versión impresa ISSN 1026-8774
Rev. mex. cienc. geol v.27 n.1 México abr. 2010
OligoceneMiocene ramp system (Asmari Formation) in the NW of the Zagros basin, Iran: Microfacies, paleoenvironment and depositional sequence
Sistema de rampa del OligocenoMioceno (Formación Asmari) en el NW de la cuenca de Zagros, Irán: microfacies, paleoambiente y secuencia de depósito
Hossein VaziriMoghaddam1,*, Ali Seyrafian1, Azizolah Taheri2, and Homayoon Motiei3
1 Department of Geology, Faculty of Sciences, University of Isfahan, Isfahan, Iran, 8174673441. *Email: firstname.lastname@example.org.
2 Geology Department, Faculty of Earth Science, Shahrood University of Technology, Shahroud, Iran.
3 National Iranian Oil Company Research and Development Division, Tehran, Iran.
Manuscript received: September 09, 2009.
Corrected manuscript received: October 8, 2009.
Manuscript accepted: October 14, 2009.
The Asmari Formation deposited in the Zagros foreland basin during the OligoceneMiocene. Four different measured sections were studied in this area in order to interpret the facies, depositional environment and sequence stratigraphy of the Asmari Formation. In this study, thirteen different microfacies types have been recognized, which can be grouped into six depositional environments: tidal flat, restricted lagoon, open lagoon, shoal, slope and basin. The Asmari Formation represents sedimentation on a carbonate ramp. Four thirdorder sequences are identified, on the basis of deepening and shallowing patterns in the microfacies and the distribution of the OligoceneMiocene foraminifers. The depositional sequences 1, 2 and 3 were observed in Dehluran and KabirkuhDarrehshahr areas, and are synchronous with a period of either erosion or nondeposition represented by unconformities in Mamulan and Sepid Dasht areas.
Key words: microfacies, paleoenvironment, ramp, Asmari Formation, Zagros basin, Iran.
La Formación Asmari se depositó en el antepaís de la cuenca Zagros durante el OligocenoMioceno. Se estudiaron y midieron cuatro secciones diferentes en esta área para interpretar las facies, ambiente de depósito y la secuencia estratigráfica de la Formación Asmari. En este estudio, trece tipos diferentes de microfacies han sido reconocidos, los cuales pueden ser agrupados en seis ambientes de depósito: planicie de marea, laguna restringida, laguna abierta, mar somero (bancos de arena), pendiente marina y cuenca. La Formación Asmari representa sedimentación en una rampa carbonatada. Cuatro secuencias de tercer orden se identificaron, según patrones de profundidad y superficialidad de las microfacies y la distribución de los foraminíferos del OligocenoMioceno. Las secuencias de depósito 1, 2 y 3 se observaron en las áreas de Dehluran y KabirkuhDarrehshahr, y son sincrónicas con un período de erosión o bien de no depósito, representado por discordancias en las áreas de Mamulan y Sepid Dasht.
Palabras clave: microfacies, paleoambiente, rampa, Formación Asmari, cuenca Zagros, Irán.
This paper deals with the Asmari Formation, an OligoceneMiocene carbonate succession in the northwestern Zagros basin, southwest Iran (Figure 1). The area is excellent to establish the geometrical relationship between sedimentary facies and sequence stratigraphy of a carbonate platform.
The Asmari Formation, a thick carbonate sequence of the OligoceneMiocene, is one of the best known carbonate reservoirs in the world. It is present in most of the Zagros basin. Lithologically, the Asmari Formation consists of limestone, dolomitic limestone, dolomite and marly limestone. Some anhydrite (Kalhur Member) and lithic and limy sandstones (Ahwaz Member) also occur within the Asmari Formation (Motiei, 1993).
The Asmari Formation was originally defined in primary works by Busk and Mayo (1918), Richardson (1924), Van Boeck et al. (1929), and Thomas (1948). Later, James and Wynd (1965), Wynd (1965), Adams and Bourgeois (1967), Kalantary (1986), and Jalali (1987) introduced the microfaunal characteristics and assemblage zones for the Asmari Formation. More recent studies of the Asmari Formation have been conducted on biostratigraphic criteria (Seyrafian et al., 1996; Seyrafian and Mojikhalifeh, 2005; Hakimzadeh and Seyrafian, 2008; Laursen et al., 2009), microfacies and depositional environments (Seyrafian and Hamedani, 1998, 2003; Seyrafian, 2000) and depositional environment and sequence stratigraphy (VaziriMoghaddam et al., 2006; Amirshahkarami et al., 2007a, 2007b; Ehrenberg et al., 2007).
This paper reports on a sedimentological study of Asmari Fm. outcrops, whose results could contribute to a better understanding of the subsurface Asmari Formation in adjacent oilfield areas. The main objectives of this reseach were foused on (1) a description of the facies and their distribution on the OligoceneMiocene carbonate platform, (2) the palaeoenvironmental reconstruction of the carbonate platform, and (3) the origin of sequences that developed in the study area mainly based on the distribution of the foraminifera.
Based on the sedimentary sequence, magmatism, metamorphism, structural setting and intensity of deformation, the Iranian Plateau has been subdivided into eight continental fragments, including Zagros, SanandajSyrjan, UrumiehDokhtar, Central Iran, Alborz, KopehDagh, Lut, and Makran (Heydari et al, 2003; Figure 2). The study area is located in the northwestern part of the Zagros basin and include four sections: 1) Dehluran, 2) KabirkuhDarrehshahr, 3) Mamulam and 4) Sepid Dasht (Figure 1).
The Zagros basin is composed of a thick sedimentary sequence that covers the Precambrian basement formed during the PanAfrican orogeny (AlHusseini, 2000). The total thickness of the sedimentary column deposited above the Neoproterozoic Hormuz salt before the Neogene Zagros folding can reach over 8 to 10 km (Alavi, 2004; Sherkati and Letouzey, 2004). The Zagros basin has evolved through a number of different tectonic settings since the end of Precambrian. The basin was part of the stable Gondwana supercontinent in the Paleozoic, a passive margin in the Mesozoic, and became a convergent orogen in the Cenozoic.
During the Palaeozoic, Iran, Turkey and the Arabian plate (which now has the Zagros belt situated along its northeastern border) together with Afghanistan and India, made up the long, very wide and stable passive margin of Gondwana, which borderered the PaleoTethys Ocean to the north (Berberian and King, 1981).
By the Late Triassic, the NeoTethys ocean had opened up between Arabia (which included the present Zagros region as its northeastern margin) and Iran, with two different sedimentary basins on both sides of the ocean (Berberian and King, 1981).
The closure of the NeoTethys basin, mostly during the Late Cretaceous, was due to the convergence and northeast subduction of the Arabian plate beneath the Iranian subplate (Berberian and King, 1981; Stoneley, 1981; Beydoun et al., 1992; Berberian, 1995). The closure led to the emplacement of pieces of the NeoTethyan oceanic lithosphere (i.e., ophiolites) onto the northeastern margin of the AfroArabian plate (e.g., Babaie et al., 2001; Babaei et al., 2005; Babaie et al., 2006).
Continentcontinent collision starting in the Cenozoic has led to the formation of the Zagros foldandthrust belt, continued shortening of the mountain range, and creation of the Zagros foreland basin. The Late Cretaceous to Miocene rocks represent deposits of the foreland basin prior to the Zagros orogeny, and subsequent incorporation into the colliding rock sequences. This sequence unconformably overlies Jurassic to Upper Cretaceous rocks.
Compressional folding began during or soon after the deposition of the OligoceneMiocene Asmari Formation (Mapstone, 1978; Sepehr and Cosgrove, 2004).
During the Palaeocene and Eocene, the Pabdeh (pelagic marls and argillaceous limestones) and the Jahrum (shallow marine carbonates) formations were, respectively, deposited in the middle part and on both sides of the Zagros basin axis (Motiei, 1993). During the OligoceneMiocene this basin was gradually narrowed and the Asmari Formation was deposited. Different facies, including lithic sandstone (Ahwaz Member) and evaporites (Kalhur Member) were deposited during late Oligoceneearly Miocene times (Ahmadhadi et al., 2007). In the southwestern part of the Zagros basin, the Asmari Formation overlies the Pabdeh Formation, whereas in the Fars and Lurestan regions it covers the Jahrum and Shahbazan formations (Figure 3). Although the lower part of the Asmari Formation interfingers with the Pabdeh Formation in the Dezful Embayment (Motiei, 1993), its upper part covers the entire Zagros basin. The maximum thickness of the Asmari Formation is found in the northeastern corner of the Dezful Embayment.
METHODS AND STUDY AREA
Four sections of the Asmari Formation were measured bed by bed, and sampled in four areas (Dehluran, 180; KabirkuhDarrehshahr, 260; Mamulan, 69/5; and Sepid Dasht, 82/5 m thick; Figures 1 and 4), and sedimentologically investigated. The sections were described in the field, including their weathering profiles, facies and bedding surfaces. Fossils and facies characteristics were described in thin sections from 408 samples. Test shapes of the largest benthic foraminifera were taken into account for the facies interpretation, as their differences depend on the environment (Hottinger, 1980, 1983; Reiss and Hottinger,1984; Leutenegger, 1984; Hohenegger, 1996; Hallock, 1999; Hohenegger et al., 1999; Geel, 2000; Brandano and Corda, 2002; Corda and Brandano, 2003; Barattolo et al., 2007). The lithology and the microfacies types were described according to the schemes porposed by Dunham (1962) and Embry and Klovan (1971). Also, the same 408 samples were used for sequence stratigraphy analyses.
Biozonation and age determinations are based on strontium isotope stratigraphy recently established for the Asmari Formation by Laursen et al. (2009). Results from the foraminifera data are summarized in Table 1.
Three assemblages of foraminifera were recognized in the studied areas and were discussed in ascending stratigraphic order as follows:
Assemblage 1. This assemblage occurs only at KabirkuhDarrehshahr area (Section 2). The most important foraminifera are: Eulepidina sp., Eulepidina dilatata, Eulepidina elephantine, Lepidocyclina sp., Nephrolepidina sp., Operculina sp., Operculina complanata, Austrotrillina howchini, Austrotrillina asmaricus, Peneroplis sp., Triloculina trigonula, Spiroclypeus blanckenhorni, miliolids and globigerinids. This assemblage is correlated with LepidocyclinaOperculinaDitrupa assemblage zone of Laursen et al. (2009) (Table 1) and is attributed to the Chattian time.
Assemblage 2. This assemblage is present in Dehluran (Section 1) and KabirkuhDarrehshahr (section 2) areas. The most diagnostic species in both studied sections include: Miogypsina sp., Elphidium sp. 14, Lepidocyclina sp., Operculina complanata, Austrotrillina sp., Austrotrillina asmaricus, Peneroplis sp., Peneroplis thomasi , Triloculina trigonula, Miogypsinoides sp., Borelis sp., Meandropsina iranica, Meandropsina anahensis, Dendritina rangi, Amphistegina sp., miliolids, Discorbis sp., Valvulinid sp. and Neorotalia viennoti. This assemblage corresponds to the MiogypsinaElphidium sp. 14 Peneroplis farsensis assemblage zone of Laursen et al. (2009) (Table 1). The assemblage is considered to be Aquitanian in age.
Assemblage 3. This assemblage is recordable in all studied sections and consists of Borelis melo curdica, Borelis sp., Peneroplis sp., Neorotalia sp., Elphidium sp., Meandropsina iranica, Dendritina rangi, Dendritina sp., miliolids, Discorbis sp. and globigerinids. The assemblage represents the Borelis melo curdicaBorelis melo melo assemblage zone of Burdigalian age (Laursen et al., 2009).
Facies analysis of the Asmari Formation in the study areas resulted in the definition of thirteen facies types (Figure 5), which characterize platform development. Each of the microfacies exhibits typical skeletal and nonskeletal components and textures. The general environmental interpretations of the microfacies are discussed in the following paragraphs.
Microfacies A. Stromatolitic boundstone (Figure 5.1)
This microfacies, with finely or moderately crinkled horizontal lamination, consists of alternating calcilutitic laminae and calcisiltic bioclastic laminae. Microfacies A is only present at Dehluran area (Section 1) and intercalates with mudstone facies.
Interpretation. This facies type is common in tidal flat sediments (Flügel, 2004; Hardie, 1986; Steinhauff and Walker, 1996; Lasemi, 1995; HernándezRomano, 1999; AguileraFranco and HernándezRomano, 2004). Today, flat laminated structures of microbial origin are found in intertidal settings. In regions with an arid climate (e.g., Persian Gulf or Shark Bay) stromatolites with smooth mats are located in the lower intertidal zone (Davies, 1970a, 1970b; Kinsman and Park, 1976; Hoffman, 1976).
Microfacies B. Fenestrate mudstone (Figure 5.2)
This facies consists of fine grained microcrystalline limestone. Bioclasts are lacking and the fenestrate structures are well developed. Microfacies B was identified at KabirkuhDarrehshahr area (Section 2) and mostly occurs with quartz mudstone.
Interpretation. Fenestrate structures are typical products of shrinkage and expansion, gas bubbles, and air escape during flooding, or may even result from burrowing activity of worms or insects. Shinn (1983) considered similar facies representative of a tidal flat environment, where trapped air between irregularlyshaped deposits leads to the development of birdseyes.
This microfacies is composed of dense lime mudstones. Sediments also contain sparse unidentified fauna. In some samples, subordinate amounts of detrital quartz grains and gypsum are also present. This facies occurs in middle and upper parts of the Asmari Formation. Microfacies C occurs at Dehluran (Section 1), KabirkuhDarrehshahr (Section 2) and Mamulan (Section 3) areas. It is either next to anhydrite or intercalates with lagoonal facies.
Interpretation. Lime mudstone, with gypsum blades and small quartz grains and no evidence of subaerial exposure, was deposited in a restricted shelf lagoon. This facies indicates hypersaline conditions within a shelf lagoon.
Microfacies D. Anhydrite (Figure 5.5)
Anhydrite facies have been observed in the upper part of the Pabdeh Formation and in the lower part of the Asmari Formation. The first anhydrite deposit is surrounded by marly limestones containing pelagic fauna. There is a sharp contact with the carbonates above and below. The second anhydrite deposit is intercalated between shallow water carbonates. Microfacies D is only present at Dehluran area (Section 1) and mostly associates with mudstone facies.
Interpretation. Considering the thickness of the anhydrite deposits, their vertical stacking and lateral continuity, it is assumed that they are submarine deposits formed in an isolated saline basin. The deposition of anhydrite implicates that the depositional environment became isolated from the open ocean at that time, which allowed for the concentration and submarine precipitation of salt. An eustatic sea level drop is invoked as the most likely cause. This event took place around the OligoceneMiocene boundary. Ehrenberg et al. (2007) noted that strontium dates obtained from anhydrite in the Asmari Formation were close to the expected depositional ages and suggested that the anhydrite formed as an evaporate rather than as a later diagenetic product.
Microfacies E. Dendritina miliolids peloids wackestonepackstonegrainstone (Figure 5.6)
Identifiable components of this facies include benthic imperforate foraminifera (Dendritina and miliolids) and peloids. Borelis, bivalves and gastropods (whole shell and broken fragments) are less common. The grains are poorly to medium sorted, are fineto medium size and vary from subangular to semirounded. Textures are dominantly packstone, but range from wackestone to grainstone. In some samples, the predominant nonskeletal carbonate grains are intraclasts. Microfacies E is present in all sections and mostly intercalates with open lagoonal facies.
Interpretation. This facies was deposited in a restricted shelf lagoon. The restricted condition is suggested by the rare to absent normal marine biota and abundant skeletal components of restricted biota (imperforate foraminifera such as miliolids and Dendritina). The subtidal origin is supported by the lack of subaerial exposure and stratigraphic position. This microfacies represents the shallowest upper part of the photic zone, with very light, highly translucent and soft muddy substrate (Geel, 2000; Romero et al., 2002; Corda and Brandano, 2003; VaziriMoghaddam et al., 2006; Bassi et al., 2007).
Microfacies F. Bioclastic rotaliids miliolids bioclast wackestonepackstone (Figure 5.7)
Skeletal grains consist of diverse fauna, including benthic foraminifera (miliolids, rotaliids), echinoid, corallinacean and bivalve fragments. Texture varies from wackestone to packstone. Microfacies F was identified at Dehluran (Section 1), KabirkuhDarrehshahr (Section 2) and Mamulan (Section 3) areas.
Interpretation. The cooccurrence of normal marine biota such as rotaliids, corallinaceans and echinoids with lagoonal biota such as miliolids, indicates that sedimentation took place in an open shelf lagoon. A similar facies with imperforate foraminifers and perforate foraminifers was reported from the inner ramp of the OligoceneMiocene sediments of the Zagros basin (VaziriMoghaddam et al., 2006).
Microfacies G. Bioclastic miliolids coral floatstonerudstone (Figure 5.8)
This facies is predominantly composed of miliolids and corallite fragments or fragments of coral colonies. Additional components are echinoderm fragments, recrystallized bivalve fragments and small benthic foraminifers (Austratrillina and Dendritina). Grains are poorly sorted and are medium to coarse sand to granule in size. Microfacies G is present at KabirkuhDarrehshahr (Section 2) and Mamulan (Section 3) areas.
Interpretation. Cooccurrence of normal marine (perforate foraminifera and corals) and platforminterior (imperforate foraminifera) components in facies F and G suggests the absence of an effective barrier. Restricted shelf organisms are effectively separated from the normal marine environment by barriers.
Microfacies H. Bioclastic ooids packstonegrainstone (Figure 5.9)
The predominant grain types are skeletal fragments and ooids. Biotic grain types include echinid and gastropods. Ooid nuclei consist of recrystallized bivalve fragments, miliolids and rotaliids, with oval, circular or elongate outlines. Grains are fine to coarsesand size and sorting is moderate. Microfacies H was only identified at KabirkuhDarrehshahr area (Section 2) and mostly intercalates with imperforated coral rudstone to bioclastic Miogypsina corallinacea packstone facies.
Interpretation. The features of this facies indicate moderate to high energy shallow waters with much movement and reworking of bioclasts and the production of ooids. Sediments are interpreted to have been deposited in sand shoal (Wilson, 1975; Flügel, 2004).
The main characteristic of this microfacies is abundant fragments of corallinacean and corals. Echinoid and bryozoan fragments are also present. The fragments are coarse sand to granule in size. Due to changes in the type of fauna in some samples, the name of this facies changes to bioclastic Miogypsina coral floatstonerudstone. Microfacies I referred to Sepid Dasht (Section 4) and mostly intercalates with bioclastic Miogypsina foraminifera corallinacea wackestonepackstone.
Interpretation. This facies is interpreted as an open marine facies that formed seaward of the platform margin and within the storm wave base. Open marine, welloxygenated conditions are indicated by the diverse fauna. A similar microfacies was reported by Wilson (1975), Longman (1981), Flügel (1982), Riding et al. (1991), and Melim and Scholle (1995).
A diverse assemblage of poorly to moderately sorted, fragmented and whole fossils in lime mud is characteristic of this microfacies. Miogypsina and corallinacean fragments are the dominant bioclasts. Less common bioclasts include bryozoan and fragments of recrystallized bivalves and echinoderm. In a few samples with increasing nummulitids, the name of this microfacies changes to bioclast nummulitids corallinacean wackestonepackstone. Microfacies J was identified at Dehluran (Section 1), KabirkuhDarrehshahr (Section 2) and Sepid Dasht (Section 4) areas and intercalates with open marine facies.
Interpretation. The presence of high diverse stenohaline fauna such as red algae, bryozoan, echinoid and larger foraminifera (Miogypsina and nummulitids) indicate that the sedimentary environment was situated in the oligophotic zone in a shallow open marine environment or near a fairwater wave base on the proximal middle shelf (Pomar, 2001a, 2001b; Brandano and Corda, 2002; Corda and Brandano, 2003; Cosovic et al., 2004). In open marine, shallow waters, foraminifera produce robust, ovate tests with thick walls, as a protection against photo inhibition of symbiotic algae inside the test in bright sunlight, and/or as a protection against test damage in turbulent water.
Microfacies K. Bioclastic lepidocyclinids nummulitids wackestonepackstone (Figure 5.14)
The main components are bioclasts and large perforate foraminifera. Bioclasts include bivalve, corallinacean (including articulated and crustose fragments), echinoderm and bryozoan fragments. The foraminifera are characterized by a relatively diverse assemblage of nummulitids (Operculina, Hetorestegina and Spiroclypeus) and lepidocyclinids (Eulepidina and Nephrolepidina). This facies is most prominent in lower parts of the Asmari Formation. Grains are coarse sand to granule in size and are in a finegrained carbonate matrix. Fragmentation of larger foraminifera is rare. In a few samples, Amphistegina are more or less equal to lepidocyclinids in abundance, therefore, the name of the microfacies changes to bioclast Amphistegina nummulitids wackestonepackstone. Microfacies K referred to KabirkuhDarrehshahr (Section 2), Mamulan (Section 3) and Sepid Dasht (Section 4) areas.
Interpretation. The presence of large flat lepidocyclinids and nummulitids indicate that sedimentation took place in relatively deep water. Flatter test and thinner walls with increasing water depth reflect the decreased light levels at greater depths (Geel, 2000; Beavington and Racey, 2004; Nebelsick et al., 2005; Bassi et al., 2007; Barattolo et al., 2007).
Microfacies L. Bioclastic planktonic foraminifera lepidocyclinids wackestonepackstone (Figure 5.15)
The most frequent skeletal components of this microfacies are test fragments of echinoids, bryozoan, corallinacean, larger benthic foraminifera (lepidocyclinids) and entire tests of planktonic foraminifers. Bioclasts are angular to rounded and size ranges from silt to granule. Bioclastic planktonic foraminifera nummulitids wackestonepackstone and bioclastic planktonic foraminifera Miogypsina wackestonepackstone are similar to the microfacies described above in overall character, but differ from each other by their larger foraminifera. Microfacies L occurs at KabirkuhDarrehshahr (Section 2) and Sepid Dasht (Section 4) areas and intercalates with bioclastic planktonic foraminifera wackestone facies.
Interpretation. In general, the observed higher faunal diversity and the associated benthic foraminifers (lepidocyclinids, nummulitids and Miogypsina) and planktonic foraminifers, as well as bioclasts, indicate an open marine environment. Poorly washed matrix and mudsupported textures suggest environments below wavebase influenced by bottomcurrents (Geel, 2000; VaziriMoghaddam et al., 2006; Amirshahkarami et al., 2007a).
Microfacies M. Bioclastic planktonic foraminifera wackestone (Figure 5.16)
In this microfacies, planktonic foraminifera are the dominant biotic components, but fine fragments of bryozoan and echinoid are also present. The planktonic foraminifers include nonkeeled globorotalids and globigerinids. Some planktonic tests are filled with sparry cement. This facies occurs mostly in lower parts of the Asmari Formaton in most sections; however, it is recorded in the upper part of the formation at SepidDasht area. Microfacies M is present at Dehluran (Section 1), KabirkuhDarrehshahr (Section 2) and Sepid Dasht (Section 4) areas.
Interpretation. The general lack of sedimentary structures, the finegrained character, and the presence of undisturbed whole fossils from planktonic foraminifera suggest that this facies was deposited in calm, deep, normalsalinity water (Buxton and Pedley, 1989; Cosovic et al., 2004; Flügel, 2004).
The recognized microfacies have allowed the differentiation of several carbonate marine system environments including tidal flat, restricted lagoon, open lagoon, shoal, slope and basin. These six depositional environments of the OligoceneMiocene in the study area are similar to those found in many modern carbonate depositional settings (Read, 1985; Jones and Desrochers, 1992). Of these, the Persian Gulf is perhaps the best modern analogue for inference of ancient water depths, because it shares many similarities with the Zagros foreland basin during the OligoceneMiocene. Therefore, sedimentological and paleontological studies show that a ramp type carbonate platform sedimentary model can be fully applied to these ancient carbonate deposits (Read, 1982; Tucker, 1985; Tucker and Wright, 1990). According to Burchette and Wright (1992), carbonate ramp environments are separated into inner ramp, middle ramp and outer ramp. Outer ramp facies are characterized by marl and marly limestone lithologies. Wackestones predominate with abundant planktonic foraminifera. The presence of mudsupported textures and the apparent absence of wave and current structures suggest a low energy environment below storm wave base (Burchette and Wright, 1992).
Larger perforate foraminifera are abundant biogenic components of the shallow water carbonate succession in the Asmari Formation. A proliferation of perforate foraminifera is indicative of normal marine conditions (Geel, 2000). The lack of abrasion of the foraminifera indicates autochthonous accumulations, thus wackestonepackstone with lepidocyclinids and nummulitids were deposited under low energy conditions, below fair weather wave base (FWWB) and above storm wave base (SWB) in the middle ramp setting. The variation in the shape of the test reflects the differences in water depth. The sediments with perforate robust and ovate specimens reflect the presence of shallower water than those containing large and flat lepidocyclinids and nummulitids. Larger foraminifera are limited geographically to temperate to tropical/subtropical environments (Hohenegger et al., 2000; Langer and Hottinger, 2000).
The common association of symbiotic algae with perforate foraminifera implies that light is a main factor in determining the depth distribution (Hansen and Buchardet, 1977; Hallock, 1979, 1981; Bignot, 1985; Hallock and Glenn, 1986).
Inner ramp deposits represent a wider spectrum of marginal marine deposits, indicating highenergy shoal, open lagoon and protected lagoon. In the restricted lagoon environment, the faunal diversity is low and the normal marine fauna are lacking, except for imperforate benthic foraminifera (miliolids, Dendritina, borelisids), which indicates quiet, sheltered conditions. A large number of porcellaneous imperforate foraminifera points to the presence of slightly hypersaline waters (Geel, 2000). Open lagoonal conditions are characterized by mixed open marine fauna (such as red algae, echinoids and perforate foraminifera) and protected environment fauna (such as miliolids). The shallow subtidal environment above the fairweather wave base is characterized by the presence of a facies association showing signs of longterm water agitation (packing, sorting, poor taphonomic preservation and ooids). Such highenergy deposits are typically associated with carbonate shoals on carbonate platforms (Figure 6).
During the Chattian, outer ramp facies (Pabdeh Formation) was predominant at the Dehluran area (Section 1, Figure 7). Simultaneously, outer to middle ramp conditions occurred at the KabirkuhDarrehshahr area (Section 2). The Dehluran area was experiencing outermiddle ramp conditions during the Early Aquitanian. At the same time, sedimentation at the KabirkuhDrarrehshahr area took placed in the middle and inner ramp environments. These areas experienced inner ramp (mostly lagoon subenvironment) condition during the Late Aquitanian (Figure 7). Eastern parts of the study area (Mamulan and Sepid Dasht were sites of nondeposition or erosion during Chattian through Aquitanian. In Mamulan area (section 3), middle and inner ramp environments prevailed through Burdigalian, whereas middle and outer ramp conditions were predominant in Sepid Dasht area (section 4) during the Burdigalian (Figure 7).
The studied succession can be framed in a sequence stratigraphic context. As a guide, we used the principal sequence stratigraphic concepts developed by many workers (e.g., Sarg, 1988, Posamentier et al., 1988; Van Wagoner et al., 1988, 1990, Read and Hrbury, 1993; Emery and Myers, 1996; Coe and Church, 2003; Catuneanu, 2006) to recognize TST (transgressive systems tract), mfs (maximum flooding surface), HST (highstand systems tract) and sequence boundaries.
Based on the distribution of planktonic and benthonic foraminifera, and on the detailed sedimentological and stratigraphical study, we defined four thirdorder sequences.
The depositional sequence 1 is present in sections 1 (Dehluran, 17 m thick) and 2 (KabirkuhDarrehshahr, 60 m thick) of the study area (Figures 8 and 9). The sediments of sequence 1 are Chattian in age. Sequence 1 includes the upper part of the Pabdeh Formation at Dehluran area, whereas at KabirkuhDarrehshahr area it encompasses the upper part of the Pabdeh Formation and the lower part of the Asmari Formation. At Dehluran area, TST and HST could not be differentiated because the relatively uniform deep subtidal succession is composed of planktonic foraminifera wackestone without distinct changes in microfacies. TST was clearly recognized at KabirkuhDarrehshahr area. Shale and marly limestone of the TST contain abundant planktonic foraminifera and document a deepsubtidal, low energy environment during the TST. The maximum flooding surface (mfs) coincides with the boundary between the Pabdeh and Asmari formations. The highstand systems tract (HST) comprises the lower part of the Asmari Formation. The early HST was characterized by constant shallow open marine environmental conditions (wackestonepackstone with perforate foraminifera). The late HST shows a trend toward more protected sediments (wackestonepackstone with imperforate foraminifera), expressing a filling of the accommodation space. The sequence boundary is characterized by abrupt facies changes from subtidallagoonal to tidal flat environments. Such changes reflect a significant decrease in water depth (Figures 8 and 9).
The depositional sequence 2 formed during the late Chattianearly Aquitanian transgression. At Dehluran area, this sequence is 21 m thick, (Figure 8), and begins with 9 mthick sediments of the anhydrite facies. These are interpreted as the lowstand systems tract (LST) of this sequence. The contact between the LST and the basinal deposits with pelagic fauna (Pabdeh Formation) below is sharp. At this section, the TST and HST comprise an 11 mthick, monotonous succession of open marine deposits, demonstrating that prograding shallowwater sediments did not reach far west. At KabirkuhDarrehshahr area, 130/5 m thick (Figure 9), the vertical variations in the facies during the transgression are different from those described in sequence 1. An increase in thirdorder accommodation space is indicated by shallow lagoonal facies overlain by shallowopen marine facies. Wackestone with abundant planktonic foraminifers represent deepwater facies; this is, therefore, interpreted as the mfs. An upwardshallowing facies trend (HST) is indicated by shallow open marine gradational facies, overlain by shallowlagoonal facies (Figures 8 and 9).
This sequence is late Aquitanian in age and is present in Dehluran area (48/5 m thick) and in KabirkuhDarrehshahr area (14/5 m thick). At Dehluran (Figure 8), the lowstand deposits of this sequence consist of a well developed anhydrite. A temporary isolation of the sedimentary environment would be necessary in order to be able to precipitate the anhydrite. At the base, the anhydrite is homogenous, but passes up into a more heterogenous composition and interdigitates with shallow water carbonates. The sea level transgression caused the deposition of shallow subtidal facies within an aggradational staking pattern in Dehluran and KabirkuhDarrehshahr areas. The sequence boundary is characterized at the top by stromatolitic boundstone (Dehluran area) and mudstone with quartz (KabirkuhDarrehshahr area), which marks the end of a shallowingupward trend (Figures 8 and 9).
The development of a long, narrow, evaporitic intrabasin, during the latest Oligoceneearliest Miocene (ChattianAquitanian) likely indicates an abrupt facies change (both laterally and vertically), which seems to be difficult to interpret simply by eustasy or any sedimentological process alone, without any tectonic control (Ahmadhadi et al., 2007). An abrupt facies change from marls to evaporites suggests a direct relationship between this restricted intrabasin lagoon and the deepseated basement faults. Nevertheless, eustatic control cannot be ruled out. Ahmadhadi et al. (2007), suggest that the genesis of this subbasin has been, at least, partly tectonically controlled.
The sequence 4 is present in all sections (Dehluran, 117/5; KabirkuhDarrehshahr, 55; Mamulan, 69/5; and Sepid Dasht, 82/5 m thick).
The lower boundary of Sequence 4 in Dehluran and KabirkuhDarrehshahr areas is characterized by a type 2 sequence boundary (Figures 8 and 9), whereas in Mamulan, (section 3) and Sepid Dasht, (section 4) areas it is defined by a type 1 sequence boundary (Figures 10 and 11). A long period of lagoonal conditions reflecting a balanced situation between accommodation and sedimentation characterizes the sequence 4 in Dehluran and KabirkuhDarrehshahr areas. Following the very shallow subtidal deposition of the uppermost part of the sequence 4 at Mamulan area, a clearly marine deepening occurred and led to the deposition of shallow lagoonal facies, forming a TST. The overlying wackestonepackstone with diverse fauna reflects a mfs, and the beginning of deposition of a HST. The overlying mfs, rich in imperforate foraminifera, have been deposited in a calm and shallowlagoonal environment; this part is interpreted as a HST. Above type 1 sequence boundary at Sepid Dasht area, there are limestones of open marine facies with a rich planktonic foraminifera, perforate larger benthic foraminifera, corallinacean and coral fragments. These sediments were characterized by constant open marine environmental conditions, representing constant accommodation at Sepid Dasht area.
Ehrenberg et al. (2007) recognized some surfaces in well sections from the Bibi Hakimeh, Marun, and Ahwaz oilfields and interpreted them as sequence boundaries (Ch 20 SB, Ch 30 SB, Aq 10 SB, intraAq10 SB, Aq20/Bu10 SB, Bu 20 SB). Because these sequence boundaries were not recognized in the study area, the sequence stratigraphy of Ehrenberg et al. (2007) can not be confidently applied to these sections.
On the basis of facies changes (Figures 8 and 9), in both sections (1 and 2), sequence boundaries recognized in the upper part of the Chattian and the middle part of the Aquitanian, may be associated with the Aq 10 and Aq20/Bu10 sequence boundaries recognized by Ehrenberg et al. (2007).
The depositional sequences 1, 2 and 3 were observed in Dehluran and KabirkuhDarrehshahr areas (sections 1 and 2), and are synchronous with a period of either erosion or nondeposition represented by unconformities in Mamulan and Sepid Dasht areas (sections 3 and 4) (Figures 7,12).
The OligoceneMiocene Asmari Formation of the Zagros basin is a thick sequence of shallow water carbonate. The outcrops of the Asmari Formation in northwest of the Zagros (Dehluran, Kabirkuh Darreshahr, Sepid Dasht and Mamulan areas) allow the recognition of different depositional environments, on the basis of sedimentological analysis, distribution of foraminifera and microfacies studies. These depositional environments correspond to inner, middle and outer ramp. In the inner ramp, the most abundant lithofacies are mediumgrained wackestonepackstone with imperforated foraminifera. The middle ramp is represented by packstonegrainstone to floatstone with a diverse assemblage of larger foraminifera with perforate wall, red algae, bryozoa, and echinoids. The outer ramp is dominated by argillaceous wackestone characterized by planktonic foraminifera and large and flat nummulitidae and lepidocyclinidae. Four thirdorder sequences are identified on the basis of deepening and shallowing microfacies patterns and on the distribution of OligoceneMiocene foraminifers.
The authors wish to thank the National Iranian Oil Company Research and Development Management for their financial support and permission to publish this work. We also thank Dr. Mohammad Ali Emadi for his support and thoughtful comments. The authors appreciate the National Iranian Oil Company Exploration Division for providing surface geological maps. Also we thanks the Revista Mexicana de Ciencias Geológicas reviewers for their constructive comments.
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