| چکیده انگلیسی مقاله |
Extended abstract 1- Introduction Granulometric analysis is a common tool used to understand and determine the origin of wind sands, and is applied in desertification studies; to realize such knowledge, the parameters of sand particles size as well as statistical analysis such as mean, standard deviation, kurtosis, skewness, etc. are used. One of the most common types of sand dunes formed by accumulation of sand in desert areas is crescent-shape dune known as Barchan. The most prominent features of such dunes are mobility and lateral movements which transform them into the heart of crisis and threat to the urban and rural residential areas as well as economic, army and communication centers. Barchans are produced in the lands where the sand fails to fully cover the surface area and the wind action is predominant from one direction. The main location of Barchan dunes is between the original wind source and the main dune concentration areas. They are crescent-shape, and possess two "horns" that face downwind and two gentle and sleeper slopes. There is a ridge in between the two slopes. Barchans are produced by the moving sands, appearing convex, and wind action predominately from one direction, and might gradually migrate. The most prominent features of this phenomenon are mobility and lateral movements with specific annual trends. 3- Methodology The research have investigated the granulometric characteristics of 7 barchans in Pashuiyeh, western Lut. Up to 42 samples have been taken from different parts of the dunes. These parts are right horn, left horn, lee slope, two samples from windward slope, and the crest. From each sample, 100 g of sediments have been separately sieved by the shaker, and then weighed, and the measurements have been transferred into GRADISTAT to analyze the data. In this step, various coefficients and granulometric components of the grains, such as K2f coefficient (chromobin sorting index), percentage of component particle size, uniformity or alignment, middle, faces, mean particle size, bending and stretching of particles using statistical relationships of Folk and Ward (1957) were applied. 4- Results The greatest mean size belongs to Barchan number 3 (2.502 j). This is because the dunes are located in alluvial fans. The greatest proportion of particle size refers to sand particles. All dunes have good sorting and due to fine grained particles they have also a symmetrical skewness. All Barchans, from the origin point of view are single-origin with a relatively good particle size and the type of texture in all is sand. The components of Barchans are all single-origin (Single-origin grain stipulates that the sediment, the water, or the wind were single-origin). Flooding from the mountains of the west to the Yardang creates streams in the region with fine-grained sediments which form a part of the sedimentation of the Barchans. For this reason, the chromaticity coefficient is based on the river sediments. The statistical results indicate that Barchans number 1 and 6 are more of a microfiber. Barchan number 6 is smaller than all the others which might be attributed to the fact that it can be split from asymmetrical Barchan behind it. Barchans numbers 2 and 7 contain the highest amount of sand. The reason of this very fact in Barchan 2 might be attributed to the tilled and abandoned farmlands around the village. These lands, which have destroyed the hard surface of the desert and are abandoned, provide a very good source of sand accessible for regional wind, traveling from the northwest. Barchan number 7, in addition to being in line with an asymmetrical and large Barchan, is located in the dry path of the streams caused by a large conifer, which provides a lot of sandy sediments for this Barchan. The least amount of sand belongs to Barchans 4 and 5. These barchans are exposed to the dry, large rivers of the area, and therefore, it is provided with more fine-grained sand material, which confirms the presence of mud aggregates and fine-grains. The amount of mud in Barchans 4 and 5 was higher, first, due to the presence of floods stuck behind these Barchans with fine-grained sediments, leading to an increase in finer materials in these Barchans, and second, due to the fact that these Barchans are on the track or behind the cloves, which are generally fine-grained. 4- Discussion and Conclusions According to the results, including crumbin coefficient, despite common perceptions of the origin of Barchan sediments, the sediments of under-study Barchans were mostly water and river sediments and due to their location by the end of the alluvial fans, they are mostly fine-grained and mobile. In most of the studies of sand dunes, as it is already known in the background, either types of sand dunes including Barchans is studied in terms of their morphometric and granulometric measurements. Most of these studies consider the source of these desert sand dunes as wind sediments. However, in this research, in addition to the morphometric and granulometric studies of Barchans, the wind sediment element in these dunes was ignored to achieve new results; upon the completion of laboratory and software processes, interesting results were gained which were different from what is attained concerning desert dunes and particle size and grain origins. These results indicate that in the study area, the Barchans have no wind origin but are the results of water and stream sediments. This result, prior to this study, in the desert area of study was unperceivable and the result is achieved through in-depth studies and scrutiny over these sand dunes within the area. |