南海珠江口外海底峡谷形成的控制因素及过程

Formation process and controlling factors of the Pearl River Canyon in the South China Sea

珠江口外海底峡谷是南海北部陆坡海底峡谷群的典型代表。该海底峡谷始于南海北部陆架,向下走向经历多次转折,峡谷末端进入南海深海盆后,发育为小型海底扇形体。该峡谷特征独特,地貌多变,不仅是陆架-陆坡-海盆物质交换的通道,也是陆缘沉积物堆积的场所。利用在该区获得的多波束数据,结合多道地震数据,研究该海底峡谷的地貌和沉积充填特征,分析珠江口外海底峡谷形成发育的主控因素和发育过程。研究发现,珠江口外海底峡谷的形成受到新生代构造作用的控制。中新世以来,白云凹陷开始发生强烈的沉降作用,变为陆坡盆地,研究区陆架陆坡深水盆地的沉积格局开始形成,陆架坡折带北迁至白云凹陷北侧。21Ma以来海平面多次下降至陆架坡折带附近,陆架出露,古珠江可以直接穿越陆架到达坡折带,并向下陆坡和深水盆地倾泻物质,从而开始了珠江口外峡谷和冲沟群的发育。研究区发育的北北西—北西向(NNW—NW)断裂控制了部分水道的展布。北侧陆坡区大量发育的小型峡谷群是陆源物质向下陆坡和深水盆地输送的主要通道。海底峡谷中下段主要依托于白云凹陷古地形发育,长期处于堆积状态,接受了经由峡谷输送的绝大部分物质。

The Pearl River Canyon is a typical canyon system on the northern continental slope of the South China Sea. The canyon head lies in the continental shelf of the South China Sea, and the course changes its direction several times before finally entering the abyssal basin. Only a small sedimentary fan is developed in the canyon mouth. The Pearl River Canyon acts not only as the main conduit for transferring terrestrial sediments towards the deep sea but also as a trap for the along-shelf transport of sediments. Through swath bathymetry in the canyon area combined with different types of seismic data, we studied the morphotectonics and controlling factors by analyzing the morphology and sedimentary structure of the canyon, as well as the main features of the continental slope around the canyon. Results show that the geomorphologic characteristics of the pearl River Canyon are closely related to the Cenozoic structural and sedimentary processes and sea level change. Since the Miocene, the Baiyun Sag has undergone a rapid thermal subsidence and become a long-term negative landform. The shelf break belt has migrated northward, to north of the Baiyun Sag. At 21 Ma a major lowstand event occurred, in which sea level fell to the shelf break, exposing the continental shelf. The paleo-Pearl River reached the shelf break, which enhanced the sediment transport with frequent slides, slumps, and other sediment flows. This resulted in an incised slope with multiple canyons. The NW-trending faults probably pre-determined the position of the canyons and caused heavy downcutting erosion. Subsequent episodic sea level changes made vertically-stacked buried channels. The canyons on the northern slope of the Baiyun Sag are the main conduits for transporting terrigenous materials to the lower slope and abyssal basin. The middle and lower reaches of the Pearl River Canyon were developed upon the paleo-terrain of the Baiyun Sag, and have been a depositional environment for a long time, receiving most of the materials transported via the canyon/channel.