Pliocene channel sedimentary system influenced by contour currents in the Beijiao Sag, Qiongdongnan Basin
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摘要: 等深流影响的水道沉积体系的沉积特征及其沉积过程是当前深水沉积学研究的热点、难点和前沿科学问题，但研究程度较为薄弱。该文以北礁凹陷上新统（地震反射T20−T30）为研究对象，利用覆盖北礁凹陷局部的三维地震资料，采用均方根属性、相干属性、时间域构造，再结合地震切片等方法，研究北礁凹陷深水区上新统斜交斜坡（走向）的特殊水道沉积体系特征及其沉积过程。研究发现，该水道沉积体系分为早、晚两期，早期发育水道和片状、扇状溢堤沉积，晚期仅发育水道和片状溢堤沉积，其中扇状溢堤沉积仅发育在水道右侧弯曲处，片状溢堤沉积仅分布在水道左侧，水道始终与区域斜坡斜交，水道对称分布且无明显迁移现象。结合该时期北礁凸起发育等深流相关的丘状漂积体和环槽，认为该水道沉积体系特殊的形态主要受控于等深流与浊流交互作用的沉积结果：浊流流经水道，其上覆浊流溢出水道，形成溢岸浊流，在水道左侧，该溢岸浊流与等深流发生相向运动，被等深流“吹拂”到单侧，大面积分布，延伸千米，形成片状溢堤沉积；而在水道弯曲处（右侧），溢岸浊流与等深流发生相对运动，抑制溢岸浊流进一步扩展，形成相对小范围扇状溢堤沉积，该沉积结果与前人水槽实验结果相一致。Abstract: Depositional characteristics and process of deep-water channel sedimentary system influenced by contour currents is a current hotspot and new scientific issue in the study of deposits resulted from interaction between contour currents and turbidity flows. Using root mean square (RMS) attribute, coherenceseismic attribute, time structure and stratal slice, we focus on depositional characteristics and process of the channel sedimentary system in Pliocene strata in the Beijiao Sag, Qiongdongnan Basin. The results show that the channel sedimentary systems are divided into the early and late channel sedimentary systems. The depositonal units of the early system include channel, flake-shaped and fan-shaped overbank deposits. The units of the late systems incorporate: channel and flank-shaped overbank deposits. Fan-shaped overbank deposits are only distributed on the right side of the bend of the channel. Flake-shaped overbank deposits are only located on the left side of the channel. The channels are persistently oblique to the strike of the slope and they are symmetrical and characterized by vertical aggradations. Combined with the development features of mounded drifts and moats over the Beijiao uplift, it is inferred that the geometry of the channel sedimentary systems are mainly controlled by depositional products of the interaction between turbidity flows and contour currents. Contour currents flow across channels, forcing the upper part (low velocity and low density) of turbidity flows to deflect towards the left (downstream) side of channels and then form overspill currents. The direction of overspills is the same to that of contour currents, in this case, generating wide flake-shaped overbank deposits extending for several kmlometers. In the bend of channels, the direction of overspill currents is in contrast to that of contour currents, blocking or confining the range of the overbank deposits and then creating fan-shaped overbank deposits. This result are consistent with the previous results of flume-tank experiment.
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Key words:
- Bejiao Sag /
- channel sedimentary system /
- overbank deposits /
- contour currents /
- turbidity flows
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图 1 等深流与浊流交互作用的两种沉积类型
a. 向等深流下游方向单向迁移的水道沉积体系模式（引自文献[3]）；b. 向等深流上游方向单向迁移的水道沉积体系模式（改自文献[16-18]）
Fig. 1 Two types of sediment models for the interaction between contour currents and turbidity flows
a. Channel sedimentary system model for unidirectional downstream migrating of contour current flows (cited from reference [3]); b: channel sedimentary system for unidirectional upstream migrating of contour current flows (modified from references [16-18])
图 3 琼东南盆地深水区地层综合柱状图
此图展示了沉积相（引自文献[29]）、构造运动及演化（引自文献[24]）、强振幅水道发育/分布层位（引自文献[15]）及海平面变化（引自文献[27]）
Fig. 3 Comprehensive stratigraphic histogram in the deep-water area of the Qiongdongnan Basin
It shows characteristics of sedimentary facies (cited from reference [29]), tectonic movement and evolution (cited from reference [24]), development or distribution location of channels with high amplitudes (cited from reference [15]), and eustatic sea level change (cited from reference [27])
图 4 琼东南盆地北礁凹陷上上新统早−晚期水道沉积体系剖面特征（a−c）及其沉积单元划分（b1，c1）
绿色实线“S”为早晚期水道沉积体系分界线
Fig. 4 Profiles characteristics (a−c) and sediment units divisions (b1, c1) of the early and late channel sedimentary systems in the upper Pliocene in the Beijiao Sag, Qiongdongnan Basin
Solid green line “S” presenting the boundary between early and late channel sedimentary systems
图 5 琼东南盆地北礁凹陷上新统早−晚期水道沉积体系均方根属性（a，b）及其沉积相解释（a1，b1）
a. 时窗为分界面S至S下移（+）40 ms；b. 时窗为分界面S至S上移（−）40 ms
Fig. 5 RMS attributes (a, b) and their sedimentary facies interpretation (a1, b1) of the early and late channel sedimentary systems in the Pliocene in the Beijiao Sag, Qiongdongnan Basin
a. Time window from boundary “S” to the downward shift (+) 40 ms of “S”; b. time window from boundary “S” to the upward shift (−) 40 ms of “S”
图 6 琼东南盆地北礁凹陷早−晚期水道沉积体系相干属性（a，b）及环槽与丘状漂积体发育特征（c）
Fig. 6 Coherence attribute of the early and late channel sedimentary systems (a, b) and the characteristics of moats and contourites (c) in the Beijiao Sag, Qiongdongnan Basin
图 7 等深流与浊流交互作用下浊流溢出堤岸的沉积模式及其水槽实验
a. 沿斜坡等深流与跨斜坡浊流的交互作用平面形态（引自文献[8]）；b. 由于浊流受到等深流的影响发生溢岸浊流，致哥伦比亚水道（扇）末端朵叶体向等深流下游方向迁移（引自文献[8]）；c. 等深流与浊流交互作用的水槽实验（引自文献[34]）
Fig. 7 Sedimentary models and flume-tank experiment of turbidity flows overflowing levees under the interaction of contour currents and turbidity flows
a. Interaction planform morphology of along-slope contour currents and across-slope turbidity flows (cited from reference [8]); b. lobe at the end of Columbia channel (fan) migrating downstream of contour currents, due to overspills from turbidity flows affected by contour currents (cited from reference [8]); c. flume-tank experiment of the interaction between contour currents and turbidity flows (cited from reference [34])
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