Chronostratigraphic and palaeogeographic interpretation of Nakdong deltaic sequences in the south-eastern Korean Peninsula

Abstract

We investigated the chronostratigraphy of six boreholes in the Nakdong River delta area in South Korea based on optically stimulated luminescence (OSL) and 14C dating. The standardised growth curves (SGCs) produced from single and multiple boreholes performed well in determining low equivalent dose (De; < 40 Gy). 14C dates from shell and wood fragments were mostly representative of depositional time. Based on 14C and OSL ages, the lowermost fluvial sediments showed relatively low sediment accumulation rates (0.1–0.8 m/kyr−1) prior to 12 ka. A hiatal discontinuity ranging from 12 to a maximum of 50 ka resulted in a break in sedimentation between the Last Glacial Maximum and the Holocene. A thick fluvio-estuarine unit in the lower part indicated relatively high accumulation rates (3.3–9.4 m/kyr−1) from 12 to 9 ka, which corresponded to rapid sea level rise. In the middle part, decreased accumulation rates (1.7–5.5 m/kyr−1) from approximately 9 to 6 ka were interpreted as transgressive deepening. Lower accumulation rates (0.9–3.2 m/kyr−1) during the middle to late Holocene were caused by relative sea level fall and a horizontally extended sedimentation area. In the upper part, the highest sediment accumulation rates (3.5–8.3 m/kyr−1) and rapid delta progradation during the last 2.0 ka might have been caused by widespread human influence and monsoonal climatic change. Isochrons of 1-kyr scale were identified based on the lithologic boundary and age results for each borehole, which represent palaeoshoreline positions during the transgressive cycle and boundaries of considerable delta progradation.