Pockmarks, as depression morphology related to fluid escape on the seafloor, are revealed by three-dimension (3D) seismic data on the northwestern South China Sea (SCS) margin. The pockmarks can be classified into two groups by their various shapes in plan-view, which are circular group and elongating group. These pockmarks in the study area could be defined as mega-pockmarks, as their maximum diameters can reach to 7.5 km. They commonly develop more than one crater, which are central crater and secondary crater. The seismic data illuminated their complicated internal architectures in the subsurface, as well as their evolution periods, such as initiation stage, mature stage and abandonment stage. According to the buried structures and their genesis mechanism, the mega-pockmarks could be classified into linear faults-associated pockmarks and volcano-associated pockmarks. The linear faults-associated pockmarks root on the top Middle Miocene, where the linear faults distribute. The linear faults on the top of fluid reservoir in Middle Miocene act as conduits for fluid seepage. The fluid seepage is driven by the break of balance between the hydrostatic and pore pressure. When the fluid seepage initiate, they will migrate along the linear faults, making the linear feature of pockmarks on the seafloor. Both thermogenic gas from deep intervals and biogenic gas in shallow intervals may be fluid source for the genesis of pockmarks. On the other hand, the volcanic activities control the genesis and evolution of volcano-associated pockmarks. The volcano-associated pockmarks root on the craters of volcanoes. The volcanoes underneath the pockmarks provide volcanic hydrothermal solutions, such as phreatomagmatic eruptions through the volcanic craters. The confined fluid seepages make the pockmarks on exhibiting more circular shape on the seafloor. Long-term, multi-episode fluid expulsions generate the complicated internal architecture that leads to multi-cratered mega-pockmarks on the northwestern margin of SCS.
The early-middle Miocene submarine fan system in the Pearl River Mouth Basin, South China Sea