Abstract
Southern Guangxi Province is one of the important areas for the study of the Silurian/Devonian boundary (SDB) in South China. Despite attempts to define the exact level of the SDB in the Yulin (Guangxi Province) area using biostratigraphy, no consensus has been reached as the indicator fossils (e.g. graptolite Monograptus uniformis uniformis, conodont Icriodus woschmidti woschmidti and trilobite Warburgella rugulosa rugosa) are not coexistent in South China. Since Mann et al. (2001: Dynamics of the Silurian/Devonian boundary sequence: sedimentary cycles vs. organic matter variation. Terra Nostra, 2001, 44-48) reported the first isotope curve based on organic carbon for the SDB at Klonk in the Czech Republic (GSSP), some comparable variation curves of δ 13 Corg across the SDB have been obtained at several locations including sections in Turkey and China. This distinct variation curve of the isotopic composition of organic carbon across the SDB provides a chemostratigraphic reference for a worldwide correlation of the SDB. In this study, organic geochemistry together with graptolite biostratigraphy is applied as a tool for identifying the SDB at the Changwantang Section in Yulin (Guangxi Province). The results suggest that the variations in some indexes of organic geochemistry can be correlated to the representative curve of the SDB in the Klonk and Esenyali sections. The exact level of the SDB in the Changwantang Section is positioned within the upper part of the Fangcheng Formation, which is consistent with the available palaeontological data from graptolites (e.g. Colonograptus colonus, Plectograptus sp., Monograptus uniformis, M. cf. M. praehercynicus and M. aequabilis).
Highlights
The well-exposed Silurian/Devonian boundary (SDB) sequences in China are mainly preserved in three areas, i.e. Qujing (Yunnan Province), West Qinling (Sichuang Province) and Yulin (Guangxi Province) areas in the South China Block (Zhao et al 2010; Zhao & Zhu 2014)
Many attempts have been made to locate the SDB in these areas (Mu et al 1983, 1988; Luo et al 1985; Rong et al 1987, 1990; Cai et al 1994; Fang et al 1994; Rong & Chen 2000; Wang 2000), the SDB in South China remains contentious because the indicator fossils in the Klonk Section are not coexistent nor even found (Rong et al 1987; Zhao & Zhu 2014)
We provide new geochemical and palaeontological data that will help in resolving the long-standing debates over the position of the SDB in the Changwantang Section exposed in Yulin, Guangxi Province, China, and will be conducive to the worldwide correlation of the SDB in different facies
Summary
The well-exposed Silurian/Devonian boundary (SDB) sequences in China are mainly preserved in three areas, i.e. Qujing (Yunnan Province), West Qinling (Sichuang Province) and Yulin (Guangxi Province) areas in the South China Block (Zhao et al 2010; Zhao & Zhu 2014). We have conducted studies on the chemostratigraphy and biostratigraphy of the SDB sequences in the Xishancun Section (Qujing area in Yunnan Province), Putonggou Section (West Qinling area in Sichuan Province) and Changwantang Section (Yulin area in Guangxi Province) for pinpointing the SDB within different sedimentary facies in the South China Block (Fig. 1) The findings from both the Xishancun Section and the Putonggou Section have proved useful towards obtaining a better definition of the SDB in these two sections and are mainly based on the distinct δ13Corg trend and occurrence of microvertebrate remains across the Silurian/Devonian transition (Zhao et al 2010, 2011, 2012). Zhao et al.: Silurian/Devonian boundary in the Changwantang Section, China
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