شواهدی از ثبت رخداد غیرهوازی اقیانوسی آپسین پیشین (oae1a) در کپه‌داغ مرکزی (برش قلعه‌زو) با تاکید بر اجتماعات نانوفسیلی

در مطالعۀ حاضر، بخش میانی رسوبات سازند سرچشمه در برش قلعه‌زو واقع در مرکز حوضۀ کپه‌داغ ازنظر ثبت رخداد غیرهوازی اقیانوسی آپسین پیشین (oae1a) بررسی شد؛ این رخداد در بازۀ زمانی آپسین پیشین در زون nc6 از بایوزون‌های نانوفسیلی و در مقیاسی جهانی ثبت شده است. در مطالعۀ حاضر، بخش‌های انتهایی بایوزون نانوفسیلی nc6a تا بخش‌های ابتدایی بایوزون nc7a (سن آپسین پیشین) در سازند سرچشمه بررسی شدند. در برش موردمطالعه، پالئواکولوژی نانوفسیل‌های آهکی و میزان درصد کربنات‌کلسیم به‌منظور تعیین محدودۀ رخداد غیرهوازی اقیانوسی آپسین پیشین oae1a بررسی شدند. کاهش درخور توجه فراوانی نانوکونیدهای آهکی یا حضورنداشتن آنها در زیرزون nc6b –هم‌زمان با کاهش درصد کربنات‌کلسیم نمونه‌ها موید تاثیر رخداد غیرهوازی اقیانوسی در محدودۀ موردمطالعه (55 تا 180 متری توالی موردمطالعه) است. بر اساس داده‌های یادشده (داده‌های نانوفسیلی و کلسیمتری)، ثبت رخداد غیرهوازی اقیانوسی ابتدای آپسین در بخش‌های میانی سازند سرچشمه درخور مقایسه با ثبت این رخداد از غرب حوضۀ کپه‌داغ (برش تکل‌کوه) است.

Evidence of early Aptian Oceanic Anoxic Event 1a (OAE1a) in the Central KopetDagh (QalehZoo section), with special reference to the calcareous nannofossil assemblages

Abstract In the current study, a succession from the middle part of the Sarcheshmeh Formation was investigated to record the early Aptian oceanic anoxic event 1a (OAE1a) at the QalehZoo section in the central part of the Kopetdagh Basin. The OAE1a has been recorded on a global scale during the early Aptian in the NC6B nannofossil biozone. In this study the interval from the uppermost part of NC6A to the lowermost part of NC7A (early Aptian) was investigated at Sarcheshmeh Formation. Calcareous nannofossil paleoecology and carbonate calcium contents were analyzed to identify the OAE1a interval. Rare presence or absence of nannoconids at NC6B along with the lowest values of carbonate calcium contents indicate the effect of OAE1a in the study section (55 to 180 meters of the studied interval). Regarding these data (calcareous nannofossils and carbonate calcium contents), the OAE1a was recorded at the middle part of Sarcheshmeh Formation which is equivalent with this event at the western part of the Kopet Dagh Basin (Takal Kuh section). Keywords: CalcareousNannofossils; KopetDagh Basin; Oceanic Anoxic Event 1a (OAE1a); Sarcheshmeh Formation.   Introduction The early Aptian OAE1a is one of the most important events in terms of temperature and paleoceanography during the Cretaceous. OAE1a is regarded as an anoxic condition that is continued for at least one million years. This event is recorded in the Early Aptian time and NC6 calcareous nanofossil biozone on a global scale (Erba 2004). Calcareous nanoplanktons are one of the most abundant planktonic groups in the seas that changes in their assemblages reflecting changes in temperature and nutrient content of the oceans (Herrle and Mutterlose 2003; Herle 2003; Tremolada et al. 2006). In terms of calcareous nanofossil distribution, the nanoconid crisis (being rare or absent in fossil assemblage) is a global event that is an evidence of OAE1a (Erba 1994, 2004). Calcium carbonate content in the sediments is also very low during this time interval which is considered as a signal for this event (Erba and Tremolada 2004). At the current study, the OAE1a is investigated based on calcareous nannofossils at the KopetDagh Basin. The KopetDagh Basin is located on the northeast of Iran, extending over Iran, Turkmenistan and Afghanistan with a westnorthwest to eastsoutheast trend. The Iranian part of the basin is located between 61°14ˊ and 54°00ˊE and 38°15ˊ and 35°38ˊ N. Cretaceous sediments are well developed and extensively exposed in the KopetDagh Basin and the Sarcheshmeh Formation was precipitated during the Early Cretaceous in this basin. In order to determine the OAE1a interval, the middle part of the Sarcheshmeh Formation in the QaleZoo section at the central part of the KopetDagh Basin has been investigated based on calcareous nanofossils.   Material & Methods The QalehZoo section is located 15 km to the northwest of Shirvan and 2.5 km to the west of Ziarat village at N 37°29ˊ39˝ and E 57°48ˊ07˝. The Sarcheshmeh Formation with 713 m thickness in QalehZoo section is conformably overlies the Tirgan Formation and is in turn overlain by the Sanganeh Formation. Regarding calcareous nannofossil biostratigraphic analysis of the Sarcheshmeh Formation (Gholamifard et al. 2016), 305 m thickness (from sample 110 to 196) with 86 samples from NC6A to NC7A biozones were analyzed in detail. Calcareous nannofossil paleoecology and calcimetry were performed in these samples to determine the OAE1a event. The studied interval mainly consists of shale, marl, argillaceous limestone and limestone. For biostratigraphic analysis, all of the samples were processed using the gravity settling technique (Bown and Young 1998) and the prepared slides were observed under a Leica DMLP Pol light microscope at ×1000 magnification. For paleoecological analysis a semiquantitative count was done according to Burnett et al. (1998) and the taxa are classified into abundant (A= more than 10 species per field of view), common (C= 1 to 10 species per field of view), Few (F= one species in 2 to 50 fields of view) and rare (R= one species in more than 50 fields of view). The carbonate calcium content of the sediments is also measured with Schibler calcimeter by measuring the amount of CO2 gases released during the analysis as follow: CaCO3% =1/2 × 0/2 × δH(sample) / weight(sample) / δH(standard) × 100   Discussion of Results & Conclusions The calcareous nannofossils of the studied interval include species with global distribution (such as Watznaueria spp., Rhaghodiscus spp., Zeugrhabdotus spp., Micrantholithus spp. and so on) and nanoconids (commonly found in the Tethys Basin). According to calcareous nanofossil data, the oceanic anoxic event 1a of the early Aptian has been reported in various parts of the world from the upper parts of the Chiastozygus litterarius (NC6) and the first part of Rhagodiscus angustus (NC7A) biozones (Erba 1994, 2004). Regarding calcareous nannofossil biostratigraphy of the studied interval, the presence of these biozones at the studied succession was determined. Paleoecological studies indicate of nanoconid crisis and low calcium carbonate content of sediments from sample 128 to 157 at the studied succession (upper part of NC6 zone and beginning of NC7A subzone) that must be the result of early Aptian OAE1a at the middle part of Sarcheshmeh Formation. This interval can be correlated with OAE1a that is identified at the western Kopetdagh Basin (Takal Kuh section) in the first part of Sanganeh Formation by Mahanipour et al. (2011).