Late Paleocene to early Oligocene dinoflagellate cysts of the Zagros basin, west Iran (palaeopalynology and palynostratography)

: Palynological investigation about 120 samples from 460m thick outcrop section of late Paleocene to early Oligocene of the Pabdeh Formation, southwestern in Iran yield 55 species of dinoflagellate results to defined 7 biozone. The quantities of marine palynomorph elements are indicated of open marine at this time, but a low increase number of spore and pollen in some of the sample indicated the condition for growth forest are suitable that consequent increase the harm climate and humidity. The species are common with different latitude and most of them cosmopolitan. Thermal maturity index measurement indicates oil prone in most of the samples.


Introduction
The first studies on fossil dinoflagellates from Western Iran were made by Zahiryin 1982(Zahiri, 1982).The principal intent of these investigations was identification of dinoflagellate cysts.The stratigraphic application of dinoflagellate cysts in this area started only at the end of 1982 when a few boreholes were investigated and the first dinoflagellate cyst zones were e r e c t e d (Zahiri, 1982).Later on more studies conducted on palynostratigraphy of some parts of the Zagros basin (e.g.Ghasemi-Nejad et al.,2006, Rabbani et al., 2013).
As the Pabdeh Formation which is lithologically made up of shale, calcareous shale and limestone, is a relatively known source rock in the Zagros basin, palynological studies on these strata could help to establish a stratigraphic framework for the formation to be further investigated in terms of potential for petroleum generation.

MATERIALS ANDMETHODS
A total of 125 rock samples from the Siah anticline section were collected.The samples were processed using palynological techniques in several steps according to palynological standard methods (Traverse,1988,pp.456-479).About 30 grams of rock samples was used for processing.The samples were first crushed and washed, then treated with 33% HCl and later with 40% HF.After these chemical preparation steps,the material was sieved through a 20 micron nylon mesh.The materials residue coarser than 20 micron were used for palynological study.(Motiei, 2003).For this reason,it is important to study palynology and palynostratigraphy of the formation and to establish a precise stratigraphic framework for this formation.(Bujak et al.,1980), Middle Eocene of Mexico (Helenes, 1984).Operculodinium cf.microtrainum, Melitasphaeridiumasterium, Impagidiniumsp., Fig. 4. Thalassiphora delicata has been reported from Late Eocene of offshore West Greenland (Nøhr-Hansen,2002), Homotriblium tenuispinosum has also been stratigraphically reported from Lower Oligocene of CentralItaly (Biffi and Manum, 1988), Upper Eocene, Egypt (El-Beialy, 1988b), Lower-Upper Eocene of England (Eaton,1976) and Lower-Upper Eocene of off shore East Canada (Williams and Bujak, 1977) Biozone7: Selenopemphix nephroides Interval zone This local biozone is being introduced here.The index species Selenopemphi nephroide shas been reported from European region.Selenopemphix nephroides has been stratigraphically reported from Lower Oligocene of Egypt (El-Bassiounietal.,1988),Upper Eocene -Lower Oligocene of Netherlands (de Coninck, 1986b), Oligocene of Nigeria (Biffi and Grignani,1983) and Oligocene of North West Germany (Benedek, 1972) Age: Early Oligocene(Rupelian), Occurrence: from 421.33 m to 457 m Definition: this zone has been defined as the interval from the LAD of Polysphaeridium zohary to the LAD of Selenopemphix nephroides.The accompanying taxon is Memranophoridium aspinatum, Fig. 4.
Polysphaeridium zohary has stratigraphically been reported from Lower Eocene-Oligocene of off shore East Canada (Williams and Brideaux,1975) and Middle Eocene of Pakistan (Köthe et al.,1988).

Table1.Correlation of Paleogene dinoflagellate cyst biozones established for Zagros basin with those of Europe and Ural
Thermalmaturity: The thermal maturity discussed and used in this study is basen on changes in spore or pollen color.Thermal maturitywas used for oil exploration as it is capable of characterizing organic matter type based on which kerogen type in the source rocks could be identified.The yellow color usually indicates a degree of maturation for the rocks that yet not attained a thermal maturation degree to generate petroleum.The brown range of color indicates a well maturation degree for oil generation.Seven samples that contain diverse assemglages of spore and pollen are selected to study for color change of their spore and pollen contents.Of these, five samples are located with in the oil prone sector and two samples indicated an over mature condition(Table 2).In general, these indicate that the Pabdeh Formation is a good source rock for the big reservoir rock unit, the Asmari Formation.Benedek, 1972;21. Spiniferites pseudofurcatus (Klumpp, 1953) Sarjeant, 197022. Hystrichokolpoma cinctum Klumpp, 1953;23. Thalassiphora pelagica (Eisenack, 1954) Eisenack andGocht, 1960;24. Systematophora placacantha (Deflandre andCookson, 1955) Davey et al., 1969 Previous Studies: There is afew record of dinoflagellate cysts from the Pabdeh Formation of the Zagros basin in the published literature (e.g.Rabbani et al., 2013).However, several good works have been published on foraminiferal biostratigraphy of the formation (e. g.Biranvand et al. ) presenting precise biostratigraphy and giving in general an age of Paleocene to Miocene to the formation.The Pabdeh Formationisa known surcerock for the Asmarire servoir, one of the largest source rocks in the Middle East Fig1.Location map of the studied section.

Biozone7:
Polysphaeridium zohary Interval zone Age: Early Oligocene (Rupelian), Occurrence: from350.46 m to 423.6 m Definition: this zone has been defined as the interval from the LAD of Thalassiphora delicate to the LAD of Polysphaeridium zohary.The accompanying taxa are:
Danian and Ypresian intervals.The zones erected here can be compared with those of different parts in Europe and Urals (Table1) generally but a complete correlation and zonation has yet to be erected for the Tertiary basins around the world.The zonation established here is discussed below: Biozone1: Apectodinium parvum interval zone Age: late Paleocene (thanetian),Occurrence:from 45.55 to 50.91m Definition: this zone has been defined as the interval from the LAD of Apectodinium parvum to the LAD of Areoligera gippingensis.Acompanying taxa include: Apectodinium , homomorphus, Melitasphaeridium , pseudorecurvatum, Areosphaeridium capricornum.

Table 2 .
Spore and pollen Color changesused for evaluation of thermal maturity The Siah anticline, located in the south western Iran, contains a rich Paleocene-Oligocene record of dinoflagellate cysts.The dinoflagellate cysts are abundant in the lower and upper parts of the section but, their abundance decreases in some parts of the section because of the limestone lithology of the layers.Seven biozones are established based on the presence of dinoflagellate cysts.These Thezones in general confirm the late Paleocene to early Oligocene age forthe Pabdeh Formation at this section which has also been gained from studies on foraminifera and nannofossils.Dinoflagellate cyst zones erected here are compatible with those of European countries.