Jahrum Formation Research Articles

Eocene larger benthic Foraminifera from the Jahrum Formation (Kalbibak Section, Boldaji Region) in High Zagros, Iran: an approach on paleontology and biostratigraphy

The Jahrum Formation is a major Eocene carbonate deposit of High Zagros in the west of Iran. It consists of thin bedded to thick massive limestone, accumulated on a shallow marine platform within the Neo-Tethys Ocean realm. Fifty-five surface samples were collected throughout a 215 m section in the Kalbibak area (Boldaji region, Chahar Mahal Bakhtiari province). The stratigraphic distribution of benthic foraminifers allows characterizing five foraminiferal biozones, which are in the following order: Alveolina aff. schwageri Taxon Range Zone, A. decastroi Taxon Range Zone, A. aff. frumentiformis–A. cf. stercusmuris Interval Zone, A. aff. stercusmuris–A. aff. fusiformis Interval Zone and Nummulites–Alveolina Assemblage Zone. The benthic foraminiferal associations in the studied area are similar to those recorded in Tethys realms and enable us to correlate the Jahrum Formation with the early to middle Eocene. Keywords: benthic foraminifers, biostratigraphy, Jahrum Formation, Kalbibak section, West Iran.

New biostratigraphy and microfacies analysis of Eocene Jahrum Formation (Shahrekord region, High Zagros, West Iran). A carbonate platform within the Neo-Tethys oceanic realm

The Eocene Jahrum Formation in High Zagros was studied in Kuh-e- Soukhteh and North Gahrou sections (southwest of Shahrekord region, Chahar-mahal Bakhtiari Province). This formation, composed of limestone, marl, and dolomitic limestone (dolostone), accumulated on a marine platform within the Neo-Tethys ocean realm. It yields a rich foraminiferal fauna, in which three larger benthic foraminiferal assemblage zones were identified. Two assemblage zones in the North Gahrou section were correlated to the Ypresian and Bartonian, and one assemblage zone is represented in the Kuh-e- Soukhteh section and assigned to the Bartonian. In addition, three other groups of benthic foraminiferal associations have been identified based on test wall type (porcellaneous, agglutinate, hyaline) and paleogeographical significance. A discontinuity marked by a hiatus from Cuisian to Lutetian in the North Gahrou section was most probably due to a concealed fault. According to microscopic textures and distribution of benthic foraminifera and other components (peloids, intraclasts, etc.), a gentle depth gradient from the inner ramp to the proximal outer ramp may be reconstructed.

Biostratigraphy and paleoenvironmental significance of Paleogene foraminiferal assemblages from Dashte Zari area in High Zagros, West Iran

The Paleogene carbonate deposits of Pabdeh and Jahrum formations are widespread from the northwest (Dashte Zari area) to the southwest of the Shahrekord region in the High Zagros Mountains of Iran and record the lateral and upward transition from open marine into the shallow water environment. The Pabdeh Formation shows a succession of open marine pelagic and hemipelagic limestone, argillaceous limestone, and argillaceous chert. It consists of planktonic wackestone, pellet-planktonic wackestone, mudstone with planktonic foraminifera, and radiolarian siliceous wackestone, which accumulated within the Zagros Foreland Basin. The planktonic foraminifers are assigned to the Late Paleocene–Late Eocene and correspond to subtropical and tropical Zones P4b–E15. The Jahrum Formation is represented by bioclast-bearing limestone and calcarenite. It consists of benthic foraminiferal wackestone, benthic foraminiferal-red algal packstone, and bioclast-intraclast packstone deposited in a shallow platform environment. The Jahrum Formation is inter-fingered in the upper part of the Pabdeh Formation and finally overlies it conformably during the Bartonian–Priabonian. Shallowing and off-lap relationships record basin shrinking, while repeated inter-fingering signals moderate tectonic subsidence. Both formations are disconformably covered by the Late Oligocene–Miocene Asmari Formation. Keywords: biostratigraphy, Pabdeh Formation, Zagros, Paleogene, Iran.

Systematic Paleontology of Bartonian Larger Benthic Foraminifera from Shahrekord Region in High Zagros, Iran

The Jahrum Formation is characterized by abundant benthic Foraminifera in carbonate beds, partly marly and dolomitic limestones in the Kuh-e-Soukhteh (Shahrekord region). This formation covers a huge stretch of the Zagros Zone which is a part of the central Tethyian realm during the Paleogene time. Bio-stratigraphic analysis of the larger benthic Foraminifera distinguishes one assemblage zone assigned to the late middle Eocene (Bartonian). This new biostratigraphic range is represented by the index fossil Rhabdorites malatyaensis (Sirel) and is correlated with calcareous rocks in the Shiraz area (south Iran), Dhofar section (Oman), and Socotra Island (Yemen). The Jahrum Formation is dominated by rich miliolids-agglutinated Foraminifera with rare small rotaliids and without Nummulites Lamarck and Alveolina d'Orbigny indicating that the formation was deposited in a shallow water environment (nearshore lagoonal zone) with low energy.

Microfacies and sedimentary environment of the Jahrum formation, south-central Zagros basin

Microfacies and sedimentary environment of the Jahrum formation, south-central Zagros basin

Reticulotaberina n. gen. jahrumiana n. sp., a new soritoidean (Praerhapydioninidae) from the Eocene of Iran

A new porcelaneous larger benthic foraminifer, Reticulotaberina jahrumiana new genus and new species, from the latest middle Eocene shallow-water carbonate platform deposits of SW Iran (Jahrum Formation, Zagros Mountains) is described. Its structure is characterized by the development of a thin alveolar layer (exoskeleton) in the marginal area of the chamber lumen and by septula and interseptal pillars (endoskeleton) in the central area. The septula build a network, whereas the interseptal pillars rise from its intersections. The intercameral foramina are concentrically arranged and are intercalated with septula. R. jahrumiana n. sp. displays a strong dimorphism: A-form has a planispiral-uncoiling subcylindrical morphology, whereas the B-form presents a flared shape with oval or compressed outline. The new taxon is included in the Praerhapydioninidae family and is so far known from the south-west of Iran. The age is late Bartonian.

Porosity classification from thin sections using image analysis and neural networks including shallow and deep learning in Jahrum formation

The porosity within a reservoir rock is a basic parameter for the reservoir characterization. The present paper introduces two intelligent models for identification of the porosity types using image analysis. For this aim, firstly, thirteen geometrical parameters of pores of each image were extracted using the image analysis techniques. The extracted features and their corresponding pore types of 682 pores were used for training two intelligent models, BPN (back-propagation network) and SAE (stacked autoencoder). The trained models take the geometrical properties of pores to classify the type of six porosity types including intra-particle, inter-particle, vuggy, moldic, biomoldic, and fracture. The MSE values for the BPN and SAE models were found to be 0.0042 and 0.0038, respectively. The precision, recall, and accuracy of the intelligent models for classifying the types of pores were calculated. The BPN model was able to correctly recognize 193 intra-particle pores out of 197 ones, 45 inter-particle pores out of 50 ones, 7 vuggy pores out of 9 ones, 10 moldic pores out of 12 ones, 2 biomoldic pores out of 3 ones, and 6 fractures out of 7 ones. Also the SAE model was able to correctly classify 193 intra-particle pores out of 197 ones, 46 inter-particle pores out of 50 ones, 8 vuggy pores out of 9 ones, 10 moldic pores out of 12 ones, 3 biomoldic pores out of 3 ones, and 7 fractures out of 7 ones. The results obtained showed that the SAE model carried out a bit more accuracy for classification of the inter-particle, vuggy, biomoldic, and fracture pores.

Microbiostratigraphy of the Upper Paleocene to Middle Eocene Jahrum Formation in the Folded Zagros Zone, SW Iran

The Jahrum Formation (Upper Paleocene to Middle Eocene) is composed of carbonate and dolomitic carbonate rocks in the Zagros Basin. The Zagros is located at the boundary between the Arabian and Eurasian lithosphere plates and represent the orogenic response to a collision between Eurasia and advancing Arabia during the Cenozoic. The study area is located in the northern part of Kuh-E-Tudej, ~175 km southeast of Shiraz in the Folded Zagros Zone. The Jahrum Formation at Kuh-E-Tudej, with a thickness of 190 m, consists of medium to massive bedded limestone. The following foraminiferal index species are identified in the studied section: Fallotella alavensis, Kathina sp., Miscellanea sp., Lockhartia sp.,Orbitolites shirazeinsis, Nummulites sp., Opertorbitolites sp., Dictyoconus cf. egyptiensis, Orbitolites cf. complanatus, Dictyoconus sp., Coskinolina sp., Somalina stefaninii, Discocyclina sp., Praerhapydionina sp., Coskinolina cf. liburnica, Nummulites cf. globulus, Nummulites cf. aturicus, and Alveolina sp. The age of the studied sediments ranges from Upper Paleocene to Middle Eocene. The microbiostratigraphic studies revealed four biozones based on the foraminifers identified in the studied section.

Erratum to: Biostratigraphy and paleoecology of Upper Paleocene to Lower Oligocene deposits in Southeast of the Hajiabad Area (Zagros Basin, SW Iran)

This study deals with foraminiferal biostratigraphy and paleoecological interpretations for the Late Paleocene to the Early Oligocene carbonate sequence in the Zagros Basin, the Hajiabad Area, SW Iran. This sequence could be divided into three parts, basal fossil-less strata have related to Sachun Formation base on their stratigraphical position, the middle part involves dolomitic to dolomitic limestone of Jahrum Formation, and the upper part consists of the Lower Asmari Formation. In general, the bulk of the section consists of marine deposits, and it is possible to divide them in to several parts base on benthic foraminiferal biozones. The Late Paleocene to the Early Eocene (the Lower Jahrum Formation) has been distinguished by the Miscellanea, Kathina, Alveolina, Fallotella, Lokhartia, Orbitolites, and small rotalids Assemblage Zone. The Middle Eocene (the Middle Jahrum Formation) has been characterized by the Somalina Total Range Zone and the Nummulites-Alveolina Assemblage Zone. The Late Eocene (the Upper Jahrum Formation) has been distinguished by the Chapmanina, Pllatispira, Baccullogypsinoides Assemblage zone, and the outcrop of the Asmari Formation belongs to the Early Oligocene Base on the Nummulites vascus, Nummulites fichteli Assemblage Zone. Clustering content benthic foraminifera in the studied section base on their morphology, coiling, feeding habits, and habitats lead to identification of 15 distinct assemblages with similar ecological niches. Paleoecological reconstructions base on benthic foraminiferal communities indicate that five major ecological habitats include tidal flat and shore, restricted shelf, nonrestricted shelf, oligotrophic shelf, and open marine.

Testing the controls on the seismic sequence stratigraphy of the Eocene–Oligocene boundary in Southern Iran with a Wheeler diagram derived from outcrops, seismic and well logs data

In this study of Southern Iran the timing of the boundary between the Eocene to Oligo–Miocene sections was determined along with a better understanding of the accumulation of the Paleocene to Eocene sediments. This was established by generating Wheeler diagrams from local seismic, well log data and surface data. This boundary was found to be mainly erosional and the time gap between Eocene to Oligo–Miocene displayed by the Wheeler diagram suggests a “degradational vacuity” formed. Relative sea level changes were found to be responsible for the seaward progradational character of the Jahrum Formation sediments. Red sediments and an intraformational conglomerate overlie this erosional boundary between the Paleocene to the Eocene Jahrum Formation and the Oligo–Miocene Asmari Formation.Long-term lower frequency trends in both regional tectonic and global sea-level curves determined from the Paleocene–Eocene sediments of south Iran, when compared to the coastal plain sediments of US New Jersey and the global coastal onlap chart, suggest that contemporaneous eustastic signals in lower Eocene time produced matching sedimentary patterns.The results of the study recorded in this paper are intended to be used as the foundation of the study of petroleum related facies and petroleum system components (source, reservoir and seal rocks) in the Tertiary portion of sedimentary section.

Sequence Microbiostratigraphy of Jahrum and Asmari Formation in Shiraz Area, Zagros, Fars, Iran

The study of sequence microbiostratigraphy of Jahrum and Asmari formation in Shiraz, Fars, Zagros, SW of Iran and recognition of relationship between depositional sequence parameters and microbiostratigraphy 3 stratigraphic sections (Beyza, Sarvestan A, B and Estahban sections) have been selected and studied. Jahrum and Asmari formations in study area with unconformity overlying together have been exposed. Based on Field and laboratory studies such as microfossils, microbiostratigraphy, microfaceis, sedimentary environment models and sequence stratigraphy evi- dences recognized 3 depositional sequences in study area that presented during Paleocene-Oli- gomiocene. Jahrum formation consists of one depositional sequence and Asmari formation consists of two depositional sequences. Microfossils and microbiostratigraphy studies will lead to the identification of 6 biozones in the studied area. In Beyza and Estahban sections biozones 1, 2 and 3 are related to Jahrum formation and suggest Paleocene to Middle Eocene age for this formation and biozone 4 is related to Asmari formation and suggests Oligocene age for this formation. In Sar- vestan section biozones 1, 2 and 3 are related to Jahrum formation and suggest Paleocene to Middle Eocene age for this formation and biozones 4 and 5 are related to Asmari formation and these biozones indicate Oligocene-Miocene (Aquitanian) age for Asmari formation. Sequence microbiostratigraphy studies indicate that biozones of Jahrum formation are formed in DS1 and biozones of Asmari formation are formed in DS2 and DS3. Also, some relationship between depositional sequence parameters and distribution of microfossils and biozones as in the case of in SB1 and LST facies didn’t found any microfossil and we can’t suggest biozonation. In study area upper part of HST of DS1 (Jahrum formation) and some biozones related to Upper Eocene have been eliminated, for example in Estahban, Sarvestan B and Beyza sections all biozones related to Upper Paleocene to Middle Eocene and eliminated biozones of Upper Eocene. In study area SB2 existed in lower boundary of DS1 (between Sachun and Jahrum formation) and between DS2 and DS3 of Asmari formation. Some parts of biozones in study area could correlate with SB2. For example for upper limit of biozones no 4 in Estahban section overlies to SB2 of DS2 in Asmari formation. Paleoecological situation in TST systems tract suitable for reproduction and growth large foraminifera and biozones 1, 2 and 3 are formed in systems tract. MFS and MFZ in all sections are recognized by mixed pelagic and benthonic foraminifera wackestone. More biozones in Jahrum and As- mari formations have been identified in HST system tract. For example in Beyza section biozones 2 and 3 (Jahrum formation) and biozone 4 (Asmari formation) are in HST. In Saevestan B section bi- ozones 3 and 4 (Jahrum formation) and biozones 5 and 6 (Asmari formation) are in this system tract. In Estahban section biozones 3 and 4 (Jahrum formation) and biozones 4 and 5 (Asmari formation) have been in HST. Also, tests of most identified large foraminifera in HST such as Alveolona sp, Alveolina aragonensis, Austrotrilina sp, Austrotrilina asmaricus and Archias sp are made of porcelaneous calcite and foraminiferal tests in TST systems tract, such as Nummulites sp. Nummulites intermedius, Nummulites fichtelli and Operculina sp are made of hyaline calcite.

Sedimentary Environments and Correlative Sequence Stratigraphy of Upper Cretaceous—Paleogene Succession in Shiraz Area, Fars, SW IRAN

Sequence stratigraphic evidences have been used in this research to investigate the stratigraphic correlation of cretaceous paleogene succession in Shiraz area, Fars, SW Iran. For this purpose, 4 stratigraphic sections have been chosen and provided. According to petrographic study and field evidences, 4 depositional sequences were identified including the Tarbur, Sachun and Jahrum formations in the east and the Gurpi, Pabadeh and Asmari formations in the west of study area. After identifying the boundaries and systems tract facies, it has been found that the mentioned formations have been deposited among the continuous/discontinuous boundaries in an equivalent manner. In this case, the purple shale (between the Gurpi and Pabdeh formations) could be considered equivalent to the upper part of the Sachun formation. From the point of Paleogeography, the plat-form sedimentary environment covered almost the whole area during the Oligo-Miocene age, so that the Asmari formation would have been deposited, which is marked by sharp contact as disconformity on Jahrum formation in the east of the basin and with continuous contact (C.C) on the Pabdeh formation in the west and southwest.

Petrography, Sedimentary Environments and Sequence Stratigraphy of Asmari Formation in Central Fars, Zagros, SW (Iran)

To study petrography, microfacies, sedimentary environments and sequence stratigraphy of Asmari formation in central Fars, 4 stratigraphic sections have benn chosen and studied (Estahban, Shams-Abad, Sarvestan and Akbar-Abad Sections). Field and laboratories study recognized 16 carbonate microfacies and 2 clastic-carbanate facies. Study of microfacies, relative sea level change and sequence stratigraphy evidences show these microfacies are deposited in carbonate ramp sedimentary environment in 5 facies belt zones: Open marine (A), Restricted (B), Shoal (C), Lagoon (D) and Tidal flat (E) during Oligomiocene. Petrography, microfacies analysis and sedimentary environment of Asmari formation shows this formation in study area consist of 3 depositional sequences. Lower sequence boundary of DS1 with Jahrum formation is erosional (disconformity) SB1. Lower and upper sequence boundary of DS2 is SB2. The upper sequence boundary of DS3 is SB1 and ended under the Razak clastic formation. In central Fars only lower part of Asmari is deposited and upper part of this formation have been eroded and disappeared.

Evolution, paleoecology and sequence architecture of an Eocene carbonate ramp, southeast Zagros Basin, Iran

ABSTRACTWe document and discuss the results of detailed fieldwork, facies analysis and the subsequent integration of paleoecological evidence from the Middle Eocene carbonate ramp succession in the southeast Zagros Basin (Jahrum Formation). A combination of a sea-level fall and tectonic and diapiric basement uplift favored the initiation of the Jahrum carbonate platform. The lower portions are affected by pervasive, probably early diagenetic dolomitization, whilst the upper Jahrum consists mainly of limestone. Here, the focus is on the limestone portions of the Jahrum Formation. Based on the abundance, diversity and rapid evolutionary turnover of the alveolinids and nummulitids, the limestone intervals of the Jahrum Formation are interpreted for the upper Middle Eocene (Bartonian). The Jahrum Formation is capped by a major unconformity and overlain by the Lower Oligocene mixed clastic/carbonate Razak Formation.Based on data from field sections, eight facies associations and a series of sub-types have been established, which correspond to inner-, middle-and outer-ramp depositional environments. In their overall context, these data show a southward-dipping inner-ramp-to-basin transect. Towards the Coastal Fars (e.g. Hulur-01 Well) the Jahrum grades laterally into deep-marine Pabdeh foredeep shale units. Based on facies analysis and paleoecological evidence from larger benthic foraminifera, a major transgressive-regressive pattern is recognized in all outcrop sections of the Jahrum. The lowermost stratigraphic units of the formation are here interpreted as a distally steepened ramp. Evidence comes from abundant allochthonous shallow-water facies in the distal, deeper-ramp setting. Shallow-water carbonate clasts were exported towards the basin, a feature that is probably linked to relative sea-level fall control. Furthermore, local to regional basement instabilities by salt diapir-related basement reorganization was arguably of significance. Upsection, evidence is found that the ramp system evolved from a distally steepened to a homoclinal geometry with an overall very gentle slope geometry during the Late Bartonian.The data shown here are significant for those concerned with the Paleogene evolution of the southeast Zagros Basin and provide a well-exposed case example of a Middle Eocene carbonate ramp factory.

Microfacies and geochemical evidence for original aragonite mineralogy of a foraminifera-dominated carbonate ramp system in the late Paleocene to Middle Eocene, Alborz basin, Iran

The Ziyarat Formation is a Late Paleocene to Middle Eocene carbonate sequence, located in the north of Tochal Village (south-east of Tehran). The Ziyarat Formation with a total thickness of 212.5 m conformably overlies the Fajan conglomerate and is overlain by greenish tuffaceous siltstone of the Karj Formation. Petrographic studies have led to the recognition of 11 microfacies. Of these microfacies, five belong to inner ramp, four belong to middle ramp and two are located in the outer ramp. Due to the great diversity and abundance of large benthic foraminifera (e.g., Nummulites, Alveolina, Discocyclina), the Ziyarat Formation represents an example of “foraminifera-dominated carbonate ramp settings”. Paleobathymetry, light level and water energy were determined during deposition of Ziyarat carbonates based on the test shape and size variation of different types of benthic foraminifera and other components in different facies. Micritization, cementation, compaction, dissolution, dolomitization and fracturing are important diagenetic processes in the Ziyarat Formation, occurring in marine to meteoric and burial diagenetic environments. Geochemical studies indicate that Ziyarat carbonates were deposited in a shallow warm-water sub-tropical environment and aragonite was the original carbonate mineralogy with mean Sr values of 3,470 ppm. It should be noted that the maximum Sr content in abiotic calcite is 1,000 ppm. In Ziyarat limestone, dissolution as leaching is the most important diagenetic event in the evolution of porosity, particularly where Nummulite and other benthic foraminifera are abundant. This may indicate that the original carbonate mineralogy of Nummulites might be aragonite, rather than low-Mg calcite. In the Jahrum Formation of Iran and in the El Garia Formation in north-east of Libya, the widespread moldic porosity in Nummulite beds enhanced the reservoir potential of the carbonate sequence. Thus, it is proposed that the original carbonate mineralogy of nummulitid shells may be varied in different environments, and those with metastable mineralogy are excellent for forming hydrocarbon reservoirs. Bivariate plots of Mn versus Sr/Ca and δ18O values illustrated that Ziyarat limestones were affected by open diagenetic system with high water/rock interaction. Early burial diagenetic temperature calculation based on heaviest oxygen isotope values of micrite and δw of Eocene seawater of −0.85 SMOW showed that temperature was around 39 °C. Cathodluminescence studies of carbonate cements illustrated dull luminescence, because these carbonates were affected mainly by burial diagenesis. This statement is confirmed by δ18O and δ13C isotope evidences. Based on different facies and evidences such as absence of reefal facies, calciturbidite deposits, the occurrence of widespread tidal flat deposits and abundant micrites in all microfacies indicate that the Ziyarat Formation was deposited in a homoclinal carbonate ramp environment. In addition, many researchers suggested that due to extreme global warming during the Paleocene–Eocene time, platform margin coral reefs declined in low latitude and heterozoan calcitic foraminifers became abundant and, thus, widespread carbonate ramp systems developed.

Microfacies, sedimentary environment and paleoecology of Paleocene–Eocene deposits in the Zagros Basin, north of Shiraz, southwest Iran

The Jahrum Formation act as reservoir rocks in the Zagros Mountains west of Iran. For the study of this formation, a stratigraphic section of Lapoee which is situated north of Shiraz has been examined. Petrographic and stratigraphic results along with field observations show that the Jahrum Formation consists of cream-grey thin-to-medium-bedded limestone so that at the top of the formation, they are locally changed to dolomite. The Jahrum Formation overlies the Sachun Formation. We also found Nubecularids as paleoecology indicators in middle parts of the Jahrum Formation. The presence of Nubecularids in the Lapoee stratigraphic section (i.e. the Jahrum Formation) indicates a lagoon depositional environment.

Relationships between foraminiferal assemblages and depositional sequences in Jahrum Formation, Ardal area (Zagros Basin, SW Iran)

The Jahrum Formation was deposited in the foreland basin in southwest Iran (Zagros Basin). The Zagros mountain belt of Iran, a part of the Alpine–Himalayan system, extends from the NW Iranian border through to SW Iran, up to the strait of Hormuz. The various facies of the Jahrum Formation were deposited in four main genetically related depositional environments, including: tidal flat, lagoon, shoal and open marine. These are represented by 14 microfacies. The Jahrum Formation represents sedimentation on a carbonate ramp. Tidal flat facies are represented by fenestral fabric, stromatolitic boundstone and thin-bedded planes. Carbonate deposition in a shallow marine lagoon was characterised by wacke–packstone, dominated by various taxa of imperforate foraminifer. The shoals are made up of medium- to coarse-grained skeletal and peloidal grainstone. This facies was deposited predominantly in an active high energy wave and current regime, and grades basinward into middle ramps facies are represented by wackestones–packstones with a diverse assemblage of echinoderm and large benthic foraminifers with perforate wall. Outer ramp facies consist of alternating marl and limestones rich in pelagic foraminifera. There is no evidence for resedimentation processes in this facies belt. The sequence stratigraphy study has led to recognition of three third-order depositional sequences.

Biostratigraphy of the Late Paleogene-Early Neogene succession, north-central border of Persian Gulf, Iran

The Late Paleogene-Early Neogene succession studied at the Khurmug area, on the northern flank of the Persian Gulf, includes upper part of the Jahrum, lower and middle parts of the Asmari and the most lowest part of the Gachsaran Formations. The upper part of the Jahrum Formation is characterized by limestones from theDictyoconus-Coskinolina-Orbitolites andGloborotalia renzi-Globigerina assemblage zones of the Middle Eocene. The lower part of the Asmari Formation is gypsiferous marly, dolomitic limestones, and limestones from theNummulites-Operculina assemblage zone of the Middle Oligocene (Rupelian) age. The middle part of the Asmari Formation is gypsiferous dolomitic limestone and limestone from theArchaias-Peneroplis assemblage zone of Early Miocene (Aquitanian) age. The lower part of the Gachsaran Formation is characterized by thick-bedded anhydrite and thin beds of dolomitic marls, with miliolids, reflects deposition during to Late Aquitanian through Burdigalian. Within the above succession, two conformable non-conformities correlatable to Priaborian-Lattorfian and Chattian times is recognized. Also, unlike most of the Zagros Basin, marine carbonate deposition (correlatable to the upper parts of the Asmari Formation) is replaced by the evaporitic facies of the Gachsaran Formation during that accumulated the Burdigalian time.

DEPOSITIONAL HISTORY AND SEQUENCE STRATIGRAPHY OF OUTCROPPING TERTIARY CARBONATES IN THE JAHRUM AND ASMARI FORMATIONS, SHIRAZ AREA (SW IRAN)

The Oligo‐Miocene Asmari Formation is one of the most important petroleum reservoir units in the Zagros Basin of south and SW Iran. It mainly consists of limestones and dolomitic limestones with interbedded shales, together with a few intervals of sandstone and gypsum assigned to the Ahwaz and Kalhur Members, respectively. The Asmari Formation rests on the thin‐bedded limestones of the Jahrum Formation (Paleocene‐Eocene). In this paper, we report on the lithofacies characteristics of these two formations using data from three measured outcrop sections near Shiraz in SW Iran. From field and petrographic data, we have identified four major lithofacies and twelve subfacies which are interpreted to have been deposited in open‐marine, shoal, lagoon and tidal flat settings.We show that the Asmari and Jahrum Formations constitute two separate depositional sequences which are separated by a thin palaeosol, representing a type‐one sequence boundary which can be correlated with global curves of relative sea‐level. Each depositional sequence is composed of many metre‐scale shallowing‐upward parasequences. This is the first time that the Asmari and Jahrum Formations have been differentiated in the study area. We hope that this study will lead to a better understanding of the Asmari Formation in the subsurface in other parts of the Zagros Basin.

Petrofacies analysis and depositional environment of the Jahrum Formation (Eocene), south-southwest of Burujen, Iran

A progressive carbonate environment from supratidal to near-shore, retricted to intertidal and restricted shallow marine environment existed during deposition of the Jahrum Formation. The main facies characteristics of the Jahrum Formation in the study area are: (1) a laminated, unfossiliferous dolomite and pelleted dolomitic mudstone with solution collapse breccias; (2) a pelleted laminated fossiliferous dolomitic mudstone; (3) reworked skeletal wackestone to packstone; (4) highly reworked biointraclastic grainstone; (5) fossiliferous wackestone to packstone. As a result, two major depositional systems tracts can be defined: (1) the lower portion, are transgressive systems tract; and (2) the upper portion, as highstand systems tract. Diagenetic processes involved in supratidal rocks of the Jahrum Formation reflect dolomitization of initial sediments, leaching of calcium sulfate and carbonate grains, replacement of dolomite and stylolitziation. Voids are the most common pore type, and porosity was controlled by diagenesis that was influenced by primary depositional texture.