U-Th Dating of Holocene Sea Level Highstands Using Fringing Coral Reefs along the Vietnamese Coast

The U-Th dating technique by thermal ionization mass spectrometry (TIMS) was used to determine the ages of 27 massive coral samples of mainly Porites sp. collected from fossil reef terrace on the Vietnamese coast. Four episodes of coral growth were uncovered in these transects: 8.8 – 8.0; 7.2 – 7.0; 3.6 – 3.0; and 2.5 – 2.0 ka BP; indicating that the sea level highstands in the Holocene were about 1-3 m above the present low tidal level. The last three episodes are well correlated with the previous data in the South China Sea, whereas the first episode remains questionable because the sea level at > 8000 years ago is expected to be 10 – 20 m below the present sea level. However, it is indicatable that coral blocks were dislogded and transported landwards from the coral terrace at the depth of 20-25 m below the modern sea level by high-energy wave events such as tsunamis. Age determination of fossil corals from other transects are in progress to address this issue