Late Pleistocene to Holocene formation process of Mekong River Delta, South Vietnam

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Detail analysis of core samples from 11 bore holes in the Lower Mekong River Delta show the sedimentation during the postglacial transgression and the Holocene evolution of the Mekong River Delta. The incised valley that formed during LGM (the Last Glacial Maximum) was mostly filled during the rise of sca-level after the LGM. The interfluvial zones are about -20 m in altitude and are covered unconformably by Holocene deltaic sediments. The deltaic sediments and detailed chlonostratigraphic analyses clearly show the delta progradation for the last 4 kyr in terms of the sediment facies, tide influence, progradation rate, and subaerial and subaqueous topography. As the delta has evolved, the delta environment has become more wave-influenced with the development of a beach-ridge field, longshore sediment dispersal, and a steep face of the delta front. Seismic profiles get by uni-boom along Mekong River and channels show clearly the unconformity within the Upper Pleistocene sediments. The a … More lluvial sediments lying on the unconformity is generally 20 to 40 m in thickness, partly upto 70m in incised valley.Lower part of the Mekong River Delta is topographically characterized by development of several lines of active and relict beach sand ridges. The geomorphologic and sedimentologic features of these active and relict beach ridges were described primarily together with some ages by OSL dating from quartz grains of beach ridges. Samples for sedimento-logic and chlonologic analysis were collected from 8 lines of beach ridges and one boring core. Eight lines of relict and active sand ridges are observed to the south of Tra Vinh City. Although they are arranged generally parallel to the recent river and coast line, are distinctively developed in the eastern side of the island. Especially four lines inland converge to the south of the City, and elongated in the N-S direction parallel to the recent river. All beach ridges are diverged southward and westward to several narrow and low ridges. Some ages are gotten from relict beach sands by OSL dating method. Their ages are classified generally into two groups, that is, sands older than 4 kyr and ones younger than 1.2 kyr. Less