بحران زیستی انتهای پرمین میانی در برش دره‌ همبست، ناحیه‌ آباده، جنوب غرب ایران و مقایسه‌ آن با رسوبات همزمان در شرق تتیس

بررسی گسترش تنوع زیستی در نهشته‌های نزدیک به مرز پرمین میانی و بالایی در برش دره‌ همبست نشان می‌دهد با توجه به تغییرات بارز محیطی و فونایی در افق‌های راسی زیر واحد 4a سازند آباده، انقراض انتهای گوادالوپین در این برش پیش از مرز گوادالوپین لوپینگین روی داده است. تغییرات جالب توجه نزدیک به مرز گوادالوپین لوپینگین در این برش، حضور دولومیت و سنگ آهک استروماتولیتی است که در بالاترین بخش زیر واحد 4a وجود دارد و کم‌عمق‌شدگی محیط رسوبی و نیز کاهش چشمگیر تنوع زیستی را نشان می‌دهد. عامل بحران زیستی انتهای گوادالوپین در برش دره‌ همبست به احتمال زیاد متاثر از کاهش سطح آب دریا بوده است. بررسی افق انقراض انتهای گوادالوپین در جنوب چین و ژاپن نشان‌دهنده‌ قرارگرفتن آن در انتهای کپیتانین است. در جنوب چین (برش پن گلیتن) و آباده (برش دره‌ همبست) پس‌روی جهانی انتهای پرمین میانی که یکی از عوامل مهم انقراض در انتهای کپیتانین است، فقط آثاری از کم‌عمق‌شدن را نشان می‌دهد که همراه با تغییرات محیطی و فونایی و شاهدی بر انقراض انتهای گوادالوپین است. در برش‌های انتهای پرمین میانی در ژاپن آثار پس‌روی و بیرون‌زدگی رسوبات در این فاصله‌ زمانی مشهود است. در برش دره‌ همبست، زون فرامینیفری hemigordius irregulariformis به سن بالاترین بخش کپیتانین دارای تعداد کمی از گونه‌های codonofusiella که در قسمت قاعده‌ای زیر واحد 4b قرار دارد، نشان‌دهنده‌ ظهور دوباره‌ فونای فرامینیفری پس از افق انقراض انتهای گوادالوپین است؛ در حالی که در جنوب چین و ژاپن زون فوزولینیدی codonofusiellareichelina به سن وچیاپینگین آغازی نشان‌دهنده‌ بهبود و ازسرگیری ظهور فرامینیفرها پس از انقراض پرمین میانی است.

The biotic crisis of end-Guadalupian in the Hambast Valley section, Abadeh, southwest Iran and its comparison with coeval deposits in eastern Tethys

AbstractThe study of Faunal biodiversity distribution in the strata close to the Middle and Upper Permian boundary in the Hambast Valley section was carried out. Considering obvious environmental and faunal changes in the topmost horizon of the subunit 4a of the Abadeh Formation, the endGuadalupian extinction has happened before Guadalupian–Lopingian boundary. The significant changes near the GuadalupianLopingian boundary in this section are the presence of dolomite and stromatolitic limestone at the topmost part of the subunit 4a, implying the shallowness of the depositional environment as well as the noticeable decline of faunal biodiversity. The cause of endGuadalupian biocrisis in the Hambast Valley section is more likely due to the falling sea level. The examination of the endGuadalupian extinction horizon in South China and Japan shows its position at the end of Capitanian. In South China (Penglaitan section) and Abadeh (Hambast Valley section), the endGuadalupian eustatic regression has only left shallowing effects with environmental and faunal changes implying the endGuadalupian extinction evidence. In the Hambast Valley section, the latest Capitanian Hemigordius irregulariformis Zone with some Codonofusiella species in the basal part of the subunit 4b are representative of the reoccurrence of the foraminiferal fauna after the endGuadalupian extinction, while fusulinid Zone of early Wuchiapingian CodonofusiellaReichelina is indicative of postextinction foraminiferal recovery.Keywords: Hambast Valley section, Biodiversity, EndGuadalupian extinction, EndGuadalupian regression, Tethys         IntroductionThe biotic crisis at the end of Permian was the biggest event throughout of Phanerozoic life history as Paleozoic faunas were replaced by new ones (Ervin 2006). This event occurred in two phases, one at Guadalupian–Lopingian boundary, ca. 260 m.y. ago, and the other at PermianTriassic boundary about 252 m.y. ago (Stanly and Yang 1994; Bambach 2006). The PermianTriassic extinction was the most severe Phanerozoic extinction, which gave rise to the loss of more than 90% of the marine species (Shen et al. 2011). It has been thought that the main cause of this extinction was linked to the high environmental disturbances caused by the Siberian Large Igneous Province (Bond and Wignall 2014). Compared to the Permian–Triassic extinction, the endGuadalupian extinction was less severe. Its effects were mostly selective (Payne and Clapham 2012) so that algalsymbiont organisms including largetest fusulinids, rugose corals and large bivalves Alatochochidae were the most affected faunas (Isozaki et al. 2007a,b). Other organisms such as brachiopods, crinoids, ostracods and bryozoans were less influenced by endCapitanian extinction. Recent studies (Ota and Isozaki 2006; Retallack et al. 2008) reveal that despite previous thoughts, biodiversity decline at the GuadalupianLopingian boundary was not fast and the main extinction happened in late Guadalupian and before GuadalupianLopingian boundary. There are different opinions about the cause of endGuadalupian extinction including global regression (Jin et al. 1994; Arefifard 2012; Kolodka et al. 2012), the gigantic eruptions of Emeishan basaltic lavas in South China (Wignall et al. 2009; Bond et al. 2010; He et al. 2010), widespread methane emissions (Retallack et al. 2008) and global cooling (Isozaki 2007a) but no definitive cause has been proposed for it. However, the incidence of massive eruptions both in Permian–Triassic boundary and endCapitanian is a common feature that might be the most effective cause of these two extinctions. On the other hand, the volcanic events as the main causal mechanism of the extinction at endChanghsingian and endCapitanian might have influenced the regions close to these two activities and for the areas further away the other causes should be invoked. The continuous Permian–Triassic deposits in Iran crop out in the NE flank of the Hambast Mountain at 60 km East of Abadeh. In order to examine the endGuadalupian extinction in the Hambast Valley, one stratigraphic section has been measured and samples which include the middle and upper parts of the unit 3 of the Surmaq Formation of Capitanian in age, and the upper Capitanian subunit 4a and uppermost Capitanian and Wuchiapingian subunit 4b of the Abadeh Formation. The main objective of this research is the study of effect of the endGuadalupian extinction on faunal diversity in the Hambast valley section and its comparison with other coeval deposits within the Tethys basin, especially in South China and Japan.           Materials MethodsIn order to examine the endGuadalupian extinction in strata before, after and across Guadalupian–Lopingian boundary in the Hambast Valley section, a stratigraphic section was measured and 274 samples were collected from the middle and upper parts of the Unit 3 of the Surmaq Formation and Abadeh Formation. In addition to the identification of foraminifer genera and species to determine precise age assignments, foraminiferal faunas and other fossil groups abundance in thin sections were evaluated, especially before and after Guadalupian–Lopingian boundary. Discussion of Results ConclusionsThe middle and upper parts of the unit 3 of the Surmaq Formation based on Altineria bacillaeformisBaisalina pulchraHemigordiopsis luquensis biozone could be assigned to lower Capitanian. The examination of the foraminiferal content in subunits 4a and 4b and unit 5 of the Abadeh Formation reveal upper Capitanian Baisalina cf. guizhouensi, uppermost Capitanian Hemigordius irregulariformis and Wuchiapingian PseudodunbarulaCodonofusiellaReichelina biozones. In the Hambast Valley section, the most biodiversity is observed in the main part of the upper Capitanian subunit 4a of the Abadeh Formation; in contrast, its uppermost portion, which is composed of dolomite and stromatolitic limestone, lacks fossil. In the Panglaitan and Tieqiao sections in South China, fossil remains are traceable to the upper member of the Maokou Formation known as Laibin and only its uppermost parts have no fossil (Shen et al. 2007). In Japan, the upper part of the Akasaka Limestone is mainly composed of black limestone belongs to the late Capitanian (Kafukuda et al. 2014). The uppermost portion of the Akasaka Limestone is followed by a greenish very thinbedded of extremely finegrained claystone. In this thinbedded clayed bed, no fossil remain is recorded. In the Hambast Valley section, the disappearance of the upper Capitanian biota considered as Barren Interval Zone occurred in one step and is documented in the dolomitic beds and stromatolitic limestones of the uppermost portion of the subunit 4a of the Abadeh Formation. This Barren Interval Zone is located in the uppermost part of the Laibin Member in South China and occurred in the very thin clayed bed in Japan, which is considered as a subaerial exposure at the top of the Akasaka Limestone resulting from the short time sealevel fall and as one of the possible causes of the endGuadalupian extinction. Considering the faunal distribution and biodiversity in the Hambast Valley section, the extinction horizon is located before the latest Capitanian and within the uppermost portion of the subunit 4a of the Abadeh Formation where no fossil is found. The examination of extinction horizon in other coeval deposits with the Hambast Valley section in South China and Japan reveals its position at the latest Capitanian in the Penglaitan and Tieqiao sections in South China and Akasaka section in central Japan. Although evidence of shallowing has been reported at the end of Capitanian in both South Chinese sections and the Hambast Valley section,  there is no indication of regression during this time interval in these two regions. However, there is a record of regression in Japan as a plausible cause of endGuadalupian extinction.