琼东南盆地北礁凹陷上新统等深流影响的水道沉积体系

Pliocene channel sedimentary system influenced by contour currents in the Beijiao Sag,Qiongdongnan Basin

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。

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.