Carbonate sedimentary characteristics of the beach rocks around Qilian Islets and Cays, Xisha Islands: Implication for coral reef development and decline

Beach rocks, developed on the coast of Qilian Islets and Cays, Xisha Islands, China, represent carbonate sedimentary formation on a typical biogenic coast adjacent to coral reef areas. The subaerial and submarine distribution and natural outcrop sections of beach rocks around three islets were surveyed, traced, measured and mapped to determine their development and depositional successions. Carbonate sedimentary characteristics of the beach rocks show that more than 95% of sediments are sourced from adjacent reef flat and reef crest, and are dominated by reef-building coral skeletal fragments and detritus, calcareous algae, and other skeletal grains of reef dwellers. Bioclastic content, algal and microbial content, sorting and roundness of skeletal grains and porosity in depositional successions change with sediment source, sediment accumulation rate, grain characteristics and textures of sediments, hydrodynamic intensity and cementation features. Cementation of the beach rocks is characterized by the binding action of algae and microbes, the micritization caused by boring of endolithic algae and microbes, the early marine cementation, and the early meteoric cementation. Algal and microbial activity played a major role in micritization of skeletal grains of beach rocks. The beach rock formation is mainly affected by the algal and microbial content in beach sediments, the sediment supply from reef flat, the grain characteristics of beach sediments, cementation and diagenesis process, the hydrodynamic intensity driven by wind and monsoons, and sea-level changes. The developmental dimensions and spatial extent of coral reef architecture, the flourishing and extinction of reef-building organisms in reef ecosystem, the abundance of algae and microbes and the diversity of reef dwellers may influence wave energy dissipation and sediment transport patterns in reef flat, thus alter the carbonate sediment budget of the coastal environment.