A laminated microbial ecosystem at the summit of the Cambrian Explosion

The Guojiaba Formation (Cambrian Stage 3), composed of laminated sandstones, siltstones, and mudstones mainly, is characterized by two units of MISS (biolamination) in the north margin of Yangzi plate. Biolamination arises from the microbial growth and it is characterized by alternation of light layers and dark layers in polished slides and thin sections. The light layers represent the normal background deposition where the development of microbial community is inhibited; whereas, the dark layers are rich in organic materials. SEM examination on the dark layers identified seven types of microorganisms in morphology confirmed the development of microbial community. The occurrence of the biolamination in Guojiaba Formation indicates a complex microbial ecosystem within epibenthic microbial mats in a relatively semi-closed to closed sedimentary environments at the summit of the Cambrian Explosion.