Biostratigraphy and Paleoecological Study of Pabdeh Formation in Interior Fars , Zagros Basin , Iran

The Zagros basin, which trends NW-SE, is covered by a sea during the Paleogene, resulting in deposition of the Pabdeh Formation in the deeper parts of the basin. The thickness of the Pabdeh Formation in Tang-e-Abolhayat is about 221 m and it consists of purple shale at the base, gray shale, marl and marly limestone with a few horizons of nodular limestone toward the top. Seventytwo samples were collected and processed for nannofossil studies. Based on the identified nannofossil zones the age of the formation in Tang-e-Abolhayat is Late Paleocene-Late Oligocene (NP9NP24). Changes in the abundance of different genera over the studied interval have resulted in identification of three ecozones: Ecozone 1 is characterized by oligotrophic conditions with an abundance of warm water taxa; Ecozone 2 marks when conditions became more eutrophic and the abundance of temperate taxa increased; Ecozone 3 represents a return to oligotrophic conditions with an increase in warm water taxa.


Introduction
The Zagros Mountain belt, which trends NW-SE, extends over 1800 km from southeastern Turkey and north Iraq border in the NW and W Iran to the Makran area in the SE, where oceanic subduction is still active [1].The Zagros Mountains are tectonically a part of the Alpine-Himalayan belt and formed as a result of convergence between Eurasia and remnants of Gondwana.The Zagros formed after collision between the Eurasian and Arabian plates, with closure of the Neo-Tethys Ocean in Late Cretaceous and later times [2] [3].The Zagros region contains a large part of global thrust belt-related hydrocarbons, which are trapped in anticlinal structures.The Zagros fold-and-thrust belt is composed of two provinces (Lurestan and Fars), separated by the Dezful Embayment [2]- [5].After Late Cretaceous movement, sea level rose and the Zagros basin was progressively inundated during the Paleogene, resulting in deposition of the Pabdeh Formation in deeper parts of the basin [6].
The Pabdeh Formation is recognized both in outcrop and the subsurface in Dezful Embayment and Izeh Zone, Fars and Lurestan in Iran.This formation, like other units in the Zagros basin, has been studied in most areas as a source rock for petroleum [7].The studied section is located on Shiraz-Kazeron road along the Tang-e-Abolhayat (29˚42'17"N, 51˚57'26"E), with a thickness of about 221 m, and consists of 18 lithological units as shown in Figure 1 and  The aim of this study was to use calcareous nannofossil biostratigraphy to determine the age of this formation and identify paleoecological and paleoceanographic conditions during deposition based on nannoplankton assemblages.Coccoliths are tiny (micron-sized) calcite plates that form the covering of marine phytoplankton known as coccolithophores and when preserved as fossils are commonly referred to as calcareous nannofossils [9].Coccoliths are abundant in seafloor sediments above the calcite compensation depth (CCD), and are usually transported to the seafloor in fecal pellets in marine snow.Their effect on deep-ocean CO 2 budgets plays an important role in the global carbon cycle [9].The ubiquitous nature of these algae, found in the photic zone in tropical to sub-polar waters, has credited them as one the largest producers of calcium carbonate in the open ocean today [10].

Material and Methods
Seventy-two samples were collected from fresh surfaces of outcrops for calcareous nannofossil studies based on lithological changes.We dug into the formation approximately 0.5 m before collecting each sample to ensure that fresh material was recovered.
Small amount of samples was used for studies of nannofossils.The standard smear-slide technique was used to prepare samples for light microscopy [11], which is one of the fastest methods used to prepare samples for micropaleontological investigations.No chemicals were used except for distilled water.Small amount of material scraped on to coverslip, drop of water added, mixed and smeared using a toothpick, dried on a hotplate, and the coverslip was attacehed with opaxi glu to the slide.Then the slides were studied with plane-polarized and cross-polarized light at ×1000 magnification by Olympus B12 microscopy and using a gypsum plate when necessary.Species concepts are those described in Perch-Nielsen [12] and Bown [9] [13].A count of 300 specimens for each sample was completed for paleoenvironmental analysis, with the collected data transformed to percent abundance.
The preparation technique for scanning electron microscope (SEM) study is that described by Donnally [14]: 0.5 g of sample was powdered and dissolved in 50 ml of distilled water, and then placed for 30 s in an ultrasonic bath.The solution was then allowed to settle for 1 min.The supernatant was removed and set aside for an hour, poured off, the residue diluted with distilled water, and one drop was dried on the filter for SEM examination.In this study, the biostratigraphic zonation is based primarily upon the calcareous nannofossil zonation of Martini [15] and secondarily upon the zonation of Okada and Bukry [16] and the timescale used is that of Gradstein et al. [17].

Results
The distribution of nannofossil taxa is shown in Figure 3 with selected species illustrated in Figure 4. Nannofossils are generally aboundant throughout the studied interval.
Preservation of calcareous nannofossils is moderate and specimens exhibit the effects of secondary alteration from etching and/or overgrowth.A total of 52 species were identified.The most common species are: Coccolithus pelagicus, Cyclicargolithus floridanus, Cyclicargolithus abisectus, Reticulofenestra dictyoda, Reticulofenestra bisecta, Sphenolithus radians, Sphenolithus moriformis and Zygrhablithus bijugatus.

Biostratigraphy
The studied section spans calcareous nannofossil Zones NP9-NP24 (late Paleocene to Oligocene) based on the distribution of species.We present the biostratigraphic results by zone below, first indicating how the zone is defined and then identifying which samples from the studied section are assigned to that zone.We describe the taxa used to identify the zone in our studied section, and also describe the general assemblage present in each zone.
Age: Late Paleocene (Thanetian).Interval: Samples 1-5 (0 -4.5 m).Comments: The marker for the top of this zone is absent from our studied material and so we therefore use secondary taxa to approximate the top of the zone.The last occurrence of Fasciculithus spp. is a lower NP10 event, occurring ~220,000 k. y. after the evolution of T. bramlettei [17].In addition, the first occurrence of D. diastypus marks the base of Zone CP9 [16] and is an event that also occurs within Zone NP10.
The calcareous nannofossil assemblage of Zone NP9 is characterized by the presence of rare Fasciculithus spp., although F. tympaniformis is more abundant, Discoaster multiradiatus is common and the last occurrence of Toweius eminens is at the top of this zone.The first occurrences of Sphenolithus editus, S. radians and Discoaster barbadiensis also occur within these zones.
Comments: Tribrachiatus orthostylus, whose extinction marks the top of this zone, is absent from the study area; however, the first appearance of Reticulofenestra dictyoda occurs at approximately the same time [17] and can therefore be used a secondary marker for the NP12/13 boundary.Discoaster spp.are common.Discoaster diastypus disappears near the top of this zone.This interval includes some reworking, as Fasciculithus tympaniformis is present in these samples.
Interval: Samples 16-20 (17.5 -29 m).Comments: As mentioned above, the first appearance of Reticulofenestra is used to approximate the base of Zone NP13, since T. orthostylus is absent from the study area.Discoaster sublodoensis is also not found, and therefore Zones NP13 and NP14 are combined.The last occurrence of D. lodoensis occurs near the top of Zone Interval: Samples 57-58 (187 -194 m).Comments: In this study the top of zone is identified by the last occurrence of Reticulofenestra hillae, which is considered a junior synonym of R. umbilicus by some [19].It is distinguished from R. umbilicus by its wide collar and narrow central aea.The nannofossil assemblage is similar to that of NP21.
Sphenolithus predistentus zone (NP23).Definition: This zone extends from the last occurrence of Reticulofenestra umbilicus to the first occurrence of Sphenolithus ciperoensis (Martini, 1971).
Comments: In this study the top of zone is approximated by the first occurrence of Helicosphaera recta, which is close to the first occurrence of Sphenolithus ciperoensis and thus can be used to approximate the NP23/ NP24 boundry [12].The nannofossil assemblage is dominated by Coccolithus pelagicus, Sphenolithus moriformis, Cyclicargolithus floridanus, and Reticulofenestra dictyoda.The diversity in Oligocene compared with Eocene assemblages is clearly low.
Sphenolithus distentus zone (NP24).Definition: This zone extends from the first occurrence of Sphenolithus ciperoensis to the last occurrence of Sphenolithus distentus (Martini, 1971).
Comments: The top of this zone is approximated by the first occurrence of Pontosphaera enormis which has been shown to be a useful event for the Late Oligocene can approximate the NP24/NP25 boundary [19] [20].Discoaster and Sphenolithus increase in abundance within this interval, and Cyclicargolithus floridanus, Reticulofenestra dictyoda and Helicosphaera recta are common.

Paleoenvironment
Changes in sea-surface temperature can induce changes in nannoplankton assemblage composition, which are then recorded in sediments [21]- [24].Warm water taxa such as Discoaster, Fasciculithus and Sphenolithus [22] [26]- [29] are abundant in the lower part of the studied section.Temperate species such as Cyclicargolithus floridanus, Cyclicargolithus abisectus, and Reticulofenestra bisecta [30] are abundant in some parts of the section, and Reticulofenestra umbilicus is relatively common.Cold-water species such as Chiasmolithus, Isthmolithus recurvus, and Reticulofenestra daviesii [31] are rare or absent; however, Reticulofenestra lockeri is common.Three ecozones are identified based on changes in the percent abundance of different taxa with known paleoecological affinities. Ecozone

Discussion
Nannofossil preservation is moderate.Biostratigraphic events are first occurrence and last occurrence, and as mentioned previously, the nannofossil zones used in this study refer to Martini [15] and Okada and Bukry, [16].Eight zones and three combined zones is identified.Based on identified zones, the age of these sediments is Late Paleocene-Late Oligocene.
Calcareous nannoplankton are an excellent proxy for reconstruction of paleoecological conditions that result from paleoenvironmental change [33].The presence and distribution of calcareous nannofossils depends on a number of factors, including temperature [30] [34] and nutrient availability [35] as well as ocean circulation and salinity [36].Using nannofossils with known paleoecological preferences, we have divided the studied section into three ecozones: Ecozone 1 is characterized by abundant Discoasters and Sphenolithus.These genera are associated with warm water, oligotrophic conditions [29].Ecozone 1 spans nannofossil zones NP9 to NP13 (late Paleocene to early Eocene), which corresponds to a time of global warmth, including the Paleocene-Eocene Thermal Maximum and Early Eocoene Climatic Optimum.The Late Paleocene to Early Eocene spanned an interval when global oceanic deep waters warmed [37] and some studies suggest that the open ocean photic zones were characterized by oligotrophic conditions [22] [33] [38].Discoaster, Fasciculithus and Sphenolithus were adapted to warmer, more oligotrophic environments [22] [27]- [30] [32] [39].In the studied area increases in the abundance of Discoaster, Sphenolithus, and Fasciculithus suggest increased oligotrophic conditions in the surface water at that time.Although Discoaster and sphenolith diversity is low, species such as D. multiradiatus and S. moriformis are quite abundant through the upper Paleocene.The number and diversity of Discoaster species declined through the early Eocene and by the middle Eocene, only a few species of Discoaster remained in the region, and Discoaster multiradiatus, Discoaster diastypus, Fasciculithus spp.had gone extinct Ecozone 2 is identified by abundant reticulofenestrids and Cyclicargolithus.The abundance of these genera suggest a change from oligotrophic to eutrophic conditions [34].Ecozone 2 spans nannofossil Zones NP14-NP23 (Middle Eocene to Early Oligocene) which corresponds to a long term cooling trend that occurred after the Early Eocene [39] [40].In the studied section, the numbers of warm water, oligotrophic taxa decrease in abundance from the Middle toward the Late Eocene.The gradual disappearance of warm water nannofossil species during the Middle and Late Eocene and the occurrence of cold to temperate water species in the Late Eocene indicate progressive cooling through this interval time and show a change from oligotrophic to eutrophic conditions [33].The early Oligocene is characterized as a time of great cooling, which is expressed in the calcareous nannofossil assemblage with an expansion of cold water placolith forms with larger sized coccoliths [12] [42].Cyclicargolithus floridanus, Cyclicargolithus abisectus, Reticulofenetra dictyoda, R. wadeae, R. bisecta, and R. reticulata dominate the nannofossil assemblages.Zygrhablithus bijugatus and Coccolithus pelagicus are common, whereas warm water genera Discoaster and Sphenolithus are rare or absent during this time.
Ecozone 3 is marked by increase in the abundance of Discoaster and Sphenolithus, suggesting a return of warmer water.Ecozone 3 includes nannofossil zone NP24 and NP25 (Late Oligocene) The Late Oligocene was characterized by an increase in indicators of warm surface waters such as Discoaster and Sphenolithus [21] [40] [43]- [45].The increased occurrences of the genera Sphenolithus, Helicosphaera and Pontosphaera could indicate near shore and warm water conditions, as the latter two genera appear to be more common in nearshore areas [46].These changes correlate with global sea level curve (Figure 5).

Conclusion
The Pabdeh Formation in Tang-e-Abolhayat was deposited under open marine conditions and spans Zones NP9-NP24 (late Paleocene to late Oligocene) based on nannofossil biostratigraphy.The section is divided into three ecozones based on changes in nannofossil assemblage that are correlated to changes in paleoenvironment.Ecozone 1 correlates to the upper Paleocene to upper Eocene interval and is characterized by a warm-water, oligotrophic assemblage.Lower abundances and diversity of middle Eocene Discoaster and Sphenolithus assemblages reflect a cooling trend of the surface waters that began after the Early Eocene.The overall nannofossil assemblage shows a change from oligotrophic to eutrophic conditions from the middle Eocene to the middle Oligocene (Ecozone 2).The Late Oligocene assemblages are characterized by an increase in warm-water, oligotrophic taxa (Ecozone 3).

Figure 2 :
Unit 1 consists of 18.5 m of purple shale [8], overlain by 11.5 m of dark gray marl of Unit 2. Unit 3 is divided based on the occurrence of trace fossils in the marl over a 4.4-m interval.Unit 4 is characterized by 16 m of alternating marl and shale, which is overlain by 4 m of limestone assigned to Unit 5. Unit 6 includes 10 m of shale and marl.Unit 7 contains 32.5 m of limy marl with interbedded shale, which is overlain by 23.5 m of gray to light brown marly limestone of Unit 8. Unit 9 is identified by nodular limestone at the base with shale and marly limestone above, and is overlain by 10 m of gray limy shale of Unit 10.Unit 11 is marked with 8.5 m of light gray limestone, which is overlain by 10 m dark shaly limestone of Unit 12. Unit 13 is characterized by 10.5 m of thin-bedded light gray limestone and is overlain by 12.5 m light gray thick-bedded limestone of Unit 14. Unit 15 consists of 9.5 m of light brown marly limestone, overlain by Units 16-18 that consist of thick (8 m), medium (7.5 m) and thin (9.5 m) bedded limestone, respectively.
Tribrachiatus contortus (NP10)/discoaster binodosus (NP11) zones.Defintion: Zone NP10 is defined from the first occurrence of T. bramlettei to the last occurrence of Tribrachiatus contortus, and the top of Zone NP11 is defined by the first occurrence of Discoaster lodoensis (Hay, 1964; Mohler & Hay, 1967).Age: Early Eocene (Ypresian).Interval: Samples 6-13 (4.5 -11.5 m).Comments: Members of the Tribrachiatus lineage are absent in the study area and so the NP10/NP11 boundary could not be identified and thus these zones are combined.These zones include the first occurrence of Discoaster diastypus, which falls within mid-NP10.Coccolithus pelagicus is abundant and Zygrhablithus bijugatus and Sphenolithus moriformis are common in this interval.
This ecozone contains the species Cyclicargolithus abisectus.C. floridanus, and Coccolithus pelagicus which are considered oceanic forms that prefer mainly temperate waters[28] [32]are abundant.In this ecozone Reticulofenestra umbilicus, Zygrhablithus bijugatus and Braarudosphaera biglowii are also relatively common in lower part with Middle Eocene age.The most abundant species of the calcareous nannofossils in this interval are Sphenolithus moriformis and Discoaster spp., which are warm water taxa and suggest a return to warmer conditions.