تحلیل دینامیکی و ژنتیکی رسوبات بادی به‏منظور تعیین منشا و منبع ماسه (مطالعه موردی: ارگ جدید رضاآباد، سبزوار)

طی شصت سال گذشته، تجمعی از تپه‏‏های ماسه‏‏ای (ارگerg ) با مساحت حدود 600 هکتار در رضاآباد خارتوران ایجاد شده است. هدف از این تحقیق تفسیر منبع و منشا ماسه بر پایه ویژگی‏های مورفولوژی تپه‏های ماسه‏‏ای و‏‏ دانه‏بندی و ‏‏کانی‏‏شناسی رسوبات بادی است. تفسیر تصاویر ماهواره نشان داد که ارگ رضاآباد دارای تپه‏‏های هلالی‏شکل ابتدایی شامل اشکال پیش‏بارخانی، تپه‏‏های بارخانی، و بارخانوئید با کشیدگی شمال‏ جنوب است. پس می‏توان نتیجه گرفت که وزش باد شکل‏‏دهنده شمالی جنوبی است. آنالیز دانه‏‏بندی با روش متداول الک خشک و ‏‏کانی‏‏شناسی به‏وسیله میکروسکوپ بینوکولر مطالعه شد. نتایج نشان داد که کشیدگی و فرم تپه‏‏ها از الگوی گل‏‏طوفان پیروی می‏‏کند. در نتیجه، باد قوی شمالی مهم‏ترین باد در تامین ماسه و توسعه تپه‏‏های ماسه‏‏ای است. آنالیزهای آماری توزیع اندازه دانه نشان داد که میانگین و میانه قطر اندازه ذرات 205 میکرون هستند که می‏توان براساس رابطه بین اندازه دانه و مسافت طی‏شده استدلال کرد که تغییرات مکانی این ذرات 20 تا 50 کیلومتر از منابع ماسه است. وجود کلسیت و ماسه‏سنگ کوارتزی در نمونه‏‏های رسوب نشان می‏دهد که واحدهای سنگی آهک مارنی و مارن و کنگلومرای آتش‏فشانی منشا‏یی برای رسوبات بادی است.

Dynamic and genetic analysis of aeolian deposition to order determining origin and source of sand (Case study: Reza Abad, Sabzevar)

Extended Abstract Introduction In desert environments, new sand dune formation or dune reactivation occurs due to a combination of sediment supply, sufficient wind strength and reduced vegetation cover, or extreme dominance of one of those factors. Studding Sediment Properties (stratigraphy and sedimentology) of desert dune can provide important information about the type and materials of sand resources, sedimentation environments and palaeoenvironmental reconstruction in the arid zone. Genetic or dynamic relationships have been used to determine removal areas. The genetic relationship between sediment source and aeolian materials has been identified from observations of the distribution of mineralogy patterns, also dynamic relationships between sediment source and aeolian materials in desert environments are compositionally and texturally controlled by Grain Size, Grain Shape and Specific Gravity. Field surveys plays a significant role in the determination of sand sources. Thus, it is very important to understand the erosion patterns of any desert region. During the past 60 years, an accumulation of sand dunes (erg) is created in the Reza Abad of Khartouran with an area of approximately 600 ha. The study area is located between 35º 52ˊ 44″ to 35º 54 ˊ 24″N and 56º 38 ˊ 13″ to 56º 38 ˊ 55″ E. The aim of this research is to interpret source and origin of sand based upon morphology of sand dunes, granulometry and mineralogy of aeolian sediments. Methodology The study has two components: firstly, Comparison of the aerial photographs with satellite images of the area, to represent the creation and dunes geomorphic features, and secondly, the study of grnulometery and mineralogy of sand samples to order the determination of sand sources and origin. The aerial photographs (1954) and terra satellite images was used for this Comparison. The comparison showed that created dunes are about 600 hectares during 60 years. Sand sampling was performed on a transect at the strong wind direction. Samples were collected from the middle part of windward. Twenty four sand samples were collected. Approximately 100 g of sand s were used for grain size analysis. Statistical analysis calculated by GRADISTAT software, according to folk graphic method (1980). Samples were analyzed for mineralogy using Binocular microscope on 100 grains in 10 sample of sand dunes sands. Mineralogy of sand sources was done According to the importance of percent of mineral in lithology. Results and Discussion Since the sand dunes have been created over the last 60 years, they are young and active. Based on Mainguet classification (1990), they are classified to one main type involve crescent dunes and chains. Crescent dunes are pre barchan, Barkhoid and transverse dunes. The transformation of sand dunes are from the north ‌‌‌‌‌‌‌‌‌‌‌to the south, from pre barchan form to transverse dunes. Aeolian sands of the studied area are fine grained and moderately to well sorted. The grain size distribution of all samples are unimodal. McLaren believes that this feature of the deposit indicates that a source sediment undergoing erosion. The dune sands have an average grain size of 2.358φ (195.8μm). Average sorting value is 0.53 φ (1.444 μm). The Average skewness and kurtosis values are 0.17 and 1.11 respectively. The grainsize values show Incremental changes from the Prebarchanic forms to Transverse dunes. The grainsize variations between dunes can be related to the height of the dune. This is due to the fact that coarse grains do not reach the crest in dunes with great heights, leading to coarse and fine grains in the flank and crest, respectively. The studies have shown that prevailing wind direction is east to west, the strong wind being in northsouth direction. It can also be said that sand dunes are formed by strong wind. The result of minerals analysis of the dunes sand showed that calcite is the dominant mineral. The highest percentage of rock fragments is related to quartz sandstone. Conclusion The mechanical analysis indicates that the sand has been transported from 20 to 50 km from sand sources. It can be argued that the most sources of sands of Reza Abad are in the quaternary materials of the derived bedrock from the North Mountains. The high contains of calcite and quartz in the samples from the Reza Abad dunes strongly indicates the majority of sand in the Reza Abad Dunefield is derived from the lithologic units of marl, calcareous marl and agglomerate which are sand origin.