guidebook for structural transect along highway 33...
TRANSCRIPT
Guidebook for Structural transect along Highway 33, Ventura to the Cuyama Badlands, California (April 2, 2017) Leader: Thom Davis, www.thomasldavisgeologist.com; www.geologicmapsfoundation.org.
Participants: Coast Geological Society
COPYRIGHT 2017 © by Thomas L. Davis, Geologist: any commercial use of this guidebook is
prohibited without prior permission.
Start: 8:00 AM sharp, April 2, 2017. Meet at the west end of parking lot behind Century 10 Movie
Theater (555 Main St.) in downtown Ventura. Parking lot is downhill of Poli Street and between
California and Chestnut Avenues. We will return to Ventura by 4:00 PM. This trip is along paved
roads. We will drive to Stop 1 first.
Figure 1. Map of a portion of the western Transverse Ranges, southern California, showing cross section lines 6-6’ and 7-7’ and the April 2, 2017 Coast Geological Society field trip stops. Cross sections are available for download at: www.thomasldavisgeologist.com.
Regional cross Section 6-6’ and 7-7’:
Regional cross section 6-6’ (Figure 2A) starts at the eastern end of the Santa Barbara
Channel near Port Hueneme, crosses the Oak Ridge fault, the deep Pliocene age
sedimentary trough of the Ventura basin, the Ventura Avenue anticline, Sulphur
Mountain and Ojai Valley, Santa Ynez Range, Pine Mountain ridge, and ends at the Big
Pine fault. The structural interpretation shown in cross section 6-6’ is from Namson
(1986 and 1987) where it first appeared in publication as cross section C-C’ (Plate II in
the 1986 publication). The cross section restoration is shown in Figure 2B. Cross
section 7-7’ (Figure 4), crosses many of the same structures as cross section 6-6’, and
the two sections reveal the variation in geometry and kinematics of the various
structures along strike. Cross section 7-7’ was incorporated into Namson and Davis’s
cross section across the entire western Transverse Ranges (Namson and Davis, 1988).
A generalized stratigraphic column of the Ventura basin is included at the end of this
guidebook (Figure 7). See Dibblee (1982a, b) for additional field descriptions.
Southern portion of regional cross Section 6-6’:
The southern portion of cross section 6-6’ (Figure 3) begins on a structural shelf just
south of the West Montalvo oil field which is part of the 60 km long Oak Ridge-Montalvo
anticlinal trend. On the basis of the asymmetric shape of the anticlinal fold trend it is
interpreted to be part of a north-vergent, fault-propagation fold above the postulated
South Mountain thrust. The Oak Ridge fault is the southern boundary of the deep trough
of the Pliocene and Pleistocene Ventura basin and the fault cuts the north limb of the
anticlinal trend. The Oak Ridge fault is interpreted to be a Pliocene age normal fault
whose shallow portion has been rotated northward and into a reverse-fault geometry by
folding of the Oak Ridge-Montalvo anticlinal trend. Excess slip from the deeper South
Mountain thrust has reactivated shallow segments of the Oak Ridge fault. This
structural interpretation can be applied to all of the Oak Ridge-Montalvo fold trend which
contains anticlinal oil traps at Sheills Canyon, Bardsdale, South Mountain, and West
Montalvo oil fields.
Cross section 6-6’ shows how convergent deformation within the deep trough of the
Ventura basin and along its southern margin is linked to San Cayetano thrust system.
Southward directed slip along the Lion Mountain detachment extends across the deep
Ventura basin and cuts and translates the shallow portion of the Oak Ridge normal fault
southward. Approximately 4.7 km of back slip coming off the Lion Mountain detachment
and up the South Mountain thrust creates a thrust wedge between the south-dipping
thrust and the deeper detachment. Northward and deeper the 4.7 kms of offset of the
Oak Ridge fault creates a structural shelf just below the Lion Mountain detachment and
between the two portions of the older normal fault. This structural shelf along the
southern margin of the deep Ventura basin and between the offset shallow and deep
segments of the Oak Ridge fault is shown in the other regional cross sections by
Namson to the east (Davis, et al, 2015). This structural shelf is a regional feature of the
southern margin of the deep Ventura basin and extends eastward to at least the Aliso
Canyon area of the eastern Ventura basin.
Fig
ure
2A
. C
ross s
ectio
n 6
-6’ fr
om
Port
Hu
en
em
e t
o t
he B
ig P
ine f
ault (
Na
mso
n, 1
98
6, 1
98
7).
Ap
pro
xim
ate
lo
catio
ns o
f fie
ld trip
sto
ps 1
-5 a
re s
how
n a
bo
ve
th
e t
opog
rap
hic
pro
file
. T
he c
ross s
ectio
n s
how
s a
fo
ld a
nd
th
rust b
elt s
tyle
inte
rpre
tation
of
defo
rma
tion
. M
uch o
f th
e d
efo
rmatio
n is la
te P
liocen
e a
nd
Qu
ate
rnary
in
ag
e a
nd
eart
hq
uakes in
dic
ate
th
e b
elt is te
cto
nic
ally
a
ctive
.
Fig
ure
2B
. T
he c
ross s
ectio
n r
esto
ratio
n r
eve
als
tw
o e
arlie
r p
ha
ses o
f d
efo
rmatio
n:
late
Mio
cen
e a
nd
Plio
cen
e n
orm
al fa
ult
mo
ve
me
nt o
n th
e O
ak R
idg
e f
ault a
nd
Olig
oce
ne
mo
ve
me
nt o
n th
e S
anta
Yn
ez f
ault.
Re
sto
ratio
n o
f th
e s
ectio
n y
ield
s 3
1.4
km
of
co
nve
rge
nce a
nd
a r
ate
is 7
.9-1
5.0
km
/yr
assu
min
g d
efo
rmatio
n s
tart
ed b
etw
een
2.0
an
d 4
.0 M
a.
Fig
ure
3. P
ort
ion o
f cro
ss s
ectio
n 6
-6’ acro
ss O
ak R
idg
e f
ault tre
nd,
Ve
ntu
ra A
ve
nu
e a
nd
Lio
n M
oun
tain
an
ticlin
es a
nd fro
nta
l b
elt o
f
the S
anta
Yn
ez M
oun
tain
s (
Nam
so
n, 1
98
6, 1
98
7).
Ge
olo
gic
unit a
bb
revia
tion
s a
re s
how
n in F
igu
re 2
A.
Nam
so
n’s
inte
rpre
tatio
n
links d
efo
rma
tion a
cro
ss th
e d
ee
p V
entu
ra b
asin
via
a m
id-le
ve
l d
eta
chm
ent
at th
e b
ase o
f th
e R
incon
Fo
rma
tion
(T
r).
Slip
on m
id-
leve
l d
eta
chm
ent
is d
erive
d fro
m t
he lo
we
r sp
lay o
f th
e S
an C
aye
tan
o t
hru
st (S
CT
1).
Th
e d
eta
chm
ent
off
sets
th
e o
lder
Oak R
idg
e
norm
al fa
ult,
exce
ss s
lip o
n t
he d
eta
chm
ent fo
rms a
fa
ult-p
ropag
ation f
old
(W
est
Mo
nta
lvo
str
uctu
re),
and
fo
ldin
g r
ota
tes t
he u
pp
er
Oak R
idg
e fa
ult in
to a
reve
rse
ge
om
etr
y. V
entu
ra A
ve
nu
e a
nticlin
e r
esults fro
m s
tacke
d fa
ult-w
ed
ge
s r
oote
d in
to th
e m
id-le
ve
l
deta
chm
ent. T
he L
ion M
oun
tain
an
ticlin
e is s
how
n a
s a
fa
ult-b
en
d f
old
re
su
ltin
g fro
m a
ra
mp in
the
SC
T1
. T
he L
ion fa
ult is s
how
n
as a
hang
ing
-wall
flat
develo
ped a
bove t
he s
ou
thw
ard
-directe
d fault w
edge a
long t
he fro
nt lim
b o
f th
e f
ault-b
end fold
. T
he c
ross
se
ction
cro
sses t
he lo
catio
n o
f th
e V
entu
ra f
ault a
s p
rop
osed b
y H
ubb
ard
, e
t a
l (2
01
2).
Su
bseq
uent re
vie
w o
f th
is a
rea r
eve
al no
we
ll a
nd
fa
ult inte
rse
ction
, w
ell
to w
ell
str
atig
raphic
co
rre
latio
n,
or
se
ism
ic r
efle
ction im
ag
e e
vid
en
ce t
o s
upp
ort
th
e p
resen
ce
of
the
Ve
ntu
ra fa
ult.
Stop #1, Ventura Avenue Anticline oil field and Red Mountain fault:
Stop is near the axis of the Ventura Avenue Anticline that at the surface has folded
deep-marine clastic deposits of the Pico Formation (Figure 3). To the northwest and
across the Ventura River wash are outcrops of north-dipping sandstone, siltstone, and
shale of the Pico Formation. Further northwest and on the ridge above the Pico
outcrops are red beds of the Sespe Formation that are thrust southward over the Pico
beds by the Red Mountain fault. The Red Mountain fault is a major east-west trending,
north-dipping, reverse fault just north of the Ventura Avenue anticlinal trend. To the
north and along the Ventura River course the surface trace of the Red Mountain fault
bends northward towards the western termination of the Sulphur Mountain anticline, and
the fault’s surface trace dies out in beds of the lower Monterey Formation. At the bend
Sespe, Vaqueros, and lower Monterey beds dip eastward towards the wide south-limb of
the Sulphur Mountain anticline (a smaller anticline on the Lion Mountain anticline). The
subsurface geometry and structural cause of this northward bend is poorly understood.
Red Mountain fault dies-out in the subsurface before reaching cross section 6-6’. The
deep Canada Larga syncline separates the Lion Mountains and Ventura Avenue
anticlines and along the south limb of the Lion Mountain anticline +5 km of Pliocene
marine strata are exposed.
Figure 3 shows the Ventura Avenue anticline is a very large east-west trending fold
between the Canada Larga syncline to the north and the thick Pliocene and Quaternary
age Ventura basin trough to the south. Both anticlinal limbs have steep dips and in map
view the north limb has a much narrower north-south extent than the south limb. The
anticline has a steep east-plunge into the Ventura trough where it terminates, but to the
west the anticlinal structure continues for some distance and, eventually, into the
offshore. Onshore the Ventura Avenue anticline traps the Ventura and San Miguelito oil
fields. Further westward the anticlinal trend continues and traps oil offshore at three
culminations: Rincon (partially offshore), Carpinteria Offshore, and Dos Cuadras
(offshore).
Ventura anticline portion of regional cross Section 6-6’:
Figure 3 shows Namson interpretation (1986) of the deep structure at Ventura Avenue
anticline and the anticline’s geometric and kinematic connection to the San Cayetano
thrust system and the Lion Mountain fault that is exposed along the north flank of
Sulphur Mountain. In this interpretation the Ventura Avenue anticline is shown as a
detached fold above the Lion fault that is a detachment fault at the base of the Rincon
Formation, and also a higher thrust flat to the San Cayetano thrust system. The Ventura
Avenue anticline is shown to be folded and uplifted by a series of wedge-shaped
imbricated thrust faults that step up from the Lion Mountain fault detachment surface.
Under the Canada Larga syncline, that is just north of Ventura Avenue anticline, the
lower splay of the San Cayetano thrust (SCT 1) intersects the Lion Mountain detachment
to form a southward-directed fault wedge in the front limb of the Lion Mountain anticline.
Stop #2, Overview of the Ojai Valley, Topa-Topa Range, Matilija overturn, and San
Cayetano fault:
Stop is in a large parking area along the northwest side of Highway 150 with an excellent
overview of the Ojai Valley and town of Ojai. Adjacent outcrops are Sespe Formation
red beds along the north flank of the Lion Mountain anticlinal trend. East of the stop the
Lion Mountain anticline plunges eastward under upper Ojai Valley. Small amounts of oil
have been produced from sandstone beds in the Coldwater and lower Sespe Formations
at the Lion Mountain area located about two miles west of the stop. The south-dipping
Santa Ana fault whose surface trace is along the southern margin of Ojai Valley is not
shown in cross section 6-6’ (Figure 3). At that location the Santa Ana fault is either not
present or a very small fault within the Sespe Formation.
From the stop, and looking northward across Ojai Valley, is the south flank of the
Topatopa Range. The various Eocene formations are overturned to a north-dip with the
Oligocene Sespe Formation red beds exposed along the base of the range. This
structure is well known, at least geologically, as the Matilija overturn. The Matilija
overturn is the frontal structure to the Santa Ynez Mountains anticlinorium (Figure 3).
The overturn is the easternmost segment of a steeply-folded panel of rocks that to the
west is generally south-dipping. The panel consists of a thick sequence of Cretaceous
and Tertiary formations and extends from here to Point Conception (a distance of about
60 miles). Eastward, the Matilija overturn merges into a more complex series of smaller
folds: the Echo Canyon and Santa Paula Ridge anticlines, and several unnamed
anticlines with overturned south limbs. There the overturn is broken by the San
Cayetano thrust which places the Eocene section over Miocene and Pliocene strata
along the north flank of the Lion Mountain anticline.
As shown in cross section 6-6’ (Figures 2A, 3) the Santa Ynez Mountains anticlinorium is
asymmetric and south-vergent. The crest and north limb of the anticlinorium are
deformed by several small folds and faults that dip steeply to the south. Namson (1987)
interprets the anticlinorium as the result of two phases of deformation: one during the
late Eocene and early Oligocene (Ynezian orogeny) and the other during the late
Cenozoic. The late Cenozoic deformation is interpreted to result from fault-bend folding
and fault-propagation folding. The fault-bend fold is related to a ramp in the lower splay
of San Cayetano thrust (SCT 1) that extends from the basal detachment at about 12 km
depth upward to a higher-level detachment in the Cretaceous section. The slip on the
SCT 1 is 17.7 km which is transferred upward and southward along the thrust ramp
causing the Lion Mountain anticline. At the top of the ramp is another detachment that is
the same mid-level detachment under the Ventura Avenue anticline. The shallower-level
fault-propagation fold above the upper splay of the San Cayetano thrust fault (SCT 2)
has 3.3 km of slip. In Namson’s 1987 interpretation the shallow-level folding has
deformed and rotated the older Santa Ynez fault which was originally a north-vergent
back-thrust associated with a south-vergent Oligocene thrust system that uplifted the
ancestral San Rafael Mountains (Figure 5) (Reed and Hollister, 1936).
Regional cross section 7-7’:
Cross section 7-7’ (Figure 4A) lies about 15-20 km east of cross section 6-6’ (Figure 2A).
Cross section 7-7’ crosses many of the same structures as cross section 6-6’ and
comparison of the two sections shows their gross similarity and the geometric and
kinematic variations along strike. Cross section 7-7’ begins offshore at the western end
of the Santa Monica Mountains and crosses South Mountain, Topatopa Range, Pine
Mountain, Frazier Mountain and ends at the San Andreas fault (In the Namson and
Davis 1991 report the cross section is labeled 6-6’). Cross section 7-7’ was incorporated
into Namson and Davis’s cross section across the entire western Transverse Ranges
(Namson and Davis, 1988).
The first structure shown on the south end of cross section 7-7’ is the Santa Monica
Mountains anticlinorium. The geometry of the anticlinorium is constrained by the surface
geology of the Santa Monica Mountains and some subsurface drilling. The fold structure
is asymmetric with a steep south limb that is only partially onshore and extends into the
offshore. The crest of the fold occurs along the south flank of the Santa Monica
Mountains and the moderately-dipping north limb is well exposed along the crest of the
range and northward in the Conejo Valley. The Santa Monica Mountains anticlinorium is
interpreted to be a fault-propagation fold caused by the Elysian Park thrust (Davis, et al.
1989) which ramps up from a basal detachment at 15 km depth and terminates in an
offshore synclinal axis at about 9 km depth. The slip on the Elysian Park thrust is 11.3
km. Within the anticlinorium are several older, down-to-the-north normal faults that
probably controlled deposition of the thick section of lower and middle Miocene
volcaniclastic and other clastic rocks that occur throughout the Santa Monica Mountains.
The Malibu Coast fault is projected offshore into the cross section, where it cuts the
south limb of the anticlinorium. The fault juxtaposes contrasting stratigraphic sections:
south of the fault the Miocene strata lie unconformably on metamorphic basement rock
and north of the fault is a thicker Miocene section as well as lower Tertiary and
Cretaceous strata which rest unconformably on metamorphic or Franciscan basement.
These relationships suggest that the Malibu Coast fault had older Miocene movement
that was down-to-the-north, and had a similar basinal control as the previously
mentioned normal faults located further north. The Malibu Coast fault has been
reactivated as a reverse fault during late Cenozoic convergence and uplift of the Santa
Monica Mountains anticlinorium.
The next structures to the north and intersected by cross section 7-7’ are a pair of
anticlines that include the Oak Ridge-Montalvo anticlinal trend and an unnamed anticline
to the south that underlie the western portion of the Simi Hills. The Oak Ridge-Montalvo
anticlinal trend is defined by surface geology and subsurface drilling and is asymmetric
with a gently-dipping south limb and steeply-dipping north limb. The north- limb is cut by
the steeply, south-dipping Oak Ridge fault. The unnamed anticline is primarily defined
by subsurface data. The fold has moderately dipping limbs and the crest is broken up by
several normal faults. The normal faults are predominantly down-to the-south and
control thick accumulations of volcaniclastic and clastic strata. The cross section
interpretation shows the two anticlines to be related to ramps on the South Mountain
thrust which is a back-thrust off the lower splay in San Cayetano thrust fault (SCT 1).
The unnamed anticline is interpreted to be a fault-bend fold associated with a ramp that
steps up from a lower detachment at 8 km to an upper detachment at 6 km. The Oak
Ridge-Montalvo anticlinal trend is interpreted to be a fault-propagation fold associated
with the second ramp on the South Mountain thrust. The Oak Ridge fault is shown as a
rotated normal fault that was active during late Miocene and Pliocene time and originally
dipped to the north. The South Mountain thrust translated slip up the rotated segment of
the normal fault reactivating the fault as a high-angle reverse fault. The slip translated
up the ramp for the unnamed fold is 4.0 km and slip translated up the ramp below the
Oak Ridge-Montalvo anticlinal trend is 3.9 km.
Cross section 7-7’ crosses the Santa Ynez Mountains anticlinorium north of the deep
trough of the Ventura basin. The anticlinorium includes the structures underlying
Sulphur Mountain and the Topatopa Range. In cross section 7-7’ Sulphur Mountain lies
above the Sisar thrust fault, and the thrust is north-vergent with an asymmetric anticline
in the hanging wall. The Sisar fault, which is similar to the Lion Mountain fault to the
west, is interpreted to be a back-thrust off the San Cayetano thrust, and the hanging wall
anticline under Sulfur Mountain is interpreted to be a fault-propagation fold. At the
surface the Sisar thrust is truncated by the upper splay of the San Cayetano thrust
system. The Santa Ynez Mountains anticlinorium is composed of several stacked folds
and complicated faults that are observed at the surface and encountered in subsurface
drilling. The deepest structure is a fault-bend fold that is in the footwalls of the Sisar
thrust and upper San Cayetano thrust splay (SCT 2). The fault-bend fold is associated
with a ramp in the deeper San Cayetano thrust splay (SCT 1). The ramp connects a
lower detachment near the base of the Eocene strata to the upper detachment at the
base of the Rincon shale. The SCT 1 cuts and translates the Big Canyon fault and the
Oak Ridge fault which are older normal faults. The Big Canyon fault is translated and
folded as it moved from the lower part of the ramp onto the upper detachment whereas
the Oak Ridge fault is only translated along the upper detachment. Approximately 8.0
km of slip on the upper detachment of the SCT 1 is divided and consumed equally
between two back thrusts, the South Mountain thrust and Sisar thrust.
The surface geology of the Santa Ynez Mountains anticlinorium along cross section 7-7’
(Figure 4A) is dominated by a thick Eocene section that is deformed into an overturned
fold in the hanging wall of the SCT 2. This overturned fold is interpreted to be an older
fault-propagation fold that has been cut and translated on the San Cayetano thrust
system that breaks through to the surface. The ramp making the fault-propagation fold
steps up from a basal detachment within the Franciscan basement at about 13 km to the
top of the Eocene. The upper part of the ramp has been cut and translated by the SCT 1
and continued slip on SCT 2 fault ruptured to the surface up the frontal synclinal axis of
the fault-propagation fold. Approximately 19.1 km of slip has been translated up the
ramp on the San Cayetano thrust system. In Namson’s 1987 interpretation the Santa
Ynez fault is shown to be a late Eocene fault associated with the Ynezian orogeny
(Figure 5). This interpretation shows it as a north-verging back thrust from a south-
verging fold and thrust structure. Subsequent late Pliocene and Quaternary folding in
the hanging wall of the San Cayetano thrust system further deformed the Santa Ynez
fault geometry.
Along cross section 7-7’ (Figure 4A) and north of the Santa Ynez anticlinorium and
Sespe Creek synclinorium are Pine Mountain and Frazier Mountain. Late Cenozoic
uplift and folding of Pine Mountain and Frazier Mountain are interpreted to be related to
the Pine Mountain fault. The Pine Mountain fault probably has two phases of movement.
The fault juxtaposes the Salinian and Franciscan basement terranes. This juxtaposition
must have occurred prior to Eocene time because the Eocene units occur
unconformably on both blocks. Late Cenozoic movement on the Pine Mountain fault
ruptured though the steep north limb of the syncline to the surface. There is
approximately 2.4 km of shortening associated with the blind thrust and fault propagation
fold and about 1.0 km on the Pine Mountain fault splay that rupture to the surface. The
Pine Mountain fault is interpreted to root into a north-verging ramp on the Pleito thrust
system which causes the uplift and folding of the San Emigdio Mountains north of the
San Andreas fault and offset of the San Andreas in the deep crust. The relationships
between thrusts of the western Transverse Ranges and San Andreas fault were first
discussed in Namson and Davis (1988). In this interpretation the San Andreas fault and
the thrust systems share a common displacement surface that dips at low angles and
offsets shallow and deep crustal parts of the plate boundary. In the big bend segment of
the San Andreas fault this displacement surface is a south-dipping fault that has
accommodated both right lateral strike-slip on the San Andreas fault and north-directed
convergence along the Pleito thrust system during the late Cenozoic. Gravity data and
the high topography of the Mount Pinos and Frazier Mountain areas support this model
(Namson and Davis, 1988). Fuis, et al. (2012), unaware of Namson and Davis’ 1988
work, proposed that the San Andreas fault is dipping southwest from 55-75 degrees
along the western big bend segment based on studies of potential field data, active-
source imaging, and seismicity.
The present-day length of regional cross section 7-7’ (Figure 4A) is 95.2 km and the
restored cross section length is 128.4 km (Figure 4B) which yields 33.2 km of
convergence. The convergence rate from the western Santa Monica Mountains to the
San Andreas fault is 8.3-16.6 mm/yr., assuming convergent deformation started between
2.0-4.0 Ma.
Fig
ure
4A
. C
ross s
ection
7-7
’ fr
om
th
e w
este
rn S
anta
Mo
nic
a M
oun
tain
s t
o t
he S
an A
ndre
as fa
ult (
Na
mso
n a
nd D
avis
, 1
99
1).
Th
e
cro
ss s
ection s
how
s a
fo
ld a
nd
th
rust
belt s
tyle
inte
rpre
tation
of
defo
rmatio
n.
Mu
ch o
f th
e d
efo
rmatio
n is la
te P
liocen
e a
nd
Qu
ate
rnary
in
ag
e a
nd
ea
rthq
uake
s s
how
th
e b
elt is t
ecto
nic
ally
active
. A
n e
arlie
r ve
rison
of cro
ss s
ection
wa
s u
se
d in N
am
so
n
and
Davis
’ (1
98
8)
cro
ss s
ection
acro
ss t
he w
este
rn T
ransve
rse
Ra
ng
es.
Fig
ure
4B
is th
e c
ross s
ectio
n r
esto
ration
. C
ross s
ection
re
sto
ration
re
ve
als
tw
o e
arlie
r p
ha
ses o
f defo
rma
tion
: la
te M
iocen
e a
nd
Plio
cen
e n
orm
al fa
ult m
ove
me
nt
on th
e O
ak R
idge
fa
ult a
nd
Olig
oce
ne
move
me
nt
on th
e S
anta
Yn
ez f
ault.
Re
sto
ratio
n o
f th
e
se
ction y
ield
s 3
3.2
km
of
co
nve
rge
nce
an
d r
ate
is 8
.3-1
6.6
km
/yr
assu
min
g d
efo
rmatio
n s
tart
ed b
etw
een
2.0
an
d 4
.0 M
a.
Stop #3, Santa Ynez Range and Santa Ynez fault:
Stop is along Matilija Creek Road and west of Hwy 33. Good exposures of the Santa
Ynez fault occur along the road-cuts. Probable Cretaceous Jalama Formation
conglomerate (south block) is faulted against Eocene Juncal Formation (north block).
McCulloh (1981) hypothesized that the Santa Ynez fault had 37 km of left-lateral
displacement mostly during the Miocene. In contrast Hall (1981) suggested the fault has
significant right-lateral displacement during the Miocene. Gordon (1978) and Dibblee
(1982) believed the fault displacement was primarily dip-slip and dismissed the large
strike-slip displacements as the fault terminated at both western and eastern ends of the
Santa Ynez-Topatopa Mountains.
Figure 5. In Namson’s 1987 interpretation the Santa Ynez fault is shown to have two
phases of activity: Late Eocene and Oligocene movement associated with the Ynezian
orogeny and late Cenozoic movement associated with the Pasadenan orogeny. This
interpretation shows it as a north-verging back thrust from a south-verging fold and
thrust structure. Subsequent late Pliocene and Quaternary folding in the hanging wall
of the San Cayetano thrust system further deformed the Santa Ynez fault geometry.
This interpretation explains the origin of the regional angular unconformity between the
upper Oligocene through Miocene strata and the upper Mesozoic through Eocene
strata, and provides a structural explanation for the Ynezian orogeny (Dibblee, 1982).
Stop #4, Overview of the Sespe Creek synclinorium and Pine Mountain fault and
anticlinorium:
Stop is along the east side of Highway 33 just south of Sespe Creek (Figure 1). Road-
cut outcrops just west of the stop are north-dipping Matilija Sandstone and Cozy Dell
Shale along the north limb of the Santa Ynez Mountains anticlinorium. To the north and
east is the Sespe Creek drainage. Prominent light-colored sandstone outcrops north of
Sespe Creek are Sespe and Coldwater Formations along the trough of an east-plunging
syncline. The steeply-dipping north-limb of the syncline is interpreted as a portion of the
lower limb of a fault-propagation fold cut by the Pine Mountain thrust (Figure 6). The
high ridge above (north) the syncline is Pine Mountain Ridge which is underlain by a
thick sequence of Eocene strata in the hanging wall of the Pine Mountain fault. Our
location is between the Neogene age Ventura and Cuyama basins. Older rocks
exposed along this portion of Highway 33 are forearc deposits predating the Ynezian
orogeny and belong to the regional forearc basin that covered much of central and
southern California from Cretaceous to Eocene time.
San Rafael Mountains portion of regional cross Section 6-6’: Immediately north of
the Sespe Creek synclinorium there are two anticlines: one in the hanging wall and one
in the footwall of the Pine Mountain fault (Figure 6). The asymmetric, subthrust anticline
in the footwall of the Pine Mountain fault is interpreted to be a fault- propagation fold
caused by a ramp in a deeper thrust with 3.5 km of slip. This deeper thrust ramps up
from an intermediate detachment at the top of the Cretaceous section and terminates in
the subsurface at the axis of Sespe Creek synclinorium and within Eocene strata. The
Pine Mountain fault is interpreted to be a higher splay that ramps up across the back
limb of the fault-propagation fold. The slip on the Pine Mountain fault is 2.4 km.
Figure 6. Portion of cross section 6-6’ across the San Rafael Mountains, Sespe
Creek drainage, Pine Mountain, and Lockwood Valley (Namson, 1986, 1987).
Coast Geological Society field trip stops 3-5 are shown above the topographic
profile. Geologic unit abbreviations are shown in Figure 2A.
Stop #5, Lunch and crest of the Pine Mountain Ridge:
Stop is within Matilija sandstone and shale beds along the north limb (backlimb) of the
Pine Mountain anticline (Figure 6). The Pine Mountain anticline lies in the hanging wall
of the Pine Mountain fault and much of its south limb has been removed by erosion
where cross section 6-6’ crosses the anticline. Several miles west of the stop the Pine
Mountain thrust merges with the left-lateral Big Pine fault (Figure 1).
Stop #6, Overview of the Cuyama basin and Badlands, Big Pine fault, and north
limb of the Pine Mountain anticlinorium:
Directly below the stop is the trace of the Big Pine fault that has had about 13 km of left
lateral displacement, mostly during the Quaternary (Davis, et al, 1987). In the distance
is the easternmost portion of the Cuyama basin and Badlands. The basin is mostly
Cenozoic nonmarine rocks of the Simmler, Caliente, Quatal, and Morales Formations.
The nonmarine strata consist of a thick sequence of alluvial fan, fluvial and lacustrine
deposits of Oligocene through Pliocene age. To the west is the Sierra Madre which is
being uplifted by the southwest-dipping Ozena reverse fault. The hanging wall of the
Ozena fault consists of a +10 km thick clastic section of upper Cretaceous through
Eocene marine rocks. The thick section is folded, partially eroded by an angular
unconformity, and capped by mid-Tertiary strata. Left-lateral displacement of 13 km on
the Big Pine fault may have offset the eastern Cuyama basin (Cuyama Badlands) from
the small Lockwood basin and the Ozena fault from the south-dipping San Guillermo
reverse fault (Dibblee, 1982b). Both basins share a similar stratigraphy and history and
the Ozena and San Guillermo faults have a similar geometry, movement, and timing.
Northern portion of regional cross Section 6-6’:
Cross section 6-6’ lies several miles east of stop 6 and ends at the Big Pine fault (Figure
1). The present-day length of the cross section is 67.6 km and the restored cross
section is 99.0 km which yields 31.4 km of convergence (Figures 2A & B). The
convergence rate from Port Hueneme to the Pine Mountain fault is 7.9-15.7 mm/yr.,
assuming convergent deformation started between 2.0 and 4.0 Ma. Cross section 7-7’
(Figure 4A) extends northward to the San Andreas fault and is part of crustal-scale cross
section across the western Transverse Ranges published in Geology (Namson and
Davis, 1988).
Comment on rotation of the Transverse Ranges:
Jay Namson and I are asked occasionally about how the large, clockwise rotation of the
Transverse Ranges, supported by paleomagnetic data, impacts our cross sections and
restorations. There is nothing in our work that supports or denies the rotations, as long
as most of the rotation is completed before 4.0 ma. It is curious to us that we have
found no structural record of such large-scale block motions recorded in the folds and
faults in the cross sections and in the field and subsurface data and mapping used to
make the cross sections.
Figure 7. Generalized stratigraphic column of the Ventura basin showing ages,
stratigraphic nomenclature, rock column, water depth, and tectonic events (Martin
Lagoe, unpublished).
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