南海白云凹陷东南部两种不同类型块体搬运沉积体系的地震响应及成因分析

文章利用三维地震数据揭示了南海白云凹陷东南部两种不同类型的块体搬运沉积体系的内部反射特征、外部形态及运动指示标志, 并且探讨了其成因机制。结果表明, 自晚中新世以来研究区共发育4种地震相: 弱振幅水平状连续地震相、强振幅波状连续地震相、弱振幅半透明杂乱反射地震相和中-强振幅丘状连续反射地震相。通过地震相分析可知, 研究区自晚中新世以来共发育两种不同类型的块体搬运沉积体系: 1) 多期块体搬运沉积复合体, 主要由弱振幅半透明杂乱反射地震相组成, 边界模糊; 2) 单期块体搬运沉积体, 主要由弱振幅半透明杂乱反射地震相和中-强振幅丘状连续反射地震相组成, 边界清晰明显。另外, 研究结果发现高沉积速率和地震活动使得研究区的块体搬运沉积体系表现出内部运动指示特征发育程度低的特征, 而东沙构造活动导致该块体搬运沉积体系具有频发性。

Seismic characteristics and triggering mechanism analysis of two types of mass-transport complexes in the southeast of Baiyun Sag,South China Sea

Mass-transport complexes (MTCs) are one of the gravity-driven depositional process. MTCs contain slides, slumps, and debris deposits in submarine settings. The occurrence of MTCs can generate submarine geo-hazards (i.e., tsunami), and the fast traveling mass can destroy offshore constructions (e.g., pipelines and communication cables). MTCs can also act as seals for submarine hydrocarbon accumulation, due to their over-compacted nature. In this study, we use 3D seismic reflection data, to reveal the internal reflection characteristics, external morphology, and kinematic indicators of two types of MTCs in the southeast of the Baiyun Sag, the South China Sea. We interpreted four distinct seismic facies in the study area, including: 1) low amplitude, horizontal and continuous seismic reflections, 2) high amplitude, undulating and continuous seismic reflections, 3) low amplitude, semi-transparent to chaotic seismic reflections, and 4) moderate-to-high amplitude continuous seismic reflections. We classified two different types of MTCs based on their scale (i.e., thickness) and seismic facies characteristics in the Upper Miocene assemblage. Type 1 MTCs consist of several small-scale MTCs, showing low amplitude, semitransparent to chaotic seismic reflections. Type 2 MTCs have larger scale, consisting of low amplitude, semitransparent to chaotic seismic facies moderate-to-high amplitude continuous seismic reflections. The widely distributed MTCs in the study area could be corelated with the high sedimentation rates and earthquakes during the Late Miocene. Our study indicates that the Dongsha event (associated seismic activity and formation uplift) is the major trigger for the occurrence of MTCs. Our result can help better understand the triggers and formation process of MTCs during the Late Miocene in the Baiyun Sag. A better understating of the distribution and occurrence of MTCs in the study area will help us better determine the location of buried MTCs, as well as conducting research for the formation mechanism of paleo-MTCs in the South China Sea.