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Formation Environment of Quaternary Fluvial Deposits of Namhan River in Danyang Country, Korea

The Suyanggae paleolithic site is situated on either the floodplain(Site-Ⅱ), or old river bottom(Sites- I, III) of Namhan River. They are composed of soil-sedimentary deposits of overbank silt and sand deposits, and old river-bed sands and gravels, and slope sediments with paleosol layers. Suyanggae site is characterized by lower fluvial deposits, which is composed of old fluvial sand and intercalated muds, and upper slope sediments with scatterings of gravels in the matrix of sandy muds. Grain size analysis indicates silt-dominant up to the altitude of 160m from top, and fluvial sand in the lower part. The vertical distribution of mean grain size, median and the size of coarsest-one percentile shows similar fluctuation patterns typical in fluvial sedimentary sequence, while the standard deviation of grain size shows opposite to them. The fluvial sedimenary deposits in sector- III of Suyanggae site are distributed on the middle terrace at the levels of 42-47m above the previous river-bed (before dam construction). The formation age of this fluvial terrace gravels in Nam Han River is assumed to be related to glacial ages before last interglacial, i.e., older than l25Ka. The stone artefacts, commonly found in the interfaces between the lower fluvial sand/gravel and slope sediments with paleosol layers of fossil frost cracks. The upper slope sediments are typified by fragipan, polygonal texture and frost cracks, which were interpreted to be formed under extreme dry and cold condition during the last glaciation, representative by LGM, ca 16-22Ka.