High-resolution carbon isotope stratigraphy of the Lower and Middle Ordovician succession of the Yangtze Platform, China

Abstract

Variation in the relative abundance of the stable carbon isotopes has been widely used to correlate Ordovician marine successions over the past two decades. To date, only a few of studies of Ordovician carbon chemostratigraphy have been conducted in South China. Most of the previous studies in this field have focused on specific time intervals and/or events in the Middle and Upper Ordovician. The Lower and Middle Ordovician of the Yangtze Platform is typically represented by a sedimentary succession dominated by carbonate rocks, which is ideal for studying the carbon chemostratigraphy. 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 δ13C chemostratigraphy. Our new δ13C data reveal five tie-points with the potential for global correlation: (1) a positive δ13C 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 δ13C shift in the lower Hunghuayuan Formation, within the early Floian Serratognathus diversus Zone; (4) a gradual positive δ13C 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. The δ13C data from these sections show successively heavier (higher) δ13C values with increasing depositional depth, which can be interpreted as due to remineralization of organic carbon within the carbonate rocks formed in the shallow-water environment.

Keywords

• lower and middle ordovician
• carbon isotopes
• chemostratigraphy
• yangtze platform
• south china