季节性河道与暂时性河道的沉积特征——以新疆白杨河冲积扇为例

Sedimentary Characteristics of Ephemeral and Intermittent Channels:A case study of the Baiyanghe fan,Xinjiang,China

通过对新疆现代白杨河冲积扇的地貌和现代沉积进行调查,发现冲积扇表面发育两种不同类型的河道,一种为季节性河道,另一种为暂时性河道。暂时性河道内水流占有率小于50%直至接近于0,主要由暴雨形成突发性洪水造成,季节性河流河道内流水占据率为50%左右,输出水流特征介于暂时性河道和常年性河流河道之间。白杨河主河道属于季节性河道,河道占冲积扇表面面积2.1%,沉积物以砾石质为主,颗粒粗、磨圆度高、分选较好,泥质含量低、叠瓦状排列特征明显,沉积物具有向下游变细的趋势,河道形态沿程变化明显。暂时性河道占冲积扇表面面积97.9%,沉积物粒度相对较细,磨圆度低、分选差、泥质含量高,河道规模向下游减小,分叉增多。季节性河道以河道径流为主,暂时性河道主要以片流、泥石流等方式搬运沉积物。季节性河道沉积物主要来自上游较远的源区,暂时性河道的沉积物来源于冲积扇扇根附近基岩风化形成的倒石锥,塌积扇和山地泥石流沉积,一部分来自于对冲积扇原有沉积物的改造、搬运和再沉积。季节性河道是形成冲积扇扇体的主要动力,暂时性河道主要对冲积扇起改造作用。研究深化了对干旱地区冲积扇沉积过程和沉积特征的认识,丰富了冲积扇的沉积模式。

A topographical investigation of modern deposits of the Baiyanghe fan in Xinjiang show that two types of fluvial channels are developed on the surface of the fan:intermittent channels and ephemeral channels.The latter,which have flow occupancy between<50%and zero,are mainly formed by occasional flooding during summer rains.The occupancy rate of intermittent channel flow is about 50%,and their flow output is between that of ephemeral and perennial channels.The main Baiyanghe channel is intermittent,occupying 2.1%of the fan surface area.Its deposits are mainly coarse-grained gravels,highly rounded,well sorted,and with low mud content and higher imbricated characteristics.The course of the channels change,with sediments fining downstream.Ephemeral channels,which are distributed over about 97.9%of the fan surface area,contain finer sediment with low roundness,poor sorting and higher mud content.The scale of ephemeral channels decreases downstream and bifurcation increases.Intermittent channels are dominated by seasonal runoff,and the source of the sediment load is mainly remote from the fan.The sediment carried by ephemeral channels is dominated by sheet flow and debris flow from colluvium and debris-flow deposits near the apex of the fan,together with some sediment from earlier deposits within the fan itself.The fan lobe are mainly constructed by intermittent channels,whereas ephemeral channels play a role in re-shaping the fan.This study has increased the understanding of the sedimentary process and characteristics of alluvial fans in arid areas and enriched the sedimentary model of fan research.