میکروفاسیس، محیط رسوبی و ژئوشیمی رسوبات کرتاسه بالایی در مقطع دمبک کوه اطراف سد لار، شمال شرق تهران

نوع مقاله : علمی -پژوهشی

نویسندگان

گروه زمین‌شناسی، دانشکده علوم زمین، دانشگاه شهید بهشتی، تهران، ایران

10.52547/esrj.13.1.84

چکیده

این مطالعه با هدف بررسی محیط رسوبی و ریزرخساره و ژئوشیمی رسوبات کرتاسه بالایی در مقطع دمبک­کوه در اطراف سد لار صورت گرفته است. در این منطقه رسوبات کرتاسه بالایی (سنومانین- کامپانین) دارای ضخامتی معادل 7/327 متر می­باشد که مرز زیرین آن با سازند تیزکوه و از نوع دگرشیبی می­باشد و مرز بالایی با گسل قطع شده است. رسوبات سنومانین که ضخامت آنها 26/54 متر است با یک کنگلومرای پیشرونده شروع می‌شوند، رسوبات تورونین که ضخامت آنها 85/22 متر می­باشد ابتدا با سنگ­آهک ضخیم لایه و سپس به سنگ­آهک چرت­دار و در ادامه به سنگ ­آهک­های الیگوستژین­دار تغییر رخساره می­دهند. رسوبات کنیاسین دارای ضخامت 85/19 متر می­باشند که از سنگ ­آهک­های الیگوستژین­دار به سنگ­آهک­های مارنی تغییر رخساره می­دهند. رسوبات سانتونین دارای ضخامت 80/182 متر می­باشند که در قاعده با آهک مارنی و الیگوستژین­دار شروع می­شود و در ادامه به سنگ­آهک متوسط لایه تغییر رخساره داده­اند. رسوبات کامپانین به­طور کامل در مقطع مورد مطالعه وجود ندارند، زیرا یک گسل آن را بریده است و تنها 55/48 متر سنگ ­آهک ­چرت­دار از این رسوبات باقی ­مانده است. براساس مطالعات پتروگرافی، تعداد 11 ریز رخساره شناسایی گردید که پهنه­ی جزرومدی 2 ریزرخساره، لاگون یک ریزرخساره، سد یک ریزرخساره و دریای باز 7 ریزرخساره را شامل می­شود  با توجه به عدم وجود ریف‌های سدی بزرگ، ساخت­های ریزشی، کلسی توربیدایت، آنکویید و پیزوئید به احتمال زیاد این نهشته‌‌‌ها در یک رمپ هموکلاین بر جای گذاشته شده­اند. نتایج حاصل از آنالیز­های عنصری و ترسیم این مقادیر در کنار یکدیگر حاکی از آن است که رسوبات آهکی کرتاسه بالایی دارای کانی شناسی اولیه آراگونیتی بوده­اند. ترسیم مقادیر Mn در برابر Sr/Ca نشان­دهنده یک محیط دیاژنتیک نیمه بسته تا باز با نسبت آب به سنگ بالا می­باشد.

کلیدواژه‌ها


عنوان مقاله [English]

Microfacies, sedimentary environment and geochemical of the Upper Cretaceous deposits in Lar Region, Dombak Kuh, Northeast of Tehran

نویسندگان [English]

  • dana shavasi
  • Mohammad Hossein Adabi
  • Abbas Sadeghi
Department of Geology, Faculty of Earth Sciences, Shahid Beheshti University, Tehran, Iran
چکیده [English]

ke the same sedimentary conditions in the Lower Cretaceous, the Upper Cretaceous sediments of Iran do not have the same facies features. Therefore, in this study, we try to study the sedimentary environment and microfacies. Using data and geochemical methods of Upper Cretaceous sediments led to valuable information about the environment and conditions of formation and primary mineralogy of Upper Cretaceous sediments across Dombak-kuh.
materials and methods
In Dombak-kuh section, sampling has been done based on lithological and facies changes in the direction perpendicular to the layers and at distances of less than 0.5 m to more than 2.5 m. These sections were stained with red alizarin solution and potassium ferrocyanide to distinguish calcite mineralogy from dolomite as well as iron content and the amount of iron in it with Dickson method (Dickson, 1965). The classification and naming of rocks is based on Dunham's method (Dunham, 1962). In sample naming, an attempt has been made to include all major allocations in sample naming in order of frequency. The percentage of alluvial abundance was obtained ocularly by comparing with Baccelle and Bosellini (1965) comparison tables. The known facies have been compared with the Flugel (2010) facies belt. In presenting the sedimentary model, the terms used in Burchett and Wright (1992) have been used.
Results and discussion
Petrogeraphic studies of this sections revealed 11 microfacies (2 microfacies from tidal flat, 1 microfacies from lagoon, 1 microfacies from shoal and 7 microfacies from open marine). All of them belong to carbonate ramp: 1. Dolomicrites – Dolomicrosparaite, 2.  Intraclast Grainstone, 3. Bioclact Wackestone, 4. Bioclast Pelloidal Grainston, 5. Oligosteginids Packstone, 6. Nezzazatinella, Dictyoconella Wackestone-Packston, 7. Lenticulina/Marginotruncana/ Oligosteginids Wackestone-Packstone, 8. Heterohelix /Globotruncana/Macroglobigerielloides Wackestone, 9. Oligosteginids/ Macroglobigerielloides / Heterohelix Packstone, 10. Heterohelix /Globotruncana/Marginotruncana packstone, 11. Globotruncana/ Heterohelix /Macroglobigerielloides Wackestone-packstone.
 The characteristics of these deposits indicate that the sedimentary environment was a ramp (hemocline). The absence of calcite turbidite deposits, fall structures and large reef and oncoid and piezoidal dams confirms the carbonate ramp environment. The results of elemental analysis and plotting of these values against each other indicate that the Upper Cretaceous sediment limestones primarily had aragonite mineralogy. The plotting of Mn values against Sr / Ca indicates a semi-closed to open diagenetic environment with a high water/rock ratio.
Conclusion
According to the studies carried out in the Dombak-kuh region of the Upper Cretaceous sediments (Cenomanian-Campanian) they have a thickness of 327.7 m, the lower boundary of which is the Tizkuh Formation and is of a steep type. Its upper boundary is a fault which has been disconnected. Cenomanian sediments, which are 54.26 m thick, start with a progressive conglomerate. Then the thoronine sediments, which are 22.85 m thick, first start with thick-layered limestone and then with chert limestone, and then with Oligosthenic limestones which change the facies and settle on it. Kenyasin sediments with a thickness of 19.85 m were observed with a combination of oligoesterated limestones followed by Santonin sediments with a thickness of 182.80 m and then Campanian sediments with a thickness of 48.5 m form the Upper Cretaceous stratigraphic sequence. In these sediments, 11 micro-facies were identified, which include tidal zone with 2 micro-losses, lagoon with 1 micro-loss, dam with 1 micro-loss and open sea with 7 micro-losses. The characteristics of these deposits indicate that the sedimentary environment was a ramp (hemocline). The absence of calcite turbidite deposits, fall structures, and large reef and oncoid and piezoidal dams confirms the carbonate ramp environment. The results of elemental analysis and mapping these values together, indicate that the Upper Cretaceous sediments in this section has aragonite primary compound which is placed in a semi-closed to semi-open diagenetic region with high proportion of water to rock (W/R).

کلیدواژه‌ها [English]

  • Dombak Kuh
  • geochemical
  • Upper Cretaceous
  • Sedimentary environment
  • Microfacies
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