DATING LATE PLEISTOCENE MARINE TERRACE SYSTEM IN SOUTHEASTERN KOREA

In the last few years, marine terrace system in southeastern Korea has been intensively investigated in order to evaluate the capability of Quaternary faults near the site of nuclear power plants and heavy industrial complexes. The threat from a particular tectonic source is principally defined by the recurrence interval and slip rate which should be measured on the basis of sound geochronological data. We tried OSL (optically stimulated luminescence) and radiocarbon measurements on the marine terrace deposits in southeastern Korea. Five to six marine terraces have been recognized in southeastern Korea, but among these, only the age of the two well-developed terraces (Q₂ and Q₃) can help us to evaluate the capability of the tectonic source because the last fault movement in this area is believed to have occurred between formation periods of these two terraces. Of the two terraces, the lower one (Q₂, c.20m above sea level) has been dated to be 48 to 71 ka by the OSL SAR (single-aliquot regenerative-dose) protocol using quartz grains, which is supported by radiocarbon age of around 35 ka for the overlain terrestrial deposits. We correlate the age of the Q₂ platform with 60 or 80 ka high sea stand (MIS 3 or 5a) considering the variation in pore water content over time. Although the difference in weathering degree of beach gravels clearly demonstrates the time gap between the lower and middle terraces, previously measured OSL age of the latter (Q₃, c.50m above sea level) was in the range of 20 to 90 ka with considerable scatter and stratigraphic inconsistency. However, the SAR protocol at 220^oC cut-heat treatment, instead of 160^oC as usually used, yields stratigraphically consistent OSL ages ranging from 77 to 104 ka for five samples collected from the Q₃ terrace. This result was obtained by considering saturated water content of the samples which should be still underestimation in comparison with that during deposition underwater, because now the clays formed by severe weathering of pebbles fill the spaces among the pebbles. Our new OSL results for the Q₃ terrace are supported by optical and radiocarbon dating of aeolian sand dunes deposited on the terrace platforms. We correlate the age of the Q₃ terrace with MIS 5e or 5c and it can be concluded that the last fault movement occurred during MIS 5a or 3 in the southeastern part of the Korean peninsula.