Jurassic Sediments Research Articles

The Ammonite Catacadoceras Barnstoni Beds and the Problem of Separation of the Middle and Upper Substages of the Bathonian Stage in Northern Siberia

The paper presents the results of studying the taxonomic composition of ammonites of the family Cardioceratidae in the Bathonian of northern Siberia in the range of distribution of the last representatives of the genus Arcticoceras and the first representatives of the genus Catacadoceras to solve the problem of distinguishing between the Middle and Upper Bathonian. The incorrectness of all previous identifications of the Middle Bathonian species Arcticoceras cranocephaloide Callomon et Birkelund, 1985 based on the northern Siberian material has been established. These determinations were mainly based on Late Bathonian ammonites belonging to the species Catacadoceras barnstoni (Meek, 1859) or the form characterized by a narrow umbilicus and described herein as Cat. aff. barnstoni. An updated zonal scale of the Middle–Upper Bathonian of Siberia based on ammonites is proposed, which is recommended for use both in the regional stratigraphic schemes of the Jurassic deposits of Eastern (Middle) Siberia and in the Boreal (Siberian) zonal standard of the Bathonian Stage. The extreme importance of preserving an auxiliary biostratigraphic subdivision in this scale, the Cat. barnstoni Beds, is noted, which is due to the wide distribution and abundance of the index species in Siberian sections, as well as its significance for the correlation of the lower part of the Upper Bathonian of northern Canada, northern Siberia, Franz Josef Land and East Greenland. In addition, two intervals, namely interval with Cat. aff. barnstoni and interval with Cat. perrarum, are fixed as benchmarks, which are useful for the rapid age estimation and correlation of the Upper Bathonian deposits in Arctic sections within the upper part of the Cat. barnstoni Beds.

Determination of the conditions of sedimentation of the Jurassic deposits of the Karaton field based on a comparison of electrofacies and sedimentological description of the core

This paper presents the results of lithofacies analysis of log curves and sedimentological description of the core material of wells from the Karaton field to determine the environmental condition and determine the similarity of the behavior of log curves. The field is located in the Karaton-Tengiz zone of uplifts, which are characterized by a rather complicated environmental conditions. Based on typical models of facies groups and a description of the core material of the Jurassic deposits, it was possible to identify zones of development of sandy deposits and determine the main environmental condition. Keywords: lithology; sedimentology; facies; electrofacies; generic model; depositional environment.

The Upper Jurassic sediments, Marib-Shabwa Basin, Yemen: Lithofacies aspects and sequence stratigraphic analysis

The present study is devoted to understanding the evolution of the Upper Jurassic Sab'atayn Formation in the Marib-Shabwa Basin, Yemen, through a sequence stratigraphic analysis based on integrating datasets of sedimentology, seismic sections, and well logs. The Sab'atayn Formation (Tithonian age) is represented by a series of clastic and evaporites that were deposited under fluvio-deltaic to prodeltaic settings. It is divided into four members including Yah (at the base), upwards to Seen, Alif, and Safir at the top. Two third-order depositional sequences were determined for the Tithonian succession which were separated by three sequence boundaries. These sequences were classified into their systems tracts signifying several sedimentation patterns of progradational, aggradational, and retrogradational parasequence sets. The first depositional sequence corresponds to the early-middle Tithonian Yah and Seen units that can be classified into lowstand, transgressive, and highstand systems tracts. The second sequence comprises the late Tithonian Alif unit that can be subdivided into transgressive and highstand systems tracts. The sandy deposits of the Alif Member (highstand deposits) represent the most productive hydrocarbon reservoir in the basin. The Upper Jurassic sediments in the study area were resulted from a combination of eustatic and tectonic effects.

Гидротермально изменённые породы – индикаторы зон разгрузки углеводородов Красноленинского свода (Западная Сибирь)

Our own research data as well as numerous publications of recent years on modern problems of oil potential in the West Siberian oil and gas province were used in this study. Based on the Palyanovskaya, Talinskaya and adjacent areas of the Krasnoleninsky arch, the issues of deposits structure and confinement to certain regional levels of the section, the problems of unconventional reservoirs formation in the zones of tectonic deformations and hydrothermal-metasomatic changes in pre-Jurassic basement rocks (Paleozoic granites), as well as problems of highly permeable “supercollector” (the lower horizons of the Jurassiccomplex – Sherkalinskaya formation) and an almost impermeable reservoir with difficult to recover huge hydrocarbon reserves (its upper horizons – Bazhenov formation) are discussed. The oil deposits distribution in the Jurassic part of the section was shown to comply with the fluid migration model with the participation of deep hydrocarbon fluid. New types of unconventional reservoirs have been identified in the basement granites and in the unloading zones in the Jurassic deposits of the sedimentary cover.

Geological structure and occurrence conditions of the Bazhenov formation within THE South Tambey gas condensate field

Background. In the 21st century, exploration works in the previously discovered Jurassic deposits has been launched in the north of the Yamal Peninsula. The South Tambey field was no exception, where 3D seismic surveys and exploration drilling have been carried out over the past 10 years to study of Jurassic sediments and rocks of the Bazhenov formation. Due to the lack of a data sufficient sample in the northern regions of the Western Siberia, formation was poorly studied. The main ideas were formed on wells scattered at great distances from each other without the possibility of a clear correlation. After receiving new drilling materials, it became possible to examine the deposits of the Bazhenov formation in more detail.Aim. Determination of the structural features and occurrence conditions of the Bazhenov formation.Materials and methods. The research was based on the logging data of exploration and production drilling. A detailed correlation of well sections was performed, which was compared and analyzed with the core description and interpretation of seismic survey materials.Results. The lithological characteristics of rocks obtained by core description are presented, the atypical neighborhood of stromatolites and radiolarians in the section is noted. Based on a detailed correlation, the Bazhenov formation was divided into three intervals with characteristic geophysical logging values and distribution features over the area. The clinoform occurrence with development in the north-west direction is determined. Based on the areal variability, it is proposed to distinguish two types of the Bazhenov formation, and approaches to their tracing are considered.Conclusion. The study of the Bazhenov formation revealed a co-directionality of the spatial orientation of the faults, the progradation of the Achimov clinoforms, and changes in the thickness of the Bazhenov formation. A hypothesis has been put forward about a single tectonic regime that prevailed in the Upper Jurassic and Lower Cretaceous. It is proposed to consider flower (keyboard) tectonics as the main factor that determined the structure and occurrence conditions of the Bazhenov formation.

Compositional and taphonomic gradients show fluctuating depositional conditions on Middle Jurassic brachiopod-rich accumulations from Northern Calcareous Alps (Austria–Germany)

ABSTRACT Brachiopod-rich accumulations of the well-known ‘VilserKalk’ calcareous units from the Middle Jurassic of Zehrer (Allgäuer Alps) have been recognised and analysed from a taphonomic standpoint. These brachiopod concentrations are frequent in the Jurassic deposits of various Alpine-Mediterranean localities, usually related to resedimentation processes generated by high-hydrodynamic episodes. In the four brachiopod-bearing accumulations recorded, high concentrations of mainly articulate shells are preserved in pocket-like deposits made up of brachiopod communities with quite different structure, size-frequency distribution, diversity and taxonomic composition. The analysed taphocharacters indicate different hydrodynamic regimes of diverse intensity for each accumulation, resulting in a gradation of the resedimentation processes. The inferred transport of the shells from the source area is minimal, the bioclastic levels constituting para-autochthonous assemblages without significant spatial averaging. The four defined taphofacies include different assemblages that chronostratigraphycally span the latest Toarcian-early Bajocian interval. These taphofacies are interpreted in terms of depth-related variations concurring with environmental disturbance events in several phases of the evolutionary history of the basin, progressively ranging from shallower proximal accumulations to shell-beds originated in more distal depositional environments.

Peculiarities of the Formation of Upper Jurassic Deposits on the Example of the Object US1

In this work, we analyzed the stock materials on the CS field to obtain an information base to characterize the geological structure of the Vasyugan formation oil deposit (object US1). According to GIS data, the section types of the formation of US1, which determine the distribution of initial oil reserves and the pattern of low rates of oil reserves in the localized dead-end and stagnant zones are highlighted. The characteristics of the displacement of the main reservoirs of the object US1 are presented. The most favorable groups of reservoirs by type of reservoirs (monolithic and irregularly layered) are identified.

Fluid inclusions geochemical characteristics as indicators of the organic matter transformation degree in Jurassic sediments of the Em-Ega crest (Krasnoleninsky arch, Western Siberia)

Fluid inclusions in quartz crystals from the Abalak formation carbonate rocks of 5 wells on the Em-Egovskaya summit of the Krasnoleninsky arch (Western Siberia) were investigated. Two-phase fluid inclusions with water-salt solutions, inclusions containing organic liquid, and single-phase gas fluid inclusions were found. For fluid inclusions with two-phases, the values of the homogenization temperature were obtained. For single-phase inclusions, the temperature of the second phase forming was revealed. It was established that two types of primary inclusions are present in one well, with oil and with water-salt solution. The homogenization temperature of the primary fluid inclusion containing a water-salt solution is 267°C, the homogenization of the oil-containing inclusion exceeds 300°C. In primary-secondary inclusions, the homogenization temperature reached 136 °C. Studies of the composition of inclusions by infrared spectrometry have shown that in single-phase inclusions, the gas is represented by methane. In primary-secondary inclusions, the composition may vary, while in the gas phase, the predominant components may be hydrocarbons with two or more carbon atoms. Studies of organic matter from the deposits of the Tutleim source-rock formation overlying the Abalak formation have shown that the stage of catagenetic transformation of kerogen in the studied wells changes. It was found that in wells in which organic matter is most transformed, the homogenization temperature of fluid inclusions exceeds 130°C. Thus, it is proved that on the territory of the Em-Egovskaya summit of the Krasnoleninsky arch, Jurassic deposits were effected by heat flows created by high-temperature fluids. Not only rocks of the Abalak formation were exposed by thermal processes, but also organic matter from the Tutleim formation was changed, as a result its maturity increased and the generation of hydrocarbon compounds was stimulated. The results obtained explain the difference in the catagenetic transformation of the organic matter of the Tutleim formation in this area. The study of the molecular composition of inclusions will allow us to more accurately reconstruct the conditions of hydrothermal processes. The study of fluid inclusions makes it possible to develop new criteria for the search for oil and gas deposits.

Fertile Goeppertella from the Jurassic of Patagonia: mosaic evolution in the Dipteridaceae-Matoniaceae lineage.

Goeppertella has been postulated as a monophyletic group, whose precise position within the Gleichenoid families Dipteriaceae and Matoniaceae, remains poorly understood. Previously described Goeppertella specimens are based on frond fragments and its fertile morphology is represented by a few, poorly preserved specimens. We describe a new species based on the largest collection of fertile specimens known to date and discuss the evolutionary history of the genus based on the additional reproductive characters provided by the fossils described. Plant impressions were collected in Early Jurassic sediments of Patagonia, Argentina. The specimens were described, and silicone rubber casts were developed to examine in detail vegetative and reproductive features. The new species was compared with other Goeppertella species. Finally, a backbone analysis was performed in the context of a previously published combined matrix of Dipteridaceae, using the maximum parsimony criterion. The new species is described based on a combination of features that have not been previously reported. The vegetative morphology shows affinities with most fossil and extant Dipteriaceae, contrasting with the reproductive morphology which is more comparable with the scarce number of fossil dipteridaceous forms and it is more spread in the sister family, Matoniaceae. The backbone analysis indicates that the position of the new species vary among different positions among Dipteridaceae and Matoniaceae. Additional analyses, discriminating the signal of reproductive and vegetative character, are provided to discuss the base of this uncertainty. We consider Goeppertella as a member of the family Dipteridaceae since we interpret most shared features with Matoniaceae as plesiomorphic conditions for the family. In contrast, most shared features with Dipteridaceae represent apomorphies for the group. Thus, Goeppertella would represent an early diverging genus in Dipteridaceae, considering the venation characters as the most important in order to define the family.

Jurassic system stratigraphy at the Kamianka village. Part 1. The Kozhulinska formation

Introduction. Despite the active research of Jurassic system deposits of Donbas during the last two centuries the issue of their stratigraphy principles is still debatable. The authors` analysis of the available scientific material revealed a conflict between the normatively established requirement to rely on lithostratigraphy during the sediment research, and the fact that the existing common system relies on biostratigraphy. This contradiction could be solved by the study of lithostratigraphic units of the Donetsk Jurassic deposits. Therefore, the purpose of the article was to research the Kozhulinska formation, establish its composition and describe its main features. The Lower Jurassic deposits from the outcrops near the village of Kamyanka (Izyum district) were studied, since the studyable stratotypes of most of the Jurassic strata, which are well exposed in the studied area, are available in this area. Material and methods. The scientific search was based on the analysis of literature, results of previous drilling and mapping works, as well as own field, laboratory and analytical studies. Clay samples were taken from different stratigraphic levels of the formation and from different lithological varieties. Laboratory research was carried out in two directions: using an electron microscope and a spectrometer to detect the material composition of clays. A palynological analysis of these deposits was also carried out, namely, stratified deposits with the help of microphytofossils, including, for the first time, according to the orthostratigraphic group – dinocysts. Results and discussion. The results showed that the Kozhulinska formation was mainly represented by shaly clays (gray, dark gray, sometimes blue-gray) that contained relatively thin layers of sandstones. The main mass of clays consisted of hydromicas, with an insignificant content of montmorillonite, and an even smaller amount of kaolin. The layers of quartz sandstones cemented with clay material were separately traced along the strike. Paleontological remains in the clays of Kozhulinska formation were rare. Palynological groups are mainly represented by pollen and spores, as well as remains of dinophytes, green algae and acritarchs. According to paleoalgological analysis, the sediments of the Kozhulinska formation are dated for the first time. Stratigraphy. Kozhulinska formation is represented by a fairly uniform layer of montmorillonite clays of hydromica composition. The clays are mainly siltstone, but there are, especially in the lower part, washed clean varieties. It is impossible to divide the formation into sub formation based on lithological features. According to biostratigraphic data, deposits are divided into Plinsbachian, Toarian and Aalenian. Paleogeography. At the end of the Early Jurassic and the beginning of the Middle Jurassic era, the territory of the Kamianska area was a low-lying part of the seabed that was drained during low tides. The land that surrounded it was also low-lying and almost did not supply detrital material, however, there is a regular removal of plant detritus. The source of the terrigenous material was the sedimentary complexes of the Donetsk Ridge. Features of the chemical composition of the world are a significant amount of iron (siderite, ocher, chamosite) and the absence of calcium and magnesium carbonates. At the end of the early Aalene, this unstable basin finally regresses, which leads to the accumulation of a powerful subaerial pack of sandstones, which weigh and border the cores of anticlines - elevated areas of the bottom or small islands.

The Geochemical Characteristics of Fluid Inclusions as Indicators of the Degree of Organic Matter Transformation in Jurassic Sediments of the Em-Egovskaya Summit (Krasnoleninsky Arch, Western Siberia)

The Geochemical Characteristics of Fluid Inclusions as Indicators of the Degree of Organic Matter Transformation in Jurassic Sediments of the Em-Egovskaya Summit (Krasnoleninsky Arch, Western Siberia)

APPLICATION OF PHASE-ENERGY ANALYSIS OF SEISMIC WAVES FOR GEOLOGICAL SECTION PREDICTION

Link for citation: Kochegurov A.I., Kochegurova E.A., Orlov O.V. Application of phase-energy analysis of seismic waves for geological section prediction. Bulletin of the Tomsk Polytechnic University. Geo Аssets Engineering, 2023, vol. 334, no. 5, рр.54-62.In Rus. Relevance. One of the main tasks in predicting a geological section is the task of determining the type of the section and mapping types over the area under study, as well as assessing the absorbing and dispersive properties of geological media. To solve these problems, specialized software-algorithmic complexes for processing and interpreting seismic data are currently widely used. In most modern complexes for forecasting a geological section, the information extracted from the kinematic and dynamic characteristics of seismic waves is used. Based on the information received and the data on the geophysical survey of wells, ideas are formed about the material composition of sediments, the presence of productive strata in the study area. This approach made it possible in a number of regions of Russia, including Western Siberia, to obtain interesting and important results related to the discovery of productive sedimentary strata and the assessment of their oil and gas content, which was confirmed by practice. At the same time, the results of practical activities have shown that when conducting studies of many areas, there are areas where the percentage of identifying false anomalies in geological environments and making erroneous decisions in the forecast is still quite large. This is due, among other things, to the fact that in the analysis of the recorded seismic field, for predicting geological section, the energy characteristics of the reflected waves are mainly used, and changes in the phase spectra are practically not taken into account. Thus, the integrated use of the characteristics of reflected seismic waves (amplitude and phase) in the prediction of a geological section is a very urgent task. The aim of the work is to carry out phase-energy analysis for various models of seismic fields based on the method of phase-frequency deconvolution for predicting the geological section. Methods: computational experiment, inverse filtering, discrete Fourier transform. Results. The authors developed the algorithm for phase-energy analysis of seismic waves, which is based on the results of phase-frequency deconvolution. The algorithm was studied on various models of geological environments, and phase-energy sections were constructed, which make it possible to identify zones with anomalously low energy values for subsequent oil and gas potential diagnostics. The paper introduces the example of constructing a phase-energy section based on real data in the interval of Upper Jurassic deposits.

Assessing the palaeobiology of Vespersaurus paranaensis (Theropoda, Noasauridae), Cretaceous, Bauru Basin – Brazil, using Finite Element Analysis

Noasauridae is a group of theropod dinosaurs mostly present in Gondwanan deposits of Jurassic and Cretaceous age. Vespersaurus paranaensis from the Bauru Basin (Caiuá Group, Cretaceous) was the first Brazilian taxon assigned to the clade. This work applied Finite Element Analysis (FEA) to investigate the functional morphology of some skeletal elements assigned to Ves. paranaensis (one tooth and two pedal ungual phalanges). The tooth was modeled and tested in six different scenarios to infer the performance of its crown in different putative feeding conditions. Three different extrinsic scenarios were tested on the ungual phalanges to simulate potential habits in which these structures were involved (piercing, scratch-digging, and hook-and-pull). The scenarios tested on the tooth suggest an ideal bite angle of 45°, with higher von Mises stress/element in the other angles. This indicates that the dentition of this noasaurid was not adapted for struggling prey, nor for harder food items. The FEA results of the ungual phalanges of Ves. paranaensis suggest a similar performance in the three tested scenarios, therefore not specifically adapted for any of those specific functions. Additionally, these phalanges are similar in shape to those of living mammals with scansorial, fossorial, and terrestrial habits. Collectively, this information suggests that, Ves. paranaensis had a generalist diet, seeking to hunt small vertebrates, invertebrates, or immobile prey, such as carcass, and did not feed on larger animals.

Hydrogeochemistry of productive Lower Jurassic deposits of the Talinskoye oil field in Western Siberia

The aim of the study is to determine, in terms of the prevalence of influence of natural or technogenic factors, the factors of formation of the hydrogeochemical composition of productive Lower Jurassic deposits of the Talinskoye oil field. During the study, the authors of the article evaluated the effects of sedimentogenesis, elutriation, neotectonic processes and oilfield water injection on the chemical composition of the waters studied. Water injection is known to cause changes in the hydrogeochemical balance and the formation of technogenic hydrogeological systems. Formation water salinities currently range from 3.7 to 15.3 g/dm3. More than 8960.3 thou-sand m3 of water was injected from overlying sediments into productive formations to maintain reservoir pressure (2014-2021). The authors analysed the genetic coefficients of different types of water (including injected water). Then they compared the coefficients. It was concluded that injected water currently doesn't have a significant impact on the hydrogeochemical appearance of the productive Lower Jurassic reservoirs of the Talinskoye oil field.

Geochemistry and origin of natural gas in the arctic area of West Siberia

Results obtained in the studies of gas composition in the hydrocarbon pools of the Jurassic–Cretaceous sediments in the Arctic regions of West Siberia are reported. The specific features of gas composition in hydrocarbon pools are shown. The main features of the vertical and lateral gas zoning in the Aptian–Albian–Cenomanian, Berriasian–Hauterivian and Jurassic oil and gas bearing complexes are revealed. Generalization of the data on the geochemistry of hydrocarbon gases (the features of component composition, the isotope composition of methane), along with the published data of the geological and geochemical investigation of the Arctic regions of West Siberia, allows us to consider the highly mature Triassic–Middle Jurassic coaly matter as the source of the dry hydrocarbon gases of the Lower and Middle Jurassic complex. Either the Triassic–Middle Jurassic sediments or the Upper Jurassic beds, enriched with the aquatic organic matter, could be involved in the formation of Upper Jurassic and Lower Cretaceous pools. The low-maturity terrestrial (carbonaceous) organic matter of the Cretaceous sediments generated the major amount of light-isotope gases in the Aptian–Albian–Cenomanian sediments. In addition, the source of those gases could be methane formed during oil biodegradation.

A new methodology using borehole data to measure angular distances between geological interfaces

The study presents a new triangulation-based workflow to assess the degree of parallelism between geological surfaces. This workflow consists of producing and analyzing angular distance distributions as well as conducting spatial analysis using grid maps applicable for subsurface environments with sparse data. We tested our approach using a set of interfaces from Kraków-Silesian Homocline, a geological unit with preferred subhorizontal dip to NE. The pairs of interfaces for angular distance measurements can be divided into two groups: i) separating only Jurassic homocline-related units and ii) separating Jurassic homocline-related units from homocline-unrelated units. To observe potential differences for these two groups, we used bootstrap methods and estimated confidence intervals for summary statistics. In our case, the mean of angular distances turned out to be in general smaller for the pair separating only homocline-related Jurassic sediments. The results also show that the method can be more sensitive to the identification of small-scale structures which are developed only in some of the analyzed surfaces. We provided open-source and freely available computer code to allow reproducibility of the results.

THE FIRST DATA ON MIDDLE JURASSIC DINOFLAGELLATE CYSTS AND FORAMINIFERS OF THE RUSSIAN FAR EAST

Information about the first records of dinocysts and foraminifers in the Middle Jurassic deposits of the Russian Far East and their taxonomic affiliation is given. The use of foraminiferal and ostracod assemblages as a tool for biostratigraphic subdivision and correlation of the Middle Jurassic sections of this region is very difficult due to the poor preservation and limited occurrence of these microfossils. Findings of dinocysts Endoscrinium galeritum, Tubotuberella cf. apatela, Wanaea fimbriata, Meiourogonyaulax cf. caytonensis raise the question of correcting the stratigraphic range of the lithological formations recognized in the Middle Jurassic.

Sequential restoration of Paradox Basin salt anticlines: Reconstruction of younger, eroded datums

Structural restoration is often accomplished by taking a folded, faulted stratigraphic horizon and transforming it back to its original, horizontal geometry. 2D sequential restoration uses a range of older to younger stratigraphic datums to reconstruct the geometric evolution of structural features through a history of multiple deformation events. For surface geology exposures, erosion may have eliminated some portions of younger horizons; these eroded gaps in stratigraphy can be mended using thickness trends to bridge the gaps. If a horizon has been fully exhumed, reconstruction of the missing stratigraphy can be inferred from burial history indicators in older, preserved units and from regional bridging from surrounding, less uplifted areas. In the Paradox Basin, late Cenozoic uplift of the Colorado Plateau has removed 1–2 km of Upper Cretaceous and Lower Tertiary stratigraphic units. Previous structural restorations have followed the structural evolution of the salt anticlines contained in the Paradox Basin from the late Paleozoic deposition of the salt-bearing Paradox Formation to end Jurassic deposition of the Morrison Formation. Yet, significant faulting and folding events continued through the Cretaceous and Tertiary. In order to capture the development of these structural features, this study reconstructs an inferred picture of the missing Upper Cretaceous and Lower Tertiary stratigraphy for Paleocene and mid-Eocene datums. Three Paradox Basin sections give an interpreted structural deformation of the salt anticlines for the Laramide event that affected much of the nearby Colorado Plateau and central Rocky Mountains. Restoration of unfaulted fold shapes show an early and very modest contractional shortening; interpreted Laramide fold amplification of the salt anticlines (∼100–150 m shortening per anticline) is followed by a later and more significant normal faulted extension (∼500 m extension per anticline). Both contraction and extension are detached from subjacent basement (= sub-Paradox) involvement. Location of basement shortening/stretching is beyond the extent of the interpreted sections and may lie within the structural features at the margins of the Paradox Basin.

Gymnosperm Foliage from the Lower Jurassic of Jirande Area, Western Alborz Mountain, North Iran

This paper describes five gymnospermous macro-remains, including bennettitalean (Otozamites furoni), coniferalean (Pityophyllum longifolium and Stachyotaxus septentrionalis), ginkgoalean (Sphenobaiera gyron), and czekanowskialean (Phoenicopsis speciosa) leaves, from the Jirandeh area in the western Alborz, Iran. Materials were collected from a Lower Jurassic coal-bearing horizon within the Shemshak Group in the Jirande locality. The morphological characters of encountered gymnospermous remains, together with their palaeogeographical distribution and stratigraphical occurrences, are discussed herein. The species Sphenobaiera gyron is recorded here for the first time from Asia. Pityophyllum longifolium and Stachyotaxus septentrionalis are described for the first time in Iran. This is also the first report of Phoenicopsis speciosa from the western Alborz region. The upland environment under a subtropical to warm-temperate climate is suggested for the Lower Jurassic deposits in the Jirande area based on the occurrence of the encountered gymnospermous taxa.

Jurassic Evolution of the Dunhuang Basin and Its Implications for the Early History of the Altyn Tagh Fault, Northeast Tibet Plateau

AbstractBasin evolution along the northeastern Tibetan Plateau related to the Altyn Tagh Fault (ATF) has significant implications for deciphering the formation of the Tibetan Plateau. However, the Mesozoic history of the ATF is still not well understood. In this contribution, we present a comprehensive sedimentologic and provenance analysis of the Jurassic Dunhuang Basin (DHB) to reconstruct the basin prototype and evolution and to discuss the early history of the ATF. The basin infill consists of late‐Early through Late Jurassic sediments interbedded with basalt, which were deposited in alluvial to lacustrine environments during the Early to Middle Jurassic, and in fluvial to fan delta environments during the Late Jurassic. The analyses of gravel component, paleocurrent and detrital zircon U‒Pb geochronology indicate that the clastic sediments were mainly derived from peripheral areas of the Bei Shan (north), Altyn Tagh—Qilian Shan (south), and the internal Sanwei Shan. Combined with seismic reflection data interpretations, our results suggest that rifting started by the activation of bilateral boundary faults during the Early Jurassic. This was followed by extension and associated magmatism due to subcrustal ductile necking and increased heat flux during the Middle Jurassic, and by final transformation into subsidence sag related to boundary fault die‐out or weakening during the Late Jurassic. We conclude that the DHB formed in an extensional setting generated by the far‐field effects of the Bangong‐Nujiang Ocean closure during the Jurassic, and that the ATF consisted of three subordinate normal faults rather than a sinistral strike‐slip fault during the Jurassic.