底流与重力流交互作用下的朵体沉积特征:以东非鲁伍马盆地中新统为例

Sedimentary characteristics of lobes formed by interacting bottom currents and gravity flows:a case study from the Miocene in Rovuma Basin,East Africa

底流与重力流的交互作用是重要的深水沉积作用类型,对海底沉积物再分配及海底地貌改造具有重要控制作用。目前对底流与水道中重力流的交互作用研究较成熟,但对底流影响下朵体的沉积特征研究较少。本研究以东非鲁伍马盆地古近系中新统朵体为目标,综合利用三维地震和测井资料,通过地震相和地震属性分析,确定朵体分布及内部岩性特征。在定量恢复重力流流速基础上,结合区域洋流资料,揭示底流与重力流交互作用对朵体沉积的控制作用。结果表明受向北流动的底流影响,早期水道向南迁移,北侧天然堤发育,在水道南侧形成负向地形,导致后期朵体沉积于水道南侧。底流作用下的朵体沉积特征受底流和朵体内重力流流向的影响。当底流与朵体延伸方向相反且斜交时,由于重力流流速远大于底流,朵体在逆底流流向一翼(南侧),重力流与底流流向几乎相反,底流影响被抑制,朵体从轴部向末端逐渐变薄,泥质含量增加;在顺底流流向一翼(北侧),重力流方向与底流斜交,重力流顶部泥质被底流淘洗出去,朵体以砂质沉积为主。当底流与重力流流向接近正交时,朵体受底流影响整体上一致,重力流顶部泥质被淘洗,逆底流流向一翼朵体以厚层砂质沉积为主,向顺底流流向一翼砂体厚度减小、泥质含量增加。

The interplay of bottom currents and gravity flows is an important deep-water sedimentary process,which plays an important role in redistributing submarine sediments and reshaping seafloor bedforms.At present,research on bottom currents and gravity flows interaction in deep-water channels is relatively mature,but little work has been done on the sedimentary characteristics of lobes influenced by bottom currents.Based on an integrated study of seismic facies and seismic attributes using 3D seismic and logging data,this study documents the distribution and lithological characteristics of the Miocene lobes in the Rovuma Basin(East Africa).The effect of bottom currents and gravity flows interaction on lobe sedimentation is revealed through quantitative restoration of gravity flow velocity,combined with regional ocean current data analysis.The results indicate that,early channels migrated southward due to the northward-directed bottom current,and their northern levees are well developed with a negative terrain present to the south of the channels,which led subsequent lobes to deposit to the south of these channels.Deposition of the lobes is influenced by bottom current flow direction and gravity flows forming the lobes.Velocity of the gravity flows is much higher than that of the bottom currents.Therefore,when the directions of bottom currents and the lobe propogation are opposite and oblique,the influence of bottom currents is suppressed on the side against bottom current flow direction(south side),and the lobe gradually thins from the axis to the margin,accompanied by increasing mud content;On the side downstream of bottom current flow direction(north side),gravity flows intersect obliquely with bottom currents,and the mud at the top of turbidity currents is stripped off by the bottom current,leaving sand deposited in the lobe.When bottom current flow direction and lobe propogration direction are close to orthogonal,the influence of bottom current on the lobe is consistent on the whole,and the mud at the top of gravity flows is stripped.Therefore,the lobe is mainly composed of thick sandy deposits at the side against the bottom current flow direction,and the sand content decreases towards the side downstream of the bottom current,accompanied by an increasing mud content.