The Baghamshah Formation was previously assigned to Jurassic age in Iran, based on stratigraphic distribution of ammonites. But, recent studies of calcareous nannofossils in the Lut Block show that the age of this formation should be assigned to the Early Cretaceous (early Berriasian to early Barremian). This study analyzed stratigraphic distribution of calcareous nannofossils of the Baghamshah Formation in the Esfandiar section located in the southeast of the Tabas Block (close to the type section). Examinination of samples identified 39 calcareous nannofossil and 6 didemnid ascidian spicules species belonging to 19 genera, corresponding from CC1 to CC5 biozones with the age of early Berriasian to early Barremian. Index nannofossil species in the succession indicate that the sedimentary basin of the Baghamshah Formation in the Esfandiar section was located in low latitudes of the Tethyan realm with warm surface water toward the top of the section. Also, the oceanic basin was an oligotrophic type with low-fertility, as its nutrient supply dropped toward the top of the section. The oligotrophic paleoenvironment during the late Berriasian to early Valanginian in the study area in eastern Iran may have corresponded to a global low sea-level.
Introduction
Stöcklin (1961) and Stöcklin et al. (1965) described the Baghamshah Formation for the first time in the east of Tabas city (central Iran). At the type section, the Baghamshah Formation conformably overlies the Parvadeh Formation and is conformably overlaid by the Esfandiar Formation (Aghanabati, 1977). The investigations on Baghamshah Formation biostratigraphy (Iran) are by Alavi-Naini (1972) and Seyed-Emami et al. (1997, 1998, 2002), based on ammonites. In some areas, an age ranging from middle–late Bathonian to early Callovian was established (Seyed-Emami et al., 1997, 1998), extending to the middle Callovian (Seyed-Emami et al., 2002). Furthermore, according to recent studies of the Baghamshah Formation by Kallanxhi et al. (2015, 2016) in the Damghan area (based on calcareous nannofossils and ammonites), the age of this formation is determined as early to middle Callovian. Baghamshah Formation palynomorphs were investigated by Hashemi-Yazdi and Sajjadi (2015) which collectively indicated a Middle Jurassic (Bajocian–Bathonian–Callovian) age. Mukherjee and Fürsich (2014) studied this formation's brachiopods and the age of the Baghmshah Formation was determined as Bathonian to early Callovian. Pandey and Fürsich (2003) studied the corals of these sediments. According to this study (corals), this formation's age is considered Bathonian–middle Callovian (Figure 1).
These previous studies show asynchronous ages for this formation. Some studies have been done on the calcareous nannofossils of the Baghamshah Formation in the Lut Block, and show that the age of this formation in the Baghdadeh and Deheshk sections was assigned to the early Berriasian to early Barremian (e.g. Khodashenas et al., 2019; Behdani et al., 2019) (Figure 1). Baghamshah Formation in Tabas Block is so widespread, and no paleontological studies have been done on the Esfandiar section in this Block. Thus, we introduced calcareous nannofossils for age determination of the Baghamshah Formation in this section for the first time. In total, this study aims to investigate the biostratigraphy and paleoenvironmental conditions of the formation based on calcareous nannofossils.
Geological settings
Baghamshah Formation is distributed in Central Iran Basin, which is one of the most important and complicated structural units in Iran's Plate (Stöcklin, 1968) and is located as a triangle in the middle of Iran. The paleogeographical position of Iran Plate during sedimentation of Baghamshah Formation was at the northern margin which is approximately 20–30°N (Wilmsen et al., 2009). Tabas and Yazd Blocks in the west, and Lut Block in the east constituted so-called Central Iranian Microcontinent (Takin, 1972). Nayband Fault separates Tabas Block from Lut Block. The western margin of Tabas Block has located at Anar Fault that this block separates from Yazd Block (Figure 2).
The Baghamshah Formation is the second formation of the Magu Group which was described by Aghanabati (1975) for the second cycle of the Jurassic successions of the Central Iran Basin. This formation was deposited in open-marine, middle–outer shelf to upper-slope settings (Kluyver et al., 1983).
The Baghamshah Formation in the Esfandiar section, which is located at 33°08′21″N and 57°26′31″E coordinates, west of Kuh-e-Esfandiar, southeastern Tabas Block (East of Iran) (Figures 2 and 3), was sampled and investigated. This formation is about 621 m thick in this region and consists of dark shales with interbedded sandstone and green calcareous shale. The Baghamshah Formation overlies the Shemshak Formation which is conformably overlain by the Esfandiar Formation (Figures 4 and 5).
Figure 2.
General Map of Iran including main structural units and Geographical situation studied section.
The overlying unit of the Baghamshah Formation (Esfandiar Limestone Formation) entirely consists of medium-bedded to massive carbonates of light-grey–brownish-yellow color deposited on an extensive carbonate platform, the ‘Esfandiar Platform’ of Fürsich et al. (2003). The Esfandiar Limestone Formation occurs in a north-south-trending outcrop close to the eastern margin of the Tabas Block (Shotori Swell). The Baghamshah or Sikhor Formation underlies the Esfandiar Limestone Formation in the type section, and the contact is often gradational. There is no Parvadeh Formation in the west of Kuh-e-Esfandiar, so the Baghmashah Formation directly overlies the Shemshak Formation. The underlying unit of the Baghamshah Formation (Shemshak Formation) contains shale and sandstone. Facies and thicknesses of formations in Shotori Range show striking differences in the areas west and east of the Shotari Range. Figure 5 shows the facies distribution of these formations on both sides of the Shotori Swell.
Material and methods
Fifty-three samples were collected from different exposures of the Baghamshah Formation in the Esfandiar section (mostly shales and green calcareous shales) (Table 1; Figure 6). Calcareous nannofossils were prepared in 53 smear slides using standard techniques described by Bown and Young (1998). They were analyzed under crossed Nicols equipped with the gypsum plate by Olympus BX50 light microscopy at 1250 × magnification. Digital images of nannofossil specimens were taken with a Sony digital camera. All calcareous nannofossil specimens were identified following taxonomic schemes of Thierstein (1976); Perch-Nielsen (1985); Crux (1989); Bown (1998); Varol (2006); Young (2008). The images of some important nannofossils are shown in Figure 7. Biostratigraphic data were interpreted using the zonation scheme of Sissingh (1977) commonly used for the lower and upper Cretaceous in the Tethyan area. Index nannofossil species found in the Iran Cretaceous deposits in this zonation are cosmopolitan species. The biozonation scheme for the early Berriasian–early Barremian interval is illustrated in Figure 8. Based on this figure, the biozones provided by different authors are correlated. The samples contain most of the index nannofossil species of Sissingh's (1977) zonation scheme; hence this zonation has been selected for the biozonation. Because of the greater temporal similarity among zonation schemes by Roth (1973), Sissingh (1977) and Applegate and Bergen (1988), these zonation schemes are compared in the text.
Figure 3.
Geological map of the area (Part of the Geological map 1:250,000 Boshruyeh; Eftekhar-nezhad and Zahedi, 1969. redrawn after Geological Survey of Iran). Fm., Formation.
Figure 4.
A, Overview of the research area; B, Close view of the Baghamshah Formation in the Esfandiar section. Fm., Formation.
Figure 5.
Facies distribution of the formations on both sides of the Shotori Swell. A, Adapted from Aghanabati (1975); B, Adapted from Stöcklin (1965). Q., Qal,eh; lst., limestone; calc., calcareous; Fm., Formation.
Figure 6.
Lithostratigraphy, biozonation and calcareous nannofossil abundance of the Baghamshah Formation in the Esfandiar section.
Relative abundances of the calcareous nannofossil taxa were recorded in 10 fields of view (Table 2) and estimated using the following categories (Figure 6):
A = abundant, (> 10 specimens per field of view); C = common, (2–10 specimens per field of view); F = few, (1 specimen per 1–5 fields of view); R = rare, (1 specimen per 6–10 fields of view); VR = very rare, (1 specimen in >10 fields of view). Diversity was considered as the number of identified taxa.
Table 1.
Number and stratigraphic levels of samples collected from the Baghamshah Formation in the Esfandiar section.
To determine the preservational degree of species, Flores and Marino's (2002) visual estimation scale was used. This scale was established based on the degree of etching and overgrowth. Rock samples and smear slides are stored at the Paleontology Laboratory of Department of Geology, Faculty of Science of Ferdowsi University, Mashhad City (Iran).
Results and discussion
This study presents data and discusses the biostratigraphy and palaeoecology of the Baghamshah Formation's calcareous nannofossils in the Esfandiar section in eastern Iran.
Nannofossil preservation, diversity, and abundance
Calcareous nannofossil preservation in our samples was rated as moderate to poor (Table 2). Specimens with moderate preservation were affected by dissolution and secondary overgrowth which partially altered primary morphological characteristics, but nearly all specimens can be identified at the species level. Moreover, poor (P) preservation is characterized by “severe dissolution, fragmentation, and secondary overgrowth with primary features mostly destroyed; many specimens cannot be recognized at species level and generic level” (Flores and Marino, 2002). Nannofossil preservation is poor in the lower part of the section, but upwards, specimens showed better preservation (Table 2). Poor preservation usually coincides with an insufficient quantity of nannofossils in smear slides. Therefore, all abundance nannofossils does not follow a general pattern as some species tend to increase or decline from base to top. Farrell and Prell (1989) believe that distribution, preservation, and accumulation of coccoliths on the seafloor depend on the balance between the biogenic production of CaCO3 in supersaturated surface waters and its dissolution in undersaturated deep waters. Therefore, the dissolution rate in the lower parts of studying section is higher than biogenic production of CaCO3, and in the upper parts of section, dissolution rate has decreased. On the other hand, Erba and Tremolada (2004) noted that nannoconids are generally the predominant fine-fraction constituent of sediments associated with high CaCO3 which decreases in nannoconids correlated with low calcium carbonate lithologies. The increase in nannoconid abundance in the upper parts of the section indicates an increase in CaCO3 and thus an increase in preservational quality.
Many authors (e.g. Armando and Eduardo, 1998; Mutterlose et al., 2005; Tremolada et al., 2009) noted that the most deposits rich in nannoconids usually indicate low-diversity coccolith assemblages. Street and Bown (2000) believe that nannoconids are neritic, warm water, r-selected taxa. This explains their presence in low diversity and their occurrence in shallow, neritic settings in the Early Cretaceous tropics. Low diversity nannofossil assemblages are probably indicative of unfavorable conditions in the water column (increased acidity and dissolution of delicate and fragile species). The results of Busson and Noёl (1991) indicate that there is an inverse relationship between nannoconid abundance and other coccoliths. Erba (1994) believes that fluctuation of the nutricline depth can explain the inverse abundance relationship between nannoconids and coccolithophores. The present findings similarly indicate that the nannoconids are more abundant than other nannofossils, therefore the diversity of coccolith assemblages in our samples is relatively low.
Watznaueria barnesiae and Nannoconus spp. are considered to be resistant to dissolution (e.g. Thierstein, 1976; Roth and Krumbach, 1986; Coccioni et al., 1992; Erba, 1992; Williams and Bralower, 1995). The enrichment of nannoconids and W. barneasae can be explained by the dissolution of other less resistant coccoliths. The fluctuations of nannofossil abundance in this sequence may thus be attributed to variable dissolution.
In total, 45 species were identified with 6 species belonging to didemnid ascidian spicules. According to the determined calcareous nannofossils in Esfandiar section, dominant nannofossil assemblages included Nannoconus, Conusphaera, Lithraphidites genera, and didemnid ascidian spicules. These genera were observed through the studied section (Figure 6). The most common and diverse genera within the assemblage are Nannoconus spp. (21 species in 1 genus). The most common Nannoconus species in the Esfandiar section are N. steinmannii steinmannii (0–60%), N. globulus (0–33%), N. kamptneri kamptneri (0–25%), N. circularis (0–42%) and N. bucheri (0–25%). Also, some species are present in the assemblage occurred sporadically with a relatively low percentage (Figure 6). Six didemnid ascidian spicules species of 3 genera were found in studying samples (Table 2).
Figure 7.
Calcareous nannofossils from the Esfandiar section. XPL, cross-polarized light; PPL, plane-polarized light. A–C, Nannoconus steinmannii steinmannii; A, sample 24; B, C, sample 4; D, E, Nannoconus kamptneri kamptneri; D, sample 46; E, sample 27; F, Nannoconus abundanc, sample 41; G, H, Nannovonus bucheri, sample 30; I, Nannoconus quadricanalis, sample 26; J, Nannoconus globulus globulus, sample 6; K, Calcicalathina oblongata, sample 27; L, Diazomatolithus lehmanii, sample 13; M, Cretarhabdus loriei, sample 30; N, Nannoconus circularis, sample 44; O, Nannoconus quadratus, sample 19; P, Rhagodiscus asper, sample 21; Q, Watznaueria barnesiae, sample 12; R, S, Retecapsa angustiforata, sample 19; T, Lithraphidites carniolensis, sample 11; U, Lithraphidites bollii, sample 46; V, Conusphaera mexicana mexicana, sample 34; W, X, Velasquezia praegothica, sample 16; Y, Paleodidemnum metaxy, sample 6.
Among the identified species in Esfandiar section, N. steinmannii, Retecapsa angustiforata, Calcicalathina oblongata, Certarhabdus loriei, Lithraphidites bollii, and Nannoconus abundans are biostratigraphical markers of the calcareous nannofossil zonation.
Table 2.
Preservation (P=poor, M=moderat) and relative abundances (%) of studied samples in 10 fields of view.
Biostratigraphy and zonation
Considering the first appearance of marker and associated nannofossils, CC1–CC5 biozones were recognized based on the Sissingh (1977) nannofossil biozonation scheme. The early Berriasian to early Barremian age of the deposits was assigned based on the Tethyan calcareous nannofossil zonations. Figure 6 shows the biostratigraphy and lithostratigraphy of Baghamshah Formation in the Esfandiar section, while the biozonation scheme for the early Berriasian–early Barremian interval is illustrated in Figure 8. According to the determined species, the biozones are as follows:
Nannoconus steinmannii zone (CC1)
This zone was explained as the interval from the first occurrence (FO) of N. steinmannii to FO of S. crenulata (= Cretarhabdus crenulatus = Retecapsa angustiforata) by Worsley (1971), amended by Thierstein (1971) and Sissingh (1977). Its age is Latest Tithonian to Early Berriasian (Perch-Nielsen, 1985). This zone is identified in the lower part of the Baghamshah Formation in the Esfandiar section. The earliest appearance of N. steinmannii steinmannii in the section is in sample 1 about 1 m from the base, and the last appearance of this species is in sample 53 at 621 m (Figure 7). Species appearances of N. colomii, N. dolomiticus, N. globulus, N. steinmannii minor, and N. steinmannii steinmannii and Polycostella beckmannii at the base of this section corresponded to the CC1 Calcareous Nannofossil Zone (Sissingh, 1977). Perch-Nielsen (1985) believed the first appearance of Lithraphidites carniolensis indicates the basal Cretaceous and the lowest part Berriasian. Therefore, the earliest appearance of L. carniolensis and N. steinmannii steinmannii in the first sample of the section confirms an early Berriasian age for the base of the Esfandiar section. The FO of N. steinmannii steinmannii and the lower part of this zone are also unknown. Therefore, its exact thickness of this zone was not determined. S. crenulata was not observed; hence according to Thierstein (1976) and Applegate and Bergen (1988), the FO of S. crenulata was replaced by the FO of R. angustiforata. Retecapsa angustiforata range occurs from sample 14 (165 m) to sample 44 (∼607 m). This zone is roughly equivalent to the NC1 and CC1 zones from Roth's (1973) zonation scheme and Applegate and Bergen's (1988) zonation scheme respectively (Figure 8).
Stradneria crenulata zone (CC2)
Thierstein (1971) proposed the S. crenulata Zone. The age of this zone is Late Berriasian to Early Valanginian (Perch-Nielsen, 1985), and based on the Geological time scale 2012, the age of the CC2 zone is Early to Late Berriasian (Ogg and Hinnov, 2012). This zone is the interval from the FO of S. crenulata or R. angustiforata (sample 14 at about 165 m) to the FO of C. oblongata (sample 24 at about 367 m). The most dominant species in this zone, besides the marker species, are W. barnesiae, W. ovata, C. margerelii, C. mexicana, N. dolomiticus, N. broennimannii, N. colomii, N. globulus, N. kamptneri, N. steinmannii, Rhagodiscus asper, and with L. carniolensis. C. oblongata is a marker species in the Tethyan Realm. The thickness of this zone is 202 m. This zone is relatively equivalent to the NC2 and CC2 zones from Roth's (1973) zonation scheme and Applegate and Bergen's (1988) zonation scheme, respectively (Figure 8).
Calcicalathina oblongata zone (CC3)
Thierstein (1971) proposed this zone and was emended by Sissingh (1977). The age of this zone is late Valanginian (Perch-Nielsen, 1985). Its age in the 2012 Geological time scale is Valanginian (Ogg and Hinnov, 2012). It includes the interval from the FO of C. oblongata to the FO of C. loriei (= Cretarhabdus striatus). In addition to the marker species, the most dominant species in this zone are W. barnesiae, C. mexicana, N. kamptneri, N. steinmannii, N. circularis, N. quadricunalis, Rhagodiscus asper, and L. carniolensis, C. oblongata is appeared in sample 24 at about 367 m from the base of section. The FO of C. loriei is in sample 30 at about 438 m from the base and marks the end of this zone. Therefore, this biozone's thickness is 71 m, and is nearly equivalent to NC3 and CC3 zones from Roth's (1973) zonation scheme and Applegate and Bergen's (1988) zonation scheme, respectively (Figure 8).
Cretarhabdus loriei zone (CC4)
Sissingh (1977) proposed the C. loriei Zone. This zone's age is Early Hauterivian (Perch-Nielsen, 1985), and in the Geological time scale of 2012 is late Valanginian to late Hauterivian (Ogg and Hinnov, 2012). This zone is identified from the FO of C. loriei to the last occurrence (LO) of Speetonia colligata by Sissingh (1977). Thierstein (1976) has used the FO of L. bollii for this zone's subdivisions in the Tethyan Realm. Applegate and Bergen (1988) stated that FO of L. bollii divides this biozone into two subzones of CC4a and CC4b. CC4a subzone is the interval from the FO of C. loriei (sample 30 at about 438 m from the base) to the FO of L. bollii (sample 36 at about 525 m from the base). Thus, this subzone's thickness is 87 m. By the appearance of L. bollii at 525 m, an Early Hauterivian age was suggested for this part of section. As Speetonia colligata were not found, Taylor (1982) believes that the LO of S. colligata can be replaced by the FO of N. abundans. The FO and LO of N. abundans is in sample 41 at 600 m from the base and sample 53 (621 m from the base). The FO of N. abundans at sample 41 at 600 m from the base could confirm a Late Hauterivian age for this part of section. According to Mutterlose (1992), N. abundans is an endemic species making it complicated to determine the upper boundary of CC4b. Thus, two CC4b Subzones and CC5 zones are merged here, and from 525 m to the top of the section is attributed to the CC4b Subzone–CC5 zone.
Lithraphidites bollii zone (CC5)
Thierstein (1971) and Sissingh (1977) believed that this biozone range is from the LO of S. colligata to LO of C. oblongata with an age of late Hauterivian to Early Barremian (Perch-Nielsen, 1985). Considering the lack of S. colligata in our sequence, the boundary between CC4b and CC5 is not exactly clear. The LO of C. oblongata and the upper part of this zone is also unknown. Therefore, its thickness was not determined. Moreover, the appearance of N. borealis at about 616 m (sample of 48) from the base of the section indicated a Barremian age for this part of the section. The appearance of Nannoconus wassallii at about 619 m could also confirm a Barremian age for the top of the Esfandiar section (Deres and Acheriteguy, 1980). The CC4b Subzone and CC5 zone are nearly equivalent to the NC4b Subzone and NC5 zone of Roth's (1973) zonation scheme (Figure 8).
Paleoecology
Calcareous nannofossils are widespread in the current oceans, from coastal areas to open ocean settings. In general, calcareous nannofossils supply only scattered information about the paleoecological conditions and the sedimentary region's paleogeographic position. The main factors controlling nannofossil distribution are latitude, water temperature, water depth, climate changes, and nutrients (Okada and Honjo, 1973; Baumann et al., 2005; Lees et al., 2005; Erba, 2006; Kҿdzierski, 2012). Studying these factors can help interpret and reconstruct palaeoclimate and paleosedimentary basin conditions. The presence of calcareous nannofossils documents marine water of medium salinity.
Tethyan forms dominate the assemblages recorded in this study. Relative abundance of nannofossils is determined for investigating paleoecological conditions. The relative abundances of our samples were recorded in 10 fields of view for the paleoecological interpretation of the Baghamshah Formation in the Esfandiar section. The vertical variation in these taxa's relative abundances is given in Figure 9, with data counts presented in Table 2. The most common species used for palaeoecological studies are N. globulus, N. kamptneri kamptneri, N. steinmannii steinmannii, N. circularis, N. bucheri, N. abundans, N. quadratus, W. barnesiae, C. mexicana, Rhagodiscus asper, L. carniolensis and also Didemnid ascidian spicules. Among these species, the species which belong to Nanocanus are more abundant in our section.
Nannoconids are an unusual group of calcareous nannofossils ranging from the late Tithonian to Campanian, and are found in pelagic/hemipelagic sediments together with coccoliths, and their affinity is uncertain, because they do not have any modern analog (Erba, 1994). Nannoconus spp. have long been considered to have somewhat different palaeoecological preferences from other calcareous nannofossil groups. Busson and Noёl (1991) synthesized the distribution of Early Cretaceous nannoconids at a global scale, and they noted that based on a high abundance of nannoconids in marginal seas and widespread occurrence at oceanic sites, the genus Nannoconus proliferated mainly in epicontinental basins. Lower Cretaceous limestones of Tethyan area often contain nannoconids (Farinacci, 1964; Dufour and Noёl, 1970; Erba, 1989, 1994).
Nannoconus spp. is the most critical group with relative abundances varying from 0 to 85%. In some samples, nannoconids observed in the Esfandiar section are dominated by wide canal forms. The most common species of nannnoconids are N. kamptneri (Figure 7d and 7e), N. steinmannii steinmannii (Figure 7a–c), N. globulus (Figure 7j), N. bucheri (Figure 7g and 7h), and N. circularis (Figure 7n). Nannoconids are rare in samples 10 and 14 from Baghamshah Formation, and their relative abundances increase up to 85% in samples 38 and 47. The average relative abundance for Nannoconus spp. was 0% about 165 m and 85% the upper part of section (based on 10 fields of view).
In general, the diversity and abundance of nannoflora reflect the nutrients of surface water masses. Most calcareous nannoplanktons are mesotrophic to oligotrophic (Brand, 1994). W. barnesiae is abundant in oligotrophic conditions (Kędzierski, 2012). Busson and Noёl (1991), Coccioni et al. (1992), and Erba (1994) proposed that nannoconids were oligotrophic, characteristic of highly oligotrophic pelagic environments. Furthermore, Nannoconus spp., C. mexicana, and L. carniolensis were interpreted as abundant species in oligotrophic conditions by Bornemann et al. (2003). Besides, Erba (1987) and Erba et al. (1992) interpreted L. carniolensis and R. asper as abundant taxa in sediments with poor nutrients. Abundance of Nannoconus spp. was controlled by fluctuations in nutricline depth (Erba, 1994; Herrle, 2003; Mutterlose et al., 2005). Some studies indicate that nutrient levels are the main limiting factor in nannnoconid occurrence. A high abundance of Nannoconus spp. may indicate a deep chlorophyll maximum zone (DCM) with increased productivity in the lower photic zone (∼80–200 m) (Erba, 1994).
Furthermore, the abundance of Nannoconus spp. and W. barneasae increase toward the top of the sequence. In the sequence investigated, only in the interval upper lower Berriasian, does nanoconid abundance decrease which may indicate a decrease in oligotrophy in this part of the section investigated. Therefore, our findings indicate an environment with oligotrophic conditions and a reduction of nutriients toward the top of this section.
Relative abundance and diversity of nannoconids increase in the late Berriasian–earliest Valanginian interval. Based on Erba (1994), nannoconids are oligotrophic characteristic of highly oligotrophic environments (nutrient-poor); therefore, according to Haq et al. (1987), a late Berriasian–earliest Valanginian oligotrophic palaeoenvironment is assumed for the study area corresponding to a global low stand sea-level.
Some nannofossil species such as Biscutum spp. and Zeugrhabdotus spp. (mainly Z. erectus), Cretarhabdus spp., Tranolithus orionatus, and Nannoconus spp. are good indicators of surface water fertility. Biscutum spp. and Zeugrhabdotus spp. are classified as indicators of high surface water fertility in unstable environments, but Nannoconus spp., Cretarhabdus spp., and T. orionatus are considered to be indicators of low fertility conditions (Roth and Krumbach, 1986; Watkins, 1989). Nannoconus spp. is the most abundant nannofossil group in our samples but Biscutum spp. is absent, and Zeugrhabdotus spp. is very rare. Indicators of high fertility are missing, thus it is not clear here; therefore, based on Nannoconus and mentioned coccoliths, that sedimentary basin had low fertility conditions during the sedimentation of the Baghamshah Formation of surface waters. Furthermore, due to the increased abundance of Nannoconus spp. upwards it is suggested that basin fertility decreased.
Since coccolithophores need sunlight for photosynthesis, they live at different depths in the photic zone (Winter and Siesser, 1994). Roth and Krumbach (1986) interpreted Nannoconus as an index of the lower photic zone (Erba, 1994; Herrle, 2003; Mutterlose et al., 2005). Mutterlose et al. (2005) believe that nannoconids have a K-selected life mode by which they can grow at lower light intensities. The occasional occurrence of Nannoconus can confirm depth fluctuation which shallow in the depositional area (Svobodova et al., 2011).
Didemnid ascidians prefer marine waters rich in carbonate (particularly coral reef environments). These environments are especially favorable for developing didemnid ascidian colonies. They indicate the most significant abundances in the shallow depths of warm seas in the world (Varol, 2006). Furthermore, according to Varol (2006), nannofossil abundance is reduced when didemnid ascidians abundance increases, suggesting shallowing waters. We can see that didemnid ascidians are increasing wherever nannofossil abundance decreases (Figure 9). Therefore, the high abundances of Nannoconus spp. and didemnid ascidians suggested that the Esfandiar section was deposited in a relatively shallow marine environment. Upward increase in abundance of Nannoconus spp. suggested that basin depth increased with some fluctuations of depth through the section (due to their life in lower photic zone with lower light intensities).
W. barnesiae abundance was found to have an inverse relationship with depth (Thierstein, 1976). Watznaueria barnesiae abundance increases from the base to the top of the sequence investigated. From 0%, within the lower part of studying section, its abundance increases up to 17% (sample 39) and 12.5% (sample 44), then decreases in the upper part of section. Considering the increase in W. barnesiae abundance in only 2 samples (39 and 44) and also the inverse relationship between depth and abundance of this species (Thierstein, 1976), it can be concluded that basin depth has slightly decreased. This is confirmed by the decrease in the abundance of Nanoconus spp. in the mentioned samples, indicating a lower depth of the photic zone (Figure 9).
W. barnesiae was a cosmopolitan taxon which covered a broad temperature range in the low and high latitudes of the Mesozoic (Mutterlose, 1996). Thibault and Gardin (2007) believed that W. barnesiae was a warm-water indicator during Cretaceous time. Nannoconus spp. was dominant in low latitudes showing warm waters of the Tethyan realm (Mutterlose et al., 2005). They were rare in Boreal realm (Mutterlose, 1992). Melinte and Mutterlose (2001) noted that high numbers of nannoconids reflect warmer conditions and a rather stable surface stratification. The success of Nannoconus in warm, neritic settings suggests that it was the most successful nannoplankton group in this environment (Street and Bown, 2000). C. oblongata and L. bollii are reported as Tethyan nanno-floras (Mutterlose, 1991). Other nannofossils such as Watznaueria spp., R. asper, Micrantholithus spp. and Conusphaera spp. are the assemblages of low latitudes. These thermophilic warm water taxa (Erba, 1987; Erba et al., 1992) indicate relatively warm surface waters of tropic and subtropic areas (Mutterlose et al., 2005). The occurrences of R. asper and L. carniolensis are recorded in samples 12 and 1, respectively. Erba (1987) and Erba et al. (1992) interpreted these species as thermophilic warm-water taxa indicating warm surface water.
Considering relative abundance of Nannoconus spp. and other warm-water indicators, nannofossils such as W. barnesiae, C. mexicana, L. carniolensis, and R. asper suggest warm surface water conditions and a relatively low-middle latitude of the Esfandiar section of the Baghamshah Formation. Furthermore, Street and Bown (2000) believe that N. abundans and N. borealis represent endemic species adapted to the cooler waters of North Sea Basin and adjacent seaways. Hence, the occurrence of both species in the Barremian (In samples of 44 and 49, respectively) show that surface water temperature in the uppermost part of Baghamshah Formation in Esfandiar section had slightly decreased.
Conclusions
Nineteen genera and 45 species of calcareous nannofossils and didemnid ascidian spicules were observed in the Baghamshah Formation in the Esfandiar section. These species correspond to the CC1–CC5 biozones, according to the Sissingh (1977) biozonation. Considering the distinguished biozones, the age of the Baghamshah Formation in this section is Early Berriasian to Early Barremian. According to the studies, calcareous nannofossil preservation in studying samples is from moderate to poor. Palaeoecological investigations based on calcareous nannofossil abundance in the Esfandiar section suggests that the Baghamshah Formation was deposited in relatively shallow marine environments from low to middle latitudes of the Tethyan realm with warm surface water and low fertility conditions. Our studies also suggest an increase in the depth and reduction of nutrients and fertility toward the top of this section.
References
Appendices
Author contributions
Idea for the article: [Fatemeh Hadavi]; Conceptualization: [Fatemeh Hadavi], [Ensieh Behdani], [Marziyeh Notghi Moghaddam]; Material Preparation: [Ensieh Behdani]; Data collection: [Ensieh Behdani]; Formal analysis and investigation: [Fatemeh Hadavi], [Ensieh Behdani], [Marziyeh Notghi Moghaddam]; Researching the literature: [Ensieh Behdani]; Writing - original draft preparation: [Ensieh Behdani]; Writing - review and editing: [Fatemeh Hadavi], [Ensieh Behdani], [Marziyeh Notghi Moghaddam]; Resources: [Ensieh Behdani]; Supervision: [Fatemeh Hadavi]. All the authors contributed to write this paper. All authors read and approve the manuscript. The content of the manuscript has not been published or submitted for publication elsewhere.
