geohistory Analysis of Santa Maria Basin, California, and Its Relationship to Transfer of Central California to the Pacific Plate J17
To yield a complete model of past tectonism, these results 

need to be integrated with regional faulting and folding 

histories. Furthermore, additional study sites are needed to 

detail the spatial and temporal evolution of the structurally 

complex Santa Maria basin region.

LATE CENOZOIC PLATE TECTONIC SETTING 

OF CENTRAL CALIFORNIA

The history of plate interactions along the central California margin appears to be reflected in the timing and nature

of tectonism within Santa Maria province. During early 

Cenozoic time, the central California continental margin was 

part of the North America Plate, and the Farallon Plate was 

being subducted eastward underneath the margin. About 

30 Ma, two microplates detached from the Farallon Plate 

between the Pioneer and Murray Fracture Zones (Atwater, 

1989;Lonsdale, 1991;FernandezandHey, 1991). The Pacific 

Plate, just south of the Pioneer Fracture Zone (Wilson, 1988), 

interacted directly with North America sometime after 30 Ma 

(Atwater, 1970) at a latitude north of Point Arguello (Atwater 

and Molnar, 1973; Stock and Molnar, 1988), with the Gulf 

of California closed by 310 km of back slip along the San

1000

2000

3000

20 16 12 8

AGE, IN MILLIONS OF YEARS BEFORE PRESENT

Figure SB. Age-depth plot for Tidewater Davis No. 1 well (Zaca Field) showing inflection 

points indicating changes in rate of sediment accumulation. Steeper slopes indicate rapid 

rates of sediment accumulation; flatter slopes indicate slow rates of sediment accumulation. Solid line, interpolated rate of sediment accumulation; dashed line, extrapolated rate; 

wavy line, hiatus or condensed interval. Shaded areas represent benthic foraminiferal 

stages (see fig. 3).

2000

4000

6000

on 20 15 10 5 

AGE, IN MILLIONS OF YEARS OF YEARS BEFORE PRESENT

Figure 8C. Backstrip plot for Tidewater Davis No. 1 well (Zaca Field) showing isostatic 

loading and components of vertical movement. Upper curve represents paleobathymetry 

inferred from benthic foraminifers (Ingle, 1985a; Garrison, 1985). Lower curve represents 

total (decompacted) movement of Franciscan Complex rocks. Middle curve represents 

residual tectonic movement after sediment and water loading are removed from lower 

curve. Shaded area on each curve represents range in paleobathymetric estimates inferred 

from benthic foraminifers (Ingle, 1985a; Garrison, 1985). Positive slope indicates uplift; 

negative slope indicates subsidence; cumulative sediment thickness represented by vertical 

distance between basement (lower) and paleobathymetry (upper) curves.

J18 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
Andreas transform. The newly formed Pacific-North America transform boundary separated the Farallon Plate into 

northern and southern remnants (see Dickinson and Snyder, 

1979a); however, sabduction of Farallon-derived microplates, between these Farallon remnants, continued into the 

early Miocene.

Thus, the initial segment of the Pacific-North America 

transform boundary formed along the margin between the 

Mendocino and Pioneer Fracture Zones when this ridge 

segment stopped spreading, with Juan de Fuca-North

America subduction occurring to the north and microplateNorth America subduction occurring to the south of the 

initial transform boundary (Lonsdale, 1991; Fernandez and 

Hey, 1991). The length of the central California transform 

boundary increased during the early Miocene by northward migration of the Mendocino Triple Junction, episodic 

ridge-segment deaths, and transfer of microplates to the 

Pacific Plate (Lonsdale, 1991; Fernandez and Hey, 1991).

The composites of Oligocene and younger plate configurations in the central California region (figs. 9 and 10)

40°

38° _

36° _

34 C

32e

30°

128 C 126° 124° 122° 120° 118e

Figure 9A. Reconstruction of Pacific, Farallon, and North America Plates with coastal California block fixed, 33.0 Ma. Age scale (Kent and Gradstein; 1985) at lower left shows age in Ma 

of selected isochrons. In figures 9/A through J, spreading ridges are thick black lines, transform 

faults are medium black lines, and fracture zones are thin black lines; isochrons east of California coastline are shown in gray; coastal California block has diagonal lines; grid shows 

extension between interior California block and North America through time. See fig. 10 for 

names of plates and fracture zones.

Neogene Geohistory Analysis of Santa Maria Basin, California, and Its Relationship to Transfer of Central California to the Pacific Plate J19
are based on global plate-circuit reconstructions of the 

Pacific Plate to a rigid North America Plate (Stock and 

Molnar, 1988) and forward-modeling reconstructions of 

microplates relative to the Pacific Plate (Fernandez and 

Hey, 1991; see same for explanation of forward-modeling 

technique). In a few aspects, the magnetic anomaly interpretations of Fernandez and Hey (1991) differ from those 

of Lonsdale (1991). We agree with Lonsdale that there is 

only evidence for one microplate, the Monterey Microplate, between the Pioneer and Morro Fracture Zones (fig. 

10) and that the anomaly sequence on the Pacific Plate 

recording Monterey Microplate spreading has no significant gaps or duplications. Therefore, we have made the 

following modifications to the Fernandez and Hey (1991)

reconstructions of plate configurations younger than 30 

Ma. We removed the Reyes Microplate north of the Monterey Microplate and, in part, replaced it by a northward 

extension of the Monterey Microplate to what Lonsdale 

terms the Farallon Fracture Zone (aligning approximately 

with the Farallon Islands). We changed the details of the 

reorientation from a north-south to a northeast-southwest 

oceanic ridge orientation to conform to Lonsdale's (1991) 

age identifications, and we slowed the Pacific-Monterey 

ridge spreading rate slightly to yield the same spreading 

distance as Lonsdale (1991) without assuming that anomaly 7 (fig. 9A) is missing from the Pacific Plate.

Our reconstructions differ from previous ones (for 

example, Atwater, 1989; Lonsdale, 1991) because of the

126° 124 122° 120° 118°

Figure 9B. Reconstruction of Pacific Plate, Monterey Microplate, Arguello Microplate, and 

North America Plate with coastal California block fixed, 30.0 Ma. See fig. 9/4 for explanation 

of patterns.

J20 Evolution of Sedimentary Basins/Onshore Oil and Gas I nvestigations Santa Maria Province
additional attempt to model shortening of coastal California, its lateral movement along the California continental 

margin by way of the San Andreas Fault, and extension in 

the Basin and Range Province. Because each composite 

(fig. 9A-I) represents a specific time in the past, the difference between two reconstructions indicates the net displacement, not the actual displacement path, for plate 

motion between specific times. Also, plate circuits have 

only been completed for 35, 20, and 10 Ma (Stock and 

Molnar, 1988); therefore, any other reconstructions represent interpolations between 35 and 20 Ma, 20 and 10 Ma, 

or 10 and 0 Ma. These interpolations result in artifacts, 

such as the large apparent change in plate-velocity vectors 

at 20 Ma (fig. 11), which could have occurred anytime

between 30 and 15 Ma, although most likely they occurred 

about 26 Ma (Cande and others, 1992). More tightly constrained displacement histories will require shorter time 

intervals between global plate-circuit reconstructions and 

more detailed chronologies of late Cenozoic tectonism and 

volcanism along the California continental margin.

Reconstructed Positions of Central California

During the Cenozoic, the plate tectonic setting of the 

central California continental margin shifted from an 

accretionary North America continental margin overlying 

Farallon Plate subduction to oblique subduction between

40°

38°

36° _

34°

32° _

30°

128° 126° 124° 122° 120° 118°

Figure 9C Reconstruction of Pacific Plate, Monterey Microplate, Arguello Microplate, and 

North America Plate with coastal California block fixed, 24.0 Ma. See fig. 9/4 for explanation 

of patterns.

Neogene Geohistory Analysis of Santa Maria Basin, California, and Its Relationship to Transfer of Central California to the Pacific Plate J21
the microplates and North America and then to transform 

motion with modest amounts of divergence or convergence between the Pacific and North America Plates. Currently, the central California continental margin appears to 

be almost completely coupled to the Pacific Plate, with the 

major portion of transform motion between the Pacific and 

North America Plates occurring along the San Andreas 

Fault to the east. Plate motion estimates for the last 3 m.y. 

and geodetic measurements for the last 50 years require 

little or no strike-slip displacement west of the San 

Andreas Fault Zone (DeMets and others, 1990; Feigl and 

others, 1990; Me Williams, 1991; Baksi and others, 1992; 

Feigl and others, 1993). Minor amounts of modern northeast-directed shortening (4-6 mm/yr) have been documented in the Santa Maria province (Harris and Segall, 

1987; Feigl and others, 1990; Feigl and others, 1993), 

indicating that this area is still part of the Pacific-North 

America plate boundary, at least for the normal component 

of relative motion.

For our reconstructions, several simplifying assumptions were made: (1) The reconstructions are fixed in the 

frame of a California central coast microplate, defined as 

the continental material west of the San Andreas Fault and 

north of the Transverse Ranges; (2) the motion of this 

microplate relative to an interior California microplate 

across the San Andreas Fault was specified to produce

40°

38°

36°

34°

32 C

30°

128° 126° 124° 122° 120° 118°

Figure 9D. Reconstruction of Pacific Plate, Monterey Microplate, Arguello Microplate, and 

North America Plate with coastal California block fixed, 19.7 Ma. See fig. 9/4 for explanation 

of patterns.

J22 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
310 km of right-lateral displacement and several kilometers 

of convergence since 10 Ma; (3) the interior California 

microplate is defined as bounded by the San Andreas and 

Garlock faults, and the frontal faults of the Eastern Sierra 

Nevada; (4) for simplicity, we assumed these microplates 

had no internal deformation, and their motion rates were 

constant; and (5) the motion of the California microplates 

relative to North America was specified such that at 30 Ma 

interior California did not overlap with the Colorado Plateau and that coastal California did not overlap with the 

Pacific Plate, as reconstructed relative to North America by 

Stock and Molnar (1988). Avoiding both of those overlaps 

requires either a larger translation or a larger clockwise 

rotation relative to geographic north than most workers

would expect. The motion specified in table 2 corresponds 

to a motion of the southern Sierra Nevada of 15 mm/yr in 

the direction of N. 65° W., rotating with respect to local 

North American north at about 0.3°/m.y. The motion for 30 

to 0 Ma exceeds the maximum estimate of Wernicke and 

others (1988) for Sierra Nevada-Colorado Plateau motion 

by a factor of about 1.5. Several potential sources of this 

discrepancy (for example, internal deformation of the California microplates or errors in estimating either the Basin 

and Range motion or the global circuit motion) could be 

resolved with little effect on the relative positions of the 

Santa Maria province and the offshore plates.

The assumption of rigid block behavior is clearly in 

error, and as better constrained displacement histories are

40°

38C

36° _

34° _

32°

30°

128° 126C 124° 122° 120° 118°

Figure 9F. Reconstruction of Pacific Plate, Arguello Microplate, and North America Plate with 

coastal California block fixed, 17.0 Ma. See fig. 9/4 for explanation of patterns.

Neogene Geohistory Analysis of Santa Maria Basin, California, and Its Relationship to Transfer of Central California to the Pacific Plate J23
developed, this oversimplification can be remedied. The 

residual component of lateral displacement between the 

Pacific and North America Plates (not accommodated 

along the San Andreas Fault) is placed at the base of the 

continental margin in our reconstructions (2,500-m isobath), as can be seen in the sequential northward progression of the magnetic isochrons shown in figure 9.

Reconstructed Positions of the 

Faraflon Microplates

Reconstructions of the Pacific and Farallon Plates, 

Farallon remnants and microplates, the interior and coastal

40°

California blocks, and the Basin and Range Province to a 

rigid North America (fig. 9) from 35.6 to 10.6 Ma (magnetic anomalies 13-5; fig. 9A) were interpolated from 

Stock and Molnar (1988) poles assuming constant relative 

motion between the Pacific and North America Plates 

between poles. Reconstructions from 5.5 Ma to present 

(magnetic anomalies 3-0; fig. 9A) were extrapolated from 

DeMets and others (1990) poles. Uncertainties in these 

reconstructions are at least ±50 km in latitude and ±70 km 

in longitude (Stock and Hodges, 1989).

In the Stock and Hodges (1989) reconstructions, the 

plates in the global plate circuit (Pacific, Antarctica, 

Africa, and North America) are assumed to have been 

rigid since 20 Ma. For some of their reconstructions, the

38C

36° _

34e

32° _

30°

128° 126° 124e 122° 120° 118°

Figure 9F. Reconstruction of Pacific and North America Plates with coastal California block 

fixed, 15.0 Ma. Note that easternmost isochron shown on Pacific Plate counterpart to Arguello 

Microplate is Anomaly 5E (fig. 9/4). See fig. 9A for explanation of patterns.

J24 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
Pacific Plate overlaps continental crust of the California 

continental margin. This overlap is attributed to nonrigid 

deformation in the North America Plate (the several hundred kilometers of Neogene extension (Wernicke and others, 1988) in the Basin and Range Province). We account 

for Basin and Range extension in our composites (fig. 9); 

however, overlaps between plates still occur around 10 

Ma. These overlaps give an indirect estimate of the magnitude of the uncertainties in model parameters. Extension 

and strike-slip displacement across the entire plate boundary limit the precision of these reconstructions.

Despite these limitations, some general inferences can 

be made about the location and length of the Pacific-North 

America transform boundary through time. This boundary

initiated south of the Pioneer Fracture Zone, apparently 

shortly after 30 Ma (Atwater, 1970). This age might be a 

few million years too old if relatively slow Pacific-Monterey ridge spreading extended north to the Pioneer Fracture 

Zone and if several tens of kilometers of Pacific Plate 

have subsequently been subducted. By 25 Ma, contact 

between the Pacific Plate and the North America continental margin had been established between the Mendocino 

and Pioneer Fracture Zones, yielding a plate boundary 

with a length of 200 to 300 km (fig. 9Q. As spreading 

between the Mendocino and Pioneer Fracture Zones 

appears to have recorded the same plate pair (Pacific-Juan 

de Fuca) as is recorded north of the Mendocino Fracture 

Zone, we expect that the downgoing oceanic slab south of

40°

38°

36° _

34°

32° _

30°

128° 126° 124e 122e 120° 118°

Figure 9C. Reconstruction of Pacific and North America Plates with coastal California block 

fixed, 10.0 Ma. See fig. 9A for explanation of patterns.

Neogene Geohistory Analysis of Santa Maria Basin, California, and Its Relationship to Transfer of Central California to the Pacific Plate J25
Mendocino Fracture Zone continued to subduct at the 

fairly fast rate of the Juan de Fuca Plate (>50 km/m.y.; 

Wilson, 1988), opening a significant "slabless window." 

We find it encouraging that this window reconstructs 

underneath a regionally extensive pulse of volcanism in 

the California Coast Ranges, mostly west of the San 

Andreas Fault, dated at 24 to 22 Ma (Stanley, 1987). 

According to the model of Zandt and Furlong (1982), 

Coast Ranges volcanism is a result of horizontal motion of 

the subducted slab causing upwelling of the trailing asthenosphere, a process that produces basaltic melt by the 

same mechanism that operates at midocean ridges. This 

model predicts extensive, nearly synchronous volcanism as 

the subducted slab south of the Mendocino Fracture Zone

moves eastward, followed by more localized volcanism 

produced by the northward movement of the Juan de Fuca 

slab edge at Mendocino. This pattern of volcanism is 

exactly that observed by Stanley (1987). A prediction of 

the Zandt and Furlong's (1982) model not yet observed is 

an age progression of volcanism that youngs from west to 

east by 1 to 2 m.y. within the originally extensive volcanic 

province.

South of the Farallon Fracture Zone, magnetic anomaly 6B (about 23 Ma; figs. IA and 9A) is the youngest 

continuous anomaly (Lonsdale, 1991). Assuming no subduction of the Pacific Plate subsequent to the formation of 

anomaly 6B, the Pacific-North America transform boundary would have lengthened by about 100 km from 22 to

40°

38°

36°

34°

32°

30°

128° 126° 124e 122° 120° 118°

Figure 9H. Reconstruction of Pacific and North America Plates with coastal California block 

fixed, 5.0 Ma. See fig. 9/4 for explanation of patterns.

J26 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
20 Ma. By 20 Ma, a boundary approximately 400 km in 

length with mostly transform motion stretched between the 

Mendocino and Monterey Fracture Zones. We presume 

that this boundary was at or near the continental margin, 

perhaps at low angle within the former subduction zone, 

from a lack of evidence for an inland location. A second 

transform initiated south of the Monterey Microplate at 

about 22 Ma, reaching a length of about 100 km by 19 to 

20 Ma (Lonsdale, 1991). The Santa Lucia escarpment at 

the site of this second transform is the only place where 

the continental margin of California appears to have been 

modified by high-angle faulting (Lonsdale, 1991; Miller 

and others, 1992; Howie and others, 1993).

Subduction of the Monterey Microplate continued 

until about 19 Ma, when the Pacific-Monterey ridge seg40°

ment, adjacent to the Santa Maria basin area, stopped 

spreading, and the relict Monterey Microplate was transferred to the Pacific Plate. With this transfer, the PacificNorth America Plate boundary again became contiguous 

from the Mendocino Fracture Zone to near the Arguello 

Fracture Zone, a distance of about 700 km. At this time, 

the plate boundary appears to move into the North American Continent.

Lengthening of the transform boundary to the southeast continued as spreading at the north end of the East 

Pacific Rise brought more of the Pacific Plate into contact 

with North America. The transform boundary rapidly 

lengthened by about 300 km over the interval from 18 to 

16 Ma, encompassing all the area now offshore from the 

California Continental Borderland. Offshore from much of

38C

36°

34°

32°

30°

128° 126° 124° 122° 120° 118°

Figure 9/. Modern configuration of Pacific Plate, relict Monterey Microplate, and North America Plate. See fig. 9A for explanation of patterns.

Neogene Ceohistory Analysis of Santa Maria Basin, California, and Its Relationship to Transfer of Central California to the Pacific Plate J27
the California Continental Borderland, magnetic anomaly 

5D (about 17.5 Ma) is the youngest identifiable isochron 

constraining the cessation of spreading to post-17.5 Ma, 

likely about 16 Ma (Fernandez and Hey, 1991). The voluminous Conejo Volcanics erupted in the Santa Monica 

Mountains (15.5-13.5 Ma; Dibblee and Ehrenspeck, 1993;

Weigand and Savage, 1993) following cessation of spreading in the borderland area. In summary, the Pacific-North 

America transform margin lengthened by about 1,000 km 

between 30 and 16 Ma, even though relative displacement 

between the Pacific and North America Plates was only 

about 400 km during this interval (fig. 10).

33 Ma

PLATE 1

30 Ma

PACIFIC 

PLATE

JUAN

DE 

FUCA

PLATE

PACIFIC 

PLATE

24 Ma

NORTH

AMERICA

PLATE

"Murray FZ

JUANDE 

FUCA PLATE

19.7 Ma

NORTH

AMERICA

PLATE

17 Ma 15 Ma

10Ma 5 Ma Present

Figure 10. Simplified summary diagram of figure 9 showing change in plate boundaries through 

time and transfer of coastal California (stippled) from North America Plate to Pacific Plate. Active 

boundaries are shown with thick lines (paired arrows show relative motion) and inactive boundaries are shown with dotted lines: Pa leo-s preading ridge shown by double lines; paleo-subduction zone shown by barbed line. FZ, fracture zone; M, Monterey Microplate; A, Arguello 

Microplate; SAP, San Andreas Fault. Reappearance of relict Monterey Microplate at 5 Ma is an 

artifact reflecting insufficient data.

J28 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
DISCUSSION

The early Cenozoic record of tectonism in the Santa 

Maria province is fragmentary and will not be addressed 

here. However, the early Cenozoic was apparently a 

period of oblique subduction of young oceanic crust, and 

if the southern Cascadia margin can be used as an analog 

for margin response to oblique subduction of young buoyant plates, strong coupling between the accretionary margin and underlying oceanic lithosphere can be inferred, 

resulting in folding and uplift of the margin (McCrory, 

1989).

35.6-30.0 Ma

Recent plate motion studies document a clockwise 

reorientation of the Pacific Plate velocity vector (Cande 

and others, 1992) and adjacent microplate vectors^(Fernandez and Hey, 1991) in the latest Oligocene (about 26 Ma). 

This reorientation increased the obliquity of convergence 

across the microplate-North America plate boundary 

Oblique relative motion in central California during the 

earliest Miocene may have been partitioned into normal 

and parallel components of relative motion. For example, 

the Russell Fault, in Cuyama basin, underwent a relatively 

short, intense period of right-lateral movement (23-19 

Ma) followed by quiescence (Yeats and others, 1989),

19.0-10.6 Ma

-66.6

30.0-25.8 Ma

10.6-5.0 Ma

25.8-19.9 Ma

speeds in mm/yr.

5.0-0.0 Ma

Ci

NA

122° W. with coastal California

motion ceased about 16.0 Ma. A late; Farallon Plate; M, .-

J29
Table 2. Model for relative motion of western North America 

microplates

[positive rotation angle rotates plate 2 counterclockwise, forward in time, 

relative to plate 1. Lat, latitude; Lon, longitude; Ang, angle; deg, degrees; 

N, north; E, east]

Rotation pole

Time Lat Lon Ang 

Plate 1 Plate 2 (Ma) (deg N) (deg E) (deg)

North 

America.

Interior 

Calif.

Coastal 

Calif.

Interior 

Calif.

Coastal 

Calif.

Santa 

Barbara.

36-0 53.4 -106.4 -14.2

10-0 47.8 -82.8 -5.7

15-0 34.4 -120.4 -80.0

implying that shear strain was accommodated in this area 

until Monterey subduction ceased at about 19 Ma.

The onset of late early Miocene extension along the 

central California continental margin is likely linked to the 

transition from subduction to strike-slip tectonics as 

inferred for other extensional events along the western 

North American continental margin (see Dickinson and 

Snyder, 1979b; Glazner and Hartley, 1984). If this is the 

case, other basins along the central and southern California continental margin should contain records of early 

Miocene extension and subsidence following the cessation 

of spreading on the adjacent oceanic-ridge segment.

The onset of volcanism in Santa Maria basin also 

appears to be linked with the cessation of microplate subduction; however, there was a lag time for the transit of 

magma up through the accretionary margin. Bimodal volcanism dated at about 18 to 16 Ma, which accompanied 

initiation of late early Miocene extension, has a mixed 

magma source composed of oceanic-ridge and continentalcrust materials (Cole and Basu, 1992). This period of volcanism is attributed to upwelling of oceanic magma into 

the "slabless window" formed behind the subducted Monterey Microplate. However, this was not a simple "slabless 

window," as a fragment of relict Monterey Microplate 

remains beneath central California and can be traced as far 

east as the San Andreas Fault (Howie, 1991). The precise 

location of this remnant oceanic crust beneath a reconstructed California continental margin when spreading 

ceased at about 19 Ma is unknown given both the uncertainties in the global-circuit reconstructions and the uncertainties in the reconstruction of strike-slip faults such as 

the Hosgri Fault. Our California continental margin reconstructions show the Monterey Microplate remnant to be 

just south of the Santa Maria basin area (fig. 9£) during 

this pulse of volcanism, with the volcanic material erupting above a true "slabless window" followed by northwestward movement of the remnant beneath the margin 

with the Pacific Plate. However, available constraints 

allow the possibility that the microplate remnant was 

already beneath the central California continental margin 

at 19 Ma and that magma upwelled into a "slabless window" formed by delamination of the oceanic crust and 

subduction of the lower lithosphere (Howie, 1991).

Transfer of the Monterey Microplate to the Pacific 

Plate when spreading ceased (about 19 Ma) would have 

created a wide Pacific-North America subhorizontal 

boundary zone at depth. The low angle boundary perhaps 

allowed shear to be distributed between the oceanic crust 

and overlying relict accretionary complex (Howie and others, 1993), until the oceanic lithosphere cooled and coupled to the overlying margin, completing transfer of the 

margin to the Pacific Plate. This postulated distributed 

shear is a possible mechanism for in place rotation of 

fault-bounded blocks, such as the western Transverse 

Ranges, above a subhorizontal shear zone. In fact, dilatation and initial subsidence of the Santa Maria basin may 

have been accompanied by oblique normal slip, also 

beginning about 18 Ma, associated with initial clockwise 

rotation of the western Transverse Ranges about a pivot 

located near Point Arguello (Hornafius, 1985; Luyendyk 

and Hornafius, 1987).

Miocene subsidence occurred in two stages. The initial rapid phase from about 18 to 16 Ma, associated with 

volcanism, is attributed to extreme local extension of the 

continental crust associated with the beginning of western 

Transverse Range rotation and triggered by Monterey 

Microplate capture. The subsequent slower phase (about 

16 to 7 Ma) is attributed to thermal subsidence associated 

with cooling of underplated young oceanic lithosphere that 

had moved northwestward with the Pacific Plate to a location beneath the Santa Maria basin area.

Recent plate-motion studies shift the Pliocene clockwise reorientation of the Pacific Plate velocity vector and 

onset of transpression across the Pacific-North America 

transform boundary from about 3.9 to 3.4 Ma (Harbert and 

Cox, 1989; Harbert, 1991) back to about 6 to 5 Ma 

(Cande and others, 1992). This older age for onset of 

transpression is approximately coeval with the time of dipslip faulting and bathymetric inversion seen in Santa 

Maria basin. This inversion had previously been attributed 

to normal faulting (Namson and Davis, 1990), but the pattern of uplift and subsidence documented by backstrip 

analysis is more easily explained by crustal shortening, an 

explanation that agrees with a transpressive stress regime. 

This shift in timing of the Pacific velocity change leaves 

the uplift event that occurred at about 3.5 Ma, documented 

in our backstrip plots (figs. 4C, 5C, 6C, 1C, and 8Q, 

unexplained.

Subhorizontal detachment faults or distributed ductile 

shear within the lower part of the Franciscan Complex 

near the base of the seismogenic zone (9-14 km deep) are

J30 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
inferred beneath the southern Coast Ranges as far east as 

the Nacimiento Fault (Crouch and others, 1984; Howie, 

1991; Miller and others, 1992). This postulated detachment zone could provide a mechanism for continued distribution of transform motion across a broad area.

SUMMARY

Integration of backstrip plots with new, more complete reconstructions of plate motions shows that Neogene 

sedimentary and volcanic events in central California 

basins can be interpreted in terms of the interactions of 

microplates with each other and the larger Pacific, Farallon, and North America Plates. When oblique subduction 

ceased west of central California about 19 Ma by cessation of spreading on the Pacific-Monterey ridge segment, 

Pacific-North America transform motion was accommodated by distributed strike-slip motion across a broad 

zone. About 6 to 5 Ma, the direction of Pacific-North 

America relative plate motion became oblique to strikeslip fault zones and should have caused transpression 

along them, producing up to 10 mm/yr of shortening in 

central California. Since about 3 Ma, the plate boundary 

has been partitioned between strike-slip movement along 

the San Andreas Fault system and shortening along 

reverse faults and associated folds to the west and east of 

the San Andreas Fault system.

The continuation of microplate subduction along the 

California continental margin in the Miocene has significant implications for the evolution of the San Andreas 

Fault system. In particular, this system clearly began as 

two separate segments, one in northern central California 

and the other in southern Baja California. These segments 

propagated toward each other over a period of several million years and finally linked about 16 Ma, at the termination of Arguello subduction.

The central California continental margin experienced a period of volcanism, normal faulting, and rapid 

tectonic subsidence starting about 18 Ma. A 10-m.y. 

interval of slow subsidence followed this period. About 

6 Ma, the margin underwent a period of shortening, 

reflected in rapid bathymetric inversion. A 2-m.y. interval 

of slow uplift followed this period. About 3 Ma, the margin again experienced a period of intense shortening 

reflected in large-scale folding of Neogene strata. This 

latest shortening event is ongoing, as documented by 

modern geodetic measurements and earthquakes with 

reverse-fault mechanisms.

Documented geologic constraints are consistent with 

a plate kinematic mechanism for tectonism along the central California margin during the Neogene. However, 

uncertainties about palinspastic restoration of extension 

across western North America and uncertainties about 

reconstruction of tectonically disrupted California hamper

the reliability of these correlations. The implications of 

this kinematic model for the dynamics of transform plate 

margins can be tested with detailed backstrip modeling of 

La Honda, Santa Cruz, Salinas, and other central California continental margin basins.

Clearly much better constraints are needed on the 

timing and amount of extension and shearing in Santa 

Maria province. Currently, strike-slip faulting is not well 

enough constrained to be tied with specific plate tectonic 

events. Also, there is not enough data to verify or discount 

the model of eastward migration of the transform boundary, but it appears that this shift of transform motion from 

the western edge of the continental margin to the San 

Andreas Fault is linked to behavior of the relict Monterey 

slab.

In summary, available data indicate that Pacific Plate 

behavior in the late Cenozoic dominated tectonic events 

along the central California continental margin. While subduction of the Farallon Plate and its derivatives was ongoing, Pacific Plate influence was indirect, expressed by 

reorientation of spreading ridges and subsequent rotation 

of microplates. Since subduction ceased, Pacific Plate 

behavior has affected the margin directly, initially by capture of the underthrust relict slab and later by changes in 

relative motion that induced a component of compression 

across the transform boundary, compression expressed as 

regional uplift as well as reverse faulting and folding.

REFERENCES CITED

Armentrout, J.M., Echols, R.J., and Ingle, Jr., J.C., 1984, Neogene 

biostratigraphic scale for the northeastern Pacific margin, in 

Ikebe, Nobuo, and Tsuchi, Ryuichi, eds., Pacific Neogene 

Datum Planes: University of Tokyo Press, p. 171-178.

Atwater, T.M., 1970, Implications of plate tectonics for the 

Cenozoic tectonic evolution of western North America: Geological Society of America Bulletin, v. 81, p. 3513-3536.

 1989, Plate tectonic history of the northeast Pacific and 

western North America, in Winterer, E.L., Hussong, D.M., 

and Decker, R.W., eds., The geology of North America, volume N The eastern Pacific Ocean and Hawaii: Geological 

Society of America, p. 22-72.

Atwater, T.M., and Molnar, Peter, 1973, Relative motions of the 

Pacific and North American plates deduced from sea floor 

spreading in the Atlantic, Indian, and South Pacific Oceans, 

in Kovach, R.L., and Nur, Amos, eds., Proceedings of the 

conference of tectonic problems of the San Andreas Fault 

system: Stanford University Publications in Geological Sciences, v. 13, p. 136-148.

Baksi, A.K., Hsu, V., McWilliams, M.O., and Farrar, E., 1992, 

40Ar/39Ar dating of the Brunhes-Matuyama geomagnetic 

field reversal: Science, v. 256, p. 356-357.

Barron, J.A., 1986, Paleoceanographic and tectonic controls on 

deposition of the Monterey Formation and related siliceous 

rocks in California: Palaeogeography, Palaeoclimatology, 

Palaeoecology, v. 53, p. 27-45.

Neogene Ceohistory Analysis of Santa Maria Basin, California, and Its Relationship to Transfer of Central California to the Pacific Plate J31
 1989, The late Cenozoic stratigraphic record and hiatuses 

of the northeast Pacific Results from the Deep Sea Drilling 

Project, in Winterer, E.L., Hussong, D.M., and Decker, R.W., 

eds., The geology of North America, volume N The eastern Pacific Ocean and Hawaii: Geological Society of America, p. 311-322.

Bilham, Roger, and Bodin, Paul, 1992, Fault zone connectivity Slip rates on faults in the San Francisco Bay Area, California: Science, v. 258. p. 281-284.

Bond, G.C., and Komitz, M.A., 1984, Construction of tectonic 

subsidence curves for the early Paleozoic miogeocline, southern Canadian Rocky Mountains Implications for subsidence 

mechanisms, age of breakup, and crustal thinning: Geological 

Society of America Bulletin, v. 95, no. 2, p. 155-173.

Cande, S.C., Stock, J.M., Raymond, C.A., and Haxby, W.F., 

1992, Changes in Pacific-Antarctic plate motion and the age 

of the bend in the Hawaiian-Emperor chain: Eos, v. 73, 

no. 43, p. 507-508.

Canfield, C.R., 1939, Subsurface stratigraphy of Santa Maria 

Valley oil field and adjacent parts of Santa Maria Valley, 

California: American Association of Petroleum Geologists 

Bulletin, v. 23, no. 1, p. 45-81.

Clague, D.A., and Jarrard, R.D., 1973, Tertiary Pacific plate 

motion deduced from the Hawaiian-Emperor chain: Geological Society of America Bulletin, v. 84, p. 1135-1154.

Clark, D.H., Hall, N.T., Hamilton, D.H., and Heck, R.G., 1991, 

Structural analysis of late Neogene deformation in the central offshore Santa Maria basin, California: Journal of Geophysical Research, v. 96, no. B4, p. 6435-6457.

Clark, J.C., Brabb, E.E., Greene, H.G., and Ross, D.C., 1984, 

Geology of Point Reyes Peninsula and implications for San 

Gregorio fault history, in Crouch, J.K., and Bachman, S.B., 

eds., Tectonics and sedimentation along the California margin: 

Society of Economic Paleontologists and Mineralogists, Pacific 

Section, Los Angeles, p. 67-86.

Cole, R.B., and Basu, A.R., 1992, Middle Tertiary volcanism during 

ridge-trench interactions in western California: Science, v. 258, 

p. 793-796.

Cook, H.E., ed., 1979, Geologic studies of the Point Conception 

deep stratigraphic test well, OCS-CAL 78-164 No. 1, outer 

continental shelf, southern California, United States: U.S. Geological Survey Open-File Report 79-1218, 148 p.

Crouch, J.K., Bachman, S.B., and Shay, J.T., 1984, Post-Miocene 

compressional tectonics along the central California margin, 

in Crouch, J.K., and Bachman, S.B., eds., Tectonics and sedimentation along the California margin: Society of Economic 

Paleontologists and Mineralogists, Pacific Section, Los Angeles, p. 37-54.

DeMets, Charles, Gordon, R.G., Argus, D.F., and Stein, Seth, 1990, 

Current plate motions: Geophysical Journal International, 

v. 101, p. 425^78.

Dibblee, T.W., Jr., 1950, Geology of southwestern Santa Barbara 

County, California: California Division of Mines Bulletin 150, 

95 p.

 1966, Geology of the central Santa Ynez Mountains, Santa 

Barbara County, California: California Division of Mines and 

Geology Bulletin 186, 99 p.

Dibblee, T.W., Jr. and Ehrenspeck, H.E., 1993, Field relations of 

Miocene volcanic and sedimentary rocks of the western Santa 

Monica Mountains, California, in Weigand, P.W., Fritsche,

A.E., and Davis, G.E., eds., Depositional and volcanic environments of middle Tertiary rocks in the Santa Monica Mountains, southern California: Society of Economic Paleontologists 

and Mineralogists, Pacific Section, Los Angeles, p. 75-92.

Dickinson, W.R., Armin, R.A., Beckvar, Nancy, Goodlin, T.C., 

Janecke, S.U., Mark, R.A., Norris, R.D., Radel, Gregg, and 

Wortman, A.A., 1987, Geohistory analysis of rates of sediment 

accumulation and subsidence for selected California basins, in 

Ingersoll, R.V, and Ernst, W.G., eds., Cenozoic Basin Development of Coastal California, Rubey volume 6: Englewood 

Cliffs, N.J., Prentice-Hall, p. 1-23.

 1979a, Geometry of triple junctions related to San Andreas 

transform: Journal of Geophysical Research, v. 84, p. 561-572.

Dickinson, W.R., and Snyder, W.S., 1979b, Geometry of subducted 

slabs related to San Andreas transform: Journal of Geology, 

v. 87, p. 609-627.

Ernst, W.G., and Hall, C.A., Jr., 1974, Geology and petrology of 

the Cambria Felsite, a new Oligocene formation, west-central 

California Coast Ranges: Geological Society of America Bulletin, v. 85, p. 523-532.

Feigl, K.L., Agnew, D.C., Bock, Yehuda, Dong, Danan, Donnellan, 

Andrea, Hager, B.H., Herring, T.A., Jackson, D.D., Jordan, 

T.H., King, R.W., Larsen, Shawn, Larson, K.M., Murray, M.H., 

Shen, Zhengkang, and Webb, F.H., 1993, Space geodetic measurement of crustal deformation in central and southern California, 1984-1992: Journal of Geophysical Research, v. 98, 

no. B12, p. 21677-21712.

Feigl, K.L., King, R.W., and Jordan, T.H., 1990, Geodetic measurement of tectonic deformation in the Santa Maria fold and 

thrust belt, California: Journal of Geophysical Research, v. 95, 

no. B3, p. 2679-2699.

Fernandez, L.S., and Hey, R.N., 1991, Late Tertiary tectonic evolution of the sea floor spreading system off the coast of California between the Mendocino and Murray Fracture Zones: 

Journal of Geophysical Research, v. 96, no. fill, p. 17955- 

17979.

Garrison, R.E., 1985, Sedimentological studies of the Monterey 

Formation in the Santa Maria-Santa Barbara region, in The 

geochemical and paleoenvironmental history of the Monterey 

Formation Sediments and hydrocarbons: Canoga Park, Calif., 

Global Geochemistry Corporation, p. 15-87.

Glazner, A.F., and Bartley, J.M., 1984, Timing and tectonic setting 

of Tertiary low-angle normal faulting and associated magmatism in the southwestern United States: Tectonics, v. 3, p. 385- 

396.

Graham, S.A., 1978, The role of the Salinian block in evolution of 

San Andreas Fault system, California: American Association 

of Petroleum Geologists Bulletin, v. 62, p. 2214-2231.

Graham, S.A., and Dickinson, W.R., 1978, Evidence for 115 kilometers of right slip on the San Gregorio-Hosgri Fault trend: 

Science, v. 199, p. 179-181.

Greenhaus, M.R., and Cox, A.V., 1979, Paleomagnetism of the 

Morro Rock-Islay Hill Complex as evidence for crystal block 

rotations in central coastal California: Journal of Geophysical 

Research, v. 84, no. B5, p. 2393-2400.

Haq, B.U., Hardenbol, J., and Vail, PR., 1987, Chronology of 

fluctuating sea levels since the Triassic: Science, v. 235, 

p. 1156-1167.

Harbert, W.P., 1991, Late Neogene relative motions of the Pacific 

and North American plates: Tectonics, v. 10, no. 1, p. 1-6.

J32 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
Harbert, W.P., and Cox, A.V., 1989, Late Neogene motion of the 

Pacific plate: Journal of Geophysical Research, v. 94, 

p. 3052-3064.

Harris, R.A., and Segall, Paul, 1987, Detection of a locked zone 

at depth on the Parkfield California segment of the San 

Andreas Fault: Journal of Geophysical Research, v. 92, 

p. 7945-7962.

Hopson, C.A., and Frano, C.J., 1977, Igneous history of the 

Point Sal ophiolite, southern California, in Coleman, R.G., 

and Irwin, W.P., eds., North American ophiolites: Oregon 

Department of Geology and Mining Industries Bulletin 95, 

p. 161-183.

Hornafius, J.S., 1985, Neogene tectonic rotation of the Santa 

Ynez Range, western Transverse Ranges, California, suggested by paleomagnetic investigation of the Monterey Formation: Journal of Geophysical Research, v. 90, p. 12503- 

12522.

Hornafius,' J.S., Luyendyk, D.P., Terres, R.R., and Kamerling, 

M.J., 1986, Timing and extent of Neogene tectonic rotation 

in the western Transverse Ranges, California: Geological 

Society of America Bulletin, v. 97, no. 12, p. 1476-1487.

Howell, D.G., Champion, D.E., and Vedder, J.G., 1987, Terrane 

accretion, crustal kinematics, and basin evolution, southern 

California, in Ingersoll, R.V., and Ernst, W.G., eds., Cenozoic basin development of coastal California, Rubey volume 

6: Englewood Cliffs, N.J., Prentice-Hall, p. 242-258.

Howie, J.M., 1991, Seismic studies of crustal structure and tectonic evolution across the central California margin and the 

Colorado plateau margin: Stanford University, Calif., Ph.D. 

dissertation, 273 p.

Howie, J.M., Miller, K.C., and Savage, W.U., 1993, Integrated 

crustal structure across the south central California margin Santa Lucia Escarpment to the San Andreas Fault: 

Journal of Geophysical Research, v. 98, no. B5, p. 8173- 

8196.

Ingle, J.C., Jr., 1980, Cenozoic paleobathymetry and depositional 

history of selected sequences within the southern California 

Continental Borderland: Cushman Foundation for Foraminiferal Research Special Publication, no. 19, p, 163-195.

 1985a, Paleobathymetric, paleoceanographic, and biostratigraphic studies, in The geochemical and paleoenvironmental 

history of the Monterey Formation Sediments and hydrocarbons: Canoga Park, Calif., Global Geochemistry Corporation, p. 88-161.

 1985b, Geohistory/subsidence analysis, in The geochemical and paleoenvironmental history of the Monterey Formation Sediments and hydrocarbons: Canoga Park, Calif., 

Global Geochemistry Corporation, p. 162-175.

Isaacs, C.M., Pisciotto, K.A., and Garrison, R.E., 1983, Facies 

and diagenesis of the Miocene Monterey Formation, California A summary, in lijima, A., Hein, J.R., and Siever, R., 

eds., Siliceous deposits in the Pacific region: Elsevier, 

Amsterdam, Developments in Sedimentology, v. 36, p. 247- 

282.

Isaacs, C.M., Jackson, L.L., Stewart, K.C., and Scott, Norman, 

III, 1989, Analytical reproducibility and abundances of 

major oxides, total carbon, organic carbon, and sedimentary 

components of Miocene and early Pliocene cuttings from 

the Point Conception deep stratigraphic test well, OCSCAL 78-164 No. 1, offshore Santa Maria basin, southern

California: U.S. Geological Survey Open-File Report 87- 

75, 28 p.

James, E.W., and Mattinson, J.M., 1985, Evidence for 160 km 

post mid-Cretaceous slip on the San Gregorio Fault, coastal 

California: Eos, v. 66, p. 1093.

Jennings, C.W., compiler, 1977, Geologic map of California: 

California Division of Mines and Geology, Geologic Data 

Map Series, Map 2, scale 1:750,000.

Kent, D.V., and Gradstein, P.M., 1985, A Jurassic to recent geochronology, in Vogt, P.R., and Tucholke, B.E., eds,, The 

geology of North America, volume M The western North 

Atlantic region: Geological Society of America, p, 45-58.

Kleinpell, R.M., 1938, Miocene stratigraphy of California: 

American Association of Petroleum Geologists, Tulsa, 450 p.

 1980, The Miocene stratigraphy of California revisited: 

American Association of Petroleum Geologists Studies in 

Geology No. 11, p. 1-349.

Lagoe, M.B., 1987, Middle Cenozoic basin development, 

Cuyama basin, California, in Ingersoll, R.V., and Ernst, 

W.G., eds., Cenozoic basin development of coastal California, Rubey volume 6: Englewood Cliffs, N.J., Prentice-Hall, 

p. 172-206.

Lonsdale, Peter, 1991, Structural patterns of the Pacific floor offshore of Peninsular California, in Dauphin, J.P., and Simoneit, B.R.T., eds., The Gulf and Peninsular Province of the 

Californias: American Association of Petroleum Geologists 

Memoir 47, p. 87-125.

Luyendyk, B.P., 1991, A model for Neogene crustal rotations, 

transtension, and transpression in southern California: Geological Society of America Bulletin, v. 103, p. 1528-1536.

Luyendyk, B.P., and Hornafius, J.S., 1987, Neogene crustal rotations, fault slip, and basin development in southern California, in Ingersoll, R.V., and Ernst, W.G., eds., Cenozoic basin 

development of coastal California, Rubey volume 6: Englewood Cliffs, N.J., Prentice-Hall, p. 260-283.

Mattinson, J.M., and James, E.W., 1985, Salinian block U/Pb age 

and isotopic variations Implications for origin and 

emplacement of the Salinian terrain, in Howell, D.G., ed., 

Tectonostratigraphic terranes of the Circum-Pacific region: 

Circum-Pacific Council for Energy and Mineral Resources, 

Earth Science Series, no. 1, p. 215-226.

McCrory, P.A., 1989, Late Neogene geohistory analysis of the 

Humboldt basin and its relationship to convergence of the 

Juan de Fuca plate: Journal of Geophysical Research, v. 94, 

no. B3, p. 3126-3138.

McCulloch, D.S., 1987, Regional geology and hydrocarbon 

potential of offshore central California, in Scholl, D.W., 

Grantz, Arthur, and Vedder, J.G., eds., Geology and resource 

potential of the continental margin of western North America 

and adjacent ocean basins Beaufort Sea to Baja California: 

Circum-Pacific Council for Energy and Mineral Resources, 

Earth Sciences Series, no. 6, p. 353-401.

Mclntosh, K.D., Reed, D.L., Silver, E.A., and Metzer, A.S., 

1991, Deep structure and structural inversion along the central California continental margin from EDGE seismic profile RU-3: Journal of Geophysical Research, v. 96, no. B4, 

p. 6459-6474.

McLean, Hugh, 1991, Distribution and juxtaposition of Mesozoic 

lithotectonic elements in the basement of the Santa Maria basin, 

California: U.S. Geological Survey Bulletin 1995-B, 12 p.

Neogene Geohistory Analysis of Santa Maria Basin, California, and Its Relationship to Transfer of Central California to the Pacific Plate J33
 1993, Miocene lavas constrain right slip movement on the 

west Huasna Fault in San Luis Obispo County to less than 8 

km: American Association of Petroleum Geologists Bulletin, 

v. 77, no. 4, p. 708-709.

Me Williams, M.O., 1991, Revised polarity time scale provides 

partial resolution of the San Andreas discrepancy: Eos, v. 72, 

no. 44, p. 442.

Miller, K.C., Howie, J.M., and Ruppert, S.D., 1992, Shortening 

within underplated oceanic crust beneath the central California margin: Journal of Geophysical Research, v. 97, no. B13, 

p. 19961-19980.

Namson, J.S., and Davis, T.L., 1990, Late Cenozoic fold and 

thrust belt of the southern Coast Ranges and Santa Maria 

basin, California: American Association of Petroleum Geologists Bulletin, v. 74, no. 4, p. 467-492.

Page, B.M., 1981, The southern Coast Ranges, in Ernst, W.G., 

ed., The tectonic development of California, Rubey volume 

1: Englewood Cliffs, N.J., Prentice-Hall, p. 329-417.

Page, B.M., and Brocher, T.M., 1993, Thrusting of the central 

California margin over the edge of the Pacific plate during 

the transform regime: Geology, v. 21, p. 635-638.

Page, B.M., Wagner, H.C., McCulloch, D.S., Silver, E.A., and 

Spotts, J.H., 1979a, Geologic cross section of the continental 

margin off San Luis Obispo, southern Coast Ranges, and the 

San Joaquin Valley, California: Geological Society of America Map and Chart Series, MC-28G, 12 p., scale 1:250,000.

 1979b, Tectonic interpretation of a geologic section of the 

continental margin off San Luis Obispo, southern Coast 

Ranges, and the San Joaquin Valley, California: Cross-section summary: Geological Society of America Bulletin, 

p. 808-812.

Ross, D.C., 1984, Possible correlations of basement rocks across 

the San Andreas, San Gregorio-Hosgri, and RinconadaReliz-King City Faults, California: U.S. Geological Survey 

Professional Paper 1317, 37 p.

Sedlock, R.L., and Hamilton, D.H., 1991, Late Cenozoic tectonic 

evolution of southwestern California: Journal of Geophysical 

Research, v. 96, no. B2, p. 2325-2351.

Stanley, R.G., 1987, Implications of the northwestwardly 

younger age of the volcanic rocks of west-central California Alternative interpretation: Geological Society of America Bulletin, v. 98, p. 612-614.

Stanley, R.G., Johnson, S.Y., Cole, R.B., Mason, M.A., Swisher, 

C.C., III, M.L., Thornton, Filewicz, M.V., Vork, D.R., Tuttle, 

M.C., and Obradovich, J.D., 1992, Origin of the Santa Maria 

basin, California: V.E. McKelvey Forum on Mineral and 

Energy Resources, U.S. Geological Survey Circular C-1074, 

p. 73.

Stanley, R.G., Johnson, S.Y., Obradovich, J.D., Tuttle, M.L., 

Thorton, M.C., Vork, D.R., Filewicz, M.V, Mason, M.A., 

and Swisher, C.C., III, 1990, Age, facies, and depositional 

environments of the lower Miocene Lospe Formation, Santa 

Maria basin, central California: V.E. McKelvey Forum on 

Mineral and Energy Resources, U.S. Geological Survey Circular C-1060, p. 78-79.

Stanley, R.G., Johnson, S.Y., Tuttle, M.L., Mason, M.A., 

Swisher, C.C., III, Thornton, M.C., Vork, D.R., Filewicz, 

M.V, Cole, R.B., and Obradovich, J.D., 1991, Age, correlation, and origin of the type Lospe Formation (lower Miocene), Santa Maria basin, central California: American

Association of Petroleum Geologists, Pacific Section, 

Bakersfield, Calif., Program and Abstracts, p. 49.

Sleekier, M.S., and Watts, A.B., 1978, Subsidence of the Atlantic-type continental margin off New York: Earth and Planetary Science Letters, v. 41, p. 1-13.

Steritz, J.W., 1986, The southern termination of the Hosgri Fault 

Zone, offshore south-central California: University of California, Santa Barbara, M.S. thesis, 78 p.

Stock, J.M., and Hodges, K.V., 1989, Pre-Pliocene extension 

around the Gulf of California and the transfer of Baja California to the Pacific Plate, Tectonics, v. 8, no. 1, p. 99-115.

Stock, J.M., and Molnar, Peter, 1988, Uncertainties and implications of the Late Cretaceous and Tertiary position of North 

America relative to the Farallon, Kula, and Pacific plates: 

Tectonics, v. 7, no. 6., p. 1339-1384.

Tennyson, M.E., 1989, Pre-transform early Miocene extension in 

western California: Geology, v. 17, p. 792-796.

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

Thornton, M.L., 1991, Contrasts in early Miocene subsidence history across Oceanic-West Huasna Fault system, 

northern Santa Maria province, California: American Association of Petroleum Geologists Bulletin, v. 75, no. 2, p. 383.

van Hinte, J.E., 1978, Geohistory analysis application of micropaleontology in exploration geology: American Association 

of Petroleum Geologists Bulletin, v. 62, p. 201-222.

Vedder, J.G., Howell, D.G., and McLean, Hugh, 1983, Stratigraphy, 

sedimentation, and tectonic accretion of exotic terranes, southern Coast Ranges, California, in Watkins, J.S. and Drake, C.L., 

eds., Studies in continental margin geology: American Association of Petroleum Geologists Memoir 34, p. 471-496.

Vedder, J.G., McLean, Hugh, Stanley, R.G., and Wiley, T.J., 

1991, Paleogeographic implications of an erosional remnant 

of Paleogene rocks southwest of the Sur-Nacimiento Fault 

Zone, southern Coast Ranges, California: Geological Society 

of America Bulletin, v. 103, p. 941-952.

Weigand, R.W, and Savage, K.L., 1993, Review of the petrology 

and geochemistry of the Miocene Conejo Volcanics of the 

Santa Monica Mountains, California, in Weigand, P.W., 

Fritsche, A.E., and Davis, G.E., eds., Depositional and volcanic environments of middle Tertiary rocks in the Santa 

Monica Mountains, southern California: Society of Economic Paleontologists and Mineralogists, Pacific Section, 

Los Angeles, Calif., p. 93-112.

Wernicke, Bruce, Axen, G.J., and Snow, J.K., 1988, Basin and 

Range extensional tectonics at the latitude of Las Vegas, 

Nevada: Geological Society of America Bulletin, v. 100, 

1738-1757

Wilson, D.S., 1988, Tectonic history of the Juan de Fuca Ridge 

over the last 40 million years: Journal of Geophysical 

Research, v. 33, no. BIO, p. 11863-11876.

Woodring, W.P., and Bramlette, M.N., 1950, Geology and paleontology of the Santa Maria district: U.S. Geological Survey 

Professional Paper 222, 185 p.

Yeats, R.S., Calhoun, J.A., Nevins, B.B., Schwing, H.F., and 

Spitz, H.M., 1989, Russell Fault Early strike-slip fault of 

California Coast Ranges: American Association of Petroleum 

Geologists Bulletin, v. 73, p. 1089-1102.

Zandt, George, and Furlong, K.R., 1982, Evolution and thickness 

of the lithosphere beneath coastal California: Geology, v. 10, 

p. 376-381.

J34 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
APPENDIX DATA USED IN BACKSTRIP PROGRAMS

Data used in backstrip program for Cost well (OCS-CAL 78-164-1)

[Age and depth refer to top of unit listed unless otherwise noted. See figure 4 for sources of data. Water depth at well is 435 m. Units enclosed within quotes 

are considered to be offshore stratigraphic equivalents of named onshore units. Mud, mudstone/shale; Sand, undifferentiated sandstone; Carb, undifferentiated limestone; Sil, biosiliceous sediment; Mic, micritic carbonate

Unit Age 

(Ma)

Depth

(m)

Lithology 

(percent)

Mud Sand Carb Sil Mic

Water depth 

(m)

Min Max

Eustatic 

sea level

(m)

Quaternary deposits

Volcaniclastic conglomerate

and sandstone.

Horizon A

Horizon B

Sandstone

Siltstone

"Careaga" Sandstone

Horizon A

Horizon B

"Foxen" Mudstone

Horizon A

Horizon B

Sisquoc Formation

Monterey Formation

Horizon A

Horizon B

Horizon C

Horizon D

Horizon E

Horizon F

Conglomerate and

sandstone.

Base of Neogene

section.

0.0

1.5

1.7

1.8

2.0

2.2

2.5

2.6

4.2

6.5

7.2

8.2

8.8

9.7

14.0

15.5

17.5

18.0?

0.0

163.1

172.2

224.0

373.4

511.2

663.0

681.2

845.9

1004.3

1321.3

1614.0

1778.5

1812.7

1970.6

2345.5

2434.5

2520.7

80?

0

0

20

80

20

20

75

60

60

60

20

20

70

70

70

100

100

20?

100

100

80

20

80

80

10

10

5

5

5

5

10

10

10

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

15

20

20

75

75

0

0

0

0

0

0

0

0

0

0

0

0

15

15

15

15

0

0

20

20

20

0

0

450

500?

1500

1500

1500

1500

2000

2000

500

500

500

500

500

500

500

500

0?

0?

450

2000?

2000

2000

2000

2000

2500

2500

2000

1500

1500

1500

1500

1500

1500

1500

500?

500?

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

Neogene Geohistory Analysis of Santa Maria Basin, California, and Its Relationship to Transfer of Central California to the Pacific Plate J35
Data used in backstrip program for Texaco Nautilus well (OCS-P 496-1), Santa Maria basin, offshore

[Age and depth refer to age at top of unit listed unless otherwise noted. Hiatus between Monterey Formation and unnamed volcaniclastic 

rocks is estimated to represent time interval between 17.5 and 9.5 Ma. See figure 5 for sources of data. Water depth at well is 375 m. Units 

enclosed within quotes are considered to be offshore stratigraphic equivalents of named onshore units. Mud, mudstone/shale; Sand, undifferentiated sandstone; Carb, undifferentiated limestone; Sil, biosiliceous sediment; Mic, micritic carbonate]

Unit

Quaternary and upper

Pliocene deposits.

"Foxen" Mudstone

Horizon A

Horizon B

Horizon C

Sisquoc Formation

Horizon A

Horizon B

Horizon C

Horizon D

Horizon E

Horizon F

Monterey Formation,

upper part.

Horizon A

Horizon B

Horizon C

Horizon D

Monterey Formation,

middle part.

Hiatus

Volcaniclastic deposits

Horizon A

Horizon B

Horizon C

Conglomerate and

sandstone

Horizon A

HorizonB , Base of conglomerate

and sandstone unit.

Age 

(Ma)

0.0

3.4

3.8

4.0

4.1

4.3

4.7

4.9

5.1

5.7

6.5

6.9

7.3

7.7

9.3

9.5

17.5

17.6

17.8

18.4

20.4

24.5

Depth 

W

0

198

259

289

300

335

390

420

442

530

650

670

686

704

779

789

789

795

804

834

926

1124

Lithology 

(percent)

Mud

75

100

90

100

100

100

95

95

80

80

80

55

40

20

20

10

10

10

0

30

15

0

Sand

25

0

5

0

0

0

5

5

20

20

20

20

20

15

15

60

60

60

100

70

70

100

Carb

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

Sil

0

0

0

0

0

0

0

0

0

0

0

25

40

55

55

25

25

25

0

0

0

0

Mic

0

0

5

0

0

0

10

0

0

0

0

0

0

10

10

5

5

5

0

0

15

0

Water depth 

W

Min

375

2000

2000

2000

1500

500

500

150

150

150

2000

2000

2000

2000

2000

2000

0

0

0

0

0

0

Max

375

2500

2500

2500

2000

1500

1500

500

500

2500

2500

2500

2500

2500

2500

2500

500

150

150

150

150

150

Bustatic 

sea level

w

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

J36 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
Data used in backstrip program for Point Sal composite section (North Beach surface section, Mussel Rock surface section, and 

Union Los Nietos well subsurface section), Santa Maria basin, onshore

[Age and depth refer to top of unit listed unless otherwise noted. Hiatus within Monterey Formation is estimated to represent time interval between 15.5 and 

14.0 Ma. See figure 6 for sources of data. Surface elevation of Union Los Nietos (Leroy A-2) well is 0 m. Mud, mudstone/shale; Sand, undifferentiated 

sandstone; Carb, undifferentiated limestone; Sil, biosiliceous sediment; Mic, micritic carbonate]

Unit

Holocene deposits and

Paso Robles Formation.

Careaga Sandstone

Foxen Mudstone

Horizon A

Horizon B

Horizon C

Sisquoc Formation

Horizon A

Horizon B

Monterey Formation,

upper part.

Monterey Formation,

middle part.

Hiatus

Monterey Formation,

lower part.

Point Sal Formation

Horizon A

Horizon B

Horizon C

Lospe Formation,

upper and middle parts.

Lospe Formation,

lower part.

Base of Lospe

Formation.

Age 

(Ma)

0.0

1.5

2.8

3.1

3.8

4.0

5.8

6.5

7.3

14.0

15.5

16.0

16.7

17.07

17.14

17.7

17.72

Depth 

(m)

0

137

206

350

750

791

1030

1191

1548

1739

1740

1991

2300

2413

2430

2703

2843

Lithology 

(percent)

Mud

50

30

90

90

90

60

60

25

20

20

50

60

60

90

40

0

0

Sand

50

70

10

10

10

30

30

0

0

0

15

40

40

10

60

100

100

Carb

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

Sil

0

0

0

0

0

10

10

55

70

70

10

0

0

0

0

0

0

Mic

0

0

0

0

0

0

10

20

10

10

40

0

0

0

0

0

0

Water depth 

(m)

Min

0

0

100

500

100

400

400

600

900

900

700

500

500

300

0

0

0

Max

0

100

150

600

150

500

500

700

1300

1300

1100

700

700

700

150

0

0

Eustatic 

sea level

(m)

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

Data used in backstrip program for Union Newlove No. 51 well, Orcutt Field, Santa Maria basin, onshore

[Age and depth refer to top of unit listed unless otherwise noted. See figure 7 for sources of data. Surface elevation of well is 243 m. Mud, mudstone/shale; 

Sand, undifferentiated sandstone; Carb, undifferentiated limestone; Sil, biosiliceous sediment; Mic, micritic carbonate]

Unit

Careaga Sandstone

Horizon A

Horizon B

Sisquoc Formation

Horizon A

Horizon B

Monterey Formation

Point Sal Formation

Lospe Formation

Base of Lospe

Formation.

Age 

(Ma)

1.4

4.0

4.0

6.5

6.5

16.0

17.0

17.7

Depth

(m)

0

150

150

594

594

859

1253

1300

Lithology 

(percent)

Mud

0

0

50

50

0

60

0

0

Sand

100

100

0

0

0

40

100

100

Carb

0

0

0

0

0

0

0

0

Sil

0

0

50

50

90

0

0

0

Mic

0

0

0

0

10

0

0

0

Water depth 

(m)

Min

0

0

150

150

1000

1000

0

0

Max

10

10

500

500

1500

1500

0

0

Eustatic 

sea level

(m)

0

0

0

0

0

0

0

0

Neogene Geohistory Analysis of Santa Maria Basin, California, and Its Relationship to Transfer of Central California to the Pacific Plate J37
Data used in backstrip program for Tidewater Davis No. 1 well, Zaca Field, Santa Maria basin, onshore.

[Age and depth refer to top of unit listed unless otherwise noted. Hiatus between Sisquoc Formation and Careaga Sandstone is estimated to represent time 

interval between 4.0 and 2.8 Ma. See figure 8 for sources of data. Surface elevation at well is 311 m. Mud, mudstone/shale; Sand, undifferentiated 

sandstone; Carb, undifferentiated limestone; Sil, biosiliceous sediment; Mic, micritic carbonate

Unit Age Depth Lithology Water depth Eustatic

(Ma) (m) (percent) (m) sea level

Mud Sand Carb Sil Mic Min Max (m)

Holocene deposits and

Paso Robles Formation.

Careaga Sandstone

Hiatus

Sisquoc Formation

Monterey Formation

Point Sal Formation

Base of Point Sal

Formation.

0.0

1.5

2.8

4.0

6.5

16.0

17.0

0

61

415

416

965

1819

1859

0

0

100

0

0

50

50

100

100

0

15

0

50

50

0

0

0

0

0

0

0

0

0

0

85

100

0

0

0

0

0

0

0

0

0

-311

0

100

400

600

500

500

-311

100

150

500

700

700

700

0

0

0

0

0

0

0

J38 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
Chapter K

Diatom Biochronology of the 

Sisquoc Formation in the Santa Maria Basin, 

California, and Its Paleoceanographic and 

Tectonic Implications

By MICHAEL P. DUMONT and JOHN A. BARRON

U.S. GEOLOGICAL SURVEY BULLETIN 1995

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

Edited by Margaret A. Keller
CONTENTS

Abstract Kl 

Introduction Kl

Study of the Miocene-Pliocene boundary interval in California Kl

Sisquoc Formation diatom biostratigraphy K2 

Diatom zonation K2 

Diatom ranges K3

Acknowledgments K4 

Diatom stratigraphy of Sisquoc Formation sections K5

Sweeney Road K5

Lompoc quarry K9

Lompoc hills K10

Point Pedernales K10

Harris Grade K10

Casmalia K10

Mussel Rock Kll

Comparison of Sisquoc Formation sections Kll 

The Monterey-Sisquoc boundary problem K12 

Paleoceanographic implications K13 

Tectonic implications K13 

Conclusions K13 

References K14

FIGURES

1. Map of Santa Maria and Santa Barbara basins in southern California showing location of Sisquoc Formation sections discussed in text K2

2. Correlation of California microfossil zones and California benthic foraminiferal 

stages of Kleinpell (1938, 1980) with geologic time scale of Berggren and others 

(1985) in vicinity of Miocene-Pliocene boundary K3

3. Ranges of biostratigraphically useful diatoms in latest Miocene and earliest Pliocene of California K4

4. Map of Sweeney Road section showing the samples studied and location of phosphatic conglomerate beds K5

5. Sweeney Road lithologic section of Ramirez (1990) showing samples studied for 

diatoms (Dumont, 1986; this report) and diatom zones of Dumont (1986), Barron 

and Baldauf (1986), and Barron (1986a) K6

6. Age versus stratigraphic height plot for Sweeney Road section K8

7. Correlation of various Sisquoc Formation sections in Santa Maria basin to geologic 

time K9

8. Correlation of various Sisquoc Formation sections in Santa Maria basin showing 

stratigraphic thickness and generalized lithology Kll

9. Comparison of key Sisquoc Formation sections to eustatic sea level curve of Haq 

and others (1987) and changes in motion of Pacific Plate after Harbert and Cox 

(1989) and Sedlock and Hamilton (1991) K14

TABLE 

1. Stratigraphically important diatoms in Sweeney Road section K7

IV Contents
Diatom Biochronology of the Sisquoc Formation in the 

Santa Maria Basin, California, and Its Paleoceanographic 

and Tectonic Implications

By Michael P. Dumont1 anc/John A. Barren

Abstract

Diatom biostratigraphy allows correlation and dating of 

the Sisquoc Formation within the Santa Maria basin, Calif. 

The diatom stratigraphy of the Sweeney Road section, east 

of Lompoc, is presented and that of six additional sections 

in the Santa Maria basin is summarized. The base of the 

Sisquoc Formation coincides with the base of the Thalassiosira 

miocenica/Nitzschia miocenica Interval Zone and is 6.0 Ma 

(latest Miocene) in age. Most exposures of the Sisquoc Formation in the Santa Maria basin do not range younger than 

the basal part of the Thalassiosira oestrupii Partial Range Zone 

or 5.0 Ma. However, the contact of the Sisquoc Formation 

with the overlying Foxen Mudstone in the Harris Grade section 

in the center of the basin falls higher in the T. oestrupii Zone 

and is estimated to be about 3.8 Ma in age.

The basal contact of the Sisquoc Formation with the 

underlying Monterey Formation is variable and may be difficult 

to recognize. Typically, the contact is a paraconformity, and 

occasionally, the contact coincides with the break between 

underlying porcellaneous rocks and overlying diatomaceous 

rocks. Such a diagenetic contact, however, can be diachronous 

over relatively short distances (3 km or less) in the Santa Maria 

basin and is a poor choice for distinguishing the Sisquoc 

Formation from the Monterey Formation. Although the Sisquoc 

Formation may closely resemble the Monterey Formation, it 

has a greater terrigenous component overall, is typically not 

as finely laminated, and has more prevalent massive diatomaceous mudstone than the Monterey Formation. Such criteria, 

however, may be difficult to establish in the subsurface or in 

the field. Diatoms, on the other hand, offer a reliable means 

for distinguishing younger (post-6.0 Ma) biosiliceous rocks that 

are usually age-equivalent to the Sisquoc Formation from the 

older (pre-6.0 Ma) rocks of the Monterey Formation.

The Sisquoc Formation was deposited during a time of 

relatively high eustatic sea level, which was bracketed by

^RCO Oil and Gas Company, Exploration Services, P.O. Box 147 

Bakersfield, California 93302 (present address Vastar Resources, Inc., 

Exploration Analysis, 15375 Memorial Dr., Houston, TX 77079.)

Manuscript approved for publication April 11, 1995.

sea level falls that occurred at approximately 6.3 and 3.8 

Ma. The tectonic reorganization of the California continental 

margin was intensified in the early Pliocene during Sisquoc 

deposition, causing uplift of the Coast Ranges and leading to 

an increased supply of clastic material to the Santa Maria 

basin.

INTRODUCTION

Study of the Miocene-Pliocene boundary 

Interval in California

Throughout much of the Santa Maria and Santa Barbara 

basins, the Monterey Formation is overlain by diatom-bearing mudstones and sandstones of the Sisquoc Formation. 

Woodring and Bramlette (1950) recognized both a finegrained basin facies and a marginal sandstone facies in the 

Sisquoc Formation, and they stated that within the Santa 

Maria basin as much as 1,524 m (5,000 ft) of rocks assignable 

to the formation are exposed. The soft diatomaceous and 

hard porcellaneous strata of the basin facies of the Sisquoc 

may seem indistinguishable from those of the underlying 

Monterey Formation; however, massive diatomaceous mudstone is more prevalent in the Sisquoc Formation, and the 

two formations are often separated by an unconformity in 

the Santa Maria basin (Woodring and Bramlette, 1950).

The Sisquoc Formation has been traditionally correlated 

with the "Delmontian" benthic foraminiferal stage of Kleinpell (1938, 1980). Recent diatom biostratigraphic studies 

(Barron, 1986a; Dumont, 1986; Barron and Baldauf, 1986; 

Dumont and others, 1986) suggest that rocks of the Sisquoc 

Formation contain the Miocene-Pliocene boundary.

Traditionally in California, the Miocene-Pliocene 

boundary has been drawn between Kleinpell's (1938) late 

Miocene "Delmontian" Stage and Natland's (1953) early 

Pliocene Repettian Stage. Subsequent work by Pierce 

(1972), Ruth (1972), and Barron (1976) demonstrated that 

the benthic foraminifers and diatoms found in the type 

"Delmontian" Stage section are middle and early late

Neogene Geohistory Analysis of Santa Maria Basin, California, and Its Relationship to Transfer of Central California to the Pacific Plate K1
Miocene in age. Consequently, the term "Delmontian" has 

been modified (now generally placed in quotation marks) 

to refer to the interval above the Mohnian Stage and 

below the Repettian Stage. This "Delmontian" interval is 

equivalent to the type Bolivina obliqua Zone of Kleinpell 

(1938). Kleinpell (1980), however, modified his "Delmontian" Stage to also include the Bolivina foraminata Zone, 

which underlies the B. obliqua Zone.

The California benthic foraminiferal stages, however, 

are time transgressive when compared to the radiolarian, 

diatom, and planktonic foraminiferal biostratigraphies 

(Ingle, 1967; Bandy, 1972; Crouch and Bukry, 1979). 

Moreover, studies during the past 30 years have shown 

that benthic foraminiferal faunas migrate up and down the 

continental slope in response to water mass changes.

The great majority of calcareous planktonic zonations 

(planktonic foraminifers and calcareous nannofossils) were 

developed in low latitudes using tropical and subtropical 

species. As a result of high-latitude climatic cooling during 

the late middle Miocene (Kennett, 1977, Savin, 1977), calcareous plankton declined in diversity in California waters 

and developed a cool to warm-temperate affinity (Ingle, 

1967, 1981), making use of low latitude planktic zonations 

difficult. Consequently, correlations based on tropical calcareous planktonic microfossils are generally problematic in 

California sections after the early middle Miocene. Also, 

planktonic foraminifers and calcareous nannofossils are generally lacking in uppermost Miocene and lowermost Pliocene 

("Delmontian" Stage) rocks in coastal California.

Diatoms, on the other hand, are generally abundant 

and diverse in the uppermost Miocene and lowermost Pliocene hemipelagic rocks of coastal California. Studies of 

the diatoms of the Sisquoc Formation have included those 

of Woodring and others (1943), Simonsen and Kanaya 

(1961), Wornardt (1963, 1967, 1986), Barren (1974, 1975, 

1976), Hornafius and others (1982), Dumont (1986), Akers 

and others (1987), Barren and Baldauf (1986), Arends and 

Blake (1986), and Barron and Ramirez (1992). During the 

last ten years, diatom biostratigraphy has undergone considerable refinement (Barron, 1992a), and it seems appropriate at this time to synthesize the diatom biostratigraphy 

of the Sisquoc Formation and to provide an updated correlation with the geologic time scale. Such a refined correlation can provide the necessary framework for recognizing 

the effect of regional and global tectonic and eustatic sea 

level events on the deposition of the Sisquoc Formation 

(Ramirez and Garrison, written commun., 1994).

Sisquoc Formation Diatom Biostratigraphy

Diatom Zonation

Studies of diatoms from the Sisquoc Formation in 

stratigraphic sections at Sweeney Road near Lompoc

(Dumont, 1986), at Harris Grade north of Lompoc (Barron 

and Baldauf, 1986), and at the Point Pedernales, Mussel 

Rock, Lompoc Hills, and Casmalia sections (Barren and 

Ramirez, 1992) (fig. 1) provide the framework for constructing a diatom biostratigraphy for the Sisquoc Formation. The diatom zonation we use merges the California 

diatom zonation of Barron (1986a) and Barron and 

Baldauf (1986) with zones proposed by Dumont (1986).

Dumont (1986) subdivided Barron and Baldaufs 

(1986) Subzone (b) of the Nitzschia reinholdii Zone into 

three zones. In ascending order these three zones are the 

Thalassiosira miocenicalNitzschia miocenica Interval 

Zone, the Thalassiosira hyalinopsis Partial Range Zone, 

and the Thalassiosira praeoestrupii Partial Range Zone. 

The T. miocenica/N. miocenica Interval Zone is defined as 

the interval above the last common occurrence of Rouxia 

californica and below the first occurrence of Thalassiosira 

hyalinopsis that contains the nominative species. The overlying T. hyalinopsis Partial Range Zone is the interval 

from the first occurrence of T. hyalinopsis to the first 

occurrence of T. praeoestrupii. The interval between the 

first occurrence of T. praeoestrupii and the first occurrence 

of T. oestrupii is referred to as the T. praeoestrupii Partial 

Range Zone.

Barren's (1986a) Subzone (a) of the Nitzschia reinholdii 

Zone was renamed by Dumont (1986) as the Rouxia 

californica Partial Range Zone. This zone encompasses the 

range of the common or consistent presence of Rouxia 

californica above the last occurrence of Thalassionema 

schraderi.

The diatom zonations of Dumont (1986) and Barron 

(1986a) are correlated with the geologic time scale of

SANTA BARBARA BASIN

Figure 1. Map of Santa Maria and Santa Barbara basins in 

southern California showing location of Sisquoc Formation 

sections (X) discussed in text.

K2 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
Berggren and others (1985) and with other microfossil 

zonations in figure 2. Age estimates for the zones of 

Dumont (1986) are derived from the paleomagnetic stratigraphy of the Santa Cruz section of the Purisima Formation in northern California, which was completed by 

Madrid and others (1986) (see Dumont and others, 1986). 

Age estimates for older diatom zones are mostly derived 

through indirect correlation to paleomagnetic stratigraphy 

and (or) radiometric ages (Barron, 1986a, 1992a).

The radiolarian zonation of Weaver and others (1981) 

and the calcareous nannofossil zonation of Okada and 

Bukry (1980) as recognized by Bukry (1973, 1981) off 

California are correlated to the diatom zones after Barron 

(1986a, 1992a). Low diversity and low abundance of agediagnostic calcareous nannofossil taxa prevents further 

subdivision of the calcareous nannofossil zones. The benthic foraminiferal zones of Kleinpell (1938,1980) and the 

corresponding benthic foraminiferal stage boundaries are 

correlated to the geologic time scale following Barron 

(1986a) and Blake (1991).

Two alternatives for placement of the MiocenePliocene boundary are shown on figure 2 based on the 

results of Berggren and others (1985) and the results of

Zijderveld and others (1986). The placement of Berggren 

and others (1985) is followed in this report because it is 

currently the standard accepted correlation. However, we 

recognize that the younger placement of Zijderveld and 

others (1986) is receiving increasing support from biostratigraphers and paleoceanographers and probably represents the correct correlation.

Recently, a new magnetic-polarity time scale was proposed by Cande and Kent (1992) that resulted in slightly 

older age estimates for events near the Miocene-Pliocene 

boundary. The Cande and Kent (1992) time scale, however, is not followed in this paper because detailed recalibration of the diatom biostratigraphy of Barron (1981, 

1986a) and Dumont (1986) is beyond the scope of this 

paper.

Diatom Ranges

Figure 3 shows the stratigraphic ranges of important 

diatom taxa in the uppermost Miocene and lowermost Pliocene rocks of California. This figure was compiled from 

the results of Barron and Baldauf (1986), Dumont (1986), 

Barron and Ramirez (1992), and Barron (1992a, b).

I 

o

o

Q. 

Ul

Ul

Z 

Ul

o

O

Q.

UJ

z

UJ

0

o

X

o

o

Q. 

UJ

CO 

CO

0)

c

0>

o 

o

a

>

a>

Ul

(1)

a>

c

a>

o

o

s

a>

CO

(2)

n

~

Ul 

0

-3.5

-4.0

-4.5-

-5.0

-5.5

-6.0

-6.5

-7.0

PALEOMAGNETIC

0

6

C2A

_

C3

C3A

f\mC4

EPOCHS

Magnetic 

polarity

__^_

M

__

 

= 

_^M

__ 

 

>,

Anoma

q

4

EUROPEAN

STAGE

Berggren 

and others

(1985) = (1) 

Zijerveld 

and others 

(1986) = (2)

ZANCLEAN

r-(2) 

(D

MESSINIAN

TORTONIAN

DIATOM ZONATION

Barron

Dumont (1986)

Thalassiosira

oestrupii

T. praeoestrupii

T. hyalinopsis

T. miocenica /

N. miocenica

Rouxia

caiifornica

UNZONED

(1986); 

Barron and 

Baldauf 

(1986)

!<n

§

 to

.0

8

S

.efc

!§

« b

.c

si

 §

V) 3

^

1

K: a

RADIOLARIAN

ZONATION

Weaver and 

others (1981)

Lamprocyrtis

heteroporos

Stichocorys

peregrins

Stichocorys 

deimontensis

CALCAREOUS

NANNOFOSSIL

ZONATION

Okada and 

Bukry (1980)

CN12

CN11

CN10

-

CN9b

Discoaster 

mendomobensis

BENTHIC FORAMINIFERAL

ZONATION

Kleinpell 

(1938)

REPETTIAN

^

^

UNZONED

 _

Bolivina

obliqua

- -

Boiivina

hughesi

Kleinpell 

(1980)

REPETTIAN

x'

UNZONED

 

Bolivina

obiiqua

 

Bolivina

foraminata

Bolivina

goudkoffi

STAGE

Kleinpell 

(1938)= (3) 

(1980)= (4)

REPETTIAN

^

.

^

Z< 

1-

c

2

_

UJ

p"

-(3)-

Z

Z

o

5 

Ul J-

_l

W

Figure 2. Correlation of California microfossil zones and 

California benthic foraminiferal stages of Kleinpell (1938, 

1980) with geologic time scale of Berggren and others 

(1985) in vicinity of Miocene-Pliocene boundary. Miocene-Pliocene boundary (1) of Berggren and others (1985) 

is followed in this paper; (2), Miocene-Pliocene boundary 

of Zijderveld and others (1986). Dark bars, normal 

polarity.

Neogene Geohistory Analysis of Santa Maria Basin, California, and Its Relationship to Transfer of Central California to the Pacific Plate K3
The last occurrences of Synedra jouseana and 

Asteromphalus darwinii are secondary markers that approximate the base of the Thalassiosira miocenicalNitzschia 

miocenica Interval Zone (Barron, 1976, 1986a), as do the 

first occurrences ofAzpeitia vetustissimus sensu Barron and 

Baldauf (1986) and Coscinodiscus subtilis. Together, these 

datum levels and the last common occurrence of Rouxia 

californica typically correspond with the Monterey-Sisquoc 

boundary (Dumont, 1986; Barron, 1986a; Barron and 

Baldauf, 1986; Barron and Ramirez, 1992). As noted by 

Dumont (1986) and Barron and Baldauf (1986), the first 

occurrence of Lithodesmium cornigerum is a secondary 

marker for the base of the Thalassiosira praeoestrupii Partial 

Range Zone. Rossiella tatsunokuchiensis, Hemidiscus ovalis, 

and Rhaphoneis fatula appear to be restricted to the lower 

part of the Thalassiosira oestrupii Partial Range Zone in 

California (Barron and Baldauf, 1986; Dumont, 1986; Barron 

and Ramirez, 1992). The upper part of the T. oestrupii Zone

is distinguished by the absence of L. cornigerum and 

L. minusculum, which have last occurrences in California 

sections at about 4.2 to 4.4 Ma (Barron and Ramirez, 1992; 

Barron, 1992b).

Acknowledgments

We thank Jack Baldauf of Texas A and M University 

and Tom McKinnon and Tom Dignes of Chevron Overseas 

Petroleum, Inc., for their helpful reviews. Scott Starratt of 

the U.S. Geological Survey (Menlo Park) also provided 

useful suggestions to improve this paper, and Margaret 

Keller of the U.S. Geological Survey (Menlo Park) did a 

very thorough job of editing the manuscript. Appreciation 

is due to Pedro Ramirez of California State University, 

Los Angeles, for sharing his stratigraphic data on Sisquoc 

Formation sections. ARCO Oil and Gas Company is

X 

o 

o

QL 

UJ

I 

O

o

QL 

UJ 

CO 

D 

</)

CO

UJ 

O

PALEOMAGNETIC 

EPOCHS

E £ Magnetic 5

o Polarity <

DIATOM 

ZONATION

Barron (1986) 

Dumont (1986)

Thalassiosira 

oestrupii

T. praeoestrupii

T. hyalinopsis

7". miocenica/ 

N. miocenica

Rouxia 

californica

Thalassiosira 

antiqua b

Figure 3. Ranges of biostratigraphically useful diatoms in latest Miocene and earliest Pliocene of California. (1), MiocenePliocene boundary of Berggren and others (1985); (2), Miocene-Pliocene boundary of Zijderveld and others (1986).

K4 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
thanked for the permission to publish this paper. Finally, 

the senior author is grateful for the inspiration provided by 

his mother, [the late] Laddie Dumont.

DIATOM STRATIGRAPHY OF 

SISQUOC FORMATION SECTIONS

Sweeney Road

The Sweeney Road section is located about 3 km east 

of Lompoc, Santa Barbara County, along the north bank of 

the Santa Ynez River (Lompoc Quadrangle 7.5-minute 

series U.S. Geological Survey topographic map). The outcrop is composed of a continuous sequence of finely laminated porcellanites of the siliceous member of the Monterey 

Formation (Isaacs, 1981; Dumont, 1986; Ramirez, 1990) 

and overlying diatomaceous mudstones and shales of the 

Sisquoc Formation (Dibblee, 1950; Dumont, 1986; 

Ramirez, 1990). The base of the section studied by Dumont

(1986) and Ramirez (1990) is located at the apex of a small 

anticline within the finely laminated porcellanites of the 

upper part of the Monterey Formation. This is about 2.4 km 

southeast of the junction of Sweeney Road and Highway 1, 

east of Lompoc, just north of the confluence of Salsipuedes 

Creek and the Santa Ynez River where the river starts to 

bend to the northwest (fig. 4).

A detailed study of the diatom stratigraphy of the 

Sweeney Road is presented in Dumont (1986); because 

this study is not readily available in the published literature, it is summarized in some detail in this report.

Twenty-nine samples were collected initially at 30.48 

m (100 ft) horizontally-measured intervals along the diatomaceous exposure at Sweeney Road (fig. 4). To refine the 

diatom biostratigraphy across the Miocene-Pliocene 

boundary, six additional samples were collected from an 

interval lying 275 to 355 m above the base of the same 

section. The lithologic column of Sweeney Road section 

after Ramirez (1990) showing Dumont's (1986) samples 1 

through 27 is shown in figure 5. The stratigraphic level of

0 KILOMETERS Q 5 

I I I I I

Figure 4. Map of Sweeney Road section (fig. 1), east of Lompoc, showing the samples studied 

(Dumont, 1986; this report) and location of phosphatic conglomerate beds. Contour interval 

equals 200 ft.

Neogene Geohistory Analysis of Santa Maria Basin, California, and Its Relationship to Transfer of Central California to the Pacific Plate K5
these samples is shown according to the measured section 

of Ramirez (1990) because his measurements were more 

detailed and employed more measurements of bedding 

attitudes than those of Dumont (1986).

All the samples used for this study were prepared 

employing a modified technique of Schrader (1973b) and 

counted using the techniques of Schrader and Gersonde 

(1978) (see Dumont, 1986, for details).

The 37 m of finely laminated porcellanites lying at 

the base of the section are correlated with the Rouxia 

californica Partial Range Zone on the basis of the occurrence of Rouxia californica in samples 1 (6 m) and 3 (31 

m), the first occurrence of Nitzschia reinholdii in sample 5

(37 m), and the absence of Thalassionema schraderi. The 

lower porcellanite interval of the Sweeney Road section 

may also be correlated to the upper part of magnetic polarity Chron 6 (Subchron C3Ar.2r of Cande and Kent, 1992) 

on the basis of an estimated age for the last consistent 

occurrence of Rouxia californica as 6.0 Ma (Barron and 

Keller, 1983; Dumont, 1984), an occurrence which 

approximates the first occurrence of Nitzschia reinholdii in 

the California Continental Borderland (Barron, 1981).

The first occurrence of the warm-water species Thalassiosira miocenica was recorded in sample 6 (72 m), followed 

by the first occurrence of Nitzschia miocenica in sample 7 

(76 m) (table 1, fig. 5), which is within laminated diatoma-

<* 

5

LU

O 

<

5.1- 

5.35-

5.6-

6.0 -

SERIES

LU

z

LU 

O 

O

-1

D.

LU

Z

m 

O 

0

^

H 

2 

D

0

1- 

< 

^

or 

0

LL 

O

O

O 

CO

CO

MONTEREY FORMATION

DIATOM ZONE

jN. reinholdii subzone b

i

0

i>

| lyalinopsis *

j T. miocenica N./T

Rouxia californica

SWEENEY 

ROAD

^ s^ x-^ -

S*\S ^~" ^X^ ^ ^X^

^ x%^ s^, 

<*\s s^s s^s 

*s **\s *^*r 

**\s **^s **^s

^ <*\s s^s 

x\^ />^ x\^

r\s ^\^ **\s

 ^ *s*> \S* 

 ~V^ *"N^

^» xv^. i xv_ 

COVERED

 ""^ ^^ I>K^ ^*-

COVERED

111

M

CO

26 2/ 

r25 

M

O*3

I 22

,....,.._ 21"- *Q 

 19/20

 19 

 ^g- 18/19

17 17/18 

 16/17

____ !L 15/16 

 15

14 U/15 

 13

11

50 meters

f==5-10

- 8

= 6 7

-r- -r- -T- \

T- r~ -T~ ~IT o 4 ^

-r- -r- -r- / 2

L- -r- -r- -y

f-1

EXPLANATION

Laminated diatomaceous

Massive to discontinuously 

laminated diatomaceous 

mudstone

Nodular (or) pelletal 

phosphate conglomerate

Stratiform dolomite

Figure 5. Sweeney Road lithologic section of Ramirez (1990) showing samples studied for 

diatoms (Dumont, 1986; this report) and diatom zones of Dumont (1986), Barron and Baldauf 

(1986), and Barron (1986a). *, Thalassiosira praeoestrupii.

K6 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
Table 1. Stratigraphically important diatoms (per sample) in Sweeney Road section after Dumont (1986)

[Stratigraphic position of Dumont's (1986) samples have been redetermined in order to conform with lithologic column of Ramirez (1990) (fig. 5) and 

was accomplished by identification of marker beds and extent of exposed outcrops]

.

6

z

0>

Q.

(0

0)

27

26

25 

24

23

22

21

20

19/20

19

18/19

18

17/18

17

16/17

16

15/16

15

14/15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

 _ ^

E,

c

g '55

8.

g

"o.

5

D)

I

tf)

440

439

430 

422

393

379

367

360

355

345

336

332

327

322

307

294

288

282

275

271

257

244

230

99

94

84

76

72

37

35

31

26

6

</> E HIT :^ 5

GO 55 o ~~~ *Q- Q

2m ,| = '« 2 m O ._ is "a s§ 8, § -5 E§8-?§~'l-8-|

1 1 1 1 3 41 I S l I I 1 t 1 1 I I

w°Jco-c"§m§cw"m-c> S£ coo.w'

g | m.o.c^g^w 0^1 mg 55 gm mg

'"§ 1 '^ .5 .is .5 c ^ "8 -2 -2 -^ Q Q Q Q Q Q

_c-§ §'o'§'S-c!®-Sw ww low ww ww

111 I III"! !!!!!! 111! oCi2;?;§:goctt ccKKKKh- KI^KK

29 10090 21 100 321 30403 17

9200302500 363 1050500

6 19 00 100 100 562 8050700 

0 12 0 0 1 1 0 0 0 0 397 172 0 44 6 34 5

0 7 0 0 0 2 0 0 0 1 284 131 1 2 1 6 0 2

0 16 0 0 1 3 0 0 0 0 379 132 0 0 0 1 0 1

020000 1000 233 33 0 10 4704

02000 10000 353 64 09 1402

6310000000 207 33 071 134

13000 10000 234 10 040043

8100030000 130 36 010011

00001 12 0002 189 46 000000

2 0 0 0 0 18 0 0 0 0 454 461 0 5 1 0 0 0

0000460002 330 204 02 1000

9 0 2 0 1 31 0 0 0 0 616 125 0 2 5 0 0 0

9000 1 13 0003 77 55 06500 1

14 0 0 0 4 7 0 0 0 0 548 153 0 1 2 0 0 1

2000090001 335 14 007005

13 0 0 0 4 34 0 0 0 4 104 195 0 6 2 0 0 1

15 0001 10 0000 295 2344000

00204 19 0001 176 4072000

0000070009 127 0000000

24 0 0 4 1 18 0 0 0 25 185 0 1 0 0 0 0 0

20 0002 28 000 21 188 0700000

29 0006 21 0000 165 01 05000

0002000000 44 0402000

0001230000 68 000 16 000

50000 10000 48 050 15 000

00001 10000 80 000 14 000

000000000050000000

000000003000000000

000000000000000000

000000001020002000

(P

c

3

o

o

§*~

948

799

953 

1144

766

971

733

820

612

724

412

525

1422

790

1095

685

995

555

942

646

582

587

988

734

704

329

263

285

329

82

155

11

36

DIATOM

ZONE

Thalassiosira

oestrupll

Thalassiosira

praeoestrupii

Thalassiosira

hyalinopsis

T. miocenica

/

N. miocenica

Rouxla

callfomica

ceous shale of the basal Sisquoc Formation (Dumont, 1984, 

1986). Sample 11 (230 m) contains the last occurrences of 

T. miocenica and N. miocenica within a finely-laminated 

diatomaceous shale (laminae 1-2 cm thick). The last occurrence datum of T. miocenica occurs at the base of the 

reversed-polarity event of Chron 5 (Subchron CSAr.lr of 

Cande and Kent, 1992) within the Purisima Formation at 

Santa Cruz in northern California (Dumont, 1986; Dumont 

and others, 1986), suggesting an age of 5.68 Ma for sample 

11 (Berggren and others, 1985).

Between 244 and 257 m above the base of the section, 

the first of six phosphatic conglomerate beds is found (fig.

5). Sample 13 (257 m), which was collected within a massive 

diatomaceous shale, contains the first occurrence of Thalassiosira hyalinopsis. Correlation to the first occurrence of T. 

hyalinopsis in the Purisima section at Santa Cruz suggests 

that sample 13 lies within the middle of the reversed-polarity 

event of Chron 5 (Subchron C3Ar.lr of Cande and Kent, 

1992), with an estimated age of 5.6 Ma (Dumont, 1984; 

Berggren and others, 1985; Dumont and others, 1986).

The first occurrence of Thalassiosira praeoestrupii and 

Lithodesmium cornigerum are found in sample 18/19 (336 

m). The first occurrence of T. praeoestrupii coincides closely 

with the top of polarity Chron 5 (Chron C3A of Cande and

Neogene Geohistory Analysis of Santa Maria Basin, California, and Its Relationship to Transfer of Central California to the Pacific Plate K7
Kent, 1992) (Dumont, 1984; Dumont and others, 1986; 

Madrid and others, 1986) and the Miocene-Pliocene boundary within the Santa Cruz section (5.35 Ma). A precise 

correlation of the £ oestrupii datum at Sweeney Road with 

the Santa Cruz section cannot be made because of the apparent unconformity identified within the Gilbert ReversedPolarity Chron (Chron C3) at Santa Cruz (Madrid, 1982). 

However, the first occurrence of T. oestrupii was correlated 

to a level within the lower reversed-polarity event of the 

Gilbert Reversed-Polarity Chron (Subchron C3Ar.4r of 

Cande and Kent, 1992) in the equatorial Pacific Ocean by 

Burckle (1978) and has an estimated age of 5.1 Ma (Baldauf, 

1985; Barron and others, 1985). Available correlations 

(Burckle, 1978; Barron, 1981) suggest that the T. oestrupii 

datum is isochronous between the tropics and California.

A benthic, cosmopolitan diatom marker for the Pliocene, Rossiella tatsunokuchiensis, was first recorded in 

sample 25 (430 m) in the Sweeney Road section. This diatom was also recorded in the last two samples, 26 (439 m) 

and 27 (440 m), at the top of the section. Another Pliocene 

benthic species, Rhaphoneis fatula (Barron and Baldauf, 

1986), was first recorded in sample 21 (379 m).

A multisiliceous microfossil study (radiolaria, diatoms, 

and silicoflagellates) of the Sweeney Road section has been 

published by Akers and others (1987). The radiolarian biostratigraphy suggests that the Sweeney Road section ranges 

in age from late Miocene to undifferentiated Pliocene, on 

the basis of the occurrences of Stichocorys peregrina, Lychnocanoma grande, and Lamprocyrtis heteroporos. However, the diatom stratigraphy presented in Akers and others 

(1987) is difficult to interpret. Basically, Akers and others 

(1987) recorded diatom species (for example, Pseudoeunotia doliolus, Rhiiosolenia praebergonii, and Actinocyclus 

oculatus) from the Sweeney Road section that are younger 

than the ages represented by the radiolarians they report. 

Aside from the internal inconsistencies among the microfossil groups (radiolaria, diatoms, and silicoflagellates), 

their report suggests that radiolaria may be useful in the 

time interval represented in this section.

In figure 6 an age versus stratigraphic-height (sedimentation rate) curve is plotted for the Sweeney Road section on the basis of the stratigraphic placement of diatom 

datum levels that have been calibrated to paleomagnetic 

stratigraphy at Santa Cruz by Dumont and others (1986) 

(fig. 6). Two alternatives are given for placement of the 

first occurrence of Thalassiosira oestrupii within the 

Sweeney Road section one, between sample 19 (345 m) 

and 19/20 (355 m), based on Dumont's (1986) study (table 

1), and the other, between samples 22 (379 m) and 23 

(393 m) based on Barron's previous studies (1975; unpub. 

data, 1993) and preliminary examination of Dumont's 

(1986) samples. The sedimentation-rate curve in figure 6 

is drawn through the latter alternative because it fits on a 

linear extrapolation of the line through the first occurrences of T. hyalinopsis and T. praeoestrupii. The two

alternative stratigraphic placements are probably due to 

slight differences in the two author's taxonomic concept of 

T. oestrupii.

The sedimentation curve in figure 6 indicates that the 

lower part of the Sisquoc Formation at Sweeney Road 

(50-230 m; about 6.0 to 5.65 Ma) accumulated at a postcompaction sedimentation rate of about 600 m/m.y. This is 

in good agreement with sedimentation rates estimated for

m

450 

400 - 

350 - 

300 

250 

200 

150 - 

100 - 

50 

0 J

Ma

B.7. oestrupii *

I 

B. T. oestrupii

B. T. praeoestrupii

280 m/m.y.

B.7. hyalinopsis 

T. N. miocenica

600 m/m.y.

B. T. miocenica 

TC. R. californica

o 

o

Q.

CD

C D 

=> CO

C

.2

CO

E

o

LL

O 

0

13 O"

CO

^

>,§ o .2

o5 75

CD

c

"Z o

OJ O

HI ~

Q_

CD 

C 

CD 

O 

O

^

CD

 i 

co

Figure 6. Age versus stratigraphic height plot for Sweeney 

Road section. Estimated sedimentation rates for lower and 

upper segments of plot are 600 and 280 m/m.y., respectively. Ages used are those of figure 2; bar shows stratigraphic constraint of datum level; B, first occurrence; T, last 

occurrence; TC, last common occurrence; *, lowest Thalassiosira oestrupii observed by Barron (unpub. data, 1993, differs from that of Dumont, 1986). Hachured area represents 

covered interval that obscures Monterey-Sisquoc contact.

K8 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
the Sisquoc Formation at Casmalia and Point Pedernales 

(600 and 580 m/m.y., respectively) by Ramirez (1990) and 

at Harris Grade (about 540 m/m.y.) by Barron and 

Ramirez (1992). Diatom stratigraphy suggests that the 

post-compaction sedimentation rate in the upper part of 

the Sisquoc Formation at Sweeney Road (230-440 m) 

declines to 280 m/m.y at about 5.6 Ma. The extrapolated 

age for the top of the exposed Sisquoc Formation at 

Sweeney Road is 4.9 Ma. The lower part of this upper 

interval is marked by numerous phosphatic conglomerate 

beds (fig. 5) (Dumont, 1986; Ramirez, 1990). Therefore, it 

is possible that these beds correspond to unconformities 

where relatively brief intervals of time, beyond the resolution of diatom biostratigraphy, are missing.

Lompoc Quarry

At the Lompoc (Johns-Mansville now Cellite) diatomite 

quarry about 4 km south of Lompoc (fig. 1), about 23 m of 

massive Sisquoc Formation rocks overlie a 50-cm-thick 

phosphatic conglomerate bed at the top of the Monterey 

Formation. This phosphatic conglomerate was recognized as 

an unconformity by Barron (1975, 1986a) and Dumont 

(1984) at which Sisquoc rocks assignable to the Thalassiosira 

hyalinopsis Partial Range Zone overlie Monterey rocks 

assignable to the Rouxia californica Partial Range Zone. A 

minimal sediment interval corresponding to the T. miocenica/N. miocenica Interval Zone (6.0 to 5.6 Ma) is missing 

at this unconformity (fig. 7).

X

o

0

0. 

UJ

UJ

z

UJ 

O 

0

UJ 

Z 

UJ 

O 

O

X

o

0

0. 

UJ

m

3

UJ

d)

4.0

4.5-

5.0

5.5-

6.5-

Paleomagnetic 

Epochs

0

Diatom 

Zonation

Barron 

(1986) 

Dumont 

(1986)

Thalassiosira

oestrupii

T. praeoestrupii

T. hyalinopsis

T. miocenica / 

N. miocenica

Rouxia 

californica

Ma

3.8?

5.1

6.0

6.7-

Sections

Harris 

Grade

Point 

Pedernales Lompoc 

Quarry

Lompoc 

Hills

wx/

AAAA

I

I1

m^%&

Sweeney 

Road

Casmalia

4A£

I m

Mussel 

Rock

VXA/

WV.

AAAA

Hiatus

EXPLANATION

AAAA Hiatus Monterey 

Formation

Sisquoc 

Formation

Foxen Mudstone

Figure 7. Correlation of various Sisquoc Formation 

ary (1) of Berggren and others (1985) is followed 

(1986). Dark bars, normal polarity.

sections in Santa Maria basin to geologic time. Miocene-Pliocene boundin this paper; (2), Miocene-Pliocene boundary of Zijderveld and others

Neogene Geohistory Analysis of Santa Maria Basin, California, and Its Relationship to Transfer of Central California to the Pacific Plate K9
Lompoc Hills

The Lompoc Hills section lies in the hills approximately 2.5 km west of Lompoc (fig. 1). A 200-m section 

of interbedded massive and laminated beds of the Sisquoc 

Formation overlying porcellaneous beds of the Monterey 

Formation was measured at this locality by Ramirez 

(1990). The lower 20 m of the measured Monterey section 

consists of finely laminated lenticular to continuous porcellaneous beds that are 3 to 8 cm thick (Ramirez, 1990). 

These porcellaneous beds alternate with 30 to 50 cm thick 

packages of massive and laminated diatomaceous strata. 

Above the porcellanites, banded and massive beds of the 

Sisquoc Formation dominate the section. The MontereySisquoc boundary is placed at the change from finely laminated to banded rocks, a change which also approximates 

the change from opal-CT rocks below to opal-A rocks 

above. The Sisquoc Formation is truncated by sandstones 

and siltstones of the Pliocene Careaga Sandstone (Woodring and Bramlette, 1950).

The absence of Thalassiosira hyalinopsis, T. praeoestrupii, and T. oestrupii in the Lompoc Hills section 

suggests that the section is older than the T. hyalinopsis 

Partial Range Zone (Barron and Ramirez, 1992). Sporadic 

occurrences of Nitzschia miocenica and the absence of 

Rouxia californica and Synedra jouseana in all samples 

above the basal samples of the section argues for correlation of the bulk of the Sisquoc Formation in the Lompoc 

Hills section with the Thalassiosira miocenicalNitzschia 

miocenica Interval Zone. The basal samples are tentatively 

correlated with the Rouxia californica Zone.

Point Pedernales

The Point Pedernales section is located along the 

coast approximately 5 to 6 km southwest of the southeast 

entrance to Vandenberg Air Force Base (fig. 1). At Point 

Pedernales, Ramirez (1990) measured and described 

approximately 400 m of mostly well-exposed strata of the 

Monterey and Sisquoc Formations along steep seacliffs 

containing exposures up to 4 m in height. The strata consist largely of alternating massive and laminated diatomaceous units ranging from less than 0.5 to greater than 5 m 

in thickness (Ramirez, 1990). The lower 10 m of the section was assigned to the Monterey Formation by Ramirez 

(1990) on the basis of biostratigraphic correlation with the 

Monterey-Sisquoc boundary at Mussel Rock (fig. 1; see 

below). Biostratigraphic determination of the MontereySisquoc formational contact was utilized because the Sisquoc Formation grades into and is lithologically similar to 

the underlying Monterey Formation and because an insufficient number of samples were taken from the Monterey 

Formation to compositionally distinguish it from the Sisquoc Formation (Ramirez, 1990).

The basal sample of the Point Pedernales section was 

assigned to the Rouxia californica Partial Range Zone on 

the basis of the rare occurrence of Rouxia californica and 

Synedra jouseana (Barron and Ramirez, 1992). The interval from 10 to 350 m is placed in the T. miocenicalN. 

miocenica Interval Zone, below the first occurrence of 

Thalassiosira hyalinopsis at 355 m. The T. hyalinopsis 

Zone then extends from 355 m upsection to a level immediately below the first occurrence of Thalassiosira praeoestrupii at 456 m, the level of the highest sample taken in 

the section. Whereas Hornafius and others (1982) reported 

Thalassiosira oestrupii from the upper part of the Point 

Pedernales section, Barron and Ramirez (1992) did not 

observe this diatom and assign the top of the Pedernales 

section to the older Thalassiosira praeoestrupii Partial 

Range Zone (5.35-5.1 Ma).

Harris Grade

The Harris Grade section, which is located about 10 

km north of Lompoc on old California Highway 1 (fig. 1), 

contains a 900-m-thick section of the Sisquoc Formation 

overlying a faulted anticline. The Sisquoc Formation is 

conformably overlain by the relatively diatom-poor Foxen 

Mudstone in the Harris Grade section. Barron and Baldauf 

(1986) recognized a 50-m section of the Thalassiosira 

praeoestrupii Partial Range Zone above a 140-m-thick 

interval of porcellanites at the base of the Harris Grade 

section. The bulk of the Harris Grade section is assignable 

to the Thalassiosira oestrupii Partial Range Zone, and 

Barron and Baldauf (1986) demonstrated that the top of 

the Sisquoc Formation is well above (at least 900 m) the 

Miocene-Pliocene boundary. Barron and Baldauf (1986) 

followed Stanley and Surdham (1984) in estimating that 

the Sisquoc-Foxen contact at the top of the Harris Grade 

section corresponded with the 4.2-Ma sea level fall identified in the global eustatic sea level curve by Vail and 

Hardenbol (1979), but Barron and Ramirez (1992) suggested that this boundary may correlate with the 3.8 Ma 

sea level fall of Haq and others (1987). This 3.8-Ma age 

for the top of the Sisquoc Formation in the Harris Grade 

section is based on an estimated sedimentation rate of 540 

m/m.y., which was obtained by Barron and Ramirez 

(1992) for the Harris Grade section by applying Barron's 

(1992b) independent age estimates to the diatom events of 

Barron and Baldauf (1986) in the Harris Grade section.

Casmalia

A 700-m-thick composite section containing the 

uppermost Monterey Formation, the Sisquoc Formation, 

and the basal Foxen Mudstone was measured along Black 

Road and along the railroad tracks that run parallel to the

K10 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
road (Ramirez, 1990) north of the town of Casmalia (fig. 

1). The lowermost 30 m of the measured section consists 

of continuously to discontinuously banded and finely laminated porcellaneous rocks assigned to the Monterey Formation. Upsection, platy-weathering rocks of the 

Monterey Formation pass into massive, dense, siliceous 

mudstones of the Todos Santos Clay stone Member of the 

Sisquoc Formation (Ramirez, 1990).

The successive first occurrences of T. hyalinopsis 

(437 m), T. praeoestrupii (557 m), and T. oestrupii (677 

m) mark the respective bases of the T. hyalinopsis, T. 

praeoestrupii, and T. oestrupii Partial Range Ziones 

according to the diatom stratigraphy of Barron and 

Ramirez (1992). The Sisquoc-Foxen contact is estimated 

to be 5.0 Ma in age at the Casmalia section on the basis of 

the extrapolation upsection of a 600 m/m.y. post-compaction sedimentation rate suggested by diatom biostratigraphy (Ramirez, 1990; Barron and Ramirez, 1992). Thus, 

the Sisquoc-Foxen contact at Casmalia may be as much as 

1.2 m.y. older than it is at Harris Grade (figs. 7, 8). This 

diachroneity may reflect a more basin-marginal depositional setting for the Casmalia section than for the Harris 

Grade section and an earlier appearance of the more clastic-rich, diatom-poor sediments of the Foxen Mudstone. 

Alternatively, this 1.2 m.y. interval (about. 5.0-3.8 Ma) 

may have been removed at a yet to be recognized unconformity in the Casmalia section.

Mussel Rock

The Mussel Rock section is exposed along the coast 

(fig. 1) southwest of the town of Guadalupe. A 400-mthick section was measured by Ramirez (1990) from the 

opal-A to opal-CT transitional zone of the upper part of 

the Monterey Formation to the top of exposed rocks of the 

Sisquoc Formation. An additional 300+ m of Monterey 

Formation strata crops out to the south along the shore 

below the measured section according to Pisciotto (1981). 

The layered rocks of the Monterey Formation contrast 

sharply with the overlying massive and intensively bioturbated rocks of the Sisquoc Formation, which become 

increasingly conglomeratic upsection. A distinct phosphatic conglomerate marks the contact between the layered Monterey and massive Sisquoc Formations. Ramirez 

(1990) measured this phosphatic conglomerate as 23 cm in 

thickness, but it apparently varies laterally in thickness.

The last common occurrence of R. californica in the 

uppermost part of the Monterey Formation at this section 

defines the top of the R. californica Partial Range Zone, a 

correlation which is supported by the last occurrences of 

Synedra jouseana and Hemiaulus polymorphus in the 

same sample. About 30 m of £ miocenicalN. miocenica 

Interval Range Zone is present in the basal Sisquoc Formation at Mussel Rock according to the diatom biostratigraphy of Barron and Ramirez (1992). The T. hyalinopsis, 

T. praeoestrupii, and T. oestrupii Partial Range Zones are 

recognizable by the successive first occurrences of the 

nominative taxa in the Sisquoc Formation at Mussel Rock 

(Barron and Ramirez, 1992).

Comparison of Sisquoc Formation Sections

Based on the comparison of the ages of seven sections 

of the Sisquoc Formation in the Santa Maria basin (fig. 7), 

it is apparent that the Monterey-Sisquoc formational boundary typically coincides with the boundary between the Rouxia 

californica Partial Range Zone and the Thalassiosira miocenica/Nitzschia miocenica Interval Zone (6.0 Ma). However, in some sections, such as the Point Pedernales section, 

no distinct lithologic break occurs at this 6.0 Ma time horizon, 

and the Monterey Formation is difficult to distinguish from 

the Sisquoc Formation. Most of the Sisquoc Formation outcrop sections do not extend much younger than the basal 

Thalassiosira oestrupii Partial Range Zone (about 5.0 Ma).

Massive 

Laminated 

ixfn?) Porcellanite Foxen Mudstone

Rx. calif.

Monterey Formation

Point Lompoc Lompoc 

Pedernales Hills Quarry

Sweeney Harris 

Road Grade

Casmalia Mussel 

Rock

w NW

Figure 8. Correlation of various Sisquoc Formation sections 

in Santa Maria basin showing stratigraphic thickness and 

generalized lithology. W, west; NW, northwest; Rx. calif., 

Rouxia californica.

Neogene Geohistory Analysis of Santa Maria Basin, California, and Its Relationship to Transfer of Central California to the Pacific Plate K11
The Harris Grade section, however, extends considerably 

younger according to the diatom studies of Barron and 

Baldauf (1986).

There is considerable variation in the thickness of individual diatom zones in Sisquoc Formation sections (fig. 8). 

For example, the Thalassiosira miocenicalNitzschia miocenica Interval Zone is over 330 m thick in the Point Pedernales 

section, but it is totally removed at an unconformity in the 

Lompoc Quarry section. Generalized lithology is also quite 

variable (Ramirez and Garrison, written commun., 1994); 

for example, the interval of the Thalassiosira hyalinopsis 

Partial Range Zone is generally represented by laminated 

sediments in the Point Pedernales section, massive diatomaceous sediments in the Sweeney Road, Casmalia, and Mussel 

Rock sections, and by porcellanites in the Harris Grade 

section. On the other hand, laminated beds are consistently 

common within the T. miocenicalN. miocenica Interval Zone 

in all Santa Maria basin sections studied. Similarly, phosphatic conglomerate beds, which often represent unconformities, are more typical of an interval embracing the upper T. 

miocenicalN. miocenica Interval Zone to lower T. hyalinopsis 

Partial Range Zone.

THE MONTEREY-SISQUOC BOUNDARY 

PROBLEM

As early as 1913, Louderback (1913) recognized the 

problem of distinguishing the upper part of the Monterey 

Formation from the overlying Sisquoc Formation in the 

Lompoc-Santa Maria-Santa Barbara region. At several 

localities in this region, the depositional sequence appears 

continuous (Arnold and Anderson, 1907; Kleinpell, 1938; 

Bramlette, 1946; Dibblee, 1950), but at the Lompoc 

(Johns-Mansville now Cellite) diatomite quarry south of 

Lompoc, an unconformity is recognized at the top of the 

Monterey Formation (fig. 2) (Barron, 1975; Dumont, 

1984). This unconformity was recognized and interpreted 

by earlier workers as relatively brief and coincident with 

both the Miocene-Pliocene boundary and the MontereySisquoc formational contact (Bramlette, 1946; Wornardt, 

1963; Barron, 1975). These observations were the origin 

of the hypothesis equating the Miocene-Pliocene boundary 

with the top of the Monterey Formation, a simple formula 

which some workers continue to apply.

Bramlette's (1946) classic paper, the first definitive 

geologic study of the Monterey Formation, emphasized 

the Monterey Formation's characteristic rhythmic bedding 

and extremely high organic content and speculated on its 

origin. At about the same time, biostratigraphic studies 

referred to the basal Sisquoc Formation as being either latest Miocene (benthic foraminifers) or earliest Pliocene 

(mollusks) in age (Woodring and Bramlette, 1950).

In the past, different lithologic features have been 

used to delineate the contact between the Monterey and

Sisquoc Formations in coastal southern California (Arnold 

and Anderson, 1907; Mulryan, 1936; Canfield, 1939; 

Bramlette, 1946; Woodring and Bramlette, 1950; Dibblee, 

1950; Wornardt, 1963; Isaacs, 1981). One approach was to 

differentiate the Monterey Formation from the overlying 

Sisquoc Formation on the basis that the Monterey Formation was finely laminated (Canfield, 1939),whereas the 

Sisquoc Formation contained a higher proportion of mudstone and was characterized by cruder bedding (Isaacs, 

1981). However, interbedded, finely laminated and massive diatomaceous shales are often present in the basal 

Sisquoc Formation (Ramirez and Garrison, written commun., 1994) (figs. 5, 8), prompting Woodring and Bramlette (1950) to state that the Monterey-Sisquoc boundary is 

gradational in some places in the Santa Maria basin and 

difficult to place.

Another approach was to place the contact between 

the Monterey and Sisquoc Formations at a depositional 

break marked by the unconformity, represented by the thin 

phosphatic conglomerate unit as seen at the Lompoc diatomite quarry (Mulryan, 1936; Bramlette, 1946; Wornardt, 

1963, 1983). However, in some sections, such as the 

Naples Beach section of the Santa Barbara basin (fig. 1) 

(Isaacs, 1981), there is no apparent unconformity or phosphatic conglomerate at the formational boundary, whereas 

in other sections (for example, the Sweeney Road section) 

there may be numerous phosphatic conglomerate units 

(fig. 5) (Dumont, 1984, 1986; Ramirez, 1990).

Dibblee (1950) placed the contact between the Monterey and Sisquoc Formations at the break between the 

porcellaneous shales (porcellanite/chert) and overlying diatomaceous shales. Isaacs (1981) described the porcellanites as "an aphanitic rock with a somewhat rough, matte 

surface texture; *** even where laminated or well bedded." Application of this description would appear to be 

the most consistent field method to differentiate the shales 

that determine the Monterey and Sisquoc formational 

boundary in the Santa Maria-Lompoc region. However, 

Isaacs (1981) pointed out that the top of the porcellaneous 

section migrates upward across different lithologic units of 

the Monterey Formation, becoming successively younger 

as one proceeds westward along the coast from Santa Barbara to Point Conception.

Diatom studies have placed the age of the top of the 

porcellanite unit at the Sweeney Road section at approximately 6.0 Ma (figs. 5, 7) (Dumont, 1984, 1986; Dumont 

and others, 1986; Ramirez, 1990). However, the top of the 

porcellanite in the nearby Lompoc diatomite quarry section is over 1 m.y. older (Barron, 1975, 1986a; Dumont, 

1984, 1986), as it contains diatoms assignable to Barron's 

(1986a) northeastern Pacific Diatom Subzone (a) of the 

Thalassiosira antiqua Partial Range Zone (7.6-7.0 Ma). 

This represents an age difference of as much as 1.5 m.y. 

for the top of the porcellanite at these two localities 

(Dumont, 1986).

K12 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
An additional approach, proposed by Isaacs (1981) 

and emphasized by Ramirez and Garrison (written comm., 

1994), focused on the much greater detrital mineral content of the Sisquoc Formation compared to that of the 

Monterey Formation. The Sisquoc Formation contains a 

"greater average abundance of detrital minerals," and a 

"much higher proportion of massive mudstone," than the 

laminated Monterey Formation (Issacs, 1981). This 

method, however, may be difficult to apply in the field, as 

the diatomaceous rocks at the base of the Sisquoc Formation and the top of the Monterey Formation can be visually indistinguishable (Woodring and Bramlette, 1950).

All of the above criteria can be applied, with varying 

success, on isolated sections to distinguish the upper part of 

the Monterey from the overlying lower part of the Sisquoc 

Formation. It can be demonstrated by diatom biostratigraphy 

that none of the above criteria can be used independently to 

distinguish the Miocene-Pliocene boundary.

In summary, the stratigraphic position of the Monterey-Sisquoc formational contact can differ at many of the 

sections found in the Santa Maria and Santa Barbara 

basins depending on the lithologic criteria employed to 

determine it. However, the Miocene-Pliocene boundary 

can be consistently defined within this region by diatom 

biostratigraphy. Where the basal Sisquoc Formation is 

visually distinguishable from the underlying Monterey 

Formation, the Monterey-Sisquoc formational contact consistently coincides with the base of the Thalassiosira miocenica/Nitzschia miocenica Interval Zone of Dumont 

(1986) (latest Miocene). Where a more gradational transition occurs between the Monterey and Sisquoc Formations, this 6.0 Ma time horizon can be accurately placed 

by diatom biostratigraphy (Barron, 1975; Dumont, 1984, 

1986) (fig. 7).

PALEOCEANOGRAPHIC IMPLICATIONS

Middle to late Miocene climatic cooling accompanied 

by the intensification of upwelling and increased primary 

productivity within the eastern boundary current region of 

coastal California was ultimately responsible for the 

increased diatom blooms that contributed to the biosiliceous richness of the rocks of the upper part of the Monterey Formation and the overlying Sisquoc Formation 

(Ingle, 1981; Barron, 1986b). Superimposed on this biogenie sedimentation pattern in coastal California is an 

increase in clastic deposition that began in the latest Miocene and intensified in the early Pliocene (Crouch, 1979; 

Ingle, 1980; Isaacs, 1981, 1983; Dumont, 1986; Teng and 

Gorsline, 1989; Barron, 1992b).

High-latitude cooling and a fall in global eustatic sea 

level at 5.5 Ma (fig. 9) are marked by the last occurrence 

of a warm-water biofacies containing Thalassiosira miocenica and Nitzschia miocenica in the Sweeney Road section

(5.6 Ma), an occurrence which is followed by an increase 

of neritic versus planktonic diatoms in this section (table 

1). A similar increase in neritic diatoms occurs in other 

Sisquoc sections in the basal T. hyalinopsis Partial Range 

Zone within the Santa Maria basin (Barron, unpub. data, 

1993), as well as in the Purisima Formation at Santa Cruz 

(Dumont, 1986). The increase of neritic diatoms in coastal 

California is coupled with a marked increase of siliciclastic debris in the samples (Dumont, 1986). This agrees with 

the findings of Isaacs (1981) and Ramirez and Garrison 

(written commun., 1994) who noted an abrupt increase in 

sedimentation of detrital minerals during Sisquoc Formation depositional time.

Other potential evidence for the 5.5 Ma sea level fall 

includes the presence of phosphatic conglomerate beds in 

the lower T. hyalinopsis Partial Range Zone at Sweeney 

Road (fig. 5), as well as the phosphatic conglomerates 

associated with unconformities between the Monterey and 

Sisquoc Formation in the Lompoc quarry and Mussel 

Rock sections (fig. 8).

In a broader sense, these events, which occurred 

approximately the time of deposition of the Monterey-Sisquoc contact, may be considered a response to the late 

Miocene climatic deterioration and buildup of polar ice 

(Kennett, 1977; Savin, 1977; Barron and Keller, 1983; 

Dumont, 1986), which in turn caused a fall in sea level.

TECTONIC IMPLICATIONS

The lithologic character and diatom content of the 

onshore Pliocene sections in California were strongly influenced by tectonic events (Barron, 1992b). Tectonic reorganization of the California Continental Borderland began in 

the latest Miocene (Crouch, 1979) and was intensified during 

the Pliocene as the Coast Ranges of California were uplifted 

and adjoining basins underwent subsidence (Ingle, 1980; 

Teng and Gorsline, 1989). An eastward jump in the San 

Andreas Fault system at 5.5 Ma (Sedlock and Hamilton, 

1991) and a major change in the motion of the Pacific Plate 

between 3.9 and 3.4 Ma (Harbert and Cox, 1989) appear to 

have been responsible for this tectonic reorganization (fig. 

9). Increased deposition of clastic-rich sediments followed 

these tectonic events and also coincided with global falls in 

sea level at 5.5 and 3.8 Ma. Basins formed during the Miocene in California were rapidly filled with Pliocene clastic 

sediments, and diatoms persisted as a major component only 

in sediments deposited in the center of basins and only until 

about 4 Ma.

CONCLUSIONS

Diatom biostratigraphy offers a valuable means for 

correlating and subdividing the Sisquoc Formation as well

Neogene Geohistory Analysis of Santa Maria Basin, California, and Its Relationship to Transfer of Central California to the Pacific Plate K13
as recognizing the Miocene-Pliocene boundary. Our study 

of the diatom biostratigraphy of the Sweeney Road section, east of Lompoc, Calif., and summaries of the diatom 

biostratigraphy of the Lompoc quarry, Lompoc Hills, Point 

Pedernales, Harris Grade, and Mussel Rock sections of the 

Santa Maria basin reveal that the base of the Sisquoc Formation typically coincides with the base of the Thalassiosira miocenica/Nitzschia miocenica Partial Range Zone 

and has an age of approximately 6.0 Ma. Diatom biostratigraphy, thus, represents a reliable means for recognizing 

the 6.0 Ma time horizon that typically coincides with the 

Monterey-Sisquoc boundary. Other criteria for recognizing 

the Monterey-Sisquoc boundary, such as the laminated 

(Monterey Formation) versus massive (Sisquoc Formation) 

expression of the rocks or their porcellaneous (Monterey 

Formation) versus diatomaceous (Sisquoc Formation) 

character are either inconsistent or time transgressive.

The top of the Sisquoc Formation in outcrop sections 

of the Santa Maria basin is typically about 5.0 Ma; however, the Sisquoc Formation may be as young as 3.8 Ma in 

the Harris Grade section in the center of the basin.

The Sisquoc Formation was deposited during a relatively high stand of eustatic sea level bracketed by lowstands dated at 6.3 and 3.8 Ma. Clastic materials, however, 

increase in many Sisquoc sections beginning at 5.0 Ma in 

response to regional tectonism.

REFERENCES

Akiba, Fumio, 1986, Middle Miocene to Quaternary diatom biostratigraphy in the Nankai Trough and Japan Trench and 

modified lower Miocene through Quaternary diatom zones 

for middle-to-high latitudes of the North Pacific, in Kagami,

UJ

z

UJ

o 

o

5

(0

-4.0

4.5

^-5.0

-5.5

-6.0

-6.5

Paleomagnetic

Epoch

Diatom

Zonation

Barron 

(1986)

Dumont 

(1986)

Thalassiosira

oestrupii

Thalassiosira 

praeoestrupii

Thalassiosira 

hyalinopsis

T. miocenica 

N. miocenica

Rouxia 

californica

Ma

3.8?

5.1

6.0

Changes in

Pacific 

Plate Motion

Harbert and Cox 

(1989); Sedlock 

and Hamilton 

(1991)

Harris 

Grade 100

Eustatic

Sea Level

Curve

Haq and

others

(1987)

50

W»J

11.4- 3.9 Ma

- - 3.8 Ma +-

4.2 Ma-*-

Sweeney Casmalia 

Road

W 

uxn

TjTt 

ftft

 5.5 Ma 5.5 Ma -* 

6.3 Ma -*-

EXPLANATION Falling *- 

Sea Level

Monterey 

Formation

Figure 9. Comparison of key Sisquoc Formation sections to eustatic sea level curve of Haq and others (1987) and changes in 

motion of Pacific Plate after Harbert and Cox (1989) and Sedlock and Hamilton (1991).

K14 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
H., Karig, D.E., Coulbourn, W.T., and others, Initial Reports, 

Deep Sea Drilling Project, Volume, 87: Washington, D.C., 

U.S. Government Printing Office, p. 393-481.

Akers, W.H., Marolt, R.E., and Navarette, R.R., 1987, Late Miocene and early Pliocene siliceous microfossils from the 

upper Monterey and lower Sisquoc Formations, Sweeney 

Road, Santa Barbara County, California: Tulane Studies in 

Geology and Paleontology, v. 20, no. 1-3, p. 1-112, 8 pis.

Arends, R.G., and Blake, G.H., 1986, Biostratigraphy and paleoecology of the Naples Bluff coastal section based on diatoms 

and benthic foraminifera, in Casey, R.E., and Barron, J.A., 

eds., Siliceous microfossil and microplankton of the Monterey Formation and modern analogs: Pacific Section, Society 

of Economic Paleontologists and Mineralogists, Book 45, 

p. 121-135.

Arnold, Ralph, and Anderson, Robert, 1907, Geology and oil 

resources of the Santa Maria oil district, Santa Barbara 

County, California: U.S. Geological Survey Bulletin 322, 

161 p.

Bandy, O.L., 1972, Neogene planktonic foraminiferal zones, California, and some geologic implications: Paleogeography, 

Paleoclimatology, Paleoecology, v. 12, p. 131-150.

Barron, J.A., 1974, The Late Miocene-Early Pliocene diatom 

assemblage of southern California Biostratigraphy 'and 

paleoecology: University of California, Los Angeles, Ph.D. 

dissertation, 289 p.

 1975, Late Miocene-Early Pliocene marine diatoms from 

southern California: Palaeontographica, v. 151(B), p. 97-170.

 1976, Marine diatom and silicoflagellate biostratigraphy 

of the type Delmontian Stage and the type Bolivina obliqua 

Zone, California: Journal of Research, U.S. Geological Survey, v. 4(3), p. 339-351.

 1981, Late Cenozoic diatom biostratigraphy and paleoceanography of the middle-latitude North Pacific, Deep Sea 

Drilling Project Leg 63, in Yeats, R.L., and Haq, B.U., eds., 

Initial Reports, Deep Sea Drilling Program, Volume 63: 

Washington, D.C., U.S. Government Printing Office, p. 507- 

538.

 1985, Miocene to Holocene planktic diatom stratigraphy, 

in Bolli, H.M., Saunders, J.B., and Perch-Nielsen, K., eds., 

Plankton stratigraphy: Cambridge, U.K., Cambridge University Press, p. 763-809.

 1986a, Updated diatom biostratigraphy for the Monterey 

Formation of California, in Casey, R.E., and Barron, J.A., 

eds., Siliceous microfossils and microplankton studies of the 

Monterey Formation and modern analogs: Pacific Section 

Society of Economic Paleontologists and Mineralogists, 

Annual Meeting, Bakersfield, California. Book 45, p. 105- 

119.

 1986b, Paleoceanographic and tectonic controls on deposition of the Monterey Formation and related siliceous rocks 

in California: Palaeogeography, Paleoclimatology, Paleoecology, v. 53, p. 27-45.

 1992a, Neogene diatom datum levels in the equatorial 

and North Pacific, in Ishizaki, K., and Saito, T., eds., The 

centenary of Japanese Micropaleontology: Tokyo, Terra Scientific Publishing Company, p. 413-425.

 1992b, Paleoceanographic and tectonic controls on the 

Pliocene diatom record of the northeastern Pacific, in Tsuchi, 

R., and Ingle, J.C., Jr., eds., Pacific Neogene Environment,

evolution, and events: Tokyo, Tokyo University Press, p. 25- 

41.

Barron, J.A., and Baldauf, J.G., 1986, Diatom stratigraphy of the 

lower Pliocene part of the Sisquoc Formation, Harris Grade 

section, California: Micropaleontology, v. 32(4), p. 357-371.

Barron, J.A., and Keller, G., 1983, Paleotemperature oscillations 

in the middle and late Miocene of the northeastern Pacific: 

Micropaleontology, v. 29(2), p. 150-181.

Barron, J.A., and Ramirez, PC., 1992, Diatom stratigraphy of 

selected Sisquoc Formation sections, Santa Maria basin, California. U.S. Geological Survey Open-File Report 92-197, 

23 p.

Berggren, W.A., Kent, D.V., Flynn, J.J., and Van Couvering, J.A., 

1985, Cenozoic geochronology, Geological Society of America Bulletin, v. 96, no. 11, p. 1407-1418.

Blake, G.H., 1991, Review of the Neogene biostratigraphy and 

stratigraphy of the Los Angeles basin and implications for 

basin evolution, in Biddle, K.T., ed., Active margin basins, 

American Association of Petroleum Geologists Memoir 52, 

p. 135-184.

Bramlette, M.N., 1946, The Monterey Formation of California 

and the origin of its siliceous rocks: U.S. Geological Survey 

Professional Paper 212, 57 p.

Bukry, David, 1973, Coccolith and silicoflagellate stratigraphy, 

Deep Sea Drilling Project Leg 18, in Kulm, L.D., and von 

Huene, R., eds., Initial Reports, Deep Sea Drilling Program 

Volume 18: Washington, D.C., U.S. Government Printing 

Office, p. 817-831.

 1981, Pacific Coast coccolith stratigraphy between Point 

Conception and Cabo Corrienties, Deep Sea Drilling Project, 

in Yeats, R.L., and Haq, B.U., eds., Initial Reports, Deep Sea 

Drilling Program, Volume 63: Washington, D.C., U.S. Government Printing Office, p. 445-471.

Burckle, L.H., 1978, Early Miocene to Pliocene diatom datum 

levels for the equatorial Pacific: Republic of Indonesia, Geologic Research and Development Centre, Special Publication 

1, p. 25-44.

Cande, S.C., and Kent, D.V., 1992, A new geomagnetic polarity 

time scale for the Late Cretaceous and Cenozoic: Journal of 

Geophysical Research, v. 97(B10), p. 13917-13951.

Canfield, C.R., 1939, Subsurface stratigraphy of Santa Maria 

Valley oil field and adjacent parts of Santa Maria Valley, 

California: Association of American Petroleum Geologists 

Bulletin, v. 23(1), p. 45-81.

Crouch, J.K., 1979, Neogene tectonic evolution of the California 

Continental Borderland and western Transverse Ranges: 

Geological Society of America Bulletin Part 1, v. 90, p. 338- 

345.

Crouch, J.K., and Bukry, D., 1979, Comparison of Miocene provincial foraminiferal stages to coccoliths in the California 

Continental Borderland: Geology, v.7, p. 211-215.

Dibblee, T.W., Jr., 1950. Geology of the southwestern Santa Barbara County, California: California Division of Mines and 

Geology Bulletin 150, 95 p.

Dumont, M.P., 1984, Miocene/Pliocene Epoch boundary from 

diatom biostratigraphy in the Lompoc, California, region: 

Abstracts with Programs, 1984, 97th Annual Meeting, Geological Society of America, Reno, Nevada, v. 16(6), p. 496.

 1986, The Miocene/Pliocene Epoch boundary as determined by diatom biostratigraphy in the California coastal

Neogene Geohistory Analysis of Santa Maria Basin, California, and Its Relationship to Transfer of Central California to the Pacific Plate K15
region: University of California, Berkeley, Masters thesis, 

115 p. 

-1989, The Monterey Formation and biostratigraphy An

overview, in MacKinnon, T.C., ed., Oil in the California 

Monterey Formation: Field trip Guidebook T311, 28th International Geological Congress, Washington, D.C., 1989, 

American Geophysical Union, p. 28-32.

Dumont, M.P., Baldauf, J.G., and Barron, J.A., 1986, Thalassiosira praeoestrupii A new diatom species for recognizing 

the Miocene/Pliocene Epoch boundary in coastal California: 

Micropaleontology, v. 32(4), p. 372-377.

Haq, B.U., Hardenbol, J., and Vail, P.R., 1987, Chronology of 

fluctuating sea level since the Triassic: Science, v. 235, 

p. 1156-1167.

Harbert, W., and Cox, A., 1989, Late Neogene motion of the 

Pacific plate: Journal of Geophysical Research, v. 94(B3), 

p. 3052-3064.

Harper, H.E., Jr., 1977, Diatom biostratigraphy of the MiocenePliocene boundary in marine strata of the circum-North 

Pacific: Harvard University, Ph.D. dissertation, 112 p.

Hornafius, J.S., Grivetti, M.C., and Lagle, C.W., 1982, Structure 

and stratigraphy of the Monterey Formation near Point Pedernales, Santa Barbara County, California in Geology and 

Mineral Wealth of the California Transverse Ranges: South 

Coast Geological Society, Guidebook No. 10, p. 331-336.

Ingle, J.C., Jr., 1967, Foraminiferal biofacies variation and the 

Miocene-Pliocene boundary in southern California: Bulletins 

of American Paleontology, v. 52(236), p. 217-394.

 1980, Cenozoic paleobathymetry and depositional history 

of selected sequences within the southern California borderland: Cushman Foundation for Foraminiferal Research Special Publication 19, p. 163-195.

-1981, Origin of Neogene diatomites around the north

Pacific rim, in Garrison, R.E., and Douglas, R.G., eds., The 

Monterey Formation and related siliceous rocks of California: Pacific Section Society of Economic Paleontologists and 

Mineralogists Special Publication 16, p. 159-179. 

Isaacs, C.M., 1981, Lithostratigraphy of the Monterey Formation, 

Goleta to Point Conception, Santa Barbara Coast, California, 

in Isaacs, C.M., ed., Guide to the Monterey Formation in the 

California coastal area, Ventura to San Luis Obispo: Pacific 

Section American Association of Petroleum Geologists, 

Camarillo, California, p. 9-23.

 1983, Compositional variation and sequence in the Miocene Monterey Formation, Santa Barbara coastal area, California, in Larue, D.K., and Steel, R.J., eds., Cenozoic marine 

sedimentation, Pacific margin, U.S.A.: Pacific Section Society of Economic Paleontologists and Mineralogists Special 

Publication, p. 117-132.

Kennett, J.P., 1977, Cenozoic evolution of Antarctic glaciation, the 

circum-Antarctic Ocean, and their impact on global paleoceanography: Journal Geophysical Research, v. 82, p. 3843-3859.

Kleinpell, R.M., 1938, Miocene stratigraphy of California: Tulsa, 

Oklahoma, American Association of Petroleum Geologists, 

450 p.

 1980, The Miocene stratigraphy of California revisited: 

American Association of Petroleum Geologists Studies in Geology, no. 11, 349 p.

Koizumi, Itaru, 1973, The late Cenozoic diatoms of Sites 183- 

193, Leg 19 Deep Sea Drilling Project, in Creager, J.S.,

Scholl, D.W., and others, Initial Reports, Deep Sea Drilling 

Project, Volume 19: Washington, D.C., U.S. Government 

Printing Office, p. 805-855.

Louderback, G.D., 1913, The Monterey series in California: University of California Publications in Geology, v. 7(10), 

p. 177-241.

Madrid, V.M., 1982, Magnetostratigraphy of the late Neogene 

Purisima Formation, Santa Cruz County, California: University of California, Davis, Masters thesis, 103 p.

Madrid, V.M., Stuart, R.M., and Verosub, K.L., 1986, Magnetostratigraphy of the late Neogene Purisima Formation, Santa 

Cruz County, California: Earth and Planetary Science Letters, v. 79, p. 431-440.

Mulryan, H., 1936, Geology, mining and processing of diatomite 

at Lompoc, Santa Barbara County, California: California 

Department of Natural Resources, Division of Mines and 

Geology, v. 32(2), p. 133-166.

Natland, M.L., 1953, Correlation of the Pleistocene and Pliocene 

stages in southern California: Pacific Petroleum Geologist, 

v. 7(2), p. 2.

Pierce, R.L., 1972, Revaluation of the late Miocene biostratigraphy of California; summary of evidence, in Stinemeyer, 

E.H., ed., The Pacific Coast Miocene Biostratigraphic Symposium: Bakersfield, California, Pacific Section Society of 

Economic Paleontologists and Mineralogists Symposium 

Proceedings, p. 334-340.

Okada, Hisatake, and Bukry, David, 1980, Supplementary modification and introduction of code numbers to the low-latitude 

coccolith biostratigraphic zonation [Bukry, 1973, 1975]. 

Marine Micropaleontology, v. 5, p. 321-325.

Pisciotto, K.A., 1981, Notes on Monterey rocks near Santa 

Maria, California, in Isaacs, C. M., ed., Guide to the Monterey Formation in the California Coastal Area, Ventura to San 

Luis Obispo: Pacific Section American Association of Petroleum Geologists, no. 52, p. 73-81.

Ramirez, PC., 1990, Stratigraphy, sedimentology and paleoceanographic implications of the Late Miocene to Early Pliocene 

Sisquoc Formation, Santa Maria area, California: University 

of California, Santa Cruz, Ph.D. dissertation, 388 p.

Ruth, J.W., 1972, Remarks made at Society of Economic Paleontologists and Mineralogists Miocene symposium, March 10, 

1972, in Stinemeyer, E.H., ed., The Pacific Coast Miocene 

Biostratigraphic Symposium: Bakersfield, California, Pacific 

Section Society of Economic Paleontologists and Mineralogists Symposium Proceedings, p. 352-353.

Savin, S.M., 1977, The history of the Earth's surface temperature 

during the past 100 million years: Annual Review Earth 

Planetary Science, v. 5, p. 319-355.

Schrader, H.J., 1973, Proposal for a standardized method of 

cleaning diatom-bearing deep-sea and land-exposed marine 

sediments: Nova Hedwigia, Beihetfe, v. 45, p. 403-409.

Schrader, H.J., and Gersonde, R., 1978, Diatoms and Silicoflagellates: Utrecht Micropaleontology Bulletin, no. 17, p. 129- 

176.

Sedlock, R.L., and Hamilton, D.H., 1991, Late Cenozoic tectonic 

evolution of southwestern California: Journal of Geophysical 

Research, v. 96(B2), p. 2325-2351.

Simonsen, Reimer, and Kanaya, Taro, 1961, Notes on the marine 

species of the diatom genus Denticula Kiitz: Internationale 

Revue der Gesamten Hydrobiologie, v. 46(4), p. 498-513.

K16 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
Stanley, K.O., and Surdham, R.C., 1984, The role of wrench 

fault tectonics and relative changes of sea level on deposition of the upper Miocene-Pliocene Pismo Formation, Pismo 

Syncline, California: in Society of Economic Paleontologists 

and Mineralogists Guidebook No. 2, Stratigraphic, tectonic, 

thermal, and diagenetic histories of the Monterey Formation, 

Pismo and Huasna Basin, California, Mid-year meeting, 

Society of Economic Paleontologists and Mineralogists, 

August 13-16, 1984, San Jose, California, p. 21-34.

Teng, L.S., and Gorsline, D.S., 1989, Late Cenozoic sedimentation in California Continental Borderland basins as revealed 

by seismic facies analysis: Geological Society of America 

Bulletin, v. 101(1), p. 27-41.

Vail, PR., and Hardenbol, J., 1979, Sea-level changes during the 

Tertiary: Oceanus, v. 22, p. 71-79.

Weaver, P.M., Casey, R.E., and Perez, A.M., 1981, Stratigraphic 

and paleoceanographic significance of early Pliocene to middle Miocene radiolarian assemblages from northern to Baja 

California, in Garrison, R.E., and Douglas, R.G., eds., The 

Monterey Formation and related siliceous rocks of California: Pacific Section Society of Economic Paleontologists and 

Mineralogists Special Publication 16, p. 71-86.

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

Woodring, W.P., Bramlette, M.N., and Lohman, K.E., 1943, Stratigraphy and paleontology of the Santa Maria district, California: American Association of Petroleum Geologists Bulletin 

27, no. 10, p. 1335-1360.

Wornardt, W.W., Jr., 1963, Stratigraphic distribution of diatom floras 

from the "Mio-Pliocene" of California: University of California, Berkeley, Ph.D. dissertation, 208 p.

 1967, Miocene and Pliocene marine diatoms from California: California Academy of Science Occasional Papers, no. 63, 

108 p.

 1983, The Sisquoc Formation is Pliocene in age and the 

Monterey Formation is Miocene in age in the Santa Maria area, 

Santa Barbara County, California: Programs and Abstracts, 

58th Annual Meeting, Pacific Section American AAPG, SEG, 

SEPM, Sacramento, California, May 18-21, 1983, p. 58.

 1986, Diatom biostratigraphy from diatomaceous dolomites

and shales in the Monterey Formation, Rodeo Canyon, Point 

Pedernales and Lion's Head sections, southwest Santa Barbara 

County, California, in Casey, R.E., and Barren, J.A., eds., 

Siliceous microfossil and microplankton of the Monterey Formation and modern analogs: Pacific Section, Society of Economic Paleontologists and Mineralogists, Book 45, p. 141-147. 

Zijderveld, J.D.A., Zachariasse, J.W, Verhallen, P.J.J.M., and 

Hilgen, F.J., 1986, The age of the Miocene-Pliocene boundary: 

Newsletter on Stratigraphy, v. 16(3), p. 169-181.

Neogene Geohistory Analysis of Santa Maria Basin, California, and Its Relationship to Transfer of Central California to the Pacific Plate K17

GPO 683-048/20004

SELECTED SERIES OF U.S. GEOLOGICAL SURVEY PUBLICATIONS

Periodicals

Earthquakes & Volcanoes (issued bimonthly). 

Preliminary Determination of Epicenters (issued monthly).

Technical Books and Reports

Professional Papers are mainly comprehensive scientific reports 

of wide and lasting interest and importance to professional scientists 

and engineers. Included are reports on the results of resource studies 

and of topographic, hydrologic, and geologic investigations. They also 

include collections of related papers addressing different aspects of a 

single scientific topic.

Bulletins contain significant data and interpretations that are of lasting scientific interest but are generally more limited in scope or geographic coverage than Professional Papers. They include the results of 

resource studies and of geologic and topographic investigations, as well 

as collections of short papers related to a specific topic.

Water-Supply Papers are comprehensive reports that present significant interpretive results of hydrologic investigations of wide interest 

to professional geologists, hydrologists, and engineers. The series covers 

investigations in all phases of hydrology, including hydrogeology, availability of water, quality of water, and use of water.

Circulars present administrative information or important scientific information of wide popular interest in a format designed for distribution at no cost to the public. Information is usually of short-term 

interest.

Water-Resource Investigations Reports are papers of an 

interpretive nature made available to the public outside the formal 

USGS publications series. Copies are reproduced on request unlike 

formal USGS publications, and they are also available for public 

inspection at depositories indicated in USGS catalogs.

Open-File Reports include unpublished manuscript reports, maps, 

and other material that are made available for public consultation at 

depositories. They are a nonpermanent form of publication that may 

be cited in other publications as sources of information.

Maps

Geologic Quadrangle Maps are multicolor geologic maps on topographic bases in 7 )4- or 15-minute quadrangle formats (scales mainly 

1:24,000 or 1:62,500) showing bedrock, surficial, or engineering geology. Maps generally include brief texts; some maps include structure 

and columnar sections only.

Geophysical Investigations Maps are on topographic or planimetric bases at various scales; they show results of surveys using 

geophysical techniques, such as gravity, magnetic, seismic, or radioactivity, which reflect subsurface structures that are of economic or geologic significance. Many maps include correlations with the geology.

Miscellaneous Investigations Series Maps are on planimetric or 

topographic bases of regular and irregular areas at various scales; they 

present a wide variety of format and subject matter. The series also 

includes 7 ^-minute quadrangle photogeologic maps on planimetric 

bases that show geology as interpreted from aerial photographs. Series 

also includes maps of Mars and the Moon.

Coal Investigations Maps are geologic maps on topographic or 

planimetric bases at various scales showing bedrock or surficial geology, stratigraphy, and structural relations in certain coal-resource 

areas.

Oil and Gas Investigations Charts show stratigraphic information for certain oil and gas fields and other areas having petroleum 

potential.

Miscellaneous Field Studies Maps are multicolor or black-andwhite maps on topographic or planimetric bases on quadrangle or irregular areas at various scales. Pre-1971 maps show bedrock geology 

in relation to specific mining or mineral-deposit problems; post-1971 

maps are primarily black-and-white maps on various subjects, such as 

environmental studies or wilderness mineral investigations.

Hydrologic Investigations Atlases are multicolor or black-andwhite maps on topographic or planimetric bases presenting a wide range 

of geohydrologic data of both regular and irregular areas; principal 

scale is 1:24,000, and regional studies are at 1:250,000 scale or smaller.

Catalogs

Permanent catalogs, as well as some others, giving comprehensive 

listings of U.S. Geological Survey publications are available under the 

conditions indicated below from the U.S. Geological Survey, Books 

and Open-File Reports Sales, Federal Center, Box 25286, Denver, CO 

80225. (See latest Price and Availability List.)

"Publications of the Geological Survey, 1879-1961" may be purchased by mail and over the counter in paperback book form and as a 

set of microfiche.

"Publications of the Geological Survey, 1962-1970" may be purchased by mail and over the counter in paperback book form and as a 

set of microfiche.

"Publications of the Geological Survey, 1971-1981" may be purchased by mail and over the counter in paperback book form (two 

volumes, publications listing and index) and as a set of microfiche.

Supplements for 1982, 1983, 1984, 1985, 1986, and for subsequent years since the last permanent catalog may be purchased by mail 

and over the counter in paperback book form.

State catalogs, "List of U.S. Geological Survey Geologic and 

Water-Supply Reports and Maps For (State)," may be purchased by 

mail and over the counter in paperback booklet form only.

"Price and Availability List of U.S. Geological Survey Publications," issued annually, is available free of charge in paperback booklet form only.

Selected copies of a monthly catalog "New Publications of the 

U.S. Geological Survey" are available free of charge by mail or may 

be obtained over the counter in paperback booklet form only. Those 

wishing a free subscription to the monthly catalog "New Publications 

of the U.S. Geological Survey" should write to the U.S. Geological 

Survey, 582 National Center, Reston, VA 22092.

Note. Prices of Government publications listed in older catalogs, 

announcements, and publications may be incorrect. Therefore, the prices 

charged may differ from the prices in catalogs, announcements, and 

publications.