Formation Mechanism of Beach Rocks and Its Controlling Factors in Coral Reef Area, Qilian Islets and Cays, Xisha Islands, China

Beach rock, which forms at the intertidal zone, is a natural barrier to protect beach and island from wave erosion. The formation mechanism of beach rocks is significant in the study of surface carbonate diagenesis and protection of island. Beach rocks in Qilian Islets and Cays were formed about 500–700 a BP, neither its composition nor sedimentary structure experienced intense post-reformation. Beach rocks in Qilian Islets and Cays are mainly composed of reef-building skeletal fragments and bioclasts without terrigenous sediments. This study focused on the types and morphologies of cements in beach rocks of Qilian Islets and Cays, and its cementation mechanisms and influencing factors. Biological activities, such as micritization caused by microbial activities and algae binding action, play an important role in the initial stage of bioclast transformation and promote the subsequent early marine and early meteoric cementation. Acicular aragonite induced by early marine cementation is well developed in beach rocks, especially in those samples from Medium Islet. Early meteoric cementation is not common as marine cementation, generally presenting granular and meniscus cements. The main factors affecting the formation and development of beach rocks in Qilian Islets and Cays include sediment source, hydrodynamic condition, climate, sea level change and anthropogenic impacts.