Renato Oscar Kowsmann

Tectonics and stratigraphy of the East Brazil Rift system: an overview

Chang, H.K., Kowsmann, R.O., Figueiredo, A.M.F. and Bender, A.A., 1992. Tectonics and stratigraphy of the East Brazil Rift system: an overview. In: P.A. Ziegler (Editor), Geodynamics of Rifting, Volume II. Case History Studies on Rifts: North and South America and Africa. Tectonophysics, 213: 97–138. The East Brazilian Rift system (EBRIS) constitutes the northern segment of the South Atlantic rift system which developed during the Mesozoic breakup of South America and Africa. Following crustal separation in the Late Aptian, it evolved into a passive continental margin. Along the continental margin six basins are recognized, while three onshore basins form part of an aborted rift. Three continental syn-rift stratigraphic sequences are recognized, spanning Jurassic to Barremian times. The Jurassic (Syn-rift I) and Neocomian (Syn-rift II) phases were most active in the interior rift basins. During the Barremian (Syn-rift III), rift subsidence rates were twice as large as during the Neocomian (Syn-rift II), both in the interior rift and in the marginal rift segments, indicating that rift axis did not migrate from the interior to the marginal setting. Rift magmatism was centered on the southern EBRIS and peaked between 130 and 120 Ma during syn-rift phase II. Rift phase III was followed by a transitional marine, evaporitic megasequence of Aptian age, which directly overlies the rift unconformity and a marine drift megasequence which spans Albian to Recent times. During the Late Cretaceous, sedimentation rates responded to first-order eustatic sea-level fluctuations. Tertiary accelerated sedimentation rates can be related to local clastic supply which filled in spaces inherited from previous starved conditions. Between 60 and 40 Ma, post-rift magmatism, centered on the Abrolhos and Royal Charlotte banks, is probably related to development of a hot spot associated with the Vitoria-Trindade Seamount Chain. Although crossing three distinct Precambrian tectono-thermal provinces, ranging from Archean through Late Proterozoic, rift structures follow a general NE trend, subparallel to the principal basement fabric. A NW-SE oriented stress field appears to be compatible with both Neocomian and Barremian phases of crustal extension. Profiles transverse to the rift axis indicate crustal stretching factors ranging between β = 2.16 and 2.88. In the shallow portions of the rift, surface extension and crustal thinning seem to be compatible; however, in the deep portions of the basins, this relationship could not be tested. Reinterpretation of refraction profiles, north and south of the Walvis-Sao Paulo Ridge transform, indicates that seafloor spreading, from M3 anomaly to Aptian off Pelotas Basin, was taken up by crustal extension in the Sao Paulo Plateau. Differences in stretching rates may have been accommodated by extension across the Ponta Grossa Arch. The Early Aptian syn-rift/post-rift transition in the EBRIS marginal basins does not coincide with the onset of the drift phase during the Early Albian. This apparent discrepancy may be explained by a change from distributed margin-wide extension to a focused mode of extension near the future continent/ocean boundary.

HYDROLOGY, MORPHOLOGY AND SEDIMENTOLOGY OF THE CAMPOS CONTINENTAL MARGIN, OFFSHORE BRAZIL

Abstract Slope sand deposits have accumulated from at least the Neogene to the Present on the southeastern Brazilian continental margin (Campos Basin area). This region shows sand accumulations concentrated on the upper portion and on the base of the continental slope with a middle to lower slope bypass zone. A synthesis of preliminary results, supported by recent cores, high-resolution geophysical surveys, geotechnical investigations and environmental research, is presented and permits a prelitrunary analysis of the sedimentological mechanisms operational in this area. These point toward a temporal and spatial multiscale set of phenomena responsible for sand deposits. At any sea-level stand these deposits are dependent on: (1) a suitable sediment source; (2) offshelf transport mechanisms; (3) a morphostructural and hydrodynamic context responsible for the deposition of these sands in the upper portion of continental slopes. The proposed scenario of depositional processes concerns: (1) a set of hydrological processes such as surface currents and counter-currents, waves, tides and eddies with sufficient energy to form submarine sand dune fields at the outer shelf; (2) the offshelf export of this sediment under a combined action of spillover, internal waves, eddies ‘seafloor polishing effect’ and gravity processes (turbidity currents); and (3) the slope sand deposits and their distribution controlled by the action of contour currents, mass movements and the morphological context, such as canyons, gullies or scarps.

Distributary channel meandering and bifurcation patterns on the Amazon deep-sea fan as revealed by long-range side-scan sonar (GLORIA)

We mapped the distributary channel system of the Amazon deep-sea fan using the GLORIA long-range side-scan sonar. Individual channels were continuously traced for distances of up to 150 km. Channel bifurcation, although observed in only a few places, results in many cases from breaching of channel levees on the outsides of meander loops. Whether both channels remain active after branching or the original channel is abandoned by avulsion generally cannot be determined. The most striking channel characteristic is high sinuosity that results in extensive, intricate, often recurving meanders. Cutoffs and abandoned meander loops (oxbows) are observed in a few places. These meandering channels are comparable in size and appearance to those of mature fluvial systems on land, such as on the lower Mississippi River. The formation, maintenance, and modification of such extensive, well-developed meander systems would seem to require large volumes of continuous turbidity flow through the channels for relatively long time periods. This may challenge the traditional concept that channel formation and modification are accomplished by intermittent or sporadic turbidity-current events.

Seismic Facies and Late Quaternary Growth of Amazon Submarine Fan

The Amazon Fan contains sedimentary/acoustic sequences characteristic of many large and small modern mud-rich fans. Analyses of high-resolution single-channel seismic-reflection profiles and 3.5-kHz profiles suggest that fan growth is in part related to sea-level fluctuations and in part related to events such as channel bifurcations and large debris flows that appear unrelated to sea-level position. Sinuous fan channels are perched on top of lens-shaped overbank deposits to form channel-levee systems in the upper and middle fan. Individual channel-levee systems overlap and coalesce to build levee complexes that also stack and overlap, but that are bounded by large debris-flow deposits. Because both channel-levee systems and debris flows can be active at the same time, this depositional pattern does not necessarily develop as a result of sea-level change. The sinuous fan channels appear to be nearly at grade because channel sinuosity varies downfan to keep the along-channel gradient uniformly decreasing downfan. Flat-lying, high-amplitude reflection packets that underlie a channel-levee system and extend downfan to form part of the lower fan may develop when new, oversteepened channels are created as a result of avulsion on the middle fan. This suggests that portions of the lower fan are formed concurrently with channel-levee systems. Piston cores from near the most recently active channel suggest that the locus of sedimentation shifted landward as sea level rose at the end of the last glaciation.

Age relationships of distributary channels on Amazon deep-sea fan: Implications for fan growth pattern

Seismic-reflection profiles and long-range side-scan (GLORIA) sonographs reveal the relative age relationships between the major distributary channels of the upper and middle Amazon deep-sea fan. These age relationships, in turn, suggest that the fan grows through formation of a succession of broad levee complexes, each of which consists of several individual leveed distributary channels whose levee systems overlap or coalesce with one another. Only one channel is apparently active at any given time on an actively growing levee complex. Eventually this channel is abandoned, probably through avulsion, and a new channel and associated levee system form nearby, thereby enlarging the levee complex. Occasionally, the formation of a new channel will cause the present levee complex to be abandoned and will initiate a new levee complex at a different location. In this manner, a succession of overlapping levee complexes is formed through time, and the fan grows upward as well as radially outward downslope.

Features Indicating Geological Instability in the Continental Slope and the São Paulo Plateau

ABSTRACT A map of the bottom morphological features that indicate sedimentary activity and submarine instability, based on 3D seismic imaging of the Campos Basin, was made for the safety of the production facilities that will be installed on the Campos Basin seabed. The nature of these features is described in light of the studies carried out in recent decades in the Campos Basin, and their ages are inferred through the analysis and dating of the piston corers available. It was concluded that most of the geological risk features mapped are relict and were formed during periods of sea level lowstand of the Pleistocene. From this study, however, the persistence of sediment transport by some submarine canyons and also of salt tectonic activity in the distal portion of the basin, during the Holocene is inferred.

FEIÇÕES INDICADORAS DE INSTABILIDADE GEOLÓGICA NO TALUDE CONTINENTAL E NO PLATÔ DE SÃO PAULO

RESUMO Visando a seguranca das facilidades de producao a serem assentadas no leito marinho da Bacia de Campos, foi confeccionado um mapa das feicoes morfologicas de fundo indicadoras de atividade sedimentar e instabilidade submarina, com base no imageamento de sismica 3D da Bacia de Campos. As naturezas dessas feicoes sao descritas a luz dos estudos realizados nas ultimas decadas na Bacia de Campos, e suas idades sao inferidas atraves da analise e da datacao dos testemunhos a pistao disponiveis. Conclui-se que a maior parte das feicoes de risco geologico mapeadas sao reliquias e foram formadas em periodos de nivel de mar baixo do Pleistoceno. Atraves deste estudo, no entanto, infere-se a persistencia do transporte de sedimentos atraves de alguns cânions submarinos e tambem da atividade da tectonica salifera na porcao distal da bacia, durante o Holoceno.

Areas Susceptible to Landsliding on the Continental Slope

ABSTRACT The areas susceptible to submarine mass movement occurrence on the continental slope and Sao Paulo Plateau were evaluated. In this evaluation, we used a spatial analysis tool, available in the Geographic Information System (GIS), together with the application of a mathematical model developed to predict areas susceptible to slides, based on a formulation of physical laws. The analysis of slope stability was carried out under undrained conditions, in terms of total stress, considering a normally-consolidated clay soil, and was based on the deterministic method for safety factor calculation through the limit equilibrium approach – one-dimensional infinite slope formulation in submerged condition. In this manner, it was possible to take into consideration the data on seafloor geometry (slope gradient) and the seafloor mechanical properties (undrained shear strength and submerged unit weight) in the safety factor calculation. As a result, we obtained regional maps of static safety factors for the evaluation of the susceptibility for submarine shallow translational slides on the slope and Sao Paulo Plateau of Campos Basin, considering two profiles of undrained shear strength of the soil: one corresponding to a lower strength limit, and the other one to an intermediary strength value.

ÁREAS PROPENSAS A ESCORREGAMENTOS NO TALUDE CONTINENTAL

RESUMO Realizou-se uma avaliacao regional das areas suscetiveis a ocorrencia de movimentos de massa submarinos no talude continental e no Plato de Sao Paulo da Bacia de Campos. Para essa avaliacao, foi utilizada uma ferramenta de analise espacial disponivel em um sistema de informacao geografica (SIG), em conjunto com a aplicacao de um modelo matematico desenvolvido para a previsao de areas propensas a deslizamentos, baseado em uma formulacao de leis fisicas. A analise de estabilidade de taludes foi feita sob condicoes nao drenadas, em termos de tensoes totais, considerando um solo argiloso normalmente adensado, e baseou-se no metodo deterministico para o calculo de fatores de seguranca atraves da abordagem de equilibrio limite – formulacao de talude infinito-unidimensional para a condicao submersa. Dessa forma, foi possivel levar em consideracao, no calculo dos fatores de seguranca, o dado referente a geometria do fundo do mar (declividade) e as propriedades mecânicas do solo marinho (resistencia ao cisalhamento nao drenada e peso especifico submerso). O resultado foi a obtencao de mapas regionais de fatores de seguranca estaticos para avaliacao de propensao a deslizamentos submarinos translacionais rasos no talude e no Plato de Sao Paulo da Bacia de Campos, considerando dois perfis de resistencia ao cisalhamento nao drenada do solo: um correspondente a um limite inferior de resistencia, e o outro referente a um valor de resistencia intermediario.

Thermal Subsidence of Santos and Sergipe-Alagoas Basins, Brazil--Application to Hydrocarbon Exploration: ABSTRACT

A uniform lithospheric extension model has been applied to the subsidence and thermal history of the Santos and Sergipe-Alagoas basins. Thermal subsidence curves were derived from more than 40 exploratory wells, complemented by seismic and gravity sections. In the Santos basin, the region landward of the hinge zone is dominated by Tertiary marine onlapping sediments. Seaward, rift (continental) and postrift (transitional and marine) sequences are present with predominance of the latter. Crustal thinning by 35% exists just east of the hinge zone and increases to 70% at the shelf break. Likewise, the hinge zone in the Sergipe-Alagoas basin overlies the largest gradient in crustal thinning, but unlike the Santos basin, rift-stage sediments dominate landward of this hinge zone. Seaward, the ratio of rift to postrift sediments is larger than in the Santos basin. Crustal thinning ranges from 20% to 60%, but this transition occurs over a short distance. Thermal and burial histories were used to estimate oil windows for potential source rocks in various parts of these basins. Maturation profiles are in good agreement with observed vitrinite reflectance and hydrocarbon maturity data. The areal distribution of maturation stages through time is variable and largely controlled by crustal thinning. Information on timing of hydrocarbon generation was applied to the prediction and evaluation of hydrocarbon accumulation. End_of_Article - Last_Page 243------------