Delta Evolution Model Inferred from the Holocene Mekong Delta, Southern Vietnam
Susumu Tanabe, Thi Kim Oanh Ta, Van Lap Nguyen, Masaaki Tateishi, Iwao Kobayashi, Yoshiki Saito, 2003. "Delta Evolution Model Inferred from the Holocene Mekong Delta, Southern Vietnam", Tropical Deltas of Southeast Asia—Sedimentology, Stratigraphy, and Petroleum Geology, F. Hasan Sidi, Dag Nummedal, Patrice Imbert, Herman Darman, Henry W. Posamentier
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The Holocene Mekong Delta is a good example of a delta that has evolved from a tide-dominated to a tide–wave mixed-type delta in its shape, sediment facies, and progradation rate during the regression after the maximum flooding during the Holocene. This evolution indicates that tide-dominated deltas developed well in the early phase of the highstand systems tract and that they changed into more wave-dominated deltas in response to subsequent regression.
The Mekong Delta, which is located at the southeast tip of the Indochina Peninsula, is one of the largest deltas in Southeast Asia, with a large delta plain ranked third largest in the world. The present Mekong Delta is classified as a tide-dominated delta, but close to a wave-dominated delta, in the triangular deltaic depositional classification system. Two rotary-drilled cores, VL1 and TV1, were obtained from the flood plain and strand plain, respectively, of the delta plain. Detailed core descriptions, high-resolution 14C dating by accelerator mass spectrometry (AMS), and topographical analysis of the delta plain indicate that the Mekong Delta evolved from a tide-dominated delta from 6.5 to 2.5 ka to a tide–wave mixed delta from 2.5 ka to the present. The present delta is characterized by beach ridges on the strand plain, dominantly wave-influenced sediments, and a low progradation rate due to longshore sediment dispersal by waves.
As the sea level rises, marine flooding causes coastal inundation and the formation of embayments, which amplifies tides and shelters deltas from waves, so that they are more tide dominated. However, as a delta progrades and its shoreline migrates seaward, the resultant smoothed coastline, narrow shelf, and increased wave energy create a more wave-dominated coastal setting. This change will become more pronounced as the delta continues to prograde towards shelf margins.
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It is the objective of this volume to bring to the fore a category of deltas with which many sedimentologists and stratigraphers are, at best, vaguely familiar. It is expected that this volume also will stimulate new research on tropical deltas by highlighting how their facies and stratigraphic architectures differ from mid- and high-latitude ones, by emphasizing their significance to the global sediment budget, and by stressing their uniqueness within a petroleum systems framework. This special publication emphasizes the need for models intrinsic to tropical deltas of Southeast Asia to supplement the more conventional general models currently in vogue, based on past studies of large and small mid-latitude deltas. The papers in this book explore how the combination of these complex factors has shaped deltas in this region. Sedimentological surprises such as distributary channels floored by thick accumulations of fluid mud lend a bit of “mystery” to tropical deltas. We hope that, rather than being merely a summary of tropical deltas, this book may open the door to a new and active phase of sedimentological and stratigraphic research in tropical environments across the globe.