Macrotidal Mudflats of the Southwestern Korean Coast: A Model for Interpretation of Intertidal Deposits

Abstract

ABSTRACT Extensive muddy tidal flats along the western coast of Korea are distinctly different from other intertidal environments used as standard models for intertidal sedimentation (e.g., the North Sea). The muddy Korean tidal flats lack extensive, dendritic drainage systems, seaward barriers, landward salt marshes, and most sedimentary structures considered diagnostic of intertidal environments (i.e., flaser, wavy, and ripple-laminated bedding). Fifty-one cores were collected from intertidal and subtidal areas of Nam Yang Bay to investigate the distribution of texture, composition, structure, and accumulation rate of these muddy sediments. Sediments coarsen seaward (from about 8.8 phi to 4.6 phi) and downcore, indicating that the tidal flats are actively accreting, with finer-grained, high-tidal- and mid-tidal-flat sediments prograding over the coarser-grained, low-tidal-flat sediments. Sedimentary structures show a trend of increasing preservation of physical stratification (mm-scale parallel to slightly wavy-bedded laminae) toward the mid-tidal flat. Pb-210 accumulation rates are highest on the mid-tidal flat (5-9 mm/yr) and decrease both seaward and landward to 1-2 mm/yr. Upper portions of intertidal cores are laminated and lower portions are mottled; the opposite is true of subtidal cores. Different processes dominate these two environments on both short and long timescales. The most significant difference between the muddy Korean tidal flats and the sandier North Sea setting is the lack of intricate drainage networks. Drainage channels on the Korean tidal flats seem to be relatively stable. Thus, channel-fill sequences do not compose a major part of muddy intertidal deposits. Muddy intertidal deposits would be more difficult to identify in the rock record compared to sandy intertidal deposits because of the lack of channel-fill sequences (which compose the bulk of sandy intertidal deposits) and because the gradients in sedimentological properties are gradual and subtle.