بايواستراتيگرافي و پالئواكولوژي سازند فرخي در شمال باختري خور، ايران مركزي با استفاده از پالينومورف‌ها و فرامينيفرها

New finding on the biostratigraphy and paleoecology of the Farrokhi Formation in northwest of Khur, Central Iran, based on palynomorphs and foraminifera

به‌ منظور مطالعه‌هاي چينه‌نگاري سنگي و زيستي سازند غيررسمي فرخي، برش عزيزآباد واقع در شمال باختري شهر خور انتخاب شد. ستبراي سازند فرخي در اين برش 233 متر است، به‌شكل پيوسته روي سنگ‌‌آهك‌هاي سازند هفتومان قرار گرفته و همبري بالايي آن با سازند چوپانان ناپيوسته از نوع موازي است و به چهار بخش غيررسمي تقسيم شده است. درمجموع، 81 نمونۀ سنگي شامل 25 نمونۀ مارني براي مطالعۀ پالينولوژيكي، 17 نمونۀ مارني براي مطالعۀ فرامينيفرهاي ايزوله و 39 نمونۀ سنگي براي تهيۀ مقاطع نازك از اين سازند برداشت شدند. از مجموع نمونه‌هاي آماده‌شده براي مطالعه‌هاي پالينولوژيكي تنها 2 نمونه داراي سيست داينوفلاژله بودند. با مطالعۀ اسلايدهاي پالينولوژيكي، مقاطع نازك و نمونه‌هاي شسته‌شده درنهايت 46 جنس و 71 گونه از داينوفلاژله‌ها، 13 جنس و 40 گونه از روزن‌بران آزاد و 9 جنس و 3 گونه از روزن‌بران بنتيك متعلق به كامپانين مياني- ماستريشتين شناسايي شدند. همچنين با مطالعه و شمارش عناصر پالينولوژيكي مربوط به 2 نمونۀ مدنظر تنها يك پالينوفاسيس (IV) تشخيص داده شد كه نشان‌دهندۀ محيط دريايي باز و كم‌عمق براي اين بخش از سازند و بيان‌كنندۀ يك زون انتقالي به بخش‌هاي عميق‌تر حوضه است. بر ‌پايۀ داينوفلاژله‌هاي شاخص محيط، محيط گرم و به‌نسبت كم‌ژرفا با انرژي متوسط براي اين قسمت از بخش مارني سازند فرخي در زمان انبايش رسوبات پيشنهاد مي‌شود.

Upper Cretaceous deposits of Iran do not have the same facial features everywhere. Indeed it seems that contrary to the same conditions of the lower Cretaceous the upper Cretaceous sedimentary basins were separated from each other and had a special condition on each basin accordingly, except for the Zagros and Koppeh-Dagh they have not been officially named (Aghanabati, 2006). Several sedimentary facies have been reported to be existing during the Cretaceous period. The sequences formed in these basins in different regions, although are similar in terms of fossil content but, they are very different in terms of petrography, thickness and deposition time. Also, Cretaceous rocks are diverse in Central Iran, with varying thickness and age, and have not been officially named yet and are generally investigated. Since the most complete Cretaceous sequence in Central Iran was in the Yazd block and in the Khur area, we decided that the Farrokhi formation (Aistove et al., 1984), which is the youngest Cretaceous rock unit in the Khur region, should be studied. The Farrokhi formation is one of the Upper Cretaceous rocky units, with a variable thickness of 65 to 180 meters (Aistove et al., 1984), and made up of limestone, limestone with rubble or strips which is characterized by abundance of echinoid and bivalves. The lowest part of this formation is reportedly 45 to 120 meters of marl with sandstone layers. Based on the brachiopods, bivalves and echinoid, the formation has been aged as Senonian-Danian (Aistove et al., 1984). Therefore, in order to complete the stratigraphic and paleontological data, the Farrokhi formation is selected and studied. The formation is the youngest Cretaceous rock unit in the Khur, Anarak and Jandag areas, which in most areas there is a marl body in its base. The Farrokhi formation in this section is 233 meters thick and is placed continuously on limestones of Haftuman formation while, it upper contact with the limestones of the Chupanan formation is conformable. The formation has been divided into four informal divisions.