Formation and sedimentary evolution characteristics of Yongshu Atoll in the South China Sea Islands

An on-site comparison is made on the reef body shape and landform of the Yongshu Atoll in the South China Sea. Accordingly, the reef top of the Yongshu Atoll in the transverse direction is divided into six facies zones. In the longitudinal direction, the developmental model of the Yongshu Atoll has a stepwise, tower-like structure. The results of engineering geological drilling show that since the Miocene the Yongshu Reef development has passed through 9 sedimentary cycles which merge into 6 large evolutionary stages (separated by 5 hiatuses of the reef body). This implies that the whole development and evolution process has undergone 6 periods of great sea level rises and 5 periods of sea level drops. The resedimentation of the reef body after several large hiatuses changed from lagoon facies to inner and outer reef-flat facies. These cyclical characteristics reflect the development process of the coral reef when the sea level was gradually rising in the interglacial stage. During different sedimentary evolution stages, the coral reef growth, sediment accumulation and topographic relief were all different, so that significant differences between facies zones emerged.