Brazilian Equatorial Margin Research Articles

Influence of the continental margin on the stress field and seismicity in the intraplate Acaraú Seismic Zone, NE Brazil

The Borborema province in NE Brazil is characterized by seismic sequences with small earthquakes that can last 10 yr or more. The seismicity in this region is concentrated in three main seismic zones. In this work, we investigate the stress field in one of these zones, the Acaraú Seismic Zone, which is located in the NW part of the Borborema province. This seismic zone exhibits earthquake sequences that contain repeated earthquakes with similar waveforms and a shallow depth. Using a local network, we investigated a seismic sequence close to the town of Santana do Acaraú from December 2009 to December 2010, and we present detailed results (velocity model, hypocentres and focal mechanism) from this network. In addition, we inverted seven focal mechanisms, including six that were used in previous studies, and determined the directions of the three main axes of the regional stress field. Selecting a very precise set of 12 earthquakes, we found an active seismic zone with a depth between 3.5 and 4.8 km and with a horizontal dimension of approximately 2.5 km in the NW–SE direction (azimuth of 118°) and a strike-slip focal mechanism. The new seismic fault and some of the previous seismic faults determined in previous studies occur near the continental-scale Transbrasiliano lineament, but they exhibit no direct relationship with that ancient structure. The stress field is characterized by NW–SE trending compression and NE–SW trending extension. This result suggests that the rheological contrast between the continental–oceanic crusts created flexural stresses with maximum horizontal compression parallel to the continental margin. This stress pattern occurs along the Potiguar basin and continues west as far as the Amazon fan along the Equatorial margin of Brazil. This stress field and related seismicity may be a characteristic of this type of passive margin that is generated during the transform shearing between the South America and Africa plates and that exhibits an abrupt oceanic–continent transition, steep continental slopes and high bathymetric gradients.

Diagenetic processes in cretaceous sandstones from occidental Brazilian Equatorial Margin

Despite a great interest in Brazilian Equatorial Margin exploration, very little was published on the diagenesis of sandstones from that area. A wide recognition petrographic study was performed to identify the major diagenetic processes that impacted the porosity of Lower Cretaceous sandstones of the Pará-Maranhão, São Luís, Bragança-Viseu and Barreirinhas basins. Arkoses from the Pará-Maranhão Basin show neoformed or infiltrated clay coatings, mica replacement and expansion by kaolinite and vermiculite, and precipitation of grain-replacive and pore-filling quartz, kaolinite, albite, chlorite, calcite, dolomite, siderite, pyrite and titanium oxides. Compaction, quartz and calcite cementation were the main porosity-reducing processes. Barreirinhas Basin lithic arkoses and subarkoses display clay coatings, compaction of metamorphic fragments into pseudomatrix, and precipitation of grain-replacive and pore-filling kaolinite, quartz, albite, chlorite, calcite, dolomite, TiO2 and pyrite. The main porosity-reducing processes were calcite cementation in the subarkoses, and compaction and quartz cementation in lithic arkoses. Quartzarenites from this basin were early- and pervasively cemented by dolomite. Arkoses and lithic arkoses of the São Luís and Bragança-Viseu basins show clay coatings, pseudomatrix from mud intraclasts compaction, and precipitation of pore-filling and grain-replacive kaolinite, vermiculite, smectite, quartz, albite, chlorite, illite, calcite, dolomite, hematite, TiO2 and pyrite. Compaction of mud intraclasts and dissolution of feldspars and heavy minerals were the main porosity-modification processes. These preliminary results may contribute to the understanding of the spatial and temporal distribution of the diagenetic processes and their impacts on the porosity of the sandstones from these basins.

Sediment studies associated with drilling activity on a tropical shallow shelf

Environmental monitoring studies were developed in an area located on the outer shelf in the Potiguar Basin, Brazilian equatorial margin. This tropical shelf represents a modern, highly dynamic mixed carbonate-siliciclastic system. Field sampling was carried out during 3 cruises surrounding a shallow-water exploratory well to compare sediment properties of the seafloor, including grain size, texture, mineral composition, carbonate content, and organic matter, prior to drilling with samples obtained 3 and 12 months after drilling. The sample grid used had 16 stations located along 4 radials from 50 m the well up to a distance of 500 m. Sediments were analyzed in the first 0-2 cm and 0-10 cm layers. The results show that sedimentary cover around the well is dominated by bioclastic sediments, poor to very poorly sorted. Only minor sedimentological variations occurred in the area affected by drilling operations. The most noticeable effects were observed during the second cruise, in terms of a change in grain size distribution associated to a slight increase in siliciclastic content. This impact occurred in the most surficial sediment (0-2 cm), in the radials closest to the well (50 m), and could suggest the effects of drilling. However, in the third cruise, 1 year after drilling, the sediments return to show the same characteristics as in the first cruise. These results show no significant sedimentological variations due to drilling activity and indicate that ocean dynamics in this area was high enough to recover the environment original characteristics.

Deep crustal structures interpreted from potential field data along deep seismic sounding transects in Australia

Energy spectral analysis techniques have been applied to magnetic and gravity data acquired across the Olympic Dam cratonic area in Australia and sedimentary basins along the Equatorial Margin of Brazil. Analysis has been conducted along two Deep Seismic Sounding lines (DSS) acquired by Geoscience Australia. There is a good correlation between interfaces found in this analysis and structures interpreted from the seismic data. Interpretation of gravity data using energy spectral analysis along the DSS survey lines show a number of deep crustal structures are evident, including the Moho which was detected using gravity data, while similar analysis of the magnetic data show indications of the Curie isotherm. In addition, the analysis was extended away from the seismic lines to detect many deep crustal horizons and structures at considerable distances from the DSS lines. The results obtained from energy spectral analysis across this area in Australia encouraged the application of this technique on the Equatorial Margin of Brazil, where the potential field data is of much lower resolution. This suggests that a much wider application of this approach could be highly valuable to investigate the deep structure under other sedimentary basins and to assist heat flow studies.

The interplay between structural inheritance and morphology in the Equatorial Continental Shelf of Brazil

Abstract The primary objective of this study is to assess the control of faults of the rift and post-rift stages on the shelf morphology of the Potiguar Basin in northeastern Brazil. This aborted rift basin was generated during the opening of the Equatorial Atlantic in the Aptian. The offshore portion of the continental margin consists of a narrow (~ 40 km) and shallow (~ 70 m below present sea-level) continental shelf with a very steep continental slope (1:11). Our dataset encompasses gravity, bathymetric, shallow seismic and structural data. The results indicate that low sedimentation rates during the Quaternary period contributed to the identification of structural controls in pre-Holocene rocks. The key evidence for fault reactivation on the seafloor is the link between coastal and shelf features associated with pre-Cenozoic structures of the Potiguar Basin. During periods of low sea level, the incision of shelf valleys was readjusted longitudinally and transversally due to the structural controls. Shelf gradient breaks are associated with the occurrences of coplanar ESE–WNW-oriented faults, and uplifted and subsided areas occur in between these fault systems. The results indicate a strong correlation between the margin geometry, modern shelf surface, near-surface expression, and the rift-phase faults, which appear to be reactivated in concordance with the present-day margin stress field. We conclude that neotectonics has influenced both the sediment deposition and morphology of the NE Brazilian Equatorial margin during Quaternary times.

Deposition and deformation in the deepwater sediment of the offshore Barreirinhas Basin, Brazil

The Barreirinhas Basin is an ideal location to study shale-dominated gravity-driven thrusting systems because of the limited areal extent of the deformed areas compared to other areas in the world. Regional seismic reflection profiles across the Barreirinhas Basin on the Brazilian Equatorial margin show two major deepwater fold and thrust belts linked landward to extensional fault systems. Thrust faults are interpreted to be products of shortening caused by gravity driven extension on the continental margin that involve rocks of both the shelf and the slope. Results show two main deformation events during the Cretaceous (between ca. 89 and 84 Ma) and several episodes during the Later Cenozoic (ca. 55–0 Ma). All events were characterized by displacement along a detachment fault linking a landward system of normal faults to a basinward system of folds and thrust faults. The Cretaceous deformation involved a

O Cone do Amazonas, bacia da Foz do Amazonas: uma nova discussão

O Cone do Amazonas, situado na porção profunda da bacia da Foz do Amazonas, Margem Equatorial Brasileira, constitui um dos maiores leques submarinos do mundo. É caracterizado por processos de argilocinese e pela presença de um sistema de falhas distensivo e compressivo conectado entre si por um descolamento basal. Nesse trabalho, re-examinaram-se seis linhas sísmicas de reflexão com o objetivo de contribuir para um melhor conhecimento da tectônica gravitacional, de idade controversa. A análise das feições sismoestratigráficas permitiu individualizar o embasamento acústico e seis sequências sedimentares. Nas seções sísmicas, reconheceram-se falhas normais com idade decrescente do domínio proximal à região central. Estas falhas são pré- a sin-tectônicas em relação ao processo de deslizamento gravitacional, com raras falhas ativas até o Recente. O descolamento basal tornou-se ativo durante o Mioceno Superior. Sugere-se que isto tenha ocorrido em função de a porção basal da Formação Travosas, paleocênica, ter sofrido uma sobrecarga causada pela deposição do Grupo Pará.

Facies, δ13C, δ15N and C/N analyses in a late Quaternary compound estuarine fill, northern Brazil and relation to sea level

The present work integrates sedimentary facies, 14C dating, δ 13C, δ 15N, and C/N with geologic and geomorphologic data available from literature. The aim was to characterize the depositional settings of a late Quaternary estuary in northeastern Marajó Island and analyze its evolution within the context of relative sea level fluctuations. The data derive from four continuous cores along a proximal-to-distal transect of a paleoestuary, previously recognized using remote sensing information. Fifteen sediment samples recorded ages ranging from 42,580 ± 1430 to 3184 ± 37 14C yr B.P. Facies analysis indicated fine- to coarse-grained sands with parallel lamination or cross stratification, massive or laminated muds and heterolithic deposits. δ 13C (− 28.1‰ to − 19.7‰, mean = − 23.0‰), δ 15N (+ 14.8‰ to + 4.7‰, mean = + 9.2‰) and C/N (14.5 to 1.5, mean = 7.9) indicate mostly marine and freshwater phytoplankton sources for the organic matter. The results confirm a large late Quaternary paleoestuary in northeastern Marajó Island. The distribution of δ 13C, δ 15N, and C/N, together with facies associations, led to identify depositional settings related to fluvial channel, floodplain, tidal channel/tidal flat, central basin, tidal delta, and tidal inlet/sand barrier. These deposits are consistent with a wave-dominated estuary. Variations in stratigraphy and geochemistry are controlled by changes in relative sea level, revealing a main transgression from an undetermined time around 42,000 14C yr B.P. and 29,340 (± 200) 14C yr B.P., which is synchronous to the overall drop in sea level after the last interglacial. Following this period, and probably until 9110 ± 37 14C yr B.P., i.e., during a time interval encompassing two glacial episodes including the Last Glacial and the Younger Dryas, there was a pronounced drop in sea level, recorded by the development of a major erosional discontinuity due to valley re-incision. Sea level rose again until 5464 ± 40 14C yr B.P, just before the main worldwide mid-Holocene transgressive peak. Mid to late Holocene coastal progradation ended the Marajó paleoestuarine history, and promoted the establishment of continental conditions throughout the island. The divergence comparing the Marajó sea level behavior with the eustatic curve allows hypothesizing that post-rifting tectonics along the Brazilian Equatorial margin influenced the sedimentary evolution of the studied paleoestuary. Considering that sedimentary facies in estuarine settings are highly variable both laterally and vertically, the present integration of facies with isotope and elemental analyses was crucial to provide a more precise interpretation of the Late Pleistocene and Holocene Marajó paleoestuary, and analyze its sea level history within the eustatic and tectonic context.

Quaternary reactivation of a basement structure in the Barreirinhas Basin, Brazilian Equatorial Margin

The Pirapemas Lineament is a remarkable 200-km-long, NE–SW trending structure in the Barreirinhas Basin, one of the several Brazilian coastal basins. This lineament splits the study area in two sectors of distinctive morphology, drainage patterns, and sedimentary covers. Terrain northward of the lineament presents a smooth topography with sub-parallel to sub-dendritic drainage patterns, whereas a dissected plateau characterized by incised valleys and rectangular drainage pattern occurs southward, suggesting a structural control by joints and faults. Geological field data, crossed with thermal luminescence (TL) and optically stimulated luminescence (OSL) dating, revealed that the surface southward of the lineament consists mostly of Miocene and late Pleistocene sedimentary deposits, represented by the Barreiras Formation and the Post-Barreiras sediments, respectively. In contrast, relatively younger sands mantle most of the northward terrain, as indicated by well-preserved paleodune deposits that grade into active aeolian dunes of the Lençóis Maranhenses National Park. Geomorphological and geological data analysis suggests that the northern sector is under the effect of subsidence, proving that the Pirapemas Lineament is an active agent modeling the landscape in the region. Geophysical data (gravity and seismic) confirm that such a structural feature is the surface expression of an active deep-seated basement fault.

Paleohydrology of an Upper Aptian lacustrine system from northeastern Brazil: Integration of facies and isotopic geochemistry

The Codó Formation records the initial evolutionary stages of an intracontinental rift system formed along the Brazilian equatorial margin in the late Aptian. Deposits of this unit exposed on the eastern margin of the Grajaú Basin include gypsum, bituminous black shales and limestones. These lithologies were formed in a low energy, well stratified, anoxic and hypersaline lake system developed in a dominantly arid/semi-arid climate. This lacustrine succession is internally organized into three categories of shallowing-upward cycles, with the first- and second-order cycles being related to seismic activity associated with fault reactivations, and the third-order cycles recording climatic fluctuations. Studies emphasizing petrography and analysis of the geochemical tracers Fe, Mg, Sr, Mn, Na and Ca helped to identify the sedimentary facies that kept a primary signal, which were thus appropriate for isotopic investigations aiming paleoenvironmental and paleohydrologic reconstructions. The results of this study revealed a wide distribution of dominantly low carbon and oxygen isotope values in carbonates, ranging from − 5.69‰ to − 13.02‰ and from − 2.71‰ to − 10.80‰, respectively. This paper demonstrates that at least in the particular case of oxygen, the isotope ratios vary according to seismically-induced shallowing-upward cycles, with values in general lower at their bases, where central lake deposits dominate, and progressively higher upward, where marginal lake deposits are more widespread. In addition to confirming a depositional signature for the analysed samples, this behavior allowed the development of a seismic-induced isotope model. The lighter isotope ratios appear to be related to flooding events promoted by subsidence, which resulted in the development of a perennial lake system, while heavier isotope values are related to ephemeral lake phases favored by uplift and/or increased stability. Furthermore, the results show that a closed lake system dominated, as indicated by the overall good positive covariance (i.e., + 0.42 to + 0.43) between the carbon and oxygen isotopes, though open phases are also recorded by negative covariance values of − 0.36. During closed phases, the δ 18O displayed the highest range of variation (i.e., − 3.63‰ to − 4.89‰) due to increased residence time, while this variation was low (i.e., − 0.09‰ to − 1.87‰) during open lake phases, when there was a balance in the water isotope composition maintained by continuous basin inflow.

An Upper Aptian saline pan/lake system from the Brazilian equatorial margin: integration of facies and isotopes

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

RADARSAT-1 images in support of petroleum exploration: the offshore Amazon River mouth example

RADARSAT-1 images were used as an attempt to identify seepage slicks in the Foz do Amazonas Basin, one of the most promising oil and gas exploration frontiers in the Brazilian equatorial margin, a region that is cloud-covered for most of the year. An unsupervised semivariogram textural classifier algorithm was used to enhance areas of smooth texture and low radar backscatter, indicative of these seepage slick targets. Complementary information related to sea surface temperature, cloud top temperature, wind velocity, and modelling for the tidal regime (all obtained as close as possible to the RADARSAT-1 acquisition) was used to support image interpretation and to exclude false targets also characterized by low radar backscatter (e.g., local upwelling, heavy rain cells). Twenty-one target areas were interpreted as seepage slicks. Considering the influence of the oceanographic and environmental conditions on drifting of the interpreted seepage slicks, they were spatially compared with available geological information, including seismically derived structural and isopach maps, and bathymetric data. Most of the interpreted seepage slicks occur in an extensional structural domain of growth faults. These structures are related to Cenozoic gravity tectonics that result from tensional stress in the continental slope due to the huge load of fluvial sediments transported by the Amazon River. Together with previously discovered subcommercial shallow water oil accumulations, the remote detection of seepage slicks is additional evidence of present-day generation and migration phenomena. The results indicate that the use of a remote, swift method to identify offshore natural oil seepage related to active petroleum systems may constitute a well-accepted approach to support exploration in frontier areas like the Foz do Amazonas Basin.

Proposição de um modelo crustal para a região do Cone do Amazonas baseado em dados gravimétricos

O Cone do Amazonas, situado na Bacia da Foz do Amazonas, Margem Equatorial Brasileira, e caracterizado por um espesso pacote sedimentar terrigeno depositado a partir do Mioceno Medio, como consequencia da reversao da drenagem do rio Amazonas decorrente da orogenia Andina. Este trabalho investigou, por meio de uma analise integrada de dados gravimetricos, a arquitetura crustal da regiao do Cone do Amazonas. Modelagens gravimetricas 2D, baseadas em modelos de densidades construidos a partir de informacoes sismicas, mostraram a espessura sedimentar, a espessura crustal, a morfologia e a profundidade da interface crosta-manto. Aplicou-se um algoritmo de inversao nao-linear, adotando-se um modelo de densidades variando exponencialmente com a profundidade para a crosta. Assim, os valores de profundidade da interface crosta-manto, bem como de espessura crustal, foram ajustados, tornando-se compativeis com valores obtidos por sismica de refracao. Os resultados alcancados neste estudo permitiram a caracterizacao da crosta na regiao do Cone do Amazonas, possibilitando a observacao das principais caracteristicas de sua estruturacao geologica.

Ages of the Magmatism and the Opening of the South Atlantic Ocean

The analysis of published and unpublished 368 K/Ar radiometric ages of basic, intermediate and alkaline volcanic rocks, related to the post-Paleozoic magmatism linked to the opening of the South Atlantic Ocean, yields some important evidence concerning the break up of the Gondwana supercontinent. At the Brazilian Equatorial margin, the Gondwana break up started in the Permo-Triassic, when the opening of the Equatorial South Atlantic Ocean began and spread out south-eastward up to the present day Amazon River mouth. During the middle Jurassic/lower Cretaceous (pre-Aptian), the continuity of this separation, towards the Potiguar Basin, was coeval with the northward opening of the south-east Brazilian margin, up to the Espírito Santo State latitude. The relationship between large volcanic events in the basins and the resistance to the rifting process development offered by the cratonic area was shown by the trend of the magmatic age. Along the equatorial margin, the fragmentation resistance caused by the São Luis / West African craton is manifested by a large basic magmatism described in the Tacutu, Acre, Solimões, Amazonas and Parnaíba basins. A similar mechanism along the south-east margin, is proposed for the magmatism described in the Paraná Basin which is associated with the fracturing resistance offered by the São Francisco/Congo cratonic area. The integration of geochronological, micropalentological, sedimentological and geochemical data from the basins of the east Brazilian continental margin supports a model to explain the final disruption between South America and Africa during Cenonian/Turonian time. This model implies that 90 Ma basic magmatic rocks, related to the oceanic crust formation, probably occur offshore from the present-day eastern Brazilian coast line.

Tectonic Evolution of Brazilian Equatorial Continental Margin Basins

The structural style and stratigraphic relationships of sedimentary basins along the Brazilian Equatorial Atlantic Continental Margin were used to construct an empirical tectonic model for the development of ancient transform margins. The model is constrained by detailed structural and subsidence analyses of several basins along the margin. The structural framework of the basins was defined at shallow and deep levels by the integration of many geophysical and geological data sets. The Barreirinhas and Para-Maranhao Basins were divided in three tectonic domains: the Tutoia, Caete, and Tromai subbasins. The Caete area is characterized by northwest-southeast striking and northeast-dipping normal faults. A pure shear mechanism of basin formation is suggested for its development. The structure of the Tutoia and Tromai subbasins are more complex and indicative of a major strike-slip component with dextral sense of displacement, during early stages of basin evolution. These two later subbasins were developed on a lithosphere characterized by an abrupt transition (<50 km wide) from an unstretched continent to an oceanic lithosphere. The subsidence history of these basins do not comply with the classical models developed for passive margins or continental rifting. The thermo-mechanical model proposed for the Brazilian equatorial margin includes heterogeneous stretching combined with shearingmore » at the plate margin. The tectonic history comprises: (1) Triassic-Jurassic limited extension associated with the Central Atlantic evolution; (2) Neocomian intraplate deformation consisting of strike-slip reactivation of preexisting shear zones; (3) Aptian-Cenomanian two-phase period of dextral shearing; and (4) Late Cretaceous-Cenozoic sea-floor spreading.« less

Structural Styles in the IcaraI Basin, Brazilian Equatorial Margin

The Icarai Basin, one of the compartments of the larger Ceara Basin located on the Brazilian equatorial margin, presents two distinct structural styles, clearly identifiable in seismic data. On the western side of the basin structures are characterized by intense folding associated with low to high angle thrust faults, whereas on the eastern side of the basin structures are represented by normal faults and tilted half graben. The change from one style to another is gradual, as both decrease in intensity towards the center of the basin, where they coexist. The distribution and relationship of structures along the basin suggests that the Icarai Basin was a transition zone between two distinct stress regimes, one predominantly transtensive and the other predominantly transpressive, which were simultaneously active in adjacent areas during the Lower Cretaceous. The existence of such distinct stress regimes is probably related to the initial geometry of development of the South American equatorial margin.

Sistemática, bioestratigrafia e paleogeografia do gênero Elateropolienites Herngreen 1973 na margem equatorial brasileira

Elateropollenites HERNGREEN 1973, originally monoespecific, was described in Lower and Middle Albian sediments of the Barreirinhas basin, Brazil. The genus diagnosis is now emended to include the new species Elateropollenites bicornis n. sp. and Elateropollenites dissimilis n. sp. The biostratigraphy and paleobiogeography of three ones are presented for Brazilian Equatorial Margin.

Structural Style and Petroleum Habitat Related to Tectonic Evolution of Potiguar Basin, Brazil: ABSTRACT

The Potiguar basin is the eastermost basin on the Brazilian equatorial margin and covers an area of 48,000 km/sup 2/. The basin extends from about 200 km landward of the coastline out beneath the present continental shelf and slope to a water depth of 2,000 m. It contains a sedimentary sequence more than 8 kim thick, which filled the asymmetric grabens that developed at an early stage of South American/Africa breakup in the Neocomian. Presently, 28 oil fields have been discovered; 23 onshore and 5 offshore. These fields contain proved oil reserves of 35 x 10/sup 6/ m/sup 3/ and occur in structures formed during the stretching and cooling stages of the tectonic evolution of the basin. Two main groups of petroleum habitats are distinguished according to structure type and reservoir-source rock association. During the stretching stage, grabens were formed and filled with lacustrine, deltaic, and deltaic fan sediments. These sediments were affected by gravity sliding, northeast-southwest normal faulting, and east-west strike-slip faulting. These movements created the closures that formed the oil accumulations. Producing reservoirs consist of coarse to fine-grained sandstones closely associated with the source rocks.

EVOLUÇÃO TECTÔNICA DA MARGEM EQUATORIAL BRASILEIRA

Rifting and tectonic evolution along the Equatorial margin of Brazil were highly complex. ln offshore Amapa, an early stage of rifting started ln Permo-Trlaseic time, accompanied by diabase intrusions. Volcanic activity extended eastward to the Maranhao Basin, without leading to continental breakup. ln early Cretaceous time (144-119 Ma) the clockwise rotation of South America relative to Africa, about a pole situated near 39°W and 7°S, caused compression west of Fortaleza, forming the Ferrer-Urbano Santos Arch, and eastward increasing extension east of Fortaleza, leading to the formation of the Potiguar Basin. ln this eastern portion, N-S extension was accompanied by E-W compression as the eastern tip of Brazil was pressed against Africa. The resulting stresses reactivated Precambrian ductile shear zones by transtensional movement, creating several inland half grabens including the Pendencia, Oros, and Rio do Peixe grabens. During Alagoas (Aptian) time, rifting propagated westward along the Equatorial margin and eastward along the Benue trough, both of which became marine seaways. From Albian time on, right lateral transtensional movement created NW trending rhomb grabens along the Equatorial margin. From Campanian through Eocene time, this right lateral movement was repeatedly interrupted by N-S compressional pulses which formed E-W trending folds and thrusts along the Equatorial margin and closed the seaway of the Benue trough. This compression may have been a distant reflex of the continental collision between Africa and Europe.

Potiguar Basin: Geologic Model and Habitat of Oil of a Brazilian Equatorial Basin: ABSTRACT

The Potiguar basin integrates the eastern part of the Brazilian equatorial Atlantic-type margin. The rifting stage of this basin occurred during the Neocomian and Aptian. The drifting stage and sea-floor spreading began in the Late Albian. The rifting stage clearly was intracratonic during the Neocomian and is recognized as a mosaic of half-grabens trending mostly northeast-southwest and filled with syntectonic lacustrine siliciclastics. The half-graben pattern exhibits rotation of beds into the major fault zone, and the preserved uplifted margins display either paleostructures or paleogeomorphic features with hydrocarbons. A regional pre-Aptian unconformity preceded the Aptian proto-oceanic rifting stage which was characterized by syntectonic fluvio-deltaic sediments. Th Aptian tectonics were represented by reactivation of former lineaments superimposed by predominant east-west normal faulting. Structural highs during this stage are so far the most prolific oil accumulations. The most important source beds and reservoir rocks are both Neocomian and Aptian sediments. Geochemistry and hydrodynamics have shown that hydrocarbon migration was driven through fracture or fault zones in both Aptian or Albian plays. Lithofacies maps support this interpretation because pools occur whenever adjacent downthrown blocks present a high shale content. End_of_Article - Last_Page 475------------