رخساره‎‍های پالینولوژیکی و دیرینه‎‍‎‍بوم‎‍شناسی سازند سنگانه در شرق و مرکز حوضۀ رسوبی کپه‎‍داغ

به‌منظور شناسایی رخساره‎‍های پالینولوژیکی و بازسازی محیط‎‍‎‍های دیرینه، در این مطالعه یکی از مهم‌ترین سازندهای شیلی کرتاسۀ زیرین در حوضۀ رسوبی کپه‎‍داغ، نمونه‌برداری و بررسی شده است. به ‌این منظور، تعداد چهار برش چینه‎‍شناسی از سازند سنگانه برای تطابق و مقایسه انتخاب شده است. برش‎‍ها از شرق به غرب شامل کاریزک، مزدوران، سنگانه و قره‌سو است. گروه‎‍های پالینومورف‎‍های دریایی، فیتوکلست‎‍ها، مواد آلی بی‎‍شکل، نسبت مواد آلی بی‎‍شکل شفاف به تیره، نسبت فیتوکلست قهوه‎‍ای به تیره، نسبت سیست‎‍های پریدینوئید به گونیالاکوئید، فراوانی و تنوع پالینومورف‎‍های دریایی و داینوفلاژله‎‍های شاخص شمارش و بررسی شده‎‍ است. در این راستا، چهار رخسارۀ پالینولوژیکی در برش‎‍های کاریزک و مزدوران شناسایی شد. براساس رخساره‎‍های پالینولوژیکی شناسایی‎‍شده و با توجه ‌به ترسیم نمودارها، افزایش میزان فیتوکلست‎‍ها و کاهش میزان پالینومورف‎‍های دریایی به‌سمت شرق مشاهده شد. این مسئله بیانگر روند افزایش عمق و کاهش میزان اکسیژن در سازند سنگانه از شرق حوضۀ رسوبی کپه‎‍داغ به‌سمت غرب است که با عدم حضور گروه پریدینیوئید، در برش قره‌سو می‌توان تطبیق داد. میزان رسوب‌گذاری سازند در منطقۀ موردمطالعه، کم در نظر گرفته شده است.

Palynofacies and Palaeoenvironment of the Sanganeh Formation, East and the Center of the KopetDagh Basin

AbstractIn order to identify the palynofacies and to reconstruct the paleoenvironment of the Lower Cretaceous depositions in the KopetDagh sedimentary basin, the Sanganeh Formation was sampled and studied. The four sections of the Sanganeh Formation were selected for correlation and comparison in the east and the center of the Kopet Dagh Basin, respectively included Karizak, Mozduran, Sanganeh and QarahSu. The study of slides was done with an optical microscope. Marine palynomorphs groups, phytoclasts, amorphous organic matter, transparent to opaque amorphous organic matter ratio, brown to opaque phytoclast ratio, Peridinioid to Gonyaulacoid dinocyst ratio, abundance, and diversity of marine palynomorphs and dinoflagellates were counted and investigated.  In this study, a quantitative analysis of the main palynologic elements (AOM, marine palynomorph, phytoclast) led to characterize four palynofacies types that were identified in the Karizak and Mozduran sections. Based on the palynofacies analyses and according to the plotting to Tyson diagram, we have recorded an increase in the number of phytoclasts and a decrease in the number of marine palynomorphs to the east indicating a decrease in the depth and oxygen trend in the Sanganeh Formation in these part of the KopetDagh sedimentary Basin. The sedimentation rate of the Sanganeh Formation was also considered low in the studied sections.Keywords: Palynofacies, Dinoflagellate, KopetDagh, Sanganeh Formation, Lower Cretaceous  IntroductionAccording to Tyson (1993, 1995) and Batten (1996b), identification of palynofacies plays an essential role in determining depositional environments as well as studying the petroleum potential in sediments. The components of palynologic slides, which are used to determine palynofacies and paleoenvironmental interpretation, can be grouped into three groups: amorphous organic matter (AOM), marine palynomorphs (MP), and phytoclasts (P).  Due to the Sanganeh Formation lithology, paleontologists have been interested in working the shale samples. Several studies have been done to determine the age and identify the paleoenvironment on this formation in the KopetDagh basin based on different fossil groups including foraminifera, calcareous nannofossil, ammonite, and palynomorph. Four stratigraphic sections have been selected from the Sanganeh Formation in the KopetDagh Basin. The studied sections from the east to the center of the Basin are included Karizak, Mozduran, Sanganeh and QarahSu, respectively with a thickness of 320, 355, 550, and 580 meters. In the studied sections, the lower and upper boundaries of the Sanganeh Formation are continuance with Sarcheshmeh and Aitamir formations. The aim of this study was to identify the palynofacies and the paleoenvironment of the Sanganeh Formation based on the counting and frequency of palynological content in the studied sections. Material & Methods    A total of 370 samples were taken from the Sanganeh Formation in four sections, (Karizak, Mozduran, Sanganeh, and QarahSu). Samples were prepared based on the acid treating technique (HCl/HF) (Travers 2007). In this standard method, dry sediment samples were degraded with 30% hydrochloric acid (HCl) and 40% hydrofluoric acid (HF) to extract acidresistant organic matter. These acids had been added to remove carbonates and silicates materials. In the following steps, after sieving (10 µm mesh), the samples were centrifuged with heavy liquid zinc chloride (ZnCl2), and the residual material was finally transferred to 3–5 lamellas for palynological studies. It should be noted that the samples were not oxidized. Discussion of Results & Conclusions  Based on the relative frequency of three palynological matter categories, three types of palynofacies are recognized at the QarahSu section,and four types of palynofacies at the Karizak and Mozduran sections. According to these plots, we could subdivide the Sanganeh Formation into two parts in the studied sections. The first part is characterized by abundant phytoclast matter indicating a highly proximal dysoxicanoxic basin.while the second part of the studied succession shows a change in organic matter in the samples composed of AOM and marine palynomorphs. This part is indicated by the proximal suboxicanoxic shelf. The depositional depth showed an increase in this part. However, an exception could be observed in the Karizak section so there could be a third part and we could have observed a decrease in the depositional depth again.The relative abundance of marine palynomorphs, phytoclasts, and amorphous organic matter, the ratio of transparent/opaque amorphous organic matter, brown/opaque phytoclast and Peridinioid/Gonyaulacoid dinocyst, and also the density and diversity of marine palynomorphs and dinoflagellates were considered at this study. The ratio of the  transparent AOM to opaque AOM indicates low oxygen conditions have fluctuated over time. The absence of dinocysts at the base of succession may be due to the lack of preservation of distributed organic matter at the sediment surface, diagenesis, or the initial lack of production of dinoflagellate cysts in the marine environments (Ogg 1992). The upward trend in the peridinioid group is also considered to be nearcoastal or transitional environments (Habib and Miller 1989).The distribution of palynological data on the Tyson diagram indicates the relative high amount of phytoclasts for this formation, especially in the eastern sections of the KopetDagh Basin. According to the diagrams, the shallowing trend to the east can be observed, clearly in the Karizak section. Therefore, the depositional environment of the Sanganeh Formation can be considered as a shallow, coastal, and low oxygen environment in the eastern part of the study area. Towards the center of the Basin, the amount of marine palynomorphs has increased which indicates deeper and distal coast, and a relatively lack of oxygen.The paleoenvironmental and palynological elements analysis of the Sanganeh Formation in the studied area generally show the following results:Comparison of the three main groups of palynological components (phytoclasts, amorphous organic matter, and marine palynomorphs) in the studied sections shows that the amount of phytoclasts in all sections is almost high. The amount of amorphous organic matter and marine palynomorphs are in the next degree of abundance. Due to the relative amount of these three groups, the base of the Sanganeh Formation has shown the shallow marine deposit and proximal setting. In the middle part, the amount of organic matter is increased and the amount of phytoclasts is reduced. The amount of marine palynomorphs also shows a relative increase in this part.Therefore, the increase in amorphous organic matters, the presence of marine palynomorphs, and the relative decrease of phytoclasts, palaeoenvironment of the sequences can be considered mud dominate distal shelf. The upper part of the studied sections, which can be seen just only in the Karizak section, indicates a decrease in depth and nearshore environments, accompanied by an increase in phytoclasts.The Karizak section, which is the most eastern section, has the lowest marine palynomorphs and the highest phytoclasts. The Mozduran section, which is the second section from the east shows  an increase in the marine palynomorph and AOM. Finally, marine palynomorphs and AOM are increased in the Sanganeh and QarahSu sections,indicating an increase in depth and a decrease in oxygen to the center of the KopetDagh basin.