The Asmari Formation is a

geological formation A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics (lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock expo ...

in western

Iran Iran, officially the Islamic Republic of Iran, and also called Persia, is a country located in Western Asia. It is bordered by Iraq and Turkey to the west, by Azerbaijan and Armenia to the northwest, by the Caspian Sea and Turkm ...

. Located in the western

Zagros Mountains The Zagros Mountains ( ar, جبال زاغروس, translit=Jibal Zaghrus; fa, کوه‌های زاگرس, Kuh hā-ye Zāgros; ku, چیاکانی زاگرۆس, translit=Çiyakani Zagros; Turkish: ''Zagros Dağları''; Luri: ''Kuh hā-ye Zāgro ...

, it is a

carbonate A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonat ...

formation that largely consists of

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms wh ...

, dolomitic limestone, dolomite, and marly limestone. There are also smaller amounts of

anhydrite Anhydrite, or anhydrous calcium sulfate, is a mineral with the chemical formula CaSO4. It is in the orthorhombic crystal system, with three directions of perfect cleavage parallel to the three planes of symmetry. It is not isomorphous with t ...

lithic sandstone Lithic sandstones, or lithic arenites, or litharenites, are sandstones with a significant (>5%) component of lithic fragments, though quartz and feldspar are usually present as well, along with some clayey matrix. Lithic sandstones can have ...

, and limey sandstone. The formation dates to the

Oligocene The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but ...

and

Miocene The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recent" ...

periods and is a major

hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ...

reserve. It has been producing oil since the 1930s. It also forms a major

aquifer An aquifer is an underground layer of water-bearing, permeable rock, rock fractures, or unconsolidated materials ( gravel, sand, or silt). Groundwater from aquifers can be extracted using a water well. Aquifers vary greatly in their characteri ...

, which discharges at various springs in the Zagros region. The Asmari Formation's basic

biostratigraphy Biostratigraphy is the branch of stratigraphy which focuses on correlating and assigning relative ages of rock strata by using the fossil assemblages contained within them.Hine, Robert. “Biostratigraphy.” ''Oxford Reference: Dictionary of Bio ...

was established in the 1950s and it was formally described in 1965. It consists of

carbonate platform A carbonate platform is a sedimentary body which possesses topographic relief, and is composed of autochthonic calcareous deposits. Platform growth is mediated by sessile organisms whose skeletons build up the reef or by organisms (usually mic ...

s that were formed in 6 distinct stages. The formation is named after the Asmari Mountains southeast of

Masjed Soleyman Masjed Soleyman ( fa, مسجدسلیمان, also Romanized as Masjedsoleimān, Masjed-e Soleymān, Masjed Soleiman, and Masjid-i-Sulaiman) is a city and capital of Masjed Soleyman County, Khuzestan Province, Iran. At the 2006 census, its populat ...

, and its type sample was taken from the Tang-e Gel Torsh in these mountains. The Asmari Formation was deposited at a time when the

Tethys Ocean The Tethys Ocean ( el, Τηθύς ''Tēthús''), also called the Tethys Sea or the Neo-Tethys, was a prehistoric ocean that covered most of the Earth during much of the Mesozoic Era and early Cenozoic Era, located between the ancient continents ...

was finally closing and the Zagros Mountains were first rising. During this period, the region was a shallow ocean gradually getting less deep. By the time of the succeeding Gachsaran Formation, this process had culminated in the sea shrinking to lagoons. In the southwestern Zagros, the Asmari Formation sits on top of the Pabdeh Formation. In the Fars and Luristan regions, the Asmari Formation sits on top of the Jahrum and Shahbazan Formations. The Asmari Formation is thickest in the northeastern part of the Dezful Embayment. The formation has various

microfossil A microfossil is a fossil that is generally between 0.001 mm and 1 mm in size, the visual study of which requires the use of light or electron microscopy. A fossil which can be studied with the naked eye or low-powered magnification, ...

s including

plankton Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a cr ...

foraminifera Foraminifera (; Latin for "hole bearers"; informally called "forams") are single-celled organisms, members of a phylum or class of amoeboid protists characterized by streaming granular ectoplasm for catching food and other uses; and commonly ...

, echinoids, red algae,

ostracoda Ostracods, or ostracodes, are a class of the Crustacea (class Ostracoda), sometimes known as seed shrimp. Some 70,000 species (only 13,000 of which are extant) have been identified, grouped into several orders. They are small crustaceans, typic ...

, and

oncoid Oncolites are sedimentary structures composed of oncoids, which are layered structures formed by cyanobacterial growth. Oncolites are very similar to stromatolites, but, instead of forming columns, they form approximately spherical structures. ...

s. Individual species include ''

Globigerina ''Globigerina'' () is a genus of planktonic Foraminifera, in the order of Rotaliida.Gl ...

'', '' Turborotalia cerroazulensis'', ''

Hantkenina ''Hantkenina'' is a genus of planktonic foraminifera that lived from the Middle Eocene up to late Eocene, cirka 49 Ma-33.9 Ma. There have been 11 morphospecies described, including one of Cribrohantkenina Coxal, H.K. y Pearson, P.N. (2006). Chapt ...

'', '' Nummulites'', '' Lepidocyclina'', '' Miogypsina'', ''

Elphidium ''Elphidium'' is an abundant genus of foraminifera. Species can be found from coastal regions out to the continental slope, and in all temperature ranges. Like other forams, fossils from different species are used to date rocks. The taxonomy of ...

'', '' Peneroplis farsenensis'', and '' Borelis melo curdica'' and ''Borelis melo melo''.''

References

{{coord missing, Iran Geologic formations of Iran Zagros Mountains Oligocene Series Miocene Series Limestone formations Dolomite formations Oil fields of Iran

See also

References