Evolution of deep-water synkinematic sedimentation in a piggyback basin, determined from three-dimensional seismic reflection data

Three-dimensional seismic data have revealed the interaction between synkinematic deposition and active folds in a deep-water piggyback basin. Background aggradational deposition is punctuated by debris flows and/or landslides, channels, canyons, fans, and degradation complexes. Gravitydriven hummocky strata-bound folds hundreds of meters to 2 km in wavelength, with tens of meters amplitude developed on the unstable slopes of large fans. At the base of the debris flows and/or landslides, long, curved, erosive furrows indicate flow transport direction and help demonstrate changing flow directions as the basin evolved. In flows that traversed several anticlines and synclines, sediments were transported as sheet-type flows in the syncline and focused back into narrow channels or canyons at anticline-related topographic ridges. Basin evolution is characterized by early synkinematic sedimentation, where growing folds are in a distal position to gravity flows and display a relatively intact antiformal geometry. Flows entering the basin tend to be axial, either from the southwest or northeast. Anticline growth triggered instability on the fold crests and caused local landslides. Later flows entered the basin perpendicular to the anticlines and crossed the seafloor ridges by either exploited weak points at the crests, e.g., overpressured mud/fluid pipes, or structurally related low points, e.g., saddle regions where two plunging anticlines linked along strike. At the exit point of some anticline-traversing canyons, high-relief fans developed where deposition locally exceeded subsidence rate. Such fans can ultimately block and divert flow from the original fan feeder canyons to other piggyback-basin entry points.