Conodont Biostratigraphy and Age Determination of the Lower-Middle Triassic Boundary in South Guizhou Province, China

The demarcation of the Lower-Middle Triassic boundary is a disputed problem in global stratigraphic research. Lower-Middle Triassic strata of different types, from platform to basin facies,are well developed in Southwest China. This is favorable for the study of the Olenekian-Anisian boundary and establishing a stratotype for the Qingyan Stage. Based on research at the Ganheqiao section in Wangmo county and the Qingyan section in Guiyang city, Guizhou province, six conodont zones have been recognized, which can be correlated with those in other regions, in ascending order as follows: 1, Neospathodus 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; and 6, Neogongdolella regalis Range-Zone. An evolutionary series of the Early-Middle Triassic conodont genera Neospathodus-Chiosella-Neogongdolella discovered in the Ganheqiao and Qingyan sections has an intermediate type named Neospathodus qingyanensis that appears between Neospathodus homeri and Chiosella timorensis in the upper part of the Neospathodus homeri-N, triangularis Zone, showing an excellent evolutionary relationship of conodonts near the Lower-Middle Triassic boundary. The Lower-Middle Triassic boundary is located at 1.5 m below the top of the Ziyun Formation, where Chiosella timorensis Zone first appears in the Qingyan section,whereas this boundary is located 0.5 m below the top of the Ziyun Formation, where Chiosella timorensis Zone first appears in the Ganheqiao section. There exists one nearly 6-m thick vitric tuff bed at the bottom of the Xinyuan Formation in the Ganheqiao section, which is usually regarded as a lithologic symbol of the Lower-Middle Triassic boundary in South China. Based on the analysis of high -precision and high-sensitivity Secondary Ion Mass Spectrum data ,the zircon age of this tuff has a weighted mean 206Pb/238U age of 239.0±2.9 Ma (2σ), which is a directly measured zircon U-Pb age of the Lower-Middle Triassic boundary. The Ganheqiao section in Wangmo county can therefore provide an excellent section through the Lower-Middle Triassic because it is continuous, the evolution of the conodonts is distinctive and the regionally stable distributed vitric tuff near the Lower-Middle Triassic boundary can be regarded as a regional key isochronal layer. This section can be regarded not only as a standard section for the establishment of the Qingyan Stage in China, but also as a reference section for the GSSP of the Lower-Middle Triassic boundary.