radioisotopic and biostratigraphic age relations in the coast … - my...

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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 gab-bro, wehrlite-pyroxenite, quartz diorite), a mafi c to felsic dike complex, and mafi c-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 hori-zons 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 sedimen-tary strata range in age from Bathonian (Uni-tary 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 SFVC sedimentary record preserves evidence of a major faunal change wherein relatively small sized, polytaxic radiolarian faunas were replaced by very robust, oligo-taxic, nassellarian-dominated faunas that included Praeparvicingula spp.

We suggest that CRO formation began after the early Middle Jurassic (172–180 Ma) colli-sion of an exotic or fringing arc with North America and initiation of a new or reconfi g-ured 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 underly-ing Franciscan assemblage. We further sug-gest that the “classic” Nevadan orogeny repre-sents a response to spreading center collision, with shallow subduction of young lithosphere causing the initial compressional deforma-tion 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). None-theless, 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 Klem-perer, 1998; Schweickert, 1997; Schweickert et al., 1984); and (3) formation by forearc rifting above an east-dipping, proto–Franciscan subduc-tion 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;

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. MurcheyU.S. Geological Survey, 345 Middlefi eld Road, Menlo Park, California 94025, USA

David L. KimbroughDepartment of Geological Sciences, San Diego State University, San Diego, California 92182-1020, USA

Paul R. RenneBerkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709 and Department of Earth and Planetary Science, University of California, Berkeley, California 94720, USA

Barry HananDepartment of Geological Sciences, San Diego State University, San Diego, California 92182-1020, USA

GSA Bulletin; May/June 2005; v. 117; no. 5/6; p. 633–653; doi: 10.1130/B25443.1; 10 fi gures; 1 table; Data Repository item 2005079.

†E-mail: [email protected].

SHERVAIS et al.

634 Geological Society of America Bulletin, May/June 2005

Shervais, 1990; Shervais and Kimbrough, 1985), but other data, such as sedimentary cover associa-tions and seismic imaging of the continental mar-gin, 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 radiolar-ian 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 signifi cant problems with this suggestion: (1) Integration of the Jurassic time scale and standard ammo-nite zones is still in fl ux, 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 unal-tered volcanic glass from the Stonyford volcanic complex. We also present new biostratigraphic data for radiolarian cherts interbedded with vol-canic rocks of the Stonyford volcanic complex.

For purposes of comparison between radioiso-tope 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 fi eld relations, these data allow us to construct a synthesis for the origin and evolution of the ophiolite, its rela-tionship to high-grade metamorphism in the Franciscan assemblage, and the tectonic evolu-tion of the western Cordillera during the Middle and Late Jurassic.

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 fore-arc 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 Paleo-zoic 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 rep-resent 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 occur-rence of Buchia piochii throughout the section (Brown, 1964a, 1964b). Farther north, how-ever, the basal Great Valley Sequence contains Buchia rugosa and a few specimens of Buchia with very fi ne ribs, transitional between B. concentrica and B. rugosa (Imlay, 1980; Jones, 1975). Imlay (1980) dated the transitional inter-val as late Kimmeridgian to early Tithonian, though he favored the earlier range.

The Franciscan assemblage is a classic Meso-zoic and Cenozoic accretionary complex char-acterized by a heterogenous mixture of litholo-gies 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

(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

The Coast Range ophiolite in northern Cali-fornia is represented by a thin selvage of serpen-tinite matrix mélange along most of the bound-ary 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 inter-preted 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 modifi ed 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 bound-ary is a complex high-angle fault that may have components of strike-slip, normal faulting, and reverse faulting.

There are two large ophiolite remnants pre-served 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 tec-tonic association (Shervais et al., 2004).

Elder Creek Ophiolite

The Elder Creek ophiolite is one of the larg-est 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 progres-sively deeper levels of the ophiolite from south to north (Fig. 2). The Elder Creek ophiolite pre-serves most of the pseudostratigraphy associated

AGE RELATIONS IN THE COAST RANGE OPHIOLITE, CALIFORNIA

Geological Society of America Bulletin, May/June 2005 635

with “true” ophiolites: Cumulate ultramafi c 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 seafl oor and rede-posited 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 fi rst magma series is represented by dunite, layered or foliated cumulate gabbro, isotropic gabbro, and a dike complex. The sec-ond magmatic episode consists of wehrlite and clinopyroxenite intrusions into the older layered complex, with less common isotropic gabbro and gabbro pegmatoid. The third magmatic epi-sode comprises stocks and dikes of hornblende diorite and hornblende quartz diorite, with fel-site 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 fi rst three magma series in a suprasubduction zone (arc) environment; rare dikes of the fi nal magma series are characterized by MORB-like major and trace element compo-sitions (Shervais, 2001; Shervais and Beaman, 1991; Shervais et al., 2004).

Volcanic rocks are most commonly pre-served 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 mafi c and felsic volcanic rocks, gabbro, pyroxenite-wehrlite, and diorite. This unit was deposited on an eroded surface that cuts all other units of the ophiolite (from cumulate ultramafi cs through dike complex). Additional volcanic rocks crop out in fault-bounded blocks and in the dike com-plex. With the exception of the late, MORB-like dikes, all volcanic and hypabyssal rocks associ-ated 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 fi rst association appears to represent residual magma related to the fi rst or second magma series; the second association forms the bulk of the third, calc-alkaline 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,

38º N

36º N

40º N

124º 122º

40º N

38º N

36º N

Point Sal

Llanada

Cuesta Ridge

Quinto Creek

Mount Diablo

Sierra Azul

Geyser Peak / Black Mountain

Stonyford

Elder Creek

SAF

SNF

Modoc Plateau

Coast Range Ophiolite

Great Valley Sequence

Franciscan Complex

Sierra Nevada

Klamath terranes

Tertiary

Salinia

165-172 Ma U/Pb New

165-173 Ma U/Pb MH92

166+/-2Ma U/Pb P93

164 Ma U/Pb HMP81

163+/-5 Ma K/Ar Lan71

156+/-2 Ma U/Pb HMP81

166-172 Ma U/Pb New164 1Ma Ar/Ar New

Stanley Mtn

153 Ma U/Pb HMP81

Leona Rhyolite

Del Puerto

165 Ma U/Pb Man91

124º W 120º W122º 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.

SHERVAIS et al.


in the low-lying hills surrounding the com-munity 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 serpenti-nite 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 fl ows, diabase, and hyaloclastite breccia. The volcanic rocks are exceptionally fresh for the Coast Range ophiolite, as shown by the preservation of pri-mary 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 fi re fountain depos-its, 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 Pacifi c oceanic basalts currently found in off-axis sea-mounts 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 hetero-geneous mantle source with at least two compo-nents: 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 infl uence, which overprints generally depleted trace element characteristics (Hanan et al., 1992; Shervais et al., 2004; Zoglman and Shervais, 1991).

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 inter-bedded 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 well-preserved 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

Wacke and siltstone

Crowfoot Point Breccia

Volcanic rocksElder Creek Ophiolite

Dike complex

Felsic dikes

Quartz diorite & diorite

Isotropic gabbro

Cumulate gabbro

Wehrlite-PyroxeniteDunite & dunite broken formation

Sheared serpentinite

Volcanic blocks in melange

Foliated metasediments(Galice?)

South Fork Mountain schist

Franciscan Assemblage

Serpentinite mélange

Great Valley Series

vc

GVS

North

0 1 2miles

wc

wc

wc

wc

wc

wc

wc

wc

GVS

GVS

GVS

GVS

fmg

ig

du

dufdcvc

vc

fdc

du

du

fdc

vc

gs

gs

fmg

du

fdc

vc

gs

gs

gs

qdi

cg

qdi

cpb

dc

sfms

cg

cg

igqdi

cpb

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ig

dc

qdicg

sfms

ig

qdissp

cg

ssp

ssp qdi

cpbdc

cpb

ig

sfms

ssp

cpb

ssp

cg

EC148-2

EC107-3

Figure 2. Geologic sketch map of Elder Creek ophiolite showing sample locations collected and dated for this study (small stars).



largest block of SFVC radiolarian chert is over-lain 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 dio-rite occurs as individual blocks ranging in size from a few meters to several tens of meters and as dikes within mélange blocks of isotropic gab-bro. Other tectonic blocks within the mélange include unmetamorphosed volcanogenic sand-stones, 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

STONYFORD VOLCANIC COMPLEXSFV: Pillow lavas & massive basaltsVolcaniclastic/hyaloclasticbrecciasChert, limestone, & siliceous mudstone

GVS: Interbedded sandstone, siltstone, and shale

Volcanogenic sandstone

Qtz-lawsonite-mica schist: metagreywacke, metasiltstone, metachertMetavolcanic

SSP: Sheared serpentiniteHarz: Massive harzburgite:partially serpentinized, shearedSchist blocks in serpentinite

Metavolcanic blocksHigh-grade blocks

Meta-plutonic rocks blocks (wehrlite, pyroxenite, gabbro, diorite)

Qal: AlluviumConglomerate

GREAT VALLEY SEQUENCE

FRANCISCAN COMPLEX (JKf)

SERPENTINITE MATRIX MELANGE

311 MILS

6 MILS

0 17'

17 1/2

GNMN

Quaternary landslide

QUATERNARY

Qal

0 1 2 km

0 7000 feet

0 1 2 miles

Harzburgite

SFVC

JKf

GVS

SSP

GVS

JKf

SSPHarz

Glass 5, 8

Glass 1-4

Chert B

Chert C

Chert A

122º 40' W39º 20' N

122º 40' W39º 25' N

122º 32' 30" W39º 25' N

122º 32' 30" W39º 20' N

Stony Creek

Stonyford

Legend

Geologic map of the Stonyford Volcanic Complex, California.Mapping by John W. Shervais and Marchell Z. Schuman

SFVC

SFV109-2

SFV141-1

SFV58-1

Chert D

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.

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 assem-blage consists largely of mélange with a per-vasively sheared matrix of mudstone and grey-wacke 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 refl ect polymetamorphism with initial high temperatures and low pressures followed by lower temperature and higher pressure meta-morphism (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 (McDow-ell 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

some southern CRO locations to 165–170 Ma, based on new data obtained with a modern mul-ticollector 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 sup-porting data, so it is not possible to evaluate their precision or accuracy.

Radiolarian biostratigraphic data from bed-ded chert at Elder Creek and Stonyford have been reported by Pessagno and Louvion-Trellu (Hopson et al., 1981; Louvion-Trellu, 1986; Pes-sagno, 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 deposi-tionally on basalt. Hopson et al. (1981) assigned radiolarians from the chert blocks to the upper part of 1977 Zone 1 (undifferentiated) of Pessa-gno, 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

U-Pb Zircon

Zircon was separated by conventional tech-niques using a Wilfl ey 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

contaminating grains. Zircon dissolution and ion exchange chemistry for separation of ura-nium and lead followed procedures modifi ed from Krogh (1973). Isotope ratios were mea-sured with the MAT 261 multicollector instru-ment 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 refl ect the low metamorphic grade of the samples and negligible or absent inherited components of radiogenic lead.

40Ar/ 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 refl ectiv-ity, 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 fl uence monitor Fish Can-yon sanidine (28.02 Ma; Renne et al., 1998). Only G-2 glass (laboratory numbers 3524–1, −3, and −4) was irradiated in the fi rst 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 auto-mated facilities and procedures described by Renne (1995). Data are presented in Data Repository document DR-1.1

Biostratigraphic

Radiolarian-bearing chert and siliceous mud-stone 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 hydrofl uoric acid (10%

1GSA Data Repository item 2005079, Ar release spectra for volcanic glasses of the Stonyford vol-canic complex technical notes on biostratigraphic calibrations and correlations, is available on the Web at http://www.geosociety.org/pubs/ft2005.htm. Re-quests may also be sent to [email protected].



of ~50% concentrate), commonly for ~24 h, fol-lowing procedures modifi ed from Dumi trica (1970) and Pessagno and Newport (1972). Then the fossils were washed off the etched rock surfaces and collected on Tyler-equivalent 250-mesh (63 µm openings) and 80-mesh (180 µm openings) screens. The fossils were identifi ed based on examination with a binocular micro-scope. 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 cal-ibration. The formal zonation uses a relatively small number of biostratigraphic events, the fi rst 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.

TABLE 1. COAST RANGE OPHIOLITE: U-PB ZIRCON ISOTOPIC DATA FOR ELDER CREEK AND STONYFORD

Fraction Weight Pb U Lead isotopic compositions Radiogenic ratios Apparent ages (Ma)(g) (ppm) (ppm) 206/208 206/207 206/204 207*/235 %err 206*/238 %err 207*/206* %err 206*/238 207*/235 207*/206* ±

Elder Ck EC-107–3

>200 0.0031 3.91 132.5 4.276 15.250 912 0.1770 0.39 0.0260 0.29 0.04945 0.26 165.2 165.5 169 6.1<100L 0.0007 3.78 129.9 4.749 17.656 2034 0.1836 0.33 0.0270 0.30 0.04940 0.13 171.5 171.2 167 3.0<200L 0.0017 4.31 149.2 4.853 19.313 6199 0.1825 0.32 0.0268 0.28 0.04940 0.16 170.4 170.4 167 3.6<100>200 0.0051 4.58 159.7 4.646 19.513 8583 0.1796 0.31 0.0263 0.29 0.04953 0.10 167.4 167.4 173 2.2<200 0.0038 5.48 192.0 4.595 19.533 9553 0.1790 0.31 0.0261 0.30 0.04965 0.06 166.4 166.4 178 1.4bulk L 0.0052 3.83 139.8 5.163 19.675 11076 0.1749 0.31 0.0256 0.28 0.04950 0.14 163.1 163.1 172 3.4

Brush Mtn EC-148–2B

<100>200 0.0045 4.69 162.1 4.407 19.310 6340 0.1793 0.30 0.0263 0.28 0.04947 0.08 167.2 167.4 170 1.6bulk 0.0029 3.41 123.8 4.562 19.565 8447 0.1721 0.30 0.0253 0.29 0.04937 0.09 161.0 161.2 165 1.7<200 0.0040 5.75 199.6 4.472 19.581 9672 0.1793 0.30 0.0262 0.28 0.04955 0.10 167.0 167.4 174 2.1>100 0.0018 18.57 637.2 4.348 18.820 4783 0.1815 0.30 0.0263 0.28 0.05006 0.10 167.4 169.4 198 2.2<325L 0.0007 7.00 242.7 4.338 17.787 2111 0.1772 0.38 0.0261 0.29 0.04925 0.24 166.0 165.6 160 5.4<200L 0.0025 5.26 183.6 4.553 19.674 10687 0.1789 0.30 0.0262 0.28 0.04945 0.09 167.0 167.1 169 1.9>100 0.0021 4.94 168.0 4.118 19.791 14094 0.1808 0.29 0.0265 0.28 0.04948 0.06 168.6 168.7 171 1.4>200L 0.0017 4.86 163.7 4.427 19.671 10744 0.1844 0.30 0.0270 0.28 0.04947 0.07 171.9 171.8 170 1.5

Brush Mtn EC-148–2A

bulk L 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

Auk Auk SFV-109–2

<200L 0.0036 4.89 192.3 3.112 19.777 11414 0.1463 0.33 0.0215 0.28 0.04928 0.18 137.3 137.3 161 4.1<325 0.0045 4.84 158.7 2.955 17.986 2381 0.1723 0.33 0.0253 0.28 0.04942 0.18 160.9 160.9 168 4.0>200Fm 0.0012 7.52 247.9 3.083 18.715 3570 0.1739 0.42 0.0256 0.29 0.04931 0.32 162.8 162.8 163 7.3<200Fm 0.0025 8.13 269.6 2.841 18.856 4111 0.1695 0.30 0.0249 0.28 0.04946 0.11 158.3 159.0 169 2.6>200L 0.0016 6.38 211.3 3.219 19.237 5542 0.1752 0.31 0.0258 0.28 0.04933 0.15 164.0 164.0 163 3.3bulk Fm 0.0041 7.12 230.7 2.911 18.937 4387 0.1746 0.30 0.0256 0.28 0.04945 0.10 163.0 163.4 169 2.2L 0.0007 6.17 208.7 3.299 18.436 3121 0.1736 0.30 0.0254 0.28 0.04953 0.09 161.8 162.5 173 2.0

Dry Creek SFV-141

bulk L 0.0007 7.62 222.8 2.094 18.437 3957 0.1813 0.31 0.0260 0.28 0.05052 0.12 165.7 169.2 219 2.7bulk 0.0005 11.77 330.4 1.864 19.454 1319 0.1759 0.32 0.0257 0.28 0.04963 0.16 163.6 164.5 177 3.5

Dry Creek SFV-58–2

<200L 0.0018 2.50 96.3 7.248 19.438 6954 0.1734 0.35 0.0255 0.34 0.04933 0.09 162.3 162.3 163 2.0<325L 0.0023 4.81 183.1 6.935 19.643 9664 0.1745 0.30 0.0256 0.29 0.04939 0.07 163.1 163.3 166 1.5<200 0.0029 4.91 188.6 6.857 19.328 6234 0.1722 0.30 0.0253 0.29 0.04938 0.07 161.0 161.3 166 1.6L 0.0018 4.61 180.6 7.540 19.234 5898 0.1713 0.30 0.0251 0.28 0.04950 0.07 159.8 160.5 171 1.5

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.

SHERVAIS et al.


RESULTS

U-Pb Zircon Ages

Elder CreekZircon 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 rep-resent 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 discor-dant, with 238U/206Pb ages ranging from 161 to 172 Ma and 207Pb/206Pb ages ranging from 165 to 198 Ma. Concordia plots suggest crystalliza-tion 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 signifi cantly 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, suggest-ing inheritance of an older zircon component, perhaps from a continental crustal source (cf. Wright and Wyld, 1986).

StonyfordZircon 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-cm-thick quartz diorite dike that crosscuts an isotro-pic 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 discor-dant 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

to the north and to the 40Ar/39Ar ages obtained on the samples of volcanic glass from the Stony-ford volcanic complex (see below). Two zircon fractions from SFV-141 are discordant and have 207Pb/206Pb ages of 177 ± 3.5 and 219 ± 2.7 Ma, suggesting inheritance of an older zircon compo-nent, perhaps from a continental crustal source (cf. Wright and Wyld, 1986). Similar results were obtained by Bickford and Day (2004), who identifi ed the presence of ca. 2153 ± 1 Ma inherited zircon in plutons of the 164–160 Ma Smartville ophiolite.

40Ar/ 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-preci-sion 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 mark-edly 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 refl ects the age of an outgassing event that completely rejuvenated the K-Ar system in all the samples. While such a sce-nario cannot be completely excluded, we inter-pret the 40Ar/39Ar ages to refl ect eruption ages in view of their within- and between-sample repro-ducibility 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 locali-ties 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 hyaloclas-tic 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 ultra-sincerum (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 offi cerense 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 confi rms 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 Pacifi c 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 prob-ably not older than Bathonian. The B1 fauna is younger than or correlative with the faunas at locality A, and the small, polytaxic assemblages



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SHERVAIS et al.


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 transi-tional 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-domi-nated 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 fi rst appearance of M. dianae dianae (worldwide range is UAZ 7–12), but it was fol-lowed 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) col-lected 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 sanfi lippoae (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 identifi cations 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 Juras-sic, Bathonian, and continued into the early Late Jurassic, Oxfordian or early Kimmeridg-ian. Between eruptions, siliceous radiolarian-rich strata accumulated slowly on basalt base-ment. Volcanic glass within the sequence of radiolarites and basalt yields dates of 164 Ma, as discussed previously.

DISCUSSION

Regional Biostratigraphic Correlations

The biostratigraphic succession in the Stonyford volcanic complex has parallels in sedimentary sequences overlying Jurassic ophi-olites 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

120

140

160

180

170

150

130

120

140

160

180

170

150

130

Cumulative Fraction 39Ar Released

120

140

160

180

170

150

130

)aM( eg

A tnerappA

120

140

160

180

170

150

130

120

140

160

180

170

150

130

0 0.5 1.0 0 0.5 1.0

SFVG-2-1 SFVG-2-2

SFVG-2-3 SFVG-2-4

SFVG-4-1 SFVG-4-2

SFVG-5-1 SFVG-5-2

SFVG-8-1 SFVG-8-2

Figure 5. 40Ar/39Ar apparent age spectra for glass samples from the Stonyford volcanic complex.



(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. Radiolar-ians 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 sec-tion 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 radiolar-ian 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

UALOCALITY TAXA ZONES ZONE

BIOSTRATIGRAPHY OF THE STONYFORD VOLCANIC COMPLEX

1

RADIOLARIAN FOSSIL DATA Calibrations based on Tethyan Unitary Association Zones (UAZ) of Baumgartner et al., 1995.

nainelaA

cissaruJ elddiM

Central Block Northern Block

Other

CALIBRATION

AGE

MAXIMUM & MINIMUM UAZ RANGES

A2

B2

B1

A3

DIVERSION DAM

C

B1

A1

9-10

8-10

7-10

7-8

3-8

6-6

6-6

3-6

B3

B2 B3 DAMA1 A2 A3 C

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1 2

1 1

1 0

9

8

4

5

6

7

2

3

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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.

Mirifusus d. baileyi, Tritrabs hayi, Pseudocrucella sanfilippoae, Archaeodictyomitra rigida, Transhsuum maxwelli, Parvicingula (undifferentiated, sensu 1977), Pantanellium riedeli. From Pessagno (1977).

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

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.

Mirifusus fragilis, Hsuum brevicostatum gp., Acanthocircus suboblongus, Podobursa helvetica, Archaeodictyomitra sp. cf. A. suzukii, Turanta sp., Saitoium sp.

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, Eucyrtidiellum u. pustulatum, Hisocapsa convexa gp., Hsuum brevicostatum gp., Acanthocircus suboblongus, Ristola procera, Paronaella bandyi, Xiphostylus gasquetensis gp., Leugeo hexacubicus.

Mirifusus fragilis, Praecaneta decora, Praecaneta turpicula, Guexella nudata, Parvicingula (?) dhimenaensis, Spongocapsula palmerae, Podobursa helvetica, Pantanellium ultrasincerum/ P. foveatum.

Mirifusus fragilis, Hisocapsa convexa gp., Hsuum brevicostatum gp., Acanthocircus suboblongus, Pantanellium ultrasincerum/ P. foveatum, Saitoium sp.

D

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.

SHERVAIS et al.


UAZ

Age

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me

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le, M

a1

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ian

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ount

ains

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2000

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IC C

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b

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corr

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bas

ed o

n U

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f Bau

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ian

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es.

Bau

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l.,

19

95

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el ac S o N

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The

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R

ange

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Fig

ure

7. R

egio

nal

bios

trat

igra

phic

cor

rela

tion

s of

key

sec

tion

s in

Cal

ifor

nia

and

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usi

ng r

adio

lari

ans.

The

cor

rela

tion

s ar

e ba

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on t

he T

ethy

an U

A Z

onat

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(Bau

mga

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r et

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5b)

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ose

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usus

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. For

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al, U

A

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sign

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ts a

re s

how

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(e.

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AZ

7–8

), s

how

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max

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and

min

imum

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. The

foss

il-ba

sed

age

calib

rati

ons

for

the

UA

Zon

es a

re il

lust

rate

d on

the

rig

ht s

ide

of t

he fi

gure

. The

tim

e sc

ale

of P

alfy

et

al. (

2000

), w

hich

is n

ot d

irec

tly

linke

d to

the

UA

cal

ibra

tion

s, is

sho

wn

for

refe

renc

e pu

rpos

es.



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 Pes-sagno 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 inter-val 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 Super-group are constrained as no younger than UAZ 12 (early to early late Tithonian). The lower part of the Great Valley in this geographic area con-tains the late Tithonian ammonite Micracantho-ceras and bivalve Buchia piochii (Hull, 1997).

Hull and Pessagno (1995) illustrated sig-nifi cant 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 calibra-tion is not tightly constrained (see Data Reposi-tory document DR-2 discussion). Pessagno et al. (1993, p. 113) arbitrarily assigned all the underlying pelagic strata in the Stanley Moun-tain section to the middle Oxfordian as well, which led them to conclude that a proposed hiatus between the base of the sedimentary sec-tion and underlying basalt spanned 8–11 m.y. (Dickinson et al., 1996; Pessagno et al., 2000). In contrast, the UAZ calibrations shrink the pos-sible 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 well-described 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 Stony-ford 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 begin-ning of this event at Stanley Mountain.

Point Sal Ophiolite, Southern Coast RangesAt Point Sal, along the southern California

coast, a 21 m condensed sequence of tuffa-ceous 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 sec-tion 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 Callo-vian 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 correla-tive 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. mediodila-tatus), 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 Praeparvic-ingula 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 West-ern Klamath Mountains of Oregon and Califor-nia formed at approximately the same time as, or slightly later than, the ophiolites of the Cali-fornia 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 40Ar/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 deforma-tion, 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 fi ne-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 interbed-ded 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 Bajo-cian) (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 inter-val between 13 and 46 m is herein assigned a possible range of UAZ 6–6/7 (middle Batho-nian to early Callovian). These direct, radiolar-ian-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 radio-larians 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

SHERVAIS et al.


River sequence. Data Repository document DR-2 contains an expanded discussion of the correlations. As previously noted in the descrip-tion 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 infl ux of the nassellarian Praeparvicingula.

Signifi cance of Biostratigraphic Correlations

Our recorrelation of fi ve key Jurassic sec-tions in California and Oregon (Fig. 7) reveals a common history of concurrent late Middle Jurassic pelagic or hemipelagic and volcaniclas-tic sedimentation and parallel patterns of faunal turnover that began slightly earlier in the north-ern 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-fl oored 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 fi rst based on a proposed multi-million-year hiatus following the cre-ation of the ophiolites and the second based on proposed syntectonic northeastward transport across hundreds of kilometers of the eastern Pacifi c 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 accumu-lated above the ophiolites for 8–10 m.y. until plate motions carried the sites into the deposi-tional 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 elim-inate 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 rap-idly northward relative to North America. The

faunal turnovers within the sections were used as evidence for northward-directed changes in paleo latitude, 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 fi ve 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 fi rst 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 rela-tive 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 Pacifi c 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 fi ve 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 cli-mate, 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 bound-ary 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 syndeposi-tional faunal changes, may be grossly correct if the fi ve 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 (Pes-sagno 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) prob-ably 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.

Signifi cance of Radioisotopic and Biostratigraphic Ages, Stonyford Volcanic Complex

40Ar/39Ar plateau ages for basalt glasses from four distinct localities within the Stonyford vol-canic 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 else-where (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 signifi -cant 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



indistinguishable from all but the oldest of the above mentioned U/Pb-based ages.

The Stonyford data and the inferred struc-tural 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 crystal-lized 165–170 Ma—consistent with the results presented here.

The age relationships presented here are con-sistent 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 Stony-ford 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 rang-ing from 0.34 to 1.78). High-Al, low-Ti basalts are found interbedded with ocean island tholei-ites 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 high-Al, 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; Sher-vais 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 locali-ties 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 unspecifi ed, very poorly preserved radiolarians associated with stage 3 volcanic rocks in the Diablo Range with the Diversion

Dam fauna in the SFVC, leaving open the pos-sibility 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 mud-stones deposited within the Stonyford volca-nic 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 com-plex and the Coast Range ophio lite 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 sub-strate 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 incorpo-rated into the Franciscan and Klamath Moun-tains 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 fi rst to show that orogenesis in the Jurassic was not confi ned 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

0

50

100

150

200

250

300

050100150200

After Murchey and Blake (1993)

Spreading AgeResidence Time

Res

iden

ce T

ime/

Rid

ge A

ge

Arrival time (Ma)

Rattlesnake Creek

Elder Creek, SFVC, W. Klamath

Van Ardsdale,Yolla Bolly

Pickett Peak

Marin Headlands,The Geyers

Burnt Hills,Permanente

Coastal Belt

King Range

Ridge Collision ≈155 Ma

Ridge?

Ridge

North Fork, East Hayfork

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.

SHERVAIS et al.


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 deforma-tion (post-169, pre-164 Ma) was coincident with

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 docu-mented 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, Emi-grant 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,

140 Ma

150 Ma

160 Ma

170 Ma

180 Ma

190 Ma

200 Ma

Tithonian

Kimmeridgian

OxfordianCallovian

Bathonian

Bajocian

Aalenian

Berriasian

Toarcian

Pliensbachian

Sinemurian

150.5 (+3.4/-2.8)

154.7 (+3.8/-3.3)

156.5 (+3.1/-5.1)

160.4 (+1.1/-0.5)

166.0 (+3.8/-5.6)

174.0 (+1.2/-7.9)

178.0 (+1.0/-1.5)

183.6 (+1.7/-1.1)

191.5 (+1.9/-4.7)

Low

er J

uras

sic

Mid

dle

Jura

ssic

Upp

er J

uras

sic 141.8 (+2.5/-1.8)

196.5 (+1.7/-5.7)

Coast Range Ophiolite

Sierra NevadaFoothills

Stonyford Glass

Elder CreekOphiolite

Stonyford MélangeBlocks

Sinistral Ductile Deformation 145-123 Ma

Foothills Event 174-185 MaCompressive, Arc Collision (?)

West East

JKf High Grade Blocks

HCPEGCScPStP

HCP = Haypress Creek Pluton 166 MaEGC = Emigrant Gap Complex 168 Ma

StP = Standard Pluton 166 MaScP = Scales Pluton 168 MaPost Deformational

Plutons

YRP

YRP = Yuba Rivers Pluton 159 Ma

Smartville Ophiolite

TuttleLake Fm

Slate Creek Complex/ Smartville Wallrock

Fiddle Creek Complex

Owens MtnDike Swarm BMFZ

-MFZ

BMFZ-MFZ = Ductile Deformation along Bear Mtn/Melones Fault Zones

Time-scale correlation from Palfy et al., 2000

Nevadan Event

Sailor Canyon Fm

PeñonBlanco

Logtown Ridge

CalaverasComplex

"Ove

rlap"

"Late Nevadan"

UC

IndependenceDike Swarm

exotic arc terranes?

Hettangien

Norian199.6 (±0.4)

Tria

ssic

BRT

BRT = Bloody Run Tonalite 165 Ma

Foot

hills

Sut

ure

Zone

?

Figure 9. Timing of igneous and metamorphic events in the Coast Ranges and Sierra Nevada foothills during the Jurassic to earliest Cre-taceous. 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).



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 over-printed 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 (Bick-ford 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 ter-ranes 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 identifi ed in the Smartville plu-tons was derived from the underlying Shoo Fly complex, and that both the Smartville ophiolite and the older Slate Creek complex formed proxi-mal to the continental margin.

Penon Blanco/ Slate Creek Arc

Sierran Arc

or ???

Rapid Formation of CRO as Oceanic Slab Sinks:

174-164 Ma

Early Mid-Jurassic Collision (?): 176-180 Ma

Ridge Collision/Subduction: 164-160 Ma

Stonyford Volcanics; MORB Dikes at Elder Creek, Mt. Diablo, Del Puerto, Sierra Azul, Cuesta Ridge

Post-collision Plutons: ≈160 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

Late Triassic to Early Jurassic: 206-180 Ma

Mid-Jurassic:180-164 Ma

Late Mid-Jurassic:163-160 Ma

Late Jurassic:159-150 Ma

A.

B.

C.

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 mar-gin; followed by formation of CRO during Bajocian-Bathonian (165–172+) by hinge roll-back 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 Foot-hills, continued chert deposition in CRO, and fi nally by Kimmeridgian transition to ductile, sinistral shear deformation in Sierra foothills, deposition of GVS on top of CRO.

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 Fran-ciscan 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 colli-sion 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 signifi cant thermal event that requires high thermal fl ux 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 Mari-posa Formation, a probable syntectonic fl ysch 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 col-lision, as proposed previously.

The main phase of Late Jurassic contrac-tional 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 oce-anic 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,

or transtension). This is essentially the same conclusion reached by Murchey and Blake (1992; 1993). Cloos (1993, page 733) summa-rized his fi ndings 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 loca-tion 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 signifi cantly 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 Pacifi c basin in the Middle Jurassic to left lateral transpression (in the upper plate) dur-ing 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 orog-eny 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 Foot-hills terrane during the latest Jurassic and Early Cretaceous (that is, post-Nevadan) is consis-tent with the onset of left lateral strike-slip motion within the arc terrane predicted by the ridge collision model. Prior to this transpres-sional 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 Sier-ras (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 ophio-lite 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 forma tion began in response to this collision during the establishment of a nascent, east-dipping, proto–Franciscan sub-duction 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-Creta-ceous; Murchey and Blake, 1992, 1993). Taken together with fi eld and geochemical observa-tions 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 mud-stones intercalated within the Stonyford volca-nic complex are correlative with those found in cherts that overlie the Coast Range ophiolite in the southern Coast Ranges. The radiolar-ians 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 hyalo-clastite breccias containing volcanic glass that we have dated at ≈164 Ma, using high-preci-sion 40Ar/39Ar laser-heating methods applied to carefully selected samples. These data show that, contrary to the interpretations of Pessa-gno 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



in the open ocean far from its current loca-tion. 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, refl ect 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 Juras-sic 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 duc-tile 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 ophiol-ite formation published by Shervais (2001). This model proposes that suprasubduction zone ophio-lites form in response to hinge retreat in nascent or newly reorganized subduction zones, and that they display a consistent life cycle as the subduc-tion zone matures. The fi nal stage of ophiolite for-mation 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 tec-tonic 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 signifi cance 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, On-land 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 Baumgart-ner, 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 Creta-ceous 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 Creta-ceous Radiolaria of Tethys: Occurrences, Systematics, Biochronology: Lausanne, Switzerland, Memoires de Geologie (Lausanne): p. 1013–1048.

Baumgartner, P.O., O’Dogherty, L., Gorican, S., Dumitrica-Jud, 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, Radiolar-ian 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 Cre-taceous 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 Caro-lina, 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 Bul-letin, 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, north-ern 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.: Engle-wood 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; Sedimenta-tion and tectonics on modern and ancient active plate margins: Geological Society of London Special Publi-cation 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 fi eld 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 Cali-fornia, in Ernst, W.G., ed., Metamorphism and crustal evolution of the Western United States: Rubey Vol-ume 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: Washing-ton, D.C., American Geological Institute, p. 3–18.

Blome, C.D., 1984, Middle Jurassic (Callovian) Radiolaria from southern Alaska and eastern Oregon: Micropale-ontology, 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 north-ern 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 mecha-nisms, 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 oro-genesis; 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 meta-morphic 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: Engle-wood 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 Inter-vals 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 Nas-sellaria 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: Geologi-cal 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., Mur-phy, 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, Rel-ative motions between oceanic and continental plates in the Pacifi c basin: Geological Society of America Special Paper 206, 59 p.

Ernst, W.G., 1970, Tectonic contact between the francis-can 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; Impli-cations for the timing and controls of Jurassic orogen-esis, northern Sierra Nevada, California, in Miller, D.M., and Busby, C., eds., Jurassic magmatism and

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: Tec-tonics, 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: 10.1130/0016-7606(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 Nico-las 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: Tec-tonics, 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 impli-cations 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 unroofi ng 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, Forma-tion and emplacement of the Josephine Ophiolite and the Nevadan Orogeny in the Klamath Mountains, California-Oregon; U/Pb zircon and 40Ar/ 39Ar geo-chronology: 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 Cali-fornia, 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 tec-tonic 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; Paleo-equatorial ocean-ridge lithosphere: AAPG Bulletin, v. 81, no. 4, p. 687.

Hull, D.M., 1995, Morphologic diversity and paleogeo-graphic signifi cance 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 Juras-sic 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, Uni-versité 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

of volcanopelagic strata above the Coast Range Ophiol-ite, in Dunne, G.C., and McDougall, K., eds., Mesozoic paleogeography of the Western United States; II. Field Trip Guidebook—Pacifi c Section, SEPM: Los Angeles, California, Society of Economic Paleontologists and Mineralogists, Pacifi c Section, p. 157–169.

Imlay, R.W., 1980, Jurassic paleobiogeography of the con-terminous 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 Califor-nia: 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 Califor-nia, in Wagner, D.L., and Graham, S., eds., Geologic fi eld trips in Northern California; Centennial meeting of the Cordilleran Section of the Geological Society of America: Special Publication 119: Sacramento, Cali-fornia, State of California, Department of Conserva-tion, 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 exten-sional faulting, and nature of the Coast Range “thrust,” California: Tectonics, v. 6, p. 475–488.

Jones, D.L., 1975, Discovery of Buchia rugosa of Kimmer-idgian age from the base of the Great Valley sequence: Geological Society of America, Abstracts with Pro-grams, v. 7, no. 3, p. 330.

Kiessling, W., 1999, Late Jurassic radiolarians from the Antarc-tic Peninsula: Micropaleontology, v. 45, no. 1, p. 1–96.

Kimbrough, D.L., and Moore, T.E., 2003, Ophiolite and vol-canic 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 hydro-thermal 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 concor-dia-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 com-plex; 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 Geol-ogy, 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: Tecto-nophysics, v. 198, no. 1, p. 1–21, doi: 10.1016/0040-1951(91)90127-E.

Mattinson, J.M., 1986, Geochronology of high-pres-sure–low-temperature Franciscan metabasite; A new approach using the U-Pb system, in Evans, B.W., and Brown, E.H., eds., Blueschists and eclogites: Geologi-cal Society of America Memoir 164, p. 95–105.

Mattinson, J.M., 1988, Constraints on the timing of Francis-can 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, Pren-tice-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 left-oblique 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 ophiol-ites of the Northern California Coast Ranges; Isotopic ages and their tectonic implications: Geological Soci-ety 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 geochronol-ogy 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 blue-schist exotic block from the Franciscan Complex, Califor-nia: 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 Geol-ogy, v. 7, no. 2, p. 211–228.

Murchey, B.L., and Baumgartner, P.O., 1995, Biostratigra-phy 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., Gori-can, 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 (Lau-sanne), p. 997–1010.

Murchey, B.L., and Blake, M.C., Jr., 1992, Postulated ridge subduction in California during the Late Jurassic; Uni-fying 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 sub-duction 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, Pacifi c 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 impli-cations for its paleogeographic origins; Reply: Geolog-ical 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 40Ar/ 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 affi ni-ties 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. Geologi-cal 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



western North America: Bulletin of American Paleon-tology, 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 Spe-cial 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: Micro-paleontology, 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 signifi cance 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: Boul-der, Colorado, Geological Society of America.

Platt, J.P., 1986, Dynamics of orogenic wedges and the uplift of high-pressure metamorphic rocks: Geological Soci-ety 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: Tecton-ics, 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 fault-ing, ductile fl ow 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: 10.1016/0264-8172(89)90001-9.

Robertson, A.H.F., 1990, Sedimentology and tectonic implica-tions 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 Geophysi-cal Union: Washington, D.C., American Geophysical Union), v. 67, no. 44, p. 1249.

Ross, J.A., and Sharp, W.D., 1988, The effects of sub-blocking temperature metamorphism on the K/Ar systematics of hornblendes; Ar-40/Ar-39 dating of polymetamorphic garnet amphibolite from the Francis-can 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 fl oor accretion and volcanoplu-tonic 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 tec-tonostratigraphic 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/ANNUREV.EA.11.050183.000401.

Saleeby, J.B., 1983b, Recognition and signifi cance 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-Pacifi c terrane conference: Stanford, California, Stanford University Publications, Geologi-cal 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 geochro-nological 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: Geo-logical 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., Meta-morphism and crustal evolution of the Western United States: Rubey Volume 7: Englewood Cliffs, New Jer-sey, 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: Geologi-cal 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 assem-blage, 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 Bea-man, J., 2004, Multi-stage origin of the Coast Range Ophiolite, California: Implications for the life cycle of supra-subduction zone ophiolites: International Geol-ogy Review, v. 46, p. 289–315.

Stacey, J.S., and Kramers, J.D., 1975, Approximation of terres-trial 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 geo-chronology: Convention on uniform decay constants in geo- and cosmochronology: Earth and Planetary Science Letters, v. 36, p. 359–362, doi: 10.1016/0012-821X(77)90060-7.

Stern, R.J., and Bloomer, S.H., 1992, Subduction zone infancy; Examples from the Eocene Izu-Bonin-Mari-ana 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, Pro-gressive arcward contraction of a Mesozoic-Tertiary fore-arc basin, southwestern Sacramento Valley, Cali-fornia: 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 metamor-phism: 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: Geo-logical 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 Orog-eny 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: Geolog-ical 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, Signifi cance of xenocrys-tic 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]: Colum-bia, 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 Juras-sic seamount in the Northern California Coast Ranges: Geological Society of America, Abstracts with Pro-grams, v. 23, no. 5, p. A395.

MANUSCRIPT RECEIVED BY THE SOCIETY 10 JULY 2003REVISED MANUSCRIPT RECEIVED 1 JULY 2004MANUSCRIPT ACCEPTED 10 JULY 2004

Printed in the USA

Data Repository Table DR-1. Ar release spectra for volcanic glasses of the Stonyford volcaniccomplex.

Laser 40Ar/39Ar 37Ar/39Ar 36Ar/39Ar 40Ar*/39ArK %40Ar* Age ±2s(W) (Ma) (Ma)

G-2/3524-01 J= 0.01950 ±0.000041.4 5.4505 5.9027 0.0044 4.6056 84.2 155.2 4.61.8 5.1465 5.8310 0.0033 4.6299 89.6 156.0 4.12.2 5.3367 5.7358 0.0040 4.6148 86.1 155.5 2.02.5 5.1368 5.8798 0.0028 4.7716 92.5 160.5 2.02.6 4.9393 5.7819 0.0017 4.8953 98.7 164.5 1.62.7 4.9145 5.8171 0.0018 4.8262 97.8 162.3 3.52.8 4.9158 5.8244 0.0017 4.8758 98.8 163.9 1.02.9 4.9315 5.7907 0.0017 4.8810 98.6 164.0 1.13.0 4.9872 5.8463 0.0019 4.8764 97.4 163.9 2.53.2 4.9254 5.7464 0.0016 4.9005 99.1 164.7 1.43.4 4.9692 5.8184 0.0017 4.9313 98.9 165.7 1.83.6 4.9866 5.7856 0.0019 4.8833 97.6 164.1 1.73.9 4.9923 5.7711 0.0019 4.8952 97.7 164.5 1.64.3 5.0119 5.7722 0.0020 4.8723 96.8 163.8 4.1

Plateau Age (Ma) 164.3 1.2

G-2/3524-03 J= 0.01950 ±0.000040.5 5.8165 6.0492 0.0057 4.6141 79.0 155.4 2.00.7 5.0914 5.8338 0.0027 4.7625 93.2 160.2 2.10.9 5.0515 5.8597 0.0018 4.9777 98.2 167.1 3.61.1 4.9347 5.8352 0.0018 4.8596 98.1 163.4 1.81.2 4.9441 5.8824 0.0017 4.9117 99.0 165.0 3.11.3 4.9320 5.8431 0.0017 4.8792 98.5 164.0 1.81.4 4.9759 5.5631 0.0017 4.9222 98.6 165.4 1.71.5 5.0186 5.8939 0.0019 4.9072 97.4 164.9 1.01.6 4.9765 5.9813 0.0019 4.8764 97.6 163.9 1.81.7 5.3531 6.0566 -0.0128 9.6262 179.1 310.4 112.81.8 6.2775 5.5654 0.0103 3.6629 58.1 124.5 40.81.9 5.7823 5.7991 0.0106 3.0966 53.3 105.8 50.92.0 5.3008 6.0194 0.0008 5.5443 104.2 185.2 198.4


G-2/3524-04 J= 0.01950 ±0.000041.9 8.1989 5.8326 0.0175 3.4911 42.4 118.8 6.92.2 6.6895 4.9392 0.0174 1.9232 28.7 66.4 26.23.0 5.0310 5.8458 0.0024 4.7939 94.9 161.2 1.33.2 5.1426 5.8311 0.0029 4.7334 91.7 159.3 5.03.5 5.9477 5.8031 0.0068 4.3954 73.6 148.4 7.73.8 5.5090 5.7228 0.0037 4.8654 88.0 163.5 2.74.1 5.1469 5.9894 0.0046 4.2581 82.4 143.9 3.34.4 5.0473 5.8604 0.0021 4.8819 96.3 164.1 4.94.7 5.0708 5.8570 0.0025 4.8000 94.3 161.4 2.85.0 5.1480 5.9151 0.0045 4.2916 83.0 145.0 3.65.3 5.0510 5.8031 0.0053 3.9279 77.5 133.2 8.65.6 5.1025 5.8927 0.0048 4.1401 80.8 140.1 2.95.9 5.1409 5.9067 0.0038 4.4774 86.8 151.0 4.96.2 5.0528 5.8676 0.0023 4.8442 95.5 162.9 1.66.5 5.0354 5.8699 0.0020 4.8933 96.8 164.4 0.90.0 5.2367 5.8662 0.0027 4.9013 93.2 164.7 0.9


G-2/4445-01 J= 0.02320 ±0.000040.4 14.9898 6.4896 0.0420 3.0740 20.4 124.3 17.90.5 7.0663 5.9758 0.0135 3.5433 49.9 142.5 13.20.6 5.8320 5.8604 0.0087 3.7203 63.5 149.4 18.30.7 4.9880 5.9442 0.0062 3.6141 72.2 145.3 10.50.9 4.5307 5.9899 0.0033 4.0361 88.7 161.5 3.91.1 4.6507 5.9222 0.0034 4.1178 88.2 164.6 3.31.2 4.6642 5.8001 0.0034 4.1010 87.6 164.0 2.91.3 4.5726 5.8537 0.0031 4.1131 89.6 164.4 2.11.4 4.5978 5.8065 0.0033 4.0744 88.3 162.9 8.21.5 4.5179 5.8320 0.0032 4.0398 89.1 161.6 7.61.7 4.9304 6.0073 0.0045 4.0540 81.9 162.2 4.12.0 4.6137 6.0569 0.0033 4.1163 88.9 164.6 2.9


G-4/4446-01 J= 0.02320 ±0.000040.7 6.1796 4.9144 0.0087 3.9823 64.2 159.4 5.00.9 4.8111 4.7686 0.0036 4.1068 85.1 164.2 11.41.1 4.6384 4.7346 0.0028 4.1661 89.5 166.4 8.61.2 4.6640 4.6821 0.0026 4.2567 91.0 169.9 9.61.3 4.5743 4.7266 0.0031 4.0330 87.9 161.4 7.81.4 4.4977 4.5846 0.0025 4.1136 91.2 164.4 10.71.5 4.3730 4.6235 0.0024 4.0171 91.6 160.8 10.01.6 4.6900 4.7208 0.0034 4.0564 86.2 162.3 13.71.7 4.2593 4.8112 0.0018 4.1147 96.3 164.5 1.11.8 4.2702 4.7580 0.0019 4.0849 95.4 163.3 1.22.0 4.4373 4.8954 0.0023 4.1324 92.8 165.2 1.32.2 4.3579 4.8693 0.0022 4.0752 93.2 163.0 1.8


G-4/4446-02 J= 0.02320 ±0.000040.5 8.8120 4.5843 0.0214 2.8435 32.2 115.3 13.40.7 4.7322 4.4952 0.0051 3.5664 75.1 143.4 4.50.9 4.6485 4.5305 0.0032 4.0662 87.2 162.6 2.51.1 4.5236 4.6022 0.0027 4.0813 89.9 163.2 1.81.2 4.4912 4.6356 0.0028 4.0265 89.4 161.1 4.81.3 4.3555 4.7008 0.0021 4.1115 94.1 164.4 1.01.4 4.9700 4.7932 0.0042 4.1000 82.2 163.9 2.91.6 5.7182 5.0282 0.0069 4.0568 70.7 162.3 3.91.8 5.2128 4.9837 0.0050 4.1109 78.6 164.3 1.92.0 4.7982 5.0509 0.0041 3.9678 82.4 158.9 1.6


G-8/4447-01 J= 0.02320 ±0.000040.4 24.9170 4.7251 0.0804 1.5144 6.1 62.3 84.60.5 9.5320 7.5391 0.0234 3.1886 33.3 128.8 45.60.6 6.0813 7.9462 0.0082 4.2920 70.2 171.2 25.70.7 5.4556 7.7418 0.0061 4.2502 77.5 169.7 22.50.8 5.0828 8.0700 0.0052 4.1769 81.7 166.9 10.00.9 4.8225 8.0146 0.0045 4.1264 85.1 164.9 10.01.0 4.8080 8.1830 0.0048 4.0208 83.2 160.9 8.71.1 4.9352 8.4140 0.0050 4.1038 82.7 164.1 8.61.2 4.5395 8.2200 0.0037 4.0964 89.7 163.8 5.01.3 4.4243 8.0973 0.0032 4.1144 92.5 164.5 2.41.4 4.5303 8.1366 0.0037 4.0734 89.4 162.9 8.41.5 4.4056 8.1577 0.0030 4.1460 93.6 165.7 5.51.6 4.3596 8.0227 0.0029 4.1409 94.5 165.5 4.91.7 4.3607 8.0886 0.0028 4.1719 95.2 166.7 3.12.0 4.2702 8.1002 0.0027 4.1034 95.6 164.1 1.6


G-8/4447-02 J= 0.02320 ±0.000040.5 10.0276 6.0168 0.0273 2.4216 24.1 98.6 20.80.7 4.7019 7.6446 0.0051 3.8004 80.4 152.4 7.30.9 4.4458 8.1867 0.0045 3.7552 84.0 150.7 8.11.1 4.2323 8.1239 0.0029 3.9944 93.9 159.9 3.41.2 4.2298 8.2379 0.0026 4.0954 96.3 163.8 2.31.3 4.1699 7.8929 0.0022 4.1365 98.7 165.3 1.61.4 4.2747 7.8427 0.0022 4.2298 98.4 168.9 5.51.6 4.2262 8.1032 0.0025 4.1080 96.7 164.2 1.91.8 4.2093 8.0233 0.0025 4.0860 96.6 163.4 2.42.0 4.2553 8.2549 0.0027 4.1049 95.9 164.1 1.62.2 4.2010 8.2295 0.0024 4.1433 98.1 165.6 1.7


G-5/4448-01 J= 0.02320 ±0.000040.5 9.0467 4.8369 0.0178 4.1577 45.8 166.1 3.50.7 4.7931 4.8670 0.0038 4.0606 84.4 162.4 2.80.9 4.5715 4.8002 0.0028 4.1045 89.5 164.1 2.11.1 4.4804 4.7716 0.0025 4.1041 91.3 164.1 2.41.2 4.4203 4.7868 0.0023 4.1013 92.5 164.0 1.91.3 4.7403 4.7958 0.0035 4.0734 85.7 162.9 2.41.4 4.7329 4.6454 0.0031 4.1662 87.8 166.5 5.31.5 4.5361 4.8257 0.0028 4.0969 90.0 163.8 5.52.0 4.7146 4.8864 0.0036 4.0396 85.4 161.6 4.22.2 4.7927 4.8555 0.0037 4.0857 85.0 163.4 2.0


G-5/4448-01 J= 0.02320 ±0.000040.4 30.9391 3.1457 0.0912 4.2446 13.7 169.4 26.10.5 17.7776 3.3412 0.0468 4.2220 23.7 168.6 15.70.6 20.6942 4.3307 0.0574 4.0573 19.5 162.3 27.40.7 16.3524 4.4359 0.0435 3.8323 23.4 153.7 18.00.8 13.9827 4.4730 0.0350 3.9962 28.5 160.0 11.20.9 11.1687 4.5153 0.0251 4.1117 36.7 164.4 10.71.0 12.9163 4.7041 0.0306 4.2524 32.8 169.7 8.01.1 12.1102 4.4312 0.0282 4.1128 33.9 164.4 7.21.2 10.4991 4.3873 0.0230 4.0534 38.5 162.1 7.81.3 8.5395 4.6100 0.0165 4.0325 47.1 161.3 4.91.4 9.7409 4.7710 0.0201 4.1727 42.7 166.7 4.51.5 11.3687 5.0148 0.0255 4.2135 36.9 168.3 8.81.6 11.9626 5.0010 0.0285 3.9399 32.8 157.8 11.41.7 12.0371 5.0567 0.0280 4.1569 34.4 166.1 9.4


Notes:All errors reported are 2sJ-values are based on 28.02 Ma for Fish Canyon sanidine; uncertainty does not include age errorPower 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 decayAges are based on nucleogenic corrections reported by Renne et al. (1998) and decay constants of Steigerand Jager (1977)Age errors do not include systematic contributions from decay constants or J valueErrors 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 206Pb/238U and 207Pb/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 calc-

alkaline 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 10-

22; 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 3-

10; 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 5-

9). 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 9-

11).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).

radioisotopic and biostratigraphic age relations in the coast … - my...

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