Neogene carbonate platform development in the NW South China Sea: Litho-, bio- and chemo-stratigraphic evidence

The variations of lithostratigraphy, foraminifera, geochemical elements, and organic biomarkers in well XK1 from the Xisha (Paracel) Islands reveal the history of carbonate platform development since the early Miocene in the South China Sea. Our results indicate that reef growth was more active in the middle Miocene than in any earlier periods, decreased in the late Miocene, revitalized in the Pliocene, and active again in the Pleistocene interglacial periods. Unlike those limestone-rich intervals, the late Miocene succession in well XK1 is characterized by dolomitized limestones with a low deposition rate (30-60 m/Ma post-compaction). These late Miocene rocks also contain relative high MgO (~ 20%), more ∑ REE, abundant algae and some small sized foraminifera, but rare larger benthic foraminifera, very low land-sourced brGDGTs (0-0.5 ng/20 g), low BIT index (0.1-0.5), and low TEX86-derived sea temperature (~ 20-25 °C), indicating limited active reef growth in relative cooler mesophotic environments at > 30 m water depths. Similar dolomitized limestones of late Miocene age also occur in other carbonate platform areas including the Nansha (Spratly) Islands in the southern South China Sea. Together, these South China Sea records provide regional evidence of a recession in global carbonate platform development starting from the late Miocene. While carbonate platform reduction in other regions may have been triggered by global cooling and changing oceanic chemistry, accelerated basin subsidence coupled with monsoon cooling are considered as the main factors causing the reef decline in the late Miocene South China Sea.