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Hybrid carbonates, defined as the combinations of in situ abiotic, microbial, and skeletal precipitates, have changed with the biological and environmental conditions throughout geological history. After a long interval of dual-hybrid carbonates, the triple hybrid carbonates became predominant in the late Pennsylvanian and lasted until the Middle Triassic. The organic reefs of the upper Moscovian–Kasimovian (mid–late Pennsylvanian) strata of Guizhou, South China, are composed of abundant microbial carbonates, abiotic marine cements, and skeletal organisms, which are classified as triple hybrid carbonates. Microbial boundstones consisting of aragonitic cements (e.g., botryoidal cements) and microbial micrite are the major contributors to the reefs, as framework constructors, cavity fillers, and hard substrate for the encrusting metazoans. Skeletal components (e.g., the colonial rugose coral Ivanovia) and stromatolitic microbial carbonates could build simple frameworks with the growth cavities filled by syndepositional marine cements, automicrite, and carbonate particulate sediments. In situ phylloid algae normally form patch reefs and biostromes with abundant microbial micrite and peloids, while algal fragments contribute to the skeletal–cement–microbial reefs as large grains encrusted by microbial carbonates or marine cements. Petrographic features suggest the reefs formed at a shelf margin or upper slope setting with active water circulation. The evolution of Carboniferous organic buildups was primarily driven by changes in reef-building organisms, closely linked to external conditions, specifically seawater chemistry and paleoclimate. The ubiquitous triple hybrid carbonates in South China reefs, in agreement with Pennsylvanian tropical shallow-water reefs in their components, are proposed to derive from the interaction between decreasing atmospheric CO2 concentration, high seawater Mg2+/Ca2+ ratio, warm tropical water, and the evolution of skeletal organisms in the Pennsylvanian. [ABSTRACT FROM AUTHOR]

The Pennsylvanian is characterized by intense paleoenvironmental changes related to glacio-eustatic sea-level fluctuations and major tectonic events, which affected the evolution of biocommunities. Most known Pennsylvanian tropical reefs and mounds are predominantly composed of calcareous algae (e.g. phylloid algae, Archaeolithophyllum), calcareous sponges, fenestrate bryozoans, Tubiphytes, and microbialites. However, in Houchang (southern China), the Late Pennsylvanian carbonate platform records a large coral reef lacking any analogs in age (Gzhelian), size (80–100 m thick) and composition (high biodiversity). The large coral reef developed at the border of the Luodian intraplatform basin. The intraplatform basin is characterized by the deposition of green algal grainstone, coated grain grainstone and bioclastic packstone, grainstone, floatstone and rudstone in shallow-waters. In the deep-water shelf, lithofacies are composed of burrowed bioclastic wackestone, microbioclastic peloidal packstone, grainstone, and fine-grained burrowed wackestone and packstone. In this context, the coral reef developed on a deep-shelf margin, in a moderate to low energy depositional environment, below the FWWB. The scarcity of Pennsylvanian coral reefs suggests global unfavorable conditions, which can be attributed to a complex pattern of several environmental factors, including seawater chemistry (aragonite seas), paleoclimatic cooling related to continental glaciation, and the biological competition with the more opportunistic and adaptive phylloid algal community that occupied similar platform margin paleoenvironments. The existence of the large Bianping coral reef in southern China, as well as a few additional examples of Pennsylvanian coralliferous bioconstructions, provides evidence that coral communities were able to endure the Late Paleozoic fluctuating paleoenvironmental conditions in specific settings. One of such settings appears to have been the deep shelf margin, where low light levels decreased competition with the phylloid algal community. [ABSTRACT FROM AUTHOR]