Geochemical and palaeontological evidence for the definition of the Silurian/Devonian boundary in the Changwantang Section, Guangxi Province, China

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 δ13Corg 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).