On the diachronous nature of the top of the Ordovician Kuniutan Formation on the Yangtze Platform: Implications of the conodont biostratigraphy of the Dacao section, Chongqing

The top part of the Kuniutan Formation in the Dacao section at Chengkou, Chongqing in the northern part of the Yangtze Platform contains the conodonts Baltoniodus variabilis (Bergström), B. variabilis(Bergström)-B. gerdae (Bergström) transitional form, B. cf. alobatus (Bergström), Complexodus pugionifer (Drygant), Dapsilodus viruensis (FÃ¥hraeus), Panderodus gracilis (Branson et Mehl), Periodon aculeatus Hadding, Protopanderodus varicostatus (Sweet et Bergström), Scabbardella altipes (Henningsmoen), and Yangtzeplacognathus jianyeensis (An et Ding). This species association is referred to the B. variabilis Zone. The transitional form between Baltoniodus variabilis and its descendent B. gerdae is recorded for the first time in China but it has been reported previously from a thin (0.5 m thick) interval at the top of the B. variabilis Subzone in Sweden and a coeval interval in much thicker successions in the Appalachians of eastern United States. This form is important because it is present in a vertically limited stratigraphic interval near the top of the Nemagraptus gracilis graptolite Zone. The new conodont data indicate that the top part of the Kuniutan Formation at the Dacao section is about three conodont zones higher than that of the same formation at the well-known Huanghuachang section in the Yichang area, Hubei and the Waigangou section at Tangshan in Nanjing, Jiangsu. This illustrates the conspicuous diachronous nature of the Kuniutan/Miaopo formation contact in the Yangtze Platform region, which has not been recognized previously in China. The widely present gap at the top of the Kuniutan Formation is at least due partly to the late Darriwilian eustatic event that can be recognized also in Baltoscandia, North America, and Argentina.