Early and Middle Ordovician chitinozoans from the Dapingian type sections, Yichang area, China
Abstract
A detailed study of chitinozoans based on a systematic bed-by-bed sampling of the Dawan Formation was carried out in the Yichang area (West Hubei, China) on two representative sections located at Chenjiahe and at Huanghuachang, where the GSSP of the base of the Dapingian (i.e. the base of the Middle Ordovician) has been defined. Four chitinozoan zones, in ascending order the Conochitina raymondii biozone, the Conochitina langei biozone including the Lagenochitina lata and the Conochitina pseudocarinata sub-biozones, the Lagenochitina combazi biozone, and the Sagenachitina dapingensis biozone, are recognized in the Dawan Formation, which ranges in age from the Floian to the early Darriwilian. In both sections the chitinozoan biozones benefit from a direct calibration with conodont and graptolite biozones. The base of the Lagenochitina combazi biozone is very close, or coincides with, the base of the Dapingian, as defined by the FAD of the conodont B. triangularis. For global correlation purposes three important Ordovician chitinozoan genera, i.e. Belonechitina, Sagenachitina and Cyathochitina have their FAD in the Dapingian.