The base of the Middle Ordovician Series and Dapingian is defined at a point 10.57m above the base of the Dawan Formation at the base of Bed SHOD-16 in the Huanghuachang Section, 22km NE of Yichang City, Hubei Province, South China. It coincides with the first appearance datum of the conodont Baltoniodus triangularis. It is associated with the FAD of conodont Periodon sp. A and followed closely by the FAD of conodont Microzarkina flabellum. This level nearly coincides with the base of the Belonechitina cf. henryi chitinozoan biozone.
The Huanghuachang Section is located along the major road from Yichang City to Xinshan County, 22km NE of Yichang City, Hubei Province, China. The section is well exposed at the center of the Ordovician Geopark, which is part of the National Geopark of the Yangtze Gorges. The section is located at a latitude of 30°51’ 37.8"N and a longitude of 110°22’ 26.5"E. It is 10.57 m above the base of the Dawan Formation (base of Bed SHod-16).
The Huanghuachang Section comprises mainly the Dawan Formation. The Dawan Formation is subdivided into three units. The Lower Member is 12.97m thick and composed of gray, thin-bedded and minor medium-bedded limestones, bioclastic limestones and glauconitic limestones with yellow-green shale interbeds. The GSSP is located at the base of Bed SHOD-16 in the upper part of the Lower Member. This is 0.6m above the top of the distinctive 0.9m thick, medium-bedded dolomitic micritic limestone (SHOD 9-11) and 10.57m above the base of the Dawan Formation. The Middle Member of the Dawan Formation is 13 m thick. It is composed of purple medium-bedded micrite with yellow-green mudstone and shale interbeds. The Upper Member of the Dawan Formation of 28 m thick, it consists of yellow-green shale intercalated with thin-bedded nodular limestone and mudstone
The GSSP coincides with the first appearance datum of Baltoniodus triangularis. It is further associated with the FAD of Periodon sp. A and followed closely by the FAD of Microzarkina flabellum.
The base of the Belonechitina cf. henryi chitinozoan biozone is very close to the base of the Baltoniodus triangularis
Conodont FAD of Baltoniodus triangularis
The base of the Baltoniodus triangularis conodont biozone can be easily recognized in the North Atlantic Realm, North American Midcontinent Realm and in the Baltoscandinavian Region and northwestern Russia.
Wang, X., Stouge, S., Erdtmann, B-D., Chen, X., Li, Z., Wang, C., Zeng, Q., Zhou, Z., and Chen, H., 2005. A proposed GSSP for the base of the Middle Ordovician Series: the Huanghuachang section, Yichang, China. Episodes 28/2, p. 105 - 117.
Chen, X., Bergström, S.M., Zhang, Y-D. & Fan, J-X. 2009: The base of the Middle Ordovician in China with special reference to the succession at Hengtang near Jiangshan, Zhejiang Province, Southern China. Lethaia, Vol. 42, pp. 218-231.
Xiaofeng W., Stouge, S., Xiaohong, C., Zhihong, L., Chuanshang, W., Finney, S.C., Qingluan, Z., Zhiqiang, Z., Huiming, C., and Erdtmann, B-D., 2009. The Global Stratotype Section and Point for the base of the Middle Ordovician Series and the Third Stage (Dapingian). Episodes 32/2, p. 96-113.