International Journal of Earth Sciences最新文献

A new model of the thermal structure of the easternmost portion of the Arabian continental crust is presented. Detailed heat flow models based on more than 660 temperature measurements from 230 exploration wells have been performed over an area of 166,000 km² in size, spanning from the Arabian Gulf to the Eastern Arabian Shield. Geothermal gradients exhibit an increase from 22 ºC/km on the Arabian Platform, to 35 ºC/km in the Faydah-Jafurah Basin. Related surface heat flow (SHF) increases in the same direction from 44 to 72 mW/m². Heat flow analysis reveals that the radiogenic heat contribution to the total surface heat flux accounts for up to 58%, and the Moho heat flux for 42%, accordingly. From thermal modeling constraints, i.e., matching borehole temperature data and resulting heat flow distribution, it can be inferred that the crust underneath the easternmost Arabian Platform (east of En Nala terrane suture) is significantly more felsic (~ 2.5 µW/m³) than the central Arabian Platform and Arabian Shield (~ 0.9 µW/m³). This is supported by deep wells intersecting rocks of granitoid composition east of the Arabian Shield. Reconstructions of lithosphere geotherms has revealed Moho temperatures around 850–900 °C. Moho heat flow is in the order of 26 mW/m². Thermal modeling revealed a spatial relationship between regional surface heat flow distribution, crustal structure and the extension and composition of basement terranes. The study demonstrates that the Proterozoic crustal configuration has an impact on the Phanerozoic thermal evolution and its subsidence pattern.

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Modeled temperature (C) at the top of the basement.

The Jurassic–Cretaceous source rocks and their exploratory operations in the Mesopotamian Foredeep Basin are limited, and the origins of recovered hydrocarbons have not been comprehensively investigated. Comprehensive geochemical analyses and 1-D basin models were performed on the Middle Jurassic–Lower Cretaceous succession. Additionally, geochemical analyses of eight crude oils from Upper Cretaceous reservoirs were used to evaluate the conventional petroleum resource potential and petroleum exploration and development. The total organic carbon (TOC) and Rock–Eval results reveal the Middle Jurassic–Lower Cretaceous succession to be characterized by fair to excellent source rock potential and consists mainly of Types II/III and III kerogens. Consequently, the Middle Jurassic–Lower Cretaceous succession can generate both oil and gas, with high oil generation potential. The dominance of such kerogen is confirmed by the substantial lipids derived from phytoplanktonic, bacterial, and algal organic matter, as indicated by biomarker compositions. Furthermore, biomarker parameters and isotopic compositions of oil samples provide evidence of a genetic link between the Middle Jurassic–Lower Cretaceous source rocks and crude oils. The data suggest that the non-biodegraded oils were generated from mature marine source rocks deposited under reducing conditions. One-dimensional basin models show that the oil generation from the organic matter-rich intervals within the Sargelu, Najmah/Naokelekan, and Gotina formations started during the Upper Cretaceous and continued into the Miocene (83–21 Ma) at low maturity levels (EASY%R_o > 0.50%). Oil expulsion from the Sargelu, Najmah/Naokelekan, and Gotina formations has taken place since the Miocene until now (21–0 Ma) with higher conversion ratios (%TR > 50%) and migrating through vertical pathways provided by faults and being trapped within Upper Cretaceous reservoirs.

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Groundwater discharge from natural springs commonly involves gas emissions, providing valuable insights into the origin of spring and subsurface geology. Here, we report substantial carbon dioxide (CO₂) emissions, instead of methane (CH₄), from Aiken Spring, which is located in a desert region of the western Qaidam Basin, a Mars-analog environment within the Tibetan Plateau. The CO₂ fluxes from the spring water surface reach up to 43.7 g/m²/h, with the estimated total emissions from the entire spring reaching at least 207 tons in the summer (90 days). The carbon (C) isotopic composition of the CO₂ released from the spring is − 8.9 ± 1.6‰, which corresponds to an estimated value for dissolved inorganic carbon (DIC) of − 4.6 ± 1.6‰ in the original spring water, suggesting a mixture of mantle-derived CO₂ and sedimentary carbonates. The mantle-derived CO₂ at Aiken Spring may indicate active subsurface magmatic degassing within the intersection of the Altyn Tagh Fault and the Kunlun Fault, but it is more likely linked to a subsurface CO₂ reservoir from ancient magmatic activity. Overall, our results indicate that Aiken Spring provides insights into deep subsurface geological processes and potentially the terrestrial subsurface biosphere.

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Before the rise of the Northern Andes in Cenozoic time, Triassic to Jurassic extensional basins in northwestern South America accommodated predominantly continental strata partly intercalated with volcanic rocks. Coeval plutonism is attributed to a magmatic arc related to the subduction of the Farallon plate beneath South America. The basins later became involved in the Andean orogeny and are now partially exposed in the Eastern Cordillera and Middle Magdalena Valley of Colombia. We have employed (U/Pb) geochronology on zircons from Triassic-Jurassic felsic to intermediate volcanic and volcaniclastic rocks. Most of the ten samples have a substantial proportion of detrital zircons, but only three had no Mesozoic grains. The Mesozoic ages obtained range from ca. 201 Ma to ca. 177 Ma and overlap with published crystallization ages (K/Ar; Ar/Ar; U/Pb) from plutonic bodies. Volcanics from the Jordán and Girón formations are latest Triassic to Early Jurassic and synchronous with major plutonic activity. These ages constrain the early evolution of the extensional basins that formed from about the Triassic-Jurassic transition in an intra-arc position and facilitated the preservation of sediment and arc-derived volcanics. Middle Jurassic ages from the Noreán Fm. are synchronous with sparse plutonism west of the Middle Magdalena Valley. At this time, the magmatic arc had migrated westward, while intrusive activity in the Eastern Cordillera ceased. A geochemical rift signature only appears in scarce Early Cretaceous mafic intrusions that resumed magmatic activity in the Eastern Cordillera. This magmatism, now in a back-arc position, coincides with maximum subsidence of the large Cretaceous basin that extended across the older intra-arc rift basins. Extension and lithospheric thinning ceased by the end of the Early Cretaceous.

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The Sierra de Famatina of northwestern Argentina contains one of the best Cambro-Ordovician stratigraphic records of the SW Gondwana margin. Two lithotectonic belts (Calalaste–Narváez and Famatina–Valle Fértil), separated by master faults, preserve evidence of two former volcano-sedimentary basins (Eastern and Western, respectively). The Calalaste–Narváez Lithotectonic Belt consists of an Ediacaran to early Cambrian basement unconformably overlain by a 490–480 Ma cover of very low-grade volcano-sedimentary and volcanic succession that presumably formed in an extensional tectonic regime. In contrast, the Famatina–Valle Fértil Lithotectonic Belt comprises a basement consisting of the late-lower-to-middle Cambrian metasedimentary Achavil and Negro Peinado formations unconformably overlain by meta/sedimentary and metavolcanic rocks ranging in age from the late Cambrian to the Middle Ordovician (ca. 490–460 Ma). This belt includes the Famatinian Cordilleran-type magmatic arc active mainly at ca. 473–468 Ma, coeval with andesite to rhyolitic volcanism (Suri and Las Planchadas formations). Rhyolitic tuffs of ca. 473 Ma (εNdi = − 4.1) were found in the La Aguadita Formation, allowing this unit to be re-assigned to the late Floian. The oldest magmatism of the Sierra de Famatina is characterized by isotopically evolved (εNdi = − 5.1) rhyolitic tuffs of ca. 490 Ma in the Bordo Atravesado Formation, which was coeval with deposition of Mn-enriched hydrothermal cherts. This early Famatinan volcanism contrast with that of similar age and isotopically less evolved occurred in the Calalaste–Narváez Lithotectonic Belt suggesting variations of the source of magmas across the space and time within the Famatinan Orogenic Cycle. We propose that both described lithotectonic belts likely diverge northwards into Chile and Peru, wrapping around the Arequipa–Antofalla Proterozoic block.

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The Lower Kura Depression (LKD) in Azerbaijan is a unique place on Earth where enormous oil and oil–gas-condensate deposits coexist with mud volcanoes. Large mud volcanoes developed in this area as a result of favorable tectonic processes, depositional settings, and subsurface pressure conditions. Disequilibrium compaction leading to overpressurization of mudrocks, as well as gas generation, have been previously proposed as the main factors that cause overpressure and trigger mud volcanism. To assess the mechanisms contributing to mudrock overpressure, we conducted a 2D basin modeling work to simulate the burial, temperature, maturation, and pressure histories of the sedimentary pile along a 120-km-long geological cross-section in the ENE‒WSW direction perpendicular to major structures in the LKD. The results of the calibrated model suggest that the main petroleum source rocks of the LKD, namely the Oligocene–Middle Miocene Maykop and the Eocene Middle Koun mudrock formations, are still in the oil generation zone. Therefore, previously speculated gas generation effect on overpressurization is insignificant in the LKD. Modeling also predicts overpressure of varying magnitude in the potential hydrocarbon source rocks of dominantly mudstone lithology. We have verified that disequilibrium compaction caused by rapid sedimentation in the last 3 million years has led to mudrock overpressurization that exceeded rock strength. We take model-predicted fracturing as a proxy indicator of mud ascent and suggest that fracturing of the mudrocks enabled ascend of the mud via fault-associated weakness zones.

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A sample of micaceous schist of the Cushamen Metamorphic Complex in the Cushamen area (northwestern North Patagonia, Argentina) preserves a complex mineral assemblage, including staurolite, andalusite, garnet, sillimanite, biotite, quartz, and plagioclase. This unit proves an opportunity to analyse a complex mineral association often related to disequilibrium stages or polymetamorphic contexts. Through detailed petrological analysis combining mineral chemistry, X-ray compositional maps, conventional thermobarometry, and phase equilibria analysis, we reconstructed the pressure–temperature (P–T) path of this schist. The schist unit preserves a polymetamorphic history characterized by M₁, M₂, and M₃ events. The M₁ event is represented by biotite, muscovite, quartz, and plagioclase. The M₂ event, associated with local mid-Carboniferous pluton intrusion, is characterized by andalusite and garnet assemblages, with peak conditions at ~ 3.3 kbar and ~ 563 °C. The main M₃ event, at the time of the Carboniferous–Permian boundary, is defined by garnet, staurolite, sillimanite, biotite, muscovite, plagioclase, and quartz. This event records a progressive P–T evolution from ~ 3.5 kbar and ~ 553 °C to ~ 4.9–5.6 kbar and ~ 620–635 °C, nearing peak conditions. This work highlights the importance of comprehensive approaches in P–T trajectory reconstructions and the critical role of selecting the reactive bulk composition, particularly in rocks with complex mineral assemblages. In addition, this study significantly contributes to understanding the metamorphic evolution of the Cushamen Complex, a unit for which there is limited knowledge regarding its structural and metamorphic evolution. This complex is part of the igneous-metamorphic basement of North Patagonia region (Argentina), which records the Paleozoic evolution of the southwestern margin of Gondwana.

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The Pliensbachian/Toarcian boundary interval represents a transition from a coldhouse into a hothouse climate state, involving the demise of a land-based cryosphere, initiating a third-order global sea-level rise. Within the intensely studied Northwest Tethyan shelf region, the South-German Basin has been investigated in more detail than the North-German Basin (NGB). We here provide a palaeoenvironmental reconstruction of the Pliensbachian/Toarcian transition from the Hondelage fossil excavation site located in the NGB employing organic, isotope, and major/trace element proxies. Here, the late Pliensbachian was characterized by cold climate, low sea level, and a slow hydrological cycle, causing minor terrigenous sediment and nutrient fluxes to the basin, instigating low marine productivity. Shallow, well-mixed shelf waters of normal salinity favored aerobic degradation of planktonic biomass, preventing sedimentary accumulation of organic matter. These conditions changed in the earliest Toarcian, where increased temperatures led to sea-level rise via meltdown of land-based ice and accelerated the hydrological cycle, causing salinity stratification. Enhanced riverine sediment and nutrient supply from nearby landmasses promoted marine primary productivity, which caused anoxic conditions in bottom and pore waters favoring enhanced preservation and accumulation of organic matter. A short-lived sea-level fall at the Lower Elegans Bed coincided with lowered productivity and enhanced carbonate precipitation, due to reduced runoff and recovery of the carbonate factory. Increased redox-sensitive trace element concentrations above the Lower Elegans Bed suggest a renewed inflow of low-salinity arctic water masses via the Viking Corridor and potentially increased freshwater input, promoting water column stratification, enhanced planktonic productivity, and re-establishment of bottom water anoxia/euxinia.

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Abstract

The marine Upper Jurassic rocks of the Franconian Alb consist largely of micritic carbonate of partly dolomitized reef mounds and bedded basinal limestone. All carbonates were lithified in the shallow (centimeters, meters) subsurface and have a wide range of ∂¹³C (≤ + 3‰ to − 10‰VPDB) but always negative ∂¹⁸O (− 1 to − 6‰VPDB). Dolomite and reef limestone show the highest ∂¹⁸O and ∂¹³C values. The most negative ∂¹³C (≥ − 10‰) occurs mainly as cement in dolomite of a basinal, partly dolomitic, biostrome interval. Basinal limestone shows intermediate ∂¹³C values. Because freshwater diagenesis and elevated temperatures cannot explain the observed isotope values, pH is here considered a major factor influencing the isotope signal of micritic limestone. The bulk sediment isotope signal was reset to lower values, from an original lime mud with ∂¹³C ≥ 3‰ and a ∂¹⁸O of ≥ + 1‰, as a result of biochemically induced diagenesis. Carbonate, probably mostly aragonite but occasionally including dolomite, was dissolved in a zone where low pH developed as a result of organic matter degradation. Dissolved carbonate was translocated by diffusion and re-precipitated as cement (ca. 50vol%) in a zone with elevated pH where all in situ lime mud ∂¹⁸O was reset. Imported cement carbonate precipitated in equilibrium with the pore fluid with negative isotope values, whereas ∂¹³C of the in situ lime mud remained unmodified. The negative shift of the bulk ∂¹³C and ∂¹⁸O is variable and depends on pH and the contribution of ¹²C from anaerobic sulfate reduction in the zone of cement precipitation. This produced an ubiquitous covariance of ∂¹⁸O and ∂¹³C. Incorporation of seawater-derived Mg²⁺ during recrystallization of carbonate can account for the local dolomitization. Elevated ⁸⁷Sr/⁸⁶Sr ratios are explained as a result of interaction of clay minerals with the stationary pore fluids. This study shows that the isotopic signal produced by biochemically induced shallow submarine subsurface carbonate diagenesis can be indistinguishable from freshwater diagenesis, that ∂¹⁸O and ∂¹³C of the bulk rock are always reset, and that carbonates can show, in the presence of clay minerals, elevated ⁸⁷Sr/⁸⁶Sr ratios even when the pore fluids were never exchanged.

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The depositional environment, hydrology and vegetational history of the Lower Radnice Coal (Duckmantian) in the Kladno coalfield was studied using sedimentary geology, coal petrology and paleobotanical/palynological methods. The peat accumulating wetland of the coal formed in a fluvial paleovalley approximately 15 km long and 2–5 km wide, bordered by basement paleohighs and landlocked in the interior of the central European Variscides. The peat swamp evolved on top of mud-dominated floodplain successions pedogenically modified to a vertic gleyed Protosol. Probably climatically controlled rising ground water table resulted in paludification that from downstream part gradually spread upstream. Most clastic load was deposited in the upper part of the valley, whereas only mud suspension was dispersed downstream throughout the vegetated swamp. The best conditions for peat accumulation were situated in the eastern part of the paleovalley, where up to 1.5 m thick coal with thin bands of impure coal and carbonaceous mudstone formed in an occasionally inundated rheotrophic system with peat accretion controlled by regional ground water table. The peat swamp was vegetated mainly by lepidodendrid lycopsids with Lepidodendron and Paralycopodites being dominant genera. Shrubby to ground cover vegetation was represented by medulosallean pteridosperms, small shrubby lycopsids, sphenopsids, and herbaceous ferns. Tree ferns were locally abundant, especially in mineral-rich substrates. The rheotrophic character of the peat swamp may indicate higher seasonality of the Variscan interior, compared to coastal areas in the North Variscan foreland with contemporaneous ombrotrophic peats. Modern equivalents of the Lower Radnice Coal swamp are inland planar tropical peat swamps in tributary paleovalleys of the Tasek Bera in peninsular Malaysia and central Congo basins.

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Lower Radnice Coal peat swamp.