The Relationship between the ＂Green-Bean Rock＂ Layers and Conodont Chiosella timorensis and Implications on Defining the Early-Middle Triassic Boundary in the Nanpanjiang Basin, South China

The Triassic “Green-bean Rock” (GBR) layers were widely recognized around the Early-Middle Triassic boundary interval in the Nanpanjiang Basin, South China. To determine the precise relationship between the GBR layers and the first appearance datum (FAD) of the conodont Chiosella timorensis, four Lower-Middle Triassic sections from the Nanpanjiang Basin, including the Gaimao, Bianyang II, Zuodeng and Wantou sections have been studied in detail. Detailed conodont biostratigraphy convinces us that there is no exact temporal relationship between the GBR layers and first occurrence of Ch. timorensis. Moreover, the numbers of the GBR layers are different from the place to place within the Nanpanjiang Basin, and the time span of the GBR layers was much longer than previously estimated. Global correlations show that the FAD of Ch. timorensis is contemporaneous basinwide and worldwide and more suitable marker defining the Olenekian-Anisian boundary (Early-Middle Triassic boundary) than any other proxies.