Using cosmogenic ¹⁰Be dating to unravel the antiquity of a rocky shore platform on the West Coast of Korea

Shore platforms commonly occur at the base of coastal cliffs on rocky shores. It is generally accepted that they form by the retreat of such cliffs through wave action and weathering processes. Although a handful of previous studies have sought to directly constrain the rate of shore platform development over annualdecadal scales, there remains little consensus on the rate of their long-term (>millennial) evolution, as a result of the complex nature of shielding processes that operate in parallel with variable sea levels. Some platforms show contemporary features; however, the possibility that some features were inherited from the last interglacial or earlier stages always exists. The occurrence of wide shore platforms in resistant rocks is often regarded as evidence of such inheritance because the short stillstands of the Holocene offered insufficient time for the sculpting of extensive platforms on resistant rocky coasts. This study focuses on dating the wide shore platforms, which are common on headlands along the west coast of the Korean Peninsula, based on the abundance of cosmogenic ¹⁰Be. This macrotidal coast is dominated by tidal processes with tidal ranges of up to 6 m. The very gently sloping (<2°) shore platform of the study site is up to 150 m wide at low tide and composed mainly of metamorphosed sandstones. Samples for cosmogenic ¹⁰Be surface exposure dating were collected along two transects oriented perpendicular to the cliff line. The apparent surface exposure ages of the samples ranged from the Holocene (ca. 4 ka) to the Pleistocene (ca. 148 ka) and increased with increasing distance from the sea cliff along both transects. Given the apparent antiquity and range of the surface exposure ages, it seems likely that this shore platform originated in the Pleistocene, when sea levels were similar to those of today, and that the present shore platform is cutting into its interglacial predecessor.