Study On The Characteristics And Biopalaeogeography Of The Tabulatomorpha And Conodont Of The Shiyanhe Formation(Upper Ordovician)in Xichuan,Henan

This paper presents the preliminary comprehensive study on the depositional facies and microfacies,the taxonomic and biopalaeogeographic characteristics of the tabulatomorpha and conodont fauna of the Shiyanhe Formation(Upper Ordovician),both at the Shiyanhe section,Xichuan County,and the Sigang section,Neixiang County,Henan Province.At the Shiyanhe section,the Ordovician succession was dominated by the carbonate deposits.With the observation of the outcrops and the thin-sections,two depositional facies and the related nine microfacies can be recognized,which include the platform-marginal biohermal facies and restricted platform facies.It formed a small cycle of the sea-level change,which was represented by a continuous depositional process from the platform-marginal biohermal into the restricted platform facies,then back to the platform-marginal biohermal ones.From the Shiyanhe Formation,23 conodont samples and 76 coral samples were collected,10 genera with 14 species of conodont and 10 genera with 16 species of coral were identified herein.Based on the conodont data,three conodont biozones can be established in ascending order as Aphelognathus grandis Zone,Aphelognathus divergens Zone and Aphelognathus zuoquensis Zone,which is as similar as those from the North America Mid-continent biogeographic realm.In addition,it indicates the Shiyanhe Formation should belong to the upper Katian Stage and lower Hinantian Stage.Simultaneously,two coral assemblages also can be established in the Shiyanhe Formation,namely the Agetolitella-Agetolitoides Assemblage Zone and Agetolitoides-Fletcheriella Assemblage Zone.Further,the current corals in South Qinling have been compared not only with that of the Zhejiang-Jiangxi border area,Ordos,East Kunlun-Qilian-Northern Qaidam,Junggar,Tarim in China,but also with that of Baltic,Kazakhstan,Siberia,North America,Laurentian,Australia globally.By means of the taxonomic analysis,the Otsuka coefficient of similarity values and Q-mode cluster analysis of the coral data,it suggests that the late Ordovician tabulata and heliolitoidea corals of the Shiyanhe Fm in South Qinling should belong to the North China-Qilian Province,Chinese Region,Euroasiatic Realm.With the analyses mentioned-above and the earlier research in Zhejiang-Jiangxi,Junggar and Tarim,it could be concluded that the first appearance of the Agetolitidae coral should be in the Jinhe Formation(upper Sanbian Stage,late Ordovician),southwest margin of the Ordos.Hence the Agetolitidae may originate from the Ordos region at first,and then gradually migrated to the Junggar or the South Qinling-Zhejiang-Jiangxi and other areas.