Arabian Platform Research Articles

A new thin-section based micropaleontological biozonation for the Saudi Arabian Jurassic carbonates

Micropaleontological biozonation of the Jurassic of the Middle East is poorly developed and shown to have limited regional applicability. This difficulty is based on the inability to apply global schemes based on open marine forms, such as planktonic foraminifera and calpionellids, to the extensive and relatively shallow, often hypersaline carbonate platforms that persisted throughout much of the Jurassic. Intrashelf basins with restricted open marine access further inhibited the use of standard global tropical planktonic biozonations. The Arabian Platform was a shallow marine, predominantly carbonate, setting during most of the Jurassic and peri-Tethyan age-indicative foraminiferal species are relatively poorly represented. Pioneer fieldwork established chronostratigraphic constraint to the various Jurassic formations and members, using ammonite and nautiloid stratigraphy but such macropaleontological schemes are impossible to use in subsurface-derived limited rock sample volumes such as those from cuttings, sidewall cores and cores of lithified carbonates. A thin-section reassessment of the Saudi Arabian Jurassic using field samples and core-based micropaleontology has revealed a hybrid benthic foraminiferal and microfossil bioevent scheme that should assist assignment to the various formations, members and reservoirs as well as assisting in paleoenvironmental interpretation. Twenty-three thin-section based local biozones (Him-3–Mm) are proposed, and serve as an initial scheme with an alphanumeric arrangement to permit refinement when additional events are encountered. These biozones are applied to the Marrat Formation (Mm; Toarcian), the Dhruma Formation (Dm-1 to Dm-6; Bajocian–Bathonian), the Tuwaiq Mountain Formation (Tm-1 to Tm-5; Callovian), the Hanifa Formation (Hm-1 to Hm-2; Oxfordian), the Jubaila Formation (Jm-1 to Jm-2; Kimmeridgian), Arab Formation (Am-1 to Am-4; Kimmeridgian), and Hith Formation (Him-1 to Him-3; Tithonian). In the absence of other disciplines, this scheme is intended to assist chronostratigraphic and lithostratigraphic assignment to field samples and for exploration single well studies to provide calibration and correlation for multiwell single field and regional reservoir characterisation studies. Rapid age and lithostratigraphic determinations of “hot-shot†exploration samples will also be facilitated using this scheme.

Paleomagnetic results from Northeast Anatolia: remagnetization in Late Cretaceous sandstones and tectonic rotation at the Eastern extension of the Izmir–Ankara–Erzincan suture zone

Paleomagnetic results obtained from Upper Cretaceous sandstones in Northeastern Anatolia demonstrate that the entire area from Erzincan to Kars has been remagnetised. The remagnetisation was acquired before the Middle Eocene collision between the Eastern Pontides and the Arabian Platform because Middle Eocene sandstones carry primary natural remanent magnetisations. The post-folding in situ mean direction of the Upper Cretaceous sandstones is compared with mean directions of younger, Middle Eocene to present rock formations. As a result, a two-stage antagonistic rotation mechanism is proposed. First, the collision between the Pontides and the Taurides between Late Cretaceous and Middle Eocene was associated by clockwise rotation of ~ 26°. In the second stage between Middle Eocene and Middle Miocene and beyond, counterclockwise rotations up to ~ 52° of the Pontide and Anatolide blocks and clockwise rotations of the Van Block were characterised by regional shortening and westward escape.

Lithospheric flexural strength and effective elastic thicknesses of the Eastern Anatolia (Turkey) and surrounding region

The Lithospheric structure of Eastern Anatolia and the surrounding region, including the northern part of the Arabian platform is investigated via the analysis and modeling of Bouguer anomalies from the Earth Gravitational Model EGM08. The effective elastic thickness of the lithosphere (EET) that corresponds to the mechanical cores of the crust and lithospheric mantle is determined from the spectral coherence between Bouguer anomalies and surface elevation data. Its average value is 18.7km. From the logarithmic amplitude spectra of Bouguer anomalies, average depths of the lithosphere-asthenosphere boundary (LAB), Moho, Conrad and basement in the study area are constrained at 84km, 39km, 16km and 7km, respectively. The geometries of the LAB and Moho are then estimated using the Parker-Oldenburg inversion algorithm. We also present a lithospheric strength map obtained from the spatial variations of EET determined by Yield Stress Envelopes (YSE). The EET varies in the range of 12–23km, which is in good agreement with the average value obtained from spectral analysis. Low EET values are interpreted as resulting from thermal and flexural lithospheric weakening. According to the lithospheric strength of the Eastern Anatolian region, the rheology model consists of a strong but brittle upper crust, a weak and ductile lower crust, and a weak lower part of the lithosphere. On the other hand, lithosphere strength corresponds to weak and ductile lower crust, a strong upper crust and a strong uppermost lithospheric mantle for the northern part of the Arabian platform.

The upper Coniacian to upper Santonian drowned Arabian carbonate platform, the Mardin-Mazidag area, SE Turkey: Sedimentological, stratigraphic, and ichthyofaunal records

A measured stratigraphic section including the upper Coniacian – upper Santonian interval in the Mardin-Mazidag area in Southeastern Turkey lies on the Arabian Platform and documents the drowning of the Arabian platform. The studied succession transgressively overlies the exposed carbonate platform and exhibits a relatively condensed sequence characterized by presence of fish remains, phosphate nodules, oncoids, phosphatic fragments, glauconites and planktonic foraminifera. Calcareous nannofossils attest a late Coniacian to late Santonian age (zones UC11 UC12). Numerous elasmobranch teeth are also present in this pelagic facies, representing mackerel sharks (Squalicorax, Pseudocorax and Scapanorhynchus) and a ray (Ctenopristis). The presence of pelagic facies and phosphate deposits on the drowned platform may indicate a sudden rise of sea level and increase of nutrient due to upwelling currents. The collapse parts of the Arabian carbonate platform occurred earlier than previously reported, starting from late Coniacian-Santonian onwards.

Hypogenic karst at the Arabian platform margins: Implications for far-field groundwater systems

Hypogenic karst at the Arabian platform margins: Implications for far-field groundwater systems

The drowning of a siliciclastic shelf: insights into oceanographic reconstructions of the northern Arabian Platform during the Early Cretaceous

AbstractBarremian‐Aptian sedimentary successions along the northern Arabian margin have been described as a transition from a siliciclastic to a carbonate‐dominated marine environment, deposited upon a low‐relief shelf or platform formed as a consequence of continuous regional subsidence. A long (360 m) core from northern Israel offers a unique look at this transition, providing valuable insights for the palaeoceanography, geometry and ventilation conditions that lead to Oceanic Anoxic Event 1 (OAE1) in this region. Results from high‐resolution elemental, mineralogical, sedimentological and petrophysical analyses carried out revealed the emplacement of abundant mass‐transport deposits (MTDs) during the Late Barremian and the Aptian. The transplanted units are characterized by fine grained calcareous shales with elevated organic matter, sulphur and iron contents. The scarcity or absence of bioturbation in the disturbed sequences provides a hint to the sediment/water interface conditions. However, a decrease in sulphur and iron occurring at the contact between the shales and the MTDs is explained as increased oxic conditions at the sediment‐water interface as a result of turbulence and mixing associated with the descending sediment masses. Such recurrent events ventilation of the low‐energy basinal environment during the Late Barremian and Aptian, predate the wide‐scale establishment of OAE1 in the northern Arabian margin. Moreover, the identification of coarse‐grained MTDs within deep‐water calcareous sediments indicates a much steeper gradient of the northern Arabian margin, challenging previous studies.

Stratigraphic Correlation between Subsurface Maastrichtian Digma Formation and Safra Unit from Outcrop Sections, Western Desert of Iraq

In this work we try to clarify, and correlate between two Maastrichtian aged units, namely Digma and Safra Units from selected sections in western desert of Iraq. Taking in consideration the stratigraphic code of NASCENT (2005), and the results of previous studies, nine criteria have been used for the correlation, as follow: 1- From lithostratigraphic point of view, Digma Formation (Fm.) mostly composed of dark grey to blackish gray, uniform Carbonates-dominated facies, while Safra unit is characterized by light yellowish brown mixed phosphatic- siliceous-siliciclastics and carbonates facies. 2- Digma Fm. is characterized by Benthonic foraminifera assemblages (Bulimmina sp., Siphogerinoides sp. & Bolivinoides sp. ), while Safra phosphatic unit is flourished by macrofossils assemblages and vertebrates (Oyster, fish teeth’s and bones). 3- The upper and lower boundaries of the Digma Fm. are conformable, and vice versa for Safra Unit. 4- Illite/Semctite mixed layers-Illite clays assemblage and Glauconitic are predominates in Digma Fm., while Sepiolite-palygorskite-Montmorillonite, are common in Safra Unit, without any Glauconitic mineralization. 5- Digma Fm. slightly influenced by neomorphism and micritization. On other hand, Safra facies extremely influenced by phosphatization, silicification, dolomitization, and neomorphism and carbonate-silica replacement of the original evaporate minerals. 6- The predominated foraminiferal wackestone and mudstone microfacies, within Digma facies associations, mostly indicate deep trough marine condition (Anah Graben). The mixed phosphatic-carbonates (Oyster coquina band) and siliciclastics facies, (occasionally cross laminated), mostly deposited in shallower water depth, on the Arabian platform and distal steepened ramp towards Rutba flanks. 7- Sequence boundary of type one and three is bounded the Safra Unit respectively from upper and lower parts. 8- Continuously deposited Sequence was detected from the parasequences of the Safra facies, and across Maastrichtian/Danian boundary (Trafawi Unit). 9- The paleo-configuration of the basin almost influenced by the reactivity of the Anah basement faults started in the Late Campanian, and combined by shallowing upwards sequences. The conclusion is that, Digma facies can be considered as local facial changes of the Shiranish Formation, while Safra unit can be considered as a lower Member of (L. Maastrichtian-Danian) Jeed Formation, which is coincides with the whole stratigraphic frame work of the Mediterranean segments of the Tethyan phosphatic realm.

Late Jurassic to Cretaceous evolution of the eastern Tethyan Hawasina Basin (Oman Mountains)

AbstractRemnants of a Mesozoic continental margin can be studied today in the nappe pile of the Oman Mountains. Successions of the Arabian carbonate platform and the adjacent deep Hawasina Basin are preserved in the nappe pile and in the foothills of this Mountain range. The Jurassic–Cretaceous sediment successions of the Hawasina Basin (Sumeini and Hamrat Duru Group) are focus of this study. These basinal archives contain information on the response of an eastern Tethyan equatorial ocean system to multiple perturbations of the carbon cycle and of climate during the Cretaceous. Turbiditic continental slope and basinal successions formed near the Calcite Compensation Depth are difficult to date with biostratigraphy and sequence stratigraphy. The available stratigraphic framework for the Hawasina successions was not sufficient for tracing palaeoceanography through the time window of interest in this study. Therefore, existing sequence stratigraphy and biostratigraphy are complemented by additional biostratigraphic data and with a newly established carbon and strontium isotope chemostratigraphy. The Hawasina Basin was affected by sea‐level variations, by changes in oceanography and also by regional tectonics. A first major modification of oceanography occurred at the end of the Jurassic when pelagic Maiolica‐type sediments were accumulated in the deep basin and on adjacent submarine highs (Lower Member of Huwar and Sid'r formations). Pelagic to hemipelagic conditions existed until the Valanginian, marked by a major carbon isotope excursion. Pelagic sediments were replaced afterwards by a succession of fine to coarse‐grained turbidites of Hauterivian to Aptian age. The transition into the mid‐Cretaceous is marked by a sudden shift to fine‐grained siliceous or chert deposits, at a time when sediments enriched in organic carbon were accumulated in the western Tethys and Atlantic Oceans. The continental slope as well as the Hawasina Basin seemed to have been well‐ventilated during Early and mid‐Cretaceous time. Siliceous limestones and chert are indicators of well‐mixed and nutrient‐rich surface water, while the absence of black shales suggests young and oxygenated deep water with a possible source on the vast Arabian platform. These peculiar oceanographic conditions were most pronounced during the onset of the extreme greenhouse episodes of the Oceanic Anoxic Event 1a.

Basin evolution model during cretaceous in the northeastern of Arabian plate in Kurdistan region

The research area concentrates in a part of the main Zagros fold and thrust belt in the Kurdistan region (Northern Iraq). From study tectono-stratigraphy we constrain the story of the basin evolution of Kurdistan during Cretaceous. However we mainly investigated the evolution of the pre-Subduction and Pre-collision periods, focusing on the relationship between tectonics and sedimentation. For this purposes we developed (1) a biostratigraphic approach using nannofossil analysis, (2) a fault tectonic analysis, and (3) a stratigraphic study. The Zagros fold belt in Kurdistan exhibits many lateral and vertical environmental and facies changes, especially during the Cretaceous times. During the Jurassic period the Kurdistan is occupied by the restricted Gotnia Basin. This basin disappeared and the Kurdistan area changed to open marine of a southwest Kermanshah Basin during the Cretaceous. During the Berriasian to Barremian the Kurdistan was covered by the carbonates of the Balambo and Sarmord formations. In the east and southeast the neritic Sarmord Formation gradationally and laterally passes to the basinal facies of the Balambo Formation. In the Aptian to Cenomanian period shallow massive reefal limestone of the Qamchuqa Formation deposited. The normal faulting that initiates during the Aptian is associated with an abrupt lateral change of the reefal Qamchuqa Formation to the Aptian-Cenomanian part of the Balambo Formation. During the Cenomanian-Early Turonian periods the graben formed in the Dokan Lake in eastern Kurdistan, where developed a deeper restricted environment (Dokan and Gulneri formations) surrounded by a shallow marine platform. During the Turonian the marine pelagic micritic cherty limestones of Kometan Formation covered northeast of Kurdistan, whereas in the Safeen, Shakrok and Harir anticlines the formation was totally, or partially, weathered during the Coniacian-Early Campanian period. The deposition during the Late Cretaceous is very heterogeneous with a gap in the Coniacian-Santonian times probably related to a non-deposition. Associated with extensive tectonics a basin developed during the Campanian with the deposition of shales, marls and marly limestones of the Shiranish Formation. The first appearance is the Kurdistan of the flysch facies of the Tanjero Formation was precisely dated of the Upper Campanian in northeastern Kurdistan. The Tanjero Formation conformably overlaying the Shiranish Formation and was deposited in the foredeep basin associated with the obduction of Tethyan ophiolites onto the Arabian Platform. The Early to Late Campanian period is a time of non-deposition in Central Kurdistan (Safeen, Shakrok and Harir anticlines). During the Late Campanian the Bekhme carbonate platform in the north disappeared when the marly limestones of the Shiranish Formation transgressed over the Bekmeh Platform. In the Aqra area the Maastrichtian Tanjero Formation laterally changed to the thick reefal sequence of the Aqra Formation that unconformably overlies by the Late Paleocene-Early Eocene lagoonal carbonate of the Khurmala Formation. The Campanian sedimentation is mainly controlled by NE- oriented normal faults forming Grabens in Dokan, Spilk and Soran areas. During the Maastrichtian in the extreme northeastern Kurdistan the NE-SW and NNW-SSE normal faults developed in the foredeep basin and originated horsts and grabens.

Three‐dimensional density model of the upper mantle in the Middle East: Interaction of diverse tectonic processes

AbstractWe present a three‐dimensional density model of the lithosphere and upper mantle for the Middle East and surroundings based on seismic, gravity, and seismic tomography data and analyze the main factors responsible for the density variations. The gravity effect of the crust is calculated and removed from the observed field using the most recent crustal model. The residual gravity anomalies are jointly inverted with the residual topography to image the density distribution within the upper mantle. The inversion is constrained by an initial density model based on seismic tomography. The obtained density variations span in a large range (±60 kg/m3), revealing strong asymmetry in the density structure of the Arabian plate. The uppermost mantle layer in the Arabian Shield is relatively dense. However, below a depth of ~100 km we observe a strong low‐density anomaly. In contrast, the mantle density in the Arabian platform increases at the same depths. The most pronounced decrease of the mantle density occurs in the Gulf of Aden, Red Sea, and East African Rift. Underneath the northern Red Sea the low‐density anomaly is limited to the depth ~150 km, while in the southern part it is likely linked to a mantle plume. The densest mantle material is found under the South Caspian basin, which is likely associated with an eclogite body in the uppermost mantle. In the collision zones (the Zagros Belt and the Hellenic Arc), the high‐density lithosphere shows the location of the subducting plates.

The lithospheric shear-wave velocity structure of Saudi Arabia: Young volcanism in an old shield

We investigate the lithospheric shear-wave velocity structure of Saudi Arabia by conducting H-κ stacking analysis and jointly inverting teleseismic P-receiver functions and fundamental-mode Rayleigh wave group velocities at 56 broadband stations deployed by the Saudi Geological Survey (SGS). The study region, the Arabian plate, is traditionally divided into the western Arabian shield and the eastern Arabian platform: The Arabian shield itself is a complicated mélange of crustal material, composed of several Proterozoic terrains separated by ophiolite-bearing suture zones and dotted by outcropping Cenozoic volcanic rocks (locally known as harrats). The Arabian platform is primarily covered by 8 to 10km of Paleozoic, Mesozoic and Cenozoic sedimentary rocks. Our results reveal high Vp/Vs ratios in the region of Harrat Lunayyir, which are interpreted as solidified magma intrusions from old magmatic episodes in the shield. Our results also indicate slow velocities and large upper mantle lid temperatures below the southern and northern tips of the Arabian shield, when compared with the values obtained for the central shield. We argue that our inferred patterns of lid velocity and temperature are due to heating by thermal conduction from the Afar plume (and, possibly, the Jordan plume), and that volcanism in western Arabia may result from small-scale adiabatic ascent of magma diapirs.

Chemostratigraphy of the Upper Albian to mid‐Turonian Natih Formation (Oman) – how authigenic carbonate changes a global pattern

AbstractThe Oman Mountains preserve a Cretaceous continental margin transect with the proximal Arabian carbonate shelf and the adjacent deep Hawasina Basin. Today, the sediments from the Arabian Platform outcrop in the Oman Mountains (Jabal Akdhar and Saih Hatat) and in the Adam Foothills. The western part of the Adam Foothills provides insight into the evolution of a Late Cretaceous intra‐platform basin with organic‐rich sediments in the central part of this basin. The aims of this study are (a) to establish a biostratigraphy and chemostratigraphy of the Natih Formation and (b) to reconstruct depositional conditions of organic‐rich sediments in an intra‐platform basin during Cenomanian–Turonian times. The hypothesis that local black shale formation is an expression of global perturbations of the global carbon cycle will be tested. Reconstruction of the depositional history of the Arabian Platform and its intra‐platform basin within a global palaeoclimatic framework requires an accurate time frame. The Upper Albian to mid‐Turonian biostratigraphy of the Natih Formation has resulted in controversial age models that will be integrated into a solid chemostratigraphic framework with additional biostratigraphic data. A major positive δ13C excursion (+4·6‰) has been identified as of Middle Cenomanian age, which is confirmed by an ammonite datum. A second positive δ13C excursion (+4·5‰) following a major negative excursion (−1·0‰) confirms the existence of the Cenomanian/Turonian Boundary Event. The accurate chemostratigraphy and biostratigraphy confirms that major source rocks in the Mishrif‐Natih Basin precede OAE2. Low δ13C values measured in the sediments of the Natih B member are considered a consequence of diagenetic alteration. Elevated organic carbon contents and argillaceous sediments alternating with limestones resulted in diagenetic conditions favouring formation of authigenic calcite depleted in C‐13.

The temporal evolution of the active margin along the Southeast Anatolian Orogenic Belt (SE Turkey): Evidence from U–Pb, Ar–Ar and fission track chronology

The Southeast Anatolian Orogenic Belt (SAOB) resulted from the north-dipping subduction of the southern Neotethyan oceanic lithosphere in late Mesozoic and early Cenozoic. However, the timing and the rate of the continental collision are still under debate. Here, we present new U–Pb, Ar–Ar and Fission Track ages from the I-type calc-alkaline granitoids (Esence, Doğanşehir and Baskil) cutting the Göksun, İspendere and Kömürhan ophiolites and also the Malatya–Keban metamorphics representing the Tauride active continental margin along the SAOB. In this study, high- to low-temperature thermochronological methods, applied to the granitoids, are used to understand the temporal and geodynamic evolution of the Tauride active continental margin and continental collision between Tauride and Arabian platforms in the frame of the Neotethyan convergence during Cretaceous and Miocene. The U–Pb zircon ages range from 83 to 88Ma for the Baskil and from 81 to 83Ma for the Esence granitoids. The 39Ar–40Ar ages indicate that these late Cretaceous granitoids cooled below 300°C in 6–10Ma. The formation ages and the timing of the gradual cooling of the late Cretaceous granitoids are similar to metamorphism age and the timing of the exhumation of the HP/LT Bitlis metamorphics during this stage. The combined field, geochemistry and geo-thermochronological data suggest that the first continental collision event occurred between Bitlis–Pütürge micro-continents to the south and Tauride platform to the north in an oblique subduction zone between 84 and 74Ma. The granitoids continued uplifting during early-middle Eocene together with the exhumation of HP/UHT Berit metaophiolite in an extensional regime related to the opening of the Maden back-arc basin. This period also let the intrusion of the Doğanşehir arc magmatism, intruding the Pütürge and Malatya–Keban metamorphics, Berit metaophiolite, and Maden Complex. The Eocene Doğanşehir granitoid has similar U–Pb and 39Ar–40Ar ages, suggesting a fast cooling. The apatite fission track (AFT) ages for all granitoid bodies suggest that they were mainly cooled or uplifted in two episodes, where the first one is in the Eocene in an extensional setting. The AFT data marked the final continental collision between the Taurides and the Arabian platform first in Oligocene and the break-off of the subducted slab and the delamination process caused fast uplift of the Eastern Anatolia during middle to late Miocene.

Stratigraphic evidence for development of Aptian intrashelf basin in the Zagros area, eastern Fars Province, SW Iran

?The Lower Cretaceous of the Arabian Platform is renowned for the development of intrashelf basins. The study area is located in eastern Fars Province on the northeast Arabian plate and has experienced this environmental condition. The present study investigated two surface sections (Genau and Anguru) of the Dariyan Formation and compared the results with three neighboring wells in eastern Fars (eastern Zagros Fold Belt) in southern Iran to determine the development of rudist-bearing carbonates and intrashelf basin sediments. The Dariyan Formation was deposited in the Aptian as shallow-to-deep water carbonate facies. Lagoon, rudist banks/barriers, and open marine/basinal facies and six microfacies types have been distinguished. In the Genau section, the upper formation contains shallow rudistid limestone separated from the orbitolinid limestone of the lower formation by a deep pelagic unit. The pelagic sets disappear toward the Anguru section, but the rudistid unit remains. Abrupt changes were observed from shallow rudist facies to deep planktonic foraminifer facies. Comparison of the surface and subsurface sections (wells) suggests an intrashelf carbonate depositional model for the Dariyan Formation in the eastern Fars area. The term "Genau intrashelf" is proposed for this intrashelf basin surrounded by a shallow ramp. This intrashelf appears to be a northern subbasin of the Bab basin of the southern Persian Gulf. Extension of the Genau intrashelf subbasin and development of rudists around the margins of this intrashelf can be defined by tracing changes in the lateral facies of the Dariyan Formation.

Evidence for anomalous mantle upwelling beneath the Arabian Platform from travel time tomography inversion

We present a new model of P-velocity anomalies in the upper mantle beneath the Arabian Peninsula, Red Sea, and surrounding regions. This model was computed with the use of travel time data from the global catalogue of the International Seismological Center (ISC) for the years of 1980–2011. The reliability of the model was tested with several synthetic tests. In the resulting seismic model, the Red Sea is clearly associated with a higher P-velocity anomaly in the upper mantle at least down to 300km depth. This anomaly might be caused by upward deviation of the main mantle interfaces caused by extension and thinning of the lithosphere due to passive rifting. Thick lithosphere of the Arabian Platform is imaged as a high-velocity anomaly down to 200–250km depth. Below this plate, we observe a low-velocity structure that is interpreted as a hot mantle upwelling. Based on the tomography results, we propose that this upper mantle anomaly may represent hot material that migrates westward and play a major role in the formation of Cenozoic basaltic lava fields in western Arabia. On the northeastern side of the Arabian Plate, we clearly observe a dipping high-velocity zone beneath Zagros and Makran, which is interpreted as a trace of subduction or delamination of the Arabian Plate lithosphere.

Dachstein-type Avroman Formation: An indicator of the Harsin Basin in Iraq

ABSTRACTA field survey was carried out in 2012 focusing on the tectonic position and the role of Upper Triassic (Upper Norian–Rhaetian) Avroman Formation outcrops located in the Zalm area of Iraq, close to the Iraq-Iran border. At this location, the Cretaceous chert-bearing strata of the Qulqula Formation are overlain by sheared mafic bodies, which are in turn topped by the cliffs of the megalodontaceae-bearing Upper Triassic Avroman Formation. Similarities in lithology, sequence and tectonics position, suggest that the Triassic section of the Bisotoun Unit from the Kermanshah Zone of Iran can be used as a tectonic analogue of the Avroman Formation. According to our model, both the Avroman and the Bisotoun units formed an intra-oceanic carbonate platform, built-up by a characteristic megalodontaceae-bearing carbonate platform assemblage during the Late Triassic.The Harsin oceanic basin, which separated the Avroman-Bisotoun Platform from the Arabian Platform, was characterised by deep-marine sedimentation, the remnants of which form the Qulqula Formation in Iraq, and the Radiolaritic Nappe and the Harsin Mélange in the Kermanshah Zone. This tectonic setting is not unique; numerous authors suggest the existence of an oceanic rim basin, separating carbonate platform units (e.g. Oman ‘exotics’) from the Arabian Platform. The age of the deformation could be Late Cretaceous (Maastrichtian), but using analogues from Iran, a Palaeogene deformation also seems possible.The Avroman Formation was interpreted to be a Dachstein-type sediment, similar to the well-studied Dachstein Formation of the Northern Calcareous Alps, Austria. Rock units, with similar lithology, or identical depositional environment and macroscopic fauna, were described by numerous authors along the Neo-Tethys suture zone from Austria to Japan, and from several tectonic units along the Panthalassa margin. The implication of this correlation is important for future studies: using well-described type localities of the marine units from the Northern Calcareous Alps as a reference, it is possible to significantly extend the available background knowledge, and to gain better insight into the Triassic regional depositional environment of the Middle East.

Isostatic Model and Isostatic Gravity Anomalies of the Arabian Plate and Surroundings

The isostatic modeling represents one of the most useful “geological” reduction methods of the gravity field. With the isostatic correction, it is possible to remove a significant part of the effect of deep density heterogeneity, which dominates in the Bouguer gravity anomalies. Although there exist several isostatic compensation schemes, it is usually supposed that a choice of the model is not an important factor to first order, since the total weight of compensating masses remains the same. We compare two alternative models for the Arabian plate and surrounding area. The Airy model gives very significant regional isostatic anomalies, which cannot be explained by the upper crust structure or disturbances of the isostatic equilibrium. Also, the predicted “isostatic” Moho is very different from existing seismic observations. The second isostatic model includes the Moho, which is based on seismic determinations. Additional compensation is provided by density variations within the lithosphere (chiefly in the upper mantle). According to this model, the upper mantle under the Arabian Shield is less dense than under the Platform. In the Arabian platform, the maximum density coincides with the Rub’ al Khali, one of the richest oil basin in the world. This finding agrees with previous studies, showing that such basins are often underlain by dense mantle, possibly related to an eclogite layer that has caused their subsidence. The mantle density variations might be also a result of variations of the lithosphere thickness. With the combined isostatic model, it is possible to minimize regional anomalies over the Arabian plate. The residual local anomalies correspond well to tectonic structure of the plate. Still very significant anomalies, showing isostatic disturbances of the lithosphere, are associated with the Zagros fold belt, the collision zone of the Arabian and Eurasian plates.

Late Triassic rifting and Jurassic–Cretaceous passive margin development of the Southern Neotethys: evidence from the Adıyaman area, SE Turkey

Evidence of rifting and continental break-up to form the S Neotethys is found within the volcanic-sedimentary Kocali Complex. This is a folded, thrust-imbricated succession that includes lavas, volcaniclastic sediments, pelagic carbonates, radiolarites and manganiferous deposits. Interbedded ribbon cherts contain radiolarians of Late Triassic to Late Jurassic age. The lower part of the succession of Mid?-Late Triassic age (Tarasa Formation) is dominated by enriched mid-ocean ridge basalt (E-MORB). The overlying Late Triassic to Mid-Jurassic interval (Konak Formation) is characterised by intercalations of ocean island basalt and E-MORB. Taking account of structural position, the basalts erupted within the outer part of a continent–ocean transition zone. Continental break-up probably occurred during the Late Triassic (Carnian–Norian). Early to Mid-Jurassic lavas and volcaniclastic sediments record volcanism probably after continental break-up. In addition, the Karadut Complex is a broken formation that is located at a relatively low structural position just above the Arabian foreland. Pelagic carbonates, redeposited carbonates and radiolarites predominate. Radiolarians are dated as Early to Mid-Jurassic and Late Cretaceous in age. The pelagic carbonates include planktic foraminifera of Late Cretaceous age. The Karadut Complex resulted from the accumulation of calcareous gravity flows, pelagic carbonate and radiolarites in a relatively proximal, base-of-slope setting. After continental break-up, MORB and ophiolitic rocks formed within the S Neotethys further north. Tectonic emplacement onto the Arabian platform took place by earliest Maastrichtian time. Regional interpretation is facilitated by comparisons with examples of Triassic rifting and continental break-up in the eastern Mediterranean region and elsewhere.

Steranes in neoproterozoic oils from the Nepa-Botuoba anteclise in the Siberian platform and the South Oman salt basin in the Arabian platform

A number of rare sterane hydrocarbons untypical of younger Phanerozoic petroleum systems have been found by gas chromatography-mass spectrometry in crude oils from the Nepa-Botuoba anteclise of the Siberian platform and the South Oman salt basin of the Arabian platform (Huqf family oils). Among regular steranes, a continuous series of C21 to C30 hydrocarbons including the previously unknown C23-C25 isomers have been identified. It has been noted that short-chain C21 and C22 steranes are represented by the most thermally stable pregnane species diginane and methyldiginane. Another series of steroids is represented by A-norsteranes, the structures demethylated at C10 (main fragment ion in the mass spectrum at m/z 203). Tricyclic hydrocarbons C26-C29 identified by m/z 219 have been attributed to the homologous series of 8,14-secosteranes. These chemofossils have a wide stratigraphic range of occurrence and, like modern minor secosterynes, can be associated with lipid membranes of sponge and their bacterial synbiotics.

Geochemistry, petrography and tectonic significance of the ophiolitic rocks, felsic intrusions and Eocene volcanic rocks of an imbrication zone (Helete area, Southeast Turkey)

The study area is situated in the imbrication zone (northwest of Adıyaman–Gölbaşı, Helete–Savran) of the Southeast Anatolian orogen, part of the Mediterranean–Himalayan orogenic belt. This area is characterized by NEE/SWW-trending, northward-dipping imbricated thrust slices located low in the nappe zone that was finally emplaced onto the Arabian platform during the Mid-Miocene. From top to bottom the main units in the study area are: (1) the Malatya Metamorphics- (2) the Harami Formation- (3) the Göksun Ophiolite–Esence Granitoid- (4) the Helete volcanics and (5) the Savran–Alacık Formation, Hompur–Lice Formation and Midyat Formation. This study contains the Upper Cretaceous–Eocene tectono-magmatic succession (ophiolitic rocks and related felsic intrusions and volcanic rocks), which are located in the Helete area. The unmetamorphosed ophiolitic rocks display an irregular ophiolitic sequence. These rocks are represented by tectonites, cumulated gabbros, isotropic gabbros and sheeted dyke complex and were intruded locally by felsic intrusions (granite, granodiorite, tonalite, dacite, and aplite). These ophiolites locally contain tectonically disrupted thin amphibolitic rocks. The mineral chemistry and geochemical features of these ophiolites indicate that these rocks exhibit sub-alkaline characteristics, and they are similar to those of island arc tholeiites formed in a supra-subduction zone. These ophiolitic rocks, located in the imbrication zone, are interpreted as being the equivalent of the Göksun Ophiolites. The felsic intrusions, which cut these ophiolitic rocks, correspond to the calc-alkaline Esence Granitoids which formed in an arc setting. Based on the 40Ar/39Ar dating method, a plateau age of 80.92±0.29Ma was obtained from a granite sample that cuts the gabbroic rocks. The Helete volcanics, which have a tectonic relationship with the Göksun Ophiolite and the Esence Granitoid, are represented by andesite-basalt (locally pillow lavas), and are overlain by nummulitic carbonate sediments. Geochemical data indicated that, the Helete volcanics evolved from a tholeiitic parental magma and formed in a subduction-related setting.