Reconstruction of bayhead delta in response to the Holocene sea-level changes in confined morphology, the Nakdong valley fill, southeastern Korea

Bayhead delta is one of the links between fluvial and estuarine system where terrestrial sediments are prograding into the estuary. Despite the critical position of the bayhead delta, the Holocene reconstructions of coastal linkage area are poorly documented based on bayhead delta data from direct cores. This study provides sedimentary characteristics, stratigraphic interpretations, and also a new concept of a full-filled bayhead delta. Five cores are acquired in the Nakdong valley fills consisting of late Pleistocene to Holocene deposits. Sixteen facies are defined and grouped into eight facies association; alluvial plain, fluvial channel and floodplain, oxidized fluvial channel, tidal flat, tidal bar, central basin and bayhead prodelta, bayhead delta front, and bayhead delta plain. The associations are compiled up three depositional units in the Nakdong valley fills: Unit 1, 2, and 3. Unit I consists of alluvial plain and fluvial channel in the lowermost part of cores. Unit II contains tidal flats, tidal bars, and central basin in an estuarine environment adjacent to Unit I. The topmost Unit III is bayhead delta deposits consisting of bayhead delta plain, bayhead delta front, and bayhead prodelta after the depositions of Unit II. Age data for the Nakdong valley fills, suggest that these depositional environments developed in depositional stages (before about 12 ka, 12 and 9 ka, 9 and 7 ka, 7 and 5 ka, 5 and 3, and after 3 ka). The sequence stratigraphy of the Nakdong valley fills can be divided into three systems tract; a lowstand systems tract (LST), a transgressive systems tract (TST), and highstand systems tract (HST). The uppermost deposits of the Nakdong valley show a bayhead deltaic succession string imprint of fluvial processes in an embayed environment. Most bayhead deltas are limitedly developed within the head of the estuarine systems, whereas the Nakdong deltaic deposits fully filled in the estuary system. This full-filled type is interpreted to represent that the relative rate of fluvial sediment input exceeds the rate of local accommodation creation.