Cyclostratigraphy Research On The Middle Ordovician Of Huangnitang Section In Changshan, Zhejiang, China

In the past decades, the discipline of cyclostratigraphy has been rapidly developed. In the Geological Time Scale of 2012, the Cenozoic Period has almost completely calibrated to the full astronomical solution, and most of the Mesozoic Period calibrated by the stable 405-kyr orbital eccentricity cycles. It is expected that the Paleozoic astrochronology will be the next frontier in the cyclostratigraphy.In this study, we conduct cyclostratigraphic analysis on the Huangnitang section, which is the Global Stratotype Section and Point (GSSP) of the Darriwilian stage of Middle Ordovician, and a CJ3 core near the section. A total of 2253 and 1670 specimens were collected from the section and core, respectively. High-resolution magnetic susceptibility (MS) series were obtained and used as paleoclimate proxy. MTM spectral analysis and ASM (Average Spectral Misfit) analysis shows that both of the section and core record well Milankovitch signals. According to the results of 405 kyr tuned MS series, two floating astronomical time scales are established for the section and core. This time framework suggests that the duration of Darriwilian is 8.7 myr, and the durations of member 2 of Hule formation (O2h), member 4 and 3 of Ningkuo formation (O2n4 and O2n3) are 3.18 myr,5.91 myr and 0.87 myr respectively. In ascending order, the durations of complete graptolite biozones are Isograptus caduceus imitates (2.82 myr), Exigraptus clavus (0.36 myr), Arienigraptus zhejiangensis subzone (2.64 myr), Undulograptus sinicus subzone (1.78 myr), Acrograptus ellesae(2.39 myr), Nicholsograpts fasciculatus(0.94 myr), Pterograptus elegans (0.66 myr) and Hustedograptus (0.31 myr). In addition, the suggested base of Middle Ordovician in the CJ3 core lies at the depth of 65.45m, above the FAD of Isograptus caduceus imitates～400 kyr.Amplitude modulations of interpreted 405 kyr and 30 kyr cycles of the MS time series show the existence of long-period of～4 myr and～1.2 myr in the section and core. The～4 myr cycles may be ontrolled by the larger time scale transgressive-regressive, and the～1.2 myr cycles originate from S4-S3.