Search

Large and hot intrusions may imprint relatively wide aureoles of ultra‐high temperature (UHT >900°C) contact metamorphism even on granulite facies rocks at mid‐crustal levels, especially on those still somewhat hydrated rocks like biotite‐rich paragneisses. After reaching UHT conditions, even followed by rapid cooling, such contact aureoles record reaction textures and newly formed mineral assemblages. Near intrusive contacts, high‐T to UHT contact aureoles present highly recrystallized hornfelsic rocks exhibiting conchoidal fracture that may also preserve inherited features. This paper describes a contact metamorphic aureole developed under high to ultra‐high temperatures at intermediate P conditions, imposed by a mafic intrusion on granulite facies paragneiss. In the Espírito Santo State (SE Brazil), many post‐collisional intrusions (the 525–480 Ma G5 supersuite) occur in the back‐arc region of the Araçuaí orogen. Among them, a greatly homogeneous norite forms a large intrusion hosted by Al‐rich migmatitic paragneisses previously metamorphosed to granulite facies during the collisional stage. Zircon U‐Pb (SHRIMP) data yield the age of 523 ± 7 Ma for the norite crystallization. This intrusion imprinted a contact metamorphism aureole on the migmatitic paragneiss, forming Al‐rich hornfelses free of biotite, near the intrusive contact, followed by recrystallized garnet‐rich rocks with relict gneissic fabrics progressively enriched in biotite towards the distal aureole. The Al‐rich hornfelses are very tenacious, massive, fine‐grained rocks with conchoidal fracture, resinous lustre, intergrowth of feldspars, simplectites of cordierite and hercynite, and show an increase calcium content in plagioclase towards the intrusion. The hornfelses comprise two mineral assemblages: (I) plagioclase–garnet–orthopyroxene–quartz and (II) plagioclase–garnet–quartz–cordierite–sillimanite–hercynite–K‐feldspar. Calculated temperatures for host rocks range from ~1,010°C, for a hornfels collected at 1 m from the intrusive contact, to ~732°C, for a paragneiss located at 262 m from the intrusion. Using Theriak‐Domino software, phase‐equilibrium modelling for the hornfels assemblage (II) provides temperatures between 970 and 1,010°C, at pressures from 5.6 to 6.0 kbar, characterizing UHT contact metamorphism at mid‐crustal level (21–22 km depth). Neocrystallized zircons with soccer ball morphology aged at 520 ± 19 Ma, and P–T conditions of 845°C at 6.35 kbar for a recrystallized paragneiss record the contact metamorphism at least up to 250 m distance from the intrusion. The presented data constrain a P–T–t peak ~5.8 kbar – 990°C – 520 Ma for the contact metamorphism while the P–T–t peak at 7.5 kbar – 785°C – 560 Ma was related to the collisional metamorphism of regional paragneisses in the literature. Therefore, the pressure–time relation between those metamorphic peaks gives a pressure difference of 1.7 kbar (~6.3 km thickness of continental crust) over 40 Ma, corresponding to low decompression (42.5 bar/Ma) and exhumation (157 m/Ma) rates from the collisional peak to the post‐collisional stage, in the studied region. [ABSTRACT FROM AUTHOR]

Coquinas are significant producing facies in the pre-salt reservoirs of Brazil and Angola. This paper describes an analogue model study to help understand the reservoir characterization of coquina reservoirs. Porosity and permeability measured in 39 core plugs from a single coquina bed in a Lower Cretaceous outcrop in NE Brazil have been combined in a threedimensional geological model for use in reservoir simulation. A training image derived from the geometry of a Holocene coquina analogue inWestern Australia controlled the distribution of the bed-scale petrophysical properties from the outcrop. A synthetic well test showed that the effective permeability of the system lies between the geometrical and harmonic averages of the plug data in the layer. There is also moderate flow anisotropy, with preferential fluid flow aligned along the beach ridges. This paper demonstrates how the combination of outcrop data and an appropriate modern environment might be used to improve our understanding of the behaviour of coquina reservoirs and to guide future reservoir studies. [ABSTRACT FROM AUTHOR]

This paper presents data on the fragments of ancient sedimentary deposits placed around the Piedade and Casa Branca streams, important right bank tributaries of the Paraopeba River. From these data, it was possible to establish a picture regarding the paleomorphology and paleodrainage of the area. The methodological steps adopted consisted of the definition of sedimentary facies, as well as the establishment of their sedimentary provenance (U/Pb method in detrital zircons) and deposition age (OSL in quartz). Through the facies analyses, it was possible to show channel facies (Gt) associated with debris flows (Gmm) and sieve deposits (Gh). In the sedimentary provenance stage, spectra of ages congruent with those found in literature for the Moeda and Cauê Formations were identified, suggesting that the Moeda mountain range is a source area. We also analyzed data on the sediments of the current bed, which indicate the same provenance, showing that the current courses constitute an embedded analog of the paleochannels. The data herein obtained corroborate the hypothesis that the eastern periphery of the Bonfim Complex was dominated by a system of alluvial fans between 9.5 ± 1 and 15.5 ± 1.6 ka before present, whose remnants are preserved on the current hilltops and mid slopes of the basement landscape. • The ancient deposits are fragments of alluvial fans that dominated the region. • The Moeda and Cauê Formations are the main source of your sediments. • The current fluvial courses constitute an embedded analogue of the paleochannels. • The fluvial incision in blocks surrounded by faults preserved adjacent deposits. [ABSTRACT FROM AUTHOR]

Grounding-line fans and their associated supercritical-flow bedforms are still poorly recognized in the upper Paleozoic glacial record. In order to fill this gap and to explore the products of the transition between upper and lower flow regimes, we examined a gravel-rich succession composed of conglomerates, sandstones, and diamictites deposited during the Late Paleozoic Ice Age (LPIA) in southeastern Paraná Basin, Brazil. To investigate this succession, we described the main bedforms and associated facies in laterally persistent outcrops and measured several vertical logs that were subsequently merged into a composite log. From base to top, the succession is composed of a muddy glacitectonized substrate, and a subglacial to ice-marginal facies association including a basal, boulder-rich, subglacial lag of local extent, and submarine deposits belonging to a grounding-line fan. In the grounding-line fan association, we identified bedforms related to concentrated flows that represent the complete spectrum of flow regime transition from supercritical to subcritical in a waning wall jet model. The proximal zone was dominated by highly turbulent supercritical flows, resulting in highly scoured conglomerates and deposition of unstable and stable antidunes. A transitional zone under transcritical flow conditions resulted in deposits represented by humpback dunes. In the distal zone, the flow behaved as subcritical, resulting in the deposition of dunes and climbing ripples. Debris flows were also active. These cohesive flows underwent dilution by water entrainment and erosion of the head, leading to the generation of turbulent portions in the head of the debris flows. This bipartite behavior of the flows resulted in thin sandstones capping diamictites, which compose the fabric of the glacigenic debrites observed in the study area. The described facies and bedforms point to the existence of a grounded ice margin at the final stages of the LPIA in the SE portion of the Paraná Basin. • The first paper proposing a detailed investigation for the characterization of upper-flow regime bedforms in LPIA deposits. • Froude supercritical flow bedforms deposited under glacimarine wall jet density flows. • The grounding-line fan exhibits a distinct bimodal sedimentation pattern. • Debris flow dilution leads to the generation of turbidity currents. • Existence of a grounded ice margin at the final stages of the LPIA in the SE Paraná Basin. [ABSTRACT FROM AUTHOR]

Banded Iron Formations (BIF) developed strongly during the Neoarchean – Paleoproterozoic Eras linked with global environmental processes such as the Great Oxidation Event (GOE) and changes in geodynamics, sea level, geobiology, hydrothermal systems, and sediment deposition conditions. BIF development and distribution worldwide are also related to supercontinent cycles, being a powerful tool to understand the evolution of plate tectonics. In this paper, we used field-based geological-structural data, petrography, X-ray diffraction, and lithogeochemical analyses to perform the first characterization of Banded Iron Formations at the Luis Alves Craton (LAC), Southern Brazil. The LAC BIF are classified as oxide and silicate facies, both with granoblastic texture. Oxide facies consists of quartz and magnetite. Besides these minerals, silicate facies contains olivine and ferrosilite, evidencing granulite metamorphism. Based on major elements oxide facies has a hydrothermal signature, with low content of Al 2 O 3 and TiO 2 , whereas silicate facies register a continental sediment contribution been more enriched in these elements, MgO, MnO, and CaO. Trace-elements also indicate the main source for both silicate and oxide facies, which is seawater with a major influence of low-T hydrothermal sources, besides continental sedimentation sources. The Ce/Ce × anomaly is true negative, representing oxidizing waters, which also may represent a shallow water column depth. The depositional environment is inferred as a shallow, restricted basin. Oxide facies represents sedimentation closer to hydrothermal vents and with weak influence from continental clastic sedimentation, while silicate facies represent sedimentation closer to an adjoining continental landmass. Oxide facies has more Algoma-like characteristics, such as higher Fe and Eu contents, Sm/Yb and Eu/Sm typical of higher hydrothermal fluid inputs, and low siliciclastic contribution. On the other hand, silicate facies resembles Superior-type BIF, with higher siliciclastic contribution, negative Ce/Ce × anomalies, and Eu/Eu* <1.8. In summary, we propose the LAC BIF is representative of a transitional environment from Algoma-to Superior-type. The first depositions of the LAC BIFs may have developed within an Algoma-like environment during the Neoarchean. The waning tectonic and volcanic activity during the Siderian shifted to a platformal environment, with shallower waters that may induce the deposition of carbonates typical of Superior-type deposits. The depositional basin may have been closed by the Rhyacian and those sediments were then pushed down to the lower crust and underwent granulite facies metamorphism. • Oxide and silicate facies BIFs were recognized in the Luis Alves Craton. • These BIFs register granulite metamorphism, having ferrosilite and olivine in their composition. • Through the geochemical data, the depositional paleoenvironment was recognized. • The sedimentary sources of BIFs are both hydrothermal and continental. • These BIFs have characteristics of both Algoma and Superior type deposits. [ABSTRACT FROM AUTHOR]

This paper presents Nd–Hf isotopic data for granitic rocks of both the Matupá (∼1859–1881 Ma) and Teles Pires (∼1790–1793 Ma) intrusive suites located in the south-central region of the Amazonian Craton, central Brazil. The Matupá Intrusive Suite consists of two facies of I-type granites with calc-alkaline affinity emplaced in volcanic arc settings. Facies 1 is composed of syenogranite and monzogranite, and facies 2 is composed of granodiorite. The Teles Pires Intrusive Suite varies from syenogranite to monzogranite with calc-alkaline signature that is consistent with mature magmatic arc settings. The Matupá Intrusive Suite derives from a source with Hf model ages ranging between 2.4–1.91 Ga and εHf (t) between +5.71 and −5.50. Zircon grains in the Matupá Intrusive Suite that are correlatable with the Creporizão Intrusive Suite show Hf model ages of between 2.0 to 2.4, and εHf (t) of −3.76 to + 4.10, while the zircon grains correlatable with the Tropas Intrusive Suite show Hf model ages of 2.1 to 2.4 Ga and εHf(t) of −3.93 to + 2.50. Both facies of the Matupá Intrusive Suite provide εNd (t) values varying from −0.61 to −1.66. The wide range of εHf (t) values (from −5.50 to +5.71) in contrast to the uniformly negative εNd (t) values (from −1.66 to −0.61) of the Matupá Intrusive Suite suggest that Nd–Hf decoupling took place in the magma source during to magma mixing processes. Nd–Hf isotope analyses of the Teles Pires Intrusive Suite reveal a source with Hf model age of between 2.3 and 1.9 Ga. This is indicative of mantle contribution and crustal reworking with values of εHf (t) ranging from +7.17 to −3.3, while other values suggest a more crustal contribution with εNd (t) from −0.07 to −0.71. Crustal contribution in the genesis and magmatic evolution of the Teles Pires Intrusive Suite is also evidenced due to the presence zircon grains inherited from Matupá Intrusive Suite. Zircon inheritance record a distant parental source coeval with the Matupá Intrusive Suite. This paper proposes that the origin and evolution of the Matupá Intrusive Suite is linked to the Cuiú-Cuiú Magmatic Arc while the origin and evolution of the Teles Pires Intrusive Suite is linked to the Juruena Magmatic Arc. Another hypothesis is that both suites were generated and evolved from a single magmatic arc. The Juruena Magmatic Arc is divided into two geographic segments. An eastern segment with a magmatic evolution marked by the interaction of crustal components during the orogenic phase of the Cuiú-Cuiú Magmatic Arc (Tapajós Domain of the Tapajós-Parimá Tectonic Province), and a western segment whose magmatic evolution is marked by rebound of the Juruena Magmatic Arc. • Amazonian Craton. • Teles Pires and Matupá intrusive suites. • Paleoproterozoic granites. • Nd–Hf isotope. • Magmatic arc. [ABSTRACT FROM AUTHOR]

Abstract Accommodation rates in rift basins are mainly controlled by tectonics. However, during early rift stages, normal fault activity is dispersed and incipient, which makes it difficult to recognize the effects of early crustal extension. The sedimentary stacking pattern of rift-related depositional environments can be decisive to address this question. This paper aimed to evaluate the stratigraphic record of the Tithonian-Berriasian (?) Serraria Formation (Sergipe-Alagoas Basin, Brazil), which was deposited in the Afro-Brazilian Depression, as a result of the early extension of West Gondwana. In total, 14 lithofacies were described, interpreted and arranged into 7 facies associations. At the base of the formation, bars and sand sheets were deposited by low discharge braided channels, which were deposited on top of lacustrine mudstones of the Bananeiras Formation. There is a change in stratigraphic architecture in the middle of the section, where fluvial channels become deeper due to increasing discharge, promoting an increase in grain size of the transported bedload and greater rates of coarse sand and gravel bar stacking. Rare crevasse channels and restricted aeolian dune fields occur adjacent to braided channel belts. Upwards, the Serraria Formation becomes dominated by delta front and delta plain deposits. The fluvial and aeolian facies associations represent a progressive decrease in accommodation, while the upper fluvio-deltaic interval means an increase in accommodation, pointing to the early rift stage climax. The fluvial "Caioba" sandstone on top of the Serraria Formation indicates reestablishment of fluvial drainage either due to decreasing tectonism or due to low accommodation at the time of deposition. Thus, the sedimentary stacking pattern of Serraria Formation represents higher frequency cycles within an early rift tectonic system tract. [ABSTRACT FROM AUTHOR]

The Southern Brasília Orogen is the result of Ediacaran continental collision between the active margin of the Paranapanema paleocontinent and the passive margin of the São Francisco paleocontinent. The active margin was deformed into the Socorro-Guaxupé Nappe and the associated Embu Domain. This paper presents new U–Pb zircon data and geothermobarometric estimates that allow to reconstruct the metamorphic evolution in these upper plate units. A pre-collisional metamorphism M1 with an age peak in metamorphic rims of zircon grains in the range 680–630 Ma is related to the development of the Socorro-Guaxupé magmatic arc with batholiths dated in a similar time span, 690 - 630 Ma. This metamorphism reached medium pressure granulite facies, now exposed by the tilted uplift and erosion due to the collision. Another peak is detected in metamorphic rims of zircon grains, in the range 630–590 Ma, labeled M2, related to the collision and corresponds to the main metamorphism in the lower plate and accretionary wedge-like nappe stack, underlying the Socorro-Guaxupé Nappe. This nappe stack was affected by greenschist facies metamorphism in the autochthonous underlying craton, grading to high pressure granulite facies in the upper nappe, with local occurrences of retroeclogite lenses. In the Socorro-Guaxupé Nappe and Embu Domain this main metamorphism (M2) is characterized by three metamorphic zones decreasing in grade with distance from the suture: a granulite facies orthopyroxene bearing zone; a high temperature amphibolite facies zone and a lower amphibolite facies zone. Geothermobarometric results in mineral associations related to M2 show temperatures decreasing from about 770 °C to ∼620 °C and pressures decreasing from about 9 kbar close to the suture to about 5 kbar in the southern and western parts of the studied region. On the upper Paranapanema plate, relative to the Socorro-Guaxupé Nappe and Embu Domain units, at the base of the plate, an older metamorphism (M1) is verified with temperatures between 960 and 890 °C and pressures between 11 and 9 kbar, reequilibrated at approximately 700 °C with a pressure of about 9 kbar (M2, continental collision). For this reason, it can be assumed that the rim of the Paranapanema plate was exhumed during the continental collision, since the temperature and pressure of the metamorphic peak determined during the continental collision are lower than those determined for the pre-collisional magmatic arc metamorphism. • Active margin in the Paranapanema Paleocontinent. • Metamorphism related to magmatic arc stage. • Exhumation of the upper plate during the Brasilia Meridional continental collision. • Arc metamorphism superimposed by collisional metamorphism of lesser degree. [ABSTRACT FROM AUTHOR]

Large-volume silicic eruptions in large igneous provinces are unique in the geological record because there are no observed analogues. Their facies architecture do not strictly follow the diagnostic features proposed in the literature for rheomorphic ignimbrites and lava flows. The models proposed for their emplacement depend on conditions such as high temperature (> 950 °C) and low viscosity (< 106 Pa s) coupled with unusually high effusion rates. Low-Ti dacitic lavas from the Paraná-Etendeka Large Igneous Province (LIP) have a wide variety of morphologies and lithofacies on both sides of the Atlantic Ocean. A common facies association framework for the Caxias do Sul/Grootberg dacites (outcropping in Namibia and southern Brazil) include the presence of platy and thinly jointed facies with a columnar jointed massive core and an amygdaloidal upper facies, with an absence of basal breccia. In this work, we describe and interpret features observed in the basal portion of these silicic units, focusing on the thinly jointed facies. This basal facies includes what has been termed as "zebra-like banding." Zebra-like banding is an apparent structure that originates from a fracture network which involves horizontal and long first-order fractures, oblique and short second-order fractures, oblique short to long third-order fractures, and intersection zones of fourth-order structures. First-order fractures are related to pure shear, the second- and third-order fractures are Riedel shears, and the fourth-order structures represent angular contacts between different sets containing first-, second-, and third-order fractures. The combination of first-, second-, and third-order fractures evolves to shear lenses. The zebra-banding" is caused by oxidation/reduction halos developing in the host rock during post-emplacement fluid circulation along the fractures, culminating in precipitation of silica polymorphs and zeolite. The fracture system, including all orders of fracture, formed during late to post emplacement stages, and accommodated pure and simple shear in a ductile–brittle basal zone during oscillations in effusion rate. Fracturing of the basal zone, rather than the formation of basal breccia, demonstrates stress accumulation in this part of the flow and helps to explain why the presence of basal breccia is not a diagnostic feature in distinguishing large-volume silicic lava flows from rheomorphic ignimbrites. [ABSTRACT FROM AUTHOR]

The lateral and vertical distribution of sedimentary facies and ichnofacies in lacustrine systems records sensitive climatic variations and rates of subsidence. The Upper Jurassic–Lower Cretaceous lacustrine system of the Pastos Bons Formation in the Parnaíba Basin, northeastern Brazil, is an excellent opportunity to evaluate the depositional history of the post-CAMP thermal subsidence. This subsidence event succeeded the voluminous emplacement of mafic magma linked to the Central Atlantic magmatic province (CAMP), ∼201 Ma, forming a large subsiding area in West Gondwana. In this sense, this paper developed outcrop-based facies and ichnofacies analysis, allowing the interpretation of two depositional phases and four facies associations (FAs): 1) The lower depositional phase comprises lacustrine deposits showing an aggradational stacking pattern. FA1 - central lake records fibrous gypsum interbedded with fossiliferous shale beds, forming shallowing-upward cycles. 2) The upper depositional phase reveals a fluvio-lacustrine succession, forming a progradational stacking pattern. Central lake (FA1) comprises laminated mudstone and sandstone beds hosting a suite representative of Mermia ichnofacies, forming coarsening-upward cycles. Lakeshore (FA2) comprising mudstone beds, limestone lenses, and sandstone beds hosting a suite representative of Scoyenia ichnofacies, forming shallowing-upward cycles; and conglomerate lenses overlaid by sandstone beds hosting a suite representative of Skolithos ichnofacies, forming fining-upward cycles. A braided fluvial system (FA3) comprises conglomerate and sandstone beds, forming fining-upward cycles. Delta front (FA4) comprises mudstone lenses and sandstone beds hosting a suite representative of Skolithos ichnofacies, organized in coarsening-upward cycles. The fluvial discharge and wave activity oxygenated the water lake in the progradational phase, creating chemical and biological conditions that allowed the establishment of the epifaunal and infaunal benthic communities, represented by the well-preserved ichnological record. The decrease of the post-CAMP thermal subsidence rates (accommodation potential) related with the increase of the wet periods to the top of the succession (sedimentary supply) indicate a depositional evolution from underfilled to overfilled lake basin, during Upper Jurassic–Lower Cretaceous, in West Gondwana. [ABSTRACT FROM AUTHOR]

The visualization, analysis and interpretation of sedimentary data can be optimized with techniques such as the Digital Outcrop Models (DOMs) generated with the Structure from Motion – Multi-View Stereo (SfM-MVS) technique. In this paper, to demonstrate the usage of DOMs in the characterization of the depositional architecture of a braided fluvial system, we developed a case study in Jaicós Formation, Parnaíba Basin, NE Brazil, where a high-resolution stratigraphic analysis of a braided fluvial system was performed, and the potentials and limitations of the method were evaluated. This multi-scale reservoir analysis allowed the identification, description and interpretation of facies, bounding surfaces and architectural elements. In addition, it was also possible to predict quantitative attributes of the fluvial system that deposited this sequence and evaluate the potential of this formation as a reservoir. • The usage of Virtual Outcrop Models in High Resolution Stratigraphy. • Analysis of facies, bounding surfaces and architectural elements. • Quantitative estimation of the fluvial system that deposited Jaicós Formation. [ABSTRACT FROM AUTHOR]

The classic fill‐and‐spill model is widely applied to interpret topographic controls on depositional architecture and facies distributions in slope successions with complicated topography. However, this model implies a constant topographic configuration over the lifespan of a turbidite system. In contrast, the impact on patterns of erosion and deposition above dynamic slopes whose topographic configuration varies spatially over time remains poorly investigated. Here, using high‐resolution 3D seismic reflection data and more than 100 wells from a 40 km long stepped slope system (Campos Basin, offshore Brazil), we document the evolution of a sand‐prone turbidite system active during the Oligocene–Miocene transition. This turbidite system was influenced by vertical and lateral deformation, and we propose a new stratigraphic model to explain the resultant depositional architecture. Two depocentres were identified as steps, with channels on the proximal step, and channel–lobe complexes on the distal step, bounded by sediment bypass‐dominated ramps. Lateral stepping of channels on the proximal step, and oblique stacking of the down‐dip lobe complexes, each cut by through‐going channels, indicate multiple fill‐and‐spill cycles. A persistent north‐east‐ward stepping and thickening on the steps are interpreted to reflect lateral tilting of the seafloor driven by salt tectonics. The dynamic substrate prevented the establishment of a single long‐lived conduit across the proximal step, as recorded in systems with fixed topographic configurations. The filling of through‐going channels with mud at the end of each cycle suggests waxing‐to‐waning sediment supply cycles and periods of sand starvation when the lateral tilting dominated and drove avulsion of the feeder channels towards topographic lows. This study demonstrates that subtle dynamic slope deformation punctuated by discrete sediment supply cycles results in complex stratigraphic patterns with multiple phases, and multiple entry and exit points. Repeated cycles of fill‐and‐spill, tilt‐and‐repeat are likely to be present in other stepped slope systems. [ABSTRACT FROM AUTHOR]

The historical variation of sedimentation rate was investigated in five cores collected from the Itanhaém watershed main rivers, the second largest coastal watershed of the São Paulo state, Brazil, using the lead-210 radioisotope as a geochronometer. The main characteristics of the rivers' waters and sediments were determined in order to support the geochronological analysis results and associate sediments with possible source areas. In this context, the fluvial waters' general classification indicated the facies sulfated or chlorinated sodium in the winter and summer seasons, except for the Branco river waters in summer, which were classified as calcium or magnesium bicarbonate. A longitudinal salinity gradient was found in the downstream river courses, under greater marine influence, with the ions Cl−, Na+, SO42+, Mg2+, Ca2+, and K+ being most common in the waters. Silica is the predominant constituent in the sediment cores and is inversely related to the organic matter (OM) presence. Inverse correlations were also found between silica and other constituents. The watershed sedimentation rates were determined in the range of 0.31 up to 3.97 g/cm2/year and 0.30 up to 3.40 cm/year, highlighting the core extracted from Branco river, which showed the highest sedimentation rate. The profiles corresponding to Preto and Aguapeú rivers presented discontinuities in the sedimentation rates. The discontinuities were dated and would probably be related to the anthropic activities, which intensified in the municipality of Itanhaém around the middle of the twentieth century. [ABSTRACT FROM AUTHOR]

Copyright of Anuario do Instituto de Geociencias is the property of Universidade Federal do Rio de Janeiro, Instituto de Geociencias and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The Quadrilátero Ferrífero region, in the State of Minas Gerais, became internationally known due to the exploitation of gold in the eighteenth century. In this paper, we will present background values for arsenic in alluvial sediments from areas that were heavily affected by the exploitation of gold in the past, in the Carmo River Basin. The analyses of the facies succession of the alluvial profiles were carried out in the sedimentary deposits of flood plains, cutbank and alluvial terraces; this being the foundation for obtaining the arsenic background in the facies of the channel and flood plains. From the knowledge of the background obtained by the statistical method, and based on the construction of cumulative frequency curves on a linear scale, geochemical maps were generated in the SIG system. The acquisition of the background values and the geochemical map for arsenic in the Carmo River Basin, representing areas with anomalous concentrations of this element, are essential for the diagnosis and environmental planning of a mining district potentially contaminated by arsenic. [ABSTRACT FROM AUTHOR]

Drowning unconformities are surfaces characterized on seismic data by high amplitude contrast between deepwater and shallow water facies. Such surfaces are created by the covering of the platform by onlapping siliciclastic sediments or other non-platform material. The key aspect of this paper is the application of the drowning unconformity concept, commonly used in marine settings, to a more restrictive one: the coquinas of the Itapema Formation, in the pre-salt succession of the Santos Basin (Brazil). In the studied area the sequence boundaries of the deposits related to the Itapema Formation are often associated with high reflectivity seismic horizons, which are tied to radioactive peaks of low-energy sedimentary facies in the gamma ray logs and also to the siliciclastic supply rate increase. However, the stratigraphic significance of these reflectors still lacks further investigation and an adequate description. In this way, it is proposed that the space-time filling of the Itapema Formation occurred by the successive migration of proximal facies towards the structural highs, under continuous accommodation space generation conditions and high carbonate production rate, with the depocenters limited by rapid expansions of the basin. The results presented here, therefore, suggest that the sedimentary record of this section was punctuated by instantaneous transgressive surfaces that altered the ecological accommodation, forcing the backstepping of the coquina deposits and the development of a drowning unconformity in the final stage of the Itapema Formation deposition. This configuration produced at the top of this unit a specific facies association characterized by packstones and mudstones composed of micritized grains, which increase in volume and thickness towards the structural lows and tend to be thinner on the way up to the structural highs, where they may be eroded by the Pre-Alagoas Unconformity. The correct identification of such surfaces is essential for understanding the sedimentary filling of the studied area. Depending on which surface is mapped, they may be related to different processes and occur in opposite positions in the base-level cycles. Thus, we propose the existence of two surfaces at the end of the deposition of the Itapema Formation. The first limits the top of the coquinas and is represented by the contrast of low-energy sediments (packstones and micritized mudstones) and the coquinas deposits, configuring the platform drowning itself. The second one, in turn, is understood here as the Pre-Alagoas Unconformity, a subaerial unconformity associated with the regional platform exposure, developed after the drowning events recognized here. • Seismic and well characterization of drowning unconformities in the pre-salt succession. • Drowning unconformities in a non-marine environment setting. • Shut-down of the coquina factory associated with a rapid drowning in early Cretaceous. [ABSTRACT FROM AUTHOR]

Abstract: The upper portion of the Pedra Pintada Alloformation includes about 100 m of mostly eolian deposits. This paper emphasizes the vertical succession and lateral association of sedimentary facies, based on analysis of outcrop data and aerial photographs, as well as the hierarchy and origin of bounding surfaces. It aims to propose a high-resolution stratigraphic and depositional model that may be useful to exploitation of eolian reservoirs. The succession has been preserved due to basin subsidence, and is described in terms of four facies associations that constitute three dominantly eolian units. These units are sharply bounded by major flooding surfaces (super surfaces) that, in turn, are overlain by 1 to 2 m thick, dominantly water-laid facies (lacustrine, fluvial, deltaic and eolian). Both their internal organization and boundaries were controlled by changes in the base level rise rate. The basal Eolian Unit is composed of crescentic eolian dunes and damp interdune deposits ascribed to a wet eolian system. On the other hand, eolian units II and III, also characterized by crescentic eolian dunes (simple and compound) deposits, were related to dry eolian systems, since they comprise dry (eventually wet) interdune facies. Eolian Unit III is truncated by basinwide unconformity, which is then overlain by the ephemeral fluvial deposits (Varzinha Alloformation). This second type of super surface is related to climate-induced wind erosion (deflation) down to the water table level (regional Stokes surface) followed by fluvial incision linked to tectonic activity. [Copyright &y& Elsevier]

Metagreywacke is a very common rock type in orogenic settings. It occurs in several domains within the Neoproterozoic Southern Brasília Orogen, that registers convergent events related to the Western Gondwana formation. Studied rocks derive from the Luminárias Nappe, which is composed of metapelite (Campestre Unit), quartzite and metagreywacke (Santo Antônio Unit) that occur in the northern portion of the structure. The potential use of the metagreywacke in metamorphic studies and the metamorphic P–T path of the Santo Antônio Unit was constrained using isochemical phase diagram, quantitative compositional mapping, and composition mineral contours. The metamorphic peak mineral assemblage of the metagreywacke is biotite + plagioclase + garnet + muscovite + ilmenite + quartz. Rutile is a prograde phase, but is replaced by ilmenite at metamorphic peak, while chlorite is a retrograde phase, as it replaced biotite and garnet. The garnet is almandine rich, preserves zoning, with lower almandine and pyrope contents in the core and lower spessartine and grossular contents in the rim. The peak assemblage is stable at temperatures of 570 ˚C to 650 ˚C and pressures of 0.7 GPa to 1.05 GPa. Prograde conditions of 540 ˚C and 0.9 GPa are obtained using the garnet core spessartine composition and the rutile occurrence. The compositional mineral contours of garnet rim, combined with those of muscovite and plagioclase, intercept at 630 ˚C and 1.0 GPa and indicate peak metamorphic conditions. Later crystallization of chlorite took place during retrograde conditions at 580 ˚C and 0.7 GPa, defining a single clockwise P–T path. The calculated topology of metagreywacke pseudosection, in this study, resembles those of a typical metapelite, and the crystallization sequence of metamorphic index minerals, along a Barrovian gradient, with slightly distinguished mineral assemblages. The similarity and differences for the two rock types occurs, specially under amphibolite facies conditions. Therefore, the metagreywacke is a promising protolith for metamorphic studies, even if it presents marking differences from pelites. The constrained P–T path is similar to those presented in previous works carried out in the Luminárias Nappe, associated with the collisional setting in the Southern Brasília Orogen. [ABSTRACT FROM AUTHOR]

The Bauru Basin of SE Brazil is a large (ca. 370,000 km2) Upper Cretaceous intracratonic feature, important for its fossil remains and of particular value as a source of regional palaeoclimatic information. Historically, lithostratigraphic reconstructions have been performed mainly for successions of the central and southern parts of the basin, resulting in a lithostratigraphic scheme that is not applicable to the northernmost regions. In particular, the northeastern deposits of the Marília Formation (Serra da Galga and Ponte Alta members) reveal lithological, stratigraphic, and palaeontological differences from southeastern and northwestern counterparts (Echaporã Member). Nevertheless, these deposits are considered as a single lithostratigraphic formation in the literature. To address this problem, this study demonstrates how the northeastern deposits of the Marília Formation do not show affinity to the rest of the unit. A more suitable lithostratigraphic model is proposed for the northeastern succession as a distinct and independent unit. Lithofacies and palaeopedological analysis, combined with lithostratigraphic mapping of the northeastern deposits, reveal 11 distinct lithofacies and three pedotypes over an area of ~450 km2. Sedimentary facies and pedotypes were assigned to six interbedded architectural elements: (a) type 1 channel fill, (b) type 2 channel fill, (c) type 3 channel fill, (d) interchannels, (e) palaeosols, and (f) calcrete beds. The succession is interpreted as a distributive fluvial system with overall direction of flow to the NNW, and which developed under the influence of a semiarid climate regime. This contrasts with deposits of the southeastern and northwestern Marília Formation, previously suggested to be of fine‐grained aeolian affinity with interbedded poorly channelised deposits assigned to an aeolian sand sheet environment. By revising the existing lithostratigraphic scheme for the northeastern deposits, and contrasting them with laterally equivalent strata, this work demonstrates how the previously named Serra da Galga and Ponte Alta members reveal a unique set of lithological, architectural, and genetic signatures that permits to separate them from the Marília Formation. Finally, a new lithostratigraphic classification for the unit is proposed: the Serra da Galga Formation, whose deposition relates to an ancient distributive fluvial system. [ABSTRACT FROM AUTHOR]

This paper aims to show the filling and stratigraphy of the units that constitute the Cambambe Basin, that is located at the mid-southern portion of the State of Mato Grosso, Brazil. The rocks of this vulcano-sedimentary unit have been deposited during Upper Cretaceous, between the Santonian and Maastrichtian Stages. The scientific literature has been associated these units to different basins, such as the Paraná Basin, the Parecis Basin and the Bauru Basin and also to other different lithostratigraphic units. Such confusion was the motivation to carry out this study in which the description and interpretation of facies associations was the main tool to clarify some of the aspects related to the Cambambe basin evolution. It has been observed five facies: Polymytic Conglomerate (PCF), Oligomitic Conglomerate (OCF), Siltstone and Mudstone (SMF), Litharenite (LF), Quartzarenite Facies (QF). This facies points to an alluvial fan system, with facies associations interpreted as corresponding to three portions: proximal, immature plains and mature plains. Such facies associations corresponds to the lithostratigrphic units that constitutes the Cambambe basin: the lower portion is the Ribeirão Boiadeiro Group, consisting of the interfingering Quilombinho and Cachoeira do Bom Jardim formations and the upper portion is the Cambambe Formation. An erosive unconfomity separates both portions. The sedimentary record exceeds 400 m in thickness and demonstrates a long-term deposition marked by numerous non-deposition surfaces, including paleosols. • Contributes to the understanding of Cretaceous stratigraphy. • Presents a proposal for the evolution of the Cambambe Basin. • Displays the distribution of facies by lithostratigraphic units. • Interprets four paleoenvironments for Cambambe Basin. [ABSTRACT FROM AUTHOR]

Mixed carbonate–siliciclastic deposits of the Tamengo Formation (terminal Ediacaran), record the rise of calcifying metazoans and the origin of exoskeletons in animals. To explore the relationships between environmental setting and the first appearance of calcified metazoans and their ecology, this study presents detailed sedimentological and stratigraphic data of eight sections that capture the final stages of the Ediacaran (550 to 543 Ma). This study combines stratigraphic characterization with detailed facies descriptions and evaluates lateral heterogeneity and overall ramp sedimentation integrated with fossil distribution. The Tamengo Formation represents a storm‐dominated ramp. The outer to mid‐ramp is composed of very fine‐grained siliciclastic rocks containing Corumbella body fossils and thin‐bedded mudstone/wackestone containing Cloudina. The mid‐inner ramp is dominated by wackestone/packstone with abundant Cloudina skeletal debris and ooid packstone/grainstone shoal deposits. Locally, fragments of Corumbella and Cloudina are found on the same horizon, which is a result of their high accumulation rate, resulting from the reworking and mixing of epifaunal organisms. In spite of taphonomic biases, the general distribution of Corumbella and Cloudina across the unit suggests that these organisms have been transported across ramp and/or probably show a differential response of fauna preservation. When compared to other occurrences worldwide, this dataset indicates an already complex ecosystem in the Ediacaran, where these early animals were capable of adapting to specific environmental niches. [ABSTRACT FROM AUTHOR]

The Passos Nappe is an important tectonic unit of the Southern Brasília Orogen with inverted metamorphism, grading from greenschist facies at the base to granulite facies in the upper part. In the central part, of amphibolite facies, mica schist layers contain garnet megaporphyroblasts (3–9 cm diameter) that contrast with the common-size garnet (0.2–1.5 cm diameter) present in other mica schist layers above and below. This paper intends to describe this textural feature in detail and to discuss its genesis. The megaporphyroblasts are ellipsoidal, with major axis parallel to the mineral and stretching lineation, associated with the main foliation (S 2). Inclusion trails analysis in forty-six megaporphyroblasts, demonstrate straight internal foliation (S i = S 1) in the core, progressively crenulated towards the rim by D 2. S i is discontinuous with the external foliation (S e = S 2), which wraps around the megaporphyroblasts, indicating early syn-D 2 growth. The angle between the axial planes of the included crenulations and S 2 in the matrix indicates that the D 2 deformation continued after porphyroblast growth. Asymmetric deformation shadows reveal non-coaxial D 2 deformation, associated with the emplacement of the nappe (top-to-the SE). The megaporphyroblasts are almandine-rich (73–82%) and spessartine-poor (0.1–2.5%) exhibiting prograde growth zonation. The common-size garnet porphyroblasts of the adjacent micaschists show similar inclusion patterns and chemical zonation indicating a similar tectonometamorphic evolution. A possible origin of the megaporphyroblasts by introduction of fluids along shear zones is discarded because the megaporphyroblasts are distributed along S 0 //S 1 planes that make an angle with S 2. Since the porphyroblasts grew early syn-D 2 , these shear zones should be parallel to S 2 which does not match the observations. The chemical data indicate that megaporphyroblast formation is related to a high-Al protolith composition, associated with scarce nucleation sites. • A first description of garnet megaporphyroblasts from the Passos Nappe. • Developed in the context of a greenschist to granulite inverse metamorphic gradient. • The main control for the large crystal growth was the low garnet nucleation density. • Megaporphyroblasts and common-size garnets share the same tectono-thermal evolution. • Megaporphyroblasts growth-rate was 20 times greater than common-size garnets. [ABSTRACT FROM AUTHOR]

The Glossifungites Ichnofacies is a substrate‐controlled ichnofacies that occurs in omission surfaces and has been used in the identification and interpretation of stratigraphic discontinuities. The Early Permian Rio Bonito–Palermo sedimentary succession from the Paraná Basin (southern Brazil) offers an opportunity to discuss the genesis of the omission surfaces demarked by the occurrence of Glossifungites suites. These suites are composed of sharp, unlined burrows with irregular boundaries and passive infill by generally medium‐ to coarse‐grained sandstones. Thalassinoides is the dominant component in the Glossifungites suites in the studied deposits, with local occurrences of Diplocraterion, Gyrolithes, and Skolithos. Based on the ichnological signatures of the distinct Glossifungites suites observed in these deposits and the characteristics of the facies in which they occur, the associated discontinuity surfaces were interpreted as autogenic or allogenic. The morphological distinction between allogenic and autogenic expressions of the Glossifungites Ichnofacies is observed. Glossifungites suites demarking autogenic surfaces are associated with dewatered muddy substrates from the estuarine settings. Glossifungites suites demarking allogenic surfaces are characterized by large vertical burrows and occur in association with a sequence boundary and coplanar surfaces registered in lower shoreface settings. [ABSTRACT FROM AUTHOR]

This paper describes facies relationships and stratigraphic architecture of early Permian deglacial deepwater to fluvio-deltaic deposits exposed in the eastern border of the Paraná Basin, southern Brazil. The studied succession is located in the Rio do Sul depocenter and comprises an upward transition between the topmost interval of the Itararé Group (Rio do Sul Formation) and the lowermost unit of the Rio Bonito Formation (Triunfo Member). Geological mapping and facies analysis allowed the recognition of the following facies associations, vertically stacked: thin and thick-bedded turbidites, hyperconcentrated and concentrated density-flow deposits (hyperpycnites), mass-transport deposits, fine-grained heterolithic beds of prodelta, delta-front, shoreface and delta-plain deposits, as well as fluvial beds resting on incised valleys. The stratigraphic analysis through the interpretation of the facies associations, stacking pattern and paleocurrent dispersal trends were enough to define the clinoform stratigraphic architecture, composing a progradational-aggradational pattern. The paleoflows for all facies associations show a main trend towards the south-southwest. In this context, hyperpycnal beds can represent a glacial source from the northeast, because of striated and faceted clasts immersed in the conglomeratic facies, whereas turbidites have been understood as the products of their dilution. Likewise, mass-transport deposits are related to the collapse of the shelf-margin delta, represented by allochthonous blocks composed of deltaic beds within the mass-flow diamictites. Thus, these mass-transport deposits do not evolve downslope to turbidity currents. This configuration was likely for building the relief of the slope system, probably a shelf-edge delta susceptible to mass failure. Paleo-ice flows towards SW recorded in the Rio do Sul Formation, genetic related to fluvio-deltaic deposits of Rio Bonito Formation, highlight the scenario of final stages of the Late Paleozoic Ice Age in the SW of Gondwana and support the previous researches which point to the presence of diachronous ice-spreading centers from paleohighs. • Evidence of glacial source to the northeast is recorded in the LPIA in Brazil. • Clinoform stratigraphic architecture is reconstructed from disconnected outcrops. • Slope clinoform progradation is a possible basin-fill mechanism during deglaciation. • Meltwater discharge is a probable triggering factor for the development of mass-transport deposits. • Mass-transport deposits are related to shelf-margin deltas susceptible to mass-failure. [ABSTRACT FROM AUTHOR]

Gneiss–migmatitic rocks and granites between the Cubatão and Itariri shear zones characterize the Mongaguá Complex-Coastal Terrane. Three major units are defined: (1) the Itariri Suite (IS) composed of monzogranites, granodiorites, tonalites, and locally gneiss–migmatitic rocks; (2) the Areado pluton composed of monzogranite and locally tonalite; and (3) the Ribeirão do Óleo Pluton mainly composed of monzogranites, which is the subject of this article. Combined field observations, petrography, Sr–Nd–Hf isotopes, U–Pb zircon data, and preliminary geochemical studies are presented for the Ribeirão do Óleo Pluton (ROP), intrusive in granite-gneisses of the IS. It is a plutonic association of weakly peraluminous, medium-to-high-K biotite monzogranites that surround microgranular intermediate enclaves, which are products of magma mingling. Different from the most examples of magma mingling described in the literature, the ROP mingling did not result from the intrusion of hot mafic magma into colder host felsic magma. Conversely, the emplacement of the monzogranite started most likely during the medium-to-final stage of crystallization of the quartz microdiorite. A detailed study of LA-ICP-MS U–Pb zircon ages, including zircon morphology and internal structure, was useful to determine more precisely the emplacement sequence and the relation between the units of the ROP. Intermediate and acid facies have Paleoproterozoic and Neoproterozoic inheritances, moreover a mixture of xenocrysts, antecrysts, and autocrysts, indicative of a complex evolution of the ROP. Several magma crystallization pulses are recorded at 655–624 Ma, 627–601 Ma, and 613–586 Ma. The negative εHf(t) ranging from − 17.26 to − 12.51 in monzogranites, εNd(t) values of − 10.66 to − 7.83 at t = 600 Ma in both facies, 87Sr/86Sr of ca. 0.708 for the monzogranite and 0.706 for the quartz microdiorite indicate a strong crustal contribution in the genesis of intermediate and acid magmas of this pluton. [ABSTRACT FROM AUTHOR]

The Sergipe–Alagoas Basin, in northeastern Brazil, has extensive exposures of carbonate rocks that may be studied as analogues to hydrocarbon reservoirs. The outcrops are equivalent to the pre-salt and post-salt reservoirs of the Campos and Santos basins, the most important hydrocarbon exploration targets nowadays in Brazil. Outcrops and well logs of the Riachuelo Formation, the most important for analogue studies of Cretaceous marine carbonate reservoirs of the Campos Basin due to the great variability of facies, are described and interpreted to identify the depositional cycles and stratigraphic sequences. Three unconformities formed during the initial drift phase of the basin are recognized. The depositional setting of the studied section changes from a siliciclastic-dominated continental to shallow-marine mixed siliciclastic-carbonate at the base to a shallow-marine carbonate platform in the upper part. The transgressive–regressive (T–R) sequence model was used for the stratigraphic analysis, leading to a stratigraphic framework of three third-order T–R sequences, with an estimated duration of approximately 2.0, 4.5 and 1.5 Ma, respectively. Consequently, the higher frequency cycles identified in the well logs represent a timing of circa 140,000 years, and are interpreted as fourth-order depositional cycles. [ABSTRACT FROM AUTHOR]

The Cerro Chato region is located in southernmost Brazil and is characterized by associations of acid volcanic and subvolcanic rocks. The region is affected by NW and NE‐ trending faults and is grouped into 2 geomorphologically distinct features: Cerro Chato and Cerro Partido. Cerro Chato is represented by ignimbrites that occur in 2 main facies: lithic‐rich ignimbrites and crystal‐rich ignimbrites. They are poorly sorted and consist of lapilli‐sized pyroclasts in a tuffaceous matrix with eutaxitic texture. Hemi‐crystalline rhyolitic flows represent effusive events, with porphyritic texture, flow structures, and spherulites. Cerro Partido is characterized by a subvolcanic rhyolitic body, with porphyritic texture, elongated in the NE–SW direction. Through geochemical data, the lithologies were characterized as high‐silica type rhyolites, correlated to the alkaline series, close to the subalkaline series limit; display metaluminous to peraluminous character and high contents of alkalis, FeOt/FeOt + MgO and agpaitic index. The Cerro Partido rhyolites were classified as high‐Ti with higher CaO, P2O5, FeOt, MgO, and K2O contents than the Cerro Chato low‐Ti rhyolites. The REE pattern is slightly enriched in LREE in relation to the HREE and has a strong negative Eu anomaly. U–Pb zircon dating indicates an age of 561 ± 2 Ma for Cerro Partido, suggesting contemporaneity with the granitoids of Dom Feliciano Suite. Additionally, Cerro Chato rhyolites indicate an age of 630.4 ± 2.8 Ma. These ages are in agreement with those obtained in the syn‐sedimentary volcanism at the base of the Maricá Formation, Camaquã Basin. [ABSTRACT FROM AUTHOR]

The Upper Cretaceous Bauru Group in south‐east Brazil consists of alluvial strata whose characteristics and distribution indicate a fluvial system developed in a semi‐arid to arid climate. Sections exposed within a 90 000 km2 study area in Minas Gerais State (in south‐eastern Brazil) were evaluated using facies and palaeosol analysis to formulate depositional and pedogenic models that may account for geomorphic and climate features. From east to west, the study succession records a gradual decrease in grain size, an increase in the width/thickness ratio in channels, a decrease in the lateral and vertical connectivity of channel deposits, and an increase in overbank deposits. The fluvial architecture indicates a braided channel belt, ephemeral ribbon–channels, and an unconfined fluvial facies from east to west in the study area. The lateral and vertical distribution of facies, stratigraphic architecture and palaeocurrent data suggest proximal, medial and distal portions of a progradational distributive fluvial system. The sedimentary dynamics were marked by the building and abandonment of channels related to processes of aggradation, vegetation growth and palaeosol generation. Macromorphological and micromorphological analyses have identified pedological and mineralogical features that indicate an arid to semi‐arid climate with a provenance from the north‐eastern part of the basin (Alto Paranaiba Uplift). From the proximal to the distal portions of the distributive fluvial system, the palaeosol development is different. In the proximal portion, the palaeosols are absent or poorly developed, allowing a possible general comparison with the present soil order: Inceptisols and Aridisols. In the medial portion of the fluvial system, the palaeosols are well‐developed and characterized by Bt, Btk, C and Ck horizons (Alfisols, Aridisols, Inceptisols and Entisols). Poorly drained to well‐drained palaeosols from the base to the top in the distal plain (Aridisols and Inceptisols) are associated with geomorphic and hydromorphic changes in the fluvial system due to progradational evolution. The genetic relationship between the fluvial architecture and the palaeosols enhances the understanding that the sedimentation and pedogenesis that occurs in different portions of the distributive fluvial system are related to the tectonic and climatic evolution of the basin. [ABSTRACT FROM AUTHOR]

The aim of this study was to investigate the influence of mining on the geochemical composition of the alluvial sediments of the Gualaxo do Norte River Basin, located in the eastern-southeastern portion of the Quadrilátero Ferrífero (Minas Gerais State, Brazil). The analysis of the sedimentological succession was carried out using 16 stratigraphic sections (cutbanks and alluvial terraces) and analysis of 111 sediment samples for major- and trace-elements by ICP-OES. The stratigraphic analysis revealed the deposition history of the collected sediments. Anomalous concentrations of certain elements such as As and Pb were found, which are associated with stratigraphic facies with evidences of gold artisanal mining, and Fe anomalies associated with iron ore mining. The classification of the sections was possible by means of principal component analysis. From the sediment characteristics, three groups were identified: (1) those influenced by iron ore mining and gold artisanal mining; (2) those influenced by iron ore mining and (3) those with no influence of human activities. [ABSTRACT FROM AUTHOR]

The rift section of the Brazilian basins represent the sedimentary record associated with the first stages of Gondwana break-up in the Early Cretaceous phase (Berriasian to Aptian). The rift succession of the Campos Basin constitutes one of the main petroleum systems of Brazil's marginal basins. This interval contains the main source rock and important reservoirs in the Lagoa Feia Group deposits. The Lagoa Feia Group is characterized by siliciclastic, carbonate and evaporite sediments deposited during the rift and post-rift phases. Despite the economic relevance, little is known in stratigraphic terms regarding this rift interval. To date, most studies of the Lagoa Feia Group have adopted a lithostratigraphic approach, while this study proposes a tectonostratigraphic framework for the deep-rift succession of the Campos Basin (Lagoa Feia Group), using the fundamentals of seismic sequence stratigraphy. This work also aims to establish a methodological and practical procedure for the stratigraphic analysis of rift basins, using seismic data and seismofacies, and focusing on tectonicstratigraphic analysis. The dataset comprised 2D seismic lines, core and lithological logs from exploration wells. Three seismic facies were identified based on reflector patterns and lithologic data from well cores, providing an improved subdivision of the pre-, syn- and post-rift stages. The syn-rift stage was further subdivided based on the geometric patterns of the reflectors. Tectonics was the main controlling factor in the sedimentary succession, and the pattern and geometry of the seismic reflectors of the syn-rift interval in the Campos Basin allowed the identification of three tectonic systems tracts: (i) a Rift Initiation Systems Tract; (ii) a High Tectonic Activity Systems Tract and (iii) a Low Tectonic Activity Systems Tract. [ABSTRACT FROM AUTHOR]

Facies and isotope (Sr and S) data were combined in order to discuss the genesis and palaeohydrology of an Upper Aptian evaporite succession formed during the initial development of an intra-continental rift from northern Brazil. Comparison of facies distribution between two investigated areas shows: (i) stable, well-stratified and hypersaline lakes with periods of anoxia and closure in the eastern margin of the basin, where there was evaporite precipitation only in central lake environments; and (ii) more ephemeral conditions in the eastern margin of the basin, where a saline pan complex developed and evaporite precipitation took place mainly in marginal salinas and surrounding mudflats. In both areas, expansion/contraction cycles developed as sedimentation took place, accompanied by a decrease and then an increase in 87Sr/86Sr- and δ34S-isotope values. This, combined with the wide distribution of Sr- and S-isotope data within individual depositional cycles, as well as petrographic and scanning electronic microscopic (SEM) images, led to the conclusion that diagenesis was not enough to obscure the primary geochemical signature. A non-marine brine source is suggested by 87Sr/86Sr ratios between 0·707824 and 0·709280, which are higher than those from Upper Aptian sea water (i.e. between 0·70720 and 0·70735). The δ34S value varies from 16·12 to 17·89‰ in the eastern margin of the basin, which is in disagreement with Upper Aptian marine values (13 to 16‰). Both geochemical tracers were influenced by facies characteristics, and thus a model is provided, where 87Sr/86Sr values respond to changes in expansion and contraction episodes of saline pan/lake systems. [ABSTRACT FROM AUTHOR]

The Barro Alto complex, central Brazil, is a layered mafic–ultramafic intrusion, which was subjected to granulite facies metamorphism during the Neoproterozoic. Ultra-high-temperature conditions are recorded by parageneses that occur in some lenses of quartz-bearing rock (metagranite, metapelite and impure quartzite). The peak paragenesis consists of spinel+quartz±cordierite±leucosome (recording the former presence of melt with quartz in excess), which is replaced by either orthopyroxene+sillimanite or garnet+sillimanite. Quartz+biotite±sillimanite±garnet symplectites are ubiquitous and indicate reactions between Fe–Mg phases and melt. Late kyanite porphyroblasts have overgrown these symplectites. The direct replacement of spinel+quartz±cordierite by orthopyroxene+sillimanite or garnet+sillimanite occurred around the [Sa] invariant point, which appears only in a petrogenetic grid with inverted topology. The topology inversion occurs at conditions of high oxygen fugacity or due to the presence of ZnO-bearing spinel. Minimum peak conditions of ultra-high-temperature metamorphism were calculated as c. 980 °C and c. 7.9 kbar. The succession of observed mineral textures can be explained by a near-isobaric cooling P–T path, with a cooling stage occurring between c. 980 and 750 °C. [ABSTRACT FROM AUTHOR]