Environmental and biological controls on the initiation and growth of a Middle Triassic (Anisian) reef complex on the Great Bank of Guizhou, Guizhou Province, China

The Great Bank of Guizhou (GBG) is an isolated Late Permian to Late Triassic carbonate platform in the Nanpanjiang Basin of Guizhou Province, southwest China. A faulted syncline exposes a cross section of the platform margin, including a well-preserved Anisian (earliest Middle Triassic) reef complex approximately I km wide and 800 meters thick. Geochronologic constraints from associated basin-margin strata indicate that reef development initiated late in the Early Triassic, making it the oldest-known platform-margin reef complex of the Mesozoic Era. The reef framework consists primarily of microspar-filled tubes similar to 100 mu m wide and up to a few cm long that are embedded in irregular to branching, mm-scale masses of micrite, traditionally assigned to the problematic genus Tubiphytes. Based on preserved sporangia, the Nanpanjiang structures are interpreted as microbially induced micritic precipitates that formed in association with an otherwise uncalcified alga. A low-diversity metazoan and algal community also occurs within the reef complex, but these organisms did not contribute significantly to the reef framework or to the accretion of the reef complex. Rather, reef development is interpreted to have resulted largely from the stabilization of platform-margin sediments by algae and associated microbial mats. Only gradually, through the Middle and Late Triassic, did framework-building metazoans evolve to occupy and then construct reefs on the margins of carbonate platforms.