96 by Wang Xiaofeng1, Svend Stouge2, Chen Xiaohong1, Li Zhihong1, Wang Chuanshang1, Stan C. Finney3, Zeng Qingluan1, Zhou Zhiqiang4, Chen Huiming1 and Bernd-D. Erdtmann5 The Global Stratotype Section and Point for the base of the Middle Ordovician Series and the Third Stage (Dapingian)

1 Yichang Institute of Geology & Mineral Resources, Yichang, Hubei 443003, China. E-mail: [email protected]; [email protected] 2 Geological Museum, University of Copenhagen, Øster Voldgade 5–7, DK-1350 Copenhagen K, Denmark. E-mail: [email protected] 3 Department of Geological Sciences, California State University, Long Beach, CA90840, USA. E-mail: [email protected] 4 Xian Institute of Geology & Mineral Resources, Xian, Shangxi, 710054, China. E-mail: [email protected] 5 Institut für Angewandte Geowissenschaften, Technical University, Berlin, Germany. E-mail: [email protected]

The Huanghuachang section near Yichang, water carbonate facies as well as in deep-water graptolite southern China was successively approved by ISOS, facies. Its geographic coordinates are: latitude recognized by ICS and then officially ratified as 30°51'37.8"N; Longitude 110°22'26.5" E of Greenwich. being the Global Stratotype Section and point (GSSP) for the base of the Middle Ordovician Series and the Introduction Third Stage, named as Dapingian, of the Ordovician System. The conodont succession at the section is Since 1989, the Subcommission on Ordovician Stratigraphy (ISOS) of the International Commission on Stratigraphy (ICS) has complete across the Lower to Middle Ordovician series been involved in an extensive project with the goal of establishing boundary and several excellent phylogenetic lineages globally applicable stage and series subdivisions for the Ordovician of Baltoniodus, Gothodus, Microzarkodina and Periodon System (Webby, 1994). The goal is now completed with the ratification are represented. The base of the Middle Ordovician Series of the last Global Stratotype Sections and Points (GSSPs) of the Ordovician System for the base of the Middle Ordovician Series and and the Daping Stage is defined at a point 10.57 m above the Third Stag named Dapingian at the Huanghuachang section. The the base of the Dawan Formation at the base of Bed new Ordovician time-scale is composed of Lower, Middle and Upper SHod-16 of the Huanghuachang section, 22km NE Ordovician Series and seven stages – Tremadocian, Floian, Dapingian, of Yichang City, Hubei Province, South China. It Darriwilian, Sandbian, Katian and Hirnantian (Fig. 1). coincides with the first appearance datum (FAD ) of the The GSSPs for the base of the Middle Ordovician and the Third Stage of the Ordovician System have been sought and discussed for conodont species Baltoniodus triangularis within the more than 12 years (Webby, 1994, 1995). In the early 90s of last Baltoniodus lineage starting from Baltoniodus cf. century two biozone levels that appeared to have potential for B. triangularis to Baltoniodus navis. It is associated with reliable global correlation were proposed as definitions of the base of the FAD of Periodon sp. A and followed closely by the the Middle Ordovician Series and the Third Stage: (1) the base of the Tripodus leavis conodont Biozone, considered correlative with the FAD of Microzarkodina flabellum, which is taken as a base of the Isograptus v. lunatus graptolite Biozone, and (2) the base reasonable proxy for the boundary. This level approxi- of the Baltoniodus triangularis conodont Biozone. The former was mates the boundary between the lower and upper thought to coincide with the base of the North American Whiterock Azygograptus suecicus graptolite Biozone, and nearly Series (Ross and Ethington, 1992; Ross et al., 1997), and the base of coincides with the base of the Belonechitina cf. henryi B. triangularis Biozone with the base of the Baltoscandian Volkhov Stage (Webby, 1994, 1995). After long discussions and a set of ballots chitinozoan Biozone. The same faunal succession is also and questionnaire circulated to the Voting Membership a formal recorded from the Chenjiahe section, near Daping village, Ordovician Subcommission postal ballot was held during October— 5 km to the north of the Huanghuachang section. December 1995 together with the ballot on the tripartite division of The proposed boundary level, which has long been the Ordovician. The base of T. laevis Biozone was finally adopted by majority (82%) of voting members and then approved by the ISOS as used in other important Ordovician successions the biohorizon on which to define the base of the Middle Ordovician worldwide, can be easily recognized and correlated Series (Webby, 1997, 1998). Afterwards, the Whiterock Narrows globally with high precision in both relatively shallow- section in the Monitor Range, Nevada was recommended as the

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reasons for the positive votes are as follows: (1) The proposed level KEY GRAPTOLITE/ with the FAD of Baltoniodus triangularis at the Huanghuachang CONODONT(C) GLOBAL section is a speciation event, where the marker species evolved from STAGES BIOHORIZONS its immediate ancestor, B. cf. B. triangularis and the horizon nearly

GLOBAL STAGES P. acuminatus 443.7Ma coincides or is close to speciation events within several other HIRNANTIAN (GSSP-Dobs Linn) 445.6Ma N. extraordinarius conodont lineages – those of Gothodus, Microzarkodina, Periodon (GSSP-Wangjiawan) and Paroistodus. The level with FAD of C. aranda at the Niquivil KATIAN section is not controlled by a speciation event in this section, and it is possible that the first appearance in the Niquivil section is not

455.8Ma UPPER D. caudatus the true FAD of this species (Bergström, 2006). (2) The presence of (GSSP-Black Knob R.) SANDBIAN graptolite and chitinozoan zones gives the Huanghuachang section 460.9Ma N. gracilis significantly higher potential for global correlation. The associated (GSSP-Fsgelsang) graptolite fauna indicates that the base of the B. triangularis Biozone DARRIWILIAN is very close to, if not coeval with, the base of the Isograptus v. U. austrodentatus 468.1Ma victoriae Graptolite Zone (Maletz 2005). Graptolites have not been (GSSP-Huangnitang) MIDDLE DAPINGIAN found at the Niquivil section, probably because of deposition in too B. triangularis(c) 471.8Ma ORDOVICIAN GSSP-Huanghuachang shallow water or low preservation potential. (3) The level with the FLOIAN FAD of Baltoniodus triangularis correlates with major stratigraphic 478.6Ma T. approximatus boundaries elsewhere, and the taxon has been used in other important (GSSP-Diabasbrottet) Ordovician successions worldwide for a long time, e.g., the horizon lies at or very close to the base of the Whiterock Series in North

LOWER TREMADOCIAN America and the Volkhov Stage in Baltoscandia (Bergström, 1995; I. fluctivagus(c) 488.3Ma GSSP-Green Point Finney, 2006). In contrast, the lower boundary of the Middle Ordovician Series, if it is defined on the basis of the level with the Figure 1. Ordovician choronostratigraphic chart showing global first occurrence of C. aranda in the Niquivil section, cannot be Series and Stages with their GSSPs (modified from Finney, 2005). correlated with any precision into graptolite successions of either the Atlantic or Pacific province. stratotype section for the base of the global Middle Ordovician Series The Huanghuachang GSSP has excellent and well-known with the boundary placed 3 m below the top of the Ninemile Formation conodont, chitinozoan, graptolite, acritarch and shelly fossil records at the FAD of Tripodus laevis (Finney and Ethington, 2000, 2001). in a stratigraphically continuous succession (Figs. 2, 3). The Lower With further study, however, the proposed boundary in the Whiterock and Middle Ordovician boundary coinciding by the FAD of B. Narrows section was shown to correlate with a higher level in the triangularis and followed by the FAD of Microzarkodina flabellum graptolite zonation than expected (Mitchell, 2001), which suggested can be easily recognized and correlated globally with high precision also that the first appearance of T. laevis was diachronous. A in both relatively shallow-water carbonate facies as well as in subsequent search for potential stratotype sections in western deep-water graptolite facies. Newfoundland, where the graptolites Isograptus v. lunatus and I. (v.) victoriae occur with conodonts was unsuccessful. Thus, the Geographical location, access and Subcommission was forced to consider alternative biohorizons. Albanesi and Carrera (2001, 2003) proposed the Niquivil section, geologic setting Argentina as the global stratotype for the base of the Middle Ordovician with the FAD of the conodont Protoprioniodus aranda The Huanghuachang GSSP is located along the major road from (= Cooperignathus aranda sensu Zhen et al., 2003) as an alternative Yichang City to Xinshan County, 22 km NE of Yichang City, Hubei index species for the boundary level, and Wang et al. (2002, 2003a) Province, China (Figs. 2, 3), and it can easily be accessed by car. The recommended the Huanghuachang section, China with the FAD of section is well exposed at the center of the Ordovician Geopark, which the conodont Baltoniodus triangularis as the boundary bio- is part of the National Geopark of the Yangtze Gorges. Yichang City marker. is the largest hydroelectric city and one of the most well-known tourist The two GSSP proposals (Wang et al., 2004, 2005; Albanesi et cities in China. It can be reached by major highway, train, and regular al., 2006), were discussed and debated at length on the Ordovician direct flights from Beijing, Shanghai, Guangzhou, Shenzhen and Hong Subcommission website. A postal ballot completed in October 2006, Kong. 75% of the votes favored the Huanghuachang proposal over the Geographically this area lies on the east limb of the Huangling Niquivil proposal, and 94% of the voting members then approved Arch, and geologically belongs to the north-central Yangtze carbonate the Huanghuachang GSSP in a ballot completed in December 2006. platform. The Ordovician rocks, together with the underlying And then the International Subcommission of Ordovician System Cambrian and overlying Silurian deposits, are widely distributed (November, 2006) and International Commission on Stratigraphy around the Huangling Arch (Fig.2). The Huanghuachang section is (ICS) of the IUGS (April, 2007) recognized and the International structurally undeformed and the strata dip at 11o to the east. The Union of Geological Sciences (IUGS) (March, 2009) officially ratified Ordovician section at Huanghuachang and the GSSP for the the Huanghuachang section, as being the global boundary stratotype Hirnantian Stage with the Ordovician-Silurian boundary section at and point (GSSP) for the base of Middle Ordovician Series and Third Wangjiawan, 22 km NE of the Huanghuachang, are attracting attention stage, named as the Dapingian Stage (Wang et al., 2007). The principal to stratigraphic subdivision and correlation on a worldwide scale.

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Ar F Wufeng Fm. 2-3 3 T Linxiang Fm. Xinshan Pt Goujiaya Baota Fm. F 1 Miaopo Fm. 2 Z Shuiyuesi Guniutan Fm. F D-P D-P 1 Yuanan Upper Dalaoling S Middle O K Wangjiawan Fm. Dawan Lower T Z Honghuayuan Fm. Zigui Jianyangping Ar2-3 S Fenxiang Fm. Xintan Fenxiang Daping D-P F Pt1 4 S O Miaohe Chenjiahe T Liantuo O Huanghuachang Beijing T Yangtze Z River T Yichang K Fm. Nanjingguan D-P O Yichang 0 900km 0 10 20km 10m T S Xilingxia Fm. 0

algae reef rimous Dolomite with Outcrop of shale limestone dolomite Section limestone limestone banded chert or Ordovician chert nodules Figure 2. Geological map of the Yichang Area, Hubei, China. The localities of the Huanghuachang and adjacent Chenjiahe sections, and the Ordovician succession of this area are shown.

Description of the Huanghuachang et al., 1995, 2003). It is in the type area for the subdivision of the Ordovician System in China, and it can be correlated confidently section with other sections in the region. The section is characterized by a continuous series of strata across the Lower/Middle Ordovician The Huanghuachang section has been studied in great detail and boundary. The succession is mainly composed of carbonate rocks for almost 50 years (Zhang et al., 1957; Mu et al., 1979; Wang, 1980; intercalated with some clastic sedimentary rocks bearing limestone Zeng et al., 1983; Wang et al., 1987, 1992a, b, 1995; An, 1987; Chen lenses and yielding various fossils (Zeng et al., 1983; Wang et al., 1987). Many Ordovician specialists from China and abroad have visited the section since 1978 (Fig.4). The Ordovician stratigraphic succession of the Yangtze Gorges area is subdivided into 10 formations shown in Fig.2. The Huanghuachang section with the Lower/Middle Ordovician boundary interval begins in the Honghuayuan Formation, which is a highstand deposit composed of thick-bedded limestone with Archaeocyphia and Calathium. Conodonts of the Serratognathus diversus Biozone and the Oepikodus communis Biozone separately occur in the lower-middle parts and the uppermost part (Fig. 5). The Oepikodus communis Biozone ranges into the overlying Dawan Formation.

Lithostratigraphic subdivision of Dawan Formation The Dawan Formation was defined by Zhang Figure 3. The Huanghuachang GSSP showing the monument of the GSSP on the left (1962) on the basis of the Dawan Bed (Zhang et side, the model of Baltoniodus triangularis on the right side and the Lower and Middle al., 1957) and the Yangtzeella Bed established by Ordovician boundary, i.e the base of the Dapingian, defined by the FAD of B. triangulates Lee (1924). Zeng et al. (1983) subdivided the at the base of Shod-16 and indicated by a red line with a small memorial table, 0.6 m Dawan Formation at the Huanghuachang section above the characterietic grayish purple medium limestone of 0.9 m , or 10.57 m above into three lithological units: the Lower Member, the base of the Dawan Formation the Middle Member and the Upper Member i.e.

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the Dawan Formation, composing most of the Huanghuachang section, have been described by Wang et al. (2005). The symbols Hod and SHod, used in the text, and figures, refer to beds or thin stratigraphic intervals sampled for fossils. If the sampling interval (Hod or SHod) possesses similar or identical lithological rock characters with a slightly bigger thickness it is subdivided into several secondary sampling intervals (i.e. SHod-17A, SHod-17B…) as shown in the section description (Figs. 3,5). The Lower Member is subdivided informally into lower, middle, and upper parts (Wang et al., 2005), which are terms used in the following description and shown on the figures. The lower part of 5.45 m (Hod-29–Hod-4) comprises gray thick-bedded glauconitic limestone, 0.93 m thick (Hod-29–Hod-27), overlain by thin-bedded glauconitic limestone and nodular limestone with yellow green shale Figure 4. More than 80 Ordovician and Silurian workers participated in the post-excursion interbeds. The middle 5.52 m of the Lower rd of the 10th International Ordovician, 3 Silurian and IGCP Project 508 Conference held Member (Hod-3–SHod-11) is composed of gray in Nanjing in July 6, 2007, and visited the Huanghuachang section. thin-bedded limestone and bioclastic limestone with yellow-green shale interbeds overlain by the Lower, Middle and Upper Dawan Formation, or Lower, Middle 0.9 m of the light purple-gray medium-bedded dolomitic micritic and Upper Units (Figs. 2, 5) (Wang et al., 1987, 1995, 2005). limestone at its top (SHod–9-11). The upper part of the Lower The Lower Member is 12.97 m thick. It consists of gray, thin- Member, which includes the GSSP, is 3m thick and composed of bedded and minor medium-bedded limestone, bioclastic limestone light purple thin-bedded nodular limestone and medium-bedded and glauconitic limestone with yellow-green shale interbeds (Fig.5). limestone interbedded with yellow-green shale with limestone lenses The strata yield abundant conodonts with graptolites, chitinozoans, (SHod-12–24). acritarchs, brachiopods, trilobites and cepholopods, representing a The proposed boundary biohorizon of the Middle Ordovician mixed biota of cool-water with warm-water biota (Wang, 1980; Wang Series, i.e. defined by the FAD of Baltoniodus triangularis, and its and Chen, 1999). Three conodont biozones, the Oepikodus communis under- and over-lying beds are described in detail as follows in Biozone, Oepikodus evae Biozone (sensu Lindström, 1971, descending order: Bergström, 1995) and Baltoniodus triangularis Biozone, can be recognized in the Member (Wang et al., 2005). The GSSP is located Lower Member of Dawan Formation (or Lower Dawan in the upper part of the Lower Member. Formation): 12.97m The Middle Member of the Dawan Formation is 13 m thick. It is Upper part: 3m composed of purple medium-bedded micrite with yellow-green mudstone and shale interbeds. The strata yield abundant cephalopods SHod-24–22 Grayish-green shale with nodular limestone 0.80 m of the Protocycloceras Biozone and brachiopods including Yangtzeella SHod-24 (0.4 m) - conodonts: Drepanoistodus forceps, Scolopodus poloi. Recent studies show that the conodonts are characterisic for princeps, Baltoniodus triangularis, Periodon sp.A, the Baltoniodus navis Biozone in the middle–upper parts and upper Microzarkodina flabellum, Paroistodus cf. proteus; trilobites: Baltoniodus triangularis Biozone in the lower part (Wang et al., 2005). Rhombampyx yii and brachiopods: Pseudoporambonites The Upper Member of the Dawan Formation of 28 m thick, it yichangensis, Euorthisina orthiformis, E. kobayashii, Leptella consists of yellow-green shale intercalated with thin-bedded nodular grandis, Sinorthis typical, Nereidella sinuate and Yangtzeella limestone and mudstone at Huanghuachang section. The Upper yichangensis. Member correlates with the upper part of the Third Stage and the SHod-23 (0.2 m) - conodonts: Baltoniodus triangularis, lower Darriwilian Stage based on the occurrence of graptolites of the Microzarkodina flabellum, Periodon sp. A, Protopanderodus Undulograptus sinodentatus Biozone at the base and representatives flouridus, Drepanoistodus forceps, Scolopodus princeps, Oistodus of the U. austrodentatus Biozone at the top, together with trilobites lanceolatus, Gothodus cf. costulatus; graptolites: Expansograptus of the Hanchugolithus Biozone, brachiopods of the Euorthisina cf. suecicus, Phyllograptus sp., Tetragraptus reclinatus reclinatus; Biozone and conodonts of the Baltoniodus norrlandicus and Lenodus chitinozoans: Belonechitina cf. henryi, Conochitina decipiens, antivariabilis Biozones (An 1987; Wang et al., 1987; Stouge, in prep, C. cf. parvulgata, C. sp. aff. langei; trilobites: Carolinites Chen et al., 2003). genacinaca, Rhombampyx yii, Liomegalaspides similis, Agerina elongata, Ningkianolithus sp. and brachiopods: same as in Detailed description of the succession across the proposed SHod-24. boundary horizon SHod-22 (A, B) (0.2 m): conodonts: Baltoniodus triangularis, The Lower Member and the lower part of the Middle Member of Microzarkodina flabellum, Periodon sp. A, Drepanoistodus

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section sample trilobite brachiopod chitinozoan graptolite conodont Biozone

SHod-29

Shbod-2ch sp. SHod-28 Hod- E-4

15m navis SHod-27 Hod- E-3

Baltoniodus

SHod-26 Hod-

f.

Middle Member Middle E-2 14

SHod-25B SHod-25A Hod-

E-1 13 Upper SHod-24

SHod-23 Hod- Pseudotrigonograptus SHod-22-B d -3 SHod-22-A SHod-21-B Hod- 12 cBelonechitina henryi

SHod-21-A Gothodus costulatus d -2

SHod-20-C navis Baltoniodus SHod-20-B Hod- SHod-20-A d -1 SHod-19-B SHod-19-A

upper part upper SHod-18 Hod- 11 c-2-3 triangularis Baltoniodus Hod- SHod-17 c-2-2 Hod- originalis Paroistodus SHod-16 c-2-1 10.57 SHod-15 Hod- c-1-4 costulatus Hod- SHod-14 c-1-3 SHod-13 Hod- cf.

c-1-2 sp.A SHod-12 Hod- c-1-1 10 suecicus Azygograptus

SHod-11 Hod-B-3 cf.

Euorthisina SHod-10 Hod-B-2 Gothodus

hd22 Periodon SHod-9 Hod-B-1

Agerina elongata Agerina

9 cf. hd21 sp.A

SHod8 Hbod1 hd20 svalbardensis Xiphograptus

Microzarkodina flabellum Microzarkodina

SHod7-3 Hbod2-3 ellesi Azygograptus SHod7-2 Hbod2-2

SHod7-1 Hbod2-1 hd19 Belonechitina henryi SHod6-2 Hbod3-2 8

Dawan Fm. SHod6-1 Hbod3-1 Belonechitina micracantha Belonechitina SHod5-4 Hbod4-4 Lower SHod5-3 Hbod4-3 hd18 SHod5-2 Hbod4-2 Paroistodus proteus SHod5-1 Hbod4-1 flouridus Protopanderodus

Conochitina pseudocarinata Conochitina

upper part upper

hd17 yichangensis Pseudoporambonites SHod4 SHod4 Microzarkodina 7m triangularis Baltoniodus e SHod3 SHod3 SHod2 SHod2

Zone

e SHod1 SHod1 princeps Scolopodus

Middle part Middle Lower Member e # Hod1 Hod1 e e e # 6 Hod2 Hod2 e e e e e e Hod3 Hod3 e

e Hod4 Hod4 n sp.B e # e # Hod5 Hod5 # e Hod6 Hod6 5 evae Oepikodus e # # e e Hod7 Hod7 e # e Hod8 Hod8

e # # Hod9 Hod9 Conochitina pseudocarinata Conochitina Hod10 Hod10

e Hod11 Hod11 4 Baltoniodus? . e #

Hod12 langei Conochitina e # Hod12 e e Hod13 Hod13 e # e e Hod14 Hod14

Hod15 Hod15 suecicus Azygograptus e # e Hod16 Hod16

e # deflexus Corymbograptus # e Hod17 Hod17 3

Carolinites genacinaca e # Hod18 Hod18

e e cf.

# e # part lower Hod19 Hod19

e e Didymograptellus bifidus Didymograptellus

e # Hod20 Hod20 forceps Drepanoistodus e Hod21 # e Hod21 e # Hod22 Hod22 2 e e e # e Hod23 Hod23 part Lower # Baltoniodus triangularis B.

e # Hod24 Hod24 flabellum Periodon e e # Hod25 Hod25

# intermedius Oepikodus e e Hod26 Hod26 1

# e # communis Oepikodus Hod27

Hod27 stola Stolodus e e Hod28-3 Hod28-3 n? sp.A # Hod28-2 Hod28-2 e e Hod28-1 Hod28-1 selenopsis Periodon # # Hod29 Hod29 # 0m ee Hoh30-1 Hoh30-1 Hoh30-2 Hoh30-2 diprion Trapezognathus

# Didymograptellus bifidus Didymograptellus

-1.5 Baltoniodus .

Conochitina langei Conochitina Conochitina raymondii Conochitina e

e e Zone Calcirudite e bioclastic shale with e e limestone e e limestone nodular limestone

e evae Oepikodus

Leptella grandis Leptella Honghua- Fm. yuan

Pseudocalymene transversa # # e e esthonica Lagenochitina glauconite

# shale strata omitted esthonica Lagenochitina ##limestone

Leptella hubeiensis Leptella

Reutterodus andinus

Oepikodus communis Oepikodus Figure 5. Stratigraphical ranges of principal conodonts, graptolites, chitinozoans, brachiopods, trilobites through the Lower Dawan Formation at the Huanghuachang section.

forceps, Scolopodus princeps, Paroistodus cf. proteus, Oistodus sp. aff. langei, C. longei, Loganochitina esthonica, trilobites lanceolatus, Protopanderodus flouridus, Gothodus cf. costulatus; Pseudocalymene transversa; trilobites: Carolinites sp. and graptolites: Xiphograptus svalbardensis, Pseudotrigonograptus brachiopods: Pseudoporambonites yichangensis, Euorthisina sp. sp.; trilobites: Phorocephala genalata, Ningkianolithus sp., SHod-20A–C (0.37 m) - trilobites: Carolinites sp.; brachiopods: Liomegalaspides similes, Geragnostus sp., Agerina elongata and Pseudoporambonites sp. and conodonts: same as below. brachiopods: Pseudoporambonites yichangensis, Euorthisina SHod-20C (0.11 m)-conodonts: Drepanoistodus forceps, Periodon orthiformis, E. kobayashii. sp. A, Scolopodus princeps, Protopanderodus flouridus, SHod-21–20 Gray medium-bedded limestone with grayish-green shale Baltoniodus triangularis, Gothodus costulatus. interbeds, dominated by conodonts brachiopods in limestone SHod-20B (0.11 m): Baltoniodus triangularis, Microzarkodina (SHod-21A-E -SHod-20A-C) and a few graptolites, chitinozoans flabellum, Drepanoistodus forceps, Periodon sp. A, Oistodus and trilobites in shale 0.60 m lanceolatus, Protopanderodus flouridus. SHod-21 (A–B) (0.23 m) - conodonts: Baltoniodus triangularis, SHod-20A (0.15m): Microzarkodina flabellum, Periodon sp., A, Microzarkodina flabellum, Periodon sp. A., Drepanoistodus Drepanoistodus forceps, Oistodus lanceolatus, Scolopodus forceps, Gothodus cf. costulatus, Scolopodus princeps, princeps. Paroistodus cf. proteus, Protopanderodus flouridus, Oistodus SHod-19A–B Grayish-green shale with a few nodular limestone, lanceolatus; graptolites: Tetragraptus reclinatus reclinatus; containing conodonts, the trilobites Liomegalaspides similes, chitinozoans: Belonechitina cf. henryi, B. micracantha, Rhombampyx yii and brachiopods Pseudoporambonites Conochitina kryos, Conochitina decipiens, C. cf. parvulgata, C. yichangensis, Euorthisina kobayashi, Sinorthis typical;

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graptolites, chitinozoans, and brachiopods are present in the lower SHod-11 (0.3 m) - conodonts: Baltoniodus cf. B. triangularis, part, 0.50 m Periodon flabellum, Paroistodus cf. proteus, Reutterodus andinus, SHod-19B (0.2 m) - conodonts: Microzarkodina flabellum, Drepanoistodus forceps, D. spp., Scolopodus princeps, Gothodus Microzarkodina sp. A, Drepanoistodus forceps, Scolopodus costulatus. princeps. Periodon sp. A., Gothodus cf. costulatus. SHod-10 (0.3 m) - conodonts: Periodon flabellum, Baltoniodus cf. SHod-19A (0.3m) - conodonts: Periodon sp. A.; graptolites: B. triangularis, Oepikodus intermedius, Reutterodus andinus, Azygograptus suecicus, A.ellesi, Xiphograptus svalbardensis; Scolopodus princeps, Gothodus costulatus, Oistodus lanceolatus, chitinozoans: Belonechitina micracantha, Conochitina brevis, C. Drepanoistodus forceps, Trapezognathus diprion, Proto- pseudocarinata, C. decipiens, prioniodus yapu, Nasusgnathus dolonus. SHod-18–12 Gray thin-bedded limestone intercalated with gray- SHod-9 (0.3 m) - conodonts: Periodon flabellum, Baltoniodus cf. greenish shale with limestone lenses, bearing conodonts, B. triangularis, Drepanoistodus forceps, Reutterodus andinus, graptolites, chitinozoans and acritarchs. 1.10 m Scolopodus princeps, Gothodus costulatus, Oistodus lanceolatus. SHod-18 (0.3 m) - conodonts: Baltoniodus triangularis, Microzarkodina flabellum (FAD), Microzarkodina sp. A., The proposed Lower/Middle Ordovician boundary is defined at Periodon sp. A., Drepanoistodus forceps, Protopanderodus the base of SHod-16 with the FAD of Baltoniodus triangularis. This flouridus, Scolopodus princeps, Oistodus lanceolatus; trilobites: level is 0.6 m above the top of the distinctive 0.9 m-thick, medium- Liomegalaspides similes, Pseudocalymene transversa, bedded dolomitic micritic limestone at the uppermost middle part Hungioides sp. Aulacopleura (Paraaulacopleura) dawanensis (SHod– 9–11) (Fig.3), 10.57 m above the base of the Dawan and brachiopods: Pseudoporambonites yichangensis, Euorthisina Formation and 2.4 m below the base of the Middle Member of the orthiformis, E. kobayashii, Leptella grandis, Sinorthis typica. Dawan Formation (Figs.3,5). This level nearly coincides with the SHod-17 (0.1 m) - conodonts: Baltoniodus triangularis, Baltoniodus first appearance of Periodon sp. A (sensu Bagnoli and Stouge, 1997) cf. B. triangularis, Microzarkodina sp. A, Periodon sp. A, P. in SHod-16 and is 0.2 m below the FAD of Microzarkodina flabellum flabellum, Drepanoistodus forceps, Paroistodus cf. proteus, (i.e. SHod-18; Fig. 5). Scolopodus princeps; graptolites: Azygograptus suecicus; trilobites: Amphyx yii and brachiopods: Pseudoporambonites Biostratigraphy yichangensis, Euorthisina orthiformis, Leptella grandis, Sinorthis typica. The Dawan Formation is widely distributed on the Yangtze SHod-16 (0.1 m) - conodonts: Baltoniodus triangularis (FAD), Platform. In the Yichang area there are at least two other relevant Baltoniodus cf. B. triangularis, Microzarkodina sp. A. Periodon sections that have been studied, the Chenjiahe section (Fig. 6) and sp. A (FAD), P. flabellum, Baltoniodus? n. sp. B., Nasusgnathus the Jianyangping section. They are exposed, respectively, in the eastern dolonus, Drepanoistodus forceps, Scolopodus princeps, Oistodus and western limbs of the Huangling Arch, and correlation with the lanceolatus, Gothodus costulatus; chitinozoans: Belonechitina Huanghuachang section is straightforward. The Chenjiahe section is cf. henryi, B. micracantha, Conochitina pseudocarinata, C. cf. located 5 km to the north of the Huanghuachang section. It is well parvulgata, C. longei, C. decipiens; trilobites: Liomegalaspides exposed along a country roadside. The Jianyangping section is well similis and brachiopods: Pseudoporambonites yichangensi, exposed in main roadside, close to a small town named Jianyangping Euorthisina sp. in the Xinshan County. The conodonts of the Chenjiahe section have SHod-15 (0.10 m) - conodonts: Microzarkodina sp. A, Baltoniodus been studied in detail by Stouge (2002, in prep.). The conodont, cf. B. triangularis, Periodon flabellum, Drepanoistodus forceps, graptolite, and chitinozoan biostratigraphic successions and their Scolopodus princeps; graptolites: Azygograptus ellesi; trilobites: relationship across the Lower and Middle Ordovician boundary Agerina elongata and brachiopods: Pseudoporambonites yichangensi. SHod-14 (0.15 m) - conodonts: Microzarkodina sp. A, Periodon flabellum, Baltoniodus cf. B. triangularis, Protopanderodus flouridus, Scolopodus princeps, Gothodus costulatus; and brachiopods: Leptella sp., Sinorthis tipica, Pseudomimella formosa. SHod-13 (0.2 m) - Microzarkodina sp. A, Periodon flabellum, Baltoniodus cf. B. triangularis, Drepanoistodus forceps, Scolopodus princeps, Oistodus lanceolatus, Gothodus costulatus. SHod-12 (0.15 m)- conodonts: Microzarkodina sp. A, Baltoniodus cf. B. triangularis, Periodon flabellum, Paroistodus cf. proteus, Scolopodus princeps, Gothodus costulatus, Drepanoistodus forceps; trilobites: Liomegalaspides similes; and brachiopods: Sinorthis tipica.

Underlying beds: uppermost middle part of the Lower Dawan Formation with 4.52 m Figure 6. The Chenjiahe section. The level indicated by a red line, SHod-11–9 gray purplish medium-bedded dolomitic limestone, with FAD of B.? triangulates is 0.6m above the grayish purple medium containing conodonts 0.9 m limestone of 0.9 m thick, same as the Huanghuachang section.

Episodes, Vol. 32, no. 2 102 interval at the Huanghuachang section are also documented in the succeeded by Oepikodus intermedius at base of Hod 16 and together Chenjiahe and Jianyangping sections (Fig. 6). with Periodon flabellum (3.03 m above the base of the Dawan Formation) (Figs. 5,7B). Advanced O. evae specimens typical for the Conodonts Baltic region and described by Lindström (1971) follow and represent the lower O. evae Biozone sensu (Lindström, 1971). Associated The conodonts from the Lower Member of the Dawan Formation important conodont species in the zone include Bergstroemognathus have been studied in detail by several specialists (Zeng et al., 1983; extensus, Gothodus costulatus, Juanognathus variabilis, Lundodus An, 1987; Ni in Wang et al., 1987; Wang Z. et al., 1995; Wang and gladiatus, Periodon spp., Reutterodus andinus, Tropodus spp., Bergström, 1998, Chen X. et al., 1995, Stouge, 2002 in prep.). The Baltoniodus? n. sp. A and Baltoniodus? n. sp. B. Reutterodus andinus conodont species association and succession from the Lower Member is present throughout the biozone. of the Dawan Formation mark a prominent change from the underlying The biozone comprises most of the Lower Member of the Dawan fauna of the Honghuayuan Formation, which includes taxa of the Formation (Fig. 2). O. evae first appears within the thick-bedded Serratognathus diversus and Oepikodus communis Biozones (An, glauconitic bioclastic limestone (Hod 28-3), 0.6 m above the base of 1987). The Oepikodus communis Biozone continues into the lowest the Dawan Formation, and ranges upwards to the base of Hod 9, 4.35 Dawan Formation where the Oepikodus evae Biozone (sensu m above the base of the Dawan Formation. Lindström, 1971, Bergström, 1995) and Baltoniodus triangularis The upper part of the O. evae Biozone is the interval where the Biozone are present in the Lower Member (Wang et al., 2004, 2005) nominated species is rare to absent. The biozone is recorded in the (Figs. 3, 5). interval from Hod 9 to SHod-15 and is characterized by Trapezognathus diprion, Periodon flabellum, Gothodus costulatus, Subdivision of conodont biozone in the Lower Dawan Tropodus spp., Drepanoistodus forceps, Oepikodus intermedius, Formation Periodon selenopsis, Baltoniodus cf. B. triangularis, Tripodus cf. Oepikodus communis Biozone. - The base of the Oepikodus leavis (=Tripodus leavis, sensu Wang et al., 2003a, b; Chen X.H. et communis Biozone is defined by the interval from the FAD of the al., 2003; Li et al., 2004), Gothodus costulatus, Protoprioniodus yapu nominate species to the first appearance of the early type of Oepikodus and Microzarkodina sp. A. The last species occurs in the uppermost evae (Hod 28-3). The Oepikodus communis Biozone spans about 1.73 part of the biozone. The conodont assemblage and succession in the m in the uppermost Honghuayuan Formation (Hoh31–Hod 30) and upper O. evae Biozone (Figs.5, 7) are similar to those of the Gothodus the thick-bedded glauconitic bioclastic limestone of 0.93 m thickness crassulus (= G. costatus), Trapezognathus diprion and (Hod 29–28) at the base of the Dawan Formation. Two new conodonts Microzarkodina sp. A zones reported from Sweden (Löfgren 1993; species reported by Zhen et al. (2006) from the Honghuayuan Stouge et al., 1995; Bagnoli and Stouge, 1997). The base of the upper Formation of Guizhou probably appear in the Serratognathus diversus O. evae Biozone herein is also indicated by the first apperance of Biozone in the lower-middle parts of the Honghuayuan Formation Drepanoistodus forceps, Baltoniodus cf. B. triangularis and Tripodus here. cf. leavis at the base of Hod 9 (Fig. 5). The former is a very common Oepikodus evae Biozone. - The Oepikodus evae Biozone is species reported from the O. evae to B. navis Zone in the Baltoscandic defined by the FAD of the early Oepikodus evae to the FAD of region (Lindström 1971; Löfgren 1978; Rasmussen 2001) and NW Baltoniodus triangularis. The biozone can be subdivided into two Russia (Tolmacheva and Federov, 2001). Baltoniodus cf. B. parts, where Oepikodus evae is represented first by an early type triangularis (Fig.8), which was named Baltoniodus? cf. B.? triangularis by Wang et al. (2005) and identified as Baltoniodus triangularis by Wang et al. (2002, 2003a, b), Chen X.H. et al. (2002, 2003) and Li et al. (2004). It is similar to B. triangularis (Lindström) from the Baltic region in most aspects, but the main difference is the lack of confluent denticles on the anterior process of the Pb element. It is the ancestor of B. triangularis within the Baltoniodus lineage (Fig.9). The O. evae Biozone in the Cow Head Group, W. Newfoundland (Johnston and Barnes, 1999) is equivalent to the O. evae Biozone of the Baltic region or the lower Figure 7. (A) Indicating the level with FAD of O. intermedius at the Huanghuachang. (B) Showing the O. evae Biozone of the boundary between the Upper and Lower Oepikodus evae Biozone. Huanghuachang section. The

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(sensu Lindström, 1971) of the Baltic region, the B. triangularis to M. flabellum interval zone of Öland, south Sweden (Bagnoli and Stouge, 1997), and the T. leavis- M. flabellum interval in the Ibex, Utah section of North America (Ethington and Clark, 1982, Ross et al., 1997). Baltoniodus navis Biozone. The Baltoniodus navis Biozone is defined by the first appearance of Baltoniodus navis. The base is at SHod-26, 1.5 m above the base of the Middle Member of the Dawan Formation. The following faunal succession is mainly composed of Oistodus, Periodon and Paroistodus and characteristic associated species includes Periodon sp. A, Oistodus lanceolatus, and Paroistodus originalis. The same faunal associa- tion continues into the upper part of the Middle Member of the Dawan Formation.

Evolutionary conodont lineages × Figure 8. Baltoniodus cf. B. triangularis (Lindström). 1. Pa element, lateral view, 150, sample, Hod- in the Huanghuachang section 9-0/6092, Hod-9; 2. Pb element, lateral view, ×150, sample, Hod-9-0/6078, Hod-9; 3. Pb element, lateral view, ×180, sample, Hod-9/1486, Hod-9; 4. Pb element, lateral view, ×150, sample, Hod-9-2/ Significant speciation events 1340, Hod-9-2; 5. Pb element, lateral view, ×150, sample, Hod-9-1/1378, Hod-9-1; 6. Pb element, occur in the Baltoniodus, Gothodus, inter-lateral view, ×100, sample, Hod-c-2/6006, SHod-16; 7. Pb element, lateral view, ×180, sample, Microzarkodina and Periodon Hod-9-0/6090, Hod-9; 8. Pb element, lateral view, ×120, sample, Hod-9-0/6080, Hod-9; 9. Sa element, lineages across the Lower and posterior view, ×150, sample, Hod-8/6088, Hod-8; 10. Sb element, lateral view, ×100, sample, Hod-c- Middle Ordovician Series boundary, 1/5995, SHod-12; 11. Sc element, lateral view, ×180, sample, Hod-c-1/5992, SHod-12; 12. Sd element, where species of the taxa other than posterior-lateral view, ×150, sample, Hod-c-1/5994, SHod-12; 13 M element, lateral view, ×120, sample, Baltoniodus triangularis are good Hod-8/2287, Hod-8. proxies for the defined boundary level (Fig. 10). In addition, species succeeding T. leavis Biozone in the Cow Head Group is likely to be within the Paroistodus and Trapezognathus lineages change across equivalent to the lower part of the upper O. evae Biozone from the the series boundary. Huanghuachang section based on the co-occurrence of Oepikodus The Baltoniodus lineage is the most significant lineage and the evae, O. intermedius and Tripodus cf. leavis. In the Niquivil FAD of Baltoniodus triangularis is proposed as the boundary section near San Juan, Argentina the FAD of Cooperignathus biomarker for the base of the Middle Ordovician. Based on aranda basically coincides with the boundary between the O. evae Baltoscandian material the lineage was considered to begin with and O. intermedius Biozones as these biozones have been defined by Baltoniodus triangularis and followed by Baltoniodus navis Albanesi et al. (2006). This level should be a little below the top of (Lindström, 1971; Löfgren, 1978; Dzik 1976, 1994). Bagnoli and the lower O. evae Biozone in the Huanghuachang section (Wang et Stouge (1997) were cautious because some specimens of Baltoniodus al. 2005). triangularis in their collection resemble Trapezognathus elements Baltoniodus triangularis Biozone. - The Baltoniodus triangularis and there were no known taxa that could be assigned as the obvious Biozone is defined by the FAD of B. triangularis (Figs. 3, 5, 9) together ancestor to Baltoniodus triangularis. Bagnoli and Stouge (1997) with first appearance of Periodon sp. A (sensu Bagnoli & Stouge, therefore tentatively assigned the triangularis elements to the genus 1997) and extends upwards to the FAD of Baltoniodus navis at the Baltoniodus with a question mark. The discovery of the species base of SHod-26. The Baltoniodus triangularis Biozone is same as Baltoniodus cf. triangularis in the South China sections solved the the Baltoniodus? triangularis - Microzarkodina flabellum Biozone problem, because this species is the obvious ancestor to Baltoniodus (sensu Wang et al., 2005). Microzarkodina flabellum (Wang et al. triangularis (Figs.8-9). The Baltoniodus lineage continues with the 2005, p.108, Fig. 5) first appears at SHod-18, 0.2 m above the base more advanced successor Baltoniodus navis via the transitional forms of the biozone. The important newcomers across the boundary referred to Baltoniodus cf. navis by Bagnoli and Stouge (1997). interval are first Microzarkodina sp. A (sensu Bagnoli and Stouge, The Gothodus lineage begins with Gothodus sp., which is 1997) followed by Baltonionus triangularis and Periodon sp. A, and recorded in the sections from South China. It is succeeded by then Microzarkodina flabellum. This faunal succession suggests the Gothodus costulatus in the Lower Ordovician Oepikodus evae correlation of present biozone with the B. triangularis Biozone Biozone and is followed by Gothodus cf. costulatus in the Baltoniodus

Episodes, Vol. 32, no. 2 104

include M. corpulenta, M. parva, M. bella, M. hagetiana and M. ozarkodella. The Periodon lineage has been outlined by Dzik (1976), Löfgren (1978), Stouge and Bagnoli (1988), Bagnoli and Stouge (1997), Johnson and Barnes (1999) and Rasmussen (2001). The oldest known species is Periodon primus followed by Periodon selenopsis, Periodon flabellum and Periodon sp. A. Periodon sp. A appears just at the base of the Baltoniodus tri- angularis Biozone of the Middle Ordovician and ranges into Baltoniodus navis Biozone. Further species of the lineage in the Middle Ordovician have most recently been revised and outlined by Rasmussen (2001). Younger species includes Periodon aculeatus and P. grandis from the Upper Ordovician. The Trapezognathus lineage was described by Bagnoli & Stouge (1997) and the present study confirms the status of the lineage. In the Huanghuachang section T. diprion appears in the upper Opikodus evae Biozone of the Lower Ordovician and is succeeded by T. quadranculum in the Baltoniodus navis Biozone of the Middle Ordovician. The precise shift of species has not yet been observed or recorded as the Figure 9. Baltoniodus triangularis (Lindström). 1. Pa element, lateral view, ×150, sample, Hod-c-2/ two species are separated by a 6009, SHod-16; 2. Pa element, oral view, ×200, sample, SHod-16/1464, SHod-16; 3. Pb element, lateral stratigraphical interval without view, ×120, sample, SHod-16/1469, SHod-16; 4A. Pb element, lateral view, ×120, sample, Hod-c-2/6007, representatives of the taxon both SHod-17; 4B. Pb element, lateral view, characteristics of the denticles in the anterior process, ×800, in South China and Baltoscandia. sample, Hod-c-2/6008, SHod-17; 5A. Pb element, lateral view, ×90, sample, Jod-23/6053; 5B. Pb element, Lindström (1971) established anterior view, ×90, Jod-23/6054; 6A. Pb element, lateral view, ×90, Jod-23/6049; 6B. Pb element, oral the Paroistodus lineage, and he view, ×90, Jod-23/6048; 7. Sc element, lateral view, ×180, sample, Hod-c-2/6000, SHod-17; 8. M element, (Lindström, 1971) used species of lateral view, ×120, sample, SHod-16/1467, SHod-16; 9. Sb element, lateral view, ×120, sample, SHod- the genus as zonal index species. 16/1468, SHod-16; 10. Sa element, posterior view, ×180, sample, Hod-c-2/5999, SHod-16; 11. Sd element, In the Huanghuachang section, posterior view, ×180, sample, SHod-2/6012, SHod-2. Paroistodus proteus is present in triangularis Biozone i.e. in the basal Middle Ordovician. Younger the lower part of the succession and is succeeded by Paroistodus cf. species exist but are not yet formalized. The Gothodus lineage proteus in the upper part of the Lower Ordovician (Wang et al., 2005). probably is the ancestor to the Phragmodus lineage, which Paroistodus originalis is present in Baltoniodus navis Biozone of appeared in the Middle Ordovician (Dzik, 1994; Stouge and Bagnoli, the Middle Ordovician, where it can be abundant. Paroistodus 1999). originalis has been used as a zonal index species in South China The Microzarkodina lineage is well displayed in the sections in (Wang and Bergström, 1995) and in Baltoscandia (Lindström, 1971). China and Baltoscandia. The oldest known representative of the genus The P. originalis Biozone is an acme zone and the zones bearing the is Microzarkodina sp. A from the top of the Lower Ordovician and same name from respectively China and Baltoscandia are not Microzarkodina flabellum is the immediate successor (Bagnoli and isochronous. In South China the zone corresponds to the Baltoniodus Stouge, 1997, Wang et al., 2005). M. flabellum appears slightly above navis Biozone and in Baltoscandia the P. originalis Biozone is younger the base of the Middle Ordovician. Younger forms of the lineage and succeeds the B. navis Biozone (Lindström, 1971). A younger

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with C. deflexus, Tetragraptus bigsbyi,

Zone Acrograptus kurki, and directly section sample underlie the bed (Hod3) with the first

SHod-29 appearance of Azygograptus suecicus.

Shbod-2ch

SHod-28 Hod- E-4

sp.A The conodonts from the underlying 15m SHod-27 Hod- E-3 navis Baltoniodus and overlying limestones indicate that SHod-26 Hod-

Middle Member Middle E-2 14

Zone the boundary between the D. bifidus

Baltoniodus navis Baltoniodus

cf.

Paroistodus originalis Paroistodus SHod-25B SHod-25A Hod- and A. suecicus Biozones is within E-1 13

Middle Ordovician Middle SHod-24 SHod-23 Hod- the lower part of the upper O. evae SHod-22-B d-3

SHod-22-A Microzarkodina SHod-21-B 12 SHod-21-A Hod- SHod-20-C d-2 Biozone (Fig. 4).

SHod-20-B Hod- Gothodus costulatus SHod-20-A d-1

cf. sensu SHod-19-B Microzarkodina flabellum Microzarkodina The Azygograptus suecicus SHod-19-A

Hod- 11

Baltoniodus SHod-18 c-2-3 triangularis

Hod- triangularis Baltoniodus Biozone here is defined by the first SHod-17 c-2-2 Hod- part upper SHod-16 c-2-1 Hod- 10.57 SHod-15 c-1-4 appearance of the index fossil at the Hod- SHod-14 c-1-3 SHod-13 Hod- c-1-2

Paroistodus proteus P. SHod-12 Hod- Stouge,1997 & Bagnoli c-1-1 10 base of Hod3, 5.45 m above the base

sp.A SHod-11 Hod-B-3 of the Dawan Formation, and it ranges SHod-10 Hod-B-2 hd22 SHod-9 Hod-B-1 9 upwards to the lower part of the hd21 Microzarkodina SHod8 Hbod1 hd20 Paroistodus

Periodon SHod7-3 Hbod2-3 Middle Member of the Dawan SHod7-2 Hbod2-2 SHod7-1 Hbod2-1 hd19 SHod6-2 Hbod3-2 8 Dawan Fm. Dawan SHod6-1 Hbod3-1 Formation. It may be subdivided into SHod5-4 Hbod4-4 SHod5-3 Hbod4-3 hd18 SHod5-2 Hbod4-2 SHod5-1 Hbod4-1 two intervals based on the species hd17 SHod4 SHod4 7m

e SHod3 SHod3 cf.

upper part upper association of Azygograptus. The SHod2 SHod2

e SHod1 SHod1 Lower Member e # part middle lower interval, located between the Hod1 Hod1 e e e # 6 Hod2 Hod2 Zone e e e Hod3 and SHod-14, yields Azygo- e e e Hod3 Hod3 costulatus Gothodus e e Hod4 Hod4 Baltoniodus triangularis B. graptus suecicus, A. eivionicus, and e # e # Hod5 Hod5 # e Hod6 Hod6 5 e # Phyllograptus anna, and the upper # e e Hod7 Hod7 e # e Hod8 Hod8 e # # Hod9 Hod9 interval (SHod-15–29) Azygograptus Hod10 Hod10 evae Oepikodus

Periodon selenopsis Periodon

e Hod11 Hod11 4 e ellesi, A. suecicus, horizontal # Hod12 e # Hod12 e e Hod13 Hod13 e # Xiphograptus svalbardensis, Pseudo- e e Hod14 Hod14 Hod15 Hod15 e # e Hod16 Hod16 Baltoniodus e # trigonograptus sp. and Tetragraptus # e Hod17 Hod17 3

Periodon flabellum Periodon e # e e Hod18 Hod18

Lower Ordovician Lower # e # part lower sp. Associated conodonts in the Hod19 Hod19 e e e # Hod20 Hod20 e Hod21 section indicate correlation of the # e Hod21 e # Hod22 Hod22 2 e e e lower A. suecicus Biozone with the # e Hod23 Hod23 # e # Hod24 Hod24 e

e # Hod25 Hod25 part Lower major part of the upper O. evae # e e Hod26 Hod26 1 # e # Hod27 Hod27 conodont Biozone and the upper e e Hod28-3 Hod28-3 Periodon # Hod28-2 Hod28-2 e e Hod28-1 Hod28-1 # A. suecicus Biozone (SHod-15–29) # Hod29 Hod29 # 0m ee Hoh30-1 Hoh30-1 Hoh30-2 Hoh30-2 # occurs in the uppermost Lower

-1.5 e Gothodus Member to the lower Middle Member e e e Honghua- Fm. yuan of the Dawan Formation and roughly e e

Zone e e # # Calcirudite e bioclastic glauconite Shale with # # shale strata omitted limestone e limestone nodular limestone communis e # limestone Oepikodus corresponds to the B. triangularis to the lower Baltoniodus navis Biozones. Figure 10. Evolutionary conodont lineages across the Lower / Middle Ordovician boundary in the The graptolite succession docu- Huanghuachang section (modified from Wang et al. 2005). mented in the Lower Member of the species is Paroistodus horridus, which appears to be the youngest Dawan Formation and its relationship with relevant conodont and representative of the genus. graptolite biozones of the world have been discussed in detail by In the Chenjiahe section the same conodont assemblages and Wang et al. (2005, Table 1). The Didymograptellus bifidus Biozone lineages mentioned above are found across the suggested boundary herein is suggested to correspond to the D. “protobifidus” and the C. (Wang et al. 2005). deflexus Biozones in the Jiangnan slope, South China (Zhang and Chen, 2003), the Australian protobifidus and I. primulus Biozones Graptolites (Webby and Nicoll, 1989; Cooper and Lindholm, 1990), and the North American bifidus Biozone (Berry, 1960, Finney and Ethington, 1992). The graptolites of the Dawan Formation of the Huanghuachang It is correlated with the upper varicosus to lower simulans Biozones section have been studied by Wang et al. (1987, 1992a, 1995). The of Britain (Cooper et al. 1995) and the upper balticus to densus revised two graptolite biozones, Didymograptellus bifidus and Biozones of Baltoscandia on the basis of the presence of D. bifidus Azygograptus suecicus Biozones, can be recognized in the Lower to and C. deflexus. The overlying Azygograptus suecicus Biozone is lower Middle Members of the Dawan Formation (Figs.2-4) (Wang et widely recorded in the interval between the D. bifidus and lower al. 2004, 2005). The index fossil of Didymograptellus bifidus Biozone E. hirundo Biozones in the Yangtze platform and between the deflexus is present in four interbeds of yellow-green shale (Hod22, Hod18, and I. cauduceus imitatus Biozones in the Jiangnan slope of South Hod8 and Hod4) between thin-bedded limestones bearing conodonts China plate (Chen et al. 2003; Zhang and Chen, 2003). Azygograptus in the lower part to lower middle part of the Lower Member. The is considered to be diagnostic of the Isograptus-Azygograptus uppermost shale interbeds (Hod4) of 0.17m thick have D. bifidus radiation (Chen and Bergström, 1995) and one of the most

Episodes, Vol. 32, no. 2 106

Table 1. Correlation of the graptolite biozones across the Lower/Middle Ordovician boundary of the Huanghuachang section (modified from Wang et al. 2005)

South China Northwest Europe Huanghuachang section Jiangnan W. Newfound- N. America land, Canada Australasia graptolite conodont Slope Baltoscandia Britain Unit Fm Wang et al. 2005; Wang et al. 2005; Chen et al.2003; William & Stevens, Berry, 1960;Finney Webby & Nicoll, this paper Cooper et al. , 1995; Maletz, 2005 this paper Zhang &Chen,2003 1988 & Ethington,1992 1989

Baltoniodus Isograptus sp.n.2 Ca3 Isograptus Isograptus v. Isograptus v. Isograptus v. navis & Maeandrogr. gibberelus maximus maximus maximus schmalenseei

Middle Mb. Middle

Isograptus Baltoniodus caduceus Isograptus Isograptus Isograptus Isograptus Ca2 Upper trangularis imitatus v. victoriae v. victoriae v. victoriae v.victoriae Isograptus v. victoriae

Middle Ordovician Middle

Dawan Formation Dawan Ca1 Azygograptus Pseudophyllogr. D.(s.l.) Isograptus Isograptus Isograptus s uecicus angustifolius v. lunatus v.lunatus v. lunatus Lower elongatus simulans

Azygograptus suecicus

Lower Member

Upper

C. deflexus Oepikodus evae Oepikodus Ch2I . primulus Didymogr. Pseudophyllogr Didymogr. Didymogr. bifidus D. densus bifidus bifidus Ch1 D. "protobifidus" D. (s.l.) ¡¡protobifidus varicosus

Lower Lower Ordovician Lower P. fruticosus P. fruticosus D.(s.l.) balticus Be1-4 P. fruticosus P. fruticosus ? T. akzharensis T.akzharensis No Oepikodus Honghua- Phyllograp- T. T. yuan Fm. graptolite communis approximatus Phyllograptoides La3 toides approximatus approximatus T. approximatus

characteristic genera of the Atlantic Province during the “Arenig” 1992, 2005). These indicate that the boundary between the lunatus (Beckly and Maletz, 1991). It is widely reported from Britain, and victoriae Biozones in these localities mentioned above is nearly Scandinavia, Bohemia, northern Spain, Gorny Altai, southern coincident with the boundary between the lower and upper A. suecicus Bolivia and South China (Beckly and Maletz, 1991; Cooper and Biozone and close to the base of the Baltoniodus triangularis Biozone Lindholm, 1990). The A. suecicus Biozone of the Huanghuachang (SHod-16) in the Huanghuachang section (Table 1). This correlation section lies directly above the D. bifidus Biozone and about 9 m is also suggested by the graphic correlation of conodonts and below the U. sinodentatus Biozone at the base of the Upper Dawan graptolites in the boundary inteval from several sections worldwide Formation. This suggests the A. suecicus Biozone here is coeval with (Stouge et al., 2005, 2006). the lunatus to victoriae Biozones of Australia (Webby and Nicoll, 1989), North America (Berry, 1960) and Canada (Williams and Chitinozoans and acritarchs Stevens, 1988), and approximately equivalent to the Scandinavian elongatus to lowest hirundo (or victoriae) Zones, and the British The chitinozoans in the Dawan Formation were reported by Chen upper simulans Zone to the lower gibberulus Biozone or victoriae X.H. et al. (1996, 2002, 2003). Further sampling has shown that four Biozone (Cooper et al. 1995; Maletz, 2005). The upper Middle chitinozoan biozones can be recognized in the Lower and lower Member of the Dawan Formation, located between the A. suecicus Middle Members of the Dawan Formation (Wang et al., 2005). and U. sinodentatus Biozones in the Huanghuachang section, should The Lagenochitina esthonica Biozone is defined by the incoming correspond to Ca 3–Ca 4 of the Australian standard. It is important to of the index fossil in lowest O. evae Biozone (Fig.5). This species, note that the base of upper A. suecicus interval, characterized by the however, is widely recorded from the lowest Dawan Formation and occurrence of Azygograptus ellesi, is close to, or only 0.1m below correlative strata throughout the Yangtze platform with Conochitina the level with FAD of B. triangularis (SHod-16) in the raymondii, C. ordinaria, and these represent the primary radiation of Huanghuachang section. The succeeding graptolites, including the the chitinozoan fauna during the Early Ordovician (Wang and Chen, horizontal and declined Xiphograptus svalbardensis, Expansograptus 2003). Based on the co-occurrence of nominated species below the sp., Pseudotrigonograptus sp. found in its lowest part (SHod-15– first appearance of Conochitina langei the L. esthonica Biozone herein 22) are very common in the lower E. hirundo Biozone of Baltoscandia is suggested to correlate with the L. esthonica-Conochitina raymondii (Cooper et al., 1995) and the base of latter is considered very close Zone reported from Quebec and western Newfoundland, Canada to, if not coeval with, the base of the I. v. victoriae Biozone (Maletz, (Achab, 1989)

June 2009 107

The overlying Conochitina langei Biozone is defined by the FAD The common brachiopods include Leptella, Sinorthis, Pseudomimella, of the index fossil at bed Hod 24 and extends upwards to the FAD of Tritoechia, Noreidella, Pseudoporambonites, Euorthisina, Conochitina pseudocarinata at the bed of SHod-4. The chitinozoan Yangtzeella, Martellia and Lepidorthis (Wang et al., 2005; Zhen and biozone in the Lower Dawan Formation covers the interval of the Harper, 2006). Restudy further confirms that two biozones, the D. bifidus to the lower part of the A. suecicus Biozones or the lower Leptella Biozone and Euorthisina Biozone, are present in the Lower to lower upper O. evae conodont Biozone (Fig.5). The principal Member (Fig.5). The Leptella Biozone is characterized by the chitinozoans include Lagenochitina esthonica, Conochitina brevis, appearance of the index fossil at the base of the Dawan Formation. C. decipiens, C. poumoti, C. ordinaria, C. langei, Tanuchitina sp. Its upper limit is defined by the appearance of Euorthisina, although aff. achabae and Eremochitina baculata. Although the last species is Leptella itself may extend upwards to the uppermost part of the Lower considered as a diagnostic for northern Gondwana (Paris, 1990), the Dawan Formation. The Euorthisina Biozone is indicated by the chitinozoan assemblage is similar to that of the C. langei / C. brevis incoming of the zonal fossil with Pseudoporambonites in the upper Biozone reported from the Lévis Formation, Canada (Achab, 1986, part of the Lower Dawan Formation, close to the base of the (SHod- 1989). The C. langei Biozone at Huanghuachang is correlative only 16) of B. triangularis Biozone. Associated graptolites indicate the with the lower part of the C. langei / C. brevis Biozone based on correlation of Leptella Biozone with the D. bifidus Biozone to the associated graptolites and conodonts. lower interval of the A. suecicus Biozone. The overlying Euorthisina The Conochitina pseudocarinata Biozone is defined by the FAD Biozone corresponds to the upper interval of the A. suecicus Biozone of the nominate fossil at the bed of SHod–4. Most of the associated and extends upwards to the U. austrodentatus Biozone. chitinozoans range into the Biozone from below, except The trilobites from the Lower Member of the Dawan Formation Sagenachititina oblonga and Cyathochitina? sp. C. pseudocarinata have been reported by Zhou (in Wang et al., 1987), Turvey and Zhou and S. oblonga, which first appear in the biozone, are considered to (2002) and Turvey (2005). Re-study suggests the principal trilobites be representative of the D. ornensis Biozone in north Gondwana include Pseudocalymene, Carolinites, Agerina, Phorocephala, (Paris, 1990, 1996). The C. pseudocarinata Biozone at Ningkianolithus, Megalaspides, Liomegalaspides, Ovalocephalus, Huanghuachang, therefore, is correlated with the D. ornensis Biozone Taihuangshania, Aulacopleura (Paraaulacopleura) and in north Gondwana. The upper limit of the C. pseudocarinata Biozone Rhombampyx. Most of them have a long range, except of Agerina, is defined by the first appearance of Belonechitina cf. henryi at the which is mainly found in upper part of the Lower Member (Fig. 5). bed of SHod-16. Among these trilobites Carolinites genacenaca, which has been The B. cf. henryi Biozone is characterized by the first appearance reported from the Reutterodus andinus Biozone in North American and development of the nominated fossil together with B. micracantha (Ross et al., 1997), is found in the Lower and Upper Members of the and Conochitina kryos. This chitinozoan association indicates that Dawan Formation, respectively (Turvey and Zhou, 2002). Agerina the B. cf. henryi Biozone of the section is equivalent to the elongata first appears in the bed of SHod-15 at the Huanghuachang Belonechitina henryi Biozone in north Gondwana (Paris, 1990, 1996). section. This level is basically coincident with the boundary between In the Huanghuachang section the base of the B. cf. henryi Biozone the Lower and Upper A. suecicus Biozone and 0.1m below the base is very close to or coincident with the base of B. triangularis conodont (SHod-16) of B. triangularis Biozone, so that this species, together Biozone. with the index fossil and Pseudoporambonites of the Euorthisina Well-preserved and diversified acritarchs have been reported brachiopod Biozone, seem to be also considered as an auxiliary from the Lower Dawan Formation by Tongiorgi et al. (1995), Brock biomark indicating the ratified Lower and Middle Ordovician and Li (1999), Yin et al. (1998) and Li et al. (2003). The latest study boundary. indicates the correlation of the Arbusculidium filamentosum– Aureotesta clatirata Assemblage Zone with Didymograptellus bifidus Sequence- and eco-stratigraphy Biozone and the overlying Ampullula–Barakella felix Assemblage The study of sequence and ecostratigraphy facilitate a subdivision Zone (Li et al., 2003) with the Azygograptus suecicus Biozone the Lower Member of the Dawan Formation into three transgressive (Wang et al., 2004, 2005). to regressive cycles or sequences and related two brachiopod The chitinozoan sequences recorded in the continuous Lower to communities, namely, the Leptella (L.) - Sinorthis Community and lower Middle Dawan Formation and its relation with relevant Pseudoporambonites-Euorthisina Community (Zeng, 1991, 2006 in conodont and graptolite biozones in the Huanghuachang section prep.). The first and second cycles, showing a generally transgressive suggest that it is necessary to re-examine the time scales of the C. trend and deepening upwards (Wang et al., 2005), occupy the lower- langei / C. brevis Biozone in North America and the D. ornensis and middle parts of the Lower Member with the Leptella (L.) - Sinorthis B. henryi Biozones in north Gondwana, and their correlation with Community, indicating Benthic Assemblages 1-2 (BA1 in lower part relevant conodonts and graptolite Biozones (Webby et al., 2004, and BA2 in middle part) (Figs.5, 11). Main brachiopods include figures 2.1 and 2.2). In the Huanghuachang section the base of B. Leptella, Sinorthis, Martellia, Pseudomimella, Yangtzeella, triangularis conodont Biozone approved for the base of the Middle Nereidella, Tritoechia, Westonia and Nushbiella etc. An obvious Ordovician Series is close to the boundary between the lower and change in lithofacies, sequence and biological assemblage first appears upper A. suecicus Biozone, nearly coincident with the base of the between the glauconitic bioclastic limestone of 0.93 m with rich Belonechitina cf. henryi chitinozoan Biozone and within the conodonts and brachiopods at the base of the Dawan Formation Ampullula-Barakella felix acritarch Assemblage Zone. (Hod29–27) and the underlying thick-bedded dolomitic limestone Other fossils (Hoh 30-31) with Archaeocyphia, Calathium at the top of the Honghuayuan Formation. This sequence turnover was associated with Brachiopods are very common in the Lower Member of the Dawan the replacement of warm water biota by the cool-water mixed with Formation, and were studied in detail by Zeng et al. (1983, 1991). warm-water biota (Figs. 5, 11). The second cycle covered the middle

Episodes, Vol. 32, no. 2 108

part of the Lower Member, and sample section 13

C/% Zone contained brachiopods of BA2 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 T/R cycle community (Zeng, 2001, 2006 in prep). SHod-29 It starts at the base of yellow-green shale SHod-28 15m interbeds (Hod 5) with first appearance SHod-27

Baltoniodus navis Baltoniodus of Azygograptus suecicus. The SHod-26 SHod-26 R

Middle Member Middle 9 underlying highstand regressive thin- 14 upper

SHod-25B Zone SHod-25A cycle 3 bedded bioclastic limestone yields 13 SHod-24

SHod-23 Ordovician Middle numerous brachiopods. The second SHod-22-B

SHod-22-A Zone SHod-21-B 12 cycle was characterized by a smaller SHod-21-A SHod-21 8 SHod-20-C SHod-20-B sea-level rise succeeded by a gentle SHod-20-A SHod-19-B regression and terminated at the top of SHod-19-A T SHod-18 11

Baltoniodus triangularis Baltoniodus the 0.9 m-thick, highstand dolomitic SHod-17 SHod-16 10.57 SHod-15 limestone in the uppermost middle part SHod-14

Azygograptus suecicus Azygograptus SHod-13 SHod-12 of the Lower Dawan Formation (SHod- SHod-12 7 10 9–11). The lowstand deposit of the third SHod-11 sequence, marking the base of the upper SHod-10 SHod-9 part of the Lower Dawan Formation, is 9 SHod8 R SHod7-3 recognized as an important worldwide SHod7-2 SHod7-1 SHod6-2 8 Dawan Fm. Dawan SHod6-1 sea-level lowstand event in the Lower/ Lower SHod5-4 SHod5-3 SHod5-2 cycle 2 Middle Ordovician boundary interval SHod5-1 SHod5 6 SHod4 upper part (Ross & Ross, 1992, Nielsen, 1992, 7m e SHod3 2004). In the Huanghuachang section SHod2 e SHod1

Lower Member e # T Zone this event is recognized by the sequence Hod1 e e e # 6 Hod2 e e Hod2 change that occurred between SHod-11 e 5 e e e Hod3 and SHod-12. The succeeding e e Hod4 e # e # Hod5 transgressive deposits, covering the # e Hod6 5 evae Oepikodus e # # e e Hod7 interval from the uppermost Oepikodus e # Hod7 4 e Hod8 e #

# Hod9 Zone evae Biozone to the lowest Baltoniodus Hod10

Lower Ordovician

e Hod11 4 e triangularis Biozone (SHod-12–18), # Hod12 e # e e Hod13 e R are characterized by thin-bedded # e e Hod14 Hod15 e # nodular limestone with yellow-green e Hod16 lower part e # shale interbeds. The fauna includes # e Hod17 3 Hod17 3 e # e e Hod18 # e # Hod19 e conodonts i.e. Baltoniodus triangularis, e cycle 1 bifidus Didymograptellus e # Hod20 e Hod21 Microzarkodina sp. A and Periodon sp. # e e # Hod22 2 e e A, graptolites Azygograptus suecicus, e # e Hod23 # e # Hod24 Hod24 A. ellesi, some trilobites, and well- e 2 Lower part e # Hod25 T # e e Hod26 preserved brachiopods of the 1 # e # Hod27 e e Pseudoporambonites-Euorthisina Hod28-3 # Hod28-2 e e Hod28-1 Hod28 1 Community. The faunal associations # # Hod29 # 0m eeHoh30-1 Hoh30-2 indicate that a relatively deep and calm #

-1.5 depositional environment prevailed e

Zone e e

communis

Oepikodus upwards and equivalent to BA3. Among e Honghua- Fm. yuan e e # # Calcirudite e bioclastic glauconite e e # shale strata omitted brachiopod community Euorthisina is limestone e e limestone ##limestone a characteristic genus of the Euorthisina Figure 11. δ13C data on the carbonates from the Lower Dawan Formation and their relation with Community for Benthic Assemblages 3 sea-level changes in the Huanghuachang section. to 5 (Havlícek, 1982, Zeng, 1991). The maximum flooding is here interpreted to correspond to yellow-green Middle Member. The Upper Member of the Dawan Formation has a shale of 0.5 m (SHod-19) with graptolites in the lower part of the transgressive succession, deepening and fining upwards. A sequence upper Azygograptus suecicus Biozone (Fig.11). A minor regression change is recognized at a level between the Middle and Upper (SHod-20–21), in turn succeeded by a gentle sea-level rise, is Members of the Dawan Formation and supported by a change in the interpreted for the upper part of the upper A. suecicus Biozone, and it brachiopod communities from BA 2 in the Middle Member to BA 3 extends up to the uppermost part of the Lower Member (SHod-22– in the Upper Member (Zeng, 1991, 2006 in prep). 24). The purple medium-bedded limestone of the Middle Member of the Dawan Formation represents the highstand regressive deposits. A Stable isotopic record smaller sea-level rise is coincident with the last appearance of Azygograptus suecicus in the 1.5 m thick of yellow green shale with During the Early–Middle Ordovician, the Yangtze platform was limestone lens (SHod-29), 2.6 m above the base of the Middle covered by an epicontinental sea dominated by temperate to cool Member. Following regression covered the middle–upper part of the water carbonate accumulation. The Huanghuachang section, located

June 2009 109 in the north-central shallow to middle-shelf area of the basin, has a approximates the boundary between the lower and upper intervals of continuous Lower to Middle Ordovician sequence dominated by the Azygograptus suecicus Biozone and is nearly coincident with the carbonates with minor argillaceous deposits. The carbon isotopic data base of the Belonechitina henryi chitinozoan Biozone (Fig.5). obtained from whole rock samples are analyzed bythe MAT 251 mass The Huanghuachang GSSP is located in a roadside exposure at spectrometer in the Isotopic Geochemistry Laboratory of Yichang the base of the bed SHod-16. The boundary level is 10.57m above Institute Geology & Mineral Resources. The result shows in general the base of the Dawan Formation, and 0.6 m above the top of the a steady rising trend during most of the O. evae Biozone (Fig.11). distinctive 0.9 m-thick, highstand dolomitic limestone (SHod-9–11) The δ13C curve begins with a low excursion of around 0‰ in the with the maximum carbon isotopic excursion related with the lowest Dawan Formation and then follows a steady rise of δ13C values worldwide lowstand event in the uppermost part of the Lower Member until the maximum value of +0.68 ‰ close to the boundary between of the Dawan Formation. The boundary level falls within a the middle and upper parts of the Lower Member of the Dawan transgressive succession characterized by thin-bedded nodular Formation. Higher in the section, the δ13C values are constant, but limestones with graptolite-bearing yellow green shale interbeds define two small peaks in the upper part of the Lower Member to the (Fig.3). Its geographic coordinates are: latitude 30°51'37.8"N; lower part of the Middle Member. The isotopic composition of the Longitude 110°22'26.5" E of Greenwich. Lower Member reveals a significant relation of positive isotopic shifts with biotic diversification and sea-level fluctuation. The lowest Global correlation of the GSSP δ13C value is found to be associated with the transgressive facies at the base of the Dawan Formation and the related biotic change (Fig.11, The base of the Middle Ordovician Series and the Dapingian Stage No.1). The warm water biota was replaced by a predominately is within a stratigraphic interval that is characterized worldwide by a cool-water biota. The two succeeding positive excursions (Fig.11, very high degree of biogeographic provincialism and palaeoecological Nos.2–3) during the first transgressive to regressive cycle may reflect differentiation (Ross & Ross, 1992; Webby 1998, 2004; Finney 2005). changes in carbonate sedimentation and organic productivity. A Conodonts and graptolites are the fossil groups most used for relatively higher δ13C value (No. 4) is coincident with the sea-level correlation of Ordovician strata, in large part because of their wide fall preceding the FAD of Azygograptus suecicus. global distributions. Nevertheless, in the boundary interval North An interesting change is the maximum carbon isotopic positive Atlantic and Midcontinent province conodont faunas and Atlantic excursion (Fig.11, No.7), which is coincident with a sea-level lowstand and Pacific province graptolite faunas characterize stratigraphic and an accumulation of various organic-shelly fossils at the successions of different paleoplates and paleoecological setting. The Huanghuachang section. This is interpreted to be related to the eustatic conodont and graptolite faunas in the Huanghuachang section are sea-level lowstand event found in the lowest Whiterockian of North dominated by conodont species of the North Atlantic province and America (Ross & Ross, 1992) and the lowest Volkhov of Baltoscandia graptolite species of the Atlantic province. Nevertheless, the (Nielsen, 1992, 2004). It is worth noting that this event in the Huanghuachang GSSP can be correlated into the zonations of continuous sedimentary succession of the Huanghuachang section Midcontinent province conodonts and Pacific province graptolites was preceded by the speciation event represented by the FAD of with confidence and considerable precision. B. triangularis. Further up section are two smaller positive isotopic The base of the B. triangularis Zone in the Baltoscandia (North excursions (Fig.11, Nos.8–9), which may be related to changes in Atlantic province) zonation was suggested to coincide with the base carbonate sedimentation and produced by the smaller sea-level pulses. of Tripodus laevis Biozone in the North American (Midcontinent province) zonation based on considerable available evidence Organic maturity (Bergström, 1995). Both were proposed as the biohorizon for the definition of the base of the global Middle Ordovician Series (Webby, The organic maturity of the Huanghuachang section was studied 1994). The conodont succession identified in the Lower Member of using CAI, reflectance of graptolites and chitinozoans, and IR-studies the Dawan Formation of the Huanghuachang section further (Wang et al., 1992b, 1993). The vitrinite reflectance equivalent is demonstrates that the base of the B. triangularis Biozone can easily estimated 0.9–1.1% Ro, and the CAI is 2–3, indicating that the be recognized in both the North Atlantic Realm and the Mid- Huanghuachang section is suitable for the palaeomagnetic and continent Realm (Table 2). The occurrence of M. flabellum in the geochemical study. Huanghuachang section demonstrates that the boundary coincides with the base of the T. leavis Biozone in type localities for the North Identification of the Huanghuachang GSSP American Whiterockian and the Ibexian sections (Ethington & Clark, 1982, Ross et al., 1997). It also correlates with the base of the T. The Huanghuachang GSSP has excellent and well-known leavis Biozone in the upper San Juan Limestone of Precordillera, conodont, chitinozoan, graptolite, acritarch, and trilobite, brachiopod San Juan, Argentina (Albanesi et al., 2003, 2006) based on the fossil records in a stratigraphically continuous succession. Its conodont presence of diagnostic fossils typical of the underlying Oepikodus succession across the Lower and Middle Ordovician boundary interval evae Biozone, e.g. Reutterodus andinus, Oepikodus intermedius and is well developed with several evolving lineages (Figs.3,10). The Microzarkodina sp. A. The conodont succession across Lower–Middle stratigraphic level of the GSSP coincides with the FAD of Baltoniodus Ordovician boundary at Huanghuachang section clearly shows that triangularis, where this species develops from its immediate ancestor, the base of the B. triangularis Biozone correlates with the base of B. B. cf. triangularis. This datum is also coincident with the FAD of triangularis Biozone at the base of the Volkhov Stage of the Periodon sp. A, and followed by the FAD of Microzarkodina Baltoscandian region (Bagnoli and Stouge, 1997) and northwestern flabellum, which all are very useful for global correlation of the Russia (Tolmacheva et al., 2001, 2003, Dronov et al., 2003). The boundary. The proposed boundary level in the Huanghuachang section occurrence of Azygograptus ellesi, together with Xiphograptus

Episodes, Vol. 32, no. 2 110

Table 2. The biostratigraphic correlation across the Lower / Middle Ordovician boundary between the recommended GSSP with the Jiangnan Slope, China, North Europe, North America, South America and Australia (Modified from Wang et al. 2005).

South China Northeast Europe North America South America Australasia

Huanghuachang section Jiangnan Slope Baltoscandia NW. Russia Great Basin W. Newfoundland Argentina

Chitinozoan Fm. Unit Conodont zones Graptolite zones Graptolite zones Conodont zones Conodont zones Conodont zones Conodont zones Conodont zones Graptolite zones

Ordovician Series Ordovician zones (An 1987; (Lindström 1971; (Wang et al. 2005; (Wang et al. 2005; (Chen X. et al.,2003; (Viira et al. 2001; (Ross et al. (Stouge & Bagnoli 1988; (Albanesi 1998; Wang et al. 2005; Löfgren 1995; (Webby & Nicoll, this report) this report) Zhang and Chen,2003 ) Tolmacheva 2001) 1997) Johnston & Barnes 1999) Albanesi et al. 2003) this report) Bagnoli & Stouge 1997) 1989)

Ca3 Baltoniodus Histiodella Baltoniodus Baltoniodus Not Zoned Baltoniodus Isograptus (v.). navis altifrons navis navis navis maximus

Belonechitina Isograptus cf. henryi cadeus imitatus

Ca2 Tripodus Tripodus Middle Ordovician Baltoniodus Baltoniodus Baltoniodus Isograptus (v.). triangularis triangularis triangularis laevis laevis Tripodus victoriae laevis suecicus Azygograptus Dawan Dawan Microzar- Ca1 Conochitina Azygograptus kodina Isograptus (V.) pseudocarinata suecicus sp. A. lunatus

Lower Upper Oepikodus Ch2 Corymbograptus intermedius Isograptus primulus deflexus Trapezo- Oepikodus Reutterodus gnathus Conochitina evae andinus Lower MemberLower Didymograp- Middle Member diprion Ch1 langei Didymograp- Oepikodus tellus Didymograptellus tellus evae Oepikodus bifidus evae Oepikodus evae Oepikodus protobifidus protobifidus Oepikodus

Lower Upper Oepikodus evae Be1-4 Pendeograptus evae Pendeograptus fruticosus fruticosus Lower Ordovician Lower Lagenochitina Oepikodus estonica Prioniodus Prioniodus Oepikodus Prioniodus Prioniodus communis No graptolite elegans elegans communis elegans elegans data La3 Hong- Tetragraptus Tetragraptus approximatus Acodus deltatus Prioniodus adami approximatus huayuan Serratognathus Conochitina Paroistodus Paroistodus - Paroistodus diversus symmetrica proteus proteus Oneotodus proteus costatus Prioniodus oepiki svalbardensis, Expansograptus sp., Pseudotrigonograptus sp. in the developed with several evolving lineages (Figs.5, 10). The proposed upper Azygograptus suecicus Biozone, moreover, allows for the boundary coincides with the FAD of B. triangularis, which has long correlation of the base of B. triangularis Biozone at the been used in other important Ordovician successions worldwide. It Huanghuachang section with the level close to the base of Isograptus can be directly correlated with zonal level precision into the graptolite v. victoriae graptolite Biozone (Ca2) of the Australia standard (Webby succession in both Atlantic and Pacific provinces on the occurrence and Nicoll, 1989; Wang et al., 2005). of graptolites in the Huanghuachang section (Finney, 2006), so that The worldwide lowstand event preceding the FAD of the B. it can be recognizable worldwide in both shelly and shale facies. triangularis in the Huanghuachang section and the related carbon isotopic positive excursions are auxiliary physical and geochemical Summary and Formal Proposal global markers. Graphic correlation was carried out for conodont and graptolite The proposed Global Stratotype Section and Point for the base of data from 20 sections that represent a great range of biotic provinces the Middle Ordovician Series and the lower stage of the Middle and depositional environments for conodonts and both Atlantic and Ordovician Series at the Huanghuachang section, near Yichang, Hubei, Pacific provinces for graptolites. The correlations resulting from the China fulfills all the basic requirements for a GSSP as follows: graphic correlation are consistent with those proposed by Cooper and Lindholm’s (1990) for graptolites, and further confirm that the 1. The proposed GSSP is accessible and well protected. It is 22 km FAD of Baltoniodus triangularis in the Huanghuachang section away from Yichang City and well exposed in the Ordovician Geo- correlates with a level close to the base of the I. v. victoriae graptolite park near the main road. A stone marker indicating the base of Biozone (Fig.12). This permits the series boundary to be correlated the Dawan Formation was set up in 1978 and a new big protective with precision and confidence into graptolite successions with Pacific marker erected on the upper Lower Dawan Formation for ensuring province faunas (Stouge et al., 2005, 2006). its protection in 2004. It is concluded that the Huanghuachang GSSP has excellent and 2. The boundary interval is well exposed with adequate thickness. well-known conodont, chitinozoan, graptolite, acritarch and other The structure is simple and the section is without serious structural shelly fossil records in a stratigraphically continuous succession. Its disturbances. conodont succession across the proposed boundary interval is well 3. The boundary is within a transgressive sequence and within an

June 2009 111

Microzarkodina flabellum, close to the boundary between the m a u d l l n m e lower and upper Azygograptus suecicus Biozone and nearly a s u b i l r v a u a l

f a g

s n n Belonechitina henryi a coincident with the base of the cf.

u a s n h r i t u d

25 d a d a o n

o chitinozoan Biozone. Thus, this biohorizon at the GSSP can be k u i g r i q n r

a o e s z t l p u o recognized and correlated globally with high precision on the a r o h t c B o i a C M n s basis of a number of speciation events. g u c o i z d e n sp. A sp. p laevis a l a r r cf. r T o

n The present paper, modified from the proposal to ISOS (Wang

Arienigraptus gracilis Arienigraptus Isograptus (v.) maximodivergens (v.) Isograptus a s v u Periodon Periodon r

20 d a et al. 2005) and final report to ICS and IUGS in January, 2007, was Tripodus Tripodus o p i

n a o n s t i i l submitted in August 2007 after the Huanghuachang section was m s d a u n o l B s l i k e r e s z r a b a approved as the GSSP. Azygraptus suecicus z e a b i l i o f g v

r n.sp. B n.sp.

z c e n

i ? r B. triangularis B. o n b M d

o s o cf. cf. d i u r Baltoniodus triangularis Arienigraptus hastatus Arienigraptus o sp. A i d e r a o P e l Isograptus (v.) victoriae (v.) Isograptus n i u P s d g

15 u Baltoniodus o i n k d a

r Acknowledgements i e r a Baltoniodus s T z m u r t o r aranda e a t c i n n cf. Xiphograptus svalbardensis Xiphograptus i u

l M Research on the Huanghuachang GSSP and preparation of this

s ) . u v d (

sp. o proposal was financially supported by the China Geological Survey s k s i u u t p d p e Expansograptus hirundo Expansograptus dumosus Pseudisograptus o a s

O (NO: 200113900072), National Natural Science Foundation of China r p u i g r s o T Protoprioniodus Protoprioniodus o s c I i (NO£º40272020), the Germany Academic exchange Centre (DAAD), t

10 e c a u v r f e

Chinese Commission of Stratigraphy and Yichang Institute of Geology s s u u t d p o Didymograptus bifidus Didymograptus

a and Mineral Resources (YIGMR). We are greatly indebted to Chen r k i g p o e e Xu and S.M. Bergström for his suggestions and comments on the O d n e

P proposal and to S.M. Bergström, F. Paris and Zhou Zhiyi for their Conodonts examinations of conodonts, chitinozoans and trilobites, respectively.

5 filiformis Acrograptus Wang Xiaofeng and his research group also desire to express their grateful thanks to YIGMR for their long-term support to this research. S. Stouge is grateful for the financial support provided by the Carlsberg foundation, Denmark. 1 Graptolites 0 References Figure 12. Ranges of selected species from the composite section Achab, A., 1986. Assemblages de chitinozoaires dans I’Ordovicien inferieur achieved by means of graphic correlation between 20 Lower–Middle de l’est du Canada. Canadian Journal of Earth Sciences, 23: 682-695. Achab, A., 1989. Ordovician chitinozoan zonation of Quebec and Western Ordovician Series boundary sections with the high palaeolatitude Newfounland. Journal of Palaeontology, 63(1): 14-24. faunas or low palaeolatitudinal faunas, respectively Albanesi, G. L. and Carrera, M. G. 2001. Niquivil section of Central Precordillera, Argentina, proposed as the Global Stratotype for the base of the Middle Ordovician Series. Geological Society of America. Abstracts interval of uniform lithology, indicating continuous deposition with Programs, 33(6): 446-447. in the boundary interval. Albanesi, G. L., Carrera, M.G., Cañas, F.L. and Saltzman, M. 2003. The Niquivil section, Precordillera of San Juan, Argentina, proposed GSSP 4. Various fossil groups, including conodonts, graptolites, for Lower/ Middle Ordovician boundary. In: Albanesi G.L., Beresi M.S. chitinozoans, acritarchs, trilobites and brachiopods are well and Peralta S.H. (eds): Ordovician from the Andes, INSUGEO, Serie represented in the boundary interval and allow for correlation Correlación Geológica, 17: 33–40. into both North Atlantic and Midcontinent conodont provinces, Albanesi, G.L., Carrera, M.G., Cañas, F.L. and Saltzman, M. 2006. A proposed and Atlantic and Pacific graptolite provinces. Global Boundary Stratotype Section and Point for the base of the Middle Ordovician Series: the Niquivil section, Precordillera of San Juan, 5. The conodonts are very abundant and diversified with several Argentina. Episodes, 29(1): 1–15. evolving lineages in the boundary interval. Associated graptolites An, T. X. 1987. Paleozoic conodonts of southern China. Beijing: Science plus well-preserved and abundant chitinozoans provide auxiliary Press: 1–238. markers for the recognition and correlation of the boundary Bagnoli, G. and Stouge, S. 1997. Lower Ordovician (Billingenian-Kunda) conodont zonation and provinces based on sections from Horns Udde, biohorizon in both shelly and graptolite facies. north Öland, Sweden. Bollettino della Società Paleontologica Italiana, 6. The continuation of several evolving conodont lineages and the 35(2), 109–163, 8 pls. range of many additional species of conodonts, graptolites and Beckly, A.J. and Maletz, J. 1991. The Ordovician graptolites Azygograptus chitinozoans across the boundary indicate the absence of any and Jishougraptus in Scandinavia and Britain. Palaeontology, 31(4): 887– significant hiatus at the boundary. 925. Bergström, S.M. 1995. The search for global biostratigraphic reference levels 7. The CAI values and the reflectance of organic-walled fossils in the Ordovican System: regional correlation potential of the base of the indicate that the section is suitable for palaeomagnetic and North American Whiterockian Stage. In: Cooper, I.D. Droser, M.L. & chemostratigraphic studies. Finney, S.C. (eds.). Ordovican Odyssey: Short papers for the Seventy 8. The base of the Middle Ordovician Series and the Third Stage International Symposium on the Ordovician System. SEPM, pacific Section, Book 77: 149–152. named Dapingian Stage coincides with the first appearance of B. Bergström, S.M. 2006. Comments on GSSP candidate sections and the triangularis at the base of Shod16, 10.57m above the base of the placement of the Lower/Middle Ordovician Series boundary. ISOS Dawan Formation. This level is 0.2 m below the FAD of Ordovician websites, 1–3.

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