珠江口盆地白云凹陷陆坡限制型海底峡谷群成因机制探讨

珠江口盆地白云凹陷北部陆坡发育着17条近似NNW-SSE走向的海底峡谷，构成了区域内的陆坡限制型海底峡谷群。基于研究区内高密度覆盖的2D地震资料，通过外部形态、内部结构等反射特征的描述和刻画，建立了第四纪以来的高精度层序地层格架，将沉积充填序列划分为三个体系域，即低位体系域（LST）、海侵体系域（TST）和高位体系域（HST）。根据陆坡进积特征、垂向地层叠加样式、侵蚀特征变化、连续性强振幅同相轴的识别和空间追踪，将高位体系域进一步划分为两个沉积旋回单元，HST-I和HST-II。研究表明，白云凹陷陆坡限制型海底峡谷群发育在高位体系域晚期沉积旋回（HST-II）中。在此基础上，分别讨论了沉积物供给、沉积物失稳作用、限制型地形和流体渗漏作用对峡谷群形成和演化的影响。第四纪以来珠江水系携带的大量沉积物经由陆架进入到陆坡区域，为海底峡谷群的发育和演化提供了充足的沉积物来源。在第四纪高位体系域早期水道形成的限制型“负地形”基础上，大量的沉积物在沿着陆坡坡降方向自北向南的输送过程中发生向下的侵蚀、沉积物失稳，导致了陆坡限制型海底峡谷群的发育。研究区内广泛分布的气烟囱构造，暗示了含气流体的垂向运移和渗漏，可能促进了海底峡谷群的进一步演化。

Forming Mechanism of the Slope-confined Submarine Canyons in the Baiyun Sag,Pearl River Mouth Basin

The bathymetric map shows that there are 17 NNW-SSE trending submarine canyons in the northern slope of the Baiyun Sag in the Pearl River Mouth Basin, with the heads terminated on the slope, composing the slope-confined submarine canyons. Based on the high resolution 2D seismic profiles, the Quaternary sequence stratigraphic framework was established by external morphology, internal structure and other reflection characters. The depositional fillings can be divided into three systems tracts, including LST (lowstand systems tract, LST), TST (transgressive systems tract, TST) and HST (highstand systems tract, HST).The HST could be further divided into two sedimentary units, HST-I and HST-Ⅱ, according to the prograding styles of the slope, the vertical stacking patterns, the change of erosion features, and the continuity of high-amplitude seismic reflectors. The results illustrated that, the slope-confined submarine canyons were developed in the later HST sedimentary unit (HST-Ⅱ). Besides, some factors controlling the formation and development of canyons were also discussed, such as sediment supply, sedimentary failures, seafloor topographic features, and fluids leakage. Since Quaternary, abundant sediments from the Pearl River in the north would be transported via shelf to slope and basinfloor, supplying the sufficient sediments for slope-confined submarine canyons. Controlled by some negative paleo-topographic features formed in the earlier HST sedimentary unit (HST-I), during the northward transportation of these sediments, erosions and sedimentary failures were developed, inducing the formations of slope-confined submarine canyons. In the study area, the widely distributed gas chimneys, implied the vertical migrations and leakages of gas-bearing fluids, which might contribute to the evolution of submarine canyons.