بررسی منشا کوهریگ‌های حاشیه دغ ارنان-جنوب یزد

کوهریگ یکی از انواع تپه های ماسه ای است که بر روی دامنه برخی کوههای مناطق خشک و نیمه خشک قابل مشاهده است. این رسوبات اغلب توسط باد حمل و پس از برخورد با موانع توپوگرافی محلی بر جای گذاشته می شوند. بر خلاف تپه های ماسه ای بیابانی که تنها توسط فرایند بادی بوجود می آیند، فرایندهای آبی و دامنه ای نیز در شکل گیری و تحول کوهریگ ها نقش دارند. هدف این پژوهش بررسی منشا کوهریگ های واقع در دامنه کوههای تنگ حوضکی و تنگ چنار در حاشیه دغ ارنان است. برای انجام این پژوهش از نقشه های زمین شناسی، توپوگرافی، نقشه رقومی ارتفاعی و مشاهدات میدانی استفاده شده است. در بازدید میدانی چند نمونه رسوب برداشت و مطالعات دانه سنجی، کانی شناسی و مورفوسکپی روی آنها انجام شد. یافته ها نشان داد که دانه های رسوبات کوهریگ از ماسه های درشت دانه و کانی های تشکیل دهنده آنها منطبق بر مونزوگرانیت های دامنه های شرقی شیرکوه هستند. مورفوسکپی آنها نشان داد که دانه های رسوبی زاویه دار و گردنشده بوده و خردگی و سائیدگی سطح آنها بسیار ناچیز است. نتایج نشان داد که گرانیت های شیرکوه در شرایط جنب یخچالی پلئیستوسن به شدت هوازده و این مواد همراه زبانه های یخی و یا جریان آب رودخانه تنگ چنار به سمت سطح اساس، دریاچه ارنان انتقال پیدا کرده و در ساحل رودخانه و دریاچه ته نشست پیدا کرده و توسط بادهای جنوب و جنوب شرقی دوره گذشته بر روی کوههای تنگ حوضکی و تنگ چنار پاشیده شده و از دامنه کوهها بالا رفته اند. در برخی موارد آنها از قله عبور و در دامنه بادپناه پایین آمده اند.

Study of sand ramps genesis around the Ernan Playa in southern Yazd

IntroductionSand ramp is one of a kind of sand dunes which is visible on the slopes of some mountains of arid and semiarid regions. These sediments are often carried by the wind and are deposited after encountering with local topographic barriers. 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. In according to, there are fluvial and colluvial sediments amongst sand ramps and grain size of sands are greater. Therefore, these landforms are more complex in terms of structure, processes, genesis, sediments and evolution than desert sand dunes. The main goal of the research is to investigate sand ramps genesis located on the slopes of Tange Chenar and Tange Hozaki Mountains in the around of Ernan Playa. These sand ramps located around the Tange Chenar River after Tange Caehnar Village in the Southern of Mehriz City in the Yazd province and also around the YazdMarvast road. After Sand ramp, there is Ernan plain and in the lowest altitude of the plain, Ernan Playa has located.Materials and methodsFor 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, one of the large sand ramp of the area with 34 meters height was selected. It is located in the 54˚17 acute; 55 eastern longitude and 31˚ 23 ́ 6 northern latitude. From this case, 18 sedimentary samples were taken along the vertical and horizontal profiles. Also, three sediment samples were taken from one of the developing sand ramp at altitudes of 24002500 meters. To ensure the results of the work, a granite sample was taken from the area as well. Then. granolumetry, mineralogy and morphoscopy analysis were performed on them. In granolumetry analysis, grain size diameter were determined and displayed using normal and cumulative distribution graphs. In mineralogy, using XRD method, minerals of sediments were determined. Further, Thin section of a number of samples were provided and analyzed by polarizing microscope for observation many characteristics of minerals including volume, composition, weathering and sediments transport rates analyzed. In the morphoscopy test, sphericity and roundness of grains were analyzed using scanning transmission electron microscope.Discussion and ResultsFindings showed that the size of the sedimentary grains of the sand ramps are much larger than the sand dunes in arid and semiarid region, so that the average of grains diameter were about 1 millimeter. Nevertheless, the analysis of vertical and horizontal profiles of the sand ramp sediments showed that they have sorted and transferred by winds alone. Even though, the volume of the very coarse sands is very high, but alluvial and colluvial deposits are not observed inside them. Mineralogical analysis of sand ramp sediments and granite stone as well showed that sand ramps sedimentary minerals correspond to the monzogranites of the eastern slopes of Shirkouh Mountain. In addition, the high volume of feldspar minerals and unweathered biotite and amphibole minerals suggest that sand ramps sediments are not far from their origins. Also, morphoscopy analysis suggest that Sediments are not rounded and have sharp angles. Therefore, they have not been transferred long distance by winds. Also, the surface of grains have not been eroded. However, the sand ramp sediments were not the same. Some have sharper angels. Others, however, show some roundness. ConclusionsResults showed that the origin of the sand ramp sediments have been monzogranites in the eastern slopes of the Shirkouh Mountain. The granites have been weathered strongly in the preglacier condition of Pleistocene. Then, weathered sediments have been transferred by glacial lobe and fluvial processes to the base level, which had been Ernan Lake. After transition, they have deposited on the shore of the lake as well as on the Tange Chenar river bank. Further, these sediments have been moved by southeastern winds and after encountering with the mountains have ascended from windward slopes and created sand ramps. Because of intense winds or low altitude of the mountains, sometimes, they have descended from leeward. In that case, sand ramps are smaller with finer grains.