南海北部白云凹陷21Ma深水重力流沉积体系

21 Ma deepwater gravity flow depositional system in Baiyun sag, northern South China Sea

深水重力流沉积是南海北部陆坡主要的沉积类型,以高分辨率三维地震、钻测井等资料为基础,通过地震资料解释和地质分析,对南海北部白云凹陷21Ma所发育的深水沉积体系进行识别、解释和预测。识别出块体搬运沉积体系、水道堤岸复合体以及朵体3类深水重力流沉积单元。块体搬运沉积体系呈杂乱反射特征,底部发育侵蚀擦痕;水道作为重力流的搬运通道,呈弱振幅U或V形特征;重力流溢流形成的堤岸呈强振幅楔形特征;水道与堤岸构成水道堤岸复合体具有典型的海鸥翼状特征;位于水道前端的朵体表现为强振幅平行—亚平行反射,在平面上具有朵状特征。块体搬运沉积体系逐渐向水道、水道堤岸复合体以及朵体演化的过程中伴随着不同流态的深水重力流之间的相互转化。综合地球物理学、沉积学以及海洋地质学等方法,对南海北部陆坡21Ma开展重力流沉积体系研究,建立深水沉积模式,对研究深水重力流沉积过程具有重要的理论意义。

Deepwater gravity flow deposits are one of the main depositional types on the slope of the northern South China Sea. Based on high-resolution 3D seismic data, drilling data and logging data, and integrated with geophysical date interpretation and geological analysis, we identified, interpreted and forecasted a 21 Ma deepwater depositional system developed in Baiyun sag, northern South China Sea. The results showed that three types of deepwater gravity-flow deposits were identified, i.e. mass-transport, channel-lev- ee complex and lobate deposits. The mass-transport deposit （MTD） has a chaotic seismic reflection with basal erosion scratches. The channel-levee complex deposit is characteristic of a ＂gull-wing＂ seismic reflection, of which the former （channel） shows a low-ampli- tude U-or V-shaped seismic reflection and acts as a main pathway for deepwater gravity-flow deposits, while the latter （levee） formed by the overflow of gravity flow is of a high-amplitude wedge-shaped reflection. A high-amplitude parallel to sub-parallel seis- mic reflection is interpreted to represent the lobate deposit, which is located in front of the channel and comes in a lobe-like shape on plane. Gradual evolution from the mass-transport deposit to the channel, channel-levee complex or labate deposit is often accompa- nied with a reciprocal transformation of various flow patterns of deepwater gravity flow. The research integrated geophysics, sedim- entology and marine geology with the observation of the 21 Ma deepwater gravity-flow depositional system in Baiyun sag, northern South China Sea to establish a deepwater sedimentary model, which is of the theoretic significance in studying various depositional processes of deepwater gravity flow.