The Qom Formation deposits, located at 12 km southeast of Salafchegan (N: 34° 21′ 26″ and E: 50° 32′ 14″), have a thickness of 110 m. The formation includes thin, and medium-to-thick bedded limestone, as well as shale, overlying the Lower Red Formation (early Oligocene) above an erosional unconformity. Its upper boundary is covered by alluvium sediments. Biostratigraphic distributions of benthic Foraminifera were used to determine the age and paleoenvironmental conditions of the Qom Formation. Two assemblage zones were recognized. Assemblage zones 1 and 2 were indicative of a Rupelian–Chattian age. The inner shelf (restricted and semi-restricted lagoons) and middle shelf (open marine) settings were formed on an open shelf platform. Euphotic conditions were dominant during the early and late Rupelian in the studied area. During the middle Rupelian, photic conditions were variable between euphotic and mesophotic to oligophotic. During the early and middle Chattian, photic conditions varied between oligophotic, mesophotic to oligophotic, and euphotic. During the late Chattian, mesophotic-oligophotic conditions were dominant. Additionally, a high level of salinity (40–50 and > 50 psu) was present in the studied area during the early Rupelian. The level of salinity varied from normal (30–40 psu) to hypersaline (40–50 psu) during the late Rupelian. Salinity during early and late Chattian subages was normal (30–40 psu). However, hypersaline (40–50 and > 50 psu) and normal (30–40 psu) conditions were present in the middle Chattian. Eutrophic to mesotrophic-oligotrophic conditions were found in the early Rupelian age. However, mesotrophy-oligotrophy dominated during the middle and late Rupelian and Chattian ages. In addition, the paleo-water depth of the Qom Sea fluctuated from < 10 m to > 20 m. Foralgal and foramol associations are dominant in the studied area during the Rupelian-Chattian ages. The general depositional environment of the Qom Formation is associated with seagrass meadows.
Introduction
Larger benthic Foraminifera were abundant at the margins of the Tethys Ocean during the Palaeogene Period (Yordanova and Hohenegger, 2007). Many researchers have used larger benthic Foraminifera such as miogypsinids, lepidocyclinids, and nummulitids of the Oligocene for biostratigraphic correlations in different areas of the Tethys Ocean (Cahuzac and Poignant, 1997; Verrubbi and Schiavinotto, 2005; Hakimzadeh and Seyrafian, 2008; Sadeghi et al., 2009; Özcan et al., 2009; Yazdi Moghadam, 2011; Mohammadi et al., 2015; Ferràndez-Cañadell and Bover-Arnal, 2017). The Western Tethys region was defined on the distribution of larger benthic Foraminifera of the Palaeogene Period (BouDagher-Fadel and Price, 2014), and today contains the Pyrenean mountains (or West Africa), the Mediterranean, the Middle East (including Iranian plateau), and Tibet. BouDagher–Fadel and Price (2014) suggested that the Western Tethys was arid/subtropical during the Palaeogene Period. The Qom Formation was deposited during the Oligocene–Miocene age, in the Iranian plateau (Aghanabati, 2006; Reuter et al., 2009b). Schuster and Wielandt (1999), Yazdi et al. (2012), and Rahiminejad et al. (2017) studied the paleoecology and paleoenvironments of the coral assemblage of the Qom Formation. Zágoršek et al. (2017) showed that bryozoan faunas of the Qom Formation lived in tropical conditions. Paleoecological studies were performed on gastropod and ostracod assemblages of the Qom Formation (Hasani and Vaziri, 2011; Hassani and Hosseinipour, 2018). Nouradini et al. (2015, 2017, 2019) studied the paleoecology of Foraminifera belonging to the Qom Formation. In addition, many researchers performed studies (biostratigraphy and sedimentology) on the Qom Formation (Mohammadi et al., 2011; Behforouzi and Safari, 2011; Seddighi et al., 2012; Mohammadi et al., 2019). The interpretation of paleoenvironmental conditions during deposition of the Qom Formation is absent in previous studies. These studies on the relationship between biostratigraphy and paleoenvironmental are very important in the Qom basin (north of Tethys). In fact, the study of the changes in the paleoenvironmental parameters during this geological age has been considered less in this basin. The main objectives of this paper are (1) to investigate the biostratigraphy of the Qom Formation in the Neyzar area based on its benthic Foraminifera and (2) to interpret paleoecological and paleoenviromental conditions for the Oligocene–Miocene deposits, based on the distribution and taxonomic composition of the benthic Foraminifera, and (3) to improve our understanding of changes in paleoenvironments of the Qom Back-arc Basin during the Oligocene.
Geological setting
The Qom Formation is located 12 km south of East Salafchegan in the Neyzar area (N: 34° 22′ 10″, E: 50° 33′ 25″; Figure 1). The Qom depositional basin was a part of the Tethyan Seaway (Reuter et al., 2009a). Because of facies variations in the Qom basin, the type section of the Qom Formation was not designated by researchers. The Qom Formation was investigated by Loftus in 1855 (Loftus, 1855), and thereafter, by researchers such as Abich (1858), Tietze (1875), and Stahl (1911). The Qom Formation commonly includes thick successions of marine marls, limestone, gypsum, and siliciclastic rocks (Aghanabati, 2006; Reuter et al., 2009a). The thickness of this formation is 110 meters. The boundary between the Lower Red and Qom formations is an erosional unconformity. In the studied area, alluvium covers the Qom Formation. In the studied area, this formation was composed of thin, medium, and thick-bedded carbonate sediments and shale. The lower 45 m of the Qom Formation consists of thin and medium-bedded limestone that alternates with shale, and the upper 65 m is limestone.
Material and methods
A total of 101 rock samples were collected for biostratigraphy and paleoenvironmental analysis. Identification of Foraminifera at the genus and species levels was conducted, following studies by Adams and Bourgeois (1967), Adams (1969), Rögl and Brandstätter (1993), Loeblich and Tappan (1988), and Ferràndez-Cañadell and Bover-Arnal (2017). The distribution of larger benthic Foraminifera is uniform in the Western Tethyan region (Less and Özcan, 2012), and they were used by Cahuzac and Poignant (1997) to define the Oligocene–Miocene biozone. The benthic Foraminifera of the Qom Formation (central Iran) are very similar to those of the Asmari Formation (southwest of Iran) (Stöcklin, 1952; Bozorgnia, 1966; Kashfi, 1988). Adams and Bourgeois (1967) identified three assemblage zones and two sub-assemblage zones in the south-west of Iran (Khuzestan and Lurestan areas, Figure 2). This biozonation was further studied in association with strontium isotopes, which resulted in identifying six assemblage zones and one indeterminate zone in the Asmari Formation (van Buchem et al., 2010) (Figure 2). Biozonations of the Qom Formation in the studied area were defined by Adams and Bourgeois (1967), Cahuzac and Poignant (1997), and van Buchem et al. (2010).
Microfacies analysis and paleoenvironmental conditions were interpreted according to the studies of Wilson and Evans (2002), Romero et al. (2002), Brandano et al. (2009), Flügel (2010), and Pomar et al. (2014). In this study, we use benthic Foraminifera to interpret photic zones, nutrient condition and salinity. Different parameters such as light, salinity, nutrient supply, hydrodynamic energy, depth, and substrate nature are effective for the distribution of benthic organisms such as benthic Foraminifera, coral, coralline red algae (Hallock and Schlager, 1986; Carannante et al., 1988; Mutti and Hallock, 2003; Pomar et al., 2004; Wilson and Vecsei, 2005). Light intensity reduces with increasing depth (Hallock, 1987). Pomar (2001) defined four photic zones: euphotic, mesophotic, oligophotic, and aphotic. Nutrients such as phosphorus, nitrogen, iron, and silicon also control the distribution of the Foraminifera (Brasier, 1975; Flügel, 2010). In tropical areas, nutrient conditions are classified as oligotrophy, mesotrophy, eutrophy, and hypertrophy (Mutti and Hallock, 2003). Seawater salinity mainly results from concentrations of sodium, magnesium, calcium, potassium, and strontium (Flügel, 2010). Mossadegh et al. (2009) considered three salinity ranges (40–50, 40–50, > 50 psu) for the Asmari Formation.
This study uses foraminiferal test shape as an indicator of water depth. Hallock and Hansen (1979), Hallock and Glenn (1986), Hallock (1999), Mateu-Vicens et al. (2009) suggested that test shape (thickness and diameter) of Amphistegina vary with increasing water depth. The Thickness-to-Diameter ratio (T/D) of the Amphistegina tests have been used for interpreting water depth (Mateu-Vicens et al., 2009). Mateu-Vicens et al. (2009) measured the T/D ratio in axial and subaxial sections in thin-sections and the trend of morphological changes was calculated according to the following equation Zom = 2.046 T/D – 2.293 (in oligo-mesotrophic conditions). Yet Mateu-Vicens et al. (2009) drew a diagram for determination of the depth based on the morphological changes (genus Amphistegina). Amphistegina is found abundantly in the studied area, so bathymetric interpretations were performed based on the T/D ratios. Flügel (2010) considered two groups of grain associations. These grain associations were divided into photozoan (chlorozoan, and choralgal) and heterozoan (foramol, bryomol, rhodalgal, and foralgal). Grain associations of Foraminifera of the Qom Formation were described based on the study by Flügel (2010). All pictures belong to the studied area is drawn by CorelDraw Graphics Suite X6 (v16.4.0.1280 SP4 software, Corel Company).
Figure 1.
Maps of the area. A, Road map of the area under study in the southeast of Salafchegan. B, Geological map of the area under study at the Neyzar area, southeast Salafchegan (modified from Ghalamghash and Babakhani, 1996).
Results
Foraminiferal assemblage zones
The assemblage zones in the studied area were identified according to the studies by van Buchem et al. (2010). Two assemblages were considered in the Qom Formation (Figure 3).
Assemblage Zone 1.—This assemblage zone is up to 48 meters in thickness (the base to 48 meters of the studied section). In this assemblage zone, the most important and common Foraminifera are listed below (Figures 3, 4, and 5).
Dominant species in the Neyzar area are Neorotalia sp., textularids, miliolids, Planorbulina sp., Pyrgo sp., Elphidium sp.1, Quinqueloculina cf. seminulum, Amphistegina bohdanowiczi, Amphistegina mammilla, Amphistegina sp. (Figure 4C), Heterostegina sp., Eulepidina sp., Heterostegina assilinoides (Figure 4E), Operculina sp., Lepidocyclina sp. (Figure 4H), and Operculina complanata (Figure 4D). Species only observed in this assemblage zone are Peneroplis sp., Quinqueloculina sp., Dendritina rangi, Austrotrilina sp., Valvulina sp., Halkyardia minima, Triloculina trigonula, Quinqueloculina cf. buchiana, Neorotalia viennoti (Figure 4B), Archaias sp., and index species: Nummulites sp. and Nummulites vascus (Figure 4A).
Van Buchem et al. (2010) and Mohammadi and Ameri (2015) indicated that Nummulites spp. became extinct at the end of the Rupelian in the Asmari and Qom formations. The Assemblage Zone 1 in the studied area is correlated with the Nummulites vascus-Nummulites fichteli, SBZ 21, and SBZ 22A assemblage zones identified by Van Buchem et al. (2010) and Cahuzac and Poignant (1997), respectively. The Assemblage Zone 1, therefore, has a Rupelian age.
Assemblage Zone 2.—This assemblage zone is identified as ranging from 48 to 110 m of the studied section and is composed of the taxa listed below (Figures 3, 4, and 5).
Dominant species in the Neyzar area are Neorotalia sp., textularids, miliolids, Planorbulina sp., Pyrgo sp., Elphidium sp.1, Quinqueloculina cf. seminulum, Amphistegina bohdanowiczi, Amphistegina mammilla, Amphistegina sp. (Figure 4C), Heterostegina sp., Operculina sp., Lepidocyclina sp. (Figure 4H), Eulepidina sp., Heterostegina assilinoides (Figure 4E), and Operculina complanata (Figure 4D). Species observed only in this assemblage zone are Ditrupa sp. (Figure 4G), Nephrolepidina marginata, Nephrolepidina tournoueri (Figure 4F), Amphistegina lessonii, Valvulina sp.1, Pyrgo cf. truncata, Pyrgo cf. subsphaerica, Triloculina sp., Miogypsinoides sp., Nephrolepidina sp., Planorbulinella cf. larvata, and Tayamaia cf. marianensis.
This assemblage zone includes benthic Foraminifera that is equivalent to the Lepidocyclina-Operculina-Ditrupa, SBZ 22 B and SBZ 23 assemblage zones of van Buchem et al. (2010) and Cahuzac and Poignant (1997), respectively. This assemblage zone is indicative of the Rupelian–Chattian age. The position of assemblage zone 2 in above of the assemblage 1 indicates that this assemblage zone 2 was formed during the Chattian age. In summary, the Qom Formation in the studied area can be considered to be Rupelian–Chattian age.
Microfacies characterization
Seven different microfacies (MF) of carbonate rocks, namely MF 1 to MF 7, are recognized in the studied section of the Qom Formation at the Neyzar area, based on the petrography, sedimentological features, and relative abundance of benthic Foraminifera (Figure 6). The stratigraphic occurrences are shown in Figure 5.
Figure 4.
Selected Foraminifera of the Qom Formation in the Neyzar section (southeast Salafchegan). A, Nummulites vascus; B, Neorotalia viennoti; C, Amphistegina sp.; D, Operculina complanata; E, Hetrostegina assilinoides; F, Nephrolepidina tournoueri; G, Ditrupa sp.; H, Lepidocyclina sp.
Figure 5.
The position of microfacies, their placement in zones of light and salinity ranges, and limits of their exposure during eutrophy to oligotrophy during deposition of the Qom Formation in the Neyzar area, SE Salafchegan.
MF 1: The major components of this gravelly/sandy bioclastic packstone-grainstone are miliolids, gastropods, bryozoans, and siliciclastics (fine quartz grains and glauconite) (Figure 6A and Table 1). MF 2: This bioclastic imperforate Foraminiferal wackestone-packstone consists of imperforate Foraminifera (miliolids and Peneroplis), coral, and bryozoan fragments (Figure 6B and Table 1). MF 3: Imperforate Foraminifera (miliolids and Peneroplis) and perforate Foraminifera (Neorotalia, Nummulites, Heterostegina, Operculina, and Lepidocyclina) are the main components of this bioclastic perforate and imperforate foraminiferal packstone (Figure 6C and Table 1). MF 4: This coral boundstone is mainly composed of coral (Figure 6D and Table 1). MF 5: This coral corallinacea packstone is composed of corallinacean algae and coral debris (Figure 6E and Table 1). MF 6: The abundant components of this corallinacean perforate foraminiferal packstone-grainstone are corallinacean algae and perforate Foraminifera (Neorotalia, Nummulites, Heterostegina, Operculina, and Lepidocyclina) (Figure 6F and Table 1). MF 7: This bioclastic perforate foraminiferal packstone-grainstone contains major components such as large, flat forms of Lepidocyclina, Operculina, Heterostegina, and Amphistegina (Figure 6G and Table 1).
Figure 6.
Microfacies types of the Qom Formation. A, Gravelly/Sandy bioclastic packstone-grainstone, Q: quartz, B: Bioclast, M: Miliolids; B, bioclastic imperforate Foraminiferal wackestone-packstone, M: Miliolids; C, bioclastic perforate imperforate Foraminiferal packstone, M: Miliolids, Ne: Neorotalia; D, Coral boundstone, C: Coral; E, coral corallinacean packstone, C: Coral, Co: corallinacean algae; F, Corallinacea perforate Foraminiferal packstone-grainstone, Co: corallinacean algae, H: Heterostegina, N: Nephrolepidina; G, Bioclastic perforate Foraminiferal packstone-grainstone, L: Lepidocyclina, O: Operculina.
Table 1.
Description and depositional environments of microfacies of the Qom Formation in the studied area.
Amphistegina morphometry
One Hundred-one thin sections were chosen in the studied area. The test thickness (T) and diameter (D) of twenty-two individuals of Amphistegina from the thin-sections were measured for determinations of water depth. Twelve individuals were obtained from the lower section (from the base to 48 meters, Rupelian stage). In addition, ten individuals were attained from the upper section (48 to 110 meters, Chattian stage, Table 2). The T/D ratio in the Rupelian stage varies between 0.42 to 0.62 and in the Chattian stage fluctuates between 0.37 to 0.65 (Table 2). In the studied area, the oligo-mesotrophic conditions are dominant. We used the results and diagram of Mateu-Vicens et al. (2009) for the determination of the paleowater depth. This diagram shows the Amphistegina test-shape distribution along with the depth under oligo-mesotrophic conditions during the Rupelian and Chattian stages (Table 2 and Figure 7).
Grain associations
Two grain associations were identified in the Qom Formation of the studied area. These grain associations belong to the heterozoan association as shown below.
Foramol.—This association is composed of gastropods, bryozoans, coral, imperforate Foraminifera (miliolids and Peneroplis), and perforate Foraminifera (Neorotalia, Nummulites, Heterostegina, Operculina, and Lepidocyclina) (Figures 6A, B, C, G). Microfacies (MF) 1, 2, 3, and 7 belong to this association.
Foralgal.—This grain association consists of larger benthic Foraminifera, red algae and coral (Figures 6D, E, F). The MFs 4, 5, and 6 are abundant in this association.
Discussion
Palaeoenvironmental conditions
Benthic Foraminifera, especially larger benthic Foraminifera, are sensitive to environmental conditions (Toler and Hallock, 1998; Mutti and Hallock, 2003; Pomar et al., 2004; Wilson and Vecsei, 2005). Flügel (2010) related microfacies to particular environmental conditions, so the interpretation of microfacies in the studied area was used for the reconstruction of paleo-environments.
Table 2.
Amphistegina T/D measurements in the area under study.
Figure 7.
Diagram for the determination of depth under oligo-mesotrophic conditions (from Mateu-Vicens et al., 2009), with plotted T/D ratios of individuals of Amphistegina. A, Rupelian stage; B, Chattian stage.
Depositional environments.—The lagoonal environment at the margin of the carbonate platform (from open marine to beach) was divided into two sub-environments, restricted and semi-restricted lagoons, based on connection with the open marine environment (Kaplin, 1982; Kjerfve, 1994; Flügel, 2010). The abundance of imperforate Foraminifera (e.g. Peneroplis) is indicative of a restricted lagoon environment (Geel, 2000; Romero et al., 2002). Characteristics such as the abundance of imperforate Foraminifera (e.g. Peneroplis and miliolids) in the gravelly/sandy bioclastic packstone-grainstone (MF 1) and bioclastic imperforate Foraminiferal wackestone-packstone (MF 2) are indicative of a restricted lagoon environment (Figure 8). Imperforate and perforate Foraminifera are abundant in a semirestricted lagoon environment with a seagrass substrate (Romero et al., 2002; Beavington-Penney et al., 2006; Afzal et al., 2011; Nebelsick et al., 2013). Bioclastic perforate and imperforate Foraminiferal packstone (MF 3) sediment was deposited in semi-restricted lagoon environment with a seagrass substrate (Figure 8). Coral boundstone (MF 4) in the studied area consisted of a series of small patch reefs that can be observed today in lagoonal environments (Beresi et al., 2017) (Figure 8). The presence of the large benthic Foraminifera (Neorotalia, Nummulites, Heterostegina, Operculina, and Lepidocyclina), corallinaceaen algae, and coral indicates a middle shelf environment (Brandano et al., 2009, 2012, 2016; Quaranta et al., 2012; Sarkar, 2017; Pomar et al., 2017). The MFs 5, 6, and 7 (containing perforate Foraminifera, corallinaceae algae, and coral) were deposited on the middle shelf (Figure 8).
Figure 8.
Sedimentary and paleoenvironmental profile of the Qom Formation and abundance of benthic organisms (benthic Foraminifera and coralline red algae) based on paleoenvironmental conditions.
Salinity conditions.—The presence of siliciclastics (e.g. fine quartz grains), miliolids, and fragments of gastropods suggest a restricted lagoon environment with a high level of salinity (Romero et al., 2002; Wilson and Evans, 2002). Imperforate Foraminifera (especially Peneroplidae and miliolids) are abundant in hypersaline environments (> 50 psu) (Mossadegh et al., 2009; Brandano et al., 2009; Flügel, 2010). The presence of perforate Foraminifera (Operculina and Nummulites) and imperforate Foraminifera (miliolids and Alveolina) reflects a higher level of water salinity in a semi-restricted lagoon environment (40–50 psu) (Mossadegh et al., 2009). Coral is abundant in normal water salinity (30–40 psu) (Mossadegh et al., 2009). The MFs 1 and 2 (containing imperforate Foraminifera such as Peneroplis and miliolids) were formed in a hypersaline environment (> 50 psu). The abundance of imperforate Foraminifera (miliolids, Peneroplis, and Archaias) and perforate Foraminifera (Neorotalia, Nummulites, Heterostegina, Operculina, and Lepidocyclina) indicates an environment with higher salinity (40–50 psu). The MF 4 (containing coral patch reefs) was deposited in an environment with normal salinity (30–40 psu). Salinity varied in the lagoon paleoenvironment from normal (30–40 psu) to hypersaline (> 50 psu). The occurrence of coral, corallinacean algae, Neorotalia, Nummulites, and Amphistegina indicates normal water salinity (30–40 psu) (Hallock and Glenn, 1986; Mossadegh et al., 2009; Flügel, 2010). The condition of normal water salinity (30–40 psu) was dominant in the open marine environment (MFs 5, 6, and 7) of the studied area.
Photic conditions.—Imperforate Foraminifera (e.g. symbiont-bearing imperforate Foraminifera, and miliolids), perforate Foraminifera (e.g. Amphistegina), dasycladaceaen algae, and coral are dominant in the euphotic zone (Hottinger, 1983; Schuster and Wielandt, 1999; Geel, 2000; Pomar, 2001; Romero et al., 2002; Beavington-Penney and Racey, 2004; Pomar et al., 2014). Euphotic conditions were dominant in a lagoonal environment of the studied area (MFs 1, 2, 3, and 4). Mesophotic to oligophotic conditions are associated with abundant larger symbiont-bearing Foraminifera, red algae, and coral debris (Pomar et al., 2014; Pomar et al., 2017). Larger nummulitids live under oligophotic conditions (Pomar et al., 2014). The greater abundance of larger symbiont-bearing Foraminifera, red algae, and coral in MFs 5 and 6 suggests mesophotic to oligophotic conditions in the studied area. The bioclastic perforate Foraminiferal packstone-grainstone (MF 7) is composed of larger benthic Foraminifera and thus was probably deposited under oligophotic conditions.
Trophic conditions.—The absence of light-dependent organisms, such as well-developed reefs, symbiont-bearing hyaline Foraminifera (orthophragminids; lepidocyclinids; and nummulitids) and red algae indicate a eutrophic condition (Renema, 2002; Beavington-Penney and Racey, 2004). Increasing nutrients create a eutrophic condition in which there are abundant bivalve, echinoid, bryozoan, and opportunistic Foraminiferal species (e.g. miliolids) (Mutti and Hallock, 2003; Beavington-Penney and Racey, 2004). The presence of imperforate Foraminifera and opportunistic Foraminiferal species (e.g. miliolids) in MFs 1 and 2 indicates deposition under eutrophic conditions. Imperforate and perforate Foraminifera and red algae associations are abundant in oligotrophic to mesotrophic conditions (Langer and Hottinger, 2000; Halfar et al., 2004; Payros et al., 2010). The abundance of coral indicates oligotrophic to mesotrophic conditions (Hottinger, 2000; Halfar et al., 2004). The great abundance of imperforate and perforate Foraminifera and corals suggest oligotrophic to mesotrophic conditions for MFs 3 and 4. The abundance of coral indicates mesotrophic to oligotrophic conditions (Hottinger, 2000; Halfar et al., 2004). The corallinacean algae are abundant under mesotrophic conditions (Payros et al., 2010). Larger benthic Foraminifera live in oligotrophic environments while nutrient-deficiency conditions limit the abundance of opportunistic Foraminifera (Mutti and Hallock, 2003; Whidden and Jones, 2012). The occurrence of the larger benthic Foraminifera, coral, and corallinaceae algae in MFs 5 and 6 suggests mesotrophic to oligotrophic conditions. Bioclastic perforate foraminiferal packstone-grainstone (MF 7) was formed under oligotrophic conditions.
Substrate.—Abundant imperforate Foraminifera (especially Archaias, Peneroplis, and miliolids) and red (corallinacean) algae can be observed in seagrass meadows today (Beavington-Penney and Racey, 2004; Pomar et al., 2014). In addition, coral, as well as corallinaceae algae, are dominant in an environment with a seagrass-bearing substrate (Pomar et al., 2017). The larger benthic Foraminifera such as Operculina, Neorotalia, and Nummulites live on a sandy seafloor (Pomar et al., 2014). Thus, the microfacies MFs 1, 2, 3, 4, 5, and 6 (containing imperforate Foraminifera, coral, and corallinacean algae) were deposited in an environment with a seagrass-bearing substrate. The abundance of the larger Foraminifera in MF 7 suggests an environment including a sandy seafloor.
Paleoenvironmental changes in the Neyzar area
Microfacies can be used for interpretation of paleoenvironmental conditions. In the studied area, the environmental parameters such as light, salinity, nutrient supply, and depth were interpreted for the Rupelian and Chattian stages (Figures 5, 7 and 8).
Rupelian stage.—During the early Rupelian, a lagoonal paleoenvironment (MFs 1, 2, 3, and 4) developed in the studied area. In the late Rupelian, paleoenvironments varied between those of lagoon (MFs 1, 2, 3, and 4) and open marine (MFs 5 and 6, Figure 5). The Rupelian stage can be divided into three substages in terms of the variation of photic conditions (Figure 5), as follows. Sediments of the lower section (0–26.5 m), the first substage, were deposited under euphotic, high-salinity (40–50 and > 50 psu) conditions, and (as observed 0–22.5 m), nutrients varied between eutrophic and mesotrophic to oligotrophic (Figure 5). Benthic Foraminifera (MFs 1, 2, and 3) were abundant (0–26 m), and the foramol association is observed (Figure 5). Photic conditions as seen in the second substage (26.5–40) varied between euphotic and mesophotic to oligophotic. During deposition of the third substage (40–48 m, upper section), euphotic conditions dominated. Salinity during the time represented by the second and third substages (26.5–48 m) varied from normal (30–40 psu) to high (40–50 psu), and (as observed 22.5–48 m) mesotrophic to oligotrophic conditions dominated. The abundance of red algae (MFs 4, 5, and 6) increases in the upper section (26 to 48 m) (Figure 5), where the foramol and foralgal associations are observed. Results for the genus Amphistegina indicated that the depth of the Qom Sea in the studied area fluctuated between more than ∼7 m to < 20 m during the Rupelian (Figure 7). The MFs 1, 2, 3, 4, 5, and 6 were developed during the Rupelian stage in the studied area, indicating seagrass meadows.
Chattian stage.—Three substages of deposition are observed for the Chattian in the studied area (Figure 5). The first substage indicates variable conditions (between euphotic and mesophotic to oligophotic (48 to 61 m), and normal salinity (30–40 psu) (48 to 79 m). In the time represented by the second substage (61 to 96 m), photic conditions were variable between oligophotic, mesophotic to oligophotic, and euphotic (Figure 5). With decreased water depth indicated by the second substage (79 to 94 m), hypersaline (40–50 and > 50 psu) and normal (30–40 psu) conditions prevailed. During the time represented by the third Chattian substage, mesophotic to oligophotic conditions (96 to 110 m) dominated and salinity was normal (30–40 psu) (94 to 110 m) (Figure 5). Thus, mesotrophic to oligotrophic conditions were dominant during the Chattian in the studied area (Figure 5). Paleoenvironmental parameters such as light, salinity, nutrient supply and temperature influenced the distribution of the grain associations (Lees and Buller, 1972; Lees, 1975; Wilson and Vecsei, 2005). The paleoenvironmental parameters (light, and salinity) changed in the Chattian stage. Compared to the Rupelian stage, the nutrient, salinity, and light levels decreased in the studied area during the Chattian stage (Figure 5). Corallinacean algae, and larger benthic Foraminifera with hyaline walls are dominant in the upper section of this sequence and foramol association changes to foralgal association (Figure 5). Yet, the distribution of the grain association is variable during the Chattian stage in the studied area (Figure 5). Therefore, three different substages were identified during the Chattian stage based on the distribution of the grain associations. The foralgal association is dominant in the first substage (48 to 61 m) and third substage (96 to 110 m), and both of the grain associations can be observed in the second substage (61 to 96 m) in the studied area. During the Chattian, the depth of the Qom Sea < 10 m during the relative sea level fall, with a maximum > 20 m at the relative sea level rise (Figure 7), as indicated by the Amphistegina study. Therefore, the Qom Sea during the Chattian was deeper than during the Rupelian. The deposits belonging to the MFs 1, 2, 3, 4, 5, and 6 were developed during the Chattian stage in the studied area (Figure 5) within a paleoenvironment that included seagrass meadows.
Conclusions
Understanding the changes in paleoenvironmental conditions of the Qom Back-arc Basin (for correlations with other basins) during the Oligocene age is important. Based on the distribution of benthic Foraminifera in the Neyzar area, southeast of Salafchegan, two benthic foraminiferal assemblage zones can be recognized in the Qom Formation. Assemblage zones 1 and 2 were indicative of the Rupelian–Chattian ages.
The Qom Formation was deposited on an open shelf platform and this platform was divided into the inner shelf (restricted and semi-restricted lagoons) and middle shelf (open marine). Three substages were identified for the changes of photic conditions during the Rupelian stage. Euphotic conditions dominated in the first and third substages. Subsequently, photic conditions were variable between euphotic and mesophotic to oligophotic in the second substage. The Chattian stage was also divided into three substages. Variable photic conditions are indicated by the first substage (variation between euphotic and mesophotic to oligophotic) and second substage (variation between oligophotic, mesophotic to oligophotic, and euphotic). Mesophotic-oligophotic conditions were observed in the third substage during the Chattian stage. Salinity in the Rupelian was high (40–50 and > 50 psu) during the time represented by the lower section (as observed 0–22.5 m) of the studied area, and then varied from normal (30–40 psu) to hypersaline (40–50 psu) during deposition of the upper section (as observed 22.5–48 m) of the studied area. In the Chattian, salinity was normal (30–40 psu) during deposition of the first and third Chattian substages, but, conditions were hypersaline (40–50 and > 50 psu), ranging to normal (30–40 psu) as observed in the second substage. During deposition of the lower section of the Rupelian stage, eutrophic to mesotrophic-oligotrophic conditions were predominant. During the Chattian, mesotrophic-oligotrophic conditions dominated in the studied area. The depth of the Qom sea in the studied area was between < 10 m and > 20 m. However, the depth of this sea fluctuated between less than 10 m to more than 20 m during the Chattian. Foralgal and foramol associations were considered for the studied area during the Rupelian–Chattian ages. The Qom Formation sediments were deposited in an environment with seagrass meadows.
Acknowledgments
The authors would like to thank the University of Isfahan for the financial support. We are thankful to Laurel Collins and Mary Karen Solomon (for improving our manuscript).
