محیط رسوبی، چینه‌نگاری سکانسی و ژئوشیمی عنصری سازند تله‌زنگ در جنوب کرمانشاه

در پژوهش حاضر برای نخستین‌بار، سازند تله‌زنگ به سن پالئوسن تا ائوسن زیرین در استان کرمانشاه (برش کبوتربالا) از دیدگاه شرایط رسوب‌گذاری، چینه‌نگاری سکانسی و ژئوشیمی عنصری ارزیابی شد. سازند تله‌زنگ در این برش به‌شکل هم‌شیب و پیوسته روی سازند امیران و به‌طور هم‌شیب و با ناپیوستگی فرسایشی زیر سازند کشکان قرار دارد. بررسی‌های صحرایی و مطالعه‌های آزمایشگاهی به شناسایی شش ریزرخساره در نهشته‌های این سازند منجر شدند. تغییرات تدریجی ریزرخساره‌ها، نبود کلسی‌توربیدایت‌ها و نبود ریف‌های سدی گسترش‌یافته با ضخامت درخور توجه تاییدکنندۀ وجود پلت‌فرم کربناته‌ای از نوع رمپ است. مطالعه‌های چینه‌نگاری سکانسی به شناسایی یک سکانس رسوبی ردۀ سوم و دسته رخساره‌های hst و tst منجر شدند؛ مرز زیرین این سکانس از نوع دوم و مرز بالایی با شواهد خروج از آب، از نوع اول است. سطح بیشترین پیشروی با ریزرخسارۀ فلوتستون تا رودستون حاوی کورالیناسه‌آ، مرجان و بایوکلاست مربوط به ابتدای رمپ میانی مشخص شد. نتایج بررسی‌های عناصر اصلی (کلسیم و منیزیم) و فرعی (استرانسیوم، سدیم، منگنز و آهن) بیان‌کنندۀترکیب کانی‌شناسی اولیۀ آراگونیتی و سیستم دیاژنزی نیمه‌بسته تا باز است. روند تغییرات عناصر در طول این برش به‌گونه‌ای است که در رسوبات بخش hst، مقدار استرانسیوم کاهش یافته است؛ درحالی‌که مقادیر آهن و منگنز به‌علت تاثیر بیشتر دیاژنز متائوریک افزایش یافته‌اند.

Sedimentary environment, sequence stratigraphy and elemental geochemistry of the Taleh Zang Formation in the South of Kermanshah

AbstractIn this study, for the first time depositional conditions, sequence stratigraphy and elemental geochemistry of the Taleh Zang Formation with Paleocene to Lower Eocene age in the Kermanshah province (Kaboutar Bala section) have been evaluated. In this section, the Taleh Zang Formation conformably overlies the Amiran Formation and is overlain by the Kashkan Formation with a disconformity surface. Field and microscopic studies led to recognition of six microfacies within the succession. Gradual microfacies change, the absence of calciturbidites and lack of extensive barrier reefs with considerable thickness, confirms a carbonate ramp. Sequence stratigraphic studies led to the identification of onethird order depositional sequence which includes TST and HST facies association. The lower boundary of this sequence, specified as type II and the upper boundary with evidences of subaerial exposure is type I. The maximum flooding surface (MFS) is determined with bioclastic coral corallinacea floatstone/rudstone microfacies of proximal middle ramp. The results of geochemical analysis of major elements (Ca, Mg) and minor elements (Sr, Na, Mn and Fe) indicate that the original carbonate mineralogy is aragonite and diagenetic system is semiclose to open system. Trend of elements changes along this stratigraphic section shows that in the HST sediments, the amount of Sr is decreased, while Fe and Mn are increased due to more meteoric diagenetic effect.Keywords: Taleh Zang Formation, Sedimentary Environment, Sequence Stratigraphy, Elemental Geochemistry, Kermanshah.   IntroductionThe Taleh Zang Formation is part of Lower Paleogene (Upper Paleocene–Middle Eocene) in the Lorestan zone of Zagros basin (Aghanabati 2010). At the type locality, this formation conformably overlies Cretaceous beds of marl and siltstone belonging to the Amiran Formation and is overlain by the Kashkan Formation with an unconformity (Aghanabati 2010). The thickness of the Taleh Zang Formation is very variable, as in some areas, this formation is absent and the Kashkan Formation directly overlies the Amiran Formation (Rajabi et al. 2011). In this paper the carbonate deposits of the Taleh Zang Formation in the Kermanshah province (Kaboutar Bala section) have been studied for the first time. The previous study of the Taleh Zang Formation in the Kermanshah province are focused on the biostratigraphy and also the components of this current study is different from other sections in the Lorestan zone (such as the absence of larger benthic foraminifera e.g. nummulitidae, alveolinidae and discocyclinidae and presence of corals and algae). So in this research for the first time depositional conditions, sequence stratigraphy and elemental geochemistry of the Taleh Zang Formation in the Kermanshah province (Kaboutar Bala section) have been evaluated. Material & Methods:For the present study, one surface section of the Taleh Zang Formation in the south of the Kermanshah region (Kaboutar Bala section) has been studied. The section measured a total thickness of 65 m and consists of limestone, dolomitic limestone, dolostone and minor amount of marl. During the fieldwork studies, 33 rock samples from carbonate deposits (limestone and dolostone) have been taken for petrographic studies. In order to differentiate ferroan and nonferroan calcite from ferroan and nonferroan dolomite in thin sections, the staining method of Dickson (1965) was applied. According to the methods of some researchers such as Haq et al. (1987) and Posamentier et al. (1988), depositional sequences were recognized. Elemental geochemistry analysis were performed form 12 samples of carbonates at the geochemistry laboratory of Shahid Beheshti University, Iran. Discussion of Results & ConclusionsBased on the field and petrographic studies, the microfacies and depositional environment of the Taleh Zang Formation were recognized. This formation in the Kaboutar Bala section have been made of six microfacies which occur in four facies belts. The tidal flat sediment is composed of dolomicrite with ranging in size from 4 to 16 µm and contain siltsize quartz grains. Many researchers believe that dolomicrite forms during very early diagenesis in supratidal to intertidal environments (e.g. Sibley and Gregg 1987; Adabi 2009). From the shoreline towards the seas, the lagoon facies belt consists of two microfacies; bioclastic benthic foraminifera peloidal wackestone, and bioclastic benthic foraminifera dasycladacea wackestone to packstone. The lack of marine biota and abundant components of restricted biota (such as miliolids and dasycladacea), represent a restricted conditions in the lagoon environment (Bachmann and Hirsch 2006; Adabi et al. 2015; Kahsnitz et al. 2018). Barrier facies belt is composed of coral boundstone microfacies with limited lateral extension. The main components of this microfacies are only corals. The dominance of coral colonies indicate a high energy environment in the barrier facies belt (Vescogni et al. 2016; Ghafari et al. 2017). The open marine facies belt includes coral floatstone and bioclastic coral corallinacea floatstone/rudstone microfacies. The common coral debris may have derived from erosion of preexisting coral colonies (in the barrier facies belt) by currents and/or storms (Roozpeykar and Maghfouri Moghaddam 2015; Shabafrooz et al. 2015). Gradual microfacies change, the absence of calciturbidites and lack of extensive barrier reefs with considerable thickness, confirms a carbonate ramp .Based on the sequence stratigraphic studies, one depositional sequence was identified in the studied sequence. The lower boundary of this sequence is located at the base of the formation and specified as type II. The upper boundary with evidences of subaerial exposure and the presence of siliciclastic Kashkan Formation specified as type I. The MFS is determined with bioclastic coral corallinacea floatstone/rudstone microfacies belonging to the proximal middle ramp.Geochemical analysis in limestone parts of the formation including Ca (37.57–39.25%), Mg (0.25–1.3%), Sr (964–1833 ppm), Na (65–160 ppm), Mn (161–421 ppm) and Fe (281–2107 ppm), and their variations indicate that the original carbonate mineralogy is aragonite and diagenetic system is semiclose to open system.Variation trends of elements along the stratigraphic column shows that the amounts of elements in the TST and HST are different from each other. Generally in the HST sediments, the amount of Sr is decreased, while Fe and Mn are increased due to more meteoric diagenetic effect at this part of the sequence.