Cambrian Miaolingian microbial reefs dominated by calcified microbes at the Jinzhou section, Dalian, Liaoning Province
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Abstract:
The Cambrian Miaolingian microbial reefs exposed at the Jinzhou section, Dalian, Liaoning Province were developed in the highstand system tract of third-order depositional sequences. Macroscopically, these microbial reefs lack distinctive internal structures and mainly consist of micrites, forming carbonate rock complexes with associated stromatolites, oolitic grainstones, and oncolitic grainstones. Microscopically, observations under a polarizing microscope reveal the presence of microclots, calcified microbes, benthic oolites, and various micritized structures within the microbial reefs. The calcified microbes include Girvanella, Renalcis, Tarthina, Epiphyton, and possible Angulocellularia. Girvanella exhibits four growth habits, and both Girvanella and Renalcis aggregates are present. Variations in the composition of calcified microbial groups in different layers may be related to the changes of sedimentary environments. The calcified microbes in the study area, together with the micritized or dolomitized calcified microbes and associated pyrites in different microbial structures, indicate that both photoautotrophs and heterotrophic sulfate-reducing bacteria played important roles in the formation of the microbial reefs. These features also suggest that the preservation of calcified microbes is not only controlled by early lithification but also is closely linked to heterotrophic bacterial activities during diagenesis. The Cambrian Miaolingian microbial reefs at the Jinzhou section provide an example of reefs dominated by calcified microbes.
