Late Quaternary Sedimentation on a Macrotidal Mudflat Deposit in Namyang Bay, West Coast of Korea

Abstract

Three late Quaternary stratigraphic units are present in the macrotidal-flat deposit in Namyang Bay, west coast of Korea: a mid-to-late Holocene greenish gray sand/mud deposit (unit M1), an early Holocene reddish brown clay deposit (unit T1), and a late Pleistocene yellowish, semi-consolidated mud deposit (unit M2), in descending order. Unit M1, representing a Holocene intertidal-flat deposit, shows a coarsening-upward trend in texture, which is resulted from the continual retrogradation of the tidal flat during the mid-to-late Holocene sea-level rise. This retrogradational coarsening-upward succession, elucidating a reverse trend to the progradational fining-upward facies model generalized for tidal flats worldwide, may be attributable to a very low accumulation rate (< 1 mm/yr) in this sediment-barren coastal setting. Reddish brown-colored unit T1, clearly separated from overlying unit M1 by a sharp lithologic boundary, consists of homogeneous clay with abundant freshwater siderite grains. Radiocarbon age, siderite grains, and lithologic features indicate that unit T1 is the early Holocene deposit in freshwater bog or swamp, infilling topographic lows. Overlain unconformably by the early Holocene clay deposit, unit M2 is characterized by dewatered, oxidized mud, which is interpreted as being the late Pleistocene tidal deposit that was subaerially exposed during the sea-level lowstand, possibly LGM. The unconformity between units M2 and T1 is probably related to the latest Pleistocene or earliest Holocene sea-level lowstand.