内蒙巴丹吉林沙漠成因的粒度分析和热发光测年新证据

巴丹吉林沙漠位于中国内蒙古自治区的西部，是中国第三大沙漠。过去通常认为该沙漠中沙丘的可能沙源来自附近的各种沉积物和岩石，包括弱水河扇的沉积物，弱固结的二叠纪页岩、白垩纪砂岩和砾岩以及戈壁阿尔泰的古老岩石。根据对风成砂的粒度观测，有证据表明在沙漠内部的风成砂沉积朝东南方向具有较弱的但又确实的变细趋势。这与沙丘横脊线的SW-NE向排列以及坡面的南东指向是一致的。因此，可以认为西北部的弱水河冲积扇是巴丹吉林沙漠风成砂的最可能的沙源。在西北部风成床沙覆盖了较老弱水河扇起源的冲积物。这种沉积物的热发光测年范围在190～100 ka BP之间。由于测量中对沉积物漂白可能不完全以及对其中平均古水含量估算均存在不确定性，同时由于对沙漠该地区风成作用开始之前堆积的沉积物进行了测年，这一数据范围代表了最大的年龄。另外三个风成砂测年结果分别接近133.66和22 ka，代表有关巴丹吉林沙漠西北部风成作用开始的最小年龄。沙漠的沙丘高度平均为200～300m，但在东南部偶尔可达450m。一些学者曾提出风成床沙覆盖了一个陡峭岛山突起的假说，来解释这些异常的空间。本次研究发现，在研究区之下是一个产状水平的白垩纪扇砾岩和砂岩的台地，可以断定在沙漠东南部呈现台地地貌，但进一步向北该台地明显延伸到这些沙丘之下。因此现在可以认为巴丹吉林沙漠中高的沙丘是不同区域气候和地貌因素相互作用的结果，而不是覆盖一个陡峭的岛山突起。对弱水河冲积扇作为巴丹吉林沙漠的主要源区的证实强调区域环境的重要性。在全新世，沿河流的绿洲植被在某种程度上阻碍了冲积扇提供沙源。现在，河西走廊的农业用水量极大地危及沿河森林，因此沿着作为天然拦沙阱的弱水河，维持足够的河水流量来保护区域性密集的沙丘植被，一定会避免具有重大威胁的沙的活动性增加。

New evidence for origin of Badain Jaran Desert of Inner Mongolia from granulometry and thermoluminescence dating

The Badain Jaran Desert(巴丹吉林沙漠),located in the western part of Inner Mongolia,is the third largest desert in China.A variety of sediments and sedimentary rocks in the vicinity of the Badain Jaran Desert ranging from sediments of the Ruoshui River (弱水河) fan to slightly consolidated Permian shales,Cretaceous sandstones and conglomerates and ancient rocks of the Gobi Altay mountains (壁阿尔泰山) have been suggested to be possible sand sources for the dunes of the desert in previous years.According to a granulometric survey of aeolian sands,there is evidence for a slight but distinct aeolian fining towards the southeast within the desert which is in agreement with the SW-NE alignment of the transverse dune ridges and slope faces pointing towards the southeast.Therefore the alluvial fan of the Ruoshui River in the northwest of the desert has to be considered as the most probable source area for the aeolian sands of the Badain Jaran Desert.In the northwestern part of the desert the aeolian bedforms cover alluvial sediments of an older Ruoshui fan generation.Results of thermoluminescence dating of such sediments range between approximately 190 ka and 100 ka BP which represent maximum ages due to uncertainties related to the dating of probably incompletely bleached sediments and the estimation of the palaeo-mean water content of the sediments and due to the dating of deposits which were accumulated prior to the onset of aeolian processes in this part of the desert.Three additional datings of aeolian sands yielded ages of approximately 133,66 and 22 ka BP respectively which represent minimum ages for the onset of aeolian processes in the related north-western part of the Badain Jaran Desert.The dunes of the desert are between 200 and 300 m high in average but attain elevations up to 450 m in the southeastern part occasionally.A steep inselberg relief which is covered by the aeolian bedforms was proposed by several authors to explain the extraordinary dimensions (Petrov,1966; Zhu et al.,1986;Yang,1991;Jkel,1996).As a result of this study,the part of the desert under consideration was found to be underlain by a platform topography of Cretaceous fanglomerates and sandstones which rest generally more or less horizontally.This platform morphology is pronounced in the southeastern vicinity of the desert but obviously extents further to the north under the dunes of the desert.Therefore it is assumed now that the high dunes of the Badain Jaran Desert are the result of the interaction of different regional climatic and geomorphologic factors rather than covering a steep inselberg relief.The verification of the alluvial fan of the Ruoshui River as the main source area of the Badain Jaran Desert emphasizes the environmental importance of this region.During the Holocene,sand supply from the alluvial fan was probably hampered to some degree by oasis vegetation along river courses.At present,the riverine forests are greatly endangered by water withdrawal for irrigation purposes in the Hexi Corridor (河西走廊).Therefore,a threatening significant increase in sand mobilization has to be avoided by maintaining a sufficient river discharge to preserve the locally dense vegetation along the Ruoshui River acting as a natural sand trap.