Stratigraphy

Precambrian

The consolidation of the Iranian basement by metamorphism, partial granitization and partly by intense folding took place in the Late Precambrian. This event has been attributed to the ‘Baikalian’ or Pan–African Orogeny by various authors. Isotopic data of Iranian basement rocks give ages between 600 and 900 Ma. A similar range of isotopic data had been obtained for Arabian Shield rocks. An important post-Pan-African magmatism is documented by the widespread Doran Granite, which cuts the Upper Precambrian rocks and is covered by Lower Cambrian sediments. Late Precambrian postorogenic volcanics, mainly alkali rhyolite, rhyolite tuff and basic dikes are known in the Eocambrian formations. In North and Central Iran, Kahar and Gharehdash Formations and the lower half of the Soltanieh Formation are of Precambrian age. The oldest rocks in Iran belong to the Kushk Series consisting of clastic sediments, acidic volcanic, tuff, and carbonates (mainly dolomite). Other formations of Late Precambrian–Early Cambrian ages include Rizu volcanic-sedimentary Formation, Dezu and Tashk Formations, Aghda limestone, Kalmard Series, Shorm Beds, and Anarak metamorphic units. The sedimentary facies of Precambrian–Lower Cambrian rocks in Northern Iran is different from that of Central Iran.

Paleozoic

With the Pan-African orogeny and following this orogenic episode, shallow marine sediments formed in Late Vendian. The influence of the orogenic episode is evident at the base of the Vendian sediments. Deposition of shallow marine sediments covered large areas in Iran during Paleozoic (e.g., Alborz, East of Iran, Zagros). There is strong stratigraphic evidence that transition from Vendian to Lower Cambrian was a progressive one, without hiatuses; there is no evidence for any orogenic or epeirogenic movements in Iran at this time (e.g., south of Zanjan, Valiabad Chalus, Shahin Dezh). Early Cambrian started with an alternation of shale, phosphate-bearing limestone, and dolomite sitting conformably and transitionally over Vendian dolomites. Transition from Soltanieh Formation to Barut, Zaigoon, and Laloon Formations is very difficult to recognize in the field. Middle Cambrian is characterized by uplift and regression; however, a renewed progression at this time led to the deposition of Mila and Kuhbonan Formations, consisting of limestone, dolomite, and shale, over older units. These formations bear trilobites and brachiopods of Middle and Late Cambrian. In some areas, the Late Cambrian carbonate facies turns transitionally into Ordovician graptolite shales, known as Lashkarak Formation in Alborz, Shirgasht Formation in Central Iran and Ilbeyk and Zardkuh Formations in Zagros. In Kalmard area, Ordovician sediments are sitting on the Vendian sediments through an angular unconformity. In Late Ordovician, most parts of Iran were affected by epeirogenic movements; this coincides with Caledonian orogeny in Europe and some other parts of the earth. The epeirogeny caused a distinct hiatus at the Ordovician–Silurian boundary. Where present, the Silurian rocks in Iran consist mainly of limestone, sandstone, shale and volcanic materials, known as Niur Formation in Central Iran. The Lower Devonian rocks have been reported from several localities in Central Iran (e.g., Tabas, Sourian, Kerman, Zagros); however, they seem to be missing in Alborz and parts of Zagros. Upper Devonian is characterized by marine transgression, particularly in Alborz, that extends into Lower Carboniferous. With exception of Tabas area, no record of Middle Carboniferous marine deposits has yet been discovered in Iran. Upper Carboniferous deposits are not significantly present in Iran and have only been identified in several localities from index goniatites. After a general regression and a distinct hiatus in Upper Carboniferous, Permian marine transgression deposits cover most parts of Iran (e.g., Alborz, Zagros, Central Iran); The Permian sediments are represented by Dorood sandstones, Ruteh and Nesen limestones in Alborz.

Mesozoic

The Lower Triassic sediments in Iran are mainly of shallow marine or continental shelf nature (e.g., Doroud sandstones and Elika dolomites in Alborz, Sorkh shales and Shotori dolomites in Central Iran. A continuous Permian–Triassic sequence has been reported from several areas in Iran, including Jolfa (northwest of Iran), Abadeh (Southern Central Iran), and Southern Urumiyeh (the continuation of the Taurus in Turkey), north of Kandovan and Southern Amol. Transition from Middle to Upper Triassic coincides with Early Cimmerian orogenic episode, which led to the segmentation of the sedimentary basin into three sub-basins: Zagros in the south and southwest, Alborz in the north, and Central Iran. The Lower Jurassic rocks conformably overlie the Upper Triassic units; so are the Early Cretaceous deposits over the Upper Jurassic strata (e.g., Zagros). In North and Central Iran, the Upper Triassic and Lower–Middle Jurassic sediments have a detrital nature, consisting mainly of shale and sandstone with thicknesses varying from a few meters to more than 3,000 m. The presence of plant remains and coal beds suggest a continental or lagoon environment for the deposits. The Cretaceous deposits, characterized by diverse sedimentary facies, are widespread all over Iran. In Late Cretaceous, tectonic movements related to the Laramide orogeny affects most parts of Iran, leading to uplift, folding, and faulting. This is a prelude to significant developments in the geological evolution of Iran.

Cenozoic

In Iran the Cenozoic begins with the Cretaceous–Paleocene boundary that is characterized by striking changes in sedimentary environments (e.g., Alborz, Central Iran). An unconformity has been reported from many locations in Iran. Both continuous and discontinuous transitions have been discovered between Paleocene and Eocene strata; as is the case with Eocene and Oligocene (e.g., Central Iran). The Oligocene and Miocene stages are characterized by rapid subsidence, deposition, and facies changes in both marine and continental sedimentary basins (e.g., Mahneshan and Halab south of Zanjan). Oligocene sediments in most parts of Iran are of shallow marine character, turning into marine facies in Upper Oligocene through Lower Miocene (e.g., Qom). The Middle–Upper Miocene sediments are mostly of continental nature. The Quaternary is the prominent feature of the plains of Iran.

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