بررسی نوسانات سطح نسبی آب دریا طی نهشته‌شدن سازند گورپی برمبنای داده‌های پالینولوژیکی در غرب دهلران

به‌منظور بررسی محیط رسوبی سازند گورپی در برش بیشه‌دراز در جنوب استان ایلام از عوامل مختلفی ازجمله تنوع و فراوانی داینوسیست‌ها، نسبت داینوسیست‌های پریدینوئید به گونیالاکوئید (p/g)، مطالعات پالینوفاسیسی و فاکتورهای حفاظت از مواد آلی استفاده شد. از ابتدای سازند گورپی به سن کمپانین تا لایه‌های شیلی قسمت فوقانی بخش آهکی سیمره (لوفا) شامل دوکفه‌ای‌های ریز و خرده‌های خشکی، شرایط پس‌رونده را شاهد هستیم. بخش آهکی سیمره خرده‌های ماکروفسیلی در زمینه‌ای از رسوبات تخریبی سست و در جهات مختلف به‌صورت خردشده و در لایه مارنی به‌هم‌خورده دارد که احتمالاً این عضو آهکی نابرجا بوده و روی سطح بالاآمدگی حوضه نهشته شده است. افزایش سطح نسبی آب در ابتدای ماستریشتین باعث ایجاد شرایط پایدار برای حفظ‌شدگی مواد ارگانیکی می‌شود. روی آن عضو آهکی امام حسن با درصد زیادی از مواد آمورف تیره‌رنگ قرار می‌گیرد که نشان‌دهنده محیطی عمیق است. این عضو با مارن‌های انتهای ماستریشتین پوشیده می‌شود. بررسی‌های فسیل‌شناسی، رسوب‌شناسی و مشاهدات صحرایی در این پژوهش، وقفه‌ای رسوبی را در گذر از مرز کرتاسه– پالئوژن در این برش نشان می‌دهد. پس از مرز برمبنای شواهد لیتولوژیکی و پالینولوژیکی تا شیل‌های ارغوانی‌رنگ سازند پابده، پیشروی و بالاآمدگی سطح آب دیده می‌شود. انطباق نسبتاً خوب منحنی تغییرات سطح آب با منحنی‌های تغییرات جهانی سطح آب حاکی از تاثیرپذیری زیاد این نهشته‌ها از تغییرات ائوستازی است.

Study of waterlevel fluctuations throughout the deposition of the Gurpi Formation based on palynological data in the west of Dehrolan, south of Ilam

Abstract To analyze the sedimentary palaeoenvironment of the Gurpi Formation in Bisheh–Deraz section in south of Ilam, various factors including diversity and abundance of dinocysts, a ratio of Peridinioid to Gonyaulacoid (P/G) dinoflagellate cysts, palynofacies parameters and selective preservation of organic matter have been used. In lower parts of the Gurpi Formation of Campanian age is very argillaceous and then shallows up in the upper part of Lopha Limestone that contains the small size of bivalve shell debris. This is followed by a stagnant condition due to sealevel rise that favored the preservation of organic matter in early Maastrichtian. The EmamHassan Limestone with high percentages of nonfluorescent amorphous organic matter represents a basin environment. The Latter member is overlain by the marls of Late Maastrichtian. Microbiostratigraphy, sedimentology, and palynofacies observation indicate sealevel fall and a 3.5 Ma hiatus at the K/P boundary. This is followed by deepening and the deposition of the basal purple shale of the Pabdeh Formation. The relative sealevel changes curve drawn based on palynomorphs in this study is well in accord with the existing eustasy curves that indicate the great effect of eustasy factor relative to other factors. Key Words: Sea level change, Palynology, Gurpi Formation, South of Ilam, Zagros   Introduction The reconstruction of water depth is an important factor to explain the palaeoecology/palaeoenvironment. Qualitative and Quantitative palynological analysis provides a highresolution database for biostratigraphy and determines the main palaeoenvironment parameters, these parameters include diversity and abundance of dinocysts, a ratio of Peridinioid to Gonyaulacoid (P/G) dinoflagellate cysts, palynofacies parameters and selective the preservation of organic matter factors. The Gurpi Formation in Bisheh–Deraz section in south of Ilam with a thickness of 201 meters consisting of an alternation of blueishgray shales and marls, with two main members of Seymareh (Lopha) and EmamHassan limestones. The lower boundary with the Ilam Formation is continuous and without any erosion and its upper boundary with the Pabdeh Formation is gradual. The Bisheh–Deraz section, with grid coordinates of N 32° 49´ 52˝ and E 46° 59´ 01˝, is located in the southwest flank of Anaran Anticline in the west of Dehloran.  The main aim of this paper is the study of palynofacies analysis and paleontological data (foraminifera and index dinocyst) to elucidate sedimentary environments and sealevel changes of the Gurpi Formation.   Material & Methods The present study is based on seventyfour samples that were collected from the Gurpi Formation. The samples were processed by standard (HClHF) palynological preparation technique, without oxidation or ultrasonic treatments. The palynological residue was sieved through 20 μm nylon sieves. Three to five permanent slides were prepared using glycerin jelly as a mounting medium. The palynofacies analysis is based on the percentage frequency of different POM constituents categorized by Tyson (1995).   Discussion of Results & Conclusions      The changes in the palynofacies composition in the Bishederaz section and the AOMpalynomorphphytoclast (APP) ternary plot reveal seven (II, IV, IX, V, VI, VII, VIII) palynofacies types in the Gurpi Formation. The percentage of each palynomaceral component is derived from the total POM frequencies. However, abundance (the number of all dinocyst taxa counted from two slides) and a simple diversity index (the number of species of dinocyst counted from two slides) and the ratio of Peridinioid to Gonyaulacoid (P/G) dinoflagellate cysts, were obtained from the total dinoflagellate cysts frequencies. The difference in the organic matter found in these regressive and transgressive offshore marine sediments have been documented and assessed within a sealevel change. The lower part of the Gurpi Formation of middle Campanian age is very argillaceous and is characterized by an abundance of fluorescence AOM and marine palynomorphs. The minimal size of the black equant phytoclasts and dinoflagellate cyst show that this interval was deposited in a quiet water environment under anoxic conditions in a pelagic environment.  This formation shallows upward to boundstones of the Lopha Member in uppermost Campanian with very low amounts of organic matter. The Lopha Limestone consists of macrofossil assemblages (bivalves, oysters (Exogyra type) and echinoids) in different directions and recessive, bioturbated marly layers with scattered large Lopha in life position showing that this member was probably reworked and deposited on the foreland bulge depocenter.  This is followed by an abrupt deepening and the deposition of pelagic marl. The high contents of transparent AOM, as well as the presence of pyrite and phosphates in these layers, suggest a dysoxic to the anoxic condition due to sealevel rise that favored accumulation and preservation of organic matter in early Maastrichtian. The EmamHassan Limestone represents a more pelagic fauna in a carbonaterich deepwater setting. The high percentages of nonfluorescent AOM and black phytoclast indicate oxic conditions that were unsuitable for the preservation of marine organic matter. This member is overlain by the marls of Late Maastrichtian. Microbiostratigraphy, sedimentology, and palynofacies observation indicate sealevel fall and a hiatus at the K/P boundary. This is followed by deepening and the deposition of the purple shale of the base of the Pabdeh Formation. This interpretation is supported by sedimentological observation.  The Gurpi Formation is entirely influenced by the compressional regime of the foreland basin.