Alborz Basin Research Articles

Middle Jurassic syn-sedimentary tectonics and palaeoenvironmental evolution in the eastern part of the Alborz Mountains (N Iran)

The facies variations in the Middle Jurassic deposits of northern Iran (northeastern Neotethys) were studied in two stratigraphic sections (Tapal and Kalariz) in the eastern part of the Alborz Basin to reconstruct the palaeoenvironmental evolution in this region. Six different palaeoenvironments were recognized based on alternation of sedimentary facies in the four formations of the studied sections (upper part of the Fillzamin, Dansirit, Dalichai and lower part of the Lar formations). The upper Fillzamin Formation (lower Bajocian) records a gradual shallowing-upward sequence from seaward to landward margin of a prodelta environment in the study area. This shallowing trend terminated with the Mid-Cimmerian tectonic event and unconformity, which is characterized by a thin limonite layer at the base of the Dansirit Formation in the Tapal section. The Dansirit Formation (lower upper Bajocian) represents heterolithic marginal marine, sand and gravel bars and terrestrial environments. The subsequent deposits of the Dalichai Formation (upper Bajocian–Callovian) represent a deepening sequence, which is associated with the appearance of pelagic strata and ammonitico rosso facies in the upper part of the formation. The ammonitico rosso facies, which is related to the development of pelagic swells in the rift basin, gives direct evidence of an extension phase and presence of a swell/trough system in the region. The rifting and subsidence are attributed to the opening of the South Caspian Basin (north of the Alborz region). The lack of correlation between the sea-level changes recorded by the upper Bajocian strata of the sections and the long-term eustatic sea-level changes suggest tectonic influence on the facies variations and sea-level fluctuations in the Alborz region. From the upper Bajocian upwards, a deepening trend in the studied sections can be correlated with the global sea-level changes, while tectonic (rifting and subsidence) overprinted the eustatic sea-level rise.

Reconstruction of Tournaisian-Viséan tectonic and climatically induced event histories of the Mobarak carbonate platform along depositional strike in the northeastern margin of Gondwana: Constraints from high-resolution cycle and sequence stratigraphy

Cyclic successions are one of the most important types of deposits in the carbonate platform of the Mississippian (Tournaisian-Viséan) in Gondwanan and Laurussian margins. The importance of climatically induced events impact on the amplitudes and frequencies of sea-level fluctuations in Mississippian of Iran is significant because the beginning, extent, duration, and significance of periods of climatically induced events in northern margin of Gondwana are not well recognized. A high-resolution sequence architecture in integration with paleoenvironmental reconstruction and foraminiferal biozonation from the Tournaisian-early Visean carbonate successions of the Mobarak Formation in Alborz basin along depositional strike (E-W- a distance of c. 340 km) suggests a mosaic-like pattern of deposition and compartmentalization of the depositional settings (homocline ramp to distally steepened ramp). According to the recognized facies, the stacking pattern of high-frequency cycles across the platform, and major accommodation events recognized on the correlated Fischer plots, six 3rd-order sequences (DS1–DS6) were identified. Detailed analysis on high-frequency cycles representing a highly variable number of shallowing-upward cycles, types, thicknesses, and facies heterogeneity along depositional strike. A combination of the episodic tectonic events along faults interacting with autocyclic processes, and inherited topographic irregularities are suggested as the driving mechanism for the cycle development. Six stages of Mobarak carbonate platform evolution based on understand the interplay of autocyclic and allocyclic controls are recognized in a hierarchical 3rd-order sequence stratigraphic framework and a chronological order: (1) early Hastarian corresponds to post-glacial transgressive episode with progressive backstepping of facies belts of early TST of DS1 throughout the region; (2) middle to late Hastarian corresponds to a major transgressional phase of DS1 following global temperature rise superimposed on different subsidence/uplift rates with the deformation of the platform along depositional strike; (3) Hastarian–Ivorian boundary corresponds to TST of DS2 with facies heterogeneity along depositional strike related to continuation of different subsidence/uplift rates. The absence of any systematic changes in the amplitude and period of cyclicity or lowstand deposits and subaerial surfaces during this phase shows the lack of influence of mid-Tournaisian glaciation and concurrent eustatic regression in northern margin of Gondwana. (4) early to middle Ivorian coincides with the RST of DS2 and TST of DS3 showing by remarkable topographic irregularities in the basin physiography and change in the position of the depocenter axis from a E–W trend to the center; (5) late Ivorian-early Moliniacian coincides with the RST of DS3 and DS4 showing by the Late Tournaisian thermal period and eustatic rise superimposed on uplift and subaerial exposure of the western part and subsidence in the central and eastern parts of the basin; (6) Moliniacian coincides with DS5-DS6 sequences showing by change from fault-related topography to major tilted fault-block topography accompanied by eustatic sea-level fall caused indirectly by the Viséan and Serpukhovian glaciations phases.

Palaeogeographic and palaeoclimatic reconstruction of the Permian sediments in the Alborz Basin, Iran: Sedimentological and geochemical approaches

The development of the Permian succession in the Alborz Basin has been dramatically affected by tectonic movement and change in the Cimmerian microcontinent position during the Pangea configuration. The Permian succession in the Alborz Basin consists mainly of siliciclastic-carbonate rocks, and is represented by two rock units: the Doroud Formation (Asselian-Middle Sakmarian) at the base, the Ruteh Formation (Artinskian-Roadian) in the middle, and lateritic horizon (Permian-Triassic boundary) at the top. The Doroud Formation was deposited in fluvial and beach environments. The integrated petrographical and geochemical studies indicate a felsic and intermediate origin for the sandstones of the Doroud Formation that were affected by semi-oxidizing to oxidizing environments under warm-humid climatic conditions. These sandstones, mainly quartz-arenites and sub-litharenites, were deposited in a passive continental margin and mostly sourced from a quartzose sedimentary provenance. The geochemical signatures reflect that the Doroud Formation has Gondwanan characteristics – the siliciclastic nature and deposition in a tectonic quiescence before rifting – so it is presumed that the Neo-Tethys spreading occurred after the deposition of the Doroud. Simultaneous with the deposition of the Ruteh Formation, the sea level was gradually increased, and a carbonate ramp was developed. Meanwhile, the configuration of continents of the Pangea began to change from Irvingian to Wegenerian, and Alborz block, as a part of the Cimmerian microcontinent, was detached from the Gondwana and moved northward through the Palaeo-Tethys Ocean. Furthermore, the Ruteh Formation shows Tethayn characteristics, including a warm climate, the presence of large fusulinid, and the dominance of carbonate sediments. A gradual regression occurred following the deposition of the Ruteh Formation and tectonic uplift in the Permian-Triassic boundary, leading to the formation of a lateritic horizon. Around this time, as the Cimmerian approached Eurasia, the Palaeo-Tethys became smaller, while the Neo-Tethys began to develop considerably.

A comment on overlooked storm sensitivity of the carbonate factory recorded in the Mississippian Mobarak Formation (Alborz Mountains, Iran)

New interpretations of depositional palaeoenvironments in the Mississippian (Lower Carboniferous) Mobarak Formation (Alborz Mountains, Iran) suggest a significant and persistent influence of storms. This deviates from previous conclusions that these deposits recorded mounds, patch reefs, and extensive lagoons deposited under stagnant environmental conditions. We here clarify and discuss the origin and nature of this misconception by explaining “unexpected” observations that are informed by outdated interpretations of the depositional environment of the Mobarak Formation. This evaluation offers the context required for appropriately interpreting and correlating Mississippian depositional records across the Alborz Basin.

Sequence stratigraphy of a southern Paleotethys distally-steepened ramp during the Early Carboniferous (Alborz Basin, Iran): Deciphering Tectonic and Eustatic controls

ABSTRACT The Lower Carboniferous carbonate sediments of the Mobarak Formation have been analysed to evaluate the role of allogenic and autogenic factors on the development of a distally steepened carbonate platform. Thirteen facies are identified in the studied succession and are grouped into four facies associations: inner ramp FA‒FB; middle ramp FC; ramp slope FD; and deep ramp FE. The rifting event at the northern margin of the Gondwana supercontinent caused a remarkable change in the depositional system so that the precursor homoclinal ramp (Khosheilagh Formation, Devonian) turned into the distally steepened ramp of the Mobarak Formation. Six depositional sequences are identified in the succession, characterized by varieties the normal transgression‒regression cycles, the TST‒HST, or the transgression‒forced regression cycles consisting of the TST‒HST‒FSST. The main controlling elements in the stacking pattern of sedimentary packages, relative sea-level changes and the formation of depositional sequences were tectonic activity and eustatic sea-level changes. Development of the FSST package was caused by the increasing tectonic activity and the formation of turbiditic sediments, amplified by wave-induced currents as the autogenic factor on formation of the re-sedimented deposits. Development of the HST deposits displays a relative balance between sedimentation rate and the creation of accommodation space. Thick packages of deep ramp calcareous shale were developed during quiescence in the tectonic activity and increase in accommodation space, created by thermal subsidence and rise in eustatic sea-level.

Understanding along-strike variability in controlling mechanisms of paleoenvironmental conditions and stratigraphic architecture: Ordovician successions in the Alborz Mountains of Iran at the northern Gondwana margin

Changes in stacking patterns of facies, stratal architecture and shoreline trajectory along depositional strike are mainly dependent on along-strike differences in factors such as physical processes (rivers, waves, tides), tectonics, eustatic changes, sediment supply, and local physiography. The lower-upper Ordovician successions (Lashkerak Formation) deposited in a Peri-Gondwana shallow continental shelf have been interpreted using the sequence stratigraphic concepts. This approach allowed to highlight main differences along-strike in the co-existing depositional systems (coastal plains, estuaries, deltas, and shoreline to shelf systems) in the Alborz basin along the northern edge of Gondwana. On the basis of sedimentary and ichnological characteristics, the siliciclastic successions are divided into four facies associations: (i) Wave-dominated estuarine, (ii) Wave-dominated shoreface-offshore complex, (iii) Mixed fluvial (F)- and wave (W)-influenced delta (Fw and Wf), and (iv) Tide-dominated, fluvial-influenced delta (Tf). Six third-order depositional sequences (DS1-DS6) were recognized, each of them are bounded by unconformities and pronounced shifts in the facies. Along depositional strike (NE/SW- a distance of c. 150 km), significant intrabasinal changes in sediment supply produced evident lateral changes in the shoreline trajectory and stratal architecture, of the DS1-DS3 (Tremadocian-Floian), may be attributed to differences in physical processes (rivers, waves, tides activity) and topographic irregularities derive probably by different rates of subsidence in the basin. During the deposition of the DS1-DS3, a significant along-strike differences in the composition of ichnological suites and the lifestyle of benthonic organisms, dependent on the relative supremacy of wave, storm, river, and tidal processes. Sedimentary supply and processes are the main controlling factors for the stratigraphic architecture rather than the tectonic activity between Tremadocian and Floian. While during the Darriwilian-Katian interval (DS4-DS6), the tectonic activity predominates with the beginning of extensional phases related to the rifting in the Alborz basin causing the activation of syn-depositional normal faults and the development of block-faulted craton-ward margin. Typically, differential subsidence due to extensional tectonic activity causes the uplift of tectonic blocks west of the Alborz Basin into the emerging areas, on one hand, and simultaneous deepening-upward stacking patterns (i.e., increased subsidence rates) in the hanging-wall depocenters of east of the Alborz Basin, on the other hand. This outcrop-based study suggests that the identification of genetically related depositional units in a larger paleogeographic context is fundamental for understanding along-strike variability in controlling mechanisms of the spatial distribution and stratigraphic architecture of coexisting and linked depositional systems.

Exotic facies episodes of a carbonate platform: implications for middle and late Cambrian ecosystems and impact of bioturbation in the Alborz Basin, Iran

Abstract The sedimentological and ichnological data of the Mila Group in the Alborz Basin, northern Iran indicate that the appearance of exotic facies and resurgence of microbialites resulted from the development of extraordinary palaeoceanographic conditions with widespread environmental stress and enhanced precipitation of CaCO 3 . Inhospitable environmental conditions in the Mila Group platform led to a considerable reduction in ichnodiversity, bioturbation intensity and depth and size of burrows, which led to periods of dominating calcimicrobe ecology. The development of exotic or anachronistic facies started around the Cambrian Stage 4–Wuliuan boundary, probably simultaneous with the SPICE event, whereas the Furongian exotic facies period probably took place simultaneous with the HERB event. Biotic and physico-chemical changes in the ocean during these events may have led as well to the virtual absence of the mixed layer and infaunal ecospace utilization in the deposits of the Mila Group. Nevertheless, the local presence of metazoan-microbial build-ups that established between the two suggested exotic facies developments indicates that these periods started at the Cambrian Stage 4–Wuliuan boundary interval, but a fluctuation of abnormal palaeoceanographic conditions and ambient normal marine conditions took place during the Miaolingian and Furongian in the Alborz Basin as a representative of the northern Gondwanan margin.

Ichnofabric analysis of shallow to deep marine Carboniferous sediments, from the southern Paleotethys margin, Alborz Basin (northern Iran): approaching autogenic and allogenic environmental controls

ABSTRACT This study integrates ichnological and sedimentological data to interpret depositional environments in the carbonate sediments of the Mobarak Fm. (Lower Carboniferous) in the Alborz Mountains of northern Iran. Seven ichnofabrics including Thalassinoides (Th), Zoophycos‒Phycosiphon (Zo-Ph), Skolithos‒Thalassinoides (Sk-Th), Zoophycos‒Diplocraterion (Zo-Di), Thalassinoides‒Rhizocorallium (Th-Rh), Zoophycos‒Arenicolites (Zo-Ar) and Chondrites‒Phycosiphon (Ch-Ph) have been identified in the studied succession. Th ichnofabric is characterised by the dominance of the deposit-feeding trace fossils, revealing low sedimentation rate and activity of the organisms in a loose substrate. Vertical succession of the Zo-Ph, Sk-Th and Zo-Di ichnofabrics indicates change in the firmness of the substrate. Th-Rh and Zo-Ar ichnofabrics reveal two different energy phases including a lower energy condition with the activity of deposit-feeders and a higher energy phase identified by the activity of suspension-feeders. Ch-Ph ichnofabric is characterised by low diversity and low abundance of trace fossils that were identified in black, limy shale, displaying a long lasting outer ramp setting with low energy, low sedimentation rate, and dysoxic-anoxic conditions of the substrate. Ichnological features of the Mobarak Fm. are controlled by subsidence-inducing changes in the accommodation space of the depositional system dictating variations in paleoenvironmental conditions such as oxygen levels and hydrodynamic energy.

The biostratigraphy of the Upper Devonian and lowermost Carboniferous of the Khoshyeilagh area, northeastern Alborz, Iran

Diverse and well-preserved marine palynomorphs occur in the Padeha, Khoshyeilagh, and Mobarak formations in the Khoshyeilagh area, northeastern Alborz, northern Iran. The marine palynomorph assemblages consist of acritarchs, prasinophytes, and chitinozoans, comprising 27 genera and 46 species. In addition, the studied sediments contain 20 species of algae, foraminifera, and corals allocated to 11 genera. The vertical distribution of acritarchs and chitinozoans allows the erection within these sediments of three local assemblage zones. Biozone I occurs in the upper part of the Padeha Formation and the basal part of the Khoshyeilagh Formation, suggesting a Frasnian–early Famennian age; Biozone II is present in the Khoshyeilagh Formation and indicates a Famennian age. Biozone III, which occurs in the basal part of the Mobarak Formation, suggests a Tournaisian age for this interval. Many of the palynomorph species, such as Crassiangulina tessellita, Daillydium pentaster, Deltotosoma intonsum, Gorgonisphaeridium discissum, Helosphaeridium microclavatum, Horologinella horologia, Papulogabata annulata, Somphophragma miscellum, and Veryhachium downiei, are closely comparable with coeval assemblages recorded from Australia, Ireland, Portugal, Saudi Arabia, North Africa, South America, and North America. This indicates the close relationship of the Iranian Platform to other parts of the northern Gondwana and southern Laurentian domain during the Late Devonian–Early Carboniferous. Moreover, results of similarity indices of marine microphytoplankton indicate a relatively high degree of similarity between the assemblages of the Khoshyeilagh area and those elsewhere in the Alborz Mountains and the Kerman Basin. Palaeontological evidence indicates that the Khoshyeilagh Formation was deposited in a low-energy, somewhat offshore, normal marine environment within the Alborz Basin, along the Palaeo-Tethys Ocean during this time interval. Furthermore, based on palynological data gathered from Iran, it can be concluded that Iran was an island during this interval of time.

Zoophycos ichnogenus distribution and paleoenvironmental analysis: examples from the Mississippian Mobarak formation (Alborz Basin, Iran)

ABSTRACT Zoophycos is a characteristic trace fossil in the Mississippian in the Alborz Basin deposits of north Iran, and is described for the first time in the Mobarak Formation. The vertical distribution of the carbonate deposits bearing Zoophycos provided the basis for distinguishing three distinct types of sedimentary sequences including Zoophycos (type A, B, and C) in shallow marine sediments of Mobarak Formation. Type A is composed of rare to common density of small to medium lobed Zoophycos. Type B is characterized by common density of medium lobed Zoophycos. This ichnogenus in type A and B is associated with Palaeophycus. The low diversity and bioturbation of the trace fossils would be the result of a stressful environment characterized by optimal water circulation and high-energy conditions. Type C consists of abundant density of large lobed Zoophycos. Type C is associated with Chondrites and planolites. The high diversity and bioturbation of the trace fossils in type C than type A and B would be the result of a calm environment characterized low-energy conditions. Under low sedimentation rate, stable paleoenvironment, dysaerobic conditions, and high amount of benthic food, the size of Zoophycos is the biggest, whereas in the high sedimentation rate, unstable paleoenvironment, aerobic conditions, and shortage of benthic food, the size of Zoophycos is small.

Warm-water carbonates in proximity to Gondwanan ice–sheets: A record from the Upper Paleozoic of Iran

Sedimentologic and paleontologic proxies recovered from the Alborz Basin, the Sanandaj – Sirjan Zone, and additional Iranian peri–Gondwanan continental blocks near the Arabian margin of northeastern Gondwana record the formation of warm-water, shallow-marine carbonates throughout the Late Pennsylvanian – earliest Permian. This reflects a prevailing favorable climatic and environmental regime that contrasts with the cold conditions of slightly more southerly Gondwanan regions that experienced profound glaciations. The sedimentologic and paleobiologic properties of these Iranian carbonates also differ greatly from coeval cool-water carbonates of Bolivia that developed at equivalent paleolatitudes along the northwestern margin of Gondwana. The complex climatic patterns reflected in the dispersal of marine taxa and their colonization of ecosystems relatively remote to their customary habitats and climate zones are interpreted to record the influence of ocean circulation. Based on these sedimentologic and paleobiologic data, we utilized a climate model (Community Climate System Model Version 3 (CCSM3)) to help resolve the mechanisms responsible for enabling transport of warm water to higher latitudes in northeastern Gondwana. These simulations reveal that spatial variations in sea surface temperatures exerted a strong forcing on Paleo-Tethyan oceanic circulation and thereby governed the distribution, diversity, and abundance of taxa in late Paleozoic marine ecosystems. Based on this outcome, we conclude that the climatic contrasts registered across Upper Pennsylvanian – Lower Permian carbonate platforms of the Paleo-Tethys and Panthalassic Ocean originated through oceanic circulation driven by geographic differences in sea surface temperature.

Exhumation of the Deylaman fault trend and its effects on the deformation style of the western Alborz belt in Iran

The Alborz range in northern Iran stretches along the southern coast of the Caspian Sea and finally runs northeast and merges into the Pamir mountains in Afghanistan. Alborz mountain belt is a doubly vergent orogen formed along the northern edge of the Iranian plateau in response to the closure of the Neo-Tethys ocean and continental collision between Arabia and Eurasia. The south Caspian depression—the Alborz basin of Mesozoic age (with W–E trend) in northern Iran—inverted in response to the Arabia–Eurasia collision. Pre-existing extensional faults of the south Caspian–Alborz system preferentially reactivated as contractional faults because of tectonic inversion. These contractional structures tend to run parallel to the trends of pre-existing extensional faults and acquire W and WNW–ESE orientations across the previous accommodation zones that were imposed by the reactivation of adjacent extensional faults with different directions. The NNE to N dipping faults show evidences of reactivation. The Deylaman fault is one of the important faults of western Alborz in Iran and is an example of inversion tectonic style of deformation in the western Alborz mountain range. The Deylaman fault, with an E–W trend, contains three discontinuous fault segments in the area under investigation. These fault segments have evidence of oblique right-lateral reverse motion and links eastward to the dextral Kandavan thrust. The importance of this fault is due to its effect on sedimentation of several rock units from the Jurassic to Neogene in western Alborz; the rock facies on each side of this fault are very different and illustrate different parts of tectonic history.

Foraminiferal proliferations in the Alborz Basin (northern Iran): global responses to Early Carboniferous glaciations

The Tournaisian interval of the Mobarak Formation in the Alborz Basin (Iran) preserves a specific bed with Earlandiidae and three foraminiferal zones that are restricted to specific intervals within the late Tournaisian and correlate with northern Eurasian biostratigraphic units. The bed with Earlandiidae dates to the early Tournaisian and corresponds with the lower Tournaisian and lower part of the upper Tournaisian of the Russian Stratigraphic Scale. The Granuliferella latispiralis–Latiendothyranopsis zone dates back to the earliest Ivorian (MFZ4?–MFZ5) and correlates with the G. latispiralis and Spinoendothyra costifera zones of the Urals. The Eotextularia diversa zone is of earliest late Ivorian age (MFZ6) and corresponds to the lower part of the E. diversa zone of the Russian Stratigraphic Scale. The Endospiroplectammina venusta–Eoparastaffella ex gr. rotunda zone is of latest Ivorian (MFZ7–MFZ8) age and correlates with the upper part of the E. diversa zone and the E. rotunda zone of the Russian Stratigraphic Scale. The entire early Tournaisian (Hastarian) portion is devoid of recognisable foraminiferal material, which is likely linked to a faunal shift of subtropical and temperate taxa to tropical latitudes in response to the glaciations at the Devonian–Carboniferous boundary. The establishment of the G. latispiralis–Latiendothyranopsis zone coincides with the first mondial Tournaisian foraminiferal radiation. The second and third episodes of foraminiferal diversification (E. diversa and E. venusta–Eoparastaffella ex gr. rotunda) are congruent with major foraminiferal shifts from Tethyan realms to higher latitudes in response to thermal periods. The occurrence of specific foraminiferal taxa in Alborz is strongly linked to transgressions and migrations of North Palaeotethyan biotic elements. The described Tournaisian cyclic patterns in the Alborz Basin share significant similarities with those in the North American, western European and Siberian realms, indicating a link with large-scale palaeoclimatic patterns. This cyclic system correlates directly with the pacing of global eustatic sea-level fluctuations caused by climate oscillations and follows the fourth-order ocean-level fluctuations as described from other, independent proxies.

Sedimentologic and paleoclimatic reconstructions of carbonate factory evolution in the Alborz Basin (northern Iran) indicate a global response to Early Carboniferous (Tournaisian) glaciations

The Lower Carboniferous Mobarak Formation records the development of a storm-sensitive pervasive carbonate factory on the southern Paleo-Tethyan passive margin following the opening of the Paleo-Tethys Ocean into the Alborz Basin along the northern margin of Gondwana. Its depositional facies encompass inner ramp peritidal environments, peloidal to crinoidal shoals, storm to fair-weather influenced mid-ramps, proximal to distal shell beds and low energy outer ramps. Sedimentological analyses and foraminiferal biostratigraphy reveal four events affecting carbonate platform evolution in the Alborz Basin during the Lower Carboniferous: (1) A transgression following global temperature rise in the Early Tournaisian (middle Hastarian) caused the formation of thick-bedded argillaceous limestones. This interval correlates with Early Tournaisian nodular to argillaceous limestones in the Moravia Basin (Lisen Formation, Czech Republic), the Dinant Basin (Pont d'Arcole Formation, Belgium), and at the Rhenish Slate Mountains (Lower Alum shale, Germany). (2) Late Hastarian–early Ivorian glaciations previously identified in Southern Gondwana but had not yet recognized in Northern Gondwana were recorded through a sequence boundary. (3) During the Late Tournaisian–Early Visean?, a differential block faulting regime along the basin's margin caused uplift of the westernmost parts of the Alborz Basin and resulted in subsidence in the eastern part of the central basin. This tectonically controlled shift in depositional regime caused vast sub-aerial exposure and brecciation preserved in the top of the Mobarak Formation in the western portion of the Central Alborz Basin. (4) Tectonic activity coinciding with a progressive, multiphase sea level drop caused indirectly by the Viséan and Serpukhovian glaciations phases ultimately led to the stagnation of the carbonate factory. Paleothermometry proxies, the presence of foraminiferal taxa with a northern Paleo-Tethyan affinity and evidence for arid conditions in the terrestrial hinterland place the Alborz Basin at lower latitudes than the approximately 45ο–50ο southern paleolatitude reported thus far.

Characterization of REE geochemistry of coals from the Sangrud coal mine, Alborz coalfield, Iran

The Sangrud underground mine is located in the west of the Alborz basin that has been mined since 1971. The concentrations and distribution patterns of rare earth elements (REEs) in coal samples from the Mazino mine were studied by inductively coupled plasma mass spectrometry (ICP-MS). The concentrations of ∑REEs in coal samples range from 2.1 to 117.22 ppm with average of 34.74 ppm. The concentration and distribution of REEs in coal samples of Sangrud mine are effected by intrusive rocks. The coal samples from the nearer source rock have higher ash yield and REE contents than the other. The REE patterns of the coal samples exhibit two slightly different types. The differential relative enrichment patterns of light rare earth element (LREE) may have resulted in the high background LREEs in the primary mineral matter.

Lower Carboniferous ramp sedimentation of the Central Alborz Basin, northern Iran: integrated sedimentological and rock–magnetic studies

Abstract The Lower Carboniferous Mobarak Formation in the Alborz Basin (northern Iran) was deposited along the northeastern margin of Gondwana in a carbonate ramp setting. This paper focuses on the Tournaisian stratigraphic interval of this formation that crops out at the Jaban section in the southwestern Central Alborz Basin. The following facies associations, representing different ramp palaeoenvironments, have been identified: (1) mudstone–wackestone outer-ramp facies; (2) crinoidal to skeletal grainstone–packstone mid-ramp facies; (3) peloidal to crinoidal grainstone–packstone inner-ramp facies; and (4) coastal facies, which include a variety of microbial laminated to oncoidal grainstones and mudstones with evaporitic pseudomorphs. This ramp profile was affected by frequent storms that were responsible for the formation of several skeletal to non-skeletal shoals in the distal mid-ramp to the most proximal inner-ramp areas. The development of the skeletal to non-skeletal shoals along the sea side of the ramp formed a semi-enclosed lagoon sensitive to the influence of both high tides and storm surges. The magnetic susceptibility (χ in ) of all the samples was measured and compared with that of the facies from which the sample was taken. There is a clear link between χ in and the facies; the average χ in values were higher for the distal facies than for the proximal facies. The χ in profile of this Lower Carboniferous carbonate sequence reflects stratigraphic variations in response to relative changes in sea level and the input of detrital materials. In the context of the sequence stratigraphic framework, the average χ in values for lowstand and transgressive systems tract deposits are higher than for the highstand systems tract deposits. The clear link between χ in and facies indicates at least a partly preserved primary χ in signal related to the detrital inputs. However, to obtain a better understanding of the nature and origin of the minerals carrying the χ in , we performed hysteresis measurements on selected samples. It appears that the χ in signal is mainly carried by low-coercivity ferromagnetic minerals such as magnetite, with a mixture of relatively coarse grains (detrital fraction) and ultra-fine grains (probably formed during diagenesis).

New Permian Aliyak and Kariz Now formations, Alborz Basin, NE Iran: correlation with the Zagros Mountains and Oman

Two new Permian-aged formations ‘Kariz Now Formation’ and ‘Aliyak Formation’ are proposed for a 65–150 m-thick succession in the Kariz Now area, with the type section for both (79.5 m thick) located 9 km northeast of Aliyak village ca. 100 km southeast of Mashhad city, northeastern Iran. The lower Kariz Now Formation is composed of siliciclastics. The age of this Formation is poorly constrained but its correlation with the Shah Zeid Formation in the Central Alborz suggests a possible Asselian-Hermagorian age for the Kariz Now Formation, which implies a hiatus of Yakhtashian–mid Midian (Artinskian–mid Capitanian) age between the siliciclastics of the Kariz Now Formation and carbonates of the disconformably overlying Aliyak Formation. There is also the possibility of a potential correlation of this Formation with the Kungurian Faraghan Formation in the Zagros area. The succeeding Aliyak Formation is mostly composed of carbonate rocks capped by a thin basaltic lava flow. The Aliyak Formation is unconformably overlain by dolostones that are correlated with the Middle Triassic Shotori Formation. Samples were collected from the Kariz Now and Aliyak formations, but fossils were only recovered from the Aliyak Formation. These include calcareous algae, small foraminiferans, fusulinids, crinoid stems and brachiopods. The recovered fusulinid assemblage from the Aliyak Formation is consistent with that of the upper Capitanian Monodiexodina kattaensis–Codonofusiella erki and Afghanella schencki–Sumatrina brevis zones of the Zagros Mountains and with the upper part of the Ruteh Fm in the Alborz Mountains. Although not radiometrically dated, the basaltic lava flow most probably corresponds to similar basaltic lava flows occurring in the uppermost part of the Ruteh Formation in Central Alborz. Thus, the Permian in the studied region developed in a basin that extended westward as far as the Central Alborz. A late Capitanian age for the Aliyak Formation implies it correlates with the Capitanian KS5 in Al Jabal Al-Akhdar in Oman, with Aliyak Unit 5 potentially representing the Permian maximum flooding surface MFS P25. Copyright © 2014 John Wiley & Sons, Ltd.

Geochemical properties of rare earth elements (REE) in coals of Abyek coalfield, North Iran

In Iran, coal deposits from the upper Triassic-lower Jurassic era are limited to the Tabas and Alborz coal basins and three small independent carboniferous regions. One of the coalfields in the Alborz basin is the Abyek coalfield, the focus of our investigation. It is located in the northern mountain ranges of Iran, and we investigate the geochemical behavior of the rare earth elements in it. Five samples were collected and analyzed for trace and rare earth elements (REE) using inductively coupled plasma (ICP). The Abyek area is characterized by very high ash value. The content of total REY (REEs + Y) in the coals varies from 143.19 to 254.39 ppm and ranged 28–43.83 ppm in argillite. The abundance of light rare earth elements (LREES) is higher relative to heavy rare earth elements (HREEs).

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.

Thermal maturity of the Upper Triassic–Middle Jurassic Shemshak Group (Alborz Range, Northern Iran) based on organic petrography, geochemistry and basin modelling: implications for source rock evaluation and petroleum exploration

AbstractOrganic petrography and geochemical analyses have been carried out on shales, carbonaceous shales and coals of the Shemshak Group (Upper Triassic–Middle Jurassic) from 15 localities along the Alborz Range of Northern Iran. Thermal maturity of organic matter (OM) has been investigated using vitrinite reflectance, Rock-Eval pyrolysis and elemental analysis of kerogen. Reflectance of autochthonous vitrinite varies from 0.6 to 2.2% indicating thermally early-mature to over-mature OM in the Shemshak Group, in agreement with other maturity parameters used. The shales of the Shemshak Group are characterized by poor to high residual organic carbon contents (0.13 to 5.84%) and the presence of hydrogen-depleted OM, predominantly as a consequence of oxidation of OM at the time of deposition and the hydrogen loss during petroleum generation. According to light-reflected microscopy results, vitrinite/vitrinite-like macerals are dominant in the kerogen concentrates from the shaly facies. The coals and carbonaceous shales of the Shemshak Group show a wide range in organic carbon concentration (3.5 to 88.6%) and composition (inertinite- and vitrinite-rich types), and thereby different petroleum potentials. Thermal modelling results suggest that low to moderate palaeo-heat flow, ranging from 47 to 79 mW m−2 (57 mW m−2 on average), affected the Central-Eastern Alborz basin during Tertiary time, the time of maximum burial of the Shemshak Group. The maximum temperature that induced OM maturation of the Shemshak Group seems to be related to its deep burial rather than to a very strong heat flow related to an uppermost Triassic–Liassic rifting. The interval of petroleum generation in the most deeply buried part of the Shemshak Group (i.e. Tazareh section) corresponds to Middle Jurassic–Early Cretaceous times. Exhumation of the Alborz Range during Late Neogene time, especially along the axis of the Central-Eastern Alborz, where maximum vitrinite reflectance values are recorded, probably destroyed possible petroleum accumulations. However, on the northern flank of the Central-Eastern Alborz, preservation of petroleum accumulations may be expected. The northern part of the basin therefore seems the best target for petroleum exploration.