High-resolution carbon isotope stratigraphy of the Lower and Middle Ordovician succession of the Yangtze Platform, China: implications for global correlation

摘要

The Lower and Middle Ordovician of the Yangtze Platform, China, is characterized by a sedimentary succession dominated by carbonate rocks. Three sections spanning the Nantsinkuan/Lunshan, Fenhsiang, Hunghuayuan, and Dawan/Zitai formations, corresponding to the Tremadocian-Dapingian in age, have been sampled for high-resolution delta C-13 chemostratigraphy (542 samples in total). Our new delta C-13 data reveal five tie-points with the potential for global correlation: (1) a positive delta C-13 excursion in the lower Nantsinkuan Formation within the Tremadocian Rossodus manitouensis Zone; (2) an excursion with two peaks roughly within the late Tremadocian Paltodus 'deltifer' Zone; (3) a positive delta C-13 shift in the lower Hunghuayuan Formation, within the early Floian Serratognathus diversus Zone; (4) a gradual positive delta C-13 shift in the late Floian, ranging from the uppermost S. diversus Zone to the basal Oepikodus evae Zone; (5) a minor negative shift in the lower Dawan/Zitai Formation, within the early Dapingian Baltoniodus triangularis Zone. These excursions are herein used for correlation of the Yangtze Platform strata with successions from South China, North China, the Argentine Precordillera, North America and Baltica. From a palaeogeographical perspective the Gudongkou, Xiangshuidong and Daling sections represent depositional environments along an inner to outer ramp profile. delta C-13 data from these sections show successively heavier (higher) delta C-13 values with increasing depositional depth. This is interpreted as due to remineralization of organic carbon within the carbonate rocks.