新疆额尔齐斯河边滩砾石定向性特征及其意义

Orientation of Irtysh River point bar gravels and its significance in Xinjiang

综合运用玫瑰图以及Curray的计算方法,对额尔齐斯河某处边滩上的20个观测点的砾石定向性进行了研究。结果表明,研究区砾石长轴存在定向排列且最优方向和河流流向平行,但是砾石定向性特征还和其他因素有关。颗粒支撑方式对砾石定向性有一定影响,多级颗粒支撑的砾石定向性最好,其次是颗粒支撑的砾石。水动力强弱会影响砾石定向排列,当水动力比较弱时,砾石在流水作用下以滚动的方式被搬运,砾石长轴和河流流向垂直;水动力比较强时,砾石在流水作用下以跳跃方式被搬运并且长轴和流向平行,在这种情况下其所受阻力最小,能够保持稳定状态。砾石粒径也会影响砾石定向性,在水动力比较弱时,小砾石(粒径小于2cm)长轴和河流流向垂直,大砾石(粒径大于2 cm)和河流流向平行;但是当水动力较强时,砾石粒径大小对颗粒定向性特征的影响就可以忽略不计。因此运用砾石长轴来判断古流向时,要综合考虑研究区的古气候、沉积时的水动力等其他因素,然后确定河流流向和砾石长轴的关系,最后再确定研究区的古流向。

In this paper,the rosette diagram and Curray＇s research method were used to study the orientation of gravels at 20 observation points in the Irtysh River point bar. It was found that the long axis ofgravels in the study area had a directional arrangement,and the optimal direction was parallel to the flow direction of Irtysh River. However,the oriention of gravels was also influenced by other factors. Grain support mode affected gravels orientation in some degree, e. g.,the multi-size particles supported gravels showed the best oriention,followed by particles support. The strength of water also had an influence on gravels orientation. The long axis of the gravels was perpendicular to the flow direction under the weak hydrodynamic force. When the hydrodynamic force became strong,the long axis of the gravels would be parallel to the flow direction. The orientation characteristics of the gravels are also related to their grain size. If the hydrodynamic force was weak,the long axis of smaller gravels（ grain size less than 2 cm） was perpendicular to the flow direction and the larger gravels（ with grain size greater than 2 cm） were parallel to the flow direction. However,when the hydrodynamic force became strong,the influence of the gravels size on orientation characteristics could be negligible. Therefore,when using long axis of gravel to determine the palaeo direction of flow,it is better to take into consideration other factors first,such as palaeoclimate and hydrodynamic force in the study area,and then a more accurate recovery of the relationship between river flow and the long axis of gravels can be obtained. Finally,you can determine the palaeo-flow direction of the study area comprehensively.