Introduction

The Asmari Formation in southwest Iran comprises approximately 400 m of cyclic platform limestones and dolostone with subordinate intervals of sandstone and shale. It contains most of Iran’s recoverable oil reservoirs, which are trapped mainly in large anticlines in the Zagros Mountain chain (Fig. 1a) (Hull and Warman 1970; Mcquillan 1973, 1974, 1985).

Fig. 1
figure 1

Location and geological map of the study area. a General map of Iran showing eight geologic provinces. The Shahneshin, Sefidar, and Jahrum Anticlines are located in the folded Zagros, adapted from Lacombe et al. (2006) and Mobasher and Babaie (2008). b Subdivisions of the Zagros Mountains and Fars Sub-basin, after Motiei (1993), with situation of the study sections in Fars province

Full size image

At the type section in Tang-e Gel-e Tursh (Valley of Sour Earth) on the southwestern flank of the Kuh-e Asmari anticline, the Asmari Formation consists of 314 m of mainly limestones, dolomitic limestones, and argillaceous limestones (Motiei 1993). The shallow-marine limestones of the Asmari Formation were deposited over the Pabdeh Formation in the southwestern part of the Zagros Basin (Fig. 2), and also covered the Jahrum and Shahbazan Formations to the northeastern part in the Fars and Lurestan regions (Fig. 2), respectively. The Asmari Formation in the Tang-e Abolhayat section is 328 m thick and conformably overlies the Pabdeh Formation with a transitional contact. The contact with the overlying Gachsaran Formation (i.e., evaporates rocks) is conformable and gradual (Fig. 4). In the 407-m-thick Tang-e Zanjiran and 157-m-thick Tang-e Ab sections, the lower contact of this formation is sharp and unconformable with underlying formation (Jahrum Formation) and upper contact is conformable and gradual with overlying formation (Razak Formation) (Figs. 5, 6). Primary works concerning the Asmari Formation are attributed to Busk and Mayo (1918), Richardson (1924), Boeckh et al. (1929), and Thomas (1948), where the Asmari Formation was originally defined. Later, James and Wynd (1965), Wynd (1965), Adams and Bourgeois (1967), Kalantari (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; Laursen et al. 2009; Sadeghi et al. 2009), microfacies and depositional environments (Seyrafian and Hamedani 1998, 2003; Seyrafian 2000; Rahmani et al. 2009) and depositional environment and sequence stratigraphy (Vaziri-Moghaddam et al. 2006; Amirshahkarami et al. 2007a, b; Ehrenberg et al. 2007; Van Buchem et al. 2010; Dill et al. 2010; Sadeghi 2010). The main aim of this paper is to describe and interpret the different microfacies using both field and petrographic observations. Moreover, recognition of the depositional environment of the Asmari Formation is another objective of this research work.

Fig. 2
figure 2

Correlation chart of the Tertiary of southwest Iran (adopted from Ala 1982)

Full size image

Geographic and geological setting

The study is based on three outcrop sections (Tang-e Abolhayat, Tang-e Zanjiran, and Tang-e Ab) (Fig. 3) from the Asmari Formation in the Zagros fold belt (Fig. 1a). Deposition of the Asmari Formation started in the Early Rupelian in a NW–SE-trending basin generally and was follow in mid-Burdigalian time by the deposition of evaporates and marls of the Gachsaran Formation (Motiei 1993). The study area is located in the southeastern part of the Zagros Basin (Figs. 1b, 2). The Zagros Mountain Belt of Iran is part of the Alpine-Himalayan system and extends from the NW Iranian border through to SE Iran and up to the Strait of Hormuz (Heydari 2008) (Fig. 1a). The Zagros fold-and-thrust belt can be divided into a number of zones (Lurestan, Izeh, Dezful Embayment, Fars, High Zagros), which differ according to their structural style and sedimentary history (Berberian and King 1981; Falcon 1974; Motiei 1993; Stocklin 1968). The study area is located in the Fars province (sub-zones of Coastal/Sub-coastal, Sub-interior/Interior and Interior Fars) (Fig. 1b). The Tang-e Abolhayat section (Shahneshin Anticline) is located about 98 km west of Shiraz (Coastal/Sub-coastal Fars Sub-basin) and 55 km northeast of Kazerun city (Figs. 1b, 3). The section was measured in detail at 29°42′17″N and 51°47′00″E. The Tang-e Zanjiran section (Sefidar Anticline) is located about 75 km south-southeast of Shiraz (Sub-interior/Interior Fars Sub-basin) and 40 km north-northeast of Firuzabad city (Figs. 1b, 3). The section was measured in detail at 29°04′16″N and 52°39′02″E. The Tang-e Ab section (Jahrum Anticline) is located about 202 km southeast of Shiraz (Interior Fars Sub-basin) and 30 km east of Jahrum city (Figs. 1b, 3). The section was measured in detail at 28°26′15″N and 53°45′17″E.

Fig. 3
figure 3

Location of the study area in Fars province, southwest Iran

Full size image

Methods

Each section was measured bed by bed. Samples were analyzed in approximately 581 thin-sections. All thin-sections were analyzed under the microscope for biostratigraphy and facies. The classification of carbonate rocks followed the nomenclature of Dunham (1962) and Embry and Klovan (1971). Facies definition is based on microfacies characters including: depositional texture, grain size, grain composition, and fossil content.

Biostratigraphy

Laursen et al. (2009) and Van Buchem et al. (2010) have established a new biozonation for the Asmari Formation based on strontium isotope stratigraphy (Table 1).

Table 1 Biozonation of the Asmari Formation (Laursen et al. 2009 and Van Buchem et al. 2010)
Full size table

Two assemblages of foraminifera recognized in the studied areas and were discussed in ascending stratigraphic order as follows:

  1. 1.

    The most important foraminifera in this assemblage are: Nummulites vascus-incrassatus group, Nummulites fichteli-intermedius group, Eulepidina dilatata, Eulepidina elephantina, Eulepidina sp., Nephrolepidina tournoueri, Nephrolepidina morgani, Nephrolepidina sp., Lepidocyclina sp., Heterostegina assilinoides, Heterostegina praecursor, Heterostegina costata, Spiroclypeus cf. ranjanae, Amphistegina bohdanowiczi/lessoni, Miogypsinoides sp., Operculina complanata, Neorotalia viennoti, and Sphaerogypsina globulosa. This assemblage is correlated with Nummulites vascus-Nummulites fichteli Assemblage zone of Laursen et al. (2009) and Van Buchem et al. (2010) (Table 1) and is attributed to the Rupelian time.

  2. 2.

    The most diagnostic species in studied sections include: Archaias hensoni, Archaias asmaricus, Archaias kirkukensis, Archaias operculiniformis, Miogypsinoides complanatus, Miogypsinoides formosensis, Miogypsinoides dehaarti, Borelis melo, Borelis pygmaea, Borelis haueri, Peneroplis evolutus, Peneroplis thomasi, Austrotrillina asmariensis, Austrotrillina howchini, Austrotrillina striata, Dendritina rangi, Praerhapydionina delicata, and miliolids. This assemblage corresponds to the Archaias asmaricus-Archaias hensoni-Miogypsinoides complanatus assemblage zone of Laursen et al. (2009) and Van Buchem et al. (2010) (Table 1). The assemblage is considered to be Chattian in age.

Microfacies analysis

Facies analysis of the Asmari Formation in study areas has resulted in the definition of 12 microfacies types (Figs. 4, 5, 6).

Fig. 4
figure 4

Microfacies changes of the Asmari Formation at the Tang-e Abolhayat section

Full size image
Fig. 5
figure 5

Microfacies changes of the Asmari Formation at the Tang-e Zanjiran section

Full size image
Fig. 6
figure 6

Microfacies changes of the Asmari Formation at the Tang-e Ab section

Full size image

MF 1, bioclastic planktonic foraminifera wackestone–packstone

This facies is dominated by planktonic foraminifera, represented by globigerinids and globorotalids in a muddy matrix. Less common skeletal constituents include bioclasts deriving from bryozoans, echinoids, and bivalves shells. Depositional textures are represented by wackestone-packstone (Fig. 7a).

Fig. 7
figure 7

a MF 1: Bioclastic planktonic foraminifera packstone. b MF 2: nummulitid lepidocyclinid Neorotalia bioclast wackestone–packstone. c MF 2: Bioclastic nummulitid lepidocyclinid packstone. d MF 3: Bioclastic nummulitid corallinacean coral rudstone. e MF 4: Bioclastic nummulitid wackestone–packstone

Full size image

Interpretation

The general lack of sedimentary structures, the fine-grained matrix, and the presence of whole fossils of planktonic foraminifera suggest that this facies was deposited in calm and deep, normal-salinity seawater below the stormwave base with sporadic contribution of skeletal debris of benthic fauna (Wilson 1975; Flügel 2004). A similar microfacies, present in the Pabdeh Formation at Chaman-Bolbol area, has been interpreted by Amirshahkarami et al. (2007a) as outer slope deposits.

MF 2, bioclastic nummulitid lepidocyclinid wackestone–packstone

This facies is represented by wackestone-packstone with large and flat (0.8–3 cm) perforate benthic foraminifera (Nummulitidae and Lepidocyclinidae) (Fig. 7b). The foraminifera are characterized by a relatively diverse assemblage of nummulitids (Operculina, Hetorestegina, and Spiroclypeous) and lepidocyclinids (Eulepidina and Nephrolepidina). This facies is most prominent in the lower parts of the Asmari Formation. Grains are coarse sand to granule size and are in a finer-grained carbonate matrix. Other bioclasts include echinoderms, bivalves, gastropods, red algae (Lithothamnium and Lithophyllum), bryozoans, and small benthic foraminifera (Fig. 7c). Occasionally, Neorotalia occur as major constituents.

Interpretation

The combination of micritic matrix and abundance of typical open-marine skeletal fauna including bryozoans, echinoids, flat and large Nummulitidae, and Lepidocyclinidae suggest a low–medium energy, open-marine environment, and between the stormwave base and fair-weather wave base for deposition of this microfacies (Wilson 1975; Flügel 2004). The presence of large and flat nummulitids and lepidocyclinids allowed us to interpret this facies as having been deposited in the lower photic/oligophotic zone (Geel 2000; Pomar 2001a; Romero et al. 2002; Nebelsick et al. 2005; Renema 2006; Bassi et al. 2007; Barattolo et al. 2007).

MF 3, bioclastic nummulitid corallinacean coral floatstone–rudstone

This facies consist of floatstone–rudstone with wackestone–packstone–grainstone matrix dominated by Nummulitidae, coralline algae and corals. Coralline algae occur as Lithothamnium and Lithophyllum. Nummulitidae are represented by Nummulites, Heterostegina, Operculina, and Spiroclypeus. Additional components are fragments of echinoids, bryozoans, oysters, Kuphus, gastropods, and lepidocyclinids. Amphistegina, Bigenerina, Lenticulina, valvulinids, textularids, planktonic foraminifera, and intraclasts are rarely observed (Fig. 7d).

Interpretation

The diverse fauna suggests that deposition occurred in a marine environment of normal salinity. Abundant open-marine skeletal fauna reflect well-lit water and oxygen contents within the water column and at the sediment surface. The presence of corallinacean and larger foraminifera suggest a middle ramp position and indicate oligotrophic conditions (Pomar 2001a, b; Brandano and Corda 2002; Mutti and Hallock 2003; Pomar et al. 2004; Brandano et al. 2009b).

MF 4, bioclastic nummulitid wackestone–packstone

This facies is characterized by coarse-grained wackestone–packstone dominated by large benthic foraminifera. The larger foraminifera consist of small lens-shaped Nummulites sp. and Operculina sp. Fragmentation of larger foraminifera is common. Other bioclasts include echinoderms, bivalves, gastropods, and bryozoans (Fig. 7e).

Interpretation

The presence of stenohaline fauna such as perforate foraminifera and echinoids, stratigraphic position below open-marine facies and the moderate sorted components in this facies suggest deposition in shallow open-marine environment. The grainy texture and the fragmented fauna suggest a relatively high-energy environment, probably near fair-weather wave base (Flügel 2004; Bassi et al. 2007; Rahmani et al. 2009).

MF 5, Neorotalia corallinacean bioclast wackestone–packstone–grainstone

This facies are composed of skeletal grains in variable quantities; they are represented by benthic foraminifera and red algae. Benthic foraminifera consist of common small lens-shaped and robust Neorotalia, Amphistegina, and Asterigerina. Associated with foraminifera and calcareous red algae are fragments of corals, bryozoans, echinoids, bivalves, and gastropods (Fig. 8a).

Fig. 8
figure 8

a MF 5: Neorotalia corallinacean bioclast packstone–grainstone. b MF 6: Corallinacean coral boundstone. c MF 7: Corallinacean echinoid packstone–grainstone. d MF 8: Nummulitid miliolid peloidal bioclast packstone–grainstone. e MF 8: Imperforate foraminifera bioclastic nummulitid packstone. f MF 9: Coral corallinacean miliolid bioclast rudstone

Full size image

Interpretation

Abundance of red algae and larger foraminifera such as Neorotalia and Amphistegina indicate that the sedimentary environment was situated in the mesophotic to oligophotic zone in shallow open-marine environment or near and below fair-water wave base on the proximal middle ramp (Geel 2000; Pomar 2001a, b; Brandano and Corda 2002; Corda and Brandano 2003; Cosovic et al. 2004). Test morphology of larger foraminifera also suggests deposition in shallow-water open-marine environments. Tests of Neorotalia sp. are robust and ovate. The sediments with robust and lens specimens are reflecting shallower water than those containing larger and flat nummulitids and lepidocyclinids (Beavington-Penney and Racey 2004; Barattolo et al. 2007).

MF 6, corallinacean coral boundstone

This facies is characterized by the abundance of scleractinian coral colonies that are mostly in growth position. Some corals are coated by crustose coralline algae. The crustose coralline algae are important as both sediment and framework-building organisms in this microfacies (Fig 8b).

Interpretation

This microfacies is interpreted to be formed by in situ organisms as an organic reef (Bioherm) in margin of platform and was located above the fair-weather wave base (FWWB) (Wilson 1975).

MF 7, corallinacean echinoid packstone–grainstone

This facies consists of medium to thick bedded packstone–grainstone. The principal biogenic components are represented by echinoids, corallinaceans, bryozoans, and bivalves. The groundmass is mostly dominated by sparite. The grains are fine- to coarse-sand size. The grains are well sorted (Fig. 8c).

Interpretation

This facies is interpreted to have been deposited under shallow-water, high-to moderate energy and above the fair-weather wave base condition based on grainy texture, the fragmented fauna and well-sorted components. The presence of rare of micrite also suggests winnowing of the sediments and removing of the micritic matrix. The sediments would have been deposited in sand shoals (Wilson 1975; Flügel 2004).

MF 8, nummulitid miliolid peloidal bioclast wackestone–packstone–grainstone

This microfacies consist of thin to medium bedded wackestone–packstone to grainstone. The biota consist of abundant and diversified benthic foraminifera (Nummulites, Operculina, Heterostegina, lens-shaped Lepidocyclina, Neorotalia, Amphistegina, Asterigerina, Ammonia, Sphaerogypsina, Miogypsinoides, Discorbis, Elphidium, Austrotrillina, Archaias, Peneroplis, Borelis, Dendritina, Spirolina, Praerhapydionina, Planorbulina, miliolids, Textularia, Bigenerina, Pseudolituonella, valvulinids). Macrofauna in this microfacies is mainly represented by fragments of corals, echinoids, bryozoans, bivalves, and gastropods. Additional biogenic components are red algae and dasycladaceans. Common peloids and rare intraclasts are also present (Fig. 8d). Occasionally, imperforate foraminifera occur as major constituents (Fig. 8e).

Interpretation

The faunal composition and stratigraphic position above the lagoonal facies indicate that sedimentation took place in a shelf lagoon with normal circulation and well-oxygenated waters (Romero et al. 2002). The presence of large porcelaneous forms associated in variable proportions with lenticular/ovate hyaline forms (small Nummulites and Lepidocyclina) and dasycladaceans also suggests deposition in the euphotic zone in an open-lagoonal environment (Pomar 2001b; Romero et al. 2002; Renema 2006). A similar foraminiferal assemblage has been interpreted by Vaziri-Moghaddam et al. (2006) as deposited in an open-lagoonal environment.

MF 9, coral corallinacean miliolid bioclast floatstone–rudstone

This facies consists of floatstone-rudstone with wackestone–packstone to grainstone matrix. The main biotic components are coral, corallinaceans, and miliolids. Fragments of green algae, bryozoans, echinoids, bivalves, and gastropods are also present. The grains are poorly sorted. They are medium to coarse sand to granule size (Fig. 8f).

Interpretation

The high biota diversity and stratigraphic position of microfacies 8 and 9 show that the primary environment had a good water circulation and the sedimentation took place in an open-lagoon environment adjacent to the platform margin. Vaziri-Moghaddam et al. (2006), Corda and Brandano (2003), Nebelsick et al. (2001), and Rasser and Nebelsick (2003) considered the similar facies are representative of a shelf lagoon.

MF 10, imperforate foraminifera bioclast peloids packstone–grainstone

The main characteristic of this microfacies is the maximum diversification of imperforate foraminifera in grain-supported textures. Several genera of imperforate foraminifera (Austrotrillina, Archaias, Peneroplis, Borelis, Meandropsina, Dendritina, Coscinospira, miliolids, Praerhapydionina, Planorbulina, Spirolina, Pseudolituonella, Textularia, valvulinids, Bigenerina) have been recognized. Pellet, peloid, intraclast and fragments of bivalves, echinoids, bryozoans, gastropods, corals, and red and green algae are also present. In some samples, charophytes occur in small amounts. The grains are poorly sorted to medium sorted (Fig. 9a). Due to changes in the type of allochems on some samples, the name of this facies changes to intraclast bioclastic imperforate foraminifera wackestone–packstone–grainstone (Fig. 9b).

Fig. 9
figure 9

a MF 10: Imperforate foraminifera bioclast peloid packstone. b MF 10: intraclast bioclastic imperforate foraminifera packstone. c, d MF 11: Peloid miliolid bioclast packstone–grainstone. e MF 11: Peloid packstone. f MF 12: Mudstone

Full size image

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 occurrence of a large number of porcelaneous imperforate foraminiferal tests may point to the depositional environment being slightly hypersaline. Such an assemblage is described as being associated with a shelf lagoon environment (Wilson 1975; Flügel 2004; Vaziri-Moghaddam et al. 2006; Brandano et al. 2009a). Furthermore, MF 10 could have originated in sea grass-dominated environments due to the presence of epiphytic foraminifera such as Borelis, Archaias and Peneroplis. The local brackish-water conditions are suggested by the local presence of charophyte remains in some samples.

MF 11, peloidal miliolid bioclast wackestone–packstone–grainstone

Common skeletal components in this microfacies are fragments of bivalves, gastropods, echinoids, bryozoans, dasycladaceans, corallinaceans, and corals. Miliolids and peloids are also abundant. Rare intraclasts are also present. The grains are poorly sorted to medium sorted. They are fine to medium size and vary from sub-angular to round. Textures are dominantly packstone but range from wackestone to grainstone (Fig. 9c, d). In some samples, benthic foraminifers and bioclasts are rare to absent (Fig. 9e).

Interpretation

The depositional environment of this microfacies is interpreted as the restricted shallow subtidal environments. This interpretation is supported by the low diversity and abundance of imperforate foraminifera (Geel 2000; Romero et al. 2002; Schulze et al. 2005). The scarcity or absence of benthic foraminifers, bioclasts, and dominance of peloids (Fig. 9e) indicate deposition in low-energy, restricted shallow lagoonal setting with poor connection with the open-marine environment (Tomasovych 2004).

MF 12, bioclastic mudstone (mudstone)

This facies consists of fine-grained microcrystalline limestone. This is poor in skeletal fragments and non-skeletal grains. In some samples very rare bioclasts and pellets are observed (Fig. 9f).

Interpretation

A high percentage of carbonate mud, the rare faunal elements, and stratigraphic position below suggest that deposition occurred in a lagoonal-peritidal environment. The low diversity of fauna indicates unfavorable life conditions for many benthic organisms and a fluctuating salinity can be assumed.

Sedimentary model

The analyzed sections represent the development of a carbonate ramp during the Rupelian-Chattian. The facies model presented here shows a depth gradient from the inner ramp to the outer ramp with distribution of foraminifera and other important components (Fig. 10). Intertidal deposits have not been observed. The absence of a shelf break is corroborated by the lack of re-sedimented lowstand deposits. Inner ramp deposits represent a wider spectrum of marginal marine deposits, indicative of a high-energy reef (MF 6), shoal (MF 7), open lagoon (MF 8 and 9), and protected lagoon (MF 10–12). In the restricted lagoon environment (MF 10–12), faunal diversity is low, and normal marine fauna are lacking, except for imperforate benthic foraminifera (Archaias, Peneroplis, Austrotrillina, miliolids, Dendritina, borelisids), which indicate quite sheltered conditions. This assemblage is regarded as well adapted to the paleoenvironmental conditions such as low turbidity, high light intensity, and low-substrate stability (Fig. 10). The low turbidity is ascribed to the high diversity of the porcelaneous foraminiferal fauna, which develops in meso-to-oligotrophic settings at shallow depth (Hallock 1984, 1988; Reiss and Hottinger 1984; Buxton and Pedley 1989; Romero et al. 2002; Barattolo et al. 2007). Today, porcelaneous larger foraminifera thrive in tropical carbonate platforms within the upper part of the photic zone (Leutenegger 1984; Reiss and Hottinger 1984; Hohenegger et al. 2000; Romero et al. 2002). Some biogenic components characterize stress conditions within restricted environments. Miliolid-dominated benthic foraminifer assemblages reflect decreased circulation and probably reduced oxygen contents or euryhaline conditions (Geel 2000). In contrast, a well-lit and oxygenated shallow open subtidal setting is characterized by mixed open-marine (such as echinoids and perforate foraminifera) and protected environment fauna (such as miliolids, Borelis and Austrotrillina) (Geel 2000; Romero et al. 2002; Corda and Brandano 2003; Vaziri-Moghaddam et al. 2006; Barattolo et al. 2007). Certainly, a shallow-marine ecosystem with normal circulation and well-oxygenated waters is a very suitable condition for a maximum diversification of benthic fauna. Washed-out micritic matrix is grainstones, which is replaced by sparitic cement implies increased wave or current energy. These conditions are characteristic for high-energetic environments, such as sand bars or shoals in the shallow subtidal environment. The shallow subtidal environment above the fair-weather wave base is characterized by the presence of a facies association showing signs of long-term water agitation (packing, well sorting, and poor taphonomic preservation) (Fig. 10). The platform margin is represented by coral boundstone. The main site of reef carbonate production was located above the fair-weather wave base (Wilson 1975) (Fig. 10). The middle ramp setting is represented by the medium to fine-grained foraminiferal–bioclastic wacke-packstone dominated by assemblages of larger foraminifera with perforate walls such as Amphistegina, Heterostegina, Operculina, and Nummulites (Fig. 10). The faunal association suggests that the depositional environment was situated in the mesophotic to oligophotic zone (Hottinger 1997; Pomar 2001b). Larger benthic foraminifera of the genera Heterostegina and Amphistegina (MF 3 and 5) are of particular ecological importance (Brandano and Corda 2002). These live in tropical to subtropical environments over a wide bathymetric range, but are particularly frequent between depths of 40 and 70 m (Hottinger 1983, 1997). Moreover, the red algae association with these larger foraminifera places the middle ramp in an oligophotic (Brandano et al. 2009a, b; Corda and Brandano 2003; Brandano and Corda 2002) to mesophotic zone (Hottinger 1997; Pomar 2001a, b). The lower photic zone is dominated by large, flat, and perforated foraminifera (such as lepidocyclinids and nummulitids) associated with symbiont-bearing diatoms (Leutenegger 1984; Renema and Troelstra 2001; Romero et al. 2002) (Fig. 10). Nutrient influx promoted the growth of coralline algae and bryozoans of MF 3 in a mesotrophic to mildly eutrophic environment, whereas the dominance of large benthic foraminifera suggests oligotrophic conditions were prevailing in the MF 2. Outer ramp facies are characterized by marl and marly limestone lithologies (Fig. 10). Wackestones predominate with abundant planktonic foraminifera in the MF 1. The presence of mud-supported textures and the apparent absence of wave and current structures suggest a low-energy environment below the stormwave base (Wilson 1975; Burchette and Wright 1992).

Fig. 10
figure 10

Depositional model of the Oligocene Asmari Formation in study areas (Fars sub-basin)

Full size image

Sedimentary development of the Oligocene Fars sub-basin

During the Paleocene and Eocene, the Pabdeh (basinal marls and argillaceous limestones) and the Jahrum (massive shallow-marine carbonates) formations were deposited in the middle and on both sides of the Zagros basinal axis, respectively (Motiei 1993) (Fig. 2). The shallow-marine limestones of the Asmari Formation were deposited above the Pabdeh Formation in the Abolhayat section (Fig. 4), and also covered the Pabdeh and Jahrum Formations in the Tang-e Zanjiran section (Fig. 5) and Jahrum Formation in the Tang-e Ab section (Fig. 6), respectively. During the Rupelian, outer ramp facies (Pabdeh Formation) (Fig. 10) was predominant at the Tang-e Abolhayat section (Kazerun area) (Fig. 4). At the same time, sedimentation at the Tang-e Zanjiran section (Firuzabad area) took placed in the middle and inner ramp environments (Fig. 5). The Tang-e Ab section (Jahrum area) represents the restricted environment, compared to the Tang-e Abolhayat and Tang-e Zanjiran sections (Fig. 6). This is visible in the lower part of the Asmari Formation where deep-marine environment characterizes the formation in Kazerun area (Tang-e Abolhayat), becoming less common in Tang-e Zanjiran and completely absent in the Tang-e Ab section (Figs. 4, 5, 6). On the other hand, deeper-water environments, which are characterized by planktonic and large and flat perforate foraminifera in Tang-e Abolhayat section (Fig. 4), have no equivalent in the Tang-e Zanjiran and Tang-e Ab sections (Figs. 5, 6). Restricted shallow subtidal environments are observed during Chattian times exclusively in the all study areas (Figs. 4, 5, 6). The restriction is indicated by an assemblage of abundant imperforate benthic foraminifera (Fig. 10). The Rupelian sediments of the Asmari Formation in Abolhayat section overlie gradationally the Pabdeh Formation (Fig. 4). As a result, the Abolhayat section was situated in the basinal position in Oligocene (Figs. 1b, 2, 3). Oligocene carbonates deposited around the edge of this basin are assigned to the Asmari Formation, whereas deep-water, basinal facies of the Pabdeh Formation continued to accumulate in the basin centre (James and Wynd 1965; Motiei 1993). With progressive infilling of this basin, the Asmari Formation prograded over the Pabdeh Formation such that the Asmari–Pabdeh contacts is diachronous, becoming younger basin ward (Thomas 1950).

Conclusions

The Asmari Formation was deposited on a tropical to subtropical shallow carbonate ramp in inner, middle, and outer ramp settings. The open-marine environment was separated from the shelf lagoon by a platform margin. The gradual change in fauna and the co-occurrence of normal marine and lagoonal fauna suggest that there was no effective barrier. In the inner ramp, the most abundant microfacies are wackestone–packstone with imperforate and perforate benthic foraminifera (such as miliolids, Austrotrillina, Archaias, nummulitids, and Neorotalia) and corallinaceans, dasycladaceans, echinoid, bivalve, gastropod, and bryozoan fragments. The shoal facies is marked by corallinacean echinoid packstone–grainstone. The proximal middle ramp is dominated by wackestone–packstone–grainstone with robust and ovate tests of Nummulites, Amphistegina, Neorotalia, and corralinacean algae, while wackestone–packstone with large and flat nummulitids and lepidocyclinids present in the distal middle ramp. Outer-ramp facies are characterized by marl and marly limestone lithologies with abundant planktonic foraminifera.