Kasimovian (Late Pennsylvanian) Phylloid Algal Reef in Guizhou, South China: Construction Model and Paleoenvironmental Significance

Abstract Phylloid algae are the dominant reef-building organisms during the Pennsylvanian, and they have a widespread distribution. However, the construction model and the controlling factors of the phylloid algal reef during this period were not well understood. In this study, a well-developed Kasimovian phylloid algal reef is reported from the Houchang area, southern Guizhou Province, South China. The reef is primarily composed of cement-rich phylloid algal boundstones. The reef developed in a shelf margin setting within the photic zone under well-circulated marine conditions, likely near the fair-weather water base. Phylloid algae and marine cements were dominant contributors to the studied reef. Phylloid algae could build simple frameworks. Early marine cement stabilized the reef framework and filled ~ 30–60% of primary cavities. Microbial carbonates occurred locally in the reef, as subordinate framework stabilizer. This cement-rich phylloid algal reef is classified as a dual hybrid carbonate in the tropical region of eastern Paleo-Tethys. It was controlled by long-term and regionally universal factors throughout the Pennsylvanian, including eustatic fluctuations, atmospheric pCO₂ levels, and seawater chemistry (specifically the Mg²⁺/Ca²⁺ ratio). The studied reef can be regarded as a transitional stage from dual hybrid carbonates to triple hybrid carbonates, which provides insight for the evolution of hybrid carbonates.