نوع مقاله : مقاله پژوهشی

نویسندگان

1 ژئومورفولوژی، دانشگاه اصفهان

2 دانشکده علوم جغرافیا و برنامه ریزی، دانشگاه اصفهان

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

4 دفتر بررسی های زمین شناسی دریایی، سازمان زمین شناسی و اکتشافات معدنی کشور

چکیده

در این پژوهش، ضمن جمع‌آوری اطلاعات قبلی در ارتباط با مطالعات اقلیم دیرینه و ‌رسوب‌شناسی، اقدام به برداشت 16 مغزه رسوبی با بیشینه ژرفای 5/11 متر به صورت دست‌نخورده از رسوبات بستر پلایای گاوخونی توسط مغزه‌گیر اوگر شد. رخساره‌های رسوبی بر اساس رنگ، ‌اندازه دانه، ساخت رسوبی، شکل دانه، و نوع کانی‌های تبخیری تفکیک شدند. نمونه‌برداری بر اساس تغییر در نوع رسوبات و رخساره‌های رسوبی انجام گرفت. تعداد 80 عدد نمونه رسوبی جهت آنالیز ‌دانه‌بندی تفکیک شد. ‌دانه‌بندی نمونه‌ها به دو روش الک مرطوب و ذرات ریزتر از ماسه (سیلت و رس) توسط دستگاه لیزر انجام گرفت و با استفاده از نرم‌افزار سدی لایزر پارامترهای آماری رسوب‏شناسی محاسبه شد و در نرم‌افزار SPSS مورد تحلیل قرار گرفت. تیپ غالب رسوبات بستر پلایا گل و محیط‌های رسوبی بادی، رودخانه، دلتا، دریاچه، و پلایا است. با توجه به آنالیز سن‌سنجی به روش ایزوتوپی C14-AMS، میزان رسوب‌گذاری در بخش‏های غربی حدود 4/0 میلی‌متر و بخش‏های شرقی 25/0 میلی‏متر در سال بوده است. در این مطالعه دو دوره مهم خشک (8200 و 4500 سال پیش) در رسوبات هولوسن بستر پلایای گاوخونی شناسایی شد که حاکی از خشک شدن و گسترش رسوبات بادی در بستر پلایاست.

کلیدواژه‌ها

موضوعات

عنوان مقاله [English]

Investigating Aeolian Deposits in Gavkhouni playa located in central of Iran during Holocene Period

نویسندگان [English]

  • Tahira Jalilian 1
  • Alireza Taghian 2
  • Razia Lak 3
  • Javad Darvishi Khatouni 4

1 Geomorphology, Isfahan University

2 Faculty of Geography and Planning, Isfahan University

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

4 Bureau of Marine Geological Surveys,, Geological Survey of Iran

چکیده [English]

Introduction
Identification of the climate changes is important for understanding the paleoenvironmental characteristics. Sedimentary evidence of the closed basins can be one of the most important methods for identifying the climatic periods and changes in the sedimentary environments. Gavkhouni Playa, as the lowland of the ZayandehRud River catchment area is located in an inter-mountainous basin in the Central Iran. This area is one of the pits separating the Sanandaj-Sirjan tectonic zone from the Central Iran, located parallel to the Zagros over thrust. A total of 16 sedimentary cores were taken from the inner part of the Gavkhouni Basin, the outer part of the basin, and the surrounding lands in order to investigate the sedimentary environments of the Gavkhouni Playa during the Late Quaternary.

Methods and Findings

The steps of this research include book studies, field works, laboratory works, data processing and then interpretation and conclusion. To investigate the paleontological climate and conditions of the sedimentary environment, 16 intact sediment cores with the mean depth of the sedimentary cores was equal to 7 m and its maximum depth was equal to 11.5 m. Sedimentary facies were identified and separated based on the sedimentary texture, type of evaporite minerals, presence of organic materials, color changes, and other microscopic components, and sampling was performed based on the changes in types of sediment and sedimentary facies. A total of 80 sediment samples were selected for granulation analysis by two methods of wet sieving for coarse-grained particles and laser particle Sizer Analysette for particles smaller than sand (Silt and Clay). Sedimentology parameters were calculated by the SediLizer software and the results were analyzed statistically by the SPSS software. Also, 3 sediment samples sent to the Marmara Research Center, Earth and Marine Sciences Research Institute in Turkey to age measurement analyzing by the radiocarbon method of organic matters.

Results and discussion

In the studied cores, 8 major sedimentary facies identified. The facies often contains clay, silt, sand, gravel, and interstitial sediments along with vegetation and shell fossils which in most cases frequency and interference in the mentioned sediments are often seen. Changes were in particle size between clay to gravel due to process type changes, environment's energy and river input's amounts and sediment color variations concerning sedimentation conditions, presence of organic matter, pH amount, salinity and temperature variations, and drought occurrence, throughout the cores. The results of the analyses indicated the identification of eight sedimentary facies, five sedimentary environments, and two types of wind and water processes, which are effective on the sedimentation of the Gavkhouni Playa sub-surface. The major types of sediments in the Gavkhouni Playa sub-surface, mud, and sedimentary environments included the aeolian, river, deltaic, lake, and playa environments. According to the C14-AMS isotope dating analysis, sedimentation rate is about 0.4 and 0.25 mm in the western and eastern parts per year, respectively. According to dating analysis, the paleogeography of the Gavkhouni Playa and the extent of its expansion have been reconstructed at least 40,000 BP. According to the studies, about 18-40 ka BP, the Gavkhouni area has been influenced by the heavy precipitation so that, the amount of sediment load imported from the west (ZayandehRud River) has been high. The great Delta of ZayandehRud has been formed and the coarse-grained particles have been deposited in the gravel and sand sizes. Since, 11.6 -18 ka BP, along with the coastline regress, the great Delta of ZayandehRud has flowed into north of the basin. The end of Delta stretches until the center of the Playa, and sand dunes have been formed to the west of the basin. With the onset of Holocene and the increase in the temperature, it has led to an increase in the size of the aeolian sand deposits and the expansion of the playa environment until about 8.2 ka BP on the margin of the lake. Since, 4.2-8.2 ka BP, it has been accompanied by a temporary progress of the Gavkhouni Lake, and 4.2 ka BP, it has regressed again with a drought occurred about 200 years ago. The progress of the lake facies has taken place about 3.5 ka BP, after which regression has occurred and playa and aeolian facies have replaced with the lake facies. According to the type of the sediments in the Gavkhouni sub-surface, it can be said that, the conditions of the northern and eastern parts of the Gavkhouni Playa have been similar to the current conditions for at least the last 3000 years and also the conditions of the western part for at least the last thousand years. The increase in the thickness of the layers of evaporite sediments, which are mainly salt, in the upper layers of the playa indicated that the Gavkhouni Playa has become drier than before in the recent centuries.

Conclusion

According to the results the Gavkhouni Basin has extended to the western part of the sand dunes during the Late Pleistocene. During the Early-Middle Holocene, coastline regress and the ZayandehRud Delta progress toward the Playa have occurred in the northern part of the area. During this time, aeolian sands have spread to the Gavkhouni Playa. In the Late Holocene, semi-arid conditions have reduced the water levels, coastline regress has occurred, and playa environment has been dominated in the Gavkhouni margin. But, there were still lake conditions in the center of the Playa. Gavkhouni Playa has experienced an increase in the level and progress of the coast towards the land about 3 ka BP. The creation of arid conditions about 1000 cal BP has turned the entire Gavkhouni zone into a playa environment, and this situation has been dominated the area almost to this day.

کلیدواژه‌ها [English]

  • Gavkhouni playa
  • Paleoclimatology
  • Paleogeography
  • Sedimentary core
  • Holocene