Graptolite Biozonation of the Wufeng and Longmaxi Formations and Its Environmental Implications from the Xindi 2 Borehole in Yongshan-Daguan Area, NE Yunnan

Due to the lack of detailed division and comparison of graptolite biozonation in Wufeng and Longmaxi formations of Yongshan-Daguan area, the distribution time of black shale and its sedimentary background are unclear. Nine graptolite biozones of the Wufeng and Longmaxi formations are recognized in the Xindi 2 borehole from the Yongshan-Daguan area, NE Yunnan in this paper. They are Dicellograptus complanatus, D. complexus, P. pacificus-M. extraordinarius, A. ascensus-P. acuminatus, C. vesiculosus, C. cyphus, D. triangulatus, L. convolutes, and S. sedgwickii biozones. A correlation of the Wufeng-Longmaxi graptolite biozones among the Xingdi 2, Ning 211, Jiaoye 1, Wei 202, and Wuxi 2 boreholes are indicated. The top of the Longmaxi black shale is diachronous, from which the top of the Longmaxi black shale of the Xindi 2, Ning 211, and Jiaoye 1 boreholes are one biozone lower than those of the Wei 202 and Wuxi 2 boreholes. High TOC content in the Longmaxi black shale is measured from WF2 (D. complexus Biozone) to LM6 (D. triangulatus Biozone) in the Xindi 2 borehole but only up to LM 5 (C. cyphus Biozone) in the Ning 211 borehole near the Central Guizhou uplift. However, the top of the Longmaxi black shale in Wei 202 and Wuxi 2 boreholes may reach to the level of LM9 (S. guerichi Biozone).According to our study, the transgression to maximum flooding period is shortened near the uplift or the old land, and the upper limit of high-quality black shale can be as low as LM5 (or below). Sedimentary differentiation occurred in Weiyuan, Jiaoshiba and Wuxi areas, which was affected by sedimentary water depth and the debris input from the uplift.