An integrated facies, diagenesis and geochemical analysis along with sequence stratigraphy of the Lower Triassic Aghe-Darband basin (north-east Iran)

https://doi.org/10.1016/j.jafrearsci.2020.103952Get rights and content

Highlights

  • The Sefid Kuh Formation, as one of the complex carbonate formations of the Middle East.

  • Depositional environment indicates the a homoclinal-ramp in this formation.

  • Four 3rd order depositional sequences determined in this formation.

  • Geochemical analysis shows open diagenetic system and high water-rock interactions.

Abstract

The Sefid Kuh Formation (Lower Triassic), in Aghe-Darband tectonic window (NE of Iran) reflects a complex setting as is deducible from intercalation of carbonates with volcanic deposits. A detailed sediment petrological, sequence stratigraphical and geochemical study of this formation was carried out to develop a comprehensive knowledge of the complexity in this formation. Field observations of four stratigraphic sections complemented by sediment petrographical analysis led to the identification of microfacies that are grouped into four facies associations. The predominance of tidal flat facies along with oolitic shoal facies and absence of any large barrier/reef features supports the existence of a homoclinal-ramp in an arc basin setting during Sefid Kuh Formation deposition. Calcite cementation and dolomitization are the main diagenetic processes that affected the original strata. Stratigraphic and field studies revealed four 3rd order depositional sequences in this formation which architecture as reflects tectonic control. The measured elemental concentrations (Mn, Sr, Na, Fe, Mg and Ca) along with petrographic studies (e.g. existence of fibrous isopachous cement) suggest aragonite as a precursor mineralogy of the Sefid Kuh carbonates that were deposited in a shallow warm-water subtropical environment. Detailed petrographic and geochemical analysis reflect marine, meteoric and burial diagenetic overprint. Variations in Sr/Ca ratio suggest that diagenetic alteration occurred in an open diagenetic system, with high rock-water interaction.

Introduction

Sedimentation and structural development during the Triassic in Iran relates to the opening of the Neotethys Ocean from Late Permian onwards, and the northward drift of the Iran plate away from the Arabian Plate (Gondwana) as well as the collision and accretion to the southern active margin of the Turan Plate (Eurasia) (Seyed-Emami, 2003). The Aghe-Darband tectonic window is located along the southern margin of the Turan Plate located in northeastern Iran. The latter shows Eo-Cimmerian structural features that are not disturbed by subsequent deformation, allowing to study a small section of the Eo-Cimmerian structural pattern at the southern edge of Laurasia (Ruttner, 1993; Zanchi et al., 2016). This window, which is located at the eastern Kopet-Dagh Range, has been first studied in 1956 (Goldschmid, 1956). Analysis of the Aghe-Darband strata have been the subject of extensive field studies, mapping and laboratory investigations (Seyed-Emami, 1971; Ruttner, 1983, 1984, 1988, 1991a,b, 1993; Baud and Stämpfli, 1989; Baud et al., 1991a,b; Boersma and van Konijnenburg-Van Crittert, 1991; Eftekharnezhad and Behroozi, 1991; Krystyn and Tatzreiter, 1991; Afshar-Harb, 1994a,b; Alavi et al., 1997; Zanchi et al., 2016; Balini et al., 2019). The Triassic sequence of this structural zone comprises four formations, i.e. Sefid Kuh, Nazarkardeh, Sina and Miankuhi Formations that altogether form the Aghe-Darband Group (Aghanabati, 2009). The Sefid Kuh Formation in the Triassic Aghe-Darband successions have not yet been studied from a basin analysis perspective. Only a few scattered investigations carried out in the 1970s and 1990s, 2006 and 2019 exist. The formation age ranges from Olenekian to the Middle Anisian age (ca. ~249.8–244.2 Ma) (Ogg et al., 2016) Balini et al. (2019) corresponding to the Lower Triassic strata in the Aghe-Darband window.

Of key importance is that the Sefid Kuh Formation is time-equivalent to the Kangan Formation (Aghanabati, 2009), which is regarded as one of the main carbonate petroleum reservoir units in Zagros Basin. Moreover, stratigraphically, the Sefid Kuh Formation is equivalent to the Elika Formations in Alborz, Sorkh shales and Shotori dolomites in Central Iran, Khaneh Kat carbonates in High Zagros and members 1 and 2 of the Alam Formation, exposed far away in the Nakhlak area (Fig. 1) (Ruttner, 1993).

In the present study four sections in the eastern Kopet-Dagh Range (Aghe-Darband window) were selected with the aim to examine the Sefid Kuh Formation. The aims of this study are to: (1) evaluate the depositional setting along with microfacies of this formation; (2) determine the sea level fluctuations during deposition of the Sefid Kuh Formation; (3) study of the diagenetic processes and (4) analyse the sedimentary geochemistry of major (Ca, Mg) and minor elements (Mn, Fe, Sr, Na) as key to address the original carbonate mineralogy of the Sefid Kuh Formation in the Kale Faqir section as well as its diagenetic overprint to study the relationship of the element variations within a sequence stratigraphic context.

Section snippets

Geological setting

The Kopet-Dagh Range, which stretches over nearly 700 km in a WNW–ESE direction east of the Caspian Sea of Iran (Davoudzadeh and Schmidt, 1984; Foroughi et al., 2017) and into Afghanistan, NE of the Palaeotethys suture (Fig. 2A) (Berberian and King, 1981; Ruttner, 1991a; Alavi et al., 1997; Buryakovsky et al., 2001; Ezampanah et al., 2018a,b), is caused by the convergence of the Iranian and Turanian Plates (Stöcklin, 1974; Sengör, 1984; Ruttner, 1993; Alavi et al., 1997; Wilmsen et al., 2009;

Stratigraphy

Triassic successions in Iran are exposed in Jolfa, Central Iran, Zagros, Abadeh, Nakhlak and the tectonic window of Aghe-Darband (Fig. 3A) (Berberian and King, 1981; Ruttner, 1991a; Alavi et al., 1997; Buryakovsky et al., 2001; Davoudzadeh and Schmidt, 1984) in NE Iran (Fig. 3B). The 1500 m thick Triassic succession of Aghe-Darband window consists, in ascending order, of the Olenekian to the Middle Anisian Sefid Kuh Formation (white, micritic limestones with andesitic pyroclastic rocks at the

Materials and methods

About 180 hand specimens were collected in a systematic way (with 2 m spacing) after which thin sections were prepared by the National Iranian Oil Company (NIOC) (i.e. 121 thin section from the Kale Faqir section, 25 thin sections from the Kale Angur section, 11 thin section from Kale Anabeh and 23 thin sections from the Kale Ghalak section). The latter were examined by conventional petrographic microscopy. In order to distinguish between calcite, dolomite, and their ferroan counterparts,

Facies

The general lithofacies as well as the stratigraphic relationship of continental and marginal marine strata as well as 180 thin section belonging four stratigraphy sections in the Sefid Kuh Formation revealed four facies association (FA), tempestites and petrofacies in this formation and discussed below.

FA-1: tidal flat setting

The Sefid Kuh Formation deposits are characterized by a variety of fabrics displaying fenestral (birds eye), stromatolite textures and also clotted fabrics in relation to microbial mats as well

Discussion

When reconstructing ancient carbonate depositional environments, a comprehensive study requires integration of field and laboratory data. Paleogeographic studies indicate that the Triassic sea in Aghe-Darband and Central Iran corresponded to an epicontinental sea (Seyed-Eemami, 2003) in which in the northern marginal part carbonate sediments were deposited in a quiet environment marked by several transgression and regression phases (Baud et al., 1991a). The Sefid Kuh settings studied in four

Conclusions

The data presented in this research show that the deposition of the Sefid Kuh Formation in the Aghe-Darband window occurred under shallow marine conditions within a homoclinal carbonate ramp environment in the Olenekian to the Middle Anisian. In the Sefid Kuh Formation, four main facies association were identified: (i) tidal flat facies carbonates mainly consisting of very early diagenetic dolomites or dolomicrite with bioturbation, lithologies with fenestrae fabrics and microbial mats

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the National Iranian Oil Company (NIOC) and University of Shahid Beheshti. The Department of Earth Sciences, University of Shahid Beheshti (Iran) provided facilities for this research, for which the authors are grateful. Special thanks are expressed to Mr. Behroze Aryafar and professor Abdol Hossein Amini for their valuable helps during the sequence stratigraphic interpretations. The journal reviewer Sherif Farouk and the editor in chief Damien Delvaux are thanked for

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      The Aghdarband Group has been divided into four formations based on their lithological and paleontological content (Ruttner, 1991) (Fig. 3). The lowermost formation of the Aghdarband Group is the Olenekian to middle Anisian Sefid-Kuh Limestone (Ruttner, 1991; Balini et al., 2019; Liaghat et al., 2021) overlying the?Upper Permian–Lower Triassic volcanoclastic conglomerates of the Qara-Geithan Formation (Ruttner, 1991; Eftekharnezhad and Behroozi, 1991; Alavi et al., 1997; Balini et al., 2009; Zanchi et al., 2016) (Fig. 3). The former unit is generally overlain by the relatively deep-water fossiliferous cherty limestones of the Nazar-Kardeh Formation (Nzkd.

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