Formation Mechanism And Paleoenvironmental Significance Of The Mesoproterozoic Microbial Reef

In recent years, the study of Precambrian microbial reefs is becoming one of the major concerns in geologic research, especially for geobiology. A large scaled massive microbial reef has been found in the fourth member of the Mesoproterozoic Gaoyuzhuang Fm at the Gangou section, Yanqing County, which forms the main topic of the present study. It is well known that microbes can trigger carbonate precipitation and mineralize organic matter through various biological activities in both modern and ancient ocean environments. The interactions between microbes and environments are of two directional. Therefore, the study of Precambrian microbial reefs has significance in revealing the microbial communities, ancient environment changes, and the microbe-environment interactions.In this thesis, the microbial reefs in the fourth member of the Mesoproterozoic Gaoyuzhuang Fm at the Gangou and Malugou sections were studied from the angles of their morphology, structures, mineral compositions, and depositional facies. Three major components, such as various columnar stromatolites as reef frames, herringbone calcites as cements, and variably developed pores as internal reef openings, can be widely recognizable throughout the reefs. Both field and microscopic observations of the specimens have revealed the existence of condensed pores resulted from gas bubbles along the central axis of the conical stromatolites, suggesting that cyanobacterial photosynthesis was one of the important microbial processes facilitating formation of the stromatolites and carbonate precipitation. This also provides a more plausible explanation for the oxygenation event recognized recently between the third and fourth member of the Gaoyuzhuang Fm. It is worth to note that herringbone calcites are quite abundant in the middle parts of the reefs both in the Malugou and Gangou sections, but are rare in the reefs of other times. It has been suggested that as a kind of authigenic carbonate precipitates, herringbone calcites were commonly formed in anoxic to dysaerial environments in which Fe2+ and Mn2+ ions were highly enriched in waters. Our study shows that microbial process and degradation of organic matter under anoxic conditions may have prompted the precipitation of fan-shaped and fiberous calcites and the herringbone calcites along margins of the large sized pores in the core reefs.Both microscopic and SEM observations have revealed abundant remnant microbial filaments. Therefore, it seems reasonable to infer that microbial processes and organo-mineralization have played important roles in the formation of the carbonates and the microbial reefs. In this study, the redox characteristics of the Mesoproterozoic ocean in the north China platform are also dealt with, based on the development of various organo-minerals and the potential bio-chemical processes that might be involved.As an important geologic archive, microbial reefs have recorded the interactions between microbes and marine environments in remote earth history. And due to their enrichment in organic matter and porosity, microbial reefs have great potential for oil and gas exploration.