Abstract:

The northern slope of the South China Sea is one of the most active areas in the world oceans with modern sedimentary process. A high-deposition-rate sediment drift was found on the down slope of Dongsha islands. The material sources of this sediment drift were examined and discussed by several other papers, but not yet solved. Based on the multi-channel seismic data and the long term bottom current observation data, this paper finds a new source for this sediment drift. The seismic profiles show that a Miocene drape formation which was started from the end of Oligocene and ended to the Early Pliocene was very well and originally developed as a horizontal layer in the study area. Since about 3 Ma, the Miocene drape formation was uplifted and eroded along Dongsha area. The amount of the eroded materials from the Miocene drape formation could be as much as that from Pearl River and Hanjiang River to the northern South China Sea, and that from the rivers of southwest Taiwan to north South China Sea. The calculated result shows that a sediment rate of only 12 cm/ka could be reached if all the materials from Pearl River and Hanjiang River deposited on the northern slope of the South China Sea. So, it seems impossible that the sediments on the high-deposition-rate sediment drift are mainly from Pearl River and Hanjiang River. Considering the special oceanic current regime of the northern South China Sea and its adjacent areas, most part of the sediments from rivers of southwestern Taiwan was trapped in the area around Taiwan, and only small part could move far to and deposit on the high-deposition-rate sediment drift. Bottom current observation data show that the hydrodynamic environment of the northern South China Sea Shelf is powerful enough to erode the uplifted Miocene drape formation at the sea floor and transfers the eroded materials along the northern shelf of the South China Sea southward to the slope area, and these materials become the main source of the high-deposition-rate sediment drift.

Key words: the northern slope of the South China Sea, high-deposition-rate sediment drift, Miocene drape formation, Dongsha uplift, tectonic erosion, sedimentary source