2005•Dicengxue zazhiRequires access
ADVANCES ON THE POTENTIAL GSSP FOR THE BASE OF MIDDLE ORDOVICIAN SERIES——HUANGHUACHANG SECTION
Xiaofeng Wang, Xiaohong Chen, Zhihong Li, Chuanshang Wang, Zeng Qingluan, Zhou Zhi-qiang, Chen Hui-ming, Miao Zhang, XU Guang-hong
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Abstract
Integrated high-resolution bio-, litho-, sequence-stratigraphic and carbon isotopic geochemical studies suggest that the Huanghuachang section near Yichang, southern China meets the requirements a Global Stratotype Section and point (GSSP) for the base of the Middle Ordovician Series and the yet-to-be-named third stage of the Ordovician System (or lower stage of Middle Ordovician Series). The definition of the base of the Middle Ordovician is proposed to be the first appearance datum (FAD) of Baltoniodus? triangularis at the base of Bed (SHod) 16, 10.57m above the base of the Dawan Formation in the measured Huanghuachang section.The proposed GSSP is well exposed at the rodeside, NE 22 km from Yichang City and easy accessibility. Microzarkodina flabellum first appears at 0.2m above the boundary bio-horinzon and can be taken as a reasonable proxy for the boundary. The conodont succession at the section is complete across the Lower to Middle Ordovician series boundary and (several) excellent phylogenetic lineages of Baltoniodus, Trapezognathus, Periodon, and Microzarkodina are represented. The proposed boundary approximates the boundary between the lower and upper intervals of the Azygograptus suecicus graptolite Biozone, and nearly coincides with the base of the Belonechitina cf. henryi chitinozoan Biozone. It is within a transgressive sequence. The worldwide lowstand event indicated by the sequence change and the related carbon isotopic maxium positive excursion, occurred 0.6m below of the level with FAD of the B.? triangularis, in the Huanghuachang section are auxiliary physical and geochemical global marker. The proposed boundary horizon can be recognized and correlated globally with high precision in both relatively shallow-water carbonate facies as well as in deep-water graptolite facies. The same faunal succession is also recorded from the Chenjiahe (formerly Daping) section, 5 km to the north of the Huanghuachang section.