基于三维地震资料和底边界层潜标观测的珠江迁移峡谷的沉积过程研究

Zhujiang canyons are the largest migrating canyons that are deflecting northwestward from mid-Miocene, indicating a peculiar sedimental and hydrodynamic environment. Previous work mainly focused on the seismic studies of sedimental evolution of Zhujiang canyons, but missed an in-situ observation. As a result, we plan to use deep-sea subsurface moorings to have a long-term and real-time monitoring on features of turbidity flow and ocean current, such as the classification, flow rate, flow direction, current thickness, current coverage and particle concentration. Based on the results from mooring observation, seimic interpretation and sediment sample analysis, we attempt to creat a depositional model of Zhujiang canyons. Our studies will pay attention to three items. (1) Studies on how gravity flow, especially turbidity flow, eroses seabed and transports particles. (2) Influences that ocean current exert on sediment process in the canyon region. (3) Interactions between turbidity flow and ocean current. Eventually, we will disscus the forming mechanism of lateral inclined deposits and try to discover the genesis of the migrating pattern of Zhujiang canyons. A combination of analysis of in-situ moorings and 3D seismic interpretation in this study provides a new method to explore the sedimental process of Zhujiang canyons and alternatively verify the previous conclusions based on seismic interpretations. This study will also help to learn more about sediment transporting, predict natural hazards and explore industrial oil & gas in the northern South China Sea.

珠江峡谷自中中新统以来继承性北东方向迁移发育，是世界上规模最大的迁移峡谷群，具有独特的沉积和水动力环境。前人基于反射地震资料研究峡谷沉积演化过程，缺少峡谷的现场观测研究。本课题拟使用多套锚系潜标长期实时观测珠江峡谷区底边界层内浊流和底流、内波等各类海流的运动学性质（类型、流速、流向、厚度、流经、沉积物悬浮浓度变化等参数）。通过潜标观测和模拟、三维地震解释和沉积样品测试分析，建立珠江峡谷的沉积模式。分析底边界层内浊流和各类海流对物质输运和沉积过程的影响，研究浊流和海流的相互作用机制，最终探讨侧向沉积体的形成机制，揭示峡谷迁移发育的原因。本课题拟采用的潜标观测和三维反射地震资料相结合的方法为探讨峡谷沉积过程提供了新的研究手段，同时观测结果可以验证前人的地震解释结果。本课题将为研究南海深海物质输运过程、地质灾害分析预测和深水油气勘探提供重要的理论基础和科学依据。

本项目在2017-2021年，成功进行了4个航次10套潜标的投放和回收，有效记录了珠江口盆地陆坡峡谷内部和脊部底边界层的流场信息。研究表明峡谷内部以内潮为主，月平均后的流速较小，一般小于0.05m/s，沿着峡谷走向朝峡谷头部流动。峡谷脊部天平均的结果表明海流总是隔几天到一个月不等时段会发生方向的彻底改变，约70%月份峡谷脊部的海流为垂直峡谷走向的西南向，30%月份是东北向。这与以往前人认为东北向恒定的底流是不一致的。温度和盐度的剧烈变化指示可能存在非线性的内波，有些振幅大的非线性内波可以引起海底沉积物的悬浮。地震资料解释表明峡谷的迁移并不总是东北向的侧向迁移，也存在相反方向的迁移，峡谷的发育受到长期的区域构造和沉积过程的重要影响。AUV资料揭示峡谷谷底广泛发育了大量来自峡谷头部的MTDs，在特定条件下比如高海平面时期，峡谷内部物质输运的主要营力不只是浊流，还应该考虑峡谷谷底不断重复进行的块体搬运沉积过程。