Cyclostratigraphy Research On The Late Ordovician-Early Silurian Marine Strata In Yichang,Hubei,South China

The Late Ordovician is characterized by the massive global glaciation,the first mass extinction in Phanerozoic,multiple stable carbon isotope excursions.A precise chronostratigraphic framework plays an important role in improving our understanding of these major geological events and their possible relationships.In this study,detailed cyclostratigraphy analysis is conducted on the shale interval of core Yihuang-1 from Huanghuachang,Yichang,South China.The shale part of Yihuang-1 core recovered Wufeng and Longmaxi formations in an ascending order,and contains Ordovician and Silurian boundary.Biostratigraphic constraint for this core from 10 graptolite biozones indicates an age of Katian to Earlay Aeronian.A total of 2576 sample was collected with an interval of about 1 centimeter.High-resolution magnetic susceptibility(MS)series was obtained and used as paleoclimate proxy.MTM power spectra and evolutionary fast Fourier transformation(efft)results reveal whole package of Milankovitch cycle signals.A floating astronomical time scale is constructed for this core based on the extracted stable 405 ka long eccentricity signals.New astrochronology framework suggests that the durations of Hirnantian and Rhuddanian stages are 0.94 Ma and 1.36 Ma,respectively.In ascending order,the calculated duration of graptolite biozone was 1.85 Ma for Dicellograptus complexus,0.80 Ma for Paraorthograptus pacificus,0.63 Ma for Normalograptus extraordinarius,0.22 Ma for Persculptograptus persculptus,1.15 Ma for Demirastrites triangulates and 1.46 Ma for Lituigraptus cinvolutus,respectively.The age of Ordovician-Silurian boundary is estimated as 442.98 ± 1.6 Ma by anchoring the floating ATS to an U-Pb age(443.2 ± 1.6 Ma)of bentonite from the upper of Guanyinqiao bed.The duration of the Late Ordovician mass extinction and the carbon isotope excursion(HICE)was estimated as ~0.60 Ma and ~1.08 Ma,respectively.