Incised-valley filling sedimentation in a small river valley of a wave-dominated, embayed coast in response to Holocene sea level rise, Yeongil Bay, Southeastern Korea

Abstract

Incised-valley infill deposits are closely related to sea level changes during the post-glacial period of the late Quaternary and are an important topic in sea level history. In Asia, there are many large river basins that receive large amounts of sediment from the land, and the infilling deposits of incised valley systems have been well studied. Yeongil Bay is a semi-enclosed, funnel-shaped topographic feature on the east coast of the Korean Peninsula, located in the southwestern part of the East Sea (Sea of Japan). The Hyeongsan River, a small river flowing into Yeongil Bay, is flows perpendicular to the East sea. During the last glacial maximum (LGM), when the sea level was >120–130 m lower than at present in the Korea Strait, inflowing river processes are thought to have formed incised valleys. In this study, the sediment depositional patterns of incised valley fill since the LGM were examined by analyzing the sedimentary facies, optically stimulated luminescence (OSL) dates, and radiocarbon dates by accelerated mass spectrometry (AMS) of three sediment cores in Yeongil Bay. The infilling sequence consists of sediments from channel lag (>12 ka) and meandering river (12–10 ka), wave-dominated estuary (12–7.8 ka), and wave-dominated delta (7.8–0 ka) systems in ascending order. The barrier appear only in the outer part of Yeongil Bay. The barrier emerged from eroding estuary systems at 8.5 ka. Millennial-scale isochrons based on OSL and radiocarbon dates showed that fluvial and estuary systems were deposited in retrogradation and delta systems were deposited in progradation since LGM. The modern delta of the Asian large rivers were initiated by a decreasing rate of sea level rise during the mid-Holocene, and consideration of this is important for RSL. This study emphasizes the importance of accommodation capacity as much as sediment discharge as a factor in the timing of delta initiation. This study provides important insights into the incised-valley infill deposits and past environmental conditions, and may help to gain a better understanding of how coastal environments have changed with sea level changes.