بررسي الگوي تجمع رسوبات كواترنر مسير رودخانه هاي ديرين كرج و شور در دشت تهران و كرج با استفاده از مطالعات ژئوالكتريك

Investigation of the Quaternary Sediment Accumulation Pattern of the Track of Karaj and Shoor Rivers in Tehran and Karaj Plains Using Geoelectric Studies

مطالعات رسوبات آبرفتي كواترنر از جنبه هاي مختلف حائز اهميت فراواني مي باشد. هدف ازاين پژوهش بررسي ساختار زمين شناسي زير سطحي رسوبات آبرفتي كواترنر در قسمتي از دشت تهران و كرج به روش توصيفي – تحليلي و با استفاده از داده هاي ژئوفيزيك به روش ژئوالكتريك مي باشد. بدين منظور بر اساس مشخصات دشت و وضعيت زمين شناسي تهران و كرج، در ابتدا مرز لايه هاي آبرفت و سنگ بستر، با استفاده از مقادير مقاومت الكتريكي بدست آمده از تفسيرها مشخص گرديد. در مرحله بعد پروفيل هاي منفك شده براساس جنس لايه ها، با استفاده از GIS بر خطوط پيمايش اصلي در دشت منطبق شده و براي هريك از نقاط سونداژ، ضخامت و مقاومت الكتريكي لايه هاي زيرسطحي تعيين شد. براي تفسير از تحليل نقشه هاي زمين شناسي، ژئومورفولوژي، هم ضخامت آبرفت و نقشه هاي پيزومتري استفاده شده است. نتايج حاصل از تحقيق يك ساختار ناوديس شكل كشيده بين رودخانه شور و كرج را نشان مي دهد كه با توجه به مقادير كم مقاومت ويژه در درون اين ناوديس كشيده و تحليل هاي ژئومورفولوژيكي، احتمالا اين ناوديس مسير قديمي رودخانه شور بوده كه در گذشته در قسمت هاي شمالي تر دشت جريان داشته است. بعلاوه مورفولوژي سنگ بستر در انتهاي دشت تهران يك فرورفتگي كه از اطراف با برآمدگي سنگ بستر محاط شده و شيبي به سمت جنوب شرق دارد را نشان مي دهد.

Quaternary is part of the history of the Earth's development، which lasted two million years. Quaternary sediments are of great importance as human's activities bed، and it is important to study these sediments and requires a comprehensive studies in this field. In general in Iran the quaternary sediment is alluvial fan.The aim of this research is to investigate the sub - surface geological structure of quaternary alluvial deposits in a part of Tehran and Karaj plain by descriptive - analytical method and using geophysical data using geoelectric method. Methodology In this research، 551 vertical electric sondaj with Schlumberger arrangement in the direction of 29 profiles with the direction of southwest - northeast have been used. Initially، based on geological conditions in plain of Tehran and Karaj، the boundary layer of alluvial and bedrock، using electrical resistance values were determined. In the next step، the profiles were Separated according to the genus of the layer، then they aligned with the main navigation lines in the plain، and each of the points، in addition to the positions x، y، had a height. Finally، a map showing the topography of the bedrock was produced. For interpretation، geological maps، geomorphology، Sediments iso thickness and piezometric maps were used. Results and discussion According to the produced map، two major ditches in the study area were observed: one of Syncline that started from Mahdasht and extends to the end of the plain، and the other Syncline end of the southeast of Tehran plain. In relation to these deformations، two hypotheses are: The first hypothesis: It seems that the existence of a syncline between the Shoor and Karaj rivers is the primary route of the Shoor River، and the initial route of the Shoor river has probably crossed the south of Mallard and the margin of the Karaj alluvial fan. Second hypothesis: It seems that with regard to the northwest slope to the south - east of Tehran's plain، underground water is gathered at its extreme point in the south - east of Tehran، and these waters leave their only way out of the way to the Qarchak of Varamin. The first hypothesis was proved with regard to the specific strength of the profiles، surface geomorphology، and geological and tectonic data Special resistance numbers in the Syncline indicate that this numbers are very low in depth and is added to these numbers on the surface. Surface morphology shows different levels of Alluvial fan in the studied area. Geological maps also show that the Shoor River passes from the Miocene formations and the Karaj River passes through the Eocene formations. The second hypothesis was analyzed by applying the map of the iso thickness of the sediment، bedrock topography and piezometric maps. Iso Thickness maps، indicate that sediments at the end of the plain، In other words، at the end of the hole، have a high thickness. The topography of the bedrock shows well the end holes. Also، piezometric maps show an increase in the aquifer level at the bottom of the plain. Conclusion The results of the research show that the Karaj River sedimentation in the Mahdasht area has led the Shoor River to be driven to the southern part and the fault building of the area facilitates this change. In other words، the river of the river has a north-south transition. In the final stage، the Karaj River، with a sedimentation at the surface of the cone، has caused the surface of the cone to rise. The rising of the convex surface caused frequent channel movements to the southeast، and the large and present cones of Karaj in the area of Shahriar Qods were caused by these displacements.Concerning the second hypothesis، the results indicate that groundwater is gathered in the south-east hole of Tehran. These waters are limited by surrounding heights، are drained through a pass way whose gradient towards Qarchak of Varamin، and the pressure caused by the movement of the groundwater of the Jajrud alluvial fan facilitates this movement. That is why in recent years we have witnessed rising basin water levels in the Qarchak area of Varamin. The presence of plenty of abuses in recent years has shown this.