Fill-and-Spill, Tilt-and-Repeat (FaSTaR) cycles: stratigraphic evolution above a dynamic submarine stepped slope

The classic fill-and-spill model is widely applied to interpret topographic controls on depositional architecture and facies distributions in slope successions with complicated topography. However, this model implies a constant topographic configuration over the lifespan of a turbidite system. In contrast, the impact on patterns of erosion and deposition above dynamic slopes whose topographic configuration varies spatially over time remains poorly investigated. Here, using high-resolution 3D seismic reflection data and more than 100 wells from a 40 km long stepped slope system (Campos Basin, offshore Brazil), we document the evolution of a sand-prone turbidite system active during the Oligocene-Miocene transition. This turbidite system was influenced by vertical and lateral deformation, and we propose a new stratigraphic model to explain the resultant depositional architecture.

Two depocentres were identified as steps, with channels on the proximal step, and channel-lobe complexes on the distal step, bounded by sediment bypass-dominated ramps. Lateral stepping of channels on the proximal step, and oblique stacking of the down-dip lobe complexes, each cut by through-going channels, indicate multiple fill-and-spill cycles. A persistent NE-ward stepping and thickening on the steps is interpreted to reflect lateral tilting of the seafloor driven by salt tectonics. The dynamic substrate prevented the establishment of a single long-lived conduit across the proximal step, as recorded in systems with fixed topographic configurations. The filling of through-going channels with mud at the end of each cycle suggests waxing-to-waning sediment supply cycles and periods of sand starvation, when the lateral tilting dominated and drove avulsion of the feeder channels towards topographic lows. This study demonstrates that subtle dynamic slope deformation punctuated by discrete sediment supply cycles results in complex stratigraphic patterns with multiple phases, and multiple entry and exit points, during repeated cycles of fill-and-spill, tilt-and-repeat (FaSTaR). These are likely to be present in other stepped slope systems.