Late Miocene carbonate-system evolution in the Xisha area, northern South China Sea

Abstract

Carbonate systems in tropical regions reliably record abundant palaeoenvironmental information. Sedimentological and paleontological data from the Upper Miocene interval of core XK-1, obtained after lithofacies classification and semi-quantitative fossil analysis, allow for a detailed documentation of the carbonate-system evolution in the Xisha area, South China Sea. The lower portion of the Upper Miocene interval is characterized by an alternation of patch-reef deposits and lagoonal bioclastic deposits, whereas the upper portion is dominated by lagoonal bioclastic deposits. Reefs increased gradually during the early Late Miocene as a result of eustatic sea level falls. The assemblage of red algae (Mesophyllum, Lithothamnion), benthic foraminifera (Amphistegina radiata, Cycloclypeus, Heterostegina, Operculina), and planktonic foraminifera preferring deeper waters in the upper portion of the Upper Miocene interval suggests a deepening trend, resulting from moderate increases in eustatic sea level and basement subsidence. Moreover, the prevalence of red algae, Halimeda, acervulinids, and bryozoans over corals after 7.1 Ma indicates the influence of elevated nutrient availability. The upwelling due to intensified East Asian Winter Monsoon could have contributed to the increase in nutrient availability. Our results demonstrate that eustatic sea level, tectonics, and monsoonal conditions jointly influenced the carbonate system in the Xisha area, South China Sea. These findings have broader implications for understanding the Late Miocene development of carbonate systems in this region.