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Research ArticleCalcareous nannofossil biostratigraphic study of forearc basin

sediments: Lower to Upper Cretaceous Budden Canyon Formation(Great Valley Group), northern California, USAiar_770 1..25

ALLAN GIL S. FERNANDO,1,2* HIROSHI NISHI,3 KAZUSHIGE TANABE,4 KAZUYOSHI MORIYA,2

YASUHIRO IBA,4 KAZUTO KODAMA,5 MICHAEL A. MURPHY6 AND HISATAKE OKADA2,7

1Nannoworks Laboratory, National Institute of Geological Sciences, University of the Philippines, Diliman,Quezon City 1101, Philippines (email: [email protected]; [email protected]); 2Division of Earthand Planetary Sciences, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan; 3The Centerfor Academic Resources and Archives, Tohoku University Museum, Tohoku University, Sendai 980-8578, Japan;

4Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, Tokyo113-0033, Japan; 5Center for Advanced Marine Core Research, Kochi University, B200 Monobe, Nankoku, Kochi783-8502, Japan; 6University of California, Riverside, California 92021, USA; and 7Office of the Vice President,

Hokkaido University, Sapporo 060-0808, Japan

Abstract The results of a calcareous nannofossil biostratigraphic investigation of the NorthFork Cottonwood Creek section of the Budden Canyon Formation (BCF; Hauterivian–Turonian) in northern California are summarized using the Boreal – cosmopolitan BorealNannofossil Biostratigraphy (BC) – Upper Cretaceous Nannofossil Biostratigraphy (UC)nannofossil zonal schemes of Bown et al. and Burnett et al. Sixteen intervals, ranging fromthe BC15 to UC8 zones, were established in the section. Combined biostratigraphic andmagnetostratigraphic studies suggest a Hauterivian to mid-Turonian age for the studiedsequence. The Hauterivian–Barremian, Barremian–Aptian, Aptian–Albian, Albian–Cenomanian, and Cenomanian–Turonian stage boundaries were delineated near the top ofthe Ogo Member, below the Huling Sandstone Member, within the upper ChickaballyMember, in the upper portion of the Bald Hills Member and within the Gas Point Member,respectively. Unconformities probably exist at the base of the Huling Sandstone Memberand the upper part of the upper Chickabally Member. The nannofossil assemblage in theNorth Fork Cottonwood Creek suggests that the study area was under the influence ofcold-water conditions during the Barremian to Lower Aptian interval, shifting to tropical/warm-water conditions during the Albian to Turonian interval as a result of the mid-Cretaceous global warming. Although oceanic anoxic events have not yet been reported inthe BCF, preliminary total organic carbon, along with nannofossil data, suggest the presenceof the global Cenomanian–Turonian boundary oceanic anoxic event 2.

Key words: biostratigraphy, Budden Canyon Formation (BCF), calcareous nannofossils,California, Cretaceous, Great Valley Group (GVG), oceanic anoxic event 2 (OAE2).

INTRODUCTION

The Great Valley Group (GVG, e.g. Surplesset al. 2006), formerly known as the Great ValleySequence, is a thick (~15 km) sedimentary

sequence that formed in a forearc basin alongthe western border of the Sacramento Valley innorthern California, between the Sierra Nevadavolcano–plutonic arc and the Franciscan subduc-tion complex, which presently underlies the CoastRanges of California (Ingersoll 1979; Suchecki1984; Fig. 1). The formations of the GVG aredivided into two regional sequences by the Elder

*Correspondence.

Received 20 July 2010; accepted for publication 11 March 2011.

Island Arc (2011)

© 2011 Blackwell Publishing Asia Pty Ltd doi:10.1111/j.1440-1738.2011.00770.x

Creek Fault. The study area is in the region northof the fault known as the Cottonwood district andexhibits fine-grained, deep-water marine sedi-ments (i.e. mudstones and siltstones) of Creta-ceous age associated with turbiditic sandstonesand conglomerates that are interpreted as subma-rine fan complexes and tectonic-related compo-nents. The Cottonwood district encompasses thedrainage basin of all the forks of CottonwoodCreek. The present study is along the North ForkCottonwood Creek, the northernmost main streamof the district.

Biostratigraphic studies of the Cottonwood dis-trict document its geological age to range from theHauterivian to Turonian based on mollusks, fora-minifera, radiolarians, and calcareous nannofos-sils. Among microfossils, calcareous nannofossilsare very useful for correlation between the GVGand the stratotype sections in the Europeanregions, and in dating of each lithological unit.This is best illustrated in the study of Bralower

(1990) in the McCarty Creek section (northernpart of GVG), wherein the NK2–NK8 biozonesof Bralower et al. (1989), established based onEuropean land sections and western North Atlan-tic Deep Sea Drilling Project (DSDP) sites, wererecognized.

Exposures of the GVG north of the Elder CreekFault belong to the Budden Canyon Formation(BCF), which is the most continuous and fossi-liferous Barremian–Turonian sequence in thenorthwestern part of the Sacramento Valley,northern California (Murphy 1956; Murphy et al.1964; Dailey 1973; Murphy & Rodda 1996). Thepresent study discusses the results of the com-bined calcareous nannofossil biostratigraphy andmagnetostratigraphic study of the North ForkCottonwood Creek section of the BCF, which spansthe Hauterivian to Middle Turonian. In addi-tion, the positions of several Cretaceous stageboundaries in the BCF and the possible existenceof oceanic anoxic events (OAEs) in the eastern

Fig. 1 Study area. (a) California index map and location of the study area in the Great Valley Group (GVG); (b) location of the North Fork CottonwoodCreek (NFCC) section in the Ono Quadrangle of Shasta County (map modified from Murphy & Rodda 1996). Solid box in Figure 1b corresponds toFigure 2.

2 A. G. S. Fernando et al.

© 2011 Blackwell Publishing Asia Pty Ltd

Pacific region will also be discussed. The studiedsection is divided into sixteen intervals using theBoreal – cosmopolitan BC – UC zonal schemesof Bown et al. (1998) for the Lower Cretaceous andBurnett et al. (1998) for the Upper Cretaceous,respectively.

GEOLOGICAL BACKGROUND

LITHOSTRATIGRAPHY

Murphy et al. (1964) revised the lithostratigra-phic nomenclature of previously defined for-mational units into member status, which weresubsequently incorporated within a single forma-tion, called the Budden Canyon Formation (BCF).The revised formation consists of seven mem-bers: Rector Conglomerate, Ogo, Roaring River,

Chickabally, Huling Sandstone, Bald Hills, andGas Point Members, in ascending order (Figs 2,3).Dailey (1973) proposed an additional memberunit, the Aiken Member, which is basically equiva-lent to the upper Chickabally Member (Murphyet al. 1964; Peterson 1967). In the present study,the lithostratigraphic nomenclature of Murphyet al. (1964, 1969) was followed. Only the upper sixmembers of the BCF were studied in the NorthFork Cottonwood Creek section (Fig. 3), each ofwhich is briefly described below.

The Ogo Member, the oldest lithological unitsampled for calcareous nannofossils, attains athickness of about 200 m in the studied section.The member consists of massive siltstones andmudstones, which are frequently associated withfine-grained laminated sandstones. The approxi-mately 150-m interval between the uppermost

Fig. 2 Geological map of a portion of the Ono Quadrangle (Fig. 1b) and the location of the sections analyzed for calcareous nannofossils along the NorthFork Cottonwood Creek (modified from Murphy et al. 1969). Note the greenish sandstone and acidic tuff outcrop units.

Cretaceous Nannofossils from California 3


Fig. 3 Stratigraphic levels of the samples used in the North Fork Cottonwood Creek section. Numbers on the left side of the column refer to samplenumbers. Sections along the creek, as shown in Figure 2, are represented by the roman numerals on the right side of the column.



conglomerate in the Ogo Member and the lower-most mudstone of the Chickabally Member isconsidered in this study as the Roaring RiverMember. The member consists of laminated sand-stones and conglomerates, with siltstone ormudstone interbeds. In the North Fork Cotton-wood Creek section, the Chickabally Member isseparated into the lower and upper ChickaballyMembers by the Huling Sandstone Member, a50-m-thick unit of thinly bedded sandstones andconglomerate. The lower Chickabally Member isabout 300 m thick and is composed mostly of mud-stones, with alternating beds of mudstones andsandstones near the base of the Huling Sandstone.The upper Chickabally Member, on the otherhand, consists of mudstones and intercalationsof thinly to thickly bedded sandstones. Based onthe dominant lithology, the upper ChickaballyMember is separated further into the mudstone-dominated unit (ms-unit) and the alternating sand-stone unit (ss-unit; Fig. 3). The ms-unit comprisesthe lower 410 m of the upper Chickabally Member,and includes a tuff bed approximately 250 m abovethe Huling Sandstone Member and alternatingbeds of sandstone and mudstone units near thetop. The ss-unit, on the other hand, includes theupper 270 m of the member and, as the name sug-gests, consists of thin to thick beds of sandstonesintercalated with mudstones. A prominent massivesandy mudstone bed occurs at the base of thess-unit. Overlying the Chickabally Member isthe generally coarse-grained Bald Hills Member,consisting predominantly of mudstones and inter-calated sandstones, conglomerates and pebblymudstones. The Bald Hills Member attains athickness of 160 m in the studied section. Theyoungest and uppermost unit of the BuddenCanyon Formation is the Gas Point Member, whichhas a thickness of about 730 m in the North ForkCottonwood Creek section. The Gas Point Memberconsists of siltstones and mudstones, with sand-stone intercalations becoming more common in theupper part of the section.

BIOSTRATIGRAPHY

Many biostratigraphic studies have been con-ducted in the BCF using ammonites, radiolarians,foraminifers, and, to a lesser extent, calcare-ous nannofossils. Murphy (1956) studied theammonites in the Lower Cretaceous sequenceand established nine zones, from the BarremianSimbirskites (Hollisites) aguila zone to the AlbianMortoniceras hulenanum zone (Fig. 4). Despite

some revisions on the age assignments of Mur-phy’s ammonite zones by subsequent studies(i.e. Popenoe et al. 1960; Murphy 1969, 1975;Murphy et al. 1969; Dailey 1973), the zones remainas useful subdivisions of the record on the westerncoast of North America, along with the otherfossil guides proposed and compiled by Popenoeet al. (1960). More recent ammonite studies inthe GVG include descriptions of new taxa (Rodda& Murphy 1992; Murphy et al. 1995; Amédro &Robaszynski 2005) and discussions on the positionof the Albian–Cenomanian boundary in northernCalifornia (Murphy & Rodda 1996; Amédro &Robaszynski 2005).

Marianos and Zingula (1966) reported theoccurrence of selected planktonic foraminifersfrom the Hauterivian to Turonian interval, andbriefly discussed the geological age assignment ofthe BCF in the Dry Creek section (Fig. 4). Dailey(1973), on the other hand, established four fauni-zones (Faunizones I–IV) based on benthic foramin-iferal assemblages. These faunizones were used tosubdivide the Hauterivian to Cenomanian inter-val of the BCF in the Ono Quadrangle. Pessagno(1976, 1977) established 12 radiolarian zones in theGVG, ranging from the Berriasian Obesacapsularotunda zone to the Maastrichtian Orbiculiformarenillaeformis zone (Fig. 4). These radiolarianzones were correlated to ammonite and planktonicforaminiferal zones and are closely tied to theEuropean stages.

Calcareous nannofossil studies in the GVGare limited to the studies of Worsley (1979) andBralower (1990; Fig. 4). These studies focused onthe Lower Cretaceous sections along McCarty,Dry, Stony, and Grindstone Creeks in Tehama andGlenn Counties, which are all located south of theCottonwood district. Despite the limited numberof samples (9 samples), Worsley (1979) demon-strated that calcareous nannofossil biostratigra-phy is effective in dating of sediments in this area.Bralower (1990), on the other hand, further illus-trated Worsley’s initial work and, in addition,showed the possibility of correlating the calcare-ous nannofossil zones of the Late Berriasian toAlbian interval between the GVG and the standardsections in Europe.

MAGNETOSTRATIGRAPHY

The magnetostratigraphy of the North ForkCottonwood Creek section was investigated atthe Center for Advanced Marine Core Researchat Kochi University, Japan. At least four reversed



magnetic polarity intervals were determined fromthe Ogo to the upper Chickabally Members.Correlation of these reversed polarity intervalswith the Geomagnetic Polarity Time Scale (GPTS)of Ogg and Smith (2004), and combined calcareousnannofossil and ammonite biostratigraphy, are dis-cussed in Magnetostratigraphic Interpretation.

MATERIALS AND METHODS

SAMPLE PROCESSING, MICROSCOPY,AND COUNTING PROCEDURES

A total of 135 samples were collected from theNorth Fork Cottonwood Creek, encompassing theOgo, Roaring River, lower and upper Chickabally,Bald Hills, and Gas Point Members of the BuddenCanyon Formation (Fig. 3). No samples were

taken within the Huling Sandstone Member,although samples were collected near its lower andupper boundaries.

To analyze the calcareous nannofossil assem-blage and establish the calcareous nannofossilbiostratigraphy of the section, smear slides wereprepared from the samples using standard prepa-ration techniques, and examined under a lightmicroscope using both cross-polarized light (XPL)and phase contrast (PC) methods at 1000 to 1600¥magnification. More than 500 fields of view (FOV)were investigated per sample. With the exceptionof some fossiliferous intervals, most of the sampleshave low nannofloral densities (<100 specimensper 500 FOV), probably as a result of dilutionof siliciclastic materials. Additional FOV werechecked to verify rare occurrences of biostrati-graphic marker species. Calcareous nannofossil

Fig. 4 Previous biostratigraphic studies in the Great Valley Group (GVG). The ammonite zonation of Murphy (1956) encompasses the Hauterivian–Albian interval. Ammonite fossil guides for the Cenomanian–Turonian interval are from Popenoe et al. (1960). Murphy (1975) proposed a more detailedzonation of the Barremian–Lower Aptian interval using taxa of the genus Shasticrioceras, but is not included in this figure.



relative abundance categories were adapted andslightly modified from Bralower (1990), Bown et al.(1998), and Burnett et al. (1998), and are given asfollows: (i) common (C), 1 specimen per 1–2 FOV;(ii) few (F) 1 specimen per >2–50 FOV; (iii) veryfew (VF), 1 specimen per >50–100 FOV; and (iv)rare (R), 1 specimen per >100 FOV.

CALCAREOUS NANNOFOSSIL ZONATION SCHEMES

Several calcareous nannofossil zonation schemeshave already been proposed in the Cretaceous,with the zonation schemes of Sissingh (1977; CCzones) and Roth (1978; NC zones) being the mostwidely used (Fig. 5). The CC zonation scheme wasbased on data from a geographically wide area,including European and North African sections.

The NC zonation scheme, on the other hand, wasestablished in the low-latitude region of the North-western Atlantic Ocean (DSDP Leg 44). Despitethis, the CC and NC zonation schemes are basi-cally similar in definition and characterization. Themost recent zonation scheme proposed for theCretaceous is the composite zonal scheme of Bownet al. (1998; BC zones) for the Lower Cretaceousand Burnett et al. (1998; UC zones) for the UpperCretaceous (Fig. 5). The BC zones were defined inthe North Sea Basin. While many of the zonalmarkers used there are also applicable in low tointermediate latitudes, several taxon range zonesare diachronous. This results in differences inthe zonal characterization between the BC andCC/NC zones, especially prior to the Albian.The UC zones, on the other hand, are derived

Fig. 5 Comparison of the Lower and Upper Cretaceous nannofossil biostratigraphic zonation schemes of Bown et al. (1998) and Burnett et al. (1998)with the CC and NC zonation schemes. Radiometric dates for the stage boundaries are from Gradstein et al. (2004). Calcareous nannofossil zoneassignments of the studied section are shown on the rightmost column.



from the CC and NC zones. The UC zonationscheme, however, was established based on recentadvances in evolutionary understanding and bio-geographic distribution of calcareous nannofossilsin high- and intermediate-latitudes, and uses morewidely distributed marker taxa compared with theCC/NC zones (Burnett et al. 1998).

Many zonal markers in the Albian–Turonianinterval are similar between the BC/UC andCC/NC zones. This is in contrast to theBarremian–Aptian interval, wherein several zonalmarkers used for the BC zonation scheme are dif-ferent from the markers used in the CC/NCscheme. This implies that provincialism/endemismoccurred during the early Cretaceous, making cor-relation between the Boreal and tropical/Tethyanregions difficult. During the mid-Cretaceous,global correlation through calcareous nannofossilswas achieved. This was attributed to the presenceof a low sea-surface temperature gradient betweenthe low- and high-latitude regions and the con-sequent expansion in the range of tropical/subtropical species to middle- and high-latituderegions (Burnett et al. 1998).

RESULTS

CALCAREOUS NANNOFOSSIL ABUNDANCEAND PRESERVATION

Calcareous nannofossil abundance values arevariable and the specimens observed in thesamples generally exhibit poor to moderate pres-ervation. Species richness (i.e. total number ofspecies) also appear to be very low comparedwith other Lower to Upper Cretaceous sections,although Mutterlose et al. (2003) noted similarspecies richness values (1–40 species/sample) inthe Vohrum section in northwest Germany. Allthese observations are probably the consequenceof the relatively high amount of siliciclastic inputin the study area (dilution effect). The low speciesrichness may also be attributed to dissolution (orpoor preservation of nannofossils), although thepresence of moderate- to well-preserved ‘fragile’forms like Biscutum spp., and Discorhabdusignotus suggests otherwise.

Calcareous nannofossil abundance and speciesdiversity values display fluctuations in the NorthFork Cottonwood Creek section. In general,however, higher nannofossil abundance and spe-cies diversity characterize the Albian–Turonianinterval compared to the Barremian–Aptian inter-val (Fig. 6). Calcareous nannofossils in the BCF

include some Boreal-diagnostic taxa, but mainlyconsist of Tethyan species, allowing recognitionof Tethyan bioevents and correlation to Tethyan(low-latitude CC/NC) zonation schemes. This isparticularly evident for the Albian and youngerunits in the studied section. Commonly occurringspecies in the assemblage include long-rangingtaxa such as Watznaueria barnesiae, Biscutumconstans, Biscutum ellipticum, and D. ignotus. Alist of the 129 nannofossil species and subspeciesobserved in the North Fork Cottonwood Creeksection is provided in Table S1 (Supporting Infor-mation). Two new calcareous nannofossil specieswere described by Fernando and Okada (2007) inthe North Fork Cottonwood Creek section of theBudden Canyon Formation: Diloma californicaand Prediscosphaera quasispinosa. Optical micro-graphs of selected nannofossil taxa, on the otherhand, are illustrated in Figures 7 and 8.

CALCAREOUS NANNOFOSSIL BIOSTRATIGRAPHY

Nine major and 21 supplementary nannofossilbioevents were recognized in the North Fork Cot-tonwood Creek section (Fig. 9). During the earlyCretaceous, however, several species that are con-sidered important markers of the CC/NC zonesin other areas are either missing or have beenrecorded at different levels in the studied section(diachroneity). In this interval, the bioevents usedin the recognition of the BC zones of Bown et al.(1998) become invaluable. In cases where zonalmarkers are not observed, supplementary nanno-fossil events were used instead. Based on the BCand UC nannofossil zonation schemes, the NorthFork Cottonwood Creek section can be dividedinto 16 intervals, ranging from the BC15 and olderzones to the UC8 zone (Figs 5,9). The recognizedzones suggest that the geological age assignmentof the section is Hauterivian – Middle Turonian,which is in agreement with previously publishedammonite and foraminiferal data. The zones arediscussed in detail below.

BC15 and older zones (undifferentiated Hauterivianand Barremian)

The lowermost zone recognized in the presentstudy includes the Ogo, Roaring River, and thelowermost part of the lower Chickabally Member,represented by the interval from the lowestsample (OGO-46) to the first significant (probablylocal) nannofossil event in the section, which is thefirst nannoconid abundance peak (US-719). This



bioevent occurs above the lowermost occurrenceof Rhagodiscus gallagheri (US-717). Calcareousnannofossils are rare to few in abundance andare poorly preserved, consisting mostly of longranging taxa such as Biscutum spp. (B. constans,

B. ellipticum), D. ignotus, and W. barnesiae. Nan-nofossil markers indicative of Hauterivian andBarremian ages were not observed within theinterval. Thus, the lower boundary of the undiffer-entiated zone is not recognized.

Fig. 6 Calcareous nannofossil abundance and species richness in the North Fork Cottonwood Creek section. The interval with the highest total organiccarbon (TOC) values is represented by the area shaded gray, while the interval containing the main d13C isotope excursion is represented by the verticalarrow (T. Tomosugi [TOC] and T. Yamanaka [d13C], unpubl. data, 2006).



Fig. 7 Optical micrographs of calcareous nannofossils from the North Fork Cottonwood Creek section of the Great Valley Group in northern California.All micrographs are crossed-polar images unless otherwise indicated. Scale bar,1 mm. (a) Axopodorhabdus albianus, US-041; (b) Biscutum ellipticum,US-019; (c) Braarudosphaera hockwoldensis, US-052; (d) small Braarudosphaera sp., US-040; (e) Bukrylithus ambiguus, US-074; (f) Calculites percenis,US-052; (g) Calculites sp. A, US-091; (h) Corollithion kennedyi, US-063; (i) C. madagaskarensis, US-092; (j,k) Diloma californica, US-040; (l)Discorhabdus ignotus, US-019; (m) Eiffellithus gorkae, US-091; (n) E. turriseiffelii, US-064; (o) Eprolithus eptapetalus, US-092; (p) E. floralis, US-090;(q,r) Farhania varolii, US-040, [r] phase contrast (PC); (s) Flabellites oblongus, US-019a; (t) Gartnerago obliquum, US-094; (u) Hayesites albiensis,US-018; (v) H. irregularis, US-019; (w) Helenea chiastia, US-721; (x) Helicolithus compactus, US-019; (y) H. trabeculatus, US-092; (z) Micrantholithusobtusus, US-719; (aa) Nannoconus inconspicuus, US-719; (ab) N. inornatus, US-718; (ac) N. minutus, US-721; (ad) N. cf. N. steinmannii, US-721.



Fig. 8 Optical micrographs of calcareous nannofossils from the North Fork Cottonwood Creek section of the Great Valley Group in northern California.All micrographs are crossed-polar images unless otherwise indicated. Scale bar, 1 mm. (a–d) Nannoconus spp. ‘discs’, [a–c] US-719, [c] PC, [d] US-011;(e) Orastrum perspicuum, US-079; (f) Owenia cf. O. hillii, US-069; (g) Prediscosphaera columnata, US-019; (h) P. cretacea, US-089; (i) P. quasispinosa,US-062; (j) P. spinosa, US-049; (k) Quadrum gartneri, US-091; (l) Radiolithus orbiculatus, US-066; (m) Retecapsa crenulata, US-089; (n) Rhagodiscusachlyostaurion, US-074; (o) R. angustus, US-052; (p) R. asper, US-069; (q) R. gallagheri, US-047; (r) Seribiscutum gaultensis, US-019; (s) S. cf. S.gaultensis, US-092; (t) Sollasites horticus, US-069; (u) Staurolithites aenigma, US-046; (v) S. gausorhethium, US-046; (w) S. cf. S. mutterlosei, US-076;(x) Tranolithus gabalus, US-046; (y) T. orionatus, US-072; (z) Watznaueria barnesiae, US-046; (aa) W. britannica, US-016; (ab) W. fossacincta, US-046;(ac) Zeugrhabdotus bicrescenticus, US-063; (ad) Z. diplogrammus, US-088.



Ammonites collected within the interval includeCrioceratites sp. (Rodda & Murphy 1987) andShasticrioceras sp. S. patricki. The former rangesfrom the Late Valanginian through the Barremian,but are mostly recorded within the Hauterivian(Popenoe et al. 1960), while the latter ranges fromthe Middle Barremian to the Early Aptian(Murphy 1975). Based on combined ammonite andcalcareous nannofossil biostratigraphy, the inter-val is probably Hauterivian to Late Barremian inage, and could correspond to the BC15 and olderzones. The undifferentiated zone is correlated tothe NC5 and CC5 – lower CC6 zones.

BC16–BC17 undifferentiated zone (Late Barremian)

The undifferentiated zone is the interval betweenthe first nannoconid abundance peak and the first

occurrence (FO) of Flabellites oblongus (US-013),and includes most of the lower ChickaballyMember. Similar to the previous interval, calcare-ous nannofossils are rare to few and are poorlypreserved. The calcareous nannofossil assemblageof this interval, however, is characterized bythe continuous occurrence and slightly higherabundance of nannoconids compared to theprevious zone (Fig. 9). Commonly occurring taxawithin the zone, aside from Nannoconus spp.,include B. constans, D. ignotus, Rhagodiscusasper, Rotelapillus crenulatus, W. barnesiae, andWatznaueria fossacincta.

The FO of F. oblongus was used in the presentstudy to approximate the Barremian–Aptian(B–A) boundary (see discussion in Stage Bound-aries in Budden Canyon Formation). Below thisbioevent, the second nannoconid abundance peak

Fig. 9 Calcareous nannofossil biostratigraphic zonation and distribution of calcareous nannofossil zonal markers and selected taxa in the North ForkCottonwood Creek section. The Nannoconus spp. abundance graph shows the number of Nannoconus specimens counted in approximately 500 fields ofview (FOV) (smear slide data). Figure 3 shows lithology and sample numbers.



was observed (Fig. 9). Another nannofossil eventrecognized within the zone is the uppermost occur-rence of Micrantholithus spp. (M. hoschulzii, M.obtusus; US-011). The last occurrence (LO) ofMicrantholithus spp. is generally recorded athigher levels, within the BC21 nannofossil zone(Bown et al. 1998; NC7 zone of Roth 1978).Barremian-indicative nannofossil taxa such asNannoconus inornatus and Tegumentum octifor-mis were also observed within the lower part ofthe zone. The BC16–BC17 undifferentiated zone,which is equivalent to the upper NC5 and CC6zones, is considered Late Barremian in age.

BC18 zone (Early to Late Aptian)

The BC18 zone of Bown et al. (1998) is originallydefined as the interval zone from the FO of R.gallagheri to the FO of Farhania varolii (Fig. 5).In the studied section, however, typical specimensand forms similar to R. gallagheri were observedin the uppermost part of the Roaring RiverMember. In the present study, the BC18 zone isconsidered as the interval between the FOs ofF. oblongus and F. varolii (US-040), both of whichoccur immediately below and above the HulingSandstone Member, respectively (Fig. 9). The FOof F. oblongus is considered as a supplementarydatum within the BC18 zone in Boreal regions andis located above the FO of R. gallagheri (Bownet al. 1998).

The zone includes, by default, the Huling Sand-stone Member, where no samples were collectedfor nannofossil analysis. The base of the HulingSandstone Member has been recognized in earlierstudies to be an erosional surface (Peterson 1967;Murphy 1975). The immediate occurrence ofF. varolii (an Early Aptian bioevent) above theHuling Sandstone Member (Late Aptian based onammonites) suggests possible erosion of earlierdeposited sediments, probably ranging from theuppermost Barremian to Lower Aptian, in thestudy area. This possibility is further supported bythe observation of the Late Aptian ammonitesEotetragonites wintunius and Parahoplites spp.near the base of the Huling Sandstone Member.The BC18 zone corresponds to the NC6A andCC7a subzones, and is considered Early to LateAptian.

BC19–BC20 undifferentiated zone (Late Aptian)

The zone includes the lowermost upper Chicka-bally Member, and is the interval between the FO

of F. varolii and the Hayesites irregularis- andRadiolithus orbiculatus-containing sample (US-037; Fig. 9). Hayesites irregularis is used as a B–Aboundary marker in low latitude regions (e.g.Thierstein 1976), and is placed within the BC18zone. The occurrence of H. irregularis at the topof the composite zone therefore suggests that thelocal range of the taxon in the North Fork Cotton-wood Creek section is less than its true range.A similar observation in the GVG has been illus-trated in Channell et al. (1995), wherein Rucino-lithus (=Hayesites) irregularis was not observedabove the B–A boundary. Radiolithus orbiculatus,on the other hand, is a secondary nannofossil eventnear the boundary of the BC20 and BC21 nanno-fossil zones. The undifferentiated zone is corre-lated to the NC6B–NC7A and CC7a–CC7b zones,and is considered Late Aptian in age.

BC21 zone (Late Aptian)

The zone includes the middle part of the mudstoneunit (ms-unit) of the upper Chickabally Member,and is the interval between the H. irregularis- andR. orbiculatus-containing sample and the LO ofF. varolii (US-033; Fig. 9). Calcareous nannofossilsare few to common and are poorly to moderatelypreserved. High nannofossil abundance character-izes the lower and upper parts of the zone (Fig. 6).Common species within the zone include Biscutumspp., D. ignotus, F. varolii, R. asper, Watznaueriaspp., and Zeugrhabdotus spp. The FO of Rhago-discus achlyostaurion is an important supplemen-tary event recognized in the upper part of the zone(same level as the LO of F. varolii). This bioeventis used to separate the NC7B and 7C subzonesof Bralower et al. (1995), and is present within theupper part of the CC7b subzone of Perch-Nielsen(1985). The BC21 zone therefore is correlatedto the NC7A–NC7B and CC7b subzones, and isconsidered as Late Aptian in age.

BC22 zone (latest Aptian to earliest Albian)

The zone is the interval from the LO of F. varolii tothe FO of Prediscosphaera columnata (US-020),within the upper Chickabally Member (Fig. 9).This zone is correlated to the NC7C and upper-most CC7b subzones, which are both assigned tothe uppermost Aptian. Since the Aptian–Albian(A–A) boundary is delineated just below the FO ofP. columnata in the Tethyan and Boreal regions(e.g. Bown et al. 1998; Gradstein et al. 2004), theage of this zone is considered in this study to be



from the latest Aptian to the earliest Albian. Mut-terlose et al. (2003) in their study of the Vohrumsection in northwest Germany, however, find theuse of Prediscosphaera unsuitable as a markerfor defining the A–A boundary since the FO ofP. columnata is rare in the sections they investi-gated (see discussion on stage boundaries).

BC23–BC24 undifferentiated zone (Early to Middle Albian)

The zone is the interval from the FO of P. colum-nata to the FO of Axopodorhabdus albianus (US-041), within the upper part of the ms-unit and thelower part of the ss-unit of the upper ChickaballyMember (Fig. 9). Calcareous nannofossils arerare to few in abundance and mostly composedof D. ignotus and species of Biscutum andWatznaueria. Other bioevents within the zone arethe FOs of Hayesites albiensis and Tranolithusorionatus. The FO of H. albiensis defines theboundary between the NC8A and 8B subzones ofBralower et al. (1995). In the studied section, theFO of H. albiensis was recorded in sample US-016,below a sandstone bed and near the boundarybetween the ms and ss units of the upper Chicka-bally Member (Figs 3,9). The FO of T. orionatus,on the other hand, is used to define the boundarybetween the BC23 and BC24 zones and betweenthe NC8B–8C and CC8a–8b subzones (Fig. 5). TheFO of T. orionatus, in addition, appears close to theEarly–Middle Albian boundary (Bown et al. 1998;Gradstein et al. 2004). In the North Fork Cotton-wood Creek section, however, this datum has beenrecorded together with the FO of A. albianus,above a prominent massive sandy mudstone bed atthe base of the ss-unit (Figs 3,9). The BC23–BC24undifferentiated zone corresponds to the NC8zone and CC8a–8b subzones, and is consideredEarly–Middle Albian in age.

BC25–BC26 undifferentiated zone (Middle to Late Albian)

The zone is the interval from the FO of A. albianusto the FO of Eiffellithus turriseiffelii (US-046),within the ss-unit of the upper ChickaballyMember (Fig. 9). Species diversity and nanno-fossil abundance are high in this zone (Fig. 6).Commonly occurring species are D. ignotus,F. oblongus, Repagulum parvidentatum, Seri-biscutum gaultensis, and species of Biscutum,Corollithion, Prediscosphaera, Rhagodiscus,Watznaueria, and Zeugrhabdotus. The uppermostoccurrence of the local range of H. albiensis wasobserved within the zone (US-019a). The LO of

H. albiensis is placed at higher stratigraphic levelsin standard sections, at the base of the BC27b–UC0b subzone, within the NC10A subzone of Bral-ower et al. (1995) and at the boundary between theCC9a and 9b subzones of Perch-Nielsen (1985;Fig. 5). The undifferentiated zone is consideredin this study as Middle–Late Albian in age andcorresponds to the NC9 zone and the upper part ofthe CC8b subzone.

BC27–UC0 zone (Late Albian to Early Cenomanian)

This zone is the interval from the FO of E. tur-riseiffelii to the FO of Corollithion kennedyi(US-058; Fig. 9), and includes samples from theupper Chickabally Member, Bald Hills Member,and the lowermost part of the Gas Point Member.Commonly occurring species are D. ignotus, T. ori-onatus, and species of Biscutum, Watznaueria,and Zeugrhabdotus. The zone is correlated to theNC10A and CC9a–9b subzones, and is consideredLate Albian to Early Cenomanian in age.

UC1a–UC1d undifferentiated zone (Early Cenomanian)

This zone is the interval from the FO of C. kennedyito the FO of Prediscosphaera cretacea (US-074),and occurs in the lower part of the Gas PointMember (Fig. 9). The undifferentiated zone isslightly modified from the UC1a–UC1d subzones ofBurnett et al. (1998). The FO of P. cretacea, whichis used as the upper boundary of the zone in thepresent study, is an event recognized within theUC1d subzone in South England. Calcareous nan-nofossils are rare to few, and are mostly composedof D. ignotus, Retecapsa crenulata, T. orionatus,and species of Biscutum, Corollithion, Eiffellit-hus, Helicolithus, Prediscosphaera, Rhagodiscus,Staurolithites, Watznaueria, and Zeugrhabdotus.Based on the FO of C. kennedyi, this interval canbe correlated to the NC10B and CC9c subzones.The correlation of the upper boundary with thestandard Tethyan zonation schemes, however, isdifficult, since the FO of P. cretacea has not beenused in the NC/CC zonation schemes. The age ofthe undifferentiated zone is considered in this studyas Early Cenomanian.

UC1d–UC3d undifferentiated zone(Early to Late Cenomanian)

This zone is the interval from the FO of P. creta-cea to the LO of C. kennedyi (US-063), withinthe lower part of the Gas Point Member (Fig. 9).



The LO of C. kennedyi is used as a boundarymarker for the UC3d and 3e subzones. Calcareousnannofossils are generally abundant and are wellpreserved in this interval (Fig. 6). Commonlyoccurring species are the same as in the previouszone.

The stratigraphic position of the LO of C.kennedyi is uncertain in the NC and CC zonationschemes. Correlation between the UC zones andthe NC/CC zones, however, can be achievedthrough the use of the FO of Lithraphiditesacutus and the LO of Gartnerago nanum, neitherof which were observed in the studied section. TheFO of L. acutus is widely used as the zonal bound-ary between the UC2–UC3, CC9–CC10, andNC10–NC11 zones (Fig. 5). In the UC zonationscheme, the FO of L. acutus is placed above theFO of P. cretacea, while the LO of G. nanum, whichdefines the UC3c and 3d boundary, is located belowthe LO of C. kennedyi. The LO of G. nanum isplaced within the upper CC10 zone by Perch-Nielsen (1979). The UC1d–UC3d undifferentiatedzone therefore probably corresponds to the inter-val ranging from the upper part of the CC9csubzone to the lower part of the CC10a subzone.Although the LO of G. nanum is not considered abioevent in the NC zonation scheme, hence thedifficulty in the correlation between the UC andNC zones, the undifferentiated zone is probablycorrelated to the interval from the upper part ofthe NC10 zone to the lower part of the NC11 zone.The age of the undifferentiated zone is consideredin this study as Early–Late Cenomanian.

UC3e–UC5c undifferentiated zone (Late Cenomanian toearliest Turonian)

This zone is the interval from the LO of C.kennedyi to the FO of Eprolithus octopetalus (US-085), and occurs within the Gas Point Member(Fig. 9). Compared to the previous zone, nanno-fossil species diversity and abundance valuesare low (Fig. 6). Commonly occurring species areD. ignotus, T. orionatusi, and species of Biscutum,Watznaueria, and Zeugrhabdotus. The FO ofE. octopetalus is placed within the UC5c subzone,just below the UC5–UC6 boundary in Europe(Burnett et al. 1998).

The LO of L. acutus is important for correla-tion between the Tethyan and Boreal zonationschemes, defining the boundary between the UC4–UC5 and the NC11–NC12 zones (Fig. 5). This bio-event, along with the LO of Helenea chiastia,however, were not recognized in the North Fork

Cottonwood Creek section. The LO of H. chiastia,which is placed just above the FO of E. octopetalus(e.g. Burnett et al. 1998), defines the boundaries ofthe UC5–UC6 zones and CC10a–CC10b subzones,and occurs within the lower part of the NC12 zone(M. chiastius subzone of Bralower et al. 1995). Theundifferentiated zone therefore corresponds to theinterval from the upper part of the NC11 zone tothe lower part of NC12 zone and to the upper partof the CC10a subzone. The interval is consideredLate Cenomanian to earliest Turonian in age.

UC5c–UC6a undifferentiated zone (Early Turonian)

The zone is the interval from the FO of E. octo-petalus to the FO of Eprolithus moratus (US-088),within the Gas Point Member (Fig. 9). Calcareousnannofossil abundance and diversity values arelow and specimens are poorly preserved (Fig. 6).Commonly occurring taxa are B. constans, D.ignotus, and Zeugrhabdotus spp. The zone isconsidered Early Turonian in age.

UC6b zone (Early Turonian)

This interval ranges from the FO of E. moratus tothe FO of Quadrum gartneri (US-091), and occurswithin the Gas Point Member (Fig. 9). Comparedto the previous zones, calcareous nannofossils areabundant and the preservation is good (Fig. 6).Commonly occurring taxa include T. orionatusand species of Biscutum, Calculites, Corolli-thion, Eprolithus, Prediscosphaera, Retecapsa,Rhagodiscus, Staurolithites, Watznaueria, andZeugrhabdotus. Three bioevents are recognizedwithin the interval: (i) FO of Helicolithus turoni-cus (US-089); (ii) LO of R. achlyostaurion (US-089); and (iii) LO of R. asper (US-090; Fig. 9). Thesequence in which these events were recognized inthe studied section is similar to the North Seazonal scheme of Mortimer (1987). All bioeventsoccur prior to the FO of Q. gartneri.

Using the FO of Q. gartneri, the UC6b zone canbe correlated to the CC10b subzone. The LO ofR. asper is used as an upper boundary marker ofthe IC52 zone of Bralower et al. (1995). The IC52zone is correlated to the upper part of the NC12zone (=Eiffellithus eximius subzone of Braloweret al. 1995). In the studied section, the LO ofR. asper is located just below the FO of Q. gart-neri. The UC6b zone therefore can be correlatedto the NC12 zone.

The age of the zone is considered in this studyas Early Turonian. It is important to note,



however, that in earlier studies, the FO of Q. gart-neri is frequently used as a post Cenomanian–Turonian (C–T) boundary marker (e.g. Tsikoset al. 2004; Kolonic et al. 2005; see discussion onstage boundaries).

UC7 zone (Early to Middle Turonian)

The lower and upper boundaries of the zone aredefined by the FO of Q. gartneri and the FO ofE. eximius (US-092; Fig. 9). The calcareous nan-nofossil assemblage is similar to the previous zone.The short duration of the UC7 zone could be dueto the presence of condensed parts within theinterval in the North Fork Cottonwood Creeksection. The lower and upper boundary markersare both recognized in Tethyan and Boreal regions(Fig. 5; Roth 1978; Perch-Nielsen 1979, 1985;Luciani & Cobianchi 1999). The zone correspondsto the CC11 and NC13–NC14 zones, and is consid-ered Early–Middle Turonian in age.

UC8 zone (Middle Turonian)

The zone is the interval from the FO of E. eximiusto the uppermost sample (US-200) of the GasPoint Member in the investigated section(Figs 3,9). Calcareous nannofossils are few tocommon, comprised mainly of T. orionatus, D.ignotus, and species of Eiffellithus, Eprolithus,Prediscosphaera, Watznaueria, and Zeugrhab-dotus. The observation of the lowermost anduppermost occurrences of Kamptnerius magnifi-cus (US-097) and E. octopetalus (US-098), respec-tively, within the interval suggests correspondenceto the UC8 zone of Burnett et al. (1998). Theformer taxon occurs within the UC8a subzone,while the latter occurs within the UC8b subzone innorthern Europe (Burnett et al. 1998). Becauseof the poor preservation and rare to few occur-rences of nannofossils in the interval, however, theinterval can not be subdivided into subzones. Nev-ertheless, using these supplementary bioevents,the interval can be correlated to the CC12 andNC14 zones, and is considered Middle Turonianin age.

DISCUSSION

CORRELATION WITH AMMONITE ZONES ANDGEOLOGICAL AGE OF BUDDEN CANYON FORMATION INTHE NORTH FORK COTTONWOOD CREEK SECTION

In the North Fork Cottonwood Creek section,several age-indicative ammonite taxa were col-

lected from the Ogo to upper ChickaballyMembers. The biostratigraphic ranges of theseammonites, along with previously establishedammonite zones in the study area, are generally ingood agreement with the established nannofossilzonation in the studied section. This suggests thatthe North Fork Cottonwood Creek section ofthe BCF, like the Dry Creek section in TehamaCounty, may also provide information on zonal cor-relation between macro- and microfossils in theNorth Pacific region.

In the lowermost nannofossil zone established inthe present study, ammonites belonging to thegenera Crioceratites (Rodda & Murphy 1987) andShasticrioceras (S. patricki) were collected, sug-gesting Hauterivian–Early Barremian and Earlyto Late Barremian ages for the Ogo and RoaringRiver Members, respectively. The ages are similarto those proposed by Murphy et al. (1969). TheOgo and lower Chickabally Members, on the otherhand, correspond to the Simbirskites (Hollisites)aguila and Shasticrioceras poniente zones, whichare both considered Barremian in age, althoughthe S. poniente zone could also partially includethe Lower Aptian samples below the Huling Sand-stone Member (Murphy 1969, 1975).

The nannofossil-derived Late Barremian toEarly Aptian age of the lower ChickaballyMember is further supported by the reportedpresence of the Late Barremian ammonitesHeinzia, Heteroceras, and Lytoceras phestus andthe Late Barremian–Early Aptian ammonitesToxoceratoides spp. near the top of the lowerChickabally Member (approximately the top of theBC16–BC17 undifferentiated zone; Murphy 1956,1975).

The assignment of the Huling SandstoneMember to the BC18 nannofossil zone (Early–Late Aptian) is supported by the observationof the Late Aptian ammonites Eotetragonites win-tunius and Parahoplites spp. near the base of themember. Murphy (1956), in addition, reported theammonites Tropeum percostatum, Phyllocerasonoense, and E. wintunius, which are all sugges-tive of the Aptian E. wintunius zone, within theHuling Sandstone Member. Following the sugges-tion of Murphy (1975), the Huling SandstoneMember is considered in the present study as LateAptian in age.

The Late Aptian to Late Albian age (BC21 toBC26 zones) of the upper Chickabally Memberbased on calcareous nannofossils is also consistentwith the Aptian to Albian age suggested by ammo-nites. According to Murphy (1956) and Murphy



et al. (1969), the ammonite zones Acanthohoplitesgardneri to Mortoniceras hulenanum zones occurwithin the upper Chickabally Member. Within theBC21 nannofossil zone, A. gardneri and A. reesideiwere collected. Both ammonite taxa are consid-ered as Middle to Late Aptian in age (Popenoeet al. 1960; Murphy et al. 1969; Dailey 1973). Inthe BC23–BC24 undifferentiated zone, the EarlyAlbian ammonite Leconteites lecontei and theMiddle Albian Douvilleiceras mammillatum werecollected near the base and in the middle part ofthe interval, respectively. Murphy (1956), in addi-tion, observed D. cf. D. mammillatum in the low-ermost Albian L. lecontei zone within the upperChickabally Member. Within the Middle to UpperAlbian BC25–BC26 undifferentiated zone, theammonite taxa Oxytropidoceras packardi (MiddleAlbian) and Mortoniceras rostratum (UpperAlbian) were collected.

The Bald Hills and Gas Point Members,based on calcareous nannofossils, are consi-dered Late Albian–Early Cenomanian and EarlyCenomanian–Middle Turonian in age, respectively.

MAGNETOSTRATIGRAPHIC INTERPRETATION

Four reversed magnetic polarity zones wereobserved from the Ogo Member to the upperChickabally Member interval (Fig. 10). The lowestthree are found below the Huling SandstoneMember at the following levels, in ascendingorder: (i) lowermost Ogo Member; (ii) OgoMember and Roaring River Member boundaryinterval; and (iii) Roaring River Member andlower Chickabally Member boundary interval. Thereversed polarity interval recognized above theHuling Sandstone Member, on the other hand,occurs within the lower part of the upper Chicka-bally Member (Fig. 10). The interval containingthe three reversed polarity zones below the HulingSandstone Member is Hauterivian to Barremian inage. Comparison of the sample distribution mapsof Rodda and Murphy (1987) and the presentstudy suggests that Crioceratites sp. was collectedwithin the lowest reversed polarity zone in the OgoMember. Using this information, the reversedpolarity zone within the Ogo Member can be con-sidered as M5r. Consequently, the upper tworeversed polarity zones within the Lower andUpper Barremian can be correlated to the M3rand M1r polarity zones, respectively (Fig. 10).

The reversed polarity zone recognized withinthe lower part of the upper Chickabally Member,on the other hand, occurs above the FOs of

H. irregularis and R. orbiculatus, within the BC21zone (Figs 9,10). In terms of planktonic foramin-iferal biostratigraphy, the polarity zone occurswithin the G. algerianus zone, suggesting a LateAptian age. The reversed polarity zone thereforecould be correlated to the M-1r zone of the stan-dard GPTS (Gradstein et al. 2004). It should benoted that although the B–A boundary has beendelineated and approximated below the HulingSandstone Member, the M0r polarity zone appearsto be missing in the North Fork Cottonwood Creeksection. The possibility that the reversed polarityzone above the Huling Sandstone is correlated tothe M0r polarity zone, however, is very low, sincebased on ammonite and nannofossil biostratigra-phy (discussed in the previous section) as well asearlier studies in the area, it appears that theuppermost Barremian to Lower Aptian intervalis not represented in the stratigraphic record.This could be due to the possible erosion of theuppermost Barremian to Lower Aptian sedimentsduring the deposition of the Huling SandstoneMember (see Hauterivian-Barremian andBarremian-Aptian boundaries and Possible HiatusLevels).

STAGE BOUNDARIES IN BUDDEN CANYON FORMATION

Hauterivian–Barremian and Barremian–Aptian boundaries

In standard integrated time scales (i.e. Channellet al. 1995; Gradstein et al. 1995; 2004; Erba1996), the Hauterivian–Barremian (H–B) andBarremian–Aptian (B–A) boundaries are placed atthe uppermost M5 polarity zone and at the base ofthe M0r polarity zone, respectively (Fig. 5). Basedon the combined nannofossil and ammonite bios-tratigraphy and magnetostratigraphy, the H–Bboundary was delineated in the uppermost part ofthe Ogo Member. The Lower–Upper Barremianboundary, on the other hand, was delineated in thelowermost part of the Roaring River Member, atthe top of the M3r polarity zone (Figs 9,10).

In conjunction with magnetostratigraphy, anumber of nannofossil events have also been usedas B–A boundary markers: H. irregularis (Thier-stein 1976), Chiastozygus litterarius (Mutterlose1991), and R. gallagheri (Bown et al. 1998). Bral-ower et al. (1994) defined the base of the NC6 nan-nofossil zone (i.e. the B–A boundary) using the FOof H. irregularis. In the same paper, the FO of F.oblongus, which we are using in the present studyto approximate the B–A boundary, is located belowthe FO of H. irregularis. This succession is similar



to that of the Tethyan nannofossil zonation schemeof Bown et al. (1998). In the paper of Braloweret al. (1995), the FO of F. oblongus was used as thebase of the NC5E zone, which is within the UpperBarremian.

As mentioned earlier in the paper, the baseof the BC18 zone of Bown et al. (1998), that is theB-A boundary, is defined by the FO of R. gallag-heri. In the North Fork Cottonwood Creek section,however, typical specimens and forms similar toR. gallagheri were observed in the uppermost partof the Roaring River Member, which is consideredin this study as Early to Late Barremian in age. Inaddition, the FO of H. irregularis was observedabove the Huling Sandstone Member, which isLate Aptian in age based on previous and morerecent ammonite biostratigraphic studies. The FOof H. irregularis was also observed between therange of F. varolii, which is a Lower–Upper Aptianmarker in the Boreal scheme of Bown et al. (1998).The immediate occurrence of F. varolii above theHuling Sandstone Member suggests that the FOof F. varolii in the North Fork Cottonwood Creeksection may not represent the true range of thetaxon. This possibility is further supported by theoccurrence of the Late Aptian ammonites Eotet-ragonites wintunius and Parahoplites spp. nearthe base of the Huling Sandstone Member. Simi-larly, the local range of H. irregularis in the NorthFork Cottonwood Creek section is less than its

true range. This difference in the stratigraphicoccurrence of the taxon in the North Fork Cotton-wood Creek section and in other sections can bedue to a number of factors such as poor pre-servation and difference in the paleogeographicpositions of the study areas.

The FO of H. irregularis is often used as a B–Aboundary marker in low latitude regions. However,it is not reported in high latitude sites (e.g. DSDPSites 511 and 763, Bralower et al. 1994). Channellet al. (1995) illustrated (Fig. 8) the absence ofH. irregularis above the FO of C. litterarius in theGreat Valley Group, although Bralower et al. (1994)observed H. irregularis in two DSDP Sites in theCentral Pacific (DSDP Sites 167 and 463). Braloweret al. (1994) observed that the FOs of H. irregularisand F. oblongus coincide at the same level in DSDPSite 167. With these observations and problemsconcerning H. irregularis and R. gallagheri, theuse of the FO of F. oblongus to approximate the B–Aboundary in the North Fork Cottonwood Creeksection is more or less justified. In the Borealscheme of Bown et al. (1998), the FO of F. oblongusis a supplementary datum within the BC18 zone(Early Aptian), above the FO of R. gallagheri(Fig. 5). The FO of F. oblongus is slightly older thanthe FO of H. irregularis and is younger than the FOof C. litterarius in northwestern Europe (Erba1996). The occurrence of the Early Aptian genusToxoceratoides near the top of the lower Chicka-

Fig. 10 Combined magnetostratigraphy, and ammonite and nannofossil biostratigraphy of the lower portion of the Budden Canyon Formation in theNorth Fork Cottonwood Creek section. Horizontal lines on the right side of the stratigraphic column show the location of the samples for magnetostrati-graphic analysis. Figure 3 shows lithological explanation.



bally Member further supports the placement ofthe B–A boundary in the section.

Aptian–Albian boundary

The Aptian–Albian (A–A) boundary in the studiedsection was delineated using the FO of P. colum-nata near the uppermost part of the ms-unit ofthe upper Chickabally Member (Fig. 9). The FO ofP. columnata is considered a marker for the A–Aboundary (Fig. 5) and has already been recognizedin the Vocontian Basin, southern France (e.g.Bréhéret et al. 1986; Hart et al. 1996; Kennedyet al. 2000). In northwestern Germany, however,the FO of P. columnata appears within the lower-most Albian ammonite zone (L. tardefurcata zone;Mutterlose 1992; Bown et al. 1998), implying diffi-culty in using the taxon as a marker for the A–Aboundary, which was discussed by Mutterloseet al. (2003). The problem arises mainly fromthe varying taxonomic concepts of the speciesbelonging to the genus by different workers. It isalready widely known that early species of Pre-discosphaera appeared during the Early Aptianare represented by the elliptical P. spinosa, fol-lowed by the more circular forms above the A–Aboundary, which are represented by P. columnata(Perch-Nielsen 1979; Mutterlose 1996 in Mutter-lose et al. 2003). According to Bown (in Kennedyet al. 2000), the transition from elliptical to circularforms appears to be gradual, thus the difficulty inusing the FO of P. columnata as a reliable markerfor the A–A boundary. In the North Fork Cotton-wood Creek section, the identified specimens ofP. columnata are more or less circular in morphol-ogy as opposed to the other species of Predis-cosphaera encountered in the samples (e.g. P.cretacea, P. quasispinosa, and P. spinosa; Fig. 8).

In terms of ammonite biostratigraphy, the A–Aboundary is delineated using the FOs of Leymeri-ella tardefurcata and the genus Douvilleiceras, orby the LO of Hypacanthoplites jacobi (Bréhéretet al. 1986; Hart et al. 1996). In the study area,Douvilleiceras was observed near the lower partof the ss-unit of the upper Chickabally Member,within the Early Albian Leconteites lecontei Zone(Murphy 1956). The A–A boundary as determinedby nannofossil biostratigraphy therefore appearsto be stratigraphically lower than the boundarydetermined by previous ammonite studies. A morerecent ammonite study by Amédro and Robaszyn-ski (2005) in the North Fork Cottonwood Creek, onthe other hand, delineated the A–A boundaryapproximately 80 m below the nannofossil-derivedA–A boundary.

Following previous studies on ammonites (e.g.Murphy 1975), the Early–Late Aptian boundaryis placed near the base of the Huling SandstoneMember (Fig. 9). Delineation of the Early–Middle–Late Albian boundaries using calcareousnannofossils, on the other hand, is problematic asseveral marker taxa were not observed in thesection. The FO of A. albianus was used toapproximate the Middle–Late Albian boundary(Figs 5,9; Bralower et al. 1995; Bown et al. 1998;Gradstein et al. 2004). Amédro and Robaszynski(2005) used the base of the Oxytropidoceras Pac-kardi zone as the Early–Middle Albian boundaryand the base of the Douvilleiceras cristatum zoneas the Middle–Late Albian boundary. The formercorresponds to the lower part of section IV of thepresent study and is more or less consistent withthe delineated Early–Middle Albian boundaryusing calcareous nannofossils (Figs 3,9). The baseof the D. cristatum zone, unfortunately, was notrecognized in the North Fork Cottonwood Creeksection (Amédro & Robaszynski 2005).

Albian–Cenomanian boundary

Using the FOs of E. turriseiffelii and C. kennedyi,the Albian–Cenomanian (A–C) boundary wasdelineated within the Bald Hills Member, consis-tent with results from the nearby Dry Creeksection and subsequently collected Albian ammo-nites of the genera Mortoniceras and Lechites inthe lower beds of the Bald Hills Member (Murphy& Rodda 1996). The FO of C. kennedyi is widelyused in both the Tethyan and Boreal zonalschemes, and has been used as a post boundarymarker nannofossil taxon (Fig. 5; Perch-Nielsen1979, 1985; Tröger & Kennedy 1996; Burnettet al. 1998). The sedimentation of the Bald HillsMember in the North Fork Cottonwood Creek isbelieved to have begun earlier than at Dry Creek(Amédro & Robaszynski 2005). This explains theobservation of Murphy and Rodda (1996) in theDry Creek section that the A–C boundary is con-strained to a thin interval encompassing the baseof the Bald Hills Member. This boundary wasdelineated using the LOs of the typical Albiangenera Mortoniceras and Stoliczkaia and theentry of mantelliceratine juveniles (probablyGraysonites) and Mariella.

Cenomanian–Turonian boundary

A number of calcareous nannofossil markers havebeen suggested to delineate the Cenomanian–



Turonian (C–T) boundary. These include the LOsof L. acutus, A. albianus, and H. chiastia and theFO of Q. gartneri (Fig. 5). These events have beenrecognized in Europe (England, Spain, southeast-ern France, Italy), North America, Jordan, andAfrica (e.g. Bralower 1988; Jarvis et al. 1988;Lamolda et al. 1997; Luciani & Cobianchi 1999;Paul et al. 1999; Schulze et al. 2004; Fernando et al.2010). The FO of Q. gartneri is considered impor-tant to the delineation of the C–T boundary. Thisevent occurs within the W. devonense zone andabove the carbon isotope excursion in Pueblo(Colorado), South England and the northwesternAfrican Shelf (Lüning et al. 2004; Tsikos et al.2004; Kolonic et al. 2005). Other studies, however,suggest that the FO of E. octopetalus is a bettermarker for the C–T boundary. This has beenobserved earlier by Bralower et al. (1988) in theWestern Interior Basin as an unidentified speciesof Eprolithus common at the C–T boundary. Thishas been illustrated further in the studies of Varol(1992), Lamolda et al. (1997), Bralower andBergen (1998), and Hardas and Mutterlose (2006).In the UC zonation scheme of Burnett et al. (1998),the FO of E. octopetalus is placed prior to the FOof Q. gartneri as a supplementary event along withthe FO of E. moratus. This event, in addition,occurs prior to the LO of H. chiastia near the C–Tboundary.

In the North Fork Cottonwood Creek section,the LOs of A. albianus and H. chiastia were rec-ognized below the LO of C. kennedyi (Late Cen-omanian) suggesting that both events cannot beused to delineate the C–T boundary, probably as aresult of diachroneity. Following the UC zonationscheme therefore the C–T boundary was delin-eated using the FO of E. octopetalus, within theGas Point Member (Fig. 9).

COMPARISON BETWEEN NORTH FORK COTTONWOODCREEK SECTION AND WESTERN INTERIORBASIN SECTIONS

Comparison between the North Fork CottonwoodCreek section and the Western Interior Basin islimited to the Cenomanian–Turonian interval, asthe youngest nannofossil zone recognized in thestudy area is UC8 (Middle Turonian). Relevant forthis discussion therefore will be the works of Bral-ower (1988) and Bralower and Bergen (1998) onthe nannofossil biostratigraphy of several sections,including the Rock Canyon stratotype in Pueblo,Colorado. In the study of Bralower (1988), a nan-nofossil zonation scheme tied up with ammonite

and foraminiferal zonations was established forthe Western Interior Basin. This was supple-mented by an informal scheme by Bralower andBergen (1998) for the Cenomanian–Santonianinterval. In these zonation schemes, the C–Tboundary is delineated using the LO of H. chiastia(=Microstaurus chiastus) which also defines theboundary between the Microstaurus chiastussubzone (Late Cenomanian) and the Eiffellithuseximius subzone (Early Turonian). These two sub-zones comprise the Rhagodiscus asper (=Parhab-dolithus asper) zone, which straddles the C–Tboundary. The succession of bioevents across theC–T boundary was also determined in the WesternInterior Basin. Above the LO of H. chiastia, theevents include (from oldest to youngest): LO R.asper, LO Cretarhabdus loriei, FO E. octopetalus,FO Eprolithus eptapetalus, and LO E. octopetalus(Bralower & Bergen 1998). Below the LO ofH. chiastia, on the other hand, the bioevents (fromoldest to youngest) are as follows: FO Q. gartneri,FO Gartnerago segmentatum, FO Microrhab-dulus decoratus, LO Gartnerago nanum, LOC. kennedyi, FO Corollithion exiguum, LO A.albianus, LO L. acutum, and FO E. eximius(Bralower 1988). Not all of these bioevents arerecognized in the North Fork Cottonwood Creeksection. Moreover, in terms of the stratigraphicsuccession of bioevents between the two areas,several discrepancies can be observed. Theseinclude the earlier disappearance of H. chiastiaand A. albianus (occurring below the LO ofC. kennedyi), later appearance of Q. gartneri, G.segmentatum, and E. eximius (above the FO ofE. eptapetalus) and later disappearance of R.asper (above the FO of E. eptapetalus). The earlydisappearance of H. chiastia in the North ForkCottonwood Creek section is also the reason fordelineating the C–T boundary near the FO ofE. octopetalus (see discussion in the previoussection). The differences observed between theWestern Interior Basin and the North ForkCottonwood Creek section can be attributed to dif-ferences in the paleogeographic position and depo-sitional environment (i.e. epeiric sea vs forearcbasin) that could have influenced the distribu-tion, abundance, and preservation of calcare-ous nannofossils in the two areas during theCenomanian–Turonian interval.

POSSIBLE HIATUS LEVELS

The occurrence of a widespread Early Cretaceousunconformity has already been documented in



northern California and Oregon Provinces (Peter-son 1967). This unconformity corresponds to thelithological boundary between the lower Chicka-bally Member and the Huling Sandstone Member.The base of the Huling Sandstone consists ofpoorly sorted, relatively massive sandstones withminor amount of conglomerates (Murphy et al.1964, 1969). The onset of the deposition of thesandstone member coincided with a sealevel dropduring the earliest Aptian (Peterson 1967; Dailey1973; Hardenbol et al. 1998; Gradstein et al. 2004),and was probably preceded by the erosion of amajor portion of the Lower Aptian, resulting in thevery short Lower Aptian interval in the section.This erosion could be the reason for the non-recognition of the M0r reversed polarity zone inthe studied section (Fig. 10).

A possible hiatus could also be present withinthe Lower Albian interval, between the BC23–24and BC25–26 undifferentiated zones. This possiblehiatus is suggested by the co-occurrence of the twomarker taxa T. orionatus and A. albianus abovea prominent massive sandy mudstone bed atthe base of the ss-unit in the upper ChickaballyMember (Figs 3,9). The presence of this uncon-formity, however, is not supported by recentammonite studies and previously conducted geo-logical mapping in the study area (e.g. Amédro &Robaszynski 2005).

NANNOFOSSIL BIOPROVINCE IMPLICATIONS ANDPOSSIBLE OCEANIC ANOXIC EVENT

The development of the sedimentary basin alongthe coast of California coincided with an eastwardtransgression, triggered by the mid-Cretaceousglobal warming (Ojakangas 1968). The resultingexpansion of the tropical to subtropical climateinduced intense chemical weathering, resultingin the deposition of abundant clay materials inthe basin (Ojakangas 1968). Sedimentation rates inthe BCF are estimated in the present study torange from 15 to 54 mm/kyr during the LowerCretaceous and 12 to 220 mm/kyr during theUpper Cretaceous. These values are comparableto the values estimated by Ojakangas (1968), whichwere 107 and 137 mm/kyr in the Lower and UpperCretaceous, respectively.

The mid-Cretaceous global warming alsoaffected the faunal and floral bioprovinces in theNorth Pacific region. Although the paleolatitudeof northern California has been estimated to belocated at about 40–50° latitude (e.g. Barron et al.1981; Smith et al. 1994), the ammonite and nanno-

fossil assemblages contain many Tethyan features.The ammonites in the BCF consist mainly of wide-spread Pacific fauna, associated with some indig-enous species in California and several typicalTethyan-diagnostic taxa, suggesting a temperateto Boreal bioprovince (Murphy 1956, 1969;Murphy & Rodda 1996). Calcareous nannofossilsin the BCF, on the other hand, include someBoreal-diagnostic taxa, but mainly consist ofTethyan species, allowing recognition of Tethyanbioevents and correlation to Tethyan (low-latitude)zonation schemes possible. This is particularlyevident for the Albian and younger units in thestudied section. Global correlation of nannofossilzones between the Tethyan and Boreal realms,however, becomes difficult during the Barremianto Lower Aptian interval due to pronounced bio-geographic separation of taxa and species diachro-neity (Bown et al. 1998; Street & Bown 2000). Inthe study area, this is evident in the presence oftypical Boreal calcareous nannofossils such as N.inornatus and F. varolii in the lower part of thestudied section, within the Barremian to EarlyAptian interval (Figs 7,9). Both taxa have beenreported in the North Sea and other Boreal sec-tions, but not in Tethyan regions (Bown et al.1998), suggesting that the nannofossil bioprovincein the BCF had been under the influence of coolertemperate to cold water conditions during theinterval.

In the North Fork Cottonwood Creek section,nannoconids occur within the Late Barremianlower Chickabally Member (Fig. 9). Speciesbelonging to the group include N. inconspicuus,N. inornatus, N. minutus, N. cf. N. steinmannii,and N. truitti truitti, although most of the nanno-conids observed in the samples are narrow-canalled ‘discs’ 6–9.5 mm (average 7.6 mm) indiameter (Fig. 8). As mentioned earlier, two nan-noconid abundance peaks were observed in theUpper Barremian. The first peak occurs within theM1r polarity zone, while the second occurs nearthe top of the lower Chickabally Member, belowthe Huling Sandstone Member and the FO of F.oblongus. The decline of nannoconids after thesecond peak in the studied section therefore couldbe related to the uppermost Barremian decline innannoconid abundance prior to the ‘nannoconidcrisis’ in the Early Aptian (Erba 1994, 2004; Erbaet al. 2004; Erba & Tremolada 2004).

Compared with the Cretaceous sections inEurope, which are mostly composed of marl–limestone facies, the BCF is composed of monoto-nous gray to dark gray terrigenous mudstones and



sandstones. For this reason, lithological recogni-tion of black shales on outcrop scale is difficult, andthe presence of OAEs, not surprisingly, has not yetbeen reported in the BCF. Preliminary results,however, show distinct excursions in the totalorganic carbon (TOC) data between the UC3e–UC5c to UC5c–UC6a zones, across the C–Tboundary (T. Tomosugi, unpubl. data, 2006; Fig. 6).This excursion in TOC, along with the drasticdecline in nannofossil abundance across the C/Tboundary, is believed to be correlated to the OAE2.

CONCLUSIONS

The work is summarized as follows.1. The present study provides a mid-Cretaceous

standard calcareous nannofossil biostratigra-phy in the North Pacific region. Using theBoreal BC zonation scheme of Bown et al.(1998) for the Lower Cretaceous and the cosmo-politan UC zonation scheme of Burnett et al.(1998) for the Upper Cretaceous, 16 nannofossilzones were recognized in the North Fork Cot-tonwood Creek section of the BCF in ShastaCounty, northern California. The zones cor-respond to the BC15 and older zones to theUC8 zone, suggesting an age of Hauterivian toMiddle Turonian, which is basically in goodagreement with the ammonite biochronology ofMurphy (1956, 1969).

2. The age of the members of the BCF in thepresent study using combined biostratigra-phic and magnetostratigraphic data confirmsthe results of previous studies done in the area:(i) Ogo–Hauterivian to Lower Barremian; (ii)Roaring River–Upper Barremian; (iii) lowerChickabally–Upper Barremian to lowermostAptian; (iv) Huling Sandstone–Upper Aptian;(v) upper Chickabally–Upper Aptian to UpperAlbian; (vi) Bald Hills–Upper Albian to LowerCenomanian; and (vii) Gas Point–Lower Cen-omanian to Middle Turonian.

3. Stage boundaries were likewise established inthe North Fork Cottonwood Creek section. TheH–B and Lower–Upper Barremian boundarieswere placed in the uppermost M5 and M3rpolarity zones, near the upper part of the OgoMember and the lowermost portion of theRoaring River Member, respectively. The B–Aboundary, on the other hand, was placed atthe topmost portion of the lower ChickaballyMember. The A–A, A–C, and C–T boundarieswere delineated within the upper Chickabally

Member, at the top of the Bald Hills Memberand within the Gas Point Member.

4. An unconformity is documented in the studiedsection, represented by the base of the HulingSandstone Member. This could have resultedin the erosion of a portion of the Lower Aptian,and probably the interval containing theM0r reversed polarity zone, and thus its non-recognition in the studied section. A secondunconformity is postulated at the base ofthe ss-unit of the upper Chickabally Member(Lower Albian), between the BC23–24 andBC25–26 undifferentiated zones.

5. The nannofossil assemblage in the North ForkCottonwood Creek suggests that the study areawas under the influence of cold water conditionsduring the Barremian to Lower Aptian inter-val. During the Albian to Turonian inter-val, however, the assemblage is dominated byTethyan taxa, probably resulting from themid-Cretaceous global warming.

6. The presence of OAEs has not yet beenreported in northern California. PreliminaryTOC and d13C isotope data, however, show thatthe global C–T boundary OAE2 can be presentin the studied section.

ACKNOWLEDGEMENTS

A.G.S. Fernando conducted and completed thestudy while at Hokkaido University under theMonbukagakusho Scholarship Program. Thisstudy is supported by the 21st Century COEProgram ‘The Neoscience of Natural History’of Hokkaido University and by the Overseas Sci-entific Research Fund of the Japan Society forthe Promotion of Science (nos. 14255001 and18403013). The authors thank Takashige Tomosugi(Hokkaido University) for providing the preli-minary TOC data. The authors acknowledge DrYasufumi Iryu and Dr Timothy J. Bralower forreviewing the initial draft and for their helpfulcomments and suggestions.

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SUPPORTING INFORMATION

Additional Supporting Information may be foundin the online version of this article:

Table S1 Calcareous nannofossil distribution inthe North Fork Cottonwood Creek section. Biblio-graphic references can be found in Perch-Nielsen(1985) and Bown (1998, 2005). Two new calcareousnannofossil species were described by Fernandoand Okada (2007) in the North Fork CottonwoodCreek section of the Budden Canyon Forma-tion: Diloma californica and Prediscosphaeraquasispinosa

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