Investigating the paleoecological characteristics of Abtalkh Formation at Bahadorkhan Section (Central Kopet-Dagh) based on planktonic and benthic foraminifera

Document Type: Research Paper

Authors

1 ferdowsi university of Mashhad

2 ferdowsi University of Mashhad

Abstract

Study of a late Late Santonian to Late Campanian hemipelagic succession from Abtalkh Formation at the Bahadorkhan section (Central Kopet-Dagh) enabled us to verify paleoecology changes based on planktonic and benthic foraminifera assemblage. Bahadorkhan section is consisted of calcareous shale, lime marl, marl, and a few dispersed chalky limestone beds. Upper and lower boundaries of Abtalkh Formation are conformable with Abderaz and Neyzar formations. Since foraminifera are proper tools for paleoecological investigations, we used them in three methods to analyze the changes of paleobathymetry: 1) Van der Zwaan’s equations of determining depth and method (the ratio of planktonic foraminifera to benthic), 2) Leckie's morphotype model, and 3) investigating the changes of benthic foraminifera and the ratio of agglutinated benthic foraminifera to the utilized calcareous benthic foraminifera. Trox model (the ratio of epifaunal to infaunal (Ep/In)) was also used with the goal of identifying oxygen level and nutrients. According to the statistical counting, four stages of changes in depth and environmental conditions in this section with age range of late Late Santonian- Late Campanian were observed. According to the analysis, in the Early Campanian, P/B ratio and infaunal benthic are high which indicates the high level of water and eutrophic environment. In such an environment, nutrient and oxygen levels are respectively high and low. Then, at end of the Early Campanian and the beginning of Middle Campanian, water level decreases, and the environment moves toward oligotrophic conditions. During Middle Campanian and the early Late Campanian, water level increases again. Finally, during Late Campanian and along with the change of lithology to Neyzar Formation sandstones, we will witness the decrease of P/B ratio, water level drop, oxygen level increase, and nutrient decrease.

Keywords


Article Title [Persian]

بررسی شاخصه های بوم شناسی دیرینه سازند آب تلخ دربرش بهادرخان (کپه داغ مرکزی) برمبنای روزن داران شناور وکف زی

Authors [Persian]

  • محمد وحیدی نیا 2
  • عباس قادری 2
2 دانشگاه مشهد
Abstract [Persian]

مطالعه توالی زسوبات همی پلاژیک اواخر سانتونین پسین- کامپانین پسین سازند آب تلخ در برش بهادرخان (مرکز حوضه کپه داغ) ما را قادر ساخت تا براساس مجموعه روزن داران شناور و کف زی تغییرات بوم شناسی دیرینه را بررسی نماییم. سنگ شناسی برش بهادرخان متشکل از شیل آهکی، مارن آهکی، مارن و بصورت پراکنده چندین واحد سنگ آهک چاکی می باشد. وضعیت مرزهای زیرین و بالایی آن با سازندهای آبدراز و نیزارهمشیب و پیوسته است. از آنجا که روزن داران ابزار مناسبی برای بررسی های بوم شناسی دیرینه می باشند به منظور بررسی تغییرات عمق دیرینه از آنها به 3 روش استفاده شد: 1- معادلات تعیین عمق و روش Van derz van (نسبت روزن داران شناور به کف زی)(P/B، 2- مدل مورفوتایپی Leckie (1987) و 3- بررسی تغییرات پوسته روزن داران کف زی و نسبت روزن داران کف زی آگلوتینه به آهکی، همچنین با هدف بررسی میزان اکسیژن و مواد مغذی از مدل Trox نسبت روزن داران کف زی سطح زی به درون زی (Ep/In) استفاده شده است. بر اساس این شمارش های آماری 4 مرحله تغییرات عمق و شرایط محیطی را در این برش در محدوده سنی اواخرسانتونین پسین- کامپانین پسین مشاهده شد. طبق بررسیها در زمان کامپانین پیشین نسبت P/B و کف زی درون زی بالاست که این نشانه بالا بودن سطح آب و محیط یوتروفیک است که درچنین محیطی میزان اکسیژن کم و مواد مغذی بالا می باشد. سپس در اواخرکامپانین پیشین و ابتدای کامپانین میانی سطح آب پایین می آید و محیط به سمت شرایط الیگوتروفیک پیش میرود، طی کامپانین میانی و ابتدای کامپانین پسین سطح آب مجدداً افزایش می یابد و نهایتاً در زمان کامپانین پسین و با تغییر سنگ شناسی به ماسه سنگهای سازند نیزار ما شاهدکاهش در نسبت P/B و پایین آمدن سطح آب و افزایش در سطح اکسیژن وکاهش مواد مغذی می باشیم

Keywords [Persian]

  • سازند آب تلخ
  • بوم شناسی دیرینه
  • روزن داران شناور و کف زی
  • حوضه کپه داغ
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