Geological history

Tabas UNESCO Global Geopark is one of the components of Central-East Iranian Microcontinent (CEIM), named and known as Tabas Block among geologists. CEIM is surrounded by Sistan and Nain-Baft ophiolite sutures, Doruneh Fault, and Kashmar-Sabzevar ophiolite. It is dividable from the east to the west into Lut Block, Shotori swell, Tabas depression, Kalmard Block, Posht-e Badam Block, Bayazeh-Bardsir Embayment, and Yazd Block by Nayband Fault, and Kuhbanan, Kalmard, and Posht-e Badam long faults, which are curved to the west (Aghanabati, 2006). This region has endured several important orogenic phases and epirogenic movements from the Permian to the Quaternary. Its main features include the occurrence of metamorphic and magmatic events simultaneously with tectonics, especially during the Late Paleozoic, Middle Triassic, Late Jurassic, and Late Cretaceous on its southwestern margin, and also the extensive Paleogene volcanic activity throughout it. In fact, the CEIM has been active as a platform for tectonic activities during the Paleozoic to Late Jurassic-Early Cretaceous. The function of these faults along the Paleozoic and Mesozoic have led to the formation of numerous sedimentary basins, sometimes with rapid depression, mainly controlled by tectonics, in this platform (Davoudzade et al., 1981).

The following is a brief description of the geological and structural features of different blocks in the CEIM.

Lut Block

The easternmost part of CEIM is Lut Block, whose eastern border is determined by Nehbandan fault and flysch basin in eastern Iran, and its western border is determined by Nayband fault and Tabas Block. This block extends about 900 km from Doruneh fault in the north to Jaz Murian basin in the south and is about 200 km wide (Stocklin & Nabavi, 1973). The Lut Block consists of pre-Jurassic metamorphic bedrock, Jurassic sedimentary rocks, and several generations of Late Mesozoic and Cenozoic intrusive or volcanic rocks (Camp & Griffis, 1982; Tirrul et al., 1983). Radiometry data also show that the oldest magmatic activity in the Central Lute Block occurred in the Middle Jurassic (Tarkian et al., 1983), and the tectonic and magmatic evolution of the Lute Block has been interpreted in a extensional environment (Jung et al., 1983; Samani & Ashtari, 1992; Tarkian et al., 1983).

Tabas Block

Tabas block with spindle-shaped configuration is located between Lut and Yazd blocks. This block reaches Nayband fault from the east, while its northern part reaches the Shotori mountain range formed on the sub-branches of this long fault, and Kalmard-Kuhbanan fault separates it from Posht-e Badam and Yazd blocks in the west (Seyed -Emami et al., 2004; Fürsich et al., 2009; Wilmsen et al., 2010; Aghanabati, 2006). These large north-south faults have been formed since the beginning of the structural evolution of Central Iran and divide different facies and sedimentary basins from the Infra-Cambrian period (Stocklin, 1968; Berberian & king, 1981). This block is part of the vast territory of Central Iran and one of the most complex and turbulent geological units in Iran. In fact, this zone can be considered as a series of suspect terrain connected to each other during long geological periods and various geotectonic activities and movements, leading to the formation of a single territory. Different deformation and orogenic phases, along with secondary processes such as the formation of large faults, volcanic and plutonic activities, erosion and sedimentation, formation of new sedimentary basins and mountain ranges following the closure of older sedimentation basins. The continuation of these activities even after amalgamation of the mentioned lands has added to the complexity of their geological conditions, leading to a highly disturbed, turbulent, and deformed area with evidence of various tectonic environments (Nazemi, 2013).

Tabas Block was formed between the Late Triassic and the Jurassic in the west of Iran, and numerous and diverse sedimentation basins with different lithofacies have been formed in it due to global fluctuations in the sea level and numerous tectonic movements on a global, regional, and local scale, especially during the Middle and Late Jurassic (Seyed -Emami et al., 2004). Given the special geological and tectonic conditions of this block, its geological and structural evolution in the Paleozoic does not correspond with the surrounding areas. The lack of obvious traces of Eifelian Hiatus (the first Middle Devonian stage) is a good instance, according to which siliciclastic and evaporate sediments (Padeha Formation) only indicate marine regression in this block. Also, there have been reports of rocks related to Late Carboniferous (Sardar Formation) that do not exist in other parts of Central Iran. Severe depression of the whole Tabas Block during the Paleozoic and Mesozoic to Cretaceous is another feature of this area. Accordingly, about 7000 meters of Paleozoic sediments without significant unconformity (in the mountainous region of Derinjal, north of Tabas) and about 10000 meters of Mesozoic sediments (Shotori sedimentary basin) have been deposited in it. (Aghanabati, 2006). Paleozoic deposits in this area are the most complete sequence of Paleozoic sediments in Iran and West Asia.

From the tectonic point of view, Tabas Block is not a uniform and integrated territory and can be divided into four different sub-zones due to structural-sedimentary inconsistencies: Shotori folded belt (Shotori swell), Tabas depression basin, Ravar – Mazino sub-block, Nayband sub- block and Kalmard Block (Aghanabati, 2006).

Shotori Fold Belt (Shotori Swell)

The first comprehensive studies in this area were performed by Stocklin et al. (1965). Shotori mountain range is a relatively narrow strip with a maximum width of 30 km and a length of more than 200 km and a north-northwest and south-southeast trend in the western part of Lut Block and the northeastern parts of Tabas Block, in the northern parts (Ozbak Kuh Mountain) of which curvature and inclination toward the north can be observed. According to Berberian (1979), the Shotori sedimentation basin is formed as a narrow trench along the Nayband fault due to the breaking of the Precambrian bedrock. In this basin, platform and shallow marine sediments have been formed during the Paleozoic to Triassic period. The thickness of these sediments in the eastern parts of the mountain range reaches 7000 meters, while in the western parts a maximum of 950 meters has been reported (Aghanabati, 2006). The maximum altitude of this mountain range is about 2900 meters above sea level (at the peak of Shotori mountain, east of Tabas city), creating an altitude difference of about 2200 meters with the city of Tabas. From the geomorphological point of view, this mountain range consists of several parallel strips (mainly from sediments of the second geological period) separated by low-lying erosive and elongated plains (Nazemi, 2013). The oldest rock outcrops of this mountain range are related to the fossiliferous limestones of the Bahram and Shishtu formations in the Middle Devonian? – Early Carboniferous observed in the Howz-e-Dorah area. Most of the heights of this mountain range have a rough and precipice appearance and are often formed in calcareous-dolomitic rocks of Jamal (Permian), Shotori (Middle Triassic), and Esfandiar (Late Jurassic) formations. Limited outcrops of Paleogene volcanic rocks are found on the western slope of the Shotori Mountain Range and its central parts, which are less frequent compared to the eastern slope.

Tabas Depression Basin

This part of Tabas Block has a triangular structure and is also known as Tabas pit. Most of it is covered by alluvial and playa sediments of the Quaternary, hiding its geological complexity. The thickness of alluvial sediments covering the plain reaches about 600 meters in some excavations, indicating severe erosion of the heights and the transport of sediments to the Tabas pit. Tabas depression basin is limited by Shotori heights and ridges composed of Neogene sediments in the eastern part, the hidden Parvadeh-Darband fault in the west, and the Cheshmeh Rostam fault in the south. In the northern part, thick rows of relatively continuous Precambrian-Paleozoic deposits have emerged in the form of ridges to scattered heights called Derenjal Mountains, Bagh-e Vang, and Shesh-Angosht. Its central and southern part is where the current Tabas Playa, called ” Rooh Marghoom playa or Jomjomeh (skull)”, is formed. The function of wind erosion and the effects of wind on natural trees and shrubs or artificial forests have led to beautiful erosive phenomena and structures in the form of nabkha and even salt polygons. According to Nazemi (1998), this zone is a contraction and tectonic depression formed due to the elevation of adjacent areas (Shotori, Kalmard, and Parvadeh heights).

Ravar – Mazino Subblock

This sub-block covers a large part of the west of Tabas Block and is limited to the hidden fault of Parvadeh-Darband from the east and Kalmard-Kuhbanan faults from the west. Many features of this zone, including the lack of outcrop of Precambrian metamorphic rocks, forming a platform with successive and long Paleozoic-Middle Triassic sediments, and considerably thick Late Triassic-Late Cretaceous? rocks, are similar to Nayband subblock. However, the only main difference between these two regions is the north-south structural pattern of the Ravar-Mazino subblock, differing significantly from the east-west trends of the Nayband subblock, which are perpendicular to each other (Aghanabati, 2006). One of the structural features of this area is the presence of large folds with a north-south trend. This zone is mainly composed of Paleozoic and Mesozoic deposits. Paleozoic-Triassic sequence of this area is similar to other parts of the block, often deposited in platform and shallow marine conditions and characterized by the absence of frequent and prolonged sedimentation. Other features of this zone include considerable thickness of Mesozoic deposits (Late Triassic – Upper Cretaceous) and the absence of Tertiary rocks.

Nayband Subblock

This subblock is located in the south of Tabas depression, and its northern border is separated from Tabas depression by Cheshmeh Rostam fault. This subblock is separated from the volcanic outcrops of Lut Block by the large Nayband fault from the east. The western border of this subblock in the northern parts (such as Parvadeh coal mines) corresponds to a precipice with a height difference of about 500 meters (called Kamar-e-Ghasemi and Qale-Kah Mountain), which is located at the border of the mountain and the plain with north-south trend and is known as the hidden fault or Parvadeh-Darband fault (Nazemi, 2013). The oldest rock outcrop in this area belongs to Morad series from Pre-Cambrian, which has an outcrop on the northern slope of Nayband Mountain. Morad series sediments are covered by carbonate deposits of Jamal Formation (Permian) with an angular unconformity, and Jamal Formation is covered by thick layers of Middle-Late Triassic deposits. Complete sequences of Jurassic strata are outcropped in this subblock, and there are outcrops of Early Cretaceous limestones in the southwestern (Chehel Payeh mountains) and western parts (Abdoughi region). It should be noted that the rate of total and tectonic subsidence in Abdoughi-Parvadeh basins increases from east to west and from south to north, and the high thickness (about 6000 m) of Late Triassic-Cretaceous deposits can indicate the subsidence of Nayband basin in the Mesozoic (Naimi Ghassabiyan et al., 2010).

Kalmard Block

Kalmard Block covers a small part of the west of Tabas Geopark and CEIM and is located between Kalmard fault in the east and Naini fault in the west. The history of this block refers to two long phase related to Pan-African orogeny and Middle Cimmerian events. In the northern parts of the Kalmard block, a relatively wide outcrop of slightly metamorphic Precambrian basement can be observed with numerous intrusives from intermediate to basic igneous rocks, covered with angular unconformity and nonconformity by siliciclastic deposits of Shirgesht and Rahdar Formation and indicating the first long sedimentary gap. Consecutive and frequent sedimentary gaps are a limited tectonic-stratigraphic unit between Pan-African orogeny and early Cimmerian events, accumulated in shallow environments and representing a clearly different evolutionary course from Tabas Block. Paleozoic sediments of this region, especially Devonian-Carboniferous and Permian deposits, are very different from the surrounding areas, leading to the separate and specific labeling of this region (For example: Rahdar, Gachal, and Khan formations). These differences are mainly due to short and frequent sedimentary gaps in Paleozoic-Triassic sediments. Accordingly, Aghanabati (2006) has separated Kalmard area from Tabas Block and introduced it as a separate block. The Jurassic stratas of this block are limited to Early-Middle Jurassic sedimentary sequences, and the absence of deposits younger than Middle Jurassic (Badamu Formation) indicates that the second longest gap of this block was after the Middle Jurassic, with the Mid-Cimmerian event as its underlying cause (Aghanabati, 2006).

Stratigraphy of Tabas Geopark

As mentioned, this area has a very high geological importance, introduced by geological experts and thinkers as a geological paradise and fossil museum of Iran. The most prominent feature of this area is its stratigraphy and the presence of the most complete Paleozoic sequences in Iran and West Asia. Therefore, many researchers have sought to answer questions leading to a better understanding of the geological history of this region of Iran and the world. This geopark is mainly covered with Precambrian to Quaternary sedimentary rocks, while scattered and partial outcrops of igneous and metamorphic rocks are also observed in different parts of the geopark. Similar characteristics of Precambrian-Paleozoic lithology and Paleomagnetism data, along with identical tectonic events in Tabas Geopark, indicate that this area and other similar parts of Iran were interconnected during the Late Precambrian to Late Paleozoic and included the northern part of Gondwanaland (Stocklin, 1977; Berberian & King, 1981; Alavi, 1994; Alsharhan & Narin, 1997; Sharland et al., 2001; Wendt et al., 2005; Ruban et al., 2007). The formation of different sedimentary-structural basins (from continental conditions to deep sea environments) due to the activity of basement faults has given this region of Iran a unique diversity of fossils and sedimentary rocks. So far, over 20 type sections and reference sections of Central Iran formations from this area have been introduced to the geology of Iran and the world.

– The scientific resources are available in the management of Tabas Geopark.
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