Abstract:

The sedimentary evolution and modification of braided alluvial fans under episodic flood conditions play a key role in shaping the reservoir architecture and distribution patterns of these dynamic systems. However,the evolutionary processes,formation mechanisms,and controlling factors of the architecture of this type of alluvial fan remain unclear. Through flume simulation experiments,this study simulated the evolution and reworking process of different geomorphic units and sedimentary architectures of braided alluvial fans under episodic flood conditions. It also investigated various construction and reworking mechanisms of episodic flood fluids with different flow states,clarifying the evolutionary process and architectural characteristics of braided alluvial fans under episodic flood conditions. The research shows that: (1)Under episodic flood conditions,the topographic slope and sediment supply are the main controlling factors affecting the morphology of the fan. As the slope increases,the fan tends to grow axially,with an increasing length-to-width ratio. An increase in sediment supply leads to an expansion in fan area but has little effect on the length-to-width ratio. An increase in the number of sediment sources results in more planar intersection areas between individual fans,reducing the length-to-width ratio of each fan.(2)The transformation of fluid properties between flood and inter-flood periods is a crucial factor controlling sedimentary architecture. During the flood period, sheet flow deposits predominantly occur in the fan apex and upper-middle parts. As the flood disperses and weakens,the lower-middle parts and fan edges transition to braided stream deposits. During the inter-flood period,persistent trickling streams flow through confined braided channels,reworking the flood gravity flow deposits formed during the previous flood period,resulting in strip-shaped braided channel deposits. The positions of transformation from sheet flow to unrestricted braided flow and from unrestricted braided flow to runoff are controlled by the flood flow velocity,sediment supply amount,and topographic slope.(3)The sheet flow deposits formed during the flood period are frequently reworked and superimposed by braided channel deposits formed during the inter-flood period,forming a complex sedimentary architecture of braided alluvial fans under episodic flood conditions in both time and space. Based on the results of flume simulation experiments,combined with the field investigation and anatomy of the Baiyang River alluvial fan,a sedimentary architecture model for braided alluvial fans under episodic flood conditions was established. This research not only enriches the sedimentological theory of alluvial fans and improves the analysis of their formation mechanisms,but also has significant practical value for the prediction of reservoirs of this type of alluvial fan.

Key words: episodic flood, braided alluvial fan, flume simulation experiment, controlling factor, sedimentary evolution, sedimentary architecture, architecture model

摘要：

阵发性洪水条件下辫状河型冲积扇沉积演化和改造过程影响该类型冲积扇储层构型及其分布,但是该类型冲积扇构型的演化过程、形成机理和控制因素尚不清楚。本研究通过水槽模拟实验,模拟了阵发性洪水条件下辫状河型冲积扇体不同地貌单元和沉积构型的演化及改造过程,研究了不同流态的阵发性洪水流体的各种建造和改造机制,明确了阵发性洪水条件下辫状河型冲积扇的演化过程及构型特征。研究表明: (1)阵发性洪水条件下辫状河型冲积扇的地形坡度、物源供应是影响扇体形态的主控因素。随着坡度的增加,扇体更倾向轴向生长,长宽比增大; 物源供应量增加使得扇体面积增大,但对扇体长宽比影响不大; 物源数量的增加导致扇体平面交汇区增多,单个扇体的长宽比降低。(2)洪水期和间洪期流体性质的转化是控制沉积构型的重要因素。在洪水期,扇根及扇中上部多发生席状化的片流沉积; 随着洪水的分散和强度的减弱,扇中下部和扇缘,转变为辫流沉积; 在间洪期,持续的涓涓细流在限制性的辫状水道中流动,改造前期洪水形成的洪水重力流沉积物,形成条带状的辫状河道沉积。(3)洪水期形成的片流沉积物被间洪期形成的辫状河道沉积物频繁改造和叠置,在时空上形成了阵发性洪水条件下辫状河型冲积扇的复杂沉积构型。根据水槽模拟实验的结果,结合对白杨河冲积扇的野外考察和解剖,构建了阵发性洪水条件下辫状河型冲积扇的沉积构型模式。该项研究不仅丰富了冲积扇的沉积学理论,完善了冲积扇的成因机理分析,而且对该类型冲积扇储集层的预测具有重要的实用价值。

关键词: 阵发性洪水, 辫状河型冲积扇, 水槽模拟实验, 控制因素, 沉积演化, 沉积构型, 构型模式

CLC Number: 

• P512.2