Opening Of Atlantic Ocean Research Articles

Petrophysical Evaluation of Arkoses Reservoirs Based on Well Logs and Plug Samples: A Case Study From Alagamar Formation, Southeastern Portion of Onshore Potiguar Basin, Brazil

The Potiguar Basin is a sedimentary basin located in northeastern Brazil. It covers about 48,000 km² and extends from onshore to offshore. The onshore basin is thought to have formed during the Late Cretaceous, as a result of the opening of the Equatorial South Atlantic Ocean, filled with a thick sequence of sedimentary rocks, including sandstones, shales, and limestones, which were deposited in a variety of processes, including marine, deltaic, and fluvial. The study area is in the southern portion of the basin and the reservoir is composed of arkose sandstones deposited by alluvial fan and fluvial systems of the Upanema Member of the Alagamar Formation, which belongs to the post-rift phase of the Potiguar Basin. Recently, new drilling campaigns revealed a reservoir heterogeneity in these deposits, which requested a new petrophysical approach. Using conventional well logs and petrophysical laboratory analysis of plug samples, this study evaluated the best way to calculate the effective porosity, and shale volume and to understand the stratigraphic controls in the distribution of these properties. Three reservoir zones were mapped: the basal one, Zone 3, has disconnected sand bodies, low porosity, and high shale content; Zone 2 has better petrophysical properties, lateral distribution, and connectivity between the fans and Zone 1 is the better reservoir zone with larger sand bodies, higher porosity values, and well-connected fans.

Provenance of the late Jurassic to Cenomanian sedimentary succession of the Araripe Basin (NE Brazil) and implication for the geodynamic evolution of Western Gondwana

The separation of Gondwana was controlled by preexisting Proterozoic structures and by the development of rift aborted basins, of which the Araripe Basin (NE Brazil) is one of the best examples. Previous studies have focused on the presence and provenance of Aptian–Albian shallow-marine incursions in the Araripe Basin but to date, little attention has been given to its paleodrainage evolution during sedimentation stages. Understanding the paleodrainage evolution is crucial for determining sediment sources and how topographic changes relate to the geodynamic development of the northern part of South America during the fragmentation of Gondwana, and this study investigates the provenance of Mesozoic rift and post-rift sedimentary rocks in the Araripe Basin using a multi-proxy dataset comprising major and trace element concentrations, Sm–Nd isotopic composition, and detrital zircon U–Pb ages. The low Eu/Eu* ratios (0.3–0.9) and high Th/Sc ratios (>0.64) in the most of analyzed samples suggest a felsic and silicic source. The εNd(0) values (−12.3 to −23.7) and TDM ages (1.68 to 2.55 Ga) of analyzed samples suggest overall ancient crustal sources. The presence of oval and elongated zircon grains suggests a major contribution of first-cycle transport sediments. The presence of 2.3–1.8 Ga and 0.63–0.58 Ga U–Pb zircon ages further indicates the dominant contribution of the Borborema Province influenced by the Brasiliano cycle (650–520 Ma). The low contribution of Tonian (∼940 Ma) zircons to the U–Pb zircon age distribution of the rift-beginning stage sample associated with published paleocurrent direction suggests sources located in the northern and northwestern terranes of the Borborema Province. The increase of Tonian (0.9–1.0 Ga) zircon grains during the rift stage suggests a provenance change with a dominant source in the eastern terranes during the rift stage. During the post-rift I stage, the decrease of 1.2–0.72 Ga zircon ages suggests a change in the source areas, with the paleodrainage coming from northern Borborema Province, similar to that of the rift-beginning stage. The samples of the post-rift II stage exhibit dominant contributions of Paleoproterozoic and Neoproterozoic U–Pb ages related to the Albian to Cenomanian uplift of the Borborema plateau during the opening of Equatorial Atlantic Ocean. Together with previous published studies, these findings highlight the significant role played by the post-rift continental uplift to the paleodrainage of the northern part of South America.

Tithonian mafic intrusions in north-central Newfoundland: link to Atlantic rifting?

The small volume, Mesozoic alkali gabbro intrusions of the Budgell Harbour and Dildo Pond stocks and associated alkaline lamprophyre dykes in Notre Dame Bay in Newfoundland are an example of onshore magmatism that may be associated with North Atlantic Ocean opening. Chemical abrasion isotope dilution thermal ionization mass spectrometry U–Pb dating of zircon from Budgell Harbour Stock drill core samples yielded a weighted average 206Pb/238U age of 147.9 ± 0.5 Ma (95% confidence interval, mean square weighted deviation = 0.10). Five 40Ar/39Ar laser step-heating, single phlogopite grain analyses from the Budgell Harbour and Dildo Pond stocks, plus a lamprophyre dyke, yielded ages ranging from 146.3 ± 0.2 to 149.5 ± 0.5 Ma. The data demonstrate a ca. 148 Ma (Jurassic and Tithonian) alkaline magmatic event in Newfoundland, contemporaneous with rifting and offshore basin formation. These new age data and published determinations from magmatic rocks on conjugate margins and adjacent regions reveal long-lived episodic magmatism in the embryonic North Atlantic rift environment. We propose a new model whereby magmatism occurred in Notre Dame Bay at the convergence of older crustal-scale faults and localized Moho depth variations that may have triggered lithospheric mantle melting through isothermal, distal, and edge-driven upwelling related to regional extension.

Volcanoclastic and epiclastic diagenesis of sandstones associated with volcano-sedimentary deposits from the upper Jurassic, Lower cretaceous, Paraná Basin, southern Brazil

The opening of South Atlantic Ocean produced an immense volume of lava that covered an active aeolian system during Lower Cretaceous. The contact of Botucatu Formation sandstones with the Serra Geral volcanic flows generated a variety of volcano-sedimentary features and deposits. During volcanism, the temperature contrast between lava and wet or water-saturated sediments formed volcanoclastic features and related sedimentary deposits. In moments of magmatic quiescence, epiclastic deposits were generated due to the return of sedimentation. Volcanoclastic deposits include volcanic breccias with sandy matrix, and epiclastic deposits include conglomerates and conglomeratic sandstones with intraclasts. Quantitative petrography of the sandy matrix samples of these deposits was analyzed, in order to discuss the paragenesis of volcano-sedimentary interactions. Regular burial diagenetic processes were discussed for epiclastic sandstones and contact diagenesis processes for volcanoclastic sandstones. The original composition of volcanoclastic and epiclastic sandstones is heterogeneous. However, the main difference between them is the presence of textural fluidization indicators found in the volcanoclastic sandstones, along the lava-sediment interface. These textural characteristics indicate that the unconsolidated sediments were probably wet or saturated in water during the interaction with the volcanic flow. Epiclastic sandstones have infiltrated clays and neoformed smectites as their main diagenetic constituent due to alteration of available volcanic lithoclasts. For volcanoclastic sandstones, the formation of opal, chalcedony and megaquartz cements through burial is unlikely, due to the intergranular volume occupied by these cements, indicating near-surface early diagenetic conditions. Therefore, the mechanisms of dissolution and repreciptation of the siliceous fluids, incorporated in the system, formed mainly the chalcedony cementation. Furthermore, zeolite cementation, which occurs along the lava-sediment contact, is another important indicator of contact diagenesis for volcanoclastic sandstones. During telodiagenesis, the formation of secondary porosity by dissolution of volcanic lithoclasts and detrital K-feldspar grains was of great importance in volcanoclastic and epiclastic sandstones.

Origin of Cretaceous alkaline annular structures in the peri-cratonic terranes of the Reguibat Rise in Morocco: New constraints from carbonatite–silicate pairs and subsolidus processes

We report an integrated field, petrographic, and geochemical study of three major Cretaceous carbonatite annular occurrences from the Oulad Dlim massif (Adrar Souttouf, NW margin of the West African Craton, south Morocco): Twihinate, Lamlaga, and Lahjayra. These complexes are commonly considered isolated carbonatite structures partially to completely overlain by silica breccias and iron oxide rocks. The carbonatites include two calcio‑carbonatites facies with distinct textural features and bulk-rock and mineral geochemistry: the “white” facies present characteristics consistent with a primary magmatic stage, while the “grey” facies likely resulted from hydrothermal alteration of the white carbonatites. We provide evidence that the annular structures include ultrapotassic alkaline syenites, probably derived from the same primary carbonated/silicated melt as the carbonatites, after partial melting of a K-rich lithospheric mantle source. The overlying silica breccias retain sandstone/quartzite-like stratifications, whereas their trace element signature is analogous to those of the carbonatites, suggesting that they were subjected to intense hydrothermal activity during carbonatite magmas emplacement. The iron-oxide rocks are chemically comparable to worldwide references of carbonatite-related laterite profiles, indicating a similar origin. All subsolidus (hydrothermal and weathering) alteration stages were accompanied by significant REE-Nb concentration resulting in high-grade deposits of monazite, synchysite, apatite and pyrochlore. Like other peri-Atlantic alkaline occurrences, the investigated Cretaceous alkaline‑carbonatite occurrences are likely related to the Atlantic Ocean opening and reactivation of pre-existing Pan-African tectonic structures.

Effects of Tectonic Quiescence Between Orogeny and Rifting

AbstractTectonic quiescence is a period of nearly null relative plate motion. During the Atlantic Ocean opening, rifting cut across continental segments of supercontinent Pangea that had been through different periods of quiescence, which varied from tens to hundreds of Myr. Using numerical models, we explored the effects of quiescence between orogeny and rifting on the pre‐rift lithosphere and subsequent conjugate rifted margin configuration. Our model results showed that quiescence of ∼30–60 Myr caused wide orogenic wedges (>300 km) to be heated beneath their centers, which led to the breakup of the lithospheric mantle before the breakup of the continental crust during rifting, resulting in hyperextended conjugate rifted margins. In these scenarios, continental lithosphere breakup away from the suture contributed to the preservation of previously subducted lower crust along the subduction zone. Long periods of tectonic quiescence (∼100 Myr) experienced by a wide orogen permitted efficient orogenic collapse and lateral spreading, with pronounced lithospheric thermal weakening. Consequently, post‐quiescence rifting reactivated the suture and effectively drove plate eduction, exhuming most of the subducted continental lower crust and mantle lithosphere. Ultimately, ultra‐wide domains developed into asymmetric conjugate margins. Contrastingly, in narrow orogenic wedges (<200 km), different quiescence durations had minor effects on the margin architecture. In these cases, a longer quiescence time contributed to a larger preservation of subducted crust in the fossil subduction zone after rifting. Our models agree with conjugate rifted margins in the North and South Atlantic, which probably developed after the collapse of wide orogens and subsequent periods of tectonic quiescence.

Delineating the main structural outlines and the petrophysical properties of the Albian-upper Cretaceous Reservoirs using seismic and well log data, Shushan Basin, north Western Desert, Egypt

This study concerns with delineating and analyzing the subsurface structural setting of Khalda Oil Field at Shushan Basin as a key factor for evaluating the petrophysical properties and the deliverability of the Bahariya Formation. Shushan Basin is considered one of the most prospective basins in the Western Desert. The oil is trapped within a gentle seismically defined anticline that contains a series of coastal sandstone reservoirs interbedded with a neritic and tidal flat mudstone and a few carbonate interbeds of the Albian-Cenomanian Bahariya Formation forming multiple oil-bearing sandstone reservoirs. A set of borehole logging and seismic data were available, tied, processed, and mapped to delineate the predominant structures in the study area. Results show the implication of the NW–SE, NE–SW, and E–W gravity faulting on the Bahariya and the overlying Abu Roash formations. This set of gravity faults is attributed to the Mesozoic tensional stresses. The depth structure contour maps on the upper surfaces of the Lower and Upper Bahariya members reveal the presence of some NE–SW gravity faults that dissected the Khalda Field into some slightly tilted blocks, shallowing in depth to the south as deepening to the north. Besides, the study area has been affected later in the Jurassic by E–W gravity faults during the Atlantic Ocean opening. This faulting trend was changed later into the NE–SW direction as a result of the movement of North Africa against Laurasia. The petrophysical analysis indicated that the Lower Bahariya sandstone reservoir has a good reservoir quality (16 ≤ ∅ ≤ 25%, 53 ≤ Sw ≤ 59%, 6.0 ≤ Vsh ≤ 32.2%, and good net-pay thickness 18.0–38.0 ft). It is revealed that the drilled wells penetrating the crest of the anticline are prospective while that located in the anticline flanks are water-bearing recommending exploration away from the anticline flanks. By applying this workflow, it is possible to explore for the similar subsurface hydrocarbons-bearing sequences in the Western Desert and North Africa in future exploitation plans.

Origin and evolution of a CO2-Rich gas reservoir offshore Angola: Insights from the Gas Composition and isotope analysis

The pre-salt sections of the offshore Kwanza Basin, Angola are potentially petroliferous reservoirs like their Brazilian counterparts on the other side of the South Atlantic Ocean. In this study, a pre-salt reservoir of the offshore Kwanza Basin was found to contain solid bitumen and gas of high portions of CO2 (ca. 83%) and methane (ca. 16%). The chemical and isotopic (C, H and He) compositions of the gas indicate that the CO2 is primarily mantle-derived while the gaseous hydrocarbons are mainly late mature thermogenic gas. The Barremian-Aptian Red Cuvo Formation with sapropelic and humic organic matter and an average TOC content of 1.7% could be the principal source of the bitumen and gaseous hydrocarbons in the reservoir. Rapid oil generation since the source rock sedimentation could have occurred as a result of the fast and thick salt deposition or the high thermal gradient created by the crustal thinning and magmatism during the South Atlantic Ocean opening. The δ13C of gaseous hydrocarbons also indicates their mixed origins of sapropelic and humic organic matters and possible abiotic processes.This reservoir is similar in the origin of CO2 and geological background to some pre-salt reservoirs in the Brazilian basins. Such similarities with prolific petroleum systems may imply great petroleum potential in the pre-salt sections of the offshore Kwanza Basin, Angola.

A new method for evaluating the spatial correspondence between surface and subsurface geological lineaments: A case from the Potiguar Basin, NE Brazil

This paper provides a new technique for evaluating lineaments that focuses on the spatial link between surface and subsurface elements in a single layer. We chose a test area in northeast Brazil's Borborema Province. We extracted magnetic and relief lineaments from aeromagnetic and SRTM data. We defined the lineaments that agree on both layers using GIS operations. To identify the concordance between lineaments, we constructed kernel density maps that combined the data from each layer. We detected NE-SW solid conditioning of the structures. Even when evaluating only the concordant lineaments in other directions, they are expressed in groupings in NE-SW, as with NW-SE features, that associates with transfer failures and are contained by large NE-SW structures. The Atlantic Ocean's opening and expansion phases were linked to the E-W direction, also truncated by NE-SW lower density sections. The N–S trend was inexpressive. Areas with a higher density of concordant lineaments were associated with regions more susceptible to the Neotectonics of landform inversion, forming domes and arches-oriented NE-SW, as in Serra do Mel, Macau High, and Ponta Grossa High. Lower lineament density areas indicated visual correspondence with the most significant number of production wells in the Potiguar Basin, indicating that these regions are more favorable for the accumulation and trapping of hydrocarbons.

The Brazilian Midwest lateritic domains: Regolith features and dynamic

Intracontinental deformations cause the uplift of erosive surfaces and are consequence of lithosphere destabilization processes in cratonic domains. The Atlantic Ocean opening influenced processes 2000 km away from the coast with reflections in the regolith. The Brazilian Midwest regolith is characterised by a stepwise landscape of highlands, mountains, plateaus, inselbergs, canyons, and lowlands that form four morphostratigraphic surfaces. Surface 1 is sustained by quartzite mountains. Surfaces 2 and 3 are mantled by ferruginous lateritic duricrusts. The duricrusts in surface 2 are quartz-hematitic, hardened, columnar, reddish-pink to yellowish-red, have protonodular, fragmental, and pisolitic textures, and reach up to 3 m thick, whereas the duricrusts in surface 3 are more friable, quartz-goethitic, columnar, yellowish-red to orangish-yellow, have a vermiform texture with bioturbated micro-aggregated clay-ferruginous matrix, reaching up to 2 m thick. Surface 4 includes the lower alluvial plains. The purpose of this study was to understand the formation of these regolith surfaces, their relationships, the impact of the lateritic dynamic, and the continental influences as a landscape-shaping agent. The area was studied using SRTM images, swath profiles, and the Roughness Concentration Index, as well as fieldwork control, textures, mineralogy, and geochemistry analysis. Surface 1 marks the regional erosional event, while surfaces 2 and 3 mark two stable well-drained ferruginization weathering episodes and a seasonal drier to humid climate change. Surface 4 sculpts the lowlands. These regolith sequential landforms surfaces were produced in a complex landscape process modelled by an extensive erosional regime as a result of tectonic movements during the Atlantic Ocean evolution evolution and tropical weathering climate since the Neogene, as indicated by the lateritic duricrusts features (textures, mineral, and chemical composition). This landscape is younger than expected for Archean to Paleozoic terrains subjected to tropical weathering, dating back at least to the Paleogene, the age of the majority of older lateritic duricrusts in Brazil and around the world. These younger morphostratigraphic surfaces in cratonic domains, as well as the geological processes involved in tectonic and climate influence, had significant implications for the Brazilian South American Platform regolith.

Tectonic and stratigraphic evolution of the Santos Basin rift phase: New insights from seismic interpretation on Tupi oil field area

Most recent published work aims to understand pre-salt carbonates interval development and depositional environment conditions. Previous contributions which debate about rift extension usually consist of regional analysis regarding the South Atlantic Ocean opening and keep the debate about syn-rift and sag stage limits. This work uses 3D seismic volume analysis, structural contour maps and isopach maps analysis in order to understand and establish rift extension development in Santos Basin pre-salt interval. Our analysis concludes that the pre-salt interval is inserted into the syn-rift context, divided into 3 phases: Lower Rift, Upper Rift and Later Rift. Fault analysis shows that the 3 main fault directions act as a structural control for the rift phases. Lower and Upper Rifts are controlled by SW-NE and N–S trend faults, while Later Rift is controlled by SE-NW trend faults. Seismic interpretation and well log analysis indicates that, at Santos Basin, Lower Rift includes Camboriú and Piçarras Formation, Upper Rift comprehend Itapema and Lower Barra Velha Formation, while Later Rift contains Upper Barra Velha Formation, as sag-type basin.

Exhumation of the passive margin of the DR Congo during pre- and post- Gondwana breakup: Evidence from low-temperature thermochronology, geology and geomorphology

The Lower Congo region encompasses the South Atlantic passive margin of the Democratic Republic of Congo (Central Africa). It is host to the lowermost course of the Congo River, cutting through the Central African Atlantic Swell (CAAS). The region is underlain by litho-structural units of the Pan-African West Congo Belt, which consists of different tectono-metamorphic domains. The Precambrian basement is covered to the west by marine deposits of the South Atlantic Ocean and to the east by continental deposits of the Congo Basin. In this study we aim to constrain the timing of uplift and exhumation of the CAAS by using apatite fission track (AFT) thermochronology in combination with an updated overview of the geology and geomorphology of the Lower Congo region. AFT ages vary widely between 108 and 312 Ma. Short track lengths (11–12 μm) and broad, complex track length distributions indicate mixed ages between multiple thermal events. We derive the timing of exhumation from inverse thermal history models, that show that the Lower Congo region experienced a first exhumation event before Gondwana breakup during the Carboniferous to Middle Jurassic. This event is probably related to compressive forces at the boundaries of Gondwana. Both rifting and subsequent opening of the South Atlantic Ocean do not seem to have had a pronounced effect on the CAAS region. During the Late Cretaceous to Palaeogene, a slight reheating is suggested and could be due to subsidence and consequential modest reburial of the Lower Congo rocks. A second phase of exhumation initiated around the Palaeogene–Neogene and eventually emplaced the sampled rocks at surface temperatures. The multi-phased differential denudation results from reactivation of fault-bounded tectono-metamorphic blocks of the Precambrian basement, controlled by the combination of two systems of faults related to the Cretaceous South Atlantic Ocean opening and Pan-African orogeny respectively. Differential denudation of the Lower Congo region is also well-marked by independent qualitative indicators of the present-day geomorphology including distinct knickpoints and steep valleys along the course of the Lower Congo River, reworking of erosion surfaces and associated laterite crust and contrasting karst morphology.

An exotic Cretaceous kimberlite linked to metasomatized lithospheric mantle beneath the southwestern margin of the São Francisco Craton, Brazil

We present major and trace element compositions of mineral concentrates comprising garnet xenocrysts, ilmenite, phlogopite, spinel, zircon, and uncommon minerals (titanite, calzirtite, anatase, baddeleyite and pyrochlore) of a newly discovered Late Cretaceous kimberlite (U-Pb zircon age 90.0 ± 1.3 Ma; 2σ) named Osvaldo França 1, located in the Alto Paranaíba Igneous Province (APIP), southeastern Brazil. Pyrope grains are lherzolitic (Lherz-1, Lherz-2 and Lherz-3), harzburgitic (Harz-3) and wehrlitic (Wehr-2). The pyrope xenocrysts cover a wide mantle column in the subcratonic lithosphere (66–143 km; 20–43 kbar) at relatively low temperatures (811–875 °C). The shallowest part of this mantle is represented by Lherz-1 pyropes (20–32 kbar), which have low-Cr (Cr2O3 = 1.74–6.89 wt.%) and fractionated middle to heavy rare earth elements (MREE-HREE) pattern. The deepest samples are represented by Lherz-2, Lherz-3, Harz-3, and Wehr-2 pyropes (36–43 kbar). They contain high-Cr contents (Cr2O3 = 7.36–11.19 wt.%) and are characterized by sinusoidal (Lherz-2 and Wehr-2) and spoon-like (Lherz-3 and Harz-3) REE patterns. According to their REE and trace elements, pyrope xenocrysts have enriched nature (e.g., Ce and Yb vs. Cr2O3), indicating that the cratonic lithosphere has been affected by a silicate melt with subalkaline/tholeiite composition due to their low Zr, Ti and Y concentrations. Besides minerals with typical kimberlitic signatures, such as ilmenite and zircon, the exotic compositions of phlogopite and ulvöspinel suggest an enriched component in the magma source. The formation of rare mineral phases with strong enrichment of light-REE (LREE) and high field strength elements (HFSE) is attributed to the late-stage kimberlitic melt. We propose a tectonic model where a thermal anomaly, represented by the low-velocity seismic anomaly observed in P-wave seismic tomography images, supplied heat to activate the alkaline magmatism from a metasomatized cratonic mantle source during the late-stages of Gondwana fragmentation and consequent South Atlantic Ocean opening. The metasomatism recorded by mineral phases is a product of long-lived recycling of subducted oceanic plates since the Neoproterozoic (Brasiliano Orogeny) or even older collisional events, contributing to the exotic character of the Osvaldo França 1 kimberlite, as well as to the cratonic lithospheric mantle.

Provenance constraints on the Cretaceous-Paleocene erosional history of the Guiana Shield as determined from the geochemistry of clay-size fraction of sediments from the Arapaima-1 well (Guyana-Suriname basin)

Recent studies have emphasized the role of the Equatorial Atlantic Ocean opening on the drainage reorganization and erosion of the West African and northeastern Brazilian cratons during the Cretaceous. However, such studies are lacking regarding the Guiana Shield. To fill this gap, we determined the provenance of sediments deposited in the Suriname-Guyana basin during the Cretaceous-Paleocene based on the total organic carbon (TOC) content, δ13CTOC, major and trace element concentrations, and Sr-Nd isotopic data of nineteen clay-size fraction samples from one industrial well (Arapaima-1). Overall, the absence of correlation between the TOC content, δ13CTOC and provenance proxies as well as the lack of Ce anomaly indicate that marine authigenesis, reducing conditions, and weathering have not markedly affected the trace element ratios (Eu/Eu*, Cr/Th, Th/Sc) and Sr-Nd isotopic compositions used as provenance proxies. Three samples (Early Cretaceous, Cenomanian and Late Campanian) have radiogenic εNd(0) values > − 2.2. The Early Cretaceous sample may have been sourced by Precambrian or Mesozoic mafic rocks of the Guiana Shield or contemporaneous basic volcanic rocks. The Cenomanian sample is likely to have been sourced by basic volcanic rocks of the Caribbean Large Igneous Province or the Takutu rift whereas the Late Campanian sample may have recorded more acid volcanism in the Takutu Rift. The other samples have an overall Guiana cratonic provenance with variable contributions of the Paleoproterozoic and Mesoproterozoic terranes and Jurassic or Precambrian mafic dykes. Increases in 87Sr/86Sr ratios and trace element ratios characteristic of more differentiated sources suggest an increasing contribution from the inner part of the Guiana Shield after the Coniacian. Because of the possible presence of active volcanism during the deposition of analyzed samples, post rift mantle upwelling under the equatorial margin is the favored mechanism to explain the increasing erosion of the Guiana Shield inner parts.

Evaporitic brines and copper-sulphide ore genesis at Jbel Haïmer (Central Jebilet, Morocco)

The Jbel Haïmer copper-sulphide mineralisation occurs at 20 km north of Marrakesh in the Variscan Jebilet massif, Morocco. Most of the ores (up to 3.9 wt% Cu, ≤38 ppm Ag, and up to 2.9 ppm Au) occur as impregnations of NE-SW fault/fracture zones and related tectonic breccia. Two independent stages of fluid circulation and mineral deposition are distinguished. First, Late Variscan high temperature-low pressure metamorphism synchronous of granite intrusion induced percolation of C–H-O-N hydrothermal fluids throughout faults and shear zones. The drop in pressure from lithostatic down to hydrostatic values at temperatures around 350 ± 50 °C triggered quartz precipitation associated with minor Sn-As-(Co-Ni) deposits and finally brecciation. The second stage, more recent than Triassic, consists in the deposition of quartz + carbonates, followed by Cu-(Pb-Zn) sulphides (±Ag-Au alloys) in fractures crosscutting 240 Ma microdiorite dikes. Mixing of evaporitic brines likely coming from Triassic formations with low salinity aqueous fluids was responsible for Cu-sulphide deposition at temperatures of around 220–280 °C at a depth of 4–5 km. These base metal-rich brines are similar to those from the nearby Roc Blanc Ag-deposit and several other silver and base metal deposits in Morocco, considered as having circulated during the Central Atlantic Ocean opening.

Geophysical-structural framework of the Campo Alegre basin (Santa Catarina State, southern Brazil)

Numerous syn- to post-collisional basins originated as a response to crust stabilization processes after the collisions between Apiaí, Curitiba and Luis Alves Cratons during the Neoproterozoic in Brazil. The late Neoproterozoic Campo Alegre Basin represents one of these so called transitional-stage basin in the southern Brazil. The structural record of this basin is an important clue for understanding the regional geological development after the Western Gondwana supercontinent agglutination. An integrated study of airborne geophysical data, ground gravimetry and structural analysis of the basin and its basement outlined its magnetic-structural framework and its tectonic evolution. Thus, it was possible to present a basin 3-event tectonic evolution history. The first event (E1) is marked by WNW and ENE normal faults as a response to conjugated extension stress fields (σ3 and σ2). The normal faults generated in (E1) conditioned the deposition of the coarse-grained basal deposits close to basin borders. A second event (E2) was recognized through EW normal faults and ENE-WSW right-lateral strike-slip faults on basin intermediate formations. Hydrothermalism from volcanism final fluids is associated with these faults, especially on the south portion. The last tectonic event (E3) that affected basin rocks is characterized by the reactivation of E1 normal faults as inverse and occurred in Mesozoic Era associated with South Atlantic Ocean opening. This event is also marked by NNW e EW strike-slips faults. Besides the gravimetric model and the magnetic interpretation, field data allow us to conclude that the nucleation of Campo Alegre Basin began as a conjugated rift system.

Structural controls and 40Ar/39Ar geochronological data of basic dike swarms in the eastern domain of the Parnaíba Basin, northeast Brazil

Two important magmatic events related to the breakup of the Pangea supercontinent and the opening of the Atlantic Ocean are represented, in the Parnaíba Basin (Northeast Brazil), by volcanic intrusive and subordinate extrusive rocks ascribed to the Mosquito (Early Jurassic) and Sardinha (Early Cretaceous) magmatic suites based on K–Ar and Ar/Ar dates. In the southeastern and central-eastern regions of the basin, the basic dike swarms display either a consistent or different orientations at given places. These rocks were elected for Ar/Ar geochronological dating to verify whether they have different ages or different emplacement models, integrating geochronological and tectonic-structural approaches. Structural analysis of remote sensor images and field data allowed to characterize (i) a family of consistent NE-trending lineaments in the southeastern border of the basin and (ii) three sets striking NE-SW, ENE-WSW, and NW-SE, in the central-eastern region. In outcrops, these swarms correspond to diabase dikes and sills associated with fracture and fault systems also recorded in their older host sedimentary rocks of the basin. In the southeastern region, the NE-trending dike system, correlated to the Sardinha Suite was controlled by extensional fractures and faults with the same orientation, arranged adjacent to and along the basin border. These structures indicate a NW extension, well documented further east in the continental rift basins of Northeast Brazil, related to the onset of the South Atlantic opening in the Early Cretaceous. Further north, in the central-eastern region of the basin, the emplacement of a WNW-trending dike, dated at 135 Ma, is interpreted as controlled by a strike-slip sinistral fault also related to the NW extension. The second group of dikes in this region presents older ages (190–200 Ma range), being thus correlated to the Mosquito Suite, controlled by NNW extension and well exposed in the central-western region of the basin. These older dikes, previously poorly known in the eastern domain, display variable orientations but following a NE-trending swarm of fractures along the NE-trending Transbrasiliano Lineament. It is proposed that the Mosquito Suite dikes in this domain were emplaced along coeval and older (Permian to early Triassic) structures reactivated by the NNW-SSE extension during the opening of the Central Atlantic Ocean, at the transition between the Triassic and the Jurassic.

Emplacement of dike swarms from the island of Ilhabela (SE Brazil) and its relationship with the South Atlantic Ocean opening revealed by magnetic fabrics

Magnetic fabric and rock-magnetism studies were performed on 153 dikes from diabase, lamprophyre and alkaline dike swarms that outcrop on the island of Ilhabela, NE São Paulo State. The dikes crosscut Archean and Proterozoic polymetamorphic rocks. Their thicknesses range from a few centimeters to 4 or 5 m. They trend predominantly N30-60E with a very steep (~60°) to vertical dip. Magnetic fabrics were determined using anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM). Rock-magnetism measurements reveal that the magnetic mineral of the three swarms is magnetite mainly with grains in the range of 2–5 μm. For a few dikes, these grains are the carriers of bulk magnetic susceptibility but are not responsible for the AMS, which is due to Fe-bearing minerals, as shown by AARM. The main AMS fabric recognized in the swarms is due to magma flow, in which the Kmax–Kint plane is parallel to the dike's plane, and the magnetic foliation pole (Kmin) is perpendicular to it. The analysis of the Kmax inclination showed that the dikes were fed by horizontal to vertical flows. However, for the minority of the dikes, the AMS and AARM tensors are not coaxial. The AARMmax is oriented N30–60 W, approximately perpendicular to the Kmax. The AARMmax orientation is similar to the direction of a fault system located mainly in the Santos marginal basin which, was formed in the Cretaceous rifting during the South Atlantic opening. The AARM fabric is tectonic in origin, and is parallel with the extensional paleostress responsible for the Gondwana break-up. The comparison of AMS and AARM fabrics suggests that diabase and lamprophyre dikes were emplaced in three distinct events in the earliest stages of the South Atlantic opening. Later, the alkaline dikes were emplaced in the final stages of the Atlantic opening.

Gravity and Magnetic Constraints on the Crustal Structure of the Ceará Plateau, Brazilian Equatorial Margin

The Brazilian Equatorial Margin is a transform passive margin with long fracture zones and several seamounts, including the largest one, the Ceara Plateau. This is a complex and poorly recognized area, with few available geophysical data. Gravity and magnetic data from the Equant I Project, seismic published lines and other previous studies are used to make several models and analyses. We recognized gravity and magnetic anomalies related to a denser, magnetized basement, internally comprised of two magmatic highs. Through gravity and magnetic forward modeling, we estimated the basement surface between 800 and 6000 m and the Mohorovicic discontinuity about 22-23 km below the Ceara Plateau. We also present and discuss the position of the continent-ocean boundary 40 km away from the continental shelf, placing the plateau at the oceanic crust domain, and the area of the transitional crust, with an extension of 40-50 km, formed during the rifting phase of the Atlantic Ocean opening.

Shallow necking depth and differential denudation linked to post‐rift continental reactivation: The origin of the Cenozoic basins in southeastern Brazil

AbstractThe southeastern Brazilian margin presents post‐breakup Cenozoic tectonism that created a series of grabens and small sedimentary basins, known as the Continental Rift of Southeastern Brazil. The formation of this rift occurred long after the South Atlantic ocean opening and has been attributed to different mechanisms like regional uplift induced by hotspot activity, pulses of Andean orogeny and reactivation of pre‐existing faults. However, the proposed models lack an analytical or numerical verification from a geodynamic point of view. Based on finite element modelling of the lithospheric stress field evolution we conclude that a shallow necking depth, consistent with the hyperextended southeastern Brazilian margin, combined with differential denudation of the continent, resulted in an extensional stress field in the upper crust that induced the observed Cenozoic tectonism.