Precambrian basement

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 1100 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.

     The basement is exposed only in limited parts of the platform area. It consist of low–grade (greenschist facies) metamorphosed ,fine-clastic sediments :the Kahar Formation in N Iran ,the Morad Series in the Kerman area and Shorm Beds in E (Fig. 325) intruded by a lecogranite (Doran Granite) .This slightly metamorphosed Precambrian basement is overlain with questionable unconformity by non- metamorphosed sedimentary rocks of Eocambrain –Cambrian age.

Paleozoic of platform area

The platform deposits are well developed in the main and S Alborz Range, in the greater part of Central and E Iran as well as in SW Iran. The north Iran Suture marks the N termination of the platform regime of the Arabian –Iranian Plate.

       The pan-African Orogeny in Iran was followed by a long period of tectonic calm during most of the interval from the Infracambrian to the middle Triassic .The rock sequence deposited during this time displays all the characteristics of a platform cover. It indicates an epicontinental environment with alternating shallow- marine, lagoon- al and continental deposits. The thickness of the whole sequence is between 3000 and 4000 m and increases exceptionally up to 8000 m in  the region of Tabas in E Iran (stِcklin, Eftekhar –Nezhad and Hushmand –Zadeh ,1965; Ruttner ,Nabavi and Hajian ,1968) .important sedimentary gaps and regional disconformities are common (Fig. 326) .they reveal considerable Silurian and early Devonian epeirogenic movements and a period of emergence in the carboniferous, followed by a marine transgression at the base of the Permian throughout the Iranian plate. Significant crustal deformation, how- ever, is not known? The Permian to Middle Triassic section of the stratigraphics remarkable uniformity .It consists mainly of shallow –water limestone and dolomite. The Paleozoic rock units of the Iranian Platform are summarized in Fig. 326.

        Following the pan-African Orogeny, in S Iran and the SE of central Iran, evaporates (salt and gypsum) were deposited, which accumulated later  in domes and ridges under the large anticlinal structures .The distribution of these evaporites facies suggests that during the late Precambrian –Cambrian ,Central  Iran and Zagros were part of the same landmass. Red clastic units, the Zaigun Formation and Lalun Sandstone, both of continental deposition, reflect the terrane configuration after the epeirogenic uplifting and emergence.

        For the Paleozoic time interval, four cycles of sedimentation bounded by an unconformity or lithological break are distinguished; an Eocambrian-Ordovicion, Silurian to Lower Devonian, Upper Devonian –Carboniferous and a Permian sedimentary cycle .Devonian rocks often rest disconformably on a carved surface of older formations. The transgression of the Lower Permian in Alborz is marked by a basal conglomerate .The real marine condition was established again in Middle Permian by the deposition of the cherty limestone (Rute Formation). Permian rocks are widely exposed in carbonate facies in many parts of central Iran .The uppermost permain and the permo- Triassic transitional beds are so far known from the Julfa region in NW Iran (stepanov, Golshany and stِocklin, 1969) and from the Abadeh region in W central Iran (Taraz, ,1969).

       The pre- upper Triassic history of the Kopet Dagh and the S Caspian area turned out differently .This region was part of Eurasia and was separated from other parts of Iran until the late Middle Triassic.

Precambrian –Cambrian boundary

The Precambrian –Cambrian boundary is now known from many places in Iran, notably in Alborz, near Kerman etc. The Lower Dolomite Member of the Soltanieh Formation contains an assemblage of phosphatic tubes and other poorly preserved remains. The succeeding Lower Shale Member bears macroscopic chuaria –like algae .Early skeletal fossil diversity raises through the middle Dolomite Member, with the successive appearance of Protohertzina anabarica. The overlying Upper Shale Member may represent lower Tommatian strata (Hamdi, Brasier and Jiang Zhiwen, 1989).

Infracambrian –Ordovician

Infracambrian rock units in Iran were deposited over large areas after the Pan–African tectonic phase .These movements gently folded and uplifted Precambrian fine clastic, flysch –type sediments (Kahar Formation .Morad Series and Shorm Beds) along Pan- African trends and caused epeirogenic uplifting, and emergence of lowland areas in W Iran, in E central Iran and probably also in E Iran.

        In Central Iran and NW Iran the Eocombrian rock units are mainly shallow water shelf platform carbonates, which interfinger towards the North Iran Suture with brown, fine–grained sandstone –shale sequence. The latter were defined as the Bayandor Formation (Fig. 326). The carbonates are stromatolitic dolomites of the lower parts of the Soltanieh Formation.

        The Infracambrian –Cambrian boundary is within the Soltanieh Formation .The Upper Shale Member  of the Soltanieh Formation is suggested as lower Tommotian strata (Fig. 327) The lower Cambrian red clastic units ,the Zaigun Formation and Lualn Sandstone both of continental deposition ,reflect the terranne configuration after the Infracambrian epeirogenic uplifting and emergence .In the lower Cambrian a beginning shallow marine transgression is indicated by red-green shales and dolomites with Cruziana traces. The Mila Group (in East Iran) and the Mila Formation (in Alborz) represent beds of shallow marine condition .They range in age from early Middle Cambrian to Ordovician and indicate a countinuous marine platform sequence. Depocenters of the Mila Group are in the NE Central Iranian embayment and in the shirgesht area (N of Tabas), where more than 2000 m of dolomites, limestones, siltstones, marls and shales with a rich fauna of trilobites, brachiopods, bryozones, cephalopods and rare graptolites were recorded (Ruttner, Navabi and Hajian, 1968). Other depocenters are in the SE central Alborz (500-700m), in the Soltanieh Mountains (NW Iran, 500-600 m) and in the area N of Esfahan.

       In SW Iran ,equivalents of the Mila Group are represented by the Zard-Kuh Formation with aggregate  thicknesses of more than 1000 m, but thins from a depocenter in the Zard-Kuh (80 km SW of Golpaygan) to the SE (Kuh-e-Dinar) and NW(Oshtoran –Kuh). The greater amount of detrital mica points to a metamorphic basement high on the pre-Zagros swell. In addition to the Mila Group, Ordovician to Silurian shales and sandstones with graptolites are reported from the Kuh-e-Faraghan (N of Bandar-Abbas), with a thickness of up to 800 m.

Silurian to Lower Devonian
Early Caledonian tectonic movements resulted in an uplifting of W and S Iran and caused emergence of the S Caspian and central Alborz area, which were temporarily linked to the West Iranian land platform.

The pre-Zagros swell emerged with a large low land area in SW Iran and remained free of marine ingressions in Silurian-early Devonian time. Central and NW central Iran was depositional areas in Silurian-early Devonian time. After a period of regression in late Ordovician –early Silurian time, tensional stress opened the way for the extrusion of basaltic lava sheets in the E Alborz, in Central Iran, and in the Kerman and Esfahan-Soh areas. In E central Iran basalts are at the base of the Niur Formation, a predominantly shallow water platform limestone and dolomite sequence with corals, brachiopods, conodonts (early Silurian to early Devonian; Weddige, 1984). This marine fossiliferous facies, which laterally interfingers with fine clastics and sandstones, is only developed in E central Iran in a thickness of about 500 m. The Niur Formation is conformably succeeded in the type section area by lower Devonian sandstone and dolomite with gypsum intercalations, 500 to 700 m thick, which were defined as the padeha Formation. The upper contact is sharp and an erosion surface terminates this sedimentary cycle of the Gushkamar group (Niur and Padeha Formations).West of Ardekan–Yazd, marine beds (coral limestones and dolomites)–equivalents of the Niur Formation with a thickness of 200 m –are at the base of the Old Red-type sandstones .In this area as well as in the area SW of Kerman (Kuh-e- Daviran), the Silurian–Lower Devonian sequence reaches a thickness of about 1000m.  In NW Iran pre-Permian early Upper Paleozoic rocks are restricted to a basin area NE of the Tabriz –Zanjan Line.

      The Silurian to Lower Devonian sedimentary cycle on the platform area was concluded by the Eifelian hiatus.

Middle Devonian to Carboniferous

Marine deposits of Devonian age are known in the Maku area (NW Iran). They consist of dolomite with some intercalations of sandstone and quartzite and are called the Muli Formation (Alavi- Naini and Bolourchi, 1973). The thickness of the Muli Formation is 1250 m and it is unconformably underlain by metamorphic rocks. In central Alborz, the Upper Paleozoic sedimentary cycle began with the transgression of the sandy Geirud Formation (Upper Devonian). The Guired Formation  consist in its lower part of sandstone, shale,  fossiliferous sandy limestones and several phosphatic layers, followed by sandstones and shale containing plant remains . In its middle part the formation consists of  150 m thick basalts .The basalts are overlain by conglomerates and sandstones followed by a fossiliferous sandy limestone. Lower Carboniferous of Central Alborz is made up of dark Limestone with subordinate marl intercalations in the lower part (Mobark Formation) .Both the Tourniasian and Visean Stages are well documented in these rocks by a rich brachiopod and coral fauna. The Mobark Formation is about 450 m thick and disconformably overlies the Geirud Formation. In E Alborz the khosh –Yelagh Formation transgresswd with a basal conglomerate on the Padeha Formation . The Khosh-Yelagh Formation consists of a 15 m basal conglomerate followed by shale, limestone, sandstone and basic volcanics in the lower part and dolomite, limestone and marly limestone in the upper part. The Khosh –Yelagh Formation is overlain by the Mobarak Formation. In khosh –Yelagh area the Mobarak Formation is overlain by the lower Permian Dorud Formation.

Central Iran

In east Central Iran the cycle of middle Devonian to late Carboniferous marine sediments reaches a thickness of 1500 to 2000 m and was defined as the Ozbak-Kuh Group in E Iran (Ruttner, Navabi and Hajian, 1968, stِcklin, Eftekhar-nezhad and-Hushmand-zadeh 1965). It consists of the Sibzar Dolomite at the base (100m thick), the Bahram Formation (300-500 m thick of late Devonian age) and the Shishtu Formation (300-400 m thick, of alternating marls, shale and limestone of the late Devonian to early carboniferous age); the cycle is concluded by the Sardar Formation, shales, sandstones and sandy limestone of the late Visean to late Carboniferous age.

Zagros area

The Devonian is nowhere present in SW Iran. The Carboniferous sandstone is very consistent, being present beneath the Permian limestone wherever the base of this limestone is exposed (Setudehnia, 1972) .This Carboniferous sandstone is discomformably underlain by Cambrian sediments in the Kuh-e-Dinar, Zard –Kuh and Oshtoran–Kuh areas .It is also disconformably underlain by Ordovician and Silurian sediments in the Kuh-e-Surmeh and Gahkum -Faraghun areas, respectively. The upper boundary of the Carboniferous with the Permian is not clear. These two units are often combined and referred to as Permo-Carboniferouse.

Permian sedimentary cycle

A lacuna and disconformity separates the Upper Devonian-Carboniferous and the Permian sedimentary cycles. Over wide areas in NW Iran and in the main Alborz Range, a red-clastic shallow marine transgressive phase, the Dorud Formation, indicates Permian sedimentation. The Dorud Formation interfingers towards the E Alborz with shelf carbonates, and towards central Iran it change into a thinner, basal quartz sandstone member .All over the Alborz Mountain Range the Dorud Formation is succeeded by the uniform shelf-platform limestones and dolomites of the Ruteh Formation, which contain a fauna of Neoschwagerina indicating  Lower to Middle Permian age . The Jamal Formation in E central Iran overlies the shale and siltstone of the Sardar Formation with a sharp lithological break which may include a lacuna. The middle Permian marine transgression may have extended also to parts of SE Iran .Laterite zone in the Upper Permian sequences indicates temperory  regressions and fluctuation of sea level in the Yazd and Golpayegan areas. In S and W Iran, the Upper Perrmian sea transgressed the lowland area of the Arabian Platform with the deposition of a basal quartz sandstone member (Farghar, Sandstone Member) followed by up to 1000 m of lower carbonates middle evaporates and upper limestone and dolomite .A predominandy dolomitic facies is developed in the S Persian Gulf and Strait of Hormoz area.

Permian –Triassic boundary

The lowermost Triassic, as well as a continuous section between Permian and Triassic, is so far only known from Julfa (NW Iran) and Abadeh (W central Iran). At Julfa the dark, brachiopod –bearing limestone of Guadalupian age is conformably overlain by 33 m of shales, marls and thin limestones of whitish gray and purple color; these are the Julfa Beds (Stepanoy, Golshany and Stِcklin, 1969), which contain a peculiar Late Permian fauna and correlate in minute detail with the stratotype of the Dzhulfian.

The Julfa Beds grade into a soft-weathering unit of dark-purple shale and marlstones, the Permian-Triassic transitional beds. They are 17 m thick and contain a mixed fauna of Permian –type dwarfed brachiopods and Triassic type ammonites. The transitional beds thus encompass the Paleozoic-Mesozoic boundary. They are followed by the red, 4 m thick paratirolites limestone, which contains abundant Eotriassic ammonites, together with the last survivors of Permian–type brachiopods. This limestone is overlain with a sharp lithologic and faunal break by light –colored, platy limestones and shales 280 m thick containing Claraia  and Meekoceras; these Claraia beds are prefect lithological equivalents of the Early Triassic lower Elikah Formation of the Alborz Mountains. In other parts of N Iran the lowermost Triassic is missing, and the Lower to Middle Triassic Elikah Formation disconformably overlies Upper Permian (Nesen Formation) or Middle-Upper Permian (Ruteh Formation).

Paleozoic of mobile belt

 The platform condition, during the Paleozoic time interval does not apply to the entire area of Iran .In N Iran fundamental lithological differences appear on both sides of the North Iran Suture .This suture line was introduced as a Paleozoic to Middle Triassic plate boundary between Scytho- Turanian plates in the N (Eurasian) and the Iran Plate in the S (Gondwana) by Davouzadeh and Schmidt (1982). The North Iran Suture passes from the N slope of the Binalud Mountian(W of Mashhad) along the kopet Dagh-East Alborz border into the Gorgan area and from there along the S Caspian into the Talesh area (west Alborz) and continues northwestward into the Minor Caucasus .. The significance of this suture is the clear Paleozoic –Triassic facies changes on both side the lineament and the Paleozoic ophiolite outcrops along this lineament. On the N side of this Suture, several uplifts of Hercynian and/or Caledonian basement associated with basic and ultrabsic rocks are exposed (Davoudzadeh, Lensch and weber –Diefenbach 1986). The South Caspian –Minor Caucasus North Anatolian ophiolitic belt is considered to be major suture of the Paleozoic Tethys, dividing, its S carbonate shelf from the Turanian –Caucasus –Pontian active margin (Adamia et al. 1981). From the Binalud area (NE Iran) towards the E, the main Hindu kush lineament or 'Hindu Kush Fualt' is conventionally considered to be the N limit of the Paleozoic platform (stِocklin, 1977).

In contrast to the platform areas with epicontinental–shallow marine sediments, the Paleozoic rocks of the mobile belt have a greater thickness of detrital sediments (shale, greywacke and sandstone), minor carbonate but hug amounts of basic volcanic rocks and in places basic intrusions and ultrabasic rocks (ophiolite).

       North of the North Iran Suture.) On the Paleozoic active margin of Eurasia characteristic geosynclinal facies prevail. North of the Binalud Range (Mashhad area) a NW-SE trending fault separates sharply the Paleozoic platform facies in the S from a Paleozoic metamorphic complex in the N. The metamorphic complex consists of sedimentary and volcanic rocks, interbedded with ultrabasic units. The sedimentary rocks are mainly pelitic, now partly staurolitegarent schist. Some calcareous intercalations also occur. Stratigraphic sections of the Mashhad and Talesh areas (SW Caspian) are correlated with the Paleozoic metamorphics of the mobile belt in Central Iran.

        Geosynclinal facies- type succession comparable to the N foot of the Alborz occur in parts of Central Iran. A rather large belt is in the Sirjan area. This belt may continue eastward to the Makran area. Another belt is in the Anarak area (NE of Nain). Smaller occurrences of Saghand area (NE of Yazd) may have belonged to the Anarak belt and were later dislocated tectonically.

        The geodynamic interpretation of such Paleozoic metamorphic complexes within the Iranian platform is still open for discussion. However, they could have been rift zones within the continental platform or they were a narrow branch of Paleotethys which existed in the N. Another possibility is that these blocks were dislocated from their orginal position (N of the North Iran Suture) by the rotation of the Central –East Iran microplate.