Permian-Triassic boundary (PTB) sections from isolated carbonate platforms in the Nanpanjiang Basin of south China contain Upper Permian skeletal packstones with diverse open-marine fossils overlain by a 7–15 m thick boundary horizon composed of calcimicrobial framestone constructed by globular to tufted, calcified cyanobacteria similar to Renalcis. The framestone contains interbeds of lime-grainstone with abundant thin-shelled bivalves and brachiopods. The overlying Lower Triassic strata contain microgastropod lime-packstones followed by a thick succession of thin-bedded lime-mudstones.

The PTB event horizon is interpreted to occur at the top of the packstone containing diverse, open-marine fauna and Palaeofusulina and coincident with the abrupt change to calcimicrobial framestone lacking Permian macrofossils. The conformable biostratigraphic boundary occurs at the first appearance of Hindeodus parvus within the basal meter of the calcimicrobial framestone.

Intensively studied PTB sections in south China, such as the GSSP at Meishan, primarily are condensed sections from deep-water, basin environments that contain a thin (< 30 cm) boundary horizon of claystone and lime-mudstone or marl. The sections reported herein are fundamentally different in that they consist of shallow-marine carbonate facies, contain a thick boundary horizon composed of calcimicrobial framestone, and lack evidence of an abrupt shift in depositional environment or water depth. The calcimicrobial framestone boundary horizon occurs in all of the isolated carbonate platforms in the Nanpanjiang Basin. A similar microbial facies has been found in the basal Triassic H. parvus zone in the Sichuan Basin and in Japan. Distribution of the calcimicrobial framestone suggests that it may represent an anomalous oceanic event that affected a vast area of the equatorial eastern Tethys and Panthalassa during and/or immediately following the end-Permian mass extinction. The persistence of similar calcimicrobial framestone horizons into the Upper Scythian suggests that detrimental environmental conditions associated with the extinction persisted until the end of the Scythian. Further study of these sections promises to provide constraints on causes of the extinction and the environments in the aftermath.

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