Distribution and Juxtaposition of 

Mesozoic Lithotectonic Elements in the 

Basement of the Santa Maria Basin, 

California

U.S. GEOLOGICAL SURVEY BULLETIN 1995-B
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Chapter B

Distribution and Juxtaposition of 

Mesozoic Lithotectonic Elements in the 

Basement of the Santa Maria Basin, 

California

By HUGH McLEAN

U.S. GEOLOGICAL SURVEY BULLETIN 1995

EVOLUTION OF SEDIMENTARY BASINS SANTA MARIA PROVINCE
U.S. DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Any use of trade, product, or firm names

in this publication is for descriptive purposes only

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

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1991

For sale by the

Books and Open-File Reports Section

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Library of Congress Cataloging-in-Publication Data

McLean, Hugh, 1939-

Distribution and juxtaposition of Mesozoic lithotectonic elements in the 

basement of the Santa Maria Basin, California / by Hugh McLean.

p. cm. (Evolution of sedimentary basins Santa Maria Province ; ch. B) 

(U.S. Geological Survey bulletin ; 1995) 

Includes bibliographical references.

1. Geology, Stratigraphic Mesozoic. 2. Sedimentary basins California 

Santa Maria Basin, 3. Geology California Santa Maria Basin. I. Title. II. Series. 

III. Series: U.S. Geological Survey bulletin ; 1995. 

QE75.B9 no. 1995-B

[QE675] 91-22861

CIP
CONTENTS

Abstract Bl 

Introduction Bl

Acknowledgments B2

Previous work B2

Methods of study B4 

Petrographic characteristics and detrital modes of outcrop samples B4

General textural characteristics B4

Lower petrofacies B5

Upper petrofacies B6

Franciscan petrofacies B6 

Subsurface distribution of Mesozoic rocks B7 

Comments on the absence of K-feldspar in sandstone knockers of Franciscan

m61ange B9

Discussion and conclusions BIO 

References BIO

FIGURES

1. Index map of Salinian and Nacimiento blocks, Cambria and Point San Luis slabs, 

and Santa Maria basin B2

2. Map showing locations of basement wells used in this study, and traces of major 

faults B3

3. Map showing distribution of Mesozoic rocks in the subsurface of the Santa Maria

basin B5 

4-7. Diagrams showing 

4. Detrital modes of Great Valley sequence outcrop samples B7

5. Detrital modes of sandstone in outcrops from the Point San Luis slab and from 

the upper part of the Great Valley sequence B7

6. Detrital modes of gray wacke in outcrops of Franciscan assemblage m61ange 

added to data shown in figure 4 B8

7. Detrital modes of sandstone samples from subsurface core samples B8

8. Map showing subsurface distribution of lower Miocene Lospe Formation B9

9. Diagram showing subsurface samples interpreted as Franciscan assemblage 

gray wacke BIO

TABLE

1. Composition of Mesozoic sandstones of the Santa Maria Basin B6

Contents III

Distribution and Juxtaposition of

Mesozoic Lithotectonic Elements in the Basement of the

Santa Maria Basin, California

ByHUGHMcLEAN

Abstract

Approximately 150 wells in the Santa Maria basin 

have been drilled to basement. Core samples from more 

than 80 wells indicate that basement is composed mainly 

of melange of the Mesozoic Franciscan assemblage, ophiolitic rocks, and smaller areas underlain by Great Valley sequence sedimentary rocks. Upper Cretaceous sandstone 

and mudstone underlie the western part of the Santa Maria 

Valley oil field and most of the Guadalupe oil field. The 

latter is on structural trend with outcrops of lithologically 

similar rocks of the Point San Luis slab, located approximately 25 km to the northwest.

Petrographic data from surface rocks exposed around 

the margin of the basin define three sandstone petrofacies: 

(1) a quartz- and feldspar-rich assemblage that contains 

abundant K-feldspar and ranges in age from Turonian(?) to 

Maestrichtian; (2) a lithic-rich assemblage that contains a 

low percentage of K-feldspar and locally ranges in age 

from Tithonian to Cenomanian; and (3) Franciscan sandstone (so-called graywacke) that lacks K-feldspar and overlaps the quartz-feldspar-lithic (QFL) detrital modes of the 

two Great Valley sequence petrofacies. Petrologic similarities between sandstone knockers in Franciscan melange 

and Great Valley sequence sandstones suggest that the 

Franciscan rocks may represent variably metamorphosed 

Upper Jurassic to Upper Cretaceous Great Valley sequence 

rocks.

The distribution of basement petrofacies does not 

constrain the direction and amount of offset on the concealed Lompoc fault (the Santa Ynez Valley fault), but 

shows that the lower and upper part of the Great Valley 

sequence exists on both sides of the fault. The PezzoniCasmalia fault juxtaposes the Point Sal ophiolite and the 

conformably overlying lower Great Valley sequence 

petrofacies on its south side against an outlier of the upper 

petrofacies of the Great Valley sequence on the north side. 

The juxtaposition of different petrofacies suggests lateral 

offset along the fault, but provides no piercing points. Areally restricted subcrops of the lower Miocene Lospe ForManuscript approved for publication, May 3, 1991.

mation suggest that Neogene motion on the PezzoniCasmalia fault has been mainly dip slip with the north side 

down.

INTRODUCTION

The Santa Maria basin (SMB) is an onshore and 

offshore Cenozoic structural depression located along the 

central California coast (fig. 1). Several fields within the 

basin produce oil from the Miocene Monterey and Miocene and Pliocene Sisquoc Formations. Economic and 

acoustic basement in the SMB and adjacent areas consists of several late Mesozoic lithotectonic units. According to data published by the State of California Division of Oil and Gas, approximately 150 wells in the 

SMB bottomed in basement rocks variously described as 

the Franciscan assemblage, Knoxville Formation, Jurassic rocks, Cretaceous rocks, serpentine, and granite 

(wells used in this study are shown in fig. 2). Most of 

these names (except granite, which is unknown west of 

the Sur-Nacimiento fault) apply to either a heterogeneous assemblage of chaotically mixed rock bodies 

(m61ange) of the Franciscan assemblage (also called the 

Franciscan Complex by some workers), or to the more 

structurally coherent submarine-fan complex called the 

Great Valley sequence by Bailey and others (1964), and 

the Great Valley Group by some workers (see Ingersoll, 

1990); the Great Valley sequence overlies the Point Sal 

ophiolite described by Hopson (1977). Outliers of Great 

Valley sequence strata and a tectonic sliver of dismembered ophiolitic rocks locally overlie Franciscan 

melange.

Using the Mesozoic sandstone petrofacies west of 

the Sur-Nacimiento fault zone described by previous 

workers such as Gilbert and Dickinson (1970), Gilbert 

(1973), and Dickinson and others (1982), this report expands the pioneering work of Gray (1980) and further 

delineates the distribution and composition of the 

Franciscan, Great Valley sequence, and ophiolitic units 

in the subsurface of the SMB. Tectonic implications suggested by the petrographic similarities of Great Valley

Introduction B1
sequence sandstones and sandstone knockers in the 

Franciscan melange are also explored herein.

Acknowledgments

The pioneering work of Lynn Gray provided the 

foundation for this study. Terry Buddin, Reon Moag, and 

Vie Rosato of UNOCAL, Larry Knauer of the California 

Core Repository; and Larry Beyer of the U.S. Geological 

Survey provided access to core material. Caroline Isaacs 

and M. Clark Blake, Jr., also provided helpful and

timely assistance. The early Miocene age for the Lospe 

Formation was kindly provided by R.G. Stanley. Early 

drafts of the manuscript were reviewed by T.C. 

MacKinnon and J.G. Vedder. Later versions profited 

from critical reviews by R.V. Ingersoll, J.A. Miller, M.E. 

Milling, T.H. Nilson, and an anonymous reviewer.

Previous Work

Gray (1980) petrographically examined 117 thin 

sections sampled from approximately 50 wells using

I I9°00'W

36°00'N

San Francisco*

Figure 1, Index map of Salinian and Nacimiento blocks, Cambria and Point San Luis slabs, and Santa 

Maria basin (outlined) as shown in figures 2, 3, and 8. Faults dashed where approximate; dotted where 

concealed.

B2 Evolution of Sedimentary Basins Santa Maria Province
core collections of the UNOCAL Corporation. Also reported were Late Cretaceous fossils, and textures of 

ophiolitic basement rocks. Detrital modes of 19 Cretaceous Great Valley sequence sandstone samples were 

also reported. Hall (1982) subsequently published a map 

of the pre-Miocene subcrops using generalized lithologies reported by the California State Division of Oil and 

Gas. Lithologies listed in the DOG reports, however, do 

not generally recognize the subtle differences in composition between rocks of the upper and lower parts of the

Great Valley sequence and sandstones in the Franciscan 

melange.

Visual inspection of basement core from 42 wells 

in this study augments the work of Gray (1980). Microfiche records from 77 wells that were drilled to basement 

provide additional data.

Mesozoic sandstone petrofacies described in the 

Nacimiento block by Brown (1968), Gilbert and 

Dickinson (1970), MacKinnon (1978), Lee-Wong and 

Howell (1977), Smith (1978), Nelson (1979), Dickinson

120° 30' 120'

35'

32S

12N

PT

PT. 

ARGUELLO

PT. CONCEPTION

6 KILOMETERS

Figure 2. Locations of basement wells used in this study, and traces of major faults. Map 

symbols indicating rock unit at total depth of exploratory wells: open squares, lower Miocene Lospe Formation; open circles, upper Great Valley sequence petrofacies; solid circles, 

lower Great Valley sequence petrofacies; open triangles, Franciscan assemblage; solid triangles, ultramafic or ophiolitic(?) volcanic rocks, and (or) serpentine. Queries indicate uncertain lithologic affinity. Fault symbols: BPF, Big Pine Mountain fault; EHF, East Huasna 

fault; HF, Honda fault; LF, Lompoc fault (also called Santa Ynez River fault); LHF, Lions 

Head fault; LPF, Little Pine fault; NF, Sur-Nacimiento fault; P-CF, Pezzoni-Casmalia fault; 

SF, Suey fault; SMVF, Santa Maria Valley fault; WHF, West Huasna fault. Faults dashed 

where approximate; dotted where concealed. Fault names and locations from Gray (1980) 

and Hall (1981). Numbers indicate township and range.

Introduction B3
and others (1982), Seiders (1983), and Toyne (1987) 

were used to identify the various Mesozoic lithotectonic 

units of this study. The published detrital modes consistently show that composition varies with age and that the 

concept of petrofacies as applied to the Great Valley sequence of the Sacramento and San Joaquin Valleys of 

California by Ojakangas (1968),'lngersoll (1978), and 

Dickinson and Rich (1972) also applies to the Great Valley sequence west of the Sur-Nacimiento fault.

Petrologic studies of sandstone knockers in the 

Franciscan melange in the SMB area are less numerous; 

however, Brown (1968), Gilbert (1973), Ernst (1980), 

Dickinson and others (1982), and Seiders (1983) have 

reported on some of the Franciscan rocks. The most 

comprehensive geologic map of the SMB is by 

Woodring and Bramlette (1950); adjacent areas have 

been mapped by Dibblee (1950), Hall and Corbatd 

(1967), Hall (1973, 1978, 1981), Hall and others (1979), 

and Vedder and others (1989).

Methods Of Study

The petrofacies established by the previous workers cited above indicate that detrital modes of quartz (Q), 

feldspar (F), and lithic fragments (L) provide a reliable 

means of differentiating Upper Jurassic to middle Upper 

Cretaceous Great Valley sequence rocks from uppermost 

Cretaceous (mainly Campanian and Maestrichtian) rocks 

in the Nacimiento block. To correctly identify Great 

Valley sequence and Franciscan sandstones in the subsurface of the SMB (fig. 3), QFL modes were determined for 45 Great Valley sequence and 17 Franciscan 

knocker outcrop samples from previously unstudied 

areas adjacent to the SMB (table 1). To insure consistency with previous studies, outcrop samples from the 

Point San Luis slab (fig. 1) of Lee-Wong and Howell 

(1977) and Gray (1980) were recounted.

The suite of outcrop sandstones was then compared 

with 39 subsurface sandstones selected from 24 wells. 

Detrital modes for this study were obtained using a 

slightly modified version of the point-counting techniques outlined by Dickinson (1970). Standard thin sections were stained for K-feldspar. Furthermore, because 

this study focused on comparing petrofacies rather than 

provenance, the Gazzi-Dickinson method described by 

Ingersoll and others (1984) was not used. However, medium-grained samples were counted where possible to 

reduce the effects of grain-size variation on composition. 

A minimum of 300 grains of quartz, feldspar, and rock 

fragments were counted in each specimen and plotted on 

ternary diagrams (figs. 4-8). Q includes monocrystalline 

and coarsely polycrystalline quartz (grains with up to 

four crystals). F includes plagioclase and potassium feldspar. L includes all rock fragments including chert and

quartzite, which in most cases are minor components. 

Grains of mica, epidote, and opaque oxides were 

counted in addition to the minimum 300 QFL grains.

Other detrital modes such as QmPK monocrystalline quartz (Qm), plagioclase feldspar (P), potassium 

feldspar (K) and QmFLt monocrystalline quartz 

(Qm), plagioclase feldspar (F), and total lithic fragments 

(Lt) were not deemed relevant to the purposes of this 

study, because potassium feldspar was not detected in 

any of the Franciscan sandstones. QFL as used herein is 

essentially the same as QmFLt because chert and quartzite are included with lithic fragments (L).

Outcrop samples were collected during geologic 

mapping by the U.S. Geological Survey in the Los Padres National Forest and the Santa Lucia Wilderness. 

Subsurface core samples were obtained from the California Core Repository (Bakersfield); from UNOCAL 

Corp., Brea, California; and from L.A. Beyer of the U.S. 

Geological Survey, Menlo Park. The thin-section collection used by Gray (1980) was also examined, and several samples were restained for K-feldspar and recounted.

PETROCRAPHIC CHARACTERISTICS AND 

DETRITAL MODES OF OUTCROP SAMPLES

To simplify the terminology used herein, Upper Jurassic (Tithonian) to lower Upper Cretaceous (possibly 

as young as Turonian) quartz-poor sandstones are informally called the lower Great Valley sequence 

petrofacies. These rocks are mapped locally as either the 

Espada Formation of Dibblee (1950) or the Toro Formation. Upper Cretaceous rocks that are largely Campanian 

and Maestrichtian in age and tend to contain abundant 

quartz and plagioclase, as well as abundant K-feldspar, 

are informally called herein the upper Great Valley sequence petrofacies. In contrast to the well-documented 

age constraints of Great Valley sequence petrofacies, 

little is known about the relationship between age and 

composition of sandstone (so-called graywacke) knockers in Franciscan melange. As shown below, however, 

the QFL modes of Great Valley sequence rocks and 

knockers of Franciscan sandstone closely overlap.

General Textural Characteristics

Great Valley sequence and Franciscan sandstones 

in the area around the SMB share several general textural characteristics. Quartz and most lithic fragments tend 

to be subangular to subrounded, whereas feldspar grains 

tend to be more angular. Framework grains are moderately poor to moderately well sorted and tend to be 

tightly packed. Grain contacts are mainly long and

B4 Evolution of Sedimentary Basins Santa Maria Province
concavo-convex, and sutured grains are rare. Intergranular clayey matrix in most rocks ranges from 10 to approximately 15 percent. Figure 4 compares the QFL 

modes of the upper and lower Great Valley sequence 

sandstone petrofacies. Sandstone knockers in Franciscan 

m61ange typically contain less than 15 percent intergranular matrix and do not classify as graywacke as defined by Pettijohn and others (1972, p. 159)

1200 30'

Lower Petrofacies

Mean detrital modes for the lower Great Valley sequence petrofacies sandstones are approximately Q16, 

F21, and L63 (table 1). Percentages of K-feldspar, biotite, 

and epidote are comparatively low, commonly less than 1 

percent for each component. Rock fragments consist 

mainly of aphanitic andesite and basaltic andesite in

1200

350 -

PT. CONCEPTION

EXPLANATION

l'';'-"'-'-';l Upper Great Valley sequence petrofacies 

kr-rj Upper and (or) lower Great Valley sequence petrofacies 

ffiHD Point Sal ophiolite 

V//\ Franciscan assemblage 

I^H Outcrops of Mesozoic basement

Figure 3. Distribution of Mesozoic rocks in the subsurface of the Santa Maria basin. 

Solid dots represent locations of wells yielding Cretaceous palynomorphs as reported by 

Gray (1980). Fault symbols as shown in figure 2.

Petrographic Characteristics and Detrital Modes of Outcrop Samples B5
Table 1 . Composition of Mesozoic sandstones of the Santa Maria Basin

[Detrital modes for QFL and QmPK; average percentages for K-feldspar, mica, and epidote, for sandstones of the Great Valley sequence, for 

Jurassic and Cretaceous Franciscan assembkge (KJf), and for well cores, n is the size of the sample population except as follows: (') n=3, C') 

n=ll,(*) n=14. QFL is quartz, feldspar, and lithic fragments including microcrystalline quartz and chert. QmPK is monocrystalline quartz, 

plagioclase, and potassium feldspar. Mica includes muscovite and biotite. Ku, Upper Cretaceous rocks; Kl, Lower Cretaceous rocks]

Qm K-spar Mica Epidote

Ku-outcrop

Ku-wells

Kl-outcrop

Kl-weUs

KJf-outcrop

KJf-wells

27

21

18

7

17

11

36.5

30.4

15.6

19.1

24.7

27.7

44

39.9

21.3

26.8

32.2

35

19.5

29.7

63.1

54.1

43.1

37.3

42.3

39.7

31.4*

45.3*

40.4"

45.2

42.8

49.1

67.7*

50.6'

59.6"

54.6

14.9

11.2

0.9

4.1

0

0

10.2

6.8

0.2

1.1

0

0

5.9

5.5

1.1

1.8

4.8

4.6

0.5

0.8

1.2

3.5

0.3

1.9

Point San Luis 

slab 33.5 45.3 21.2 38.8 45.4 15.8 10.8 5.6 0.9

which the glassy groundmass is altered to a paste of 

semiopaque clay minerals. Sedimentary rock fragments 

include very fine grained quartzofeldspathic sandstone, 

siltstone, and argillite. Other lithics include rare 

microgranitic fragments (chiefly diorite), quartz-mica 

schist, quartzite, and assorted (recrystallized?) lime clasts. 

Lower petrofacies rocks, mapped as the Espada 

Formation south of the Lompoc fault by Dibblee (1950), 

contain unusually high percentages of little-altered mafic 

volcanic rock fragments and abundant clinopyroxene, 

components that are notably absent in the lower 

petrofacies rocks mapped as the Espada Formation, on 

the east side of the basin. The abundance of clinopyroxene-rich volcanic debris suggests proximity to a volcanic 

(arc?) source.

Upper Petrofacies

Upper Great Valley sequence petrofacies sandstones characteristically contain abundant quartz and 

feldspar, and significant percentages of K-feldspar and 

biotite. Mean detrital modes of the upper Great Valley 

sequence petrofacies suite are approximately Q36, F44, 

and L20 (table 1). Coarse-grained sandstones contain 

abundant granitic rock fragments, and weathered coarsegrained rocks often have a grussy texture, similar to that 

of a decomposed granitic rock.

QFL modes from the Cambria and Point San Luis 

slabs (fig. 1) correspond closely with the QFL modes of 

the other upper petrofacies outcrop samples (fig. 5); textures and accessory-mineral assemblages are also similar. Paleontologic age control in the Cambria slab is 

poor, but rare palynomorphs and dinoflagellates suggest 

a Late Cretaceous (no younger than Campanian) age 

(Smith, 1978, p. 21). Fossils have not been reported 

from the Point San Luis slab, but a Late Cretaceous age 

is supported by the petrographic data of Lee-Wong and 

Howell (1977), Smith (1978), Gray (1980), and Gray's 

samples recounted in this report.

Franciscan Petrofacies

Hsii (1968) noted that so-called graywacke sandstone knockers in the Franciscan m61ange of the San 

Francisco Peninsula range widely in color, composition, 

degree of deformation, and metamorphic grade. Sandstone knockers in the Franciscan m61ange in areas adjacent to the SMB exhibit similar kinds of variation. Some 

knockers superficially resemble upper and (or) lower 

Great Valley sequence petrofacies sandstones in color 

and texture; however, the Franciscan rocks tend to be 

harder, denser, and more highly fractured, with abundant 

calcite- or quartz-filled veins. Deformation in knockers 

ranges from mild fracturing to penetrative deformation

B6 Evolution of Sedimentary Basins Santa Maria Province
with complex shearing and development of incipient S1 

foliation.

Figure 6 compares QFL modes for Franciscan 

sandstone knockers with upper and lower petrofacies 

outcrop suites. The Franciscan rocks overlap most of the 

lower petrofacies suite, and the relatively quartz-poor 

upper petrofacies suite. Although ages of individual 

knockers of Franciscan sandstone are unknown, their 

QFL modes overlap more with the lower petrofacies 

suite than with the upper petrofacies suite. Possible implications of the overlapping QFL modes of Franciscan 

and Great Valley sequence sandstones are discussed below (see section "Discussion and Conclusions").

SUBSURFACE DISTRIBUTION OF MESOZOIC 

ROCKS

Several 8MB wells that contain K-feldspar-bearing 

sandstones also contain Late Cretaceous palynomorphs 

(Gray, 1980) (figs. 3, 7). Similar upper Great Valley sequence petrofacies rocks were found in this study to underlie significant parts of the Guadalupe and Santa Maria 

Valley oil fields (stippled pattern of fig. 3). Basement 

rock cores indicate that Great Valley sequence strata 

structurally overlie Franciscan assemblage melange and 

that the Great Valley sequence is locally truncated by 

protrusions of serpentinite and (or) serpentinized

o

Upper Great Valley sequence 

petrofacies

Lower Great Valley sequence 

petrofacies

Figure 4. Detrital modes of Great Valley sequence outcrop samples. Q-monocrystalline and polycrystalline quartz; F-plagioclase and potassium feldspar; L-lithic rock fragments including chert.

Q

Upper Great Valley sequence 

petrofacies

Point San Luis slab

L

Figure 5. Detrital modes of sandstone in outcrops from the Point San Luis slab (fig. 1) and from the 

upper part of the Great Valley sequence.

Petrographic Characteristics and Detrital Modes of Outcrop Samples B7
melange that contains incorporated fragments of ultramafic and spilitic volcanic rocks (Brown, 1968; Vedder 

and others, 1989). The subsurface relations in the 

Guadalupe and Santa Maria oil fields are similar to field 

relations visible in the Cambria and Point San Luis 

slabs.

Upper Jurassic and Lower Cretaceous sandstone, 

mudstone, and shale exposed in the seacliffs of Point Sal 

were mapped as the Knoxville Formation by Woodring 

and Bramlette (1950) and are part of the lower Great 

Valley sequence petrofacies as defined herein. These 

Knoxville strata overlie a succession of chert, pillow basalt, and ultramafic rocks of Jurassic age called the Point 

Sal ophiolite, (fig. 3) The lower petrofacies (Knoxville) 

and the ophiolitic rocks are in turn unconformably overlain by up to 800 m of lower Miocene sandstone, conglomerate, mudstone, and tuff, called the Lospe Formation by Stanley and others (1990). Much of the reddish 

and greenish colored sedimentary detritus in the Lospe is 

derived from Franciscan and ophiolitic rocks.

Data from widely spaced wells in the coastal lowlands between the Lions Head fault and Lompoc faults 

indicate that the Franciscan m61ange is locally overlain 

by both upper and lower petrofacies rocks that are in 

turn unconformably overlain by local deposits of the 

Lospe Formation (figs. 2, 3, 8).

The results of this study indicate that the SMB is 

largely underlain by Franciscan m61ange with smaller 

areas underlain by Great Valley sequence and tectonic 

slivers of ophiolitic rocks (fig. 3). Franciscan sandstone,

Q

Franciscan assemblage 

(graywacke)

Upper Great Valley sequence 

petrofacies

Lower Great Valley sequence 

petrofacies

Figure 6. Detrital modes of graywacke in outcrops of Franciscan assemblage melange added to data shown 

in figure 4.

x Santa Maria basin wells

Figure 7. Detrital modes of sandstone samples from subsurface core samples. 

B8 Evolution of Sedimentary Basins Santa Maria Province
presumably enclosed in subsurface melange, was identified in only 8 wells. The QFL modes of 11 specimens of 

rocks interpreted to be Franciscan sandstone overlap the 

fields of both upper and lower Great Valley sequence 

petrofacies (fig. 9). The Franciscan rocks that plot in the 

upper petrofacies field represent four wells within a radius of 5 km located in the southeasternmost part of the 

basin, whereas sandstones that plot in the lower 

petrofacies field represent widely spaced wells located 

along the eastern edge of the basin.

COMMENTS ON THE ABSENCE OF K-FELDSPAR 

IN SANDSTONE KNOCKERS OF FRANCISCAN 

MELANGE

Although the Franciscan and Great Valley sequence sandstones studied here share similar grain 

shapes, size sorting, accessory minerals, and framework

modes, none of the Franciscan rocks contain K-feldspar. 

Conversely, upper Great Valley sequence petrofacies 

sandstones generally contain 5 to 10 percent orthoclase, 

and even lower petrofacies rocks commonly contain at 

least trace amounts of detrital K-feldspar (table 1). Thus, 

in subsurface rocks, the absence of K-feldspar is an important criterion for distinguishing Franciscan from 

Great Valley sequence sandstones, especially in rocks 

that plot in the upper petrofacies field. Feldspar grains in 

the Franciscan suite, however, commonly display chessboard texture suggesting that albite has replaced detrital 

K-feldspar as reported by Walker (1984). Parallel extinction of albite twins suggests that detrital plagioclase has 

likewise been albitized. Trends of increasing albitization 

of K-feldspar with depth have been documented in the 

Denver Basin by Walker (1984) and in the North Sea 

adjacent to Norway by Saigal and others (1988). Increasing albitization of plagioclase with burial depth has also 

been reported by Pittman (1988). Neither detrital K-feld120° 30' 1200

350

PT.

PT. 

ARGUELLO

PT. CONCEPTION

0 6 KILOMETERS

Figure 8. Subsurface distribution map of lower Miocene Lospe Formation (stippled area); 

outcrops shown as dark shaded areas. Fault symbols as shown in figure 2.

Comments on the Absence of K-feldspar in Sandstone Knockers of Franciscan Melange B9
spar nor calcic plagioclase would be expected to survive 

in sandstone knockers in Franciscan melange, which are 

commonly metamorphosed to greenschist facies and in 

some cases blueschist facies, although textural evidence 

in some rocks indicates that K-feldspar may have existed 

prior to metamorphism.

DISCUSSION AND CONCLUSIONS

The 8MB is floored mainly by melange of the 

Franciscan assemblage and locally by strata of the upper 

and (or) lower petrofacies of the Great Valley sequence. 

Slivers of altered ophiolitic rocks are probably enclosed 

in melange (fig. 3). The lower Miocene Lospe Formation 

contains abundant Franciscan and ophiolitic detritus and 

overlaps the Pezzoni-Casmalia fault, which suggests that 

Neogene movement on the fault has been mainly vertical 

with the north side down relative to the south side.

Mesozoic sandstone petrofacies established by previous workers for rocks in the Nacimiento block and 

elsewhere in California provide a useful means of differentiating Franciscan and Great Valley sequence rocks in 

the subsurface. The overall petrographic similarities of 

sandstone knockers in Franciscan melange and sandstone 

of the lower part of the Great Valley sequence suggest 

that quartz-poor, lithic-rich Franciscan sandstone may 

represent the subducted lower Great Valley sequence 

petrofacies. Conversely, quartz and feldspar-rich 

Franciscan sandstone knockers may represent the subducted upper Great Valley sequence petrofacies. Although the Franciscan rocks no longer contain K-feldspar, their range in composition and associated feldspar 

textures suggest (but do not prove) that the melange 

sandstones may represent subducted Great Valley sequence that ranges in age from Late Jurassic and (or) 

Early Cretaceous to middle Late Cretaceous. Analogs of 

quartz- and feldspar-rich Campanian and Maestrichtian 

Great Valley sequence sandstone petrofacies are unknown in the Franciscan melange of the Santa Maria 

basin area. Conversely lithic-rich lower petrofacies rocks 

are unknown in the Cambria and Point San Luis slabs 

and in the northern floor of the SMB. The observed distribution of Great Valley sequence petrofacies suggests 

that a depocenter located on the continental slope may 

have moved progressively seaward during Mesozoic subduction in central California. Previous workers such as 

Howell and others (1977), Smith (1978), and Smith and 

others (1979) interpreted the Cambria and Point San 

Luis slabs as trench or slope-basin deposits that accumulated outboard from the main fore-arc basin. Complex 

isoclinal folds, faults, and broken formation in the two 

slabs suggest that sediment accumulated on a structurally 

active zone of accretion. Preservation of abundant detrital K-feldspar in the Cambria and Point San Luis slabs 

suggests that although the rocks were tectonically 

"kneaded," they were not subducted into the subjacent 

accretionary complex, which is now represented by 

Franciscan melange.

REFERENCES

Bailey, E.H., Irwin, W.P., and Jones D.L., 1964, Franciscan 

and related rocks, and their significance in the geology of 

western California: California Division of Mines and Geology Bulletin 183, 177 p.

Brown, J.A., Jr., 1968, Thrust contact between Franciscan 

Group and Great Valley sequence, northeast of Santa 

Maria, California, [Ph.D. dissertation]: Los Angeles, Uni-

<g) Franciscan assemblage 

(graywacke)

x Santa Maria basin wells

Figure 9. Subsurface samples interpreted as Franciscan assemblage graywacke. Dashed line is 

upper Great Valley sequence petrofacies field and dotted line is lower Great Valley sequence 

petrofacies field shown in figure 3.

B10 Evolution of Sedimentary Basins Santa Maria Province
versity of Southern California, 234 p.

Dibblee, T.W., Jr., 1950, Geology of southwestern Santa Barbara County, California: California Divison of Mines Bulletin 150, 95 p.

Dickinson, W.R., 1970, Interpreting detrital modes of graywacke and arkose: Journal of Sedimentary Petrology, v. 

40, p. 695-707.

Dickinson, W.R., Ingersoll, R.V., Coan, D.S., Helmold, K.P., 

and Suczek, C.A., 1982, Provenance of Franciscan graywackes in coastal California: Geological Society of 

America Bulletin, v. 93, p. 95-100.

Dickinson, W.R., and Rich, E.I., 1972, Petrologic intervals and 

petrofacies in the Great Valley sequence, Sacramento Valley, California: Geological Society of America Bulletin, v. 

83, p. 3007-3024.

Ernst, W.G., 1980, Mineral paragenesis in Franciscan 

metagraywackes of the Nacimiento block, a subduction 

complex of the southern California Coast Ranges: Journal 

of Geophysical Research, v. 85, p. 7045-7055.

Gilbert, W.G., 1973, Franciscan rocks near Sur fault zone, 

northern Santa Lucia Range, California: Geological Society of America Bulletin, v. 84, p. 3317-3328.

Gilbert, W.G., and Dickinson, W.R., 1970, Stratigraphic variations in sandstone petrology, late Mesozoic Great Valley 

sequence, southern Santa Lucia Range, California: Geological Society of America Bulletin, v. 81, p. 949-954.

Gray, L.D., 1980, Geology of Mesozoic basement rocks in the 

Santa Maria basin, Santa Barbara and San Luis Obispo 

counties, California [M.S. thesis]: San Diego, California, 

San Diego State University, 78 p.

Hall, C.A., Jr., 1973, Geologic map of the Morro Bay South 

and Port San Luis quadrangles, San Luis Obispo County, 

California: U.S. Geological Survey Miscellaneous Field 

Studies Map MF-511, scale 1:24,000.

 1978, Geologic map of Twitchell Dam and parts of 

Santa Maria and Tepusquet Canyon quadrangles, Santa 

Barbara County, California: U.S. Geological Survey Miscellaneous Field Studies Map MF-933, scale 1:24,000.

 1981, Map of geology along the Little Pine fault, parts 

of the Sisquoc, Foxen Canyon, Zaca Lake, Bald Mountain, 

Los Olivos, and Figueroa Mountain quadrangles, Santa 

Barbara County, California: U.S. Geological Survey Miscellaneous Field Studies Map MF-1255, 2 sheets, scale 

1:24,000. 

-1982, Pre-Monterey subcrop and structure contour

maps, western San Luis Obispo and Santa Barbara Counties, south-central California: U.S. Geological Survey Miscellaneous Field Studies Map MF-1384, scale 1:24,000.

Hall, C.A., Jr., and Corbato, C.E., 1967, Stratigraphy and sedimentology of Mesozoic and Cenozoic rocks, Nipomo 

quadrangle, southern Coast Ranges, California: Geological 

Society of America Bulletin, v. 78, p. 559-582.

Hall, C.A., Jr., Ernst, W.G., Prior, S.W., Wiese, J.W., 1979, 

Geologic map of the San Luis Obispo-San Simeon region, 

California: U.S. Geological Survey Miscellaneous Field 

Investigations Map 1-1097, scale 1:48,000.

Hopson, C.A., 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 Mineral Industries Bulletin, 9S, p. 161-183.

Howell, D.G., Vedder J.G., McLean, Hugh, Joyce, J.M., Clark, 

S.H., Jr., and Smith, Greg, 1977, Review of Cretaceous 

geology, Salinian and Nacimiento blocks, Coast Ranges of 

central California, in Howell, D.G., Vedder, J.G., and 

McDougall, K., eds, Cretaceous geology of the California 

Coast Ranges, west of the San Andreas fault: Society of 

Economic Paleontogists and Mineralogists, Pacific Section, Pacific Coast Paleogeography Field Guide 2, p. 1-46.

Hsii, K.J., 1968, The principles of melanges and their bearing 

on the Franciscan-Knoxville problem: Geological Society 

of America Bulletin, v. 79, p. 1063-1074.

Ingersoll, R.V., 1978, Petrofacies and petrologic evolution of 

the Late Cretaceous fore-arc basin of northern and central 

California: Journal of Geology, v. 86, p. 335-352.

 1990, Nomenclature of Upper Mesozoic strata of the 

Sacramento Valley of California: Review and recommendations, in Ingersoll, R.V., and Nilsen T.H., eds., Sacramento Valley Symposium and Guidebook: Society of Economic Paleontologists and Mineralogists, Pacific Section, 

Book No. 65 p. 1-3.

Ingersoll, R.V., Bullard, T.F., Ford, R.L., Pickle, J.D., and 

Sares, S.W., 1984, The effect of grain size on detrital 

modes: a test of the Gazzi-Dickinson point-counting method: Journal of Sedimentary Petrology, v. 54, p. 103-116.

Lee-Wong, Florence, and Howell, D.G., 1977, Petrography of 

Upper Cretaceous sandstone in the Coast Ranges of central California, in Howell, D.G., Vedder, J.G., and 

McDougall, K., eds., Cretaceous geology of the California 

Coast Ranges, west of the San Andreas fault: Society of 

Economic Paleontologists and Mineralogists, Pacific Section, Pacific Coast Paleogeography Field Guide 2, p. 483- 

491.

MacKinnon, T.C., 1978, The Great Valley sequence near Santa 

Barbara, California, in Howell, D.G., and McDougall, K., 

eds., Mesozoic paleogeography of the western United 

States: Society of Economic Paleontogists and Mineralogists, Pacific Section, Pacific Coast Paleogeogaphy Symposium 2, p. 483-491.

Nelson, A.S., 1979, Upper Cretaceous depositional environments and provenance indicators in the central San Rafael 

Mountains, Santa Barbara County, California [M.A. thesis]: Santa Barbara, University of California, Santa Barbara, 153 p.

Ojakangas, R.W., 1968, Cretaceous sedimentation, Sacramento 

Valley, California: Geological Society of America Bulletin, v. 79, p. 973-1008.

Pettijohn, F.J., Potter, P.E., and Siever, Raymond, 1972, Sand 

and sandstone: New York, Springer-Verlag, 618 p.

Pittman, E.D., 1988, Diagenesis of Terry sandstone (Upper 

Cretaceous), Spindle field, Colorado: Journal of Sedimentary Petrology, v. 58, no. 5, p. 785-800.

Saigal, G.C., Morad, S., Bj0rlykke, K., Egeberg, P.K., and 

Aagaard, P., 1988, Diagenetic albitization of detrital Kfeldspar in Jurassic, Lower Cretaceous, and Tertiary clastic reservoir rocks from offshore Norway, I. Textures and 

origin: Journal of Sedimentary Petrology, v. 58, no. 6, p. 

1003-1013.

Seiders, V.M., 1983, Correlation and provenance of upper 

Mesozoic chert-rich conglomerate of California: Geological Society of America Bulletin, v. 94, p. 875-888.

References B11
Smith, G.W., 1978, Stratigraphy, sedimentology, and petrography of the Cambria slab, San Luis Obispo County, California [M.S thesis]: Albuquerque, University of New 

Mexico, 123 p.

Smith, G.W., Howell, D.G., and Ingersoll, R.V., 1979, Late 

Cretaceous trench-slope basins of central California: Geology. v- 7, p. 303-306.

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

Cotton Thomton, M.L., York, D.R., Filewicz, M.V., Mason, M.A., and Swisher, C.C., HI, 1990, Age and depositional setting of the type Lospe Formation (Lower Miocene), Santa Maria basin, central California: American Association of Petroleum Geologists Bulletin, v. 74, p. 770- 

771.

Toyne, Cameron, 1987, Petrology and provenance of Upper 

Cretaceous sandstone, southern San Rafael Mountains, 

Santa Barbara County, California: American Association 

of Petroleum Geologists Bulletin, v. 71, no. 5, p. 622.

Vedder, J.G., Howell, D.G., and McLean, Hugh, 1989, Geologic map of Chimney Canyon quadrangle and part of 

Huasna Peak quadrangle, California: U.S. Geological Survey Open-File Report 89-161, scale 1:24,000.

Walker, T.R., 1984, Diagenetic albitization of potassium feldspar in arkosic sandstones: Journal of Sedimentary Petrology, v. 54, p. 3-16.

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.

B12 Evolution of Sedimentary Basins Santa Maria Province
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