Investigation of gravity flow deposits on the Lingshui slope of the northern South China Sea

Deep-water environment is rather complex. Within the shallow part of the environment, there occur various gravity flow deposits, that become new challenges to the design, construction and operation of underwater facilities. The landslides on scale of hundreds of meters can be identified by 2D/3D digital seismic survey, but the recognition of gravity flow depositional system on the shallow seabed, which is concerned by marine engineers, is limited. Taking the Lingshui block in the north of the South China Sea as an example, this paper discussed the formation and distribution models of shallow gravity flow deposits in the deep sea bottom with the results of MBES backscattering on board, the sub-bottom profile data carried by the Autonomous Underwater Vehicle (AUV), combined with the data of Jumbo Piston Corer (JPC) sampling and AMS¹⁴C dating. The results show that the MBES backscatter result can accurately identify the region of gravity flow deposition and the changes of surface sediments, the Sub-bottom profiler carried by AUV can accurately identify the vertical differences of strata, and JPC sampling can obtain the gravity flow deposits as well as their physical and mechanical parameters. The comprehensive analysis of the above data founded the basis to accurately identify, study and recognize the present situation of the gravity flow depositional system. The latest two periods of gravity flow deposition in the study area occurred about 5.5 ka and 45 ka ago, respectively. Generally, the gravity flow is caused by the activity of submarine canyon on the upper slope. In the latest gravity flow event, turbidity current deposition and MTDS deposition took place successively, and the MTDS deposition produced obvious erosion to the underlying strata. In the design and construction of underwater structures in the gravity flow active area, special attention should be paid to the change of terrain slope caused by gravity flow deposition and the difference of soil composition in the stratum, which may affect the installation and maintenance of underwater facilities.