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

Community structure and carbonate production of a temperate rhodolith bank from Arvoredo Island, southern Brazil

A small (100,000 m²) rhodolith bank located at the Arvoredo Marine Biological Reserve (Santa Catarina, Brazil) has been surveyed to determine the main bank components, the community structure, and carbonate production rates. Data from five photographic transects perpendicular to Arvoredo Island shore were complemented with sediment samples and shallow cores, all collected by scuba diving. The main bank component is the unattached, nongeniculate, coralline red algae Lithophyllum sp., used as substrate by the zoanthid Zoanthus sp. Percentage cover of living and dead coralline algae, zoanthids and sediment patches account for nearly 98% of the investigated area. Classification and ordination of samples showed that differences in the proportion of live and dead thalli of Lithophyllum sp. determine the relative abundances of zoanthids. Results also indicate that similarity of samples is high and community gradients are subtle. Significant differences in percentage cover along transects are concentrated in the central portion of the bank. Low carbonate content of sediments from deeper samples suggests low rates of recruitment and dispersal of coralline algae via fragmentation. However, carbonate production of Lithophyllum sp ranging from 55-136.3 g m-2 yr-1 agrees with production rates reported for other temperate settings. In the long run, rhodolith density at Arvoredo Is. is likely to be dependent upon random dispersal of spores and/or fragments from other source areas.<br>Investigou-se um pequeno (100,000 m²) banco de rodolitos localizado na Reserva Biológica Marinha do Arvoredo (Santa Catarina, Brasil) para se determinar os pricipais componentes do banco, a estrutura da comunidade e a produção de carbonato de cálcio. Dados de cobertura relativa foram obtidos ao longo de cinco transectos fotográficos perpendiculares à ilha do Arvoredo, e complementados com amostras de sedimento superficial e testemunhos rasos. O principal componente do banco é a alga calcária não articulada, de vida livre, Lithophyllum sp., que é usada como substrato pelo zoantídeo Zoanthus sp. A cobertura relativa de algas vivas, mortas, zoantídeos e manchas de sedimento perfazem quase 98 % da superfície investigada. A classificação e ordenação das amostras mostrou que diferenças nas proporções de talo vivo e morto de Lithophyllum sp. determinam a abundância relativa de zoantídeos. Os resultados também indicam que a similaridade entre as amostras é alta e que gradientes da comunidade são sutis. Diferenças significativas da cobertura relativa ao longo dos transectos concentram-se na porção central do banco. A baixa concentração de carbonato de cálcio nos sedimentos mais profundos sugere baixo recrutamento por fragmentação. Entretanto, a taxa de produção de carbonato de cálcio determinada entre 55-136.3 g m-2 yr-1 encontra-se dentro da faixa observada para outros ambientes temperados. No longo prazo, a densidade de rodolitos da ilha do Arvoredo pode depender da dispersão aleatória de esporos e/ou fragmentos de outras áreas fonte