Introduction

Nummulites and other large benthic foraminifera (LBF) such as Assilina, Operculina and Alveolina are abundant in Paleocene to Late Eocene carbonate sediments, particularly in shallow, oligotrophic, Circum-Tethyan platforms (Buxton and Pedley 1989). In the Middle East, nummulitic limestones form hydrocarbon reservoir and thus are potential exploration targets (Beavington-Penney et al. 2005). Larger Foraminifera cover a wide range of platform environments and are influenced by global and local factors such as temperature, water chemistry, nutrient, sea level and plate tectonics (Hallock and Glenn 1986; Hottinger 1997; Scheibner et al. 2005). Romero et al. (2002) proposed a model for the paleoenvironmental distribution of larger foraminifera based on late Middle Eocene deposits on the margin of the South Pyrenean basin. The Early–Middle Eocene was characterized by high atmospheric CO2 content (mean ~ 900 ppmv; Zachos et al. 2001, 2008) and the warmest global temperature of the last 100 million years (Zachos et al. 2001). Payros et al. (2010) suggested that during the Middle Eocene time, the atmospheric buildup of greenhouse (CO2) gases caused an increase in global temperature and eustatic sea-level rise. Increased atmospheric CO2 concentrations will lead to ocean acidification and dissolution of aragonitic coral reefs. Increased sea-surface temperature causes zooxanthellae to be expelled from coral tissue, resulting in a decline in coral reefs and widespread mortality (Payros et al. 2010). For all these reasons, larger foraminiferal Eocene limestones have been regarded as analogs of future shallow water carbonate sedimentary environments if the ongoing global warming proceeds; thus, a reduction in carbonate platform fringing aragonitic coral reefs will occur (Hallock 2005; Pomar and Hallock 2008; Scheibner and Speijer 2008; Payros et al. 2010). Scheibner et al. (2005) also suggested that the larger foraminifer’s turnover (LFT) during the Paleocene–Eocene transition closely correlated with the Paleocene–Eocene thermal maximum (PETM). Larger Eocene foraminifers were abundant when the Paleocene–Eocene hyperthermal global warming event caused severe environmental perturbations and, thus, a decline in fringing coral reef in the tropical shallow seas, because corals were particularly affected by extreme global warming (Schmitz and Pujalte 2007; White and Schiebout 2008; Payros et al. 2010). Therefore coral reefs in platform margin declined in low latitude warm water, and heterozoan calcitic foraminifers, better adapted to environmentally stressed conditions and thus widespread carbonate ramp systems developed (Hallock 2002; Pomar and Hallock 2008; Scheibner and Speijer 2008; Pujalte et al. 2009; Payros et al. 2010). A similar example has been reported from the Mesozoic extreme warming events due to intense volcanic CO2 outgassing (Jenkyns 2003) from the Aptian Oceanic Anoxic Event 1 (AOAE), in which photozoan carbonate characterized by coral and rudistids was replaced by a heterozoan agglutinated orbitolinid foraminifer (Burlar et al. 2008; Folmi and Gainon 2008). In all extreme greenhouse intervals, previous reef-rimmed carbonate platforms were replaced by gently dipping carbonate ramp systems, in which large benthic foraminifera were dominant (Payros et al. 2010).

The goals of this study are: to present the different types of microfacies, to interpret the depositional environment based on facies variations and morphology of foraminifers, and to determine the original carbonate mineralogy and different diagenetic processes.

Geological setting and stratigraphy

The type section of the Ziyarat Formation is located about 36 km south-east of Tehran, close to Tochal Village in Alborz Mountains, Iran (Fig. 1). The Ziyarat Formation was studied at 51°42′E, and 36°5′N. Tochal Village is located east of Anti-Alborz and is tectonically active. Thus, many major faults such as Tehran North, Qarehcheshmeh and North Tochal were recognized.

Fig. 1
figure 1

Location map of the study area in Alborz, Iran (Atlas road of Iran, Bakhtiyari 2011)

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The Ziyarat Formation (Late Paleocene to Middle Eocene) was deposited as a local facies in Tochal Village. This formation has a total thickness of 212.5 m, conformably overlies the Fajan conglomerate and is overlain by greenish tuffaceous siltstone of the Karaj Formation. The age of the Late Paleocene–Middle Eocene was considered for the Ziyarat Formation at the type section, based on different foraminifera (Dellenbach 1964).

Stratigraphic sequences from base to the top of the formation consist of 13 m of finely laminated evaporates (gypsum), 65 m of finely laminated marl with gray to greenish color, 109 m of gray cliff forming thick bedded limestone and marly limestone, and finally 26 m of greenish limy marl (Fig. 2).

Fig. 2
figure 2

Stratigraphic column of the Ziyarat Formation in the study area

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Methods

This study is based on 87 thin sections from samples collected near the type section of the Ziyarat Formation. Petrographic analysis was carried out on stained thin sections, using the method of Dickson (1965) to distinguish ferron and non-ferron calcite and dolomite. The petrographic classification for carbonates is based on Dunham limestone classification (Dunham 1962). Flügel (2004) facies belts were also used. Measurements of nummulitid, alveolinid and operculina size, as well as their test shapes [diameter/thickness (D/T) ratio] show variation along the paleoenvironmental gradient (Beavington-Penney et al. 2006).

Twenty powdered samples (18 micrites and 2 samples from the test of Nummulite) were analyzed by atomic absorption spectrometer for Ca, Mg, Sr, Na, Mn and Fe at the Geology Department of the Shahid Beheshti University, Tehran, Iran, using 0.125 g samples. Because of low permeability of micrites, these samples are more reliable components for geochemical analysis (Asmerom et al. 1991; Adabi 2009). The precision was ± 0.5 % for Ca and Mg and ± 5 ppm for Sr, Na, Mn and Fe (Robinson 1980). Standard samples were used during elemental analysis to monitor the precision. Twelve powdered samples (eight micrites and four samples from test of Nummulite) which had previously been analyzed for major and minor elements were analyzed with a VG STRA Series II for oxygen and carbon isotopes at the Central Science Laboratory, University of Tasmania, Australia. Fifteen milligrams of powdered samples was allowed to react with anhydrous phosphoric acid in reaction tubes under vacuum at 25 °C for 24 h. The CO2 extract from each sample was analyzed for δ18O and δ13C by mass spectrometry. The precision of data were established with duplicate analysis as ± 0.1 ‰ for both δ18O and δ13C, and these values were reported relative to PDB. Selected samples were observed with a cathodoluminescence microscope (Nikon CL, CCL 8200) at the Research Institute of Petroleum Industry (RIPI) in Tehran, Iran to determine different diagenetic sequences.

Facies analysis and depositional environment

Ziyarat carbonates consist of a large variety of skeletal and non-skeletal grains, calcite cements, micrites, and early and late diagenetic dolomites. Skeletal grains are mostly bivalves, crinoids, abundant benthic and some pelagic foraminifera, radiolitidae, red algae and bryozoan fragments. Miliolids, Discocyclina sp., Actinocyclina sp., Astrocyclina sp., Operculina sp., Alveolina sp. and Rotalia sp. are common and Nummulites sp. is the major type of benthic foraminifera in the Ziyarat Formation. Non-skeletal grains consist mainly of ooids, intraclasts and peloids.

Based on lithology, texture, sedimentary characteristics and association of benthic foraminifera, 11 shallow marine facies occur in the Ziyarat Formation. These facies consist of the inner carbonate ramp (including coastal tidal flat), the middle and the outer carbonate ramp facies belts. The characteristics of the inner and middle ramp facies belts are similar to those described in other Eocene carbonate ramps (Bassi 1998, 2005; Rasser 2000; Scheibner et al. 2003; Beavington-Penney and Racey 2004; Cosovic et al. 2004; Beavington-Penney et al. 2005; Rasser et al. 2005; Barattolo et al. 2007; Adabi et al. 2008; Payros et al. 2010; Racey et al. 2001). However, the outer ramp facies in this study are different with the Eocene storm dominated for algal ramp of the western Pyrenees described by Payros et al. (2010). In this study, an absence of calciturbidite deposits, reefal facies, gradual facies changes, widespread tidal flat deposits and abundant heterozoan calcitic foraminifers support that the microfacies in the Ziayart Formation were deposited in a homocline carbonate ramp environment. Due to the great diversity and abundance of larger benthic foraminifera, this carbonate ramp is referred to as a “foraminifera-dominated carbonate ramp system”. These microfacies are described briefly below from the shallowest to the deepest environment.

Inner ramp microfacies

Five microfacies were recognized in this sub-environment, composed mainly of evaporite, dolomicrite, intraclast, ooid and benthic foraminifera.

Evaporate facies

This facies consists of thin to thick bedded evaporite (Figs. 3a, b). Evaporate minerals and their pseudomorphs are common components of shallow marine carbonates. Evaporite has been defined as a rock that was originally precipitated from a saturated surface or near-surface brine in hydrological systems driven by solar evaporation (Warren 2006). The main site of modern marine sulfate precipitation is in the high intertidal and supratidal zones (Tucker 1991), where seawater is brought close to the surface by capillary action or from surface flooding (flood recharge).

Fig. 3
figure 3

a Evaporite facies in outcrop. b Evaporite fabric in thin section,WA. XPL. c Dolomicrite or dolomudstone, Z86.5, XPL. d Ooid intraclast packstone to grainstone, Z85, PPL. e Miliolid wackestone, Z87, PPL. f Alveolina Nummulite packstone, Z108, XPL

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In the Ziyarat Formation (Tochal section), evaporite sequence has a total thickness of 12.5 m. These evaporates are overlain by thick, heavily to weakly weathered yellowish to greenish argillaceous limestone (marl). Thin section studies of a few samples show that the crystals have low relief and weak birefringence, confirming gypsum mineralogy. Gypsum crystals in the form of swallow-tail twin crystals (Spencer and Lowenstein 1990) are present in many thin section samples. In addition, sulfate minerals in the Ziyarat Formation support the gypsum mineralogy, due to convincing structural evidence, such as chicken wire texture, to prove that these evaporates formed in subaerial dominated settings (such as coastal sabkhas). The sedimentary associations such as calcite pseudomorphs after evaporates, the lack of marine organisms (fossils), presence of bird’s eye textures and few scattered fine silt-sized quartz grains within the micritic matrix may suggest that these evaporates formed in high intertidal to supratidal zones in a sabkha type of setting.

Dolomudstone

This microfacies consists mainly of dense, tightly packed, very fine to finely crystalline dolomite, ranging in size from less than 10 to about 16 μm (Fig. 3c). The presence of some traces of original depositional textures such as intraclasts, scattered silt- to sand-sized quartz grains, lamination, fenestral fabric, trace of calcite pseudomorph after evaporates and absence of fossil are the main features of this microfacies. On the basis of the above features, it is considered that this microfacies formed during very early diagenesis, under near-surface, low-energy condition, possibly in supratidal to intertidal sub-environments (Adabi 2009). These features indicate subaerial conditions, possibly in a zone with oscillating water table (Payros et al. 2010), with a subaerial exposure index higher than 60 % (Shinn 1983; Tucker and Wright 1990).

Ooid intraclast packstone to grainstone

The most common allochems in this microfacies include ooids and intraclasts (Fig. 3d). Ooids are commonly sub-spherical to sub-elongate, with mean size of 0.2 mm. Some ooids have undergone micritization, while others are affected by dissolution and therefore moldic porosity (oomold) is created. Intraclasts are generally polymodal in size, ranging from 0.2 to 0.4 mm with mean of 2.5 mm. Intraclasts are often rounded and are internally homogeneous and consist of micrites. The lack of bioclasts, widespread dissolution features and relatively high quartz content indicate proximity to terrestrial source areas and suggest that this facies possibly formed in low- to high-energy environment, above fair-weathered wave base (FWWB) near coastal zone.

Miliolid wackestone

The predominant skeletal grains in this microfacies are miliolids (~ 25 %). Other minor biogenic components include bryozoan fragments and red algae (Fig. 3e). Most foraminifera with imperforate porcelaneous wall (such as miliolids) suggest eutrophic condition, with increasing light level and high nutrient content. The high abundance of miliolids and the presence of a low-diversity foraminiferal association may be indicative of saline to hypersaline shallow marine conditions (Geel 2000). The presence of miliolids within mud matrix shows deposition in low-energy, shallow marine (lagoon) settings (Flügel 2004; Vaziri-Moghaddam et al. 2006; Brandano et al. 2009).

Alveolina Nummulite packstone

This microfacies is composed mainly of large benthic foraminifera, such as nummulitids (20–25 %) with mean size of 1.5 mm as well as porcelaneous alveolinids (10–15 %) (Fig. 3f). Other components include miliolids and green algae. Nummulites in this microfacies have robust to ovate test and thick shelled, indicating shallow low-energy marine environment with high light intensity and sufficient nutrients (Beavington-Penney and Racey 2004; Barattolo et al. 2007). This is supported by the dominance of the micritic matrix and abundance of alveolinids. The inner ramp environment is characterized by the predominance of mud-rich lithology with low-diversity faunal association, indicating a very shallow environment with low to moderate energy.

Middle ramp microfacies

Nummulite packstone

This microfacies is characterized by the dominance of nummulitids (~ 40 %) (Fig. 4a). The other bioclastic components such as Discocyclina sp. and alveolinids are very rare, since alveolinids and Discocyclina thrive in different environments (Hohenegger et al. 1999; Adabi et al. 2008). The Nummulites are well preserved and the lack of abrasion of the Nummulite tests indicates that they were of autochthonous accumulation (Adabi et al. 2008). However, in some nummulitids tests breakage and re-orientation are present due to physical compaction.

Fig. 4
figure 4

a Nummulite packstone, Z107.5, PPL. b Red algal Nummulite packstone, Z105, PPL. c Discocyclina Nummulite wackestone, Z117, XPL. d Nummulite Discocyclina wackestone to packstone, Z118, PPL. e Benthic foraminifera packstone, ZA, PPL. f Radiolar sponge spicule wackestone, Z141, PPL

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Nummulitids thrived in oligotrophic (Langer and Hottinger 2000) to possibly slightly mesotrophic (Halfar et al. 2004) open marine environment, 40–80 m deep (Hottinger 1997; Beavington-Penney and Racey 2004). Micritic mud, chaotic fabric, scarcity of alveolinids and orbitolitids in the matrix and general lack of sedimentary structures suggest nummulitid packstone deposited in a generally low-energy environment, below a fair-weather wave base (Scholle and Ulmer-Scholle 2006), (Adabi et al. 2008; Payros et al. 2010). Therefore, these facies are believed to represent the mid-ramp environment in which nummulitids flourished (Aigner 1985).

Red algal Nummulite packstone

This microfacies is dominated by the Nummulites (~ 32 %) and subordinate red algal fragments (~ 20 %). Other components are scarce Discocyclina sp., bryozoan fragments, echinoid plates and small benthic foraminifers (e.g., rotaliids). Nummulites with both large- and small-sized tests are well preserved, but irregularly fragmented specimens also occur (Fig. 4b). Bioturbation structures are relatively high in this microfacies. Nummulitids was probably characterized by random scatter of nutrients or preferred lower nutrient level, while red algae tend to dominate in mesotrophic conditions (Bassi 2005). Red algae formed in oligophotic environments ~ 60 m deep, possibly near fair-weather wave base (Payros et al. 2010). Thus, the middle ramp was mostly located just below fair-weather wave base, at water depths between 40 and 80 m. This statement is very similar to that made by Barattolo et al. (2007) in Greece and by Bassi (2005) in northern Italy for Late Eocene middle ramp deposits, accumulated below fair-weather wave base.

Discocyclina Nummulite wackestone

The predominant skeletal grains are lenticular nummulitids (~ 25 %) and orthophragminids (mainly Discocyclina sp., Actinocyclina sp. (Fig. 5a), Asterocyclina sp. (Fig. 5b) and Operculina sp. (Fig. 5c) embedded in a mud matrix (Fig. 4c). Other components are echinoids, bryozoan fragments and alveolinids. No preferred particle orientation or imbricated fabric is present. The abundance of micritic matrix and the fine-grained texture suggest a low-energy environment located below the fair-weather wave base.

Fig. 5
figure 5

a Actinecyclina sp., Z68, XPL. b Asterocyclina sp., Z120, PPL. c Operculina sp., Z118, XPL

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Nummulite Discocyclina wackestone to packstone

This microfacies is dominated by fragments of Discocyclina (25–30 %), and Nummulites (20–25 %), (Fig. 4d). Some benthic foraminifera tests show breakage of their marginal cord. Most bioclasts are aligned parallel to the bedding plane. Elongated test shape, thinner test walls and abundance of mud matrix suggest that this microfacies deposited in much deeper oligophotic water, below fair-weather wave base (Pomar et al. 2004), because these organisms, with a mean D/T (e.g., diameter/thickness) ratio of 7.4, prefer decreasing light level and water energy (Rasser 2000; Beavington-Penney 2002). Thus, the sedimentary environment of this microfacies is in the deeper part of the middle ramp (Romero et al. 2002; Corda and Brandano 2003).

To sum up, the biotic assemblage presented mostly by nummulitids and discocyclinids suggests sedimentation in the oligophotic zone of the middle ramp. The middle ramp in this study was mostly located just below the fair-weather wave base, at water depths between 40 and 80 m (Payros et al. 2010). This conclusion is very similar to that made by Bassi (2005) in northern Italy, Barattolo et al. (2007) in Greece and Payros et al. (2010) in the western Pyreness (Urbasa-Andia Formation) for Eocene middle ramp deposits, accumulated below the fair-weather wave base.

Outer ramp microfacies

Benthic foraminifera packstone

The predominant skeletal grains are (~ 40 %) fragments of Nummulites sp., Discocyclina sp., echinoids, brachiopods and thin pelecypod shells, belonging to open marine environment (Fig. 4e). Subordinate biogenic components include crinoid debris and spong spicules. Abraded broken tests of Nummulites sp., Discocyclina sp., echinoids, brachiopods and thin pelecypod shells reflect prolonged hydrodynamic transport (Beavington-Penney and Racey 2004). There is no preferred bioclast orientation; a random, chaotic biofabric is very common. Quartz is generally absent in this facies.

Radiolar sponge spicule wackestone

The main components of this microfacies are sponge spicules (~ 12 %) and radiolaria (~ 10 %). The occurrence of radiolaria indicates that this microfacies belongs to the deepest part of the open marine carbonate ramp settings (Flügel 2004; Ghabeishavi et al. 2010; Adabi et al. 2010) (Fig. 4f). Light-independent and oligophotic organisms such as radiolarian indicate the deepest part of the photic zone, in the outer ramp area between 80 and 130 m (Hottinger 1997; Romero et al. 2002; Beavington-Penney and Racey 2004). The abundance of micritic matrix and the fine-grained texture show that current winnowing was mild suggesting a low-energy environment located below the fair-weather wave base and possibly below storm wave base. The most distinctive feature of the Ziyarat carbonate system was the outer ramp low-energy conditions (Table 1). In this microfacies, there is no evidence of current-induced imbricated fabric, grainstone facies or downslope-migrating sand wave by storm-induced currents described from a Miocene carbonate ramp from southern Spain by Puga-Bernabeu et al. (2010), from the Urbasa-Andia Formation (western Pyrenees) by Payros et al. (2010) and from the Eocene EL Garia Formation from Tunisia by Beavington-Penney et al. (2005). Thus, Ziyarat carbonates cannot be interpreted as a storm-dominated carbonate ramp.

Table 1 Facies of the Ziyarat Formation
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Paleoenvironmental model

Eocene was a time of abundance of miliolid and larger benthic foraminifera (LBF), particularly nummulitids, alveolinids and Discocyclina sp. Nummulites occupied a broad range of open marine environments on both ramp and shelf settings, and was generally absent from more restricted waters (Racey 2001). Smaller lenticular Nummulites occur in shallower, inner ramp/shelf settings, often along with alveolinids, whilst large flat Nummulites with similarly shaped Assilina and Discocyclina sp. occur in relatively deeper water (middle to outer ramp) environments (Beavington-Penney 2002; Vaziri-Moghaddam et al. 2006; Payros et al. 2010). Medium- to large-sized, lenticular to globular shaped Nummulites tend to occupy intermediate environments (Adabi et al. 2008). This broad pattern (with minor modification) is also presented in several studies of ancient ramps (e.g., Sinclair et al. 1998). The summary of key faunal associations on idealized carbonate ramp successions during the Eocene was suggested by Racey (1994). Based on this model, Textulariids, miliolids and Orbitolites occur in the shallowest, while Discocyclina sp. and Assilina occur in the deepest part of the ramp environment. This biotic association is typical of shallow, benthic calcareous communities in modern tropical to sub-tropical marine carbonate environment. Thus, larger foraminifers are valuable tools to construct paleoenvironmental models in the warm shallow marine settings (Geel 2000) and excellent indicators for facies interpretation (Rasser et al. 2005). The presence of larger foraminifera in the carbonate rocks of the Ziyarat Formation in Alborz shows the persistence of an equatorial climate throughout deposition of this formation, similar to other Tethyan carbonate platforms (Buxton and Pedley 1989).

Based on different microfacies associations recognized in the Late Paleocene to Middle Eocene of the Ziyarat Formation, a foraminifera-dominated gently dipping carbonate ramp model is proposed for this formation (Fig. 6). The larger foraminifers along with red algae (for algal deposits) were main carbonate producers in the Eocene shallow marine ramp setting throughout the world (Wilson and Vecsei 2005; Payros et al. 2010). The lack of any reefal development, absence of high-energy grainstone, gradual deepening trend from the shallow platform to the basin, no evidence of resedimentation, (e.g., calciturbidite) and the occurrence of abundant micrite in all microfacies are more compatible with a ramp setting than a shelf (Wright 1986; Tucker et al. 1993). This suggests a low-energy setting, which is more likely to occur on low-gradient slope characteristic of a ramp environment. The Ziyarat Formation in this study is separated into the inner ramp, the middle ramp and the outer ramp according to Burchette and Wright (1992). The inner ramp settings are characterized by alveolinid and miliolids dominated microfacies types, and middle ramp settings are dominated by nummulitids and discocyclinids. The outer ramp settings consist mainly of Radiolaria and spong spicules. In this study, there was no evidence of high-energy oceanic currents and strong storm waves suggested for a few Eocene carbonate ramp settings (e.g., Puga-Bernabeu et al. 2010; Payros et al. 2010). The biotic assemblage and paleolatitude of the Ziyarat Formation clearly suggest a tropical–subtropical depositional setting.

Fig. 6
figure 6

Depositional model of a carbonates ramp platform of the Ziyarat Formation, Alborz Basin, Iran, and position of fair-weather wave base (FWWB); storm wave base (SWB)

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Foraminifera that live in shallow, wave-influenced environments exhibit ‘robust’, ovate tests with thick walls (decreasing D/T values), whilst tests from a facies deposited in much deeper, oligophotic water show increasing D/T ratio (i.e., flatter tests) and thinner test walls, reflecting decreased light levels and water energy at greater depths (Hallock and Glenn 1986; Beavington-Penney 2002). The test shape and wall thickness of the Nummulites and other LBF in the Ziyarat Formation along the paleoenvironmental gradient varied from inner to outer ramp sub-environment. In the inner ramp, the test shape is ovate with thick walls to prevent photo-inhibition of symbiotic algae within the test in bright sunlight and/or test damage in turbulent water. By increasing depth and decreasing sunlight and water energy, the test shape is elongate and flatter and test walls are thinner.

Diagenesis

The diagenetic processes that affected the Ziyarat carbonates were micritization, physical and chemical compaction, tectonic fracturing, cementation and dissolution (porosity development). Cementation and dissolution, which are discussed below, were the main diagenetic processes that affected the original texture, especially overprinting the porosity of this formation (Table 2).

Table 2 Paragenetic sequence of the Ziyarat Formation, Iran
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Cementation

Different generations of sparry calcite cements were recognized in Ziyarat limestone, ranging from meteoric to burial cements. These include: clear syntaxial cements forming around echinoid fragments (Fig. 7a). These are the most common cements. Equant cements occur mainly as interparticle and sometimes within cavities (Fig. 7c). Vein calcite cements mostly occur throughout the limestone sequences as equant or platy fabric (Fig. 7e).

Fig. 7
figure 7

a Clear syntaxial overgrowth in Z103, PPL; b same area as in a under cathodoluminescence. c Equant calcite cement as void filling in Z98, PPL; d same area as in c under cathodoluminescence, indicating burial origin. e Coarse grain vein calcite cement, in Z102, ppl; f same area as in e under cathodoluminescence. Vein calcite cement shows bright yellow to brown luminescence, possibly of meteoric origin, and dark area (red arrow) non-luminescence, indicating burial origin

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Dissolution

Porosity evolution

Dissolution is a destructive diagenetic process which increases porosity and permeability of the rocks and has major control on reservoir development (Moore 1989). Dissolution in carbonates occur in near surface (vadose), phreatic and also burial diagenetic environment. In the Ziyarat Formation, dissolution as leaching is the most important diagenetic factor in the evolution of porosity, particularly in the inner to middle ramp facies. The primary porosity is very rare and the porous intervals exhibit mainly secondary porosity. The dominant porosity in this section is inter and intraparticle, moldic, vuggy and fracture types (Fig. 8). Most of these porosities were filled by later stage calcite cements.

Fig. 8
figure 8

a Interparticle porosity in packstone sample, W1, XPL. b Intraparticle porosity in Asselina sp., Z107.5, XPL. c Intercrystalline porosity in dolomite, Z66.5, XPL. d Moldic porosity that filled with spary calcite, Z85, XPL. e Vuggy to channel porosity in bioclast packstone, Z118, XPL

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Discussion

Cathodoluminescence petrography illustrates that clear syntaxial cements are dull to non-luminescent (Fig. 7b), indicating a burial origin (Tucker and Wright 1990). Under cathodoluminescence, the coarsely crystalline equant calcite cements are mostly of burial origin (Fig. 7d). Vein calcite cements are characterized by both bright luminescence to dull (dark) non-luminescence indicating meteoric and burial origin (Fig. 7f).

The porosity percentages and distributions are variable from minor (< 1 %) in the lower part of the section to common (between 1 and > 10 %) in the middle and upper parts of the sections where Nummulites and other benthic foraminifera are abundant. This variation is facies controlled because it is lowest in outer ramp mud-supported facies. Thus, dissolution in grain-supported facies is a major control on the evolution of porosity in this study. Similarly, Moallemi (2010) showed that in the Jahrum Formation (Eocene in age) in Mond Oil Field in east of Busher (south–west of Iran), porosity is predominant in nummulitic facies as vuggy, intraparticle and moldic (Fig. 9; Moallemi 2010). Dissolution of nummulitic layers is the most important diagenetic event in the Jahrum Formation (Fig. 9). The widespread Nummulite beds with vuggy and moldic porosity, due to dissolution of benthic foraminifera, enhanced reservoir potential of the carbonate sequence in the Jahrum Formation.

Fig. 9
figure 9

a, b Moldic and intraparticle porosity in Nummulite tests in the Jahrum Formation, Gisakan Mountain and Mound #6 Oil Field, in East of Bushehr, Iran respectively. The blue color is due to injection of Epoxy resin, PPL (Moallemi 2010)

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In the El Garia Formation in north-east of Libya, Nummulite beds are partially to completely dissolved and have major effect on porosity evolution and thus development of suitable hydrocarbon reservoirs (Jorry 2004). This may indicate that the original carbonate mineralogy of Nummulites might be aragonite/high-Mg calcite, rather than low-Mg calcite. It is proposed that the original carbonate mineralogy of Nummulite shells might be varied in different environments, and those with metastable mineralogy are excellent for forming hydrocarbon reservoirs.

Bioturbation, boring and micritization, geopetal fabric, breakage and deformation are also present in different microfacies of the Ziyart Formation.

Determination of original carbonate mineralogy

Aragonite is the predominant mineral, along with some high-Mg calcite, forming in modern shallow sub-tropical warm waters (Milliman 1974; James and Clarke 1997). In modern temperate carbonates, high-Mg calcite predominates over low-Mg calcite, and in polar cold-water carbonates, low-Mg calcite is the dominant mineral (Rao 1981, 1996; Nelson 1988; Rao and Adabi 1992; James and Clarke 1997).

Studies of the original carbonate mineralogy during Phanerozoic was debated and discussed by many researchers (e.g., Sandberg 1983; Wilkinson et al. 1985; Wilkinson and Algeo 1989; Hardie 1996; Stanly and Hardie 1998; Dickson 2004). These workers have argued that the mineralogy of ancient carbonates may have been different from that of modern sediments, with calcite being considered the dominant mineral during the Ordovician, Devonian–mid-Carboniferous and Jurassic–Cretaceous to Early/Middle Cenozoic. However, some other researchers suggested that the assumption of change of original carbonate mineralogy through times needs to be re-evaluated (e.g., Nelson 1988; Rao 1991; Morse et al. 1997; Adabi 2004; Adabi and Asadi Mehmandosti 2008).

Determination of Ordovician Gordon Group sub-tropical shallow marine carbonates of Tasmania, Australia (Rao 1990); the Upper Jurassic carbonates (Mozduran Formation) in the Kopet-Dagh Basin in north-east Iran (Adabi and Rao 1991); the Cretaceous limestone of the Illam Formation in the Tange-Rashid area, Izeh, south-west of Iran (Adabi and Asadi Mehmandosti 2008); the Lower Cretaceous carbonates (Fahliyan Formation), south-west Iran (Adabi et al. 2010), based on petrographic and geochemical evidences, indicates that aragonite, not calcite, was the dominant mineral in these warm-water sub-tropical carbonates. Recently, Westphal and Munneck (2003) also concluded that during Ordovician, Jurassic and Cretaceous periods, the original carbonate mineralogy was aragonite, similar to that of warm-water shallow marine carbonates.

In this study, petrographic, elemental and isotopic evidences were used to compare this information with modern aragonitic warm-water and calcitic cool to cold temperate carbonates and originally aragonitic mineralogy of Ordovician sub-tropical carbonates, the calcitic mineralogy of Permian sub-polar cold water of Tasmania, and the Upper Jurassic Mozduran carbonates of Iran to understand original carbonate mineralogy.

Geochemistry

Major and minor elements

Strontium The Sr concentration in recent tropical abiotic aragonite ranges from 8,000 to 10,000 ppm (Milliman 1974), whereas the maximum Sr contents in biotic calcite is 1000 ppm, without any diagenetic alteration (Adabi and Rao 1991; Rao 1996). The Sr contents increase with increase in aragonite content (Adabi and Rao 1991) and increase in water temperature (Morse and Mackenzie 1990). The concentration of Sr in Ziyarat limestone samples ranges from 383 to 3,472 ppm (mean 3,470 ppm) in micrite and from 1,026 to 1,092 ppm (mean 1,059 ppm) in the shell of Nummulites (Fig. 10a). Sr values of micritic samples show moderate diagenetic alteration, due to the replacement of aragonite by calcite during the two stages of diagenetic stabilization (Al-Aasm and Veizer 1986). The Nummulite shells have higher Sr value when compared with modern biotic calcite.

Fig. 10
figure 10

a Sr and Mn variations in Ziyarat limestones, compared with fields of sub-tropical warm-water Ordovician aragonite (Rao 1991), Recent tropical shallow marine aragonite (Milliman 1974) and altered original aragonite mineralogy of the Mozduran Formation (Adabi and Rao 1991). b Sr and Na variations in Ziyarat limestones, compared with fields of recent temperate bulk carbonate (Rao and Adabi 1992; Rao and Ammini 1995), Recent tropical shallow marine aragonite (Milliman 1974), sub-tropical warm-water Ordovician aragonite (Rao 1991) and altered original aragonite mineralogy of the Mozduran Formation. c Sr/Ca versus Mn ratios in Ziyarat limestones. This trend shows that these carbonates were affected by non-marine fluids in an open diagenetic system (Brand and Veizer 1980). d Variation of δ18O versus Mn also confirmed open diagenetic system. e Mn and Sr/Na variations in Ziyarat limestones. Note that the Sr/Na ratios of most samples are >1, indicating the original aragonite mineralogy

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Sodium The Na concentration in recent tropical abiotic aragonite ranges from 1,500 to 2,700 ppm (mean 2,500 ppm; Land and Hoops 1973; Veizer 1983; Rao and Adabi 1992), whereas modern temperate calcitic sediments on have high mean Na content (~ 5,000 ppm). The concentration of Na in carbonate sediments are related to salinity, biological fractionation, kinetics, mineralogy and water depth (Land and Hoops 1973; Morrison and Brand 1986; Rao and Adabi 1992). The concentration of Na in the Ziyarat micrite samples ranges from 513 to 1,553 ppm (mean 911 ppm) and in shell of Nummulites from 572 to 619 ppm (mean 596 ppm). Na values are lower than those of modern warm-water aragonitic and calcitic counterparts, due to moderate diagenetic alteration. The plot of Sr–Na values shows that most limestone samples fall within or close to the warm-water sub-tropical aragonite fields of the Mozduran Formation (Adabi and Rao 1991) and the Gordon Group limestone of Tasmania, Australia (Rao 1991; Fig. 10b).

Manganese The concentration of Mn in Ziyarat limestone ranges from 430 to 1,475 ppm (mean 998 ppm) in micrites and 568 to 724 ppm (mean 646 ppm) in Nummulite tests. In modern warm-water aragonite, Mn and Fe concentrations are < 20 ppm (Milliman 1974), and in modern temperate shallow marine calcite sediments mean Mn is about 150 ppm (Rao and Adabi 1992). The higher Mn contents in both micrite samples and Nummulite shells, when compared with modern tropical and temperate carbonates, are due to diagenetic alteration by non-marine fluids. The Sr versus Mn values show that most samples fall close to the aragonite field of Mozduran and warm-water sub-tropical Gordon Group (Fig. 10a). The appreciable gain of Mn and loss of Sr and Na in these samples are due to high water/rock interaction in an open diagenetic system.

The bivariate plot of Sr/Ca versus Mn shows that the limestone samples have been stabilized by fluids in an open diagenetic system (Brand and Veizer 1980; Fig. 10c). Variation of δ18O versus Mn (Fig. 10d) also confirmed the open diagenetic system.

Sr/Na ratio Modern and ancient tropical carbonates are differentiated from their non-tropical counterparts by their Sr/Na ratio and Mn contents (Rao 1991; Winefield et al. 1996; Adabi and Asadi Mehmandosti 2008; Adabi et al. 2010). Modern tropical aragonitic sediments have low Mn and high Sr/Na ratio from 3 to ~ 5 (mean 4); in contrast, modern temperate bulk carbonates have high Mn and low Sr/Na ratios < 1 to ~ 1 (Adabi and Rao 1991; Rao 1996, Fig. 10e). Sub-polar Permian cold-water fossils with calcite mineralogy and the Permian sub-polar bulk cold-water calcitic limestones have also an Sr/Na ratio of ~ 1 (Rao 1991, 1996). In the Ziyarat Formation, the mean Sr/Na ratio in both micritic samples and Nummulite shells is ~ 1.5 (Fig. 10e). This value is within the range of warm-water aragonitic carbonates. All the above evidences along with other petrographic and diagenetic features such as dissolution and evolution of porosity particularly in grain-supported facies, and presence of ooids, dolomites and evaporates support the interpretation that the original carbonate mineralogy of Ziyarat limestone was aragonite.

Oxygen and carbon isotopes

δ18O and δ13C are valuable tools for determination of paleotemperature, diagenetic trend, distinction between tropical, temperate and polar carbonates and recognition of the stratigraphic boundary between the two formations (e.g., Marshall 1992). The δ18O values of the micritic samples of the Ziyarat Formation vary between −5.76 and –9.71 ‰ PDB (mean –7.73 ‰ PDB), and Nummulite shells ranges between −7.46 and −7.92 ‰ PDB (mean −7.69 ‰ PDB). The δ13C values vary between +1.2 and −1.58 ‰ PDB (mean −1.39 ‰ PDB) in micrite samples and between −0.12 and +0.1 ‰ PDB (mean −0.11 ‰ PDB) in Nummulite shells. In Fig. 11, the isotope values of limestone samples have been compared with different isotopic carbonate fields, including recent warm shallow marine bulk carbonates (Milliman 1974), recent benthic forams of the Great Barrier reefs (Rao 1996), altered aragonitic carbonates of the Mozduran Formation (Adabi and Rao 1991), recent bulk temperate carbonates of Tasmania and New Zealand (Rao and Nelson 1992), altered micritic and benthic forams of Eocene deposits of the Tale-Zang Formation in Zagros Basin, south-west of Iran (Zohdi 2007) and altered aragonitic bulk carbonates of Gordon Limestone of Tasmania, Australia (Rao 1991). Most isotope data are plotted close or within isotopic field of altered aragonitic bulk carbonates of Gordon Limestone, probably due to similar original carbonate mineralogy and diagenetic system.

Fig. 11
figure 11

Comparison of δ18O and δ13C values of Ziyarat limestone samples with fields of recent calcitic temperate bulk carbonates (Rao and Adabi 1992), Recent warm shallow marine bulk carbonates (Milliman 1974), Eocene carbonate deposit of the Taleh-Zang Formation, Iran (Zohdi 2007), aragonitic Gordon Limestone of Tasmania, Australia (Rao 1991), aragonitic carbonates of the Mozduran Formation of Iran (Adabi and Rao 1991) and benthic forams of the Great Barrier reefs of Australia (Rao 1996)

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δ18O versus δ13C values from Ziyarat limestones indicate that diagenetic alteration occurred mainly in a burial diagenetic environment. The least-altered carbonate sample in this study, with a δ18O value of −5.76 ‰ PDB, was used to calculate paleotemperature, using the equation of Anderson and Arthur (1983) and δ18O value for Eocene seawater −0.85 ‰ SMOW)Veizer et al. 1999). This calculation gives an early shallow burial fluid temperature of about 39 °C.

Conclusions

  1. 1.

    The Ziyarat Formation is a shallow warm-water sequence of the Late Paleocene to Middle Eocene in age, overlies the Fajan conglomerate and is overlain by tuffaceous siltstone of the Karj Formation.

  2. 2.

    In the Ziyarat Formation, 11 microfacies were recognized from the distal to proximal part of the platform. of the 11 microfacies, 5 belong to the inner ramp, 4 to the middle ramp and 2 are located in the outer ramp settings. The lack of evidence of resedimentation, e.g., calciturbidite related to steep slope, absence of reefal facies and widespread tidal flat deposits, along with abundance of large benthic foraminifera indicate that the Ziyarat Formation was deposited in a homocline carbonate ramp environment.

  3. 3.

    The evaporite facies, dolomicrite, ooid intraclast packstone to grainstone, miliolid wackestone and Alveolina Nummulite packstone belong to the inner ramp, euphotic, sub-environment; the middle ramp microfacies, indicating an oligophotic zone, is composed of Nummulite packstone, red algae Nummulite packstone, Discocyclina Nummulite wackestone and Nummulite discocyclina wackestone to packstone; and outer ramp microfacies, showing the lower limit of the photic zone, consist of benthic foraminifera packstone and radiolar sponge spicule wackestone. Nummulites in the Ziyarat Formation show variation in test shape, along the paleoenvironmental gradient. Nummulites from the inner ramp have robust ovate shape with thick walls, while with increasing water depth and decreasing light levels, the test shape becomes flatter and elongate.

  4. 4.

    Bivariate plots of minor elements such as Sr/Na versus Mn (~ 1.5), high Sr content in micritic facies (mean 3,470 ppm), along with petrographic studies (such as dissolution of nummulitic beds, leading to widespread moldic porosity) indicate that the original carbonate mineralogy was dominantly aragonite in Ziyarat limestone. This is in contrast with Payros et al.’s (2010) statement that increased atmospheric CO2 concentrations lead to ocean acidification and dissolution of aragonitic coral reefs. We believe that the decline of coral reefs in the Eocene is mainly due to hyperthermal global warming event (not acidification of water), as corals are vulnerable to thermal stress, leading to a rise of heterozoan calcitic larger foraminifers (Hallock 2005). Larger foraminifera can tolerate high seawater temperatures and mesotrophic conditions (Hallock 2005; Pomar and Hallock 2008; Scheibner and Speijer 2008).

  5. 5.

    Variations of Sr/Ca and also δ18O values versus Mn suggest that diagenetic alteration occurred in an open system with high water/rock interaction.

  6. 6.

    Variations of δ18O and δ13C values and CL studies of carbonate samples (dull to dark and typical yellow to orange luminescence) suggest that Ziyarat limestones were stabilized both in burial and meteoric diagenetic environment.

  7. 7.

    Cementation and dissolution were the main diagenetic processes that affected the original texture, and especially overprinted the porosity of this formation. The porosity percentages and distributions are facies controlled, because they are lowest in the outer ramp mud-supported facies. The dominant porosity in this study is inter and intraparticle, moldic, vuggy and fracture types. Porosity is common in the middle and upper parts of the sections where Nummulites and other benthic foraminifera are abundant. The widespread moldic porosity in Nummulite beds enhanced reservoir potential of carbonate sequence in both Jahrum (Iran) and the El Garia (Libya) formations. This may indicate that the original carbonate mineralogy of Nummulites might be aragonite, rather than low-Mg calcite. Thus, it is proposed that the original carbonate mineralogy of nummulitid shells might be varied in different environments, and those with metastable mineralogy are excellent for forming hydrocarbon reservoirs.

  8. 8.

    A temperature calculation based on the heaviest oxygen isotope value of the least-altered sample indicates that the early shallow burial diagenetic temperature was around 39 °C.