A late-Devonian (Famennian) Renalcis-Epiphyton reef at Zhaijiang, Guilin, South China

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Microbial reefs, together with stromatolitic mounds and ooid shoals, constitute massive limestones in Famennian platform marginal strata in Guilin, in sharp contrast to the well-known coral-stromatoporoid reefs in the Givetian and Frasnian. Microbes played a significant and important role as stabilizers in the Famennian carbonate deposits of Guilin. A reef at Zhaijiang was constructed by Epiphyton and Renalcis, and is representative of such carbonate buildups. The reef is situated 10 km west of Guilin and corresponds to a microbe-dominanted platform margin carbonate complex. Organisms in the Zhaijiang microbial reef are low diversity and dominated by ostracods and two genera of microbes, Epiphyton and Renalcis. Other microbial genera such as Sphaerocodium and Wetheredella occur in most of reef facies in Guilin, but their role as reef builder is doubtful because they occur only in minor amounts. The same four genera occur in volumetrically significant amounts in the upper Devonian carbonate complexes of Alberta, Canada and Western Australia. However, Epiphyton is more abundant in the Guilin reefs. The Zhaijiang microbial reef developed above Famennian proximal slope facies, as suggested by reef architecture and paleogeographic setting. The facies sequence of the microbial reef can be divided into three parts. The lower part is composed of medium-bedded bioclastic grainstones with a few microbial framestone lithoclasts, representing a proximal slope facies. The middle part consists of thin-bedded mudstone and shale with limestone lenses that are thought to be low stand deposits. In some cross sections, mudstone and shale infilled tidal channels that developed in the bioclastic grainstones. Renalcis-Epiphyton framestone constitutes the upper part with massive stacking patterns. The reef is 35 m thick and over 50 m in width. Nine litho- and biofacies are recognized. Zhaijiang reef provides an example of a binder guild-dominated buildup in the almost vacant reef ecosystem of the Famennian and represents a characteristic kind of reef after the Frasnian/Famennian extinction.

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