Upper Ordovician (Sandbian) conodonts from the Hoedongri Formation of western Jeongseon, Korea

摘要

Five sections in western Jeongseon-eastern Pyeongchang provide a relatively complete composite section of carbonate platform facies through the upper part of the Hoedongri Formation. A total of 29 samples (125 kg) yielded 669 relatively well-preserved conodont specimens, which are classified into 16 species representing 10 genera. The fauna is dominated by Panderodus gracilis (Branson and Mehl) (55%), which generally is believed to have been pelagic and eurytopic, and comprises characteristic elements of North China faunal affinities. Other taxa include Aphelognathus solidum Pei, Belodina sp., Eoligonodina prima (Branson and Mehl), Erismodus asymmetricus (Branson and Mehl), E. quadridactylus (Stauffer), Plectodina aculeata (Stauffer), P. alatheta An, Tasmanognathus careyi Burrett, and T. sishuiensis Zhang. This assemblage was assigned to the Tasmanognathus sishuiensis-Erismodus asymmetricus Zone herein. The interval of the zone is between the first and last occurrences of the eponymous species. The lower limit of the zone is unclear due to the absence of conodont data from underlying dolomitic limestone beds. The T. sishuiensis-E. asymmetricus Zone is the youngest conodont zone in the Taebaeksan Basin, Korea and was correlated to the 'upper fauna' of the Yeongheung Formation in Yeongwol, Korea, to the Belodina compressa-Microcoelodus symmetricus Zone in Shandong, to the T. sishuiensis-Erismodus typus Zone in Gansu, and to the T. sishuiensis Zone in the Ordos Basin, North China, representing the interior platform and slope facies of Sandbian age. Based on the recent recognition of the Aurilobodus serratus Zone and T. sishuiensis-E. asymmetricus Zone, the Hoedongri Formation was assigned to the Middle Ordovician to early Late Ordovician (Darriwilian, D3 to Sandbian, Sa2).