Sequence biostratigraphy and paleoenvironmental ...

Arab J Geosci7 :9 )6102( DOI 10.1007/s12517-015-2064-4

ORIGINAL PAPER

Sequence biostratigraphy and paleoenvironmental reconstruction of the Oligocene-early Miocene deposits of the Zagros Basin (Dehdasht area, South West Iran) Asghar Roozpeykar 1 & Iraj Maghfouri Moghaddam 1

Received: 16 October 2014 / Accepted: 10 September 2015 # Saudi Society for Geosciences 2015

Abstract The Asmari succession is a predominantly carbonate-lithostratigraphic unit corresponding to an Oligocene-Lower Miocene shallow and homoclinal ramp system that developed in the southwest of the Zagros Basin. The sedimentation occurs within a transition from a mixed carbonate–siliciclastic to carbonate-dominated depositional environment. Carbonate production in this system is dominated by diverse benthic foraminifera, coralline algae, mollusks, bryozoans, echinoids, and corals. Based on fossil content and texture, 16 different facies types have been distinguished that accumulated in three interpreted paleoenvironments: The euphotic inner ramp is characterized by two facies zones: an inner and an outer shallow water zone. The inner shallow water zone is characterized by the occurrence of imperforated foraminifera together with mollusks in restricted parts (foramol assemblage). The outer shallow water zone is dominated by wackestones-packstones with benthic foraminifera and coralline red algae, minor mollusks, bryozoans, echinoids, and small epiphytic biota (LB-foralgal assemblage). The higher-energy parts on platform margin are represented by ooids, scattered corals, and robust foraminifera-rich grainstones (coralgal assemblage). The middle ramp is characterized by small coral patch reefs that developed in mesophotic conditions, and larger foraminiferal packstone facies often associated with coralline red algal debris (LBforalgal assemblages) characterize the sediments of the deeper oligophotic zone, whereas the outer ramp facies–aphotic zone–are represented by bryozoans, mollusks, and echinoid

* Asghar Roozpeykar [email protected] 1

Geology Department, Faculty of Sciences, Lorestan University, Lorestan, Iran

association (bryomol assemblage). This facies passes basinward into wackestone, packstone, and rarely mudstone with pelagic and small benthic foraminifera, sponge spicule, and echinoid fragments (echinofor assemblages). Changes in the depositional facies and cycle stacking patterns indicate four transgressive–regressive sea-level cycles from the bottom to the top of the section. These depositional sequences have been separated by type 2 sequence boundaries. Sea-level fluctuations that cause changes in light penetration which may produce shifts from aphotic to oligophotic and euphotic conditions are an important factor in skeletal production and spatial distribution of carbonate factories. The presence of nonskeletal grains (ooids and ploeids), the occurrence of Porites patch reefs, and the existence of LBF such as Heterostegina, Operculina, Amphistegina, Archaias, and Lepidocyclina in the larger foraminifera associations together with Lithoporella mellobesioides and Spirolithon in coralline red algae assemblage all suggest that carbonate sedimentation took place in tropical waters under oligotrophic to slightly mesotrophic conditions. Keywords Asmari Formation . Carbonate ramp . Dehdasht . Sequence stratigraphy . Tropical zone . Oligotrophic to mesotropic

Introduction The Asmari Formation is a thick carbonate sequence of the Oligocene-lower Miocene in the south west (SW) Iran. Deposition took place on a carbonate platform developed across the foreland Zagros Basin. Due to the abundance of fractures and porosities, it is one of the most important petroleum reservoir rocks in the world. The lithology of this Formation mainly consists of thin, medium to thick, and

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massive limestones. These limestones are highly fossiliferous, and non-skeletal grains are also common. Biogenic components include diverse benthic foraminifera, coralline red algae, corals, mollusks, echinoids, bryozoans, and serpulid. Nonskeletal grains, larger benthic foraminifera, and zooxanthellate corals suggest deposition in warm tropical waters. The Asmari Formation consists of two members: the Ahvaz member and Kalhur member. The Ahvaz member occurs in the south west portion of the region and is composed of calcareous sandstones. Three main paleoenvironments have been interpreted for it: coastal plain to terrestrial environment, distal deltaic subtidal environment, and offshore marine to basinal setting (Van Buchem et al. 2010). The Kalhur member is located in the north western (NW) region. Lithologically, it consists of anhydrite. The depositional environment of this unit has been described as a submarine-isolated saline basin that is caused by a big eustatic sea-level drop (Van Buchem et al. 2010). The main purposes of this paper are to (1) conduct facies analysis and interpret paleoenvironments of deposition and (2) describe and interpret the sequence stratigraphic model.

Geological setting Asmari Formation is part of the geological history of the Zagros fold-thrust Belt. This belt is part of AlpineHimalayan orogenic belt (Takin 1972; Berberian and King 1981) and lies on the northeastern margin of the Arabian Plate (Sepehr and Cosgrove 2005). It extends from the NW Iranian border through to SW Iran up to the Strait of Hormuz (Sherkati and Letouzey 2004) and is dominated by NW–SE trending folds and thrusts (Sepehr and Cosgrove 2005). The tectonic history of this belt includes the platform phase in Paleozoic, rifting in Triassic, passive continental margin in Jurassic-middle Cretaceous, subduction to the northeast (beneath the Iranian Lithospheric plates), and emplacement some of Neo-Tethyan ophiolite sheets over the Afro-Arabian passive continental margin in Turonian to Campanian and finally, collision and shortening during the last Alpine phases (Valashian to Pasadnian) (Falcon 1974; Alavi 1994, 2004; Jackson and McKenzie 1984). From south west to north east, the Zagros Basin is divided into three zones: 1-Khozestan Plain, 2-Simply Folded Zagros and 3-Imbricated Thrust Zone (Motiei 1993). Falcon (1961) based on the sedimentary history and structural style separated the Simply Folded Zagros into several zones: Fars, Lurestan, Abadan plain, Thrust Zone, Dezful Embayment, Izeh, Bandar Abas Hinterland, and Complex Structure with metamorphic rocks (Fig. 1). These were all part of the continental margin of the Arabian platform and are now separated from each other by N–S and E–W trending fault zones (Sepehr and Cosgrove 2004). These fault zones, which include the Mountain Front, Kazerun, Bala Rud, and Izeh Fault Zones played, and still play, an important

role in controlling sedimentation of the Basin, and as a result these regions have different sedimentary successions (Sepehr and Cosgrove 2004; Sepehr et al. 2006). The study area is located in the Izeh Zone (Fig. 1). The Izeh zone is situated in the Central Zagros between the High Zagros Fault to the northwest and the Mountain Front Fault to the southeast (Sherkati et al. 2006). This zone consists of a variety of structures of variable sizes and geometrical character (Sherkati et al. 2005).

Study area and methods A total of 157 samples were collected. Thin sections were prepared from the collected samples and subsequently analyzed using an optical microscope. Facies description was based on field observation and microfacies characteristics including skeletal and non-skeletal components, depositional texture, and grain size. The textural classification follows Embry and Klovan (1971) and Dunham (1962) classifications. The work of Pomar (2001a) had been used for the interpretation of light conditions in carbonate platform. The study area is located in Zargham Abad Village about 80 km north east of Behbahan and 15 km north of Dehdasht (Izeh Zone). The section was measured in detail at 30°52′N and 50°38′E (Fig. 1). Biogenic components Biogenic components are dominated by larger and small foraminifera, coralline algae, coral, bryozoan, echinoids, and mollusks. Subordinate components are polychaete worm tubes (Ditrupa), green algae, sponge, and pelagic foraminifera. Larger foraminifera are represented by both hyaline and porcellaneous forms. The hyaline foraminifera are dominated b y E u l e p i d i n a , N e p h ro l e p i d i n a , A m p h i s t e g i n a , Miogypsinoides, Miogypsina, Asterigerina, Neorotalia, Heterostegina, Operculina, and Spiroclypeus. Nummulites can be locally present. Larger porcellaneous forms are represented by Archaias, Peneroplis, Dendritina, Marginopora, Spirolina, Meandropsina, Borelis, and larger miliolids Austrotrilina. Small foraminifera forms are represented by Textularia, Bigenerina, Planorbulina, Neorotalia, Ammonia, Valvulinid, Elphidium, Discorbis, and small miliolids Triloculina, Pyrgo, and Quincueloculina. The calcareous red algae are dominated by members of corallinaceaen families with the subfamilies of mastophoroideae and melobesioideae and the family of sporolithaceae. Six genera were found: Subfamily mastophoroideae is characterized by Lithoporella, Lithothamnion, and Mesophyllum. Melobesioids are represented by Lithophyllum and Titanoderma. Family sporolithaceae is dominated by Sporolithon.


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Fig. 1 General map of Iran showing eight geologic provinces (A). The study area is located in Zagros province (adapted from Heydari 2003). Subdivisions of the Simply Folded Zagros Belt (after Falcon 1961) (B)

Bivalves are represented by pectinids, oysters, and aragonitic-shelled bivalves. Echinoderms are recognized mostly as spine cross-sections or test fragments. Bryozoans are represented by erect branching and encrusting growth forms belonging to cyclostomate and cheilostomate families. Corals are diverse and include zooxanthellate and azooxanthellate groups. Zooxanthellate corals are mainly dominated by faviids and

poritids. The main genera recognized are Porites, Hydnophora, Tarbellastrea, Thegioastrea, and Favites. Biostratigraphy According to (Adams CG, Bourgeois E, unpublished). Eulepidina-Nephrolepidina-Nummulites assemblage zone

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represent Oligocene. But they could not recognize the individual stages of Oligocene. Also, Laursen et al. (2009) reported that the occurrence of Nummulites and Lepidocyclinia (Eulepidina and Nephrolepidina) indicates the Oligocene. They stated that the occurrence of Nummulites is restricted to the Rupelian. Racey (1995) placed the extinction of Nummulites near the end of Rupelian age. Ehrenberg et al. (2007) also believed that the last occurrence of Nummulites was about 1 Ma before the end of Rupelian age. In our material (Fig. 4), the last occurrence of Nummulites vascus was in sample no. 10, which therefore represented the end of Rupelian age. Laursen et al. (2009) stated that the species of Spiroclypeous blankenhorni only indicates the Oligocene age and the Miogypsinoides complanatus characterized the Chattian stage. Ehrenberg et al. (2007) also reported that the occurrence of Spiroclypeous blankenhorni was only in the Chattian age. According to Sharland et al. (2004). M. complanatus is the most indicative of the late Oligocene (Chattian) in the Middle East. Sharland et al. (2004) redefined the circum Arabian maximum flooding surface of the late Paleogene and compared clean carbonates with M. complanatus of the northern Iraq and Sinai of Egypt with a new maximum flooding surface (MFS) Pg50 of Chattian age. Cahuzac and Poignant (1997) also indicated that the presence of M. complanatus determined the late Chattian (Shallow Benthic Zone SBZ 23). The Aquitanian is more difficult to identify with marker species (Van Buchem et al. 2010). A general observation was that when Miogypsina spp. and Elphidium sp. 14 occur together, an Aquitanian age is likely (Van Buchem et al. 2010). The occurrence of Miogypsina is used by other workers to mark the Aquitanian age. Elphidium sp. 14 and Miogypsins sp. also indicate the biozone of Miogypsina-Elphidium sp. 14-Peneroplis farsensis assemblage zone of Laursen et al. (2009) and boundary between subzones 2a and 2b of Adams and Bourgeois (1967). The marker species of Borelis melo curdica is the most indicative of the Neogene in the Zagros Zone. Its first occurrence indicates the beginning of Burdigalian stage (Adams and Bourgeois 1967; Ehrenberg et al. 2007; Laursen et al. 2009). The first occurrence of B. melo curdica in our sample was in sample no. 124, which therefore characterized the beginning of Burdigalian age.

Facies description and interpretation The studied facies mainly consist of packstones, wackestones, and grainstones, while mudstones, floatstones, and boundstones are subordinate. Sixteen facies were distinguished on the basis of textures, dominant and subordinated components, and grain size. The identified sedimentary facies are attributed to sub-environments of basinal, open marine, shoal, restricted to semi-restricted shelf lagoon, and tidal flat, which were deposited on a carbonate ramp environment.


These environments are represented by the following facies associations: MF1: spiculate mudstone This facies is composed of mudstone. Bioclasts are lacking and the uniaxial siliceous sponge spicules are the only biogenic elements (Fig. 3). In accordance with the standard microfacies type described by Wilson (1975). microfacies 1 accumulated below the wave base in deep and quite water of basin. The scarcity of fauna association implies stressed paleo-ecological conditions such as dysoxic conditions. MF 2: bioclast, pelagic foraminifera wackestone-packstone This facies is dominated by the pelagic foraminifera (globigerinid sand globorotalids) and echinoid fragments. Small benthic foraminifera (Textularia, Elphidium, and Rotalia) are also present. Other components are mollusk fragments, Ditrupa, and sponge spicules. The texture is characterized by a wackestone-packstone facies (Fig. 3). The occurrence of pelagic taxa indicates deep offshore setting with more pelagic and calm conditions (Vecsei and Sanders 2003; Mateu-Vicens et al. 2008). The presence of Elphidium documents down-shelf sediment transport from inner settings (Pomar et al. 2012). The occurrence of Ditrupa worm tubes indicates soft and unstable substrates (Barbera et al. 1978). The abundance of carbonate mud also suggests deposition below the influence of wave and current action (Beavington-Penney et al. 2005). MF 3: bioclast wackestone-packstone This microfacies is characterized by erect branching bryozoans, bivalves (pectinids), gastropods, and echinoids. Very rare pelagic foraminifera are also present. The matrix is finegrained micrite (Fig. 3). Abundant erect branching bryozoans suggest low water energy conditions in deep offshore habitats below the storm wave base (e.g., Smith 1995). The absence of light-dependent biota (corallinacean and larger benthic foraminifera (LBF)) places this environment in the aphotic zone (Pomar et al. 2004). The low presence of pelagic foraminifera may point to shallower water conditions. MF 4: bioclast, lepidocyclinidae wackestone-packstone This facies is characterized by dominance of Eulepidina with large and flat tests which can reach size of up to more than 10 cm in diameter (Figs. 2 and 3). Amphistegina, Operculina, and Heterostegina with elongate and thin-walled tests are less


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Fig. 2 Outcrop view of the of the representative facies lower part of the Asmari Formation. a–c Bioclast, lepidocyclinidae wackestone-packstone facies. The large and discoidal tests of the foraminifera point to a deep low light, turbid environment. d corallinacea, nummulitidae, lepidocyclinidae wackestonepackstone facies characterized by thin and elongated foraminifera tests in a packstone matrix. e Platy coral colony. f Scattered corals in the foraminifera, coral floatstone/ rudstone lithofacies. Pen for scale

abundant. Very rare Archaias and miliolids are also present in some samples. Other components are pelagic foraminifera, corallinacean, bryozoans, and echinoids. The space between the fauna is occupied by clay matrix (Figs. 2 and 3). The lepidocyclinids with large size and discoidal flat tests (Eulepidina) are typical for a low-energy, turbid, and low light environment on outer ramp (Hallock 1985; Hallock and Glenn 1986). In the Buxton and Pedley (1989) model, Lepidocyclina facies is also characterizing the distal environment (outer ramp) in the deepest part of photic zone. Larger benthic foraminifera are able to clean small amounts of sediment from their surface (Van Der Zwaan et al. 1999). allowing them to survive small episodic influxes of siliciclastic sediments (Lokier et al. 2009). Therefore, these fauna produced flat and thin tests allowing increased light capture. The cooccurrence of oligophotic with euphotic factories reflects an offshore transport from shallower inner ramp by currents and waves (Mateu-Vicens et al. 2008).

MF 5: corallinacea, nummulitidae, lepidocyclinidae wackestone-packstone The main components are corallinacean and large benthic foraminifera. The LBF skeletons are mostly more than 3 cm in diameter (Figs. 2 and 4). The nummulitidae are represented by

Spiroclypeus, Heterostegina, Nummulites, and Operculina. Corallinacean mainly correspond to subfamily melobesioideae genera (Lithothamnium and Mesophyllum) and rarely Lithoporella melobesioides and Lithophyllum. Other components are bryozoans, echinoids, Amphystegina, and Asterigerina. In some samples, pelagic foraminifera are also present (Fig. 4). The dominance of flat-shaped larger benthic foraminifera combined with melobesioid coralline algal assemblage indicates low light intensity and quiet habitats (Novak et al. 2013) in the oligophotic zone (Brandano et al. 2010). The predominance of large foraminifers may indicate a highly oligotrophic paleoenvironment dominated by K-strategists (Bassi 2005).

MF 6: coral boundstone This facies is characterized by platy corals embedded in a muddy matrix. The platy colonies are mainly represented by poritid and faviid coral associations in growth position. The main genera are Porites sp., Favites sp., Turbellastraea sp., Thegioastrea sp., Pavona sp., Favia sp., as well as Hydnophora sp. No reworked and scattered biotas were present. The space between corals fauna is occupied by clay matrix (Figs. 2 and 4).

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Fig. 3 Photomicrographs of the 16 microfacies recognized within the Asmari Formation. a MF1: spiculate mudstone. b MF2: bioclast, pelagic foraminifera wackestone-packstone. c, d MF3: bioclast wackestone-packstone. e, f MF4: planktonic foraminifera, bioclast, lepidocyclinidae wackestone-packstone. SS sponge spicule, Mol mollusk, Ech echinoid, Bry Bryozoan, Rot small rotaliid. Spi Spiroclepeus, Oper Operculina, Eu Eulepidina, Amp Amphistegina, PF pelagic foraminifers

The dominance of corals colonies with platy morphologies is indicative of muddy substrate type (Novak et al. 2013). with turbid and low-lit conditions (Wilson and Lokier 2002). The adoption of platy forms by corals in such environment has been widely interpreted as a strategy to receive more incident light than vertical surfaces (Graus and Macintyre 1976;

Titlyanov and Latypov 1991; Rosen et al. 2002). The absence of reworked lithofacies in this zone indicates that production/ deposition occurred in a quiet environment (Morsilli et al. 2012). The high diversity of coral fauna may reflect oligotrophic conditions (Hallock and Glenn 1986) and relatively stable substrates in this setting (Lokier et al. 2009).


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Fig. 4 Photomicrographs of the 16 microfacies recognized within the Asmari Formation. a–d MF5: bioclast, nummulitidae, lepidocyclinidae wackestonepackstone. e MF6: coral boundstone. f MF7: bioclast, coral floatstone/rudstone. g, h MF8: bioclast, foraminifera grainstone. Mol mollusk, Ech echinoid, Cor coral, Bry Bryozoan, RA red algae, Spi Spiroclepeus, Oper Operculina, Eu Eulepidina, Amp Amphistegina, Het Heterostegina, Nep Nephrolepidina, Mio Miogypsinoides, Mil miliolid, PF pelagic foraminifers, BF benthic foraminifers

MF 7: bioclast coral floatstone/rudstone This facies consists of floatstone/rudstone with wackestone/ packstone matrix dominated by scattered encrusting corals. Rare coral colonies are found in growth position. Other skeletal components are represented by fragments of echinoid spine, bryozoans, coralline red algae, Textularia, and LBF with flat shapes such as Spiroclypeus, Eulepidina, and Nephrolepidina (Figs. 2 and 4). The foraminiferal assemblage consisting of Spiroclypeus, Eulepidina, and Nephrolepidina suggests that this facies was deposited in the lower part of the photic zone in middle ramp setting. The high abundance and diversity of benthic foraminifera and other bioclasts indicate stabilization of environmental conditions, most likely caused by a decrease of terrigenous input and increased light intensity (e.g., Novak et al. 2013). The micritic matrix throughout the facies indicates relatively low-energy conditions. The common coral debris may have derived

from erosion of pre-existing coral colonies by episodic currents and/or storms. MF 8: bioclast, foraminifera grainstone This facies comprise well-sorted, abraded, and highly fragmented bioclasts dominated by benthic foraminifera and small mollusk fragments. Other constituents include echinoids, corallinacean, and bryozoans. The taxonomic diversity of foraminifera assemblage is very high that is dominated by thick-shelled, robust, and round-shaped Neorotalia, Amphistegina, Miogypsinoides, and miliolids. Dendritina and Peneroplis are also present with abraded walls but are rare. Components are highly micritized (Fig. 4). The absence of mud is regarded as indicative of moderate to high bottom current conditions, an interpretation that is further supported by the abundance of thick walls and robust tests of the benthic foraminifera such as Rotalia, Amphistegina, and Miogypsinoides (Hallock

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and Glenn 1986; Fournier et al. 2004) as well as highly abraded and fragmented benthic foraminifera and other biogenic components. Based on the biotic components, good sorting and the grainstone texture, these highenergy marine deposits have been interpreted as subtidal offshore bars in a shallow marine environment.

MF 9: ooid grainstone This facies is dominated by high abundance of ooids with good sorting. In some samples, the foraminifera (Miogypsina, Rotalia, miliolids, Peneroplis, and Dendritina) are also present and most of them are nucleus of ooids (Fig. 5). The high abundance, good sorting, and overpacking of ooids suggest deposition in shallow, high-energy sand shoals (Flügel 2004). The presences of fossils within ooids indicate short-term stable conditions during the growth of ooids (Flügel 2004).

Fig. 5 Photomicrographs of the 16 microfacies recognized within the Asmari Formation. a, b MF9: ooid grainstone. c–e MF10: foraminifera, coral floatstone/ rudstone. f–h MF11: bioclast, corallinacean, larger foraminifera packstone–floatstone. Mol mollusk, Ech echinoid, Cor coral, Bry Bryozoan, RA red algae, Oper Operculina, Amp Amphistegina, Nep Nephrolepidina, Mio Miogypsinoides, Mil miliolid, Rot small rotaliids, Dis Discorbis, Den Dendritina


MF 10: foraminifera, coral floatstone/rudstone Skeletal components of this facies are abundant coral fragments in a wackestone to packstone–grainstone matrix; the corals are mainly represented by Porites. Some are encrusted by corallinaceans. The foraminiferal assemblage is dominated by LBF (Peneroplis, Dendritia, Austotrilina, and rarely Marginopora) and small benthic forms (miliolid, Elphidium, Textularia, and Discorbis). Other components are mollusks, echinoids, and green and red algae fragments (Fig. 5). The biotic association (coral, corallinacean, and larger foraminifera) and sedimentologic characteristics of this facies indicate deposition in a low to high hydrodynamic energy conditions and clear water, with good light penetration and normal marine salinity value. The presence of epiphytic foraminifera such as Peneroplis, Austrotrilina, miliolids, and discorbids suggested that these fauna originated in a seagrass-dominated environment (Brandano et al. 2009).


The common coral debris could also have been produced in situ from isolated coral colonies that are known to grow in seagrass environments (Brasier 1975). The grainstone texture and highly abraded and fragmented coral, benthic foraminifera, and other biogenic in some thin sections indicate that somewhat the sediments of this facies are reworked and transported. MF 11: bioclast, corallinacean, larger foraminifera packstone–floatstone Biogenic components are dominated by larger foraminifera and coralline red algae fragments. The larger foraminifera assemblage represented by the thick-walled genera Lepidocyclinidae, Miogypsinoides, Amphistegina, Archaias, Austrotrilina, and Miogypsina. Red algae mostly correspond to fragments of mastophoroids (Lithoporella) and lithophylloids (Lthophyllum and Titanoderma). Melobesioids (Lithothamnion and Mesophyllum) are rare. Corals, small benthic foraminifera, echinoids, mollusks, and bryozoans are present as subordinate components. Small benthic foraminifers were characterized by textulariids, miliolids, discorbids, Neorotalia, and rare encrusting forms Sphaerogypsina and Planorbulina (Fig. 5). Abundance of larger benthic foraminifera suggests shallow, well-illuminated, warm, oligotrophic waters with suitable substrate (Hottinger 1983; Murray 1991). The abundance of lithophylloideae and mastophoroids together with low proportions of melobesioids are also indicative of shallow water conditions (Braga and Aguirre 2001). The prevalence of epifaunal forms in the benthic assemblages indicates a vegetated, mobile, and well-oxygenated substratum in a shallow water environment (Bicchi et al. 2006). The mixture of large rotaliids along with the seagrass-associated bioclasts (porcellaneous forms and small rotaliids) indicates meso-euphotic conditions (Pomar et al. 2014). The local abundance of micrite indicated that deposition took place in a low-water-energy environment. MF 12: foraminifera, corallinacea, bryozoan packstone/floatstone This facies is dominated by bryozoans (Celleporaria sp.) and coralline red algae (L. melobesioides). Growth forms are mainly restricted to encrusting types. Other biogenic components are represented by benthonic foraminifera with low taxonomic diversity and echinoid fragments. Theses foraminifera are characterized by larger and small forms such as Miogypsina, Elphidium, and miliolids (Fig. 6). Celleporid thickets can be found at different depths and settings, but all are associated with mesotrophic conditions, moderate sedimentation rates, and relatively low energy (below swell-wave base) (Hageman et al. 2003). Miogypsina,

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Elphidium, and miliolids are known to be epiphytic organisms that are associated to shallow water, well-illuminated conditions, and living on the leaves of sea grasses (Murray 2006; Reuter et al. 2011; Brandano and Policicchio 2011). The occurrence of encrusting forms of corallinacean red algae (melebisoid) and Celleporaria bryozoan is also representing a seagrass facies (Hageman et al. 2003; Cigliano et al. 2006). Further evidence for seagrass can be given by high micrite content and poor grain sorting of this facies (Reuter et al. 2011; Pomar et al. 2014). This facies is therefore regarded as reflecting deposition within a shallow and well-illuminated inner-ramp environment colonized by seagrass meadows. The decreasing number of foraminiferal species and individuals is most probably related to the increasing nutrient level (mesotrophic regime). MF 13: bioclast wackestone-packstone This facies is dominated a low-diversity assemblage of fauna including bivalves (epifaunal forms: oysters) and small benthic foraminifers Ammonia. In some samples, the diversity is high and small benthic foraminifers and fragments of macrofossils also occur. Texture varies from wackestone to packstone (Fig. 6). This facies is surrounded by limestones containing larger imperforated foraminifera. Ammonia is a widespread euryhaline species that occurs in almost all marginal marine assemblages, reaching from brackish lagoons and estuaries to marine waters, as well as hypersaline environments (Murray 2006). The predominance of opportunistic species Ammonia and low-diversity benthic foraminifer fauna is generally typical for unstable shallow marginal marine environments (brackish) exposed to salinity fluctuations (Sen Gupta 1999; Murray 1991, 2006). Furthermore, seasonal fluctuations in food supply may result in foraminifera blooms of great abundance but low diversity. These opportunistic species must reach maturity quickly; hence, they are relatively small in size (Phleger 1960). In summary, smallsized and low-diversity benthic foraminifer fauna pointing to a marginal marine shallow water environment with unfavorable conditions; major food supply and salinity value fluctuations (short-time fluctuations) seem plausible. Conditions change with some samples, which yielded more diverse micro- and macrofossil assemblages. The identified benthic foraminiferal associations are dominated by rotaliide genera (Elphidium, Ammonia) and less abundant agglutinated and porcellaneous forms (Textularia, Quinqueloculina). Macrofossils are represented by fragments of gastropods, bivalve (pectinids and ostrea), bryozoans (Tubucellaria and Celleporaria), red algae, and echinoids. Mixohaline assemblage of Ammonia and Elphidium (Patacca et al. 2013). the presence of bryozoans, and relatively high values of diversity are indicative of a subtidal environment with more stable conditions and water salinities, close to

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Fig. 6 Photomicrographs of the 16 microfacies recognized within the Asmari Formation. a, b MF12: foraminifera, corallinacea, bryozoa wackestone-packstone. c, d MF13: bioclast wackestonepackstone e, f MF14: foraminifera wackestone-packstonegrainstone. g MF15: mudstone. h MF16: stromatolitic boundstone. Mol mollusk, Ech echinoid, Bry Bryozoan, RA red algae, Amm Ammonia, Mio Miogypsina, Mil miliolid, Pen Peneroplis, Bor Borelis, Dis Discorbis, Den Dendritina, Qg quartz grain

normal marine values (Filipescu et al. 2014). Normal marine salinity is also supported by the occurrence of echinoid fragments. Abundant heterotrophic organisms (bryozoans, mollusks, and echinoid) and replacement of miliolids and smaller rotaliines instead of larger foraminifera (soritids) indicate increased food supplies (Hirchfield et al. 1968; Hallock 1985). MF 14: foraminifera wackestone-packstone-grainstone This microfacies is characterized by a high diversity of imperforate foraminifera assemblage including Dendritina, Meandropsina, Spirolina, Borelis, Peneroplis, and miliolids. Subordinate components are mollusks shell fragments, Textularia, and Discorbis. Peloieds are also present. Textures range from wackestone-packstone to grainstone (Fig. 6). The dominance of imperforated foraminifers is interpreted to have been deposited in a shallow, high light, high-energy restricted environment, consistent with a seagrass meadow (Reuter et al. 2007; Bassi and Nebelsick 2010). The occurrence of a large number of porcelaneous imperforate foraminiferal tests may point to the depositional environment being

slightly hypersaline (Brandano et al. 2009). Episodes of low to moderate energy are suggested by the occurrence of wackestone and packstone. MF 15: mudstone This facies is represented by mud-supported texture. Rare scattered detrital silt-size quartz grains and shell fragments in some samples are also present. No subaerial exposure features in this facies were observed (Fig. 6). The dominant carbonate mud and paucity of fossils indicate deposition in restricted low-energy lagoon or embayment facies that formed near shore, locally under elevated salinities (e.g., Collins et al. 2006). MF 16: stromatolitic boundstone This facies consists of stromatolitic lamina. No fossils in this facies were observed (Fig. 6). Today, in regions with an arid climate (such as Persian Gulf), stromatolites form in the lower intertidal zone (Purser


1973; Shinn 1983). Laminated stromatolites develop in upper intertidal to lower supratidal settings (Palma et al. 2007). Taking the previous findings altogether, this facies represent a deposition in the tidal flat environment.

Depositional models Skeletal components indicate decreasing depth from the aphotic heterotrophic deposits to the oligophotic large foraminifers’ wackestone-packstone to euphotic seagrass deposits (Figs. 7 and 8). No gravity deposits and break on the slope angle were observed. This clearly indicates that the Asmari carbonate system was a homoclinal ramp in which small coral buildups were not able to build three-dimensional wave-resistant structures up to sea level. The terminology proposed by Burchette and Wright (1992) is used here to subdivide the ramp facies. Inner ramp: euphotic to mesophotic zone Inner-ramp environment spans from upper shore face to fairweather wave base and constantly agitated by wave agitations. Two facies zones have been recognized among these sediments: an inner and an outer shallow water zone: The inner shallow water zone is characterized by the occurrence of poorly fossiliferous mudstone and mollusks-porcellaneous foraminiferal grainstone to packstone (foramol association). The porcellaneous foraminifera were the best adapted fauna to the paleoenvironmental conditions such as low turbidity, high light intensity, and low substrate stability in meso- to oligotrophic settings, at shallow depth. The outer shallow water zone regroups a high-diversity assemblage of larger benthic foraminifera, coralline red algae, and small benthic foraminifera (small rotaliids, miliolids, and textularids) (LB-foralgal association). Mollusks, corals, echinoids, and bryozoans are common to abundant. Small rotaliids, Textulariidae, and heterotrophic organisms (bryozoans, echinoids, and mollusks) dominate the shallowest associations. The deeper associations are represented by larger benthic foraminifera and coralline red algae-rich wackestone-packstone. Compared to the shallowest association, small foraminifera, bryozoans, mollusks, and echinoid fragments are less common. A shoal belt of grainstones to rudstones occurs seawards of the shore deposits. In shoal deposits, skeletal grains usually display a high degree of fragmentation and abrasion. Coral debris, robust foraminifera, and corallinacean fragments were the predominant biotic components. Non-skeletal grains are also a common feature, especially tangential ooids (coralgal association). The occurrence of larger benthic foraminifera such as Elphidium, Operculina, Lepidocyclina, and corallinacean association indicates that deposition of inner-ramp environment

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took place in shallow waters of euphotic-mesophotic zone (Pomar 2001). Middle ramp: mesophotic to oligophotic zone The main biota producing carbonate sediment of the middle ramp are larger foraminifera and red algae (LB-foralgal associations), although patches of corals are present in some intervals. In these intervals, corals did not form wave-resistant buildups. Instead, they built, along with red algae and some LBF crusts, small patch reefs on the ramp slope. The absence of wave-resistant growth fabrics along with the associated skeletal components suggests the bathymetric position of the coral buildups to have been below wave base in the mesophotic zone (Pomar et al. 2014). In the lower photic zone, large and flat lepidocyclinid Eulepidina abundantly thrived along with nummulitids and Nephrolepidina. No wave-related sedimentary structures were observed in the middle ramp deposits. The absence of wave-related structures and the abundance of red algae and larger benthic foraminifers such as Heterostegina, Operculina, and Amphistegina place the middle ramp settings in the oligophotic zone, below the base of wave action (Pomar et al. 2004). Outer ramp: oligophotic to aphotic zone On the basis of the biota associations and lithological characteristics, the outer ramp can be subdivided in to a proximal, an intermediate, and distal outer ramp sector. The proximal outer ramp facies are represented by wackestone, packstone with larger, flatness foraminifera (Eulepidina), and pelagic forms. The intermediate outer ramp facies are constituted by bryozoan colonies and mollusks (bryomol association). This Heterozoan association does not require light; therefore, these biota are independent of water depth and are often associated with greater water depth and low water energy condition (Pomar 2001; Brandano and Corda 2002). The distal sector of the outer ramp is marked by marls and marly limestone alternations with echinoids, pelagic, small benthic foraminifers, and sponge spicules (echinofor association). The Heterozoan dominance and the absence of light-related organisms place this outer-ramp carbonate production in the aphotic zone, where reduced rates of carbonate supply from the ramp and some siliciclastic influence caused marly facies to be deposited (Pomar et al. 2012). The presence of small coral patch reefs in the lower and middle parts of section, the existence of LBF such as Heterostegina, Operculina, Amphistegina, Archaias, and Lepidocyclina in the larger foraminifera associations (Betzler et al. 1997). and the occurrence of non-skeletal grains and green algae (Pomar et al. 2004) all suggest that carbonate sedimentation took place in tropical waters and oligotrophic to slightly mesotrophic conditions.

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Fig. 7 Vertical facies distribution and sequences of the Asmari Formation at Dehdasht area, Zagros. The vertical distributions of these facies indicate a shallowing upward trend from the deep, aphotic, outer ramp to oligophotic, middle ramp, and into shallow water, euphotic, inner ramp


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Fig. 8 Reconstruction of the depositional environments for the platform carbonate of the Asmari Formation at Dehdasht area, Zagros Basin, SW Iran

Sequence stratigraphy Sequence stratigraphy is the study of the deposits of a basin placed between unconformity levels and/or their correlative conformity levels (Emery and Myers 1996). Sequence is the fundamental unit of sequence stratigraphy that is produced during an important cycle of rising and falling of relative sea level and includes the different facies bounds (Lasemi 2000). Litho- and biofacies analyses are acknowledged tools in sequence stratigraphy (e.g., Sarg 1988; Brett 1995) in this context. Functional morphology and lifestyle of foraminifers can be used to determine upward shallowing and deepening trends in sedimentary sequence (Geel 2000). The facies architecture of the Asmari deposits in the study section includes four longterm depositional sequence (Fig. 7). Sequence 1 The thickness of sequence 1 (Rupelin-Chattian (between 34 and 23.03 Ma)) is 118 m. Considering the facies and depositional geometries, this sequence can contain a transgressive system tract and a highstand system tract. The Transgressive Systems Tract (TST) predominantly consists of outer and middle ramp communities

(pelagic and benthic foraminifers) and shows a deepening upward trend. The mfs is evidenced by the super imposition of deeper facies (spiculate mudstone) on corallinacea, nummulitidae, piledocyclinidae wackestonepackstone facies. In the Highstand Systems Tract (HST), a gradual shift from deeper facies of open marine to shallower inner ramp facies can be observed. The HST consists mainly of lagoonal microfacies. The boundary between sequence 1 and 2 is a type 2 of sequence boundary (SB2) and characterized by mudstone. Sequence 2 This sequence with Aquitanian age (between 23.03 and 21.4 Ma) has about 70 m thickness. The lower part of TST consists of lagoonal facies that is dominated by imperforate foraminifers. The upper part of TST is characterized by a change from lagoonal facies to shoal facies that is dominantly represented by ooid facies. The mfs is marked by ooid grainstone which indicates a high-energy shoal environment. The HST is represented by imperforate foraminifers. The boundary between sequences 2 and 3 is a

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type 2 of sequence boundary (SB2) and characterized by stromatolitic boundstone. Sequence 3 This sequence with Upper Aquitanian age (between 21.4 and 20.4 Ma) is 46 m thick. The HST consists mainly of lagoonal microfacies and shows a shallowing upward trend. The basal part (TST) of sequence 3 is marked by imperforated foraminifera. The upper part of TST consists of shelf lagoon which is characterized by the simultaneous perforate and imperforate foraminifers. The mfs is marked by packstone–grainstone with numerous perforate and imperforate foraminifera together with ooids, which indicate a moderate to highenergy shoal environment. The HST of sequence 3 consists of limestones with numerous lagoonal imperforate and small foraminifers. Sequence 4 The thickness of sequence 4, Burdigalian in age (between 20.43 and 18.5 Ma), is 84 m. The TST of this sequence is characterized by inner ramp facies which contain imperforate foraminifera in the lower part and small hyaline foraminifera with mollusks in the upper part. The mfs is defined by foraminifera and corallinacea-bryozoan facies. Restricted lagoonal facies rich in larger imperforated foraminifera composed the HST. The upper boundary of this sequence was defined by evaporates of Gachsaran Formation that shown disconformity (SB1).

Comparison of sequence stratigraphy with other regions of Zagros Basin The sequence stratigraphy of Asmari Formation in various regions of Zagros Basin was proposed by many researchers. The current study compares the sequence stratigraphy of the Asmari Formation in the study area with Tang-e-Gurguda, Chaman Bolbol, and Dill sections that were studied by Amirshahkarami et al. 2007a, b and Allahkarampour Dill et al. 2010. The identified biota assemblages and microfacies trends in each study section represent deposition in an environment of a carbonate platform of homoclinal ramp type (Fig. 9). The identified sequences are compared below with the Tang-e-Gurguda, Chaman Bolbol, and Dill sections. Rupelian-Chattian: This sequence is recognized in study and Chamman Bolbol sections. In the Dill section, the Rupelian deposits are not exposed and therefore this sequence is only recognized in Chattian deposits. In Tang-e-Gurguda section, the Rupelian-Chattian deposits show a shallowing upward trend (HST), and mfs and TST of this sequence is characterized by pelagic facies that are placed in Pabdeh

Formation (lower boundary of Asmari Formation). In Chaman Bolbol and study sections, the TST is characterized by the occurrence of planktonic and larger hyaline foraminifers (open marine facies). The mfs in study section is marked by basinal facies (sponge spicule) and in Chamman Bolbol section, the mfs is defined by slope facies. The HST represents a shallowing upward trend from open marine to lagoonal facies. The TST in the Dill section is different from two previous sections and is composed of open-shelf lagoonal facies, rich in small benthic hyaline and porcellaneous foraminifers. The mfs is marked by open marine facies (larger hyaline foraminifers). The HST is defined by a shallowing upward trend from open marine facies to shoal facies and finally into lagoonal facies. The upper boundary of this sequence is marked by SB2 and is mainly placed in Aquitanian deposits. The lower boundary of this sequence in study and Chamman Bolbol sections is placed in Pabdeh Formation. Aquitanian: The Aquitanian sequence is recognized in the study, Dill, and Chaman Bolbol sections: in the study section, two sequence, and in Dill and Chaman Bolbol sections, only one sequence is recognized. In Dill and Chaman Bolbol sections, the lagoonal shelf facies composed the TST. The HST is mainly composed of restricted lagoonal facies rich in porcellaneous foraminifers. In the study section, in early sequence, the TST is composed of open-shelf lagoonal and shoal facies. The mfs is marked by shoal facies (ooid grainstone). The restricted lagoonal facies composed the HST. In the late sequence, the TST is composed from inner ramp facies. The mfs is marked by shoal facies. Restricted lagoonal facies composed the HST. The upper and lower boundaries of this sequence in all sections are marked by SB2. Burdigalian: This sequence is recognized in the study and Dill sections. In both sections, the inner ramp facies composed the TST. The mfs in both sections also is characterized by open lagoonal shelf facies. The HST of this sequence is marked by the occurrence of restricted lagoonal facies rich in porcellaneous foraminifers. In Tang-e-Gurguda and Chaman Bolbol sections, the Burdigalian deposits are composed of restricted and semi-restricted lagoonal facies with no evidence of deepening. They have been aggradational stacking pattern. In all sections, this sequence is overlain by Gachsaran Formation with a disconformity surface (SB1).

Summary The studied mixed carbonate–siliciclastic units are represented by biogenic components mainly of foraminifera, corallinacean, coral, bryozoan, and mollusks. Based on the foraminifera distribution the Asmari Formation is Oligocene-Lower Miocene in age. Sixteen facies were identified on the basis of the sedimentary features of the study sediments and the identified faunal assemblages. Based on


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Fig. 9 Comparison of sequence stratigraphy in studied section with some regions of Zagros Basin

the facies groups and the faunal constituents, the carbonate sediments of the Asmari Formation were deposited in a homoclinal ramp. Based on facies analysis and dependence of biota to light, the ramp is divided into three parts: an inner ramp, a middle ramp, and an outer ramp. The inner ramp is characterized by wackestone–-packstone and grainstone with diverse assemblage of imperforated foraminifera in shallow protected areas, whereas the deepest parts

of the inner ramp are dominated by wackestones-packstones with benthic foraminifera and coralline red algae, minor mollusks, bryozoans, echinoids, and small epiphytic biota. The shoal facies is marked by robust and thick-wall foraminifera and corals in packstone–grainstone texture as well as ooid grainstone. The shallower parts of middle ramp are characterized by the occurrence of platy corals embedded in clay matrix. Corallinacean and hyaline benthic foraminifera

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(nummulitidae and lepidocyclinidae ) with thin and elongated tests are predominant components in deeper parts of middle ramp. The proximal outer ramp was dominated by thin and discoidal tests of lepidocyclinidae (Eulepidina). The middle parts of outer ramp are characterized by bryozoan, mollusks, and echinoids. Pelagic foraminifera and sponge spicules are the most important components of distal outer ramp to basin facies. Detailed facies analysis revealed that the Asmari Formation consists of four depositional sequences. Sequence 1 is composed of a TST and a shallowing upward HST. The TST predominantly consists of outer and middle ramp communities (pelagic and benthic foraminifers), and the HST consists mainly of lagoonal microfacies. Sequence 2 is composed of transgressive and highstand systems tracts. The lower part of TST consists of lagoonal facies that is dominated by imperforate foraminifers. The upper part of TST is characterized by a change from lagoonal facies to shoal facies that is dominantly represented by ooid facies. The HST is represented by imperforate foraminifers. Sequence 3 is characterized by aggradational lagoonal facies. The basal part (TST) of sequence 3 is marked by imperforated foraminifera. The upper part of TST consists of shelf lagoon which is characterized by the simultaneous perforate and imperforate foraminifers. The boundaries between sequence 1 and sequence 2 as well as sequence 2 and sequence 3 are characterized by mudstone (SB1) and stromatolitic boundstone (SB2), respectively. In sequence 4, the TST is characterized by inner ramp facies which contain imperforate foraminifera in the lower part and small hyaline foraminifera with mollusks in the upper part. The mfs is defined by foraminifera, corallinacea-bryozoan facies. Restricted lagoonal facies rich in larger imperforated foraminifera composed the HST. Acknowledgments This paper is part of a M.Sc. study by A. Roozpeykar at Isfahan University. I am thankful to Prof. Marco Brandano from di Roma “La Sapienza” University for his valuable suggestions and Kazem Sherafati for his help during the field work.

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