南海西北次海盆深水扇系统沉积演化特征

Depositional Architecture and Evolution of Deepwater Fan System in the Northwestern Sub-Basin,South China Sea

基于高品质二维地震资料的分析解释,在南海西北次海盆深海平原区识别出大规模深水扇系统。深水扇系统上扇为限制性水道复合体发育区,中扇为多期扇体垂向叠置区,下扇则以水道—朵体沉积为主。琼东南中央峡谷—水道是本区深水扇系统的主要物源通道,沉积物主要来源于红河、北部陆架—陆坡以及中西沙隆起区。本区深水扇系统可基本划分为晚中新世(Ⅰ)、中新世(Ⅱ和Ⅲ)以及第四纪(Ⅳ和Ⅴ)五期。各期深水扇的空间展布受到先存地形和物源供给强度的控制,双峰海山将深水扇系统分隔为南北两部分,早期沉积的扇体改变了后期扇体沉积地形。在丰富物源供给下,水道的冲溢频率较高,各个深水扇之间存在明显的侧向上叠迁移特征。沉积物源和南海北部"三段式"陆坡地形控制着整个南海北部深水扇系统的发育和演化。

Based on the existing high-resolution 2D seismic data,we identified a large deep-water fan system in the Northwest sub-basin,the abyssal plain of northern South China Sea.The results show a canyon-channel transition zone with channel avulsions in the upper fan,channel-levee systems in the middle fan and channel-lobe systems in the lower fan.The Qiongdongnan Center canyon-channel is considered as the main sediment supply system,with sediments mainly from the Red River and Indochina,northern shelf and slope,and Zhongsha and Xisha uplift regions.A combination of the pre-existing topography and sediment supply strength appears to have a control on the temperospatial distribution of the deep-water fan system.Geographically the fan system is divided into north and south parts by the Shuangfeng seamount,and geologically I to V sub-fans can be identified,respectively corresponding to the late Miocene（I）,Pliocene（II and III）,and Quaternary（IV and V）.The early-deposited sub-fans altered the topography,while strong sediment supplies improved the overflow frequency,with obvious lateral migration in individual fans.The ＂three step＂ continental slope of northern South China Sea affected the development and evolution of deepwater fan systems.