Age and morphodynamics of a sandy beach fronted by a macrotidal mud flat along the west coast of Korea: a lateral headland bypass model for beach-dune formation

摘要

The Dasari beach-dune system fronted by an intertidal mud flat is a typical example of numerous small beaches found both in embayments and along the open macrotidal west coast of Korea. The beach is frequently exposed to energetic wave action at high tide in winter. Although this coastal dune-sandy beach-intertidal mud flat system has previously been described, its origin and morphodynamic behavior has to date not been firmly established. To clarify these issues, elevation profiles and surficial sediment samples were collected seasonally along five monitoring transects across the tidal flat. In addition, box-cores as well as vibro- and drill-cores were acquired along the middle transect. Optically stimulated luminescence (OSL) and C-14- AMS (accelerator mass spectrometry) dating methods were applied to determine the age of the tidal flat, the beach and the dune deposits. The results show that Dasari beach is topographically composed of two distinct morphological and sedimentological sectors, comprising a high-tide sandy beach that merges seaward into an extensive low-tide tidal flat composed of mud. The transition between the two sectors is marked by a sharp break in slope and change in internal sedimentary structures. At the boundary, the subtle shoreward fining trend in mean grain size on the intertidal flat switches to a pronounced shoreward coarsening trend. Near the transition, mixing between the beach sand and the mud is observed. Another striking feature is a seasonal rotation of the beach system centered on the middle sector, with the northern sector eroding in winter and accreting in summer, and the southern sector accreting in winter and eroding in summer. The spatial grain-size pattern reveals that the beach is fed from the neighboring beach in the north by lateral headland bypassing, rather than onshore transport across the tidal flat, the intermittent lateral supply of sand explaining the seasonal rotation of the beach. Stratigraphically, the beach-dune deposits are underlain by muddy tidal flat deposits, which results in a clear upward coarsening grain-size trend and thus intimates transgressive deposition associated with sea-level rise over the past 7-8 ka. However, a time gap of 4-5 thousand years between the tidal flat and the beach-dune deposits indicates that there is no genetic link between the two depositional systems. As the modern beach-dune system is remote from the former landward limit of the tidal flat, a continuous retreat model must be rejected. Instead, a lateral headland bypassing model, in which sand supplied alongshore progressively encroached the tidal flat, is more plausible. In the light of this interpretation, many of the barrier beach-lagoon models proposed for the macrotidal west coast of Korea need to be reconsidered.