Cyclostratigraphy Study On A Marine Sequence Of Kasimovian-Gzhelian Stages In Narao Section,South China

The Late Carboniferous is a critical period in the evolution of the earth system,when carbon dioxide reduction,continental aridification,marine biodiversification and negative excursion of carbon isotope occurred.The reconstruction of the times and rates of these events require an accurate and comparable global chronostratigraphic framework.Calibrating the cycle in the geological record to the astronomical orbital periods using cyclostratigraphy study can obtain a 20-400 kyr resolution continuous time scale and estimate the duration of the geological events.Narao section in South China is located in the Qian-Gui Basin during Carboniferous.The accumulated carbonate in a tectonically stable environment act as an ideal area for the study of Carboniferous stratigraphy.In this study,the cyclostratigraphy study was conducted on the Kasimovian-Gzhelian stages in Narao section,which shows 3.8-4.1 m,0.72-0.85 m,0.28-0.3 m and 0.15-0.22 m sedimentary cycles from the obtained GR(gamma ray)and MS(magnetic susceptibility)series.The calculated sedimentation rate range from 0.91 to 0.95 cm/kyr.The complete Milankovitch signal was recorded in this interval,including long eccentricity period of405 kyr,short eccentricity period of 84.5-96.3 kyr,obliquity period of 30.8-31.4 kyr,and precession period of 15.8-21.7 kyr.Based on 405 kyr calibration,a floating astronomical time scale of?7.7 Ma has been established,which provides an estimated duration of the Kasimovian and Gzhelian stages as 2.94 Myr and 4.76 Myr,and duration of Idiognathodus eudoraensis,Streptognathodus zethus,I.simulator,and I.nashuiensis as 39 kyr,792 kyr 132 kyr and 1221 kyr.The modulation in short eccentricity and obliquity cycles was?2.4 Myr and?1.2 Myr cycle,which can be ascribed as the g₄-g₃term and s₄-s₃ term,respectively.The Bayesian inversion approach estimated long eccentricity period as 403.7 kyr,short eccentricity period as 95.5 kyr to 131.0 kyr and precession period as 16.9 kyr to 20.5 kyr at?303 Ma.It is estimated that the precession constant is 59.03±3.502 arcsec/y,the distance between the earth and the moon is371.59±4.88×10⁶ m,and the length of the day is 22.19±0.64 h.The comparation among the sea-level trend from Narao and the published record from Donets Basin in Ukraine and Great Basin in USA indicates that the sea-level changes in low latitude were synchronously driven by the 405 kyr and 2.4 Myr eccentricity cycle.