Abstract in English:Abstract The organic geochemistry of organic-rich facies including shales, marls, and carbonates (Assistência Member) of the Irati Formation, Paraná Basin, Brazil, were analyzed to evaluate the thermal effects of igneous intrusions upon the kerogen present in these facies. Total organic carbon (TOC) content and hydrocarbon source potential (S2) of the Irati source rocks range from 0.03 to 20.4% and 0.01 to 112.1 mg HC/g rock, respectively, indicating excellent potential as a source for hydrocarbon generation. Hydrogen index (HI) values reveal that the kerogen is predominantly type I (HI: up to 892.6 mg HC/g TOC) and, therefore, an oil source, except for samples having low TOC content due to severe maturation caused by the heat from diabase intrusions. The thickness of igneous intrusions in the 64 wells investigated in this study ranged from 2 to 231 m. They clearly had a major impact on TOC, HI, and S2 values, which decrease in the vicinity of intrusions, indicating a gradual increase in maturation toward the igneous body. In wells without the influence of igneous intrusions, Tmax values of Rock-Eval pyrolysis and %Ro indicate that the organic matter is immature for the generation of hydrocarbons.
Abstract in English:Abstract Mass movements naturally occurring along slopes, especially after heavy rains, cause heavy losses of materials and life and affect highways, pipelines, and bridges, among other structures. Among the most common mass movements, landslides and subsequent mass flows especially stand out. Remote sensing and geoprocessing techniques are useful tools for geological and geotechnical analysis, here applied to characterization of mass movements from the analysis of the influence of slopes, lineaments or fractures, and geological-geotechnical at the junction of the Curral homocline and the Moeda syncline, Quadrilátero Ferrífero, Southeast Brazil. The area consists of Archean granitic-gneissic basement and the greenstone belt sequence of the Rio das Velhas Supergroup with Proterozoic metasedimentary rocks of the Minas Supergroup covering these two older successions. Based on satellite images, a preliminary map of lineaments was created, and fractures were grouped according to their direction before being rasterized. Each lineament group was associated with a previously known geological and/or structural feature. Geological units mapped in the area were numerically classified according to their mass movement susceptibility and related geotechnical units. Using a Shuttle Radar Topography Mission (SRTM) image, a terrain slope raster was generated. The objective was to obtain a raster indicating low and high probabilities of mass movements. Four sets of lineaments were identified: the first set associated with the northeast strike stratification of the rocks of the Moeda Formation; the second set related to eastward-trending warp axis lineations; the third set related to the southeast trend of stretch lineaments or mafic dykes directions; and the fourth set evidently associated with the watershed pattern of the Curral Range crest, perpendicular to the strike of the rocks in the area. Simulations with different combinations of parameters produced eight scenarios were generated were constructed to classify the area as to its susceptibility to rupture through changes in slope, lithotype, and lineament density; the first four (1 to 4) were classified by using the slope percentage raster without interval reclassification, so its values in the cartographic algebra were not grouped. The last for (5 to 8), consider the slope percentage classified. This methodology successfully combines mathematical models for predicting mass movements and determining areas in the urban landscape most susceptible to these phenomena. As such, it comprises a useful tool for government planning of preventative actions for areas of high risk.
Abstract in English:Abstract The Rio dos Remédios Group comprises a supracrustal sequence that occupies the base of the Espinhaço Supergroup, São Francisco Craton, Brazil. Its basal formation, Novo Horizonte, crops out in the Paramirim region mainly as metavolcanic rocks that represent one of the fewer occurrences of blue quartz phenocrysts in South America. Their mineralogy consists of quartz and K-feldspar phenocrysts, whereas biotite, muscovite, fluorite, allanite, chlorite, sericite, zircon, and opaque phases occur immersed in a quartz-feldspar-rich groundmass. Such heterogeneous composition is also supported by x-ray diffraction and chemical data. Electron probe microanalysis in some samples revealed the presence of two distinct groups of biotite (magmatic and neoformed), in addition to the presence of iron-rich white mica and almost pure orthoclase feldspar. Our data suggest that the studied metavolcanic rocks have maintained their magmatic characteristics, which were progressively overprinted by hydrothermal fluids and ductile-to-brittle deformation. The magmatic mineralogy is akin to strongly peraluminous and alkaline magmas, common in anorogenic settings – a fertile site for the origin of blue quartz-bearing rocks worldwide.
Abstract in English:Abstract The Geological Strength Index (GSI) system is the basis of parameters used in the Hoek-Brown failure criterion for rock mass strength estimation. The author tested this system and here suggests a modified GSI called Geological Strength Index-slope (GSIslope). The modified system combines two different existing approaches: the GSI system and Slope Mass Rating (SMR). The purpose of GSIslope is to allow engineering geologists to quickly evaluate the stability of natural and excavated slopes or open-pit mining in the field. GSIslope is computed by subtracting a constant value of 10 and the multiplication of adjustment factors for discontinuity orientation and slope (F1, F2, and F3, based on the parallelism of discontinuity and slope, discontinuity dip angle, and the difference between the inclination angle of discontinuity and slope) from GSI, and adding field groundwater rating to it. Modified curves are also proposed in this work to determine the accurate ratings of the adjustment factors. The results of this work are compared to the values obtained from equations of continuous-SMR and SMR-value itself for both the adjustment factors and GSIslope values. The comparison showed that the proposed curves and GSIslope equation are valid and easy to use for estimating the adjustment factors’ ratings and GSIslope value.
Abstract in English:Abstract Sinter overlying the first quartz andesite flow at the base of the Serra Geral Group is a surface manifestation of intense hydrothermal processes operating in the Paraná Basin during the Early Cretaceous. The coarse quartz sinter from western Rio Grande do Sul state was studied in satellite images, field surveying, optical petrography, scanning electron microscopy, and electron probe microanalyses, including backscattered electron images and chemical analyses of rocks. Quartz forms large crystals (10 cm) because it was either deposited in a dilute aqueous solution or recrystallized from fine-grained sinter. Well-crystallized chamosite — an iron aluminosilicate (Fe-chlorite) — occurs in quartz crystals, partly associated with fractures. The composition of chamosite is akin to that in ore deposit associations. Noble metals in two sinter samples are present in concentrations of 0.1 ppm Ag and 15 ppb Au. The contents of Ba, Bi, Cu, Mo, S, and W are low but are significant. The present description of sinter quartz signals the presence of a major paleo-hotspring field in the Fronteira Oeste Rift, Rio Grande do Sul, Brazil, that mertis further study to fully characterize the extent and metallogenetic endowment (Au-Ag-Cu) of the epithermal province.
Abstract in English:Abstract Seismic data, borehole image logs, and conventional well logs were used to investigate the distribution and characteristics of paleokarst features in the Aptian carbonates of the Barra Velha Formation in a pilot area of the Santos Basin, Brazil. Multiple seismic attributes were used to enhance details on the seismic data and highlight key seismic parameters including strata deformation and geometry, continuity of seismic events, and fault patterns. The study found that karst structures are controlled by faults and fractures along structural highs, which served as a conduit for the flow of dynamic fluids that dissolved the carbonate materials. Several closed, circular depressions and bright spots identified in the northeastern portion of the study area represent possible sinkhole structures. Epigenic and hypogenic processes due to the action of meteoric water, hydrothermal activity, and intra-formation acidity along regional unconformities in the Barremian-Aptian may have been responsible for the dissolution. Limitations of this study are related to the difficulty of integrating multiple datasets with various scales. However, the higher confidence for the occurrence of the karst features is provided by borehole images at the sub-seismic scale. The findings of this study hold significant relevance for the strategic planning of energy development and carbon sequestration initiatives in the Brazilian continental margins, thereby aiding in informed decision-making.
Abstract in English:Abstract Disarticulated shells of three bivalve mollusk species (Anomalocardia brasiliana, Codakia orbicularis, and Divaricella quadrisulcata) were experimentally tested in laboratory flumes to determine the threshold of motion and final orientation of the valves. A total of 150 current flow experiments were conducted on single shells resting on a fixed sand bed. This study demonstrated that shells in the convex-up position are more resistant to flow when the umbo is pointing downstream rather than upstream. Moreover, species with higher frontal areas were more likely to be entrained at lower flow velocities. Results of dimensionless shear stress exhibited values far below the threshold of grains movement for beds of uniform roughness (Shields curve). It was observed that circular shells in convex-up positions were mostly orientated with the umbo pointing downstream. Conversely, elliptical shells in convex-up position tended to align their longer axis parallel to the flow with the posterior side of the valve pointing downstream. These results are not only directly applicable in interpretations of incipient shell motions and in paleocurrent analyses from field and sample data, but also support construction of accurate geological models.
Abstract in English:Abstract Central Brazil hosts Paleo-Mesoproterozoic A-type granitic suites related to the Goiás Tin Province (GTP) that contain Sn (±W, Nb-Ta, REE) mineralization associated with greisen, veins, and small pegmatite bodies. The Pedra Branca granite massif (1.77–1.74 Ga) is the main representative of GTP, marked by important cassiterite (±wolframite) contents. The cassiterite contains SnO2 = 96–100 wt. %, with the sum FeOtotal, TiO2, WO3, Ta2O5, Nb2O5, In2O3, and UO2 content below 4 wt.%, while wolframite contains WO3 = 71.5–74.5 wt.%, FeOtotal = 14.3–17.4 wt.% and MnO = 6.3–9.9 wt.%, as well as Sn, Ca, Ti, Ta, Nb, Pb, In, and U as trace elements. The δ18O data on the quartz-cassiterite pair (quartz = 9.4–10.4‰, cassiterite = 2.6–2.9 ‰) from greisen reveal a magmatic-hydrothermal signature with calculated crystallization temperatures between 410 and 485°C. However, during the Neoproterozoic Brasiliano/Pan-African Orogeny (800–500 Ma), all lithologies and ore sites were subjected to flattening, fragmentation, and mylonitic deformation. Fluid inclusion data revealed the presence of low-salinity aqueous solutions with homogenization temperatures between 215 and 100°C related to Neoproterozoic deformation. Finally, during the Phanerozoic, prolonged erosive produced Sn (±W)-rich alluvium around the Pedra Branca granitic massif.
Abstract in English:Abstract Benthic foraminifera, ostracods and pteropods are reliable paleoenvironmental indicators in Quaternary deposits. However, in the Ceará Basin, on the Brazilian Equatorial Margin, these microfossils are poorly studied. This paper investigates environmental changes during the Pleistocene–Holocene transition in the Icaraí subbasin based on micropaleontological analysis of the core ANP 1011. Seventy-four taxa of benthic foraminifera, represented predominantly by Globocassidulina, Uvigerina, Pyrgo and Melonis, have been identified. The ostracod assemblages are composed mainly by the families Macrocyprididae, Cytheruridae, Trachyleberididae, Pontocyprididae and Krithidae, of which the genus Krithe was the most abundant. The composition of the ostracod assemblages identified in this study area differs somewhat from other regions of the Brazilian Margin. The assemblages of foraminifera and ostracod characterize a typical bathyal paleoenvironment. The occurrence of pteropods and dominance of epifaunal foraminifera taxa, mainly Pyrgo sp. and Miliolinella sp. in the lower portion of the core (Pleistocene), indicates higher phytodetritus input and oxygen concentration. A conspicuous environmental change was observed in the upper portion of the core, which corresponds to the Holocene, where the increase of infaunal foraminifera (e.g., Uvigerina, Globocassidulina and Melonis) suggests reduction in the organic matter input and, probably, increased bacterial density and depletion in dissolved oxygen in the sediment.
Abstract in English:Abstract Stromatolites are laminated biosedimentary structures of great importance for paleobiological, paleoecological, and paleoenvironmental analyses, mainly in Precambrian rocks. Their value is related to the glimpse of past life recorded in their lamination, fabric, and, eventually, due to the preservation of organic matter, including microfossils, and because their deposition is directly influenced by environmental conditions. Although stromatolites are widely described in microscopic scale, there is a lack of standardization of their nomenclature, precluding better paleoenvironmental and paleobiological interpretations. In this study, we propose a guide for the microscopic analysis of fossil stromatolites and, possibly, thrombolites, and provide a review of specialized literature and the bibliometric context of main terms. The goal is to contribute to the improvement of their application through systematization of microscopic data, in the face of novel paleoecological and paleobiological approaches and for astrobiological prospection for microbialites in therock record of Mars.