Dan Matsumoto

Origin and evolution of interdistributary delta plains; insights from Mekong River delta

The geometry of river deltas is considered to refl ect the interplay of coastal processes, river discharge, and sediment supply. As ~25% of the world’s population lives on deltaic lowlands, prediction of delta growth is critical. Knowledge of processes responsible for delta geometries is not well established, although such knowledge is critical for the risk management of land use and settlements on deltas. The Mekong River delta of southern Vietnam is one of the largest deltas in the world and offers a unique opportunity to understand the sedimentary evolution of mixed tide- and wave-dominated large river deltas. We constrained the three-dimensional sedimentary evolution of the delta plains, based on optically stimulated luminescence dating of beach ridges coupled with radiocarbon-dated sediment cores. Results show that the beach shoreline in the lower delta plain initiated ca. 3.5 ka by aggradation on basement shoals. The delta plain propagated laterally during the late Holocene, evolving from bars that resulted in asymmetric bifurcation of the river mouth. Asymmetry was caused by the southwestward longshore sediment drift enhanced by the winter monsoon. Bars were successively formed on the wider side of the bifurcated river mouths, and subsequently accreted seaward, being stabilized by tide effects to result in shore-perpendicular elongate delta plains. Our work, based on the Mekong delta, demonstrates for the fi rst time that bar emergence is a key process in the long-term evolution of mixed tide- and wave-dominated deltas.

Mid- to late-Holocene marine inundations inferred from coastal deposits facing the Nankai Trough in Nankoku, Kochi Prefecture, southern Japan

This study investigates the Holocene sedimentary history of a small coastal lowland in Nankoku, Kochi Prefecture, on the coast of southern Japan facing the Nankai Trough. The sedimentary fill of the lowland area consists mainly of marine-brackish clay overlain by beds of freshwater clay and peat. We found four laterally extensive sand sheets, one directly underlying the freshwater deposits and the other three interbedded with them. Radiocarbon dates show that these sand sheets were deposited between 5970 and 2440 cal. BP. Although the sand sheets contained few marine-brackish diatoms, they were concentrated in the seaward part of the study site, suggesting that they were deposited by marine inundations. These sand sheets were formed as a result of tsunamis or unusually large storm surges. The apparent frequency of marine inundations during 5970–2440 cal. BP was much lower than that of megathrust earthquakes along the Nankai Trough recorded during the last 1300 years. Event deposits were absent between 2440 and 960 cal. BP, a gap that we attribute to the development of beach ridges. The new marine inundation records reported here will aid efforts to reconstruct the timing and recurrence intervals of megathrust earthquakes in the western Nankai Trough.