زمین¬ریخت¬شناسی و نحوه¬ی پیدایش پلایای سهل‌آباد- خاور ایران دکتر سیدناصر رئیس¬السادات ، دکتر محمدحسین زرین¬کوب ، دکتر محمدمهدی خطیب

نوع مقاله: مقاله پژوهشی

10.22111/gdij.2012.326

چکیده

پلایای دشت سهل‌آباد در 120 کیلومتری جنوب بیرجند در مسیر جاده­ی بیرجند- نهبندان قرار دارد. منطقه­ی مورد مطالعه در منطقه­ای خشک و کویری قرار گرفته است. نهشته­های کواترنری آن شامل تراس‌های آبرفتی، رسوبات مخروط­افکنه، آبرفت­های رودخانه­ای، تپه‌های ماسه‌ای و رسوبات کفه نمکی می‌باشد. دو تیپ کفه های گلی و نمکی در این پلایا شناخته می­شود. ضمناً سه بخش اصلی  پهنه رسی، ناحیه مرطوب و قشر نمک بر روی عکس هوایی قابل جدایش می­باشند. این منطقه از نظر زمین ساختی گسل­خورده، خرد شده و چین­خورده بوده که در یک پهنه­ی همگرا ایجاد شده است. مطالعات اولیه­ی زمین­شناسی ساختمانی نشان می­دهد که کفه­ی نمکی سهل­آباد یک حوضه­ی فروافتاده رومبوئدر شکل تقریباً متقارن بوده که بر اثر عملکرد دو سری از گسل­های امتداد لغز راستگرد که از شاخه­های سیستم گسلی نهبندان می­باشند به وجود آمده­اند.
کفه نمکی سهل­آباد یک ساختار پول­آپارت می­­باشد. این ساختار در اثر خمیدگی گسل امتداد لغز سهل­آباد شکل گرفته است. محاسبه­ی سینوسیتی کانال رودها در گستره­ی  منتهی به کفه نیز نشان­دهنده­ی زمین ساخت فعال در این ناحیه می­باشد. بر اساس نتایج حاصل از این پژوهش، کفه نمکی سهل­آباد در حال گسترش می­باشد. نتایج این تحقیق می­تواند در سایر مطالعات منطقه مانند تحقیقات جغرافیای طبیعی، آبخیزداری و کنترل سیلاب و پی­جویی منابع معدنی مورد استفاده قرار گیرد.

کلیدواژه‌ها


عنوان مقاله [English]

Geomorphology and Genesis of Sahl Abad Playa – East of Iran Dr. Seyyed Naser Raisossadat Assistant Professor of Geology University of Birjand Dr. Mohammad Hossein Zarrinkoub Associate Professor of Geology University of Birjand Dr. Mohammad Mahdi Khatib Associate Professor of Geology University of Birjand

چکیده [English]

Introduction
Sahl Abad playa is located at about 120 kilometers south of Birjand at Birjand-Nehbandan road in South Khorasan province. This playa is geographically located between 58o 40′ to 60o 10′ east longitude and 31o 50′ to 32o 15′ north latitude , the area of playa is about 1116 km2 (Figure 1).
 The study area is located in dry and arid climatic conditions. Since there is no meteorology station in Sahl Abad playa, therefore based on the figures of the surrounding areas, it is possible to estimate  the annual average of precipitation of all the area about 150 mm (Eshagian, 1990).
Formation  of Sahl Abad salt flats has made a new ecology in the north east area of Nehbandan, in this area, extraction of salt storages and different kinds of industrial salts creates a suitable opportunity for sustained job opportunity in the deprived area of Sahl Abad. Therefore recognizing its geomorphology features and natural geography will be a great help for development planning of the area. It is hoped that this study leads to a better recognition of the said flat and be used in the studies for geographical investigations of the area, watershed management, flood control, exploring natural and mineral resources and other similar cases.
Sahl Abad playa is an active sedimentary basin in quaternary. Geomorphology and morpho-tectonic of this playa and the mechanism of its formation are the main goals of the present study.
 
Research Methodology
Methods include studying aerial in 1/55000 scales, satellite photos in 1/1000000 scales, field works and sampling from sediments and salts up to two meters depth.
 
 
 
Figure 1: position of Sahl Abad playa on satellite image that shows main faults and position of
topographical cross sections of Figure 6. (From Landsat 8).
 
Discussion and Results
First works were done by Krinsley (1970) and Samani (1973). However this is the first work on Sahl Abad playa. Many works have been done on playas around the world. Here some works that related to our work can be referred here such as Castaneda et al. (2005) on facies identification within the playa-lakes of the Monegros desert in Spain, Gutirrez-Elorza et al., (2005) on Origin and evolution of playas and blowouts in the semiarid zone of Tierra de Pinares, Duero Basin, Spain, Messina et al., (2005) on Macropolygon morphology, development, and classification on North Panamint and Eureka playas, Death Valley National Park CA and Mann et al., 1983 on the effects of structure geology on playas.
Quaternary sediments are important in formation of Sahl Abad playa. These sediments include recent alluviums, alluvial terraces, alluvial fans, sand dunes and salt plain. The study area is a folded, faulted and crushed zone that is formed in a convergent area. Faults are mostly trust and strike slip. Trends of folds and faults are mostly in north west- south east. The tensional stress with this trend has formed a Pull-apart basin (Figure 1). The most important faults are Shir Shotor, Sahl Abad and Esmaiel Abad (Figure 1).
Sahl Abad playa extended 1880 kilometers squares. Based on geological and topographical maps, aerial photos, field observation and Krinsely (1970), Samani (1973), Ahmadi (1988) and Torshizian (1994), two types and few faces have been identified.
1- Mud flats types: There are five facies including puffy and soft faces, plough surfaces (Figure 2), clay plain, Nebka and clay-slat polygons (Figure 4).





 
Geomorphology and Genesis of Sahl Abad Playa – East of Iran





2- Salt plain type: there are five faces including clay polygons, salt faces with clay –salt blossoms, salt polygons and black salt faces, salt polygons with salt blossoms (Figure 5) and salt faces (Figure 6).
There are three distinct parts from margin to center of playa.
1- Clay flat: this part is formed by clay, silt and a little salt. Margin of this part is frequently dry and show a puffy ground. This part is about 37.5% of the salt plain (Figure 7).
2- Wet zone: in this part moreover Sodium chloride, gypsum and other salts are present. This part has more salt than the previous part. The extension of this area changes during year and depend on precipitation. This part is about 18.6% of the salt plain.
3-Salt crust: this part has perfectly white colour because salt crust is thick here. This part has lowest elevation and therefore ground water surface is high in this part. Due to evaporation and capillary pressure salt coming up to surface and forms salt crust. This area is about 17.45% of the salt plain.
 
 
Figure 2: Plough surfaces in north west margin of Sahl Abad
 
 
Figure 4: Clay-salt polygons
 
 
 
 
 



 
 



Figure 5: Polygons with salt blossoms


Figure 6: Salt faces with a thick layer of salt that is formed in high evaporation of brackish waters




 
 
Figure 7: Sahl Abad Geomorphologic zones that is prepared with aerial photos
 
Structural elements in the study area shows Sahl Abad playa is surrounded by strike slip faults and activity of these faults has effected on formation of salt plain. Mann et al., 1983 believes that strike slip faults are seldom straight and will be bent or split. They probably are connected together or far away from each other. Such transform faults will lead to formation of ellipsoid or spherical basins.





 
Geomorphology and Genesis of Sahl Abad Playa – East of Iran





Strike slip basins have been discussed (Balance, 1980; Crowell, 1976; Mann & Burke, 1982; Burke et al., 1982).
Paull-apart term was introduced by Burchfiel and Stwewart (1960) for the first time and then applied for about 60 quaternary basins that are formed along strike slip faults (Aydin and Nur, 1982).
Enechelan faults normally produced a regional movement. This might lead to formation of extensional and compression zones. Where there is an extension, sedimentary basins are formed and where there is compression, high lands are formed and will be under erosion. These materials will fill the formed basins. Shape of these basins is related to faults patterns. Pull-apart term has been introduced for all strike slip basins.
Based on the above explanations, structural geology of study area, strike slip faults with thick terrigenous sediments in their margins, shape of basin and faulted margin, a Pull-apart basin is suggested for Sahl Abad playa (Figures 1 and 8).
 
 
Figure 8: A development model of Pull-apart basin formation
 
In addition to the above factors, river sinuosity increases with slope decrease that sinuosity changes could be related to tectonic activities. Therefore with increase of slope, channel morphology will change to distributed channels shape and this could confirm tectonic rising of the area.
Based on definition, channel sinuosity is channel length to down valley length (Adams et al., 1999.)
 
V= length of channel
C= straight length of channels along valley
For investigation of this morphometeric index in sahl Abad playa, 33 channels have been selected (Figure 9) and their sinousity have been calculated (Table 1). The results could be intrepreted as tectonic activity of the study area and uprising of the margins of Sahl Abad playa and depression in center.
 
 
Figure 9: channels position for sinuosity
 
Table 1: Calculated channel sinuosity in study area





Channel


Calculated sinuosity


Channel


Calculated sinuosity


Channel


Calculated sinuosity




A


1.1


X


1.03


Y


1.14




B


1.09


M


1.06


Z


1.07




C


1.05


N


1.11


AA


1.09




D


1.03


O


1.07


BB


1.08




E


1.09


P


1.05


CC


1.11




F


1.02


Q


1.03


DD


1.04




G


1.06


R


1.09


EE


1.08




H


1.05


S


1.06


FF


1.08




I


1.1


T


1.05


GG


1.12




J


1.09


U


1.07


Position of channels are platted in figure 13.
 




K


1.06


V


1.09




L


1.11


W


1.02





 





 
Geomorphology and Genesis of Sahl Abad Playa – East of Iran





Conclusion
Two types of clay and salt plains have been identified in Sahl Abad playa. In Mud flats types, four faces including puffy and soft faces, plough surfaces clay plain, Nebka and clay-slat polygons and in Salt plain type five faces including clay polygons, salt faces with clay –salt blossoms, salt polygons and black salt faces, salt polygons with salt blossoms and salt faces have been recognized.
Based on structural setting, strike slip faults and thick clastic sediments beside the faults, shape of basin and faulted margins can probably suggest that Sahl Abad is a Pull-apart basin. Generation of this basin is interpreted due to strike slip fault actions (operation) that are branched from Nehbandan fault. However, it could be mentioned that this basin is in young stage and there is no volcanic action.
Based on Sahl Abad active fault and morpho-tectonic interpretation it is suggested that the basin is in extension state. Calculation of channel sinuosity also confirmed an active tectonic in the area and it is expected that Sahl Abad plain is going to be bigger.
Further studies on Sahl Abad area are suggested and it is hoped that the result of this study could be used in natural geography, water management, flood control and mineral resources.
 
Acknowledgment: This research has been supported by the University of Birjand (Research affairs). We thank M. Bardeh (previous student of Geology Dept. of Birjand University) and Mr. E. Yazadan panah (PhD student of geology Dept. of Birjand University) for their help in field and lab works.
 
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