湖北宜昌黄花场中/下奥陶统界线附近几丁虫组合及其地层学意义

THE CHITINOZOAN ASSEMBLAGES ACROSS THE LOWER-MIDDLE ORDOVICIAN BOUNDARY AT THE HUANGHUACHANG SECTION OF YICHANG, HUBEI, AND ITS STRATIGRAPHICAL SIGNIFICANCE

对宜昌黄花场大湾组下段牙形石 Oepikodus evae带至 Baltoniodus triangularis带系统采集与分析几丁虫样品 ,共发现几丁虫 6属 12种 ,其中包括 1个相似种、1个未定种 ,它们是 Conochitina raymondii,C.ordinaria,C.primitiva,C.langei,C.brevis,C.poumoti,C.decipiens,Belonechitina cf.micracantha,L agenochitina esthoni-ca,L.sp.,Eremochitina baculata和 Cyathochitina dispar等。根据上述几丁虫的地质和地理分布特征 ,自下而上可划分为 Conochitina langei带和 Eremochitina baculata带 ,讨论并指出了他们与相关笔石。

The GSSP of Lower Middle Ordovician boundary has not been finally decided yet. Following “the Stratigraphic Guidebook in China”, the Lower Middle Ordovician boundary is defined on the basis of the FAD of Baltoniodus triangularis in this paper. The chitinozoans from the typical section of the lower Dawan Stage at the Huanghuachang section of Yichang, Hubei Province, have been investigated. Twelve species belonging to six genera were found in the interval between the Oepikodus evae and Baltoniodus triangularis Zones. They are Conochitina raymondii, C. ordinaria, C. primitiva, C. langei, C. brevis, C. poumoti, C. decipiens, Belonechitina cf. micracantha, Lagenochitina sp., L. esthonica, Eremochitina brevis and Cyathochitina dispar . The geological distribution of each chitinozoan species and its relationships with graptolite and conodont zones are discussed. Two chitinozoan zones can be recognized in this interval.The Conochitina langei Zone is characterized by the appearance of the zonal fossil. Associated main elements include Lagenochitina esthonica, Conochitina brevis, C. raymondii, C. ordinaria, C. decipens, C. poumoti, C. primitiva and conodonts of the Oepikodus evae Zones. The Eremochitina baculata Zone is marked by the appearance of the zonal fossil in association with Cyathochitina dispar . These two species together with Conochitina ordinaria, C. poumoti, C. primitiva and Belonechitina cf. micracantha were found in the lower part of the Baltoniodus triangularis conodont Zone with graptolites Corymbograptus deflexus, Tetragraptus bigsby etc. This shows that the Eremochitina baculata Zone may be of the earliest Middle Ordovician. The Lower Middle Ordovician boundary, therefore, is fixed between the chitinozoan Eremochitina baculata Zone and Conochitina langei Zone.