Anomalous depressions in the northern Yellow Sea Basin: Evidences for their evolution processes

Depressions of both known and unknown origin are observed in continental margin basins and ocean floors worldwide. A field of anomalous depressions with dome structures within the centers are documented for the first time using sub-bottom seismic profiles, multibeam bathymetry data, and well data in the northern Yellow Sea Basin. They have complex seabed morphologies (circular, elliptical, polygonal, linear and crescent shapes) with kilometer-scale diameters and shallow depths (10 s decimeters to meters). Focused fluid flow pathways, such as gas chimneys, faults and pipes, and free gas (enhanced amplitude anomalies) are observed within the shallow strata. The depressions are interpreted as 'immature pockmarks', possibly caused by insufficient fluid supply compared to fully developed 'normal pockmarks'. They are suspected to still be in the middle stage of pockmark evolution. Their formation processes are suspected to be controlled by sub-seabed plumbing/fluid flux and bottom currents. This study is important for geologic hazard assessments in the study area and it also contributes to the full understanding of pockmark evolution.