رخساره‌ها‌ و‌ محیط‌ رسوبی‌ سازند‌ تیرگان‌ در‌ ناحیه‌ بزنگان،‌ شرق‌ حوضه‌ رسوبی‌ کپه‌داغ

نهشته‌های‌ سازند‌ تیرگان که‌ در‌ گستره‌‌ وسیعی‌ از‌ حوضه‌ رسوبی‌ کپه‌داغ‌ رخنمون‌ دارند،‌ در‌ ناحیه‌ بزنگان‌ از‌ شیل‌های‌ سبز‌ تا‌ خاکستری‌ با‌ میان‌ لایه‌های‌ سنگ آهک‌ نازک‌ لایه‌ در‌ بخش‌ پایینی‌ و‌ تناوب‌ سنگ‌ آهک‌های‌ نازک‌‌ ‌ متوسط‌ لایه‌ و‌ سنگ آهک‌ مارنی‌ در‌ بخش‌ بالایی‌ تشکیل‌ شده‌ است.‌ بررسی‌ مشخصات‌ سنگ‌ شناسی،‌ فرم‌ هندسی‌ و‌ محتوای‌ فسیلی‌ رسوبات‌ مورد‌ مطالعه‌ در‌ مطالعات‌ صحرایی‌ همراه‌ با‌ مشخصه‌های‌ بافتی‌ و‌ محتوای‌ میکروفسیلی‌ در‌ بررسی‌های‌ آزمایشگاهی،‌ امکان‌ تفکیک‌ 16‌ رخساره‌ کربناته‌ و‌ سیلیسی‌‌ ‌ آواری‌ را‌ میسر‌ نموده‌ است.‌ این‌ رخساره‌ها‌ در‌ پنج‌ کمربند‌ رخساره‌ای‌ پهنه‌ کشندی،‌ لاگون‌ و‌ پشته‌های‌ ‌ماسه‌ای‌ متعلق‌ به‌ رمپ‌ درونی،‌ بخش‌ کم‌ ژرفای دریای‌ باز‌ متعلق‌ به‌ رمپ‌ میانی‌ و‌ بخش‌ عمیق‌ دور‌ از‌ ساحل‌ متعلق‌ به‌ رمپ‌ بیرونی‌ قرار‌ ‌می‌گیرند.‌ الگوهای‌ رخساره‌ای‌ مشاهده‌ شده‌ از‌ جمله‌ ‌نبود‌ رخساره ‌ توربیدایتی‌ مربوط‌ به‌ شکستگی‌ در‌ شیب‌ حوضه‌ و‌ تغییرات‌ تدریجی‌ رخساره‌ ها‌ از‌ شیل‌های‌ سبز‌‌ ‌ خاکستری‌ مربوط‌ به‌ رمپ‌ بیرونی،‌ فلوتستون‌‌ ‌ ردستون‌ بیوکلاست‌ و‌ ‌اینتراکلست‌دار‌ (رمپ‌ میانی)‌ و‌ سپس‌ گرینستون‌ اُاُئید‌ و‌ ‌اینتراکلست‌دار‌ و‌ پکستون‌‌ ‌ گرینستون‌ اُاُئیدی‌ (رمپ‌ درونی)‌ نشانگر‌ نهشته‌ شدن‌ این‌ توالی‌ کربناته‌ بر‌ روی‌ یک‌ پلتفرم‌ کربناته‌ نوع‌ رمپ‌ با‌ شیب‌ یکنواخت‌ است.‌ بخش‌ میانی‌ این‌ سیستم‌ کربناته‌ تحت‌ نفوذ‌ امواج‌ و‌ بخش‌ درونی‌ (به‌ ویژه‌ پشته‌ ماسه‌ای)‌ تحت‌ نفوذ‌ جریانات‌ کشندی است.‌ ‌عوامل‌ اصلی‌ در‌ ایجاد‌ تنوع‌ زیستی‌ و‌ رخساره‌ای‌ در‌ رسوبات‌ سازند‌ تیرگان‌ سطح‌ و‌ نوع‌ انرژی‌ (کشند و‌ امواج)،‌ میزان‌ مواد‌ غذایی،‌ سطح‌ اکسیژن،‌ میزان‌ گردش‌ آب‌ و‌ آشفتگی‌ بستر‌ رسوبی‌ بوده‌ است.‌ ‌

Sedimentary facies and Depositional environment of the Tirgan Formation in the Bazangan Area, E Kopet-Dagh Basin

IntroductionThe KopetDagh sedimentary Basin that crops out from NE Iran to Turkmenistan was formed as an intracontinental Basin due to the southeastern extension of the South Caspian Basin by Neotethyan backarc rifting after the closure of the Palaeotethys and the early Cimmerian Orogeny (middle Triassic). A thick sedimentary package (10 kilometers) consisting of five transgressive regressive supersequences from Jurassic to Miocene time is deposited in the Eastern part of the sedimentary basin that is mainly controlled by NWSW running major faults. The Cretaceous sequence in the KopetDagh Basin is divided into nine formations, mainly composed of sandstones, conglomerates, mudstones, limestones, and dolomites with minor amounts of evaporates (AfsharHarb, 1979). The regional trend of the KopetDagh sedimentary basin was northwestsoutheast during the Cretaceous. After the deposition of thick, red siliciclastic sediments of the fluvial system during the Early Cretaceous, a suitable condition for the deposition of carbonate sediments (named as Tirgan Formation) was provided as a result of major marine transgression during the BarremianAptian stages. The Tirgan Formation is one of the most widespread Upper Cretaceous formations in the eastern KopetDagh that unconformably overlies the fluvial sediments of the Shurijeh Formation and is overlain by a sharp contact of the Sarcheshmeh Formation.Material and MethodsThis study is focused on two stratigraphic sections in the Eastern KopetDagh in northern Iran. Ninety thin sections were examined to identify finescale physical characteristics (mineralogical composition and fossil contents). Lithology, grain size, and sedimentary structures were recorded.DiscussionBased on sedimentological features, sixteen facies are recognized that grouped within five facies associations consist of deep and shallow open marine (FA), shoal (FB), lagoon (FC), and Tidal flat (FD). Deep open marine facies include greengray fissile (sandysilty) shale and shallow open marine facies are mainly consist of bioclastic wackestone, ooid/peloid bioclastic floatstone, sandy intraclastic floatstone, sandy intraclastic bioclastic floatrudstone, and sandy peloid bioclastic grainstone. The main constituents in this association are brachiopods, bryozoans, echinoderms, oysters, orbitolinids, intraclasts, ooids, and peloids. Micrite envelopes and borings are the common features in this association. The skeletal elements display high fragmentation, preferentially horizontal orientation, and fining upward fabrics. Greengray fissile shale is deposited in a low energy depositional setting below weather wave base (SWWB), periodically affected by storm waves, suggested by the presence of the siliciclastic grains. The skeletal elements of the shallow open marine facies offer a shallow full open marine setting between SWWB and fairweather wave base (FWWB) (BovarArnal et al., 2009). High fragmentation, preferentially horizontal orientation, and fining upward skeletal elements suggest the stormgenerated shell concentrations. Low energy periods of the sedimentary environment (post and prestorm phase) are indicated by micrite envelopes and borings. Shoal association consists of ooid grainstone and sandy ooid grainstone facies with predominantly wellsorted fabric. The main sedimentary structures in this association are sigmoidal crossbeds, wave ripples, cross lamination, and planar crossbeds with the erosional surface. These sediments are deposited in mediumhigh energy shoal settings above FWWB in the inner ramp environment suggested by well sorting of the elements and predominantly grainstone facies (Bachmann Hirsch, 2006; Brandano et al., 2012). The sedimentary structures clearly show that tidal currents controlled the association’s deposition in a sandy shoal environment. Lagoon association includes sandy mudstone, bioclastic wackpackstone, sandy ooid wackestone, sandy peloid packstone, intraclast, ooid floatrudstone, ooid/peloid packgrainstone, ooidal bioclastic floatrudstone with main ooids, peloids, benthic foraminifers, echinoderms, and minor content of green algae, bivalves, brachiopods as well as siliciclastic grains that are commonly floated in the micritic matrix. These sediments are mainly deposited in a semirestricted to restricted (sandy mudstone) lagoon setting above FWWB. The periodical water circulation suggested by the mixture of the open marine (echinoderm, brachiopods) to more restricted and brackish water elements (ostracodes, benthic forams), floated within the micritic matrix (Colombie Strasser, 2005; Bachmann Hirsch, 2006; BovarArnal et al., 2009). The tidal flat association mainly consists of peloids, benthic foraminifers, and ostracods surrounded by flat microbial laminations. The flat geometry of the microbial lamination and the presence of the peloids and benthic foraminifers as the main elements in this association demonstrate a flat substrate in a tidal flat setting of the most internal part of the carbonate platform.ConclusionIn a general view, petrography and field observations, facies associations relationship, and vertical trend of the studied successions suggest Tirgan sediments in the KopetDagh basin are deposited in shallow to the deep marine environment with tidal flat, lagoon, shoal, and shallow to deep marine facies zones. These sediments were deposited in a homoclinal ramp characterized by gradationally vertical changes in the facies associations and abundant storm deposits. This carbonate system was influenced by storm (shallow marine zone) and tidal (shoal zone) currents suggested by the stormgenerated shell concentrations, wave ripples, cross laminations, and sigmoidal crossbeds.ReferencesAfsharHarb, A., 1979. The stratigraphy, tectonics and petroleum geology of the Kopet Dagh region, Northern Iran. Ph.D. thesis, Imperial College of Science and Technology, London, 316 p. (Unpublished).Bachmann,‌ M.,‌ ‌ ‌ Hirsch,‌ F.,‌ 2006.‌ Lower‌ Cretaceous‌ carbonate‌ platform‌ of‌ the‌ eastern‌ Levant‌ (Galilee‌ and‌ the‌ Golan‌ Heights):‌ stratigraphy‌ and‌ secondorder‌ sealevel‌ change.‌ Cretaceous‌ Research,‌ 27:‌ 487512.‌BoverArnal,‌ T.,‌ Salas,‌ R.,‌ MorenoBedmar,‌ J.A.,‌ ‌ ‌ Bitzer,‌ K.,‌ 2009.‌ Sequence‌ stratigraphy‌ and‌ architecture‌ of‌ a‌ late‌ Early–Middle‌ Aptian‌ carbonate‌ platform‌ succession‌ from‌ the‌ western‌ Maestrat‌ Basin‌ (Iberian‌ Chain,‌ Spain).‌ Sedimentary‌ Geology‌, 219: 280–301.Brandano,‌ M.,‌ Lipparini,‌ L.,‌ Campagnoni,‌ V.,‌ ‌ ‌ Tomassetti,‌ L.,‌ 2012.‌ Downslopemigrating‌ large‌ dunes‌ in‌ the‌ Chattian‌ carbonate‌ ramp‌ of‌ the‌ Majella‌ Mountains‌ (Central‌ Apennines,‌ Italy).‌ Sedimentary‌ Geology,‌ 255256: 2941.‌Colombie,‌ C.,‌ ‌ ‌ Strasser,‌ A.,‌ 2005,‌ Facies,‌ cycles,‌ and‌ controls‌ on‌ the‌ evolution‌ of‌ a‌ keepup‌ carbonate‌ platform‌ (Kimmeridgian,‌ Swiss‌ Jura).‌ Sedimentology,‌ 52:‌ 12071227.