牵引流态洪水条件差异对阵发性河流型冲积扇沉积控制机理研究

In the nature world, episodic flood events not only can result in the formation of debris flows but also can generate tractional flood flows. However, the key issues of the controlling mechanisms of the time-space variability in the property of tractional flood flows on the deposition of alluvial fans have been rarely addressed in the world. Furthermore, depositional model of episodic fluvial fans originated from tractional flood flows can be quite different from traditional continuous fluvial fans, which are constructed by different river systems. Accordingly, the Baiyang Fan, which was developed in the northern boundary of Heshituoluogai Basin under arid conditions, is chosen for the field study. Moreover, flume tank experiments are designed for the simulation study. Based on the analysis of lithofacies and microfacies in the field and the paleohydraulic reconstruction, tractional flood flows with different properties are simulated in the flume tank for the comparative studies of sedimentary-geomorphic evolution path of alluvial fans. The objectives of this study are to investigate the sedimentary fluid mechanisms of the tractional flood flows; to clear the sedimentary characteristics of the episodic fluvial fans; to establish the detailed evolution model of the episodic fluvial fans; and to discuss the passive factor on the deposition of the episodic fluvial fans. Overall, this study can not only deepen the basic geological theory of sedimentology, expend the classification of alluvial fans, but also provide a reasonable theoretical guiding significance in the recognition, comparison and prediction of buried alluvial fan conglomerate bodies.

自然界中阵发性洪水事件除形成泥石流外还可形成牵引流态洪水，但针对牵引流态洪水在时间-空间上的流体条件差异对冲积扇沉积控制机理这一关键问题，国内外尚未开展系统研究。而由阵发性牵引流态洪水建造的河流型冲积扇与传统的由不同河流体系连续性建造而成的河流型冲积扇在沉积特征及沉积模式上存在较大差异。据此，本项目选取和什托洛盖盆地北缘干旱气候下发育的现代白杨扇为野外沉积解剖对象，并结合室内水槽实验模拟，在静态岩相、微相分析及古水动力计算恢复基础上，从动态角度模拟对比不同牵引流态洪水条件下扇体沉积-地貌演化过程。进而阐明牵引流态洪水可存在的不同沉积流体机制，明确受牵引流态洪水控制的阵发性河流型冲积扇沉积特征，建立其系统的沉积-地貌演化模式，并探讨其沉积演化的静态控制因素。本项目不仅可丰富沉积学理论，拓展冲积扇成因分类，并且对埋藏冲积扇砂砾岩体的成因识别、对比及预测具有重要的实际指导意义。

冲积扇作为一特殊的山前沉积体，其是“源-汇”系统研究中的关键，同时也是重要的油气储集体类型。其复杂的沉积演化过程是揭示构造活动、气候变迁及盆地充填的重要线索。然而国际上有关牵引流态洪水条件差异对阵发性河流型冲积扇沉积控制机理仍缺乏系统认识。据此，为解决这一关键科学论题本项目主要进行五方面研究内容，即：（1）阵发性河流型冲积扇发育岩相类型、岩相成因及组合关系研究；（2）阵发性河流型冲积扇微相单元、相带展布及古水动力恢复研究；（3）阵发性河流型冲积扇不同洪水条件下沉积-地貌水槽实验模拟研究；（4）阵发性河流型冲积扇沉积-地貌演化静态控制因素水槽实验研究；（5）阵发性河流型冲积扇沉积演化模式综合研究。而研究结果表明：（1）牵引流态洪水控制的阵发性河流型冲积扇演化具有阶段性，其主体由席状化和河道化洪水流体共同建造完成。其中洪水期发育席状下切洪流微相、片流微相、非限制性洪流微相及间歇性扇面水洼微相；洪退期发育限制性河道微相；间洪期则发育侵蚀滞留砾石层、流沟、风成砂及河流阶地微相。（2）古水动力条件恢复揭示扇体内部阵发性河道体系各河道宽度、深度、满岸流量和水动力向下游均不断减小，且沉积地形坡度显著高于传统的河流体系。此外，阵发性河流体系单支河道呈带状，而多支河道平面呈分流特征，且向下游方向由于水流损失将会逐渐消失于扇面。（3）受控于周期性湿润-干旱气候波动所导致的洪水流量及沉积物供给量差异，席状化洪水流体建造往往形成于相对湿润的气候周期；河道化洪水流体建造阶段则对应于相对干旱的气候周期；而废弃阶段则出现于两次阵发性洪水事件之间或代表洪水难以形成的极度干旱时期。上述研究成果不仅丰富了沉积学理论，同时拓展了冲积扇成因分类，并且对山前埋藏冲积扇砂砾岩储层的识别、对比及预测具有重要的理论和实际意义。