پديد آورندگان :
نايب زاده، فرين دانشگاه محقق اردبيلي , مددي، عقيل دانشگاه محقق اردبيلي , عزيزي، قاسم دانشگاه تهران - دانشكدة جغرافيا
كليدواژه :
اشتهارد و پادگانه , تاقديس و درياچه , قزوين و كواترنر , شواهد ژئومورفولوژي كواترنري
چكيده فارسي :
آگاهي از سر گذشت و چگونگي تحول شكل هاي زمين، از ديرباز ذهن بسياري از دانشمندان علوم زمين را به خود مشغول ساخته و اين كار با شواهد ثبت شده در حافظه تاريخي زمين شكل ها امكان پذير است. در اين مقاله شواهد ژئومورفولوژي كواترنري دشت اشتهارد و قزوين به منظور پي بردن به چگونگي تحول اين منطقه، با استفاده از بازديدهاي ميداني و تهيه نقشه ژئومورفولوژي منطقه در نرم افزار ArcMap نمونه برداري از رسوبات از محل ترانشه ها و لندفرم ها، و استفاده از مدل ارتفاع رقومي با دقت 12
/ 5 متر بررسي و تجزيه و تحليل شده است. نخست، شواهد پالئوژئومورفولوژي منطقه شناسايي و نمونه برداري شد. سپس، نمونه ها به آزمايشگاه منتقل و آزمايش هاي گرانولومتري، مورفوسكوپي و هيدرومتري انجام شد. براي نمودارها و آناليزهاي رسوبي از نرم افزار 4 GRADISTAT استفاده شد. در پايان، با برقراري ارتباط ميان نتايج به دست آمده از تحليل ژئومورفولوژيكي منطقه، نتايج آزمايشگاه رسوب شناسي و تحليل هاي ارتفاعي چگونگي تغيير و تحول دشت اشتهارد بررسي شد. بر اين اساس، درياچه اي در گذشته در شمال اشتهارد و جنوب غربي دشت قزوين وجود داشته است كه با طاقديس كشيده كم ارتفاعي به نام تاقديس حلقه در از جنوب محدود بوده است. در اثر بالا آمدن آب اين درياچه، آب از اين ارتفاعات سرريز و شروع به باز كردن مسير خود كرده است. آب تخليه شده از درياچه در پشت ارتفاعات درياچه ديگري به عمق 10 متر به وجود مي آورد. در نهايت، سرريز آن در منحني ارتفاعي 1140 متر، مسير خود را تا درياچه قم كنوني باز مي كند. بعد از آن، درياچه در شمال و جنوب تاقديس حلقه در از بين رفته و كف آن تحت تأثير فرسايش رسوبات قرار گرفته است.
چكيده لاتين :
Qazvin and Eshtehard plains is quite visible in the region’s landforms. Being the function of geological structure, tectonic movements and geomorphological processes, these effects have shaped the geomorphology of the region in a way that Eshtehard plain forms a saline depression with a gentle slope entering from the south into another hole via Shoor River that reaches the end of Mahdasht alluvial fan. In the middle of this depression, parallel hills can be seen drawn from west to the east. The existence of eroding loose hills in a low slope area covered in evaporite sediments and water stroke in southern hole, has formed the geomorphic features of the region in a way that is not justified with the current situation. Researchers who have studied this region, have achieved different results. Sadough et al (1396) have concluded thattectonic movements caused by Eshtehard Fault and the development of Mahdasht alluvial fan, have changed Shoor River’s direction towards south. Ramesht et al (1393) have concluded that there has been a paleo-lake behind Mahdasht Alluvial fan and its overflow, has caused the lake to tear through Mega-Floods. It seems that to explain the paleo-geomorphology of the area the evidence of the upstream location of these events should be investigated with a wider look to the region.
Materials and Methods
Resources used in this research, include two categories of documentary and library sources including articles, books, sites, theses, all written resources and field data including field visit, sampling and investigating geomorphological evidence. The tools used in this research include two groups as well. Physical instruments like maps, diagrams, laboratory tools and field sampling equipment, conceptual tools such as ArcGIS software, GRADISTAT, Google Earth and etc. First, geomorphology map of the area was created and the history of past processes was identified. Then landforms of area were sampled. Samples were transferred to the laboratory in order for some experiments to be done, such as granulometry for extracting statistical parameters of sediments, morphoscopy to identify the amount of rounding, origin and sedimentation process, hydrometry for indentifying particle diameter in silt and clay size. At the end by connecting the results, the evolution process of Qazvin and Eshtehard Plains, was explained.
Results
The results showed that the existence of stretched and fine-grained hills in the north of Qazvin Plain and Eshtehard, represents a relatively stable environment in the past and cutting it down by water-ways, shows a change in situation from sedimentary environment to erosion environment. Earth’s current surface in the area is lower than the past sedimentary floor and the little difference between the heads of northern and southern hills indicates that there has been a paleo-lake in this region with an insignificant slope from north to the south. Forming alluvial terraces in south-western part of the basincould be related with changing the river base level, in addition to climate change.
This is because the reverse and gentle slope of sedimentary layers indicates a very slow tectonic movements in southern mountains. Because of that, forming of terraces in relation with baseline changes could be considered as sign of descending in lake level in the current location of Qazvin Plain. The sign of water in the southern slopes of halghedar anticline are seen like significant water strokes. The existence of clay hills in Qazvin Plain and waterstrokes in Eshtehard Plain indicate that full opening of the lake which has been situated in the north of halghedar hills, has occurred in transition from glacier to inter-glacier. Buried sand hills and gypsum crystallized particles among the sediments confirms the existence of an evaporate environment in the studied area.
Conclusion
The obtained sedimentary evidence indicates periodicity of high and low water courses in a closed sediment basin. These evidence include layered fine grained deposits, also layers with coarser sediments, evidence of the formation of sand dunes in the region and the presence of crystalline particles of salt and gypsum among the sediments. Terraces of Kharroud River in southwestern part of the basin show the change in base level. Loose and parallel clay hills, can prove that these sediments have been deposited in a calm and closed area. Water strokes in southern slope of Halghedar anticline shows the sudden exit of water from the area and the formation of a lake in the southern depression.
Thus, the results of this study proves the existence of a lake in Qazvin plain in the last glacial period and shows that in transition from glacier to inter-glacier period, as the ice melted in heights and mega-floods occurred, the lake’s water level rised and overflowed from the southern heights, and meanwhile a part of anticline eroded. This causes a reduction in depth of the lake and in inter-glacier period, when the weather was much warmer, evaporation created playa environment in the lake and this happened frequently until the front of lake is completely open in the last period of transition from glacier to inter-glacier
