A laminated microbial ecosystem at the summit of the Cambrian Explosion

Microbially induced sedimentary structures (MISS) are microbial traces in sandy deposits. They are formed by various modes of microbial behaviour in response to the prevailing physical dynamics in shallow marine environments since the early Archean. The Guojiaba Formation (Cambrian Stage 3), composed of laminated sandstones, siltstones, and mudstones mainly, is characterized by two units of MISS (biolaminite) in the north margin of the Yangtze Block. Biolaminite arises from the microbial growth and it is characterized by alternation of light layers and dark layers in polished slides and the 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 identified 7 types of morphologies of microorganisms of a microbenthic community accompanied by a large number of Extracellular Polymeric Substances (EPS) and pyrite framboids from the dark layers. The occurrence of the Biolaminite in the Guojiaba Formation indicates a complex microbial ecosystem within epibenthic microbial mats in a relatively semi-closed to closed sedimentary environment with a high salinity, such as a lagoon, at the summit of the Cambrian Explosion.