Oligocene carbonate reef in northwestern South China Sea and its significant implication for the opening of the South China Sea

An industry well in the northwestern South China Sea (SCS) encountered thick bioclastic limestone deposits. Through seismic interpretation， well calibration， and micropaleontological analysis， the bioclastic limestone is confirmed as Oligocene in age. Lithologic analysis suggests that the bioclastic limestone represent shallow marine shoal deposits that developed in-situ on relatively high horst structures within rift system. It is inferred that the reef-shoal system developed in paleo-high topographic areas， which implies the occurrence of marine environment in the Oligocene. This is the direct evidence which represents the earliest known marine reef-shoal sediments on the northern margin of SCS. This discovery enables correlation between shallow marine reefs in northern and southern margins of SCS， and highlights the conjugate feature of the continental margin of SCS. Along with the opening of northeast sub-basin， some rifts transition to shallow marine environment， driven by extensional movements. These rifts could connect to northeast sub-basin of SCS， where the internal horsts provided suitable condition for reef-shoal development. Following the opening of central sub-basin of SCS， these rifts drifted apart: some remained on the northern margin， such as Zhongjiannan Basin， while others developed on the southern margin， such as Liyue and Palawan Basin.