Michael J. Steventon, Christopher Aiden-Lee Jackson , David Hodgson, Howard D. Johnson

Construction of continental margins is driven by sediment transported across the shelf to the shelf-edge, where it is reworked by wave-, tide- and river-influenced processes within deltas and flanking clastic shorelines. Stalling of continental margin progradation often results in degradation of the outer shelf to upper slope, with re-sedimentation to the lower slope and basin-floor via a range of sediment gravity-flows and mass-movement processes. Our understanding of how these processes contribute to the long-term development of continental margins has typically been limited to observations from broadly two-dimensional, subsurface and outcrop datasets. Consequently, the three-dimensional, particularly along-strike variability in process regime and margin evolution is poorly constrained and often underappreciated. We use a large (90 km by 30 km, parallel to depositional strike and dip, respectively) post-stack time-migrated 3D seismic-reflection dataset to investigate along-strike variations in shelf margin progradation and outer-shelf to upper-slope collapse in the Santos Basin, offshore SE Brazil. Early Palaeogene to Eocene progradation of the shelf margin is recorded by spectacularly imaged, SE-dipping clinoforms. Periodic failure of the outer-shelf and upper slope formed c.30 km-wide (parallel to shelf margin strike) slump scars, which resulted in a strongly scalloped upper slope. Margin collapse caused (1) the emplacement of slope-attached mass-transport complexes (MTCs) (up to ca. 375 m thick, 12+ km long, 20 km wide) on the proximal basin-floor, and (2) accommodation creation on the outer shelf to upper slope. This newly formed accommodation was infilled by shelf-edge-delta clinoforms (up to 685 m thick), that nucleated and prograded basinward from the margin-collapse headwall scarp, downlapping onto the underlying slump scar and/or MTCs. Trajectory analysis of the shelf-edge deltas suggests that slope degradation-created accommodation was generated mainly during times of base-level rise rather than, as would be predicted by most sequence-stratigraphic models, during base-level fall. Our results highlight the significant along-strike variability in depositional style, geometry and evolution of that can occur on this and other continental margins. Coeval strata, separated by only a few kilometres, display strikingly different stratigraphic architectures; this variability could be missed in 2D datasets and is not currently captured in conventional 2D sequence stratigraphic models.