平移型点坝形成机理与沉积特征

作为曲流河沉积中主要结构单元，平移型点坝（Translational point bar）的研究还相对滞后。通过文献调研和现代沉积观察对平移型点坝的成因机理及沉积特征进行了总结。研究表明，低位时期的下切谷及同沉积断层限制条件是平移型点坝发育的基础。根据平面形态变化，平移型点坝可划分为点坝主体和点坝尾部两部分。点坝主体以粗粒沉积为主，且在顺水流方向上沉积物保存程度增大、粒度逐渐减小；当水流以不同角度冲击外岸时，会形成两种不同模式的点坝尾部沉积，即以细粒沉积为主的反向点坝沉积（Counter-point-bar deposits）和以粗粒沉积为主的涡流堆积（Eddy-accretion deposit）。最后，通过三维岩相模型，平移型点坝内泥质侧积层的分布范围和砂体厚度的差异性进一步明确，点坝主体部分的砂体具有连通性好、砂体厚度大的特征，而点坝尾部砂体连通性较差、砂体厚度较薄。

Formation Mechanism and Sedimentary Characteristics of Translational Point Bars

Researches on translational point bars, which are the main structural units in meandering river sediment, is inadequate lagging. This paper summarizes the formation mechanism and sedimentary characteristics of translational point bars through literature analysis and modern sedimentary observation. Studies have shown that an incised valley in the lowstand period and the limiting conditions of synsedimentary faults are the basis for the development of translational point bars. According to the variations in plane shape, a translational point bar can be divided into two parts: the point bar body and the point bar tail. The point bar body is mainly composed of coarse-grained sediments, and along the direction of water flow, the sediment preservation increases while granularity gradually decreases. Two different modes of point bar tail deposition can be formed, depending on the angle at which the water flow hits the outer bank: counter-point-bar, mainly composed of fine-grained sediments, and eddyaccretion deposits, mainly composed of coarse-grained sediments. By establishing a three-dimensional (3D) lithofacies model, we further clarified the distribution range of the lateral mud interlayer and the thickness of the sand body in the translational point bar. The sand body in the point bar body was characterized by good connectivity and great thickness, whereas the sand body in the point bar tail was poorly connected and relatively thin.