Search

The Deh‐Molla Formation of the Alborz Zone in northern Iran is a key unit to understanding global sea‐level fluctuations, and likely climate changes during the Furongian Epoch. The sedimentological characteristics of the formation indicate that the succession was deposited on a gently sloping epeiric platform on the northern margin of Gondwana. The Paibian Member 1 of the Deh‐Molla Formation was deposited on a carbonate platform with stable environmental conditions, probably during a (semi)arid climate condition, as indicated by microbial–sponge patch reefs, thick‐bedded units formed in broad facies belts, and a thick dolomite unit formed under evaporative conditions. The overlying Member 2, considered to span the Jiangshanian–Age 10, includes thin‐bedded wavy, and rippled strata known as ribbon rocks. This sequence records considerable environmental fluctuations. Thin‐bedded strata with flat‐pebble conglomerates, hummocky cross‐stratification, lenticular, flaser, and wavy bedding, ripple cross‐lamination, and graded bedding signify that deposition of Member 2 took place in a depositional environment repeatedly affected by tidal currents, and storm waves. A comparison of the Deh‐Molla Formation, and particularly its ribbon rocks with coeval strata in Laurentia and North China reveals secular controls on their development. In contrast to the Furongian strata of the Great American Carbonate Bank, deposited under tropical conditions, the ribbon rocks of the Deh‐Molla Formation suggest deposition in a temperate latitude zone with dominant cool‐water conditions. However, the Deh‐Molla platform was seasonally subjected to monsoons that brought warm tropical waters to the region, and led to rapidly changing environments. Ooids, peloids, stromatolites, and other criteria suggestive of a tropical climate are frequent in Laurentian successions, but absent in the Deh‐Molla Member 2. However, both Laurentian and Gondwanan examples share characters that indicate comparable controls on their development. Significant environmental changes during the Furongian are indicated by the SPICE and HERB events, and the latter is taken as showing biotic and physico‐chemical changes in the ocean and atmosphere, possibly concurrent with the formation of ribbon rocks on different continental blocks. [ABSTRACT FROM AUTHOR]

Abstract The lower Cambrian Lalun Formation of the Tabas Block, Central Iran, allows to generate a sequence stratigraphic model which characterises the sedimentary evolution and improves palaeoenvironmental interpretation of sedimentary successions along the northern margin of the Prototethys. This formation provides a key for understanding processes that took place during the late early Cambrian in Iran and is the best representative for the complex mosaic of mixed siliciclastic-carbonate systems which illustrate the development and evolution of fluvial and tide-dominated estuarine systems adjacent to an open coast during this interval in the region. Detailed facies analysis allows to differentiate five vertically-stacked groups of facies associations (fluvial facies associations; estuarine facies association; open coast tidal flat facies association; carbonate ramp facies association; and delta facies association) in the uppermost interval of the Lalun Formation. The conceptual depositional models for the siliciclastic-carbonate successions of the Lalun Formation are summarised in a sequence stratigraphic framework, including three third-order depositional sequences (braided fluvial systems; incised-valley fill; and carbonate ramp and tide-influenced delta systems). The data suggest that the Central Iranian Microcontinent broke up during latest Neoproterozoic–early Cambrian times, resulting in the Proto-/Palaeotethys rifting, with considerable differential subsidence recognizable from the reactivation of faults in the early Cambrian which led to new syn-rift successions. The mosaic pattern of deposition observed among the studied sections suggests a compartmentalization of the depositional setting as a result of the interplay between tectonics, sediment supply, and eustasy. Abrupt lateral changes in facies and thickness within the sequences depositional sequences suggest that deposition was controlled by repeated tectonic downdrop and uplift as a consequence of local syndepositional, extensional fault movements. Tectonic activities in the studied region controlled sedimentation and accommodation as well as the position of depocentres within the basin, and the stratal geometries within the sequences. However, the stratigraphic architectural style of the depositional sequences and the asymmetrical trend of the transgressive and regressive second-order sequences imply that the deposition of the late early Cambrian successions is at least partly a result of eustatic sea level changes and cannot be linked exclusively to differential subsidence due to fault movements. Our study indicates that stratigraphic relationship between carbonate and siliciclastic facies in the depositional sequences was also controlled by subsidence/uplift in the hinterland and the areas of siliciclastic input rather than climatic changes. The late early Cambrian depositional environments evolved from braided rivers and an incised valley system (lowstand) to transgressive incised-valley-fill successions which are capped by transgressive open coast tidal flat and carbonate open marine facies. Depositional history of the formation terminated with a regression that ceased with progradation of tide-influenced delta systems. The stratigraphic evolution described in this study documents an eustatic sea-level fall interposed between the top of the Lalun Formation and the base of transgressive tidal bars of the overlying Fasham Formation reflecting the complex (sub-)globally recognizable regressive–transgressive event at the lower–middle Cambrian boundary interval, which led to the establishment of an extensive middle–late Cambrian carbonate platform in present-day northern and central Iran. Highlights • Stratigraphic analyses of the early Cambrian Lalun Formation has been conducted. • Synthesis of tectonic and eustatic controls on stratigraphic architecture. • The depositional models were proposed based on sequence stratigraphic framework. • Results are useful for correlation to Cambrian strata in other parts of Gondwana. [ABSTRACT FROM AUTHOR]

This study focuses on reservoir rock typing of the Oligo-Miocene Asmari Formation in order to introduce a field-scaled reservoir zonation for the strata in one petroleum field of SW Iran, where the formation is composed of mixed marginal marine sand/sandstones and shallow marine carbonates, capped by the evaporite-bearing Gachsaran Formation. On the way, sedimentary and diagenetic aspects of the formation are presented to characterize the main geologic factors controlling reservoir attributes of the petroleum field. Then, an integration of three different reservoir rock classification methods is applied to identify speed and/or baffle zones of the reservoir. Based on the results, the main speed zones of the field are comparable to the siliciclastics of the reservoir whose distribution is the product of sea-level-controlled terrigenous supply to the basin. Although the carbonates have high storage capacity owing to noticeable early diagenetic dissolution, they possess low flow capacity due to isolated network of pore spaces; therefore, they make the baffle zones of the field. The five reservoir zones determined can be located in other parts of the field via well logs and/or the sequence stratigraphic framework. [ABSTRACT FROM AUTHOR]

The siliciclastic Gadvan Formation from Abadan Plain, southwestern Iran, is highly bioturbated and allows relationships between changes in ichnocoenoses within a depositional system to be documented and placed in a high-resolution sequence stratigraphic framework. Relying on the sedimentary and ichnological characteristics, the siliciclastic succession is divided into two facies associations: a wave-dominated offshore-shoreface complex and a tide-river influenced delta. The first includes facies that have been deposited in shelf-offshore, upper offshore, lower shoreface and upper/middle shoreface environments, the latter includes facies that have been deposited in prodelta and delta front. Integrated ichnologic and sedimentologic studies of the Gadvan Formation, allow distinction between prodelta and delta front and open marine deposits. With the identification of maximum flooding and ravinement surfaces as bounding surfaces of the stratal units, detailed analysis on systematic changes in the stacking pattern (cycle thickness, cycle type, and facies proportion) are made. Eight ichnocoenoses could be differentiated in the studied sections. The positions of the ichnocoenoses within genetically related stratal units (genetically related ichnocoenoses), indicate three large-scale cycles (DS1 to DS3, from oldest to youngest). The cyclical nature of the Gadvan Formation is attributed to low-amplitude eustasy in greenhouse conditions formed under interaction of eustatic high-frequency cycles and longer term tectonically driven sea-level variations during the long-term transgressive sea-level trend of the early Cretaceous. Stratigraphic architectural style of sequences DS1 to DS3 (which includes scarce evidence of lowstand deposits, partial or total truncation of the HST, and predominance of thick transgressive deposits), is remarkably similar to long-term transgressive sea-level trend of the Early Cretaceous across the Arabian Plate. This study suggests a more relatively seaward position of the siliciclastic successions of the Gadvan Formation of Abadan Plain than the Mesopotamian Basin (upper Zubair Formation equivalent in western Iraq and Kuwait), which would be concordant with the prevailing view of an easterly prograding coastline across the Arabian Plate. This study reveals important sedimentological and ichnological features and permits the development of predictive models for the paleoenvironmental and sequence stratigraphical significance of trace fossil assemblages that can be readily compared or translated to analogous depositional systems worldwide. The ichnological analysis is based on cores and can be especially applied to evaluate the applicability of current ichnological models to the study of Cretaceous reservoirs of western Iraq, Kuwait and western Saudi Arabia. [ABSTRACT FROM AUTHOR]

The Middle-Late Ordovician Ghelli Formation in the Kopet Dagh region northeastern Alborz range of Iran is composed of siliciclastic rocks deposited in a variety of turbidite to marginal marine environments (deep marine clastic fan and related environments and prodelta, distal delta front, proximal delta front and distributary channels). The ichnology of the middle member of the Ghelli Formation is here reported. Combined sedimentological and ichnological analysis allows recognition of a tide-influenced deltaic succession in the middle member of the Ghelli Formation consisting of three main facies associations: lower distributary channels, delta front and prodelta facies. Twenty-two ichnogenera have been identified in this marginal marine succession: Arenicolites, Arthrophycus, Bergaueria, Chondrites, Conichnus, Cruziana, Cylindrichnus, Didymaulichnus, Diplichnites, Diplocraterion, Helminthopsis, Gyrochorte, Lockeia, Macaronichnus, Monomorphichnus, Palaeophycus, Planolites, Rosselia, Rusophcus, Skolithos, Teichichnus and ?Scolicia. Their distribution is clearly linked with lithofacies and depositional palaeoenvironments. The ichnological analysis reveals that the trace fossil suites developed in these environments indicate low diversity and low to moderate abundance of burrows, poor development of tiering and sporadic distribution. Low ichnodiversity and low bioturbation intensities with higher bioturbation clusters consist of facies-crossing ichnogenera, and the impoverishment of suspension-feeding trophic types indicates 'stressed', non-archetypal expression of the Cruziana ichnofacies. The periodic higher intensities of bioturbation due to variations in hydrodynamic regimes of tidal currents reflect the archetypal of the Cruziana ichnofacies (and rare Skolithos ichnofacies). [ABSTRACT FROM AUTHOR]

The trace fossil assemblages from the Upper Cretaceous Neyzar Formation are described for the first time from Kopet-Dagh, Iran, enhancing the record of this fossil group in the Cretaceous of the Middle East. Thirty-one ichnogenera have been identified in open marine successions: Agrichnium , Asteriacites , Asterosoma , Bergaueria , Chondrites , Cylindrichnus , Halopoa , Helminthopsis , Gordia , Gyrochorte , Laevicyclus , Lockeia , Megagrapton , Nereites , Neonereites , Ophioichnus , Ophiomorpha , Palaeophycus , Phycodes , Phycosiphon , Planolites , Protovirgularia , Rhizocorallium , Rosselia , Scolicia , Sinusichnus , Skolithos , Spongeliomorpha , Taenidium , Teichichnus , and Thalassinoides. The Neyzar Formation accumulated on a gently dipping shelf dominated by storm- and fair weather-wave processes and includes shelf, lower offshore, upper offshore, lower shoreface-proximal offshore, middle-upper shoreface and foreshore deposits. Identification and interpretation of ichnological signatures and the spatial arrangement of sedimentary structures in the successions are used to further refine sedimentary interpretations of parameters such as wave energy, substrate properties, the nature of the available food supply, variability in sedimentation rates and proximal–distal trends of the wave-dominated shoreface–offshore complex. The prevalent palaeoceanographic situation during the deposition of the studied successions was ideal for tropical storms, thus promoting tempestite deposition and the occurrence of a tropical and subtropical trace fossil suite. According to this study, increasing mobility and infaunality with more complex trace systems or sophisticated feeding strategies, climax population strategies and the high diversity, associated with rapid increase in the abundance and depth of infaunal structures, indicate a major reorganization of the shallow-marine benthic communities that occurred in response to the Mesozoic marine revolution. [ABSTRACT FROM AUTHOR]

The Albian–Turonian Sarvak Formation is the second most important reservoir of Iran. In this study, an integrated field-scale reservoir zonation is applied to the Sarvak reservoir in one oil field of the Dezful Embayment, SW Iran to understand the distribution of speed, barrier, and baffle zones. The geologic controls of depositional and post-depositional factors on the suggested reservoir zonation are also discussed here. Based on the results, the studied reservoir is composed of seven zones with different reservoir qualities. The best reservoir zones show considerable amount of dissolution pores, and are developed beneath regional-scale paleo-exposure surfaces that are also recognizable in coeval strata on the Arabian Plate. These surfaces represent recurring subaerial exposure episodes during the development of the Sarvak Formation through which meteoric diagenesis highly affected the rock record leading to the development of paleo-karst profiles with noticeable dissolution pores in the strata beneath the paleo-exposure surfaces. However, the long-lasting emergence time of the mid-Turonian exposure (mT-ES; the interface between the Sarvak Formation and overlying Ilam Formation) caused the over-maturation of the related paleo-karst profile and the deterioration of the created reservoir quality. The results of this study reveal the effects of regional-scale geologic controls on the reservoir characteristics of the Sarvak Formation and may be also applicable to the reservoir characterization of other time-equivalent strata in the Arabian Plate. • Diagenesis acted as the superior control on reservoir quality of the Sarvak Fm. • Seven reservoir zones are recognized in the Sarvak Formation. • Best reservoir zones are beneath some regional unconformity surfaces. • Subaerial exposures led to the development of secondary dissolution pores. • Emergence duration determines the reservoir quality of subaerially exposed strata. [ABSTRACT FROM AUTHOR]