نوع مقاله : مقاله پژوهشی

نویسندگان

1 رسوب شناسی و سنگ شناسی رسوبی، دانشگاه آزاد اسلامی، واحد تهران شمال

2 پژوهشکده علوم زمین، سازمان زمین شناسی و اکتشافات معدنی کشور

3 دانشگاه آزاد اسلامی، واحد تهران شمال

4 ژئوفیزیک، گرایش الکترومغناطیس، موسسه ژئوفیزیک، دانشگاه تهران

چکیده

ماندای عینک در استان گیلان،غرب شهر رشت و در نزدیکی رودخانه گوهر رود واقع‌شده است. جهت بررسی محیط رسوبی این مانداب، تعداد  44 نمونه رسوب سطحی توسط دستگاه گرب برداشت شد. این نمونه‌ها مورد آنالیز دانه‌بندی و کانی‌شناسی قرار گرفتند و پارامترهای آماری از قبیل جورشدگی، کج شدگی، کشیدگی و همچنین میانگین اندازه ذرات محاسبه شد. مطالعات ژئوفیزیک با استفاده از 2 روش ژئو‌ الکتریک و رادار نفوذی زمین، با‌هدف تعیین ساختار و عمق رسوبات،  نزدیکی به منشأ و میزان انرژی محیط‌رسوبی، بررسی حضور ناپیوستگی‌های زیرسطحی و ارتباط این مانداب با رودخانه گوهررود صورت گرفت. نتایج دانه سنجی  و پارامترهای آماری حضور13 تیپ رسوبی با منشأ  رودخانه ای را در منطقه مورد‌‌مطالعه نشان می‌دهد. نتایج کانی‌شناسی حاصل از آنالیز XRD، حاکی از یک محیط رسوبی با مواد آواری است. در پروفیل‌های ژئوفیزیک مشخص گردید که حد‌فاصل مانداب با رودخانه گوهر رود  ناحیه‌ای با مقاومت ویژه نسبی بالاست که این مسئله ناشی از وجود گسل رشت در حاشیه مانداب و تغییر جنس رسوبات در این ناحیه است. بررسی پارامترهای آماری رسوبی و مطالعات ژئوفیزیک نشان می‌دهد که مانداب عینک یک شاخه از رودخانه گوهر رود  است که توسط گسل رشت قطع‌شده و به شکل کنونی درآمده است.

کلیدواژه‌ها

موضوعات

عنوان مقاله [English]

Reconstruction of Holocene Sedimentary Environment of Eynak marsh By Sedimentological and Geophysical Evidences (West of Rasht, Gilan province)

نویسندگان [English]

  • Ayda Hazer Moshar 1
  • Razieh Lak 2
  • Mohammad Reza Spahbod 3
  • Nader Kohansal Ghadim vand 3
  • Mahdi Mohammadi vizheh 4
  • Mahdi Pasha zadeh 1

1 Sedimentology and Sedimentary Petrology, Islamic Azad University, North Tehran Branch

2 Research Institute of Earth Sciences,, Geological Survey of Iran

3 Islamic Azad University, North Tehran branch

4 Geophysics, Electromagnetism Department, Institute of Geophysics, University of Tehran

چکیده [English]

Improvement of our understanding in the environmental and geomorphological changes’ effects on marshes andisolated waters is a critical step to address issues related to continental marshes and their responses to thesechanges. Also, sedimentological studies are proper tools to interpret the evolution of sedimentary environment(Ward et al,1998). Additionally, to assess the ecological impact of contamination to the environment, it is vitally important tounderstand the full extent and the level of pollution into the background in the area.Regarding its capabilities in providing goods, marshes environments are classified as the most precious ecosystemson earth(Moreno,2015). Considering the effects of climate changes and human interferences, today the future ofthese important landforms and ecosystems seems to be at risk and it may cause possibly irreparabletransformations(Murray et al,2011). Also characterizing of the composition and the sedimentology of surfacesediments is vital not only from geochemical point of view, but also from an environmental perspective. Thusvariations in mineral compositions, trace elements and lithogenic components should be considered as valuabletools to find out the possible sediment sources and physico-chemicalprocess affecting the geological records) Bernardez,2012).Nowadays, Eynak marsh is strictly isolated from any riverine and oceanic sediment input. Anthropogenic effects (as an instance intense construction operation, sewage input to marsh and etc.) likely contribute to changing the circumstances and its natural habitat. Despite the importance of this area, there is no worthy investigation devoted to the study of, geophysical, mineralogical and sedimentological signature of Eynak marsh. This investigation could be more momentous if we spot contamination of sewage entrance from the urban areas and also underground linking to GoharRood River. It is noteworthy that, GoharRood by itself is a fully contaminated river transporting sewage from upstream.
Materials and methods
Eynak Marsh is located in west of the Rasht city, North of the Guilanprovince.Eynak naming comes from Persian translation of word Glasses, because of similarity between glasses and aerial photos of EynakMarsh.Eynak Marsh is located on Gohar Rood river near by at the urban area with dimensions of more than two thousand meters in length and more than 150 meters in width.
To study the sedimentary environment of the Eynak Marsh, 44surface sediment samples were collected using of a sediment sampling device (Van Ween Grab). Afterward, on the lab, samples were dried at 70˚ and dried bulk sediment was sieved to separate various fractionsusing wet sieving (based on standard test ASTM for determining average grain size). Grain size analysis of the 44 surficial sediment samples performed by Analysette 19 wet sieving instrument. Therefore, samples grouped into mud, sand and gravel fraction according to Udden and Wentworth. A detailed description of grain size <63 μm fractions executed by Laser Particle Seizer (model analysette 22).Collected samples analyzed for granolometery and it calculated same statistical parameters such as mean, sorting, skewness, and  kurtosis. Moreover, X-Ray diffractions in 44 sediment samples in the area have been performed using an automaticpowder EQUINOX 3000 X-ray diffractometer.Geophysical studies done using of Geoelectric and ground penetration radar techniques to determine depth and material of surrounding sediments and checking for sub-surface discontinuities existence and connection of this marsh with Gohar Rood River.
Results and discussion
mineralogicaland physical characteristics of the sediments in Eynak marsh can be influenced by rock assemblages inupstream such as basaltic-andesitic lava, dark grey limestone, slate and Arkozic sandstone which has been subjectedinto weathering, eroded and transported to downstream. In fact, due to terrestrial sources of sediments inEynak marsh, it is characterized by similar mineralogical and physical features.According to grading studies, 13 sedimentary types existed in surface sediments including: Gravelly Mud,Muddy Gravel, Gravelly Sand, Muddy Sandy Gravel, Gravelly Muddy Sand, Muddy Sand with a Little Gravel,Sandy Mud with a Little Gravel, Mud with a Little Gravel, Silty Sand, Muddy Sand, Sandy silt, Silt, Sandy Mud.Various statistical parameters of Eynak marsh sediments computed. Mean grain size: Different values obtained for textural statistical parameters varying from minimum 1.5 to maximum6.6, i.e. thusit falls between coarse sand and medium silt. Sorting: In the study area sedimentsranges in 3 sorts: poorly sorted,very poorly sorted, and moderately sorted. The closer to GoharRood, the sorting number increases and sedimentspoorly sorted. Skewness: In the present study skewness values ranges −0.43 to 0.68 with an average−0.02 representsfive sorts: strongly fine skewed, fine skewed, near symmetrical, coarse skewed, strongly coarseskewed.Kurtosis: Many curves designated to minute Kurtosis and it varies from platykurtic to mesokurtic. Also thevalues are among 0.5 to 2 with an average of 0.99. Scatter plot with mean, standard deviation and skewness can be used successfully for the distinction of the sedimentaryenvironments, always using a large number of samples for each sedimentary body sampled(Martins,1997).The scatter diagram proved that the distribution of grains belongs to fluvial and riverinesediments.
Conclusion
The results of granolometery shows thirteen dispositional types in the region and major component mineral were quartz, calcite,feldspar, and mica and minor mineral were pyroxene, evaporates along with some heavy minerals.bad sorting in Eynak Marsh shows that the source of sediments near the basin pass through a short transportation route. Skewness generally seen as near symmetrical shows the abundance of coarse grains in an energetic environment, where as kurtosis is generally seen as platykurtic and mesokurtic. The essence is revealed that between the Eynak Marsh and the Gohar Rood River exists a high resistive anomaly due to Rasht fault on the sidelines of  Marsh and change the kind of sediments in this area. Therefore, sedimentary statistical parameters and geophysical studies show that the Eynak Marsh Tributary of Gohar Rood Riverwere cut-off By  Rashtfault and it has shaped in current form.

کلیدواژه‌ها [English]

  • Eynak Marsh
  • GoharRood River
  • Rasht Fault
  • Sedimentary Environment
  • statistical parameters