پالينواستراتيگرافي سازند سنگانه در شرق و مركز حوضه‌رسوبي كپه داغ بر مبناي سيست داينوفلاژله‌ها

Palynostratigraphy of the Sanganeh Formation at the East and Centre of Kopeh-Dagh Basin based on dinoflagellate cysts

سازند سنگانه با سن كرتاسه زيرين كه در سرتاسر حوضه‌رسوبي كپه داغ رخنمون دارد، به طور پيوسته بين دو سازند سرچشمه و آيتامير قرار گرفته است. اين سازند در شرق و مركز حوضه از شيل‌هاي تيره با ميان لايه‌هاي آهك و سيلت تشكيل شده است. براي بررسي داينوفلاژله‌هاي اين سازند، چهار برش چينه‌شناسي از شرق به سمت مركز شامل برش‌هاي كاريزك، مزدوران، سنگانه و قره‌سو انتخاب شدند. ضخامت برش‌هاي انتخاب شده به ترتيب، 320، 355، 550 و 580 متر است. از تعداد 370 نمونه برداشت شده از اين چهار برش، دو برش كاريزك و مزدوران به طور كلي فاقد داينوسيست‌هاي شاخص براي تعيين سن بودند. داينوسيست‌هاي شناسايي شده در دو برش سنگانه و قره‌سو به تفكيك دو زون DZ1 و DZ2 در اين برش‌ها منجر شد. داينوزون‌هاي تفكيك شده منطبق بر زون‌هاي Odontochitina operculata و Pseudoceratium turneri بوده و معرف سن آپتين پسين- آلبين آغازين براي سازند سنگانه در برش‌هاي مذكور است. همچنين توزيع و پراكندگي داينوفلاژله‌ها، افزايش عمق حوضه از شرق به مركز را بيان مي‌كند.

Introduction The Kopeh-Dagh basin is located in northeast of Iran with WNW-ESE trend. This sedimentary basin contains a great gas field which is in common with Iran, Turkistan and Afghanistan. The recent topography and morphology has formed during the Alpine tectonic phase (Aghanabati, 2004). This basin is especially interesting for geologists because it is located between the two super continents of Eurasia and Gondowana. The Sanganeh Formation is one of the Early Cretaceous rock units in the Kopet-Dagh basin (northeast of Iran). The studied formation conformably overly the Aptian Sarcheshmeh Formation and is in turn overlain conformably by glauconitic sandstone of the Aitamir Formation. The considered Formation in eastern and central parts of the basin comprises predominantly dark shale with some interbeds of limestone and siltstone. In order to analyse dinoflagellate content of this rock unit, four stratigraphic sections from east to the centre of the basin including Karizak (320m), Mozduran (355m), Sanganeh (550m), and Qare-Su (580m) were measured and sampled palynologically. Material and Methods A total of 370 rock samples were taken from the four sections and processed palynologically and scanned under a light microscope. The samples were processed for palynomorphs based on Traverse (2007) technique. After washing and drying, the sediment samples were prepared for palynological analysis following standard extraction techniques (Travers, 2007) involving hydrochloric (HCl) and hydrofluoric acid (HF) treatment in order to dissolve carbonate and siliceous contents. The concentration of palynomorphs was filtered by decantation over a 10 µm nylon mesh sieve, and the samples were centrifuged to concentrate the residues. The residues were not oxidized or stained. Three to five slides were prepared from each sample. The slides were investigated under a light microscope with magnifications of 200× -1000×, to identify and count the palynomorphs. Discussion of Results and Conclusions Dinoflagellate cysts are helpful to age determination and biostratigraphic studies. They are also a useful tool for interpreting paleoenvironments. Dinoflagellate cysts are good index for paleoproductivity of the surface and oxygen changes at the bottom of the sea water. Diversity of living dinoflagellates is under control of the nutrient availability, temperature, vertical stratification, salinity and initial environmental stress. Of these sections, two sections including Karizak and Mozduran were barren of any preserved dinocysts while, the samples from other two sections (Sanganeh and Qarah-Su section) yielded a relatively diverse and well preserved assemblage. The absence of dinocysts may be due to disruption maintain organic matter distribution in sedimentary surface, diagenesis and original lack of dinoflagellate cysts in the open marine environment. According to the recognized dinocysts, two zones DZ1 and DZ2 were established and differentiated in Sanganeh and Qarah-Su sections. The differentiated dinozones were coincident to Odontochitina operculata and Pseudoceratium turneri which are indexes for Late Aptian-Early Albian age for the Sanganeh Formation at the mentioned stratigraphic sections. DZ1. This dinozone corresponds to the total-range occurrence of the dinoflagellate cyst species Odontochitina operculata, which is considered to be diagnostic of it (Evans, 1966). It encompasses the interval from the base to 230m (for Sanganeh section) and 330 m (for Qarah-su Section). DZ2. This dinozone corresponds to the total-range occurrence of the dinoflagellate cyst species Pseudoceratium turneri with an early Albian age (Morgan, 1980). This biozones encompassed the interval from 230m (for Sanganeh section) and 330m (Qarah-su section) to the top of the formation. At the Sanganeh section, Spiniferites species was recorded abundant; and Florentinia sp., Subtilisphaera sp. and Oligosphaeridium sp. are in the next in the slides. Thick-walled cysts and cavate forms are shown high energy and shallow marine. Chorate forms prefer to live in open marine environments with low energy conditions. In addition, distribution and dispersion of dinoflagellate cyst contents indicate a deepening trend of from east to the central part. Keywords: Sanganeh Formation, East and Centrl of Kopeh-Dagh, Dinoflagellate cysts, Early Cretaceous