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

Thick dolostones developed widely in the South China Sea (SCS) during the Cenozoic. However, the dolostone types and the formation mechanism of these dolostones remain controversial. The core Nanke-1, drilled on Meiji Reef in the southern SCS, recovers thick dolostone formed during the Neogene, providing a good opportunity to reveal the mechanism of various SCS dolostones. The dolostones in this core exist in two forms: microsucrosic and mimetic dolostones. Microsucrosic dolostone, composed of high-calcium dolomite (HCD), shows larger crystal size and partial iron stains, while mimetic dolostone, composed of low-calcium dolomite (LCD), shows crystals with smaller size. The dolomitization was mediated by the normal to slightly evaporated seawater. The differences in microsucrosic and mimetic dolostones are mostly attributed to rock porosity and Mg/Ca ratio of the dolomitizing fluid. During the Late Miocene, coarse sediments with rich pores accumulated on the platform. These sediments were exposed and heavily dissolved during the sea-level lowstands, producing abundant dissolution pores. The fast circulation during the highstands mediated the dolomitization process, favoring the development of microsucrosic dolostone. In contrast, during the Pliocene, the relatively higher sea-level caused the accumulation of bioclasts with fewer primary pores and formation of limited dissolution pores. These conditions favored the development of mimetic dolostone. The above processes suggest that sea-level fluctuation played an important role in causing the differences in Upper Miocene and Pliocene dolostones. Moreover, the increase in seawater Mg/Ca ratio facilitated the occurrence of HCD during the Late Miocene and LCD during the Pliocene in the SCS. The development of Neogene dolostone in the SCS was roughly synchronous with the dolomitization events elsewhere, which hints that the formation of these dolostones could be controlled by similar triggers (e.g., sea-level fluctuation). The dolomitization mechanisms in core Nanke-1 discussed herein can thus also benefit further explanation for the Cenozoic dolomitization events in other regions worldwide.