CALIFORNIA STATE UNIVERSITY NORTHRIDGE
STRATIGRAPHY AND DEPOSITIONAL ENVIRONMENTS OF THE VAQUEROS FORMATION, CENTRAL SANTA MONICA MOUNTAINS, CALIFORNIA
A thesis submitted in partial satisfaction of the requirements for the degree of Master of Science in Geology by Juli G. Oborne
January, 1987 is approved:
Dr. A.~ritsche (CSUN), Chair
California State University, Northridge
ii To Mark and Alex whose constant encouragement, love, and impending arrival made this possible
iii _, .
ACKNOWLEDGMENTS
I would like to thank my thesis chairman, Dr. Eugene
Fritsche for all his help and guidance throughout the project. I am appreciative of the participation of Dr.
Frank Kilmer in ·this study. Special thanks goes to Dr.
Richard Squires for critically reading the thesis and making helpful comments. Dr. Eugene Fritsche also aided in the macrofossil identification. Dr. Judith Smith, U. S.
Geological Survey, provided assistance in macrofossil iden tifications and also provided many helpful suggestions. I would like to thank Jeff Kennedy, of the Minerals Manage ment Service, for first introducing me to the area.
A very special thanks go to all of my field assis tants, Mark Oborne, Steve Anderson, and Mike Blundell.
These people were kind enough to offer their time, coopera tion, and much encouragement to help complete the field portion of this study.
I would like to thank Hark Oborne and Barbara
Henschel for all of their help with the typing of the thesis. I am especially grateful for the continual moral support and encouragement given to me by my husband and parents.
iv r .
TABLE OF CONTENTS
Page
Abstract...... xi
In trod uc tion...... 1
Purpose and Location...... 1
Previous Work...... 4
Geologic Setting...... 5
Stratigraphy...... 5
Age and Correlation...... 11
StruGture...... 12
Lithosome Descriptions and Process Interpretations.. 22
Methods...... 22
Introduction to Lithosomes...... 23
Lithosome Descriptions and Interpretations...... 25
Parallel-Laminated Siltstone (A)...... 25
Description...... 25
Interpretation.. . . • ...... • . . . . . 2 7
Parallel-Laminated Fine-Grained Sandstone (B)... 28
Description...... 28
Interpretation...... 30-
Structureless Fine-Grained Sandstone (C)...... 32
Description...... 32
Interpretation...... • . . . . . 33
Cross-Bedded Sandstone (D)...... 34
Description...... 34
Interpretation...... • ...... 39
v Page
Parallel-Laminated Coarse-Grained Sandstone (E). 39
Description ...... ·...... 39
Interpretatign...... • ...... 41
Structureless Coarse-Grained Sandstone (F)...... 42
Description...... 42
Interpretation...... • ...... 43
Bioturbated Fine-Grained Sandstone (G)...... 44
Description ...... -·... . . 44
Inter pretatign...... • . • ...... 47
Parallel-Laminated Medium-Grained Sandstone (H). 48
])escript ion ...... ~ ...... • ...... 48
In terpreta ti0n...... • . . . . 49
Interbedded Red-and-Green Siltstone (I)...... 50
Description. Cl...... 50
Interpretation...... 52
Paleogeographic Interpretation...... 53
Paleoenvironmental Analysis...... 53
Introduction. • ...... • ...... 53
Offshore/Prodelta Environments...... 56
Transition Zone Environments...... 58
Shoreface/Delta-Front Environments...... 59
Distributary Mouth Bar Environments...... 61
Foreshore Environments...... 62
Backshore Environments...... 63
Lower Delta-Plain Environments...... 65
vi Page
Upper Delta~Plain Environments...... 66
Paleocurrent Analysis...... 69
Provenance. . . . • ...... 7 2
Paleogeographic Map and Conclusions...... 76
References ...... 83
Appendix...... 92
vii LIST OF ILLUSTRATIONS
Figure Page
1. Map showing the study area in the central
Santa Monica Mountains...... 2
2. Map of the central Santa Monica Mountains
showing Vaqueros Formation outcrops...... 3
3. Summary of the stratigraphic nomenclature
used by previous workers in the Santa Monica
Mountains...... 6
4. Composite vertical diagram of the sedimen-
tary rock units of the central Santa Monica
Mountains ...... 7
5. Age correlation chart for the Vaqueros
Formation in the central Santa Monica
Mountains...... 13
6. Approximate location of the "detachment
surfaces" in the central Santa Monica
Mountains ...... 2-1
7. Photograph of swales and cross bedding in
the fine-grained, cross-bedded sub-
lithesome (D)...... 37
8. Photograph of the bioturbation in the
b~oturbated, fine-grained sandstone
lithesome (G)...... 46
9. Lateral facies diagram of the Vaqueros
Formation in the study area ...... 54
viii Page
10. Study area interpreted epositional environ-
ments and associated lithosomes...... 57
11. Paleocurrent data from the Vaqueros
Formation in the central Santa Monica
Mountains...... 71
12. Ternary diagrams showing the composition of
the sandstones in the Vaqueros Formation.... 73
13. Pre-rotation geometry in southern California
about late Ol~gocene time...... 77
14. Possible orientations of the shoreline
during the deposition of the Vaqueros
Formation in the Santa Monica Mountains ..... 78
15. Paleogeographic map ...... •...... 80
Table Page
1. List of macrofossils collected from the
Vaqueros Formation by the United States
(;eological Survey...... • ...... 15
2. List of microfossils collected from the
Vaqueros Formation by the United States
(;eological Survey...... 19
ix Plate
1. Measured stratigraphic columns of the
Vaqueros Formation, central Santa Monica
Mountains, California ...... •.••...... (in pocket)
2. Petrology of sandstone samples collected
during this study from the Vaqueros
Formation .....•...... •. (in pocket)
X p '
ABSTRACT
STRATIGRAPHY AND DEPOSITIONAL ENVIRONMENTS OF THE
VAQUEROS FORMATION, CENTRAL SANTA MONICA MOUNTAINS,
CALIFORNIA
by
Juli G. Oborne
Master of Science in Geology
The Vaqueros Formation along the southern flank of the central Santa Monica Mountains is a sequence of lower upper Oligocene through lower middle Miocene sandstone and siltstone that records a rapid transgression due to rising sea level and a subsequent progradation of a deltaic system.
Study of these rocks reveals nine lithosome units that are process controlled. The lithosomes, from lower to upper, include: a parallel-laminated siltstone (A); parallel-laminated, fine-grained sandstone (B): structure less, fine-grained sandstone (C); cross-bedded sandstone
(D); parallel-laminated, coarse-grained sandstone (E); structureless, coarse-grained sandstone (F); bioturbated,
xi <1 '
fine-grained sandstone (G); parallel-laminated, medium- grained sandstone (H); and interbedded red-and-green siltstone (I).
Provenance and paleocurrent data are scarce, but source terrains that contributed high-grade metamorphic and reworked sedimentary clastic material to the study area are indicated. The river that supplied sediment probably flowed from a northern or eastern direction. There are two possible orientations of the shoreline: one that trended north-south and another that trended east-west. A river- dominated deltaic depositional system existed in the east ern portion of the study area, whereas wave-dominated deposition along an erosional coastline took place in the western region.
The western stratigraphic sections - San Nicholas
Canyon, Encinal Canyon, and Latigo Canyon contain strictly marine environments from offshore to backshore. A large amount of sediment from increasing tectonics caused the shoreline to prograde and the rocks generally reflect a shallowing upward from the bottom to the top of the strati graphic sections. The eastern sections Corral Canyon,
Puerco Canyon, Malibu Canyon, and Piuma Road - contain rocks which also reflect a general shallowing upward, however, this is due to the progradation of a River- dominated deltaic system. The inferred environments deter- mined from these rocks are prodelta, delta front, and
xii lower and upper delta plain. Although it is difficult to distinguish between the two, the deltaic and marine strandline deposits probably interfinger with each other between Corral and Latigo Canyons.
During the early late Oligocene there was a rapid transgression of the sea. In the west, offshore and shoreface sediment of the Vaqueros was deposited unconfor- mably on the nonmarine Sespe Formation. At the same time, deltaic deposits of the Vaqueros and fluvial deposits of the Sespe were interfingering in the east. The greatest extension of the transgression was just east of the Puerco
Canyon section. Continued subsidence and deposition al- lowed a thick section of sand to accumulate. Although sea levels continued to rise, the delta prograded due to the high sedimentation rate. After the early medial Miocene, uplift was followed by erosion and the Topanga Canyon For mation was deposited unconformably onto much of the
Vaqueros.
xiii INTRODUCTION
PURPOSE AND LOCATION
The purpose of this thesis is to examine the stratig raphy of the upper Oligocene through lower middle Miocene
Vaqueros Formation exposed in the central Santa Monica
Mountains, California, in order to determine the Vaqueros environments of deposition and to reconstruct the paleo geography of the study area during Vaqueros deposition.
The Santa Monica Mountains trend along the southern margin of the western Transverse Ranges and extend for ap proximately 80 km across portions of Los Angeles and Ven- tura Counties (Fig. 1). The mountains are bounded by the
Malibu Coast fault to the south, and the Simi Hills and San
Fernando Valley to the north. Westward the range ter- minates abruptly at the Oxnard Plain. The study area is approximately 25 km in length and is within the Malibu
Beach, Point Dume, and Triunfo Pass 7.5-minute quadrangles,
Los Angeles County, California (Fig. 2). Although inter rupted by cross-cutting faults, the Vaqueros Formation trends east-west across the southern flank of the central
Santa Monica Mountains and forms a disrupted but continuous band of outcrops (Fig. 2). Southerly draining canyons and fire-road cuts provide good exposures.
1 I I s-411; '-..., ''-..., 41VDf?24s ~\~ ~) --- SESPE?::P.EiK--', ~\~ ---- ) ;..>.\Cl ~~ wESTERN\ T" lt A I /~~ ~V IE R SE ,__,_.------/ . 4&1?1f:z / s!>if' aft' •"'"' ' \ l{A..Jvoss \ // \ ___ // ) ZONE
OXNARD SIMI HILLsJ I PLAIN I ' I I ', SANTA MONICA FAULT ,'
LO~ ANGELES // I ,,~1 \ dl:t) FIGUREAREA OF 2 - ', /-.p- ~~~~i~~~~~~'~ ) ,- 1 { il j 1cl Sanla Monica I h / / r-;,'t' Bay )
Q IQ 2Q 30 MILES ' l I I I I I I 0 IQ 20 30 KLOMETERS
Figure 1. Map showing location of Central Santa Monic~ Mountains.
N THOUSAND OAKS lliUAORANGLE / CALABASAS \. \ / "UACflANil_E " / ~--...:! I / "\ 'y' \ Cfly / I \ / - .... ,Dr> (Os / Los~..._ ..._ ~ltt-~'-Es / ,£ Cf!L~s co.- ...... / )w-~ /
POJNT IJJME MALIBU BEACH TOPANGA ' / lii.UACflANGLE lliUADRANGLE (\Uii:f!ANGLE } I _..)_,-...... _ / I !w ,/ X ~o ~-'r 1.5 ,-' 1 I r, lr ~<{o ".1--';-;"?~ I r- I -~'9' I ) I l}~r••·•·~(/~~ I -, )
")
I I \
~~THRUST FAULTS NORMAL FAULTS 0 1 2 3 4 5 MILES PO[NT DUME I I I I I I VAQUEROS FORMATION 0 l ~ ~ l ~ ~ t KILOMETERS OUTCROPS
Figure 2. Map of the central Santa Monica Mountains showing Vaqueros out crops (stipled pattern) .. Geology modified from Jennings and Strand (1975).
w 4
PREVIOUS WORK
Geologic study of the Santa Monica Mountains has been ongoing for many years during which time much of the ran- gehas been mapped in detail. Eldridge and Arnold (1907) studied a portion of the Ventura Basin and assigned the name "Vaqueros Sandstone" to lower Miocene rocks which possessed Turritella inezana macrofauna. Kew (1923, 1924) mapped, in greater detail, an area which included the Santa
Monica Mountains, and further defined the stratigraphy of the Vaqueros Formation. Soper ( 1938) mapped the central portion of the Santa Monica Mountains. Exposures of the
Vaqueros east of Topanga Canyon were mapped by Hoots
(1931). Durrell (1954) published a preliminary map of the entire Santa Monica Mountain range and subsequently sum- marized the geology and stratigraphy of the area (Durrell,
1956). Detailed mapping projects in the study area were undertaken by the following Master's students at University of California, Los Angeles: Bass (1960) mapped the western portion of the Point Dume quadrangle, Carter (1958) studied the Malibu Bowl fault area, Guynes (1959) mapped the Tri- unfo Pass area, and a portion of his map was used as a geologic reference map in this study.
More recently, a comprehensive stratigraphic study and synthesis of the strata exposed in the central Santa
Monica Mountains was done by workers of the United States
Geological Survey {Campbell and others, 1970; Yerkes and 5
others, 1971; Yerkes and Campbell, 1979, 1980). The most recent studies (Yerkes and Campbell, 1979, 1980) include detailed geologic mapping and a regional stratigraphic syn thesis of the Tertiary-aged formations exposed in the range. A thorough paleoenvironmental analysis of these rocks, however, has not been accomplished. A summary of the history of stratigraphic nomenclature in the study area is shown on Figure 3.
GEOLOGIC SETTING
Stratigraphy
In general, the central Santa Monica Mountains con tain rocks that range in age from Jurassic to Quaternary.
Nomenclature of the Tertiary rock units in the area has recently been updated by Yerkes and Campbell (1979). They describe a section of Cretaceous and younger rocks that rest upon the Jurassic Santa Monica Slate, in most of the area, and on a quartz diorite pluton in the most eastern part.
The rocks record several transgressions and regres sions, the first of which began in the Late Cretaceous.
From the Late Cretaceous up to the late Miocene, there was nearly continuous marine and nonmarine deposition in the
Santa Monica Mountains area. Over 3,750 m of sediment were deposited during this time. Figure 4 shows the formations that occur in the study area and illustrates the strati- HOOTS. 1"131 I SOPER. 1"138 SYSTEM I SER[ES KOI, 1"!24 SANTA MIJ'l[CA MTS.
w I T(PANGA I TCJ'ANGA I TOPANGA ...1 Cl TIFANGA GROUP Cl >- ...... I < I INTRUSIVE __5. I FCAMATION a: I :I: ([ I~ ..... I- -~, ~ ~ ~ I TOPANGA a: V~tlERoS Iii~~.:::~~ PlUM A w VA~EROS VA~EROS VA~UEROSI NON- !FORMATION' I- SANOSTIJ'lE FCAMATION _. AND I CAN[ELSOM ...... _ MARINE HEH!ER~ -F.t" ----- ::::: UJz SESPE C?l UJ I SESPE I SESPE I SESPE u SESPE 0 Cl FORMAHONS ...... FORMATION FORMAT[ON ...1 I FCAMA T I il'l I I FORMAHON I 0 Figure 3. A summary of the stratigraphic nomenclature in the Santa Monica Mountains (modified from Yerkes and Campbell, 1979).
0\
-:;; TR[UNFO PASS CANYON PO[NT DUME ~UAORANGLE MAL[BU BEACH ~UADRANGLE ~UADRANGLE ANGLE w E nt:i J>("") nrn nr nn n-o z:c(J) J>Z DJ> J>o J>C -
EXPLANATION
500 M
Ts1 I ,~ K1; IT uno Con yon F m, Figure 4. Composite vertical diagram of the sedimentary rock units in the central Santa Monica Mountains. Shows approximate locations of measured sections (modified from Yerkes and Campbell, 1979).
-....! 8
graphic relationships between them. This study adheres to the stratigraphic nomenclature of Yerkes and Campbell
(1979).
The Sespe Formation of Oligocene and, in part,
Miocene age conformably underlies the Vaqueros Formation in the eastern part of the study area and unconformably un- derlies the Vaqueros in the western part. The Sespe was deposited in a fluvial environment (Yerkes and Campbell,
1979) and in general consists of red, fine- to coarse- grained sandstone and interbedded conglomerate. In the study area, no fossils were found in the Sespe Formation.
Complete stratigraphic sections in the Santa Monica Moun- tains are rare due to faulting. The Piuma Member of the
Sespe Formation is named for a section of nonmarine rocks that conformably underlie and overlie a tongue of Vaqueros
Formation sandstone (Fig. 4). These rocks are exposed along Piuma Road in the Malibu Beach Quadrangle.
In their stratigraphic study, Yerkes and Campbell
(1979) have reserved the name "Vaqueros Formation" for the sequence of strata which lies above the Sespe Formation and below the Topanga Group. As shown on Figure 3, the
Vaqueros Formation has been treated differently by various workers in the Santa Monica Mountains. The most recent work of Yerkes and Campbell (1979) recognizes Vaqueros un differentiated in the eastern part of the central Santa
Monica Mountains and two members of the Vaqueros Formation 9
in the westernmost portion. The Vaqueros undifferentiated consists primarily of fine- to coarse-grained sandstone.
The lower of the two members in the west, the Daniel son Member, consists mostly of dark siltstone and occurs in the Point Dume and Triunfo Pass quadrangles (Fig. 4) and farther west. It was named by Sonneman (1956) for ex posures of the Vaqueros Formation on the Danielson Ranch.
Sonneman reports that the dark siltstone and claystone of the Danielson contain Turritella inezana which is charac- teristic of the West Coast provincial molluscan "Vaqueros
Stage".
The San Nicholas Member is composed of a thick sandstone sequence which conformably overlies the Danielson
Member. The San Nicholas Member is exposed west of Trancas
Canyon. The name San Nicholas Member was proposed by
Yerkes and Campbell for a sandstone sequence which lies stratigraphically above the Danielson at San Nicholas
Canyon in the eastern part of the Triunfo Pass quadrangle.
This member is reported to contain Turritella inezana and
Turritella ocoyana at various localities in the Point Dume quadrangle (Yerkes and Campbell, 1979).
Field work in the study area indicates that the
Vaqueros Formation stratigraphy proposed by Yerkes and
Campbell (1979) is applicable. The Danielson Member, as described by Yerkes and Campbell (1979), is easily mappable in the field because of its distinct lithology difference 10
from the overlying San Nicholas Member. The difference be
tween the San Nicholas Member and the Vaqueros Formation undifferentiated is more difficult to determine in the field. The Vaqueros Formation undifferentiated as noted by
Yerkes and Campbell, contains interbeds of siltstone, whereas the San Nicholas Member is more massive with very few interbeds.
The Topanga Canyon Formation unc on£ arm ably aver 1 ies the Vaqueros in the study area. This formation is divid~d into several members by Yerkes and Campbell (1979). Be tween Trancas and Malibu Canyons, undivided Topanga Canyon
Formation overlies the Vaqueros. It consists of medium- to coarse-grained, pebbly sandstone and has a basal con- glomerate. West of Trancas Canyon, the Encinal member of the Topanga Canyon Formation is a dark gray siltstone and conformably overlies the Vaqueros. There may be some evidence of discordance, but poor exposure makes it dif ficult to distinguish between a local unconformity or a fault contact.
Miocene-aged volcanic flows, dikes, and sills of basalt and andesite are common in the study area. The val- canics comprise a stratigraphic sequence which Yerkes and
Campbell (1979) have named the "Conejo Volcanics". This formation, along with the Topanga Canyon and Calabasas For mations, are included within the Topanga Group. These vol canics rest concordantly on the Topanga Canyon Formation 11
and locally on the Vaqueros Formation, but over most of the area the lower contact of the volcanics is sharply discor- dant with the underlying strata. The Calabasas Formation rests conformably upon the volcanics. Results from radio- metric age dating of the Conejo Volcanics range from 13.9 to 15.6 Ma (Turner, 1968, 1970). These ages have been recalculated, and the age range of the Conejo Volcanics is now considered to be 13.4 to 16.6 Ma (Weigand, 1982).
~ and Correlation
Macrofossils were collected by the author from four localities in the study area (Appendix 1) and were iden- tified by Judith Smith and Eugene Fritsche. The fossil localities were assigned CSUN numbers and the collection is housed at California State University, Northridge, Depart- ment of Geological Sciences. A species of large pectinid identified as Macrochlamis magnolia ojaiensis Smith (in press) was found in the Latigo Canyon section (CSUN 897):
This taxon has a geologic age range from early late through medial late Oligocene (late early through early medial
"Vaqueros Stage") (Smith, in press).
Macrofossil collections were acquired throughout the
Vaqueros Formation in the central Santa Monica Mountains during the mapping projects of the U. S. Geological Survey
(Yerkes and others, 1971; Campbell and others, 1970) and a faunal list of the species collected by them is shown on \
\ ;'\ )'~ 12 il '
nia. The samples collected during field work were barren except for the presence of "sporbo". Although "sporbo" is common in Miocene rocks, it is not restricted to that age.
Macro- and microfossil age ranges were plotted on an age-correlation chart for the Vaqueros Formation in the study area (Fig. 5). It indicates that the age of the
Vaqueros in the central Santa Monica Mountains ranges from
29 Ma to 16 Ma, which corresponds to a range of middle
Chattian through middle Langhian Stages, according to the revised European time chart (Berggren and others, 1985;
Palner, 1983). Therefore, based on molluscan fauna col- lected by the author and benthic foraminifera collected by previous workers, the Vaqueros Formation is believed to range in age from early late Oligocene through early medial
Miocene.
Structure
In general, the structure of the Santa Honica Moun- tains has been interpreted to be an asymmetric anticlinal uplift along a major fault zone which is on the southern border of the mountains (Hoots, 1931; Durrell, 1954 1956;
Truex, 1976, 1977; Truex and Hall, 1969). The anticline is broken up by many high-angle reverse faults. Other workers
(Campbell and others, 1966; Yerkes and Campbell, 1980) have mapped several low-angle "detachment" faults upon which
Cretaceous and Tertiary Formations in the Santa Monica THrs sTuov w 1R5oVE~~~u d)~~~mwA CALCAREous WEST coAsT EUROPEAN •.• ·- s~E:RiE:s· --- ·.\ sTA MONICA MTS o (J MoLLusCAN BLOW.l969 NANNoFossiLs PRovENciAL BENTHic sTAGEs t:l (J MorRo- ;. MTCRo~ossu •
'JACALITOS' Nl? ffi V N16 -~ NNll TORTONIAN ~ 15 NN NNlel MOHNIAN 112l W -10-1------+V:~ 4 ~~ MARGARITAN" 1 / N13 .~ NN9 Z 8 = o:: t==-_ N12 _;:; ~~ R W 0:: w ~ Nll '// NNR SE RYALLIAN ~
g: N - ,_ Nlel ;;; NN5 LUISIAN 15 u 15 3 ~ ~ B ~ w CO r------1 "" N9 / ------LANGHIAN 0 2: 4 a~ r-- -- ~ ffi ~ N8 / NN RELIZIAN 1---i g: - V) I-- 3 ~ ~ ~en a = 3 ~ N7 / NN3 2 u Ui wo:: 2 I -...... _ / ...s gj ~~~ - r- LL BURDIGALIAN o:: ~ ~ 1l:! 82i5 20- o:: N6 / NN2 212l ~ · [~ ~ ~~ (f) ~ SAUCESIAN :J >- ~ wu 0 a.. N5 :i t-W
I--' w
'C 14
Table 1. Yerkes and Campbell (1979) stated that both the
Vaqueros Formation undifferentiated and its two formal mem- bers contain Turritella inezana, which is a diagnostic species of the \vest Coast provincial molluscan "Vaqueros
Stage" of California (Loel and Corey, 1932; Weaver and others, 1944). Yerkes and Campbell (1979) found Vertipec- ten bowersi and Lyropecten miguelensis in the San Nicholas
Member of the Vaqueros Formation. These species are in- dicative of the upper "Vaqueros Stage" (Smith, in press).
Addicott (1979) stated that the "Vaqueros Stage" age range corresponds to late Oligocene through early Miocene.
Vaqueros Formation microfauna are rare to absent in the central Santa Monica Mountains, and when present, are poorly preserved. Yerkes and Campbell (1979) reported that microfossils were collected from a siltstone that occurs stratigraphically above the San Nicholas Member (Table 2).
The benthic foraminifera are of late Saucesian through late
Relizian age (early medial Miocene). Sonneman (1956) col- lected foraminifera from the Danielson Member that ar~ Df
Zemorrian age. Barron and others ( 1985) reported the
Zemorrian Stage age range to be equivalent to the late
Oligocene.
Vaqueros siltstone samples were collected by the author in several localities for microfossil analysis. The samples were processed and analyzed by Union Oil Company,
Ventura, California and Texaco, Inc., Los Angeles, Califor- TABLE 1 List of macrofossils collected by the U. S. Geological Survey
~ro ro u ro~ ~~~~ ~~00 ~~~~O~~~~~OO~OO~~~~~~~N~~OON~OOO~~~~O ~~N~~~~~~N~~N~~~~~~~~~~~~~~N~~N~OOOOOOOO~ uuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuzzzzuI I I I I I I I I I ij I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I :::;::, :::;::, :::;::, :::;::, I ~~~~~~~~~~~~~~~~NNNNN~~~~~~~~~~~OOOOOOOO~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~uvuu~ GASTROPODS
Crepidula pinceps X X Conrad Crepidula pinceps X Crepidula sp. X Forreria gabbiana X (Anderson) Muricid X Naticid X X Neverita sp. X Ocenebra X Rapana vaquerosensis X X (Arnold) Thylaeodus sp. X Turritell inezana X X X X Conrad T. inezana X X ~ inezana pantana X xxxxxx Loel and Corey ~ inezana sespeensis X Arnold T. inezana bicarini X X X X X X Loel and Corey
1-' Vl TABLE 1 (continued)
~m m 0 m~ ~~~~ ~~00 ~~~~O~~~~~OO~OO~~~~~~~N~~OON~OOO~~~~O ~~N~~~~~~N~~N~~~~~~~~~~~~~~N~~N~OOOOOOOO~ uuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuzzzzuI I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I pppp I ~~~~~~~~~~~~~~~~NNNNN~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~u~_uu~
T. inezana hoffmani X Gabb T. inezana ss X Conrad T. ocoyana Conrad X X X X X Turritella sp. X T. variata Conrad X
PELECYPODS
Aequipecten andersoni X (Arnold) Chione sp. X Chlamys hericia X (Gould) ~ sespeensis x X X X X Clementia sp. X Dosinia sp. X Lucina excavata X Carpenter Lucinoma sp. X X Lyropecten estrellanus X (Conrad) L. magnolia X
...... ~ TABLE 1 (continued)
~m m u mA ~~~~ ~~00 ~~~~O~~~~~OO~OO~~~~~~~N~~OON~OOO~~~~O ~~N~~~~~~NN~N~~~~~~~~M~~~~~N~~N~OOOOOOOO~ uuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuzzzzuI I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I :::> :::> :::> :::> I ~~~~~~~~~~~~~~~~NNNNN~~~~~~~~~~~mmmm~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ '-0_ \0_\0 ~~Ct \0_-,o_\,0__ \0._\,Q_ >,0_\.0_\.0 l.!l >,0_\.!l_\.0 l.!l ..n_..n _U _r ~ L 1 ~ r ~1 \.C'l ~ magnolia Conrad X X X ~ miguelensis X X (Arnold) Macoma sp. X Macrochlamis magnolia X X ojaiensis Miltha sanctaecrucis x Arnold Miltha sp. x ~ expansus Arnold X X Mytilus sp. X Ostrea eldridgei X ynezana Loel and Corey Panope abrupta X Pectinid X X X X X X Saxidomis sp. X Semele n. sp. X Spondifus perrini X Wiedey Tivela sp. X Trachycardium X vaquerosensis ~ vaquerosensis X (Arnold)
...... '-I TABLE 1 (continued)
~ro ro ro ro~ ~~~~ ~~00 ~~~~O~~~~~OO~OO~~~~~~~N~~OON~OOO~~~~O ~~N~~~~~~NN~N~~~~~~~~~~~~~~N~~N~OOOOOOOO~ uuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuzzzzuI I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I i:Ji:Ji:Ji:J I ~~~~~~~~~~~~~~~~NNNNN~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~uuuu~ Vertipecten X X nevadanus (Conrad) V. nevadanus X perrini (Arnold) Undet. X X X X
ECHINOIDS
Vaqueros ella X vaquerosensis
BARNACLES
Balanus concavus x Bronn Balanus sp. X X X Balanus? sp. X X
(Please note that list is preliminary and has not been approved for publication by the U. S. Geological Survey.)
!--' 00 19
TABLE 2 List of microfossils collected by the U. S. Geological Survey
cU 0 l.() .--I 0 0 (") l.() 1.()0 .--I .--I .--I .--I N N N .--I N .--I I I I I I I I I I I u u u u u u u uu y I I I I I I I I I \0 ...... \0 N \0 ...... r--N FORAMINIFERA \0 \0 \0 \0 \0 \0 \0 \0 \0\0 Bolivina advena Cushman X X Bolivina advena advena Cushman X Bad vena striatella Cushman X B. conica Cushman X 12..:._ marginata X 12..:._ marginata adelaidana X 12..:._ marginata Cushman X X 12..:._ perrini X Bolivina sp. X B. tumida Cushman X X B. tumida tumida Cushman X Bulimia sp. X Buliminella curta Cushman X X Buliminella sp. X X B. subfusiformis X B. subfusiformis Cushman X E2istominella sub2eruviana X sub2eruviana Cushman Globigerina dubia Egger X Globigerina sp. X X Nonian costiferum (Cushman) X N. incisum X N. incisum Cushman X X Si2hogenerina branneri (Bag g) X Si2hogenerina sp. X Uvigerinella obesa Cushman X Virgulina californiensis Cushman X X
(Please note that this list is preliminary and has not been approved for publication by the U. S. Geological Survey.) 20
Mountains moved southward as gravity slides from an elevated area in the north.
Campbell and others (1966) describe the central Santa
Monica Jvlountains as an autochthon with three allochthons, superimposed upon one another. The allochthons, from bot tom to top, are named the Tuna Canyon, Zuma, and Malibu
Bowl thrust sheets (Fig. 6). The thrust sheets are named for the faults beneath them.
Dibblee (1982) summarized the structural history of the area as follows. Earliest uplift occurred in the early
Miocene, but subsidence in the medial Miocene allowed great thicknesses of marine sediment and volcanic rocks to be deposited. Northward tilting on the north-dipping thrust faults, local folding and volcanic injections into the faults created another uplift in the late medial Miocene.
The local gravity sliding on the south side of the moun tains may have occurred at this time.
This upheaval was followed by subsidence and marine deposition of the Modelo Formation in the late Miocene.
The absence of Pliocene rocks indicates the range was uplifted and eroded throughout or after Pliocene time. " .. ;:1-f
POINT DUME MALIBU BEACH TOPANGA ~UADRANGLE i!l,UADRANGLE ~UADRANGLE X
EXPLANATION
MALIBU BOWL TUNA CANYON THRUST SHEET NORTH OF 1-AALIBU THRUST SHEET POINT OUME CJ COAST FAULT ~
0 1 2 3 4 5 MILES I I . I I I I ZUMA 0 l 1 ! ~ ~ ~ ~ KILOMETERS THRUST SHEET ~ AUTOCHTHON
Figure 6. Shows approximate locations of the "detachment surfaces" in the central Santa Monica Mountains (modified from Campbell and others, 1966).
N f--' LITHOSOME DESCRIPTIONS AND PROCESS INTERPRETATIONS
METHODS
Seven stratigraphic sections were measured by means of the tape-and-brun ton method (Compton, 1962) along an east-west transect of the central Santa Monica Mountains, west of Topanga Canyon (Appendix 1). Reconnaissance work was done in the areas around the measured sections. The field work for this study was completed in 30 days during the fall and winter of 1984. Plate 1 contains the drafted stratigraphic columns of the Vaqueros Formation in the study area.
A total of 60 days was spent in the laboratory to perform thin section analysis, paleocurrent analysis, hand sample description, and some macrofossil identification.
The compositions and rock names of the samples were deter- mined after 38 thin sections were analyzed. The location of thin section samples and macrofossils are denoted beside the columns on Plate 1. One-half of each slide was stained for potassium feldspar and 400 po~nts per thin section were counted to determine the percent composition, Lithologic description of 120 hand samples was also performed: Litho- logic colors used in the lithosome descriptions are from the rock color chart of the Geological Society of America
(Goddard, 1970). The grain-size classification used in this paper is taken from Wentworth (1922), rounding class ification is taken from Powers (1953), and the shape
22 r . 23
descriptions are from Zingg (1953). The rock names, sort- ing, and maturity classification are from Folk (1974).
INTRODUCTION TO LITHOSOMES
During field work for this study, the rocks of the
Vaqueros Formation in the central Santa Monica Mountains were divided into nine major lithosomes. These lithosomes are distinguished from one another by obvious differences in grain size and sedimentary structures. Also, the lithosomes were picked without regard to the environments in which they might have been deposited.
Plate 1 is an important key in understanding this study and must be referred to while reading the text of the next two major sections: Lithosome Descriptions and Process
Interpretations and Environmental Analysis. On Plate 1, the lithosomes have been assigned a color and on the ex planation there is a brief description of the lithology and sedimentary structures that define each lithosome. Each lithosome was deposited by a distinctive process. A process, as used here, is any activity that affects the sediment. This includes physical, chemical, and biological activities (Friedman and Sanders, 1978, p. 83). Commonly, a particular process is unique to one environment, so near the beginning of this study an attempt was made to equate each litho some with a single environment. It became ap- parent, however, that the processes that deposited the 24 Q .
lithosomes of the Vaqueros in the study area were not
restricted to one environment (i.e., rocks deposited in
prodelta and in offshore environments are both siltstone
deposits governed by suspension processes). Therefore, the environmental interpretation of the lithosomes is based on their stratigraphic association to one another, as well as
their depositional process characteristics.
The following description and interpretation section contains the lithosome descriptions and includes any physi cal or biological characteristics that were noted in the
field. It must be emphasized again that Plate 1 is an es- sential reference to use as these descriptions are read.
Following the description of the lithosomes is the process interpretation discussion of the lithosome. The inter- pretation section indicates the processes which might have deposited the previously described lithosome. No environ- mental interpretations are made in this depositional pro cess interpretation section.
The lithosomes are discussed in the order that they occur from the base to the top of stratigraphic sections and from the western to the eastern sections. Environmen- tal interpretations are discussed in the paleogeographic interpretation section. 25
LITHOSOME DESCRIPTIONS AND INTERPRETATIONS
Parallel-laminated siltstone (A)
Description
The parallel-laminated siltstone lithosome is a thinly parallel-laminated siltstone to silty shale. It oc curs mainly in the lower part of the formation, in the western part of the field area. Its lowest occurrence is in the lower part of the Corral Canyon section (Plate 1).
Thicknesses of this lithosome increase to the west, where continuous exposures reach a thickness of 20 m at the base of the San Nicholas Canyon section. Its maximum thickness is not known at this location because the base of the
Vaqueros cannot be seen. The l±thosome is usually as- sociated with the parallel-laminated, fine-grained sand stone (lithosome B) and the structureless, fine-grained sandstone (lithosome C). The color of the siltstone on a fresh surface ranges from yellowish gray (5Y 7/2) to olive black (5Y 2/1). The weathered surface color is usually yellowish gray (5Y 7 /2). Petrographic analysis revealed that this lithosome is predominantly a sandy siltstone.
Laminated beds are laterally continuous and range in thick- ness from 1 to 5 em. The average bed thickness is about
1. 5 em. Upper and lower bed contacts are sharp, whereas upper and lower lithosome contacts are gradational.
The laminations are predominantly parallel, but in the Corral Canyon section this unit displays some un- 26
dulatory lamination. The best exposures of the lithosome are at the Latigo Canyon section and many features such as carbonaceous matter and limestone concretions are present there. Fragments of carbonaceous matter are rare to abun- dant, and at the Latigo Canyon section, a piece of petrified wood was found. The wood fragment is 20 em in length and 2 to 5 em in diameter. It was situated parallel to bedding. Several other pieces of wood are present along the same horizon. The siltstone in the Latigo Canyon sec- tion also contains large limestone concretions which range in color from gray to orange. The concretions are oval in shape and are up to 30 em in length. A nucleus is not ap- parent in these nodules.
Although rich in carbonaceous matter, macrofossils are rare. A small, articulated unidentif:led bivalve was found at the Latigo Canyon section. Lenses of Turritella sp. fragments are present in Latigo Canyon and at the base of the Corral Canyon section. These lenses contain only apical portions of shells all the same size (approximately
5 em long). Several microfossil samples were collected at the Latigo Canyon and San Nicholas Canyon sections, and in
Trancas Canyon, but all were barren except for "sporbo" in one of the samples from the Latigo Canyon section.
"Sporbo" are very small concretions of phosphatic material
(Galliher, 1931, p. 257). The term originated from the combination of the underlined letters ~mooth-R£lished- 27
~ound-llack-Qbjects.
Interpretation of Depositional Process
The parallel-laminated siltstone lithosome is inter-
preted to be a suspension deposit. In the absence of a
current, sediment which ranges in size from very fine sand
to clay can deposit from suspension, and such deposits form
planar-laminated beds (Harms and others, 1982, p. 3.44).
Collinson and Thompson (1982, p. 56) stated that fluctua-
tions in sediment type can produce laminations. The rare,
wavy bedding observed in the Corral Canyon section may have
formed by current influence from storms or floods.
"Sporbo" are commonly found in lenses of thin marine
siltstone and mudstone (Galliher, 1931, p. 258) The
mechanism of formation requires the decomposition of or-
ganic material. Sand grains have been found in the center
of some "sporbo", but the concretions found in the Vaqueros
siltstone of the study area lack the internal structure
which would suggest sand served as a nucleus. The nodules
probably formed in situ, but may have been transported par-
ticles. The formation of phosphate implies a continental-
shelf environment of warm water with upwelling, low
sedimentation, and slightly reducing conditions (Reineck
and Singh, 1975, p. 132). 28
Parallel-laminated, fine-grained sandstone~
Description
The lithesome consists of very fine- to medium grained, arkosic sandstone with medium, parallel lamina- tions. It is present, in abundance, in the Puerco Canyon,
Corral Canyon, Latigo Canyon, and Encinal Canyon sections
(Plate 1). The thickness of the lithesome ranges from 1 to
59 m and averages 6 m. Rocks which belong to this litho some generally are associated with the structureless, fine grained sandstone (lithesome C) and parallel-laminated siltstone (lithesome A) and occur in the lower and middle parts of the formation. The color of a fresh surface ranges from pinkish gray (SYR 8/1) to pale yellowish orange
(10YR 8/6). The weathered surface colors range from grayish orange (10YR 7/4) to dark yellowish orange (lOYR
6/6). Petrographic analysis of these rocks indicates that they consist of poorly to moderately sorted, fine- to medium-grained, micaceous arkose and lithic arkose (Plate
2). Rock fragments (less than 2 mm in diameter) in the thin sections are mainly granite (3%-5%), chert (2%-5%) and volcanic rock ( 1%-2%). This lithesome is thin to thick bedded, and beds range in thickness from 2 em to 1 m. The average bedding thickness is 10 em. Bed contacts observed are gradational.
Parallel laminations within the beds of this lithesome are the result of alternating grain size and 29
changes in the abundance of mafic minerals. The lamina- tions are predominantly parallel, with rare lensoidal laminations in the Corral Canyon section. Layers of thin- laminated, olive-green siltstone can be observed in the
Corral Canyon and Latigo Canyon sections. The siltstone layers range in thickness from 1 to 5 em. Sandstone con- cretions, which average 2 em in diameter, are common in the
Puerco Canyon and the Corral Canyon sections. These concretions are randomly oriented with respect to bedding.
In the Corral Canyon section, the deposits contain thin lenses of coarse- to very coarse-grained sandstone, which are 1 to 2 m in length and approximately 10 em thick. The lenses commonly display fining-upward gradation.
The unit, for the most part, contains no body fos sils, however, weak to moderate bioturbation is present.
The burrows are usually vertical and range from 1 to 2 em in diameter. Pectinid impressions are rare and they occur at one location in the Corral Canyon section (409 m above the base of the section) and at one location in the Encinal
Canyon section (30 m above the base of the section). An unidentified bivalve impression was observed at the Encinal
Canyon section (34 m above the base of the section). One exposure of this unit in the Encinal Canyon section con tains an abundance of calcareous worm-tube structures~ The tubes comprise 50% to 75% of the rock, and are almost ex- elusively parallel to bedding. Fossil-hash lenses occur @ ' 30
sporadically in this lithosome in the ·corral Canyon and En cinal Canyon sections. The thickness of these lenses ranges from 20 em to over 1 m and the length ranges from
0 •. s to 3 m. The basal contacts of the lenses are sharp, and possibly erosional. The upper contacts are grada- tional. Throughout this lithosome these lenses are abun- dant and contain combinations of barnacle, Turritella sp., and pectinid fragments.
Interpretation of Depositional Process
The parallel-laminated, fine-grained sandstone lithosome is interpreted to have been deposited by one of two processes. The first process that would deposit such planar-laminated beds is a traction current in the plane bed phase of the lower flow regime (Harms and other, 1982, p. 3.26). Reineck and Singh (1975, p. 106) stated that planar-laminated beds are formed in the lower flow regime if there is adequate sediment supply. A traction current that would deposit this lithosome was probably unidirec- tional. An oscillatory current may deposit laminated beds, but this lithosome's association with relatively deep-water deposits (parallel-laminated siltstone) makes oscillatory current deposition an unfavorable choice.
The second process that deposits laminated sand is the sedimentation of grains from suspension (Reineck and
Singh, 1975, p. 106). Although the laminae produced from 31
suspension deposition can display normal grading, there is no normal grading observed in the parallel-laminated, fine- grained sandstone in the study area. Grains . can be put into suspension by storm or turbidity currents.
Wiedey (1928, p. 155) indicated that worm tubes much like the ones found in the Encinal Canyon section are prob ably formed by a type of polychaete worm. The fossil-hash
1 en s e s and c oar s e - g r a in e d s an d 1 en s e s a r e i n t e r p r e t e d a s storm-lag deposits. Brenner and Davies (1973, p. 1602-
1692) describe deposits such as these as being the result of rapid change from low- to high-energy conditions that may occur during a storm. The poorly sorted, coarse grained sandstone, fragmentation of the fossils, and the sharp, possibly erosional, basal contact are evidence of a high-energy environment which might have occurred during storms. Rocks with similar characteristics have been in- terpreted to be storm deposits by Brenner and Davies (1973, p. 1691), Squires (1981, p. 928-931) and Allen (1984, p.
487-491). Modern storm-lag deposits have been described by
Kumar and Sanders (1976). The fossils found in these storm-lag deposits have varied depth ranges. Dead Balanus sp. may occur in water depths from intertidal to 311 m
(Keen, 1963; Gasner, 1971; Ricketts and Calvin, 1968) but live specimens prefer a shallow marine rock substrate
(Yonge and Thompson, 1976). Turritella sp. has a depth range of 4 to 185 m, but are most frequently reported in 32
depths from 26 to 56 m (Squires, 1984, p. 6) and pectinids can be found in water depths from 10 to 580 m (Morris and others, 1980).
Structureless, fine-grained sandstone (C)
Description
The structureless, fine-grained sandstone unit con sists almost entirely of a fine- to medium-grained, struc- tureless arkosic sandstone. It occurs in all the sections examined and is most abundant in the Puerco Canyon and Cor~ ral Canyon areas (Plate 1). The lithosome ranges in thick- ness from 1 to 49 m and averages 6 m thick. The unit in terfingers with any of the lithosomes found in the study area. The color of fresh and weathered surfaces ranges from light gray (N7) to yellowish brown (10YR 5/4). The structureless, fine-grained sandstone consists of poorly to well-sorted, fine- to medium-grained arkose with minor amounts of clay (Plate 2). Thin-section analysis reveals that granitic rock fragments comprise 5% to 12% of the sandstone. A minor percentage of the rock fragments are volcanic. Beds of the lithosome are most commonly tabular and resistant. The beds are thin to thick bedded, and range from 5 em to over 2 m in thickness; with an average thickness of 30 to 50 em. Upper and lower bed contacts are usually gradational and diffuse over several centimeters.
Upper contacts with cross-bedded sandstone lithosomes, 33
however, are sharp and perhaps erosional.
Small sandstone concretions, approxim~tely 1 to 2 em in diameter are common in Corral Canyon. Although the litho some is essentially structureless, Ophiomorpha sp.
burrows are observed. These burrows generally are found in the Puerco Canyon and Corral Canyon sections. Rare fossil fragments, as well as complete fossils, are present. Mac rochlamis magnolia ojaiensis Smith (in press) is present in
Latigo Canyon and Encinal Canyon sections. Unidentified pectinids are present in the Corral Canyon and San Nicholas
Canyon sections. Fragments of Balanus sp. are abundant in the La tigo Canyon section. In some places the barnacles are found in the rock, but in most cases, the barnacles weather out and leave a "pock mark". Also in the Latigo
Canyon section, there is abundant carbonaceous matter.
Interpretation of Depositional Process
Structureless sandstone can form in two ways: one from primary deposition and one from secondary reworking.
Rapid deposition and burial can prevent working of the sediments by currents (Reineck and Singh, 1975, p. 113).
Poor sorting and the presence of clay in this lithosome are indicative of such rapid deposition. Currents are unable to sort the sediment and winnow out the finer-grained material.
Structureless sandstone also can form subsequent to 34
deposition by extensive bioturbation (Reineck and Singh,
1975). The sandstone may have been deposi~ed with lamina- tion or other bedding and these features were destroyed by organisms which burrowed the sediment. The author favors bioturbation over rapid deposition as the process respon sible for the structureless nature of lithosome C. Its association with fine-grained sandstone and siltstone indi cate that a low energy environment, which is conducive to biogenic activity, probably existing.
Howard (1972) reported that Ophiomorpha sp. usually occurs in a shallow, shoreface environment, however it has been found in rocks deposited in environments from non marine (Kennedy and Sellwood, 1979; Bown, 1982) to deep water turbidites (Bottjer, 1981; Link and Bottjer, 1982).
The occurrence of Ophiomorpha sp. burrows with Macrochlamis magnolia ojaiensis in this lithosome is a good indication that these burrows were formed in a marine shoreface en vironment.
Cross-bedded sandstone (D)
Description
The lithosome is divided into two sublithosomes. The first is a fine- to medium-grained sands~one that contains low-angle cross bedding and swales. The presence of this sublithosome is rare. Outcrops usually consist of one or two beds each with several sets of cross beds. In some 35
places, thin lenses of conglomerate are contained within the cross-bedded sandstone. This unit is· present in the
Piuma Road, Puerco Canyon, and Encinal Canyon sections
(Plate 1). The unit ranges in thickness from 1 to 5 m. It is usually associated with the interbedded red-and-green siltstone (lithesome I) and structureless, fine-grained sandstone (lithesome C). The deposits of this first sub- lithosome in the Encinal Canyon section are unusual in that they are composed of abundant, fine-grained barnacle £rag ments and sand. The color of this unit on fresh and weathered surfaces ranges from light olive gray (5Y 6/1) to grayish orange (10YR 7/4). The sandstone of this cross bedded, fine-grained sandstone sublithosome is, in general, moderately sorted, fine- to medium-grained arkose (Plate
2). Rock-fragment composition in thin section is predominantly granitic, with minor amounts of volcanic rocks and chert. Beds of this unit are tabular and con- tinuous. They are resistant to weathering and tend to form ridges in outcrop. The average bed thickness is ap proximately 20 em. The lower bed contacts are sharp and erosional, whereas the upper contacts are usually diffuse.
The beds generally contain low- to very low-angle trough cross beds. Several sets of cross bedding occur in each outcrop. The cross beds are tangential at the base and truncated at the top. In the Piuma Road section, low angle swales occur with the cross beds. The swales angle 36
up toward the base of the overlying bed, then roll back down in the opposite direction (Fig. 7). The conglomerate lenses are composed of matrix-supported, pebble con glomerate. These lenses are approximately 0.5 to 2 m in length and 1 to 50 em thick. The matrix consists of coarse- to very coarse-grained arkosic sandstone that is moderately to poorly sorted. The su bangular to rounded clasts are poorly sorted and range in diameter from 2 mm to
15 em. The average clast size is 1 em. The clast composi- tion is primarily quartzite and granite, with some volcanic clasts. Shale and siltstone rip-up clasts are common in this lithosome. They range in diameter from 2 mm to 15 em and occur on the slip faces of the cross beds in many of the sets. In the Piuma Road section, the base of this sub- lithosome contains two imprints of large pieces of land- derived wood. These pieces are approximately 5 em in diameter and 1 to 1.5 m long.
The second sublithosome is a coarser-grained, cross- bedded unit. This unit is a medium- to coarse-grained, cross-bedded arkosic sandstone. The sublithosome ranges in thickness from 1 to 9 m and averages 3 m thick. It is most abundant in the upper portions of the Puerco Canyon and
Corral Canyon sections (Plate 1). A few exposures are present in the Latigo Canyon and San Nicholas Canyon sec- tions. The sublithosome is most commonly associated with the interbedded red-and-green siltstone (lithosome I), but Figure 7. Swales and cross bedding in the fine - grained, cross- bedded sandstone (lithosome D). Hammer is 30 em in length.
w '-1 J can be interbedded with the parallel-laminated, coarse grained sandstone (lithosome E) and the parallel-laminated, medium-grained sandstone (lithosome H). The colors on a fresh and weathered surface range from grayish orange (lOYR
7 /4) to dark yellowish orange (lOYR 6/6). Petrographic analysis indicates that this coarse-grained sublithosome is lithologically similar to the fine-grained cross-bedded sublithosome (Plate 2). This unit is more poorly sorted, has a higher percentage of volcanic rock fragments, and less chert rock fragments. Granite comprises the dominant rock fragment percentage in thin section. Beds are tabular in shape and fairly resistant. The bed thickness ranges from 20 em to 1 m with an average of 20 to 30 em. The top contacts are predominantly diffuse, and bottom contacts tend to be sharp.
The cross beds in this unit are trough shaped. The low- to medium-angle, unidirectional cross beds are trun- cated at the top and tangential at the bottom. A cross-bed set averages about 20 em in thickness. The cross beds at the San Nicholas Canyon section are actually composed of fragments of Balanus sp. In the Corral Canyon section, these deposits contain rare, floating clasts. The clasts range in size from 2 mm to 6 em, with an average diameter of 0.5 em. Clasts are subrounded to subangular in shape, and consist predominantly of quartzite and granite. Verti- cal burrows are rare in the unit. 39
Interpretation of Depositional Process
Cross bedding is formed from the migration of bed forms (Harms and other, 1982, p. 3.37) in the transition phase between upper and lower flow regimes. The descrip- tion section of this lithosome describes two sublithosomes, however their description differs mainly in grain size.
This difference in grain size can be explained by a dif ference in energy of the current that carried the sediment.
Trough cross bedding is formed by the migration of three-dimensional dunes and megaripples (Harms and others,
1982, p. 3. 20). Cross beds formed by dune and megaripple migration dip parallel to the current direction. Reineck and Singh (1975, p. 85) stated that at the base of the cross beds there can be an accumulation of coarser material, such as pebbles, shells, or clay clasts.
Tabular cross bedding is formed by the migration of two-dimensional large ripples. In an unidirectional flow, the two-dimensional bedforms indicate lower current velocities than three-dimensional bedforms (Harms and others, 1982, p. 3.22). They also stated that the lower velocity interpretation is supported by the steeper and more planar cross laminae.
Parallel-laminated, coarse-grained sandstone (E)
Description
The unit consists predominantly of a coarse-grained, 40
parallel-laminated arkosic sandstone. It occurs most com- monly in the upper portions of the Corral Canyon and Latigo
Canyon sections with limited exposures in Encinal Canyon
(Plate 1). This unit ranges in thickness from 1 to 23 m and averages 6 m. It is commonly associated with the in- terbedded red-and-green siltstone (lithosome I) deposits and the cross-bedded sandstone (lithosome D) deposits described in the previous section. The color on fresh and weathered surfaces ranges slightly from yellowish gray (SY
8/1) to pale yellowish orange (10YR 8/6). The sandstone of this lithosome is poorly sorted, medium- to coarse-grained arkose and lithic arkose (Plate 2). The sandstone contains a percentage of rock fragments (less than 2 mm in diameter) which, as seen in thin section, consist primarily of granitoids with some chert and volcanic rock fragments.
The beds of this lithosome are tabular in shape and resis- tant to weathering. The beds are medium to thick bedded, and thickness ranges from 10 em to 1 m, with an average of
20 em. The basal and top bed contact are sharp, and pos- sibly erosional.
The laminations of this lithosome are due to the al- ternation of dark and light minerals in the rock. The unit contains some thin lenses of laminated siltstone and thin layers of siltstone with climbing-ripple, trough, cross lamination. The beds commonly contain small, green siltstone rip-up clasts. Floating clasts in the sandstone 41
are common in the Latigo Canyon sections. The clasts con- sist of subrounded to rounded granitoid and metamorphic rock fragments. These clasts range in diameter from 2 mm to 6 em, and average approximately 0.5 to 1 em.
The parallel-laminated lithosome contains abundant carbonaceous debris and fragments of Balanus sp. in the
Latigo Canyon section. "Pock marks" are also common in this section.
Interpretation of Depositional Process
The parallel-laminated, coarse-grained sandstone was deposited by traction currents, which are interpreted to have been of steady, unidirectional flow. The velocity of the traction currents was probably in the upper flow regime. Harms and others (1982, p. 3.24) stated that a large grain size usually denotes a strong current. Al though plane-laminated sand can be formed by lower flow regime currents, the depth of the transport water is always shallow. It would be difficult to deposit a substantial thickness of sediment. The presence of pebble-sized clasts and rip-up clasts in this lithosome also supports a high flow regime current.
Climbing-ripple, trough, cross lamination is produced by the migration and upward growth of wave or current ripples (Reineck and Singh, 1975, p. 95). Reineck and
Singh (1975, p. 96) suggested that the preservation of 42
these ripples is due to a large amount of available sedi ment. The fine grain size of these ripples is probably in dicative of a lower flow regime.
Structureless, coarse-grained sandstone (F)
Description
The lithesome is a poorly sorted, structureless, coarse-grained, arkosic sandstone. As illustrated on Plate
1, it is present at the top of the Puerco Canyon, Corral
Canyon, and San Nicholas Canyon sections. It ranges in thickness from 1 to 25 m and averages 4 m. The unit com- manly is associated with interbedded red-and-green siltstone (lithesome I) and locally is interbedded with structureless, fine-grained sandstone (lithesome C).
Colors on a fresh surface range from medium gray (NS) to pale yellowish orange (10YR 8/6), and the color on the weathered surface is usually pale yellowish orange ( 10YR
8/6). The rocks of this lithesome consist of sandstone which is very poorly to poorly sorted, medium- to very coarse-grained arkose (Plate 2). Granitic rock fragments comprise a large percentage of the sandstone in thin sec tion with minor amounts of volcanic, metamorphic, and sedimentary rock fragments. Barnacle fragments are a major constituent (21%) in one of the thin section samples from the San Nicholas section (81 m above the base). The average bed thickness ranges from 20 to 30 em. Upper and 43
lower contacts are diffuse over 5 em.
Thin layers of sandstone with siltstone rip-up clasts are present in this lithosome at the Corral Canyon section.
These rip-up clasts are composed of structureless, green
siltstone and occur at the base of the beds. The diameter of the rip-up clasts ranges from 1 to 2 em. Scattered clasts occur in the rock at the Corral Canyon section and are predominantly quartzite, with rare volcanic pebbles.
These clasts are subrounded and range in size from 2 mm to
8 em in diameter.
Burrows are present in this lithosome in all the sec- tions, however they are rare. The burrows are mainly per- pendicular to bedding, lined, and have a diameter which ranges from 1 to 2 em. No identification of these burrows was made. Fossil debris is present throughout the unit and is mostly made up of opercular fragments of Balanus sp. with some pectinid fragments. An unidentified articulated bivalve was observed at the San Nicholas Canyon section.
Also present in the Latigo Canyon section are "pock marks" left by weathered-out barnacle fragments.
Interpretation of Depositional Process
As discussed in the structureless, fine-grained sandstone (lithesome C) section, structureless sandstone can be the result of rapid deposition or complete bioturba- tion (Reineck and Singh, 1975, p. 112-113). The large grain size and presence of scattered pebbles and siltstone
rip-up clasts in the sandstone suggest a relatively high
velocity current. The poor to very poor sorting may be an indication of rapid sedimentation. If the deposition rate is very high, currents do not have an opportunity to rework and sort the sediment. The presence of burrows may indi- cate bioturbation as a cause for the structureless sand.
The burrows that are present in this lithosome are small, perpendicular to the bedding plane, and have no constricted tops. The small and perpendicular characteristics imply that the organisms did not take much time to rework the sediment to any great extent. The absence of constricted tops indicates that some erosion did take place and scoured the tops off.
Bioturbated, fine-grained sandstone (G)
Description
This bioturbated, fine-grained sandstone lithosome consists predominantly of moderately-bioturbated, fine- to medium-grained arkosic sandstone and contains rare lenses of limestone. The lithosome is not common in the Vaqueros
Formation of the Santa Monica Mountains and is found only in the Piuma Road, Malibu Canyon, and Corral Canyon sec tions (Plate 1). It ranges in thickness from 2 to 6 m and averages 2 m. The unit usually is associated with the parallel-laminated, medium-grained sandstone (lithosome H) 45
and the structureless, fine-grained sandstone (lithosome
C). The colors of the sandstone on both fresh and weathered surfaces are quite unique and range from a light olive gray (SY 6/1) to a dusky yellow green (SG 5/2). The sandstone of this deposit is a poorly sorted, fine- to very fine-grained lithic arkose (Plate 2). The rock fragments observed in thin section include volcanics, granitoids, and chert. The sandstone portion of this unit is generally thick bedded, and contains beds which range in thickness from 0.5 to 2 m, with an average of 1 m. The beds are resistant to weathering and fairly continuous, with sharp and planar upper and lower contacts.
Perhaps the most distinctive characteristic of this unit is its mottled appearance. No primary sedimentary structures are apparent, but individual burrows can be dis- tinguished. These burrows are lighter in color than the surrounding rock and range in diameter from 1 to 2 em. The burrows are lined and are both parallel and perpendicular to bedding. Some burrows bifurcate and have turnaround nodes (Fig. 8).
Other noted features of the rock are the abundance of organic material, predominantly wood and plant fragments, and the very rare occurrence of fossil fragments too small to identify.
The limestone of the bioturbated, fine-grained sandstone deposit is very fine grained and crystalline. It Figure otograph o ine-grained sandstone (lithosome G) exposed in the Piuma Road section. Note the vertical burrow at the right. Distance across photograph is 50 em.
.j::-- 0'1 •----··---- j 47
is observed only at the Piuma Road section and there it un- derlies the cross-bedded sandstone (lithosome D). It is lensoidal and not very continuous. The lenses range in thickness from 0.3 to 0.5 m. Color on fresh and weathered surfaces is usually very light gray (N8), but the upper part of the limestone is pinkish gray (SYR 8/1). In the lower part of the limestone lenses, fossil fragments, pos- sibly barnacles, were observed. The upper part of the lenses contains no fossil fragments and the texture changes to microcrystalline. Secondary chert replacement is ob- served in the upper part of the limestone lenses.
Interpretation of Depositional Process
The massive nature of this lithosome is the result of moderate to thorough bioturbation. The fine grain size and presence of burrows which were both horizontal and vertical at the time of deposition indicate a low-velocity current and a very low sedimentation rate at the time bioturbation occurred. The poor sorting might indicate that as the sediment was deposited, it was not reworked by currents.
The limestone in this lithosome is interpreted to be a deposit of shell fragments, possibly barnacles. The shell fragments subsequently dissolved to form the lime stone bed, which was then partially replaced by chert.
The origin of the chert is unknown. 48
Parallel-laminated, medium-grained sandstone (H)
Description
The deposit consists predominantly of very fine- to
medium-grained, arkosic sandstone with thin, parallel
laminations. This lithosome is present in all the sections
west of Latigo Canyon (Plate 1). It ranges in thickness
from 1 to 12 m and averages 6 m. It is commonly inter-
bedded with interbedded red-and-green siltstone (lithosome
I) and parallel-laminated, coarse-grained sandstone (litho-
some E) deposits. The color of a fresh surface ranges from
very light gray (N8) to dark greenish gray (SG 4/1) and the weathered surface colors range from medium gray (NS) to
greenish gray (SG 6/1). Petrographic analysis of the
deposits indicates that the sandstone is moderately to well
sorted, fine-grained arkose and lithic arkose (Plate 2).
Granitic and volcanic rock fragments comprise approximately
5% to 6% of the sandstone in thin section. Two of the thin
section samples have abundant hematite and limonite cement.
Sandstone beds of this lithosome can be tabular or len-
soidal and can form resistant ridges. Beds are thin to
medium bedded and range in thickness from 10 to 50 em, with
an average thickness of 30 em. The basal contact is
usually sharp and planar. The top contact can be either
gradational or sharp.
Dark green siltstone rip-up clasts occur in the
parallel-laminated, medium-grained sandstone lithosome, 49
primarily in the Corral Canyon (508 m above the base) and
Malibu Canyon (25 m above the base) sections. These rip-up clasts are elongate and oriented parallel to the bedding planes. They occur in the laminated sandstone within layers approximately 1 to 5 em thick. The layers of rip ups are usually found at the base of the deposits. Rip-up layers are also present in the middle of the bed. Rare, graded-bedding sequences are noted in this unit at the
Piuma Road section (29m above the base). The graded beds range from 10 to 15 em thick and fine upward from a medium grained sandstone to a very fine-grained sandstone.
The parallel-laminated, medium-grained sandstone deposits contain no body fossil material. There is evidence of bioturbation, but it is not common. The bur- rows that are preserved are 0.5 to 1 em in diameter and are either perpendicular, parallel, or curved in relation to the bedding. The burrows are usually filled with a medium- to coarse-grained, clean sandstone. In the Piuma Road and
Malibu Canyon sections, beds of this lithosome have a blot- chy appearance. This is caused by a network of tiny burrows that mix the light and dark minerals of the lamina- tions. The general laminated appearance is discernible, even though very disrupted by burrowing.
Interpretation of Depositional Process
The parallel-laminated, medium-grained sandstone 50
lithosome, like the other parallel-laminated sandstones,
was probably deposited by a steady, unidirectional traction
current. The medium grain size makes it difficult to determine whether the traction current was in the upper or
lower flow regime. The current was fast enough to pick up material from nearby siltstone beds and the presence of
graded bedding indicates that there was slowing current.
The presence of burrows and bioturbation also indicates
periods of subsequent low sedimentation rates and rela- tively calm water. Reineck and Singh (1975, p. 104) indi- cate that graded beds, which are formed in shallow-water environments do not attain substantial thickness. They also stated that the graded beds can be produced by the sedimentation of a suspension cloud, deposition in the last phase of a major flood, or deposition by waning current activity.
Interbedded red-and-green siltstone ill
Description
This lithosome is very common in the Piuma Road sec- tion. The number of exposures steadily diminish westward of Piuma Road, and they pinch out west of the Corral Canyon section (Plate 1). When the lithosome occurs more than once in a section, it usually alternates with parallel- laminated, medium-grained sandstone (lithosome H), cross-bedded sandstone (lithosome D), parallel-laminated, 51
coarse-grained sandstone (lithosome E), and structureless, coarse-grained sandstone (lithosome F). The sandstone al ways overlies the green siltstone, not the red siltstone.
The average color of the green siltstone, on both a fresh and weathered surface, is olive gray (SY 4/1). The colors of the red siltstone on both fresh and weathered surfaces range from grayish red (SR 4/2) to very dusky red (lOR
2/2). Petrographic analysis indicates that the rocks are gypsiferous and porous, immature arkose (Plate 2). Beds are moderately continuous, but lensoidal in places. Bed thickness ranges from 1 to 30 em, with an average thickness of approximately 5 em. The lithosome is not very resistant and weathers easily. The upper and lower contacts between siltstones of different colors are usually sharp, and ir- regular. The upper and lower contacts between this lithosome and the sandstone are always sharp and possibly erosional.
The siltstone commonly contains thin, laterally con- tinuous, parallel laminations. Climbing ripple lamination is present at the Piuma Road and Latigo Canyon sections but is not common in the green siltstone. Lateral continuity of the ripples is poor. Small calcium carbonate nodules, approximately 2 em in diameter, occur in this lithosome at the top of the Puerco Canyon section. At one location in the Piuma Road section, the red siltstone contains abundant rhizomorphs. Commonly, the lithosome has isolated pockets 52
of green siltstone in red siltstone. The green siltstone in this unit is very similar to the material which com prises the siltstone rip-up clasts in the cross-bedded sandstone and parallel-laminated, medium-grained sandstone deposits. No macrofossils or microfossils were found in this unit.
Interpretation of Depositional Process
The interbedded red-and-green siltstone lithosome is interpreted to have been deposited primarily from suspen- sion. The fine-grained nature and parallel laminations are characteristic of suspension deposits. The climbing-ripple lamination, which was produced by the migration of ripples, indicates that there was a low-velocity current.
Reineck and Singh (1975, p. 132) indicate that the red color of rocks is the result of iron oxidation which usually occurs when the sediment is exposed to air. Rocks with green coloring are interpreted to have been deposited in a reducing environment (Pettijohn, 1975, p. 275). Pock- ets of green siltstone in red siltstone are explained by
Blatt (1982, p. 56) to be the result of reduction from the decomposition of the plant roots in aerated sediment. The presence of rhizomorphs is also an indication of land-plant life. PALEOGEOGRAPHIC INTERPRETATION
PALEOENVIRONMENTAL ANALYSIS
Introduction
By means of study of the stratigraphic sequences across the central Santa Monica Mountains (as shown on
Plate 1) and identification of depositional processes by which the rocks were deposited, interpretations can now be made in this section of the report of the environments of deposition of the Vaqueros Formation. In general, the
Vaqueros is interpreted to be comprised of marine offshore through backshore deposits in the western part of the central Santa Monica Mountains and deltaic deposits in the eastern part of the central Santa Monica Mountains (Fig.
9). The eastern sequences of marine deltaic deposits prograde over the western offshore marine deposits, which lie unconformably above the nonmarine Sespe Formation. The marine portion of the deltaic complex (lower delta plain and deeper) extended at least as far east of Puerco Canyon as the Las Flores thrust fault. Exposures of the Vaqueros
Formation east of the fault are mostly nonmarine (upper delta plain). A clear tran~ition from lower to upper delta plain rocks is not evident across the fault. This probably is due either to movement on the fault which juxtaposed un like environments or to the fact that 80% of the formation is covered in the Malibu Canyon area.
It has not been possible to identify a time line in
53 TOPANGA C'YN FM. CORRAL_ C/INY'Cf'.l w UPPER DELTA PLA[N E FM. MEMBER
y'0· LD~ OCL TA PLAIN :'\ <)~ c{;>- 0 LATIOJ
~Gf?.. 1 ,o~~ P!t:MA ROAD
8ACY.SHO'.E ~7 Lf'PER OCL TA PLAIN SAN NICHCI.,I~ ENCINAL CANYON / ' -~-. FORESHORE -~ '\ F~~A I --...__ SHOREFOCE -~.. JQBRHoRE / <- ..,.
\.....>
SESPE FMa "' ""'\ PROOEL TA SESPE FMa "' PIUMA MEMBER .. ,1 "' "' ~./:.===--=,KM=-2 =KM= "' "' "' SESPE FM,
Figure 9 . Lateral facies diagram for the Vaqueros Formation in the central Santa Monica Mountains. Dark vertical lines denote measured stratigraphic sections.
Vl .j::-- 55
the rocks to recreate the time-space relationships of the environmental components recognized. The rocks were found to be barren of microfossils where sampled, and no con tinuous tuffaceous beds or bentonite beds were found.
Two depositional systems are interpreted to have deposited the rocks of the Vaqueros Formation in the study area. These two systems, river-dominated deltaic coastline and wave-dominated marine strandline environments, are believed to interfinger between the Latigo Canyon and Cor- ral Canyon sections. The dividing line is placed here for two reasons. One, although similar rock types and struc- tures exist in both environments, an increase in sand thickness and decrease in silt layers occurs between the two sections. This increase in sand is an indication that the river, which supplied sediment to the delta, is nearby.
The second reason for placing the line between Latigo
Canyon and Corral Canyon sections is that from Corral
Canyon to the east, the Vaqueros Formation lies conformably on the Sespe. From Latigo Canyon to the west, the Vaqueros lies unconformably on the Sespe.
The following section contains details of the en vironmental analysis and interpretations, starting at the base of the formation and proceeding upsection.
At the base of the sections, the deeper water en vironment of both the deltaic and strandline marine deposi tional systems contain similar lithosomes and are therefore 56
discussed together. The shallow-water and subaerial en- vironments (high in the sections) of the two systems differ greatly and necessitate separate discussions. Figure 10 is a diagram which plots the environments versus lithosomes and can be used as a guide while reading this section.
Reference should be made to Plate 1, which illustrates the environments and facies relationships.
Offshore/Prodelta Environments
Rocks of the offshore environment are exposed mainly in the lower parts of the San Nicholas Canyon and Latigo
Canyon sections (Plate 1). The most common lithosome that occurs in this environment is the parallel-laminated siltstone (lithosome A). Beds of the parallel-laminated, fine-grained sandstone (lithosome B), however, are also present. These coarser grained beds commonly contain lenses of Turritella sp.
Reineck and Singh (1975, p. 308) and McCubbin (1982, p. 260) stated that deposition in the offshore zone is dominated by suspension processes. These processes are believed to have deposited the fine-grained siltstone ex- posed in the San Nicholas Canyon and Latigo Canyon sections
(see Interpretation of Depositional Process on the
Parallel-laminated siltstone, lithosome A). The suspension process can be interrupted by wave and current action produced by storms (HcCubbin, 1982, p. 295). Reineck and RIVER-DOMINATED WAVE -DOMINATED DEL TAlC COASTLINE EROSIONAL COASTLINE Lat1go Canyon to East Lat1ao Canuon_, to West
ENVIRONMENT LITHOSOMES ENVIRONMENT LITHOSOMES
>90% L1 thosome A >90/. L1thosome A Prodelta Offshore (10/. L1 thosome B <10/. L1 thosome B
>70% L1 thosome C Transt tlon >50% L 1 thosome A Delta Front <20% L1 tho some B <10% L1 thosomes A & E Zone <50% L1thosome 8 D1stnbutar!:J I >95% L1 thosome D >G0/. L1 thosome C Mouth Bar (5% L1 tho some C (15/. L1 thosome F Shoreface Lower Delta I <15% Lt thosome B Plam I >50% L1thosome D <101. L1 thosomes A, 0 & E ~ts~hannell~------Lower Delta >30% L 1thosome C >60/. L1thosome 8 Plam I <20% L1thosomes I,C,B, & E Foreshore (30/. L1 thosome E T1dal Channell i <10/. L1 thosomes D & H UppePrl cnnDelta I )95% L1 tho some I ~~Mars~_L ______>60/. L1 tho some E Back shore <30/. L1 thosome H Upper Delta I ) 70% L1 tho some H (10% L1 thosomes D & I Plam 1 ------(Crevasse <30/. L1 thosomes C & G ~_Spla~_L ______
Upper Delta I )80% L1 tho some E Plo1n 1 • Figure 10. Diagram shows the interpreted environments occurring in the Vaqueros Formation and indicates which lithosomes are associated with the interpreted environments. V1 --.j 58 Singh (1975, p. 397) stated that these storm events can deposit poorly sorted, sandy layers and fossil-lag deposits similar to those beds in the Latigo Canyon section. Coleman (1981, p. 33), Reineck and Singh (1975, p. 273), and Coleman and Prior (1982, p. 158) stated that parallel lamination is the most common sedimentary struc- ture in a prodelta environment. Fisher and others (1969, p. 18-19) indicated that land-plant matter is common in the prodelta. They also state that sediment deposited in the prodelta generally has high organic content which tends to reduce the pH of the interstitial water. This acidic en- vironment may commonly leach any shells that have been in cluded in the sediment. Leaching might account for the lack of microfossils and the rare presence of any macrofos sils in the siltstone. Transition Zone Environment The transition zone environment exists between coas tal sand and shelf mud sediments (Reineck and Singh, 1975, p. 307). It is located just below normal wave base where fine-grained sediments (silt and clay) are deposited. The sand layers are the result of deposition during storms. Reineck and Singh (1975, p. 308) state that shell layers may be present and usually have been transported. Rocks of the transition zone are exposed near the base of the Latigo Canyon section. The parallel-laminated 59 siltstone (lithosome A) and fine-grained, parallel- laminated sandstone (lithosome B) occur in this environment in equal amounts. An equivalent environment in the deltaic depositional system is not noted in the literature and is not identified in the stratigraphic sections. Shoreface/Delta-Front Environments Rocks deposited in the shoreface and delta-front en vironments were recognized in all sections west of Malibu Canyon (Plate 1). There are several processes that control deposition in these environments and they are discussed in the following paragraphs. The shoreface is that environment between mean low water level and effective wave base subject to marine in- fluence only. Rocks interpreted to have formed in such an environment are exposed in the San Nicholas Canyon, Encinal Canyon, and Latigo Canyon sections (Plate 1). The dominant lithosomes include the fine-grained, structureless sandstone (lithosome C) and the coarse-grained, structure- less sandstone (lithosome F). Some of the fine-grained, parallel-laminated sandstone (lithosome B) is also present. Reineck and Singh (1975, p. 305) stated that the dominant sedimentary structure in the shoreface is parallel lamina tion, however, the rate of bioturbation can be high and burrowing organisms destroy the primary sedimentary fea- 60 tures (Reineck and Singh, 1975; McCubbin, 1982, p. 255). Thus, the structureless sandstones (lithosomes C and F) in this environment are interpreted to have been bioturbated subsequent to deposition. High-angle, cross-bedded sandstone units (lithosome D) which occur in the shoreface deposits are the result of megaripple formation and migration due to high-energy storm activity. Cross bedding is more common in the upper part of the shoreface (Reineck and Singh, 1975, p. 305). The poorly sorted, coarse-grained cross beds with barnacle fragments (lithosome D) in the San Nicholas Canyon section (Plate 1) are also an indication of a high-energy environ- ment. Swift and others (1969) indicated that lenses of fossil hash and coarse clastic material, which they describe as storm deposits, are common in the shoreface. The shoreface and delta-front deposits interfinger between the Latigo Canyon and Corral Canyon sections. The section of shoreface deposits thickens from Latigo Canyon to Corral Canyon (Plate 1) which perhaps indicates the Cor ral Canyon section is closer to the supply of sediment. The delta-front deposits are exposed in the lower and middle portions of the Corral Canyon and Puerco Canyon sec- tions (Plate 1). The predominant lithosome is the struc- tures, fine-grained sandstone (lithosome C). Minor amounts of the parallel-laminated, fine-grained sandstone (litho some B) and cross-bedded sandstone (lithosome D) are also 61 exposed. In general, the delta-front is the seaward sloping margin of an advancing delta between mean low water level and effective wave base. This environment receives sedi- ment only sporadically and burrowing organisms can thrive. Bioturbation rate is high under those conditions. Biotur- bation probably accounts for the structureless nature of the structureless, fine-grained sandstone (lithosome C). Small burrows and fossil lenses are commonly present in the delta-front environment (Coleman, 1981, p. 35; Coleman and Prior, 1982, p. 7 3)' but ichnofaunal density is usually low (Elliott, 1978, p • 12 7) . A modern analog for the delta- front environment is the Guadalupe Delta in the Texas Gulf Coast as described by Donaldson and others (1970, p. 148- 1 4 9 ) • They described a sequence which includes fine- grained sand with interbeds of silt that contain some burrows, fossil hash lenses, and coarse-grained sand lenses. This sequence of sand and silt is similar to the interpreted delta-front deposits in the Corral Canyon and Puerco Canyon sections. Distributary Mouth Bar Environment Distributary mouth bar deposits occur within the delta-front deposits in the Corral Canyon and Puerco Canyon sections (Plate 1). The dominant lithosome is the parallel-laminated, fine-grained sandstone (lithosome B). 62 The literature on bar deposits in a deltaic environment in dicates that parallel lamination is common in this type of environment (Reineck and Singh, 1975, p. 269; Coleman and Prior, 1982). The sandstone of the bar deposits contains fossil hash lenses and small, isolated burrows. Dis- tributary mouth bar deposits are at the seaward end of dis- tributary channels in a delta environment. As distributary water emerges from the channel, it suddenly decreases in velocity, and coarser sand is deposited from suspension. As the sand accumulates at the channel mouth it forms first a shoal and then develops into a bar. Reineck and Singh (1975, p. 269) report that the sedimentation rate is higher at this location than at any other place on the delta. This high sedimentation rate would account for the marked increase in the amount of sand that is deposited in the Corral Canyon and Puerco Canyon sections. Lithosome B deposited in a distributary mouth bar is distinguished from lithosome B deposited in the delta front environment by its association with lower delta plain deposits and thicker sand accumulations. Foreshore Environment The foreshore environment lies between the mean low tide mark and the mean high tide mark (Reineck and Singh, 1975, p. 301). Foreshore deposits overlie shoreface deposits at the Encinal Canyon and Latigo Canyon sections 63 (Plate 1). The lithosomes which consist mainly of moderately to well sorted, parallel-laminated sandstone (lithosomes B, E, and H) are interpreted to have been deposited in a foreshore environment. According to Reineck and Singh (1975, p. 301) and Shepard (1973, p. 156-175), the most common bedding type in the foreshore consists of parallel laminations that dip seaward at angles of 2 to 3 degrees. Oscillatory traction currents due to wave action are responsible for depositing and reworking these sedi- ments. The moderate to well sorted sediments indicate that reworking has occurred. Storm activity on the foreshore environment raises the energy of the waves and results in an increase in the size of suspension and saltating par ticles (Komar, 1976, p. 342; Russell and Mcintire, 1969, p. 314-315). The lenses of coarse-grained sandstone and fos- sil hash lenses observed in exposures of foreshore deposits at Latigo Canyon are interpreted to be storm deposits. Moderate-angle cross bedding, such as that exposed in this rock unit at the Latigo Canyon section, is also reported by Reineck and Singh (1975, p. 302) to occur in the foreshore environment. It is possible that these cross-bedded deposits are the result of a ridge-and-runnel system. Backshore Environment Parallel-laminated, medium-grained (lithosome H) and coarse-grained (lithesome E) sandstone with rare, low-angle 64 cross bedding, present in the upper portions of the Latigo Canyon and Corral Canyon sections, are interpreted to be storm washover deposits in a backshore environment. Sedi- ment transport in the backshore is dominated by waves and wave-induced traction currents (McCubbin, 1982, p. 253). Parallel-laminated sandstone (lithosome E) in the Corral Canyon section contains green siltstone rip-up clasts. Such deposits, as described by Reineck and Singh (1975, p. 298), form when storms carry detritus beyond the foreshore and deposit it landward of the beach berm. The wave energy during these storms is high and larger particles such as coarse sand, rock fragments, and body fossil material are carried over the beach berm. Bioturbate structures, such as root mottles and burrows, are common (Reineck and Singh, 1975, p. 299) and could account for the thin beds of struc tureless sandstone (lithosome C), in the Latigo Canyon sec- tion. Greenish-colored siltstone beds (lithosome I) occur interbedded with parallel-laminated sandstone (lithosomes E and H) in the Latigo Canyon section, and are interpreted to be marsh deposits which formed landward of the washover (backshore) deposits beyond the influence of storm waves. The interbedded coarse- to medium-grained, parallel- laminated sandstone (lithosome E and H, respectively) and green siltstone (lithosome I) probably represent oscilla tions of the shoreline. 65 Lower Delta-Plain Environments The lower delta-plain is defined by Coleman and Prior (1982, p. 148) as that portion of the delta which lies es sentially between mean low and mean high tides, subject to both river and marine influences. The foreshore deposits exposed in the middle portion of the formation at the Latigo Canyon section are interpreted to trend eastward and interfinger with lower delta-plain deposits at the Corral Canyon section (Plate 1). These lower delta-plain deposits consist of fine- to coarse-grained, structureless sandstone (lithosomes C and F, respectively) interbedded with red and-green siltstone (lithosome I). East of Corral Canyon, at the Puerco Canyon section, the lower delta-plain deposits thicken. Sandstone beds with low- to medium-angle, trough cross bedding (lithosome D) and rare ripple bedding are also present in this unit. Some of the sandstone beds contain a few fossil hash lenses composed mainly of barnacles. These cross-bedded units are interpreted to be distributary channels. Reineck and Singh (1975, p. 269) state that the most common sedimentary structure is cross bedding. The channels that deposited lithosome D in the study area were probably very small. East of the Las Flores thrust fault (Plate 1), a thin exposure of lower delta-plain deposits is present at Piuma Road. The bed consists of fine-grained sandstone with low angle cross bedding and swales (lithosome D). This bed 66 most likely represents the migration of megaripples and dunes in a small distributary channel. The most common feature of distributary channels is the cross bedding and lag which may consist of clay pebbles and organic debris at the base (Reineck and Singh, 1975, p. 269; Kanes, 1970, p. 95-96). The cross bedding in the Piuma Road section, al- though low-angle, contains rip-up clasts of green siltstone on the slip faces. This indicates there were periods of high velocity traction-current action, Upper Delta-Plain Environments The upper delta-plain is defined by Coleman and Prior (1982, p. 140) to be that portion of the delta which lies above the level of high tide and is unaffected by marine processes. The upper portion of the Puerco Canyon section and virtually all of the exposed portions of the Malibu Canyon and Piuma Road sections are interpreted to be upper delta-plain deposits. These deposits are divided into three subenvironments: marsh, crevasse-splay, and dis tributary channel. Parallel-laminated, red-and-green siltstone (litho- some I) is present in all sections east of the Encinal Canyon section and occurs interbedded with backshore and lower delta-plain deposits (Plate 1). These rocks are in- terpreted to be marsh deposits. The marsh interpretation is based upon the fine silt sizes, red and green colors, 67 and rare traces or evidence of roots and organic matter. Kanes (1970, p. 95) described "high" marsh and "low" marsh deposits. In the Vaqueros strata, the "high" marsh cor responds to the red siltstone and "low" marsh to the green siltstone. The red color is attributed to aerial exposure. Reineck and Singh (1975, p. 297) indicated that roots and plant debris may be present in marsh deposits. Crevasse-splay deposits occur in the lower part of the Malibu and Piuma Road sections. Three different rock types medium-grained, parallel-laminated sandstone (litho some H), fine-grained, bioturbated sandstone (lithosome G), and fine-grained, structureless sandstone (lithosome C) have been observed. The medium-grained sandstone contains parallel laminations, fining-upward sequences, basal silt stone rip-up clasts, and isolated burrows that are perpen- dicular to bedding. The fine~grained, bioturbated sandstone is structureless due to moderate to thorough reworking of the sediments by burrowing organisms. Crevasse-splay deposits characteristically fine upward. The deposits form usually during storm conditions when the water level in the distributary channels attains a level high enough to cut a small crevasse channel into the bank. Water pours through this channel and the current velocity quickly decreases. Sediment-laden water deposits rela tively coarse material into the marsh (Fisher and others, 1969; Elliott, 1978, p. 105). Deposition by traction cur- 68 rents occurs first, and as the water velocity decreases, deposition from suspension predominates. Between episodes of deposition, burrowing animals are able to rework upper portions of the crevasse-splay deposits (Coleman, 1981, p. 41) . Bioturbation is interpreted to have caused the struc- tureless sand (lithosome C and G) found in the splay deposits. The majority of studies done on crevasse-splay deposits report that the most abundant sedimentary struc ture is climbing-ripple lamination (Coleman, 1981, p. 41; Coleman and Prior, 1982, p. 180). The primary structure in the Vaqueros Formation crevasse-splay deposits is parallel lamination, which Reineck and Singh (1975, p. 248) reported can be present in such deposits. In general, siltstone rip-up clasts can also occur in crevasse-splay deposits (MacMillan, i974). Rip-up clasts are composed of the material deposited on the subaqueous levees which was subsequently picked up by fast-flowing currents when the channel bank was breached. Each layer of rip-up clasts deposited in the Vaqueros Formation in the study area might represent a breach episode in the bank. Structureless, coarse-grained sandstone (lithosome F) which interfingers with marsh deposits in the Puerco Canyon section (Plate 1) is not easily explained. Its close stratigraphic association with more positively identified upper delta-plain marsh deposits suggests that it may also 69 ,, ' have been deposited in the upper delta-plain. Such an in- terpretation is supported by the lack of marine fossils or other structures which may indicate a marine environment. Structureless sand is reported by Elliott (1978, p. 126) to form in distributary channels on the upper delta-plain en- vironment. Elliott (1978, p. 126) illustrated an ancient analog for fluvial-distributary channel deposits in south- western Wales. The sandstone he described is massive, has an erosional base, and some cross-stratified lenses. Small, erosional surfaces also are present above the base of the bed. These sandstones are interpreted by Elliott to have been deposited during flood conditions. The internal erosion planes may indicate different flood events or pulses. The deposits are interpreted, therefore, to be rapidly deposited flood sediments in a distributary channel. PALEOCURRENT ANALYSIS Paleocurrent data were collected in six localities from cross bedding attitudes in the cross-bedded sandstone (lithesome D). Because the occurrence of this lithesome is rare, only 37 measurements were taken in the field. The current directions of the beds were plotted on a Wulff stereonet and rotated to horizontal according to methods discussed by Ragan (1973). To aid in the interpretation, the data were then plotted on rose diagrams for analysis 70 (Fig. 11). Measurements from each bed of the cross-bedded sandstone were plotted separately so as to help define any large differences in current direction between measurement localities. Three localities in the San Nicholas Canyon and En cinal Canyon sections yield variable current directions which range from the northwest to the southeast. These current indicators were taken from cross bedding inter preted to have formed in a shoreface environment. The variation in current direction is probably due to the com bination of normal wave and longshore currents, and the ef fects of tides and storm waves. Current directions from cross beds formed in the lower delta-plain environment exposed in the Puerco Canyon section are variable. These cross beds are interpreted to have formed in small distributary channels. Reineck and Singh (1975, p. 269) indicate that in the lower delta-plain the channel flares out and the current direction becomes variable. The cross bedding and swales in the lower delta- plain deposits of the Piuma Road section also yield scat tered and variable current direction. Conclusions about current direction in the study area cannot be made. There is just not enough data from the rocks in the Vaqueros Formation. The data that were col- lected were too variable to yield definite paleocurrent directions. 71 COARSE-GRAINED SUBLITHOSOME PUERCO CANYON N2 N=2 DARSE-GRAINED SUBLITHOSOME COARSE-GRAINED SUBLITHOSOME PUERCO CANYON ENCINAL CANYON COARSE-GRAINED SUBLITHOSOME SAN NICHOLAS CANYON COARSE-GRAINED SUBLITHOSOME SAN NICHOLAS CANYON Figure 11. Paleocurrent data on rose diagrams from measurements taken in lithosome D. 72 PROVENANCE Thin-section analysis indicates that two types of quartz predominate in the sandstones of the Vaqueros Forma- tion in the central Santa Monica Mountains. The first type of quartz has few vacuoles and no microlites. The second, and more common, type of quartz has abundant vacuoles. There is a slight predominance of plagioclase feldspar over orthoclase in the samples (Fig. 12). This characteristic is displayed by 85% of the thin sections from the Vaqueros Formation in the central Santa Monica Mountains. Volcanic rock fragments are present to some degree in every thin section made from rocks collected in the study area. High percentages of these fragments are consistently present in samples from the Malibu Canyon, Puerco Canyon, and Corral Canyon sections. The presence of chert, rounded tourmaline and zircon crystals, and high percentage of quartz in the samples may represent reworked sediments. The suite of hornblende, epidote, and garnet, which are present in thin section, imply a high rank metamorphic source terrane (Pettijohn, 1975). Also, a majority of the quartz in thin section has undulose extinction, which is indicative of the metamorphic quartz variety. However, hornblende with biotite and granitoid rock fragments would imply a felsic igneous source area. There does not seem to be any consistent Q p K ~------~R Q=QUARTZ P= PLAGIOCL.ASB K=ORTHOCLASB 1'- FELDSP All R= ROCK: FRAGMJ!NTS Figure 12. Ternary diagrams showing composition of the sandstones in lithosomes B through H of the Vaqueros Formation, central Santa Monica Moun tains. "-.) w 74 variation of these minerals in the sections from west to east. The clasts in the pebble-conglomerate lenses of the Vaqueros Formation in the Santa Monica Mountains are com posed of mainly quartzite and granitoids with minor amounts of gneiss and volcanics. The rounded to subangular clasts are likely to have been eroded from previously deposited conglomerates. The sedimentary source for the Vaqueros in the study area was probably not the underlying Sespe Formation be cause of the conformable contact between the two in the eastern portion of the study area. In the west, however, there is an unconformity between the Vaqueros and Sespe Formation or, in places, a very thin transgressive lag. It is possible that the immediate source for sediment near the base of the stratigraphic sections in this area came from the underlying Sespe Formation. The stratigraphic relationship between the Vaqueros and Sespe in the Santa Monica Mountains indicates that the river in the fluvial system which deposited the Sespe, probably is the same one that fed sediment to the Vaqueros delta. Therefore, it is probable that the same sources existed for both the Sespe and Vaqueros. Middle Miocene Topanga Formation rests unconformably on granitoid core rocks in the easternmost part of the Santa Monica Mountains and the rocks that rest on the 75 granitoid and metamorphic core of the mountain range become progressively older towards the west (Hoots, 1931). This may indicate that the eastern Santa Monica Mountains were exposed during the time the Vaqueros was deposited. Clastic debris from these granitoid and associated Eocene through Cretaceous-aged sedimentary rocks were shed to the west into the fluvial plain. The Eocene to Cretaceous sec- tion also contains many conglomerate beds which might have provided the clasts that occur in the Vaqueros Formation. The arkosic nature of the Vaqueros Formation in the study area would require a short transport distance which also supports the eastern Santa Monica Mountains as a primary source for sediment. It is possible that the Simi Hills were exposed during the deposition of the Vaqueros Formation in the study area (Blundell, 1983, p. 169). This highland would have provided more reworked sediment onto the fluvial plain. The San Gabriel Mountains are a probable high rank metamorphic source, and exposure during the Hiocene has been suggested by Reid (1978). Crowell (1975) reported 60 km of right slip on the San Gabriel fault since the Miocene. If the 60 km of right slip are removed, the San Gabriel Mountains would be approximately 40 km to the north of the study area. The anorthosite that is exposed in the San Gabriel Mountains may have provided the plagioclase 76 that caused the predominance of plagioclase over orthoclase found in thin-section analysis. At present, there are theories that suggest there has been 90 degrees of clockwise rotation in the Santa Monica Mountains since the late Oligocene (Luyendyk and others, 1980). Figure 13 presents the configuration of areas, and the geometry of fault blocks, including the Santa Monica Mountains, as they appeared prior to rotation. The provenance and paleogeography portion of this study did not take into account this rotation. More data should be col lected from this area before incorporating the rotation theory. PALEOGEOGRAPHIC MAP AND CONCLUSIONS Previous stratigraphic study of the Vaqueros Forma tion indicates that there are two possible orientations for a shoreline in the study area. If the Simi Hills were ex- posed, a general east-west trending shoreline existed just south of the highlands. If the Simi Hills were not ex- posed, then it is possible to have a north-south shoreline east of the Santa Monica Mountains, trending north to the present day location of Big Mountain (Fig. 14). Blundell (1983, p. 167-169) mentions that stratigraphic evidence in the Big Mountain suggests a north-south shoreline existed to the east. In the western half of the study area, wave dominated marine deposition in offshore to backshore en- 77 II8°W + Faults: GF=Garloc fault, SAF= San Andreas fault, NF= Nacimiento fault SYF SYRF=Santa Yn & Santa Ynez River faults, MCF=Malibu Coast fault, SCI=Santa Cruz Island fault, NIF= Newport-Inglewood fault; Place names: DB=Diligencia Basin, SB-Soledad Basin, PR= Plush Ranch Fm., SLO=San Luis Obispo, SBA=Santa Barbara, SW1= Santa Monica Mountains., PVP=Palos Verdes Peninsula. Figure 13. Pre-rotation geometry on southern California about late Oligocene time. Study area is located on the stipled block below the circle denoting the Santa Monica Mountains (modified from Luyendyk and others, 1980). -t-"""~"'"" ...... ~".,.,.,.h'l .. ,,_ ~v ,,,,,,,,, s~ ,,,,,,,,,. i .. .,, .. ,,,,, ;- w, .. .,,,,.,,,,, i ''",,, ~ ~.,, ·~ ..,..,,..,,,, .. , 1.,, . "'''u''"••w ,... ,~... ~ .... An '"'•~n~,,,,l ~.... ~::::.Ar1 ''"",,; '-;...... ·-··· ,,,, ? .· ' r··-- .· Gr ,, ? --? ~~ .....;.?~ •• /::::: • c ., . r--:. ..,~.-... ;::::::·· Gr:1 \. /'~ :.:>- . . ..:> ( 7 \.. _)····· :.-,.. SG • ., .....,, .. ,,. ...· :I··· . ···,.:~·-...,,,,,, _../· y·· j.-· shorel1ne ·.. '''•i i ;.. ·· ~ y,,,,, .: / during"? "'··.;:_f"'"J~ I" . ' .,~ ··. "\.t""":J).,.,~ . !y··· ~ Vaqueros \ !;-,... ~ ,,,,, ... I ' ?. deposi tio~ [ ? / \1\111111/, ~···'S.Hy} .:....-.., , shoreline A ,.,..,'\'-. I~ :., ,,~' .....,~'''"'''' ~ ...... ---- ·:::j·-;:::· S H ~ ? •;·~~-=i·-~::· E>H ~ dur1ng --...-=. 11-i, i g •}~J~ '1./l:lfiUIIItlf~ 1 «'i~~-~'".I.U!''!.!! -. Vaqueros --•wd ' depOSltlO, , ~ ~ ? Figure 14. Shows two possible orientations for the shoreline that was present during the deposition of the Vaqueros Formation in the study area. An=Anorthosite, Gr=Granitic rocks, SH=Simi Hills, SM=Santa Monica Mountains, SG=San Gabriel Mountains (modified after Corey, 1954). -....) I.XJ 79 vironments took place along an erosional coastline. A large fluvial system which flowed generally from northeast to southwest entered the ocean and deposited the Vaqueros Formation as a prograding river-dominated delta in the eastern half of the study area (Fig. 15). Sedimentary structures and textures noted in the marine deltaic deposits of the Vaqueros Formation indicate that the environments probably were subjected to little tidal action and wave influence. Marine deposition in the western portion of the study area probably took place on a shallow-sloping shelf off a low-energy, possibly protected coastline. The western portion of the area was affected a little more by wave action and storm activity. Poorly sorted, immature to submature arkose found throughout the formation is an indication that sediment was not exten sively reworked by tidal or wave currents and that it was deposited after a short transport distance. The pectinids and barnacle fragments found in the Vaqueros Formation sup port deposition in a shallow-water, shoreface environment with a rocky shoreline nearby. From west to east, rocks of the Vaqueros Formation that overlie the Sespe Formation become progressively shal lower. This is an indication that the contact between the two represents a transgression of the sea. This contact may be an unconformity and there may be a thin transgres sive lag between the Vaqueros and Sespe Formations in the { ------f / ·. ----- ~ .· \;· . '• Backshore -- ',, {' . ). . ---~~~~:~~~·c.·.·.··:· ' ~ . . ---.c-.~hi>t<>'· · · · .c, " ' '< =J'~r:e"'Sh ~- ~ ~ ~ ~-- <~l~i{;~:., .... -:;\ '',' '- ( ,•' \ ~... reface , '<··.:'.. "·' ..._, : .·····¥·. \. .. ···. " , / / 0 ,, . . ·'' ' ' ...•. , ' ' .. •• / • , .. ,::~-;:-~:-.:· ... · ¥ i .., '\.,~ .. ~ ) -- ...·-- · .. ""· .... ·~...... ,~·~l·~~~,.~=t., - .. "" r -. , .. ··.- ' ' '""" •·-,,, Cre1tas \ '·- '·'· _ _ TransHion? '';:-.> • , <·" , i·-,c.~--;-~ SPlaY,·.. ;_.~·: ... :.·.::;. · =:::------...,. '-'::· .. :' ' ' : ' ' ' ' .. ·, .. / ': /· ... -::·;.:·:.:";: . --= '--~', '.: ,.,, ' • Marsh ' ' )·"~i[(,_, ':<":O·: .·; --' ,_,,,,, ' ' ¥' ,• / •· ...... Offshore ','~~ ~~'.\~.~-: ~~ ,...... } .....·· .... ~... /\! Upper ~ta /.~/·;;·. · ~ ~ ·-'t.;:.!~~:-.... _, ....·. "··' . ' .."111·,,_ Plaoa}, / /. ... / ·/ ',, ..... :~ . '<:¢~'' , :_ --c-·r' ;,; . ', ...... ~.. 'Cc~·>p.:-;-::-.:-•.·.::··.~~ /j / --,_ "' '"''--::..~~;::..:\~ / / ' ·. , , ·:.0',. , "' B , / ·'· ..... ' /' ' ' -·.... ()u·•:"'' • / / 0 ·1 2 3 ', ftC!/ 5 km ' ...... - - ..__ ..__ ----Delt C1J 0 81 western portion of the study area. As the shoreline transgressed from the area of the Corral Canyon section to the Puerco Canyon section, the Vaqueros and Sespe Forma tions began to interfinger and the contact between the two is conformable. The shoreline probably advanced a short distance northeast of the Puerco Canyon section. At this point, the delta prograded toward the southwest, and deposition from shallow-water and subaerial environments occurred over the entire study area. A width of approximately 10 km for the Vaqueros delta was estimated by measuring the distance from the eastern extent of the Vaqueros Formation outcrops in the Santa Monica Mountains to a point midway between the Latigo Canyon and Corral Canyon section. A fluvial setting similar in size to the Santa Clara River in the Oxnard Plain is postulated for the study area during the time of Vaqueros Formation deposition. Streams on the southern flank of the San Gabriel highland contributed sediment to the river that flowed southeast along the base of the moun- tain. This river probably joined others as it flowed into the fluvial valley toward the ocean. Throughout the time of deposition of the Vaqueros Formation, it is reported that sea level rose steadily (Vail and others, 1977). 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E., 1971, Preliminary geologic map of the Malibu Beach quadrangle, Los Angeles County, California: U. S. Geological Survey open file map, scale 1:12,000, 2 sheets. Yonge, C. M., and Thompson, T. E., 1976, Living marine molluscs: London, William Collins Sons and Company, Ltd., 228 p. Zingg, T., 1953, Beitrag zur schotteranalyse: Schweiz. Mineral. Petrogr. Mitt., v. 15, p. 39-140. 92 APPENDIX 1 LOCATION OF MEASURED SECTIONS AND CSUN FOSSIL LOCALITIES San Nicholas Canyon measure Topanga- Malibu Sequit just south of section 19, 1 S., R 19 W., Triunfo Pass 7.5-minute quadrangle, California. Encinal Canyon measured section (2), within Topanga-Malibu Sequit south of section 28, T. 1 S., R. 19 W., Point Dume 7.5-minute quadrangle, California. 93 Latigo Canyon measured section , S., R. 18 W., Point Dume 7.5-minute quadrangle, California. Corral Canyon section measured (4), T. 1 S., R. 18 W., Point Dume 7.5-minute quadrangle, California. 94 Puerco Canyon measured section (5), T. 1 S., R. 17 W. and T. 1 S., R. 18 W., Malibu Beach 7.5-minute quadrangle, California. 95 Nalibu Canyon measured section (6), T. 1 S., R. 17 W., Nalibu Beach 7.5-rninute quadrangle, California. Piuma Road measured section (7), T. 1 S., R. 17 W., Malibu Beach 7.5-minute quadrangle, California.