Proposal for the Terrestrial Stratotype of the Qishuihe Stage (Carnian) in the Ordos Basin, China

Triassic terrestrial deposits are widely distributed in Pangea including circum-Paleo-Tethys Ocean and Gondwana. Although the GSSP of Carnian (Upper Triassic) was nailed at the Prati di Stuores, Italy and marked by the FAD of Daxatina canadensis (ammonite) and the FO of Paragondolella polyhnathiformis(conodont), the correlations between the marine and terrestrial deposits are far from being resolved (Mietto et al., 2012). Although the FAD of Pattinasporites densus and Vallasporites ignacii was put forward as the palynological criteria, the need for terrestrial stratotype is getting more and more important, especially in Gondwana, northern and northwestern China, and the other part of the Eurasia. The Qishuihe stage was proposed to correlate with Carnian, and named after the stratotype Qishuihe profile in Tongchuan of Shaanxi, China (Tong and Yin, 2015). The profile was measured by the Geological Institute of Chinese Academy of Geological Sciences in 1980. The boundary between the Qishuihe stage and Jinshuoguan stage was set at the bottom of the layer 249 on the Qishuihe profile (Li et al., 2016). While, the base of the Yangjiaping stage (=Qishuihe stage) was defined as the FAD of Asterotheca szeiana and Danaeopsis fecundaon the Kuyanhe profile in Shenmu of Shaanxi, China (Li et al., 2016). The two profiles are 300 Km apart, and there isn’t any report of Asterotheca szeiana and Danaeopsis fecunda in the 233.1 m thick deposits above the layer 248 on the Qishuihe profile (Wang, 1980). Moreover, Asterotheca szeiana was also documented in the Lower Triassic Lingwen Formation in Hainan, China (Zhou and Li, 1979; Zhang et al., 1992). Therefore, the FAD of Danaeopsis fecunda is practicable (Deng et al., 2018). Recent studies on the palynological biostratigraphy indicated that the YC7-3 oil-shale of the Yanchang Formation on Yunmeng profile, Tongchuan of Shaanxi is aged earliest Carnian (Li et al., 2023). These species in the YC7-3, such as Cyathidites minor, Dictyophylliditesharrisii, Apiculatisporisbulliensis, Aratrisporitesxiangxiensis, Piceaepollenitesomoriciformis, Podocarpiditesornatus, Discisporitesgranulus, and Classopollis sp. were only found in the Carnian or even younger than Carnian stage, and the age of YC-3 was dated as 236.0-234.1 Ma by U-Pb isotopic dating (Sun et al., 2020). The coexistence of Dictyophylliditesharrisii, Apiculatisporisbulliensis, Aratrisporitesxiangxiensis, Piceaepollenitesomoriciformis, Podocarpiditesornatus, Discisporites and Classopollis is regarded as the criteria for the Carnian palynoflora (Li, 2023). Although the FADs of these species are not at the same level on the Yunmeng profile and the Borehole of Y1011, the attempt for terrestrial Carnian stratotype is still very valuable. This work is supported by the National Nature Science Foundation of China (41972012) and the National Special Project for Deep Earth: Helium Formation and Enrichment Mechanisms (No. 2025ZD1010601).