The microbial mud mound of the Middle Permian Qixia period in Sichuan basin and its indication significance to sedimentary environment

Abstract: A microbial mud mound was found for the first time in the Middle Permian Qixia Formation of Huayin Xikou area. This mud mound is supported by mud, the primary builder is fungi, followed by invertebrates. The species diversity is low, with high species richness. The main lithology includes bioclastic micritic limestone and thrombolite. The mud mound can be divided into mound?core, mound?flank, and mound?base. According to the results of sample analysis of the Qixia Formation, all of the paleosalinity Z values >122, δ13C > 0; δ18O values are between ?6.33‰ and ?4.42‰, the ancient water temperature values are between 33.60℃ and 24.26℃, V/(V+Ni) ratio is 0.51228, and U/Th ratio is 8.28205, suggesting that the environment of Qixia period was characterized by relatively deep water, relatively weak hydrodynamics, higher temperature, high salinity and anoxic nature. Such an environment was conducive to the development of microbial mud mound, and affected the development and growth of reef?building organisms. This was one of the reasons leading to the lack of skeleton of the reef in Qixia Formation.