南海东北部海底沉积物波的形态、粒度特征及物源、成因分析

Geomorphology,grain-size charicteristics,matter source and forming mechanism of sediment waves on the ocean bottom of the northeast South China Sea

在南海东北部广泛发育沉积物波。通过高分辨率多波束数据、地震剖面以及重力柱状样,对沉积物波的形态特征、粒度特征、物源以及形成机制进行了分析。研究表明大致以台湾浅滩南海底峡谷为界,北侧为近北东向展布,南侧为近南北向展布。对其分布规律、地貌和形态特征及重力柱状样粒度分析表明这些沉积物波为浊流成因。沉积物波的发育与新生代晚期研究区的构造活动密切相关,自距今6.5 Ma以来台湾造山运动使台湾岛强烈抬升剥蚀,这些剥蚀物为研究区提供了大量的陆源物质,而在南海东北部陆坡区大量发育的峡谷-冲沟系统为陆缘物质向下陆坡的输送提供了良好的通道。研究区西侧的东沙隆起长期处于抬升剥蚀状态,这种抬升剥蚀也为研究区沉积物波的发育提供了部分物源。随着坡度的减缓,浊流沉积物开始堆积,在台湾浅滩南海底峡谷的北侧形成了展布方向与冲沟垂直的沉积物波,而在南侧由于台湾浅滩南海底峡谷发生转向,浊流从水道中漫溢出来,沉积物堆积下来,形成了与原先水道近于垂直的近南北向的沉积物波。

Deep-sea sediment waves are regular bed undulations on a relatively large scale on the ocean bottom of the northeast South China Sea. With high-resolution multi-beam data, seismic profiles and gravity piston columns, the geomorphology, the matter source and the forming mechanism of these sediment waves are discussed. The southern Taiwan Bank canyon divides the study area into two parts in different crestline orientation; the plan view morphology of the north part indicates that the crestline orientation is roughly parallel to the regional bathymetric contours. The dominant orientation is NE. In the south part the crestline is nearly N--S oriented and is orthogonal to the channel trend. The close relation between the sediment waves and the channel-levee system shows that the former is turbidity origin, which is also identified by the material composition and internal characteristics established by the gravity piston column and seismic-stratigraphic analysis. The development of the sediment waves are controlled by the Neogene tectonic activities. The collision between the Luzon arc and the Eurasian Plate since 6.5 Ma BP had triggered the Taiwan orogeny. The uplift and erosion of Taiwan Island offered sufficient terrigenous sediments. Furthermore, the well-developed channel-canyon system on the continental slope favors the turbidity flow＇ s transportation to the deep sea, while the continuous uplift and erosion of the Dongsha uplift west of the study area also provides some materials. With the decrease of the gradient of the continental slope, these turbidity sediments begin to accumulate and form NE-oriented sediment waves at the end of the channellevee system. The NS-oriented sediment waves in the south of the south Taiwan Bank canyon are mostly resulted from flow＇s spillover caused by the sharply turn of canyon＇s course.