THE LATE HOLOCENE MEKONG RIVER DELTA AS AN AUTOGENIC GRADE SYSTEM

The profile of a river that conveys sediment without net deposition and net erosion is referred to as ‘graded’ with respect to vertical aggradation of the river segment. Recent autostratigraphic experimental studies have brought a novel view that alluvial grade can be autogenically attained by an equilibrium response to a constant base level fall in a moving boundary setting where subaqueous basin slope is equal to alluvial slope. In a graded system, whether autogenic or allogenic, the autocyclic lateral shifting and avulsion of distributary channels tends to be intensely suppressed, and a graded alluvial channel can be stabilized and seldom moves transversely from that position. Here we make the first report of a natural autogenic grade system from stratigraphic records. As demonstrated below, the 3.5 ka to subrecent Mekong River delta satisfies an array of theoretical criteria for identifying autogenic grade(Table 1; Muto and Swenson, 2006). We here make the first successful identification of an autogenic grade system in the geological record: the late Holocene Mekong River delta. From 3.5 ka to subrecent, the delta exhibits peculiar features, including (1) no trace of significant sediment accumulation and erosion on the delta plain surface, (2)a delta plain surface with the same slope as the underlying shelf surface, (3) distributary channels that are stabilized in transverse directions but extend linearly basinward, and (4) a delta set thickness that matches a theoretical value. These features in combination are indicative of autogenic grade. Coastal dispersal of river-derived sediment by tides, waves, and ocean currents, as well as tectonic features and mangrove vegetation, may have contributed to the attainment and maintenance of grade. Ongoing drastic changes in sea level and human activities have caused the downstream Mekong River to become ungraded and unstable with a much higher risk of channel avulsion and shifting than in the past.