湖北宜昌黄花场剖面中/下奥陶统界线附近的牙形剌

重点描述宜昌黄花场剖面中 /下奥陶统界线附近的重要牙形剌共 3属、5种和 2新种Bergstroe mognathussp .nov .,BaltoniodustetrachotomusLiandWangsp .nov .,并对Oepikodusevae带和Baltoniodustriangu laris带分子的首现位置及带的含义进行厘定。作者还在此剖面大湾组下段发现北美中大陆区暖水型牙形剌Tripoduslaevis和Protoprioniodusaranda ,并确定Tripoduslaevis的首现位置在Baltoniodustriangularis带底界之上 0 .3m处。Tripoduslaevis的首次出现曾提议为中奥陶统的底界的标志 (见陈旭等 ,2 0 0 0 ) ,在宜昌黄花场剖面上Tripoduslaevis的首现位置接近Baltoniodustriangularis带底界 ,对确定中 /下奥陶统界线很重要。由于在黄花场剖面可识别出Baltonioduscrassulus—B .gladiatus—B .triangularis演化谱系 ,作者建议以B .triangularis的首次出现作为中奥陶统的底界 ,宜昌黄花场剖面为中 /下奥陶统界线的层型剖面。同时 ,文中将本区的牙形刺带与世界其它地区作了对比。

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.