Controls on Mass-Wasting in Deep Water of the Campos Basin

Abstract Full sonar coverage with associated sub-bottom profiles, ubiquitous 3D seismics, tens of geotechnical boreholes and hundreds of piston cores have allowed the characterization of the seafloor geology of the Campos basin between the shelf break and the 3000m isobath. Erosion and mass wasting are associated with the regional physiography, which is largely controlled by the underlying geology and to some extent by geostrophic currents. Particularly influential are (a) the extensional faults produced by salt flowage due to focussed sediment loading on the upper slope, which often control canyon formation, (b) the clinoform shape of the thick Miocene prograding wedge that determines the gradients of this middle and lower slope. Folded creep deposits are the norm on the low-gradient middle slope while debris-flow deposits, forming a large apron, lie at the foot of the steep bypass-prone lower slope. This prograding wedge also caused regional differential salt displacement in the adjacent São Paulo Plateau (SPP) which dictates the trend of the salt ridges bounding the depositional troughs. Off the mouths of shelf-breaching canyons, these troughs become turbidite pathways. Turbidites were mostly deposited during lowstands of sealevel, but where shelf indentation is significant, deposition persisted during sea-level highstands. Mass-wasting deposits associated with the continental slope occurred preferentially during lowstands of sea-level. Those lying at the base of salt ridges on the SPP are dominantly interglacial. Introduction The discovery by Petrobras of giant oil fields on the continental slope of the Campos basin in the mid-1980's and the necessity to develop them safely, sparked a concern for the geologic hazards involved. In 1990 a team consisting of marine geologists, paleontologists, exploration geophysicists and geotechnical engineers was pooled together, under the auspices of the PROCAP Program run by the company. From the start the team adopted a regional, processoriented approach to the problem, in which tool-integration was emphasized. With the lease of blocks in the ultra-deep waters of the SPP in the late 1990's, Petrobras complemented its seafloor image database in the region with basinwide 3-D seismics. Piston coring for geochemical prospecting provided surface sediment data in ultra deep waters of the SPP. The original mission of the geohazards team was augmented to cover site-specific data analysis in support of drilling and production operations. This paper focuses on a specific type of geohazard, namely mass-wasting and its distribution in space and time in the Campos basin slope and adjacent SPP, and discusses the geological aspects that control them. Data This broad overview is based on data collected during a decade of seafloor surveys. Basin wide information from slope and from the northern part of the adjacent SPP, includes industry standard 3-D seismic coverage with extraction of seafloor bathymetry, amplitude and coherence of the time difference of the sea-floor arrivals (edge map). It also includes a mosaic based on 43,000 km 2 of surface-towed, digitally acquired sidescan sonar (SIS-9) with simultaneous 25,000 km of chirp sub-bottom profiler data

Architecture and stratigraphic framework of shelf sedimentary systems off Rio de Janeiro state, Northern Santos Basin-Brazil

Seismic analysis of sparker lines of GEOMAR cruises allowed us to address a first stratigraphic scenario for the shallow sedimentary record (~300 msec) of the continental shelf off Rio de Janeiro State, northern Santos basin. Two sets of seismic sequences were identified and interpreted as a succession of depositional sequences induced by repeated glacioeustatic cycles. Depositional sequences composing Set I (SqA-SqC) are dominantly sigmoidal, reflecting periods of increasing accommodation space that favoured the preservation of both aggradational and progradational units; sequences of Set II (Sq1-Sq5) are essentially seaward-thickening stacks of forced-regression wedges, implying periods of declining accommodation space. Comparison between seismic lines and chronostratigraphic data allowed the mapped sequences to be placed within the Plio-Quaternary. Correlations also suggest that most of Set I (SqA and lower portion of SqB) was deposited during the Pliocene (undifferentiated Pliocene), while the upper portion of sequence SqC and sequences of Set II (Sq1-Sq5) have been placed within the Quaternary. Correlation of chronostratigraphic data with &#948;18O isotopic "sea level curves" also supports the hypothesis that sequences Sq1-Sq4 are fourth-order forced-regression sequences that record 100-120 kyr glacioeustatic cycles for the last 440-500 kyr, while sedimentary units labeled Sq5 would represent the transgressive and highstand deposition during the Holocene.<br>A análise sísmica de dados sparker das Operações GEOMAR permitiu a elaboração de um primeiro arcabouço estratigráfico da seção rasa (~300 msec) da plataforma continental do Estado do Rio de Janeiro, norte da bacia de Santos. Dois conjuntos de seqüências sísmicas foram interpretados como seqüências deposicionais induzidas por oscilações glacio-eustáticas. O Conjunto I (SqA-SqC), composto por seqüências dominantemente sigmoidais, reflete condições de geração de espaço de acomodação sedimentar capaz de preservar seus componentes agradacionais-progradacionais; o Conjunto II (Sq1-Sq5), composto principalmente por prismas de regressão forçada, indica diminuição relativa de espaço de acomodação. Dados cronoestratigráficos de poços permitiram posicionar a seção sísmica investigada na janela plio-quaternária: à maior parte do Conjunto I (SqA e parte inferior da SqB) foi atribuída uma idade Plioceno (indiferenciado); à seção estratigráfica que se estende da porção superior da SqC até o Conjunto II foi atribuída uma idade quaternária. A correlação entre a base de dados e curvas globais de variações isotópicas de &#948;18O permitiu ainda sugerir que as seqüências Sq1-Sq4 registram sequências regressivas de quarta ordem (ciclos glacio-eustáticos de cerca de 100-120 ka) durante os últimos 440-500 ka. A sequência Sq5 representaria a deposição holocênica, constituída por depósitos transgressivos e de sistemas de mar alto