The Late Quaternary Carbonate sand deposits at the Xuande Atoll

Carbonate sands are the common clastic deposits formed on the surface of carbonate platforms. Understanding the depositional processes of the deposits is of great significance to the development of carbonate sand resources and the construction of modern artificial islands. On the basis of the latest drilling and geological survey data, the distribution pattern and vertical evolution of the Late Quaternary carbonate sand deposits at the Xuande Atoll, Xisha Islands are studied in this paper. The results suggest that the centennial scale depositional process is rather complex on the South Island, and the interdecadal sediments are mixed each other and not conducive to the study of centennial scale sedimentation. The boundary at 18.39m in depth on the Woody Island is defined as the boundary between Pleistocene and Holocene. The Pleistocene deposits of coral reef limestone below 18.39m were formed during the submerging period of the platform. Upon the boundary deposited the loose sediments of coral and shell bioclasts formed in the initial period, expansionary period and stable period of the coral island respectively. The bioclastic deposits at the Xuande atoll were firstly formed during glacial MIS4, mainly on the Shidao Island. A large number of coral islands appeared at the Xuande Atoll after Middle Holocene. The shape of the carbonate sand island is significantly affected by the distribution of beach rocks which are controlled by the monsoon and ocean waves.