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

초록

We investigated the chronostratigraphy of six boreholes in the Nakdong River delta area in South Korea based on optically stimulated luminescence (OSL) and C-14 dating. The standardised growth curves (SGCs) produced from single and multiple boreholes performed well in determining low equivalent dose (D-e; < 40 Gy). C-14 dates from shell and wood fragments were mostly representative of depositional time. Based on C-14 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.

키워드