Beichen Chen ^1,2 Feng Wu ^2,3* Xinong Xie ^2* Ya Gao ⁴ Xiao Wang ² Zhiyi Tang ²

• ¹ China National Offshore oil Corporation Ltd, Shen, China
• ² College of Marine Science and Technology, China University of Geosciences Wuhan, Wuhan, Hubei Province, China
• ³ College of Oceanography, Hohai University, Nanjing, Jiangsu Province, China
• ⁴ School of Marine Sciences, Sun Yat-sen University, Zhuhai Campus, Zhuhai, Guangdong Province, China

This study presents a comprehensive analysis of the late Permian platform marginal reefs in the Sichuan Basin, focusing on reefal lithofacies and sequence stratigraphic patterns. Field outcrop observations and rock sample analyses from the Jiantianba reef were conducted to establish an evolution model of sponge reef development and spatial distribution. Four stages of marginal carbonate platform were documented: open platform, gently sloping reef, steeply sloping reef, and reef bank system. Distinct lithofacies were identified in these stages, reflecting different depositional environments and growth rates. The gently sloping reef was composed of filled skeleton framestone, filled skeleton bafflestone, and micrite organism limestone, indicating limited reef-building capacity. In contrast, the lithofacies of steeply sloping reef was composed of open skeleton framestone, open skeleton bafflestone, binding skeleton bafflestone, and benthic organism bindstone, indicating stronger reef-building ability. Based on depositional features and carbon isotopic trends, the reef strata were divided into two sequences. Sequence 1 corresponds to formation unit 1, and sequence 2 can be further divided into unit 2 and unit 3. In unit 1, reef developed in a relatively deeper-water setting. It was characterized by rich micrite limestone, forming a gentle margin. The unit 2 witnessed reef development in shallower waters. Early marine cementation and microbial clots were prevalent, contributing to form a steep margin. In the early unit 3, reefs primarily developed in a tidal controlled environment. Subsequently, reef strata experienced a transition to a wave-influenced environment, leading to the formation of reef bank system. In general, sequence 1 mainly formed in heterozoan dominated factory, and reefs contributed to a relatively gently sloping platform margin. In contrast, sequence 2 formed in photozoan dominated factory, and reefs contributed to a relatively steeply sloping platform margin.