The effects of climate and lithology on the formation of sand ramps around Ernan Plain- South of Yazd

Introduction Sand ramp is a kind of sand dune located on or in the foot of some mountains slopes of arid and semi-arid regions. Although these landforms does not differ from other types of sand dunes in these areas apparently, but, there are significant differences between them from the point of view of slope, sediment grain size, Morphology, sediment homogeneous, stratigraphy, lamination, mineralogy, evolution trend, Paleoenvironment, and formation processes. Unlike desert sand dunes, which are created only by the wind process, fluvial and colluvial processes also play a large role in the formation and evolution of sand ramps. Because these landforms have a much smaller dispersion than the desert sand dunes in the world, they has less studied and known. In addition, they are buried under clastic sediments in some region and not visible. In Iran, sand ramps has been created in the in the center of the country in Yazd and Kerman Provinces. The main goal of the research is the study of how to provide high volume of sediments of sand ramps located around the Ernan Plain in the eastern slopes of Shirkouh Mountain in the south of Yazd. Therefore, two factors includes rock and its weathering were studied in specific climatic conditions. The sand ramps have been created in the past climate and they are stable and inactive at now. Materials and Method For doing the research, we have used topographic (1:50000), geologic (1:100000) and DEM (with 5 meter resolution) maps and field observations as well. In field observations, several samples of one of a sand ram and also a sample from granite stone were taken. In laboratory, using XRD method, minerals of sediments and of the granite stone were determined. Also, to see better the granite and sediments samples, we provided thin sections of them. These section was magnified 60 times with the electron microscope and using the camera placed on them, different images were taken from their different angles. Thus, the minerals of sand ramps sediments and also granite stone and their properties were defined and analyzed. After clearing that the minerals of sand ramp sediments and granite stone (Monzogranite) are relatively the same, weathering of granite stones, as main stone of sand ramp genesis, were analyzed. Therefore, climate condition of the region, especially temperature and precipitation, were analyzed. In according to, the isotherm and isohyet maps for present time were drawn. Because, sand ramps has been created in the past cold period, in quaternary, past temperature were reconstructed and its map were drawn.      Results and Discussion The study area has located in the central Iran's desert region with altitude variations from 1700 to 3800 meters. According to, it has a very specific climate with high temperature and humidity fluctuations. These fluctuations have been the main factor of stone weathering and consequently, the formation of specific landforms during quaternary. The current temperature data show that the study area has two separate warm and cold seasons. So that temperature during cold season has drop severe and in about 3 months, includes December, January and February, the average temperature is around 2 to 3 degrees Celsius. However, the temperature at altitudes above 3000 meters for a cold period reaches to below zero degree. Because of high altitudes, precipitation is more than 200 mm per year. Therefore, the preglacier morphoclimatic conditions is dominant on the region weakly at now and frost and melting ice happens, especially during the cold season.  Reconstruction of the past temperature in the region showed that the area has been 13 ° cooler in the Pleistocene cold period. Besides, the findings of the sand ramp sediment showed that Plagioclases, Alkali Feldspars, Quartzes, Biotite, Muscovite, Apatite and Amphibole are the major minerals orderly according to the volume and the abundance. These minerals create the monzogranite rocks of eastern slopes of Shirkouh Mountain. Conclusion Results showed that sand ramp sediments had been provided by monzogranite rocks weathering of the eastern slopes of the Shirkouh Mountain. The study of developing sand ramp showed that it is forming on the foot of slope of altitudes above 2400 meters. Where granite stones had been outcropped and the preglacier climate is dominant. In fact, the preglacier morphoclimatic condition had caused to weather granite stones. Results, also showed that the temperature in the past period had been 13 ° C cooler than present. In this condition, the slopes above 2500 m. had have glacial morphoclimatic and below them had been influenced by preglacial morphoclimatic conditions. These climates have been favorite for degradation and weathering of granites. The presence of moisture destroys biotite and feldspars minerals and causes to separate other minerals, such as quartz, from each other. Temperature also affects granite weathering. Because its constituent minerals have different temperature expansion points. For example, the expansion of the quartz temperature is three times that of feldspar.Therefore, temperature changes create pressures inside and between mineral grains that cause them to be separated. However, lower temperatures with minimal moisture (that is preglacier conditions) are one of the most important factors affecting the weathering of granite. Thus, with the destruction of granites, a large amount of fine-grained sediment materials is provided and carried by the water or ice tongue to the downstream. Theses sediments have been ascended on the mountain slopes by winds and created sand ramps.