Download ABSTRACT Callovian to early Kimmeridgian (Unitary Keywords: ophiolite, age, CRO, cordillera,

Radioisotopic and biostratigraphic age relations in the Coast Range
Ophiolite, northern California: Implications for the tectonic evolution
of the Western Cordillera
John W. Shervais†
Department of Geology, Utah State University, 4505 Old Main Hill, Logan, Utah 84322-4505, USA
Benita L. Murchey
U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA
David L. Kimbrough
Department of Geological Sciences, San Diego State University, San Diego, California 92182-1020, USA
Paul R. Renne
Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709 and Department of Earth and Planetary Science,
University of California, Berkeley, California 94720, USA
Barry Hanan
Department of Geological Sciences, San Diego State University, San Diego, California 92182-1020, USA
ABSTRACT
The Coast Range ophiolite (CRO) in
northern California includes two distinct
remnants. The Elder Creek ophiolite is a
classic suprasubduction zone ophiolite with
three sequential plutonic suites (layered gabbro, wehrlite-pyroxenite, quartz diorite), a
mafic to felsic dike complex, and mafic-felsic
volcanic rocks; the entire suite is cut by late
mid-oceanic-ridge basalt (MORB) dikes and
overlain by ophiolitic breccia. The Stonyford
volcanic complex (SFVC) comprises three
volcanic series with intercalated chert horizons that form a submarine volcano enclosed
in sheared serpentinite. Structurally below
this seamount are mélange blocks of CRO
similar to Elder Creek.
U/Pb zircon ages from plagiogranite and
quartz diorites at Elder Creek range in age
from 165 Ma to 172 Ma. U/Pb zircon ages
obtained from CRO mélange blocks below
the SFVC are similar (166–172 Ma). 40Ar-39Ar
ages of alkali basalt glass in the upper SFVC
are all younger at ≈164 Ma. Radiolarians
extracted from chert lenses intercalated with
basalt in the SFVC indicate that the sedimentary strata range in age from Bathonian (Unitary Association Zone 6–6 of Baumgartner et
al., 1995a) near the base of the complex to late
†
E-mail: [email protected].
Callovian to early Kimmeridgian (Unitary
Association Zones 8–10) in the upper part.
The SFVC sedimentary record preserves
evidence of a major faunal change wherein
relatively small sized, polytaxic radiolarian
faunas were replaced by very robust, oligotaxic, nassellarian-dominated faunas that
included Praeparvicingula spp.
We suggest that CRO formation began after
the early Middle Jurassic (172–180 Ma) collision of an exotic or fringing arc with North
America and initiation of a new or reconfigured east-dipping subduction zone. The data
show that the CRO formed prior to the Late
Jurassic Nevadan orogeny, probably by rapid
forearc extension above a nascent subduction
zone. We infer that CRO spreading ended
with the collision of an oceanic spreading
center ca. 164 Ma, coincident with the oldest
high-grade blocks in the structurally underlying Franciscan assemblage. We further suggest that the “classic” Nevadan orogeny represents a response to spreading center collision,
with shallow subduction of young lithosphere
causing the initial compressional deformation and with a subsequent change in North
American plate motion to rapid northward
drift (J2 cusp) causing sinistral transpression
and transtension in the Sierra foothills. These
data are not consistent with models for Late
Jurassic arc collision in the Sierra foothills or
a backarc origin for the CRO.
Keywords: ophiolite, age, CRO, cordillera,
tectonics.
INTRODUCTION
The Coast Range ophiolite of California and
the tectonically subjacent Franciscan assemblage
have played a pivotal role in plate tectonic theory
since its inception and even now are considered a
paradigm for active margin processes (Dickinson,
1971; Ernst, 1970; Ingersoll et al., 1999). Nonetheless, the origin of the Coast Range ophiolite
(CRO) is still controversial, as is its relationship
to the Franciscan assemblage (e.g., Dickinson et
al., 1996; Godfrey and Klemperer, 1998; Saleeby,
1997). Postulated origins include (1) formation at
a Middle Jurassic equatorial midoceanic ridge
followed by rapid northward transport and Late
Jurassic accretion to North America (Hopson et
al., 1997); (2) formation as backarc basin crust
behind a Middle Jurassic island arc that was
sutured to the continental margin during the Late
Jurassic Nevadan orogeny (Godfrey and Klemperer, 1998; Schweickert, 1997; Schweickert et
al., 1984); and (3) formation by forearc rifting
above an east-dipping, proto–Franciscan subduction zone during the Middle Jurassic, prior to the
Nevadan orogeny (Shervais, 1990; Shervais and
Kimbrough, 1985; Shervais et al., 2004; Stern
and Bloomer, 1992). There is abundant evidence
for arc-related geochemical signatures in the
CRO (Evarts et al., 1999; Giaramita et al., 1998;
GSA Bulletin; May/June 2005; v. 117; no. 5/6; p. 633–653; doi: 10.1130/B25443.1; 10 figures; 1 table; Data Repository item 2005079.
For permission to copy, contact [email protected]
© 2005 Geological Society of America
633
SHERVAIS et al.
Shervais, 1990; Shervais and Kimbrough, 1985),
but other data, such as sedimentary cover associations and seismic imaging of the continental margin, have been interpreted as support for the open
ocean or backarc basin models (e.g., Godfrey and
Klemperer, 1998; Hull et al., 1993; Robertson,
1989). Resolving this controversy is central to
our understanding of the tectonic evolution of
western North America during the Jurassic and to
our understanding of how ophiolites form.
Much of this debate hinges on age relations
within the CRO, and between the CRO, the
Franciscan assemblage, and the Sierra Nevada
foothills metamorphic belt. Previous work
suggests that the CRO ranges in age from ca.
163 Ma at Point Sal to ca. 154 Ma at Del Puerto
Canyon (Hopson et al., 1981), although there
is reason to believe the upper age limit may
be even older (Mattinson and Hopson, 1992).
These ages for CRO formation are similar to
the oldest age found for high-grade (amphibolite
facies) metamorphism in the northern Franciscan
assemblage, as determined by U-Pb isochron
and 40Ar/ 39Ar dating on high-grade metamorphic
blocks (Mattinson, 1986, 1988; Ross and Sharp,
1986; Ross and Sharp, 1988), leading to the
suggestion that high-grade metamorphic blocks
in the Franciscan assemblage formed during
subduction initiation beneath an existing backarc
basin (= CRO) (Wakabayashi, 1990).
Further, it has been suggested that radiolarian cherts deposited unconformably on top of
the CRO document a hiatus of some 8–11 m.y.
between ophiolite formation in the Middle
Jurassic and deposition of overlying chert in the
Late Jurassic (Oxfordian–Tithonian) (Hopson et
al., 1992; Hopson et al., 1981; Pessagno et al.,
2000). This proposed hiatus is cited as primary
evidence for an open-ocean origin to the CRO,
far from any source of arc detritus or terrigenous
sediment. However, there are two significant
problems with this suggestion: (1) Integration
of the Jurassic time scale and standard ammonite zones is still in flux, with differences in the
proposed ages for some stage boundaries of up
to 15 m.y. between alternative time scales (see
discussions in Gradstein et al., 1994; Palfy
et al., 2000); and (2) major differences in the
calibrations of different radiolarian zonations
compound the time scale uncertainties pointed
out above (Baumgartner et al., 1995a; Hull and
Pessagno, 1995; Pessagno et al., 1993; Pessagno
et al., 1987).
In this paper, we present new radioisotopic
age data for plutonic rocks of the Coast Range
ophiolite in northern California and for unaltered volcanic glass from the Stonyford volcanic
complex. We also present new biostratigraphic
data for radiolarian cherts interbedded with volcanic rocks of the Stonyford volcanic complex.
634
For purposes of comparison between radioisotope and biostratigraphic ages, we use the recent
Jurassic time scale of Palfy et al. (2000) and the
radiolarian zonation of Baumgartner (1995).
Taken together with geochemical data for rocks
of the ophiolite and their field relations, these
data allow us to construct a synthesis for the
origin and evolution of the ophiolite, its relationship to high-grade metamorphism in the
Franciscan assemblage, and the tectonic evolution of the western Cordillera during the Middle
and Late Jurassic.
(Murchey and Blake, 1993), predating the Coast
Range ophiolites. The oldest clastic rocks in
the Franciscan slightly postdate the base of the
Great Valley Sequence in northern California
(Imlay, 1980). Some high-grade metamorphic
blocks in the Franciscan assemblage probably
formed ca. 160–165 Ma (Mattinson, 1988), but
regional metamorphism of strata in the Eastern
belt, including Valanginian metagraywacke,
may have occurred in the late Early Cretaceous
(115–120 Ma) (Blake and Jones, 1981).
COAST RANGE OPHIOLITE
GEOLOGIC SETTING
The Mesozoic geology of California south of
the Klamath Mountains comprises three main
provinces, from east to west: The Sierra Nevada
magmatic arc province, the Great Valley forearc province, and the Franciscan accretionary
complex (Fig. 1). The Sierra Nevada province
consists of island arc volcanic, plutonic, and
sedimentary rocks that range in age from Paleozoic to Late Cretaceous. Many of these rocks
formed more or less in place along the western
margin of North America; others in the Sierra
Nevada Foothills metamorphic belt may represent exotic or fringing arc terranes that were
welded to the continental margin during Late
Jurassic or older collisions (Girty et al., 1995;
Saleeby, 1983a; Schweickert et al., 1984).
The Great Valley province represents a Late
Jurassic through Cretaceous forearc basin that
was underlain by the Coast Range ophiolite
(e.g., Bailey et al., 1970). The Great Valley
Sequence consists largely of distal turbidites
near its base, which become more proximal and
more potassic upsection (Dickinson and Rich,
1972; Ingersoll, 1983; Linn et al., 1992). The
lower part of the Great Valley Sequence near
Stonyford is Tithonian, based on the occurrence of Buchia piochii throughout the section
(Brown, 1964a, 1964b). Farther north, however, the basal Great Valley Sequence contains
Buchia rugosa and a few specimens of Buchia
with very fine ribs, transitional between B.
concentrica and B. rugosa (Imlay, 1980; Jones,
1975). Imlay (1980) dated the transitional interval as late Kimmeridgian to early Tithonian,
though he favored the earlier range.
The Franciscan assemblage is a classic Mesozoic and Cenozoic accretionary complex characterized by a heterogenous mixture of lithologies and metamorphic grades. In the study area,
it includes both strongly metamorphosed
blueschist facies rocks in the Eastern belt, as
well as true tectonic mélange in the Central belt
(Bailey et al., 1964). Volcanic-pelagic layers of
oceanic crust incorporated into the Franciscan
are as old as Early Jurassic, Pliensbachian
The Coast Range ophiolite in northern California is represented by a thin selvage of serpentinite matrix mélange along most of the boundary separating forearc sedimentary rocks of the
Great Valley Sequence from blueschist facies
metamorphic rocks of the Franciscan Eastern
belt or shale-matrix mélange of the Franciscan
Central belt. This boundary was originally interpreted as a fossil subduction zone (“the Coast
Range thrust,” e.g., Bailey et al., 1970), but later
work showed that this boundary may have been
modified by later low-angle detachment faults
(Harms et al., 1992; Jayko et al., 1987; Platt,
1986), out-of-sequence thrust faults (Ring and
Brandon, 1994; Ring and Brandon, 1999), and
by east-vergent Neogene thrust faults (Glen,
1990; Unruh et al., 1995). The current boundary is a complex high-angle fault that may have
components of strike-slip, normal faulting, and
reverse faulting.
There are two large ophiolite remnants preserved in the northern Coast Ranges west of the
Sacramento Valley: The Elder Creek ophiolite
and the Stonyford volcanic complex (Fig. 1).
Despite their close proximity (they are separated
by only 60 km along strike) these remnants are
distinctly different. The Elder Creek ophiolite
is a “classic” ophiolite, with cumulate plutonic
rocks and sheeted dike complex, while the
Stonyford volcanic complex consists largely of
volcanic rocks comprising a Jurassic seamount
(Shervais and Hanan, 1989). Detailed mapping
of both areas, however, has shown that they are
related and that both formed in the same tectonic association (Shervais et al., 2004).
Elder Creek Ophiolite
The Elder Creek ophiolite is one of the largest exposures of CRO in California and also the
northernmost (Fig. 1). The ophiolite is named
for outcrops along the South, Middle, and North
Forks of Elder Creek, which expose progressively deeper levels of the ophiolite from south
to north (Fig. 2). The Elder Creek ophiolite preserves most of the pseudostratigraphy associated
Geological Society of America Bulletin, May/June 2005
AGE RELATIONS IN THE COAST RANGE OPHIOLITE, CALIFORNIA
124º
122º
Elder Creek
40º N
40º N
165-172 Ma U/Pb New
Stonyford
166-172 Ma U/Pb New
164 1Ma Ar/Ar New
Geyser Peak /
Black Mountain
Mount Diablo
38º N
165 Ma U/Pb Man91
38º N
Leona Rhyolite
Del Puerto
Sierra Azul
163+/-5 Ma K/Ar Lan71
156+/-2 Ma U/Pb HMP81
Tertiary
Quinto Creek
Modoc Plateau
Salinia
SNF
Llanada
36º N
164 Ma
U/Pb HMP81 36º
Great Valley Sequence
Coast Range Ophiolite
Franciscan Complex
Cuesta Ridge
Sierra Nevada
153 Ma U/Pb HMP81
Klamath terranes
124º W
N
SAF
Stanley Mtn
166+/-2Ma U/Pb P93
Point Sal
165-173 Ma U/Pb MH92
122º W
120º W
Figure 1. Generalized geologic map of California showing major lithotectonic provinces
discussed in text, along with location of ophiolite remnants in the Coast Ranges; the CRO
is shown in black. Ages labeled “new” are from this study; others from Lanphere, 1971
(Lan71); Hopson, Mattinson, and Pessagno, 1981 (HMP81); Mattinson and Hopson, 1992
(MH92); J.M. Mattinson in Pessagno et al., 1993a (P93); and J.M. Mattinson in Mankinen
et al., 1991 (Man91). SAF—San Andreas Fault, SNF—Sur-Nacimiento Fault.
with “true” ophiolites: Cumulate ultramafic
rocks, cumulate gabbro, isotropic gabbro, and
sheeted dikes (Hopson et al., 1981; Shervais and
Beaman, 1991; Shervais et al., 2004). Prior to
deposition of the Great Valley Sequence, most
volcanic rocks were removed by erosion related
to tectonic disruption on the seafloor and redeposited as clasts within the overlying Crowfoot
Point breccia (Blake et al., 1987; Hopson et al.,
1981; Robertson, 1990).
Field relations and geochemistry of plutonic
rocks show that the Elder Creek ophiolite
formed from four magmatic episodes (Shervais,
2001; Shervais and Beaman, 1991; Shervais et
al., 2004). The first magma series is represented
by dunite, layered or foliated cumulate gabbro,
isotropic gabbro, and a dike complex. The second magmatic episode consists of wehrlite and
clinopyroxenite intrusions into the older layered
complex, with less common isotropic gabbro
and gabbro pegmatoid. The third magmatic episode comprises stocks and dikes of hornblende
diorite and hornblende quartz diorite, with felsite dikes that are marginal to the quartz diorite
plutons; rocks of this suite intrude all of the
older lithologies. The diorite stocks commonly
form magmatic breccias (“agmatites”) with
xenoliths of cumulate or foliated gabbro, dike
complex, or volcanic rock in a quartz diorite
matrix. The fourth magma series is represented
by rare basaltic dikes that crosscut rocks of the
older episodes. Geochemical data are consistent
with formation of the first three magma series in
a suprasubduction zone (arc) environment; rare
dikes of the final magma series are characterized
by MORB-like major and trace element compositions (Shervais, 2001; Shervais and Beaman,
1991; Shervais et al., 2004).
Volcanic rocks are most commonly preserved as clasts in the Crowfoot Point breccia,
a coarse, unsorted fault-scarp talus breccia that
varies from <10 m to over 1000 m in thickness
(Hopson et al., 1981; Robertson, 1990). The
Crowfoot Point breccia contains clasts of mafic
and felsic volcanic rocks, gabbro, pyroxenitewehrlite, and diorite. This unit was deposited
on an eroded surface that cuts all other units of
the ophiolite (from cumulate ultramafics through
dike complex). Additional volcanic rocks crop
out in fault-bounded blocks and in the dike complex. With the exception of the late, MORB-like
dikes, all volcanic and hypabyssal rocks associated with the Elder Creek ophiolite are island arc
tholeiite or calc-alkaline series basalts, andesites,
or dacites (Shervais and Beaman, 1991).
Felsic plutonic rocks crop out in two distinct
associations: (1) As small (<2 m across) lenses
intruded into the lower part of the dike complex,
and (2) as large stocks and sills that intrude all
other ophiolite lithologies. The first association
appears to represent residual magma related to
the first or second magma series; the second
association forms the bulk of the third, calcalkaline magma series. Both associations were
sampled for U-Pb zircon dating.
Stonyford Volcanic Complex
The Stonyford volcanic complex (SFVC)
crops out ~60 km south of Elder Creek ophiolite,
Geological Society of America Bulletin, May/June 2005
635
SHERVAIS et al.
in the low-lying hills surrounding the community of Stonyford (Fig. 1). It was originally
mapped by Brown (1964a) and later mapped
in detail by Zoglman and Shervais (Zoglman,
1991; Zoglman and Shervais, 1991). Contrary
to our earlier suggestions that the SFVC might
represent Franciscan assemblage volcanics
tectonically transferred to the CRO serpentinite mélange (Shervais and Kimbrough, 1985;
Shervais and Kimbrough, 1987), our later work
has shown that the SFVC formed as an integral
part of the CRO and that it was never part of the
subduction complex (Zoglman and Shervais,
1991). The SFVC is also distinct from the
St. Johns Mountain complex, which contains
incipient blueschist facies metamorphism and
is clearly within the Franciscan assemblage
(MacPherson, 1983).
The SFVC consists of four large blocks
within sheared serpentinite-matrix mélange; the
largest block is some 5 × 3 km in areal extent
(Fig. 3). The SFVC consists largely of pillow
lava with subordinate sheet flows, diabase,
and hyaloclastite breccia. The volcanic rocks
are exceptionally fresh for the Coast Range
ophiolite, as shown by the preservation of primary igneous plagioclase and clinopyroxene in
most of the lavas and unaltered basaltic glass in
many of the hyaloclastites (Shervais and Hanan,
1989; Shervais and Kimbrough, 1987; Zoglman
and Shervais, 1991). The hyaloclastite breccias,
which represent submarine fire fountain deposits, contain unaltered volcanic glass with relict
phenocrysts of olivine, plagioclase, and chrome
spinel (Shervais and Hanan, 1989).
Volcanic rocks of the SFVC form three
groups: (1) Enriched, oceanic tholeiite basalts,
(2) transitional alkali basalts and glasses, and
(3) high-alumina, low-Ti tholeiites (Shervais
et al., 2004; Zoglman, 1991). Pb isotopic data
for the volcanic glasses are similar to Pacific
oceanic basalts currently found in off-axis seamounts and associated with large ion lithophile
elements-rich mantle plumes (Hanan et al.,
1992). The rare earth elements, trace element,
and Pb data indicate that the oceanic tholeiites
and alkali basalts were derived from a heterogeneous mantle source with at least two components: A depleted MORB-source asthenosphere
and an enriched plumelike component (Hanan et
al., 1992). The high-Al, low-Ti basalts resemble
second-stage melts of a MORB asthenosphere
source, which form by melting plagioclase
lherzolite at low pressures; these lavas also
resemble high-Al island arc basalts. The trace
element and Pb systematics show an alkali
basalt influence, which overprints generally
depleted trace element characteristics (Hanan
et al., 1992; Shervais et al., 2004; Zoglman and
Shervais, 1991).
636
Great Valley Series
GVS
du
GVS
cpb
Wacke and siltstone
Crowfoot Point Breccia
wc
du
Elder Creek Ophiolite
cg
wc
du
qdi
fmg
EC148-2
fdc
du
ig
GVS
fdc
vc
cg
vc
Volcanic rocks
dc
Dike complex
fdc
Felsic dikes
qdi
Quartz diorite & diorite
ig
Isotropic gabbro
cg
Cumulate gabbro
wc
Wehrlite-Pyroxenite
du
Dunite & dunite
broken formation
vc
cg
sfms
wc
qdi
cg
fdc
wc
ig
EC107-3
dc
cpb
Serpentinite mélange
qdi
ssp
cg
ig
sfms
wc
gs
Sheared serpentinite
vc
wc
qdi
ssp
gs
Volcanic blocks
in melange
fmg
Foliated metasediments
(Galice?)
cg
ssp
Franciscan Assemblage
South Fork
sfms
Mountain schist
qdi
gs
GVS
cpb
dc
gs
wc cpb
ssp
North
gs
ig
0
vc cpb
1
miles
2
ssp
GVS
Figure 2. Geologic sketch map of Elder Creek ophiolite showing sample locations collected
and dated for this study (small stars).
Lensoid intercalations of red radiolarian chert
and pink siliceous mudstone up to 1 km long
and 50 m thick occur throughout the section
(Fig. 3). The cherts are typically Mn-rich and
are commonly associated with hydrothermal
alteration of the underlying basalts, suggesting
that they formed in proximity to submarine hot
springs. In places, typical ribbon cherts are interbedded with siliceous mudstones containing
rip-up clasts of chert that were entrained while
they were still soft. Both the ribbon cherts and
the siliceous mudstones contain abundant wellpreserved radiolarians.
Structural and stratigraphic relations show
that, in general, the lavas and their sedimentary
intercalations dip moderately to the northeast,
and that they become younger to the northeast as
well. Along the northern margin of the structurally
Geological Society of America Bulletin, May/June 2005
AGE RELATIONS IN THE COAST RANGE OPHIOLITE, CALIFORNIA
JKf
39º 25' N
122º 40' W
Chert B
39º 25' N
122º 32' 30" W
Glass 1-4
SSP
Harz
SFV58-1
SFV141-1
Chert C
Glass 5, 8
GVS
SFV109-2
Chert A
SFVC
Qal
reek
ny C
Sto
Stonyford
Chert D
SFVC
GVS
39º 20' N
122º 40' W
SSP
JKf
Harzburgite
39º 20' N
122º 32' 30" W
Legend
STONYFORD VOLCANIC COMPLEX
SFV: Pillow lavas & massive basalts
Volcaniclastic/
hyaloclasticbreccias
Chert, limestone, &
siliceous mudstone
QUATERNARY
Qal: Alluvium
Conglomerate
Quaternary landslide
MN
SERPENTINITE MATRIX MELANGE
GREAT VALLEY SEQUENCE
GVS: Interbedded sandstone,
siltstone, and shale
SSP: Sheared serpentinite
FRANCISCAN COMPLEX (JKf)
Qtz-lawsonite-mica schist:
metagreywacke, metasiltstone,
metachert
Metavolcanic
Schist blocks in serpentinite
GN
Harz: Massive harzburgite:
partially serpentinized, sheared
High-grade blocks
1
0
0
2 miles
7000 feet
Metavolcanic blocks
Meta-plutonic rocks blocks
(wehrlite, pyroxenite,
gabbro, diorite)
Volcanogenic sandstone
0
1
2 km
0 17'
6 MILS
17 1/2
311 MILS
Geologic map of the Stonyford Volcanic Complex, California.
Mapping by John W. Shervais and Marchell Z. Schuman
Figure 3. Geologic sketch map of the Stonyford volcanic complex showing the location of samples collected and dated for this study (small
stars), hyaloclastite layers, and chert-siliceous mudstone intercalations. GN—UTM Grid convergence, MN—Magnetic north.
largest block of SFVC radiolarian chert is overlain stratigraphically by a thick hyaloclastite layer
containing unaltered volcanic glass (Fig. 3).
Dismembered remnants of CRO plutonic
and volcanic rock, including dunite, wehrlite,
clinopyroxenite, gabbro, diorite, quartz diorite,
and keratophyre pillow lava, occur as blocks up
to 200 m across within the serpentinite matrix
mélange structurally beneath the SFVC. These
blocks are identical to lithologies found farther
north in the Elder Creek ophiolite. Quartz diorite occurs as individual blocks ranging in size
from a few meters to several tens of meters and
as dikes within mélange blocks of isotropic gabbro. Other tectonic blocks within the mélange
include unmetamorphosed volcanogenic sandstones, foliated metasediments, and pale green
metavolcanic rocks.
FRANCISCAN ASSEMBLAGE
The Franciscan assemblage in northern
California consists of two primary units: The
Eastern belt of high P/T metamorphic rocks
and the structurally underlying Central Belt
mélange (Blake et al., 1988). The Eastern
belt of the Franciscan assemblage comprises
blueschist facies metamorphic rocks in two
Geological Society of America Bulletin, May/June 2005
637
SHERVAIS et al.
distinct terranes: The structurally higher Pickett
Peak terrane and the underlying Yolla Bolly
terrane. Both terranes contain blueschist facies
metagreywacke in relatively intact, coherent
thrust sheets; the Pickett Peak terrane also
contains the South Fork Mountain schist and
its Chinquapin metabasalt member (Blake et al.,
1988). The Eastern belt is distinguished from
the Central belt by the coherent nature of thrust
sheets, its blueschist facies metamorphism, and
its foliated textures.
The Central belt of the Franciscan assemblage consists largely of mélange with a pervasively sheared matrix of mudstone and greywacke sandstone containing blocks and slabs of
intact greywacke sandstone, greenstone, chert,
and serpentinite (Berkland et al., 1972; Blake et
al., 1982; Blake et al., 1989). Some greywacke
slabs are up to several km across; greenstone
and (or) chert knockers are smaller but can also
be tens of km in extent. Also common in the
Central belt are high-grade blocks of blueschist,
eclogite, amphibolite, and garnet amphibolite
(Cloos, 1986; Moore, 1984). These high-grade
blocks reflect polymetamorphism with initial
high temperatures and low pressures followed
by lower temperature and higher pressure metamorphism (Moore, 1984; Moore and Blake,
1989; Wakabayashi, 1990).
PREVIOUS WORK
The northern CRO has received limited
attention and there are few radioisotopic or
biostratigraphic ages reported. Lanphere (1971)
reported a K-Ar age of 154 ± 5* Ma for an
isotropic hornblende gabbro dike in pyroxenite
that is overlain unconformably by the Crowfoot
Point breccia. He also reported ages of 162 ±
5* Ma and 164 ± 8* Ma for hornblendes from
an isolated peridotite and an isotropic gabbro
lens in the Del Puerto ophiolite (*note: These
ages have been recomputed by McDowell et
al. [1984] using the new decay constants of
Steiger and Jaeger [1977]). McDowell et al.
(1984) reported hornblende K-Ar ages for four
CRO gabbros: three from the Elder Creek area
and one from Wilbur Springs. One Elder Creek
hornblende has a reported age of 166 ± 3 Ma;
the others are all ca. 143–144 ± 3 Ma (McDowell et al., 1984).
Hopson et al. (1981) reported U-Pb zircon
dates for eleven samples from locales south
of Stonyford (Healdsburg, Del Puerto, Cuesta
Ridge, Pt. Sal, and Santa Cruz island). These
zircons have reported 238U/206Pb ages of 144 ±
2–165 ± 2 Ma and 207Pb/206Pb ages of 144 ± 15 to
201 ± 15 Ma (Hopson et al., 1981); none of these
are isochron ages. In an abstract, Mattinson and
Hopson (1992) revised these dates upward for
638
some southern CRO locations to 165–170 Ma,
based on new data obtained with a modern multicollector mass spectrometer. In addition, new
U-Pb zircon dates have been reported for Stanley
Mountain (166 ± 1 Ma, J.M. Mattinson, reported
in Pessagno et al., 1993) and Mount Diablo
(165 Ma, J.M. Mattinson, reported in Mankinen
et al., 1991). None of the dates reported since
Hopson et al. (1981) (either in abstracts or as a
personal commun.) has been published with supporting data, so it is not possible to evaluate their
precision or accuracy.
Radiolarian biostratigraphic data from bedded chert at Elder Creek and Stonyford have
been reported by Pessagno and Louvion-Trellu
(Hopson et al., 1981; Louvion-Trellu, 1986; Pessagno, 1977). All the of cherts sampled at Elder
Creek are from mélange blocks in the Round
Valley serpentinite mélange, including blocks of
chert only and blocks with chert resting depositionally on basalt. Hopson et al. (1981) assigned
radiolarians from the chert blocks to the upper
part of 1977 Zone 1 (undifferentiated) of Pessagno, and they calibrated the faunas as Oxfordian
to Kimmeridgian. The taxa listed by Hopson et
al. (1981, p. 477) are all long ranging in the UA
zonation of Baumgartner et al. (1995a, 1995b)
except for Parvicingula sp. C, a synonym of
Praecaneta (Ristola) turpicula, which has a
range from UA Zones 5–6, late Bajocian to
Bathonian. In a subsequent study of the mélange
(Louvion-Trellu, 1986), additional radiolarian
faunas collected from the blocks were assigned
ages of Callovian to early Oxfordian based
primarily on European-based biostratigraphic
calibrations. Radiolarians are also present in
mudstone in the lower part of the Great Valley
Sequence, which unconformably overlies the
Elder Creek ophiolite (Pessagno, 1977). The
radiolarians occur with, and were calibrated by,
the previously mentioned late Kimmeridgian or
early Tithonian bivalves. Pessagno (1977) also
documented the radiolarians from a sample near
the “Diversion Dam” along Stony Creek in the
Stonyford volcanic complex and assigned them
to his 1977 Subzone 2B (equivalent to Zone 3
of Pessagno et al., 1993), which he inferred to
be early Tithonian (Pessagno, 1977). The taxa
from Diversion Dam locality are discussed and
recalibrated below.
METHODS
contaminating grains. Zircon dissolution and
ion exchange chemistry for separation of uranium and lead followed procedures modified
from Krogh (1973). Isotope ratios were measured with the MAT 261 multicollector instrument at UC Santa Barbara and the VG Sector
54 multicollector instrument at San Diego State
University. Analytical uncertainties, blanks,
and common lead corrections are outlined in
Table 1. Most of the samples yield concordant to
near-concordant U/Pb dates that are interpreted
to closely approximate crystallization ages. The
relatively simple systematics for these samples
is interpreted to reflect the low metamorphic
grade of the samples and negligible or absent
inherited components of radiogenic lead.
40
Ar/ 39Ar
Samples of clear brown volcanic glass were
crushed into equant grains in distilled water
and then ultrasonically cleaned successively in
10% HCl and 7% HF for three minutes in each
acid. Grains 1.0–2.0 mm in dimension were
selected individually for high optical reflectivity, freedom from inclusions and veins, and
generally fresh appearance. Five to ten grains
selected from each sample were irradiated in
two batches for ~28 h each at Los Alamos
National Laboratories’ Omega West reactor,
along with neutron fluence monitor Fish Canyon sanidine (28.02 Ma; Renne et al., 1998).
Only G-2 glass (laboratory numbers 3524–1,
−3, and −4) was irradiated in the first batch,
which used Cd shielding; all four glasses were
co-irradiated in a second batch that did not use
Cd shielding. Two individual grains of glass
samples G-4, G-5, and G-8, and four of G-2,
were incrementally degassed in 10–15 steps
with an Ar-ion laser and analyzed for relative
Ar isotopic abundances using the fully automated facilities and procedures described by
Renne (1995). Data are presented in Data
Repository document DR-1.1
Biostratigraphic
Radiolarian-bearing chert and siliceous mudstone were collected from several localities in
the Stonyford volcanic complex. Radiolarians
and siliceous sponge spicules were etched from
surrounding rock matrices by bathing broken
rock fragments in diluted hydrofluoric acid (10%
U-Pb Zircon
Zircon was separated by conventional techniques using a Wilfley Table, heavy liquids, and
a Franz magnetic separator. The least magnetic
zircons from each sample were split into size
fractions and then handpicked to remove any
1
GSA Data Repository item 2005079, Ar release
spectra for volcanic glasses of the Stonyford volcanic complex technical notes on biostratigraphic
calibrations and correlations, is available on the Web
at http://www.geosociety.org/pubs/ft2005.htm. Requests may also be sent to [email protected].
Geological Society of America Bulletin, May/June 2005
AGE RELATIONS IN THE COAST RANGE OPHIOLITE, CALIFORNIA
TABLE 1. COAST RANGE OPHIOLITE: U-PB ZIRCON ISOTOPIC DATA FOR ELDER CREEK AND STONYFORD
Fraction
Weight Pb
U
(g)
(ppm) (ppm)
Lead isotopic compositions
206/208 206/207 206/204 207*/235
Radiogenic ratios
%err 206*/238 %err 207*/206* %err
Apparent ages (Ma)
206*/238 207*/235 207*/206*
±
Elder Ck EC-107–3
>200
<100L
<200L
<100>200
<200
bulk L
0.0031
0.0007
0.0017
0.0051
0.0038
0.0052
3.91
3.78
4.31
4.58
5.48
3.83
132.5
129.9
149.2
159.7
192.0
139.8
4.276
4.749
4.853
4.646
4.595
5.163
15.250
17.656
19.313
19.513
19.533
19.675
912
2034
6199
8583
9553
11076
0.1770
0.1836
0.1825
0.1796
0.1790
0.1749
0.39
0.33
0.32
0.31
0.31
0.31
0.0260
0.0270
0.0268
0.0263
0.0261
0.0256
0.29
0.30
0.28
0.29
0.30
0.28
0.04945
0.04940
0.04940
0.04953
0.04965
0.04950
0.26
0.13
0.16
0.10
0.06
0.14
165.2
171.5
170.4
167.4
166.4
163.1
165.5
171.2
170.4
167.4
166.4
163.1
169
167
167
173
178
172
6.1
3.0
3.6
2.2
1.4
3.4
162.1
123.8
199.6
637.2
242.7
183.6
168.0
163.7
4.407
4.562
4.472
4.348
4.338
4.553
4.118
4.427
19.310
19.565
19.581
18.820
17.787
19.674
19.791
19.671
6340
8447
9672
4783
2111
10687
14094
10744
0.1793
0.1721
0.1793
0.1815
0.1772
0.1789
0.1808
0.1844
0.30
0.30
0.30
0.30
0.38
0.30
0.29
0.30
0.0263
0.0253
0.0262
0.0263
0.0261
0.0262
0.0265
0.0270
0.28
0.29
0.28
0.28
0.29
0.28
0.28
0.28
0.04947
0.04937
0.04955
0.05006
0.04925
0.04945
0.04948
0.04947
0.08
0.09
0.10
0.10
0.24
0.09
0.06
0.07
167.2
161.0
167.0
167.4
166.0
167.0
168.6
171.9
167.4
161.2
167.4
169.4
165.6
167.1
168.7
171.8
170
165
174
198
160
169
171
170
1.6
1.7
2.1
2.2
5.4
1.9
1.4
1.5
0.0035 15.37 542.1
4.607
19.632
11811
0.1778
0.30
0.0260
0.28
0.04969 0.07
165.2
166.2
181
1.5
192.3
158.7
247.9
269.6
211.3
230.7
208.7
3.112
2.955
3.083
2.841
3.219
2.911
3.299
19.777
17.986
18.715
18.856
19.237
18.937
18.436
11414
2381
3570
4111
5542
4387
3121
0.1463
0.1723
0.1739
0.1695
0.1752
0.1746
0.1736
0.33
0.33
0.42
0.30
0.31
0.30
0.30
0.0215
0.0253
0.0256
0.0249
0.0258
0.0256
0.0254
0.28
0.28
0.29
0.28
0.28
0.28
0.28
0.04928
0.04942
0.04931
0.04946
0.04933
0.04945
0.04953
0.18
0.18
0.32
0.11
0.15
0.10
0.09
137.3
160.9
162.8
158.3
164.0
163.0
161.8
137.3
160.9
162.8
159.0
164.0
163.4
162.5
161
168
163
169
163
169
173
4.1
4.0
7.3
2.6
3.3
2.2
2.0
0.0007 7.62 222.8
0.0005 11.77 330.4
2.094
1.864
18.437
19.454
3957
1319
0.1813
0.1759
0.31
0.32
0.0260
0.0257
0.28
0.28
0.05052 0.12
0.04963 0.16
165.7
163.6
169.2
164.5
219
177
2.7
3.5
7.248
6.935
6.857
7.540
19.438
19.643
19.328
19.234
6954
9664
6234
5898
0.1734
0.1745
0.1722
0.1713
0.35
0.30
0.30
0.30
0.0255
0.0256
0.0253
0.0251
0.34
0.29
0.29
0.28
0.04933
0.04939
0.04938
0.04950
162.3
163.1
161.0
159.8
162.3
163.3
161.3
160.5
163
166
166
171
2.0
1.5
1.6
1.5
Brush Mtn EC-148–2B
<100>200
bulk
<200
>100
<325L
<200L
>100
>200L
0.0045
0.0029
0.0040
0.0018
0.0007
0.0025
0.0021
0.0017
4.69
3.41
5.75
18.57
7.00
5.26
4.94
4.86
Brush Mtn EC-148–2A
bulk L
Auk Auk SFV-109–2
<200L
<325
>200Fm
<200Fm
>200L
bulk Fm
L
0.0036
0.0045
0.0012
0.0025
0.0016
0.0041
0.0007
4.89
4.84
7.52
8.13
6.38
7.12
6.17
Dry Creek SFV-141
bulk L
bulk
Dry Creek SFV-58–2
<200L
<325L
<200
L
0.0018
0.0023
0.0029
0.0018
2.50
4.81
4.91
4.61
96.3
183.1
188.6
180.6
0.09
0.07
0.07
0.07
Notes: Fractions: 100, 200, 325 = mesh sizes; bulk; L—HF leach; Fm—Frantz magnetic fraction. Separation of U and Pb was done using HCl column chemistry.
Concentrations were determined using mixed 208Pb/235U and 205Pb/235U spikes. Lead isotopic compositions corrected for ~0.10% ± 0.05% per mass unit mass
fractionation. Ages calculated with following decay constants: 1.55125E-10 = 238U and 9.8485E-10 = 235U. Present day 238U/235U = 137.88. Common lead corrections
made using Stacey and Kramers (1975) model lead isotopic compositions. Total lead blanks averaged c. 20 picograms.
*Radiogenic Pb ratios.
of ~50% concentrate), commonly for ~24 h, following procedures modified from Dumitrica
(1970) and Pessagno and Newport (1972). Then
the fossils were washed off the etched rock
surfaces and collected on Tyler-equivalent 250mesh (63 µm openings) and 80-mesh (180 µm
openings) screens. The fossils were identified
based on examination with a binocular microscope. Selected specimens were also examined
with a scanning electron microscope.
The radiolarian samples were dated using the
Tethyan Unitary Association (UA) Zonation of
Baumgartner et al. (1995a), a widely utilized
international standard that is calibrated with
ammonites, nannofossils, and calpionellids. The
zonation incorporates data on the ranges of more
than 400 Jurassic and (or) Early Cretaceous
radiolarian species from scores of stratigraphic
sections around the world. For the Middle and
Late Jurassic, the unitary association zones
(UAZ) are numbered sequentially from oldest
to youngest: Zones 1–13. All zonal assignments
given in this study, even those restricted to a
single UAZ, are indicated as a range, following
the convention of the zonation.
Biostratigraphic correlations between the
Stonyford sequence and other key sequences
in California and Oregon were based on UAZ
ranges as well as direct comparisons of local
species ranges. Thus, the older Stonyford faunas
were also correlated with radiolarian faunas in
the Middle Jurassic Blue Mountains reference
localities of Pessagno et al. (1987), which are
dated with ammonites. We relied heavily on the
many excellent faunal descriptions of Pessagno
and his colleagues for the basic data used in the
interbasin correlations. However, we did not use
the formal, event-based zonation of Pessagno et
al. (1993, 1987) for either correlation or age calibration. The formal zonation uses a relatively
small number of biostratigraphic events, the first
or last occurrences of selected taxa, as marker
ties; but we wanted to maximize the number of
taxa used for correlation and thereby minimize
the effects of range diachronism. In addition,
although some zonal reference intervals for the
Jurassic zonation of Pessagno et al. (1993, 1987)
are quite well calibrated, key reference intervals
particularly relevant to this study have minimal
direct molluscan control on the ammonite-based
stage boundaries.
Geological Society of America Bulletin, May/June 2005
639
SHERVAIS et al.
RESULTS
U-Pb Zircon Ages
Elder Creek
Zircon was separated from two samples
of quartz diorite collected from the Elder
Creek ophiolite (Fig. 2). Sample EC107–3 is
a plagiogranite lens that intrudes sheeted dike
complex along the South Fork of Elder Creek.
Sample EC148–2 is from an ≈500-m-thick sill
of hornblende quartz diorite that intrudes along
the contact between isotropic gabbro and dike
complex and crops out on the summit of Brush
Mountain. As shown by Shervais and Beaman
(1991), quartz diorite sills similar to EC148–2
crosscut and intrude all other rock types in the
Coast Range ophiolite at Elder Creek and represent the last arc-related magmatic event in the
history of the ophiolite.
Zircon ages for these two samples are shown
in Table 1. Both samples are slightly discordant, with 238U/206Pb ages ranging from 161 to
172 Ma and 207Pb/206Pb ages ranging from 165
to 198 Ma. Concordia plots suggest crystallization ages of 169.7 ± 4.1 Ma for EC107–3 and
172.0 ± 4.0 Ma for EC148–2 (all errors are 2σ;
Fig. 4). These ages are significantly older than
previous hornblende K-Ar dates from gabbro
(154 ± 5 to 163 ± 5 Ma) (Lanphere, 1971;
McDowell et al., 1984), which we interpret here
as cooling or Ar-loss ages. Two zircon fractions
from EC-148 are discordant and have 207Pb/206Pb
ages of 181 ± 1.5 and 198 ± 2.2 Ma, suggesting inheritance of an older zircon component,
perhaps from a continental crustal source (cf.
Wright and Wyld, 1986).
Stonyford
Zircon was separated from three samples of
quartz diorite that occur as blocks within the
serpentinite matrix mélange beneath the SFVC.
Two of these samples are from discrete blocks,
the third is from a dike within a block of isotropic
gabbro. Sample SFV-109–2 is from a 30-cmthick quartz diorite dike that crosscuts an isotropic gabbro block below Auk-Auk Ridge (Fig. 3).
SFV-141–1 is from a large (100 m) block of
coarse-grained quartz diorite that crops out in
Dry Creek. SFV-58–2 is from a small (4 m) block
of strongly foliated quartz diorite that crops out in
serpentinite mélange 400 m north of Dry Creek.
Zircon ages for these three samples are shown
in Table 1. 207Pb/206Pb ages for the least discordant fractions range from 163 Ma to 173 Ma
with a precision of ±1.5–4.1 Ma. Concordia
intercept ages are 164.8 ± 4.8 Ma for SFV-109–2
and 163.5 ± 3.9 Ma for SFV-58–1 (Fig. 4). These
ages are essentially identical to U/Pb zircon ages
determined for the Elder Creek ophiolite 60 km
640
to the north and to the 40Ar/39Ar ages obtained
on the samples of volcanic glass from the Stonyford volcanic complex (see below). Two zircon
fractions from SFV-141 are discordant and have
207
Pb/206Pb ages of 177 ± 3.5 and 219 ± 2.7 Ma,
suggesting inheritance of an older zircon component, perhaps from a continental crustal source
(cf. Wright and Wyld, 1986). Similar results
were obtained by Bickford and Day (2004),
who identified the presence of ca. 2153 ± 1 Ma
inherited zircon in plutons of the 164–160 Ma
Smartville ophiolite.
40
Ar/ 39Ar Glass Ages
Apparent age spectra for replicate samples
of glass from four distinct hyaloclastite units
are shown in Figure 5. Many of the age spectra
are slightly discordant, with anomalously young
ages from low temperature steps, but all yield
plateaux of varying quality and precision. The
mean ages for each unit are 164.4 ± 0.4 Ma
(G-2), 164.0 ± 0.5 Ma (G-4), 163.8 ± 0.8 Ma
(G-5), and 164.6 ± 0.7 Ma (G-8), calculated as
the weighted mean of all plateau steps for each
sample, and are all mutually indistinguishable
at the 2σ level. The consistency of high-precision plateau ages provides strong evidence
that the K-Ar systems have not been disturbed
beyond minor alteration, the effects of which are
removed in low-temperature steps. Mobility of
K and/or Ar is common in glasses, particularly
those having suffered hydration (e.g., Cerling
et al., 1985), but the consistency of our data
precludes such effects unless they were markedly homogeneous both within and between
samples. Part of our success in dating these
glasses stems from the ability to date individual
small grains that could be selected according to
stringent criteria for freedom from alteration.
It could be argued that the mean age of ca.
164 Ma for the glasses reflects the age of an
outgassing event that completely rejuvenated the
K-Ar system in all the samples. While such a scenario cannot be completely excluded, we interpret the 40Ar/39Ar ages to reflect eruption ages in
view of their within- and between-sample reproducibility and their consistency with the Middle
Jurassic age of associated radiolarian faunas.
Radiolarian Biostratigraphy, Stonyford
Volcanic Complex
Red to green banded cherts and massive pink
siliceous mudstones, forming lenses up to 50 m
thick, crop out at several locations within the
Stonyford volcanic complex. Radiolarian localities A, C, and D lie within the central block of
the complex; Locality B lies within the northern
block (Fig. 3). Taxonomic lists of the radiolar-
ians in each locality are included in Figure 6,
along with the UA zonal range (Baumgartner et
al., 1995a) for each group of samples.
The oldest Stonyford samples, collected at
Locality A, are structurally below the hyaloclastic units that have yielded 40Ar/39Ar glass ages of
ca. 164 Ma. The section is divided herein into
intervals A1, A2, and A3 (Fig. 6). Interval A1
is no older than UAZ 3 based on the presence
of Mirifusus fragilis (UAZ 3–8), Acanthocircus
suboblongus suboblongus (UAZ 3–11), Hsuum
brevicostatum gp. (UAZ 3–11), and Saitoium sp.
(UAZ 3–21). Intervals A2 and A3 are assigned
to UAZ 6–6 based on the ranges of Praecaneta
turpicula (UAZ 5–6), Spongocapsula palmerae
(UAZ 6–13), and Xiphostylus (Xiphostylus
gasquetensis gp.) (UAZ 1–6). As determined
from the calibrations of the Tethyan UA zones,
the lower part of the chert section at Locality A
is Bajocian or Bathonian, and the upper part is
Bathonian. Direct correlation between Locality
A and ammonite-dated Middle Jurassic strata in
the Blue Mountains of northeastern Oregon also
favors a Bathonian age for the section (Fig. 7)
based on the ranges in the Blue Mountains of
Praecaneta turpicula (Bathonian), Praecaneta
decora (Bathonian), Eucyrtidiellum unumaense
pustulatum (Bathonian), Pantanellium ultrasincerum (Bathonian), Xiphostylus (forms with
compressed tests) (Bajocian and Bathonian),
Spongocapsula spp. (Bathonian and younger),
Leugeo hexacubicus (sensu Baumgartner et al.,
1995b, p. 296–297) (Bajocian to Callovian),
Archaeodictyomitra spp. aff. A. suzukii (genus
from late Bajocian), and Parahsuum officerense
gp. (Bajocian) (Blome, 1984; Nagai and Mizutani,
1990; Pessagno and Blome, 1980; Pessagno et al.,
1993; Pessagno and Whalen, 1982; Pessagno
et al., 1989). The good agreement in the results
derived from two different calibration methods
confirms the previous conclusions of Murchey
and Baumgartner (1995) that the calibrations of
Middle Jurassic UA Zones 1–6 (Baumgartner et
al., 1995a) compare well with ammonite-based
calibrations for radiolarian sequences in the
Pacific Northwest (Pessagno et al., 1987).
Samples at Locality B are subdivided into
three intervals, from oldest to youngest: B1,
B2, and B3. The oldest sample, B1, is assigned
a possible range of UAZ 3–8, calibrated as
Bathonian to early Oxfordian. The zonal range
is constrained by the lower and upper ranges
of Mirifusus fragilis (UAZ 3–8) and the upper
range of Turanta sp. (UAZ 1–8). A poorly
preserved specimen of probable Guexella
nudata (UAZ 5–8) also was observed in this
interval, suggesting that the sample is probably not older than Bathonian. The B1 fauna is
younger than or correlative with the faunas at
locality A, and the small, polytaxic assemblages
Geological Society of America Bulletin, May/June 2005
Geological Society of America Bulletin, May/June 2005
0.0255
0.0257
0.0259
0.0261
0.0245
0.167
0.0247
0.0249
0.0251
0.172
0.169
160
207
0.180
162
235
0.173
P b /235U
168
Pb/ U
207
0.176
0.171
164
Auk Auk SFV109-2
160
158
0.0248
0.168
0.0252
0.0256
0.0260
Pb/238U 0.0253
Pb/238U
Brush Mtn EC148-2B
Intercept at
164.8 ±4.8 Ma
MSWD = 6.5
0.175
164
0.184
172
0.177
166
0.188
159
0.0248
0.1695
0.0250
0.0252
0.0254
0.0256
0.0258
0.0252
0.172
162
0.0256
0.0260
0.0264
P b /238U
P b /238U
206
206
0.0268
0.0272
0.176
0.1705
160
0.1715
161
207
235
0.1735
163
0.182
170
Pb/ U
0.1725
162
235
0.180
Pb/ U
168
207
0.178
quartz diorite gneiss SFV58-2
0.174
164
166
Elder Ck EC107-3
0.1745
Intercept at
163.5 ±3.9 Ma
MSWD = 6.3
164
0.184
Intercept at
169.7 ± 4.1 Ma
MSWD = 14
172
0.1755
0.186
174
Figure 4. Concordia plots showing U/Pb systematics of zircons dated for this study from Elder Creek ophiolite and the Stonyford volcanic complex. MSWD—mean standard
weighted deviation.
206
206
0.0264
0.0268
0.0272
0.0276
AGE RELATIONS IN THE COAST RANGE OPHIOLITE, CALIFORNIA
641
SHERVAIS et al.
642
180
170
160
150
140
130
SFVG-2-1
SFVG-2-2
SFVG-2-3
SFVG-2-4
SFVG-4-1
SFVG-4-2
SFVG-5-1
SFVG-5-2
120
180
170
160
150
140
130
Apparent Age (Ma)
in both are similar. The B2 and B3 intervals are
younger than A1–A3 and B1. The B2 interval is
assigned to UAZ 7–8 (late Bathonian to early
Oxfordian) based on the ranges of Xitus sp.
(UAZ 7–22) and Mirifusus fragilis (late transitional form) (UAZ 3–8). The B3 interval has a
possible range from UAZ 7–10 (late Bathonian
to early Kimmeridgian), based on the ranges of
Mirifusus dianae dianae (senior synonym of
M. mediodilatatus) (UAZ 7–12), Xitus (UAZ
7–22), and Transhsuum maxwelli (3–10). A
major faunal change occurs within the Locality
B section, wherein the relatively small-sized,
polytaxic radiolarian faunas in the lower part of
the Stonyford sequence (A1–A3, B1) give way
to very robust, oligotaxic, nassellarian-dominated faunas that include Praeparvicingula spp.
(B2–B3, C). This change occurred sometime
during the span represented by UA Zones 6–8.
The initial turnover appears to have predated
the local first appearance of M. dianae dianae
(worldwide range is UAZ 7–12), but it was followed by a great increase in Praeparvicingula
associated with M. d. dianae. Therefore, faunal
turnover most likely occurred during the UAZ
6–7 interval, or mid-Bathonian to Callovian.
The radiolarians collected at locality C are
very similar to those at B3, but the samples
also contain specimens herein assigned to
Podobursa spinosa (UAZ 8–13), the basis for
assigning this interval to UAZ 8–10, Callovian
to earliest Kimmeridgian. However, closely
related forms range down to at least UAZ 7 in
the southern Coast Ranges (Hull, 1997; Hull
and Pessagno, 1994).
The Diversion Dam sample (locality D) collected by Pessagno (1977) is assigned to UAZ
9–10 based on the ranges of the following
taxa that he reported (genus names updated):
Mirifusus d. baileyi (UAZ 9–11), Tritrabs
hayi (UAZ 3–10), Pseudocrucella sanfilippoae
(UAZ 7–10), and Transhsuum maxwelli (UAZ
3–10). Other reported taxa are listed in Figure 6.
We did not recollect this locality because the
cherts here have been subjected to hydrothermal
alteration, and many are altered to yellow-ochre,
botryoidal jaspers. Assuming the taxonomic
identifications in the 1977 report are still valid,
this sample, which was originally calibrated as
early Tithonian in age, is herein recalibrated as
Oxfordian or early Kimmeridgian.
In summary, the eruptions of the Stonyford
volcanic complex began in the Middle Jurassic, Bathonian, and continued into the early
Late Jurassic, Oxfordian or early Kimmeridgian. Between eruptions, siliceous radiolarianrich strata accumulated slowly on basalt basement. Volcanic glass within the sequence of
radiolarites and basalt yields dates of 164 Ma,
as discussed previously.
120
180
170
160
150
140
130
120
180
170
160
150
140
130
120
180
170
160
150
140
130
120
SFVG-8-2
SFVG-8-1
0
0.5
Cumulative
1.0 0
Fraction 39Ar
0.5
1.0
Released
Figure 5. 40Ar/ 39Ar apparent age spectra for glass samples from the Stonyford volcanic complex.
DISCUSSION
Regional Biostratigraphic Correlations
The biostratigraphic succession in the
Stonyford volcanic complex has parallels in
sedimentary sequences overlying Jurassic ophiolites elsewhere in California (Fig. 7). In the
following discussion, the Stonyford biostrati-
graphic sequence is compared with radiolarian
sequences overlying ophiolites in the southern
Coast Ranges of California and overlying the
Josephine ophiolite in the western Klamath
Mountains (Fig. 1). Our regional correlations
are based on UAZ ranges supplemented by
direct interbasin comparisons of local ranges
such as the vertical distribution of Mirifusus
species (Fig. 7). Data Repository document DR-2
Geological Society of America Bulletin, May/June 2005
AGE RELATIONS IN THE COAST RANGE OPHIOLITE, CALIFORNIA
BIOSTRATIGRAPHY OF THE STONYFORD VOLCANIC COMPLEX
9-10
Podobursa spinosa, Mirifusus d. dianae, Mirifusus guadalupensis,
abundant Praeparvicingula, Tetraditryma corralitosensis,
Transhsuum maxwelli, Eucyrtidiellum ptyctum, Xitus sp.,
Archaeodictyomitra sp. aff. A. rigida, Acanthocircus suboblongus
(very large forms), cryptothoracic nassellarians
8-10
Northern
Block
Other
UA
A 1 A 2 A 3 C B1 B2 B3 DAM
12
B1
A3
Mirifusus guadalupensis (late form), Mirifusus fragilis (late
transitional form), rare Praeparvicingula sp., fragments of probable
Tripocyclia blakei (large), Transhsuum maxwelli, Eucyrtidiellum
ptyctum, Archaeodictyomitra sp. aff. A. rigida, Xitus sp.,
Acanthocircus suboblongus (very large forms), cryptothoracic
nassellarians.
7-8
Mirifusus fragilis, Hsuum brevicostatum gp., Acanthocircus
suboblongus, Podobursa helvetica, Archaeodictyomitra sp. cf. A.
suzukii, Turanta sp., Saitoium sp.
3-8
8
7
6
Mirifusus guadalupensis (early form: transitional from M. fragilis),
Mirifusus fragilis, Praecaneta decora, Parahsuum officerense,
Podobursa helvetica, Guexella nudata, Parvicingula (?)
?
dhimenaensis, Spongocapsula palmerae, Protonuma sp.,
Tetraditryma corralitosensis, Archaeodictyomitra sp. cf. A. suzukii,
6-6
Eucyrtidiellum u. pustulatum, Hisocapsa convexa gp., Hsuum
brevicostatum gp., Acanthocircus suboblongus, Ristola procera,
Paronaella bandyi, Xiphostylus gasquetensis gp., Leugeo
hexacubicus.
A2
Mirifusus fragilis, Praecaneta decora, Praecaneta turpicula,
Guexella nudata, Parvicingula (?) dhimenaensis, Spongocapsula
palmerae, Podobursa helvetica, Pantanellium ultrasincerum/
P. foveatum.
A1
Mirifusus fragilis, Hisocapsa convexa gp., Hsuum brevicostatum
gp., Acanthocircus suboblongus, Pantanellium ultrasincerum/
P. foveatum, Saitoium sp.
Oxfordian
9
Callov.
B2
7-10
Bathonian
B3
Kimmer.
11
10
Mirifusus d. dianae, abundant Praeparvicingula sp., Acaeniotyle
diaphoragona, Transhsuum maxwelli, Eucyrtidiellum ptyctum,
Archaeodictyomitra sp. aff. A. rigida, Xitus sp., Acanthocircus
suboblongus (very large forms), cryptothoracic nassellarians,
demosponge spicules.
AGE
ZONE
Tithonian
Mirifusus d. baileyi, Tritrabs hayi, Pseudocrucella sanfilippoae,
Archaeodictyomitra rigida, Transhsuum maxwelli, Parvicingula
(undifferentiated, sensu 1977), Pantanellium riedeli.
From Pessagno (1977).
Central Block
5
4
Middle Jurassic
C
ZONES
Bajocian
D
DIVERSION
DAM
TAXA
3
6-6
2
3-6
1
Aalenian
LOCALITY
CALIBRATION
Late Jurassic
MAXIMUM & MINIMUM UAZ RANGES
RADIOLARIAN FOSSIL DATA
Calibrations based on Tethyan Unitary Association Zones (UAZ) of Baumgartner et al., 1995.
Figure 6. Radiolarians faunal distributions within the Stonyford volcanic complex. Locations A, B, C described in paper; location D—
Diversion Dam locale along Stony Creek, UA—Unitary Association.
(see footnote 1) includes a technical discussion
of UA zonal assignments shown in Figure 7 and
discussed below.
Stanley Mountain Ophiolite, Southern Coast
Ranges, California
At Alamo Creek near Stanley Mountain,
130 m of chert, tuffaceous chert, and mudstone
overlie basalt of the Stanley Mountain ophiolite
and underlie graywacke sandstone and siliceous
shale of the Great Valley Supergroup. Radiolarians from the pelagic chert and mudstone unit
and the lower 28 m of the Great Valley have
been well described and documented (Hull,
1995, 1997; Pessagno, 1977; Pessagno et al.,
1984). We have used the published faunal lists
to assign UAZ ranges to the composite section.
Based on UAZ ranges, the lower part of the section is Middle Jurassic in age, the upper part is
Late Jurassic (Fig. 7).
The radiolarians in the basal 27 m of the
Stanley Mountain section are poorly preserved,
and the age range of the interval is poorly
constrained. Yet, such a thickness of pelagic
chert and mudstone can represent millions of
years of deposition. Eucyrtidiellum ptyctum
(UAZ 5–11) in the 3.8 m horizon indicates that
it is no older than UAZ 5 (late Bajocian or early
Bathonian). Mirifusus dianae dianae (UAZ 7–
12) constrains the maximimum age of the 21 m
horizon as no older than UAZ 7 (late Bathonian
or early Callovian). A well-described radiolarian fauna at 27.1 m is no younger than UAZ 7.
Based on the calibrations of the UA Zonation of
Baumgartner et al. (1995a), the lower part of the
Stanley Mountain section may be late Bathonian
to early Callovian age in its entirety, although
the lowest 20 m could be as old as late Bajocian
or early Bathonian. As discussed above, zircon
Geological Society of America Bulletin, May/June 2005
643
Geological Society of America Bulletin, May/June 2005
4 -5
3 -5
1 -5
5 -6
5 -8
?
Scale in m
8-11
8-10
0
10
20
30
40
x
50
3-6/7
6-6/7
6 0 7-10
6-10
70
80
90
UAZ
max. &
min.
ranges
Klamath Mtns.
of California:
strata above the
Josephine
ophiolite
ø
ø
g,f
g,f
g,f
d
ø
d
ø
ø
ø
ø
Mirifusus
?
A1: 3-6
A2: 6-6
A3: 6-6
C: 8-10
b
Mirifusus
B1: 3-8
B2: 7-8
f
f
g,f
f
g,f
B3: 7-10 d,g
d,g
D: 9-10
UAZ maximum &
minimum ranges
Northern Coast Range
STONYFORD
VOLCANIC COMPLEX
0
10
20
30
40
50
x
70
80
90
100
120
130
140
Scale in m
5-6/7
6/7-6/7
7 -8
7-10
9-10
9-10
10-10
10-12
UAZ max.
& min.
ranges.
STANLEY MTN.
sp.
ø
d
d,g,f
d,g,f
ø
b
b
b
b,d
b,d
b,d
ø
ø
ø
Mirifusus
0
4
8
12
16
20
5-8
7-8
8-8
9 - 10
8 / 9 - 8/9
9 - 11
UAZ max.
& min.
ranges
The Stanley Mtn.
and Point Sal
sections overlie
ophiolites of the
Southern Coast
Ranges of California
POINT SAL
The correlations are based on
Unitary Association Zones
(UAZ) of Baumgartner et al.
(1995a,b) as well as direct
comparisons of radiolarian
ranges.
ø
d
ø
ø
ø
d,g
d,g,f
b,d,g
b,g
b
Aalenian
Bajocian
Bathonian
Callovian
Oxfordian
Kimmeridgian
increase in Praeparvicingula
increase in Parvicingula and
hiatus or probable hiatus
correlation
no data or insufficient data
basalt, greenstone
f
ø
g
d
M. guadalupensis (UAZ 5-11)
M. fragilis (UAZ 3-8)
None reported
M. dianae dianae (UAZ 7-12)
Praeparvicingula
Mirifusus species:
b M. dianae baileyi (UAZ 9-11)
x
+1.2/-7.9
+3.8/-5.6
178
+1.0/-1.5
Palfy et al., 2000
174
166
160.5 +1.1/-0.5
156.5 +3.1/-5.1
154.7 +3.8/-3.3
150.5 +3.4/-2.8
Time Scale, Ma
141.8 +2.5/-1.8
tuffaceous chert, mudstone
Patterns and symbols
graywacke, shale
Baumgartner et al.,
1995
1
4
3
2
6
5
7
8
10
9
Tithonian
14
13
12
11
Age/Stage
Berriasian
UAZ
Figure 7. Regional biostratigraphic correlations of key sections in California and Oregon, using radiolarians. The correlations are based on the Tethyan UA Zonation
(Baumgartner et al., 1995b) supplemented by direct interbasin comparisons of local radiolarian ranges such as those of Mirifusus species. For a given sample or interval, UA
Zone assignments are shown as a range (e.g., UAZ 7–8), showing maximum and minimum possible ranges. The fossil-based age calibrations for the UA Zones are illustrated
on the right side of the figure. The time scale of Palfy et al. (2000), which is not directly linked to the UA calibrations, is shown for reference purposes.
x
x
x
x
UAZ
Blue Mountains of
northeastern
Oregon: Snowshoe
& Lonesome Fms.,
ammonite-bearing
Mirifusus
SMITH RIVER
No Scale
BLUE MTNS.
No Scale
unconformity
Stage
Oxfordian
Callovian
Bathonian
Bajocian
Scale in m
644
Mirifusus
BIOSTRATIGRAPHIC CORRELATIONS BETWEEN KEY
SECTIONS IN CALIFORNIA AND OREGON
SHERVAIS et al.
AGE RELATIONS IN THE COAST RANGE OPHIOLITE, CALIFORNIA
U/Pb ages of 166 ± 2 Ma have been reported for
plagiogranite of the Stanley Mountain ophiolite
(166 ± 1 Ma, J.M. Mattinson, reported in Pessagno et al., 1993); 166.0 +3.8/–5.6 Ma is the
boundary between the Bajocian and Bathonian
in the time scale of Palfy et al. (2000).
The boundary between strata of Middle
and Late Jurassic age lies within the interval
between 28 and 62 m above basalt basement.
A sample 45.6 m above basalt basement has a
possible range of UAZ 7–8. UAZ 8 is calibrated
as middle Callovian to early Oxfordian. The
62 m horizon is no older than UAZ 9 (middle
to late Oxfordian) because it contains Mirifusus
dianae baileyi (UAZ 9–11). The pelagic interval between 80 and 104 m above basement is
assigned a range of UAZ 10–10 (late Oxfordian
to early Kimmeridgian) while the uppermost
part of the tuffaceous pelagic section and the
lowermost part of the clastic Great Valley Supergroup are constrained as no younger than UAZ
12 (early to early late Tithonian). The lower part
of the Great Valley in this geographic area contains the late Tithonian ammonite Micracanthoceras and bivalve Buchia piochii (Hull, 1997).
Hull and Pessagno (1995) illustrated significant differences arising from calibrations
based on the UA Zonation of Baumgartner et
al. (1995a) and their own calibrations based
on the zonation of Pessagno et al. (1993). The
most important difference is that Pessagno et al.
(1993) considered the 21 m to 27.1 m interval
at Stanley Mountain to be Late Jurassic in age,
middle Oxfordian. In our opinion, their calibration is not tightly constrained (see Data Repository document DR-2 discussion). Pessagno et
al. (1993, p. 113) arbitrarily assigned all the
underlying pelagic strata in the Stanley Mountain section to the middle Oxfordian as well,
which led them to conclude that a proposed
hiatus between the base of the sedimentary section and underlying basalt spanned 8–11 m.y.
(Dickinson et al., 1996; Pessagno et al., 2000).
In contrast, the UAZ calibrations shrink the possible duration of a basal hiatus to a few million
years, if any.
The Stonyford samples collected at localities
B2, B3, and C correlate best with the 21–60 m
interval of the Stanley Mountain composite
section (Fig. 7). Intervals A1–A3 at Stonyford
also have many taxa in common with the welldescribed Stanley Mountain fauna collected at
27.1 m but may be a bit older. Accordingly, we
very tentatively correlated the A1–A3 interval
with the poorly described lower 20 m at Stanley
Mountain based solely on their respective UAZ
ranges. The Stonyford Diversion Dam (D) site
correlates best with the upper part (from 62 m)
of the Stanley Mountain section. Like the Stonyford sequence, the middle of the Stanley Moun-
tain section (62 m) records a change in faunal
character that Pessagno et al. (1984) and Hull
(1995) partly characterized as an increase in
the relative abundance of Praeparvicingula and
Parvicingula. According to Hull (1995, 1997),
there is a hiatus immediately below the beginning of this event at Stanley Mountain.
Point Sal Ophiolite, Southern Coast Ranges
At Point Sal, along the southern California
coast, a 21 m condensed sequence of tuffaceous pelagic chert overlies pillow basalt of the
Point Sal ophiolite. The UAZ ranges assigned
herein are based on published taxonomic lists
(Baumgartner, 1995, Appendix, p. 1105–1106;
Pessagno, 1977; Pessagno et al., 1984) (Fig. 7).
The poorly preserved base of the Point Sal section cannot be calibrated more precisely than
UAZ 5–8 (late Bajocian to early Oxfordian). The
middle part of the section (~11.5 m to 13.4 m) is
no older than UAZ 8 nor younger than UAZ 9, a
range calibrated as no older than middle Callovian nor younger than late Oxfordian. The upper
part of the section could be as young as early
Tithonian. A more complete discussion is found
in Data Repository document DR-2.
Intervals A and B1 at Stonyford are correlative with and (or) older than the oldest, poorly
preserved and poorly constrained samples at
Point Sal. Intervals B2, B3, and C at Stonyford
correlate best with the 10–12 m interval at Point
Sal based primarily on the ranges of Mirifusus
guadalupensis, M. d. dianae (M. mediodilatatus), Transhsuum maxwelli, and Podobursa
spinosa in both. The Diversion Dam sample
of Pessagno (1977) at Stonyford correlates best
with the upper part of the Point Sal section. In
the Point Sal section, an increase in Praeparvicingula spp. and related taxa at ~10 m (UAZ 7–8)
(Pessagno et al., 1984) parallels a similar trend
in the previously described sections (Fig. 7).
Josephine Ophiolite, Western Klamath
Mountains, Northern California and Oregon
The Josephine ophiolite complex of the Western Klamath Mountains of Oregon and California formed at approximately the same time as,
or slightly later than, the ophiolites of the California Coast Ranges. Zircon 238U/206Pb ages for
the Josephine ophiolite range from 162 ± 1 Ma
(162 +7/–2 Ma; recalculated by Palfy et al.,
2000) to 164 ± 1 Ma (Devils Elbow ophiolite)
(Wright and Wyld, 1986); the zircon 207Pb/206Pb
age is 163 ± 5 (Harper et al., 1994). Hornblende
40
Ar/39Ar ages are 160 ± 2.5 Ma, 164.5 ± 5 Ma,
and 165.3 ± 5 Ma (Harper et al., 1994; includes
two ages from Devils Elbow ophiolite). The
Josephine ophiolite probably formed by rifting
within an older Mesozoic accretionary complex
along the margin of North America (Harper et
al., 1994). Unlike the ophiolites of the Coast
Ranges, the Josephine ophiolite complex was
profoundly affected by Late Jurassic deformation, burial, and metamorphism.
The Josephine ophiolite is overlain locally
by radiolarian-bearing strata. Pessagno et al.
(1993) described the radiolarian stratigraphy
of a 95 m sequence along the Middle Fork
of the Smith River in California (Fig. 7). The
lower half of the section is predominantly
fine-grained siliceous mudstone and chert with
admixed tuffaceous material; the upper half of
the section is a metagraywacke and mudstone
unit. Pessagno et al. (1993) also described the
radiolarians in a few chert samples interbedded with basalt at the Turner-Albright mine
in Oregon, near the California border (not
included in Fig. 7). Baumgartner et al. (1995b)
subsequently assigned UA Zones to the faunas.
The tuffaceous pelagic strata at both localities
are Middle Jurassic. Baumgartner et al. (1995b)
calibrated chert interbedded with basalt at the
Turner-Albright Mine as Bajocian (UAZ 3–4),
and an argument can be made that the range can
be further constrained to UAZ 4–4 (late Bajocian) (see Data Repository document DR-2).
In the Smith River section, the pelagic interval
from 4.1 to 13 m above the basalt is constrained
only as having a possible range of UAZ 3–6/7
(Bajocian to early Callovian); the pelagic interval between 13 and 46 m is herein assigned a
possible range of UAZ 6–6/7 (middle Bathonian to early Callovian). These direct, radiolarian-based calibrations are much older than the
previous isotope-based, time-scale–dependent
calibrations of Pessagno et al. (1993) (see Data
Repository document DR-2 discussion). The
UAZ ranges for the upper, metagraywacke and
mudstone unit of the Smith River section are
imprecise and only loosely constrain the middle
part of the interval as no older than Callovian or
early Oxfordian (UAZ 8) nor younger than late
Oxfordian or early Kimmeridgian (UAZ 10).
These ranges bracket Pessagno et al.’s (1993)
previous interpretation of a middle Oxfordian
age for the clastic and hemipelagic interval.
A more complete discussion of the published
age calibrations is included in Data Repository
document DR-2.
The radiolarian faunas in intervals A1–A3
and B1 at Stonyford correlate best with the radiolarians in the lower 45 m of tuffaceous chert and
mudstone of the Smith River sequence (Fig. 7).
Stonyford interval A3 correlates particularly
well with the Smith River section between 19
and 22 m. Intervals B2, B3, and C at Stonyford
correlate best with the upper part of the Smith
River measured section. The Diversion Dam
fauna at Stonyford may be the same age as or
younger than the uppermost part of the Smith
Geological Society of America Bulletin, May/June 2005
645
SHERVAIS et al.
River sequence. Data Repository document
DR-2 contains an expanded discussion of the
correlations. As previously noted in the description of the Stonyford section, a pronounced
faunal change occurs between the B1 and B2
intervals. A similar faunal change occurs in the
Smith River section, near the top of the pelagic
section, where Pessagno et al. (1993) noted the
disappearance of pantanellid spumellarians and
an influx of the nassellarian Praeparvicingula.
Significance of Biostratigraphic Correlations
Our recorrelation of five key Jurassic sections in California and Oregon (Fig. 7) reveals
a common history of concurrent late Middle
Jurassic pelagic or hemipelagic and volcaniclastic sedimentation and parallel patterns of faunal
turnover that began slightly earlier in the northern sections. Two of the sequences, the Blue
Mountains and Smith River sections, formed
in arc-related settings: The former as part of a
long-lived island-arc complex (Pessagno and
Blome, 1986), the latter as an ophiolite-floored
rift basin in an older accretionary complex
(Harper et al., 1994). The similarities between
their histories and those of the CRO basins
counter two arguments that the ophiolite basins
of the Coast Ranges formed in an environment
unrelated to the Blue Mountains and Smith
River sections: The first based on a proposed
multi-million-year hiatus following the creation of the ophiolites and the second based on
proposed syntectonic northeastward transport
across hundreds of kilometers of the eastern
Pacific toward North America (Dickinson et al.,
1996; Pessagno et al., 2000).
Hopson et al. (in Dickinson et al., 1996) and
Pessagno et al. (2000) proposed that the Middle
Jurassic Coast Range ophiolites formed at a
spreading ridge far from the North American
margin and that no pelagic sediments accumulated above the ophiolites for 8–10 m.y. until
plate motions carried the sites into the depositional apron of an active Late Jurassic volcanic
arc. The Middle Jurassic UAZ ranges for the
Stonyford and Stanley Mountain sedimentary
sequences and the ca. 164 Ma 40Ar/ 39Ar dates on
the interleaved hyaloclastites at Stonyford eliminate the argument for a major, multi-million
year hiatus at the base of the pelagic sequences
and indicate that the ophiolites originated near
the sources of the volcanic detritus in overlying
sedimentary strata.
Hopson et al. (in Dickinson et al., 1996) and
Pessagno et al. (2000) also proposed that the
Middle Jurassic Coast Ranges ophiolites formed
near the equator, remained at low latitudes until
the Late Jurassic, and were then transported rapidly northward relative to North America. The
646
faunal turnovers within the sections were used
as evidence for northward-directed changes in
paleolatitude, following the model of Pessagno
and Blome (1986). The age calibrations and
interbasin correlations in this study lead us to
different conclusions. As previously discussed,
a distinct faunal change occurs within each of
the five sections in Figure 7. In the Stonyford
section, we describe it as a shift from relatively
small-sized, polytaxic radiolarian faunas to
very robust, oligotaxic, nassellarian-dominated
faunas that include Praeparvicingula. For
other sections, Pessagno and Blome (1986),
who first noted the phenomenon, characterized
the changes as a transition from faunas with
abundant pantanellids to faunas with abundant
Praeparvicingula or Parvicingula s.s. They
observed that the two parvicingulid genera are
common at high paleolatitudes and are also
commonly associated with megafossils that
may have preferred cool temperatures. In the
Bathonian, Praeparvicingula increased in relative abundance in the Blue Mountains sequence
in Oregon (Pessagno and Blome, 1986), and its
range began to expand southward, reaching the
basins of the southern Coast Range ophiolites
by the Callovian or Oxfordian (Fig. 7). By the
Kimmeridgian and Tithonian, Praeparvicingula
or Parvicingula s.s. was a component of virtually
all eastern Pacific faunas from high latitudes such
as Antarctica to low latitudes of central Mexico
and the future Caribbean plate (Kiessling, 1999;
Murchey and Hagstrum, 1997).
The parallelism of the faunal changes in the
five sections illustrated in Figure 7 favors a
common cause or set of causes. The two most
likely causes are major paleoceanographic
change and (or) the northward migration of the
North American plate. The Jurassic breakup
of Pangea and creation of new oceans must
have changed circulation patterns and climate, which may account for the southward
expansion of the range of Praeparvicingula
and the evolution of the even more elongated
Praeparvicingula s.s. If, for example, a strong,
invigorated, and relatively cool eastern boundary current developed in response to plate
reorganization, it might have carried the taxa
southward. At the same time, North America
moved rapidly northward following the
breakup, which would have carried associated
marginal basins to higher latitudes. Pessagno
and Blome’s (1986) basic hypothesis, that
north-directed plate motions caused syndepositional faunal changes, may be grossly correct if
the five basin sequences illustrated in Figure 7
all formed along the North American margin
and moved northward in unison. However, no
compelling evidence supports linked corollary
hypotheses that the Coast Ranges, Josephine,
and Blue Mountains basins moved northward
(1) relative to North America, (2) at different
times during the Jurassic, and (3) in trajectories
unrelated to the motions of one another (Pessagno et al., 1993; Pessagno and Blome, 1986;
Pessagno et al., 2000)—neither plate motion
models, paleomagnetic studies (Hagstrum
and Murchey, 1996; Murchey and Hagstrum,
1997), nor the recorrelations in this study.
In summary, the early paleontologic and
sedimentary histories of the Stonyford Volcanic
Complex, Stanley Mountain ophiolite, and Point
Sal ophiolite favor Middle Jurassic origins (1)
near sources of volcanic detritus, (2) probably
in proximity to coastal currents, and (3) probably traveling with the North American plate in
tandem with the basins of the Blue Mountains
arc complex and Josephine ophiolite complex.
The Coast Range ophiolite basins persisted as
pelagic depocenters into the early Late Jurassic,
even as syntectonic clastic detritus prograded
across the Sierra Nevada foothills terranes and
collapsing Josephine ophiolite basins during the
initial phase of the Nevadan orogeny.
Significance of Radioisotopic and
Biostratigraphic Ages, Stonyford Volcanic
Complex
Ar/39Ar plateau ages for basalt glasses from
four distinct localities within the Stonyford volcanic complex indicate that they erupted over
a relatively short period of time ca. 164 Ma.
These age determinations are consistent with
the occurrence of Middle and Middle to Late
Jurassic radiolarians in sedimentary layers
intercalated with the volcanic rocks. This age
overlaps previously determined U/Pb zircon
ages for plagiogranites from the CRO elsewhere (e.g., Hopson et al., 1981), but appears
to be slightly younger than ages determined
here for late quartz diorite intrusions that
now occur as blocks in serpentinite mélange
(166–172 Ma). Nearly identical U/Pb zircon
ages (165–172 Ma) are found for late quartz
diorite intrusions at Elder Creek, as reported
here. Comparison between 40Ar/39Ar and U/Pb
ages must include consideration of the significant systematic errors affecting 40Ar/39Ar ages
and some variants of U/Pb ages. In the 40Ar/39Ar
system, uncertainties related to decay constants
and standards are on the order of 2%, meaning
that the real accuracy of the data reported herein
is ~3–4 m.y. (Min et al., 2000; Renne et al.,
1998). Similarly, the large effects (~4 m.y. in
the Middle Jurassic) of U decay constant errors
on both 207Pb/206Pb and concordia-intercept ages
(Ludwig, 2000) limit the absolute accuracy of
ages determined by these means. Within such
limits, the Stonyford 40Ar/39Ar ages are probably
Geological Society of America Bulletin, May/June 2005
40
AGE RELATIONS IN THE COAST RANGE OPHIOLITE, CALIFORNIA
Dam fauna in the SFVC, leaving open the possibility that the eruption of the SFVC may have
been concurrrent with the last phase of arclike
volcanism in the CRO of the Diablo Range.
(3) Radiolarian faunas in cherts and mudstones deposited within the Stonyford volcanic complex and on top of the Coast Range
ophiolite at other locations (e.g., Pt. Sal, Stanley
Mountain) show a similar vertical progression
from polytaxic assemblages of relatively small
radiolarians to oligotaxic assemblages of large,
thick-walled species dominated by nassellarians.
This implies that the Stonyford volcanic complex and the Coast Range ophiolite underwent
similar changes in oceanographic environment
or paleolatitude and argues against formation
of the Stonyford complex in a location distant
from the Coast Range ophiolite (see discussion
in previous section).
Taken together, the evidence suggests that the
Stonyford volcanic complex was built on a substrate of “normal” Coast Range ophiolite after
most suprasubduction zone magmatism came
to an end in the northern CRO, but possibly
concurrent with “stage 3” ophiolite magmatism
(Shervais, 2001) in the Diablo Range. Shervais
(2001) and Shervais et al. (2004) have proposed
that eruption of the SFVC corresponds to a
ridge-subduction event in the northern CRO,
based on the geochemistry of the SFVC and of
late MORB dikes that intrude the Elder Creek
ophiolite. This interpretation is supported by a
Jurassic reversal in the younging directions of
ocean crustal fragments successively incorporated into the Franciscan and Klamath Mountains accretionary complexes, a phenomenon
that Murchey and Blake (1992, 1993) ascribed
to arrival of an oceanic spreading ridge along
the margin in the Middle to early Late Jurassic
(Fig. 8). The age data presented here suggest
that this event occurred ca. 160–164 Ma in the
northern CRO, although it may have been later
in the Diablo Range.
Timing of Ophiolite Formation and
Associated Tectonic Events
Late Early to Early Middle Jurassic Arc
Collision or Collapse (175–185 Ma) and the
Formation of a Nascent Subduction Zone
(ca. 172 Ma)
Wright and Fahan (1988) were among the first
to show that orogenesis in the Jurassic was not
confined to a singular event in the Late Jurassic
“Nevadan orogeny.” They were able to document
that many structures attributed to the Nevadan
orogeny in previous studies are in fact Middle
Jurassic in age and must have formed during
After Murchey and Blake (1993)
300
Residence Time/ Ridge Age
indistinguishable from all but the oldest of the
above mentioned U/Pb-based ages.
The Stonyford data and the inferred structural relationships cast doubt on the validity
of K-Ar ages younger than 165 Ma obtained
on high-level hornblende gabbros (Lanphere,
1971; McDowell et al., 1984) as crystallization
ages. It is probable that the younger K-Ar dates
represent cooling ages, argon loss, or alteration
artifacts, and that formation of the ophiolite
was complete by ca. 164 Ma, shortly before
or during eruption of the SFVC. Mattinson and
Hopson (1992) have revised U-Pb zircon dates
of plagiogranites from Coast Range ophiolite
localities south of San Francisco, with newer
data showing that most plagiogranites crystallized 165–170 Ma—consistent with the results
presented here.
The age relationships presented here are consistent with the idea that the Stonyford volcanic
complex was built on a substrate of “oceanic”
crust represented by the older Coast Range
ophiolite assemblage. This idea is supported by
three independent lines of evidence.
(1) The high-Al, low-Ti basalt suite at Stonyford resembles island arc basalts similar to those
found elsewhere in the Coast Range ophiolite
(e.g., Shervais, 1990; Shervais and Kimbrough,
1985) in their major element characteristics, but
have trace element characteristics that suggest
addition of an enriched component to a depleted
source region (for example, La/Smn ratios ranging from 0.34 to 1.78). High-Al, low-Ti basalts
are found interbedded with ocean island tholeiites and alkali basalts at all stratigraphic levels
of the volcanic complex, which requires that the
depleted, arclike source of the high-Al basalts
and the plume-enriched source of the tholeiitic
and alkali basalts were physically contiguous
(Zoglman and Shervais, 1991). Note that highAl, low-Ti basalts are not found in ocean island
basalt suites of the Franciscan assemblage,
which contain almost exclusively ocean island
tholeiites and alkali basalts (MacPherson, 1983;
MacPherson et al., 1990; Shervais, 1990; Shervais and Kimbrough, 1987).
(2) Based on the radiolarian assemblages,
eruption of the SFVC was concurrent with
deposition of the volcano-pelagic section
(tuffaceous chert) on top of the CRO at localities in the southern Coast Ranges, e.g., Pt. Sal,
Stanley Mountain (see discussion in previous
section). Thus, eruption of the SFVC must
have postdated formation of the main ophiolite
at these localities—consistent with the results
of our new, high-precision U-Pb zircon dates.
Pessagno (in Hopson et al., 1981) tentatively
correlated unspecified, very poorly preserved
radiolarians associated with stage 3 volcanic
rocks in the Diablo Range with the Diversion
North Fork, East Hayfork
Spreading Age
Residence Time
250
Rattlesnake Creek
200
150
100
Van Ardsdale,
Yolla Bolly
Marin Headlands,
The Geyers
Elder Creek,
Pickett Peak
SFVC, W. Klamath
Ridge
Collision
≈155 Ma
Burnt Hills,
Permanente
Ridge?
Coastal Belt
50
0
200
150
100
50
Arrival time (Ma)
King
Range
0
Ridge
Figure 8. Ages of accreted terranes and ages of accretion, after Murchey and Blake (1993).
Upper curve represents age of spreading center, based on fossils and age radioisotopic dates,
plotted as function of arrival time at the western margin of North America. Lower curve
represents residence time of accreted crust (= age of formation minus age of accretion) as
function of arrival time at the western margin of North America. The age reversal and
age minimum in residence time at ca. 155 ± 5 Ma, which corresponds to the arrival times,
implies ridge collision.
Geological Society of America Bulletin, May/June 2005
647
SHERVAIS et al.
Coast Range
Ophiolite
141.8 (+2.5/-1.8)
Tithonian
Owens Mtn
Dike Swarm
Kimmeridgian
154.7 (+3.8/-3.3)
Oxfordian
156.5 (+3.1/-5.1)
JKf High
Grade Blocks
Middle Jurassic
160.4 (+1.1/-0.5)
200 Ma
Lower Jurassic
Independence
Dike Swarm
Nevadan Event
YRP
Stonyford
Glass
Logtown Ridge
Bathonian
HCP
ScP EGC
StP
166.0 (+3.8/-5.6)
BRT
Bajocian
174.0 (+1.2/-7.9)
Aalenian
Smartville Ophiolite
Elder Creek
Ophiolite
Stonyford
Mélange
Blocks
Toarcian
183.6 (+1.7/-1.1)
Foothills Event 174-185 Ma
Compressive, Arc Collision (?)
Tuttle
Lake Fm
Fiddle Creek
Complex
Pliensbachian
Peñon
Blanco
191.5 (+1.9/-4.7)
Sinemurian
196.5 (+1.7/-5.7)
Hettangien
Triassic
190 Ma
BMFZ
-MFZ
"Overlap"
150.5 (+3.4/-2.8)
178.0 (+1.0/-1.5)
180 Ma
Sinistral Ductile
Deformation 145-123 Ma
"Late Nevadan"
Callovian
160 Ma
170 Ma
Berriasian
Slate Creek Complex/
Smartville Wallrock
199.6 (±0.4)
Norian
West
BMFZ-MFZ = Ductile Deformation
along Bear Mtn/Melones Fault Zones
Post Deformational
Plutons
Time-scale correlation from Palfy et al., 2000
exotic arc terranes?
UC
Foothills Suture Zone?
150 Ma
Upper Jurassic
140 Ma
Sierra Nevada
Foothills
Sailor
Canyon
Fm
Calaveras
Complex
East
HCP = Haypress Creek Pluton 166 Ma
EGC = Emigrant Gap Complex 168 Ma
ScP = Scales Pluton 168 Ma
StP = Standard Pluton 166 Ma
YRP = Yuba Rivers Pluton 159 Ma
BRT = Bloody Run Tonalite 165 Ma
Figure 9. Timing of igneous and metamorphic events in the Coast Ranges and Sierra Nevada foothills during the Jurassic to earliest Cretaceous. Foothills event in the Sierras postdates Early Jurassic arc complexes (Slate Creek, older Smartville, Penon Blanco, Fiddle Creek)
and accretionary complex (Calaveras complex) and predates formation of the CRO, the younger Smartville rocks, and the Logtown Ridge
volcanics, which form a Callovian overlap suite along with the Mariposa formation. Postdeformational plutons that crosscut structures
formerly associated with the Nevadan event include the Yuba Rivers pluton (YRP, 159 Ma), the Standard pluton (StP, 163 Ma), the Scales
pluton (ScP, 168 Ma), the Emigrant Gap complex (EGC, 165 Ma), the Bloody Run tonalite (165 Ma), and the Haypress Creek pluton (HCP,
169 Ma). High-grade knockers in the Franciscan complex (JKf) range in age up to 162 Ma. BMFZ—Bear Mountain Fault Zone, MFZ—
Melones Fault Zone. Data sources listed in the text. Time scale after Palfy et al. (2000).
an earlier orogenic event, termed the “Siskiyou
event” in the Klamaths (Coleman et al., 1988;
Hacker et al., 1995). The Siskiyou event in the
Klamaths postdates the ca. 172–169 Ma Western
Hayfork arc and predates the ca. 164–159 Ma
Wooley Creek plutonic belt (Renne and Scott,
1988). Thus, Siskiyou compressional deformation (post-169, pre-164 Ma) was coincident with
648
extension that formed the Coast Range ophiolite.
Siskiyou compression was followed by regional
extension ca. 167–155 Ma, also overlapping
in part formation of the Coast Range ophiolite
(Hacker et al., 1995).
Work in the Sierra Nevada foothills has documented the importance of an older, early Middle
Jurassic event in the Sierra Nevada arc realm
(e.g., Edelman et al., 1989a; Edelman and Sharp,
1989; Girty et al., 1995; Saleeby, 1990; Saleeby
et al., 1989). This event is bracketed in part by
Middle Jurassic plutons (Standard, Scales, Emigrant Gap, Haypress Creek, and Bloody Run
plutons), which intrude structures that deform
older, ca. 200 Ma arc volcanics and plutonics
(e.g., Peñon Blanco arc, Slate Creek complex,
Geological Society of America Bulletin, May/June 2005
AGE RELATIONS IN THE COAST RANGE OPHIOLITE, CALIFORNIA
A.
Penon Blanco/
Slate Creek Arc
Sierran Arc
Late Triassic to
Early Jurassic:
206-180 Ma
B.
or ???
Rapid Formation of CRO
as Oceanic Slab Sinks:
174-164 Ma
Early Mid-Jurassic
Collision (?): 176-180 Ma
Mid-Jurassic:
180-164 Ma
?
C.
Ridge Collision/Subduction:
164-160 Ma
Late Mid-Jurassic:
163-160 Ma
Post-collision
Plutons: ≈160 Ma
Stonyford Volcanics; MORB Dikes at Elder Creek,
Mt. Diablo, Del Puerto, Sierra Azul, Cuesta Ridge
Late Jurassic:
159-150 Ma
(1) Shallow subduction, Compression:
Classic Nevadan Orogeny 159-155
(2) Change from direct convergence to Sinistral
transtension as NA moves Northward: 156-144 Ma
Figure 10. Model for Jurassic evolution of CRO: (A) Early Jurassic fringing/exotic arc;
(B) Toarcian-Aalenian (185–175 Ma) collision of fringing/exotic arc with continental margin; followed by formation of CRO during Bajocian-Bathonian (165–172+) by hinge rollback during initiation of proto–Franciscan subduction zone; also intrusion of undeformed
plutons in Sierra Foothills, including Haypress Creek, Emigrant Gap, Standard, Scales, and
Smartville complex, that crosscut fabric and structures formed during earlier arc collision;
(C) mid-Callovian collision with active spreading ridge, overlaps arc volcanism in Foothill;
eruption of SFVC and high-grade metamorphism of oceanic crust in the proto–Franciscan
subduction zone, followed by late Callovian through Oxfordian compression in Sierra Foothills, continued chert deposition in CRO, and finally by Kimmeridgian transition to ductile,
sinistral shear deformation in Sierra foothills, deposition of GVS on top of CRO.
older wallrock of the Smartville complex;
Fig. 9). The Peñon Blanco arc has been dated at
196–200 Ma (Saleeby, 1982) and is underlain by
fault-bounded garnet amphibolites dated at 178
± 3 Ma by 40Ar/ 39Ar (Sharp, 1988). The Fiddle
Creek complex, a chert-argillite assemblage
that sits on ophiolitic mélange and ranges in
age from Late Triassic to possibly early Middle
Jurassic, is deformed by these same structures
(Edelman et al., 1989b). The Red Ant schist, a
lawsonite-bearing blueschist that has been overprinted by pumpellyite-actinolite assemblages
(Edelman et al., 1989b; Hacker, 1993), has
been dated by K-Ar as ≈174 Ma (Schweickert
et al., 1980); this is considered a minimum age
for metamorphism. The Calaveras complex
comprises a chert-argillite terrane with volcanic
inclusions that has been interpreted as a Late
Triassic–Early Jurassic accretionary complex;
it is intruded by a postkinematic pluton dated
at 177 Ma (Edelman et al., 1989b; Schweickert
et al., 1988; Sharp, 1988). The Calaveras and
Shoo Fly complexes are juxtaposed along a
west-vergent thrust fault that is dated at 176 ±
3 by 40Ar/ 39Ar on synkinematic amphibolites,
and is cut by an ≈166 Ma postkinematic pluton
(Sharp, 1988). The Fiddle Creek, Red Ant, and
Calaveras complexes all lie inboard (east) of the
Late Triassic–Early Jurassic arc complexes of
the Foothills metamorphic belt.
The most conservative interpretation of
these data is that the late Early Jurassic to early
Middle Jurassic deformation discussed above
corresponds to the collapse of a fringing arc
terrane or collision of an exotic arc terrane,
against the margin of North America in the
early Middle Jurassic, forming the Foothills
suture (Edelman et al., 1989a; Edelman et al.,
1989b; Edelman and Sharp, 1989; Girty et al.,
1995; Hacker, 1993; Saleeby, 1983b, 1990;
Saleeby et al., 1989; Sharp, 1988). This island
arc, the Foothills arc terrane, may have formed
above a west-dipping subduction zone prior
to the ca. 174–185 Ma collision, but inherited
zircons in the Slate Creek complex suggest an
origin proximal to the continental margin (Bickford and Day, 2004).
Our data show that formation of the CRO in
California began shortly after the early Middle
Jurassic deformation event documented in
the Sierra Nevada foothills at 174–185 Ma
(Fig. 9). We propose that formation of the CRO
coincided with the establishment of a new or
newly reorganized, east-dipping subduction
zone beneath the amalgamated Sierran terranes around ≈172 Ma (Fig. 10). Formation of
the CRO above this nascent subduction zone
probably proceeded in response to sinking of
the backarc basin crust and rapid extension of
the nascent forearc region into the gap created
by rollback of the subduction hinge (Kimbrough
and Moore, 2003; Shervais, 2001; Shervais et
al., 2004; Stern and Bloomer, 1992).
The suggestion of a zircon component with
Pb inherited from old continental crust, seen here
and in the essentially coeval Josephine/Devils
Elbow ophiolite (Wright and Wyld, 1986) and
Smartville ophiolite (Bickford and Day, 2001),
implies that ophiolite formation was linked to
the continental margin. Bickford and Day (2001)
conclude that the Proterozoic inherited zircon
component they identified in the Smartville plutons was derived from the underlying Shoo Fly
complex, and that both the Smartville ophiolite
and the older Slate Creek complex formed proximal to the continental margin.
Geological Society of America Bulletin, May/June 2005
649
SHERVAIS et al.
Formation of High-Grade Blocks in the
Franciscan (160–165 Ma), Ridge Collision, and
Correlation with the Coast Range Ophiolite
High-grade metamorphic blocks in the Franciscan assemblage and high-grade terranes in the
Eastern Belt of the Franciscan (e.g., Taliaferro
metamorphic complex) seemed to have formed
ca. 160–165 Ma (Mattinson, 1988)—that is,
coincident with the postulated ridge collision event in the northern CRO. We suggest
that the Franciscan high-grade blocks formed
in response to the elevated thermal gradients
caused by ridge collision. The high-grade blocks
are too young to have formed during subduction
initiation, as proposed by Wakabayashi (1990),
which we have interpreted to be at least 172 Ma
or older, based on the oldest CRO dates. As
noted by Wakabayashi (1990), 10–12 m.y. after
the initiation of subduction, the hanging wall
lithosphere would be too cold to form high-grade
amphibolites and garnet amphibolites by heating
from above. The problem of subduction-zone
refrigeration has been noted in other ophiolites
as well (e.g., Hacker and Gnos, 1997). The
high-grade blocks represent a significant thermal
event that requires high thermal flux at the base
of the ophiolite, which we believe could only be
provided by collision with an active spreading
ridge at the time required.
Ridge Collision and the Late Jurassic
Nevadan Orogeny (155 ± 5 Ma)
The classic Nevadan orogeny was a Late
Jurassic contractional event that folded, faulted,
and metamorphosed strata as young as the
Oxfordian to early Kimmeridgian (?) age Mariposa Formation, a probable syntectonic flysch
deposit in the foothills of the Sierra Nevada.
Radioisotopic dates on Nevadan structures date
deformation ca. 155 ± 5 Ma (Schweickert et al.,
1984; Tobisch et al., 1989). This event is now
generally believed to postdate suturing of the
eastern and western arc terranes of the Sierra
Nevada (Edelman and Sharp, 1989; Saleeby,
1982, 1983a; Saleeby et al., 1989; Tobisch et
al., 1989), and therefore the Nevadan orogeny
cannot represent an arc-arc or arc-continent collision, as proposed previously.
The main phase of Late Jurassic contractional deformation in the Sierra Nevada began
shortly after the ridge collision event, which
occurred ca. 160–164 Ma in the northern CRO.
We propose a direct tectonic link between the
ridge collision event and the Nevadan orogeny
(Fig. 9). Ridge collision will result in shallow
underthrusting of the buoyant subducting oceanic lithosphere (e.g., Cloos, 1993), heating of
the superjacent lithosphere, and a likely change
in relative convergence directions (e.g., from
direct convergence to strike-slip, transpression,
650
or transtension). This is essentially the same
conclusion reached by Murchey and Blake
(1992; 1993). Cloos (1993, page 733) summarized his findings thus:
The subduction of spreading ridges will cause vertical
isostatic uplift and subsidence of as much as 2 to 3 km
in the forearc region compared to when 80-m.y.-old
oceanic lithosphere is subducted. The subduction
of an active spreading center causes such a major
perturbation in the margin’s thermal structure that
evidence of the event is likely to be recorded widely in
the geology of the forearc block.
Ward (1995) analyzed long-term cyclical
changes in the style, composition, and location of magmatism along North America and
concurrent changes in inferred plate motions
of North America relative to ocean plates and
noted parallels between Miocene and Jurassic
events. Jurassic subduction rates along North
America varied in response to changes in plate
motions and convergence. Both relative and
absolute plate motions changed significantly at
the beginning of the Late Jurassic—coincident
with the ridge subduction event postulated
here and just prior to the Nevadan orogeny.
Plate motion studies (Engebretson et al., 1985;
May et al., 1989; Ward, 1995) and structural
analysis of the Foothills terrane (Tobisch et
al., 1989) both show a change from relative
convergence between North America and plates
of the Pacific basin in the Middle Jurassic to left
lateral transpression (in the upper plate) during the early Late Jurassic. Using these lines
of evidence, which are distinct from those of
Murchey and Blake (1993) or Shervais (2001),
Ward (1995) also concluded that an ocean
spreading ridge arrived along the margin in the
late Middle Jurassic and speculated that Great
Valley basement might be analagous to the Gulf
of California.
Initial shortening during the Nevadan orogeny can be attributed to shallow subduction
of the young, buoyant oceanic lithosphere
(Fig. 10). The change from contractional to
ductile shear deformation documented by
Tobisch et al. (Tobisch et al., 1989) in the Foothills terrane during the latest Jurassic and Early
Cretaceous (that is, post-Nevadan) is consistent with the onset of left lateral strike-slip
motion within the arc terrane predicted by the
ridge collision model. Prior to this transpressional deformation (Saleeby, 1978; Saleeby,
1981; Saleeby, 1982), the dominant stress was
transtensional, as documented by dike swarms
in the Sierra foothills and in the eastern Sierras (Owens Mountain and Independence dike
swarms; Fig. 9) (Wolf and Saleeby, 1992; Wolf
and Saleeby, 1995).
CONCLUSIONS
New high-precision U/Pb zircon dates from the
northern Coast Range ophiolite show that ophiolite formation began before ≈172 Ma and was
largely complete by ≈164 Ma. These ages postdate
the Toarcian to Aalenian (185–174 Ma) collision of
an exotic or fringing arc to the Cordilleran margin.
We propose that ophiolite formation began in
response to this collision during the establishment
of a nascent, east-dipping, proto–Franciscan subduction zone, as proposed by Stern and Bloomer
(1992) and Shervais (1990, 2001).
New high-precision 40Ar/ 39Ar laser fusion
dates on unaltered samples of volcanic glass
from the Stonyford volcanic complex show that
it formed during a brief interval of eruption ca.
164 Ma, shortly after completion of ophiolite
formation and just prior to the onset of “main
phase” Nevadan deformation in the Sierra
foothills (e.g., Tobisch et al., 1989). This time
period corresponds to the ages of high-grade
amphibolite and garnet amphibolite blocks in
the Franciscan assemblage (Mattinson, 1988;
Ross and Sharp, 1988) and shortly precedes a
change in plate motion for North America from
slow westward drift to rapid northward motion
(May et al., 1989; Ward, 1995). It is roughly
bracketed by a reversal in the younging polarity
of ocean crustal fragments in accreted terranes
of the Klamaths (westward younging through
the Early Jurassic) and the Franciscan (eastward
younging from Late Jurassic to mid-Cretaceous; Murchey and Blake, 1992, 1993). Taken
together with field and geochemical observations in the SFVC and at Elder Creek, these data
are interpreted to require collision with and/or
subduction of a major oceanic spreading ridge
axis (Shervais et al., 2004).
Radiolarians from cherts and siliceous mudstones intercalated within the Stonyford volcanic complex are correlative with those found
in cherts that overlie the Coast Range ophiolite
in the southern Coast Ranges. The radiolarians in the SFVC range in age from Bathonian
(Unitary Association Zone 6–6 of Baumgartner
et al., 1995a) near the base of the complex to
late Callovian to early Kimmeridgian (Unitary
Association Zones 8–10) in the upper part. The
Stonyford cherts are interbedded with hyaloclastite breccias containing volcanic glass that
we have dated at ≈164 Ma, using high-precision 40Ar/39Ar laser-heating methods applied
to carefully selected samples. These data show
that, contrary to the interpretations of Pessagno and coworkers (Hopson et al., 1997; Hull
et al., 1993; Pessagno et al., 1987; Pessagno
et al., 2000), we see no evidence of a major
depositional hiatus following formation of the
ophiolite and thus no requirement that it formed
Geological Society of America Bulletin, May/June 2005
AGE RELATIONS IN THE COAST RANGE OPHIOLITE, CALIFORNIA
in the open ocean far from its current location. Further, the recorrelations presented here
support the idea that parallel turnovers in the
character of the radiolarian assemblages in the
basins of the CRO, the Josephine ophiolite, and
the Blue Mountains arc, occurring slightly later
in the southern basins, reflect a shared history
of oceanographic change such as the expansion
of cool coastal currents and (or) synchronized
plate motion trajectories rather than a separate
history for the CRO relative to North America.
We propose that this ridge collision event was
the driving force behind the classic, Late Jurassic Nevadan orogeny. The resulting change in
relative and absolute plate motions is recorded
in deformation fabrics and dike swarms in the
Sierras (Tobisch et al., 1989; Wolf and Saleeby,
1992; Wolf and Saleeby, 1995); subsequent ductile deformation in the Sierra foothills (Tobisch
et al., 1989) resulted from sinistral transpression
following this event.
All of the events described here are consistent
with the model for suprasubduction zone ophiolite formation published by Shervais (2001). This
model proposes that suprasubduction zone ophiolites form in response to hinge retreat in nascent
or newly reorganized subduction zones, and that
they display a consistent life cycle as the subduction zone matures. The final stage of ophiolite formation typically involves subduction of an active
spreading center, as observed in the CRO.
The data presented here also show that the
problem of ophiolite genesis cannot be resolved
in isolation, without consideration of the tectonic evolution of the entire orogenic system
of which it is part. Further, it is not possible to
understand the evolution of an orogenic system
without a clear understanding of the ophiolites
found within it.
ACKNOWLEDGMENTS
This paper would not have been possible without
the pioneering work and insights of Cliff Hopson,
who introduced us (Shervais, Kimbrough, Hanan) to
the Coast Range ophiolite and who has provided the
inspiration for our continued work there. This research
was supported by NSF grants EAR8816398 and
EAR9018721 (Shervais) and EAR9018275 (Kimbrough
and Hanan). Geologic mapping of the Elder Creek and
Stonyford ophiolites formed parts of Master’s Theses by
Joe Beaman (1989) and Marchell Zoglman Schuman
(Zoglman, 1991) at the University of South Carolina.
REFERENCES CITED
Bailey, E.H., Irwin, W.P., and Jones, D.L., 1964, Franciscan
and related rocks, and their significance in the geology
of western California: California, Division of Mines
and Geology Bulletin, v. 183, p. 1–177.
Bailey, E.H., Blake, M.C., Jr., and Jones, D.L., 1970, Onland Mesozoic oceanic crust in California coast ranges,
in U.S. Geological Survey Professional Paper 700-C,
p. C70–C81.
Baumgartner, P.O., 1995, Towards a Mesozoic radiolarian
database—Updates of work 1984–1990, in Baumgartner, P.O., O’Dogherty, L., Gorican, S., Urquhart, E.,
Pillevuit, A., and De Wever, P., eds., Middle Jurassic to
Lower Cretaceous Radiolaria of Tethys: Occurrences,
Systematics, Biochronology: Lausanne, Switzerland,
Memoires de Geologie (Lausanne), v. 23, p. 689–700
and Appendix: p. 1091–1106, 1144–1150.
Baumgartner, P.O., Bartolini, A., Carter, E.S., Conti, M.,
Cortese, G., Danelian, T., De Wever, P., Dumitrica, P.,
Dumitrica-Jud, R., Gorican, S., Guex, J., Hull, D.M.,
Kito, N., Marcucci, M., Matsuoka, A., Murchey, B.L.,
O’Dogherty, L., Savary, J., Vishnevskaya, V., Widz, D.,
and Yao, A., 1995a, Middle Jurassic to Early Cretaceous radiolarian biochoronology of Tethys based
on Unitary Associations, in Baumgartner, P.O.,
O’Dogherty, L., Gorican, S., Urquhart, E., Pillevuit, A.,
and De Wever, P., eds., Middle Jurassic to Lower Cretaceous Radiolaria of Tethys: Occurrences, Systematics,
Biochronology: Lausanne, Switzerland, Memoires de
Geologie (Lausanne): p. 1013–1048.
Baumgartner, P.O., O’Dogherty, L., Gorican, S., DumitricaJud, R., Dumitra, P., Pillevuit, A., Urquhart, E., Matsuoka,
A., Danelian, T., Bartolini, A., Carter, E.S., DeWever, P.,
Kito, N., Marcucci, M., and Steiger, T., 1995b, Radiolarian catalogue and systematics of Middle Jurassic to Early
Cretaceous Tethyan genera and species, in Baumgartner,
P.O., O’Dogherty, L., Gorican, S., Urquhart, E., Pillevuit,
A., and De Wever, P., eds., Middle Jurassic to Lower Cretaceous Radiolaria of Tethys: Occurrences, Systematics,
Biochronology: Lausanne, Switzerland, Memoires de
Geologie (Lausanne), p. 37–685.
Beaman, B.J., 1989, Evolutional history of the plutonic
section of the Elder Creek ophiolite: A remnant of
the Coast Range ophiolite, California [M.S. thesis]:
Columbia, South Carolina, University of South Carolina, 185 p.
Berkland, J.O., Raymond, L.A., Kramer, J.C., Moores,
E.M., and O’Day, M., 1972, What is Franciscan?: The
American Association of Petroleum Geologists Bulletin, v. 56, no. 12, p. 2295–2302.
Bickford, M.E., and Day, H.W., 2004, Tectonic setting of the
Jurassic Smartville and Slate Creek complexes, northern Sierra Nevada, California: Geological Society of
America Bulletin, v. 116, p. 1515–1528.
Blake, M.C., Jr., and Jones, D.L., 1981, The Franciscan
assemblage and related rocks in northern California; A
reinterpretation, in Ernst, W.G., ed., The Geotectonic
Development of California; Rubey Volume I.: Englewood Cliffs, New Jersey, Prentice-Hall, p. 307–328.
Blake, M.C., Jr., Jayko, A.S., and Howell, D.G., 1982,
Sedimentation, metamorphism and tectonic accretion
of the Franciscan assemblage of Northern California,
in Leggett, J., ed., Trench-Forearc geology; Sedimentation and tectonics on modern and ancient active plate
margins: Geological Society of London Special Publication 10, p. 433–438.
Blake, M.C., Jr., Jayko, A.S., Jones, D.L., and Rogers, B.W.,
1987, Unconformity between Coast Range Ophiolite
and part of the lower Great Valley Sequence, South
Fork of Elder Creek, Tehama County, California, in
Hill, M.L., ed., Cordilleran Section of the Geological
Society of America: Boulder, Colorado, Centennial
field guide, v. 1, p. 279–282.
Blake, M.C., Jr., Jayko, A.S., McLaughlin, R.J., and
Underwood, M.B., 1988, Metamorphic and tectonic
evolution of the Franciscan Complex, Northern California, in Ernst, W.G., ed., Metamorphism and crustal
evolution of the Western United States: Rubey Volume 7: Englewood Cliffs, New Jersey, Prentice-Hall,
p. 1035–1060.
Blake, M.C., Jr., McLaughlin, R.J., and Jones, D.L., 1989,
Sedimentation and tectonics of western North America:
Terranes of the northern Coast Ranges, in Blake, M.C.,
Jr., Harwood, D.S., and Hanshaw, P.M., eds., Field trips
for the 28th International Geological Congress, Volume
2: Tectonic evolution of Northern California: Washington, D.C., American Geological Institute, p. 3–18.
Blome, C.D., 1984, Middle Jurassic (Callovian) Radiolaria
from southern Alaska and eastern Oregon: Micropaleontology, v. 30, no. 4, p. 343–389.
Brown, R.D., Jr., 1964a, Geologic map of the Stonyford
quadrangle, Glenn, Colusa, and Lake counties, Cali-
fornia: U.S. Geological Survey Mineral Investigations
Field Study Map MF-279, scale 1:48,000.
Brown, R.D., Jr., 1964b, Thrust-fault relations in the northern Coast Ranges, California, Short papers in geology
and hydrology: U.S. Geological Survey Professional
Paper 475-D, p. D7–D13.
Cerling, T.E., Brown, F.H., and Bowman, J.R., 1985, Low
temperature alteration of volcanic glass: Hydration,
Na, K, 18O and Ar mobility: Isotope Geoscience,
v. 52, p. 281–293.
Cloos, M., 1986, Blueschists in the Franciscan Complex of
California; Petrotectonic constraints on uplift mechanisms, in Evans, B.W., Brown, E.H,. eds., Blueschists
and eclogites: Geological Society of America Memoir
164, p. 77–93.
Cloos, M., 1993, Lithospheric buoyancy and collisional orogenesis; Subduction of oceanic plateaus, continental
margins, island arcs, spreading ridges, and seamounts:
Geological Society of America Bulletin, v. 105, p. 715–
737, doi: 10.1130/0016-7606(1993)1052.3.CO;2.
Coleman, R.G., Manning, C.E., Mortimer, N., Donato, M.M.,
and Hill, L.B., 1988, Tectonic and regional metamorphic framework of the Klamath Mountains and
adjacent Coast Ranges, California and Oregon, in
Ernst, W.G., ed., Rubey Volume 7: Metamorphism and
Crustal Evolution of the Western United States: Englewood Cliffs, New Jersey, Prentice-Hall, p. 1061–1097.
Dickinson, W.R., 1971, Plate tectonic models for orogeny at
continental margins: Nature, v. 232, p. 41–42.
Dickinson, W.R., and Rich, E.I., 1972, Petrologic Intervals and Petrofacies in the Great Valley Sequence,
Sacramento Valley, California: Geological Society of
America Bulletin, v. 83, p. 3007–3024.
Dickinson, W.R., Hopson, C.A., Saleeby, J.B., Schweickert,
R.A., Ingersoll, R.V., Pessagno, E.A., Jr., Mattinson,
J.M., Luyendyk, B.P., Beebe, W., Hull, D.M., Munoz,
I.M., and Blome, C.D., 1996, Alternate origins of the
Coast Range Ophiolite (California); Introduction and
implications: GSA Today, v. 6, no. 2, p. 1–10.
Dumitrica, P., 1970, Cryptocephalic and cryptothoracic Nassellaria in some Mesozoic deposits of Romania: Revue
Roumaine de Geologie, Geophysique, et Geologie,
Serie de Geologie, v. 14, p. 45–124.
Edelman, S.H., and Sharp, W.D., 1989, Terranes, early faults,
and pre-Late Jurassic amalgamation of the western
Sierra Nevada metamorphic belt, California: Geological Society of America Bulletin, v. 101, p. 1420–1433,
doi: 10.1130/0016-7606(1989)1012.3.CO;2.
Edelman, S.H., Day, H.W., and Bickford, M.E., 1989a,
Implications of U-Pb zircon ages for the Smartville
and Slate Creek complexes, northern Sierra Nevada,
California: Geology, v. 17, p. 1032–1035, doi: 10.1130/
0091-7613(1989)0172.3.CO;2.
Edelman, S.H., Day, H.W., Moores, E.M., Zigan, S., Murphy, T.P., and Hacker, B.R., 1989b, Structure across a
Mesozoic ocean-continent suture zone in the northern
Sierra Nevada, California: Geological Society of
America Special Paper 224, 56 p.
Engebretson, D.C., Cox, A.C., and Gordon, R.G., 1985, Relative motions between oceanic and continental plates
in the Pacific basin: Geological Society of America
Special Paper 206, 59 p.
Ernst, W.G., 1970, Tectonic contact between the franciscan melange and the great valley sequence—Crustal
expression of a late mesozoic benioff zone: Journal of
Geophysical Research, v. 75, no. 5, p. 886–901.
Evarts, R.C., Coleman, R.G., and Schiffman, P., 1999, The
Del Puerto ophiolite: Petrology and tectonic setting, in
Wagner, D.L., and Graham, S.A., eds., Geologic Field
Trips in Northern California: Sacramento, California
Division of Mines and Geology, p. 136–149.
Giaramita, M., MacPherson, G.J., and Phipps, S.P., 1998,
Petrologically diverse basalts from a fossil oceanic
forearc in California; The Llanada and Black Mountain
remnants of the Coast Range Ophiolite: Geological
Society of America Bulletin, v. 110, p. 553–571, doi:
10.1130/0016-7606(1998)1102.3.CO;2.
Girty, G.H., Hanson, R.E., Girty, M.S., Schweickert, R.A.,
Harwood, D.S., Yoshinobu, A.S., Bryan, K.A., Skinner,
J.E., and Hill, C.A., 1995, Timing of emplacement of
the Haypress Creek and Emigrant Gap plutons; Implications for the timing and controls of Jurassic orogenesis, northern Sierra Nevada, California, in Miller,
D.M., and Busby, C., eds., Jurassic magmatism and
Geological Society of America Bulletin, May/June 2005
651
SHERVAIS et al.
tectonics of the North American Cordillera: Geological
Society of America Special Paper 299, p. 191–202.
Glen, R.A., 1990, Formation and thrusting in some Great
Valley rocks near the Franciscan Complex, California,
and implications for the tectonic wedging hypothesis:
Tectonics, v. 9, p. 1451–1477.
Godfrey, N.J., and Klemperer, S.L., 1998, Ophiolitic basement
to a forearc basin and implications for continental growth;
The Coast Range/Great Valley Ophiolite, California: Tectonics, v. 17, p. 558–570, doi: 10.1029/98TC01536.
Gradstein, F.M., Agterberg, F.P., Ogg, J.G., Hardenbol, J.,
van, V.P., Thierry, J., and Huang, Z., 1994, A Mesozoic
time scale: Journal of Geophysical Research, B, Solid
Earth and Planets, v. 99, no. 12, p. 24,051–24,074.
Hacker, B.R., 1993, Evolution of the northern Sierra
Nevada metamorphic belt: Petrological, structural,
and Ar/Ar constraints: Geological Society of America
Bulletin, v. 105, p. 637–656, doi: 10.1130/00167606(1993)1052.3.CO;2.
Hacker, B.R., and Gnos, E., 1997, The conundrum of Samail;
Explaining the metamorphic history, in Mainprice, D.,
Boudier, F., and Bouchez, J.-L., eds., The Adolphe Nicolas volume: Tectonophysics, v. 279, p. 215–226.
Hacker, B.R., Donato, M.M., Barnes, C.G., McWilliams,
M.O., and Ernst, W.G., 1995, Timescales of orogeny:
Jurassic construction of the Klamath Mountains: Tectonics, v. 14, p. 677–703, doi: 10.1029/94TC02454.
Hagstrum, J.T., and Murchey, B.L., 1996, Paleomagnetism
of Jurassic radiolarian chert above the Coast Range
Ophiolite at Stanley Mountain, California, and implications for its paleogeographic origins: Geological
Society of America Bulletin, v. 108, p. 643–652, doi:
10.1130/0016-7606(1996)1082.3.CO;2.
Hanan, B.B., Kimbrough, D.L., and Renne, P.R., 1992, The
Stonyford volcanic complex; A Jurassic seamount in
the Northern California Coast Ranges: AAPG Bulletin,
v. 76, no. 3, p. 421.
Harms, T.A., Jayko, A.S., and Blake, M.C., Jr., 1992,
Kinematic evidence for extensional unroofing of the
Franciscan Complex along the Coast Range Fault,
northern Diablo Range, California: Tectonics, v. 11,
p. 228–241.
Harper, G.D., Saleeby, J.B., and Heizler, M., 1994, Formation and emplacement of the Josephine Ophiolite and
the Nevadan Orogeny in the Klamath Mountains,
California-Oregon; U/Pb zircon and 40Ar/ 39Ar geochronology: Journal of Geophysical Research, B, Solid
Earth and Planets, v. 99, no. B3, p. 4293–4321, doi:
10.1029/93JB02061.
Hopson, C.A., Mattinson, J.M., and Pessagno, E.A., Jr.,
1981, Coast Range ophiolite, western California, in
Ernst, W.G., ed., The geotectonic development of California, Rubey Volume I: Englewood Cliffs, New Jersey,
Prentice-Hall, p. 418–510.
Hopson, C.A., Mattinson, J.M., Luyendyk, B.P., Beebe,
W., Pessagno, E.A., Jr., Hull, D.M., and Blome, C.D.,
1992, California Coast Range Ophiolite; Jurassic tectonic history: AAPG Bulletin, v. 76, no. 3, p. 422.
Hopson, C.A., Mattinson, J.M., Luyendyk, B.P., Beebe,
W.J., Pessagno, E.A., Jr., Hull, D.M., Munoz, I.M.,
and Blome, C.D., 1997, Coast Range Ophiolite; Paleoequatorial ocean-ridge lithosphere: AAPG Bulletin,
v. 81, no. 4, p. 687.
Hull, D.M., 1995, Morphologic diversity and paleogeographic significance of the family Parvicingulidae
(Radiolaria): Micropaleontology, v. 41, no. 1, p. 1–48.
Hull, D.M., 1997, Upper Jurassic Tethyan and southern
Boreal radiolarians from western North America:
Micropaleontology, v. 43, Suppl., no. 2, p. 202.
Hull, D.M., and Pessagno, E.A., Jr., 1994, Upper Jurassic radiolarian biostratigraphy of Stanley Mountain,
Southern California Coast Range, in Cariou, E., and
Hantzpergue, P., eds., 3ème Symposium International
de Stratigraphie du Jurassique: Lyon, France, Université Claude Bernard, Departement de Geologie
Geobios, Memoire Special, p. 309–315.
Hull, D.M., and Pessagno, E.A., Jr., 1995, Radiolarian
stratigraphic study of Stanley Mountain, California, in
Baumgartner, P.O., O’Dogherty, L., Gorican, S., Urquhart,
E., Pillevuit, A., and De Wever, P., eds., Middle Jurassic to
Lower Cretaceous Radiolaria of Tethys: Occurrences,
Systematics, Biochronology: Lausanne, Switzerland,
Memoires de Geologie (Lausanne), p. 985–996.
Hull, D.M., Pessagno, E.A., Jr., Blome, C.D., Hopson, C.A.,
and Munoz, I.M., 1993, Chronostratigraphic assignment
652
of volcanopelagic strata above the Coast Range Ophiolite, in Dunne, G.C., and McDougall, K., eds., Mesozoic
paleogeography of the Western United States; II. Field
Trip Guidebook—Pacific Section, SEPM: Los Angeles,
California, Society of Economic Paleontologists and
Mineralogists, Pacific Section, p. 157–169.
Imlay, R.W., 1980, Jurassic paleobiogeography of the conterminous United States in its continental setting: U.S.
Geological Survey Professional Paper 1062, p. 1–134.
Ingersoll, R.V., 1983, Petrofacies and provenance of late
Mesozoic forearc basin, Northern and Central California: AAPG Bulletin, v. 67, no. 7, p. 1125–1142.
Ingersoll, R.V., Ratajeski, K., Glazner, A.F., and Cloos, M.,
1999, Mesozoic convergent margin of Central California, in Wagner, D.L., and Graham, S., eds., Geologic
field trips in Northern California; Centennial meeting
of the Cordilleran Section of the Geological Society of
America: Special Publication 119: Sacramento, California, State of California, Department of Conservation, Division of Mines and Geology, p. 101–135.
Jayko, A.S., Blake, M.C., Jr., and Harms, T.A., 1987,
Attenuation of the Coast Range Ophiolite by extensional faulting, and nature of the Coast Range “thrust,”
California: Tectonics, v. 6, p. 475–488.
Jones, D.L., 1975, Discovery of Buchia rugosa of Kimmeridgian age from the base of the Great Valley sequence:
Geological Society of America, Abstracts with Programs, v. 7, no. 3, p. 330.
Kiessling, W., 1999, Late Jurassic radiolarians from the Antarctic Peninsula: Micropaleontology, v. 45, no. 1, p. 1–96.
Kimbrough, D.L., and Moore, T.E., 2003, Ophiolite and volcanic arc assemblages on the Vizcaino Peninsula and
Cedros Island, Baja California Sur, Mexico: Mesozoic
forearc lithosphere of the Cordilleran magmatic arc,
in Johnson, S.E., Paterson, S.R., Fletcher, J.M., Girty,
G.H., Kimbrough, D.L., and Martin-Barajas, A., eds.,
Tectonic evolution of northwestern Mexico and the
southwestern USA: Geological Society of America
Special Paper 374, p. 43–71.
Krogh, T.E., 1973, A low-contamination method for hydrothermal decomposition of zircon and extraction of U
and Pb for isotopic age determinations: Geochimica
et Cosmochimica Acta, v. 37, no. 3, p. 485–494, doi:
10.1016/0016-7037(73)90213-5.
Lanphere, M.A., 1971, Age of the Mesozoic oceanic crust
in the California Coast Ranges: Geological Society of
America Bulletin, v. 82, p. 3209–3211.
Linn, A.M., DePaolo, D.J., and Ingersoll, R.V., 1992, Nd-Sr
isotopic, geochemical, and petrographic stratigraphy
and paleotectonic analysis; Mesozoic Great Valley
forearc sedimentary rocks of California: Geological
Society of America Bulletin, v. 104, p. 1264–1279,
doi: 10.1130/0016-7606(1992)1042.3.CO;2.
Louvion-Trellu, C., 1986, Les radiolarites des Coast Ranges,
Californie: Etude biochronologique, sedimentologique
et geochimique [DEA thesis]: Brest, France, Universite
de Bretagne Occidentale, 46 p.
Ludwig, K.R., 2000, Decay constant errors in U-Pb concordia-intercept ages: Chemical Geology, v. 166, no. 3-4,
p. 315–318, doi: 10.1016/S0009-2541(99)00219-3.
MacPherson, G.J., 1983, The Snow Mountain volcanic complex; An on-land seamount in the Franciscan terrain,
California: Journal of Geology, v. 91, p. 73–92.
MacPherson, G.J., Phipps, S.P., and Grossman, J.N., 1990,
Diverse sources for igneous blocks in Franciscan
melanges, California Coast Ranges: Journal of Geology, v. 98, p. 845–862.
Mankinen, E.A., Gromme, C.S., and Williams, K.M., 1991,
Concordant paleolatitudes from ophiolite sequences in
the Northern California Coast Ranges, USA: Tectonophysics, v. 198, no. 1, p. 1–21, doi: 10.1016/00401951(91)90127-E.
Mattinson, J.M., 1986, Geochronology of high-pressure–low-temperature Franciscan metabasite; A new
approach using the U-Pb system, in Evans, B.W., and
Brown, E.H., eds., Blueschists and eclogites: Geological Society of America Memoir 164, p. 95–105.
Mattinson, J.M., 1988, Constraints on the timing of Franciscan metamorphism; Geochronological approaches and
their limitations, in Ernst, W.G., ed., Metamorphism
and crustal evolution of the Western United States:
Rubey volume 7: Englewood Cliffs, New Jersey, Prentice-Hall, p. 1023–1034.
Mattinson, J.M., and Hopson, C.A., 1992, U/Pb ages of the
Coast Range Ophiolite; A critical reevaluation based
on new high-precision Pb/Pb ages: AAPG Bulletin,
v. 76, no. 3, p. 425.
May, S.R., Beck, M.-E.J., and Butler, R.-F., 1989, North
American apparent polar wander, plate motion, and leftoblique convergence, Late Jurassic-Early Cretaceous
orogenic consequences: Tectonics, v. 8, p. 433–451.
McDowell, F.W., Lehman, D.H., Gucwa, P.R., Fritz, D., and
Maxwell, J.C., 1984, Glaucophane schists and ophiolites of the Northern California Coast Ranges; Isotopic
ages and their tectonic implications: Geological Society of America Bulletin, v. 95, p. 1373–1382.
Min, K., Mundil, R., Renne, P.R., and Ludwig, K.R., 2000,
A test for systematic errors in 40Ar/39Ar geochronology through comparison with U-Pb analysis of a 1.1 Ga
Rhyolite: Geochimica et Cosmochimica Acta, v. 64,
no. 1, p. 73–98, doi: 10.1016/S0016-7037(99)00204-5.
Moore, D.E., 1984, Metamorphic history of a high-grade blueschist exotic block from the Franciscan Complex, California: Journal of Petrology, v. 25, no. 1, p. 126–150.
Moore, D.E., and Blake, M.C., Jr., 1989, New evidence for
polyphase metamorphism of glaucophane schist and
eclogite exotic blocks in the Franciscan Complex,
California and Oregon: Journal of Metamorphic Geology, v. 7, no. 2, p. 211–228.
Murchey, B.L., and Baumgartner, P.O., 1995, Biostratigraphy of Middle Jurassic radiolarians in the Franciscan
Complex, California: Implications for resolution of age
discrepancies between North American and European
zonations, in Baumgartner, P.O., O’Dogherty, L., Gorican, S., Urquhart, E., Pillevuit, A., and De Wever, P.,
eds., Middle Jurassic to Lower Cretaceous Radiolaria
of Tethys: Occurrences, Systematics, Biochronology:
Lausanne, Switzerland, Memoires de Geologie (Lausanne), p. 997–1010.
Murchey, B.L., and Blake, M.C., Jr., 1992, Postulated ridge
subduction in California during the Late Jurassic; Unifying hypothesis to explain the Nevadan Orogeny and
the formation of the Great Valley fore-arc sequence:
AAPG Bulletin, v. 76, no. 3, p. 426.
Murchey, B.L., and Blake, M.C., Jr., 1993, Evidence for subduction of a major ocean plate along the California margin
during the Middle to Early Late Jurassic, in Dunne, G.C.,
and McDougall, K., eds., Mesozoic paleogeography of
the Western United States II: Field Trip Guidebook: Los
Angeles, California, Society of Economic Paleontologists
and Mineralogists, Pacific Section, p. 1–17.
Murchey, B.L., and Hagstrum, J.T., 1997, Paleomagnetism
of Jurassic radiolarian chert above the Coast Range
Ophiolite at Stanley Mountain, California, and implications for its paleogeographic origins; Reply: Geological Society of America Bulletin, v. 109, p. 1633–1639,
doi: 10.1130/0016-7606(1997)1092.3.CO;2.
Nagai, H., and Mizutani, S., 1990, Jurassic Eucyrtidiellum
(Radiolaria) in the Mino Terrane: Transactions and
Proceedings of the Palaeontological Society of Japan,
New Series, v. 159, p. 587–602.
Palfy, J., Smith, P.L., and Mortensen, J.K., 2000, A U-Pb and
40
Ar/ 39Ar time scale for the Jurassic: Canadian Journal
of Earth Sciences, v. 37, no. 6, p. 923–944.
Pessagno, E.A., Jr., 1977, Upper Jurassic Radiolaria and
radiolarian biostratigraphy of the California Coast
Ranges: Micropaleontology, v. 23, no. 1, p. 56–113.
Pessagno, E.A., Jr., and Blome, C.D., 1980, Upper Triassic
and Jurassic Pantanelliinae from California, Oregon
and British Columbia: Micropaleontology, v. 26, no. 3,
p. 225–273.
Pessagno, E.A., Jr., and Blome, C.D., 1986, Faunal affinities and tectonogenesis of Mesozoic rocks in the Blue
Mountains Province of eastern Oregon and western
Idaho, in Vallier, T.L., and Brooks, H., eds., Geology
of the Blue Mountains region of Oregon, Idaho, and
Washington; Geologic implications of Paleozoic and
Mesozoic paleontology and biostratigraphy, Blue
Mountains Province, Oregon and Idaho: U.S. Geological Survey Professional Paper 1435, p. 65–78.
Pessagno, E.A., Jr., and Newport, R.L., 1972, A technique
for extracting Radiolaria from radiolarian cherts:
Micropaleontology, v. 18, no. 2, p. 231–234.
Pessagno, E.A., Jr., and Whalen, P.A., 1982, Lower and
Middle Jurassic Radiolaria (multicyrtid Nassellariina)
from California, East-central Oregon and the Queen
Charlotte Islands, B.C.: Micropaleontology, v. 28, no. 2,
p. 111–169.
Pessagno, E.A., Jr., Blome, C.D., and Longoria, J., 1984, A
revised radiolarian zonation for the Upper Jurassic of
Geological Society of America Bulletin, May/June 2005
AGE RELATIONS IN THE COAST RANGE OPHIOLITE, CALIFORNIA
western North America: Bulletin of American Paleontology, v. 87, no. 320, p. 1–51.
Pessagno, E.A., Jr., Blome, C.D., Carter, E.S., Macleod, N.,
Whalen, P.A., and Yeh, K.Y., 1987, Studies of North
American Jurassic Radiolaria; Part II, Preliminary
radiolarian zonation for the Jurassic of North America:
Cushman Foundation for Foraminiferal Research Special Publication 23, p. 1–18.
Pessagno, E.A., Jr., Six, W.M., and Yang, Q., 1989, The
Xiphostylidae Haeckel and Parvivaccidae n. fam.
(Radiolaria) from the North American Jurassic: Micropaleontology, v. 39, no. 2, p. 93–166.
Pessagno, E.A., Jr., Blome, C.D., Hull, D.M., and Six, W.M.,
1993a, Jurassic Radiolaria from the Josephine ophiolite
and overlying strata, Smith River subterrane (Klamath
Mountains), northwestern California and southwestern
Oregon: Micropaleontology, v. 39, no. 2, p. 93–166.
Pessagno, E.A., Jr., Hull, D.M., and Hopson, C.A., 2000,
Tectonostratigraphic significance of sedimentary strata
occurring within and above the Coast Range Ophiolite
(California Coast Ranges) and the Josephine Ophiolite
(Klamath Mountains), northwestern California, in
Dilek, Y., Moores, E.M., Elthon, D., and Nicolas, A.,
eds., Ophiolites and Oceanic Crust: New Insights from
Field Studies and the Ocean Drilling Program: Boulder, Colorado, Geological Society of America.
Platt, J.P., 1986, Dynamics of orogenic wedges and the uplift
of high-pressure metamorphic rocks: Geological Society of America Bulletin, v. 97, p. 1037–1053.
Renne, P.R., 1995, Excess 40Ar in biotite and hornblende
from the Norils’k 1 intrusion: Implications for the age of
the Siberian Traps: Earth and Planetary Science Letters,
v. 131, p. 165–176, doi: 10.1016/0012-821X(95)00015-5.
Renne, P.R., and Scott, G.R., 1988, Structural chronology,
oroclinal deformation, and tectonic evolution of the
Southeastern Klamath Mountains, California: Tectonics, v. 7, p. 223–242.
Renne, P.R., Swisher, C.C., Deino, A.L., Karner, D.B.,
Owens, T., and DePaolo, D.J., 1998, Intercalibration of
standards, absolute ages and uncertainties in 40Ar/ 39Ar
Dating: Chemical Geolology, v. 145, no. 1–2, p. 117–
152, doi: 10.1016/S0009-2541(97)00159-9.
Ring, U., and Brandon, M.T., 1994, Kinematic data for the
Coast Range Fault and implications for exhumation of the
Franciscan subduction complex: Geology, v. 22, p. 735–
738, doi: 10.1130/0091-7613(1994)0222.3.CO;2.
Ring, U., and Brandon, M.T., 1999, Ductile deformation
and mass loss in the Franciscan subduction complex;
Implications for exhumation processes in accretionary
wedges, in Ring, U., Brandon, M.T., Lister, G.S., and
Willett, S., eds., Exhumation processes; Normal faulting, ductile flow and erosion: Geological Society of
London Special Publication 154, p. 55–86.
Robertson, A.H.F., 1989, Palaeoceanography and tectonic
setting of the Jurassic Coast Range ophiolite, Central
California; Evidence from the extrusive rocks and the
volcaniclastic sediment cover: Marine and Petroleum
Geology, v. 6, no. 3, p. 194–220, doi: 10.1016/02648172(89)90001-9.
Robertson, A.H.F., 1990, Sedimentology and tectonic implications of ophiolite-derived clastics overlying the Jurassic
Coast Range Ophiolite, Northern California: American
Journal of Science, v. 290, no. 2, p. 109–163.
Ross, J.A., and Sharp, W.D., 1986, 40Ar/ 39Ar and Sm/Nd
dating of garnet amphibolite in the Coast Ranges,
California, EOS (Transactions, American Geophysical Union: Washington, D.C., American Geophysical
Union), v. 67, no. 44, p. 1249.
Ross, J.A., and Sharp, W.D., 1988, The effects of subblocking temperature metamorphism on the K/Ar
systematics of hornblendes; Ar-40/Ar-39 dating of
polymetamorphic garnet amphibolite from the Franciscan Complex, California: Contributions to Mineralogy
and Petrology, v. 100, no. 2, p. 213–221.
Saleeby, J., 1978, Kings River ophiolite, Southwest Sierra
Nevada foothills, California: Geological Society of
America Bulletin, v. 89, p. 617–636.
Saleeby, J., 1981, Ocean floor accretion and volcanoplutonic arc evolution of the Mesozoic Sierra Nevada,
in Ernst, W.G., ed., The geotectonic development of
California; Rubey Volume I: Englewood Cliffs, New
Jersey, Prentice-Hall, p. 132–181.
Saleeby, J.B., 1982, Polygenetic ophiolite belt of the
California Sierra Nevada; Geochronological and tectonostratigraphic development: Journal of Geophysical
Research, B, v. 87, no. 3, p. 1803–1824.
Saleeby, J.B., 1983a, Accretionary tectonics of the North
American Cordillera: Annual Review of Earth and
Planetary Sciences, v. 11, p. 45–73, doi: 10.1146/ANN
UREV.EA.11.050183.000401.
Saleeby, J.B., 1983b, Recognition and significance of
boundary transforms and rift edges within island arc
terranes of the western Sierra Nevada, in Howell, D.G.,
Jones, D.L., Cox, A., and Nur, A.M., eds., Proceedings
of the Circum-Pacific terrane conference: Stanford,
California, Stanford University Publications, Geological Sciences, v. 18, p. 167–169.
Saleeby, J.B., 1990, Geochronological and tectonostratigraphic
framework of Sierran-Klamath ophiolitic assemblages, in
Harwood, D.S., and Miller, M.M., eds., Paleozoic and
early Mesozoic paleogeographic relations; Sierra Nevada,
Klamath Mountains, and related terranes: Geological
Society of America Special Paper 255, p. 93–114.
Saleeby, J.B., 1997, The Coast Range Ophiolite (CRO) debate
is fraught with complimentarities and indeterminacy; a
few examples: AAPG Bulletin, v. 81, no. 4, p. 692.
Saleeby, J.B., Shaw, H.F., Niemeyer, S., Moores, E.M., and
Edelman, S.H., 1989, U/Pb, Sm/Nd and Rb/Sr geochronological and isotopic study of northern Sierra Nevada
ophiolitic assemblages, California: Contributions to
Mineralogy and Petrology, v. 102, no. 2, p. 205–220.
Schweickert, R.A., 1997, Critical stratigraphic, structural,
and timing relations within the western Sierra Nevada,
California, and their bearing on models for origin
of the Foothills Terrane and the Great Valley Basin:
AAPG Bulletin, v. 81, no. 4, p. 692.
Schweickert, R.A., Armstrong, R.L., and Harakal, J.E.,
1980, Lawsonite blueschist in the northern Sierra
Nevada, California: Geology, v. 8, p. 27–31.
Schweickert, R.A., Bogen, N.L., Girty, G.H., Hanson, R.E., and
Merguerian, C., 1984, Timing and structural expression
of the Nevadan Orogeny, Sierra Nevada, California: Geological Society of America Bulletin, v. 95, p. 967–979.
Schweickert, R.A., Bogen, N.L., and Merguerian, C., 1988,
Deformational and metamorphic history of Paleozoic
and Mesozoic basement terranes in the western Sierra
Nevada metamorphic belt, in Ernst, W.G., ed., Metamorphism and crustal evolution of the Western United
States: Rubey Volume 7: Englewood Cliffs, New Jersey, Prentice-Hall, p. 789–822.
Sharp, W.D., 1988, Pre-Cretaceous crustal evolution in the
Sierra Nevada region, California, in Ernst, W.G., ed.,
Metamorphism and crustal evolution of the Western
United States: Rubey Volume 7: Englewood Cliffs,
New Jersey, Prentice-Hall, p. 823–864.
Shervais, J.W., 1990, Island arc and ocean crust ophiolites:
Contrasts in the petrology, geochemistry, and tectonic
style of ophiolite assemblages in the California Coast
Ranges, in Ophiolites: Oceanic crustal analogues,
Proceedings of the Symposium Troodos 1987, Nicosia,
Cyprus, p. 507–520.
Shervais, J.W., 2001, Birth, Death, and Resurrection: The life
cycle of supra-subduction zone ophiolites: Geochemistry,
Geophysics, Geosystems, v. 2, Paper 2000GC000080,
20,925 words.
Shervais, J.W., and Beaman, B.J., 1991, The Elder Creek
Ophiolite: Multi-stage magmatic history in a fore-arc
ophiolite, northern California Coast Ranges: Geological Society of America, Abstracts with Programs, v. 23,
no. 5, p. A387.
Shervais, J.W., and Hanan, B.B., 1989, Jurassic volcanic glass
from the Stonyford volcanic complex, Franciscan assemblage, Northern California Coast Ranges: Geology, v. 17,
p. 510–514, doi: 10.1130/0091-7613(1989)0172.3.CO;2.
Shervais, J.W., and Kimbrough, D.L., 1985, Geochemical
evidence for the tectonic setting of the Coast Range
ophiolite; A composite island arc-oceanic crust terrane
in western California: Geology, v. 13, p. 35–38.
Shervais, J.W., and Kimbrough, D.L., 1987, Alkaline and
transitional subalkaline metabasalts in the Franciscan
Complex melange, California, in Morris, E.M., and
Pasteris, J.D., eds., Mantle metasomatism and alkaline
magmatism: Geological Society of America Special
Paper 215, p. 165–182.
Shervais, J.W., Murchey, B., Kimbrough, D.L., Hanan, B.B.,
Renne, P.R., Snow, C.A., Schuman, M.Z., and Beaman, J., 2004, Multi-stage origin of the Coast Range
Ophiolite, California: Implications for the life cycle of
supra-subduction zone ophiolites: International Geology Review, v. 46, p. 289–315.
Stacey, J.S., and Kramers, J.D., 1975, Approximation of terrestrial lead isotope evolution by a two-stage model: Earth
and Planetary Science Letters, v. 26, no. 2, p. 207–221.
Steiger, R., and Jaeger, E., 1977, Subcommission on geochronology: Convention on uniform decay constants
in geo- and cosmochronology: Earth and Planetary
Science Letters, v. 36, p. 359–362, doi: 10.1016/0012821X(77)90060-7.
Stern, R.J., and Bloomer, S.H., 1992, Subduction zone
infancy; Examples from the Eocene Izu-Bonin-Mariana and Jurassic California arcs: Geological Society of
America Bulletin, v. 104, p. 1621–1636, doi: 10.1130/
0016-7606(1992)1042.3.CO;2.
Tobisch, O.T., Paterson, S.R., Saleeby, J.B., and Geary, E.E.,
1989, Nature and timing of deformation in the Foothills
Terrane, central Sierra Nevada, California; Its bearing on
orogenesis: Geological Society of America Bulletin, v. 101,
p. 401–413, doi: 10.1130/0016-7606(1989)1012.3.CO;2.
Unruh, J.R., Loewen, B.A., and Moores, E.M., 1995, Progressive arcward contraction of a Mesozoic-Tertiary
fore-arc basin, southwestern Sacramento Valley, California: Geological Society of America Bulletin, v. 107,
p. 38–53, doi: 10.1130/0016-7606(1995)1072.3.CO;2.
Wakabayashi, J., 1990, Counterclockwise P-T-t paths from
amphibolites, Franciscan Complex, California; Relics
from the early stages of subduction zone metamorphism: Journal of Geology, v. 98, p. 657–680.
Ward, P.L., 1995, Subduction cycles under western North
America during the Mesozoic and Cenozoic eras, in
Miller, D.M., and Busby, C., eds., Jurassic magmatism
and tectonics of the North American Cordillera: Geological Society of America Special Paper 299, p. 1–45.
Wolf, M.B., and Saleeby, J.B., 1992, Jurassic Cordilleran dike
swarm-shear zones; Implications for the Nevadan Orogeny and North American plate motion: Geology, v. 20,
p. 745–748, doi: 10.1130/0091-7613(1992)0202.3.CO;2.
Wolf, M.B., and Saleeby, J.B., 1995, Late Jurassic dike
swarms in the southwestern Sierra Nevada Foothills
Terrane, California; Implications for the Nevadan
Orogeny and North American Plate motion, in Miller,
D.M., and Busby, C., eds., Jurassic magmatism and
tectonics of the North American Cordillera: Geological
Society of America Special Paper 299, p. 203–228.
Wright, J.E., and Fahan, M.R., 1988, An expanded view
of Jurassic orogenesis in the western United States
Cordillera; Middle Jurassic (pre-Nevadan) regional
metamorphism and thrust faulting within an active arc
environment, Klamath Mountains, California: Geological Society of America Bulletin, v. 100, p. 859–876,
doi: 10.1130/0016-7606(1988)1002.3.CO;2.
Wright, J.E., and Wyld, S.J., 1986, Significance of xenocrystic Precambrian zircon contained within the southern
continuation of the Josephine Ophiolite; Devils Elbow
Ophiolite remnant, Klamath Mountains, Northern
California: Geology, v. 14, p. 671–674.
Zoglman, M.M., 1991, The Stonyford Volcanic Complex:
Petrology and structure of a Jurassic Seamount in the
Northern California Coast Ranges [M.S. thesis]: Columbia, South Carolina, University of South Carolina, 129 p.
Zoglman, M.M., and Shervais, J.W., 1991, The Stonyford
Volcanic Complex: Petrology and structure of a Jurassic seamount in the Northern California Coast Ranges:
Geological Society of America, Abstracts with Programs, v. 23, no. 5, p. A395.
MANUSCRIPT RECEIVED BY THE SOCIETY 10 JULY 2003
REVISED MANUSCRIPT RECEIVED 1 JULY 2004
MANUSCRIPT ACCEPTED 10 JULY 2004
Printed in the USA
Geological Society of America Bulletin, May/June 2005
653
Data Repository Table DR-1. Ar release spectra for volcanic glasses of the Stonyford volcanic
complex.
Laser
(W)
40
Ar/39Ar
37
Ar/39Ar
36
Ar/39Ar
40
Ar*/39ArK
%40Ar*
Age
(Ma)
±2s
(Ma)
G-2/3524-01
1.4
1.8
2.2
2.5
2.6
2.7
2.8
2.9
3.0
3.2
3.4
3.6
3.9
4.3
Plateau Age (Ma)
5.4505
5.1465
5.3367
5.1368
4.9393
4.9145
4.9158
4.9315
4.9872
4.9254
4.9692
4.9866
4.9923
5.0119
J= 0.01950 ±0.00004
5.9027
5.8310
5.7358
5.8798
5.7819
5.8171
5.8244
5.7907
5.8463
5.7464
5.8184
5.7856
5.7711
5.7722
0.0044
0.0033
0.0040
0.0028
0.0017
0.0018
0.0017
0.0017
0.0019
0.0016
0.0017
0.0019
0.0019
0.0020
4.6056
4.6299
4.6148
4.7716
4.8953
4.8262
4.8758
4.8810
4.8764
4.9005
4.9313
4.8833
4.8952
4.8723
84.2
89.6
86.1
92.5
98.7
97.8
98.8
98.6
97.4
99.1
98.9
97.6
97.7
96.8
155.2
156.0
155.5
160.5
164.5
162.3
163.9
164.0
163.9
164.7
165.7
164.1
164.5
163.8
164.3
4.6
4.1
2.0
2.0
1.6
3.5
1.0
1.1
2.5
1.4
1.8
1.7
1.6
4.1
1.2
G-2/3524-03
0.5
0.7
0.9
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2.0
Plateau Age (Ma)
5.8165
5.0914
5.0515
4.9347
4.9441
4.9320
4.9759
5.0186
4.9765
5.3531
6.2775
5.7823
5.3008
J= 0.01950 ±0.00004
6.0492
5.8338
5.8597
5.8352
5.8824
5.8431
5.5631
5.8939
5.9813
6.0566
5.5654
5.7991
6.0194
0.0057
0.0027
0.0018
0.0018
0.0017
0.0017
0.0017
0.0019
0.0019
-0.0128
0.0103
0.0106
0.0008
4.6141
4.7625
4.9777
4.8596
4.9117
4.8792
4.9222
4.9072
4.8764
9.6262
3.6629
3.0966
5.5443
79.0
93.2
98.2
98.1
99.0
98.5
98.6
97.4
97.6
179.1
58.1
53.3
104.2
155.4
160.2
167.1
163.4
165.0
164.0
165.4
164.9
163.9
310.4
124.5
105.8
185.2
164.6
2.0
2.1
3.6
1.8
3.1
1.8
1.7
1.0
1.8
112.8
40.8
50.9
198.4
1.4
G-2/3524-04
1.9
2.2
3.0
3.2
3.5
3.8
4.1
4.4
4.7
5.0
5.3
5.6
5.9
6.2
6.5
0.0
Plateau Age (Ma)
8.1989
6.6895
5.0310
5.1426
5.9477
5.5090
5.1469
5.0473
5.0708
5.1480
5.0510
5.1025
5.1409
5.0528
5.0354
5.2367
J= 0.01950 ±0.00004
5.8326
4.9392
5.8458
5.8311
5.8031
5.7228
5.9894
5.8604
5.8570
5.9151
5.8031
5.8927
5.9067
5.8676
5.8699
5.8662
G-2/4445-01
J= 0.02320 ±0.00004
0.4
14.9898
6.4896
0.5
7.0663
5.9758
0.6
5.8320
5.8604
0.7
4.9880
5.9442
0.9
4.5307
5.9899
1.1
4.6507
5.9222
1.2
4.6642
5.8001
1.3
4.5726
5.8537
1.4
4.5978
5.8065
1.5
4.5179
5.8320
1.7
4.9304
6.0073
2.0
4.6137
6.0569
Plateau Age (Ma)
0.0175
0.0174
0.0024
0.0029
0.0068
0.0037
0.0046
0.0021
0.0025
0.0045
0.0053
0.0048
0.0038
0.0023
0.0020
0.0027
3.4911
1.9232
4.7939
4.7334
4.3954
4.8654
4.2581
4.8819
4.8000
4.2916
3.9279
4.1401
4.4774
4.8442
4.8933
4.9013
42.4
28.7
94.9
91.7
73.6
88.0
82.4
96.3
94.3
83.0
77.5
80.8
86.8
95.5
96.8
93.2
118.8
66.4
161.2
159.3
148.4
163.5
143.9
164.1
161.4
145.0
133.2
140.1
151.0
162.9
164.4
164.7
164.3
6.9
26.2
1.3
5.0
7.7
2.7
3.3
4.9
2.8
3.6
8.6
2.9
4.9
1.6
0.9
0.9
1.4
0.0420
0.0135
0.0087
0.0062
0.0033
0.0034
0.0034
0.0031
0.0033
0.0032
0.0045
0.0033
3.0740
3.5433
3.7203
3.6141
4.0361
4.1178
4.1010
4.1131
4.0744
4.0398
4.0540
4.1163
20.4
49.9
63.5
72.2
88.7
88.2
87.6
89.6
88.3
89.1
81.9
88.9
124.3
142.5
149.4
145.3
161.5
164.6
164.0
164.4
162.9
161.6
162.2
164.6
163.8
17.9
13.2
18.3
10.5
3.9
3.3
2.9
2.1
8.2
7.6
4.1
2.9
2.4
G-4/4446-01
0.7
0.9
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
2.0
2.2
Plateau Age (Ma)
6.1796
4.8111
4.6384
4.6640
4.5743
4.4977
4.3730
4.6900
4.2593
4.2702
4.4373
4.3579
J= 0.02320 ±0.00004
4.9144
4.7686
4.7346
4.6821
4.7266
4.5846
4.6235
4.7208
4.8112
4.7580
4.8954
4.8693
0.0087
0.0036
0.0028
0.0026
0.0031
0.0025
0.0024
0.0034
0.0018
0.0019
0.0023
0.0022
3.9823
4.1068
4.1661
4.2567
4.0330
4.1136
4.0171
4.0564
4.1147
4.0849
4.1324
4.0752
64.2
85.1
89.5
91.0
87.9
91.2
91.6
86.2
96.3
95.4
92.8
93.2
159.4
164.2
166.4
169.9
161.4
164.4
160.8
162.3
164.5
163.3
165.2
163.0
164.1
5.0
11.4
8.6
9.6
7.8
10.7
10.0
13.7
1.1
1.2
1.3
1.8
1.4
G-4/4446-02
0.5
0.7
0.9
1.1
1.2
1.3
1.4
1.6
1.8
2.0
Plateau Age (Ma)
8.8120
4.7322
4.6485
4.5236
4.4912
4.3555
4.9700
5.7182
5.2128
4.7982
J= 0.02320 ±0.00004
4.5843
4.4952
4.5305
4.6022
4.6356
4.7008
4.7932
5.0282
4.9837
5.0509
0.0214
0.0051
0.0032
0.0027
0.0028
0.0021
0.0042
0.0069
0.0050
0.0041
2.8435
3.5664
4.0662
4.0813
4.0265
4.1115
4.1000
4.0568
4.1109
3.9678
32.2
75.1
87.2
89.9
89.4
94.1
82.2
70.7
78.6
82.4
115.3
143.4
162.6
163.2
161.1
164.4
163.9
162.3
164.3
158.9
163.9
13.4
4.5
2.5
1.8
4.8
1.0
2.9
3.9
1.9
1.6
1.6
0.0804
0.0234
0.0082
0.0061
0.0052
0.0045
0.0048
0.0050
0.0037
0.0032
0.0037
0.0030
0.0029
0.0028
0.0027
1.5144
3.1886
4.2920
4.2502
4.1769
4.1264
4.0208
4.1038
4.0964
4.1144
4.0734
4.1460
4.1409
4.1719
4.1034
6.1
33.3
70.2
77.5
81.7
85.1
83.2
82.7
89.7
92.5
89.4
93.6
94.5
95.2
95.6
62.3
128.8
171.2
169.7
166.9
164.9
160.9
164.1
163.8
164.5
162.9
165.7
165.5
166.7
164.1
164.6
84.6
45.6
25.7
22.5
10.0
10.0
8.7
8.6
5.0
2.4
8.4
5.5
4.9
3.1
1.6
2.2
G-8/4447-01
J= 0.02320 ±0.00004
0.4
24.9170
4.7251
0.5
9.5320
7.5391
0.6
6.0813
7.9462
0.7
5.4556
7.7418
0.8
5.0828
8.0700
0.9
4.8225
8.0146
1.0
4.8080
8.1830
1.1
4.9352
8.4140
1.2
4.5395
8.2200
1.3
4.4243
8.0973
1.4
4.5303
8.1366
1.5
4.4056
8.1577
1.6
4.3596
8.0227
1.7
4.3607
8.0886
2.0
4.2702
8.1002
Plateau Age (Ma)
G-8/4447-02
J= 0.02320 ±0.00004
0.5
10.0276
6.0168
0.7
4.7019
7.6446
0.9
4.4458
8.1867
1.1
4.2323
8.1239
1.2
4.2298
8.2379
1.3
4.1699
7.8929
1.4
4.2747
7.8427
1.6
4.2262
8.1032
1.8
4.2093
8.0233
2.0
4.2553
8.2549
2.2
4.2010
8.2295
Plateau Age (Ma)
0.0273
0.0051
0.0045
0.0029
0.0026
0.0022
0.0022
0.0025
0.0025
0.0027
0.0024
2.4216
3.8004
3.7552
3.9944
4.0954
4.1365
4.2298
4.1080
4.0860
4.1049
4.1433
24.1
80.4
84.0
93.9
96.3
98.7
98.4
96.7
96.6
95.9
98.1
98.6
152.4
150.7
159.9
163.8
165.3
168.9
164.2
163.4
164.1
165.6
164.7
20.8
7.3
8.1
3.4
2.3
1.6
5.5
1.9
2.4
1.6
1.7
1.6
G-5/4448-01
0.5
0.7
0.9
1.1
1.2
1.3
1.4
1.5
2.0
2.2
Plateau Age (Ma)
J= 0.02320 ±0.00004
4.8369
4.8670
4.8002
4.7716
4.7868
4.7958
4.6454
4.8257
4.8864
4.8555
0.0178
0.0038
0.0028
0.0025
0.0023
0.0035
0.0031
0.0028
0.0036
0.0037
4.1577
4.0606
4.1045
4.1041
4.1013
4.0734
4.1662
4.0969
4.0396
4.0857
45.8
84.4
89.5
91.3
92.5
85.7
87.8
90.0
85.4
85.0
166.1
162.4
164.1
164.1
164.0
162.9
166.5
163.8
161.6
163.4
163.7
3.5
2.8
2.1
2.4
1.9
2.4
5.3
5.5
4.2
2.0
1.8
J= 0.02320 ±0.00004
30.9391
3.1457
17.7776
3.3412
20.6942
4.3307
16.3524
4.4359
13.9827
4.4730
11.1687
4.5153
12.9163
4.7041
12.1102
4.4312
10.4991
4.3873
8.5395
4.6100
9.7409
4.7710
11.3687
5.0148
11.9626
5.0010
12.0371
5.0567
0.0912
0.0468
0.0574
0.0435
0.0350
0.0251
0.0306
0.0282
0.0230
0.0165
0.0201
0.0255
0.0285
0.0280
4.2446
4.2220
4.0573
3.8323
3.9962
4.1117
4.2524
4.1128
4.0534
4.0325
4.1727
4.2135
3.9399
4.1569
13.7
23.7
19.5
23.4
28.5
36.7
32.8
33.9
38.5
47.1
42.7
36.9
32.8
34.4
169.4
168.6
162.3
153.7
160.0
164.4
169.7
164.4
162.1
161.3
166.7
168.3
157.8
166.1
164.4
26.1
15.7
27.4
18.0
11.2
10.7
8.0
7.2
7.8
4.9
4.5
8.8
11.4
9.4
2.3
G-5/4448-01
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
Plateau Age (Ma)
9.0467
4.7931
4.5715
4.4804
4.4203
4.7403
4.7329
4.5361
4.7146
4.7927
Notes:
All errors reported are 2s
J-values are based on 28.02 Ma for Fish Canyon sanidine; uncertainty does not include age error
Power refers to laser output in Watts. Note that different samples were run with different laser focus.
Isotope ratios are corrected for blanks, mass discrimination, and radioactive decay
Ages are based on nucleogenic corrections reported by Renne et al. (1998) and decay constants of Steiger
and Jager (1977)
Age errors do not include systematic contributions from decay constants or J value
Errors reported for plateau ages include analytical error in J determination
DR-2: Technical notes on biostratigraphic calibrations and correlations
Range discordancies , range diachronism, and other sources of uncertainty
Biostratigraphic zonations, like absolute time scales, are inherently works-in-progress.
Neither the zonation of Baumgartner et al. (1995a) nor of Pessagno et al. (1987, 1993) produces
perfectly concordant results in all Cordilleran sections (Murchey and Baumgartner, 1995; and
herein). Range discordancies, which are overlapping ranges that are dissonant with ranges in a
formal zonation, result from unrecognized range diachronism or reworking. In the Cordillera,
significant environmentally-controlled diachronism at the genus level is well-documented: e.g.,
Praeparvicingula, Parvicingula, Pantanellium (Pessagno and Blome, 1984), and Mirifusus
(Murchey and Baumgartner, 1995). Therefore, we took some care to note range discordancies
arising from application of the Tethyan UA Zonation of Baumgartner et al. (1995a) to the
sections discussed. In addition, we have summarized some of the assumptions and uncertainties
associated with previous calibrations based on the zonation of Pessagno et al. (1993) as they
apply to specific sections. Kiessling’s (1999) recalibrations of Subzones 2 to 4 of Pessagno et
al. (1987, 1993), which incorporate data from a well-constrained Kimmeridgian and Tithonian
section, brought the Late Jurassic calibrations of the two major zonation schemes into much
closer alignment, but did not directly address the problems of reconciling the calibrations of
Superzone 1 and Zone 2 (Subzones 2 , 2, and 2) of Pessagno et al. (1987, 1993) with the UA
Zonation.
Josephine ophiolite, Western Klamath Mountains, northern California and Oregon:
UAZ calibrations: Baumgartner et al. (1995a) used the radiolarian faunas documented by
Pessagno et al. (1993) to calibrate the sedimentary sections intercalated with and overlying basalt
of the Josephine ophiolite. At the Turner-Albright Mine in Oregon, where sedimentary rocks are
interbedded with basalt, they assigned two intervals on strike with one another to UAZ 3-4. An
argument can be made that the interval can be further constrained to UA Zone 4 based on the
presence of Unuma typicus (UAZ 3-4), Levileugeo spp. (equivalent to Leugeo hexacubicus sensu
Baumgartner at al., 1995, UAZ 4-8), and a species with affinity to Cyrtocapsa (?) kisoensis
(UAZ 3-4). UA Zone 4 is calibrated as late Bajocian. The samples also contain common to
abundant Praecaneta decora (which may fall within Baumgartner et al.’s (1995b) definition of
Parvicingula(?) spinata, UAZ 3-10); P. decora first occurs in the Bathonian of the Blue
Mountains of Oregon above a late Bajocian to early Bathonian unconformity (Pessagno and
Whalen, 1982).
In the Smith River section, 46m of tuffaceous pelagic strata overly basalt of the Josephine
ophiolite (fig. 7). The horizon 4.1 meters above the base of the basalt is no older than UAZ 3
(early to middle Bajocian) because it contains Acanthocircus suboblongus (UAZ 3-11); it is
constrained as no younger than UAZ 6 or 7 based on overlying assemblages. Praecaneta decora
(see above),is present in the horizon and in an underlying sample. In figure 7, we correlated the
base of the section with the approximate boundary between the Bajocian and Bathonian of the
Blue Mountains of Oregon.
The interval between 13 and 46 meters is assigned a range of UAZ 6-6/7. The maximum
range (UAZ 6, middle Bathonian) of the interval is based on the presence of Emiluvia hopsoni
(UAZ 6-15) in many samples from 13 meters to the top of the volcanopelagic section, and on the
presence of Spongocapsula palmerae (UAZ 6-13) at the ~21m horizon. Several other taxa in the
interval range no lower than UAZ 5 (latest Bajocian to early Bathonian): Ristola procera,
Mirifusus guadalupensis, Eucyrtidiellum ptyctum (sensu Baumgartner et al., 1995), and
Bernoullius cristatus.. The minimum age of the interval is constrained by several taxa in the
upper part of the volcanopelagic section which have their final occurrences in UAZ 7:
Tetraditryma praeplena, Bernoullius r. delnortensis, Linaresia beniderkoulensis, and Linaresia
spp. Xiphostylus spp. (UAZ 1-6) are common throughout the interval. However, we choose not
to rely completely on this genus to constrain the upper limit of the section because its upper
range in the eastern Pacific is questionable (discussion in Baumgartner et al., 1995, p. 1041). It is
present in the Bathonian but not in the Callovian of the Blue Mountains (Pessagno et al., 1989);
it is present in a single sample of the Stanley Mountain section (Hull, 1997) along with taxa
calibrated as UAZ 7 (late Bathonian to early Callovian); and it is present and highly discordant in
the uppermost sample of the Smith River section (see below). The UAZ ranges of the
pantanellids Pachyoncus (Pantanellium sp. L: UAZ 2-4) and Trillus (UAZ 1-5) are discordant;
however, in the Blue Mountains of Oregon, Pachyoncus occurs in both the Bajocian and upper
Bathonian (Pessagno and Blome, 1980). In summary, the interval between 13 and 46 meters is
illustrated in Fig. 7 as having a possible range of UAZ 6-6/7, and is calibrated as no older than
middle Bathonian and no younger than late Bathonian or early Callovian.
The radiolarian faunas in the upper half of the Smith River section are much lower in
diversity than those in the underlying pelagic section (Pessagno et al., 1993). They are dominated
by taxa whose ranges are not yet well-established elsewhere nor used in the UA Zonation of
Baumgartner et al. (1995b). The ranges that Baumgartner et al. (1995) assigned to this interval
(Fig. 7) are based on very few taxa, including Mirifusus d. dianae (UAZ 7-12), Mirifusus
guadalupensis (UAZ 5-11), and Podobursa spinosa (UAZ 8-12), so the results are imprecise.
The most tightly constrained part of the interval (UAZ 8-10) still has a possible range from
Callovian to Kimmeridgian. The reappearance of the genus Xiphostylus sp. (UAZ 1-6)(species
unidentified), in a single sample about 9.5m below the top of the Smith River section, is a highly
discordant occurrence which is not integrated into Figure 7. Either the upper UAZ range of the
genus needs to be revised (discussion in Baumgartner et al., 1995a, p. 1041) or it was reworked
from the underlying volcanopelagic section, the last reported occurrence being 51.7m below.
Pessagno et al. (1993, p. 111) reported that the “ upper 15.2m of the syntectonic flysch
succession contain common, small rip up clasts of light gray pelagic limestone.”
Age calibrations of Pessagno et al. (1993): Pessagno et al. (1993) described the radiolarians
in the sedimentary rocks overlying the Josephine ophiolite in great detail, but they did not use
them to calibrate the sections. They considered existing calibrations for Jurassic radiolarian
zonations of Europe and Japan to be erroneous or to reflect systematic range diachronism of
radiolarians or ammonites (Pessagno et al. (1993, p. 103-107).
Pessagno et al. (1993a, after Pessagno and Blome, 1990), used a radiometric date of ~162
Ma on a plagiogranite locality about 13 km from the Smith River section as a proxy for the age
of the basalt basement at the Smith River section in California and the Turner-Albright Mine
section in Oregon. This method provided a rough estimate for the maximum ages of the
sedimentary sections, but the cumulative uncertainties built into the analysis were and remain
very large. The actual zircon
206
Pb/238U and
207
Pb/235U dates are 162 Ma, with no uncertainty
quoted (revised from 157 ±2 Ma; Saleeby, 1987) and the 207Pb/206Pb date is 163 ±5 Ma (Harper et
al. 1994). Palfy et al. (2000, Appendix 1: Item 46) assigned the sample a crystallization age of
162 +7/-2 Ma, based on apparent lead loss and the absence of a duplicate concordant fraction.
Pessagno et al. (1993) considered 161 Ma to be the boundary between the Oxfordian and
Callovian. Accordingly, they calibrated basalt in both sections as late Callovian in age and
arbitrarily placed the boundary between the Callovian and Oxfordian within the lower 13 meters
of the pelagic section of the Smith River sequence. In this way, Pessagno et al. (1993) calibrated
the reference intervals for their new Zones 1H (Turner-Albright Mine section) and 1I (lower 13
m of the Smith River section). In the more recent time scale of Palfy et al. (2000), the Bathonian
ranges from 166.0 +3.8/-5.6 Ma to 160.4 +1.1/-0.5 Ma. Using this time scale and a
crystallization age of 162 +7/-2 Ma, one plagiogranite site within the Josephine ophiolite
crystallized sometime between the Bajocian and early Callovian. At best, the reanalyzed data
provide only a very indirect and crude constraint on the maximum ages of the sedimentary
sections but, interestingly, the results are compatible with the UAZ calibrations of in situ
radiolarian assemblages.
The upper half (56 meters) of the Smith River section is a transitional interval between
pelagic and hemipelagic strata below and thick bedded graywacke sandstone and conglomerate
facies above. The total thickness of the massive flysch sequence is more than 500m (Harper,
1994; Pinto-Auso and Harper, 1985). Pessagno et al. (1993, p. 116) suggested an interval of
metamorphism following deposition of the pelagic interval and preceding deposition of the
transitional interval. A few kilometers from the measured section, the flysch sequence contains
the middle Oxfordian to late Kimmeridgian bivalve Buchia concentrica (Pessagno et al., 1993
per Diller, 1907, Harper, 1983). Therefore, the upper part of the Smith River section is no
younger than B. concentrica but could be older, in part or whole. Pessagno et al. (1993) made the
interpretation that the upper part of the Smith River section lies entirely within the middle
Oxfordian. They made a lithologic correlation between the upper part of the Smith River section
and the lower part of the type Galice Formation, which contains a middle Oxfordian megafossil
assemblage of B. concentrica as well as the Oxfordian ammonite Dichotomosphinctes (Imlay,
1961, 1980). Pessagno and Blome (1990) did not identify any species-level radiolarians in the
type Galice, although they reported the presence of two long-ranging genera, Mirifusus and
Praeparvicingula (revised from Parvicingula by Pessagno et al., 1993). At the genus level,
neither is sufficient for inter-basin correlation. The type Galice lies in a separate fault-bounded
subterrane about 60 km to the north. The formation depositionally overlies a succession of calcalkaline volcanic and volcaniclastic rocks, the Rogue Formation, rather than tuffaceous pelagic
strata. Pessagno and Hull (2002, text-figure 9) illustrated a “probable hiatus and unconformity”
between the base of the type Galice and the underlying Rogue Formation. Pessagno et al..’s
(1993) correlation assumes the synchronous onset of syntectonic flysch deposition in the two
separate fault-bounded subterranes. The middle Oxfordian calibration of the upper part of the
Smith River section is a reasonable approximation but it implies a precision that does not exist
given the nature of clastic systems, the interbasin differences in pre-flysch stratigraphy, and the
inferred hiatuses or unconformities. Still, the calibration is compatible with the poorly
constrained Callovian to Kimmeridgian calibration obtained from the UA Zonation of
Baumgartner (1995a, see above) for part of the section. More recently, Pessagno and Hull (2002)
described two well-dated Oxfordian samples from the East Indies. Neither sample contains any
of the markers used in the zonation of Pessagno et al. (1993) but a few of the 50 species-level
taxa in the East Indian samples occur in the upper part of the Smith River section: Paronaella
cleopatraensis, Praeparvicingula hurdygurdyensis, and Praeparvicingula deadhorsensis. At
present, their full ranges are not well-calibrated; Paronaella deadhorsensis also occurs in the
Tithonian of Antarctica, for example (Kiessling, 1999).
Additional notes on correlations: Stonyford interval A3 correlates particularly well with the
Smith River section between 19 m and 22 m based on the presence in each of M. fragilis, M.
guadalupensis (transitional from M. fragilis), Eucyrtidiellum u. pustulatum, Hisocapsa convexa
gp., Acanthocircus suboblongus, Tetraditryma corralitosensis corralitosensis, Xiphostylus
gasquentensis gp, Praecaneta decora, Ristola procera, Levileugeo, and Paronaella bandyi
(Pessagno et al., 1993a and figure 6, this study). Stonyford intervals B3 and C and the upper part
of the Smith River sequence both contain M. guadalupensis, M. d. dianae (M. mediodilatatus in
Pessagno et al., 1993a) and abundant Praeparvicingula spp. The Diversion Dam fauna, in which
Pessagno (1977) noted M. d. baileyi, may be the same age or may be younger than the upper part
of the Smith River sequence which did not include M. d. baileyi among the few specimens of the
genus (1 or 2 each in two samples) reported from the latter (Pessagno et al., 1993a: Text-Figure
18).
Stanley Mountain ophiolite, southern Coast Ranges, California
Hull and Pessagno (1995) compared the calibrations of the Stanley Mountain section using
the Tethyan UA Zonation of Baumgartner et al. (1995) and the North American Zonation of
Pessagno et al. (1993). Based on the Tethyan UA Zonation, the lower part of the Stanley
Mountain composite section is Middle Jurassic in age and the upper part is Late Jurassic; based
on the zonation of Pessagno et al. (1993), the section is entirely Late Jurassic in age. The latter
calibration is a key element in the interpretation of Hopson et al. (2000) that a major hiatus
occurs at the base of the sedimentary sequence. In the following discussion and in figure 7, our
UAZ calibrations do not precisely correspond with the generalized ranges given by Hull and
Pessagno (1995), in part, because we incorporated some additional sample data.
The lower 28 meters of the Stanley Mountain composite section represents a substantial
accumulation of tuffaceous pelagic sediment. Eucyrtidiellum ptyctum (UAZ 5-11) in strata 3.8 m
above the base of the section (Pessagno et al. , 1984) limits the maximum possible range of the
section to UAZ 5, or no older than late Bajocian or early Bathonian. Mirifusus dianae dianae
(UAZ 7-12; senior synonym of M. mediodilatatus) occurs 21 m above the base of the Stanley
Mountain section (Pessagno et al., 1984). The only faunal assemblage in the lower 28 meters that
has been well-described lies far above the basalt basement at the 27.1 meter horizon (Hull, 1995,
1997, Hull and Pessagno, 1995). It is no younger than UAZ 7 as constrained by the ranges of
Palinandromeda depressa (UAZ 3-7), Ristola altissima major (UAZ 5-7), Stichocapsa decora
(UAZ 4-7), and Kilinora spiralis (UAZ 6-7),. In Figure 7, we have illustrated the presence of
range discordancies in the sample by showing its range as “UAZ 6/7-6/7”: the ranges of Unuma
echinatus (UAZ 1-6), Xiphostylus (UAZ 1-6), and Tricolocapsa plicarum ssp. A (UAZ 4-5,
sensu lato UAZ 3-8) are discordant and older than the ranges of co-occuring species Mirifusus d.
dianae (UAZ 7-12), Sethocapsa dorysphaeroides (UAZ 7-22), and Williriedellum carpathicum
(UAZ 7-11). UAZ 6 is calibrated as Bathonian; UAZ 7 as latest Bathonian to early Callovian. In
summary, the lower part of the Stanley Mountain composite section is no younger than early
Callovian as calibrated by the UA Zonation, and the lowest strata could be significantly older.
Pessagno et al. (1993) used the first occurrences of Mirifusus d. dianae (21 m above
basement) and E. ptyctum (3.8 m above basement) to redefine their Subzone 2, for which the
lower part of the Stanley Mountain section is a principal reference section. They calibrated the
evolutionary first occurrence of Mirifusus d. dianae as middle Oxfordian based on the species’
presence in a single sample of the Smith River section, 60.2 m above the basalt basement and
about 15 m above a possible hiatus preceding flysch deposition (Pessagno et al., 1993).
However, a single-sample range of a long-ranging taxon indicates local range truncation. No
compelling evidence indicates that the evolutionary first occurrence of M. dianae dianae is better
calibrated by the Smith River section than in the UA Zonation, where it is calibrated as latest
Bathonian or early Callovian. Pessagno et al. (1993) did not independently calibrate the first
occurrence of Eucyrtidiellum ptyctum (sensu Pessagno et al., 1993), which is not present in the
Smith River section; they estimated the event to be middle Oxfordian as well. In the UA
Zonation, this species’ (sensu Baumgartner et al., 1995) first occurrence is calibrated as latest
Bajocian or early Bathonian. (UAZ 5). In conclusion, Pessagno et al. (1993) did not make a
strong case that the base of the Stanley Mountain sedimentary section is middle Oxfordian
The calibrations of the upper part of the Stanley Mountain section are less disparate and,
therefore, less controversial. Hull (1997) assigned the interval between 28 and 62 meters above
the basalt basement to Subzone 2 of Pessagno et al. (1993), which is not independently
calibrated in a North American reference section. In the zonation of Baumgartner et al. (1995), a
sample from the 45.6 meter horizon has a possible range from UAZ 7 (late Bathonian to early
Callovian) to UAZ 8 (middle Callovian to early Oxfordian) based on the limiting ranges of
Mirifusus d. dianae (UAZ 7-12), Crucella theokaftensis (UAZ 7-11), Sethocapsa
dorysphaeroides (UAZ 7-22), Williriedellum carpathicum (UAZ 7-11), Mirifusus fragilis s.l.
(UAZ 3-8), and Tricolocapsa plicarum s.l. (UAZ 3-8). Another sample collected 59.8 meters
above basalt (Hull, 1997, p. 201) is assigned a possible range from UAZ 7 (late Bathonian to
early Callovian) to UAZ 10 (late Oxfordian to early Kimmeridgian) based on the constraining
ranges of Williriedellum carpathicum (UAZ 7-11), Paronaella bandyi (UAZ 3-10), and
Angulobracchia purisimaensis (UAZ 3-10).
Hull (1997) assigned the interval between 62 and about 80 meters to Zone 3 of Pessagno et
al. (1993). Zone 3 is middle Oxfordian to late Kimmeridgian according to the recalibrations of
Kiessling (1999). In the UA Zonation of Baumgartner et al. (1995), samples 62m and 75m above
basalt basement are no older than UAZ 9 (middle to late Oxfordian) based on the presence of
Mirifusus dianae baileyi (UAZ 9-11) and no younger than UAZ 10 (late Oxfordian to early
Kimmeridgian) based on the UAZ ranges of radiolarians in overlying strata.
Hull (1997) assigned the interval between 80 and 104 meters to Subzone 4 of Pessagno et
al. (1993). The Subzone 4 interval in the Stanley Mountain section is herein assigned a range of
UAZ 10-10 (late Oxfordian to early Kimmeridgian) based on the constraining ranges of
Acanthocircus trizonalis dicranacanthos (UAZ 10-17; 80 m), Acaeniotyle umbilicata (UAZ 1022; 90.5m), Tetraditryma corralitosensis corralitosenis (UAZ 3-10; 85.5m), Homoeoparonaella
(?) giganthea (UAZ 8-10; 90.5m), Homoeoparonaella elegans (UAZ 4-10; 90.5m), Paronaella
bandyi (UAZ 3-10; 90.5m), Paronaella mulleri (UAZ 6-10), Transhsuum maxwelli gp. (UAZ 310; 90.5m), Tritrabs casmaliaensis (UAZ 4-10; 99.1m), and Angulobracchia purisamaensis
(UAZ 3-10, 104m). The interval is secondarily constrained as no younger than UAZ 11(late
Kimmeridgian to early Tithonian) by the presence of the following taxa: Emiluvia orea orea,
Napora pyramidalis, Mirifusus dianae baileyi, Acanthocircus trizonalis trizonalis, Triactoma
blakei, Tritrabs exotica, and Perispyridium ordinarium gp.. Several taxa with discordant ranges
are also present in this interval: Gorgansium (UAZ 3-8), Hexastylus(?) tetradactylus (UAZ 1-4),
Sethocapsa(?) zweili (UAZ 14-19), Triactoma luciae (UAZ 13-21), and Acanthocircus carinatus
(UAZ 18-22). Pessagno et al. (1993) calibrated the base of Zone 4, Subzone 4, as late Tithonian
based on the apparent timing of four selected biostratigraphic events in subsiding basin
sequences in Mexico. Only one of the four events is calibrated in the UA Zonation: the first
occurrence of Acanthocircus t. dicranacanthos during UA Zone 10 (late Oxfordian to early
Kimmeridgian). More recently, Kiessling (1999) illustrated that the other three formal marker
events occurred no later than late Kimmeridgian in an ammonite-dated Antarctic section: the first
occurrences of Vallupus hopsoni, Hsuum mclaughlini, and Parvicingula colemani. Kiessling
(1999) recalibrated the range of Subzone 4 as late Kimmeridgian to early Tithonian.
The boundary between the tuffaceous pelagic section and the lowermost Great Valley
Supergroup lies about 130 meters above basalt basement. Tritrabs casmaliaensis (UAZ 4-10)
ranges at least as high as 125.2 meters above the base of the Stanley Mountain section. The
uppermost part of the tuffaceous pelagic section and the lowermost part of the clastic Great
Valley Supergroup are constrained as no younger than UAZ 12 (early to early late Tithonian)
based on the occurrences of the following taxa reported by Hull (1997, p. 202): Podobursa
spinellifera (125.2m), Ristola altissima (141.9m, 146.4m), and Loopus primitivus (145m). Hull
(1997) assigned this interval to Subzone 4 of Pessagno et al. (1993), which Kiessling (1999)
recalibrated as early to late Tithonian based on ranges in Antarctica.
Point Sal ophiolite, southern Coast Ranges
The UAZ ranges for the condensed, 21 m pelagic section at Point Sal are based on published
taxonomic lists (Baumgartner, 1995, Appendix: p. 1105-1106; Pessagno, 1977; Pessagno et al.,
1984) (figure 7). Eucyrtidiellum ptyctum (UAZ 5-11) occurs just above basalt basement and
constrains the maximum possible age of the section as no older than late Bajocian or early
Bathonian. Mirifusus d. dianae (M. mediodilatatus)(UAZ 7-12) occurs at 4.5m and further
constrains the maximum age of the lower part of the section as no older than late Bathonian or
early Callovian. A sample 11.5 meters above the basalt basement is herein assigned a range of
UAZ 8-8 (middle Callovian to early Oxfordian) based on the limiting ranges of Mirifusus fragilis
(UAZ 3-8), Parahsuum stanleyensis (UAZ 3-8), Monotrabs plenoides gp. (UAZ 5-8), Napora
lospensis (UAZ 8-13), Triactoma cornuta (UAZ 8-10), Podobursa spinosa (UAZ 8-13), and
Archaeodictyomitra apiarium (UAZ 8-22). The range of Deviatus diamphidius hipposidericus
(UAZ 9-13) is slightly discordant. This sample is a reference for Subzone 2 (undifferentiated)
of Pessagno et al. (1993), which was initially calibrated as late Kimmeridgian based, in large
part, on correlation with a sample from the base of the Great Valley sequence near Paskenta in
Northern California. The basis for the correlation, the first occurrence of Parvicingula in both
sections, was arguable owing to a major unconformity below the Great Valley sample. Subzone
2 was subsequently recalibrated by Kiessling (1999) as ranging from the early Oxfordian to
early middle Oxfordian. An overlying sample at the 13.4 m horizon is a primary reference for the
base of Subzone 3 of Pessagno et al. (1993) which is defined by the first occurrence of M. d.
baileyi. The sample is no older than UAZ 8 (middle Callovian to early Oxfordian) based on the
limiting ranges of Napora lospensis (UAZ 8-13), Emiluvia pessagnoi multipora (UAZ 8-14),
Podobursa spinosa (UAZ 8-13), and Archaeodictyomitra apiarum (UAZ 8-22). It is no younger
than UAZ 9 (middle to late Oxfordian) based on the limiting range of Ristola procera (UAZ 59). However, in Figure 7 we illustrate its range as UAZ 8/9-8/9 because the ranges of Turanta
flexa (holotype in this sample; UAZ 6-8), and M. d. baileyi (holotype in this sample; UAZ 911).are discordant. About 16m above the base, the radiolarians are assigned to UAZ 9-10 based
on the constraining ranges of M. d. baileyi (UAZ 9-11), Paronaella broennimanni (UAZ 4-10),
and Tritrabs casmaeliaensis (UAZ 4-10). This sample is a reference for the top of Subzone 3.
Pessagno et al. (1993) calibrated Subzone 3 as Tithonian; Kiessling (1999 recalibrated Subzone
3 as middle Oxfordian to late Kimmeridgian. The top of the section (about 19m above
basement) is assigned a range of UAZ 9-11 (Oxfordian to early late Tithonian) based on the
ranges of M. d. baileyi (UAZ 9-11) and Eucyrtidiellum ptyctum (UAZ 5-11).