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