Wuchiapingian Bioconstructions in the Changning‐Menglian Belt, Western Yunnan, China: A Window Into the Bioconstruction Development During the Reef Depauperate Time

ABSTRACT The Wuchiapingian (Late Permian) is commonly considered a reef barren period after the thriving of Guadalupian (Middle Permian) reefs. Here, we present the results of a detailed investigation of Wuchiapingian oncoid bed and coral biostrome of a section in the Changning‐Menglian Belt, western Yunnan, China. The coral biostrome is almost entirely composed of coral bafflestone, where corals are mostly preserved in prostrate forms and the spaces between them are filled with skeletal wackestone. The oncoid bed is characterised by abundant oncoids and green algae. The oncoids are 1–24 mm in diameter and can be classified into spherical and ellipsoidal (types A and B) and irregular (type C) ones. Oncoids‐A with a bioclastic nucleus and B with a lithoclastic nucleus are characterised by an unlaminated, micritic cortex, while oncoids‐C are represented by a lithoclastic nucleus and a laminated cortex. Filamentous microbes are observed in almost all the cortices. Oncoids‐A and B were constructed through inducing, trapping and binding lime mud by filamentous microbes on the continually rolling nuclei in a moderate‐energy setting. When water could no longer roll and overturn these already enlarged particles, filamentous microbes only grew on the upper side of the initial regular oncoids, finally forming the asymmetrically cortical oncoids‐C. Thereby, the irregular surface morphology of oncoids does not necessarily mean that they are formed in a low‐energy environment as previously thought, because the bigger the particle, the higher the water energy required to consistently overturn them. From a global perspective, the Wuchiapingian oncoid beds have been found in five places, stromatolites in two places, and coral biostromes in two places. During this period, there are also two reports of sponge bioconstructions. Thereby, the Wuchiapingian bioconstructions are more abundant than previous understanding.