Abstract Studies integrating ichnology and sedimentology in paleoenvironmental interpretations are rare in Brazilian rift successions. This study applies an integrated ichno-sedimentological analysis to a well core from the Resende and Tremembé Formations (Cenozoic Taubaté Basin) to reconstruct paleoenvironmental conditions. Three paleoenvironments were identified: alluvial fan, braided river, and lacustrine. The alluvial settings is characterized by coarse-grained deposits displaying bioturbation intensity and sporadic trace fossils, dominated by Palaeophycus and Skolithos, reflecting opportunistic colonization. Braided river deposits consist of stacked sandstones with trough cross-bedding and a trace-fossil assemblage with Palaeophycus, Skolithos, Macanopsis, and Taenidium. The lacustrine environment is characterized by dark shale interbedded with fine-grained sandstones, including ostracod-rich white beds. Sparse beds containing Planolites indicate a complex lacustrine system with potential variations in salinity and oxygenation. Comparison with previous studies suggests that ichnofossils and paleosols reflect water table fluctuations and humid conditions in the Resende Formation. For the Tremembé Formation, ichnofossil data, palynomorphs (Botryococcus and Pediastrum), and taphonomic signatures in fishes suggest a dynamic environment with fluctuating water levels, periodic salinity increases, and decreased oxygen levels.

Abstract Lateritic formations are widespread in eastern Amazon, occurring as bauxite-bearing mature profiles on plateaus or as immature ones in lowlands. Climate and landscape evolution over the Neogene were analyzed via mineralogical (XRD), textural (optical microscopy), and geochemical (ICP-OES, ICP-MS) methods carried out in a selected lateritic profile outcropping in Abel Figueiredo. The profile features columnar and nodular iron crust, overlaid by a spherulitic horizon formed of hard fragments in clayey matrices, covered by yellow topsoil. Major minerals include hematite, goethite, kaolinite, and quartz, with minor anatase and heavy minerals. Hematite and goethite dominate lower horizons, while kaolinite, quartz, Al-goethite, and anatase appear in the matrix and topsoil. Trace elements were immobilized by iron oxyhydroxides and anatase or maintained in zircon and other resistate minerals. Mineralogy and geochemical distribution indicates a transition from iron crusts to fragmented iron crust, followed by physical reworking, leading to the formation of nodules and spherulites, with clay accumulation in between. In a final stage, a topsoil developed over the profile. This set of events requires an alternation between humid tropical climate with more arid conditions, with implications to rainforest and savanna development, resulting in mineral formation and chemical or physical decomposition.

Abstract Paleohelcura tridactyla is a trackway characterized by up to three round tracks per series, typically arranged linearly or triangularly, often with a medial impression. This pattern is consistently found in several eolian deposits and has been described in the Botucatu Formation in Brazil, an eolian sandstone unit in the Paraná Basin deposited during the earliest Early Cretaceous under arid conditions. Here, we describe a new small morphotype of Paleohelcura tridactyla in the Botucatu Formation. This morphotype is smaller than previously described specimens of Paleohelcura tridactyla, and although it shares a similar size with Paleohelcura araraquarensis, it differs both in track morphology and series arrangement. We propose three hypotheses for the new morphotype: (1) The trackway may have been produced by the juvenile stage of the arthropod responsible for the larger Paleohelcura tridactyla tracks. (2) The size difference may indicate sexual dimorphism within the same species. (3) Alternatively, it could represent a different arthropod species, which is smaller in size but leaves similar track patterns. This discovery reinforces the ichnotaxonomical distinction between P. tridactyla and P. araraquarensis and raises the possibility that another arthropod species may have been present in the Botucatu Paleodesert fauna.

Abstract Poorly lithified siliciclastic rocks correspond to important post-salt reservoirs in the Southeastern Brazilian Margin offshore basins. These reservoirs present challenges regarding the prediction of the hydraulic behavior of faults due to low cohesion leading to diffuse brittle deformation. To improve the understanding of the deformation influence on the reservoir quality of these rocks, this paper presents a 3D modeling method based on an outcrop analog composed of Paleogene poorly lithified fluvial siliciclastic deposits in the Volta Redonda Basin (Continental Rift of Southeastern Brazil). A database was collected containing a 3D digital terrain model of the outcrop, a 1:50-scale geological section, permeability and porosity measurements, and lithological and sonic log from a well drilled upon the outcrop to the basement. Two models were built: one using traditional methods for industry where properties were uniformly distributed throughout the grid and a second model where sectors were delimited and populated with data from deformed rocks, highlighting the impact of tectonic structures (faults and deformation bands) on fluid flow, reducing the permeability by one order of magnitude, and increasing the flow time between wells by more than 10%. This study contributes to improving reservoir modeling accuracy, enhancing the understanding of fluid flow behavior in complex, deformed, poorly lithified rocks.

Abstract Poorly lithified siliciclastic rocks correspond to important post-salt reservoirs in the Southeastern Brazilian Margin offshore basins. These reservoirs present challenges regarding the prediction of the hydraulic behavior of faults due to low cohesion leading to diffuse brittle deformation. To improve the understanding of the deformation influence on the reservoir quality of these rocks, this paper presents a 3D modeling method based on an outcrop analog composed of Paleogene poorly lithified fluvial siliciclastic deposits in the Volta Redonda Basin (Continental Rift of Southeastern Brazil). A database was collected containing a 3D digital terrain model of the outcrop, a 1:50-scale geological section, permeability and porosity measurements, and lithological and sonic log from a well drilled upon the outcrop to the basement. Two models were built: one using traditional methods for industry where properties were uniformly distributed throughout the grid and a second model where sectors were delimited and populated with data from deformed rocks, highlighting the impact of tectonic structures (faults and deformation bands) on fluid flow, reducing the permeability by one order of magnitude, and increasing the flow time between wells by more than 10%. This study contributes to improving reservoir modeling accuracy, enhancing the understanding of fluid flow behavior in complex, deformed, poorly lithified rocks.

Abstract Silicification is a diagenetic process that affects the porosity and permeability of carbonate rocks, such as those in the Itapema and Barra Velha formations, Pre-salt of Santos Basin. This contribution integrates macroscopic and microscopic descriptions of silicified intervals, identifying controls and mechanisms of dissolution, replacement, and precipitation, relating them within the diagenetic history of the formations. The pre-compaction eodiagenetic silicification comprises tabular and nodular geometries, reflecting fine-grained laminated and coarser-grained massive lithologies. The post-compaction mesodiagenetic silicification comprises irregular silica disseminated throughout carbonates. Silica bodies consist of replacive (cryptocrystalline silica, mosaic microcrystalline quartz and spherulitic chalcedony) and pore-filling textures (chalcedony and macrocrystalline quartz). The highly selective nature of the replacement suggests that carbonate dissolution initiates silica precipitation as a fluid-mediated process that transfers crystallographic information. The successive zoning with decreasing crystallinity in filling/cement textures indicates a decrease in the fluid supersaturation as the precipitation advances. These features indicate that textures and silicification mechanisms reflect precipitation physicochemical conditions. The first silica event follows the dolomitization stage that replaces primary clays and carbonates, while the second event occurs after mechanical compaction and is later followed by replacive/pore-filling barite textures. Late macrocrystalline quartz veins host primary hydrocarbon fluid inclusions, linking the second silicification event with oil migration.

Abstract The AN-34 and QT-02 iron oxide–copper–gold (IOCG) prospects are located in the northern portion of Carajás Mineral Province (CMP), where important copper IOCG deposits occur. This region lacks basic geological information on the outskirts of the large IOCG deposits, and these small prospects might provide information regarding fluid–rock interaction and alteration-mineralization styles. In these prospects, banded iron formations (BIFs), basic volcanic rocks, and granitoids are variably deformed and hydrothermally altered. The hydrothermal alteration evolved from early silicification, potassic alteration, and Fe enrichment. All these alterations are pervasive and linked to shearing structure development. They are followed by late, ductile-brittle, chloritic alteration and sodic alteration. The quartz–magnetite oxygen isotope geothermometry (δ18Oqtz–mag) and chlorite geothermometry reveal a decrease in the temperature of the hydrothermal system from 520°C ± 30°C to 273°C ± 20°C. The calculated δ18OH2O for silicification (6.6–5.8‰) and Fe enrichment (9.2–10.47‰) suggest the involvement of magmatic fluids. δ18OH2O values from 0.27 to 0.77‰ and 1.7 to 3.3‰ from late sodic alteration and hematite are compatible with meteoric fluids. Our findings shed more light on the fluid–rock interaction, hydrothermal evolution, and mineralization styles in the IOCG deposits of the northern sector of the CMP.

Abstract The Upper Jurassic to Lower Cretaceous Botucatu Formation of the Paraná Basin consists primarily of trough cross-bedded eolian sandstone deposited in an intracontinental desert. In southeastern Brazil, an unusual occurrence of conglomerate and coarse-grained sandstone unconformably overlies the Neoproterozoic basement and is overlapped by volcanic rocks of the Serra Geral Formation. This represents one of the few documented occurrences of fluvial facies described for the Botucatu Formation in the northern part of the basin. To determine its depositional system and source area, we conducted facies and paleocurrent analysis, detrital grain modal counting, macroscopic provenance, and conventional heavy mineral analysis. The integrated analysis suggests that the studied succession was deposited by an ephemeral fluvial system at the margins of the extensive dune fields of the Botucatu paleodesert. Paleocurrent data and detrital composition indicate sediment dispersal toward the SE, with a possible source area associated with a recycled orogen to the NW (Brasília Belt). Although ephemeral rivers acted as important sediment suppliers for this continental-scale desert, recycling of eolian sediments by fluvial systems is not ruled out. Finally, using a multi-proxy approach, we present a paleogeographic scenario for the northern margin of the Botucatu paleodesert during the last period of Gondwana, prior to its initial rifting.

Abstract The structure of two superposed layers within a redox-altered basaltic flow is restricted to the Paraná Volcanic Province among large igneous provinces. World-class amethyst-geode and agate-geode deposits occur in reduced Tier 1 of the Cordillera Flow (> 100 km long). Satellite and drone-sourced imagery, several field surveys, and 11 new geochemical analyses were used in this study. The lower Tier 1 (10 m thick) consists of massive rocks without cooling joints, whereas the barren Tier 2 is massive with colonnade. The flow is identified from the stratigraphy and chemical composition (e.g., 1.3 wt.% TiO2). The presence of poikilitic microphenocrysts of magnetite (mostly plagioclase inclusions) is a distinctive feature. Tier 2 rocks are chemically similar to the original lava; Tier 1 displays a decrease in SiO2 and K2O contents and an increase in Fe2O3T. Reducing and acidic hot water formed amethyst and agate deposits in Tier 1. Curved shapes (erosional remnants with agate) were formed by silicification of the lower portion of the flow. The interaction of the Guarani Paleoaquifer with the lava flow formed the two layers: the geodes and the curved portions. Other provinces may have similar features where hot aquifer water was available below the basalt.

Abstract Situated between the Campos and Santos basins, the Cabo Frio High emerges as a focal point of heightened magmatic activity during the Santonian–Campanian and the Eocene period following the breakup of these basins. The geodynamic framework associated with these events took place during their passive margin stage, wherein magmatic occurrences could potentially be attributed to the reactivation of significant geological structures or thermal enhancement facilitated by proximal hotspots, within a context of lithospheric thinning (i.e., Trindade Plume). This investigation unveils novel Sr–Nd–Pb–Hf isotope data derived from alkaline magmatic rocks extracted from three boreholes strategically positioned within the Cabo Frio High area. The evolutionary trajectory of these rocks reflects the intricate processes of assimilation and fractional crystallization. The results of the analysis and binary mixing modeling point out to an involvement of both depleted (characterized by Depleted Mantle [DM]/Mid-Ocean Ridge Basalt [MORB] or HIMU compositions) and enriched (manifesting as the Enriched Mantle I [EMI] and the Enriched Mantle II [EMII]) sources in the genesis of these magmas. Moreover, the discerned incorporation of a fertile Ocean Island Basalt (OIB-like) reservoir hints at a plausible association with the Trindade Plume, amplifying the complexity of the magmatic genesis within the studied region.