The calcareous nannofossil group is a powerful tool for studying stratigraphic correlations on a global and regional scale, and has been discussed in detail by various researchers (e.g., Martini 1971; Okada and Bukry 1980; Perch-Nielsen 1985; Gradstein et al. 2012). In the present study, lithologies of the upper part of the Pabdeh Formation and the lower part of the Asmari Formation in the Bid-Zard section were examined. A total of 13 genera and 29 species of calcareous nannofossils were identified, which are well preserved in most slides and document general changes through the section.
The zonal boundaries were determined by the comparison with the standard biozonations of Martini (1971) and Agnini et al. (2014). Accordingly, our data based on calcareous nannofossil events allow the studied section to be assigned to the NP19/NP20 combined zone to the NP25 zone, indicating a Priabonian to Chattian interval. This study also focuses on the environmental conditions of the calcareous nannofossils, which are index fossils at low latitudes during the late Eocene to late Oligocene.
Five biozones in the studied section are introduced in the following. They are equivalent to the NP19/NP20 combined zone to NP23 zone in the upper part of the Pabdeh Formation and to the NP25 zone in the lower part of the Asmari Formation according to the Martini (1971) zonation:
3.1 Biozones in the upper part of the Pabdeh Formation
3.1.1
Isthmolithus recurvus Zone/Sphenolithus pseudoradians Zone (NP19/NP20 combined zone) — Top of CNE18, CNE19, CNE20/CP15b
The lowest nannofossil biozone recorded in this study is the Priabonian Isthmolithus recurvus Zone/Sphenolithus pseudoradians Zone (NP19/NP20 combined zone). The NP19 zone is identified from the first occurrence of Isthmolithus recurvus to the first occurrence of Sphenolithus pseudoradians in the Martini (1971) zonation. Isthmolithus recurvus was not found in the studied section, so the lower boundary of the NP19 zone has not been detected. Subsequently, the NP20 zone is defined from the first occurrence of Sphenolithus pseudoradians to the last occurrence of Discoaster saipanensis in the Martini (1971) zonation. According to Martini (1971), since the first occurrence of S. pseudoradians was reported differently and might not be an accurate indicator for determining the lower boundary of the NP20 zone, the NP19 zone cannot be separated clearly from the NP20 zone. Hence, the interval below the last occurrence of Discoaster saipanensis (the marker of the base of both NP21 and CP16 zones), can be defined as the NP19/NP20 combined zone or is equivalent to the CP15b subzone. In this zone, the calcareous nannofossil species are abundant and they are well-preserved. The nannofossil assemblages include Blackites spinosus, Cribrocentrum isabellae, C. reticulatum, Coccolithus pelagicus, Cyclicargolithus floridanus, Discoaster deflandrei, D. barbadiensis, D. saipanensis, Ericsonia formosa (=Coccolithus formosus), Helicosphaera bramlettei, H. compacta, H. euphratis, Pontosphaera multipora, Reticulofenestra reticulata, R. stavensis, R. umbilicus, R. minuta, Sphenolithus praedistentus, S. pseudoradians, S. moriformis, and Zigrhablithus bijugatus. According to the Agnini et al. (2014) zonation, the CNE18 zone is defined as the interval from the top of common Cribrocentrum erbae to the first occurrence of Cribrocentrum isabellae. In the Bid-Zard section, the base of the CNE18 zone cannot be recognized due to the absence of Cribrocentrum erbae, but the top of the zone was determined by the first occurrence of C. isabellae (at 11.15 m). The CNE19 zone is defined from the first occurrence of C. isabellae (at 11.15 m) to the last occurrence of C. reticulatum (at 33.42 m), which is restricted to the Priabonian. In addition, the CNE20 zone is defined by the last occurrence of C. reticulatum, a suitable index used to determine the boundary of the CNE19/CNE20 combined zone (Fornaciari et al. 2010; Agnini et al. 2011). The last occurrence of C. reticulatum is found at the 33.42 m level and the last occurrence of D. saipanensis at 44.56 m, which respectively mark the base of the CNE21 zone and the top of the CP15b subzone in the Okada and Bukry (1980) zonation. Therefore, the NP19/NP20 combined zone corresponds to the upper part of CNE18, CNE19, CNE20, and CP15b with a thickness of 44.56 m.
3.1.2
Ericsonia subdisticha Zone (NP21) — CNE21–CNO1/CP16a–CP16b
The next nannofossil biozone recorded in the upper part of the Pabdeh Formation is the NP21 zone dated as Priabonian/Rupelian. The Ericsonia subdisticha Zone is defined as an interval from the last occurrence of Discoaster saipanensis to the last occurrence of Ericsonia formosa (=Coccolithus formosus) (Martini 1971). The last occurrence of Discoaster saipanensis and the last occurrence of Ericsonia formosa were found at 44.56 m and 66.84 m, respectively. The most important nannofossil taxa of this zone belong to Coccolithus pelagicus, C. miopelagicus, C. formosus, Cyclicargolithus floridanus, Dictyococcites bisectus, Discoaster deflandrei, Helicosphaera bramlettei, H. compacta, H. euphratis, Pontosphaera multipora, Reticulofenestra minuta, Sphenolithus moriformis and Zygrhablithus bijugatus. It should be mentioned that across this interval, the total abundance of nannofossil increased and they are well-preserved. In the Agnini et al. (2014) zonation, the CNE21 zone is introduced as an interval from the last occurrence of Discoaster saipanensis to the first occurrence of acme Clausicoccus subdistichus. Subsequently, CNO1 is defined as an interval from the first occurrence of acme Clausicoccus subdistichus to the last occurrence of Ericsonia formosa (=Coccolithus formosus). In the present study, the base of the CNO1 zone cannot be defined due to the absence of C. subdistichus. Therefore, the CNE21/CNO1 zone boundary and the CP16a/CP16b subzone boundary are not specified. The last occurrence of E. formosa (=C. formosus), marks the top of the CNO1 zone and the CP16b subzone (66.84 m). Hence, the NP21 zone corresponds to the CNE21–CNO1 combined zones and also to the CP16a–CP16b combined subzones. The thickness of this zone is 22.28 m.
3.1.3
Helicosphaera reticulata Zone (NP22) — CNO2/CP16c
The Helicosphaera reticulata Zone (NP22) is defined as the interval from the last occurrence of Ericsonia formosa (=Coccolithus formosus) to the last occurrence of Reticulofenestra umbilicus in the Martini (1971) zonation. In this study, the NP22 zone spans an interval from the last occurrence of C. formosus (at 66.84 m) to the last occurrence of Reticulofenestra umbilicus (at 89.12 m) and falls in the Rupelian (32.95–32 Ma) (Gradstein et al. 2012). According to Martini (1971), the high occurrence of R. umbilicus defines the base of the NP23 zone. Species identified in this biozone include Braarudosphaera bigelowii, Coccolithus pelagicus, C. miopelagicus, Cyclicargolithus floridanus, Discoaster deflandrei, Dictyococcites bisectus, Helicosphaera euphratis, H. compacta, H. bramlettei, Reticulofenestra umbilicus, R. minuta, Sphenolithus praedistentus, S. moriformis, S. pseudoradians, and Zygrhablithus bijugatus. The preservation of calcareous nannofossils in this zone is moderate to good. In the Agnini et al. (2014) zonation, the CNO2 and CNO3 zones are defined by the last occurrence of E. formosa (=C. formosus) and the last occurrence of R. umbilicus, which were found in the Bid-Zard section at 66.84 m and 89.12 m, respectively. Therefore, the NP22 zone corresponds to the CNO2 zone and CP16c subzone, already shown by Agnini et al. (2014). The NP22 zone reaches a thickness of 22.2 m.
3.1.4
Sphenolithus praedistentus Zone (NP23) — CNO3 and the lower part of CNO4/CP17 and the lower part of CP18
The uppermost nannofossil biozone recorded in the Pabdeh Formation is the Sphenolithus praedistentus Zone (NP23). According to Martini (1971), the NP23 zone spans an interval from the last occurrence of Reticulofenestra umbilicus to the first occurrence of Sphenolithus ciperoensis. As mentioned above, the last occurrence of Reticulofenestra umbilicus was recorded at 89.12 m, but the first occurrence of Sphenolithus ciperoensis was not confirmed due to the absence of all calcareous nannofossil species in the interval from 111.35 m to 118 m. Nannofossils were absent from the upper part of the Pabdeh Formation composed by the tufa microbial facies, which is a bio-event that coincided with the Rupelian/Chattian interval in the studied section. Therefore, the top of NP23 zone or the transition between the Pabdeh and Asmari formations is simply lacking in nannofossils. Here, the boundary between the formations as well as the upper part of the NP23 zone and the entire NP24 zone were not determined due to the lack of index species (Fig. 3). Species such as Braarudosphaera bigelowii, Coccolithus pelagicus, C. miopelagicus, Cyclicargolithus floridanus, Discoaster deflandrei, Dictyococcites bisectus, Helicosphaera bramlettei, H. euphratis, Reticulofenestra minuta, R. reticulata, Sphenolithus distentus, S. moriformis, S. praedistentus, S. pseudoradians and Zygrhablithus bijugatus were identified in the lower part of the NP23 zone, and were moderately preserved. In the Agnini et al. (2014) zonation, the CNO3 zone is defined from the last occurrence of Reticulofenestra umbilicus to the first occurrence of Sphenolithus distentus. In this study, the first occurrence of S. distentus was recorded at 105.85 m, so the boundaries of CNO3/CNO4 and CP17/CP18 zones were specified. According to Agnini et al. (2014), the CNO4 zone spans the interval from the first occurrence of Sphenolithus distentus to the last occurrence of S. praedistentus. As mentioned above, the last occurrence of S. praedistentus and the first occurrence of S. ciperoensis were not recorded. Therefore, the NP23 zone corresponds to the CNO3 zone and the lower part of CNO4 or the CP17 zone and the lower part of CP18. In this study, the abundance of calcareous nannofossils is also suddenly reduced in this biozone, and no nannofossil was recorded at levels between 111.35 m and 118 m. The lower part of the NP23 zone is defined as Rupelian (32–29.6 Ma) (Gradstein et al. 2012).
3.2 Biozone in the lower part of the Asmari Formation
Although calcareous nannofossils were absent from the transition zone of the Pabdeh and Asmari formations, gradually their abundance increased again in the shale of the Asmari Formation during the Chattian. The biozone identified close to the base of the Asmari Formation is the Sphenolithus ciperoensis Zone.
3.2.1
Sphenolithus ciperoensis Zone (NP25) — The lower part of CNO5/the lower part of CP19b
The last biozone in the section is the Sphenolithus ciperoensis Zone. It was proposed by Bramlette and Wilcoxon (1967) and emended by Martini (1976). According to Martini (1971), this zone is defined between the last occurrence of Sphenolithus distentus and the last occurrence of Sphenolithus ciperoensis. The nannofossil assemblage of the topmost part of the section includes Braarudosphaera bigelowii, Coccolithus pelagicus, Cyclicargolithus floridanus, Pontosphaera enormis, Sphenolithus dissimilis, S. moriformis, and Zygrhablithus bijugatus. The last occurrence of S. distentus and the first occurrence of S. dissimilis were recorded at 123.5 m, but S. ciperoensis was not identified. Therefore, only the lower part of the NP25 zone was determined. The preservation of calcareous nannofossils in this part is moderate to good. Moreover, the lower part of this zone corresponds to the lower part of the CP19b subzone defined by the last occurrence of Sphenolithus distentus (at 123.5 m) according to the Okada and Bukry (1980) zonation. Based on Agnini et al. (2014), the CNO5 zone spans the interval from the last occurrence of Sphenolithus praedistentus to the last occurrence of Sphenolithus ciperoensis. At the base of the Asmari Formation, the last occurrence of S. predistentus was not recorded. However, based on the last occurrence of S. distentus, the lower part of the CNO5 zone is suggested. The age of this zone is Chattian (late Oligocene).