季节性河流体系高流态沉积构造特征:以内蒙古岱海湖半滩子河为例*

高流态尤其是超临界流动的水动力学机制及其床沙底形演化的研究相较于次临界流动具有一定的差距。季节性河流以高流态为主要沉积搬运过程,为超临界流的形成与保存提供了有利条件,是研究超临界流沉积的重要载体。在季节性河流沉积体系研究进展调研基础上,明确了其基本定义、判别标准及沉积特征。通过对内蒙古岱海湖北部典型季节性冲积体系——半滩子河流发育的沉积构造进行研究表明:随着低流态向高流态演化,沙丘底形(Dune)逐步向上部平坦床沙底形(Upper plane bed)过渡,形成了低角度/S型交错层理;在高流态初期,形成了上部平坦床沙成因的平行层理;随着流动强度逐渐增大,流动机制演变为超临界流,平坦床沙逐渐向逆行沙丘(Antidune)过渡,形成了与平行层理伴生的逆行沙丘交错层理;当流动强度进一步增大,携带沉积物的流体发生较强的水力跳跃,形成了流槽与冲坑(Chute-and-Pool)。半滩子河流现代沉积中发育的高流态沉积与区域内气候变化具有明确的响应关系,表明河流沉积中广泛发育的高流态沉积构造指示了强烈季节性变化的气候特征。

Sedimentary structures formed under upper-flow-regime in seasonal river system: A case study of Bantanzi River,Daihai Lake,Inner Mongolia

In contrast to Froude subcritical flows,understanding of the mechanism and genetic bedforms of upper-flow-regime,especially Froude supercritical flows,remains limited. Seasonal rivers have been considered as important proxy for the study of upper-flow-regime,since the highly potential of preservation and generation of bedforms produced under upper-flow-regime in this kind of depositional system. In this study,the definition,criterion and sedimentary characteristics of seasonal rivers have been summarized briefly according to literatures review. Upper-flow-regime sedimentary structures in the Bantanzi River,which is a typical seasonal alluvial system at the north margin of Daihai Lake,were identified and interpreted. Results show that low-angle and sigmoidal cross-stratifications were generated during the transformation period of dune to upper plane bed. At the initial stage of upper-flow-regime,planar laminations were formed by upper plane bed. As flow strength increasing,flow regime changed to supercritical flow and antidune cross-stratifications were produced. With consistent increasing in flow strength,hydraulic jumps arisen in sediment-laden flow,hence chute-and-pool occurred and produced related sedimentary structures. The upper-flow-regime sedimentary structures developed in the Bantanzi River correspond genetically to the climate features in this area,which infers that extensive upper-flow-regime structures produced and preserved in fluvial deposits could indicate strongly seasonal variation of paleoclimate.