Origin and evolution of widespread crescentic pockmarks on the western South China Sea margin

Abstract

Pockmarks are prominent morphological features formed by fluid escape at the seafloor that provide valuable information on past environmental conditions and ongoing sedimentary processes, and they are also biodiversity hotspots and pose potential hazards in marine environments. Crescentic pockmarks are widespread on the present seafloor of the western South China Sea margin, however, their formation mechanisms are still poorly constrained. Here, we have imaged and investigated 116 crescentic pockmarks based on multibeam bathymetry and three-dimensional seismic data to gain insights into their origin and development processes. Crescentic pockmarks show asymmetrical geometries in cross-section and their opening directions are mainly oriented NW to NNW. Morphological analysis shows the presence of two types of composite styles, and the ratios between their widths and lengths are approximately ∼0.47. Seismic profiles crossing the crescentic pockmarks illustrate that their position has a trend of downslope migration. Time slices extracted from seismic data reveal that they exhibit as circular depressions in the deep strata, while they become crescentic gradually towards the shallow strata. We consider that the crescentic pockmarks may evolve from deep circular pockmarks and the intensity of fluid escape has played a vital role in affecting their morphologies. Downslope migration of these pockmarks might be influenced by the downslope bottom currents and turbidity currents, leading to more deposition in their upstream side and erosion in the downstream counterpart. Our study demonstrates how bottom currents and gravity flows rework pockmarks’ seafloor expression to form the crescentic morphologies. Hence, it provides a world-class morphological current indicator for other modern and paleo-pockmarks on continental margins worldwide.