محیط زیست دیرینه و رخساره‌های پالینولوژیکی سازند سنگانه در برش چینه‌شناسی امیرآباد (خراسان رضوی)

سازند سنگانه یکی از سازندهای حوضه رسوبی کپه‌داغ  به سن کرتاسه پیشین است که محیط زیست دیرینه و رخساره‌های پالینولوژیکی آن در برش امیرآباد مورد مطالعه قرار گرفته ‌است. سنگ شناسی عمده سازند سنگانه در این برش شامل شیل‌های خاکستری تیره ‌است که در بعضی از افق‌ها، حاوی کنکرسیون، مخروط در مخروط و فسیل‌های آمونیت می‌باشد و 530 متر ضخامت  دارد. مرز زیرین آن با سازند سرچشمه توسط یک لایه سنگ آهک فسیل‌دار مشخص می‌شود و مرز بالایی آن با سازند آیتامیر با گذر تدریجی است. برای مطالعه رخساره پالینولوژیکی و محیط دیرینه سازند سنگانه تعداد 50 نمونه با فواصل معین برداشت شد و پس از آماده سازی نمونه‌ها بر اساس میزان خرده‌های آلی در اسلایدهای پالینولوژیکی، سه رخساره پالینولوژیکی مختلف شناسایی گردید. با توجه به بررسی گروه‌های اصلی پالینولوژیکی و نتایج مطالعات آماری می‌توان چنین استنباط کرد که محیط رسوبی سازند سنگانه در زمان رسوب‌گذاری محیطی کم‌اکسیژن تا فاقد اکسیژن بوده‌است. همچنین با توجه به داینوسیست‌های شاخص، محیط رسوبی دریای حاشیه‌ای، تا دریای باز کم ژرفا برای سازند سنگانه در زمان رسوب‌گذاری پیشنهاد می‌شود.

Palaeoecology and Palynofacies of the Sanganeh Formation in Amir Abad Stratigraphic Section (Khorasan-e- Razavi Province)

IntroductionThe Sanganeh Formation is one of the Early Cretaceous formations which is deposited in the KopetDagh sedimentary basin. It is investigated from palynofacies and palaeoecological points of view in the Amir Abad stratigraphic section. This formation has a thickness of 530 m and is mainly composed of dark grey shales. It contains septarian nodules as well as coneincone structures with ammonite in some horizons. The Sanganeh Formation overlaid the Sarcheshmeh Formation and is distinguished by a limestone richin fossil and is gradually underlain by the Aitamir Formation. Sanganeh Formation in the Amirabad section is located at 36o34‘  northern latitude and 60o 5’ eastern longitude 852 meters above sea level, in Amirabad village along Mashhad Kalat road. Discussion50 samples were systematically collected from the Sanganeh Formation. The traverse method (Traverse 1998) was used to produce Palynological slides. About 40 grams of each sample was cleaved and washed carefully and kept in 20% concentration HCL for 24 hours and afterward 24 hours in 40% concentration HF. Furthermore, samples were boiled in 20 concentration HCL for 20 minutes and sieved using 20micron nylon sieve. Moreover, in order to separate palynomorphs from heavier materials, samples were centrifuged with a zinc chloride solution between 1.9 to 2 densities. Fensome and Williams (2004), Fensome et al., (2008), Slimani et al. (2008, 2010, 2011), were used for categorizing and identification of dinoflagellates species. The following species are the most important dinoflagellates that were identified from analysis of Palynological slides from Sanganeh Formation:Achomosphaera neptuni Astrocysta sp. Batiacasphaera sp. Cerbia tabulate Cleistosphaeridium sp. Cribroperidinium sp. Cribroperidinium orthoceras Cymososphaeridium sp. Coronifera oceanica Diconodinium sp. Endoscrinium campanula  Eucladinium sp. Fromea amphora Gardodinium trabeculosum Hystrichosphaeridium sp. Kiokansium polypes Odontochitina sp. Odontochitina operculata Oligosphaeridium albertense Oligosphaeridium cf. fenestratum Oligosphaeridium complex Oligosphaeridium pulcherrimum Oligosphaeridium sp. Pseudoceratium polymorphum Pseudoceratium retusum Spiniferites sp. Spiniferites ramosus.From the identified fossils and different Palynological elements, three species of palynofacies were identified: 1) Highly proximal shelf or basin, 2) Marginal dysoxic anoxic basin, 3) Proximal suboxicanoxic shelf. A good interpretation of the environments can be obtained, using the data from Palynological Facies and the study of some conservation factors of organic materials. These results are obtained from the combination of quantitative and qualitative criteria. Most important factors are listed below:Fraction of transparent AOM to opaque AOM: This factor demonstrates the less oxygen or low oxygen (in very few slides) conditions during sedimentation.Lability factor: The Lability factor for the Sanganeh Formation samples shows low levels of Lability factor, due to lack of existence or low existence of brown palynomacerals trace in these samples. This indicates that deposition has taken place far from shore, far from the source, and extremely in the low oxygen conditions.Transparent AOM to a fraction of marine palynomorphs fraction and opaque AOM to marine palynomorphs fraction: Increases from AOMt to MPs is an indicator of low oxygen or oxygenfree conditions and slow sedimentation rate, also decreases from AOMop to MPs is an indicator of low oxygen conditions during sedimentation in Sanganeh Formation.Opaque same dimension Palynomaceralto Spiky Palynomaceral fraction: In general, a high fraction of PM4R to PM4B is an indicator of sedimentation in deeper parts of the basin.Corate to proximate Dinoflagellate fraction, proximocorate to cavate (C/PPC): High occurrence of proximate, caveat, and proximocorate Dinoflagellate is an indicator of shallow sedimentation in this Stratigraphy Section. 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