Middle to Upper Ordovician Conodont Succession from the Qiliao Section of Shizhu, Chongqing—Revealing a Depositional Hiatus between Lower Darriwilian and Sandbian
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Abstract:
Conodonts from the Padoda Formation (Katian) to the Kuniutan Formation (early Darriwilian) in the Qiliao section, Shizhu, Chongqing are documented. They can be subdivided into (in descending order) the following 7 conodont biozones, including the Hamarodus brevirameus, Amorphognathus superbus, Baltoniodus alobatus, B. variabilis, Yangtzeplacognathus jianyyensis, Yangtzeplacognathus crassus and Lenodus variabilis biozones. The boundaries of the Katian/Sandbian and the Sandbian/Darriwilian are within the Baltoniodus alobatus Biozone and the Yangtzeplacognathus jianyyensis Biozone respectively. The Guniutan Formation in this section is incomplete in comparison with the type section in Yichang of Hubei Province, with only two conodont biozones, namely the Yangtzeplacognathus crassus and Lenodus variabilis biozones of the early Darriwilian age represented. Therefore the absence of Yangtzeplacognathus protoramosus, Y. foliaceus, Eoplacognathus suecicus and Dzikodus tablepointensis biozones of the middle to upper Darriwilian in this section indicates a significant depositional hiatus between the Kuniutan Formation (early Darriwilian) and the Datianba Formation (Sandbian). Apparently, the time span of the hiatus is well corresponded to the major eustatic sea-level fall during the mid-late Darriwilian. However, this preliminary result is yet to be confirmed by further detailed studies in a wider area located near the current western and northwestern margins of the Yangtze Platform to better confine the temporal and spacial distribution of the hiatus identified herein.
