Jurassic Carbonate Research Articles

Detailed Investigation of Dolomites in the Middle Jurassic Samana Suk Formation, Kahi Section, Nizampur Basin, North-Western Himalayas (Pakistan)

Abstract —Diagenetically modified carbonate rocks are more common in the rock record. Among these modifications, multiphase dolomitization is the most common process. The Middle Jurassic carbonates of the Samana Suk Formation are extensively altered by the dolomitization process in the Kahi section, Nizampur Basin. The primary objective of this study is to investigate this multiphase dolomitization and to elucidate its possible mechanism. Field investigation shows both host limestone (oolitic, fossiliferous, and massive) and dolomites. Dolomite bodies are of both beddings: parallel to and crosscutting the bedding planes. Different types of dolomites were recognized on the basis of color contrast as dark gray replacive dolomite, light gray dolomite, brownish dolomite, and yellowish dolomite. Along with the replacive phase, void- and fracture-filling cementing saddle dolomite and cementing calcites are recognized in the field. Petrographic studies show the complex diagenetic history of the Samana Suk Formation from near-surface diagenesis, including micritization, neomorphism, and several varieties of dolomites. These varieties are as follows: RD1 is very fine- to fine-grained dolomite; RD2 is medium- to coarse-grained and anhedral to subhedral dolomite; RD3 is coarse- to very coarse-grained and planar euhedral zoned dolomite; and RD4 is coarse-grained euhedral to subhedral ferroan dolomite. In addition, cementing saddle dolomite SD consists of large crystals with curved faces showing sweeping extinction. Cementing calcite phases are as follows: CC1 is granular mosaic; CC2 is twin; CC3 is fracture-filling; and CC4 is ferroan calcite. The stable isotope values of limestone (δ18O is –7.13 to –0.73‰ V-PDB, and δ13C is –0.05 to 1.32‰ V-PDB) show depletion with respect to the Jurassic marine signature. The values of multiphase dolomites RD1–RD4 and SD (δ18O is –8.65 to –3.16‰, and δ13C is –3.56 to 2.09‰) indicate multiphase dolomitization. The CC1–CC3 values (δ18O is –11.07 to –8.97‰, and δ13C is –2.14 to 0.76‰) indicate highly depleted values of δ18O, showing hydrothermal origin. From field, petrography, and geochemistry data, it is deduced that a possible source of Mg for hydrothermal dolomites is activation of faults and fractures during active tectonic regime in the area and might be related to activation and reactivation of the Kahi Thrust system.

Tracking Depositional Architecture and Diagenetic Evolution in the Jurassic Carbonates, Trans Indus Ranges, NW Himalayas

The evolution of Jurassic carbonates is globally significant for understanding the depositional framework, diagenetic phases and sedimentary characteristics of shallow marine shelf deposits. For this purpose, two outcrop sections of the Jurassic carbonates with a road distance of 121 km in the Trans Indus Ranges, NW Himalayas, were included in this study. Geological fieldwork was conducted for sedimentological data, and representative samples were collected for microfacies analysis and diagenetic evolution complemented by carbon and oxygen isotope analysis. Results show that eight microfacies were identified in both sections where mudstone microfacies was only present in the Chichali section, whereas wackestone and packstone facies widely existed in both sections. The diagenetic evolution interpreted that dolomitization and stylolization were pronounced in the Paniala section, while micritization and calcite cementation were prevalent in the Chichali section. The interpreted depositional setting implies the wide range from supratidal to outer ramp shallow marine for the Chichali section, suggesting a wide range and relatively deeper environment, alongside merely intertidal to middle ramp settings for Paniala section. Diagenetic evolution suggests marine to meteoric influence in the Chichali section, while burial and uplift phases were dominant in the Paniala section. The diagenetic events were also validated by the isotopic analysis, where most of the samples with values up to −4‰ VPDB δ18O, corresponding to a carbon isotope range of up to +4‰, were interpreted as the burial phase of diagenesis; meanwhile, a few samples with −2 δ13C and −7‰ VPDB δ18O isotope signatures were marked as meteoric influx in the Paniala section. This study indicates the diversity of the depositional environment and diagenetic heterogeneity by integration of thin sections using isotope data, which are quite applicable to shallow marine carbonates.

Aborted oceanic basin during the Mesozoic rifting of the South-Iberian Palaeomargin and interactions with the sedimentation processes

ABSTRACT During the Mesozoic, the South-Iberian Palaeomargin experienced successive phases of extensional tectonics related to the fragmentation of Pangea and the aperture of the Central Atlantic. During the Pliensbachian, the rifting favoured the fracturing and collapse of the Early Jurassic carbonate platform and delimited a subdomain characterized by high subsidence called Median Subbetic (Betic Cordillera, S Spain). The progression of the rifting and the related extension along ~ 102 Ma, from late Pliensbachian to Santonian, favoured the activity of lithospheric scale faulting and extensional basic magmatism in the Median Subbetic with alkaline affinity. The highest volume of lava emissions, represented by pillow-lava flows interbedded with hemipelagic sediments, occurred during the Middle Jurassic. The pillow-lavas show aphanitic texture with Ca-rich plagioclase and clinopyroxene, whereas subvolcanic rocks are aphanitic with high content of plagioclase to phaneritic with plagioclase, olivine and biotite. The rocks present low SiO2 (46.6–53.5 wt.%) and relatively high K2O (locally reaching 8.7 wt.%), and the content of trace elements generally ranges between those from E-MORB and OIB composition. Values of some ratios such as (Gd/Yb)N, (La/Yb)N, (La/Sm)N, and the Nb content increase in the Upper Cretaceous pillow-lava flows, whereas Yb and Y contents decrease, that point to more alkaline affinity. Low Yb and Y contents point to the presence of garnet as residual phase in the mantle source. We interpret that during the early Late Cretaceous, hyperextensional phase of the continental rifting throughout an aborted oceanic basin occurred and allowed lithospheric deeper asthenospheric mantle up-lift and the partial melting. The phases of extensional tectonic controlled the configuration of this palaeomargin in complex tectono-palaeogeographic subdomains, the sedimentary cycles, and the volcanic activity during the Jurassic and Cretaceous. Finally, the transition from passive margin to a convergent one began during the Campanian and no more volcanism is recorded in the Median Subbetic.

Hydrologically sensitive carbonates: Tectonic and groundwater controls on synrift sedimentation in the Late Jurassic–Early Cretaceous of the western Cameros Basin, Northern Spain

AbstractBasin hydrology and subcrop are key controls on carbonate sedimentation in continental basins. Hydrologically sensitive carbonates can record groundwater fluctuations within an aquifer in deep time. Late Jurassic extension, footwall uplift, erosion and karstification of marine Jurassic carbonates in the western Cameros Basin (Spain) saw deposition of ?Upper Kimmeridgian‐Tithonian syntectonic alluvial fan deposits (Señora de Brezales Formation). Biogenic laminar calcretes and phreatophytic rhizocretions record roots exploiting capillary fringe groundwater. Progressive infill of rift topography and footwall erosion lowered sedimentary gradients and clastic supply during deposition of the ?Tithonian–Berriasian Rupelo Formation. Distal alluvial marls (Las Viñas Member) contain charophytes, with 2 m thick carbonate lenses at the top reflecting intermittent rise of groundwater in ponds on the basin floor. Stacked palustrine limestones with rare charophytes and laminar calcretes (Ladera Member) record overstep of seasonal carbonate wetlands onto basin margins and footwall highs with intense pedogenetic modification during lengthy seasonal exposure. Overlying Berriasian charophyte‐ostracod wackestones, displaying microkarst cavities and interbedded intraclastic conglomerates, with vivianite sauropod bones, footprints and polygonal desiccation cracks at the top (Mambrillas de Lara Member) record open lacustrine conditions with limited subaerial exposure and high water tables. Desiccation‐cracked limestones and marls with correlatable evaporite horizons (Rio Cabrera Member) contain marginal marine foraminifera and dasycladaceae at the top. Lagoonal conditions reflected transgression to seaward and intermittent marine connection via the Basco‐Cantabrian Basin. The distribution and thicknesses of hydrologically sensitive carbonates reflected onlap onto a faulted and karstified marine Jurassic carbonate pediment and the subtle influence on hydroperiod of fault (and potentially localised Triassic salt) controlled differential subsidence and transgressive groundwater rise. Hydrological facies evolution reflects progressive basin infilling and eustasy beyond. Transitions in this continental succession from clastic to carbonate facies and from closed to open hydrology record hydrological change over time rather than contemporaneous deposition under Walther's Law.

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.

Novel insights to unconventional carbonate mudstone reservoir with quantitative nanoporosity characterization and modeling of Tuwaiq Mountain Formation

Organic-rich carbonate mudstones have emerged as a significant unconventional play globally, particularly the Middle Jurassic carbonate mudstones of the Arabian plate. Despite their importance, these mudstones are highly heterogeneous, composed of organic-lean and organic-rich deposits, and often exhibit similar reservoir quality, making them challenging to characterize. While most studies have attributed the reservoir quality of these mudstones to the organic-related pores, little attention has been paid to the origin of the pore structure and network of the organic-lean counterparts. To address these issues, we performed high-resolution quantitative pore characterization and modeling by coupling micro-computed tomography and focused ion beam scanning electron microscopy to gain insight into the matrix-related pore structure, size, and networks on the organic-lean mudstones. Our study showed that in the organic-lean mudstones micropores are present but isolated, while the nanoscale pore structure appears to be well-connected, hence higher permeability. The pore-network model result has a median pore radius of 189–226 nm and a median throat radius of 91–112 nm, rarely exceeding 1000 nm and 500 nm, respectively. The frequency plots demonstrate positively skewed distributions indicating that large pores and throats are anomalous. Our study highlights that the organic-lean mudstone nanoscale pores are comparable to previously reported nanopores in the organic-rich zone and other mudstone plays around the globe. It emphasizes coexisting organic and matrix-related nanopores, suggesting the significance of matrix-bound nanopores' influence on carbonate mudstones unconventional play. The findings can enhance our understanding of the nano-scale pore structure and provide insights to evaluate and build development strategies of carbonate mudstones in the Arabian Intrashelf Basin and elsewhere.

Tectonic evolution of the Circum-Moesian orocline of the Carpatho-Balkanides: Paleomagnetic constraints

The areas of the present study in eastern Serbia, the Danubicum and the Timok Magmatic Complex (TMC, part of the Geticum) are situated between the Vardar Zone and Moesia. The first is Moesia derived and thrust over the Geticum during the latest Cretaceous, the second represents the central segment of the subduction related Apuseni-Banat-Timok-Srednogorie (ABTS) metallogenic belt. The new results, based on 18 geographically distributed sampling points (228 field oriented drill cores) imply large CW vertical axis rotations for the Upper Jurassic (Lower Cretaceous) carbonates of the Danubicum and a moderate one for the Upper Cretaceous igneous and sedimentary rocks from the TMC. These, together with earlier published paleomagnetic data provide kinematic constraints to test the circum-Moesian backarc-convex orocline model. The strike test plot clearly documents that it is a progressive arc. The starting situation at the time of the volcanic activity in the metallic belt (90–70 Ma) must have been a generally E-W oriented S segment, continuing in NNW-SSE oriented ABT segments. The present geometry of the circum-Moesian belt, in the context of Miocene paleomagnetic results from the Vardar Zone and the Apuseni Mts, is interpreted as the result of two main tectonic processes. The first is an about 30° vertical axis CW rotation which took place in coordination with the Vardar Zone (20–17 Ma). The second is an additional 40–65° CW rotation (17–15 Ma) involving also the Danubicum, due to the subduction pull of the E Carpathians in combination with the corner effect of Moesia.

Late Jurassic High-Pressure Metamorphism of Variscan I-Type Granitoids in the Northern Part of the Pelagonian Unit (Republic of North Macedonia)

Abstract The high P/T metamorphic Pelagonian Unit in the Republic of North Macedonia comprises (1) a Variscan basement consisting of gneisses, schists and minor meta-mafic rocks, which are all intruded by I-type granitoids and rare related dikes; (2) a metamorphosed sedimentary sequence of Permian to Lower Triassic age, and (3) a sequence of calcite and dolomite marbles resulting from Late Triassic to Middle Jurassic carbonate sediments. All these rocks underwent a common high-P/T metamorphism of Late Jurassic age. This paper deals with the metamorphism of the Variscan I-type granitoids which contained the igneous mineral assemblage plagioclase I + alkali feldspar I + quartz I + biotite I + titanite I + allanite I + zircon I + apatite I ± magnetite I. During Late Jurassic high-P/T metamorphism, these undeformed granitoids were thoroughly metamorphosed under isotropic pressure conditions as documented by undeformed granitic textures that are overgrown by metamorphic minerals such as garnet II, epidote II, and phengite II. Various, eventually metasomatic mineral reactions took place in different textural positions: (1) Former igneous plagioclase grains became completely transformed to Na-rich plagioclase IIa (An09–14) containing numerous small grains of epidote IIa and phengite IIa. Either this transformation was an allochemical one and was accompanied by the syn-metamorphic introduction of an aqueous fluid phase containing Fe, Mg and K or, alternatively, the more Ca-rich parts of plagioclase I became considerably sericitized before high-P/T metamorphism, and the resulting mixture of more Na-rich relic plagioclase with its sericite-rich domains became later metamorphosed under high-P/T conditions. In the first case, an aqueous phase is needed during metamorphism, while in the second case high-P/T metamorphism might have proceeded under H2O-undersaturated conditions; (2) igneous alkali feldspar I was changed to albite-poor orthoclase II or microcline II; (3) igneous Ti-rich biotite I reacted with plagioclase to metamorphic garnet II + Ti-poorer biotite II + titanite II + phengite II + quartz II ± epidote II ± rutile II, which is rimmed by Ttn II. At textural positions, where igneous plagioclase I was not available, igneous biotite I was transformed to Ti-poorer biotite II + titanite II ± ilmenite-hematite II; (4) during uplift, high-P/T metamorphic rutile II became marginally overgrown by titanite II ± ilmenite II; (5) igneous allanite I grains stayed unaltered, but when located near to former plagiocase I, they became partially rimmed by metamorphic epidote II. Equilibrium phase diagram calculations showed that the observed metamorphic paragenesis (plagioclase II + K-rich feldspar II + biotite II + garnet II + epidote II + phengite II + garnet II + quartz II + rutile II + titanite II) is only stable under H2O-unsaturated conditions. The I-type granitoids and their metamorphic country rocks were metamorphosed under high-P/T conditions of 1.3 to 1.5 GPa and 560 to 590 °C.

Greensand formation, siliceous earth deposition and coastal metal drawdown in the Danubian Cretaceous Basin (Bavaria, Germany)

The Late Cretaceous epoch was characterized by extreme greenhouse climates and widespread glaucony formation in shallow marine settings. However, in the Danubian Cretaceous Basin (DCB, Bavaria, SE-Germany), contemporaneous shallow marine deposition of glauconitic sandstones of the Regensburg Formation in the eastern and the glaucony-free Neuburg Siliceous Earth deposits of the Wellheim Formation in the western parts of the basin during Cenomanian–early Turonian times is puzzling. An integrated approach of sedimentology, stratigraphy, mineralogy and geochemistry reveals that the striking lithological and mineralogical differences can be attributed to the geological structure of the hinterland and the nature of element input: in the eastern DCB, deeply chemically weathered granites and gneisses of the Bohemian Massif were leached due to the warm climate and high precipitation rates as indicated by high values of the modified Chemical Index of Alteration (CIXrev). Elements crucial for glaucony formation (K, Fe, Si, Al) were amply supplied by rivers, fueling a shallow marine glaucony factory. Slightly reducing conditions and a temporally increased primary productivity further promoted favorable conditions for geologically fast shallow water glaucony authigenesis. In the western DCB, in contrast, a hinterland consisting of karstified Upper Jurassic carbonates devoid of essential elements for glaucony formation and a lack of significant fluvial input inhibited shallow marine glaucony formation. Furthermore, we suggest that the depositional environment of the Neuburg Siliceous Earth was affected by submarine discharge of silica-rich groundwater resulting in intense early diagenetic silicification. Additionally, our geochemical data provide the first evidence of a trace metal drawdown during Oceanic Anoxic Event 2 (approx. 94 Ma) in shallow water/coastal settings in Germany, as shown in very low V/Al, Ni/Al, Cu/Al and As/Al ratios, much lower than the average shale and upper continental crust.

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.

Multiscale discrete fracture network modelling of shallow-water carbonates: East Agri Valley Basin, Southern Italy

We investigate the multiscale geometrical and dimensional properties of fracture and fault networks crosscutting Lower Jurassic to Cretaceous shallow-water carbonate rocks. The carbonates are exposed along the flanks of the Viggiano Mt., on the eastern side of the High Agri Valley Basin of southern Italy. Furthermore, we employ the results of field and digital structural analyses to derive the input parameters for subsequent Discrete Fracture Network modelling (DFN) of geocellular volumes whose dimension and architecture resemble those of the investigated sites. DFN modelling is carried out for geocellular volumes representing the studied bed packages (5m-side volumes), outcrops (50m-side volumes), and reservoir-scale carbonate cliffs (500m-side volumes). Notably, modelling is obtained considering the minimum mechanical aperture value obtained in the field for porosity computations and theoretical values of fracture hydraulic aperture for permeability computations. This is because the mechanical aperture values and roughness profiles collected in the field along single fractures are unreliable due to weathering and localized karst-related dissolution. Our findings reveal that a scale-dependent geometry characterizes the Cretaceous carbonates over three orders of magnitude. These results support previously published data, and contrast with those obtained for the Lower Jurassic carbonates, which suffer of some bias due to the quality of the exposures. After DFN modelling, we show that the amount of computed fracture porosity is mainly due to the SB and NSB fractures, and that equivalent permeability is greater within faulted rock volumes. In terms of horizontal permeability, which is the most reliable result after DFN modelling, we document near isotropic horizontal permeability ellipses at all scales of observations. At individual outcrops, we note that the permeability ellipses are elongated parallel to the dominant fault sets that denotes how localized strain affects the directionality of fluid flow. Finally, we summarize the original data in a conceptual model illustrating the modalities of meteoric fluid infiltration through the vadose zone, and then subsequent horizontal flow in the phreatic zone present within the fracture carbonates at shallow depths.

Depositional Environment, Diagenesis and Source Rock Characterization of Middle Jurassic Carbonates, North Waziristan, Khyber Pakhtunkhwa, Pakistan

Depositional Environment, Diagenesis and Source Rock Characterization of Middle Jurassic Carbonates, North Waziristan, Khyber Pakhtunkhwa, Pakistan

Opening and Post-Rift Evolution of Alpine Tethys Passive Margins: Insights from 1D Numerical Modeling of the Jurassic Mikulov Formation in the Vienna Basin Region, Austria

This study employed 1D numerical pseudo models to examine the Upper Jurassic carbonate succession, focusing on the Mikulov Formation in the Vienna Basin region. It addresses the protracted and complex history of the Jurassic source rock play, revealing a transition from rapid syn-rift (>200 m/Ma) to slower post-rift sedimentation/subsidence of the overlying layers during extensional deformation (up to 120 m/Ma with a thickness of 1300 m). This provides valuable insights into the rift-to-drift stage of the central Alpine Tethys margin. The Mikulov marls exhibit characteristics of a post-rift passive margin with slow sedimentation rates. However, a crustal stretching analysis using syn-rift heat flow sensitivity suggested that thermal extension of the basement alone cannot fully explain the mid-Jurassic syn-rift stage in this segment of the Alpine Tethys. The sensitivity analysis showed that the mid-late Jurassic differential syn-rift sequences were exposed to slightly cooler temperatures than the crustal stretching model predicted. Heat flow values below 120 mW/m2 aligned with measurements from deeply settled Mesozoic successions, suggesting cold but short gravity-driven subsidence. This may account for the relatively low thermal maturation of the primary source rock interval identified by the time-chart analysis, despite the complex tectonic history and considerable sedimentary burial. The post-Mesozoic changes in the compaction trend are possibly linked to the compressional thrusting of the Alpine foreland and postdating listric faulting across the Vienna Basin.

Reservoir heterogeneities due to diagenesis in Jurassic Samana Suk Formation Kahi section Nizampur Basin North West Himalayas Pakistan

The porosity and permeability determine the reservoir quality of sedimentary rocks, which is fundamentally influenced by both depositional and diagenetic processes. The Jurassic carbonates are targeted in many regions for oil and gas exploration. The current study is carried out to elaborate on the diagenetic alterations and their effect on reservoir properties. A thick outcrop of the Jurassic Samana Suk Formation is studied in the Kahi section of Nizampur Basin Northwest Himalayas, Pakistan, to study and relate the trend of lithological variations and depositional settings. Field investigation revealed that the Samana Suk Formation is extensively distributed in the area and primarily made up of interbedded limestone and dolomite units. The original unaltered limestone has a thick-bedded and oolitic to bioclastic nature. Different types of dolomites have been recognized based on colour contrast and sedimentary features. Moreover, saddle dolomite cement, calcite cementation, and mechanical and chemical compaction have also been observed. The petrographic studies show different types of diagenetic alterations that affected the Samana Suk Formation, including micritization, bioturbation, mechanical and chemical compaction in the form of fractures and stylolites, various calcite cementation, which includes the various cement types that range from isopachous, blocky, granular equant, fibrous, and dog tooth cementation, along with dissolution that occurred in different diagenetic realms. Pyritization was rarely observed. Moreover, different phases of dolomites were identified, ranging in size and shape, i.e., finely crystalline to coarse crystalline and planar euhedral to non-planar anhedral. The stable oxygen isotope values of these dolomites show depletion from original marine signatures and suggest burial-related fault-controlled dolomitization events. Overall, diagenetic processes like dissolution, fracturing and dolomitization increased the reservoir potential. On the contrary, the overburden and cement precipitation result in a decrease in the reservoir properties.

Geochemistry and fluid inclusion study of the Jbel Tirremi fluorite-baryte deposit, Morocco: New insights into the genetic model in relation to Mesozoic tectonics

The highly fractured Jurassic carbonates at Jbel Tirremi in northeastern Morocco host fluorite-baryte deposit. The mineralization occurs both as stratabound in marl-limestone contact and as fault-hosted veins (N-S- and NNW-SSE). The mineral paragenesis consists of two fluorite and baryte generations and calcite with subordinate amounts of quartz, dolomite, traces of sulfides (chalcopyrite, pyrite, galena), and oxidized minerals. Fluid inclusion data reveal that hot moderately saline fluids derived from the Paleozoic basement mixed with Triassic brines and cooler, meteoric waters. The REY inventory, C-O-S-Pb-Sr-Nd isotope, and crush-leach data point to the Paleozoic basement as the primary source of metals with a contribution from the Triassic red beds. The refined ore genetic model developed in this study from our new geological and geochemical data includes the downward movement of Triassic-Jurassic evaporated seawaters along normal faults, followed by the leaching of metals from Paleozoic and Triassic rocks, and the subsequent upward flow of these metalliferous fluids. During the Late Cretaceous-Paleocene basin inversion, the deep-seated ore-forming fluids migrated upward, which eventually mixed with Triassic brines and cooler meteoric waters. This fluid mixing caused the precipitation of multiple generations of fluorite and baryte.

Thermal springs associated with the Melilla-Fès-Smaala-Oulmès fault (Morocco): The role of fluid geochemistry in identifying a major active geodynamic structure

The thermal springs form a hydrothermal system implanted on the active tectonic structure of the Melilla - Fès - Smaala - Oulmès fault (MFSO). This fault forms a multiple play tectonic corridor generally oriented NE-SW parallel to the Nekor fault. It crosses two structural domains, the Meseta, to the South, and the Rif, to the North, forming a highly complex deep shear zone. The thermal waters from these springs have chloride-sodium facies and sodium-calcium bicarbonate facies with emergence temperatures between 24.4 and 53.6 °C. Along this fault, the pH of these waters varies centrifugally, with acidic values in the South and North and neutral values in the intermediate sector. Important information about the origin of thermal waters and the characteristics of the reservoirs is provided by the chemical compositions (chemical elements, isotopes, and gases) of these waters involving rock-water-gas interaction during their circulation in depth and flow to the surface, through different types of reservoirs intersected by the fracture networks associated to the MFSO. These reservoirs correspond to the crystalline basement and its Triassic evaporitic clay, Jurassic carbonate, Cretaceous marl, and Miocene saliferous cover, whose temperatures have been estimated between 100 and 200 °C. The stable isotopic compositions (−8.39 to −5.2‰ for δ18O and −51.2 to 30.1‰ for δ2H) indicate a meteoric origin of these waters, whose recharge areas are located in the Rif and Atlas reliefs. The gas phase present in the waters shows δ13Cco2 (−19.3 ‰ to −4,25 ‰) and 3He/4He (0.203–3.864 Ra) with mantellic signature, implying that the fault is deeply rooted. The mantellic helium content decreases progressively in these waters from SE to NE, following the direction of the MFSO fault, from the emergencies in the Palaeozoic basement to the springs in the secondary and Tertiary formations of the Prerif and Mesorif. This decrease results probably from the mixing with gases derived from the thick secondary and Tertiary sedimentary cover. These waters would also collect these gases during their interactions with these sedimentary formations. Towards the NE, these waters, which rise within the formations of the Mediterranean arc, are also influenced by gases derived from the sediments of volcanic arcs.The MFSO fault, which is permeable due to its fracture network, favours the diffusion of heat and the upwelling of mantle gases, which are collected by the fluids that will interact with the rocks during their circulation at depth and flow to the surface. Its hydraulic character, highlighted in the North by seismic tomographic sections, confirms the presence of fluids at a depth of 5 km. This hydraulic character is continuous towards the South, implying the intimate association of the springs studied with this active tectonic structure. The pressure generated by these fluids and the precipitation of secondary clay minerals will weaken this fault, giving it an aseismic character.

Global geologic expert system and database for predicting carbonate reservoirs from seismic: Application to the Upper Jurassic

Carbonate systems are influenced by a great variety of physical and biological controlling factors that operate from global to local scales. The resulting intrinsic complexity of carbonate platforms makes them difficult to predict, especially when data are limited. Predicting geologic geometries and properties based on limited sampling or uncalibrated seismic data generally relies on a priori knowledge and equivocal interpretations that are marked by geologist perception and personal experience. To overcome these uncertain interpretations of such a complex natural system, which can become critical in frontier exploration, we developed an expert system that relies on a process-based method and a standardized data set using normalized information and parameters. The main innovation relies on the realization of knowledge- and process-based synthetic carbonate stratigraphic architectures that support seismo-stratigraphic interpretations. The workflow consists of four steps: (1) bibliographic compilation of a geologic database for each case study supported by quantitative parameters (e.g., sedimentation duration and thickness) and qualitative parameters (geodynamic context, seismic architecture, and facies model); (2) statistical analyses to establish consistent geologic classes and spatiotemporal trends; (3) process-based modeling to simulate stratigraphic architectures associated with carbonate sedimentation processes in a physically constrained numerical environment and testing different geologic hypotheses; and (4) realization of a predictive palaeogeographic map representing the global distribution of carbonate stratigraphic architectures, and estimation of controlling parameters for unconstrained case studies. The expert system is based on 77 case studies of Upper Jurassic carbonate platforms, which reveal the resemblance of these carbonate systems, in response to uniform global palaeoclimatic conditions and sea level. Significant local differences in stratigraphic architectures are related to specific geodynamic contexts and subsidence trends. The thickest carbonate platforms are developed in extensive/passive geodynamic settings such as the Central Atlantic Ocean margins, while thinner platforms form in intra- and peri-cratonic settings such as those of the Arabian region.

Identification of deep Czech Republic–Austria transboundary aquifer discharge and associated river chloride loading

The deep transboundary aquifer of regional scale along the Czech Republic–Austria border in Central Europe serves as a thermal-mineral water resource for balneotherapy and plays an important role in the region’s development. The aquifer is composed mostly of Jurassic carbonates at depths from 160 to − 3000 masl. Despite more than two decades of exploitation, no complex analysis of groundwater flow directions and groundwater fluxes ever took place. Now, cross-border cooperation enabled the research team to gather crucial information on the Jurassic aquifer. For a better understanding of the groundwater flow system, a numerical model was developed. To simulate the effect of variable density and viscosity occurring in such a deep aquifer, the SEAWAT numerical model was used. The simulation shows that there is an inflow of low mineralised groundwater from the crystalline outcrops in the northwest and inflow of saline groundwater from southeast. Aquifer discharge was identified along the zone partly corresponding to the course of the Dyje River. To check the model’s accuracy, the river water was sampled together with streamflow measurements. Detected sections of increasing chloride concentration indicate zones of the Jurassic aquifer discharge into the Dyje River. The discharge rate of 85 L/s derived from streamflow and chloride concentrations matches the value computed by the model. The relatively high discharge of the Jurassic aquifer contributes significantly to the high chloride loading observed in the Dyje River.

Late Triassic to Early Jurassic carbon isotope chemostratigraphy and organo-facies evolution in a distal to proximal transect of the North German Basin

AbstractThroughout the Latest Triassic and the Early Jurassic, major changes in paleogeography, climate and eustatic sea-level impacted on the development of shelf depositional environments. Secular trends in environmental conditions were punctuated by transient perturbations that occurred in relation to large-scale volcanic events, such as the emplacement of the Central Atlantic Magmatic Province at the Triassic/Jurassic boundary and the Karoo–Ferrar Large Igneous Province in the early Toarcian. We here present bulk organic (HAWK programmed pyrolysis) and organic carbon isotope (δ13Corg) data for three drill cores recovering Latest Triassic and Early Jurassic strata (Rhaetian to Toarcian). Study sites are located in the northeastern part of the Central European Epicontinental Sea and were positioned along a distal–proximal transect of the North German Basin. This allows discussing the differential response of depositional settings and organo-facies toward secular and transient environmental change. Biostratigraphically anchored trends in δ13Corg values allow the precise correlation along the transect, as well as with distant sites. At all North German locations, diagnostic secular trends in δ13Corg are punctuated by transient negative carbon isotope excursions, reflecting perturbations of the global carbon cycle at the Triassic/Jurassic boundary and in the early Toarcian. Stratigraphic gaps occurred during sea-level lowstands and are most pronounced at shallow proximal sites. Programmed pyrolysis data indicate spatiotemporal organo-facies trends that on a temporal scale occurred in response to sea-level and climate trends, while spatial patterns were governed by basin morphology and paleobathymetry. Substantial marine organic matter accumulations occurred at high sea level during the Toarcian only, and were most continuous at distal sites. Graphical Abstract

Seismic lithology and geological implications of pre-salt jurassic carbonate rocks in the Amu Darya Right Bank

The formation and evolution of oil and gas reservoirs are typically related to the environment where sedimentary facies occur and regional sedimentary facies are distributed. The accurate characterization of reef–shoal complexes is one of the important tasks for exploring oil and gas in carbonate reservoirs. However, most traditional methods for characterizing sedimentary facies are based on high-density drilling data, core data, and thin-section data, often making them less suitable in areas with few drilled wells or sparse well networks. Hence, they often cannot characterize sedimentary facies accurately, severely hindering the oil and gas exploration success rate. This study seeks to characterize sedimentary facies using limited amount of data from drilled wells. An innovative model representing the relationship between lithofacies and seismic attributes was established by comprehensively analyzing the relationship among lithologic associations, principal component analysis, and regression analysis based on seismic lithology and data from various sources. The sedimentary facies of the Upper Jurassic (Oxfordian) formation in the eastern region of the Amu Darya Right Bank were accurately characterized using Red-Green-Blue (RGB) fusion. These analyses indicate that the relationship among micrite, bioclastic micrite, and bioclastic limestone can effectively represent the developmental pattern of sedimentary facies in the study area. The planar distribution of sedimentary facies based on the relationship between the lithologic associations identified through seismic interpretation revealed the relationship between sedimentary facies and the paleomorphology of the region. Results show that the paleo-landform in the study area's western zone is low-lying, where shoals are poorly developed; the paleo-landform in the central zone is high, with high-energy water and well-developed and widely distributed thick shoals; and the eastern zone is far from the open sea and affected by the central-zone landform, with low-energy water and low-quality reef–shoal complexes. Controlled by the unique landforms of the study area, the western reservoirs are thin and have low quality, central reservoirs are thick with good petrophysical properties, and eastern reservoirs have differing thickness and petrophysical properties.