金字塔沙丘粒度变化及表面过程的初步研究

Grain Size and Surface Processes of the Pyramid Dune in the Mogao Grottoes of China

莫高窟顶金字塔沙丘粒径集中分布在2．44～2．94圣，属细砂。标准差介于0．24～0．65之间，偏度在0．03～0．31，属于正偏级。峰度在0．78～1．72，介于宽峰态至尖峰态。沙粒粒径级配是风况、地貌与沙物质相互均衡作用的结果。金字塔沙丘存在着沙臂顶部沙最细或最粗两种粒度模式。金字塔沙丘沙丘主臂（WNW--ESE）属于在双向风环境作用下，类似于横向沙丘粒度变化模式，两坡下部以风蚀为主，丘顶附近以堆积过程为主，导致沙臂两坡粒径与分选性由下而上变细与变好。频率高、风速不大的局地气流，可将大量细沙物质搬运到金字塔沙丘主臂，造成丘底到丘顶的粒度变化不大。副臂（SW）受较强Nw风力作用，具有横向及纵向沙丘粒度变化模式，整个坡面处于风蚀状态，沙臂顶部风蚀最为强烈，导致从底部到顶部的粒度逐渐由细变粗。

The pyramid dune at the top of the Mogao Grottoes is composed of fine sands with a diameter be tween 2.44 φ and 2.94 φ and a standard deviation in 0. 24--0. 65. The skewness varying between 0.03 and 0.31 with a mean value of 0.11 indicates that the sands belonging positive skews. The kurtosis is between 0.78 and 1.72 with a mean value of 1.28 ranging from broad to narrow scare. The pattern of grain size characteristic of the main arm （NWW--ESE direction） is similar to that of the transverse dune. Erosion is occurred at the toe and deposition around the dune top. Abundant fine blown sand is moved toward the crest of main arms by the frequent local wind with low speed, leading to homegeny grain size distributions at the toe and the top. The secondary arm （SW direction） is controlled by stronger NW wind regime and its grain size distribution pattern is similar to longitudinal dune, The whole slope, especially the dune top, suffers e-rosion and the diameter of sand increases from the toe to the top.