ABSTRACT The Cabo Frio High corresponds to a regional basement arch located on the continental platform between the Santos and Campos basins. The distal margin of the Cabo Frio Outer High is characterized by gravity and magnetic anomalies suggesting an association with magmatic centers that affected the salt basin. Volcanic rocks are observed both on the proximal margin, where the Cabo Frio Volcanic Complex is marked by several magmatic episodes, and on the distal margin, where the Cabo Frio Outer High is marked by intrusive and extrusive igneous features affecting the pre-salt and post-salt sedimentary successions. The most important magmatic events in the area are related to (a) the pre-rift phase, with massive lava flows both onshore and offshore of the incipient continental margin; (b) the synrift phase, as indicated by several wells that drilled subaerial basaltic lava flows intercalated with lacustrine sediments; (c) the sag basin and transitional evaporitic post-rift phase, as indicated by sills and laccoliths overlain by salt and also by discordant igneous structures intruded into salt layers; and (d) the post-breakup phase, with major magmatic activity registered in the Upper Cretaceous and in the Paleogene. Volcanic events in the Cabo Frio region are a major factor in basin development and greatly impact the petroleum resources assessment, particularly when igneous rocks intrude into pre-salt source rocks and reservoirs. The geochemical data from producing fields and exploratory wells in the Cabo Frio region indicate that the main source rock system for the known accumulations are the upper Barremian calcareous black shales, deposited in brackish-to-saline water lacustrine environments from the Coqueiros Formation. In the continental shelf, the oil fields are characterized by intense biodegradation, which has deteriorated the oil quality. Several factors are important elements that control the prospectivity of the Cabo Frio region, such as thermal maturity of the source rocks, reservoir depth, seal effectiveness, magmatic events, and mixing of oils generated from different maturity pulses.

Abstract The great geological success in OGX's oil prospecting at the Albian-Cenomanian carbonate shelf on the southern Campos basin involves at least three different geologic components: (1) the appropriate geographic position of the traps within a low geopressure zone, away from the high pressure oil kitchen (external rift) and beyond the most prolific oil carrier bed of the Campos basin–the Cabo Frio fault system (Papa Terra, Maromba, Polvo and Peregrino oil fields and all of the discoveries made by OGX); (2) the Late Cretaceous and mainly early Tertiary magmatic events that contributed to the generation of an intensive and extensive high secondary porosity and increased permeability by thermobaric effects on the carbonate rocks; and (3) the presence of a high energy carbonate depositional system in the Quissamã Formation (Albian) and Imbetiba Formation (Cenomanian), which involves lateral accretion [e.g. , Pipeline (1-OGX-2A-RJS well), Waimea (1-OGX-3-RJS well), Etna (1-OGX-6-RJS well), Fuji (1-OGX-8-RJS well), Illimani (1-OGX-28D-RJS well), Peró (1-OGX-14-RJS well), and Tamborá (1-OGX-52-RJS well)] and vertical stacking [e.g., Waikiki (1-OGX-25-RJS well)] of thickening and shoaling upward carbonate depositional cycles. The oil accumulations in the Quissamã Formation are well defined by logs and pressure data (Pipeline, Etna, Illimani, Fuji, Waimea and Tambora wells) and are typically related to five tilted fault block trends (faults strike direction northeast/southwest), subparallel to the Cabo Frio fault system, all of which are filled to the spill-point, and each has a different oil/ water contacts. As a general rule the antithetic faults act as the major controlling seal for the oil accumulations in these three-way dip closure structures. The last successfully tested exploration model was a stratigraphic-structural trap formed by the updip pinch out of prograding carbonate shoals in the Imbetiba Formation relative to regional structural dip. This was drilled by the 1-OGX-14-RJS well (Peró Prospect) with an oil column of approximately 60 m based on pressures and well log data.

Abstract The Cabo Frio region, offshore Rio de Janeiro, lies between two of the most prolific Brazilian oil provinces, the Campos and Santos basins. Major geologic features have been identified using a multidisciplinary approach integrating seismic, gravity, petrographic, and borehole data. The Cabo Frio frontier region is characterized by marked changes in stratigraphy and structural style and is unique among the Brazilian marginal basins. Major geologic features include the deflection of the coastline and pre-Aptian hinge line from northeast to east; a large east-striking offshore graben related to salt tectonics; a northwest-trending lineament extending from oceanic crust to the continent; basement-involved landward-dipping (antithetic) normal faults in shallow water; a stable platform in the southern Campos Basin; a thick sequence of postbreakup intrusive and extrusive rocks; and, near the Santos Basin, a mobilized sequence of deep-water postrift strata affected by landward-dipping listric normal faults. These faults are unusual in salt-related passive margins in that they dip landward, apparently detach on the Aptian salt, and show large late Tertiary offsets. Locally, the older sequences do not show substantial growth in the downthrown blocks. South of the Rio de Janeiro coast, a phenomenal landward-dipping fault system detaches blocks of the Albian platform to the north and, to the south, coincides with the depositional limit of the Albian platform. This deep-water fault system controls features that can be mapped for hundreds of kilometers along strike and forms an Albian stratigraphic gap tens of kilometers wide. Two end-member processes of salt tectonics in the Cabo Frio region result in either synthetic or antithetic basal shear along the fault weld under the overburden: (1) thin-skinned processes, in which the listric faults were caused by salt flow in response to gravity forces related to massive clastic progradation from the continent; and (2) thick-skinned processes, in which faulting was indirectly triggered by diastrophic causes or disequilibrium in the basement topography. The structural styles in the Cabo Frio region are compared with analogs from other basins and with sandbox models.

Abstract This work integrates the available geological information and geochronology data for the Cretaceous–Recent magmatism in the South Atlantic, represented by onshore and offshore magmatic events, including the oceanic islands along the transform faults and near the mid-ocean ridge. The analysis of the igneous rocks and their tectonic settings allows new insights into the evolution of the African and Brazilian continental margins during the South Atlantic opening. Following the abundant volcanism in the Early Cretaceous, the magmatic quiescence during the Aptian–Albian times is a common characteristic of almost all Brazilian and West African marginal basins. However, rocks ascribed to the Cabo Granite (104 Ma) are observed in NE Brazil. In West Africa, sparse Aptian–Albian ages are observed in a few coastal igneous centres. In the SE Brazilian margin, an east–west alkaline magmatic trend is observed from Poços de Caldas to Cabo Frio, comprising igneous intrusions dated from 87 to 64 Ma. Mafic dyke swarms trending NW also occur in the region extending from the Cabo Frio Province towards the Central Brazilian Craton. On the West African side, Early Cretaceous–Recent volcanism is observed in the Walvis Ridge (139 Ma), the St Helena Ridge (81 Ma) and the Cameroon Volcanic Line (Early Tertiary–Recent). Volcanic islands such as Ascencion (1.0–0.65 Ma), Tristão da Cunha (2.5–0.13 Ma) and the St Helena islands (12 Ma) most probably correspond to mantle plumes or hot spots presently located near the mid-Atlantic spreading centre. Within the South America platform and deep oceanic regions, the following volcanic islands are observed: the Rio Grande Rise (88–86 Ma), Abrolhos (54–44 Ma), the Vitória–Trindade Chain (no age), Trindade (2.8–1.2 Ma) and Fernando de Noronha (12–1.5 Ma). There are several volcanic features along the NW–SE-trending Cruzeiro do Sul Lineament from Cabo Frio to the Rio Grande Rise, but they have not been dated. The only known occurrence of serpentinized mantle rocks in the South Atlantic margin is associated with the Saint Peter and Saint Paul Rocks located along the São Paulo Fracture Zone. The Cameroon Volcanic Line in NW Africa is related to the magmatism that started in the Late Cretaceous and shows local manifestations up to the Present. The compilation of all available magmatic ages suggests an asymmetrical evolution between the African and South America platforms with more pre-break-up and post-break-up magmatism observed in the Brazilian margin. This is most likely to have resulted from the different geological processes operating during the South Atlantic Ocean opening, shifts in the spreading centre, and, possibly, the rising and waning of mantle plumes. Supplementary material: A complete table with radiometric dates that have been obtained by universities, government agencies and research groups is available at: www.geolsoc.org.uk/SUP18596

Abstract Salt–sediment interplay in the Santos Basin is investigated integrating seismic interpretation, kinematic restoration and analogue modelling. Deformation within the post-salt sequence results from thin-skinned gravitational gliding and spreading, driven primarily by halokinesis, greatly affected by massive sediment inflows. The impressive landward-dipping listric Cabo Frio Fault controls the major depocentres updip, whereas salt-cored folds accommodate downdip shortening. Sediment supply from confluent directions creates a complex interference pattern of superposed folds with intervening polygonal minibasins. A new structure is identified (termed the ‘Ilha Grande Gravitational Cell’), a linked system of updip extension and downdip contraction detached on salt, comprising the Cabo Frio Fault and minibasins. It moves to the SE, with eastern and western borders suggesting lateral gradients of slippage. This thin-skinned feature results from the differential load imposed by a thick prograding wedge over the ductile salt and is independent of pre-salt structures. The post-salt sequence moves basinwards due to halokinesis, thereby changing position relative to the pre-salt sequence, which implies that any present-day correspondence between pre- and post-salt structures may not attest to linkage in the past. Application of kinematic restoration techniques allows the true position and geometry of the key elements through time, improving petroleum systems assessment.

Abstract A 530–490 Ma tectono-metamorphic event, the Búzios orogeny, is recognized within the Ribeira Belt, along the coast of SE Brazil. Tectonic evolution started with a Late Neoproterozoic marine basin and volcanic activity at c . 610 Ma. The rocks in this basin were affected by high-grade metamorphism at c . 530 Ma, coeval with deformational phases D 1 –D 2 , which generated compressive low-angle tectonic structures with top-to-NW tectonic transport. Large recumbent folds with NW–SE axes parallel to the main stretching lineation formed during D 3 as the Cabo Frio tectonic domain, the focus of this study, collided with the Oriental terrane to the NW. D 4 sub-vertical shear zones are limited in extent. A new U–Pb age of 501±6 Ma is reported for zircon from an amphibolite-facies shear zone related to either D 3 or D 4 . Post-tectonic 440 Ma pegmatites mark the final stage of tectono-magmatic activity. The Cabo Frio tectonic domain has African affinities and is exotic to the Ribeira Belt. Middle Cambrian deformational and metamorphic ages are also reported from the ‘Angolan’ Pan-African belt, the southern Kaoko and Damara belts in Namibia, and the Cuchilla Dionisio–Punta Del Este terrane in Uruguay. The occurrence of Cambrian metamorphic rocks along the present African and South American coastlines shows that Mesozoic rifting closely follows Palaeozoic sutures of West Gondwana.

Abstract Changes in the oceanographic and sedimentologic regime off southern Brazil are reflected in the carbonate enrichment and foraminiferal assemblages of the shelf sediments. North of Cabo Frio, carbonate-rich sands and gravels are characterized by Halimeda , bryozoans, and coralline algae; tropical Foraminifera include various species of miliolids and peneroplids, as well as the encrusting Homotrema rubrum . Subtropical deposits of carbonate-rich sediments between Cabo Frio and Sao Paulo are primarily restricted to the outer shelf, and are dominated by coralline algae and increased amounts of agglutinated Foraminifera. Temperate-water carbonate deposits off southernmost Brazil contain mainly barnacles and mollusks, and fewer agglutinated Foraminifera. Carbonate sediments generally are not important in the middle-shelf muds and inner-shelf sands. Benthonic Foraminifera in nearshore waters (particularly in temperate areas) are characterized by paucity of both species and individuals.

Abstract The Brasília Belt comprises terranes and thrust-sheets that were tectonically transported towards the western passive margin of the São Francisco–Congo palaeocontinent during an orogenic episode resulting from collision of the Paranapanema and Goiás blocks and the Goiás magmatic arc against São Francisco–Congo at 0.64–0.61 Ga. The tectonic zones of the belt are, from east to west: a foreland zone with Archaean–Palaeoproterozoic granite–greenstone basement covered by Neoproterozoic anchimetamorphic sedimentary rocks (Bambuí Group); a low metamorphic grade thrust-fold belt of proximal shelf successions, mostly siliciclastic, containing rare basement slivers; metamorphic nappes in upper greenschist to granulite facies of distal shelf and slope metasediments and subordinate tholeiitic metabasalts; the Goiás massif, possibly a microcontinent; and the Goiás magmatic arc. The accretion of these terranes against the western margin of the São Francisco–Congo palaeocontinent took place during an early phase of Gondwana supercontinent amalgamation, when terranes accreted around São Francisco–Congo to create a proto-West Gondwana landmass, around which subsequent collisional and accretionary events followed, such as those in the Borborema–Trans-Saharan province ( c. 0.62–0.60 Ga); in the Ribeira–Araçuaí belt ( c. 0.58 Ga); along the Araguaia and Paraguay belts (collision of Amazonia, c. 0.54–0.52 Ga); and the accretion of Cabo Frio terrane in the Ribeira Belt ( c. 0.53–0.50 Ga).

Abstract Four main classes of tectonic entities may be considered for the Ribeira Belt and southwest African counterparts: (1) cratonic fragments older than 1.8 Ga and their passive margin successions, (2) reworked basement terranes with Mesoproterozoic and/or Neoproterozoic deformed cover, (3) magmatic arc associations, (4) terranes with Palaeoproterozoic basement and deformed Neoproterozoic back-arc successions. Based on comparative investigation, a tectonic model of polyphase amalgamation is proposed with c . 790 and 630–610 Ma major episodes of intra-oceanic and cordilleran arc magmatism along both sides of the Adamastor Ocean. Subsequent diachronous collision of the arc terranes and small plates followed at c . 630, 600, 580 and 530 Ma. The tectonic complexity reflects an accretionary evolution from Cryogenian to Cambrian times. The São Francisco–Congo and Angola palaeo-continents did probably not behave as one consolidated block, but rather may have accommodated considerable convergence during the Brasiliano/Pan-African episodes. The final docking of Cabo Frio and Kalahari in the Cambrian was coeval with the arrival of Amazonia on the opposite side, resulting in lateral reactivation and displacement between the previously amalgamated pieces. The transition between the Cambrian and the Ordovician is marked by the extensional collapse of the metamorphic core zones of the orogens.