Microfacies and Evolution of Sedimentary on Carbonate Rocks around the Permian-Triassic Boundary in Xianfeng, South Sichuan

Key words: 
• microbialites /
• microfacies /
• the Permian-Triassic boundary /
• sedimentary sequences /
• south of Sichuan province

Abstract: Based on the sedimentary characteristics of the Permian-Triassic boundary (PTB) strata in Xianfeng, combined with analysis from boreholes lithofacies, the stratum was divided into two kinds of sedimentary facies, including homoclinal carbonate ramp and rimmed carbonate platform, and seven kinds of subfacies, such as high-energy bioclastic shoal, inner ramp and intertidal flat and so on. Then analyzing microfacies characters of the carbonate rocks, such as biotic assemblage, grain composition and supported type. Boundary strata of the studied section can be mainly divided into fourteen microfacies types. From this, the sedimentary facies sequences and evolution of this period were established. The studied section outputs one layer of microbialites. From observations of plain microscope, the following results were obtained: microbialites consist of stromatolites; in most of these microbialites, coccoid cyanobacterias, calcified sheaths and microsparry cladding layer were found largely. Characteristics of the biotic assemblage were be analyzed after grinding the thin sections continuously for rock core of K9 limestone which is 0.58 m long and comes from borehole ZK1501. There are abundant fossils in this place where 6 cm away from the bottom of the admitted major extinction horizon, such as Trilobites, Gastropods and Phylloid Algae. Above the stratum, miniaturized Ostracods(<200 μm) were found. We concluded obviously that the major extinction horizon of Permian should be located at the place, while PTB is at the middle and lower segments of the miniaturized Ostracods layer.