Recognition and three‐dimensional characteristics of ancient supercritical flow bedforms on a submarine slope: An example from the South China Sea

Supercritical flow bedforms are important elements of sedimentary environments, but their internal three-dimensional structure has been elusive due to seismic imaging limitations. This article presents high-resolution three-dimensional seismic reflection data from Formosa Ridge - a ridge between two canyons that incise into the north-eastern South China Sea margin. The ridge consists of 300 m thick submarine deposits including sediment waves that are manifested as crescentic depressions surrounded by elevated walls on the palaeo-seafloor. Cross-sectional profiles display scour fills that turn into step-like sediment waves further downstream. These bedforms are 470 to 1370 m long and 30 to 140 m high. The three-dimensional seismic data clearly show the step-like bedforms that may be misinterpreted as faults or slumps on data with lower resolution. Despite exhibiting negative palaeo-seafloor relief, they are overall depositional structures and have constructed at least part of the Formosa Ridge. The bedforms' morphology and upslope migration suggest that they are the continuum of partially depositional to fully depositional cyclic steps formed by bottom currents travelling, based on a series of simple calculations, at least 2 m s(-1). These currents are able to transport sediments with grain size up to coarse sand, and such dynamic processes might impact seafloor infrastructure safety, oil and gas reservoir systems, and the functioning of benthic ecosystems in similar settings.