Numerical simulation of post-mass transport deposition: a case study of the margin slope of South China Sea
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摘要:
块体搬运沉积体系（MTDs）具有起伏不平的顶面，会影响后续的浊流沉积，厘清MTDs顶面对后续沉积的控制作用，有助于了解海底扇的发育规律。基于南海北部琼莺段陆坡的三维地震数据，对块体搬运沉积体系及其上覆充填物的形态进行刻画，并使用Flow3D进行数值模拟，分析其顶面对后续沉积的影响。研究认为：①MTDs的体部顶面具有低平、宽缓的特点，可形成宽广低缓的凹槽；趾部挤压区顶面起伏大,可形成孤立的凹槽。②MTDs体部顶面可形成大规模的、连续的海底扇；趾部顶面凹槽发育小型扇体；局部隆起的背流面下端可发育规模不一的扇体；MTDs末端低洼处是大型海底扇形成的有利区。③MTDs顶面起伏程度越大，越有利于沉积物卸载、沉积，形成更大规模海底扇。
Abstract:The mass transport deposits (MTDs) have largely-undulating top surface, which would affect post-MTD turbidity currents deposition and distribution. A systematic study on the MTD deposition is helpful for interpreting the deposit pattern of submarine fan. Based on 3D seismic data in the Qiongying continental slope of the northern South China Sea, the MTD sedimentary system, and its overburden deposits, and the impact of the top surface on post-MTD sedimentation were numerically simulated using the Flow3D. Results show that: ① the top surface of translational domain is flat and gentle, on which distributed broad-gentle troughs; and the top surface of the toe domain fluctuates greatly, forming an isolated troughs. ② On the top surface of the translational domain forms a large and continuous submarine fan, while a small submarine fan can form in the toe domain; the slip slope of local uplift can develop different-sized submarine fans. The terminal domain is a favorable area for large submarine fans formation. ③ The greater relief of the top surface, the more conducive to sediment unloading and deposit, and the larger submarine fan formation.
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表 1 MTDs识别标志
Table 1. Identification standards of MTDs
沉积单元类型 地震剖面特征 地质模式 特征 正断层 
发育在头部，由较连续的蠕虫状反射过渡为杂乱反射 逆冲断层 
发育在趾部，体部前端，挤压形成逆冲断层，上覆常见凹槽 侵蚀擦痕 
发育在体部，MTDs切割下伏地层，可见侵蚀凹槽 滑塌块体 
发育在体部，孤立地质体出现杂乱反射中，下伏常见侵蚀凹槽 注：剖面位置见图1a。 
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表 2 沉积数值模拟边界条件
Table 2. Boundary conditions of the numerical simulation
参数项 设定值 浊流初始流速/（m/s） 1～10 颗粒粒径/mm 0.5（砂岩）、0.04（粉砂）、0.005（黏土） 密度/（g/cm3） 500（砂岩）、2 200（粉砂岩）、1 800（黏土） 重力加速度/（m/s2） 9.81 陆坡坡度/（°） 2 模拟时间步长/s 2 网格大小/m 0.5×0.5×0.5
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