CONODONTS ACROSS THE LOWER-MIDDLE ORDOVICIAN BOUNDARY IN THE HUANGHUACHANG SECTION OF YICHANG, HUBEI

This paper describes in detail the conodont zonation and some important conodont species across the Lower Middle Ordovician boundary at the Huanghuachang section of Yichang. The first appearance datum (FAD) and the content of the Oepikodus evae , Baltoniodus triangularis zones etc. are revised. North American Midcontinent provincial conodonts Tripodus laevis and Protoprioniodus aranda referred to warm water fauna have been found for the first time in the lower part of the Dawan Formation at Huanghuachang section. The FAD of Tripodus laevis was found at a level, which is 0.3m above the base of Baltoniodus triangularis Zone. The first occurrence of Tripodus laevis was proposed as the base of the Middle Ordovician (see Chen Xu et al ., 2000). So its first occurrence very close to the base of the Baltoniodus triangularis Zone is very important. The evolutionary lineage of Baltoniodus crassulus B. gladiatus (= Acodus ? gladiatus Serpagli, 1974) B. triangularis can be recognized at the Huanghuachang section. So the first occurrence of B. triangularis is a better marker for the base of Middle Ordovician. The correlation of the conodont sequences across the Lower Middle Ordovician boundary among Yichang of Hubei, Newfoundland of Canada, Nevada of USA, Europe as well as Argentina is discussed here. The Huanghuachang section of Yichang, Hubei, can be proposed as a global stratotype for the base of the Middle Ordovician Series.