Anachronistic facies in the early Triassic successions of the Persian Gulf and its palaeoenvironmental reconstruction

    Anachronistic facies in the early Triassic successions of the Persian gulf and its palaeoenvironmental reconstruction Javad Abdolmaleki, Vahid Tavakoli PII: DOI: Reference:

S0031-0182(16)00032-8 doi: 10.1016/j.palaeo.2016.01.031 PALAEO 7661

To appear in:

Palaeogeography, Palaeoclimatology, Palaeoecology

Received date: Revised date: Accepted date:

4 July 2015 3 January 2016 8 January 2016

Please cite this article as: Abdolmaleki, Javad, Tavakoli, Vahid, Anachronistic facies in the early Triassic successions of the Persian gulf and its palaeoenvironmental reconstruction, Palaeogeography, Palaeoclimatology, Palaeoecology (2016), doi: 10.1016/j.palaeo.2016.01.031

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ACCEPTED MANUSCRIPT Anachronistic facies in the Early Triassic successions of the Persian Gulf and its palaeoenvironmental reconstruction

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Javad Abdolmaleki 1*, Vahid Tavakoli1 1

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School of Geology, College of Science, University of Tehran, Iran School of Geology, Enghelab Squ., University of Tehran, Tehran, Iran E-mail address: [email protected], [email protected]; [email protected] * Corresponding author

Abstract

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The Permian–Triassic boundary corresponds with the largest mass extinction in the Earth’s

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history, during which most living taxa were diminished. The delayed recovery time after this boundary was also one of the longest among all mass extinction events, and resulted from

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enduring unfavorable environmental conditions for living organisms. Such long-lasting harsh

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conditions during the Early Triassic resulted not only in the delayed recovery of organisms but also considerable changes in depositional processes, which formed 'anachronistic facies' all

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around the world. In this study, anachronistic facies and features are reported, for the first time, from the Early Triassic intervals of the Persian Gulf. Also, it aims to interpret the factors governing formation of these facies using sedimentological evidence. Accordingly, three types of microbial facies (including stromatolitic boundstones, oncoidal facies and thrombolytic facies), highly cemented facies/layers, large-ooid facies and coarse intraclastic facies are introduced as the anachronistic facies of the Early Triassic sequences in four giant and supergiant gas fields in the Persian Gulf region. Seemingly, the oscillations in CaCO3 saturation can be considered as the main cause for the formation of anachronistic facies in these intervals. The evidence shows that an increase in dissolved CO2 caused acidification of the sea water, CaCO3 dissolution and extinction. Carbonate dissolution causes reduction in dissolved CO2 and increase in Ca2+ and HCO3- (super-saturation situation). Therefore, sea-water under-saturation and subsequent super-

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ACCEPTED MANUSCRIPT saturation in an organism-free substrate were the main causes for development of such anachronistic facies (in the super-saturation situation phase).

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Keywords: Anachronistic facies; Early Triassic; Doozakh phase; Barzakh phase; Persian Gulf;

Introduction

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1.

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Kangan Formation

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The Permian–Triassic boundary (PTB) is marked by the world’s largest mass extinction event that led to death of the most biota (Wignall, 1992; Erwin, 2006; Coney, 2007). Some

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important mechanisms postulated for this extinction are bolide impact (Becker et al., 2001), massive volcanism (e.g. Isozaki, 2007), ocean anoxia (Wignall and Hallam, 1992; Wignall and

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Twitchett, 1996; Isozaki, 1997), oceanic acidification (e.g. Heydari and Hassanzadeh, 2003; Payne et al., 2007; Montenegro et al., 2011; Clapham et al., 2013), acid rains (Sheldon, 2006;

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Knoll et al., 2007), worldwide depletion of stratospheric ozone (e.g. Kump et al., 2005), cooling (e.g. Korte et al., 2010), warming (Bercovici et al., 2015; Retallack et al., 2003;) or synergistic combinations among some of these events (e.g. Erwin, 2006). Seemingly, anything that caused such extinction also resulted in the long-term unfavorable environmental condition and delayed adaptive radiation (organism’s recovery) after the extinction (e.g. Hallam, 1991). However, the end-Permian mass extinction marked only the beginning of a prolonged (5- to 6-million-year in some regions) biotic crisis, with low ecological complexity, and an absence of metazoan reefs, which extended through the Early Triassic in all marine environments (Hallam, 1991; Flugel, 1994; Schubert and Bottjer, 1995 Flugel, 2002; Fraiser and Bottjer, 2005). Some researchers believe that the delayed recovery time back to “normal” conditions, extending until the Middle Triassic, is exceptionally long relative to those following other mass extinctions (Hallam, 1991;

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ACCEPTED MANUSCRIPT Schubert and Bottjer, 1995). This should be mentioned, although the exact definition of “recovery” is debated (see Hallam, 1991; Twitchett et al., 2004; Fraiser and Bottjer, 2007; Tong

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et al., 2007), but in some areas it took place immediately after extinction (e.g. Chen and

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McNamara, 2006; Beatty et al., 2008; Brayard et al., 2010, 2015; Chen and Benton, 2012; Forel, 2013), although further discussion of this is beyond the scope of this study.

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Such environmental conditions prevented adaptive radiation (recovery) and resulted in some

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remarkable changes in the environment. These exceptional conditions led to considerable variations in depositional patterns and characteristics. They resulted in the formation of

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“anachronistic facies” in the sedimentary record of this time interval (e.g., Pruss et al., 2005; Baud et al., 2007). Anachronistic facies is the term used for all 'unusual facies', that are normally

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temporally restricted to periods when the Earth had a very different ocean/atmosphere chemistry. This term was introduced by Sepkoski and his co-authors for the first time for facies, or facies

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and fabrics typical of earlier periods in Earth's history (Sepkoski et al., 1991), but it seems that this term needs more revision to cover some Early Triassic facies (for example see Kershaw et al., 2009). In some studies “unusual facies” or “disaster forms” were also used (see Schubert and Bottjer, 1992; Wignall and Twitchett, 1999; Groves et al., 2003; Pruss et al., 2005; Kershaw et al., 2009). However in this study (such as Baud et al., 2005, 2007; Woods and Baud, 2008; Zhao et al., 2008; Woods, 2014; Deng et al., 2015 and etc.), anachronistic facies is used for all features which are responsible for harsh environmental conditions during Early Triassic time. Formation of anachronistic facies throughout the Early Triassic, coeval with delayed recovery, indicates that the sedimentary response to the end-Permian event was not limited to the Permian–Triassic boundary interval. The deposition of microbialites, seafloor precipitated carbonate cements, abundant flat-pebble conglomerates (unusual intraclast), which are typically absent in the marine

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ACCEPTED MANUSCRIPT environment for hundreds of millions of years (e.g. Sepkoski et al., 1991; Sumner and Grotzinger, 1996; Pruss et al., 2005), are linked to protracted ecologic and environmental

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changes that began with the end-Permian mass extinction and persisted until the Early to Middle

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Triassic in some regions (Hallam, 1991; Pruss et al., 2006). A literature survey of anachronistic facies occurring in other regions during the Early Triassic, such as central and western Tethys

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(Oman, Armenia, Iran, Hungary, Turkey, northern Italy) (Wignall and Twitchett, 1999; Heydari

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et al., 2003; Baud et al., 2005, 2007), eastern Tethys (south China) (Kershaw et al., 1999, Lehrmann et al., 2001), western Panthalassa (Japan) (Sano and Nakashima, 1997), eastern

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Panthalassa (southwestern United States) (Pruss et al., 2005) and the Boreal Seaway (Greenland) (Wignall and Twitchett, 2002) shows that anomalous deposition occurred globally following the

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extinction event and in some regions persisted to the Middle Triassic. The Triassic sequences of the Persian Gulf are named as the Kangan Formation (Fig. 1;

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Szabo and Kheradpir, 1978). Along with the Upper Permian Dalan Formation, it provides one of the lucrative hydrocarbon reserves of the world (Fig. 1; Esrafili-Dizaji and Rahimpour-Bonab, 2013, Tavakoli, 2015). The Dalan-Kangan Formations (Khuff equivalents) have been the subject of many studies in Iran and Arabian territories (e.g. Insalaco et al., 2006; Rahimpour-Bonab, 2007; Esrafili-Dizaji and Rahimpour-Bonab, 2009; Maurer et al., 2009; Rahimpour-Bonab et al., 2009; Tavakoli et al., 2011; Koehrer et al., 2010, 2012). Despite these numerous studies on the depositional environment and reservoir characteristics of these Early Triassic deposits, its unusual palaeoenvironmental characteristics have not been appreciated thoroughly in this part of the Tethyan realm. Also in the Arabian parts, there has been little publication about palaeoenvironmental conditions during the Triassic (e.g. Krystyn et al., 2003; Woods and Baud, 2008; Weidlich and Bernecker, 2007, 2010; Richoz et al., 2010; Clarkson et al., 2013). In this

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ACCEPTED MANUSCRIPT study, anachronistic facies are described from the Early Triassic sequences of the Persian Gulf. Using detailed sedimentological and palaeontological evidence, the origin and depositional

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conditions of these facies are elaborated.

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FIG1 2. Materials and methods

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The database for this study was based on six key wells in four giant and supergiant gas fields in the Persian Gulf; including the world’s largest gas field (South Pars), and other neighboring

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fields including Lavan (CF), Salman (DF) and Golshan (AF) (Fig. 2). Detailed petrographic

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studies have been carried out on nearly 1500 meters of cores, core slabs and 5000 stained thin sections. The thin section spacing was 30 centimeters, but to detect small-scale facies variations,

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the spacing was closer around facies boundaries. Facies analysis was carried out using standard

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models and microfacies descriptions (e.g. Wilson, 1975; Flugel, 2004). A modified Dunham

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(1962) texture scheme (Embry and Klovan, 1971) was used to describe facies.

Geological setting and stratigraphy

3.1. Geological setting

Permian–Triassic carbonate platforms were developed on the passive margins of the Neotethys Ocean (NE of Gondwana supercontinent) (Fig. 2) during a major tectonic-eustatic event including sea-level transgression and thermal subsidence (Sharland et al., 2001, Insalaco et al., 2006). During the Early Triassic, the platform was located around 17 to 20 degrees latitude in the Southern Hemisphere (Stampfli, 2000; Angiolini et al., 2003). At this time, the Persian Gulf was a shallow carbonate platform, which became deeper toward the Zagros area (Ziegler, 2001; Fig. 2B). This region is one of the largest hydrocarbon (mainly gas) provinces of the world in which Permian–Triassic carbonate successions host gigantic reserves (Sharland et al., 2001; Ziegler,

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ACCEPTED MANUSCRIPT 2001; Aali et al., 2006; Insalaco et al., 2006; Tavakoli et al., 2011 Tavakoli and RahimpourBonab, 2012; Esrafili-Dizaji and Rahimpour-Bonab, 2013; Tavakoli, 2015).

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In the Arabian nomenclature, the Permian–Triassic carbonates are known as the Khuff

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sequences and in Iran; they are named as the Dalan and Kangan Formations (Fig. 1) (Szabo and Kheradpir, 1978; Insalaco et al., 2006). The lower boundary of the Kangan Formation

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corresponds with the Permian–Triassic boundary (PTB). In the Arabian Plate, the nature of this

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boundary is under discussion. Some researchers (e.g. Sharland et al., 2001) believe that this boundary is unconformable, while the others consider it as a conformable surface (e.g. Insalaco

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et al., 2006). The upper limit of the Kangan Formation is marked by a sharp lithological change from carbonate-evaporate (Kangan Fm.) to shale-carbonate-evaporate of the Dashtak Formation

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(Fig. 1; Szabo and Kheradpir, 1978).

The commonly accepted facies model for these platform interior successions is a homoclinal

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ramp with gentle facies transitions from marginal marine siliciclastics to inner platform carbonate-anhydrite cycles and to open-marine platform carbonates (e.g. Sharland et al., 2001; Ziegler 2002; Insalaco et al., 2006; Esrafili-Dizaji and Rahimpour-Bonab, 2009, Weidlich and Bernecker, 2010, Tavakoli et al, 2011). Typical facies associations deposited on this ramp include supra- to intertidal sabkha, salina anhydrites and mudstones, inner-ramp bioclastic wacke- and packstones, high-energy shoal pack- and grainstones as well as outer-ramp muddy, argillaceous carbonates (see Alsharhan, 2006; Insalaco et al., 2006; Maurer et al., 2009 and Koehrer et al., 2010 for details).

3.2. PTB in the Persian Gulf and surrounding area, a paleontological approach

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ACCEPTED MANUSCRIPT Precise biostratigraphic dates are especially challenging for the Permian–Triassic boundary on the Arabian platform due to the absence of conodonts and ammonoids (Weidlich and Bearcker,

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2010). The main index foraminifera for the Changhsingian Stage and latest presence of

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foraminifera such as palaeofusulinids and colaniellids (Gaillot and Vachard, 2007) have not been recognized in most fields of the Persian Gulf (see also Insalaco et al., 2006). Insalaco et al.,

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(2006) note “The lack of ‘classic’ Late Permian markers, such advanced Colaniella and

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Palaeofusulina, may be related to provincialism and different palaeobiogeographic regimes”. Vachard et al. (2002) considered Colaniella as a reliable Changhsingian marker of open-marine

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carbonates and mentioned its absence within predominately restricted Late Permian platform carbonates of the Arabian platform. Also, Gaillot and Vachard (2007) expressed “the two

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foraminifers considered as classical Changhsingian markers are Palaeofusulina and Colaniella,

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and inhabits rather more open seas”.

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In the studied fields from the Persian Gulf (current study) the disappearance of the Permian fauna (specific foraminifera such as Paradagmarita monodi) occurs just below a highly cemented layer and with the transition of an azoic interval (differing from a few centimeters to 2 meter in the studied fields) is then followed by the Triassic fauna (such as Spirorbis phlyctena) recovery (Vachard et al., 2002; Insalaco et al., 2006; Gaillot and Vachard, 2007; Weidlich and Bearcker, 2010). Therefore, in this study the PTB is determined at the top of layer at which the Permian fauna disappeared and below the highly cemented layer (belonging to the Triassic) such as followed in other studies (Heydari et al., 2003; Heydari and Hassanzadeh, 2003; Insalaco et al., 2006). At the Arabian plate all fossils preserved in the Early Triassic are of small size and low in abundance and diversity (Gaillot and Vachard, 2007). Beside body fossils, trace fossil sizes in the Early Triassic are small and thin (see Knaust, 2010). Accordingly, although biotic

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ACCEPTED MANUSCRIPT recovery after the end-Permian mass extinction in the studied area might not be as long as in some areas (e.g. Hallam, 1991; Schubert and Bottjer, 1995; Meyer et al., 2011; Chen et al., 2010;

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Pietsch and Bottjer, 2010), but full recovery and ecosystem-stabilizing was slow and a marine

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environment with low biodiversity lasted for the entire Early Triassic (see Chen et al., 2010; Sun et al., 2012). The mentioned ecosystem characteristics indicated that unfavorable environmental

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conditions continued in the aftermath of the PTB mass extinction and persisted for most of Early

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Triassic time (Hallam, 1991; Flugel, 1994; Schubert and Bottjer, 1995; Flugel, 2002; Fraiser and Bottjer, 2005; Kiessling, 2009; Meyer et al., 2011; Chen et al., 2010; Pietsch and Bottjer, 2010,

Results (Early Triassic anachronistic facies in the Persian Gulf)

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2014).

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In this part, some anachronistic facies and features in the Early Triassic successions of the

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Persian Gulf are introduced. Such facies are reported from Early Triassic intervals in other areas all around the world, which are referred to during the Discussion. In this section, firstly, the facies and features will be introduced and described in detail. Then, their palaeoenvironmental significance will be elaborated.

4.1. Microbial facies As well-known Early Triassic anachronistic facies, microbial facies of a variety of forms have been reported from numerous parts of the world (e.g., Schubert and Bottjer, 1992; Baud et al., 1997, 2005, 2007; Sano and Nakashima, 1997; Kershaw et al., 1999; Lehrmann, 1999; Pruss and Bottjer, 2004; Pruss et al., 2006; Kershaw et al., 2007; Mary and Woods, 2008; Kershaw et al., 2011; Tavakoli et al, 2011; Kershaw et al., 2012; Tavakoli, 2015). In the Early Triassic interval of the Persian Gulf, three different forms of microbial facies have been recorded as follows:

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ACCEPTED MANUSCRIPT 4.1.1 Stromatolite boundstone In the studied intervals, the stromatolitic facies has been recorded in both the Late Permian and

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Early Triassic sequences. Nonetheless, they become more frequent in the Early Triassic Kangan

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Formation and can be refereed as "disaster forms" instead of anachronistic facies (see Kershaw et al, 2009). In the studied fields, the stromatolitic facies of the Kangan Formation and upper Dalan

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Member are generally similar in their shapes and structures. The stromatolitic boundstones of the

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Kangan Formation are mostly laminated (Fig. 3A, B) and contain rare bioclasts fragments. Esrafili-Dizaji and Rahimpour-Bonab (2009) located the stromatolitic facies of the Dalan

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Formation (Late Permian in age) in the (landward) intertidal zone. But, it seems that the facies was formed in a different environmental condition during the Early Triassic in comparison to late

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Permian time. Some observations (i.e., close association with shoal facies) showed that the facies

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can also form in back-shoal settings (see also Kershaw et al., 2011).

4.1.2. Oncoidal facies

The oncoidal facies is the other microbial constituent in the Triassic sequences of the Kangan Formation. This facies is mostly detectable on slabbed surfaces of the core samples (Fig. 3C). The facies mostly has a grain-dominant fabric (Fig. 3D, E) indicating a high-energy depositional setting. In the Permian intervals of the studied wells, oncoidal facies are rarely recorded. However, they are one of the most important shoal-building elements in the Early Triassic units.

4.1.3. Thrombolite boundstone This facies is one of the most important and typical anachronistic facies in the Early Triassic intervals that is limited to a distinct stratigraphic position and is correlatable across the platform

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ACCEPTED MANUSCRIPT (Insalaco et al., 2006; Maurer et al., 2009, Tavakoli, 2015). It is recorded in nearly all studied wells from about 1 to 5 meters above the PTB. On the core samples, this facies shows a clotted

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appearance (Fig. 3F, G) and was developed mainly in subtidal settings (Rahimpour-Bonab et al.,

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2010). Despite the stromatolite facies, it contains bioclasts and whole fossils of the Early Triassic (Fig 3 H, I, J). In some cases, it reaches a maximum thickness of about 5 meters and is associated

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with other facies, which are highly cemented in many cases (Fig. 3K).

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FIG3 4.2. Highly cemented facies/layers

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An unusual and intense cementation occurred above the PTB. The cements were mostly formed in sea-floor and have a calcitic mineralogy. A dolomitic mineralogy is also recorded in some

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cases, which formed during early diagenesis by dolomitization. In most cases, this syn-sedimentary submarine calcite cement is recorded in forms of

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cement crusts and botryoids (fan shape) (Fig. 4A-D) that comprises fibrous and bladed calcite crystals (Fig 4A, C, D, E). These types of cements are also reported from similar intervals of the Persian Gulf and other places around the world (see Insalaco et al., 2006; Baud et al., 2007, Woods and Baud, 2007, Kershaw et al., 2011). It seems that some bladed types were formed during diagenesis by the neomorphism of fibrous crystals. The cement types often grew (semi-) perpendicular to the seafloor (Woods and Baud, 2008; Fig 4B and E). However, in more cases, the highly cemented layers contain the grain’s rime cements as thick radial forms (e.g. Insalaco et al., 2006; Fig. 4F-H). Dominant grains of this facies are coarse intraclasts (see Insalaco et al., 2006; Fig. 4). This highly cemented facies/layer is located on top of a dissolution and/or erosional surface that is referred to the PTB. Besides, this facies also shows a close association with the other described

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ACCEPTED MANUSCRIPT anachronistic facies - the large-ooid facies and microbial facies. Similar unusual highly cemented layers have been reported just above the PTB both in the Persian Gulf basin (see Insalaco et al.,

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2006) and other areas in the Neotethys Ocean (e.g., Heydari et al., 2003). In addition, numerous

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researchers have reported similar highly sea-floor cemented facies/layers from other Early Triassic sequences in various parts of the world (Baud et al., 1997, 2005; Woods et al., 1999,

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2007; Heydari et al., 2003, 2008; Pruss et al., 2005; Mary and Woods, 2008; Algeo et al., 2007;

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Lehrmann, et al., 2007; Kershaw et al., 2011). FIG4

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4.3. Large-ooid facies

A facies with irregularities in the formation of ooids has been reported from many sections all

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around the world (e.g. Heydari et al., 2003; Baud et al., 2005). These irregularities occurred as frequent and sharp increases in ooid-rich layers in the sequences (named as "disaster ooids" by

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Groves et al., 2003, e.g. Heydari et al., 2003; Baud et al., 2005; Algeo et al., 2007; Groves et al., 2007; Haas et al., 2007; Lehrmann et al., 2007; Woods et al., 2013) and/or formation of unusualy large-ooid grains (named as "giant ooids") within the ooid-bearing layers (e.g., Li et al., 2010, 2013; Payne et al., 2006; Lehrmann et al., 2007; Weidlich, 2007; Woods, 2013 and references therein). The ooid-rich layers have been reported earlier from upper Permian intervals of the Persian Gulf (e.g., Esrafili-Dizaji and Rahimpour-Bonab, 2009). But, in the studied fields, the facies containing large ooids (larger than 2mm and in some case larger than 5mm), are recorded in the Early Triassic Kangan Formation (Fig. 5). However, in some cases diagenetic modifications make it difficult to distinguish between the large ooids and oncoids. The largeooid facies is mostly concomitant with the microbial facies and highly cemented layers. In more cases, the large-ooid facies have undergone intense marine cementation. The thickness,

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ACCEPTED MANUSCRIPT frequency and vertical/lateral extension of ooid-rich layers in the Triassic intervals (Kangan Formation) is remarkably lesser than the Late Permian sequences. In the studied fields of the

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Persian Gulf, ooid-bearing facies occur as thin and repetitive layers in the Early Triassic

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intervals. Therefore, although the large-ooid facies are present in the Lower Triassic intervals of the Persian Gulf, their frequency and vertical continuity decreases upward in the Triassic

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sequences.

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FIG5 4.4. Coarse intraclastic facies

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This facies has previously been reported by many researchers from the Early Triassic intervals of the Persian Gulf and other areas of the world (e.g. Wignall and Twitchett, 1999; Pruss and

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Bottjer, 2004; Pruss et al., 2005, 2006; Insalaco et al., 2006; Rahimpour-Bonab et al., 2009). In these studies, it was named as "unusual intraclast", "flat pebble", and "micro-conglomerate". We

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prefer to call it “coarse intraclastic facies” due to its large intraclasts that are not always flatted. As revealed in this study, the facies is more frequent in the Early Triassic successions than in the Permian among all studied wells. However, in the Early Triassic it is concentrated in some distinct horizons. To recognize their depositional setting, it is necessary to elaborate on their depositional characteristics and near-surface (syn-sedimentary) diagenetic features. Seemingly, the coarse intraclastic facies in the Late Permian and Triassic sequences occurs as three main sub-facies: Sub-facies 1: Angular to sub-angular micritic/dolomicritic fragments in an anhydritic cement (or matrix) with brecciated texture. Some features such as fenestral fabrics and mud cracks are also present, which are indications of subaerial exposure.

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ACCEPTED MANUSCRIPT Sub-facies 2: Sub-rounded to rounded intraclasts bearing facies juxtaposed with facies that present hummocky cross stratification and other storm features. It has been recorded mainly

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in association with shoal and mid-ramp facies. This group shows a higher frequency in the

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Early Triassic (i.e. storm deposit).

Sub-facies 3: Angular to rounded intraclasts bearing facies that formed in different depositional

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settings. It coincides with all or several unusual features including changes in cementation

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rate in underlying and overlying facies. In most cases, this facies shows intense cementation, coeval with the end Permian mass extinction and also dissolution and/or erosional surface

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(truncation bottom surfaces referred to PTB) in the Early Triassic Kangan Formation. It also shows evidence of high energy depositional settings.

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The two first sub-facies are recorded both in the Late Permian and Early Triassic sequences (Insalaco et al., 2006; Esrafili-Dizaji and Rahimpour-Bonab, 2009). Although, it seems that sub-

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facies 2 is more frequent in the Triassic intervals (may be an unusual feature). Sub-facies 3 is only recorded in the Early Triassic sequences of the studied wells and is considered here as an anachronistic facies. Sub-facies 3 of intraclast bearing units are separated from their underlying intervals by dissolved and/or eroded (Fig. 6A, B, C) surfaces. The surfaces show erosional truncation in some cases (Fig. 6D). However, unusual intraclasts become more frequent within the cemented layers. The intraclasts of studied intervals show different orientations, but they are mostly uni- or bidirectional (Fig. 6A, B, C). The other important evidence of this sub-facies 3 is its association with bioturbated bioclastic truncation layers (Fig. 6E), which are erosional in some cases. The sutured appearance of intraclast margins is an indication of a dissolution and/or erosional phase before the rip up stage and intraclast formation (Fig. 6B, D, E). In the studied intervals, no

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ACCEPTED MANUSCRIPT shallowing upward trend or any meteoric feature (i.e. meteoric cements) is recorded in the intraclastic facies (Fig. 6F, G, H).

Discussion

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5.

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FIG6

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5.1 Palaeoenvironmental interpretation of the anachronistic facies 5.1.1. Microbial facies

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Forel (2013) stated that photosynthetic cyanobacteria may have locally provided oxygen to the

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supposedly anoxic environments of the earliest Triassic, because ostracods and microbialites temporarily disappeared during the Induan. The interpretation indicates an oxygen deficiency

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during microbial facies formation. However, the formation of these facies has also been reported

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in normally oxygenated conditions (e.g. Forel et al., 2009; Kershaw et al., 2011).

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In the studied intervals, the appearance of a Triassic fauna is recognized to have occurred at an earlier time than the occurrence of microbial facies (which contains bioclasts), and it shows prevalence of oxygenic conditions even before it. Besides, in this study, no evidence for anoxic signature (e.g., early pyrite or dark color) is observed in the microbialite facies and before it. One of the most important aspects of microbialite occurrence is the origin of the carbonate calcium during formation of this facies. Woods (2013) noted that it was the result of three main factors: (1) the unusual chemistry of Early Triassic oceans; (2) run-off of nutrient-rich waters, which improved microbialite growth; and, (3) wave agitation and warm waters that led to CO2 degassing and further super-saturation of shallow waters relative to the calcium carbonate. However, the occurrence of microbial fabrics and features are also dependent on the reduction of grazing pressures (Woods, 2013) and local conditions (Kershaw et al., 2012). Thus it is necessary to note that the increase in microbial activity cannot be ascribed to the absence of

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ACCEPTED MANUSCRIPT metazoan decline (see Riding, 2006). Moreover, Riding (2005) and Riding and Liang (2005) proposed that microbialites are controlled primarily by carbonate saturation. In recent

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environments, it is also proved that carbonate saturation is the main controlling factor for the

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microbial facies (Burne and Moore, 1987; Riding, 2000). Therefore, the occurrence of the microbialite facies in several intervals of the Early Triassic sequences of the Persian Gulf

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represents seawater super-saturation of those intervals in the Early Triassic concomitant with the

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5.1.2. Highly cemented facies/layers

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low biodiversity background.

Highly cemented facies/layers are recorded on top of the PTB of the studied wells and some

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other sections (e.g., Woods et al., 1999, 2007; Heydari et al., 2003; Baud et al., 1997; Pruss et al., 2005). This facies is indicative of intense and abrupt saturation of CaCO3 in some horizons of the

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Triassic sequences (see Heydari et al., 2003; Insalaco et al., 2006).

5.1.3. Large-ooid facies

As noted above, the ooid-rich facies or layers are recorded in both the Permian and Triassic intervals of the Persian Gulf basin. But, they contain abnormally large ooid grains in the Early Triassic successions, which are considered here as an anachronistic facies. The main characteristic of this facies is the heterogeneity in ooid sizes, which range from 1mm to nearly 5mm. One of the probable causes of this phenomenon was the lack of adequate nuclei resulting from the PTB mass extinction and its aftermath (see Sumner and Grotzinger, 1993). The other important factor was intense and abrupt super-saturation of seawater with respect to calcium carbonate (Payne et al., 2006; Lehrmann et al., 2007; Weidlich, 2007; Li et al., 2013). Evidence

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ACCEPTED MANUSCRIPT from the studied intervals shows that both factors were active. However, it seems that the latter

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factor was more important.

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5.1.4. Coarse intraclastic facies

As noted above, coarse intraclastic facies can be divided into three different sub-facies. The first

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sub-facies shows brecciation along with exposure features. Although the second sub-facies

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shows an unusually high frequency in the Early Triassic intervals, the third sub-facies has only been recorded in the Early Triassic intervals. In some studies, it is mentioned that the absence of

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deep bioturbation and also different early lithification created a suitable substrate for frequent intraclast formation by storm waves (Pruss et al., 2005; Wignall and Twitchett, 1999).

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Seemingly, the absence of deep bioturbation (as body fossils and trace fossil sizes in the Early Triassic are small and thin as shown by previous studies in the Persian Gulf and Arabian plate

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(e.g. Knaust, 2010) and specifically different early lithification of carbonate seafloors coeval with storm activity are important factors for formation of these facies. Moreover, in some outcrop studies, the origin of the intraclast-bearing facies in the Early Triassic intervals is still under the shadow of doubt (Wignall and Twitchett, 1999). The existence of a suitable substrate is necessary for the formation of unusual intraclasts in the Early Triassic intervals all around the world. As noted in many studies, the lack of deep bioturbation and differential lithification can be considered as the main factor forming the intraclasts (e.g., Wignall and Twitchett, 1999; Pruss et al., 2006). In this study some other evidence for formation of these large intraclastic facies is presented. As shown (Fig. 6), in most cases this facies occurs just above the dissolution and/or erosional surface (PTB). These surfaces are associated with the bio-event and the existence of super-

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ACCEPTED MANUSCRIPT saturated facies just above it but without any evidence of shallowing-upward trends in facies associations. Therefore, they cannot be attributed to subaerial exposure and storm deposits.

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Moreover, that the facies formed during super-saturation phases above these surfaces (marine

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cemented) is also an indication of their non-meteoric origin. They mostly show sutured margins which are the indications of a dissolution and/or erosional phase before their formation and

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reworking stages. In some cases, it is possible that the dissolution surface changed to an

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erosional surface or was strongly removed by marine currents. In addition, the suspension load of marine waters, which resulted from the previous dissolution phase, could have accelerated this

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process. It is not true to attribute their overlying highly cemented layers and short-lived organisms (e.g., Knoll et al., 2007; Weidlich and Bernecker, 2010; Chen and Benton, 2012) to

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stormy conditions (see Aigner, 1985). However, the orientation of intraclasts indicates the contribution of high energy currents during deposition of these facies. Thus, it seems that other

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chemical factors (under-saturation condition) together with high energy currents (physical factor) governed intraclastic facies formation. As depicted in Fig. 7, when seawater was under-saturated with respect to calcium carbonate, formation of dissolution and/or erosional surface occurred. At these surfaces, some clasts of sea floor sediments can be dugout and re-deposited as intraclasts. A similar process for submarine intraclast formation during sea-water under-saturation has been mentioned in other studies (Flugel, 2004). FIG7 In some cases, this facies is recorded above the highly cemented layers (Fig. 8) during the lower saturation stages. The intraclastic facies formation above highly cemented layers can be attributed to differential early lithification (Fig. 8; Sepkoski et al., 1991; Wignall and Twitchett, 1999). Therefore, this evidence indicates that the intraclastic facies became more abundant after

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ACCEPTED MANUSCRIPT the under-saturation phases (PTB) and also above the highly cemented facies. Lack of deep bioturbation is another important factor controlling the intraclastic facies formation (Wignall and

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Twitchett, 1999; Pruss et al., 2006).

5.2. Factors governing the anachronistic facies and delayed recovery in the Early Triassic

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sequences of the Persian Gulf

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Most of the discussed anachronistic facies are shown on a slabbed core section of the Early Triassic intervals in one of the studied wells and also a schematic and idealized model for the

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Early Triassic intervals of the Persian Gulf is presented in Fig. 8. Accordingly, two general

FIG8

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5.2.1. Doozakh Phase

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Triassic sequences, as follows.

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phases can be introduced based upon the facies characteristics and their association in the Early

The “Doozakh Phase” was firstly introduced by Heydari and Hassanzadeh (2003) as the period in which sea water became under-saturated with respect to CaCO3. However increased concentrations of carbon dioxide (CO2) in both sea water and atmosphere has been argued as the main cause for the extinction and delayed recovery at the PTB and afterward (in many studies such as Heydari et al., 2003; Heydari and Hassanzadeh, 2003; Fraiser and Bottjer, 2007; Knoll et al., 2007; Payne et al., 2007; Clapham and Payne, 2011; Kiessling and Simpson, 2011; Montenegro et al., 2011; Heydari et al., 2013; Tavakoli, 2015). Two main sources for carbon dioxide are suggested by these researchers: 1- the Siberian Traps (e.g., Payne et al., 2007; Montenegro et al., 2011; Fraiser and Bottjer 2007; Knoll et al., 2007) and 2- methane hydrate

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ACCEPTED MANUSCRIPT oxidation during the mantle plump (e.g., Heydari et al., 2003; Heydari and Hassanzadeh, 2003; Heydari et al., 2013; Berner, 2002, 2004).

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Outgassing from continental flood basalt province volcanism in Siberia is considered as a

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major source of elevated Permian–Triassic Boundary CO2, as Siberian Trap volcanism correlates well with elevated atmospheric CO2 and with the end-Permian mass extinction (Wignall, 2001;

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Courtillot and Renn,e 2003; Grard et al., 2005). The Siberian Traps emitted between 2 and 3

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million km3 of material over 1 million years spanning the PTB, including between 7×1012 g and 4.8×1013 g of CO2 (Renne et al., 1995; Berner 2002; Reichow et al., 2002; Kamo et al., 2003).

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The other hypothesis is that the Upper Permian accumulation period of gas hydrates ended abruptly adjacent to the PTB and the dissociation event began releasing 3.2 to 4.7×1018 g CH4

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into the ocean. Oxidation of CH4 in the water column created seawater that was charged with CO2 (an oceanic acid bath) and had lower than normal O2 content (but not anoxic) (Heydari and

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Hassanzadeh, 2003).

Detailed discussion about the causative agent of extinction is not the aim of this study. However, evidence supports the assumption that “sea water acidification” occurred during the end-Permian event (see also Tavakoli, 2015). The evidence includes subaqueous dissolutions and erosional surfaces (see Heydari and Hassanzadeh, 2003; Payne et al., 2007) and highly cemented layers just above the Permian–Triassic boundary (see Heydari et al., 2003; Heydari and Hassanzadeh, 2003).

5.2.2. Barzakh Phase In the Persian terminology, "Barzakh" means “the world between death and redevelopment of the life" (Moein, 1995). In this study, the Barzakh Phase is attributed to the time span between

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ACCEPTED MANUSCRIPT the PTB mass extinction and the return to normal conditions. This time includes an interval between two huge bio-events (near extinction of organism and their recovery). No evidence of

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normal conditions are recorded in the base of Kangan Formation, thus, it seems that it has been

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deposited during the Barzakh Phase. As mentioned, in the Persian Gulf area (and even across the Arabian Plate), organism recovery time was delayed and relatively short. The existence of

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anachronistic facies in the Triassic sequences of the Kangan Formation is the other testimony for

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this interpretation.

The upper parts of the Kangan Formation are marked by fossils and traces which are

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relatively large in size and associated with more bioturbation. All of these are indications for environmental conditions being closer to the normal situation. The higher frequency of

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anachronistic facies in the lower parts of this unit also supports this idea. A review of previous work led to two generalizations about formation of these facies in unusually super-saturated

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intervals. One of the hypotheses states that sulfate reduction in organic matter rich sediments during anoxic conditions (oxygen crisis) led to an increase in the bicarbonate ion rather than waters super-saturated with calcium carbonate (e.g. Woods et al., 199; Pruss and Bottjer, 2004). In the studied intervals, no evidence of anoxic conditions (e.g. early pyrite or dark color) was recorded (see Wignall et al., 2005; Wingall et al., 2005; Shen et al., 2007; Clapham et al., 2013). Moreover, it is not possible to attribute the super-saturated facies to anoxic conditions because some researchers believe that the formation of super-saturated layers during the oxygen crisis phases was formed by anoxic waters that up-welled into shallow marine environments (e.g. Pruss and Bottjer, 2004). Nonetheless, it seems that the different oceanographic characteristics of Neotethys prevented the upwelling (see Kidder and Worsley, 2004, and Fig 2A). Additionally, the subject of Deep Ocean Anoxia at the PTB and during the Early Triassic is still under

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ACCEPTED MANUSCRIPT negotiation (for more details see Zhang et al., 2001; Heydari and Hassanzadeh, 2003; Kidder and Worsley, 2004; Montenegro et al., 2011). Also, restricted upwelling is suggested in some studies

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(e.g. Kershaw et al., 2012).

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However, in other parts of the Neotethys, evidence for anoxic condition has not been reported at the PTB nor in the Lower Triassic (e.g. Heydari et al., 2003; Kozur, 2007). In the

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second hypothesis, an oceanic acid bath has been assumed to have formed as the result of CO2

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release into the ocean (at the PTB). This oceanic acid bath first dissolved suspended fine-grained carbonate particles and small calcareous organisms, followed by extensive dissolution of

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platform carbonates, raising seawater Ca2+ and HCO3- concentrations. When CO2 release declined, the acid-bath (under-saturation) ocean became a soda-ocean (super-saturated situation)

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(Heydari et al., 2003; Heydari and Hassanzadeh, 2003). Also, normal oxygen conditions during the formation of microbial facies are also reported in some other studies (Kershaw et al., 2011).

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In the Persian Gulf, Insalaco et al. (2006) have not reported any anoxic evidence in the flatpebble bearing (equivalent of large intraclast bearing facies in this study) and highly cemented facies and accepted the interpretation of Heydari and Hassanzadeh (2003). No evidence of anoxic conditions has been recorded in the studied intervals (PTB and Early Triassic) of the Persian Gulf.

Finally, the integration of sedimentological evidence indicates that CaCO3 dissolution (at the PTB) and re-precipitation is the main factor governing the formation of super-saturated facies in the Early Triassic (e.g., Heydari and Hassanzadeh, 2003).

6. Conclusions

The Early Triassic sequences of the Persian Gulf are characterized by their especial faunal content and anachronistic facies, which are indications of unfavorable (harsh) environmental

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ACCEPTED MANUSCRIPT conditions after the Permian–Triassic boundary mass extinction. The anachronistic facies in the Early Triassic successions of the Persian Gulf include microbial facies (including stromatolitic

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boundstones, oncoidal facies and thrombolytic facies), highly cemented facies/layers, large-ooid

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facies, and coarse intraclastic facies which are located just above the PTB.

Chemical variations in Early Triassic sea water were the main cause for development of

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anachronistic facies (and their biotic effects). Unusual sea-water super-saturation, which was

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located just above the PTB, can be considered as the main factor governing the formation of anachronistic facies in the studied intervals. The lack of evidence for anoxic conditions in these

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facies shows that it is not possible to attribute their formation to bacterial sulfate reduction and/or upwelling of anoxic waters.

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Both sedimentological and palaeontological evidence show that an acidification of sea water

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was responsible for carbonate dissolution and subsequent increase in Ca2+ and HCO3- in the sea

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water. A subsequent decrease in dissolved CO2 caused the super-saturation conditions and concomitant anachronistic facies. As well, some non-skeletal grains (e.g., coarse intraclasts) of anachronistic facies may have formed during the under-saturation phases. However, all of these facies were deposited during the unusual super-saturation phases. The results of this study reveal that there were two main general conditions in the Early Triassic sequences of the Persian Gulf; 1- The Doozakh phase: under-saturation phase of calcium carbonate at the PTB and 2- The Barzakh phase; the time interval between the extinction event and the return to normal conditions. This phase thoroughly covers the facies at the base of the Kangan Formation in the studied wells of central and eastern Persian Gulf.

Acknowledgements

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ACCEPTED MANUSCRIPT We thank Y. Arghavani for her support; R. and T. Abdolmaleki, Z. and H.A. Sheykhlar are also thanked for their useful help. We further thanks H. Rahimpour-Bonab, H. Mehrabi B. Esrafili-

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Dizaji and A. Asadi-Eskandar for their valuable help and suggestions. We are grateful to the

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University of Tehran for the provision of facilities for this research and to the National Iranian Oil Company (NIOC), Pars Oil and Gas Company (POGC) and Total Company for support and

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data preparation. We thank Vivi Vajda, guest editor of Paleo3 journal and Mike Pole for their

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very useful comments.

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ACCEPTED MANUSCRIPT Figure Captions Fig.1. Stratigraphic chart of the Late Permian to Early Triassic intervals in Iran and Arabian

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territories. General 3rd and 4th order sequence stratigraphic subdivisions of these successions are also presented (after Insalaco et al., 2006).

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Fig.2. A- Palaeogeography of the Neotethys and Persian Gulf during the Early Triassic (Kershaw et al., 2011). B- Palaeo facies map of the Arabian Plate during the Early Triassic and

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approximate position of the studied subsurface sections in the Persian Gulf (after with some modification from Ziegler 2001).

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Fig.3. Core photos and photomicrographs of the microbial facies in the Kangan Formation. A, BStromatolite facies with obvious lamination. C- Oncoid bearing facies in core photo and in thin

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section (D, E). F, G- Clotted thrombolytic facies. H, I, J- Skeletal debris within the thrombolytic facies. K- Highly cemented facies intercalated with thrombolytic facies. Fig.4. Photomicrographs of highly cemented facies in the Kangan Formation. Cements of these

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facies are recorded as cement crusts and botryoids (fan shape) (A-D) that comprises fibrous and

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bladed calcite crystals (A, C, D, E). The cement types were often grown (semi-) perpendicular to

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the seafloor (B, E). The highly cemented layers contained grain’s rime cements as thick radial

Fig.5. Photomicrographs of large-ooid rich facies in the Kangan Formation. In this facies, the ooid grains are mostly larger than 2mm and reach to nearly 5mm, in some cases. Most of cements of this facies are marine types. Dissolution vugs (A, B, D, H) and anhydritic cement (E) were formed during the post-marine diagenesis, in the meteoric and shallow burial realms. Fig.6. Core photos from large intraclastic facies. A- Basal dissolution and erosional surface and bidirectional intraclasts. B- Basal dissolution and erosional surface and unidirectional intraclasts. C- Bidirectional intraclasts. D- Basal erosional surface (white arrow) and intraclasts with sutured surfaces (red arrow) indicating dissolution phase before the reworking. E- Erosional truncation surface (white arrow), which has resulted in truncation of bioturbated layer and fossils and sutured around of intraclasts (red arrow). F- Intraclast bearing layer with stylolite. G, H- Layer containing coarse intraclast grains. Fig.7. Schematic cartoon illustrating the subaqueous formation of intraclasts above the erosional/dissolution surfaces during the carbonate under-saturated phases. As shown, some of the intraclast grains have sutured surfaces indicating a dissolution phase before their ripping up

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Triassic condition phase) and Doozakh (carbonate under-saturation phase) phases are adopted from Heydari and Hassanzadeh (2003). The Barzakh phase refers to the time spans between the

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extinction and the recovery of Early Triassic organisms, which were marked by unusual

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carbonate super-saturation condition in most times. Also slabbed core sample from the Early Triassic intervals of the Kangan Formation in which most of the anachronistic facies are visible.

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The facies are marked on the photo. See to text for more details.

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We report anachronistic facies from P-T boundary successions, Iran. Microbial facies, highly cemented facies/layers, large-ooid facies and coarse intraclastic facies are introduced as the anachronistic facies. The factors governing formation of these anachronistic facies are interpreted. The oscillations in CaCO3 saturation can be considered as the main cause for the formation of anachronistic facies.

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