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

نوع مقاله: مقاله علمی پژوهشی

نویسندگان

1 استادیار مرکز تحقیقات بین‏المللی بیابان، دانشگاه تهران، تهران، ایران

2 استاد دانشکدة منابع طبیعی، پردیس کشاورزی و منابع طبیعی دانشگاه تهران، تهران، ایران

3 کارشناس ‏ارشد همزیستی با بیابان، دانشگاه تهران، تهران، ایران

چکیده

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 منشأ ماسه بر پایة 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 شکل‏‏دهندة شمالی- جنوبی است. آنالیز دانه‏‏بندی با روش متداول الک خشک و ‏‏کانی‏‏شناسی به‏وسیلة 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 تأمین ماسه و توسعة تپه‏‏های ماسه‏‏ای است. آنالیزهای آماری توزیع اندازة دانه نشان داد که میانگین و میانة قطر اندازة ذرات 205 میکرون هستند که می‏توان براساس رابطه بین اندازة 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 منشأ‏یی برای رسوبات بادی است.

کلیدواژه‌ها


عنوان مقاله [English]

Dynamic and Genetic Analysis of Aeolian Sedimentation to Determine Origin and Source of Sand Dunes (Case Study: Reza Abad, Sabzevar)

نویسندگان [English]

  • Naser Mashhadi 1
  • Sadat Feiznia 2
  • Soghra Abdi 3
1 Assistant Professor of Desert Researches, International Desert Research Center (IDRC), University of Tehran, Tehran, Iran
2 Professor of Natural Resources, Faculty of Natural Resources, University of Tehran, Tehran, Iran
3 MA in Desert Coexistence, University of Tehran, Tehran, Iran
چکیده [English]

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 prevailing dominance of one of those factors. Studding sediment properties (stratigraphy and sedimentology) of desert dunes can provide important information about the type and materials of sand resources, sedimentation environments and palaeoenvironmental reconstruction in the arid areas. 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. Dynamic relationships between sediment source and composition and texture of aeolian materials in desert environments are controlled by grain size, grain shape and specific gravity. Field surveys are essential to determine 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 detect the origins of sands based on morphology of sand dunes, granulometry and mineralogy of aeolian sediments.
Methodology
The study has two components; firstly, we have compared the aerial photographs with satellite images of the area, to represent the creation of dunes geomorphic features, and secondly, examined granulometery and mineralogy analysis of sand samples to determine sand sources and origin.  
The aerial photographs (1954) and terra satellite images was used for this comparison. The comparison showed that the dunes are about 600 hectares during 60 years. Sampling of the sands was performed on transect along the prevailing wind direction. The samples were collected from the middle part of windward. Twenty four sand samples were collected in the survey. Approximately 100 g of the samples were used for grain size analysis. Statistical analysis was calculated by GRADISTAT software according to folk graphic method (1980). The samples were analyzed for mineralogy using Binocular microscope on 100 grains in 10 samples of sand dunes. Mineralogy analysis of sand sources was performed according to the percent of mineral in lithology.
Results and discussion
Since the sand dunes have been configured over the last 60 years, they are young and active. Based on Mainguet classification (1990), they are classified into one main type involve crescent dunes and chains. Crescent dunes are pre-barchan, Barkhoid and transverse dunes. The sand dunes are gradually transformed from the north ­­­­­­­­­­­to the south, from pre barchans form to transverse-formed dunes. Aeolian sands of the study area are fine grained and moderately to well sorted. The grain size distribution of all samples is 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 grain-size values show incremental changes from the Prebarchanic forms to transverse dunes. The grain-size variations among the 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 the dunes with great heights leading to coarse grains 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 north-south direction. It can also be said that sand dunes are formed by strong wind. The result of minerals analysis of the sand dunes showed that calcite is the dominant mineral. The highest percentage of rock fragments is related to quartz sandstone. 
Conclusion
The mechanical analysis indicated that the sands have been transported from 20 to 50 km from sand sources. It can be argued that the most sources of sands in Reza Abad are developed 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 that the majority of the sand in the Reza Abad Dunefield is derived from the lithologic units of marl, calcareous marl and agglomerate.

کلیدواژه‌ها [English]

  • Erg
  • sand dunes
  • aeolian sediments
  • pre-Barchanic
  • Granulometry
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