Radioisotopic and Biostratigraphic Age Relations in the Coast Range Ophiolite, Northern California: Implications for the Tectonic Evolution of the Western Cordillera

Home , Great Valley Sequence

Radioisotopic and biostratigraphic age relations in the Coast Range Ophiolite, northern California: Implications for the tectonic evolution of the Western Cordillera

John W. Shervais† Department of Geology, Utah State University, 4505 Old Main Hill, Logan, Utah 84322-4505, USA Benita L. Murchey U.S. Geological Survey, 345 Middleﬁ eld Road, Menlo Park, California 94025, USA David L. Kimbrough Department of Geological Sciences, San Diego State University, San Diego, California 92182-1020, USA Paul R. Renne Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709 and Department of Earth and Planetary Science, University of California, Berkeley, California 94720, USA Barry Hanan Department of Geological Sciences, San Diego State University, San Diego, California 92182-1020, USA

ABSTRACT Callovian to early Kimmeridgian (Unitary Keywords: ophiolite, age, CRO, cordillera, Association Zones 8–10) in the upper part. tectonics. The Coast Range ophiolite (CRO) in The SFVC sedimentary record preserves northern California includes two distinct evidence of a major faunal change wherein INTRODUCTION remnants. The Elder Creek ophiolite is a relatively small sized, polytaxic radiolarian classic suprasubduction zone ophiolite with faunas were replaced by very robust, oligo- The Coast Range ophiolite of California and three sequential plutonic suites (layered gab- taxic, nassellarian-dominated faunas that the tectonically subjacent Franciscan assemblage bro, wehrlite-pyroxenite, quartz diorite), a included Praeparvicingula spp. have played a pivotal role in plate tectonic theory mafi c to felsic dike complex, and mafi c-felsic We suggest that CRO formation began after since its inception and even now are considered a volcanic rocks; the entire suite is cut by late the early Middle Jurassic (172–180 Ma) colli- paradigm for active margin processes (Dickinson, mid-oceanic-ridge basalt (MORB) dikes and sion of an exotic or fringing arc with North 1971; Ernst, 1970; Ingersoll et al., 1999). None- overlain by ophiolitic breccia. The Stonyford America and initiation of a new or reconfi g- theless, the origin of the Coast Range ophiolite volcanic complex (SFVC) comprises three ured east-dipping subduction zone. The data (CRO) is still controversial, as is its relationship volcanic series with intercalated chert hori- show that the CRO formed prior to the Late to the Franciscan assemblage (e.g., Dickinson et zons that form a submarine volcano enclosed Jurassic Nevadan orogeny, probably by rapid al., 1996; Godfrey and Klemperer, 1998; Saleeby, in sheared serpentinite. Structurally below forearc extension above a nascent subduction 1997). Postulated origins include (1) formation at this seamount are mélange blocks of CRO zone. We infer that CRO spreading ended a Middle Jurassic equatorial midoceanic ridge similar to Elder Creek. with the collision of an oceanic spreading followed by rapid northward transport and Late U/Pb zircon ages from plagiogranite and center ca. 164 Ma, coincident with the oldest Jurassic accretion to North America (Hopson et quartz diorites at Elder Creek range in age high-grade blocks in the structurally underly- al., 1997); (2) formation as backarc basin crust from 165 Ma to 172 Ma. U/Pb zircon ages ing Franciscan assemblage. We further sug- behind a Middle Jurassic island arc that was obtained from CRO mélange blocks below gest that the “classic” Nevadan orogeny repre- sutured to the continental margin during the Late the SFVC are similar (166–172 Ma). 40Ar-39Ar sents a response to spreading center collision, Jurassic Nevadan orogeny (Godfrey and Klem- ages of alkali basalt glass in the upper SFVC with shallow subduction of young lithosphere perer, 1998; Schweickert, 1997; Schweickert et are all younger at ≈164 Ma. Radiolarians causing the initial compressional deforma- al., 1984); and (3) formation by forearc rifting extracted from chert lenses intercalated with tion and with a subsequent change in North above an east-dipping, proto–Franciscan subduc- basalt in the SFVC indicate that the sedimen- American plate motion to rapid northward tion zone during the Middle Jurassic, prior to the tary strata range in age from Bathonian (Uni- drift (J2 cusp) causing sinistral transpression Nevadan orogeny (Shervais, 1990; Shervais and tary Association Zone 6–6 of Baumgartner et and transtension in the Sierra foothills. These Kimbrough, 1985; Shervais et al., 2004; Stern al., 1995a) near the base of the complex to late data are not consistent with models for Late and Bloomer, 1992). There is abundant evidence Jurassic arc collision in the Sierra foothills or for arc-related geochemical signatures in the †E-mail: [email protected]. a backarc origin for the CRO. CRO (Evarts et al., 1999; Giaramita et al., 1998;

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

Shervais, 1990; Shervais and Kimbrough, 1985), For purposes of comparison between radioiso- (Murchey and Blake, 1993), predating the Coast but other data, such as sedimentary cover associa- tope and biostratigraphic ages, we use the recent Range ophiolites. The oldest clastic rocks in tions and seismic imaging of the continental mar- Jurassic time scale of Palfy et al. (2000) and the the Franciscan slightly postdate the base of the gin, have been interpreted as support for the open radiolarian zonation of Baumgartner (1995). Great Valley Sequence in northern California ocean or backarc basin models (e.g., Godfrey and Taken together with geochemical data for rocks (Imlay, 1980). Some high-grade metamorphic Klemperer, 1998; Hull et al., 1993; Robertson, of the ophiolite and their fi eld relations, these blocks in the Franciscan assemblage probably 1989). Resolving this controversy is central to data allow us to construct a synthesis for the formed ca. 160–165 Ma (Mattinson, 1988), but our understanding of the tectonic evolution of origin and evolution of the ophiolite, its rela- regional metamorphism of strata in the Eastern western North America during the Jurassic and to tionship to high-grade metamorphism in the belt, including Valanginian metagraywacke, our understanding of how ophiolites form. Franciscan assemblage, and the tectonic evolu- may have occurred in the late Early Cretaceous Much of this debate hinges on age relations tion of the western Cordillera during the Middle (115–120 Ma) (Blake and Jones, 1981). within the CRO, and between the CRO, the and Late Jurassic. Franciscan assemblage, and the Sierra Nevada COAST RANGE OPHIOLITE foothills metamorphic belt. Previous work GEOLOGIC SETTING suggests that the CRO ranges in age from ca. The Coast Range ophiolite in northern Cali- 163 Ma at Point Sal to ca. 154 Ma at Del Puerto The Mesozoic geology of California south of fornia is represented by a thin selvage of serpen- Canyon (Hopson et al., 1981), although there the Klamath Mountains comprises three main tinite matrix mélange along most of the bound- is reason to believe the upper age limit may provinces, from east to west: The Sierra Nevada ary separating forearc sedimentary rocks of the be even older (Mattinson and Hopson, 1992). magmatic arc province, the Great Valley fore- Great Valley Sequence from blueschist facies These ages for CRO formation are similar to arc province, and the Franciscan accretionary metamorphic rocks of the Franciscan Eastern the oldest age found for high-grade (amphibolite complex (Fig. 1). The Sierra Nevada province belt or shale-matrix mélange of the Franciscan facies) metamorphism in the northern Franciscan consists of island arc volcanic, plutonic, and Central belt. This boundary was originally inter- assemblage, as determined by U-Pb isochron sedimentary rocks that range in age from Paleo- preted as a fossil subduction zone (“the Coast and 40Ar/ 39Ar dating on high-grade metamorphic zoic to Late Cretaceous. Many of these rocks Range thrust,” e.g., Bailey et al., 1970), but later blocks (Mattinson, 1986, 1988; Ross and Sharp, formed more or less in place along the western work showed that this boundary may have been 1986; Ross and Sharp, 1988), leading to the margin of North America; others in the Sierra modifi ed by later low-angle detachment faults suggestion that high-grade metamorphic blocks Nevada Foothills metamorphic belt may rep- (Harms et al., 1992; Jayko et al., 1987; Platt, in the Franciscan assemblage formed during resent exotic or fringing arc terranes that were 1986), out-of-sequence thrust faults (Ring and subduction initiation beneath an existing backarc welded to the continental margin during Late Brandon, 1994; Ring and Brandon, 1999), and basin (= CRO) (Wakabayashi, 1990). Jurassic or older collisions (Girty et al., 1995; by east-vergent Neogene thrust faults (Glen, Further, it has been suggested that radiolar- Saleeby, 1983a; Schweickert et al., 1984). 1990; Unruh et al., 1995). The current bound- ian cherts deposited unconformably on top of The Great Valley province represents a Late ary is a complex high-angle fault that may have the CRO document a hiatus of some 8–11 m.y. Jurassic through Cretaceous forearc basin that components of strike-slip, normal faulting, and between ophiolite formation in the Middle was underlain by the Coast Range ophiolite reverse faulting. Jurassic and deposition of overlying chert in the (e.g., Bailey et al., 1970). The Great Valley There are two large ophiolite remnants pre- Late Jurassic (Oxfordian–Tithonian) (Hopson et Sequence consists largely of distal turbidites served in the northern Coast Ranges west of the al., 1992; Hopson et al., 1981; Pessagno et al., near its base, which become more proximal and Sacramento Valley: The Elder Creek ophiolite 2000). This proposed hiatus is cited as primary more potassic upsection (Dickinson and Rich, and the Stonyford volcanic complex (Fig. 1). evidence for an open-ocean origin to the CRO, 1972; Ingersoll, 1983; Linn et al., 1992). The Despite their close proximity (they are separated far from any source of arc detritus or terrigenous lower part of the Great Valley Sequence near by only 60 km along strike) these remnants are sediment. However, there are two signifi cant Stonyford is Tithonian, based on the occur- distinctly different. The Elder Creek ophiolite problems with this suggestion: (1) Integration rence of Buchia piochii throughout the section is a “classic” ophiolite, with cumulate plutonic of the Jurassic time scale and standard ammo- (Brown, 1964a, 1964b). Farther north, how- rocks and sheeted dike complex, while the nite zones is still in fl ux, with differences in the ever, the basal Great Valley Sequence contains Stonyford volcanic complex consists largely of proposed ages for some stage boundaries of up Buchia rugosa and a few specimens of Buchia volcanic rocks comprising a Jurassic seamount to 15 m.y. between alternative time scales (see with very fi ne ribs, transitional between B. (Shervais and Hanan, 1989). Detailed mapping discussions in Gradstein et al., 1994; Palfy concentrica and B. rugosa (Imlay, 1980; Jones, of both areas, however, has shown that they are et al., 2000); and (2) major differences in the 1975). Imlay (1980) dated the transitional inter- related and that both formed in the same tec- calibrations of different radiolarian zonations val as late Kimmeridgian to early Tithonian, tonic association (Shervais et al., 2004). compound the time scale uncertainties pointed though he favored the earlier range. out above (Baumgartner et al., 1995a; Hull and The Franciscan assemblage is a classic Meso- Elder Creek Ophiolite Pessagno, 1995; Pessagno et al., 1993; Pessagno zoic and Cenozoic accretionary complex char- et al., 1987). acterized by a heterogenous mixture of litholo- The Elder Creek ophiolite is one of the larg- In this paper, we present new radioisotopic gies and metamorphic grades. In the study area, est exposures of CRO in California and also the age data for plutonic rocks of the Coast Range it includes both strongly metamorphosed northernmost (Fig. 1). The ophiolite is named ophiolite in northern California and for unal- blueschist facies rocks in the Eastern belt, as for outcrops along the South, Middle, and North tered volcanic glass from the Stonyford volcanic well as true tectonic mélange in the Central belt Forks of Elder Creek, which expose progres- complex. We also present new biostratigraphic (Bailey et al., 1964). Volcanic-pelagic layers of sively deeper levels of the ophiolite from south data for radiolarian cherts interbedded with vol- oceanic crust incorporated into the Franciscan to north (Fig. 2). The Elder Creek ophiolite pre- canic rocks of the Stonyford volcanic complex. are as old as Early Jurassic, Pliensbachian serves most of the pseudostratigraphy associated

634 Geological Society of America Bulletin, May/June 2005 AGE RELATIONS IN THE COAST RANGE OPHIOLITE, CALIFORNIA

Field relations and geochemistry of plutonic 124º 122º 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 second magmatic episode consists of wehrlite and 40º N Elder Creek clinopyroxenite intrusions into the older layered 165-172 Ma U/Pb New 40º N complex, with less common isotropic gabbro and gabbro pegmatoid. The third magmatic episode comprises stocks and dikes of hornblende diorite and hornblende quartz diorite, with fel- Stonyford site dikes that are marginal to the quartz diorite 166-172 Ma U/Pb New plutons; rocks of this suite intrude all of the 164 1Ma Ar/Ar New older lithologies. The diorite stocks commonly form magmatic breccias (“agmatites”) with Geyser Peak / xenoliths of cumulate or foliated gabbro, dike Black Mountain complex, or volcanic rock in a quartz diorite matrix. The fourth magma series is represented Mount Diablo by rare basaltic dikes that crosscut rocks of the 38º N 165 Ma U/Pb Man91 38º N older episodes. Geochemical data are consistent Leona Rhyolite with formation of the fi rst three magma series in a suprasubduction zone (arc) environment; rare Del Puerto dikes of the fi nal magma series are characterized Sierra Azul by MORB-like major and trace element compo- 163+/-5 Ma K/Ar Lan71 sitions (Shervais, 2001; Shervais and Beaman, 156+/-2 Ma U/Pb HMP81 Tertiary 1991; Shervais et al., 2004). Quinto Creek Volcanic rocks are most commonly pre- Modoc Plateau served as clasts in the Crowfoot Point breccia, a coarse, unsorted fault-scarp talus breccia that Salinia SNF varies from <10 m to over 1000 m in thickness Llanada (Hopson et al., 1981; Robertson, 1990). The 36º N 164 Ma Crowfoot Point breccia contains clasts of mafi c U/Pb HMP81 36º N and felsic volcanic rocks, gabbro, pyroxenite- Great Valley Sequence wehrlite, and diorite. This unit was deposited SAF on an eroded surface that cuts all other units of Coast Range Ophiolite the ophiolite (from cumulate ultramafi cs through Franciscan Complex dike complex). Additional volcanic rocks crop Stanley Mtn out in fault-bounded blocks and in the dike com- Cuesta Ridge 166+/-2Ma U/Pb P93 plex. With the exception of the late, MORB-like Sierra Nevada 153 Ma U/Pb HMP81 dikes, all volcanic and hypabyssal rocks associated with the Elder Creek ophiolite are island arc Klamath terranes Point Sal tholeiite or calc-alkaline series basalts, andesites, 165-173 Ma U/Pb MH92 124º W 120º W or dacites (Shervais and Beaman, 1991). 122º W Felsic plutonic rocks crop out in two distinct Figure 1. Generalized geologic map of California showing major lithotectonic provinces associations: (1) As small (<2 m across) lenses discussed in text, along with location of ophiolite remnants in the Coast Ranges; the CRO intruded into the lower part of the dike complex, is shown in black. Ages labeled “new” are from this study; others from Lanphere, 1971 and (2) as large stocks and sills that intrude all (Lan71); Hopson, Mattinson, and Pessagno, 1981 (HMP81); Mattinson and Hopson, 1992 other ophiolite lithologies. The fi rst association (MH92); J.M. Mattinson in Pessagno et al., 1993a (P93); and J.M. Mattinson in Mankinen appears to represent residual magma related to et al., 1991 (Man91). SAF—San Andreas Fault, SNF—Sur-Nacimiento Fault. 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. with “true” ophiolites: Cumulate ultramafi c volcanic rocks were removed by erosion related rocks, cumulate gabbro, isotropic gabbro, and to tectonic disruption on the seafl oor and rede- Stonyford Volcanic Complex sheeted dikes (Hopson et al., 1981; Shervais and posited as clasts within the overlying Crowfoot Beaman, 1991; Shervais et al., 2004). Prior to Point breccia (Blake et al., 1987; Hopson et al., The Stonyford volcanic complex (SFVC) deposition of the Great Valley Sequence, most 1981; Robertson, 1990). crops out ~60 km south of Elder Creek ophiolite,

Geological Society of America Bulletin, May/June 2005 635 SHERVAIS et al. in the low-lying hills surrounding the com- Great Valley Series munity of Stonyford (Fig. 1). It was originally mapped by Brown (1964a) and later mapped du GVS GVS Wacke and siltstone in detail by Zoglman and Shervais (Zoglman, cpb 1991; Zoglman and Shervais, 1991). Contrary Crowfoot Point Breccia wc to our earlier suggestions that the SFVC might du represent Franciscan assemblage volcanics cg Elder Creek Ophiolite tectonically transferred to the CRO serpenti- wc vc Volcanic rocks du nite mélange (Shervais and Kimbrough, 1985; dc Dike complex Shervais and Kimbrough, 1987), our later work qdi fdc has shown that the SFVC formed as an integral fmg Felsic dikes part of the CRO and that it was never part of the EC148-2 qdi Quartz diorite & diorite subduction complex (Zoglman and Shervais, fdc ig du GVS 1991). The SFVC is also distinct from the vc fdc ig Isotropic gabbro St. Johns Mountain complex, which contains cg cg Cumulate gabbro incipient blueschist facies metamorphism and vc wc is clearly within the Franciscan assemblage sfms cg Wehrlite-Pyroxenite (MacPherson, 1983). cg qdi wc du Dunite & dunite The SFVC consists of four large blocks fdc broken formation within sheared serpentinite-matrix mélange; the wc EC107-3 ig dc cpb largest block is some 5 × 3 km in areal extent qdi Serpentinite mélange (Fig. 3). The SFVC consists largely of pillow ssp Sheared serpentinite lava with subordinate sheet fl ows, diabase, cg vc and hyaloclastite breccia. The volcanic rocks Volcanic blocks gs are exceptionally fresh for the Coast Range ig wc in melange ophiolite, as shown by the preservation of pri- qdi fmg Foliated metasediments mary igneous plagioclase and clinopyroxene in sfms gs wc ssp (Galice?) most of the lavas and unaltered basaltic glass in cg many of the hyaloclastites (Shervais and Hanan, Franciscan Assemblage qdi 1989; Shervais and Kimbrough, 1987; Zoglman ssp South Fork sfms and Shervais, 1991). The hyaloclastite breccias, gs GVS Mountain schist which represent submarine fi re fountain depos- cpb its, contain unaltered volcanic glass with relict dc phenocrysts of olivine, plagioclase, and chrome spinel (Shervais and Hanan, 1989). gs wc cpb Volcanic rocks of the SFVC form three ssp groups: (1) Enriched, oceanic tholeiite basalts, gs North (2) transitional alkali basalts and glasses, and (3) high-alumina, low-Ti tholeiites (Shervais ig et al., 2004; Zoglman, 1991). Pb isotopic data 0 12 for the volcanic glasses are similar to Pacifi c vc cpb miles oceanic basalts currently found in off-axis seamounts and associated with large ion lithophile ssp elements-rich mantle plumes (Hanan et al., 1992). The rare earth elements, trace element, GVS and Pb data indicate that the oceanic tholeiites and alkali basalts were derived from a hetero- geneous mantle source with at least two compo- Figure 2. Geologic sketch map of Elder Creek ophiolite showing sample locations collected nents: A depleted MORB-source asthenosphere and dated for this study (small stars). and an enriched plumelike component (Hanan et al., 1992). The high-Al, low-Ti basalts resemble second-stage melts of a MORB asthenosphere Lensoid intercalations of red radiolarian chert rip-up clasts of chert that were entrained while source, which form by melting plagioclase and pink siliceous mudstone up to 1 km long they were still soft. Both the ribbon cherts and lherzolite at low pressures; these lavas also and 50 m thick occur throughout the section the siliceous mudstones contain abundant well- resemble high-Al island arc basalts. The trace (Fig. 3). The cherts are typically Mn-rich and preserved radiolarians. element and Pb systematics show an alkali are commonly associated with hydrothermal Structural and stratigraphic relations show basalt infl uence, which overprints generally alteration of the underlying basalts, suggesting that, in general, the lavas and their sedimentary depleted trace element characteristics (Hanan that they formed in proximity to submarine hot intercalations dip moderately to the northeast, et al., 1992; Shervais et al., 2004; Zoglman and springs. In places, typical ribbon cherts are inter- and that they become younger to the northeast as Shervais, 1991). bedded with siliceous mudstones containing well. Along the northern margin of the structurally

636 Geological Society of America Bulletin, May/June 2005 AGE RELATIONS IN THE COAST RANGE OPHIOLITE, CALIFORNIA

JKf 39º 25' N 122º 40' W Chert B SFV58-1 39º 25' N 122º 32' 30" W Glass 1-4 Harz SSP SFV141-1 Chert C Glass 5, 8

GVS

SFV109-2 Qal

Chert A reek SFVC y C Ston Stonyford Chert D

SFVC

GVS

SSP 39º 20' N JKf 122º 40' W Harzburgite 39º 20' N 122º 32' 30" W

Legend

STONYFORD VOLCANIC COMPLEX QUATERNARY SFV: Pillow lavas & massive basalts Qal: Alluvium Volcaniclastic/ Conglomerate hyaloclasticbreccias Chert, limestone, & Quaternary landslide siliceous mudstone SERPENTINITE MATRIX MELANGE MN GREAT VALLEY SEQUENCE SSP: Sheared serpentinite GN GVS: Interbedded sandstone, Harz: Massive harzburgite: siltstone, and shale partially serpentinized, sheared 1 2 miles FRANCISCAN COMPLEX (JKf) Schist blocks in serpentinite 0 Qtz-lawsonite-mica schist: High-grade blocks metagreywacke, metasiltstone, 0 7000 feet 17 1/2 metachert Metavolcanic blocks Metavolcanic Meta-plutonic rocks blocks 0 1 2 km 0 17' 311 MILS (wehrlite, pyroxenite, 6 MILS gabbro, diorite) Volcanogenic sandstone

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

Figure 3. Geologic sketch map of the Stonyford volcanic complex showing the location of samples collected and dated for this study (small stars), hyaloclastite layers, and chert-siliceous mudstone intercalations. GN—UTM Grid convergence, MN—Magnetic north.

largest block of SFVC radiolarian chert is over- blocks are identical to lithologies found farther FRANCISCAN ASSEMBLAGE lain stratigraphically by a thick hyaloclastite layer north in the Elder Creek ophiolite. Quartz dio- containing unaltered volcanic glass (Fig. 3). rite occurs as individual blocks ranging in size The Franciscan assemblage in northern Dismembered remnants of CRO plutonic from a few meters to several tens of meters and California consists of two primary units: The and volcanic rock, including dunite, wehrlite, as dikes within mélange blocks of isotropic gab- Eastern belt of high P/T metamorphic rocks clinopyroxenite, gabbro, diorite, quartz diorite, bro. Other tectonic blocks within the mélange and the structurally underlying Central Belt and keratophyre pillow lava, occur as blocks up include unmetamorphosed volcanogenic sand- mélange (Blake et al., 1988). The Eastern to 200 m across within the serpentinite matrix stones, foliated metasediments, and pale green belt of the Franciscan assemblage comprises mélange structurally beneath the SFVC. These metavolcanic rocks. blueschist facies metamorphic rocks in two

Geological Society of America Bulletin, May/June 2005 637 SHERVAIS et al. distinct terranes: The structurally higher Pickett some southern CRO locations to 165–170 Ma, contaminating grains. Zircon dissolution and Peak terrane and the underlying Yolla Bolly based on new data obtained with a modern mul- ion exchange chemistry for separation of ura- terrane. Both terranes contain blueschist facies ticollector mass spectrometer. In addition, new nium and lead followed procedures modifi ed metagreywacke in relatively intact, coherent U-Pb zircon dates have been reported for Stanley from Krogh (1973). Isotope ratios were mea- thrust sheets; the Pickett Peak terrane also Mountain (166 ± 1 Ma, J.M. Mattinson, reported sured with the MAT 261 multicollector instru- contains the South Fork Mountain schist and in Pessagno et al., 1993) and Mount Diablo ment at UC Santa Barbara and the VG Sector its Chinquapin metabasalt member (Blake et al., (165 Ma, J.M. Mattinson, reported in Mankinen 54 multicollector instrument at San Diego State 1988). The Eastern belt is distinguished from et al., 1991). None of the dates reported since University. Analytical uncertainties, blanks, the Central belt by the coherent nature of thrust Hopson et al. (1981) (either in abstracts or as a and common lead corrections are outlined in sheets, its blueschist facies metamorphism, and personal commun.) has been published with sup- Table 1. Most of the samples yield concordant to its foliated textures. porting data, so it is not possible to evaluate their near-concordant U/Pb dates that are interpreted The Central belt of the Franciscan assem- precision or accuracy. to closely approximate crystallization ages. The blage consists largely of mélange with a per- Radiolarian biostratigraphic data from bed- relatively simple systematics for these samples vasively sheared matrix of mudstone and grey- ded chert at Elder Creek and Stonyford have is interpreted to refl ect the low metamorphic wacke sandstone containing blocks and slabs of been reported by Pessagno and Louvion-Trellu grade of the samples and negligible or absent intact greywacke sandstone, greenstone, chert, (Hopson et al., 1981; Louvion-Trellu, 1986; Pes- inherited components of radiogenic lead. and serpentinite (Berkland et al., 1972; Blake et sagno, 1977). All the of cherts sampled at Elder al., 1982; Blake et al., 1989). Some greywacke Creek are from mélange blocks in the Round 40Ar/ 39Ar slabs are up to several km across; greenstone Valley serpentinite mélange, including blocks of and (or) chert knockers are smaller but can also chert only and blocks with chert resting deposi- Samples of clear brown volcanic glass were be tens of km in extent. Also common in the tionally on basalt. Hopson et al. (1981) assigned crushed into equant grains in distilled water Central belt are high-grade blocks of blueschist, radiolarians from the chert blocks to the upper and then ultrasonically cleaned successively in eclogite, amphibolite, and garnet amphibolite part of 1977 Zone 1 (undifferentiated) of Pessa- 10% HCl and 7% HF for three minutes in each (Cloos, 1986; Moore, 1984). These high-grade gno, and they calibrated the faunas as Oxfordian acid. Grains 1.0–2.0 mm in dimension were blocks refl ect polymetamorphism with initial to Kimmeridgian. The taxa listed by Hopson et selected individually for high optical refl ectiv- high temperatures and low pressures followed al. (1981, p. 477) are all long ranging in the UA ity, freedom from inclusions and veins, and by lower temperature and higher pressure meta- zonation of Baumgartner et al. (1995a, 1995b) generally fresh appearance. Five to ten grains morphism (Moore, 1984; Moore and Blake, except for Parvicingula sp. C, a synonym of selected from each sample were irradiated in 1989; Wakabayashi, 1990). Praecaneta (Ristola) turpicula, which has a two batches for ~28 h each at Los Alamos range from UA Zones 5–6, late Bajocian to National Laboratories’ Omega West reactor, PREVIOUS WORK Bathonian. In a subsequent study of the mélange along with neutron fl uence monitor Fish Can- (Louvion-Trellu, 1986), additional radiolarian yon sanidine (28.02 Ma; Renne et al., 1998). The northern CRO has received limited faunas collected from the blocks were assigned Only G-2 glass (laboratory numbers 3524–1, attention and there are few radioisotopic or ages of Callovian to early Oxfordian based −3, and −4) was irradiated in the fi rst batch, biostratigraphic ages reported. Lanphere (1971) primarily on European-based biostratigraphic which used Cd shielding; all four glasses were reported a K-Ar age of 154 ± 5* Ma for an calibrations. Radiolarians are also present in co-irradiated in a second batch that did not use isotropic hornblende gabbro dike in pyroxenite mudstone in the lower part of the Great Valley Cd shielding. Two individual grains of glass that is overlain unconformably by the Crowfoot Sequence, which unconformably overlies the samples G-4, G-5, and G-8, and four of G-2, Point breccia. He also reported ages of 162 ± Elder Creek ophiolite (Pessagno, 1977). The were incrementally degassed in 10–15 steps 5* Ma and 164 ± 8* Ma for hornblendes from radiolarians occur with, and were calibrated by, with an Ar-ion laser and analyzed for relative an isolated peridotite and an isotropic gabbro the previously mentioned late Kimmeridgian or Ar isotopic abundances using the fully auto- lens in the Del Puerto ophiolite (*note: These early Tithonian bivalves. Pessagno (1977) also mated facilities and procedures described by ages have been recomputed by McDowell et documented the radiolarians from a sample near Renne (1995). Data are presented in Data al. [1984] using the new decay constants of the “Diversion Dam” along Stony Creek in the Repository document DR-1.1 Steiger and Jaeger [1977]). McDowell et al. Stonyford volcanic complex and assigned them (1984) reported hornblende K-Ar ages for four to his 1977 Subzone 2B (equivalent to Zone 3 Biostratigraphic CRO gabbros: three from the Elder Creek area of Pessagno et al., 1993), which he inferred to and one from Wilbur Springs. One Elder Creek be early Tithonian (Pessagno, 1977). The taxa Radiolarian-bearing chert and siliceous mud- hornblende has a reported age of 166 ± 3 Ma; from Diversion Dam locality are discussed and stone were collected from several localities in the others are all ca. 143–144 ± 3 Ma (McDow- recalibrated below. the Stonyford volcanic complex. Radiolarians ell et al., 1984). and siliceous sponge spicules were etched from Hopson et al. (1981) reported U-Pb zircon METHODS surrounding rock matrices by bathing broken dates for eleven samples from locales south rock fragments in diluted hydrofl uoric acid (10% of Stonyford (Healdsburg, Del Puerto, Cuesta U-Pb Zircon Ridge, Pt. Sal, and Santa Cruz island). These zircons have reported 238U/206Pb ages of 144 ± Zircon was separated by conventional tech- 1GSA Data Repository item 2005079, Ar release 2–165 ± 2 Ma and 207Pb/206Pb ages of 144 ± 15 to niques using a Wilfl ey Table, heavy liquids, and spectra for volcanic glasses of the Stonyford volcanic complex technical notes on biostratigraphic 201 ± 15 Ma (Hopson et al., 1981); none of these a Franz magnetic separator. The least magnetic calibrations and correlations, is available on the Web are isochron ages. In an abstract, Mattinson and zircons from each sample were split into size at http://www.geosociety.org/pubs/ft2005.htm. Re- Hopson (1992) revised these dates upward for fractions and then handpicked to remove any quests may also be sent to [email protected].

638 Geological Society of America Bulletin, May/June 2005 AGE RELATIONS IN THE COAST RANGE OPHIOLITE, CALIFORNIA

TABLE 1. COAST RANGE OPHIOLITE: U-PB ZIRCON ISOTOPIC DATA FOR ELDER CREEK AND STONYFORD Fraction Weight Pb U 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 1.4 bulk 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 1.6 bulk 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 3.3 bulk 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 2.2 L 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 2.7 bulk 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 1.6 L 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.

of ~50% concentrate), commonly for ~24 h, fol- radiolarian species from scores of stratigraphic many excellent faunal descriptions of Pessagno lowing procedures modifi ed from Dumi trica sections around the world. For the Middle and and his colleagues for the basic data used in the (1970) and Pessagno and Newport (1972). Then Late Jurassic, the unitary association zones interbasin correlations. However, we did not use the fossils were washed off the etched rock (UAZ) are numbered sequentially from oldest the formal, event-based zonation of Pessagno et surfaces and collected on Tyler-equivalent 250- to youngest: Zones 1–13. All zonal assignments al. (1993, 1987) for either correlation or age cal- mesh (63 µm openings) and 80-mesh (180 µm given in this study, even those restricted to a ibration. The formal zonation uses a relatively openings) screens. The fossils were identifi ed single UAZ, are indicated as a range, following small number of biostratigraphic events, the fi rst based on examination with a binocular micro- the convention of the zonation. or last occurrences of selected taxa, as marker scope. Selected specimens were also examined Biostratigraphic correlations between the ties; but we wanted to maximize the number of with a scanning electron microscope. Stonyford sequence and other key sequences taxa used for correlation and thereby minimize The radiolarian samples were dated using the in California and Oregon were based on UAZ the effects of range diachronism. In addition, Tethyan Unitary Association (UA) Zonation of ranges as well as direct comparisons of local although some zonal reference intervals for the Baumgartner et al. (1995a), a widely utilized species ranges. Thus, the older Stonyford faunas Jurassic zonation of Pessagno et al. (1993, 1987) international standard that is calibrated with were also correlated with radiolarian faunas in are quite well calibrated, key reference intervals ammonites, nannofossils, and calpionellids. The the Middle Jurassic Blue Mountains reference particularly relevant to this study have minimal zonation incorporates data on the ranges of more localities of Pessagno et al. (1987), which are direct molluscan control on the ammonite-based than 400 Jurassic and (or) Early Cretaceous dated with ammonites. We relied heavily on the stage boundaries.

Geological Society of America Bulletin, May/June 2005 639 SHERVAIS et al.

RESULTS to the north and to the 40Ar/39Ar ages obtained ians in each locality are included in Figure 6, on the samples of volcanic glass from the Stony- along with the UA zonal range (Baumgartner et U-Pb Zircon Ages ford volcanic complex (see below). Two zircon al., 1995a) for each group of samples. fractions from SFV-141 are discordant and have The oldest Stonyford samples, collected at Elder Creek 207Pb/206Pb ages of 177 ± 3.5 and 219 ± 2.7 Ma, Locality A, are structurally below the hyaloclas- Zircon was separated from two samples suggesting inheritance of an older zircon compo- tic units that have yielded 40Ar/39Ar glass ages of of quartz diorite collected from the Elder nent, perhaps from a continental crustal source ca. 164 Ma. The section is divided herein into Creek ophiolite (Fig. 2). Sample EC107–3 is (cf. Wright and Wyld, 1986). Similar results intervals A1, A2, and A3 (Fig. 6). Interval A1 a plagiogranite lens that intrudes sheeted dike were obtained by Bickford and Day (2004), is no older than UAZ 3 based on the presence complex along the South Fork of Elder Creek. who identifi ed the presence of ca. 2153 ± 1 Ma of Mirifusus fragilis (UAZ 3–8), Acanthocircus Sample EC148–2 is from an ≈500-m-thick sill inherited zircon in plutons of the 164–160 Ma suboblongus suboblongus (UAZ 3–11), Hsuum of hornblende quartz diorite that intrudes along Smartville ophiolite. brevicostatum gp. (UAZ 3–11), and Saitoium sp. the contact between isotropic gabbro and dike (UAZ 3–21). Intervals A2 and A3 are assigned complex and crops out on the summit of Brush 40Ar/ 39Ar Glass Ages to UAZ 6–6 based on the ranges of Praecaneta Mountain. As shown by Shervais and Beaman turpicula (UAZ 5–6), Spongocapsula palmerae (1991), quartz diorite sills similar to EC148–2 Apparent age spectra for replicate samples (UAZ 6–13), and Xiphostylus (Xiphostylus crosscut and intrude all other rock types in the of glass from four distinct hyaloclastite units gasquetensis gp.) (UAZ 1–6). As determined Coast Range ophiolite at Elder Creek and rep- are shown in Figure 5. Many of the age spectra from the calibrations of the Tethyan UA zones, resent the last arc-related magmatic event in the are slightly discordant, with anomalously young the lower part of the chert section at Locality A history of the ophiolite. ages from low temperature steps, but all yield is Bajocian or Bathonian, and the upper part is Zircon ages for these two samples are shown plateaux of varying quality and precision. The Bathonian. Direct correlation between Locality in Table 1. Both samples are slightly discor- mean ages for each unit are 164.4 ± 0.4 Ma A and ammonite-dated Middle Jurassic strata in dant, with 238U/206Pb ages ranging from 161 to (G-2), 164.0 ± 0.5 Ma (G-4), 163.8 ± 0.8 Ma the Blue Mountains of northeastern Oregon also 172 Ma and 207Pb/206Pb ages ranging from 165 (G-5), and 164.6 ± 0.7 Ma (G-8), calculated as favors a Bathonian age for the section (Fig. 7) to 198 Ma. Concordia plots suggest crystalliza- the weighted mean of all plateau steps for each based on the ranges in the Blue Mountains of tion ages of 169.7 ± 4.1 Ma for EC107–3 and sample, and are all mutually indistinguishable Praecaneta turpicula (Bathonian), Praecaneta 172.0 ± 4.0 Ma for EC148–2 (all errors are 2σ; at the 2σ level. The consistency of high-preci- decora (Bathonian), Eucyrtidiellum unumaense Fig. 4). These ages are signifi cantly older than sion plateau ages provides strong evidence pustulatum (Bathonian), Pantanellium ultra- previous hornblende K-Ar dates from gabbro that the K-Ar systems have not been disturbed sincerum (Bathonian), Xiphostylus (forms with (154 ± 5 to 163 ± 5 Ma) (Lanphere, 1971; beyond minor alteration, the effects of which are compressed tests) (Bajocian and Bathonian), McDowell et al., 1984), which we interpret here removed in low-temperature steps. Mobility of Spongocapsula spp. (Bathonian and younger), as cooling or Ar-loss ages. Two zircon fractions K and/or Ar is common in glasses, particularly Leugeo hexacubicus (sensu Baumgartner et al., from EC-148 are discordant and have 207Pb/206Pb those having suffered hydration (e.g., Cerling 1995b, p. 296–297) (Bajocian to Callovian), ages of 181 ± 1.5 and 198 ± 2.2 Ma, suggest- et al., 1985), but the consistency of our data Archaeodictyomitra spp. aff. A. suzukii (genus ing inheritance of an older zircon component, precludes such effects unless they were mark- from late Bajocian), and Parahsuum offi cerense perhaps from a continental crustal source (cf. edly homogeneous both within and between gp. (Bajocian) (Blome, 1984; Nagai and Mizutani, Wright and Wyld, 1986). samples. Part of our success in dating these 1990; Pessagno and Blome, 1980; Pessagno et al., glasses stems from the ability to date individual 1993; Pessagno and Whalen, 1982; Pessagno Stonyford small grains that could be selected according to et al., 1989). The good agreement in the results Zircon was separated from three samples of stringent criteria for freedom from alteration. derived from two different calibration methods quartz diorite that occur as blocks within the It could be argued that the mean age of ca. confi rms the previous conclusions of Murchey serpentinite matrix mélange beneath the SFVC. 164 Ma for the glasses refl ects the age of an and Baumgartner (1995) that the calibrations of Two of these samples are from discrete blocks, outgassing event that completely rejuvenated the Middle Jurassic UA Zones 1–6 (Baumgartner et the third is from a dike within a block of isotropic K-Ar system in all the samples. While such a sce- al., 1995a) compare well with ammonite-based gabbro. Sample SFV-109–2 is from a 30-cm- nario cannot be completely excluded, we inter- calibrations for radiolarian sequences in the thick quartz diorite dike that crosscuts an isotro- pret the 40Ar/39Ar ages to refl ect eruption ages in Pacifi c Northwest (Pessagno et al., 1987). pic gabbro block below Auk-Auk Ridge (Fig. 3). view of their within- and between-sample repro- Samples at Locality B are subdivided into SFV-141–1 is from a large (100 m) block of ducibility and their consistency with the Middle three intervals, from oldest to youngest: B1, coarse-grained quartz diorite that crops out in Jurassic age of associated radiolarian faunas. B2, and B3. The oldest sample, B1, is assigned Dry Creek. SFV-58–2 is from a small (4 m) block a possible range of UAZ 3–8, calibrated as of strongly foliated quartz diorite that crops out in Radiolarian Biostratigraphy, Stonyford Bathonian to early Oxfordian. The zonal range serpentinite mélange 400 m north of Dry Creek. Volcanic Complex is constrained by the lower and upper ranges Zircon ages for these three samples are shown of Mirifusus fragilis (UAZ 3–8) and the upper in Table 1. 207Pb/206Pb ages for the least discor- Red to green banded cherts and massive pink range of Turanta sp. (UAZ 1–8). A poorly dant fractions range from 163 Ma to 173 Ma siliceous mudstones, forming lenses up to 50 m preserved specimen of probable Guexella with a precision of ±1.5–4.1 Ma. Concordia thick, crop out at several locations within the nudata (UAZ 5–8) also was observed in this intercept ages are 164.8 ± 4.8 Ma for SFV-109–2 Stonyford volcanic complex. Radiolarian locali- interval, suggesting that the sample is prob- and 163.5 ± 3.9 Ma for SFV-58–1 (Fig. 4). These ties A, C, and D lie within the central block of ably not older than Bathonian. The B1 fauna is ages are essentially identical to U/Pb zircon ages the complex; Locality B lies within the northern younger than or correlative with the faunas at determined for the Elder Creek ophiolite 60 km block (Fig. 3). Taxonomic lists of the radiolar- locality A, and the small, polytaxic assemblages

640 Geological Society of America Bulletin, May/June 2005 AGE RELATIONS IN THE COAST RANGE OPHIOLITE, CALIFORNIA 5 6 4 5 8 7 7 1 1 . 1 . 0 0 a

3 4 t . a M

6 t 6 4

M t

a 1 9

4 = 1

p . t

5 1 = e 3 8 . p

D 4 c ± 4 1 e 2

. D 7 c 5 W e ± r . 7 t

0 1 W S e 3 n . 7 t 1 I . S 6 M 0 n 9 I 1 M 6 1 2 3 8 6 5 1 . 3 1 0 0 7 7 1 . 1 0 U 5 U 0 3 5 8 2 3 1 / 2 . / 5 2 0 b 2 6 b 8 7 P 1 6 1 7 P . 1 0 7 0 2 0 8 2 7 2 - 1 . 8 0 5 V 5 F 1 6 1 volcanic complex. MSWD—mean standard volcanic S 7

6 6 1 s . s 1 1 i 6 0 3 e 7 - n 7 1 . g 0

0 1 e t i C r 5 E o

i 0 k d 4

7 C z

6 1 4 t 0 r . r 7 1 e 6 a 0 1 d u . l 1 q 0 E 9 5 5 9 1 2 2 6 7 1 6 . 1 0 8 2 4 6 8 . 1 0 5 4 5 5 5 5 0 4 6 0 2 2 8 2 2 2 2 2 2 5 7 6 6 5 6 0 0 0 0 0 0 2 2 2 2 2 2 ...... 0 0 0 0 0 0 0 0 0 0 0 0 ...... 0 0 0 0 0 0 U 8 U 3 8 2 3 / 2 / b b P 6 P 0 6 2 0 2 8 8 1 . 0 7 6 7 1 6 . 1 0 a

5 t . M a

t 4 8 = . p

8 4 e 1 D ± c .

r 2 8 W e 0 . t 7 S 4 n 5 I 1 6 M 7 1 1 . 4 0 6 1 0 8 1 . 0 3 U U 7 8 5 5 1 3 3 6 . 2 2 0 1 / / 2 b b 6 1 P P 7 7 0 0 6 2 2 7 1 . 1 0 7 1 . B 0 4 2 - 6 8 0 4 2 1 - 1 6 9 C 0 1 E 1

2 V n t 7 F 9 M 1 S 6

h 1 k 0 . s u 0 u A r

B k u A 0 8 6 5 1 1 8 7 6 6 1 1 . . 0 6 2 8 4 2 8 0 6 0 1 7 9 5 9 3 1 5 7 7 7 5 5 6 4 6 6 6 5 5 4 5 4 4 5 5 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ...... 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 U U 8 8 3 3 2 2 / / b b P P 6 6 0 0 2 2 Figure 4. Concordia plots showing U/Pb systematics of zircons dated for this study from Elder Creek ophiolite and the Stonyford Elder Creek this study from dated for U/Pb systematics of zircons 4. Concordia plots showing Figure weighted deviation.

Geological Society of America Bulletin, May/June 2005 641 SHERVAIS et al. in both are similar. The B2 and B3 intervals are 180 younger than A1–A3 and B1. The B2 interval is 170 assigned to UAZ 7–8 (late Bathonian to early 160 Oxfordian) based on the ranges of Xitus sp. (UAZ 7–22) and Mirifusus fragilis (late transi- 150 tional form) (UAZ 3–8). The B3 interval has a 140 possible range from UAZ 7–10 (late Bathonian 130 to early Kimmeridgian), based on the ranges of SFVG-2-1 SFVG-2-2 Mirifusus dianae dianae (senior synonym of 120 M. mediodilatatus) (UAZ 7–12), Xitus (UAZ 180 7–22), and Transhsuum maxwelli (3–10). A 170 major faunal change occurs within the Locality 160 B section, wherein the relatively small-sized, polytaxic radiolarian faunas in the lower part of 150 the Stonyford sequence (A1–A3, B1) give way 140 to very robust, oligotaxic, nassellarian-domi- 130 nated faunas that include Praeparvicingula spp. SFVG-2-3 SFVG-2-4 (B2–B3, C). This change occurred sometime ) 120 during the span represented by UA Zones 6–8. a 180 M

( 170

The initial turnover appears to have predated e the local ﬁ rst appearance of M. dianae dianae 160 (worldwide range is UAZ 7–12), but it was fol- g A

150 lowed by a great increase in Praeparvicingula t associated with M. d. dianae. Therefore, faunal n 140 e turnover most likely occurred during the UAZ r

a 130

6–7 interval, or mid-Bathonian to Callovian. p SFVG-4-1 SFVG-4-2

The radiolarians collected at locality C are p 120 very similar to those at B3, but the samples A 180 also contain specimens herein assigned to 170

Podobursa spinosa (UAZ 8–13), the basis for 160 assigning this interval to UAZ 8–10, Callovian to earliest Kimmeridgian. However, closely 150 related forms range down to at least UAZ 7 in 140 the southern Coast Ranges (Hull, 1997; Hull 130 and Pessagno, 1994). SFVG-5-1 SFVG-5-2 The Diversion Dam sample (locality D) col- 120 180 lected by Pessagno (1977) is assigned to UAZ 9–10 based on the ranges of the following 170 taxa that he reported (genus names updated): 160 Mirifusus d. baileyi (UAZ 9–11), Tritrabs hayi (UAZ 3–10), Pseudocrucella sanﬁ lippoae 150 (UAZ 7–10), and Transhsuum maxwelli (UAZ 140 3–10). Other reported taxa are listed in Figure 6. 130 We did not recollect this locality because the SFVG-8-1 SFVG-8-2 120 cherts here have been subjected to hydrothermal 0 0.5 1.0 0 0.5 1.0 alteration, and many are altered to yellow-ochre, 39 botryoidal jaspers. Assuming the taxonomic Cumulative Fraction Ar Released identiﬁ cations in the 1977 report are still valid, Figure 5. 40Ar/ 39Ar apparent age spectra for glass samples from the Stonyford volcanic complex. 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 DISCUSSION graphic sequence is compared with radiolarian volcanic complex began in the Middle Juras- sequences overlying ophiolites in the southern sic, Bathonian, and continued into the early Regional Biostratigraphic Correlations Coast Ranges of California and overlying the Late Jurassic, Oxfordian or early Kimmeridg- Josephine ophiolite in the western Klamath ian. Between eruptions, siliceous radiolarian- The biostratigraphic succession in the Mountains (Fig. 1). Our regional correlations rich strata accumulated slowly on basalt base- Stonyford volcanic complex has parallels in are based on UAZ ranges supplemented by ment. Volcanic glass within the sequence of sedimentary sequences overlying Jurassic ophi- direct interbasin comparisons of local ranges radiolarites and basalt yields dates of 164 Ma, olites elsewhere in California (Fig. 7). In the such as the vertical distribution of Mirifusus as discussed previously. following discussion, the Stonyford biostrati- species (Fig. 7). Data Repository document DR-2

642 Geological Society of America Bulletin, May/June 2005 AGE RELATIONS IN THE COAST RANGE OPHIOLITE, CALIFORNIA

BIOSTRATIGRAPHY OF THE STONYFORD VOLCANIC COMPLEX RADIOLARIAN FOSSIL DATA MAXIMUM & MINIMUM UAZ RANGES CALIBRATION Calibrations based on Tethyan Unitary Association Zones (UAZ) of Baumgartner et al., 1995. Northern UA AGE Central Block Other LOCALITY TAXA ZONES Block ZONE D Mirifusus d. baileyi, Tritrabs hayi, Pseudocrucella sanfilippoae, A1 A2 A3 C B1 B2 B3 DAM Archaeodictyomitra rigida, Transhsuum maxwelli, Parvicingula n

DIVERSION a 9-10 i

DAM (undifferentiated, sensu 1977), Pantanellium riedeli. 12 n o

From Pessagno (1977). h t i Podobursa spinosa, Mirifusus d. dianae, Mirifusus guadalupensis, T c abundant Praeparvicingula, Tetraditryma corralitosensis, 11 i s s C Transhsuum maxwelli, Eucyrtidiellum ptyctum, Xitus sp., 8-10 r a e

Archaeodictyomitra sp. aff. A. rigida, Acanthocircus suboblongus r m u

(very large forms), cryptothoracic nassellarians J m

i e t 10 K a

Mirifusus d. dianae, abundant Praeparvicingula sp., Acaeniotyle L diaphoragona, Transhsuum maxwelli, Eucyrtidiellum ptyctum, n

B3 Archaeodictyomitra sp. aff. A. rigida, Xitus sp., Acanthocircus 7-10 a i

suboblongus (very large forms), cryptothoracic nassellarians, 9 d r o

demosponge spicules. f x Mirifusus guadalupensis (late form), Mirifusus fragilis (late O transitional form), rare Praeparvicingula sp., fragments of probable 8 .

Tripocyclia blakei (large), Transhsuum maxwelli, Eucyrtidiellum v

B2 7-8 o l ptyctum, Archaeodictyomitra sp. aff. A. rigida, Xitus sp., l a

Acanthocircus suboblongus (very large forms), cryptothoracic C. nassellarians. 7

Mirifusus fragilis, Hsuum brevicostatum gp., Acanthocircus

suboblongus, Podobursa helvetica, Archaeodictyomitra sp. cf. A. n B1 3-8 a i

suzukii, Turanta sp., Saitoium sp. 6 n o h t

Mirifusus guadalupensis (early form: transitional from M. fragilis), a B c

Mirifusus fragilis, Praecaneta decora, Parahsuum officerense, i s

Podobursa helvetica, Guexella nudata, Parvicingula (?) ? 5 s a

dhimenaensis, Spongocapsula palmerae, Protonuma sp., r u J

A3 Tetraditryma corralitosensis, Archaeodictyomitra sp. cf. A. suzukii, 6-6 e Eucyrtidiellum u. pustulatum, Hisocapsa convexa gp., Hsuum l d n d

brevicostatum gp., Acanthocircus suboblongus, Ristola procera, 4 i a i M

Paronaella bandyi, Xiphostylus gasquetensis gp., Leugeo c o hexacubicus. j a B Mirifusus fragilis, Praecaneta decora, Praecaneta turpicula, 3 Guexella nudata, Parvicingula (?) dhimenaensis, Spongocapsula A2 6-6 palmerae, Podobursa helvetica, Pantanellium ultrasincerum/ P. foveatum. n a

2 i

Mirifusus fragilis, Hisocapsa convexa gp., Hsuum brevicostatum n e l

A1 gp., Acanthocircus suboblongus, Pantanellium ultrasincerum/ 3-6 a P. foveatum, Saitoium sp. 1 A

Figure 6. Radiolarians faunal distributions within the Stonyford volcanic complex. Locations A, B, C described in paper; location D— Diversion Dam locale along Stony Creek, UA—Unitary Association.

(see footnote 1) includes a technical discussion been well described and documented (Hull, (UAZ 5–11) in the 3.8 m horizon indicates that of UA zonal assignments shown in Figure 7 and 1995, 1997; Pessagno, 1977; Pessagno et al., it is no older than UAZ 5 (late Bajocian or early discussed below. 1984). We have used the published faunal lists Bathonian). Mirifusus dianae dianae (UAZ 7– to assign UAZ ranges to the composite section. 12) constrains the maximimum age of the 21 m Stanley Mountain Ophiolite, Southern Coast Based on UAZ ranges, the lower part of the sec- horizon as no older than UAZ 7 (late Bathonian Ranges, California tion is Middle Jurassic in age, the upper part is or early Callovian). A well-described radiolar- At Alamo Creek near Stanley Mountain, Late Jurassic (Fig. 7). ian fauna at 27.1 m is no younger than UAZ 7. 130 m of chert, tuffaceous chert, and mudstone The radiolarians in the basal 27 m of the Based on the calibrations of the UA Zonation of overlie basalt of the Stanley Mountain ophiolite Stanley Mountain section are poorly preserved, Baumgartner et al. (1995a), the lower part of the and underlie graywacke sandstone and siliceous and the age range of the interval is poorly Stanley Mountain section may be late Bathonian shale of the Great Valley Supergroup. Radiolar- constrained. Yet, such a thickness of pelagic to early Callovian age in its entirety, although ians from the pelagic chert and mudstone unit chert and mudstone can represent millions of the lowest 20 m could be as old as late Bajocian and the lower 28 m of the Great Valley have years of deposition. Eucyrtidiellum ptyctum or early Bathonian. As discussed above, zircon

Geological Society of America Bulletin, May/June 2005 643 SHERVAIS et al. and 174 +1.2/-7.9 Palfy et al., 2000 141.8 +2.5/-1.8 150.5 +3.4/-2.8 154.7 +3.8/-3.3 156.5 +3.1/-5.1 160.5 +1.1/-0.5 166 +3.8/-5.6 178 +1.0/-1.5 (UAZ 7-12) (UAZ 5-11) (UAZ 9-11)

Parvicingula (UAZ 3-8) Praeparvicingula ses. species:

Tithonian Kimmeridgian Oxfordian Callovian Bathonian Bajocian Aalenian Praeparvicingula tuffaceous chert, mudstone graywacke, shale basalt, greenstone M. dianae correlation increase in increase in hiatus or probable M. dianae baileyi M. guadalupensis M. fragilis g f b d 9 3 4 8 6 7 5 2 x 1 ø None reported None ø 1 413 12 Berriasian 11 10 ations for the UA Zones are illustrated Zones are the UA ations for Patterns and symbols UAZ Age/StageMa Time Scale, Baumgartner et al., 1995 Mirifusus are based on the Tethyan UA Zonation UA Tethyan based on the are

species. For a given sample or interval, UA UA sample or interval, a given species. For

f s u s u i r i M b ø ø d b,g d,g,f d,g b,d,g Mirifusus 7-8 5-8 8-8 9-11 9-10

8/9-8/9

POINT SAL UAZ max. & min. ranges

m n i e l a c S 4 0 8 12 The Stanley Mtn. and Point Sal sections overlie ophiolites of the Southern Coast Ranges of California

The correlations are based on Unitary Association Zones al. et (UAZ) of Baumgartner (1995a,b) as well direct comparisons of radiolarian ranges.

s u s u f i r i M ø ø ø d ø b ø sp. b b b,d b,d b,d d,g,f 7-8 d,g,f 9-10

9-10 10-12 7-10 10-10 5-6/7

6/7-6/7 UAZ max. & min. ranges.

n i e l a c S STANLEY MTN. m 0

x 40 16 5 0 2 0 ø data or insufficient no 10 20 90 80 30

70 140 130 100 120

s u s r i M u f i b f f f g,f d,g g,f d,g e g B1: 3-8 B3: 7-10 D: 9-10 STONYFORD minimum ranges UAZ maximum &

A2: 6-6 A1: 3-6 A3: 6-6

C: 8-10

VOLCANIC COMPLEX e l a c S o N

Northern Coast Ran

M u s u f i r i s ø ø ø ø ø d d ø ø

8-11 8-10 6-10 7-10

3-6/7

UAZ max. & min. ranges

m n e c S i l a x Klamath Mtns. of California: strata above the Josephine ophiolite 90 70 60 4 030 g,f20 g,f 10 g,f B2: 7-8 80 50 SMITH RIVER

gure. The time scale of Palfy et al. (2000), which is not directly linked to the UA calibrations, is shown for reference purpo reference for calibrations, is shown to the UA linked et al. (2000), which is not directly The time scale of Palfy gure. ?

s u s u f i r i M

Z A U

5-8 5-6 6-6/7 4-5 3-5 0 1-5

e l a c S o N y t i m r o f n o c n u

x x x x

n a i v o l l a C n a i o h t a B n a i c o j a B n g a t S n a i d r o f x O BLUE MTNS. e BIOSTRATIGRAPHIC CORRELATIONS BETWEEN KEY SECTIONS IN CALIFORNIA AND OREGON Blue Mountains of northeastern Oregon: Snowshoe & Lonesome Fms., ammonite-bearing Figure 7. Regional biostratigraphic correlations of key sections in California and Oregon, using radiolarians. The correlations using radiolarians. and Oregon, sections in California of key 7. Regional biostratigraphic correlations Figure interbasin comparisons of local radiolarian ranges such as those (Baumgartner et al., 1995b) supplemented by direct Zone assignments are shown as a range (e.g., UAZ 7–8), showing maximum and minimum possible ranges. The fossil-based age calibr The fossil-based maximum and minimum possible ranges. 7–8), showing UAZ as a range (e.g., shown Zone assignments are on the right side of ﬁ

644 Geological Society of America Bulletin, May/June 2005 AGE RELATIONS IN THE COAST RANGE OPHIOLITE, CALIFORNIA

U/Pb ages of 166 ± 2 Ma have been reported for tain section (62 m) records a change in faunal al., 1994). Unlike the ophiolites of the Coast plagiogranite of the Stanley Mountain ophiolite character that Pessagno et al. (1984) and Hull Ranges, the Josephine ophiolite complex was (166 ± 1 Ma, J.M. Mattinson, reported in Pes- (1995) partly characterized as an increase in profoundly affected by Late Jurassic deforma- sagno et al., 1993); 166.0 +3.8/–5.6 Ma is the the relative abundance of Praeparvicingula and tion, burial, and metamorphism. boundary between the Bajocian and Bathonian Parvicingula. According to Hull (1995, 1997), The Josephine ophiolite is overlain locally in the time scale of Palfy et al. (2000). there is a hiatus immediately below the begin- by radiolarian-bearing strata. Pessagno et al. The boundary between strata of Middle ning of this event at Stanley Mountain. (1993) described the radiolarian stratigraphy and Late Jurassic age lies within the interval of a 95 m sequence along the Middle Fork between 28 and 62 m above basalt basement. Point Sal Ophiolite, Southern Coast Ranges of the Smith River in California (Fig. 7). The A sample 45.6 m above basalt basement has a At Point Sal, along the southern California lower half of the section is predominantly possible range of UAZ 7–8. UAZ 8 is calibrated coast, a 21 m condensed sequence of tuffa- ﬁ ne-grained siliceous mudstone and chert with as middle Callovian to early Oxfordian. The ceous pelagic chert overlies pillow basalt of the admixed tuffaceous material; the upper half of 62 m horizon is no older than UAZ 9 (middle Point Sal ophiolite. The UAZ ranges assigned the section is a metagraywacke and mudstone to late Oxfordian) because it contains Mirifusus herein are based on published taxonomic lists unit. Pessagno et al. (1993) also described the dianae baileyi (UAZ 9–11). The pelagic inter- (Baumgartner, 1995, Appendix, p. 1105–1106; radiolarians in a few chert samples interbed- val between 80 and 104 m above basement is Pessagno, 1977; Pessagno et al., 1984) (Fig. 7). ded with basalt at the Turner-Albright mine assigned a range of UAZ 10–10 (late Oxfordian The poorly preserved base of the Point Sal sec- in Oregon, near the California border (not to early Kimmeridgian) while the uppermost tion cannot be calibrated more precisely than included in Fig. 7). Baumgartner et al. (1995b) part of the tuffaceous pelagic section and the UAZ 5–8 (late Bajocian to early Oxfordian). The subsequently assigned UA Zones to the faunas. lowermost part of the clastic Great Valley Super- middle part of the section (~11.5 m to 13.4 m) is The tuffaceous pelagic strata at both localities group are constrained as no younger than UAZ no older than UAZ 8 nor younger than UAZ 9, a are Middle Jurassic. Baumgartner et al. (1995b) 12 (early to early late Tithonian). The lower part range calibrated as no older than middle Callo- calibrated chert interbedded with basalt at the of the Great Valley in this geographic area con- vian nor younger than late Oxfordian. The upper Turner-Albright Mine as Bajocian (UAZ 3–4), tains the late Tithonian ammonite Micracantho- part of the section could be as young as early and an argument can be made that the range can ceras and bivalve Buchia piochii (Hull, 1997). Tithonian. A more complete discussion is found be further constrained to UAZ 4–4 (late Bajo- Hull and Pessagno (1995) illustrated sig- in Data Repository document DR-2. cian) (see Data Repository document DR-2). niﬁ cant differences arising from calibrations Intervals A and B1 at Stonyford are correla- In the Smith River section, the pelagic interval based on the UA Zonation of Baumgartner et tive with and (or) older than the oldest, poorly from 4.1 to 13 m above the basalt is constrained al. (1995a) and their own calibrations based preserved and poorly constrained samples at only as having a possible range of UAZ 3–6/7 on the zonation of Pessagno et al. (1993). The Point Sal. Intervals B2, B3, and C at Stonyford (Bajocian to early Callovian); the pelagic inter- most important difference is that Pessagno et al. correlate best with the 10–12 m interval at Point val between 13 and 46 m is herein assigned a (1993) considered the 21 m to 27.1 m interval Sal based primarily on the ranges of Mirifusus possible range of UAZ 6–6/7 (middle Batho- at Stanley Mountain to be Late Jurassic in age, guadalupensis, M. d. dianae (M. mediodila- nian to early Callovian). These direct, radiolar- middle Oxfordian. In our opinion, their calibra- tatus), Transhsuum maxwelli, and Podobursa ian-based calibrations are much older than the tion is not tightly constrained (see Data Reposi- spinosa in both. The Diversion Dam sample previous isotope-based, time-scale–dependent tory document DR-2 discussion). Pessagno et of Pessagno (1977) at Stonyford correlates best calibrations of Pessagno et al. (1993) (see Data al. (1993, p. 113) arbitrarily assigned all the with the upper part of the Point Sal section. In Repository document DR-2 discussion). The underlying pelagic strata in the Stanley Moun- the Point Sal section, an increase in Praeparvic- UAZ ranges for the upper, metagraywacke and tain section to the middle Oxfordian as well, ingula spp. and related taxa at ~10 m (UAZ 7–8) mudstone unit of the Smith River section are which led them to conclude that a proposed (Pessagno et al., 1984) parallels a similar trend imprecise and only loosely constrain the middle hiatus between the base of the sedimentary sec- in the previously described sections (Fig. 7). part of the interval as no older than Callovian or tion and underlying basalt spanned 8–11 m.y. early Oxfordian (UAZ 8) nor younger than late (Dickinson et al., 1996; Pessagno et al., 2000). Josephine Ophiolite, Western Klamath Oxfordian or early Kimmeridgian (UAZ 10). In contrast, the UAZ calibrations shrink the pos- Mountains, Northern California and Oregon These ranges bracket Pessagno et al.’s (1993) sible duration of a basal hiatus to a few million The Josephine ophiolite complex of the West- previous interpretation of a middle Oxfordian years, if any. ern Klamath Mountains of Oregon and Califor- age for the clastic and hemipelagic interval. The Stonyford samples collected at localities nia formed at approximately the same time as, A more complete discussion of the published B2, B3, and C correlate best with the 21–60 m or slightly later than, the ophiolites of the Cali- age calibrations is included in Data Repository interval of the Stanley Mountain composite fornia Coast Ranges. Zircon 238U/206Pb ages for document DR-2. section (Fig. 7). Intervals A1–A3 at Stonyford the Josephine ophiolite range from 162 ± 1 Ma The radiolarian faunas in intervals A1–A3 also have many taxa in common with the well- (162 +7/–2 Ma; recalculated by Palfy et al., and B1 at Stonyford correlate best with the radio- described Stanley Mountain fauna collected at 2000) to 164 ± 1 Ma (Devils Elbow ophiolite) larians in the lower 45 m of tuffaceous chert and 27.1 m but may be a bit older. Accordingly, we (Wright and Wyld, 1986); the zircon 207Pb/206Pb mudstone of the Smith River sequence (Fig. 7). very tentatively correlated the A1–A3 interval age is 163 ± 5 (Harper et al., 1994). Hornblende Stonyford interval A3 correlates particularly with the poorly described lower 20 m at Stanley 40Ar/39Ar ages are 160 ± 2.5 Ma, 164.5 ± 5 Ma, well with the Smith River section between 19 Mountain based solely on their respective UAZ and 165.3 ± 5 Ma (Harper et al., 1994; includes and 22 m. Intervals B2, B3, and C at Stonyford ranges. The Stonyford Diversion Dam (D) site two ages from Devils Elbow ophiolite). The correlate best with the upper part of the Smith correlates best with the upper part (from 62 m) Josephine ophiolite probably formed by rifting River measured section. The Diversion Dam of the Stanley Mountain section. Like the Stony- within an older Mesozoic accretionary complex fauna at Stonyford may be the same age as or ford sequence, the middle of the Stanley Moun- along the margin of North America (Harper et younger than the uppermost part of the Smith

Geological Society of America Bulletin, May/June 2005 645 SHERVAIS et al.

River sequence. Data Repository document faunal turnovers within the sections were used and Blue Mountains basins moved northward DR-2 contains an expanded discussion of the as evidence for northward-directed changes in (1) relative to North America, (2) at different correlations. As previously noted in the descrip- paleolatitude, following the model of Pessagno times during the Jurassic, and (3) in trajectories tion of the Stonyford section, a pronounced and Blome (1986). The age calibrations and unrelated to the motions of one another (Pes- faunal change occurs between the B1 and B2 interbasin correlations in this study lead us to sagno et al., 1993; Pessagno and Blome, 1986; intervals. A similar faunal change occurs in the different conclusions. As previously discussed, Pessagno et al., 2000)—neither plate motion Smith River section, near the top of the pelagic a distinct faunal change occurs within each of models, paleomagnetic studies (Hagstrum section, where Pessagno et al. (1993) noted the the fi ve sections in Figure 7. In the Stonyford and Murchey, 1996; Murchey and Hagstrum, disappearance of pantanellid spumellarians and section, we describe it as a shift from relatively 1997), nor the recorrelations in this study. an infl ux of the nassellarian Praeparvicingula. small-sized, polytaxic radiolarian faunas to In summary, the early paleontologic and very robust, oligotaxic, nassellarian-dominated sedimentary histories of the Stonyford Volcanic Signifi cance of Biostratigraphic Correlations faunas that include Praeparvicingula. For Complex, Stanley Mountain ophiolite, and Point other sections, Pessagno and Blome (1986), Sal ophiolite favor Middle Jurassic origins (1) Our recorrelation of fi ve key Jurassic sec- who fi rst noted the phenomenon, characterized near sources of volcanic detritus, (2) probably tions in California and Oregon (Fig. 7) reveals the changes as a transition from faunas with in proximity to coastal currents, and (3) prob- a common history of concurrent late Middle abundant pantanellids to faunas with abundant ably traveling with the North American plate in Jurassic pelagic or hemipelagic and volcaniclas- Praeparvicingula or Parvicingula s.s. They tandem with the basins of the Blue Mountains tic sedimentation and parallel patterns of faunal observed that the two parvicingulid genera are arc complex and Josephine ophiolite complex. turnover that began slightly earlier in the north- common at high paleolatitudes and are also The Coast Range ophiolite basins persisted as ern sections. Two of the sequences, the Blue commonly associated with megafossils that pelagic depocenters into the early Late Jurassic, Mountains and Smith River sections, formed may have preferred cool temperatures. In the even as syntectonic clastic detritus prograded in arc-related settings: The former as part of a Bathonian, Praeparvicingula increased in rela- across the Sierra Nevada foothills terranes and long-lived island-arc complex (Pessagno and tive abundance in the Blue Mountains sequence collapsing Josephine ophiolite basins during the Blome, 1986), the latter as an ophiolite-fl oored in Oregon (Pessagno and Blome, 1986), and its initial phase of the Nevadan orogeny. rift basin in an older accretionary complex range began to expand southward, reaching the (Harper et al., 1994). The similarities between basins of the southern Coast Range ophiolites Signifi cance of Radioisotopic and their histories and those of the CRO basins by the Callovian or Oxfordian (Fig. 7). By the Biostratigraphic Ages, Stonyford Volcanic counter two arguments that the ophiolite basins Kimmeridgian and Tithonian, Praeparvicingula Complex of the Coast Ranges formed in an environment or Parvicingula s.s. was a component of virtually unrelated to the Blue Mountains and Smith all eastern Pacifi c faunas from high latitudes such 40Ar/39Ar plateau ages for basalt glasses from River sections: The fi rst based on a proposed as Antarctica to low latitudes of central Mexico four distinct localities within the Stonyford vol- multi-million-year hiatus following the cre- and the future Caribbean plate (Kiessling, 1999; canic complex indicate that they erupted over ation of the ophiolites and the second based on Murchey and Hagstrum, 1997). a relatively short period of time ca. 164 Ma. proposed syntectonic northeastward transport The parallelism of the faunal changes in the These age determinations are consistent with across hundreds of kilometers of the eastern fi ve sections illustrated in Figure 7 favors a the occurrence of Middle and Middle to Late Pacifi c toward North America (Dickinson et al., common cause or set of causes. The two most Jurassic radiolarians in sedimentary layers 1996; Pessagno et al., 2000). likely causes are major paleoceanographic intercalated with the volcanic rocks. This age Hopson et al. (in Dickinson et al., 1996) and change and (or) the northward migration of the overlaps previously determined U/Pb zircon Pessagno et al. (2000) proposed that the Middle North American plate. The Jurassic breakup ages for plagiogranites from the CRO else- Jurassic Coast Range ophiolites formed at a of Pangea and creation of new oceans must where (e.g., Hopson et al., 1981), but appears spreading ridge far from the North American have changed circulation patterns and cli- to be slightly younger than ages determined margin and that no pelagic sediments accumu- mate, which may account for the southward here for late quartz diorite intrusions that lated above the ophiolites for 8–10 m.y. until expansion of the range of Praeparvicingula now occur as blocks in serpentinite mélange plate motions carried the sites into the deposi- and the evolution of the even more elongated (166–172 Ma). Nearly identical U/Pb zircon tional apron of an active Late Jurassic volcanic Praeparvicingula s.s. If, for example, a strong, ages (165–172 Ma) are found for late quartz arc. The Middle Jurassic UAZ ranges for the invigorated, and relatively cool eastern bound- diorite intrusions at Elder Creek, as reported Stonyford and Stanley Mountain sedimentary ary current developed in response to plate here. Comparison between 40Ar/39Ar and U/Pb sequences and the ca. 164 Ma 40Ar/ 39Ar dates on reorganization, it might have carried the taxa ages must include consideration of the signifi - the interleaved hyaloclastites at Stonyford elim- southward. At the same time, North America cant systematic errors affecting 40Ar/39Ar ages inate the argument for a major, multi-million moved rapidly northward following the and some variants of U/Pb ages. In the 40Ar/39Ar year hiatus at the base of the pelagic sequences breakup, which would have carried associated system, uncertainties related to decay constants and indicate that the ophiolites originated near marginal basins to higher latitudes. Pessagno and standards are on the order of 2%, meaning the sources of the volcanic detritus in overlying and Blome’s (1986) basic hypothesis, that that the real accuracy of the data reported herein sedimentary strata. north-directed plate motions caused syndeposi- is ~3–4 m.y. (Min et al., 2000; Renne et al., Hopson et al. (in Dickinson et al., 1996) and tional faunal changes, may be grossly correct if 1998). Similarly, the large effects (~4 m.y. in Pessagno et al. (2000) also proposed that the the fi ve basin sequences illustrated in Figure 7 the Middle Jurassic) of U decay constant errors Middle Jurassic Coast Ranges ophiolites formed all formed along the North American margin on both 207Pb/206Pb and concordia-intercept ages near the equator, remained at low latitudes until and moved northward in unison. However, no (Ludwig, 2000) limit the absolute accuracy of the Late Jurassic, and were then transported rap- compelling evidence supports linked corollary ages determined by these means. Within such idly northward relative to North America. The hypotheses that the Coast Ranges, Josephine, limits, the Stonyford 40Ar/39Ar ages are probably

646 Geological Society of America Bulletin, May/June 2005 AGE RELATIONS IN THE COAST RANGE OPHIOLITE, CALIFORNIA indistinguishable from all but the oldest of the Dam fauna in the SFVC, leaving open the pos- based on the geochemistry of the SFVC and of above mentioned U/Pb-based ages. sibility that the eruption of the SFVC may have late MORB dikes that intrude the Elder Creek The Stonyford data and the inferred struc- been concurrrent with the last phase of arclike ophiolite. This interpretation is supported by a tural relationships cast doubt on the validity volcanism in the CRO of the Diablo Range. Jurassic reversal in the younging directions of of K-Ar ages younger than 165 Ma obtained (3) Radiolarian faunas in cherts and mud- ocean crustal fragments successively incorpo- on high-level hornblende gabbros (Lanphere, stones deposited within the Stonyford volca- rated into the Franciscan and Klamath Moun- 1971; McDowell et al., 1984) as crystallization nic complex and on top of the Coast Range tains accretionary complexes, a phenomenon ages. It is probable that the younger K-Ar dates ophiolite at other locations (e.g., Pt. Sal, Stanley that Murchey and Blake (1992, 1993) ascribed represent cooling ages, argon loss, or alteration Mountain) show a similar vertical progression to arrival of an oceanic spreading ridge along artifacts, and that formation of the ophiolite from polytaxic assemblages of relatively small the margin in the Middle to early Late Jurassic was complete by ca. 164 Ma, shortly before radiolarians to oligotaxic assemblages of large, (Fig. 8). The age data presented here suggest or during eruption of the SFVC. Mattinson and thick-walled species dominated by nassellarians. that this event occurred ca. 160–164 Ma in the Hopson (1992) have revised U-Pb zircon dates This implies that the Stonyford volcanic com- northern CRO, although it may have been later of plagiogranites from Coast Range ophiolite plex and the Coast Range ophio lite underwent in the Diablo Range. localities south of San Francisco, with newer similar changes in oceanographic environment data showing that most plagiogranites crystal- or paleolatitude and argues against formation Timing of Ophiolite Formation and lized 165–170 Ma—consistent with the results of the Stonyford complex in a location distant Associated Tectonic Events presented here. from the Coast Range ophiolite (see discussion The age relationships presented here are con- in previous section). Late Early to Early Middle Jurassic Arc sistent with the idea that the Stonyford volcanic Taken together, the evidence suggests that the Collision or Collapse (175–185 Ma) and the complex was built on a substrate of “oceanic” Stonyford volcanic complex was built on a sub- Formation of a Nascent Subduction Zone crust represented by the older Coast Range strate of “normal” Coast Range ophiolite after (ca. 172 Ma) ophiolite assemblage. This idea is supported by most suprasubduction zone magmatism came Wright and Fahan (1988) were among the ﬁ rst three independent lines of evidence. to an end in the northern CRO, but possibly to show that orogenesis in the Jurassic was not (1) The high-Al, low-Ti basalt suite at Stony- concurrent with “stage 3” ophiolite magmatism conﬁ ned to a singular event in the Late Jurassic ford resembles island arc basalts similar to those (Shervais, 2001) in the Diablo Range. Shervais “Nevadan orogeny.” They were able to document found elsewhere in the Coast Range ophiolite (2001) and Shervais et al. (2004) have proposed that many structures attributed to the Nevadan (e.g., Shervais, 1990; Shervais and Kimbrough, that eruption of the SFVC corresponds to a orogeny in previous studies are in fact Middle 1985) in their major element characteristics, but ridge-subduction event in the northern CRO, Jurassic in age and must have formed during have trace element characteristics that suggest addition of an enriched component to a depleted source region (for example, La/Smn ratios rang- After Murchey and Blake (1993) ing from 0.34 to 1.78). High-Al, low-Ti basalts 300 are found interbedded with ocean island tholei- North Fork, East Hayfork ites and alkali basalts at all stratigraphic levels Spreading Age of the volcanic complex, which requires that the 250 Residence Time depleted, arclike source of the high-Al basalts and the plume-enriched source of the tholeiitic Rattlesnake Creek Marin Headlands, and alkali basalts were physically contiguous 200 Van Ardsdale, The Geyers (Zoglman and Shervais, 1991). Note that high- Yolla Bolly Al, low-Ti basalts are not found in ocean island Burnt Hills, Elder Creek, Pickett Peak Permanente basalt suites of the Franciscan assemblage, 150 SFVC, W. Klamath which contain almost exclusively ocean island tholeiites and alkali basalts (MacPherson, 1983; Ridge Ridge? MacPherson et al., 1990; Shervais, 1990; Sher- 100 Collision Coastal Belt vais and Kimbrough, 1987). ≈155 Ma (2) Based on the radiolarian assemblages, King eruption of the SFVC was concurrent with 50 Range

deposition of the volcano-pelagic section Ridge Age Residence Time/ (tuffaceous chert) on top of the CRO at locali- 0 ties in the southern Coast Ranges, e.g., Pt. Sal, 200 150 100 50 0 Stanley Mountain (see discussion in previous Ridge section). Thus, eruption of the SFVC must Arrival time (Ma) have postdated formation of the main ophiolite Figure 8. Ages of accreted terranes and ages of accretion, after Murchey and Blake (1993). at these localities—consistent with the results Upper curve represents age of spreading center, based on fossils and age radioisotopic dates, of our new, high-precision U-Pb zircon dates. plotted as function of arrival time at the western margin of North America. Lower curve Pessagno (in Hopson et al., 1981) tentatively represents residence time of accreted crust (= age of formation minus age of accretion) as correlated unspeciﬁ ed, very poorly preserved function of arrival time at the western margin of North America. The age reversal and radiolarians associated with stage 3 volcanic age minimum in residence time at ca. 155 ± 5 Ma, which corresponds to the arrival times, rocks in the Diablo Range with the Diversion implies ridge collision.

Geological Society of America Bulletin, May/June 2005 647 SHERVAIS et al.

Coast Range Sierra Nevada Ophiolite Foothills 140 Ma Berriasian 141.8 (+2.5/-1.8) "Late Nevadan" Sinistral Ductile Deformation 145-123 Ma Tithonian Owens Mtn BMFZ 150 Ma 150.5 (+3.4/-2.8) Dike Swarm Independence -MFZ Dike Swarm Kimmeridgian JKf High 154.7 (+3.8/-3.3) Grade Blocks

Upper Jurassic Oxfordian 156.5 (+3.1/-5.1) Nevadan Event YRP 160 Ma Callovian Stonyford 160.4 (+1.1/-0.5) Glass Logtown Ridge HCP Bathonian StP ScP EGC 166.0 (+3.8/-5.6)

BRT "Overlap" 170 Ma Bajocian Smartville Ophiolite

174.0 (+1.2/-7.9) Elder Creek

Middle Jurassic Ophiolite Aalenian Foothills Event 174-185 Ma 178.0 (+1.0/-1.5) Compressive, Arc Collision (?) Stonyford Tuttle 180 Ma Mélange Lake Fm Toarcian Blocks 183.6 (+1.7/-1.1) Fiddle Creek UC Complex Pliensbachian 190 Ma Peñon 191.5 (+1.9/-4.7) Blanco Sinemurian

Lower Jurassic 196.5 (+1.7/-5.7) Slate Creek Complex/ Smartville Wallrock Sailor Hettangien Canyon Foothills Suture Zone? 200 Ma 199.6 (±0.4) Fm Calaveras Norian exotic arc terranes? West Complex East Triassic BMFZ-MFZ = Ductile Deformation HCP = Haypress Creek Pluton 166 Ma along Bear Mtn/Melones Fault Zones EGC = Emigrant Gap Complex 168 Ma Post Deformational ScP = Scales Pluton 168 Ma Plutons StP = Standard Pluton 166 Ma YRP = Yuba Rivers Pluton 159 Ma Time-scale correlation from Palfy et al., 2000 BRT = Bloody Run Tonalite 165 Ma

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

an earlier orogenic event, termed the “Siskiyou extension that formed the Coast Range ophiolite. (e.g., Edelman et al., 1989a; Edelman and Sharp, event” in the Klamaths (Coleman et al., 1988; Siskiyou compression was followed by regional 1989; Girty et al., 1995; Saleeby, 1990; Saleeby Hacker et al., 1995). The Siskiyou event in the extension ca. 167–155 Ma, also overlapping et al., 1989). This event is bracketed in part by Klamaths postdates the ca. 172–169 Ma Western in part formation of the Coast Range ophiolite Middle Jurassic plutons (Standard, Scales, Emi- Hayfork arc and predates the ca. 164–159 Ma (Hacker et al., 1995). grant Gap, Haypress Creek, and Bloody Run Wooley Creek plutonic belt (Renne and Scott, Work in the Sierra Nevada foothills has docu- plutons), which intrude structures that deform 1988). Thus, Siskiyou compressional deforma- mented the importance of an older, early Middle older, ca. 200 Ma arc volcanics and plutonics tion (post-169, pre-164 Ma) was coincident with Jurassic event in the Sierra Nevada arc realm (e.g., Peñon Blanco arc, Slate Creek complex,

648 Geological Society of America Bulletin, May/June 2005 AGE RELATIONS IN THE COAST RANGE OPHIOLITE, CALIFORNIA

Penon Blanco/ comprises a chert-argillite terrane with volcanic Sierran Arc A. Slate Creek Arc 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 Late Triassic to or ??? Shoo Fly complexes are juxtaposed along a Early Jurassic: west-vergent thrust fault that is dated at 176 ± 206-180 Ma 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. Rapid Formation of CRO Early Mid-Jurassic The most conservative interpretation of as Oceanic Slab Sinks: B. Collision (?): 176-180 Ma these data is that the late Early Jurassic to early 174-164 Ma Middle Jurassic deformation discussed above Mid-Jurassic: corresponds to the collapse of a fringing arc 180-164 Ma 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; C. Ridge Collision/Subduction: Saleeby et al., 1989; Sharp, 1988). This island 164-160 Ma arc, the Foothills arc terrane, may have formed Late Mid-Jurassic: above a west-dipping subduction zone prior 163-160 Ma to the ca. 174–185 Ma collision, but inherited zircons in the Slate Creek complex suggest an Post-collision origin proximal to the continental margin (Bick- Plutons: ≈160 Ma ford and Day, 2004). Stonyford Volcanics; MORB Dikes at Elder Creek, Our data show that formation of the CRO in Mt. Diablo, Del Puerto, Sierra Azul, Cuesta Ridge California began shortly after the early Middle Jurassic deformation event documented in (1) Shallow subduction, Compression: the Sierra Nevada foothills at 174–185 Ma Late Jurassic: Classic Nevadan Orogeny 159-155 159-150 Ma (Fig. 9). We propose that formation of the CRO (2) Change from direct convergence to Sinistral coincided with the establishment of a new or transtension as NA moves Northward: 156-144 Ma newly reorganized, east-dipping subduction Figure 10. Model for Jurassic evolution of CRO: (A) Early Jurassic fringing/exotic arc; zone beneath the amalgamated Sierran ter- (B) Toarcian-Aalenian (185–175 Ma) collision of fringing/exotic arc with continental mar- ranes around ≈172 Ma (Fig. 10). Formation of gin; followed by formation of CRO during Bajocian-Bathonian (165–172+) by hinge roll- the CRO above this nascent subduction zone back during initiation of proto–Franciscan subduction zone; also intrusion of undeformed probably proceeded in response to sinking of plutons in Sierra Foothills, including Haypress Creek, Emigrant Gap, Standard, Scales, and the backarc basin crust and rapid extension of Smartville complex, that crosscut fabric and structures formed during earlier arc collision; the nascent forearc region into the gap created (C) mid-Callovian collision with active spreading ridge, overlaps arc volcanism in Foothill; by rollback of the subduction hinge (Kimbrough eruption of SFVC and high-grade metamorphism of oceanic crust in the proto–Franciscan and Moore, 2003; Shervais, 2001; Shervais et subduction zone, followed by late Callovian through Oxfordian compression in Sierra Foot- al., 2004; Stern and Bloomer, 1992). hills, continued chert deposition in CRO, and ﬁ nally by Kimmeridgian transition to ductile, The suggestion of a zircon component with sinistral shear deformation in Sierra foothills, deposition of GVS on top of CRO. 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), older wallrock of the Smartville complex; Jurassic, is deformed by these same structures implies that ophiolite formation was linked to Fig. 9). The Peñon Blanco arc has been dated at (Edelman et al., 1989b). The Red Ant schist, a the continental margin. Bickford and Day (2001) 196–200 Ma (Saleeby, 1982) and is underlain by lawsonite-bearing blueschist that has been over- conclude that the Proterozoic inherited zircon fault-bounded garnet amphibolites dated at 178 printed by pumpellyite-actinolite assemblages component they identiﬁ ed in the Smartville plu- ± 3 Ma by 40Ar/ 39Ar (Sharp, 1988). The Fiddle (Edelman et al., 1989b; Hacker, 1993), has tons was derived from the underlying Shoo Fly Creek complex, a chert-argillite assemblage been dated by K-Ar as ≈174 Ma (Schweickert complex, and that both the Smartville ophiolite that sits on ophiolitic mélange and ranges in et al., 1980); this is considered a minimum age and the older Slate Creek complex formed proxi- age from Late Triassic to possibly early Middle for metamorphism. The Calaveras complex mal to the continental margin.

Geological Society of America Bulletin, May/June 2005 649 SHERVAIS et al.

Formation of High-Grade Blocks in the or transtension). This is essentially the same CONCLUSIONS Franciscan (160–165 Ma), Ridge Collision, and conclusion reached by Murchey and Blake Correlation with the Coast Range Ophiolite (1992; 1993). Cloos (1993, page 733) summa- New high-precision U/Pb zircon dates from the High-grade metamorphic blocks in the Fran- rized his fi ndings thus: northern Coast Range ophiolite show that ophio- ciscan assemblage and high-grade terranes in the lite formation began before ≈172 Ma and was Eastern Belt of the Franciscan (e.g., Taliaferro The subduction of spreading ridges will cause vertical largely complete by ≈164 Ma. These ages postdate metamorphic complex) seemed to have formed isostatic uplift and subsidence of as much as 2 to 3 km the Toarcian to Aalenian (185–174 Ma) collision of ca. 160–165 Ma (Mattinson, 1988)—that is, in the forearc region compared to when 80-m.y.-old an exotic or fringing arc to the Cordilleran margin. coincident with the postulated ridge colli- oceanic lithosphere is subducted. The subduction We propose that ophiolite forma tion began in of an active spreading center causes such a major sion event in the northern CRO. We suggest response to this collision during the establishment perturbation in the margin’s thermal structure that that the Franciscan high-grade blocks formed evidence of the event is likely to be recorded widely in of a nascent, east-dipping, proto–Franciscan sub- in response to the elevated thermal gradients the geology of the forearc block. duction zone, as proposed by Stern and Bloomer caused by ridge collision. The high-grade blocks (1992) and Shervais (1990, 2001). are too young to have formed during subduction New high-precision 40Ar/ 39Ar laser fusion initiation, as proposed by Wakabayashi (1990), Ward (1995) analyzed long-term cyclical dates on unaltered samples of volcanic glass which we have interpreted to be at least 172 Ma changes in the style, composition, and loca- from the Stonyford volcanic complex show that or older, based on the oldest CRO dates. As tion of magmatism along North America and it formed during a brief interval of eruption ca. noted by Wakabayashi (1990), 10–12 m.y. after concurrent changes in inferred plate motions 164 Ma, shortly after completion of ophiolite the initiation of subduction, the hanging wall of North America relative to ocean plates and formation and just prior to the onset of “main lithosphere would be too cold to form high-grade noted parallels between Miocene and Jurassic phase” Nevadan deformation in the Sierra amphibolites and garnet amphibolites by heating events. Jurassic subduction rates along North foothills (e.g., Tobisch et al., 1989). This time from above. The problem of subduction-zone America varied in response to changes in plate period corresponds to the ages of high-grade refrigeration has been noted in other ophiolites motions and convergence. Both relative and amphibolite and garnet amphibolite blocks in as well (e.g., Hacker and Gnos, 1997). The absolute plate motions changed signifi cantly at the Franciscan assemblage (Mattinson, 1988; high-grade blocks represent a signifi cant thermal the beginning of the Late Jurassic—coincident Ross and Sharp, 1988) and shortly precedes a event that requires high thermal fl ux at the base with the ridge subduction event postulated change in plate motion for North America from of the ophiolite, which we believe could only be here and just prior to the Nevadan orogeny. slow westward drift to rapid northward motion provided by collision with an active spreading Plate motion studies (Engebretson et al., 1985; (May et al., 1989; Ward, 1995). It is roughly ridge at the time required. May et al., 1989; Ward, 1995) and structural bracketed by a reversal in the younging polarity analysis of the Foothills terrane (Tobisch et of ocean crustal fragments in accreted terranes Ridge Collision and the Late Jurassic al., 1989) both show a change from relative of the Klamaths (westward younging through Nevadan Orogeny (155 ± 5 Ma) convergence between North America and plates the Early Jurassic) and the Franciscan (eastward The classic Nevadan orogeny was a Late of the Pacifi c basin in the Middle Jurassic to left younging from Late Jurassic to mid-Creta- Jurassic contractional event that folded, faulted, lateral transpression (in the upper plate) dur- ceous; Murchey and Blake, 1992, 1993). Taken and metamorphosed strata as young as the ing the early Late Jurassic. Using these lines together with fi eld and geochemical observa- Oxfordian to early Kimmeridgian (?) age Mari- of evidence, which are distinct from those of tions in the SFVC and at Elder Creek, these data posa Formation, a probable syntectonic fl ysch Murchey and Blake (1993) or Shervais (2001), are interpreted to require collision with and/or deposit in the foothills of the Sierra Nevada. Ward (1995) also concluded that an ocean subduction of a major oceanic spreading ridge Radioisotopic dates on Nevadan structures date spreading ridge arrived along the margin in the axis (Shervais et al., 2004). deformation ca. 155 ± 5 Ma (Schweickert et al., late Middle Jurassic and speculated that Great Radiolarians from cherts and siliceous mud- 1984; Tobisch et al., 1989). This event is now Valley basement might be analagous to the Gulf stones intercalated within the Stonyford volca- generally believed to postdate suturing of the of California. nic complex are correlative with those found eastern and western arc terranes of the Sierra Initial shortening during the Nevadan orog- in cherts that overlie the Coast Range ophiolite Nevada (Edelman and Sharp, 1989; Saleeby, eny can be attributed to shallow subduction in the southern Coast Ranges. The radiolar- 1982, 1983a; Saleeby et al., 1989; Tobisch et of the young, buoyant oceanic lithosphere ians in the SFVC range in age from Bathonian al., 1989), and therefore the Nevadan orogeny (Fig. 10). The change from contractional to (Unitary Association Zone 6–6 of Baumgartner cannot represent an arc-arc or arc-continent col- ductile shear deformation documented by et al., 1995a) near the base of the complex to lision, as proposed previously. Tobisch et al. (Tobisch et al., 1989) in the Foot- late Callovian to early Kimmeridgian (Unitary The main phase of Late Jurassic contrac- hills terrane during the latest Jurassic and Early Association Zones 8–10) in the upper part. The tional deformation in the Sierra Nevada began Cretaceous (that is, post-Nevadan) is consis- Stonyford cherts are interbedded with hyalo- shortly after the ridge collision event, which tent with the onset of left lateral strike-slip clastite breccias containing volcanic glass that occurred ca. 160–164 Ma in the northern CRO. motion within the arc terrane predicted by the we have dated at ≈164 Ma, using high-preci- We propose a direct tectonic link between the ridge collision model. Prior to this transpression 40Ar/39Ar laser-heating methods applied ridge collision event and the Nevadan orogeny sional deformation (Saleeby, 1978; Saleeby, to carefully selected samples. These data show (Fig. 9). Ridge collision will result in shallow 1981; Saleeby, 1982), the dominant stress was that, contrary to the interpretations of Pessa- underthrusting of the buoyant subducting oce- transtensional, as documented by dike swarms gno and coworkers (Hopson et al., 1997; Hull anic lithosphere (e.g., Cloos, 1993), heating of in the Sierra foothills and in the eastern Sier- et al., 1993; Pessagno et al., 1987; Pessagno the superjacent lithosphere, and a likely change ras (Owens Mountain and Independence dike et al., 2000), we see no evidence of a major in relative convergence directions (e.g., from swarms; Fig. 9) (Wolf and Saleeby, 1992; Wolf depositional hiatus following formation of the direct convergence to strike-slip, transpression, and Saleeby, 1995). ophiolite and thus no requirement that it formed

650 Geological Society of America Bulletin, May/June 2005 AGE RELATIONS IN THE COAST RANGE OPHIOLITE, CALIFORNIA

in the open ocean far from its current loca- Baumgartner, P.O., 1995, Towards a Mesozoic radiolarian fornia: U.S. Geological Survey Mineral Investigations tion. Further, the recorrelations presented here database—Updates of work 1984–1990, in Baumgart- Field Study Map MF-279, scale 1:48,000. ner, P.O., O’Dogherty, L., Gorican, S., Urquhart, E., Brown, R.D., Jr., 1964b, Thrust-fault relations in the north- support the idea that parallel turnovers in the Pillevuit, A., and De Wever, P., eds., Middle Jurassic to ern Coast Ranges, California, Short papers in geology character of the radiolarian assemblages in the Lower Cretaceous Radiolaria of Tethys: Occurrences, and hydrology: U.S. Geological Survey Professional Systematics, Biochronology: Lausanne, Switzerland, Paper 475-D, p. D7–D13. basins of the CRO, the Josephine ophiolite, and Memoires de Geologie (Lausanne), v. 23, p. 689–700 Cerling, T.E., Brown, F.H., and Bowman, J.R., 1985, Low the Blue Mountains arc, occurring slightly later and Appendix: p. 1091–1106, 1144–1150. temperature alteration of volcanic glass: Hydration, in the southern basins, refl ect a shared history Baumgartner, P.O., Bartolini, A., Carter, E.S., Conti, M., Na, K, 18O and Ar mobility: Isotope Geoscience, Cortese, G., Danelian, T., De Wever, P., Dumitrica, P., v. 52, p. 281–293. of oceanographic change such as the expansion Dumitrica-Jud, R., Gorican, S., Guex, J., Hull, D.M., Cloos, M., 1986, Blueschists in the Franciscan Complex of of cool coastal currents and (or) synchronized Kito, N., Marcucci, M., Matsuoka, A., Murchey, B.L., California; Petrotectonic constraints on uplift mecha- O’Dogherty, L., Savary, J., Vishnevskaya, V., Widz, D., nisms, in Evans, B.W., Brown, E.H,. eds., Blueschists plate motion trajectories rather than a separate and Yao, A., 1995a, Middle Jurassic to Early Creta- and eclogites: Geological Society of America Memoir history for the CRO relative to North America. ceous radiolarian biochoronology of Tethys based 164, p. 77–93. We propose that this ridge collision event was on Unitary Associations, in Baumgartner, P.O., Cloos, M., 1993, Lithospheric buoyancy and collisional oro- O’Dogherty, L., Gorican, S., Urquhart, E., Pillevuit, A., genesis; Subduction of oceanic plateaus, continental the driving force behind the classic, Late Juras- and De Wever, P., eds., Middle Jurassic to Lower Creta- margins, island arcs, spreading ridges, and seamounts: sic Nevadan orogeny. The resulting change in ceous Radiolaria of Tethys: Occurrences, Systematics, Geological Society of America Bulletin, v. 105, p. 715– Biochronology: Lausanne, Switzerland, Memoires de 737, doi: 10.1130/0016-7606(1993)1052.3.CO;2. relative and absolute plate motions is recorded Geologie (Lausanne): p. 1013–1048. Coleman, R.G., Manning, C.E., Mortimer, N., Donato, M.M., in deformation fabrics and dike swarms in the Baumgartner, P.O., O’Dogherty, L., Gorican, S., Dumitrica- and Hill, L.B., 1988, Tectonic and regional meta- Jud, R., Dumitra, P., Pillevuit, A., Urquhart, E., Matsuoka, morphic framework of the Klamath Mountains and Sierras (Tobisch et al., 1989; Wolf and Saleeby, A., Danelian, T., Bartolini, A., Carter, E.S., DeWever, P., adjacent Coast Ranges, California and Oregon, in 1992; Wolf and Saleeby, 1995); subsequent duc- Kito, N., Marcucci, M., and Steiger, T., 1995b, Radiolar- Ernst, W.G., ed., Rubey Volume 7: Metamorphism and tile deformation in the Sierra foothills (Tobisch ian catalogue and systematics of Middle Jurassic to Early Crustal Evolution of the Western United States: Engle- Cretaceous Tethyan genera and species, in Baumgartner, wood Cliffs, New Jersey, Prentice-Hall, p. 1061–1097. et al., 1989) resulted from sinistral transpression P.O., O’Dogherty, L., Gorican, S., Urquhart, E., Pillevuit, Dickinson, W.R., 1971, Plate tectonic models for orogeny at following this event. A., and De Wever, P., eds., Middle Jurassic to Lower Cre- continental margins: Nature, v. 232, p. 41–42. taceous Radiolaria of Tethys: Occurrences, Systematics, Dickinson, W.R., and Rich, E.I., 1972, Petrologic Inter- All of the events described here are consistent Biochronology: Lausanne, Switzerland, Memoires de vals and Petrofacies in the Great Valley Sequence, with the model for suprasubduction zone ophiol- Geologie (Lausanne), p. 37–685. Sacramento Valley, California: Geological Society of ite formation published by Shervais (2001). This Beaman, B.J., 1989, Evolutional history of the plutonic America Bulletin, v. 83, p. 3007–3024. section of the Elder Creek ophiolite: A remnant of Dickinson, W.R., Hopson, C.A., Saleeby, J.B., Schweickert, model proposes that suprasubduction zone ophio- the Coast Range ophiolite, California [M.S. thesis]: R.A., Ingersoll, R.V., Pessagno, E.A., Jr., Mattinson, lites form in response to hinge retreat in nascent Columbia, South Carolina, University of South Caro- J.M., Luyendyk, B.P., Beebe, W., Hull, D.M., Munoz, lina, 185 p. I.M., and Blome, C.D., 1996, Alternate origins of the or newly reorganized subduction zones, and that Berkland, J.O., Raymond, L.A., Kramer, J.C., Moores, Coast Range Ophiolite (California); Introduction and they display a consistent life cycle as the subduc- E.M., and O’Day, M., 1972, What is Franciscan?: The implications: GSA Today, v. 6, no. 2, p. 1–10. tion zone matures. The fi nal stage of ophiolite for- American Association of Petroleum Geologists Bul- Dumitrica, P., 1970, Cryptocephalic and cryptothoracic Nas- letin, v. 56, no. 12, p. 2295–2302. sellaria in some Mesozoic deposits of Romania: Revue mation typically involves subduction of an active Bickford, M.E., and Day, H.W., 2004, Tectonic setting of the Roumaine de Geologie, Geophysique, et Geologie, spreading center, as observed in the CRO. Jurassic Smartville and Slate Creek complexes, north- Serie de Geologie, v. 14, p. 45–124. ern Sierra Nevada, California: Geological Society of Edelman, S.H., and Sharp, W.D., 1989, Terranes, early faults, The data presented here also show that the America Bulletin, v. 116, p. 1515–1528. and pre-Late Jurassic amalgamation of the western problem of ophiolite genesis cannot be resolved Blake, M.C., Jr., and Jones, D.L., 1981, The Franciscan Sierra Nevada metamorphic belt, California: Geologi- assemblage and related rocks in northern California; A cal Society of America Bulletin, v. 101, p. 1420–1433, in isolation, without consideration of the tec- reinterpretation, in Ernst, W.G., ed., The Geotectonic doi: 10.1130/0016-7606(1989)1012.3.CO;2. tonic evolution of the entire orogenic system Development of California; Rubey Volume I.: Engle- Edelman, S.H., Day, H.W., and Bickford, M.E., 1989a, of which it is part. Further, it is not possible to wood Cliffs, New Jersey, Prentice-Hall, p. 307–328. Implications of U-Pb zircon ages for the Smartville Blake, M.C., Jr., Jayko, A.S., and Howell, D.G., 1982, and Slate Creek complexes, northern Sierra Nevada, understand the evolution of an orogenic system Sedimentation, metamorphism and tectonic accretion California: Geology, v. 17, p. 1032–1035, doi: 10.1130/ without a clear understanding of the ophiolites of the Franciscan assemblage of Northern California, 0091-7613(1989)0172.3.CO;2. in Leggett, J., ed., Trench-Forearc geology; Sedimenta- Edelman, S.H., Day, H.W., Moores, E.M., Zigan, S., Mur- found within it. tion and tectonics on modern and ancient active plate phy, T.P., and Hacker, B.R., 1989b, Structure across a margins: Geological Society of London Special Publi- Mesozoic ocean-continent suture zone in the northern ACKNOWLEDGMENTS cation 10, p. 433–438. Sierra Nevada, California: Geological Society of Blake, M.C., Jr., Jayko, A.S., Jones, D.L., and Rogers, B.W., America Special Paper 224, 56 p. 1987, Unconformity between Coast Range Ophiolite Engebretson, D.C., Cox, A.C., and Gordon, R.G., 1985, Rel- This paper would not have been possible without and part of the lower Great Valley Sequence, South ative motions between oceanic and continental plates the pioneering work and insights of Cliff Hopson, Fork of Elder Creek, Tehama County, California, in in the Pacifi c basin: Geological Society of America who introduced us (Shervais, Kimbrough, Hanan) to Hill, M.L., ed., Cordilleran Section of the Geological Special Paper 206, 59 p. the Coast Range ophiolite and who has provided the Society of America: Boulder, Colorado, Centennial Ernst, W.G., 1970, Tectonic contact between the francis- inspiration for our continued work there. This research fi eld guide, v. 1, p. 279–282. can melange and the great valley sequence—Crustal Blake, M.C., Jr., Jayko, A.S., McLaughlin, R.J., and expression of a late mesozoic benioff zone: Journal of was supported by NSF grants EAR8816398 and Underwood, M.B., 1988, Metamorphic and tectonic Geophysical Research, v. 75, no. 5, p. 886–901. EAR9018721 (Shervais) and EAR9018275 (Kimbrough evolution of the Franciscan Complex, Northern Cali- Evarts, R.C., Coleman, R.G., and Schiffman, P., 1999, The and Hanan). Geologic mapping of the Elder Creek and fornia, in Ernst, W.G., ed., Metamorphism and crustal Del Puerto ophiolite: Petrology and tectonic setting, in Stonyford ophiolites formed parts of Master’s Theses by evolution of the Western United States: Rubey Vol- Wagner, D.L., and Graham, S.A., eds., Geologic Field Joe Beaman (1989) and Marchell Zoglman Schuman ume 7: Englewood Cliffs, New Jersey, Prentice-Hall, Trips in Northern California: Sacramento, California (Zoglman, 1991) at the University of South Carolina. p. 1035–1060. Division of Mines and Geology, p. 136–149. Blake, M.C., Jr., McLaughlin, R.J., and Jones, D.L., 1989, Giaramita, M., MacPherson, G.J., and Phipps, S.P., 1998, Sedimentation and tectonics of western North America: Petrologically diverse basalts from a fossil oceanic REFERENCES CITED Terranes of the northern Coast Ranges, in Blake, M.C., forearc in California; The Llanada and Black Mountain Jr., Harwood, D.S., and Hanshaw, P.M., eds., Field trips remnants of the Coast Range Ophiolite: Geological for the 28th International Geological Congress, Volume Society of America Bulletin, v. 110, p. 553–571, doi: Bailey, E.H., Irwin, W.P., and Jones, D.L., 1964, Franciscan 2: Tectonic evolution of Northern California: Washing- 10.1130/0016-7606(1998)1102.3.CO;2. and related rocks, and their signifi cance in the geology ton, D.C., American Geological Institute, p. 3–18. 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Geological Society of America Bulletin, May/June 2005 653 Data Repository Table DR-1. Ar release spectra for volcanic glasses of the Stonyford volcanic complex.

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

G-2/3524-01 J= 0.01950 ±0.00004 1.4 5.4505 5.9027 0.0044 4.6056 84.2 155.2 4.6 1.8 5.1465 5.8310 0.0033 4.6299 89.6 156.0 4.1 2.2 5.3367 5.7358 0.0040 4.6148 86.1 155.5 2.0 2.5 5.1368 5.8798 0.0028 4.7716 92.5 160.5 2.0 2.6 4.9393 5.7819 0.0017 4.8953 98.7 164.5 1.6 2.7 4.9145 5.8171 0.0018 4.8262 97.8 162.3 3.5 2.8 4.9158 5.8244 0.0017 4.8758 98.8 163.9 1.0 2.9 4.9315 5.7907 0.0017 4.8810 98.6 164.0 1.1 3.0 4.9872 5.8463 0.0019 4.8764 97.4 163.9 2.5 3.2 4.9254 5.7464 0.0016 4.9005 99.1 164.7 1.4 3.4 4.9692 5.8184 0.0017 4.9313 98.9 165.7 1.8 3.6 4.9866 5.7856 0.0019 4.8833 97.6 164.1 1.7 3.9 4.9923 5.7711 0.0019 4.8952 97.7 164.5 1.6 4.3 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 ±0.00004 0.5 5.8165 6.0492 0.0057 4.6141 79.0 155.4 2.0 0.7 5.0914 5.8338 0.0027 4.7625 93.2 160.2 2.1 0.9 5.0515 5.8597 0.0018 4.9777 98.2 167.1 3.6 1.1 4.9347 5.8352 0.0018 4.8596 98.1 163.4 1.8 1.2 4.9441 5.8824 0.0017 4.9117 99.0 165.0 3.1 1.3 4.9320 5.8431 0.0017 4.8792 98.5 164.0 1.8 1.4 4.9759 5.5631 0.0017 4.9222 98.6 165.4 1.7 1.5 5.0186 5.8939 0.0019 4.9072 97.4 164.9 1.0 1.6 4.9765 5.9813 0.0019 4.8764 97.6 163.9 1.8 1.7 5.3531 6.0566 -0.0128 9.6262 179.1 310.4 112.8 1.8 6.2775 5.5654 0.0103 3.6629 58.1 124.5 40.8 1.9 5.7823 5.7991 0.0106 3.0966 53.3 105.8 50.9 2.0 5.3008 6.0194 0.0008 5.5443 104.2 185.2 198.4 Plateau Age (Ma) 164.6 1.4 G-2/3524-04 J= 0.01950 ±0.00004 1.9 8.1989 5.8326 0.0175 3.4911 42.4 118.8 6.9 2.2 6.6895 4.9392 0.0174 1.9232 28.7 66.4 26.2 3.0 5.0310 5.8458 0.0024 4.7939 94.9 161.2 1.3 3.2 5.1426 5.8311 0.0029 4.7334 91.7 159.3 5.0 3.5 5.9477 5.8031 0.0068 4.3954 73.6 148.4 7.7 3.8 5.5090 5.7228 0.0037 4.8654 88.0 163.5 2.7 4.1 5.1469 5.9894 0.0046 4.2581 82.4 143.9 3.3 4.4 5.0473 5.8604 0.0021 4.8819 96.3 164.1 4.9 4.7 5.0708 5.8570 0.0025 4.8000 94.3 161.4 2.8 5.0 5.1480 5.9151 0.0045 4.2916 83.0 145.0 3.6 5.3 5.0510 5.8031 0.0053 3.9279 77.5 133.2 8.6 5.6 5.1025 5.8927 0.0048 4.1401 80.8 140.1 2.9 5.9 5.1409 5.9067 0.0038 4.4774 86.8 151.0 4.9 6.2 5.0528 5.8676 0.0023 4.8442 95.5 162.9 1.6 6.5 5.0354 5.8699 0.0020 4.8933 96.8 164.4 0.9 0.0 5.2367 5.8662 0.0027 4.9013 93.2 164.7 0.9 Plateau Age (Ma) 164.3 1.4

G-2/4445-01 J= 0.02320 ±0.00004 0.4 14.9898 6.4896 0.0420 3.0740 20.4 124.3 17.9 0.5 7.0663 5.9758 0.0135 3.5433 49.9 142.5 13.2 0.6 5.8320 5.8604 0.0087 3.7203 63.5 149.4 18.3 0.7 4.9880 5.9442 0.0062 3.6141 72.2 145.3 10.5 0.9 4.5307 5.9899 0.0033 4.0361 88.7 161.5 3.9 1.1 4.6507 5.9222 0.0034 4.1178 88.2 164.6 3.3 1.2 4.6642 5.8001 0.0034 4.1010 87.6 164.0 2.9 1.3 4.5726 5.8537 0.0031 4.1131 89.6 164.4 2.1 1.4 4.5978 5.8065 0.0033 4.0744 88.3 162.9 8.2 1.5 4.5179 5.8320 0.0032 4.0398 89.1 161.6 7.6 1.7 4.9304 6.0073 0.0045 4.0540 81.9 162.2 4.1 2.0 4.6137 6.0569 0.0033 4.1163 88.9 164.6 2.9 Plateau Age (Ma) 163.8 2.4 G-4/4446-01 J= 0.02320 ±0.00004 0.7 6.1796 4.9144 0.0087 3.9823 64.2 159.4 5.0 0.9 4.8111 4.7686 0.0036 4.1068 85.1 164.2 11.4 1.1 4.6384 4.7346 0.0028 4.1661 89.5 166.4 8.6 1.2 4.6640 4.6821 0.0026 4.2567 91.0 169.9 9.6 1.3 4.5743 4.7266 0.0031 4.0330 87.9 161.4 7.8 1.4 4.4977 4.5846 0.0025 4.1136 91.2 164.4 10.7 1.5 4.3730 4.6235 0.0024 4.0171 91.6 160.8 10.0 1.6 4.6900 4.7208 0.0034 4.0564 86.2 162.3 13.7 1.7 4.2593 4.8112 0.0018 4.1147 96.3 164.5 1.1 1.8 4.2702 4.7580 0.0019 4.0849 95.4 163.3 1.2 2.0 4.4373 4.8954 0.0023 4.1324 92.8 165.2 1.3 2.2 4.3579 4.8693 0.0022 4.0752 93.2 163.0 1.8 Plateau Age (Ma) 164.1 1.4

G-4/4446-02 J= 0.02320 ±0.00004 0.5 8.8120 4.5843 0.0214 2.8435 32.2 115.3 13.4 0.7 4.7322 4.4952 0.0051 3.5664 75.1 143.4 4.5 0.9 4.6485 4.5305 0.0032 4.0662 87.2 162.6 2.5 1.1 4.5236 4.6022 0.0027 4.0813 89.9 163.2 1.8 1.2 4.4912 4.6356 0.0028 4.0265 89.4 161.1 4.8 1.3 4.3555 4.7008 0.0021 4.1115 94.1 164.4 1.0 1.4 4.9700 4.7932 0.0042 4.1000 82.2 163.9 2.9 1.6 5.7182 5.0282 0.0069 4.0568 70.7 162.3 3.9 1.8 5.2128 4.9837 0.0050 4.1109 78.6 164.3 1.9 2.0 4.7982 5.0509 0.0041 3.9678 82.4 158.9 1.6 Plateau Age (Ma) 163.9 1.6

G-8/4447-01 J= 0.02320 ±0.00004 0.4 24.9170 4.7251 0.0804 1.5144 6.1 62.3 84.6 0.5 9.5320 7.5391 0.0234 3.1886 33.3 128.8 45.6 0.6 6.0813 7.9462 0.0082 4.2920 70.2 171.2 25.7 0.7 5.4556 7.7418 0.0061 4.2502 77.5 169.7 22.5 0.8 5.0828 8.0700 0.0052 4.1769 81.7 166.9 10.0 0.9 4.8225 8.0146 0.0045 4.1264 85.1 164.9 10.0 1.0 4.8080 8.1830 0.0048 4.0208 83.2 160.9 8.7 1.1 4.9352 8.4140 0.0050 4.1038 82.7 164.1 8.6 1.2 4.5395 8.2200 0.0037 4.0964 89.7 163.8 5.0 1.3 4.4243 8.0973 0.0032 4.1144 92.5 164.5 2.4 1.4 4.5303 8.1366 0.0037 4.0734 89.4 162.9 8.4 1.5 4.4056 8.1577 0.0030 4.1460 93.6 165.7 5.5 1.6 4.3596 8.0227 0.0029 4.1409 94.5 165.5 4.9 1.7 4.3607 8.0886 0.0028 4.1719 95.2 166.7 3.1 2.0 4.2702 8.1002 0.0027 4.1034 95.6 164.1 1.6 Plateau Age (Ma) 164.6 2.2 G-8/4447-02 J= 0.02320 ±0.00004 0.5 10.0276 6.0168 0.0273 2.4216 24.1 98.6 20.8 0.7 4.7019 7.6446 0.0051 3.8004 80.4 152.4 7.3 0.9 4.4458 8.1867 0.0045 3.7552 84.0 150.7 8.1 1.1 4.2323 8.1239 0.0029 3.9944 93.9 159.9 3.4 1.2 4.2298 8.2379 0.0026 4.0954 96.3 163.8 2.3 1.3 4.1699 7.8929 0.0022 4.1365 98.7 165.3 1.6 1.4 4.2747 7.8427 0.0022 4.2298 98.4 168.9 5.5 1.6 4.2262 8.1032 0.0025 4.1080 96.7 164.2 1.9 1.8 4.2093 8.0233 0.0025 4.0860 96.6 163.4 2.4 2.0 4.2553 8.2549 0.0027 4.1049 95.9 164.1 1.6 2.2 4.2010 8.2295 0.0024 4.1433 98.1 165.6 1.7 Plateau Age (Ma) 164.7 1.6

G-5/4448-01 J= 0.02320 ±0.00004 0.5 9.0467 4.8369 0.0178 4.1577 45.8 166.1 3.5 0.7 4.7931 4.8670 0.0038 4.0606 84.4 162.4 2.8 0.9 4.5715 4.8002 0.0028 4.1045 89.5 164.1 2.1 1.1 4.4804 4.7716 0.0025 4.1041 91.3 164.1 2.4 1.2 4.4203 4.7868 0.0023 4.1013 92.5 164.0 1.9 1.3 4.7403 4.7958 0.0035 4.0734 85.7 162.9 2.4 1.4 4.7329 4.6454 0.0031 4.1662 87.8 166.5 5.3 1.5 4.5361 4.8257 0.0028 4.0969 90.0 163.8 5.5 2.0 4.7146 4.8864 0.0036 4.0396 85.4 161.6 4.2 2.2 4.7927 4.8555 0.0037 4.0857 85.0 163.4 2.0 Plateau Age (Ma) 163.7 1.8

G-5/4448-01 J= 0.02320 ±0.00004 0.4 30.9391 3.1457 0.0912 4.2446 13.7 169.4 26.1 0.5 17.7776 3.3412 0.0468 4.2220 23.7 168.6 15.7 0.6 20.6942 4.3307 0.0574 4.0573 19.5 162.3 27.4 0.7 16.3524 4.4359 0.0435 3.8323 23.4 153.7 18.0 0.8 13.9827 4.4730 0.0350 3.9962 28.5 160.0 11.2 0.9 11.1687 4.5153 0.0251 4.1117 36.7 164.4 10.7 1.0 12.9163 4.7041 0.0306 4.2524 32.8 169.7 8.0 1.1 12.1102 4.4312 0.0282 4.1128 33.9 164.4 7.2 1.2 10.4991 4.3873 0.0230 4.0534 38.5 162.1 7.8 1.3 8.5395 4.6100 0.0165 4.0325 47.1 161.3 4.9 1.4 9.7409 4.7710 0.0201 4.1727 42.7 166.7 4.5 1.5 11.3687 5.0148 0.0255 4.2135 36.9 168.3 8.8 1.6 11.9626 5.0010 0.0285 3.9399 32.8 157.8 11.4 1.7 12.0371 5.0567 0.0280 4.1569 34.4 166.1 9.4 Plateau Age (Ma) 164.4 2.3 Notes: All errors reported are 2s J-values are based on 28.02 Ma for Fish Canyon sanidine; uncertainty does not include age error Power refers to laser output in Watts. Note that different samples were run with different laser focus. Isotope ratios are corrected for blanks, mass discrimination, and radioactive decay Ages are based on nucleogenic corrections reported by Renne et al. (1998) and decay constants of Steiger and Jager (1977) Age errors do not include systematic contributions from decay constants or J value Errors reported for plateau ages include analytical error in J determination DR-2: Technical notes on biostratigraphic calibrations and correlations

Range discordancies , range diachronism, and other sources of uncertainty

Biostratigraphic zonations, like absolute time scales, are inherently works-in-progress. Neither the zonation of Baumgartner et al. (1995a) nor of Pessagno et al. (1987, 1993) produces perfectly concordant results in all Cordilleran sections (Murchey and Baumgartner, 1995; and herein). Range discordancies, which are overlapping ranges that are dissonant with ranges in a formal zonation, result from unrecognized range diachronism or reworking. In the Cordillera, significant environmentally-controlled diachronism at the genus level is well-documented: e.g., Praeparvicingula, Parvicingula, Pantanellium (Pessagno and Blome, 1984), and Mirifusus (Murchey and Baumgartner, 1995). Therefore, we took some care to note range discordancies arising from application of the Tethyan UA Zonation of Baumgartner et al. (1995a) to the sections discussed. In addition, we have summarized some of the assumptions and uncertainties associated with previous calibrations based on the zonation of Pessagno et al. (1993) as they apply to specific sections. Kiessling’s (1999) recalibrations of Subzones 2 to 4 of Pessagno et al. (1987, 1993), which incorporate data from a well-constrained Kimmeridgian and Tithonian section, brought the Late Jurassic calibrations of the two major zonation schemes into much closer alignment, but did not directly address the problems of reconciling the calibrations of Superzone 1 and Zone 2 (Subzones 2 , 2, and 2) of Pessagno et al. (1987, 1993) with the UA Zonation.

Josephine ophiolite, Western Klamath Mountains, northern California and Oregon:

UAZ calibrations: Baumgartner et al. (1995a) used the radiolarian faunas documented by Pessagno et al. (1993) to calibrate the sedimentary sections intercalated with and overlying basalt of the Josephine ophiolite. At the Turner-Albright Mine in Oregon, where sedimentary rocks are interbedded with basalt, they assigned two intervals on strike with one another to UAZ 3-4. An argument can be made that the interval can be further constrained to UA Zone 4 based on the presence of Unuma typicus (UAZ 3-4), Levileugeo spp. (equivalent to Leugeo hexacubicus sensu Baumgartner at al., 1995, UAZ 4-8), and a species with affinity to Cyrtocapsa (?) kisoensis (UAZ 3-4). UA Zone 4 is calibrated as late Bajocian. The samples also contain common to abundant Praecaneta decora (which may fall within Baumgartner et al.’s (1995b) definition of Parvicingula(?) spinata, UAZ 3-10); P. decora first occurs in the Bathonian of the Blue Mountains of Oregon above a late Bajocian to early Bathonian unconformity (Pessagno and Whalen, 1982).

In the Smith River section, 46m of tuffaceous pelagic strata overly basalt of the Josephine ophiolite (fig. 7). The horizon 4.1 meters above the base of the basalt is no older than UAZ 3 (early to middle Bajocian) because it contains Acanthocircus suboblongus (UAZ 3-11); it is constrained as no younger than UAZ 6 or 7 based on overlying assemblages. Praecaneta decora (see above),is present in the horizon and in an underlying sample. In figure 7, we correlated the base of the section with the approximate boundary between the Bajocian and Bathonian of the Blue Mountains of Oregon.

The interval between 13 and 46 meters is assigned a range of UAZ 6-6/7. The maximum range (UAZ 6, middle Bathonian) of the interval is based on the presence of Emiluvia hopsoni (UAZ 6-15) in many samples from 13 meters to the top of the volcanopelagic section, and on the presence of Spongocapsula palmerae (UAZ 6-13) at the ~21m horizon. Several other taxa in the interval range no lower than UAZ 5 (latest Bajocian to early Bathonian): Ristola procera, Mirifusus guadalupensis, Eucyrtidiellum ptyctum (sensu Baumgartner et al., 1995), and Bernoullius cristatus.. The minimum age of the interval is constrained by several taxa in the upper part of the volcanopelagic section which have their final occurrences in UAZ 7: Tetraditryma praeplena, Bernoullius r. delnortensis, Linaresia beniderkoulensis, and Linaresia spp. Xiphostylus spp. (UAZ 1-6) are common throughout the interval. However, we choose not to rely completely on this genus to constrain the upper limit of the section because its upper range in the eastern Pacific is questionable (discussion in Baumgartner et al., 1995, p. 1041). It is present in the Bathonian but not in the Callovian of the Blue Mountains (Pessagno et al., 1989); it is present in a single sample of the Stanley Mountain section (Hull, 1997) along with taxa calibrated as UAZ 7 (late Bathonian to early Callovian); and it is present and highly discordant in the uppermost sample of the Smith River section (see below). The UAZ ranges of the pantanellids Pachyoncus (Pantanellium sp. L: UAZ 2-4) and Trillus (UAZ 1-5) are discordant; however, in the Blue Mountains of Oregon, Pachyoncus occurs in both the Bajocian and upper Bathonian (Pessagno and Blome, 1980). In summary, the interval between 13 and 46 meters is illustrated in Fig. 7 as having a possible range of UAZ 6-6/7, and is calibrated as no older than middle Bathonian and no younger than late Bathonian or early Callovian.

The radiolarian faunas in the upper half of the Smith River section are much lower in diversity than those in the underlying pelagic section (Pessagno et al., 1993). They are dominated by taxa whose ranges are not yet well-established elsewhere nor used in the UA Zonation of Baumgartner et al. (1995b). The ranges that Baumgartner et al. (1995) assigned to this interval (Fig. 7) are based on very few taxa, including Mirifusus d. dianae (UAZ 7-12), Mirifusus guadalupensis (UAZ 5-11), and Podobursa spinosa (UAZ 8-12), so the results are imprecise. The most tightly constrained part of the interval (UAZ 8-10) still has a possible range from Callovian to Kimmeridgian. The reappearance of the genus Xiphostylus sp. (UAZ 1-6)(species unidentified), in a single sample about 9.5m below the top of the Smith River section, is a highly discordant occurrence which is not integrated into Figure 7. Either the upper UAZ range of the genus needs to be revised (discussion in Baumgartner et al., 1995a, p. 1041) or it was reworked from the underlying volcanopelagic section, the last reported occurrence being 51.7m below. Pessagno et al. (1993, p. 111) reported that the “ upper 15.2m of the syntectonic flysch succession contain common, small rip up clasts of light gray pelagic limestone.”

Age calibrations of Pessagno et al. (1993): Pessagno et al. (1993) described the radiolarians in the sedimentary rocks overlying the Josephine ophiolite in great detail, but they did not use them to calibrate the sections. They considered existing calibrations for Jurassic radiolarian zonations of Europe and Japan to be erroneous or to reflect systematic range diachronism of radiolarians or ammonites (Pessagno et al. (1993, p. 103-107).

Pessagno et al. (1993a, after Pessagno and Blome, 1990), used a radiometric date of ~162 Ma on a plagiogranite locality about 13 km from the Smith River section as a proxy for the age of the basalt basement at the Smith River section in California and the Turner-Albright Mine section in Oregon. This method provided a rough estimate for the maximum ages of the sedimentary sections, but the cumulative uncertainties built into the analysis were and remain very large. The actual zircon 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).