THE PARADOX OF HIGH SEDIMENT SUPPLY TO THE EAST CHINA SEA CONTINENTAL MARGIN AND THE ABSENCE OF SUBMARINE FANS AND LARGE-SCALE SLOPE FAILURE IN THE OKINAWA TROUGH

The objective of this study is to characterize the paradoxical behavior of depositional systems that do not display typical features nor responses usually associated with high sediment supply slope and basin systems. The study site is the Okinawa Trough in the East China Sea, which is an active back-arc basin in the early stages of evolution. Despite the tremendous influx of sediment from the Yellow and Yangtze Rivers, the second and fourth largest rivers in the world in terms of sediment supply, there is very little evidence of thick submarine fans and major slope deformation features on the outer shelf of the East China Sea and northwestern slope of the Okinawa Trough. All these features are part of the classic stratigraphic model of high sediment supply passive margins and should be seen at this site, but the area is sediment starved. Sequence stratigraphic analyses of seismic and high-resolution chirp sonar data provide insight into why there is such a small volume of sediment reaching the Okinawa Trough. The thickness of the sediment is only tens to hundreds of meters as opposed to kilometers thick in other systems (i.e. Mississippi River system). Crystalline basement is easily seen in some areas of the data. Preliminary results show that the strong currents continuously sweep across the shelf and remove much of the sediment before it reaches the basin. An additional factor that reduces the offshelf flux of sediment is that the shelf/ slope break lies between 150 to 190 m depth and sea level fell only 120 m during the last lowstand of sea level (20 ka). Perched deltas and absence of canyons and stream incisions on the shelf suggest that the sea level did not fall beyond the shelf/slope break during the lowstand, so sediment was trapped on the shelf and very little of it was bypassed to the slope and basin. While there are few modern situations were high sediment areas are starved, there are many in the geologic record. Characterization of the Okinawa Trough area of the East China Sea provides a model of the stratigraphy that evolves in these unique circumstances.