Triassic Lower Jurassic Research Articles

Organic carbon isotope (δ13Corg) curve and extinction trends across the Triassic/Jurassic boundary at Mt. Sparagio (Italy): A tool for global correlations between peritidal and pelagic successions

The Triassic/Jurassic boundary (TJB, 201.3 Ma) is characterized by profound turnovers in both marine and terrestrial biota, known as end-Triassic mass Extinction event (ETE). During this severe event, distinct negative carbon isotope excursions (CIEs) have been globally observed, and they were linked to volcanogenic emissions or methane release by dissociation of clathrates. The triggering factor of the negative CIEs was attributed to the emplacement of the Central Atlantic Magmatic Province (CAMP) and the break-up of the Pangea. Specifically, three significant carbon-cycle disruptions named Precursor, Initial and Main CIE have been recorded in several stratigraphic successions deposited in terrestrial and pelagic environments. We investigated the organic carbon isotope curve from the subtidal facies of the Mount Sparagio section (Sicily, Italy), which is a continuous peritidal succession representing an Upper Triassic to Lower Jurassic carbonate platform edging the south-western side of the Tethys Ocean. For the first time, we achieved a complete profile of organic carbon stable isotopic composition (δ13Corg) during the end-Triassic mass Extinction event (ETE) in a carbonate shallow water environment. The δ13Corg profile highlights the 3 negative excursions that characterized the Triassic/Jurassic boundary time interval in pelagic and deep-water successions. The documented CIEs correspond to significant biotic turnovers recognized along the Mt. Sparagio section, suggesting that also the Upper Triassic-Lower Jurassic carbonate platforms were affected by the onset of the Central Atlantic Magmatic Province. Furthermore, although the Mt. Sparagio section has been studied in detail for microfacies associations and it is well biostratigraphically constrained with shallow marine macro- and microfossils, only the documented δ13Corg negative shifts allowed to correlate peritidal environments to pelagic successions, making the organic carbon curve (δ13Corg) a powerful tool for global correlations.

ABOUT THE MAIN RESULTS OF DEEP DRILLING IN THE TERRITORY OF THE PREDVERKHOYANSKAYA OIL AND GAS AREA

This article presents the results of drilling wells in the Predverkhoyanskaya oil and gas area, where deep exploratory drilling for oil and gas was started in 1954 at the Kitchan, Sangar and Ust-Vilyu structures pre-pared by the seismic survey in combination with structural core drilling.According to the results of deep drilling in the Predverkhoyanskaya oil and gas area, the section of the sedimentary cover was studied, and ideas about the material composition of the foundation upper part were obtained. The pro-spects of oil and gas potential are evaluated: the reservoir properties are possessed by rocks of the subsalt Nizhnevendsky, Vendian-Lower Cambrian terrigenous and carbonate complex-es, which are associated with industrial oil and gas potential in nearby fields.The following fluid-bearing complexes are distinguished in the sedimentary section of the Preverkhoyanskiy trough: Permian (P), Lower Triassic (T1), Triassic-Lower Jurassic (T2–3–J1), Middle Upper Jurassic (J2-3), Lower Cretaceous (K1).Deposits of the Lower Triassic aquifer complex are deposited on Permian formations. The capacity of the complex is variable, due to the structural features of the Lower Triassicsediments, and it has not been established with-in the Predverkhoyanskiy trough. Thermobaric data were not obtained during the test. Reser-voir pressures are close to hydrostatic. Reser-voir temperatures, as in the Permian complex, increase from west to south-east to the Predverkhoyanskiy trough from 50 °C to 95 °C.The deposits of the Middle-Upper Triassic – Lower Jurassic aquifer complex are more widely developed. During the sampling and test-ing of oil and gas exploration wells, water and gas inflows were obtained from the deposits of the complex, including industrial ones on Ust-Vilyuiskaya, Sobo-Khainskaya Square. The flow rates of water inflows reached 300 m3/day (Sangarskaya 2), gas inflows up to 1800000 m3/day (Ust-Vilyuiskaya area). In the Sobo-Khainskaya 1 well, an industrial gas in-flow was obtained from the interval of 1328–1349 m.The prospects for oil and gas exploration at the Preverkhoyanskaya oil and gas area are associated with its northern part (Setas struc-ture), on which the Setas parametric well No. 1 with a design depth of 4000 m is being laid, and its southern part, where it is advisable to drill 2 parametric wells with a planned depth of 3000 m.

Using the tectophase conceptual model to assess late Triassic–Early Jurassic far-field tectonism across the South-central Barents Sea shelf

The Upper Triassic–Lower Jurassic succession of the Barents Sea Shelf (BSS) represents one of Europe’s most prolific and strategic petroleum systems. This succession reflects various depositional environments and tectonostratigraphic events. Even though these strata are considered largely well-understood, connections with far-field stresses triggered by regional tectonics remain a subject of investigation. This study presents new interpretations that focus on relationships between the stratigraphic succession across the south-central BSS and Triassic–Jurassic Novaya Zemlya compressional tectonics. By applying the “tectophase model,” developed in the Appalachian Basin, to analyze this succession, the presence of foreland-basin depozones and associated far-field processes related to compressional tectonics in an adjacent orogen are suggested. This model addresses unconformity development, lithostratigraphic succession, and reactivation of structures. Use of this model suggests far-field tectonostratigraphic responses during two episodes of Novaya Zemlya tectonism, reflected in the coeval BSS stratigraphy. Overall, this tectonostratigraphic study aligns with other research suggesting a Late Triassic inception for Novaya Zemlya compressional tectonism, which influenced larger parts of the BSS through extensive clastic sedimentation, far-field structural reactivation, and flexural responses to deformational loading triggered by tectonics.

Clam shrimp Nothocarapacestheria from the Lower Jurassic of the Junggar Basin, northwestern China

Abstract Nothocarapacestheria Rohn et al. , 2014 was erected based on material from the Upper Triassic–Lower Jurassic of the Paraná Basin, Brazil. Here we describe a Nothocarapacestheria species that has been discovered in the Lower Jurassic Badaowan Formation in Xinjiang, northwestern China. The described species is characterized by: (1) almost the whole carapace being ornamented with medium reticulations, 3–6 polygons per growth band, 20–60 µm mesh diameter; (2) the gradual transition to radially elongated reticulations in the distal region of the carapace, sometimes ending as radial lirae; and (3) fine puncta in the mesh of reticulations. Through this study, we are able to infer that the distribution of Nothocarapacestheria was more widespread than previously thought, and that Euestheria sp. previously reported from the Lower Jurassic Badaowan Formation are probably poorly preserved or juvenile individuals of Nothocarapacestheria .

Characteristics and discovery significance of the Upper Triassic–Lower Jurassic marine shale oil in Qiangtang Basin, China

Mesozoic marine shale oil was found in the Qiangtang Basin by a large number of hydrocarbon geological surveys and shallow drilling sampling. Based on systematic observation and experimental analysis of outcrop and core samples, the deposition and development conditions and characteristics of marine shale are revealed, the geochemical and reservoir characteristics of marine shale are evaluated, and the layers of marine shale oil in the Mesozoic are determined. The following geological understandings are obtained. First, there are two sets of marine organic-rich shales, the Lower Jurassic Quse Formation and the Upper Triassic Bagong Formation, in the Qiangtang Basin. They are mainly composed of laminated shale with massive mudstone. The laminated organic-rich shale of the Quse Formation is located in the lower part of the stratum, with a thickness of 50–75 m, and mainly distributed in southern Qiangtang Basin and the central-west of northern Qiangtang Basin. The laminated organic-rich shale of the Bagong Formation is located in the middle of the stratum, with a thickness of 250–350 m, and distributed in both northern and southern Qiangtang Basin. Second, the two sets of laminated organic-rich shales develop foliation, and various types of micropores and microfractures. The average content of brittle minerals is 70%, implying a high fracturability. The average porosity is 5.89%, indicating good reservoir physical properties to the level of moderate–good shale oil reservoirs. Third, the organic-rich shale of the Quse Formation contains organic matters of types II1 and II2, with the average TOC of 8.34%, the average content of chloroform bitumen ‘A’ of 0.66%, the average residual hydrocarbon generation potential (S1+S2) of 29.93 mg/g, and the Ro value of 0.9%–1.3%, meeting the standard of high-quality source rock. The organic-rich shale of the Bagong Formation contains mixed organic matters, with the TOC of 0.65%–3.10% and the Ro value of 1.17%–1.59%, meeting the standard of moderate source rock. Fourth, four shallow wells (depth of 50–250 m) with oil shows have been found in the organic shales at 50–90 m in the lower part of the Bagong Formation and 30–75 m in the middle part of the Quse Formation. The crude oil contains a high content of saturated hydrocarbon. Analysis and testing of outcrop and shallow well samples confirm the presence of marine shale oil in the Bagong Formation and the Quse Formation. Good shale oil intervals in the Bagong Formation are observed in layers 18–20 in the lower part of the section, where the shales with (S0+S1) higher than 1 mg/g are 206.7 m thick, with the maximum and average (S0+S1) of 1.92 mg/g and 1.81 mg/g, respectively. Good shale oil intervals in the Quse Formation are found in layers 4–8 in the lower part of the section, where the shales with (S0+S1) higher than 1 mg/g are 58.8 m thick, with the maximum and average (S0+S1) of 6.46 mg/g and 2.23 mg/g, respectively.

Palynology of the Triassic–Jurassic transition of the Danish Basin (Denmark): a palynostratigraphic zonation of the Gassum–lower Fjerritslev formations

The Upper Triassic–Lower Jurassic succession in the Danish Basin is penetrated by many deep wells that were drilled during former hydrocarbon exploration campaigns, but it is today targeted for geothermal energy and storage of CO2. The Stenlille salt dome on Sjælland sandstones of the Gassum Formation, sealed by the overlying Fjerritslev Formation mudstones, has been used for decades as a seasonal storage for natural gas. With its comprehensive dataset of seismics, geophysical well logs and conventional core data from 20 wells, the Stenlille succession serves as a model for other salt domes currently being evaluated as potential CO2 storage sites in the basin. Over the last decade the cored Triassic–Jurassic boundary succession has contributed to the understanding of environmental and palynological events during the end-Triassic mass extinction. Core, sidewall core and cutting samples from several of the closely situated Stenlille wells are here used to establish a high-resolution palynostratigraphic zonation scheme covering the entire Rhaetian to Sinemurian succession by integrating new analyses with previously published data. The palynological dataset has allowed the recognition of nine formally described spore-pollen zones, of which eight are new, while two previously described dinoflagellate cyst zones are subdivided into three informal subzones each. The palynological zonation is integrated with a sequence stratigraphic framework and will form the basis for the dating of future well sections in the Danish Basin and other basins and for correlation to outcrops. The large palynological dataset further shows that the vegetation around the Danish Basin was remarkably stable during the early to middle Rhaetian, but that events related to the emplacement of the Central Atlantic Magmatic Province accelerated ecosystem changes for c. 175 ky in the late Rhaetian and earliest Hettangian, including ∼25 ky of successional recovery before the terrestrial ecosystem had again stabilised.

Facies character and evolution of a mixed carbonate–siliciclastic shelf: Upper Triassic–Lower Jurassic succession in the eastern Northern Calcareous Alps (Stumpfmauer, Austria)

Western Tethys sedimentary successions constitute fundamental archives of Late Triassic–Early Jurassic environmental, carbonate production and tectonic changes. During the Late Triassic, the Northern Calcareous Alps (Austria) belonged to the Western Tethys passive margin, characterised by the deposition of the early-dolomitized peritidal Hauptdolomit (Norian) adjacent basinward to the Dachstein carbonate shelf and passing upward to the mixed carbonate–siliciclastic Kössen Formation (Upper Norian–Rhaetian). The Kössen Fm. was subdivided into the lower shallow-water Hochalm Member and the upper Eiberg Member, accumulated in an intraplatform basin coeval to shallow-water carbonates (Upper Rhaetian Limestone). The Eiberg Mb. and overlying Jurassic strata were extensively studied as a continuous marine record across the Triassic/Jurassic boundary. In contrast, shallow-water successions, time-equivalent to the Eiberg Mb. and Upper Rhaetian Limestone, located North of the Eiberg Basin, are poorly investigated. This study focusses on the approximately 350 m thick Norian to Lower Jurassic succession cropping out in the eastern Northern Calcareous Alps (Stumpfmauer). The 32 distinguished lithofacies compose seven, vertically superimposed, sedimentary units (A–G), belonging to the Hochalm Mb. of the Kössen Fm. (Unit A peritidal cyclothems, Unit B claystone/marlstone with fossiliferous beds, Unit C coral boundstone to floatstone), Upper Rhaetian Limestone (Unit D subtidal cyclothems with claystone), shallow-water carbonate strata transitional to Lower Jurassic (Unit E ooidal coated grain peloidal grainstone with basal transgressive lag, Unit F bivalve-rich, microbialite and oncoidal lithofacies, previously attributed to the Upper Rhaetian Limestone) and Hettangian–Sinemurian Kalksburg Fm. (Unit G cross-laminated coated grain peloidal grainstone with quartz and chert). The detailed lithofacies characterisation presented in this study contributes to the knowledge on the Northern Calcareous Alps stratigraphy and depositional environments across the Triassic/Jurassic boundary. The identified sedimentary units can be framed in the evolution of Western Tethys and share similarities with depositional systems from the Western Carpathians, Transdanubian Range, Southern Alps and Dinarides suggesting coherent sedimentary response and environmental, climate and tectonic controls in different palaeogeographic domains.

Integrating 4D Data Into Dynamic Model Improves Johan Sverdrup Field Development

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 209541, “Integration of Johan Sverdrup PRM 4D Data Into the Dynamic Model,” by Monica A. Calvert, Yuri Tsaplin, and Hans P. Hansen, TotalEnergies. The paper has not been peer reviewed. _ The Johan Sverdrup field permanent reservoir monitoring (PRM) system now has recorded two surveys documenting the progression of the waterflood since production and injection began in October 2019, revealing a clear 4D signal linked to the increase in water saturation. The 4D signals have been used to help guide updates within the reservoir model, providing a better understanding of how the field development is progressing. Field Background and PRM System The giant Johan Sverdrup oil field was discovered in 2010 in the North Sea approximately 150 km west of Stavanger. The field is laterally extensive, covering 200 km2 and consisting primarily of Upper Jurassic intra-Draupne sandstone and Upper Triassic-Lower Jurassic Statfjord sandstones in a low relief structure. The reservoir is characterized by excellent porosity and permeability, with drillstem tests showing permeabilities of up to 70 darcies with good communication over long distances and across faults. The oil is undersaturated with a low gas/oil ratio and is normally pressured, with slight depletion from a regional drawdown. Reservoir thickness varies across the field, with the intra-Draupne sandstone ranging from 5 to 40 m. The primary drainage strategy includes initial waterflooding to be supplemented by water-alternating-gas injection at the end of the oil-production plateau. The full field development consists of 64 wells, both platform and subsea. Producers are placed on the structural highs with water injectors placed on the flanks below the initial oil/water contact (OWC). Because of full voidage replacement by water injection, reservoir pressure is expected to stay fairly constant during field life. Johan Sverdrup is the first oil field to have a PRM system installed before production startup. The PRM system is a key component of the reservoir-monitoring and data-acquisition plan and was installed in two stages in 2019 and 2020 (Fig. 1). Yearly monitor surveys are planned to monitor waterflood development. The first seismic acquisition in 2019, referred to as PRM0, covered the initial first stage of installation on the northern part of the field. Stage two of the installation was completed in 2020, and two monitor surveys have since been acquired, covering the entire field, in 2020 (PRM1) and 2021 (PRM2). Feasibility studies show that a 4D response would be detectable within the reservoir. The increase in water saturation caused by water injection and upward movement of the OWC yields an increase in acoustic impedance, or hardening, in the reservoir. For the 4D analysis, the main seismic surfaces used were the Top Aasgard formation, the Top Viking group (which contains the Draupne formation), and the Intra Upper Triassic Unconformity (IUTU). The acoustic impedance increase measured in the 4D seismic data is compared with water-saturation changes within the reservoir model. Differences between the two can highlight areas within the reservoir model that require updating.

Tectonic Evolution of the Southern Dabie Orogenic Belt, China: Insights from Peak P–T Conditions and U–Pb Zircon Dating of the Susong Metamorphic Complex

The Susong metamorphic complex (SSC) in the southern margin of the Dabie orogenic belt (DOB) in central-eastern China is a key metamorphic unit for understanding subduction and exhumation processes in the DOB. However, the formation age and metamorphic grade of the SSC remain uncertain, hampering our understanding of the mechanism of the formation of the DOB. An integrated study of field survey, regional metamorphic petrology, geothermobarometry, and U–Pb dating of zircon was carried out in this study. Our results reveal that the SSC was metamorphosed under epidote amphibolite- to amphibolite-facies conditions with average metamorphic P–T values of 0.98 ± 0.07 GPa and 531 ± 35 °C. The smooth spatial variation in peak P–T conditions and an apparent geothermal gradient of ~17 °C/km indicate that the SSC as a whole fall into Barrovian-type metamorphic environments. Zircon U–Pb dating for garnet–mica schists of sample ZT003, ZT005 and ZT006 yield five (Groups I to V), six (Groups I to VI) and five (Groups I to V) age groups, respectively, concentrating on the Meso-Neoarchean, early-middle Paleoproterozoic, middle Mesoproterozoic, early Neoproterozoic, Palaeozoic and Triassic-lower Jurassic. Therein, a 259–190 Ma (Group V) from zircons with Th/U ratios of <0.1 in sample ZT006 record the timing of both peak and retrograde metamorphism for the SSC. All other ages are detrital zircon ages, and from age provenances in the DOB or the Yangtze Block (YZB), indicating the YZB affinity of the SSC. The two youngest age populations of 427–415 Ma (Group VI) and 475–418 Ma (Group V) from samples ZT005 and ZT006, respectively, suggest that the formation age of the SSC could be Middle Devonian. The similarity of formation age and peak P-T conditions of the SSC to Foziling Group, located in the northernmost DOB, implies that both units formed the sedimentary cover on the passive continental margin of the YZB during the late Palaeozoic, and subducted into the middle-lower crust of 20–40 km depth as a whole, corresponding to the shallow subduction. Compared to the deep subduction defined by high-pressure (HP) and ultrahigh-pressure (UHP) units, larger differences in peak P–T conditions, age and geothermal gradient between two different tectonic environments happen. Accordingly, it is speculated that a transitional subduction from shallow to deep levels occurred at Moho depths during the Early Triassic, and is due to a change in subduction dip angle.

Maturity Matters in Provenance Analysis: Mineralogical Differences Explained by Sediment Transport from Fennoscandian and Variscan Sources

The significance of mineralogical maturity as a provenance indicator has long been debated and we use this study to demonstrate that it can indeed be a powerful tool to track the distribution of sandstone reservoirs. We investigate the cause of the pronounced geographic and stratigraphic differences in mineralogical composition that are found in the Upper Triassic–Lower Jurassic Gassum Formation across the Norwegian–Danish Basin and surrounding areas. Zircon U-Pb dating of 46 sandstone samples including analysis of 4816 detrital grains are combined with quantifications of the detrital mineralogical composition and placed in a sequence stratigraphic framework. The results show that the Gassum Formation can be divided into a southeastern region with high mineralogical maturity and a less mature region to the northwest with more feldspars, rock fragments, micas, and heavy minerals. Both the mineralogical assemblage and the provenance signature have been thoroughly homogenized in the SE region where sediment supplies from the Fennoscandian Shield and the Variscan Orogen are evident. In the NW region, sediment was initially supplied from Fennoscandia only, but the provenance abruptly changed from the Telemarkia Terrane to comprising also the more distant Caledonian Orogen resulting in a different mineralogical assemblage. The change occurred during a basinwide regression and may be caused by tectonic movements in the hinterland that permanently changed the composition of the sediment supplied to the basin.

Fault-bounded models of oil–Gas and gas–Hydrate accumulation in the Chaoshan Depression, the South China Sea

The Dongsha Basin is a large Mesozoic basin extended from the northern South China Sea (SCS) to onshore South China. Though long-term uplift and denudation occurred since the end of the Mesozoic, still thick Mesozoic strata (up to 5,000 m) relict over the Dongsha waters where lies the largest depression, Chaoshan Depression, covering an area of 3.7 × 104 km2. It was confirmed by a drilling hole, the well LF35-1-1, that high organic carbon-bearing marine Jurassic layers are present in the depression. However, due to the complexity of the superposed Mesozoic and Cenozoic tectonism and poor imaging quality in previous surveys, the petroleum geology remains poorly understood in view of the deep basin structure, the Mesozoic hydrocarbon migration conditions, and the oil–gas accumulation mechanism. In recent surveys, the seismic imaging quality has been significantly improved by employing long and quasi-3D seismic streamer techniques. Correlating with the regional geology onshore the South China, drilling data of the Well LF35-1-1, and well-tying seismic profiles, it is found that two sets of source rocks are developed in the semi-closed gulf during the Upper Triassic-Lower Jurassic and the Upper Jurassic. Their effective thicknesses are estimated as 495 m and 600 m, respectively, being hopeful with high hydrocarbon generation potentials. During the Dongsha Movement that occurred in the late Cenozoic, deep faults have been extensively activated to disturb the overlying sequences, even in some places breaking through the seafloor. A potential trap structure, DS-A, is found in an intra-sag bulge which is bounded by antithetic and synthetic faults. The oil and gas generated in the neighboring sags can migrate along the faults into reservoir layers at higher levels. The antithetic faults also play the role of seal for oil and gas from the hanging wall. Apparent flat bright spots appearing within the DS-A trap indicate likely entrapment of layered petroleum. The synthetic faults on the opposite side of the DS-A structure, although fails to seal oil–gas reservoirs, provide plumbing channels for oil and gas to leak to the shallow layers above which a few pockmarks and mud volcanoes are visible. As the water depth of the continental slope there ranges from 300 m to 2000 m, it is likely for the leaked gas to form natural gas hydrates. A close cogenetic interrelation exists between the natural gas hydrates at the seafloor and oil–gas reservoirs in the deep.

Provenance of the Upper Triassic–Lower Jurassic Flysch and the Middle–Upper Jurassic Coarse Clastic Sequences in the Cimmerides of the Crimean Mountains Based on the Results of U–Th–Pb Isotopic Dating of Detrital Zircon Grains

Provenance of the Upper Triassic–Lower Jurassic Flysch and the Middle–Upper Jurassic Coarse Clastic Sequences in the Cimmerides of the Crimean Mountains Based on the Results of U–Th–Pb Isotopic Dating of Detrital Zircon Grains

Interpretation of 3rd order depositional sequences in the Mesozoic of the Prebetic and the Miocene of the Gulf of Valencia: an approximation to its sedimentary architecture

In the 1980s and 1990s the interpretation of the depositional sequences and the system tracts that they are made up of was one of the most evident advances that could be applied to the geological interpretation of the platform, slope and basins. Since then, there have been no notable conceptual changes, which is why studies from the 1990s can be considered again as long as they were conceived according to the basic rules of interpretation: the existence of three sea level episodes, low (LST and SMW), transgressive (TST) and high (HST) which differentiate the areas of erosion and deposit from the sedimentary inputs to be distributed by the marine environment which receives them. The Prebetic and the Gulf of Valencia cross section that can be constructed by the interpretation of sedimentary columns provided by some deep wells are the object of this paper and allow us to differentiate a minimum of 51 or 52 depositional sequences of 2nd and 3rd order that make it possible to draw the sedimentary architecture and characterize the three stages that formed its apparent complexity: the rift phase (Triassic-Lower Jurassic, with 5 or 6 depositional sequences), the passive margin phase (Middle Jurassic-Lower Miocene, with 36 depositional sequences) and the molasses phase (Miocene, with 9 depositional sequences). Such an interpretation cannot be made for the Paleogene and Neogene of the Prebetic due to the absence of wells that could to test this characterization.

Uncharted Permian to Jurassic continental deposits in the far north of Victoria Land, East Antarctica

The remote lower reaches of the Rennick Glacier in the far north of Victoria Land hold some of the least-explored outcrop areas of the Transantarctic basin system. Following recent international field-work efforts in the Helliwell Hills, we here provide a comprehensive emendation to the regional stratigraphy. Results of geological and palaeontological reconnaissance and of petrographic, geochemical and palynostratigraphic analyses reveal a stack of three previously unknown sedimentary units in the study area: the Lower Triassic Van der Hoeven Formation (new unit, 115+ m thick) consists mainly of quartzose sandstone and non-carbonaceous mudstone rich in continental trace fossils. The Middle to Upper Triassic Helliwell Formation (new unit, 235 m thick) consists of coal-bearing overbank deposits and volcaniclastic sandstone and yielded typical plant fossils of the Gondwanan Dicroidium flora together with plant-bearing silicified peat. The succession is capped by c . 14 m of the sandstone-dominated Section Peak Formation (uppermost Triassic–Lower Jurassic). Our results enable more detailed correlation of the Palaeozoic–Mesozoic successions throughout East Antarctica and into Tasmania. Of particular interest is one section that spans the end-Permian mass extinction interval, which promises to allow detailed reconstructions of high-latitude vegetation dynamics across this critical interval in Earth history. Supplementary material: A Supplementary Data File containing supplementary information, figures S1–S7, and additional references is available at https://doi.org/10.6084/m9.figshare.c.5118431

Clay mineral signatures of fault-related fluid flows in a sandstone reservoir: A case study from the Teloua Formation, Tim Mersoї Basin, Niger

During the evolution of sedimentary basins, clay minerals may record changes in their mineralogy, chemistry, and texture as a response to diagenesis, fluid flows, and tectonic activities. Therefore, studying clay minerals in sedimentary rocks provides important information that may assist to decipher the basin history. The focus of the present study is the Teloua Sandstone Formation (Fm) which belongs to the Tim Mersoï Basin located in the NE of Niger republic (West Africa). The formation is cut by the major Arlit fault which is the largest tectonic structure impacting the Tim Mersoï Basin. The aim of the present work is to use the texture, mineralogy, and chemistry of clay minerals in the Teloua Fm as geochemical markers for burial diagenesis history and fluids flows (i.e., attributed mainly to the Arlit fault). Therefore, three drill holes (DH1, DH2, and DH3) were investigated as a function of their distance from the fault and samples have been collected all along the Teloua Fm (Upper Triassic-Lower Jurassic), at the unconformity surface with the underlaying Moradi Fm (Upper Permian), and at the top of the Moradi Formation.Petrographic observations demonstrate that the sediments of the Teloua Fm have undergone transformations induced by burial diagenesis, brittle deformation, and multiple stages of fluid flows. These transformations are marked by grain compaction, fragmentation, dissolution of detrital minerals, recrystallization of authigenic clay minerals, and modifications in the primary porosity. XRD and SEM results indicate no significant variation in the detrital mineralogy of the three drill holes; Teloua Fm samples are composed of quartz, feldspars (microcline and albite), micas (biotite and muscovite), minor heavy minerals such as hematite, anatase, phosphates (apatite and monazite), and zircon. The authigenic clay minerals observed in the three drill holes are rosette-like tosudite (i.e., a regular interstratified chlorite/smectite), saponite, montmorillonite, and kaolinite/smectite mixed layer. Regarding their vertical distribution, tosudite occurs throughout the Teloua Fm, whereas montmorillonite is mainly present at the top of the formation, and saponite is only observed at the unconformity between Teloua and Moradi formations. The Mg content of tosudite tends to decrease with increasing vertical distance from the basal unconformity and the lateral distance from the Arlit fault. This is interpreted as an evidence of Mg-rich fluid circulations related to the Arlit fault activity that penetrated the formation through the unconformity surface and fractures. This is also evidenced by the enrichment of dolomite at the unconformity when moving closer to the Arlit fault. In addition, the occurrence of both authigenic saponite (at the unconformity) and montmorillonite (upper part of the Teloua Fm) that post-dated tosudite is attributed to another stage of Mg-rich fluid circulation during late diagenesis. Furthermore, late downward circulation of meteoric fluids of low pH are evidenced by the partial alteration and replacement of montmorillonite by kaolinite/smectite mixed layer in the Teloua Fm (at shallow depth of the DH3). These results demonstrate the validity of clay minerals as reliable tools to provide information about fluid/rock interaction in complex geological and tectonic setting.

Late Mesozoic tectonic evolution of the central Bangong–Nujiang Suture Zone, central Tibetan Plateau

ABSTRACT The Bangong–Nujiang Suture Zone (BNSZ), located in the central Tibetan Plateau, experienced a Late Mesozoic transition from oceanic closure to continent–continent collision. Based on sandstone-composition analyses, U-Pb detrital-zircon dating and structural observations, we determined that this transition occurred coeval with the accretion of the Lhasa Terrane to Eurasia. According to our newly observed stratigraphic and geochronological data, we conclude the following: (1) the Upper Triassic-Lower Jurassic Muggar Gangri Group was mainly derived from the Qiangtang Terrane; (2) the Shamuluo Formation composed of Upper Jurassic-Lower Cretaceous shallow marine strata was sourced from both the Qiangtang Terrane and recycled Muggar Gangri Group; (3) the Lower Cretaceous (~125–110 Ma) Duoni Formation shows both Lhasa and Qiangtang detrital zircon fingerprints, indicating closure of the Bangong–Nujiang Ocean at ~120 Ma; and (4) the Upper Cretaceous Abushan Formation is a suite of intramontane molasse deposited after the Qiangtang–Lhasa collision. Therefore, these sedimentary strata record the transition from oceanic closure to continent–continent collision from 125 to 90 Ma within the central BNSZ. Moreover, based on relationships among strata distributions and thrust locations, we propose that thrust movements propagated southward during this transition and controlled sedimentary basin deposition. The Late Mesozoic tectonic transition within the BNSZ occurred well before the India-Asia collision and hence profoundly influenced the Cenozoic formation of the Tibetan Plateau.

The Alamos Metamorphic Complex, evidence of late Paleozoic collision between Laurentia and Gondwanan blocks in northwestern Mexico

The Alamos Complex is a metamorphosed volcano-sedimentary sequence exposed south of the late Paleozoic Ouachita–Marathon–Sonora orogen in northwestern Mexico, which in turn, is regarded as a continuation of the Alleghenian–Variscan mountain chain. Mineral assemblages in metavolcanic rocks indicate greenschist facies metamorphism. Thermobarometric studies yield an average metamorphic temperature of 380 °C, whereas andalusite in metapelites indicates pressure less than 0.4 ± 0.05 GPa. The main deformation phase generated pervasive foliation, which in turn was isoclinal to close folded and a second axial planar cleavage was developed. Metavolcanic rocks of the Alamos Complex can be separated in two suites: (1) an anorogenic suite, similar to enriched middle ocean basalts (E-MORB) in the Tesia region, and (2) an orogenic suite, characteristic of volcanic-arc setting in the Mocuzari region. U–Pb detrital zircon geochronology of one sample from Tesia yielded a maximum depositional age averaging 395 ± 28 Ma. One sample from the Mocuzari region yielded a maximum depositional age averaging 1104 ± 24 Ma, with the largest peak at 1428. A clastic sequence correlated with the Middle Triassic–Lower Jurassic Barranca Group, nonconformably cover the Alamos Complex. The basal conglomerate of that group yields a maximum depositional age averaging 372 ± 23 Ma. Accordingly, the Alamos Complex must be in part deposited after the early Devonian time and before the Middle Triassic. The main sources of detritus for the complex are the Meso- and Paleoproterozoic rocks of southwestern Laurentia, whereas the main source of detritus for the Barranca Group is the Rio Fuerte Formation exposed ~ 100 km south of the study area. The metamorphic event is ascribed to the late Paleozoic collision of Gondwanan blocks against the southern margin of Laurentia.

From orogenic collapse to rifting: A case study of the northern Porcupine Basin, offshore Ireland

Offshore Ireland, the North Atlantic opening is generally interpreted as successive, 10-to-15-Myr-long, rifting events during the Mesozoic. However, their interaction is poorly documented in terms of structural inheritance and fault reactivation. From extensive seismic and well data in the northern Porcupine Basin, we show that extension actually evolved over an abnormal period of ca. 220 Myrs with overlapping tectonic stages that, all together, reflect the Irish Atlantic Margin construction from the Palaeozoic.During the Carboniferous, extension initiated throughout the Variscan and Caledonian thickened crusts and controlled the deposition of generally slightly-tilted, continental clastics. Two subsequent rifting events, during the Triassic-Lower Jurassic and Upper-Jurassic-Lower Cretaceous, controlled shallow-to-deep marine sedimentation in separated sub-basins that progressively widen and connect once extension localised along bounding faults.We propose that extensional tectonics, first controlled by a general orogenic collapse reflecting an early stage of stretching, is then followed by several rifting periods that ultimately evolved toward the hyper-thinning of the margin and shaped an aborted rift propagator. This evolution is due to pre-existing orogenic-related structures and boundary conditions variations. Such a continuum of deformation, although pulsed, implies varying structural interactions of crustal scale that are often recorded in syn-tectonic sediments.

Geometry and Cenozoic evolution of the Crimean fold-thrust belt from cross-section balancing and kinematic forward modeling

The Crimean fold-thrust belt comprises the onshore Crimean-dome fold-thrust belt and the offshore Sorokin accretionary wedge, a sub-surface imbricate stack with high oil and gas potential. We combine geomorphological and balanced cross-section analyses with published low-temperature thermo¬chronology and offshore seismic data to constrain both its present geometry and Cenozoic structural evolution. We interpret the Crimean dome as a map-scale, ramp-related antiform above the Main Crimean thrust, the basal detachment to the onshore part of the fold-thrust belt. The Main Crimean thrust separates the continental Scythian plate from the transitional to oceanic Eastern Black-Sea basin and likely reactivates an Upper Triassic–Lower Jurassic passive continental margin structure. The Crimean fold-thrust belt has accommodated ~24 km shortening since the Eocene, with ~12 km contraction each in the thick-skinned Crimean dome and the thin-skinned Sorokin accretionary wedge. The intermediate geometries in the kinematic evolution traced by the kinematic forward model are testable by future hydrocarbon exploration and thermochronologic studies.

Predictive modelling of fossil-bearing locality distributions in the Elliot Formation (Upper Triassic–Lower Jurassic), South Africa, using a combined multivariate and spatial statistical analyses of present-day environmental data

Traditional methods of palaeontogical fieldwork often involve costly and inefficient searching for suitable fossil-bearing sites. The advent of digital spatial data offers an opportunity to improve fieldwork search efficiency. In this paper, we develop a model that identifies potential fossil localities at the reconnaissance stage of prospecting using a criterion of present-day environmental suitability for fossil discovery. This model employs techniques from analytical biology, Geographical Information Systems (GIS), and remote sensing. The model is designed to be flexible with no limit on the number of variables and can use a variety of data types (both continuous and categorical) by means of a principle coordinate analysis (PCO). We tested the model on outcrops of the Elliot Formation (Upper Triassic–Lower Jurassic) in the Free State, South Africa. Our model correctly predicted 95% of the known occurrences and reduced the potential prospective area from approximately 2300km2 of mapped total outcrop to 597km2 of prospectable outcrop. Initial field testing in 2015 identified two new sites based on model output.