Conglomeratic Lower Member of 

Lospe Formation (Lower Miocene)

n

Lower Miocene Lo

sts in the

Geophysical section offshore Santa Maria basin

Geologic section onshore Santa Maria basin
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Sedimentology of the 

Conglomeratic Lower Member of the 

Lospe Formation (Lower Miocene), 

Santa Maria Basin, California

By SAMUEL Y. JOHNSON and RICHARD G. STANLEY

Provenance of Sandstone Clasts in the

Lower Miocene Lospe Formation

Near Point Sal, California

By HUGH MCLEAN and RICHARD G. STANLEY

Chapters D and E 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 

Robert M. Hirsch, Acting Director

Any use of trade, product, or firm names

in this publication is for descriptive purposes only

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

Text and illustrations edited by James W. Hendley II

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1994

For sale by

U.S. Geological Survey, Map Distribution 

Box 25286, MS 306, Federal Center, 

Denver, CO 80225

Library of Congress Cataloging in Publication Data

Johnson, Samuel Y.

Sedimentology of the conglomeratic lower member of the Lospe 

Formation (Lower Miocene), Santa Maria Basin, California / by Samuel Y. 

Johnson and Richard G. Stanley. Provenance of sandstone clasts in the 

Lower Miocene Lospe Formation near Point Sal, California / by Hugh 

McLean and Richard G. Stanley.

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

investigations Santa Maria Province ; ch. D, E) (U.S. Geological Survey 

bulletin ; 1995)

"Chapters D and E are issued as a single volume and are not available 

separately."

Includes bibliographical references.

Supt. of Docs, no.: I 19.3: 1995-D, E

1. Geology, Stratagraphic Miocene. 2. Sedimentation and deposition 

California. 3. Sandstone California. 4. Lospe Formation (Calif.) I. Stanley, 

Richard G. II. McLean, Hugh, 1939- Provenance of sandstone clasts in the 

Lower Miocene Lospe Formation near Point Sal, California. 1994. 

III. Title. IV. Title: Provenance of sandstone clasts in the Lower Miocene 

Lospe Formation near Point Sal, California. V. Series. VI. Series: U.S. 

Geological Survey bulletin ; 1995. 

QE75.B9 no. 1995-D-E 

[QE694]

557.3 s dc20 93-39079 

[551.7'87'09794 CIP
Chapter D

Sedimentology of the 

Conglomeratic Lower Member of the 

Lospe Formation (Lower Miocene), 

Santa Maria Basin, California

By SAMUEL Y. JOHNSON 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 Dl 

Introduction Dl

Acknowledgments Dl 

The Lospe Formation Dl 

Sedimentology of the lower member of the Lospe Formation D3

Unit 1:0 to 47m D4

Unit 2: 47 to 68 m D7

Unit 3: 68 to 112m D9

Unit 4: 112 to 142m D16

Rocks overlying unit 4 in the North Beach section D16 

Discussion D19 

Conclusions D20 

References cited D20

1. Index map of Santa Maria area, showing Santa Maria basin, major faults, and distribution of the Lospe Formation D2

2. Location map of Point Sal area, showing outcrops of Lospe Formation D3

3. Stratigraphic chart showing Lospe Formation and bounding units D4

4. Photograph of the unconformity at the base of the Lospe Formation D5

5. Rose diagrams showing paleocurrent data for Lospe Formation lower member D5

6. Schematic Stratigraphic section of Lospe Formation lower member at 

North Beach D6

7. Photographs and line drawing of sedimentologic features of unit 1 of Lospe Formation lower member D8

8. Photographs and line drawing of sedimentologic features of unit 2 of Lospe Formation lower member DIG

9. Photograph and line drawing of stratification style in unit 3 of Lospe Formation 

lower member D12

10. Photograph and line drawing of stratification style in unit 3 of Lospe Formation 

lower member D13

11. Photographs showing sedimentologic features of unit 3 of Lospe Formation lower 

member D14

12. Photograph and line drawing of stratification style in unit 4 of Lospe Formation 

lower member D17

13. Photographs showing sedimentologic features of unit 4 of Lospe Formation lower 

member D18

Contents IV
Sedimentology of the Conglomeratic

Lower Member of the Lospe Formation (Lower Miocene),

Santa Maria Basin, California

By Samuel Y. Johnson and Richard C. Stanley

Abstract

The lower Miocene Lospe Formation records the initial 

subsidenceof the Santa Maria basin, a major petroleum province 

in central California. The lower member of the Lospe is best 

exposed in a 142-m-thick section at the North Beach (informal 

name) locality on Vandenberg Air Force Base. These North 

Beach exposures are divided into four units on the basis of 

distinctive sedimentary textures and structures. Unit 1 (0 to 47 

m) consists mostly of crudely stratified lenticular and channelform pebbletoboulderconglomerate interpreted as mid-alluvialfan streamflow and debris-flow deposits. Unit 2 (47 to 68 m) 

comprises mainly flat- to low-angle-bedded and channel-form 

pebble conglomerate interpreted as sheetflood and streamflow 

sediments deposited in a slightly more distal alluvial fan setting. 

Unit 3 (68 to 112 m) is a sedimentologically heterogeneous 

sequence of fine-grained to granular sandstone and pebblecobble conglomerate. Flat-bedded sandstone and poorly sorted 

lenticular conglomerate beds are interpreted as the subaqueous 

sediment-gravity-flow deposits of a lacustrine fan delta. Interbedded channel-form conglomerate beds are interpreted as 

mainlysubaerialstreamflowanddebris-flowsedimentsdeposited 

during lacustrine regressions. Unit 4 (112 to 142 m) consists 

mostly of flat- and low-angle-bedded fine-grained to granular 

sandstone interpreted as high-density turbidity current deposits. 

Overall, the North Beach section is a fining-upward sequence 

that represents a long-term lacustrine transgression punctuated 

by many short-term lacustrine regressions. Paleocurrent data 

indicate that sediment transport was to the southeast. Sedimentologic data and interpretations are consistent with a rapidly 

evolving landscape and can provide important constraints on 

the paleogeography of the Santa Maria basin and the history 

of the San Simeon-Hosgri Fault.

INTRODUCTION

The onshore part of the Santa Maria basin of central 

California is a major petroleum province with cumulative 

production of more than 800 million barrels of oil and 800

billion ft3 of associated gas (California Division of Oil and 

Gas, 199la). Subsidence of the Santa Maria basin began 

in the early Miocene during an episode of local crustal 

extension driven by right-lateral shear along the boundary 

between the North American and Pacific plates (Hornafius, 

1985; Luyendyk and Hornafius, 1987; Luyendyk, 1991; 

Stanley and others, 1992). The earliest phase of basin evolution is recorded by the lower Miocene Lospe Formation, 

an 800-m-thick sequence of nonmarine and shallow marine sedimentary rocks and interbedded tuffs. This paper 

describes the sedimentology of the lower member of the 

Lospe, composed of conglomerate and sandstone. These 

strata, interpreted herein as alluvial fan and lacustrine fandelta deposits, provide important information for reconstructing paleogeography during the earliest phase of 

evolution of the Santa Maria basin and for understanding 

the depositional processes and facies that characterize the 

initial development of transtensional basins.

Acknowledgments

This research was supported by the Evolution of Sedimentary Basins Program and the Onshore Petroleum Investigations Program of the U.S. Geological Survey. We 

thank Vandenberg Air Force Base for permitting access to 

the outcrops that are the subject of this report. We benefited from stimulating discussions with R.B. Cole (University of Rochester), R.F. Dubiel (U.S. Geological Survey), 

Hugh McLean (U.S. Geological Survey), J.P. Smoot (U.S. 

Geological Survey), and D.R. York (UNOCAL). R.B. Cole 

and R.F. Dubiel also provided constructive reviews.

THE LOSPE FORMATION

The Lospe Formation (Wissler and Dreyer, 1943) consists of nonmarine and shallow marine sedimentary rocks 

and minor rhyolitic tuffs that crop out in the Casmalia Hills 

Manuscript near Point Sal and that also occur in the subsurface of the approved for publication April 1,1993.

Sedimentology of the Conglomeratic Lower Member of the Lospe Formation (Lower Miocene), Santa Maria Basin, California D1
Santa Maria basin (figs. 1, 2) (Woodring and Bramlette, 

1950; Hall, 1978; McLean, 1991; California Division of Oil 

and Gas, 1991b). In its type area on the southwest flank of 

the Casmalia Hills (fig. 2), the Lospe Formation overlies 

the Jurassic Point Sal ophiolite of Hopson and Frano (1977) 

and is overlain by the lower Miocene (Saucesian and Relizian) Point Sal Formation (fig. 3). Woodring and Bramlette 

(1950) divided the Lospe into two mappable members: (1) 

a lower member consisting largely of reddish sandstone and 

conglomerate, and (2) an upper member consisting mainly 

of greenish sandstone, siltstone, and gypsiferous mudstone. 

The Lospe was determined to be of early Miocene age on 

the basis of microfossils and radiometric ages (Stanley and 

others, 1991). Palynomorphs suggest a temperate, "summer-wet" climate during Lospe deposition (D.R. York, oral 

commun., 1988).

The Lospe Formation crops out in several areas in 

the Casmalia Hills (fig. 2), including a 5-kilometer-wide 

outcrop belt extending east-southeast from the North 

Beach (informal name) locality on Vandenberg Air Force 

Base and smaller outcrops in the Corralitos Canyon-Point 

Sal Ridge area. The Corralitos Canyon-Point Sal Ridge

outcrops are of poor quality and are involved in several 

small folds and are cut by several faults. A 413-m-thick 

section of the Lospe, extending from the basal unconformity (fig. 6) to the overlying Point Sal Formation, is 

well exposed along the sea cliff at North Beach. This section is cut by several faults, one of which juxtaposes disparate sedimentary facies at the top of the 142-m-thick 

lower member (fig. 6). The poorly exposed Lospe section 

inland from North Beach along the informally named 

Chute creek (fig. 2) is about 830 m thick and includes 

a poorly exposed 210-m-thick lower member. These 

thickness contrasts and the inferred facies juxtaposition 

suggest that part of the Lospe (including at least the 

upper part of the lower member) has been cut out by 

faulting in the North Beach section. The lower member 

of the Lospe Formation thins substantially to the southeast of Chute creek owing to post-depositional faulting 

(Dibble, 1989) and also possibly owing to original relief 

on an irregular basement surface (Woodring and Bramlette, 1950) and(or) a lateral facies change. Despite structural complexities, the North Beach outcrops of the lower 

member of the Lospe (fig. 2) are well exposed and pro12V 120*30' 120'

35' 30' -

34' 30' -

Figure 1. Index map of Santa Maria area. Onshore Santa Maria basin is triangular area bounded by Santa Maria River Fault, Santa Ynez River Fault, and 

present shoreline. Shaded areas show surface and subsurface distribution of 

Lospe Formation in the Santa Maria basin according to Hall (1978) and 

McLean (1991) and in the San Simeon area according to Hall (1976). Location of San Simeon-Hosgri Fault zone compiled and simplified from Hall 

(1975) and Steritz (1986).

D2 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
vide the opportunity for detailed sedimentologic analysis. 

These outcrops are described and interpreted below.

SEDIMENTOLOGY OF THE LOWER MEMBER 

OF THE LOSPE FORMATION

The Lospe Formation unconformably overlies gabbros 

of the Jurassic Point Sal ophiolite (figs. 4, 6) at the base of 

the North Beach section. The overlying conglomerate and 

sandstone of the 142-m-thick lower member of the Lospe is 

here divided into four units based on distinctive sedimentary 

textures and structures. Beds in this section strike from about

N. 70° W. to due west, nearly perpendicular to the northsouth-trending shoreline; beds dip moderately to steeply 

(40° to 72°) to the north. This orientation permits a nearly 

complete description of the vertical distribution of sedimentary facies but only limited observation of the lateral distribution of facies. Paleocurrent measurements from the lower 

member (figs. 5, 6) suggest that paleoflow was to the eastsoutheast. Conglomerate clasts in the North Beach section 

consist of arkosic to lithic sandstone, gabbro, altered serpentinite, and chert, and minor carbonate nodules, blueschist, 

leucogabbro, altered volcanic rocks, unaltered green serpentinite, and tuff (Anderson, 1980; Hugh McLean, oral commun., 1992). These rocks are inferred to have been derived

EXPLANATION

Lospe Formation, 

upper member

Lospe Formation, 

lower member

Lospe Formation, 

undivided

Espada Formation 

of Dibblee(1989)

Point Sal ophiolite of 

Hopson and Frano (1977)

Geologic contact

Fault-Dotted where 

concealed or Inferred

Figure 2. Location map of Point Sal area showing outcrop localities mentioned in 

text, important faults, and distribution of Point Sal ophiolite of Hopson and Frano 

(1977), Espada Formation of Dibblee (1989), and Lospe Formation. Blank areas 

are underlain by unconsolidated deposits and by non-Lospe rocks of Cenozoic 

age.

Sedimentology of the Conglomeratic Lower Member of the Lospe Formation (Lower Miocene), Santa Maria Basin, California D3
from regional basement rocks that include the Franciscan 

assemblage, the Point Sal ophiolite, and the Great Valley 

sequence (stratigraphic names used as in McLean, 1991). 

The Lospe sandstones are mainly lithic arenites (S.Y. 

Johnson, unpub. data, 1991).

Unitl: 0 to 47m

Unit 1 consists of interbedded reddish-brown pebble 

to boulder conglomerate and minor coarse-grained to granular sandstone and tuff (figs. 6, 7). Two types of conglomerate are recognized. The first type constitutes about 80 to 

90 percent of the section and consists of crudely stratified 

(flat and low-angle bedded) to structureless pebble to cobble conglomerate. Beds range in thickness from 10 cm to 

several meters and are lenticular and irregular in shape 

(fig. ID). Lower contacts of beds are typically low-angle 

(<20°) channel scours; upper contacts are typically convex 

upward. Relief on channel scours is generally less than 

about 20 cm; however, a steep channel margin at 3 m in 

the section (fig. 6) has 150 cm of relief. In massive intervals, subtle contrasts in pebble size, shape, and sorting

define stratification. Pebbles and cobbles are typically 

moderately sorted, subangular to subrounded, and either 

clast supported or, less commonly, dispersed in a matrix of 

coarse-grained to granular sandstone. Clast diameter is 

generally less than 10 cm but ranges up to about 25 cm. 

Clast imbrication is common, particularly in thinner beds.

The second conglomerate type in unit 1 is much less 

common and consists of poorly sorted cobble to boulder 

conglomerate (figs. 6, 7). Typically, beds are internally 

structureless and fill narrow (50 to 500 cm wide), shallow 

(depth <100 cm) channels cut into crudely stratified, clastsupported conglomerate. Cobbles and boulders are typically poorly sorted, subangular to subrounded, and either 

matrix (medium-grained to granular sandstone) supported 

or clast supported. Some beds include both clast-supported and matrix-supported zones. Outsized boulders are as 

large as 80 cm in maximum dimension. Larger clasts are 

commonly concentrated at the tops of beds or on bed 

margins and in many cases project upward into overlying 

beds (fig. IB, C, D). Conglomerate clasts commonly are 

randomly oriented (fig. IB). Imbrication is uncommon.

Coarse-grained to granular sandstone forms uncommon lenses generally less than 30 cm thick (fig. 1C, D)

AGE 

(Ma)

16-

17-

17.4-

17.7-

96-

PERIOD

138

170-

EPOCH SUBEPOCH

Middle

Early

STAGE1

Luisian

Relizian

Saucesian

STRATIGRAPHY

Monterey Formation

Point Sal Formation

Lospe Formation

Upper member

Late

Early

Great Valley 

sequence

Lower member

Upper 

petrofacies2

Lower 

petrofacies2 

(Espada

Formation3)

Point Sal ophiolite4

THICKNESS, 

IN METERS

490

460

620

210

Unknown

380

Unknown

TERTIARY(?) TO JURASSIC Franciscan assemblage Unknown

1 Modified from Kleinpell (1938,1980).

2 Modified from McLean (1991).

3 Espada Formation of Dibblee (1989).

4 Point Sal ophiolite of Hopson and Frano (1977).

Figure 3. Stratigraphic chart showing Lospe Formation and bounding rock units. 

D4 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
that are internally structureless or have weakly defined 

low-angle stratification. These sandstone lenses generally 

overlie (sharply or gradationally) conglomerate beds; less 

commonly, they occur in the lower parts of channel fills. 

Tuff beds that are 20 cm thick occur at approximately 31 

and 40 m in the section (figs. 6, IB) and form laterally 

continuous (to the limits of the outcrop) sheets.

Sedimentologic textures (for example, coarse grain 

size, poor to moderate sorting, subangular clasts) and 

structures (dominantly intersecting low-angle channel fills)

described above indicate deposition on an alluvial fan (for 

example, Bull, 1972; Nilsen, 1982) proximal to source. 

Crudely stratified conglomerate and less common moderately sorted clast-supported imbricate conglomerate were 

deposited in shallow fan channels by streamflow or possibly by hyperconcentrated flood flow (Smith, 1986). Uncommon sandstone lenses mainly reflect deposition during 

waning flood stages. The lack of crossbedding and other 

features representing deposition on migrating bedforms or 

on bars with more than minimal relief indicates that sand

Figure 4. Unconformity (surface marked by arrows) between gabbros of the Point Sal 

ophiolite (right) and overlying conglomerate and sandstone of the lower member of the 

Lospe Formation (left). Hammer (center left) for scale.

NORTH NORTH 

T

Figure 5. Rose diagrams showing paleocurrent data from 

Lospe Formation lower member in the outcrop belt between 

North Beach and Chute creek (fig. 2). Labelled arrows show 

vector means. A, Clast imbrications (150 measurements). B, 

Channel axes (7 measurements).

Sedimentology of the Conglomeratic Lower Member of the Lospe Formation (Lower Miocene), Santa Maria Basin, California D5
was similarly deposited by either shallow streamflow or 

by hyperconcentrated floodflow. Slow burial may have allowed the upper, more fine-grained parts of channel fills to 

be removed by erosion.

Poorly sorted cobble to boulder conglomerate beds 

are interpreted as debris-flow deposits, despite a general

lack of clay matrix. The poor sorting and common random 

orientation of clasts, the presence of outsized clasts, the 

projection of clasts into overlying beds, and the mix of 

clast fabrics (both clast and matrix supported) within beds 

support this interpretation. These inferred debris-flow deposits generally fill channels (fig. 7), and the typical lack

EXPLANATION

Vitric-lithic tuff

Horizontal- to low-anglebedded sandstone, 

commonly graded

Structureless sandstone

Horizontal- to low-angle-bedded 

pebbly sandstone

Interbedded sandstone and 

lenticular conglomerate-filled 

channels

Alternating thin to thick beds of flat- to 

low-angle-stratified conglomerate and 

sandstone; minor channel-fill 

conglomerate

Crude horizontally to low-angle bedded 

or structureless conglomerate; 

sandstone matrix

Crude horizontal- to low-angle-bedded 

conglomerate cut by tentacular 

conglomerate-filled channels

Poorly sorted conglomerate; commonly 

matrix rich; common outsized clasts

Clast-supported structureless 

conglomerate

Fault

^ Ripple mark

_^ Paleocurrent indicator based on clast imbrication

T> Paleocurrent indicator based on channel orientation

[] Trace fossils; burrows or grazing trails

Grain size scale

f Fine-grained sandstone

m Medium-grained sandstone

c Coarse-grained sandstone

g Granular sandstone

p Pebble conglomerate

c Cobble conglomerate

b Boulder conglomerate

0 

METERS

II I I I I f m eg p c b

GRAIN SIZE

1 I II 

0 20 40 60 80

CENTIMETERS 

MAXIMUM CLAST SIZE

Figure 6. Schematic stratigraphic section of Lospe Formation lower member at North Beach.

D6 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
of fine-grained matrix may in large part represent winnowing by subsequent streamflow through these channels. 

Comparable matrix-poor debris-flow deposits of alluvial 

fans have been described by Gloppen and Steel (1981) and 

Nemec and others (1984). The dominance of streamflow 

deposits relative to debris-flow deposits in unit 1 suggests 

deposition in a mid-alluvial-fan setting below the "intersection point" (Hooke, 1967).

100-T

Unit 2: 47 to 68 m

Unit 2 consists of reddish-brown interbedded pebble 

and cobble conglomerate and less common medium-grained 

to granular sandstone. Unit 2 is distinguished from unit 1 

on the basis of finer grain size, generally more well-defined 

stratification, the occurrence of conglomerate-sandstone couplets, and the apparent paucity or absence of poorly sorted 

conglomerate of inferred debris-flow origin. Unit 2 consists 

largely of two sedimentary facies, present in relatively equal

130-

120-

100 

METERS

Lospe Formation, 

upper member

iunttimuimm»*tBtme» I

i i i i i 11

t m eg p c b 

GRAIN SIZE

I I I I I 

0 20 40 60 80

CENTIMETERS 

MAXIMUM CLAST SIZE

I I I I I n f m eg p c b

GRAIN SIZE

III 1 I 

0 20406080

CENTIMETERS 

MAXIMUM CLAST SIZE

Figure 6. Continued.

Sedimentology of the Conglomeratic Lower Member of the Lospe Formation (Lower Miocene), Santa Maria Basin, California D7
Figure 7. Selected outcrops of unit 1 of the Lospe Formation lower member (see fig. 6) 

illustrating stratification style. A, Lenticular bed of poorly sorted pebble to cobble conglomerate (arrows show base and top of bed) of inferred debris-flow origin, overlain by 

flat-bedded clast-supported locally imbricate conglomerate of inferred streamflow origin. Approximately at 45 m in section shown in figure 6. B, Section comprising (from 

lower right to upper left) lenticular channel-fill pebble conglomerate of inferred fluvial 

origin; tuff bed (approximately at 31 m in section shown in fig. 6); moderately sorted 

clast-supported pebble conglomerate of probable fluvial origin; and poorly sorted 

channel-filling (base of channel shown by arrows) cobble-boulder conglomerate of inferred debris-flow origin. Note variable orientation (vertical to horizontal) of clasts in 

inferred debris-flow bed, as well as concentration of large clasts at the top of the unit. 

C, D, Photograph (Q and line drawing (D) showing bedding lenticularity and lowrelief channels in interbedded conglomerate and sandstone. In D, stippled pattern represents sandstone; small gravel pattern shows well-sorted clast-supported conglomerate 

of inferred fluvial origin; pebble-sand pattern represents poorly sorted conglomerate of 

probable debris-flow origin. Light screen pattern represents areas of cover or poor exposure. Approximately at 46 m in section shown in figure 6.

D8 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
amounts. The first facies resembles the crudely stratified 

streamflow conglomerate described in unit 1. Clast-supported pebble to cobble conglomerate forms irregular convexupward lenticular beds bounded by low-angle (<20°) erosion

100 CENTIMETERS

Figure 7. Continued.

surfaces (figs. 8A, B) defining broad (several meters wide) 

channels. Relief on channel scours is generally less than 

about 20 cm; however, at 57.5 m in the section (fig. 6) 

there is 140 cm of relief at the base of one channel. Conglomerate beds range up to about 150 cm in thickness. 

Pebbles and cobbles are typically moderately sorted, subangular to subrounded, clast supported, and commonly imbricate (fig. 8Q. Clast diameter is generally less than 10 

cm, but outsized clasts range up to 27 cm. These outsized 

clasts (fig. 8C) may represent redeposited debris-flow material. Many of these low-relief channel-filling units grade 

upward to structureless sandstone, and there is an increased 

proportion of sandstone in this facies relative to channelfill fluvial deposits of unit 1. These sandstone beds are generally lenticular in shape and are erosionally truncated, indicating low preservation potential.

The second conglomerate facies in unit 2 consists of 

commonly graded couplets of pebble conglomerate and medium-grained to granular sandstone (fig. 8 A, B, D), interpreted as sheetflood deposits (Hogg, 1982). These couplets are 

typically 10 to 40 cm thick. The conglomerate-sandstone 

ratio in the couplets is roughly 5:1. Couplets typically have 

minor (<10 cm) low-angle (<10°) relief at their base and are 

laterally continuous for as much as 20 m. Pebbles are generally less than 10 cm in diameter, moderately to moderately well 

sorted, subrounded, clast supported, and commonly imbricated. Contacts within couplets between conglomerate and 

sandstone intervals are graded, or less commonly, sharp. The 

sandstone layer in couplets is typically thin and in many cases 

(as indicated by textural changes in amalgamated conglomerate layers) has been entirely removed by erosion. Comparable 

modern and ancient alluvial fan sheetflood deposits have 

been described by Blair (1987a, b) and many others.

Beds of inferred streamflow and sheetflood origin are 

complexly interbedded in unit 2 (fig. 8/4, B). This architecture (Miall, 1985) suggests deposition in a slightly more 

distal alluvial fan setting than for unit 1, a setting where 

low-relief channels broadened so that streamflow became 

less confined and a mix of streamflow and sheetflood deposits accumulated.

Unit3: 68 to 112 m

Unit 3 consists of reddish-brown to, less common, 

greenish-gray interbedded pebble and cobble conglomerate, fine-grained to granular sandstone, and minor tuff (fig. 

6). It is distinguished from unit 2 on the basis of much 

finer grain size (conglomerate-sandstone ratio is about 

1.2:1) and more diverse sedimentary textures and structures. Unit 3 consists of several depositional facies (figs. 9, 

10, 11) and is the most heterogeneous of the four depositional units of the lower member of the Lospe Formation.

Fine-grained to granular sandstone is mainly flat bedded 

to very low angle bedded (fig. 11 A, C, E). Bed thickness

Sedimentology of the Conglomeratic Lower Member of the Lospe Formation (Lower Miocene), Santa Maria Basin, California D9
Figure 8. Selected outcrops of unit 2 of the Lospe Formation lower member (fig. 6) 

illustrating stratification style. A, B, Photograph (A) and line drawing (B) of flat-bedded 

conglomerate-sandstone couplets interpreted as sheetflood deposits (pebble-sand pattern), overlain by intersecting, low-angle channel-fill conglomerate (small gravel pattern). Lenticular sandstone interbeds represented by stippled pattern. View is slightly 

distorted because beds are dipping approximately 60° to the north, away from viewer. 

Jacob staff in center of photograph (150 cm long) rests between 50 and 55 m in 

section (fig. 6). Light screen pattern represents areas of cover or poor exposure. C, 

Crudely stratified (beds dip from upper right to lower left) granule to cobble conglomerate, interpreted as streamflow deposits. D, Three crudely graded couplets (arrows 

show upper couplet contacts) of flat- to low-angle-bedded imbricate conglomerate and 

coarse-grained to granular sandstone of inferred sheetflood origin. Couplets overlain by 

clast-supported conglomerate of inferred streamflow origin.

D10 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
typically ranges from about 5 to 50 mm, and beds form 

intervals as thick as a few meters (fig. 6). Beds are sheetlike 

or are characterized by very low angle (<5°) contacts. Bed 

contacts are both sharp and diffuse. Sandstone is moderately 

sorted to well sorted, and grains are subrounded. Normal 

grading is common. Many flat-bedded sandstone intervals 

include thin (one or two clasts thick) sheets or lenses (20 to 

>200 cm wide) of pebble conglomerate. Conglomeratic channel fills incise into and thereby occur at the same stratigraphic 

level as many intervals of flat-bedded sandstone (fig. 10).

Primary stratification has been obscured to varying 

(generally small) degrees in sandstone beds at several horizons (at 70, 80 to 85, 88, 100, and 104 m, fig. 6) by bioturbation (fig. 1 ID). Trace-fossil forms include the following:

(1) oblique to vertical burrows that are 5 to 20 mm wide, 50 

to 110 mm deep, and internally structureless or, less commonly, have meniscate backfill; (2) irregular, bulbousshaped, horizontal to subhorizontal, single and compound, 

sand-filled burrows (fig. 11 D) that are 10 to 30 mm wide 

and as much as 100 mm long; and (3) poorly preserved, 

discontinuous epichnial tracks less than 10 mm wide. Burrows are commonly calcite cemented, weather to a greenishgray color, and are more resistant to erosion than enclosing 

reddish-brown sandstone.

Pebble and cobble conglomerate mainly occur as channel fills. There appears to be a continuum in the shapes and 

dimensions of conglomeratic channel fills from forms similar to those described in unit 2 (several tens of centimeters

Figure 8. Continued. 

Sedimentology of the Conglomeratic Lower Member of the Lospe Formation (Lower Miocene), Santa Maria Basin, California D11
thick, several meters wide) to much narrower (tens of centimeters wide) forms that in some cases have steep (>60°) 

margins (figs. 9, 10, 11A, C, F). Contacts between conglomerate and overlying sandstone beds range from gradational to sharp. Pebbles and cobbles are typically 

moderately sorted, subangular to subrounded, and clast 

supported. Clast diameter is generally less than 10 cm but 

ranges up to 30 cm.

150 CENTIMETERS

Figure 9. Photograph (A) and line drawing (B) of North Beach section at about 80 m 

(fig. 6) showing interbedded conglomerate and sandstone in unit 3 of the Lospe Formation lower member. In line drawing, stippled pattern represents coarse-grained to granular sandstone; small gravel pattern represents stratified moderately sorted 

conglomerate; pebble-sand pattern represents poorly sorted conglomerate of probable 

debris-flow origin. Jacob staffs in A are 150 cm long. Light screen pattern represents 

areas of cover or poor exposure.

D12 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
Poorly sorted pebble and cobble conglomerate occurs 

as relatively flat-based lenses (~20 to 300 cm wide) and 

sheets and, less commonly, as channel fills (figs. 9, 11B). 

Bed thickness typically ranges from about 10 to 30 cm. 

Beds show no internal stratification and clast fabric is 

weak (parallel alignment of clast long axes) to random. 

Pebbles and cobbles are generally matrix (silt to granular 

sand) supported but are locally clast supported. Inverse to 

normal grading is present in a few beds (fig. 1 IB).

Finally, couplets of conglomerate and sandstone comparable to but more fine grained than those described for 

unit 2 occur between about 93 and 100 m in the North 

Beach section (fig. 6). These couplets are 20 to 60 cm 

thick, commonly have low-relief scours at their base, and 

have a conglomerate-sandstone ratio of about 1:1.

Sedimentary facies of unit 3 collectively indicate deposition in a fan-delta setting (Holmes, 1965; McPherson and 

others, 1987, 1988; Nemec and Steel, 1988). The alluvial

fan deposits of units 1 and 2 were encroached by a standing 

body of water. Partial subaqueous deposition is indicated 

by (1) the abundance of well-sorted flat-bedded commonly 

graded sandstone, (2) evidence for a locally abundant 

burrowing infauna, and (3) the presence of uncommon inverse-to-normally graded, poorly sorted conglomerate (see 

discussion below). The paucity of evidence for significant 

wave activity indicates that the body of water occupied a 

restricted environment protected from wave energy. Crossbedding is apparently absent, and conglomerate facies lack 

features (good sorting, well-defined stratification, common 

crossbedding, laterally continuous beds, well-defined clast 

fabrics) typical of marine fan-delta and shallow-marine environments (Clifton, 1973; Bourgeois and Leithold, 1984; 

Ethridge and Wescott, 1984; Nemec and Steel, 1984; Maejima, 1988; Marzo and Anadon, 1988). A lacustrine setting 

is favored because of the absence of marine macrofossils, 

which are generally abundant in deposits of restricted

B

Figure 10. Photograph (A) and line drawing (B) of North Beach section at about 102 to 111 m (see fig. 6) 

in unit 3 of the Lospe Formation lower member showing geometry of conglomeratic channel fills (small 

gravel pattern) and flat-bedded sandstone (stippled pattern). Figures 11£and Fshow details of stratification. 

Note contrast with conglomerate bed geometry in unit 4 (fig. 12). Light screen pattern represents areas of 

cover or poor exposure.

Sedimentology of the Conglomeratic Lower Member of the Lospe Formation (Lower Miocene), Santa Maria Basin, California D13
Figure 11. Details of stratification in unit 3 of the Lospe Formation lower member (see 

fig. 6). A, Strata about 71 m in North Beach section of figure 6. Poorly sorted bed of 

probable debris-flow origin at base (arrow shows top of bed), overlain by interbedded 

channel-fill conglomerate and flat-bedded sandstone. Arrow indicates horizon of burrowed sandstone. Hammer (center left) for scale. B, Detail of stratification in rocks of 

figure 9 showing lenticular conglomerate and flat- to low-angle-bedded fine-grained to 

granular sandstone. Poorly sorted beds (arrows) are of probable debris-flow origin. 

Lowest bed displays inverse to normal grading. C, Graded flat beds of well-sorted finegrained to granular sandstone overlain by lenticular channel-fill conglomerate and 

pebbly sandstone. At about 89 m in section shown in figure 6. D, Bedding-plane view 

of top of intensely burrowed bed at 101 m in North Beach section (fig. 6). Burrows are 

calcite cemented, weather to a white color, and are more resistant to erosion than 

enclosing red sandstone. Scale bar (16.5 cm long) in lower right. E, Detail of stratification in rocks of figure 10, showing well-sorted laterally continuous thin (1 to 5 cm) 

beds of fine-grained to granular sandstone. Pencil used for scale is 14 cm long. About 

105 m in the section shown in figure 6. F, Detail of stratification in rocks of unit 3, 

showing steeply dipping (top to left) beds of lenticular channel-fill conglomerate. At 

about 110 m in section shown in figure 6(top of section shown in fig. 10).

D14 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
marine settings such as estuaries, and because sulfate isotopic data indicate that gypsum in mudstones of the overlying upper member of the Lospe Formation precipitated from 

a brine that could not have been in contact with marine 

waters (M.L. Tuttle, U.S. Geological Survey, oral commun., 

1990; Stanley and others, 1992).

Moderately well sorted clast-supported conglomeratic 

channel fills and conglomerate-sandstone couplets are interpreted as subaerial (incised channel and sheetflood) deposits of this inferred fan delta. The subaerial (channel fill 

and sheetflood) deposits of the inferred fan delta are 

sedimentologically similar and only slightly more fine 

grained than those of underlying unit 2 and probably formed 

in a similar mid-alluvial-fan setting.

The increased proportion of sandstone in unit 3 (compared to underlying units) resulted from rapid sediment deposition associated with subaqueous flow expansion where 

sediment-laden streams entered the lake. Flat-bedded sandstone of unit 3 was probably deposited in part as sediment 

gravity underflows associated with major flood events. 

Nemec and others (1984) have suggested that sandstone 

beds in a similar lacustrine fan-delta sequence were deposited

'- 'A*'

ft

Figure 11. Continued.

Sedimentology of the Conglomeratic Lower Member of the Lospe Formation (Lower Miocene), Santa Maria Basin, California D15
by flows transitional between grain flows and high-density 

turbidity currents. They suggested that deposition of these 

beds must have been rapid enough to prevent development 

of bedforms other than a diffuse plane bed but not so rapid 

that vertical segregation of grain sizes (grading) was entirely 

prevented. In some horizons, flat-stratified texturally mature 

sandstone beds may also reflect limited wave reworking 

and redeposition, some of which may have occurred during 

shoreline migrations driven by lake-level fluctuations. Horizons of abundant trace fossils reflect periods of nondeposition. Pollard and others (1982) have previously described 

shallow-lacustrine trace fossils from a similarly coarsegrained fan-delta sequence in Norway.

The poorly sorted partly matrix-supported pebble and 

cobble conglomerate beds (fig. 9, 115) are interpreted as 

debris-flow deposits. The inverse to normal grading in 

some beds suggests a subaqueous origin (Nemec and Steel, 

1984). The subaqueous debris flows probably originated 

in coarse material residing on unstable slopes (subaerial 

and subaqueous) near the alluvial fan/lake interface. Earthquake-induced shocks associated with basin-margin and regional tectonism could have provided a trigger for slope 

failure. Other poorly sorted conglomerate beds (most notably the uncommon channel fills) may have been deposited 

subaerially.

The channel-fill conglomerates of inferred subaerial origin represent significant breaks in lacustrine deposition. 

They commonly incise into inferred lacustrine sandstone 

(fig. 10), a fact which suggests fluctuating lake levels. The 

tendency toward narrower channel fills with steeper margins 

in unit 3 relative to units 1 and 2 probably reflects (1) 

the increased cohesion of the more fine-grained channelbank material, and (2) the funneling effects of very lowrelief lacustrine-shoreline bars and (or) dunes on streamflow 

(Smoot, 1991; Smoot and Lowenstein, 1991). Apart from 

the complex interfingering of inferred subaqueous and subaerial deposits, fluctuating lake levels are suggested by the 

presence of several thin (one or two clasts thick) pebble 

layers in sandstone-rich intervals, which might represent 

transgressive lags.

Unit 4: 112 to 142 m

Unit 4 consists of greenish-gray and minor reddishbrown fine-grained to granular sandstone and pebble to 

cobble conglomerate (fig. 6). It is largely distinguished 

from unit 3 on the basis of its finer grain size (conglomerate-sandstone ratio is about 1:3 to 4). Sandstone is 

mainly flat to low-angle bedded (figs. 12, 13). Sandstone 

beds are typically 1 to 15 cm thick and laterally continuous for several meters (to the limits of outcrops). Most 

beds are well sorted, but many contain dispersed granules 

and pebbles (fig. 13A). Bed contacts range from sharp to 

diffuse. Sharp contacts are planar or are characterized by

low-relief scours (fig. 13A). Many beds are internally 

graded and plane laminated; ripple lamination and crossbedding were not observed. Several beds are bioturbated 

and internally massive (fig. 135). Burrows commonly 

form horizontal to inclined sand-filled tubes as much as 

2 cm in diameter and 10 cm in length. These burrows are 

commonly calcite cemented, have a distinctive grayish 

color, and are more resistant to erosion than surrounding 

strata. Calcite cementation also characterizes many thin 

(<5 cm) planar beds (fig. 12, 13A, E). Some of these 

planar beds include calcite-cemented burrows and are 

inferred to represent relatively stable, extensively bioturbated surfaces.

Conglomerate beds are generally 5 to 50 cm thick, 

have sharp and planar lower contacts, and have sharp or 

diffuse upper contacts. Beds are internally massive or, 

less commonly, graded and form both laterally continuous sheets and discontinuous lenses (fig. 13). Some beds 

have convex-upward upper surfaces with as much as 

20 cm of low-angle relief (fig. 12), a fact which suggests 

deposition as lobes or bars. As in unit 3, there are many 

one- to two-clast thick pebbly layers (fig. 12, 13A). Conglomerate-filled channels (figs. 12, 13A) are also present 

but are smaller and less common than in unit 3. Pebbles 

and cobbles in unit 4 conglomerate beds are typically 

moderately sorted and mainly clast supported; less commonly, pebbles are dispersed in a matrix of coarsegrained to granular sandstone. Clast diameter is generally 

less than 5 cm but ranges up to 18 cm.

Unit 4 mainly represents shallow subaqueous lacustrine deposition along the front of a fan delta. Strata were 

originally deposited by sediment gravity flows, including 

high-density turbidity currents (see unit 3 discussion 

above) and possibly uncommon debris flows. Significant 

reworking by waves and fluctuating shorelines is suggested by the laterally continuous well-sorted sandstone beds 

and by the one- to two-clast thick conglomerate beds interpreted as lag deposits. The uncommon conglomeratic 

channel fills (figs. 12, 13A) were probably incised and 

filled on the lower alluvial fan surface during lacustrine 

regressions. The greenish-gray color of the predominantly 

subaqueous deposits of unit 4 contrasts markedly with 

the reddish-brown color of underlying alluvial fan deposits; the color difference probably reflects distinctly different oxidizing/reducing conditions in the early diagenetic 

environment.

Rocks Overlying Unit 4 in the 

North Beach Section

A fault in the North Beach section (fig. 6) juxtaposes 

north-dipping unit 4 with south-dipping strata (on the south 

limb of a small fold) in the lower part of the upper member 

of the Lospe Formation. These north-dipping strata consist

D16 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
of gray and brown interbedded fine- and medium-grained 

sandstone and mudstone (sandstone-mudstone ratio is about 

3:1). Coarse-grained sandstone occurs as uncommon beds 

and lenses, and conglomerate is rare. Sandstone beds are

generally 5 to 30 cm thick, have sharp bases, are laterally 

continuous (for about 10 m, the limits of outcrop), 

and commonly are internally graded. Incomplete Bouma 

sequences characterize most graded beds. The Bouma

'4*

0 100 CENTIMETERS 

I____|

Figure 12. Photograph (A) and line drawing (6) of unit 4 of the Lospe Formation lower member at about 125 to 130 m in 

North Beach section (fig. 6) showing geometry of conglomeratic lenses (small gravel pattern) interbedded with flat-bedded 

sandstone (stippled pattern). See figure 13A for close-up of one- to two-clast-thick conglomerate lens and bounding strata at 

level of Jacob staff in left center of figure 12A Arrow (at lower left) in figure 12A shows conglomerate bed with convexupward relief, suggesting deposition as a lobe or bar. Note contrast with conglomerate bed geometry in unit 3 (fig. 10). Light 

screen pattern represents areas of cover or poor exposure.

Sedimentology of the Conglomeratic Lower Member of the Lospe Formation (Lower Miocene), Santa Maria Basin, California D17
sequences indicate deposition by turbidity currents (Bouma, 

1962) of lower density than those inferred for the more 

coarse-grained beds of units 3 and 4.

Sedimentary textures and structures in these overlying 

beds and their stratigraphic position between fan-delta deposits and more fine-grained lacustrine rocks suggest deposition in a lacustrine prodelta setting. This setting is 

consistent with the fining-upward transgressive trend in 

the lower member of the Lospe. However, the abrupt fades change at the contact between the two members 

strongly suggests that a part of the overall transgressive 

sequence has been cut out by faulting.

;s

Figure 13. A, Close-up view of stratification in unit 4 of the Lospe Formation lower 

member shown in figure 12 (rotated to horizontal) showing one- to two-clast-thick 

conglomerate lens and bounding sandstone. Channel-fill conglomerate is at base of 

photograph, and granule-lined scour surface and thin conglomeratic lens are at top of 

view. B, Detail of stratification at approximately 125 m in North Beach section (fig. 6) 

showing light-colored calcite-cemented layers (above and below hammer) and trace 

fossils in lowermost cemented horizon (marked with arrow).

D18 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
DISCUSSION

The lower member of the Lospe Formation in the Casmalia Hills area is a fining-upward sequence that records 

alluvial fan (units 1 and 2) and lacustrine fan-delta (units 

3 and 4) sedimentation during initial subsidence of the Santa 

Maria basin. The limited outcrop does not allow reconstruction of the morphology of this fan and fan delta; however 

it apparently lacks the steep foresets of Gilbert-type fan 

deltas (Fisher and others, 1969), which are characteristic 

of sequences deposited along steep basin margins in many 

extensional basins (Leeder and others, 1988). The Lospe 

fan delta apparently had lesser relief and may resemble 

the "shelf-type" fan deltas described by Ethridge and Wescott 

(1984) and Colella (1988). Relatively slow initial subsidence 

of the Santa Maria basin and (or) high rates of sediment 

supply could have inhibited development of steeper depositional surfaces.

The pattern of sedimentation in the lower member of 

the Lospe indicates a long-term transgression punctuated 

by short-term regressions. There are many factors that 

could have contributed partly or wholly to the relative rise 

in lake level needed for transgression:

(1) Increased run-off into the lake, either owing to a 

climate change or to enlargement of the area of the drainage 

basin feeding the lake. There is no direct sedimentologic 

evidence (such as an upward increase in channel dimensions 

or changes in suites of sedimentary structures indicating 

greater depth of flow) for an increase in runoff during deposition of the North Beach fan. However, the lake was probably fed by more than one source; therefore, the limited 

data from the North Beach section are inadequate to test 

the climate-change hypothesis. There is similarly no data 

to test the enlarging-drainage-basin hypothesis; however, 

because the central coast of California was very active tectonically during the early Miocene (Hall, 1978; Anderson, 

1980; Luyendyk and Hornafius, 1987; Luyendyk, 1991), 

significant reorganization of drainage patterns would have 

been expected.

(2) Progressive tectonic tilting of the basin floor in 

the direction of the North Beach alluvial fan. Tilting is 

required because (all other factors being equal) uniform 

subsidence would result in equal lowering of the alluvial 

fan, lake level, and any lake outflows. Basin tilting is 

common in extensional basins (Leeder and others, 1988).

(3) Given the conditions outlined in either 1 or 2 

above, a decrease in sediment supply would contribute to 

the magnitude of the transgression.

Rocks of the North Beach section described above 

provide constraints on reconstructions of local paleogeography. The very coarse texturally immature sediments of 

units 1 and 2 suggest an alluvial fan depositional environment and a nearby source. Fan deposition, characterized 

mainly by shallow streamflow, sheetflood, and debris-flow 

deposits, was clearly ephemeral; thus the drainage basin

feeding the fan was probably small. Clast compositions 

(Anderson, 1980; Hugh McLean, oral commun., 1992) indicate that rocks exposed and eroding in the source drainage basin included the Franciscan assemblage, the Point 

Sal ophiolite, and the Great Valley sequence. These rocks 

occur over a large range of structural and stratigraphic levels (fig. 3). The drainage-basin source for the alluvial fan 

was therefore probably cut by high-angle faults along 

which diverse basement rocks were juxtaposed.

Because most of the local area is draped by post-Lospe 

strata, the exact location of the above-described structurally 

complex source area and other inferences about source-drainage-basin geometry during Lospe time are problematic. We 

recognize three possible configurations for the source of the 

North Beach alluvial fan sediments.

(1) The nearby source could have been an uplifted 

block on the west flank of an unmapped north-northwesttrending offshore fault, parallel to the San Simeon-Hosgri 

Fault. This hypothetical structure would need to have been 

located more than about 2 km from the North Beach section 

and west of Point Sal, because basement rocks at Point Sal 

are apparently not cut by such a fault (Dibblee, 1989). In this 

scenario, the paleoslope indicated by paleocurrent directions 

(figs. 5, 6; vector mean = 120°) would have been oriented 

about 45° to the basin margin. The San Simeon-Hosgri Fault 

is located about 10 km offshore (fig. 1), too far away to feed 

an alluvial fan characterized to such a degree by the coarse 

immature sediments of the North Beach section.

(2) The nearby source could have been an uplifted 

block on the southwest side of the southeast-trending 

(-130°) Lions Head Fault, located approximately 500 m 

southwest of the Lospe outcrop belt (fig. 2). In this scenario, sediment transport (figs. 5, 6) was nearly parallel to the 

basin-bounding fault. If this inferred block were the source 

of the North Beach alluvial fan sediments, then streams on 

the fan surface must have been diverted in a short distance 

to nearly follow a basin-axial trend. This diversion would 

presumably be a response to asymmetric basin subsidence.

(3) The nearby source could have been an uplifted 

block of Mesozoic basement on the north side of the Point 

Sal fault, located 1 to 2 km north of the North Beach section in the present area of Point Sal Ridge (fig. 2). Preliminary data (R.G. Stanley and others, unpub. data, 1991) 

indicate that this block was uplifted and eroding during 

Lospe time; however, the basement rocks that are presently exposed along Point Sal Ridge do not include the structurally low Franciscan assemblage, a requirement for any 

possible source. Franciscan rocks could have been exposed and completely eroded north of the present Point 

Sal Ridge outcrops but only if the base of the Lospe Formation exposed in the Corralitos Canyon area is younger 

than the lower member of the Lospe at North Beach. At 

this time none of our three hypotheses can be ruled out, 

but 1 or 2 seem more likely because of the highly speculative assumptions required for 3.

Sedimentology of the Conglomeratic Lower Member of the Lospe Formation (Lower Miocene), Santa Maria Basin, California O19
Documentation of the sedimentology of the lower 

member of the Lospe Formation is potentially an important element in understanding the amount and timing of 

right-lateral displacement along the San Simeon-Hosgri 

Fault. Hall (1975, 1978) and Graham and Dickinson 

(1978a, b) noted similarities between the lower member of 

the Lospe Formation in the Casmalia Hills area and 

coarse-grained ophiolite-derived conglomerate in the San 

Simeon area (fig.l). They inferred that these units were 

contiguous at the time of deposition and used this inference as an important factor in proposing 80 to 115 km of 

right-lateral displacement on the San Simeon-Hosgri Fault. 

These inferences were made without the benefit of detailed 

sedimentologic analysis of the relevant strata in either 

area. The data provided here on the sedimentology and 

vertical sequence of the lower member of the Lospe 

should, in combination with similar information collected 

from the San Simeon outcrops, allow more rigorous testing of displacement models on this fault system.

CONCLUSIONS

The conglomeratic lower member of the Lospe Formation records initial subsidence of the Santa Maria basin of 

central California. The lower member is best exposed in a 

140-m-thick section along North Beach on Vandenberg Air 

Force Base. Four depositional units are recognized in this 

section on the basis of distinctive sedimentary textures and 

structures. These four units comprise a fining-upward sequence that records deposition by mostly streamflow, debrisflow, and sheetflood mechanisms in an alluvial fan setting 

(units 1 and 2) and by subaerial streamflow and subaqueous 

sediment-gravity-flow mechanisms in a lacustrine fan-delta 

setting (units 3 and 4). Vertical facies trends suggest a longterm lacustrine transgression punctuated by many short-term 

regressions. Sedimentologic data and interpretations reported here are consistent with a rapidly evolving landscape and 

should ultimately provide important constraints on regional 

paleogeographic reconstructions, most notably the offset 

history of the San Simeon-Hosgri Fault.

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of Geology, v. 75, p. 438-460.

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 Mineral 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. 12,503-12,522.

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American Association of Petroleum Geologists, 450 p.

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alluvial fans and fan deltas in neotectonic extensional settings Implications for the interpretation of basin fills, in 

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p. 173-185.

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

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in Ingersoll, R.V., and Ernst, W.G., eds., Cenozoic basin development of coastal California, Rubey Volume VI: Englewood Cliffs, N. J., Prentice Hall, p. 260-283.

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and Steel, R.J., eds., Fan deltas Sedimentology and tectonic 

setting: Glasgow, Blackie and Son Ltd., p. 318-340.

McLean, Hugh, 1991, Distribution and juxtaposition of Mesozoic 

tectonic elements in the basement of the Santa Maria basin, 

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

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Geological Society of America Bulletin, v. 99, p. 331-340.

 1988, Fan deltas and braid deltas Conceptual problems, 

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Ltd., p. 14-22.

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p. 3-13.

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American Association of Petroleum Geologists Memoir 31, 

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1074, p. 73.

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of the Santa Maria district, in Jenkins, O.P., ed., Geologic 

formations and economic development of the oil and gas 

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Sedimentology of the Conglomeratic Lower Member of the Lospe Formation (Lower Miocene), Santa Maria Basin, California D21

Chapter E

Provenance of Sandstone Clasts in the 

Lower Miocene Lospe Formation 

Near Point Sal, California

By HUGH MCLEAN 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 El 

Introduction El

Previous work El

Methods E3 

Petrographic data E3

Comparison of sandstone clasts from the Lospe Formation with Mesozoic sandstones in and adjacent to the Santa Maria basin E3

Upper Jurassic and Lower Cretaceous sandstones of the lower part of the Great 

Valley sequence E3

Upper Cretaceous sandstones in the basement of Santa Maria basin and at Point San 

Luis E4

Sandstone knockers in melange of the Franciscan assemblage E6 

Discussion E6 

Conclusions E6 

References cited E7

FIGURES

1. Index map showing location of North Beach section of the Lospe Formation and 

other relevant geographic features E2

2. Comparison of detrital modes of sandstone clasts in the Lospe Formation with other 

in-place sandstones of Mesozoic age in and adjacent to the Santa Maria basin E5

TABLES

1. Point count data for sandstone clasts of the Lospe Formation E4

2. Detrital modes, volume percent, of sandstone clasts in the Lospe Formation E5
Provenance of Sandstone Clasts in the Lower Miocene 

Lospe Formation Near Point Sal, California

By Hugh McLean and Richard G. Stanley

Abstract

Conglomeratic alluvial fan and fan-delta facies of the 

lower Miocene Lospe Formation exposed in seacliffs at 

North Beach (informal name), southeast of Point Sal along 

the central California coast, contain an assemblage of clasts 

chiefly derived from Mesozoic basement rocks. Paleocurrent 

data suggest that these Lospe deposits were derived from 

sources located in the modern offshore.

Although the Lospe Formation at North Beach unconformably overlies ultramafic rocks of the Point Sal ophiolite 

and the Lospe conglomerate beds contain numerous ophiolitic clasts, the beds also contain many nonophiolitic clasts 

including an assortment of quartzofeldspathic and lithic 

sandstones and various colors of chert including red radiolarian chert. Rare clasts include blueschist, nodular lime 

clasts, and felsic volcanic rocks.

Detrital modes, textures, lithic fragment populations, 

and accessory minerals of many of the sandstone clasts suggest a petrologic affinity with rocks of the upper part of the 

Great Valley sequence. Some clasts have the same petrographic characteristics as sandstone (so-called graywacke) 

knockers in melange of the Franciscan assemblage. Clasts 

that resemble the lower part of the Great Valley sequence 

are rare. The assortment of sandstone clasts in the Lospe Formation suggests that in early Miocene time Franciscan melange, the Point Sal ophiolite, and Great Valley sequence 

rocks were uplifted and subaerially eroded. Angular clasts of 

blueschist and red radiolarian chert also suggest a provenance that includes melange of the Franciscan assemblage.

INTRODUCTION

The Lospe Formation is an 830-m-thick sequence of 

sedimentary and associated tuffaceous rocks that unconformably overlie basement rocks of Mesozoic age. Recent 

radiometric ages of tuffs reported by Stanley and others 

(1990 a, b, and 1991) indicate an early Miocene age. The 

type section of the Lospe Formation is located in the Casmalia Hills, an area in west-central California first mapped

Manuscript approved for publication April 1,1993.

in detail by Woodring and Bramlette (1950) (fig. 1). Approximately 140 m of conglomeratic alluvial fan and fandelta facies of the Lospe are well exposed in seacliffs 

along a north-trending section of coast southeast of nearby 

Point Sal where the Lospe Formation unconformably overlies the Point Sal ophiolite. The seacliff outcrops are informally called North Beach in our report. North Beach is 

located approximately 10 km southwest of the Santa 

Maria Valley oil field (fig. 1) and is located within the 

perimeter of Vandenberg Air Force Base (VAFB).

Previous Work

Anderson (1980) studied the sedimentology and composition of the Lospe Formation at several localities in the 

area of the Santa Maria basin, including North Beach, 

which she called "VAFB" section in her report. Her work 

focused on the provenance of Lospe clasts and the tectonic 

implications of clast assemblages. Andersen's pebble 

counts from the basal part of the North Beach section indicated an abundance of ophiolitic debris. Ophiolitic clasts 

included gabbro and red serpentinite (a silica carbonate 

rock that is an alteration product associated with the ophiolitic rocks). Anderson also reported chert, leucogabbro, 

carbonate nodules (called nodular lime clasts in our report), metavolcanic rocks, claystone, and rare blueschist. 

In contrast to the local abundance of ophiolitic debris, 

however, some of the North Beach conglomerate beds 

contain as much as 80 percent sandstone clasts (Anderson, 

1980, p. 185).

Anderson (1980) reported detrital modes of four sandstone clasts from the North Beach section. Two of the clasts 

contained detrital potassium (K) feldspar; the other two 

clasts contained no K-feldspar. The assemblage of sandstone clasts in the basal part of the North Beach section was 

interpreted by her to have been derived from a source other 

than the Franciscan assemblage (Anderson, 1980, p. 136). 

The sedimentary source for clasts from the lower and upper 

parts of the Lospe Formation in the Point Sal area was 

interpreted by Anderson (1980, p. 136) to be "Upper Cretaceous Great Valley sequence-equivalent and (or) Paleogene

Provenance of Sandstone Clasts in the Lower Miocene Lospe Formation Near Point Sal, California El
sedimentary rocks." North-northwest to south-southeast 

oriented trough axes at North Beach were reported by 

Anderson (1980, p. 74, 75). Anderson (1980, p. 69) also 

reported north-northeast to south-southwest oriented cutand-fill channel axes in conglomeratic alluvial fan facies of 

the Lospe on VAFB. Anderson's (1980) paleogeographic 

reconstruction for the Lospe Formation at Point Sal portrayed an eastward-directed alluvial fan system that incorporated sandstone clasts derived from a source located no 

more than 50 km to the southwest (southwest according to 

modern coordinates) at the time of deposition.

Subsequent work by Stanley and others (1990 a, b, 

and 1991) on the age and sedimentology of the Lospe Formation in the Casmalia Hills identified two discrete alluvial fan systems each with a different paleoflow and clast 

assemblage. At the North Beach section they reported 

southeast flow and a mixed-clast assemblage, whereas in 

the Corralitos Canyon (fig. 1) section they reported northeast flow and mainly ophiolitic clasts. The ophiolitic clast 

assemblage at Corralitos Canyon was also noted by 

Anderson (1980), but no paleocurrent data were cited. 

Clast assemblages at North Beach suggested to Anderson

(1980) that the conglomerates of the Lospe Formation 

were derived by subaerial erosion from at least three Mesozoic lithotectonic elements: (1) ophiolitic rocks that lithologically resemble parts of the Point Sal ophiolite of 

Hopson and others (1975), Hopson and Frano (1977), and 

Hopson and others (1981); (2) sandstones that are lithologically similar to those of the upper and lower Great 

Valley sequence of the Santa Maria basin; and (3) a variety of lithologies that occur as knockers in melange of the 

Franciscan assemblage such as blueschist and red radiolarian chert. Rare felsic volcanic clasts were interpreted by 

Anderson (1980, p. 134) as being derived from a fourth 

provenance composed of middle Tertiary rocks.

Woodring and Bramlette (1950) mapped the geology 

of the Casmalia Hills, where some Upper Jurassic sedimentary rocks were called the Knoxville Formation. Lithologically similar (and tentatively age-equivalent) rocks 

along the southern margin of the Santa Maria basin were 

called the Espada and Honda Formations by Dibblee 

(1950). Gray (1980), Hall (1982), and McLean (1991) reported the presence of correlative rock units in the subsurface of the Santa Maria basin.

121* 120°

35° -Corralitos Canyonjf GOF 

Point Sal

0 1.0 2.0 30 KILOMETERS^ 

0 i^ 20 MILES

Figure 1. Index map showing location of North Beach section of the Lospe 

Formation and other relevant geographic features. GOF, Guadalupe oil field; 

SMOF, Santa Maria Valley oil field; OCS-0427, offshore location of Pennzoil 

test well.

E2 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
This report expands on the reconnaissance sedimentary petrology of sandstone clasts reported by Anderson 

(1980) and addresses the petrologic diversity and provenance of such clasts in the Lospe Formation at North 

Beach. Provenance of the Lospe sandstone clasts was determined by comparing the detrital modes and other petrographic characteristics of different clast types with 

outcrops of the lower and upper parts of the Great Valley 

sequence, sandstone knockers in melange of the Franciscan assemblage, Upper Cretaceous rocks in the subsurface 

of the nearby Santa Maria basin, and outcrops of lithologically similar rocks at Point San Luis. Comparisons were 

made using petrographic information on outcrops and core 

material that were published by McLean (1991).

Methods

Pebble- and cobble-size sandstone clasts were collected from the Lospe Formation in the North Beach section 

between 3 m and 82.1 m stratigraphically above its basal 

contact with the Point Sal ophiolite. Samples were selected to represent as many different textures and compositions as could be determined from the study of hand 

specimens. Twenty-seven clasts were selected for thin section study and point counting; table 1 lists the raw point 

count data. Detrital modes that are tabulated in table 2 are 

also illustrated in ternary diagrams in figure 2. In most 

cases, the detrital modes were determined by counting between 300 and 400 framework grains of quartz, feldspar, 

and rock fragments in standard thin sections that had been 

stained for K-feldspar. Point counting followed the methods outlined by Dickinson (1970). The Gazzi-Dickinson 

method of Ingersoll and others (1987) was used to minimize the effects of grain size differences. Unfortunately, 

the Gazzi-Dickinson method requires that polycrystalline 

grains with individual crystals larger than 0.0625 mm be 

counted as a separate component mineral rather than as a 

rock fragment. Consequently, percentages of polycrystalline rock fragments such as those of granitic and (or) metamorphic origin in medium and coarser grained sandstones 

tend to be underrepresented.

PETROGRAPHIC DATA

Comparison of Sandstone Clasts from the 

Lospe Formation with Mesozoic Sandstones 

in and Adjacent to the Santa Maria Basin

Figure 2 compares the detrital modes of Lospe Formation sandstone clasts at North Beach with outcrops of 

the upper and lower parts of the Great Valley sequence 

and sandstones of the Franciscan assemblage. Plots of QtF-L and Qm-F-Lt in this study (fig. 2) differ only slightly 

for most rocks because of low percentages of polycrystalline quartz and chert (note that Qt and Lt both include 

chert and polycrystalline quartz). Ternary plots of volcanic, sedimentary, and metamorphic rock fragments were 

omitted in this study because their sum rarely exceeded 60 

or 70 points per thin section (table 1); only values greater 

than 100 were considered to be statistically significant.

Upper Jurassic and Lower Cretaceous 

Sandstones of the Lower Part of the 

Great Valley Sequence

Rocks of the lower part of the Great Valley sequence 

that crop out around the perimeter of the Santa Maria basin 

consist of sandstone, shale, mudstone, and chert-pebble 

conglomerate and range in age from Late Jurassic (Tithonian) to Early Cretaceous. Partly coeval rocks were mapped 

in the Casmalia Hills as the Upper Jurassic Knoxville Formation by Woodring and Bramlette (1950), whereas rocks 

of an overall similar age exposed on the south and east 

flanks of the Santa Maria basin were called the Espada 

Formation (Late Jurassic and Early Cretaceous) and the 

Honda Formation (Late Jurassic) by Dibblee (1950). The 

Knoxville and Espada Formations in the Santa Maria basin 

comprise petrofacies of the lower part of the Great Valley 

sequence. These rocks conformably overlie radiolarian 

cherty mudstones that form the upper part of the Point Sal 

ophiolite. The Knoxville and Espada sandstones around the 

Santa Maria basin are characterized by high percentages of 

spilitic volcanic rock fragments and chert and by low percentages of quartz and feldspar (fig. 2). K-feldspar abundance rarely exceeds trace amounts in these rocks. 

Plagioclase grains and volcanic rock fragments are relatively unaltered, especially when compared to sandstones with 

similar Q-F-L ratios in the Franciscan assemblage.

The Qt-F-L and Qm-F-Lt diagrams in figure 2 illustrate that the detrital modes of only two clasts of the Lospe 

Formation are lithic rich, which is a characteristic of rocks 

of the lower part of the Great Valley sequence (the undivided Espada Formation of Dibblee (1950) and Knoxville Formation). Table 1 includes data for three outcrop samples 

(L87-2B, 89C-444, and 89C-430) of lower Great Valley 

rocks. The suites of lithic rock fragments and accessory 

minerals indicate that only two Lospe sandstone clasts (L10 and L-14) might have been derived from the Knoxville 

or Espada Formation. In addition, a clast of chert-pebble 

conglomerate was also collected but not point counted; 

chert-pebble conglomerate is a common constituent of the 

lower part of the Great Valley sequence. Accessory grains 

of clinopyroxene (augite) in clast L-10 (table 1, see Cpx) 

are noteworthy because such grains are rare in the rocks of 

the lower part of the Great Valley sequence but were reportProvenance of Sandstone Clasts in the Lower Miocene Lospe Formation Near Point Sal, California E3
Table 1. Point count data for sandstone clasts of the Lospe Formation

[Inferred provenance based on petrographic data: Ku, upper part of the Great Valley sequence (Upper Cretaceous); Kl, lower part of the Great 

Valley sequence (the undivided Espada Formation of Dibblee (1950) and Knoxville Formation) (Upper Jurassic and Lower Cretaceous); OC, 

outcrop sample of Lower Cretaceous sandstone; KJf, knockers in melange of the Franciscan assemblage (Jurassic and Cretaceous). Qm, monocrystalline quartz; Qp, polycrystalline quartz; plag., plagioclase; K-spar, potassium feldspar; biot., biotite; muse., muscovite; L-unid., unidentified lithic 

(rock) fragments; L-meta., metamorphic rock fragments; L-sed., sedimentary rock fragments; L-volc., volcanic rock fragments; epid., epidote; 

opq., opaque grains; Ic, lime clasts; cpx, clinopyroxene; sph, sphene; cmt., intergranular cement and (or) matrix; NC, not counted]

Sample 

no.

89C-409A

89C-409B

89C-409C

89C-409D

89C-410C

L-5

L-6

L-7

L-9

L-ll

L-12

L-15

L-16

L-18

L-21

L-22

L-10

L-14

L87-2B

89C-444

89C-430

89C-410A

89C-410B

L-l

L-3

L-4

L-8

L-17

L-19

L-20

Infer, 

prov.

Ku

Ku

Ku

Ku

Ku

Ku

Ku

Ku

Ku

Ku

Ku

Ku

Ku

Ku

Ku

Ku

Kl

Kl

K1.OC

K1.OC

K1.OC

KJf

KJf

KJf

KJf

KJf

KJf

KJf

KJf

KJf

Qm

123

126

97

114

100

116

132

172

100

134

117

129

127

115

116

89

14

19

4

70

50

172

99

45

135

55

153

124

82

50

Qp Chert

5

12

3

11

4

5

20

14

13

13

7

7

3

12

10

7

9

11

3

31

31

36

114

0

21

19

10

10

26

21

0

0

0

0

0

2

0

0

0

3

3

1

2

0

0

0

0

2

0

0

18

2

0

0

1

1

0

3

12

6

Plag.

155

172

161

136

144

138

159

99

173

180

212

107

165

162

176

110

64

23

51

48

21

109

60

21

129

67

174

148

90

49

Kspar

34

23

34

29

35

35

30

29

18

24

49

21

23

21

30

7

0

1

0

7

6

0

0

0

0

0

0

0

0

0

Biot. Muse.

21

5

7

10

14

19

32

31

30

10

19

16

32

20

34

19

0

0

0

3

0

15

0

7

5

1

44

18

5

4

2

3

0

0

4

12

4

7

6

11

2

1

3

7

3

5

0

0

0

2

0

2

0

3

26

4

9

4

0

0

1^

unid.

24

16

35

19

28

37

31

52

44

51

27

19

21

35

40

35

186

65

125

169

73

59

65

7

51

56

43

30

93

82

Lmeta.

12

10

11

19

0

8

17

10

17

25

22

25

15

7

8

17

41

29

6

36

67

51

15

0

8

17

4

10

2

6

lr

sed.

7

0

2

2

10

14

0

1

1

0

0

3

1

1

4

3

0

0

0

4

20

19

5

3

0

0

2

0

0

0

L- Epid. Opq. 

vole.

0

3

4

0

6

0

4

0

5

5

3

12

6

3

0

9

80

78

111

11

58

2

46

0

0

2

0

2

19

19

1

3

5

6

2

3

4

11

11

7

5

2

4

2

0

4

12

7

8

0

0

0

0

0

5

8

3

3

6

4

2

2

1

0

1

0

3

4

0

1

0

4

0

0

3

2

4

0

0

3

0

0

6

4

3

1

0

5

2

2

Other

0

0

0

0

0

0

0

0

0

0

0

lc,5

lc,2

sph,2

lc,4

0

cpx, 3

0

cpx,2

lc,7

lc,3

0

lc,ll

0

0

0

0

hbl,l

lc,2

0

Cmt./ 

matrix

35

54

63

71

74

162

44

NC

83

73

64

71

33

40

85

105

111

108

56

89

59

37

48

42

53

43

73

58

80

49

Total 

count

421

429

423

417

422

551

480

430

501

537

530

423

437

427

513

412

524

343

366

480

406

504

469

132

437

274

515

416

419

292

ed locally in the Espada Formation along the southern flank 

of the Santa Maria basin by McLean (1991).

Upper Cretaceous Sandstones in the Basement 

of Santa Maria Basin and at Point San Luis

K-feldspar-bearing, quartzofeldspathic (arkosic) sandstones in core material from exploratory wells in parts of 

the Santa Maria basin basement were reported by Gray 

(1980). Palynomorphs associated with the sandstones in 

some wells were reported to be of Late Cretaceous age by 

Gray (1980). McLean (1991) reported that Upper Cretaceous sandstones were penetrated by several wells in the 

Santa Maria Valley and Guadalupe oil fields (fig. 1). The 

Upper Cretaceous sandstones in and around the Santa 

Maria basin are characterized petrographically by abundant quartz and plagioclase (McLean, 1991). McLean 

(1991, p. 6) reported that 21 Upper Cretaceous subsurface 

rocks averaged 030^40^30' whereas 27 outcrop sandstones 

averaged approximately Q^44^20- K-feldspar (mainly orthoclase and microcline) ranges from 5 to 10 percent, biotite ranges from 2 to 5 percent, and detrital epidote from 1 

to 2 percent. Lithic rock fragments typically include intermediate volcanic rocks, quartz mica schist, and granitoid 

rocks. Texturally, the rocks are characterized by subangular to subrounded grains that are moderately sorted and 

framework supported. The rocks contain little primary argillaceous intergranular matrix, although pseudomatrix 

formed from lithic rock fragment alteration is abundant in 

some rocks.

Surface rocks that are almost identical petrographically to the Upper Cretaceous subsurface sandstones in the 

Santa Maria basin crop out at Point San Luis (fig. 1). Although fossils have not been reported in the isoclinally 

folded strata at Point San Luis, previous workers such as 

Lee-Wong and Howell (1977), Smith (1978), and Gray 

(1980) have interpreted these rocks as being compositionally consistent with sandstones of the Great Valley sequence of Late Cretaceous age.

Detrital modes shown in figure 2 illustrate that Qt-FL and Qm-F-Lt for 16 Lospe clasts either overlap or are

E4 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
Table 2. Detrital modes, in volume percent, of 

sandstone clasts in the Lospe Formation

[Qt, monocrystalline and polycrystalline quartz and chert; F, 

feldspar-plagioclase (P) and K-feldspar (K); L, aphanitic lithic rock fragments excluding polycrystalline quartz and chert; 

Qm, monocrystalline quartz; Lt, aphanitic lithic rock fragments including polycrystalline quartz and chert. Ku, upper 

part of the Great Valley sequence (Upper Cretaceous); Kl, 

lower part of the Great Valley sequence (the undivided Espada 

Formation of Dibblee (1950) and Knoxville Formation) (Upper Jurassic and Lower Cretaceous); KJf, Franciscan assemblage (Jurassic and Cretaceous). ND, not determined]

Sample No.

89C-409A

89C-409B

89C-409C

89C-409D

89C-410C

L-5

L-6

L-7

L-9

L-ll

L-12

L-15

L-16

L-18

L-22

L-21

Qt,F,L

36,53,11

39,55,06

28,56,16

39,52,09

32,54,14

35,49,16

39,48,13

49,34,17

30,51,19

34,47,19

28,60,12

41,39,20

36,52,12

36,51,13

34,42,24

32,53,15

Qm,F,Lt

35,53,12

39,54,07

28,56,16

36,52,12

30,54,16

33,49,18

34,48,18

46,34,20

27,51,22

31,47,22

27,60,13

39,39,22

35,52,13

32,51,17

32,42,26

30,53,17

Qm,P,K Inferred 

provenance

39,50,11

39,54,07

33,55,12

41,49,10

36,52,12

40,48,12

41,50,09

57,33,10

34,59,07

40,53,07

31,56,13

50,42,8

40,52,8

39,54,7

43,53,4

36,55,9

Ku

Ku

Ku

Ku

Ku

Ku

Ku

Ku

Ku

Ku

Ku

Ku

Ku

Ku

Ku

Ku

Ku mean- 34,51,15 33,50,17 40,51,9

89C-410A 

89C-410B 

L-l 

L-3 

L-4 

L-8 

L-17 

L-19 

L-20

KJf mean 

L-10 

L-14

47,25,28 

53,15,32 

59,28,13 

45,38,17 

ND 

42,45,13 

41,46,13 

36,28,36 

33,21,46

44,31,25

06,16,78 

13,10,76

39,25,36 

25,15,60 

59,28,13 

39,38,23 

ND 

40,45,15 

38,46,16 

25,28,47 

21,21,58

36,31,33

04,16,80 

08,10,88

61,39,0 

62,38,0 

68,32,0 

51,49,0 

ND 

47,53,0 

46,54,0 

48,52,0 

51,49,0

54,46,0

18,82,0 

ND

KJf 

KJf 

KJf 

KJf 

KJf 

KJf 

KJf 

KJf 

KJf

Kl 

Kl

Qm

Qm

^ Figure 2. Comparison of detrital modes of sandstone 

clasts in the Lospe Formation with other in-place sandstones 

of Mesozoic age in and adjacent to the Santa Maria basin. 

Qt, monocrystalline quartz, polycrystalline quartz, and chert; 

F, plagioclase and potassium feldspar; L, lithic rock fragments; Qm, monocrystalline quartz; Lt, lithic rock fragments 

including polycrystalline quartz and chert; P, plagioclase 

feldspar; K, potassium feldspar.

EXPLANATION

Lospe Formation (clasts)

Upper Cretaceous sandstones (well-core material) 

(upper Great Valley sequence)

Point San Luis sandstones (outcrop)

Espada Formation of Dibblee (1950)

and Knoxville Formation, undivided (outcrop) 

(lower Great Valley sequence)

Franciscan melange sandstone knockers (outcrop)

Provenance of Sandstone Clasts in the Lower Miocene Lospe Formation Near Point Sal, California E5
contiguous with the more quartzofeldspathic samples of 

Upper Cretaceous sandstones and the Point San Luis 

rocks. Features common to both clasts and in-place Upper 

Cretaceous rocks include grain shapes and grain compaction, degrees of plagioclase alteration, types of lithic fragments, and accessory minerals (notably epidote). Grains 

range from angular to subrounded and are moderately to 

poorly sorted with tight and often interpenetrating grain 

boundaries. Twinned and untwinned plagioclase vary 

from clear grains with no visible alteration to moderately 

and heavily mottled grains that are almost entirely replaced by either sericite, calcite, or, in rare cases, epidote. 

The variation in plagioclase alteration and the presence of 

detrital K-feldspar and detrital epidote suggest that much 

of the plagioclase alteration might be inherited from altered (granitic?) source rocks rather than being products 

of post-burial diagenesis. K-feldspars often are partly albitized, a fact which suggests that both clasts and in-place 

rocks shared a common burial history. Lithic fragments 

common to both Lospe clasts and in-place Upper Cretaceous sandstones are characterized by abundant aphanitic 

grains of unidentifiable origin; granitic grains composed 

of various combinations of quartz, plagioclase and Kfeldspar; quartz-mica schist; weakly foliated polycrystalline quartz tectonite; and rare felsic and spilitic volcanic 

rocks.

Sandstone Knockers in Melange 

of the Franciscan Assemblage

Detrital modes of sandstone and metasandstone 

knockers (tectonic phacoids) in melange of the Franciscan 

assemblage from outcrops along the eastern and northern 

margins of the Santa Maria basin were reported by 

McLean (1991); their detrital modes are illustrated in figure 2. The Franciscan sandstone (so-called graywacke) 

knockers studied by McLean (1991) vary widely in composition from rocks composed mainly of altered mafic volcanic fragments to rocks that contain abundant quartz and 

plagioclase; none of the Franciscan sandstone knockers 

contained K-feldspar, regardless of their quartz and feldspar content. McLean (1991) reported textural evidence 

that suggested some of the Franciscan sandstones had 

originally contained K-feldspar, which had altered to albite 

(probably during subduction). The Franciscan melange 

sandstone knockers are typically crisscrossed by veinlets 

of quartz and (or) calcite, contain albitized feldspar, and 

have a matrix composed of a mix of chlorite and quartz. 

Lithic rock fragments have a bleached or washed-out appearance in plane light, which may be the result of pervasive chloritization. Clasts in the Lospe Formation that 

share the aforementioned petrographic characteristics are 

interpreted as being probably derived from Franciscan melange sandstones (table 1).

DISCUSSION

Overlapping and (or) gradational detrital modes that 

are illustrated in figure 2, as well as similar textures and 

common accessory minerals, indicate that most of the 

sandstone clasts in the Lospe Formation at North Beach 

were derived from either the upper part of the Great Valley sequence or from knockers in melange of the Franciscan assemblage. A few clasts were derived from the lower 

part of the Great Valley sequence.

McLean (1991) noted that the Q-F-L modes of sandstones in melange of the Franciscan assemblage overlapped those of most of the lower part of the Great Valley 

sequence and the more lithic-rich part of the upper Great 

Valley sequence. Compositional and textural similarities 

suggested to McLean (1991) that the Franciscan rocks 

might be composed of subducted and accreted lower Great 

Valley sequence and subducted and accreted lower parts of 

the upper Great Valley sequence. The age of the subducted 

sandstones in the Franciscan assemblage of the Santa 

Maria basin area is inferred to range from Late Jurassic to 

early Late Cretaceous (pre-Cenomanian?). Geographically, 

most of the Franciscan melange sandstones sampled by 

McLean (1991) were from outcrops located near the eastern and northern margins of the Santa Maria basin and 

possibly represent an older (landward) part of the accretionary complex than that part which lies seaward of the 

modern coastline. Relatively high percentages of quartz 

and feldspar in Lospe clasts that are interpreted to have a 

Franciscan provenance were possibly derived from younger melange that contained subducted Campanian and 

younger Great Valley sequence rocks.

CONCLUSIONS

As was noted by Anderson (1980), clasts in the Lospe 

Formation at North Beach reflect erosion of at least three 

Mesozoic lithotectonic elements as well as pre-Lospe felsic volcanic rocks of probable middle Tertiary age (Cambria Felsite or correlatives?). Radiometric ages of tuff 

reported by Stanley and others (1991) indicate an early 

Miocene age for the Lospe Formation. Much of the coarse 

Lospe conglomeratic detritus was derived from nearby 

masses of ophiolitic rocks. Some material, however, such 

as blueschist, red radiolarian chert, and altered sandstone 

(so-called graywacke) was most likely derived from Franciscan melange. Many of the sedimentary clasts in the 

Lospe Formation at North Beach near Point Sal were derived from the upper part of the Great Valley sequence and 

from melange of the Franciscan assemblage. Anderson 

(1980) reported that some of the sandstone clasts might 

have been derived from a Paleogene source, and this remains as a possibility because Paleocene and Eocene 

sandstones in the southern Coast Ranges and in the WestE6 Evolution of Sedimentary Basins/Onshore Oil and Gas Investigations Santa Maria Province
ern Transverse Range are indistinguishable petrographically from those of Upper Cretaceous age. Paleogene 

sandstones, however, are unknown in areas that have been 

penetrated by drilling in the basement of both the onshore 

and offshore parts of the Santa Maria basin (McCulloch, 

1987), except for Eocene strata that were reported by Vedder and others (1991) in the Pennzoil OCS 0427 test well, 

which is located about 11 km west of Purisima Point and 

15 km south-southwest of Point Sal. Vedder and others 

(1991, p. 947) cited an unpublished paleontologic report 

that indicated the Pennzoil well had penetrated a 27-m 

section of white micaceous sandstone and microfossil-rich 

mudstone and shale, which was "possibly equivalent to 

part of the Anita Shale."

Southeast-oriented paleocurrent indicators (as determined by modern coordinates) in the North Beach section 

suggest that Mesozoic basement now located offshore 

probably consists of ophiolite, Franciscan melange, and 

rocks of the lower and upper parts of the Great Valley sequence. The Pennzoil well data suggest that at least remnants of Paleogene rocks exist in the offshore Santa Maria 

basin.

REFERENCES CITED

Anderson, D.S., 1980, Provenance and tectonic implications of 

mid-Tertiary nonmarine deposits, Santa Maria basin and vicinity, California: Los Angeles, University of California, 

M.S. thesis, 218 p.

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

County, California: California Division 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.

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

Santa Maria basin, Santa Barbara and San Luis Obispo 

counties, California: San Diego, Calif., San Diego State University, M.S. thesis, 78 p.

Hall, C.A., Jr., 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.

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 Mineral Industries Bulletin 95, 

p. 161-183.

Hopson, C.A., Frano, C.J., Pessagno, E.A., Jr., and Mattinson, 

J.M., 1975, Preliminary report and geologic guide to the Jurassic ophiolite near Point Sal, Southern California Coast: 

Geological Society of America, Cordilleran Section Field 

Trip Guidebook 5, 36 p.

Hopson, C.A., Mattinson, J.M., and Pessagno, E.A., Jr., 1981, 

Coast Range ophiolite, western California, in Ernst, W.G., 

ed., The geotectonic development of California (Rubey volume 1): Englewood Cliffs, N.J., Prentice-Hall, p. 418-510.

Ingersoll, R.V., Bullard, T.F., Ford, R.L., Grimm, J.P., Pickle, 

J.D., and Sares, S.W., 1987, 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.

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, v. 6, p. 353-401.

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.

Smith, G.W., 1978, Stratigraphy, sedimentology, and petrography 

of the Cambria slab, San Luis Obispo County, California: 

Albuquerque, University of New Mexico, M.S. thesis, 123 p.

Stanley, R.G., Johnson, S.Y., Obradovich, J.D., Tattle, M.L., Cotton Thornton, M.L., Vork, D.R., Filewicz, M.V., Mason, 

M.A. and Swisher, C.C., III, 1990a, Age and depositional 

setting of the type Lospe Formation (Lower Miocene), Santa 

Maria basin, central California [abs.]: American Association 

of Petroleum Geologists Bulletin, v. 74, p. 770-771.

 1990b, Age, facies, and depositional environments of the 

lower Miocene Lospe Formation, Santa Maria basin, central 

California [abs.], in Carter, L.M.H., ed., USGS research on 

energy resources 1990, Program and Abstracts: U.S. Geological Survey Circular 1060, p. 78-79.

Stanley, R.G., Johnson, S.Y., Tuttle, M.L., Mason, M.A., Swisher, C.C., III, Cotton Thornton, M.L., 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 [abs.]: American 

Association of Petroleum Geologists Bulletin, v. 75, p. 382.

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.

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.

Provenance of Sandstone Clasts in the Lower Miocene Lospe Formation Near Point Sal, California E7

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