3rd International Congress of the Brazilian Geophysical Society

O Leque Submarino de Săo Tomé está localizado na Bacia de Campos-RJ e corresponde a uma protuberância sedimentar notável no talude e elevaçăo continental adjacente. Nesta regiăo, expediçőes GEOMARES observaram cânions, descritos por Brehme (84), que identificou na porçăo norte do talude um grupo deles denominados de Grupo Nordeste de Cânions, compreendendo os cânions Săo Tomé, Itapemirim e Alte, Câmara; na porçăo sul do talude, foi identificado um outro conjunto denominado Grupo Sudeste de Cânions correspondendo a uma série de cânions que foram numerados de um a cinco de sul para norte.

O Complexo do Leque Submarino de Săo Tomé caracteriza-se por uma pronunciada protuberância sedimentar localizada ao largo do talude e elevaçăo continental da Bacia de Campos-RJ (mapa 1). Esta feiçăo submarina foi primeiramente definida por Brehme (1984) nos domínios do talude continental e do Plato de Săo Paulo, entre a foz do rio Itabapoana e o Grupo Sudeste de Cânions, sendo ŕ época denominado de Leque Submarino do Paraíba do Sul.

The rapid growth of exploration in the Brazilian and west African marginal basins in the last decade can be attributed to the recognition by oil companies that these basins constitute some of the last frontiers in the world for appropriation of large oil and gas reserves. In most Brazilian and West African basins, exploration is far from being mature, since deep water probes practically have not started. However, evidences have shown that similar oil types in the southern Brazilian Santos, Campos and Espirito Santo basins, are to be found in the West African Cuanza, Cabinda and Zaire basins. Such basins fit neatly in the gig-saw puzzle if the maps of the South America and Africa continents were to be brought together (Mello et al., 1993). A geochemical survey of the most productive Brazilian and West African sedimentary basins, using a wide selection of oils and source rocks ranging from Lower Cretaceous to Miocene in age, has been carried out. The aims were to identify and characterize the oil types, to assess the respective depositional paleoenvironments and the age of their putative source rocks, and to perform the oil-oil and oil-source rock correlations in order to compare and characterize oils derived from both sides of the Atlantic. This approach was based mainly on the distributions and concentrations of biological markers (Moldowan et al., 1985; . Mello et al., 1988, Mello and Maxwell, 1991).

Approximately 47000 line kilometres of 2-D seismics has been acquired along the west coast margin of South Africa. From this data and a limited number of boreholes an understanding of the tectonic and depositional history has been derived for this Atlantic divergent plate margin. Drilling results on the shelf area have had sufficient merit to establish the significance of this region as a hydrocarbon province. Aspects of the petroleum geology and the exploration techniques applied are discussed together with examples of typical trap types. Guided by inference from geochemical, seismic and gravity studies, exploration has recently moved into deep water areas where the potential exists for the future discovery of significant hydrocarbon reserves.

We analyze magnetic data collected during 1988 and 1989 cruises in the South Atlantic run jointly by the Naval Research Laboratory (NRL) and the Diretoria de Hidrographia e Navigacao (DHN). The cruises surveyed the Bahia Seamount group that lies off the Brazil coast between latitudes 11°S and 12°S and longitudes 31°W and 35°W (Figure 1). The seamounts are on oceanic crust formed during the Cretaceous Normal Superchron, and were found to be typically small, deep. and conical. We find paleomagnetic poles for 6 seamounts by inverting magnetic anomaly data using the seminorm minimization method. We find one paleomagnetic pole for 7 seamounts that share a volcanic platform using a least-squares inversion that models seamounts with rocks of opposing polarity. We could not find poles for other seamounts owing to reversals in magnetic polarity along the seamount group and the large number of seamounts sharing volcanic platforms. We construct a South American Tertiary apparent polar wander (APW) path using our seamount poles. One of the seamounts was radiometrically dated. We estimate ages for the other seamounts by comparing their paleomagnetic poles with Late Cretaceous South American poles, and with North American Tertiary poles. Our APW path begins with a Late Cretaceous pole at 76.4 °N, 178.4 °E, then moves toward the spin axis in a slight zigzag, through a Paleocene pole at 84.2°N. 194.3°E. and an Early Eocene pole at 85.7°N, 123.9°E. We estimate that the South American paleomagnetic pole reached the spin axis in the Middle Eocene.

In the Amazon basin extensive volcanism occurred during the Mesozoic with wide-spread intrusion of diabase sills into the Palezoic sedimentary infill. In some areas there exists a close relationship between hydrocarbon accumulations and distance from Mesozoic dikes and sills: Vitrinite reflectance, spore coloration and abundance of gas increase upwards towards the sills, contrary to what would be expected from sedimentary burial by subsidence alone. At greater distances away from the sill the geothermal effect due to burial becomes dominant and an increase in vitrinite reflectances away from the sill is observed. To investigate this relationship we developed both an analytic and numerical model of a finite width heat source (dike or sill) which takes also into account the geothermal heat flux due to sediment burial. This allows us to calculate the complete temperature history of the intrusive and the surrounding sedimentary rock. The model demonstrates that the temperature history is primarily sensitive to the thickness of the intrusive, the latent heat of fusion and the thermal conductivities and diffiuivities of the intruaive and the host rocks. The maturation of organic matter is evaluated using a vitrinite reflectance model proposed by Lerche (1988), for which we determined the relevant parameters through an inverse fitting procedure to actual well data. The resulting relationship can be used to evaluate hydrocarbon maturation in areas where sill intrusions are a major thermal factor.

The seismic reflection and drilling data show that the Southern Brazilian margin (Pelotas and Santos basins. Florianopolis High and Sao Paulo Plateau) is of a volcanic type. characterized by the effusion of flood basalt sequences (seaward dipping reflectors). recognized along a 70 to 380 km wide band parallel to the coast line. The flood basalt sequences. which may be as thick as 8 krn, was emplaced between the Early Valanginian and the Late Aptian. The volcanic rocks are partially overlain by clastic sediments. Both, volcanic rocks and clastic sediments are related to the synrift phase that preceded the openning of the Southern South Atlantic. The presence of the volcanic sequences along the Southern Brazilian margin. and of similar age and of similar seismic characteristics on the Argentinean margin and on the conjugate margins of South Africa and Namibia (where the seaward dipping reflectors have also been identified) indicates that the southern portion of the South Atlantic is borded by volcanic type margin.

Many magmatic provinces (mainly flood basalts) occur along continental margins and are associated with initiation and early development of continental rifting. The causes of this magmatic activity are yet unclear and frequently related with the early activity of hotspots. The hotspots would correspond to the anomalously warm material rising from the lower mantle through conduits, the so called mantle plumes. Many hotspots are supposed to have begun with massive outpourings of basaltic magmas over broad regions (e.g. Parana Basin). Subsequent volcanism over the plume conduit produces the tracks that connect flood basalts to presently active hotspots.

Cada material natural apresenta um estado intermediário entre os estados Iíquido e sólido cristalino. Em estado líquido, o material tende a apresentar uma reologia próxima a newtonian ŕ: isto e, esforços infinitesimais săo capazes de causar deformaçőes lentas (dependendo da viscosidade) mas infinitas, que terminam por deixá-Ios com sua superfície perpendicular ao maior esforço principal. Em estado sólido cristalino, ao contrario, o comportamento dos materiais e, idealmente, puramente elástico: a deformaçăo e proporcional ao esforço aplicado, e termina, sob a açăo de esforços desviados muito grandes, em fratura brusca.

A Margem Equatorial Norte Brasileira deve ser analisada ŕ luz das aberturas do Atlântico Norto o do Atlântico Sul. Para tal, suas macroestruturas devem ser observadas no contexto brasileiro e africano: e no que tem a nos ensinar as estruturas e processos geológicos atuais e do passado geológico recente, do meio do oceano Atlántico Equatorial. O presente trabaIho tem essa finalidade, para tentar compreender a relatividade da importância do arcabouço tectônico da margem versus a sedimentaçăo que moldou a morfologia atual de seu fundo submarino.