Lower Middle Jurassic Research Articles

New occurrences of Bolonia lata Meunier from the Lower–Middle Jurassic of Germany, Algeria and France, with a stratigraphic-palaeoenvironmental review of the ichnogenus

New occurrences of Bolonia lata Meunier from the Lower–Middle Jurassic of Germany, Algeria and France, with a stratigraphic-palaeoenvironmental review of the ichnogenus

Stratigraphy and microfacies analysis of the Lower–Middle Jurassic platform carbonate succession in the Yeşilovacık (Mersin) area (Central Taurides, S Turkiye)

This study covers the detailed analyses of stratigraphy, microfacies and depositional environments of the Lower-Middle Jurassic carbonate succession in the southern part of Anatolide-Tauride Carbonate Platform in southern Türkiye (south of Central Taurides, around Yeşilovacık). The carbonate succession is composed of the Dibekli and Tokmar formations, and the Işıklıkızıtepe Member which forms the base of the Tokmar Formation. The Dibekli Formation (Upper Sinemurian-Lower Bajocian) is composed of limestones intercalated with intraformational conglomerates in the lower parts, with dolostones in the lower to middle parts and with oncoidal limestones in middle to upper parts. The Işıklıkızıtepe Member (Upper Bajocian) starts with basal conglomerates containing brown-red- iron nodules and consists of sandstone and sandy-clayey limestone alternations. These are overlain by thin limestone layers passing upward to thick-bedded dolostones of the Tokmar Formation (Upper Bajocian–Bathonian). The middle to upper parts of the formation are composed of sandy, oncoidal limestones rich in bivalve shells and end with dolostones-limestone alternations in the upper part. According to microfacies analyzes and litho-bio content; Late Sinemurian-Plesbachian is a period that low-to moderate-energy, restricted inner platform conditions dominated the depositional environments. The presence of comman intraformational conglomerates shows tectonic activity from time to time in this period. The Toarcian to the earliest Bajocian, low-energy intertidal and shallow lagoon conditions dominated the depositional environments, and in the Early Bajocian, the depositional environment changed to a high-energy sand shoal and above wave base environment. The basal conglomerate consisting of brown-red, iron-noduler and bauxite grains at the base of the earliest Late Bajocian indicates a short-term emersion and continued under low energy shallow lagoonal conditions during the Late Bajocian–Bathonian. Environmental changes during the Toarcian to Early Bajocian, and the earliest Late Bajocian short-term emersion, and the environmental changes during the Late Bajocian-Bathonian are interpreted to short-term sea level changes. Yeşilovacık Lower-Middle Jurassic carbonate sequences are correlated with those in peri-Mediterranean platforms. They show close similarities to the other Tauride carbonate sequences in terms of stratigraphy and facies. Paleontological and sedimentological analyses enable us to reconstruct a detailed dating and paleoenvironmental evolution for the Yeşilovacık carbonate deposits during the Lower-Middle Jurassic period.

A large pterosaur from the Middle Jurassic (lower Bajocian) of Rutland, United Kingdom

An incomplete, but large hollow bone is identified as pterosaurian on account of its pneumatic nature, thin bone wall, palaeohistology and the characteristic surface texture of the peridermal bone. Despite being incomplete it is tentatively identified as a ?left ulna, and is remarkable for its size. Coming from the lower Middle Jurassic Lincolnshire Limestone Formation of Rutland, eastern England, this is the first record anywhere in the World of a Bajocian pterosaur, and probably the first pterosaur recorded from Rutland. It is most likely from Discites to Laeviuscula zones part of the lower Bajocian succession, and represents one of the largest Jurassic pterosaurs yet recorded, with a wingspan tentatively estimated at between 3.5 and 4.0 m. The taxonomic identity of the new specimen is uncertain, but it is likely allied to the Pterodactyloidea on account of its size, although presently it remains Pterosauria fam. et gen., et sp. indet.

Mesozoic orogenic gold metallogenesis at the Bangong–Nujiang suture belt, central Tibet: Constraints from pyrite textures, composition and sulfur isotope of the Shangxu gold deposit

Pyrite is a good carrier to provide gold within a hydrothermal mineralization system. Under prograde metamorphism, recrystallization of sedimentary and diagenetic pyrite and transformation of pyrite into pyrrhotite in a metamorphic terrane lead to the release of trace elements from the pyrite carrier into an upward migrating liquid phase carrier. In this case study, we report the in situ trace element and δ34S compositions of the pyrite from four different stages, which chronologically comprise sedimentary Py1, diagenetic Py2, metamorphic Py3a, Py3b and hydrothermal Py4a, Py4b, Py5, and the dominant sulfides from the hydrothermal stage at Shangxu, an Early Cretaceous turbidite-hosted orogenic gold deposit in the middle Bangong–Nujiang suture belt (BNSB) in central Tibet, in order to decipher the ore-forming material source and the metallogenesis of the Mesozoic orogenic gold mineralization in the BNSB. Concentrations of trace elements, i.e., Au, Co, Cu, Zn, Mo, Ag, Sb, Te, Hg, Tl, Pb and Bi, that are easy to dissolve during pyrite recrystallization, show a general progressive decline from Py1 to Py4b (excepting in Py3b and Py4a, which have a sudden rise in trace element composition) and a plunge in Py5, indicating that Py1 and Py2 supply the parent source minerals for mineralization, and that the main ore-forming stage are contemporary with the deposition of Py3b and Py4a, ending up with the precipitation of Py5. The δ34S values of pyrite narrow from −46.7 ‰–19.3 ‰ (Py1, Py2), through −7.6 ‰–0.2 ‰ (Py3a, Py3b), to − 3.6 ‰–5.7 ‰ (Py4a, Py4b) and −7.7 ‰–5.6 ‰ (Cp, Gn), indicating that a probable coeval seawater sulfate sulfur source for Py1 and Py2 (by bacterial sulfate reduction), and an inherited sulfur source from Py1 and Py2 for Py3 and Py4. We highlight that contributions from the magmatic materials to the hydrothermal gold mineralization are limited at Shangxu, and the widely distributed Lower–Middle Jurassic pyrite-rich turbidite of the Mugagangri Group is a highly potential primary source rock for the regional gold mineralization.

Stratigraphy, depositional environments and provenance of the Shemshak Group (Upper Triassic–lower Middle Jurassic) on the northern Tabas Block; evidence for the emergent Cimmerian ridge in Central Iran

The Upper Triassic to lower Middle Jurassic Shemshak Group of the Central-East-Iranian-Microcontinent is important for understanding Mesozoic geodynamics of the Middle East during and in the aftermath of the collision of the Cimmerian microplates with Eurasia, resulting in the massive burial of organic matter and the formation of significant coal reserves. The present study thus aims to elucidate the stratigraphic patterns, lithofacies inventory, depositional environments and geodynamic significance of the Shemshak Group in the Halvan area of the Kalmard sub-block of the northern Tabas Block where the succession is most completely developed. Lithostratigraphic studies through detailed logging resulted in a differentiation of the siliciclastic Upper Triassic Nayband and Lower Jurassic Ab-e-Haji formations, bounded at their bases by the Eo- and Main Cimmerian unconformities, respectively, and the mixed carbonate-siliciclastic Badamu Formation (upper Lower to lower Middle Jurassic). The lithofacies analysis and stacking patterns of siliciclastic strata indicate deposition in a variety of environments ranging from (proximal to distal) coastal and delta plains to delta front and prodelta settings for the Nayband and Ab-e-Haji formations. Detailed litho- and microfacies studies of the Badamu Formation revealed inner to middle parts of a mixed carbonate-siliciclastic ramp system. Careful petrographic studies indicate that sandstones of the succession are generally lithic-rich (ranging from phyllarenite to sedarenite) and, up-section, grade into quartz-rich sublitharenite, providing important information for sedimentary provenance analysis. The sandstones rich in metamorphic- and sedimentary lithic fragments indicate a proximal source on the western Yazd Block as well as rocks exposed on the Kalmad sub-block itself. The integrated studies also led to the recognition of the Eo- and Main Cimmerian orogenic phases, characterized by rapid uplift and/or subsidence phases. Late Triassic and Early Jurassic deposition took place in a back-arc extensional basin characterized by normal faulting. During the Toarcian–Aalenian, the reduced input of clastic material due to source-area denudation and/or a major transgression resulted in the development of the mixed carbonate-siliciclastic system of the lower Badamu Formation in the western part of the northern Tabas Block. This ramp system developed into an attached, ocean-facing carbonate platform with a prograding margin of ooid shoals.

New record of conifer wood Agathoxylon from the Upper Jurassic in the Sichuan Basin, southern China, and its palaeobiodiversity and palaeoclimatic significance

ABSTRACT Although abundant and diverse foliage fossils are well recorded in the Upper Triassic and Lower–Middle Jurassic of the Sichuan Basin in South China, the Upper Jurassic plant fossils are rarely documented due to extensively developed red beds in this basin. Decades ago, abundant fossil wood specimens were discovered from the Upper Jurassic Penglaizhen Formation in the Sichuan Basin. However, detailed investigations on the systematic palaeobotany of these wood fossils are very limited, which hinders further understanding of the terrestrial ecosystem and biota. Here, we describe anatomically well-preserved fossil wood specimens from the Penglaizhen Formation at Wangjiagou Village of Shehong City in the central Sichuan Province. These wood fossils exhibit Agathoxylon anatomy with usually uniseriate contiguous radial tracheid pitting and araucarioid cross-field pitting, representing a new record of Agathoxylon in the Sichuan Basin, and contribute to further understanding of the vegetation composition of the southern phytoprovince in China during the Late Jurassic. Independent palaeoclimatic evidence from leaf, pollen and sediments indicates that the Agathoxylon trees from Shehong area have grown under generally hot and semi-arid or arid environmental conditions, probably in habitats at the banks of rivers and lakes with locally abundant water supply, as indicated by the indistinct growth rings.

Dolomitization of the Lower Part of the Sargelu Formation, Northeastern Iraq

The prominent characteristic of the lower part of the Sargelu Formation of the Middle Jurassic is the occurrence of destructive dolomitization. This dolomitized unit, cropped out in the high folded and Imbricated zones in Northern Iraq, is situated in the Gotnia Basin and has been distinctly identified and are documented in previous studies. Dolomite crystals are euhedral in shape; hence, they are classified as saccharoidal dolomite, resulting in the elimination of all biocontent and the destruction of the original rock fabrics. Despite the abundance of intercrystalline pores in this unit and their role in the quality of reservoirs in the oil fields, the origin or mechanism of dolomitization is not well understood yet. The aim of this investigation is to find out a suitable model for the dolomitization and to propose possible mechanisms that led to the dolomitization. At the top of the Lower Jurassic Sehkaniyan Formation, an extensive mass of destructive dolomitization has also occurred. The dolomitic unit of the Sargelu Formation, which extends for hundred kilometers constantly. Stratigraphic correlation reveals that the Sargelu Formation is juxtaposed with different successions, such as the Alan and Sehkaniyan formations, that show evidence about peritidal and sabkha environments, where the dolomitization units were associated with solution collapse breccia and microbial stromatolite. This study proposes that the reflux and burial dolomitization models for the lower part of the Sargelu Formation by comparing the paleoenvironmental conditions of the late Lower Jurassic and early Middle Jurassic in the Gotnia basin with a modern analogue model.

The facies-genetic type of triassic sediments and the main oil and gas formations of the Mangyshlak peninsula

This paper presents the results of many years of research by the authors. The facies-genetic types of Triassic deposits of the Mangyshlak region have been studied in detail.It is shown that the Triassic and especially the Middle and Upper Jurassic sediments are enriched with more dispersed organic matter than in comparison with the layers below and above. According to the analysis results, the Jurassic and Lower Cretaceous sediments are characterized by abnormal values of dispersed bituminous matter compared to other rocks.The bitumen content is generally low (from 0.01 to 0.5%). Jurassic deposits are characterized by higher values than Lower Cretaceous ones. At the same time, a detailed study of the distribution of organic residues (including faunal remains) gives grounds to classify the entire Triassic-Jurassic-Lower Cretaceous regional oil and gas complex (with the exception of the lower part of the Neocomian and Triassic) as shallow-marine sediments of stable and moderate sedimentation regimes. Oil and gas formations, regional patterns of their distribution in the sedimentary cover of the Mangyshlak region, as well as the adjacent oil and gas region, have been studied in detail. The main oil and gas bearing formations of the Mangyshlak region include the pre-Permian substrate (Devonian, carboniferous), Permian terrigenous-clay and carbonate-chemogenic formation (transitional) (Permian), terrigenous-clay colorful-gray-dark gray-colored (subglenic) (Triassic, Lower Middle Jurassic, Kellovian), carbonate-chemogenic-the clay formation (Oxford-Kimmeridge), the sandy-carbonate-clay formation (Neocom) and the clay-sand formation (apt-Alba) also distinguish them in the northern and southern regions with their common rocks, the nature of development and the degree of research. Over the next thirty years, associated with the cessation of regular parametric, exploratory drilling, and various seismic operations.Despite the large volume of scientific, technical and other research, studies of promising deposits and territories are conducted extremely poorly.One of the important reasons for this is the interdepartmental, intersectoral variance of the work performed, the low variance of the information received. In order to increase the efficiency of the exploration and research complex of productive formations, in our opinion, the solution of the issue of oil and gas exploration and exploration in the Mesozoic of the Mangyshlak region should be carried out according to a single intersectoral, interdepartmental program with an assessment of the progress of work on further stages and final results.

Molecular and Carbon Isotopic Compositions of Crude Oils from the Kekeya Area of the Southwest Depression, Tarim Basin: Implications for Oil Groups and Effective Sources

Molecular and stable carbon isotopic compositions of 32 crude oils from the Kekeya area of the Southwest Depression, Tarim Basin, were analyzed comprehensively to clarify oil groups and trace oil sources. The results indicate that lacustrine shale sequences within the Upper-Middle Permian Pusige Formation (P3–2p) are the major effective oil sources; the thermal maturation effects exert the crucial impact on geochemical compositions of crude oils. In the Kekeya structural belt, crude oils produced from the Lower-Neogene, Middle-Paleogene and Middle-Cretaceous sandstone reservoirs were generated mainly from deeply buried P3–2p at the late-to-high maturity stage. These condensates are depleted in terpanes, steranes and triaromatic steranes and enriched in adamantanes and diamantanes. The evaluated thermal maturity levels of crude oils by terpanoids and steranes are generally lower than that of diamondoids, implying at least two phases of oil charging. In the Fusha structural belt, oils produced from the Lower-Jurassic reservoirs (J1s) of Well FS8 were generated from the local P3–2p at the middle to late mature stage. On the contrary, these oils are relatively rich in molecular biomarkers such as terpanes and steranes and depleted in diamondoids with only adamantanes detectable. The P3–2p-associated oils can migrate laterally from the Kekeya to Fusha structural belt, but not to the location of Well FS8. The Middle-Lower Jurassic (J1–2) lacustrine shales as the major oil sources are limited to the area around Well KS101 in the Kekeya structural belt. Crude oils originated from J1–2 and P3–2p can mix together within the Cretaceous reservoirs of Well KS101 by presenting the concurrence of high concentrations of terpane and sterane biomarkers and diamondoids as well as 2–4% 13C-enriched n-alkanes than those of P3–2p derived oils. This study provides a better understanding of hydrocarbon sources and accumulation mechanisms and hence petroleum exploration in this region.

ДЕТАЛЬНАЯ КОРРЕЛЯЦИЯ И УТОЧНЕННОЕ ФАЦИАЛЬНОЕ РАЙОНИРОВАНИЕ НИЖНЕ-СРЕДНЕЮРСКИХ НЕФТЕГАЗОНОСНЫХ ОТЛОЖЕНИЙ СИБИРСКОГО СЕКТОРА АРКТИКИ

Lower–Middle Jurassic deposits of the Siberian sector of the Arctic shelf which prove to be promising in terms of oil and gas-bearing capacity are associated with challenges stemming from their complex structure and a paucity of drilling data. Results of the detailed correlation based on the biostratigraphic, lithostratigraphic and cyclostratigraphic methods at the scale of horizons, formations, sub-formations and cyclic sedimentary units are presented. The data used were derived from 27 correlation profiles, of which ten (encompassing all the facies regions in the study area) are presented in this article. Due to their major focus on paleontology, the sections of wells and correlation profiles used were few in number. The refined herewith variant of facies zoning of deposits in the study area is based on research results obtained from: paleontological studies, detailed correlation and lithological-paleogeographic maps and maps showing surface sediment composition. The materials allowed us, firstly, to refine the facies zonation along with stratigraphy of the sediment infill in basal formations of Lower-Middle Jurassic deposits, and secondly, to predict spatial distributions of all 19 regionally expressed producing and highly potential oil sands within the area of study.

Ilmenite Content of Different Genesis of Productive Sediments of the Trostyanytsky Deposit

The article presents information on the geological structure and ore content of the Trostyanytsky ilmenite deposit located in the northern part of the Volodarsk-Volynskyi massif of the Korostensky pluton bedrock. The deposit is localized in the weathering crust of the basement crystalline rocks and continental deluvial-alluvial and alluvial sediments of the Middle Jurassic-Lower Cretaceous, which were formed by erosion and redeposition of eluvium. Weathering crust together with deluvial-alluvial and alluvial sediments formed the productive deposits of the Trostyanytsky deposit. On the basis of the created target database containing conditional coordinates of wells, their description and testing results, mapping constructions (isohypses of the roof, sole, isopachites of the thickness of the weathering crust of crystalline rocks of the basement, alluvial sands and redeposited kaolins of the Middle Jurassic-Lower Cretaceous) were carried out; lateral distribution of the average ilmenite content in the weathering crust, in the Middle Jurassic-Lower Cretaceous fluvial deposits and in productive sediments, distribution of ilmenite content in the vertical section of wells). These mapping constructions were created based on the results of preliminary (1999) and detailed (2008) geological exploration. The direction and strength of correlations between the topography of the sole and the surface, topography of the sole and the thickness of ilmenite-bearing formations, sole relief and ilmenite content, sediment thickness and ilmenite content in the weathering crust, alluvial deposits of the Middle Jurassic-Lower Cretaceous and in productive deposits. The obtained results made it possible to clarify the structural and material parameters of the ore-bearing rocks, the lateral and vertical distribution of ore bearing in different genetic and different age's formations.

Early–middle Jurassic source to sink evolution and its tectonic significance in the northeastern Sichuan Basin

This paper discusses the sedimentary environment and source supply in the sedimentary area and their coupling relationship through a detailed description of the deposits from the Ziliujing Formation to Shaximiao Formation of the Lower–Middle Jurassic on the Tieshan section in the Dazhou City, northeastern Sichuan Province, through sedimentary characterization, determination of the paleocurrent direction, analysis of heavy minerals, and detrital zircon U–Pb dating. The results show that the Zhenzhuchong Member is sufficiently supplied with detrital sediments and is dominantly composed of fluvial-delta deposits. The Dongyuemiao Member—the first member of the Lianggaoshan Formation—is dominated by lacustrine deposits, with the detrital supply increased initially in the early Lianggaoshan. The second member of the Lianggaoshan Formation suggests a significant increase in detrital supply, with shrunken lake basin and changed paleocurrent direction. The Shaximiao Formation reveals a complete disappearance of the lake basin in the northern Sichuan Basin. The comprehensive analysis on source supply indicates that the change in source property is apparently coupled with the change in the sedimentary environment, both controlled by orogenesis around the basin. It is inferred from the zircon age distribution that the changes in the sedimentary environment and source supply during the late Ziliujing period and the middle and late Lianggaoshan period resulted from the uplifting of the Micangshan Mountain, Dabashan Mountain, and Qinling Mountain, respectively.

Duration of ammonite zones and characteristic Jurassic fossil taxa from the Pliensbachian–Aalenian successions of Bulgaria

The relative durations of ammonite biozones and selected ammonite taxa, as well as some bioevents associated with coeval Lower–Middle Jurassic benthic taxa are represented herein. Based on linked Sr-isotope data, we found a notable difference in the time spans of the zones and in the relative longevity of the guiding ammonite genera around the Pliensbachian/Toarcian (P/T) boundary. The last occurrences of the well-known late Pliensbachian bivalve taxa Gryphaea gigantea and Pseudopecten aequivalvis were dated at 184 Ma and 183.8 Ma, respectively. Recently defined Toarcian–Aalenian autochtonous brachiopod taxa (Bulgariarhynchia and Capillirhynchia brezenensis) were found to have significant taxonomic durations (2 Ma and 5 Ma). The Sr-isotope data also revealed ~2 Ma duration of seawater warming and 0.2 Ma fossil hiatus around the P/T boundary.

The Hettangian-Bajocian flora of the Shemshak group, Kerman, Iran

Numerous examples of macro plant fossils can be found in the Kerman region’s Early and Middle Jurassic successions. The Ab-e-Haji, Badamu, and Hojedk formations were among these patterns. One of the suitable regions for paleobotanical study is the Chatroud section. In this region, the Lower-Middle Jurassic is approximately 210 meters thick. 18 species of macro plant fossils from 14 genera, including Nilssonia polymorpha, Pterophyllum nathorsti, Podozamites distans, Elatocladus zamioides, Elatides thomasii, Nilssonia feriziensis, Nilssonia herriesi, and Beania sp., which are identified and described in this study, along with Pterophyllum schenki, Anomozamites marginatus, and Klukia exilis. In this region, there are two plant assemblage zones known as Anomozamites marginatus-Pterophyllum schenki Zone and Klukia exilis Zone. The Hettangian-Bajocian age is preserved at the Chatroud section.

Hydrocarbon prospectivity of pinch-out zone of Jurassic deposits in Southwestern part of South Taimyr Monocline

A comprehensive analysis has been conducted on the geological structure of the entire Jurassic interval within the southwestern part of the South Taymyr Monocline, which has allowed a more precise understanding of the structure of the Khabei gas field and the identification of prospective objects of similar geological pattern. The study utilized CDP 2D seismic data, covering a total length of 7835 kilometers, as well as results from exploratory and appraisal drilling. Based on visual analysis of temporal cross-sections, an assumption has been made regarding the correlation between reservoir development zones and paleo-channel systems of the Lower-Middle Jurassic deposits. In order to identify and locate such systems, an analysis of temporal cross-sections, maps of total Jurassic deposit thickness, and maps of dynamic attributes were conducted. As a result, areas of development of large paleo-channel systems (paleo-valleys) within the South Taymyr Monocline have been delineated through integration. These identified zones are characterized by relatively shallow burial depths of up to 2.5 km, the absence of abnormally high reservoir pressures (AHBP), and comparably high filtration-capacity properties for Jurassic reservoirs (porosity – 22%, permeability – 25–50 mD). However, considering the complexity of these objects’ structures and the uncertainties caused by the low degree of geological-geophysical exploration in the study area, it is recommended to conduct additional seismic surveys (CDP 3D) to maturate the objects for exploration drilling.

Burial history and hydrocarbon generation model of the Lower–Middle Jurassic sediments in the Tarnovo depression

This study deals with the geodynamic evolution in the Tarnovo depression, contributes to a better understanding of the burial and thermal history, and gives a model of the generation of hydrocarbons from the Lower–Middle Jurassic rocks. The implications of the complex heat flow scenario for the timing of hydrocarbon generation from the Stefanets Member of the Etropole Formation in selected sections were also analyzed. The Tarnovo depression is defined as a small tectonic unit in the central part of the South Moesian Platform margin developed mainly during the Early–Middle Jurassic. Therefore, the sediment subsidence during the Early–Middle Jurassic is largely a consequence of the tectonic processes in an extensional basin with tilted, fault-bounded, and eventually rotated blocks. There is a trend of increasing tectonic and basin subsidence in southern direction, observed in modeled well sections. The Lower–Middle Jurassic rocks enter the “oil window” in the late Barremian, Hauterivian, or even the late Valanginian towards the south, starting active generation in the middle Aptian. The model shows that gas generation is possible only in the southern and marginal zones of the Tarnovo depression. Among the sequence, the Stefanets Member of the Etropole Formation (Aalenian–early Bajocian in age), composed of shales and siltstones, possesses source rock qualities and good hydrocarbon generation potential. The research model argues for peak of the oil generation from the Stefanets Member at the end of the Barremian up to the Albian to the north and gas generation starting in the Barremian and continuing during the Cenozoic only in the southern part.

Geological and chronostratigraphic overview of the Upper Triassic and Jurassic successions of the Junggar Basin, NW China

Abstract The vast, widely exposed terrestrial (lacustrine to fluvial) Upper Triassic–Jurassic (except Tithonian) successions of the Junggar Basin not only record most of the stratigraphic boundaries of the Upper Triassic and Jurassic, including the Triassic–Jurassic boundary and the Hettangian–Sinemurian, Sinemurian–Pliensbachian, Pliensbachian–Toarcian, Lower–Middle Jurassic, Middle–Upper Jurassic and Oxfordian–Kimmeridgian boundaries, but also record a range of geological, organic, palaeogeographic and palaeoclimatic events known to have happened globally in the Late Triassic and Jurassic. The Triassic–Jurassic boundary is placed in the stratigraphic interval of the first occurrence of Retitriletes austroclavatidites and Callialasporites dampieri and the last occurrence of Lunatisporites rhaeticus . The end-Triassic mass extinction is characterized by the disappearance of most of the sporomorph and macro-plant taxa. The end-Triassic mass extinction occurred before the first occurrence of the sporomorph Cerebropollenites thiergartii , and ended after its appearance when life began to revive. The Junggar Basin was situated at a high latitude during the Late Triassic–Early Jurassic Pliensbachian ‘hothouse’ and ‘greenhouse’ periods. The Late Triassic–Mid Jurassic Bajocian was humid and warm, and rich in coal swamps, except the Toarcian, which yields little coal because it was relatively warmer and drier. It became arid from the early Late Jurassic Oxfordian.

Source-reservoir rock assemblages and hydrocarbon accumulation models in the Middle-Lower Jurassic of eastern Sichuan Basin, China

The eastern Sichuan Basin in China holds vast potential for oil and gas exploration in the Lower-Middle Jurassic strata. However, the geological characteristics and hydrocarbon accumulation patterns of this region remain largely unclear. During the deposition period of the Lower-Middle Jurassic strata, the eastern Sichuan is characterized by the formation of multiple sets of source, reservoir, and caprock assemblages through depositing lake-delta-fluvial deposits, which have great exploration potential. The Jurassic source rocks in eastern Sichuan are mainly developed in the Dongyuemiao Member, Da’anzhai Member, and Liangshan Formation. These source rocks have a total organic carbon (TOC) content greater than 1 and a varying range of organic matter maturity, with a Ro value of 0.8–2.0. The kerogen in these source rocks is primarily type II, with a smaller proportion being type III. A range of reservoir rocks can be found in the Jurassic strata of eastern Sichuan, with sandstone reservoirs being predominantly found in the Liangshan Formation, Shaximiao Formation, and Zhenzhuchong Member. Shale reservoirs are mostly present in the Dongyuemiao, Da’anzhai, Liangshan, and Maanshan Members, and there is a limited distribution of limestone reservoirs in the Da’anzhai Member and Dongyuemiao Formation. The arrangement of source rocks and reservoir rocks in eastern Sichuan has led to the formation of three types of reservoir-forming combinations, including lower generation and upper storage, self-generation and self-storage, and composite. Sandstone reservoirs are typically of lower generation and upper storage, shale reservoirs are primarily of self-generation and self-storage, and limestone reservoirs are mostly composite. The exploration of Jurassic oil and gas in eastern Sichuan should prioritize “layer and area selections.” The Da’anzhai, Dongyuemiao, and Liangshan shale reservoirs should be the primary exploration targets, with the semi-deep lake deposits in the syncline area being the most favorable. The degree of fracture development in the exploration area also has a significant impact on the shale oil and gas content. The Liangshan Formation and Shaximiao Formation sandstone reservoirs can serve as secondary exploration targets, with anticline areas that have better sealing conditions being more favorable. Limestone reservoirs have limited distribution, and exploration areas with high and steep fractures are relatively more advantageous.

Evolution of depositional environment of the Pliensbachian Stage (Lower Jurassic) coal‐bearing series in the Qaidam Basin, NW China

The Qaidam Basin is one of the major coal‐bearing basins in northeastern China with the coal seams mainly preserved in the Lower–Middle Jurassic. The Middle Jurassic sedimentary environment, sequence stratigraphy, paleoclimate, and coal accumulation in the Qaidam Basin have been extensively studied, while the sedimentary characteristics and depositional evolution of the Lower Jurassic coal‐bearing series are less well known. We systematically analysed the facies association, depositional system, basin evolution, and coal accumulation of the Early Jurassic Pliensbachian‐aged coal‐bearing series from the Dameigou section in the northern Qaidam Basin. Five lithological types have been identified, ranging from conglomerates, sandstones, siltstones, and mudstones to combustible organic rocks, which were further subdivided into 12 lithofacies. These lithofacies were grouped into four facies associations, representing the braided fluvial, meandering fluvial, lacustrine, and meandering fluvial‐lacustrine transition deposits, respectively. The Pliensbachian Stage mainly developed as a fluvial‐lacustrine depositional system and an evolution history from braided fluvial to meandering fluvial, then to lacustrine, and finally back to meandering fluvial was reconstructed. We suggest that climate change is the major controlling factor for depositional evolution during the Pliensbachian. The transition of the depositional environment from fluvial to lacustrine in the study area can be attributed to the rapid climatic change induced by volcanism at this interval.

First Discovery of the Spinicaudatan Genus Carapacestheria Shen, 1994 in Asia

The rare clam shrimp (spinicaudatan) genus Carapacestheria Shen, 1994 was originally described from the upper Lower–lower Middle Jurassic of Antarctica. Later, it was reported from the Middle Jurassic of Argentina and the lowest Cretaceous of the United Kingdom. Here, a new species, Carapacestheria cangshanensis sp. nov., is described from the Upper Jurassic Penglaizhen Formation in southwestern China. The subquadrate-shaped carapace of the new species has a distinct ornamentation pattern, which transitions from medium-sized reticulations to radial lirae. There are punctae within the polygonal cells and between the lirae. According to its geological range and geographical distribution, Carapacestheria first occurred in Antarctica, and later appeared in the northern hemisphere during the Late Jurassic.