Sediments and sedimentary sequences on a modern macrotidal flat, Inchon, Korea

Enormous, unbarred tidal flats fringe the west coast of Korea. Near Inchon, where spring tides range between 8 and 9 m, the intertidal expanse is more than 4 km wide. This low-energy regime results in three broadly intergradationai modern subfacies: l) an intensely bioturbated inner flat of slightly sandy mud; 2) a wavy-bedded middle flat of clayey sandy silt; and 3) a ripple-laminated outer flat of bioturbated sandy silt to silty sand. The mid-flat region is less distinctive sedimentologically than the mixed-flat subfacies of North Sea tidal flats, and flaser and lenticular bedding are rare. Well-developed intertidal drainage networks and landward salt marshes are absent. Vibracores reveal two additional sequences underneath the modern sequence, their contacts defined by scour horizons and shell concentrations. The basal sequence is characterized by irregularly oxidized, intensely-to-totally bioturbated argillaceous sediment with scattered wavy beds and abundant in-situ plant roots. These deposits suggest a transition from a shallow subtidal or low intertidal environment to a salt marsh developed in a protected intracoastal setting. The overlying intermediate sequence also consists principally of bioturbated fine sediment with scattered wavy beds, but plant roots are absent. Stratigraphic distributions of mollusk shells and other features ally this sequence with landward parts of the modern sequence.