Seismic sedimentary characteristics of the Ancient Pearl River system and its depositional model of confluence scours, northern shelf of the South China Sea in Early-Mid Miocene
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摘要:
河道体系是侵蚀和沉积物搬运的重要通道，控制了大陆边缘源汇体系中砂体总体的输送和分布格局，并可有效指示古代构造活动、物源供给、气候变迁和海平面变化等丰富地质信息。研究基于高分辨率的三维地震、钻测井资料，结合层序划分方法和地震沉积学综合分析手段，揭示了南海北部珠一坳陷早—中中新世古珠江分流河道体系的类型和展布特征，并且以典型层序为例精细揭示了海平面变化控制下河道类型由辫状向曲流的转换。同时，在研究区地震剖面上识别到多处异常下切现象，经研究可解释为辫状河道内的汇流冲刷，其典型特征为：① 下切深度局部增大；② 平面近似圆形或椭圆形；③ 内部为砂质充填。虽然汇流冲刷结构在地震剖面特征上与“传统”意义的下切谷极易混淆，但在沉积特征和成因上存在较大差别，因此，对它的识别有助于避免层序界面的错误判别，并且其后期充填的河道砂体具有局限分布的特点，易于在后期海侵泥岩的覆盖下形成潜在的岩性圈闭新类型。
Abstract:The river channel system is an important passage for erosion and sediment transport, which controls the overall transport and distribution pattern of sand bodies in the source-sink system of the continental margin, which indicates effectively the tectonic subsidence, sediment supply, climate change, and sea level change of the ancient sedimentary environment. Based on the high-resolution 3D seismic data, well logging and coring data, the types and distributions of the fluvial system in the Early-Mid Miocene of the Zhu I Depression in the ancient Pearl River Delta, northern South China Sea were studied in combination of the classification of the sequence stratigraphy and comprehensive seismic-sedimentary analysis. Furthermore, representative sequences were used to analyze the transition mechanism of fluvial types controlled by sea-level change. Abnormal incisions in large numbers were identified from the high-resolution seismic profiles of the study area, which can be interpreted as confluence scour in a braided channel. The typical characteristics are as follows: (1) showing local increase of cutting depth; (2) being approximately circular or elliptical in plan view; and (3) the interior being filled with sand. Although the confluence scour structure is easily confused with the incised valley in seismic profile, it is quite different in sedimentary characteristics and genesis. Therefore, the identification of confluence scour could help avoid wrong interpretation of sequence boundary, and they are filled with sediments carried by later fluvial channels, which is easy to form a new type of potential lithologic trap under the cover of the later transgressive mudstone. The confluence scours facilitated the access to the reservoirs and connected the stacked fluvial sandstones, which effectively enhanced the internal connectivity of sandstone reservoirs.
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Key words:
- braided river /
- meandering river /
- confluence scour /
- incised valley /
- the Pearl River Delta
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表 1 河道体系类型的划分
Table 1. Classification of the river system types
河道类型 河流宽度 弯曲度 地震反射特征 地震地貌特征 大型辫状分流河道 平均宽度＞10 km
最大宽度可达30 km较低 强-中振幅、好-中等连续、
平行-亚平行反射内部河道交切明显，呈现“辫状” 中型河道 0.5～7 km 中等 强-中振幅、中等连续性 辫状河道体系末端分支河；下切谷：
延伸较远且地震剖面见明显下切小型曲流河道 100～300 m 较高 单点状的强振幅且连续性较好 单支河道孤立发育 
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表 2 汇流冲刷和下切谷的识别特征
Table 2. The distinctive features between confluence scour and incised valley
平面形态特征 下切深度 充填特征 岩性特征 层序意义 汇流冲刷 近似圆形或椭圆形，分布局限，宽度约0.5～5 km 中等 单期充填 中-粗河道砂岩 无明显层序意义 下切谷 明显河道状，延伸距离远 较大 多期充填 中-粗河道砂岩至海相泥岩，岩性多样 层序边界标志
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