بررسي تغييرات سطح آب درياي خزر در اواخر هولوسن بر اساس سن سنجي و مورفولوژي پادگانه ها در محدوده جنوب خليج گرگان

Caspian Sea Base Level Changes during the Late Quaternary Based dating, and terraces morphology in the Confine of Gorgan bay

جنوب خليج گرگان به دليل دست نخوردگي نهشته ها و رسوبات سطحي آن يكي از بهترين مكان ها براي مطالعه شواهد ژيومورفولوژيك تغييرات سطح اساس درياي خزر در كواترنري پسين است. در اين پژوهش براي تعيين تغييرات خط ساحلي درياي خزر در اواخر كواترنري از شواهد ژيومورفولوژي، رسوبي و فسيلي در محدوده خليج گرگان استفاده شده است. در اين راستا براي شناسايي شواهد ژيومورفولوژي و نمونه برداري از رسوبات و فسيل ها، عمليات ميداني صورت گرفت و با سيستم تعيين موقعيت ژيودتيك، موقعيت نقاط نمونه برداري با دقت بالا تعيين گرديد. براي تعيين سن، پنچ نمونه فسيل صدف هاي دوكفه اي برجا به روش كربن 14، به آزمايشگاه شيراكاوا (ژاپن) ارسال و تعيين سن گرديد. تحليل نتايج سن سنجي و بررسي هاي ميداني، وجود 5 پادگانه دريايي را مشخص نمود كه در سطوح ارتفاعي 15/23، 8/21، 30/22، 68/22 و 05/22 متر به ترتيب با سن هاي 22±461، 22±496، 23±541، 22±594 و 24±2438 سال پيش تشكيل شده اند. مطالعه داده هاي رسوب شناسي، فسيل-شناسي در دو مقطع نوكنده و گلوگاه 2 در جنوب خزر، بيانگر وجود محيط هاي مردابي كنار ساحل در كواترنري پسين است. در مقطع گلوگاه 2 در جنوب خزر تا 496 سال قبل محيط لاگوني در منطقه حاكم بوده كه نشان دهنده شرايط پيشروي دريا بوده و بعد از آن دريا در شرايط پسروي قرارگرفته كه موجب تشكيل پادگانه هاي دريايي شده است.

Introduction: The Caspian Sea as the largest land-locked water body is susceptible to environmental changes. The Caspian Sea has experienced different sea-level fluctuations. In the Quaternary alone, sea-level cycles of five orders of magnitude were established, with a range between at least +50 m during the last Glacial and -113 m during the Early Holocene. (Kroonenberg et al., 1997 and 2000) Gorgan bay south is one of the best places for study Geomorphological evidences of the Caspian Sea base level changes during the late quaternary using field data. The study area is located along Iranian coast of the Caspian Sea in the Gorgan Bay south. Methodology: The aim of this study is Caspian Sea Base Level Changes during the Late Quaternary based dating, and terraces morphology in the confine of Gorgan bay. For this purpose, library descriptive method to examine theoretical and research background, field method to match the data of maps and find Geomorphologic evidences, sample sediments and fossils, Take notes from the height coordinates of points were applied. Further, the research uses, experimental studies to determine the age of marine terraces and examine the sediment of sections under study, and finally achieves an analytical method. Topographic maps, geological maps, aerial photographs and IRS images were used as the main instrument in this research. Extensive field observations in the coastal area in the case study were performed for discovering suitable outcrops and cores sites representing with Late Holocene records. Five marine terraces were found in the region. The location and height of the terraces were noted by an accurate method of measuring height and using geodetic GPS with cm precision. The altitudes of the dated horizons were leveled through comparison with the nearest benchmark of the Iranian Cartographic Center (Bandar-Gaz), which is a base on the Persian Gulf mean water level. To identify the age of the terraces, the Carbon 14 method was used and five samples of Bivalve fossils were sent to the Shirakawa laboratory in Japan. Results and discussion: Geomorphologic evidences resulting from the Caspian Sea Level fall in the late Quaternary and present age are marine terraces. Marine terraces are excellent morphological markers and have been used world- wide to recognize past sea-level changes (Gaki et al.,2009) Marine terraces are important markers for quantifying coastal uplift as well (Saillard et al., 2011). The Maine factor in the formation of marine terraces in khazar plain is recent tectonic movements and climate changes Five samples were selected carefully and only double-valved mollusks were considered for dating analysis. This paper also presents a reconstructed Late Holocne sea-level curve based on radiocarbon dating from outcrops and exact altitude of dated points Conclusion: The studied area represents one of the best locations to show geomorphologic evidences of Caspian Sea level change in the late Quaternary. Although the area is vastly affected by human activities, but southern part of Caspian coastal area is still considered as an undisturbed area. The results of satellite image studies and field observations indicate that five marine terraces were formed in the height levels of -23/15, -21/89, -22/30 , -22/68 and -22/05 meters and with the ages of 461±22, 496±22, 541±23, 594±22 and 2438±24 years ago respectively. The study of the data from sediments and fossils in two sections of Nokandeh with water levels, -22/30 and-22/68 and Galogah2 with water level -21/89 in Caspian Sea south indicates an increase in the water level before 541±23 , 594±22 and 496±22, years ago caused lagoon growth in the case study.