The base of the Middle Ordovician in China with special reference to the succession at Hengtang near Jiangshan, Zhejiang Province, southern China

Extensive work during the last decade has led to the recent decision by the International Subcommission on Ordovician Stratigraphy that the base of the Middle Ordovician Series should be placed at the base of the Baltoniodus triangularis Conodont Biozone in the Huanghuachang GSSP (Global Stratotype Section and Point) section near Yichang, Hubei Province, China. A review of the biostratigraphy of successions in many parts of China shows that for various reasons, it is currently difficult to recognize the precise boundary level in many regions, and additional studies are clearly needed. A newly exposed, previously poorly known, condensed section in deeperwater facies at Hengtang near Jiangshan in the Chiangnan (Jiangnan) belt has yielded a substantial number of important graptolites and conodonts through the boundary interval. It provides a more informative illustration of the relations between the ranges of several key taxa of these groups than any other section in China, and the level of the base of the Middle Ordovician appears to be within an about 1.8 m thick interval of Ningkuo Formation, and lies in the lower Isograptus caduceus imitatus Biozone.