Effect of irregular, complex underlying topography in Holocene sedimentation revealed from late Quaternary sequences in the mid-western coast of Korea

Through a detailed analysis of seismic profile and vibra-/drilling-core sediments (up to 25.5 m long) with AMS 14C ages from a tidal flat in the mid-western coast of Korea, we reconstruct development of coastal sedimentary sequences in response to sea-level changes for more than about 150 ka, and reveal effect of irregular, complex underlying topography in the Holocene coastal sedimentation. Unit I overlying rock basement and the lower part of Unit II consist of fluvial sediments deposited during sea-level lowstands prior to interglacial period of Eemian Stage (MIS 5e). During the sea-level highstand of MIS 5e, muddy tidal sediments between 7.40 and 10.76 m deep below the present tidal-flat surface were deposited in the upper to middle part of Unit II. The study area had been subaerially exposed for a long duration from MIS 5e to ca. 10 ka. During this period, the muddy tidal sediments in Unit II had been underwent oxidization and significant erosion by fluvial process, forming irregular and complex morphology of the upper boundary of Unit II with a large topographic relief (up to ca. 7 m) even over a short (ca. 560 m) distance. On the upper boundary of Unit II, topographic highs could act as barriers for weakening wave effects from ca. 8-9 ka to 3-4 ka (period of relatively rapid sea-level rise), depositing muddy tidal sediments (Unit III-B). As the muddy tidal sediments (Unit III-B) filled the irregular, complex underlying morphology, surface topography was nearly flat without barriers around about 3-4 ka. Since then, the nearly flat surface morphology without barriers, together with relatively slow sea-level rise and direct exposure of strong onshore winter waves/storms, could promote to deposit wave-/storm-driven sandy sediments (Unit III-A). This study suggests that change in surface morphology could affect stratigraphic evolution of the Holocene coastal sequence by variation in depositional regime.