罗布泊地区雅丹形态特征及演化过程

Geomorphology and origin of yardangs in Lop Nur Lake region

雅丹地貌因其多样形态类型长期以来吸引人们的关注,对其形态特征描述也成为研究热点。作为雅丹地貌形态最早命名地,罗布泊地区雅丹因其形态的独特性和复杂性闻名,然而对该地区雅丹形态特征和发育演化过程的深入研究较为缺乏。本文结合野外实测数据和UAV(小型无人机)所摄地景影像,采用地形数字化方法提取地貌原始数据,选定形态参数后识别测量得到雅丹形态参数数据,基于以上数据和前人研究成果尝试描述罗布泊地区雅丹形态特征并对该地区雅丹演化过程进行讨论。主要结论有:(1)罗布泊地区雅丹长度集中分布在20 m以下,少有长度> 160 m,宽度集中分布在8 m以下,高度多2 m以下,表明区内雅丹个体规模总体较小,低矮雅丹在数量上占绝对优势;雅丹长宽比值呈现连续增加的变化过程,伴随高度的增加长宽比值迅速增大,由2∶1逐步增大到4∶1,并持续升至10∶1以上,走向分布集中于22.5°~67.5°,与主风向相类似,雅丹个体平面形态呈现连续变化过程;雅丹的相邻雅丹数值集中分布在3~4个,平均顶间距分布在<20 m和40~80 m区间,显示罗布泊地区雅丹空间特征的多样性。在个体雅丹形态以外,初成雅丹和雅丹共基座现象是罗布泊地区主要存在的2种复杂化雅丹形态表现。(2)罗布泊地区雅丹理想演化模式经历初成期、青年期、成熟期和衰亡期4个阶段,由于存在地层岩性、风力剥蚀、流水作用等控制因素影响,造就了类型复杂、变化多样的罗布泊地区雅丹形态现状。(3)罗布泊地区雅丹发育具有多期性,第四纪以来主要存在晚更新世、早全新世、晚全新世和小冰期等4个发育期,结合形态特征和环境演化情况推测龙城和白龙堆雅丹形成于晚更新世,楼兰北部高大雅丹形成于早全新世,楼兰中北部雅丹多数形成于晚全新世,而楼兰地区的低矮雅丹形成于小冰期;依据地层年代和雅丹高度数据,计算得到0.5 ka B.P.后楼兰遗址附近平均风蚀速率为6.2 mm a^(-1)。精确描述罗布泊地区雅丹形态特征并探讨其演化过程,对于了解雅丹形态控制因素影响程度,深入分析区域风蚀过程具有重要意义。

Yardang is a typical, aeolian erosion landform widely distributed on Earth and discovered on other planets(Venus and Mars) too. Because of its various forms and volatility,yardang’s morphological characteristics have attracted attention for generations. Lop Nur is located in the eastern part of the Tarim Basin in Xinjiang, China. Although there are many yardangs with various types in the area of Lop Nur, there is a lack of study about this impressive landform there. On the basis of field investigation, the authors found that UAV drones can efficiently and effectively collect yardang morphology data. Based on collected yardang morphology data(including field survey and DEM from UAV), using the terrain digital extraction method, after measuring the morphological parameter data of yardang, this article attempts to clarify the morphological characteristics of yardangs in the Lop Nur area, and to discuss its formation processes. The main conclusions are as follows:(1) The lengths of yardangs in the Lop Nur area are often < 20 m, although a few of them are even > 160 m. The widths of yardangs in our study area are mainly < 8 m, with a height of mostly < 2 m. It shows that the sizes of yardangs in the study area are generally small, although some large ones occur too. The ratio of length vs. width(R) of yardangs ranges from 2:1 to 4:1, and even > 10:1. The orientations of the yardangs in our study area are mainly toward 22.5°-67.5°, consistent with the predominant wind directions. Our observations suggest that the morphology of yardangs in the study area initiated from a dome-shaped form and then changed to drop-shaped, and finally to linear forms. The number of yardangs occurring nearby is often 3-4, and the distance between them is often < 20 m but occasionally 40-80 m.(2) The most likely evolution processes of yardangs in the Lop Nur area can be summarized into four stages: embryonic, juvenile, mature and decay periods. However, due to the influence of various controlling factors such as stratum sediment characteristics, wind erosion, and flowing water, yardang’s morphology in the Lop Nur area become quite complex.(3) The developments of yardangs in the Lop Nur area have multiple periods. Since the Quaternary, there have been four main development periods in late Pleistocene(ca. 90 ka B.P.), early Holocene(8-9 ka B.P.), late Holocene(after 1.6 ka B.P.) and Little Ice Age(around 0.5 ka B.P.). According to morphological characteristics and environmental evolution, we infer that yardangs in Longcheng and Bailongdui were formed in the late Pleistocene, the northern Loulan grand yardangs formed in the Early Holocene,yardangs in central and northern Loulan formed in the late Pleistocene and the small yardangs in Loulan area formed probably during the Little Ice Age. Based on the chronology and yardangs’ height, the average wind erosion rate near the Loulan site after 0.5 ka B.P. was calculated to be6.2 mm a^(-1). Accurately describing the morphological characteristics of yardangs in the Lop Nur area and understanding its evolution procession is of great significance for analyzing the morphological development of yardangs and the regional wind erosion process.