The Pabdeh Formation (Paleocene–Oligocene) is located in the Zagros foreland basin. This formation was well appreciated as a source rock and somehow a potential reservoir particularly in the Dezful Embayment. Although it has been less appreciated due to overwhelming fine grained lithology in the majority of the Zagros basin. Microfacies analysis and depositional environment of the Pabdeh Formation was studied by examining 500 samples collected from three surface sections namely Malekshahi, Gandab and Pirmohammad, which are located around the Malekshahi city. Based on petrography, proportional abundance of foraminifera and other constituents, texture and microfacies characterizes, eight facies associations were recognized which were deposited in three facies belts on a ramp. As a whole, the Pabdeh Formation comprises marl and limestone beds. Based on geochemical analysis (calcimetry and XRF analysis), the marl lithofacies was established, although it was literally described as shale. Wet chemical oxidation method was employed to determine the organic matter content of some selected samples, which revealed relatively higher organic matter content in marlstones. A mixed carbonate-clastic system may develop, once clastic input exerted into a ramp environment. Such intervals are defined with shale, sandstone and carbonate beds (Tucker 2003). Marl could deposited instead of shale, if the clastic input are limited (Holland 1993). Periods of clastic input in such environments are probably linked to sea-level changes. In present research, the Pabdeh Formation is proposed as a typical carbonate-clastic sedimentation, which was deposited during Paleocene–Oligocene in the Zagros foreland basin. The Zagros basin comprises a thick sedimentary succession over the Precambrian basement (Alavi 2004). The basin was a part of Gondwana supercontinent during Paleozoic which was bordered by Paleotethys in the north, a passive margin during Mesozoic and a convergent orogenic belt in Cenozoic (Bahroudi and Koyi 2004; Sepehr and Cosgrove 2004). The Neotethys was initiated since Late Triassic between Iran and the Arabian plate (Berberian and King 1981; Sepehr and Cosgrove 2004), and ceased during late Cretaceous due converging movement of subducting Arabian Plate beneath the Iran sub-plate (Berberian and King 1981; Stoneley 1981; Berberian 1995). Continental collision in Cenozoic led to the formation a Zagros fold-thrust belt and foreland basin where since Late Cretaceous onward, sedimentation continued without any major break. The succession includes the Gurpi (shale and marl), Amiran (siltstone, sandstone, limestone and conglomerate), Pabdeh (argillaceous limestone, shale and marl), Taleh Zang (limestone), Kashkan (siltstone, sandstone and conglomerate), Shahbazan (dolomitic limestone and dolostone), Asmari (limestone), Gachsaran (anhydrite, marl and limestone) and Aghajari formations (calcareous sandstone, siltstone and sandstone) (James and Wynd 1965). During Paleocene–Eocene the argillaceous limestone and pelagic marls of the Pabdeh Formation were deposited in the middle part of the Zagros basin in a ramp setting (intrashelf basin) (Motiei 1993; Mohseni et al. 2011). The Pabdeh Formation was subject of numerous researches since it is considered as a potential source interval. For example Mohseni et al. (2011) untilized trace fossils for interpreting the depositional environment and recognizing turbidity currents (Mohseni and Al-Asam 2004; Khodabakhsh et al. 2009; Mohseni et al. 2011). Khavari et al. (2014) and Senemari (2018) examined the nanostratigraphy and paleoechology of the Pabdeh Formation. This formation is well defined by the purple shale member at the base with Gurpi Formation and conformably underlies by the Asmari Formation (Motiei 1993) and laterally passes into the Kashkan and Shahbazan formations (Alavi 2004). Although James and Wynd (1965) described the Pabdeh Formation as shale interval with subordinate limestone beds, recent studies revealed a major part of this formation includes limestones which were deposited in ramp setting (Mohseni 2003; Behbahani 2006). Since a major part of the so called shales beds are recognized here as marls, the main goal of this research is examining microfacies, depositional environment and petrographic reappraisal of the formation in three selected surface sections around the Malekshahi County using geochemical approaches.    Microfacies analysis and depositional environment of the Pabdeh Formation was studied by examining 500 samples collected from three surface sections namely Malekshahi, Gandab and Pirmohammad, which are located around the Malekshahi city (southeast Ilam Province, the Zagros basin; Iran). Petrographic investigations of the rhythmites was performed by using polarizing microscope (Zeiss, Axioscope, 40) on three intervals comprising pronounced rhythms. The nomenclatures of lithofacies are based on Dunham (1962) and Pettijohn (1987). Element composition was measured by S4 Explorer/ x-ray Spectrometry-Bruker (WD XRF) in XRF analysis laboratory at Yazd University. The carbonate content (= CaCO3%) were determined by titration method (Carver, 1971). The organic matter content was determined via wet chemical oxidation (by hydrogen peroxide) (Lewis and McConchie 1994).    As a whole, the Pabdeh Formation comprises marl and limestone beds. Distinction between marl/limy marl and limestone beds was done based on geochemical analysis (calcimetry and XRF analysis). Although marl lithofacies was literally described as shale. Wet chemical oxidation method was employed to determine the organic matter content of some selected samples, which revealed relatively higher organic matter content in marlstones. Based on petrography, proportional abundance of foraminifera and other constituents, texture and microfacies characterizes, three micro/lithofacies associations were recognized within the selected rhythms which were deposited in three facies belts on a carbonate ramp environment: A)            Inner Ramp, include the following microfacies (MF): MF A1- Bioclastic mudstone with extraclast and quartz MF A2- Oncoid bioclastic wackestone-packstone MF A3- Ooid packstone B)            Middle Ramp, include the following microfacies and lithofacies (LF): MF B1- Algal bioclastic packstone MF B2- Bioclastic mudstone-packstone with benthic foraminifera and echinoderm LF B3- Marl/calcareous marl lithofacies C)            Outer Ramp, include the following microfacies and lithofacies (LF): MF C1- Bioclastic mudstone-packstone with planktonic foraminifera and glauconite LF C2- Marl/calcareous marl lithofacies Two lithofacies (LF C2 and B3) alternate with the recognized limestone microfacies (as described above) resulting two cycle types: 1-             Alternation of LFC2 with microfacies type C (LF C-MF C cycle) 2-             Alternation of LFB3 with microfacies type B (LF B-MF B cycle)  Sedimentation in the ramp environment may be affected by clastic input (mainly as event deposits). In some cases, more than 90% of basins was filled by these evenest (i.e. tempestite/turbidites; Colombie et al., 2012). The cyclic (=rhythmite) deposition of micro/lithofacies in the study area indicate two depositional mechanisms: 1) background in-situ sedimentation (carbonate facies), 2) fine clastics transported by traction currents from shallow-marine that dumped siliciclastics into the deep-marine. The latter mechanism show characters of event deposits as it was reported by Khodabakhsh et al. (2009) and Mohseni et al. (2011). Limestone-marl alternations in the Pabdeh Formation was possibly controlled by periodic fluctuations in siliciclastic input by shallow-water derived currents (probably turbidity currents). The trace fossils of Zoophycos group are normally associated with turbidites and debris flows in ramp environments. Frequent occurrences of such trace fossils in the study area could be assigned to event deposits. This study revealed the Pabdeh Formation comprise 8 microfacies deposited in a carbonate ramp. The Zoophycos trace fossil displaying an evolutionary change in acceding order which suggest shift from more calm water setting toward relatively agitated condition upward throughout the formation. Marlstone lithofacies is consistently distributed throughout the whole section, which was described literally as shale beds. Relatively more Al2O3 content of marlstones in compare to limestone beds could imply more clastic input during deposition of these beds.

In this research, Pabde Formation (Paleocene-Oligocene) was evaluated as one of the hydrocarbon producing formations in the Zagros sedimentary basin from the point of view of organic geochemistry and hydrocarbon generation potential. In this regard, a 180-meter-thick stratigraphic section was selected on the northern side of the Kabirkoh anticline, Dere Shahr city, and 20 healthy and non-weathered rock samples were taken from this section for Rock-Eval 6 analysis. Lithology of Pabdeh formation in the interval study section6 was taken for Rock-Eval analysis. The lithology of Pabdeh Formation in the section under study is an interval of shale and marl, where the shale facies is dominant. Plotting S 2 values ​​against Toc indicates a medium to relatively good source rock for Pabdeh Formation, whose kerogen was detected as a combination of type II and type III, but type II kerogen is more abundant, which can be due to the dominance of marine facies. be the constituent of this formationThe plot of Tmax values ​​against HI showed that 25% of the samples are in the immature range and 75% of them are in the beginning to the middle of the oil generation window, which is confirmed by Tmax between 435 and 460 degrees Celsius. The diagram of HI versus Toc indicates the formation of various organic facies in Pabde formation, these facies include: organic facies C (30 percent), BC (50 percent) and to a lesser extent B (15 percent) and AB (5 percent). This facies diversity can be due to the diversity of organic materials with marine and continental origin and sedimentation in relatively regenerating environments such as the Pro-Delta, the outer shelf, and the upper parts of the continental shelf.

In order to determine stratigraphy and the introduction of biozonationes in the Pabdeh Formation based on the new planktonic foraminifera biozonationes in the Thethys strict, one stratigraphic sections in the Izeh zone at Gurpi anticline are selected and sampled. The thickness of the Pabdeh Formation in this section is 672 m., and 4 lithological units (Purple shale (106.25 m. thickness), Lower limestone and marl (50/75 m. thickness), Limestone (295 m. thickness) and Upper limestone and marl (220 m. thickness)) in Chahardeh stratigraphic section at Izeh zone. Biostratigraphic studies led to recognition of 30 genera and 141 species of planktonic foraminifera and then 21 biozones in realm (Wade et al. (2011) were identified. These identified biozones (P3-P5 (Paleocene),E1-E16 (Eocene), O1(Oligocene)) in the studied section in Izeh zone are in well correlated with biozones of Tethys region. Based on the indicated biozones and fossil contents, the age of the Pabdeh Formation is Middle Paleocene (Selandian) - Late Oligocene (Chattian).

The Pabdeh Formation with a thickness of 797.2 m. and 4 lithological units (purple shale (144.25 m.), lower limestone and marl (221.75 m.), limestone (73.5 m.) and upper limestone and marl (357.7 m.) is stretched at Chahardeh village stratigraphic section in Izeh zone.. Biostratigraphic studies led to recognition of 30 genera and 150 species of planktonic foraminifera based on which 23 biozones were identified in Tethys realm. These biozones are well correlated with biozones of Tethys region. Based on the indicated biozones and fossil contents, the age of the Pabdeh Formation is Middle Paleocene (Selandian) - Late Oligocene (Chattian). In this study, Paleocene- Eocene boundary is located in Purple shale unit and Eocene- Oligocene boundary is placed in upper marl and limestone unit.

Palygorskite is a widely distributed clay mineral that is commonly found in soils and sediments of arid regions, particularly in the Middle East. This study aimed to determine the distribution of palygorskite in major parent materials of soils developed on central Iranian Tertiary sediments. Based on field observations and geologic maps, 12 geological formations with Tertiary age were sampled and analyzed for mineralogical composition using x-ray diffraction (XRD) and transmission and scanning electron microscopy (TEM and SEM). Results showed that sediments of Miocene and Pliocene ages had the highest amount of palygorskite in the study area. In contrast, no trace of palygorskite was found in samples of Paleocene, Eocene and Oligocene ages. Smectites were highly associated with palygorskite. Stability diagrams for smectite-palygorskite system in Tertiary sediments of diff erent ages showed that geochemical conditions were not conductive for pakygorskite formation during the Late Oligocene. Occurrence of palygorskite in the Tertiary sediments of Central Iran suggests that the most possible source of this mineral in soils of the study area, and probably those of other areas with similar conditions, is the inheritance from parent materials. It seems that the geochemical conditions of the Tethys Sea became gradually suitable for palygorskite formation in the Tertiary sediments of central Iran. Today's arid environment prevailing in central, eastern and southern Iran has led to the stability of palygorskite in soils developed on Tertiary sediments.

Type section of Pabdeh Formation (Late Paleocene- Early Oligocene) is located in the Gurpi anticline, Zagros, Khouzestan Province. Lithologically Pabdeh Formation consists of purple and white to yellowish marl, marly limestone and thin- to thick- bedded limestone, rich in pelagic foraminiferas. Pabdeh Formation conformably overlies Gurpi Formation and is conformably overlained by Asmari Formation. Petrographic studies, description and microfacies analysis specially hemipelagic, pelagic and calciturbidite facies of Paleocene to Lower Oligocene indicate that Pabdeh Formation was deposited in a deep marine basin. Field observations, facies studies and sequence studies resulted in identification of stratal key surfaces of sequence stratigraphy such as sequence boundary, maximum flooding surface, transgressive surface, system tracts and 9 depositional sequences. Sequence P1, P2 and LST, FSST of sequence P3 are of Paleocene in age, based on identification of planktonic foraminifera, TST, HST and FRWST of sequence P3 and sequences P4, P6, P7 and P8 were deposited during Eocene, and sequence P9 suggest an age of Upper Eocene- Lower Oligocene. All sequences begin with SB2 and except of sequence P3 and P4 , SB2 and Ts surfaces coincide with each other. Deepest mfs is belongs to the sequence P3; due to the remarkable abundance of glauconite minerals. Relative sea- level curves in most sequences, except for sequences P3, P8 and P9 exactly correlate with standard global sea level curves. The role of synsedimentary tectonic activities and sea-level oscilations ,probably, are responsible for deposition of sequences P3, P8 and P9.

In this research, two stratigraphic sections were chosen from the late Paleocene to early Miocene (Jahrum and Asmari Formations). These sections includes Sadra and Dodaj plus 645 meters of late Paleocene to early Miocene scdiments. For this investigation 360 thin sections were prepared and 12 samples analysed by XRF. We studied Biotope that includes biozone. These index biozones and percentage of elements that appears in biotope are identified. Principally index foraminifers, identified in the studied sections, are listed bellow: Miscellanea sp. Kathina sp., Fallotella alavensis Mangin, Opertorbitionlites sp. Alveolina sp. Lituonella ruberti Stache, Rhapydionina urensis Sirel, Orbitolites complanatus Lamark, somalina Stefaninii Silvestri, Dictyoconus sp. Coskinolina liburnica Atache., Nummulites fabianii Prever, Silvestriella tetraedra Gumbel., Nummulites fichteli Michelotti, Austrotrillina howchini Schlumberger, Meandropsina anahensis Henson. Regarding the frequency, type of wall and biotope percentage ratio of each foraminiferal biozone, there are different biotopes that indicate change in the trend of paleoecologic conditions of the late Paleocene to Oligocene sediments.

Pabdeh Formation has been investigated in three sections viz. the Type Section, Tang-e Hatti and the northern limb of Gurpi Anticline within the Zagros Sedimentary Basin with emphasis on calcareous nannofossils. Study of microscope slides prepared from the 1508 collected samples resulted in determination of 256 species belonging to 59 genera from 22 families of nannofossils and biostratigraphic classification was carried out based on their stratigraphic distribution. According to Martini (1971) zonal scheme, presence of NP4-NP23 nannofossil zones at the Type Section, and NP5-NP23 nannofossil zones at both Tang-e Hatti and northern limb sections have been ascertained thus indicating ‌‌Early Paleocene to Early Oligocene age for the Pabdeh Formation at these three localities. Calcareous nannofossil assemblages reveal sedimentation of Pabdeh Formation occurred in an oligotrophic, warm water environment during Paleocene-Eocene which was later on replaced by a relatively cooler water condition in Oligocene times.