شواهد ژئومورفولوژي آثار يخچالي در عصر پلئيستوسن (نمونه موردي: حوضه دشت ابراهيم آباد - يزد)

Study of geomorphological Evidences of glacier landforms of Pleistocene period(Case study: Dashte Ebrahimabad basin-Yazd

شواهد ژئومورفولوژي يخچالي يكي از مشخص ترين ميراث تحولات اقليمي دوره كواترنر در ايران است. اين شواهد در ايران مركزي باوجود شرايط گرم و خشك كنوني، تغيير در سيستم هاي مورفوكليماتيك و به تبع آن مورفوژنتيك و مورفوديناميك را به شكل مشخص تر و دقيق تري نمايش مي دهد. در اين پژوهش، به منظور بررسي اين شواهد و بازسازي سيستم هاي شكل زايي گذشته و تغييرات در گستره و مرز اين سيستم ها در حال حاضر از نقشه هاي توپوگرافي 1:50000 منطقه، نقشه زمين شناسي، عكس هاي هوايي، تصاوير ماهواره اي، نقشه رقومي ارتفاعي و به ويژه بررسي هاي صحرايي استفاده شده است. يافته ها نشان مي دهد كه در منطقه مورد مطالعه آثار كاوشي و تراكمي يخ ها به شكل خاص قابل مشاهده است. از آثار كاوشي يخچالي وجود حدود 10 سيرك بزرگ و كوچك، دره هاي بزرگ و عريض، كه برخي از آن ها بيش از 500 متر پهنادارند، تراس سنگي به عنوان بستر زبانه يخي و از اشكال تراكمي آن مي توان به يخ رفت ها، به ويژه يخ رفت هاي انتهايي(سنگ هاي سرگردان) اشاره نمود كه قطر برخي از اين سنگ ها در ورود به دشت بيش از 10 متر و وزن آنها به بيش از 500 تن مي رسد.براساس اين ش.اهد و داده هاي اقليمي زمان حال مشخص گرديد كه يخچالها در عصر پلئيستوسن بخشي از ارتفاعات را در قلمرو خود داشته به گونه اي كه پوشش هاي يخي از ارتفاع 2200 متر به بالا را مي پوشانده اند. از اين ارتفاع تا خط تعادل آب و يخ، يعني حدود ارتفاع 1600 متر در قلمرو سيستم هاي شكل زايي جنب يخچالي قرار داشته است و پايين تر از آن تحت تاثير سيستم فلوويال و بعضاً پلوويال بوده است. اما در شرايط كنوني سيستم هاي مورفوكليماتيك گذشته تغيير پيداكرده و توسط سيستم هاي مورفوژنتيك جنب يخچالي(ارتفاعات بالاي 3000 متر)، فلوويال(1800-3000 متر) و پديمانتاسيون پايين تر از ارتفاع 1800 متر جايگزين شده است.

Quaternary، environmental conditions and its evidences has attracted researchers’ attention around the world. Although، more than half of area of Iran country has been covered by quaternary deposits، but there is a little knowledge about the events of the period in Iran. In according to، Geomorphological evidences of glaciers are one of the most specified evidences of quaternary climate changes in Iran. The evidence، in central of Iran with present warm and dry climate، represents more accurate changes in morphoclimatic systems and also morphogenetic and morphodynamic systems. Studying of Glacial evidences and the limit of their developments in the mountain regions of Iran is the best indicator for identifying the boundary of morphoclimatic and morphodynamic in quaternary. The main goal of the study is to investigate the scope of development of morphoclimatic and morphodynamic regions of glaciers and sub-glaciers in the past in the Ebrahimabad Plain. The area has located in the North-west of Mehriz city in the slope of Shirkuh Mountain along 31° 18´ to 31° 30´ of northern latitudes and 54° 14´ to 54°25´ of eastern longitudes. The range of elevation in the region is between 1600 meters in the output point of the basin and 4100 meters in the peaks of mountains. Primary studding show some landform which has created by glaciers such as cirques، U form valleys، and erratic stones. In the researches، we have tried to study the landforms in details. Material and methods This research has been performed on basis of field studies and direct observation of glacial landforms. But، we studied topographical maps (1/50000)، geological maps (1/100000)، DEM with resolution 20 meters، and also aerial photographs، at first. The resulted information of the maps controlled with GPS System in the field. Then، we measured some boulders and evaluated the glacial landforms that have been determined on the map. For example، we determined glacial cirques and valleys using contour lines at first. Then، in the field we tested determined landforms. Also، according to geological map and climatic data we investigated the effects of lithology and climate condition on the landforms. Finally، according to erratic stones، snowline، and another glacial and sub-glacial landforms، we reconstructed past morphoclimatic and morphodynamic boundaries and drew their maps using geographical information system and surfer. Results and discussion Observation of the different maps and also landforms in the field showed that glaciers dominated in the study area in the Pleistocene period. Therefore، we could observed evidences of two major erosional processes of glaciers includes deposition and abrasive processes. Deposition processes observed in the region are moraines، tills and erratic stones; and the abrasive process are broad valley and cirques as following: ◦abrasive evidences -Cirques: using topographical map and its contours characteristics we marked cirques on the map. Cirques appear on the map with stretched contours as sinusoidal form. In according، we define 10 cirques، and then checked them on the field using GPS system. More of them have laid between 2050 to 2550 meters. -Glacial valley: A valley carved by glaciers is normally U-shaped. The valley becomes visible upon the recession of the glacier that forms it. Glacier valleys are the biggest abrasive forms glacier in mountain regions that become broad towards peak. U-shaped valleys present in many parts of the world and are characteristic features of mountain glaciation. In the three sub-catchment of the region there are typically U shaped valleys. The width of some of them reach to 300 meters. They are wider in the metamorphic stones. Stone terraces are visible in parts of the valleys. Locating the erratic stones in three different place shows that the glacier tongue may have regressed in three time periods. ◦Depositional evidences -Erratic stones or frontal moraines: An erratic stone is a boulder is carried and deposited by a glacial tongue that differs from minerals and type than the bedrock or native rock to the area in which it have remained. Erratic stones are useful indicator for showing of progressing of former ice flows. Errtics in the Ebrahimabad plain have located in the three height includes 1600، 1700 and 1800 meters. The diameter of some of them when entering the plain reach to more than 10 meters with more than 10 tones. Conclusion In according the geomorphological evidences of glaciers and present climate data، it was found that glaciers in the Pleistocene age has extended above 2200 meters heights. From this line to the 1600 meters had been covered by preglacier system. Lower than had been affected by fluvial and pluvial systems.Different evidences of all systems has remained at present time especially evidences of glacial morphoclimatic system. However، past morphoclimatic systems changed at now and has been replaced by preglacier morphogenetic systems (above 3000 meters)، fluvial system (between 1800-3000 meters) and pedimentation (below 1800 meters).