Research on Conodont Biostratigraphy near the Bottom Boundary of the Middle Triassic Qingyan Stage in the Southern Guizhou Province

The demarcation of the Lower—Middle Triassic boundary is a disputed problem in stratigraphic research in the world. Lower—Middle Triassic strata of different types, from platform facies to basin facies, are well developed in Southwest China. This is helpful for the research on the Global Stratotype Section and Point (GSSP). According to the research on conodonts in the Ganheqiao section in Wangmo County and the Qingyan section in Guiyang City, Guizhou Province, six conodont zones, which can be correlated with those in other regions, have been recognized. They are in ascending order as follows: ① Neospathdus cristagalli interval-zone, ② Neospathodus pakistanensis interval-zone, ③ Neospathodus waageni interval-zone, ④ Neospathodus homeri- N. triangularis assemblage-zone, ⑤ Chiosella timorensis interval-zone, ⑥ Neogongdolella regale range-zone. An evolutionary series of conodont Neospathodus—Chiosella—Neogongdolella occurred in the Ganheqiao section and Qingyan section in the Early—Middle Triassic. In this evolutionary series, a transitional Neospathodus qingyanensis appeared between Neospathodus homeri and Chiosella timorensis in the upper part of Neospathodus homeri- N. triangularis zone. This shows an excellent Early—Middle Triassic evolutionary series of conodont across the Lower—Middle Triassic boundary. The Lower—Middle Triassic boundary is located 1.5m below the top of the Ziyun Formation, where the Chiosella timorensis zone first appears on the Qingyan section; while the Lower—Middle Triassic boundary is situated 0.5 m below the top of the Ziyun Formation, where the Chiosella timorensis zone first appears on the Ganheqiao section. Based on the above-mentioned discussion, the present authors suggest that the Qingyan section can be regarded as a candidate of the GSSP of the Lower—Middle Triassic boundary and the Ganheqiao section as a reference of the GSSP of the Lower—Middle Triassic boundary for further necessary investigation.