Structural characteristics and genesis of pockmarks in the northwest of the South China Sea derived from reflective seismic and multibeam data

Pockmarks are usually residual topographies caused by fluids escaping through the seafloor. The study area of this paper is located in the northwest of the South China Sea, including the southeast part of the Qiongdongnan basin and the north-central part of the Zhongjiannan basin. The fine-grained sediments of the seabed as well as the strong bottom current provide favorable conditions for the formation and development of pockmarks. The main factor for the development of pockmarks is the fluid flow system which is related with faults, gas chimneys, mud volcanoes, mud diapirs, acoustic blanks/acoustic turbidities and enhanced reflections in the study area. The geometric features of pockmarks vary in different regions. Firstly, the sizes of pockmarks change greatly, with widths of tens to hundreds of meters for those associated with bottom current channels and those in the Guangle uplift, and thousands of meters for other portions in the study area, which are usually called giant pockmarks. Secondly, the surface morphologies of pockmarks are complicated, including circle, ellipse, crescent and annulus, which may indicate different stages of pockmark development. Crescent pockmarks may be younger, followed by annular pockmarks, and the circular ones are relatively older. According to this point, the crescent pockmarks are possibly active, which seems to be supported by data of seismic oceanography.