زيست‌چينه‌نگاري سازند گورپي بر اساس نانو فسيل‌هاي آهكي در شمال‌ شرق شهرستان دهدشت (زون ايذه)

Biostratigraphy of the Gurpi Formation based on Calcareous Nannofossils at the north east of Deh-Dasht area (Izeh Zone)

در مطالعۀ حاضر، سازند گورپي در تاقديس كوه سياه واقع در شمال‌شرقي شهرستان دهدشت (منطقۀ كهگيلويه) ازنظر نانوفسيل‌هاي آهكي بررسي شد. مطالعۀ زيست‌چينه‌نگاري نانوفسيل‌هاي آهكي سازند گورپي در برش يادشده با 212 متر ضخامت به شناسايي 48 جنس و 115 گونه از نانوفسيل‌هاي آهكي منجر شد. بر اساس نانوفسيل‌هاي آهكي شاخص، بايوزون‌هاي CC15 تا CC25 از بايوزوناسيون (Sissingh 1977) تصحيح‌شده توسط (Perch-Nielsen 1985) و UC11a تا UC19 از بايوزوناسيون (Burnett 1998) براي اين برش تعيين و سن سازند گورپي در اين برش كنياسين پسين تا انتهاي مايستريشين پيشين پيشنهاد شد. در مطالعۀ حاضر، مرز اشكوب‌هاي موجود در مقايسه با مقاطع نمونۀ مرزي جهاني (Global Stratotype Section and Point) و ساير برش‌هاي دنيا تعيين شد؛ ازاين‌رو، مرز اشكوب‌هاي كنياسين - سانتونين در محدودۀ بين نخستين حضور گونۀ Lucianorhabdus cayeuxii و آخرين حضور گونۀ Lithastrinus septenarius، مرز سانتونين - كامپانين با نخستين حضور گونۀ Aspidolithus parcus و مرز كامپانين - مايستريشين با آخرين حضور گونۀ Quadrum trifidum مشخص شد.

The Zagros mountain belt extends over 1800 km from Kurdistan in N Iraq to the Strait of Hormuz in Iran (Lacombe and Mouthereau 2006). Zagros Mountain belt is one of the richest oil provinces in the world (Emmami 2008) and is important to paleontological, sedimentological, structural and geological studies. The Cretaceous sediments in the Zagros Mountain Belt (SW Iran), is characterized by a large diversity of rocks and facies. The Gurpi Formation is one of the source rock for petroleum in Zagros basin and outcrops extensively to southwest Iran (James and Wynd 1965). The type section of this Formation (49° 13´ 47″ E, 32° 26´ 50″ N) is measured at Tang-e Pabdeh, north of the Lali oilfield in Khuzestan (James and Wynd 1965). This section is composed of 320 meters of dark bluish gray, thin-bedded marl and shale beds and occasionally thin beds of marly limestone (Motiei 1995). The following publications have appeared for this formation (e.g. Kalantari 1976; Vaziri Moghaddam 2002; Ghasemi-Nejad et al. 2006; Darvishzad et al. 2007; Bahrami 2009; Rabbani et al. 2009; Bahrami and Parvaneh-Nejad Shirazi 2010; Bieranvand and Ghasemi-Nejad 2013; Parvaneh-Nejad Shirazi et al. 2013; Beiranvand et al. 2013, 2014 and Fereydoonpour et al. 2014). Late Cretaceous calcareous nannofossils study is still scarce in Zagros basin and is a strong motivation for investigating calcareous nannofossils in southwest Iran. Late Cretaceous calcareous nannofossils in Gurpi Formation were studied by (Senemari and Azizi 2012; Badri and Kani 2014; Razmjooei et al. 2014; Najafpour et al. 2015; Senemari 2015 and Hadavi et al. 2016). The aim of the present study was to establish the biostratigraphic zonation for the Gurpi Formation and determine age range of this formation in Izeh sub- zone by means of calcareous nannofossils. A 215-m-thick, well-exposed section was chosen for this purpose. The Kuh-e Siah section is located about 15 Km to the north of Deh-Dasht area. This section was measured in detail at 30°54'31.86˝ N and 50°36΄38.80˝ E. Lithologically, it consists of shale, shaly limestone, and shales with thin bedded limestone. It conformably overlies the Ilam Formation and is overlain unconformably by the pabdeh Formation.