A proposal for a candidate section for restudy of the base of Silurian  

Chen Xu¹, Rong Jia–yu¹ and Fan Jun–xuan¹  

¹State Key Laboratory, Nanjing Institute of Geology and Palaeontology, CAS, P.R. China. E–mail: xuchen@jlonline.com

Key word: Ascensus. Candidate section. Dob’s Linn. Silurian. Wangjiawan South.

Since the Dob’s Linn section was ratified by the IUGS as the GSSP of the base of Silurian System 17 years ago (Holland, 1984), this stratotype has been the subject of debate (Lespérance et al., 1987). Recently, most of the titular members of the Silurian Subcommission agreed that the base of the Silurian System should be restudied. Melchin and Williams (2000) re–studied the Dob’s Linn section and revised the definition of the base of the Silurian System. Based on the studies of the base of the Silurian from the Yangtze Gorges, in particular the Wangjiawan sections, Yichang, W Hubei, China, we propose here that the Wangjiawan South section may be considered to one of the candidate sections for re–study of the base of the Silurian (locality map see Chen et al., 2000).

The Ordovician and Silurian rocks from the Yangtze Gorges were investigated by Lee and Chao in 1924 and the fossils described by Grabau in 1924 and Sun in 1933(see Chen et al., 2000). The boundary between the Ordovician and Silurian and the related fossils have been studied by Yin in 1949, Mu in 1945, 1954 and 1962, Ge, Rong et al. in 1979, Mu et al. in 1984, Rong in 1984, Mu, Boucot et al. in 1986, Wang et al. in 1987, Rong and Harper in 1988 (see Chen et al., 2000). An international research group, including Chen Xu, Rong Jia–yu, Fan Jun–xuan, Zhan Ren–bin, M.J. Melchin, D.A.T. Harper, and C.E. Mitchell, has been working on the project since the last decade. The biostratigraphy through the Ordovician and Silurian transition (Chen et al., 2000), the Hirnantia fauna in time and space (Rong et al., 2002), the Hirnantian graptolite fauna (Chen et al., in press), and the extinction – survival – recovery bioevents through this time interval (Rong and Harper, 1999, Chen et al., in prep.) have been published or will be published in the coming years. This research group is working on the early Rhuddanian graptolites and the related shelly fossils from the Wangjiawan South section as well as from the other areas of the Yangtze region.

The Wangjiawan Ordovician–Silurian sections yield the Ashgillan Linhsiang Formation, Wufeng Formation, and Kuanyinchiao Beds as well as the Llandovery Lungmachi Formation in ascending order. Three continuous sections through the Ordovician–Silurian boundary, including the Wangjiawan South, Wangjiawan North and the Wangjiawan River side sections, have been studied. Two of these sections (Wangjiawan South and North) were visited by the Ordovician Subcommission delegation in 1978, by the participants of the International Symposium of the Ordovician–Silurian Boundary in 1983, of the Fourth International Graptolite Working Group in 1990, and of the IGCP 410 Project field meeting in 1998. During the past 20 years, the Wangjiawan sections have also been studied by many colleagues from different countries. The Wangjiawan South section is recommended here as a candidate section for the restudy of the base of the Silurian. We recognize the potential of the Wangjiawan South section and the nearby Wangjiawan North and Wangjiawan River side sections into the following aspects.

Figure 1. Key fossils through the Ordovician-Silurian transition at Wangjiawan South section, Yichang, Hubei, China. All of the graptolites are x4.5.
A, B. Kinnella kielanae (Temple), A. internal mould of brachial valve, x8, NIGP 82909, Kuanyinchiao Beds; B. internal mould of pedicle valve, x8, NIGP 82940, Kuanyinchiao Beds. C, F. Hirnantia sagittifera (M’Coy), C. internal mould of brachial valve, x1.5, NIGP 82930, Kuanyinchiao Beds; F. internal mould of pedicle valve, x2, NIGP 82928, Kuanyinchiao Beds. D, E. Dalmanitina yichangensis Lin (Text-fig. 4c, d, Zhu and Wu, 1984), D. cephalon, Kuanyinchiao Beds, E. hypostoma, Kuanyinchiao Beds. G, S. Neodiplograptus bicaudatus (Chen and Lin), NIGP 136060 and 136072, Base of the A. ascensus Zone. H. Normalograptus lungmaensus (Sun), NIGP 136061, Base of the A. ascensus Zone. I. Akidograptus ascensus (Davies), NIGP 136062, Base of the A. ascensus Zone. J. Normalograptus lubricus (Chen and Lin), NIGP 136063, Base of the A. ascensus Zone. K. Normalograptus rectangularis (M’Coy), NIGP 136064, Base of the A. ascensus Zone. L. Parakidograptus acuminatus (Nicholson), NIGP 136065, Base of the P. acuminatus Zone. M. Paraclimacograptus innotatus (Nicholson), NIGP 136066, Base of the A. ascensus Zone. N. Normalograptus madernii (Koren and Mikhailova), NIGP 136067, Base of the A. ascensus Zone. O, P. Normalograptus miryensis (Obut and Sobolevskaya), NIGP 136068 and 136069, Base of the A. ascensus Zone. Q. Normalograptus angustus (Perner), NIGP 136070, Base of the A. ascensus Zone. R. Normalograptus brenansi (Legrand), NIGP 136071, Base of the A. ascensus Zone. T. Atavograptus ceryx Rickards and Hutt, NIGP 136073, Base of the A. ascensus Zone.

1. The continuity of sedimentation and biozonation. As published in Chen et al. (2000) the Wangjiawan sections show continuous sedimentary sequence through the persculptus – ascensus – acuminatus zonal interval. Also, it is clear that the sedimentation and the biozonation are continuous through the whole Ashgillian to early Llandovery strata in the Wangjiawan sections.

2. Completeness of exposures. The outcrop through Ordovician and Silurian at Wangjiawan extends 200m along the roadside and about 100m along the riverside. No unconformities exist in this interval. Similar strata across the Ordovician and Silurian boundary exposed from Huanghuachang, Fenshiang, Wangjiawan, and Tangya, north of the Yichang city along the local highway between Yichang and Nanzhang within a distance of 30km.

3. Both of the graptolites and shelly fossils are well developed. The shelly Hirnantia fauna (Rong, 1984, Zeng in Wang et al., 1987) is known through middle extraordinarius–ojsuensis Zone to lower persculptus Zone. Trilobites, nautiloids, and other microfossils are also present (Zhu and Wu, 1984, Zou, 1985, Chen et al., 2000, Wang et al., 1987).

4. Adequate thickness of sediments. At Wangjiawan South section, the Hirnantian extraordinarius–ojsuensis Zone is 0.46m, the Kuanyinchiao Beds (Hirnantia beds) is 0.31m, the persculptus Zone is 0.34m, the ascensus Zone is 0.21m, and the acuminatus Zone is 1.68m. Have compared with the other well–known graptolite–bearing sections, this is certainly a condensed section.

5. Abundance and variety of well–preserved fossils. Both of the graptolites and shelly fossils are well preserved, in particular, at the Wangjiawan South section. Conodonts are rare because of the absence of the carbonates through the Ordovician and Silurian boundary. Wang et al. (1987) listed a few conodonts in the Wufeng Formation from Yichang. Chen et al. (2000) recorded Amorphognathus ordovicicus, which identified by S.M. Bergström and Wang Zhi–hao, from the N.extraordinarius–N. ojsuensis Zone at Wangjiawan.

6. Favorable facies and widespread correlation. Both of the graptolite facies Ashgillian and Llandovery strata and the shelly Hirnantia beds are well developed. The graptolite biozones and the Hirnantia fauna possess good potential for global correlation (Chen et al., 2000).

7. Freedom from structural complication. There is no fault and fold at the Wangjiawan South section.

8. Metamorphism and other alteration. It is not high degree of metamorphism and thermal alteration. We are working on the palynological, magnetostratigraphic and chemostratigarphic studies. Wang Kun et al. (1993) have published their chemostratigraphic result from this area.

9. Amenability to isotopic age determination. A few bentonites through the Ordovician and Silurian transition provide the potential for determining radiometric dates. A primitive result published by Ross and Naeser (1984) based on the sample from the Wufeng Formation of Yichang. Samples from the Wufeng Formation and the lower Lungmachi Formation have been prepared from the Yangtze region recently.

10. Accessibility. The Wangjiawan South section and the other two, Wangjiawan North and Wangjiawan River side sections are all present on both side of the local high way between Yichang and Nanzhang. There is about one hour drive from the Yichang city to Wangjiawan. Daily local bus between Yichang and Wangjiawan is also available. The traveling, including airline, railway, buses, and ships along the Yangtze River between Yichang and big cities of China are available and comfortable. Tickets are able to book or order abroad or within China.

Figure 2. A range chart of the graptolites with the graptolitic biozonation through the Ordovician–Silurian transition. HR, Hirnantia Bed.

In summary, we propose that the Wangjiawan South section would be a good candidate for the re–study of the base of Silurian. The key fossils through the Ordovician–Silurian transition at Wangjiawan South and a range chart of the graptolites with the graptolitic biozonation through the interval are shown in the Figures 1 and 2. The present study is supported by the grant (DEA20020–3 and G 20000777000–3) from the Ministry of Science and Technology of China.

Chen Xu, Rong Jia–yu, Mitchell, C.E., Harper, D.A.T., Fan Jun–xuan, Zhan Ren–bin, Zhang Yuan–dong, Li Rong–yu and Wang Yi 2000. Late Ordovician to earliest Silurian graptolite and brachiopod biozonation from the Yangtze region, South Chian with a global correlation. Geological Magazine, 137(6): 623–650.

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Received: February 15, 2003

Accepted: June 15, 2003