Famennian (Upper Devonian) calcimicrobial ( Renalcis) reef at Miaomen, Guilin, Guangxi, South China

Abstract

Famennian (Upper Devonian) reefs represent a calcimicrobial and stromatolitic reef framework with a few skeleton-dominated (stromatoporoid) examples after the skeletal metazoans in reef ecosystems were impacted on a global scale by the Frasnian–Famennian extinction event. Calcimicrobes, thrombolites, stromatolites, and biologically induced cement formed the major part of reef framework volume and contributed to rigidity of reefs. In this study, one example of Famennian non-skeletal carbonate buildups at Miaomen, Guilin, South China, has been documented in detail. Thick and massive limestones in the platform margin facies show a lateral transition to well-bedded fenestral and laminated limestone in the back-reef facies southeastward and to well-bedded intraclastic grainstone, mudstone, and shale with lithoclasts and breccias in fore-reef slope facies northwestward. The Miaomen reef is almost exclusively constructed by calcimicrobes and cement. Major reef builders are Renalcis, Izhella, Paraepiphyton, Garwoodia, and a ‘ Keega’-like microbe. Other reef builders are Wetheredella, Rivularia, Rothpletzella, Ortonella, and Girvanella. Some less common algae and calcimicrobes also occur in the Miaomen reef, including Parachaetetes, Solenopora, Tharama-like objects, and unidentifiable microbes. Cavities are well developed in the reef limestone. The Miaomen Renalcis reefs developed along leeward platform margin settings adjoined by intraplatform depressions and rimmed Famennian carbonate platforms along with stromatolite reefs, ooid shoals, brachiopod-shell shoals, nautiloid shoals, and previously described Renalcis–Epiphyton and Renalcis-cement reefs. Miaomen reefs demonstrate the significant roles of calcimicrobes and microbial carbonates in the development of Famennian carbonate systems in South China. Famennian microbial reefs in Guilin represent an interval of profound biotic change in the style and extent of carbonate buildups and in the composition of buildup communities and indicate important environmental and ecological changes within the carbonate system.