[HQR04-02] Fine-grained quartz OSL dating for reconstructing the Holocene evolution of the Mekong River floodplain
Keywords:OSL dating, floodplain, Mekong River, Holocene
Progradational avulsion and crevasse splay deposition promoted rapid aggradation before 7 ka. The abrupt cessation of aggradation is correlated to the lowering rate of eustatic sea-level rise. After 7 ka, fluvial deposition was limited near the river courses, and a hiatus between 7 and 4 ka was observed at one site. Ages obtained at the upstream end of an abandoned large distributary channel suggest that it was fully or partially abandoned at 3.5 ka. A previous study also indicated that another channel was abandoned at 4 ka. The relative sea-level fall since the middle to late Holocene in the area would have lowered the river bed and stabilized the distributary channels. A sharp precipitation decrease associated with the weakening of the EASM at 4 ka may have reduced the discharge of the Mekong River, promoting the channel abandonment. Upstream of Phnom Penh, the river actively migrated and a series of abandoned inner bank levees were preserved. The OSL ages obtained from the levee deposits are < 0.5 ka, which demonstrates rapid migration at least after 0.5 ka. The Digital Surface Model indicates that there are no older levees around the area. A human-induced increase in sediment discharge possibly promoted the rapid channel migration or older series of levees may have been eroded by recent channel migration.