Introduction

Based on geological structure, Iran has been divided into several structural units. For the understanding and good description of these areas, it is necessary to know these structural units, characterized by a relatively unique record of stratigraphy, tectonics, magmatic activities, orogenic events, and overall geological style. The first systematic geological studies in Iran started in the late 1960s. Stocklin (1968) suggested several basins for the first time. Later, using mostly the NIOC database and important investigations, Nabavi (1976), Eftekharnejad (1981), and Berberian and King (1981) classified Iran into several different structural zones (units) namely Zagros, Sanandaj-Sirjan, Central Iran, Kopet Dagh, Alborz, and Eastern Iran. However, in recent years, new interpretations and models have been offered regarding the geological setting of Iran (Nogol-e-Sadat and Almasian 1993; Alavi 1991; Aghanabati 2009). Eastern Iran can be divided into two parts namely Lut block and Flysch or colored mélange of Zabol–Baluch Zone. Lut block located to the west of Zabol–Baluch Zone is the main body of Eastern Iran. Lut block extends for about 900 km in a north–south direction. Lut block is part of the eastern microcontinental area of the Central Iran. Cretaceous sediments comprised of two different facies. In some parts of south Birjand, Cretaceous succession occurs in carbonate form and belongs to Early to Late Cretaceous (Aghanabati 2009). Flysch Zone (Zabol–Baluch) is located between Lut block to the west and Helmand (in Afghanistan) to the east. Flysch Zone is highly deformed and tectonized and consists of thick deep-sea sediments (Fig. 1).

Fig. 1
figure 1

Iranian major tectono-sedimentary units (redrawn from Berberian and King 1981): stable area, Arabian Precambrian platform in southwest and Turanian Hercynian plate in northeast (1); Zagros, including Zagros foredeep, main sector of the marginal active fold belt peripheral to stable area and High Zagros (2); Alborz Mountains (3); Central Iran lying between the two marginal active fold belts (4); Talesh, Armenian Late Hercynian belt with a possible continuation to Iranian Talesh Mountain (5); ZabolBaluch and Makran post ophiolite flysch trough (6); Kopet Dagh folded belt and foredeep (7)

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Recent detailed geological studies of the area were carried out by Fauvelet and Eftekhar-nezhad (1990), Berthiaux et al. (1991), and Raisossadat et al. (2011, 2014). The rudists were studied by Raissosadat and Skelton (2005) and Khazaei et al. (2011). The Orbitolina limestones were studied by Babazadeh et al. (2010) and Zarei et al. (2014).

The main aims of this study are to determine the benthic and planktonic foraminifera from the Late Aptian to Early Cenomanian interval of the Nimbolook succession at the east of Lut block in Eastern Iran for the first time. The biostratigraphic studies and applying to suggestion biozones, classification of foraminifera contents to morphogroups, and interpretation of the paleoechological conditions based on them are the other aims of this study.

Materials and methods

In order to study foraminifera, a total of 60 samples both hard and soft were collected from Nimbolook section. The shale and marl samples were crushed to little particles (2–3-cm diameters), soaked in tap water with diluted hydrogen peroxide 10 % during 48 h, and then washed, and the residue was sieved at 125, 80, and 60 μm; then, all of foraminifera were picked under a binocular microscope, while SEM was used at Ferdowsi Mashhad University for taxonomic identification of foraminifera assemblages. The real thickness has been calculated, and stratigraphical section was drawn by using computer software packages. Excel software has also been used for drawing charts and graphs.

Stratigraphy

The Cretaceous succession anywhere in the East and Central Iran begins with coarse grain sediments that gradually change to fine grain including sandstone, limestone, shale, marl, and thick-bedded limestone.

The measured section of Nimbolook is located about 30 km northwest of Qayen. For access to the measured section from Qayen–Gonabad road, a track road passable by car is used for about 9 km in the southwest of Nimbolook village. The exact location of the section is at 33° 52′ 9 N latitude and 59° 00′ 49 E longitude (Fig. 2). The Nimbolook succession with 222-m thickness conformably overlies and underlies the limestone beds and could be divided into four units (Fig. 3). The first unit consists of

  1. 1.

    16 m of gray to light gray medium-bedded limestone beds, in the base containing bivalvia.

  2. 2.

    20 m of gray to greenish gray sandy limestone with few meters of thin-bedded greenish gray sandstone was partly seen at the base of this unit.

  3. 3.

    122-m alternation of light brown thin-bedded shale and marl gray to greenish gray medium-bedded sandy limestone.

  4. 4.

    64-m massive to medium-bedded limestone of this unit that changed to green to greenish gray, weathered color brown thin-bedded limestone (Fig. 4)

Fig. 2
figure 2

Geographical names and their positions, location of measured section

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Fig. 3
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A field photograph showing outcrop of the section

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Fig. 4
figure 4

Stratigraphical column of Nimbolook section

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Assemblage of foraminifera and biostratigraphy

Bed-by-bed sampling at the Nimbolook succession for biostratigraphic studies provides a firm basis for a biozonation in the Late Aptian–Early Cenomanian age. This study led to the identification of 12 genera and 13 species of benthic foraminifera as well as 13 genera and 18 species of planktonic foraminifera, making a total of 6 genera and 6 species.

Many biozonations have been proposed by different authors, based on the planktonic foraminifera in Tethys realm. The most important are Caron (1985), Bolli (1966), Hardenbol et al. (1998), and Premoli-Silva and Verga (2004). In this study, the three biozones suggested coincide with global biozones. The following benthic foraminifera Berthelina baltica, Berthelina intermedia, Dorothia sp., Gaudryina rugosa, Guttulina communis, Gyroidinoides infracretaceus, Lenticulina macrodisca, Lenticulina saxocretacea, Lenticulina subalata, Laevidentalina communis, Laevidentalina cf. soluta, Marginulina sp., Marssonella sp., Mayncina orbignyi, Pseudonodosaria sp., Orthokarstenia shastaensis, Saracenaria triangularis, and Saracenaria sp. and planktonic foraminifera Biticinella subbreggiensis, Favusella washitensis, Muricohedbergella delrioensis, Pseudothalmanninella subticinensis, Planomalina buxtorfi, and Rotalipora appenninica were identified, respectively, in the studied section. Based on these foraminifera, three biozones have been suggested (Fig. 5) namely (1) B. intermedia assemblage zone, (2) B. subbreggiensis interval zone, and (3) R. appenninica interval zone (Fig. 5).

  1. 1.

    B. intermedia assemblage zone

Fig. 5
figure 5

Foraminifera range chart, proposed biozonation and stratigraphical column in Nimbolook section

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This zone is an assemblage zone that was introduced by Holbourn and Kaminski (1997). B. intermedia is mentioned by Moullade (1974) and Weidich (1990) from Late Aptian to Cenomanian age, while Holbourn and Kaminski (1997) believed that this species starts from Albian.

This biozone is defined based on first appearance of B. intermedia and is recorded from 40–102 m of studied section (Fig. 5). The assemblage foraminifera in this biozone are Marssonella sp., G. communis, G. rugosa, L. communis, Dorothia sp., B. intermedia, G. infracretaceus, O. shastaensis, L. saxocretacea, L. macrodisca, Saracenaria sp. 1, and Saracenaria sp. 2. Based on these assemblage fauna, the Early Albian age has been suggested.

  1. 2.

    B. subbreggiensis interval zone

This biozone was introduced by Postuma (1971) as interval zone for Middle Albian–Late Albian age. Researchers like Van Hinte (1976), Postuma (1971), Sigal (1977), and Premoli-Silva and Verga (2004) considered it as Middle Albian. This biozone includes 25 m (102 to 127) of the studied section. The lower boundary of this biozone is defined based on the first appearance of B. subbreggiensis, and the upper boundary of this biozone is based on the first appearance of R. appenninica (Fig. 5). The assemblage foraminifera in this biozone are Marssonella sp., L. saxocretacea, L. subalata, L. macrodisca, Saracenaria sp. 2, G. communis, and L. communis.

  1. 3.

    R. appenninica interval zone

This zone is an interval zone introduced by Bronnimann (1952). Postuma (1971) placed this zone in the lower most part of Cenomanian, while others such as Bolli (1966), Sigal (1977), Carron (1985), and Premoli-Silva and Verga (2004) placed it in Late Albian. This biozone is defined based on the first appearance of R. appenninica up to the first appearance of Rotalipora brotzeni. The thickness of this zone is 33 m (127 to 160) (Fig. 5). The assemblage fauna includes P. buxtorfi, F. washitensis, M. delrioensis, P. subticinensis, B. baltica, Pseudonodosaria sp., Marginulina sp., L. subalata, Saracenaria sp. 1, Saracenaria sp. 2, and L. communis.

Foraminifera morphogroups

Previous studies of benthic morphological groups by Koutsoukos and Hart (1990) and Nagy (1992) revealed strong relationship between environmental conditions and morphotypes. According to the study of Corliss and Chen (1988), Koutsoukes and Hart (1990), and Coccioni and Galeotti (1993), there is an acceptable relationship between test morphology and inferred habitat of foraminifera fauna. Friedrich et al. (2003) is of the opinion that benthic foraminifera assemblages as well as stable isotope data can be used to interpret the oxygen content of bottom waters, organic matter flux to the sea floor, and sea-level changes during the Late Aptian time. In their study, Erbacher et al. (1998) stated that benthic foraminifera can serve as a sensitive tool to provide a better understanding of the conditions at the sea floor, during the Lower Cretaceous time.

The planktonic morphogroups are dependent on sea-level changes. Eicher and Worstell (1970) and Eicher (1967) have shown that the globigernid and globular morphotypes were the first planktonic foraminifera that appeared in transgression and the last to disappear during regression. The globular forms such as Globigerina inhabit surface waters and are keeled bioconvex and keeled plano-convex such as globorotallids which inhabit deeper waters (Hart and Bailey 1979; Wiedmann et al. 1982; Boudagher Fadel 2015).

Another approach is the successive appearance and disappearance of species or genera along gradients, e.g., water depth, with other factors also influencing the distribution of species. These may include the rise and fall of oxygen minimum zones (Jarvis et al. 1988; Leary et al. 1989; Koutsoukos and Hart 1990; Hart 1999) or nutrient supply (Premoli-Silva and Sliter 1999).

The foraminifera assemblage of the Nimbolook succession allowed three benthic (MG1–3) and two planktonic (MG4–5) morphogroups to be distinguished (Fig. 6). This classification is based on morphology, life position, feeding habitat, and environment of foraminifera.

Fig. 6
figure 6

Distribution of morphogroups of foraminifera in Nimbolook section

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Morphogroup 1 (MG1)

This morphogroup consists of taxa with calcareous test and planispiral to trochospiral shape test which is typical for aerobic and eutrophic to mesotrophic environments. Most of these taxa are epifaunal forms and active deposit feeders that have been identified in the middle–outer shelf to upper slope deposits (Coccioni and Galeotti 1993; Lowery et al. 2014). B. baltica, B. intermedia, and Gyroidinoides infracretacea are the common examples of this group (Fig. 7 (1–3)). G. infracretaceus seems to be mainly controlled by paleobathymetry. The species is characteristic of outer shelf to middle bathyal depths in Albian strata of the South Atlantic Ocean (Kochhann et al. 2014).

Fig. 7
figure 7

Some selected SEM photographs of foraminifera that classified in morphogroups. Berthelina intermedia, sample no. MN 4-10, scale 100 μm (1a–1c); Anomalina ammonoides, sample no. MN4-29, scale 100 μm (2a–2c); Gyroidinoides infracretaceus, sample no. MN 4-12, scale 100 μm (3a and 3b); Dorothia sp., sample n. MN 4-15, scale 100 μm (4a and 4b); Gaudryina rugosa, sample no. MN 4-15, scale 100 μm (5); Marssonella sp., sample no. MN 4-17, scale 100 μm (6); Orthokarstenia shastaensis, sample no. MN 4-15, scale 100 μm (7); Laevidentalina communis, sample no. MN 4-25, scale 100 μm (8); Laevidentalina cf. soluta, sample no. MN 4-2, scale 100 μm (9); Marginulina sp., sample no. MN 4-29, scale 100 μm (10)

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Morphogroup 2 (MG2)

This morphogroup is divided into two submorphogroups including the calcareous and agglutinated benthic foraminifera with elongate or subcylindrical test shape which is indicative of the following characteristic of environmental conditions. All these taxa are potentially shallow to deep infaunal, active deposit feeder, and mesotrophic to eutrophic conditions, which are present in a wide range of marine environments from inner shelf to upper bathyal (Ruckheim et al. 2006; Bindiu and Filipescu 2011). These faunae seem to be typical for moderately low levels of oxygen. Dorothia sp., G. rugosa, and Marssonella sp., could be classified as agglutinated benthic foraminifera in this morphogroup. In some taxa with elongated flattened to straight periphery, life position is shallow to deep infaunal, deposit feeder, neritic to upper bathyal, aerobic to dysaerobic, and mesotrophic to eutrophic environment conditions (Ruckheim et al. 2006). The most taxa with calcareous test belonging to this group are O. shastaensis, L. communis, L. cf. soluta, Marginulina sp., and Pseudonodosaria sp. This morphogroup seems to be well oxygenated and to prefer meso-eutrophic living conditions (Fig. 7 (4–10)).

Morphogroup 3 (MG3)

The foraminifera fauna was dominated by taxa such as Saracenaria sp. 1, Saracenaria sp. 2, G. communis, L. macrodisca, L. subalata, and L. saxocretacea, which could have the following specific characteristics: calcareous, biconvex test shape, active deposit feeders, and eutrophic to mesotrophic conditions as is the case for epifaunal to deep infaunal assemblages. They can be found in a wide range of environment from sublithoral to upper bathyal and aerobic to dysaerobic conditions (Ruckheim et al. 2006) (Fig. 8).

Fig. 8
figure 8

Some selected SEM photographs of foraminifera that classified in morphogroups. Lenticulina macrodisca, sample no. MN 4-17, scale 100 μm (1a–1c); Lenticulina saxocretacea, sample no. MN 4-21, scale 100 μm (2a–2c); Saracenaria sp. 1, sample no. MN 4-21, scale 100 μm (3a and 3b); Saracenaria sp. 2, sample no. MN 4-12, scale 100 μm (4); Lenticulina subalata, sample no. MN 4-25, scale 100 μm (5a–5c); Guttulina communis, sample no. MN 4-21, scale 100 μm (6a and 6b)

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The diversity and specific distribution of benthic assemblage foraminifera are mainly influenced by oceanic circulation patterns, oxygenation and nutrient availability, surface fertility, changes in depth of the CCD, and variations within the seawater chemistry, such as pH (Van der Zwaan et al. 1999). The preservation of benthic foraminifera in Nimbolook section varies from good to poor throughout the studied interval.

Planktonic morphogroup (MG4)

This morphogroup includes trochospiral forms which are living in surface or near surface and suspension feeding (Price and Hart 2002; Lowery et al. 2014). They live in shallow epicontinental sea in a pelagic, aerobic, highly eutrophic environment. M. delrioensis, F. washitensis, and B. subbreggiensis are examples of these morophogroups (Leckie 1987; Kochhann et al. 2013) (Fig. 9). The oxygen isotope data supports the view that the Cenomanian globular foraminifera, such as the genus Muricohedbergella, inhabited near-surface waters that are rich in organic material (Caron and Homewood 1982; Leckie 1989; Premoli-Silva and Sliter 1999; Price and Hart 2002).

Fig. 9
figure 9

Some selected SEM photographs of foraminifera that classified in morphogroups. Muricohedbergella delrioensis, sample no. MN 4-37, scale 100 μm (1a–1c); Favusella washitensis, sample no. MN 4-37, scale 100 μm (2a–2c); Muricohedbergella delrioensis, sample no. MN 4-34, scale 100 μm (3a–3c); Biticinella subbreggiensis, sample no. MN 4-29, scale 100 μm (4a and 4b)

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From the standpoint of view of paleobiogeogaphy and distribution pattern, species of Muricohedbergella belong to shallow and open marine (Hart and Bailey 1979; Leckie 1985). According to Jarvis et al. (1988) and Corfield et al. (1990), small planktonic forms such as Muricohedbergella lived near the surface and bigger forms such as Rotalipora in deeper depth. Muricohedbergella planispira is a proxy for normal salinity surface and low-oxygen waters (Eicher and Worstell 1970; Leckie 1987).

Keller and Pardo (2004) studied foraminifera of the Pueblo basin of Colorado and concluded that the abundance of M. delrioensis coincides with increases of oxygen. This species lived in rich nutrient and normal salinity sea. Fisher and Arthur (2002) believed that hedbegellids lived in different levels of water such that M. delrioensis is located under M. planispira.

Planktonic morphogroup (MG5)

This morphogroup includes trochospiral forms floating in nearly deep water of upper part of bathyal to lower part of surface water (Norris and Wilson 1998). Pseudothalmanninella, Rotalipora, and Planomalina are the most abundant specimens belonging to this morphogroup (Fig. 10). According to Keller and Pardo (2004) and Leckie (1987), normal salinity and aerobic and oligotrophic environment are the characters in this habitat.

Fig. 10
figure 10

Some selected SEM photographs of foraminifera that classified in morphogroups. Rotalipora appenninica, sample no. MN 4-37, scale 100 μm (1a–1c); Planomalina buxtorfi, sample no. MN 4-39, scale 100 μm (2a–2c); Pseudothalmanninella subticinensis, sample no. MN 4-29, scale 100 μm (3a–3c)

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Discussion and conclusion

For the first time in eastern margin of Lut block, foraminifera contents of Albian deposits have been investigated that led to the following results.

The Nimbolook succession has a thickness of about 222 m at the studied section including sandstone, limestone, shale, and marl. It is conformably overlain, underlies the limestone beds, and could be divided into four units.

The stratigraphic distribution of the 13 benthic and 6 planktonic identified foraminiferal species enabled to suggest three biozones as follows: (1) B. intermedia assemblage zone, (2) B. subbreggiensis interval zone, and (3) R. appenninica interval zone. They coincide with the global biozones. Based on foraminifera fauna, an Early–Late Albian age is suggested for the studied area.

Palaeoecological conditions have been reconstructed. This study lead to the recognition of three benthic foraminifera and two planktonic morphogroups. The preservation of benthic species shows relatively good to moderate in lower and middle part of Nimbolook section.

A detailed analysis of the studied succession appeared a high abundance of the benthic morphogroups (MG1–3) in the lower and middle part and the planktonic morphogroups (MG4–5) in the upper part of the section, respectively. The first morphogroup is epifauna, the second is infauna, and the third is assemblage of epifauna and infauna morphogroups. It seems that living conditions for benthic morphogroups were better than in the upper part of the section.

The most abundant species of calcareous and agglutinate benthic foraminifera in morphogroups 1 to 3 are Berthelina, Lenticulina, and Laevidentalina; Dorothia; and Gaudryina, respectively. Muricohedbergella and Rotalipora are the most common species in planktonic foraminifera that belong to morphogroups 4 and 5.

Because of the planktonic morphogroups (MG4–5) that lived in deeper water to compare with benthic morphogroups (MG1–3), it could be concluded that the depth of basin from lower to upper of succession increased.

According to the identified morphogroups in Nimbolook succession, during the Early–Late Albian–Early Cenomanian time, this section exposited in shallow to fairly deep basin.