The Ages of Fault Activities of the Ilkwang Fault in Southeastern Korea, Revealed by Classification of Geomorphic Surfaces and Trench Survey

The Ilkwang Fault is NNE-striking, elongated 40 Km between Ulsan and Haendae-ku, Busan in southeastem part of the Korean Peninsula(Kim, D.H. et al., 1989; Kim, J.S. et al., 2003). This paper is mainly concemed about the ages of the fault activities especially in the Quatemary, infered from classification of geomorphic surface and trench excavation for the construction of Singori nuclear power plant. The geomorphi surfaces are classified into the Beach and the Alluvial plain, the 10 m a.s.l. Marine terrace, the 20 m a.s.l. Marine terrace, the Reworked surface of 45 m a.s.l. Marine terrace and the Low relief erosional surface, from lower to higher altitude. The Beach and the Alluvial plain are elongated to the Holocene terrace(ist terrace, choi, 2003). The 10 m a.s.l. Marine terrace is correlated to 2nd terrace (MIS 5em 125 Ka. y. B.P., Choi, 1998). The 45 m a.s.l. Marine terace is correlated to the Lower marine terrace (MIS 7,220 Ka. y. B.P., Choi, 2003 or MIS 9,320 y. B.P.) to the Gwanganri terrace(Penultimate interglacial age, 200-200 Ka. Y. B.P., Oh, 1981). The Low relief erosional surface is distributed coastal side, the Reworked surface of 45 m a.s.l. Marine terrace inland side by the Ilkwang Fault Line as the boundary line. But the former is above 10 m higher in relative height than the latter. The 20 m a.s.l. Marine terrace on the elongation line of the Ilkwang Fault reveals no dislocation. A site was trenched on the straight contract line with $N30^{\circ}$ E-striking between the 10 m a.s.l. Marine terrace and the 20 m a.s.l. Marine terrace. Fault line or dislocation was not observable in the trench excavation. Accordingly, the straight contact line is inferred as the ancient shoreline of the 10 m a.s.l. Marine terrace. The Ages of the Fault activities are inferred after the formation of the Ichonri Formation - before the formation of the 45 m a.s.l. Marine terrace (220 Ka. y. B.P. or 320 Ka. y. B.P.). The Low relief erosional surface was an island above the sea-level during the formation of the 45 m a.s.l. Marine terrace in the paleogeography.