Abstract

Three-dimensional seismic-reflection data are used to characterize the seismic expression and investigate the origin of two mass transport complexes (sensu Moscardelli and Wood, 2007) that are contained in two salt-related minibasins on the São Paulo Plateau, offshore SE Brazil. The mass transport complexes contain numerous slide blocks, which are expressed on seismic data as broadly tabular packages of weakly deformed reflections. Individual slide blocks are up to 50 m thick and up to 1 km2 in plan view. The slide blocks are flanked by laterally continuous reflections, which are interpreted as the seismic expression of either a thin mudstone-dominated debrite deposit or a thin interval of hemipelagic mudstones. The surface that bounds the base of the two mass transport complexes is planar, and this suggests that the associated gravity-flow event caused only limited erosion of the seafloor. The upper surfaces of the mass transport complexes are very rugose and onlapped by overlying strata, indicating that the slide blocks formed seabed relief of up to 20 m. The complexes occur at the base of a unit that thins toward the margins of the studied minibasins, and the slide blocks in the mass transport complexes decrease in size and density away from the salt-cored structural high that separates the two minibasins. Based on its seismic stratigraphic context and the spatial distribution of the slide blocks that they contain, the mass transport complexes are interpreted to have been derived from the failure of the margins of the salt-cored high in response to a period of relatively rapid minibasin subsidence. The results of this study indicate that large volumes of sedimentary material may be derived from the margins of salt-related minibasins in relatively distal submarine settings. From a geohazard perspective, this observation is important because the risk associated with submarine slope failure, and the transport and emplacement of large blocks, is typically thought to be low in relatively distal, deep-water environments.

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