The effect of mass-transport deposits on the younger slope morphology, offshore Brazil

The effect of Cenozoic mass-transport deposits (MTDs) on the morphology of the Late Neogene to Quaternary seafloor is investigated using a 3D seismic volume from offshore Brazil. The studied MTD shows large remnant blocks deforming the seafloor several Ma after a principal instability event marking the base of the investigated strata. Remnant blocks formed during this latter instability event were quickly buried, with differential compaction between individual blocks and adjacent debrites triggering: a) seafloor instability on the flanks of uncompacted (remnant) blocks, b) the incision of submarine channels between seafloor highs formed by buried remnant blocks, c) local uplifted areas on the seafloor that may form potential triggers for future slope instabilities. The interpreted data show that palaeo-seafloor scarps reached more than 120 m in height, with flanking strata to remnant blocks reaching angles of 15°. Angles of this magnitude caused local collapse of seafloor strata and, in some intervals, the confinement of younger MTDs sourced from the upper slope. The statistical data presented here indicate that differential compaction over heterogeneous MTDs continued well after early burial, still deforming the seafloor c. 15 Ma after the main instability event. In addition, significant structural traps are formed by forced folds on remnant blocks that not experienced substantial compaction. Therefore, we conclude that MTDs on passive margins can control seafloor topography after early burial, at the same time contributing to the formation of significant structural traps in post-MTD successions.