南海珠江口外海底峡谷内底形沉积结构与形成机理

The one of focus on bedforms mechanism is the interaction of different mechanisms which show different frequency of occurrence, spatial scale and fluid type. The submarine canyons out of the Pearl River Estuary in the north margin of South China Sea, which are controlled by the combined effect of turbidity currents, bottom currents and internal waves and where developed different-scales, multi-levels bedforms, are the idea place to analyze the bedforms characteristics and resolve coupling effect. In this study, the different vertical resolution data including meter-scale 3D seismic data and decimeter-scale AVU sub-bottom profile data will be combined to integrate their advantages of horizontal continuity and vertical resolution. The data will be analyzed to shape the fine features, quantitative identify type and get internal structure and evolution sequence of the bedforms from the seafloor to the internal part using seismic sedimentology principles and methods. The hydrodynamic process of seafloor sediments in suspension and deposition will be simulated by the deposition dynamics simulation. The emphasis of simulation are how to realize the coupling of turbidity currents, bottom currents and internal waves taking into account their occurrence time, location and frequency. It is the purpose to reveal the sedimentary architecture and evolution sequence of bedforms and to build a deposition kinetics model within the deep-water canyon where the source of supply was sufficient. The conclusion will be of benefit to providing a theoretical and scientific basis for deep-water engineering, seafloor instability and marine geological hazards.

不同发生频率、空间规模、流体类型的形成机制之间的相互作用是底形成因机理的争论焦点之一。南海珠江口外海底峡谷区受到浊流、底流和内波共同影响，发育不同尺度、多级别的深水底形。该海底峡谷区是分析底形特征、解析多种机制耦合作用的理想场所。本研究将垂向分辨率分别为米级的3D地震数据与分米级的AVU浅剖数据结合起来，整合各自的平面连续性和垂向高分辨率的优势。首先，利用地震沉积学原理与方法，从海底面开始深入到底形内部，精细刻画底形特征、定量划分底形类型，获得底形的内部结构和演化序列；其次，辅以沉积动力学模拟，重点考虑发生时间、地点、概率等因素，将浊流、底流和内波等水动力过程耦合起来，模拟海底沉积物颗粒的悬浮及沉积过程；最终，试图揭示物源供给充足、深水海底峡谷内底形的沉积特征与演化规律，建立一个深水海底峡谷内底形成因的沉积动力学模型，为深水工程、海底不稳定性及海洋地质灾害研究提供理论基础与科学依据。

本研究将垂向分辨率分别为米级的3D地震数据与分米级的AVU浅剖数据结合起来，整合各自的平面连续性和垂向高分辨率的优势。南海珠江口外海底峡谷区受到浊流、底流和内波共同影响，发育不同尺度、多级别的深水底形。不同发生频率、空间规模、流体类型的形成机制之间的相互作用是底形成因机理的争论焦点之一。研究表明，C14峡谷宽度为1.5-3.7 km，长度约为27.9 km。根据不同的地貌特征，沿下坡方向将峡谷分为上游、中游、下游和峡谷口4个部分。上游的海底非常陡峭，发育6个海底滑坡，水深750-950 m，坡度约2.76°。中游河段发育深达30m的侵蚀沟槽，水深950 m，水深1180 m，坡度减小到2.32°。下游主要发育由块状搬运沉积物填充的冲坑，水深1200-1300 m，平均坡度接近1.3°。峡谷口在水深约为1400 m，发育一系列近似平行的线性侵蚀。沿着峡谷中泓线观察到一列七个周期阶坎。周期阶坎表现为平缓的迎流面、半圆形到新月形陡峭的背流面。周期阶坎波长从851 m到3373 m，平均值为2072 m；波高4.7-172 m，平均值56 m。迎流面坡度为0.54°-2.14°，平均坡度为1°。背流面坡度为2.1°-4.83°，平均坡度3.4°。最大的周期阶坎S5出现在水深为1190 m的峡谷下游，其波长为3373 m，波高为172 m。结果表明，各周期阶坎的不对称系数Ay和宽深比W/D值随坡度的减小而增大，坡度越小，周期阶坎的不对称性越大。这项研究提供了一个独特的机会，以推进对斜坡限制型海底峡谷微观形态的理解。