CONODONT BIOSTRATIGRAPHY AND ASTRONOMICAL TUNING OF THE LOWER–MIDDLE ORDOVICIAN LIANGJIASHAN (NORTH CHINA) AND HUANGHUACHANG (SOUTH CHINA) MARINE SECTIONS

In this study, systematic conodont biostratigraphy and cyclostratigraphy are carried out on Ordovician sections from the North China and South China blocks. The Liangjiashan section, situated on the North China Block margin, accumulated shallow marine carbonate during the Early Ordovician. The Huanghuachang section (GSSP for the Dapingian Stage), situated on the South China Block margin, accumulated thinly bedded carbonates and minor clastics in a deep shelf environment. Conodont biostratigraphy and 405-kyr long-eccentricity cycles interpreted from the Liangjiashan and Huanghuachang sections are used to establish an Astronomical Time Scale (ATS) for the Early–Middle Ordovician period. The ATS indicates that the duration of the Floian Stage is 7.08±0.405 Myr. The temporal ranges of Lower–Middle Ordovician conodont biozones are also estimated: the durations of the Serratognathus bilobatus, Serratognathus extensus, Oepikodus communis, Oepikodus evae, and Baltoniodus triangularis zones are 1.0 Myr, 5.1 Myr, 0.2 Myr, 0.8 Myr, and 0.2 Myr, respectively. The evolution of the ratio of obliquity to total power and short-eccentricity power to total power depicts periodicities of 1 Myr for the s₄–s₃ term, and 1.9 Myr shifting to 1 Myr for the g₄–g₃ term. The shift in the g₄–g₃ term may indicate a chaotic transition in the Mars and Earth orbital motions from a (s₄–s₃)–2(g₄–g₃) to (s₄–s₃)–(g₄–g₃) resonance state during the Early Ordovician.

Key Words: Cyclostratigraphy; Conodont biozones; Astronomical time scale; Floian; Earth-Mars chaotic transition