Geomorphology, shallow structure, and geological hazards in the East China Sea

The inner shelf of the East China Sea is covered with a lens of fine-grained modern sediment derived from the adjacent Changjiang; modern sediments north of the Changjiang came from the Huanghe. Rapid accumulation results in a featureless seafloor, except where escaping biogenic gases cause vents. The modern mud thins towards the mid-shelf and sediment becomes increasingly sandy and locally rippled, indicative of its relict age and present-day mixing and transport. The outer shelf, particularly south of Cheju Island, is covered with fine-grained sediment that apparently reflects modern Huanghe-derived sedimentation. Shallow seismic profiles show that the Holocene sediment overlies an eroded late Pleistocene surface, dominated by a number of well-defined buried river channels formed (and filled) during the last low stand of sea level; some channels may be associated with recent faulting. These filled channels and depressions probably represent the single most important geological hazard for future emplacement of man-made structures (such as drilling platforms) on the seabed. Other potential hazards include methane gas vents on the inner shelf, possible vents of deeper gas on the mid-shelf, outcrops or near-surface expressions of the Fukien-Reinan Massif, and possibly migrating sand waves.