Evolution of holocene depositional environmentsin the coastal area fromthe Tien river tothe Hau river mouths

Abstract

The Holocene coastal zone of Mekong river plain is the result of prolonged marine-fluvial interaction. Lithofacies association in ti me and space is characterized by three depositional system tract belonged to the upper part of a sequence stratigraphy. Based on lithology should be divided 5 sedimentary types and 18 lithofacies distributed in stratigraphical column and in sea bottom varying from 25m water depth to mainland coastal area. According to sequence stratigraphy the transgressive systems tract at 5 Ky Bp, while from geochronology point of view the boundary betweenMiddle Holocene and Late Holocene is 3 Ky Bp – a regressi ve stage. During Early-Middle Holocene stage transgressive depositional system tract is characterized by two associated lithofacies upward section: delta front swamp mud rich in organic materials facies and marine shallow grey-greenish clay facies corresponded with marine flooding plain. And then Late Hol ocene regressive phase corresponding with Highstand systems tract composed of delta plain clayish silt facies in which there are different sandy ridges generations distributed younger seaward. Each sand ridge generation was mark by a coastal zone and associated lithofacies. In circumstance of global climate change and sea-level rising, the Mekong ri ver coastal zone will be changed much more in framework of modern tectonic subsidence. If the rate of sea-level rising is 2mm/year then sea bed will be subsided with a rate of 4mm/year. But recent rate of sediment accumulation is over 4mm/ year, so the modern coastline continue to pr ograde seaward with a rate of 40m/ year. The Mekong river mouths are migrating to East-North, and as a result geosystems and landscapes are changing.