BIOSTRATIGRAPHIC STUDIES OF THE LOWER CRETACEOUS (UPPER BARREMIAN-LOWER APTIAN) SARCHESHMEH AND SANGANEH FORMATIONS IN THE KOPET DAGH BASIN, NE IRAN: AN INTEGRATION OF CALCAREOUS NANNOFOSSIL AND AMMONITE STRATIGRAPHIES
© 2011 S. N. Raisossadat" and M. H. Shokri4
aGeology Department, Faculty of Sciences, Birjand University, Birjand, Iran
e-mail: snaser_rais@yahoo.co.uk, snraeisosadat@birjand.ac.ir bExploration and Production, National Iranian Oil Company, Tehran, Iran e-mail: shokri502002@yahoo.com Received June 18, 2009; in final form February 18, 2010
Abstract—Lower Cretaceous sediments of the northwestern part of the Kopet Dagh sedimentary basin have been sampled with the purpose to study stratigraphic distribution of calcareous nannofossils. A total of 87 samples from the 1900-m-thick marly limestones, shales and siltstones of the Sarcheshmeh and Sanganeh Formations (late Barremian—early Aptian) displayed diverse nannofossil assemblages. Representative species of the following genera were recorded from the Sarcheshmeh Fm.: Braarudosphaera, Calcicalathina, Calcio-solenia, Chiastozygus, Conusphaera, Cretarhabdus, Cyclagelosphaera, Eprolithus, Haqius, Hayesites, Lithraphidites, Manivitella, Micrantholithus, Nannoconus, Radiolithus, Retecapsa, Rhagodiscus, Rucinolithus, Watznaueria, and Zeugrhabdotus. In the Sanganeh Formation, Biscutum, Broinsonia, Cribrosphaerella, Cru-cicribrum, Cyclagellosphaera, Diazomatolithus, Discorhabdus, Eiffellithus, Lithraphidites, Nannoconus, Pre-discosphaera, Rhagodiscus, Tranolithus, and Watznaueria were found. The identified nannofossil assemblages enabled the recognition of NC5—NC7A zones in the studied part of the section. Paleoecologically, these nannofossil assemblages are typical for the Lower Cretaceous of the Tethyan realm and indicate warm surface water conditions.
Keywords: Lower Cretaceous, nannofossils, ammonites, Kopet Dagh, Iran.
INTRODUCTION
The Kopet Dagh (Koppeh Dagh) sedimentary basin is situated in the northeastern Iran and southern Turkmenistan. The Iranian part of the Kopet Dagh Basin is geographically located between 54°00' to 61°14'E and 36°00' to 38°16'N (Fig. 1).
The Kopet Dagh Basin was formed as an intracon-tinental basin in the northeastern Iran after the closure of Hercynian Ocean that followed Early Kimmerian orogeny (Berberian and King, 1981). From the Jurassic through Eocene, relatively continuous sedimentary succession forms five major transgressive—regressive sequences in the eastern Kopet Dagh (Afshar-Harb, 1979, 1983). A fault-controlled subsidence of the Kopet Dagh Basin started in the late mid-Jurassic (Af-shar-Harb, 1979; Seyed-Emami and Alavi-Naini, 1990; Seyed-Emami et al., 1994, 1996) and terminated in the Oligocene causing the deposition of up to 10-km-thick sedimentary succession (Berberian and King, 1981) (Fig. 2).
The Cretaceous deposits are divided into nine formations in the Kopet Dagh Mts. (Fig. 3). A single me-gasequence in the Kopet Dagh Basin embraces the whole Lower Cretaceous succession which is made up
of conglomerates and sandstones of the Shurijeh Fm. in the lower part and of dark grey shales and siltstones of the Sanganeh Fm. in the upper part (Fig. 3).
Detailed geological studies were conducted by geologists of the National Iranian Oil Company (NIOC) during the 1960s and 1970s. The most important publications were made by Afshar-Harb (1969, 1979) on general geology and petroleum geology, Kalantari (1969) on the Jurassic and Cretaceous foraminifers, and by Hubber (1976) and Afshar-Harb (1982, 1983) as to geological mapping.
Despite recent lithostratigraphic, biostratigraphic and paleontological study of the Sarcheshmeh and Sanganeh Fms. in the Kopet Dagh Basin (Dehghan, 2002; Hadavi and Shokri, 2001, 2006; Hadavi and Badaghi, 2006; Immel et al., 1997; Raisossadat, 1999, 2000, 2001, 2002, 2003a, 2003b, 2004, 2006; Raisossadat and Moussavi-Harami, 1993, 2000; Raisossadat et al., 1998; Shokri, 2001), the research in many fields needs to be continued. For instance, foraminifers and ammonites have been studied in more detail, whereas other fossil groups like calcareous nannofossils have not been considered up to now. In this paper, calcareous nannofossils from the Lower Cretaceous Sanga-
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At., Aitamir Daz., Darreh Gaz Khn., Khangiran Sar., Sarcheshmeh Shu., Shurijeh
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Fig. 1. Geographical names and their position. Location of the measured sections and gas reservoirs are shown by circles and ovals, respectively.
neh Fm. are reported for the first time and the nanno-fossil biostratigraphy and paleoecology of the Sarcheshmeh and Sanganeh Fms. are discussed.
MATERIALS AND METHODS
Two stratigraphic sections have been studied and sampled during the fieldwork in summer 1998 and 1999.
Smear-slides were prepared from 154 samples using gravity settling methods (Bown and Young, 1998). Light-microscope images were carried out from smear-slides using OLYMPUS DP10 digital camera. Scanning electronic microscopy (SEM) images were made on a LEO Gemini 1530 at the Ruhr University, Bochum. The smear-slides are stored in Geology Department of Birjand University, Iran. Bibliographic references for identified taxa are given in Perch-Nielsen (1985) and Bown et al. (1998).
STRATIGRAPHY
Sarcheshmeh Formation
Sarcheshmeh village is situated in the central Ko-pet Dagh, 14 km east of the Bojnord town. Afshar-Harb (1979) proposed to consider a succession exposed in the Khur valley along the Mashhad-Kalat road as a type section of the Sarcheshmeh Fm. It is one
of the most complete and best outcropped successions of the area (Fig. 1).
Normally, the Sarcheshmeh Fm. consists of two informal members: the lower marly and the upper shaly member. In the type section, the lower member is represented by a 178-m-thick uniform light greenish-grey to bluish-grey marl with pencil weathering. A 20-cm-thick bed rich in oyster coquina occurs at the top of this member.
The upper member includes a 98-m-thick dark bluish-grey calcareous shale weathered to light greyish-green color in the lower part, overlain by the 34-m-thick sequence ofal-ternating bluish-grey shale and thinly bedded limestones (Afshar-Harb, 1979; Raisossadat and Moussavi-Harami, 1993) (Figs. 4 and 5).
The sediments are moderately weathered and form a distinct grey rock succession disposed between the underlying ridge-forming brown-weathered Tirgan Fm. and the overlying dark grey to black Sanganeh Fm.
Kalantari (1969) and Raisossadat and Moussavi-Harami (1993) suggested the Aptian age for the Sarcheshmeh Fm. on the basis of foraminifer study. The ammonite fauna found in this formation indicates the late Barremian—early Aptian age (Immel et al., 1997; Raisossadat, 2002, 2003a, 2004). It should be emphasized that the formation shows younger age eastward.
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Fig. 2. Iranian major tectonosedimentary units (redrawn from Berberian and King, 1981): (1) Stable area, Arabian Precambrian platform in southwest and Turanian Hercynian Plate in northeast; (2) Zagros, including Zagros foredeep, main sector of the marginal active fold belt peripheral to stable area and High Zagros; (3) Alborz Mountains; (4) Central Iran lying between the two marginal active fold belts; (5) Talesh, Armenian Late Hercynian belt with a possible continuation to Iranian Talesh Mountain; (6) Zabol-Baluch and Makran post-ophiolite flysch trough; (7) Kopet Dagh folded belt and foredeep.
System Series Stage Formation Lithology
Paleogene Paleocene Pesteligh
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Fossiliferous limestone
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Shaly limestone or silty shale
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Sandy siltstone
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Fig. 3. General stratigraphic column of the Cretaceous of the eastern Kopet Dagh Basin (modified from Kalantari, 1987 and Im-mel et al., 1997).
Sanganeh Formation
The name is derived from the village of Sanganeh, 70 km northeastward from the town of Mashhad (Fig. 1). The Sanganeh Fm. consists of dark grey to
black weathered shales, includes few thin siltstone beds in some areas, and contains abundant septarian nodules and cone-in-cone structures (Raisossadat and Moussavi-Harami, 1993; Afshar-Harb, 1994). It dif-
Fig. 4. Weathered pencil marl of the lower part of the Sarcheshmeh Formation, Sanganeh section, near the Kalat Road section.
Top Sarcheshmeh Fm.
Fig. 5. The Sarcheshmeh—Sanganeh formations' boundary near the Kalat Road section, dark to black shales of the Sanganeh Formation overlying light grey shales and fossil-iferous limestones ofthe Sarcheshmeh Formation, looking north.
Fig. 6. Concretion nodule in the Sanganeh Formation.
fers from the underlying Sarcheshmeh limestone bed and the overlying ridge-forming Ai
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