The Early–Middle Ordovician graptolite sequence of the Upper Yangtze region, South China
Zhang Yuandong1 and Chen Xu1
1 State Key Laboratory of Palaeobiology and Stratigraphy (Nanjing Institute of Geology and Palaeontology, CAS), Nanjing 210008, P.R. China. E–mail: email@example.com / firstname.lastname@example.org
Key words: Graptolites. Biostratigraphy. Early–Middle Ordovician. Yangtze region. China.
The Upper Yangtze region yields rich early to middle Ordovician graptolites of ‘Atlantic’ affinity. In this region, the Tremadoc strata are dominated by carbonates, in which graptolites are rare and difficult to be encountered, whereas the strata of post–Tremadoc to early Caradoc are dominated by graptolitic shales with intercalated limestones. Since the 1960’s, considerable work has been done to discover graptolites from a variety of localities (Lee and Chen, 1962; Zhang et al., 1964; Mu et al., 1979). Based on these collections, Mu et al. (1979) were able to propose a "standard" graptolite succession for the region, although there are diverse arguments about the taxonomy of the graptolites. Wang X.F. (1982) and Fu (in Lai et al., 1982) systematically discussed the sequence and its correlation.
In recent years we, together with some of our colleagues from Nanjing Institute of Geology and Palaeontology, recollected some of these reference sections in the border area between Guizhou and Sichuan Provinces, including particularly two sections in Sichuan: the Shuanghe section in Changning (Figure1, Loc.7) and Nayangqing in Gulin (Loc.9), and four sections in northern Guizhou: the Honghuayuan section in Tongzi (Loc.14), the Shizipu (Gaoqiao) section in Zunyi (Loc.15), the Wulipo section in Meitan (Loc.16) and the Shatuodu section in Yanhe (Loc.12). Based on a study of the graptolite specimens from these sections, as well as a restudy of some of those (stored in NIGP and KUST) from other sections, a revised graptolite biozonation scheme for the Upper Yangtze region is proposed herein.
Early to Middle Ordovician graptolite biozonation of the Upper Yangtze region
From uppermost Tremadoc to the top of Darriwilian, 13 graptolite zones are recognized in the upper Yangtze region (Figure 2 and Figure 3).
1) Hunnegraptus copiosus Zone. This zone is defined by the FAD of Hunnegraptus. The confirmed specimens of Hunnegraptus were first discovered by Wang G. (1981) from the top shales of the Tungtzu Formation in the Daping section of Gulin, S. Sichuan. The zone was introduced to ‘Yangtze Craton’ by Wang X.F. and Wang C.S. (2001) in their correlation table (Table 2). Our investigation produced several additional specimens of this genus from the same level at the Nayangqing section, tens of kilometers to the east of Daping, which confirmed the identification of the zone. Although no specimens of H. copiosus have been found in the region so far, we see no reasons to create another zonal name than adopting the name from Scandinavia. Species of Hunnegraptus have also been discovered from North America and South America, making the zone internationally recognizable.
Figure 1. The main sections of early–middle Ordovician graptolites in Upper Yangtze Platform and Jiangnan Slope of South China, on which our present biozonation is based. Localities: 1. Zhaojiaba, Ningqiang, Shaanxi; 2. Zhongliangshan, Nanzheng, Shaanxi; 3. Yangjiaba, Chengkou, Sichuan; 4. Jianyangping, Xingshan, Hubei; 5. Fenxiang, Chenjiahe and Huanghuachang of Yichang, Hubei; 6. North of the Yanjin County town, Yunnan; 7. Shuanghe, Changning, Sichuan; 8. Dashiban, Weixin, Yunnan; 9. Nayangqing, Gulin, Sichuan; 10. Yanzikou, Bijie, Guizhou; 11. Qijiang, Sichuan; 12. Shatuodu and Ganxi, Yanhe, Guizhou; 13. Datianba, Xiushan, Sichuan; 14. Honghuayuan, Tongzi, Guizhou; 15. Shizipu of Gaoqiao, Zunyi, Guizhou; 16. Wulipo and Xinglongchang, Meitan, Guizhou; 17. Yinwuxi, Sinan, Guizhou; 18. Shalang, Kunming, Yunnan; 19. Datang, Sandu, Guizhou; 20. Taoyuan, Hu’nan; 21. Wuning, Jiangxi; 22. Yixian, Anhui; 23. Ningguo, Anhui; 24, Lin’an, Zhejiang, 25. Zhuji, Zhejiang; 26. Huangnitang, Changshan, Zhejiang; 27. Jiangshan, Zhejiang; 28. Chenjiawu, Yushan, Jiangxi.
2) Tetragraptus approximatus Zone. In SW China, this zone has been identified only from Datang section in Sandu of S. Guizhou, where is actually a part of Jiangnan Slope. In the entire Yangtze region, this stratigraphic interval is occupied by carbonates of Hunghuayuan Formation, and from which no graptolites have ever been found. However, the identification of the zone could be easier over the whole region, provided the formation is approximately coeval to the zone as believed. Whereas in SE China, this zone is recognizable with the leading form, T. approximatus, being rather common (Chen, 1994).
3) Acrograptus filiformis Zone: defined by the FAD of the zonal species, which is approximately equivalent to the base of the Meitan Formation in N. Guizhou. In contrast to the rarity of the zonal species in the lower part of the zone, two other species, A. pusillus and A. saukros Ni, are surprisingly rather common (Figure2G,K). At the Honghuayuan section, the FAD of A. filiformis is a bit later than that of A. pusillus (and A. saukros), a case differing from that of Scandinavia and the JCY area (Jiangshan–Changshan–Yushan, in the Jiangxi–Zhejiang border area of China) where their FAD are almost coeval (Maletz et al., 1991; Xiao et al., 1991). The zonal species has also been found in Great Britain, North America and at Yushan of NE Jiangxi Province, China (Xiao and Chen, 1990), but not yet in Australasia where the corresponding interval is dominated by pendent Pendeograptus, a genus absent on the whole Yangtze region but rarely present in Yushan of Jiangxi Province of China.
In Great Britain, the corresponding interval was designated as C. deflexus Zone, in which the zonal species itself has been proved to be very rare. Instead Didymograptus (s.l.) varicosus, a species based on specimens from Shalang of Yunnan, is found to be abundant in association with A. filiformis and probably D. protobalticus. Therefore, the former C. deflexus Zone was replaced by D. (s.l.) varicosus Zone (Cooper et al., 1995; Fortey et al., 2000). In Yunnan, the interval containing the D. (s.l.) varicosus (originally Didymograptus deflexus varicosus Wang, J.D., 1974) was for long time referred to C. deflexus Zone (Mu et al., 1979). This seems to be a mistake (see Figure2J for the distinguishing morphology of C. varicosus Wang, J.D.).
Figure 2. Camera–Lucida drawings of some critical graptolites from the Early–Middle Ordovician of Upper Yangtze Region. A, J. Corymbograptus varicosus (Wang 1974); A. proximal part of an adult form, A. filiformis Zone, Meitan Fm., NIGP135729 (AFI1005), ×10; J. holotype, Hungshihyen Fm. at the Back Mountain section nearby Xicun village of Shalang, Kunming, it was supposed to be from the C. deflexus Zone (however, according to our restudy of the type specimens and redefinition of the zone, it is more likely to be from an interval corresponding to the upper part of A. filiformis Zone. See the texts about A. filiformis Zone for more discussions), YF0756 (SF–33), ×4; B. Azygograptus suecicus Moberg 1892, A. suecicus Zone, Meitan Fm., NIGP135730 (AFI1051), ×5; C. Expansograptus hirundo (Salter 1863), broken stipes with proximal end, from the E. hirundo Zone, Meitan Fm., NIGP135731 (AFI1067), ×5; D. Undulograptus intersitus (Harris and Thomas 1935), from U. intersitus Zone, top part of Meitan Fm., NIGP135732 (AFI1096), ×10; E. Undulograptus austrodentatus (Harris and Keble 1932), U. austrodentatus Zone, Meitan Fm., NIGP135733 (AFI1092), ×10; F. Corymbograptus deflexus (Elles and Wood 1901), from the basal part of C. deflexus Zone, Meitan Fm., NIGP135734 (AFI1029e), ×5; G. Acrograptus filiformis (Tullburg 1880), growth stage from the basal part of D. eobifidus Zone, NIGP135735 (AFI1008), ×10; H. Didymograptellus obesus (Wang 1974), adult specimen from Dalieshan section of Kunming, doubtfully C. deflexus Zone, Cat.Nº YF–I (paratype), ×3; I. Didymograptellus eobifidus (Chen and Xia 1974), adult specimen with the distal parts of the stipes yet to be prepared, D. eobifidus Zone, Meitan Fm., NIGP135736 (AFI1020), ×5; K. Acrograptus pusillus (Tullburg 1880), A. filiformis Zone, top part of Hunghuayuan Fm., NIGP135737 (AFI998), ×5. Localities of the specimens: except for those of figures H and J are from the Hungshihyen Fm. in Shalang, NW Kunming, Yunnan, all the rests are from the Honghuayuan section of Tongzi, Guizhou.
Figure 3. The correlation of early to middle Ordovician graptolite zones of Yangtze region with those of other major continents and regions. TS—Time Slices (after Webby, Cooper et al. (2001). Data Source: Jiangnan Slope—Chen and Han (1964), Chen et al. (1983), Xiao and Chen (1990); Xiao et al. (1991), Chen and Bergström (1995), Chen, Rong et al. (1995). Australasia—VandenBerg and Cooper (1992), Webby, Cooper et al. (2001). Britain—Rushton (1985), Cooper et al., (1995), Fortey et al. (1995, 2000). Erdtmann (1965), Lindholm (1991), Maletz et al. (1991), Maletz (1997b), Maletz and Egenhoff (2001). North America (W. Newfoundland and Quebec)—Williams and Stevens (1988), Mitchell and Chen (1995), Maletz (1997a), Albani et al. (2001).
4) Didymograptellus eobifidus Zone. The zone is derived from the former D. protobifidus Zone identified in northern Guizhou (Chen in Zhang et al., 1964) and the D. eobifidus Subzone of the C. deflexus Zone identified in northern Sichuan (Mu et al., 1979, in which paper the differences between D. eobifidus and D. protobifidus were also discussed in details, see p.60–61), and is now defined with its base at the FAD of the zonal species (and probably of the pendent didymograptids) (Chen et al., in press). It corresponds approximately to the D. bifidus of North America, upper part of D. varicosus Zone of Great Britain. In northern Hubei, quite a few specimens of Didymograptellus was found by Li (1980) from the basal Dawan Formation, indicating a similar age.
5) Corymbograptus deflexus Zone. This zone has been misleading for long time because it was referred to intervals of considerably different age over the world. A reinvestigation of the British succession indicated that the true C. deflexus occurs abundantly in the succeeding D. (s.l.) simulans Zone and in association with Azygograptus and Acrograptus nicholsoni etc., and therefore lead to the abandonment of the zonal name (Rushton in Cooper et al., 1995). In the upper Yangtze region, this zone is retained, being redefined with its base by the FAD of true C. deflexus (not D. varicosus).
6) Azygograptus suecicus Zone. This zone is defined by the FAD of the zonal species which has proved to be rather common in both the Yangtze region and Jiangnan Slope regions, as well as in Scandinavia and Britain. However, the equivalent interval in Australasia and in the Zhujiang region of South China is characterized by the occurrence of isograptids and allied groups (VandenBerg and Cooper, 1992).
7) Expansograptus hirundo Zone. This zone is derived from the former E. cf. hirundo Zone, and is marked with its base at the FAD of the zonal species and characterized by the predominating occurrence of Expansograptus. Our recent discovery of many specimens with proximal ends preserved from the Honghuayuan section in Tongzi confirmed the recognition of this zone. However, there are difficulties with the direct correlation of this zone between South China and Britain where it indicated a younger age of Da1 and early Da2 after redefinition (Fortey et al., 2000), though it has been proposed that the type specimens of E. hirundo was not coming from the eponymous zone itself but from the preceding I. gibberullus Zone (Rushton, 1985).
8) Exigraptus clavus Zone. This zone, defined by the FAD of the zonal species which was first discovered from the upper part of the Dawan Formation at Fenxiang of Yichang (Mu et al., 1979), was identified in the JCY area (Chen and Bergström, 1995; Chen, Rong et al., 1995), and recently adopted for the upper Yangtze region (Chen et al., 2001).
9) Undulograptus austrodentatus Zone. The base of this zone is marked by the FAD of the zonal species. The zone is characterized by relatively abundant biserial graptolites, and the occurrence of Pseudobryograptus parallelus, and is correlated with the same zone in Australasia. It may correspond lithologically to the top part of the Meitan Formation in SW China or the Dawan Formation in the Yangtze Gorges area.
10) Undulograptus intersitus Zone. This zone is recognized for the first time based on specimens from the basal Shizipu Formation in the Honghuayuan section at Tongzi, Guizhou, and defined at the FAD of the zonal species. The zone is correlated with the same zone in Australasia, and may correspond to the E. dentatus Zone of North America (Maletz 1997a), and roughly the U. sinicus Subzone of SE China. However, there is a controversy regarding the synonymy of U. intersitus and E. dentatus (Maletz and Mitchell, 1995).
11) Didymograptus artus Zone. This zone is derived from the former D. cf. artus Zone (Mu et al., 1979), and is defined with its base by the FAD of the epinomous species. It is correlated to the same zone in Scandinavia and Britain, but a correlation with Australia is not so precise because the species occurs there inside the U. intersitus zone (VandenBerg and Cooper 1992). In the Yangtze region, this zone is regionally restricted to the border area of Guizhou, Sichuan and Yunnan (Loc. 6–10, dotted circle), where water is believed to have been relatively deeper at that time, whereas in the other areas the interval is occupied by conodont carbonates, namely the Kuniutan Formation (Zhang, J.H., 1998).
12) Didymograptus murchisoni Zone. This zone is defined by the first appearance of the zonal species, and is correlated to the same zone of Britain. It corresponds approximately to the Pterograptus elegans Zone and Pseudamplexograptus distichus Zone of Scandinavia (Maletz, 1995). The recognition of this zone is geographically restricted to the border area of Guizhou, Sichuan and Yunnan (e.g., Loc.7–10).
13) Gymnograptus linnarssoni Zone. This zone was derived from the former Dicellograptus sextans exilis—Gymnograptus linnarssoni Zone (Chen, in Zhang et al., 1964), and the Glossograptus hincksii / Gymnograptus linnarssoni Zone (Mu, 1974), and is now defined by the first appearance of the zonal species (e.g., Loc.15,16). It is also characterized by the occurrence of Hustedograptus teretiusculus, whose first appearance is, however, still controversial (see Maletz, 1997b), and may be correlated to the H. teretiusculus Zone of Baltoscandinavia, Britain and North America. Glossograptus hincksii, a species common in SE China within this interval, has not been found in the upper Yangtze region.
A correlation of the graptolite zones of the upper Yangtze region with those in the other main regions of the world is proposed (Figure 2).
We are indebted to Profs. J.Y. Rong, Z.Y. Zhou, colleagues R.B. Zhan, Y. Wang, J.H. Cheng, H.H. Xu, and Prof. J.B. Liu from Beijing University for their assistance in the field and helpful discussions, to Mr. L.Z. Gong from Guizhou Geological Survey and Prof. J.D. Wang from Kunming University of Science and Technology for information, and to Prof. J. Maletz for comments. Financial support from NSF (49902001, 40002001) and MST (G2000077700) of China and MPG of Germany are greatly acknowledged. We thank Prof. B.–D. Erdtmann for reading and improving the manuscript.
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Received: February 15, 2003
Accepted: June 15, 2003