A Refined Stratigraphic Framework for the Tremadocian–floian Stage Boundary (lower Ordovician) in the North China Block

A high-resolution integrated bio-chemostratigraphic framework is established for the upper Tremadocian to lower Floian (Lower Ordovician) succession of the North China Block, based on new conodont and carbon isotope (δ13Ccarb) data from the expanded Jiguanshan and Wudingshan sections in Liaoning Province, northeastern China. Thirty conodont species of 19 genera were identified from the two sections. Based on these we recognize four conodont biozones, in ascending order: the Colaptoconusquadraplicatus, the Paltodusdeltifer, the Tropodusbifidus, and the Serratognathusbilobatus biozones. Within this interval, the δ13Ccarb profile exhibits a distinct positive excursion, followed by a more protracted negative shift. This anomaly is interpreted as the Late Tremadocian Isotopic Carbon Excursion (LTICE), which was recently named to overcome challenges in regional and global chemostratigraphic correlation of the Tremadocian–Floian stage boundary interval. The LTICE extends from the upper C. quadraplicatus Biozone to the lower S. bilobatus Biozone. Integration of conodont biostratigraphy with carbon isotope chemostratigraphy refines the regional stratigraphic scheme and facilitates correlation of the upper Tremadocian–lower Floian succession between North China, South China, Argentina, Baltica, and North America. In the carbonate-dominated succession in eastern North China, the Tremadocian–Floian stage boundary is placed at the first local occurrence of the conodont Serratognathus bilobatus, a horizon closely coincides with a positive-to-negative shift in the LTICE δ13Ccarb values.