STRATIGRAPHY PRODUCED BY SMALL COASTAL RIVERS ON THE EAST CHINA SEA CONTINENTAL MARGIN

The East China Sea (ECS) is a broad, shallow epicontinental sea with a low gradient and dominated by high energy and high sediment supply. Frequent and intense storms, the Kurishio Current, and two of the world’s top four rivers, in terms of sediment discharge (the Yangtze and Yellow Rivers), dominate the current depositional environment. As part of a continuing study comparing the differing depositional processes of the ECS, a high-resolution (100- 2000 Hz) seismic data set, consisting of 1765 km of profiles, was acquired in April 2000 in the southern ECS. This high-density survey grid is located south of the present-day depositional center of the rivers in the outer-shelf and slope environments. Sequence stratigraphic interpretation of these data suggests that small coastal rivers dominated the sedimentation on an exposed shelf during the last lowstand. Evidence of river processes includes incision and chaotic seismic facies. The fluvial package of the last lowstand overlies a laterally continuous facies of flat lying, high frequency, and moderate amplitude reflections. This sheet is interpreted as the highstand systems tract deposited during the last interglacial (oxygen isotope stage 5) when sea level was at a similar elevation as today. Observations of faulting on the upper slope indicate slumping and structural variation within the environment. Also observed are variations in paleo-shelf morphology and orientation between sea level cycles. The variations over time of the paleo-shelf and sea level result in different distributions of depositional systems. The changes in distribution of deposition systems generate the heterogeneity of seismic facies observed in the margin of the southeast ECS. A significant difference between this portion of the margin and the middle and northern portions of the margin is that in the area dominated by small coastal rivers the stratigraphy is much more heterogeneous than the stratigraphy associated with the larger river systems to the north.