Backstepping patterns of an isolated carbonate platform in the northern South China Sea and its implication for paleoceanography and paleoclimate

Abstract

Backstepping of carbonate platform margins is an important process for understanding how carbonate platform responds to paleoenvironment changes. The products of backstepping, such as backstepped margin, backstepping surface and backstepping succession have been widely documented on both modern and ancient platforms around the world. However, the causes of platform backstepping have not been understood fully yet. This study presents a detailed description of variations in backstepping pattern of Xuande platform in the northern South China Sea using 2D seismic and bathymetric data. The results show two episodes of backstepping events occurred during the late Middle Miocene and the latest Miocene to the Early Pliocene. The former backstepping event occurred on the eastern margin of the platform and likely resulted from upwelling and turbulence related to the onset of contour currents in the Xisha Archipelago. The latter backstepping event shows a change of backstepping distances at different platform margins with more pronounced backstepping along the winter-monsoon windward side compared to the corresponding leeward side. It is proposed that this event was largely the result of vigorous currents and waves impinging on the platform margins related to an intensification of the East Asian monsoon and a relative sea-level rise. A difference in hydrodynamic energy was responsible for the variations in backstepping distances at platform margins. Platform margins exposed to higher-energy environment backstepped further than margins exposed to lower-energy envrionment. Results from this work indicate the morphology and stratigraphic architecture of carbonate platforms are strongly controlled by the hydrodynamic energy, here related to currents and waves driven by East Asian monsoon winds.