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International Journal of Earth Sciences
Journal Prestige (SJR): 1.125

Citation Impact (citeScore): 2
Number of Followers: 37

Hybrid journal (It can contain Open Access articles)
ISSN (Print) 1437-3262 - ISSN (Online) 1437-3254
Published by Springer-Verlag

[2469 journals]

Speleothems of the Wadi Sannur cave (Eastern Desert, Egypt): a
well-preserved archive of paleoenvironmental and palaeoclimatic changes
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  PubDate: 2022-06-01
Arc magmatism in the Nkoula granitoid suites, Central African Fold belt in
Cameroon: evidence of a metasomatized high oxidized S- and I- type magma
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  Abstract: Abstract Situated in the Tikar plain, the Nkoula granitoid suites consist of granites, quartz monzonites and syenites. The petrography, mineral chemistry and whole-rock geochemistry of the studied granitoids were done. The rocks are mainly made up of quartz, K-feldspar, plagioclase, amphibole, biotite ± sphene ± epidote. They are I-and S-type, metaluminous to peraluminous, magnesian and belong to high-K calc-alkaline to shoshonitic series. K-feldspars are orthoclase and plagioclases have the chemical composition of oligoclase. Biotites are reequilibrated primary Mg-rich while amphiboles are magmatic, calcic and essentially magnesian. All the granitoids display LREE enrichment and HREE depletion with negative Eu anomaly. The primitive mantle normalized trace element patterns display Rb, Ba, U, Th and Pb enrichment and depletion in Nb, Ta, Ti and P. The major and trace element variations indicate that fractional crystallization plays an important role during the magmatic processes of the S-type magma. The trace element ratios Ba/Rb, Nb/Ta, Y/Nb, Th/La, Sm/La, Hf/Sm and Ta/La, along with the biotite and amphibole chemistry, point to a crust-mantle mixed source for the Nkoula granitoid suites. The granitoids emplaced in the syn-collisional and volcanic arc setting. The most likely processes for the Nkoula granitoid consist of the partial melting of the subducted slab and the probable contribution of a mantle component. The break-off of the subducted slab may have favored the upwelling of the asthenosphere that supplies heat for the partial melting of the slab and crust components. The Nkoula granitoid magma displays 3–5 wt.% of water and was formed under high oxidizing conditions. They were emplaced at relatively shallow depth (3–9 km) and crystallized at about 800–900 °C in the Adamawa-Yade continental crust due to the subduction of the Yaounde oceanic crust under the Adamawa-Yade domain during the Pan-African orogeny.
  PubDate: 2022-06-01
New geophysical and geological data on the Moldanubian plutonic complex
and the Kaplice Fault, southern Bohemia
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  Abstract: Abstract The paper presents a new structural model of the granitoid intrusions of the Moldanubian plutonic complex in southern Bohemiaeast of the town of Kaplice. Valuable archival gravity data were supplemented by magnetic survey and precise profile gravity measurements to better define the gravitational effect of the granitoid body and provide an accurate model solution. Our findings, based on geophysical analyses and modelling, bring new information on the shape and deep structure of the Moldanubian plutonic complex in southern Bohemia. These results are complemented by a tectonic investigation of the Freistadt granodiorite body and the Kaplice Fault. New information on the geometry of the Kaplice Fault plane acquired from Linsser indications is discussed in relation to its possible recent seismic activity. The usefulness of the Linsser indications method for active tectonic studies is demonstrated. This is the first regional study in this insufficiently known part of the Bohemian Massif, providing new information on the extent of granites and their relationship to one of the most significant faults in this area.
  PubDate: 2022-06-01
Mineral chemistry, geochemistry and geophysical investigations of Simlipal
volcanics from Eoarchean Singhbhum Craton (Eastern India): geodynamic
implications of pervasive plume–lithosphere interaction
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  Abstract: Abstract Singhbhum craton in eastern India is one of the oldest and perhaps geologically and geophysically most complex geological terrains on the surface of the Earth, containing a number of intra-cratonic Archean volcanic suites. In the present study, we investigate Mesoarchean volcanic rocks of the Simlipal complex, a tiger reserve in the Mayurbhanj district of Odisha (India). Our results provide a new understanding of the regional geodynamic scenario of the Eoarchean Singhbhum craton, including the nature of plume–lithosphere interaction based on the detailed analysis of geological, geochemical and geophysical data. Whole-rock geochemical studies exhibit large variation in SiO2 (39.30–60.57 wt%), TiO2 (0.20–1.55 wt%), Al2O3 (4.45–15.63 wt%), MgO (5.25–37.00 wt%) and CaO (3.46–11.26 wt%) with low to moderately high Cr (50–1503 ppm) and Ni (36–550 ppm) contents. These rocks are cumulates to porphyritic and petrographically and geochemically, can be classified as ultramafic (lherzolite, pyroxenite), picrite, basalt, basaltic andesite, andesite and boninite. Their trace elements and HFSE/REE patterns indicate that they belong to the same parental magma. Rare earth elements and trace element patterns exhibit moderate fractionation with a coherent pattern of (La/Yb)n: 1.38–8.50, (Gd/Yb)n: 0.92–2.59 and (La/Sm)n: 1.11–4.40, consistent with polybaric melting. The present study suggests that these rocks were generated by decompressive melting of a mantle plume head at a depth of garnet to spinel lherzolite field together with fractional crystallization and crustal contamination in the subcontinental lithospheric mantle (SCLM), which is corroborated by the highly uplifted nature of this terrain, associated with positive gravity anomalies. This plume had a mantle potential temperature between 1400 and 1700 °C and pressure reaching 4.4 GPa. Massive upwarping of the lower crust to an extremely shallow depth of about 4 km from the surface, 21 km thick magmatic underplating above the Moho and the lithosphere asthenosphere boundary at 81 km, would support plume-induced active crust–mantle thermal interaction and extremely warm and deformed nature of the lithosphere beneath the Simlipal volcanic complex. Based on the findings of millerite (NiS), quench/spinifex texture and shock metamorphic features in these volcanic rocks, we infer that there could be a possibility that the suggested mantle plume in the study area may have been triggered by an asteroidal impact.
  PubDate: 2022-06-01
Enigmatic well-characterized remanent magnetization of silicified Lower
Devonian rocks from the Tadrart area (Murzuq basin, SE Algeria)
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  Abstract: Abstract To improve the poor Gondwana paleomagnetic database for Devonian times, detailed paleomagnetic analyses were performed on red chert-like rocks and partly silicified paleosols within the Lower Devonian Ikniouen level (fine-grained sandstones including red ironstone) in conformity within the sub-horizontal Tadrart coarse white formations of the Murzuq basin. Silicification, limited to this level that is only a few meters thick, was probably due to tropical warm climatic conditions during and shortly after the rock deposition. In two sections 40 km away each other, paleomagnetic data point out a high-temperature Characteristic Remanent Magnetization (ChRM) with very well-defined mean direction, positive reversal test and relatively high (5) Q and R scores. Rock magnetic data indicate minerals of the hematite family, but the presence of a minor amount of other mineral phases remains possible. At least part of the ChRMs are Chemical Remanent Magnetizations, likely acquired during or shortly after deposition. The corresponding paleomagnetic results (paleomagnetic pole at 28.6° E and 71.1° S, with K = 1004, A95 = 1.5°) could have major geodynamical implications for the Gondwana supercontinent. In fact, ChRM acquired in this level during or shortly after deposition should imply a much-unexpected fast latitudinal continental drift of the Gondwana during the Lower Devonian or a significant and fast true polar wander. Though much more difficult to match with the ChRM and geological characteristics, the only possible alternative interpretation for the Ikniouen data should be a chemical remagnetization acquired during the Late Cretaceous–Early Paleocene times.
  PubDate: 2022-06-01
Calcareous nannofossil biostratigraphy and paleoenvironment of the
Eocene–Oligocene interval in the Pabdeh Formation in southwestern Iran
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  Abstract: Abstract Studies of the Paleogene sequence in the Zagros Basin of Iran are crucial due to the existence of large oil fields such as the Pazanan oil field. This study examines calcareous nannofossils from the Eocene–Oligocene interval in the upper part of the Pabdeh Formation in the Zagros Basin. Samples were taken from a well drilled in the Pazanan oil field, located in the Dezful Embayment (Zagros Basin, southwestern Iran). The studied section is ~ 54 m thick and comprises mainly a succession of marls and limestones. Calcareous nannofossil biostratigraphy allows the studied interval to be assigned to Martini (Standard tertiary and quaternary calcareous nannoplankton zonation. In: Proceedings of the 2nd planktonic conference. Roma, Italy, p 739–785, 1971) Zones NP19 (Isthmolithus recurvus Zone) through NP22 (Helicosphaera reticulata Zone), which spans the Priabonian and Rupelian stages and includes the Eocene/Oligocene boundary. The nannoflora paleoecology allows us to characterize major changes in paleoproductivity during this interval. Late Eocene (Priabonian) assemblages are dominated by oligotrophic nannofossils. Above the Eocene/Oligocene boundary, there is a diverse assemblage of eutrophic calcareous nannofossils, with a significant decrease in the abundance of oligotrophic species. As well as nutrient supply, water temperature also appears to be another main controlling factor of the paleoenvironment in this region, where there is an increase in cold-water species recorded at the base of the Oligocene (Zone NP21), accompanied by a decrease in warm-water species. These variation in the calcareous nannofossil assemblages may indicate increased nutrient supply, climatic imbalances—associated with weather changes—and eventually sea-surface temperature (SST) cooling in this part of the Zagros Basin (Neo-Tethys domain) throughout the Eocene/Oligocene transition (EOT).
  PubDate: 2022-06-01
Active morphogenic faulting and paleostress analyses from the central
Nahan Salient, NW Siwalik Himalaya
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  Abstract: Abstract Tectonics of ancient foreland basins of collisional orogens are of enormous international attention from the perspectives of applied (e.g., hydrocarbon exploration) and theoretical geosciences (e.g., tectonic models). Morphogenic faulting produces identifiable surface morphology and structural patterns that provide vital insights for reconstructing paleostress scenarios and the present-day surface topography. The principal stress directions that affected the region are important to understand the polyphase tectonic zones. We perform geomorphic analyses and combine them with paleostress analysis of mesostructures from the Panchkula-Morni region from the central Nahan Salient. We reconstruct the orientation of the principal stress axes that evolved during the tectonic evolution of the brittle sheared sandstones of the Lower/Upper Siwalik units. Three deformation phases are determined that are interpreted to be coeval with the Himalayan orogeny: (i) extensional stress field with NW oriented extension, (ii) extensional stress field with NE–ENE oriented extension, and (iii) NE-directed compression. Out of these, component (iii) led to the uplift of the Himalayan orogen and produced the present-day topography. Slickenside kinematic indicators viz., PT, RM and PO structures helped in interpreting the movement of the slip planes exposed in the uniform lithology. Compressional stress field with NE compression is consistent with the geometry of the Bisiankanet Thrust (BkT) inferred from geomorphic analyses. The NE extension direction and oblique plunge of the σ1-axis may be responsible for the oblique movement with major normal slip at the western extremity of the BkT. Abrupt change in topographic swath profiles, their mutual offset in the hangingwall and knickpoint migration indicate active uplift especially across the newly identified out-of-sequence Tikkar Tal Thrust (TTT) on the map and also on an existing seismic section.
  PubDate: 2022-06-01
From Cadomian back-arc basin to Rheic Ocean closure: the geochronological
records of the Kurtoğlu Massif, eastern Sakarya Zone, Turkey
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  Abstract: Abstract New U–Pb zircon data from the Kurtoğlu Massif (eastern Sakarya Zone, Turkey) reveal a Cadomian back-arc basin, evolving to closure of the Rheic Ocean. A host metapsammite from the lower tectonic slice of the massif yielded detrital zircon ages ranging from Neoarchean (2.73 Ga) to late Neoproterozoic with a major population between 610 and 553 Ma and a peak at 592 Ma, implying that the basement of the Sakarya Zone has a Cadomian affinity. This metasedimentary sequence was intruded by an orthogneiss in the early Ordovician (481 Ma) probably during the opening of the Rheic Ocean. Zircon grains from the overlying metasedimentary sample yielded concordant ages spreading throughout the Mesoproterozoic (1608–1032 Ma) and early Paleozoic (516–351 Ma), suggesting that they were deposited in the Rheic Ocean likely prior to its final closure. The muscovite metagranite, emplaced into the host rocks of the Kurtoğlu Massif at 388 Ma, formed probably during northward closure of the Rheic Ocean. Zircon grains of a metasiltstone from the upper tectonic slice of the massif, represented mainly by phyllites, yielded similar age spectra to that of the host metapsammite of the lower tectonic slice. Such phyllites in the Pulur Massif ⁓ 35 km to the southeast display a close association with the ophiolitic complexes of the Rheic Ocean. This relationship together with the age spectra mentioned above suggests that sedimentation was on a place away from the Paleozoic igneous sources, in an analogous position to the southern passive margin of the Rheic Ocean.
  PubDate: 2022-06-01
The detrital zircon record of Variscan to post-Variscan tectonosedimentary
and magmatic processes in the Tauern Window (Eastern Alps)
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  Abstract: Abstract Coupled U–Pb and Lu–Hf LA-ICP-MS detrital and igneous zircon data were obtained from metasedimentary sequences (Kaserer Formation, Schmirntal Quartzite, Seidlwinkel Formation, Bündnerschiefer Basin, Riffler Basin) of the western Tauern Window (Eastern Alps). Results show maximum deposition ages between the Late Permian and the Triassic, indicating protracted sedimentation and magmatism between the Late Paleozoic and the Mesozoic. The Lu–Hf fingerprint shows a change from subchondritic to variable subchondritic to suprachondritic compositions at ca. 290 Ma, possibly documenting the transition from Late Paleozoic Variscan post-collisional processes to intracontinental extension. Lithospheric thinning and magmatic underplating may explain the observed Hf isotopic evolution as the result of mixing of crustal and mantellic sources. From a paleogeographical perspective, results confirm that the Tauern Window was situated between Alpine basement units (South Alpine, Austroalpine and External Massifs) and the Bohemian Massif during the Permian–Triassic.
  PubDate: 2022-06-01
Chronology, geochemical characteristics, and tectonic implications of a
Triassic complex in the Rongma Area, Southern Qiangtang, Tibet
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  Abstract: Abstract The spatio-temporal evolution of the Paleo-Tethys Ocean has been a hot and controversial issue in the world. Here we carry out petrographic, chronological, and geochemical study on garnet–phengite–quartz schist and mafic rocks in the Rongma area from the northern margin of the Southern Qiangtang block to determine the early Mesozoic tectonic evolution of the Shuanghu Paleo-Tethys Ocean in northern Tibet. The zircons from a phengite–quartz schist sample yielded concordant ages of 1936–393 Ma, indicating that its protolith deposited after ~ 393 Ma. Overgrowth zoning garnet with three stages of metamorphic evolution from garnet core to rim (i.e., Peak metamorphic, early retrograde metamorphic, and late retrograde metamorphic stages) in the schist was recognized, indicating that two subduction in a short time might be involved for its genesis. Two groups of phengite in the schist yielded 40Ar/39Ar plateau ages of 229 ± 1.4 Ma and 225 ± 1.3 Ma, respectively; thus, the late retrograde metamorphism of the schist might occur at ~ 229–225 Ma. Zircon U–Pb dating of a diabase in the area yielded crystallization age of 241 ± 1.1 Ma implying its formation in the early Triassic. The Hf-in-zircon and whole-rock Nd isotopes of the diabase show εHf(t) of − 0.6- + 19.1 and εNd(t) of − 0.8 to + 0.9, respectively. Combined with the whole-rock geochemical features of the early Triassic diabase (241 Ma) and gabbro (237 Ma), they indicate that these mafic rocks are formed in a back-arc extensional setting related to the subduction of the oceanic plate between the Northern and Southern Qiangtang blocks beneath the latter. Combined with regional data, our study favors that view bi-directional subduction of the Shuanghu Paleo-Tethys ocean in the Early Triassic and it was finally closed at ~ 237 to ~ 229 Ma. Our model will help us better understand the tectonic evolution of the Paleo-Tethys Ocean.
  PubDate: 2022-06-01
Strontium isotopes reveal Early Devonian to Middle Triassic carbonate
sedimentation in the Sakar-Strandzha Zone, SE Bulgaria
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  Abstract: Abstract We focused on the Sakar-Strandzha Zone (SASTZ) carbonate sequences in Bulgaria, where the Sakar, Subbalkanide and Strandzha Triassic facies types are presented. Strontium isotopic compositions of carbonate samples from the Triassic type section of the Sakar unit confirm an Early-Middle Triassic age of sedimentation consistent with biostratigraphic data, whereas the strontium isotopic compositions of other Sakar-type inferred Triassic sections suggest an Early Permian depositional age. The strontium isotopic compositions of a Strandzha-type Triassic carbonate sample are in agreement with Middle Triassic sedimentation. A few Subbalkanide-type Triassic carbonate samples from the northern and western SASTZ yielded strontium isotopic compositions indicating an Early Devonian sedimentation. The new results indicate late Paleozoic-early Mesozoic sedimentation, and require most of the Subbalkanide-type Triassic and some of the Sakar-type inferred Triassic carbonate sequences in the SASTZ to be revisited.
  PubDate: 2022-06-01
Elemental and isotopic tracing of mineral infillings from various
microstructures of a fault system into fine-grained sediments: which
interacting fluids'
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  Abstract: Abstract Elemental contents and Sr isotopic data of calcite and celestite infillings from various tectonic microstructures of the so-called “Main Fault” that occurs in the Opalinus Clay Formation (Switzerland) were used to elaborate on the geochemical impact of this structural system on the fine-grained host sediments. The chemical data of the leachates from vein, slickenside and gouge infillings are significantly different from those of leachates from diffuse calcite of the undeformed and of the highly deformed Opalinus Clay rock matrix called scaly clays. For instance, the fluids flowing within the microstructural veins and slickensides could not derive from pore waters of the host sediments, as the data show that the flowing waters diffused from veins into the matrix. The fluid/rock interactions in the gouges were also different from those in the veins and the slickensides, probably due to an additional pressure-solution impact in the former, complemented by a further local supply of organics from very close sediments. The infillings of the vein and slickenside micro-features and of some scaly clays that precipitated from bi-carbonated and/or sulfated flowing fluids yield an almost constant 87Sr/86Sr ratio of 0.70774 ± 0.00001 (2 \(\upsigma\) ). This constrained Sr isotopic value points to a unique homogeneous external source for the tectonic-related fluids that were of probable marine origin, signing those of the early, but also of the late faulting event. Notably different from those seeping presently throughout the host rocks, the initial flowing fluids were apparently related to a late-Eocene marine invasion into the Delémont Basin that occurred during the contemporaneous rifting of the Rhine-Graben tectonic system. The flowing fluids induced by the late Main-Fault system obviously used, at least, some if not most of these earlier tectonic microstructures, as they yield the Sr isotopic signature of their infillings. The regional evolution includes then an initial Rhine-Graben tectonic episode with fluids resulting from a flooding of the deposited sediments and into the faults of the regional active Rhine-Graben rifting by contemporaneous seawater. These fluids precipitated precisely calcite and celestite infillings in the microstructures induced by the rifting that remained then unaffected until the renewed tectonic activation during the late Eocene in the Mont Terri and its fault system. This new tectonic–thermal episode created a new generation of microstructures during the thin-skinned deformation of the Jura Belt within and around the Main Fault, in addition to those from earlier Rhine-Graben event that already existed in the host rocks. Connections between the two generations of veins could be observed at the wall of a new gallery in the rock laboratory. The flowing fluids involved in this late belt event moved apparently along these newly created tectonic microstructures, as well as along older ones when accessible. This duality allowed the preservation of the same initial Sr isotopic signature by partial dissolution of the initial infillings that precipitated about 30 million years earlier in the initial drains of the Rhine-Graben fault system.
  PubDate: 2022-06-01
Internal igneous growth, doming and rapid erosion of a mature ocean
island: the Miocene evolution of Maio (Cabo Verde)
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  Abstract: Abstract Maio Island (Cabo Verde Archipelago) is composed of uplifted Early Mesozoic MORB-type pillow lavas and deep-sea sediments, unconformably overlain and intruded by Miocene igneous rocks. Combined structural analyses and 40Ar–39Ar dating were used to constrain the Miocene evolution of Maio. Structures and ages of uplifted Mesozoic sequences and crosscutting Miocene dykes showed that numerous intrusive events were associated with the intense growth of an igneous core complex in the middle to upper crust, causing semi-circular doming and partial disruption of the Mesozoic strata. Two nosean nephelinite dykes cut the Valanginian Batalha Formation and yielded phlogopite 40Ar–39Ar ages of 10.405 ± 0.033 Ma and 10.570 ± 0.053 Ma (2σ errors). A nosean nephelinite dyke that cuts the overlying Valanginian to Early Aptian Morro Formation yielded an age of 9.273 ± 0.020 Ma. Combined with existing K–Ar and 40Ar–39Ar ages, this confirmed a main period of island growth between ~ 16 and 8.7 Ma. We re-interpreted extensive polymict conglomerates, which occur below the Late Miocene Monte Penoso Formation, as landslide deposits. A nephelinite lava clast yielded a phlogopite 40Ar–39Ar age of 8.666 ± 0.0274 Ma, which represents a maximum age for these landslides and thus confined a period of large-scale flank collapses and erosion to between 8.7 and 6.7 Ma. Flank collapses and further mass wasting during this period may have rejuvenated the igneous activity, i.e., resulting in the formation of the Tortonian/Messinian Monte Penoso and Malhada Pedra Formations, due to decompression-induced melting at upper mantle depths. Such interaction between flank collapses and rejuvenated volcanism may be a key to better understand ocean island evolution worldwide.
  PubDate: 2022-06-01
3D thermal and rheological models of the southern Río de la Plata Craton
(Argentina): implications for the initial stage of the Colorado rifting
and the evolution of Sierras Australes
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  Abstract: Abstract We present 3D thermal and rheological models of a key intra-plate locality of South America: the southernmost limit of the Río de la Plata Craton area (which encompasses the Claromecó Basin, the Sierras Australes, and the Colorado Basin). Both models were calculated on the basis of a previously published 3D lithospheric scale density model, after the population of the different units with thermal and rheological properties. Firstly, the steady-state conductive thermal field was modelled using different thermal properties in accordance with the assumed lithological composition of the units. Moving forward, the strength distribution was calculated considering the resulting thermal field and published rheological properties for common rock types as input for the different layers that compose the 3D density configuration of the area. Our main results suggest that the thickness and composition of the crust exert a first-order control on the present-day thermal field of the area and, subsequently, on the lithospheric strength. Particularly, we identified a rheologically weak lithospheric zone that coincides with previously proposed inherited Paleozoic structures of Gondwana. This inherited lithospheric fabric could have controlled the opening of the Colorado Rift Basin during the early Late Jurassic. In that sense, the present day high strength of the Rio Negro and Colorado transfer zones associated with the mafic infill of Colorado rifting in the Mesozoic suggests that the weak zones should have necessarily been developed during (at least) Paleozoic to early Mesozoic times. This supports the hypothesis that the reactivation and interaction of the latter with the Gondwanic weak lithospheric zone could have deformed and uplifted the Sierras Australes during the Gondwanides Orogeny.
  PubDate: 2022-05-25
Piperock ichnofabrics from western Henan, China: spatial distribution
pattern and environmental controls on middle Cambrian carbonates
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  Abstract: Abstract The middle Cambrian (Miaolingian Series, Wuliuan Stage) Mantou Formation Member III in the Dengfeng area, western Henan, China provides an opportunity to explore the relationship between suspension feeders and sedimentary substrates in the early Phanerozoic hydrodynamic high-energy environment. Its strata record abundant vertical burrows in the easily migrated carbonate sediments of calcarenitic shoals or oolitic beaches. Three types of piperock ichnofabrics are identified: Skolithos ichnofabric, Arenicolites ichnofabric and Skolithos-Arenicolites ichnofabric. Bedding plane expressions of these ichnofabrics show that there is a spatial distribution pattern dominated by segregated distribution between individual burrows; this pattern is shown to be genuine through nearest neighbor and spatial point process analyses. These characteristics not only indicate that burrows were constructed by contemporaneous suspension feeders, but also reflect the selection and adaptation of suspension feeders to strong hydrodynamics and mobile substrates. In addition, the interspecific competition caused by the heterogeneity of nutrients is one of the determinants of the spatial distribution pattern of vertical burrows. As ecosystem engineers, these suspension feeders significantly transform landforms, create pressure gradients and accelerate geochemical cycles, which may further promote biodiversity in benthic organisms.
  PubDate: 2022-05-25
Biomarker and compound-specific isotope records across the Toarcian CIE at
the Dormettingen section in SW Germany
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  Abstract: Abstract The Toarcian oceanic anoxic event (T-OAE) is associated with a prominent negative carbon isotope excursion (CIE; ~ 183 million years (Myr)). About 10-m-thick organic matter-rich sediments accumulated during the T-OAE in the Southwest German Basin (SWGB). Rock–Eval, maceral and biomarker analysis were used to determine variations of environmental conditions across the CIE interval. Carbon isotope records were determined for various n-alkanes, pristane and phytane to contribute to the reconstruction of the paleo-environment and to study the factors controlling molecular δ13C values. Geochemical redox indicators provide evidence for photic zone anoxia during the Toarcian CIE, which reached its maximum after deposition of the “Unterer Stein” marker horizon. The 2α-methylhopane index suggests enhanced activity of diazotrophic cyanobacteria, which is also supported by nitrogen isotope data. This distinguishes the SWGB from other basins with Toarcian black shale. Oxygen-depleted conditions, albeit with lower intensity continued after the CIE. All investigated compounds replicate the negative CIE, but the magnitudes vary considerably. The largest shift is observed for n-C27 (9‰) and reflects the combined effect of the global CIE and a major change in organic matter input (termination of terrigenous organic matter input). The shift for short-chain n-alkanes, pristane, and phytane, interpreted to reflect marine biomass, varies between 4.5 and 5.0‰. This is the highest value observed so far for any Toarcian section. δ13C values of pristane and phytane reach a minimum near the base of the CIE interval and increase upsection. Thus, the maximum negative isotope shift predates the strongest basin restriction by about 450 thousand years (kyr).
  PubDate: 2022-05-23
Genesis: early life survived in the Polar Circles by precipitating banded
iron formation (~ 3.7–1.85 Ga) followed by stratified ferruginous
siliciclasts until ~ 580 Ma, when tectonically shifted to lower
latitudes initiating the ‘Cambrian Explosion’
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  Abstract: Abstract The inclination of Planet Earth’s axis of rotation by 23½° resulted in extreme climatic changes. Weak, solar radiation upon the Polar Circles during half-a-year alternated with half-a-year of darkness, turning them into freezing terrains for nearly the whole Precambrian. Exhalant hydrothermal solutions formed huge lakes over Polar Regions, undergoing intensive evaporation and condensation. Chemical interactions incidentally created primitive live forms, surviving as chemoautotrophic bacteria under the weakest UV rays. Daily changing solar radiation emitting UV rays over low latitudes prevented any life form. Some of these polar bacteria developed photosynthesis, improving their nourishment simultaneously releasing oxygen. The high content of ferrous iron in the lakes absorbed toxic oxygen forming iron oxides as banded iron formation (BIF). Excess photosynthetic oxygen molecules escaped into the anoxic atmosphere. At ~ 1.8 Ga oxygenated meteoric water infiltrated the continental subsurface oxidizing hydrothermal fluids, precipitating underground layered iron-oxides followed by silica only during the dry summers. The dilution of the rising solutions terminated biologically induced BIF precipitation. Consequently, intensive evaporation cemented sililiciclasts into stratified ferruginous formations, becoming abundant in the Late Neoproterozoic as NIF. The co-occurrence of Paleoproterozoic BIF and Neoproterozoic NIF sites evidence the tectonic and climatic stability of the Polar Circles. Magmatic convection currents split them ~ 750 Ma ago, but only after 580 Ma shifted the individual plates radially to low latitudes with advance of ‘Plate Tectonics’. The polar bacteria connected with the open sterile sea for the first time had to adapt to daily changing ecosystems by combining into mobile primitive eukaryotes (Ediacaran Biota) and further diversify, erroneously referred to the ‘Cambrian Explosion’. Geological evidence corroborated the activity of convection currents since Earth’s consolidation controlling its inner heat budget and supplying hydrothermal solutions forming lakes on the Polar Circles where life originated and iron ores accumulated.
  PubDate: 2022-05-18
Organic geochemistry and origin of bitumen seeps in the Upper Permian
(Zechstein) bituminous anhydrite in a Cu–Ag mine in western Poland
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  Abstract: Abstract Migrating hydrocarbons in the form of bitumen seeps can provide detailed information about the overall features of a petroleum system, source rock characteristics and maturity. Here we present the results of organic geochemical analyses of five bitumen seeps collected at the Polkowice–Sieroszowice Cu–Ag mine in western Poland at the eastern margin of the Southern Permian Basin. Bitumen seeps occur in fractures in bituminous anhydrite in the Lower (Werra) Anhydrite (A1d) and in veins cutting the Zechstein Limestone (Ca1) dolomite. The A1d bituminous anhydrite is one of several anhydrite types interpreted to have been deposited in supratidal sabkha and pan settings. Thermal maturity parameters (isoprenoid-based, hopane, sterane and aromatic hydrocarbon ratios) show that the bitumen was generated in the early to peak oil window. The analysed samples are characterised by high abundances of C35 homohopanes (homohopane index, HHI 0.3–0.5), an even-over-odd predominance of C16–32 n-alkanes, high gammacerane (0.2–0.4) index and low Pr/Ph (< 0.7) ratios. These parameters together indicate (anoxic) evaporitic depositional conditions for the related source rock. The presence of C27–29 4-desmethyl steranes (C29 > C27 > C28) and bicyclic sesquiterpanes together with a DBT/P ratio of < 0.5 imply the dominance of algal material and a clay-rich source rock, respectively. The presence of characteristic biomarkers for archaeal lipids (2,6,10,15,19-pentamethylicosane, squalane and C40 head-to-head biphytane) and C40 carotenoids (β- and γ-carotane, C21 and C30 alkylbenzenes) confirms that the bitumen was generated from the A1d bituminuous anhydrite. The molecular characteristics of the analysed bitumen samples rule out the possibility that the hydrocarbons have migrated from underlying Carboniferous source rocks or from Kupferschiefer mudrocks (higher Pr/Ph ratios, lack of gammacerane, secohopanes, secosteranes, and β- and γ-carotanes).
  PubDate: 2022-04-20
Statistical analysis of the connection between geomagnetic field reversal,
a supernova, and climate change during the Plio–Pleistocene transition
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  Abstract: Abstract A significant change in the Earth’s climate occurred during the Pliocene–Pleistocene transition. Different external and internal forcings were interrelated in such a way that they promoted the cooling of the climate and environmental changes. We analyzed these changes using a mathematical and statistical approach based on a new algorithm combining Artificial Intelligence and the new multiple cross-wavelet analysis. Although the geomagnetic field (GF) paleointensity showed oscillations over time and not a uniform decay, the results of the Radial Basis Function Artificial Neural Networks showed a long periodicity oscillation with a downward trend practically throughout the Plio–Pleistocene transition. A weakened GF could have allowed an increase in the flux of galactic cosmic rays (GCR) penetrating the Earth’s atmosphere. This led us to hypothesize that, during the Plio–Pleistocene transition, the increase in GCR could have induced a gradual increase in clouds and, therefore, the albedo intensified, causing a progressive decrease in the Earth's surface temperature, as recorded in different paleoclimatic reconstructions of the Plio–Pleistocene. In addition to the known variations in orbital parameters, the GF behavior (the Gauss–Matuyama reversal) and a feasible influence of a coetaneous near supernova event (at less than 100 pc), could have reinforced the climatic forcings towards glacial conditions during the Plio–Pleistocene transition (~ 2.5 Ma).
  PubDate: 2022-04-06
How Alpine seismicity relates to lithospheric strength
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  Abstract: Abstract Despite the amount of research focussed on the Alpine orogen, different hypotheses still exist regarding varying spatial seismicity distribution patterns throughout the region. Previous measurement-constrained regional 3D models of lithospheric density distribution and thermal field facilitate the generation of a data-based rheological model of the region. In this study, we compute the long-term lithospheric strength and compare its spatial variation to observed seismicity patterns. We demonstrate how strength maxima within the crust (~ 1 GPa) and upper mantle (> 2 GPa) occur at temperatures characteristic of the onset of crystal plasticity in those rocks (crust: 200–400 °C; mantle: ~ 600 °C), with almost all seismicity occurring in these regions. Correlation in the northern and southern forelands between crustal and lithospheric strengths and seismicity show different patterns of event distribution, reflecting their different tectonic settings. Seismicity in the plate boundary setting of the southern foreland corresponds to the integrated lithospheric strength, occurring mainly in the weaker domains surrounding the strong Adriatic plate. In the intraplate setting of the northern foreland, seismicity correlates to modelled crustal strength, and it mainly occurs in the weaker and warmer crust beneath the Upper Rhine Graben. We, therefore, suggest that seismicity in the upper crust is linked to weak crustal domains, which are more prone to localise deformation promoting failure and, depending on the local properties of the fault, earthquakes at relatively lower levels of accumulated stress than their neighbouring stronger counterparts. Upper mantle seismicity at depths greater than modelled brittle conditions, can be either explained by embrittlement of the mantle due to grain-size sensitive deformation within domains of active or recent slab cooling, or by dissipative weakening mechanisms, such as thermal runaway from shear heating and/or dehydration reactions within an overly ductile mantle. Results generated in this study are available for open access use to further discussions on the region.
  PubDate: 2022-04-05