| Pyramid dunes have researched less study than other major dune types, though they are one of the most majorest aeolian geomorphologies in the world sand sea have been widely recognized because of the pyramid shape, radian sinuous arms and larger scale. The deserts of arid area in China almost all have pyramid dunes distribution, and the Badain Jaran Desert has the tallest pyramid dunes. The paper took Badain Jaran Desert as studied area and selected pyramid dunes as researched objects through. Thorough thoroughly field investigations, indoor experimentation and analysis, remote sensing image interpretation and Google earth software tools, the paper mainly identified new distribution areas of pyramid dunes and measured morphological parameter then classified the pyramid dunes. These results could provide reference data for further study of pyramid dunes in Badain Jaran Desert. It is the first time to use the wind data from Miaohaizi wind towers located inner desert studied the formation conditions and development mode of pyramid dunes, which not only verified the pre-existing research result of pyramid dunes but also was filled the gap of the formation conditions and development mode of pyramid dunes in Badain Jaran Desert. The results indicated:(1)The distributions and external characteristics of pyramid dunes. Pyramid dunes in Badain Jaran Desert have developed in four discrete areas, including the south ancient JvYan lake(zone Ⅰ), the east areas of desert near the Baoritaolegainuoer(zone Ⅱ), Northwest side of the Yabulai mountain(zone III), and Southwest edge(zone IV). Four varieties of pyramid dunes have been identified in Badain Jaran Desert:ephemeral simple, complex and compound pyramid dunes. These pyramid dunes all have the NEN-SWS main arms and another NW arm or SE arm, others of them both have NW and SE arms. The directions of arms indicated these dunes under the main influence of the corresponding NW and SW winds. The dunes’ height, bottom area and distance are various among different dunes areas. Generally, the dune volume was greater the height and distance were also greater.(2)The grain-size and structural characteristics. The grain-sizes surface pyramid dunes have apparent changes at different parts. The main components of the deposits are fine sand and account for about57.26percent, mean grain-size was2.15Φ and finer than other types dunes in Badain Jaran Desert. There are the finest sand in the crestal and nearest zone, the mean grain-size only has0.2mm. Instead the mean grain-size can reached0.25mm in the sandy land among the dunes. The sedimentary structure of pyramid dunes were very complex. The sedimentary stratums have very high angles and the cross-stratification inclination trend in multiple direction on top of the pyramid dunes, which indicated the complex wind regime. There were complicated sedimentary structure inner arms and the low angles sedimentary stratums can be seen in arms.(3) The control factors of pyramid dunes formation and evolution. The factors that control the formation and evolution of pyramid dunes may differ for different dune areas, but the wind regime, topography, and sediment availability were the three main control factors and play a important role. Wind regime is a dynamic conditions, sediment availability is a material basis, and topography play a transformation role for dune development. Around the Badain Jaran Desert was control by SE, NW, NE wind system, which was different from that desert hinterland Miaohaizi area, The maximum monthly mean wind velocity appears on May to July, and sand-driving wind account for49.1%of the annul wind record, and the monthly mean speed of sand-driving wind varies5.8m/s～6.38m/s. The annul DP in Miaohaizi area is34UV, and RDP is14.3UV, so the study area belongs to low wind energy environment. The annul net sand transport direction is WNW345.7°. The value of wind direction variability(RDP/DP) is0.41, belongs to intermediate ratio and narrow unimodal wind regimes. Topography influences pyramid dunes in the Badain Jaran Desert in two ways. The first is around desert topography influence of the local air circulation patterns. The second is through the influence of underlying topography on the near-surface air flow, and hence on sediment transport. Sediment availability has long been considered an important control factor for dune development.(4) The development model of pyramid dunes formation and evolution. The development of star dunes includes their formation process and their maintenance by secondary air flows, which also increase the dune’s size. There are three kinds development models have been proposed for pyramid dunes. Barchan dunes move to trade wind regimes, which change them external characteristics to pyramid dunes. Another development model is that pyramid dunes could be form at the intersection point of several sand flows. The third development model is that when wind direction flowing through the surface of transverse chain of dunes is perpendicular to original wind direction, the transverse dunes also could be transformed and change to be the pyramid dunes. |
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