1 Center for Stratigraphy and Palaeontology, China Geological Survey and Yichang Institute of Geology and Mineral Resources, Yichang, Hubei 443003, China. E–mail:wangxf@public.yc.hb.cn

Key words: Conodonts. Graptolites. Lower/Middle Ordovician boundary. Huanghuachang. China.

The work on the definition of global Series and Stages of the Ordovician is now focused on the selection of the GSSPs for the base of the Middle Ordovician Series. The Whiterock Narrows section (Finney and Ethington, 2001), Niqivil section (Albanesi and Cerrera, 2001) and Huanghuachang section (Wang et al., 2002) are subsequently recommended as the GSSPs for the base of the Middle Ordovician Series. Based on the detailed research and assessment of all information now available it is considered that the Huanghuachang section near Yichang, China is superior to the two other sections as a global boundary stratotype. The boundary interval has excellent and well–known conodont, chitinozoan, graptolite, acritarch, brachiopod and trilobite biostratigraphy in a stratigraphically continuous sedimentary succession, and is well exposed at main roadside, 12km from NE Yichang City. It is easily accessible. A proposed biozone boundary, regardless if it is taken as the base of the Baltoniotus triangularis Biozone or the base of Tripodus laevis Biozone, can be recognized in the section and correlated globally. The Huanghuachang section, therefore, should be given serious consideration as a global boundary stratotype for the Middle Ordovician base.

The proposed section is a typical section for the Ordovician subdivision and correlation in China with long studying history (Zhang et al., 1957; Mu et al., 1979; Zeng et al., 1983; Wang et al., 1987, 1992, 1996; Chen X. et al., 1995). Geographically the Huanghuachang area is on the east limb of the Huangling Arch, belonging to the central–north Yangtze carbonate platform. The texture is very simple. Its Ordovician sequence is complete, and composed mainly of about 330 m thick of carbonate rocks intercalated with some clastic rocks, and yields abundant and various fossils. Detailed litho–, bio– and sequence–stratigraphic logs were measured for the section and neighbour Chenjiahe (Daping) section (Zeng et al., 1983; Wang et al., 1987, 2002; Chen et al., 1996, 2002). Many Ordovician specialists at home and abroad have visited the section since 1978. The Dawan Formation at the Huanghuachang section is subdivided into 3 members (Figure1) (Zeng et al., 1983). First, the lower member is interpreted transgressive deposit of deeping and fining upward (Wang et al., 1996). It is composed of grayish thin–bedded bioclastic limestone, glauconitic limestone with yellow–green shale interbeds of 16 m thick, showing a mixed composition of shelly fossils with graptolites and of the cool water biota with warmer water biota (Wang and Chen, 1999). The conodonts, trilobites, brachiopods, cephalopods mainly occur in the thin–bedded limestone and glauconitic limestone, and graptolites, chitinozoans, acritarchs in the shale interbeds.

Figure 1. Sketch geological map in Yichang Area, Hubei, China.

Figure 2. Showing stratigraphic ranges of major conodonts (Li Zhihong), chitinozoans (Chen and Wang) and graptolites (Wang and Wang), and their sampled position at the Huanghuachang section.

 Second, the underlying Honghuayuan Formation compresses a set of highstand thick–bedded limestone, yielding conodonts of the P. proteus Biozone and archaeocyphia, Calathium in the lower– middle parts and conodonts of the O. communis Biozone in the uppermost part. The conodonts from the lower member of the Dawan Formation were ever studied by several colleagues (Zeng et al., 1983; An et al., 1987; Ni in Wang et al., 1987; Wang Z. et al., 1996). The Lower/ Middle Ordovician boundary is revised and fixed at the base (Hod 19) of the B. triangularis Biozone, 2.6 m above the base of the Dawan Formation on the basis of recent study and the proposal for the base of global Middle Ordovician. T. laevis first appears at level (Hod 17), 0.2m above the base of B. triangularis Biozone. The middle member of the Dawan Formation is characterized by the incoming of highstand purple medium–bedded micrites and mudstone, and is 13 m thick. It yields conodonts of the P. originalis Biozone with numerous cephalopods and brachiopods. The upper member of the Dawan Formation consists of 28 m thick of grayish thin–bedded nodular argillaceous limestone, mudstone of the TST, intercalated with yellow–greenish shale, and yields the graptolites of the Undulograptus sinodentatus Zone at base and of the U. austrodentatus Zone at top. Coexisted fossils include trilobites, brachiopods and conodonts of the Microzarkodina parva Zone. The Middle and Upper Dawan Formations are thought to be equivalent to the Yapeenian to lowest Darriwillian in age. The sequence of the Dawan Formation is widespread in the Yangtze platform. In the Yichang area there are at least another 3 relevant sections exposed around the Huangling Arch, such as the Chenjiahe (=Daping) section, Xintan section and Jianyangping section. They are easy to correlate with the Huanghuachang section (Figure1).

The conodont assemblage from the Lower Dawan Formation is characterized by the radiation and rapidly evolution of compound type conodonts (Figure 2). Both the warmer water (Midcontinental Realm of North American) (MRNA) conodont indicators, O. communis, O. intermedius, J. variabilis, T. laevis (Finney and Ethington, 2001) and the cool water (the N. Atlantic Realm) (NAR) ones, O. evae, B. triangularis (Lindström, 1955, Löfgren, 1994), as well as same representatives of the inter–mingling conodont each other reported from West Newfoundland, Canada (Stouge and Bagnoli, 1988, Johnston and Barnes, 1999) are all found together with the graptolites and chitinozoans etc. Recent detailed study further identifies those three conodont biozones, i.e., the O. evae Biozone, the B. triangularis Biozone, the B. navis Biozone, can be recognized in the Lower Dawan Formation. The bases of them are separately defined at 0.6m (Hod–27), 2.6m (base of Hod–19) and 6.58 m (Hod–0) above the base of Dawan Formation. The conodont succession is considered to be a unique of complete and well–represented sequence across the Early to Middle Ordovician boundary, providing an excellent evolving lineage from Baltoniodus crassulus to B. triangularis via Bltoniodus tetrastichus Li sp.nov.. In general the Baltoniodus crassulus is considered the ancestor of B. triangularis as indicated by Klapper et al. (1977, p.81). Detailed collection and study reveals that Baltoniodus triangularis would directly evolve from Bltoniodus tetrastichus Li sp. nov.. Their stratigraphic range and phylogenetic relation are discussed by Wang et al., (2003 in this volume)(Figure2). The FAD of B. triangularis, therefore, is proposed as conodont icon for the definition of the Middle Ordovician base. The FAD of T. laevis at 0.2m above the boundary can be taken as a reasonable proxy for the boundary correlation. The proposed Middle/ Lower boundary as so defined can be recognized in both the NAR and MRNA and correlated precisely.

Two graptolite zones, C. deflexus and A. suecicus Zones, were recorded in the Lower Member of the Dawan Formation by Wang et al. (1987, 1992). Recent study found that D. protobifidus is also present in the original C. deflexus Zone. The graptolites of the C. deflexus / D. protobifidus Zone revised are only found in the very thin–bedded interbeds of yellow– green shale from 2.7 m (Hod–18) to 5.4 m (Hod–4) above the base of the Dawan Formation. The interval covers the range of the most D. triangularis Biozone (Figure 2). The base of the A. suecicus Zone is overlain directly on the beds with graptolites D. deflexus and D. protobifidus in the lowest B. navis Biozone. The chitinozoans in the Dawan Formation were studied by Chen et al. (1996, 2002). Further sampling and studying identify that the C. langei / C. brevis Biozone are recognized in the interval from the O. evae to the middle B. triangularis Biozone, but the level (Hod–22) with FAD of C. brevis is about 0.8m above that of C. langei (Hod–24) and 0.6m below the base of the B. triangularis Biozone. The principal chitinozoans comprise Conochitina cf. raymondii, C. ordinaria, C. primitiva, C. brevis, C. poumoti, C. decipiens, Belonechitina cf. micracantha, Lagenochitina esthonica etc (Figure 2). The chitinozone assemblage of the biozone is similar to that reported from the C. langei / C. brevis Zone of the Levis Formation, Canada (Achab, 1989). Eremochitina baculata is a typical element for the C. deflexus Zone in the N. Gondwana (Paris, 1996). At the Huanghuachang section it occurs at the level (Hod–8), 2 m higher than the FAD of B. triangularis, together with graptolites of the C. deflexus/ D. protobifudus Zone. Numerous well–preserved and diversified acritarches of the Arbusculidium filamentosum – Aureotesta clatirata A.Z. have been frequently reported from the revised C. deflexus/ D. protobifidus Zone of Lower Dawan Formation (Yin et al., 1998; Li et al., 2002). Overlying Ampullula first appears in the A. suecicus Zone together with the FAD of Barakella felix (Li et al., 2002). These conodonts, graptolite, chitinozoan and acritarch assemblages in the Lower Dawan Formation indicate that the base of the B. triangularis Biozone may be separately close to the Middle C. langei / C. brevis Chitinozoan Biozone or the base of the C. deflexus /D. protobifidus Graptolite Biozone, and approximately corresponding to the base of the Arbusculidium filamentosum – Aureotesta clatirata A.Z.

Numerous brachiopods and trilobites are documented from the Lower Dawan Formation by Zeng et al., (1983) and Zhou (In: Wang et al., 1987). The Leptella brachiopod Zone and Pseudocalymene cyclidrica trilobite zone are proposed for the interval from the O. evae to B. triangularis Biozones.

Study of CAI value, the reflectance of graptolites, chitinozoans and IR–studies (Wang et al., 1993) indicate that the Huanghuachang section be suitable for a paleomagmatic or chemostratigraphic study with the vitrinite reflectance equivalent of 0.9–1.1 % Ro and CAI value of 2–3.

Our study has further confirmed that the Huanghuachang section fulfills the requirements for a GSSP. The proposed GSSP for the Middle Ordovician base is exposed in the roadside, at the base of Bed 19 (Hod–19), 2.6m above the base of the Dawan Formation in the measured Huanghuachang section, near Yichang, coinciding with the FAD of Baltoniodus triangularis (Lindström, 1955), and 0.2 m below the level with FAD of Tripodus laevis Bradshaw, 1969, associated with chitinozoans of the C. langei / C. brevis Biozone, graptolites of the C. deflexus/ D. protobifidus Biozone, acritarchs of the Arbusculidium filamentosum – Aureotesta clatirata A.Z., brachiopods of the Leptella Biozone and trilobites of the Pseudocalymene cyclidrica Biozone across the boundary interval. The boundary proposed, therefore, can be easily recognized and correlated globally with high precise, either in relatively shallow–water carbonate facies or deep–water or in graptolite facies.

This research is financially supported by the NSFC, CCS and CGS during last four years. Special thanks are expressed to Stan Finney, Guillermo L. Albanesi, Berry Webby, Stig Bergstöm, Wang Zhihao, Svend Stouge and Florentin Paris for their suggestions, comments and examinations of conodonts and chitinozoans respectively.

Accepted: June 15, 2003