Microfacies and sedimentary environment of the early-middle Miocene deposits (Mishan Formation) in south of Iran.
The Mishan Formations was introduced by James and Wynd  and the type section was measured in the Gachsaran oilfield. Generally, it overlies the Gachsaran Fm. and underlies the Aghajari Formation. It consists of carbonate deposits (i.e., thick-bedded to massive, feature forming, hard and fossiliferous limestones) called the Guri Member and green marl with intercalations of thin to medium bedded limestones. The studied areas are located in the Khorgu (56 29' 05.5" longitude and 27 35' 36" latitude) and Khamir anticlines (55 41' 53" longitude and 27 07' 23" latitude). The rocks were classified in the field using a hand lens and their depositional fabric were described . Indeed, more than 120 thin sections were provided from limestone and marl particles for micropaleontology and microfacies studies. Khorgu and Khamir sections are located at the southeastern end of the Zagros Folded Zone consisting of a simple anticline-syncline system . Khorgu section is 430 m in thickness with uncertain lower boundary and conformable and gradual upper boundary between the Mishan and Aghajari formations. The Khamir section is 1104 m in thickness with conformable and sharp lower boundary between the Guri Member and the Gachsaran Fm. and also with sharp upper boundary (between the Mishan and the Aghajari Formations). The age of the Mishan Fm. in the studied sections based on the occurrence of Triloculina tricarinata, Asterigerina rotula, Amphistegina lessoni, Neorotalia viennotti, Ammonia beccari, Ammonia stachi, Elphidium sp.14, Elphidium sp.1, Miogypsina sp., Operculina complanata, Orbulina universa, Globigerinoides sp., Globigerina sp., Globorotalia sp. Globigerina bulloides, Globigerinoides trilobus and Globigerinoides sicanusis Early to Middle Miocene (Aqutanian to Langhian).
Late Cenozoic and Recent tectonic and sedimentary features in the Zagros basin have been resulted by the final collision between of the African/Arabian and central Iran plates, which was started during the Mesozoic . The Zagros region is an active growth area of the mountains belt and can be divided into five structural zones: 1) magmatic arc of Urumieh-Dokhtar, 2) metamorphic of Sanandaj-Sirjan, 3) high Zagros 4) Zagros simply folded belt and 5) foreland of Persian Gulf (Alavi, 2004). The studied areas of this research are located in the Southeastern part of the Zagros simply folded belt (Fig. 1).
There are two different facies types in the Mishan Fm.; clastic and carbonate facies. Facies analysis of the carbonate part has resulted in the definition of 12 microfacies types.
Clastic facies consists of marls, but most outcrops are erroded or covered by alluvium deposits in some areas. These marls contain large amounts of quartz in size of sand and silt particles. The particles of clastic material transported from land to the sea which are exogenetic. The carbonate cycles are separated by the marls. This microfacies is highly diverse in larger benthic foraminifera with perforate wall such as Neorotalia viennotti, Ammonia beccari, A. stachi and pelagic foraminifera such as Globigerinoides sp., Globigerina sp., Globigerina bulloides, Globigerinoides trilobus and Globigerinoides sicanus. Based on lithological and paleontological evidence, it can be confirmed that the clastic facies in, comparision to the carbonate facies are formed in the deeper basin.
MF1. Ditrupa- planktonic foraminifera-quartz wackestone (Fig. 2.a):
This type is distinguished from MF11 by the presence of Globorotalia sp. and the lack of benthic foraminifera. The presence of planktonic foraminifera, abundant micrits and stratigraphic position indicate that the deposition has been taken place in a low energy deep water environment (outer shelf environment) . It occurs at the lower part of the Khorgu section.
MF2. Bentic/planktonic foraminifera wackestone (Fig. 2.b):
This type is characterized by an association of pelagic foraminifera (i.e., Globigerina, Globigerinoides and Globorotalia), small benthic foraminifera (i.e., Ammonia beccarii andNeorotalia viennoti), Ditrupa, ostracoda and small quartz grains. Quartz grains probably have been transported by wind from land to the sea. This microfacies has been observed in from of limestone, marly limestone and calcareous marl in the many parts of the Khorgu section. This facies indicates the outer shelf environment, too.
MF3. Quartz mudstone (Fig. 2.c):
This facies consists of a high percentage of carbonate mud, quartz grains and the rare faunal elements (i.e., Globogerina, Globorotalia and Ammonia). This microfacies have been observed in from of light grey limestone, white marly limestone and dark grey calcareous marl in the Khorgu section. The plankton faunal elements and stratigraphic position show that the deposition has been occurred in a deep and low energy the outershelf environment.
MF4. Operculina wackestone to packestone (Fig. 2.d,e):
The main components are bioclasts and large perforate foraminifera. Bioclasts include bivalve, red algae, echinoderm and bryozoa fragments. The foraminifera are characterized by a relatively diverse assemblage of Operculina, Amphistegina, Asterigerina and Miogypsinoides. Also, the Lack of planktonic foraminifera is the other character of this facies. This facies dominates in the Khamir section and has been observed in from of limestone and marly limestone. The presence of red algae and larger foraminifera such as Amphistegina indicates that sedimentation in the end of middle shelf environment [10,8,4].
MF5. Ditrupa packestone (Fig. 2.f):
In this type of microfacies, bioclast fragments (i.e., abundant Ditrupa sp., Neorotalia, Ammonia and Bigenerina and echinoderms) are dominant biotic components. Also, there are rarely planktonic foraminifers including non-keeled Globigerinids. This facies in from of cream-brown limestones in the middle parts of the Khorgu section. Ditrupa has an infaunal-epifaunal life-style in fine sediments, from mud to clean sand. However, it is preferentially found between 20 and 30 m depth . Accompanied planktonic foraminifers show a deeper environment. This microfacies represents deposition in middle shelf.
MF6. Rotaliids-bryozoan wackestone to packestone (Fig. 3.a):
This microfacies is characterized by the abundant occurrence of small rotaliids (i.e., Neorotalia and Ammonia), bryozoa (i.e., Tubucellaria, Memberanipora and Onychocella) and abundant micrite matrix. It occurs in the cream-brown, thin to medium-bedded limestones in the upper parts of the Khorgu and Khamir sections. Rotaliids are abundant in the high energy shallow water (inner shelf) and bryozoans are found between 60 and 120 m from the base. This microfacies represents deposition in the end of inner shelf.
MF7. Bioclast wackestone (Fig. 3.b):
The main bioclastic components are bivalve and gastropods fragments. Foraminifers such as small Rotaliids are very rare. This facies has been observed in the limestones of the lower and upper parts of the Khamir section. The presence of abundant micrite, fragments of molluska and lack of foraminifera indicate that sedimentation has been taken place at the turbulent environment in the inner shelf.
MF8. Bioclast-dendritina grainstone (Fig. 3.c):
Identifiable components of this facies include abundant Dendritina, Elphidium, Ammonia, bryozoa, interclast and quartz grains. Imperforated crusts of Dendritina have been hematitize. This facies has been observed as cream to brown limestones in the base of the Khamir section. The features of these facies indicate the moderate to high energy shallow water conditions such as inner shelf lagoonal environment.
MF9. Bioclast grainstone (Fig. 3.d,e):
This type is characterized by abundant bryozoans and fragments of bivalves. Other bioclasts are rare and include gastropods, algae and small benthic foraminifera such as Ammonia. The texture is grainstone. It has been observed as brown on suggest that deposition occurred in a shallow and high energy environmentin the innershelf environlimestone in the upper part of the Khamir section. The texture of facies, faunal elements and stratigraphic position suggest that deposition occurred in a shallow and high energy environment in the inner shelf.
MF10. Mudstone (Fig. 3.f and4.a):
This facies consists of fine-grained microcrystalline limestone. There are lacks of sedimentary structures. Very rare bioclasts (i.e., Elphidium and Neorotalia) are also present. This microfacies occurs at the lower parts of the Khamir section. This facies was deposited in a lagoon. Evidence for this interpretation includes the lack of fauna both in diversity and abundance, and a stratigraphic position below the lagoonalfacies .
MF11. Miliolid packestone (Fig. 4.b):
The most abundant benthic foraminifera in this type are milolids such as Quinquloculina, Pyrgo, Schlumbergerina and Triloculina The other fossil components of this microfacies are mainly Dendritina, Ammonia, Discorbis and fragments of echinoderms. This microfacies has been observed as grey limestones in the lower part of the Khamir section. The fossil diversity, biological perturbations and abundant micritic matrix indicate of low energy and a shelf lagoon deposition environment.
MF12. Dolomudstone(Fig. 4.c):
The abundanc of micrite matrix, lack of fossils, presence of evaporate deposits and fenestral porosity indicates the facies the formed in the tidalflat environment . This microfacies has only been observed at the base Mishan Formation in the Khamir section in from of limestones and dolomitic limestones.
On the basis of the distribution of the foraminifera and vertical facies relationships, three major depositional environments have been identified in the Early to Middle Miocene succession of the Khamir section. These include inner, shelf and outer shelf environments. There are three environments in the Khamir section in carbonate facies which are represented by 11 microfacies (Mf 2 to Mf12) (Fig. 5). Serious facies changes are specially observed at the base of Khamir section. This can be related to the salt dome activites that has been very influential in Miocene marine basin of southern Iran.
Two major depositional environments have been identified in the Khorgu section, too. These include middle and outer shelf which are represented by 6 microfacies (Mf 6 to Mf12) (Fig. 6). There are gradual transitions in the lithofacies and biofacies of this section.
Inner shelf deposits represent a wider spectrum of marginal marine deposits Faunal diversity is low and normal marine fauna are lacking, except for imperforate benthic foraminifera such as miliolids, Dendritina and perforate rotaliids which indicate quite sheltered conditions . The middle shelf environment is characterized by abundant hyaline, lamellar, perforate larger foraminifera and smaller perforate taxa (rotaliids). Larger perforate forms are represented by Amphistegina, Operculina, Asterigerina, Miogypsina and Miogypsinoides. The most common smaller foraminiferal fauna are constituted by Neorotalia and Ammonia. The outer shelf environment is characterized by hematitized, well-bedded and laterally continuous deposits those are marked by abundante planktonic foraminiferal contents .
There are two different facies types in the Mishan Formations clastic and carbonate facies. Based on biogenic and abiogenic components and texture, there are 12 microfacies in the carbonate facies, those are grouped into three depositional environments corresponding to the inner, middle and outer shelf. It can be interpreted as a carbonate platform that is developed in an open shelf situation. In the inner shelf, the most abundant lithofacies are the wackestone-packstone with rotaliids foraminifera and bryozoan. The middle shelf is represented by wackstone-packestone with larger foraminifera with perforate wall. The outer ramp is dominated by wackestone characterized by planktonic foraminifera and rotaliids. The depth changes in the Khorgu section are gradual and the extreme fluctuations in the Khamir section are probably related to the syn-depositional activities of the salt domes.
Received 21 April 2014
Received in revised form 23 May 2014
Accepted 13 June 2014
Available online 10 July 20144
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(1) Roya Fanati Rashidi, (2) Seyed Hamid Vaziri, (3) Keyvan Khaksar, (4) Hossein Gholamalian
(1) Department of Geology, science and research Branch, Islamic Azad University, Tehran, Iran.
(2) Department of Geology, Faculty of Basic Sciences, North Tehran Branch, Islamic Azad University, Tehran, Iran.
(3) Islamic Azad University, Qom, Iran.
(4) Department of Geology, Faculty of Sciences, Hormozgan University, Bandar Abbas, Iran.
Corresponding Author: Roya Fanati Rashidi, Department of Geology, science and research Branch, Islamic Azad University, Tehran, Iran.
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|Author:||Rashidi, Roya Fanati; Vaziri, Seyed Hamid; Khaksar, Keyvan; Gholamalian, Hossein|
|Publication:||Advances in Environmental Biology|
|Date:||Jun 1, 2014|
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