Petrology and Diagenetic Evolution of Beachrock in Zhaoshu Island, Xisha Islands

ABSTRACT The diagenetic characteristics and petrological analysis of beachrocks can reflect the environmental changes during their sedimentation and reveal their formation and evolution processes. Based on field work, this study analysed the evolution of beachrocks on Zhaoshu Island by using microfossil identification, AMS 14 C dating, geochemical element analysis and scanning electron microscopy (SEM). This study explored the influence of sea‐level changes and microbial activities on the beachrocks over the past 2000 years. This study suggests that the development of beachrocks on Zhaoshu Island has undergone three stages. The dominant factors influencing beachrock development vary across different periods. In the first stage, the sea level rose slowly, and the cementing materials were mainly an isopachous fringe of acicular cements, formed under a seawater environment. In the second stage, sea level remained stable, and the beachrocks cemented rapidly, with the multiple generations of carbonate, meniscus pendant and isopachous fringe of acicular cements. In the third stage, the sea level declined, and the cementing materials are meniscus cements and pendant cements, showing the characteristics of the joint action of seawater and freshwater. During this period, in the area with weak hydrodynamic conditions on the southern side, the cementation of beachrocks was significantly influenced by microbial activities. This study established a model of the diagenetic evolution of beachrocks in the study area since 2000 BP, and analysed the influence of sea‐level changes and microbial factors, which can provide a basis for future environmental research in the South China Sea region.