Three-dimensional seismic characterization of tidal sand ridges in submerged low-profile backgrounds and simulation of their depositional processes—An example from the Pinghu Formation of the Xihu Depression
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摘要:
西湖凹陷平湖组地层中广泛分布潮汐沙脊,前人对潮汐沙脊的形成过程和控制因素进行了单因素分析和推测,但未综合考虑多种因素共同对潮汐沙脊成因的影响。本文利用钻测井资料和地震资料对西湖凹陷平湖组的潮汐沙脊形态特征及空间展布进行研究,并通过沉积数值模拟的方法探究潮汐沙脊的形成过程和控制因素。研究取得以下认识:① 研究区的潮汐沙脊发育在平湖组中段的水退时期,纵向上多个沙脊叠置,在平面上可以看到孤立分布呈条带状的潮汐沙脊,潮汐沙脊展布方向和潮流方向相近。② 潮汐沙脊的形成主要以潮汐作用为主。河流作用将砂体搬运到入海口的河口湾沉积形成了三角洲,为研究区输入了物源。退潮时砂体被向海搬运,涨潮时缓坡上潮流被砂体较厚处阻挡发生偏转;潮流水动力在砂体间的薄弱处冲刷侵蚀,将砂体改造成孤立生长的细长条形潮汐沙脊。③ 潮汐沙脊的形成受到了潮流水动力和水下低凸起的影响。在潮流水动力较弱的时候,潮流将砂体平铺在水下低凸起上。随着潮流水动力增强,砂体被带向更深的地带,并被侵蚀改造形成长条形潮汐沙脊。水下低凸起起到了阻挡的作用。当水下低凸起高度较低时,河流作用较强,潮汐作用对砂体改造不明显;当高度增高时,阻挡了河流作用继续向海的搬运过程,潮汐作用将砂体搬运到缓坡上,并沉积形成了潮汐沙脊。
Abstract:Tidal sand ridges are widely distributed in the Pinghu Formation in the Xihu Sag. Previous studies were carried out on single-factor analyses and speculations regarding both the formation process and the controlling factors of tidal sand ridges; however, the combined influence of multiple factors on the genesis of tidal sand ridges was largely ignored. Using drilling and logging data and seismic data, we studied the morphological characteristics and spatial distribution of the tidal sand ridges in the Pinghu Formation of the Xihu Sag, and numerically simulated the formation process and controlling factors of the tidal sand ridges. Results show that: ① The tidal sand ridges are developed during the recession period above T33, and several sand ridges are superimposed in the longitudinal direction. The tidal sand ridges are isolated and distributed in strips, and their directions are similar to that of the tidal currents; ② The formation of tidal sand ridges is mainly affected by tidal action. The river action transports the sand bodies to the estuary bay and forms a delta, providing material sources for the study area. At ebb tide, the sand bodies are transported to the sea, and at flood tide, the tidal current on the gentle slope is blocked by thicker sand bodies and deflected; the tidal hydrodynamics scour and erode the weak points between the sand bodies, transforming the sand bodies into isolated, long, and thin tidal sand ridges; ③ The formation of tidal sand ridges is also affected by tidal hydrodynamics and underwater low rises. At weak tidal hydrodynamics, the tide spreads the sand bodies onto the underwater low rises. With the tidal hydrodynamics increase, the sand bodies are carried to deeper areas and eroded and transformed into long tidal sand ridges. The underwater low rise play a blocking role. When the height of the underwater low rise is low, river action is strong, and the tidal action does not transform the sand bodies much; when the height increases, the river action is blocked from continuing to transport the sand bodies to the sea, the tidal action transports the sand bodies to the gentle slope, and finally form tidal sand ridges.
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Key words:
- tidal sand ridges /
- numerical simulation /
- tidal hydrodynamics /
- underwater low rise
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表 1 沉积过程数值模拟参数设置
Table 1. Parameter settings for numerical simulation of deposition process
模拟参数项 模拟参数 网格单元 502×302 网格单元大小 100 m×100 m 模拟时长 30 d 时间步长 1min 地貌演化系数 100 床底坡度 0.03° 水深 20 m 沉积物组分 2(砂质组分,泥质组分) 沉积物浓度 砂质组分∶泥质组分=1∶3 砂质沉积物中值粒径 0.2 mm 河流流量 500 m3/s 潮汐(M2)/m 1.5 3 4.5 潮汐(S2)/m 1 2 3 水下低凸起高度/m 6 10 14 -
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