The 20 Journals
Geoscience e-Journals, the portal!
|
Geologica Acta, vol. 23, no. 17, 22 p. Skelton P.W. & Gili E. (2025).- Myths and models of rudist ecology- Characterizing the ecology of rudist bivalves has long proved challenging, given their baroque shell shapes and the inconvenience of their extinction. Contrasting interpretations have taken hold in the literature via two different approaches. On the one hand, the appealing but simplistic hypothesis of ‘rudist reefs’ based on analogy with recent tropical coral reefs has been uncritically accepted by many, ignoring both the contrasts between the organisms concerned and the differing effects on sedimentation of icehouse versus greenhouse patterns of sea-level change. Worse, untested speculations have been spun on this suspect foundation, including the hypotheses of widespread photosymbiosis in rudists and of their competitive displacement of corals. On the other hand, opposed to such myth-making is an approach that respects the primacy of evidence. Here, synthetic interpretative models of rudist formations are built up from all circumstantial considerations and iteratively tested for compliance with field and laboratory observations. This process involves collating evidence ranging from well-supported inferences concerning rudist and coral palaeobiology, and growth fabrics and sedimentological contexts, to larger scale reconstructions of facies anatomy. The picture that commonly emerges –in contrast to the classic actualistic barrier-reef model– is of opportunistic settlements of usually low relief on flat-topped, tabular-bedded platforms distally grading into flanking clinoforms, without distinct intervening reefal edifices. Opportunities to test such synthetic models are provided by exceptional outcrops that display the full panorama of facies relationships. The latter is one of many achievements of detailed field studies over many years by the ‘Ramon Salas School’ in the Maestrat Basin. Geologica Acta, vol. 23, no. 18, 26 p. Ossò A., Artal P., Bakel B.W.M. van & Ferrer O. (2025).- Decapod fauna from the lower Aptian in the Valencian sector of the Maestrat Basin (NE Iberia)- The Forcall Formation in the Morella and Orpesa sub-basins of the Valencian sector of the Maestrat Basin has yielded a diverse assemblage of decapod crustaceans. A total of thirteen taxa are documented, representing the infraorders Achelata, Astacidea, Glypheidea, Axiidea, Anomura and Brachyura. Among these, one new genus, three new species, and one new taxonomic combination are proposed: Viapagurus covavidrensis n. sp., Eomunidopsis rigolleti n. sp., Morellosia maestratensis n. gen., n. sp., and Distefania pimientai n. comb. Comparisons with decapod faunas from other sub-basins of the Maestrat Basin reveal a notable degree of faunal homogeneity across the region. Furthermore, the palaeobiogeographic implications of these assemblages provide new insights into the distribution and diversity of Aptian decapod crustaceans. The findings highlight a significant faunal similarity with coeval basins such as the Garraf and Basque-Cantabrian basins and underscore the potential role of the Maestrat Basin as a biogeographic corridor linking decapod faunas of the Tethyan and Boreal realms. Overall, this study contributes to a more comprehensive understanding of the decapod assemblages of the Forcall Fm. and their relationships with contemporaneous faunas in the western Maestrat Basin and beyond. Geologica Acta, vol. 23, no. 19, 18 p. TomÁs S., Parcerisa, D. Cantarero I., Baqués V., Salas R. & Travé A. (2025).- Linking early diagenesis and relative sea-level changes in ancient corals: a multiproxy approach from the Benassal Formation (Lower Cretaceous, Maestrat Basin, E Spain)- The Aptian shallow-water carbonate platform of the Benassal Formation in eastern Spain (Maestrat Basin) contains facies dominated by scleractinian corals. Corals and coral reefs are widely used as environmental archives; however, the effects of their complex diagenetic evolution are a major factor impacting reliable reconstructions of the environments. This study addresses the environmental signatures (i.e. relative sea-level changes) and burial conditions that controlled the various diagenetic minerals present in the Aptian coral facies. A multiproxy approach including petrographic and geochemical evaluation reveals the diagenetic pathways, fluid compositions and timing accounting for the syn- to post-depositional history of this ancient coral facies. Cc1, which shows low Fe and Mn content and high Na content, precipitated within the primary porosity, alongside the replacement of coral skeletons, in a mixing zone dominated by marine waters. Cement Cc2, characterized by its higher Fe and Mn content and low Na values, precipitated in the mixing zone dominated by meteoric waters related to a relative sea-level drop. Cement Cc3, distinguished by low Fe and Mn content and high Na content, reflects a subsequent phase of increased marine influence, likely associated with a relative sea-level rise. Silica and isolated rhombohedral dolomite crystals formed concurrently in this mixing zone. During intermediate burial, saddle dolomite and cement Cc4 precipitated from high-temperature formation brines. Finally, during uplift, meteoric fluids caused the calcitization of previously formed dolomite rhombohedra. Geologica Acta, vol. 23, no. 20, 17 p. Sethy P.C., Naik A. & Nayak R. (2025).- Geochemical characterization and tectonic significance of the granite gneiss in the Tikiba Granitic Complex, Northwest Odisha, India- The study explores the Precambrian tectonic evolution and the growth of continental crust in the southern region of the North Odisha Singhbhum Craton (NOSC), delving into the potential petrogenetic processes and source characteristics. The Tikiba Granitic Complex (TGC) is in the contact area of the Eastern Ghat Mobile Belt (EGMB) and the NOSC. The present study involves the geochemical characteristics of TGC granitoids and their geochemical variations. Petrographically, the TGC is comprised of alkali feldspar granite, monzogranite to granodiorite. The major element geochemistry reveals calc-alkalic, ferroan to magnesian affinities, and a metaluminous, peraluminous and peralkaline character. It is enriched in LILE and HFSE like Yb, Sm, Zr and Y. The (La/Yb)N values range from 4.0–26.1 and exhibit distinct negative Eu anomalies (Eu/Eu*) (0.4-0.6). This granitic complex shows high ?REE contents of 176-860ppm with variable enrichment in LREE. Both REE and other incompatible element compositions define an A-type affinity. Based on geochemical data, we conclude that these granitoids have probably derived from a predominant crustal source with variable mantle characteristics in a post-orogenic setting. Geologica Acta, vol. 23, no. 21, 42 p. Albert-Villanueva E., Bover-Arnal T., Ferràndez-Cañadell C., GonzÁlez L., Esteban M. & Salas R. (2025).- Lower Miocene carbonate platforms of the Falcòn Basin (NW Venezuela) compared to the offshore Perla Field reservoir- Based on field data collected in the northern and southern Falcòn Basin (Venezuela), this paper presents a comprehensive monograph that reports on the age and sedimentary evolution of the San Luis and Churuguara formations. It integrates platform-to-basin depositional models through facies distribution and stratal architecture, a palaeogeographic reconstruction, larger foraminifera biostratigraphy, sequence stratigraphy, and the evaluation of the accommodation history. An additional aim is to compare these onshore mixed carbonate-siliciclastic systems with the Oligo-Miocene subsurface carbonate reservoir of Perla in offshore Gulf of Venezuela, considered the largest gas discovery ever in Latin America. The carbonate platforms studied formed as a result of a second-order transgressive event, and corresponded to distally steepened or undifferentiated ramps dominated by coralline algae and larger foraminifera. The presence of Miosorites americanus and Annulosorites spiralis dates the San Luis and Churuguara formations to the Early Miocene. The miogypsinid associations identified further distinguish between the Aquitanian and Burdigalian stages. The San Luis and Churuguara platforms exhibit a general aggradational trend, while the Perla carbonates have been interpreted as a retrogradational unit. Third-order T-R sequences interpreted from these successions were mainly influenced by differential tectonic subsidence. Additionally, major regional regressive events in the Falcòn Basin and Perla Field coincide with global glaciations, highlighting the influence of eustatic changes. However, the Perla reservoir exhibits significant differences in thickness, spatial extent and sedimentary architecture compared to the onshore equivalents. The differences arise from variations in tectonic and palaeotopographic settings, accommodation and sedimentation rates and influence of adjacent siliciclastic systems. As a result, predicting the sedimentary architecture of carbonate reservoirs in this region using a generalized depositional model has proven to be challenging. Geologica Acta, vol. 23, no. 22, 18 p. Canales-García I., Pérez-Cruz G., Orozco-AlmazÁn L. & Urrutia-Fucugauchi J. (2025).- Basement structural controls on the Jurassic sedimentary sequences in the Tampico-Misantla Basin, northeastern Mexico from a 3-D seismic reflection study- The Tampico-Misantla basin is a mature oil rich province in northeastern Mexico with large unconventional oil plays currently being explored. Results of a 3-D seismic reflection study of the basement and Jurassic sequences are used to analyze the basement influence on the sedimentary sequence and basin structure, by mapping the seismo-stratigraphic horizons for the (Tithonian) Pimienta, (Kimmeridigian) Taman/San Andrés, (Oxfordian) Santiago, (Callovian) Tepexic/Huehuetepec and (Bathonian) Cahuasas formations. The basement is characterized by stepped NW-SE tilted blocks of horst and grabens. The horst sections to the northeast are found at depths of 3,200m and get deeper up to 6,500m to the southwest. Blocks bounded by planar curved high angle normal faults with NW-SE and NE-SW orientation form a Triassic-Jurassic system, with two major pulses during the Bajocian and Oxfordian. Vertical slips of main faults are variable and may reach 2,000m. Basement blocks play major roles in defining the depositional processes, with thickness and distribution of overlying sedimentary units. Based on our analysis the Middle Jurassic Cahuasas Formation units were deposited predominantly in continental environments and exhibit large thickness variations. The Callovian Tepexic and Huehuetepec formations were deposited in transitional and shallow marine environments, with relatively uniform thickness and distribution. The Oxfordian to Tithonian Santiago, Taman/San Andres and Pimienta formations were deposited in marine environments, showing multiple fault deformation with tendency of progressively pinching out against the topographic highs. The thickness variation in the Santiago Formation suggests episodes of increasing subsidence and fault reactivation. The seismic volume models constraint the basement structures and Jurassic sequence stratigraphy in the Tampico-Misantla basin Geologica Acta, vol. 23, no. 23, 27 p. Mutlu S. (2025).- Tectonic geomorphology of theSemdinli Yüksekova Fault Zone (SYFZ) and Baskale Fault Zone (BFZ) (SE Turkey): Evidence from morphometric analysis and neotectonic structures- The collision between the Arabian and Eurasian plates resulted in the uplift of The East Anatolian Province of Shortening (EAPS). Within the EAPS, the faulting mechanism is characterized by right- and left-lateral strike-slip faults, thrust faults, and a relatively limited occurrence of normal faults. When examined on a regional tectonic scale, about 70% of the deformation of the EAPS is controlled by strike-slip faults. However, the proportion of right- and left-lateral strike-slip faults contributing to this deformation remains unknown and is poorly understood. Two adjacent fault zones located at the southeastern margin of EAPS were selected to investigate the deformation associated with right- and left-lateral strike-slip mechanisms using morphometric analysis along with field observations. These fault zones are the Semdinli Yüksekova Fault Zone (SYFZ), which exhibits right lateral deformation and is defined as a seismic gap, and the Baskale Fault Zone (BFZ), which has a left lateral deformation and that caused a destructive earthquake in (23/02/2020, Mw: 5.9). To elucidate the relative degree of active tectonics of both fault zones, morphometric analysesis were applied alongside field surveys. An adaptation of the relative active tectonic Index (Iat) was computed for Baskale and Yüksekova-Semdinli sub-basins in the region. The results supported by field observations indicate that the drainage basins are largely controlled by fault activity and exhibit a youthful topographic character. Moreover, quantitative data derived from the Index of Active Tectonics (IAT) reveal that the SYFZ demonstrates very high tectonic activity, while the BFZ displays moderate levels of active tectonic deformation. These numerical findings, which indicate ongoing deformation, are further corroborated by field observations that clearly reflect the surface expressions of tectonic activity. Geologica Acta, vol. 23, no. 24, 17 p. Machek M., RoxerovÁ Z., Kusbach V., KucerÁkovÁ M. & Vratislav S. (2025).- Evolution of microstructure and texture in a small-scale high-temperature marble shear zone from the Ossa-Morena Zone (Iberian Variscan Belt)- This research investigates microstructures and crystallographic preferred orientation in a high-temperature Variscan small-scale shear zone within marbles from the south-western Ossa-Morena Zone (Iberian Variscan Belt). Quantitative microstructural analysis, and crystallographic preferred orientation data obtained via neutron diffraction and electron backscatter diffraction mapping allowed to infer the deformation mechanisms and active slip systems. The primary S2 foliation, formed during the Variscan D2 deformation phase, is characterized by high-temperature grain boundary migration recrystallization mechanism, coupled with intracrystalline slip on the c(0001)<-12-10> system. Ductile shear zone development, marked by a coarse-grained microstructure with pronounced shape preferred orientation parallel to the shear plane, involved both high-temperature grain boundary migration recrystallization and intracrystalline dislocation creep, accommodated by f{-1012} <10-11> and r{10-14} <-2021> slip systems. These results indicate a high-temperature extensional origin for the S2 foliation and the shear zone formation during the D2 deformation event of the Ossa-Morena Zone, linked to thermal effects from the Beja Igneous Complex plutonism and regional partial melting driven by asthenosphere upwelling (~353–335Ma). Geologica Acta, vol. 23, no. 25, 19 p. Oliva-Urcia B., RomÁn-Berdiel T., Torres-Lòpez S. & Villalaín J.J. (2025).-Shortening-controlled magnetic fabrics in the Alpine thrust front of the Cantabrian Mountains- The magnetic fabrics (or anisotropy of magnetic susceptibility) are used in this work to decipher thrust kinematics in the Cantabrian Mountains (western Pyrenees). Most of the analyzed rocks (total of 51 sites) correspond to the Mesozoic rifting and post-rifting stages (Triassic, Jurassic and Cretaceous) rocks in the Asturian-Leonese region, whose alpine structure is controlled by south-verging, basement-involved thrusts. Paramagnetic minerals are scarce as shown by the thermomagnetic analyses, and maghemite/hematite predominates as carriers of the magnetic fabric. The rifting phase is recorded by a primary fabric that remained unaltered during the alpine contraction, whereas the magnetic fabric of the Albian and Upper Cretaceous rocks show a reorientation due to contraction, which is better recorded in the fine grained continental facies of the Utrillas Group than in the carbonatic rocks overlying them. In the contractional fabrics found, the magnetic lineation delineates the thrust front enhancing the changes of the local shortening direction along strike. This reinforces the hypothesis of a strong control of the thrust direction by inherited structures. Geologica Acta, vol. 23, no. 26, 11 p. Ruh J.B. (2025).-Grain-size reduction diminishes the impact of viscous shear heating in lithospheric mantle shear zones- Active plate tectonics require weak boundaries that accommodate large strain over long time periods. Many studies argue that shear heating converted from dissipated mechanical energy may be responsible for the necessary weakening in the mantle lithosphere to initiate and sustain weak plate boundaries. Here, I present simple numerical simulations of pure olivine systems with a composite diffusion-dislocation creep rheology coupled to a self-consistent grain-size evolution model to investigate the effect of grain-size reduction on shear heating. Results demonstrate that the weakening related to grain-size reduction and the activation of grain-size-sensitive diffusion creep undermines the importance of shear heating in mantle shear zones. Depending on shear zone width, shear velocity and apparent stresses, estimations on ideal shear zone widths and viscosities are presented. | |
Click HERE for the next ten titles. |
© 2006-2025 GeoSciences e-Journals
