A significant geologic event occurred on the Oligocene/Miocene boundary at 23.8 Ma in the northern South China Sea, which is named the Baiyun (白云) movement in this article. This event strongly affected not only the South China Sea, but also East Asia. After the Baiyun event, the ridge of seafloor spreading of the South China Sea jumped southward and rotated counterclockwise, and a strong subsidence occurred in the Baiyun sag of the Pearl River Mouth basin. The shelf break shifted suddenly from the south to the north of the Baiyun sag, and the deposition environment in this sag changed from continental shelf with neritic deposition to continental slope with deep-water deposition. Sediment geochemistry study indicated that the Baiyun event played a key role in the rapid change of sediment provenance for the Pearl River Mouth basin. Between 32 and 23.8 Ma, the source of sedi-ments was mainly from the granites in South China, while after 23.8 Ma some sediments might have come from the eastern Himalaya, as the Pearl River drainage extended westward after the uplift of Tibet since that time. The Baiyun event led to a great change in the drainage framework of the paleo- Pearl River, sediment types and the depositional environments in the Pearl River Mouth basin, and relative sea level of the northern South China Sea, as well as sedimentation and hydrocarbon accumu-lation in the area. A significant geologic event occurred on the Oligocene / Miocene boundary at 23.8 Ma in the northern South China Sea, which is named the Baiyun movement in this article. This event was strongly not only the South China Sea, but also East Asia. After the Baiyun event, the ridge of seafloor spreading of the South China Sea jumped southward and rotated counterclockwise, and a strong subsidence occurred in the Baiyun sag of the Pearl River Mouth basin. The shelf break shifted suddenly from the south to the north of the Baiyun sag, and the deposition environment in this sag changed from continental shelf with neritic deposition to continental slope with deep-water deposition. Sediment geochemistry study that the Baiyun event played a key role in the rapid change of sediment provenance for the Pearl River Mouth Between 32 and 23.8 Ma, the source of sedi-ments was mainly from the granites in South China, while after 23.8 Ma some sediments might have come from the the eastern Himalaya, as the Pearl River drainage extended westward after the uplift of Tibet since that time. The Baiyun event led to a great change in the drainage framework of the paleo- Pearl River, sediment types and depositional environments in the Pearl River Mouth basin , and relative sea level of the northern South China Sea, as well as sedimentation and hydrocarbon accumulation in the area.