در مطالعۀ حاضر، سازند گورپی در تاقدیس کوه سیاه واقع در شمالشرقی شهرستان دهدشت (منطقۀ کهگیلویه) ازنظر نانوفسیلهای آهکی بررسی شد. مطالعۀ زیستچینهنگاری نانوفسیلهای آهکی سازند گورپی در برش یادشده با ۲۱۲ متر ضخامت به شناسایی 48 جنس و 115 گونه از نانوفسیلهای آهکی منجر شد. بر اساس نانوفسیلهای آهکی شاخص، بایوزونهای CC15 تا CC25 از بایوزوناسیون (Sissingh 1977) تصحیحشده توسط (Perch-Nielsen 1985) و UC11a تا UC19 از بایوزوناسیون (Burnett 1998) برای این برش تعیین و سن سازند گورپی در این برش کنیاسین پسین تا انتهای مایستریشین پیشین پیشنهاد شد. در مطالعۀ حاضر، مرز اشکوبهای موجود در مقایسه با مقاطع نمونۀ مرزی جهانی (Global Stratotype Section and Point) و سایر برشهای دنیا تعیین شد؛ ازاینرو، مرز اشکوبهای کنیاسین - سانتونین در محدودۀ بین نخستین حضور گونۀ Lucianorhabdus cayeuxii و آخرین حضور گونۀ Lithastrinus septenarius، مرز سانتونین - کامپانین با نخستین حضور گونۀ Aspidolithus parcus و مرز کامپانین - مایستریشین با آخرین حضور گونۀ Quadrum trifidum مشخص شد.
عنوان مقاله [English]
Biostratigraphy of the Gurpi Formation based on Calcareous Nannofossils at the north east of Deh-Dasht area (Izeh Zone)
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.
Material and Methods
In this study a 215-m thick of Cretaceous sediments including the upper part of Illam Formation, Gurpi Formation and the lower part of Pbdeh Formation were recognized. A total of 122 samples with ~ 1 ‒ 2 m sampling interval were collected. Calcareous nannofossils were processed by smear slides technique described by Bown and Young (1998). The slides were studied with a polarized microscope Olympous (BH2) with 1000×magnification. The zonal scheme proposed in this study is based on the ranges of the most important stratigraphically nannofossil taxa. Calcareous nannofossil taxonomy follows Perch-Neilsen (1985) and Bown, (1998). In this study the stratigraphic zonation developed by Sissingh, (1977) modified by Perch-Nielsen (1985) was applied and is compared with, Burnett, (1998) biozones.
Discussion of Result and Conclusion
According to the first and last occurrences of the index species of calcareous nannofossils, Late Coniacian to Early Maastrichtian age were recognized for the sediments of Gurpi formation in this area. The standard zones identified are listed below from the oldest to the youngest:
Reinhardtites anthophorus biozone (CC15) (Equivalent of Subzone UC11a and UC11b), late Coniacian.
Lucianorhabdus cayeuxii biozone (CC16) (Equivalent of Subzone UC11c and UC12), late Coniacian- middle Santonian.
Calculites obscurus biozone (CC17) (Equivalent of Subzone UC12 and UC13), middle Santonian- Santonian Campanian boundary.
Aspidolithus parcus biozone (CC18) (Equivalent of Subzone UC14 a, b, cTP), early Campanian.
Calculites ovalis biozone (CC19) (Equivalent of Subzone UC14dTP and UC15aTP), late early Campanian.
Ceratolithoides aculeus biozone (CC20) (Equivalent of Subzone UC15bTP), late early Campanian.
Quadrum sissinghii biozone (CC21) (Equivalent of Subzone UC15cTP), early late Campanian.
Quadrum trifidum biozone (CC22) (Equivalent of Subzone UC15dTP andUC15eTP), middle late Campanian.
Tranolithus phacelosus biozone (CC23) (Equivalent of Subzone UC16 and UC17), middle late Campanian- early Maastrichtian.
Reinhardtites levis biozone (CC24) (Equivalent of Subzone UC18), middle early Maastrichtian.
Arkhangelskiella cymbiformis biozone (CC25) (Equivalent of Subzone UC19), late early Maastrichtian.
Using of these results and their comparison to other global nanno events, which were reported on upper Cretaceous stage boundaries, can be used to better identify of these boundaries in this section. In this study the Coniacin- Santonian, Sntonian- Campanian and Campanian- Maastrichtian boundaries were identified as follows:
Coniacin/ Santonian boundary: This boundary has been reported as the interval from the FO of M. staurophora to the FO of R. anthophorus by Perch-Nielsen (1985). Bralower et al. (1995) have been shown this boundary between the FO of the M. staurophora to the FO of L. cayeuxii. Burentt (1998), Melinte and Lamolda (2002) and Melinte and Lamolda (2007) have been considered the FOs of R. anthophorus, L. grillii, Micula concav, Lucianorhabdus inflatus and L. cayeuxii in the late Coniacin and the LO of L. septenarius in the Early Santonian. The Coniacin- Santonian boundary (CSB) has been placed between the FO of L. cayeuxii and the LO of L. septenarius, within the CC16 Zone of Sissingh (1977), and in the UC11c sub-zone of Burnett (1998). Also the presence of C. obscures after the FO of L. cayeuxii and higher abundance of M. concava, has been reported the CSB (Lamolda et al. 2014). The first sample of the Kuh-e Siah section contains R. anthophorus, L. grillii, Q. gartneri and M. staurophora. The FO of L. cayeuxii and the LO of L. septenarius has been recorded at 5m and 15 m, respectively. Based on these data, the lowermost part of the Kuh-e Siah section is late Coniacian and the CSB was considered in the Kuh-e Siah section in the UC11c/ CC16/ NC17 zones, between the FO of L. cayeuxii and the LO of L. septenarius.
Santonian/ Campanian boundary:
At informal sections of GSSP (Hancock and Gale, 1996; Hampton et al. 2007)., this boundary is marked by the FO of A. parcus parcus. Two main suggested markers, i.e. the FO of A. parcus parcus and the LO of Dicarinella asymetrica occur closely to the base of Chron 33r which may be suggested as the main events for the base of the Campanian (Wagreich et al. 2015). The Santonian- Campanian boundary is described within the intetrval from the FO of A. cymbiformis (UC13a) to the FO of A. parcus parcus (UC14) (Gale et al., 2008), while Melinte and Bojar, (2010) placed this boundary within the CC17/ UC13 and the LO of A. cymbiformis is located before the boundary. The Santonian- Campanian boundary is located at the base of CC18/UC14 zone, marked by the FO of Broinsonia parca parca, (Wagreich et al., 2010; Cetean et al., 2011; Russo, 2013; Dubicka et al., 2017). In the Kuh-e Siah section, the Santonian- campanin boundary was marked by the FO of A. parcus parcus at the base of the CC18/ UC14/ NC18 at 45 m.
Campanian‒ Maastrichtian bondary: The Campanian ‒ Maastrichtian boundary (CMB) at GSSP is defined below the LO of Q. trifidum and Q. gothicus. The LO of B. parcus constricta is also recorded about 1.8 Ma above the CMB (Gardin et al. 2001) and considered as an indicator of the CMB which is observed above the LO of Q. trifidum and Q. gothicus (Odin and Lamaurelle, 2001). At Gubbio area (Gardin et al. 2012), similar to Tercis-Les-Bains (Gardin et al. 2001), the LOs of Q. gothicus and Q. trifidum are followed by the LOs of A. parcus constrictus and Q. trifidum and differs with the proposed CC and UC zonations. It must be mentioned that the LOs of T. orionatus (Gardin et al. 2012) and A. parcus constrictus (Gardin et al. 2012; do Monte Guerra et al. 2016) are diachronous between different latitudes. Thibault also used the LO of Quadrum trifidum as marker for the CMB at low latitude site. In the Kuh-e Siah section, the Campanian- Maastrichtian boundary is identified by the LO of the Quadrum trifidum.
1- The presented data suggest that the age of sediments of Gurpi Formation in Kuh-e Siah section are late Coniacian to late early Maastrichtian.In kuhe Siah section
2- According to the continuous presence of L.grillii، M. concava و M. staurophora in samples of Illam and Gurpi Formations and the presence of Q .gartneri and R. anthophorus in the first sample of the Gurpi Formation, the Illam and Gurpi formations boundary is conformably.
3- The LO of B. hayi bio- evente has a reverse order in this study relative to biozones was described by Sissingh (1977). Although the LO of B. hayi was recorded in zone CC19 (Sissingh 1977), the LO of this species has been recorded up to the middle part of the CC20. In this section, the LO of Bukryaster hayi was recorded in the CC20. Razmjooei et al., (2014) in the northeast of Kazeroon in Iran (Fars area) and Melinte and Bojar (2010) in southern Romania have also recorded the LO of B. hayi in zone CC20. Based on Melinte and Bojar, (2010), the LO of B. hayi in CC20 is attributed to diachronism or reworking.
4- Due to the presence of Biantholithus sparseus, Fasciculithus tympaniformis and Markalius inversus in the first sample of the Pabdeh Formation and the non-identification of the end zone of the CC25/ UC19 zones in this section, the Gurpi and Pabdeh Formations boundary is unconformably.