Modern geological processes in the Xisha Trough on the margin of acarbonate platform:Evidence from the geological observation by manned submersible

The Xisha Trough in the northern part of the South China Sea has existed for a long time. Deepwater processes of the South China Sea are well recorded in the trough. Therefore, it has special implications for deepwater geology. According to the geological observation of the manned submersible, the bottom plain of the trough can be subdivided into several sub-order geomorphic units, such as small grooves, large symmetrical ridges and flat-top ridges, as the results of the joint action of episodic turbidity current and near east-west bottom flow. Some small sand dunes are also observed, which are interpreted as the nozzle of overpressure gas or living caves of small animals. In the Xisha Trough, biological diversity is low since the small number of species and the harsh living conditions. It could be called the "desert of seabed". Abnormal high pressure gas is found in the argillaceous sediments in this area, probably caused by high rate of sedimentation.