The formation of concealed firmground in open-coast macrotidal flats, southwestern coast of Korea: its sedimentological and ichnological characteristics

Since wave-dominated open-coast tidal flats have sedimentologically strong similarity with typical wave-dominated shorefaces, it is very difficult to distinguish between them in ancient records. Many-year field observations along the southwestern Korean tidal flats have revealed that autogenically generated firmground muds are frequently exposed on the high-tide beach face, as a result of swash-bar migration/welding. Observed ichnology in the firmground muds have clearly shown that ichnocoenoses across the intertidal swash bars are in good agreement with the preserved firmground-mud traces, reflecting different substrate consistencies at the time of colonization. Unlined deep-tier firmground burrows, including Psilonichnus and Thalassinoides, typically crosscut the pre-existing softground suites including Skolithos-, Arenicolites- and Monocraterion-like burrows. Due to the fact that such firmground muds and the autogenic Glossifungites Ichnofacies are very uncommon in a normal wave-dominated shoreface to beach setting, the presence of firmground muds on high-energy beach face may indicate a very specific depositional environment with significant tides and considerable wave energy (i.e., open-coast tidal flats). In this context, the study results would aid in the recognition of wave-dominated tidal flat in ancient deposits.