161 / 2019-07-01 14:40:01

Autogenic grade: Theory, experiments, and field application

×Autostratigraphy, Grade, Base level, Experiment, Mekong River delta

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. Grade is a long-term, dynamic equilibrium state of the river in terms of sediment balance. A correct understanding of grade is fundamental to stratigraphy, because grade represents the critical condition that discriminates between aggradational and degradational regimes in a river system, and also because grade is a key to the exploration of fluvial response to base-level forcing. It has been a common assumption in stratigraphic studies that as long as the base level remains stationary, the alluvial river feeding the delta eventually attains a graded state. Recent theoretical and physical modeling in autostratigraphy has brought a new view of grade that in downstream alluvial rivers it 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. Criteria to identify an autogenically-graded profile in stratigraphic records include the following features: (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. The 3.5ka−subrecent Mekong River delta, satisfying these criteria, perhaps developed as an autogenic grade system. An implication from the detection of autogenic grade from the late Holocene strata is that autogenically-graded alluvial systems are much more probable and detect¬able in stratigraphic records than ever expected. In case of the Mekong River delta, coastal dispersal of river-derived sediment by tides, waves, and ocean currents, as well as tectonic features and mangrove vegetation, may also have contributed to the attainment and maintenance of grade.