Parecis Basin Research Articles

Sedimentary provenance and U-PB detrital zircon in the mesozoic record of the Parecis Basin, Rôndonia and Juruena sub-basin

Sediment provenance analysis is a tool increasingly used in the study of basins. This technique allows identifying the mineralogical and lithological composition of source areas, inferring paleogeography, climate and relief, determining the main sand distribution routes, and estimating the distance and transport time. The Mesozoic volcanic-sedimentary packages of the Parecis Basin mark a significant part of the South American Platform evolution, regarding tectonism, subsidence, sedimentation, basic and alkaline magmatism, uplift and erosion. Also, these packages registers the initial stages of Andean Orogeny subduction. Different authors have established opposite stratigraphic proposals for Parecis Basin regional correlations, and also for its tectonics evolution. Given this context, there are controversies regarding the chronostratigraphic positioning and relationships between the intervals, especially the units called Salto das Nuvens and Utiariti formations. This article presents the faciological framework and facies association, heavy mineral assemblage, ZTR index and sediment sources, and U-Pb detrital zircon ages to both sub-basins. Detrital Zircon data reveal minimum ages of 124 Ma for the Rondônia sub-basin and 245 Ma for the Juruena sub-basin for sedimentary deposition attributed to the base of the Cretaceous units. The detrital age of zircons points to a varied contribution, between elements from different provinces and ages of the Amazon Craton, the Paraguay Fold Belt and records of alkaline magmatism of ages Permo-Triassic and Cretaceous in the different kimberlithic fields of Rondônia and Mato Grosso.

The Tectonic Framework of Parecis Basin: Insights from a Multiphysics Interpretation Workflow

The Parecis Basin, one of Brazil’s most extensive intracratonic basins, holds significant potential for hydrocarbon exploration. Despite its vast size, Parecis has yet to be extensively explored, with only five wildcat wells drilled. So far, no commercial discoveries have been announced. Regional studies have suggested Paleozoic sedimentation, while recent analyses have revealed a Neoproterozoic infill. Its tectonic model is still a matter of debate, and to date, no detailed structural map for the whole basin has been published. The present work proposes a new detailed structural map of the Parecis Basin based on a four-step interpretation workflow integrating seismic and gravimetric data. The first step includes converting the public 2D seismic lines to the depth domain. The second step is estimating the residual Bouguer anomaly, where the computed residual anomalies should relate to the basin’s tectonic features. The third step comprises the 2D forward modeling of the gravimetric anomalies using the 2D seismic interpretation as a constraint. The final step compiled all the interpreted features into our new structural map. This map reveals the top of the basement, forming a complex framework of horsts and grabens. Normal faults define the main structural style in the basin. Further, we could recognize thick, high-density bodies embedded in the crystalline basement. These bodies consist of Orosian–Calimian (1.8–1.6 Ga) mafic and ultramafic rocks, which may be a potential source for hydrogen exploration in the basin. Subsequent geophysical and geochemical surveys will assess the hydrogen potential in the area.

Exploring the formation of Tangará sill: A single-pulse intrusion feeding CAMP lava flows in Parecis Basin, Brazil

The Central Atlantic Magmatic Province (CAMP) stands out as a distinctive large igneous province on the Earth. Due to the extensive outcrop area of CAMP and its complex geological setting, the links between the extrusive and intrusive units are not fully understood. In the intracratonic Parecis Basin located in Central Brazil, the CAMP rocks are represented by mafic lava flows at its south (Tapirapuã Formation) and western edges (Anari Formation). Despite the availability of substantial geophysical, petrological and geochronological data, the understanding of CAMP feeder system in this region remains unclear. In this study, our focus is on investigating the Tangará sill, a dolerite intruded into Triassic sedimentary rocks in the South region of the Parecis Basin. Our aim is to contribute to a better understanding of the plumbing system in this area by investigating the Tangará sill and its potential links to the lava flows at the surface. The sill is identified as a tabular feature in seismic data, with a discernible base and top that extend for about 100 km from northeast to southwest. The Tangará sill was drilled by the 6-ANP-MT-02 stratigraphic well, revealing a continuous igneous intrusion with a thickness of 180 m (at depths ranging from 470 to 650 m). The dolerite intrusion is composed by plagioclase, clinopyroxene, opaque minerals and mesostasis (quartz + feldspar intergrowth and apatite). It can be divided into five segments: the upper and basal margins consist of fine-grained dolerite, the central portion comprises coarse-grained gabbro, which includes pegmatoid zones. The central portion is separated from the margins by two sections of medium-grained dolerite. The dolerite is tholeiitic, with low TiO2 content (<2 wt% TiO2) and low La/Yb ratio (average of 4.4). Geochemical and petrographic data indicate that the intrusion was formed by a single magma pulse and underwent differentiation through plagioclase and pyroxene dominated crystal fractionation. Two samples dated by 40Ar/39Ar geochronology using laser step-heating technique, yield plateau ages ranging from 204.0 ± 0.4 Ma to 202.0 ± 0.5 Ma. Sr-Nd-Pb isotopic data suggest a lithospheric mantle source with possible crustal contamination. Our findings suggest an emplacement model in which a thick single-pulse magma intruded the Triassic red beds. Along a NE-SW section, the sill is truncated by faults along the southern border of the basin, near the mapped lava flows of the Tapirapuã Formation. This suggests that the magma propagated from the northeast to the southwest along the sedimentary strata and fed the Tapirapuã volcanism. This example highlights a single-pulse sill feeding lava flows of the Central Atlantic Magmatic Province (CAMP).

Sequence stratigraphy and tectonic framework of the Parecis Basin Mesozoic interval- a novel proposition

This paper presents a novel Stratigraphic and Tectonic Framework for the Mesozoic Register of the Parecis Basin, using the concept of Supply Generated Sequences. The tectonic evolution of the South American Platform during the Mesozoic was utilized to establish the significance and correlations of the proposed Parecis Sequences 1 to 4 and their bounding surfaces. Parecis Sequence 1 exhibits a complete Transgressive Regressive Climatic Induced Clastic Behavior, with a depositional age from the Juro-Triassic period, related to localized deposition in an extensional regime caused by the CAMP event. A non-depositional/erosional gap of 90 million years between Parecis Sequence 1 and 2 is attributed to regional uplift associated with the Juruá Orogeny until the Late Cretaceous. Parecis Sequence 2 displays a complete Transgressive Regressive Climatic Induced Clastic Behavior and is associated with the Mochica phase of the Andean Orogen. Parecis Sequence 3 displays a complete Transgressive Regressive Climatic Induced Clastic Behavior correlated with the Peruvian phase of the Andean Orogeny. Parecis Sequence 4 considers the Rio Tapirapé Formation as a probable fourth sequence, with lake register and age defined by Coniacian notosuchian fossils. The Parecis Supersequence comprises Sequences 2 to 4, representing a trend of basin fill from Albian to Coniacian in a back-bulge foreland basin.

A Record of the Snowball Earth Events? The Parecis Basin May Encompass Three Major Neoproterozoic Glaciations on the Amazonian Craton

The Parecis Basin is an intracratonic basin located at the south-southeast portion of the Amazonian craton. Its stratigraphic units comprise siliciclastic and carbonate rocks that potentially record intense paleoenvironmental changes related to the Sturtian (∼717–660 Ma), Marinoan (∼657–635 Ma), and Gaskiers (∼580 Ma) glaciations. We present novel X-ray fluorescence geochemistry, C, O and Sr isotopic data from carbonate rock samples retrieved from the stratigraphic well 2ANP 0006 MT. The results show two negative excursions of δ13C here correlated to post-Sturtian and Marinoan glaciations, recorded in the Salto Magessi and Mirassol d’Oeste carbonate units, respectively. Both negative excursions were followed by δ13C positive incursions. At the base of the section, Salto Magessi carbonate presents unradiogenic Sr ratios (87Sr/86Sr from 0.70655 to 0.70776) typical of Cryogenian interglacial carbonate rocks found elsewhere. The negative δ13C excursion from Mirassol d’Oeste is associated with radiogenic 87Sr/86Sr ratios (up to ∼0.7186), possibly related to continental influx from the intense erosion generated by the Marinoan deglaciation. Upward, the Nobres Formation displays preserved primary sulfate minerals (gypsum and anhydrite) and isotopically heavy oxygen compositions which point to an evaporative environment and support the scenario of enhanced oxidative weathering during early to middle Ediacaran. Finally, despite the absence of cap carbonate, diamictites related to the Serra Azul Formation occur overlying the Nobres Formation and are associated with the Gaskiers event. Therefore, the 2ANP 0006 MT may record three major glacial events from the Neoproterozoic, representing a promising section for further studies concerning chemostratigraphic and paleoenvironmental reconstructions during the Neoproterozoic Era.

High-frequency sequence stratigraphy applied to a strictly continental and fragmented basin – Case study in the Mesozoic record of the Parecis basin, western Brazil

Sequence stratigraphy applied to strictly continental alluvial system can provide better understanding about the triggering mechanism and tectono-climatic control over sedimentation behavior. The Parecis Basin, a large intracratonics basin located in mid-west Brazil, have an understudied Mesozoic continental record, with an undefined lithostratigraphy and regional correlations. We thus applied a sequence stratigraphy approach to reassess the stratigraphic framework of the Vilhena Outcropping Area, which is the uplifted western border of the Parecis Basin, situated inside the Guaporé Block. Based on new field data and stratigraphic analysis, we identified three Third Order Sequences, registering the evolution of at least a significant part of the Mesozoic register. Three basin wide bounding surfaces were identified, being: the Mesozoic Basal Surface, Bounding Surface 1 and Bouding Surface 2, all of these surfaces can be traced to another outcropping areas inside the Guaporé Block, allowing the first sub-basin correlation of the Parecis Basin.

Evolution of parallel, oblique and triple-junction rifts: Insights from analogue modelling

The study of extensional structures such as normal faults, grabens and rift systems using analogue modelling techniques has been widely used to understand the evolution of sedimentary basins. These have been particularly helpful to understand the generation of oil reservoirs and structural traps. The present work was inspired by the discussion on the origins of two sub-parallel grabens located in the intracratonic Parecis Basin in the centre-west of Brazil. Parallel, oblique or intersecting grabens may form by the reactivation of inherited basement weaknesses that can act as velocity discontinuities, with their geometry and proximity controlling the evolution of the resulting grabens. In this work, we use sandbox analogue experiments to investigate the geometric and kinematic evolution of two nearby, sometimes intersecting, grabens. We do this by varying the distance and the geometry of two basal velocity discontinuities (VDs). In all experiments, an extension of 3 cm (scaling to 12 km) was prescribed to affect a 3 cm thick sand pack. The materials were deformed using a plexiglass box with two basal velocity discontinuities attached to two lateral movable walls. In a first set of experiments, we varied systematically the distance between the VDs. In the second set, we varied the angles between the VDs with different configurations, including situations in which the VDs intersect forming triple junctions. The results show that when the VDs are initially close (<2 cm, corresponding to ~8 km in nature) there is the formation of a wide graben composed of two sub-grabens that interfere with each other, while if the spacing between the VDs is >2 cm, the grabens develop independently and do not interfere. Furthermore, it is possible to conclude that whenever (not interfering) VDs are prescribed to the base of a sand pack it always leads to the formation of asymmetric grabens, except when the two VDs are initially close to each other. In the experiments with conjugate triple-junction VDs, the extension was shared between two independent asymmetric grabens where the VDs were at a certain distance from each other, and always led to the formation of a wide graben where the VDs were close or intersect each other.

The 1.88 Ga Uatumã Magmatism in the Serra dos Magalhães region: petrology and implications to the extension of the south-eastern edge of the Amazonian Craton

The Amazonian Craton (AC) is a Pre-Cambrian tectonic segment that extends over 4.3 × 105 km2 at the northern portion of the South American Platform. Phanerozoic sedimentary basins cover most of the edges of the AC and sparse occurrences of Precambrian rocks may aid the definition of its extension. One of these occurrences is the volcano-plutonic rocks that outcrop in the Serra dos Magalhães region, which relates to the Uatumã Magmatism, an expressive Orosirian magmatic event that constitutes a Siliceous Large Igneous Province. Effusive rhyodacites and rhyolites comprise the volcanic unit and the plutonic counterpart consists of epizonal monzogranites. Whole-rock geochemistry indicates an A2-type granite affinity to all magmatic rocks. Contrasting signatures suggest a magmatic evolution from a single stratified magma chamber. U-Pb zircon crystallization ages are 1870 ± 11 Ma for the rhyodacites, 1879 ± 5 Ma for the monzogranites, and a slightly younger age of 1863 ± 14 Ma for the rhyolites. Interpretation of satellite and geophysical images allows the recognition of regional lineaments in the Parecis and Bananal basins that suggest the extension of the AC up to the Tucurui Fault in the eastern portion (Bananal Basin) and up to the Brasnorte High in the southern part (Parecis Basin).

Statistical analysis of hydrochemistry and isotopic characterization of groundwater from the Parecis Basin

ABSTRACT Knowledge of mechanism controlling the hydrochemistry of groundwater are crucial requirement to understand the hydrochemical evolution and evaluate the water quality of subsurficial water resources. Thus, to understand the process governing the hydrochemistry variability and flow dynamics of the Parecis and Ronuro Aquifers, the use of statistical analysis and isotopic characterization were combined. In the correlation matrix it was found that the highest correlation coefficient was observed for HCO3-, Ca2+, Mg2+, Sr2+, SO42-, and F-, which suggested that these parameters are derived from the same source, most likely from rock–water interactions. Additionally, we noticed moderate to high correlation among NO3-, Cl-, Na+, K+, and Ba2+, which collectively are indicative of domestic sewage contamination. Finally, a principal component analysis (PCA) identified that the most variance in hydrochemistry from the evaluated samples was controlled by HCO3-, Ca2+ and Mg2+, which is associated with rock–water interaction. Regarding the 18O and 2H isotope values, it could be observed that the aquifer recharges were of meteoric origin and that the Ronuro Aquifer samples were more enriched than the PAS samples. This work reinforces the capability of multivariate statistics to discern the main process that controls the variability of groundwater hydrochemistry.

Characterization of the Paleozoic-Neoproterozoic boundary of the Parecis Basin

Characterization of the Paleozoic-Neoproterozoic boundary of the Parecis Basin

Geology and U-Pb detrital zircon geochronology coupled with zircon fission tracks of the Jauru formation of Parecis Basin, Brazil

The Jauru Formation fills a symmetric NNW-trending graben whose basement is represented by Calymmian granitic intrusions and the paleoprotorezoic and mesoproterozoic metavolcanic sequence of the Jauru Terrane. The graben represents an intracratonic basin with glacial siliciclastic sequence dominated by lake sediments consisting of conglomerates, fine sandstones, siltstones and shales. The Late Carboniferous timing of basin filling is suggested by Cristatisporites sp. palynomorphs. The detrital zircons U-Pb data shows several ages, highlighting the source areas at 0.9 Ga (Sunsás and Aguapeí Belts), 1.3 Ga (Paraguá Terrane), 1.4 Ga (Rio Alegre and Jauru Terranes), 1.5 Ga (Jauru Terrane), and 1.8 Ga (Jauru and Paraguá Terranes), as well as ages in the Neoarchean (~2.8 Ga) and Siderian (2.1 Ga). The spectrum of U-Pb ages in detrital zircons of the Jauru Formation shows that the ages of the zircons are much older than the Mississippian age of basin formation, suggesting that the Jauru Formation was generated in an extensional continental setting without magmatism. Data from detrital zircon FTM suggest at least three continental exhumation or heating/cooling cycles related to long-lived Pangea break-up. The second and more representative cycle is probably coeval to continental flood basalts of Anari and Tapirapuã Formation, during Upper Triassic and Lower Jurassic. The first (280 Ma) and third (150 Ma) events, are not clearly related to regional or continental geological processes in the South American Platform interior.

Cretaceous succession of the Cambambe basin, Chapada dos Guimarães, Mato Grosso, Brazil

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.

Deep structure of Parecis Basin, Brazil from 3D magnetotelluric imaging

The resistivity structure beneath the Parecis basin in central Brazil has been studied using broadband (0.001–5000 Hz) magnetotelluric (MT) data for 455 stations recorded along five lines in the Juruena sub-basin and over the Precambrian basement at the northern and southern basin borders. We found that the reconstructed resistivity-versus-depth profiles from our 3D MT inversion models satisfactorily match the average values of the resistivity well logs in the top 2200 m at two exploration well locations chosen for model validation. The low resistivity Parecis Formation at shallow depth can be traced across the region while an areally restricted conductive sequence at about 2000 m depth can be imaged in the central part of Juruena sub-basin and is interpreted as synrift deposits or remnants of a buried Neoproterozoic basin. Sedimentary formations deeper than 2200 m could be sensed but not individually resolved in our resistivity models. The deeper resistivity structure consists of a resistive upper crust, severely thinned over the Neoproterozoic fold belts bounding the basin to the north and south where it is underlain by conductive zones at mid-lower crustal depths. We interpret the deep crustal steep conductors as pre-existing shear-zones possibly associated with the Upper Cretaceous to Tertiary alkaline magmatism in the region.

Mesozoic tectonic in the deposition and evolution of Cretaceous sedimentary packages of the Parecis Basin, center-western Brazil

The Cretaceous record of the Parecis Basin (central western Brazil) consists of two sedimentary packages with distinct depositional signatures, physically discontinuous, and deposited in two depocenters in the east and west of Serra Formosa Arch. The basin evolved during the Mesozoic age and includes a regional record with volcanism, tectonism, deposition and uplift with erosion. During the Upper Triassic and Lower Jurassic, volcanism and sedimentation occurred, which were followed by uplift and erosion until the Lower Cretaceous, moment in which basic and alkaline magmatism was present. Since the Upper Cretaceous, with the beginning of the compressive phase of Andean Orogeny and opening of the Atlantic Ocean, tectonism and sedimentation were developed in the Juruena Sub-basin during the Cenomanian and in the Xingu Sub-basin from the upper Coniacian. Based on facies association and regional surface recognition, a differential depositional signature was identified for each sedimentary package, influenced by the activity and behavior of the internal sub-basins. In the Juruena Sub-basin, fluvial and aeolian sedimentary package were deposited with sedimentation rates higher than that of subsidence. In the Alto Xingu Sub-basin, a sedimentary package is here named the Rio Tapirapé Formation and includes a lacustrine system that was established with an overpass of the subsidence rate in relation to the sedimentation rate. Toward the top, it was replaced by a fluvial and deltaic sedimentary system, with the decrease in the subsidence rate.

Sedimentology and stratigraphy of Neoproterozoic-lower Paleozoic carbonate-siliciclastic succession of the southwesternmost Amazon Craton, state of Rondônia, Brazil

ABSTRACT: Facies and stratigraphic analysis were carried out in Neoproterozoic-Lower Paleozoic carbonate-siliciclastic deposits of Cacoal and Pimenta Bueno formations exposed on basement rocks and into the Pimenta Bueno Graben, northwestern portion of Parecis Basin, southwesternmost Amazon Craton. The redescription and redefinion of this succession confirmed the previous interpretation for the Cacoal Formation as a Marinoan (~ 635 Ma) cap carbonate. The Cacoal Formation is subdivided here in two units separate by sharp contact found exclusively overlying Mesoproterozoic crystalline basement rocks: 1) a homonymous formation characterized by diamictites, sandstones and siltstones with dropstones interpreted as glacio-marine deposits; and 2) the Espigao d’Oeste Formation that consists of dolostone, dolomitic stromatolites, dolostone-siltstone rhythmite and siltstone interpreted as shallow to moderately deep platform deposits. The Ordovician to Silurian Pimenta Bueno Formation is a filling of Pimenta Bueno graben and overlies locally the Meso and Neoproterozoic rocks. This unit consists in diamictites, sandstones, siltstones and pelites interpreted as glacial-marine and tide- to storm-influenced platform deposits, recording a glacio-eustatic regressive-transgressive event. This new stratigraphic proposal modify the current stratigraphy for the Parecis Basin and suggest, at least, two levels of glaciation exposed in the sothwesternmost Amazon Craton related to the Marinoan and Late Ordovician-Early Silurian events.

Paleoenvironmental reconstruction and evolution of an Upper Cretaceous lacustrine-fluvial-deltaic sequence in the Parecis Basin, Brazil

The Cretaceous in the Brazilian Platform records events of magmatism, tectonism and sedimentation coupled to the Gondwana breakup. Some of these events are registered as sedimentary sequences in interior basins, such as in the Cretaceous sequence of the Alto Xingu Sub-basin, Parecis Basin, Central Brazil. This article proposes the faciologic characterization and paleoenvironmental reconstruction of the Cretaceous sequence of the eastern portion of the Parecis Basin and its relation with some reactivated structures as, for instance, the Serra Formosa Arch. Based on both data from outcrops and core drillings a paleoenvironmental and evolutionary reconstruction of the sequence is herein presented. The base of the studied section is characterized by chemical and low energy clastic sedimentation of Lake Bottom and Shoreline, in a context of fast initial subsidence and low sedimentation rate. As the subsidence process decreased, a deltaic progradation became dominant with deposition in a prodelta environment, followed by a deltaic front and deltaic plain interbedded with fluvial plain, and aeolian deposition completing the sequence. The inferred Coniacian–Santonian age is based on vertebrate (fishes and notosuchians) and ostracod fossils with regional chrono-correlates in the Adamantina (Bauru Group), Capacete (Sanfranciscana Basin), and Bajo de la Carpa (Neuquén Group, in Argentina) formations. The formation of a Coniacian depocenter in the Alto Xingu Sub-basin is associated to the Turonian–Coniacian reactivation event in the Peruvian Orogenic Phase of the Andean Orogeny, with the transference of stresses to interplate setting, reactivating Proterozoic structures of the basement.

Integrated geologic and geophysical interpretation of the Buraco da Velha copper deposit (Rondônia, Brazil): A basis for exploring in related environments

The Buraco da Velha copper deposit lies at the northern limit of the Parecis Basin in the Colorado Graben, Rondônia, Brazil. New geophysical data indicate the presence of a magnetic source below and to the north of the Buraco da Velha deposit, where it corresponds with high gamma-ray U, Th, and K counts. We have studied the source of the magnetic anomaly and delineated it through derivative transforms, Euler deconvolution, MaxiMin total magnetization direction analysis, staged inverse modeling, Th/K ratio, and radiometric ternary imaging. The magnetic anomaly is elongated in the east–west direction and measures [Formula: see text], and the top of the source is estimated to lie at depths mostly between 50 and 100 m. Based on the magnetic model, we indirectly estimate a Jurassic to Cretaceous age, which is compatible with the 180–80 Ma range dated for the copper mineralization. Gamma-ray data are consistent with the hydrothermal alteration in the sedimentary cover and with the presence of an intrusion in the subsurface. We suggest that the intrusion of the magnetic body generated the necessary thermal energy to mix an already oxidized brine and sulfide-bearing fluids at the border of the Parecis Basin, leading to copper deposition and providing a potential analog for similar environments of mineral deposits elsewhere, such as in the Kupferschiefer deposit (Poland) and the Zambia Copper Belt (Zambia).

The new occurrence of Marinoan cap carbonate in Brazil: The expansion of snowball Earth events to the southwesternmost Amazon Craton

Carbonate deposits exposed in the border of the Pimenta Bueno and Colorado grabens, western part of Parecis Basin, southwestern Amazon Craton, Brazil, have been previously considered as Paleozoic record. These deposits lying unconformably on Mesoproterozoic crystalline rocks, the basement of the grabens, and consist predominantly by pinkish dolomite overlying glacial diamictites, with average negative values of δ13C of −3,10‰VPDB. The contact between the dolostone and diamictites is sharp and deformed similarly with others Neoproterozoic cap carbonates occurrences in the Amazon Craton, also related to the Marinoan Glaciation (635 Ma). This new occurrence of Marinoan cap carbonate is composed by two facies associations. Facies Association 1 consists of pinkish peloidal dolostone with even parallel and quasi-planar laminations, wavy and megarriple bedding, macropeloid lenses associated with low-angle truncations, interpreted as fairwhether- and storm-influenced shallow platform deposits. Facies association 2 consists in dolostone rhythmically interbedded with shale underlaid by 5 m-thick laminated siltstones, interpreted as moderately deep platform deposits. This retrogradational succession is overlaid in angular unconformity by Early Paleozoic diamictites and locally by Mesozoic volcanic rocks. This cap carbonate precedes the Paleozoic deposits of Parecis Basin and represents a post-glacial event linked to the Marinoan glaciation, extending to the southwesternmost Amazon Craton the phenomena of the Snowball Earth hypothesis.

Magnetic field analysis of Morro do Leme nickel deposit

The Morro do Leme laterite nickel deposit lies inside the western border of the Parecis Basin (Brazil). This deposit is characterized by high concentrations of lateritic Ni (about 1.8%) and anomalous contents of Pd, Au, Cu, Na, Co, Zn, and Pt in a peridotite and dunite layered intrusion. Besides the existence of geochemical and drilling data, the 3D distribution in the subsurface of this layered intrusion is still unknown. An airborne magnetic survey revealed three east–west elongated magnetic anomalies, characterized by a significant remanent magnetization. The sources of these anomalies were delimitated laterally and had their depths estimated between 90 and 150 m, using techniques that use derivatives. Further, the total magnetization direction was obtained from a distortion analysis of the magnetic anomalies. All these data were united in an initial model for the 3D inversion of the magnetic data. The total and induced magnetization directions were attributed to the inverted model of 0.12 (SI) susceptibility, allowing indirect estimation of the remanence. The model, defined by the depth, the inversion, and the remanence estimates, linked the intrusion to analogue events in the Rondonian-San Ignácio Province. The results indicated that to explore for laterite Ni, the best locations are the southern part of the main anomaly and in the cover above the two smaller anomalies, whereas to explore for Pd, Au, Cu, Na, Co, Zn, and/or Pt, the indicated region is the central portion of the main anomaly.

Application of Terrestrial and Aerial Gravimetry in the Pimenta Bueno Graben, Brazil

This paper presents an application of the gravimetric method using terrestrial and aerial data collected in one sector of the Parecis Basin, Brazil. The data received latitude and elevation corrections and also the Eotvos correction in the aerial survey case. Maps of Free-Air anomaly and Bouguer anomaly were obtained. These maps allowed us to identify the Pimenta Bueno Graben, which is an attractive region for oil and gas prospecting. Finally, the maps were compared and correlated to tectonic domains of the Pimenta Bueno Graben region.