The Late Ordovician Conodont Fauna And Carbon Isotope Chemostratigraphy In The Neixiang Area,Henan Province,Central China

There is a lack of continuous Katian?Late Ordovician?carbonate-dominated succession in China,and asuccessive Katian carbon-isotope(?¹³C_carb)chemostratigraphic sequence constrained by conodont biostratigraphy has not yet been concluded.The Neixiang area in Henan Province,central China is an exception where a relatively complete Katian calcarous stratigraphic sequence is developed.Carbonate rocks of the Katian Shiyanhe Formation in the Neixiang area were systematically sampled for conodont and carbon-isotope analyses.Based on these biostratigraphical and chemostratigraphical data,the following understandings have been obtained:The conodont fauna in the Sigang section of the Neixiang area,Henan Province,central China,has been studied further,and the conodont biostratigraphic sequence in thearea were reassessed.The conodont biozones,in ascending order,are Belodina confluens-Oulodus ulrichi Biozone,Oulodus robustus Biozone,Aphelognathus grandis Biozone,Aphelognathus divergens Biozone.Zonal fossils and the associated species have been described in detail.The stable carbonisotope samples from carbonate rocksin the Shiyanhe Formation have been analyzed.Diagenetic influences on the carbon isotope composition were evaluated,indicating that the data of?¹³C record the original seawater carbon-isotope composition of the time interval between the Katian Guttenberg carbonisotope excursion?GICE?and the Hirnantian carbonisotope excursion?HICE?.Due to several obvious noisy points in the present isotope data,three-point moving average method is adopted to construct the variation of?¹³C_carb.On the basis of the constraints of conodont biostratigraphy,a comparison with the Cincinnatian composite profile leads to recognize five?¹³C positive excursions,namely the Kope?K1/2?,Fairview?Ka2?,Waynesville?Ka2/3?,Whitewater?Ka3/4?excursions,and a problematic Elkhorn?Ka4?excursion.Among others,except the Whitewater excursion,the other four positive carbon isotope anomalies were descripted in China for the first time.Correlation between carbon-isotope data curve and the relativesea-level changes shows no clear correspondance,and hence?¹³C excursions during the Katian were probably not principally driven by sea level changes.A fine comparasion of the chemostratigraphy between the Sigang section and the coeval stratigraphic sections elsewhere outside China suggests that:1)the Katian carbon-isotope excursions represent global perturbations in carbon cycle,2)carbon-isotope chemostratigraphy is very useful for improving local,regional and global stratigraphic correlations.Bio-chemostratigraphic framework established in the Neixiang area based on a integrated correlation of conodont biostratigraphy and carbon-isotope chemostratigraphy can be regarded as a standard reference for the corresponding research regarding the mid-late Katian strata in China.