Middle Ordovician) in the Yangtze Platform an D the JCY Area in Zhejiang, China

Bollettino della Società Paleontologica Italiana Modena, Novembre 1999

Conodont-graptolite biostratigraphic relations across the base of the Darriwilian Stage (Middle Ordovician) in the Yangtze Platform an d the JCY area in Zhejiang, China

Zhi-hao WANG Stig M. BERGSTROM Institute of Geology and Palaeontology Department of Geologica! Sciences Academia Sinica, Nanjing The Ohio State University

KEY WORDS- Conodonts, Graptolites, Biostratigraphy, Middle Ordovician, Darriwilian, China.

ABSTRACT- Although the recently ratifìed stratotype of the base of the global Darriwilian Stage in China has a magnificent graptolite succession, conodonts in the boundary interval are not very diagnostic biostratigraphically and the precise graptolitelconodont zone relatiom have no t been well established. In an effort to clarifj these relations, conodont samples were collected from coeva! strata in three Ytzngtze Platjòrm sections with occurrences ofgraptolites of the lowermost Darriwilian U. austrodentatus graptolite Zone. Biostratigraphically highly diagnostic Ba/tic conodonts were found that make it possble to apply the Ba/tic conodont zone scheme to the study sections. Although the restricted and spotty occurrences ofgraptolites in these sections make it difficult to recognize the exact leve! ofthe base of the Darriwilian, the new data are consistent with the idea that this leve! may be near the base ofthe Baltoniodus norrlandicus- Microzarkodina parva Zone.

RIASSUNTO- [Rapporti biostratigrafici tra conodonti e graptoliti a cavallo della base del Darriwiliano (Ordoviciano Medio) nella Piattaforma dello Yangtze e nell'area JCY dello Zhejiang, Cina] -Lo stratotipo della base del Darriwiliano è stato recentemente istituito ufficialmente in Cina. Esso presenta una ottima successione di graptoliti, mentre i conodonti non sono particolarmente significativi da un punto di vista biostratigrafico nell'intervallo attorno a/limite: ne consegue che le relazioni tra le zone a graptoliti e quelle a conodonti non sono state stabilite con precisione. Per cercare di risolvere questo problema sono stati campionati per conodonti strati coevi in tre sezioni della piattaforma dello Yengtze, dove sono presenti i graptoliti della prima biozona del Darriwiliano, la Zona a U. austrodentatus. Sono stati trovati conodonti ad affinità baltica altamente diagnostici e ciò ha reso possibile l'applicazione a queste sezioni dello schema di biozone a conodonti del Baltico. Anche se in queste sezioni i graptoliti non sono abbondanti e i loro rittrovamenti non sono regolari, tanto da rendere difficoltoso il riconoscimento della base del Darriwiliano, appare verosimile che esso sia molto prossimo alla base della biozona a conodonti a Baltoniodus norrlandicus - Microzarkodina parva.

INTRODUCTION Darriwilian Stage. lt is located at Huangnitang near Changshan in the Zhejiang Province of China. Por Up to recently, there has been little agreement about information about this section, and supplementary how to subdivide the Ordovician System imo series sections in the Jiangshan-Changshan-Yushan QCY) and stages. Local schemes have been in generai use in region, see Chen & Bergstrom (1995), Mitchell & the major outcrop regions and because of the Maletz (1995), Mitchell eta!. (1997), and Chen eta!. conspicuous biogeographic differentiation of (1997). Ordovician faunas, correlation between these provincia! The Huangnitang section has a marvellous schemes have remained diffìcult, imprecise, and in graptolite succession (Zhang, 1995; Mitchell & Chen, many cases controversia!. Recognizing the need for a 1995), clearly the bes t known in that interval anywhere precise, stable and globally applicable series and stage in the world, and limestone interbeds in the dominantly classifìcation (Webby, 1998), The lnternational shaly succession contain relatively numerous conodonts Subcommission on Ordovician Stratigraphy has during (Wang & Bergstrom, 1995; Bergstrom &Wang, 1995), the last decade established severa! working teams including several biostratigraphically useful species. charged with the task of identifying fossil-defìned However, the species diversity is somewhat limited, stratigraphic levels with a high global correlation especially compared with that reported (N i & Li, 1987) potential that may be useful as series and stage from coeval shallower-water strata in the Yangtze boundaries. A second goal of this effort has been to Platform about 700 km to the northwest and north fìnd sections in which these boundaries are particulary (for the geographic relations between the J CY area an d well-defìned that may be suitable to serve as global the centrai Yangtze Platform, see Chen, 1995, text-fìg. boundary stratotypes (GSSP). The first such 2). In the absence of conodont index species, the zonal Ordovician boundary stratotype approved by the assignment of the lower Darriwilian Stage in the Ordovician Subcommission and recently ratifìed by Huangnitang stratotype has remained uncertain (Wang The lnternational Commission on Stratigraphy and & Bergstrèim, 1995, text-fìg. 35) although based on The lnternational Union of Geologica! Sciences is what was known about the Yangtze conodont defìning the base of the lower Middle Ordovician succession (Ni & Li, 1987), the base of the Darriwilian 188 Z.H. WANG, S.M. BERGSTROM was correlateci with a leve! near the base of the China (Zeng et al., 1987). The numerous studies Microzarkodina parva Zone (Wang & Bergstri::im, carried out on the geology and paleontology of these 1995, text-fìg. 36). Although a substantial amount of rocks have made this area a classica! one in not only conodont work has been carried out in Lower and Chinese but international Lower Paleozoic geology. The Middle Ordovician sections in the Yangtze Platform lower Darriwilian and immeditely subjacent strata are (see, e.g., An, 1981, 1987; An et al., 1981, 1985; An represented in the Dawan Formation (Bergstri::im & & Ding, 1982; Chen, Chen & Zhang, 1983; Chen & Wang, 1995, text-fìg. l) that consists of green shales, Zhang, 1984, 1989; Ni, 1981; Ni & Li, 1987; Wang, nodular limestones, and reddish biocalcarenites. For a 1993; Zhang, 1998), its lower Darriwilian conodont detailed lithologic description of the Dawan Formation succession has remained incompletely known. at Chenjiahe (Daping) 4 km north ofHuanghuachang, Additional study was clearly warranted, especially for where it is about 37m thick, see Lindstri::im, Chen & the clarification of the conodont/graptolite Zhang (1991). The Dawan Formation contains nu- biostratigraphic relations across the base of the merous fossils. Of special interest for the present study Darriwilian Stage. This is of criticai importance for the is the occurrence of graptolites of the basai Darriwilian correlation of this global stage boundary into the non- Undulograptus austrodentatus Zone and, probably, the graptolitic shelly facies that characterizes most of the subjacent Exigraptus clavus Zone in the upper member cratonic successions. The goal of the present study was of the formation (Chen, Zhang & Mitchell, 199 5). to try top in down the exact position of the base of the For the purpose of the present study, 63 strati- Darriwilian Stage in the conodont zone succession graphically precisely located conodont samples were using new and carefully stratigraphically located collected at three sections through the middle and conodont collections from a few Yangtze Platform upper members of the Dawan Formation and the sections that also contain zonal graptolites. lowermost part of the overlying Kuniutan (Guniutan) Formation. These are the Huanghuachang and Chenjiahe (Daping) sections located about 18 km YANGTZE PLATFORM CONODONTS north-northeast of Yichang, and the Jianyangping section which is situated about 6 km south ofXinshan SECTIONS ANO COLLECTIONS (Text-fìg. l). Ali samples were processed using standard conodont separation methods at the lnstitute of Ordovician strata are excellendy exposed in Geology and Palaeontology, Academia Sinica, in numerous road cuts and other sections in the Yangtze Nanjing. Platform region in Hubei Province, south-eastern Conodonts proved to be moderately common in

o 25

N Fengjie Wushan t Yuanan

- .. "'.·) Hubei Province\ ·. /'

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\ Wuhan City ·. Yidu

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_/ l o 50 / L___j km

Text-fig. l - Sketch-map showing the location of study section northwest and nonh-northeasr ofYichang, Hubei Province. l) Huanghuachang; 2) Chenjiahe (Daping); 3) Jianyangping. For location of the JCY area in relarion ro rhe Yangtze Plarform, see Chen (1995, Text-fig. 2). ORDOVICIAN CONODONT/GRAPTOLITE RELATIONS, YAN GTZE AREA 189 these samples but the species ciiversity is not very great PROV1NCIAL CHARACTER OF THE CONODONT FAUNA in parts of the sruciy interval. The state of preservation of the conociont elements is, by anci large, gooci but Previous sruciies of Micicile Orciovician conocionts many specimens are broken. The fact that reasonably from the Yangtze Platform have shown the presence of complete, anci positively icientifiable, platform elements many species best known from what was referreci to as are rare makes it sometimes ciifficult to establish the the Baltic Province by Bergstrom (1990). Hence, in a precise level of the base of both the Lenodus anti- recent study of the conocionts of the Kuniutan variabilis Zone anci the Lenodus variabilis Zone. The Formation, Zhang (1998) founci that more than 90% conocionts have been mocierately heateci as shown by of the species were previously known from Balroscanciia. the fact that they exhibit a CAI (Color Alterati o n lnciex; Hence, it is not surprising that our Dawan anci Epstein, Epstein & Harris, 1977) of2.5-3. This agrees lowermost Kunuitan collections are completely well with the CAI values of2-3 of conocionts from the dominateci by such taxa. This striking similarity, which Kunuitan Formation in the same region reporteci by inclucies the occurrence of severa! taxa useci as zone Zhang (1998). Representative conocionts are illustrateci inciices in Baltoscanciia, makes it possible to recognize in Plates l anci 2. Most of these taxa are well-known the Baltoscanciic conociont zones in our sections. from ciescriptions in the literature, anci there is no neeci to re-ciescribe them herein. All specimens are ciepositeci CONODONT SPECIES RANGES AND CO NODONT ZONES in the collections of the Nanjing Institute of Geology anci Palaeontology, Acaciemia Sinica, Nanjing. AFA203, The vertical distribution of conociont species in the etc. in the plate legencis refers to sample number, anci sruciy sections is shown in Text-figs. 3-5 in which we etc. indicates reposirory number of illustrateci also classifY the succession in terms of conociont zones. specrmens. The Lower-Micicile Orciovician Balroscanciic

Uifgren Wang & Bergstrom Lindstrom & Stouge Albanesi et al. This paper 1978,1993 1995 1971 1997 1998 (Yangtze Gorges) 1994,1995 (Sweden) Q) (JCY area) (Argentina) Cl (Europe) (Sweden) <{ Conodont Graptolite Conodont Graptolite Conodont Conodont Conodont Conodont zonation zonation zonation zonation zonation zonation zonation zonation

Eoplacognathus Amolp/toQnalhus N. fasciculatus crassus v8liabllis t------L. variabilis

c A. variabilis A. variabilis L. sp.A Lenodus A. e/lesse ·;: variabilis L- --?--- ? oco 1------L. Lenodus antivariabilis antivariabilis u u. Microzarkodina austrodentatus austrodentatus M. parva Baltoniodus paNa norrfandicus B. M.parva - norrfandicus Microzarkodina 1------c PBN8 E. clavus co M. parva ·c: Paroistodus P. originalis co Paroistodus originalis ..c. originalis l. caduceus P. (/) origina/is B. navis :::s imitatus >- (p art)

Text-fìg. 2 - Conodont zones used in the present paper compared with those employed in some other regions. 190 Z.H. WANG, S.M. BERGSTROM conodont zone scheme introduced by Lindstri:im The Lenodus antivariabilis Zone (1971) has subsequently undergone only relatively minor modifìcation and refìnement (Text-fìg. 2). Following Bagnoli & Stouge (1997), the base of However, because some zones have been given a this zone is defìned as the level of fìrst appearance of L. somewhat different scope by different authors, it is antivariabilis. The scope of this zone corresponds to appropriate to briefly comment on the zone the upper part of the Microzarkodina parva Zone of defìnitions used in the present paper. Li:ifgren (1994, 1995). Apart from the zone index, important species include Baltoniodus norrlandicus, The Eoplacognathus crassus Zone Protopanderodus calceatus, Scolopodus rex, Tripodus? brevibasis, Drepanoistodus basiovalis, and Microzar- This zone was fìrst proposed by Zhang (1996) and kodina parva (cf. Zhang, 1998). has subsequently been used by Bergstri:im & Wang (1998) and Zhang (1998). lts base is taken to be the The Baltoniodus norrlandicus - Microzarkodina parva level of the fìrst appearance of the zone index, E. crassus. Zone Other important species include Lenodus variabilis, Baltoniodus norrlandicus, B. medius, B. prevariabilis, The base of this zone is taken to be the leve! of the Protopanderodus calceatus, Paroistodus liorridus, and fìrst occurrence of B. norrlandicus, which is the same Periodon aculeatus (cf. Zhang, 1998). As indicated in horizon as the base of the B. norrlandicus interval Zone Text-fìg. 2, this zone represents an interval that was of Bagnoli & Stouge (1997). The scope of this zone formerly included in the Lenodus (or Amorphognathus) corresponds to the lower portio n of the Microzarkodina variabilis Zone. parva Zone of Li:ifgren (1994, 1995) and to the B. norrlandicus interval Zone ofBagnoli & Stouge (1997). The Lenodus variabilis Zone In the Yangtze Platform sections, the fauna of this zone is relatively diverse and includes, among others, First introduced by Lindstri:im (1971), this zone Paroistodus originalis, Oistodus lanceolatus, Periodon designation is now used in a more restricted sense and jlabellum, Tripodus? brevibasis, Protopanderodus calceatus, includes only the lower/ortion ofLindstri:im's (1971) Paroistodus parallelus, an d Microzarkodina flabellum. uni t. The base is defìne as the leve! of fìst appearance of Lenodus variabilis. Other important species include The Yangtze Platform sequence immediately Baltoniodus norrlandicus, Protopanderodus cooperi, P. beneath this zone is referred to the Paroistodus originalis calceatus, Scolopodus rex, Periodon aculeatus, Zone. The upper part of this zone is correlateci with Drepanoistodus basiovalis, an d Erraticodon balticus (cf. the upper part of the Paroistodus originalis Zone of Zhang, 1998). Li:ifgren (1994, 1995) and the Microzarkodina parva interval Zone of Bagnoli & Stouge (1997).

EXPLANATION OF PLATE l

Figs. l, 2 - Cornuodus longibasis (Lindstrom). Latera! view, AFA 203/130367, x 57, x 50, respectively. Figs. 3, 4 - Microzarkodinaflabellum (Lindstrom). Latera! view ofS elements, AFA215/130368, AFA208/130369, x 50, x 60, respectively. Figs. 5-11 , 14 - Baltoniodus norrlandicus (Lofgren). 5) Latera! view of Sb element, AFA189/130370, x 80. 6) Latera! view ofPb element, AFA210/130371, x 50. 7) Postero-lateral view of Sa element, AFA213/130372, x 60. 8)Posterior view of Sa element, AFA213/130373, x 80. 9) Postero-lateral view ofSd element, AFA189/130374, x 100. lO) Latera! view of Se element, AFA213/130375, x 80. 11) Latera! view oEPb element, AFA213/130376, x 60. 14) Latera! view ofPa element, AFA189/130379, x 80. Fig. 15 - Tripodus? brevibasis (Sergeeva). Latera! view, AFA203/130380, x 80. Figs. 12, 13, 16-19- Baltoniodus navis (Lindstrom). 12) Latera! view ofPa element, AFA4/130377, x 80. 13) Postero-lateral view ofSc element, AFAl/130378, x 100. 16-19) Latera! views ofP elements, AFA210/130381, AFA204/130382, AFAl/130383, AFA3/130384, x 80, x 80, x 60, x 80, respectively. Figs. 20-22 - Oistodus lanceolatus Pander. 20) Latera! view of P element, AFA201/130385, x 50. 21) Posterior view of Sa element, AFA200/130386, x 60. 22) Latera! view ofM element, AFA200/130387, x 50. Fig. 23 - Oistodus meseaus An. Latera! view of P element, AFA2011130388, x 67. Z.H. WANG, S.MBERGSTROM, ORDOVICIAN CONODONT/GRAPTOLITE RELATIONS, YANGTZE AREA P!. l 192 Z.H. WANG, S.M BERGSTROM

CONODONT ZONES IN THE STUDY SECTIONS Huanghuachang, the appearance of this graptolite fauna is taken to be at about 7 m below the top of The Huanghuachang section the Dawan Formation, hence nearly the same level as the base of the Baltoniodus norrlandicus- Specimens of Baltoniodus norrlandicus appear in Microzarkodina parva Zone. Sample 11 (Text-fig. 3), which is located about 7 m below the top of the Dawan Formation and this level The Chenjiahe (Daping) section is taken as the base of the Baltoniodus norrlandicus- Microzarkodina parva Zone. This zone is overlain by a The lowermost part of the invesrigared succession thin development of the Lenodus antivariabilis Zone is referred to the Baltoniodus norrlandicus - that extends imo the lowermost meter of the Kuniutan Microzarkodina parva Zone that probably extends Formation. Unfortunately, the conodont fauna! below the sampled interval.The interval with U. diversity is low in the shaly interval with the austrodentatus, the base of which is about 7 m below Undulograptus austrodentatus Zone graptolite fauna, an d the top of the Dawan Formation according to a substantial portion of that interval is not available Lindstri:im, Chen & Zhang (1991 ), is virtually barren for study any more, being covered by a building. of platform index conodonts. If a fragmentary and However, based on a comparison with the section at specifically indeterminate plarform specimen found at Chenjiahe (Daping) described by Lindstri:im, Chen & about 4.7 m below the top of the Dawan Formation Zhang (1991), which is located only 4 km north of represents Lenodus antivariabilis, this would indicate

Co lumnar Conodont section Ra ng es of con odont speci es zones 20 E Eopfacognathus I.J... CfBSSUS c 19 ro ::; ·c: Lenodus :l 18 variabilis

L Q) c antivariabilis o N Cl) .2 c: Q) "O E _g N Cl) :::> <1l c :::i o :o:; eu oE u.. c eu .!!) "' eu "' o "' E o" "'c:: (.) "' ,gc:: l

Text-fìg. 3-Vertical ranges of conodont species and conodont zones in the Huanghuachang section. The interval of che principal occurrence of che Undulograptus austrodentus Zone graptolites is indicated by an arrow. The leve! of che base of this graptolite zone is currently poorly constrained bue appears to be dose to the base of the Baltoniodus norrlandicus- Microzarkodina parva Zone. ORDOVICIAN CONODONT/GRAPTOLITE RELATIONS, YANGTZE AREA 193

Columnar Ranges of conodont species C'""""' the L. antivariabilis Zone (Text-fìg. 4). Based on section specimens identifìed as Lenodus variabilis, the base of 'IF 92 the Lenodus variabilis Zone is located in the lowermost E "' LL >< I !:1 part of the Kunuitan Formation. The thickness of the l!? .o c 191 I "' latter zone appears to be greater than in the ro ·"'c: ""' Huanghuachang section a few km away. The covered c t::S!.O"' _j ::J o stratigraphic position of this zone in relation to the j t: 190 "' l!? t: Dawan/Kuniutan Formation boundary is slightly 169 = Q • O.o different than at the nearby Huanghuachang section, t-- 166 ci - "'t: suggesting that this formation boundary is diachronous. 167 . .e This is in agreement with Zhang (1998) who showed "' "' c;; Q) covered CI) c i t: that the top of the Dawan Formation is diachronous o "' .2 N 166 .o regionally. Cl) "' ·c: .2 165 "· "' c 164 c: " o "' .o t: The jianyangping section ai 163 "' 't:! "' "' E 162 t: 15" '- e c: The conodont succession in this section (Text-fig. o t5 161 "' ._J LL ;:) "' "' 5), which is located about 50 km northwest of Cll 160 c c: 179 'E"' Q) Huanghuachang, is similar to that of the ro ._J :::i Huanghuachang section in that the Baltoniodus 176 • t-? - oro norrlandicus - Microzarkodina parva Zone in the upper 177 l!? • .Q "' member of the Dawan Formation is about 6 m thick, 76 .!;; 13lt .g> "· c:"' and in that the Lenodus antivariabilis Zone, which is 175 o "' o c: about four times as thick as at Huanghuachang, 174 ""' 1>= 173 straddles the base of the Kuniutan Formation. Q) e • l$ m Baltoniodus norrlandicus fìrst appears in Sample 205 172 "'c: "' '"Si from about 9 m below the top of the Dawan Formation, 171 • Cll. 11 and this is provisionally taken as the base of the

Texr-fìg. 4 - Verrical disrriburion of conodom species and conodom zones in rhe Chenjiahe (Daping) secrion. The imerval of rhe Columnar principal occurrence of Undulograptus sect ion Ranges of conodont species austrodentus Zone graprolires is indicared by an arrow. "' R "' i ••• "'E"' "'Q. "' "' o .tlJ " " .!:1 c: "' "' "' c: :8" • .8 ".2"' c: QJ c c: "' c: "'15 .!!) o o '8" · C: N g " -'2"'"' .

Baltoniodus norrlandicus- Microzarkodina parva Zone. austrodentatus Zone, presumably the Undulograptus Specimens of Lenodus antivariabilis were found in sinicus Subzone of Chen et al. (1995). This graptolite Sample 214 from about 2 m below the top of the fauna occurs in different sections at somewhat different Dawan Formation and this level is taken as the base of levels in the uppermost few m of the formation. the Lenodus antivariabilis Zone that extends about 2 Another graptolite species association, which includes m into the overlying Kuniutan Formation. This is in Undulograptus sinodentatus, Exigraptus clavus, dose agreement with the vertical range of the latter Tetragraptus (T) bigsbyi, and T (T) reclinatus, is present zone in the Fenxiang section (Zhang, 1998, Text-fìg. in stratigraphically lower strata in the upper member 14), which is located about 7 km north-east of the of the Dawan Formation (Wang et al., 1987; Chen, Huanghuachang section. Zhang & Mitchell, 1995; Lindstrom, Chen & Zhang, 1991). lts zonal assignment is somewhat uncertain but In Text-fìg. 6 we summarize our conodont bio- it represents either the lowermost part of the stratigraphic classifìcation of the Yangtze Platform Unndulograptus austrodentatus Zone or, more likely, the sections and compare it with some previous Exigraptus clavus Zone. Chen, Zhang & Mitchell classifìcations. (1995, text-fig. 22) referred this fauna to the Undulograptus sinodentatus Zone and correlated this zone with the E. clavus Zone of the JCY area. The CONODONT- GRAPTOLITE ZONE RELATIONS stratigraphic occurrences of this fauna in relation to the top of the Dawan Formation vary a great deal; hence THE YANGTZE PLATFORM Lindstrom, Chen & Zhang (1991) recorded it from about 7 m below the top of the formation at Chenjiahe The occurrence of biostratigraphically diagnostic (Daping) whereas Wang et al. (1987) and Chen, graptolites is restricted to narrow, rather widely spaced, Zhang & Mitchell (1995) reported it from 27m below stratigraphic intervals in the upper member of the the same level at Huanghuachang. Doubt!ess, the Dawan Formation which makes the establishment of sporadic occurrences of these graptolite faunas were the precise level of the base of the Undulograptus controlled by local environmental conditions. The base austrodentatus Zone, that is, the base of the Darriwilian of the Undulograptus austrodentatus Zone most likely Stage, a challenging, if not impossible, task. A low- corresponds to a level between the intervals of these diversity graptolite fauna, which includes two graptolite faunas, and it is herein somewhat Undulograptus austrodentatus, has long been known to arbitrarily taken to be at 7.5 m below the top of the occur in the upper member of Dawan Formati o n (see, Dawan Formation at Huanghuachang (Text-fìg. 3) e.g. Mu et al., 1979). According to Chen, Zhang & based on comparison with the nearby Chenjiahe Mitchell (1995; oral communication, 1998) this fauna (Daping) section described by Lindstrom, Chen & represents a high level in the Undulograptus Zhang O991). If this is approximately correct, the base

EXPLANATION OF PLATE 2

Fig. l - Paroistodus parallelus (Pander). Latera! view of drepanodonciform element, AFA201 / 130389, x 80. Fig. 2 - Lenodus variabilis (Sergeeva). Upper view ofP element, AFA189/130390, x 60. Figs. 3, 5 - Protopanderodus calceatus Bagnoli & Srouge. Lacerai views, AFA2031130391 , AFA203/130392, x 80, x 50, respectively. Fig. 4 - Protopanderodus rectus (Lindsrrom). Latera! view, AFA189/130393, x 100. Figs. 6, 7 - Paroistodus originalis (Sergeeva). Latera! views of drepanodontiform elements, AEP210/130394, AEP210/130395, x 120 (From che Ningkuo Formation, Huangnitang section, JCY area; for sample location, see Wang & Bergstrom, 1995, text- fìg. 35.) Figs. 8, 9 - Lenodus antivariabilis (An). 8) Upper view of P element, AFA178/130396, x 80. 9) Lacerai view ofS element, AFA213/130397, x 80. Figs. 10-13 - Microzarkodina parva Lindstrom. l O) Posteri or view of Sa element, AFA205/ 130398, x l 00. 11-13) Latera! views of P elements, AFA205/130399-130401, x 120, x 120, x100, respectively. Fig. 14 - Eoplacognathus crassus Chen & Zhang. Upper view of P element, AEP250/130402, x 120 (From che Hulo Formation, Huangnitang section, JCY area; for sample location, see Wang & Bergstrom, 1995, Text-fìg. 35). Figs. 15-19 - Periodon jlabellum (Lindstrom). 15, 18) Latera! views of S elements, AEP151A1/130403, AEP156N130405, both x 100 (From che Chenjiawu and Huangnitang sections, for sample location, see Zhang & Winsron, 1995, text-fìgs. 16, 13). 17) Lacerai view ofM element, AEP156A1/130407, x 120 (sameJCYarea section as Figs. 15 and 18). 16, 19) Latera! views of Se and P elements, AFA213/130404 and AFA200/406, x 50 and x 80, respectively. Fig. 20 - Periodon aculeatus Hadding. Latera! view ofM element, AEP4-3/130408, x 80 (From che JCY area, for sample location, see Zhang & Winston, 1995, text-fìg. 11). Z.H WANG, S.M.BERGSTRGM, ORDOVICIAN CONODONT/GRAPTOLITE RELATIONS, YANGTZE AREA P!. 2 196 Z. H. WANG, S.M BERGSTROM

Wang & Bergstrom, 1995 An et al., 1981 An, 1987 & Wang, 1998 Zhang, 1998 This paper

oc: +=l co E. crassus Y. crassus E. crassus e_ Amorphognathus variabilis O t: A. U...co variabilis c:: a. - co ...... - A. antivariabilis :l A. antivariabilis L variabilis L. variabilis ·c:- A. variabilis :l ------L antivariabilis L antivariabilis c: .Q Baltoniodus Microzarkodina unnamed zone 1U aff.navis parva B. norrlandicus Eo :e-co -M. parva U...a. c: ...... Paroistodus co P. originalis P. originalis P. originalis Cl originalis

Text-fìg. 6 - Comparison between some conodont zone classifìcations of the upper Dawan Formati o n and lower Kuniutan Formation in the Yichang region of the Yangtze Platform. The base of the Darriwilian appears to correspond with a leve! in the lowermost Baltoniodus norrlandicus - Microzarkodina parva Zone.

of the Undulograptus austrodentatus Zone at Bergstrom (1995) suggested that the base ofthe latter Huanghuachang would be near the base of the graptolite zone is very dose to the base of the Baltoniodus norrlandicus-Microzarkodina parva Zone. Microzarkodina parva (now Baltoniodus norrlandicus- The corresponding leve! in the Chenjiahe (Daping) Microzarkodina parva) Zone. Although we have no section (Text-fig. 4) would be about l m above the additional data from the JCY area, the new information base of the sampled section, and most of the portion from the Yangtze Platform gained during the course of of the Dawan Formation having Undulograptus the present study appears to be consistent with this austrodentatus would represent the Lenodus antivariabilis correlation. Zone. In the Jianyangping section (Text-fig. 5) the base of the Undulograptus austrodentatus Zone, an d the base of the Darriwilian Stage, is tentatively taken to be at CONCLUDING REMARKS about 6-7 m below the top of the Dawan Formation and near the base of the Baltoniodus norrlandicus- lt is obviously important that global stage and series Microzarkodina parva Zone. boundaries are defined in such a way that they can be identified with confidence in a variety of facies using more than one fossi! group. The base of the Darriwilian COMPARISON WITH THE JCY AREA Stage is fundamentally defined on graptolites but compared to other fossil groups, conodonts appear to The ratified global stratotype of the base of the offer the best prospects to serve as secondary index Darriwilian Stage is in the Huangnitang section in the fossils to locate this leve!, especially in the dominantly ]CY area where 'the golden spike'is at the the base of calcareous successions on many continental platforms Bed AEP 184 of the Ningkuo Formation (Mitchell et in which zonal graptolites are rare or absent. The present al., 1997, Text-fig. 5). This leve! does not coincide with study supports the idea that this leve! is in the any marked change in the conodont fauna, which is of lowermost part of the Baltoniodus norrlandicus - low diversity in the boundary interval (Wang & Microzarkodina parva Zone of the Atlantic conodont Bergstrom, 1995, Text-fìg. 35). The absence of zone scheme. Furthermore, Bergstrom & Wang (1995) biostratigraphically diagnostic taxa makes it difficult showed that this leve! is likely to be dose to the base of to classity the Undulograptus austrodentatus Zone in the Histiodella sinuosa Zone of the North American terms of conodont zones in the JCY area but based on Midcontinent conodont zone scheme (Sweet, 1984). the admittedly incomplete data at hand, Wang & Hence, it is possible to recognize the approximate ORDOVJCIAN CONODONT/GRAPTOLITE RELATIONS, YANGTZE AREA 197 position of this important horizon using conodonts CHEN MIN-]UAN & ZHANGJ IAN-H UA, 1989, Ordovician conodonts in several key regions in both Europe, Asia, and North from the Shitai Regio n, Anhui: Acta Micropaleontologica Sinica, 6: 213-228. America. Furthermore, recent work on the conodonts CHEN Xu, 1995, Regional stratigraphy and paleogeography: in the San Juan Limestone in the Precordillera of Palaeoworld, 5: 7-13. Argentina (Lehnert, 1995; Albanesi, Hi.inicken & CHEN Xu & BERGSTROM, S.M., 1995: Introduction- Hisrorical Barnes, 1998) suggests that the base of the Darriwilian, review: Palaeoworld, 5:1-6. CHEN Xu, MITCHELL, C.E., Z HANG YuAN -DONG, W ANG ZHI-HAO, although not yet located precisely, is in the upper BERGSTROM, S.M., WINSTON, D. & PARis, F. , 1997, GSSP of portion of that formation. Darriwilian (Middle Ordovician) in China: Acta Palaeon- tologica Sinica, 36(4): 423-431. CHEN Xu, ZHANG YuAN -D ONG & MITCHELL , C.E., 1995, Castlemainian to Darriwilian (Late Yushanian to early ACKNOWLEDGEMENTS Zhejiangian) graptolite faunas: Palaeoworld, 5: 36-66. ErsTEIN, A., ErsTEI N, J.B. & HAruus, L. , 1977, Conodont color The senior author received grant support from the Basic alteration - an index to organic metamorphism. U.S . Science Research Program of the Chinese Academy of Sciences Geologica! Survey Professional Paper, 995: 1-27. (KZ952-Jl-401). 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