Introduction
Miocene outcrops in Central Iran trend northwest to southeast. The deposits extend to the northwest of northern Mako and Khoi and from there to the series of saline carbonate and clastic sediments of Nakhchivan in Azerbaijan and similar marine sediments in northeast Anatolia, Turkey (
Rahimzadeh, 1994;
Aghanabati, 2004). Because of the lateral facies changes and tectonic complexity of the Qom Formation, no type section has yet been defined. A type area was designated in the southern Qom plain around Dochah, where outcrops are most extensive and least lateral changes (
Stocklin and Setudehnia, 1991;
Hadavi et al., 2010;
Mohammadi et al., 2011).
The unique character of the Qom formation and the oil reservoir it hosts have prompted a variety of studies (
Furrer and Soder, 1955;
Daneshian and Ramezani Dana, 2007).
As a part of the Tethyan Seaway, Central Iran was a link between eastern Tethys (the proto-Indian Ocean) and western Tethys (the proto-Mediterranean Sea) (
Schuster and Wielandt, 1999;
Reuter et al., 2009). The Qom Formation was deposited during the final marine transgression in Central Iran during the Oligo-Miocene (
Daneshian and Ramezani Dana, 2007;
Reuter et al., 2009;
Mohammadi et al., 2011,
2013,
2015). However, uncertainties remain about the precise age and biostratigraphic evidence for correlation. Several studies have been carried out on different aspects of this formation such as biostratigraphy (benthic and planktic foraminifera, nannofossils, ostracods, gastropoda, corals), sequence stratigraphy, microfacies, tectonic, paleogeography, paleoecology, etc. More recently, studies were done by Seyrafian and Torabi (2005);
Daneshian and Ramezani Dana (2007);
Khaksar and Maghfouri Moghadam (2007);
Berning et al. (2009);
Morley et al. (2009);
Hadavi et al. (2010);
Behforouzi and Safari (2011);
Hasani and Vaziri Moghadam (2011);
Yazdi et al. (2012);
Yazdi-Moghadam (2012); Mohammadi et al. (2011, 2013, 2015);
Anjomshoa and Amirshahkarami (2014);
Amirshahkarami and Karavan (2015);
Daneshian and Ramezani Dana (2007);
Mohammadi and Ameri (2015); and
Daneshian et al. (2017). Some of these investigations specifically considered foraminiferal biostratigraphy of the Qom Formation with references to
Daneshian and Ramezani Dana (2007);
Behforouzi and Safari (2011);
Yazdi Moghadam (2011);
Mohammadi et al. (2015); and
Mohammadi and Ameri (2015).
Daneshian and Ramezani Dana (2007), after studying the biostratigraphy of the Qom Formation in the northern part of the Central Iran Basin (north of Garmsar), proposed that the biozonation of
Adams and Bourgeois (1967) is applicable to the Qom Formation based on the benthic foraminifera present in Oligocene–Miocene sediments of the Asmari Formation in Zagros. They estimated an Aquitanian to Burdigalian age for the studied section (Deh Namak).
Behforouzi and Safari (2011) by investigating larger benthic foraminifera of the Qom Formation in the Urumieh-Dokhtar magmatic arc (intra-arc basin) in northwestern Kashan, recognized the
Lepidocyclina- Operculina- Ditrupa assemblage zone of Wynd (1965) in their studied section and assigned the Oligocene age.
Yazdi Moghadam (2011) considered larger benthic foraminifera of the Qom Formation south of Uromieh (Baranduz). He correlated the foraminiferal assemblage of this section with SBZ 21, the European standard shallow benthic zonation, and proposed this biozone extends to northwestern Iran. He suggested an Early–Middle Rupelian age for the studied section.
Mohammadi et al. (2013) mentioned this age is in contrast to the results of previous studies. They studied the biostratigraphy of two stratigraphic sections (Ghohroud and Vidoja) of the Qom Formation to the south and southwest of Kashan and revised the results of previous works. They concluded that the Qom Formation is Rupelian–Burdigalian in age. They also mentioned the transgression of the Tethyan Seaway on the Iranian Plate started in the southeast and continued to the northwest gradually. This transgression indicates that due to the compressive tectonic regime in the Central Iran back-arc basin, the gates to the open ocean gradually became restricted in the Early Miocene. They suggested that the evaporitic ‘d’ Member in the Qom Formation was deposited in the small area of the Central Iran back-arc basin and was deposited totally within the early Miocene (Aquitanian–Burdigalian).
Mohammadi and Ameri (2015) and
Mohammadi et al. (2015) studied deposits of this formation in the Sanandaj–Sirjan zone (fore-arc basin in northern Abadeh) and analyzed biotic components, especially foraminiferal assemblages, and documented a Rupelian-Chattian age for deposits in the Abadeh area. They suggested obtained results are compatible with the common trend of the transgression of the Tethyan Seaway onto the Iranian Plate.
Mohammadi et al. (2015) studied the larger benthic foraminifera of the Qom Formation, mainly Lepidocyclinidae, Nummulitidae, and
Neoalveolina biostratigraphically in the Sanandaj–Sirjan fore-arc and Central Iran back-arc basins and dated the deposits to the Rupelian (SW Kashan, Varkan section), Rupelian-Chattian (E Sirjan, Bujan section) and Rupelian-Burdigalian (Qom, Khurabad section). Moreover, they suggested the
Nummulites intermedius-Nummulites vascus assemblage zone of
Wynd (1965) and the
Eulepidina-Nephrolepidina-Nummulites assemblage zone of
Adams and Bourgeois (1967), with Oligocene ages (Rupelian-Chattian), should all be ascribed to the Rupelian.
There are few studies on the foraminiferal biostratigraphy of the Qom Formation of the Dochah area to the northwest of the type area, especially based on planktic foraminifera. Therefore, the foraminiferal components of this formation, particularly planktic forms, are given a detailed study in this paper with respect to foraminiferal assemblages and recognized biozones for correlation.
Geological setting
The mountains in the Central Iran zone formed during the Late Eocene to Early Oligocene by the Pyrennian orogenic phase. Shallow depositional environments formed and continental sediments were deposited through the Oligocene;
Gansser (1955) named these evaporites and terrestrial deposits the Lower Red Formation. Evaporitic and terrestrial deposition continued up to the Early Oligocene when sea levels rose during the Savian event (
Emami, 1991;
Rahimzadeh, 1994), causing a transgression from southeast to northwest in Iran (
Emami, 1991;
Rahimzadeh, 1994; Heydari et al., 2003;
Daneshian and Ramezani Dana, 2007;
Mohammadi et al., 2013,
2015). The marine sediments of the Qom Formation mainly consist of limestone and marl. These sediments show high variation in thickness, age, and lithology in different parts of Iran (
Rahimzadeh, 1994;
Heydari et al., 2003;
Daneshian and Ramezani Dana, 2007;
Mohammadi et al., 2013).
Nogol-Sadat (1973) believed that the facies changes in the Qom basin are related to water depth differences due to vertical motion of the basement. Increasing the depth led to limestone deposition, decreasing depth led to marl.
A marine regression led to the thick red clastic- evaporitic deposition of the Upper Red Formation at the end of Burdigalian (
Rahimzadeh, 1994;
Aghanabati, 2004;
Daneshian and Ramezani Dana, 2007;
Mohammadi et al., 2011,
2013). Amini (2001) attributed the widespread and thick deposition of this rock unit to faulting that provided accommodation space in the Qom basin.
Berberian (1983) believed formation of the Qom basin in Central Iran was related to the subduction of a high Zagros (neo-Tethyan) oceanic plate beneath the active continental margin of the southwestern edge of the Central Iran plate during the Oligocene- Miocene. This subduction led to the opening of a back-arc basin in the center and north of the Central Iran Zone, in which the Qom marine sediments were deposited (
Rahimzadeh, 1994;
Aghanabati, 2004;
Mohammadi et al., 2011,
2013). In fact, this subduction, leading to the collision between the Arabian-African plate and the Iranian plate in the Mesozoic, was the closure of the Tethyan Seaway during the Miocene, known as the so-called Terminal Tethyan Event (TTE) (
Schuster and Wielandt, 1999;
Harzhauser et al., 2007;
Reuter et al., 2009;
Mohammadi et al., 2011,
2013).
One of consequences of this event was the creation of the volcanic arc of the Urumieh–Dokhtar belt during the Eocene which led to compartmentalization of the region and creating the Esfahan–Sirjan fore-arc and the Qom back-arc basins at the northern margin of the Tethyan Seaway (
Berning et al., 2009;
Reuter et al., 2009;
Mohammadi et al., 2013;
Mohammadi and Ameri, 2015). In both basins, the deposition of Qom marine deposits started in the Oligocene and continued up to the Early Miocene (
Reuter et al., 2009;
Mohammadi et al., 2011,
2013;
Seddighi et al., 2012). There has been a lot of debate pertaining the precise timing of the Tethyan Seaway closing. For instance
Adams et al. (1983) assigned it to the Aquitanain and Rogl and Steininger (1984) attributed it to the Burdigalian.
Schuster and Wielandt (1999) mentioned that the marine sedimentation began during the Early Oligocene and continued until the end of the Early Miocene.
Deposition of the Qom Formation (Rupelian–Burdigalian) took place in three NW–SE-trending basins including: the Sanandaj–Sirjan (fore-arc basin), the Urumieh–Dokhtar magmatic arc (intra-arc basin), and the Central Iran (back-arc basin). Marine conditions in the low latitudes of these three basins started in the Rupelian. But in the back-arc basin, Rupelian sediments were deposited only in a few places in close proximity to the magmatic arc
(Mohammadi et al., 2013).
The study area is located in the north-central Iran zone, near Qom city. The study section (Dochah) is located in the northwest of the Type area (Fig. 1), where shallow marine carbonates of the Qom Formation unconformably overlie the Lower Red Formation and underlie the shale and siltstone of the Upper Red Formation (Fig. 2). Generally, the Qom Formation was deposited in a shallow marine environment and consists mainly of limestone and marl layers. This rock unit was divided into members by researchers such as
Dozy (1944),
Furrer and Soder (1955),
Abaie et al. (1964), and
Bozorgnia (1966).
Stocklin and Setudehnia (1991) based on
Bozorgnia (1966), in the Stratigraphic Lexicon of Iran, defined nine members (a, b, c-1, c-2, c-3, c-4, d, e, and f) for the Qom Formation and described lithological features of these members in the Type area. The Qom Formation in the Dochah section is 894.1 meters thick, including ‘a to e’ members and lithologically consists of limestone, sandy limestone, argillaceous limestone, shale, sandstone, gypsum, marl, sandy marl, and siltstone (Fig. 2).
Material and methods
In all, 152 samples were collected, including 78 hard samples and 74 soft samples. The lithology of the top ‘e’ member of the section consisted of significant thicknesses of marl and sandy marl. Abundant planktic foraminiferal components led to sampling in the lower reaches of the member. From the hard samples, 161 thin sections were examined. The 74 soft samples were washed according to the usual procedures. Then one gram of the 35 micron and 120 micron sieve fractions was weighed out and picked for identification of foraminifera. Foraminifera were photographed and SEM images were prepared for free specimens. An ultrasonic treatment was used to remove particles from the best free specimens prior to imaging by TEScan-VagaII at the Razi Metallurgical Research Center.
For identifying genera and species of foraminifera, references such as
Papp and Schmid (1985),
Petrová (2004),
Sharaf et al. (2005),
Iaccarino and Premoli-Silva (2005),
Popescu and Crihan (2005),
Wilson (2005),
Kender et al. (2009),
Finger (2013), and
Sirel (2015) were applied and the systematics follow
Loeblich and Tappan (1988). For biozonation and correlation, the main purposes of this research, bioevents of index foraminifera were extracted. First and last occurrences of planktic foraminifera were used as limits on biozonation. Planktic foraminifera are less affected by diachroneity of provincialism than benthic foraminifera. Biozonation of planktic foraminifera follows
Wade et al. (2011), the most recent biozonation for planktic foraminifera. Previous biozonations such as
Banner and Blow (1965),
Kennett and Srinivasan (1983),
Bolli and Saunders (1985), and
Berggren et al. (1995) were compared for this section with biostratigraphy criteria schematically shown in Fig. 3. Biozonation of benthic foraminifera was considered according to the biozonation of
Adams and Bourgeois (1967) which was proposed for the Asmari Formation and were applied where planktic foraminifera were inadequate.
Results
Investigation of hard and soft samples led to the identification of 12 genera and 27 species of planktic foraminifera and 68 genera and 155 species of benthic foraminifera (Figs. 4–6). Figure 7 presents the ranges of index foraminifera and the taxa reported for the first time in this section. Table 1 lists all identified foraminifera; highlighted species are reported for the first time.
Benthic and planktic foraminifera, ostracods, bivalves, bryozoa, coralline red algae, corals, echinoids, and gastropods are biogenic components of the Qom Formation in the Dochah section, of which the main faunal components are foraminifera. Due to the availability of both benthic and planktic foraminifera in the Dochah section, biozonation was carried out in two separate parts. Although
Adams and Bourgeois (1967) built their biozonation for the Zagros basin in Iran, it can be applied to the Central Iran and the Qom Formation deposits. This biozonation is applicable to Central Iran sediments because a connection between the Qom and Zagros basins in Late Aquitanian time has been suggested (
Bozorgnia, 1966;
Adams and Bourgeois, 1967;
Kashfi, 1988). However,
Mohammadi et al. (2013) believe that there is no connection between the Oligo-Miocene deposits of Qom and the Asmari Formation. This idea was proved using faunal similarities, especially foraminifera, between the Qom and Asmari formations. New data and more investigations indicate that there are some differences in the fauna and stratigraphic distribution of species between these two basins. Besides the
Adams and Bourgeois (1967) biozonations, which were applied by some authors (
Bozorgnia, 1966;
Daneshian and Ramezani Dana, 2007;
Mohammadi et al., 2015) for the Qom Formation biostratigraphy, there are other biozonations for the Oligo- Miocene deposits of the
Zagros basin such as Wynd (1965);
Ehrenberg et al. (2007) and
Laursen et al. (2009). Considering
Adams and Bourgeois (1967), their biozonation, although it has some difficulties is more accurate than other biozonations in respect to benthic foraminiferal content of the studied section. The dissimilarity between foraminiferal assemblages and their stratigraphic distribution in both Qom and Zagros basins make it necessary to develop a more comprehensive biozonation specifically for the Qom Formation (
Daneshian and Ramezani Dana, 2007). In the Dochah section, the best data for biozonation based on planktic foraminifera occur at top of the section. Herein, we apply the biozonation of
Wade et al. (2011) together with consideration of schemes proposed earlier by
Banner and Blow (1965),
Kennett and Srinivasan (1983),
Bolli and Saunders (1985), and
Berggren et al. (1995) (Fig. 3).
Discussion
Planktic foraminifera from the Qom Basin at Dochah are considerably more abundant and diverse that at localities in central Iran (
Daneshian and RamezaniDana, 2007;
Daneshian and Aftabi, 2010;
Daneshian and Naderi, 2014;
Daneshian and Ramezani Dana, 2015;
Daneshian et al., 2017). From formation Members ‘a’ to ‘d’ the presence of benthic
genera Miogypsinoides and
Valvulina prove an Aquitanian age, equivalent to biozone 2 of
Adams and Bourgeois (1967). The lack of Borelis species at top of the section in Member ‘e’ prohibited using the Adams and Bourgeois biozonation. The diagnostic species
Borelis melo curdica is not present here due to different environmental conditions. However, based on planktic foraminifera, the age of upper part (Member ‘e’) of the section is Burdigalian.
The occurrences of planktic foraminifera such as
Globigerinoides triloba and
Globigerinoides immaturus which appear from 45 meters above the section base and the presence of Globorotalia sp. and Ammonia sp. from 5 meters in Member ‘a’, confirms an Early Miocene (Aquitanian) age. Also, the presence of
Paragloborotalia and
Globigerinoides with their abundance and diversity indicate Aquitanian age for Members ‘a’, ‘b’ and ‘c-1’ to ‘d’. The presence of
Catapsydrax parvula ,
Globorotalia archeomenardi,
Globigerinoides bisphericus,
Praeorbulina sicana,
Praeorbulina transitoria confirm a Late Burdigalian age for the beginning of Member ‘e’ at Dochah. In this, we follow the biozonation of
Wade et al. (2011) as compared with other biozonations in Fig. 8.
The biozonations of
Banner and Blow (1965) and
Blow (1969) use the first appearance of
Globigerinoides primordius to define the lower boundary of biozone N4,
Paragloborotalia kugleri for N5, and
Globigerinatell insueta for N6 biozone (Fig. 3). Because these three species are missing at Dochah, separate recognition of N4, N5, and N6 is impossible, but this does not rule out the possibility that the section extends through the top of biozone N6.
From Member ‘a’ up to middle part of Member ‘e’ (DO-120) the section likely spans biozones N4-N7. The first appearance of Praeorbulina sicana (at 719 m) represents the lower boundary of N7. Also, foraminiferal contents show a Late Burdigalian age (Fig. 8).
Definitive bioevents for the
Kennett and Srinivasan (1983) zone boundaries are lacking in Dochah. Based on the presence of species such as
Globigerinoides triloba (slightly higher than biozone (N4B),
Globigerinoides subquadratus (N4B),
Globigerinoides immaturus (N5), and also
Globigerinoides quadrilobatus (N6), and the lack of any Oligocene taxa at the base of the section, we place the bottom part of the Dochah section in the Miocene (Aquitanain) and equivalent to biozone N4. There is no index indicator for determining the upper boundary of this zone. Only the first occurrence of
Globigerinoides quadrilobatus can be used for placing biozone N5 – a species not defined as an event for this biozone, but helpful for separation of biozones. Thus the base of the section is determined to be equivalent to the N4-N5 biozones. Above that, strata from the beginning of Member ‘e’ (sample DO-79) up to the first appearance of
Praeorbulina sicana (DO-120 at 590 m) are equivalent to biozones N6-N7. According to Kennett and Srinivasan (1983), the first appearance of this species defines the lower boundary of N8 biozone (Fig. 3). There is an event for separating biozones N6 and N7 in the tropics but no event for the lower boundary of biozone N9, such as
Orbulina species. Therefore, we attribute the top of the section to biozone N8 (Fig. 8).
In
Bolli and Saunders’ (1985) biozones, an Early Miocene age is assigned to biozone N4. Its lower boundary corresponds with the last appearance of
Globigerinoides primordius and the upper boundary is determined by the last appearance of
Paragloborotalia kugleri. This event is not present at Dochah. Thus, the base of the section could not be placed into the biozonation of
Bolli and Saunders (1985). Similarly, there is no event for recognizing their N5 and N6 biozones. The single occurrence of
Catapsydrax dissimilis creates a disparity in age and we decided to ignore the presence of this species. The first appearance of
Globigerinoides bisphericus helps identify the lower boundary of biozone N7. In fact, the beginning of Member ‘e’ (DO-96 at 645 m) can be considered as an equivalent to the N7 biozone . The lower boundary of biozone N8 was defined by the presence of
Praeorbulina glomerosa, a species absent from Dochah. Therefore, the presence of
Praeorbulina transitoria,
Praeorbulina sicana, and
Catapsydrax parvula, was used to recognize this biozone based on
Bolli and Saunders (1985). From the first appearance of
Globigerinoides bisphericus up to the end of the section is considered equivalent to the N7-N8 biozones and indicates a Late Burdigalian age (Fig. 8).
In subtropical regions, the first and last appearances of
Paragloborotalia kugleri define the M1 biozone, and the last occurrence of
Globigerinatella insueta defines the upper boundary of M2 and the lower boundary of M3, of Burdigalian age (
Berggren et al., 1995). The last appearance of
Catapsydrax dissimilis defines the lower boundary of M4, also Burdigalian (Fig. 3). None of these events can be used for separating biozones in the Dochah section. Only the first appearance of
Globoquadrina dehiscens can be applied; it places biozone M1b at the base of the ‘e’ Member, but it is not definitive for the biozone. The absence of this species at the base of the section can be caused by different factors and in any case, the local first appearance contradicts other information and was not used. In summary, the lack of definitive events at Dochah prevents separation of biozones M1, M2, M3, and M4. As a consequence, the base of the section (Member ‘a’) is assigned to the Early Miocene (Aquitanian) age up to sample DO-120 and the first appearance of
Praeorbulina sicana. This Member ‘a’ can be considered the equivalent to the M1-M4 biozones of
Berggren et al. (1995).
From the first appearance of
Praeorbulina sicana up to the top of the Dochah section is considered to be an equivalent to biozone M5, which represents beginning of the Langhian. There is no evidence for the start of the next biozone (Fig. 8). The biozonation of
Wade et al. (2011) confirms that the age of succession is Aquitanian. In this way, due to absence of distinctive events, strata from the base (Member ‘a’) up to the first appearance of
Praeorbulina sicana are equivalent to biozones M1-M4 and include Members ‘a’, ‘b’, ‘c1-4’ and ‘d’ with the basal part of Member ‘e’ indicating Aquitanian- Burdigalian age. The first appearance of
Praeorbulina sicana is beginning of biozone M5a of late Late Burdigalian age. The upper boundary of this zone corresponds to the first occurrence of
Praeorbulina glomerosa which is not present at Dochah. Thus, the age of the section according to
Wade et al. (2011), ranges from Aquitanian up to late Late Burdigalian (Fig. 8).
Conclusions
Study of the Qom Formation in its type area yielded 33 taxa of Miocene foraminifera not previously reported. Among the identified species, Praeorbulina sicana, Praeorbulina transitoria, and Globigerinoides bisphericus have particular importance because of their role in age determination and biozonation. Among these species, Praeorbulina sicana indicates biozone subzone (M5a), proving a younger age than previously recognized. Therefore, the Tethyan seaway occupied this area up to the end of Burdigalian time and must have closed later than previously estimated.
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