GRAVITY-DRIVEN SEDIMENTATION ON THE NORTHWESTERN CONTINENTAL SLOPE IN THE SOUTH CHINA SEA: RESULTS FROM HIGH-RESOLUTION SEISMIC DATA AND PISTON CORES

A distinct echo-character was assigned to sedimentation processes, which were then verified using data from surface sediment samples and piston cores. Of echo types on the continental slope perfectly reflecting both sediment erosion and deposition, four sedimentary types have been recognized: (1) submarine slides distributed on the shelfbreak and characterized by high silt and water content, loose structure, poor consolidation and low shearing strength; (2) slumps occurring on the shelfbreak, middle slope channel and reef margin near Dongsha Islands, but having different origins; (3) debris flow occurring either in sea areas around Dongsha Atoll, or on the continental slope's three channels, where the transparent debris flow deposits often overlie or abruptly truncate highly stratified hemipelagic sediments; are of limited to local extent, ranging from a few square kilometers to hundreds of square kilometers in area; but on the lower slope, usually occur as 1000 km(2), about 1130 km(2) individual complexes; and (4) turbidites, limited on the continental slope; some occurring as migrating waves of sediments at the toe of the slope, and are rhythmically-bedded, coarse-grained. Their migration is a result of overbank flow downslope through the submarine channel at the west. The slope faces axe dominated by mass wasting deposition, and a few trubidite current sediments. Mass wasting is an important process. Some debris flow complexes on the west are buried by well-stratified conformable sediments, whereas others on the east appear on the present seafloor and therefore are relatively recent.