مرز کامبرین- اردوویسین در کپه‌داغ باختری و البرز خاوری

نوع مقاله: مقاله پژوهشی

نویسندگان

1 دانشجوی دکترا، گروه زمین‌شناسی، دانشکده علوم، دانشگاه فردوسی مشهد، مشهد، ایران

2 دانشیار، گروه زمین‌شناسی، دانشکده علوم، دانشگاه گلستان، گرگان، ایران

3 استاد، گروه زمین‌شناسی، دانشکده علوم، دانشگاه فردوسی مشهد، مشهد، ایران

چکیده

لایه‌های رسوبی مرز کامبرین- اردوویسین در برش کلات، در کپه‍‌داغ باختری، بیشتر متشکل از یک توالی یکنواخت از رسوبات آواری ریزدانه آرژیلیتی با میان‌لایه‌های آهکی صدف‌دار است که در یک محیط فلات قاره یا شلف بیرونی ته‌نشین شده‌اند. در محدوده این مرز، یک توالی کامل از زیست‌زون‌های کنودونتی Cordylodus intermedius،Cordylodus lindstromi  و Cordylodusangulatusدیده می‌شود. اولین ظهور گراپتولیت پلانکتونی Rhabdinoporacf. flabelliformis flabelliformis (Eichwald, 1840)، در بخش بالایی زیست‌زون کنودونتی Cordylodus lindstromi،در رسوبات آرژیلیتی سیاه رنگ موقعیت به نسبت دقیق مرز کامبرین- اردوویسین را در برش کلات آشکار می‌سازد. در البرز خاوری، به دلیل نبود کنودونت و گراپتولیت شاخص در محدوده مرز کامبرین- اردوویسین، تعیین دقیق این مرز ممکن نیست. ردیف رسوبات گذر کامبرین- اردویسین در برش‌های ده‌ملا و سیمه‌کوه از البرز خاوری، از دید سنگ‌شناسی به سه بخش قابل تقسیم است: 1) سنگ‌های آهکی به سن فورونگین که به سمت بالا به رسوبات آواری تبدیل می‌شود؛ 2) ماسه‌سنگ‌های دارای Cruziana مربوط به پایان کامبرین، نشان‌دهنده کمپلکس‌های ساحلی یا  پایابی؛ 3) آرژیلیت‌های خاکستری متعلق به ترمادوسین که گویای نواحی ژرف‌تر دور از ساحل هستند. در این ناحیه مرز تقریبی کامبرین- اردوویسین در قاعده زیست‌زون تریلوبیتی Asaphellus inflatus- Dactylocephalus قرار دارد.

کلیدواژه‌ها


عنوان مقاله [English]

Cambrian-Ordovician boundary in Western Koppeh-Dagh and Eastern Alborz

نویسندگان [English]

  • H Jahangir 1
  • M Ghobadi Pour 2
  • A.R Ashouri 3
1 Ph.D. Studentin, Department of Geology, Faculty of Science, Ferdowsi University of Mashhad, Mashhad, Iran
2 Associate Professor, Department of Geology, Faculty of Sciences, Golestan University, Gorgan, Iran
3 Professor, Department of Geology, Faculty of Science, Ferdowsi University of Mashhad, Mashhad, Iran
چکیده [English]

The Cambrian- Ordovician boundary strata in the Kalat section, western Koppeh-Dagh, comprise continuous succession of fine clastic sediments, mostly argillites with some shell beds, accumulated offshore. This interval contains  analmost complete succession of conodont biozones, including the Cordylodus intermedius, Cordylodus lindstromi and Cordylodus angulatus biozones. The first appearance of the planktonic graptolite Rhabdinopora cf. Flabelliformis flabelliformis (Eichwald, 1840) in black argillite of the upper part of the Cordylodus lindstromi Biozone indicates relatively precise position of the Cambrian- Ordovician boundary in the section. Cambrian- Ordovician boundary beds in the eastern Alborz Region can be subdivided into three lithological units: Furongian bioclastic limestone mainly replaced by fine clastic sediments in the upper part, the terminal Cambrian Cruziana sandstone representing a shoal complex and the Tremadocian grey argillite accumulated offshore. In this area the position of Cambrian- Ordovician boundary is situated somewhat below the base of the Asaphellus inflatus- Dactylocephalus trilobite Biozone. 

کلیدواژه‌ها [English]

  • Furongian
  • Tremadocian
  • Conodont
  • Trilobite
  • Graptolite
  • Biozone

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