Petrological Characteristics and Controlling Factors of Diagenetic Evolution of Beachrocks in Zhaoshu Island, Xisha Islands

Abstract

Beachrocks are widely developed in the the Xisha Islands in the South China Sea. As an important component of the coastal zone, beachrocks record rich ancient environmental information, play an important role in the evolution of the coastal zone. This study focuses on the extensive distribution of beachrocks in the coastal zone of Zhaoshu Island. Based on field geological surveys, investigate the petrological structure, elemental geochemical characteristics, and diagenetic processes of the beachrocks in Zhaoshu Island. We established a model for the diagenetic evolution of beachrocks since 2000 yr B.P. in Zhaoshu Island, the Xisha Islands. Through the analysis of petrology and diagenetic characteristics, we discussed the impact of sea level changes and microbial action on the beachrocks of Zhaoshu Island since 2000 yr before present(B.P.). There are three stages of diagenetic evolution in the beachrocks of Zhaoshu Island. During the period of 2000 yr B.P. to 1780 yr B.P., the sea level in the South China Sea where Zhaoshu Island is located slowly rose, and its beach rock cement showed characteristics of seawater environment, Between 1780-836 years ago, the sea level remained stable and beachrocks cemented rapidly, since 760 yr B.P., the sea level has started to rapidly decline, and the coastal zone has been exposed on a large scale. Influenced by atmospheric freshwater, beachrocks exhibit characteristics of joint action between seawater and freshwater. During this period, in the southern region with weaker hydrodynamic conditions, beach rock cementation has been significantly affected by microbial activity.

Keywords: Beachrocks, Petrological characteristics, Diagenesis, Xisha Islands, South China Sea