Incised channel morphology and depositional fill of the paleo-Seomjin River in the continental shelf of the South Sea, Korea

Abstract

This study is the first attempt at describing and interpreting incised-channels on the continental shelf of the South Sea, Korea. The distribution, pattern, shape, and infilling of submerged channels incised across the continental shelf of the South Sea have been investigated based on approximately 1940 line-km of chirp and sparker data. The incised-channel system of the paleo-Seomjin River on the continental shelf is approximately 109 km long, 800–5000 m wide, and is more than 890 km2 in area. Meandering and straight types of channel are dominant on the inner shelf, with a change to straight and braided planforms on the mid to outer shelf. Sparker seismic data suggest the paleo-channels formed under fluvial systems when the shelf was exposed during the Last Glacial Maximum (LGM). Channel fills are divided into five types, based on erosional surfaces and internal seismic reflectors: (I) transparent to semi-transparent incised channel fill; (II) parallel to sub-parallel incised channel fill; (III) complex incised channel fill; (IV) divergent incised channel fill; and (V) chaotic incised channel fill. The acoustically transparent facies of the channel fill type (Type I) suggest the presence of basin muddy deposits in a low energy environment, which were dominant in the inner shelf. These muddy sediments were likely during a period of higher sea level during the Holocene transgression. Types II, III, and IV are characterized by onlap, downlap, and wave or tidal ravinement surfaces, respectively, indicating that they were deposited during a transgressive stage, which type V represent basal incision surface. The evolution of paleo Seomjin River has been strongly controlled by sea level change and sediment transport processes since the LGM.