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روند تکاملی خطوط ساحلی استان هرمزگان از هولوسن میانی
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نویسنده
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غریب رضا محمدرضا ,چوپانی سعید ,معصومی حمیدرضا
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منبع
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كواترنري ايران - 1394 - دوره : 1 - شماره : 1 - صفحه:57 -67
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چکیده
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منطقه ساحلی استان هرمزگان، جایگاه شکلگیری بیش از 12 سری سواحل پسرونده است. هدف این تحقیق، دستیابی به روند تکاملی خطوط ساحلی در سواحل استان هرمزگان، از هولوسن میانی است. از اینرو از متدولوژی فراگیر شامل پهنهبندی خطوط ساحلی دیرینه، پیمایشهای میدانی و سنسنجی به روش کربن14 استفاده شد. نتایج نشان از وجود یک سطح پیشروی (ts) و سری رخنمون سطحی افقهای افت تراز دریا (fsst) در هر یک از مناطق شناسایی شده دارد. نمونههای فسیل برجای از مناطق کریان و بند معلم جهت سنسنجی به روش کربن14 انتخاب و بهدقت مساحی و آنالیز شدند. سن مطلق خطوط ساحلی دیرینه s4, s24 (منطقه کریان) و b1 (منطقه بند معلم) به ترتیب 4329±64 bp، 1787±43 bp و 1306±14 bp بدست آمد. این خطوط ساحلی در تراز ارتفاعی 10.21، 5.58 و 3.76 متر از سطح دریا و در فواصل 4570، 500 و 244 متری از نوار ساحلی کنونی واقع شدهاند. بر این اساس، روند نسبی توسعه سواحل پسرونده از s24، s4 وb1 به ترتیب1.05 ، 0.28 و 0.186 متر در سال بدست آمد. نتایج نشان از تشکیل این سواحل پسرونده در محیط رسوبی ساحلی آواری داشته که شرایط تکتونیکی، هیدرودینامیکی و بودجه رسوب آنها از 4000 تا 2000 سال گذشته ثابت بوده است.
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کلیدواژه
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استان هرمزگان، خطوط ساحلی دیرینه، سنسنجی کربن 11 ، سواحل پسرونده، هولوسن میانی
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آدرس
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پژوهشکده حفاظت خاک و آبخیزداری کشور, گروه تحقیقات مهندسی رودخانه ها و سواحل, ایران, مرکز تحقیقات کشاورزی و منابع طبیعی هرمزگان, ایران, دانشگاه آزاد اسلامی واحد بندرعباس, گروه زمین شناسی, ایران
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پست الکترونیکی
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Authors
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Gharibreza Mohammadreza ,Chopani Saeid ,Masoumi Hamid
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Abstract
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IntroductionPaleoshorelines are the most important key landforms for studying evolutionary trend of coastal sedimentary environments. Hormozgan province is one of the Iranian coastal provinces at the north of the Persian Gulf and the Oman Sea that represents high variety of geological settings influenced by regional and local geological events. According to large scale coastal landmarks, sedimentary environments have been formed mainly since the late Quaternary. Intrusion of the Precambrian salt domes, structural development of rocky coasts, and the Quaternary glacial and interglacial events have contributed in shaping of Hormozgan province’s coastal area. For instance, the permanent uplifting movement at coastal area ranging between 0.2 mm y 1 and 7 mm y 1 is controled by salt domes. Fossil beaches as landmarks of regression coasts can be observed at 12 large coastal dune fields. They demonstrate superficial evidences of sea level fall tracts especially since the late Quaternary. The study area is an active morphodynamic zone because of monsoon storm waves, neo tectonic movements and erodible marine terraces. Recognition of the Mid Holocene mechanism of land progradation and rate of shoreline mobility is required for interpretation of regional coastal sedimentary environments. Therefore, the main aim of this research was determination of evolutionary trend of paleoshorelines along the Hormozgan province.Material and methodsThe research method was included application of geographic information system (GIS), surveying of geo indicators, and dating of fossils using 14C method. Therefore, digital topographic maps (1:25,000 scales) and satellite image (IRS, 2006) of spatial resolution 6 m, and numerous aerial photos (flown in 1967 at an approximate scale of 1:20,000) as materials in order to obtain the research objectives were used. Thus, mapping of paleoshorelines and fossil beaches was carried out using GIS tools. Additionally, survey of coastal features, check of maps accuracy, and collect of in situ fossils from paleoshorelines were implemented during field observations. Survey of paleoshorelines was included geometry of each fossil beaches along the certain corss sections at Kerian, Band e Moalem, Charak, Shenas headland, Mansoori headland, Kohestak areas. Preservation of fossil beach geometry, environmental context of paleo environments, taphonomy of fossil sles and condition of redistribution of coastal materials were considered in order to select fossil beaches for further investigations. Accordingely, the Kerian and Band e Moalem were chosen as master areas for 14C dating from east and west of the study area, respectively. Topographic features of fossil sles (elevation, ± 1 cm location, ± 5 mm) were surveyed using the Leica Geosystem 300 Differential Geographical Positioning System (DGPS). The National Cartographic Center (NCC) has provided reference stations (First class, 1 m accuracy), in order to verify unknown sle geographical positions. The 14C dating (Ultra Low Level Liquid Sintillation Counting on Benzene) of fossil sles was conducted using a Quantulus 1220 instrument calibrated with oxalic acid I with a detection limit of DL = 1 2 pMC at the Laboratory of Gamma Spectrometry of Iran’s Atomic Energy Organization. Resultant data was calibrated by using CalPal 2007 HULU software, Cologn radiocarbon calibration paleoclimate research pacage, Germany. Moreover, OxCal, VALIB, and Fairbanks0107 as other well known calibration softwares were tested in order to find biases in calendric ages. Calibration of 14C dating raw results in the most of mentioned software is based on comparison of obtained data with tree rings radiocarbon curves, which involves the wide time ranges from 0 to 12410 year.Results and discussionDistribution of fossil beaches were identified at 12 dune fields wherein, superficial evidences of sea transgression (TS) and falling stage system tract (FSST) have been formed since the mid Holocene. Details of the most reliable landmarks of shoreline mobility at Kerian and Band e Moalem areas were obtained. As a results, S4, S24 and B1 were dated 4329±64 BP, 1787±43 BP, and 1306±14 BP, respectively. elevation of collected fossils were calculated to be 10.21 m, 5.58 m, and 3.76 m, respectively. Position of these sles were obtained at 4570 m, 500 m and 244 m far from the current coastline. Therefore, the mean rate of coastal progradation from S4, S24 and B1 were calculated to be 1.05 m yr 1, 0.28 m yr 1, and 0.186 m yr 1, respectively. The mean annual rate of uplifting in the Hormozgan Province (except salt domes) was calculated to be 2.75±0.38 mm. This value is in agreement with the mean rate of uplifting in the faulted coastal area that was reported by Vita Finzi (1979).ConclusionPresent research has concluded that rate of sediment supply, tectonic movements, and hydrodynamic condition had been relatively constant between 4000 BP and 2000 BP. Accordingly, coastal ridges have been distributed parallel to the shoreline uniformly without considerable disruption. Results have confirmed that regression coasts at the eastern coastal areas have been created wider and more completed than the western coasts. Superficial evidences of the Mid Holocene sea level fall in the study area have provided opportunity to find out regional evolutionary trend of paleoshorelines and to sketch sea level curve for the Persian Gulf and the Oman Sea. Therefore, importance of Quaternary landmarks along the north of the Persian Gulf and the Oman Sea was highlighted by present research for further investigation of coastal zones formation.
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Keywords
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Plaeoshorelines ,Holocene ,14C dating ,Hormozgan Province ,Retrograssive coasts
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