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1. Introduction
Coralline red algae were one of the major components of calcareous benthic communities, reef builders, and carbonate producers in the Miocene shallow marine paleoenvironments (Halfar and Mutti, 2005; Bragaet al., 2010; Sarkar and Ghosh, 2015; Sarkaret al., 2016). Several of the reported genera of coralline red algae in Miocene deposits encompass still living species and constitute a useful tool for paleoenvironmental interpretations (Bragaet al., 2010), since they are good indicators of water depth, temperature, light intensity, and hydrodynamic energy of carbonate paleoenvironments (Bosence, 1991; Aguirreet al., 2000; Brandanoet al., 2005; Bragaet al., 2010).
Generally, the Oligocene-Miocene carbonate paleoenvironments represent the highest diversity of coralline red algae in the geologic record (Aguirreet al., 2000; Bragaet al., 2010), with its maximum levels of diversity recorded in the Lower Miocene (Aquitanian) and represented by 245 species (Aguirreet al., 2000; Bragaet al., 2010). Mainly, non-geniculate coralline red algae were dominant in the Miocene paleoenvironments, whereas, geniculate corallines were subordinate (e.g.Ghosh, 2002; Rösleret al., 2015; Colettiet al., 2018; Roozpeykaret al., 2019a).
The carbonate facies of the Qom and Asmari formations in central Iran and the Zagros Basin are the principal deposits of Miocene coralline red algae in Iran (Zabihi Zoeramet al., 2014; Roozpeykaret al., 2019a). This paper focuses on the paleoecological aspects of Lower Miocene coralline red algae from the Qom Formation and contributes data from the algal-rich Cenozoic carbonates of Iran.
2. Geological Setting
In the present study, samples yielding fossil coralline red algae were collected from the Lower Miocene Qom Formation in the Vartun section (Figures 1 and 2). This section is located about 50 km northeast of the city of Esfahan (32° 56′ 13.79″ N and 52° 08′ 45.32″ E; Figure 1) and is situated in the north of the Esfahan-Sirjan fore-arc basin in central Iran (Figure 1A). The most exposed strata in the studied area comprise Quaternary terraces in addition to Triassic (massive dolomite, dolomitic limestone, limestone, shale), Jurassic (sandy shale, sandstone, conglomerate), and Oligocene-Miocene deposits (Figure 1C). Strata of the Qom Formation were deposited on the northeastern edge of the Tethyan Seaway in the Sanandaj-Sirjan fore-arc basin, and/or Esfahan-Sirjan fore-arc basin, and the Urumieh-Dokhtar magmatic arc basin in central Iran during the last marine transgression in the Oligo-Miocene (Daneshian and Ramezani Dana, 2007; Reuteret al., 2009; Zágoršeket al., 2017).
Figure 1. (A) Geological and structural map of Iran and location of the studied section (Vartun section) in the north of the Esfahan-Sirjan Basin in central Iran (modified from Rahiminejad and Hassani, 2016; Rahiminejadet al.,2017, 2018; Rahiminejad and Zand-Moghadam, 2018, 2020; Moosavizadehet al., 2020). (B) Location map of the Vartun section (modified from Zágoršeket al., 2017). (C) Geological map of the studied area (redrawn and modified from Zahedi, 1978 and Rahiminejadet al., 2020)
Figure 2 Measured stratigraphic column of the Qom Formation in the Vartun section. Age of the Qom Formation in the section is based on Rahiminejadet al., 2020. Our studies focused on a 1 m-thick interval algal limestone comprising coralline red algae-rich grainstone. The symbols illustrating the growth forms of the identified algae are redrawn and modified from Woelkerlinget al., (1993), and Nebelsick and Bassi (2000). The percentage or relative abundance of different algal growth forms in the studied grainstone facies is as follows: fruticose: 70 %, lumpy: 20%, foliose, encrusting, and arborescent: 10%.
Generally, the Oligocene and Miocene deposits of the Qom Formation in central Iran consist mainly of limestone, marl, gypsum, and subordinate siliciclastic sediments containing diverse fossil invertebrates represented by corals, gastropods, echinoids, bivalves (particularly pectinids), and coralline red algae (Seyrafian and Toraby, 2005; Reuteret al., 2009; Yazdiet al., 2012; Rahiminejadet al., 2020). These unconformably overlie the continental rocks of the Lower Red Formation (Oligocene) and are unconformably overlain by the Middle to Upper Miocene continental strata of the Upper Red Formation (Stöcklin and Setudehina, 1991; Yazdi- Moghadam, 2011; Amirshahkarami and Karavan, 2015; Rahiminejadet al., 2020). The depositional environments of the Qom Formation in the Esfahan-Sirjan Basin are consistent with mixed carbonate-siliciclastic homoclinal ramp (Reuteret al., 2009), open shelf (SE Chahriseh (Moradi, 2012)), and inner and middle ramps Vartun section (Rahiminejadet al., 2020).
3. Materials and Methods
The investigation of fossil coralline red algae and their associated facies or sedimentary texture was based on microscopic analyses of thin-section slides from thirty rock samples of algal limestone that were examined with a Stereo binocular Microscope model Yaxun YX AK21 and photographed using a CCD digital camera model KECam. The classification of carbonate facies followed that of Dunham (1962) and Flügel (2010), which is based on the types and abundance of grains and the matrix in the deposits. Taxonomic analysis of the identified coralline red algae was based on Braga (2003), Brandano and Piller (2010), and Sarkar (2017). Coralline red algal growth forms were described according to Woelkerlinget al. (1993), and Nebelsick and Bassi (2000).
Since the collected algal specimens in the studied section are present as bioclasts and partially preserved fossils, confirmation of taxa at the species level cannot be accurately established. However, detailed statistical analyses on well-preserved algal samples may improve paleoecological interpretation in future studies.
The age of the Qom Formation in the Vartun section (Figures 1 and 2) is based on previous studies carried out by Rahiminejadet al. (2020) (Figure 3).
Figure 3 Some of the identified large benthic foraminifera used for age determination of the Qom Formation in the Vartun section (Rahiminejadet al.,2020). (A)Lepidocyclinasp., (B)Nephrolepidinasp., (C)Eulepidinasp., (D)Operculinaspp., (E)Miogypsinasp., (F)Amphisteginasp. Note that these microfossil specimens were not collected from the algal grainstone facies in this study.
4. Results
4.1. STRATIGRAPHY AND FACIES ANALYSIS OF THE STUDIED ALGAE-RICH LIMESTONE
We focused our paleoecological studies on a 1 m thick interval of Lower Miocene (Aquitanian) massive and lithified coralline red algae-rich limestone in the Qom Formation (Figures 2 and 4). The stratigraphic details of the Qom Formation in the studied section are shown in Figure 2. In the field, the limestone interval (Figures 2 and 4) is laterally exposed up to ca 70 m, is discontinuous, and untraceable in a wide area. CylindricalSkolithosburrows are common and fossils of bivalves (pectinids), gastropods(Conus, Natica, and Pleurotomaria),and echinoids(Clypeasteroids)are abundant in the interval (Figure 2). Likewise, the limestone interval is composed of coralline algae-rich grainstone facies dominated by skeletal grains embedded in a sparitic matrix or sparry cement (Figures 5A and 5B). The latter (65%) include coarse-grained bioclasts and partially preserved specimens of abundant coralline red algae. Coarse-grained bioclasts of corals, echinoids, gastropods, and bivalves (22%) as well as scarce (13%) larger benthic foraminifera (Nephrolepidinasp.,Amphisteginasp., and miliolids) comprise the subordinate skeletal grains.
Figure 4 (A) Outcrop view of the studied coralline red algae-rich limestone (1 m thick) in a grainstone facies. (B) Rock sample of the studied algae-rich limestone.
4.2. THE IDENTIFIED CORALLINE RED ALGAE AND THEIR GROWTH FORMS
The micropaleontological studies revealed the presence of five genera of coralline red algae in the studied grainstone facies (Table 1 and Figures 5 to 8). Non-geniculate forms of the identified algae predominate (Table 1), whereas algae representative of geniculate forms (genusCorallina) are scarce. Several of the non-geniculate algae are highly fragmented and abraded, while geniculate forms are disarticulated and fragmented. The percentage or relative abundance of the algae is indicated in Table 1. The non-geniculate algae include melobesioids, mastophoroids, and sporolithaceans (Table 1). Melobesioids comprise the taxaLithothamnioncf.valens,Lithothamnioncf.rovereoti,Lithothamnioncf.peleense, andLithothamnionspp. Mastophoroids are represented by the taxaNeogoniolithonsp.,Spongitescf.fruticulosus, andSpongitesspp. Sporolithaceans include the taxaSporolithonspp., andSporolithoncf.airoldii.
Table 1 The identified coralline red algae and their growth forms in the algal grainstone facies. The algae are dominated by non-geniculate forms. Geniculate forms (Corallinasp.) of the algae are scarce. The percentage or relative abundance of different growth forms of the algae in the studied grainstone facies is as follows: fruticose: 70 %, lumpy: 20% foliose, encrusting, and arborescent: 10%.
| Algae: percentage (relative abundance) | Genera / species | Growth form | |
|---|---|---|---|
| Non-geniculate | Melobesioids: 60% |
Lithothamnion cf. valens Lithothamnion cf. rovereoti Lithothamnion cf. peleense Lithothamnion spp. |
Fruticose Lumpy Fruticose 1-Encrusting, 2-Foliose, 3-Fruticose |
| Mastophoroid: 25% |
Neogoniolithon sp. Spongites spp. Spongites cf. fruticulosus |
Foliose Fruticose Lumpy |
|
| Sporolithaceans: 10% |
Sporolithon spp. Sporolithon cf. airoldii |
Lumpy Lumpy |
|
| Geniculate | Corallina: 5% | Corallina | Arborescent |
Figure 6 Identified coralline red algae in the grainstone facies. (A) Fragment ofSporolithonsp. showing a lumpy growth form. (B) Fragment of a fruticose thallus of a melobesioid. (C) Foliose thalli ofNeogoniolithonsp. (D) Fragment (disarticulated) ofCorallinasp. with an arborescent growth form. (E) Fragment of a fruticose thallus ofLithothamnioncf.valens.
Figure 7 Identified coralline red algae in the grainstone facies. (A) Lumpy growth form ofLithothamnioncf.rovereoti. (B) Fragment of a fruticose thallus ofLithothamnionsp.; partially abraded. (C) Fragment of fruticose thalli ofLithothamnioncf.peleense. (D) Fruticose thallus ofLithothamnionsp. (E) Foliose thalli ofLithothamnionsp.
Figure 8 Identified coralline red algae in the grainstone facies. (A) Fragment of a fruticose thallus ofLithothamnionsp. (B) Fragment ofLithothamnionsp. (encrusting growth form). (C) Melobesioid algae with a fruticose growth form.
In the studied grainstone facies, the identified coralline red algae mainly display fruticose (70 %) and lumpy (20%) growth forms (Table 1). Algae with foliose, encrusting, and arborescent growth forms are also present (Table 1), although subordinately (10%).
5. Discussion and interpretation
5.1. PALEOECOLOGY OF THE IDENTIFIED CORALLINE RED ALGAE
The dominance of a sparitic matrix or sparry cement in the identified algal grainstone facies and the abundance of bioclasts and skeletal grains point to a shoal environment (in an inner ramp) in a high-energy water zone above the fair-weather wave base during the Aquitanian (Flügel, 2010; Sadeghiet al., 2011; Vescogniet al., 2014; Roozpeykaret al., 2019b).
The presence ofSkolithosburrows in the algal grainstone-bearing limestone indicates high hydrodynamic (water) energy (Vinn and Wilson, 2013; Sedorkoet al., 2018). Further details on the depositional environment of the Lower Miocene deposits of the Qom Formation in the Vartun section have been provided by Rahiminejadet al. (2020), whose results confirm our environmental interpretation.
The co-occurrence of melobesioids (Lithothamnion), mastophoroids (Neogoniolithon,Spongites), andSporolithonmay be representative of the basinward part of the shoal environment, although a number of these algae could have been reworked and transported in the shoal environment.
Usually, the occurrence of coralline red algae in a grainstone facies is related to nutrient influx due to encroachment of siliciclastics (Erlichet al., 1990, 1993; Roozpeykaret al., 2019a). For instance, Ghosh and Sarkar (2013) have reported a grainstone facies comprising an algal association ofCorallina,Lithothamnion, andSpongitesfrom Pliocene reef and back-reef shelf environments as well as a melobesioid coralline algal assemblage ofPhymatolithon,Mesophyllum, andLithothamnionfrom a coralline algal boundstone in a Pliocene fore-reef environment. Roozpeykaret al. (2019a, 2019b) have studied a coralline red algae-rich grainstone in a Miocene shoal environment in the Zagros Basin and have reported non-geniculate (dominantSpongitesand reworked melobesioids) and geniculate algae (Corallina).
Generally, coralline red algae were one of the major and most abundant’biotic components of photozoan and heterozoan communities of Oligocene-Miocene cool waters (non-tropic) and tropical carbonates (Bassi and Nebelsick, 2010; Bragaet al., 2010). Mastophoroids (specificallySpongites) are common in shallow tropical carbonates (Adeyet al., 1982; Braga and Aguirre, 2001, 2004), but are also present in subtropical and temperate environments. Some genera (e.g. LithothamnionandMesophyllum) of the subfamily Melobesioideae occur in both temperate and warm conditions (Braga and Aguirre, 2001; Halfar and Mutti, 2005; Bassiet al., 2009; Bragaet al., 2010; Brandano and Piller, 2010; Rösleret al., 2015; Roozpeykaret al., 2019a, 2019b). Also, sporolithaceans (Sporolithon) are common in tropical deposits (Brandanoet al., 2005). Coralline red algae can survive in different trophic conditions ranging from oligotrophic to mesotrophic environments in different latitudes (Bragaet al., 2010). The main paleoecological factors controlling their growth and development in benthic marine environments include depth, hydrodynamic energy, chemical composition, light intensity, and temperature of marine waters (Sarkar, 2017). The different growth forms of coralline red algae are useful tools in paleoecological interpretation of marine carbonate facies (Bosence and Pedley, 1982; Braga and Martín, 1988; Braga and Aguirre, 2001; Rasser and Piller, 2004; Brandanoet al., 2005; Brandano and Ronca, 2014) and include warty, lumpy, fruticose, foliose, encrusting, layered (non-geniculate), and arborescent (geniculate) (Woelkerlinget al., 1993; Nebelsick and Bassi, 2000).
As evidenced in this study, the dominance of coralline red algae with fruticose growth forms (Table 1) in the shoal carbonate environment is consistent with high water energy (Bosence, 1991; Basso, 1998; Nebelsicket al., 2000). Agitation in the shoal environment, resulted different taphonomic features such as high fragmentation, disarticulation, and abrasion of several algae (Nebelsicket al., 2000; Nebelsick and Bassi, 2000; Roozpeykaret al., 2019a, 2019b). The identified algae (Table 1 and Figures 5-8) and the presence of larger benthic foraminifera point to a well-illuminated euphotic marine zone (Vescogniet al., 2014; Roozpeykaret al., 2019a, 2019b). The overall recognition of algae indicates that the Lower Miocene grainstone was deposited in warm tropical to subtropical waters (Bosence, 1983a, 1983b; Braga and Aguirre, 2001; Brandanoet al., 2005; Roozpeykaret al., 2019a). Moreover, tropical conditions can be inferred owing to the presence of skeletal grains of corals and larger benthic foraminifera such as miliolids,Amphistegina, andNephrolepidinain the studied grainstone facies (Pomaret al., 2004; Brandanoet al., 2009; Roozpeykaret al., 2019a, 2019b).
6. Conclusions
The Lower Miocene (Aquitanian) algal-rich grainstone facies of the Qom Formation of Esfahan-Sirjan basin in central Iran is characterized by non-geniculate coralline red algae represented by melobesioids, mastophoroids, and sporolithaceans. The distribution of coralline red algae in the grainstone facies is probably linked to nutrient influx and the dominance of the fruticose growth form is indicative of high-energy conditions in a shoal environment. Due to the dominance of algae in well-illuminated waters, it may be concluded that the studied facies was deposited in the euphotic marine zone, and the Lower Miocene algal forms were inhabited in warm tropical to subtropical waters during the Aquitanian, having high-energy conditions, (i.e.increased hydrodynamic activity coupled with sediment agitation) that led to enhanced fragmentation, abrasion, and disarticulation of many algal forms recorded herein.
