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Conodonts from the Cambrian–Ordovician boundary interval in the southeast margin of the Sichuan Basin, China
Journal of Asian Earth Sciences 64 (2013) 115–124
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Conodonts from the Cambrian–Ordovician boundary interval in the southeast margin of the Sichuan Basin, China Ru Fan a,b,c, Yuan-zheng Lu a,b,c, Xue-lei Zhang a, Shi-ben Zhang a,b,c, Sheng-hui Deng a,b,c,⇑, Xin Li a,b,c a
Research Institute of Petroleum Exploration & Development, PetroChina, P.O. Box 910, Xueyuan Road 20#, Haidian District, Beijing 100083, China State Key Laboratory of Enhanced Oil Recovery, P.O. Box 910, Xueyuan Road 20#, Haidian District, Beijing 100083, China c Key Laboratory for Oil & Gas Reservoirs (KLOGR) of PetroChina, P.O. Box 910, Xueyuan Road 20#, Haidian District, Beijing 100083, China b
a r t i c l e
i n f o
Article history: Received 25 April 2011 Received in revised form 21 November 2012 Accepted 26 November 2012 Available online 26 December 2012 Keywords: Conodont Biostratigraphy Cambrian–Ordovician boundary Sichuan Basin China
a b s t r a c t Conodonts from the Cambrian–Ordovician transition at the Liangcun section in Xishui County, Guizhou and at the Huangcao section in Wulong County, Chongqing are examined for the first time. Both sections are located at the southeast margin of the Sichuan Basin. A total of 1367 specimens were recovered, representing 30 species and 15 genera. Based on the ranges of conodonts generalized from these two sections and another six sections previously studied in the same region, three conodont zones, Cordylodus proavus, Monocostodus sevierensis and Cordylodus angulatus zones are recognized. The index species of the Cambrian–Ordovician boundary at the global stratotype section and point (GSSP), Iapetognathus fluctivagus and its substitute in China Iapetognathus jilinensis are not observed in the study sections, therefore it is impossible to determine the Cambrian–Ordovician boundary exactly. However, it probably lies within the lower part of M. sevierensis zone (the upper part of the Loushanguan Group), correlating with the GSSP in Canada and the Dayangcha section in China. Chronological sequences of the FAD (First Appearance Datum) of C. angulatus, Chosonodina herfurthi and Rossodus manitouensis are not obvious in the study, so the C. angulatus zone here is correlated with zones defined by C. angulatus, Ch. herfurthi and R. manitouensis in the lower Yangtze Platform. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Conodonts from the Cambrian–Ordovician boundary interval at the southeast margin of the Sichuan Basin were reported as early as the late 1980s (Jiang and An, 1985; An, 1987; Mao, 1987), and the study of An (1987) was considered as the most representative one. In that study, six sections were referred, including the Ganxi (Yanhe County) and Honghuayuan (Tongzi County) sections in Guizhou Province, the Sanhui-Banhe (Nanchuan County), Guanyinqiao (Qijiang County), Duhui (Shizhu County) and Wanzu (Pengshui County) sections in Chongqing City (Fig. 1). As a consequence, three conodont zones, the Cordylodus proavus, Monocostodus sevierensis and Acanthodus lineatus zones were established and the FAD of M. sevierensis was taken as the proxy of the Cambrian–Ordovician boundary. Since then, there had been few researches on conodonts from the critical interval in this region (Chen and Chen, 1990; Jin, 1996). In 1999, the GSSP for the base of the Ordovician System was chosen within the Green Point section in Newfoundland, Canada, coinciding with ⇑ Corresponding author at: Key Laboratory for Oil & Gas Reservoirs (KLOGR) of PetroChina, P.O. Box 910, Xueyuan Road 20#, Haidian District, Beijing 100083, China. Tel.: +86 1083597306; fax: +86 1083597480. E-mail address: [email protected] (S.-h. Deng). 1367-9120/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jseaes.2012.11.046
the first appearance of the conodont Iapetognathus fluctivagus (Cooper et al., 2001). So far, there are only a few locations globally that contain the index conodont (Jin et al., 2008; Cooper et al., 2001). In China, with the exception of the Wushan section in Hebei Province (Cooper et al., 2001), it has been reported only in two sections, the Wa’ergang section in Hunan Province (Dong et al., 2004) and the Raowangshan section in Shandong Province (Wu et al., 2005) where it appears in low numbers and is poorly preserved. Because the index fossil is absent in most areas of China, Iapetognathus jilinensis instead of C. lindstromi (Dong et al., 2004; Zhang et al., 1999) or M. sevierensis (Ding et al., 1993; An, 1987) has been proposed as the substitute for it (Wang and Zhen, 2009) in China to correlate better with the GSSP. However, the species I. jilinensis is known only in the Dayangcha section, Jilin Province. A recent restudy of the distribution of the conodont Iapetognathus in the GSSP section shows that I. fluctivagus does not occur at the boundary interval, thus questioning its use as the key guide taxon for the GSSP horizon (Terfelt et al., 2012). In this case, it is necessary to restudy the Cambrian–Ordovician boundary. Herein two sections are selected to research the conodont fauna from the Cambrian–Ordovician transition, where the Cambrian and Ordovician Systems are exposed continuously and there is no previous study on conodonts.
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Fig. 1. Location of the reference sections at the southeast margin of the Sichuan Basin.
2. Description of sections
3. Materials and methods
The Liangcun section is exposed along the road from Liangcun Town to Houtan Village in Xishui County, Zhunyi City, Guizhou Province (Fig. 1). The Huangcao section is situated along the G104 (National highway) in Huangcao Town, Wulong County, Chongqing (Fig. 1). Both sections lie in the southeast margin of the Sichuan Basin and the linear distance between them is about 182 km. Successions of the Cambrian–Ordovician interval in the two sections are made up by the upper part of the Loushanguan Group and the lower part of the Tongzi Formation. The Loushanguan Group consists of light to dark gray dolomicrite (thick-bedded or massive). The Tongzi Formation is comprised of gray, dark gray micritic and bioclastic limestone, shale and some argillaceous and micritic dolomite. Both the limestone and dolomite contain conodonts. The additional six sections studied previously are distributed around the Liangcun and Huangcao sections (Fig. 1). Distances between the Liangcun section and the Guanyinqiao and Honghuayuan sections are approximately 46 km and 56 km, respectively, and thicknesses range from a maximum 114.9 m to a minimum 29.7 m among the Huangcao section and the other sections. Based on the latest stratigraphic division of the Ordovician System in the Sichuan Basin (Deng et al., 2010), the lithostratigraphic units in these eight sections should share the same names. So the previous ‘‘Cambrian–Ordovician Dolomite’’, ‘‘Maotian Formation’’ (An, 1987) are unified into the Loushanguan Group, and the ‘‘Nanjinguan Formation’’, together with the overlying ‘‘Fenxiang Formation’’ is merged into the Tongzi Formation.
The upper Cambrian and lower Ordovician successions have been sampled from the 23 m interval in the Liangcun section (Fig. 2) and from the 108 m interval in the Huangcao section (Fig. 3) with particular emphasis on the limestone and dolomite beds. Consequently, over 150 kg rock material of nine samples from the Liangcun section and 37 samples from the Huangcao section have been examined for conodonts. The material was processed by routine etching with about 10% acetic acid. The residues were washed through a 106 lm sieve and the material containing conodont elements was separated in bromoform (specific gravity = 2.85 g/cm3). All of the heavy residues were picked. A total of 1367 specimens were recovered (551 obtained from eight samples in the Liangcun section and 816 from eight samples in the Huangcao section), referred to 30 species and 15 genera (Figs. 4 and 5). The figured specimens were photographed by Scanning Electron Microscope. The specimens recovered from the other six sections, including the Honghuayuan, Guanyinqiao, Sanhui-Banhe, Wanzu, Duhui and Ganxi sections represent 20 species and 11 genera based on the previous record (An, 1987) (Fig. 6).
4. Conodont zonation and correlation The conodont collections from the Liangcun, Wulong and other six reference sections on the whole include three different species assemblages. Two of them are from the upper part of the Loushan-
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Fig. 2. Stratigraphical distribution of conodont taxa recovered from the Liangcun section in Xishui County, Guizhou Province.
guan Group and the third is from the top of that group and the lower part of the overlying Tongzi Formation. These assemblages represent three conodont zones, from bottom to top, the: C. proavus, M. sevierensis and Cordylodus angulatus zones. 4.1. C. proavus zone The definition of this biozone is the interval from the first appearance of C. proavus to the first occurrence of M. sevierensis. Only one sample from Bed 52 in the Huangcao section is correlated with the C. proavus zone. The sample is dominated by Teridontus nakamurai and C. proavus (Fig. 3). The zone might range into lower strata but its base is undetermined. Only two samples, 5 m and 28 m below the current FAD of the C. proavus have been collected, from Bed 50 and 51 respectively, from which only one specimen of Proconodontus sp. and seven specimens of T. nakamurai have been recovered (Fig. 3). The reason that the C. proavus zone is not observed in other reference sections of the study area might be that the sample density is insufficient. The C. proavus zone was first established in the North American carbonate platform facies in the House Range, Utah (Miller, 1969, 1980). Then, it was recognized at nearly all potential Cambrian–Ordovician boundary international stratotype sections (Bruton et al., 1988). The species, C. proavus is also well known throughout the world, such as in the Nanjinguan Formation in Hubei (Wang and Wu, 2007), the upper Ouchong Formation in Anhui (Wang and Wu, 2009) and the Shenjiawan Formation in Hunan (An, 1987;
Dong et al., 2004), South China; Chaomidian Formation in Shandong (Du et al., 2009; Wu et al., 2005) and the Yehli Formation in Jilin (Chen and Gong, 1988; Chen et al., 1988; Zhang et al., 1996, 1999), North China; Green Point Formation in Newfoundland, Canada (Barnes, 1988; Cooper et al., 2001; Terfelt et al., 2012); Ninmaroo Formation in Australia (Ripperdan et al., 1992); La Silla Formation in Argentina (Buggisch et al., 2003); Formation III in the Batyrbay section, South Kazakhstan (Appollonov et al., ˜ Formation, southern Mexico and Acerocare ecorne 1988); Tinu subzone, Norway (Bruton et al., 1988). The holotype occurs in the upper part of the Signal Mountain Limestone in Oklahoma, United States (Ding et al., 1993). Here this biozone can be correlated with the zones of the same name from the sections in South China (An, 1987; Dong et al., 2004) and the Dayangcha section in Jilin Province, North China (Chen and Gong, 1988; Chen et al., 1988; Zhang et al., 1996, 1999) (Fig. 7). It also correlates with its namesake and the Cordylodus caboti zone in the Green Point section in Newfoundland, Canada (Terfelt et al., 2012) (Fig. 7). The strata yielding the C. proavus zone are referred to the Stage 10, Furongian (Fig. 7). 4.2. M. sevierensis zone The biozone is described as the interval between the appearance of M. sevierensis and the appearance of C. angulatus, Chosonodina herfurthi or Rossodus manitouensis. The samples from the Loushanguan Group at the Liangcun section are dominated by M. sevierensis and Teridontus huanghua-
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Fig. 3. Stratigraphical distribution of conodont taxa recovered from the Huangcao section in Wulong County, Chongqing.
changensis, including a few specimens of T. erectus, Cordylodus sp. and T. nakamurai ranging from the lower biozone. In this section, the sample 6 m below the Loushanguan Group top does not contain M. sevierensis, but the presence of Drepanodus sp. (Figs. 2–1) indicates that the interval at least 4 m below the horizon where the M. sevierensis first occurs should be assigned to the M. sevierensis zone (Fig. 2). At the Huangcao section, the index species M. sevierensis does not appear until 2 m below the Loushanguan Group top (Fig. 3). The sample 30 m below the group top yields specimens of T. huanghuachangensis and T. erectus, which are common species of the M. sevierensis zone fauna in the Liangcun section (Fig. 2). T. nakamurai also appears, but with a strong decrease in number. An (1987) proposed that ‘‘T. nakamurai, the oldest species of the genus is abundant in biozones below the M. sevierensis zone, and the occurrence of T. huanghuachangensis, whose white matter is perpendicular to the growth line, indicates the conodont fauna enter into a new evolution stage’’. Consequently, the lower boundary of the M. sevierensis zone in this section might be extended downwards. To resolve the issue, more samples are required from the interval between the horizons 30 m and 2 m below the Loushanguan Group top. M. sevierensis, together with Acanthodus cf. lineatus is present 8 m below the top of the Loushanguan Group at the Honghuayuan
section (Fig. 6). The FAD of A. lineatus is higher than that of M. sevierensis, thus the fauna should be correlated to the upper part of the zone as An (1987) suggested. At the Guanyinqiao section, the sample 30 m below the Loushanguan Group top contains specimens representing M. sevierensis. Above this, there are not any diagnostic species until the specimens of Glyptoconus quadraplicatus appear in the sample 60 m above the bottom of the Tongzi Formation (Fig. 6). An (1987) thought that these specimens of M. sevierensis should be assigned to M. sevierensis zone or the successive zone (A. lineatus zone). However, we think they are more likely to be of M. sevierensis zone. Because the occurrences of conodonts in the neighboring sections such as Liangcun and Honghuayuan (Figs. 2, 3 and 6) indicate that the bottom of A. lineatus or C. angulatus zone seems as low as the bottom of the Tongzi Formation in the study area. Although the guide species M. sevierensis is not recovered from the Wanzu and Duhui sections, the appearance of A. lineatus at the bottom of the Tongzi Formation indicates that the upper boundary of the M. sevierensis zone cannot be higher than the top of the Loushanguan Group. Detailed data have not been reported on the stratigraphic range of conodonts from the Cambrian–Ordovician boundary interval in the Sanhui-Banhe section. According to the description of An (1987), in this section, the interval from the top of the ‘‘Cam-
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Fig. 4. Conodonts from the Liangcun section in Xishui County, Guizhou Province. Scale bars 100 lm. All specimens in lateral view unless otherwise noted. 1, Drepanodus sp., Loushanguan Group, sample XL-0-Y-2; 2, 3, 8, Monocostodus sevierensis (Miller, 1969), (2,8) S element, (3) P element, Loushanguan Group, sample XL-0-Y-3, XL-0-Y-4; 3, Teridontus nakamurai Nogami, 1967, Sc element, Loushanguan Group, sample XL-0-Y-4; 5–7, Teridontus huanghuachangensis (Ni, 1981), (5) Pb element, (6) Sa element, (7) Sb element, Loushanguan Group, sample XL-0-Y-4; 9, Teridontus gracilis (Furnish, 1938), P? element, Tongzi Formation, sample XL-1-Y-1; 10, 11, Scolopodus primitivus An, 1983, Tongzi Formation, sample XL-1-Y-1; 12, Teridontus erectus (Druce and Jones, 1971), posterior view, Tongzi Formation, sample XL-0-Y-4; 13, 14, Scolopodus sp. A An, 1983, Tongzi Formation, sample XL-1-Y-1, (13) anterior view; 15, 16, Aloxoconus iowensis Furnish, 1938, posterior view, Tongzi Formation, sample XL-1-Y-1; 17, 18, Acanthodus lineatus (Furnish, 1938), e? element, Tongzi Formation, sample XL-1-Y-3; 19–22, Cordylodus angulatus Pander, 1856, (19, 20) Pa element, (21, 22) Sc element, Tongzi Formation, sample XL-1-Y-1, XL-1-Y-2; 23, 26, 27, Cordylodus caseyi Druce and Jones, 1971, (23) Sc element, (26) Sd element, (27) Sb element, Tongzi Formation, sample XL-1Y-1, XL-1-Y-2; 24, Cordylodus intermedius Furnish, 1938, Tongzi Formation, sample XL-1-Y-1, XL-1-Y-2; 25, Cordylodus lindstromi Druce and Jones, 1971, M element, Tongzi Formation, sample XL-1-Y-2; 28, Chosonodina herfurthi Druce and Jones, 1971, posterior view, Tongzi Formation, sample XL-1-Y-1; 29, Drepanodus tenuis Moskalenko, 1967, Sa element, Tongzi Formation, sample XL-1-Y-2; 30–32, 34, 38, 41, Rossodus manitouensis (Repetski and Ethington, 1983), (30, 38) M element, (31, 32) Sa element, (34, 41) Sb element, Tongzi Formation, sample XL-1-Y-2, (31, 34) posterior view, (32) anterior view, (41) ventro-lateral view; 33, 37, Glyptoconus quadraplicatus (Branson and Mehl, 1933), c? element, Tongzi Formation, sample XL-2-Y-1, (33) posterior view; 35, 36, Aloxoconus staufferi Furnish, 1938, posterior view, Tongzi Formation, sample XL-1-Y-3; 39; Scolopodus restrictus An, 1983, b? element, Tongzi Formation, sample XL-1-Y-2; 40, Drepanodus liziyaensis An, 1983, Sa element, Tongzi Formation, sample XL-1-Y-2; 42, Scolopodus filiformis An, 1983, a? element, Tongzi Formation, sample XL-1-Y-2; 43, Scolopodus bassleri Furnish, 1938, a? element, Tongzi Formation, sample XL-1-Y-2. Specimens are preserved in the Key Laboratory for Oil & Gas Reservoirs (KLOGR) of PetroChina.
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Fig. 5. Conodonts from the Huangcao section in Wulong County, Chongqing. Scale bars 100 lm. All specimens in lateral view unless otherwise noted. 1, Teridontus nakamurai Nogami, 1967, Sa element, Loushanguan Group, sample WH-51-Y-1; 2, Teridontus huanghuachangensis (Ni, 1981), Sb element, Loushanguan Group, sample WH-52-Y-2; 3, Drepanodus tenuis Moskalenko, 1967, Sb element, Tongzi Formation, sample WH-54-Y-1; 4, Drepanodus lineatus Furnish, 1938, S? element, Tongzi Formation, sample WH-54Y-1; 5, Drepanodus liziyaensis An, 1983, Sb? element, Tongzi Formation, sample WH-54-Y-1; 6, Ulrichodina sp., drepanodiform element, Tongzi Formation, sample WH-54-Y-1; 7, 24, Monocostodus sevierensis (Miller, 1969), (7) P? element, (24) S element, Loushanguan Group, sample WH-52-Y-4; 8, Teridontus erectus (Druce and Jones, 1971), Pa element, Loushanguan Group, sample WH-52-Y-2; 9, Drepanodus suberectus (Branson and Mehl, 1933), P? element, Tongzi Formation, sample WH-54-Y-1; 10, Parapanderodus cf. striatus (Graves and Ellison, 1941), drepanodiform element, posterior view, Tongzi Formation, sample WH-54-Y-1; 11, 12, Scolopodus barbatus An, Du, Gao, Chen and Lee, 1981, acontiodiform element, posterior view, Tongzi Formation, sample WH-55-Y-1; 13, 14, Acanthodus lineatus (Furnish, 1938), e? element, Tongzi Formation, (13) sample WH-54-Y-1, (14) sample WH-55-Y-1; 15, 16, 26, 27, Glyptoconus quadraplicatus (Branson and Mehl, 1933), Tongzi Formation, (15, 16, 26) a? element, (27) c element, (15) sample WH-54-Y-1, (16, 26, 27) sample WH-55-Y-1, (26, 27) posterior view; 17, Scolopodus bassleri Furnish, 1938, b? element, postero-lateral view, Tongzi Formation, sample WH-54-Y-1; 18, Paltodus sp., S? element, Tongzi Formation, sample WH-55-Y-1; 19–22, Rossodus manitouensis (Repetski and Ethington, 1983), Tongzi Formation, (19) Sc element, ventro-lateral view, sample WH-55-Y-1, (20) Sb element, posterior view, (21) Sa element, posterior view, (22) M element, (20–22) sample WH-54-Y-1; 23, Proconodontus sp., compressed element, Loushanguan Group, sample WH-50-Y-2; 25, Drepanodus subarcuatus Furnish, 1938, Pa element, Tongzi Formation, sample WH-54-Y1; 28, 29, Aloxoconus aff. staufferi Furnish, 1938, posterior view, Tongzi Formation, sample WH-54-Y-1; 30, 31, Chosonodina herfurthi Druce and Jones, 1971, posterior view, Tongzi Formation, sample WH-54-Y-1; 32, Cordylodus intermedius Furnish, 1938, Tongzi Formation, sample WH-55-Y-1; 34, 35, 37–39, Cordylodus angulatus Pander, 1856, (34) Sa element, (35, 37, 38) Pa element, (39) Sc element, Tongzi Formation, sample WH-55-Y-1; 33, Cordylodus proavus Müller, 1959, Sd element, Loushanguan Group, sample WH-52-Y-1; 36, 40, 41, Cordylodus caseyi Druce and Jones, 1971, (36, 41) Sc element, (40) Sb element, Tongzi Formation, sample WH-55-Y-1. Specimens are preserved in the Key Laboratory for Oil & Gas Reservoirs (KLOGR) of PetroChina.
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Fig. 6. Stratigraphical distribution of conodont taxa recovered from other six sections previously studied (data from An, 1987).
Fig. 7. Correlations of the conodont zones at the C–O boundary interval in the southeast margin of the Sichuan Basin, the neighboring area and the typical C–O boundary sections.
brian–Ordovician Dolomite’’ to the first appearance horizon of the A. lineatus is quite thick, so it is at least partly assigned to the M. sevierensis zone. However, the zone mostly corresponds to the upper part of the Loushanguan Group in other reference sections, except the Ganxi section which is a little farther from the Sanhui-Banhe section than others (e.g. Honghuayuan, Guanyinqiao, Wanzu and Duhui sections). Thus, further study on the conodont biostratigraphy is required in the Sanhui-Banhe section. The M. sevierensis zone can be correlated with the C. intermedius zone, C. lindstromi zone and Iapetognathus? fluctivagus zone from the Green Point section in Newfoundland, Canada (Terfelt et al., 2012); the C. intermedius zone, C. lindstromi zone and I. jilinensis zone from the Dayangcha section in Jilin Province, North China (Chen and Gong, 1988; Chen et al., 1988; Zhang et al., 1996, 1999; Wang and Zhen, 2009); the C. intermedius zone, C. lindstromi zone and I. fluctivagus zone from the sections in Hunan Province, South China (Dong et al., 2004); the M. sevierensis zone and C. lindstromi zone from the lower Yangtze Platform in South China (Wang and Wu, 2009) and the homonymous zone named by An (1987) in South China (Fig. 7). Obviously, it spans the Cambrian–Ordovician boundary.
4.3. C. angulatus zone The biozone is defined as the interval from the first appearance of C. angulatus, Ch. herfurthi or R. manitouensis to the first appearance of G. quadraplicatus. The chronological sequence of the FAD of C. angulatus, Ch. herfurthi and R. manitouensis as seen in the lower Yangtze Platform in South China (Wang and Wu, 2009) has not been observed in study sections. So the interval corresponds to the C. angulatus zone, Ch. herfurthi zone and R. manitouensis zone from the lower Yangtze Platform in South China (Wang and Wu, 2009); the C. angulatus zone, Ch. herfurthi zone and the successive biozone from the Dayangcha section in Jilin Province, North China (Chen and Gong, 1988; Chen et al., 1988; Zhang et al., 1996, 1999); the C. angulatus zone and the R. manitouensis zone from the Green Point section in Newfoundland, Canada (Terfelt et al., 2012) as well as to the A. lineatus zone previously established in the study area (An, 1987) (Fig. 7). In the Liangcun, Wanzu and Ganxi sections the earliest occurrence of C. angulatus is approximately at the base of the Tongzi Formation (above the base 4 m in Ganxi), accompanied by Ch.
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herfurthi, R. manitouensis or A. lineatus (Figs. 2 and 6). It indicates that the true FAD of C. angulatus should be lower. Therefore, the base of C. angulatus zone should descend slightly assuming no unconformity, but this cannot be determined due to limited data on the stratigraphic range of the guide species. The index species C. angulatus was first recovered from the sample collected 18.5 m above the base of the Tongzi Formation in the Huangcao section, together with Ch. herfurthi, R. manitouensis, A. lineatus and G. quadraplicatus (Fig. 3). The presence of G. quadraplicatus indicates the horizon is at least correlated with G. quadraplicatus zone. So the boundaries of C. angulatus zone in the Huangcao section should be located within the 18.5 m interval, comparing with the conodont zonation in neighboring sections such as the Wanzu section which is situated only 30 km southeast of the Huangcao section, and the Duhui section. To further refine the zone in study sections, more intense sampling is needed. Besides, herein we concur with Nicoll (1990) to reassign the material previously assigned to C. rotundatus to C. angulatus.
5. The Cambrian–Ordovician boundary The Loushanguan Group in southwest China including the study area yields no macrofossils, so the Cambrian–Ordovician boundary of the region had not been defined until the conodont biostratigraphy was studied (Zhang et al., 1979). In the earliest work, the Cambrian–Ordovician boundary was located at the FAD of M. sevierensis (An, 1987). Then, with the development of the boundary studies, it became a global trend to locate the Cambrian–Ordovician boundary at the boundary between the C. intermedius zone and the C. lindstromi zone (Chen et al., 1986; Barnes, 1988; Miller, 1988), which was first established by Wang in North China (1983, 1985a,b). Simultaneously, the Dayangcha section situated at Dayangcha Town, Baishan City in Jilin Province, was proposed to serve as the global stratotype and point (GSSP) for the Cambrian–Ordovician boundary. In this section, the FAD of M. sevierensis is ca. 1.38 m higher than the FAD of C. intermedius, 3.32 m lower than that of C. lindstromi and 3.42 m lower than that of Iapetognathus sp. (jilinensis?), and its last appearance datum (LAD) is at least 0.93 m higher than the FAD of Iapetognathus sp. (jilinensis?) (Zhang et al., 1996). Therefore, based on the biozonation and the Cambrian–Ordovician boundary definition of that time, the species M. sevierensis ranges through at least two zones, the C. intermedius and C. lindstromi zones, and even the I. jilinensis zone that was established later. The proposal choosing the Dayangcha section as the GSSP for the base of the Ordovician System was not passed at the 6th International Ordovician System Congress held in Sydney in 1991 (Chen and Zhou, 2005). Then, the Green Point section in Canada was recommended as the GSSP for the boundary in the 8th International Symposium on the Ordovician System held in Prague in 1999 and the species chosen for boundary definition is I. fluctivagus instead of C. lindstromi (Chen and Zhou, 2005; Chen and Bergström, 2008). The decision was approved by the International Subcommission on Ordovician Stratigraphy (ISOS) in the same year. Then to better correlate with the GSSP, I. jilinensis zone was established within the original C. lindstromi zone at the Dayangcha section and the base of the zone was proposed to serve as the Cambrian–Ordovician boundary (Wang and Zhen, 2009). Recently, however, Terfelt et al. (2012) re-evaluated the distribution of the conodont Iapetognathus in the GSSP section and found that the GSSP horizon as now defined in the Green Point section does not correspond to the FAD of I. fluctivagus, but is based on a level part-way through the range of I. preaengensis, corresponding to the Clavohamulus hintzei conodont subz-
one of the Cordylodus intermedius conodont zone. The true FAD of I. fluctivagus is above the FAD of planktonic graptolites and well above the FAD of Cordylodus lindstromi in the Green Point section (Terfelt et al., 2012). This occurrence correlates well with that of the sections in Hunan Province, South China and even with the occurrence of I. jilinensis of the Dayangcha section, Jinlin Province, North China (Fig. 7). So the use of I. fluctivagus as the primary index species in the Green Point section is seriously challenged and as a consequence I. jilinensis also needs to be reconsidered in China. The index species I. fluctivagus, its substitute I. jilinensis and the former guide species C. lindstromi are actually all absent in the study sections. Before the GSSP is redefined formally, however, we think that the ‘‘GSSP horizon’’ in the C. intermedius zone in the Green Point section should still be a significant reference when correlating the Cambrian–Ordovician boundary from the GSSP section. Based on the ‘‘GSSP horizon’’, obviously, the species M. sevierensis spans the Cambrian–Ordovician boundary not only in China but also in Canada and the Cambrian–Ordovician boundary lies in the lower part of the M. sevierensis zone in the southeast margin of the Sichuan Basin (Fig. 7). Although the new candidates of the GSSP indicator suggested by Terfelt et al. (2012) does not include M. sevierensis, we think it is a valuable species when narrowing the interval of the Cambrian–Ordovician boundary. There are three reasons: (1) it is a truly cosmopolitan species with a distribution both in slope and platform facies; (2) it ranges through the Cambrian–Ordovician interval and (3) it appears close to the first appearance of C. intermedius (Fig. 7). According to Terfelt et al. (2012), the FAD of M. sevierensis in the slope setting (correlating well with that in platform setting) in Laurentia is somewhat below the FAD of C. intermedius and of Hirsutodontus simplex, and the LAD of M. sevierensis is close to the FAD of C. angulatus. However, based on Cooper et al. (2001), the FAD of M. sevierensis in the same region is slightly above that of Hirsutodontus simplex which appears above C. intermedius, and the LAD of M. sevierensis extends to the upper part of the C. angulatus zone. Herein we synthesize the data from both sides (Fig. 7). The stratigraphical range of M. sevierensis in South China starts from the strata correlating to the bottom of the C. intermedius zone and ends close to the bottom of the Ch. herfurthi zone based on previous data (e.g. Wang and Wu, 2009; Dong et al., 2004; An, 1987) (Fig. 7). The incomplete range of M. sevierensis in Hunan Province, we think, is probably caused by insufficient data. In North China, the FAD of M. sevierensis is somewhat above that of C. intermedius (Zhang et al., 1996). In general, the FAD of M. sevierensis seems slightly diachronous between different continents (e.g. Laurentia, South China), but it is consistent with each other in each continent. So the species could be taken at least as a regional marker species of the Cambrian–Ordovician boundary. C. intermedius is considered to be a strong candidate replacing the present primary marker taxon of the Cambrian–Ordovician boundary (Terfelt et al., 2012). However, it is only a form-species. Nicoll (1990) thought that elements previously identified as C. intermedius should be assigned as Sb elements of other species of Cordylodus, and the C. intermedius zone should be replaced by a lower Hirsutodontus simplex zone and an upper Clavohamulus hintzei zone. In light of this, it might be necessary to reconsider the suggestion to select C. intermedius as a new candidate of the GSSP indicator. Herein, we reserve the species preliminarily in view of the new relationship between it and the base of the Ordovician System GSSP, but do not divide C. intermedius zone. The present data in study sections show that it co-occurs with C. angulatus (Figs. 4–6), and thus the present range of it is incomplete and probably should be extended downwards.
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6. Conclusions The conodont fauna recovered from the Cambrian–Ordovician boundary interval in the Liangcun and Huangcao sections comprises 30 species of 15 genera, and there are 20 species of 12 genera recognized from other six sections based on the previous study. The index species for the boundary, I. fluctivagus, its substitute in China, I. jilinensis and the former guide species C. lindstromi are all not observed in the study. Three conodont zones, the C. proavus, M. sevierensis and C. angulatus zones are recognized based on the stratigraphical range of conodonts from these eight sections. The C. proavus zone is correlated with the zone of the same name from sections in Canada, North China and South China. The M. sevierensis zone is correlated with the C. intermedius zone, C. lindstromi zone and I. fluctivagus zone from Newfoundland, Canada, the C. intermedius zone, C. lindstromi zone (of which the upper part is equivalent to I. fluctivagus zone or I. jilinensis zone) from Jilin and Hunan, China, and the C. lindstromi zone and M. sevierensis zone from the lower Yangtze Platform. The C. angulatus zone corresponds to the C. angulatus zone and the successive biozone in Newfoundland, the C. angulatus–Ch. herfurthi zone and the successive biozone in Jilin, the C. angulatus, Ch. herfurthi and R. manitouensis zones in the lower Yangtze Platform and the Acanthodus lineatus zone in South China. The Cambrian–Ordovician boundary in the study area should lie within the lower part of the M. sevierensis zone based on correlations of conodont biostratigraphy with the current GSSP section. Although the GSSP horizon as now defined in the Green Point section is questioned severely, at present, it should still be the sole dependable standard of the Cambrian–Ordovician boundary. The species M. sevierensis probably is not a suitable candidate of the GSSP indicator, but might be effective as a regional marker. Acknowledgements This study is supported by national project ‘Development of the coal seam gas and large oil–gas fields’ (2008ZX05004) of China and the project of PetroChina ‘Key technologies for the exploration and development of the large oil–gas oilfields forming in marine carbonate rocks’ (2008E-0702). We thank Mr. Xiao-bo Wang and Dr. Sheng-qiang Yuan from Research Institute of Petroleum Exploration & Development (RIPED), PetroChina, Dr. Xiu-chun Jing and Dr. Yun-bo Zhang from China University of Geosciences, Mr. Hong-bin Deng, Mr. Wen-lin Zhang, Mr. Xi-nan Yang and Mr. Wen Chen from the Southwest Filiale of PetroChina, Wu-ren Xie from RIPED-Langfang, Mr. Xiao-yang Shao from Foshan Geological Survey in Guangdong Province and two students from Southwest Petroleum University for the help in the field work. Prof. Zhi-hao Wang from Nanjing Institute of Geology and Palaeontology provided help during the writing process of the paper. We also thank Prof. Lawrence H. Tanner from Le Moyne College for making valuable improvements of the language, and Prof. Yuan-dong Zhang from Nanjing Institute of Geology and Palaeontology and two anonymous reviewers for their helpful reviews. References An, T.X., 1987. The Lower Paleozoic Conodonts of South China. Peking University Press, Berlin, pp. 3–83. Appollonov, M.K., Chugaeva, M.N., Dubinina, S.V., Zhemchuzhnikov, V.G., 1988. Batyrbay Section, South Kazakhstan, USSR – potential stratotype for the Cambrian–Ordovician Boundary. Geological Magazine 125 (4), 445–449. Barnes, C.R., 1988. The proposed Cambrian–Ordovician global Boundary stratotype and point (GSSP) in Western Newfoundland, Canada. Geological Magazine 125 (4), 381–414. Bruton, D.L., Koch, L., Repetski, J.E., 1988. The Naersnes section, Oslo Region, Norway: trilobite, graptolite and conodont fossils reviewed. Geological Magazine 125 (4), 451–455.
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Conodonts from the Cambrian–Ordovician boundary interval in the southeast margin of the Sichuan Basin, China Ru Fan a,b,c, Yuan-zheng Lu a,b,c, Xue-lei Zhang a, Shi-ben Zhang a,b,c, Sheng-hui Deng a,b,c,⇑, Xin Li a,b,c a
Research Institute of Petroleum Exploration & Development, PetroChina, P.O. Box 910, Xueyuan Road 20#, Haidian District, Beijing 100083, China State Key Laboratory of Enhanced Oil Recovery, P.O. Box 910, Xueyuan Road 20#, Haidian District, Beijing 100083, China c Key Laboratory for Oil & Gas Reservoirs (KLOGR) of PetroChina, P.O. Box 910, Xueyuan Road 20#, Haidian District, Beijing 100083, China b
a r t i c l e
i n f o
Article history: Received 25 April 2011 Received in revised form 21 November 2012 Accepted 26 November 2012 Available online 26 December 2012 Keywords: Conodont Biostratigraphy Cambrian–Ordovician boundary Sichuan Basin China
a b s t r a c t Conodonts from the Cambrian–Ordovician transition at the Liangcun section in Xishui County, Guizhou and at the Huangcao section in Wulong County, Chongqing are examined for the first time. Both sections are located at the southeast margin of the Sichuan Basin. A total of 1367 specimens were recovered, representing 30 species and 15 genera. Based on the ranges of conodonts generalized from these two sections and another six sections previously studied in the same region, three conodont zones, Cordylodus proavus, Monocostodus sevierensis and Cordylodus angulatus zones are recognized. The index species of the Cambrian–Ordovician boundary at the global stratotype section and point (GSSP), Iapetognathus fluctivagus and its substitute in China Iapetognathus jilinensis are not observed in the study sections, therefore it is impossible to determine the Cambrian–Ordovician boundary exactly. However, it probably lies within the lower part of M. sevierensis zone (the upper part of the Loushanguan Group), correlating with the GSSP in Canada and the Dayangcha section in China. Chronological sequences of the FAD (First Appearance Datum) of C. angulatus, Chosonodina herfurthi and Rossodus manitouensis are not obvious in the study, so the C. angulatus zone here is correlated with zones defined by C. angulatus, Ch. herfurthi and R. manitouensis in the lower Yangtze Platform. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Conodonts from the Cambrian–Ordovician boundary interval at the southeast margin of the Sichuan Basin were reported as early as the late 1980s (Jiang and An, 1985; An, 1987; Mao, 1987), and the study of An (1987) was considered as the most representative one. In that study, six sections were referred, including the Ganxi (Yanhe County) and Honghuayuan (Tongzi County) sections in Guizhou Province, the Sanhui-Banhe (Nanchuan County), Guanyinqiao (Qijiang County), Duhui (Shizhu County) and Wanzu (Pengshui County) sections in Chongqing City (Fig. 1). As a consequence, three conodont zones, the Cordylodus proavus, Monocostodus sevierensis and Acanthodus lineatus zones were established and the FAD of M. sevierensis was taken as the proxy of the Cambrian–Ordovician boundary. Since then, there had been few researches on conodonts from the critical interval in this region (Chen and Chen, 1990; Jin, 1996). In 1999, the GSSP for the base of the Ordovician System was chosen within the Green Point section in Newfoundland, Canada, coinciding with ⇑ Corresponding author at: Key Laboratory for Oil & Gas Reservoirs (KLOGR) of PetroChina, P.O. Box 910, Xueyuan Road 20#, Haidian District, Beijing 100083, China. Tel.: +86 1083597306; fax: +86 1083597480. E-mail address: [email protected] (S.-h. Deng). 1367-9120/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jseaes.2012.11.046
the first appearance of the conodont Iapetognathus fluctivagus (Cooper et al., 2001). So far, there are only a few locations globally that contain the index conodont (Jin et al., 2008; Cooper et al., 2001). In China, with the exception of the Wushan section in Hebei Province (Cooper et al., 2001), it has been reported only in two sections, the Wa’ergang section in Hunan Province (Dong et al., 2004) and the Raowangshan section in Shandong Province (Wu et al., 2005) where it appears in low numbers and is poorly preserved. Because the index fossil is absent in most areas of China, Iapetognathus jilinensis instead of C. lindstromi (Dong et al., 2004; Zhang et al., 1999) or M. sevierensis (Ding et al., 1993; An, 1987) has been proposed as the substitute for it (Wang and Zhen, 2009) in China to correlate better with the GSSP. However, the species I. jilinensis is known only in the Dayangcha section, Jilin Province. A recent restudy of the distribution of the conodont Iapetognathus in the GSSP section shows that I. fluctivagus does not occur at the boundary interval, thus questioning its use as the key guide taxon for the GSSP horizon (Terfelt et al., 2012). In this case, it is necessary to restudy the Cambrian–Ordovician boundary. Herein two sections are selected to research the conodont fauna from the Cambrian–Ordovician transition, where the Cambrian and Ordovician Systems are exposed continuously and there is no previous study on conodonts.
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Fig. 1. Location of the reference sections at the southeast margin of the Sichuan Basin.
2. Description of sections
3. Materials and methods
The Liangcun section is exposed along the road from Liangcun Town to Houtan Village in Xishui County, Zhunyi City, Guizhou Province (Fig. 1). The Huangcao section is situated along the G104 (National highway) in Huangcao Town, Wulong County, Chongqing (Fig. 1). Both sections lie in the southeast margin of the Sichuan Basin and the linear distance between them is about 182 km. Successions of the Cambrian–Ordovician interval in the two sections are made up by the upper part of the Loushanguan Group and the lower part of the Tongzi Formation. The Loushanguan Group consists of light to dark gray dolomicrite (thick-bedded or massive). The Tongzi Formation is comprised of gray, dark gray micritic and bioclastic limestone, shale and some argillaceous and micritic dolomite. Both the limestone and dolomite contain conodonts. The additional six sections studied previously are distributed around the Liangcun and Huangcao sections (Fig. 1). Distances between the Liangcun section and the Guanyinqiao and Honghuayuan sections are approximately 46 km and 56 km, respectively, and thicknesses range from a maximum 114.9 m to a minimum 29.7 m among the Huangcao section and the other sections. Based on the latest stratigraphic division of the Ordovician System in the Sichuan Basin (Deng et al., 2010), the lithostratigraphic units in these eight sections should share the same names. So the previous ‘‘Cambrian–Ordovician Dolomite’’, ‘‘Maotian Formation’’ (An, 1987) are unified into the Loushanguan Group, and the ‘‘Nanjinguan Formation’’, together with the overlying ‘‘Fenxiang Formation’’ is merged into the Tongzi Formation.
The upper Cambrian and lower Ordovician successions have been sampled from the 23 m interval in the Liangcun section (Fig. 2) and from the 108 m interval in the Huangcao section (Fig. 3) with particular emphasis on the limestone and dolomite beds. Consequently, over 150 kg rock material of nine samples from the Liangcun section and 37 samples from the Huangcao section have been examined for conodonts. The material was processed by routine etching with about 10% acetic acid. The residues were washed through a 106 lm sieve and the material containing conodont elements was separated in bromoform (specific gravity = 2.85 g/cm3). All of the heavy residues were picked. A total of 1367 specimens were recovered (551 obtained from eight samples in the Liangcun section and 816 from eight samples in the Huangcao section), referred to 30 species and 15 genera (Figs. 4 and 5). The figured specimens were photographed by Scanning Electron Microscope. The specimens recovered from the other six sections, including the Honghuayuan, Guanyinqiao, Sanhui-Banhe, Wanzu, Duhui and Ganxi sections represent 20 species and 11 genera based on the previous record (An, 1987) (Fig. 6).
4. Conodont zonation and correlation The conodont collections from the Liangcun, Wulong and other six reference sections on the whole include three different species assemblages. Two of them are from the upper part of the Loushan-
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Fig. 2. Stratigraphical distribution of conodont taxa recovered from the Liangcun section in Xishui County, Guizhou Province.
guan Group and the third is from the top of that group and the lower part of the overlying Tongzi Formation. These assemblages represent three conodont zones, from bottom to top, the: C. proavus, M. sevierensis and Cordylodus angulatus zones. 4.1. C. proavus zone The definition of this biozone is the interval from the first appearance of C. proavus to the first occurrence of M. sevierensis. Only one sample from Bed 52 in the Huangcao section is correlated with the C. proavus zone. The sample is dominated by Teridontus nakamurai and C. proavus (Fig. 3). The zone might range into lower strata but its base is undetermined. Only two samples, 5 m and 28 m below the current FAD of the C. proavus have been collected, from Bed 50 and 51 respectively, from which only one specimen of Proconodontus sp. and seven specimens of T. nakamurai have been recovered (Fig. 3). The reason that the C. proavus zone is not observed in other reference sections of the study area might be that the sample density is insufficient. The C. proavus zone was first established in the North American carbonate platform facies in the House Range, Utah (Miller, 1969, 1980). Then, it was recognized at nearly all potential Cambrian–Ordovician boundary international stratotype sections (Bruton et al., 1988). The species, C. proavus is also well known throughout the world, such as in the Nanjinguan Formation in Hubei (Wang and Wu, 2007), the upper Ouchong Formation in Anhui (Wang and Wu, 2009) and the Shenjiawan Formation in Hunan (An, 1987;
Dong et al., 2004), South China; Chaomidian Formation in Shandong (Du et al., 2009; Wu et al., 2005) and the Yehli Formation in Jilin (Chen and Gong, 1988; Chen et al., 1988; Zhang et al., 1996, 1999), North China; Green Point Formation in Newfoundland, Canada (Barnes, 1988; Cooper et al., 2001; Terfelt et al., 2012); Ninmaroo Formation in Australia (Ripperdan et al., 1992); La Silla Formation in Argentina (Buggisch et al., 2003); Formation III in the Batyrbay section, South Kazakhstan (Appollonov et al., ˜ Formation, southern Mexico and Acerocare ecorne 1988); Tinu subzone, Norway (Bruton et al., 1988). The holotype occurs in the upper part of the Signal Mountain Limestone in Oklahoma, United States (Ding et al., 1993). Here this biozone can be correlated with the zones of the same name from the sections in South China (An, 1987; Dong et al., 2004) and the Dayangcha section in Jilin Province, North China (Chen and Gong, 1988; Chen et al., 1988; Zhang et al., 1996, 1999) (Fig. 7). It also correlates with its namesake and the Cordylodus caboti zone in the Green Point section in Newfoundland, Canada (Terfelt et al., 2012) (Fig. 7). The strata yielding the C. proavus zone are referred to the Stage 10, Furongian (Fig. 7). 4.2. M. sevierensis zone The biozone is described as the interval between the appearance of M. sevierensis and the appearance of C. angulatus, Chosonodina herfurthi or Rossodus manitouensis. The samples from the Loushanguan Group at the Liangcun section are dominated by M. sevierensis and Teridontus huanghua-
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Fig. 3. Stratigraphical distribution of conodont taxa recovered from the Huangcao section in Wulong County, Chongqing.
changensis, including a few specimens of T. erectus, Cordylodus sp. and T. nakamurai ranging from the lower biozone. In this section, the sample 6 m below the Loushanguan Group top does not contain M. sevierensis, but the presence of Drepanodus sp. (Figs. 2–1) indicates that the interval at least 4 m below the horizon where the M. sevierensis first occurs should be assigned to the M. sevierensis zone (Fig. 2). At the Huangcao section, the index species M. sevierensis does not appear until 2 m below the Loushanguan Group top (Fig. 3). The sample 30 m below the group top yields specimens of T. huanghuachangensis and T. erectus, which are common species of the M. sevierensis zone fauna in the Liangcun section (Fig. 2). T. nakamurai also appears, but with a strong decrease in number. An (1987) proposed that ‘‘T. nakamurai, the oldest species of the genus is abundant in biozones below the M. sevierensis zone, and the occurrence of T. huanghuachangensis, whose white matter is perpendicular to the growth line, indicates the conodont fauna enter into a new evolution stage’’. Consequently, the lower boundary of the M. sevierensis zone in this section might be extended downwards. To resolve the issue, more samples are required from the interval between the horizons 30 m and 2 m below the Loushanguan Group top. M. sevierensis, together with Acanthodus cf. lineatus is present 8 m below the top of the Loushanguan Group at the Honghuayuan
section (Fig. 6). The FAD of A. lineatus is higher than that of M. sevierensis, thus the fauna should be correlated to the upper part of the zone as An (1987) suggested. At the Guanyinqiao section, the sample 30 m below the Loushanguan Group top contains specimens representing M. sevierensis. Above this, there are not any diagnostic species until the specimens of Glyptoconus quadraplicatus appear in the sample 60 m above the bottom of the Tongzi Formation (Fig. 6). An (1987) thought that these specimens of M. sevierensis should be assigned to M. sevierensis zone or the successive zone (A. lineatus zone). However, we think they are more likely to be of M. sevierensis zone. Because the occurrences of conodonts in the neighboring sections such as Liangcun and Honghuayuan (Figs. 2, 3 and 6) indicate that the bottom of A. lineatus or C. angulatus zone seems as low as the bottom of the Tongzi Formation in the study area. Although the guide species M. sevierensis is not recovered from the Wanzu and Duhui sections, the appearance of A. lineatus at the bottom of the Tongzi Formation indicates that the upper boundary of the M. sevierensis zone cannot be higher than the top of the Loushanguan Group. Detailed data have not been reported on the stratigraphic range of conodonts from the Cambrian–Ordovician boundary interval in the Sanhui-Banhe section. According to the description of An (1987), in this section, the interval from the top of the ‘‘Cam-
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Fig. 4. Conodonts from the Liangcun section in Xishui County, Guizhou Province. Scale bars 100 lm. All specimens in lateral view unless otherwise noted. 1, Drepanodus sp., Loushanguan Group, sample XL-0-Y-2; 2, 3, 8, Monocostodus sevierensis (Miller, 1969), (2,8) S element, (3) P element, Loushanguan Group, sample XL-0-Y-3, XL-0-Y-4; 3, Teridontus nakamurai Nogami, 1967, Sc element, Loushanguan Group, sample XL-0-Y-4; 5–7, Teridontus huanghuachangensis (Ni, 1981), (5) Pb element, (6) Sa element, (7) Sb element, Loushanguan Group, sample XL-0-Y-4; 9, Teridontus gracilis (Furnish, 1938), P? element, Tongzi Formation, sample XL-1-Y-1; 10, 11, Scolopodus primitivus An, 1983, Tongzi Formation, sample XL-1-Y-1; 12, Teridontus erectus (Druce and Jones, 1971), posterior view, Tongzi Formation, sample XL-0-Y-4; 13, 14, Scolopodus sp. A An, 1983, Tongzi Formation, sample XL-1-Y-1, (13) anterior view; 15, 16, Aloxoconus iowensis Furnish, 1938, posterior view, Tongzi Formation, sample XL-1-Y-1; 17, 18, Acanthodus lineatus (Furnish, 1938), e? element, Tongzi Formation, sample XL-1-Y-3; 19–22, Cordylodus angulatus Pander, 1856, (19, 20) Pa element, (21, 22) Sc element, Tongzi Formation, sample XL-1-Y-1, XL-1-Y-2; 23, 26, 27, Cordylodus caseyi Druce and Jones, 1971, (23) Sc element, (26) Sd element, (27) Sb element, Tongzi Formation, sample XL-1Y-1, XL-1-Y-2; 24, Cordylodus intermedius Furnish, 1938, Tongzi Formation, sample XL-1-Y-1, XL-1-Y-2; 25, Cordylodus lindstromi Druce and Jones, 1971, M element, Tongzi Formation, sample XL-1-Y-2; 28, Chosonodina herfurthi Druce and Jones, 1971, posterior view, Tongzi Formation, sample XL-1-Y-1; 29, Drepanodus tenuis Moskalenko, 1967, Sa element, Tongzi Formation, sample XL-1-Y-2; 30–32, 34, 38, 41, Rossodus manitouensis (Repetski and Ethington, 1983), (30, 38) M element, (31, 32) Sa element, (34, 41) Sb element, Tongzi Formation, sample XL-1-Y-2, (31, 34) posterior view, (32) anterior view, (41) ventro-lateral view; 33, 37, Glyptoconus quadraplicatus (Branson and Mehl, 1933), c? element, Tongzi Formation, sample XL-2-Y-1, (33) posterior view; 35, 36, Aloxoconus staufferi Furnish, 1938, posterior view, Tongzi Formation, sample XL-1-Y-3; 39; Scolopodus restrictus An, 1983, b? element, Tongzi Formation, sample XL-1-Y-2; 40, Drepanodus liziyaensis An, 1983, Sa element, Tongzi Formation, sample XL-1-Y-2; 42, Scolopodus filiformis An, 1983, a? element, Tongzi Formation, sample XL-1-Y-2; 43, Scolopodus bassleri Furnish, 1938, a? element, Tongzi Formation, sample XL-1-Y-2. Specimens are preserved in the Key Laboratory for Oil & Gas Reservoirs (KLOGR) of PetroChina.
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Fig. 5. Conodonts from the Huangcao section in Wulong County, Chongqing. Scale bars 100 lm. All specimens in lateral view unless otherwise noted. 1, Teridontus nakamurai Nogami, 1967, Sa element, Loushanguan Group, sample WH-51-Y-1; 2, Teridontus huanghuachangensis (Ni, 1981), Sb element, Loushanguan Group, sample WH-52-Y-2; 3, Drepanodus tenuis Moskalenko, 1967, Sb element, Tongzi Formation, sample WH-54-Y-1; 4, Drepanodus lineatus Furnish, 1938, S? element, Tongzi Formation, sample WH-54Y-1; 5, Drepanodus liziyaensis An, 1983, Sb? element, Tongzi Formation, sample WH-54-Y-1; 6, Ulrichodina sp., drepanodiform element, Tongzi Formation, sample WH-54-Y-1; 7, 24, Monocostodus sevierensis (Miller, 1969), (7) P? element, (24) S element, Loushanguan Group, sample WH-52-Y-4; 8, Teridontus erectus (Druce and Jones, 1971), Pa element, Loushanguan Group, sample WH-52-Y-2; 9, Drepanodus suberectus (Branson and Mehl, 1933), P? element, Tongzi Formation, sample WH-54-Y-1; 10, Parapanderodus cf. striatus (Graves and Ellison, 1941), drepanodiform element, posterior view, Tongzi Formation, sample WH-54-Y-1; 11, 12, Scolopodus barbatus An, Du, Gao, Chen and Lee, 1981, acontiodiform element, posterior view, Tongzi Formation, sample WH-55-Y-1; 13, 14, Acanthodus lineatus (Furnish, 1938), e? element, Tongzi Formation, (13) sample WH-54-Y-1, (14) sample WH-55-Y-1; 15, 16, 26, 27, Glyptoconus quadraplicatus (Branson and Mehl, 1933), Tongzi Formation, (15, 16, 26) a? element, (27) c element, (15) sample WH-54-Y-1, (16, 26, 27) sample WH-55-Y-1, (26, 27) posterior view; 17, Scolopodus bassleri Furnish, 1938, b? element, postero-lateral view, Tongzi Formation, sample WH-54-Y-1; 18, Paltodus sp., S? element, Tongzi Formation, sample WH-55-Y-1; 19–22, Rossodus manitouensis (Repetski and Ethington, 1983), Tongzi Formation, (19) Sc element, ventro-lateral view, sample WH-55-Y-1, (20) Sb element, posterior view, (21) Sa element, posterior view, (22) M element, (20–22) sample WH-54-Y-1; 23, Proconodontus sp., compressed element, Loushanguan Group, sample WH-50-Y-2; 25, Drepanodus subarcuatus Furnish, 1938, Pa element, Tongzi Formation, sample WH-54-Y1; 28, 29, Aloxoconus aff. staufferi Furnish, 1938, posterior view, Tongzi Formation, sample WH-54-Y-1; 30, 31, Chosonodina herfurthi Druce and Jones, 1971, posterior view, Tongzi Formation, sample WH-54-Y-1; 32, Cordylodus intermedius Furnish, 1938, Tongzi Formation, sample WH-55-Y-1; 34, 35, 37–39, Cordylodus angulatus Pander, 1856, (34) Sa element, (35, 37, 38) Pa element, (39) Sc element, Tongzi Formation, sample WH-55-Y-1; 33, Cordylodus proavus Müller, 1959, Sd element, Loushanguan Group, sample WH-52-Y-1; 36, 40, 41, Cordylodus caseyi Druce and Jones, 1971, (36, 41) Sc element, (40) Sb element, Tongzi Formation, sample WH-55-Y-1. Specimens are preserved in the Key Laboratory for Oil & Gas Reservoirs (KLOGR) of PetroChina.
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Fig. 6. Stratigraphical distribution of conodont taxa recovered from other six sections previously studied (data from An, 1987).
Fig. 7. Correlations of the conodont zones at the C–O boundary interval in the southeast margin of the Sichuan Basin, the neighboring area and the typical C–O boundary sections.
brian–Ordovician Dolomite’’ to the first appearance horizon of the A. lineatus is quite thick, so it is at least partly assigned to the M. sevierensis zone. However, the zone mostly corresponds to the upper part of the Loushanguan Group in other reference sections, except the Ganxi section which is a little farther from the Sanhui-Banhe section than others (e.g. Honghuayuan, Guanyinqiao, Wanzu and Duhui sections). Thus, further study on the conodont biostratigraphy is required in the Sanhui-Banhe section. The M. sevierensis zone can be correlated with the C. intermedius zone, C. lindstromi zone and Iapetognathus? fluctivagus zone from the Green Point section in Newfoundland, Canada (Terfelt et al., 2012); the C. intermedius zone, C. lindstromi zone and I. jilinensis zone from the Dayangcha section in Jilin Province, North China (Chen and Gong, 1988; Chen et al., 1988; Zhang et al., 1996, 1999; Wang and Zhen, 2009); the C. intermedius zone, C. lindstromi zone and I. fluctivagus zone from the sections in Hunan Province, South China (Dong et al., 2004); the M. sevierensis zone and C. lindstromi zone from the lower Yangtze Platform in South China (Wang and Wu, 2009) and the homonymous zone named by An (1987) in South China (Fig. 7). Obviously, it spans the Cambrian–Ordovician boundary.
4.3. C. angulatus zone The biozone is defined as the interval from the first appearance of C. angulatus, Ch. herfurthi or R. manitouensis to the first appearance of G. quadraplicatus. The chronological sequence of the FAD of C. angulatus, Ch. herfurthi and R. manitouensis as seen in the lower Yangtze Platform in South China (Wang and Wu, 2009) has not been observed in study sections. So the interval corresponds to the C. angulatus zone, Ch. herfurthi zone and R. manitouensis zone from the lower Yangtze Platform in South China (Wang and Wu, 2009); the C. angulatus zone, Ch. herfurthi zone and the successive biozone from the Dayangcha section in Jilin Province, North China (Chen and Gong, 1988; Chen et al., 1988; Zhang et al., 1996, 1999); the C. angulatus zone and the R. manitouensis zone from the Green Point section in Newfoundland, Canada (Terfelt et al., 2012) as well as to the A. lineatus zone previously established in the study area (An, 1987) (Fig. 7). In the Liangcun, Wanzu and Ganxi sections the earliest occurrence of C. angulatus is approximately at the base of the Tongzi Formation (above the base 4 m in Ganxi), accompanied by Ch.
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herfurthi, R. manitouensis or A. lineatus (Figs. 2 and 6). It indicates that the true FAD of C. angulatus should be lower. Therefore, the base of C. angulatus zone should descend slightly assuming no unconformity, but this cannot be determined due to limited data on the stratigraphic range of the guide species. The index species C. angulatus was first recovered from the sample collected 18.5 m above the base of the Tongzi Formation in the Huangcao section, together with Ch. herfurthi, R. manitouensis, A. lineatus and G. quadraplicatus (Fig. 3). The presence of G. quadraplicatus indicates the horizon is at least correlated with G. quadraplicatus zone. So the boundaries of C. angulatus zone in the Huangcao section should be located within the 18.5 m interval, comparing with the conodont zonation in neighboring sections such as the Wanzu section which is situated only 30 km southeast of the Huangcao section, and the Duhui section. To further refine the zone in study sections, more intense sampling is needed. Besides, herein we concur with Nicoll (1990) to reassign the material previously assigned to C. rotundatus to C. angulatus.
5. The Cambrian–Ordovician boundary The Loushanguan Group in southwest China including the study area yields no macrofossils, so the Cambrian–Ordovician boundary of the region had not been defined until the conodont biostratigraphy was studied (Zhang et al., 1979). In the earliest work, the Cambrian–Ordovician boundary was located at the FAD of M. sevierensis (An, 1987). Then, with the development of the boundary studies, it became a global trend to locate the Cambrian–Ordovician boundary at the boundary between the C. intermedius zone and the C. lindstromi zone (Chen et al., 1986; Barnes, 1988; Miller, 1988), which was first established by Wang in North China (1983, 1985a,b). Simultaneously, the Dayangcha section situated at Dayangcha Town, Baishan City in Jilin Province, was proposed to serve as the global stratotype and point (GSSP) for the Cambrian–Ordovician boundary. In this section, the FAD of M. sevierensis is ca. 1.38 m higher than the FAD of C. intermedius, 3.32 m lower than that of C. lindstromi and 3.42 m lower than that of Iapetognathus sp. (jilinensis?), and its last appearance datum (LAD) is at least 0.93 m higher than the FAD of Iapetognathus sp. (jilinensis?) (Zhang et al., 1996). Therefore, based on the biozonation and the Cambrian–Ordovician boundary definition of that time, the species M. sevierensis ranges through at least two zones, the C. intermedius and C. lindstromi zones, and even the I. jilinensis zone that was established later. The proposal choosing the Dayangcha section as the GSSP for the base of the Ordovician System was not passed at the 6th International Ordovician System Congress held in Sydney in 1991 (Chen and Zhou, 2005). Then, the Green Point section in Canada was recommended as the GSSP for the boundary in the 8th International Symposium on the Ordovician System held in Prague in 1999 and the species chosen for boundary definition is I. fluctivagus instead of C. lindstromi (Chen and Zhou, 2005; Chen and Bergström, 2008). The decision was approved by the International Subcommission on Ordovician Stratigraphy (ISOS) in the same year. Then to better correlate with the GSSP, I. jilinensis zone was established within the original C. lindstromi zone at the Dayangcha section and the base of the zone was proposed to serve as the Cambrian–Ordovician boundary (Wang and Zhen, 2009). Recently, however, Terfelt et al. (2012) re-evaluated the distribution of the conodont Iapetognathus in the GSSP section and found that the GSSP horizon as now defined in the Green Point section does not correspond to the FAD of I. fluctivagus, but is based on a level part-way through the range of I. preaengensis, corresponding to the Clavohamulus hintzei conodont subz-
one of the Cordylodus intermedius conodont zone. The true FAD of I. fluctivagus is above the FAD of planktonic graptolites and well above the FAD of Cordylodus lindstromi in the Green Point section (Terfelt et al., 2012). This occurrence correlates well with that of the sections in Hunan Province, South China and even with the occurrence of I. jilinensis of the Dayangcha section, Jinlin Province, North China (Fig. 7). So the use of I. fluctivagus as the primary index species in the Green Point section is seriously challenged and as a consequence I. jilinensis also needs to be reconsidered in China. The index species I. fluctivagus, its substitute I. jilinensis and the former guide species C. lindstromi are actually all absent in the study sections. Before the GSSP is redefined formally, however, we think that the ‘‘GSSP horizon’’ in the C. intermedius zone in the Green Point section should still be a significant reference when correlating the Cambrian–Ordovician boundary from the GSSP section. Based on the ‘‘GSSP horizon’’, obviously, the species M. sevierensis spans the Cambrian–Ordovician boundary not only in China but also in Canada and the Cambrian–Ordovician boundary lies in the lower part of the M. sevierensis zone in the southeast margin of the Sichuan Basin (Fig. 7). Although the new candidates of the GSSP indicator suggested by Terfelt et al. (2012) does not include M. sevierensis, we think it is a valuable species when narrowing the interval of the Cambrian–Ordovician boundary. There are three reasons: (1) it is a truly cosmopolitan species with a distribution both in slope and platform facies; (2) it ranges through the Cambrian–Ordovician interval and (3) it appears close to the first appearance of C. intermedius (Fig. 7). According to Terfelt et al. (2012), the FAD of M. sevierensis in the slope setting (correlating well with that in platform setting) in Laurentia is somewhat below the FAD of C. intermedius and of Hirsutodontus simplex, and the LAD of M. sevierensis is close to the FAD of C. angulatus. However, based on Cooper et al. (2001), the FAD of M. sevierensis in the same region is slightly above that of Hirsutodontus simplex which appears above C. intermedius, and the LAD of M. sevierensis extends to the upper part of the C. angulatus zone. Herein we synthesize the data from both sides (Fig. 7). The stratigraphical range of M. sevierensis in South China starts from the strata correlating to the bottom of the C. intermedius zone and ends close to the bottom of the Ch. herfurthi zone based on previous data (e.g. Wang and Wu, 2009; Dong et al., 2004; An, 1987) (Fig. 7). The incomplete range of M. sevierensis in Hunan Province, we think, is probably caused by insufficient data. In North China, the FAD of M. sevierensis is somewhat above that of C. intermedius (Zhang et al., 1996). In general, the FAD of M. sevierensis seems slightly diachronous between different continents (e.g. Laurentia, South China), but it is consistent with each other in each continent. So the species could be taken at least as a regional marker species of the Cambrian–Ordovician boundary. C. intermedius is considered to be a strong candidate replacing the present primary marker taxon of the Cambrian–Ordovician boundary (Terfelt et al., 2012). However, it is only a form-species. Nicoll (1990) thought that elements previously identified as C. intermedius should be assigned as Sb elements of other species of Cordylodus, and the C. intermedius zone should be replaced by a lower Hirsutodontus simplex zone and an upper Clavohamulus hintzei zone. In light of this, it might be necessary to reconsider the suggestion to select C. intermedius as a new candidate of the GSSP indicator. Herein, we reserve the species preliminarily in view of the new relationship between it and the base of the Ordovician System GSSP, but do not divide C. intermedius zone. The present data in study sections show that it co-occurs with C. angulatus (Figs. 4–6), and thus the present range of it is incomplete and probably should be extended downwards.
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6. Conclusions The conodont fauna recovered from the Cambrian–Ordovician boundary interval in the Liangcun and Huangcao sections comprises 30 species of 15 genera, and there are 20 species of 12 genera recognized from other six sections based on the previous study. The index species for the boundary, I. fluctivagus, its substitute in China, I. jilinensis and the former guide species C. lindstromi are all not observed in the study. Three conodont zones, the C. proavus, M. sevierensis and C. angulatus zones are recognized based on the stratigraphical range of conodonts from these eight sections. The C. proavus zone is correlated with the zone of the same name from sections in Canada, North China and South China. The M. sevierensis zone is correlated with the C. intermedius zone, C. lindstromi zone and I. fluctivagus zone from Newfoundland, Canada, the C. intermedius zone, C. lindstromi zone (of which the upper part is equivalent to I. fluctivagus zone or I. jilinensis zone) from Jilin and Hunan, China, and the C. lindstromi zone and M. sevierensis zone from the lower Yangtze Platform. The C. angulatus zone corresponds to the C. angulatus zone and the successive biozone in Newfoundland, the C. angulatus–Ch. herfurthi zone and the successive biozone in Jilin, the C. angulatus, Ch. herfurthi and R. manitouensis zones in the lower Yangtze Platform and the Acanthodus lineatus zone in South China. The Cambrian–Ordovician boundary in the study area should lie within the lower part of the M. sevierensis zone based on correlations of conodont biostratigraphy with the current GSSP section. Although the GSSP horizon as now defined in the Green Point section is questioned severely, at present, it should still be the sole dependable standard of the Cambrian–Ordovician boundary. The species M. sevierensis probably is not a suitable candidate of the GSSP indicator, but might be effective as a regional marker. Acknowledgements This study is supported by national project ‘Development of the coal seam gas and large oil–gas fields’ (2008ZX05004) of China and the project of PetroChina ‘Key technologies for the exploration and development of the large oil–gas oilfields forming in marine carbonate rocks’ (2008E-0702). We thank Mr. Xiao-bo Wang and Dr. Sheng-qiang Yuan from Research Institute of Petroleum Exploration & Development (RIPED), PetroChina, Dr. Xiu-chun Jing and Dr. Yun-bo Zhang from China University of Geosciences, Mr. Hong-bin Deng, Mr. Wen-lin Zhang, Mr. Xi-nan Yang and Mr. Wen Chen from the Southwest Filiale of PetroChina, Wu-ren Xie from RIPED-Langfang, Mr. Xiao-yang Shao from Foshan Geological Survey in Guangdong Province and two students from Southwest Petroleum University for the help in the field work. Prof. Zhi-hao Wang from Nanjing Institute of Geology and Palaeontology provided help during the writing process of the paper. We also thank Prof. Lawrence H. Tanner from Le Moyne College for making valuable improvements of the language, and Prof. Yuan-dong Zhang from Nanjing Institute of Geology and Palaeontology and two anonymous reviewers for their helpful reviews. References An, T.X., 1987. The Lower Paleozoic Conodonts of South China. Peking University Press, Berlin, pp. 3–83. Appollonov, M.K., Chugaeva, M.N., Dubinina, S.V., Zhemchuzhnikov, V.G., 1988. Batyrbay Section, South Kazakhstan, USSR – potential stratotype for the Cambrian–Ordovician Boundary. Geological Magazine 125 (4), 445–449. Barnes, C.R., 1988. The proposed Cambrian–Ordovician global Boundary stratotype and point (GSSP) in Western Newfoundland, Canada. Geological Magazine 125 (4), 381–414. Bruton, D.L., Koch, L., Repetski, J.E., 1988. The Naersnes section, Oslo Region, Norway: trilobite, graptolite and conodont fossils reviewed. Geological Magazine 125 (4), 451–455.
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