نوع مقاله : مقاله پژوهشی

نویسندگان

1 گروه علوم زمین، دانشگاه آزاد اسلامی، واحد علوم و تحقیقات، تهران

2 گروه احیای مناطق خشک، دانشکده منابع طبیعی، دانشگاه تهران، کرج

3 گروه علوم زمین دانشگاه آزاد اسلامی، واحد علوم و تحقیقات، تهران

4 گروه زمین شناسی، دانشکده علوم، دانشگاه آزاد اسلامی، واحد تهران شمال، تهران

چکیده

دریای خزر بزرگ‌ترین توده آبی محصور در خشکی است که از حدود سه‌میلیون سال پیش از دریاهای جهان جدا شد. مغزه‏های مورد مطالعه در بخش‏های شرقی بابلسر، شمال شرقی جویبار، و شرق زاغمرز در سواحل جنوبی دریای خزر واقع شده است. این پژوهش با هدف تفکیک و بررسی رخساره‏ها و محیط رسوبی و نیز ژئوشیمی عناصر با مطالعه مغزه‏های رسوبی انجام شد. طی بررسی‏های میدانی سه مغزه BAG و AZD و AM با تناوبی از رسوبات آواری ریز تا متوسط دانه به ترتیب با عمق‏های تقریبی 700 و 500 و 900 سانتی‏متری انتخاب شدند. در مجموع 92 نمونه برای مطالعات رسوب‏شناسی (46 نمونه) و ژئوشیمی (46 نمونه) مورد تجزیه‌وتحلیل قرار گرفتند. دانه‏بندی رسوبات درشت و ریز دانه به ترتیب با الک مرطوب و لیزری صورت پذیرفت. تحلیل ژئوشیمی عناصر اصلی و فرعی رسوبات نیز با استفاده از ICP-OES انجام شد. بر اساس مطالعات رسوب‏شناسی و بافتی، رخساره‏های گل ماسه‏ای قهوه‏ای، ماسه‏ای تا ماسه‏گلی خاکستری، و گلی تا رس سیلتی ماسه‏ای سیاه شناسایی شدند. در توالی مغزه‏ای نسبت‏های عنصری Ti/Al و Fe/Al در افق‏های رسوبی دانه‏ریز و Si/Al و Zr/Al در افق‏های رسوبی دانه‌درشت هماهنگی و انطباق خوبی نشان می‏دهند که به ترتیب حاکی از کاهش و افزایش انرژی محیطی است. همچنین انطباق قوی بین Ca و Sr در برخی افق‏های مغزه‏ها حاکی از رسوب‌گذاری کانی‏های کربناته در درون دریا یا دریاچه به صورت زیستی است.

کلیدواژه‌ها

موضوعات

عنوان مقاله [English]

Facies analysis, sedimentary environment and geochemistry of core sediments from the southern Coast of Caspian Sea, Mazandaran Province, Iran

نویسندگان [English]

  • Afsaneh Dehghan Chenari 1
  • Sadat Faiz Nia 2
  • Seyyed Mohsen Al Ali 3
  • Manouchehr Ghorashi 4

1 Department of Earth Sciences, Islamic Azad University, Science and Research Unit, Tehran

2 Department of Reclamation of Dry Areas, Faculty of Natural Resources, University of Tehran, Karaj

3 Department of Earth Sciences, Islamic Azad University, Science and Research Unit, Tehran

4 Department of Geology, Faculty of Science, Islamic Azad University, North Tehran Branch, Tehran

چکیده [English]

1- Introduction
The sediment cores BAG, AZD and AM in the Babolsar, Jouybar and Zaghmarz areas are located at the southern Iranian part of the coastal plain of the Caspian Sea. These areas are contained by major mountain ridge, the Alborz in the south (Central Alborz Structural Zone). The Alborz Mountain represent the main source of terrigenous materials in the South basin of the Caspian Sea. The geology of the southern part of the Caspian Sea catchment include sedimentary deposits, loess and terrasses, volcanic rocks, Precambrian metamorphics and Jurassic massive limestones (Aghanabati 2004; Lahijani and Tavakoli 2012).
Sedimentary facies are controlled by physical processes such as velocity, depth and type of flow and sediment transport rates (Catuneanu 2006). Sediments are deposited in various depositional environments based on physical, chemical, and biological processes (Fursich, 1995; Walker, 2006). This study focuses on the interpretation of sedimentary facies, sedimentary environment and geochemistry of the sediments from the core successions. The results of this study can be useful in the palaeoenvironmental reconstruction in the southern Iranian part of the coastal plain of the Caspian Sea through the Quaternary time.

2- Materials and methods
Detailed field investigations were carried out on the three core sediments from the Babolsar, Jouybar and Zaghmarz areas. Using the rotary drilling machine, cores BAG, AZD and AM were obtained during 2016 fieldworks. A total of 88 representative samples were collected from three cores at different depths for the sedimentological (46 samples) and whole rock geochemical (42 samples) analyses. After collection, selected samples were sealed in polyethylene bags and then air-dried in the laboratory. The granulometric characteristics and textural properties were determined using Sieve analysis. Then, 46 samples were classified following Folk’s (1974). Subsequently, the 42 samples were powdered to less than 200 mesh using an agate mortar. The selected major and trace elements concentrations were measured using an Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES) at the Geological Survey and Mineral Exploration, Tehran, Iran.

3- Results and discussion
Based on the sedimentological characteristics, cores BAG, AZD and AM have been divided into three lithofacies. They include sandy mud, muddy and sandy to silty sand lithofacies. Sandy mud lithofacies is characterized by pale-yellow to light-brown (partly mottled), poor sorting with a maximum thickness. This facies is also dominated by massive to rarely thin lamination, abundant plant fragments and roots, individual shell fragments, and lack of marine fossils. The presence of plant roots and pale-yellow to light-brown colour and massive to thin lamination claystone facies indicate suspension fallout from standing water in floodplain setting (Selim 2017).
Sandy/ silty sand lithofacies is supported by the presence of unconsolidated lightly gray silty sand, moderate to well sorting, with high bivalve contents and the deposition of alternating layers of silt and fine to coarse sand. This lithofacies has an unclear and irregular boundary with the underlying facies and has up to 1 m thickness. Coarse-grained texture with moderate to well sorted particles indicate deposition under high-energy conditions in a beach setting (Sim and Lee, 2006)
Muddy facies consists of poorly sorted, alternative horizons of light green to dark grey-coloured silty clay, with the thickness varies from 0.5 to 2 m. In this lithofacies, foraminifera and ostracod shells, endogenic gypsum mineral as well as calcite were also observed. The fine-grained texture, dark gray to black colored and the presence of foraminifer and ostracod shells indicates deposition may have taken place under low-energy environment in a relatively reduced oxygen contents lagoon (Geel, 2000; Corda and Brandano, 2003; Amao et al., 2016)
The behaviour of Ti/Al, Fe/Al, Zr/Al and Si/Al can be exploited to acquire information on grain size (Dypvik and Harris, 2001). In the studied cores, Ti/Al and Fe/Al, and Zr/Al and Si/Al ratios show well correlations in the fine- and coarse grained horizons, which is matched by the muddy and sandy to sandy mud facies, respectively.
In finer-grained sediments, Rb/Zr can be used as a proxy for changes in grain size, with lower values representing fine-grained material and higher values representing coarse-grained material. Increases in Zr/Rb proxy is matched by an increase in the coarse fraction in the sandy and sandy mud facies, as evidenced by the grain size analysis. A decrease in the Zr/Rb ratio is matched by an increase in clay fractions, in the muddy facies.

4- Conclusion
- The sediment cores BAG, AZD and AM in the southern Iranian part of the coastal plain of the Caspian Sea, are dominated by mud and sand fractions.
- These sediment cores represent the Sandy mud, Sandy/silty sand and Muddy lithofacies.
- These lithofacies were deposited in the flood plain, beach and lagoon environments, respectively.
- The geochemical behaviour of most elements, in a general way, is related to the grain size in the sediments.
- Samples with high Si/Al and Zr/Al ratios are generally coarser grained, while high Ti/Al, Fe/Al and Rb/Zr are related to fine-grained samples.

کلیدواژه‌ها [English]

  • Grain size
  • sedimentary facies
  • sea level changes
  • Caspian Sea