The Importance of Incised Valley Abandonment and Widespread Fluvial Deposition on the High Energy and High Sediment Supply East China Sea Continental Shelf

Louis R. Bartek, Robert W. Wellner

In order to access the shallow (<100 m), Late Quaternary record of the East China Sea (ECS) shelf, a high-resolution (<1 m) Geopulse seismic reflection (2,825 km), X-Starr "Chirp" sonar (3,428 km) and coring (28 stations and 39 gravity cores) survey was conducted aboard the USNS Wilkes. The ECS is part of a large, semi-enclosed epicontinental sea that is characterized by high tidal currents, frequent and intense storms, and an extremely high sediment supply. This setting provides an unique opportunity to observe distal "end-member" type depositional conditions. In contrast to less dynamic shelves (e.g. Atlantic Coast, Gulf of Mexico), that have lower sediment supplies and subsidence rates, and compressed and complex stratigraphic records, the ECS shelf has elatively thick, complete depositional sequences with well preserved lithofacies.

On the inner to middle portions of the ECS Margin we have identified extensive areas, outside of several large incised valleys, that are characterized by chaotic, channel-fill seismic facies. We interpret this facies as fluvial sediments that were deposited prior to the late Wisconsin (oxygen isotope stage 2) maximum sea level lowstand. Historical records suggest that the tremendous sediment load of the Yellow River caused it to avulse frequently and meander over hundreds of kilometers, depositing a thick sheet of fluvial sand and silt in "interfluvial" areas. The upper portions of these "interfluvial" fluvial deposits were reworked during the ensuing transgression (i.e. development of the transgressive surface (TS)) while the lower fluvial facies is preserved in place. The adjacent i cised valleys contain a thick fill sequence with several different seismic facies, but most notable are the large tidal ridges above the TS that developed during the ensuing late glacial/Holocene transgression.

The dramatic difference between the stratigraphy and facies distributions associated with this "end-member" type depositional setting and others, suggests that further characterization of the relationships between dominant depositional processes, sequence stratigraphic architecture, and facies distribution is needed. This in turn will facilitate a more reliable assessment of hydrocarbon recovery.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995