رخساره‌ها و محیط رسوبی سازند آیتامیر در دشت گرگان: مدلی از محیط حاشیه‌ ساحلی زیر نفوذ امواج

رسوبات سیلیسی -آواری سازند آیتامیر (آلبین سنومانین) در برش اقچی در ناحیه‌ دشت گرگان شامل توالی ضخیمی از رسوبات شیل خاکستری تیره و ماسه‌سنگ‌های نازک تا بسیار ضخیم‌لایه است که در یک سیستم رسوبی حاشیه‌ ساحلی تا دور از ساحل نهشته شده‌اند. چهار مجموعه‌ رخساره‌ای شامل دور از ساحل، انتقالی دور از ساحل، پایین حاشیه‌ ساحلی و میانه بالای حاشیه‌ ساحلی در این رسوبات شناسایی شده است. مجموعه‌ دور از ساحل شامل توالی ضخیمی از شیل‌های متورق تیره‌رنگ با مقادیر فراوان پیریت خوشه‌ای است که ته‌نشینی در زیر حد موجسار طوفانی و شرایط کم‌اکسیژن را نشان می‌دهد. مجموعه‌ انتقالی دور از ساحل شامل مجموعه‌ ناجور سنگی از شیل و ماسه‌سنگ و سیلتستون گلاکونیتی بسیار نازک‌لایه است. در مجموعه‌های پایین و میانه بالای حاشیه‌ ساحلی به‌تدریج بر میزان و ضخامت طبقات ماسه‌سنگی افزوده می‌شود و به‌طور کلی یک الگوی ضخیم‌شونده به طرف بالا را نشان می‌دهند که الگوی تیپیک سیستم حاشیه‌ ساحلی است. انطباق جانبی مجموعه‌های رسوبی برش مطالعه‌شده با برش‌های مرکزی سازند آیتامیر در حوضه‌ کپه‌داغ، افزایش تاثیر فرونشینی را در ایجاد فضای رسوب‌گذاری و کاهش میزان رسوب‌گذاری/ تامین رسوب را در بخش‌های شمال شمال غربی نشان می‌دهد. وجود ضخامت زیادی از رسوبات شیل تیره‌رنگ با مقادیر فراوان پیریت‌های خوشه‌ای و حجره پرکن، رسوبات سازند آیتامیر را به‌مثابه‌ رسوبات مستعد مطالعات اکتشاف هیدروکربن مطرح می‌کند.

Sedimentary facies and depositional environment of the Aitamir Formation in the Gorgan Plain: A wave-dominated shoreface model

Abstract: The siliciclastic sediments of the Aitamir Formation (Albian–Cenomanian) at the Oghchi locality of the Gorgan Plain consist of a thick succession of the blackgrey shales and thin to very thickbedded sandstone, deposited in a shoreface to offshore depositional system. Four facies associations including offshore, offshore transition, lower and midupper shoreface associations are identified in this succession. Offshore facies association consisting of a thick package of the fissile dark shales with the abundance of the framboidal pyrites displays deposition below the SWWB and dysoxic condition. The offshore transition facies association includes the heterolithic sediments of the shale and very thinbedded sandstones/siltstones. The lower and midupper associations are characterized by an increase in abundance and the thickness of the sandstone layers and display a generally thickening and coarsening upward trend of a classical shoreface depositional system. The lateral correlation of the studied section and central sections of the Aitamir Formation of the KopetDagh Basin indicates an increase in subsidence effect in the creation of the accommodation space and a decrease in the sediment supply of the NNW parts of the basin. The presence of a thick package of  dark shales with abundant framboidal and chamber filling pyrites of the Aitamir Formation suggests these sediments as a prone candidate for hydrocarbon exploration.Keywords: Aitamir, KopetDagh, shoreface, tectonic, hydrocarbon  Introduction: The KopetDagh Basin, located in the NE Iran and 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) (Stampfli et al., 1991). A continuous sedimentary package (up to seven kilometers), including five transgressiveregressive supersequences from the Jurassic to Miocene were deposited in the Eastern part of the sedimentary basin (MoussaviHarami and Berner 1992) that is mainly controlled by NWSW running major faults. The Cretaceous succession in the KopetDagh Basin is divided into nine formations, composed mainly of sandstones, conglomerates, mudstones, limestones and dolomites with minor amounts of evaporates (AfsharHarb 1979). The considerable lateral changes of the sedimentary packages and the thickness of the Aitamir Formation in the KopetDagh basin are related to the tectonic and eustatic sealevel changes that are the main controlling factors on the sediment supply and available accommodation space.  The objectives of this paper are: (1) to describe lithofacies/facies associations and the role of waveinduced currents on the sediment distribution as the main autogenic controlling factor in the Aitamir Formation and (2) to use sedimentological data to interpret the depositional environment of the Aitamir Formation. Representing a more detailed depositional model for the studied succession could provide valuable insights into basin evolution of KopetDagh Basin during Albian–Cenomanian. Material Methods:This study focused on one measured section in the western KopetDagh Basin in the northwest of Iran. The geographical coordinates of the Oghchi section is 37° 45´ 00̋ N and 55° 25´ 33̋ E (about 35 km northeast from GonbadKavous City). Lithology, macrofossils, grainsize, bedding geometry, bedding contact, stacking pattern, and sedimentary structures were recorded in detail.  Lithofacies were classified/entitled following Miall (2006) and petrofacies according to Folk’s (1980) classification. One hundred and forty thin sections were examined to identify finescale physical characteristics (mineralogical composition and fossil contents). Discussion of Results Conclusions:The Aitamir Formation in the studied section is about 1630 m thick and subdivided into two lower and upper parts. The lower part of  includes medium to the thickbedded sheetlike glauconitic sandstones and thick packages of  dark grey shales (up to 80 m) and thick packages sandstone constitute the uppermost part of this unit. The upper part of the Aitamir Formation mainly consists of a thick package of  dark grey shales intercalated with the thinbedded sandstones. Based on sedimentological features, four facies associations are recognized in the studied succession of the Aitamir Formation as follows: offshore (FA), offshore transition (FB), lower shoreface (FC), and midupper shoreface (FD). The offshore facies association consists of a thick package of  dark grey fissile shales intercalated with the thinbedded, very finegrained sandstones. The offshore facies association includes abundant framboidal and chamber filling pyrites displaying a dysoxic condition in the deepest part of the sedimentary environment suggesting these sediments as a prone candidate for hydrocarbon exploration. The offshore transition facies association includes a thick package of  grey shales with several thin to the mediumbedded sheetlike sandstones, displayed by Sm, St, Sp., Shl. The lower shoreface facies association consists of an alternation of  dark grey shales and thin to the mediumbedded sheetlike sandstones with sharp lower and upper boundaries. Sm, Shl., Sp, and HCS are the most common facies in this facies association. The midupper shoreface facies association includes amalgamated, sheetlike, medium to  thickbedded sandstones with large scale St, Sp, and Sm, Shl. The sedimentary structures such as HCS, Shl, and sheet like geometry of the sandstone layers with the sharp and/or erosional boundaries as well as thickening and coarsening upward stacking pattern of the sediments of the Aitamir Formation display a wavedominated shoreface depositional system. The structural morphology and normal faults direction of the KopetDagh Basin are the allogenic and large scale elements that were directly controlled the sedimentary processes and hydrodynamic regime and type of the lithofacies/facies associations (indirectly) of the Aitamir depositional system. In this context, the KopetDagh basin in the Barremian–Aptian time was located in the northern part of the NeoTethys Ocean and was characterized by the normal faults with ESE to WNW direction. The formation of the riftrelated faults led to the development of the elongated, open coast morphology of the KopetDagh Basin, where wavegenerated currents  increased as a sedimentary mechanism of the sediment distribution.  In a general view, the petrography and field observations, facies associations relationship, and vertical trend of the studied successions suggest Aitamir sediments in the KopetDagh basin were deposited in a wavedominated shoreface environment with midupper shoreface, lower shoreface, offshore transition, and offshore zones. These sediments were deposited in a shallow marine depositional setting, characterized by gradationally vertical changes in the facies associations and abundant storm deposits. This depositional system was influenced by storm waves, suggested by the abundant storminduced facies such HCS, gradedbedded sandstone with erosional lower surface and Shl. The presence of a thick package of the dark shales with abundant framboidal and chamber filling pyrites of the Aitamir Formation suggests these sediments as a prone candidate for hydrocarbon exploration.