Introduction

The Sachun Formation has an extensive distribution in the south of the Fars province in the Zagros Basin and consists mainly of thick successions of evaporite, red marl, thin-bedded dolomite and limestone (Fig. 1). The stratigraphic column of phanerozoic sediment in the Zagros Basin contains evaporites at different stratigraphic levels in different parts. In Iran, evaporites are mainly present in three major horizons including: (1) Precambrian Hormoz series, (2) Upper Jurassic and (3) Cenozoic deposits. In many cases in southern Iran and the Persian Gulf region, these evaporites play an important role for hydrocarbon entrapment and reservoir seals (Bahroudi and Koyi 2003).

Fig. 1
figure 1

Cenozoic stratigraphic correlation chart of the Iranian sector of the Zagros Basin, adapted from James and Wynd (1965)

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A few previous studies have focused on biostratigraphy and lithostratigraphy (James and Wynd 1965; Wynd 1965; Amiri-Bakhtiar 2007) of the Sachun Formation and there are no detailed investigations on sedimentological and sequence stratigraphy. In the current paper, a detailed facies analysis, paleoenvironmental interpretation, depositional models and sequence stratigraphic approach are used to understand the relationship between paleoenvironmental parameters and change of sedimentary facies. Considering evaporites within a sequence stratigraphic framework allows stratigraphers to identify critical evaporite–carbonate facies transitions in time and space, and therefore to delineate a more complete basin history. These results could provide important information about the sedimentary setting of the Zagros Basin area.

Geological setting

The Iranian plateau extends over a number of continental fragments welded together along suture zones of oceanic character (Alavi 1994; Heydari 2008). The fragments are delineated by major boundary faults, which appear to be inherited from older geological periods. Each fragment differs in its depositional sequence, nature and age of magmatism and metamorphism, its structural character and intensity of deformation (Berberian and King 1981). These fragments formed the Zagros, Sanandaj-Sirjan, Urumieh-Dokhtar, Central Iran, Alborz, Kopet Dagh, Lut and Makran provinces (Alavi 1994; Fig. 2a).

Fig. 2
figure 2

Location map of the studied area. a General map of Iran showing eight geologic provinces. The study area is located in the Zagros province (modified from Heydari 2008). b Main structural subdivisions of the Zagros fold and thrust belt (modified from Farzipour-Saein et al. 2009). c Location and geological map of the type locality of the Sachun Formation, SE Shiraz, Iran

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The Zagros Basin of SW Iran is one of the most prolific oil-producing basins in the Middle East (Murris 1980). The Zagros fold-and-thrust belt (ZFTB) is a NW–SE trending orogenic belt, which extends over about 2,000 km from Turkey through southwest Iran down to the Strait of Hormuz. It is bound to the north by the Main Zagros Fault interpreted as the suture zone of the Neo-Tethys ocean (Sherkati et al. 2006; Fig. 2b).

The ZFTB 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 (Falcon 1974; Berberian and King 1981). The study area is located in the Fars province (interior Fars, Fig. 2b). This basin developed during the Paleogene on the eastern continental margin of the Tethys and is a typical Cenozoic carbonate platform on which alternating carbonates–evaporites and siliciclastic were deposited (Alavi 1994, 2004; Heydari 2008). The main Cenozoic sedimentary sequences of the Zagros Basin consist of Paleocene–Eocene shallow water carbonate–evaporite and dolostones (i.e., Sachun and Jahrum Formations) and basinal marl and marly limestone (i.e., Pabdeh Formation) (Alavi 2004).

Lithofacies maps of the Sachun Formation indicate that the formation was deposited in a narrow (about 150 km wide) basin that existed continuously from Paleocene to Lower Eocene times (Fig. 3; Ziegler 2001).

Fig. 3
figure 3

Litho-facies map of the Paleocene to Early Eocene covering the depositional time of the Sachun, Jahrum and Pabdeh Formations, and their regional equivalents (modified from James and Wynd 1965; Ziegler 2001)

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Methods

Most of the data in this research were obtained from detailed outcrop studies of the Sachun Formation at the type locality. Field and petrographic studies were carried out for facies analysis and paleoenvironmental reconstruction. More than 465 samples were taken at less than 2 m intervals and in some cases, sampling distance was reduced to 20 cm, based on field observations. Thin sections (235) were prepared and stained with Alizarin Red-S to differentiate dolomite from limestone (Dickson 1966). The textural classification of Dunham (1962) was used with modifications for the description of the facies types. Wilson (1975) and Flügel (2004) facies belts and depositional models were also used. Facies studies include the analysis of matrix and grains, texture, fossil content, petrography and standard facies zone. The composition of the associated fauna (presence of benthic and pelagic foraminifera) and non-skeletal grains (e.g. ooids, intraclasts and peloids) was considered. Sedimentologic texture and structure (e.g. dolomitization, presence of silt-size quartz grains, boring, burrowing and encrustation) have been considered qualitatively.

The sequence stratigraphic interpretation of the Sachun Formation is based on detailed description of facies associations as well as the nature of the contacts that separate them. Sequence stratigraphic nomenclature adopted in this paper follows the terminology summarized by Catuneanu et al. (2009). In this manuscript, following the subdivision of the Sachun Formation, ‘‘units’’ (i.e., unit 1, 2 and 3) refer to lithostratigraphic units, whereas ‘‘sequences’’ (i.e., sequence A, B and C) are sequence stratigraphic.

Lithostratigraphy

The Sachun Formation takes its name from, at type locality, Kuh-e Sachun, near the Sachun village, in Fars province (James and Wynd 1965). This formation consist mainly of evaporites (mainly gypsum), carbonate and also argillite marls, intercalated with thin-bedded dolostones and is Paleocene to Eocene in age (Wynd 1965; Falcon 1974; Amiri-Bakhtiar 2007).

The Sachun Formation unconformably overlies the rudistic limestone of the Tarbur Formation. The upper contact with the dolomitic Jahrum Formation is unconformable. The area of development of the Sachun Formation, which is present only in the interior Fars province, is coincident with an area of a marked thickening of the Cretaceous–Tertiary section from coastal Fars to interior Fars (James and Wynd 1965; Sepehr and Cosgrove 2004). At the type locality, this formation is divided into three units including lower evaporite (unit 1, 125 m), middle carbonate (unit 2, 355 m) and upper evaporite (unit 3, 466 m) that will be described below in ascending order (Fig. 4)

Fig. 4
figure 4

Generalized section of the Sachun Formation at the type locality illustrating lithostratigraphic units and biozones (modified after James and Wynd 1965)

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Lower evaporite (unit 1)

This unit unconformably overlies the rudistic limestones of the Tarbur Formation (Fig. 5a). It is mainly composed of 125 m of bedded gypsum (Fig. 5b), thin-bedded dolomite intercalation (Fig. 5c), gray marl and gypsiferous red-color marl (Fig. 5d). This unit is barren of any index fossil mainly due to evaporitic nature of the deposits as well as diagenetic effects. The various kinds of benthic foraminifera such as textularid and valvulinid were seen in this part. Based on macrofossils and palynologic data obtained from the interbedded mudstone within the Sachun Formation, this unit is Paleocene in age (Wynd 1965; Amiri-Bakhtiar 2007).

Fig. 5
figure 5

Lithostratigraphic aspects of the Sachun Formation at the type locality. a Field view of the basal contact of the Sachun Formation with the Tarbur Formation; a paleosol horizon sits on top of the dolomitized rudistic limestones of the Tarbur Formation (arrow). b Field view of the Sachun Formation at the type locality; a thick limestone succession (unit 2) overlies an evaporitic succession (unit 1). c Field view of thin-bedded dolomite in the lower evaporite unit. d Field view of gypsiferous red marl in the lower evaporite unit. e Field view of a horizon with dispersed large carbonate nodules in the middle carbonate unit 2. f Erosional surface and stromatolitic lamination with residual iron-oxide bands representing paleosol at contact between units 2 and 3. g Upper evaporite succession with several small-scale shallowing-upward hemicycles. h General view of the middle carbonate and upper evaporite units of the Sachun Formation and overlying dolomitic Jahrum Formation

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Middle carbonate (unit 2)

This unit consists of light gray, medium-thick bedded, highly fossiliferous limestone and marl interbeds with 355 m thickness. Most of the limestones are medium bedded (0.25–0.5 m thick), planar to undulating, and include numerous thinning-upward beds. Bedding gradually thins (2–10 cm) towards the top of the succession. Based on presence of Alveolina, Glomalveolinids, Textularia, Discocyclina, Kathina and Miscellanea, this unit is Late Paleocene in age (Wynd 1965). A similar foraminiferal assemblage of this age occurs in the other regions of the Zagros Basin (Wynd 1965). Spherical to sub-spherical calcareous nodules, which range in diameter from 0.5 to 1.5 m, are recorded at two stratigraphic horizons within this part of the section. They are irregularly disseminated and oriented parallel to bedding planes and show sharp contacts with the host carbonate rocks (Fig. 5e). A thin paleosol separates this middle carbonate unit from the overlying evaporitic unit (Fig. 5f).

Upper evaporite (unit 3)

This unit forms a 466 m of white-colored bedded and brecciated gypsum, which contains several shallowing-upward cycles that can be clearly recognized in the field (Fig. 5g). The upper evaporite of the Sachun Formation is almost barren of fossils, but rare occurrence of Opertorbitolites sp. indicates Lower Eocene age. This unit conformably underlies the dolomitic limestone of the Jahrum Formation (Fig. 5h).

Facies analysis

Three lithofacies types are recognized in the Sachun Formation. These are evaporites, carbonates and mixed carbonate–siliciclastic lithofacies, which are described and interpreted as follows:

Evaporite lithofacies

The term ‘evaporite facies’ or ‘evaporite sediment type’ is used here in the same sense as used by Arakel (1980), and refers to a distinct sediment or sedimentary rock deposited under specific environmental conditions, irrespective of age or physiographic stratigraphic position. The evaporite-dominated lithofacies is the most dominant lithofacies in the study area. The Sachun Formation is mainly composed of secondary gypsum with porphyroblastic and alabastrine fabrics, which originated from the hydration of a precursor anhydrite rock. Secondary gypsum rocks, formed from anhydrite by the action of ground waters and/or surface weathering, are divided into two main petrographic groups, porphyroblastic and alabastrine secondary gypsum (Holliday 1970). The evaporite lithofacies in the Sachun Formation are described according to the classification of Maiklem et al. (1969), which is based on the relationship between anhydrite and associated matrix, such as pseudomorphing crystalline, nodular and laminated-banded. Based on this classification and thickness of the evaporite beds, this lithofacies can be subdivided into the following three groups

Nodular, nodular-banded evaporite facies

Description: This lithofacies consists of medium to thick bedded, nodular, mosaic, chicken-wire, enterolithic and massive anhydrite with a red to brown siltstone background. Its thickness ranges from 0.3 to 30 m. The beds are composed of nodules that may be vertically elongated, or subspherical. The nodular horizons are made up of small and medium anhydrite nodules. The size of anhydrite nodules in this facies ranges from 0.5 to 10 cm. More translucent gypsum and/or satin spar gypsum veins are present at the contacts between the nodules (Fig. 6a). In some cases, dolomicrite fragments irregularly filled and intercalated the inter-nodular spaces of this lithofacies (Fig. 5c). The matrix around the anhydrite crystals is composed of detrital silty quartz and clay patches of interbedded gypsiferous red-colored marls (Fig. 5d). The coalescing nodules exhibit thin enterolithic and chicken-wire fabrics with micro-nodules enveloped by cohesive clayey carbonate mud.

Fig. 6
figure 6

Lithostratigraphic aspects of evaporites of the Sachun Formation at the type locality. a Nodular anhydrite lithofacies with chicken-wire structure within the upper evaporite unit 3. b Brecciated gypsum within lower evaporite unit 1; the fabric is densely packed and clast-supported, with subordinated fine-grained matrix surrounding the clasts. c Breccia composed of gypsum (white material) and red clastic (dark material) within the upper evaporite unit 3. The breccia consists of angular to subangular clasts and blocks that is irregularly on the bedded upper evaporite unit. d Laminated-banded gypsum lithofacies with regular alternation of gray and white gypsum laminae; some of them display stromatolite-like undulations. e Alteration of well-preserved laminated texture of stromatolitic algal network and laminated gypsum. f Petrographic view of gypsum laminae, which are intercalated with networks of algal micrite, micrite laminae and granular gypsum that is interpreted to have formed during periods of dilution

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Interpretation: Anhydrite characteristically occurs as nodules in the sabkha facies (Hardie and Eugster 1971; Handford 1991; Alsharhan and Kendall 1994). Chicken-wire and enterolithic fabrics in the nodular secondary gypsum horizons indicate that the intra-sediment growth of anhydrite took place above the water table of a completely exposed sabkha. These anhydrite nodules grew during the early diagenetic (synsedimentary) stage and were partly or completely hydrated to secondary gypsum during early and/or late diagenetic (exhumation) processes (Gundogana et al. 2005). Similar synsedimentary anhydrite nodules occur at present in sabkha environments on the southern coast of the Persian Gulf in Abu Dhabi (Alsharhan and Kendall 1994). In some levels, resedimented, intraclastic, brecciated lithofacies alternate with laminated-banded gypsum. Based on above evidences, this facies indicates a sabkha and very shallow-water depositional environment.

Brecciated gypsum

Description: Gypsum breccias with variable amounts of gypsum components occur in both evaporitic units (units 1 and 3). This type of evaporite commonly inter-grades with the nodular-banded evaporite facies. The brecciated fabric of this lithofacies is characterized by matrix-free, clast-supported, gypsum (Fig. 6b). Such breccias are defined by stacked horizons (Fig. 6c), made up of clast-to-clast angular to subangular fabric with sharp basal contacts to the underlying dolostones and sharp to gradational upper contacts with the overlying subtidal carbonates. The beds of this facies are generally 40 to 60 cm in thick. Occasionally, they are up to 1 m or more thick. Rippled surfaces, erosional and wash-out surfaces, imbricated lithoclasts, microfaults, angular or subangular fragments of gypsum and gypsiferous carbonate rocks, clay chips, fine-grained clastic gypsum, gypsum pseudomorphs, graded-bedded and mud cracks are distinctive features of this lithofacies.

Interpretation: This lithofacies shows common features (convolute lamination, graded-bedded laminae, erosional boundaries) characteristic of redeposited sediments. Brecciated gypsum is interpreted as originally gypsum-dominated clastic deposits derived from the reworking and redeposition of the earlier deposited sediments (Kasprzyk and Ortı 1998). Evidence for this comes from the abundance of pseudomorphs after gypsum, the presence of intraclasts, common synsedimentary redeposition features and the incorporation of terrigenous material within this lithofacies. Brecciated gypsum in various depositional environments in the Messinian evaporite units has been mostly described as resedimented evaporates (Parea and Ricci-Lucchi 1972; Warren 2006).

Regular laminated gypsum

Description: This lithofacies is laminated, algal-like morphology as well as wavy ripples overprinted on the gypsum laminae. Gypsum laminites are generally associated with marly-silty algal layers and microcrystalline dolomudstone. Each bed is made up of plane-parallel stratified gypsum laminae with intercalations of dolomitic laminae and satin spar gypsum (Fig. 6d). In some cases, the laminated evaporite lithofacies consists of thin layers of networks of algal micrite alternating with granular gypsum laminae (Fig. 6e). The laminites commonly contain intercalations of decimeter-thick calcitic or dolomitic mudstone (Fig. 6f). The horizontal algal laminations are included in the gypsum crystals and have not experienced deformation. Gypsum crystals that are surrounded with a network of algal micrite were probably entrapped by the algal filaments.

Interpretation: The laminated, banded and massive lithofacies represent primary, subaqueous precipitates (Orti and Alonso 2000). The associated sedimentary structures indicate a restricted shallow water environment. Although diagenesis has partially obliterated the primary laminated structure and the internal crystal fabrics, the observed laminated structure and texture of the gypsum, as well as the structure of the algal micrite, are typical of sedimentation in a subtidal lagoonal environment. Similar features were recorded from the Messinian evaporite of the Mediterranean basin and have been discussed by many authors (Aref et al. 1997; Pope et al. 2000; Warren 2006; Fiduk 2009). As the salinity of the marine water increases, gypsum was precipitated from the water body on the top of algal mats. Continuous precipitation of gypsum and its reworking at the sediment surface was also accompanied by the growth of large gypsum crystals below the sediment-brine interface. During phases of dilution of the lagoonal water, gypsum sedimentation ceased and was replaced by flourishing algal mats on the bottom sediment. Further influxes of muddy flood water into the lagoon led to deposition of lime mud over the gypsum-algal micrite lamination.

Carbonate lithofacies

Field observations, fossil contents, textural and sedimentological features led to recognition of 12 facies as follows.

Facies 1: dolostone

Description: This facies consists of fine to very fine dolomite crystals. The main features of this facies are ghost structures of intraclasts (Fig. 7a), scattered microcrystalline anhydrite nodules (Fig. 7b, c), detrital quartz grains, thin-bedding, fenestral fabrics, irregularly shaped dissolution voids up to several centimeters in size (Fig. 7d). The facies might be burrowed and burrow-mottled or laminated. Primary sedimentary structures are generally obliterated. Occasionally, skeletal fragments (mostly small unidentifiable foraminifera) are still preserved, while the former micritic matrix is completely replaced.

Fig. 7
figure 7

a Photomicrograph of well-preserved intraclasts in fine grain dolostone ground mass in cross-polarized light (XPL). b, c Micronodular anhydrite laths (arrows) within dolomitic host sediment in the lower evaporite unit, (XPL). d Fine crystalline dolomite with fenestral fabric, which is filled with granular sparite, partly satin-spar gypsum in plane polarized light (PPL). e Field view of laminar stromatolite. f Vertical section of microstromatolites showing their internal lamination. Thin laminae are porous and they contain common microbial traces; thin section is stained with Alizarin Red-s (XPL). g Mudstone with fenestral fabric; the right half of the thin section is stained with Alizarin Red-S (XPL). h Photomicrograph of evaporate pseudomorphs (arrow) within mudstone facies (XPL)

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Interpretation: The fine-grained dolomite probably resulted from the early dolomitization of precursor lime-mud (Rameil 2008). This took place in an intertidal to supratidal environment by evaporation in shallow ponds. The evaporation of seawater raised Mg concentration and Mg/Ca ratio to the level needed for dolomitization (Fu et al. 2006). The very fine crystal size, the presence of scattered detrital silt-size quartz grains, evaporitic nodules, preservation of original depositional textures and absence of fossils, reveal that this dolomite formed under near surface, low-energy conditions, possibly in a tidal flat setting (Rameil 2008).

Facies 2: stromatolitic boundstone

Description: The thickness of this facies varies from 0.2 to 1.8 m and has a gradual contact with the underlying dolomitic facies (Fig. 5f). Basinward the stromatolitic facies grades into dolomitic, cemented, peloidal facies. A typical stromatolite is made up of numerous successive laminae that are stacked forming a variety of morphotypes. These deposits are represented by mud-supported structures formed by millimeter thick laminae, generally without fossils, irregularly undulating and laterally continuous (Fig. 7e). Generally, the laminae in stromatolites occur in low-relief domes or are planar to slightly undulated (Fig. 7f).

Interpretation: Stromatolites are interpreted to have grown largely by cyanobacteria on firm substrates in water depths sufficient for their complete submergence. Modern stromatolites grow in shallow waters under hyper-salinity conditions, in supratidal to subtidal zones, (Hardie and Eugster 1971; Flügel 2004; Riding and Tomas 2006; Oliveri et al. 2010). Hypersalinity may result from a prolonged arid climate coupled with evaporation. The laminated stromatolite structures are formed by trapping and binding activities of phototrophic microbes (Logan et al. 1964; Riding and Tomas 2006). The planar to slightly undulated stromatolite types probably formed in a protected upper intertidal flat under relatively low-energy conditions.

Facies 3: fenestral mudstone

Description: This facies basically consists of very fine-grained, partly silty and generally component-poor deposits. It mainly comprises gray but also reddish or grayish color mudstones and minor wackestone and includes somewhat different types of limestone, characterized by abundant micritic matrix. In all cases, there are few recognizable bioclasts, mainly because of very small size. The deposits are preferably formed of micrite or microspar with evaporite pseudomorphs and have a fenestral fabric (Fig. 7g). They are mainly massive, but occasionally show a nodular fabric or wavy horizontal to flaser lamination due to the accumulation of angular quartz grains in thin layers or horizons. Scattered shell fragments, evaporite pseudomorphs (Fig. 7h) and bioturbation are present. Peloids or ostracods are embedded in a commonly slightly recrystallized, microsparitic matrix. Occasionally, isolated, euhedral dolomite crystals also occur.

Interpretation: The formation of mudstones is generally explained by the deposition of very fine, suspended sediment particles under quiet-water conditions. The occurrence of mudstones at the base of individual cycles took place subsequent to the renewed flooding of the platform, when the carbonate factory was about to start up again. Hence, this facies probably formed under sediment-starved conditions when the production of biotically controlled sediment material was still low. The nearly complete absence of skeletal fragments and presence of evaporite should imply restricted marine and hypersaline conditions, similar to what has been described from the Mississippian Midale beds of the Charles Formation in SE Saskatchewan (Qing and Nimegeers 2008).

Facies 4: intraclastic breccia

Description: The intraclastic breccia is dark gray in color and consists of micrite to microspar with an appreciable amount of poorly sorted intraclasts. It is characterized by a bioturbated matrix and dark angular and irregular micritic intraclasts (several mm to 1 cm). Some intraclasts contain fragments of ostracod valves and occasional benthonic foraminifera (e.g. miliolids; Fig. 8a). Fenestral pores with internal sediment may be locally present.

Fig. 8
figure 8

Photomicrographs of carbonate facies recorded in the studied section. a Facies 4: intraclastic breccia (floatstone) with poorly sorted, angular to subrounded intraclasts. b Facies 5: miliolid packstone. c Facies 6: peloidal packstone. d Facies 7: Alveolina wackestone; note the well-preserved Alveolina foraminifers. e Facies 8: dasycladacean wackestone; note that dasycladacean algae have been dissolved and their molds were filled by satin-spar gypsum. f Facies 9: ooid grainstone; some ooids are micritized and a few are dissolved (arrow). Interparticle pores are filled with calcitic cement. g Facies 10: bioclastic grainstone; this bioclastic grainstone consists of skeletal grains and some micritic ooids. Bioclasts are predominantly Alveolina and miliolid foraminifera. The chambers of the miliolid foraminifera were partly occluded by cement (arrows). h Facies 11: small rotalia-bioclastic wacke/packstone. i, j Facies 12: planktonic foraminifer mud/wackestone

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Interpretation: Intraclastic breccia could have been deposited in various environments on a shallow-marine carbonate platform (Orti and Alonso 2000). Among them, the tidal flat deposition of this facies is inferred from the close stratigraphic relationship with the stromatolitic and dolomitic facies and the presence of subaerial exposure horizons (Fig. 5f). The micritic intraclasts in this facies have been probably derived from disrupted stromatolitic laminae, which were exhumed and buried again.

Facies 5: miliolid packstone

Description: This facies (Fig. 8b) is dominated by the occurrence of small benthic miliolids foraminifera in a micritic matrix. Other foraminifera such as alveolinids, textularia and austrotrillina are also present. Other components are intraclasts, echinoderm fragments and coralline algae.

Interpretation: Recent miliolids are euryhaline forms living in shallow, restricted/lagoonal environments with low turbulence thriving on soft substrates (Zamagni et al. 2008). The occurrence of a large number of porcellaneous imperforate foraminifer tests points to a slightly hypersaline environment (Alcicek et al. 2007). Such an assemblage and the similarity with the standard facies types described by Wilson (1975) and Flügel (2004) indicate a shelf lagoon environment. The textural characteristics and abundance of miliolids and intraclasts suggest a restricted lagoon towards a nearby tidal flat depositional environment. This facies is likely to have accumulated in broad lagoons, possibly proximal to the shoreline where elevated temperatures and/or salinities may result from reduced water circulation, on shelf tops or in an inner ramp location.

Facies 6: peloidal packstone

Description: This facies (Fig. 8c) is characterized by abundant peloids (maximum size 1 mm). It also includes variable proportions of poorly rounded bioclasts (mainly miliolids, gastropods, bivalves and minor amounts of textulariids, scattered gastropod shells, echinoderms and dasycladacean algae), micritic intraclasts and micritized ooids. The peloids are irregular and poorly sorted showing gradation into micritic intraclasts.

Interpretation: The majority of the peloids of this facies most likely originate from the disintegration and abrasion of encrusting foraminifera and in minor amounts from the micritization of other bioclasts and ooids. In general, well-sorted grainstones and packstones were deposited above the fair weather wave base (FWWB) within the turbulent, shallow-submarine to intertidal areas of the inner platform. Here, peloids accumulated due to winnowing caused by weak tidal currents or wave action within semi-protected, lagoonal areas, probably in the vicinity of a smaller barrier. The predominance of muddy textures and the skeletal content (especially the presence of miliolids and dasycladacean algae but also of open-marine fauna) reflect low-energy conditions in protected (not restricted) lagoon areas.

Facies 7: Alveolina wackestone/packstone

Description: The main feature of this facies is the dominance of alveolinids and glomalveolinids (Fig. 8d). Other minor biogenic components include miliolids, Opertorbitolites, molluscs (mainly gastropods) and dasycladacean algae. This facies contain well-preserved foraminifers, both large (especially Alveolina) and small (often miliolids). Fragmentation of larger foraminifera is rare.

Interpretation: Alveolina are main components in Early and Middle Eocene shallow-water carbonates and they were most prominent in protected shelf and higher-energetic shoal environments (Rasser et al. 2005). The lack of rotaliids and the high abundance but low diversity of miliolids, alveolinids and glomalveolinids can be indicative of hypersaline conditions, a typical of a lagoonal setting. This mud-supported facies shows a low to moderate energy environment. Episodes of high energy are suggested by the local occurrence of alveolinids grainstone. The presence of the dasycladacean green algae suggests a very shallow marine setting (Cosovic et al. 2004). Living and fossil alveolinids are shown to occur in a variety of shallow-marine settings with distributions independent of the substrate. Living alveolinids, such as Borelis sp., proliferate to depths less than 35 m (Zamagni et al. 2008).

Facies 8: Dasycladacean wackestone

Description: This facies is dominated by dasycladacean algae and shell fragments in a micritic groundmass (Fig. 8e). In addition, gastropods, alveolinids and smaller miliolids are present. In some sample, the dasycladacean algae are associated with intraclasts, ooids and abundant bioclastic debris, mostly composed of echinoderm plates, spines and unidentified foraminifera. Many dasycladacean algae have been dissolved and their molds were filled by granular calcite and satin-spar gypsum.

Interpretation: Dasycladacean algae can be accumulated in the shallow-lagoon, adjacent to or between oolitic bars and barriers that offered good back-reef-like protection (Bucur and Sasaran 2005). The association of the biogenic components suggests deposition in a more restricted environment with a more onshore position than the Alveolina wackestone facies (Bucur and Sasaran 2005).

Facies 9: Ooid grainstone

Description: This facies is dominated by ooids with grain sizes of up to 1 mm and peloids (Fig. 8f). Minor amounts of small foraminifera, intraclasts, shell fragments and probably molluscs (recrystallized) are present. Many ooids have been dissolved and their molds were filled by single crystals of granular calcite and satin-spar (gypsum). The central nuclei of the ooid grains consist mainly of smaller, recrystallized skeletal fragments, such as small foraminifera and peloids. The grainstones exhibit vaguely or clearly graded, parallel and low-angle cross lamination and may show erosive basal surfaces develop on the underlying micrite dominated strata.

Interpretation: The presence of a grain-supported fabric as well as ooids, peloids and fossil content suggest a high energy and shallow-water depositional setting probably in the inner ramp. This facies reflects low-diverse, winnowed sands of barriers, which formed under higher energy conditions within the intertidal to shallow marine inner ramp. The formation of ooidal sands requires supersaturated and agitated water conditions, enabling the growth of concentric coatings, embracing the nuclei, while the grains are in permanent movement and rotation. Altered ooids probably originate from primary aragonitic radial-fibrous ooids due to early diagenetic dissolution and recrystallization processes (Flügel 2004). The presence of disarticulated and broken bioclasts as well as intraclasts are indicators of intense reworking and deposition from storm-induced currents transporting coated grains and algae from the inner ramp barrier and lagoon.

Facies 10: Bioclastic grainstone

Description: This facies is characterized by abundant well-sorted bioclasts and some ooids. Peloids are also present. The main bioclastic components are benthic foraminifera (Alveolina, rotaliids and miliolids), gastropods and green algae (Fig. 8g). This facies is generally arranged in tabular and irregular levels up to 1 m-thick, with local planar cross-bedding.

Interpretation: Good sorting and the absence of matrix indicate high energy conditions. In accordance to the standard facies types described by Wilson (1975) and Flügel (2004), this facies is interpreted as a barrier environment above the normal wave base, which was located at inner ramp, separating the open-marine from the more restricted marine environments. The features of this facies indicate high energy conditions with significant movement and reworking of bioclasts in a shallow-water environment. This facies show a mixture of skeletal and non-skeletal grains (mainly peloids and ooids) partly derived from the interior lagoonal areas and from the active shoal facies belt. Compared to the lagoon areas, the bioclastic content indicates a more open-marine condition, with lower proportion of the bioclasts typical of the lagoon environment (i.e., Alveolina, miliolids, textulariids, ostracods and dasycladacean algae).

Facies 11: Miscellanea-bioclastic wackestone-packstone

Description: The main components of this facies are miscellanea accompanied by other larger and smaller benthic foraminifera. The other skeletal grains consist of benthic foraminifera, corallinaceans, fragments of echinoderms and bryozoans. The benthic foraminifera include Nummulites, small rotalia and valvulinids (Fig. 8h). A few scattered planktonic foraminifera are present.

Interpretation: The faunal association (miscellanea, rotalia, Nummulites and scattered planktonic foraminifera) of this facies indicates shallower environments of mid ramp, probably near high energy barriers. The presence of rotalia and other foraminifera such as Nummulites and operculina suggest a middle ramp position and points to oligotrophic conditions (Brandano et al. 2009). This facies represents parautochthonous, reworked and re-sedimented inner platform deposits (Nebelsick et al. 2005).

Facies 12: Planktonic foraminifera mudstone-wackestone

Description: the predominant skeletal grains are planktonic foraminifera (Astrorotalia palmera, Globorotalia and Globigerina; Fig. 8i, j). Subordinate taxa belong to the smaller benthic foraminifera (rotalia and Nummulites). This facies is lime mud-dominated and lacks a shallow-water neritic fauna. No sedimentary features that are indicative of shallow-water and high energy sedimentation were observed.

Interpretation: The abundance of planktonic foraminifera, the occurrence of smaller benthic foraminifera and the micritic matrix suggest that deposition may have taken place in a lower part of a mid-ramp environment. The low hydrodynamic energy regime indicates deposition below the normal wave base (Wilson 1975; Geel 2000; Flügel 2004).

Mixed carbonate and siliciclastic lithofacies

These lithofacies commonly occupies the middle intervals of the unit 2 and comprise up to 5 to 10 % of the Sachun Formation, which can be subdivided into two major facies, including red marl and gray marl facies.

Red marl facies

Description: This marly facies consists of reddish to red and ocher-color non-fossiliferous gypsiferous marl that is thinly laminated. In addition, this lithofacies is associated with gypsum crystals and beds (Fig. 5d). Thin intervals of red marl are present between the evaporites successions in upper and lower units which are represented by a cyclic succession of shallowing-upward meter-scale. Occasionally this facies contains scattered remains of dasycladacean green algae and shell fragments. This facies, in some parts, represents the basal part of the gypsum cycles in the unit 1.

Interpretation: The association of this lithofacies with other evaporite and shallow-water carbonates, and the presence of gypsum crystals and dasycladacean algae indicate deposition in very shallow water, representing a lagoonal environment (Mahboubi et al. 2001, 2010). Calm-water conditions allow fine particles to settle down from suspension, under protected conditions, were the site of deposition of these facies. The facies might have occurred in intertidal and supratidal environments, as the marls intercalated with gypsum bands.

Gray marl lithofacies

Description: This facies is homogeneous marl (ranging in thickness from 2 to 17 m) with interbedded carbonate layers ranging in thickness from 0.5 to 1 m. This facies is distinguished by its gray, green color, medium-bedded, presence of marine fauna (molluscs, shell fragments and sometimes Alveolina and Nummulites) and partially bioturbated properties in the field. Infillings of bioturbation are composed of satin-spar gypsum and muddy materials. Some beds of this facies show thin parallel lamination and contain ironstone nodules.

Interpretation: In addition to carbonate rocks, this facies with marine fauna may have been deposited in relatively deeper parts of the platform. The presence of marls with well-preserved sparsely distributed isolated marine fauna may reflect low hydrodynamic settings, probably, below storm wave-base.

Depositional model

Lithofacies maps (Fig. 3) indicate that the Sachun Formation was deposited in a narrow and continuous basin during Paleocene to Early Eocene times (Ziegler 2001). The Sachun succession at the type locality represents a shallow inner and middle ramp (Fig. 9). In the studied section the predominance of mud-supported lithologies and the lack of wave-related fabrics indicate that deposition occurred along a protected carbonate ramp. As a result of the facies interpretation given above, it is suggested that higher portions of inner ramp are more common in the study area. The inner ramp includes sabkha-salina, tidal flat, lagoon and shoal. Sabkha-salina sediments formed in the supratidal environments that were periodically flooded by seawater. Most of the Sachun evaporite facies displays features characteristic of secondary deposition, which were diagenetically formed within host sediments (e.g., nodular anhydrite), or have obliterated the original laminated structure (e.g., nodulization of the laminated structure, resedimentation breccias probably due to halite dissolution, etc.).

Fig. 9
figure 9

a Schematic profile of the mixed carbonate-evaporate platform illustrating the distribution of the twelve carbonate facies types, which are represented by numbers. b Schematic depositional model for the restricted platform carbonate-evaporate of the Sachun Formation at the type locality

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The fenestral fabric, stromatolitic boundstone and thin-bedded dolomites can be interpreted as algal or microbial mat deposits related to upper intertidal to subtidal environment (Flügel 2004). Wackestones with imperforate foraminifera, green algae and dominance of peloids are interpreted to have deposited in the restricted lagoon settings with poor connection with the middle ramp. Lagoonal facies types are highly variable but contain abundant imperforated tests of foraminifera (miliolids, Alveolina, textularia and valvulinid), gastropod and algal debris. Towards the shoal, imperforated foraminifers and perforated foraminifera (lens shaped nummulitids and rotalids) occur together (Geel 2000).

The shoal is characterized by a high abundance of ooids and rounded bioclasts. The presence of a grain-supported fabric as well as ooids and fossil content suggests a high-energy and shallow-water depositional setting in the inner ramp.

The open marine setting is marked by high planktonic foraminifera contents embedded in a mudstone–wackestone, thus representing deposition in quiet-water environmental conditions (Cosovic et al. 2004; Rasser et al. 2005).

Sequence stratigraphy

According to the vertical arrangement of the different lithofacies types, the mixed carbonate-evaporite succession of the Sachun Formation can be grouped into three-third-order depositional sequences (A, B and C; Fig. 10). These sequences generally have a shallowing-upward trend. Most of the sediments are deposited during the highstand. The transgressive systems tract (TST) of the Paleocene-Early Eocene Sachun Formation show relatively deeper facies (subtidal-lower intertidal). These facies are often heterogeneous and are composed of several facies types of varying thickness. The highstand systems tract (HST) is divided into an early highstand systems tract (EHST), which fills the vertical accommodation space and a progradational (regressive) late highstand systems tract (LHST). Several exposure surfaces occur throughout the Sachun sequences and are commonly represented by iron-stained anhydrite and an undulate surface. Some of these surfaces are associated with extensive dissolution.

Fig. 10
figure 10

Stratigraphic column showing the lithofacies, depositional environments and depositional sequences of the Sachun Formation at the type locality

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Based on lithofacies analysis, 29 of superimposed higher order cycles have been distinguished within the Sachun Formation (Fig. 10). Most of these cycles consist of carbonate lithofacies at the base, which are overlain by evaporite sediments, and exhibit a shift from normal marine to evaporitic condition. The thickness of each of these cycles range from a few tens to several tens of meters, dependent on subsidence and palaeogeographic position within the basin. Cycle tops are sharp, recording a period of subaerial exposure and/or no deposition. Most cycles record an upward shallowing and are interpreted as base-level rise or fall hemicycles. A set of base-level rise hemicycles consists of an upward-deepening facies succession; e.g. the upward transition from evaporite-dominated tidal and mud flat deposits to lagoonal and barrier deposits. The base-level fall hemicycles are composed of an upward-shallowing facies succession; e.g. the upward shift from carbonate tidal flat deposits to evaporite-dominated salina and sabkha deposits. As mentioned above, three-third-order depositional sequences at the type locality of the Sachun Formation are identified and describe as follow.

Sequence A

The Sachun/Tarbur boundary in most parts of the SE Zagros is marked by a subaerial exposure surface. This surface is irregular with leaching and dissolution pipes penetrating the underlying bed of the uppermost part of the Tarbur Formation (Fig. 5a). Such features suggest a paleosol development at the time of subaerial exposure (Retallack 2001). Thus, this surface belongs to a subaerial unconformity (Catuneanu 2002; Catuneanu et al. 2009). Also, the occurrence of shallow marine deposits above the recognized subaerial unconformity surface refers to a transgressive surface of erosion (Catuneanu 2002; Catuneanu et al. 2009). Sequence A commences with burrowed and burrow-mottled, laminated, sucrosic dolomite with gypsum pseudomorphs and good intercrystalline porosity (TST). It is overlain by one meter of dense layered ooid-grainstone as a maximum flooding zone (MFZ), which is capped by thick successions (HST) of gypsum and anhydrite (lower evaporite unit). The latter were deposited in a shallow marine and/or lagoon sabkha environment as a result of the regression of sea at the beginning of the Paleocene. This sequence frequently pass vertically up from barrier facies (oolitic and bioclastic grainstones) to lagoonal and tidal-flat facies. The uppermost part of the lower evaporite unit is characterized by the brecciated gypsum lithofacies, associated with an alternation of stromatolite boundstone and red stained thin dolomitic lamina.

Sequence B

Sequence B is defined by a carbonate-dominate succession (unit 2 and the lower part of unit 3). The boundary between A and B sequences is on top of a dolomitic stromatolite boundstone with fenestral fabric (SB1). The lower part of this sequence (TST) consists of shelf lagoon deposits, which are mostly characterized by benthic foraminifera and dasycladacean. These deposits overlie the lower evaporite unit and they represent a major interruption in evaporite precipitation. Bioclastic grainstone and small rotalia-bioclastic wackestone/packstone with occasional cross-bedding and scouring follow. The succession gradually passes upward to gray and greenish marl, probably indicating a long-term maximum increase in base level rise. This horizon might represent a maximum flooding surface in this area. The evaporite-dominated cycles are composed of successions of repetitive lagoonal marl with intermittent tidal flat and evaporite salina anhydrite. These cycles record an upward-brining trend, which indicates the EHST of this sequence. The sequence terminates with a nodular to bedded anhydrite cap (LHST). Direct transitions from subtidal to supratidal facies (dolomites and stromatolite boundstone capped by an erosional surface) and sedimentary pockets with residual iron-oxide bands have been found with minor occurrence (Fig. 5f).

Sequence C

This sequence display numerous shallowing upward meter-scale cycles composed of carbonate and evaporite sediments. Evaporites form part of the sedimentary sequences that begin with normal open marine and shelf deposits and pass upward into thickly bedded and massive-nodular anhydrite that alternates with thinly bedded limestone (dolomitized with anhydrite nodules), and that is unconformably overlaid by massive bedded dolomitic limestones of the Jahrum Formation.

Carbonate-dominated depositional cycles consist of basal lagoonal deposites, which grade upward into tidal-flat dolostones and finally into evaporitic sabkha. The TST in this sequence begins with a dolomitized peloidal packstone facies and is bounded by a pelagic foraminiferal wackestone facies representing the MFZ. Based on the occurrence of planktonic foraminifera (Astrorotalia palmera) this MFZ is Early Eocene in age (Wynd 1965). It can be correlated with the maximum flooding surface Pg20 of Sharland et al. (2001) on the Arabian plate. The MFS Pg20, which is the most prominent surface for correlation in Arabian plate, was defined based on a strong influx of an open marine fauna in the Early Eocene Midra Shales (Sharland et al. 2001). The lower part of the HST consists of barrier to lagoonal facies, while the upper part of this shallowing parasequence is composed of lagoonal supratidal to sabkha lithofacies (LHST). Mosaic, massive and deformed anhydrites occur at the upper parts of this sequence and are interpreted as sabkha-salina deposits. A thin layer of subaerial sabkha sediments, consisting of microcrystalline dolomite with anhydrite nodules overlies the succession. This sequence is overlain by dolomitic limestones of the Jahrum Formation.

Conclusion

The Sachun Formation in the study area consists of cyclic interbedded evaporites, carbonates, and mixed carbonate–siliciclastic lithofacies formed in the arid climatic setting on a restricted carbonate ramp. Evaporite lithofacies analysis reveals three facies: nodular-banded gypsum, brecciated gypsum and laminated gypsum, which probably represent a sabkha and very shallow-water depositional environment. Twelve carbonate facies types are recognized in the carbonates of Sachun Formation, indicating deposition from shallow to deeper parts of the platform. Dolostone, stromatolitic boundstone, mudstone and intraclastic breccia might have occurred in the tidal flat to supratidal sabkha environment; miliolid, peloidal, Alveolina and dasycladacean wackestone/packstone are probably deposited in the shallow subtidal lagoon; ooid and bioclastic grainstone are related to a barrier setting; small rotalia-bioclastic and planktonic foraminifera mudstone/wackestone are deposited in the open marine environment. The siliciclastic lithofacies is subdivided into two major facies; red marl with evaporite is considered to have formed in a restricted lagoonal inner ramp environment and gray marl with marine fauna might have deposited in relatively deeper parts of the platform. The Sachun Formation is interpreted to have been deposited on a low-angle (homoclinal), relatively low-energy carbonate ramp with no significant break in slope. Deposition took place under varied conditions such as restricted, partly open marine circulation, low to high energy and normal to raised salinity during alternating periods of extensive and ceased evaporite sedimentation.

Four sequence boundaries, marking regional relative sea-level falls, divide the Sachun Formation into three depositional sequences. Each sequence is marked by a number of sedimentary facies that designate repeated small-scale transgressive–regressive cycles. Sequences A and C consist of evaporite-dominated successions while sequence B is dominated by shallow-water carbonate succession. TSTs of all three depositional sequences are associated with shallow-water carbonate, while HSTs are characterized by shallowing cycles composed of lagoonal-supratidal to sabkha lithofacies.