古尔班通古特沙漠西南缘新月形沙丘内部沉积构造特征研究

Internal sedimentary structure of barchan dune in the southwest of Gurbantunggut Desert

沙丘内部沉积构造保存了沙丘动力演变过程的重要信息。以古尔班通古特沙漠西南缘固定、半固定新月形沙丘区为研究区域,利用探地雷达在春、秋两季对固定、半固定新月形沙丘内部构造进行探测,获取了不同规模形态的固定、半固定新月形沙丘深约8 m的内部构造图像信息。通过图像增益处理、解译和对比分析表明:(1)古尔班通古特沙漠固定、半固定新月形沙丘共有5种沙丘内部构造雷达相,即高倾角斜层理、楔状交错层理、上凸形交错层理、低倾角-近水平层理和块状层理,其中前4种主要分布在3~5 m的浅层,而块状层理主要分布在4~5 m以下的深度。(2)高倾角斜层理、楔状交错层理主要分布在高大新月形沙丘(链)的迎风坡上部和丘顶地带,前者为背风坡前积层埋藏而成,反映高大新月形沙丘迎风坡上部和丘顶风沙活动较频繁,沙丘“固身缩顶”后埋藏的前积纹层因风蚀而出露,后者为迎风坡风蚀坑和风蚀槽中由风沙流充填而成的构造或在丘顶风向的季节性变化形成的构造。(3)与灌丛沙丘相关的上凸形交错层理广泛分布在新月形沙丘的迎风坡中下部,在背风坡也有局部出现,表明灌丛沙丘在沙丘表层的风沙过程中占有重要地位;而深部的块状层理可能是早期风积层受到强烈的生物扰动,原生层理消失而产生。(4)以上沙丘内部构造的类型与组合分布特点,反映了研究区新月形沙丘总体上趋于稳定或衰退状态,这与现代沙丘“固身缩顶”的地貌变化特征相一致。例如,迎风坡中上部和丘顶常见风蚀槽,背风坡因坡度变缓、前积层发育趋缓,现代风沙活动主要集中于新月形沙丘的上部和丘顶等。由此可见,研究区固定、半固定新月形沙丘内部构造及其组合分布特征异于流动新月形沙丘,也与半个多世纪以来北疆沙漠气候变暖变湿、平均风速减弱、植被盖度增加的区域自然地理环境变化趋势有一定的吻合性。

The internal sedimentary structure of aeolian dunes preserves important information on the dynamic evolution of dune morphology.In this study,we detected the internal structure of fixed and semi-fixed barchan dunes with various sizes and shapes using ground-penetrating radar in spring and autumn in southwest Gurbantunggut Desert in Xinjiang,China.The depth of the internal structure image information reached 8 m.Through image gain processing,interpretation,and comparative analysis,the following observations are made:(1)This detection revealed five radar facies of the internal structure of dunes:high-angle dipping oblique bedding,wedge crossbedding,convex cross-bedding,low-angle to sub-horizontal bedding,and massive bedding.The first four bedding types are mainly distributed in shallow layers at 3-5 m,whereas the massive bedding is mainly distributed at depths below 4 m to 5 m.(2)High-angle dipping oblique bedding and wedge cross-bedding are mainly distributed in the upper part of the windward slope and the crest of the high barchan dune(chain).The former is buried in the fore-deposit of the leeward slope,reflecting the frequent sand drift activities on the upper and top of the windward slope of the tall barchan dune,and the buried fore-deposit lamina is exposed because of wind erosion after the dune is“fixed and reduced”.The latter is formed by the filling of wind and sand flow in the windward pit and wind erosion groove of the windward slope or the seasonal variation in the wind direction at the crest.(3)The convex cross-bedding of shrub dunes is widely distributed in the windward slope of barchan dunes,and there are local occurrences in the leeward slope,indicating that the shrub dunes are important in the process of wind and sand on the surface of dunes;the massive stratification may be caused by the disappearance of the original stratification caused by the strong bioturbation of the early aeolian stratification.(4)The sedimentary structural types and combined distribution characteristics reflect the overall stable or declining state of barchan dunes in the study area,which is consistent with the topography characteristics of modern dunes.For example,wind erosion troughs are common in the upper part of the windward slope and the crest,and the development of fore-deposit is slow.Modern sand drift activities are mainly concentrated in the upper part of the sand dune and the crest.Therefore,the internal structural characteristics of barchan dunes differ from mobile barchan dunes and are consistent with the change in the regional physical geography environment of northern Xinjiang characterized by the warming and wetting of the desert climate,the weakening of the average wind speed,and the increase in vegetation coverage.