Sedimentation on a wave-dominated, open-coast tidal flat, south-western Korea: summer tidal flat – winter shoreface

Sedimentation on the open-coast tidal flats of south-western Korea is controlled by seasonal variation in the intensity of onshore-directed winds and waves. As a result, an environmental oscillation takes place between tide-dominated conditions in summer and wave-dominated conditions in winter. In summer, thick muddy deposits, including sporadic storm deposits, accumulate in response to low wave energy, weak currents, and intense solar insolation that promotes consolidation of the mud at low tide. Bioturbation is minimal because of rapid sedimentation and soft substrate. During the autumn, the summer mud deposits experience erosion due to increasingly strong onshore winds and waves, until only small mud patches and mud pebbles remain. The concentration of ebb runoff between the mud patches produces small, ephemeral tidal creeks. In winter, storm waves occur frequently (ca 10 days a month) and dominate sedimentation in the intertidal zone, producing extensive wave-generated parallel lamination and short-wavelength (0·3–2 m) hummocky cross-stratification. The prevalence of strong onshore winds decreases in spring, allowing longer and more frequent intervals of calm weather, during which time muddy sediments are deposited by tidal processes. Over the long term, winter storm waves dominate sedimentation and the preserved deposits consist of amalgamated storm beds that resemble those generally associated with shorefaces. This raises the question of how many ancient ‘shorefaces’ are, in fact, open-coast tidal flats.