Restudy of the microbialite from the Permian-Triassic boundary section, Chongqing

Restudy of the microbialite in the Permian-Triassic boundary section in Laolongdong, Chongqing, reveals that the main factors controlling the formation of the dendroid structure of the microbialite were diagenesis and microbes. Diagenesis led to formation of cements of coarse calcite in the pores between carbonate grains and inner pores of fossils, and local recrystallization of carbonate components. The irregular coarsely-crystalline part of the rock resulted from the two diagenetic processes appears as dark-colored dendroid part of the rock outcrop. The presents or abundance of microbes in the microbialite formed pores after they died and decomposed, which provided pathway for migration of diagenetic fluids and pore spaces for cements. The microbial fossils are irregular in shapes, and are butterfly, cloudlet, irregular round and lunular shapes in thin sections. They generally vary from 100 to 600 mu m in size. Without any original inner structure being preserved, they are generally internally filled by calcite cements. In terms of their shapes and sizes, the microbial fossils are presumed to be formed from some bacterial colony.