A Preliminary Study of the Depositional Environment of a Neotectonic Sedimentary Basin in the Korean Peninsula

The formation of post-Miocene sedimentary basins in the Korean Peninsula has not been well understood, largely due to the lack of terrestrial stratigraphic evidence. However, recent studies conducted in the eastern offshore area of the Korean Peninsula have identified a previously unrecognized post-Miocene tectonic process by which piggyback basins, hereafter referred to as Neotectonic Basins, were formed. The Bukpyeong Basin is known as a terrestrial Tertiary basin located in the eastern coast of the Korean Peninsula, but its detailed post-Miocene tectonic and depositional history remains uncertain. This study presents a preliminary reconstruction of the depositional environment of the Bukpyeong Basin. Based on sedimentary features, we classified the semi- or unconsolidated basinfills of the Bukpyeong Basin into twelve sedimentary facies and grouped them into the five facies associations. Temporal variation in facies associations indicates that the depositional environment of the Bukpyeong Basin evolved from a river-mouth channel to a subaqueous environment, probably a lake, lagoon, or estuary, in response to the base-level rise. Subsequently, such a low-energy environment progressively shifted to a fluvial system characterized by small channels and a floodplain, in response to the base-level fall. We infer that the base-level fluctuations recorded in the basinfills were likely related to major tectonic events in the East Sea (Sea of Japan), such as the Miocene pull-apart extension or the post-Miocene compression. To clarify which tectonic events controlled the changes in depositional environment, additional microfossil analyses of the basinfills are required to constrain the precise depositional ages.