اثرات اقلیم و لیتولوژی بر شکل‌گیری کوهریگ‌های پیرامون دشت ارنان مقالة

کوهریگ؛ نوعی تپه‌ ماسه‌ای است که بر روی یا پای دامنه‌ی برخی کوه‌های مناطق خشک و نیمه‌خشک قرار می‌گیرد. اگرچه در ظاهر این نوع لندفرم‌ها تفاوتی با سایر انواع تپه‌های ماسه‌ای در این مناطق را ندارد، اما شیب، اندازه‌ی قطر دانه‌های رسوبی، مورفولوژی، همگونی رسوبات، چینه‌بندی، لامیناسیون، کانی‌های تشکیل‌دهنده‌، شرایط محیط شکل‌گیری و تحول، محل رسوب‌گذاری و فرآیندهای به‌وجودآورنده‌ی آن‌ها بسیار متفاوت از دیگر تپه‌های ماسه‌ای بیابانی و ساحلی است. بر خلاف تپه‌های ماسه‌ای بادی که تنها توسط باد حمل و رسوب‌گذاری شده‌اند، در داخل کوهریگ‌ها، رسوبات آبرفتی و کوه‌ریختی نیز قابل‌مشاهده است. هدف این پژوهش، بررسی چگونگی فراهم‌آوری حجم زیاد رسوب در یک منطقه‌ی محدود برای شکل‌گیری این عارضه است. بر این اساس، دو مؤلفه‌ی سنگ و هوازدگی سنگ‌ها در شرایط اقلیمی خاص موردتوجه قرار گرفت. برای انجام این پژوهش، رسوبات با روش XRD کانی‌سنجی شدند که فلدسپار، پلاژیوکلاز، کوارتز، بیوتیت، آمفیبول، مسکویت و آپاتیت عمده‌ی کانی‌های تشکیل‌دهنده‌ی رسوبات بودند و پس از کانی‌شناسی، نمونه‌ای از مونزوگرانیت‌های منطقه‌ مشاهده گردید که کوهریگ‌ها مواد رسوبی تخریب‌شده‌ی این سنگ‌ها هستند. سپس برای تحلیل بهتر ویژگی‌های رسوب و سنگ، برش نازکی (Thin section) از آن‌ها فراهم شد و در زیر میکروسکپ الکترونی با 60 بار بزرگنمایی مورد تحلیل قرار گرفتند. در ادامه، شرایط بارش و دمای حال حاضر بررسی و با بازسازی برف‌مرزهای منطقه‌ بر اساس روش‌های رایت و پورتر، دمای دوره‌ی سرد بازسازی گردید. نتایج نشان داد که گرانیت‌های منطقه‌ در شرایط اقلیمی سرد و مرطوب دوره‌ی سرد پلئیستوسن (شرایط جنب ‌یخچالی) به شدت هوازده شده و حجم زیادی از رسوبات را در دسترس باد برای انتقال بر روی یا پای دامنه‌ فراهم آورده است. همچنین، مشاهدات میدانی نشان داد که در ارتفاعات بالای 2400 متر با بارش خوب و دمای پایین زمستان، هر جا گرانیت‌ها برون‌زدگی دارند، کوهریگ‌ها در حال گسترش هستند.

Introduction Sand ramp is a kind of sand dune located on or at the foot of some mountains slopes of arid and semi-arid regions. Although these types of landforms do not differ in appearance from other types of sand dunes in these areas, But the slope, size of sedimentary grains, morphology, sediment homogeneity, stratification, lamination, constituent minerals, environmental conditions of formation and evolution, location of sedimentation and their processes are very different from other hills of desert and coastal sand. Unlike wind sand dunes, which are only transported and deposited by the wind, alluvial and mountainous sediments are also visible within the mountain ranges. Because this type of landform is much less scattered than desert and coastal sand dunes across the globe, it is less well studied. Also, in some areas under clastic sediments the slopes are buried and are not visible. In Iran, this complication is only limited in the areas of central Iran and in the provinces of Yazd and more narrowly in Kerman. The main purpose of this study was to investigate the preparation of mountain sediments located around the Ernan Plain in the eastern slopes of Shirkouh Mountain in the south of Yazd city. The sand ramps were created in past climatic conditions and are currently inactive. Materials and Methods To study mountain ranges around the plain of Arnan and their sediment sources, 1: 50000 topographic maps, 1: 100,000 geological maps, satellite imagery, digital elevation maps, Google Earth imagery, as well as field observations and surveys have also been used. In the first three consecutive years, the sand ramps geographical area was visited more than 5 times. Then, some sedimentary samples were taken from the granite rocks of the area as well as from the mountain peaks and analyzed in the laboratory using XRD method. In order to better see the granite rocks and sediments of the sand ramps, thin sections were made and magnified 60 times using an electron microscope and different angles were taken using a camera mounted on them. Thus, the minerals of sand ramps sediments and also granite stone and their properties were defined and analyzed. After it was found that the minerals that form the sand ramp sediments are in relatively complete agreement with the granite (monzogranite) of the study area, Degradation and weathering of granites as the main stone for the preparation of sand ramp sediments was investigated. From this point of view, the climatic characteristics of the study area, including the present temperature and humidity, were studied to investigate the environmental conditions. Since most of the sand ramps have been formed in the past and in different climatic conditions, so the past weather (temperature) has been rebuilt. Thus, the maps of simultaneous rainfall and temperature of present time and the maps of simultaneous temperature were prepared and plotted. Discussion and Results The study area is located in the desert area of Central Iran with altitude variations of about 1700 to 3800 m. Accordingly, it has a very specific climate with high temperature fluctuations and humidity. These fluctuations have been the main cause of weathering and destruction of rocks and consequently the formation of unique landforms throughout geological history. The present temperature data show that the study area has two distinct warm and cold seasons. So that the cold season is accompanied by severe temperature drop and about 3 months of 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 and the average number of frost days is more than one month a year, and in some years it is more than 40 days. Because it is a relatively high area, it has more than 200 mm of rainfall per year. As such, the region still enjoys weak and short-lived transgressive climatic conditions, and ice and thawing are characteristic of the region, especially during the cold season. Reconstruction of past temperatures in the region indicates that the area was 13 degrees colder than it was in the Pleistocene cold period. Sand ramp sedimentological findings showed that plagioclases, alkaline feldspar, quartz, biotite, muscovite, and apatite and amphibole sub-minerals are the major constituents of the sand ramp sediments, respectively. These minerals form the monzogranite rock that forms the basin of the Tang Chenar catchment in the eastern slopes of Shirkouh. Conclusions The results of this study showed that the Kohrigi sediments were obtained from the destruction of monzogranites of the eastern slopes of Shirkouh Mountain. Developing sand ramp study showed that it is forming at altitudes above 2400 meters. Where monzogranites are outcrops and morphoclimatic conditions dominated by glaciers. These morphoclimatic conditions have led to the destruction and disintegration of granites. The results show that in the last cold period the temperature was 13 degrees colder than the present. In these conditions, the slopes above 2500 m were under glacier dominance and below that were influenced by glacial morphoclimatic conditions. These conditions were favorable for the destruction 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 has a great effect on the destruction of granites. Because the minerals that form it have different temperature expansion points. The thermal volume expansion of quartz is three times that of feldspar. Therefore, temperature changes create pressures inside and between mineral grains that cause them to be separated. However, low temperatures with minimal humidity (ie, the conditions of the glacier) are the most important factors affecting the weathering of granite. As a result, granite degradation provided large volumes of fine-grained sedimentary material and was transported downstream of the basin by water or ice tab. The winds transported these sediments up the slopes and created the sand ramps.

(رجوع شود به تصویر صفحه) ٤-٣- نقشه ی هم دمای حال حاضر منطقه ی موردمطالعه به دلیل آنکه گرانیت ها در دماهای پایین ، از حساسیت بسیار بیش تری نسبت به تخریب و هوازدگی دارند؛ بنابراین در اینجا به جای تهیه نقشه ی هم دمای متوسط کل سال ، به ترسیم نقشه ی هم دمای ماه های سرد سال (آذر، دی و بهمن ) مبادرت گردید. (رجوع شود به تصویر صفحه) بر پایه ی کانیهای رسوبات کوهریگ ، میتوان بیان داشت که این عارضه ها از تخریب و هوازدگی مونزوگرانیت های دامنه های شرقی شیرکوه پدید آمده است ؛ چراکه بررسی نقشه ی زمین شناسی منطقه ی موردمطالعه نشان میدهد سنگ های تشکیل دهنده ی منطقه ی پیرامونی این عارضه ها نیز گرانیت ، آهک و دولومیت است . (رجوع شود به تصویر صفحه) به دلیل تخریب و فرسایش بخش هایی از لایه های آهکی دامنه های شرقی شیرکوه ، سنگ های گرانیت (مونزوگرانیت ) در پهنه ی وسیعی رخنمون پیدا کرده اند و این سنگ ها نیز با شدت و سرعت به ویژه در دوره های آب وهوایی گذشته ، هوازده شده و رسوبات حاصل پس از تجمیع و انباشته شدن بر روی دامنه ی کوه ها، کوهریگ ها را پدید آورده است . Late Quaternary sand ramps in south-western Namibia-Nature, origin and palaeoclimatological significance. Geochemistry of sediments from Quaternary sand ramps in the southeastern Mojave Desert, California.