Mass-occurrence of oncoids in the Early-Middle Cambrian transition at western margin of North China Platform : A response of microbial community to shallow marine anoxia
Geological records showed that an important biological extinction event happened during the Early-Middle Cambrian transition period, but there are controversial views about its orign. In order to reveal the response of microbial community to the biotic crisis and the marine environment change,micro-fabrics and organo-minerals in the oncoid cortices formed in that period at the North China Platform were studied. The study revealed that abundant microbial fossils exist in the oncoids. Study of microfabrics and organomineralization patterns in the oncoid cortices suggested that the oncoids were likely formed in seawater with high alkalinity and active bacterial sulfate reduction (BSR). Rich pyrite framboids and purported heterotrophic remains in the cortices represented an anoxic bottom-water condition, and facies and depositional sequence analyses suggested a shallow shelf environment. A wide correlation indicated that the time-equivalent oncoids or related microbialites were widespread in the North China Platform, and also well recorded in some other continents or plates. This may imply an extensive bloom of microbial communities in the aftermath of terminal Early Cambrian biotic crisis, which was likely related to a pervasive anoxia in shallow marine environments caused by LIP (Large igneous province) eruptions,and subsequently global warming and invasion of anoxic deep seawater onto the shelves at that time. The broad distribution of oncoids and other related microbialites at the Early-Middle Cambrian transition may have reflected an ecological response and rapid expansion of microbial community to the high-stressed environments where benthic metazoans had been largely suppressed during the biotic crises.