Confirmation of the Floian-Darriwilian (Lower to Middle Ordovician) hiatus in the Taebaek Group, Korea: integration of conodont biostratigraphy and sedimentology

摘要

An Ordovician depositional hiatus spanning the middle Floian to lower Darriwilian, estimated to represent a period of at least 3 Myr, is confirmed for the first time in the Taebaek Group, Korea, based on integrated conodont biostratigraphic and sedimentologic data. The conodont biozones of the uppermost Dumugol and lower Makgol formations are revised and redefined with presentation of a new biozonal framework, which contains, the (Serratognathus bilobatus) Biozone, the (Serratognathus extensus) Biozone, the (Paraserratognathus obesus) Biozone, and the (Tangshanodus tangshanensis) Biozone in ascending order. The hiatus lies within the barren interval between the middle Floian (Paraserratognathus obesus) Biozone and the early Darriwilian (Tangshanodus tangshanensis) Biozone. The unconformity surface is located at the boundary between the basal and lower members of the Makgol Formation, 1.5 meters above the base of the barren interval, on the basis of sedimentary evidence such as differences in the depositional and diagenetic features as well as a sharp contact between the two members with subaerial exposure feature including vuggy to channel pore and dolostone breccia. The barren interval above the hiatus is considered due to environmental factor: the peritidal condition is regarded unfavorable for inhabitation of conodont animals. The newly established Floian and Darriwilian conodont biozones and depositional hiatus within the lower Makgol Formation indicate that the Ordovician succession of the Taebaek Group is essentially an extension of that of the North China Block.