Abstract：A significant change in composition of sediments of the northern South China Sea took place in the terminal Oligocene (23.8 Ma BP), accompanied by an abrupt change in geochemical composition, and meanwhile there occurred a depositional hiatus and olistostromes in abyssal sediments. These sedimentary events coincided in time with the jump of direction of the seafloor spreading axis of the South China from E-W to NE-SW, representing the most important tectonic event in the tectonic evolution of the South China Sea since the Oligocene. Through this tectonic event, the sedimentary environment in the Baiyun subbasin was transformed from the neritic continental shelf in the late Oligocene to the continental slope since the early Miocene and the provenance of sediments in the northern South China changed from mainly proximal areas in the Oligocene to mainly distal areas in the Miocene, and furthermore, the region has generally exhibited the feature of sea transgression since the early Miocene in the area. This abrupt change event has far-reaching influence on the formation of petroleum accumulations in the northern South China Sea.