Conodont Biostratigraphy and Carbonate Carbon Isotope Chemostratigraphy of the Smithian-Spathian Boundary Interval (upper Lower Triassic)

The Smithian-Spathian transition is a crucial period of the Early Triassic that witnessed significant biological, climatic and environmental events. A precise Lower Triassic timescale is essential to understand the sequence of these events. The conodont biostratigraphy of the Olenekian is not as refined as that of the Induan stage. The sub-stage boundary within the Olenekian, the Smithian-Spathian boundary (SSB), has not yet been formally defined, which has hindered the understanding of these geological events and their global correlation during the late Early Triassic. The conodont Novispathodus pingdingshanensis was first described from the West Pingdingshan section, Chaohu, South China, and has been proposed to mark the SSB by its first appearance. It has a wide paleogeographic distribution and has been verified in numerous sections from different water depth environments and low to middle latitude areas in both Tethys Ocean and Panthalassa Ocean, including Vietnam, Slovenia, Japan, Oman, Spiti, India, Kashmir, Pakistan, western Canada, western USA, and China (e.g., Guizhou, Guangxi, Hubei, Jiangsu, and Xizang). Besides its wide paleogeographic distribution, Nv. pingdingshanensis is usually abundant in these study sections. In addition, the δ13Ccarb signal changes rapidly from the middle Smithian (N3 minimum) to the early Spathian (P3 maximum) through the Smithian-Spathian transition interval, which has been reported from many SSB sections globally. On the basis of the correlation between carbon isotopes and the FO of Nv. pingdingshanensis summarized by Zhang et al. (2019), the midpoint of the positive carbon isotope shift mdpt (N3-P3) is generally close to the level of the SSB based on biostratigraphic constraints, especially for those well-studied SSB sections in the Tethys region. At the West Pingdingshan section, mdpt (N3-P3) is located at the same level as the first occurrences of Nv. pingdingshanensis, 5.33 m below P3, whereas the mdpt (N3-P3) is ~5.5 m below P3, a little higher than the FO of Nv. pingdingshanensis at the Guryul Ravine section, Kashmir, and similar results reported from the Zuodeng section, Guangxi, South China. Therefore, the mdpt (N3-P3) can serve as an auxiliary marker for the SSB defined by the first appearance of Nv. pingdingshanensis.