Formation mechanism of deep-sea giant pockmarks: A case study of the Reed Basin in the South China Sea

Abstract

Pockmarks are widely distributed in the seabed around the world, however, mega pockmarks with diameters >1 km are rare, and their formation mechanisms remain enigmatic. Using high-resolution multi-beam bathymetric data, we firstly identified mega pockmarks in the Reed Basin in the South China Sea (SCS). These mega pockmarks of various shapes occur in groups, primarily along the sides of submarine canyons. We propose a new hypothesis about the formation mechanism of the deep-sea pockmarks that pockmarks in the Reed Basin are formed by seafloor gas explosions, widely developed carbonates store buried gas that continuously seeps along NE-SW trending faults, cap layers are undercut by submarine canyons forming lines of mechanical weakness, during periods of rapid sea level fall, depressurization causes buried gas to be rapidly ejected along these lines, forming mega pockmark groups. Submarine pockmarks reported in 21 regions globally have been analyzed, and it is found that the average median of the ratio of depth to radius (d/r) of the pockmarks is close to 0.3. Based on the theory of crater formation, it is found that the d/r ratio of the crater is also close to 0.3, which provides a new idea for the formation mechanism of deep-sea pockmarks. It is also found that the average size of the pockmarks is controlled by the water depth, and giant pockmarks are more likely to form in the deep sea. Because these results correlate easily observed bathymetric features to the presence of buried gas deposits, they have important implications for the exploration and research of deep-sea gas resources.