Record of the end-Permian extinction and Triassic biotic recovery in the Chongzuo-Pingguo platform, southern Nanpanjiang basin, Guangxi, south China

The Chongzuo-Pingguo platform is a vast isolated platform, 180 km across, in the southern part of the Nanpanjiang basin. The end-Permian extinction is recorded in conformable sections in the northern part of the platform (Pingguo area). Upper Permian skeletal packstone contains diverse open-marine fossils including Nankinella and Sphaerulina. It is overlain by a 4.6 m thick horizon of calcimicrobial framestone constructed by globular calcified microbial framework similar to Renalcis. The PTB event horizon is interpreted to occur at the top of the packstone, coincident with the abrupt change to calcimicrobial framestone lacking Permian macrofossils. The transition from Hindeodus latidentatus to H. parvus occurs 1 m above the base of the calcimicrobial framestone, marking the conformable biostratigraphic boundary.During the Early Triassic the platform developed as a low-relief bank rimmed with oolite shoals but was bordered by a high-relief, fault-controlled escarpment along its southern margin. Platform interior facies of the Majiaoling and Beisi formations are 900 m thick and consist of thin-bedded lime mudstone, oolite, and dolostone with restricted marine biota in four shallowing-upward packages that define depositional sequences. In the Chongzuo area the platform was terminated at the end of the Early Triassic by burial with up to 1600 m of felsic pyroclastic volcanics and lava flows. Transgression and shallow-marine carbonate sedimentation resumed early in the Middle Triassic followed by drowning in the Bithynian as indicated by a shift to deep-marine carbonate and clastic deposition.In the Pingguo area volcanic deposits are much thinner and only briefly interrupted carbonate accumulation. Here the platform interior drowned during a major back step early in the Middle Triassic (Anisian, Bithynian). Smaller pinnacle platforms of the Guohua Fm., up to 1700 m thick, continued to accumulate above the Early Triassic platform margin and were eventually drowned by the Late Pelsonian. In the Anisian, the pinnacle platforms shifted to sharply defined, reef-rimmed margins with Tubiphytes reefs. The shift coincides with accelerated metazoan biotic recovery in the Anisian. However, the reefs lack metazoan frameworks and are composed almost entirely of Tubiphytes reinforced by large volumes of marine cement. These observations indicate that controls beyond the recovery of framework-building metazoans governed the re-establishment platform-margin reefs in the Middle Triassic.