1 Introduction

The Iranian Plate is similar to other parts of the Middle East terranes and was affected by the evolution of the Paleozoic Tethyan oceans, the Hun and Cimmerian superterranes, and the Gondwana and Pangea supercontinents (Stampfli et al., 2001; Stampfli and Borel, 2002; Torsvik and Cocks, 2004; Natal’in and Sengör, 2005). The present-day Iran is part of the Alpine-Himalayan orogenic belt that is limited by the Turan Plate in the northeast and the Arabian Shield in the southwest (Alavi, 1991). The Iranian Plate consists of Central Iran Microcontinent, Alborz, Sanandaj-Sirjan and Zagros, of probable Gondwanan or peri- Gondwanan affinity, known as a part of the larger Cimmerian continent during evolution of the Paleo-Tethys and Neo-Tethys oceans (Sengör, 1987). Most studies consider the Paleozoic of Iran representing of shallow marine deposits developed in a passive margin until the Mid-Permian - Triassic, when Cimmeria started to rift away and the Neo-Tethys Ocean to open (e.g., Stampfli et al., 2001). Despite of this shallow marine regime, the Late Ordovician (Early

Katian) to Early Silurian (Late Telychian) succession recorded some global glacial events (Page et al., 2007) that are associated with a decrease in marine biodiversity and a major mass extinction during the Hirnantian Glacial Maximum (Armstrong, 2007). According to this, the Late Ordovician -Early Silurian deposits of Gondwana correspond to glacio-fluviatile sequence overlaid by post-glacial Silurian characterized by thick, organic-rich and fine-grained clastic marine deposits in the Middle East except in the Derenjal Mountains in Central Iran (Al-Husseini, 1991). The lithological and faunal similarities of the Silurian deposits of Zagros and Sanandaj-Sirjan with the Saudi Arabian domain demonstrate these terranes were attached to the northern margin of Gondwana during this time (e.g., Wendt et al., 2002, 2005; Ruban et al., 2007) whereas the Silurian deposits of Central Iran are different respect to faunal (Hairapetian et al., 2008 and 2011) and lithological properties (Al-Husseini, 1991). The present study concentrates in the sedimentary analysis of the two stratigraphic sections of the Silurian Niur Formation at the Central Iranian Microcontinent (CEIM) and the Kopeh-Dagh sedimentary basin (Turan Plate). Many researchers believe that the Kopeh-Dagh basin was part of the Turan Plate and the Hun superterranes (Stampfli et al., 2001;

Facies Analysis and Sequence Stratigraphy of Silurian Carbonate Ramps in the Turan (Kopeh-Dagh) and Central Iran Plates with Emphasis on

Gondwana Tectonic Event

Zohreh Nowrouzi*, Asadollah Mahboubi, Reza Moussavi-Harami, Mohammad Hossein Mahmudy Gharaie, and Farzin Ghaemi

Department of Geology, Faculty of Sciences, Ferdowsi University of Mashhad, Mashhad 91735, Iran Abstract: Two sections from the Silurian deposits in the Central Iran Micro and Turan Plates were measured and sampled. These deposits are mostly composed of submarine volcanic rocks, skeletal and non-skeletal limestone, shale and sandstone that were deposited in low to high energy conditions (from tidal flat to deep open marine). According to gradual deepening trend, wide lateral distribution of facies as well as absence of resedimentation deposits, a depositional model of a homoclinal ramp was proposed for these deposits. Field observations and facies distribution indicate that, two depositional sequences were recognized in both sections. These sections show similarities in facies and depositional sequence during the Early Silurian in the area. Although there are some opinions and evidences that demonstrated Paleo-Tethys rifting phase started at the Late Ordovician-Early Silurian, similarities suggest that the Turan and Iran Plates were not completely detached tectonic block during this time, and that their depositional conditions were affected by global sea level changes and tectonic events. Key words: facies, sequence stratigraphy, Silurian, Central Iran Plate, Turan Plate

Vol. 89 No. 4 pp.1801–1840 ACTA GEOLOGICA SINICA (English Edition) Aug. 2015

2015-7-23打印

* Corresponding author. E-mail: Z_Novroozi85@yahoo.com

完成作者一校并发出二校

© 2015 Geological Society of China

1802 Vol. 89 No. 4 Aug. 2015 ACTA GEOLOGICA SINICA (English Edition) http://www.geojournals.cn/dzxben/ch/index.aspx http://mc.manuscriptcentral.com/ags

Stampfli and Borel, 2002; Torsvik and Cocks, 2004; Ruben et al., 2007). The Hun superterranes were part of the Gondwana supercontinent until the Paleo-Tethys started to opening at the north and the Hun superterranes rifted away from the north of Gondwana (Ruban et al., 2007). Submarine volcanic rocks at the base of Silurian deposits in Central Iran and Kopeh-Dagh basin and also Alborz realm demonstrated an intra-continental rifting phase during Early Silurian (Alavi, 1991). This rifting phase demonstrated Paleo-Tethys likely started to opening at the Early Silurian time (Lasemi, 2001). The aim of this study is twofold: 1) interpretation of facies and sequence distribution, paleoenvironment as well as paleogeographic conditions of the Central Iran and Turan Plates during the Silurian, and 2) interpretation and discussion of the role of tectonic events and climatic factors and global sea-level changes on the facies and sequence variations of Central Iran, Turan Plates and Middle East. This data will help us to improve the understanding about sedimentary, tectonic and paleogeographic conditions of the North Gondwana and Middle East during this time and to obtain more comprehensive models of their evolution. 2 Geological Setting

The CEIM consists of three N to S-oriented structural units, i.e., the Lut, Tabas, and Yazd Blocks (Fig. 1). The first studied section (Dahaneh-Kalut) with 646m thickness, locates in the eastern side of Dahaneh-Kalut gorge in the south of Derenjal Mountains, at the Tabas Block. This succession was first described by Rutner et al. (1968) as the Niur Formation and is mainly composed of limestones, dolomitic limestones, sandstones and shales with submarine volcanic rocks at its lowermost part. Dolostone and milky sandstones are more common at the lower part of this succession, whereas bioclastic and Corallian limestones dominate at the upper part. Although Ruttner et al. (1968) considered the lower contact of the Niur Formation with the Early–Middle Ordovician Shirgesht Formation to be a transitional boundary; Ghobadi Pour et al. (2006) believed to be faulted. The upper boundary is also conformable with the the Devonian Padeha Formation (Ruttner et al., 1968). According to ostracod fauna in the lower part of the Niur Formation at Derenjal Mountains, this unit is Llandovery in age, whereas coral and ostracode fauna indicates a Wenlock to Peridoli age for the upper part of this succession (Flügel and Saleh, 1970; Hairapetian et al., 2011). The second studied section (Bojnourd) locates at approximately 35 Km south of Bojnourd city at the Kopeh-Dagh basin (Fig. 1). This section is 425m in thickness and Afshar-Harb (1979) subdivided it into lower and upper members. The

lower member is similar to the Silurian deposits of Zagros and the Arabian Plate. It startes with submarine volcanic rocks, graptolite and trilobite-bearing dark shales (bioclastic shale), siltstones and also Orthoceras-bearing nodular limestones, followed by shallower carbonate deposits. The upper part member corresponds to sandy limestones and milky sandstones. Ghavidel-Syooki and Vekoli (2007) believed the lower part is Llandovery in age and the upper part is Wenlock in age.

3 Materials and Methods

Two stratigraphic sections were measured and sampled (Fig.1) with emphasis on lithology and sedimentary structures. Petrographic studies were performed on approximately 230 thin sections that were stained with Alizarin Red S and potassium ferricyanide solution (Dickson, 1966) for the determination of calcite from dolomite. Facies textures were classified based on Folk (1962, 1980), Dunham (1962) and Embry and Klovan (1971). In some facies, clasticity index (Carozzi 1993) was measured for ooids, intraclasts and echinoderm fragments. In this study, completely micritized grains have been considered as micritized coated grains. Since the lower parts of the Dahaneh-Kalut section were dolomitized, facies interpretation was carried out according to field characteristics and their fossil contents. Description of the siliciclastic lithofacies is based on facies codes presented by Miall (2006). Facies types and stacking patterns were classified based on Schlager (2005) and Catuneanu et al. (2009). Based on facies trends, stacking pattern and lithological changes, we present depositional sequence stratigraphic system tract distribution and named following T/R sequence (Catuneanu et al., 2009).

4 Results and Discussions 4.1 Facies associations

Based on lithology, textures and characteristics and textures, four facies associations including siliciclastic, mixed siliciclastic-carbonate, carbonate and also volcanic facies were identified for the Silurian deposits of Dahaneh-Kalut and Bojnourd sections (Figs. 2, 3 and 4). Facies characteristics of each association have been summarized in Tables 1 to 3. These associations were deposited in tidal flat, lagoon, shoal and open marine environments. 4.1.1 Tidal flat

These deposits consist of sandstones, mudrocks

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(siltstone and silty limestone) and sandstone with interbedded siltstones that are identified in the middle and upper parts of Dahaneh-Kalut section and also in the upper part of Bojnourd section. Five sandstone (St, Sp, Sh, Sl and Sr), one mudrock (Fl) and one sandstone with interbedded siltstones (Sl/Fl) lithofacies were identified

(Table 1 and Fig. 5). Sandstones are mainly medium to fine grained, subrounded to well-rounded and well-sorted quartzarenites and litharenites (Fig. 5a; Nowrouzi et al., 2013). In some cases, quartzarenites have bioclastic debrise of brachiopods and echinoderms (Fig. 5b) that usually are dolomitized and silicified. Sandstone (St, Sp,

Fig. 1. (a), Geographic location of the two studied Silurian successions in Central Iran and Turan Plate (black stars) and (b), their structural context (shaded area in a), at (c), the Bojnourd section and (d), the Dahaneh-kalut section.

1804 Vol. 89 No. 4 Aug. 2015 ACTA GEOLOGICA SINICA (English Edition) http://www.geojournals.cn/dzxben/ch/index.aspx http://mc.manuscriptcentral.com/ags

Sh, Sl, and Sr) lithofacies formed sandy bedforms (SB) architectural element (Miall, 2006) with linear form in plane view in Dahaneh-Kalut and Bojnourd sections. Herringbone cross-beds (Fig. 5c) and wavy ripples were observed in Sp and Sr lithofacies respectively (Fig. 5e and Fig. 5f). Mudcracks also existence in siltstones and Sl lithofacies (Fig. 5g). Sandstone with interbedded siltstones consists of laminated fine grain sublitharenites with interbedded laminated siltstones (Sl/Fl) (Fig. 5h). Interpretation

Herringbone cross-beds and wavy ripples were dominant in an intertidal zone (Strand 2005) whereas mudcracks are the major sedimentary structures that are related to supratidal or upper-intertidal zone (e.g., Eriksson and Simpson, 2012). Well rounded mature-super mature quartzarenites indicate high energy conditions such as intertidal to subtidal environments (El-Azaby and El-Araby, 2005; Wanas, 2008). Presence of bioclasts in the

well rounded, super mature quartzarenites show that these sandstones may have been deposited in the subtidal environment with high energy conditions (Wanas, 2008), whereas sandstones alternating with siltstones (Sl/Fl), laminated siltstones and silty lime mudstones (Fl) demonstrated deposition in the low energy conditions of the subtidal environment (Strand, 2005). According to identified sedimentary structures and lithofacies, this sandy bedform (SB) element formed in the high to low energy tidal flat environment. 4.1.2 Lagoon

Lagoonal deposits consist of four facies (Table 2), which are mainly composed of peloids and include: Bioclastic peloidal wackestone/packstone (L1) (Fig. 6a), Peloidal packstone (L2) (Fig. 6b), Oncoidal rudstone (L3) (Fig. 6c), and Peloidal spiculitic packstone (L4) (Fig. 6d). L1 mostly composed of spherical pellets, micritized grains and also bivalves and gastropods. Spherical and very

Fig. 2. Legend of abbreviations for Figs. 3 and 4.

Table 1 Description of tidal flat siliciclastic and mixed siliciclastic-carbonate lithofacies Lithofacies Description petrography Sandstone St Trough cross-bedded, Medium sand, Set thickness: 10-30 cm Quartzarenite, sublitharenite Sp Planar cross-bedded, Coarse to fine sand,Herring bone cross bedding Quartzarenite, sublitharenite Sh Horizontally bedded or laminated, Fine sand, in some cases there are mudcracks, Set bed thickness:

10 to 50 cm Quartzarenite, litharenite

Sl Low-angle cross bedded, Fine sand, small sets Quartzarenite, subarkos, sublitharenite

Sr Rippled (wave and interference ripples), Medium to fine sand Quartzarenite, litharenite, subarkos

Mudrock Fl Laminated or rippled, Silt size, 1 to 2 cm layers, in some cases with mudcracks Siltstone, Silty (or sandy)

lime mudstone Sandstone with interbedded siltstone

Sl/Fl Laminated sandstone and siltstone, Fine sand and Silt , 5 cm to 10 cm layers of sandstone with 1 to 2 cm layers of siltstone

Sandstone with interbedded siltstone  

Aug. 2015 Vol. 89 No. 4 1805 ACTA GEOLOGICA SINICA (English Edition) http://www.geojournals.cn/dzxben/ch/index.aspx http://mc.manuscriptcentral.com/ags

rounded pellet, micritized coated grains and bivalves are very common in L2. L3 mainly composed of micritized and microbial concentric oncoids (C-type). Ellipsoidal to angular pellets and sponge spicules are more frequent grains in L4. Interpretation

Due to low energy conditions, lagoonal facies are characterized by large proportions of mud with gastropods, bivalves and microbial oncoids (Bachmann and Hirsch, 2006; Palma et al., 2007; Lasemi et al., 2012). Protected lagoons with varying salinities behind barriers are sites for the deposition of fine grained sediments with low-diversity biota (Lasemi et al., 2012). The presence of

peloids (pellets) in the packstone and wackestone facies (L1 and L2) demonstrated low energy conditions and also restricted depositional environment. Absence of open marine bioclasts and presence of euryhaline biota (such as bivalves and gastropods) in L1 demonstrated a very shallow marine back shoal environment with large fluctuation in salinity. Open marine bioclasts such as echinoderms, brachiopods, coral and trilobite in L1 and also sponge spicules in L4 could be reworked by high energy storms and tides from outer ramp (Delecat et al., 2001) to lagoon conditions. Oncoidal rusdstone (L3) with microbial concentric (C-type) oncoids is related to back barrier and shallow inner ramp settings with low energy and sedimentation rate (Lasemi et al., 2012; Chen and Lee,

Fig. 3. Lithology and sequence stratigraphy interpretation of Silurian deposits (Niur Formation) at the Dahaneh-Kalut section.

1806 Vol. 89 No. 4 Aug. 2015 ACTA GEOLOGICA SINICA (English Edition) http://www.geojournals.cn/dzxben/ch/index.aspx http://mc.manuscriptcentral.com/ags

2014). 4.1.3 Shoal

Shoal facies association is composed of 4 facies (Table 3) that are mainly grainstones and consist of Peloidal grainstone (S1) (Fig. 7a), Bioclastic grainstone (S2) (Fig. 7b), Intraclast grainstone (S3) (Fig. 7c) and Bioclstic packstone/grainstone (S4) (Fig. 7d). Lithic pelloids, micritized grains and fine sand size extraclasts are more common in S1. S2 extremely composed of bioclasts such as echinoderms, brachiopods, bryozoans and coral in spary calcite. Intraclasts in S3 are composed of sand size grains that situated in spary calcite. Lithic pelloids, micritized grains and marine bioclasts are more common in S3.

Echinoderms, brachiopods, bryozoans and coral are more common in S4 that are situated in mixed spary and micritic matrix. Interpretation

Shoal environments are characterized by high wave energy, relatively high tidal range and low portions of mud (Lasemi et al., 2012). Since peloidal grainstones were deposited in warm and shallow water with slight circulation environment (e.g., Jank et al., 2006), S1 was probably deposited in barrier near the lagoon setting. Presence of open marine bioclasts in S2 and S4 facies indicated deposition at the vicinity of open marine setting with normal marine salinity conditions (Flügel 2010).

Fig. 4. Lithology and sequence stratigraphy interpretation of Silurian deposits at the Bojnourd section.

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Layers containing intraclasts (S3) indicate episodic high-energy events that are usually caused by wave actions (Hips and Hass, 2009). Intraclastic grainstone (S3) are often interpreted as deposits formed by storm erosion and reworking of various sediment types occurring in shallow-marine environments (Bádenas and Aurell, 2010). This facies association with variable portions of bioclasts (S2 and S3) and the presence of large intraclasts (S3) with high clasticity index and also cross bedding was deposited in a relatively high energy shoal environment with normal marine conditions (Cortes et al., 2009). The presence of mud in S4 indicated a relatively low energy setting under a shoal (near the open marine) environment (Bádenas and Aurell, 2010). 4.1.4 Patch reef

The patch reef facies (Figs. 8a and 8b)(R= boundstone) is present at the upper part of Dahaneh-Kalut section and is mainly formed by branching and encrusting Bryozoansns (Table 3) that are preserved in situ. Base of patch reef composed of very thin encrusted bryozoans and corals meadows. Inter particle space in patch reef and meadow mainly filled by bladed isopachus or drusy calcite cements and/or mud, clast-supported internal sediment and peloidal wackestone (Fig. 8c). Internal sediments are mostly composed of well rounded clast-supported intraclasts and bioclasts. Patch reef is about 0.5 m in mean thickness and pinches out or grades into bioclastic shoal deposits. Interpretation

Bryozoans and corals meadows at the base of patch reef made a hard substrate for the growth of branching byozoans and corals. The presence of in situ preserved corals and bryozoans suggests that they formed in a shallow subtidal setting (Chablais et al., 2010). Clast-supported internal sediment such as intraclasts and bioclasts are mainly well rounded that indicating redeposition. Peloidal wackestone or isopachous bladed and drusy calcite that filled cavities, indicated syndepositional extensive water circulation in high energy conditions of shallow subtidal platform interior environment (Atkinson et al. 1981). This early cementation led to early lithification and also preservation or reef structures (Onoue and Stanley, 2008). Additionally, cross-reef currents and tidal exchange across reef margin can be enhanced water circulation (Atkinson et al., 1981). Limited lateral extension and also its variable thickness indicated reef facies is not continuous and is asymmetric. The presence of open-marine biota such as brachiopods and echinoderm within some lagoon facies demonstrated tidal exchange between platform and ocean (Chablais et T

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1808 Vol. 89 No. 4 Aug. 2015 ACTA GEOLOGICA SINICA (English Edition) http://www.geojournals.cn/dzxben/ch/index.aspx http://mc.manuscriptcentral.com/ags

Fig. 5. Tidal flat facies. (a), Super mature quartzarenite (XPL); (b), Quartzarenites with echinoderm debris (XPL); (c), Herringbone cross bedding in Sp lithofacies, Da-haneh-Kalut section; (d), low angle cross bedding sandstone (Sl), Dahaneh-Kalut section; (e), St and Sl lithofacies at the Dahaneh-Kalut section; (f), interference ripplemarks in the Sr lithofacies, Bojnourd section; (g), polygonal mudcracks in the fine grain litharenite, Bojnourd section; (h), Alter-nation of siltstones (Fl lithofacies) and sandstones (S lithofacies), Dahaneh-Kalut section.

Aug. 2015 Vol. 89 No. 4 1809 ACTA GEOLOGICA SINICA (English Edition) http://www.geojournals.cn/dzxben/ch/index.aspx http://mc.manuscriptcentral.com/ags

Fig. 6. Photomicrograph of lagoon facies in XPL. (a), Bioclastic peloidal wackestone/packstone (L1), Bojnourd section; (b), Peloidal packstone (L2), Bojnourd section; (c), Oncoidal rudstone (L3) with c-type oncoid, Dahaneh-kalut section; (d), Peloidal spiculitic packstone (L4) with monoaxon (green arrow) and tetraxon (orange arrow) sponge spicules, Bojnourd section.

Fig. 7. Photomicrograph of shoal facies in XPL. (a), Peloidal grainstones (S1), Dahaneh-Kalut section; (b), Bioclastic grainstones (S2), Dahaneh-Kalut and Bojnourd sections respectively; (c), Intraclastic grain-stone (S3), Dahaneh Kalut section; (d), Bioclastic packstone/grainstone (S4), Bojnourd section.

1810 Vol. 89 No. 4 Aug. 2015 ACTA GEOLOGICA SINICA (English Edition) http://www.geojournals.cn/dzxben/ch/index.aspx http://mc.manuscriptcentral.com/ags

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al., 2010). Since R facies is not thick and continuous and graded to bioclastic shoal facies, we suggested that this facies was located at the vicinity of the shoal environment with high energy conditions.

4.1.5 Open marine

This facies association consists of eight facies that are mainly composed of corals, echinoderms, brachiopods, bryozoans and also Orthoceras, trilobites and graptolites (Table 3). These facies consist of Brachiopod rudstone (O1) (Fig. 9a), Bryozoans rudstone (O2) (Fig. 9b), Echinoderm packstone (O3) (Fig. 9c), Bioclastic packstone/wackestone (O4) (Fig. 9d), Bioclastic and iron ooid-bearing siltstone (O5) (Fig. 9e), Orthoceras-bearing nodular limestone (O6) (Fig. 9f), Bioclastic shale (O7) (Fig. 9g) and Submarine volcanic rocks (O8) (Figs. 9h and 10). Marine biota that is situated in micritic matrix is the special characteristic of O1-O7 facies. However, these facies are mostly medium to thin bedded, pillow structures existence in O8. Additionally, there are some strata in the Dahaneh-Kalut section that are completely dolomitized. This dolostone, due to field properties and open marine biota contents (such as echinoderms, corals, brachiopods and bryozoans), demonstrates as open marine facies in the facies column (Fig. 3). Interpretation

Open marine biota are very common in this facies association. The presence of this biota and the higher proportion of mud compared to shoal facies, indicate an open marine setting (Bádenas and Aurell, 2010). The presence of biota that lived in normal saline water, well-oxygenated and euphotic conditions (such as corals, brachiopods, bryozoans and echinoderms) in the O1-O4 facies demonstrate shallow open marine conditions (Hips and Hass, 2009). Facies carbonate dominated by Orthoceras, graptolites and trilobites (O5-O7) indicate a pelagic-carbonate platform environment (Flugel, 2010; Sachanski et al., 2010). Whereas O1-O3 indicated inner-mid ramp transition conditions, mud supported facies deposited in mid (O4) and outer-ramp (O5-O7) with greater water depth (Aghaei et al., 2012). Since siltstones and shales (O5 and O7) were likely deposited in low-energy conditions, bioclastic and iron ooid-bearing siltstones (O5) that have outer ramp bioclasts (such as trilobites) deposited as distal tempestites so that ooids were redeposited via storms from shallower areas (Hips and Hass, 2009). These iron ooids can be explained by the replacement of carbonate ooids by iron compounds, probably during very early diagenetic stages (Flügel 2010). Nodular bedding in O6 is explained as the result of early lithification in the muddy calcitic sediments and T

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nd

Boj

nour

d  

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intense burrowing in O4 shows low rate of sedimentation in open marine environment (Flügel, 2010). Sub marine volcanic rocks (O8) that are presented as pillow lava were related to an intra-continental rifting phase during Early Silurian time (Lasemi, 2001) and were place in deep open marine and also basin environments (Gibson et al., 1999). These sub marine volcanic rocks presented the deepest facies at the studied areas and show a porphyric texture (Fig. 9h).

4.3 Sequence stratigraphy Facies analysis and field observations led to the

identification of two depositional sequences (DS) with correlative sequence boundary (SB2) at both sections that are a part of third-order depositional sequence (Fig. 11). Since the lower part of Dahaneh-Kalut section was affected by dolomitization, system tracts and depositional sequence were mainly determined based on field

observations and fossils contents. The upper boundary of the second depositional sequence at the Dahaneh-Kalut section, is likely located in the Devonian Padeha Formation, but there is a stratigraphic gap between the Niur Formation and Padeha Formation in the Bojnourd section. 4.3.1 Dahaneh-Kalut section

The Niur Formation at the Dahaneh-Kalut section was deposited during the Early (Llandovery) to Late Silurian (Pridoli) (Flügel, 1962; Hairapetian et al., 2011). In this section (Fig. 3), the Niur Formation is composed of two depositional sequences. The first depositional sequence was deposited during Llandovery, while TST and lower most HST were deposited during Wenlock to Pridoli. The rest of the HST of DS2 was likely to continue to the Devonian Padeha Formation. Transgressive system tract (TST) of the first depositional sequence consists of submarine volcanic rocks (basin facies), dolostone and

Fig. 8. Patch reef facies (R: Boundstones). (a), Field photo of patch reef facies at the Dahanh-Kalut section with bryozoans, corals and echinoderms; (b) close up view of (a); (c), Photomicro-graph of boundstone in PPL that is showing biota colony of bryozoans, gastropods, brachiopods and also intraclasts and microbial oncoids.

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Fig. 9. Open marine facies. (a), Brachiopod rudstone (O1), XPL, Bojnourd section; (b), Bryozoans rudstone (O2), XPL, Bojnourd section; (c), Echinoderm packstone (O3), XPL, Bo-jnourd section; (d), Bioclastic packstone/wackestone (O4), XPL, Dahaneh-Kalut section; (e), bioclastic and iron ooid-bearing siltstone (O5), XPL, Bojnourd section; (f), Field photo of Orthoceras-bearing nodular limestone (O6), Bojnourd section; (g), Field photo of bioclastic shale (O7), arrow shows trilobite, Bojnourd section; (h), Porphyric texture of submarine volcanic rocks, XPL, Dahaneh-Kalut section.

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shale. Field observations and biota contents of dolostone and shale such as echinoderms, brachiopods, corals and bryozoans biota demonstrate open marine facies for these deposits. Highstand system tract (HST) of DS1 is

composed of tidal flat facies (sandstones, sandy (dolomitized) limestones and sandstones with interbedded siltstones). The absence of fossils in sandy dolomitized limestone and also intercalation with tidal flat facies can

Fig. 10. Field photo of Submarine volcanic rocks (O8) with pillow structures at the lower part at the Dahaneh-Kalut section.

Fig. 11. Outcrop image of the Silurian deposits at the Bojnourd section indicating depositional (DS1 and DS2), trangressive and highstand systems tracts (TST and HST), sequence boundary (SB2) and maximum flooding surface (mfs).

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be assigned sandy dolomitized limestone as tidal flat facies. MFS of DS1 which represent a sharp progradation of these tidal facies over the open marine facies located at the top of bioclastic shale. HST of DS1 with a correlative sequence boundary graded into second depositional sequence. First SB located between DS1 and DS2 at the top of the last package of sandy dolomitized limestone which is overlaid by the first open lagoon carbonate deposite. Second depositional sequence at this section started with domination of carbonate regime of the TST deposits. TST consists of dolostone (with open marine biota and minor terrigenous input during the relative sea level rise), bioclastic peloidal wackestone/packstone, oncoidal rudstone, bioclastic grainstone, intraclastic grainstone, peloidal grainstone, boundstone, bryozoans rudstone and bioclastic packstone/wackstone. Open marine facies overlaid by the tidal flat facies (silty lime mudstone and sandstones). Tidal flat facies demonstrate highstand system tract of DS2. Sharp change from open marine to tidal flat facies shows progradation and demonstrates MFS located at the top of bioclastic packstone/wackstone facies. The HST deposits are conformably overlain by Devonian Padeha Formation sandstones and the Niur Formation records only the lowermost part of HST of DS2. 4.3.2 Bojnourd Section

According to paleontological studies, lower and upper part of Niur Formation at the Bojnourd section deposited during Early (Llandovery) and Middle (Wenlock) Silurian respectively (Ghavidel-syooky and Vecoli, 2007). These deposits (Fig. 4) similar to Dahaneh-Kalut section, started with transgressive system tract (TST). TST consists of sub marine volcanic rocks and deep open marine facies such as bioclastic and iron ooid-bearing siltstone, Orthoceras nodular limestone and bioclastic shale. TST deposits overlaid by HST of DS1 that composed of bioclastic packstone/wackstone and bioclastic peloidal wackstone/packstone (lagoon facies). MFS of DS1 located at the top of the last open marine facies which is overlaid by lagoon facies and represent a sharp progradation of these lagoon facies over the open marine facies. First SB is correlative and located between DS1 and DS2 at the top of the last package of lagoon facies which is overlaid by the first open marine facies of TST of DS2. TST of DS2 consists of open marine facies such as bryozoans rudstone, echinoderm packstone, bioclastic wackestone/packstone and brachiopod rudstone. MFS of DS2 located at the top of the last open marine facies which is overlaid by shoal facies. HST of DS2 consists of shoal facies bioclastic packstone/grainstone and bioclastic grainstone with bryozoans, echinoderm and coral that grade into lagoonal

peloidal spiculitic packstone and tidal flat sandy limestones, siltstones and sandstones. According to Ghavidel-syooky and Vecoli (2007), TST and HST phase of DS1 and also TST phase of DS2 deposited during Llandovery whereas HST of DS2 (sandstone and sandy limestone) deposited in Wenlock. The HST deposits overlain by Devonian Padeha Formation sandstones. 4.3.3 Sea level change interpretation and depositional model

Our studies revealed that the Dahaneh-Kalut section was composed of two DS that were deposited during the Llandovery-Pridoli periods. Thus, TST and HST of DS1 were deposited during Llandovery and TST and lower most HST of DS2 deposited during Wenlock to Pridoli. The rest of the HST (of DS2) continued till the Devonian Padeha Formation. At the Bojnourd section, the Niur Formation was also composed of two DS that deposited during Llandovery-Wenlock time. In this section, whereas TST and HST of DS1 and also TST of DS2 deposited during Llandovery, HST of DS2 deposited during Wenlock and overlain by the Devonian Padeha Formation. Comparison of the sequences in two studied sections (> 400km apart) has shown in figure 12 with approximately chronostartigarphy location of SB and MFS. This comparison revealed that there is a gap encompassing Ludlow and Pridoli stages in the Bojnourd section, and DS1 in the Dahaneh-Kalut has to include the two DS (DS1 and DS2) of Bojnourd section. This comparison also shows TST of DS1 in two sections is similar respect to the time of deposition, facies type and facies association. Depositional sequences of the Niur Formation followed four sedimentary stages in the studied area (Fig. 12).

(1) The first sedimentary stage related to post-glacial transgressive phase (Llandovery) recorded in both section. During the Late Ordovician and Early Silurian with final Hirnantian stage, polar glaciers advanced over regions of Gondwana from northern Africa to Saudi-Arabia (Al-Husseini, 1991) and some parts of Iran (Ghavidel-Syooky et al., 2011). This phase marked by a sharp sea level fall during the latest Ordovician (Haq and Schutter, 2008). At the Early Silurian (Llandovery) time, polar glacial was melted and sea level rose sharply (Haq and Schutter, 2008). Post glacial transgressive phase (Llandovery) marked by graptolite organic rich shale on the Arabian and adjoining Plates except for the Silurian Niur Formation of Central Iran (Al-Husseini 1991). This TST phase followed by shallowing upward trend in the Saudi-Arabia (Qusaiba member of the Tabuk Formation), Syria (Tanf Formation), Jordan and Zagros (Sarchahan Formation)( Al-Husseini, 1991) similar to HST phase of DS1 at the Central Iran and Turan Plate that described here.

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(2) The second sedimentary stage took place at the Middle-Llandovery to Middle-Late Wenlock. Some geologists believed that Paleo-Tethys rifting phase started at the Mid-Silurian to Early Devonian (e.g. Stampfli et al., 2001; Stampfli and Borel, 2002;) but the presence of submarine volcanic rocks at the base of Silurian deposits in the Central Iran and Turan Plate demonstrated an intracontinental rifting phase and ripen separation of Turan from Central Iran Plate during this time (Berberian and King, 1981; Lasemi 2001) that led to separation of Kopeh-Dagh (Turan Plate) from Iranian Plate (Lasemi 2001). This separation caused different facies and sequence stratigraphy patterns between two sedimentary basins (Central Iran and Turan Plate) and Dahaneh-Kalut and Bojnourd section. At the end of this stage, sedimentation has been completed in Bojnourd section.

(3) The third sedimentary stage started with TST of DS2 in Dahaneh-Kalut section and was simultaneous with global sea level rise (Haq and Schutter, 2008) at Middle-Wenlock to Ludlow.

(4) At the Late-Silurian took place a major global sea level drop (Haq and Schutter, 2008). This event and

regional Silurian progradation caused major Hiatus at the Middle East (Al-Husseini, 1991). Berberian and King (1981) believed that this hiatus related to Caledonian Orogeny but Ruban et al. (2007) cited the pre-rift thermal swelling or post-rift isostatic rebound associated with the Hun Superterrane break way as effective agent for distinct Upper Silurian hiatuses (Syria, Iraq and Oman) in the Middle East. This phase of sea level regression is similar to HST of DS2 at the Dahaneh-Kalut section in Central Iran.

Previous studies considered the Central Iran Plate as a part of Gondwana that completely attached to it, but new researches suggested this position only for the Zagros basin (Hairapetian et al, 2008 and 2011). According to these researches, there is a small sea between the Central Iran, Turan and Gondwana supercontinent. Figure 13 shows the position of Central Iran and Turan Plate during Earliest and Late Silurian time. Field observations and facies analysis of the Silurian deposits demonstrate a gradual deepening trend from tidal flat environment to the deep marine facies this is more compatible with a ramp setting than a shelf. The absence of resedimentation evidences such as turbidite deposits (related to a steep

Fig. 12. Correlation chart of depositional sequences at the Dahaneh-Kalut and Bojnourd sec-tions that showing more similarities during Early and Middle Llandovery (Early Silurian), when Iran and Turan Plates were not separated. The chronostartigraphic location of SB and MFS is approximate.

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slope) and lack of breccias, slope and talus debris at the front of non-continuous patch reef and the existence of open marine biota in the lagoon facies suggested a homoclinal ramp (Read, 1982 and 1985) as depositional model for Silurian deposits in Dahaneh-Kalut and Bojnourd sections. Lasemi (2001) suggested that, the separation of the Turan Plate from the Iran Plate took place during the Late Ordovician to Early Silurian. Therefore, the Silurian Niur Formation was formed in two distinct depositional models at the Dahaneh-Kalut and Bojnourd sections. Since these basins were affected by Paleotethys, our field observations demonstrated a northward deepening trend for the Central Iran and Dahaneh-Kalut section, but the Turan Plate and Bojnourd section show a southward deepening trend (Fig. 14).

A comparison of the Silurian deposits in the Central Iran and Turan Plates with other places in the world revealed that global sea level plays an important role in the observed facies and regional relative sea level changes although local tectonic events and fault activities could provide complications on these patterns. The Paleozoic plate tectonic reconstruction of Iran required much more tectono-stratigraphy, paleontology and paleo-magnetite data and investigations. It is hopeful that this study will improve the understanding of sedimentary and tectonic conditions of the north Gondwana and Tethyan side of Middle East during the Silurian and can be used for paleogeographic reconstruction. 5 Conclusions

The Silurian deposits in the Central Iran and Turan

(Kopeh-Dagh) Plates are mainly composed of limestones, shale, sandstones and submarine volcanic rocks at the base. Facies analysis demonstrated these strata were deposited in low to high energy conditions from tidal flat to deep open marine environments at the continental passive margin basin. Twenty three facies were distinguished in two measured sections based on sedimentary structures, texture and lithology. Field observations and facies distributions demonstrated these strata were deposited in a homoclinal ramp that preserved two third-order depositional sequences (DS1 and DS2) with non-erosional sequence boundary. The Niur Formation in the Dahaneh-Kalut section were deposited during Llandovery-Pridoli (TST and HST of DS1: Llandovery, TST and lowermost HST of DS2: Llandovery, rest of HST continuing in the Padeha Formation). The Bojnourd section was deposited in Llandovery-Wenlock so that TST and HST of DS1 and TST of DS2 deposited during Llandovery and HST of DS2 deposited in Wenlock. Thus, both sections started with open marine facies as TST stage of DS1. Although there are evidences of passive rifting activities during the Early Silurian that led to the separation of Turan from Iran Plate, similarities between depositional sequences of Central Iran and Turan Plate demonstrated these plates were not completely detached basins during the Early Silurian (Llandovery) and facies differences in two section during this time may be related to local tectonic events. From the Middle Llandovery time with formation of different depositional sequence at both sections, the rifting likely led to the separation of Turan from Iran Plate and formation of different depositional sequence. Despite of

Fig.13. Paleogeographic maps of Central Iran (1) and Turan Plate (2) during earliest (A) and late (B) Silurian (Modified from Hairapetian et al., 2008 and 2011).

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differences and similarities, sequence stratigraphy patterns of these sections are very similar to other places at the world and Middle East region. These similarities revealed that global eustatic changes were premium stimulus of observed relative sea level changes during the Silurian although local tectonic events and fault activities could provide complications on these patterns.

Acknowledgements

We would like to thank the logistical and financial support given to this study by the Department of Geology of Ferdowsi University of Mashhad-Iran. Thorough and

constructive reviews by Beatrix Bádenas Lago, and greatly improved the manuscript.

Manuscript received accepted

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About the first author Zohreh Nowrouzi, born in Mashhad, Iran in 1984, received his

master’s degree in sedimentology and sedimentary petrology from Ferdowsi University of Mashhad in 2008 and is now a candidate for a Ph.D. of the department of Geology from the Ferdowsi University of Mashhad, Mashhad, Iran. Her current interests include facies analysis, diagenetic history and evolution of sedimentary rocks.

E-mail: Z_Novroozi85@yahoo.com.