Conodonts from the Cambrian￢ﾀﾓordovician Boundary Interval In

Journal of Asian Earth Sciences 64 (2013) 115–124

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Journal of Asian Earth Sciences

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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 b 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 article info abstract

Article history: Conodonts from the Cambrian–Ordovician transition at the Liangcun section in Xishui County, Guizhou Received 25 April 2011 and at the Huangcao section in Wulong County, Chongqing are examined for the first time. Both sections Received in revised form 21 November 2012 are located at the southeast margin of the Sichuan Basin. A total of 1367 specimens were recovered, rep- Accepted 26 November 2012 resenting 30 species and 15 genera. Based on the ranges of conodonts generalized from these two sec- Available online 26 December 2012 tions 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 Keywords: the Cambrian–Ordovician boundary at the global stratotype section and point (GSSP), Iapetognathus fluc- Conodont tivagus and its substitute in China Iapetognathus jilinensis are not observed in the study sections, therefore Biostratigraphy Cambrian–Ordovician boundary it is impossible to determine the Cambrian–Ordovician boundary exactly. However, it probably lies Sichuan Basin within the lower part of M. sevierensis zone (the upper part of the Loushanguan Group), correlating with China 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 the first appearance of the conodont Iapetognathus fluctivagus (Cooper et al., 2001). So far, there are only a few locations glob- Conodonts from the Cambrian–Ordovician boundary interval ally that contain the index conodont (Jin et al., 2008; Cooper at the southeast margin of the Sichuan Basin were reported as et al., 2001). In China, with the exception of the Wushan section early as the late 1980s (Jiang and An, 1985; An, 1987; Mao, in Hebei Province (Cooper et al., 2001), it has been reported only 1987), and the study of An (1987) was considered as the most in two sections, the Wa’ergang section in Hunan Province (Dong representative one. In that study, six sections were referred, et al., 2004) and the Raowangshan section in Shandong Province including the Ganxi (Yanhe County) and Honghuayuan (Tongzi (Wu et al., 2005) where it appears in low numbers and is poorly County) sections in Guizhou Province, the Sanhui-Banhe (Nanch- preserved. Because the index fossil is absent in most areas of Chi- uan County), Guanyinqiao (Qijiang County), Duhui (Shizhu na, Iapetognathus jilinensis instead of C. lindstromi (Dong et al., County) and Wanzu (Pengshui County) sections in Chongqing City 2004; Zhang et al., 1999)orM. sevierensis (Ding et al., 1993; (Fig. 1). As a consequence, three conodont zones, the Cordylodus An, 1987) has been proposed as the substitute for it (Wang and proavus, Monocostodus sevierensis and Acanthodus lineatus zones Zhen, 2009) in China to correlate better with the GSSP. However, were established and the FAD of M. sevierensis was taken as the the species I. jilinensis is known only in the Dayangcha section, Ji- proxy of the Cambrian–Ordovician boundary. Since then, there lin Province. A recent restudy of the distribution of the conodont had been few researches on conodonts from the critical interval Iapetognathus in the GSSP section shows that I. fluctivagus does in this region (Chen and Chen, 1990; Jin, 1996). In 1999, the GSSP not occur at the boundary interval, thus questioning its use as for the base of the Ordovician System was chosen within the the key guide taxon for the GSSP horizon (Terfelt et al., 2012). Green Point section in Newfoundland, Canada, coinciding with In this case, it is necessary to restudy the Cambrian–Ordovician boundary. Herein two sections are selected to research the conodont fauna ⇑ Corresponding author at: Key Laboratory for Oil & Gas Reservoirs (KLOGR) of from the Cambrian–Ordovician transition, where the Cambrian and PetroChina, P.O. Box 910, Xueyuan Road 20#, Haidian District, Beijing 100083, China. Tel.: +86 1083597306; fax: +86 1083597480. Ordovician Systems are exposed continuously and there is no pre- E-mail address: [email protected] (S.-h. Deng). vious study on conodonts.

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 The upper Cambrian and lower Ordovician successions have Town to Houtan Village in Xishui County, Zhunyi City, Guizhou been sampled from the 23 m interval in the Liangcun section Province (Fig. 1). The Huangcao section is situated along the (Fig. 2) and from the 108 m interval in the Huangcao section G104 (National highway) in Huangcao Town, Wulong County, (Fig. 3) with particular emphasis on the limestone and dolomite Chongqing (Fig. 1). Both sections lie in the southeast margin of beds. Consequently, over 150 kg rock material of nine samples the Sichuan Basin and the linear distance between them is about from the Liangcun section and 37 samples from the Huangcao sec- 182 km. Successions of the Cambrian–Ordovician interval in the tion have been examined for conodonts. The material was pro- two sections are made up by the upper part of the Loushanguan cessed by routine etching with about 10% acetic acid. The Group and the lower part of the Tongzi Formation. The Loushan- residues were washed through a 106 lm sieve and the material guan Group consists of light to dark gray dolomicrite (thick-bed- containing conodont elements was separated in bromoform (spe- ded or massive). The Tongzi Formation is comprised of gray, dark cific gravity = 2.85 g/cm3). All of the heavy residues were picked. gray micritic and bioclastic limestone, shale and some argillaceous A total of 1367 specimens were recovered (551 obtained from and micritic dolomite. Both the limestone and dolomite contain eight samples in the Liangcun section and 816 from eight samples conodonts. in the Huangcao section), referred to 30 species and 15 genera The additional six sections studied previously are distributed (Figs. 4 and 5). The figured specimens were photographed by Scan- around the Liangcun and Huangcao sections (Fig. 1). Distances be- ning Electron Microscope. tween the Liangcun section and the Guanyinqiao and Honghuayu- The specimens recovered from the other six sections, including an sections are approximately 46 km and 56 km, respectively, and the Honghuayuan, Guanyinqiao, Sanhui-Banhe, Wanzu, Duhui and thicknesses range from a maximum 114.9 m to a minimum 29.7 m Ganxi sections represent 20 species and 11 genera based on the among the Huangcao section and the other sections. Based on the previous record (An, 1987)(Fig. 6). latest stratigraphic division of the Ordovician System in the Sichu- an Basin (Deng et al., 2010), the lithostratigraphic units in these eight sections should share the same names. So the previous ‘‘Cam- 4. Conodont zonation and correlation brian–Ordovician Dolomite’’, ‘‘Maotian Formation’’ (An, 1987) are unified into the Loushanguan Group, and the ‘‘Nanjinguan Forma- The conodont collections from the Liangcun, Wulong and other tion’’, together with the overlying ‘‘Fenxiang Formation’’ is merged six reference sections on the whole include three different species into the Tongzi Formation. assemblages. Two of them are from the upper part of the Loushan- Download English Version: https://daneshyari.com/en/article/4731237

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