Genesis and controlling factors of Cenozoic dolostones in the South China Sea: A case study from core Nanke-1 in the Nansha Islands

Abstract

Abstract The "dolomite problem", the origin of dolomite in natural environment, remains controversial after being a topic of debate in geology for more than two century. In the South China Sea (SCS), the dolostone widely occurs in the Cenozoic reef-bearing successions. Based on mineralogical and geochemical data from the Nanke-1 core at Meiji Reef, Nansha Islands, the processes of dolomitization and its associated diagenesis are investigated in this study. The Neogene dolomites in the Nanke-1 core exist in two forms: microsucrosic and mimetic dolomites. Microsucrosic dolomite is commonly characterized by high calcium content (HCD) and shows the larger crystal size with partial iron stains. In contrast, the mol% Ca content is more depeleted in the mimetic fabric with the smaller size of rhombs. The Upper Miocene dolostone is dominantly composed of thick-layered microsucrosic dolomite, while the Pliocene dolostone is mainly composed of mimetic dolomite. The discrepancy of petrographic occurrence between two types of dolomite rhombs is most attributed to the porosity and Mg/Ca ratio of the parent dolomitizing fluid. During the Late Miocene, the global sea level was generally low, and the platform top was flooded mainly during the highstands, allowing the formation of coarse sediments with abundant primary pores in the shallow-water setting. These platform top carbonates were exposed and experienced meteoric dissolutions during the subsequent sea level falls, favoring the development of secondary pores. The above processes may cause the Upper Miocene strata porous. The higher porosity in the Upper Miocene stratum facilitated more extensive diagenetic alteration, leading to the production of microsucrosic dolomite. In contrast, during the Pliocene, the rising sea-level and the resultant fine-grain sediments may provide a relatively restricted diagenetic environment which produces the mimetic fabric in the dolostone. The gradual rise in seawater Mg/Ca ratio from the Late Miocene to the Pliocene led to the preferential formation of HCD in the Upper Miocene stratum. Additionally, the development of Neogene dolostone in the SCS was synchronous with the dolomitization events elsewhere, which hints that these dolostones could be controlled by similar triggers. The dolomitization mechanisms at core Nanke-1 discussed herein can benefit further explanation for the Cenozoic dolomitization events in other regions worldwide.