Late Early Cambrian archaeocyath reefs in Hubei Province, South China: modes of construction during their period of demise.

The Lower Cambrian (lower Toyonian) Tianheban Formation of Hubei Province, South China, includes small archaeocyath-calcimicrobial reefs that formed by combinations of archaeocyaths (e.g., Archaeocyathus yichangensis) and calcimicrobes (including Epiphyton, Renalcis, and Girvanella). The archaeocyaths provided substrates onto which the calcimicrobes became attached. In particular, Girvanella encrusts directly upon the surfaces of archaeocyaths, and bush-shaped aggregations of Epiphyton, growing downward, spread over the Girvanella. The remaining spaces among these reef framework-builders are infilled by chambered forms of Epiphyton and/or Renalcis. These calcimicrobes made a strong contribution to reinforcement of the reef frameworks. The archaeocyath-bearing reefs in South China first appeared during the Atdabanian and are last seen in the early Toyonian Tianheban Formation in Hubei Province. Subsequent reefs are purely microbial reefs without archaeocyaths. The Tianheban reefs are therefore the last representatives of archaeocyath-bearing reefs in South China. These reefs, reported herein, record the transition from archaeocyath-calcimicrobial reefs to purely microbial reefs during the Toyonian. Further studies involving integrated geobiological and geochemical analyses are needed to identify the factors that led to the demise of archaeocyath-bearing reefs and that subsequently hindered the development of skeletal-dominated reefs for 40 million years. [ABSTRACT FROM AUTHOR]