Cyclostratigraphy Study Of The Middle-late Ordovician In Yichang,South China

The evolution of the biological system and its relationship with the earth's environment in deep time is one of the hottest spots of the earth science research,which not only provides the straightforward evidence for the evolution history,but also affords enlightenment for the future environmental changes.The Middle-Late Ordovician transition is one of the important periods of the earth's environmental changes and biotic turnovers.The biodiversification accompanying with excursion of carbon stable isotopes had occurred.Geological time scale is important for understanding earth's evolution,especially for the eventful Mid-Late Ordovician.A large number of classical Ordovician sections are well-exposed in the Yangtze platform,Yichang area,South China.Here,we used magnetic susceptibility as the paleoclimate proxy to conduct cyclostratigraphic analysis on the Zhenjin and Puxihe sections in Yichang,which provides accurate temporal constraints for the positive isotope carbon excursion events and bio-evolution during the Darriwilian to early Katian.Spectra analyses of the magnetic susceptibility sequence show that the ratios of different sedimentary cycles in Kunuitan,Miaopo and Pagoda formations,are consistent with those of Milankovitch cycles during the Mid-Late Ordovician.It indicates that astronomical forcing has been well-recorded in these sediments.A ~7.94 Myr floating astronomical time scale(ATS)is constructed based on the 405 kyr long eccentricity cycles calibration,which indicates the durations of middle Darriwilian isotope carbon excursion(MDICE)and Guttenberg isotope carbon excursion(GICE)are ~460 kyr and ~700 kyr,respectively.The amplitude modulation analysis in Zhenjin demonstrates that the trigger of the MDICE was alignment of minimum of amplitude of obliquity and 405 kyr eccentricity.Based on the high-precision astronomical time scale,characteristics of amplitude of astronomical forcing,and the detailed biostratigraphy,we have conducted continental correlation between Yichang area in South China and the Dawangou section in Tarim,which revealed that the duration of hiatus on the top of the Kunuitan formation is ~2.87 Myr.