／凡}

bY C. E. Mitchell', Chen X[12, S. M. BergstrbM3, Zhang Yuan-dong2, Wang ZhI-ha02,B. D. Web勿4 and S. C. Finne-y5

Definition of a global boundary stratotype for theDarriwilian Stage of the Ordovician SystemI I)epartment qf'Geologv, dic State Uiiiversit)‘，／New York at Byff}do, Buflato, NY 14200-3050, USA2 Van.jing Institute of'Geolog) and Paleontolog}，39 Be}png East Road, Nanjing 2 /0008, China3 I)epartment (?f*Geologi‘二／S( iolc}、Ohio,State Universitv, Coluttibus, OH43210-1,397,USA4 Centre／‘，，，Ecostratigraph.N and Palaeobiology, Macquarie UniversiA-, NSW 2109, A ustralia,5 Department‘，／Geological Sciences (itGdiftmaa State Universitv, Lo，ng Beach, Cahfi)rnia 90840, USA

The International Conunission on Stratigraphic Nomen-clature and the lUGS Executive Committee have

recenth, approvedselection。，／‘，global stratot}vpe sec-

tion and point defining the base of ． the second stage ofthe Middle Ordovician Series. This stage is nained the

Darriwilian Stage“力er the Australian regional stage o／

thesaine name, with which itshares identical definitionand scope. The base o0he Darriwilian Stage is． fixed at

the base可the Undulograptus austrodentatus Biozone inC}osection through the Ningkuo Shale exposed at Huang-niiang, Changshan CountY, Zhejiang Province, south-eo.s t China. This boundarY is defined bY the.万rst appear-ance datum (Y'the U. }iustrodentatus within a densel-v

sampled su〔工cession可．graptolite万rst occurrences thatfacilitate precise global correlation of' this boundarv.

Conodont species (ire also present within this section

and indicate that the base可. the Darriwilion is likelv tolie within the lower part ol'the Microzarkodina parva

Bio7one1, 可．the North Atlantic‘onodout 7,onal succes-

SiOn. Correlation ol'this boundarY with well studiedsuc-(TASIOns elsewhere indicates that it is coeval with a level

；，，the mid Whiterockian in North America. and in the

lowet- Tennian Stage‘，／the uppe，一A二，iig of Wales. Thebase‘）／the Dat-rAvilian is ahout 407.5州 材“

Introduction

Advances in our knowlcd},e of the Earil-i's history, including devel-opincrit ol'an increasingly refined geological time scale, have led tothe posing of' questions in inany areas of' geological research thatreCjUirc yet further increase,,, in the precision and reliability of thistime scale. To address this need in the Ordovician, the IUGSACS

Subcommission on Ordovician Stratigraphy since 1989 has beenengaged in an active prograin anned at identifying those biologicalc\cru horizon,, that exhibit the greatest potential for chronostrati-graphic subdivision of the Ordo\ ician Systern for world-wide use.In particular, this effort has focused on especially widely recogniz-able biozones within the pelagic graptolite and conodont faunas. Thehi-li dc-ree of biogeographic differentiation which characterizesOrdovician biotas has made development of a precise. unified globalstandard a slow and difficult process. In most regions。们he world,

provincially based, regional subdivisions are still actively used. TheOrdovician System was first established in the British IsIcs and con-sequently the British series, although also based on local and oftenendemic biotas, have been widely employed as a linguaftanca forinternational correlation within the Ordovician. Given these facts,

adoption ofthe new global subdivisions and nomenclature will be a-raclual process. Nevertheless, several important first steps havebeen accomplished, including the first. formally ratified proposal fora global Ordovician stage.

Early in 1996. by a 90% majority, the Titular members of theOrdovician Subcommission voted in favor of a division of the sys-tern into three series named the Lower, Middle. and Upper Ordovi-cian. This provides the basis for the next step} that is, to adopt a sim-ple six-fold global stage level division in which each series isdivided into two stages. Most of the stages and series have yet to beformally defined or named, but potential levels for their bases havebeen identified with particular conodont or graptolite biozones withpotential for long range correlation. I he general framework of theseglobal stages and series ofthe Ordovician System has now emerged(Figure I）．It is within this context that the Darriwilian GSSP hasbeen proposed.

The task of identifying suitable levels for series and syste，二sub-division has been conducted by means of a series ofintra-0rclovicianworking groups. Within the interval（们he Middle Ordovician (Fig-ure I），the Ordovician Subcommission workim, ,roups identifiedtwo biozonal levels that appeared to have potential for subdivision ofthe series: the base of the I)id.vinograptiis arnis (=L). b功dus ofElles), and the Undidograptus austrodentalus biozones. I he formerlevel is coincident with the base of the British I.lanvirn Series, and

the second with the base ofthe Darriwilian Stage ofAustralia. 'thescope. rank, and correlation of the Llanvirn Series have been the sub-ject of much discussion and disagreement (among recent papers see,for example, Fortey and others. 1990: Cooper and Lindholin, 1990}Maletz.］992: Berry, 1995). Fortey and others门995). in an effort toaddress the need for it better unit in this interval, recently revised the［lanvirn Series. Although they identified a possible stratotype sec-tion for the hase of' the Llanvirn in Wales (see Fortey and others,1991, p. 13), no Welsh candidate sections for a Ldobal stratotype atthis level have been SUbmitted to the Subcommission lot- Ordovician

Stratigraphy fo）r consideration. Members of the Artits WorkingGroup discussed several additional sections from other regions ofthe world as a potential stratotype for a stadia] boundary at the baseof the D. artas Biozone, but none appear ideal and again none havebeen submitted forinally to the Subcommission for consideration.

The Ordovician succession in Australia has：、long history ofbiostl飞ltigl}I phic study. The Darriwi han Staae was erected by Hal Ibiostrati"r i I（1899). redefined by Hari-is（1916, 1935). Hari-is and Keble（1932),and most recently by VandenBer- and Cooper门991 P. 53) for theinterval between the base ofthe Uiidu1ogr(ipms austrodetitattis Bio-zone and the Nemagraptus gra(ilis Biozone. Since the I 930's, the

September 1997

59

│芝 │二 ＿一 │（ │一 ││口日 │Cc │一 G LO BA LST A GES │ 一 K：丫 GRA PT O LITE/ │

│卜 │ │ │｝ CONODONT (C) ││V) │ │ │一 BIO HO RIZO N S ││＞－－ │ │ ├─────────────┤

│(A │ │ │一： ＿＿P- dcum inatus 、 │

├───┼────┼────────┤一“’approximalu's │

│2 │Q!! │一 二 二 ？二 二 ｛ │一‘ lapetognathus n.sp. I │

│＜ │口习 │ │｝， }C) ││乙 │氏 │ │ │

│＞ │a- │ │ ││O │习 │ │ │

│O ├────┼────────┤ │

│匕 │口习 │】 DA RRIW ILIA N ！ │ │

│0 │__j ├────────┤ ││ │O │【 一 │ │

│ │O │ │ ││ │网门． ． │ │ │

│ │乏 │ │ │

│ │比 │一 一 │ │

│ │日J 一 ├────────┤ ││ │弓 │一 一 │ │

│ │0 │ │ ││ │－ │ │ │

│ TRADITIONAL REGIONAL SUBDIVISIONS │

│(SOME UNITS LIKELY TO BE USED IN GLOBAL STAGE NOMENCLAIURE) │

│BR！丁AIN │BALTOSCANDIA │NORTH │AUSTRALASIA │CHINA │

│ │ │AMERICA │ │ │

│Ashgill │Harju │Cincinnatian│Bolindian │Chientang-││ │ │ │Eastonian │ kianqian│

│ │ │ │Gisbornian ├─────┤

├─────┤ │ │ │Neichian- ││Caradoc ├─────────┤ │ │ shanian │

│ │vir。xxxxViru BB├──────┤ │ │

│ │ │ Mohawkian │ │ ││ │ │IXX X │ │ │

│ │ │ M日 │ │ │

├─────┤ │Whiterockian│ │ │

│Lianvirn │ │ ├───────┼─────┤│ ├──┬──────┤ │Darriwilian │Zhepangian│├─────┤1口 │ Kunda │ │ │ │

│Arenig │〔 ├──────┤ │ │ │

│ │门 │V olkhov │ ├───────┼─────┤│ │Cu │ │ │ Yapeenian │Yushanian │

│ │0 │ │ │Castlem ainian │ │

│ │ │Latorp │lbexian │Chewtonian ├─────┤

├─────┤ │ │ │Bendigonian │Ichangian │

│Tremadoc │ ├──────┤ │Lancefieldian │ ││ │ │Trennadoc │ ├───────┤ │

443

Ma

495

Figure I Chart illustrating stratigraphic relationships between global Ordovician Series, Stages, key fi7unal markers (two of which，，。、have rat访ed GSSPs) and some regional subdivisions (after Webby, 1997). Volcanic ash event marker (the Big (=Kinnekulle) Bentonite,BB, and correlative Millbrig Bentonae, MB), as well as approximate radiometric dates (after Tucker & McKerrow 1995), also shown.

"raptOlile Zonation Upon which the DarriAilian is bascd hj,; beenwidely used as a ba.,,is 1or internalional correlation. particularly inthe Pacific Pro＼一ince, and，、reaclil} recognizable in graptolite faciesaround the -lobe. The Darriwilian Sta},e sl)ans a strati-raphic inter-val which is similar to but sligghtly larger than that of、 the revisedLlanvirn Scrics ot'Fortey and、）山ers（1995).

ISSLICS raised by山e need to choose one or the other of' the D.arms or I人‘my,rodelffilluv lc}cls were dkeusscd at length clUringworkshoj)s held at the Sixth International S}rnposiuni on the Ordo-vician Systern in Sydney, ALISII-aha. In 1991．and at a subcommissionfield niecting in Nanjin- in 1993. and apain at the Seventh Intei-nj-fional Symposium on the Ordovician System at Las Ve-as. USA. in1995, all minuted ill issues of' Ordovician News, nos. 9（1992), 11

（1994), ai记13（1996). In 1992-93, in response to discussions at theSydney F11CCtiu,' Ofthe Subcommission, Chen XU or,,anized an intei-national workinL, oroup to investigate sections spanning the Yapeen-ian-Darriwilian interval in llic Jl：1：19、11：、，1一Cl：：lllgsh：，1卜Yush：In area(JCY area) alono the border between Zhejiang and Jiangxiprovince,;, China, and an open in\itation was made for participantsto join the working -LrOLIP (Ordoviciati News. no. 9. p. 20, 1992)Jhe rcstilting, JCY working }'FOUJ) included specialists trom Chinathe I ISA, France, Germany. and Australia. Based on this joint inves-OL'ation, as vclI：、、the pre\ ions work Ot'OUl- Chinese COlICal"Hes, thebase ot the Undulograptus ati.modewams Biozone of the Huan-ni-tant-1 section in this arca has been identified its：、suitable GSSP for theDarriv, ilian（（hen and others 1995b:Chcn and 13crostrorn, 1995}Mitchell是 in(] Nlalctz 1995). This section was chosen as the GSSP

I)CCZIU}C it represents the best a\ailable section thl-OLI,,h the intervalIincludim, the hase of the U. i nistrodenla no Biozone. Five othcr sec-tions in the Y Y re,,ion were included in the Study, and several ofIthese come near to rivaling the Huangnitan" section ill their expo-sure, fossil content, and (2011611161V Of Succession. With this work ininind. R. D. Webby. Chan（们lie Ordovician Subcommission. sentan advisory questionnaire to Titular members of’ the Ordovician Sub-commission in early 1995 (Oidoviciwi News, no. 13. 1). 30-3 1）．Theresults of this survc} provided further guidance for clecision-ruakingin I'- or Of usi'llu the hasc ofille U. ousirodcmants Bio/onc

The name Darriwilian (Hall, 1899). based on the Australian

re2ional stage. was chosen by the JCY workin" group because of itspriority for the interval extending upwards from the base of一 U. ou}-Itrodentants Biozone. FUl山erniore, it has been agreed by a vote ofthe Titular members of the SUbCOMMISSiOll (8214 in favoi）川Febi Ll-

ary 1996 that the overlyin2 global stage (and series) will be definedby the base of the刀e｝1，‘191了、1）了1，、又1丫，（、111＼ Biozone. Thus, the concell-tion ofthe global Darriwilian Stage intcr\ it], from its base to its top,conforms exactly with prior Australian re.010nal LlSa(-1C

In July Ot 1996 the Darriwilian GSSP proposal was ovci-whelmingly approved by the Titular members of' the OrdovicianSubcommission（17 votes in favor, I against, 2 abstentions. and I noresponse, for a 94(/t majority). Subsequently, the Darriwilian GSSPwas also approved by the ICS, and finally. ratitied by a unaninIOLINvote ofthe JUGS Executive Committee at its Januarv. 1997 nicetii),-,.

Recommended global stratotype sectionand point

The recommended GSSP is located along, an unpaved road on theriverside near Huan-nitan- }illaoe, 3.-} kin southwest of the (’ fiam2shan County Town of Zhejian, Province (1-i,,urcs 2. 3). The sectionis accessible by, vehicle (in fair weathct）front the town oj'Erduqi}io.500rn east of Huanonitaiw,. A state lii-h\}aN connects the Cham-,shan County Town with Hangzhou and Shanghai（onscrvation ,ilidprotection ofthe section has been a',SL1FC,-I by the Changshan（01,1111\Government and the Zliciian9 Provincial Go、厂ernment to,-,clhci- \A, iththe Nanjin2 Institute of' Geology and Palacontology, Acadenii}iSinica

The section exposed at FlUangnitam, provides the thickest }tndImost fassififerous succession across the base of the Unelulograpitt}austrodeniatits Biozone of any that we hjvc cxaniined (Fi}-'Llie 4)The Huangnitang section compri,;es mainly the NinAuo Formationwhich at this locality spans the interval froin Bendigonian confin［卜

["pisolle、，认）／}0. Ile). 3

j60

〔少

j刁shan County Town

卜惬

）／认

口 ,因

，，日, 凡 11日 30

beds, the limestone constituents are well一、Orled, tine-rainedIinicritic intraclasts. superficial ooliths. and skeletal 1'ra-men卜set Incalcite spin- matrix. Analysis by graphic correlation based oil 11'al-inaldata fi-oin Huangnitang, and tile fi\e Supplementary sections Studiedin the JCY area indicates that the Fluammitano section has a consis-Itcntly high scdimentation rate and no detectable stratigraphic hreaks.Graptolites are common to abundant in the tipper Ningki-10 Forma-tion. Above Bed AEP 178 many graptolites are pyritized and pre-served in three dimensions, and I、一（)m AEP 179 to about AEP 199 the

rocks are essentially continuously tossilifcrOLIS. The base of the U. ouvrodentants Biozonc in the Huan-nitan-

section is selected as the base ol'Darriwilian Stage, and is defined asthe lc}el of the first appearance datu川（FAD) of the name bearer:Ulldlllograptit} austrodentatio (Figure 0). This levcl is coincidentwith the base of Bed AEP 184 ot'thc Fluangnitang section, which is221n below the top of、 the Ningkuo Formation (FiLlUres 4, 5).Graphic correlation aniong the six sections examined in the JCY areaindicates that this appearance in the Fluangnitang section corre-sponds to the oldest known first appearance of' the species (Zhang,1995). Thus, it is probable that the hase of' the U. au,m-odentatusBiozone in this section represents the time of'evolutionary origin ofIthis key species.

The base of the Darriwilian is formally defined its the base ofthe U. austrodentams Biozone, however, it is important to note thatidentification of this boundary is a distinct process 1'rom its dchni-tion. The hase of the Darriwilian should not he identified oil the

basis of the first appearance of' U. aum-odentams alone, apart fromits faunal context, but only on the basis of the species' hi-st appear-ance within a well controlled succession of other first-appearariccs,A second highly distinctive species, ArienigralmiV 7he'／'1 1. Itangensis（＝Pseudisograj)tus angel Jenkins), first occurs in bed AEP 182, 0.5 mbelow the FAD of U. austrodentatus. III the other JCY sections, the

A. zhe.ji(I1I,qe17sis FAD occupies a similar position at or just below theU. au.strodentatits FAD. Within the precision of strjtigraphic corre-lations in the region, the Arieniqj-(j1)tjjS 7/1(1jiall‘gcl，、1、 -AD is svn-iiangetisis fchronous with the base of the口 austrodentatio Biozone. Thus, the

appearance of A. 1-hefiangensis serves as important collateral evi-dence of the location of the base of the Darriwilian. For this reason.

we employ this species as the name-bearer of the lower subzone (itthe U. austrodentatus Biozone (see below).

．一

．｜｜！11！｜卜比队厂：﹂

higtirre2 Map showing location of the Huangnitang-GSSPsection relative to the nearby Changshan County Town.

otrdy to Darriwilian. The NinAuci Formation yields abundant grap-tolitcs from the shales and conodonts from interbedded limestones.

Ranges ofinain graptolite and conodont species in the Fluarignitangscclion are shown in Figure 5.

The Nin,,kLIo Formation is dominantly a well-sorted, line-}_,rained. thinly and evenly laminatcd black shale (Figure 4). The baseOf U. austrodematits Bio）zone in the FIuan2nitan2 section. as well asthe supplementary sections in the JCY area. lies within a black shaleseClUence that is entirely consistent in its sediment character andrellects apparently uniform depositional conditions throughout theYapeenian and early Darrixvilian. Tabular beds of dark gray lime-stone interleave with the shale. I hin sections reveal that in many

0.:两ngkuo FormationOY: Yenwashan Formation

I&,- Hulo Formation

Oi,g: Huangnigang Formation

活妞

Huangnitang Village

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48'

协产钦 黔Shale

Oh /X OY50,

Nodular limestone Ohg

Chert P=}:q LimestoneM" 睡〕Mudstone OR Cover 0气

50 m

日Un

；

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e

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三

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e

三

三

l

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工J‘LL七

尸卜L．卜‘

r‘卜r卜‘

翻 1 1 ． 1 」

Figure 3 6eological sketch map q/ the Huangnitang section, Changshan, China. Rocks dip moderately to the east-so u tit -east, andvoting in Mat direction.

Scptember 1997

/61

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Figure 4 A. View of the Huangnitang section (looking west), and the surrounding country side. GSSP is near star along south batik ofthe Natimen Stream. B. Close-up of section showing beds in the boundary interval. Base of the U. austrodentatus Biozone andDarriwilian Stage lies at base of bed AEP 184 (indicated勿double line), just above limestone bed AEP 183. Scale markedin decimeters.

The graptolite record

The presence of several short-ranging species and the pattern ofgraptolite species succession through the interval of the Darriwilianboundary are critical to reliable recognition of the base of the Darri-wilian as well as to its precise international chronostrati graphic cor-relation. Several of the critical species are illustrated in Figure 6.The graptolite fauna present in the 3 in interval just below the baseof the Darriwilian at Huangnitang is a typical Yapeenian assemblagethat includes Pseudisograptus manubriatus harnsi, P. manubriatuskoi, Exigraptus uniformis, and E. clavus (which is closely allied toApiograptus crudus). The composite standard constructed for theJCY region (Zhang, 1995) indicates that strata in this interval in theJCY area also contain Oncograptus magnus, Artentgraptus dumo-sus, Isograptus victortae maximodivergens, and L victoriae diver-gens (see also Mao and Chen, 1990). We refer the interval from AEP180 to 183 to the Exigraptus clavus Biozone, the base of which iscovered in this section. Shales of the mid to uppermost E. clavusBiozone (AEP 181 at Huangnitang) contain the first appearance ofCardiograptus obesus followed in bed AEP 182 by C amplus,Undulograptus sinodentatus, and Arienigraptus zhejiangensis (Fig-ure 5).

Based on the Huangnitang stratotype section, as well as othersupplementary sections in the JCY area, we recognize two subzones,the Arienigraptus zhejiangensis (lower) and Undulograptus sinicus(upper) subzones, within the U. austrodentatus Biozone (Chen andothers, 1995b; Mitchell and Maletz, 1995). The base of the A. zhe-liangensis Subzone, like that of the zone itself, is defined by the firstappearance of U. austrodentatus. The fauna of the A. zhejiangensisSubzone is very diverse and includes a rapid succession of firstappearances of species of Undulograptus, Cardiograptus, and Exi-graptus (Figure 5), including Undulograptus formosus, and U.primus. The base of the Undulograptus sinicus Subzone, is markedby the first appearance of U. sinicus. This level is very nearly coin-cident with the first appearance of Cryptograptus antennarius andParaglossograptus tentaculatus, as well. The base of the upper sub-zone defines the top of the lower subzone. The last appearance ofseveral of characteristically Yapeenian species that continue into the

lower U. austrodentatus Biozone occurs near the base of the U. sini-

cus Subzone. These species include Pseudisograptus manitbriatus,Isograplus victoriae divergens, and Oncograptus upsilon. Overlapof these species with U. sinicus provides a very precise control of thebase of the subzone.

The Undidograptus austrodentatus Biozone is overlain in theJCY region by the Acrograptus ellesae Biozone (Xiao and Chen,1990). Preliminary data (Zhang Yuan-dong, unpublished) indicatethat species of Tylograptus, Sinograptus and other graptolites char-acteristic of A. ellesae Biozone appear in the uppermost 5.2 m of theNingkuo Formation and continue into the lower part of the Hulo For-mation at the Huangnitang section (Figure 5).

Accurate identification -of the index species Undulograptusaustrodentatus and Arienigraptus zhejiangensis (see Figure 6) isessential to recognition of the base of this biozone. In the past, con-siderable controversy has surrounded the issue of whether or not theappearance of "biserial graptolites" was actually synchronousaround the globe (e.g., Skevington, 1963; Berry, 1968; Cooper andLindholm, 1990; Fortey and others, 1990; Maletz, 1992). Much ofthe confusion arose as a consequence of application of the term "his-erial" to a wide variety of species (including glossograptids) andconfusion about the identification of particular species such as Didy-mograptus bifidus and U. austrodentatus. This problem has beensolved by recent systematic work based on especially well preservedmaterial (e.g., Cooper and Fortey, 1982; Fortey and Cooper, 1986;Mitchell, 1992, 1994; Mitchell and Maletz, 1995), although contraryinterpretations continue to have advocates (e.g., Berry, 1995). Chenand others (1995a) have redescribed Undulograptus austrodentatus(Harris＆Keble) and Arienigraptus zhejiangensis Yu and Fang indetail.

The conodont record

Conadonts from limestone interbeds within the Huangnitang section(commencing with sample AEP 167) are referable to the Paroisto-this originalis Biozone (Wang and Bergstr6m, 1995). Although thefauna is of low diversity, elements are numerous and well preserved(Figure 4). They are typical of the low diversity North Atlantic

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163

Figure‘Scanning electron micrographs ofthree-dimensionally preserved, pyritized specimensof some critical graptolite species. A. Exigraptusclavus (x功．B. Undulograptus austrodentatus(,00). C Undulograptus sinicus (note that all ofthe thecae of this species bearprominent spineslocated at the point of maximum curvaturejustbelow their apertures, however, these spines arenot preserved in this internal mold; X30).D. Arienigraptus zhejiangensis ()00).

Province faunas usually obtained in off-shore settings, and containnone of the shallow water endemics found in rocks of the YangtzePlatform. Based on these faunas as well as occurrences of conodonts

and graptolites together in the Dawan Formation of the Yichangregion, Yangtze Platform (about 700 km WNN of the JCY region;see Wang and Bergstr6m, 1995), and in the Vinini Formation of cen-tral Nevada (Finney and Ethington, 1992; Mitchell, 1992), the baseof the U austrodentatus Biozone lies just above the base of theMicrozarkodina parva Biozone in the North Atlantic conodont zona-tion and within the upper part of the Histiodella altifrons Biozone inthe North American midcontinent conodont zonation.

Conodonts from bed AEP 250 in the Huangnitang section

include Eoplacognathus crassus. In terms of graptolite zones, thisbed lies within the Acrograptus ellesae Biozone. The AEP 250 con-odont fauna indicates that these strata belong to the lower, but notlowermost, part of the Amorphognathus variabilis Biozone of theNorth Atlantic conodont zonation, consistent with an M. parva Bio-zone age suggested for the base of the U. austrodentatus Biozone

JCY area, D. ancoriforme has been previously reported widely fromthe Llanvirn and younger strata in Europe and northern Africa. Thisspecies is also present in late Arenig rocks within the Sdrka Forma-tion in Bohemia together with a D. bulla Biozone chitinozoan fauna(Vavrdov6, 1993).

Clearly, additional work is required on these fossils and theirdistribution. Further research into their occurrence in the JCY and

Yangtze Platform regions is underway currently. However, theavailable data are sufficient to indicate that clutinozoans and acri-

tarchs may offer the opportunity to make an accurate correlation ofthe base of the Darriwifian with late Arenig successions of the pen-Gondwanan realm, where U. austrodentatus and its associates are

absent

Chitinozoan and acritarch records

Prospects for paleomagnetic reversalstratigraphy and radiometric agedeterminations

Correlation between the Lower Ordovician low latitude faunas of the

Pacific Province and the dominantly cool to cold water faunas ofGondwana has been a persistently difficult problem. Correlation viathe trilobite faunas in Wales represents one possibility, albeit largelyuntested at present. Recent attempts to obtain chitinozoans from theU. austrodentatus boundary interval suggest another potential solu-tion. Paris and Chen (1995, 1996) report discovery of chitinozoansfrom Huangnitang and three other sections in the JCY area. Taxapresent include species of the genera Conochitina, Cyathochitina,Rhabdochitina, Tanuchitina, Laufeldochitina, Belonechitina,Desmochitina and Sagenachitina. Specimens of the first three gen-era are the most common in their collections. Several of these gen-era are known from Gondwanan locales and Sagenochitina in partic-ular is、 a characteristic element of upper Arenig (Whitlandian) tolower Llanvirn strata in northern Gondwana (e.g., France, Portugal,Algeria, and Libya; see Paris, 1990).

Servais and others (1995), in an thorough revision of the dis-finctive acritarch Dicrodiacrodium ancoriforme, report that its firstappearance lies within the latest Yapeenian U. sinodentatus Biozonein the Dawan Formation, China. Although not yet known from the

No paleomagnetic work has yet been carried out in the Middle Ordo-vician sections of the JCY area. However, in view of the fact that the

Conodont Color Alteration Index (CAI) is only 3-4, indicating heat-ing of the rocks to a maximum of about 100'C-300*C (Wang andBergstr6m, 1995), there is clearly potential for successful establish-ment of magnetic reversal stratigraphy across the base of the Darri-wilian. The only rocks in the Ningkuo Formation in the study regionthat are potentially useful for radiometric age determinations are twothin K-bentonite beds in the Azygograptus suecicus Biozonedescribed by Huff and Bergstr6m (1995) in a section at Hengtang.No radiometric age data are yet available from these beds, but in a

companion paper to the present contribution, Huff and others (1997)report a U-Pb age of 464t2 Ma from a K-bentonite located withinthe lower Darriwilian U. austrodentatus Biozone in the Pre-

cordilleran Terrane of Argentina.

Episodes, Vol. 20, no. 3

2(54

Australasia I S．E．China NorthAmerica Baltoscandia Wales

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F}qure7 Correlation chart showing chronostratigraphic relations between the base qj． the recently ratified global Damwilian Stage ojthe Middle Ordovician Series and several well-known regional series and stages based on data and interpretations cited in MachellandChen (1995) and Mitchell and Maletz. (1995), with radiometric ages interpolated based on datafi-om Tacker et al. (1995) and Hqffet A(1997), and relative stage durations./roin Cooper (1992).

Correlation of the base of the

Darriwilian Stage

1hc distinctive fauna of the I一‘mstrodenialirs Bitmonc has been

reported f’rom Australia. New Zealand, North America. Europe,South America,（hina- Mon},,olia, and Ka/akhstan permitting confi-clcni correlation with these re,_,ions (Fil-ItHe 7). A. zheiiim,,,ensis Sub-lone faunas occur in the Marathon rc(_,ion. 'f exas, and simila：一laUnasarc also present clseMicic alon.,_, Oic ,kestern Noi-ih ArneiicanCoidillera (Mitchell and Chen Xu 1995). An U. Silli(11N SUb/one

fauna has been recorded froul Quebec and Newfoundland (Mitchelland Maletz 199}). (orrelations with f,aunas from North America

indicate that the base、）｝、the V. S111h 11A SUhzone is ncarlv coincident

vJth the base of the widely reco-ni/'edlality Bio},orie. Similar relationships are present in the ArgentinePrecordillera (Orte-a and others 1993} Albanesi and others, 1995Ed,,el Brussa and C.E. Mitchell, unpublished data).

The oldest diplograptacean known from Wales is Undulograp-tu.} cumbrensi.v from the Ber}omia ruslitoni Biozone (Figure 7). ItsFAD is certainly within the U. sinhus Subzone (Mitchell andMaletz. 1995). 1 he base of the U. em,wrodentano Biozone, therefore,should lie below the oldest occurrence ot'diplograptaccans }e( dis-co%cred in Wales, within the lower part of the Fennian Stage. Theinicrval of the lower U. austrodentatus Biozone is represented inNorth Wales (as at Nant y Gadwen) and in the Skiddaw Group of theLuke District, based on the presence there of- A. 711eiiangensts(reported as P. an,}el）川the L甲per lso}raptus caduccus }ihberuhf.sBio/.one (Fortey and others- 1990). 1 he U. sinicus Subzone is pre-sent in vounLcr rocks（们fie Lake District, \,\,here it is equivalent to山L: Didvmo,}raptus hirmido Bio/.one as used by Jackson‘1962) and

in North Wales neat（acinail-on and Bangor (Forte} and othersII99（））．

In Sweden, ,A, Jie.fi(mgensi.} Occurs in tile Illiddle part、）！tile 1),hirmido Bio/.onc looctlicr with Pscildisograpilts monlibliallo jamoand 、or. U. sinic ms and U. C11/H/))VIISi.S Occurin the LIPI)el' 1). hirundo hio/one (Mitchell and Malet／一1995). Thebase of the Hanvirn most likcl} coincides with a level slight]" abmethe base of the A．一夕（Ilesm, Bio/one of SOLItheast China. }khich i,,

closely comparable to the Umlidogi,apno intei几、1111's Blozouc（Dal ri -wilian 2) in Australia and New Zealand. Conodow C\'iL[C.IICC Il-0111

the JCY area indicalcs that the ba.,,c of1he A.'少 ellcsac Bio/olle col

responds to：、le}cl near tile base of' the Amorphogiatilms variabli}sBiozone, which is in aorcement \AiIh the conodont aLc ofilic，一（)cks inthe Areni--Hanvirn boundary interval in the Baltic region

As mentioned preViOLISII,, COIIO(]Oflt CVi(]CIICC fr(）川tile JCYarea indicates that the base of创．alls/ m dento Ill's Biozone is in strata

that prohabk correspond to either the Upper part of tile Bakoscan-clian八iroi.slodivs ori-bialisBiozone or lokker part of the盯；‘ro,-.arkodina parva Biozone.〔ornparison with the Yanum/.c Plat-form conodoill Succession favors the latter interpretafioll (13cl-str6rual记Wan-. 1995). The conodont /onal assignment of rocks fromboth Sweden and Nevada that. hased oil their graplolite faLluaS, arecorrelatives of the lower 11. austroden tal ti.% Biozone also fall within

the解．parva Biozonc. Strata ofthe A. ellestie Biozone川the JCYarea (e.g., AEP 250) as well as rocks from just ahove the U. misit'o-dentottis Bio/one川Quebec an(] Sweden belong to tile A. varioblisBiozone（Bergstr6in and Wang,, 1995: Mitchell and Maletz, 1995).The A. variablis Biozone fauna at Huan-nitarn, includes the distinc-tive (albeit newly recognized) conodont,万oplwognalhu.s（R I.S.S USChen and Zhang, which is also present in this saine interval in theYangtze platform region, as well as at localities in Sweden, Estonia,and Poland (see Zhang, 1997). E. iTO.SSUS appeal、to he：、short ran-

September 1997

265

ing species

variabilis

restricted to the lower. but not lowei川（)st

Bio/,one (i.e., the upper part of the A

part of the A. variabiliv-

M1C1-O,-0I-k0diI10 flObe/h/111 SUbZO11C). Miscd on these conodow11raptolite relafions we aic confident that the a.,2e ot the base of the I/auvrodcntams Biozone is consistent

Atlantic prov i nccs.

across both fire Pacific and

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Webbv. B D. 1994. To"ards c}tablishinL -flohally applicable boundaric, tol the Ordo\ ician System: Major intermitiOnal dIN i}ion} and crituria 1ol (Ict initions: Ordmician NcA}- \. I I．pp. 0-- 15

Wehhv} B 1). 1997. Some comments oil 01-ClO\ ICKIll SLlhdi} isions and Ll'}LLC

Ordo\ ician Ncx\ }.、14. (ill prc}s)Wan- Zhi-hao, and Bergstr6m. S M} 199};一CastIcillainiall (1-ate、Lr}hajuan)

to Darimilian (Zhepanpan) collodolu taullas. ill（bell XLL .11111 Bergstrom, S M} eds．The base of the au,fro(h,watits Zone as }i le}、｝tol

tpi.wdc},＼；（，／-l 0，）＿矛

166

global subdivision of the Ordovician System: Palaeoworld, no. 5, pp 86-91.

Williams, A, Strachan, 1, Bassett, D A, Dean, W T, Ingham, J K, Wright, A D, and Whittington, H B, 1972, A correlation of the Ordovician rocks in

the British Isles: Geological Society of London Special Report no. 3, 74p.Xiao Chengxie and Chen Hongye, 1990, Some graptolite faunas of the Lower

and Middle Ordovician from Gucheng area, Yushan: Geology of Jiangxi v. 4, no么244 p. (in Chinese with English summary).

Zhang, Jian-hua, 1997, The Lower Ordovician conodont Eoplacognathus

M. Bergstrbm is professor of Geological Sciences at Ohio State Uni-versity, USA where he has been teaching -since 1968 after havingreceived his graduate training at Lund- University, Sweden. He hasbeen a voting member of the Subcommission on Ordovician }Stratig-raphy since 1974 and served as subcommission secretary in 1976-1982. His principal research interest is global Ordovician geology,especially aspects of the evolution of the Iapetus, the significance ofLower Paleozoic K-bentonites, and conodonts and graptolite mor-

crassus Chen and Zhang, 1993; GFF, v. 119, pp. 61-65.Zhang Yuan-dong, 1995, Graptolite composite standard sequence (GCSS), in

phology and biostratigraphy.

Chen Xu, and Bergstr6m, S M, eds., The base of the austrodentatus Zoneas a level for global subdivision of the Ordovician System: Palaeoworld,

Zhang Yuan-dong, Born in 1967, Graduated户-om the ChangcunCollege of Geology in 1987, Received Ph.D. from the Nanjing Insti-

no.5,pp.67-74.

Charles E. Mitchell is an associate

professor and Assistant Chair in theDepartment of Geology at the StateUniversity of New York at Buffalo(cem @ acsit. buffalo. edu). Researchinterests include graptolite evolutionand biostratigraphy, as well as appli-cation of stratigraphy to understand-ing basin history and tectonics. He isa Titular member of the ICS Subcom-mission on Ordovician Stratigrap勺(since 1996), and Secretary andnewsletter editor for the GraptoliteWorking Group of the InternationalPalaeonto logical Association．

Chen Xu, Born in 1936, Graduatedfrom the Beijing College of Geologyin 1959, employed by the NanjingInstitute of Geology and Palaeontol-ogy, Academia Sinica from 1959 topresent. Research professor from1989, Vice-Chairman of the Ordovi-cian Subcommission from 1991.Research field: Ordovician and Sil-urian straligrap勺and graptolites.

tute of Geology and Palaeontology, Academia Sinica in 1993 andemployed勿the Institute. Associate professor of the Institutefrom1994. Working on Ordovician and Silurian biostratigrap勿andgraptolites.

Wang Zhi-hao, Born in 1941, Graduateelfrom the Nanjing Univer-sity in 1965, employed勿the Nanjing Institute of Geology and Pal-aeontology, Academia Smica from 1965 to present. Research pro-

fessor from 1996. Working on Ordovician stratigraphy and cono-donts.

Barry D Webby is aversity Centre for senior research geologist at the Macquarie Uni- Ecostratigraphy and Palaeobiology, Australia,

with active interests in a number of aspects of Ordovician geologyand the palaeontology of calcified sponges. He was Chairman of theICS Subcommission on Ordovician Stratigraphyfrom 1989 to 1996,

and is now one of the leaders of IGCP project 410focusing on thegreat Ordovician biodiversification event.

Stanley C. Finney is Professor and Chair in the Department of Geo-logical Sciences at Cal扣rma State University, Long Beach. Ilbresearch interests include graptolite systematics and biostratig-raphy, the Late Ordovician extinction, and stratigraphy andpaleon-tology of Ordovician rocks of the Great Basin. Finney is Chair o}the ICS Subcommission on Ordovician Stratigraphy.

September 199