Role of calcimicrobes and microbial carbonates in the Late Carboniferous (Moscovian) mounds in southern Guizhou, South China

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Various microbial fabrics characterize late Moscovian mounds in Houchang Town, southern Guizhou, South China. The dominant components of the mounds are microbial boundstones with stromatolitic structures, irregular oncoid-like forms, and wrinkle structures. Calcimicrobes recognized in the mounds include Girvanella, Ortonella, Wetheredella-like, Palaeomicrocodium-like, and some problematic calcimicrobes occurring in deposits between microbial boundstones, in thrombolitic textures, and in some intraclasts. Microbial carbonates are common in the substrate and interior of the mounds, including thrombolitic textures, microstromatolites, microbial ooids, oncoids, irregular encrusted layers, microbial mat debris, and microbial micrite. Calcimicrobes and microbial carbonates played an important role in the construction of the mounds: Girvanella might have contributed as a source of lime mud that formed the mound and stabilized the coral frame; thrombolitic mats could trap and fix sediments and bioclasts, contributing to the stabilization of substrate and mound limestones; and, microbial boundstone, clotted micrite and micritized bioclast could provide a hard substrate for encrusting metazoans (e.g., bryozoans, Ivanovia). The abundant microbial fabrics in the mounds indicate that microbial activity was widespread in Moscovian reef mounds in southern Guizhou, and this suggests that microbial organisms were the primary mound builders in the study area.