Skeletal–cement–microbial reefs in the Pennsylvanian: a case study in Guizhou, South China.

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]