عنوان مقاله [English]
Sand ramps are a kind of sedimentary accumulation found at the foot of the slopes of some desert and semi-desert region in the world. Sand banked up against mountain fronts or other topographical obstacle, forming a ramp (Bertram, 2003). For this reason they called sand ramps. The land form among the people of central of Iran is called Kuh-e-Rig, means a mountain of sands. Until the early 90s, sand ramps they was considered as sand dune. But more exactly studying showed despite more materials of sand ramps are sands, but there are huge differences between sand ramps and sand dune in terms of form, materials, and processes. Despite researches in the past, Sand ramps not form just only by Aeolian processes, but another geomorphological processes such as fluvial and colluvium processes. In fact, sands mound up against mountain slopes. Thus they may be seen as a falling or climbing deposits. Therefore, they are actually very complex landform and different of another sand dunes. Sand ramps contain a “variety of sediments formed in different environment including Aeolian, fluvial and talus deposits” (Bertram, 2003). The aim of this research is studying of sand ramps of Tange-Chenar Catchment located in the south of Mehriz city in the Yazd province. The study region is located in the west slopes of Shirkuh mountain with more than 4000 meters high between 31° 18´ to 31° 26´ of northern latitudes and 54° 18´ to 54°29´ of eastern longitudes.
Material and methodology
This research has been performed on basis of field studies and direct observation of sand ramps. But, we studied topographical and geological maps, DEM, satellite images as well as aerial photographs, at first. Therefore, the resulted information of the maps controlled with GPS System in the field. For studying sand ramps in the Tange-Chenar catchment, we visited the area in tow time periods involving 13/11/2015 and 30/9/2014. When visiting the area, we picked up some sample of sediments. Then, we took them to the laboratory of geomorphology, and put about 450 grams inter shaker set for any case for about15 minutes. The diameter of used meshes were 2000, 1000, 550, 355, 250, 125, and 63 mm in orderly. Then, using Gradistat software we calculated statistics parameters includes Mz, Md, SDI, SKI, and Ku. In addition, because of some sample were similar in terms of statistical parameters, we put them inside five statistical community. Finally, using SPSS software, One-Way ANOVA method and Tukey test, we did statistical comparisons and significance level between variables, to compare the relationship between these variable together and understand effective processes on the formation and evolution of the sand ramps.
As mentioned, for analyzing picked samples we have used of granulometry method and determined statistical parameters of sediments. Initial analysis show that some of sample are similar from point of statistical view. Thus, we placed them in five groups. Finding showed that the sample and the communities are different; Even though some of them have located in adjacent. The first community of sand deposited above the height of 2200 meters and coming down on steep slopes. Field studies showed that these deposits have descending under the influence of gravity. Thus, winds had no role in creating them. The sample of 7 and 8 are part of these community. The average diameter of sands are less than 600 microns in the sample 7, but it is about 1000 microns in a sample 8. The second community was picked from a river valley cut with 20 meters thickness. Four samples of it were taken from 2230, 2235, 2245 and 2250 meters heights. From sedimentology point of view, sands includes very coarse sand and gravels. Upper sediment were coarse size. Their histogram are bimodal and multi modal. The sediments have not sorted very well. The most of them are platykurtic and very fine skewed. The third community of sediments has located in the middle part of catchments next to a big river and big alluvial fans. Therefore, the volume of the sediment in this segment is very much that has brought by waters and transfer by winds on slow slopes and in some cases on the slopes. The samples of the community moderately well sorted but are fine skewed. The fourth community of sediments has placed in the downstream of the catchment where the slope is low. The sediment is affected by wind processes. The granulometric analysis also confirmed it. The samples of 6, 11, 12, 13 and 15 are in this community. The samples are very fine skewed, moderately sorted and very leptokurtic. The mean diameter of the sands is high, because mass movement processes have brought down gravel, pebble and boulder and deposited inside the sands. Finally, the fifth community includes just one sample. Because the sediments of the segment were seemed similar and uniform. The sample was taken from above 2400 meters height. The sediments of the sample has placed as debris cone on the slopes. The domain process to create them is physical weathering. Then come down influence by gravity.
Results show that, contrary to initial thinking, winds have not been major role in the formation and development of sand ramps. Even though, the sediment characteristics and statistical parameters of this type of sediments is similar to sand dune that is created by wind process. Most of these sediments has located on steep slopes; they are scattered in different geographical directions; there are gravels and boulders between sediment layers that implicate hillslope processes. In addition, the sediments diameter, sorting, skewness and kurtosis are different with the sediment of sand dunes.