نوع مقاله : مقاله پژوهشی

نویسندگان

1 گروه زمین شناسی، دانشگاه زنجان، زنجان، ایران

2 گروه زمین شناسی، دانشگاه لرستان، ایران

چکیده

هفده برش از نهشته ­های کواترنری شمال باختری زنجان برای مطالعات سنگ چینه­ نگاری و دیرینه ­شناسی مورد پیمایش قرار گرفتند. هدف از این مطالعات، شناسایی و تفکیک لایه های رسوبی در قالب واحدهای سنگ چینه ای، بررسی تغییرات پارامترهای محیط های رسوبی و در نهایت ارائه یک مدل رسوبی برای محیط رسوبی توالی مورد مطالعه است. بدین منظور شانزده برش در حاشیه آزاد­راه زنجان-تبریز از محل عوارضی تا کیلومتر 40 و یک برش در کیلومتر 8 بلوار فرودگاه برداشت شدند. این نهشته ­ها به طور ناپیوستگی فرسایشی بر روی مارن­ های رنگین پلیو-پلیستوسن قرار می­ گیرند و مرز بالایی آن­ها فرسایشی است. پنج واحد سنگ چینه­ نگاری در قالب توالی دانه درشت شونده به بالا تشخیص داده شدند که از پایین به بالا شامل: (a) توالی رسوبات گل قهوه ­ای تیره تا روشن با میان لایه­های نازک ماسه و گراول دانه ریز؛ (b) ماسه و گروال ریزدانه با جورشدگی متوسط تا خوب با لایه­بندی مورب به رنگ بنفش تا خاکستری با میان لایه­ های پراکنده گل، ماسه و گراول کرم رنگ؛ (c) تناوب منظمی از رسوبات گل، ماسه و گراول به رنگ کرم تا قهوه­ای روشن؛ (d)  گراول بسیار ستبر و دانه درشت و به رنگ روشن با میان لایه­ های گل قهوه­ای-قرمز روشن و (e) گراول سست گل پشتیبان به رنگ کرم تا سبز روشن. با توجه به موقعیت چینه ­نگاری، سن این رسوبات به پلیستوسن-هولوسن است. پارامترهای شاخص محیط رسوبی مانند ژئومتری لایه های رسوبی، نوع رسوبات و ساخت های رسوبی نشان می دهند که این رسوبات در یک محیط مخروط افکنه متاثر از فعالیت­های گسل­های پیرامون نهشته شده ­اند.

کلیدواژه‌ها

موضوعات

عنوان مقاله [English]

Stratigraphy and architecture elements of Quaternary sediments of west Zanjan

نویسندگان [English]

  • Setareh Rahimi 1
  • Nasrolah Abbasi 1
  • Bijan Yousefi Yeganeh 2

1 Department of Geology, Zanjan University, Zanjan, Iran

2 Department of Geology, Lorestan University, Iran

چکیده [English]

1- Introduction
There are well exposures of Quaternary sediments along the beside of Zanjanrud river, west Zanjan province, northwest Iran. The Zanjanrud river watershed is restricted by two northwest-southeast trend mountain ranges: The Soltanieh Mountains in the south and Tarom Mountains in the north. The Quaternary sediments were divided as the terrace and alluvial units in the previously studies. Here, we survived these sediments in the stratigraphic viewpoint, and for dividing theminto lithostratigraphic units with age determination.   
2- Material and Methods
Zanjanrud watershed is 120 km in long and 60-70 km in width, which terminates to Qezel Ozan river in the northwest Zanjan. Seventeen sections were surveyed for evaluating and distinguishing the lithostratigraphic units, with studies of sedimentological characteristics, paleontological features and architectural elements of the sediments. 16 sections were survived besides the Zanjan-Tabriz highways from toll station to Mahneshan cross-road, located in the 40 Km distance. The good exposures of sediments are available in the tranches in both sides of the highway. The last section (seventeenth section) studied in the eight kilometers of Zanjan’s airport highway. The thickness of surveyed sections differ from 5 m, in section number 14 to 26.4 m, in section number 12. The Quaternary deposits of the west of Zanjan overlay on the colored Plio-Pleistocene marls as an erosional unconformity and their upper boundary is erosional surface. Provenance of particles of sediments is considerable from Tarom and Soltanieh mountains. Tarom Mountains comprise volcaniclastic sediments of Karaj Formation, Eocene in age. Soltanieh Mountains, however, includes diverse rock units from Precambrian to Eocene. The rock units of Soltanieh Mountains are Kahar, Bayandor formations (Precambrian), Soltanieh, Barut, Zagon, Lalun and Mila formations (Cambrian), Dorud and Ruteh formations (Permian). Mesozoic rock units are scarce as Shemshak, Dalichai and Lar formations (Jurassic). Fajan, Ziyarat, and Karaj formations are rock units of Cenozoic and are Paleocene to Eocene in age.   
 
 
 
3- Results and Discussion
Five lithostratigraphic units have been identified which include coarsening upward sequence as following as:
Upper boundary; erosion surface.
Unit (e), eroded mud-supported gravel, cream to green in colure.
Unit (d), light colored and thick-bedded coarse-grained gravels with brown to purple mud intercalations, and finally.
Unit (c), regular alternations of light brown to cream mud, sand, and gravels.
Unit (b), gray to violet and moderate to well-sorted sand and fine-grained gravels with cross-bedding and sparse cream mud, sand and gravel intercalations.
Unit (a), alternations of dark to light brown muds with thin sand and fine-grained gravel intercalations.
Lower boundary; concordance with Plio-Pleistocene colored marls.
Sedimentology studies of the 176 sampled specimens show particle mean size variations as 1.80 to 2.91 ɸ, with statistical analysis as very poorly sorted, strongly coarse-grained skewed, near symmetrical and fine skewed, and platykurtic to mesokurticin the textural parameters. Heavy minerals of the 16 sampled specimens include Apatite, Anatase, Epidote, Sphen, Ilmenite, Barite, Titanomagentite, Rutile, Zircon, Limonite, Amphibole group, Pyroxene group, Magnetite and Hematite as main heavy minerals. Sphalerite, Biotite, Pyrite, Chalcopyrite, Garnet, Jarosite and Leucoxene are secondary heavy minerals in these samples. The frequency of main heavy minerals shows sinus variations in the scrolling distance and secondary heavy minerals frequency, however, are linear in the distribution in this distance. Architectural elements of the studied sections include five gravel facies (Gm, Gms, Gp, Gt, Gh), four sandstone facies (Sm, Sp, St, Sh), two mudstone facies (Fm, Fl), and a calcareous facies (p). In the studied sections, any significant body fossils were found, and only single trace fossil, Skolithos, was observed. According to the stratigraphic position, the age of the studied sections attributable to Pleistocene to Holocene.
 
4-conclusion
Quaternary sediments of Zanjanrud watershed in the west of Zanjan, include coarse-grained gravel, sands, silt and clay deposits. The thickness of sediments varies from 5 to 24 meters. Five distinctive lithostratigraphic units are distinguishable as coarsening upward sequence. The lower part of the sequence includes dark to light brown muds with thin sand and fine-grained gravel intercalations, gradually sand size sediments increase in upper parts. The alternations of mud-gravel layers are well developed in the mid parts of the studied sequence. Uppermost layers of sequences comprise thick bedded coarse-grained matrix-supported gravels. No macrofossils were found in the studied section, and they are Pleistocene to Holocene in age at all. It seems that studied sediments deposited in the Alluvial fan system, which was fault controlled environment between Soltanieh and Tarom mountains.         

کلیدواژه‌ها [English]

  • Zanjan
  • Lithostratigraphy
  • Quaternary
  • Alluvial fan
  • Stratigraphic unit