graptolite biostratigraphy of China and its international correlation
Yuandong Zhang1, Bernd–D. Erdtmann2 and Hongzhen Feng3
1 Nanjing Institute of Geology
and Palaeontology, Nanjing 210008, China. E-mail: firstname.lastname@example.org
2 Technische Universität
Berlin, Institut für Angewandte Geowissenschaften, Ernst-Reuter-Platz 1, Sekr.EB10,
D-10587, Germany. E-mail: email@example.com
3 Department of Earth Sciences, Nanjing University, China. E-mail: firstname.lastname@example.org
Key words: Graptolites. Biozonation. Tremadocian. Ordovician. China.
Tremadocian is the first stage of the Ordovician System, with its base defined on the FAD of the conodont Iapetognathus fluctivagus and its top on the FAD of the graptolite Tetragraptus approximatus (Cooper et al., 2001). This interval is confined by two Global Stratotype Sections and Points (GSSPs), the basal one at Green Point, western Newfoundland, Canada, approved in early 2000, and the upper one at Diabasbrottet Quarry of Hunneberg, Sweden, established in 2001.
In China, there have been controversies on the definition of the basal boundary of Tremadocian, although much attention had been concentrated on this topic and many investigations have been conducted in the past twenty years (Chen et al., 1983, 1988; Zhou et al., 1984; Mu and Lin, 1984; Chen, 1986; Lin, 1986, 1992; Wang et al., 1998). On the other hand, the Tremadocian graptolite successions have not yet been adequately studied and clearly established (Mu et al., 1979; Wang and Erdtmann, 1987; Zhao et al., 1988; Wang, 1997; Feng and Erdtmann, 1999; Wang and Wang, 2001) – even outside China.
The most important regions in China for Tremadocian biostratigraphy include the North China and South China Palaeoplates. For those classification schemes of North China, a major problem is with the biozonation of the late Tremadocian; most Chinese biostratigraphers regarded the Adelograptus–Clonograptus Zone as belonging to the top part of the Yehli Formation and thus representing a long range up to the top of the Tremadocian, whereas some foreign experts proposed that the latest Tremadocian graptolites do not exist in China (e.g., Cooper, 1999; Maletz and Egenhoff, 2001).
Our recent study is mainly based on three graptolite collections: 1) Tremadocian graptolite specimens collected by the authors in 1999 from the Dayangcha section, Jilin, NE China; 2) late Tremadocian graptolite specimens collected by the first author (YDZ) in 2001 from the Nayangqing section in Gulin of S. Sichuan, China; and 3) re–examination of those Tremadocian graptolite specimens collected in the past two decades from a variety of localities in South China and North China, and that are now stored at the NIGPAS and TUB repositories. Our study indicates that the former Adelograptus–Clonograptus Zone has been misleading and should be abandoned. A new graptolite zonation, that includes eight zones, is proposed for the Tremadocian (Xinchangian) of China.
All the localities for the graptolites discussed in the present paper are shown in the index map (Figure 1).
Figure 1. Index map of the localities of Tremadocian graptolites in North and South China. 1–Dayangcha, Baishan, S. Jilin; 2–Muxiantougou, Baishan, S. Jilin; 3–Tianshifu, Benxi, Liaoning; 4–Huolianzhai, Benxi, Liaoning; 5–Liu Zhuang, Qinhuangdao, Hebei; 6–Kaiping, Tangshan, Hebei; 7–Kouquan, Datong, Shanxi; 8–Zhongzhuangpu, Hunyuan, Shanxi; 9–Liangdu, Jingxin, Shanxi; 10–Yanzhuang, Xintai, Shandong; 11–Power station, Huaibei, Anhui; 12–Shuanghe, Ziyang, S. Shaanxi; 13–Emeishan, SW Sichuan; 14–Shuanghe, Changning, Sichuan; 15–Nayangqing and Daping, Gulin, Sichuan; 16–Ganxi, Yanhe, Guizhou; 17–Siyangqiao, Badong, Hubei; 18–Jianyangping, Xingshan, Hubei; 19–Fenxiang, Huanghuachang and Chenjiahe, Yichang, Hubei; 20–Changyang, Hubei; 21–Dayong, Hunan; 22–Chuxian, Anhui; 23–Tangshan, Nanjing, Jiangsu; 24–Datang, Sandu, Guizhou; 25–Xinhua, Hunan; 26–Panjiazui, Taoyuan, Hunan; 27–Tianjingshan, Taojiang, Hunan; 28–Xiushui, Wuning, Jiangxi; 29–Jiangshan, W. Zhejiang, 30–Changshan, W. Zhejiang, 31–Yushan, E. Jiangxi, 32–Yixian, S. Anhui; 33–Ningguo, S. Anhui; 34–Pingnan, E. Guangxi; 35–Qidong, Hunan; 36–Yongxin, W. Jiangxi; 37–Chongyi, S. Jiangxi; 38–Xinchang, Taishan, Guangdong.
The base and top of the Tremadocian in China
The base of the Tremadocian (Xinchangian of Mu, 1974) in China was traditionally drawn to be in accordance with a lithological boundary. For example, on the Yangtze Platform (South China) this base was arbitrarily set between the Sanyiutung Formation, which is composed of massive dolomites with syngenetic breccia, and the superjacent Nantsinkuan Formation, the lower part of which is composed of limestones intercalated with some yellowish green shales. In North China the boundary was drawn between the Fengshan Formation and the overlying Yehli Formation, the latter characterized by the occurrence of flat–pebble limestone conglomerate. This division was followed by most Chinese geologists until the early 1980’s when multi–disciplinary stratigraphical methods became widely applied and consequently the definition of the Cambrian/Ordovician boundary did not necessitate a coincidence with lithological boundaries. In Dayangcha, Jilin, NE China, several levels were suggested to define this boundary, one of them being the first appearance of the conodont Cordylodus lindstromi, which is 2.23 m below the FAD of planktic graptolites (Chen et al., 1988). Subsequently this boundary position was widely adopted in China.
The identification of the base of the Ordovician in China is a little problematic because Iapetognathus fluctivagus has so far been reported only from the Wushan section, Hebei Province, although this species, as defined, can be found in a variety of facies and depositional environments ranging from shallow shelf to lower continental slope. A re–examination of the conodont biostratigraphy of this boundary interval in the Dayangcha section of NE China shows that the first appearance of Iapetognathus is later than that of the earliest planktic graptolites (80cm above the latter), a case contrasting with respect to the position at the Green Point section. However, at Green Point this horizon coincides with the FAD of Cordylodus lindstromi, that occurs 4.8m below the first appearance of the first planktic (nematophorous) graptolites, Rhabdinopora praeparabola and Staurograptus dichotomus (Cooper et al., 2001). These occurrences might help to indicate more precisely the Cambrian/Ordovician boundary in China. Although the FAD of Cordylodus lindstromi in the Dayangcha section was found to coincide with that of Iapetognathus and consequently is a bit younger, some other indeterminate specimens assigned to C. aff. lindstromi occur 2.1m below the FAD of nematophorous graptolites (Nowlan and Nicoll, 1995), a level probably best indicating the Cambrian/Ordovician boundary.
The occurrences of R. praeparabola at Dayangcha, China involve several taxonomic problems. The species R. proparabola, identified by Lin (1986) as the earliest planktic graptolite observed in the Dayangcha section, has been regarded as a junior synonym of R. praeparabola and R. f. parabola (Cooper et al., 1998; Wang et al., 1998). Our investigation also confirm that the R. praeparabola occurs together with R. f. parabola in the Xiaoyangqiao (Dayangcha) section and, accordingly the relevant horizon should belong to the R. f. parabola Zone. There is also evidence from the Green Point section pointing to the overlapping of the ranges of R. praeparabola and R. f. parabola in the basal R. f. parabola Zone (Cooper et al., 2001). Therefore, the R. praeparabola Zone cannot be recognized in the Dayangcha section, nor can it currently be identified in any other area of North China.
In South China, the boundary is even more difficult to identify because carbonates dominate this interval in most areas. Traditionally, the boundary was drawn at the base of the graptolite Rhabdinopora f. parabola / Staurograptus dichotomus Zone which can be recognized in both the Jiangnan transitional region and in the Zhujiang deep–water region. So far, no planktic graptolites below this biozone have been confirmed. Indeed, R. praeparabola was found existing at many localities in South China, but it occurs together with R. f. parabola and thus belongs to the parabola Zone (Feng and Erdtmann, 1999). Recently, however, Wang and Wang (2001) proposed that Rhabdinopora? taojiangensis is a senior synonym of R. praeparabola and consequently, claimed recognition of the R. praeparabola Zone in South China. The synonymy could be correct, providing more details about R.? taojiangensis can be presented (e.g., location of type specimen, precise position within secured graptolitic and conodont succession, etc.). As Jin and Wang (1977) described in the original paper, the species is in association with Rhabdinopora f. socialis, a subspecies occurring in the upper part of the R. f. parabola Zone and lower part of the Anisograptus matanensis Zone (see Cooper et al., 1998, p.18), and Staurograptus diffissus, a species based on poorly preserved specimens from Australia (La1) and believed to probably belong to Anisograptus (see Cooper and Stewart, 1979, p.779). For these reasons, we questionably position the occurrence of R.? taojiangensis below that of R. f. parabola; thus, the zone named after this species is yet to be fully confirmed.
The top of the Tremadocian in China is more readily recognized, compared with its base. In South China, there are many localities along the Jiangnan Slope, in the Zhujiang Basin and along the northern margin of the Yangtze Platform (Figure1–12,24,27,29,35,37), where T. approximatus or equivalent graptolites have been found from continuous sections crosssing the boundary interval (Chen and Wang, 1993). In Huocheng, Xinjiang of western China, this boundary can also be recognized (Hsu and Huang, 1979).
Figure 2. Composite range chart of Tremadocian graptolites from South China and illustrations of some species. A, B, Kiaerograptus stelcki (Jackson 1974) from Yinchufu Fm. of W. Zhejiang, drawings after Chen (1985), NIGP76529, NIGP76531, x4, x4. C. Hunnegraptus novus (Berry) from the top part of the Tungtzu Formation, Nayangqing of Gulin county, S. Sichuan, immature specimen showing the delayed secondary branching at the fifth theca, NIGP134706, x6. D–F. ‘Didymograptus’ priscus Mu 1963 from Yinchufu Fm. at Huangnigang section of Jiangshan, W. Zhejiang, NIGP53914a, NIGP53914d, NIGP53914e, x6, x8, x8. G, H, Bryograptus tintinniformis Mu 1979 from Nantsinkuan Fm. at Siyangqiao section of Badong, Hubei, NIGP31726 (paratype), NIGP31724 (holotype), x8, x8. I, Staurograptus apertus Ruedemann from Yinchufu Fm. of W. Zhejiang, NIGP76489, Col. no.ADA38, x4. J. Bryograptus kjerulfi orientalis Chen, drawing after Chen (1985) from Yinchufu Fm. of W. Zhejiang, NIGP76520 (holotype), Col. no.ADA32, x4. K. Rhabdinopora flabelliformis (Eichwald) (s.l.), NIGP53903, Yinchufu Formation at Shanbeiling of Changshan, SE China, x3.
Unfortunately, from the central part of the Yangtze Platform and the whole of North China, T. approximatus Zone graptolites have not been discovered because the relevant interval is dominated by carbonates. Thus the identification of the top boundary in this region can only be accomplished by using conodonts. From the Hunghuayuan Formation of the Yangtze Platform, the Serratognathus conodont Zone, (which includes two subzones, in ascending order, of Paroistodus proteus and Paracordylodus gracilis) was identified and correlated with the T. approximatus graptolite Zone (An, 1987). The boundary lies approximately at the base of the Hunghuayuan Formation. In Scandinavia the top occurrence of P. proteus has also been proved to be roughly coeval with, or slightly below the FAD of T. approximatus (Maletz et al., 1996). A similar solution can also be applied to North China, where the Serratognathus bilobatus Zone was recognized from the lower part of the Liangchiashan Formation and whose base was also believed to be equivalent to that of the P. proteus Zone in Scandinavia (An and Zheng, 1990, p.116).
Tremadocian graptolite successions of China
So far there have been a variety of proposals for the Tremadocian graptolite zonation in China, most of them being modifications of that by Mu (1974). However, no composite range chart has been available for the Tremadocian graptolites in China, partially due to the intermittent occurrences of graptolites from base to top in almost every section. Here, based mainly on a restudy of the graptolites collected from North China and South China, as well as on new collections made by the authors, composite graptolite zonal successions are proposed.
In North China, four graptolite zones are suggested, which in ascending order are: 1) Rhabdinopora flabelliformis parabola Zone, 2) Anisograptus matanensis Zone, 3) Psigraptus jacksoni Zone, and 4) Aorograptus victoriae Zone. Unfortunately, no separate R. praeparabola Zone can be identified in this region, because the species is always found in association with R. f. parabola. The base of the A. matanensis Zone is defined on the FAD of the zonal species, indicating the appearance of triradiate forms. The Psigraptus jacksoni Zone, derived from the former Psigraptus Zone (or Bed), is defined by the FAD of the zonal species. Other species of Psigraptus have proven to be slightly younger and fall into the middle and upper interval of this zone. The Aorograptus victoriae Zone is introduced for this region, because the former Adelograptus–Clonograptus Zone is misleading as no true Clonograptus has been confirmed from this interval.
In South China, seven graptolite zones are proposed for the Tremadocian (Figure 2). The lowest zone, that of R.? taojiangensis, was proposed by Wang and Wang (2001) as corresponding to the R. praeparabola Zone of Newfoundland, though the validity of the zonal species has yet to be confirmed. The R. f. parabola and A. matanensis zones are defined by the FAD of their zonal subspecies and species, respectively. The R. f. anglica Zone was indicated by the occurrence of the zonal species as well as the triradiate R. hunanensis and A. matanensis, and has so far been recognized only in the Panjiazui section in Taoyuan of Hunan (Figure1–26). No graptolite fauna corresponding to that of the Psigraptus jacksoni Zone has, as yet, been found in South China.
The A. victoriae Zone contains relatively abundant graptolites, including mainly Aorograptus, Kiaerograptus, Bryograptus and Adelograptus, and can be recognized in a variety of sections in the Jiangnan slope facies region. The species described by Mu and Lin (1984) as ‘Didymograptus’ priscus is diagramed here (Figure 2 D–F) and is believed to belong to Kiaerograptus. Wang and Wang (2001) regarded specimens of B. tintinniformis as possibly juvenile forms of Rhabdinopora f. anglica which would indicate an older age, a suggestion rejected herein. The holotype and one of the paratypes of the species are diagramed (Figure 2 G,H) to show the pendent rhabdosome, extremely long sicula (characteristic of Bryograptus) and a typical triradiate proximal end. The Acanthograptus sinensis Zone was originally proposed by Hsu and Ma (1948) on the basis of the abundant dendroids including Acanthograptus, Callograptus, Dendrograptus and Aspidograptus etc. from the lower part of the Fenghsiang Formation in the Yangtze Gorges region. The horizon is above that of B. tintinniformis, and is believed to indicate an interval younger than the A. victoriae Zone, and might correspond approximately to the Araneograptus murrayi Zone of Scandinavia. The occurrence of A. murrayi and a species reminiscent of T. longus in Ziyang of southern Shaanxi probably indicates a similar or slightly younger age (Figure1–12; Mu et al., 1982). The recognition of the Adelograptus–Kiaerograptus Zone by Wang et al. (1987) from the top part of Fenghsiang Formation, is rather peculiar and needs to be confirmed. The identification of H. copiosus Zone in South China is based on the discovery of Hunnegraptus in Gulin of southern Sichuan, from the top part of Tungtzu Formation (Figure1–15; Wang G., 1981; Lindholm, 1991; Wang and Wang, 2001; Zhang et al., in prep.).
Figure 3. Correlation for the Tremadocian graptolite succession of South China and North China with other continents.
Our proposed Tremadocian graptolite zonation of China includes eight zones in total: 1) Rhabdinopora? taojiangensis Zone, 2) R. flabelliformis parabola Zone, 3) Anisograptus matanensis Zone, 4) R. flabelliformis anglica Zone, 5) Psigraptus jacksoni Zone, 6) Aorograptus victoriae Zone, 7) Acanthograptus sinensis Zone and 8) Hunnegraptus copiosus Zone; these are based mainly on the composite graptolite successions in North China and South China. A correlation of these zones with those of other main continents is also suggested herein (Figure 3).
We are indebted to J. Maletz and X. Chen for helpful discussions, to X.L. Zhao, P.J. Liu of Jilin University, and to J.M. Zhang and M.Y. Zhu at NIGPAS for suggestions. Financial supports from NSF (49902001,40002001), CAS (KZCX2–SW–129) and MST (G2000077700) of China and MPG of Germany are greatly acknowledged.
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Received: February 15, 2003
Accepted: June 15, 2003