CALIFORNIA STATE UNIVERSITY, NORTHRIDGE
DEPOSITIONAL ENVIRONMENTS OF THE EOCENE THROUGH OLIGOCENE SESPE FORMATION IN THE NORTHERN SIMI VALLEY AREA VENTURA COUNTY, SOUTHERN CALIFORNIA
A thesis submitted in partial satisfaction of the requirements for the degree of Master of Science in
Geology
by
Gary Edward Taylor
May 1984 The Thesis of Gary Edward Taylor is approved:
D£7. I v'a'K P. Col burn
Dr. A. Eugeqe Fritsche
Dr. Richard t. Squ-fres, Ct;ommittee- Chairman
California State University, Northridge
ii CONTENTS
Page
ABSTRACT vii INTRODUCTION 1 PREVIOUS WORK 4 AGE 5 METHODS 5 STRATIGRAPHY 9 MEASURED SECTIONS 10 SEDIMENTATION RATE 12 ACKNOWLEDGEMENTS 13 DESCRIPTION AND DEPOSITIONAL ENVIRONMENTS 14 SANDY BRAIDED-RIVER DEPOSITS 14 BRAIDED-RIVER SANDSTONE LITHOSOMES 16 BRAIDED-RIVER CONGLOMERATE LITHOSOMES 24 BRAIDED-RIVER MUDROCK LITHOSOMES 28 DISCUSSION OF BRAIDED-RIVER DEPOSITS 29 MEANDERING-RIVER FLOODBASIN DEPOSITS 30 MEANDERING-RIVER SANDSTONE LITHOSOMES 31 MEANDERING-RIVER CLAYSTONE LITHOSOMES 35 MEANDERING-RIVER CONGLOMERATE LITHOSOME 38 DISCUSSION OF MEANDERING-RIVER FLOODBASIN DEPOSITS 39
iii Page
LOWER-DELTA PLAIN DEPOSITS 42 LOWER-DELTA PLAIN FINE SANDSTONE LITHOSOME 42 LOWER-DELTA PLAIN CLAYSTONE LITHOSOME 46 LOWER-DELTA PLAIN, MEDIUM TO COARSE SANDSTONE LITHOSOME 47 DISCUSSION OF LOWER-DELTA PLAIN DEPOSITS 48
PALEOCURRENTS AND PALEOCLIMATE 50 PALEOCURRENTS 50 PALEOCLIMATE 50 PROVENANCE 53 DESCRIPTION 53 DISCUSSION 55 PALEOGEOGRAPHY 59 LOWER DEPOSITIONAL PHASE 59 MIDDLE DEPOSITIONAL PHASE 61 UPPER DEPOSITIONAL PHASE 61 SESPE/VAQUEROS TRANSITION PHASE 65 REFERENCES CITED 67
iv LIST OF ILLUSTRATIONS
Figure Page
1. Index map of the Simi Valley area 2
2. Generalized stratigraphic section of Simi Valley area 3
3. Explanation for stratigraphic sections of the Sespe Formation north of Simi Valley 7
4. Measured sections of the Sespe Formation north of Simi Valley 8
5. Generalized stratigraphic relationships of the Sespe Formation north of Simi Valley 11
6. Stratigraphic position of the Sespe/Vaqueros transition north of Simi Valley 13
7. Sketch of Lenticular Conglomerate Lithosome 22
8. Photo of Parallel and Low-Angle Cross-Bedded Lithosome 22
9. Photo of Lenticular Conglomerate Lithosome 25
10. Photo of Stratified Pebble-Cobble Conglomerate Lithosome 27
11. Photo of typical fining upward sandstone/claystone sequences of the middle member 30
12. Photo of botryoidal, fibrous-calcite concretion 34
13. Photo of claystone intraclasts within Bedded Sandstone Lithosome 36
14. Photo of Lenticular Sandy Claystone Lithosome 38
15. Photo of Matrix-Supported Pebble-Cobble Conglomerate Lithosome near Canada de la Brea fault 40
16. Photo of calcareous rhizomorph in Mottled Rhizomorph-Bearing Claystone Lithosome 47
17. Paleocurrent rose diagrams from lower and upper members 51
18. Composite stratigraphic section of the Sespe Formation showing stratigraphic position of depositional phases 60
v Figure Page
19. Diagrammatic paleogeographic reconstruction of the Sespe Formation north of Simi Valley 62
20. Diagrammatic paleogeographic reconstruction of the lower and middle depsoitional phases 64
21. Diagrammatic paleogeographic reconstruction of the upper and Sespe/Vaqueros transition depositional phases 66
Table Page
1. Vertebrate megafossils of the Sespe Formation north of Simi Valley 6
2. Comparison of calculated stratigraphic thicknesses of the members of the Sespe Formation 11
3. Distinguishing characteristics of the lithosomes seen in the Sespe Formation north of Simi Valley 15
4. Petrology of lower member samples 17
5. Petrology of upper member samples 18
6. Heavy mineral analysis of samples from the lower and middle members 19
7. Clast counts from the lower and upper members 25
8. Petrology of middle member samples 32
9. Heavy mineral analysis of samples from the middle member 33
10. Clast count from one locality in the middle member 40
11. Petrology of Sespe/Vaqueros transition samples 43
12. Heavy mineral analysis of samples from the Sespe/Vaqueros transition 44
13. Comparison of compositional assembleges of the Sespe Formation 54
Geologic Map In pocket
vi ABSTRACT
DEPOSITIONAL ENVIRONMENTS OF THE EOCENE THROUGH OLIGOCENE
SESPE FORMATION IN THE NORTHERN SIMI VALLEY AREA
VENTURA COUNTY, SOUTHERN CALIFORNIA
by
Gary Edward Taylor
Master of Science in Geology
The nonmarine Sespe Formation (early late Eocene through late
Oligocene age) is well exposed in a northward-dipping, homoclinal
sequence of strata north of the Simi anticline, northern Simi Valley,
Ventura County, California. Three members and one transitional zone
were recognized in the formation.
The lower member and lower three-fourths of the upper member
represent sandy braided-river deposits. Braided-bar deposits consist
of parallel, low-angle, and trough cross bedded, medium to coarse
lithic arkose and cross-bedded pebble conglomerate. Structureless and
graded lithic arkose and structureless, silty claystone represent overbank-sheetflood and braided-river floodplain deposits. Migrating,
primary channel-fill deposits consist of intraclast-bearing sandstone
vii and clast supported, pebble-cobble conglomerate. Anabranch channel- lag deposits are represented by discontinuous, stratified pebble- cobble conglomerate beds. Climbing-ripple laminated, carbonaceous, mudstone indicates braided-river swamp (pond) deposits.
The middle member consists predominantly of cyclical sandstone and claystone with lesser amounts of conglomerate and represents meandering-river floodbasin deposits. Structureless and graded, medium to coarse lithic arkose and mottled, structureless, claystone represent overbank-sheetflood and suspension deposits. Crevasse-splay deposits consist of cross-bedded, intraclast-bearing, medium to coarse lithic arkose and lenticular sandy claystone beds. Rare matrix- supported pebble-cobble conglomerate represents meandering channel- fill deposits.
The upper one-fourth of the upper member (Sespe/Vaqueros transition) represents lower-delta-plain deposits that are gradational ,.., with overlying wave-dominated delta-front deposits of the Vaqueros
Formation. Parallel-bedded, medium to coarse, intraclast-bearing lithic arkose represents distributary channel-fill deposits. Laminated well-sorted fine arkose occurs near the top of the formation and represents foreshore deposits. Mottled, rhizomorph-bearing claystone occurs near the top of the formation and represents salt-marsh deposits.
The provenance appears to be primarily from reworked Paleogene formations which crop out in the Simi Hills area. Less dominant and more distant mafic and high-rank metamorphic source areas may have originated from the southern San Gabriel Mountains. In addition, the
viii rare occurrence of anorthosite clasts in the upper member may indicate a northern San Gabriel Mountains source.
The Sespe Formation north of Simi Valley represents a south eastward-transgressive fluvial to deltaic sequence, which is indicated by four depositional phases. The lower phase represents a braided river floodplain which onlapped onto a pre-Sespe erosion surface in the Simi Valley area. The middle phase consists of meandering-river floodbasin deposits resulting from infilling of a stable depositional basin. The upper phase consists of northwesterly-flowing braided river deposits which, based on stratigraphic position, were deposited in a upper-delta plain environment. The Sespe/Vaqueros transition phase represents lower-delta plain deposits which moved into the area by continued southeastward transgression of the "Vaqueros shoreline".
ix INTRODUCTION
The nonmarine Sespe Formation in northern Simi Valley, Ventura
County, California (Fig. 1; Geologic Map in pocket) is well exposed in
a broad, homoclinal sequence of strata that crops out between the Simi
Valley and the southern flank of Big Mountain.
The formation is of particular economic interest in the Simi
Valley area because it is the major oil-producing horizon in the
Simi, Canada de la Brea (C.D.L.B.). and Big Mountain oil fields.
The scope of this paper is to describe, in detail, the Sespe
sedimentary deposits and to determine their depositional environments
by the use of modern stratigraphic analysis techniques. Based on a
study of detailed measured sections as well as on a lateral tracing of
beds in a 38 km 2 area, the following depositional environments are recognized: 1) sandy braided river, 2) meandering-river floodbasin, and 3) lower delta plain.
Early workers did not discuss depositional environments of the
formation, and they obtained stratigraphic thicknesses from ortho
graphic projections, cross sections, and/or well logs. Later workers
discussed only the generalized depositional environments of the formation in the Simi Valley area.
The Sespe Formation attains a maximum thickness of 1,656 m, which comprises approximately one-fourth of the entire thickness of the
Cenozoic section found in the Simi Valley area (Fig. 2).
1 Rl'ler Piru ··-··. .../··· "'ountaln •.• --···-· Fillmore~ \Cl --···-c••'···- ... - Cl\0 Newhall .. 0 ~>Jdl• ~ 0 oak .. ~\::1 Sa171 :l s \ s~s "'ountaln \ a,., [·:· ·sruar· AREA'.(.:pll \ - 1+~~~-i~/.:\:/;::: ~·····'''' Valley ,,. '~-' 0 Chatsworth sun I ...J r Canoga Park 0 Reaeda I o ~it c,.,. 'c N sa,,il
'Monica Mountains
0 5 10
Miles 0 5 10 20 34° Point Dume Kilometers 119° 118.30'
Figure 1. Index map of Simi Valley and vicinity.
N 3
SERIES/ SYSTEM Sub series ,:G:.:R.:.:A:;P:-;H:.:I:.::C~L.::O:;G;.-____=.:..::..:..:.:::..:::=...:.....:::....:==:!:::!!.....!.!.!;~~:.!:..::::....______, .,; Up . Ill ~ c Gi ~.-. II NONMARINE SANDSTONE & CONGLOMERATE 0::: ...0 . .w-· ... c Lower SAUGUS FORMATION .2 c. c. MARINE SANDSTONE & COOUINITE a: ::1 ? ? Up? MODELO FORMATION ? MARINE DIATOMACEOUS MUDSTONE & SANDSTONE Q) 1: c Q) :;; CALABASAS FORMATION 0 ... 0 i :i tow ?- ? MARINE SANDSTONE AND SILTSTONE Q) 1: Q) SESPE FORMATION 0 0 NONMARINE SANDSTONE, CONGLOMERATE, & !?} MUDSTONE 0 ? ? a:> < ... •a. i= a. a: ::> w Cl) 1: ... Cl) 0 0 w 7... LLAJAS FORMATION (545m) ... NONMARINE BASAL CONGLOMERATE; MARINE SANDSTONE & i- SILTSTONE Low SANTA SUSANA FORMATION (10 MARINE SANDSTONE AND MUDSTONE, LOCALLY CHANNELIZED CONGLOMERATE li c. Q) c. 1: ::1 Cl) ·.:.:._··.:. ·. ·: ·.. 0 :·. 0 ? .! (195m) a! LAS VIRGENES SANDSTONE ... NONMARINE TO MARINE SANDSTONE AND MUDSTONE 0.. c II 0 SIMI CONGLOMERATE (150m) ... NONMARINE TO MARINE CONGLOMERATE AND SANDSTONE
::. (1830•m) "'0 CHATSWORTH FORMATION ...... Ill Q.U MARINE SANDSTONE, CONGLOMERATIC; MUDSTONE ::>5o..E
Figure 2. Generalized stratigraphic section of the Simi Valley area (after Squires and Filewicz, 1983). 4
PREVIOUS WORK
Watts (1897) first named and described the Sespe "brownstone"
Formation at the type section along lower Sespe Creek, approximately
27 km northwest of Simi Valley, California (Fig. 1). Eldridge and
Arnold (1907) discarded the term '~rownston~' and redefined the upper boundary which Kew (1924) later considered as within the lowermost portion of the Vaqueros Formation. Kew (1919, 1924) mapped the formation in the Simi Valley area and described lower, middle, and upper members, which were later mapped in detail by Hetherington
(1957), Pasta (1958), and Van Camp (1959).
Subsurface and structural geologic studies were conducted by
Canter (1973) and Hanson (1981). Economic geology was discussed by
Johnson (1913), Kew (1919, 1924), Stipp (1943), Hall and others
(1967), and Kimmel and others (1983).
Vertebrate faunal studies were done by Stock (1932, 1948), Wilson
(1949), Savage and Downs (1954), Golz (1976), Golz and Lillegraven
(1977), and Lander (1983).
Regional sedimentological studies of the Sespe Formation that involve the Simi Valley area are those of Gianella (1928), Reed
(1929), Baily (194 7), Flemal (1966), McCracken (1972), and Black
(1982). Gottsdanker (1939) conducted a more localized study on the sedimentation of the Sespe Formation north of Simi Valley. He empha sized clast composition, and his stratigraphic thicknesses were deter mined from cross sections that did not include the uppermost portion of the formation. 5
This paper represents an expanded version of the depositional
environments discussed in Taylor (1983a). Field trip stops in the
Sespe Formation in the northern Simi Valley area are discussed in
Taylor (1983b).
AGE
The age of the Sespe Formation north of Simi Valley, based on
vertebrate fauna (Table 1) is early late Eocene (Uinta "C" North
American Land Mammal "Age"; Stock, 1948; Golz, 1976; Golz and
Lillegraven, 1978) through late Oligocene (early late Arikareean North
American Land Mammal "Age"; Wilson, 1949; Lander, 1983). Blake
(1983), based on benthonic foraminifera, states the base of the
Vaqueros Formation at Big Mountain is late Zemorrian (late Oligocene)
in age.
METHODS
Approximately 62 days were spent in the field between late spring
of 1982 and late summer of 1983. During these days, six stratigraphic
sections were measured by the Jacob's staff method in well exposed
areas of approximately equal distance along strike (Figs. 3 and 4;
Geologic Map in pocket). In addition, outcrops were studied between
the measured sections in order to provide information on the lateral
continuity and variation of strata within the formation. Geologic
contacts, mapped by previous workers, were field checked and addi tional contacts were added (Geologic Map in pocket).
Thin sections were cut from 25 rock samples and orthoclase grains were stained. Heavy mineral analysis was done on ten samples using 6
TABLE!. VERTEBRATE MEGAFOSSILS OF THE SESPE FORMATION IN NORTHERN SIMI VALLEY (COMPILED FROM GOLZ AND LILLEGRAVEN, 1978; AND LANDER, 1983)
LOCAL FAUNA": TAPO AND BREA PEARSON ALAMOS CANYON RANCH CANYON NORTH AMERICAN UNITIAN "C" DUCHESNEAN L. ARIKAREEAN LAND MAMMAL "AGE": early late Eoc. late Eocene late Oli~ocene STRATIGRAPHIC LOWER PORTION UPPER PORTION SESPE/VAQUEROS HORIZON: MIDDLE MEMBER MIDDLE MEMBER TRANSITION ENVIRONMENT: MEANDERING RIVER FLOODBASIN LOWER-DELTA PLAIN TAXA COMMON NAME FISH Chondrichthyan Shark REPTILES Chelonia Fresh-Water Turtles Crocodilia Crocodile Peltosaurus macrodon Lizard Boavus affinis SnakeI BIRDS Aves indet. Bird MARSUPIALS Nanode1Qh:£S californicus Opossum like Peratherium sp. Opossum like INSECTIVORES Simidectes merriami Otter like AQatemys bell us Otter like PRIMATES D:tseolemur 12acificus Lemur Craseo12s sylvestris Tarsier Chumashius balchi Lemur RODENTS Simimys simplex Jumping Mouse Leidymys nematodon · Mouse Micro12aramys tricus Squirrel like Le12totomus taQensis Squirrel like EohaQlomys matutinus Squirrel like GriQhomys alacer Gopher Archaeolagus? sp. Rabbit Ses12edectes singularis Porcupine like Proterixoides J2Umilus Porcupine like CARNIVORES Plesiomiacis I!rogressus Dog like Ha:tenodon venturae Dog like UNGULATES Perissodactyls Duchesneodus californicus Brontothere TripluQus? woodi Primitive Rhino Am:tnodon sp. Primitive Rhino DiloEhodon sp. Tapir Artiodactyls Ta12ochoerus egressus Primitive Swine Simimer:tx hudsoni Mouse Deer H:tJ:!ertragulus hesperius Mouse Deer Protoreodon pumilus Oreodont ProtyloQUS stocki Primitive Camel ProtylOJ2US 12earsonensis Primitive Camel 7
9 '
EXPLANATION
GRAIN SIZE
MUD ROCK
SANDSTONE
EXTRAFORMATIONAL CONGLOMERATE
INTRACLASTIC CONGLOMERATE
SEDIMENTARY STRUCTURES
PARALLEL BEDDING
LOW-ANGLE CROSS BEDDING
TROUGH-CROSS BEDDING
RIPPLE LAMINATION
DESICCATION CRACKS
BURROWS
PLANT OR WOOD FRAGMENT
CONTACTS
SCOUR CHANNEL UNCONFORMITY TRANSITIONAL ••••••••••• INTRAFORMATIONAL
SV-51 CSUN THIN SECTION LOCALITY
CSUN 565 CSUN FOSSIL LOCALITY
Figure 3. Explanation for stratigraphic sections of the Sespe Formation north of Simi Valley (nodified after Selley, 1978). 8
UPPER MEMBER
(SANDY BRAIDED-RIVER DEPOSITS)
TAPO RANCH Modelo Fm.
0 BASE NOT BEEN
600
MIDDLE MEMBER
(MEANDERING-RIVER FLOODBASIN DEPOSITS) MARA RANCH Modele Fm. 497
~~!ili:f-':"' Canada da Ia Br .. fault
( Repellllon of Strata
...,._..."-;:_:..atrathern fault 200- ,.-.... ,:•.: •...... ··················· SV-1515 ..·····•·····•···· ...... SV-66 LOWER MEMBER SV-72 :-;:,,1:,; 0 ~:!::~~ ...... ··········· BASE NOT SEEN (SANDY BRAIDED-RIVER DEPOSITS) SV-152
Lla)as Fm. !1}\!.{; ,_., 0 BASE NOT METERS SEEN
Figure 4. Measured sections of the Sespe Formation north of Simi Valley (see Geologic Map in pocket for location of sections and Figure 3 for explanation of symbols). the techniques of Friedman and Johnson (1982). Percent composition of thin sections and heavy minerals was determined by point counts of 100 to 350 grains. The above samples are on file at California State
University, Northridge (CSUN).
Rock sorting, maturity, and classification are based on the terminology of Folk (1980). Grain size is according to the scale of
Wentworth (1922). Rock colors follow those of Goddard (1970).
Five clast counts were done in the field by identifying 100 contiguous clasts. Clast shape is based on the classification of
Zingg (1935).
Nomenclature and definitions of depositional environments are from Allen (1965), Reineck and Singh (1980), and Coleman and Prior
(1982). Lithofacies codes of Miall (1977, 1978) and Rust (1978) also were utilized to aid in determining fluvial environments.
Paleocurrent data from the lower and upper members were collected in the field by utilizing the methods of Rust (1972). His methods consist of measuring imbrication-surface-attitude of disc-shaped clasts and measuring trend and plunge of elongated-clast axes. The data were processed using "Orient" (Cooper and Marshall, 1981) and
"Rosenet" (Williams, 1980) computer programs, which are designed for determining paleocurrent directions (program modified by Stan Popelar,
CSUN).
STRATIGRAPHY
The Sespe Formation in the Simi Valley area unconformably overlies the late early through early medial Eocene age Llajas Formation 10
(Squires, 1981, 1983a; Filewicz and Hill, 1983). The basal contact is well exposed east of Tapo Canyon, whereas, to the west, the base is not seen and occurs only in the subsurface of the Simi anticline
(Geologic Map in pocket).
The late Oligocene to early Miocene age Vaqueros Formation
(Blake, 1983) is transitional with the uppermost strata of the Sespe
Formation in the Big Mountain and Alamos Canyon areas. The medial
Miocene age Calabasas Formation (Fritsche and others, 1983) overlaps the Sespe Formation in the Oak Canyon area, whereas the medial Miocene age Modelo Formation (Clark, 1983) overlaps it in the Tapo Ranch and
Marr Ranch areas. The late Pliocene age lower Saugus Formation
(White, in preparation) overlaps the Sespe Formation in the Las Llajas
Canyon area and west of the study area. Eastward thinning of the
Sespe Formation is created by the overlapping Neogene deposits. A generalized summary of the stratigraphic relationships of the Sespe
Formation in northern Simi Valley area is shown in Figure 5.
MEASURED SECTIONS
The cumulative stratigraphic thickness of the Sespe Formation north of Simi Valley is 1,656 m. Maximum thicknesses of the lower, middle, and upper members are 292 m, 610 m, and 754 m, respectively.
Discrepancies in cumulative and individual member thicknesses exist between previous workers and this paper (Table 2). The discrepancies are probably the result of 1) constructing cross sections through areas where faults were not previously mapped or 2) including strata of the Calabasas Formation within the Vaqueros Formation. In the Brea 11
WEST EAST
Modelo Forrndon
upper member
S.ape Formation middle member ... --.. --·----·---- lower member
Uajaa Formation
Figure 5. Generalized stratigraphic relationships of the Sespe Formation in the northern Simi Valley area.
TABLE 2. COMPARISON OF CALCULATED STRATIGRAPHIC THICKNESSES OF THE MEMBERS OF THE SESPE FORMATION, NORTH OF SIMI VALLEY (THICKNESS IN METERS)
KEW STOCK GOTTSOANKER HETHERINGTON PASTA VAN CAMP THIS 1924 1932 1939 1957 1958 1959 PAPER UPPER MEMBER §853 1,280 §792 §457 §610 §556 754 m MIDDLE MEMBER §366 439 §671 §640 §537 610 m LOWER §244* MEMBER §671 558 §427 §518 §51 B** 292 m TOTAL §1,890 2,277 §1890 §1 ,615 §1391 * 1,656 m METHOD: CROSS NOT CROSS CROSS CROSS CROSS JACOB'S SECTION KNOWN SECTION SECTION SECTION SECTION STAFF KEY: §= Approximate thickness. *= Brea Canyon area (base not seen). **= Marr Ranch area (thickness from Hetherington, 1957). 12
Canyon area (Fig. 3; Geologic Map in pocket), repetition of strata occurs between the Strathern and Canada de la Brea faults. These faults do not appear on Kew's (1919, 1924) geologic maps which were used by Gottsdanker (1939) for constructing cross sections.
Correlation between the Marr Ranch and Tapo Ranch sections (Fig.
3; Geologic Map in pocket) was determined by comparing well-log data from Tapo Fee 15-A (provided by M. Katz, Getty Oil Company, Ventura) with the surface sections and by tracing strata laterally. Tapo
Ranch, Oak Canyon, and Brea Canyon sections (Fig. 3) were correlated laterally along the basal contact of the upper member. Correlation from the Oak Canyon section to the Alamos Canyon section was done by tracing a distinct sandstone bed which contains abundant, horizontally stratified, pebbles.
The transitional zone between the upper member and the overlying
Vaqueros Formation in the Alamos Canyon and Big Mountain areas is marked at the base by the lowermost appearance of fine, well sorted, marine megafossil-bearing sandstone and at the top by the uppermost appearance of red-green mottled, rhizomorph-bearing claystone (Fig. 6).
SEDIMENTATION RATE
The average sedimentation rate of the Sespe Formation in the northern Simi Valley area, based on the time scale of Berggen and others (in press), is approximately 9 em / 1,000 years, and indicates a slow to moderate rate of sedimentation. 13
Vaquero• Formation
aeape Formation (upper member) ...... ~
STUDY AREA
Figure 6. Stratigraphic position of the Sespe/Vaqueros transition zone in the northern Simi Valley area. The dashed line indicates the uppermost occurrence of red-green mottled, rhizomorph bearing claystone, and the dotted line indicates the lowermost occurrence of marine fossils.
ACKNOWLEDGEMENTS
I would like to thank Dr. R. L. Squires for introducing me to the
Sespe Formation north of Simi Valley and for his helpful guidance in the field work and writing stages of this thesis. Dr. A. E. Fritsche provided valuable comments on sedimentary structures and their origin and critically read the manuscript. Dr. I. P. Colburn critically read the manuscript and offered helpful comments. Financial assistance was, in part, provided by the CSUN Geology Alumni Trust Fund and the
Getty Oil Company Grant-In-Aid Program. D. R. White, J. D. Parker, D.
A. Yamashiro, S. J. Popelar, and J. Friedman assisted in field and laboratory work. DESCRIPTION AND DEPOSITIONAL ENVIRONMENTS
The Sespe Formation north of Simi Valley represents deposition in three distinct environments: 1) sandy braided river, 2) meandering river floodbasin, and 3) lower-delta plain. The upper one-fourth of the upper member represents transitional nonmarine to marine deposits that indicate the transgression of the Vaqueros Formation (see
Blundell, 1981, 1983). These transitional deposits will be referred to herein as the "Sespe/Vaqueros transition".
The deposits that represent the three depositional environments were subdivided into lithosomes which are based on differences in grain size, composition, sedimentary structures, and color. Table 3 shows some of the distinguishing characteristics of the lithosomes, which are arranged in order of decreasing grain size.
SANDY BRAIDED-RIVER DEPOSITS
The sandy braided-river deposits occur in the lower member and lower three-fourths of the upper member (Fig. 4). The deposits consist of interbedded medium to coarse sandstone (80%), pebble-cobble conglomerate (10%), and mudrock (10%). Sandstone and conglomerate are most common in the lower portions of the members, whereas sandstone and mudrock are more abundant in the upper portions. The lower member unconformably overlies the Llajas Formation and is gradational with overlying meandering-river floodbasin deposits
(middle member). The braided-river deposits of the upper member are gradational with both underlying meandering-river floodbasin deposits
14 TABLE 3. DISTINGUISHING CHARACTERISTICS OF LITHOSOMES SEEN IN THE SESPE FORMATION
LITHOSOME DISTINGUISHING CHARACTERISTICS Lenticular Conglomeraj:;e__ _ __ l-_entjcular, clast_support_ec:h__§uctureless__ to vaguely bedded, pebble-cobble conglomerate. Stratified Pebble- Isolated, discontinuous, tabular to somewhat lenticular beds consisting of matrix supported Cobble Conglomerate pebbles and cobbles. Cr.oss-Bedded Pebble Low-angle cross-bedded pebble conglomerate beds consisting of a single layer of imbricated Conglomerate bladed pebbles. Matrix-Supported Pebble- Lenticular, matrix-supported, pebble-cobble conglomerate. Cobble Conglomerate Yellow-Orange Moderately sorted, limonitic, lithic arkose. Color ranges from grayish orange (10YR 7/4) to Structureless Sandstone dark yellowish orange ( 1OYR _6/_§) • Light-Gray Moderate- to well sorted, biotitic, lithic arkose. Color is light gray (N7). Commonly Structureless Sandstone associated with Mottled Claystone. Claystone Intraclast Rounded claystone intraclasts within sandstone matrix. Yellow-Orange Graded beds within Yellow-Orange Structureless Sandstone. Color ranges from grayish orange Graded Sandstone (10YR 7/4) to dark yellowishorange (10YR_6/6). Light-Gray Color is light gray (N7). Occurs within Light Gray Structureless Sandstone. Graded Sandstone Parallel and Low-Angle Lenticular units containing parallel and low-angle cross bedding. Cross Bedded Sandstone Low-Angle and Trough Lenticular'units containing low-angle and trough cross bedding. Cross Bedded Sandstone Bedded Sandstone Parallel, low-angle and planar cross bedding. Contorted claystone intraclasts. Parallel Bedded Sandstone Parallel bedded lithic arkose containing floating and vaguely stratified pebbles, cobbles and claystone intraclasts. Laminated Fine Sandstone Parallel and low-angle cross laminated, well-sorted arkose. Rare marine megafossils. Climbing-Ripple Climbing-ripple laminations. Abundant carbonized plant fragments. Laminated Mudstone Structureless Silty Claystone Structureless silty claystone. No mottlesor rhizomo_lJ)hs. Mottled Claystone Grayish red (5R 4/2) claystone 1J.lith pale_y_E)llowish_green (1DGY _7}_2j mottles. Lenticular Sandy Claystone Lenticular_ s_C!_Il_d_y__S:laystone. Floating pebbles and cobbles occur near basal contact. Mottled Rhizomorph- Color is grayish red (5R 4/2) with locally abundant pale yellowish green (10GY 7/2) mottles. Bearing Claystone Locally abundant, clacareous rhizomorphs. Interbedded with Laminated Fine Sandstone.
1-' Vl 16
and overlying lower-delta plain deposits (Sespe/Vaqueros transition).
Braided-River Sandstone Lithosomes
The braided-river sandstone deposits consist of five lithosomes.
The lithosomes are similar in texture, composition, and color, but differ in sedimentary structures presenL Most lithosomes randomly interfinger with each other. Graded sandstone beds are most common where the braided-river sandstone deposits are transitional with overlying ~eandering-river and lower-delta plain deposits. Tables 4 and 5 show some of the textural and compositional properties of the sandstone li thosomes. Table 6 shows composition of heavy minerals present.
The sandstone is a moderately sorted, medium to coarse, limonitic, lithic arkose which contains rare floating pebbles. Beds are primarily tabular and range in thickness from 50 em to 30 m. Some lenticular beds occur within conglomerate lenses and range in thickness from 10 em to 1.5 m and are up to 15 m in length. The sandstone lithosomes are gradational with underlying conglomerate lithosomes. The basal contacts with claystone lithosomes are sharp and rarely erosional. Top contacts with conglomerate are gradational, whereas top contacts with claystone are sharp to gradational.
Calcareous and petroliferous concretions are common in the lower member. Calcareous and limonitic concretions are common in the upper member. Fresh-surface color ranges from very pale orange (10YR 8/2) to grayish orange (lOYR 7/4) in the lower member and yellowish gray
(SY 7/2) to dark yellowish orange (10YR 6/6) in the upper member. TABLE 4. PETROLOGY OF LOWER MEMBER SAMPLES FROM THE SESPE FORMATION NORTH OF SIMI VALLEY (SEE FIGURE 4 AND GEOLOGIC MAP IN POCKET FOR LOCATION OF SAMPLES)
SAMPLE SECTION PERCENT COMPOSITION TEXTURE ROCK NAME (after Folk, 1980) QTZ ORT PLG RKF OTH SER CCM LCM POR SRT RNO SV-51 BREA 25.5 23.2 5.5 18.0 7.4 3.3 o.o 10.0 6.8 M SA Medium sandstone: limonitic submature volcanite-bearing biotitic lithic arkose SV-52 BREA 29.8 21.6 5.0 18.4 6.8 2.0 9.6 5.2 4.8 M SR Conglomeratic coarse sandstone: cal- citic slightly limonitic submature biotitic lithic arkose SV-66 TAPO 26.8 24.6 2.4 25.0 6.4 o.o 14.0 o.o 0.8 M SR Coarse sandstone: calcitic submature volcanite-bearing biotitic lithic arkose SV-67 TAPO 29.6 26.8 1.6 13.6 2.4 o.o 14.4 o.o 11.6 M SR Coarse sandstone: calcitic submature volcanite-bearing lithic arkose SV-68 TAPO 22.8 25.2 3.6 22.4 7.6 o.o 9.6 7.6 1.2 M SR Medium sandstone: calcitic slightly limoniticsubmaturevolcanite-bearing biotitic lithic arkose SV-70 MARR 23.0 19.0 3.0 29.0 4.0 o.o o.o 24.0 2.0 p SR Conglomeratic coarse sandstone: li- monitic submature plant fossilif- erous felds~athic arenite SV-71 MARR 24.0 28.4 2.4 15.6 6.4 o.o 20.4 o.o 2.8 M SR Very coarse sandstone: calcitic sub- mature petroliferous lithic arkose SV-72 MARR 36.4 24.2 4.0 13.8 1.2 0.0 0.0 12.4 8.0 P SR Coarse sandstone: limonitic subma ture lithic arkose KEY: PERCENT COMPOSITION: QTZ= Quartz, ORT = Orthoclase, PLG= Plagioclase, RKF= Rock Fragments, OTH= Other, SER= Sericite, CCM= Calcite Cement, LCM= Limonite Cement, POR= Porosity. AVERAGE TEXTURE: SRT = Sorting, P= Poor, M= Moderate, W= Well. RND= Roundness, SA= Subangular, SR= Subrounded.
I-' -.J TABLE 5. PETROLOGY OF UPPER MEMBER SAMPLES FROM THE SESPE FORMATION NORTH OF SIMI VALLEY (SEE FIGURE 4 AND GEOLOGIC MAP IN POCKET FOR LOCATION OF SAMPLES)
SAMPLE SECTION PERCENT COMPOSITION TEXTURE ROCK NAME (after Folk, 1980) QTZ ORT PLG RKF OTH SER CCM LCM POR SRT RND SV-60 ALAMOS 24.5 19.5 7.0 13.0 9.5 o.o 14.5 4.5 6.5 M SA Very coarse sandstone: calcitic slightly limonitic submature biotitic lithic arkose SV-61 ALAMOS 24.6 20.8 7.4 15.6 12.4 o.o o.o 18.5 0.7 M SR Fine sandstone: limonitic submature volcanite-bearing biotitic lithic arkose SV-62 ALAMOS 22.3 18.6 6.8 9.0 11.6 2.2 9.5 14.2 5.8 M SA Medium sandstone: limonitic slightly calcitic submature biotitic lithic arkose SV-65 OAK 27.5 24.0 6.5 11.5 6.0 0.5 0.0 7.0 17.0 M SA Coarse sandstone: limonitic subma ture biotitic lithic arkose KEY: PERCENT COMPOSITION: QTZ= Quartz, ORT= Orthoclase, PLG= Plagioclase, RKF= Rock Fragments, OTH= Other, SER= Sericite, CCM= Calcite Cement, 'LCM= Umonite Cement, POR= Porosity. A\'i'ERAGE TEXTURE: SRT= Sorting, P= Poor, M= Moderate, W= Well. RND= Roundness, SA= Subangular, SR= Subrounded.
1-' CXl .L7
TABLE 6. HEAVY MINERAL ANALYSIS OF THE LOWER AND UPPER MEMBERS OF THE SESPE FORMATION NORTH OF SIMI VALLEY (CMP = CUMULATIVE MEAN PERCENT) LOWER MEMBER UPPER MEMBER
Sample No.: SV-51 SV-66 SV-72 SV-61 SV-65 Percent Heavy Minerals: 7.4 4.4 17.0 7.0 25.3 GRAINS COUNTED CMP GRAINS COUNTED CMP OPAQUES Magnetite 13 30 80 19.7% 20 15 11.3% Ilmenite 0 9 33 6.7% 7 3 3.2% Hematite 5 11 4 3.2% 4 4 2.6% Limonite 7 10 59 12.2% 42 31 23.4% Leucoxene 8 5 23 5.8% 28 18 14.7% Unknown 1 4 0 0.8% 0 3 1.0% ULTRASTABLES Zircon 22 20 27 11.1% 17 10 8. 7% Tourmaline (Brn) 3 0 2 0.8% 11 2 4.2% Rutile 5 3 0 1.3% 1 0 0.3% MET AS TABLES Apatite 7 9 6 3.5% 6 4 3.2% Hornblende 2 2 5 1.4% 1 0 0.3% Augite 21 29 25 12.0% 5 9 4.5% Enstitite 2 9 0 1.8% 0 0 0.0% Garnet 1 16 9 4.2% 20 8 9.0% Epidote 4 20 12 5.8% 8 23 10.0% OTHERS Phosphate 6 0 9 2.4% 0 0 0.0% Spinel 0 0 0 0.0% 1 0 0.3% Sphene 2 0 0 0.3% 0 0 0.0% MICAS Biotite 2 12 7 3.4% 0 3 1.0% Muscovite 3 9 4 2.6% 3 1 1.3% Chlorite 0 6 0 1.0% 0 3 1.0% TOTAL GRAINS COUNTED: 115 204 305 624 174 137 311 20
p '
Yellow-Orange Structureless Sandstone Lithesome
The yellow-orange structureless sandstone lithesome (lithofacies
Sh of Miall, 1977, 1978) contains rare floating pebbles ranging in diameter from 2 mm to 6 em and is the most common lithesome found in the braided river deposits. Beds are tabular and range in thickness from 50 em to 30 m. Very rare, unlined vertical burrows, defined by limonite/hematite halos, are present near the base of the Marr Ranch section (Fig. 5). Burrows average 1 em in diameter and are up to 15 em in length.
The structureless sandstone lithesome is similar to sheetflood sediments described by Picard and High (1973) and Cotter (1978). The sheetflood sediments are commonly deposited within and adjacent to the river bed of a braided river during waning flood stage. Vertical burrows in nonmarine, pebbly sandstone may indicate bioturbation by plant roots, animals, or insects (Williams and Rust, 1969).
Claystone Intraclasts Lithesome
The structureless sandstone with claystone intraclasts lithosome
(Se) is most common near the base of the Marr Ranch section.
Intraclasts are subrounded and range in diameter from 1 to 25 em.
Clasts are floating and their composition is identical to the structureless silty claystone lithesome described below. Basal contacts with adjacent sandstone lithosomes are erosional.
The intraclast-bearing structureless sandstone with scoured basal surfaces may represent primary channel-fill deposits in a sandy 21
braided river (Smith, 1970; Cant and Walker, 1978; Cant, 1982).
Claystone intraclasts are gouged out of stream channels and/or
previous overbank deposits and indicate vigorous currents within the
channel during peak flood stage (Picard and High, 1973; Reineck and
Singh, 1980).
Yellow-Orange Graded Sandstone Lithosome
The yellow-orange graded sandstone lithosome commonly occurs
within the structureless sandstone lithosome and is most abundant near
the base of the upper member. Grain size ranges from very coarse to
medium sand. Bed thickness ranges from 50 em to 3 m. Basal contacts are sharp to gradational. Top contacts are gradational with adjacent
sandstone lithosomes and sharp to gradational with overlying mudrock
lithosomes.
Graded sandstone beds associated with structureless sandstone
beds are common in sheetflood sediments deposited during waning floodstage (Picard and High, 1973; Cotter, 1978). Sheetflood deposits containing graded beds occur primarily in floodplain areas of sandy
braided rivers (Allen, 1965).
Parallel and Low-Angle Cross-Bedded Sandstone Lithosome
Parallel (Sh) and low-angle cross (Sl) bedded sandstone occurs primarily in lenses within and adjacent to lenticular conglomerate beds (Figs. 7 and 8). Bedding is defined by a change in grain size and ranges in thickness from 5 mm to 1.5 em. Sandstone lenses range in thickness from 20 em to 1.5 m and are up to 15 m in length. Basal and top contacts are gradational. 22
Figure 7. Sketch of Lenticular Conglomerate Lithosome (Channel fill), near base of Marr Ranch section (lower member), showing Parallel and Low-Angle Cross Bedded Sandstone Lithosome, and Claystone Intraclast Lithosome (see Figure 3 for explanation of symbols).
Figure 8. Parallel and Low-Angle Cross Bedded Sandstone Lithosome (longitudinal? bar deposits) within sandy braided-river deposits. Near base of Brea Canyon section (lower membe r). 23
Lenticular sandstone containing parallel and low- angle cross
bedding may represent channel bars (longitudinal and/or transverse)
deposited during fluctuating flood currents (Picard and High, 1973).
Lenticular, parallel and low- angle cross- bedded sandstone adjacent to
lenticular conglomerate beds may represent longitudinal- bar deposits
(Williams, 1971; McDonald and Banerjee, 1971).
Low- Angle and Trough Cross- Bedded Sandstone Lithosome
The low- angle (Sl) and trough (St) cross- bedded lithosome is rare
and was only observed as lenses adjacent to conglomerate lenses within
the upper member. Beds are defined by a change in grain size and
range in thickness from 4 mm to 2 em. Trough cross beds range in
length from 5 em to 65 em and average 2.5 em in depth. Lenses range
from 50 em to 12 m in length and 25 em to 1.25 m in thickness. Top
and basal contacts are gradational.
Low-angle and trough cross-bedded sandstone may represent
transverse- bar deposits within a braided river during waning flood
stage (Smith, 1970; Miall, 1977).
Differentiating between bar types is often difficult to determine
in ancient sequences, and Smith (1978) suggests the term "unit bar" be
used where uncertainty exists. The lateral relationships between
various bar types is complex and contacts are difficult to determine
(Smith, 1978). A generalized geographic area, containing superimposed
bar types over a large areal extent, is referred to as "braided river
bed" (Doeglas, 1962) or "sand flat" (Cant and Walker, 1978) .
Therefore, both the low-angle and trough cross- bedded, and the 24 p .
parallel and low-angle cross bedded sandstone lithosomes may represent
"unit-bar" deposits, and the more generalized terms "braided river bed" or "sand flat" can be applied when conglomerate lithosomes are included.
Braided-River Conglomerate Lithosomes
The braided-river conglomerate lithosomes consist of three lithosomes. The lithosomes are most common near the base of the lower member and occur randomly throughout the lower portion of the upper member (Fig. 4).
Lenticular Conglomerate Lithosome
The lenticular, pebble-cobble conglomerate (Gm) beds are commonly clast supported and structureless (Figs. 7, 9). Poorly defined low angle cross bedding and moderately defined clast imbrication occur in some of the larger lenses. Boulders, up to 75 em in diameter, are rarely present in the Oak Canyon section and upper portion of the Tapo
Ranch section. Lenses range in thickness from 30 em to 2.5 m and are up to 35m in length. The matrix/clast ratio ranges from 1:9 to 2:3 and averages 1:4. The sandstone matrix is structureless and is similar in composition and texture to the braided-river sandstone lithosomes discussed above. Basal contacts are erosional, whereas top contacts are gradational with sandstone. Very rare carbonized wood fragments and molds are present in larger lenses near the base of the lower member. Molds range in diameter from 5 to 25 em and are up to
50 em in length. Typical clast composition is shown in Table 7. 25
Figure 9. Structureless, clast-supported, pebble-cobble conglomerate lens (Lenticular Conglomerate Lithosome) near base of Brea Canyon section (lower member). Interpreted as primary, migrating, channel-fill deposits within sandy braided-river deposits.
TABLE 7. CLAST COUNTS OF THE LOWER AND UPPER MEMBERS OF THE SESPE FORMATION (CLAST COUNTS CONSIST OF 100 CONTIGUOUS CLASTS AT ONE LOCALITY IN EACH SECTION)
LOWER MEMBER UPPER MEMBER MEASURED SECTION: BREA MARR OAK TAPO CLAST CANYON RANCH CANYON RANCH SHAPE CLAST COMPOSITION PERCENT COMPOSITION Felsic plutonic 12 18 21 31 equant Mafic plutonic 1 2 1 0 equant Anorthosite 0 0 1 2 equant Felsic volcanic 30 24 17 12 prolate Intermediate volcanic 13 12 16 7 prolate Gneiss 5 7 14 12 equant Schist 0 1 5 8 bladed Quartzite white 8 10 4 5 equant red 8 2 9 7 equant black 9 11 2 1 equant banded 7 1 6 5 equant Sandstone (intraclast) 4 2 0 1 equant Mud rock (intraclast) 2 8 4 10 equant Chert 1 2 0 0 equant 26
Compositional and textural properties of the matrix are shown in
Tables 4 and 5.
The lenticular, pebble-cobble conglomerate beds which have an erosional base are similar to migrating, primary channel-fill deposits described by Williams and Rust (1969), Smith (1974), and Miall (1977).
Structureless to crudely-bedded pebbles and cobbles, which may show imbrication, represent flood and low-water conditions (Doeglas, 1962;
Miall, 1977). Drifted-wood fragments are common in braided river channel-fill deposits (Reineck and Singh, 1980).
Stratified Pebble-Cobble Conglomerate Lithosome
The stratified pebble-cobble conglomerate (Gm) consists of isolated, discontinuous, tabular to somewhat lenticular beds which are surrounded by medium to coarse, yellow-orange structureless sandstone.
Clasts are matrix supported and rarely imbricated. Beds commonly consist of a single horizontal layer of matrix-supported clasts (Fig.
10) which have an average diameter of 10 em. Beds range in length from 50 em to 12 m and average 3 m. The matrix/clast ratio ranges from 3:7 to 4:1 and averages 1:1. Basal contacts are gradational and rarely erosional. Top contacts are gradational.
Discontinuous, tabular to lenticular, pebble-cobble conglomerate beds represent anabranch (low-order) channel-lag deposits. Anabranch channel-lag deposits most commonly dissect bar surfaces in sandy braided rivers and form during waning floodstage (Smith, 1970; Miall,
1977; Cant and Walker, 1978). Anabranch channel-lag deposits consist of horizontally bedded, primarily matrix-supported pebbles and cobbles 27
Figure 10. Stratified Pebble-Cobble Conglomerate Lithosome overlain by Parallel and Low-Angle Cross Bedded Sandstone Lithosome near base of Brea Canyon section (lower member). which may show slight imbrication (Doeglas, 1962; Picard and High,
1973).
Cross-Bedded Pebble-Conglomerate Lithosome
The low-angle, cross-bedded pebble conglomerate commonly consists of a single, tabular layer of moderately imbricated, bladed pebbles and is commonly interbedded with parallel and low-angle cross-bedded sandstone. Beds range in thickness from 1 em to 5 em and average 2 em. The length of beds ranges from 5 m to 150 m and averages 25 m.
Claystone intraclasts are common. The matrix/clast ratio ranges from
3:7 to 4:1 and averages 1:1. Basal and top contacts are gradational to sharp.
Longitudinal bar-lag deposits commonly have greater continuity than anabranch channel-lag deposits and show well defined low-angle 28
stratification (McDonald and Banerjee, 1971; Williams, 1971; Smith,
1974). Bar-lag deposits grade downstream into finer sediments and are commonly interbedded with horizon tally stratified sand, which indicates fluctuating flood currents (Doeglas, 1962; Miall, 1977).
Braided- River Mudrock Lithosomes
Braided-river mudrock lithosomes consist of two distinct litho somes which differ in texture, composition, and sedimentary structures. Both represent braided- river floodplain deposits, but they may differ in distance from the river- channel complex.
Climbing-Ripple-Laminated Mudstone Lithosome
Climbing-ripple-laminated mudstone (Fl) is rare and occurs only near the base of the Marr Ranch section. Laminations range from 1 to
8 mm and average 4 mm. Starved, fine- sand-filled, ripples are common and range in length from 1 to 5 em and average 8 mm in thickness.
Climbing-ripple laminated beds are lenticular and range in thickness from 0 to 30 em and up to 35m in length. Abundant carbonized plant fragments (lithofacies C of Miall, 1978) form lamination surfaces which produce a fissile texture. Color ranges from brownish gray (5YR
4/1) to pale yellowish brown (lOYR 6/2).
The climbing- ripple laminated mudstone is similiar to swamp
(pond) deposits described by Miall (1978) and Reineck and Singh
(1980). Climbing-ripple laminated swamp (pond) deposits are rarely preserved in braided- river systems, but are common in floodplain deposits (Reineck and Singh, 1980). Because these deposits are 29
associated with conglomerate and sandstone lithosomes, they probably represent swamps (ponds) within the river-channel system.
Structureless Silty Claystone Lithosome
Structureless silty claystone (lithofacies Fsc of Miall, 1978) is commonly associated with yellow-orange structureless and graded sandstone lithosomes and rarely occurs adjacent to conglomerate lithosomes. Claystone beds are most abundant near the top of the braided-river deposits. Beds are tabular to lenticular and range in thickness from 30 em to 1.5 m. Basal and top contacts are primarily sharp, but can be gradational into sandstone lithosomes. The structureless claystone ranges in color from grayish red (SR 4/2) to grayish yellow green (SGY 7 /2).
The structureless, mottled claystone is interpreted as braided river floodplain sediments deposited by suspension during waning flood stage. The structureless nature can occur from rapid sedimentation
(Smith, 1974) or from intensive bioturbation (Doeglas, 1962; Miall,
1978).
Discussion of Sandy Braided River Deposits
According to the classifications of Miall (1977, 1978) and Rust
(1978), the sandy braided-river deposits seem transitional between the
Donjek (G111), South Saskatchewan cs11), and in part the Platte (s11) type rivers. These are modern, primarily cyclic, distal, braided rivers. Whether or not the Sespe Formation in northern Simi Valley represents a proximal or a distal braided river cannot be accurately determined without regional analysis of the formation. 30
MEANDERING-RIVER FLOODBASIN DEPOSITS
Meandering river floodbasin deposits consist of interbedded medium to coarse sandstone (50%), claystone (49%), and pebble-cobble conglomerate (1%), which together make up the middle member (Figs. 4 and 11). Upward-fining sequences are common and are produce wherever claystone lithosomes are underlain by structureless and/or graded sandstone lithosomes (Fig. 11). The sandstone is moderately resistant and interbedded with the claystone, and the two together display badlands topography. The deposits are gradational with underlying and overlying braided river deposits. Vertebrate remains (Table 1) are most common in the middle member.
Figure 11. Typical fining upward sandstone/claystone sequences of the middle member near Brea Canyon (meandering-river floodbasin deposits). 31
Meandering-River Sandstone Lithosomes
The meandering-river sandstone lithosomes consist of calcitic,
moderately- to well sorted, medium to coarse, micaceous, lithic arkose
(Table 8). Color ranges from light gray (N7) to grayish orange (10YR
7/4). Three lithosomes are recognized on the basis of sedimentary
structures, grain size, and color. The lithosomes randomly inter
finger throughout the middle member. Heavy mineral analysis of
sandstone lithosomes is shown in Table 9.
Light-Gray Structureless Sandstone Lithosome
The structureless, medium to coarse sandstone lithosome contains
very rare, unlined, inclined to vertical, claystone-filled burrows.
Burrows range in diameter from 8 mm to 2 em and are only pre.sent in
the upper portions of beds. Length of burrows ranges from 3 to 12 em.
Floating and vaguely stratified pebbles are rare and occur near the
base of beds. Botryoidal, fibrous-calcite concretions (Fig·. 12) are
brownish gray (SYR 4/1) on weathered surfac~s and occur in localized
concentrations near some vertebrate fossil localities. Beds are
primarily tabular, but occur also in lenses up to 500 m in length.
Beds range in thickness from 1 m to 30 m and average 4 m. Basal
contacts are sharp, whereas top contacts are sharp to gradational with
overlying mottled claystone. Structureless sandstone is more common
than other sandstone lithosomes.
The structureless sandstone beds are similar to those described
' by Schumm and Lichty (1963), McKee and others (1967), Taylor and
Woodyer (1978), and Cant (1982). The sandstone represents overbank- TABLE 8. PETROLOGY OF MIDDLE MEMBER SAMPLES OF THE SESPE FORMATION NORTH OF SIMI VALLEY (SEE FIGURE 4 AND GEOLOGIC MAP IN POCKET FOR LOCATION OF SAMPLES)
SAMPLE SECTION PERCENT COMPOSITION TEXTURE ROCK NAME (after Folk, 1980) QTZ ORT PLG RKF OTH CLA SER CCM LCM POR SRT RND SV-53 BREA 27.0 20.8 5.4 19.6 2.4 0.0 0.8 13.6 6.0 4.4 M SR Coarse sandstone: calcitic slightly limonitic submature volcanite-bearing biotitic lithic arkose SV -54 BREA 13.5 1.5 0.0 0.0 0.0 56.0 0.0 6.5 0.0 2.0 P R Clay-pisolite arenite
SV-55 BREA 31.0 20.6 4.2 13.8 4.4 0.0 0.0 17.6 4.4 4.0 M SR Coarse sandstone: calcitic submature biotitic lithic arkose SV-56 BREA 0.5 0.0 0.0 0.0 0.0 99.5 0.0 0.0 0.0 0.0 Claystone
SV -57 BREA 29.0 22.5 4.0 10.5 10.0 0.0 14.0 0.0 0.0 10.0 M SR Coarse sandstone: sericitic subma ture biotitic lithic arkose SV-58 BREA 14.5 6.5 4.5 3.5 2.5 68.5 0.0 0.0 0.0 0.0 P SR Biotiticarkosicsandyclaystone
SV -69 TAPO 24.4 22.4 2.0 8.0 16.6 0.0 3.2 8.8 0.0 13.6 W SR Medium sandstone calcitic submature micaceous arkose SV-73 MARR 18.4 21.2 3.6 2.8 18.4 0.0 0.0 28.4 0.0 7.2 W SR Medium sandstone: calcitic mature micaceous arkose SV-74 MARR 2.0 o.o o.o o.o o.o o.o o.o 97.0 o.o 1.0 Fiberous calcite- coprolite?
SV-75 MARR 44.0 26.5 0.5 18.0 1.0 0.0 0.0 7.0 1.5 1.5 M SA Coarse sandstone: calcitic slightly limonitic submature lithic arkose KEY: PERCENT COMPOSITION: QTZ= Quartz, ORT= Orthoclase, PLG= Plagioclase, RKF= Rock Fragments, OTH= Other, CLA= Clay, SER= Sericite, CCM= Calcite Cement, LCM= Limonite Cement, POR= Porosity. AVERAGE TEXTURE: SRT= Sorting, P= Poor, M= Moderate, W= Well. RND= Roundness, SA= Subangular, SR= Subrounded, R=Rounded.
w N 33
TABLE 9. HEAVY MINERAL ANALYSIS OF THE MIDDLE MEMBER OF THE SESPE FORMATION NORTH OF SIMI VALLEY (CMP = CUMULATIVE MEAN PERCENT)
Sample No.: SV-57 SV-69 SV-73 Percent Heavy Minerals: 6.2 18.1 16.3 GRAINS COUNTED CMP OPAQUES Magnetite 12 32 30 14.3% Ilmenite 0 5 16 4.1% Hematite 6 9 11 5.0% Limonite 1 2 3 1.2% Leucoxene 13 9 15 7.1% Unknown 4 1 2 1.4% ULTRASTABLES Zircon 10 26 21 11.0% Tourmaline (Brn) 5 2 0 1.4% Rutile 2 1 0 0.6% MET AS TABLES Apatite 5 10 8 4.3% Augite 5 10 8 4.3% Garnet 3 11 12 5.0% Epidote 21 33 22 14.7% OTHERS Spinel 2 0 2 0.8% Sphene 3 2 0 1.0% MICAS Biotite 2 22 39 12.2% Muscovite 3 12 29 8.5% Chlorite 1 8 7 3.1% TOTAL GRAINS COUNTED: 98 195 225 518 34
Figure 12. Botryoidal, fibrous-calcite concretions which occur within Light-Gray Structureless Sandstone Lithosome. X 0.75 sheetflood sediments that were deposited during maximum floodstage within a meandering-river floodbasin. The claystone-filled burrows probably represent root casts which originated from the overlying claystone beds. Botryoidal calcite concretions, based on their unusual shape, may represent coprolites.
Light-Gray Graded Sandstone Lithosome
The light-gray graded sandstone lithosome occurs interbedded with the structureless sandstone lithosome and ranges from medium to very coarse sand. Beds range in thickness from 6 em to 1 m and seem tabular. Basal contacts are sharp and top contacts are gradational.
Graded-sandstone deposits are commonly associated with structureless, overbank-sheetflood deposits in meandering-river 3S
floodbasins (Schumm and Lichty, 1963; McKee and others, 1967; Taylor and Woodyer, 1978). The graded beds may occur from suspension during fluctuations of maximum floodstage currents.
Bedded Sandstone Lithosome
The bedded sandstone lithosome contains parallel beds, low-angle
(Sl), and planar (Sp) cross beds, and trough-shaped scour surfaces
(Ss). Subrounded and contorted claystone intraclasts (Se) are present and range in diameter from 1 to SO em (Fig. 13). Floating and vaguely stratified pebbles are common. Beds range from S mm to SO em and are defined by a grain size change. Bedded units are tabular to somewhat lenticular, up to 2SO m in length, and range in thickness from 1 m to
1S m. Basal contacts are sharp to erosional and top contacts are sharp.
Cross-bedded, medium to coarse sandstone that contains large intraclasts and scour-and-fill structures is similar to recent crevasse- splay deposits described by Allen (196S), Coleman (1969), and
Reineck and Singh (1980). Crevasse splays form during maximum flood stage when natural levees are breached.
Meandering-River Claystone Lithosomes
Meandering-river claystone deposits are structureless and consist of two lithosomes which differ primarily in grain size, bed form, and color. Compositional and textural properties are shown in Table 8.
Mottled Claystone Lithosome
The mottled claystone (Fm) is grayish red (SR 4/2) in color with 36
Figure 13. Claystone intraclasts within Bedded Sandstone Lithosome near the top of the middle member in the Brea Canyon section. pale yellowish green (lOGY 7/2) mottles. Rare calcrete (lithofacies P of Miall, 1978) and pi soli tic-arenite concretions are present in the uppermost portions of some beds and are up to 60 em in diameter.
Mudcracks are very rare and are infilled with medium to coarse sandstone. Mud chips range in diameter from 1 to 5 em and have an average thickness of 8 mm. Beds are tabular and range in thickness from 1 to 15 m and average 3 m. Basal contacts with sandstone are sharp to gradational within 1 to 3 em intervals. Top contacts are sharp when overlain by bedded sandstone. The claystone is pervasively fractured. Vertebrate remains are most common in mottled claystone beds.
Thick, continuous, claystone beds are commonly deposited by 37
suspension during waning floodstage in meandering-river floodbasins
(Jahns, 1947; Taylor and Woodyer, 1978; Reineck and Singh, 1980). Red pig mentation may indicate oxidizing conditions, whereas green pigmentation may indicate oxidation of organic material such as plants or rootlets (Blatt, 1982; Cant, 1982). Sedimentary structures may not be present due to rapid sedimentation from suspension (Allen, 1970), bioturbation by plants, animals, and insects (Gustavson, 1978; Cant,
1982), and/or continued wetting and drying over a long duration
(Jahns, 1947; Gustavson, 1978) • Mudcracks are commonly present on the surface of fine-grained sediments of floodbasins after floodwaters recede (Taylor and Woodyer, 1978; Reineck and Singh, 1980). Partial pedogenesis may be indicated by the presence of calcrete and pisolitic concretions, which can be abundant in floodbasin clays (Gustavson,
1978; Taylor and Woodyer, 1978).
Lenticular Sandy Claystone Lithosome
The lenticular sandy claystone is arkosic, structureless, and is associated only with bedded, medium to coarse sandstone. Lenses range in thickness from 50 em to 1.5 m and are up to 100 m in length (Fig.
14). Floating pebbles and cobbles commonly occur in a thin (up to 15 em thick) interval above the basal contact. Basal contacts are sharp to gradational within 1 to 3 em intervals. Top contacts are sharp.
The sandy claystone ranges from grayish red (SR 4/2) to grayish green
(1 OGY 5 I 2) in color.
The lenticular, sandy claystone represents crevasse-splay channel- fill deposits (plugs) similar to those described by Coleman 38
Figure 14. Lenticular Sandi Claystone Lithesome interbedded with Bedded Sandstone Lithesome near the top of the middle member in the Brea Canyon section.
(1969) and Reineck and Singh (1980). The floating pebbles and cobbles near the basal contact probably represent crevasse-splay channel-lag deposits similar to those described by Coleman (1969) and Reineck and
Singh (1980). The lag deposits formed during floodstage, and as the floodstage waned, the abandoned channel (lined with pebbles and cobbles) became infilled with overbank clay.
Meandering-River Conglomerate Lithesome
Meandering-river conglomerate deposits are very rare and consist of one lithesome. The lithesome is interbedded with structureless, medium to coarse sandstone. 39
Matrix-Supported Pebble-Cobble Conglomerate Lithosome
The matrix-supported, pebble-cobble conglomerate lithosome (Fig.
15) occurs only in the Brea Canyon section near the Canada de la Brea fault and seems more abundant southwest of the study area. The matrix consists of medium to coarse sandstone, lacks sedimentary structures, and is commonly petroliferous. The matrix/ clast ratio ranges from
3:7 to 3:2 and averages 1:1. Subrounded claystone intraclasts are rare. Beds are lenticular and range in length from from 15 m to 25 m.
Lenses range in thickness from 1.5 to 8 m and average 2 m. Basal and top contacts are gradational into adjacent light-gray structureless sandstone. Typical clast composition is shown in Table 10.
The matrix-supported conglomerate is similar to meandering-river channel-fill deposits described by Lattman (1960) and Reineck and
Singh (1980), Intraclasts, within recent deposits, are derived from adjacent, overbank-floodbasin deposits and reworked into the channel fill deposits (Reineck and Singh, 1980). In meandering rivers that do not have recognizable point-bar deposits, the thickness of channel fill deposits may represent maximum depth of scouring during floodstage (Mackin, 1937). Therefore, the maximum floodstage-scouring depth during deposition of the middle member of the Sespe Formation in northern Simi Valley is approximately 8 m.
Discussion £i Meandering-River Floodbasin Deposits
The lithofacies classifications of Miall (1977, 1978) and Rust
(1978) for braided-river deposits are applied to the sandstone and claystone lithosomes because the mechanisms of deposition are somewhat 40
Figure 15. Matrix-Supported Pebble-Cobble Conglomerate Lithosome near the Canada de la Brea fault in the Brea Canyon section (middle member).
TABLE 10. CLAST COUNT OF 100 CONTIGUOUS CLASTS FROM ONE LOCALITY IN THE BREA CANYON SECTION OF THE MIDDLE MEMBER OF THE SESPE FORMATION NORTH OF SIMI VALLEY
CLAST COMPOSITION PERCENT COMPOSITION CLAST SHAPE Felsic plutonic 22 equant Mafic plutonic 1 equant Felsic volcanic 35 prolate Intermediate volcanic 22 prolate Gneiss 3 equant Schist 1 bladed Quartzite white 8 equant red 3 equant black 1 equant banded 1 equant Sandstone (intraclast) 1 equant Mudrock (intraclast) 2 equant 41 ~ .
similar. Sp (sandstone with planar crossbeds) may occur under low flow-regime conditions. Se (erosional scours with intraclasts), Ss
(shallow scour fills), and Sl (low-angle crossbeds), may represent scour-and-fill deposits associated with crevasse splays. Fm
(structureless mud with desiccation cracks) indicates overbank deposits, and P (carbonate nodules) indicates pedogenic features.
Cyclical upward-fining sequences of structureless and/or graded sandstone overlain by claystone, similar to those of the middle member, are common deposits within the recent meandering-river floodbasins described by Jahns (1947), Schumm and Lichty (1963), and
Taylor and Woodyer (1978). These recent cyclical sequences are indicative of deposition by suspension with some local current activities. The thickness of these deposits depends on the distance from the channel, flood duration, and caliber of stream load (Allen,
1965). Thick, recent deposits (similar to the middle member) are commonly present when the meandering-river channel is in a fixed position and previous deposits are difficult to erode (Reineck and
Singh, 1980; Cant, 1982). Therefore, similar conditions, consisting of fixed channel positions and long flood durations, may have existed during deposition of the middle member. 42
LOWER-DELTA PLAIN DEPOSITS
The lower-delta plain deposits represent the uppermost 100 m of the upper member (Sespe/Vaqueros transition) and lowermost portion of the Vaqueros Formation. The deposits consist of interbedded laminated fine sandstone (55%), mottled rhizomorph-bearing claystone (25%), and bedded, medium to coarse sandstone (20%). The bedded sandstone is abundant in the lower portion of the Sespe/Vaqueros transition, whereas laminated sandstone is abundant in the upper portion.
Contacts are gradational with both the underlying braided-river deposits (lower three-fourths of the upper member) and overlying nearshore (foreshore and shoreface) deposits of the Vaqueros Formation
(Blundell, 1981). Lower-delta-plain deposits are poorly exposed and crop out only in the Alamos Canyon and Big Mountain areas (Figs. 4,
Geologic Map in pocket). Vertebrate fossils are very rare in the lower-delta-plain deposits and are listed in Table 1.
Lower-Delta Plain Fine Sandstone Lithosome
Fine sands tone deposits occur only at the top of the formation and represent the first appearance of marine sediment. Sandstone units are tabular with sharp basal and top contacts. Color ranges from grayish-yellow green (SGY 7/2) to grayish orange (10YR 7/4).
Fine sandstone is represented by one lithosome. Textural and compositional properties are shown in Table 11. Heavy mineral composition is shown in Table 12. TABLE 11. PETROLOGY OF THE SESPE/VAQUEROS TRANSITION SAMPLES FROM THE SESPE FORMATION (SEE FIGURE 4 AND GEOLOGIC MAP IN POCKET FOR LOCATION OF SAMPLES)
SAMPLE SECTION PERCENT COMPOSITION TEXTURE ROCK NAME (after Folk, 1980) QTZ ORT PLG RKF OTH CLA SER CCM LCM POR SRT RND SV -59 BIG MT 21.0 17.5 7.5 0.0 33.5 7.5 2.0 0.0 6.5 4.5 W SR Fine sandstone: slightly limonitic mature micaceous heavy mineral bearing arkose SV-63 ALAMOS 27.8 16.4 6.2 0.8 26.8 1.6 0.0 15.6 2.0 2.8 W SR Fine sandstone: calcitic slightly limonitic mature micaceous heavy mineral- bearing arkose SV -64 ALAMOS 28.0 12.6 5.8 0.8 4.4 37.6 0.0 0.8 0.0 8.0 P SR Muddy fine sandstone: immature mus covitic plant fossiliferous arkose KEY: PERCENT COMPOSITION: QTZ= Quartz, ORT = Orthoclase, PLG= Plagioclase, RKF = Rock Fragments, OTH= Other, CLA = Clay, SER= Sericite, CCM= Calcite Cement, LCM= Limonite Cement, POR= Porosity. AVERAGE TEXTURE: SRT= Sorting, P= Poor, M= Moderate, W= Well . RND= Roundness, SA= Subangular, SR= Subrounded, R=Rounded.
.1:: w 44
TABLE 12. HEAVY MINERAL ANALYSIS OF THE SESPE/VAQUEROS TRANSITION OF THE SESPE FORMATION NORTH OF SIMI VALLEY (CMP = CUMULATIVE MEAN PERCENT)
Sample No.: SV-59 SV-63 Percent Heavy Minerals: 6.3 6.0 GRAINS COUNTED CMP OPAQUES Magnetite 18 5 4.7% Ilmenite 8 9 3.5% Hematite 5 2 1.5% Limonite 3 0 0.6% Leucoxene 13 0 2.7% Unknown 1 0 0.2% ULTRASTABLES Zircon 10 9 3.9% Tourmaline (Brn) 7 0 1.5% Tourmaline (Blu) 5 7 2.5% Rutile 14 6 4.1% METASTABLES Apatite 42 36 16.3% Augite 10 5 3.1% Enstitite 0 9 1.9% Garnet 23 19 8.8% Epidote 34 25 12.2% MICAS Biotite 27 68 19.7% Muscovite 7 35 8.7% Chlorite 0 20 4.1% TOTAL GRAINS COUNTED: 227 255 482 45
Laminated Fine Sandstone Lithosome
Laminated fine sandstone is primarily well sorted, calcareous, biotitic arkose. Sedimentary structures consist primarily of parallel and low-angle cross laminations with some climbing-ripple cross laminations. Laminations are defined by grain size change and heavy mineral (primarily biotite) placers and range in thickness from 2 to
15 mm. Rare claystone intraclasts range from 10 to 25 em in diameter and occur at the base of some beds. Rare lenses containing marine gastropod fragments and disarticulated marine bivalves are present in uppermost beds in the Big Mountain section (CSUN fossil locality 565,
Fig.4). Burrows are rare and are poorly defined by limonite/hematite halos and consist of unlined, vertical, inclined, and complex shafts.
Leaf impressions occur very rarely within a parallel laminated bed at the top of the Alamos Canyon section (SV-64, Fig. 4). Laminated units range in thickness from 50 em to 3 m.
The laminated, well sorted fine sandstone probably represents foreshore sedimentation along a high energy coast. Parallel laminations, low-angle and climbing-ripple cross laminations, defined by heavy-mineral placers, are commonly the result of storm-wave and wind- current deposition (Davis, 1978; Reineck and Singh, 1980).
Claystone intraclasts, derived from adjacent salt marshes, are deposited in the foreshore during storm conditions (Howard and Frey,
1975). Localized shell concentrations are carried by storm waves and deposited in the foreshore (Kumar and Sanders, 1976). Plant fragments are commonly transported into the foreshore, but are rarely preserved
(Howard and Frey, 1975). 46
Lower-Delta Plain Claystone Lithosome
Lower-delta plain claystone consists of one lithosome that is similar to the structureless, silty claystone found in underlying braided-river deposits. However, lower-delta plain claystone beds are thinner, less silty, mottled, and contain rhizomorphs.
Mottled Rhizomorph-Bearing Claystone Lithosome
The mottled rhizomorph-bearing claystone is structureless and consists of tabular beds that range in thickness from 30 em to 1 m.
Color is primarily grayish red (SR 4/2) with locally abundant, pale yellowish green (lOGY 7/2) mottles. Rare rhizomorphs consist of calcitic, complex shafts (Fig. 16) that average 8 mm in diameter, and they occur in the Big Mountain and Alamos Canyon sections. Basal contacts are sharp to gradational within 1 to 5 em intervals. Top contacts are sharp and locally irregular. The lithosome is commonly interbedded with laminated fine sandstone and bedded medium to coarse sandstone.
Based on stratigraphic position between foreshore deposits and on lithology, the mottled claystone is similar to delta-plain, salt-marsh deposits described by Coleman and Wright (1975) and Frey and Basan
(1978). The structureless nature of salt-marsh deposits can be due to in tense bioturbation by plant roots, animals, and insects (Frey and
Basan, 1978), or due to rapid sedimentation (Davis, 1978). Plant debris and roots are abundant on most salt marshes, but they are rarely preserved as molds or rhizomorphs. The plant remains are commonly winnowed out by tidal flushing or decay from biological and 47
Figure 16. Calcareous rhizomorph in Mottled Rhizomorph-Bearing Claystone Lithosome from Big Mountain section. X 0.50 chemical activity (Redfield, 1972; Frey and Basan, 1978). The mottled appearance may indicate reduction of ferric iron adjacent to plant- root material and the removal of ferrous ions from groundwater. Salt- marsh clay is deposited within the backshore by suspension resulting from marine storm conditions, floodstage of local rivers, and/or spring tides (Frey and Basan, 1978; Reineck and Singh, 1980).
Lower-Delta Plain, Medium to Coarse Sandstone Lithosome
The medium to coarse sandstone lithosome is moderately sorted, limonitic, biotitic, lithic arkose. Sandstone beds range from tabular to wedge shaped. Textural and compositional properties are similar to braided-river deposits within the lower portion of the upper member.
Limonitic concretions are rare. Color ranges from dark yellowish 48
orange (lOYR 6/6) to grayish orange (lOYR 7/ 4).
Parallel-Bedded Sandstone Lithesome
Parallel-bedded sandstone (Sh) contains floating and vaguely stratified pebbles, cobbles, and claystone intraclasts. Intraclsts are subrounded and range in diameter from 1 to 30 em and average 8 em.
The parallel-bedded sandstone primarily occurs in the upper half of the delta-plain deposits. Beds range in thickness from 5 mm to 2 em and average 1 em. Sandstone units are tabular, but can be lenticular up to 75 m in length ranging from 2 to 30 m in thickness. Basal contacts are primarily erosional. Top contacts are sharp but can be gradational.
The parallel-bedded sandstone is similar to channel-fill sediments of the sandy braided rivers described by Doeglas (1962),
Williams and Rust (1969), and Smith (1974). Claystone intraclasts are common in channel-fill deposits of sandy braided rivers and are derived from adjacent overbank-floodplain deposits. Based on stratigraphic position and lithology, the parallel-bedded sandstone units may represent lower-delta plain, distributary, channel-fill deposits, similar to those described by Coleman and Wright (197 5),
Naidu and Mowatt (1975), and Coleman (1981).
Discussion £f Lower-Delta Plain Deposits
Based on stratigraphic position between the underlying braided river deposits of the Sespe Formation and the overlying nearshore
(foreshore) deposits of the Vaqueros Formation, and on lithology, the deposits of the Sespe/Vaqueros transition are similar to the sequences 49
and lithologies of the lower-delta plains (the area seaward of the
limit of tidal inundation; Coleman and Prior, 1982) of the Senegal and
Sao Francisco Rivers, which are described by Coleman and Wright (1975) and Coleman (1981). Both of these modern-river deltas are wave dominated with straight coast lines and both have braided-delta plains. The Senegal Delta has stronger longshore currents than the
Sao Francisco Delta and is more similar in lithology to Sespe/Vaqueros transition deposits. Therefore, the "Vaqueros shoreline" may represent a wave-dominated delta front with a straight coast line.
Marine currents along a wave-dominated delta front rework fluvial sediments and produce typical nearshore (foreshore- and shoreface like) sequences of madera te- to high energy coasts (Elliott, 1980).
Blundell (1981) recognized similar nearshore deposits in the lower portion of the Vaqueros Formation at Big Mountain. These nearshore deposits probably represent wave-dominated delta-front deposits. PALEOCURRENTS AND PALEOCLIMATE
PALEOCURRENTS
Paleocurrent data from the lower and upper members were collected by measuring imbrication-surface attitudes of disc-shaped clasts and measuring trend and plunge of elongated-clast axes. Paleocurrents in the lower and upper members generally trend to the northwest and show slight dispersion (Fig. 17). The small amount of dispersion supports the interpretation of low-sinuosity (braided) rivers which represent deposition of the lower and upper members. Paleocurrent data were not collected from the middle member because clasts lacked any preferred orientations.
Kamerling and Luyendyk (1979), using paleomagnetic data, proposed that clockwise rotation, from 64 to 81 degrees, occurred during the
Neogene in the Simi Valley area. This counterclockwise-correction factor, when applied to the data, would create south- to southwest trending paleocurrents.
PALEOCLIMATE
The paleoclimate during deposition of the Sespe Formation is controversial. Flemal (1966) states the climate was arid based on sedimentary structures, abundance of fresh feldsp~r and biotite, presence of evaporites, and analogies with modern "red bed" environments. McCracken (1972) disagrees with Flemal and states that a semiarid climate existed during deposition of the "lower Sespe" and a warm temperate climate during deposition of the "upper Sespe".
McCracken's conclusions for both the "lower and upper Sespe" are based
so \ \\ > \ .\ ,. n=38 (I) \ ,c·· \ KILOMETEAI \ .. ·~··\ 0 1 N n=60 (A)
MILES ~(0 0 1 r ~nz42 n=number of mea...... ,ta
Figure 17. Paleocurrent rose diagrams (not corrected for rotation) of the lower and upper members of Sespe Formation north of Simi Valley. Unshaded rose diagrams represent lower member paleocurrent directions, shaded rose diagrams represent upper member paleo current directions. Dashed and dotted lines represent measured sections. (A) = data obtained from measuring trend and plunge of elongated-clast axea. (I)= data obtained from measuring imbrication-surface attitudes of disc-shaped clasts.
Vl 1-' 52
on clay mineralogy, scarcity of plant and vertebrate remains, and lack of eolian sedimentary structures.
Evidence of paleoclimatic changes may be indicated by the change from meandering to braided channel geometries, but changes in stream gradient seem a more likely explanation because other evidence of paleoclimatic changes (i.e. abrupt changes in fresh plagioclase content and flora) were not observed in the study area. However, based on the lack of desert-like deposits (i.e. evaporites), comparison of modern vertebrate occurrences with fossil vertebrates
(Table 1), and perhaps the low abundance of fresh plagioclase, it seems unlikely that an arid environment existed throughout deposition of the Sespe Formation as stated by Flemal (1966). PROVENANCE
DESCRIPTION
Four composition assemblages consisting of the combined heavy mineral and extraformational-clast populations (Selley, 1978; clasts derived from an extrabasinal origin) were recognized in the Sespe
Formation north of Simi Valley. The assemblages correspond with the lower member, middle member, upper member, and Sespe/Vaqueros transition. The dominant and/ or characteristic heavy minerals and clast compositions for the four assembleges are shown on Table 13.
Limonite, hematite, and leucoxene are common heavy minerals in most samples, but they are not used in the interpretations because they can be authigenic. Micas are also common, but were not used because of unreliable hydraulic settling.
The oldest assemblage, called the lower suite, which consists of all the extraformational clasts (Table 7) and heavy minerals (Table 6) found in the lower member. The clasts are well rounded and identical in composition to those found in the underlying Eocene Llajas
Formation, and Paleocene Santa Susana Formation and Simi Conglomerate.
Euhedral magnetite, subhedral augite, and rounded zircon grains are the dominant heavy minerals present. Phosphate grains are also present in some samples.
Heavy mineral composition of the middle suite (Table 9) is similar to that of the lower suite, except that subhedral augite decreases in abundance and is replaced by subhedral epidote as a dominant mineral. Extraformational-clast composition is also similar
53 TABLE 13. COMPARISON OF COMPOSITION ASSEMBLEGES OF THE SESPE FORMATON
LOWER SUITE MIDDLE SUITE UPPER SUITE SESPE/VAQUEROS TRANSITION SUITE
DOMINANT AND CHARACTERISTIC Euhedral Magnetite Euhedral Magnetite Euhedral Magnetite HEAVY MINERALS Rounded Zircon Rounded Zircon Rounded Zircon Subhedral Augite Phosphate Subhedral Epidote Subhedral Epidote Subhedral Epidote Subhedral Garnet Subhedral Garnet Subhedral Apatite Subhedral Rutile Blue Tourmaline
DOMINANT AND CHARACTERISTIC Felsic Volcanic Felsic Volcanic Felsic Volcanic Felsic Volcanic EXTRAFORMATIONAL CLASTS Felsic Plutonic Felsic Plutonic Felsic Plutonic Felsic Plutonic Quartzite Quartzite Quartzite Quartzite Intermediate Intermediate Intermediate Intermediate Volcanic Volcanic Volcanic Volcanic Gneiss Gneiss Schist Schist Anorthosite
l/1 -1>- 55
to that found in the lower suite. Phosphate grains are absent.
Clast composition of the upper suite (Table 7) is similar to that found in previous suites, except that large gneiss boulders (up to 75 em) and rare anorthosite cobbles are present is some channel fill deposits. Heavy mineral composition (Table 6) is similar to that of the middle suite, except that subhedral garnet increases in abundance.
The Sespe/Vaqueros transition suite (Table 12) differs from previous suites in that euhedral magnetite and rounded zircon decrease in abundance and are replaced by subhedral apatite and subhedral rutile. The suite also marks the first appearance of blue tourmaline. Clast composition is similar to that of the upper suite.
DISCUSSION
The slightly angular unconformity at the base of the Sespe
Formation at Simi Valley and to the south indicates that the underlying sedimentary rocks were part of an eroding highland just previous to being covered by Sespe Formation deposits. North of Simi
Valley in the Fillmore area was a site of continuous deposition from
Eocene through Oligocene (Kew, 1924), based on the fact that there are no unconformities in the stratigraphic record. To the northeast, in the Canton Canyon area, the Sespe Formation is angularly unconformable on older rocks and evidence indicates that the San Gabriel Mountains were an eroding highland throughout the upper Oligocene and the
Miocene (Crowell, 1954). It is not certain, however, if the San
Gabriel highland existed during the time of deposition of the lower 56
Sespe at Simi Valley.
Based on northwestern paleocurrent directions for the lower member, a clast population in the lower suite that is similar to that of the underlying sedimentary formations, and the presence of rounded zircon grains, it seems apparent that the major source rock for the lower member was the underlying Paleocene and Eocene sedimentary rocks. A second, more distant, mafic igneous source may may be indicated by the abundance of euhedral magnetite and subhedral augite.
Based on paleocurrent directions, such a mafic igneous source could have originated in the southern San Gabriel Mountains, approximately
35 km southeast of Simi Valley, providing that these mountains existed at that time.
Based on the apparent onlapping of middle-Miocene marine rocks over Paleogene formations in the western Simi Hills, the existence of an eroding highland in the Simi Hills area seems likely throughout deposition of the Sespe Formation at Simi Valley. Therefore, the middle suite may have a similar reworked sedimentary source as that of the lower suite. However, some uncertainity exists because middle member paleocurrent data are unavailable and the basal unconformity is at some distance from the base of the middle member. In addition, the abundance of subhedral epidote indicates a second, more distant, high rank metamorphic source. Assuming that northwesterly paleocurrents existed, it seems likely that the high-rank metamorphic source may also represent the southern San Gabriel Mountains, providing that these mountains existed at that time. 57
The provenance of the upper suite, based of northwesterly paleocurrent directions, may also be primarily reworked Paleogene strata which crop out in the Simi Hills. However uncertainity exists because the basal unconformity is several hundred meters below the upper member. The increased abundance of first-cycle? gneiss and schist clasts, subhedral epidote, and subhedral garnet indicate a second, more distant, high-rank metamorphic source. Based on north westerly paleocurrents, this high-rank metamorphic source may have originated in the southern San Gabriel Mountains. The existance of the San Gabriel Mountains, during deposition of the upper member, is documented by the rare presence of anorthosite clasts, which probably were derived from the San Gabriel Anorthosite, presently 35 km northeast of Simi Valley. The presence of anorthosite indicates a northeastern mafic igneous source, and furthermore, casts some uncertainity on the reliability of the northwesterly trending paleo current data. Paleocurrent data should indicate southwesterly trends in order to justify a northern San Gabriel Mountains source. Such a northern San Gabriel source may indicate southerly trending trib utaries and/or a river system which came from the northeast, but flowed northwesterly in the Simi Valley area.
The Sespe/Vaqueros transition suite differs from previous suites in that a reworked sedimentary source seems to be less dominant.
Rounded zircon grains and extraformational clasts, similar in composition to those of underlying formations, are less abundant. A high-rank metamorphic source area may be indicated by the abundance of first-cycle? gneiss and schist clasts, subhedral epidote, and 58
subhedral garnet. A felsic igneous source may be indicated by the increased abundance of subhedral apatite, and a mafic igneous source may be indicated by increased abundance of subhedral rutile. The abrupt appearance of blue tourmaline and possibly the abundance of
subhedral garnet may indicate a pegmatite source area.
The source area of the Sespe/Vaqueros transition suite is
presently unresolved due to a lack of paleocurrent data, and perhaps due to the introduction of longshore or other marine currents along the "Vaqueros shoreline". Perhaps the reworked sedimentary, high-rank
metamorphic, and mafic igneous sources, are similar to those of the upper suite, that is, the Simi Hills area and San Gabriel Mountains area. The felsic igneous and pegmatite source areas could also have originated in the San Gabriel Mountains, but supporting evidence is not presently available. PALEOGEOGRAPHY
The Sespe Formation in northern Simi Valley can be divided into
four distinct depositional phases (Fig. 18) which represent the lower
member (sandy-braided river), middle member (meandering-river
floodbasin), upper member (sandy-braided river), and Sespe/Vaqueros
transition (lower-delta plain).
Intercalated braided- and meandering-river deposits commonly occur in the ancient-rock record (Miall, 1977). The cyclic nature of
these deposits may indicate fluctuations in climate, provenance, and/or tectonic activity (Miall, 1977). Climatic changes are an unlikely explanation because, as stated in the discussion of paleo climate, indications of climatic changes were not observed in the study area. Tectonic activity is also unlikely because intraforma tional unconformities do not exist within the Sespe Formation. Since tectonic acitvity is unlikely and abrubt changes in source areas were not observed, then it seems unlikely that changes in provenance created the cyclic nature of the Sespe deposits. In addition, many modern rivers show a continuous gradation between braided- and meandering-channel patterns along their lengths primarily due to changes in physiography (stream gradient) and to a lesser extent by the availability of suspended-sediment load (Allen, 1965).
LOWER DEPOSITIONAL PHASE
Prior to deposition of the lower depositional phase the Simi
Valley area seems to have undergone uplift and erosion, which is
59 60
::.i~~:~d VAOLEROS FORMATlON 1858 ·.... ,,.,, .., .. ------..... , ...... :·:· SESPE/VAOUEROS TRANSITION PHASE 11100- :;:::-~.\:'_,~if;',\. (LOWER-DELTA PLAIN)
UPPER PHASE
(IJPI"ER--ELTA PLAIN)
1000 •t:?~?! ·.{• ..;,~ .•• ~;3 lhl-~:-:::.~:-~ ... ~-~
..... co::.·.·.: :-x ···········-···············--·····-···-·-...... ::/=/:.~~~ ·:,.:: ..... :·.·, ·":!. -~·:·. .':'·. -,;:; 'J aoo :.;;·-:'::~
:.:~.;;,:::<) - ·.•.:.;:'.-",;·:· :,'.->:::~
=~r=c::.: .. : .:.:) :·.: .. ; .. MIDDLE PHASE eoo : :·.·.. ·
•.•. b ~ . ·.t:.; (MEANDERING-RIVER FLOOOBAMO
i>:··.-:·.. -~ ::;:_;:~·.-.:-~ :•:.·.. · ....
400 ·.·.·. ·.·:;:o.. . ,.. , ..•. .,., -·····-----·-····-·····---·-----··
LOWER PHASE
(sANDY BRAIDED RIVER)
...:::.:; ·.·:~: .":::::> .;·:;;.:~::-f.-·,:> o~~~·v,~~~~=:'-~~~~-~~,·~~------~------
llAJAS FORMATION
Figure 18. Composite stratigraphic section of the Sespe Formation north of Simi Valley, showing stratigraphic position of the depositional phases (see Figure 3 for explanation of symbols). 61
indicated by the presence of a basal unconformity, whereas continuous
deposition occurred throughout the Eocene and Oligocene to the north
of Simi Valley (Kew, 1924; Fig. 19a). The lower phase begins when
continued filling of the floodplain to the north produces overlapping
of floodplain deposits onto the erosion surface in the Simi Valley
area. The lower phase represents migrating, northwesterly flowing
channels within a broad, sandy river bed, which was periodically
inundated by large floods (Figs. 19b and 20).
MIDDLE DEPOSITIONAL PHASE
The middle depositional phase represents meandering-river
floodbasin deposits which consist of overbank, sheetflood, crevasse
splay, and very rare channel-fill deposits (Figs. 19c and 20).
Physiography (stream gradient) appears to be the best explanation for
the change in channel geometry between the lower and middle depo
sitional phases. Such a decrease in channel gradient can occur by
infilling of a tectonically-stable depositional basin and would result
in a sequence where meandering-river deposits (lower gradient
geometry) overlie braided-river deposits (higher gradient geometry).
Change in the availability of suspended-sediment load is an unlikely
explanation for the change in channel geometry of the Sespe deposits
because both members have a similar sediment-size continuum and the
source areas seem constant.
UPPER DEPOSITIONAL PHASE
The upper depositional phase consists of sandy, braided-river deposits which represent migrating, westerly flowing channels that are 62
A N BRAIDED RIVER FLOODPLAIN ?•
Simi Valley HIGHLAND 1
MEANDERING RIVER FLOODBASIN B
?•
UPPER DELTA PLAIN c
BRAIDED RIVER FLOODPLAIN Simi Valley
MEANDERING RIVER FLOODBASIN
Figure 19. Diagrammatic paleogeographic reconstruction of the Sespe Formation north of Simi Valley. A) Pre-Sespe erosion, continuous Eocene deposition in the vicinity of the type section. B) Lower depositional phase (braided-river floodplain). C) Middle depositional phase (meandering-river floodbasin). North arrow indi cates present day north. Continued next page. 63
E
OCEAN
Valley
MEANDERING RIVER FLOODBASIN
.o
SHORELINE OCEAN
Simi Valley
UPPER DELTA PLAIN
LOWER DELTA PLAIN
Figure 19 (cont.). Diagrammatic paleogeographic reconstruction of the Sespe Formation north of Simi Valley. D) Upper depositional phase (upper-delta plain). E) Sespe/Vaqueros transition phase (lower-delta plain). Figure 20. Diagrammatic paleogeographic reconstruction of the lower and middle depositional phases of the Sespe Formation (lower and middle members) north of Simi Valley. Key: lm = lower member, mm = middle member; 1 = anabranch channels, 2 = braided-river bars, 3 = sheetfloods, 4 = floodplain, 5 = crevasse splays.
0' -1> 65
similar to those of the the lower depositional phase. Because the
upper member is underlain by meandering-river floodbasin deposits and
overlain by lower-delta-plain deposits, it seems likely that the upper
phase represents braided (distributary) river deposits within the
upper-delta plain environment (Figs. 19d and 21). The transition from
a lower gradient geometry (meandering-river floodbasin) to a higher
gradient geometry (braided river) is probably the result of trans
gression of a braided, upper-delta plain, which is characterized by
higher downstream gradients (Coleman and Prior, 1982), on top of a
meandering-river floodbasin. Based on northwesterly paleocurrent
directions the upper-delta plain appears to have transgressed toward
the southeast.
SESPE/VAQUEROS TRANSITION DEPOSITIONAL PHASE
The Sespe/Vaqueros transition depositional phase consists of
foreshore, salt marsh, and distributary channel-fill deposits of a
lower-delta-plain environment (Figs. 19e and 21) which moved into the
Simi Valley area by continued southeastward transgression of the
Vaqueros shoreline. Lower-delta-plain deposits are overlain by near
shore deposits of the Vaqueros Formation. 111oc•11•
Figure 21. Diagrammatic paleogeographic reconstruction of the upper and Sespe/Vaqueros transition depositional phases of the Sespe Formation (upper member and Sespe/Vaqueros transition) north of Simi Valley. Key: mm =middle member, urn= upper member, svt = Sespe/Vaqueros transition, vf = Vaqueros Formation; 1 = anabranch channels, 2 = braided-river bars, 3 = sheetfloods, 4 = floodplain and salt marshes, 5 = foreshore.
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