Investigation of gravity flow deposits on the Lingshui slope of the northern South China Sea
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摘要:
深水沉积环境复杂、浅层沉积物土质差异明显、重力流沉积多样，对深水海洋工程水下设施的设计、施工和运营安全提出了新的挑战。通过二维/三维数字地震等方式能够识别出数百米的滑坡体，但是对于海洋工程上所关注的浅层海底的重力流沉积体系，其分析能力有限。以南海北部陵水区块为例，通过船载多波束的后向散射成果、自主水下航行器（AUV）搭载的浅地层剖面资料，结合重力活塞取样器（JPC）取样和碳14测年（AMS14C）等资料综合分析，对深水海底浅层重力流沉积的形成和分布模式进行探讨。研究表明，船载多波束后向散射图能够较为准确地识别区域性重力流沉积区域，以及表层沉积物的变化，AUV搭载的浅地层剖面能够精确地识别地层的纵向差异，JPC取样能够获取重力流沉积样品及其物理力学参数，以上资料的综合分析，是准确识别、研究和认识现代重力流沉积体系的重要方法。研究区内最近的2期重力流沉积分别发生于5.5 kaBP左右和45 kaBP前，重力流的发生一般都是由上陆坡区海底峡谷的活动引起的。最近的一期重力流事件中，先后发生了浊流沉积和块状搬运体（MTDs）沉积，MTDs沉积过程中会对下伏地层产生明显的冲蚀现象。在重力流沉积区进行水下结构物设计和施工时，应特别关注重力流沉积引起的地形坡度变化，以及地层中土质成分的差异，可能对水下结构物的安装和维护产生的影响。
Abstract: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.
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Key words:
- backscatter /
- gravity flow deposition /
- turbidity /
- deep-water sedimentary model
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表 1 浊流沉积JPC取样土质参数
Table 1. Soil parameters for JPC Sampling in turbidity area
样品深度/ m 含水/ % 容重/（kN/m3） 界限含水量/% 200#/ % 颗粒密度 液限 塑限 塑性指数 0.5 113 13.8 89.2 39.4 49.8 94 2.7 1.3 80 15.2 71.1 31.6 39.5 96 2.71 2 117 13.7 81 40.7 40.3 93 2.69 2.7 130 13.4 101.7 45.1 56.6 95 2.7 3.5 133 13.1 105.1 45.5 59.6 93 2.7 4.2 112 13.6 104 44.6 59.4 96 2.7 4.9 119 13.5 94.3 40.4 53.9 97 2.7 
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