南海北部狭窄陆架-断裂陆坡控制的大型深水扇体系

Large Submarine Fan System Controlled by Narrow Continental Shelf-Faulted Continental Slope in Northern South China Sea

南海北部狭窄陆架-断裂陆坡控制的深水沉积体系研究程度极低.以“源-渠-汇”耦合思想为指导,基于岩石学特征、测井相、地震相分析,刻画了揭阳凹陷珠海组深水扇砂体的垂向叠置和横向迁移特征,并将其形成演化划分为珠海组四段初始形成期、珠海组三段-二段发展扩大期、珠海组一段萎缩消亡期.揭阳凹陷珠海组大型盆底扇的形成受控于易剥蚀的中生界物源、多期的相对海平面快速下降、狭窄陆架-断裂陆坡的有利地貌这3个关键因素的耦合作用;在区域三级相对海平面快速下降的背景下,揭阳凹陷北侧东沙隆起富砂的下白垩统快速剥蚀,沉积物直接沿狭窄陆架区的侵蚀下切谷或小型陆架边缘三角洲的水下分流河道搬运,顺断裂坡折带或构造转换带调节,以重力流形式经峡谷水道继续搬运至下陆坡盆地形成大型海底扇.

The sedimentary system controlled by the narrow continental shelf-faulted continental slope in the northern South China Sea is very poorly studied.Guided by the coupling idea of“source-conduit-sink”and based on petrological characteristics,logging facies and seismic facies analysis,the vertical superposition and lateral migration characteristics of submarine fan sand bodies of the Zhuhai Formation in Jieyang Sag are characterized,and its evolution history is divided into the initial formation period of Member Ⅳ of Zhuhai Formation,the development and expansion period of Member Ⅲ-Ⅱ of Zhuhai Formation and the shrinkage and extinction period of Member Ⅰ of Zhuhai Formation.It is considered that the formation of large submarine fan of Zhuhai Formation in Jieyang Sag was controlled by the coupling effect of three key factors:easily eroded Mesozoic provenance,multi-stage rapid decline of relative sea level,favorable geomorphology of the narrow continental shelf-faulted continental slope.Under the background of rapid decline of regional third order relative sea level,the sand-prone Lower Cretaceous was rapidly eroded from the Dongsha uplift on the north side of Jieyang Sag.The sediments were transported directly along the erosion downcut valley or the underwater distributary channel of the small shelf-margin delta in the narrow continental shelf area,adjusted along the fault slope break zone or structural transition zone,and continued to be transported to the lower slope basin in the form of gravity flow through the canyon channels to form a large submarine fan.