Geomorphological characteristics and formation mechanisms of large pockmarks in the Baiyun submarine channel system, northern South China Sea

Hydrothermal vents and pockmarks in the global ocean are direct evidence for extensive seabed fluid emission and fluid seepage. Pockmarks are seafloor crater-like depressions associated with fluid escape from the seabed and are considered to contribute to the transfer of methane into the ocean and ultimately into the atmosphere. In the South China Sea， several-meters to kilometer-scale pockmarks lie in different sedimentary basins around the continental margin and are related to local subsurface structures such as mud diapirs， gas chimneys， mud volcanoes and faults. The formation of pockmarks in the Baiyun submarine channel system was accompanied with the evolution of unidirectional migrating channels in the Pear River Mouth Basin (PRMB) that produced and developed under the interaction between downslope turbidity currents and along-slope bottom currents since the Middle Miocene. Based on new acquisitioned multibeam bathymetry and sub-bottom profiler data in the Baiyun submarine channel system in the PRMB， twelve large pockmarks with 500-1500 m long， 200-900 m wide and 10-30 m deep are recognized on the top of inter-channel ridges (ICRs) in the Baiyun submarine channel system. These pockmarks in the channel system indicate mainly circular， elliptical， crescent-shaped， elongated and horseshoe shaped in plan view. The crescent-shaped depressions at the head part of channels and the ‘pond-like’ depressions between the individual channel are found and interpreted from slope variations of the bathymetry data. We proposed that channel-related pockmarks occurred at the ICRs were produced by the upward migration of overpressure gas and fluid along gas chimneys and faults caused by the gas hydrate dissolution in the deep reservoirs since the Middle Miocene and the ICRs underwent the collapse and erosion process by bottom currents in the shallow part. The sources of fluid and gas induced to the formation of pockmarks are from deep to shallow reservoirs in the submarine channel system.